Psychopharmacology PDF

UNIT 1. Structure and Function of the Nervous tirsdag 10. september 2024 16:01 Structure and Function of the Nervous System 1. Cells of the Nervous System 2. Electrical Transmission within a Neuron 3. Organization of the Nervous System UNIT 1 Structure and Function...

UNIT 1. Structure and Function of the Nervous tirsdag 10. september 2024 16:01 Structure and Function of the Nervous System 1. Cells of the Nervous System 2. Electrical Transmission within a Neuron 3. Organization of the Nervous System UNIT 1 Structure and Function of the NS Page 1 UNIT 1 Structure and Function of the NS Page 2 1. Cells of the Nervous System onsdag 11. september 2024 19:04 CELLS OF THE NERVOUS SYSTEM Cells of the Nervous System Stem cells are undifferentiated cells that have the ability to proliferate and the capacity to become specific cell types. Embryonic stem cells are derived from very early-stage embryos and are valuable as research tools and potential treatments for many diseases. Embryonic stem cells that form the nervous system become two primary cell types: Neurons: Transmit information in the form of electrical signaling. Glial cells: Provide metabolic support, protection, and insulation for neurons. Types of Glial Cells in the CNS This image shows the four different types of glial cells found in the central nervous system: Ependymal cells (light pink) The outer layer, and it is touching the CSF Astrocytes (green) Microglial cells (red) Oligodendrocytes (light blue; functionally similar to Schwann cells in the PNS). By Artwork by Holly Fischer http://open.umich.edu/education/med/resources/second-look-series/materials - CNS Slide 4, CC BY 3.0 Types of Neurons in the CNS In the CNS, there are different types of neurons: UNIT 1 Structure and Function of the NS Page 3 In the CNS, there are different types of neurons: Motor neurons: Direct biobehavioral responses appropriate for the situation. Sensory neurons: Convert physical stimuli into electrical signals. Interneurons: Neurons in the brain and spinal cord; form interacting neural circuits responsible for conscious sensations, recognition, memory, decision-making, and cognition. Neuron Structure Neurons have three parts: Soma: The cell body that contains the nucleus and other organelles. Dendrites: Projections from the soma that receive information. Axon: An extension that conducts electrical signals from the cell body to the terminal buttons. Dendrites and soma receive information from other cells across the gap between them, the synapse. There are thousands of receptors on the dendrites and soma. Effect on receiving cell may be excitatory or inhibitory. Convergence: A neuron receives and integrates vast amounts of information from many cells. Divergence: The integrated information can be transmitted to a few or thousands of other neurons. UNIT 1 Structure and Function of the NS Page 4 Dendrites are covered with short dendritic spines that increase surface area. Dendrites and their spines are constantly modified and can change shape rapidly in response to changes in synaptic transmission. Prolonged synaptic activity can also change the number, size, and shape of dendritic spines, supporting lifelong learning and new associations. Axonal Transmission Axons transmit electrical signals from the axon hillock (at the soma) to the terminals. A neuron usually has one axon, but it may branch to form axon collaterals. Terminal buttons have synaptic vesicles containing neurotransmitter chemicals. Most axons are wrapped with a myelin sheath, a fatty insulating coating created by layers of glial cells. *The thicker the myelin sheath, the better it insulates the axon, reducing the loss of electrical current. This helps maintain the strength and speed of the signal as it travels. Myelin prevents ions from leaking out of the axon, which helps to preserve the electrical signal. In a thicker myelin sheath, this insulation is even more effective, reducing the energy needed to regenerate the signal and allowing it to move more efficiently. UNIT 1 Structure and Function of the NS Page 5 regenerate the signal and allowing it to move more efficiently. With thicker myelin, the distance between the Nodes of Ranvier is greater, allowing the signal to jump further between nodes. This reduces the number of times the signal needs to regenerate, making the process faster. Nodes of Ranvier are breaks in the myelin sheath where action potentials are regenerated. The myelin sheath increases the speed of conduction along the axon; the thicker the myelin, the faster the conduction. Cell Membrane Proteins Many cell membrane proteins are: Receptors: The initial sites of action for neurotransmitters, hormones, and drugs. Enzymes: Catalyze biochemical reactions. (meaning they help speed up chemical reactions inside or near the membrane. These reactions might involve breaking down or building up molecules, aiding in processes like digestion, metabolism, or cellular repair.) Ion channels and transporters: Facilitate the movement of charged molecules such as amino acids, glucose, and metabolic products. Ion Channels Ions such as potassium (K+), sodium (Na+), chloride (Cl–), and calcium (Ca2+), necessary for neuron function, can move only through ion channels. Ion channels are specific for one or a few ions. Gated channels are normally closed and open in response to specific stimuli: Ligand-gated channels: Open when a ligand binds to a receptor. UNIT 1 Structure and Function of the NS Page 6  Voltage-gated channels: Open when the electrical potential across the membrane is altered. Neuronal Energy Requirements Neurons require large amounts of ATP, so mitochondria are especially abundant where energy needs are greatest. The rate of ATP synthesis reflects neuron activity, which can be visualized using techniques that show brain function. ATP also functions as an extracellular signaling molecule between neurons and glia. Functions of Glial Cells UNIT 1 Structure and Function of the NS Page 7 Oligodendroglia are more efficient UNIT 1 Structure and Function of the NS Page 8 2. Electrical Transmission Within a Neuron torsdag 12. september 2024 12:31 ELECTRICAL TRANSMISSION WITHIN A NEURON Membrane Permeability The membrane is selectively permeable, allowing some ions to move in and out of the membrane via ion channels. The resting membrane potential is the difference between the electrical potential inside and outside of the neuron. Electrically charged molecules (ions) can be either positively or negatively charged. When a neuron is at "rest" (not sending a signal), its membrane potential is –70 millivolts (mV), meaning the inside of the cell is polarized. There are more negative ions (and amino acids) inside the cell, while there are more positive ions outside. Ion Channel Function Most ion channels are gated, but some K+ channels are not, allowing K+ to move freely. K+ moves into the cell due to electrostatic pressure (attraction to negatively charged particles) K+ moves back out when its concentration rises (concentration gradient). The equilibrium potential for potassium is reached when these forces are balanced. The membrane potential still at –70 mV. The Na+–K+ pump moves 3 Na+ out of the cell for every 2 K+ moved in, maintaining the resting potential. This process requires energy as ions are pumped against their concentration gradients. UNIT 1 Structure and Function of the NS Page 9 Action Potential (AP) An action potential (AP) is a rapid change in membrane potential that propagates down the length of the axon. The threshold for firing is –50 mV, at which point voltage-gated Na+ channels open, generating a rapid change in membrane potential. Neurotransmitters act on the postsynaptic membrane, the effects are called: Excitatory postsynaptic potentials (EPSPs): Increase the likelihood of firing. Inhibitory postsynaptic potentials (IPSPs): Decrease the likelihood of firing. APs are "all or none" events, meaning their size is unrelated to the amount of stimulation. AP moves along the axon because Na+ ions spread passively to nearby regions, which change the membrane potential to threshold, which opens more Na+ channels. Local Potentials Local potentials: Ion channels open momentarily, causing small, local changes in ion distribution and potential differences. If Na+ channels open, Na+ enters the cell and causes local depolarization. If Cl– channels open, Cl– enters the cell and causes hyperpolarization (inhibitory). UNIT 1 Structure and Function of the NS Page 10 Local potentials are graded—the larger the stimulus, the greater the magnitude of hyperpolarization or depolarization. They also show summation, or integration—several small depolarizations can add up to larger changes in membrane potential. If hyperpolarization and depolarization occur simultaneously, they cancel each other out. Steps of Action Potential 1. Resting potential: K+ channels open and Na+ channels closed (membrane potential = –70 mV). 2. If the summation of local potentials reaches the threshold, Na+ channels open and Na+ rushes into the cell. 3. Na+ change in membrane potential from –50 mV to +40 mV, creating an action potential. 4. Na+ channels close and cannot be opened for a fixed period (absolute refractory period). 5. Rising phase: Changing membrane potential opens voltage-gated K+ channels, and K+ moves out of the cell, returning the membrane to resting potential. 6. Relative refractory period: The membrane overshoots resting potential and is hyperpolarized until excess K+ diffuses away. During this time, it is more difficult to generate another action potential. UNIT 1 Structure and Function of the NS Page 11 Saltatory Conduction In myelinated axons, regeneration of the action potential occurs only at the nodes of Ranvier. The conduction seems to jump along the axon—this is known as saltatory conduction. Some drugs alter action potential conductance by blocking voltage-gated Na+ channels (e.g., Novocain). Local vs. Action Potentials Local potentials are graded and vary based on stimulus strength, while action potentials are all-or- none events and do not vary in size. Electrical Transmission within a Neuron: Questions UNIT 1 Structure and Function of the NS Page 12 UNIT 1 Structure and Function of the NS Page 13 3. Organization of the Nervous System torsdag 12. september 2024 12:32 ORGANIZATION OF THE NERVOUS SYSTEM Central and Peripheral Nervous Systems (CNS and PNS) Peripheral Nervous System (PNS) *Taken from physiology content Many psychotherapeutic drugs alter norepinephrine or acetylcholine in the brain to relieve symptoms. However, altering these neurotransmitters in the peripheral nerves can produce side effects, such UNIT 1 Structure and Function of the NS Page 14 However, altering these neurotransmitters in the peripheral nerves can produce side effects, such as elevated blood pressure, dry mouth, and urinary problems. CNS and PNS Spinal cord: gray matter contains neuron cell bodies; white matter is myelinated axons. Where it enters the skull, it enlarges into the brain stem—medulla, pons, and midbrain. Spinal nerves carry both sensory and motor information (mixed nerves). Sensory afferent nerves carry information to the spinal cord; soma are in the dorsal root ganglia. Motor efferent nerves result in voluntary movements; soma are in ventral horn. Central Nervous System (CNS) Meninges: layers of tissue that cover and protect the brain and spinal cord. ○ Dura mater: outermost layer; toughest. ○ Arachnoid has a weblike sublayer (the subarachnoid space) filled with cerebrospinal fluid (CSF) ○ Pia mater: thin layer on top of nerve tissue. The Meninges: Neuroanatomy Video Lab - Brain Dissections CSF surrounds brain and spinal cord, fills the cerebral ventricles and the central channel in the spinal cord. It protects the brain and aids exchange of nutrients and waste products between brain and blood. CSF is formed in the choroid plexus. UNIT 1 Structure and Function of the NS Page 15 CSF is formed in the choroid plexus. Divisions of the CNS (embryonic) Understanding brain development is important in understanding effects of drugs or toxins at different stages. ○ Massive proliferation of neurons begins at week 5 after conception neurogenesis ○ 6 to 24 weeks after conception: neuronal migration (different brain structures are formed) ○ At 23rd prenatal week: connections between neurons begin to form ○ Birth: 100 billion neurons ○ https://www.youtube.com/watch?v=XdN9i_ZWGho The CNS begins as the neural tube, and develops into telencephalon, diencephalon, forebrain, midbrain, hindbrain, and spinal cord Divisions of the CNS (adult) UNIT 1 Structure and Function of the NS Page 16 Directions in the nervous system UNIT 1 Structure and Function of the NS Page 17 Questions: UNIT 1 Structure and Function of the NS Page 18 UNIT 2 Research Methods in Neuropsychopharmacology onsdag 18. september 2024 11:09 RESEARCH METHODS FOR EVALUATING THE BRAIN AND BEHAVIOR Techniques in Behavioral Pharmacology Evaluating Animal Behavior Techniques in Neuropharmacology Multiple Neurobiological Techniques for Assessing the CNS Neuroscience: Multidisciplinary field looks at the structure and function of the human brain and nervous system. Understanding the Brain: Requires coordinated effort from many disciplines as: cellular and molecular biology, anatomy and physiology, human behavior and cognition, biochemistry, neuropharmacology, neuroanatomy, endocrinology, computer science, and others. Behavior, mood, and cognitive function are the focus of neuropsychopharmacology. It is important to understand and critically evaluate the techniques used to quantify behavioral changes. Scientists must be able to evaluate the relationship between experimental manipulations and changes in behavior. Behavior of the experimental treatment group is compared with that of placebo control subjects. EVALUATING ANMIMAL BEHAVIOR Behavioral measures are crucial for: ○ Understanding the neurochemical basis of behavior, and drug-induced changes in that behavior. ○ Developing animal models of psychiatric disorders. ○ Screening newly designed drugs in preclinical pharmaceutical settings. Studies using animals because: ○ Rigorous control of living conditions, food, etc. is possible. ○ Histories and genetic backgrounds are well known. ○ Many research methods are unethical with human subjects. Brains and behaviors of nonhuman mammals and humans are similar enough to allow generalization across species. Results from controlled animal studies can validate correlational relationships. Example: Data collected on alcohol consumption during pregnancy and occurrence of fetal alcohol syndrome (FAS) suggest a relationship, but does not show that alcohol causes FAS. Research institutions have animal care committees that review protocols: ○ Three “R-s” in animal research: i. Replacement ii. Reduction iii. Refinement Research should be relevant to human or animal health, advancement of knowledge, or UNIT 2 Research Methods in Neuropsychopharmacology Page 19  ○ Research should be relevant to human or animal health, advancement of knowledge, or the good of society. ○ Alternative methods that do not require animal subjects must be considered. ○ Procedures must avoid or minimize discomfort, distress, and pain. Animal tests with high face validity—tests closely resemble those used on humans (e.g., blood pressure). To compare symptoms normally described in human terms, such as guilt, delusions, altered mood etc., a quantifiable measure of animal behavior is substituted (low face validity). Predictive validity—if the drug effects observed in laboratory tests closely parallel or predict the clinical effect. Construct validity—extent to which the animal measurement tool actually measures the characteristic being investigated. An optimal animal behavioral test should: ○ Have predictive validity. ○ Be specific for the class of drug being screened. ○ Be sensitive to a normal therapeutic range of dose and show a dose–response relationship. ○ Demonstrate the same rank order of potency in the animal test as the order of potency of the therapeutic action of the drugs. ○ Have high reliability—the same results will be recorded each time the test is used. Behavioral Tests: ○ Simple observation of tremors, drooping eyelids, salivation, catalepsy (e.g., from motor side effects), etc. ○ Measures of motor behavior: identify drugs that produce sleep, loss of coordination, or drugs that stimulate activity. ○ Measures of emotion-like behavior: anxiety-like behavior, depression-like behavior, fear- like behavior, etc. ○ Measures of cognition: effects on learning & memory test, flexibility and inhibition test. Operant Conditioning: ○ Operant conditioning → animal learns a response to obtain rewards. ○ In a training session, the animal learns the response (e.g., pressing a lever) to receive a food reinforcer. ○ Once the behavior is learned the requirements for reinforcement can be altered according to a schedule of reinforcement. ○ In a fixed-ratio (FR) schedule, reinforcement is delivered after a fixed number of responses. ○ Fixed-interval (FI) schedule—reinforcement after a fixed amount of time. Rat in an Operant Chamber UNIT 2 Research Methods in Neuropsychopharmacology Page 20 Image description: A rat in an operant chamber. MEASURES OF ANALGESIA Measures of analgesia (reduction of perceived pain without loss of consciousness). Tail-flick test measures response of the animal to heat applied to the tail. Time between onset of stimulus and animal’s removal of its tail is assumed to be correlated with pain intensity. Tail-Flick Test of Analgesia Image description: A rat in a tail-flick test of analgesia. Measures of Analgesia: A variation of the operant fixed ratio schedule uses negative reinforcement. This increases the probability of a response that terminates an aversive condition. The technique can be applied to operant analgesia testing. The animal is trained to turn off a foot shock by pressing the lever. In testing, the lowest shock intensity at which the animal first presses is considered the aversive threshold. Analgesic drugs would be expected to raise the threshold. MEASURES OF LEARNING AND MEMORY ○ Mazes: ▪ Mazes have a start box at the beginning of an alley with one (T-maze) or more (multiple T-maze) choice points that lead to the goal box, which contains food or other reward. ▪ Drugs may alter the animal’s memory or behavior, but can also make it more or less hungry, disoriented, etc. ○ Radial Arm Maze: ▪ A radial arm maze has a piece of food at the end of each arm. UNIT 2 Research Methods in Neuropsychopharmacology Page 21  ▪ A radial arm maze has a piece of food at the end of each arm. ▪ Normal rats learn to visit each arm only once on a given day, indicating effective spatial memory (working memory). ▪ Drugs can interfere with spatial memory. Radial Arm Maze Image description: A rat in a radial arm maze. ○ Morris Water Maze: ▪ A circular pool of water made opaque by addition of milk or a dye. ▪ Animals must swim until they find the escape platform hidden from view just below the water surface. ▪ The animals learn the spatial position of the platform relative to landmarks outside the pool. Morris Water Maze Image description: A rat in a Morris Water Maze. ○ Delayed-Response Test: ▪ Reward is placed in one of two food wells, the food wells are covered, and the subject cannot see where the reward is hidden. ▪ After a delay period of several seconds, the subject is required to select the correct food well. ▪ Location of reward varies randomly from trial to trial, the subject needs to remember the spatial information specific for the bait location and use this information to make a correct choice The Delayed-Response Task UNIT 2 Research Methods in Neuropsychopharmacology Page 22 The Delayed-Response Task Image description: A monkey performing the delayed-response test. MEASURES OF ANXIETY Most tests use induced fear as an analogy to human anxiety, using unconditioned animal reactions or traditional learning. Light-Dark Box Test Rodents naturally avoid brightly lit places. Uses a box with one side brightly lit and the other side dark. Measures include number of crossings, time spent on each side, and total motor activity. Open Field Test Rodents naturally avoid open places. Animal is allowed to explore a novel and brightly lit test chamber. Rodents typically stay near the walls. Antianxiety drugs increase the amount of time spent in the unprotected center. Elevated Plus-Maze Cross-shaped maze raised more or less 50 cm off the floor. Two arms are open (normally avoided because of aversion to heights) and two arms have enclosed sides. Antianxiety drugs increase amount of exploration of the open arms; drugs that increase anxiety decrease exploration of open arms. Novelty Suppressed Feeding Paradigm Animals are presented their usual food in a new, potentially threatening environment, or are provided novel food in their usual environment. In either case, the novelty prolongs the latency to begin eating. Ultrasonic Vocalizations Very young rats make ultrasonic vocalizations (which can be measured) when separated from their mother. Antianxiety drugs suppress vocalizations. *Less common test Geller-Seifter Conflict Test Animal learns a behavior in an operant chamber, then the reinforcer (food) is accompanied by a foot shock—producing a “conflict” for the animal. Antianxiety drugs have no effect on the reinforced schedule but increase lever pressing during the conflict procedure. *Less common test MEASURES OF FEAR Conditioned Emotional Response (Freezing Response) Presentation of a signal (a light or tone) is followed by an unavoidable electric shock. After conditioning, the signal will suppress behavior (“freezing”). Fear-Potentiated Startle UNIT 2 Research Methods in Neuropsychopharmacology Page 23 Fear-Potentiated Startle Enhancement of basic startle response when the stimulus is preceded by presentation of a conditioned fear stimulus. Example: If a light has been paired with a foot shock, presentation of the light increases the magnitude of the startle response to a novel stimulus, such as a loud clap. MEASURES OF DEPRESSION Forced Swimming Test (Porsolt Test) Rodents naturally avoid water; they are placed in an inescapable transparent tank that is filled with water and their escape-related mobility behavior is measured. Used for evaluation of antidepressant drugs, antidepressant efficacy of new compounds, and experimental manipulations that are aimed at rendering or preventing depressive-like states. Escape-related mobility behavior increasing with antidepressant drug administration. Grooming Test Self-grooming is a complex innate behavior with an evolutionary conserved sequencing pattern and is one of the most frequently performed behavioral activities in rodents. Saccharose substance is sprayed on rodents’ fur and self-grooming behavior is measured. Self-grooming behavior increasing with antidepressant drug administration. Expected self-grooming behavior in rodent models of neuropsychiatric and neurodegenerative disorders Tail Suspension Test Rodents (mostly mice) are suspended by their tails with tape, in such a position that it cannot escape or hold on to nearby surfaces. The resulting escape-oriented behaviors are quantified (5-6 min). Useful in the screening of potential antidepressant drugs, and assessing other manipulations that are expected to affect depression-related behaviors. Escape-oriented behaviors increasing with antidepressant drug administration. ASSESSING DRUG REWARD AND REINFORCEMENT Operant Conditioning FR (Fixed Ratio) schedules have been used to identify drugs capable of inducing dependence. We assume that if an animal will press a lever to receive an injection of drug, the drug must have reinforcing properties. UNIT 2 Research Methods in Neuropsychopharmacology Page 24  have reinforcing properties. The drug self-administration method is an accurate indicator of abuse potential in humans. Animals can also be given a choice between two drugs with two different levers. Operant Conditioning Varying the schedule of reinforcement determines how hard the animal will work to get the injection. Breaking point: point at which effort required exceeds reinforcing value—the higher the breaking point, the higher the abuse potential. Link to additional resource: https://www.jove.com/science-education/5427/self- Drug Self-Administration Method Image description: A rat in drug self-administration chamber. Drug Discrimination Test With conditioning, the animal learns to discriminate between drug and saline treatment based on internal cues produced by the drug. Novel drugs can be tested based on the similarity of internal cues for known drugs. Image description: A rat performing drug discrimination test. MULTIPLE NEUROBIOLOGICAL TECHNIQUES FOR ASSESSING THE CNS Genetic Engineering Genetic altered animal models: ○ DNA of an organism is altered (knockouts or knock-ins), or a foreign gene is added to the DNA of an organism (transgenic). ○ Results in highly specific mutations in the mouse genome to study subsequent changes in brain function and behavior. ○ Genes of interest are extracted and modified, then injected into fertilized mouse eggs, which are then implanted into a surrogate mother. ○ Chimeric offspring (The transgene is expressed in some cells but not others) are then mated with wild-type mice. ○ Ultimately, mice that are homozygous for the transgene are produced. UNIT 2 Research Methods in Neuropsychopharmacology Page 25 Image description: Breeding of transgenic mice. Knockout Mice Model of lack gene for making a particular protein or. Comparing behavior and drug response of altered mice with normal animals provides information about the function of the protein that has been deleted. Allows study of relationship between protein structure and function. In neuropharmacology, the protein of interest is often a receptor subtype or an enzyme. Transgenic Mice One gene is substituted for another. Human genes can be inserted into mice to produce models for diseases such as Huntington’s and Alzheimer’s. Genetic engineering. Example 1 knockout mice The serotonin (5-hydroxytryptamine; 5-HT) receptor 1A is found on serotonergic neurons, where it acts as an autoreceptor, and on nonserotonergic neurons. 5-HT1A receptor agonists are currently used in the treatment of anxiety disorders, and antagonists of this receptor have been suggested to improve the efficacy of certain antidepressant drugs. However, the clinical value of these drugs, as well as their mechanism of action, is still unclear. To study the role of the 5-HT1A receptor in mood control, we have generated mice lacking this receptor What is the main role and why are they silencing this receptor? What are they wanting to study in humans? 5-HT1A receptor agonists are used in the treatment of anxiety disorders—they help to reduce feelings of anxiety. By preventing the receptor from dampening serotonin signals, these antagonists may boost the antidepressant effects. In summary, activating the 5-HT1A receptor helps reduce anxiety, while blocking it might enhance the action of antidepressants, potentially improving their effectiveness in treating depression. Genetic engineering. Example 2 knockout mice Sigma-1 receptor is a protein encoded by the SIGMAR1 gene. Sig-1R is expressed at high levels in the central nervous system and has been shown to have neuroprotective effects. Mutations in the SIGMAR1 gene have been linked to an increased risk of cognitive dysfunction and are strongly associated with Alzheimer’s disease (AD). In addition, two studies have found that Sig-1R may be involved in the development of depression through its effects on neurotrophic and growth factor signaling pathways. Selective serotonin reuptake inhibitors (SSRIs) have a moderate affinity for UNIT 2 Research Methods in Neuropsychopharmacology Page 26 signaling pathways. Selective serotonin reuptake inhibitors (SSRIs) have a moderate affinity for Sig-1R and the antidepressant effect of SSRIs may be mediated by Sig-1R. One potential mechanism through which Sig-1R may exert its neuroprotective effects is by promoting brain-derived neurotrophic factor (BDNF) expression. For example, Sig-1R agonists have been shown to reverse the downregulation of BDNF and improve symptoms of post-traumatic stress disorder (PTSD). Another Sig-1R agonist has been demonstrated to have neuroprotective effects in a mouse model of alpha-thalassemia X-linked intellectual disability. The brain’s BDNF protein was elevated by chronic antidepressant treatment, including SSRIs. Activation of Sig-1R enhanced the conversion of pro- BDNF to mature BDNF, as well as the release of mature BDNF into the extracellular space. These pieces of evidence suggest that activation of the Sig-1R promotes chaperone activity, which influences BDNF secretion, exerting antidepressant-like effects Potential Problems with Transgenic Mice Behaviors are controlled by multiple genes; changing or eliminating one gene alters only a small part of the overall behavioral trait. Compensation by other genes for the missing or overexpressed gene may mask effect of the mutation. Altered gene function occurs in all tissues at all stages of development; changes in other organs or in other brain areas may be responsible for behavioral changes. These mice are developing organisms, so environmental factors also affect the ultimate gene expression. TRANSLATIONAL RESEARCH Translational research: transforming discoveries from basic neuroscience research into clinical applications. Because human clinical trials are lengthy and expensive, recent emphasis has been on improving predictability of therapeutic drug effects based on animal research. All new drugs produced and sold in the U.S. must be approved by the Food and Drug Administration (FDA). They must be demonstrated to be both effective and safe. About 20% of new drugs being tested reach final approval; 80% are eliminated. Image description: Term planning of drug testing for FDA approval. The cost of developing new drugs has increased greatly, driven mostly by expenses associated with Phase III human clinical trials. In Phase III, a drug is tested in large trials involving thousands of patients at multiple testing sites around the country. Improving predictability of therapeutic effects would reduce the cost of drug development. As genetic differences associated with psychiatric disorders are identified, animal models with specific mutations will be developed. MULTIPLE NEUROBIOLOGICAL TECHNIQUES FOR ASSESSING THE CNS Psychopharmacology research uses multiple approaches. Techniques of behavioral analysis are combined with studies of the anatomy, physiology, and neurochemistry of the brain. UNIT 2 Research Methods in Neuropsychopharmacology Page 27 Stereotaxic Surgery Stereotaxic surgery allows precise implantation of devices in specific brain areas. The precise target is defined by the intersection of three planes. A similar apparatus is used for human brain surgery. Link to additional resource Stereotaxic Surgery (Part 1) Stereotaxic Surgery (Part 2) Stereotaxic Surgery (Part 3) UNIT 2 Research Methods in Neuropsychopharmacology Page 28 Lesioning Lesioning uses a stereotaxic device to position an electrode in a specific region. A very high-frequency radio current is passed through the electrode to heat the cells and destroy the tissue. Comparison of the animal’s behavior before and after lesioning provides information about the function of that brain area. Chemical Lesions Chemical lesions can be produced by injecting neurotoxins via a cannula to destroy specific cells. Specific neurotoxins injected directly into the brain are taken up by the normal reuptake mechanism of neurons and destroy cell terminals. Behavioral measures obtained before and after a neurotoxic lesion tell us about the role of the neurotransmitter in a particular behavior. Computer Reconstruction Lesions cannot be induced in humans, but accidents, trauma, strokes, and tumors have provided insights into brain functions. Microdialysis Microdialysis can measure neurotransmitters released in a specific brain region while the subject is actively engaged in behavior. Extracellular fluid is collected via a cannula at precise sites, even deep within the brain. Microdialysis requires only tiny amounts of fluid, analyzed by highly sensitive techniques that separate the sample into components depending on molecular size or ionic charge. Then concentration of molecules of interest can be determined. UNIT 2 Research Methods in Neuropsychopharmacology Page 29  concentration of molecules of interest can be determined. Collection of Extracellular Fluid with Microdialysis (Part 2) In Vivo Voltammetry In vivo voltammetry: measures neurotransmitter release using stereotaxically implanted microelectrodes. Changes in current flow at the electrode tip reflect changes in concentration of neurotransmitters or their metabolites. Measurements are made continuously. Implanted Macroelectrodes Implanted macroelectrodes: electrical stimulation produces action potentials and neurotransmitter release. Electrical stimulation should produce effects similar to those seen when neurotransmitter or an agonist is injected. Stimulation of a given cell group should produce effects opposite to those caused by a lesion at the same site. Electrical Brain Stimulation and Recording (A) Tremor Control System for Parkinson’s Research using stereotaxic implantation of electrodes led to development of the Tremor Control System for Parkinson’s. Electrodes in the brain are connected to a pulse generator that delivers enough electrical current to reduce tremors. Link to video UNIT 2 Research Methods in Neuropsychopharmacology Page 30 Quiz Questions 1. Microdialysis measures: ○ A. The amount of neurotransmitters in intracellular fluid ○ B. The amount of oxygen in intracellular fluid ○ C. The amount of neurotransmitters in extracellular fluid 2. Which one of the following options is an animal test for measuring learning and memory? ○ a) Elevated plus-maze ○ b) Morris Water-maze ○ c) Open Field Test 3. Which of the following animal tests does not measure anxiety-like behavior? ○ a) Open field test ○ b) Light-dark box test ○ c) Tail suspension test 4. In the context of drug reward and reinforcement, what is the breaking point? When they stop pressing the lever, which means that the cost is higher than the reward. 5. In the context of the techniques used for evaluating animal behavior, what is the predictive validity? The similarity between animal research and clinical trials in humans. UNIT 2 Research Methods in Neuropsychopharmacology Page 31 UNIT 3 Principles of Pharmacology tirsdag 1. oktober 2024 17:47 1. PRINCIPLES OF PHARMACOLOGY Pharmacology: The Science of Drug Action Pharmacokinetic Factors Determining Drug Action Therapeutic Drug Monitoring Pharmacodynamics: Drug–Receptor Interactions Biobehavioral Effects of Chronic Drug Use PHARMACOLOGY: THE SCIENCE OF DRUG ACTION Pharmacology: study of the actions of drugs and their effects on living organisms. Introduction of drugs by the pharmaceutical industry forced development of specialized fields: Neuropharmacology: study of drug-induced changes in nervous system cell functioning. Psychopharmacology: emphasizes drug-induced changes in mood, thinking, and behavior. Neuropsychopharmacology: ○ Identifies chemical substances that act on the nervous system to alter behavior. ○ Uses chemical agents as probes to gain an understanding of the neurobiology of behavior. Drug action: molecular changes produced by a drug when it binds to a target site or receptor. The molecular changes alter physiological or psychological functions, called drug effects. Drugs act at several target sites and have multiple effects. ○ Therapeutic effects: the drug–receptor interaction produces desired physical or behavioral changes. ○ All other effects are side effects, from mildly annoying to distressing and dangerous. Specific drug effects are based on physical and biochemical interactions of a drug with a target site in living tissue. Nonspecific drug effects are based on certain unique characteristics of the individual (e.g., mood, expectations, perceptions, attitudes). A placebo → nonspecific effect. ○ It is a pharmacologically inert compound but can have therapeutic and side effects. ○ Belief in a drug may produce real physiological effects despite the lack of chemical activity. Placebos are used to evaluate the effectiveness of new medications. ○ Double-blind experiment: neither patient nor observer knows which treatment the patient has received. Bioavailability: amount of drug in the blood that is free to bind at target sites. Pharmacokinetic component of drug action: the dynamic factors that contribute to bioavailability. Pharmacokinetic factors: 1. Routes of administration 2. Absorption and distribution 3. Binding 4. Inactivation (or biotransformation) 5. Excretion UNIT 3 Principles of Pharmacology Page 32  5. Excretion PHARMACOKINETIC FACTORS DETERMINING DRUG ACTION Drug effect also depends on: ○ How rapidly the drug reaches its target ○ Frequency and history of prior drug use (tolerance) ○ Nonspecific factors characteristic of individuals and their environment Methods of drug administration influence the onset of drug action. ○ Enteral methods use the gastrointestinal (GI) tract; generally slow and produces variable blood levels. ○ Parenteral methods: injection, pulmonary, and topical administration. ○ Oral administration (PO) is safe, self-administered, economical. ▪ The drug must be resistant to destruction by stomach acid and enzymes. ▪ Insulin is not resistant and cannot be given orally. *Examples of enteral method: ○ Oral (PO): Taking pills or liquids by mouth. ○ Sublingual: Placing a drug under the tongue for quick absorption. ○ Rectal: Using suppositories or enemas to deliver a drug through the rectum *Parenteral Methods Definition: Parenteral methods refer to drug administration that bypasses the gastrointestinal tract Absorption: movement of the drug from the site of administration to the blood circulation. ○ Factors that influence absorption include the amount of food in the stomach, type of food, physical activity, etc. UNIT 3 Principles of Pharmacology Page 33 ROUTES OF DRUG ADMINISTRATION Rectal administration: placement of a drug-filled suppository in the rectum. ○ Used for infants or patients who are vomiting, unconscious, or unable to take medication orally. Intravenous (IV) injection: most rapid and accurate method. ○ Quick onset of drug effect can be a hazard—little time to correct an overdose or allergic reaction; the drug cannot be removed from the body as it can by stomach pumping. Intramuscular (IM) injection: slower, more even absorption over a period of time. ○ Absorption can be slowed by combining the drug with another drug that constricts blood vessels, or with vegetable oil. Subcutaneous (SC) injection: drug is injected just below the skin. ○ Absorption is slow but can be variable; can be slowed by vegetable oils. Intraperitoneal (IP) injection: rarely used with humans, but common for small laboratory animals. ○ Drug is injected through the abdominal wall into the space surrounding the abdominal organs. Inhalation: drug is absorbed from the lungs. ○ Absorption is rapid because the lungs have large surface area and many capillaries. ○ Drug effect is rapid (e.g., nicotine, crack cocaine). Topical: drug is applied to skin or mucous membranes. ○ Mostly local effects. Intranasal administration can cause local effects such as relieving nasal congestion, but can also have systemic effects. UNIT 3 Principles of Pharmacology Page 34  also have systemic effects. ○ Intranasal absorption occurs with cocaine “snorting.” Transdermal (through the skin) administration with skin patches: controlled and sustained delivery of drug. Epidural injection: spinal anesthetics are delivered directly to the cerebrospinal fluid; bypasses blood–brain barrier. ○ Research techniques include microinjection of drugs to discrete brain areas (intracranial) or ventricles (intracerebroventricular). Route of administration alters rate of absorption and thus blood levels of a drug. Each method of administration has advantages and disadvantages. THE TIME COURSE OF DRUG BLOOD LEVEL DEPENDS ON ROUTE OF ADMINISTRATION ADVANTAGES AND DISADVANTAGES OF SELECTED ROUTES OF DRUG ADMINISTRATION The most important factor in determining plasma drug levels is the rate of passage of the drug UNIT 3 Principles of Pharmacology Page 35 The most important factor in determining plasma drug levels is the rate of passage of the drug through cell membranes. Cell membranes are primarily phospholipids, which have a negatively charged region (hydrophilic) and two uncharged tails (hydrophobic), arranged in a bilayer. *Plasma drug levels refer to the concentration or amount of a drug present in the blood plasma (the liquid part of blood) at a given time. This is an important measurement because it helps determine how much of the drug is available to act on the body and produce its intended effect. Blood plasma is the fluid in which blood cells are suspended, and it carries substances like nutrients, hormones, and drugs. Plasma drug levels indicate how much drug is circulating in the blood, available to reach target tissues or organs CELL MEMBRANES Proteins are inserted into the phospholipid bilayer and serve as receptors or channels. The molecular characteristics prevent most molecules from passing through unless they are fat-soluble. Lipid-soluble drugs can pass through cell membranes by passive diffusion. Movement is in a direction of higher to lower concentration. The larger the concentration gradient, the faster the diffusion. Many substances in blood fluctuate significantly and would have disruptive effects on the brain if materials could move freely between the blood and the brain/cerebrospinal fluid. Cerebrospinal fluid (CSF) fills the subarachnoid space around the brain and spinal cord, ventricles, and canals. Blood–brain barrier: the separation between brain capillaries and the brain/CSF. Many substances that diffuse out of the blood do not enter the CSF. UNIT 3 Principles of Pharmacology Page 36  Many substances that diffuse out of the blood do not enter the CSF. DISTRIBUTION OF CEREBROSPINAL FLUID (PART 2) Brain capillaries have distinct morphology. Other capillaries are designed to allow movement of materials between the blood and surrounding cells. Brain capillaries have no clefts or pinocytic sites, movement of water-soluble molecules is minimized. UNIT 3 Principles of Pharmacology Page 37  Brain capillaries are surrounded by glial feet—extensions of glial cells called astrocytes. The feet help maintain tight junctions between capillary endothelial cells. Blood-brain barrier It is important to know whether a drug can cross the blood–brain barrier. Example: Physostigmine crosses the barrier; used to treat intoxication from agricultural pesticides; increases availability of the neurotransmitter acetylcholine. Closely related neostigmine does not cross the blood–brain barrier. ○ It increases acetylcholine peripherally (as in myasthenia gravis). The placenta forms a barrier between the maternal blood circulation and the fetus. It is important to know whether drugs can cross this barrier. Potentially damaging effects include acute toxicity and teratogenic effects. Teratogens induce developmental abnormalities, including drugs, X-rays, and maternal infections. Effects depend on timing of exposure: most damage occurs in the first trimester when fetal organs are formed. Drugs remaining in the newborn can have prolonged action due to slow, incomplete metabolism. Opiates such as heroin easily enter fetal circulation. Tranquilizers, anesthetics, alcohol, and cocaine can cause acute toxicity in the fetus. PERIODS OF MAXIMUM TERATOGENIC SENSITIVITY FOR SEVERAL ORGAN SYSTEMS IN THE UNIT 3 Principles of Pharmacology Page 38 PERIODS OF MAXIMUM TERATOGENIC SENSITIVITY FOR SEVERAL ORGAN SYSTEMS IN THE HUMAN FETUS Drug depots: binding at inactive sites where no biological effect is initiated. Includes plasma proteins (e.g., albumin), muscle, and fat. Drug molecules tied up in these depots cannot reach active sites or be metabolized by the liver, but binding is reversible. Depot binding affects magnitude and duration of drug action: ○ Reduces concentration of drug at its sites of action and delays effects. ○ Individuals vary in amount of depot binding, which explains in part why some people are more sensitive to a drug than others. EFFECTS OF DRUG DEPOT BINDING ON THERAPEUTIC OUTCOME Depot binding is nonselective; similar drugs can compete for binding sites. This can result in higher than expected blood levels of a displaced drug, and overdose. Depot binding can result in drugs remaining in the body for extended periods. Drugs are eliminated via biotransformation (metabolism), and metabolites are excreted. Drug clearance from the blood is usually exponential (first-order kinetics). The rate is concentration-dependent. UNIT 3 Principles of Pharmacology Page 39  Half-life: amount of time required for removal of 50% of the drug (t½). Half-life determines interval between doses. A drug given once a day should have a half-life of about 8 hours. Longer half-life could lead to accumulation, which increases potential for side effects and toxicity. HALF-LIFE OF SOME COMMON DRUGS Goal: Maintain concentration of a drug in blood plasma at a constant level. ○ Target therapeutic concentration is achieved only after multiple doses. ○ Half-life determines the time needed to reach the steady state plasma level: absorption/distribution phase is equal to the metabolism/excretion phase. Some drugs are eliminated according to zero-order kinetics: molecules are cleared at a constant rate regardless of concentration. ○ It occurs when drug levels are high and routes of metabolism or elimination are saturated. ○ Example: alcohol UNIT 3 Principles of Pharmacology Page 40 *First-order: The rate of drug elimination depends on how much is in your body (a constant percentage is removed). Example: Imagine you have 100 units of a drug, and your body removes 10% per hour. In the first hour, you'd lose 10 units (10% of 100). In the second hour, you'd lose 9 units (10% of 90), and so on. The rate slows down as the drug amount decreases. Zero-order: The rate of drug elimination is constant (the same amount is removed regardless of how much you have). Example: If your body eliminates 5 units per hour, and you start with 100 units, after 1 hour, you'll have 95 units. After 2 hours, you'll have 90 units, and so on. The rate doesn’t change, no matter how much drug is left Most biotransformation (drug metabolism) occurs in the liver. ○ Type I or Phase I: modification by oxidation, reduction, or hydrolysis. ○ Type II or Phase II: reactions requiring combination (conjugation) of the drug with a small molecule such as glucuronide, sulfate, or methyl groups. ▪ The products are ionized and less lipid-soluble, and usually biologically inactive. ▪ Inactive metabolites are excreted via kidneys or bile. ▪ Active metabolites return to the blood and have prolonged action. *Phase I (Type I) – First step of metabolism: The liver changes the drug by breaking it down through chemical reactions like adding oxygen (oxidation), removing oxygen (reduction), or using water to split the drug apart (hydrolysis). This process makes the drug more ready for the next phase of metabolism, and sometimes, the drug is still active and can affect the body after this step. Phase II (Type II) – Second step of metabolism: The liver adds other substances like glucuronide (a sugar molecule), sulfate, or methyl groups to the drug. This step usually makes the drug bigger, less fat-soluble, and easier to eliminate from the body. After this phase, the drug is usually inactive (won’t affect you anymore) and is ready to be removed. What happens next? Inactive metabolites (drug breakdown products) get eliminated by the kidneys in urine or through bile in feces. Sometimes, the body produces active metabolites, meaning the drug (or part of it) is still working. These active forms can stay in the blood and have longer-lasting effects, making the drug's action last longer Microsomal enzymes: liver enzymes that metabolize psychoactive drugs. They lack strict specificity and can metabolize a wide variety of foreign chemicals UNIT 3 Principles of Pharmacology Page 41  ○ They lack strict specificity and can metabolize a wide variety of foreign chemicals (xenobiotics). ○ The cytochrome P450 (CYP450) enzyme family is responsible for oxidizing most psychoactive drugs. Factors that modify biotransformation capacity include: 1. Enzyme induction 2. Enzyme inhibition 3. Drug competition 4. Individual differences in age, gender, and genetics. ENZYME INDUCTION Enzyme induction: Repeated use of a drug increases the number of enzyme molecules and speeds biotransformation. ○ Also speeds transformation of other drugs that the enzyme acts on, and reduces their effect. ○ Drug tolerance and cross tolerance: Drugs lose effectiveness with repeated use. Failed Birth Control Pills Women taking certain medications such as carbamazepine to control their epileptic seizures show a significantly higher occurrence of “failures” in their oral contraceptives, leading to unwanted pregnancies. ○ Question: Under what circumstances could using certain prescription drugs increase a woman’s probability of pregnancy? ENZYME INHIBITION Enzyme inhibition: A drug may inhibit an enzyme, also reducing metabolism of other drugs. ○ Effects are more intense or prolonged; toxicity is possible. ○ Example: Monoamine oxidase inhibitors (MAOIs) inhibit several enzymes and can have many side effects. Ordinary Food as Poison Question: Have you heard of strange cases in which food becomes quite toxic? ○ Foods rich in tyramine—Cheddar, Roquefort, and Camembert cheeses, pickled herring, red wine, beer, and chicken liver—may suddenly cause a dangerous increase in blood pressure, cardiac arrhythmias, and fatal cerebral hemorrhage if consumed in combination with certain antidepressant medications. ○ Question: How can a normal diet interact with prescription drugs in such a dangerous manner? DRUG COMPETITION Drug competition for an enzyme: Elevated levels of one drug reduce metabolism of the second, causing potentially toxic levels. ○ Example: Alcohol plus sedatives such as Valium compete for cytochrome P450. Drug metabolism also depends on genetic and environmental factors. THERAPEUTIC DRUG MONITORING Therapeutic drug monitoring: Multiple blood samples are taken after drug administration and plasma levels measured. ○ This helps determine effective dose for an individual, with minimal side effects. ○ Examples: Antiepileptic drugs, antidepressants, mood stabilizers Some drugs have a narrow therapeutic index (effective dose is close to dose that causes serious side effects). ○ Monitoring is especially important. ACTIVITY UNIT 3 Principles of Pharmacology Page 42 ACTIVITY You work in a laboratory where you are studying a substance (substance A) that could be useful as a medication. In preclinical studies, you have administered this substance to guinea pigs to evaluate its pharmacokinetic characteristics. The first animal (animal 1) weighs 900 grams, and we administered a single intravenous dose of 50 mg/kg (45 mg in total). The second animal (animal 2) weighs 825 grams, and we administered a single oral dose of 50 mg/kg (41.25 mg in total). We obtained several blood and urine samples indicating the following data: Table 1. Concentration of Substance A in Blood (μg/ml) based on time since administration (time in hours) Table 2. Concentration of substance A in urine (mg) based on time since administration (time in hours) 1. Indicate which pharmacokinetic values can be obtained and the similarities and differences between both routes of administration. The maximum concentration (C-max) ○ Animal 1: 1.61 (obtained almost immediately) ○ Animal 2: 0.75 (obtained after 4hrs, T-max, which is the time needed to obtain the C- max) Half-lives ○ Animal 1: The half-life is approximately 1.5 hrs. ○ Animal 2: The half-life is approximately 7 hrs. It is a first-order kinetics in the animals. (It's difficulty to see this in the first animal, but we can see it in the second animal) In this case, it seems to be a first-order kinetic. However, sometimes the metabolizing starts with a zero-order, then it becomes a first-order. It depends on how many enzymes that are available. 2. Would you consider that the elimination route of this substance is primarily urinary? No, because only about 4mgs of the total drug (the total was 45mg and 41.25mg) was eliminated through the urine. UNIT 3 Principles of Pharmacology Page 43  eliminated through the urine. *Answers to question by ChatGPT: 1. Pharmacokinetic Values and Differences Between Routes of Administration Pharmacokinetic values that can be obtained: Maximum concentration (C-max): ○ Animal 1 (Intravenous): C-max = 1.61 μg/ml at 0.5 hours (immediate). ○ Animal 2 (Oral): C-max = 0.75 μg/ml at 4 hours (delayed due to oral absorption). Half-life: ○ Animal 1: The half-life is approximately 1.5 hours (seen by the drop from 0.85 μg/ml to 0.33 μg/ml between 1.5 and 3 hours). ○ Animal 2: The half-life is approximately 7 hours (drop from 0.75 μg/ml at 4 hours to 0.27 μg/ml at 8 hours). T-max (Time to maximum concentration): ○ Animal 1: The T-max is 0.5 hours (rapid absorption due to intravenous route). ○ Animal 2: The T-max is around 4 hours (delayed absorption via oral route). Elimination patterns: ○ Animal 1: The concentration in blood drops quickly after reaching the peak, suggesting faster elimination, which is typical for intravenous administration. ○ Animal 2: The concentration decreases more slowly, indicative of slower absorption and elimination with oral dosing. Similarities and differences between the routes: Similarity: Both animals exhibit a decline in concentration over time, indicating elimination of the substance, with the first animal showing a faster rate. Difference: The intravenous route (Animal 1) leads to immediate absorption and peak concentration, while the oral route (Animal 2) involves delayed absorption and a lower C-max due to first-pass metabolism. 2. Is the Elimination Route Primarily Urinary? No, the elimination route is likely not primarily urinary. From the urine data: Animal 1: Only 3.97 mg out of 45 mg (administered dose) was excreted in the urine, representing a small percentage of the total drug. Animal 2: 3.35 mg out of 41.25 mg was excreted, also a small portion of the dose. This suggests that most of the drug is eliminated via non-urinary routes (e.g., hepatic metabolism), as urinary excretion accounts for a small fraction of the administered dose. PHARMACODYNAMICS: DRUG–RECEPTOR INTERACTIONS Pharmacodynamics: Study of the physiological and biochemical interaction of drug molecules with cell receptors in target tissue. ○ Receptors: Proteins on cell surfaces or within cells. ○ Ligand: Molecule that binds to a receptor with some selectivity. Most drugs do not pass into neurons but act on surface receptors. Neuropharmacology identifies drugs that act at neurotransmitter receptors to enhance or reduce normal functioning of the cell. Most hormones that act on the brain bind to receptors in the cytoplasm or nucleus. ○ Hormone binding alters cell function by triggering changes in gene expression. TWO PRINCIPAL TYPES OF RECEPTORS (PART 1) UNIT 3 Principles of Pharmacology Page 44 TWO PRINCIPAL TYPES OF RECEPTORS (PART 2) Receptors have specificity for ligands due to their molecular shapes. ○ Receptor agonist: Has the best chemical “fit” (highest affinity); attaches readily to the receptor and produces a significant biological effect. ○ Receptor antagonists: Also fit receptors but produce no cellular effect (low efficacy). AGONIST AND ANTAGONIST INTERACTIONS WITH RECEPTORS Receptor antagonists can prevent the active ligands from binding. Partial agonists have intermediate efficacy. Inverse agonists initiate a biological action that is opposite to that produced by an agonist. *The natural occurring neurotransmitters are full agonists UNIT 3 Principles of Pharmacology Page 45  *The natural occurring neurotransmitters are full agonists *It is possible to make drugs that mimic the effects of full agonists and create the same effects. *For instance, if someone has hyperdopaminergic activity in the brain, an antagonist can be used to prevent the dopamine (naturally in the brain) from binding to the receptor. CONTINUUM OF DRUG EFFICACY CONTINUUM OF DRUG EFFICACY Drugs can also act as agonists by: ○ Enhancing synaptic function by increasing neurotransmitter synthesis or release. ○ Prolonging action of the neurotransmitter within the synapse. Ligand–receptor binding is temporary. After ligand separates, the receptor is free to bind again. UNIT 3 Principles of Pharmacology Page 46  again. ○ Ligand binding causes a change in receptor shape that initiates a series of events in the cell. Competitive antagonists: Drugs that compete with agonists to bind receptors but do not initiate intracellular effects, reducing the effect of the agonist. ○ Competitive antagonists can be replaced by an excess of agonist. ○ Example: Naloxone is a competitive antagonist of morphine. *The graph shows that even when a competitive antagonist is present, the agonist can still achieve the same maximum effect (Emax). The key difference is that more of the agonist is required to reach that effect. Without the antagonist (red curve), a lower dose of the agonist produces the desired effect. With the antagonist (blue curve), the agonist's potency is reduced, meaning you need a higher dose of the agonist to achieve the same effect. But the maximum effect itself (Emax) remains unchanged; it just takes more of the drug to get there when the antagonist is present. EC50 (the concentration of the agonist needed to produce 50% of its maximum effect) is shifted. The blue curve shows a higher EC50, meaning it takes more of the agonist to reach 50% of the effect when the competitive antagonist is present. Noncompetitive antagonists reduce the effect of agonists by: ○ Binding to the receptor at a site other than the agonist binding site. ○ Disturbing the cell membrane supporting the receptor. ○ Interfering with cell processes that were initiated by the agonist. *Here's how they can do that: Binding to a different site: UNIT 3 Principles of Pharmacology Page 47  Binding to a different site: ○ The noncompetitive antagonist binds to the receptor at a different site than the agonist. This changes the shape or function of the receptor, making it harder or impossible for the agonist to activate the receptor fully, even if the agonist is present in high concentrations. ○ Example: A drug like ketamine acts as a noncompetitive antagonist at NMDA receptors. It binds to a separate site, altering the receptor function, which reduces the effect of glutamate (the agonist). Disturbing the cell membrane: ○ Noncompetitive antagonists can also disrupt the environment around the receptor (like the cell membrane) in a way that prevents the receptor from functioning properly. This makes it harder for the agonist to produce its usual effect, even though the receptor itself may still be present. ○ Example: Certain anesthetics work by disturbing membrane proteins, which indirectly affect receptor activity. Interfering with cell processes: ○ Some noncompetitive antagonists block or disrupt the cellular processes initiated after the agonist binds to the receptor. Even if the agonist activates the receptor, the antagonist interferes with the downstream signals, reducing the overall effect. ○ Example: Some drugs inhibit enzymes or ion channels that are part of the cell's response to the agonist, stopping the effect further down the signaling pathway. Receptor proteins have a particular lifespan; the number and sensitivity of receptors can also change. ○ Up-regulation: Number of receptors increases in response to the absence of ligands. ○ Down-regulation: Number of receptors is reduced in response to chronic activation. Receptor subtypes: Receptors with different characteristics in different tissues. ○ A goal of neuropharmacology—design drugs that bind with greater affinity to one receptor subtype to initiate a selective effect, without acting on related receptor subtypes and producing side effects. Dose–response curves describe the extent of effect (response) produced by a given drug concentration. ○ Threshold dose: Smallest dose that produces a measurable effect. ○ ED50 (50% effective dose): Dose that produces half the maximal effect. ○ Maximum response: Assume all receptors are occupied. DOSE–RESPONSE CURVE UNIT 3 Principles of Pharmacology Page 48 PHARMACODYNAMICS: DRUG-RECEPTOR INTERACTIONS Absolute amount of drug necessary to produce a specific effect indicates the potency of a drug. ○ Comparing ED50 of different drugs shows differences in potency. ○ Dose-response curves for fours analgesic agents: TD50 (50% toxic dose): Dose at which 50% of the population experiences a toxic effect. ○ Comparing TD50 with ED50 helps determine appropriate doses to prevent toxic effects. ○ Therapeutic index (TI) = TD50/ED50 *Interpretation: A high TI means there's a large difference between the effective dose and the toxic dose, indicating that the drug is relatively safe (e.g., TI of 10 means the toxic dose is 10 times higher than the effective dose). A low TI means there's a small margin between effective and toxic doses, indicating a higher risk of toxicity (e.g., TI of 2 means the toxic dose is only twice the effective dose). BIOBEHAVIORAL EFFECTS OF CHRONIC DRUG USE Drug tolerance: Diminished response to a drug after repeated exposure. Increasing dosages must be administered to obtain the same magnitude of biological effect. UNIT 3 Principles of Pharmacology Page 49 Increasing dosages must be administered to obtain the same magnitude of biological effect. Cross tolerance: Tolerance to one drug can diminish the effectiveness of a second drug. Metabolic tolerance (drug disposition tolerance): Repeated use of a drug reduces the amount of the drug available at the target tissue. ○ Example: When drugs increase their own rate of metabolism by inducing liver microsomal enzymes. Pharmacodynamic tolerance: Changes in nerve cell function compensate for the continued presence of the drug. ○ Examples: Receptor down-regulation and up-regulation. Behavioral tolerance (context-specific tolerance): Tolerance is not apparent or is reduced in a novel environment. BIOBEHAVIORAL EFFECTS OF CHRONIC DRUG USE Pavlovian, or classical conditioning ○ Many psychoactive drugs elicit reflexive effects such as cortical arousal, elevated blood pressure, or euphoria, and act as unconditioned stimuli. ○ The drug-taking procedure or the environment may elicit a conditioned response even before the drug is taken. CLASSICAL CONDITIONING OF DRUG-RELATED CUES State-dependent learning: Tasks learned in the presence of a psychoactive drug may subsequently be performed better in the drugged than in the non-drugged state. ○ Conversely, learning acquired in the non-drugged state may be more available in the non-drugged state. UNIT 3 Principles of Pharmacology Page 50 UNIT 4 Chemical Signaling by Neurotransmitters and Hormones onsdag 9. oktober 2024 15:50 CONTENT: Chemical Signaling between Nerve Cells Neurotransmitter Synthesis, Release, and Inactivation Neurotransmitter Receptors and Second-Messenger Systems Pharmacology of Synaptic Transmission Synaptic Plasticity The Endocrine System CHEMICAL SIGNALING BETWEEN NERVE CELLS Synapse — the point of communication between neurons. ○ Transmission occurs in only one direction (from the presynaptic cell to the postsynaptic cell). Axodendritic synapses: a presynaptic axon terminal communicates with a dendrite of the postsynaptic cell. AN ELECTRON MICROGRAPH OF AN AXODENDRITIC SYNAPSE The gap between cells, or synaptic cleft, is very small, about 20 nm. Synaptic vesicles in the axon terminals are filled with several thousand molecules of a neurotransmitter. Numerous mitochondria in the axon provide ATP for ion pumping and transmitter release. The synapse is surrounded by processes (fibers) from astrocytes (glial cells). CHEMICAL SIGNALING BETWEEN NERVE CELLS Axosomatic synapses—between an axon terminal and a nerve cell body. Axoaxonic synapses—one axon synapses on the terminal of another axon. ○ This permits the presynaptic cell to alter neurotransmitter release from the postsynaptic cell directly at the terminals. THE THREE TYPES OF SYNAPTIC CONNECTIONS BETWEEN NEURONS UNIT 4 Chemical Signaling Page 51 MAJOR CATEGORIES OF NEUROTRANSMITTERS NEUROTRANSMITTER SYNTHESIS, RELEASE, AND INACTIVATION Criteria to determine whether a chemical is a neurotransmitter: Presynaptic cell contains the chemical plus a mechanism to make it. A mechanism for inactivating the chemical should also be present. Chemical is released from the axon terminal when the neuron is stimulated. Receptors for the chemical are present on the postsynaptic cell. Direct application of chemical or an agonist drug has the same effect on the postsynaptic cell as stimulating the presynaptic neuron. Applying an antagonist drug that blocks the receptors inhibits both the chemical’s action and the effect of stimulating the presynaptic neuron. *Summary: These criteria collectively ensure that the substance in question: Is produced and released by neurons. Has a specific mechanism of action on other neurons. Is part of a regulated system of chemical signaling that allows for controlled communication between neurons AXON TERMINAL OF A NEURON THAT SYNTHESIZES BOTH A CLASSICAL NEUROTRANSMITTER AND A NEUROPEPTIDE An individual neuron can make one to several different neurotransmitters. Vesicles can hold one or both of the transmitters. UNIT 4 Chemical Signaling Page 52  Most neurotransmitters are made in the axon terminals; enzymes for their synthesis are transferred to the terminals. Neuropeptides are made from protein precursors. Precursors are shipped from the cell body to axon terminals in large vesicles. Precursor proteins are broken down by enzymes during shipment to axon terminals. Neuropeptides can’t be replenished very quickly. FEATURES OF NEUROTRANSMISSION USING NEUROPEPTIDES NEUROTRANSMITTER SYNTHESIS, RELEASE, AND INACTIVATION When a wave of depolarization reaches the axon terminals, voltage-gated Ca²⁺ channels open and Ca²⁺ rushes into the cell. High Ca²⁺ levels trigger neurotransmitter release. Ca²⁺ channels are concentrated in active zones of the terminal membrane. Vesicles “dock” with the cell membrane in the active zone and are primed for release. Various proteins are involved in these steps. Exocytosis: Ca²⁺ binds to receptors and results in fusion of vesicle membranes with the cell membrane, releasing neurotransmitter molecules into the synaptic cleft. PROCESSES INVOLVED IN NEUROTRANSMISSION AT A TYPICAL SYNAPSE USING A CLASSICAL UNIT 4 Chemical Signaling Page 53 PROCESSES INVOLVED IN NEUROTRANSMISSION AT A TYPICAL SYNAPSE USING A CLASSICAL NEUROTRANSMITTER Vesicle recycling: During exocytosis, vesicle membranes are added to the cell membrane. The vesicle membrane is recycled by endocytosis—the cell membrane buds and pinches off a portion, which then fuses with an endosome. THE LIFE CYCLE OF THE SYNAPTIC VESICLE Neurotransmitter release is regulated by: ○ Rate of neuron firing ○ Probability that vesicles will undergo exocytosis ○ Autoreceptors: receptors for the same transmitter released by the neuron. Terminal autoreceptors: activated by the neurotransmitter; inhibit further transmitter release. UNIT 4 Chemical Signaling Page 54  release. Some drugs block or stimulate autoreceptors. Heteroreceptors: receptors for other transmitters released at adjacent neurons or cells. To stop signal transmission, the neurotransmitter molecules must be removed from the synaptic cleft. ○ Transmitter may be broken down by enzymes—common for ACh, lipid and gaseous transmitters, and neuropeptides. ○ Transporter proteins in the cell membrane take up the transmitter molecules. Reuptake—transmitters are taken up by the cell that released them. Transmitters may be taken up by the postsynaptic cell or astrocytes. NEUROTRANSMITTER INACTIVATION Some psychoactive drugs block the transporters, and signal transmission is enhanced. ○ Cocaine blocks transporters for DA, 5-HT, and NE. ○ Many antidepressants block the 5-HT transporter, the NE transporter, or both. NEUROTRANSMITTER RECEPTORS AND SECOND-MESSENGER SYSTEMS Neurotransmitter receptors are proteins located in cell membranes. The transmitter binds to the receptor to activate it. The resulting effect may be excitatory or inhibitory. UNIT 4 Chemical Signaling Page 55  The resulting effect may be excitatory or inhibitory. Neurotransmitters bind to more than one type of receptor—receptor subtypes. There are two major categories of transmitter receptors: ionotropic and metabotropic. Ionotropic receptors consist of 4 or 5 subunits with an ion channel in the center. ○ When the transmitter binds to the receptor, the channel opens and allows ion flow (ligand-gated channel receptors). ○ Some ionotropic receptors are Na⁺ channels, resulting in depolarization and an excitatory response (e.g., nicotinic receptor for ACh). ○ Others allow the flow of Cl⁻ (e.g., GABA₊ receptor), leading to hyperpolarization (inhibitory). Metabotropic receptors: ○ Act more slowly, but the response lasts longer. ○ Consist of one subunit, with 7 trans-membrane domains (7-TM receptors). ○ Work by activating G proteins (G protein-coupled receptors). STRUCTURE OF METABOTROPIC RECEPTORS G proteins act in two ways: Inhibit or activate ion channels (e.g., K⁺ channels open, K⁺ moves out of the cell and hyperpolarization results). Stimulate or inhibit effector enzymes in the cell membrane that synthesize or break down second messenger molecules. FUNCTIONS OF METABOTROPIC RECEPTORS UNIT 4 Chemical Signaling Page 56  The neurotransmitter molecule is the first messenger. Second messengers are molecules inside the cell that activate protein kinases to cause phosphorylation of substrate proteins. Phosphorylation of nuclear proteins can turn gene expression on or off. *Protein kinases are enzymes that catalyze the transfer of a phosphate group from a high-energy molecule (usually ATP) to a specific amino acid in a protein. This process is known as phosphorylation. THE MECHANISM OF ACTION OF SECOND MESSENGERS Some second messengers: Cyclic adenosine monophosphate (cAMP)—stimulates protein kinase A (PKA). Cyclic guanosine monophosphate (cGMP)—regulated by the gas NO; stimulates protein kinase G (PKG). Phosphoinositide—breaks down a phospholipid in the cell membrane, and liberates two second messengers: diacylglycerol (DAG) and inositol triphosphate (IP₃). Together, they cause an increase in Ca²⁺ ions in the postsynaptic cell and activation of UNIT 4 Chemical Signaling Page 57  ○ Together, they cause an increase in Ca²⁺ ions in the postsynaptic cell and activation of protein kinase C (PKC). Ca²⁺ acts as a second messenger. Activation of the calcium/calmodulin kinase (CaMK). *Phosphoinositide is a type of fat in the cell membrane that, when broken down, releases DAG and IP₃. DAG and IP₃ increase calcium ions (Ca²⁺) in the cell and activate protein kinase C (PKC). Ca²⁺ itself also acts as a second messenger, activating calcium/calmodulin kinase (CaMK) to carry out important functions in the cell. SECOND-MESSENGER SYSTEMS AND PROTEIN KINASES NEUROTRANSMITTER RECEPTORS AND SECOND-MESSENGER SYSTEMS Increased Ca2+ also stimulates the enzyme nitric oxide synthase (NOS). NOS in neurons and endothelial cells catalyzes formation of NO from arginine. NO acts differently from other transmitters. It is not stored in vesicles and must be made on demand; it diffuses through cell membrane. PHARMACOLOGY OF SYNAPTIC TRANSMISSION Synaptic effects form the basis of almost all actions of psychoactive drugs, including drugs of abuse. Drug is a neurotransmitter (NT) precursor; can increase the rate of synthesis: ○ L-DOPA—precursor to DA ○ 5-HTP—precursor to 5-HT Drug inhibits enzyme for synthesis of NT and reduces levels of NT. ○ Alpha-methyl-para-tyrosine inhibits tyrosine hydroxylase, involved in making DA and NE. Drug blocks enzyme involved in NT breakdown (enhances NT action). ○ Physostigmine blocks acetylcholinesterase, which breaks down Ach. ▪ Anticholinergic poisoning ○ Phenelzine blocks monoamine oxidase (MAO), important in breakdown of DA, NE, and 5- HT. Drug blocks drugs transporters for NT uptake (enhances NT action): ○ Cocaine blocks the transporters for DA, NE, and 5-HT. ○ Antidepressants block transporters for 5-HT. Drugs stimulate or inhibit autoreceptors: ○ Clonidine stimulates autoreceptors for NE and 5-HT; reduces NT release. ○ Pindolol inhibits 5-HT autoreceptors; increases NT release. Drug acts on postsynaptic receptors: Agonist: mimics NT effect on receptor. UNIT 4 Chemical Signaling Page 58  ○ Agonist: mimics NT effect on receptor. ▪ Benzodiazepines, heroin, morphine, nicotine, THC ○ Antagonist: inhibits effect of NT on the receptor. ▪ Antagonists at D2 receptor subtype for DA used to treat schizophrenia. *Summary Psychoactive drugs can have a wide range of effects on neurotransmitter systems by: Increasing or decreasing the synthesis of neurotransmitters. Blocking the breakdown or reuptake of neurotransmitters, enhancing their effects. Stimulating or inhibiting autoreceptors to modulate neurotransmitter release. Acting directly on postsynaptic receptors as agonists or antagonists A. Myeline, axon B. Synaptic vesicle C. Endosome - responsible for recycling of vesicles D. Transmitter molecules, neurotransmitters E. Ca+ ion channel F. Postsynaptic receptor THE ENDOCRINE SYSTEM Hormones are another form of cellular communication. Hormones are secreted by endocrine glands into the bloodstream. They affect target organs that can be far from their point of manufacture. To respond to a hormone, a target cell must have appropriate receptors. COMPARISON OF SYNAPTIC AND ENDOCRINE COMMUNICATION UNIT 4 Chemical Signaling Page 59 MAJOR ENDOCRINE GLANDS AND THEIR LOCATION IN THE BODY Adrenal glands: two glands that come together during embryonic development. ○ Adrenal medulla—chromaffin cells secrete the monoamines epinephrine (EPI) and norepinephrine (NE). ○ Stressors stimulate release of EPI and NE as part of the “fight-or-flight” response. STRUCTURE OF THE ADRENAL GLAND, SHOWING THE OUTER CORTEX AND THE INNER MEDULLA UNIT 4 Chemical Signaling Page 60  Adrenal cortex cells secrete glucocorticoids: ○ Cortisol (hydrocortisone) in humans ○ Corticosterone in rats and mice Glucocorticoids maintain normal blood glucose levels while helping store excess glucose for future use. Secreted in increased amounts during stress (they help us to cope with stressful experiences). Other steroid hormones: Gonads (ovaries and testes) secrete estrogens (such as estradiol); progestins (mainly progesterone); and androgens (such as testosterone). These hormones determine secondary sex characteristics. Pancreas ○ Islets of Langerhans cells secrete insulin and glucagon, which regulate glucose levels. ○ Lack of insulin results in diabetes. ○ Both are peptide hormones. Thyroid gland ○ Secretes thyroxine (T4) and triiodothyronine (T3), important for normal energy metabolism. Pineal gland ○ Secretes melatonin → controls rhythmic functions. In humans, most secretion is at night, suggesting control of sleep rhythms. Pituitary gland ○ Secretes many hormones, some control secretion of hormones from other glands. ○ Actually two separate glands connected to the hypothalamus by a thin stalk: anterior and posterior pituitary. *Summary Gonads: Produce hormones critical for sexual characteristics and reproduction. Pancreas: Regulates blood sugar with insulin and glucagon. Thyroid Gland: Controls metabolism with T3 and T4. Pineal Gland: Regulates sleep cycles with melatonin. Pituitary Gland: A master gland that influences other glands and various body functions. Adrenal Glands: Secrete hormones involved in stress response (EPI and NE) and glucocorticoids UNIT 4 Chemical Signaling Page 61  Adrenal Glands: Secrete hormones involved in stress response (EPI and NE) and glucocorticoids (like cortisol) that regulate glucose metabolism. ORGANIZATION OF THE HYPOTHALAMIC–PITUITARY AXIS Anterior pituitary—connected to hypothalamus via blood vessels. ○ Hypothalamic-releasing hormones made in the hypothalamus travel to the anterior pituitary in the blood and stimulate release of TSH, FSH, and LH, and ACTH. Axons from neurons in the hypothalamus run through the stalk and secrete hormones into blood vessels in the posterior pituitary. ○ Vasopressin acts on the kidneys to increase water retention. ○ Oxytocin stimulates uterine contractions during childbirth, and milk letdown during lactation. *Posterior pituitary: Stores and releases hormones made in the hypothalamus, controlled via nerve signals. Hormone receptors are on cell surfaces or intracellular. Peptide hormones have surface receptors. Steroid and thyroid hormones have intracellular receptors. Most are in the cell nucleus and function as transcription factors, turning gene expression on or off. Importance of the endocrine system to pharmacologists: 1. Drugs can alter hormone secretion and cause physiological abnormalities. 2. Hormones can alter responses to drugs. 3. Secretions from the pituitary are controlled by neurotransmitter systems in the brain. These systems can be altered by disease, injury, or drugs HPA AXIS UNIT 4 Chemical Signaling Page 62  CRF: corticotropin releasing factor Exam: you have to know the names of the hormones! *1. Stress Detection Stress is detected by the brain, particularly by regions like the hippocampus and hypothalamus. 2. Hypothalamus Response The hypothalamus responds by releasing a hormone called Corticotropin-Releasing Factor (CRF) (sometimes referred to as CRH, Corticotropin-Releasing Hormone). 3. Anterior Pituitary Activation The CRF travels through the blood to the anterior pituitary gland. In response, the anterior pituitary secretes Adrenocorticotropic Hormone (ACTH) into the bloodstream. 4. Adrenal Gland Activation The ACTH travels to the adrenal glands, which are located on top of the kidneys. This stimulates the adrenal glands to produce glucocorticoids, mainly cortisol (a stress hormone). 5. Cortisol’s Effect Cortisol helps the body manage stress by increasing blood sugar, enhancing the brain's use of glucose, and suppressing non-essential functions (like digestion or immune responses). 6. Negative Feedback Once enough cortisol is produced, it sends a signal back to the hypothalamus and hippocampus to reduce CRF production, thus lowering the whole stress response system. This is known as negative feedback. UNIT 4 Chemical Signaling Page 63  is known as negative feedback. Hormones Involved: CRF (Corticotropin-Releasing Factor): Released by the hypothalamus. ACTH (Adrenocorticotropic Hormone): Released by the anterior pituitary. Cortisol: Released by the adrenal glands in response to ACTH. UNIT 4 Chemical Signaling Page 64 UNIT 5 Major Neurotransmitter Systems onsdag 23. oktober 2024 14:14 1. Catecholamines: Dopamine (DA) Norepinephrine (NE) Epinephrine (EPI) 2. Serotonin 3. Acetylcholine 4. Glutamate and GABA UNIT 5 Major Neurotransmitter Systems Page 65 1. Catecholamines onsdag 16. oktober 2024 16:36 CATECHOLAMINES Content of first section: Catecholamines Catecholamine Synthesis, Release, and Inactivation Organization and Function of the Dopaminergic System Organization and Function of the Noradrenergic System INTRODUCTION Catecholamines include: Dopamine (DA) Norepinephrine (NE) Epinephrine (EPI) They belong to the group of neurotransmitters called monoamines. *Monoamines are a broad class of neurotransmitters. They are called "monoamines" because their chemical structure includes one amine group (-NH₂) attached to an aromatic ring. *Catecholamines: A subcategory of monoamines, which includes dopamine (DA), norepinephrine (NE), and epinephrine (EPI). These are especially important in functions like movement, stress responses, and alertness. *An amine group is a functional group commonly found in organic chemistry and biology. It consists of a nitrogen atom (N) bonded to one or more hydrogen atoms (H). The basic structure of an amine group is -NH₂, where the nitrogen atom has a lone pair of electrons, making it reactive. Structural features of catecholamines EPI and NE are also known as adrenaline and noradrenaline. Adjective forms—adrenergic, noradrenergic, and dopaminergic. The adrenal medulla secretes EPI and NE into the bloodstream. The neurotransmitter function of EPI is relatively minor. CATECHOLAMINE SYNTHESIS, RELEASE, AND INACTIVATION Catecholamine synthesis begins with the amino acid tyrosine. Tyrosine hydroxylase (TH) and aromatic amino acid decarboxylase (AADC) are found in neurons that make DA. Neurons that synthesize NE also have dopamine β-hydroxylase (DBH). UNIT 5 Major Neurotransmitter Systems Page 66 Multistep pathway of catecholamine synthesis TH determines the overall rate of DA or NE synthesis. Activity of TH is regulated by several factors: High catecholamine levels tend to inhibit TH. Rate of cell firing—when neurons are firing at a high rate, such as during stress, TH is stimulated, and catecholamine synthesis accelerates. *When neurons are firing quickly, especially during stressful situations, it stimulates TH, meaning it encourages the enzyme to work faster. This leads to an increase in the production of dopamine and norepinephrine. Catecholamine synthesis can be increased by administering a precursor such as L-DOPA, which has been used to treat Parkinson’s disease. α-methyl-para-tyrosine (AMPT) blocks TH, preventing overall catecholamine synthesis. After synthesis, catecholamines are packaged into vesicles. A specific transporter in the vesicle membrane recognizes monoamines – the vesicular monoamine transporter (VMAT). VMAT can be blocked by the drug reserpine. Catecholaminergic neurons use a vesicular monoamine transporter protein (VMAT2) UNIT 5 Major Neurotransmitter Systems Page 67  Catecholamines are normally released by exocytosis when a nerve impulse reaches the terminal. Release leads to behavioral activation. In lab animals, increased locomotor activity occurs. At higher doses, this is replaced by stereotyped behaviors—intense sniffing, repetitive head and limb movements, licking, and biting. This continuum of behavioral activation stems from increasing stimulation of DA receptors in the nucleus accumbens and striatum. The DA and NE systems have several receptor subtypes. The DA autoreceptor is D2 subtype; the NE autoreceptor is α2 subtype. Mutant mice with no D2 autoreceptors (but normal postsynaptic D2 receptors in non-DA neurons) were more active than controls and more sensitive to cocaine. Drugs that stimulate autoreceptors inhibit catecholamine release; autoreceptor antagonists enhance the rate of release. Inactivation of catecholamines: 1. Reuptake: DA and NE move from the synaptic cleft into the nerve terminal via specific membrane transporter proteins. The molecules are then repackaged into vesicles or broken down. Transporter-blocking drugs: Tricyclic antidepressants inhibit reuptake of both NE and serotonin (5-HT). Cocaine inhibits reuptake of all the monoamine transmitters: DA, NE, and 5-HT. 2. Breakdown of catecholamines by catechol-O- methyltransferase (COMT) and monoamine oxidase (MAO). The metabolites enter the CSF and the bloodstream and are eliminated with urine. *Catecholamines are broken down by COMT and MAO enzymes. The products of this UNIT 5 Major Neurotransmitter Systems Page 68  ○ *Catecholamines are broken down by COMT and MAO enzymes. The products of this breakdown are called metabolites. These are the substances left after catecholamines have been processed by COMT and MAO ○ These metabolites enter the cerebrospinal fluid and the bloodstream, and they are eliminated from the body via urine. ○ This process is essential for regulating catecholamine levels and ensuring that neurotransmission is appropriately controlled. Drugs that inhibit breakdown enzymes: MAO inhibitors are used to treat clinical depression. COMT inhibitors enhance the effectiveness of L-DOPA in treating Parkinson’s disease by preventing breakdown of DOPA. ORGANIZATION AND FUNCTION OF THE DOPAMINERGIC SYSTEM The dopaminergic system originates in several cell groups, mostly in the brainstem. Classification system for DA and NE neurons: Cell groups A1 to A7 are noradrenergic. Cell groups A8 to A16 are dopaminergic. The ascending DA system is divided into three pathways: 1. Nigrostriatal tract—axons from the A9 cell group in the substantia nigra extend to the caudate-putamen or striatum. The A10 cell group is in the ventral tegmental area (VTA). It gives rise to two pathways: 2. Mesolimbic dopamine pathway from the VTA to various structures of the limbic system. 3. Mesocortical dopamine pathway from the VTA to the prefrontal cerebral cortex. *The ascending dopamine system is divided into three important pathways: The nigrostriatal tract, involved in movement. The mesolimbic pathway, linked to reward and emotion. The mesocortical pathway, associated with cognitive functions and decision-making. *The ventral tegmental area is important for the reward system in the brain. The ascending DA system can be divided into three pathways (Part 1) The ascending DA system can be divided into three pathways (Part 2) UNIT 5 Major Neurotransmitter Systems Page 69 The ascending DA system can be divided into three pathways (Part 3) *The lateral septum is important for emotions like anxiety The Nigrostriatal Tract The nigrostriatal tract facilitates voluntary movement. Parkinson’s disease involves loss of DA neurons in the substantia nigra and consequent denervation of the striatum. Neurotoxins 6-OHDA and MPTP have been used in animals to damage this pathway, as a model for Parkinson’s. Damage to the nigrostriatal pathway on one side of the brain UNIT 5 Major Neurotransmitter Systems Page 70 DOPAMINE RECEPTORS DA uses five receptor subtypes, D1 to D5. All are metabotropic: they interact with G proteins and function via second messengers. D1 and D5 are similar (D1-like receptors) D2, D3, and D4 are a separate family (D2-like receptors). *Metabotropic Receptors: All dopamine receptors are metabotropic. This means they don’t directly open ion channels; instead, they interact with G proteins inside the cell. When dopamine binds to these receptors, it activates G proteins, which then initiate a series of intracellular events called second messenger pathways. These pathways can alter various cellular functions and responses. D1-like and D2-like Receptors: D1 and D5 receptors are grouped together as D1-like receptors. They generally have similar effects on cells and are involved in processes like increasing cellular activity. D2, D3, and D4 receptors form a separate family known as D2-like receptors. These receptors usually have opposite effects, such as inhibiting cellular activity. D2 receptors function as autoreceptors and as postsynaptic receptors. Receptor Activations Effect D1 receptor activation stimulates adenylyl cyclase and synthesis of cAMP; D2 receptor activation has the opposite effect. ○ *D1 Receptors: When activated, D1 receptors stimulate the enzyme adenylyl cyclase, leading to an increase in cyclic adenosine monophosphate (cAMP) levels. This process generally promotes cellular activity ○ D2 Receptors: In contrast, when D2 receptors are activated, they inhibit adenylyl cyclase, resulting in decreased cAMP levels. This has a calming effect on cellular activity. DA receptor antagonists suppress exploratory and locomotor behavior. At higher doses, such drugs can result in catalepsy—lack of spontaneous movement. UNIT 5 Major Neurotransmitter Systems Page 71  DA receptor agonists and antagonists have provided much information about the behavioral functions of DA. A newer approach: manipulate genes for individual components of the system using knockout mice in which the gene of interest has been inactivated (“knocked out”). Gene Knockout Each gene knockout produces a unique behavioral phenotype. ○ DA transporter knockout mice are extremely hyperactive (DA can’t be removed from the synaptic cleft). ○ D1 receptor knockout mice exhibit deficits in several kinds of cognitive tasks. ○ D2 knockout mice show impairment in spontaneous movement, coordination, and posture control. ○ Double knockout of both D1 and D2 receptor genes leads to fatality during the second or third week of life. ORGANIZATION AND FUNCTION OF THE NORADRENERGIC SYSTEM The noradrenergic system has central and peripheral components. Central: cell bodies in the brainstem; their ascending fibers go to many forebrain structures. Peripheral: part of the sympathetic nervous system. NE neurons in the brain are in the pons and medulla of the brainstem. Locus coeruleus (LC) in the pons: a dense collection of NE neurons corresponding roughly to the A6 cell group. The fibers extend to nearly all areas of the forebrain, as well as the cerebellum and spinal cord. The locus coeruleus (LC) UNIT 5 Major Neurotransmitter Systems Page 72 Peripheral component Many neurons in the sympathetic nervous system use NE as the transmitter. Their fibers extend to target organs throughout the body. Adrenergic receptors (adrenoceptors) are metabotropic. There are two subtypes: α and β. α2-receptors reduce synthesis of cAMP (like D2 receptors). α1-receptors operate via the phosphoinositide second-messenger system. β1- and β2-adrenoceptors stimulate adenylyl cyclase and enhance synthesis of cAMP (like D1 receptors). *α2-receptors: Decrease cAMP (inhibiting activity). *α1-receptors: Use the phosphoinositide system (increasing calcium levels). *β1- and β2-receptors: Increase cAMP (enhancing activity). Reducing cAMP means that a receptor is telling the cell to slow down its activities, decreasing the effects of excitatory signals. In-depth-explanation: 1. α-Adrenergic Receptors There are two main types of α-receptors: α1-receptors and α2-receptors. α2-receptors: These work similarly to D2 dopamine re