Neuropharmacology Intro & Principles PDF

*Intro to Neuropharmacology.* **Neuropharmacology** is the study of how drugs affect the nervous system, including the brain, spinal cord, and peripheral nerves. It focuses on understanding the interactions between drugs and neural cells, such as neurons and glial cells, to influence their function...

*Intro to Neuropharmacology.* **Neuropharmacology** is the study of how drugs affect the nervous system, including the brain, spinal cord, and peripheral nerves. It focuses on understanding the interactions between drugs and neural cells, such as neurons and glial cells, to influence their function, signalling, and behaviour. **Psychopharmacology** (effects of drugs on psychologic parameters such as emotion\ and cognition).\ **Neuropsychopharmacology** (all sorts of drug effects on nervous system).\ **Medical neuropharmacology** (effects of medicines and their side-effects).\ **In vitro neuropharmacology** (effects of drugs on isolated tissues or neurones -- determining concentration (in M) response relationships).\ **In vivo neuropharmacology** - (effects of drugs in organisms and animals;- determining dose (e.g. mg/kg) response relationships). **CASE STORY 1**. Picrotoxin acts as a **GABA A receptor antagonist**, meaning it inhibits the function of **GABA A receptors**, which are responsible for mediating the inhibitory effects of GABA in the brain. By blocking these receptors, picrotoxin reduces GABAergic inhibition and increases neural excitability. Picrotoxin is derived from a plant (commonly referred to as the \"fish plant\"). Its seeds are traditionally used to **stun or stupefy fish**. Since **GABA** is the brain\'s major inhibitory neurotransmitter, inhibiting its receptors leads to increased neural activity, which can result in effects like seizures if taken in high doses. **Picrotoxin** can be used as an **antidote for barbiturate toxicity**, as it counteracts the excessive inhibitory effects of barbiturates on the central nervous system. **CASE STORY 2.** A 52-year-old man presented to the ER with nausea, vomiting, and acute dyspnea after consuming the liver and gonads of a fish he had caught. Minutes after eating, he experienced perioral paresthesias, muscle weakness, and progressive respiratory failure, leading to bradypnea, bradycardia, and cardiac arrest. He was resuscitated, intubated, and stabilized but remained in a deep coma (GCS 3) with complete paralysis, absent motor responses, and unreactive pupils. His family provided the fish he had consumed, which was identified as **Lagocephalus sceleratus** (a toxic pufferfish) by a marine biologist and poisoning centre. The clinical features and timing confirmed a diagnosis of **tetrodotoxin (TTX) poisoning**. He was treated with supportive care in the ICU. TTX **blocks sodium channels**. TTX binds to voltage gated sodium channels, blocks the path of action potential mediated by sodium. No neurotransmitter release. A **depolarisation** of the membrane means the **sodium channels open** and **conduct current** into the cell. When **100 nM** of TTX is supplied the **depolarisation** of the membrane is **blocked**, sodium action disappears. TTX is an antagonist against the sodium channels.  At **30 nM** the **sodium** current completely **disappears**, after wash there is a recovery. As we **increase concentration of TTX**, the **conductance** through the channel **decreases**, more inhibited. TTX can also be used as a pain treatment. When **TTX binds to sodium channels**, it **blocks the influx of sodium ions (Na⁺)** into neurons, which prevents the generation and propagation of action potentials. This inhibition disrupts pain signalling in sensory neurons, making TTX a potential analgesic in conditions involving severe or chronic pain. **CASE STORY 3.** **Curare vine**, a liana used by Amazonian Indigenous people for making arrow and dart poisons, contains alkaloids with paralytic effects. The primary active compound is **d-tubocurarine**, an **antagonist at acetylcholine receptors** (specifically **nicotinic acetylcholine receptors** at the neuromuscular junction). By blocking these receptors, d-tubocurarine prevents acetylcholine from binding, thereby inhibiting muscle contraction and causing paralysis. The graph shows the effects of **d-tubocurarine** on muscle twitch tension in two different muscles: **EDL (extensor digitorum longus)** and **DIA (diaphragm)**, with IC50 values of 0.3 µM and 1.8 µM, respectively. The **instant death** occurs because the diaphragm (DIA) is essential for breathing. Although the diaphragm is less sensitive to d-tubocurarine (higher IC50), at sufficiently high doses, the drug causes complete paralysis of the diaphragm, leading to respiratory failure and death. In contrast, the EDL muscle, which is not vital for survival, is more sensitive (lower IC50), so it becomes paralyzed first. However, paralysis of the diaphragm directly impacts breathing, causing **instant death** when the drug concentration exceeds the threshold for diaphragm paralysis.  **d-Tubocurarine** reduces muscle contraction by blocking ACh receptors. **Neostigmine** reverses this effect by increasing ACh levels, partially overcoming the blockade caused by d-tubocurarine. **The leach test and the discovery of acetylcholine:** The leech test was based on the use of **eserine** (physostigmine), a **potent inhibitor** of the enzyme **acetylcholinesterase**.\ The technique was based on the discovery by the German pharmacologist Fühner (1918), who showed that when eserine was added to an organ bath in which a leech muscle was suspended, the muscle became extremely sensitive to acetylcholine, and he suggested the preparation as an assay system for eserine.\ Wilhelm Feldberg merely reversed the procedure and used the eserinised muscle as a sensitive and simple assay for acetylcholine.\ The use of eserine in experiments by adding it to the perfusion fluid, reduced the circulating levels of acetylcholinesterase, increased the amounts of acetylcholine and thus made the accurate measurements possible (Kymograph). Concluded that acetylcholine was the substance which was mediating muscle contraction. **Neurotransmitter Criteria:** Should be able to **pharmacologically isolate** them and **replicate** them synthetically to produce same action. Should be some type of **antagonist**, to prove the action of the molecules. **Peripheral vs Central Excitatory Transmission:** **Neuromuscular Junction (Peripheral)**: **Multi-vesicular release** ensures strong signals. **Acetylcholine (ACh)** is the neurotransmitter. Signal terminated by **acetylcholinesterase (AChE)** breaking down ACh. **Central Synapse**: **Single vesicle release** allows fine control. **Glutamate (Glu)** is the neurotransmitter. Signal terminated by **glutamate re-uptake** via transporters. Neurotransmitters are either **ionotropic** or **metabotropic** G protein couples (GPCR). **Major neurotransmitters:** **Classical neurotransmitters** act quickly and directly mediate synaptic transmission. **Non-classical neurotransmitters** are slower, modulate signalling, and often have broader, long-term effects. *Basic Principles of Neuropharmacology.* - **Ligand** is any chemical that **binds** to (or combines with) a **receptor**. - A **Receptor** is a **cellular macromolecule** - or assembly of macromolecules - concerned directly and specifically in chemical signalling between and within cells. **Binding** of ligands to receptors is an **active process**, which happens due to the alignment of 3D shape and biophysical properties (forces) between the ligand and the binding site on the receptor: --Multiple points of interaction may be needed within a binding site for binding to occur.\ --Hydrophilic and hydrophobic, charge sites.\ --Van der Waals, electrostatic, covalent. Binding happens with a **force**. Binding of one part of the molecule can **facilitate or prevent** the binding of another part (e.g. through shape changes). **Ligands** that are **produced naturally** by the body (e.g. neurotransmitters and other molecules that can bind to receptors) are referred to as **endogenous ligands**. Endogenous ligands can also be **synthesised** in the lab or **modified** to **change** their **properties**. Endogenous ligands that are modified or designed by the chemists are termed **exogenous ligands**. The concept of "binding" of ligands to their receptors in tissues was developed in 1960s using radioligand binding assays to quantify the amount of ligand bound to receptors. A **ligand** (e.g., neurotransmitter) is **radiolabelled** and incubated with a **tissue preparation**, which is then **extensively washed** to remove loosely bound drug molecules. A **radioactive atom**---typically **³H**, **¹⁴C**, or **¹²⁵I**---is added to the ligand in a way that does not alter its **binding properties**. In a **radioligand binding assay**, the binding of molecules is not entirely **specific**, as many molecules exhibit both **specific** and **nonspecific binding sites**. For instance, binding at the **membrane** may result in **nonspecific labelling** and generate **nonspecific signals** from the tissue. Washing the preparation can reduce some of these nonspecific interactions; however, molecules that bind with a certain **binding force** may persist, leading to **inaccurate results**. As we **increase ligand concentration**, more and more ligands are bound, linear part of the curve tells us about the total binding of the ligand to the binding sites which includes both specific and nonspecific binding sites. **Quantification of radioligand binding:** Looking at specific binding: Plotting the log values gives us the second graph and makes it easy to work out the Kd value. **Kd means dissociation constant** it is the equilibrium of the compound with the receptor. The **specific binding** of a **ligand** to a **tissue preparation**, when the ligand is at **equilibrium** with the receptors, is quantified by two key properties: 1. **Affinity of binding**, expressed as the **dissociation constant (Kₐ)**, which measures how tightly the ligand binds to the receptor. 2. **Total binding capacity (Bmax)**, which represents the **maximum binding** achievable with the available receptors. As the **ligand concentration increases**, **non-specific binding** also rises, which can interfere with the accurate measurement of specific binding. **Autoradiography:** Tissue is incubated with a radiolabelled ligand to allow it to bind to its targets. The tissue is then exposed to photo film. The radiation emitted by the radioactive ligand exposes the film and creates a picture highlighting the specific pattern of binding in the brain. **Functional classification of ligands: agonists and antagonists:** Ligands, in simple terms, can be defined as agonists or antagonists, described in terms of effects and their ability to bind to their targets. **Agonists** evoke (produce) effects in biological tissue; they can be full, partial or inverse.\ **Antagonists** do not have effects of their own on biological tissue but can block effects evoked by the agonists. Thus, their effect is to antagonise the action of an agonist. Antagonists can be competitive and non-competitive. All neurotransmitters are agonists. **Quantification of an agonist effect:** Increasing conc of agonist, increase in biological response reaching E max which is the maximum biological response. **Concentration-Response Curve** - **Emax (Maximum Response)**: The maximal effect the agonist can produce is indicated by the plateau on the graph. Adding more agonist beyond this point does not increase the response. - **EC50 (Effective Concentration 50)**: The concentration of the agonist required to produce **50% of the maximum response**. In this graph, EC50 = 10mM. - The **x-axis** represents the **agonist concentration**, while the **y-axis** shows the **biological response**. - Increasing agonist concentration initially increases the response, but at higher levels, the response plateaus, indicating receptor saturation. - Displays the agonist concentration (x-axis) and response (y-axis) on a linear scale. - Useful for visualizing changes at low agonist concentrations but less effective for identifying EC50 or trends over wide ranges. **Logarithmic Plot**: - The x-axis represents the **log of agonist concentration** rather than the concentration itself. - Preferred for **accurately determining EC50** and better visualizing **sigmoidal concentration-response relationships**. - EC50 = 10mM is easily identifiable as the point corresponding to 50% of the maximum response.   **Potency**: Determined by **EC50** (concentration needed for 50% of the maximum response). Agonists with **lower EC50** are more potent. In the first graph, all agonists have the same **Emax** but differ in potency (A \> B \> C). **Efficacy**: Defined by **Emax** (maximum response an agonist can achieve). Agonists with higher **Emax** are more effective. In the second graph, all agonists have the same **EC50** but differ in efficacy (A \> B \> C). Potency reflects **how much** agonist is needed, while efficacy reflects **how effective** the agonist is at its maximum potential. Some partial agonists are more potent than full agonists. e.g. partial agonist is twice as potent as full agonist. Both bind to same receptor, but one has higher affinity than other agonist. **Maximal Drug Responses and Spare Receptors:** **Non-Linear Relationship:** **Linear Relationship:** **Hyperbolic Relationship:** In most systems, the relationship between receptor occupancy and response is hyperbolic. Spare receptors allow maximal responses to be achieved without full receptor occupancy, enhancing sensitivity to agonists. **Amplification of Efficacy:** **Intracellular Pathways:** **Response Potency**: Potency depends on **signal amplification**: **Assay close to receptor interaction** shows small differences between agonists. **Intracellular amplification** enhances differences, making weaker agonists appear as full agonists at the response level. **Other types of concentration response curves:** An inverted U-shaped curve indicates that the biological response elicited by an agonist **progressively increases** as the agonist concentration **increases** and subsequently peaks at a moderate concentration; **higher concentrations elicit progressively smaller responses.** **Competitive Antagonists:** A **competitive antagonist** competes with an **agonist** (or **endogenous ligand**) for the **same binding site** on the receptor. The **antagonist** does not reduce the **efficacy** of the agonist because the **same number of receptors** remain available for binding. This is why an **increased concentration** of the agonist can **overcome the effects** of a competitive antagonist. **Competitive antagonists** cause a **rightward shift** in the agonist\'s dose-response curve without affecting its **maximum response**. **Non-competitive antagonist:**  A **non-competitive antagonist** binds to a **different site** on the receptor, altering its **configuration** and reducing the **number of available receptors** for the agonist. While the **potency** of the agonist remains unchanged, the **efficacy** is significantly reduced because the maximum response cannot be achieved, regardless of agonist concentration. **How do inert antagonists produce behavioural responses?** By preventing the agonist action and therefore preventing the biological effect.\ **Competitive antagonists** compete with agonists (or endogenous ligands) for the same binding site on the receptor.\ **Noncompetitive antagonists** bind to an allosteric (non-agonist) site on the receptor to prevent activation of the receptor.\ In the presence of a **constant concentration of an agonist** (endogenous or exogenous), and by systematically changing the antagonist concentration we can quantify the inhibitory effect of the antagonist action on the agonist evoked response. The IC 50 value (half maximal inhibitory concentration) indicates how much antagonist is needed to inhibit a biological process by half. **SUMMARY:**  Most **ligands** exhibit **affinity** for multiple **receptor types**, meaning they can bind to and activate or block more than one receptor. **Selectivity** refers to the **preference** of a ligand for binding to a specific receptor over others. - A **highly selective ligand** interacts predominantly with one receptor type, minimizing off-target effects. - A **non-selective ligand** binds to multiple receptors, which can lead to broader effects but may also increase the risk of **side effects**. *Basic Principles of Neuropharmacology 2.* Basic Terms: **Ligand** is any chemical that binds to a receptor, they can be agonists or antagonists.\ **Drug** is any substance (other than food) that is used to prevent, diagnose, treat, or relieve symptoms of a disease or abnormal condition. Drugs can also affect how the brain and the rest of the body work and cause changes in mood, awareness, thoughts, feelings, or behaviour. Some types of drugs, such as opioids, may be abused or lead to addiction.\ **Medicine** refers to the practices and procedures used for the prevention, treatment, or relief of symptoms of diseases or abnormal conditions. This term may also refer to a legal drug used for the same purpose.\ Drugs that influence behaviour are known as psychotropic agents. But there are many other terms: chemical, compound, agent, etc... **Antidepressant**, antianxiety, anticonvulsant and antipsychotic agents are among the most\ widely prescribed medications.\ Some of these acts on other organ systems and are associated with unpleasant side effects.\ Many people use common substances, such as caffeine, alcohol and nicotine, that also act on\ the central nervous system. In some people, drugs are used compulsively, in a manner that constitutes an addiction. The initial target of a drug determines the cells and circuits on which the drug acts, and at the same time the potential efficacy and side effects. The initial binding of a drug to its target is only the beginning of a signalling cascade that affects\ the behaviour of cells, neural circuits and animals. **Pharmacodynamics** (grk, medicine & power).\ --The time course of the effect of a drug, and the intensity (power) of the effects.\ --"What a drug does to the body?" The ability of a drug to produce an effect on an organism is dependent on the underlying mechanisms of drug action.\ Affinity, efficacy, potency, concentration-response relationships, spare receptors & amplification. **Pharmacokinetics** (grk, medicine & movement)\ --The time course of a drug in the body\ --"What the body does to a drug". The ability of a drug to produce an effect on an organism is dependent on many of its properties (in addition to its mechanism of action), from its absorption to its stability to its elimination - its bioavailability! **Stages of Pharmacokinetics:** 1.Route of administration\ 2. Release / liberation\ 3. Absorption (dosing regiments)\ 4. Distribution (compartments)\ 5. Metabolism (metabolite kinetics, clearance)\ 6. Excretion (clearance) **Route of administration:** Route of administration must be considered, which can determine whether or\ how rapidly a drug reaches its target organ and which organs it affects. Enteral (through intestine): Oral administration typically results in a relatively slow onset of action. Sublingual / buccal / rectal. Parenteral describes all other routes of administration, including:\ Intravenous (into the venous system)\ Intramuscular (into a muscle)\ Subcutaneous (under the skin)\ Inhalation / nasal\ Intraperitoneal (into the peritoneal-- abdominal cavity)\ Intracerebroventricular (into the cerebral ventricular system)\ Intracerebral (into the brain parenchyma) delivery. **Oral bioavailability:** When drugs reach the stomach, it is broken down into fragments, this increases the surface area of the drug and the fragments can be dissolved into the solution and can reach intestine for absorption, Ph in stomach is low (1-2), drugs have to withstand this acidity. Pepsin and drugs can be broken down. Most drugs are absorbed within duodenum / jejunum of small intestine (first and second sections of the small intestine)\ Villi increase surface area.\ Highest concentration of villi in duodenum/ jejunum.\ Lined with epithelial cells.\ Supportive network of capillaries draining into portal vein. Drug absorption involves a variety of processes:\ Passive diffusion, Convective absorption, Active transport, Facilitated transport, Ion pair formation, Pinocytosis. Drug absorption depends on intestinal motility:\ Food, Exercise, Disease, Drugs, Time of day. Absorption is the disappearance of drug from its site of administration and not the appearance of the drug in the general circulation. **First pass metabolism:** Drug goes through the intestine and might go out to faeces or might get absorbed by cytochrome p50 enzyme. The drug which is absorbed and delivered into the liver is inactive and, in the liver, it gets converted into active an passed onto organ. **Bioavailability is affected by binding to plasma proteins:**  **Bioavailability is also affected by barriers:** Bioavailability is regulated through intestinal transport. There is also bioavailability to BBB, two types of drugs either water soluble or lipid soluble can pass. **Getting drugs into the brain -- bioavailability:** The bioavailability of a drug determines how much of the drug that is administered actually reaches its target. **Influenced** by absorption of the drug (from the gut if administered orally), **Affected** by metabolism and excretion, **Affected** by binding of the drug to plasma proteins, which makes the drug unavailable to bind to its target**, Influenced** by a drug's ability to penetrate the blood-brain barrier, or its ability to permeate cell membranes. **Drug pharmacokinetics decides the dosing regimen:** When drug is taken up and reaches maximum availability, then decays exponentially. When repeated we can reach a steady state of drug concentration.  **Pharmacokinetics & dosing - terms used:** **Volume of Distribution:** Injecting 286 mg/L of a substance into 20grams in tank. Volume of tank = 20,000/ 286= 70L. This number tells us where the drug is.  **Fate Of Ethanol In Body:** **Ethanol Dose and Dosage Form:** Ethanol can be consumed through various beverages like beer, wine, neat spirits, cocktails, or mixed drinks. **Route of Administration:** Ethanol enters the body via different routes: - **Oral:** The most common method, drinking. - **Intravenous:** Rare and medical cases. - **Rectal or inhalation (lungs):** Uncommon and alternative methods. **Pharmacokinetics: Describes how the body handles ethanol through four steps:** - **Absorption:** Ethanol is absorbed into the bloodstream, primarily through the stomach and intestines. - **Distribution:** It spreads throughout the body, including vital organs like the brain. - **Metabolism:** The liver processes ethanol primarily, converting it into acetaldehyde and then acetic acid. - **Excretion:** Ethanol is eliminated from the body, mainly through the urine, breath, and sweat**.** **Concentration-Time Profile:** Shows the Blood Alcohol Concentration (BAC) over time. BAC increases after consumption and decreases as ethanol is metabolized and excreted **Passage Through the Blood-Brain Barrier:** Ethanol crosses the blood-brain barrier due to its chemical properties, affecting the central nervous system (CNS). **Pharmacodynamics** Refers to how ethanol interacts with the body to produce effects. For example: Ethanol binds to GABA-A receptors in the brain, enhancing inhibitory signals and causing sedation or relaxation. **Effects (Based on BAC Levels)** - **Low BAC (\~50 mg%):** Leads to effects like excitement, relaxation, disinhibition, and euphoria. - **High BAC (\>400 mg%):** Causes severe impairments such as ataxia (loss of coordination), unconsciousness, coma, or even death.