Neurotransmitters and Synaptic Transmission

Neurotransmitters

• Many different types exist including amino acids, amines, peptides, gases, and opioids
• Synthesized and degraded by enzymes
• Some neurons can use more than one, known as co-transmission
• Affect targets: neurons, muscles, and glands
• Commonly interacting with receptors
• Agonist vs. antagonist

Synaptic Transmitters and Families of Transmitters

• Amines include Quaternary amines with Acetylcholine (ACh)
• Monoamines are further divided into Catecholamines, with Norepinephrine(NE), Epinephrine(adrenaline), and Dopamine (DA)
• Also, Indoleamines, with Serotonin(5-hydroxytryptamine;5-HT), and Melatonin
• Amino Acids include Gamma-aminobutyric acid (GABA), Glutamate, Glycine, and Histamine
• Neuropeptides include Opioid peptides: Enkephalins(Met-enkephalin and Leu-enkephalin), Endorphins(ß-endorphin), and Dynorphins(Dynorphin A)
• Peptide Hormones include Oxytocin, Substance P, Cholecystokinin (CCK), Vasopressin, Neuropeptide Y (NPY), and Hypothalamic releasing hormones
• Gases include Nitric oxide and Carbon monoxide

Neurotransmitter Synthesis

• Choline, found in milk, eggs, and nuts, is used for ACh
• Tryptophan, in soy, is a precursor for 5-HT
• Phenylalanine, from protein-rich foods like meat, fish, cottage cheese, and nuts, is needed to make DA, NE, and EP

Neurotransmitter Systems

• Neurotransmitter systems are defined by their chemical molecule, synthetic machinery, packaging, reuptake and degradation and action
• Acetylcholine(ACh) was first identified by Loewi
• Nomenclature is based on -ergic
• Dale discovered Cholinergic and noradrenergic neurotransmitters
• Loewi and Dale shared the 1936 Nobel Prize in physiology or medicine

Criteria for identifying Neurotransmitters

• Synthesis and storage must occur in the presynaptic neuron
• Must be released by the presynaptic axon terminal
• When applied, it must mimic the postsynaptic cell response produced by the release of neurotransmitter from the presynaptic neuron
• Receptors must exist
• Must have an inactivation mechanism
• Activity varies based on developmental stage, daily variations and drug variations

Studying Neurotransmitter Systems

• Immunocytochemistry (IHC) is used for localizing transmitters and transmitter-synthesizing enzymes to cells, using antibodies to show protein locations
• In situ hybridization localizes the synthesis of protein or peptide to a cell by detecting mRNA sequence and creating a probe
• Shows which neurons produce which mRNA transcripts

Studying Transmitter Release

• Used to confirm if a transmitter candidate is synthesized and localized in the terminal, and whether it's released upon stimulation
• Brain slice acts as a model
  • Kept alive in vitro to stimulate synapses, and collect and measure released chemicals
  • Like Loewi, but often difficult
• Molecules may seem a neurotransmitter
• Microiontophoresis: assesses postsynaptic actions by applying substances to cells in small amounts
• Microelectrode: probe used in measuring effects on membrane potential

Studying Receptor Subtypes

• Methods to study Receptor Subtypes include Neuropharmacological analysis
• Agonists and antagonists are used such as Nicotine/Curare agonists and antagonists for ACh receptors
• ACh receptors are Nicotinic and muscarinic
• Glutamate receptors include agonists such as: AMPA, NMDA, and kainate

Acetylcholine(ACh)

• First neurotransmitter discovered by Otto Loewi
• Found at NMJs
• Linked to Black widow venom, Botulin, Botox, Nerve gasses and insecticides, and Alzheimer's Disease
• ChAT makes ACh in terminal, packaged by ACh transporters
• Degraded in cleft by acetylcholinesterase (AChE)

ACh Receptors

• Nicotinic receptors are found at NMJs, are ionotropic and usually excitatory, and Curare is an antagonist
• Muscarinic receptors affect the Heart, are Metabotropic, can be Excitatory or inhibitory, and Atropine is an antagonist

Study of Receptors

• Use ligand-binding methods
  • Identifies natural receptors using radioactive ligands
  • Ligands used can be agonists, antagonists, or chemical neurotransmitters
• Opiate receptors are an example of receptors which were discovered before the endogenous chemical that binds to them
• Naloxone antagonist

Receptor Subunit Properties

• Molecular analysis is used to determine receptor protein classes
• Different polypeptides can make up subunits
• Transmitter-gated ion channels
• Nicotinic receptors and GABAA receptors are examples

Other Amine Neurotransmitters: Catecholamines

• Catecholamines are involved in movement, mood, attention, and visceral function
• Tyrosine is the precursor amino acid, found in high-protein foods
• Dopamine is a major wanting NT
  • ~1 million neurons
  • Receptors: D1, D2, D3, D4, D5
• Norepinephrine(noradrenaline)
  • Metabotropic: B1, B2
• Epinephrine(adrenaline)
  • Metabotropic: A1, A2
• The reuptake from the cleft by Na+-dependent transporters
• Reloaded or degraded by MAO

Serotonergic (5-HT) Neurons

• Serotonergic Neurons are amine neurotransmitters
• Only ~200,000 neurons
• Cell bodies concentrated in brainstem/raphe nuclei
• Linked to Sleep states, depression, OCD, and eating
• Selective serotonin reuptake inhibitors (SSRIs)—antidepressants
• Hallucinogens (e.g. MDMA, LSD)
• 15 receptor subtypes with different types
  • 5HT1..
• MAO or reloaded
• 2-step synthesis of serotonin derives from tryptophan

Amino Acids as Neurotransmitters

• Differences among amino acidergic neurons are quantitative, not qualitative
• Terminated by reuptake into terminals and astrocytes
• Glutamate is a major excitatory neurotransmitter
  • Glutamate can cause a glutamate cascade/excitotoxicity
  • Astrocytes clear cleft
  • Ionotropic receptors
    • AMPA, kainate, NMDA (learning & memory)
  • Metabotropic
    • mGluR
• GABA is present in 40-50% of inhibitory synapses
  • Receptors = GABAA, В, С
  • Keeps brain from excessive arousal(anxiety, insomnia)
  • Agonists = tranquilizers

Amino Acid-Gated Channels

• Features Glutamate-gated channels such as AMPA, NMDA, and kainite
• NMDA channels are both voltage and ligand-dependent
• Important for learning and long-term potentiation
• GABA receptors have multiple binding sites
  • Binding sites for GABA, ethanol, benzodiazepines, and barbiturates
  • Cl- channel
  • Never mix alcohol & benzodiazepines

Other Neurotransmitter Candidates and Intercellular Messengers

• ATP excites some neurons and is often a co-transmitter. Degraded to make adenosine and binds to purinergic receptors
• Endocannabinoids are retrograde messengers
  • presynaptic CB receptors
• Nitric Oxide is a retrograde messenger
  • Viagra increases NO's ability to relax vessels

Transmitter-Gated Channels

• Fast synaptic transmission
• Sensitive detectors of chemicals and voltage
• Regulate flow of large currents
• Differentiate between similar ions
• 4-6 protein subunits form a pore
• Examples
  • nicotinic ACh receptor at NMJ
  • GABAA receptor
  • NMDA receptor for Glutamate

G-Protein-Coupled Receptors and Effectors:Transmissions

• Three steps in transmission:
  • Binding of the neurotransmitter to the receptor protein
  • Activation of G-proteins
  • Activation of effector systems can either stimulate or inhibit effectors
• Signal is passed from receptor to effector proteins
• Basic structure of G-protein-coupled receptors (GPCRs)
  • Single polypeptide with several alpha helices

Five Steps in G-Protein Operation

• Inactive: 3 subunits-α, β, and γ-"float" in membrane (a bound to GDP)
• Active: bumps into activated receptor and exchanges GDP for GTP
• Ga-GTP and GBY-influence effector proteins
• Ga inactivates by slowly converting GTP to GDP
• Ga and Gẞy recombine to start the cycle again

G-Protein-Coupled Effector Systems

• The shortcut pathway is a direct, fast, and localized route from receptor to G-protein to ion channel
• Second messenger cascades involves G-protein coupling neurotransmitter with downstream enzyme activation
• Some cascades branch
• Phosphate groups are added to or removed from a protein, which changes the conformation and biological activity
• Signals can be amplified, longer-lasting and more varied than in ligand-gated receptors

Summation and Concluding Remarks

• Neurotransmitters transmit information
• They are an essential link between neurons and effector cells
• One transmitter activates more than one receptor subtype for a greater postsynaptic response (divergence)
• Different transmitters converge to affect same effector system (convergence)
• Signaling pathways are inputs which vary temporally and spatially and drugs can shift the balance of signaling power

Psychopharmacology

• Is a branch of pharmacology and the study of Psychological effects of drugs
• Pharmakon means Charm, poison, scapegoat, remedy/medicine
• Virtually all ingested substances can alter bodily functions
• A drug is a chemical administered to bring about desired change with pronounced effects in small quantities
• Drugs are exogenous ligands

Drugs and Mechanisms

• No drug has a single effect
• Higher concentrations interact with variety of molecules
• Few bind to a single target
• Drugs do not 'seek' targets
• Drugs work by one or more of the following mechanisms
• Increasing neurotransmitter synthesis
• Causing vesicles to leak
• Increasing neurotransmitter release
• Decreasing reuptake, thus increasing the time the NT is in the synapse
• Blocking the breakdown of the neurotransmitter, thus increasing the concentration
• Directly stimulating or blocking postsynaptic receptors

Drug Types

• Opiates is an umbrella term of drugs, including natural, synthetic and semi-synthetics
• Cocaine is a powerfully addictive stimulant drug
• Alter neural messaging and functionality

Opiates

• Includes Painkillers, narcotics, Heroin(artificially modified form, 3 x morphine), Fentanyl, OxyContin, and Percocet
• Opiates bind to receptors in limbic, hypothalamus, locus coeruleus, and in periaqueductual gray
• They are G-protein coupled receptors that are responsible for the strong potential for addiction/severe withdrawal, and overdose(respiratory depression)

Cocaine

• Originates from the Coca plant and traditionally consumed in chew leaves
• In chew form, provides endurance, hunger alleviation, and well-being
• A Cocaine powder, crack, is purified form addictive
• Blocks monoamine transporters for NE and DA (particularly in nucleus accumbens)
• Increases cAMP production and postsynaptic activity

Reward Circuitry

• Reward Pathway features the ventral tegmental area to nucleus accumbens and connects to the prefrontal cortex

Consequences of Drug Use

• A Drug Tolerance, or habituation, is a process where more drugs are required to achieve a certain response
• Withdrawal occurs when the body does not have a drug present
• Craving is a strong urge to do drugs
• Addiction is a state that involves Craving, seeking, and use (psychological & behavioral)

Drug Vulnerabilities

• People differ in their vulnerability to drug use
• Biological factors (brain organization, receptor distribution)
• Personal characteristics
• Family situation
• Environmental factors
• Prenatal environment
• Prevent use, abuse and dependence
• No single approach is uniformly effective