Neuropharmacology of the Nervous System

• The following notes pertain to the neuropharmacology of the peripheral nervous system (PNS) and central nervous system (CNS).

Learning Outcomes

• Recognize and classify the major neurotransmitters and transmitter substances within the CNS. These include amino acid transmitters, monoamines, neuropeptides, gases, and purines.
• Explain how neurotransmitters act as excitatory/inhibitory, referring to receptor effects on membrane excitability.
• Describe normal synaptic neurotransmitter events like synthesis, storage, release, receptor action, reuptake, and degradation.
• Consider drug actions at synaptic targets to modify cellular responses.

Anatomy of The Nervous System

• The nervous system is divided into the central nervous system (CNS) and the peripheral nervous system (PNS).
• The CNS consists of the brain and spinal cord.
• The PNS is divided into the somatic (voluntary) and autonomic (involuntary) nervous systems.
• The autonomic nervous system includes the parasympathetic and sympathetic branches, as well as the enteric nervous system.

Neurotransmitter Substances

• Classical transmitters include GABA, glutamate, glycine, noradrenaline, dopamine, serotonin (5-HT), histamine, and acetylcholine.
• Neuropeptides include opioids (e.g., enkephalins), neurohypophyseals (e.g., oxytocin), somatostatins, and tachykinins (e.g., substance P).
• Gases include nitric oxide and carbon monoxide.
• Other transmitter substances are neurosteroids (e.g., allopregnanolone, DHEA), purinergics (e.g., ATP, adenosine), and lipid metabolites (e.g., anandamide, 2-AG).

Classical Neurotransmitters

• Monoamines are catecholamines (noradrenaline, adrenaline, dopamine), indoleamines (serotonin), and histamine.
• Amino acid transmitters include GABA, glutamate, and glycine.
• Acetylcholine is a quarternary amine neurotransmitter.

Neurotransmitters of the PNS

• Acetylcholine (ACh) acting on nicotinic acetylcholine receptors (nAChR) mediates somatic efferent system activity on skeletal muscle.
• In the sympathetic nervous system (SNS), ACh (nAChR) acts. Smooth muscle and salivary glands are affected via noradrenaline (NA). Sweat glands are innervated by ACh (mAChR), and adrenal medulla by ACh (nAChR).
• The parasympathetic nervous system (PSNS) uses ACh (nAChR) to communicate. ACh (mAChR) affects salivary glands and smooth muscle.

Fast vs Slow Neurotransmitters

• Fast neurotransmitters act in milliseconds to seconds through ligand-gated ion channels
• Examples of fast neurotransmitters are GABA (GABAA, GABAC), glutamate (AMPA, NMDA), glycine, serotonin (5-HT3) and acetylcholine (nAChR).
• Slow neurotransmitters act in seconds to minutes through G-protein coupled receptors.
• Examples of slow neurotransmitters are GABA (GABAB), glutamate (mGluR), serotonin (5-HT1/2/4/5/6/7), dopamine (D1, D2), noradrenaline (α, β), acetylcholine (mAChR) and histamine (H1-4).

Inhibitory vs Excitatory Neurotransmitters

• Inhibitory neurotransmitters include GABA (all), glycine (strychnine-sensitive), dopamine (D2), serotonin (5-HT1), acetylcholine (M2), histamine (H3, H4), and noradrenaline (α2).
• Excitatory neurotransmitters include glutamate (most), glycine (NMDA co-agonist), dopamine (D1), serotonin (5-HT2/3/4/5/6/7), acetylcholine (nACh, M1, M3), histamine (H1, H2), and noradrenaline (α1, β).

Ion Gradients and Excitability

• Resting membrane potential of a neuron is ~-70 mV.
• Ion conductance influences cell excitability: cation permeability increases the probability of cell firing, while anion permeability decreases it.

Synaptic Transmission

• Is electrochemical.
• Action potentials are generated by Na+ influx through voltage-gated Na+ channels that cause terminal depolarization.
• Na+ entry increases membrane potential from resting (-70 mV) to threshold potential (-55 mV).
• Spatial and temporal summation leads to terminal depolarization.

Characteristics of a Neurotransmitter

• Chemicals transmit information to control cell behavior at short distances.
• Substances are synthesized in neurons and stored in vesicles in nerve terminals.
• When neurons are activated, substances are released via a Ca2+-dependent mechanism (exocytotic vesicular release).
• The effect of nerve stimulation is mimicked or blocked by exogenous agents (agonists or antagonists).
• There is a mechanism present for termination of action (enzyme degradation or uptake).

Synaptic Targets of Neurotransmission

• 1 Precursor
• 2 Neurotransmitter
• 3 Heteroreceptor
• 4 Autoreceptor
• 5 Transporter
• 6 NT Breakdown

Drug Targets

• Target enzymes, substrates, metabolites, proteins, receptors, ion channels, transport proteins, DNA/RNA, ribosomes, monoclonal antibodies, physicochemical mechanisms, and unknown mechanisms of action.

Gene Family Distribution of Current Drugs

• GPCRs (G protein-coupled receptors) make up a significant portion of drug targets.
• Other categories are nuclear receptors (NRs), ligand-gated ion channels (LGICs), and voltage-gated ion channels (VGICs).
• Many drugs still have unknown mechanisms of action.

Monoamine Synthesis

• L-tyrosine is converted to L-dihydroxyphenylalanine (L-DOPA) by tyrosine hydroxylase (cytoplasm, rate limiting).
• L-DOPA is converted to dopamine by L-aromatic acid decarboxylase (dopa decarboxylase, cytoplasm).
• Dopamine is converted to noradrenaline by dopamine β-hydroxylase (vesicles).
• Noradrenaline can be converted to adrenaline by phenylethanol-amine N-methyl-transferase (PNMT)(cytoplasm).
• L-tryptophan is converted to 5-hydroxytryptophan by tryptophan hydroxylase (cytoplasm, rate limiting).
• 5-hydroxytryptophan is converted to 5-hydroxytryptamine (serotonin) by L-aromatic acid decarboxylase.

Amino Acid Transmitter Synthesis

• In neurons and glial cells, glucose is converted to pyruvate and then to Acetyl-CoA. A-ketoglutarate is converted to glutamate and then to GABA.
• In neurons only, glutamine is converted to glutamate and then to GABA.

Acetylcholine Synthesis (and Degradation)

• Acetyl CoA + Choline are converted to Acetylcholine via Choline acetyltransferase.
• Acetylcholine is broken into Acetate + Choline, via Acetylcholinesterase.

Transmitter Storage

• Active transport into vesicles (approx. 1.1M) occurs via vesicular transporters.
• Vesicles prevent leakage of neurotransmitters into the cytoplasm.
• Transport is driven by an H+ electrochemical gradient across the vesicular membrane generated by an ATP-dependent H+ pump.
• VMAT selectively transports monoamines by H+ gradient.
• VAChT selectively transports ACh by H+ gradient.
• VGAT selectively transports GABA and glycine by H+ and electrical gradient.
• VGLUT selectively transports glutamate and inorganic phosphate ions by voltage gradient.
• Small, clear vesicles store glutamate, GABA, glycine, and ACh in nerve terminals.
• Intermediate dense-core vesicles store monoamines in nerve terminals.
• Large dense-core vesicles store neuropeptides in the cell body and undergo fast axonal transport to nerve terminals.

Traditional Neurotransmitter Release

• Terminal depolarization occurs.
• Voltage-gated Ca2+ channels open, and Ca2+ enters.
• Ca2+ entry promotes the fusion of vesicles to the terminal membrane leading to exocytosis.

GABA and Glutamate Information

• Almost all brain neurons have receptors for glutamate and GABA, approximately 50% of neurons release glutamate as an excitatory neurotransmitter. About 30-40% release GABA as an inhibitory neurotransmitter.

Histamine Information

• Cell bodies are in a small hypothalamic area and project to almost all brain parts.

Nomenclature of Drugs that Target Ion Channels

• Modulators increase ion channel opening.
• Blockers bind to and block ion channels.

Neurotransmitter Binding and Receptor Activation

• Released neurotransmitter binds to postsynaptic receptors.
• Neurotransmitter binding changes the conformation of the receptor. The receptor becomes "activated".
• Ligand-gated ion channels permit cations or anions through the membrane.
• G-protein coupled receptors activate second messengers, causing changes to ion channels and other cell signaling pathways.

Ligand-Gated Ion Channels

• 'Ionotropic' receptors gate ion channels permeable to cations or anions.
• Effects observed in milliseconds due to ion channel opening.

G Protein-Coupled Receptors

• ‘Metabotropic’ receptors activate in seconds to minutes.
• Second messenger activation leads to ion channel opening, the control of protein phosphorylation, and the release of intracellular calcium stores.

Receptors

• Dopamine receptors (D1-like and D2-like) use G protein-coupled receptors and are mainly excitatory or inhibitory.
• Noradrenaline receptors (a1, a2, and b-adrenoceptors) use G protein-coupled receptors and are excitatory or inhibitory.
• Serotonin receptors (5-HT1, 5-HT2, 5-HT4, 5-HT5, 5-HT6, 5-HT7) use G protein-coupled receptors and are inhibitory or excitatory.
• 5-HT3 receptors are ligand-gated ion channels and are excitatory.
• Acetylcholine receptors (Muscarinic M1-like, Muscarinic M2-like) are G protein-coupled and excitatory or inhibitory.
• Nicotinic AChRs are ligand-gated ion channels and are excitatory.
• GABA receptors such as GABAA and GABAC are ligand-gated ion channels (Cl-) and inhibitory. GABA receptors such as GABAB G protein-coupled (K+ efflux, inhibits Ca2+ influx) and inhibitory.
• Glutamate receptors (AMPA, Kainate, NMDA) are ligand-gated ion channels.
• Group I mGluRs are G protein-coupled and mobilize Ca2+, inhibiting K+ efflux and excitatory.
• Group II and III mGluRs are G protein-coupled (K+ efflux) and inhibitory.

Orthosteric and Allosteric Binding Sites

• Orthosteric binding sites are the recognition sites of endogenous molecules on the receptor, where agonists and antagonists bind.
• Allosteric binding sites are "other" binding sites on the receptor where modulators bind.

Nomenclature for Drugs that Target Receptors

• Agonists bind to and activate a receptor.
• Antagonists bind to and block a receptor.
• Allosteric modulators bind to another site on the receptor to increase or decrease the response to the endogenous molecule or agonist. Positive allosteric modulators dial the response up, and negative allosteric modulators dial the response down.

Receptors as Drug Targets

• Drug targets include ionotropic (ligand-gated ion channels), metabotropic (G protein-coupled receptors), kinase-linked, and nuclear receptors.

Transmitter Re-uptake

• Active transport into neurons occurs via high-affinity Na+-dependent membrane transporter proteins.
• Transmitter re-uptake is the primary mechanism for terminating synaptic action.

Nomenclature for Drugs that Target Transporters

• Substrates are transported by transporters.
• Inhibitors bind to and block transporters.

Monoamine Degradation

• Occurs through oxidative deamination by monoamine oxidase (MAO) in neurons.
• MAO is bound to neuronal and non-neuronal cell mitochondria.
• MAO-A degrades 5-HT, NA, Adr, and DA, while MAO-B degrades DA.
• Catecholamine degradation involves catechol-O-methyl transferase (COMT), generally at extraneuronal locations.

Amino Acid Transmitter Degradation

• Glutamate is degraded into a-ketoglutarate and Glutamine
• GABA is degraded into Succinic acid

ACh Synthesis, Storage, Degradation & Reuptake

• ACh is synthesized in the pre-synapse from choline.
• Degradation in the synapse is the primary mechanism for terminating ACh synaptic action.
• Choline is actively transported into the pre-synapse into neurons via choline transporters. Is then stored into small clear vesicles via vAChT.

Nomenclature for Drugs that Target Enzymes

• False substrates mimic the action of endogenous substances.
• Inhibitors stop a false substrate or endogenous substance from being catalyzed.
• Pro-drugs form a biologically active product when catalyzed by an enzyme.

Summary

• Neurons communicate via electrochemical transmission involving neurotransmitters and transmitter substances.
• Neurotransmitters can be excitatory or inhibitory, based on receptor effects on membrane excitability.
• Normal synaptic neurotransmission events include synthesis, storage, release, receptor action, reuptake, and degradation.
• These processes can be modified by drugs acting on different synaptic events.
• Transmitters have Major Functions, Therapeutic Targets, and Examples of Drugs for:
  • Dopamine
  • Noradrenaline
  • Serotonin (5-HT)
  • Acetylcholine
  • GABA
  • Glutamate
  • Endocannabinoids
  • Opioids