Neurotransmission and Pharmacological Modification in the Peripheral Nervous System

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Neurotransmission in the Peripheral Nervous System

Overview of Neurojunctional Transmission

Neurotransmission in the peripheral nervous system involves the transfer of chemical signals (neurotransmitters) across synapses or neuroeffector junctions. The arrival of an action potential at the axonal terminal initiates a cascade of events leading to the release of excitatory or inhibitory neurotransmitters.

Action Potential: Electrical impulse that travels along the neuron to the terminal.
Synapse: Junction between two neurons or a neuron and an effector cell.
Neurotransmitter: Chemical messenger released from the presynaptic neuron to transmit signals to the postsynaptic cell.
Example: Acetylcholine (ACh) and noradrenaline (NA) are major neurotransmitters in the autonomic nervous system.

Anatomy and Neurotransmitter Features

The autonomic and somatic motor nerves differ in their neurotransmitter usage and anatomical organization. Parasympathetic and sympathetic pathways utilize acetylcholine and noradrenaline at different synaptic sites.

Parasympathetic: Uses ACh at both pre- and postganglionic synapses.
Sympathetic: Uses ACh at preganglionic and NA at postganglionic synapses (except sweat glands, which use ACh).
Somatic: Uses ACh at neuromuscular junctions.

Processes Involved in Synthesis, Storage, and Release of Neurotransmitters

Key Steps in Neurotransmitter Handling

Neurotransmitter synthesis, storage, and release involve several coordinated steps within the nerve terminal.

Uptake of Precursors: Neurons absorb precursor molecules required for neurotransmitter synthesis.
Synthesis of Transmitter: Enzymatic reactions produce neurotransmitters from precursors.
Uptake/Transport into Vesicles: Neurotransmitters are packaged into synaptic vesicles for storage.
Degradation of Surplus Transmitter: Excess neurotransmitter is broken down to prevent overaccumulation.
Depolarization by Action Potential: Electrical signal triggers influx of Ca2+ ions.
Release by Exocytosis: Vesicles fuse with the membrane, releasing neurotransmitter into the synaptic cleft.
Diffusion to Postsynaptic Membrane: Neurotransmitter diffuses and binds to receptors.
Interaction with Postsynaptic Receptors: Initiates cellular response in the postsynaptic cell.
Inactivation of Transmitter: Enzymatic breakdown or reuptake terminates the signal.
Reuptake of Degradation Products: By nerve terminals for recycling.
Uptake by Non-Neuronal Cells: For further metabolism or clearance.
Interaction with Presynaptic Receptors: Modulates further neurotransmitter release.

Cholinergic Transmission

Synthesis and Packaging of Acetylcholine (ACh)

Acetylcholine is synthesized and stored in cholinergic neurons through a series of biochemical steps.

Choline Uptake: Choline enters the neuron via a carrier protein.
Acetylation of Choline: Choline is acetylated by acetyl CoA, catalyzed by choline acetyltransferase (CAT).

Equation:

Packaging: ACh is transported into synaptic vesicles by the vesicular acetylcholine transporter (VAT).

Release and Action of Acetylcholine

Exocytosis: Action potential triggers Ca2+ influx, causing vesicle fusion (via VAMPs and SNAPs) and ACh release.
Postsynaptic Receptors: ACh binds to nicotinic and muscarinic receptors, initiating responses such as muscle contraction.
Presynaptic Receptors: ACh can also bind to presynaptic receptors to regulate its own release.

Termination and Recycling

Hydrolysis: Acetylcholine is broken down by acetylcholinesterase (AChE) in the synapse.

Equation:

Reuptake: Choline is taken up by the nerve terminal for resynthesis of ACh.

Drugs Affecting Cholinergic Transmission

Various drugs can modulate cholinergic transmission at the neuromuscular junction.

Acetylcholinesterase inhibitors: Increase ACh levels by preventing breakdown (e.g., neostigmine).
Hemicholinium: Blocks choline uptake.
Botulinum toxin: Inhibits ACh release.

Noradrenergic Transmission

Synthesis of Noradrenaline and Adrenaline

Noradrenaline (NA) and adrenaline are synthesized from tyrosine through a series of enzymatic steps.

Tyrosine Transport: Tyrosine enters the nerve ending via a sodium-dependent carrier.
Rate-Limiting Step: Conversion of tyrosine to DOPA by tyrosine hydroxylase.
Further Steps: DOPA → Dopamine → Noradrenaline → Adrenaline (in adrenal medulla).

Storage and Release

Storage: Dopamine is transported into vesicles by vesicular monoamine transporter (VMAT) and converted to noradrenaline.
Release: Action potential opens Na+ and K+ channels, depolarizes the membrane, Ca2+ influx triggers exocytosis and NA release.

Fate of Released Noradrenaline and Adrenaline

Postsynaptic Action: NA/Adrenaline binds to α and/or β adrenoceptors.
Presynaptic Action: α2 receptors provide negative feedback; β receptors provide positive feedback.
Reuptake: Uptake 1 (presynaptic terminal, NET-mediated) and Uptake 2 (postsynaptic cell).
Metabolism: Monoamine oxidase (MAO) in axoplasm and catechol-O-methyltransferase (COMT) in synapse metabolize NA/Adrenaline.

Metabolic Pathways

MAO: Converts noradrenaline and adrenaline to inactive metabolites. COMT: Methylates catecholamines, further inactivating them.

Drug-Induced Modification of Transmitter Release

Role of Ion Channels

Ion channels are critical in the regulation of neurotransmitter release at nerve terminals.

Na+ Channels: Influx initiates depolarization.
Ca2+ Channels: Influx triggers vesicle fusion and neurotransmitter release.
K+ Channels: Efflux causes repolarization, restoring resting membrane potential.

Drugs Modifying Neurotransmitter Release

Drugs Decreasing Release

Ca2+ Channel Blockers: High concentrations of divalent cations (Cd2+, Mg2+, Mn2+) compete with Ca2+ and prevent entry, inhibiting exocytosis.

Drugs Increasing Release

K+ Channel Blockers: Blocking K+ channels (e.g., 4-aminopyridine) prolongs depolarization, increasing neurotransmitter release.

Drugs Acting at Noradrenergic Nerve Terminal

Drug Action	Example
Block synthesis	α-methyl-p-tyrosine
Block transport into vesicle	Reserpine
Block release	Bretylium, Guanethidine
Stimulate release	Tyramine, Amphetamine
Block reuptake	Cocaine, Tricyclic antidepressants
Block receptor (antagonist)	α, β-blocking drugs
Stimulate receptor (agonist)	α, β-agonists
Block monoamine oxidase	MAO inhibitors
Block catechol-O-methyltransferase	COMT inhibitors

Summary of Key Concepts

Major neurotransmitters: Acetylcholine and noradrenaline in the autonomic nervous system.
Release mechanism: Depolarization and Ca2+ entry trigger exocytosis.
Receptor action: Neurotransmitters act on post- and presynaptic receptors to conduct and regulate impulses.
Drug effects: Drugs can act at various stages of neurotransmitter synthesis, storage, release, degradation, and receptor interaction.
Ion channel blockers: Potassium channel blockers increase neurotransmitter release; divalent ion blockers inhibit release.

References

Rang, H.P., Dale M.M., Ritter J., Flower R. (2012). Pharmacology, 7th Edition, Churchill Livingstone.
Bertram G. Katzung, Susan B. Masters, Anthony J. Trevor. Basic & Clinical Pharmacology, 13th Edition, Lange.
Laurence L. Brunton, Bruce A. Chabner, Björn C. Knollmann (2010). Goodman and Gilman's The Pharmacological Basis of Therapeutics, 12th Edition, McGraw-Hill.
Bennett, P.N & Brown, M.J (2012). Clinical Pharmacology, 11th Edition, Churchill Livingstone.

Additional info: The notes expand on the biochemistry of neurotransmitter synthesis, release, and pharmacological modulation, which are foundational for understanding biosignaling and membrane transport in biochemistry.