Neurotransmission B.Sc 2002

Archetypal neurotransmitter is acetylcholine; We know more about cholinergic transmission than any other. However, cholinergic transmission is the exception not the rule in the CNS

Acetylcholine can be very rapidly hydrolysed, that is broken down by reaction with water. It may be this characteristic that makes it suitable as a transmitter

Why is ACh a transmitter? What makes it suitable? Ach is a quaternary amine; it has a nitrogen atom that is positively charged at one end of the molecule Most compounds that are agonists at cholinergic synapses also have a positively charged nitrogen

Some General Cholinergic agonists

Two types of cholinergic agonists; Nicotinic and Muscarinic Examples of Specific Nicotinic agonists: Nicotine, Lobeline

Muscarinic agonists do not need positively charged nitrogen;

Nicotinic: ion channel opening Muscarinic: G-protein coupled M1; ‘neural’; slow epsps in ganglia; increase IP 3,DAG M2; ‘cardiac’ decrease heart rate; decrease cAMP M3;’glandular’ increase secretion increase IP 3

Atropine & scopolamine are antagonists at all muscarinic synapses

Transmitter release Acetylcholine is found in synaptic vesicles (50 nm diameter), clustered around release zones in the presynaptic membrane. Voltage-gated N- or P- type calcium channels are found in large numbers in the presynaptic membrane Blockade of these channels (black widow spider venom, conotoxin) prevents transmitter release

After stimulation of axon, muscles normally show an ‘end plate potential’ that triggers AP. In quiescent muscles, spontaneous ‘miniature’ end plate potentials are observed.

Fact that mepps were quantized (2 mV, 4mV, 6 mV, not e.g. 3.5 mV, was evidence that transmitter was released in packets, or quanta. This led to idea that transmitter was released from vesicles. Even in absence of stimulation, some vesicles were releasing transmitter

Methods to study transmitter release mechanisms Toxins (mostly proteases) eg botulinus toxin, tetanus toxin Capacitance measurements of secreting cells Intracellular ion-sensitive dyes Mutation of intracellular vesicular & membrane proteins

Synaptic docking proteins On vesicle: synaptotagmin (sytg), synaptobrevin (syb) On plasma membrane: SNAP-25, syntaxin (sytx) In cytoplasm: NSF (contains  -SNAP)

Syb: synaptobrevin, Sytg: synaptotagmin,sytx: syntaxin

Transmitter can ‘leak’ from fusion complexes before proper calcium triggering Spontaneous release can occur from fluctuations in intracellular calcium

Studies on ACh receptors have used the electric eel‘torpedo’ as it contains very large amount of receptor protein in the electroplax organ.

Transmitter inactivation ACh uses enzymic breakdown as mechanism of inactivation All other transmitters use reuptake as inactivation mechanism

UNSOLVED PROBLEMS: How does ACh unbind from binding site? What is actual mechanism of allosteric distortion?

Cholinegic antagonists Depolarisation blockers eg suxamethonium Do not unbind from receptor, leave muscle depolarised and unable to fire second AP Curare-like agents bind to receptor but do not open ion channel