Acetylcholine receptors

The Nicotinic receptors (nAChR) are ligand-gated ion channels found all the way through the CNS. They mediate rapid acetylcholine transmission from the septum to the GABAergic inhibitory interneurons in the hippocampus. This is considered to help in synchronizing the rhythmic firing of hippocampal pyramidal cells. They also mediate rapid ACh transmission from brainstem nuclei to the ventral tegmental region, stimulating the dopamine reward pathways. This might be the route by which nicotine is addictive. The Presynaptic nicotinic receptors enhance transmitter release at numerous sites. Interestingly they potentiate the glutamate transmission through NMDA but not AMPA receptors in the prefrontal cortex, suggesting a role in the memory.

The Slow cholinergic transmission by acetylcholine in the CNS is mediated by muscarinic receptors (mAChR). All the mAChR responses can be blocked by atropine. There are five mAChR subtypes, in all GPCRs. M1, M3, and M5 are all attached through Gq to the phospholipase second messenger system therefore are excitatory by elevating the inositol trisphosphate and diac-ylglycerol. M2 and M4 reduce cAMP, however the M2 also has other effects.

The Postsynaptic mAChR are generally M1 that can be selectively blocked by the pirenzepine, whereas the presynaptic autoreceptors are M2 receptors, reduce the release of ACh, and can be blocked by the methoctamine. M1 receptors facilitate cortical neuron responses to excitatory input (by closing the KM potassium channels), and learning.

In the peripheral nervous system, both the muscarinic and nicotinic receptors are included in cholinergic transmission in autonomic ganglia, while muscarinic receptors are found only at the neuroeffector junctions of the ANS; that is, on cardiac muscle, smooth muscle, and glands. The Nicotinic receptors mediate rapid transmission at the neuromuscular junction among motor neurons and the skeletal muscle.