Cholinergic signaling

Cholinergic signaling is a fundamental neurotransmitter system operating in both the central and peripheral nervous systems, mediating fast transmission at the neuromuscular junction, autonomic ganglia, and within the brain, where it regulates attention, learning, memory, and arousal.

Acetylcholine (ACh) is synthesized from choline and acetyl-CoA by choline acetyltransferase (ChAT) and packaged into vesicles by VAChT (Oda, 1999; Parsons et al., 1993). Signaling is terminated by acetylcholinesterase (AChE), which hydrolyzes ACh within milliseconds; the released choline is recycled via the high-affinity choline transporter (CHT1) (Soreq & Seidman, 2001). ACh acts through two receptor classes: ionotropic nicotinic receptors (nAChRs), which are pentameric cation channels, and metabotropic muscarinic receptors (mAChRs), which are G protein-coupled (Changeux, 2010; Wess et al., 2007). Muscle-type nAChRs mediate neuromuscular contraction, while neuronal nAChRs (e.g., α4β2, α7) mediate fast transmission and presynaptic modulation in the brain (Dani & Bertrand, 2007).

The five muscarinic subtypes (M1-M5) couple to Gαq (M1, M3, M5) or Gαi/o (M2, M4); M1 receptors are critical for cognition, M2 are presynaptic autoreceptors, and M3 mediate peripheral parasympathetic responses (Caulfield & Birdsall, 1998). Central cholinergic neurons originate in the basal forebrain (innervating cortex and hippocampus) and the pedunculopontine tegmentum (regulating arousal), while peripheral cholinergic signaling controls all parasympathetic postganglionic and somatic motor functions (Mesulam, 2004; Picciotto et al., 2012). Cholinergic dysfunction underlies Alzheimer's disease, where basal forebrain degeneration causes cognitive decline, treated symptomatically with cholinesterase inhibitors (Francis et al., 1999). Myasthenia gravis results from autoimmune attack on muscle nAChRs, treated with pyridostigmine (Drachman, 1994). Organophosphate poisoning irreversibly inhibits AChE, managed with atropine and pralidoxime (Eddleston et al., 2008). Nicotine addiction is driven by nAChRs in the ventral tegmental area (Picciotto et al., 1998). Subtype-selective muscarinic modulators are under investigation for Alzheimer's and schizophrenia (Conn et al., 2009).

In conclusion, cholinergic signaling, through fast nicotinic and slower muscarinic actions, controls neuromuscular, autonomic, and cognitive functions, with dysfunction central to Alzheimer's, myasthenia, and addiction.

References

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Conn, P. J., Jones, C. K., & Lindsley, C. W. (2009). Subtype-selective allosteric modulators of muscarinic receptors for the treatment of CNS disorders. Trends in Pharmacological Sciences, 30(3), 148–155.

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Mesulam, M. M. (2004). The cholinergic innervation of the human cerebral cortex. Progress in Brain Research, 145, 67–78.

Oda, Y. (1999). Choline acetyltransferase: The structure, distribution and pathologic changes in the central nervous system. Pathology International, 49(11), 921–937.

Parsons, S. M., Prior, C., & Marshall, I. G. (1993). Acetylcholine transport, storage, and release. International Review of Neurobiology, 35, 279–390.

Picciotto, M. R., Zoli, M., Rimondini, R., et al. (1998). Acetylcholine receptors containing the β2 subunit are involved in the reinforcing properties of nicotine. Nature, 391(6663), 173–177.

Picciotto, M. R., Higley, M. J., & Mineur, Y. S. (2012). Acetylcholine as a neuromodulator: Cholinergic signaling shapes nervous system function and behavior. Neuron, 76(1), 116–129.

Soreq, H., & Seidman, S. (2001). Acetylcholinesterase—New roles for an old actor. Nature Reviews Neuroscience, 2(4), 294–302.

Wess, J., Eglen, R. M., & Gautam, D. (2007). Muscarinic acetylcholine receptors: Mutant mice provide new insights for drug development. Nature Reviews Drug Discovery, 6(9), 721–733.