Cholinergic Signalling

Acetylcholine is synthesised by choline acetyltransferase from choline and acetyl-CoA and hydrolysed within milliseconds by acetylcholinesterase in the synaptic cleft. It acts on two receptor families with entirely different kinetics: nicotinic receptors are ligand-gated ion channels producing fast depolarisation, while the five muscarinic subtypes are G protein-coupled receptors producing slower modulation. Central projections arise mainly in the basal forebrain, including the nucleus basalis of Meynert to cortex and the medial septum to hippocampus, where cholinergic tone gates attention and the encoding of new information.

The cholinergic hypothesis of Alzheimer's disease grew from the loss of those basal forebrain neurons early in the illness and the fall in cortical choline acetyltransferase activity. It produced donepezil, rivastigmine and galantamine, all approved acetylcholinesterase inhibitors and all symptomatic: they raise synaptic acetylcholine, improve cognitive scale scores modestly, and do not alter the underlying pathology. Their dose ceiling is peripheral muscarinic activity, which produces nausea, diarrhoea, bradycardia and vivid dreams.

The distinction that matters is symptomatic versus disease-modifying. Because cholinergic tone directly modulates attention, such a drug can lift performance on a test battery without touching disease trajectory, and the gain disappears with the drug.

Two claims recur and neither survives scrutiny. Choline-containing supplements are marketed as raising acetylcholine, but substrate supply is rarely rate-limiting in a well-nourished brain. And nootropic claims are built on rodent scopolamine-reversal experiments, which show only that a compound opposes a pharmacologically induced deficit, not that it enhances anything in an intact brain.