1. Acetylcholine (ACh)

ACh is formed in a single enzymatic step. The enzyme, choline acetyltransferase, catalyzes the esterification of choline by acetyl-CoA.

Acetyl CoA + choline

CH3— C — O — CH2 — CH2— N — (CH3)3 + CoA Acetylcholine

The transferase is specific to cholinergic neurons and is not expressed in any other cell type. (The term cholinergic is used to denote a cell that releases ACh as a neurotransmitter. Similarly, glutaminergic, dopami-nergic, and serotonergic indicate that a neuron releases glutamate, dopamine, or serotonin, respectively. If a cell responds to ACh, that cell is called cholinoceptive, a term used infrequently for the other neurotransmit-ters; e.g., "dopaminoceptive" is unusual.) The formation of ACh is limited by the supply of choline. Choline is not made in nervous tissue, but must be obtained through the cerebrospinal fluid from dietary sources or recaptured from the synaptic cleft from the ACh released and hydrolyzed by the enzyme acetylcholi-nesterase (see later discussion).

There are two general classes of acetylcholine receptors (AChR): nicotinic, responding to the alkaloid nicotine, and muscarinic, responding to the mushroom poison, muscarine. ACh is excitatory at the neuromuscular junction, where it binds to post-synaptic nicotinic AChRs. As we saw with Loewi's experiment, it is an inhibitory (parasympathetic) transmitter to the heart through muscarinic AChRs. In the periphery, ACh is also the transmitter for all preganglionic neurons of the autonomic nervous system. In the brain, there are many cholinergic systems, for example, cholinergic neurons in the nucleus basalis have widespread projections to the cerebral cortex.

Nicotinic AChRs are ionotropic, meaning that, when they bind ACh, they open up to pass ions from the extracellular space into the postsynaptic neuron. Muscarinic AChRs are metabotropic. These receptors activate various second messenger pathways to

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