Removal of ACh is primarily enzymatic. Acetylcholi-nesterase is anchored extracellularly to the postsynap-tic membrane near the AChRs and hydrolyzes the transmitter to acetate and choline. The enzyme is so effective that on average an ACh molecule binds only once to a receptor. Thus, hydrolysis of ACh serves to punctuate the message; it also is a conservation mechanism (see later discussion).

Both small-molecule and peptide transmitters are degraded by enzymes, but these do not play an important role in synaptic transmission. A clinically important example is monoamine oxidase (MAO), which is located in the outer membrane of mitochondria. MAO catalyzes the oxidative deamination of catecholamines to the corresponding aldehyde. Another nonspecific degradative enzyme is catechol-O-methyltransferase (COMT), which is present in the blood. The transferase inactivates catechols by methy-lating one of the ring hydroxyl groups. Unlike acetylcholinesterase, these enzymes are not located strategically at synapses, and they operate over a long time relative to synaptic transmission. Nevertheless, physiologically they are important because they ultimately contribute to the regulation of the amount of transmitters in the brain. Regulation of transmitter concentration is thought to be important in some psychiatric conditions, for example, aggression and depression.

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