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produce biochemical changes within the postsynaptic neuron. Thus, as with other neurotransmitters, ACh can excite or inhibit depending on the postsynaptic receptor.

2. Biogenic Amines

The biogenic amine transmitters include the catechola-mines [dopamine, norepinephrine (NE), and epinephr-ine], the indoleamine serotonin (5-hydroxytryptamine, 5-HT), and the imidazole histamine. Synthesis of the catecholamines and 5-HT is catalyzed by a set of enzymes that are similar in structure and mechanism. Because of amino acid sequence similarity, these enzymes are believed to come from a common ancestral molecule. The first enzymes in the synthesis of catecho-lamines and 5-HT are oxidases that require a pterin (tetrahydrobiopterin, THB) cofactor. These enzymes are specific and rate-limiting.

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