Any specific chemical agent (including acetylcholine, five amines, four amino acids, two purines, and more than 28 peptides) released by a presynaptic cell, on excitation, that crosses the synapse to stimulate or inhibit the postsynaptic cell. More than one may be released at any given synapse. The neurotransmitters released by presynaptic cells may modulate transmitter release from presynaptic cells. Nitric oxide may be a retrograde neurotransmitter, released from postsynaptic cells, to act on presynaptic cells.
[neuro- + L. transmitto, to send across]
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Examples: glitazone, GI cocktail, etc.