Interactions between glutamate, GABA, acetylcholine and histamine in the periaqueductal gray's control of vocalization in the squirrel monkey.

In the squirrel monkey (Saimiri sciureus), vocalization was elicited by periaqueductal injections of the glutamate agonist homocysteic acid, the acetylcholine agonist carbachol, the GABA antagonist bicuculline and the monoaminergic transmitter histamine. All chemically induced vocalizations could be blocked completely by prior periaqueductal injections of the non-specific glutamate antagonist kynurenic acid and the GABA agonist muscimol. The acetylcholine antagonist scopolamine and the histamine antagonist diphenhydramine, in contrast, blocked their respective transmitter agonists' vocalizations but had only a minor effect on glutamatergically induced vocalizations. It is concluded that the periaqueductal control of vocalization is brought about by the simultaneous activation of glutamatergic and inhibition of GABAergic mechanisms. Acetylcholine and histamine seem to play only a modulatory role.