Effects of chemical stimulation in the periaqueductal gray on vocalization in the squirrel monkey

Twenty-nine agonists and 32 antagonists of more than 10 transmitters known to be present in the periaqueductal gray (PAG) have been injected into the squirrel monkey's PAG in order to test their effects on spontaneous vocalization at sites yielding vocalization with electrical stimulation. Vocalization could be elicited with the glutamate agonists sodium-L-glutamate, L-aspartic acid, L-homocysteic acid, N-methyl-D-aspartic acid, quisqualic acid, and kainic acid, the cholinergic agonists acetylcholine, carbachol, and muscarine, the monoaminergic agonist histamine, and the GABA antagonists bicuculline and picrotoxin. No vocalizations could be obtained with agonists of dopamine, noradrenaline, adrenaline, serotonin, GABA, glycine, nicotinic receptors, and endogenous opioids, as well as with antagonists of glutamate, acetylcholine, dopamine, noradrenaline, adrenaline, serotonin, histamine, glycine, GABA-B, delta- and mu-receptors. Blocking of spontaneous vocalization was obtained with the nonspecific glutamate antagonist kynurenic acid and the GABA-A receptor agonist muscimol. The results indicate that the production of vocalization depends upon the activation of glutamatergic synapses in the PAG. GABAergic afferents seem to have a tonic inhibitory control on the periaqueductal vocalization mechanism, while acetylcholine and histamine seem to exert only a transient modulatory control.