Limbic influence on the periaqueductal gray: a single unit study in the awake squirrel monkey.

Single neurons of the periaqueductal gray (PAG) were studied during electrical stimulation of the amygdala and hippocampus. Fifty-one percent (34/67) of the units sampled throughout the rostrocaudal extent of the PAG were found to have a limbic influence. PAG neurons were characterized by low spontaneous firing rates (means = 4.94 spikes/sec). Units responded to basolateral amygdala stimulation primarily with short duration excitatory responses having a mean latency of 30 ms (range: 13.3-110 ms). Responses to corticomedial and lateral amygdala stimulation produced different patterns of activation including complex excitatory and inhibitory sequences. Only 10 units (15%) sampled in PAG responded to hippocampal stimulation with excitatory or tonic-inhibitory responses. The majority of responsive units (8) were to anterior hippocampal stimulation (latency range: = 20-75 ms). High frequency (9 Hz) basolateral amygdala stimulation recruited responses with increases in the probability of firing and a decrease in initial latency and latency variability.