Hippocampal influence on amygdala unit activity in awake squirrel monkeys.

The influence of hippocampal electrical stimulation on amygdala extracellular unit activity was studied in the awake squirrel monkey. Hippocampal influence was topographically organized. Stimulation elicited responses in 20% of 476 units tested. Ipsilateral anterior stimulation was more effective than posterior or contralateral stimulation. In the 6 areas with more than 25 tested units, the basomedial nucleus had the highest percentage of responsive units (39%), followed by the accessory basolateral (33%), central (22%y, basolateral (21%), and lateral (5%) nuclei, and the anterior amygdala area (4%). Initial excitation (E) was more prevalent than initial inhibition (I) in the central (90% E vs 10% I) and basomedial (82% E vs 18% I) nuclei; but initial inhibition was more common in basolateral (37% E vs 63% I) and accessory basolateral (33% E vs 67% I) nuclei. The mean response latency was 30.8 msec, ranging from 12 to 130 msec. The basomedial nucleus appears to receive the most potent hippocampal influence. Response characteristics are consistent with a hypothesized relay of nonfornix hippocampal influences on basal forebrain and hypothalamus via the basomedial nucleus. Possible pathways of hippocampal influence and the implications of our results for concepts of hippocampal and amygdala function are discussed.