Limbic projections to the ventromedial hypothalamus of the opossum

Limbic projection systems which influence neuronal activity of the ventromedial nucleus of the hypothalamus of the opossum were studied by an evoked potential method. Bipolar concentric recording electrodes were placed in the ventromedial nucleus to monitor activity while exploring the forebrain with a stimulating electrode delivering single rectangular pulses (0.5 msec, 0.4 ma, 0.5–1/sec). Responses in the ventromedial nucleus were consistently evoked by stimulating three forebrain systems. From the dorsal septum, responses with one monophasic component were evoked (latency 3–9 msec). Stimulation more ventrally in the septum evoked, in addition, a second slower component. Stimulation of the corticomedial amygdala produced biphasic responses in the ventromedial nucleus at latencies of 4–9 msec; stimulation of the stria terminalis, the efferent pathway from this division of the amygdaloid complex, yielded almost identical responses at 4- to 6-msec latencies. Responses in the ventromedial nucleus were also recorded to stimulation of the dorsal hippocampus and adjacent fimbria (latencies, 3–7 msec). These responses demonstrate three possibly direct limbic influences on the ventromedial nucleus of a metatherian mammal, and provide new data on the anatomic relationships of this behaviorally significant hypothalamic area.