Amygdala afferents from the mediobasal hypothalamus: an electrophysiological and neuroanatomical study in the rat.

Electrophysiological techniques and the retrograde transport of horseradish peroxidase (HRP) were used to determine the efferent projections from the caudal mediobasal hypothalamus to the amygdala. In pentobarbital anesthetized rats, the activity of 1780 mediobasal hypothalamic neurons was examined for response to stimulation sites in the amygdala and stria terminalis. Evidence of orthodromic activation from both stimulation sties was commonly observed. Sixty-five cells mostly located in the ventromedial nucleus displayed antidromic invasion from the basolateral, basomedial or cortical amygdala over a latency range of 5-34 msec (mean 15.3 +/- 6.7 msec S.D.). Three of 440 tested cells displayed antidromic activation from stimulation on the stria terminalis. Amygdala evoked antidromic responses were still present after lesions of the stria terminalis. May ipsilateral ventromedial hypothalamic neurons and a few cells in the ipsilateral arcuate nucelus and periventricular region and contralateral ventromedial nucleus displayed retrograde transport of HRP after an infection into the amygdala. Lesions of the stria terminalis had little effect on the numbers of HRP labeled neurons. Relatively more neurons were labeled retrogradely after medial injections than after lateral injections in the amygdala. Data from both electrophysiological and anatomical techniques therefore indicate that certain mediobasal hypothalamic neurons, particularly those located in the ipsilateral ventromedial nucleus, project to the amygdala probably via a route other than the stria terminalis. Thus there is substantial evidence in the rat for reciprocal connections between the amygdala and the hypothalamic ventromedial nucleus.