Evidence that the medial amygdala projects to the anterior/ventromedial hypothalamic nuclei to inhibit maternal behavior in rats

The maternal behaviors shown by a rat that has given birth are not shown by a virgin female rat when she is first presented with young. This absence o f maternal behavior in virgins has been attributed to the activity of a neu ral circuit that inhibits maternal behavior in nulliparae. The medial amygd ala and regions of the medial hypothalamus such as the anterior and ventrom edial hypothalamic nuclei have previously been shown to inhibit maternal be havior, in that lesions to these regions promote maternal responding. Furth ermore, we have recently shown that these and other regions, such as the pr incipal bed nucleus of the stria terminalis, the ventral lateral septum, an d the dorsal premammillary nucleus, show higher pup-induced Fos-immunoreact ivity in non-maternal rats exposed to pups than during the performance of m aternal behavior, indicating that they too could be involved in preventing maternal responsiveness. The current study tested whether the medial amygda la projects to the anterior/ventromedial hypothalamic nuclei in a neural ci rcuit that inhibits maternal behavior, as well as to see what other brain r egions could participate in this circuit. Bilateral excitotoxic lesions of the medial amygdala, or of the anterior/ve ntromedial hypothalamic nuclei, promoted maternal behavior. Unilateral medi al amygdala lesions caused a reduction of pup-induced Fos-immunoreactivity in the anterior/ventromedial hypothalamic nuclei in non-maternal rats ipsil ateral to the lesion, as well as in the principal bed nucleus of the stria terminalis, ventral lateral septum, and dorsal premammillary nucleus. Final ly, unilateral medial amygdala lesions paired with contralateral anterior/v entromedial hypothalamic nuclei lesions promoted maternal behavior, althoug h ipsilateral lesion placements were also effective. Together, these results indicate that the medial amygdala projects to the a nterior/ventromedial hypothalamic nuclei in a neural circuit that inhibits maternal behavior, and that the principal bed nucleus of the stria terminal is, ventral lateral septum, and dorsal premammillary nucleus could also be involved in this circuit. (C) 2001 IBRO. Published by Elsevier Science Ltd. All rights reserved.