Projections from the anteroventral part of the medial amygdaloid nucleus in the rat.

The medial amygdaloid nucleus (Me) integrates pheromonal and olfactory information with gonadal hormone cues, being implicated in social behaviors. It is divided cytoarchitectonically in an anterodorsal, anteroventral (MeAV), posterodorsal and posteroventral part, whose projections are well characterized, except for those of the tiny MeAV. Here, MeAV efferents were examined in the rat with the anterograde Phaseolus vulgaris leucoagglutinin (PHA-L) and retrograde Fluoro-Gold (FG) tracers and compared with those of other Me parts. The present PHA-L observations show that the MeAV projects profusely to itself, but its projections to other Me parts are modest. In conjunction with FG experiments, they suggest that the MeAV innervates robustly a restricted set of structures it shares with the anterodorsal and/or posteroventral Me. Its major targets are the core of the ventromedial hypothalamic nucleus (especially the dorsomedial and central parts), reached mainly via the stria terminalis, and the amygdalostriatal transition area. In addition, the MeAV innervates substantially the lateral and posterior basomedial amygdaloid nuclei and the intraamygdaloid bed nucleus of the stria terminalis. In contrast to other Me parts, it provides only modest inputs to the main and accessory olfactory systems, medial bed nucleus of the stria terminalis and reproductive hypothalamic nuclei. This anatomical framework suggests that the MeAV may play a role in orienting responses to chemosensory cues and defensive behaviors elicited by the odor of predators.