Bidirectional connections of the medial amygdaloid nucleus in the Syrian hamster brain: simultaneous anterograde and retrograde tract tracing.

In the male Syrian hamster, mating is dependent on chemosensory and hormonal stimuli, and interruption of either input prevents copulation. The medial amygdaloid nucleus (Me) is a key nodal point in the neural circuitry controlling male sexual behavior because it relays both odor and steroid cues. Me is comprised of two major subdivisions, anterior (MeA) and posterior (MeP), which have distinct, although overlapping efferent projections. The present study investigated the afferents and efferents of MeA and MeP by using combined anterograde and retrograde tract tracing. Phaseolus vulgaris-leucoagglutinin and cholera toxin B were injected by iontophoresis through a single glass micropipette and detected by immunohistochemistry. MeA has widespread connections with olfactory structures, whereas MeP is heavily interconnected with steroid-responsive brain regions. The efferent projections of MeA and MeP were similar to those reported previously for the rat and hamster. In particular, MeP projects to the posteromedial subdivision of the bed nucleus of the stria terminalis (BNST) and to the medial preoptic nucleus, whereas MeA projects to adjacent subnuclei in BNST and the preoptic area. MeA and MeP also have distinct patterns of afferent input. Furthermore, the combination of anterograde and retrograde tract tracers shows that MeA and MeP are each bidirectionally connected with each other and with limbic nuclei. These results demonstrate that subnuclei of Me are interconnected with limbic structures in hamster brain. These connections may contribute to chemosensory and hormonal integration to control male sexual behavior.