Differential Projections of the Anterior and Posterior Regions of the Medial Amygdaloid Nucleus in the Syrian Hamster

The medial nucleus of the amygdala is important for the neural control of reproductive behavior in the adult male Syrian hamster. Two types of signals are essential for this behavior, chemosensory stimuli and gonadal steroids; these signals appear to be received in different parts of the medial nucleus. The anterior region receives input from olfactory and vomeronasal systems, both of which are required for this behavior, whereas the posterior region receives gonadal hormone inputs. Behavioral studies have also suggested a functional differentiation of these two areas; electrolytic lesions of the anterior, but not the posterior, part eliminates normal sexual behavior. In this study, the efferent projections of the anterior and posterior divisions of the medial nucleus of the amygdala in the Syrian hamster were analyzed throughout the forebrain after injections of the anterograde neuronal tracer, Phaseolus vulgaris-leucoagglutinin. Neurons of the anterior, but not the posterior, medial nucleus, were found to project to numerous olfactory bulb projection areas and to the ventral striatopallidal complex. Within areas of the chemosensory circuitry that control reproductive behavior, the anterior region of the medial nucleus projects to the intermediate part of the posterior bed nucleus of the stria terminalis and the lateral part of the medial preoptic area, whereas the posterior region of the medial nucleus projects to the medial parts of these areas. Differences in targets were also observed in the ventromedial nucleus of the hypothalamus where the anterior region projects to the core while the posterior part projects to the shell of this nucleus. Furthermore, reciprocal projections between the anterior and posterior regions of the medial nucleus were observed. Taken together, these studies support the hypothesis that the anterior and posterior regions of the medial amygdaloid nucleus provide substantially different contributions to the control of reproductive behaviors.