The cytoarchitecture, neuronal morphology and connections of the medial amygdaloid nucleus and the influence of gonadal steroids on neuronal structure in the adult male Syrian hamster.

Abstract

The medial amygdaloid nucleus in the adult male Syrian hamster (Mesocricetus auratus) is an important area along the chemosensory pathway where sensory and hormonal signals are processed for control of reproductive behavior. The studies described herein are a neuroanatomical description and a quantitative analysis of the influence of gonadal steroids on neuronal morphology in this nucleus. Four separate cytoarchitectural subdivisions were identified on the basis of neuronal density and the orientation of somata: anterodorsal, anteroventral, posterodorsal and posteroventral. All of these areas, however, contained a population of neurons with one to four primary dendrites and sparse dendritic arborizations. These neurons had a range of somal shapes and sizes. The influence of gonadal steroids on the structure of these neurons was revealed by comparing Golgi-impregnated neurons from reproductively intact and castrated adult animals. Neurons in the posterior, but not the anterior, region of the medial nucleus were altered by castration. Somal area and degree of dendritic branching were significantly reduced in castrated compared to intact animals. The importance of testosterone and its metabolites, dihydrotestosterone and estradiol, for maintaining neuronal structure was assessed by comparing neurons from intact to castrated males and castrates treated with exogenous hormones. Neurons from animals treated with dihydrotestosterone had significantly reduced somal area, highest dendritic branch level, total dendritic length and spine density. Treatment with testosterone, estradiol, or the combination of both metabolites, on the other hand, maintained these structural characteristics similar to age-matched controls. To further evaluate the functional circuitry of the chemosensory system, the efferent projections of the anterior and posterior parts of the medial nucleus were analyzed using an anterograde neuronal tract tracer, Phaseolus vulgaris-leucoagglutinin. Neurons of the anterior medial nucleus project to numerous olfactory areas, the ventral striatopallidal complex, and areas within the vomeronasal circuitry, whereas projections of neurons in the posterior medial nucleus were limited to areas within the vomeronasal circuitry. Furthermore, anterior and posterior divisions project to different regions of the vomeronasal circuitry: the anterior region projects to the lateral part of the bed nucleus of the stria terminalis and the preoptic area whereas the posterior region projects to the medial parts of these nuclei. Taken together, these studies support the hypothesis that the anterior and posterior regions make substantially different contributions to the control of reproductive behaviors.