Medial nucleus of the amygdala in the adult Syrian hamster: A quantitative Golgi analysis of gonadal hormonal regulation of neuronal morphology

The medial nucleus of the amygdala (Me) processes both chemosensory and hormonal input. In the male Syrian hamster the integrity of this nucleus is essential for normal reproductive behavior. To determine if gonadal steroids modulate neuronal structure in this nucleus, the morphology of Golgi- stained neurons in the anterior and posterior regions of Me were compared in castrated and reproductively intact adult hamsters. In castrated males, neurons in the posterior, but not the anterior, region of Me undergo structural changes, as indicated by a decrease in the mean highest den- dritic branch level and mean somal area compared to intact males. To further elucidate the importance of testosterone and its metabolites in maintenance of neuronal structure in the adult, seven groups of male Syrian hamsters were stud- ied. Animals were castrated and received a blank silastic capsule or a capsule filled with either testosterone, dihydrotestosterone, or estradiol, or two capsules contain- ing both metabolites, dihydrotestosterone and estradiol. Two groups of reproduc- tively intact animals were included: brains from one group were processed simul- taneously with the castrated and hormone-treated groups (control intact group), and the other group was processed at the beginning of the experiment. Animals were tested for mating behavior and flank glands were measured to test whether the capsules were effective in releasing the hormones into circulation. After a 12-week survival period, the brains were processed with Golgi stain and well- impregnated neurons from the posterior Me were quantitatively analyzed for so- mal area, highest dendritic branch, total dendritic length, and density of spines. All the measures analyzed revealed a consistent pattern of response to the differ- ent gonadal steroids. Castration resulted in a decrease in the mean somal area, the mean highest dendritic branch, and the percentage of neurons with tertiary branch segments. Dihydrotestosterone treatment also resulted in a significant decrease in somal area, mean highest dendritic branch, and percentage of neurons with ter- tiary dendritic branches. In addition) the total dendritic length and spine density on terminal dendrites were reduced in these brains. The remaining hormone treat- ment groups were not significantly different from the control group. These results suggest that in orchidectomized male Syrian hamsters) testosterone or its aroma- tized form, estradiol, but not dihydrotestosterone, is sufficient to maintain the normal morphology of the neurons in the posterior part of the medial nucleus of the amygdala. Gonadal hormones have been shown to be involved in organizing the neural circuits of the developing cen- tral nervous system (Goy and McEwen, 1980; Arnold and Breedlove, 1985; Arnold and Jordan, 1988). As summarized by Torand-Allerand (19841, the exposure of the brain to circulating gonadal steroids perma- nently influences a variety of morphological character- istics such as neuronal size, neuron number, dendritic branching, and the synaptic connectivity within differ- ent brain areas and in different species. Recent evi- dence has emerged demonstrating that the structural