Pubertal and seasonal plasticity in the amygdala.

The present experiments investigated the effects of pubertal maturation and photoperiod on the size of brain regions that mediate mating behavior in the male Syrian hamster. We hypothesized that the low levels of reproductive behavior exhibited by prepubertal and photoinhibited males would be correlated with morphological changes in the neural circuit that mediates mating behavior. We found that the Nissl-stained cross-sectional area of the posterodorsal subdivision of the medial amygdala was significantly smaller in prepubertal and photoinhibited males compared to photostimulated adult males. These differences appear to be caused by a decrease in somal size of individual cells in the ventral aspect of this nucleus. We also found that prepubertal males have a larger anterior subdivision of the medial amygdala (MeA) compared to adults. This difference in the MeA does not appear to be caused by alteration in somal size since somal size did not differ significantly between juveniles and adults. It is concluded that the neural circuit that mediates male mating behavior in this species is capable of significant morphological plasticity during both pubertal development and in adulthood. Furthermore, these alterations may reflect underlying mechanisms of the deficits in sexual behavior exhibited by prepubertal and photoinhibited males.