Pubertal testosterone organizes regional volume and neuronal number within the medial amygdala of adult male Syrian hamsters.

The medial amygdala plays a key role in regulating adult social behavior and undergoes structural changes during puberty that may be driven by gonadal hormone secretion during this developmental period. The current study sought to investigate potential organizational effects of testosterone during puberty, activational effects of testosterone in adulthood, and any interactions on regional volume and neuronal number of the medial amygdala. Male Syrian hamsters either did or did not experience endogenous testosterone during pubertal brain development, and then received either testosterone-filled or blank capsules during adulthood 2 weeks before tissue collection. The results show that pubertal testosterone has long-term organizational effects on volume of specific subregions of the medial amygdala such that the presence of pubertal testosterone resulted in 1) decreased volume of the anterior ventral amygdala and, to a lesser extent, the anterior dorsal medial amygdala; and 2) increased volume of the posterior dorsal medial amygdala. Both effects were independent of the presence of testosterone during adulthood. Pubertal testosterone also decreased neuronal number in the anterior dorsal medial amygdala, suggesting a possible mechanism by which pubertal testosterone decreases volume in this subregion. In addition, there was a significant interaction between pubertal and adult testosterone, such that testosterone in adulthood increased the number of neurons in the posterior ventral medial amygdala only in males that did not experience endogenous pubertal testosterone. In conclusion, pubertal testosterone organizes the medial amygdala in a subregion-specific manner, which may contribute to the maturation of adult-typical social behavior.