Sexual difference in nuclear volume and its ontogeny in the rat amygdala.

Nuclear volume of the medial and lateral nuclei of the amygdala was investigated in male and female rats. The volume of the medial nucleus in the adult male rat was significantly greater than that of the female rat. Treatment of female rats with estrogen for the first 30 days of postnatal life increased the nuclear size, and the volume became comparable to that of the males. Although there was no significant difference in volume of this nucleus between male and female rats from days 1 (day of birth) to 11, the difference became evident at day 21, and thereafter it persisted. In contrast, the nuclear volume of the lateral nucleus was not different between the two sexes. Estrogen treatment did not influence the volume of the lateral nucleus in the female animals. These data indicate that the nuclear volume is sexually different in the medial nucleus of the amygdala, and its differentiation occurs during the early postnatal period under the influence of the organizational action of sex steroids.