The sexually differentiated brain contains sexually
dimorphic nuclei (SDNs). The formation of SDNs in rodents is affected by estrogen,
which is synthesized from testicular androgen and binds with estrogen receptor α
(ERα) during perinatal period. In this study, we carried out morphometrical analyses using the brain of the transgenic
mice, which express green fluorescent protein (GFP) under the control of ERα
promoter, and the wild-type littermates. As a result, we discovered a female-dominant sexually
dimorphic area in the hypothalamus of mice. Analysis of GFP- and calbindin-D28K
(Calb)-immunostained brain sections of tg mice showed that female tg mice had
greater number of GFP-expressing cells in an area sandwiched between two Calb-immunoreactive
SDNs, the principal nucleus of the bed nucleus of the stria terminalis (BNSTp)
and Calb-SDN. Next, we studied the sandwiched area in wild-type mice and observed
that the number of neurons in the sandwiched area of females was greater than
that of males, but no sex difference in the number of glia cells. In addition,
we found that the sandwiched area of wild-type female mice contained larger number of ERα-immunoreactive cells. To evaluate the effect of postnatal exposure to gonadal hormones, we measured
the number of neurons in the sandwiched area of adult males bearing neonatal
castration and adult females that were injected with either
estradiol or testosterone in the postnatal period. The neuron number of males
was significantly increased by neonatal castration.
By contrast, postnatal treatment with estradiol and testosterone significantly
decreased the neuron number of females. These results suggest that the area
sandwiched between the BNSTp and Calb-SDN of the mouse brain exhibits female-dominant
morphological sex differences and that the sex differences of the sandwiched
area arise under the influence of testicular androgen in the postnatal period.