Abstract
The medial preoptic area of the rat is one of the sexually dimorphic regions. The medial preoptic nucleus (MPN) is an intensely stained neuron group in the rostroventral periventricular gray of the preoptic area, and its volume is markedly greater in females than males.
In the present study, as one step to elucidate the possible mechanism involved in the development of sexual dimorphism in the MPN, we attempted to determine whether the perinatal administration of testosterone propionate (TP) could influence the structure of the MPN.
The pregnant rats were injected subcutaneously with 4 mg of TP on day 17 or 21 of pregnancy. Female offspring from non-injected mothers were injected with 40 p, g of TP. for 7 days from the day of birth. Male offspring not exposed to prenatally were castrated on the day of birth.
TP-exposed males and females and neonatally castrated males were sacrificed at 90 days of age. In addition, normal males and females were sacrificed at the same age as the control.
The volume of the MPN was significantly greater in the females than in the males, the female volume being 2.2-fold as large as that of the male. However, neonatal treatment of female rats with TP for the first 7 days of life effectively reduced the nuclear volume to a level comparable with that of normal males. These results suggest that the development of the MPN is dependent on the neonatal sex steroid environment. Althogh neonatal castration of males reversed the nuclear volume to a female size, this may be due to the organizational action of endogeneous prenatal androgen exerted before neonatal castration.
