Differentiation between neural and hormonal control of sexual behavior and gonadotropin secretion in the female rat

The purpose of this study was to assess the relative roles of the neural and endocrine feed-back systems in the regulation of gonadotropin secretion and behavior in the female rat. Retrochiasmatic knife cuts were placed in animals which were tested for mating behavior on five consecutive days, subsequent to the development of cornified vaginal smears. A knife-cut control group was autopsied immediately after behavioral tests. A second knife-cut group and a group of cycling controls were acutely ovariectomized, and their behavior was assessed until mating ceased. Some of each of the two groups were administered small daily amounts of estradiol cypionate (estrogen) under two-dose regimens. Mating behavior and vaginal cytology were followed, and autopsies were performed 70 days after ovariectomy. At autopsy, serum was collected for LH, FSH, estradiol, and progesterone radioimmunoassays. The knife cuts caused high levels of mating on five consecutive days of testing in all animals. After ovariectomy, the surgical group mated at higher rates for longer time periods than did the controls, and the vaginal smears were predominately leukocytic. Knife cut controls had significantly elevated serum estrogen titers over cycling controls except for proestrous morning values. Exogenous estrogen was retained longer in the serum in the surgical than in the control group. Progesterone was relatively low in the retrochiasmatic cut groups. Serum LH was not elevated in the surgical groups except following ovariectomy; exogenous estrogen not only stimulated more mating sooner in the surgical group, but inhibited high levels of LH caused by ovariectomy. FSH was elevated in the surgical and control ovariectomized groups, and exogenous estrogen lowered FSH in the surgical but not the control group. Estrogen and LH levels were compared between the control and knife cut groups, and it was determined that the set-point for negative feedback effects of estrogen on LH was comparable for the two groups. It was concluded that retrochiasmatic knife cuts cause high estrogen titers, which may be responsible for the continuous behavior in animals with intact ovaries, whereas, in the absence of ovaries, the surgical treatment may reduce fore-brain inhibition on behavior. The data also suggest that the mediobasal hypothalamus in animals with retrochiasmatic knife cuts responds normally to exogenous estrogen by reducing LH secretion in the ovariectomized animal.