[The mechanism of the pineal gland in inhibiting sexual receptivity of female rats: (II) Does it concern progesterone?].

The present study was designed to find the role of progesterone in modulating the effect of pineal gland upon reproductive behavior in the female rat. Monosodium L-glutamate (MSG) was used as a neurotoxin to induce hypogonadal function. In the first part of the experiment, rats were divided into four groups: (1) normal, (2) pinealectomized (Px), (3) MSG-treated (MSG), and (4) pinealectomized MSG-treated (Px-MSG), as immature rats became mature, the IEC (Incidence of Estrous Cycle) in each group was determined by vaginal smear for three weeks. Serum samples for estimating the levels of progesterone were collected at estrous. Then, ovariectomy was performed. Two weeks later, estradiol was primed for three days following progesterone administration and sexual receptivity as well as solicitation were observed. The results showed that, the serum progesterone levels and IEC of Px-MSG rats were significantly higher than that of the MSG group. In addition, Px and Px-MSG rats showed a higher sexual receptivity than normal and MSG rats, respectively. In the second part of the experiment, rats were divided into two groups: (1) normal and (2) Px. All rats were ovariectomized at 2.5 months of age and implanted with a 2-mm 20% estradiol benzoate-filled silastic capsule. Then, sexual receptivity to various doses of progesterone (0, 0.02, 0.1, 0.5 mg/rat) were observed. The results showed that receptivity of Px rats was significantly higher than that of the normal ones in a dose-dependent manner. It is concluded that pineal gland plays an important role in the regulation of ovary function as well as sexual receptivity in female rats. Also, progesterone may be one of the factors modulating the effect of pineal gland on sexual receptivity.