Mechanisms by which 5HT stimulates ovulation in the immature rat treated with pregnant mare serum gonadotrophin.

Pregnant mare serum gonadotrophin (PMS) normally induces a pre-ovulatory immunoreactive LH surge followed by ovulation in 27 day old rats provided they weigh more than 60 g; ovulation does not occur in the lighter rats although a pre-ovulatory LH surge can still be measured. 5HT injected into the paraventricular nucleus (PVN) can induce ovulation in these smaller rats treated with PMS and does so without appearing to stimulate extra immunoreactive LH. The LH secreted in the under 60 g PMS-treated rats is a pleiomorphic form since it cannot be detected by cytochemical bioassay, indicating abnormal biological activity, which may account for its failure to induce ovulation. After unilateral injection of 5HT into PVN, the LH was detectable by the bioassay. Injection of 5HT also stimulated the release of growth hormone and prolactin but had no significant effect on the plasma concentration of the gonadotrophins or corticosterone. Since an injection of exogenous GH converted the LH to a fully bioactive form it is suggested that GH is important in inducing the secretion of an "adult' form of LH in the peripubertal rat necessary for first ovulation and that 5HT exerts its ovulatory effect via stimulating the release of this hormone.