Stress and the hypothalamus-pituitary-gonadal axis in the cyclic rat

Plasma profiles of luteinizing hormone (LH) and follicle stimulating hormone (FSH) were measured during restraint stress on the day of pro-oestrus; these profiles were considered in relation to ovulation rate on the next day. Rats bearing a permanent jugular vein cannula were subjected to restraint which was started 0, 1 or 2 h before the presumed onset of the LH surge and ended just before the beginning of the dark period. Exposure to restraint resulted in a suppression of the secretion of both gonadotropins on the day of pro-oestrus. Suppression of the LH surge was virtually complete (plasma LH < 0.2 ng/ml) in 1 5 out of 32 stressed rats, and the ovaries of these rats contained graafian follicles with oocytes in germinal vesicle stage. In these rats, the LH surge did not occur 24 h later. In the remaining 17 rats, restraint resulted in a considerable suppression of the LH surge. Of these rats, 5 had an ovulation rate of 100% and 4 ovulated partially. In unruptured follicles of the latter, the oocyte had not resumed meiosis and the follicle wall was not luteinized. In the remaining 8 rats with a reduced LH surge, ovulations had not occurred and graafian follicles were unaffected. The results of this study indicate that during pro-oestrus restraint stress suppresses and does not delay the release of preovulatory gonadotropins. Partial suppression of LH by restraint does not result in induction of meiotic resumption without subsequent ovulation or in luteinized unruptured follicles.