EFFECTS OF PINEALECTOMY AND MELATONIN ON THE TIMING OF THE PROESTROUSLUTEINIZING-HORMONE SURGE IN THE RAT

Effects of pinealectomy and melatonin replacement on the timing of the preovulatory luteinizing hormone (LH) surge were examined in female r ats housed under a light-dark cycle (lights on 06:30-18:30 h). Animals were pinealectomized or sham-operated 21-28 days before bleeding. The y were sequentially bled in the afternoon of proestrus through the ind welling cardiac cannula without anesthesia. Proestrous LH surges were observed in all pinealectomized rats, but the onset times of the LH su rge in these animals showed a significantly greater variance than thos e in sham-operated controls. A single melatonin injection at 18:00 or 21:00, shortly before or after the light-dark transition, on the day b efore proestrus (diestrus II) reduced the variance in the LH surge ons et in pinealectomized rats in a dose-dependent manner. In contrast, me latonin injected at 14:00 or 23:00 on diestrus II or at 04:00 on proes trus was ineffective. These results showed that the pineal gland is in volved in controlling the timing of the proestrous LH surge in the rat . The pineal signal may be transmitted by melatonin, a major product o f the gland.