The Influence of Melatonin Administered Subcutaneously, Intravenously, or Intraocularly upon Ovulation in the PMS-Treated Immature Rat

Immature female rats were induced to ovulate 72 hours after an injection of pregnant mare serum (PMS). Melatonin, a compound synthesized in the pineal gland, was injected by 3 different routes; subcutaneously, intravenously, and intraocularly. 5 experiments were conducted to determine the influence of melatonin on the ovulation in the PMS-treated immature rat when given through the 3 different methods mentioned. In experiment 1, 10 mcg of melatonin, injected at 12, 2, and 4 p.m. on Day 2 after PMS, was ineffective in preventing ovulation, whether injected subcutaneously or into the eyeball. However, 100 mcg of melatonin placed into the eyeball reduced the number of rats ovulating, as well as ovarian weights, relative to controls. Uterine weights of these rats were increased compared to controls. In experiment 2, the intraocular injection of 100 mcg of melatonin inhibited ovulation in the greatest number of rats, compared with the intravenous or subcutaneous routes. Experiment 3 was a repeat of experiment 2, with comparable results. In experiment 4, with a dosage of 75 mcg of melatonin, the intraocular administration appeared to be the most effective route for inhibiting ovulation. In experiment 5, subcutaneous administration of 50 mcg of melatonin was completely ineffective in blocking ovulation or altering ovarian weights. The findings of this study confirm the inhibitory effect of melatonin upon ovulation in the immature PMS-induced ovulating rat and demonstrate that melatonin is more effective in inhibiting ovulation when administered intraocularly than subcutaneously, and more effective subcutaneously than intravenously.