Luteinizing Hormone Releasing Hormone (LHRH) Levels in Pituitary Stalk Plasma During the Preovulatory Gonadotropin Surge of Rabbits1

Mating in the rabbit evokes a surge of LH secretion and ovulation. Hypothalamic luteinizing hormone releasing hormone (LHRH) secretion is believed to link mating and the LH surge humorally but the pattern of LHRH secretion after mating remains unknown. In the present investigation, we studied the pattern of LHRH secretion into pituitary stalk blood after an iv injection of cupric acetate, a chemical stimulus previously established to evoke typical post-coital patterns of gonadotropin secretion and ovulation, to have a hypothalamic site of action and to be effective in anesthetized rabbits. Pituitary stalk blood for LHRH measurements was collected by a trans-palatal, trans-sphenoidal surgical approach; blood was collected continuously for 5 h into a cannula at a rate of 20 μl/min. Cupric acetate was administered after a 1 h control collection period. In 5 mature female rabbits, immunoassayable LHRH levels in pituitary stalk plasma were undetectable (<5 pg/ml) during the hour before and the hour after cupric acetate treatment but increased to 30–45 pg/ml during the 2nd, 3rd, and 4th h. This pattern of LHRH secretion was significantly correlated (r = 0.96; P < 0.05) with the pattern of LH levels in peripheral plasma determined in a companion group of rabbits treated only with cupric acetate. Three control rabbits treated with saline instead of cupric acetate, exhibited undetectable stalk plasma LHRH concentrations (<5 pg/ml) throughout the 5 h collection period. These results suggest that mating evokes a preovulatory LH surge by stimulating an antecedent rise in LHRH secretion.