Possible changes in the regulatory mechanisms of pulsatile luteinizing hormone secretion in adult pituitary-grafted female rats.

It is well known that LH is released in an episodic fashion. However, the effects of hyperprolactinemia on luteinizing hormone (LH) pulsatility are not fully understood. The present study was undertaken to describe hyperprolactinemia effects on the regulatory mechanism of LH pulsatility. For this purpose hyperprolactinemia was induced in female rats by the transplantation of two pituitary glands beneath the kidney capsule. Age-matched sham-operated animals were used as controls. We have evaluated the pulsatile pattern of LH in both groups of animals. As expected, pituitary grafting increased mean serum prolactin levels compared with the values found in control animals. Luteinizing hormone-releasing hormone (LHRH) administration did not change serum prolactin levels in control or in pituitary-grafted rats. Thyrotropin-releasing hormone (TRH) administration increased prolactin secretion in both groups. There is an increase in mean values of LH and in the absolute amplitude of LH peaks in pituitary-grafted compared with control rats. No other parameters of LH pulsatility were changed by pituitary grafting. After LHRH administration, LH release was increased and a priming effect after the second administration of LHRH is observed in control rats. In pituitary-grafted animals the responses of LH to LHRH administration was diminished compared with the response observed in control rats. The administration of two pulses of TRH to control rats only decreases the duration of LH pulses. However, in pituitary-grafted animals, TRH administration was followed by a decrease in the mean values of LH and in the absolute amplitude of the LH peaks. Vasoactive intestinal peptide (VIP) administration increased mean values of LH and the absolute and relative amplitudes of LH pulses in sham-operated animals, whereas only the relative amplitude of LH peaks was modified by VIP in pituitary-grafted rats. All these data suggest that hyperprolactinemia induced by pituitary-grafting interfere with the neuromodulator effects on the pulsatile regulatory center of the hypothalamus.