Neuroendocrine regulation of GnRH release in induced ovulators

GnRH is the key neuropeptide controlling reproductive function in all verte brate species. Two different neuroendocrine mechanisms have evolved among f emale mammals to regulate the mediobasal hypothalamic (MBH) release of GnRH lending to the preovulatory secretion of LH by the anterior pituitary glan d. In females of spontaneously ovulating species, including rats, mice, gui nea pigs, sheep, monkeys, and women, ovarian steroids secreted by maturing ovarian follicles induce a pulsatile pattern of GnRH release in the median eminence that, in turn, stimulates a preovulatory LH surge. In females of i nduced ovulating species, including rabbits, ferrets, cats, and camels, the preovulatory release of GnRH, and the resultant preovulatory LH surge, is induced by the receipt of genital somatosensory stimuli during mating. Indu ced ovulators generally do not show "spontaneous" steroid-induced LH surges during their reproductive cycles, suggesting that the positive feedback ac tions of steroid hormones on GnRH release are reduced or absent in these sp ecies. By contrast, mating-induced preovulatory surges occasionally occur i n some spontaneously ovulating species. Most research in the field of GnRH neurobiology has been performed using spontaneous ovulators including. rat, guinea pig, sheep, and rhesus monkey. This review summarizes the literatur e concerning the neuroendocrine mechanisms controlling GnRH biosynthesis an d release in females of several induced ovulating species, and whenever pos sible it contrasts the results with those obtained for spontaneously ovulat ing species. It also considers the adaptive, evolutionary benefits and disa dvantages of each type of ovulatory control mechanism. In females of induce d ovulating species estradiol acts in the brain to induce aspects of procep tive and receptive sexual behavior. The primary mechanism involved in the p reovulatory release of GnRH among induced ovulators involves the activation of midbrain and brainstem noradrenergic neurons in response to genital-som atosensory signals generated by receipt of an intromission from a male duri ng mating. These noradrenergic neurons project to the MBH and, when activat ed, promote the release of GnRH from nerve terminals in the median eminence . In contrast to spontaneous ovulators, there is little evidence that endog enous opioid peptides normally inhibit MBH GnRH release among induced ovula tors. Instead, the? neural signals that induce a preovulatory LH surge in t hese species seem to be primarily excitatory. A complete understanding of t he neuroendocrine control of ovulation will only be achieved in the future by comparative studies of several animal model systems in which mating-indu ced as well as spontaneous, hormonally stimulated activation of GnRH neuron s drives the preovulatory LH surge. (C) 2000 Academic Press.