Mandatory Neuropeptide-Steroid Signaling for the Preovulatory Luteinizing Hormone-Releasing Hormone Discharge*

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Abstract
I. Introduction PITUITARY hormones are secreted in the form of episodic bursts. At least two modalities of these periodic hormonal events have been identified in the rat. Generally, low amplitude episodes occurring at more or less constant frequency dominate the pituitary secretory pattern. On occasion, however, these ultradian intermittent secretory bursts are interrupted by high amplitude surges of hormone secretion lasting for a considerable length of time (1, 2). Well characterized examples of these latter events are the preovulatory gonadotropin (3, 4) and PRL (4, 5) surges in the female rat, the daily burst of ACTH secretion (6), and the periodic high mass and longer duration episodes of GH and PRL in both female and male rats (7, 8). The importance of these pituitary hormone surges in sustaining the internal milieu and the perceived corollary that subtle and progressive derangements in the periodicity and amplitude of these secretory events impel premature aging of reproductive processes are well recognized. Consequently, the neuroendocrine control of each of these seemingly abrupt massive outpourings of pituitary hormones has been the focus of expanding research efforts over several years. This article details the steady progress made toward deciphering the neurochemical pathways involved in augmenting the hypothalamic discharge of LHRH, the primary trigger for the preovulatory surge of pituitary LH secretion on proestrus, since the publication of a similar survey and related concepts a decade ago (3).