Central aspects of systemic oestradiol negative‐ and positive‐feedback on the reproductive neuroendocrine system

The central nervous system regulates fertility via the release of gonadotrophin‐releasing hormone (GnRH). This control revolves around the hypothalamic‐pituitary‐gonadal axis, which operates under traditional homeostatic feedback by sex steroids from the gonads in males and most of the time in females. An exception is the late follicular phase in females, when homeostatic feedback is suspended and a positive‐feedback response to oestradiol initiates the preovulatory surges of GnRH and luteinising hormone. Here, we briefly review the history of how mechanisms underlying central control of ovulation by circulating steroids have been studied, discuss the relative merit of different model systems and integrate some of the more recent findings in this area into an overall picture of how this phenomenon occurs.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/153639/1/jne12724.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/153639/2/jne12724_am.pd

Circannual Alterations in the Circadian Rhythm of Melatonin Secretion

To determine if a circadian rhythm known to be functionally related to the reproductive axis varies on a circannual basis, we monitored the circadian secretion of melatonin at monthly intervals for 2 years in four ovariectomized, estradiol-implanted ewes held in a constant short-day photoperiod. Prior to the study, ewes had been housed in a short-day (8L:16D) photoperiod for 4 years and were exhibiting circannual reproductive rhythms as assessed by serum luteinizing hormone (LH) levels. Three of the four sheep showed unambiguous deviations from the expected nocturnal melatonin secretion at two different times approximately 1 year apart. Nocturnal rises in melatonin, which usually last the duration of the dark phase, were delayed by 3-14 h or were missing. Altogether, five of the seven melatonin alterations observed in these three ewes occurred during the nadir of the circannual LH cycle. In the remaining ewe, we did not observe an altered melatonin secretory pattern during this period, and this ewe also failed to show a high amplitude circannual cycle of LH. The results provide evidence for a circannual change in the circadian rhythm of melatonin secretion. This alteration in melatonin secretion may serve as a "functional" change in daylength, and thereby may influence the expression of the circannual reproductive rhythm of sheep held in a fixed photoperiod for an extended time.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/68029/2/10.1177_074873049501000104.pd

Live view of gonadotropin-releasing hormone containing neuron migration

Neurons that synthesize GnRH control the reproductive axis and migrate over long distances and through different environments during development. Prior studies provided strong clues for the types of molecules encountered and movements expected along the migratory route. However, our studies provide the first real-time views of the behavior of GnRH neurons in the context of an in vitro preparation that maintains conditions comparable to those in vivo. The live views provide direct evidence of the changing behavior of GnRH neurons in their different environments, showing that GnRH neurons move with greater frequency and with more changes in direction after they enter the brain. Perturbations of guiding fibers distal to moving GnRH neurons in the nasal compartment influenced movement without detectable changes in the fibers in the immediate vicinity of moving GnRH neurons. This suggests that the use of fibers by GnRH neurons for guidance may entail selective signaling in addition to mechanical guidance. These studies establish a model to evaluate the influences of specific molecules that are important for their migration

Targeted expression of a dominant-negative fibroblast growth factor (FGF) receptor in gonadotropin-releasing hormone (GnRH) neurons reduces FGF responsiveness and the size of GnRH neuronal population

Increasing evidence suggests that fibroblast growth factors (FGFs) are neurotrophic in GnRH neurons. However, the extent to which FGFs are involved in establishing a functional GnRH system in the whole organism has not been investigated. In this study, transgenic mice with the expression of a dominant-negative FGF receptor mutant (FGFRm) targeted to GnRH neurons were generated to examine the consequence of disrupted FGF signaling on the formation of the GnRH system. To first test the effectiveness of this strategy, GT1 cells, a GnRH neuronal cell line, were stably transfected with FGFRm. The transfected cells showed attenuated neurite outgrowth, diminished FGF-2 responsiveness in a cell survival assay, and blunted activation of the signaling pathway in response to FGF-2. Transgenic mice expressing FGFRm in a GnRH neuron-specific manner exhibited a 30% reduction in GnRH neuron number, but the anatomical distribution of GnRH neurons was unaltered. Although these mice were initially fertile, they displayed several reproductive defects, including delayed puberty, reduced litter size, and early reproductive senescence. Overall, our results are the first to show, at the level of the organism, that FGFs are one of the important components involved in the formation and maintenance of the GnRH system

Discovery of potent kisspeptin antagonists delineate physiological mechanisms of gonadotropin regulation

Neurons that produce gonadotropin-releasing hormone (GnRH) are the final common pathway by which the brain regulates reproduction. GnRH neurons are regulated by an afferent network of kisspeptin-producing neurons. Kisspeptin binds to its cognate receptor on GnRH neurons and stimulates their activity, which in turn provides an obligatory signal for GnRH secretion—thus gating down-stream events supporting reproduction. We have developed kisspeptin antagonists to facilitate the direct determination of the role of kisspeptin neurons in the neuroendocrine regulation of reproduction. In vitro and in vivo studies of analogues of kisspeptin-10 with amino substitutions have identified several potent and specific antagonists. A selected antagonist was shown to inhibit the firing of GnRH neurons in the brain of the mouse and to reduce pulsatile GnRH secretion in female pubertal monkeys; the later supporting a key role of kisspeptin in puberty onset. This analogue also inhibited the kisspeptin-induced release of luteinizing hormone (LH) in rats and mice and blocked the post-castration rise in LH in sheep, rats and mice, suggesting that kisspeptin neurons mediate the negative feedback effect of sex steroids on gonadotropin secretion in mammals. The development of kisspeptin antagonists provides a valuable tool for investigating the physiological and pathophysiological roles of kisspeptin in the regulation of reproduction and could offer a unique therapeutic agent for treating hormone-dependent disorders of reproduction, including precocious puberty, endometriosis, and metastatic prostate cancer

The neuroendocrine signal for ovulation: Definition and regulation.

Ovulation is caused by a surge of luteinizing hormone (LH) release from the pituitary. The nature and regulation of the neuroendocrine signal for ovulation were defined in sheep. First, the neuroendocrine factor regulating reproduction, gonadotropin-releasing hormone (GnRH), was measured in pituitary portal blood during the preovulatory period. Low frequency GnRH pulses occurred during the luteal phase. Pulse frequency increased and amplitude decreased during the follicular phase. A robust GnRH surge began coincident with the preovulatory LH surge; the GnRH surge outlasted that of LH by several hours. Whether estradiol elicits these changes in GnRH release was investigated using a physiologic model for the follicular phase, in which estradiol was manipulated (removed or increased to peak follicular phase level) 16 hours after progesterone removal. Following estradiol removal, GnRH and LH were secreted in coincident bursts. Estradiol had a biphasic effect on GnRH release: first, it suppressed pulsatile GnRH release, second, it induced a GnRH surge. These results support the hypothesis estradiol regulates GnRH release, and that this central action is part of the mechanism by which estradiol induces the LH surge. Two studies were performed to examine the mechanism by which estradiol evokes the GnRH surge. Very frequent samples of pituitary portal blood were obtained to determine the moment-to-moment GnRH-release pattern during the surge. Estradiol shifted to pattern of GnRH release, permitting continuous elevation in portal blood, possibly by desynchronizing GnRH neurons, accelerating pulses beyond resolution of the technique, or recruiting an asynchronous surge-specific population of GnRH neurons. The latter was investigated in the final study in which active cells were localized by immunocytochemical staining for cFos--a marker of neural activity. A striking induction of cFos expression occurred in GnRH neurons during the surge. These cells, however, were not located in an anatomically distinct region, precluding identification of a surge-specific population of GnRH neurons. A surge-specific population of non-GnRH-positive cells was identified, raising the possibility these cells convey the positive feedback signal to the GnRH neurons. We conclude estradiol activates several neural populations and thereby induces the GnRH surge, which is a prerequisite neuroendocrine signal for ovulation in sheep.Ph.D.PhysiologyUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/105576/1/9135655.pdfDescription of 9135655.pdf : Restricted to UM users only

Gonadotropin‐releasing hormone (GnRH) measurements in pituitary portal blood: a history

Much about the neuroendocrine control of reproduction is inferred from changes in the episodic release of luteinizing hormone (LH), as measured in samples of peripheral blood. This, however, assumes that LH precisely mirrors gonadotropin-releasing hormone (GnRH) release from the hypothalamus. As GnRH is not measurable in peripheral blood, characterization of the relationship between these two hormones required the simultaneous measurement of GnRH and LH in pituitary portal and peripheral blood, respectively. Here, we review the history of why and how portal blood collection was developed, the aspects of the true output of the central component of the hypothalamo-pituitary-gonadal axis that this methodology helped clarify, and conditions under which the pituitary fails to serve as an adequate bioassay for the release pattern of GnRH