The GnRH pulse generator refers to the collective functioning of GnRH neurons residing in the mediobasal hypothalamus that secrete GnRH into the portal vasculature and thereby regulate LH and FSH release from pituitary gonadotrophs (Figure 1). GnRH neurons are endogenously pulsatile. However, for the bolus of GnRH to be sufficient to trigger gonadotropins release, a group of GnRH neurons must secrete synchronously. Bolus size reflects the cohort size of GnRH- to- GnRH synapses or appositions. The GnRH pulse generator is active during fetal life and then is suppressed by central processes that cause enhanced negative feedback sensitivity to oestradiol until the onset of puberty. HPA activation increases the sensitivity of GnRH to oestradiol inhibition. Figure 2 illustrates a hypothetical scheme to explain how different pulse frequencies might result from varying the size of the GnRH cohort that synchronously releases GnRH pulses into the portal circulation to drive the pituitary release of LH and FSH (Figures 3 and 4).
Fig1. Neuroanatomical photomicrograph of gonadotrophin- releasing hormone pulse generator. Shown is the hypothalamic region from a rhesus monkey. Median eminence and adjacent basal hypothalamus are stained for gonadotrophin- releasing hormone in brown and counterstained with a methyl-grey Nissl stain. Gonadotrophin- releasing hormone neurons are visible at the border of the median eminence, within the median eminence, and within the hypothalamus. The dense accumulation of gonadotrophin- releasing hormone axons occurs where gonadotrophin- releasing hormone axons converge upon the portal loops that carry the gonadotrophin- releasing hormone to the pituitary. Reproduced with permission from Berga SL et al. Secondary amenorrhoea, Chapter 2. In: Atlas of Clinical Gynecology, Reproductive Endocrinology, Vol III (Stenchever MA, Series Editor; Mishell DR Jr, Volume Editor). Current Medicine, Inc., Philadelphia, PA, USA, 1999, with special thanks to Gloria Hoffman, PhD who performed and contributed the work.
Fig2. Diagram of the gonadotropin- releasing hormone pulse generator and its modulation. The gonadotrophin- releasing hormone pulse generator and mechanisms of its control are illustrated. The hypothetical activity of three gonadotrophin- releasing hormone neurons is shown in the lower right. When the activity of the neurons is summed at the level of the median eminence, bursts of activity that result in pulses of gonadotrophin- releasing hormone (short arrows) are produced. Some of the factors that influence the firing or release of gonadotrophin- releasing hormone are also illustrated. The gonadotrophin- releasing hormone neurons shown in this figure receive input from other gonadotrophin- releasing hormone neurons, both dendritic (in the form of bridges) and axonal (indicated by the terminal from the unlabelled neuron to cell 3). These cells also receive extrinsic excitatory and inhibitory inputs from neurons in various regions of the brain whose activity, in turn, is greatly modified by gonadal steroids and nutritional signals. In addition, pulse generator activity can be affected by changes in glial investment of gonadotrophin- releasing hormone soma and terminals that influence cohort size. Glia also can reversibly isolate gonadotrophin- releasing hormone cells from extrinsic influences or prevent gonadotrophin- releasing hormone from gaining easy access to the portal vasculature. Reproduced with permission from Berga SL et al. Secondary amenorrhoea, Chapter 2. In: Atlas of Clinical Gynecology, Reproductive Endocrinology, Vol III (Stenchever MA, Series Editor; Mishell DR Jr, Volume Editor). Current Medicine, Inc., Philadelphia, PA, USA, 1999, with special thanks to Gloria Hoffman, PhD who contributed this diagram.
Fig3. Luteinizing hormone pulse patterns in two women with FHA are shown in the top two panels and in two eumenorrheic, ovulatory women who were studied in the early follicular (EF) phase in the bottom two panels. Luteinizing hormone pulse patterns are the best in vivo surrogate for assessing gonadotrophin- releasing hormone activity in humans. To assess luteinizing hormone pulsatility in conditions in which slower frequencies are anticipated, one generally obtains blood samples from a forearm vein at 15- min intervals for 24 h. Computer- driven algorithms are then used to resolve the string of luteinizing hormone determinations into pulses so that frequency and amplitude can be computed. The eumenorrheic women regularly display regular luteinizing hormone pulses of even amplitude whereas the women with FHA display irregular luteinizing hormone pulses with an overall slower frequency and variable amplitude. In general, women with FHA have luteinizing hormone pulse frequency that is roughly half of that observed in eumenorrheic women. The expected nocturnal slowing is seen in the eumenorrheic women whereas some augmentation of luteinizing hormone is seen in women with FHA, a pattern that is somewhat similar to that seen in puberty.
Fig4. Serum oestradiol (o) and progesterone (▪) levels and vaginal bleeding (•) in a woman with functional hypothalamic amenorrhoea (FHA) who was observed and did not recover (top panel) compared with a woman with FHA who was treated with cognitive behaviour therapy (CBT) and recovered (lower panel). The inset depicts the proportion of women with FHA who had evidence of ovarian recovery following randomization to observation (OBS, n = 8; o) or CBT (n = 8; ▪). Adapted with permission from Berga SL, Marcus MD, Loucks TL, Hlastala S, Ringham R, Krohn MA. Recovery of ovarian activity in women with functional hypothalamic amenorrhea who were treated with cognitive behavior therapy. Fertil Steril, 2003; 80: 976– 81. Copyright © 2003 American Society for Reproductive Medicine.
Figure 5 illustrates the proximate neuromodulatory cascade that alters GnRH function. Behavioural activation of the HPA axis reduces GnRH drive and results in FHH. Hypercortisolaemia, as reflected by increased urinary free cortisol and elevated cerebrospinal fluid levels of cortisol, has been demonstrated in amenorrhoeic athletes and non- athletes. Acute nutritional deprivation activates the HPA axis and reduces LH pulsatility. Given the energetic expense of repro duction, it is not surprising that metabolic factors gate reproductive function. Psychosocial influences, including external stressors and attitudes, heighten behavioural reactivity and alter the neuroendocrine cascade that modulates GnRH drive. Endocrine- disrupting chemicals, such as bisphenol A and some polychlorinated biphenyls, may sensitize GnRH and kisspeptin neuroregulatory cascades to stressors by modulating GnRH gene transcription and/ or by acting as oestrogen agonists or antagonists. Behavioural modifications that reduce cortisol levels reversed the associated neuroendocrine cascade and restored ovulation in women with FHA.
Fig5. Schematic representation of neural interactions between metabolic and reproductive functions depicting likely sites of action of leptin, insulin, and ghrelin to control the release of gonadotropin- releasing hormone. Abbreviations: 3V, third ventricle; AgRP, agouti- related peptide; ARC, arcuate nucleus; CART, cocaine- and amphetamine- regulated transcript; GABA, gamma-aminobutyric acid; GHSR, growth hormone secretagogue receptor; IR, insulin receptor; Kiss1r, kisspeptin receptor; KNDy, kisspeptin/ neurokinin B/ dynorphin; LepR, leptin receptor; MC3r, melanocortin receptor 3; MC4r, melanocortin receptor 4; ME, median eminence; NPY, neuropeptide Y; PMV, ventral premammillary nucleus; POA, preoptic area; POMC, pro- opiomelanocortin.–2–1 3 5 7 9 Reproduced with permission from Navarro VM1, Kaiser UB. Metabolic influences on neuroendocrine regulation of reproduction. Curr Opin Endocrinol Diabetes Obes. 2013 Aug;20(4):335– 41.
As shown in Figure 5, kisspeptin is the G protein– coupled receptor ligand for its receptor, GPR54. Kisspeptin– GPR54 signalling plays a critical role in the initiation of GnRH secretion during puberty. Kisspeptin/ neurokinin B/ dynorphin (KNDy) neurons within the arcuate nucleus secrete kisspeptin, which stimulates GnRH neurons. KNDy neurons integrate other neuromodulatory signalling systems linked to reduced GnRH pulsatility. Stressors, regardless of type, activate the HPA axis and elicit a constellation of neuroendocrine alterations, including hypothalamic hypothyroidism that conserves and diverts energetic expenditure. Metabolic signals such as leptin regulate KNDy neurons via a GABAergic mechanism. In monkey models, ad ministration of a CRH antagonist, astressin reversed the impact of social stress upon central GABA- a binding in prefrontal cortex and altered responsivity to ghrelin.
Mechanisms implicated in the regulation of GnRH pulsatility include glial interposition into the synaptic cleft to reduce appositions and effective GnRH cohort size [30] and multiple neuromodulatory factors including corticotropin- releasing hormone, endogenous opioids, neuropeptide Y, leptin, γ- aminobutyric acid (GABA) and ghrelin. In monkeys, challenge paradigms have included restraint stress, exercise, nutritional restriction, injection of insulin, endotoxin, and social instability; stressors are synergistic. However, monkey studies do not reveal why humans initiate deleterious coping strategies or persistently engage in stressful behaviours. The response to some stressors, such as endotoxin, is blunted by sex steroids, while the response to restraint is heightened if the ovaries are present. Further, some individuals are more sensitive to the same stressor than others. The neuro endocrine and endocrine responses to exercise were exaggerated in men whose cortisol levels did not suppress when given dexamethasone when compared with the responses in men whose cortisol levels did suppress. In women with FHA, a 20- minute exercise challenge not only provoked a drop in glucose that was not seen in eumenorrheic women but also elicited a proportionally larger rise in cortisol. In contrast, in lactational amenorrhoea, stress responses to challenge are blunted. Reactivity to challenge may be mediated in part by the serotonergic system, particularly variants in the serotonin transporter. Overall, the impact of stressors is species- specific, context- dependent, and idiosyncratic.
Although the hypothalamus synthesizes and secretes specific re leasing hormones that govern the synthesis and secretion of specific pituitary hormones, it nonetheless functions as an integrated unit to coordinate the endocrine responses to internal and external signals. Because of these complex interdependent relationships, it is simplistic to view FHA as an isolated disturbance in GnRH drive.
