Clarke, Iain J.

SummaryBackgroundGonadotropin‐inhibitory hormone (GnIH, human homologue of RFRP‐3) suppresses gonadotropin secretion in animal models, but its effects have not been studied in the human.ObjectiveWe tested the hypotheses that exogenous GnIH inhibits LH secretion (i) in postmenopausal women and (ii) in men concurrently administered exogenous kisspeptin.DesignFollowing in vitro and in vivo preclinical studies to functionally characterize the GnIH peptide, a dose‐finding study (human GnIH: 1·5–150 μg/kg/h, iv for 3 h) was undertaken, and 50 μg/kg/h selected for further evaluation. Five postmenopausal women were administered 50 μg/kg/h iv infusion for 3 h or vehicle on two separate days. Four men were administered kisspeptin‐10 (0·3 μg/kg iv bolus) with simultaneous infusion of GnIH (50 μg/kg/h, iv for 3 h) or vehicle.ParticipantsHealthy postmenopausal women (mean age 58 ± 2 years, LH: 30·8 ± 2·9 IU/l, FSH: 78·7 ± 6·4 IU/l, oestradiol: <50 pmol/l) and men (39·8 ± 2·1 years, mean total testosterone 12·1 ± 1·8 nmol/l, LH 2·2 ± 0·2 IU/l).Primary outcomeChange in area under curve (AUC) of LH during GnIHvs vehicle.ResultsDuring GnIH administration in postmenopausal women, LH secretion decreased (ΔAUC: −9·9 ± 1·8 IU/3 h) vs vehicle (ΔAUC: −0·5 ± 1·7 IU/3 h; P = 0·02). Kisspeptin‐10‐stimulated LH responses in men were not affected by GnIH co‐administration (60‐min AUC of LH 6·2 ± 0·8 IU/h with kisspeptin‐10 alone, 6·3 ± 1·0 IU/h, kisspeptin‐10 with GnIH, P = 0·72). Exogenous GnIH was well tolerated, with no adverse events reported.ConclusionsGonadotropin‐inhibitory hormone decreased LH secretion in postmenopausal women in this first‐in‐human study. Kisspeptin‐stimulated LH secretion in men was not inhibited during concomitant administration of GnIH.

GnIH was first identified in avian species, and there is now strong evidence that it is operant in mammals as an inhibitor of reproduction. Mammalian gonadotropin-inhibitory hormone (GnIH)-3 is encoded by the RFRP gene in neurons of the dorsomedial nucleus. These neurons project to the median eminence, predicting a role as a secreted neurohormone and regulation of the pituitary gonadotropes. To determine whether GnIH-3 is a secreted neurohormone, we measured its concentration in hypophyseal portal blood in ewes during the nonbreeding (anestrous) season and during the luteal and follicular phases of the estrous cycle in the breeding season. Paired portal and jugular blood samples were collected and plasma prepared for RIA using an ovine GnIH-3 antibody. Pulsatile GnIH-3 secretion was observed in the portal blood of all animals. Mean GnIH-3 pulse amplitude and pulse frequency was higher during the nonbreeding season. GnIH-3 was virtually undetectable in peripheral blood plasma. There was a lack of association between secretory pulses of GnIH-3 (portal) and LH (peripheral). To determine the role of secreted GnIH-3, we examined its effects on GnRH-stimulated LH secretion in hypothalamo-pituitary-disconnected ewes; a significant reduction in the LH response to GnRH was observed. Finally, to identify cellular targets in the pituitary, the expression of GnIH receptor [G protein-coupled receptor 147 (GPR147)] in fractions enriched for gonadotropes somatotropes, and lactotropes was examined; expression was observed in each cell type. These data show GnIH-3 is secreted into portal blood to act on pituitary gonadotropes, reducing the action of GnRH.

Homeostasis in the intact organism is achieved implicitly by repeated incremental feedback (inhibitory) and feedforward (stimulatory) adjustments enforced via intermittent signal exchange. In separated systems, neurohormone signals act deterministically on target cells via quantifiable effector-response functions. On the other hand, in vivo interglandular signaling dynamics have not been estimable to date. Indeed, experimentally isolating components of an interactive network definitionally disrupts time-sensitive linkages. We implement and validate analytical reconstruction of endogenous effector-response properties via a composite model comprising ( i ) a deterministic basic feedback and feedforward ensemble structure; ( ii ) judicious statistical allowance for possible stochastic variability in individual biologically interpretable dose–response properties; and ( iii ) the sole data requirement of serially observed concentrations of a paired signal (input) and response (output). Application of this analytical strategy to a prototypical neuroendocrine axis in the conscious uninjected horse, sheep, and human ( i ) illustrates probabilistic estimation of endogenous effector dose–response properties; and ( ii ) unmasks statistically vivid (2- to 5-fold) random fluctuations in inferred target-gland responsivity within any given pulse train. In conclusion, balanced mathematical formalism allows one to ( i ) reconstruct deterministic properties of interglandular signaling in the intact mammal and ( ii ) quantify apparent signal-response variability over short time scales in vivo . The present proof-of-principle experiments introduce a previously undescribed means to estimate time-evolving signal-response relationships without isotope infusion or pathway disruption.