The hypophysiotropic gonadotropin-releasing hormone (GnRH) and its neurons are crucial for vertebrate reproduction, primarily in regulating luteinizing hormone (LH) secretion and ovulation. However, in zebrafish, which lack GnRH1, and instead possess GnRH3 as the hypophysiotropic form, GnRH3 gene knockout did not affect reproduction. However, early-stage ablation of all GnRH3 neurons causes infertility in females, implicating GnRH3 neurons, rather than GnRH3 peptides in female reproduction. To determine the role of GnRH3 neurons in the reproduction of adult females, a Tg(gnrh3:Gal4ff; UAS:nfsb-mCherry) line was generated to facilitate a chemogenetic conditional ablation of GnRH3 neurons. Following ablation, there was a reduction of preoptic area GnRH3 neurons by an average of 85.3%, which was associated with reduced pituitary projections and gnrh3 mRNA levels. However, plasma LH levels were unaffected, and the ablated females displayed normal reproductive capacity. There was no correlation between the number of remaining GnRH3 neurons and reproductive performance. Though it is possible that the few remaining GnRH3 neurons can still induce an LH surge, our findings are consistent with the idea that GnRH and its neurons are likely dispensable for LH surge in zebrafish. Altogether, our results resurrected questions regarding the functional homology of the hypophysiotropic GnRH1 and GnRH3 in controlling ovulation.
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Vasoactive Intestinal Peptide Indirectly Elicits Pituitary LH Secretion Independent of GnRH in Female Zebrafish
Vasoactive intestinal peptide (Vip) regulates luteinizing hormone (LH) release through the direct regulation of gonadotropin-releasing hormone (GnRH) neurons at the level of the brain in female rodents. However, little is known regarding the roles of Vip in teleost reproduction. Although GnRH is critical for fertility through the regulation of LH secretion in vertebrates, the exact role of the hypophysiotropic GnRH (GnRH3) in zebrafish is unclear since GnRH3 null fish are reproductively fertile. This phenomenon raises the possibility of a redundant regulatory pathway(s) for LH secretion in zebrafish. Here, we demonstrate that VipA (homologues of mammalian Vip) both inhibits and induces LH secretion in zebrafish. Despite the observation that VipA axons may reach the pituitary proximal pars distalis including LH cells, pituitary incubation with VipA in vitro, and intraperitoneal injection of VipA, did not induce LH secretion and lhβ mRNA expression in sexually mature females, respectively. On the other hand, intracerebroventricular administration of VipA augmented plasma LH levels in both wild-type and gnrh3-/- females at 1 hour posttreatment, with no observed changes in pituitary GnRH2 and GnRH3 contents and gnrh3 mRNA levels in the brains. While VipA’s manner of inhibition of LH secretion has yet to be explored, the stimulation seems to occur via a different pathway than GnRH3, dopamine, and 17β-estradiol in regulating LH secretion. The results indicate that VipA induces LH release possibly by acting with or through a non-GnRH factor(s), providing proof for the existence of functional redundancy of LH release in sexually mature female zebrafish.
more » « less- Award ID(s):
- 1947541
- NSF-PAR ID:
- 10361787
- Publisher / Repository:
- DOI PREFIX: 10.1210
- Date Published:
- Journal Name:
- Endocrinology
- Volume:
- 163
- Issue:
- 2
- ISSN:
- 0013-7227
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract In spontaneously ovulating rodent species, the timing of the luteinising hormone (LH) surge is controlled by the master circadian pacemaker in the suprachiasmatic nucleus (SCN). The SCN initiates the LH surge via the coordinated control of two opposing neuropeptidergic systems that lie upstream of the gonadotrophin‐releasing hormone (GnRH) neuronal system: the stimulatory peptide, kisspeptin, and the inhibitory peptide, RFamide‐related peptide‐3 (RFRP‐3; the mammalian orthologue of avian gonadotrophin‐inhibitory hormone [GnIH]). We have previously shown that the GnRH system exhibits time‐dependent sensitivity to kisspeptin stimulation, further contributing to the precise timing of the LH surge. To examine whether this time‐dependent sensitivity of the GnRH system is unique to kisspeptin or a more common mechanism of regulatory control, we explored daily changes in the response of the GnRH system to RFRP‐3 inhibition. Female Syrian hamsters were ovariectomised to eliminate oestradiol (E2)‐negative‐feedback and RFRP‐3 or saline was centrally administered in the morning or late afternoon. LH concentrations and
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Abstract Stress suppresses pulsatile luteinising hormone (LH) secretion in a variety of species, although the mechanism underlying this inhibition of reproductive function remains unclear. Metabolic stress, particularly hypoglycaemia, is a clinically‐relevant stress type that is modelled with bolus insulin injection (insulin‐induced hypoglycaemia). The present study utilised ovariectomised C57BL/6 mice to test the hypothesis that acute hypoglycaemia suppresses pulsatile LH secretion via central mechanisms. Pulsatile LH secretion was measured in 90‐minute sampling periods immediately prior to and following i.p. injection of saline or insulin. The secretion of LH was not altered over time in fed animals or acutely fasted (5 hours) animals following an i.p. saline injection. By contrast, insulin elicited a robust suppression of pulsatile LH secretion in fasted animals, preventing LH pulses in five of six mice. To identify the neuroendocrine site of impairment, a kisspeptin challenge was performed in saline or insulin pre‐treated animals in a cross‐over design. LH secretion in response to exogenous kisspeptin was not different between animals pre‐treated with saline or insulin, indicating normal gonadotrophin‐releasing hormone cell and pituitary responses during acute hypoglycaemia. Based on this finding, the effect of insulin‐induced hypoglycaemia on arcuate kisspeptin (Kiss1) cell function was determined using c‐Fos as a marker of neuronal activation. Insulin caused a significant suppression in the percentage of Kiss1 cells in the arcuate nucleus that contained c‐Fos compared to saline‐injected controls. Taken together, these data support the hypothesis that insulin‐induced hypoglycaemia suppresses pulsatile LH secretion in the female mouse via predominantly central mechanisms, which culminates in the suppression of the arcuate Kiss1 population.
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