Reproductive effects of ghrelin in the hypothalamus

Abstract

Ghrelin is a metabolic hormone released by the stomach at increased levels during times of fasting. It is known that ghrelin has suppressive effects on reproduction, for example by decreasing luteinizing hormone (LH) pulsatility. Whether it also inhibits the neuroendocrine control of ovulation is unknown. We set up three experiments to delve further into ghrelin’s actions on the control of reproduction by the hypothalamus, the region of the brain critical for control of the reproductive axis. In the first experiment we established a protocol to abolish the preovulatory LH surge in mice by fasting them for 36 hours or treating with 20mg/kg ghrelin at 2 hourly intervals on the afternoon of the surge. We then examined whether restoration of ghrelin levels to a non-fasted state by using mice with a knocked-out ghrl gene, restoration of leptin levels by administration of 1mg leptin/kg in at ~12 hourly intervals, or restoration of both would restore the surge in the fasted mice. We found that neither ghrelin knockout or leptin replacement nor their combination was able to restore the surge in fasted mice, while ghrelin injections prior to the expected LH surge, given to fed control mice, were able to abolish the preovulatory LH surge. It is proposed that the effects of ghrl knockout and leptin replacement be re-examined in a shorter (and therefore more physiologically relevant) fast model. This suggests that ghrelin is sufficient but not necessary to inhibit the preovulatory LH surge in fasted mice. In the second experiment we attempted to systematically examine various potential signalling markers of ghrelin activity within the hypothalamus. We implanted intracerebroventricular (ICV) cannulae into two groups of Sprague-Dawley rats and administered 3nm ghrelin into one group, and vehicle only into the other group, and perfused the rats an hour later. The brains were then stained immunohistochemically for either phosphorylated extracellular signal-related kinase 1/2 (pERK 1/2) or phosphorylated cAMP-response element-binding protein (pCREB). The series were then co-stained for either tyrosine hydroxylase (as a surrogate for anteroventral periventricular kisspeptin neurons), gonadotrophin-releasing hormone (GnRH), corticotrophin-releasing hormone (CRH), RF-amide related peptide 3 (RFRP-3), or kisspeptin. No change was found in CREB phosphorylation with ghrelin administration, but did find statistically significant decreases of ERK 1/2 phosphorylation in the arcuate nucleus (ARC), ventromedial hypothalamic nucleus (VMH), as well as the dorsomedial hypothalamic nucleus (DMH). The significance of these findings are difficult to speculate, save to say that ERK 1/2 phosphorylation appears to be negatively regulated by ghrelin activity within the hypothalamus. For our third experiment, we proposed a model through which ghrelin decreases LH pulsatility, namely that it stimulates CRH neurons which then suppress arcuate kisspeptin neurons which are thought to drive the GnRH pulse generator in rodents. To test this hypothesised pathway 3nmol ghrelin with or without 27nmol of the CRH-R2 antagonist astressin-2B, the antagonist alone, or 5μg CRH alone was administered ICV into groups of Sprague-Dawley rats. LH levels in the blood were measured 1 hour later as well as Kiss1 mRNA levels in the caudal and rostral hypothalami of the rats. Our results were statistically insignificant but trends in the LH levels suggest that CRH-R2 is a mediator of ghrelin’s suppression of LH pulsatility. Overall, we have examined varying facets of ghrelin’s neuroendocrine reproductive roles and found several interesting new additions to these.