Emily L. Thompson

Free fatty acid receptor 2 and nutrient sensing: a proposed role for fibre, fermentable carbohydrates and short-chain fatty acids in appetite regulation

The way in which the composition of the diet may affect appetite, food intake and body weight is now receiving considerable attention in a bid to halt the global year-on-year rise in obesity prevalence. Epidemiological evidence suggests that populations who follow a fibre-rich, traditional diet are likely to have a lower body weight and improved metabolic parameters than their Western-diet counterparts. The colonic effects of fibre, and more specifically the SCFA that the fermentation process produces, may play a role in maintaining energy homeostasis via their action on the G-coupled protein receptor free fatty acid receptor 2 (FFA2; formerly GPR43). In the present review, we summarise the evidence for and against the role of FFA2 in energy homeostasis circuits and the possible ways that these could be exploited therapeutically. We also propose that the decline in fibre content of the diet since the Industrial Revolution, particularly fermentable fractions, may have resulted in the FFA2-mediated circuits being under-utilised and hence play a role in the current obesity epidemic.

Administration of kisspeptin-54 into discrete regions of the hypothalamus potently increases plasma luteinising hormone and testosterone in male adult rats

Kisspeptin‐54 is the peptide product of the KiSS‐1 gene and an endogenous agonist of the GPR54 receptor. KiSS‐1 was initially discovered as a metastasis suppressor gene, but recent studies demonstrate that the kisspeptin/GPR54 system is a key regulator of the reproductive system. Disrupted GPR54 signalling causes hypogonadotrophic hypogonadism in rodents and man. Intracerebroventricular or peripheral administration of kisspeptin potently stimulates the hypothalamic‐pituitary‐gonadal (HPG) axis via the hypothalamic gonadotrophin‐releasing hormone system. We have investigated the effect of injection of kisspeptin‐54 into discrete hypothalamic regions on the HPG axis. To construct a dose–response curve for the effects of intrahypothalamic kisspeptin administration, adult male Wistar rats were cannulated into the medial preoptic area (MPOA) at the level of the organum vasculosum laminae terminalis (OVLT). Kisspeptin‐54 was injected into the MPOA at doses of 0.01, 0.1, 1, 10 and 100 pmol. At 60 min following injection of 1, 10 or 100 pmol kisspeptin‐54, plasma luteinising hormone (LH) and total testosterone levels were significantly increased. Adult male Wistar rats were then cannulated into the rostral preoptic area at the level of the OVLT (RPOA), the MPOA, the paraventricular (PVN), dorsomedial (DMN) and arcuate hypothalamic nuclei, and the lateral hypothalamic area. A dose of 1 pmol kisspeptin‐54 was administered into all areas. The circulating levels of LH and total testosterone were significantly increased 60 min postinjection of kisspeptin‐54 into the RPOA, MPOA, PVN and arcuate nucleus. Our results suggest that kisspeptin may mediate its effects on the HPG axis via these regions of the hypothalamus.

Relaxin-3 stimulates the hypothalamic-pituitary-gonadal axis

The hypothalamus plays a key role in the regulation of both energy homeostasis and reproduction. Evidence suggests that relaxin-3, a recently discovered member of the insulin superfamily, is an orexigenic hypothalamic neuropeptide. Relaxin-3 is thought to act in the brain via the RXFP3 receptor, although the RXFP1 receptor may also play a role. Relaxin-3, RXFP3, and RXFP1 are present in the hypothalamic paraventricular nucleus, an area with a well-characterized role in the regulation of energy balance that also modulates reproductive function by providing inputs to hypothalamic gonadotropin-releasing hormone (GnRH) neurons. Other members of the relaxin family are known to play a role in the regulation of reproduction. However, the effects of relaxin-3 on reproductive function are unknown. We studied the role of relaxin-3 in the regulation of the hypothalamo-pituitary-gonadal (HPG) axis. Intracerebroventricular (5 nmol) and intraparaventricular (540–1,620 pmol) administration of human relaxin-3 (H3) in adult male Wistar rats significantly increased plasma luteinizing hormone (LH) 30 min postinjection. This effect was blocked by pretreatment with a peripheral GnRH antagonist. Central administration of human relaxin-2 showed no significant effect on plasma LH. H3 dose-dependently stimulated the release of GnRH from hypothalamic explants and GT1-7 cells, which express RXFP1 and RXFP3, but did not influence LH or follicle-stimulating hormone release from pituitary fragments in vitro. We have demonstrated a novel role for relaxin-3 in the stimulation of the HPG axis, putatively via hypothalamic GnRH neurons. Relaxin-3 may act as a central signal linking nutritional status and reproductive function.

Pyroglutamylated RFamide Peptide 43 Stimulates the Hypothalamic-Pituitary-Gonadal Axis via Gonadotropin-Releasing Hormone in Rats

Although it is established that other members of the RFamide family stimulate the hypothalamic-pituitary-gonadal axis, the influence of the novel pyroglutamylated RFamide peptide 43 (QRFP43) is not known. We show intracerebroventricular (icv) administration of QRFP43 (2 nmol) to male rats increased plasma LH and FSH levels at 40 min after injection. icv administration of 3 nmol QRFP43 did not affect food intake in ad-libitum-fed male rats. The icv administration of 2 nmol QRFP43 did not significantly influence behavior in male rats. Intraperitoneal administration of doses up to 1200 nmol/kg QRFP43 in male rats did not significantly influence circulating gonadotropin or sex steroid levels. In vitro, QRFP43 stimulated GnRH release from hypothalamic explants from male rats and from GT1-7 cells. Pretreatment with a GnRH receptor antagonist, cetrorelix, blocked the increase in plasma LH levels after icv administration of QRFP43 (2 nmol). These results suggest that icv QRFP43 activates the hypothalamic-pituitary-gonadal axis via GnRH.

The thyroid hormone derivative 3-iodothyronamine increases food intake in rodents

Background:  The thyroid hormone derivative 3‐iodothyronamine (T1AM), an endogenous biogenic amine, is a potent agonist of the G protein–coupled trace amine‐associated receptor 1 (TAAR1). T1AM is present in rat brain, and TAAR1 is expressed in hypothalamic nuclei associated with the regulation of energy homeostasis.

Kisspeptin-54 at high doses acutely induces testicular degeneration in adult male rats via central mechanisms

Background and purpose:  The kisspeptins are critical regulators of reproduction and a therapeutic target for reproductive disease. Intracerebroventricular (i.c.v.) or peripheral injection of kisspeptin potently stimulates the hypothalamic‐pituitary gonadal (HPG) axis via gonadotrophin‐releasing hormone (GnRH). However, little is known regarding the effects of kisspeptin administration on testicular function. We investigated the mechanism(s) of kisspeptin‐induced testicular degeneration in the rat.

Microinjection of Galanin‐Like Peptide into the Medial Preoptic Area Stimulates Food Intake in Adult Male Rats

Galanin‐like peptide (GALP) is a neuropeptide implicated in the regulation of feeding behaviour, metabolism and reproduction. GALP is an endogenous ligand of the galanin receptors, which are widely expressed in the hypothalamus. GALP is predominantly expressed in arcuate nucleus (ARC) neurones, which project to the paraventricular nucleus (PVN) and medial preoptic area (mPOA). Intracerebroventricular or intraparaventricular (iPVN) injection of GALP acutely increases food intake in rats. The effect of GALP injection into the mPOA on feeding behaviour has not previously been studied. In the present study, intra‐mPOA (imPOA) injection of GALP potently increased 0–1‐h food intake in rats. The dose–response effect of imPOA GALP administration on food intake was similar to that previously observed following iPVN administration. The effects of GALP (1 nmol) or galanin (1 nmol) on food intake were then compared following injection into the PVN, mPOA, ARC, dorsal medial nucleus (DMN), lateral hypothalamus and rostral preoptic area (rPOA). GALP (1 nmol) increased food intake to a similar degree when injected into the imPOA or iPVN, but produced no significant effect when injected into the ARC, DMN, lateral hypothalamus or rPOA. Similarly, galanin (1 nmol) significantly increased food intake following injection imPOA and iPVN. However, the effect was significantly smaller than that following administration of GALP (1 nmol). Galanin also had no significant effect on food intake when administered into the ARC, DMN, lateral hypothalamus and rPOA. These data suggest that the mPOA and the PVN may have specific roles in mediating the orexigenic effect of GALP and galanin.

The effects of kisspeptin‐54 on blood pressure in humans and plasma kisspeptin concentrations in hypertensive diseases of pregnancy

AIMS To investigate (i) if kisspeptin administration alters heart rate (HR) or blood pressure (BP) in healthy male and female volunteers, (ii) whether circulating plasma kisspeptin concentrations in healthy pregnant women and women with hypertensive diseases of pregnancy correlate with BP and (iii) whether women with hypertensive diseases of pregnancy have altered plasma kisspeptin concentrations. METHODS We have previously reported the effects of administration of kisspeptin-54 on gonadotrophin secretion in healthy male and female volunteers. In these studies, cardiovascular parameters were not a primary endpoint. However, data were also collected on BP and HR for 4h post administration of kisspeptin-54. Blood samples were taken from 105 women in the third trimester of pregnancy (27 women with hypertensive diseases of pregnancy and 78 controls). Samples were assayed for plasma kisspeptin immunoreactivity (IR). RESULTS Administration of kisspeptin was not associated with significant changes in HR or BP in healthy men or women. There was no significant correlation between plasma kisspeptin concentration and BP in healthy pregnant women or in those with hypertensive diseases of pregnancy. No significant differences in plasma kisspeptin-IR concentrations were observed between women with hypertensive diseases of pregnancy and normotensive pregnant controls, plasma kisspeptin concentrations ±SE: controls 2878 ± 157pmol l(-1) ; pregnancy-induced hypertension 2696 ± 299pmoll(-1) (95% CI vs. controls -514, 878pmoll(-1) ); pre-eclampsia 3519 ± 357 (95% CI vs. controls -1644, 362pmoll(-1) ). CONCLUSIONS Elevation of plasma kisspeptin-IR is not associated with an alteration in BP in humans.

Neuromedin B stimulates the hypothalamic-pituitary-gonadal axis in male rats.

Neuromedin B (NMB) is a highly conserved bombesin-related peptide found in mammals. NMB mRNA is detected in the central nervous system (CNS) and is highly expressed in the rat hypothalamus, in particular the medial preoptic area and the arcuate nucleus. The mammalian bombesin family of receptors consists of three closely related G protein coupled receptors, BB1, BB2 and BB3. The BB1 receptor subtype has the highest affinity for NMB. NMB has well documented roles in the regulation of the thyroid axis and the stress axis in rats. However, there is little available data regarding the role of NMB in the regulation of the hypothalamic-pituitary-gonadal (HPG) axis. It is known that the NMB receptor is expressed in immortalised gonadotrophin releasing hormone (GnRH) releasing GT1-7 cells and murine forebrain GnRH neurons, and that anterior pituitary NMB-immunoreactivity is altered by changes in the sex steroid environment. The objective of these studies was thus to further investigate the effects of NMB on the HPG axis. Intracerebroventricular (ICV) administration of NMB (10 nmol) to adult male rats significantly increased plasma luteinising hormone (LH) levels 30 min after injection (plasma LH ng/ml; saline 0.69±0.07, 10 nmol NMB 1.33±0.17, P<0.01). In vitro, NMB stimulated GnRH release from hypothalamic explants from male rats and from hypothalamic GT1-7 cells. NMB had no significant effect on LH release from anterior pituitary explants from male rats, or from pituitary LβT2 cells in vitro. These results suggest a previously unreported role for NMB in the stimulation of the HPG axis via hypothalamic GnRH. Further work is now required to determine the receptor mediating the effects of NMB on the reproductive axis and the physiological role of NMB in reproduction.