RFamide-related peptide 3 signaling via neuropeptide FF receptor stimulates prolactin secretion in female rats

The RF-amide peptides comprise a family of neuropeptides that includes the kisspeptin (Kp), the natural ligand of kisspeptin receptor (Kiss1r), and the RFamide-related peptide 3 (RFRP-3) that binds preferentially to the neuropeptide FF receptor 1 (Npffr1). Kp stimulates prolactin (PRL) secretion through the inhibition of tuberoinfundibular dopaminergic (TIDA) neurons. Because Kp has also affinity to Npffr1, we investigated the role of Npffr1 in the control of PRL secretion by Kp and RFRP-3. Intracerebroventricular (i.c.v.) injection of Kp increased PRL and luteinizing hormone (LH) secretion in ovariectomized, estradiol-treated rats (OVX+E). The unselective Npffr1 antagonist RF9 prevented these responses, whereas the selective antagonist GJ14 altered PRL but not LH levels. The i.c.v. injection of RFRP-3 in OVX+E rats increased PRL secretion, associated with a rise in the dopaminergic activity in the median eminence, and had no effect on LH levels. The RFRP-3-induced increase in PRL secretion was prevented by GJ14. Moreover, the estradiol-induced PRL surge in female rats was blunted by GJ14, alongside the amplification of the LH surge. Nevertheless, whole-cell patch clamp recordings showed no effect of RFRP-3 on the electrical activity of TIDA neurons in dopamine transporter-Cre recombinase transgenic female mice. We provide evidence that RFRP-3 binds to Npffr1 to stimulate PRL release, which plays a role in the estradiol-induced PRL surge. This effect of RFRP-3 is apparently not mediated by a reduction in the inhibitory tone of TIDA neurons but possibly involves the activation of a hypothalamic PRL-releasing factor.

Pattern of gonadotropin secretion along the estrous cycle of C57BL/6 female mice

The pattern of gonadotropin secretion along the estrous cycle was elegantly described in rats. Less information exists about the pattern of gonadotropin secretion in gonad-intact mice, particularly regarding the follicle-stimulating hormone (FSH). Using serial blood collections from the tail-tip of gonad-intact C57BL/6 mice on the first day of cornification (transition from diestrus to estrus; hereafter called proestrus), we observed that the luteinizing hormone (LH) and FSH surge cannot be consistently detected since only one out of eight females (12%) showed increased LH levels. In contrast, a high percentage of mice (15 out of 21 animals; 71%) exhibited LH and FSH surges on the proestrus when a single serum sample was collected. Mice that exhibited LH and FSH surges on the proestrus showed c-Fos expression in gonadotropin-releasing hormone- (GnRH; 83.4% of co-localization) and kisspeptin-expressing neurons (42.3% of co-localization) of the anteroventral periventricular nucleus (AVPV). Noteworthy, mice perfused on proestrus, but that failed to exhibit LH surge, showed a smaller, but significant expression of c-Fos in GnRH (32.7%) and AVPVKisspeptin (14.0%) neurons. Finally, 96 serial blood samples were collected hourly in eight regular cycling C57BL/6 females to describe the pattern of LH and FSH secretion along the estrous cycle. Small elevations in LH and FSH levels were detected at the time expected for the LH surge. In summary, the present study improves our understanding of the pattern of gonadotropin secretion and the activation of central components of the hypothalamic-pituitary-gonadal axis along the estrous cycle of C57BL/6 female mice.

Effects of the Isolated and Combined Ablation of Growth Hormone and IGF-1 Receptors in Somatostatin Neurons

Hypophysiotropic somatostatin (SST) neurons in the periventricular hypothalamic area express growth hormone (GH) receptor (GHR) and are frequently considered as the key neuronal population that mediates the negative feedback loop controlling the hypothalamic-GH axis. Additionally, insulin-like growth factor-1 (IGF-1) may also act at the hypothalamic level to control pituitary GH secretion via long-loop negative feedback. However, to the best of our knowledge, no study so far has tested whether GHR or IGF-1 receptor (IGF1R) signaling specifically in SST neurons is required for the homeostatic control of GH secretion. Here we show that GHR ablation in SST neurons did not impact the negative feedback mechanisms that control pulsatile GH secretion or body growth in male and female mice. The sex difference in hepatic gene expression profile was only mildly affected by GHR ablation in SST neurons. Similarly, IGF1R ablation in SST neurons did not affect pulsatile GH secretion, body growth, or hepatic gene expression. In contrast, simultaneous ablation of both GHR and IGF1R in SST-expressing cells increased mean GH levels and pulse amplitude in male and female mice, and partially disrupted the sex differences in hepatic gene expression. Despite the increased GH secretion in double knockout mice, no alterations in body growth and serum or liver IGF-1 levels were observed. In summary, GHR and IGF1R signaling in SST neurons play a redundant role in the control of GH secretion. Furthermore, our results reveal the importance of GH/IGF-1 negative feedback mechanisms on SST neurons for the establishment of sex differences in hepatic gene expression profile.

Distinct effects of growth hormone deficiency and disruption of hypothalamic kisspeptin system on reproduction of male mice

Growth hormone (GH) deficiency is a common cause of late sexual maturation and fertility issues. To determine whether GH-induced effects on reproduction are associated with alterations in hypothalamic kisspeptin system, we studied the male reproduction in two distinct GH deficiency mouse models. In the first model, mice present GH deficiency secondary to arcuate nucleus of the hypothalamus (ARH) lesions induced by posnatal monosodium glutamate (MSG) injections. MSG-induced ARH lesions led to significant reductions in hypothalamic Ghrh mRNA expression and consequently growth. Hypothalamic Kiss1 mRNA expression and Kiss1-expressing cells in the ARH were disrupted in the MSG-treated mice. In contrast, kisspeptin immunoreactivity remained preserved in the anteroventral periventricular and rostral periventricular nuclei (AVPV/PeN) of MSG-treated mice. Importantly, ARH lesions caused late sexual maturation and infertility in male mice. In our second mouse model, we studied animals profound GH deficiency due to a loss-of-function mutation in the Ghrhr gene (Ghrhr^lit/lit mice). Interestingly, although Ghrhr^lit/lit mice exhibited late puberty onset, hypothalamic Kiss1 mRNA expression and hypothalamic kisspeptin fiber density were normal in Ghrhr^lit/lit mice. Despite presenting dwarfism, the majority of Ghrhr^lit/lit male mice were fertile. These findings suggest that spontaneous GH deficiency during development does not compromise the kisspeptin system. Furthermore, ARH Kiss1-expressing neurons are required for fertility, while AVPV/PeN kisspeptin expression is sufficient to allow maturation of the hypothalamic-pituitary-gonadal axis in male mice.

Leptin Receptor Expression in GABAergic Cells is Not Sufficient to Normalize Metabolism and Reproduction in Mice

Previous studies indicate that leptin receptor (LepR) expression in GABAergic neurons is necessary for the biological effects of leptin. However, it is not clear whether LepR expression only in GABAergic neurons is sufficient to prevent the metabolic and neuroendocrine imbalances caused by LepR deficiency. In the present study, we produced mice that express the LepR exclusively in GABAergic cells (LepRVGAT mice) and compared them with wild-type (LepR+/+) and LepR-deficient (LepRNull/Null) mice. Although LepRVGAT mice showed a pronounced reduction in body weight and fat mass, as compared with LepRNull/Null mice, male and female LepRVGAT mice exhibited an obese phenotype relative to LepR+/+ mice. Food intake was normalized in LepRVGAT mice; however, LepRVGAT mice still exhibited lower energy expenditure in both sexes and reduced ambulatory activity in the females, compared with LepR+/+ mice. The acute anorexigenic effect of leptin and hedonic feeding were normalized in LepRVGAT mice despite the hyperleptinemia they present. Although LepRVGAT mice showed improved glucose homeostasis compared with LepRNull/Null mice, both male and female LepRVGAT mice exhibited insulin resistance. In contrast, LepR expression only in GABAergic cells was sufficient to normalize the density of agouti-related peptide (AgRP) and α-MSH immunoreactive fibers in the paraventricular nucleus of the hypothalamus. However, LepRVGAT mice exhibited reproductive dysfunctions, including subfertility in males and alterations in the estrous cycle of females. Taken together, our findings indicate that LepR expression in GABAergic cells, although critical to the physiology of leptin, is insufficient to normalize several metabolic aspects and the reproductive function in mice.

Growth hormone receptor in dopaminergic neurones regulates stress-induced prolactin release in male mice

Arcuate nucleus (ARH) dopaminergic neurones regulate several biological functions, including prolactin secretion and metabolism. These cells are responsive to growth hormone (GH), although it is still unknown whether GH action on ARH dopaminergic neurones is required to regulate different physiological aspects. Mice carrying specific deletion of GH receptor (GHR) in tyrosine hydroxylase (TH)- or dopamine transporter (DAT)-expressing cells were produced. We investigated possible changes in energy balance, glucose homeostasis, fertility, pup survival and restraint stress-induced prolactin release. GHR deletion in DAT- or TH-expressing cells did not cause changes in food intake, energy expenditure, ambulatory activity, nutrient oxidation, glucose tolerance, insulin sensitivity and counter-regulatory response to hypoglycaemia in male and female mice. In addition, GHR deletion in dopaminergic cells caused no gross effects on reproduction and pup survival. However, restraint stress-induced prolactin release was significantly impaired in DAT- and TH-specific GHR knockout male mice, as well as in pegvisomant-treated wild-type males, whereas an intact response was observed in females. Patch clamp recordings were performed in ARH DAT neurones and, in contrast to prolactin, GH did not cause acute changes in the electrical activity of DAT neurones. Furthermore, TH phosphorylation at Ser40 in ARH neurones and median eminence axonal terminals was not altered in DAT-specific GHR knockout male mice during restraint stress. In conclusion, GH action in dopaminergic neurones is required for stress-induced prolactin release in male mice, suggesting the existence of sex differences in the capacity of GHR signalling to affect prolactin secretion. The mechanism behind this regulation still needs to be identified.

Tumor Necrosis Factor α and Interleukin-1β Acutely Inhibit AgRP Neurons in the Arcuate Nucleus of the Hypothalamus

Obesity-associated low-grade inflammation favors weight gain, whereas systemic infection frequently leads to anorexia. Thus, inflammatory signals can either induce positive or negative energy balance. In this study, we used whole-cell patch-clamp to investigate the acute effects of three important proinflammatory cytokines, tumor necrosis factor α (TNF-α), interleukin-6, and interleukin-1β (IL-1β) on the membrane excitability of agouti-related peptide (AgRP)- or proopiomelanocortin (POMC)-producing neurons. We found that both TNF-α and IL-1β acutely inhibited the activity of 35-42% of AgRP-producing neurons, whereas very few POMC neurons were depolarized by TNF-α. Interleukin-6 induced no acute changes in the activity of AgRP or POMC neurons. Our findings indicate that the effect of TNF-α and IL-1β, especially on the activity of AgRP-producing neurons, may contribute to inflammation-induced anorexia observed during acute inflammatory conditions.

STAT5 ablation in AgRP neurons increases female adiposity and blunts food restriction adaptations

AgRP neurons are important players in the control of energy homeostasis and are responsive to several hormones. In addition, STAT5 signalling in the brain, which is activated by metabolic hormones and growth factors, modulates food intake, body fat and glucose homeostasis. Given that, and the absence of studies that describe STAT5 function in AgRP cells, the present study investigated the metabolic effects of Stat5a/b gene ablation in these neurons. We observed that STAT5 signalling in AgRP neurons regulates body fat in female mice. However, male and female STAT5-knockout mice did not exhibit altered food intake, energy expenditure or glucose homeostasis compared to control mice. The counter-regulatory response or glucoprivic hyperphagia induced by 2-deoxy-d-glucose treatment were also not affected by AgRP-specific STAT5 ablation. However, under 60% food restriction, AgRP STAT5-knockout mice had a blunted upregulation of hypothalamic Agrp mRNA expression and corticosterone serum levels compared to control mice, suggesting a possible role for STAT5 in AgRP neurons for neuroendocrine adaptations to food restriction. Interestingly, ad libitum fed knockout male mice had reduced Pomc and Ucp-1 mRNA expression compared to control group. Taken together, these results suggest that STAT5 signalling in AgRP neurons regulates body adiposity in female mice, as well as some neuroendocrine adaptations to food restriction.