Interleukin-6 regulates the expression of hypothalamic neuropeptides involved in body weight in a gender-dependent way

Interleukin (IL)-6 has been involved in the control of body weight and body fat. Nevertheless, the mechanisms underlying these effects are not completely understood because central and peripheral actions of IL-6 are plausible. To gain further insight into the central effects of IL-6, we used transgenic mice expressing the IL-6 gene under the control of the glial fibrillary acidic protein (GFAP) promoter (GFAP-IL-6 mice), therefore with central nervous system-restricted over-expression of IL-6, and we studied the expression of the main neuropeptides responsible for energy homeostasis in specific hypothalamic nuclei. Neuropeptide Y (NPY), agouti-related peptide (AgRP), melanin-concentrating hormone (MCH), prepro-orexin (preproOX) (orexigenic and anabolic neuropeptides), pro-opiomelanocortin (POMC) and corticotrophin-releasing hormone (CRH) (anorexigenic and catabolic peptides) mRNA levels were determined using in situ hybridisation in young (2-4 month-old) and old (10-12 month-old) female and male mice under different feeding conditions: normal diet (control) and high-fat diet (HFD), and 24 h-food deprivation. In GFAP-IL-6 females fed a control diet (GFAP-IL-6-control), we showed a significant decrease in NPY and AgRP mRNA levels at all ages, and a late increase in POMC expression (only significant in older animals). These differences were blunted in HFD mice. By contrast, GFAP-IL-6-control males showed a decrease in CRH mRNA content at early ages (2-4 months), and an increase in older mice (10-12 months). Interestingly, these differences were again blunted in HFD mice. Finally, central IL-6 was not able to counteract the effects of 24 h of fasting on body weight, plasma glucose levels and the mRNA content of the peptides evaluated in the present study. Our results demonstrate that IL-6 may regulate the expression of hypothalamic neuropeptides involved in the control of body weight and body fat acting at the central level in a gender- and age-dependent way.

Perinatal overfeeding in rats results in increased levels of plasma leptin but unchanged cerebrospinal leptin in adulthood

OBJECTIVE

To study the effect of perinatal programming and overfeeding on the hypothalamic control mechanisms of food intake in adult rats.

DESIGN

Neonatal programming effects on body weight, food intake, central and peripheral leptin levels, hypothalamic neuropeptides, leptin receptors and central leptin responsiveness in adult rats.

MEASUREMENTS

Plasma and cerebrospinal fluid (CSF) leptin levels were analyzed using radioimmunoassay. Neuropeptide mRNA levels were analyzed using in situ hybridization. Leptin receptor mRNA levels were analyzed using reverse transcriptase-polymerase chain reaction.

RESULTS

Perinatally overfed rats growing up in small litters (SL) maintain their obese and hyperleptinemic phenotype in adulthood. However, leptin levels in CSF are abnormally low considering the plasmatic hyperleptinemia. In contrast to the already reported changes in perinatally overfed juvenile rats, perinatally overfed adult rats did not show any alteration in the expression of leptin receptor isoforms and evaluated neuropeptides. Moreover, SL adult rats showed a normal sensitivity regarding the inhibitory effect of intracerebroventricular leptin administration on food intake.

CONCLUSION

Perinatal overfeeding does not induce alterations in either the anorectic response to central leptin administration or expression of leptin receptors and neuropeptides in adulthood. The leptin resistance to peripheral leptin in SL adult rats may be related to impaired leptin transport across the blood-brain barrier.

A possible role of neuropeptide Y, agouti-related protein and leptin receptor isoforms in hypothalamic programming by perinatal feeding in the rat

AIM/HYPOTHESIS

Perinatal overfeeding predisposes humans and rats to obesity and diabetes in later life. One classical model for studying the effect of early feeding is manipulation of the size of rat litters. Rats growing up in small litters gain more weight than rats growing up in normal-sized litters. Interestingly, these obese rats maintain this phenotype in adulthood. Conversely, rats raised in large litters show a delay in growth and a decrease in body weight. The aim of this work was to assess the hypothalamic control mechanisms of food intake regulated by perinatal feeding.

METHODS

Leptin levels were analysed using RIA. Leptin receptor mRNA levels were analysed using RT-PCR. Neuropeptide mRNA levels were analysed using in situ hybridisation.

RESULTS

Perinatally overfed neonatal male rats exhibited hyperleptinaemia and a decrease in hypothalamic mRNA levels of the long isoform of the leptin receptor (OB-Rb), explaining their leptin resistance. Moreover, this obese model showed an increase in the mRNA expression of cocaine- and amphetamine-regulated transcript, neuropeptide Y and agouti-related protein in the hypothalamic arcuate nucleus (ARC). In contrast, perinatally underfed neonatal male rats with hypoleptinaemia showed an increase in hypothalamic mRNA of the short isoforms of the leptin receptor. Furthermore, they exhibited an increase in expression of neuropeptide Y and agouti-related protein in the ARC.

CONCLUSIONS/INTERPRETATION

Rats overfed during early postnatal life show a leptin-resistant state mediated by down-regulation of the hypothalamic OB-Rb. These data, together with the increased expression of neuropeptide Y and agouti-related protein in specific neurons in the ARC, might indicate the existence of regulated programming in this nucleus and may provide a new aetiopathogenic concept in susceptibility to obesity.

Hypothalamic levels of NPY, MCH, and prepro-orexin mRNA during pregnancy and lactation in the rat: role of prolactin

Pregnancy and lactation provide excellent models of physiological hyperphagia and hyperprolactinemia. To identify possible factors associated with the increased feeding in these situations, we measured hypothalamic mRNA levels of three orexigenic neuropeptides--NPY, MCH, and orexins--in nonpregnant, pregnant, and lactating rats by in situ hybridization. NPY mRNA content in the arcuate nucleus was significantly increased during pregnancy and lactation. However, MCH and prepro-orexin expression was decreased in both states. 48 or 72 h of fasting in pregnant and lactating rats further elevated NPY mRNA levels and increased the low MCH mRNA content. Surprisingly, no effect was observed in prepro-orexin mRNA levels. Finally, we investigated the possible effect of high PRL levels on these orexigenic signals using a model of hyperprolactinemia induced by pituitary graft. NPY mRNA content was unchanged, but MCH and prepro-orexin mRNA levels were significantly decreased. Our results suggest that the increased NPY expression might be partly responsible for the hyperphagia observed during pregnancy and lactation. MCH and prepro-orexin may be involved in the adaptation of other homeostatic mechanisms and their decreased levels in these physiological settings could be mediated by the elevated circulating PRL levels.

Effect of cyclic 3',5'-adenosine monophosphate, glucocorticoids, and insulin on leptin messenger RNA levels and leptin secretion in cultured human trophoblast

Leptin is a polypeptide hormone originally thought to be produced exclusively by adipocytes. However, both leptin mRNA and leptin protein were identified in human placental trophoblast cells, suggesting a potential role in human pregnancy. In the present report, we examined the regulation of leptin mRNA levels and secretion by cAMP, glucocorticoids, and insulin in term human placental tissue. Placentae were obtained immediately after delivery from mothers with uncomplicated pregnancies. Leptin concentrations were measured by ELISA in the cultured media of trophoblast maintained in monolayer culture for 24, 48, and 72 h. Likewise leptin mRNA levels in these cultured human trophoblast cells were determined by reverse transcription-polymerase chain reaction. Treatment with forskolin and (Bu)(2) cAMP led to a time- and dose-dependent increase in leptin release, significant after 48 and 72 h. Moreover, incubation with forskolin for 48 h also clearly increased leptin mRNA concentration. Leptin secretion and mRNA levels were also assessed after treatment with insulin or dexamethasone. We found a time- and dose-dependent increase in leptin release, significant after 48 and 72 h. Leptin mRNA levels were also increased after these treatments. All this supports a stimulatory role of cAMP pathway, insulin and dexamethasone in the leptin mRNA levels, and leptin release in trophoblast cells in vitro.

Prepro-orexin mRNA levels in the rat hypothalamus, and orexin receptors mRNA levels in the rat hypothalamus and adrenal gland are not influenced by the thyroid status

Orexins are two recently discovered neuropeptides that play an important role in the regulation of food intake and in the regulation of the sleep-wake cycle. In this work we examined the effects of thyroid hormones on prepro-OX expression in the rat hypothalamus, and OXRs expression in the rat hypothalamus and adrenal gland. Hypo- and hyperthyroidism were induced in adult male rats, and the levels of hypothalamic prepro-OX and OXRs mRNA, and adrenal OXRs mRNA were determined using semiquantitative reverse transcription-polymerase chain reaction and/or in situ hybridization. Our results indicate that thyroid status affects neither prepro-OX in the hypothalamus nor hypothalamic and adrenal gland OXRs expression.

Gestational profile of leptin messenger ribonucleic acid (mRNA) content in the placenta and adipose tissue in the rat, and regulation of the mRNA levels of the leptin receptor subtypes in the hypothalamus during pregnancy and lactation

Serum leptin levels were significantly increased during rat gestation. Our data showed that leptin mRNA levels in both the adipose tissue and placenta were higher as pregnancy progressed, suggesting a role for both tissues in the hyperproduction of leptin. This paradoxical increase in leptin concentration during gestation suggests that a physiological state of leptin resistance may exist at the hypothalamic level that may explain the hyperphagia observed in pregnant rats. In order to study this issue further, levels of the mRNA encoding the different leptin receptor isoforms were determined in the hypothalamus of pregnant and nonpregnant rats. We found a specific reduction of the mRNA levels encoding the leptin receptor isoform Ob-Rb in the hypothalamus of pregnant rats compared to nonpregnant animals, suggesting that during pregnancy the hypothalamus shows a physiological resistance to the high levels of leptin due, at least in part, to a decrease in the expression of the long, biologically active form of the leptin receptor (Ob-Rb). During lactation, serum leptin levels returned to values observed in nonpregnant rats. In the hypothalami of these animals, Ob-Rb mRNA content was similar to that observed in nonpregnant rats, but we found an increased expression of some of the short forms of the leptin receptor (Ob-Re and Ob-Rf). This could contribute to induction of the hyperphagia present during lactation. These data provide new insights into the adaptive mechanisms that take place during pregnancy and lactation in order to meet increased metabolic requirements.

Regulation of hypothalamic somatostatin and growth hormone releasing hormone mRNA levels by inhibin

Although it is well established that inhibin plays a major role in the regulation of the hypothalamic-pituitary-gonadal axis, its influence in the regulation of other neuroendocrine functions is still poorly understood. Recent results indicate that inhibin suppresses plasma GH levels, but its site of action is yet unknown. Therefore, in the present work we investigated the effects of inhibin on somatostatin and growth hormone releasing hormone (GHRH) mRNA levels in the hypothalamus by 'in situ' hybridization. We found that inhibin administration (4, 12 and 24 h, i.c.v.) led to an increase in somatostatin mRNA levels in the periventricular nucleus, and to a decrease in GHRH mRNA content in the arcuate nucleus of the hypothalamus. These findings indicate that inhibin regulates the hypothalamic levels of somatostatin and GHRH mRNA.

Role of growth hormone (GH)-releasing hormone and somatostatin on leptin-induced GH secretion

Leptin is a hormone secreted by the adipocytes that regulates food intake and energy expenditure. It is known that growth hormone (GH) secretion is markedly influenced by body weight, being suppressed in obesity and cachexia, and recent data have demonstrated that GH release is regulated by leptin levels. Although one of the sites of action of leptin is likely to be the hypothalamus, since leptin receptor mRNA is particularly abundant in several hypothalamic nuclei, the mechanisms by which leptin regulates GH secretion are not yet known. The aim of the present study was to investigate whether leptin could act at the hypothalamic level modulating somatostatin and GH-releasing hormone (GHRH) expression. The administration of anti-GHRH serum (500 microl, i.v.) completely blocked leptin-induced GH release in fasting rats. In contrast, the treatment with anti-somatostatin serum (500 microl, i.v.) significantly increased GH release in this condition. Furthermore, leptin administration (10 microg, i.c.v.) to intact fasting animals reversed the inhibitory effect produced by fasting on GHRH mRNA levels in the arcuate nucleus of the hypothalamus, and increased somatostatin mRNA content in the periventricular nucleus. Finally, leptin administration (10 microgram, i.c.v.) to hypophysectomized fasting rats increased GHRH mRNA levels, and decreased somatostatin mRNA content, indicating an effect of leptin on hypothalamic GHRH- and somatostatin-producing neurons. These findings suggest a role for GHRH and somatostatin as mediators of leptin-induced GH secretion.

Inhibin suppresses in vivo growth hormone secretion

Although the effects of inhibin on gonadotropin synthesis and secretion have been extensively studied, the role of inhibin in the neuroregulation of in vivo growth hormone (GH) secretion still remains to be elucidated. In the present work, we investigated the effects of inhibin on spontaneous GH secretion in three different groups of conscious adult male rats: intact, gonadectomized, and dihydrotestosterone (DHT)-treated gonadectomized animals. We found that inhibin administration (100 microg/kg, i.v.) led to a marked suppression in spontaneous GH secretion in all the groups studied. This significant decrease was assessed by the area under the curve in intact (311.1 +/- 163.3 vs. 3,882.1 +/- 1,084.6 ng/ml/6 h, p < 0. 01), gonadectomized (416.6 +/- 120.9 vs. 2,078.5 +/- 298.4 ng/ml/6 h, p < 0.01) and gondadectomized rats treated with DHT (755.0 +/- 102. 3 vs. 4,539.3 +/- 1,670.6 ng/ml/6 h, p < 0.01). Furthermore, intravenous inhibin significantly reduced in vivo GH responses to GHRH (10 microg/kg, i.v.) in both intact and gonadectomized rats. These findings suggest a role of inhibin on in vivo GH secretion in the male rat.

Transforming growth factor-betas inhibit somatostatin messenger ribonucleic acid levels and somatostatin secretion in hypothalamic cells in culture

Treatment of hypothalamic cells in monolayer culture with transforming growth factor-beta1 (TGFbeta1) significantly reduced both basal and cAMP-induced somatostatin messenger RNA (mRNA) levels and somatostatin secretion. This inhibitory effect was dose- and time-dependent and not mediated by glial cells, as it was also observed in glial-free hypothalamic cell cultures treated with cytosine arabinonucleoside. TGFbeta2 and -beta3 mimicked the actions of TGFbeta1, which indicated that the three isoforms of the TGFbeta family expressed in the central nervous system displayed similar effects on the somatostatinergic neurons. The blockade of synthesis of proteins with either cycloheximide or puromycin for 24 h prevented the inhibitory effect of TGFbeta1 on somatostatin mRNA. This implied that the reduction of this mRNA by TGFbeta1 required de novo protein synthesis. We next studied whether TGFbeta1 acted at the transcriptional or posttranscriptional level by altering the stability of somatostatin mRNA. Examination of the rate of disappearance of somatostatin mRNA by Northern blot, after inhibition of mRNA transcription with either actinomycin D (AcD) or 5,6-dichloro-1beta-ribofuranosyl benzimidazole revealed that TGFbeta1 did reduce the stability of somatostatin mRNA. This effect was observed when we pretreated the cultures with TGFbeta1 4 h before the addition of AcD, but not when we administered TGFbeta1 simultaneously with AcD or 5,6-dichloro-1beta-ribofuranosyl benzimidazole. Altogether these results demonstrated that the treatment of hypothalamic cells in culture with TGFbeta1, TGFbeta2, or TGFbeta3 resulted in a decrease in somatostatin mRNA levels and somatostatin secretion. TGFbeta1 reduced the steady state levels of somatostatin mRNA by inducing the synthesis of a protein (s), that appears to accelerate the degradation of the mRNA of somatostatin. Whether TGFbeta1 has additional effects on the transcription of the somatostatin gene will require further study.

Regulation of hypothalamic somatostatin, growth hormone-releasing hormone, and growth hormone receptor messenger ribonucleic acid by glucocorticoids

Although it is well known that chronic treatment with glucocorticoids inhibits somatic growth, the mechanism of action of this inhibitory effect is not completely understood. It is likely that glucocorticoids act at various levels, including pituitary, hypothalamus, and peripheral organs modulating GH synthesis, secretion, and action. In this work, we evaluated the effect of dexamethasone on hypothalamic somatostatin and GH-releasing hormone (GHRH) messenger RNA (mRNA) levels by in situ hybridization. We found a significant decrease of somatostatin mRNA content in the periventricular nucleus of the hypothalamus after 3, 8, and 15 days of treatment with dexamethasone. Furthermore, we observed a reduction in GHRH mRNA levels in the arcuate nucleus after 8 and 15 days of treatment with this steroid. As it has been shown that GH feeds back to regulate somatostatin and GHRH expression at the hypothalamic level through high affinity GH receptors, we evaluated the possibility of a GH-mediated action in the inhibitory effect of glucocorticoids on somatostatin and GHRH mRNA levels. To address this issue, we first studied the GH receptor mRNA content in both the periventricular and arcuate nuclei of the hypothalamus after dexamethasone treatment. Secondly, the effect of dexamethasone on somatostatin and GHRH mRNA levels in hypophysectomized animals also was assessed. We found a significant decrease in GH receptor mRNA levels in the periventricular nucleus and in the arcuate nucleus after 1, 3, 8, and 15 days of glucocorticoid administration. Finally, in hypophysectomized rats, dexamethasone treatment for 15 days did not reduce somatostatin mRNA levels in the periventricular nucleus but significantly decreased GHRH mRNA content in the arcuate nucleus. In summary, our results suggest an inhibitory GH-mediated effect of dexamethasone on somatostatin mRNA levels in the periventricular nucleus and an inhibitory direct effect of dexamethasone on GHRH neurones in the arcuate nucleus.

Gonadal regulation of somatostatin receptor 1, 2 and 3 mRNA levels in the rat anterior pituitary

In order to investigate the gonadal regulation of pituitary somatostatin receptors (SSTRs)-gene expression, we studied the existence of sexual dimorphism on SSTR1, SSTR2, and SSTR3 mRNA levels in the rat anterior pituitary. The roles of testosterone and other non-androgenic testicular factors were also evaluated. SSTR1, SSTR2 and SSTR3 mRNA content were assessed by in-situ hybridization in male rats lacking the influence of androgenic and/or non-androgenic testicular factors. We found that SSTR1 and SSTR3 mRNA levels in the anterior pituitary exhibited sexual dimorphism. Moreover, our data indicate that SSTR1 mRNA levels are regulated by testosterone, while SSTR3 mRNA content is regulated by non-androgenic testicular factors. In contrast, SSTR2 levels are not influenced by gonadal function.

Evidence for the involvement of non-androgenic testicular factors in the regulation of hypothalamic somatostatin and GHRH mRNA levels

The growth hormone (GH) secretory pattern is dependent on sex and developmental stage. It is generally accepted that in the male rat this pattern is markedly influenced by androgens secreted by the Leydig cells. Recent findings, however, point to the existence of other non-androgenic testicular factors produced by the Sertoli cells and which regulate in vivo the GH responses to growth hormone releasing hormone (GHRH). The aim of this work was to investigate the role played by non-androgenic testicular factors on hypothalamic somatostatin (SST) and GHRH mRNA levels. Seventy-day-old male Sprague-Dawley rats were used throughout the work. They were divided into five groups: (1) control rats; (2) gonadectomized rats; (3) gonadectomized rats supplemented with exogenous administration of dihydrotestosterone (DHT); (4) ethylene dimethane sulphonate (EDS)-treated rats; (5) EDS-treated rats supplemented with exogenous administration of DHT. EDS is a cytotoxic agent that specifically destroys the Leydig cells. The rats were killed after 15 days of treatment. Hypothalamic SST mRNA levels were determined by Northern blot and by in situ hybridization, and GHRH mRNA levels assessed by Northern blot. We found that selective removal of Leydig cells with EDS greatly reduced the SST mRNA content in the periventricular nucleus of the hypothalamus. These levels were significantly lower than those found in gonadectomized rats. Furthermore, replacement treatment with dihydrotesterone (DHT) did not completely restore SST mRNA levels in EDS-treated rats, contrasting with the complete recovery of SST mRNA levels in gonadectomized rats. On the other hand, gonadectomy and EDS treatment produced a significant reduction in GHRH mRNA levels. DHT administration reversed the action of gonadectomy, but did not restore GHRH mRNA content in EDS-treated rats. These data suggest that, in addition to testosterone, as yet unidentified non-androgenic testicular factors can significantly influence SST and GHRH mRNA levels. This may indicate that non-androgenic testicular factors acting at hypothalamic level may be important in the neuroregulation of GH secretion and in the maintenance of sexual dimorphism in GH secretory pattern.

Distribution of somatostatin receptors 1, 2 and 3 mRNA in rat brain and pituitary

In this study sequence-specific antisense oligonucleotide probes have been used to investigate the distribution of the mRNAs coding for the somatostatin receptor subtypes termed somatostatin receptor 1, somatostatin receptor 2 and somatostatin receptor 3 in the rat brain and pituitary using in situ hybridization techniques. The three receptor subtype mRNAs were found to be widely distributed in the brain with different patterns of expression, but with some overlap. Somatostatin receptor 1 mRNA was particularly concentrated in the cerebral and piriform cortex, magnocellular preoptic nucleus, hypothalamus, amygdala, hippocampus, and several nuclei of the brainstem. Somatostatin receptor 3 mRNA was very abundant in the cerebellum and pituitary (in contrast to somatostatin receptor 1), but it was also found in hippocampus, amygdala, hypothalamus and in motor nuclei of the brainstem. Somatostatin receptor 2 mRNA levels were very low relative to the other two mRNAs evaluated. Receptor 2 mRNA was observed in the anterior pituitary, and in the brain it was found in the medial habenular nucleus, claustrum, endopiriform nucleus, hippocampus some amygdala nuclei, cerebral cortex and hypothalamus. None of the three somatostatin receptor mRNAs studied here was found in the caudate nucleus. Northern analysis revealed distinct sizes of mRNAs for each subtype, and displacement experiments showed that each probe sequence was subtype-specific.

Effects of free fatty acids on somatostatin secretion, content and mRNA levels in cortical and hypothalamic fetal rat neurones in monolayer culture

The effects of free fatty acids on somatostatin secretion, content and mRNA levels in fetal rat hypothalamic and cortical cell cultures were investigated. Somatostatin secretion and content were quantified by radioimmunoassay. Somatostatin mRNA levels were measured by Northern blot hybridization using a cRNA probe. Treatment with either caprylic acid (5 x 10(-3) M) or oleic acid (5 x 10(-5) M) for 90 min inhibited basal somatostatin secretion in both hypothalamic and cortical cell cultures. In addition, the increase in somatostatin secretion induced by incubation with veratridine (10(-4) M) or carbachol (10(-4) M) for 90 min was significantly reduced by the addition of caprylic acid, but somatostatin release stimulated by 5.6 x 10(-2) M KCl was not affected. Furthermore, treatment with these free fatty acids for 90 min markedly decreased somatostatin mRNA levels in both types of neurone culture. These inhibitory effects were transient, being observed after 90 min, but not after 5 h. These results support the probability that there is a role for free fatty acids in the regulation of somatostatin mRNA levels and somatostatin secretion in both hypothalamic and cortical cell cultures.