Insulin treatment stimulates the rat melanin-concentrating hormone-producing neurons

Identification of IGF-1 Effects on White Adipose Tissue and Hippocampus in Alzheimer's Disease Mice via Transcriptomic and Cellular Analysis

Alzheimer's disease (AD) stands as the most prevalent neurodegenerative disorder, characterized by a multitude of pathological manifestations, prominently marked by the aggregation of amyloid beta. Recent investigations have revealed a compelling association between excessive adiposity and glial activation, further correlating with cognitive impairments. Additionally, alterations in levels of insulin-like growth factor 1 (IGF-1) have been reported in individuals with metabolic conditions accompanied by memory dysfunction. Hence, our research endeavors to comprehensively explore the impact of IGF-1 on the hippocampus and adipose tissue in the context of Alzheimer's disease. To address this, we have conducted an in-depth analysis utilizing APP/PS2 transgenic mice, recognized as a well-established mouse model for Alzheimer's disease. Upon administering IGF-1 injections to the APP/PS2 mice, we observed notable alterations in their behavioral patterns, prompting us to undertake a comprehensive transcriptomic analysis of both the hippocampal and adipose tissues. Our data unveiled significant modifications in the functional profiles of these tissues. Specifically, in the hippocampus, we identified changes associated with synaptic activity and neuroinflammation. Concurrently, the adipose tissue displayed shifts in processes related to fat browning and cell death signaling. In addition to these findings, our analysis enabled the identification of a collection of long non-coding RNAs and circular RNAs that exhibited significant changes in expression subsequent to the administration of IGF-1 injections. Furthermore, we endeavored to predict the potential roles of these identified RNA molecules within the context of our study. In summary, our study offers valuable transcriptome data for hippocampal and adipose tissues within an Alzheimer's disease model and posits a significant role for IGF-1 within both the hippocampus and adipose tissue.

Crosstalk between Melanin Concentrating Hormone and Endocrine Factors: Implications for Obesity

Melanin-concentrating hormone (MCH) is a 19aa cyclic peptide exclusively expressed in the lateral hypothalamic area, which is an area of the brain involved in a large number of physiological functions and vital processes such as nutrient sensing, food intake, sleep-wake arousal, memory formation, and reproduction. However, the role of the lateral hypothalamic area in metabolic regulation stands out as the most relevant function. MCH regulates energy balance and glucose homeostasis by controlling food intake and peripheral lipid metabolism, energy expenditure, locomotor activity and brown adipose tissue thermogenesis. However, the MCH control of energy balance is a complex mechanism that involves the interaction of several neuroendocrine systems. The aim of the present work is to describe the current knowledge of the crosstalk of MCH with different endocrine factors. We also provide our view about the possible use of melanin-concentrating hormone receptor antagonists for the treatment of metabolic complications. In light of the data provided here and based on its actions and function, we believe that the MCH system emerges as an important target for the treatment of obesity and its comorbidities.

Melanin-concentrating hormone and food intake control: Sites of action, peptide interactions, and appetition

Given the increased prevalence of obesity and its associated comorbidities, understanding the mechanisms through which the brain regulates energy balance is of critical importance. The neuropeptide melanin-concentrating hormone (MCH) is produced in the lateral hypothalamic area and the adjacent incerto-hypothalamic area and promotes both food intake and energy conservation, overall contributing to body weight gain. Decades of research into this system has provided insight into the neural pathways and mechanisms (behavioral and neurobiological) through which MCH stimulates food intake. Recent technological advancements that allow for selective manipulation of MCH neuron activity have elucidated novel mechanisms of action for the hyperphagic effects of MCH, implicating neural "volume" transmission in the cerebrospinal fluid and sex-specific effects of MCH on food intake control as understudied areas for future investigation. Highlighted here are historical and recent findings that illuminate the neurobiological mechanisms through which MCH promotes food intake, including the identification of various specific neural signaling pathways and interactions with other peptide systems. We conclude with a framework that the hyperphagic effects of MCH signaling are predominantly mediated through enhancement of an "appetition" process in which early postoral prandial signals promote further caloric consumption.

Dissociated Pmch and Cre Expression in Lactating Pmch-Cre BAC Transgenic Mice

The melanin-concentrating hormone (MCH) system plays a role in many physiological processes including reproduction and lactation. However, research regarding the function of MCH on different aspects of the reproductive function lags, due in part to a lack of validated genetic models with which to interrogate the system. This is particularly true in the case of female reproduction, as the anatomy and function of the MCH system is not well-characterized in the female mouse. We set out to determine whether the commercially available Pmch-Cre transgenic mouse line is a viable model to study the role of MCH neurons in distinct female reproductive states. We found that Pmch is transiently expressed in several nuclei of the rostral forebrain at the end of lactation. This includes the medial subdivision of the medial preoptic nucleus, the paraventricular nucleus of the hypothalamus, the ventral subdivision of the lateral septum, the anterodorsal preoptic nucleus and the anterodorsal nucleus of the thalamus. The Pmch expression in these sites, however, does not reliably induce Cre expression in the Pmch-Cre (BAC) transgenic mouse, making this line an inadequate model with which to study the role of MCH in behavioral and/or neuroendocrine adaptations of lactation. We also contribute to the general knowledge of the anatomy of the murine MCH system by showing that lactation-induced Pmch expression in the rostral forebrain is mostly observed in GABAergic (VGAT) neurons, in contrast to the typical MCH neurons of the tuberal and posterior hypothalamus which are glutamatergic (VGLUT2).

Involvement of orexin-A neurons but not melanin-concentrating hormone neurons in the short-term regulation of food intake in rats

In order to elucidate the involvement of melanin-concentrating hormone (MCH) and orexin-A (ORX-A) neurons of the perifornical/lateral hypothalamic areas (PF/LH) in the regulation of food intake induced by acutely reduced glucose availability, we examined the food intake response and c-Fos expression in the MCH and ORX-A neurons in the PF/LH during 2-deoxy-D-glucose (2DG)-induced glucoprivation (400 mg/kg; i.v.) and systemic insulin-induced hypoglycemia (5 U/kg; s.c.) in male Wistar rats. The administration of both 2DG and insulin stimulated food intake and induced c-Fos expression in the ORX-A neurons corresponding to food intake, but not in the MCH neurons. These data indicate that ORX-A neurons, but not MCH neurons, play a role in the short-term regulation of food intake, and that the input signals for the neurons containing MCH and ORX-A are different, and these neurons play different roles in the regulation of feeding behavior.

Melanin-concentrating hormone producing neurons: Activities and modulations

Regulation of energy homeostasis in animals involves adaptation of energy intake to its loss, through a perfect regulation of feeding behavior and energy storage/expenditure. Factors from the periphery modulate brain activity in order to adjust food intake as needed. Particularly, "first order" neurons from arcuate nucleus are able to detect modifications in homeostatic parameters and to transmit information to "second order" neurons, partly located in the lateral hypothalamic area. These "second order" neurons have widespread projections throughout the brain and their proper activation leads them to a coordinated response associated to an adapted behavior. Among these neurons, melanin-concentrating hormone (MCH) expressing neurons play an integrative role of the various factors arising from periphery, first order neurons and extra-hypothalamic arousal systems neurons and modulate regulation of feeding, drinking and seeking behaviors. As regulation of MCH release is correlated to regulation of MCH neuronal activity, we focused this review on the electrophysiological properties of MCH neurons from the lateral hypothalamic area. We first reviewed the knowledge on the endogenous electrical properties of MCH neurons identified according to various criteria which are described. Then, we dealt with the modulations of the electrical activity of MCH neurons by different factors such as glucose, glutamate and GABA, peptides and hormones regulating feeding and transmitters of extra-hypothalamic arousal systems. Finally, we described the current knowledge on the modulation of MCH neuronal activity by cytokines and chemokines. Because of such regulation, MCH neurons are some of the best candidate to account for infection-induced anorexia, but also obesity.

Peripheral ghrelin interacts with orexin neurons in glucostatic signalling

Ghrelin interactions with glycemia in appetite control as well as the potential mechanisms involving the orexin and melanin-concentrating hormone (MCH) neurons in the orexigenic ghrelin signals were investigated by using a specific anti-ghrelin antibody (AGA). Our results confirm that peripheral ghrelin is an important signal in meal initiation and appetite. Employing immunohistochemistry techniques, we found that c-fos positive neurons in the lateral hypothalamus (LH) and perifornical area (PFA) increased after insulin or 2-deoxyglucose administration. Moreover, we have also demonstrated that peripheral ghrelin blockade by the AGA, reduces the orexigenic signal induced by insulin and 2-DG administration probably partly producing a decrease of c-fos immunoreactivity in the LH and PFA as well as a lower activation of orexin neurons. In contrast, the c-fos positive MCH neurons were not apparently affected. In summary, our findings suggest that peripheral ghrelin plays an important role in regulatory "glucostatic" feeding mechanisms by means of its role as a "hunger" signal affecting the LH and PFA areas, which may contribute to energy homeostasis through orexin neurons.

Rapid and preferential activation of Fos protein in hypocretin/orexin neurons following the reversal of dehydration-anorexia

Dehydration (DE)-anorexia is stimulated by chronic consumption of hypertonic saline. Spontaneous nocturnal food intake is markedly reduced with this treatment but is rapidly reversed upon the return of drinking water. Here we examined the neurons in the lateral hypothalamic area (LHA) of chronically dehydrated rats for their peptidergic phenotype, colocalization, and activation profiles following the rapid reversal of anorexia. To do this, we used double-labeling combinations of Fos immunocytochemistry and radioisotopic- and digoxigenin-labeled in situ hybridization. We found that lateral hypothalamic corticotropin-releasing hormone (CRH) neurons show extensive coexpression with neurotensin mRNA, but they are distinct from hypocretin/orexin and melanin-concentrating hormone (MCH) neurons. Chronic dehydration increases Fos-ir in large numbers of neurons in dorsal regions of the LHA. Some of these LHA neurons also show increased CRH, but not hypocretin/orexin or MCH gene expression, as dehydration-anorexia develops. Furthermore, the behavioral sequence of eating and increased activity exhibited by DE animals in the minutes following water drinking is accompanied by a further increase in the number of Fos-ir nuclei in the LHA. Increased Fos activation occurs in a significant number of LHA hypocretin/orexin neurons, but not CRH or MCH neurons, in the LHA. Together these data implicate CRH but not hypocretin/orexin or MCH neurons in the LHA in the motor events associated with dehydration. However, when water is returned, contributions to the network controlling responses evidently come from hypocretin/orexin, but not CRH or MCH, neurons in the LHA.

Sex difference in the response of melanin-concentrating hormone neurons in the lateral hypothalamic area to glucose, as revealed by the expression of phosphorylated cyclic adenosine 3',5'-monophosphate response element-binding protein

Because there are sex differences in feeding behavior in rats, we looked for a possible sex difference in the response to glucose of melanin-concentrating hormone (MCH) neurons in the lateral hypothalamic area using phosphorylated cAMP response element-binding protein (pCREB) as a marker of neural activity. Intact male rats and female rats at diestrus 2, proestrus, or estrus were fed normally or fasted for 48 h and injected with saline or glucose (400 mg/kg). Thereafter, preparations were subjected to immunohistochemical processing for the double staining of MCH and pCREB. Fasting increased the ratio of MCH neurons with pCREB (double-stained cells) in both male and female rats. In fasted rats, glucose injection decreased the ratio of double-stained cells more promptly in females than in males. The magnitude of decrease caused by glucose was greater at proestrus and estrus than at diestrus 2. Gonadectomy in males enhanced and in females attenuated the response of MCH neurons to glucose. Testosterone and estrogen replacement in males and females, respectively, restored the response of MCH neurons to glucose. The demonstrated sex differences in the response of MCH neurons to glucose correlated well with the gonadal steroid milieu; thus, MCH neurons may play an important role in sex differences in feeding behavior.

Effect of CCK-8 on insulin-induced hyperphagia and hypothalamic orexigenic neuropeptide expression in the rat

The influence of cholecystokinin octapeptide (CCK-8) on normal and insulin-induced feeding and expression of orexigenic hypothalamic neuropeptides was investigated in male rats. CCK-8, administered during meals (4 microg/kg) or continuously (32 microg/kg over 60 min), blunted the stimulating effect of insulin (50 IU/kg) on feeding by reducing meal size (-60%; P<0.05 or -86%; P<0.0001, respectively). Rats without access to food and injected with IP insulin (50 IU/kg) showed increased hypothalamic mRNA levels of orexin (+30%; P<0.05) and melanin-concentrating hormone (+52%; P<0.05), as compared with ad libitum-fed and saline-injected control rats. Continuous IP infusion of CCK-8 (32 microg/kg) blunted these increases. Our results suggest that both orexin and melanin-concentrating hormone participate in the response to insulin hypoglycemia without food being present; these neurons may be involved in mechanisms related to insulin-induced hyperphagia. Signals triggered by peripheral CCK-8 act to decrease the expression of orexin and melanin-concentrating hormone. This may be associated with a reduction in hyperphagia.

Increases in melanin-concentrating hormone and MCH receptor levels in the hypothalamus of dietary-obese rats

Melanin-concentrating hormone (MCH) is a hypothalamic neuropeptide that stimulates feeding and increases body weight in rodents. We studied the role of the system in energy homeostasis and its regulation by the satiety signals, leptin and insulin. We used real-time PCR to measure the hypothalamic expression of MCH and its receptor (MCHR1) in two contrasting models of altered nutritional status, namely, obesity induced by 8 weeks' voluntary overeating and food restriction for 10 days. Diet-fed rats were stratified according to final total fat-pad mass into a 'high fat gain' group (HG) and 'low fat gain' group (LG). MCH mRNA levels were increased by 31% (p>0.05) and 49% (p<0.05) in the LG and HG, respectively, compared with controls. MCHR1 mRNA levels rose by 118% in the LG (p<0.01) and 85% in the HG (p<0.01). There were significant positive correlations (p<0.05) between plasma leptin concentration and both MCH and MCHR1 mRNA levels, and between plasma insulin and MCHR1 expression. A positive correlation was also observed between MCH and MCHR1 mRNA levels (p<0.05). Food-restricted rats showed no significant alterations in the levels of either MCH mRNA or MCHR1 mRNA. In a second experiment, we measured MCH peptide levels in five discrete hypothalamic areas of dietary-obese rats. MCH concentrations were significantly increased in the arcuate nuclei of the HG (p<0.05) and the paraventricular nuclei of both the LG (p<0.05) and HG (p<0.05), compared with their lean counterparts. These results suggest that the MCH system becomes more active in dietary obesity and could be involved in enhancing appetite for palatable food. The possibility that MCH and MCHR1 expression are positively regulated by leptin and insulin, which normally inhibit feeding, is a putative explanation for how appetite for palatable food is able to override mechanisms that prevent the development of obesity.

Septopreoptic mu opioid receptor mediation of hindbrain glucoprivic inhibition of reproductive neuroendocrine function in the female rat

Central glucostasis is a critical monitored variable in neuroendocrine regulation of pituitary LH secretion. Glucoprivic signals originating within the caudal hindbrain suppress LH. Septopreoptic mu opioid receptors (mu-R) function within neural pathways maintaining basal LH levels and mediate the effects of diverse physiological stimuli on hormone release. To identify potential sites in the septopreoptic area where ligand neuromodulatory actions may occur in response to hindbrain glucoprivic signaling, the present studies evaluated the distribution of mu-R-immunoreactive (-ir) neurons in the septopreoptic area that are genomically activated in response to caudal fourth ventricular (CV4) delivery of the glucose antimetabolite, 5-thioglucose (5TG). The effects of lateral ventricular pretreatment with the selective mu-R antagonist, d-Phe-Cys-Tyr-d-Trp-Orn-Thr-Pen-Thr-NH(2) (CTOP), on LH secretory and GnRH neuronal transcriptional responses to hindbrain glucoprivation were also evaluated. Estradiol benzoate- and progesterone-primed, ovariectomized female rats were treated by CV4 administration of 5TG or the vehicle, saline, at the onset of the afternoon LH surge. The inhibitory effects of hindbrain glucoprivation on mean plasma LH levels as well as colabeling of rostral preoptic GnRH neurons for Fos-ir were attenuated in animals pretreated by lateral ventricular delivery of CTOP. Dual immunocytochemical labeling for septopreoptic mu-R-ir and Fos-ir demonstrated a robust induction of Fos expression by receptor-positive neurons within discrete septopreoptic sites in response to CV4 5TG, a genomic response that was diminished by CTOP pretreatment. The current studies provide novel evidence for the transcriptional activation of neuroanatomically characterized, mu-R-expressing neurons by decreased hindbrain glucose utilization and show that the functional status of mu-R is critical for maximal induction of the Fos stimulus-transcription cascade in these cells by central glucoprivic signaling. The finding that receptor antagonist-mediated suppression of this genomic response is correlated with increased reproductive neuroendocrine output supports a role for these discrete mu-R-expressing neuron populations as substrates for ligand regulatory effects on the GnRH-pituitary LH axis during neuroglucopenia.

Hypothalamic control of energy balance: different peptides, different functions

Energy balance is maintained via a homeostatic system involving both the brain and the periphery. A key component of this system is the hypothalamus. Over the past two decades, major advances have been made in identifying an increasing number of peptides within the hypothalamus that contribute to the process of energy homeostasis. Under stable conditions, equilibrium exists between anabolic peptides that stimulate feeding behavior, as well as decrease energy expenditure and lipid utilization in favor of fat storage, and catabolic peptides that attenuate food intake, while stimulating sympathetic nervous system (SNS) activity and restricting fat deposition by increasing lipid metabolism. The equilibrium between these neuropeptides is dynamic in nature. It shifts across the day-night cycle and from day to day and also in response to dietary challenges as well as peripheral energy stores. These shifts occur in close relation to circulating levels of the hormones, leptin, insulin, ghrelin and corticosterone, and also the nutrients, glucose and lipids. These circulating factors together with neural processes are primary signals relaying information regarding the availability of fuels needed for current cellular demand, in addition to the level of stored fuels needed for long-term use. Together, these signals have profound impact on the expression and production of neuropeptides that, in turn, initiate the appropriate anabolic or catabolic responses for restoring equilibrium. In this review, we summarize the evidence obtained on nine peptides in the hypothalamus that have emerged as key players in this process. Data from behavioral, physiological, pharmacological and genetic studies are described and consolidated in an attempt to formulate a clear statement on the underlying function of each of these peptides and also on how they work together to create and maintain energy homeostasis.

Cell and molecular cell biology of melanin-concentrating hormone

Recent advances in the study of melanin-concentrating hormone (MCH) have depended largely on molecular biological techniques. In mammals, which have attracted the most attention, novel findings concern (i) the MCH gene, which can yield several peptides by either posttranslational cleavage or alternative splicing, as well as bidirectional transcription; (ii) the identification of two G protein-coupled MCH receptors in the brain and peripheral tissues; and (iii) the evidence for subpopulations of MCH neurons in the central nervous system, characterized by their chemical phenotypes, connections, and individual physiological responses to different physiological paradigms. The involvement of central MCH in various functions, including feeding, reproduction, stress, and behavior patterns, is reviewed. The stage during evolution at which MCH may have acquired hypophysiotrophic and hormonal functions in lower vertebrates is considered in light of morphological data. Evidence that MCH also has peripheral paracrine/autocrine effects in mammals is provided.

Neurochemical phenotype of hypothalamic neurons showing Fos expression 23 h after intracranial AgRP

Agouti-related protein (AgRP) is coexpressed with neuropeptide Y (NPY) in a population of neurons in the arcuate nucleus (ARC) of the hypothalamus and stimulates food intake for up to 7 days if injected intracerebroventricularly. The prolonged food intake stimulation does not seem to depend on continued competition at the melanocortin-4 receptor (MC4R), because the relatively specific MC4R agonist MTII regains its ability to suppress food intake 24 h after AgRP injection. Intracerebroventricular AgRP also stimulates c-Fos expression 24 h after injection in several brain areas, so the neurons exhibiting delayed Fos expression might be particularly important in feeding behavior. Thus we aimed to identify the neurochemical phenotype of some of these neurons in select hypothalamic areas, using double-label immunohistochemistry. AgRP-injected rats ingested significantly more chow (10.2 +/- 0.6 g) vs. saline controls (3.4 +/- 0.7 g) in the first 9 h (light phase) after injection. In the lateral hypothalamus (particularly the perifornical area) 23 h after injection, AgRP induced significantly more Fos vs. saline in orexin-A (OXA) neurons (25.6 +/- 4.9 vs. 4.8 +/- 3.1%), but not in melanin-concentrating hormone (MCH) or cocaine- and amphetamine-regulated transcript (CART) neurons. In the ARC, AgRP induced significantly more Fos in CART (40.6 +/- 5.9 vs. 13.4 +/- 1.8%) but not NPY neurons. In the paraventricular nucleus, there was no significant difference in Fos expression induced by AgRP vs. saline in oxytocin and CART neurons. We conclude that the long-lasting hyperphagia induced by AgRP is correlated with and possibly partially mediated by hyperactive OXA neurons in the lateral hypothalamus and CART neurons in the ARC, but not by NPY and MCH neurons. The substantial increase in light-phase food intake by AgRP supports a role for the arousing effects of OXA. Activation of CART neurons in the ARC (which likely coexpress proopiomelanocortin) could indicate attempts to activate counterregulatory decreases in food intake.

To eat or to sleep? Orexin in the regulation of feeding and wakefulness

Orexin-A and orexin-B are neuropeptides originally identified as endogenous ligands for two orphan G-protein-coupled receptors. Orexin neuropeptides (also known as hypocretins) are produced by a small group of neurons in the lateral hypothalamic and perifornical areas, a region classically implicated in the control of mammalian feeding behavior. Orexin neurons project throughout the central nervous system (CNS) to nuclei known to be important in the control of feeding, sleep-wakefulness, neuroendocrine homeostasis, and autonomic regulation. orexin mRNA expression is upregulated by fasting and insulin-induced hypoglycemia. C-fos expression in orexin neurons, an indicator of neuronal activation, is positively correlated with wakefulness and negatively correlated with rapid eye movement (REM) and non-REM sleep states. Intracerebroventricular administration of orexins has been shown to significantly increase food consumption, wakefulness, and locomotor activity in rodent models. Conversely, an orexin receptor antagonist inhibits food consumption. Targeted disruption of the orexin gene in mice produces a syndrome remarkably similar to human and canine narcolepsy, a sleep disorder characterized by excessive daytime sleepiness, cataplexy, and other pathological manifestations of the intrusion of REM sleep-related features into wakefulness. Furthermore, orexin knockout mice are hypophagic compared with weight and age-matched littermates, suggesting a role in modulating energy metabolism. These findings suggest that the orexin neuropeptide system plays a significant role in feeding and sleep-wakefulness regulation, possibly by coordinating the complex behavioral and physiologic responses of these complementary homeostatic functions.

Central and peripheral dysregulation of melanin-concentrating hormone in obese Zucker rats

Melanin concentrating hormone (MCH) is a peptide synthesized in the lateral hypothalamus which stimulates food ingestion and leptin secretion in rodents. In this experiment, we measured the expressions of MCH as well as of its receptor (SLC-1) in the hypothalamus of obese hyperphagic and lean Zucker rats by quantitative real time RT-PCR. MCH mRNA expression in the obese rats was significantly increased by a factor of five (P<0.01) whereas expression of SLC-1 was decreased by more than 50% (P<0.05). Circulating levels of leptin and MCH were increased in the plasma of obese Zucker rats when compared to lean rats (38-fold and 1.7-fold, respectively, P<0.001 and P<0.01). However, individual MCH levels were not directly correlated to leptin levels in the lean (functional leptin receptor) or in the obese (non-functional leptin receptor) Zucker rats. These results indicate that the absence of leptin signaling in rats is associated with an increased hypothalamic expression and circulating release of MCH, contributing to their obesity syndrome.

Hypoglycemia activates orexin neurons and selectively increases hypothalamic orexin-B levels: responses inhibited by feeding and possibly mediated by the nucleus of the solitary tract

Orexins are novel appetite-stimulating peptides expressed in the lateral hypothalamic area (LHA), and their expression is stimulated by hypoglycemia in fasted rats. We investigated activation of orexin and other neurons during insulin-induced hypoglycemia using the immediate early gene product Fos. Insulin (50 U/kg) lowered plasma glucose by >50% after 5 h and stimulated feeding sixfold compared with saline-injected controls. Hypoglycemic rats allowed to feed and normoglycemic controls both showed sparse Fos-positive (Fos+) neurons in the LHA and the paraventricular nucleus (PVN) and arcuate nucleus (ARC) and showed none in the nucleus of the solitary tract (NTS), which relays visceral feeding signals to the LHA. In the LHA, total numbers of Fos+ neurons were comparable in fed hypoglycemic and control groups (60 +/- 6 vs. 52 +/- 4 cells/mm2, P > 0.05), as were Fos+ neurons immunoreactive for orexin (1.4 +/- 0.4 vs. 0.6 +/- 0.4 cells/mm2, P > 0.05). By contrast, hypoglycemic rats that were fasted showed significantly more Fos+ nuclei in the LHA (96 +/- 10 cells/mm2, P < 0.05, vs. both other groups) and Fos+ orexin neurons (8.4 +/- 3.3 cells/mm2, P < 0.001, vs. both other groups). They also showed two- to threefold more Fos+ nuclei (P < 0.001) in the PVN and ARC than both fed hypoglycemic rats and controls and showed strikingly abundant Fos+ neurons in the NTS and dorsal motor nucleus of the vagus. In parallel studies, whole hypothalamic orexin-A levels were not changed in hypoglycemic rats, whether fasted or freely fed, whereas orexin-B levels were 10-fold higher in hypoglycemic fasted rats than in control and hypoglycemic fed groups. These data support our hypothesis that orexin neurons are stimulated by falling glucose levels but are readily inhibited by signals related to nutrient ingestion and suggest that they may functionally link with neuronal activity in the NTS. Orexin-A and -B may play specific roles in behavioral or neuroendocrine responses to hypoglycemia.

Phe(13),Tyr(19)-melanin-concentration hormone and the blood-brain barrier: role of protein binding

Melanin-concentrating hormone (MCH), found both peripherally and centrally, is involved in food ingestion. Although its expression in brain is increased by fasting, it is not known whether it crosses the blood-brain barrier (BBB). Use of the sensitive method of multiple-time regression analysis has shown that almost all of the peptides and polypeptides tested cross the BBB at a rate faster than the vascular marker albumin. With this same method, however, we found that the 19-amino acid 125I-Phe13,Tyr19-MCH did not cross faster than 99mTc-albumin. Several mechanisms were excluded as possible explanations for the slow rate of influx. These included degradation, association with capillary endothelial cells, and transport from brain to blood. When Phe13,Tyr19-MCH was perfused in blood-free buffer, however, it entered the brain significantly faster than albumin. This suggested protein binding as an explanation for the slow rate of influx when the MCH was administered in blood. Protein binding was confirmed by capillary zone electrophoresis, which showed that almost all of the Phe13,Tyr19-MCH added to blood migrated with a large-molecular-weight substance. Sodium dodecyl sulfate-capillary gel electrophoresis of Phe13,Tyr19-MCH in buffer additionally showed that the MCH aggregated as a trimer, a factor not preventing its influx by blood-free perfusion. Thus, the results show that blood-borne Phe13,Tyr19-MCH does not significantly cross the BBB, probably because of its binding to serum proteins.

Melanin-concentrating hormone expression in slice cultures of rat hypothalamus is not affected by 2-deoxyglucose

In rats, melanin-concentrating hormone (MCH) neurons are mainly located within the lateral hypothalamic area (LHA). This area is known to be involved in the control of feeding and to contain glucose-sensitive cells. As a role for MCH in the regulation of food intake has been reported, we investigated the effects of 2-deoxyglucose (2DG) on MCH expression in cultured LHA slices, to verify if MCH neurons are sensitive to local glucoprivation through a modulation of MCH synthesis. After a 2-10 h 2DG incubation, competitive reverse transcription-polymerase chain reaction (RT-PCR) did not show any variation of MCH mRNA; no change was also observed in MCH immunocytochemical labeling. A slight decrease of MCH mRNA (5-15%) after a 17 h 2DG treatment might be due to a general degradation of neurons induced by long-term glucoprivation. In conclusion, we suggest that MCH neurons are not the glucose-sensitive cells previously described in the LHA and that the signals inducing their previously reported response to glycemia variations do not arise from the LHA itself.

Neurons containing orexin in the lateral hypothalamic area of the adult rat brain are activated by insulin-induced acute hypoglycemia

Orexin-A and -B (also known as hypocretin-1 and -2) are neuropeptides which stimulate food intake when administered intracerebroventricularly. Orexins are specifically localized in neurons within and around the lateral hypothalamic area (LHA). Previous electrophysiological studies have demonstrated that some neurons in the LHA are activated by hypoglycemia, and are therefore termed 'glucose-sensitive neurons'. In the present study, we examined whether orexin-containing neurons are activated in the hypoglycemic states, using Fos-like immunoreactivity (FLI) as a marker of neuronal activation. We observed that FLI was induced in the LHA by acute insulin treatment. Double staining with anti-Fos and anti-orexin antibodies revealed that up to 33% of the orexin-containing neurons in the LHA also expressed FLI under the hypoglycemic condition. These results suggest that some populations of neurons which contain orexins are activated under hypoglycemic conditions.

CNS sites involved in sympathetic and parasympathetic control of the pancreas: a viral tracing study

The viral transneuronal tracing method was used to identify the CNS cell groups that regulate the parasympathetic and sympathetic outflow systems of the pancreas. Pseudorabies virus (PRV) was injected into the pancreas of vagotomized rats and after 6 days survival, the pattern of transneuronal labeling in the CNS sympathetic regulatory regions was determined. The converse experiment was performed in order to elucidate the central parasympathetic cell groups that regulate the pancreas. Immunohistochemical methods were used to identify putative neuropeptide- and catecholamine-containing CNS neurons involved in these regulatory circuits. The major finding of this study indicates that five brain regions, viz., paraventricular hypothalamic nucleus, perifornical hypothalamic region, A5 catecholamine cell group, rostral ventrolateral medulla, and lateral paragigantocellular reticular nucleus, contain a considerable amount of overlap in cell body labeling. In addition, the ventrolateral part of the periaqueductal gray matter and gigantocellular reticular nucleus, ventral part also showed a similar overlap, but the numbers of neurons found in these areas were considerably lower than the five major regions. These data suggest that these brain regions may provide parallel and possibly redundant, autonomic pathways affecting glucagon and adrenaline release.

Changes in rat melanin-concentrating hormone and dynorphin messenger ribonucleic acids induced by food deprivation

Melanin-concentrating hormone (MCH) and dynorphin genes are expressed in two discrete neuron populations of the rat lateral hypothalamus. Their roles remain hypothetical in mammals. In order to analyse changes in MCH and dynorphin gene expression, a multiplex competitive semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR) was developed to assay their mRNAs. This technique was used to examine MCH and dynorphin mRNA content in 24-h and 48-h food-deprived rats compared to controls. A two-fold induction of dynorphin mRNA by 24 h, followed by a sharp decrease at 48 h were observed. A moderate increase in MCH mRNA content was noticed by 24 h; 48 h of fasting restored the control levels.

Exploring the expression of the melanin-concentrating hormone messenger RNA in the rat lateral hypothalamus after goldthioglucose injection

Melanin-concentrating hormone (MCH) is expressed in a large neuronal population of the rat lateral hypothalamus. This area is known to be implicated in the regulation of thirst and hunger and to contain glucose-sensitive cells. In the present study, we investigated the effects of goldthioglucose (GTG), a toxic form of glucose, on the expression of the MCH gene in the rat lateral hypothalamus by immunocytochemistry, in situ hybridization and competitive RT-PCR. We observed that the MCH immunoreactivity and the level of MCH mRNA were not changed after intraperitoneal GTG injection (0.35 mg/g body weight). These results together with previous data suggest that the glucose-sensitive cells of the lateral hypothalamus are different from the MCH neurons and remain to be identified.

Melanin-concentrating hormone is a potent anorectic peptide regulated by food-deprivation and glucopenia in the rat

Melanin-concentrating hormone is a cyclic nonadecapeptide that is produced almost exclusively in neurons of the lateral hypothalamus and sub zona incerta areas while fibers are widespread in the rat brain. Such a localization strongly suggests that this peptide might participate as neurotransmitter/neuromodulator in the control of feeding behavior. In this study we examined first the influence of rat melanin-concentrating hormone on feeding behavior at different times either in the light or in the dark period (light off at 18.00 h) of the day in fed Wistar rats. Intracerebroventricular injection of rat melanin-concentrating hormone (1-100 ng per rat) at 18.00 h reduced food consumption as early as 2 h after injection and for the next 24 h. In addition, similar anorectic effect was noted after bilateral administration of 1 ng melanin-concentrating hormone into the lateral hypothalamic area at 11.30 h but not at 16.30 h. These findings strongly suggest that rat melanin-concentrating hormone may exert inhibitory control over food intake behavior depending on the circadian rhythm. Second, we investigated the modifications induced by food deprivation/refeeding on melanin-concentrating hormone messenger RNA levels in Wistar rats. Total RNA was isolated from whole hypothalamic dissections and the contents of melanin-concentrating hormone messenger RNA, beta-actin messenger RNA (taken as sample control) and neuropeptide Y messenger RNA (taken as control of food-deprivation paradigms) were assessed by using northern blotting. The time-course of messenger RNA expression was determined in groups of rats deprived for 24, 48 and 72 h and revealed a three-fold induction of melanin-concentrating hormone messenger RNA by 24 and 48 h, with reduced increase at 72 h. As expected, the same treatment led to a three-fold increase in neuropeptide Y messenger RNA content by 48 and 72 h. Refeeding groups of animals for up to 72 h after 24 h of food deprivation resulted in full restoration of melanin-concentrating hormone messenger RNA levels by 24 h. Strikingly, a large range of variations in melanin-concentrating hormone messenger RNA content between individuals was observed in food-deprived versus controls or refed rats suggesting that genetic or environmental factors may alter response in melanin-concentrating hormone gene activity after food deprivation. Finally, we investigated the effects of short-term glucoprivation induced by intraperitoneal administration of either 2-deoxy-D-glucose or insulin on melanin-concentrating hormone messenger RNA expression. A transitory increase in melanin-concentrating hormone messenger RNA content was noted 1 h after 2-deoxy-D-glucose injection while melanin-concentrating hormone messenger RNA levels rose two-fold only 5 h after insulin treatment. These results indicate that 2-deoxy-D-glucose and insulin activate melanin-concentrating hormone gene expression through likely distinct regulatory pathways.

Melanotropic peptides in the mammalian brain: the melanin-concentrating hormone

Melanin-concentrating hormone (MCH) has been identified in neurons of the mammalian brain. This review summarizes some current information regarding the cell biology of this neuropeptide and the topography of MCH-immunoreactive (-IR) neurons in several species including mouse, rat, hamster, guinea pig, rabbit, dog and monkey; and atlas of MCH-IR neurons in the hypothalamus and subthalamus of the brain of guinea pig is presented. Based upon the location of this MCH cell group, it is hypothesized that they may be functionally involved in circuits of extrapyramidal motor systems from striatal centers to the thalamus and cerebral cortex and to the midbrain and spinal cord.