Intraventricular melanin-concentrating hormone stimulates water intake independent of food intake

Neuroanatomical, electrophysiological, and morphological characterization of melanin-concentrating hormone cells coexpressing cocaine- and amphetamine-regulated transcript

Melanin-concentrating hormone (MCH) cells in the hypothalamus regulate fundamental physiological functions like energy balance, sleep, and reproduction. This diversity may be ascribed to the neurochemical heterogeneity among MCH cells. One prominent subpopulation of MCH cells coexpresses cocaine- and amphetamine-regulated transcript (CART), and as MCH and CART can have opposing actions, MCH/CART+ and MCH/CART- cells may differentially modulate behavioral outcomes. However, it is not known if there are differences in the cellular properties underlying their functional differences; thus, we compared the neuroanatomical, electrophysiological, and morphological properties of MCH cells in male and female Mch-cre;L10-Egfp reporter mice. Half of MCH cells expressed CART and were most prominent in the medial hypothalamus. Whole-cell patch-clamp recordings revealed differences in their passive and active membrane properties in a sex-dependent manner. Female MCH/CART+ cells had lower input resistances, but male cells largely differed in their firing properties. All MCH cells increased firing when stimulated, but their firing frequency decreases with sustained stimulation. MCH/CART+ cells showed stronger spike rate adaptation than MCH/CART- cells. The kinetics of excitatory events at MCH cells also differed by cell type, as the rising rate of excitatory events was slower at MCH/CART+ cells. By reconstructing the dendritic arborization of our recorded cells, we found no sex differences, but male MCH/CART+ cells had less dendritic length and fewer branch points. Overall, distinctions in topographical division and cellular properties between MCH cells add to their heterogeneity and help elucidate their response to stimuli or effect on modulating their respective neural networks.

Melanin-concentrating hormone and orexin shape social affective behavior via action in the insular cortex of rat

RATIONALE

In a social context, individuals are able to detect external information from others and coordinate behavioral responses according to the situation, a phenomenon called social decision-making. Social decision-making is multifaceted, influenced by emotional and motivational factors like stress, sickness, and hunger. However, the neurobiological basis for motivational state competition and interaction is not well known.

OBJECTIVE

We investigated possible neural mechanisms through which internal states could shape social behavior in a social affective preference (SAP) test. In the SAP test, experimental rats given a choice to interact with naïve or stressed conspecifics exhibit an age-dependent preference to interact with stressed juvenile conspecifics, but avoid stressed adult conspecifics. First, we assessed the effect of food and water deprivation on SAP behavior. Behavior in the SAP test requires the insular cortex, which receives input from the ingestion-related peptides melanin-concentrating hormone (MCH) and orexin neurons of the lateral hypothalamus (LH). This study aimed to evaluate the role of LH and insular MCH and orexin in SAP test.

METHODS

SAP tests were conducted in rats that were sated, food and water deprived or allowed 1 h of access to food and water after 14 h of deprivation (relieved condition). Separate cohorts of sated rats received cannula implants for microinjection of drugs to inhibit the LH or to block or stimulate MCH or orexin receptors in the insula prior to SAP tests or social interaction tests.

RESULTS

Food and water deprivation prior to SAP tests with juvenile rats caused a shift in preference away from the stressed rat toward the naïve juveniles. Pharmacological inhibition of LH with muscimol (100 ng/side) abolished the preference for the juvenile-stressed conspecific, as well as the preference for the adult naïve conspecific. The blockade of MCH receptor 1or orexin receptors in the insular cortex with SNAP94847 (50 μM) or TCS1102 (1 μM), respectively, also abolished the preference for the stressed juvenile conspecific, but only the antagonism of orexin receptors was able to abolish the preference for the adult naïve conspecific. Microinjection of increasing doses (50 or 500 nM) of MCH or orexin-A in the insular cortex increased the interaction time in the one-on-one social interaction test with juvenile conspecifics; however, only the microinjection of orexin-A increased the interaction time with adult naïve conspecifics.

CONCLUSIONS

Taken together, these results suggest that lateral hypothalamus peptides shape the direction of social approach or avoidance via actions MCH and orexin neurotransmission in the insular cortex.

Time to drink: Activating lateral hypothalamic area neurotensin neurons promotes intake of fluid over food in a time-dependent manner

The lateral hypothalamic area (LHA) is essential for ingestive behavior but has primarily been studied in modulating feeding, with comparatively scant attention on drinking. This is partly because most LHA neurons simultaneously promote feeding and drinking, suggesting that ingestive behaviors track together. A notable exception are LHA neurons expressing neurotensin (LHANts neurons): activating these neurons promotes water intake but modestly restrains feeding. Here we investigated the connectivity of LHANts neurons, their necessity and sufficiency for drinking and feeding, and how timing and resource availability influence their modulation of these behaviors. LHANts neurons project broadly throughout the brain, including to the lateral preoptic area (LPO), a brain region implicated in modulating drinking behavior. LHANts neurons also receive inputs from brain regions implicated in sensing hydration and energy status. While activation of LHANts neurons is not required to maintain homeostatic water or food intake, it selectively promotes drinking during the light cycle, when ingestive drive is low. Activating LHANts neurons during this period also increases willingness to work for water or palatable fluids, regardless of their caloric content. By contrast, LHANts neuronal activation during the dark cycle does not promote drinking, but suppresses feeding during this time. Finally, we demonstrate that the activation of the LHANts → LPO projection is sufficient to mediate drinking behavior, but does not suppress feeding as observed after generally activating all LHANts neurons. Overall, our work suggests how and when LHANts neurons oppositely modulate ingestive behaviors.

The Lateral Hypothalamus: An Uncharted Territory for Processing Peripheral Neurogenic Inflammation

The roles of the hypothalamus and particularly the lateral hypothalamus (LH) in the regulation of inflammation and pain have been widely studied. The LH consists of a parasympathetic area that has connections with all the major parts of the brain. It controls the autonomic nervous system (ANS), regulates feeding behavior and wakeful cycles, and is a part of the reward system. In addition, it contains different types of neurons, most importantly the orexin neurons. These neurons, though few in number, perform critical functions such as inhibiting pain transmission and interfering with the reward system, feeding behavior and the hypothalamic pituitary axis (HPA). Recent evidence has identified a new role for orexin neurons in the modulation of pain transmission associated with several inflammatory diseases, including rheumatoid arthritis and ulcerative colitis. Here, we review recent findings on the various physiological functions of the LH with special emphasis on the orexin/receptor system and its role in mediating inflammatory pain.

Distinct Subsets of Lateral Hypothalamic Neurotensin Neurons are Activated by Leptin or Dehydration

The lateral hypothalamic area (LHA) is essential for ingestive behavior but it remains unclear how LHA neurons coordinate feeding vs. drinking. Most LHA populations promote food and water consumption but LHA neurotensin (Nts) neurons preferentially induce water intake while suppressing feeding. We identified two molecularly and projection-specified subpopulations of LHA Nts neurons that are positioned to coordinate either feeding or drinking. One subpopulation co-expresses the long form of the leptin receptor (LepRb) and is activated by the anorectic hormone leptin (Nts^LepRb neurons). A separate subpopulation lacks LepRb and is activated by dehydration (Nts^Dehy neurons). These molecularly distinct LHA Nts subpopulations also differ in connectivity: Nts^LepRb neurons project to the ventral tegmental area and substantia nigra compacta but Nts^Dehy neurons do not. Intriguingly, the LHA Nts subpopulations cannot be discriminated via their classical neurotransmitter content, as we found that all LHA Nts neurons are GABAergic. Collectively, our data identify two molecularly- and projection-specified subpopulations of LHA Nts neurons that intercept either leptin or dehydration cues, and which conceivably could regulate feeding vs. drinking behavior. Selective regulation of these LHA Nts subpopulations might be useful to specialize treatment for ingestive disorders such as polydipsia or obesity.

New insight into GABAergic neurons in the hypothalamic feeding regulation

Several lines of study have suggested that GABA in the hypothalamic feeding center plays a role in promoting food intake. Recent studies revealed that not only NPY/AgRP neurons in the hypothalamic arcuate nucleus (ARC) that co-express GABA but also other GABAergic neurons act as an orexigenic. Here, we review the progress of studies on hypothalamic GABAergic neurons distributed in ARC, dorsomedial hypothalamus (DMH), and lateral hypothalamus (LH). Three advanced technologies have been applied and greatly contributed to the recent progress. Optogenetic (and chemogenetic) approaches map input and output pathways of particular subpopulations of GABAergic neurons. In vivo Ca2+ imaging using GRIN lens and GCaMP can correlate the activity of GABAergic neuron subpopulations with feeding behavior. Single-cell RNA-seq approach clarifies precise transcriptional profiles of GABAergic neuron subpopulations. These approaches have shown diversity of GABAergic neurons and the subpopulation-dependent role in feeding regulation.

Activation of lateral hypothalamic area neurotensin-expressing neurons promotes drinking

Animals must ingest water via drinking to maintain fluid homeostasis, yet the neurons that specifically promote drinking behavior are incompletely characterized. The lateral hypothalamic area (LHA) as a whole is essential for drinking behavior but most LHA neurons indiscriminately promote drinking and feeding. By contrast, activating neurotensin (Nts)-expressing LHA neurons (termed LHA Nts neurons) causes mice to immediately drink water with a delayed suppression of feeding. We therefore hypothesized that LHA Nts neurons are sufficient to induce drinking behavior and that these neurons specifically bias for fluid intake over food intake. To test this hypothesis we used designer receptors exclusively activated by designer drugs (DREADDs) to selectively activate LHA Nts neurons and studied the impact on fluid intake, fluid preference and feeding. Activation of LHA Nts neurons stimulated drinking in water-replete and dehydrated mice, indicating that these neurons are sufficient to promote water intake regardless of homeostatic need. Interestingly, mice with activated LHA Nts neurons drank any fluid that was provided regardless of its palatability, but if given a choice they preferred water or palatable solutions over unpalatable (quinine) or dehydrating (hypertonic saline) solutions. Notably, acute activation of LHA Nts neurons robustly promoted fluid but not food intake. Overall, our study confirms that activation of LHA Nts neurons is sufficient to induce drinking behavior and biases for fluid intake. Hence, LHA Nts neurons may be important targets for orchestrating the appropriate ingestive behavior necessary to maintain fluid homeostasis. This article is part of the Special Issue entitled 'Hypothalamic Control of Homeostasis'.

Orexin neurons couple neural systems mediating fluid balance with motivation-related circuits

During extracellular dehydration, neural systems that sense deficits in body fluid homeostasis operate in tandem with those that mediate motivation and reward in order to promote ingestive behaviors that restore fluid balance. We hypothesized that hypothalamic orexin (Ox) neurons act as an interface to couple brain regions sensing and processing information about body fluid status with central nervous system motivation and reward systems. An initial set of anterograde and retrograde tracing experiments suggested that structures along the lamina terminalis (LT), a region of the forebrain that serves to monitor and integrate information reflecting body fluid balance, project to hypothalamic Ox neurons that, in turn, project to dopamine neurons in the ventral tegmental area (VTA). A second set of experiments determined whether Ox neuron activation is associated with extracellular dehydration and the seeking out and consumption of water and saline. An elevation of Fos-like immunoreactivity in Ox neurons was observed in fluid-depleted rats that were allowed to ingest water and sodium. A final experiment was conducted to determine whether Ox release in the VTA promotes thirst and salt appetite. Bilateral microinjection of the Ox Type I receptor antagonist SB-408124 into the VTA prior to acute extracellular dehydration attenuated fluid intake. Together, these studies support the hypothesis that structures along the LT modulate activity in the VTA through actions of orexinergic neurons that have cell bodies in the hypothalamus. This pathway may function to facilitate sustained consumption of fluids necessary for restoration of fluid balance. (PsycINFO Database Record

The Melanin-Concentrating Hormone as an Integrative Peptide Driving Motivated Behaviors

The melanin-concentrating hormone (MCH) is an important peptide implicated in the control of motivated behaviors. History, however, made this peptide first known for its participation in the control of skin pigmentation, from which its name derives. In addition to this peripheral role, MCH is strongly implicated in motivated behaviors, such as feeding, drinking, mating and, more recently, maternal behavior. It is suggested that MCH acts as an integrative peptide, converging sensory information and contributing to a general arousal of the organism. In this review, we will discuss the various aspects of energy homeostasis to which MCH has been associated to, focusing on the different inputs that feed the MCH peptidergic system with information regarding the homeostatic status of the organism and the exogenous sensory information that drives this system, as well as the outputs that allow MCH to act over a wide range of homeostatic and behavioral controls, highlighting the available morphological and hodological aspects that underlie these integrative actions. Besides the well-described role of MCH in feeding behavior, a prime example of hypothalamic-mediated integration, we will also examine those functions in which the participation of MCH has not yet been extensively characterized, including sexual, maternal, and defensive behaviors. We also evaluated the available data on the distribution of MCH and its function in the context of animals in their natural environment. Finally, we briefly comment on the evidence for MCH acting as a coordinator between different modalities of motivated behaviors, highlighting the most pressing open questions that are open for investigations and that could provide us with important insights about hypothalamic-dependent homeostatic integration.

Type 1 cannabinoid receptor modulates water deprivation-induced homeostatic responses

The present study investigated the type 1 cannabinoid receptor (CB1R) as a potential candidate to mediate the homeostatic responses triggered by 24 h of water deprivation, which constitutes primarily a hydroelectrolytic challenge and also significantly impacts energy homeostasis. The present results demonstrated for the first time that CB1R mRNA expression is increased in the hypothalamus of water-deprived (WD) rats. Furthermore, the administration of ACEA, a CB1R selective agonist, potentiated WD-induced dipsogenic effect, whereas AM251, a CB1R antagonist, attenuated not only water but also salt intake in response to WD. In parallel with the modulation of thirst and salt appetite, we confirmed that CB1Rs are essential for the development of appropriated neuroendocrine responses. Although the administration of ACEA or AM251 did not produce any effects on WD-induced arginine vasopressin (AVP) secretion, oxytocin (OXT) plasma concentrations were significantly decreased in WD rats treated with ACEA. At the genomic level, ACEA significantly decreased AVP and OXT mRNA expression in the hypothalamus of WD rats, whereas AM251 potentiated both basal and WD-induced stimulatory effects on the transcription of AVP and OXT genes. In addition, we showed that water deprivation alone upregulated proopiomelanocortin, Agouti-related peptide, melanin-concentrating hormone, and orexin A mRNA levels in the hypothalamus, and that CB1Rs regulate main central peptidergic pathways controlling food intake, being that most of these effects were also significantly influenced by the hydration status. In conclusion, the present study demonstrated that CB1Rs participate in the homeostatic responses regulating fluid balance and energy homeostasis during water deprivation.

To ingest or rest? Specialized roles of lateral hypothalamic area neurons in coordinating energy balance

Survival depends on an organism’s ability to sense nutrient status and accordingly regulate intake and energy expenditure behaviors. Uncoupling of energy sensing and behavior, however, underlies energy balance disorders such as anorexia or obesity. The hypothalamus regulates energy balance, and in particular the lateral hypothalamic area (LHA) is poised to coordinate peripheral cues of energy status and behaviors that impact weight, such as drinking, locomotor behavior, arousal/sleep and autonomic output. There are several populations of LHA neurons that are defined by their neuropeptide content and contribute to energy balance. LHA neurons that express the neuropeptides melanin-concentrating hormone (MCH) or orexins/hypocretins (OX) are best characterized and these neurons play important roles in regulating ingestion, arousal, locomotor behavior and autonomic function via distinct neuronal circuits. Recently, another population of LHA neurons containing the neuropeptide Neurotensin (Nts) has been implicated in coordinating anorectic stimuli and behavior to regulate hydration and energy balance. Understanding the specific roles of MCH, OX and Nts neurons in harmonizing energy sensing and behavior thus has the potential to inform pharmacological strategies to modify behaviors and treat energy balance disorders.

The role of melanin concentrating hormone (MCH) in the central chemoreflex: a knockdown study by siRNA in the lateral hypothalamus in rats

Melanin concentrating hormone (MCH), a neuropeptide produced mainly in neurons localized to the lateral hypothalamic area (LHA), has been implicated in the regulation of food intake, energy balance, sleep state, and the cardiovascular system. Hypothalamic MCH neurons also have multisynaptic connections with diaphragmatic motoneurons and project to many central chemoreceptor sites. However, there are few studies of MCH involvement in central respiratory control. To test the hypothesis that MCH plays a role in the central chemoreflex, we induced a down regulation of MCH in the central nervous system by knocking down the MCH precursor (pMCH) mRNA in the LHA using a pool of small interfering RNA (siRNA), and measured the resultant changes in breathing, metabolic rate, body weight, and blood glucose levels in conscious rats. The injections of pMCH-siRNA into the LHA successfully produced a ∼62% reduction of pMCH mRNA expression in the LHA and a ∼43% decrease of MCH levels in the cerebrospinal fluid relative to scrambled-siRNA treatment (P = 0.006 and P = 0.02 respectively). Compared to the pretreatment baseline and the scrambled-siRNA treated control rats, knockdown of MCH resulted in: 1) an enhanced hypercapnic chemoreflex (∼42 & 47% respectively; P < 0.05) only in wakefulness; 2) a decrease in body weight and basal glucose levels; and 3) an unchanged metabolic rate. Our results indicate that MCH participates not only in the regulation of glucose and sleep-wake homeostasis but also the vigilance-state dependent regulation of the central hypercapnic chemoreflex and respiratory control.

Effects of a specific MCHR1 antagonist (GW803430) on energy budget and glucose metabolism in diet-induced obese mice

OBJECTIVE

The melanin-concentrating hormone (MCH) is a centrally acting peptide implicated in the regulation of energy homeostasis and body weight, although its role in glucose homeostasis is uncertain. Our objective was to determine effects of MCHR1 antagonism on energy budgets and glucose homeostasis in mice.

METHODS

Effects of chronic oral administration of a specific MCHR1 antagonist (GW803430) on energy budgets and glucose homeostasis in diet-induced obese (DIO) C57BL/6J mice were examined.

RESULTS

Oral administration of GW803430 for 30 days reduced food intake, body weight, and body fat. Circulating leptin and triglycerides were reduced but insulin and nonesterified fatty acids were unaffected. Despite weight loss there was no improvement in glucose homeostasis (insulin levels and intraperitoneal glucose tolerance tests). On day 4-6, mice receiving MCHR1 antagonist exhibited decreased metabolisable energy intake and increased daily energy expenditure. However these effects had disappeared by day 22-24. Physical activity during the dark phase was increased by MCHR1 antagonist treatment throughout the 30-day treatment.

CONCLUSIONS

GW803430 produced a persistent anti-obesity effect due to both a decrease in energy intake and an increase in energy expenditure via physical activity but did not improve glucose homeostasis.

Estrogen, astrocytes and the neuroendocrine control of metabolism

Obesity, and its associated comorbidities such as type 2 diabetes, cardiovascular diseases, and certain cancers, represent major health challenges. Importantly, there is a sexual dimorphism with respect to the prevalence of obesity and its associated metabolic diseases, implicating a role for gonadal hormones. Specifically, estrogens have been demonstrated to regulate metabolism perhaps by acting as a leptin mimetic in the central nervous system (CNS). CNS estrogen receptors (ERs) include ER alpha (ERα) and ER beta (ERβ), which are found in nuclear, cytoplasmic and membrane sites throughout the brain. Additionally, estrogens can bind to and activate a G protein-coupled estrogen receptor (GPER), which is a membrane-associated ER. ERs are expressed on neurons as well as glia, which are known to play a major role in providing nutrient supply for neurons and have recently received increasing attention for their potentially important involvement in the CNS regulation of systemic metabolism and energy balance. This brief overview summarizes data focusing on the potential role of astrocytic estrogen action as a key component of estrogenic modulation responsible for mediating the sexual dimorphism in body weight regulation and obesity.

Neurochemical characterization of neurons expressing melanin-concentrating hormone receptor 1 in the mouse hypothalamus

Melanin-concentrating hormone (MCH) is a hypothalamic neuropeptide that acts via MCH receptor 1 (MCHR1) in the mouse. It promotes positive energy balance; thus, mice lacking MCH or MCHR1 are lean, hyperactive, and resistant to diet-induced obesity. Identifying the cellular targets of MCH is an important step to understanding the mechanisms underlying MCH actions. We generated the Mchr1-cre mouse that expresses cre recombinase driven by the MCHR1 promoter and crossed it with a tdTomato reporter mouse. The resulting Mchr1-cre/tdTomato progeny expressed easily detectable tdTomato fluorescence in MCHR1 neurons, which were found throughout the olfactory system, striatum, and hypothalamus. To chemically identify MCH-targeted cell populations that play a role in energy balance, MCHR1 hypothalamic neurons were characterized by colabeling select hypothalamic neuropeptides with tdTomato fluorescence. TdTomato fluorescence colocalized with dynorphin, oxytocin, vasopressin, enkephalin, thyrothropin-releasing hormone, and corticotropin-releasing factor immunoreactive cells in the paraventricular nucleus. In the lateral hypothalamus, neurotensin, but neither orexin nor MCH neurons, expressed tdTomato. In the arcuate nucleus, both Neuropeptide Y and proopiomelanocortin cells expressed tdTomato. We further demonstrated that some of these arcuate neurons were also targets of leptin action. Interestingly, MCHR1 was expressed in the vast majority of leptin-sensitive proopiomelanocortin neurons, highlighting their importance for the orexigenic actions of MCH. Taken together, this study supports the use of the Mchr1-cre mouse for outlining the neuroanatomical distribution and neurochemical phenotype of MCHR1 neurons.

Control of food intake by MC4-R signaling in the lateral hypothalamus, nucleus accumbens shell and ventral tegmental area: interactions with ethanol

The melanocortin system is involved in animal models of obesity and anorexia-cachexia and MC4 receptors (MC4-R) are currently a target system for the development of drugs aimed to treat obesity and eating disorders in humans. Previous evidence suggest that feeding peptides might lack their orexigenic activity while stimulate ethanol intake. The present study comparatively evaluated food intake (4-h interval) in Sprague-Dawley (SD) rats drinking ethanol (6% w/v, 2 bottle choice paradigm) (EE group) and ethanol-naïve (EN) rats in response to bilateral infusion of the selective MC4-R antagonist HS014 (0, 0.02 or 0.05 μg/0.5 μl/site) or the selective MC4-R agonist cyclo(NH-CH(2)-CH(2)-CO-His-d-Phe-Arg-Trp-Glu)-NH(2) (0, 0.75 or 1.5 μg/0.5 μl/site), into the lateral hypothalamus (LH), the nucleus accumbens (NAc), or the ventral tegmental area (VTA). The main findings in the study are: (1) LH-infusions of the MC4-R antagonist increased and the agonist reduced feeding and total calories consumed, while ethanol intake remained unaltered. (2) NAc- and VTA-infusions of the selective agonist reduced food, ethanol and total calories intake. (3) NAc- and VTA-infusions of the MC4-R antagonist increased feeding in EN rats, but not in EE animals which showed a mild increase in ethanol intake, while total calories consumed remained unaltered. Present data show that having ethanol available reduces feeding elicited by NAc and VTA-MC4-R blockade. Additionally, while MC4-R signaling in the LH appears to modulate homeostatic aspects of feeding, it may contribute to non-homeostatic aspects of ingestive behaviors in the VTA and the NAc.

Glucocorticoids are required for meal-induced changes in the expression of hypothalamic neuropeptides

Glucocorticoid deficiency is associated with a decrease of food intake. Orexigenic peptides, neuropeptide Y (NPY) and agouti related protein (AgRP), and the anorexigenic peptide proopiomelanocortin (POMC), expressed in the arcuate nucleus of the hypothalamus (ARC), are regulated by meal-induced signals. Orexigenic neuropeptides, melanin-concentrating hormone (MCH) and orexin, expressed in the lateral hypothalamic area (LHA), also control food intake. Thus, the present study was designed to test the hypothesis that glucocorticoids are required for changes in the expression of hypothalamic neuropeptides induced by feeding. Male Wistar rats (230-280 g) were subjected to ADX or sham surgery. ADX animals received 0.9% NaCl in the drinking water, and half of them received corticosterone in the drinking water (B: 25 mg/L, ADX+B). Six days after surgery, animals were fasted for 16 h and they were decapitated before or 2 h after refeeding for brain tissue and blood collections. Adrenalectomy decreased NPY/AgRP and POMC expression in the ARC in fasted and refed animals, respectively. Refeeding decreased NPY/AgRP and increased POMC mRNA expression in the ARC of sham and ADX+B groups, with no effects in ADX animals. The expression of MCH and orexin mRNA expression in the LHA was increased in ADX and ADX+B groups in fasted condition, however there was no effect of refeeding on the expression of MCH and orexin in the LHA in the three experimental groups. Refeeding increased plasma leptin and insulin levels in sham and ADX+B animals, with no changes in leptin concentrations in ADX group, and insulin response to feeding was lower in this group. Taken together, these data demonstrated that circulating glucocorticoids are required for meal-induced changes in NPY, AgRP and POMC mRNA expression in the ARC. The lower leptin and insulin responses to feeding may contribute to the altered hypothalamic neuropeptide expression after adrenalectomy.

Vasopressin and oxytocin excite MCH neurons, but not other lateral hypothalamic GABA neurons

Neurons that synthesize melanin-concentrating hormone (MCH) colocalize GABA, regulate energy homeostasis, modulate water intake, and influence anxiety, stress, and social interaction. Similarly, vasopressin and oxytocin can influence the same behaviors and states, suggesting that these neuropeptides may exert part of their effect by modulating MCH neurons. Using whole cell recording in MCH-green fluorescent protein (GFP) transgenic mouse hypothalamic brain slices, we found that both vasopressin and oxytocin evoked a substantial excitatory effect. Both peptides reversibly increased spike frequency and depolarized the membrane potential in a concentration-dependent and tetrodotoxin-resistant manner, indicating a direct effect. Substitution of lithium for extracellular sodium, Na(+)/Ca(2+) exchanger blockers KB-R7943 and SN-6, and intracellular calcium chelator BAPTA, all substantially reduced the vasopressin-mediated depolarization, suggesting activation of the Na(+)/Ca(2+) exchanger. Vasopressin reduced input resistance, and the vasopressin-mediated depolarization was attenuated by SKF-96265, suggesting a second mechanism based on opening nonselective cation channels. Neither vasopressin nor oxytocin showed substantial excitatory actions on lateral hypothalamic inhibitory neurons identified in a glutamate decarboxylase 67 (GAD67)-GFP mouse. The primary vasopressin receptor was vasopressin receptor 1a (V1aR), as suggested by the excitation by V1aR agonist [Arg(8)]vasotocin, the selective V1aR agonist [Phe(2)]OVT and by the presence of V1aR mRNA in MCH cells, but not in other nearby GABA cells, as detected with single-cell RT-PCR. Oxytocin receptor mRNA was also detected in MCH neurons. Together, these data suggest that vasopressin or oxytocin exert a minimal effect on most GABA neurons in the lateral hypothalamus but exert a robust excitatory effect on presumptive GABA cells that contain MCH. Thus, some of the central actions of vasopressin and oxytocin may be mediated through MCH cells.

Lateral thinking about leptin: a review of leptin action via the lateral hypothalamus

The lateral hypothalamic area (LHA) was initially described as a "feeding center" but we are now beginning to understand that the LHA contributes to other aspects of physiology as well. Indeed, the best-characterized neuronal populations of the LHA (which contain melanin-concentrating hormone (MCH) or the hypocretins/orexins (OX)) are not strictly orexigenic, but also have roles in regulation of the autonomic and sympathetic nervous systems as well as in modulating motivated behavior. Leptin is an anorectic hormone that regulates energy homeostasis and the mesolimbic DA system (which transduces the wanting of food, drugs of abuse, and sex) in part, via actions at the LHA. At least three populations of LHA neurons are regulated by leptin: those containing MCH, OX or the long form of the leptin receptor, LepRb. The emerging picture of leptin interaction with these LHA populations suggests that the LHA is not merely regulating feeding, but is a crucial integrator of energy balance and motivated behavior.

Animals models of MCH function and what they can tell us about its role in energy balance

Melanin-concentrating hormone (MCH) has attracted considerable attention because of its effects on food intake and body weight and the MCH receptor (MCHR1) remains one of the viable targets for obesity therapy. This review summarizes the literature examining the effects of MCH on body weight, food intake and energy expenditure in rodent models, and the central sites where MCH acts in regulating energy homeostasis. Emphasis is given on the discrepancies between the genetic and pharmacologic models of MCHR1 inactivation. We propose some solutions to resolve these discrepancies and discuss some future directions in MCH research.

Electrophysiological effects of MCH on neurons in the hypothalamus

Melanin concentrating hormone (MCH) has been implicated in many brain functions and behaviors essential to the survival of animals. The hypothalamus is one of the primary targets where MCH-containing nerve fibers and MCH receptors are extensively expressed and its actions in the brain are exerted. Since the identification of MCH receptors as orphan G protein coupled receptors, the cellular effects of MCH have been revealed in many non-neuronal expression systems (including Xenopus oocytes and cell lines), however, the mechanism by which MCH modulates the activity in the neuronal circuitry of the brain is still under investigation. This review summarizes our current knowledge of electrophysiological effects of MCH on neurons in the hypothalamus, particularly in the lateral hypothalamus. Generally, MCH exerts inhibitory effects on neurons in this structure and may serve as a homeostatic regulator in the lateral hypothalamic area. Given the contrast between the limited data on cellular functions of MCH in the hypothalamus versus a fast growing body of evidence on the vital role of MCH in animal behavior, further investigations of the former are warranted.

Orexin-A hyperphagia: hindbrain participation in consummatory feeding responses

Orexin-A (ORXA) is an orexigenic neuropeptide produced by the lateral hypothalamus that increases food intake when injected into the brain ventricles or forebrain nuclei. We used a licking microstructure analysis to evaluate hindbrain and forebrain ORXA effects in intact and hindbrain-lesioned rats, to identify the motivational and anatomical bases of ORXA hyperphagia. Intact rats with cannulas in the fourth brain ventricle (4V) received vehicle (artificial cerebrospinal fluid) or ORXA (0.1, 0.4, 1, or 10 nm) injections before 90 min access to 0.1 m sucrose. Meal size and frequency were increased in a double-dissociated manner by the 1 and 10 nm doses, respectively. In experiment 2, 4V 1 nm ORXA was applied to rats offered solutions varied in caloric and gustatory intensity (water and 0.1 and 1 m sucrose). ORXA increased meal frequency for all tastants. ORXA increased meal size only for 0.1 m sucrose, by prolonging the meal without affecting early ingestion rate or lick burst size, suggesting that 4V ORXA influenced inhibitory postingestive feedback rather than taste evaluation. In experiment 3, rats with cannulas in the third ventricle (3V) received dorsal medullary lesions centered on the area postrema (APX group) or sham procedures, and licking for water and 0.1 and 1 m sucrose was evaluated after 1 nm 3V ORXA/artificial cerebrospinal fluid injections. The 3V ORXA increased 0.1 m sucrose meal size and meal frequency for all tastants in the sham group, as observed after 4V ORXA in experiment 2. In the APX group, 3V ORXA injections influenced meal frequency, but they no longer increased meal size. However, the APX rats increased meal size for 0.1 m sucrose after food and water deprivation and after 3V angiotensin II injection. They also showed meal size suppression after 3V injection of the melanocortin-3/4 receptor agonist melanotan II (1 nm). These findings suggest that the area postrema and subjacent nucleus of the solitary tract are necessary for increases in consummatory (meal size) but not appetitive (meal frequency) responses to 3V ORXA. The meal size increases may be due to reduced postingestive feedback inhibition induced by ORXA delivered to either the hindbrain or forebrain ventricles.

Estradiol decreases the orexigenic effect of neuropeptide Y, but not agouti-related protein, in ovariectomized rats

Available data suggest that estradiol exerts an inhibitory effect on food intake by modulating the actions of multiple gut- and brain-derived peptides implicated in the control of food intake. For example, recent studies have shown that estradiol decreases the orexigenic effects of ghrelin and melanin-concentrating hormone. In the present study, we examined estradiol's ability to decrease the actions of two additional orexigenic peptides, neuropeptide Y (NPY) and agouti-related protein (AgRP). Food intake was monitored following lateral ventricular infusions of 5 microg NPY, 10 microg AgRP, or saline vehicle in ovariectomized rats treated with either 1 microg estradiol or sesame oil vehicle. NPY increased food intake for 2h in both oil- and estradiol-treated ovariectomized rats. During this interval, the orexigenic effect of NPY was significantly greater in oil-treated rats, relative to estradiol-treated rats. In contrast to the short-term action of NPY, a single injection of AgRP increased food intake for 3 days in oil- and estradiol-treated rats. Meal pattern analysis revealed that the orexigenic effect of AgRP is mediated by an increase in meal size, not meal number. Unlike that observed following NPY treatment, estradiol failed to modulate the magnitude by which AgRP increased food intake and meal size. We conclude that a physiological regimen of estradiol treatment decreases the orexigenic effect of NPY, but not AgRP, in ovariectomized rats.

The orexigenic effect of melanin-concentrating hormone (MCH) is influenced by sex and stage of the estrous cycle

Recently, it was shown that the orexigenic effect of melanin-concentrating hormone (MCH) is attenuated by estradiol treatment in ovariectomized (OVX) rats. This suggests that female rats may be less responsive than male rats to the behavioral effects of MCH. To investigate this hypothesis, the effects of lateral ventricular infusions of MCH on food intake, water intake, meal patterns, and running wheel activity were examined in male and female rats. To further characterize the impact of estradiol on MCH-induced food intake, female rats were OVX and tested with and without 17-beta-estradiol benzoate (EB) replacement. In support of our hypothesis, food and water intakes following MCH treatment were greater in male rats, relative to female rats. Specifically, the orexigenic effect of MCH was maximal in male rats and minimal in EB-treated OVX rats. In both sexes, the orexigenic effect of MCH was mediated by a selective increase in meal size, which was attenuated in EB-treated OVX rats. MCH-induced a short-term (2 h) decrease in wheel running that, unlike its effects on ingestive behavior, was similar in males and females. Thus, estradiol decreases some, but not all, of the behavioral effects of MCH. To examine the influence of endogenous estradiol, food intake was monitored following MCH treatment in ovarian-intact, cycling rats. As predicted by our findings in OVX rats, the orexigenic effect of MCH was attenuated in estrous rats, relative to diestrous rats. We conclude that the female rat's reduced sensitivity to the orexigenic effect of MCH may contribute to sex- and estrous cycle-related differences in food intake.

Effects of hindbrain melanin-concentrating hormone and neuropeptide Y administration on licking for water, saccharin, and sucrose solutions

Melanin-concentrating hormone (MCH) and neuropeptide Y (NPY) are orexigenic peptides found in hypothalamic neurons that project throughout the forebrain and hindbrain. The effects of fourth ventricle (4V) infusions of NPY (5 microg) and MCH (5 microg) on licking for water, 4 mM saccharin, and sucrose (0.1 and 1.0 M) solutions were compared to identify the contributions of each peptide to hindbrain-stimulated feeding. NPY increased mean meal size only for the sucrose solutions, suggesting that caloric feedback or taste quality is pertinent to the orexigenic effect; MCH infusions under identical testing conditions failed to produce increases for any tastant. A second experiment also observed no intake or licking effects after MCH doses up to 15 microg, supporting the conclusion that MCH-induced orexigenic responses require forebrain stimulation. A third experiment compared the 4V NPY results with those obtained after NPY infusions (5 microg) into the third ventricle (3V). In contrast to the effects observed after the 3V NPY injections and previously reported forebrain intracerebroventricular (ICV) NPY infusion studies, 4V NPY failed to increase meal frequency for any taste solution or ingestion rate in the early phases of the sucrose meals. Overall, 4V NPY responses were limited to intrameal behavioral processes, whereas forebrain ICV NPY stimulation elicited both consummatory and appetitive responses. The dissociation between MCH and NPY effects observed for 4V injections is consistent with reports that forebrain ICV injections of MCH and NPY produced nearly dichotomous effects on the pattern of licking microstructure, and, collectively, the results indicate that the two peptides have separate sites of feeding action in the brain.

Alcohol Drinking in MCH Receptor-1-Deficient Mice

BACKGROUND

Recently, we demonstrated that exogenous melanin-concentrating hormone (MCH) increases alcohol drinking in rats when administered into the brain. However, because the physiological relevance of this finding is unclear, we tested the hypothesis that endogenous MCH signaling enhances alcohol consumption.

METHODS

Alcohol intake was assessed in male and female wildtype (WT), heterozygous (HET), and homozygous MCH receptor-1-deficient (KO) mice. Mice were given 24-hour access to a series of alcohol-containing solutions. Following this, the mice were given limited (1-hour) access to 10% alcohol. Finally, mice were allowed 24-hour access to sucrose/quinine as a caloric control and a means to assess taste preference. A naïve cohort of male WT and KO mice was tested for alcohol clearance following intraperitoneal administration of 3 g/kg alcohol. Another naïve cohort of female mice was utilized to confirm that intracerebroventricular administration of MCH (5 microg) would augment alcohol drinking in mice.

RESULTS

Exogenous MCH enhanced 10% alcohol consumption in mice (saline=0.45+/-0.08 g/kg, 5 microg MCH=0.94+/-0.20 g/kg). Male KO mice consumed more 10% alcohol (11.50+/-1.31 g/kg) than WT (6.26+/-1.23 g/kg) and HET mice (6.49+/-1.23 g/kg) during ad libitum access. However, alcohol intake was similar among genotypes during 1 hour daily access. Male KO mice tended to consume less 17.75% sucrose+1.3 mM quinine than controls (WT=10.5+/-3.6, HET=7.5+/-1.7, KO=4.4+/-0.9 g/kg). Alcohol metabolism was similar between WT and KO mice.

CONCLUSIONS

The finding that male KO consume more alcohol than WT and HET mice, are reminiscent of the counterintuitive reports that KO mice are hyperphagic and yet eat more when administered exogenous MCH. Changes in taste preference or alcohol metabolism do not appear to be important for the increased alcohol drinking in KO mice.

Cardiovascular effects of melanin-concentrating hormone

Melanin-concentrating hormone (MCH) is a cyclic 19-amino acid neuropeptide exclusively synthesized in the lateral hypothalamic area (LHA) and the zona incerta (ZI) that has been implicated in the regulation of energy balance. Despite what is known about the orexigenic effect of MCH, whether MCH has distinct cardiovascular and metabolic effects has yet to be determined. Thus, our goal here was to characterize the concurrent cardiovascular, metabolic, and behavioral responses of male rats to chronic intracerebroventricular (icv) infusion of MCH. Male Long-Evans rats were instrumented with telemetry transmitters for measurement of heart rate (HR) and housed in room calorimeters for assessment of food intake and oxygen consumption (VO(2)) at standard lab ambient temperature (23 degrees C) in order to examine physiological responses to chronic infusion of MCH (8 microg/d and 16 microg/d). Our findings provide the first evidence that chronic administration of MCH induces bradycardia and reduced mean arterial pressure, while it did not affect VO(2). A second experiment was performed in which the physiological responses to an acute icv infusion of MCH were observed. The results of experiment 2 indicate that MCH leads to a low HR that is maintained during the first 2 h post-infusion, the time period during which MCH acutely stimulated feeding. Collectively, these findings confirm that MCH may be an important modulator of sympathetic nervous system activity and thus may play a critical role in coordinating normal responses to negative energy balance.

The regulation of alcohol intake by melanin-concentrating hormone in rats

Given into the brain, melanin-concentrating hormone (MCH) increases alcohol consumption, but the mechanism and physiological relevance of this effect are unclear. We hypothesized that endogenous MCH will enhance alcohol drinking and that MCH increases alcohol's reinforcing properties. An MCH receptor 1 (MCHR1) antagonist, or saline was administered centrally alone, or preceding MCH or saline to rats trained to drink 10% alcohol using sucrose fading. Blocking MCHR1 neither reduced alcohol intake (saline=0.4+/-0.1 g, 30 microg MCHR1 antagonist=0.4+/-0.1 g/kg alcohol), nor attenuated MCH-induced alcohol drinking (MCHR1 antagonist/saline=0.7+/-0.1 g/kg, MCHR1 antagonist/MCH=0.9+/-0.1 g/kg alcohol). Another cohort of rats was trained to lever press for alcohol on a progressive ratio schedule. MCH or saline was administered centrally and lever presses were measured. MCH had no effect prior to the break point, but increased total responding during the session (saline=87.2+/-32.0, MCH=315.4+/-61.0 presses). In conclusion, these data suggest that MCH augments alcohol drinking partly by enhancing the drug's reinforcing value. Further, endogenous MCH does not seem to regulate alcohol drinking, however because the antagonist failed to attenuate MCH-induced alcohol intake this conclusion is tentative.

Involvement of specific orexigenic neuropeptides in sweetener-induced overconsumption in rats

Palatability is one of the factors that regulates food and fluid intake and contributes to overconsumption in turn contributing to obesity. To elucidate the brain mechanisms of the palatability-induced ingestion, we explored the roles of six hypothalamic orexigenic neuropeptides, orexin, melanin-concentrating hormone (MCH), neuropeptide Y (NPY), agouti-related protein (AgRP), ghrelin and dynorphin, in the intake of a palatable solution, saccharin. Of the six peptides, intracerebroventricular (i.c.v.) administrations of orexin, MCH and NPY increased the intake of saccharin. Drinking of saccharin in turn elevated the mRNA levels of orexin and NPY, but not MCH. Pre-treatments of naloxone, an opioid antagonist, blocked the orexigenic effects of orexin and NPY. Specific gastric motor responses induced by central orexin-A and NPY are well known, however, MCH did not induce such responses. The i.c.v. administration of orexin-A facilitated gastric emptying. These results suggest that the overconsumption promoted by sweet and palatable tastes is attributed to the activation of orexigenic neuropeptides, such as orexin and NPY, and a downstream opioid system together with enhanced digestive functions.

Expanding the scales: The multiple roles of MCH in regulating energy balance and other biological functions

Melanin-concentrating hormone (MCH) is a cyclic peptide originally identified as a 17-amino-acid circulating hormone in teleost fish, where it is secreted by the pituitary in response to stress and environmental stimuli. In fish, MCH lightens skin color by stimulating aggregation of melanosomes, pigment-containing granules in melanophores, cells of neuroectodermal origin found in fish scales. Although the peptide structure between fish and mammals is highly conserved, in mammals, MCH has no demonstrable effects on pigmentation; instead, based on a series of pharmacological and genetic experiments, MCH has emerged as a critical hypothalamic regulator of energy homeostasis, having effects on both feeding behavior and energy expenditure.

The melanocortins and melanin-concentrating hormone in the central regulation of feeding behavior and energy homeostasis

A number of different neuropeptides exert powerful concerted controls on feeding behavior and energy balance, most of them being produced in hypothalamic neuronal networks under stimulation by anabolic and catabolic peripheral hormones such as ghrelin and leptin, respectively. These peptide-expressing neurons interconnect extensively to integrate the multiple opposing signals that mediate changes in energy expenditure. In the present review I have summarized our current knowledge about two key peptidic systems involved in regulating appetite and energy homeostasis, the melanocortin system (alpha-MSH, agouti and Agouti-related peptides, MC receptors and mahogany protein) and the melanin-concentrating hormone system (proMCH-derived peptides and MCH receptors) that contribute to satiety and feeding-initiation, respectively, with concurrent effects on energy expenditure. I have focused particularly on recent data concerning transgenic mice and the ongoing development of MC/MCH receptor antagonists/agonists that may represent promising drugs to treat human eating disorders on both sides of the energy balance (anorexia, obesity).

Effects of melanin-concentrating hormone on licking microstructure and brief-access taste responses

The effects of intracerebroventricular application of melanin-concentrating hormone (MCH) on licking for sucrose, quinine hydrochloride (QHCl), and water solutions were evaluated in two experiments. In experiment 1, rats received 90-min access to sucrose and water solutions after MCH or vehicle microinjection to the third ventricle (3V). MCH increased intake largely through increases in the rate of licking early in the meal and in the mean duration of lick bursts, suggesting an effect on gustatory evaluation. Therefore, in experiment 2, brief access tests were used with a series of sucrose and QHCl concentrations to behaviorally isolate the effects of intracerebroventricular MCH on gustatory evaluation. MCH uniformly increased licking for all sucrose solutions, water, and weak concentrations of QHCl; however, it had no effect on licking for the strongest concentrations of QHCl, which were generally avoided under control conditions. Thus MCH did not produce nonspecific increases in oromotor activity, nor did it change the perceived intensity of the tastants. We conclude that MCH enhanced the gain of responses to normally accepted stimuli at a phase of processing after initial gustatory detection and after the decision to accept or reject the taste stimulus. A comparison of 3V NPY and MCH effects on licking microstructure indicated that these two peptides increased intake via dichotomous behavioral processes; although NPY suppressed measures associated with inhibitory feedback from the gut, MCH appeared instead to enhance measures associated with hedonic taste evaluation.

Gonadal hormones determine sensitivity to central leptin and insulin

Males have proportionally more visceral fat and are more likely to develop complications associated with obesity than females, and the male brain is relatively more sensitive to the catabolic action of insulin and less sensitive to that of leptin than the female brain. To understand the underlying mechanism, we manipulated estrogen through ovariectomy (OVX) and estradiol administration. Rats with relatively high systemic estrogen (intact females and OVX females and males administered estrogen subcutaneously) were significantly more sensitive to leptin's anorexic action in the brain (i3vt), as well as significantly less sensitive to insulin's i3vt action, than intact males. Administering estradiol directly into the brain of our females increased i3vt leptin sensitivity while decreasing i3vt insulin sensitivity and changed the body fat distribution of our females to resemble that of intact females. These data indicate that estrogen acts within the brain to increase leptin sensitivity, decrease insulin sensitivity, and favor subcutaneous over visceral fat.

MCH-/- mice are resistant to aging-associated increases in body weight and insulin resistance

Ablation of the hypothalamic peptide, melanin-concentrating hormone (MCH), leads to a lean phenotype and resistance to diet-induced obesity. Observation of MCH(-/-) mice at older ages suggested that these effects persist in mice >1 year old. Leanness secondary to caloric restriction is known to be associated with improved glucose tolerance as well as an overall increase in life span. Because the MCH(-/-) model represents leanness secondary to increased energy expenditure rather than caloric restriction, we were interested in determining whether this model of leanness would be associated with beneficial metabolic effects at older ages. To assess the effects of MCH ablation over a more prolonged period, we monitored male and female MCH(-/-) mice up to 19 months. The lean phenotype of MCH(-/-) mice persisted over the duration of the study. At 19 months, MCH(-/-) male and female mice weighed 23.4 and 30.8% less than their wild-type counterparts, a result of reduced fat mass in MCH(-/-) mice. Aged MCH(-/-) mice exhibited better glucose tolerance and were more insulin sensitive compared with wild-type controls. Aging-associated decreases in locomotor activity were also attenuated in MCH(-/-) mice. We also evaluated two molecules implicated in the pathophysiology of aging, p53 and silent inflammatory regulator 2 (Sir2). We found that expression of the tumor suppressor protein p53 was higher in MCH(-/-) mice at 9 and 19 months of age. In contrast, expression of Sir2 was unchanged. In aggregate, these findings suggest that MCH ablation improves the long-term outcome for several indicators of the aging process.

Melanin concentrating hormone innervation of caudal brainstem areas involved in gastrointestinal functions and energy balance

Neural signaling by melanin-concentrating hormone and its receptor (SLC-1) has been implicated in the control of energy balance, but due to the wide distribution of melanin-concentrating hormone-containing fibers throughout the neuraxis, its critical sites of action for a particular effect have not been identified. The present study aimed to anatomically and functionally characterize melanin-concentrating hormone innervation of the rat caudal brainstem, as this brain area plays an important role in the neural control of ingestive behavior and autonomic outflow. Using retrograde tracing we demonstrate that a significant proportion (5-15%) of primarily perifornical and far-lateral hypothalamic melanin-concentrating hormone neurons projects to the dorsal vagal complex. In the caudal brainstem, melanin-concentrating hormone-ir axon profiles are distributed densely in most areas including the nucleus of the solitary tract, dorsal motor nucleus of the vagus, and sympathetic premotor areas in the ventral medulla. Close anatomical appositions can be demonstrated between melanin-concentrating hormone-ir axon profiles and tyrosine hydroxylase, GABA, GLP-1, NOS-expressing, and nucleus of the solitary tract neurons activated by gastric nutrient infusion. In medulla slice preparations, bath application of melanin-concentrating hormone inhibited in a concentration-dependent manner the amplitude of excitatory postsynaptic currents evoked by solitary tract stimulation via a pre-synaptic mechanism. Fourth ventricular administration of melanin-concentrating hormone (10 microg) in freely moving rats decreased core body temperature but did not change locomotor activity and food and water intake. We conclude that the rich hypothalamo-medullary melanin-concentrating hormone projections in the rat are mainly inhibitory to nucleus of the solitary tract neurons, but are not involved in the control of food intake. Projections to ventral medullary sites may play a role in the inhibitory effect of melanin-concentrating hormone on energy expenditure.

Central administration of melanin-concentrating hormone increases alcohol and sucrose/quinine intake in rats

BACKGROUND

Alcohol is a caloric compound that can contribute to energy intake. Therefore, peptides that regulate energy balance likely modify the motivation to consume alcohol. Melanin-concentrating hormone (MCH) regulates energy homeostasis and has been implicated in other behaviors that impact alcohol consumption (i.e., anxiety, fluid balance, and reward). We tested the hypothesis that MCH would decrease the motivation to consume alcohol secondarily to reducing anxiety.

METHODS

Rats were trained to drink 10% ethanol or an isocaloric concentration of sucrose with use of a sucrose-fading technique. MCH (1, 5, or 10 microg) or its saline vehicle was administered into the third cerebral ventricle (i3vt), and intake of ethanol or sucrose and chow was assessed for 2 hr. Alcohol-naïve rats were evaluated in an elevated plus maze after i3vt MCH (10 microg), neuropeptide Y, or saline administration.

RESULTS

Contrary to the hypothesis, MCH dose-dependently increased alcohol intake: saline = 0.7 +/- 0.1 g/kg, 1 microg MCH = 1.0 +/- 0.1 g/kg, 5 microg MCH = 1.2 +/- 0.1 g/kg, and 10 microg MCH = 1.8 +/- 0.3 g/kg (p < 0.01), and this was true whether water was simultaneously available or not. MCH also significantly increased sucrose intake (saline = 1.0 +/- 0.3 g/kg, 10 mug MCH = 1.4 +/- 0.5 g/kg; p < 0.05). MCH had no effect on time spent in the open arms (54.3 +/- 11.5 sec) relative to saline (58.2 +/- 23.8 sec), whereas neuropeptide Y, a known anxiolytic, increased time spent on the open arms (119.2 +/- 22 sec, p < 0.05).

CONCLUSIONS

We conclude that MCH nonspecifically increases ingestive behavior. Furthermore, MCH had no apparent effect on anxiety. The ability of MCH to increase alcohol and/or sucrose intake may be explained by the effect of MCH on energy balance and/or reward processes.

Enhanced running wheel activity of both Mch1r- and Pmch-deficient mice

Mch1r-deficient (Mch1r(-/-)) mice are hyperphagic, hyperactive, lean, and resistant to diet-induced obesity. To examine whether the MCH1R is involved in regulating activity-based energy expenditure, we investigated voluntary wheel running (WR) activity of wild-type (WT) and Mch1r(-/-) mice basally, in response to diets with different caloric density and with different feeding schedules. We also evaluated WR activity of mice with ablation of the prepro-MCH gene (Pmch(-/-) mice). Dark cycle WR activity of Mch1r(-/-) mice fed low fat (LF) chow was increased significantly relative to WT mice. Transition to moderate high-fat (MHF) diet was associated with an increase in nocturnal WR activity in both genotypes. Both Mch1r(-/-) and WT mice exhibited food anticipatory activity (FAA) before the daily scheduled feeding time, indicating that MCH1R is not required for FAA. Naloxone (3 mg/kg, i.p.) suppressed WR activity of both genotypes, suggesting opioid regulation of locomotor activity. WR increased nocturnal dynorphin mRNA levels in Mch1r(-/-) brain. Importantly, Pmch-deficient mice had significantly enhanced WR activity relative to WT controls. These results suggest that endogenous MCH plays an inhibitory role in regulating locomotor activity. In summary, we demonstrated enhanced WR activities in mice lacking either MCH or its cognate receptor.

Comparative analysis of melanin-concentrating hormone structure and activity in fishes and mammals

A comparative analysis of the structure of the melanin-concentrating hormone (MCH) precursor reveals that this sequence has been subjected to a higher selection pressure in mammals than in teleosts, suggesting that the structural constraints have not been the same throughout the vertebrate lineage. In contrast, the MCH peptide sequence has been very well conserved in all species. A sensitive and reproducible eel skin assay was developed and allowed us to define the structural features needed for a full MCH bioactivity. It was shown that the minimal structure carrying the critical residues was the same in fishes and in mammals. A pharmacological approach confirmed that MCH receptor activation decreased the cAMP levels in the fish skin, but this effect appeared to be independent from a Galphai protein. We propose that one of the intracellular signaling pathways of the MCH receptor in fish skin is the activation of one or several cellular phosphodiesterases.

Melanin-concentrating hormone signaling systems in fish

Melanin-concentrating hormone (MCH) is a cyclic neuropeptide synthesized as a preprohormone in the hypothalamus of all vertebrates. This neuropeptide binds to G-protein-coupled seven transmembrane receptor(s) to mediate its function. MCH was named after its function in teleosts, in which it causes aggregation or concentration of melanin granules in melanophores, thus regulating body color. The function of central MCH that has attracted most attention is its involvement in regulating food intake and energy homeostasis in mammals, a role confirmed through a series of experiments, including central administration of MCH or MCH receptor blockers, and genetic manipulation of MCH and its receptors. The aim of this article is to review the recent data on MCH and MCH receptor signaling systems in fish.