Cardiovascular effects of melanin-concentrating hormone

Melanin-concentrating hormone regulates the hypercapnic chemoreflex by acting in the lateral hypothalamic area

NEW FINDINGS

What is the central question of this study? MCH suppresses the hypercapnic chemoreflex but the mechanism by which this effect is produced has not been previously explored. What is the main finding and its importance? MCH acting in the lateral hypothalamic area but not in the locus coeruleus in rats, in the light period, attenuates the hypercapnic chemoreflex. Our data provide new insight regarding the role of MCH in the modulation of the hypercapnic ventilatory response.

ABSTRACT

Melanin-concentrating hormone (MCH) is a hypothalamic neuropeptide involved in a broad range of homeostatic functions including regulation of the hypercapnic chemoreflex. We evaluated whether MCH modulates the hypercapnic ventilatory response by acting in the lateral hypothalamic area (LHA) and/or in the locus coeruleus (LC). Here, we measured pulmonary ventilation (V_E ), body temperature, electroencephalogram (EEG) and electromyogram (EMG) of unanesthetized adult male Wistar rats before and after microinjection of MCH [0.4 mM] or MCH1-R antagonist (SNAP-94847 [63 mM]) into the LHA and LC, in room air and 7% CO₂ conditions during wakefulness and sleep, in the dark and light periods. MCH intra-LHA caused a decreased CO₂ ventilatory response during wakefulness and sleep in the light period, while SNAP-94847 intra-LHA increased this response, during wakefulness in the light period. In the LC, MCH or the MCH1-R antagonist caused no change in the hypercapnic ventilatory response. Our results suggest that MCH, in the LHA, exerts an inhibitory modulation of the hypercapnic ventilatory response during the light-inactive period in rats. This article is protected by copyright. All rights reserved.

Bioactive Signaling in Next-Generation Pharmacotherapies for Heart Failure: A Review

Importance

The standard pharmacotherapy for heart failure (HF), particularly HF with reduced ejection fraction (HFrEF), is primarily through the use of receptor antagonists, notably inhibition of the renin-angiotensin system by either angiotensin-converting enzyme inhibition or angiotensin II receptor blockade (ARB). However, the completed Prospective Comparison of ARNI With an ACE-Inhibitor to Determine Impact on Global Mortality and Morbidity in Heart Failure (PARADIGM-HF) trial identified that the use of a single molecule (sacubitril/valsartan), which is an ARB and the neutral endopeptidase inhibitor (NEPi) neprilysin, yielded improved clinical outcomes in HFrEF compared with angiotensin-converting enzyme inhibition alone.

Observations

This review examined specific bioactive signaling pathways that would be potentiated by NEPi and how these would affect key cardiovascular processes relevant to HFrEF. It also addressed potential additive/synergistic effects of ARB. A number of biological signaling pathways that may be potentiated by sacubitril/valsartan were identified, including some novel candidate molecules, which will act in a synergistic manner to favorably alter the natural history of HFrEF.

Conclusions and Relevance

This review identified that activation rather than inhibition of specific receptor pathways provided favorable cardiovascular effects that cannot be achieved by renin-angiotensin system inhibition alone. Thus, an entirely new avenue of translational and clinical research lies ahead in which HF pharmacotherapies will move beyond receptor antagonist strategies.

GABAA but not GABAB receptors in the lateral hypothalamus modulate the tachycardic response to emotional stress in rats

The lateral hypothalamus (LH) has been described as one of the hypothalamic areas involved in the behavioral and physiological responses triggered by aversive stimuli. Previous studies indicated involvement of the LH in cardiovascular responses to stress. Despite this evidence, the local neurochemical mechanisms involved in LH control of stress responses is still poorly understood. Therefore, in the present study, we investigated the role of GABAergic neurotransmission within the LH in cardiovascular responses induced by an acute session of restraint stress in rats. For this, we evaluated the effect of bilateral microinjection of selective antagonists of either GABA_A or GABA_B receptors into the LH on arterial pressure increase, heart rate (HR) increase and reduction in tail skin temperature induced by restraint stress. We found that microinjection of the selective GABA_A receptor antagonist SR95531 into the LH decreased the increase in HR caused by restraint stress, but without affecting the increase in arterial pressure increase or the reduction in tail skin temperature. Conversely, LH treatment with the selective GABA_B receptor antagonist CGP35348 did not affect the restraint-evoked cardiovascular changes. These findings indicate that GABAergic neurotransmission in the LH, acting through activation of local GABA_A receptors, plays a facilitatory role in the tachycardic response observed during aversive threats.

Nesfatin-1 regulates the lateral hypothalamic area melanin-concentrating hormone-responsive gastric distension-sensitive neurons and gastric function via arcuate nucleus innervation

Nesfatin-1, a recently discovered neuropeptide involved in satiety. Recent studies have revealed that central nesfatin-1 inhibits gastric emptying and gastric acid secretion, though the mechanisms involved in these processes are not known. We aim to explore the effects of nesfatin-1 on a population of gastric distension (GD)-sensitive neurons in the lateral hypothalamus (LHA), gastric motility, and gastric secretion and the role for an arcuate nucleus (Arc)-LHA neural pathway in these processes. Single unit extracellular discharge recordings were made in of LHA. Further, gastric motility and gastric secretion in rats were monitored. Retrograde tracing and fluorescent immunohistochemical staining were used to explore nesfatin-1 neuron projection. The results revealed that administration of nesfatin-1 to the LHA or electric stimulation of the Arc could alter the neuronal activity of melanin-concentrating hormone (MCH)-responsive, GD-responsive neurons in LHA, which could be blocked by pretreatment with MCH receptor-1 antagonist PMC-3881-PI or weakened by pretreatment of a nesfatin-1 antibody in LHA. Administration of nesfatin-1 into LHA could inhibit gastric motility and gastric secretion, and these effects could be enhanced by administration of PMC-3881-PI. Electrical stimulation of Arc promoted the gastric motility and gastric secretion. Nesfatin-1 antibody or PMC-3881-PI pretreatment to LHA had no effect on Arc stimulation-induced gastric motility, but these pretreatments did alter Arc stimulation-induced effects on gastric secretion. Our findings suggest that nesfatin-1 signaling in LHA participates in the regulation of efferent information from the gastrointestinal tract and gastric secretion which also involve MCH signaling. Further, they show that a nesfatin-1-positive Arc to LHA pathway is critical for these effects.

The role of food intake regulating peptides in cardiovascular regulation

Obesity is a risk factor that worsens cardiovascular events leading to higher morbidity and mortality. However, the exact mechanisms of relation between obesity and cardiovascular events are unclear. Nevertheless, it has been demonstrated that pharmacological therapy for obesity has great potential to improve some cardiovascular problems. Therefore, it is important to determine the common mechanisms regulating both food intake and blood pressure. Several hormones produced by peripheral tissues work together with neuropeptides involved in the regulation of both food intake and blood pressure. Anorexigenic (food intake lowering) hormones such as leptin, glucagon-like peptide-1 and cholecystokinin cooperate with α-melanocyte-stimulating hormone, cocaine- and amphetamine-regulated peptide as well as prolactin-releasing peptide. Curiously their collective actions result in increased sympathetic activity, especially in the kidney, which could be one of the factors responsible for the blood pressure increases seen in obesity. On the other hand, orexigenic (food intake enhancing) peptides, especially ghrelin released from the stomach and acting in the brain, cooperates with orexins, neuropeptide Y, melanin-concentrating hormone and galanin, which leads to decreased sympathetic activity and blood pressure. This paradox should be intensively studied in the future. Moreover, it is important to know that the hypothalamus together with the brainstem seem to be major structures in the regulation of food intake and blood pressure. Thus, the above mentioned regions might be essential brain components in the transmission of peripheral signals to the central effects. In this short review, we summarize the current information on cardiovascular effects of food intake regulating peptides.

Chronic heart failure alters orexin and melanin concentrating hormone but not corticotrophin releasing hormone-related gene expression in the brain of male Lewis rats

OBJECTIVE

The aim of this study was to investigate the effect of chronic heart failure (HF; 16 weeks post left coronary artery ligation) on the brain's orexin (ORX) and related neuropeptide systems.

METHODS

Indicators of cardiac function, including the percent fractional shortening (%FS) left ventricular posterior wall shortening velocity (LVPWSV) were assessed via echocardiography at 16 weeks post myocardial infarction or sham treatment in male Lewis rats (n=5/group). Changes in gene expression in HF versus control (CON) groups were quantified by real-time PCR in the hypothalamus, amygdala and dorsal pons.

RESULTS

HF significantly reduced both the %FS and LVPWSV when compared to CON animals (P<0.02). In the hypothalamus ORX gene expression was significantly reduced in HF and correlated with changes in cardiac function when compared to CON (P<0.02). No significant changes in hypothalamic ORX receptor (type 1 or type 2) gene expression were identified. Alternatively hypothalamic melanin concentrating hormone (MCH) gene expression was significantly upregulated in HF animals and negatively correlated with LVPWSV (P<0.006). In both the amygdala and dorsal pons ORX type 2 receptor expression was significantly down-regulated in HF compared to CON. ORX receptor type 1, CRH and CRH type 1 and type 2 receptor expressions were unchanged by HF in all brain regions analyzed.

CONCLUSION

These observations support previous work demonstrating that cardiovascular disease modulates the ORX system and identify that in the case of chronic HF the ORX system is altered in parallel with changes in MCH expression but independent of any significant changes in the central CRH system. This raises the new possibility that ORX and MCH systems may play an important role in the pathophysiology of HF.

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.

The hypocretins (orexins) mediate the "phasic" components of REM sleep: A new hypothesis

In 1998, a group of phenotypically distinct neurons were discovered in the postero-lateral hypothalamus which contained the neuropeptides hypocretin 1 and hypocretin 2 (also called orexin A and orexin B), which are excitatory neuromodulators. Hypocretinergic neurons project throughout the central nervous system and have been involved in the generation and maintenance of wakefulness. The sleep disorder narcolepsy, characterized by hypersomnia and cataplexy, is produced by degeneration of these neurons. The hypocretinergic neurons are active during wakefulness in conjunction with the presence of motor activity that occurs during survival-related behaviors. These neurons decrease their firing rate during non-REM sleep; however there is still controversy upon the activity and role of these neurons during REM sleep. Hence, in the present report we conducted a critical review of the literature of the hypocretinergic system during REM sleep, and hypothesize a possible role of this system in the generation of REM sleep.

Orexin, cardio-respiratory function, and hypertension

In this review we focus on the role of orexin in cardio-respiratory functions and its potential link to hypertension. (1) Orexin, cardiovascular function, and hypertension. In normal rats, central administration of orexin can induce significant increases in arterial blood pressure (ABP) and sympathetic nerve activity (SNA), which can be blocked by orexin receptor antagonists. In spontaneously hypertensive rats (SHRs), antagonizing orexin receptors can significantly lower blood pressure under anesthetized or conscious conditions. (2) Orexin, respiratory function, and central chemoreception. The prepro-orexin knockout mouse has a significantly attenuated ventilatory CO2 chemoreflex, and in normal rats, central application of orexin stimulates breathing while blocking orexin receptors decreases the ventilatory CO2 chemoreflex. Interestingly, SHRs have a significantly increased ventilatory CO2 chemoreflex relative to normotensive WKY rats and blocking both orexin receptors can normalize this exaggerated response. (3) Orexin, central chemoreception, and hypertension. SHRs have higher ABP and SNA along with an enhanced ventilatory CO2 chemoreflex. Treating SHRs by blocking both orexin receptors with oral administration of an antagonist, almorexant (Almxt), can normalize the CO2 chemoreflex and significantly lower ABP and SNA. We interpret these results to suggest that the orexin system participates in the pathogenesis and maintenance of high blood pressure in SHRs, and the central chemoreflex may be a causal link to the increased SNA and ABP in SHRs. Modulation of the orexin system could be a potential target in treating some forms of hypertension.

Orexin, cardio-respiratory function, and hypertension

In this review we focus on the role of orexin in cardio-respiratory functions and its potential link to hypertension. (1) Orexin, cardiovascular function, and hypertension. In normal rats, central administration of orexin can induce significant increases in arterial blood pressure (ABP) and sympathetic nerve activity (SNA), which can be blocked by orexin receptor antagonists. In spontaneously hypertensive rats (SHRs), antagonizing orexin receptors can significantly lower blood pressure under anesthetized or conscious conditions. (2) Orexin, respiratory function, and central chemoreception. The prepro-orexin knockout mouse has a significantly attenuated ventilatory CO₂ chemoreflex, and in normal rats, central application of orexin stimulates breathing while blocking orexin receptors decreases the ventilatory CO₂ chemoreflex. Interestingly, SHRs have a significantly increased ventilatory CO₂ chemoreflex relative to normotensive WKY rats and blocking both orexin receptors can normalize this exaggerated response. (3) Orexin, central chemoreception, and hypertension. SHRs have higher ABP and SNA along with an enhanced ventilatory CO₂ chemoreflex. Treating SHRs by blocking both orexin receptors with oral administration of an antagonist, almorexant (Almxt), can normalize the CO₂ chemoreflex and significantly lower ABP and SNA. We interpret these results to suggest that the orexin system participates in the pathogenesis and maintenance of high blood pressure in SHRs, and the central chemoreflex may be a causal link to the increased SNA and ABP in SHRs. Modulation of the orexin system could be a potential target in treating some forms of hypertension.

A subset of presympathetic-premotor neurons within the centrally projecting Edinger-Westphal nucleus expresses urocortin-1

Numerous motivated behaviors require simultaneous activation of somatomotor and autonomic functions. We have previously characterized the organization of brain circuits that may mediate this integration. Presympathetic premotor neurons (PSPMNs) that are part of such circuits are distributed across multiple brain regions, which mediate stress-elicited behavioral and physiological responses, including the Edinger-Westphal nucleus (EW). Based on its connectivity and function, EW has recently been re-classified into a preganglionic (EWpg) and a centrally projecting (EWcp) population. Neurons within EWcp are the major source of urocortin 1 (Ucn-1), an analog of the corticotropin-releasing factor that binds the CRFR1 and CRFR2 receptors and has been implicated in mediating homeostatic responses to stress. We hypothesized that a subset of EWcp PSPMNs expresses Ucn-1. Utilizing dual-label immunofluorescence, we initially mapped the distribution of Ucn-1 and cholinergic neurons within EW in colchicine pre-treated rats. Based on this labeling we divided EWcp into three neuroanatomical levels. To examine connections of EWcp neurons to the gastrocnemius muscle and the adrenal gland, we next employed trans-synaptic tract-tracing in a second group of rats, utilizing two pseudorabies virus (PRV) recombinants that express unique reporter proteins. Using multi-label immunofluorescent staining, we identified the presence of Ucn-1-positive PSPMNs, dually labeled with PRV and present throughout the entire extent of EWcp and intermingled with Ucn-1 neurons infected with one or neither of the viral recombinants. Compared to rats pretreated with colchicine, we observed significantly fewer Ucn-1 neurons in animals that received PRV injections. Post hoc analyses revealed significantly fewer Ucn-1 neurons at the rostral level as compared to the caudal and middle levels. These data suggest functional and anatomic heterogeneity within EWcp; this organization may coordinate various aspects of stress-elicited and emotionally salient behaviors.

Intrathecal melanin-concentrating hormone reduces sympathetic tone and blocks cardiovascular reflexes

Melanin-concentrating hormone (MCH) is a neuropeptide that acts to increase feeding behavior and decrease energy expenditure. The role of MCH in central cardiorespiratory regulation is still poorly understood. Experiments were conducted on urethane-anesthetized, vagotomized, and artificially ventilated male Sprague-Dawley rats ( n = 22) to ascertain whether MCH modulates sympathetic vasomotor tone, as well as barosympathetic, chemosympathetic, and somatosympathetic reflexes at the level of the spinal cord. Intrathecal injection of 10 μl of MCH produced a dose-dependent hypotension, bradycardia, and sympathoinhibition. Peak response was observed following administration of 1 mM MCH, causing a decrease in mean arterial pressure of 39 ± 2 mmHg ( P < 0.001), splanchnic sympathetic nerve activity of 78 ± 11% ( P < 0.001), and heart rate of 87 ± 11 beats per minute (bpm) ( P < 0.01). The two peaks of the somatosympathetic reflex were decreased by intrathecal MCH, 7 ± 3% ( P < 0.01) and 31 ± 6% ( P < 0.01), respectively, and the spinal component of the reflex was accentuated 96 ± 23% ( P < 0.05), with respect to the baseline for MCH, compared with the two peaks and spinal component of the somatosympathetic reflex elicited following saline injection with respect to the baseline for saline. MCH decreased the sympathetic gain to 120 s of hyperoxic hypercapnea (10% CO2 in 90% O2) and to 10–12 s poikilocapneic anoxia (100% N2) from 0.74 ± 0.14%/s to 0.23 ± 0.04%/s ( P < 0.05) and 16.47 ± 3.2% to 4.35 ± 1.56% ( P < 0.05), respectively. There was a 34% decrease in gain and a 62% decrease in range of the sympathetic baroreflex with intrathecal MCH. These data demonstrate that spinal MCH blunts the central regulation of sympathetic tone and adaptive sympathetic reflexes.

Central control of thermogenesis

In mammals and birds, conservation of body heat at around 37 °C is vital to life. Thermogenesis is the production of this heat which can be obligatory, as in basal metabolic rate, or it can be facultative such as the response to cold. A complex regulatory system has evolved which senses environmental or core temperature and integrates this information in hypothalamic regions such as the preoptic area and dorsomedial hypothalamus. These areas then send the appropriate signals to generate and conserve heat (or dissipate it). In this review, the importance of the sympathetic nervous system is discussed in relation to its role in basal metabolic rate and adaptive thermogenesis with a particular emphasis to human obesity. The efferent sympathetic pathway does not uniformly act on all tissues; different tissues can receive different levels of sympathetic drive at the same time. This is an important concept in the discussion of the pharmacotherapy of obesity. Despite decades of work the medicine chest contains only one pill for the long term treatment of obesity, orlistat, a lipase inhibitor that prevents the absorption of lipid from the gut and is itself not systemically absorbed. The central controlling system for thermogenesis has many potential intervention points. Several drugs, previously marketed, awaiting approval or in the earlier stages of development may have a thermogenic effect via activation of the sympathetic nervous system at some point in the thermoregulatory circuit and are discussed in this review. If the balance is weighted to the "wrong" side there is the burden of increased cardiovascular risk while a shift to the "right" side, if possible, will afford a thermogenic benefit that is conducive to weight loss maintenance. This article is part of a Special Issue entitled 'Central Control Food Intake'

Female mice and rats exhibit species-specific metabolic and behavioral responses to ovariectomy

Ovariectomy (OVX) leads to hyperphagia and weight gain in rats, which can be prevented by estradiol (E2) replacement; however, the role of endogenous E2 on feeding and energy homeostasis in female mice has not been well characterized. The primary goal of this study was to assess the relative contribution of increased energy intake and decreased energy expenditure to OVX-induced weight gain in female rats and mice. OVX led to hyperphagia in rats, but did not produce daily, nor cumulative, hyperphagia in mice. OVX decreased mass-specific metabolic rate in mice, but not in rats. OVX decreased home cage locomotor activity in both species. Pair-feeding attenuated OVX-induced weight gain in rats and produced both short- and long-term changes in expression of key hypothalamic genes involved in food intake and energy homeostasis, i.e., the anorexigenic neuropeptide pro-opiomelanocortin (POMC) and the orexigenic neuropeptides: melanin-concentrating hormone (MCH) and agouti-related peptide (AgRP). No differences in hypothalamic gene expression were observed between OVX'd and sham mice. The results suggest that OVX-induced weight gain is mediated by hyperphagia and reduced locomotor activity in rats, but that in mice, it is primarily mediated by reduced locomotor activity and metabolic rate.

Chronic MCH infusion causes a decrease in energy expenditure and body temperature, and an increase in serum IGF-1 levels in mice

Melanin concentrating hormone (MCH) is an orexigenic peptide secreted from the lateral hypothalamus. Various observations suggest a role for MCH in energy expenditure in transgenic mice; however, the influence of MCH on energy expenditure and body temperature in WT mice was inadequately studied. Therefore, our first goal was to characterize the influence of chronic intracerebroventrical MCH infusion on energy homeostasis in mice. Our second goal was to explore the effect of MCH on the GH-insulin like growth factor 1 (IGF-1) axis in vivo. We have recently published that MCH directly increased GH-secretion from pituitary cells in vitro, suggesting that MCH may exert part of its effects on energy balance via direct pituitary hormone regulation. Mice were centrally infused with MCH for 14 days, resulting in a significant increase in food intake, body weight, fat mass and plasma IGF-1 levels, while decreasing body temperature and energy expenditure. Our data emphasize the role of MCH as a key regulator of energy homeostasis by means of appetite regulation, regulation of energy expenditure, and an integrator of energy balance with the neuroendocrine system regulating pituitary hormone secretion. They also support the notion that MCH may have a physiologic role in GH regulation that may, in turn, contribute to its effect on body weight.

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.

MCH receptor peptide agonists and antagonists

Melanin-concentrating hormone (MCH) is an important neuropeptide hormone involved in multiple physiological processes. Peptide derivatives of MCH have been developed as tools to aid research including potent radioligands, receptor selective agonists, and potent antagonists. These tools have been used to further understand the role of MCH in physiology, primarily in rodents. However, the tools could also help elucidate the role for MCHR1 and MCHR2 in mediating MCH signaling in higher species.

Effects of intracerebroventricular and intra-accumbens melanin-concentrating hormone agonism on food intake and energy expenditure

The brain melanin-concentrating hormone (MCH) system represents an anabolic system involved in energy balance regulation through influences exerted on the homeostatic and nonhomeostatic controls of food intake and energy expenditure. The present study was designed to further delineate the effect of the MCH system on energy balance regulation by assessing the actions of the MCH receptor 1 (MCHR1) agonism on both food intake and energy expenditure after intracerebroventricular (third ventricle) and intra-nucleus-accumbens-shell (intraNAcSH) injections of a MCHR1 agonist. Total energy expenditure and substrate oxidation were assessed following injections in male Wistar rats using indirect calorimetry. Food intake was also measured. Pair-fed groups were added to evaluate changes in thermogenesis that would occur regardless of the meal size and its thermogenic response. Using such experimental conditions, we were able to demonstrate that acute MCH agonism in the brain, besides its orexigenic effect, induced a noticeable change in the utilization of the main metabolic fuels. In pair-fed animals, MCH significantly reduced lipid oxidation when it was injected in the third ventricle. Such an effect was not observed following the injection of MCH in the NAcSH, where MCH nonetheless strongly stimulated appetite. The present results further delineate the influence of MCH on energy expenditure and substrate oxidation while confirming the key role of the NAcSH in the effects of the MCH system on food intake.

Melanin-concentrating hormone neurons discharge in a reciprocal manner to orexin neurons across the sleep-wake cycle

Neurons containing melanin-concentrating hormone (MCH) are codistributed with neurons containing orexin (Orx or hypocretin) in the lateral hypothalamus, a peptide and region known to be critical for maintaining wakefulness. Evidence from knockout and c-Fos studies suggests, however, that the MCH neurons might play a different role than Orx neurons in regulating activity and sleep-wake states. To examine this possibility, neurons were recorded across natural sleep-wake states in head-fixed rats and labeled by using the juxtacellular technique for subsequent immunohistochemical identification. Neurons identified as MCH+ did not fire during wake (W); they fired selectively during sleep, occasionally during slow wave sleep (SWS) and maximally during paradoxical sleep (PS). As W-Off/Sleep-On, the MCH neurons discharged in a reciprocal manner to the W-On/Sleep-Off Orx neurons and could accordingly play a complementary role to Orx neurons in sleep-wake state regulation and contribute to the pathophysiology of certain sleep disorders, such as narcolepsy with cataplexy.

Regulation of synaptic efficacy in hypocretin/orexin-containing neurons by melanin concentrating hormone in the lateral hypothalamus

The lateral hypothalamus (LH) is a central hub that integrates inputs from, and sends outputs to, many other brain areas. Two groups of neurons in the LH, expressing hypocretin/orexin or melanin concentrating hormone (MCH), have been shown to participate in sleep regulation, energy homeostasis, drug addiction, motor regulation, stress response, and social behaviors. The elucidation of crosstalk between these two systems is essential to understand these behaviors and functions because there is evidence that there are reciprocal innervations between hypocretin/orexin and MCH neurons. In this study, we used MCH receptor-1 knock-out (MCHR1 KO) and wild-type (WT) mice expressing green fluorescent protein in hypocretin/orexin-containing neurons to examine the hypothesis that MCH modulates hypocretin/orexin-mediated effects on behavioral state and synaptic transmission in the LH. In MCHR1 KO mice, the efficacy of glutamatergic synapses on hypocretin/orexin neurons is potentiated and hypocretin-1-induced action potential firing is facilitated, potentially explaining an increased effect of modafinil observed in MCHR1 KO mice. In wild-type mice with intact MCHR1 signaling, MCH significantly attenuated the hypocretin-1-induced enhancement of spike frequency in hypocretin/orexin neurons. The MCH effect was dose dependent, pertussis toxin sensitive, and was abolished in MCHR1 KO mice. Consistent with this effect, MCH attenuated hypocretin-1-induced enhancement of the frequency of miniature EPSCs in hypocretin/orexin neurons. These data from MCHR1 KO and WT mice demonstrate a novel interaction between these two systems, implying that MCH may exert a unique inhibitory influence on hypocretin/orexin signaling as a way to fine-tune the output of the LH.

Organization of brain somatomotor-sympathetic circuits

Numerous physiological and emotionally motivated behaviors require concomitant activation of somatomotor and sympathetic efferents. Likewise, adaptive and maladaptive responses to stress are often characterized by simultaneous recruitment of these efferent systems. This review describes recent literature that outlines the organization of somatomotor-sympathetic circuitry in the rat. These circuits were delineated by employing recombinant pseudorabies (PRV) viral vectors as retrograde trans-synaptic tract tracers. In these studies PRV-152, a strain that expresses enhanced green fluorescent protein, was injected into sympathectomized hindlimb muscle, while PRV-BaBlu, which expresses beta-galactosidase, was injected into the adrenal gland in the same animals. Immunofluorescent methods were then used to determine the presence of putative dual-function neurons that were infected with both viral strains. These somatomotor-sympathetic neurons (SMSNs) were detected in a number of brain regions. However, the most prominent nodes in this circuitry included the paraventricular, dorsomedial, and lateral nuclei of the hypothalamus, ventrolateral periaqueductal grey and ventromedial medulla. Phenotypic studies revealed subsets of SMSNs to be capable of synthesizing serotonin, or to contain neuroactive peptides vasopressin, oxytocin, orexins, or melanin-concentrating hormone. Based on these data and the results of studies employing monosynaptic tracers a central somatomotor-sympathetic circuit is proposed. This circuitry is likely recruited in diverse situations, including stress responses, cold defense, exercise and sleep. Furthermore, activation of specific classes of SMSNs likely shapes distinct stress-coping strategies. Dysregulation in the organization and function of this circuit may also contribute to the expression of physical symptoms of affective disorders, such as major depression, anxiety and panic.

Distinct populations of presympathetic-premotor neurons express orexin or melanin-concentrating hormone in the rat lateral hypothalamus

Orexin and melanin-concentrating hormone (MCH) have been implicated in mediating a variety of different behaviors. These include sleep and wakefulness, locomotion, ingestive behaviors, and fight-or-flight response, as well as anxiety- and panic-like behaviors in rodents. Despite such diversity, all these processes require coordinated recruitment of the autonomic and somatomotor efferents. We have previously mapped the locations of presympathetic-premotor neurons (PSPMNs) in the rat brain. These putative dual-function neurons send trans-synaptic projections to somatomotor and sympathetic targets and likely participate in somatomotor-sympathetic integration. A significant portion of these neurons is found within the dorsomedial (DMH) and lateral hypothalamus (LH), areas of the brain that contain MCH- and orexin- synthesizing neurons in the central nervous system. Thus, we hypothesized that hypothalamic PSPMNs utilize MCH or orexin as their neurotransmitter. To test this hypothesis, we identified PSPMNs by using recombinant strains of the pseudorabies virus (PRV) for trans-synaptic tract tracing. PRV-152, a strain that expresses enhanced green fluorescent protein, was injected into sympathectomized gastrocnemius muscle, whereas PRV-BaBlu, which expresses beta-galactosidase, was injected into the adrenal gland in the same animals. By using immunofluorescent methods, we determined whether co-infected neurons express MCH or orexin. Our findings demonstrate that PSPMNs synthesizing either MCH or orexin are present within LH, where they form two separate populations. PSPMNs located around the fornix express orexin, whereas those located around the cerebral peduncle are more likely to express MCH. These two clusters of PSPMNs within LH likely play distinct functional roles in autonomic homeostasis and stress coping mechanisms.

Characterization of brainstem peptide YY (PYY) neurons

Peptide YY (PYY), a member of the NPY superfamily of peptides, is predominantly synthesized by the colon and is thought to act on both the gut and brain to modulate energy homeostasis. Although neurons expressing PYY mRNA have also been reported in the brainstem, little is known about their physiological role and study of their projections has been problematic due to crossreactivity of PYY antibodies with NPY. In the present study we examined the localization of central PYY cell bodies in the mouse, rat, and monkey. In addition, efferent projections and afferent inputs of central PYY neurons were examined in rodents. Central PYY projections were examined by immunohistochemistry in the NPY knockout mouse, or with an NPY-preabsorbed PYY antibody in the rat to avoid any crossreactivity with NPY. In all species investigated PYY-immunoreactive (ir) cell bodies were localized exclusively to the gigantocellular reticular nucleus (Gi) of the rostral medulla. The highest density of PYY fibers was present within the solitary tract nucleus, specifically within the dorsal and lateral aspects. PYY fibers were also concentrated within the dorsal motor nucleus of the vagus and the hypoglossal nucleus. In addition, both orexin and melanin-concentrating hormone fibers made numerous close appositions with PYY cell bodies in the Gi. Collectively, the projection pattern and association with orexigenic neuropeptides suggest that brainstem PYY neurons may play a role in energy homeostasis through a coordinated effect on visceral, motor, and sympathetic output targets.

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.

Microinjections of melanin concentrating hormone into the nucleus tractus solitarius of the rat elicit depressor and bradycardic responses

The presence of melanin-concentrating hormone (MCH) containing processes, projecting from the lateral hypothalamus to the medial nucleus tractus solitarius (mNTS), has been reported in the rat. It was hypothesized that MCH acting within the mNTS may modulate the central regulation of cardiovascular function. This hypothesis was tested in urethane-anesthetized, artificially ventilated, adult male Wistar rats. Microinjections (100 nl) of MCH (0.25, 0.5, 0.75, and 1 mM) into the mNTS of anesthetized rats elicited decreases in mean arterial pressure (20.4+/-1.6, 50.7+/-3.3, 35.7+/-2.8 and 30.0+/-2.6 mm Hg, respectively). The decreases in heart rate in response to these concentrations of MCH were 40.0+/-8.7, 90.0+/-13.0, 48.0+/-7.3 and 48.0+/-8.0 beats/min, respectively. Maximum cardiovascular responses were elicited by a 0.5 mM concentration of MCH. Cardiovascular responses to MCH were similar in unanesthetized mid-collicular decerebrate rats. Control microinjections of normal saline (100 nl) did not elicit any cardiovascular response. Ipsilateral or bilateral vagotomy significantly attenuated MCH-induced bradycardia. Prior microinjections of PMC-3881-PI (2 mM; MCH-1 receptor antagonist) into the mNTS blocked the cardiovascular responses to microinjections of MCH. Microinjection of MCH (0.5 mM) into the mNTS decreased efferent greater splanchnic nerve activity. Direct application of MCH (0.5 mM; 4 nl) to barosensitive nucleus tractus solitarius (NTS) neurons increased their firing rate. These results indicate that: 1) MCH microinjections into the mNTS activate MCH-1 receptors and excite barosensitive NTS neurons, causing a decrease in efferent sympathetic activity and blood pressure, and 2) MCH-induced bradycardia is mediated via the activation of the vagus nerves.