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

Reversed synaptic effects of hypocretin and NPY mediated by excitatory GABA-dependent synaptic activity in developing MCH neurons

In mature neurons, GABA is the primary inhibitory neurotransmitter. In contrast, in developing neurons, GABA exerts excitatory actions, and in some neurons GABA-mediated excitatory synaptic activity is more prevalent than glutamate-mediated excitation. Hypothalamic neuropeptides that modulate cognitive arousal and energy homeostasis, hypocretin/orexin and neuropeptide Y (NPY), evoked reversed effects on synaptic actions that were dependent on presynaptic GABA release onto melanin-concentrating hormone (MCH) neurons. MCH neurons were identified by selective green fluorescent protein (GFP) expression in transgenic mice. In adults, hypocretin increased GABA release leading to reduced excitation. In contrast, in the developing brain as studied here with analysis of miniature excitatory postsynaptic currents, paired-pulse ratios, and evoked potentials, hypocretin acted presynaptically to enhance the excitatory actions of GABA. The ability of hypocretin to enhance GABA release increases inhibition in adult neurons but paradoxically enhances excitation in developing MCH neurons. In contrast, NPY attenuation of GABA release reduced inhibition in mature neurons but enhanced inhibition during development by attenuating GABA excitation. Both hypocretin and NPY also evoked direct actions on developing MCH neurons. Hypocretin excited MCH cells by activating a sodium-calcium exchanger and by reducing potassium currents; NPY reduced activity by increasing an inwardly rectifying potassium current. These data for the first time show that both hypocretin and NPY receptors are functional presynaptically during early postnatal hypothalamic development and that both neuropeptides modulate GABA actions during development with a valence of enhanced excitation or inhibition opposite to that of the adult state, potentially allowing neuropeptide modulation of use-dependent synapse stabilization.

Projections from melanin-concentrating hormone (MCH) neurons to the dorsal raphe or the nuclear core of the locus coeruleus in the rat

Brainstem aminergic and cholinergic nuclei are essential components of reticular activating system which are under the control of hypothalamic sleep/arousal centers. In contrast to well-known role of hypocretin (Hcrt) as a potent wake-promoting substance, only recent reports stated that melanin-concentrating hormone (MCH) plays a role in the maintenance of rapid eye movement (REM) sleep. As the sequel to our report concerning the MCH/Hcrt projection to the brainstem cholinergic nuclei (Hong et al., 2011), in the present study we examined the differential projection from MCH/Hcrt neurons in medial and lateral subdivisions of the lateral hypothalamus (LH) to the dorsal raphe (DR) or the nuclear core of the locus coeruleus (LC) of the rat. Following the injection of Red Retrobeads into the LC core (n=6), the proportions of retrogradely labeled (retro-) MCH neurons over the total retro-cells were 4.4% ± 0.5% (medial subdivision) and 7.4% ±0 .6% (lateral one), whereas those of retro-Hcrt cells over the total retro-cells were 69.4% ± 3.6% (medial) and 64.4% ± 5.2% (lateral). Following midline-DR injections (n=6), the proportions of retro-MCH neurons over the total retro-cells were 14.3% ± 2.9% (medial) and 12.3% ± 1.6% (lateral), while those of retro-Hcrt cells over the total retro-cells were 46.5% ± 6.2% (medial) and 51.3% ± 9.5% (lateral). Following lateral wing-DR injections (n=3), the proportions of retro-MCH neurons over the total retro-cells were 15.5% ± 1.2% (medial) and 11.9% ± 3.1% (lateral), while those of retro-Hcrt cells over the total retro-cells were 48.5% ± 2.7% (medial) and 52.8% ± 2.3% (lateral). The statistical analysis showed that MCH neurons projecting to the LC core or DR were outnumbered by Hcrt cells (P<0.01) and that retro-MCH cells exhibited lateral predominance in LC injection cases (P<0.05). Based on our present as well as previous (Hong et al., 2011) observations, we suggested that MCH and Hcrt neurons in the LH provide preferential projections to the brainstem cholinergic and aminergic nuclei, respectively and that MCH projections to the nuclear core of the LC exhibit differential distribution within LH subdivisions.

Immunohistochemical evidence for synaptic release of GABA from melanin-concentrating hormone containing varicosities in the locus coeruleus

Melanin-concentrating hormone (MCH) is synthesized by neurons located in the hypothalamus and projecting to widespread regions of the brain, including the locus coeruleus (LC), through which MCH could modulate sleep-wake states. Yet MCH does not appear to exert direct postsynaptic effects on target neurons, including the noradrenergic LC neurons. Previous studies using in situ hybridization showed that MCH neurons synthesize glutamic acid decarboxylase (GAD) and could thus utilize GABA as a neurotransmitter. To determine whether MCH varicosities can release GABA, we examined by fluorescent microscopy in the LC, whether their terminals also contain the vesicular transporter for GABA (VGAT). In dual-immunostained sections, we found that approximately 6% of MCH varicosities was immunopositive for VGAT and a similar proportion for synaptophysin, the presynaptic marker for small synaptic vesicles, whereas <1% was positive for the vesicular glutamate transporter (VGluT2). Moreover, of the MCH varicosities, ∼5% abutted puncta that were immunostained for gephyrin, the postsynaptic marker for GABAergic synapses. In triple-immunostained sections viewed with confocal laser scanning microscopy, we established that MCH varicosities that also contained VGAT or abutted upon gephyrin puncta contacted the tyrosine hydroxylase-immunostained neurons of the LC. Our results suggest that although MCH neurons can influence noradrenergic LC neurons through paracrine release and indirect effects of their peptide, they can also do so through synaptic release and direct postsynaptic effects of GABA and thus serve to inhibit the LC neurons during sleep, when they are silent, and the MCH neurons discharge.

Glutamatergic input to the lateral hypothalamus stimulates ethanol intake: role of orexin and melanin-concentrating hormone

BACKGROUND

Glutamate (GLUT) in the lateral hypothalamus (LH) has been suggested to mediate reward behaviors and may promote the ingestion of drugs of abuse. This study tested the hypothesis that GLUT in the LH stimulates consumption of ethanol ( EtOH ) and that this effect occurs, in part, via its interaction with local peptides, hypocretin/orexin (OX), and melanin-concentrating hormone (MCH).

METHODS

In Experiments 1 and 2, male Sprague-Dawley rats, after being trained to drink 9% EtOH , were microinjected in the LH with N-methyl-d-aspartate (NMDA) or its antagonist, D-AP5, or with alpha-amino-5-methyl-3-hydroxy-4-isoxazole propionic acid (AMPA) or its antagonist, CNQX-ds. Consumption of EtOH , chow, and water was then measured. To provide an anatomical control, a separate set of rats was injected 2 mm dorsal to the LH. In Experiment 3, the effect of LH injection of NMDA and AMPA on the expression of OX and MCH was measured using radiolabeled in situ hybridization (ISH) and also using digoxigenin-labeled ISH, to distinguish effects on OX and MCH cells in the LH and the nearby perifornical area (PF) and zona incerta (ZI).

RESULTS

When injected into the LH, NMDA and AMPA both significantly increased EtOH intake while having no effect on chow or water intake. The GLUT receptor antagonists had the opposite effect, significantly reducing EtOH consumption. No effects were observed with injections 2 mm dorsal to the LH. In addition to these behavioral effects, LH injection of NMDA significantly stimulated expression of OX in both the LH and PF while reducing MCH in the ZI, whereas AMPA increased OX only in the LH and had no effect on MCH.

CONCLUSIONS

Glutamatergic inputs to the LH, acting through NMDA and AMPA receptors, appear to have a stimulatory effect on EtOH consumption, mediated in part by increased OX in LH and PF and reduced MCH in ZI.

Control of sleep and wakefulness

This review summarizes the brain mechanisms controlling sleep and wakefulness. Wakefulness promoting systems cause low-voltage, fast activity in the electroencephalogram (EEG). Multiple interacting neurotransmitter systems in the brain stem, hypothalamus, and basal forebrain converge onto common effector systems in the thalamus and cortex. Sleep results from the inhibition of wake-promoting systems by homeostatic sleep factors such as adenosine and nitric oxide and GABAergic neurons in the preoptic area of the hypothalamus, resulting in large-amplitude, slow EEG oscillations. Local, activity-dependent factors modulate the amplitude and frequency of cortical slow oscillations. Non-rapid-eye-movement (NREM) sleep results in conservation of brain energy and facilitates memory consolidation through the modulation of synaptic weights. Rapid-eye-movement (REM) sleep results from the interaction of brain stem cholinergic, aminergic, and GABAergic neurons which control the activity of glutamatergic reticular formation neurons leading to REM sleep phenomena such as muscle atonia, REMs, dreaming, and cortical activation. Strong activation of limbic regions during REM sleep suggests a role in regulation of emotion. Genetic studies suggest that brain mechanisms controlling waking and NREM sleep are strongly conserved throughout evolution, underscoring their enormous importance for brain function. Sleep disruption interferes with the normal restorative functions of NREM and REM sleep, resulting in disruptions of breathing and cardiovascular function, changes in emotional reactivity, and cognitive impairments in attention, memory, and decision making.

Transitions between sleep and feeding states in rat ventral striatum neurons

Neurons in the nucleus accumbens (NAc) have been shown to participate in several behavioral states, including feeding and sleep. However, it is not known if the same neuron participates in both states and, if so, how similar are the responses. In addition, since the NAc contains several cell types, it is not known if each type participates in the transitions associated with feeding and sleep. Such knowledge is important for understanding the interaction between two different neural networks. For these reasons we recorded ensembles of NAc neurons while individual rats volitionally transitioned between the following states: awake and goal directed, feeding, quiet-awake, and sleeping. We found that during both feeding and sleep states, the same neurons could increase their activity (be activated) or decrease their activity (be inactivated) by feeding and/or during sleep, thus indicating that the vast majority of NAc neurons integrate sleep and feeding signals arising from spatially distinct neural networks. In contrast, a smaller population was modulated by only one of the states. For the majority of neurons in either state, we found that when one population was excited, the other was inhibited, suggesting that they act as a local circuit. Classification of neurons into putative interneurons [fast-spiking interneurons (pFSI) and choline acetyltransferase interneurons (pChAT)] and projection medium spiny neurons (pMSN) showed that all three types are modulated by transitions to and from feeding and sleep states. These results show, for the first time, that in the NAc, those putative inhibitory interneurons respond similarly to pMSN projection neurons and demonstrate interactions between NAc networks involved in sleep and feeding.

Shining light on wakefulness and arousal

Alterations in arousal states are associated with multiple neuropsychiatric disorders, including generalized anxiety disorders, addiction, schizophrenia, and depression. Therefore, elucidating the neurobiological mechanisms controlling the boundaries between arousal, hyperarousal, and hypoarousal is a crucial endeavor in biological psychiatry. Substantial research over several decades has identified distinct arousal-promoting neural populations in the brain; however, how these nuclei act individually and collectively to promote and maintain wakefulness and various arousal states is unknown. We have recently applied optogenetic technology to the repertoire of techniques used to study arousal. Here, we discuss the recent results of these experiments and propose future use of this approach as a way to understand the complex dynamics of neural circuits controlling arousal and arousal-related behaviors.

Interleukin-6 receptor α is co-localised with melanin-concentrating hormone in human and mouse hypothalamus

Interleukin (IL)-6 deficient mice develop mature-onset obesity. Furthermore, i.c.v. administration of IL-6 increases energy expenditure, suggesting that IL-6 centrally regulates energy homeostasis. To investigate whether it would be possible for IL-6 to directly influence the energy homeostasis via hypothalamic regulation in humans and rodents, we mapped the distribution of the ligand binding IL-6 receptor α (IL-6Rα) in this brain region. In the human hypothalamus, IL-6Rα-immunoreactivity was detected in perikarya and first-order dendrites of neurones. The IL-6Rα-immunoreactive (-IR) neurones were observed posterior to the level of the interventricular foramen. There, IL-6Rα-IR neurones were located in the lateral hypothalamic, perifornical, dorsal and posterior hypothalamic areas, the hypothalamic dorsomedial nucleus and in the zona incerta. In the caudal part of the hypothalamus, the density of the IL-6Rα-IR neurones gradually increased. Double-labelling immunofluorescent studies demonstrated that IL-6Rα immunoreactivity was localised in the same neurones as the orexigenic neuropeptide, melanin-concentrating hormone (MCH). By contrast, IL-6Rα-immunoreactivity was not observed in the orexin B-IR neurones. To determine whether the observed expression of IL-6Rα is evolutionary conserved, we studied the co-localisation of IL-6Rα with MCH and orexin in the mouse hypothalamus, where IL-6Rα-immunoreactivity was present in numerous MCH-IR and orexin-IR neurones. Our data demonstrate that the MCH neurones of the human hypothalamus, as well as the MCH and orexin neurones of the mouse hypothalamus, contain IL-6Rα. This opens up the possibility that IL-6 influences the energy balance through the MCH neurones in humans, and both MCH and orexin neurones in mice.

Microinjection of the melanin-concentrating hormone into the lateral basal forebrain increases REM sleep and reduces wakefulness in the rat

AIMS

To examine the effects of bilateral microinjection of melanin-concentrating hormone (MCH) 50 and 100 ng into the horizontal limb of the diagonal band of Broca (HDB) on sleep variables during the light phase of the light-dark cycle of the rat.

MAIN METHODS

Male Wistar rats were implanted for chronic sleep recordings. In addition, a guide cannula was implanted above the right and left HDB. Following the microinjection of MCH or control solution the electroencephalogram and the electromyogram were recorded for 6 h. Data was collected and classified as either wakefulness (W), light sleep, slow wave sleep (SWS) or REM sleep (REMS). Latencies for SWS and REMS, as well as the number of REM periods and the mean duration of REM episodes were also determined.

KEY FINDINGS

MCH 50 and 100 ng significantly decreased W during the first 2-h of recording. Moreover, MCH 100 ng significantly reduced REMS latency and increased REMS time during the first 2-h block of the recording, due to an increase in the number of REM periods.

SIGNIFICANCE

Our findings tend to suggest that the basal forebrain participates in the effects of MCH on W and REMS through the deactivation of cholinergic, glutamatergic and γ-aminobutyric acid (GABA)-ergic cells.

Thyrotropin-releasing hormone (TRH) inhibits melanin-concentrating hormone neurons: implications for TRH-mediated anorexic and arousal actions

Thyrotropin-releasing hormone (TRH) increases activity and decreases food intake, body weight, and sleep, in part through hypothalamic actions. The mechanism of this action is unknown. Melanin-concentrating hormone (MCH) and hypocretin/orexin neurons in the lateral hypothalamus (LH) together with neuropeptide Y (NPY) and proopiomelanocortin (POMC) neurons in the arcuate nucleus play central roles in energy homeostasis. Here, we provide electrophysiological evidence from GFP-reporter transgenic mouse brain slices that shows TRH modulates the activity of MCH neurons. Using whole-cell current-clamp recording, we unexpectedly found that TRH and its agonist, montrelin, dose-dependently inhibited MCH neurons. Consistent with previous reports, TRH excited hypocretin/orexin neurons. No effect was observed on arcuate nucleus POMC or NPY neurons. The TRH inhibition of MCH neurons was eliminated by bicuculline and tetrodotoxin, suggesting that the effect was mediated indirectly through synaptic mechanisms. TRH increased spontaneous IPSC frequency without affecting amplitude and had no effect on miniature IPSCs or EPSCs. Immunocytochemistry revealed little interaction between TRH axons and MCH neurons, but showed TRH axons terminating on or near GABA neurons. TRH inhibition of MCH neurons was attenuated by Na(+)-Ca(2+) exchanger (NCX) inhibitors, TRPC channel blockers and the phospholipase C inhibitor U-73122. TRH excited LH GABA neurons, and this was also reduced by NCX inhibitors. Finally, TRH attenuated the excitation of MCH neurons by hypocretin. Together, our data suggest that TRH inhibits MCH neurons by increasing synaptic inhibition from local GABA neurons. Inhibition of MCH neurons may contribute to the TRH-mediated reduction in food intake and sleep.

Role of the lateral paragigantocellular nucleus in the network of paradoxical (REM) sleep: an electrophysiological and anatomical study in the rat

The lateral paragigantocellular nucleus (LPGi) is located in the ventrolateral medulla and is known as a sympathoexcitatory area involved in the control of blood pressure. In recent experiments, we showed that the LPGi contains a large number of neurons activated during PS hypersomnia following a selective deprivation. Among these neurons, more than two-thirds are GABAergic and more than one fourth send efferent fibers to the wake-active locus coeruleus nucleus. To get more insight into the role of the LPGi in PS regulation, we combined an electrophysiological and anatomical approach in the rat, using extracellular recordings in the head-restrained model and injections of tracers followed by the immunohistochemical detection of Fos in control, PS-deprived and PS-recovery animals. With the head-restrained preparation, we showed that the LPGi contains neurons specifically active during PS (PS-On neurons), neurons inactive during PS (PS-Off neurons) and neurons indifferent to the sleep-waking cycle. After injection of CTb in the facial nucleus, the neurons of which are hyperpolarized during PS, the largest population of Fos/CTb neurons visualized in the medulla in the PS-recovery condition was observed in the LPGi. After injection of CTb in the LPGi itself and PS-recovery, the nucleus containing the highest number of Fos/CTb neurons, moreover bilaterally, was the sublaterodorsal nucleus (SLD). The SLD is known as the pontine executive PS area and triggers PS through glutamatergic neurons. We propose that, during PS, the LPGi is strongly excited by the SLD and hyperpolarizes the motoneurons of the facial nucleus in addition to local and locus coeruleus PS-Off neurons, and by this means contributes to PS genesis.

Orexins, feeding, and energy balance

In this chapter, we give an overview of the current status of the role of orexins in feeding and energy homeostasis. Orexins, also known as hypocretins, initially were discovered in 1998 as hypothalamic regulators of food intake. A little later, their far more important function as regulators of sleep and arousal came to light. Despite their restricted distribution, orexin neurons have projections throughout the entire brain, with dense projections especially to the paraventricular nucleus of the thalamus, the arcuate nucleus of the hypothalamus, and the locus coeruleus and tuberomammillary nucleus. Its two receptors are orexin receptor 1 and orexin receptor 2. These receptors show a specific and localized distribution in a number of brain regions, and a variety of different actions has been demonstrated upon their binding. Our group showed that through the autonomic nervous system, the orexin system plays a key role in the control of glucose metabolism, but it has also been shown to stimulate sympathetic outflow, to increase body temperature, heart rate, blood pressure, and renal sympathetic nerve activity. The well-known effects of orexin on the control of food intake, arousal, and wakefulness appear to be more extensive than originally thought, with additional effects on the autonomic nervous system, that is, to increase body temperature and energy metabolism.

Development of posterior hypothalamic neurons enlightens a switch in the prosencephalic basic plan

In rats and mice, ascending and descending axons from neurons producing melanin-concentrating hormone (MCH) reach the cerebral cortex and spinal cord. However, these ascending and descending projections originate from distinct sub-populations expressing or not “Cocaine-and-Amphetamine-Regulated-Transcript” (CART) peptide. Using a BrdU approach, MCH cell bodies are among the very first generated in the hypothalamus, within a longitudinal cell cord made of earliest delaminating neuroblasts in the diencephalon and extending from the chiasmatic region to the ventral midbrain. This region also specifically expresses the regulatory genes Sonic hedgehog (Shh) and Nkx2.2. First MCH axons run through the tractus postopticus (tpoc) which gathers pioneer axons from the cell cord and courses parallel to the Shh/Nkx2.2 expression domain. Subsequently generated MCH neurons and ascending MCH axons differentiate while neurogenesis and mantle layer differentiation are generalized in the prosencephalon, including telencephalon. Ascending MCH axons follow dopaminergic axons of the mesotelencephalic tract, both being an initial component of the medial forebrain bundle (mfb). Netrin1 and Slit2 proteins that are involved in the establishment of the tpoc and mfb, respectively attract or repulse MCH axons.

We conclude that first generated MCH neurons develop in a diencephalic segment of a longitudinal Shh/Nkx2.2 domain. This region can be seen as a prosencephalic segment of a medial neurogenic column extending from the chiasmatic region through the ventral neural tube. However, as the telencephalon expends, it exerts a trophic action and the mfb expands, inducing a switch in the longitudinal axial organization of the prosencephalon.

Differential distribution of melanin-concentrating hormone (MCH)- and hypocretin (Hcrt)-immunoreactive neurons projecting to the mesopontine cholinergic complex in the rat

Hypocretin (Hcrt or orexin) and melanin-concentrating hormone (MCH) containing neurons are located in the hypothalamus and are implicated in the regulation of feeding behavior, energy homeostasis, and sleep-wake cycle. MCH and Hcrt are not co-localized within the same neuron, but these neurons project widely throughout the brain, especially to brain regions regulating arousal. Recent data indicate that HCRT and MCH neurons located medially with respect to the fornix have a differential projection pattern compared to those located lateral to the fornix. To further elucidate the projection of these neurons in the present study we use retrograde tracing methods combined with double immunofluorescence to determine the differential distribution of Hcrt- and MCH-immunoreactive neurons projecting to the pedunculopontine tegmental (PPTg) or laterodorsal tegmental (LDTg) nuclei. In rats where the retrograde tracer was confined to the PPTg/LDTg we found that there were more MCH neurons projecting to these targets compared to HCRT neurons (P<0.01). When the retrograde tracer was confined to the PPTg, there were more retrogradely labeled MCH neurons lateral to the fornix compared to MCH neurons in the medial LH subdivision (P<0.05). On the average, only about 4.5% of MCH neurons versus 6.1% of HCRT neurons project to PPTg/LDTg. Thus, very few of the MCH or HCRT neurons project to these arousal populations. Although there were significantly more MCH neurons projecting to the mesopontine cholinergic arousal zone compared to the HCRT neurons, the HCRT neurons also exert an indirect influence via the tuberomammillary nucleus. Based on the present and previous (Hong and Lee, 2011) observations, we suggest that both MCH and HCRT neurons exert a potent influence on the PPTg/LDTg, which might play an important role in arousal.

c-Fos expression in neurons projecting from the preoptic and lateral hypothalamic areas to the ventrolateral periaqueductal gray in relation to sleep states

The ventrolateral division of the periaqueductal gray (vlPAG) and the adjacent deep mesencephalic reticular nucleus have been implicated in the control of sleep. The preoptic hypothalamus, which contains populations of sleep-active neurons, is an important source of afferents to the vlPAG. The perifornical lateral hypothalamus (LH) contains populations of wake-active neurons and also projects strongly to the vlPAG. We examined nonREM and REM sleep-dependent expression of c-Fos protein in preoptic-vlPAG and LH-vlPAG projection neurons identified by retrograde labeling with Fluorogold (FG). Separate groups of rats (n=5) were subjected to 3 h total sleep deprivation (TSD) followed by 1 h recovery sleep (RS), or to 3 h of selective REM sleep deprivation (RSD) followed by RS. A third group of rats (n=5) was subjected to TSD without opportunity for RS (awake group). In the median preoptic nucleus (MnPN), the percentage of FG+ neurons that were also Fos+ was higher in TSD-RS animals compared to both RSD-RS rats and awake rats. There were significant correlations between time spent in deep nonREM sleep during the 1 h prior to sacrifice across groups and the percentage of double-labeled cells in MnPN and ventrolateral preoptic area (VLPO). There were no significant correlations between percentage of double-labeled neurons and time spent in REM sleep for any of the preoptic nuclei examined. In the LH, percentage of double-labeled neurons was highest in awake rats, intermediate in TSD-RS rats and lowest in the RSD-RS group. These results suggest that neurons projecting from MnPN and VLPO to the vlPAG are activated during nonREM sleep and support the hypothesis that preoptic neurons provide inhibitory input to vlPAG during sleep. Suppression of excitatory input to the vlPAG from the LH during sleep may have a permissive effect on REM sleep generation.

Orexin gene transfer into zona incerta neurons suppresses muscle paralysis in narcoleptic mice

Cataplexy, a sudden unexpected muscle paralysis, is a debilitating symptom of the neurodegenerative sleep disorder, narcolepsy. During these attacks, the person is paralyzed, but fully conscious and aware of their surroundings. To identify potential neurons that might serve as surrogate orexin neurons to suppress such attacks, the gene for orexin (hypocretin), a peptide lost in most human narcoleptics, was delivered into the brains of the orexin-ataxin-3 transgenic mouse model of human narcolepsy. Three weeks after the recombinant adenoassociated virus (rAAV)-mediated orexin gene transfer, sleep-wake behavior was assessed. rAAV-orexin gene delivery into neurons of the zona incerta (ZI), or the lateral hypothalamus (LH) blocked cataplexy. Orexin gene transfer into the striatum or in the melanin-concentrating hormone neurons in the ZI or LH had no such effect, indicating site specificity. In transgenic mice lacking orexin neurons but given rAAV-orexin, detectable levels of orexin-A were evident in the CSF, indicating release of the peptide from the surrogate neurons. Retrograde tracer studies showed that the amygdala innervates the ZI consistent with evidence that strong emotions trigger cataplexy. In turn, the ZI projects to the locus ceruleus, indicating that the ZI is part of a circuit that stabilizes motor tone. Our results indicate that these neurons might also be recruited to block the muscle paralysis in narcolepsy.

Association between the activation of MCH and orexin immunorective neurons and REM sleep architecture during REM rebound after a three day long REM deprivation

Rapid eye movement (REM) sleep rebound following REM deprivation using the platform-on-water method is characterized by increased time spent in REM sleep and activation of melanin-concentrating hormone (MCH) expressing neurons. Orexinergic neurons discharge reciprocally to MCH-ergic neurons across the sleep-wake cycle. However, the relation between REM architecture and the aforementioned neuropeptides remained unclear. MCH-ergic neurons can be divided into two subpopulations regarding their cocaine- and amphetamine-regulated transcript (CART) immunoreactivity, and among them the activation of CART-immunoreactive subpopulation is higher during the REM rebound. However, the possible role of stress in this association has not been elucidated. Our aims were to analyze the relationship between the architecture of REM rebound and the activation of hypothalamic MCH-ergic and orexinergic neurons. We also intended to separate the effect of stress and REM deprivation on the subsequent activation of subpopulations of MCH-ergic neurons. In order to detect neuronal activity, we performed MCH/cFos and orexin/cFos double immunohistochemistry on home cage, sleep deprived and sleep-rebound rats using the platform-on-water method with small and large (stress control) platforms. Furthermore, REM architecture was analyzed and a triple MCH/CART/cFos immunohistochemistry was also performed on the rebound groups in the same animals. We found that the activity of MCH- and orexin-immunoreactive neurons during REM rebound was positively and negatively correlated with the number of REM bouts, respectively. A negative reciprocal correlation was also found between the activation of MCH- and orexin-immunoreactive neurons during REM rebound. Furthermore, difference between the activation of CART-immunoreactive (CART-IR) and non-CART-immunoreactive MCH-ergic neuron subpopulations was found only after selective REM deprivation, it was absent in the large platform (stress control) rebound group. These results support the role of CART-IR subpopulation of MCH-ergic neurons and the inverse relationship of MCH and orexin in the regulation of REM sleep after REM sleep deprivation.

Local network regulation of orexin neurons in the lateral hypothalamus

Obesity and inadequate sleep are among the most common causes of health problems in modern society. Thus, the discovery that orexin (hypocretin) neurons play a pivotal role in sleep/wake regulation, energy balance, and consummatory behaviors has sparked immense interest in understanding the regulatory mechanisms of these neurons. The local network consisting of neurons and astrocytes within the lateral hypothalamus and perifornical area (LH/PFA), where orexin neurons reside, shapes the output of orexin neurons and the LH/PFA. Orexin neurons not only send projections to remote brain areas but also contribute to the local network where they release multiple neurotransmitters to modulate its activity. These neurotransmitters have opposing actions, whose balance is determined by the amount released and postsynaptic receptor desensitization. Modulation and negative feedback regulation of excitatory glutamatergic inputs as well as release of astrocyte-derived factors, such as lactate and ATP, can also affect the excitability of orexin neurons. Furthermore, distinct populations of LH/PFA neurons express neurotransmitters with known electrophysiological actions on orexin neurons, such as melanin-concentrating hormone, corticotropin-releasing factor, thyrotropin-releasing hormone, neurotensin, and GABA. These LH/PFA-specific mechanisms may be important for fine tuning the firing activity of orexin neurons to maintain optimal levels of prolonged output to sustain wakefulness and stimulate consummatory behaviors. Building on these exciting findings should shed further light onto the cellular mechanisms of energy balance and sleep-wake regulation.

Relationship between clinical characteristics of narcolepsy and CSF orexin-A levels

Although an abnormally low cerebrospinal fluid (CSF)-orexin level is well known to be a specific finding in narcoleptic patients, the relationships between the severity of the core symptoms of narcolepsy [i.e. daytime sleepiness and increased rapid eye movement (REM) propensity], as well as levels of obesity, and CSF-orexin levels have not been well elucidated. The aim of this study was to examine the relationship between these characteristic symptoms of narcolepsy and CSF-orexin level. Fifty-three patients with narcolepsy with cataplexy (NA/CA) and 17 without cataplexy (NA w/o CA) were enrolled. Sleep latency and sleep onset REM latency were measured using the multiple sleep latency test (MSLT). Multiple linear regression analysis was used to determine factors associated with both mean sleep latency and mean sleep onset REM latency on MSLT, with %body mass index (BMI), gender, onset age, length of excessive daytime sleepiness (EDS) morbidity, diagnostic subgroup and CSF-orexin levels being used as independent variables. The NA/CA group included a significantly higher number of patients with undetectable CSF-orexin levels and shorter sleep onset and rapid eye movement (SOREM) latency, as well as a higher %BMI, versus NA w/o CA. Multiple linear regression analysis revealed that sleep latency was associated significantly with CSF-orexin levels and gender. With regard to sleep onset REM latency and %BMI, only CSF-orexin levels appeared to be a significantly associated factor. In narcoleptic patients, the severity of both excessive daytime sleepiness and increased REM propensity, as well as the level of obesity, could be associated with CSF-orexin deficiency, rather than with subcategories of the disorder.

Similarities in hypothalamic and mesocorticolimbic circuits regulating the overconsumption of food and alcohol

Historically, studies of food intake regulation started with the hypothalamus and gradually expanded to mesocorticolimbic regions, while studies of drug use began with mesocorticolimbic regions and now include the hypothalamus. As research on ingestive behavior has progressed, it has uncovered more and more similarities between the regulation of palatable food and drug intake. It has also identified specific neurochemicals involved in palatable food and drug intake. Hypothalamic orexigenic neurochemicals specifically involved in controlling fat ingestion, including galanin, enkephalin, orexin and melanin-concentrating hormone, show positive feedback with this macronutrient, with these peptides both increasing fat intake and being further stimulated by its intake. This positive relationship offers some explanation for why foods high in fat are so often overconsumed. Research in Bart Hoebel's laboratory in conjunction with our own has shown that consumption of ethanol, a drug of abuse that also contains calories, is similarly driven by these neurochemical systems involved in fat intake, consistent with evidence closely relating fat and ethanol consumption. Both fat and ethanol intake are also regulated by dopamine and acetylcholine acting in mesocorticolimbic nuclei. This close relationship of fat and ethanol is likely driven in part by circulating lipids, which are increased by fat and ethanol intake, known to increase expression and levels of the neurochemicals, and found to promote further intake of fat and ethanol. Compellingly, recent studies suggest that these systems may already be dysregulated in animals prone to consuming excess fat or ethanol, even before they have ever been exposed to these substances. Further understanding of these systems involved in consummatory behavior will allow researchers to develop effective therapies for the treatment of overeating as well as drug abuse.

Activation of orexin neurons in dorsomedial/perifornical hypothalamus and antidepressant reversal in a rodent model of depression

Chronic stressful life events are risk factors for depression often accompanied by homeostatic disturbances. Hypothalamic neuropeptides, such as orexins (OXs) and melanin-concentrating hormone (MCH), are involved in regulation of several autonomic functions that are altered in depression. However, little is known about the link between orexinergic or MCH-ergic systems and depression. Using double immunohistochemical labeling for OX- or MCH-containing neurons and Fos protein, we studied the effects of a chronic selective serotonin reuptake inhibitor antidepressant treatment (fluoxetine) on the OX and MCH neuronal activation in mice exposed to unpredictable chronic mild stress (UCMS), a rodent model of depression. Western blot was also performed to assess OX and MCH receptor expression in various brain areas. Finally, almorexant, a dual OX receptor antagonist, was assessed in the tail suspension test. UCMS induced physical and behavioral disturbances in mice reversed by 6-week fluoxetine treatment. Orexinergic neurons were more activated in the dorsomedial and perifornical hypothalamic area (DMH-PFA) of UCMS-subjected mice compared to the lateral hypothalamus (LH), and this increase was reversed by 6-week fluoxetine treatment. UCMS also reduced expression of OX-receptor 2 in the thalamus and hypothalamus, but not in animals chronically treated with fluoxetine. MCH neurons were neither affected by UCMS nor by antidepressant treatment, while UCMS modulated MCH receptor 1 expression in thalamus and hippocampus. Finally, chronic but not acute administration of almorexant, induced antidepressant-like effect in the tail suspension test. These data suggest that OX neurons in the DMH-PFA and MCH-ergic system may contribute to the pathophysiology of depressive disorders.

Nitric oxide production in the perifornical-lateral hypothalamic area and its influences on the modulation of perifornical-lateral hypothalamic area neurons

The perifornical-lateral hypothalamic area (PF-LHA) is a major wake-promoting structure. It predominantly contains neurons that are active during behavioral and cortical activation. PF-LHA stimulation produces arousal and PF-LHA lesions produce somnolence. Nitric oxide (NO) is a gaseous neurotransmitter that has been implicated in the regulation of multiple pathological and physiological processes including the regulation of sleep. NO levels are higher in the cortex and in the basal forebrain (BF) during arousal. In this study we determined whether NO levels increase in the PF-LHA during prolonged arousal and whether increased NO modulates the discharge activity of PF-LHA neurons. Experiments were conducted during lights-on phase between 8.00 and 20.00 h (lights-on at 8.00 h). First, we quantified levels of NO metabolites, NO2- and NO3- (collectively called NOx-) in the microdialysis dialysates collected from the PF-LHA during baseline (undisturbed rats), 6 h of sleep deprivation (SD), and recovery after SD. We further determined the effects of a NO donor, NOC-18, on the discharge activity of PF-LHA neurons in urethane-anesthetized rats. Overall, SD significantly affected NOx- production in the PF-LHA (one way repeated measures ANOVA, F=7.827, P=0.004). The levels of NOx- increased progressively in animals that were subjected to prolonged arousal as compared to the undisturbed predominantly sleeping animals and decreased during the recovery period. Local application of NOC-18 significantly suppressed the discharge of PF-LHA neurons including a majority of stimulus-on neurons or neurons exhibiting activation during electroencephalogram (EEG) desynchronization. The findings of this study suggest that in the PF-LHA, NO production is elevated during prolonged waking and that NO exerts predominantly inhibitory effects on PF-LHA neurons, especially on those neurons that are active during cortical activation. These findings are consistent with a hypothesis that NO in the PF-LHA plays a role in sleep regulation by inhibiting its neurons.

Evidence that neurons of the sublaterodorsal tegmental nucleus triggering paradoxical (REM) sleep are glutamatergic

STUDY OBJECTIVES

To determine whether sublaterodorsal tegmental nucleus (SLD) neurons triggering paradoxical (REM) sleep (PS) are glutamatergic.

DESIGN

Three groups of rats were used: controls, rats deprived of PS for 72 h, and rats allowed to recover for 3 h after deprivation. Brain sections were processed for double labeling combining Fos immunohistochemistry and vesicular glutamate transporter 2 (vGLUT2) in situ hybridization.

MEASUREMENTS AND RESULTS

The number of single Fos+ and Fos/vGLUT2+ double-labeled neurons was counted for each experimental condition. A very large number of Fos+ neurons expressing vGLUT2 mRNA specifically after PS hypersomnia was counted in the SLD. These double-labeled cells accounted for 84% of the total number of Fos+ cells.

CONCLUSIONS

This finding adds further evidence to the concept that PS-on neurons of the SLD generating PS are of small size and glutamatergic in nature. By means of their descending projections to medullary and/or spinal glycinergic/GABAergic premotoneurons, they may be especially important for the induction of muscle atonia during PS, a disturbed phenomenon in narcolepsy and REM sleep behavior disorder.

Melanin-concentrating hormone: a new sleep factor?

Neurons containing the neuropeptide melanin-concentrating hormone (MCH) are mainly located in the lateral hypothalamus and the incerto-hypothalamic area, and have widespread projections throughout the brain. While the biological functions of this neuropeptide are exerted in humans through two metabotropic receptors, the MCHR1 and MCHR2, only the MCHR1 is present in rodents. Recently, it has been shown that the MCHergic system is involved in the control of sleep. We can summarize the experimental findings as follows: (1) The areas related to the control of sleep and wakefulness have a high density of MCHergic fibers and receptors. (2) MCHergic neurons are active during sleep, especially during rapid eye movement (REM) sleep. (3) MCH knockout mice have less REM sleep, notably under conditions of negative energy balance. Animals with genetically inactivated MCHR1 also exhibit altered vigilance state architecture and sleep homeostasis. (4) Systemically administered MCHR1 antagonists reduce sleep. (5) Intraventricular microinjection of MCH increases both slow wave sleep (SWS) and REM sleep; however, the increment in REM sleep is more pronounced. (6) Microinjection of MCH into the dorsal raphe nucleus increases REM sleep time. REM seep is inhibited by immunoneutralization of MCH within this nucleus. (7) Microinjection of MCH in the nucleus pontis oralis of the cat enhances REM sleep time and reduces REM sleep latency. All these data strongly suggest that MCH has a potent role in the promotion of sleep. Although both SWS and REM sleep are facilitated by MCH, REM sleep seems to be more sensitive to MCH modulation.

The waking brain: an update

Wakefulness and consciousness depend on perturbation of the cortical soliloquy. Ascending activation of the cerebral cortex is characteristic for both waking and paradoxical (REM) sleep. These evolutionary conserved activating systems build a network in the brainstem, midbrain, and diencephalon that contains the neurotransmitters and neuromodulators glutamate, histamine, acetylcholine, the catecholamines, serotonin, and some neuropeptides orchestrating the different behavioral states. Inhibition of these waking systems by GABAergic neurons allows sleep. Over the past decades, a prominent role became evident for the histaminergic and the orexinergic neurons as a hypothalamic waking center.

Sleep state switching

We take for granted the ability to fall asleep or to snap out of sleep into wakefulness, but these changes in behavioral state require specific switching mechanisms in the brain that allow well-defined state transitions. In this review, we examine the basic circuitry underlying the regulation of sleep and wakefulness and discuss a theoretical framework wherein the interactions between reciprocal neuronal circuits enable relatively rapid and complete state transitions. We also review how homeostatic, circadian, and allostatic drives help regulate sleep state switching and discuss how breakdown of the switching mechanism may contribute to sleep disorders such as narcolepsy.

A comparative analysis shows morphofunctional differences between the rat and mouse melanin-concentrating hormone systems

Sub-populations of neurons producing melanin-concentrating hormone (MCH) are characterized by distinct projection patterns, birthdates and CART/NK3 expression in rat. Evidence for such sub-populations has not been reported in other species. However, given that genetically engineered mouse lines are now commonly used as experimental models, a better characterization of the anatomy and morphofunctionnal organization of MCH system in this species is then necessary. Combining multiple immunohistochemistry experiments with in situ hybridization, tract tracing or BrdU injections, evidence supporting the hypothesis that rat and mouse MCH systems are not identical was obtained: sub-populations of MCH neurons also exist in mouse, but their relative abundance is different. Furthermore, divergences in the distribution of MCH axons were observed, in particular in the ventromedial hypothalamus. These differences suggest that rat and mouse MCH neurons are differentially involved in anatomical networks that control feeding and the sleep/wake cycle.

Immunoneutralization of melanin-concentrating hormone (MCH) in the dorsal raphe nucleus: effects on sleep and wakefulness

Hypothalamic neurons that utilize melanin-concentrating hormone (MCH) as a neuromodulator exert a positive control over energy homeostasis, inducing feeding and decreasing metabolism. Recent studies have shown also that this system plays a role in the generation and/or maintenance of sleep. MCHergic neurons project to the serotonergic dorsal raphe nucleus (DR), a neuroanatomical structure involved in several functions during wakefulness (W), and in the regulation of rapid-eye movements (REM) sleep. Recently, we determined the effect of MCH microinjected into the DR on sleep variables in the rat. MCH produced a marked increment of REM sleep, whereas slow wave sleep (SWS) showed only a moderate increase. In the present study, we analyze the effect of immunoneutralization of MCH in the DR on sleep and W in the rat. Compared to the control solution, microinjections of anti-MCH antibodies (1/100 solution in 0.2 μl) induced a significant increase in REM sleep latency (31.2±7.1 vs. 84.2±24.8 min, p<0.05) and a decrease of REM sleep time (37.8±5.4 vs. 17.8±2.9 min, p<0.05) that was related to the reduction in the number of REM sleep episodes. In addition, there was an increase of total W time (49.8±4.6 vs. 72.0±5.7 min, p<0.01). Light sleep and SWS remained unchanged. The intra-DR administration of a more diluted solution of anti-MCH antibodies (1/500) or rabbit pre-immune serum did not modify neither W nor REM sleep variables. Our findings strongly suggest that MCH released in the DR facilitates the occurrence of REM sleep.

The endogenous actions of hypothalamic peptides on brown adipose tissue thermogenesis in the rat

Although the neuronal pathways within the hypothalamus critical in controlling feeding and energy expenditure and projecting to brown adipose tissue (BAT) have been identified and their peptidergic content characterized, endogenous action of such peptides in the control of BAT activity has not been elucidated. Here male Sprague Dawley rats received infusions of either melanin-concentrating hormone antagonist (SNAP-7941) (1 microg/microl x h), orexin A receptor antagonist (SB-334867-A; 1 microg/microl x h), combined SB-334867-A (1 microg/microl x h), and SNAP-7941 (1 microg/microl x h), or melanocortin-3/4 receptor antagonist (SHU9119) (1 microg/microl x h) via an indwelling cannula in the lateral ventricle attached to s.c. implanted osmotic minipump. BAT temperature, physical activity, body weight, food intake, and changes in uncoupling protein (UCP)-1 were measured. SB-334867-A and SNAP-7941 significantly increased BAT temperature and UCP1 expression and reduced food intake and body weight. Combined infusion of SB-334867-A and SNAP-7941 produced a pronounced response that was greater than the addition of the individual effects in all parameters measured. SHU9119 significantly decreased BAT temperature and UCP1 expression and increased feeding and body weight. In a second series of experiments, the effect of SB-334867-A and SNAP-7941 alone or combination on the expression of the Fos protein was determined. SB-334867-A and SNAP-7941 increased Fos expression in key hypothalamic and brainstem feeding-related regions. In combination, these antagonists produced a greater than additive elevation of Fos expression in most of the regions evaluated. These findings support a role for endogenous orexigenic and anorexigenic hypothalamic peptides acting in concert to create a thermogenic tone via BAT activity.

A very large number of GABAergic neurons are activated in the tuberal hypothalamus during paradoxical (REM) sleep hypersomnia

We recently discovered, using Fos immunostaining, that the tuberal and mammillary hypothalamus contain a massive population of neurons specifically activated during paradoxical sleep (PS) hypersomnia. We further showed that some of the activated neurons of the tuberal hypothalamus express the melanin concentrating hormone (MCH) neuropeptide and that icv injection of MCH induces a strong increase in PS quantity. However, the chemical nature of the majority of the neurons activated during PS had not been characterized. To determine whether these neurons are GABAergic, we combined in situ hybridization of GAD₆₇ mRNA with immunohistochemical detection of Fos in control, PS deprived and PS hypersomniac rats. We found that 74% of the very large population of Fos-labeled neurons located in the tuberal hypothalamus after PS hypersomnia were GAD-positive. We further demonstrated combining MCH immunohistochemistry and GAD₆₇in situ hybridization that 85% of the MCH neurons were also GAD-positive. Finally, based on the number of Fos-ir/GAD⁺, Fos-ir/MCH⁺, and GAD⁺/MCH⁺ double-labeled neurons counted from three sets of double-staining, we uncovered that around 80% of the large number of the Fos-ir/GAD⁺ neurons located in the tuberal hypothalamus after PS hypersomnia do not contain MCH. Based on these and previous results, we propose that the non-MCH Fos/GABAergic neuronal population could be involved in PS induction and maintenance while the Fos/MCH/GABAergic neurons could be involved in the homeostatic regulation of PS. Further investigations will be needed to corroborate this original hypothesis.

Major impairments of glutamatergic transmission and long-term synaptic plasticity in the hippocampus of mice lacking the melanin-concentrating hormone receptor-1

The hypothalamic neuropeptide melanin-concentrating hormone (MCH) plays important roles in energy homeostasis, anxiety, and sleep regulation. Since the MCH receptor-1 (MCH-R1), the only functional receptor that mediates MCH functions in rodents, facilitates behavioral performance in hippocampus-dependent learning tasks, we investigated whether glutamatergic transmission in CA1 pyramidal cells could be modulated in mice lacking the MCH-R1 gene (MCH-R1(-/-)). We found that both α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and N-methyl-d-aspartate (NMDA) receptor-mediated transmissions were diminished in the mutant mice compared with their controls. This deficit was explained, at least in part, by a postsynaptic down-regulation of these receptors since the amplitude of miniature excitatory postsynaptic currents and the NMDA/AMPA ratio were decreased. Long-term synaptic potentiation (LTP) was also impaired in MCH-R1(-/-) mice. This was due to an altered induction, rather than an impaired, expression because repeating the induction stimulus restored LTP to a normal magnitude. In addition, long-term synaptic depression was strongly diminished in MCH-R1(-/-) mice. These results suggest that MCH exerts a facilitatory effect on CA1 glutamatergic synaptic transmission and long-term synaptic plasticity. Recently, it has been shown that MCH neurons fire exclusively during sleep and mainly during rapid eye movement sleep. Thus these findings provide a mechanism by which sleep might facilitate memory consolidation.

GABAergic neurons intermingled with orexin and MCH neurons in the lateral hypothalamus discharge maximally during sleep

The lateral hypothalamus (LH), where wake-active orexin (Orx)-containing neurons are located, has been considered a waking center. Yet, melanin-concentrating hormone (MCH)-containing neurons are codistributed therein with Orx neurons and, in contrast to them, are active during sleep, not waking. In the present study employing juxtacellular recording and labeling of neurons with Neurobiotin (Nb) in naturally sleeping-waking head-fixed rats, we identified another population of intermingled sleep-active cells, which do not contain MCH (or Orx), but utilize gamma-aminobutyric acid (GABA) as a neurotransmitter. The 'sleep-max' active neurons represented 53% of Nb-labeled MCH-(and Orx) immunonegative (-) cells recorded in the LH. For identification of their neurotransmitter, Nb-labeled varicosities of the Nb-labeled/MCH- neurons were sought within sections adjacent to the Nb-labeled soma and immunostained for the vesicular transporter for GABA (VGAT) or for glutamate. A small proportion of sleep-max Nb+/MCH- neurons (19%) discharged maximally during slow-wave sleep (called 'S-max') in positive correlation with delta electroencephalogram activity, and from VGAT staining of Nb-labeled varicosities appeared to be GABAergic. The vast proportion of sleep-max Nb+/MCH- neurons (81%) discharged maximally during paradoxical sleep (PS, called 'P-max') in negative correlation with electromyogram amplitude, and from Nb-labeled varicosities also appeared to be predominantly GABAergic. Given their discharge profiles across the sleep-wake cycle, P-max together with S-max GABAergic neurons could thus serve to inhibit other neurons of the arousal systems, including local Orx neurons in the LH. They could accordingly dampen arousal with muscle tone and promote sleep, including PS with muscle atonia.

Recent updates on the melanin-concentrating hormone (MCH) and its receptor system: lessons from MCH1R antagonists

Melanin-concentrating hormone (MCH) is a 19-amino-acid cyclic peptide which was originally found to lighten skin color in fish that is highly conserved among many species. MCH interacts with two G-protein-coupled receptors, MCH1R and MCH2R, but only MCH1R is expressed in rodents. MCH is mainly synthesized in the lateral hypothalamus and zona incerta, while MCH1R is widely expressed throughout the brain. Thus, MCH signaling is implicated in the regulation of many physiological functions. The identification of MCH1R has led to the development of small-molecule MCH1R antagonists that can block MCH signaling. MCH1R antagonists are useful not only for their potential therapeutic value, but also for understanding the physiological functions of the endogenous MCH system. Here, we review the physiological functions of the MCH system which have been investigated using MCH1R antagonists such as food intake, anxiety, depression, reward, and sleep. This will help us understand the physiological functions of the MCH system and suggest some of the potential applications of MCH1R antagonists in human disorders.

GABAergic regulation of the perifornical-lateral hypothalamic neurons during non-rapid eye movement sleep in rats

The perifornical-lateral hypothalamic area (PF-LHA) has been implicated in the regulation of behavioral arousal. The PF-LHA predominantly contains neurons that are active during behavioral and cortical activation and quiescent during non-rapid eye movement (nonREM) sleep, that is, are nonREM-off neurons. Some in vitro and in vivo studies indicate that PF-LHA neurons, including hypocretin-expressing neurons, are under GABAergic control. However, a role of GABA in suppressing the discharge of PF-LHA neurons during spontaneous nonREM sleep has not been confirmed. We recorded the sleep-wake discharge profiles of PF-LHA neurons and simultaneously assessed the contributions of local GABA(A) receptor activation and blockade on their wake- and nonREM sleep-related discharge activities by delivering GABA(A) receptor agonist, muscimol (500 nm, 5 microM, and 10 microM) and its antagonist, bicuculline (5 microM, 10 microM, and 20 microM), adjacent to the recorded neurons via reverse microdialysis. Muscimol dose-dependently decreased the discharge of PF-LHA neurons including nonREM-off neurons. Muscimol-induced suppression of discharge during nonREM sleep was significantly weaker than the suppression produced during waking. In the presence of bicuculline, PF-LHA neurons, including nonREM-off neurons, exhibited elevated discharge, which was dose-dependent and was significantly higher during nonREM sleep, compared to waking. These results suggest that GABA(A) receptor mediated increased GABAergic tone contributes to the suppression of PF-LHA neurons, including nonREM-off neurons, during spontaneous nonREM sleep.

Melanin-concentrating hormone facilitates migration of preadipocytes

Adipose tissue develops from differentiating preadipocytes that expand and migrate. 3T3-L1 preadipocytes respond to melanin-concentrating hormone (MCH) by increasing leptin production. Here, we investigate whether MCH elicits remodeling of the actin cytoskeleton and whether this translates into altered migratory capacity of these cells. Incubation with MCH resulted in a loss of actin stress fibers accompanied by a change in morphology from a stretched-out fibroblast to a rounded cell. PMC-3881-PI, a MCH receptor 1 antagonist blocked the effect, confirming this receptor is solely responsible for MCH-mediated actin rearrangements. Both a pharmacological activator and inhibitor of phospholipase C were used to demonstrate this molecule's importance to the signaling pathway. Finally, MCH was shown to facilitate preadipocyte migration into a scratch wound, revealing a previously unknown role for MCH in the regulation of cellular migration. We conclude that MCH could influence the expansion of adipose tissue through its ability to enhance preadipocyte migration.

The action of leptin on appetite-regulating cells in the ovine hypothalamus: demonstration of direct action in the absence of the arcuate nucleus

It is widely accepted that leptin acts on first-order neurons in the arcuate nucleus (ARC) with information then relayed to other hypothalamic centers. However, the extent to which leptin mediates its central actions solely, or even primarily, via this route is unclear. We used a model of hypothalamo-pituitary disconnection (HPD) to determine whether leptin action on appetite-regulating systems requires the ARC. This surgical preparation eliminates the ARC. We measured effects of iv leptin to activate hypothalamic neurons (Fos labeling). In ARC-intact animals, leptin increased the percentage of Fos-positive melanocortin neurons and reduced percentages of Fos-positive neuropeptide Y neurons compared with saline-treated animals. HPD itself increased Fos labeling in the lateral hypothalamic area (LHA). Leptin influenced Fos labeling in the dorsomedial nucleus (DMH), ventromedial nucleus, and paraventricular nucleus (PVN) in HPD and normal animals, with effects on particular cell types varying. In the LHA and DMH, leptin decreased orexin cell activation in HPD and ARC-intact sheep. HPD abolished leptin-induced expression of Fos in melanin-concentrating hormone cells in the LHA and in CRH cells in the PVN. In contrast, HPD accentuated activation in oxytocin neurons. Our data from sheep with lesions encompassing the ARC do not suggest a primacy of action of leptin in this nucleus. We demonstrate that first order to second order signaling may not represent the predominant means by which leptin acts in the brain to generate integrated responses. We provide evidence that leptin exerts direct action on cells of the DMH, ventromedial nucleus, and PVN.

A1 receptor mediated adenosinergic regulation of perifornical-lateral hypothalamic area neurons in freely behaving rats

The perifornical-lateral hypothalamic area (PF-LHA) plays a central role in the regulation of behavioral arousal. The PF-LHA contains several neuronal types including wake-active hypocretin (HCRT) neurons that have been implicated in the promotion and/or maintenance of behavioral arousal. Adenosine is an endogenous sleep factor and recent evidence suggests that activation and blockade of adenosine A(1) receptors within the PF-LHA promote and suppress sleep, respectively. Although, an in vitro study indicates that adenosine inhibits HCRT neurons via A(1) receptor, the in vivo effects of A(1) receptor mediated adenosinergic transmission on PF-LHA neurons including HCRT neurons are not known. First, we determined the effects of N(6)-cyclopentyladenosine (CPA), an adenosine A(1) receptor agonist, on the sleep-wake discharge activity of the PF-LHA neurons recorded via microwires placed adjacent to the microdialysis probe used for its delivery. Second, we determined the effects of CPA and that of an A(1) receptor antagonist, 1,3-dipropyl-8-phenylxanthine (CPDX) into the PF-LHA on cFos-protein immunoreactivity (Fos-IR) in HCRT and non-HCRT neurons around the microdialysis probe used for their delivery. The effect of CPA on Fos-IR was studied in rats that were kept awake during lights-off phase, whereas the effect of CPDX was examined in undisturbed rats during lights-on phase. CPA significantly suppressed the sleep-wake discharge activity of PF-LHA neurons. Doses of CPA (50 muM) and CPDX (50 muM) that suppressed and induced arousal, respectively, in our earlier study [Alam MN, Kumar S, Rai S, Methippara M, Szymusiak R, McGinty D (2009) Brain Res 1304:96-104], significantly suppressed and increased Fos-IR in HCRT and non-HCRT neurons. These findings suggest that wake-promoting PF-LHA system is subject to increased endogenous adenosinergic inhibition and that adenosine acting via A(1) receptors, in part, inhibits HCRT neurons to promote sleep.

A decade of hypocretins: past, present and future of the neurobiology of arousal

In 1998, two groups independently identified the hypocretins, also known as orexins, as two hypothalamic peptides derived from the same precursor expressed in a few thousand neurones restricted to the perifornical area. A decade later, an amazing set of discoveries has demonstrated a key role for this neurotransmitter system in arousal and beyond. Here I review some of the experiments that led to these discoveries and the implications in the neurobiology of the hypothalamus and our understanding of brain arousal.

Activation of the basal forebrain by the orexin/hypocretin neurones

The orexin neurones play an essential role in driving arousal and in maintaining normal wakefulness. Lack of orexin neurotransmission produces a chronic state of hypoarousal characterized by excessive sleepiness, frequent transitions between wake and sleep, and episodes of cataplexy. A growing body of research now suggests that the basal forebrain (BF) may be a key site through which the orexin-producing neurones promote arousal. Here we review anatomical, pharmacological and electrophysiological studies on how the orexin neurones may promote arousal by exciting cortically projecting neurones of the BF. Orexin fibres synapse on BF cholinergic neurones and orexin-A is released in the BF during waking. Local application of orexins excites BF cholinergic neurones, induces cortical release of acetylcholine and promotes wakefulness. The orexin neurones also contain and probably co-release the inhibitory neuropeptide dynorphin. We found that orexin-A and dynorphin have specific effects on different classes of BF neurones that project to the cortex. Cholinergic neurones were directly excited by orexin-A, but did not respond to dynorphin. Non-cholinergic BF neurones that project to the cortex seem to comprise at least two populations with some directly excited by orexin-A that may represent wake-active, GABAergic neurones, whereas others did not respond to orexin-A but were inhibited by dynorphin and may be sleep-active, GABAergic neurones. This evidence suggests that the BF is a key site through which orexins activate the cortex and promote behavioural arousal. In addition, orexins and dynorphin may act synergistically in the BF to promote arousal and improve cognitive performance.

Role of orexin in the regulation of glucose homeostasis

Orexin-A (hypocretin-1) and orexin-B (hypocretin-2) are hypothalamic neuropeptides that play key roles in the regulation of wakefulness, feeding, reward, autonomic functions and energy homeostasis. To control these functions indispensable for survival, orexin-expressing neurones integrate peripheral metabolic signals, interact with many types of neurones in the brain and modulate their activities via the activation of orexin-1 receptor or orexin-2 receptor. In addition, a new functional role of orexin is emerging in the regulation of insulin and leptin sensitivities responsible for whole-body glucose metabolism. Recent evidence indicates that orexin efficiently protects against the development of peripheral insulin resistance induced by ageing or high-fat feeding in mice. In particular, the orexin receptor-2 signalling appears to confer resistance to diet-induced obesity and insulin insensitivity by improving leptin sensitivity. In fact, the expression of orexin gene is known to be down-regulated by hyperglycaemia in the rodent model of diabetes, such as ob/ob and db/db mice. Moreover, the levels of orexin receptor-2 mRNA have been shown to decline in the brain of mice along with ageing. These suggest that hyperglycaemia due to insulin insensitivity during ageing or by habitual consumption of a high-fat diet leads to the reduction in orexin expression in the hypothalamus, thereby further exacerbating peripheral insulin resistance. Therefore, orexin receptor controlling hypothalamic insulin/leptin actions may be a new target for possible future treatment of hyperglycaemia in patients with type 2 diabetes.