Immunoreactivities for antisera to three putative neuropeptides of the rat melanin-concentrating hormone precursor are coexpressed in neurons of the rat lateral dorsal hypothalamus

Structural and molecular characterization of paraventricular thalamic glucokinase-expressing neuronal circuits in the mouse

The thalamic paraventricular nucleus (PVT) is a structure highly interconnected with several nuclei ranging from forebrain to hypothalamus and brainstem. Numerous rodent studies have examined afferent and efferent connections of the PVT and their contribution to behavior, revealing its important role in the integration of arousal cues. However, the majority of these studies used a region‐oriented approach, without considering the neuronal subtype diversity of the nucleus. In the present study, we provide the anatomical and transcriptomic characterization of a subpopulation of PVT neurons molecularly defined by the expression of glucokinase (Gck). Combining a genetically modified mouse model with viral tracing approaches, we mapped both the anterograde and the retrograde projections of Gck‐positive neurons of the anterior PVT (Gck^aPVT). Our results demonstrated that Gck^aPVT neurons innervate several nuclei throughout the brain axis. The strongest connections are with forebrain areas associated with reward and stress and with hypothalamic structures involved in energy balance and feeding regulation. Furthermore, transcriptomic analysis of the Gck‐expressing neurons revealed that they are enriched in receptors for hypothalamic‐derived neuropeptides, adhesion molecules, and obesity and diabetes susceptibility transcription factors. Using retrograde labeling combined with immunohistochemistry and in situ hybridization, we identify that Gck^aPVT neurons receive direct inputs from well‐defined hypothalamic populations, including arginine‐vasopressin‐, melanin‐concentrating hormone‐, orexin‐, and proopiomelanocortin‐expressing neurons. This detailed anatomical and transcriptomic characterization of Gck^aPVT neurons provides a basis for functional studies of the integration of homeostatic and hedonic aspects of energy homeostasis, and for deciphering the potential role of these neurons in obesity and diabetes development.

Projections from the dorsomedial division of the bed nucleus of the stria terminalis to hypothalamic nuclei in the mouse

As stressful environment is a potent modulator of feeding, we seek in the present work to decipher the neuroanatomical basis for an interplay between stress and feeding behaviors. For this, we combined anterograde and retrograde tracing with immunohistochemical approaches to investigate the patterns of projections between the dorsomedial division of the bed nucleus of the stria terminalis (BNST), well connected to the amygdala, and hypothalamic structures such as the paraventricular (PVH) and dorsomedial (DMH), the arcuate (ARH) nuclei and the lateral hypothalamic areas (LHA) known to control feeding and motivated behaviors. We particularly focused our study on afferences to proopiomelanocortin (POMC), agouti-related peptide (AgRP), melanin-concentrating-hormone (MCH) and orexin (ORX) neurons characteristics of the ARH and the LHA, respectively. We found light to intense innervation of all these hypothalamic nuclei. We particularly showed an innervation of POMC, AgRP, MCH and ORX neurons by the dorsomedial and dorsolateral divisions of the BNST. Therefore, these results lay the foundation for a better understanding of the neuroanatomical basis of the stress-related feeding behaviors.

The zona incerta system: Involvement in attention and movement

The zona incerta (ZI) is a large structure made of four neurochemically defined regions (at least, in rodents). It is globally involved in complex connections with telencephalic and brainstem centers. In this work, we focus on some of the anatomical links this structure develops with the cerebral cortex and the tectum. We also point to its integration within a larger basal ganglia network. The functions of this region are still mysterious, even if recent works suggest its participation in behavioral expression. Studies about the functional organization of the vibrissal system have provided the first integrated model, illustrating the ZI's role in sensory-motor programing. In addition, ZI connections with the superior colliculus and the cerebral cortex as well as recent behavioral studies point to this region playing a role in cognitive processes related to attention toward salient stimuli.

The claustrum is a target for projections from the supramammillary nucleus in the rat

Injection of the anterograde tracer Phaseolus vulgaris leucoagglutinin (PHAL) into the rat rostral and caudal supramammillary nucleus (SUM) provided expected patterns of projections into the hippocampus and the septal region. In addition, unexpectedly intense projections were observed into the claustrum defined by parvalbumin expression. Injections of the retrograde tracer fluorogold (FG) into the hippocampus and the region of the claustrum showed that the cells of origin of these projections distributed similarly within the borders of the SUM. The SUM is usually involved in control of hippocampal theta activity, but the observation of intense projections into the claustrum indicates that it may also influence isocortical processes. Therefore, the SUM may coordinate sensory processing in the isocortex with memory formation in the hippocampus.

Parasubthalamic and calbindin nuclei in the posterior lateral hypothalamus are the major hypothalamic targets for projections from the central and anterior basomedial nuclei of the amygdala

The parasubthalamic nucleus (PSTN) and the ventrally adjacent calbindin nucleus (CbN) form a nuclear complex in the posterior lateral hypothalamic area (LHA), recently characterized as connected with the central nucleus of the amygdala (CEA). The aim of the present work is to analyze in detail the projections from the amygdala into the PSTN/CbN, also focusing on pathways into the LHA. After fluorogold injections into the PSTN/CbN, the medial part of the CEA (CEAm) appears to be the main supplier of projections from the CEA. Other amygdalar nuclei contribute to the innervation of the PSTN/CbN complex, including the anterior part of the basomedial nucleus (BMAa). Injections of the anterograde tracer, Phaseolus vulgaris leucoagglutinin (PHAL), into the CEAm and BMAa revealed that projections from the CEAm follow two pathways into the LHA: a dorsal pathway formed by axons that also innervate the paraventricular hypothalamic nucleus, the anterior perifornical LHA and the PSTN, and a ventral pathway that runs laterally adjacent to the ventrolateral hypothalamic tract (vlt) and ends in the CbN. By contrast, the BMAa and other telencephalic structures, such as the fundus striatum project to the CbN via the ventral pathway. Confirming the microscopic observation, a semi-quantitative analysis of the density of these projections showed that the PSTN and the CbN are the major hypothalamic targets for the projections from the CEAm and the BMAa, respectively. PSTN and CbN receive these projections through distinct dorsal and ventral routes in the LHA. The ventral pathway forms a differentiated tract, named here the ventrolateral amygdalo-hypothalamic tract (vlah), that is distinct from, but runs adjacent to, the vlt. Both the vlt and the vlah had been previously described as forming an olfactory path into the LHA. These results help to better characterize the CbN within the PSTN/CbN complex and are discussed in terms of the functional organization of the network involving the PSTN and the CbN as well as the CEA and the BMAa.

Melanin-concentrating hormone axons, but not orexin or tyrosine hydroxylase axons, innervate the claustrum in the rat: An immunohistochemical study

The claustrum is a small, elongated nucleus close to the external capsule and deep in the insular cortex. In rodents, this nucleus is characterized by a dense cluster of parvalbumin labeling. The claustrum is connected with the cerebral cortex. It does not project to the brainstem, but brainstem structures can influence this nucleus. To identify some specific projections from the lateral hypothalamus and midbrain, we analyzed the distribution of projections labeled with antibodies against tyrosine hydroxylase (TH), melanin-concentrating hormone (MCH), and hypocretin (Hcrt) in the region of the claustrum. The claustrum contains a significant projection by MCH axons, whereas it is devoid of TH projections. Unlike TH and MCH axons, Hcrt axons are scattered throughout the region. This observation is discussed mainly with regard to the role of the claustrum in cognitive functions and that of MCH in REM sleep. J. Comp. Neurol. 525:1489-1498, 2017. © 2016 Wiley Periodicals, Inc.

A premammillary lateral hypothalamic nuclear complex responds to hedonic but not aversive tastes in the male rat

The lateral hypothalamic area (LHA) has two major roles: arousal/waking and food intake controls. Here, it is shown that a premammillary part of the LHA is neurochemically and cytoarchitectonically distinct from the tuberal LHA in male rats. This part contains nuclear masses, namely the parasubthalamic nucleus and the calbindin nucleus, involved in pathways that predict its participation in the control of food intake. Analyzing c-Fos expression in experiments related to feeding behavior, this region responded specifically to the ingestion of palatable nutriments.

Anatomical organization of MCH connections with the pallidum and dorsal striatum in the rat

Neurons producing the melanin-concentrating hormone (MCH) are distributed in the posterior hypothalamus, but project massively throughout the forebrain. Many aspects regarding the anatomical organization of these projections are still obscure. The present study has two goals: first to characterize the topographical organization of neurons projecting into the cholinergic basal forebrain (globus pallidus, medial septal complex), and second to verify if MCH neurons may indirectly influence the dorsal striatum (caudoputamen) by innervating afferent sources to this structure. In the first series of experiments, the retrograde tracer fluorogold was injected into multiple sites in the pallidal and medial septal regions and the distribution of retrogradely labeled neurons were analyzed in the posterior lateral hypothalamus. In the second series of experiments, fluorogold was injected into the caudoputamen, and the innervation by MCH axons of retrogradely labeled cells was analyzed. Our results revealed that the MCH system is able to interact with the basal nuclei in several different ways. First, MCH neurons provide topographic inputs to the globus pallidus, medial septal complex, and substantia innominata. Second, striatal projecting neurons in the cortex, thalamus, and substantia nigra presumably receive only sparse inputs from MCH neurons. Third, the subthalamic nucleus is heavily innervated by MCH projections, thus, presumably serves as one important intermediate station to mediate MCH influence on other parts of the basal nuclei.

Different distributions of preproMCH and hypocretin/orexin in the forebrain of the pig (Sus scrofa domesticus)

Neurons producing melanin-concentrating hormone (MCH) or hypocretin/orexin (Hcrt) have been implicated in the sleep/wake cycle and feeding behavior. Sleep and feeding habits vary greatly among mammalian species, depending in part of the prey/predatory status of animals. However, the distribution of both peptides has been described in only a limited number of species. In this work, we describe the distribution of MCH neurons in the brain of the domestic pig. Using in situ hybridization and immunohistochemistry, their cell bodies are shown to be located in the posterior lateral hypothalamic area (LHA), as expected. They form a dense cluster ventro-lateral to the fornix while only scattered cells are present dorsal to this tract. By comparison, Hcrt cell bodies are located mainly dorsal to the fornix. Therefore, the two populations of neurons display complementary distributions in the posterior LHA. MCH projections are, as indicated by MCH-positive axons, very abundant in all cortical fields ventral to the rhinal sulcus, as well as in the lateral, basolateral and basomedial amygdala. In contrast, most of the isocortex is sparsely innervated. To conclude, the distribution of MCH cell bodies and projections shows some very specific features in the pig brain, that are clearly different of that described in the rat, mouse or human. In contrast, the Hcrt pattern seems more similar to that in these species, i.e. more conserved. These results suggest that the LHA anatomic organization shows some very significant interspecies differences, which may be related to the different behavioral repertoires of animals with regard to feeding and sleep/wake cycles.

Melanin-concentrating hormone expression in the rat hypothalamus is not affected in an experiment of prenatal alcohol exposure

Alcohol consumption during pregnancy can cause a "fetal alcoholic syndrome" (FAS) in the progeny. This syndrome is characterized by important brain defects often associated to a decreased expression of the morphogenic protein sonic hedgehog (Shh). The goal of this study was to verify if a FAS could modify the differentiation of hypothalamic neurons producing MCH. Indeed, the expression of this peptide and neurons producing it are dependent of a Shh controlled genetic cascade in the embryo. To address this question, female rats received a 15% ethanol solution to drink during pregnancy and lactation. Higher abortion rate and smaller pups at birth confirmed that descendants were affected by this experimental condition. MCH expression was analyzed by RT-qPCR and immunohistochemistry in embryos taken at E11 and E13, or in pups and young adults born from control and alcoholic mothers. MCH expression level, number of MCH neurons or ratio of MCH sub-populations were not modified by our experimental conditions. However, Shh expression was significantly lover at E11 and we also observed that hindbrain serotonergic neurons were affected as reported in the literature. These findings as well as other data from the literature suggest that protective mechanisms are involved to maintain peptide expressions and differentiation of some specific neuron populations in the ventral diencephalon in surviving embryos exposed to ethanol during pregnancy.

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.

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.

The development of the MCH system

Although a great deal is published on the MCH neurons, very few works were devoted to the study of their development. However, existing literature points out two important traits: first, these neurons differentiate a MCH phenotype very early in all species studied so far, which might suggest a role for the MCH peptide during development; second, in the rat, birth date greatly influence the phenotype of MCH neurons. At least two sub-populations were described on the basis of their chemical phenotype, projection pattern and birth date. The understanding of processes involved in the differentiation of these sub-populations may help understand the medio-lateral differentiation of the tuberal hypothalamus.

Distribution and genesis of the RFRP-producing neurons in the rat brain: comparison with melanin-concentrating hormone- and hypocretin-containing neurons

Prepro-RFRP-containing neurons have recently been described in the mammalian brain. These neurons are only found in the tuberal hypothalamus. In this work, we have provided a detailed analysis of the distribution of cells expressing the RFRP mRNA, and found them in seven anatomical structures of the tuberal hypothalamus. No co-expression with melanin-concentrating hormone (MCH) or hypocretin (Hcrt), that are also described in neurons of the tuberal hypothalamus, was observed. Using the BrdU method, we found that all RFRP cell bodies are generated between E13 and E14. Thus, RFRP neurons form a specific cell population with a complex distribution pattern in the tuberal hypothalamus. However, they are generated in one peak. These observations are discussed with data concerning the distribution and genesis of the MCH and Hcrt cell populations that are also distributed in the tuberal hypothalamus.

Differential c-Fos immunoreactivity in arousal-promoting cell groups following systemic administration of caffeine in rats

Despite the widespread use of caffeine, the neuronal mechanisms underlying its stimulatory effects are not completely understood. By using c-Fos immunohistochemistry as a marker of neuronal activation, we recently showed that stimulant doses of caffeine activate arousal-promoting hypothalamic orexin (hypocretin) neurons. In the present study, we investigated whether other key neurons of the arousal system are also activated by caffeine, via dual immunostaining for c-Fos and transmitter markers. Rats were administered three doses of caffeine or saline vehicle during the light phase. Caffeine at 10 and 30 mg/kg, i.p., increased motor activities, including locomotion, compared with after saline or a higher dose, 75 mg/kg. The three doses of caffeine induced distinct dose-related patterns of c-Fos immunoreactivity in several arousal-promoting areas, including orexin neurons and adjacent neurons containing neither orexin nor melanin-concentrating hormone; tuberomammillary histaminergic neurons; locus coeruleus noradrenergic neurons; noncholinergic basal forebrain neurons that do not contain parvalbumin; and nondopaminergic neurons in the ventral tegmental area. At any dose used, caffeine induced little or no c-Fos expression in cholinergic neurons of the basal forebrain and mesopontine tegmentum; dopaminergic neurons of the ventral tegmental area, central gray, and substantia nigra pars compacta; and serotonergic neurons in the dorsal raphe nucleus. Saline controls exhibited only few c-Fos-positive cells in most of the cell groups examined. These results indicate that motor-stimulatory doses of caffeine induce a remarkably restricted pattern of c-Fos expression in the arousal-promoting system and suggest that this specific neuronal activation may be involved in the behavioral arousal by caffeine.

Specific alteration of the expression of selected hypothalamic neuropeptides during acute and late mouse brain infection using a morbillivirus: relevance to the late-onset obesity?

Neurotropic viruses are involved in pathologies of the central nervous system, triggering transient or irreversible disorders, such as neurological diseases or homeostasis imbalance. In experimental animals, viruses have been shown to cause obesity, a complex disease depending on multiple factors, including genetic susceptibility and environmental components. Using a mouse model of virally induced obesity following brain infection by the Canine Distemper Virus (CDV), a morbillivirus closely related to the human measles virus, we investigated the modulation of expression of several hypothalamic neuropeptides known to intervene in the regulation of body weight and energy expenditure, both during the acute and late stages of infection. During the acute stage, while viral replication occurs, we found a dramatic decrease of expressions of neuropeptides, in particular neuropeptide Y, melanin-concentrating hormone (MCH), hypocretin, vasopressin and tachykinins, the magnitude of which seemed to be linked to the viral burden and the individual susceptibility. The effect of the virus, however, varied with the hypothalamic nucleus and neuropeptide involved, suggesting that certain circuits were affected while others remained intact. During the late stage of infection, marked recovery to the initial hypothalamic levels of peptide expression was seen in a number of lean animals, suggesting recovery of homeostasis equilibrium. Interestingly, some neuropeptidergic systems remained disturbed in mice exhibiting obese phenotype, arguing for their involvement in triggering/maintaining obesity. Even though our data could not fully explain the viral-induced obesity, they may be helpful in understanding the molecular events associated with obesity and in investigating therapeutic alternatives.

Immunohistochemical localization of cocaine- and amphetamine-regulated transcript peptide in the central nervous system of the frogRana esculenta

The distribution of cocaine- and amphetamine-regulated transcript peptide (CARTp)- like immunoreactivity was studied only in the rat central nervous system (CNS). In mammals, CART peptides occur among others in brain areas that control feeding behavior. We mapped CARTp-immunoreactive structures in the CNS of the frog Rana esculenta and assumed that differences may exist in the CARTp-containing neuronal populations between the frog, which does not feed in winter, and the rat. In the forebrain, immunoreactive cells and fibers were found in the olfactory bulb, nucleus accumbens, amygdala, medial pallium, septum, striatum, the preoptic nuclei, ventromedial nucleus, central thalamic nucleus, and the hypothalamus. The optic pathway was free of immunoreactivity. The neurohypophysis showed intense immunostaining. In the mesencephalon, many cells were stained in the Edinger-Westphal nucleus, and a few in the optic tectum, where fibers were stained in all plexiform layers. In the retina, some cells in the inner nuclear layer contained CARTp. In the rhombencephalon, cells were stained in the raphe nuclei, central gray, nucleus of the solitary tract, and the vicinity of motor nuclei. Neurons of the motor cranial nerves were densely innervated by CARTp-positive fibers originating from the spinal cord. In the spinal cord, preganglionic cells were stained, and motoneurons were surrounded by immunoreactive varicose axon terminals. Major differences were found between the frog and the rat brains in the distribution of CARTp in the visual system, olfactory bulb, preoptic area, and the motor nuclei. Some of these differences may be related to feeding behavior of these animals.

Diencephalic neurons producing melanin-concentrating hormone are influenced by local and multiple extra-hypothalamic tachykininergic projections through the neurokinin 3 receptor

As melanin-concentrating hormone (MCH) neurons express the neurokinin 3 receptor (NK3) in the rat diencephalon, their innervation by tachykininergic fibers, the origin of this innervation and the effect of a NK3 agonist on MCH mRNA expression were researched. The obtained results show that the tachykininergic system develops complex relationships with MCH neurons. Overall, MCH cell bodies appeared targeted by both NKB- and SP-inputs. These afferents have multiple hypothalamic and extra-hypothalamic origins, but a local (intra-lateral hypothalamic area) origin from small interneurons was suspected as well. MCH cell bodies do not express NK1, but around 2.7% of the MCH neurons contained SP after colchicine injection. Senktide, a NK3 agonist, produced an increase of the MCH mRNA expression in cultured hypothalamic slices. This effect was reversed by two NK3 antagonists. Tachykinins enhance MCH mRNA expression, and, thus, may modulate the effect of MCH in functions such as feeding and reproductive behaviors in which this peptide has been experimentally involved.

Evidence of melanin-concentrating hormone-containing neurons supplying both cortical and neuroendocrine projections

In the rat, melanin-concentrating hormone-containing projections are detected in the median eminence and in the neural lobe of the pituitary. After vascular injections of the retrograde tracers fluorogold or fastblue, melanin-concentrating hormone neurons are retrogradely labeled in the rostromedial zona incerta and adjacent perifornical region. These neurons may be the source of the melanin-concentrating hormone projections toward the median eminence and posterior pituitary, and may release their secretory products into the bloodstream. After fastblue injections in the cerebral cortex and vascular fluorogold injections, some melaninconcentrating hormone neurons contain both tracers, indicating that they send collaterals in the cerebral cortex and in the median eminence/posterior pituitary. No such collaterals have been described for the classical neuroendocrine systems. The melanin-concentrating hormone system is thought to play a role in arousal in correlation with specific goal oriented behaviors such as feeding or reproduction. Some MCH neurons may be involved in such functions by modulating directly cortical activity as well as being neuroendocrine.

Sensitivity of galanin- and melanin-concentrating hormone-containing neurones to nutritional status: an immunohistochemical study in the ovariectomized ewe

The sensitivities of galanin and melanin-concentrating hormone (MCH) neuronal systems to nutrition are poorly understood in sheep compared to rodents. The aim of this study was to describe the changes in the numbers of galanin and MCH neurones in ovariectomized ewes submitted to different nutritional levels. In the first experiment, ewes were fed ad libitum or food deprived for 24 h. In the second experiment, two groups of ewes were fed at maintenance level (group 100) or undernourished (group 40) for 167 days, after which one-half of each group was killed or refed ad libitum (group 100R and 40R) for 4 days. The MCH neuronal population located in the lateral hypothalamic area was not affected by these nutritional changes. Long-term undernutrition enhanced the number of galanin neurones located in the infundibular nucleus and the dorsal hypothalamic area (DHA), refeeding resulted in an increase of neurones in the DHA and preoptic area, but short-term starvation had no effect on any galanin subpopulations. Our data suggest that the sensitivity of MCH neuronal populations to nutrition in sheep differs from that of rodents. Various populations of galanin-containing neurones differ in sensitivity in ewes subjected to long undernutrition and refeeding but not to short starvation.

Time of genesis determines projection and neurokinin-3 expression patterns of diencephalic neurons containing melanin-concentrating hormone

Anatomical and functional evidence suggests that the diencephalic melanin-concentrating hormone- (MCH-) containing neurons do not form a homogeneous population. In this work, the expression of the neurokinin-3 receptor (NK3) has been researched in MCH neurons which have been retrogradely labelled following fast blue injections into either the spinal cord or the cerebral cortex. The birth-date of these cortically and spinally projecting cells has been determined using the bromodeoxyuridine method. The results obtained show that neurons projecting to the spinal cord are born early (E11) and most of them (78,7%) do not express NK3, but neurons that send axons to the cerebral cortex are born later (E12-E13) and most of them (84,8%) express NK3. Both neuronal types are largely intermingled in the lateral hypothalamic area proper. These results are discussed in terms of the functional organization of the MCH neuronal population.

Orexin/hypocretin neurons: chemical phenotype and possible interactions with melanin-concentrating hormone neurons

We showed earlier that a specific neuron population of the rat lateral hypothalamus, differing from the codistributed melanin-concentrating hormone (MCH) neurons, express both dynorphin (DYN) and secretogranin II (SgII) genes. We demonstrated later that this population corresponds in fact to the newly identified orexin/hypocretin (OX/Hcrt) neurons. In the present study, by revisiting the chemical phenotype of these neurons, we confirm that all of them contain DYN B- and SgII-immunoreactive materials. The roles played by these peptide/protein in OX/Hcrt neurons are still unclear. Double immunocytochemical stainings highlight putative somasomatic, axosomatic and axodendritic contacts between OX/Hcrt and MCH neurons. Adding OX/Hcrt to the culture medium of hypothalamic slices from 8-day-old rats results either in a significant increase of MCH mRNA after 24 h survival or a strong fall after 10 days culture. These results taken together suggest that OX/Hcrt can directly and/or indirectly affect MCH expression, and that both OX/Hcrt and MCH neuron populations interact to respond in a coordinated manner to central and peripheral signals.

Distribution of melanin-concentrating hormone-like immunoreactivity in the central nervous system of Rana esculenta

The distribution of salmon and rat melanin-concentrating hormone (MCH)-like and neuropeptide glutamate-isoleucine (NEI)-like immunoreactivity in the brain and spinal cord of the frog Rana esculenta was studied with immunohistochemistry. In the telencephalon, only fibers showed immunoreactivity in the olfactory bulb, lateral pallium, diagonal band, septum, and the amygdala. Immunoreactive fibers were abundant in all diencephalic structures, except the optic tract, the visual neuropils, and the habenula. Several cells in the central thalamic nucleus and a few in the suprachiasmatic nucleus were stained with the MCH antisera. Cells and their processes were intensely stained in the dorsal hypothalamus with the MCH and NEI antisera. Immunoreactive fibers were found in all tegmental nuclei and the white matter of the mesencephalon. They formed terminal plexuses in the deep layers of the optic tectum and the laminar nucleus of the torus semicircularis. Immunoreactive fibers were sparse in the rhombencephalon and the spinal cord.

Down regulation of melanin concentrating hormone in virally induced obesity

Obesity is a complex disease involving genetic components and environmental factors and probably associated with the dysregulation of central homeostasis normally maintained by the hypothalamic neuroendocrine/neurotransmitter network. We previously reported that canine distemper virus (CDV), which is closely related to human measles virus, can target hypothalamic nuclei, and lead to obesity syndrome in the late stages of infection. Here, using differential display PCR, we demonstrate specific down-regulation of melanin-concentrating hormone precursor mRNA (ppMCH) in infected-obese mice. Although ppMCH was down-regulated in all infected mice during the acute stage of infection, this was only seen during the late stage of infection in infected-obese mice. In addition, ppMCH mRNA and protein expression in the lateral hypothalamus was decreased in the absence of neuronal death. These results show the importance of ppMCH in the establishment and maintenance of obesity and the involvement of a virus as an environmental factor.

Melanin-concentrating hormone-producing neurons in birds

The peptidergic melanin-concentrating hormone (MCH) system was investigated by immunocytochemistry in several birds. MCH perikarya were found in the periventricular hypothalamic nucleus near the paraventricular organ and in the lateral hypothalamic areas. Immunoreactive fibers were very abundant in the ventral pallidum, in the nucleus of the stria terminalis, and in the septum/diagonal band complex, where immunoreactive pericellular nets were prominent. Many fibers innervated the whole preoptic area, the lateral hypothalamic area, and the infundibular region. Some fibers also reached the dorsal thalamus and the epithalamus. The median eminence contained only sparse projections, and the posterior pituitary was not labeled. Thus, in birds, a neurohormonal role for MCH is not likely. Immunoreactive fibers were observed in other regions, such as the intercollicular nucleus, stratum griseum periventriculare (mesencephalic tectum), central gray, nigral complex (especially the ventral tegmental area), reticular areas, and raphe nuclei. Although no physiological investigation concerning the role of MCH has been performed in birds, the distribution patterns of the immunoreactive perikarya and fibers observed suggest that MCH may be involved in functions similar to those described in rats. In particular, the projections to parts of the limbic system (ventropallidal ganglia, septal complex, hypothalamus, dorsal thalamus, and epithalamus) and to structures concerned with visceral and other sensory information integration suggest that MCH acts as a neuromodulator involved in a wide variety of physiological and behavioral adaptations (arousal) with regard to feeding, drinking, and reproduction.

Cellular localization and role of prohormone convertases in the processing of pro-melanin concentrating hormone in mammals

Melanin concentrating hormone (MCH) and neuropeptide EI (NEI) are two peptides produced from the same precursor in mammals, by cleavage at the Arg145-Arg146 site and the Lys129-Arg130 site, respectively. We performed co-localization studies to reveal simultaneously the expression of MCH mRNA and proconvertases (PCs) such as PC1/3 or PC2. In the rat hypothalamus, PC2 was present in all MCH neurons, and PC1/3 was present in about 15-20% of these cells. PC1/3 or PC2 was not found in MCH-positive cells in the spleen. In GH4C1 cells co-infected with vaccinia virus (VV):pro-MCH along with VV:furin, PACE4, PC1/3, PC2, PC5/6A, PC5/6B, or PC7, we observed only efficient cleavage at the Arg145-Arg146 site to generate mature MCH. Co-expression of pro-MCH together with PC2 and 7B2 resulted in very weak processing to NEI. Comparison of pro-MCH processing patterns in PC1/3- or PC2-transfected PC12 cells showed that PC2 but not PC1/3 generated NEI. Finally, we analyzed the pattern of pro-MCH processing in PC2 null mice. In the brain of homozygotic mutants, the production of mature NEI was dramatically reduced. In contrast, MCH content was increased in the hypothalamus of PC2 null mice. In the spleen, a single large MCH-containing peptide was identified in both wild type and PC2 null mice. Together, our data suggest that pro-MCH is processed differently in the brain and in peripheral organs of mammals. PC2 is the key enzyme that produces NEI, whereas several PCs may cleave at the Arg145-Arg146 site to generate MCH in neuronal cell types.

17beta-estradiol regulation of melanin-concentrating hormone and neuropeptide-E-I contents in cynomolgus monkeys: a preliminary study

Melanin-concentrating hormone (MCH) and neuropeptide-E-I (NEI) regulate several behaviors and neuroendocrine functions in rats. Possible influence of these peptides on sexual behavior and reproduction in mammals other than rodents prompted us to investigate: 1) The sites of synthesis of MCH and NEI in the brain of a non-human primate (M. fascicularis); 2) The effect of 17 beta-estradiol (E2) benzoate (E2B) on pro-MCH-derived peptide concentrations in the hypothalamus of the ovariectomized (OVX) cynomolgus monkeys (M. fascicularis). Expression of MCH mRNA and peptides was examined by Northern blotting, RT-PCR and RP-HPLC/RIA. Our results demonstrate that the MCH gene is predominantly expressed in hypothalamus of macaque. E2B exposure of OVX monkeys provoked parallel phasic variations in the MCH-immunoreactivity (IR) and NEI-IR. NEI-IR and to a lesser extent MCH-IR, showed a transient increase (associated with the estradiol peak) at 30 h with a final rise of both MCH-IR and NEI-IR observed at the time (72 h post E2B) of the luteinizing hormone (LH) surge. RP-HPLC analysis of peptide extracts revealed the presence, in addition to mature MCH and NEI, of different MCH-IR and NEI-IR forms in the hypothalami of control and E2B-treated monkeys. Taken together, our results indicated that hypothalamic MCH and NEI contents are regulated after E2B treatment and they suggest the possible involvement of these peptides in the regulation of the pre-ovulatory midcycle LH surge in primates.

Polyethylene glycol-induced hypovolemia affects the expression of MCH mRNA, but not dynorphin or secretogranin II mRNAs, in the rat lateral hypothalamus

Two prominent neuron populations of the rat lateral hypothalamus express genes encoding respectively the prepromelanin-concentrating hormone (MCH) or dynorphin (DYN) and secretogranin II (SGII). Their roles remain hypothetical in mammals. In the present study, we examined the changes in MCH, DYN and SGII gene expression in dehydrated rats compared to controls. Dehydration was obtained by subcutaneous injection of polyethylene glycol (PEG) resulting in a large reduction of the extracellular fluid volume. Using competitive semi-quantitative RT-PCR and in situ hybridization methods, PEG-injected animals showed a significant increase of MCH mRNA level but no variation of DYN and SGII mRNA levels. These results confirm previous observations suggesting that intra- and extracellular dehydration challenges affect different regulation circuits; they indicate that both neuron populations could be involved in the maintenance of body fluid homeostasis, directly, or indirectly, as integrators of various information leading to goal-oriented behaviour.

Melanin-concentrating hormone system in the brain of the lungfish Protopterus annectens

The neurochemical anatomy of the lungfish brain is of particular interest, because many features in these animals might be representative of the common ancestor of land vertebrates. In the present study, we have investigated the localization and biochemical characteristics of melanin-concentrating hormone (MCH)-immunoreactive material in the central nervous system of the African lungfish, Protopterus annectens. The most prominent group of MCH-immunoreactive cell bodies was found in the dorsal hypothalamus. Additional groups of MCH-immunoreactive perikarya were detected in the telencephalon within the medial and dorsal pallium, the medial subpallium, and the ventral part of the lateral subpallium. Brightly immunofluorescent nerve fibers were seen in the anterior olfactory nucleus, the ventral part of the medial pallium, the medial subpallium, and the anterior preoptic area. In the diencephalon, the hypothalamus and the medial region of the dorsal thalamus exhibited a dense accumulation of fibers. MCH-immunoreactive fibers were also found in the tectum and the tegmentum of the mesencephalon and within the reticular formation of the rhombencephalon. In the pituitary, several small groups of cells of the intermediate lobe showed a bright fluorescence. Reversed-phase high-performance liquid chromatography (HPLC) analysis of diencephalon and pituitary extracts resolved a major MCH-immunoreactive peak that coeluted with synthetic salmon MCH. The distribution of MCH in the brain of P. annectens suggests that, in lungfishes, this peptide may exert neuromodulator or neurotransmitter functions. The presence of MCH-like immunoreactivity in the intermediate lobe of the pituitary indicates that, in dipnoans, MCH may also act as a typical pituitary hormone.

Lithium increases melanin-concentrating hormone mRNA stability and inhibits tyrosine hydroxylase gene expression in PC12 cells

Melanin-concentrating hormone (MCH) is a cyclic peptide involved in the regulation of food-intake behaviour and stress response in mammals. Expression of the MCH gene predominates in hypothalamic neurons. Mechanisms governing the regulation of expression of MCH gene in established cell lines were not explored yet. Here, we analysed the actions of nerve growth factor (NGF), dexamethasone, forskolin and lithium on MCH mRNA levels in the PC12 pheochromocytoma cell line. We compared them with those observed on tyrosine hydroxylase (TH) mRNA, constitutively expressed in PC12 cells, and neurotensin (NT) mRNA, taken as a control. In untreated cells, MCH RNA species of high molecular weight were found. Exposure of cells at a combination of NGF and lithium resulted in decreased expression of these MCH RNAs and in the transient production of mature MCH mRNA. Strikingly, after short exposure of PC12 cells to NGF, lithium per se elicited a marked increase in MCH mRNA levels whilst it exerted a potent inhibitory action on TH mRNA expression. Detailed investigations revealed that lithium enhanced MCH mRNA expression through post-transcriptional mechanisms whereas it regulated TH gene expression mainly at the level of transcription. These results demonstrate that lithium, an agent widely used for treatment of manic depressive illness, can exert an opposite effect on MCH and TH mRNA production in PC12 cells. The MCH gene system in NGF-treated PC12 cells provides a good opportunity for studying the effect of lithium on gene expression at post-transcriptional levels in a neuron-like cellular model.

The structural organization of connections between hypothalamus and cerebral cortex

Motivated behavior requires coordinated somatic, autonomic, and endocrine responses, and may be divided into initiation, procurement, and consummatory phases (Swanson, L.W. and Mogenson, G.J., Neural mechanisms for the functional coupling of autonomic, endocrine and somatomotor responses in adaptative behavior, Brain Res. Rev., 3 (1981) 1-34). Obviously, such behavior may involve the entire central nervous system, although it is important to identify circuitry or systems that mediate the behavior directed toward specific goal objects. This problem has recently been clarified by the identification of hypothalamic subsystems important for the execution of instinctive behaviors related to ingestion, reproduction, and defense. These subsystems are modulated by sensory (reflex), central control (e.g., circadian), and voluntary (cortical) inputs. The latter are dominated by inputs from the ventral temporal lobe and medial prefrontal region, which are both direct and via associated parts of the basal nuclei (ganglia). Hypothalamic output is characterized by descending projections to brainstem and spinal motor systems, and by projections back to the cerebral cortex, which are both direct and via a continuous rostromedial part of the dorsal thalamus. This thalamic region includes the anterior, medial, and midline groups, which in turn innervate a continuous ring of cortex that includes the hippocampal formation and the cingulate, prefrontal, and insular regions. Parts of this thalamic region also innervate the ventral striatum, which receives a massive input from the cortical rings as well.

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

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

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

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

Immunocytochemical detection of the neurokinin B receptor (NK3) on melanin-concentrating hormone (MCH) neurons in rat brain

The presence of the neurokinin B receptor (NK3 receptor) in the rat lateral hypothalamus and the zona incerta was previously reported. The aim of the present study was to define its cellular localization in these areas. Investigations, coupling immunocytochemical and in situ hybridization techniques, focussed on two neuron populations: the melanin-concentrating hormone (MCH) neurons and a population of neurons recognized by an ovine prolactin antiserum (PRL-ir neurons). While PRL-ir neurons did not exhibit NK3 immunoreactivity, 57% +/- 6% of MCH neurons were strongly stained by the NK3 antiserum. These results suggest that neurokinin B is involved in the regulation of MCH neuron activity via the NK3 receptor; they provide new bases for further investigations on MCH role in the control of food and water intake.

Distribution of melanin-concentrating hormone (MCH)-like immunoreactivity in neurons of the diencephalon of sheep

An immunohistochemical study with an antiserum raised against salmon melanin concentrating-hormone has demonstrated the presence of numerous melanin concentrating-hormone-immunoreactive neurons in the lateral hypothalamic areas of the sheep. The pattern of distribution of these perikarya is similar to that of rodents and primates. In sheep, however, melanin concentrating-hormone-immunoreactive neurons appeared to form two gatherings: the first is situated ventromedially to the internal capsule and the second in the dorsolateral hypothalamus. In these areas, numerous immunostained perikarya are observed. Compared to the rats, labelled neurons extended more caudally in the ventral tegmental area and more rostrally above the optic chiasma. Compared to primates, these neurons are less numerous in the periventricular area. In our study, dense networks of melanin concentrating-hormone-immunoreactive varicose fibers were observed in the supramamillary nucleus, the lateral hypothalamus, the nucleus medialis thalami and nucleus reuniens and in the bed nucleus of the stria terminalis.

A role for melanin-concentrating hormone in the central regulation of feeding behaviour

The hypothalamus plays a central role in the integrated regulation of energy homeostasis and body weight, and a number of hypothalamic neuropeptides, such as neuropeptide Y (ref. 1), galanin, CRH (ref. 3) and GLP-1 (ref. 4), have been implicated in the mediation of these effects. To discover new hypothalmic peptides involved in the regulation of body weight, we used differential display polymerase chain reaction to identify messenger RNAs that are differentially expressed in the hypothalamus of ob/+ compared with ob/ob C57B1/6J mice. We show here that one mRNA that is overexpressed in the hypothalamus of ob/ob mice encodes the neuropeptide melanin-concentrating hormone (MCH). Fasting further increased expression of MCH mRNA in both normal and obese animals. Neurons containing MCH are located in the zona incerta and in the lateral hypothalamus. These areas are involved in regulation of ingestive behaviour, but the role of MCH in mammalian physiology is unknown. To determine whether MCH is involved in the regulation of feeding, we injected MCH into the lateral ventricles of rats and found that their food consumption increased. These findings suggest that MCH participates in the hypothalamic regulation of body weight.

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

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

The synthesis of melanin-concentrating hormone is stimulated by ventromedial hypothalamic lesions in the rat lateral hypothalamus: a time-course study

The activity of melanin-concentrating (MCH) neurons, was investigated by immunocytochemical and hybridocytochemical techniques in male rats bearing limited lesions of the ventromedial hypothalamic nuclei (VMN). 2 days after operation, the abundance of immunoreactive cell bodies and fibres and the intensity of labelling seemed slightly decreased in lesioned rats as compared to controls while no significant difference could be detected in MCH gene expression. After 8 days, synthesis, storage and transport of MCH appeared strongly stimulated and this stimulation lasted until the end of the experiment (day 35), suggesting that VMN plays a physiological role in controlling MCH neuron activity.

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

Melanin-concentrating hormone (MCH) is involved in the regulation of body colour in teleost fish. A peptide highly homologous to salmon MCH has been found in the rat brain, but its physiological functions have not yet been precisely defined. The location of MCH neurons in the lateral hypothalamus (LHT) of the rat suggests possible implication in feeding behaviour. In the present study, immunohistochemical and in situ hybridization methods were used to investigate MCH gene expression following insulin injections. Five hours after insulin injection, a significant increase in the abundance and staining intensity of MCH immunoreactive perikarya and fibres was observed. Concurrently the level of MCH mRNA significantly increased (50%). Insulin-treatment also induced a marked and progressive increase in the number and staining intensity of nuclei detected by a Fos antiserum in LHT and other brain areas. Double labelling technique demonstrated that very few if any MCH neurons exhibited Fos-like immunoreactivity. These results demonstrate that an insulin-treatment stimulates MCH neuron activity without the mediation of the proto-oncogene c-fos. The mechanisms triggering this activation remain to be elucidated.

Activation of the rat melanin-concentrating hormone neurons by ventromedial hypothalamic lesions

The occurrence of a melanin-concentrating hormone-like peptide (MCH) was previously reported in the lateral hypothalamus of the rat. The sequence of this peptide was determined but its role as well as its regulation remain unclear. In the present study, we examined the effects of minor electrolytic lesions of the ventromedial nuclei (VMN) on MCH neurons by using immunocytochemical and in situ hybridization procedures. We report that VMN lesions resulted in (1) a clear elevation in the number and staining intensity of MCH immunoreactive perikarya and fibres, (2) a significant increase in the level of hybridocytochemical signal obtained with an oligonucleotide probe complementary to rMCH mRNA. These data provide evidence for a role of VMN in modulating the MCH gene a peptide expression.

Induction of Fos-immunoreactivity in prolactin-like containing neurons of the rat lateral hypothalamus after insulin treatment

The role of the lateral hypothalamus (LHT) in the regulation of feeding behavior has long been established. The major contribution of LHT glucose-sensitive neurons, activated by low glucose concentration, is well accepted. Two peptidergic neuron populations, whose perikarya are exclusively located within this area, have been recently described. They respectively produce peptides derived from the melanin-concentrating hormone (MCH) precursor and prolactin-like immunoreactive peptide (PLI). In the present study, hypoglycemia-induced neuron stimulation was assessed in the rat LHT, by using Fos immunocytochemistry 1 to 5 h after a single insulin injection. In control animals, very scarce Fos-like immunoreactivity (FLI)-containing nuclei were observed in LHT. Insulin treatment induced an important and progressive increase in the number and staining intensity of neuron nuclei detected by Fos antiserum. Five hours after injection, few MCH neurons exhibited FLI, but about 80% of PLI-containing neurons expressed FLI and more than 50% of the FLI-containing nuclei belonged to PLI neurons. It was thus concluded that PLI-expressing neurons, which dramatically responded to insulin treatment, might correspond to a subpopulation of the glucose-sensitive LHT neurons.

Diffusible factors from rat arcuate nucleus and Broca's diagonal band nucleus increase size and neurite outgrowth, respectively, of cultured melanin-concentrating hormone containing neurons

Using a co-culture model, we showed that diffusible factors from arcuate nucleus (AN) specifically increased the number and the size of hypothalamic neurons producing melanin-concentrating hormone (MCH). In this model neurite outgrowth and contacts between MCH neurons and dopaminergic neurons were also prominently increased, as compared to control lateral areas of the posterior hypothalamus (LH) primary cultures. These effects were mediated in part by AN glia but also by neurons of both fetal and adult AN. AN glia produced diffusible factor(s) mainly responsible for an important MCH neurite outgrowth and expressed inhibiting factors, preventing the adhesion of LH cells on AN glial cells. Furthermore, we report here a nerve growth factor-like effect from Broca's diagonal band on MCH hypothalamic neurons.