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.     

Establishment of mouse-immortalized hybrid clones expressing characteristics of differentiated neurons derived from the cerebellar and brain stem regions.

Two clonal immortalized neurons designated CL8c4.7 and CL8a5.2 were established by somatic cell fusion between a hypoxanthine phosphoribosyltransferase-(HPRT-) deficient neuroblastoma N18TG2 and newborn mouse cerebellar/brain stem neurons. In the serum-containing medium without extra differentiating agents, both clones exhibited a morphology of differentiated neurons. They contained high levels of glutamate but no gamma-aminobutyric acid (GABA). The CL8a5.2 clone synthesized choline acetyltransferase and serotonin. In immunocytochemical studies, both clones expressed 200 kD neurofilament protein, neuron-specific enolase, microtubule-associated protein 2 (MAP2), tau protein, neuronal cell adhesion molecule (N-CAM), HNK-1, Thy-1.2, saxitoxin-binding sodium channel protein, and glutamate. Synaptophysin immunoreactivity was identified in the neuritic terminals of CL8c4.7 cells. Most of these antigens were barely detectable on N18TG2 cells. Electrophysiologically, both clones generated action potentials in response to electrical stimuli. The hybrid clones that express characteristics of differentiated neurons derived from the cerebellar and brain stem regions might be invaluable for the study of the molecular basis of neuronal differentiation and degeneration in these regions.     

Establishment of mouse-immortalized hybrid clones expressing characteristics of differentiated neurons derived from the cerebellar and brain stem regions

Two clonal immortalized neurons designated CL8c47 and CL8a5.2 were established by somatic cell fusion between a hypoxanthine phosphoribosyltransferase (HPRT^?) deficient neuroblastoma N18TG2 and newborn mouse cerebellar/brain stem neurons. In the serum-containing medium without extra differentiating agents, both clones exhibited a morphology of differentiated neurons. They contained high levels of glutamate but no gamma-aminobutyric acid (GABA). The CL8a5.2 clone synthesized choline aceryltransferase and serotonin. In immunocytochemical studies, both clones expressed 200 kD neurofilament protein neuron-specific enolase, microtubule-associated protein 2 (MAP2), tau protein, neuronal cell adhesion molecule (N-CAM), HNK-1, Thy-1.2, saxitoxin-binding sodium channel protein, and glutamate. Synaptophysin immunoreactivity was identified in the neuritic terminals of CL8c4.7 cells. Most of these antigens were barely detectable on N18TG2 cells. Electrophysiologically, both clones generated action potentials in response to electrical stimuli. The hybrid clones that express characteristics of differentiated neurons derived from the cerebellar and brain stem regions might be invaluable for the study of the molecular basis of neuronal differentiation and degeneration in these regions. © 1992 John Wiley & Sons, Inc.     

Selective expression of Narp,a secreted neuronal pentraxin,in orexin neurons

Recent studies have provided compelling evidence demonstrating that orexin (also known as hypocretin) neurons play a central role in the pathophysiology of narcolepsy. However, targeted deletion of orexin does not fully mimic the functional deficits induced by selective ablation of these neurons; implying that other secreted signaling molecules expressed in these neurons mediate key aspects of their function. In this study, we demonstrate that orexin neurons display robust expression of neuronal activity-regulated pentraxin (Narp), a secreted neuronal pentraxin, implicated in regulating clustering of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptors. Furthermore, we have found that hypothalamic melanin-concentrating hormone (MCH) neurons, which form a peptidergic pathway thought to oppose the effects of the orexin system, express another neuronal pentraxin, NP1. Thus, these findings suggest that these pathways utilize neuronal pentraxins, in addition to neuropeptides, as synaptic signaling molecules.     

Physiological properties of hypothalamic MCH neurons identified with selective expression of reporter gene after recombinant virus infection

Neurons that synthesize melanin-concentrating hormone (MCH) may modulate arousal and energy homeostasis. The scattered MCH neurons have been difficult to study, as they have no defining morphological characteristics. We have developed a viral approach with AAV for selective long-term reporter gene (GFP) expression in MCH neurons, allowing the study of their cellular physiology in hypothalamic slices. MCH neurons showed distinct membrane properties compared to other neurons infected with the same virus with a cytomegalovirus promoter. Transmitters of extrahypothalamic arousal systems, including norepinephrine, serotonin, and the acetylcholine agonist muscarine, evoked direct inhibitory actions. Orexigenic neuropeptide Y was inhibitory by pre- and postsynaptic mechanisms; an anorexigenic melanocortin agonist had no effect. In contrast, the hypothalamic arousal peptide hypocretin/orexin evoked a direct inward current and increased excitatory synaptic activity and spike frequency in the normally silent MCH neurons. Together, these data support the view that MCH neurons may integrate information within the arousal system in favor of energy conservation.     

Immunocytochemical localization and ontogenic development of melanin-concentrating hormone in the brain of a pleuronectiform fish,the barfin flounder

Melanin-concentrating hormone (MCH) was first discovered in the pituitary of chum salmon because of its role in the regulation of skin pallor. Later, it was found that MCH could also play a role as a central neurotransmitter or neuromodulator in the brain. However, knowledge of the function of MCH in fish has been restricted to certain fish species. Therefore, in the present study, the immunocytochemical localization and ontogenic development of MCH in the brain of a pleuronectiform fish, the barfin flounder Verasper moseri, were examined to obtain a better understanding of this hormone. In adult barfin flounder, MCH-immunoreactive (ir) neuronal somata were most prevalent in the magnocellular neurons of the nucleus tuberis lateralis (NLT), which project to the pituitary. In the pituitary, MCH-ir fibers were distributed in the neurohypophysial tissues within the pars intermedia and, to a lesser extent, into the pars distalis. MCH-ir neuronal somata were also present in dorsally projecting parvocellular neurons, located more posteriorly in the area above the lateral ventricular recess (LVR). LVR-MCH neurons did not seem to project to the pituitary. In the brain, MCH-ir fibers were detected not only in the hypothalamus but also in areas such as the optic tectum and thalamus. MCH-ir neuronal somata and fibers were not detected on the day of hatching. MCH-ir neuronal somata and fibers were first detected in the hypothalamus and the pituitary, respectively, 7 days after hatching. Subsequently, MCH-ir neuronal somata were observed in the NLT and in the area above the LVR 14 days after hatching. The distribution of MCH-ir neuronal somata and fibers showed a pattern similar to that in the adult fish 35-42 days after hatching. These results indicate that MCH neurons were located in the NLT and in the area above the LVR and that NLT-MCH neurons project to the pituitary. MCH neurons were first detected 7 days after hatching, suggesting that MCH plays some physiological role in the early development of barfin flounder.     

Melanin-concentrating hormone producing neurons: Activities and modulations

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

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

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

Production of GABA by cultured hippocampal glial cells

Medium conditioned by cultured hippocampal glial contains an inhibitory factor that can hyperpolarize and suppress neuronal activity. Using biochemistry, electrophysiology, pharmacology, and mass spectrometry, we have identified the inhibitory factor as GABA (gamma-aminobutyric acid). Like GABA, the inhibitory factor increases chloride and potassium currents in neurons, which can be blocked by bicuculline. Mass spectrometry analysis of conditioned medium reveals peaks that are identical to that for GABA. Up to 500 micromolar GABA is found in conditioned medium from glial cultures. No GABA is found in conditioned medium from neuronal cultures. Hippocampal glia make much more GABA than cortical glia or glia from other brain regions. It is not clear how hippocampal glia synthesize GABA. Although they express GAD mRNA and adding glutamate to the culture medium increases the amount of GABA produced, other data suggest that glia do not use GAD to make GABA. Identifying the mechanism(s) by which GABA is produced by hippocampal glia would help clarify its role in modulating neuronal activity in the brain.     

Effects of Arachidonic Acid on Glutamate and γ-Aminobutyric Acid Uptake in Primary Cultures of Rat Cerebral Cortical Astrocytes and Neurons

The effects of arachidonic acid on glutamate and gamma-aminobutyric acid (GABA) uptake were studied in primary cultures of astrocytes and neurons prepared from rat cerebral cortex. The uptake rates of glutamate and GABA in astrocytic cultures were 10.4 nmol/mg protein/min and 0.125 nmol/mg protein/min, respectively. The uptake rates of glutamate and GABA in neuronal cultures were 3.37 nmol/mg protein/min and 1.53 nmol/mg protein/min. Arachidonic acid inhibited glutamate uptake in both astrocytes and neurons. The inhibitory effect was observed within 10 min of incubation with arachidonic acid and reached approximately 80% within 120 min in both types of culture. The arachidonic acid effect was not only time-dependent, but also dose-related. Arachidonic acid, at concentrations of 0.015 and 0.03 mumol/mg protein, significantly inhibited glutamate uptake in neurons, whereas 20 times higher concentrations were required for astrocytes. The effects of arachidonic acid were not as deleterious on GABA uptake as on glutamate uptake in both astrocytes and neurons. In astrocytes, GABA uptake was not affected by any of the doses of arachidonic acid studied (0.015-0.6 mumol/mg protein). In neuronal cultures, GABA uptake was inhibited, but not to the same degree observed with glutamate uptake. Lower doses of arachidonic acid (0.03 and 0.015 mumol/mg protein) did not affect neuronal GABA uptake. Other polyunsaturated fatty acids, such as docosahexaenoic acid, affected amino acid uptake in a manner similar to arachidonic acid in both astrocytes and neurons. However, saturated fatty acids, such as palmitic acid, exerted no such effect. The significance of the arachidonic acid-induced inhibition of neurotransmitter uptake in cultured brain cells in various pathological states is discussed.     

Molecular annotation of integrative feeding neural circuits

The identity of higher-order neurons and circuits playing an associative role to control feeding is unknown. We injected pseudorabies virus, a retrograde tracer, into masseter muscle, salivary gland, and tongue of BAC-transgenic mice expressing GFP in specific neural populations and identified several CNS regions that project multisynaptically to the periphery. MCH and orexin neurons were identified in the lateral hypothalamus, and Nurr1 and Cnr1 in the amygdala and insular/rhinal cortices. Cholera toxin β tracing showed that insular Nurr1(+) and Cnr1(+) neurons project to the amygdala or lateral hypothalamus, respectively. Finally, we show that cortical Cnr1(+) neurons show increased Cnr1 mRNA and c-Fos expression after fasting, consistent with a possible role for Cnr1(+) neurons in feeding. Overall, these studies define a general approach for identifying specific molecular markers for neurons in complex neural circuits. These markers now provide a means for functional studies of specific neuronal populations in feeding or other complex behaviors.     

Electrophysiological effects of MCH on neurons in the hypothalamus

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

Immunocytochemical detection of orexin A in endocrine cells of the developing mouse gut.

Orexins are novel neuropeptides that were originally localized in neurons of the hypothalamus and neuronal fibers of the brain. Recently orexin A and its receptor have also been reported in neurons and endocrine cells of the gastrointestinal tract. Because no studies have been done at the embryonic period, we studied the appearance and distribution of orexin A during the development of mouse gastrointestinal tract using immunocytochemical methods. Immunoreactivity to orexin A was detected in neuroendocrine cells of the pyloric region of the stomach at gestational Day 14 and 1 day after in the small intestine. The numbers of immunoreactive cells progressively increased through development until the adult pattern was reached. Staining of reverse-face sections demonstrated that orexin A and serotonin co-localized in some endocrine cells of the mouse stomach and small intestine. These findings suggest that orexin A may be relevant in the growth and maturation of the gastrointestinal tract during intrauterine life.     

GABAA Receptors Modulate Early Spontaneous Excitatory Activity in Differentiating P19 Neurons

Abstract: P19 embryonic carcinoma (EC) stem cells are pluripotent and are efficiently induced to differentiate into neurons and glia with retinoic acid (RA) treatment. Within 5 days, a substantial number of differentiating P19 cells express gene products that are characteristic of a neuronal phenotype. P19 neurons were used as a model to explore the relationship between neuronal “differentiation” in vitro and the acquisition of γ-aminobutyric acid (GABA_A) receptors and functional GABA responses. Pulse-labeling experiments using bromodeoxyuridine indicated that all neurons had become postmitotic within 3–4 days after treatment with RA. This was confirmed by a reduction in the immunocytochemical detection of the undifferentiated stem cell antigen SSEA-1. Subsequently, a transient expression of nestin was observed during the first 5 days in vitro (DIV) after exposure to RA. By 5–10 DIV after RA, a significant number of neurons (～80–90%) expressed immunocytochemically detectable glutamate decarboxylase and GABA coincident with the acquisition of membrane binding sites for tetanus toxin. These phenotypic markers were maintained for >30 DIV after RA. Under current-clamp conditions, random, low-amplitude, spontaneous electrical activity appeared in neurons within the first few days after RA treatment and this was blocked by the specific GABA_A receptor antagonist bicuculline. Thereafter, the appearance and progressive increases in the frequency of spontaneous action potentials in P19 neurons were observed that were similarly attenuated by bicuculline. In neurons > 5 DIV after RA, exogenous application of GABA elicited similar action potentials. The onset of excitatory responses to GABA or muscimol in voltage-clamped neurons appeared immediately after the cessation of neuronal mitosis and before the previously reported acquisition of responses to glutamate. In fura-2 imaging studies, the exogenous application of GABA resulted in neuron-specific increases in intracellular Ca²⁺. Thus, P19 neurons provide an in vitro model for the study of the early acquisition and properties of electrical excitability to GABA and the expression of functional GABA_A receptors.     

Development of the glutamate system in rabbit retina

We have investigated two characteristics of the glutamate system in the developing rabbit retina. 1) Glutamate immunoreactivity was observed at birth within developing processes of four cell types; two of which, photoreceptors and ganglion cells, are known to be glutamatergic in the adult. Two other cell types, type A horizontal cells and amacrine cells, are immunoreactive to both glutamate and GABA at birth, suggesting that endogenous pools of glutamate in GABAergic neurons serve as precursor for GABA synthesis. Thus it appears that endogenous glutamate pools are present within neurons prior to synaptogenesis as part of the early expression of either the glutamate or GABA transmitter phenotype. 2) Analysis of³H-glutamate metabolism during retinal development showed that rapid conversion of glutamate to glutamine does not occur until the second postnatal week, coincident with the expression of Muller (glial) cell activity. In the absence of glial metabolism in the neonate, extracellular concentrations of glutamate remain relatively high and are likely to have major effects on neuronal maturation.Special issue dedicated to Dr. Frederick E. Samson     

Hypothalamic action of glutamate leads to body mass reduction through a mechanism partially dependent on JAK2

Glutamate acts in the hypothalamus promoting region-, and cell-dependent effects on feeding. Part of these effects are mediated by NMDA receptors, which are up regulated in conditions known to promote increased food intake and thermogenesis, such as exposure to cold and consumption of highly caloric diets. Here, we hypothesized that at least part of the effect of glutamate on hypothalamic control of energy homeostasis would depend on the control of neurotransmitter expression and JAK2 signaling. The expression of NMDA receptors was co-localized to NPY/AgRP, POMC, CRH, and MCH but not to TRH and orexin neurons of the hypothalamus. The acute intracerebroventricular injection of glutamate promoted a dose-dependent increase in JAK2 tyrosine phosphorylation. In obese rats, 5 days intracerebroventricular treatment with glutamate resulted in the reduction of food intake, accompanied by a reduction of spontaneous motility and reduction of body mass, without affecting oxygen consumption. The reduction of food intake and body mass were partially restrained by the inhibition of JAK2. In addition, glutamate produced an increased hypothalamic expression of NPY, POMC, CART, MCH, orexin, CRH, and TRH, and the reduction of AgRP. All these effects on neurotransmitters were hindered by the inhibition of JAK2. Thus, the intracerebroventricular injection of glutamate results in the reduction of body mass through a mechanism, at least in part, dependent on JAK2, and on the broad regulation of neurotransmitter expression. These effects are not impaired by obesity, which suggest that glutamate actions in the hypothalamus may be pharmacologically explored to treat this disease.