Hypothalamic Neuronal Populations Research Articles

GnRH Neurons Directly Listen to the Periphery

GnRH Neurons Directly Listen to the Periphery

The hypothalamus and the neurobiology of drug seeking.

The hypothalamus is a neural structure critical for expression of motivated behaviours that ensure survival of the individual and the species. It is a heterogeneous structure, generally recognised to have four distinct regions in the rostrocaudal axis (preoptic, supraoptic, tuberal and mammillary). The tuberal hypothalamus in particular has been implicated in the neural control of appetitive motivation, including feeding and drug seeking. Here we review the role of the tuberal hypothalamus in appetitive motivation. First, we review evidence that different regions of the hypothalamus exert opposing control over feeding. We then review evidence that a similar bi-directional regulation characterises hypothalamic contributions to drug seeking and reward seeking. Lateral regions of the dorsal tuberal hypothalamus are important for promoting reinstatement of drug seeking, whereas medial regions of the dorsal tuberal hypothalamus are important for inhibiting this drug seeking after extinction training. Finally, we review evidence that these different roles for medial versus lateral dorsal tuberal hypothalamus in promoting or preventing reinstatement of drug seeking are mediated, at least in part, by different populations of hypothalamic neurons as well as the neural circuits in which they are located.

Inflaming Hypothalamic Neurons Raises Blood Pressure

Obesity and hypertension are strongly associated, and neural dysfunction has been implicated in both. The hypothalamus integrates signals regulating blood pressure and energy homeostasis. A recent paper in Nature Medicine (Purkayastha etal., 2011) suggests that obesity and hypertension are caused by inflammation in distinct hypothalamic neuronal populations.

Mutant Huntingtin Causes Metabolic Imbalance by Disruption of Hypothalamic Neurocircuits

In Huntington's disease (HD), the mutant huntingtin protein is ubiquitously expressed. The disease was considered to be limited to the basal ganglia, but recent studies have suggested a more widespread pathology involving hypothalamic dysfunction. Here we tested the hypothesis that expression of mutant huntingtin in the hypothalamus causes metabolic abnormalities. First, we showed that bacterial artificial chromosome-mediated transgenic HD (BACHD) mice developed impaired glucose metabolism and pronounced insulin and leptin resistance. Selective hypothalamic expression of a short fragment of mutant huntingtin using adeno-associated viral vectors was sufficient to recapitulate these metabolic disturbances. Finally, selective hypothalamic inactivation of the mutant gene prevented the development of the metabolic phenotype in BACHD mice. Our findings establish a causal link between mutant huntingtin expression in the hypothalamus and metabolic dysfunction, and indicate that metabolic parameters are powerful readouts to assess therapies aimed at correcting dysfunction in HD by silencing huntingtin expression in the brain.

AGRP neurons are sufficient to orchestrate feeding behavior rapidly and without training.

Two intermingled hypothalamic neuron populations, specified by expression of agouti-related peptide (AGRP) or pro-opiomelanocortin (POMC), positively and negatively influence feeding behavior respectively, possibly by reciprocally regulating downstream melanocortin receptors. However, the sufficiency of these neurons to control behavior, and the relationship of their activity to the magnitude and dynamics of feeding are unknown. To measure this, we used channelrhodopsin-2 for cell type-specific photostimulation. Activation of only 800 AGRP neurons in mice evoked voracious feeding within minutes. The behavioral response increased with photoexcitable neuron number, photostimulation frequency, and stimulus duration. Conversely, POMC neuron stimulation reduced food intake and body weight, which required melanocortin receptor signaling. However, AGRP neuron-mediated feeding was not dependent on suppressing this melanocortin pathway, indicating that AGRP neurons directly engage feeding circuits. Furthermore, feeding was evoked selectively over drinking without training or prior photostimulus exposure, which suggests that AGRP neurons serve a dedicated role coordinating this complex behavior.

Kisspeptins and the neuroendocrine control of reproduction

Reproductive function, as essential for the survival of species, is under the control of a vast array of regulatory factors that ultimately modulate the release of GnRH. However, GnRH neurons lack the ability to directly sense most of these signals; hence, intermediate pathways are required. Kisspeptins have recently emerged as a pivotal piece in the reproductive brain, serving primarily as conduits for central and peripheral regulatory cues of GnRH release. Different populations of hypothalamic Kiss1 neurons have been described, which mediate either the positive or negative feedback of sex steroids in the sexually differentiated brain of rodents. Kisspeptins, however, are not the only recently-appointed contributors to this integrative process. Indeed, neurokinin B (NKB) and dynorphin have been described to co-localize within Kiss1 neurons at the arcuate nucleus in different species, and may contribute to the regulation of kisspeptin release. In this work, we provide a concise overview of the major reproductive headlines of kisspeptins, focusing on their role as mediators of sex steroid feedback and their interaction with key neurotransmitters, such as NKB and dynorphin.

Localization of plasminogen in mouse hippocampus, cerebral cortex, and hypothalamus

Although the tissue plasminogen activator/plasminogen system contributes to numerous brain functions, such as learning, memory, and anxiety behavior, little attention has as yet been given to the localization of plasminogen in the brain. We have investigated the localization of plasminogen in the adult mouse brain by using immunohistochemistry. In the hippocampus, plasminogen immunoreactivity was seen in the pyramidal cell layer as numerous punctate structures in neuronal somata. An electron-microscopic study further demonstrated that the plasminogen-immunoreactive punctate structures represented secretory vesicles and/or vesicle clusters. In the cerebral cortex, plasminogen immunoreactivity was evident in the somata of the layer II/III and V neurons. A quantitative analysis revealed that parvalbumin (PV)-positive neurons had more plasminogen-immunoreactive puncta compared with those of PV-negative neurons in the hippocampus and cerebral cortex. Plasminogen immunoreactivity was present throughout the hypothalamus, being particularly prominent in the neuronal somata of the organum vasculosum laminae terminalis, ventromedial preoptic nucleus, supraoptic nucleus, subfornical organ, medial part of the paraventricular nucleus (PVN), posterior part of the PVN, and arcuate hypothalamic nucleus. Thus, plasminogen is highly expressed in specific populations of hippocampal, cortical, and hypothalamic neurons, and plasminogen-containing vesicles are mainly observed at neuronal somata.

Differential Changes in Responses of Hypothalamic and Brainstem Neuronal Populations to Prolactin During Lactation in the Mouse

During lactation, there are numerous functional adaptations in the maternal brain. There is evidence that the high levels of circulating prolactin present during lactation might contribute to these adaptive changes. The present study aimed to investigate levels of functional prolactin-mediated signal transduction in the brain of lactating mice, using prolactin-induced phosphorylation of signal transducer and activator of transcription 5 (pSTAT5) as a marker, and compare these to the effect of exogenous prolactin during diestrus. On Day 7 of lactation, widespread induction of pSTAT5 was observed in numerous regions of the mouse forebrain and brainstem. In the medial preoptic nucleus, bed nuclei stria terminalis, paraventricular nucleus, and medial amygdala of the forebrain, and in the rostral periaqueductal gray, parabrachial nucleus, dorsal raphe, and the raphe obscurus nucleus of the brainstem, pSTAT5 expression was markedly increased during lactation compared with the response to exogenous prolactin during diestrus. In the anteroventral periventricular nucleus, arcuate nucleus, ventromedial nucleus, and dorsomedial nucleus, responses in lactation were comparable to diestrus. Conversely, in the area postrema of the brainstem, there was a reduction in response to prolactin, with a loss of pSTAT5 expression, during lactation. These differential responses following either acute or chronic elevations in prolactin were not accompanied by any changes in levels of prolactin receptor mRNA, when measured by in situ hybridization. These data are consistent with the hypothesis that prolactin might mediate widespread adaptive responses in the maternal brain.

Gonadotropin-Releasing Hormone Fibers Contact POMC Neurons in the Hypothalamic Arcuate Nucleus

The metabolic state has long been shown to affect reproduction. Peripheral signals and hormones from the reproductive organs are also known to regulate energy metabolism and feeding and energy expenditure. Much attention has been paid to determine the signaling flow from key hypothalamic neuronal populations, including those producing the anorexigenic proopiomelanocortin (POMC) derivate, α-melanocyte stimulating hormone (α-MSH), to the medial preoptic area gonadotropin-releasing hormone (GnRH) neurons, cells that are the drivers of ovulation and reproduction in general. In this study, the authors explored whether a reverse signaling modality may also exist. Specifically, the authors analyzed GnRH efferents in the arcuate nucleus with particular emphasis on their anatomical proximity to arcuate nucleus melanocortin perikarya. Using correlated light and electron microscopy, the authors observed direct apposition between GnRH-containing axon terminals and POMC cell bodies. These data provide the first experimental evidence to suggest that GnRH may have a direct influence on feeding, energy expenditure, and glucose homeostasis, independent of the activity of the gonadal axis.

Evidence of hypothalamic degeneration in the anorectic anx/anx mouse

Mice homozygous for the anorexia (anx) mutation are characterized by poor food intake and death by three to five weeks after birth. By P21 these mice display lower density of hypothalamic neuropeptides, including Agouti gene-related protein (AGRP). The AGRP/neuropeptide Y (NPY) system of the anx/anx mice develops normally until postnatal day (P) 12, then the normal increase in fiber density ceases, in some areas even distinctly decreases. This overlaps with activation of microglia, indicating an inflammatory and/or degenerative process. Here we studied, by in situ hybridization and immunohistochemistry (IHC), the expression of major histocompatibility complex (MHC) class I-related molecules and markers for cellular reactivity in hypothalamus of anx/anx mice. MHC class I transcript and -related proteins were found in arcuate nucleus (Arc), presumably both in neurons and glia, the latter also in areas innervated by AGRP (NPY) neurons. In the anx/anx hypothalamus, using TUNEL labeling, significantly higher number of apoptotic cells were found compared with +/+ mice, and active caspase 6 immunoreactivity was detected in degenerating NPY-fibers as well as signs of "microglia-associated cell death". In addition, Y1 receptor-labeled processes and soma of pro-opiomelanocortin (POMC) neurons, were markedly decreased at P21. These results support the hypothesis of degeneration of hypothalamic arcuate neuron populations in the anx/anx mice, whereby the AGRP system may be first affected, the changes in the POMC system being secondary in this process.

Distribution and neurochemical characterization of protein kinase C-theta and -delta in the rodent hypothalamus

PKC-theta (PKC-θ), a member of the novel protein kinase C family (nPKC), regulates a wide variety of functions in the periphery. However, its presence and role in the CNS has remained largely unknown. Recently, we demonstrated the presence of PKC-θ in the arcuate hypothalamic nucleus (ARC) and knockdown of PKC-θ from the ARC protected mice from developing diet-induced obesity. Another isoform of the nPKC group, PKC-delta (PKC-δ), is expressed in several non-hypothalamic brain sites including the thalamus and hippocampus. Although PKC-δ has been implicated in regulating hypothalamic glucose homeostasis, its distribution in the hypothalamus has not previously been described. In the current study, we used immunohistochemistry to examine the distribution of PKC-θ and -δ immunoreactivity in rat and mouse hypothalamus. We found PKC-θ immunoreactive neurons in several hypothalamic nuclei including the ARC, lateral hypothalamic area, perifornical area and tuberomammillary nucleus. PKC-δ immunoreactive neurons were found in the paraventricular and supraoptic nuclei. Double-label immunohistochemisty in mice expressing green fluorescent protein either with the long form of leptin receptor (LepR-b) or in orexin (ORX) neurons indicated that PKC-θ is highly colocalized in lateral hypothalamic ORX neurons but not in lateral hypothalamic LepR-b neurons. Double-label immunohistochemistry in oxytocin-enhanced yellow fluorescent protein mice or arginine vasopressin-enhanced green fluorescent protein (AVP-EGFP) transgenic rats revealed a high degree of colocalization of PKC-δ within paraventricular and supraoptic oxytocin neurons but not the vasopressinergic neurons. We conclude that PKC-θ and -δ are expressed in different hypothalamic neuronal populations.

Molecular Aspects of Adipokine-Bone Interactions

Adipose tissue is an endocrine organ able to produce a wide series of pleiotropic molecules, defined "adipokines". In addition to the regulation of food intake and energy metabolism, adipokines are also implicated in the complex control of bone biology and specifically of bone remodeling. Leptin, the most studied adipokine, promotes satiety and energy expenditure and its circulating levels are proportional to fat mass. Some paradoxical findings originally suggested the involvement of leptin in controlling bone mass. For example, obese postmenopausal women, with elevated circulating leptin and leptin resistance, appear protected against the development of osteoporosis. Moreover, genetically leptin-deficient mice, which are hypogonadal and obese, display a decreased trabecular volume in long bones, but an increased vertebral bone mass, which is reduced by leptin administration. The complex mechanisms of leptin regulation of bone mass appear to involve selected hypothalamic neuronal populations and the sympathetic outflow, with an important role of osteoblastic beta2-adrenergic receptors. Adiponectin is another adipokine, which promotes insulin sensitivity and is reduced in obese and diabetic subjects. Adiponectin appears to exert a negative effect on bone mass and seems to be an independent predictor of lower bone mass. Although the adipokines resistin and visfatin do not seem to significantly affect bone metabolism, the potential impact of them and other adipokines is still to be determined. Moreover, the molecular adipokine-bone interactions should also be considered in the context of the adipokine changes observed in diseases such as obesity and the metabolic syndrome.

Loss of AMP-activated protein kinase α2 subunit in mouse β-cells impairs glucose-stimulated insulin secretion and inhibits their sensitivity to hypoglycaemia

AMPK (AMP-activated protein kinase) signalling plays a key role in whole-body energy homoeostasis, although its precise role in pancreatic β-cell function remains unclear. In the present stusy, we therefore investigated whether AMPK plays a critical function in β-cell glucose sensing and is required for the maintenance of normal glucose homoeostasis. Mice lacking AMPKα2 in β-cells and a population of hypothalamic neurons (RIPCreα2KO mice) and RIPCreα2KO mice lacking AMPKα1 (α1KORIPCreα2KO) globally were assessed for whole-body glucose homoeostasis and insulin secretion. Isolated pancreatic islets from these mice were assessed for glucose-stimulated insulin secretion and gene expression changes. Cultured β-cells were examined electrophysiologically for their electrical responsiveness to hypoglycaemia. RIPCreα2KO mice exhibited glucose intolerance and impaired GSIS (glucose-stimulated insulin secretion) and this was exacerbated in α1KORIPCreα2KO mice. Reduced glucose concentrations failed to completely suppress insulin secretion in islets from RIPCreα2KO and α1KORIPCreα2KO mice, and conversely GSIS was impaired. β-Cells lacking AMPKα2 or expressing a kinase-dead AMPKα2 failed to hyperpolarize in response to low glucose, although KATP (ATP-sensitive potassium) channel function was intact. We could detect no alteration of GLUT2 (glucose transporter 2), glucose uptake or glucokinase that could explain this glucose insensitivity. UCP2 (uncoupling protein 2) expression was reduced in RIPCreα2KO islets and the UCP2 inhibitor genipin suppressed low-glucose-mediated wild-type mouse β-cell hyperpolarization, mimicking the effect of AMPKα2 loss. These results show that AMPKα2 activity is necessary to maintain normal pancreatic β-cell glucose sensing, possibly by maintaining high β-cell levels of UCP2.

Identification of neuronal subpopulations that project from hypothalamus to both liver and adipose tissue polysynaptically

The autonomic nervous system regulates fuel availability and energy storage in the liver, adipose tissue, and other organs; however, the molecular components of this neural circuit are poorly understood. We sought to identify neural populations that project from the CNS indirectly through multisynaptic pathways to liver and epididymal white fat in mice using pseudorabies virus strains expressing different reporters together with BAC transgenesis and immunohistochemistry. Neurons common to both circuits were identified in subpopulations of the paraventricular nucleus of the hypothalamus (PVH) by double labeling with markers expressed in viruses injected in both sites. The lateral hypothalamus and arcuate nucleus of the hypothalamus and brainstem regions (nucleus of the solitary tract and A5 region) also project to both tissues but are labeled at later times. Connections from these same sites to the PVH were evident after direct injection of virus into the PVH, suggesting that these regions lie upstream of the PVH in a common pathway to liver and adipose tissue (two metabolically active organs). These common populations of brainstem and hypothalamic neurons express neuropeptide Y and proopiomelanocortin in the arcuate nucleus, melanin-concentrating hormone, and orexin in the lateral hypothalamus and in the corticotrophin-releasing hormone and oxytocin in the PVH. The delineation of this circuitry will facilitate a functional analysis of the possible role of these potential command-like neurons to modulate autonomic outflow and coordinate metabolic responses in liver and adipose tissue.

Neuronostatin is co-expressed with somatostatin and mobilizes calcium in cultured rat hypothalamic neurons

Neuronostatin (NST) is a newly identified peptide of 13-amino acids encoded by the somatostatin (SST) gene. Using a rabbit polyclonal antiserum against the human NST, neuronostatin-immunoreactive (irNST) cells comparable in number and intensity to somatostatin immunoreactive (irSST) cells were detected in the hypothalamic periventricular nucleus. Fewer and/or less intensely labeled irNST cells were noted in other regions such as the hippocampus, cortex, amygdala, and cerebellum. Double-labeling hypothalamic sections with NST- and SST-antiserum revealed an extensive overlapping of irNST and irSST cells in the periventricular nucleus. Pre-absorption of the NST-antiserum with NST (1 μg/ml) but not with SST (1 μg/ml) abrogated irNST and vice versa. The activity of NST on dissociated and cultured hypothalamic neurons was assessed by the Ca 2+ imaging method. NST (10, 100, 1000 nM) concentration-dependently elevated intracellular Ca 2+ concentrations [Ca 2+] i in a population of hypothalamic neurons with two distinct profiles: (1) a fast and transitory increase in [Ca 2+] i, and (2) an oscillatory response. Whereas, SST (100 nM) reduced the basal [Ca 2+] i in 21 of 61 hypothalamic neurons examined; an increase was not observed in any of the cells. Optical imaging with a slow-responding voltage sensitive dye DiBAC 4(3) showed that NST (100 nM) depolarized or hyperpolarized; whereas, SST (100 nM) hyperpolarized a population of hypothalamic neurons. The result shows that NST and SST, though derived from the same precursor protein, exert different calcium mobilizing effects on cultured rat hypothalamic neurons, resulting in diverse cellular activities.

EM66-containing neurons in the hypothalamic paraventricular nucleus of the rat: No plasticity related to acute immune and thermal stress

Event Abstract Back to Event EM66-containing neurons in the hypothalamic paraventricular nucleus of the rat: No plasticity related to acute immune and thermal stress Fatima-Zohra E. Yamani1, Laurent Yon2, Marlène Guérin2, Abdelilah Alaoui1, Seloua E. Ouezzani1, Nicolas Chartrel2, Youssef Anouar2 and Rabia Magoul1* 1 Université Sidi Mohamed Ben Abdallah, Laboratoire de Neuroendocrinologie et Environnement Nutritionnel et Climatique, Faculté des Sciences Dhar El Mahraz, Morocco 2 Université de Rouen, INSERM U982, Laboratoire Différenciation et Communication Neuronale et Neuroendocrine (DC2N), IFRMP 23, France Neuropeptides, main neuroendocrine system effectors, notably regulate the response to different stressors via a secretory plasticity within their respective hypothalamic neuronal populations. Indeed, the hypothalamic response to an environmental stress implicates the corticotropin-releasing hormone (CRH) neuroendocrine system of the hypothalamic parvocellular paraventricular nucleus (pPVN) in addition to other neuropeptides coexpressed within CRH neurones and controlling the hypothalamo-pituitary-adrenal axis activity as well. The aim of the present study was to explore by immunocytochemistry the occurrence of the neuropeptide EM66 within the parvocellular neurones of the PVN of rat and its potential expression plasticity following stress. The present results have shown that the secretogranin II (SgII)-derived peptide EM66 is strongly expressed within hypothalamic neuroendocrine areas such as the pPVN as well as the median eminence, suggesting a probable hypohysiotropic effect of this peptide. As a first approach to investigate such a role, we evaluated by immunocytochemistry EM66 expression within the pPVN following acute immune stress induced by lipopolysaccharide (LPS, ip) or interleukin – 1 (l ï€­ï€ ï€±ï¢ï€ ï€¬ï€ icv) ï€ injection in rat. The results showed that the number of EM66-immunolabeled cells did not fluctuate in this structure following LPS peripheral injection. In line with this observation, an intracerebroventricular injection of l ï€­ï€ ï€±ï¢ did not provoke any significant variation of the number of EM66 neurones in the pPVN. Furthermore, EM66 expression level did not change upon an acute thermal stress. In conclusion, the present study revealed for the first time that EM66 expression within the neurosecretory hypothalamic pPVN is not responsive to central and peripheral l ï€­ï€ ï€±ï¢. Consequently, EM66 does not participate in the phenotypic plasticity of hypothalamic parvocellular neurons in response to acute inflammatory or thermal stress. Acknowledgment: This study was supported by an exchange program from the INSERM (France) and the CNRST (Morocco) Conference: 2nd NEUROMED Workshop, Fez, Morocco, 10 Jun - 12 Jun, 2010. Presentation Type: Oral Presentation Topic: Oral Session 2: Adaptation to environmental stress Citation: Yamani FE, Yon L, Guérin M, Alaoui A, Ouezzani SE, Chartrel N, Anouar Y and Magoul R (2010). EM66-containing neurons in the hypothalamic paraventricular nucleus of the rat: No plasticity related to acute immune and thermal stress. Front. Neurosci. Conference Abstract: 2nd NEUROMED Workshop. doi: 10.3389/conf.fnins.2010.12.00028 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 04 Jun 2010; Published Online: 04 Jun 2010. * Correspondence: Rabia Magoul, Université Sidi Mohamed Ben Abdallah, Laboratoire de Neuroendocrinologie et Environnement Nutritionnel et Climatique, Faculté des Sciences Dhar El Mahraz, Fès, Morocco, rabea_magoul@yahoo.com Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Fatima-Zohra E Yamani Laurent Yon Marlène Guérin Abdelilah Alaoui Seloua E Ouezzani Nicolas Chartrel Youssef Anouar Rabia Magoul Google Fatima-Zohra E Yamani Laurent Yon Marlène Guérin Abdelilah Alaoui Seloua E Ouezzani Nicolas Chartrel Youssef Anouar Rabia Magoul Google Scholar Fatima-Zohra E Yamani Laurent Yon Marlène Guérin Abdelilah Alaoui Seloua E Ouezzani Nicolas Chartrel Youssef Anouar Rabia Magoul PubMed Fatima-Zohra E Yamani Laurent Yon Marlène Guérin Abdelilah Alaoui Seloua E Ouezzani Nicolas Chartrel Youssef Anouar Rabia Magoul Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Functional and evolutionary insights into vertebrate kisspeptin systems from studies of fish brain

The kiss1 gene product kisspeptin is now considered to be an essential regulator of the hypothalamic-pituitary-gonadal (HPG) axis in most vertebrate species. Recent findings in fishes are beginning to set a new stage for the kisspeptin study; the existence of paralogous kisspeptin genes as well as kisspeptin receptor (formerly called GPR54) genes has quite recently been reported in several fish and amphibian species. The fishes may provide excellent animal models for the study of general principles underlying the kisspeptin and kisspeptin receptor systems of vertebrates from the evolutionary viewpoint. Unlike placental and marsupial mammalian species mainly studied so far, many teleost species have two paralogous genes of kisspeptin, kiss1 and kiss2. Medaka, Oryzias latipes, in which kiss1 and kiss2 are expressed in distinctive hypothalamic neuron populations, is a good model system for the study of central regulation of reproduction. Here, the kiss1 system but not the kiss2 system shows expression dynamics strongly indicative of its direct involvement in the HPG axis regulation via its actions on GnRH1 neurons. On the other hand, the kiss1 gene is missing, and only kiss2 is expressed in some fish species. Also, there are some recent reports that Kiss2 peptide may be a potent regulator of reproduction in some fish species. The ancestral vertebrate probably already had two paralogous kiss genes, and their main function was the HPG axis regulation. In the species that retained both paralogues during evolution, either Kiss1 or Kiss2 predominantly retains its ability for the HPG axis regulation, while the other may assume new non-reproductive functions (neofunctionalization). Alternatively, both the paralogues may assume complementary functions in the HPG axis regulation (subfunctionalization). After the divergence of teleost and tetrapod lineages, either one of the two paralogues, or even both in birds, have been lost (degradation) or became a pseudogene (non-functionalization), but the remaining paralogue retained its original function of HPG axis regulation. The identification of multiple forms of kisspeptin receptors and the rather promiscuous ligand-receptor relationships has led to the further proposal that such promiscuousness may be the basis for the functional robustness of kisspeptin and kisspeptin receptor systems in the HPG axis regulation, when one or both paralogous genes are lost or functionally partitioned during evolution.

VA Opsin-Based Photoreceptors in the Hypothalamus of Birds

Studies in the 1930s demonstrated that birds possess photoreceptors that are located within the hypothalamus and regulate photoperiodic responses to day length. Most recently, photoperiod has been shown to alter the activity of the pars tuberalis to release thyrotrophin, which ultimately drives a reproductive response. Despite these significant findings, the cellular and molecular identity of the hypothalamic photoreceptors has remained a mystery. Action spectra implicated an opsin-based photopigment system, but further identification based on rod- or cone-opsin probes failed, suggesting the utilization of a novel opsin. The vertebrate ancient (VA) opsin photopigments were isolated in 1997 but were thought to have a restricted taxonomic distribution, confined to the agnatha and teleost fish. Here, we report the isolation of VA opsin from chicken and show that the two isoforms spliced from this gene (cVAL and cVA) are capable of forming functional photopigments. Further, we show that VA opsin is expressed within a population of hypothalamic neurons with extensive projections to the median eminence. These results provide the most complete cellular and molecular description of a deep brain photoreceptor in any vertebrate and strongly implicate VA opsin in mediating the avian photoperiodic response.

Leptin: Taking the Path Less Traveled

The metabolic hormone leptin influences feeding and metabolism through action on a distributed brain network. In this issue, Leinninger et al. (2009) describe a novel lateral hypothalamic neuronal population that is responsive to leptin and interfaces with dopamine circuits.

Inhibition of neural activity depletes orexin from rat hypothalamic slice culture

Orexins (hypocretins) are neuropeptides produced by a small population of hypothalamic neurons whose dysregulation may lead to narcolepsy, a neurological disorder characterized by disorganization of sleep and wakefulness. Excessive stimulation of the N-methyl-D-aspartate (NMDA) subtype of glutamate receptors causes preferential loss of orexin neurons in the hypothalamus, whereas an adequate level of neuronal excitatory activities is generally known to be important for the maintenance of central neurons. By examining the effect of manipulation of neural activity, we found that 24-72 hr application of tetrodotoxin (TTX) caused a substantial decrease in the number of orexin-immunoreactive neurons, but not of melanin-concentrating hormone-immunoreactive neurons, in hypothalamic slice culture. Similar results were obtained when neural activity was arrested by added extracellular Mg(2+). Reduction of orexin expression by TTX and Mg(2+) was also observed at mRNA level. The decrease of orexin-immunoreactive neurons was attributable to depletion of orexin, because it was reversible after washout of TTX or elevated extracellular Mg(2+) and was not associated with induction of cell death. Blockers of voltage-dependent Ca(2+) channels as well as of NMDA receptors also induced a significant and selective decrease of orexin-immunoreactive neurons. Moreover, TTX-induced decrease of orexin immunoreactivity was largely abrogated by concurrent application of a moderate concentration of NMDA. These results suggest that Ca(2+) entry associated with nontoxic levels of spontaneous activity of glutamatergic inputs plays an important role in the maintenance of orexin neurons in a tissue culture model.