The hypothalamic arcuate nucleus-median eminence complex: Immunohistochemistry of transmitters, peptides and DARPP-32 with special reference to coexistence in dopamine neurons

Hypothalamic neurons fully or partially expressing the dopaminergic phenotype: development, distribution, functioning and functional significance. A review

The hypothalamus is a key link in neuroendocrine regulations, which are provided by neuropeptides and dopamine. Until the late 1980 s, it was believed that, along with peptidergic neurons, hypothalamus contained dopaminergic neurons. Over time, it has been shown that besides dopaminergic neurons expressing the dopamine transporter and dopamine-synthesizing enzymes - tyrosine hydroxylase (TH) and aromatic L-amino acid decarboxylase (AADC) - the hypothalamus contains neurons expressing only TH, only AADC, both enzymes or only dopamine transporter. The end secretory product of TH neurons is L-3,4-dihydroxyphenylalanine, while that of AADC neurons and bienzymatic neurons lacking the dopamine transporter is dopamine. During ontogenesis, especially in the perinatal period, monoenzymatic neurons predominate in the hypothalamic neuroendocrine centers. It is assumed that L-3,4-dihydroxyphenylalanine and dopamine are released into the neuropil, cerebral ventricles, and blood vessels, participating in the regulation of target cell differentiation in the perinatal period and the functioning of target cells in adulthood.

Advances in steroid research from the pioneering neurosteroid concept to metabolomics: New insights into pregnenolone function

Advances in neuroendocrinology have led to major discoveries since the 19th century, identifying adaptive loops for maintaining homeostasis. One of the most remarkable discoveries was the concept of neurosteroids, according to which the brain is not only a target but also a source of steroid production. The identification of new membrane steroid targets now underpins the neuromodulatory effects of neurosteroids such as pregnenolone, which is involved in functions mediated by the GPCR CB1 receptor. Structural analysis of steroids is a key feature of their interactions with the phospholipid membrane, receptors and resulting activity. Therefore, mass spectrometry-based methods have been developed to elucidate the metabolic pathways of steroids, the ultimate approach being metabolomics, which allows the identification of a large number of metabolites in a single sample. This approach should enable us to make progress in understanding the role of neurosteroids in the functioning of physiological and pathological processes.

Class A and C GPCR Dimers in Neurodegenerative Diseases

Neurodegenerative diseases affect over 30 million people worldwide with an ascending trend. Most individuals suffering from these irreversible brain damages belong to the elderly population, with onset between 50 and 60 years. Although the pathophysiology of such diseases is partially known, it remains unclear upon which point a disease turns degenerative. Moreover, current therapeutics can treat some of the symptoms but often have severe side effects and become less effective in long-term treatment. For many neurodegenerative diseases, the involvement of G proteincoupled receptors (GPCRs), which are key players of neuronal transmission and plasticity, has become clearer and holds great promise in elucidating their biological mechanism. With this review, we introduce and summarize class A and class C GPCRs, known to form heterodimers or oligomers to increase their signalling repertoire. Additionally, the examples discussed here were shown to display relevant alterations in brain signalling and had already been associated with the pathophysiology of certain neurodegenerative diseases. Lastly, we classified the heterodimers into two categories of crosstalk, positive or negative, for which there is known evidence.

The physiological control of eating: signals, neurons, and networks

During the past 30 yr, investigating the physiology of eating behaviors has generated a truly vast literature. This is fueled in part by a dramatic increase in obesity and its comorbidities that has coincided with an ever increasing sophistication of genetically based manipulations. These techniques have produced results with a remarkable degree of cell specificity, particularly at the cell signaling level, and have played a lead role in advancing the field. However, putting these findings into a brain-wide context that connects physiological signals and neurons to behavior and somatic physiology requires a thorough consideration of neuronal connections: a field that has also seen an extraordinary technological revolution. Our goal is to present a comprehensive and balanced assessment of how physiological signals associated with energy homeostasis interact at many brain levels to control eating behaviors. A major theme is that these signals engage sets of interacting neural networks throughout the brain that are defined by specific neural connections. We begin by discussing some fundamental concepts, including ones that still engender vigorous debate, that provide the necessary frameworks for understanding how the brain controls meal initiation and termination. These include key word definitions, ATP availability as the pivotal regulated variable in energy homeostasis, neuropeptide signaling, homeostatic and hedonic eating, and meal structure. Within this context, we discuss network models of how key regions in the endbrain (or telencephalon), hypothalamus, hindbrain, medulla, vagus nerve, and spinal cord work together with the gastrointestinal tract to enable the complex motor events that permit animals to eat in diverse situations.

Satb2 regulates the development of dopaminergic neurons in the arcuate nucleus by Dlx1

Dopaminergic (DA) neurons in the arcuate nucleus (ARC) of the hypothalamus play essential roles in the secretion of prolactin and the regulation of energy homeostasis. However, the gene regulatory network responsible for the development of the DA neurons remains poorly understood. Here we report that the transcription factor special AT-rich binding protein 2 (Satb2) is required for the development of ARC DA neurons. Satb2 is expressed in a large proportion of DA neurons without colocalization with proopiomelanocortin (POMC), orexigenic agouti-related peptide (AgRP), neuropeptide-Y (NPY), somatostatin (Sst), growth hormone-releasing hormone (GHRH), or galanin in the ARC. Nestin-Cre;Satb2flox/flox (Satb2 CKO) mice show a reduced number of ARC DA neurons with unchanged numbers of the other types of ARC neurons, and exhibit an increase of serum prolactin level and an elevated metabolic rate. The reduction of ARC DA neurons in the CKO mice is observed at an embryonic stage and Dlx1 is identified as a potential downstream gene of Satb2 in regulating the development of ARC DA neurons. Together, our study demonstrates that Satb2 plays a critical role in the gene regulatory network directing the development of DA neurons in ARC.

Reduced Levels of Intestinal Neuropeptides and Neurotrophins in Neurotoxin-Induced Parkinson Disease Mouse Models

Intestinal neuropeptides and neurotrophins as endocrine messengers play a key role in the bidirectional gut-brain interaction both in health and disease status. Their alterations in several neurological disorders have been reported, but whether a remarkable change occurs in Parkinson disease (PD) remains unexplored. In this study, we aimed to investigate the levels of 13 neuropeptides and 4 neurotrophins in the intestine of neurotoxin-induced PD mice. The PD mice were obtained by chronic injection of 1-methyl-4-phenyl-1,2,3,6 tetrahydropyridine (MPTP) or MPTP/probenecid (MPTP/p). The levels of mRNA and protein expression in mouse intestines were measured by using real-time reverse transcription polymerase chain reaction and Western blotting, respectively. We found that the mRNA expression of 2 neuropeptides (cholecystokinin [CCK] and dynorphin A [Dyn A]) and 2 neurotrophins (brain-derived neurotrophic factor [BDNF] and neurotrophin-5) was significantly decreased in the colon of MPTP group compared to the vehicle-treated group. The protein levels of CCK, Dyn A, and BDNF were reduced in the colon of MPTP- or MPTP/p-treated mice compared to those of the vehicle-treated group. These data suggest that the intestinal expression of CCK, Dyn A, and BDNF was significantly reduced in PD animal models, and may play a role in the gut-brain axis in PD.

Extra-forebrain impact of antipsychotics indicated by c-Fos or FosB/ΔFosB expression: A minireview

It is apparent that the c-Fos and FosB/ΔFosB immunohistochemistry has generally become a useful tool for determining the different antipsychotic (AP) drugs activities in the brain. It is also noteworthy that there are no spatial limits, while to the extent of their identification over the whole brain axis. In addition, they can be in a parallel manner utilized in the unmasking of the brain cell phenotype character activated by APs and by this way also to identify the possible brain circuits underwent to the APs action. However, up to date, the number of APs involved in the extra-striatal studies is still limited, what prevents the possibility to fully understand their extra-striatal effects as a complex as well as differentiate their extra-striatal impact in qualitative and quantitative dimensions. Actually, it is very believable that more and more anatomical/functional knowledge might bring new insights into the APs extra-striatal actions by identifying new region-specific activities of APs as well as novel cellular targets affected by APs, which might reveal more details of their possible side effects of the extra-striatal origin.

Arcuate nucleus, median eminence, and hypophysial pars tuberalis

The arcuate nucleus (ARC) is located in the mediobasal hypothalamus and forms a morphological and functional entity with the median eminence (ME), the ARC-ME. The ARC comprises several distinct types of neurons controlling prolactin release, food intake, and metabolism as well as reproduction and onset of puberty. The ME lacks a blood-brain barrier and provides an entry for peripheral signals (nutrients, leptin, ghrelin). ARC neurons are adjacent to the wall of the third ventricle. This facilitates the exchange of signals from and to the cerebrospinal fluid. The ventricular wall is composed of tanycytes that serve different functions. Axons of ARC neurons contribute to the tuberoinfundibular tract terminating in the ME on the hypophysial portal vessels (HPV) and establish one of the neurohumoral links between the hypothalamus and the pituitary. ARC neurons are reciprocally connected with several other hypothalamic nuclei, the brainstem, and reward pathways. The hypophysial pars tuberalis (PT) is attached to the ME and the HPV. The PT, an important interface of the neuroendocrine system, is mandatory for the control of seasonal functions. This contribution provides an update of our knowledge about the ARC-ME complex and the PT which, inter alia, is needed to understand the pathophysiology of metabolic diseases and reproduction.

Obesity induced by Borna disease virus in rats: key roles of hypothalamic fast-acting neurotransmitters and inflammatory infiltrates

Human obesity epidemic is increasing worldwide with major adverse consequences on health. Among other possible causes, the hypothesis of an infectious contribution is worth it to be considered. Here, we report on an animal model of virus-induced obesity which might help to better understand underlying processes in human obesity. Eighty Wistar rats, between 30 and 60 days of age, were intracerebrally inoculated with Borna disease virus (BDV-1), a neurotropic negative-strand RNA virus infecting an unusually broad host spectrum including humans. Half of the rats developed fatal encephalitis, while the other half, after 3-4 months, continuously gained weight. At tripled weights, rats were sacrificed by trans-cardial fixative perfusion. Neuropathology revealed prevailing inflammatory infiltrates in the median eminence (ME), progressive degeneration of neurons of the paraventricular nucleus, the entorhinal cortex and the amygdala, and a strikingly high-grade involution of the hippocampus with hydrocephalus. Immune histology revealed that major BDV-1 antigens were preferentially present at glutamatergic receptor sites, while GABAergic areas remained free from BDV-1. Virus-induced suppression of the glutamatergic system caused GABAergic predominance. In the hypothalamus, this shifted the energy balance to the anabolic appetite-stimulating side governed by GABA, allowing for excessive fat accumulation in obese rats. Furthermore, inflammatory infiltrates in the ME and ventro-medial arcuate nucleus hindered free access of appetite-suppressing hormones leptin and insulin. The hormone transport system in hypothalamic areas outside the ME became blocked by excessively produced leptin, leading to leptin resistance. The resulting hyperleptinemic milieu combined with suppressed glutamatergic mechanisms was a characteristic feature of the found metabolic pathology. In conclusion, the study provided clear evidence that BDV-1 induced obesity in the rat model is the result of interdependent structural and functional metabolic changes. They can be explained by an immunologically induced hypothalamic microcirculation-defect, combined with a disturbance of neurotransmitter regulatory systems. The proposed mechanism may also have implications for human health. BDV-1 infection has been frequently found in depressive patients. Independently, comorbidity between depression and obesity has been reported, either. Future studies should address the exciting question of whether BDV-1 infection could be a link, whatsoever, between these two conditions.

Distributions of hypothalamic neuron populations coexpressing tyrosine hydroxylase and the vesicular GABA transporter in the mouse

The hypothalamus contains catecholaminergic neurons marked by the expression of tyrosine hydroxylase (TH). As multiple chemical messengers coexist in each neuron, we determined if hypothalamic TH-immunoreactive (ir) neurons express vesicular glutamate or GABA transporters. We used Cre/loxP recombination to express enhanced GFP (EGFP) in neurons expressing the vesicular glutamate (vGLUT2) or GABA transporter (vGAT), then determined whether TH-ir neurons colocalized with native EGFP^Vglut2 - or EGFP^Vgat -fluorescence, respectively. EGFP^Vglut2 neurons were not TH-ir. However, discrete TH-ir signals colocalized with EGFP^Vgat neurons, which we validated by in situ hybridization for Vgat mRNA. To contextualize the observed pattern of colocalization between TH-ir and EGFP^Vgat , we first performed Nissl-based parcellation and plane-of-section analysis, and then mapped the distribution of TH-ir EGFP^Vgat neurons onto atlas templates from the Allen Reference Atlas (ARA) for the mouse brain. TH-ir EGFP^Vgat neurons were distributed throughout the rostrocaudal extent of the hypothalamus. Within the ARA ontology of gray matter regions, TH-ir neurons localized primarily to the periventricular hypothalamic zone, periventricular hypothalamic region, and lateral hypothalamic zone. There was a strong presence of EGFP^Vgat fluorescence in TH-ir neurons across all brain regions, but the most striking colocalization was found in a circumscribed portion of the zona incerta (ZI)-a region assigned to the hypothalamus in the ARA-where every TH-ir neuron expressed EGFP^Vgat . Neurochemical characterization of these ZI neurons revealed that they display immunoreactivity for dopamine but not dopamine β-hydroxylase. Collectively, these findings indicate the existence of a novel mouse hypothalamic population that may signal through the release of GABA and/or dopamine.

Sugar Promotes Feeding in Flies via the Serine Protease Homolog scarface

A balanced diet of macronutrients is critical for animal health. A lack of specific elements can have profound effects on behavior, reproduction, and lifespan. Here, we used Drosophila to understand how the brain responds to carbohydrate deprivation. We found that serine protease homologs (SPHs) are enriched among genes that are transcriptionally regulated in flies deprived of carbohydrates. Stimulation of neurons expressing one of these SPHs, Scarface (Scaf), or overexpression of scaf positively regulates feeding on nutritious sugars, whereas inhibition of these neurons or knockdown of scaf reduces feeding. This modulation of food intake occurs only in sated flies while hunger-induced feeding is unaffected. Furthermore, scaf expression correlates with the presence of sugar in the food. As Scaf and Scaf neurons promote feeding independent of the hunger state, and the levels of scaf are positively regulated by the presence of sugar, we conclude that scaf mediates the hedonic control of feeding.

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The fly brain responds to specific macronutrients via distinct signaling pathways

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Serine protease homologs act as neuromodulators under sugar deprivation

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Sugar is both necessary and sufficient to maintain expression levels of scarface

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Scarface and Scarface neurons mediate hedonic control of feeding in flies

Unified Classification of Molecular, Network, and Endocrine Features of Hypothalamic Neurons

Peripheral endocrine output relies on either direct or feed-forward multi-order command from the hypothalamus. Efficient coding of endocrine responses is made possible by the many neuronal cell types that coexist in intercalated hypothalamic nuclei and communicate through extensive synaptic connectivity. Although general anatomical and neurochemical features of hypothalamic neurons were described during the past decades, they have yet to be reconciled with recently discovered molecular classifiers and neurogenetic function determination. By interrogating magnocellular as well as parvocellular dopamine, GABA, glutamate, and phenotypically mixed neurons, we integrate available information at the molecular, cellular, network, and endocrine output levels to propose a framework for the comprehensive classification of hypothalamic neurons. Simultaneously, we single out putative neuronal subclasses for which future research can fill in existing gaps of knowledge to rationalize cellular diversity through function-determinant molecular marks in the hypothalamus.

Mapping the co-localization of the circadian proteins PER2 and BMAL1 with enkephalin and substance P throughout the rodent forebrain

Despite rhythmic expression of clock genes being found throughout the central nervous system, very little is known about their function outside of the suprachiasmatic nucleus. Determining the pattern of clock gene expression across neuronal subpopulations is a key step in understanding their regulation and how they may influence the functions of various brain structures. Using immunofluorescence and confocal microscopy, we quantified the co-expression of the clock proteins BMAL1 and PER2 with two neuropeptides, Substance P (SubP) and Enkephalin (Enk), expressed in distinct neuronal populations throughout the forebrain. Regions examined included the limbic forebrain (dorsal striatum, nucleus accumbens, amygdala, stria terminalis), thalamus medial habenula of the thalamus, paraventricular nucleus and arcuate nucleus of the hypothalamus and the olfactory bulb. In most regions examined, BMAL1 was homogeneously expressed in nearly all neurons (~90%), and PER2 was expressed in a slightly lower proportion of cells. There was no specific correlation to SubP- or Enk- expressing subpopulations. The olfactory bulb was unique in that PER2 and BMAL1 were expressed in a much smaller percentage of cells, and Enk was rarely found in the same cells that expressed the clock proteins (SubP was undetectable). These results indicate that clock genes are not unique to specific cell types, and further studies will be required to determine the factors that contribute to the regulation of clock gene expression throughout the brain.

A molecular census of arcuate hypothalamus and median eminence cell types

The hypothalamic arcuate-median eminence complex (Arc-ME) controls energy balance, fertility, and growth through molecularly distinct cell types, many of which remain unknown. To catalog cell types in an unbiased way, we profiled gene expression in 20,921 individual cells in and around the adult mouse Arc-ME using Drop-seq. We identify 50 transcriptionally distinct Arc-ME cell populations, including a rare tanycyte population at the Arc-ME diffusion barrier, a novel leptin-sensing neuronal population, multiple AgRP and POMC subtypes, and an orexigenic somatostatin neuronal population. We extended Drop-seq to detect dynamic expression changes across relevant physiological perturbations, revealing cell type-specific responses to energy status, including distinctly responsive subtypes of AgRP and POMC neurons. Finally, integrating our data with human GWAS data implicates two previously unknown neuronal subtypes in the genetic control of obesity. This resource will accelerate biological discovery by providing insights into molecular and cell type diversity from which function can be inferred.

Hypocretin/Orexin Peptides Excite Rat Neuroendocrine Dopamine Neurons through Orexin 2 Receptor-Mediated Activation of a Mixed Cation Current

Hypocretin/Orexin (H/O) neurons of the lateral hypothalamus are compelling modulator candidates for the chronobiology of neuroendocrine output and, as a consequence, hormone release from the anterior pituitary. Here we investigate the effects of H/O peptides upon tuberoinfundibular dopamine (TIDA) neurons – cells which control, via inhibition, the pituitary secretion of prolactin. In whole cell recordings performed in male rat hypothalamic slices, application of H/O-A, as well as H/O-B, excited oscillating TIDA neurons, inducing a reversible depolarising switch from phasic to tonic discharge. The H/O-induced inward current underpinning this effect was post-synaptic (as it endured in the presence of tetrodotoxin), appeared to be carried by a Na⁺-dependent transient receptor potential-like channel (as it was blocked by 2-APB and was diminished by removal of extracellular Na⁺), and was a consequence of OX2R receptor activation (as it was blocked by the OX2R receptor antagonist TCS OX2 29, but not the OX1R receptor antagonist SB 334867). Application of the hormone, melatonin, failed to alter TIDA membrane potential or oscillatory activity. This first description of the electrophysiological effects of H/Os upon the TIDA network identifies cellular mechanisms that may contribute to the circadian rhythmicity of prolactin secretion.

Hypothalamic arcuate nucleus tyrosine hydroxylase neurons play orexigenic role in energy homeostasis

Energy homeostasis, food intake, and body weight are regulated by specific brain circuits. Here we introduce an unexpected neuron, the tyrosine hydroxylase (TH) neuron of the arcuate nucleus (ARC), that we show makes an orexigenic contribution. Optogenetic stimulation of mouse ARC TH neurons increased food intake; attenuating transmitter release reduced body weight. Optogenetic stimulation of ARC TH cells inhibited pro-opiomelanocortin (POMC) neurons through synaptic mechanisms. ARC TH cells project to the hypothalamic paraventricular nucleus; optogenetic stimulation of ARC TH axons inhibited paraventricular nucleus neurons by dopamine and GABA co-release. Dopamine excited orexigenic neurons that synthesize agouti-related peptide and neuropeptide Y but inhibited anorexigenic neurons that synthesize POMC, as determined by whole cell recording. Food deprivation increased c-fos expression and spike frequency in ARC TH neurons. The gut peptide ghrelin evoked direct excitatory effects, suggesting these neurons monitor metabolic cues. Together these data support the view that ARC TH cells play an unrecognized and influential positive role in energy homeostasis.

Functional expression of P2 purinoceptors in a primary neuroglial cell culture of the rat arcuate nucleus

The arcuate nucleus (ARC) plays an important role in the hypothalamic control of energy homeostasis. Expression of various purinoceptor subtypes in the rat ARC and physiological studies suggest a modulatory function of P2 receptors within the neuroglial ARC circuitry. A differentiated mixed neuronal and glial microculture was therefore established from postnatal rat ARC, revealing neuronal expression of ARC-specific transmitters involved in food intake regulation (neuropeptide Y (NPY), proopiomelanocortin (POMC), tyrosine hydroxylase (TH)). Some NPYergic neurons cosynthesized TH, while POMC and TH expression proved to be mutually exclusive. Stimulation with the general purinoceptor agonists 2-methylthioadenosine-5'triphosphate (2-MeSATP) and ATP but not the P2X1/P2X3 receptor subtype agonist α,β-methyleneadenosine-5'triphosphate (α,β-meATP) induced intracellular calcium signals in ARC neurons and astrocytes. Some 5-10% each of 2-MeSATP responsive neurons expressed POMC, NYP or TH. Supporting the calcium imaging data, radioligand binding studies to hypothalamic membranes showed high affinity for 2-MeSATP, ATP but not α,β-meATP to displace [α-(35)S]deoxyadenosine-5'thiotriphosphate ([(35)S]dATPαS) from P2 receptors. Repetitive superfusion with equimolar 2-MeSATP allowed categorization of ARC cells into groups with a high or low (LDD) degree of purinoceptor desensitization, the latter allowing further receptor characterization. Calcium imaging experiments performed at 37°C vs. room temperature showed further reduction of desensitization. Agonist-mediated intracellular calcium signals were suppressed in all LDD neurons but only 25% of astrocytes in the absence of extracellular calcium, suggestive of metabotropic P2Y receptor expression in the majority of ARC astrocytes. The highly P2Y1-selective receptor agonists MRS2365 and 2-methylthioadenosine-5'diphosphate (2-MeSADP) activated 75-85% of all 2-MeSATP-responsive ARC astrocytes. Taking into consideration the high potency to dose-dependently stimulate ARC cells of the LDD group, the high affinity for rat P2X(1-3) and low affinity for rat P2X4, P2X7 and P2Y receptor subtypes except P2Y1 and P2Y13, the agonist 2-MeSATP primarily acted upon P2X2 and P2Y1 purinoceptors to trigger intracellular calcium signaling in ARC neurons and astrocytes.

Central injection of CDP-choline suppresses serum ghrelin levels while increasing serum leptin levels in rats

In this study we aimed to test central administration of CDP-choline on serum ghrelin, leptin, glucose and corticosterone levels in rats. Intracerebroventricular (i.c.v.) 0.5, 1.0 and 2.0 µmol CDP-choline and saline were administered to male Wistar-Albino rats. For the measurement of serum leptin and ghrelin levels, blood samples were obtained baseline and at 5, 15, 30, 60 and 120 min following i.c.v. CDP-choline injection. Equimolar doses of i.c.v. choline (1.0 µmol) and cytidine (1.0 µmol) were administered and measurements were repeated throughout the second round of the experiment. Atropine (10 µg) and mecamylamine (50 µg) were injected intracerebroventricularly prior to CDP-choline and measurements repeated in the third round of the experiment. After 1 µmol CDP-choline injection, serum ghrelin levels were suppressed significantly at 60 min (P=0.025), whereas serum leptin levels were increased at 60 and 120 min (P=0.012 and P=0.017 respectively). CDP-choline injections also induced a dose- and time-dependent increase in serum glucose and corticosterone levels. The effect of choline on serum leptin and ghrelin levels was similar with CDP-choline while no effect was seen with cytidine. Suppression of serum ghrelin levels was eliminated through mecamylamine pretreatment while a rise in leptin was prevented by both atropine and mecamylamine pretreatments. In conclusion; centrally injected CDP-choline suppressed serum ghrelin levels while increasing serum leptin levels. The observed effects following receptor antagonist treatment suggest that nicotinic receptors play a role in suppression of serum ghrelin levels,whereas nicotinic and muscarinic receptors both play a part in the increase of serum leptin levels.

A secretagogin locus of the mammalian hypothalamus controls stress hormone release

A hierarchical hormonal cascade along the hypothalamic-pituitary-adrenal axis orchestrates bodily responses to stress. Although corticotropin-releasing hormone (CRH), produced by parvocellular neurons of the hypothalamic paraventricular nucleus (PVN) and released into the portal circulation at the median eminence, is known to prime downstream hormone release, the molecular mechanism regulating phasic CRH release remains poorly understood. Here, we find a cohort of parvocellular cells interspersed with magnocellular PVN neurons expressing secretagogin. Single-cell transcriptome analysis combined with protein interactome profiling identifies secretagogin neurons as a distinct CRH-releasing neuron population reliant on secretagogin's Ca(2+) sensor properties and protein interactions with the vesicular traffic and exocytosis release machineries to liberate this key hypothalamic releasing hormone. Pharmacological tools combined with RNA interference demonstrate that secretagogin's loss of function occludes adrenocorticotropic hormone release from the pituitary and lowers peripheral corticosterone levels in response to acute stress. Cumulatively, these data define a novel secretagogin neuronal locus and molecular axis underpinning stress responsiveness.

TIDAL WAVES: Network mechanisms in the neuroendocrine control of prolactin release

Neuroendocrine tuberoinfundibular dopamine (TIDA) neurons tonically inhibit pituitary release of the hormone, prolactin. Through the powerful actions of prolactin in promoting lactation and maternal behaviour while suppressing sexual drive and fertility, TIDA neurons play a key role in reproduction. We summarize insights from recent in vitro studies into the membrane properties and network behaviour of TIDA neurons including the observations that TIDA neurons exhibit a robust oscillation that is synchronized between cells and depends on intact gap junction communication. Comparisons are made with phasic firing patterns in other neuronal populations. Modulators involved in the control of lactation - including serotonin, thyrotropin-releasing hormone and prolactin itself - have been shown to change the electrical behaviour of TIDA cells. We propose that TIDA discharge mode may play a central role in tuning the amount of dopamine delivered to the pituitary and hence circulating prolactin concentrations in different reproductive states and pathological conditions.

Expression and colocalization patterns of calbindin-D28k, calretinin and parvalbumin in the rat hypothalamic arcuate nucleus

Calcium binding proteins (CaBPs) form a diverse group of molecules that function as signal transducers or as intracellular buffers of Ca(2+) concentration. They have been extensively used to histochemically categorize cell types throughout the brain. One region which has not yet been characterized with regard to CaBP expression is the hypothalamic arcuate nucleus, which plays a vital role in neuroendocrine control and the central regulation of energy metabolism. Using in situ hybridization and immunofluorescence, we have investigated the cellular distribution of the three CaBPs, calbindin-D28k (CB), calretinin (CR) and parvalbumin (PV) in the rat arcuate nucleus. Both mRNA and immunoreactivity was detected in the arcuate nucleus for CB - located in the medial aspects - and CR - located ventrolaterally. No PV mRNA was detected in the arcuate nucleus. Immunofluorescence results for PV were ambiguous; while one antibody detected a group of cell somata, a different antibody failed to visualize any arcuate nucleus cell profiles. Using double-labeling, neither of the examined CaBPs were observed in cells immunoreactive for the signaling molecules agouti gene-related protein, tyrosine hydroxylase, neurotensin, growth hormone-releasing hormone, somatostatin, enkephalin, dynorphin or galanin. We did, however, observe CB- and CR-immunoreactivity, in two distinct populations of neurons immunoreactive for the melanocortin peptide α-melanocyte-stimulating hormone. These data identify distinct subpopulations of arcuate neurons defined by their expression of CaBPs and provide further support for differentiation between subpopulations of anorexigenic melanocortin neurons.

Neural input is critical for arcuate hypothalamic neurons to mount intracellular signaling responses to systemic insulin and deoxyglucose challenges in male rats: implications for communication within feeding and metabolic control networks

The hypothalamic arcuate nucleus (ARH) controls rat feeding behavior in part through peptidergic neurons projecting to the hypothalamic paraventricular nucleus (PVH). Hindbrain catecholaminergic (CA) neurons innervate both the PVH and ARH, and ablation of CA afferents to PVH neuroendocrine neurons prevents them from mounting cellular responses to systemic metabolic challenges such as insulin or 2-deoxy-d-glucose (2-DG). Here, we asked whether ablating CA afferents also limits their ARH responses to the same challenges or alters ARH connectivity with the PVH. We examined ARH neurons for three features: (1) CA afferents, visualized by dopamine-β-hydroxylase (DBH)- immunoreactivity; (2) activation by systemic metabolic challenge, as measured by increased numbers of neurons immunoreactive (ir) for phosphorylated ERK1/2 (pERK1/2); and (3) density of PVH-targeted axons immunoreactive for the feeding control peptides Agouti-related peptide and α-melanocyte-stimulating hormone (αMSH). Loss of PVH DBH immunoreactivity resulted in concomitant ARH reductions of DBH-ir and pERK1/2-ir neurons in the medial ARH, where AgRP neurons are enriched. In contrast, pERK1/2 immunoreactivity after systemic metabolic challenge was absent in αMSH-ir ARH neurons. Yet surprisingly, axonal αMSH immunoreactivity in the PVH was markedly increased in CA-ablated animals. These results indicate that (1) intrinsic ARH activity is insufficient to recruit pERK1/2-ir ARH neurons during systemic metabolic challenges (rather, hindbrain-originating CA neurons are required); and (2) rats may compensate for a loss of CA innervation to the ARH and PVH by increased expression of αMSH. These findings highlight the existence of a hierarchical dependence for ARH responses to neural and humoral signals that influence feeding behavior and metabolism.

The multilingual nature of dopamine neurons

The ability of dopamine (DA) neurons to release other transmitters in addition to DA itself has been increasingly recognized, hence the concept of their multilingual nature. A subset of DA neurons, mainly found in the ventral tegmental area, express VGLUT2, allowing them to package and release glutamate onto striatal spiny projection neurons and cholinergic interneurons. Some dopaminergic axon terminals release GABA. Glutamate release by DA neurons has a developmental role, facilitating axonal growth and survival, and may determine in part the critical contribution of the ventral striatum to psychostimulant-induced behavior. Vesicular glutamate coentry may have synergistic effects on vesicular DA filling. The multilingual transmission of DA neurons across multiple striatal domains and the increasing insight into the role of glutamate cotransmission in the ventral striatum highlight the importance of analyzing DA neuron transmission at the synaptic level.

Brain neurons partly expressing dopaminergic phenotype: location, development, functional significance, and regulation

In addition to catecholaminergic neurons possessing all the enzymes of catecholamine synthesis and the specific membrane transporters, neurons partly expressing the catecholaminergic phenotype have been found a quarter of a century ago. Most of them express individual enzymes of dopamine (DA) synthesis, tyrosine hydroxylase (TH), or aromatic l-amino acid decarboxylase (AADC), lacking the DA membrane transporter and the vesicular monoamine transporter, type 2. These so-called monoenzymatic neurons are widely distributed throughout the brain in ontogenesis and adulthood being in some brain regions even more numerous than dopaminergic (DA-ergic) neurons. Individual enzymes of DA synthesis are expressed in these neurons continuously or transiently in norm and pathology. It has been proven that monoenzymatic TH neurons and AADC neurons are capable of producing DA in cooperation. It means that l-3,4-dihydroxyphenylalanine (l-DOPA) synthesized from l-tyrosine in monoenzymatic TH neurons is transported to monoenzymatic AADC neurons for DA synthesis. Such cooperative synthesis of DA is considered as a compensatory reaction under a failure of DA-ergic neurons, for example, in neurodegenerative diseases like hyperprolactinemia and Parkinson's disease. Moreover, l-DOPA, produced in monoenzymatic TH neurons, is assumed to play a role of a neurotransmitter or neuromodulator affecting the target neurons via catecholamine receptors. Thus, numerous widespread neurons expressing individual complementary enzymes of DA synthesis serve to produce DA in cooperation that is a compensatory reaction at failure of DA-ergic neurons.

Neuropeptide transmission in brain circuits

Neuropeptides are found in many mammalian CNS neurons where they play key roles in modulating neuronal activity. In contrast to amino acid transmitter release at the synapse, neuropeptide release is not restricted to the synaptic specialization, and after release, a neuropeptide may diffuse some distance to exert its action through a G protein-coupled receptor. Some neuropeptides such as hypocretin/orexin are synthesized only in single regions of the brain, and the neurons releasing these peptides probably have similar functional roles. Other peptides such as neuropeptide Y (NPY) are synthesized throughout the brain, and neurons that synthesize the peptide in one region have no anatomical or functional connection with NPY neurons in other brain regions. Here, I review converging data revealing a complex interaction between slow-acting neuromodulator peptides and fast-acting amino acid transmitters in the control of energy homeostasis, drug addiction, mood and motivation, sleep-wake states, and neuroendocrine regulation.

Studies on the relationship of tyrosine hydroxylase, dopamine and cyclic amp-regulated phosphoprotein-32 immunoreactive neuronal structures and d1 receptor antagonist binding sites in various brain regions of the male rat-mismatches indicate a role of d1 receptors in volume transmission

The relationship between tyrosine hydroxylase (TH), dopamine (DA) and cyclic AMP-regulated phosphoprotein-32 (DARPP-32) immunoreactive (IR) neuronal structures and D1 receptor antagonist binding sites has been analysed in various brain regions in the male rat, using immunocytochemistry and receptor autoradiography with the iodinated analogue of SCH 23390 ([(125)I]SCH 23982) as radioligand. Two-colour immunocytochemistry was used to establish in detail the relationship between DARPP-32 and the TH IR neuronal structures in mes-, di- and telencephalon. The analysis reveals complex matches and mismatches between central DARPP-32 immunoreactive neurones, DA neurones and D1 DA receptors. The results inter alia indicate a probable release of DA from the dendritic plexus of the zona reticulata of the substantia nigra to reach D1 DA receptors via extracellular pathways. DA released from the few DA terminals present in the entopeduncular nucleus and from adjacent dopamine axons may also reach D1 DA receptors in this nucleus by extracellular diffusion. A similar situation may also exist in the globus pallidus. Thus, DA may in some regions be released as a paracrine signal to reach distant D1 DA receptors. This type of chemical transmission has been called volume transmission and D1 receptors may thus participate in volume transmission. The mismatch obtained in, for example, the amygdaloid cortex and hypothalamus between D1 receptor antagonist binding sites and DARPP-32 IR nerve cell profiles, is compatible with the possibility that some D1 receptors linked to adenylate cyclase may not involve DARPP-32 as a substrate protein for the cyclic AMP-dependent protein kinase. In addition the possibility should be considered that D1 receptors may not always be linked to adenylate cyclase. Finally, the mismatch in the median eminence between [(125)I]SCH 23982 binding sites and DARPP-32 IR profiles may indicate the existence of D1 receptors which are masked under basal conditions in the male rat.

Expression and distribution of TRPV2 in rat brain

Transient receptor potential (TRP) proteins are non-selective cation channels that mediate sensory transduction. The neuroanatomical localization and the physiological roles of isoform TRPV2 in the rodent brain are largely unknown. We report here the neuroanatomical distribution of TRPV2 in the adult male rat brain focusing on the hypothalamus and hindbrain regions involved in osmoregulation, autonomic function and energy metabolism. For this we utilized immunohistochemistry combined with brightfield microscopy. In the forebrain, the densest immunostaining was seen in both the supraoptic nucleus (SON) and the magnocellular division of the paraventricular nucleus (PVN) of the hypothalamus. TRPV2 immunoreactivity was also seen in the organum vasculosum of the lamina terminalis, the median preoptic nucleus and the subfornical organ, in addition to the arcuate nucleus of the hypothalamus (ARH), the medial forebrain bundle, the cingulate cortex and the globus pallidus to name a few. In the hindbrain, intense staining was seen in the nucleus of the solitary tract, hypoglossal nucleus, nucleus ambiguous, and the rostral division of the ventrolateral medulla (RVLM) and some mild staining in the area prostrema. To ascertain the specificity of the TRPV2 antibody used in this paper, we compared the TRPV2 immunoreactivity of wildtype (WT) and knockout (KO) mouse brain tissue. Double immunostaining with arginine vasopressin (AVP) using confocal microscopy showed a high degree of colocalization of TRPV2 in the magnocellular SON and PVN. Using laser capture microdissection (LCM) we also show that AVP neurons in the SON contain TRPV2 mRNA. TRPV2 was also co-localized with dopamine beta hydroxylase (DBH) in the NTS and the RVLM of the hindbrain. Based on our results, TRPV2 may play an important role in several CNS networks that regulate body fluid homeostasis, autonomic function, and metabolism.

Non-dopaminergic neurons partly expressing dopaminergic phenotype: distribution in the brain, development and functional significance

Besides the dopaminergic (DA-ergic) neurons possessing the whole set of enzymes of DA synthesis from l-tyrosine and the DA membrane transporter (DAT), the neurons partly expressing the DA-ergic phenotype have been first discovered two decades ago. Most of the neurons express individual enzymes of DA synthesis, tyrosine hydroxylase (TH) or aromatic l-amino acid decarboxylase (AADC) and lack the DAT. A list of the neurons partly expressing the DA-ergic phenotype is not restricted to so-called monoenzymatic neurons, e.g. it includes some neurons co-expressing both enzymes of DA synthesis but lacking the DAT. In contrast to true DA-ergic neurons, monoenzymatic neurons and bienzymatic non-dopaminergic neurons lack the vesicular monoamine transporter 2 (VMAT2) that raises a question about the mechanisms of storing and release of their final synthetic products. Monoenzymatic neurons are widely distributed all through the brain in adulthood being in some brain regions even more numerous than DA-ergic neurons. Individual enzymes of DA synthesis are expressed in these neurons continuously or transiently in norm or under certain physiological conditions. Monoenzymatic neurons, particularly those expressing TH, appear to be even more numerous and more widely distributed in the brain during ontogenesis than in adulthood. Most populations of monoenzymatic TH neurons decrease in number or even disappear by puberty. Functional significance of monoenzymatic neurons remained uncertain for a long time after their discovery. Nevertheless, it has been shown that most monoenzymatic TH neurons and AADC neurons are capable to produce l-3,4-dihydroxyphenylalanine (L-DOPA) from l-tyrosine and DA from L-DOPA, respectively. L-DOPA produced in monoenzymatic TH neurons is assumed to play a role of a neurotransmitter or neuromodulator acting on target neurons via catecholamine receptors. Moreover, according to our hypothesis L-DOPA released from monoenzymatic TH neurons is captured by monoenzymatic AADC neurons for DA synthesis. Such cooperative synthesis of DA is considered as a compensatory reaction under a failure of DA-ergic neurons, e.g. in neurodegenerative diseases like hyperprolactinemia and Parkinson's disease.Thus, a substantial number of the brain neurons express partly the DA-ergic phenotype, mostly individual complementary enzymes of DA synthesis, serving to produce DA in cooperation that is supposed to be a compensatory reaction under the failure of DA-ergic neurons.

Stress-induced activation of neurons in the ventromedial arcuate nucleus: a blood-brain-CSF interface of the hypothalamus

In response to a pain-related acute stress, the expression of c-fos protein (Fos), a marker of acute neuronal excitation, was investigated in the hypothalamus of rats. Few Fos-immunopositive cells were seen 15 min after a single subcutaneous injection of 4% formalin in the hypothalamus, but only in the paraventricular nucleus (PVN). Fifteen minutes later, a high number of parvocellular neurons of the PVN showed Fos expression. By 60 min after injection, strong immunoreactivity appeared in the arcuate nucleus, but the Fos-positive neurons distributed almost exclusively in the ventromedial subdivision of the nucleus. Neurons in this part of the arcuate nucleus express mainly neuropeptide Y (NPY) that projects to the medial parvocellular subdivision of the PVN. It has been demonstrated by previous studies that this part of the arcuate nucleus receives blood partly from the anterior pituitary through the subependymal plexus of the median eminence, and that it establishes, together with the median eminence, a blood-brain barrier-free area in the medial basal hypothalamus. Since the PVN-projecting NPY neurons in the arcuate neurons are sensitive to alterations in circulating corticosterone levels, the existence of a possible short feedback route in the stress-activated hypothalamo-pituitary-adrenocortical system is discussed.

Protein components of the blood-brain barrier (BBB) in the mediobasal hypothalamus

The blood-brain barrier (BBB) plays an important role in controlling the access of substances to the brain. Of the circumventricular organs (CVO), i.e. areas that lack a BBB, the median eminence and its close relationship with the hypothalamic arcuate nucleus plays an important role in controlling the entry of blood-borne substances to neurons of the mediobasal hypothalamus. In order to clarify the nature of the BBB in the median eminence-arcuate nucleus complex, we have used immunohistochemistry and antisera to protein components of the BBB-(1) tight junctions, claudin-5 and zona occludens-1 (ZO-1); (2) endothelial cells: (a) all endothelial cells: rat endothelial cell antigen-1 (RECA-1), (b) endothelial cells at BBB: endothelial barrier antigen (EBA), glucose transporter 1 (GLUT1) and transferrin receptor (TfR), and (c) endothelial cells at CVOs: dysferlin; (3) basal lamina: laminin; (4) vascular smooth muscle cells: smooth muscle actin (SMA); (5) pericytes: chondroitin sulfate proteoglycan (NG2); (6) glial cells: (a) astrocytes: glial fibrillary acidic protein (GFAP), (b) tanycytes: dopamine- and cAMP-regulated phosphoprotein of 32kDA (DARPP-32), (c) microglia: CD11b. Neuronal cell bodies located in the ventromedial aspect of the arcuate nucleus were visualized by antiserum to agouti-related protein (AgRP). The study provides a detailed analysis on the cellular localization of BBB components in the mediobasal hypothalamus. Some vessels in the ventromedial aspect of the arcuate nucleus lacked the BBB markers EBA and TfR, suggesting an absence of an intact BBB. These vessels may represent a route of entry for circulating substances to a subpopulation of arcuate nucleus neurons.

Aberrant agouti-related protein system in the hypothalamus of the anx/anx mouse is associated with activation of microglia

Agouti-related protein (AgRP) is a key orexigenic neuropeptide expressed in the hypothalamic arcuate nucleus and a marker for neurons conveying hormonal signals of hunger to the brain. Mice homozygous for the anorexia (anx) mutation are characterized by decreased food intake, starvation, and death by 3-5 weeks of age. At this stage immunoreactivity for AgRP is increased in cell bodies but decreased in the nerve terminals. We studied when during early postnatal development the aberrant phenotype of the AgRP system becomes apparent in anx/anx mice and possible underlying mechanisms. AgRP and ionized calcium binding adapter molecule (Iba1), a marker for activated microglia, as well as Toll-like receptor 2 (TLR-2), were studied by immunohistochemistry at postnatal days P1, P5, P10, P12, P15 and P21 in anx/anx and wild-type mice. We found that the AgRP system in the anx/anx mouse develops similarly to the wild type until P12, when AgRP fibers in anx/anx mice cease to increase in density in the main projection areas. At P21, AgRP fiber density in anx/anx mice was significantly reduced vs. P15, in certain regions. At P21, many strongly AgRP-positive cell bodies were observed in the anx/anx arcuate nucleus vs. only few and weakly fluorescent ones in the wild type. The decrease in AgRP fiber density in anx/anx mice overlapped with an increase in Iba1 and TLR-2 immunoreactivities. Thus, the aberrant appearance of the AgRP system in the anx/anx mouse in the early postnatal development could involve a microglia-associated process and the innate immune system.

Challenging the omnipotence of the suprachiasmatic timekeeper: are circadian oscillators present throughout the mammalian brain?

The suprachiasmatic nucleus of the hypothalamus (SCN) is the master circadian pacemaker or clock in the mammalian brain. Canonical theory holds that the output from this single, dominant clock is responsible for driving most daily rhythms in physiology and behaviour. However, important recent findings challenge this uniclock model and reveal clock-like activities in many neural and non-neural tissues. Thus, in addition to the SCN, a number of areas of the mammalian brain including the olfactory bulb, amygdala, lateral habenula and a variety of nuclei in the hypothalamus, express circadian rhythms in core clock gene expression, hormone output and electrical activity. This review examines the evidence for extra-SCN circadian oscillators in the mammalian brain and highlights some of the essential properties and key differences between brain oscillators. The demonstration of neural pacemakers outside the SCN has wide-ranging implications for models of the circadian system at a whole-organism level.

Evidence for hypothalamic dysregulation in mouse models of anorexia as well as in humans

Eating disorders constitute major medical health problems in the western world. Even though little is known about the molecular mechanisms behind abnormal eating behavior, it has become clear that the central nervous system (CNS), particularly the hypothalamus, plays a significant role. The anorexic anx/anx mouse is a unique model for studying food intake and energy expenditure. The anx mutation is linked to marked alterations in hypothalamic distributions of signal substances known to have potent regulatory roles in the control of food intake. Another mouse model that displays an anorectic phenotype similar to the anx/anx mouse is the Contactin KO mouse. This model displays very similar hypothalamic alterations as seen in the anx/anx mouse, arguing for a role of these specific hypothalamic changes in an anorectic phenotype. In human eating disorders, hypothalamic systems corresponding to those defective in mouse models could be compromised since autoantibodies against melanocortin peptides have been detected in anorectic and bulimic patients. These findings represent research avenues that may lead to a better understanding of eating disorders and development of targeted therapeutic approaches.

Hypothalamic proopiomelanocortin (POMC) neurons have a cholinergic phenotype

Neuronal networks originating in the hypothalamic arcuate nucleus play fundamental roles in the control of energy balance. Neuropeptide Y (NPY)-producing neurons in the arcuate nucleus stimulate food intake, whereas arcuate nucleus neurons that release the proopiomelanocortin (POMC)-derived peptide alpha-melanocyte-stimulating hormone (alpha-MSH) potently reduce food intake. Relatively little attention has been focused on classical neurotransmitters in regulation of food intake. Here, we have investigated the potential presence of acetylcholine (ACh) in NPY- and POMC-containing neuronal populations of the arcuate nucleus. Antisera to proteins required for cholinergic neurotransmission, including choline acetyltransferase (ChAT) and the vesicular acetylcholine transporter (VAChT), were employed in double-labeling immunohistochemical experiments. In colchicine-treated rats, ChAT- and VAChT-immunopositive cell bodies were located in the ventral aspect of the arcuate nucleus. ChAT and VAChT immunoreactivities were demonstrated in alpha-MSH- and cocaine- and amphetamine-regulated transcript (CART)-containing cell bodies of the arcuate nucleus, whereas cell bodies containing NPY or agouti-related peptide (AGRP) were distinct from VAChT-immunoreactive neuronal perikarya. VAChT immunoreactivity was also present in a large number of alpha-MSH-containing nerve fiber varicosities throughout the central nervous system. In the commissural part of the nucleus tractus solitarius, no alpha-MSH-containing cell bodies were found to have ChAT or VAChT immunoreactivity. The presence of markers for cholinergic neurotransmission in a subpopulation of hypothalamic POMC/CART neurons suggests co-release of ACh with peptides derived from the POMC precursor and CART. The results indicate a role for ACh in control of energy balance, mediating the effects of peripheral hormones such as leptin and insulin.

Multiple co-localizations in arcuate GHRH-eGFP neurons in the mouse hypothalamus

In the present work, we took advantage of a recently described model of GHRH-enhanced green fluorescent protein (eGFP) transgenic mice to evaluate the extent of co-localization of GHRH neurons with galanin (GAL), neurotensin (NT) and tyrosine hydroxylase (TH) in 3- and 8-month-old male and female mice. The total number of GHRH-eGFP neurons along the rostro-caudal axis of the arcuate nucleus did not differ according to gender or age. GAL-immunoreactivity was present in 40-44% of 3-month-old GHRH-eGFP neurons in male and female arcuate nucleus, respectively, but only 25-22% in 8-month-old mice. TH immunoreactivity occurred in 36-35% of GHRH-eGFP neurons in male and female arcuate nucleus from 3-month-old mice and these proportions increased to 40 and 45% in 8-month-old mice. NT immunoreactivity was present in 14 and 24% of GHRH-eGFP neurons in male and female arcuate nucleus from 3-month-old mice up to 28 and 26% in 8-month-old mice. Thus, co-localization of peptides and enzyme in GHRH-eGFP neurons displays a sexual dimorphism at 3-month of age for NT, and at 8-month for TH, while the total number of GHRH-eGFP neurons does not exhibit gender difference at either age. In summary, it appears that changes in co-localized (and presumably co-released) peptides, rather than GHRH per se, may contribute to the changes in sexually dimorphic GH secretion with aging in the mouse.

Growth hormone-releasing hormone (GHRH) neurons in the arcuate nucleus (Arc) of the hypothalamus are decreased in transgenic rats whose expression of ghrelin receptor is attenuated: Evidence that ghrelin receptor is involved in the up-regulation of GHRH expression in the arc

GH secretagogue (GHS)/ghrelin stimulates GH secretion by binding mainly to its receptor (GHS-R) on GHRH neurons in the arcuate nucleus (Arc) of the hypothalamus. GHRH, somatostatin, and neuropeptide Y (NPY) in the hypothalamus are involved in the regulatory mechanism of GH secretion. We previously created transgenic (Tg) rats whose GHS-R expression is reduced in the Arc, showing lower body weight and shorter nose-tail length. GH secretion is decreased in female Tg rats. To clarify how GHS-R affects GHRH expression in the Arc, we compared the numbers of GHS-R-positive, GHRH, and NPY neurons between Tg and wild-type rats. Immunohistochemical analysis showed that the numbers of GHS-R-positive neurons, GHRH neurons, and GHS-R-positive GHRH neurons were reduced in Tg rats, whereas the numbers of NPY neurons and GHS-R-positive NPY neurons did not differ between the two groups. The numbers of Fos-positive neurons and Fos-positive GHRH neurons in response to KP-102 were decreased in Tg rats. Competitive RT-PCR analysis of GHRH mRNA expression in the cultured hypothalamic neurons showed that KP-102 increased NPY mRNA expression level and that NPY decreased GHRH mRNA expression level. KP-102 increased GHRH mRNA expression level in the presence of anti-NPY IgG. GH increased somatostatin mRNA expression. Furthermore, GH and somatostatin decreased GHRH mRNA expression, whereas KP-102 showed no significant effect on somatostatin mRNA expression. These results suggest that GHS-R is involved in the up-regulation of GHRH and NPY expression and that NPY, somatostatin, and GH suppress GHRH expression. It is also suggested that the reduction of GHRH neurons of Tg rats is induced by a decrease in GHS-R expression.

Sexual dimorphism in the organization of the rat hypothalamic infundibular area

The hypothalamic infundibular area is located outside the blood-brain barrier and includes, the ventromedial arcuate nucleus (vmARC) sensing circulating substances, and the median eminence (ME) where neurohormones are released into the hypothalamo-hypophysial vasculature. This integrated functional unit, pivotal in endocrine control, adjusts neuroendocrine output to feedback information. Despite a differing physiology in males and females, this functional unit has not appeared differently organized between sexes. Using immunocytochemistry, we describe here for the first time in adult rats, a conspicuous sex-difference in its axonal wiring by intrinsic glutamatergic neurons containing the neuropeptides neurokinin B (NKB) and dynorphin. In the male, NKB neurons send axons to capillary vessels of the vmARC and of the ME (only where gonadotropin-releasing hormone (GnRH) axons terminate). Electron microscopy revealed that NKB axons target the barrier of tanycytes around fenestrated capillary vessels (in addition to GnRH axons), suggesting a control of regional bidirectional permeability. In the female, NKB neurons send axons to the neuropile of the vmARC, suggesting a direct control of its sensor neurons. The other projections of NKB neurons, studied by surgical isolation of the ARC-ME complex and confocal microscopy, are not sexually dimorphic and target both integrative and neuroendocrine centers controlling reproduction and metabolism, suggesting a broad influence over endocrine function. These observations demonstrate that the mechanisms subserving hypothalamic permeability and sensitivity to feedback information are sexually dimorphic, making the infundibular area a privileged site of generation of the male-to-female differences in the adult pattern of pulsatile hormonal secretions.

Anatomy of the hypophysiotropic somatostatinergic and growth hormone-releasing hormone system minireview

The central control of growth hormone (GH) secretion from the pituitary gland is ultimately achieved by the interaction between two hypothalamic neurohormones, somatostatin which inhibits and growth hormone-releasing hormone (GHRH) which stimulates GH release. The regulation of the somatostatin and GHRH release from the hypothalamus is regulated by a range of other neuropeptides, neurotransmitters, neurohormones. In this mini review we attempt to provide a short summary covering the anatomy and chemical characteristics of the various cell populations regulating GH secretion as a tribute to Miklós Palkovits who pioneered the field of functional neuroanatomy of hypothalamic networks.

Tyramine-immunoreactive neuronal structures in the rat brain: Abundance in the median eminence of the mediobasal hypothalamus

Immunoreactivity to p-tyramine, one of the natural trace amines, was studied in the rat brain by an anti-p-tyramine antibody. Immunoreactivity to this amine is very weak in the nigrostriatal dopaminergic neurons and terminals, and weak in the locus coeruleus noradrenergic ones. It was intensified in these structures after monoamine oxidase inhibition. On the other hand, this amine was highly concentrated in the median eminence of the mediobasal hypothalamus, in which its physiological function on prolactin release has been demonstrated.

Glutamatergic innervation of neuropeptide Y and pro-opiomelanocortin-containing neurons in the hypothalamic arcuate nucleus of the rat

Abstract The hypothalamic arcuate nucleus contains a number of neurochemically different cell populations, among others neuropeptide Y (NPY)- and pro-opiomelanocortin (POMC)-derived peptide-expressing neurons; both are involved in the regulation of feeding and energy homeostasis, NPY neurons also in the release of hypophysiotropic hormones, sexual behaviour and thermogenesis. Recent observations indicate that there is a dense plexus of glutamatergic fibres in the arcuate nucleus. The aim of the present studies was to examine the relationship of these fibres to the NPY and POMC neurons in the arcuate nucleus. Double-label immunoelectron microscopy was used. Glutamatergic elements were identified by the presence of vesicular glutamate transporter 1 (VGluT1) or 2 (VGluT2) (selective markers of glutamatergic elements) immunoreactivity. A significant number of VGluT2-immunoreactive terminals was observed to make asymmetric type of synapses with NPY and with beta-endorphin (a marker of POMC neurons)-immunostained nerve cells of the arcuate nucleus. About 15% of VGluT2 synapsing terminals established asymmetric synapses with NPY-positive cells and more than 40% of VGlut2-positive terminals formed synapse on beta-endorphin-positive neurons. VGluT2-positive perikarya were also observed, part of them also contained beta-endorphin. Nerve terminals containing both VGluT2 and beta-endorphin were demonstrated in the cell group. Only very few VGluT1 fibres were detected. Our observations provide the first direct neuromorphological evidence for the existence of glutamatergic innervation of NPY and POMC neurons of the arcuate nucleus.

Physiology of ghrelin and related peptides

Growth hormone (GH) released from pituitary under direct control of hypothalamic releasing (i.e., GHRH) and inhibiting (i.e., sst or SRIF) hormones is an anabolic hormone that regulates metabolism of proteins, fats, sugars and minerals in mammals. Cyril Bowers' discovery of GH-releasing peptide (GHRP-6) was followed by a search for synthetic peptide and nonpeptide GH-secretagogues (GHSs) that stimulate GH release, as well as a receptor(s) unique from GHRH receptor. GHRH and GHSs operate through distinct G protein-coupled receptors to release GH. Signal transduction pathways activated by GHS increase intracellular Ca2+ concentration in somatotrophs, whereas GHRH increases cAMP. Isolation and characterization of ghrelin, the natural ligand for GHS receptor, has opened a new era of understanding to physiology of anabolism, feeding behavior, and nutritional homeostasis for GH secretion and gastrointestinal motility through gut-brain interactions. Other peptide hormones (i.e., motilin, TRH, PACAP, GnRH, leptin, FMRF amide, galanin, NPY, NPW) from gut, brain and other tissues also play a role in modulating GH secretion in livestock and lower vertebrate species. Physiological processes, such as neurotransmission, and secretion of hormones or enzymes, require fusion of secretory vesicles at the cell plasma membrane and expulsion of vesicular contents. This process for GH release from porcine somatotrophs was revealed by atomic force microscopy (AFM), transmission electron microscopy (TEM) and immunohistochemical distribution of the cells in pituitary during stages of development.

Maturation of the hypothalamic arcuate agouti-related protein system during postnatal development in the mouse

The hypothalamic arcuate nucleus (Arc) and its neurons expressing agouti-related protein (AgRP) are key components of the forebrain circuitry involved in long-term regulation of energy homeostasis, including conveying leptin signaling to other hypothalamic and extrahypothalamic regions. In the present work, we investigated the postnatal development (P0, P5, P10, P15, and P21) of this system (AgRP transcript and peptide) in the mouse brain using in situ hybridization and immunohistochemistry. At all stages, AgRP mRNA expression was detected exclusively in the Arc. At P0, AgRP mRNA levels were low, and only a few AgRP-immunoreactive fibers were present reaching, rostrally, the bed nucleus of the stria terminalis and, caudally, the dorsal raphe nucleus. During the following period (P5-P21), the levels of AgRP mRNA gradually increased in the Arc along with a parallel increase in the AgRP fiber density in the hypothalamic regions responsible for control of appetite, including the paraventricular nucleus, as well as in extrahypothalamic regions, including locus coeruleus. These data provide evidence that, in the mouse, the maturation of the AgRP Arc system occurs mainly during the first three postnatal weeks. Together with the existing data on the physiology of appetite and body weight, our data suggest that the first three postnatal weeks in the mouse represents a critical period for the formation of brain mechanisms underlying appetite control via peripheral hormones.