Localization of neuroactive substances in the hypothalamus with special reference to coexistence of messenger molecules

Single cell transcriptomics of hypothalamic warm sensitive neurons that control core body temperature and fever response Signaling asymmetry and an extension of chemical neuroanatomy

We report on an 'unbiased' molecular characterization of individual, adult neurons, active in a central, anterior hypothalamic neuronal circuit, by establishing cDNA libraries from each individual, electrophysiologically identified warm sensitive neuron (WSN). The cDNA libraries were analyzed by Affymetrix microarray. The presence and frequency of cDNAs were confirmed and enhanced with Illumina sequencing of each single cell cDNA library. cDNAs encoding the GABA biosynthetic enzyme Gad1 and of adrenomedullin, galanin, prodynorphin, somatostatin, and tachykinin were found in the WSNs. The functional cellular and in vivo studies on dozens of the more than 500 neurotransmitters, hormone receptors and ion channels, whose cDNA was identified and sequence confirmed, suggest little or no discrepancy between the transcriptional and functional data in WSNs; whenever agonists were available for a receptor whose cDNA was identified, a functional response was found. Sequencing single neuron libraries permitted identification of rarely expressed receptors like the insulin receptor, adiponectin receptor 2 and of receptor heterodimers; information that is lost when pooling cells leads to dilution of signals and mixing signals. Despite the common electrophysiological phenotype and uniform Gad1 expression, WSN transcriptomes show heterogeneity, suggesting strong epigenetic influence on the transcriptome. Our study suggests that it is well-worth interrogating the cDNA libraries of single neurons by sequencing and chipping.

The wake-promoting peptide orexin-B inhibits glutamatergic transmission to dorsal raphe nucleus serotonin neurons through retrograde endocannabinoid signaling

The wake-promoting neuropeptides orexins (hypocretins) play a crucial role in controlling neuronal excitability and synaptic transmission in the CNS. In this study, using whole-cell patch-clamp recordings in an acute dorsal raphe nucleus (DRN) slice preparation, we report that orexin B (Orx-B) depresses the evoked glutamate-mediated synaptic currents in DRN 5-HT neurons. The Orx-B-induced depression is accompanied by an increase in the paired-pulse ratio and the coefficient of variance, suggesting a presynaptic site of action. Orx-B also reduces the frequency but not the amplitude of miniature EPSCs, indicating that depression of glutamatergic transmission is mediated by a decrease in glutamate release. Surprisingly, the Orx-B-induced inhibition of glutamatergic transmission is abolished by postsynaptic inhibition of G-protein signaling with GDPbetaS, suggesting that this effect is signaled by postsynaptic orexin receptors and expressed presynaptically, presumably through a retrograde messenger. Interestingly, the Orx-B-induced depression of glutamate release is mimicked and occluded by the cannabinoid receptor agonist WIN 55,212-2, and is abolished by the CB1 cannabinoid receptor antagonist AM 251. These results imply that the Orx-B-induced depression of glutamatergic transmission to DRN 5-HT neurons is mediated by retrograde endocannabinoid release. Examination of downstream signaling pathways involved in this response indicates that the effect of Orx-B requires the activation of phospholipase C and DAG lipase enzymatic pathways but not a rise in postsynaptic intracellular calcium. Therefore, our findings reveal a previously unsuspected mechanism by which postsynaptic orexin receptors can modulate glutamatergic synaptic transmission to DRN 5-HT neurons.

Hypocretin/Orexin excites hypocretin neurons via a local glutamate neuron-A potential mechanism for orchestrating the hypothalamic arousal system

Neurons that release hypocretin/orexin modulate sleep, arousal, and energy homeostasis; the absence of hypocretin results in narcolepsy. Here we present data on the physiological characteristics of these cells, identified with GFP in transgenic mouse brain slices. Hypocretin-1 and -2 depolarized hypocretin neurons by 15mV and evoked an increase in spike frequency (+366% from a 1-3 Hz baseline). The mechanism for this appears to be hypocretin-mediated excitation of local glutamatergic neurons that regulate hypocretin neuron activity, in part by presynaptic facilitation of glutamate release. This represents a possible mechanism for orchestrating the output of the diffuse hypothalamic arousal system. No direct effect of hypocretin on membrane properties of hypocretin cells was detected. Norepinephrine and serotonin, transmitters of other arousal systems, decreased spike frequency and evoked outward currents, whereas acetylcholine and histamine had little effect.

LHRH and sexual dimorphism

An increasing amount of evidences suggests that galanin plays an important role in the regulation of reproduction in the rat. Galanin is colocalized with luteinizing hormone-releasing hormone (LHRH) in a subset of LHRH neurons, and the pattern of coexpression is sexually dimorphic, with a higher incidence of colocalization and level of galanin mRNA and peptide expression in females than in males. Therefore, the role of galanin may be unique to females, as are the LH surge and ovulation. The colocalization of LHRH and galanin is neonatally determined by an epigenetic mechanism involving the testis, whereas the expression of galanin in adult LHRH neurons is upregulated by estrogen and modulated by progesterone. The action of these sex steroids requires intact neurotransmission towards LHRH neurons, indicating that their action is mediated by interneurons.

Corticotropin-releasing hormone neurons in the paraventricular nucleus project to the external zone of the median eminence: a study combining retrograde labeling with immunocytochemistry

Corticotropin-releasing hormone (CRH) is the major regulator of the pituitary-adrenal axis. CRH-immunoreactive perikarya are widely distributed in the central nervous system; however, only those which participate directly in the regulation of adrenocorticotropin are connected to the portal circulation in the external zone of the median eminence. The present study describes the identification of these hypophysiotropic neurons using retrograde labeling and CRH immunocytochemistry. Fluoro-Gold was injected peripherally then, 5 days later, the animals were treated with colchicine. Twenty-four hours later the animals were sacrificed, and their brains were immunostained for CRH with the indirect immunofluorescence technique. The results indicate that the vast majority of the Fluoro-Gold-accumulating and CRH-immunopositive perikarya (hypophysiotropic neurons) are located in the medial parvicellular subdivision of the paraventricular nucleus (PVN). However, not each CRH-immunoreactive neuron contains Fluoro-Gold, i.e. a small portion of these neurons project to areas of the brain other than the median eminence. The anterior, lateral and periventricular subdivisions of the PVN also contain hypophysiotropic CRH-immunoreactive perikarya, however, their number is much less than in the medial parvicellular subdivision. Scattered double-labeled cells are also present in the medial preoptic area and the dorsal hypothalamus, just behind the PVN. These results support previous observations that the PVN, particularly the medial parvicellular subdivision, is the predominant source of the hypophysiotropic CRH neurons.

Current status of brain hypophysiotropic factors: morphologic aspects

Nearly 40 putative neurotransmitters and other chemical messengers, mostly peptides, are present in the median eminence that constitutes the final common pathway for signals from the brain to the pituitary. The majority of them are produced in perikarya located in different nuclei of the hypothalamus; however, some of them arise from the brainstem. The neurons contacting capillaries of the median eminence (hypophysiotropic neurons) are intermixed with neurons containing the same transmitter (hypophysiotropic factor(1)), but projecting to other areas of the brain. Depending on their site of release, the hypophysiotropic factors may function as neurohormones acting an the pituitary or neurotransmitters affecting the activity of other neurons in the central nervous system. Based on retrograde tracing studies in combination with immunocytochemistry, the origin of many nerve terminals in the median eminence has been determined.

Colocalization of galanin and luteinizing hormone-releasing hormone in a subset of preoptic hypothalamic neurons: anatomical and functional correlates

Colocalization of neurotransmitters, including neuropeptides and amines, in the same neuron of certain areas or well-defined nuclei of the central and peripheral nervous systems appears to be the rule rather than the exception. The coexistent neurotransmitters can be coreleased and interact at pre- and postsynaptic levels in a synergistic or antagonistic manner. Galanin is a recently isolated and characterized "gut-brain" peptide. It is colocalized with many neurotransmitters in both the central and the peripheral nervous systems. Among other regions in the central nervous system, galanin is present in neuronal perikarya of the septum and the hypothalamus. The dense accumulation of nerve terminals in the external zone of the median eminence suggests that galanin is an important peptide regulating neuroendocrine functions. Although most galanin and luteinizing hormone-releasing hormone (LHRH) neurons have a distinctly different morphology, a subset of galanin-immunoreactive perikarya in the diagonal band of Broca and the medial preoptic area, near the organum vasculosum of the lamina terminalis, have morphological features similar to those of LHRH neurons. By using double-labeling immunocytochemistry, we have found that in the preoptic region of the male rat brain approximately 15-20% of these "LHRH-like" galanin-immunoreactive neurons are also immunopositive for LHRH. Moreover, in the medial preoptic area and the diagonal band of Broca, some of the single-labeled LHRH cells are surrounded with galanin-immunoreactive nerve terminals, suggesting that LHRH perikarya have synaptic contacts with galanin-immunoreactive terminals. Additional studies indicated that galanin can readily enhance in vitro release of LHRH from nerve terminals in the median eminence. The observations that (i) galanin is coexpressed with LHRH, (ii) galanin seems to innervate LHRH-producing neurons, and (iii) galanin acts as a putative neurotransmitter to enhance the release of LHRH suggest that galanin should be considered an important regulator of LHRH-containing neurons and, therefore, of reproductive functions.