Gene expression and chemical diversity in hypothalamic neurosecretory neurons

Kisspeptin and lactational anestrus: Current understanding and future prospects

Lactational anestrus, characterized by the suppression of pulsatile gonadotropin-releasing hormone (GnRH)/luteinizing hormone (LH) release, would be a strategic adaptation to ensure survival by avoiding pregnancy during lactation in mammals. In the present article, we first provide a current understanding of the central regulation of reproduction in mammals, i.e., a fundamental role of arcuate kisspeptin neurons in mammalian reproduction by driving GnRH/LH pulses. Second, we discuss the central mechanism inhibiting arcuate Kiss1 (encoding kisspeptin) expression and GnRH/LH pulses during lactation with a focus on suckling stimulus, negative energy balance due to milk production, and the role of circulating estrogen in rats. We also discuss upper regulators that control arcuate kisspeptin neurons in rats during the early and late lactation periods based on the findings obtained by a lactating rat model. Finally, we discuss potential reproductive technology for the improvement of reproductive performance in milking cows.

Neuropeptide and Small Transmitter Coexistence: Fundamental Studies and Relevance to Mental Illness

Neuropeptides are auxiliary messenger molecules that always co-exist in nerve cells with one or more small molecule (classic) neurotransmitters. Neuropeptides act both as transmitters and trophic factors, and play a role particularly when the nervous system is challenged, as by injury, pain or stress. Here neuropeptides and coexistence in mammals are reviewed, but with special focus on the 29/30 amino acid galanin and its three receptors GalR1, -R2 and -R3. In particular, galanin's role as a co-transmitter in both rodent and human noradrenergic locus coeruleus (LC) neurons is addressed. Extensive experimental animal data strongly suggest a role for the galanin system in depression-like behavior. The translational potential of these results was tested by studying the galanin system in postmortem human brains, first in normal brains, and then in a comparison of five regions of brains obtained from depressed people who committed suicide, and from matched controls. The distribution of galanin and the four galanin system transcripts in the normal human brain was determined, and selective and parallel changes in levels of transcripts and DNA methylation for galanin and its three receptors were assessed in depressed patients who committed suicide: upregulation of transcripts, e.g., for galanin and GalR3 in LC, paralleled by a decrease in DNA methylation, suggesting involvement of epigenetic mechanisms. It is hypothesized that, when exposed to severe stress, the noradrenergic LC neurons fire in bursts and release galanin from their soma/dendrites. Galanin then acts on somato-dendritic, inhibitory galanin autoreceptors, opening potassium channels and inhibiting firing. The purpose of these autoreceptors is to act as a 'brake' to prevent overexcitation, a brake that is also part of resilience to stress that protects against depression. Depression then arises when the inhibition is too strong and long lasting - a maladaption, allostatic load, leading to depletion of NA levels in the forebrain. It is suggested that disinhibition by a galanin antagonist may have antidepressant activity by restoring forebrain NA levels. A role of galanin in depression is also supported by a recent candidate gene study, showing that variants in genes for galanin and its three receptors confer increased risk of depression and anxiety in people who experienced childhood adversity or recent negative life events. In summary, galanin, a neuropeptide coexisting in LC neurons, may participate in the mechanism underlying resilience against a serious and common disorder, MDD. Existing and further results may lead to an increased understanding of how this illness develops, which in turn could provide a basis for its treatment.

Maternal stress and the MPOA: Activation of CRF receptor 1 impairs maternal behavior and triggers local oxytocin release in lactating rats

Maternal behavior and anxiety are potently modulated by the brain corticotropin-releasing factor (CRF) system postpartum. Downregulation of CRF in limbic brain regions is essential for appropriate maternal behavior and an adaptive anxiety response. Here, we focus our attention on arguably the most important brain region for maternal behavior, the hypothalamic medial preoptic area (MPOA). Within the MPOA, mRNA for CRF receptor subtype 1 (protein: CRFR1, gene: Crhr1) was more abundantly expressed than for subtype 2 (protein: CRFR2, gene: Crhr2), however expression of Crhr1, Crhr2 and CRF-binding protein (protein: CRFBP, gene: Crhbp) mRNA was similar between virgin and lactating rats. Subtype-specific activation of CRFR, predominantly CRFR1, in the MPOA decreased arched back nursing and total nursing under non-stress conditions. Following acute stressor exposure, only CRFR1 inhibition rescued the stress-induced reduction in arched back nursing while CRFR1 activation prolonged the decline in nursing. Furthermore, inhibition of CRFR1 strongly increased maternal aggression in the maternal defense test. CRFR1 activation had anxiogenic actions and reduced locomotion on the elevated plus-maze, however neither CRFR1 nor R2 manipulation affected maternal motivation. In addition, activation of CRFR1, either centrally or locally in the MPOA, increased local oxytocin release. Finally, inhibition of CRFBP (a potent regulator of CRFR activity) in the MPOA did not affect any of the maternal parameters investigated. In conclusion, activity of CRFR in the MPOA, particularly of subtype 1, needs to be dampened during lactation to ensure appropriate maternal behavior. Furthermore, oxytocin release in the MPOA may provide a regulatory mechanism to counteract the negative impact of CRFR activation on maternal behavior.

Potential of Oxytocin in the Treatment of Schizophrenia

Schizophrenia is a heterogeneous, debilitating disorder characterized by three distinct sets of clinical features: positive symptoms, negative symptoms, and cognitive deficits. Extant antipsychotic drugs have been most successful at treating the positive symptoms of patients with schizophrenia but have minimal therapeutic effects on negative symptoms and cognitive deficits, which are the symptoms that best predict the poor prognosis of these patients. Therefore, there has been a major effort towards identifying compounds that alleviate these symptoms. Oxytocin (OT) is a nonapeptide that regulates peripheral reproductive-relevant functions, and also acts as a neurotransmitter in the brain. Converging evidence from both preclinical and clinical research suggests that OT may have therapeutic efficacy for the positive symptoms, negative symptoms, and cognitive deficits of schizophrenia. In the majority of the small, randomized, placebo-controlled clinical trials conducted to date, OT has shown particular promise in its potential to treat the intractable negative symptoms and social cognitive deficits exhibited by most of the patients with this debilitating disorder. In this leading article, we summarize the clinical evidence relevant to (1) endogenous OT and schizophrenia, and (2) the putative therapeutic effects of OT on each of the three clinical domains.

The secretogranin-II derived peptide secretoneurin modulates electric behavior in the weakly pulse type electric fish, Brachyhypopomus gauderio

Secretoneurin (SN) in the preoptic area and pituitary of mammals and fish has a conserved close association with the vasopressin and oxytocin systems, members of a peptide family that are key in the modulation of sexual and social behaviors. Here we show the presence of SN-immunoreactive cells and projections in the brain of the electric fish, Brachyhypopomus gauderio. Secretoneurin colocalized with vasotocin (AVT) and isotocin in cells and fibers of the preoptic area. In the rostral pars distalis of the pituitary, many cells were both SN and prolactin-positive. In the hindbrain, at the level of the command nucleus of the electric behavior (pacemaker nucleus; PN), some of SN-positive fibers colocalized with AVT. We also explored the potential neuromodulatory role of SN on electric behavior, specifically on the rate of the electric organ discharge (EOD) that signals arousal, dominance and subordinate status. Each EOD is triggered by the command discharge of the PN, ultimately responsible for the basal EOD rate. SN modulated diurnal basal EOD rate in freely swimming fish in a context-dependent manner; determined by the initial value of EOD rate. In brainstem slices, SN partially mimicked the in vivo behavioral effects acting on PN firing rate. Taken together, our results suggest that SN may regulate electric behavior, and that its effect on EOD rate may be explained by direct action of SN at the PN level through either neuroendocrine and/or endocrine mechanisms.

Distribution of Fos-immunoreactive cells in rat forebrain and midbrain following social defeat stress and diazepam treatment

The anxiolytic diazepam selectively inhibits psychological stress-induced autonomic and behavioral responses without causing noticeable suppression of other central performances. This pharmacological property of diazepam led us to the idea that neurons that exhibit diazepam-sensitive, psychological stress-induced activation are potentially those recruited for stress responses. To obtain neuroanatomical clues for the central stress circuitries, we examined the effects of diazepam on psychological stress-induced neuronal activation in broad brain regions. Rats were exposed to a social defeat stress, which caused an abrupt increase in body temperature by up to 2°C. Pretreatment with diazepam (4mg/kg, i.p.) attenuated the stress-induced hyperthermia, confirming an inhibitory physiological effect of diazepam on the autonomic stress response. Subsequently, the distribution of cells expressing Fos, a marker of neuronal activation, was examined in 113 forebrain and midbrain regions of these rats after the stress exposure and diazepam treatment. The stress following vehicle treatment markedly increased Fos-immunoreactive (IR) cells in most regions of the cerebral cortex, limbic system, thalamus, hypothalamus and midbrain, which included parts of the autonomic, neuroendocrine, emotional and arousal systems. The diazepam treatment significantly reduced the stress-induced Fos expression in many brain regions including the prefrontal, sensory and motor cortices, septum, medial amygdaloid nucleus, medial and lateral preoptic areas, parvicellular paraventricular hypothalamic nucleus, dorsomedial hypothalamus, perifornical nucleus, tuberomammillary nucleus, association, midline and intralaminar thalami, and median and dorsal raphe nuclei. In contrast, diazepam increased Fos-IR cells in the central amygdaloid nucleus, medial habenular nucleus, ventromedial hypothalamic nucleus and magnocellular lateral hypothalamus. These results provide important information for elucidating the neural circuitries that mediate the autonomic and behavioral responses to psychosocial stressors.

Extracellular pH modulates GABAergic neurotransmission in rat hypothalamus

Changes in extracellular pH have a modulatory effect on GABAA receptor function. It has been reported that pH sensitivity of the GABA receptor is dependent on subunit composition and GABA concentration. Most of previous investigations focused on GABA-evoked currents, which only reflect the postsynaptic receptors. The physiological relevance of pH modulation of GABAergic neurotransmission is not fully elucidated. In the present studies, we examined the influence of extracellular pH on the GABAA receptor-mediated inhibitory neurotransmission in rat hypothalamic neurons. The inhibitory postsynaptic currents (IPSCs), tonic currents, and the GABA-evoked currents were recorded with whole-cell patch techniques on the hypothalamic slices from Sprague-Dawley rats at 15-26 postnatal days. The amplitude and frequency of spontaneous GABA IPSCs were significantly increased while the external pH was changed from 7.3 to 8.4. In the acidic pH (6.4), the spontaneous GABA IPSCs were reduced in amplitude and frequency. The pH induced changes in miniature GABA IPSCs (mIPSCs) similar to that in spontaneous IPSCs. The pH effect on the postsynaptic GABA receptors was assessed with exogenously applied varying concentrations of GABA. The tonic currents and the currents evoked by sub-saturating concentration of GABA ([GABA]) (10 μM) were inhibited by acidic pH and potentiated by alkaline pH. In contrast, the currents evoked by saturating [GABA] (1mM) were not affected by pH changes. We also investigated the influence of pH buffers and buffering capacity on pH sensitivity of GABAA receptors on human recombinant α1β2γ2 GABAA receptors stably expressed in HEK 293 cells. The pH influence on GABAA receptors was similar in HEPES- and MES-buffered media, and not dependent on protonated buffers, suggesting that the observed pH effect on GABA response is a specific consequence of changes in extracellular protons. Our data suggest that the hydrogen ions suppress the GABAergic neurotransmission, which is mediated by both presynaptic and postsynaptic mechanisms.

Galanin and the orexin 2 receptor as possible regulators of enkephalin in the paraventricular nucleus of the hypothalamus: relation to dietary fat

Recent studies show that the non-opioid peptides, galanin (GAL) and orexin (OX), are similar to the opioid enkephalin (ENK) in being stimulated by dietary fat and also in enhancing the consumption of a high-fat diet (HFD). This suggests that, when an HFD is provided, these non-opioids may stimulate the opioid system to promote excess consumption of this diet. Using single- and double-labeling immunohistochemistry, the present study sought to identify possible neuroanatomical substrates for this close relationship. Focusing on the hypothalamic paraventricular nucleus (PVN), and particularly its anterior (aPVN), middle (mPVN) and posterior (pPVN) parts, the experiments examined whether GAL itself or the receptors for GAL and OX are stimulated by an HFD in the same areas and possibly the same neurons as ENK. Compared to animals fed a standard chow diet, rats consuming an HFD exhibited an increased density of medial parvocellular neurons immunoreactive (IR) for GAL in the mPVN and aPVN and for ENK in the mPVN and pPVN, distinguishing the mPVN as an area where both peptides were affected. While showing little evidence for GAL and ENK colocalization with a chow diet, double-labeling studies in HFD-fed rats revealed significant colocalization specifically in medial parvocellular neurons of the mPVN. Immediately posterior to this site, further analyses revealed a similar relationship between the OX 2 receptor (OX(2)R) and ENK in HFD-treated animals. While increasing the density of neurons immunoreactive for OX(2)R as well as for the GAL 1 receptor but not OX 1 receptor, HFD consumption increased the colocalization only of OX(2)R and ENK, specifically in the medial parvocellular neurons of the pPVN. These changes in HFD-fed rats, showing GAL and OX(2)R to colocalize with ENK exclusively in neurons of the medial parvocellular mPVN and pPVN, respectively, suggest possible neural substrates through which the non-opioid peptides may functionally interact with ENK when exposed to an HFD.

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.

Plasticity of nonapeptidergic neurosecretory cells in fish hypothalamus and neurohypophysis

The structure and function of nonapeptidergic neurosecretory cells (NP-NSC) are considered in terms of comparative morphology. Among NSC of different ergicity for NP-NSC the most characteristic involve massive accumulation and storage of neurohormonal products. Only in NP-NSC are the secretory cycles of functioning clearly expressed. Their highest reactivity is established during experimental and physiological stresses. In contrast, liberinergic, statinergic, and monoaminergic NSC, unlike NP-NSC, are characterized even in the "norm" by a constantly high level of extrusion processes. As signs of maximum NP-NSC plasticity, we consider the largest size of elementary neurosecretory granules, the diversity of secretion forms, and the maximum development of Herring bodies-clear manifestations of secretory cycles of functioning. In particular, phases of massive storage of neurosecretory granules in the extrusion cycle of NP-NSC neurosecretory terminals express accumulation of neurosecretory products. It is concluded that a particularly high degree of plasticity of NP-NSC is provided by their capability for functional reversion. This reversion is manifested first in the form of the restoration of the initial moderate level of functioning and especially in the accumulation of neurosecretory products. The reversion is considered an important mechanism providing a high degree of NSC plasticity. This degree turns out to be sufficient for participation of NP-NSC in the integration of fish reproduction. It is shown that NP-NSC are organized by the principle of a triad of the balanced system. This system consists of two alternative states: accumulation and release of neurosecretory products and the center of control of dynamics of their interrelations, the self-regulating center. In the latter, the key role is probably played by the Golgi complex.

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.

APeg3, a novel paternally expressed gene 3 antisense RNA transcript specifically expressed in vasopressinergic magnocellular neurons in the rat supraoptic nucleus

Vasopressin (VP) and oxytocin (OT) play critical roles in the regulation of salt and water balance, lactation, and various behaviors and are expressed at very high levels in specific magnocellular neurons (MCNs) in the hypothalamo-neurohypophysial system (HNS). In addition to the cell-specific expression of the VP and OT genes in these cells, there are other transcripts that are preferentially expressed in the VP or OT MCNs. One such gene, paternally expressed gene 3 (Peg3), is an imprinted gene expressed exclusively from the paternal allele that encodes a Kruppel-type zinc finger-containing protein involved in maternal behavior and is abundantly expressed in the VP-MCNs. We report here the robust expression in the VP-MCNs of an RNA, which we designate APeg3 that is transcribed in the antisense direction to the 3' untranslated region of the Peg3 gene. The APeg3 mRNA is about 1 kb in size, and the full-length sequence of APeg3, as determined by 5' and 3' RACE, contains an open reading frame that predicts a protein of 93 amino acids and is predominantly expressed in VP-MCNs. Both Peg3 and APeg3 gene expression in the VP-MCNs increase during systemic hyperosmolality in vivo, demonstrating that both of these genes are osmoregulated.

Regulation of rat APJ receptor messenger ribonucleic acid expression in magnocellular neurones of the paraventricular and supraopric nuclei by osmotic stimuli

The novel apelin receptor (APJ receptor, APJR) has a restricted expression in the central nervous system suggestive of an involvement in the regulation of body fluid homeostasis. The endogenous ligand for APJR, apelin, is also highly concentrated in regions that are involved in the control of drinking behaviour. While the physiological roles of APJR and apelin are not fully known, apelin has been shown to stimulate drinking behaviour in rats and to have a regulatory effect on vasopressin release from magnocellular neurones of the hypothalamic paraventricular (PVN) and supraoptic (SON) nuclei. To determine the role of APJR in the regulation of water balance, this study examined the effects of osmotic stimulation on the expression of APJR mRNA in the magnocellular PVN (mPVN) and SON of salt-loaded and water-deprived rats. Intake of 2% NaCl and water deprivation for 48 h induced expression of APJR mRNA in the mPVN and SON. Using dual-label in situ hybridization histochemistry, we also investigated whether APJR is colocalized within vasopressin neurones in control, salt-loaded and water-deprived rats. APJR mRNA was found to colocalize with a small population of vasopressin-containing magnocellular neurones in control and water-deprived rats. Salt-loading resulted in an increased colocalization of APJR and vasopressin mRNAs in the SON. These data verify a role for APJ receptors in body fluid regulation and suggest a role for apelin in the regulation of vasopressin-containing neurones via a local autocrine/paracrine action of the peptide.

The magnocellular neuronal phenotype: cell-specific gene expression in the hypothalamo-neurohypophysial system

The magnocellular oxytocin (OT) and vasopressin (VP) neurons of the hypothalamo-neurohypophysial system are exceptional cell biological models to study mechanisms of cell-specific gene expression and neurosecretion of neuropeptides in the central nervous system. Single cell differential gene expression experiments have further defined these phenotypes by identifying novel and distinct regulatory molecules in these neurons. Transgenic mouse studies have led to the intergenic region (IGR) hypothesis, which states that the DNA sequences between the OT- and VP-genes contain critical enhancer sites for their cell-specific expression. The recent cloning and sequencing of the human IGR, and its comparison with the mouse IGR sequence has identified conserved sequences as putative, cell-specific enhancer sites which are now being evaluated by biolistic transfections of organotypic hypothalamic cultures. With these data, it is possible to target the gene expression of specific molecules to magnocellular neurons both in vivo and in vitro, in order to perturb and/or visualize neurosecretory and other processes.

Functional characterization of GABA(A) receptors in neonatal hypothalamic brain slice

The hypothalamus influences a number of autonomic functions. The activity of hypothalamic neurons is modulated in part by release of the inhibitory neurotransmitter GABA onto these neurons. GABA(A) receptors are formed from a number of distinct subunits, designated alpha, beta, gamma, delta, epsilon, and theta, many of which have multiple isoforms. Little data exist, however, on the functional characteristics of the GABA(A) receptors present on hypothalamic neurons. To gain insight into which GABA(A) receptor subunits are functionally expressed in the hypothalamus, we used an array of pharmacologic assessments. Whole cell recordings were made from thin hypothalamic slices obtained from 1- to 14-day-old rats. GABA(A) receptor-mediated currents were detected in all neurons tested and had an average EC(50) of 20 +/- 1.6 microM. Hypothalamic GABA(A) receptors were modulated by diazepam (EC(50) = 0.060 microM), zolpidem (EC(50) = 0.19 microM), loreclezole (EC(50) = 4.4 microM), methyl-6,7-dimethoxy-4-ethyl-beta-carboline (EC(50) = 7.7 microM), and 5alpha-pregnan-3alpha-hydroxy-20-one (3alpha-OH-DHP). Conversely, these receptors were inhibited by Zn(2+) (IC(50) = 70.5 microM), dehydroepiandrosterone sulfate (IC(50) = 16.7 microM), and picrotoxin (IC(50) = 2.6 microM). The alpha4/6-selective antagonist furosemide (10-1,000 microM) was ineffective in all hypothalamic neurons tested. The results of our pharmacological analysis suggest that hypothalamic neurons express functional GABA(A) receptor subtypes that incorporate alpha1 and/or alpha2 subunits, beta2 and/or beta3 subunits, and the gamma2 subunit. Our results suggest receptors expressing alpha3-alpha6, beta1, gamma1, and delta, if present, represent a minor component of functional hypothalamic GABA(A) receptors.

Gene expression in the supraoptic nucleus

The magnocellular neurosecretory cells (MNCs) in the supraoptic nucleus (SON) express multiple kinds of genes, including not only the classical hormones arginine vasopressin (AVP) and oxytocin (OXT), but also other physiologically active substances including neuropeptides, their receptors, and nitric oxide (NO) synthase, the rate-limiting enzyme in the synthesis of NO under physiological condition. For example, osmotic stimuli such as dehydration and chronic salt loading cause a wide range of changes of the expression levels of the genes and marked induction of the expression of the genes in the SON. The expression of the NO synthase gene in the SON under physiological conditions is reviewed.

Cell-specific expression and subcellular localization of neurophysin-CAT-fusion proteins expressed from oxytocin and vasopressin gene promoter-driven constructs in transgenic mice

The cell-specific expression of both the oxytocin (OT) and vasopressin (VP) genes in magnocellular neurons (MCNs) of the hypothalamus has been proposed to be under the control of cis-elements in an intergenic region downstream of the VP gene. We examined this hypothesis using transgenic mice containing mouse genomic DNA-derived constructs linked to chloramphenicol acetyltransferase (CAT) reporters. VP gene expression was studied using constructs containing 3.8 kbp of the 5' flanking region and all the exons and introns in the mouse VP gene, which was fused at the end of exon 3 to a CAT reporter. The two VP-transgene constructs differed by the lengths of their VP gene 3' flanking regions (2.1 versus 3.6 kbp). A similar construct for the oxytocin CAT transgene was used which contained the full-length (3.6 kbp) downstream intergenic region between the mouse genes. All three transgenic constructs produced cell-specific expression of the CAT-reporter in the magnocellular neurons as determined by CAT-immunoreactivity. Oxytocin transgene expression was restricted to OT cells in two founders, and the two VP transgenes to VP cells in five founders. Electron microscopic immunocytochemistry showed that the CAT fusion proteins produced from the OT- and VP-transgenes were efficiently trafficked through the regulated secretory pathways in their respective magnocellular neurons, packaged into large dense core vesicles, and transported to nerve terminals in the posterior pituitary.

Gene regulation in the magnocellular hypothalamo-neurohypophysial system

The hypothalamo-neurohypophysial system (HNS) is the major peptidergic neurosecretory system through which the brain controls peripheral physiology. The hormones vasopressin and oxytocin released from the HNS at the neurohypophysis serve homeostatic functions of water balance and reproduction. From a physiological viewpoint, the core question on the HNS has always been, "How is the rate of hormone production controlled?" Despite a clear description of the physiology, anatomy, cell biology, and biochemistry of the HNS gained over the last 100 years, this question has remained largely unanswered. However, recently, significant progress has been made through studies of gene identity and gene expression in the magnocellular neurons (MCNs) that constitute the HNS. These are keys to mechanisms and events that exist in the HNS. This review is an inventory of what we know about genes expressed in the HNS, about the regulation of their expression in response to physiological stimuli, and about their function. Genes relevant to the central question include receptors and signal transduction components that receive and process the message that the organism is in demand of a neurohypophysial hormone. The key players in gene regulatory events, the transcription factors, deserve special attention. They do not only control rates of hormone production at the level of the gene, but also determine the molecular make-up of the cell essential for appropriate development and physiological functioning. Finally, the HNS neurons are equipped with a machinery to produce and secrete hormones in a regulated manner. With the availability of several gene transfer approaches applicable to the HNS, it is anticipated that new insights will be obtained on how the HNS is able to respond to the physiological demands for its hormones.

Regulation by osmotic stimuli of galanin-R1 receptor expression in magnocellular neurones of the paraventricular and supraoptic nuclei of the rat

Neurones of the supraoptic nucleus (SON) and the magnocellular and parvocellular divisions of the paraventricular nucleus (PVN) express galanin and [125I]galanin binding sites. Although the precise role(s) of galanin in these different cell populations is still unknown, it has been shown to regulate the electrophysiological, neurochemical and secretory activity of magnocellular neurones. In light of the well-described effects of hyperosmotic stimuli, such as salt-loading on magnocellular neurone activity and galanin synthesis and release, and the recent identification of multiple galanin receptors in brain, this study assessed the possible regulation of galanin receptor subtype expression in the PVN/SON of salt-loaded, dehydrated and food-deprived rats. Gal-R1 mRNA was abundant in the SON (and magnocellular PVN) of control rats and levels were increased in these same cells after 4 days of salt-loading (2% NaCl solution as drinking water) or water deprivation. The density of specific [125I]galanin(1-29) binding and the intensity of Gal-R1-like immunostaining were also increased in the characteristically enlarged, magnocellular neurones of the PVN and SON after these treatments. Gal-R2 mRNA was detected in the parvocellular PVN, but levels were not altered by the hyperosmotic stimuli. In contrast, food deprivation (4 days), which has been shown to reduce levels of several neurochemical markers in magnocellular neurones, produced a significant reduction in Gal-R1 (and galanin) mRNA levels in the SON, but no consistent change in neurone size, [125I]galanin binding levels, or Gal-R1 immunostaining. Along with previous findings from this and other laboratories, these data suggest that the expression of galanin and Gal-R1 receptors is regulated in parallel with functional and morphological changes in hypothalamic magnocellular neurones. Furthermore, Gal-R1 immunoreactivity was primarily detected in somatodendritic areas and thus galanin may influence the activity of these cells, particularly vasopressin synthesis/release, via autocrine or paracrine activation of Gal-R1 receptors, especially during long-lasting stimulation.

Phenotypical segregation among female rat hypothalamic gonadotropin-releasing hormone neurons as revealed by the sexually dimorphic coexpression of cholecystokinin and neurotensin

The neuroendocrine control of the gonad is exerted primarily by the gonadotropin-releasing hormone neurons located in the septum and the hypothalamus. Despite their sexually dimorphic activity, tonic in males and phasic in females, these neurons have not appeared qualitatively different between sexes in intrinsic organization or chemical phenotype. Here, by using multiple-label immunocytochemistry, it is demonstrated that the phenotype of gonadotropin-releasing hormone neurons is sex specific. In females only, 54.5% of them co-expressed cholecystokinin immunoreactivity and 29.4% additionally expressed neurotensin immunoreactivity. These multipeptidergic neurons were observed in the hypothalamus but not in the septum. During postnatal development, cholecystokinin and neurotensin immunoreactivities were first detected in gonadotropin-releasing hormone-containing axons of the median eminence at vaginal opening, suggesting an involvement of the neuropeptides in peri-ovulatory events. This peptidergic phenotype was not apparent in females ovariectomized as adults but was reinstated by estradiol treatment. In adult males, the testicle does not control this phenotype because orchidectomized adults did not display it, whatever the post-operative delay (one to five weeks) or substitutive chronic steroid treatment (testosterone or estradiol). The testicle may, however, masculinize the phenotype neonatally because estradiol or testosterone treatment in adulthood induced an expression of cholecystokinin immunoreactivity in gonadotropin-releasing hormone-containing axons of the median eminence in both males and females that were gonadectomized at birth. This procedure, however, failed to significantly induce an expression of neurotensin immunoreactivity, suggesting a role of the postnatal ovary on this element of the chemistry of gonadotropin-releasing hormone neurons.Thus, the gonad permanently organizes the gonadotropin-releasing hormone neuronal population, resulting, at least in females, in a mosaic of phenotypically distinct, functional subunits.

Gene expression in the rat supraoptic nucleus induced by chronic hyperosmolality versus hyposmolality

Magnocellular neurons of the hypothalamo-neurohypophysial system play a fundamental role in the maintenance of body homeostasis by secreting vasopressin and oxytocin in response to systemic osmotic perturbations. During chronic hyperosmolality, vasopressin and oxytocin mRNA levels increase twofold, whereas, during chronic hyposmolality, these mRNA levels decrease to 10-20% of that of normoosmolar control animals. To determine what other genes respond to these osmotic perturbations, we have analyzed gene expression during chronic hyper- versus hyponatremia. Thirty-seven cDNA clones were isolated by differentially screening cDNA libraries that were generated from supraoptic nucleus tissue punches from hyper- or hyponatremic rats. Further analysis of 12 of these cDNAs by in situ hybridization histochemistry confirmed that they are osmotically regulated. These cDNAs represent a variety of functional classes and include cytochrome oxidase, tubulin, Na(+)-K(+)-ATPase, spectrin, PEP-19, calmodulin, GTPase, DnaJ-like, clathrin-associated, synaptic glycoprotein, regulator of GTPase stimulation, and gene for oligodendrocyte lineage-myelin basic proteins. This analysis therefore suggests that adaptation to chronic osmotic stress results in global changes in gene expression in the magnocellular neurons of the supraoptic nucleus.

Control of food intake via leptin receptors in the hypothalamus

Food intake is regulated via neural circuits located in the hypothalamus. During the past decade our knowledge on the specific mediators and neuronal networks that regulate food intake and body weight has increased dramatically. An important contribution to the understanding of hypothalamic control of food intake has been the characterization of the ob gene product (leptin) via positional cloning. Absence of circulating, functionally active, leptin hormone results in massive obesity as seen in ob/ob mice. Leptin inhibits food intake and increases energy expenditure via an interaction with specific leptin receptors located in the hypothalamus. Leptin receptors, of which there are several splice variants (Ob-Ra through Ob-Re), belong to the superfamily of cytokine receptors, which use the JAK-STAT pathway of signal transduction. Obese db/db mice, which have a mutation in the db locus, are unable to perform JAK-STAT signal transduction due to absence of functionally active (long form; Ob-Rb) leptin receptors. Ob-Rb is primarily expressed in the hypothalamus, with particularly high levels in the arcuate, paraventricular, and dorsomedial nuclei and in the lateral hypothalamic area. The abundance of leptin receptors in the ventromedial and lateral hypothalamus supports early observations that these two regions are intimately associated with the regulation of food intake. Leptin receptors have been identified in neuropeptide Y (NPY)/lagouti-related peptide (AgRP)- and proopiomelanocortin (POMC)/cocaine- and amphetamine-regulated transcript (CART)-containing neurons of the ventromedial and ventrolateral arcuate nucleus, respectively, and in melanin-concentrating hormone (MCH)- and hypocretin/orexin-containing neurons of the lateral hypothalamus, suggesting that the above-mentioned messengers are mediators of leptin's action in the hypothalamus. Indeed, functional studies show that NPY, AgRP, POMC-derived peptides, CART, MCH, and hypocretins/orexins all are important regulators of food intake. Leptin is essential for normal body weight balance, but the exact mechanisms by which leptin activates hypothalamic neuronal circuitries is known to a limited extent. In order to find pharmaceutical approaches to treat obesity, further studies will be needed to reveal the exact mechanisms by which leptin lowers body weight and which role leptin and leptin receptors have in the pathogenesis of human obesity.

GHRP-6-induced changes in electrical activity of single cells in the arcuate, ventromedial and periventricular nucleus neurones [correction of nuclei] of a hypothalamic slice preparation in vitro

Previously, we demonstrated that systemic injection of the growth hormone secretagogue, growth hormone-releasing peptide (GHRP)-6, selectively activated cells in the hypothalamic arcuate nucleus, as reflected by increased electrical activity and induction of the immediate early gene c-fos. The growth hormone secretagogue receptor distribution is not confined to the arcuate nucleus, suggesting that additional sites of action may exist. In the present study we characterized the electrophysiological responses of cells in the arcuate nucleus, ventromedial nucleus and periventricular nucleus in an in-vitro hypothalamic slice preparation, following bath application of GHRP-6. Additionally, since central somatostatin administration has been shown to attenuate the induction of the c-fos gene by GHRP-6, we sought to determine whether the arcuate cells activated by GHRP-6 are also somatostatin-sensitive. Male Wistar rats (100-150 g body weight (BW)) were anaesthetized (urethane; 1.2 g/kg BW) and the brains removed. Coronal sections (400 microm thickness) were cut through a block of hypothalamus and were transferred to a slice chamber perfused with artificial cerebrospinal fluid. Forty-one arcuate nucleus cells were tested with bath application of 15 microm GHRP-6 for 10 min, 16 of which were tested subsequently (>30 min later) with application of 10 microM somatostatin. Following GHRP-6 administration, 19 cells (46. 3%) showed a significant increase in firing rate during the 15-min period after GHRP-6 application (P<0.001), 17 cells (41.5%) did not respond and the remaining five cells (12.2%) were significantly inhibited. Six of the eight arcuate nucleus cells that were excited by GHRP-6 were significantly inhibited by somatostatin. By contrast, five of the six arcuate nucleus cells that were unresponsive to GHRP-6 were also unresponsive to somatostatin. In the ventromedial nucleus, of 19 cells tested, eight cells (42.1%) were excited by GHRP-6, eight cells (42.1%) were unresponsive and the remaining three cells (15.8%) were significantly inhibited. Of 19 cells recorded in the periventricular nucleus, 13 (68.4%) were unresponsive to GHRP-6 and six (31.6%) were significantly inhibited. Thus, electrophysiological studies in vitro suggest that: (1) neurones in the hypothalamic arcuate nucleus, ventromedial nucleus and periventricular nucleus show changes in electrical activity in response to GHRP-6; and (2) the arcuate nucleus cells excited by GHRP-6 are also subject to inhibitory control by somatostatin.

Single cell reverse transcription-polymerase chain reaction analysis of rat supraoptic magnocellular neurons: neuropeptide phenotypes and high voltage-gated calcium channel subtypes

Magnocellular neurosecretory cells (MNCs) in the hypothalamo-neurohypophysial system that express and secrete the nonapeptides oxytocin (OT) and vasopressin (VP) were evaluated for the expression of multiple genes in single magnocellular neurons from the rat supraoptic nucleus using a single cell RT-PCR protocol. We found that all cells representing the two major phenotypes, the OT and VP MNCs, express a small, but significant, amount of the other nonapeptide's messenger RNA (mRNA). In situ hybridization histochemical analyses confirmed this observation. A third phenotype, containing equivalent amounts of OT and VP mRNA, was detected in about 19% of the MNCs from lactating female supraoptic nuclei. Analyses of these phenotypes for other coexisting peptide mRNAs (e.g. CRH, cholecystokinin, galanin, dynorphin, and the calcium-binding protein, calbindin) generally confirmed expectations from the literature, but revealed cell to cell variation in their coexpression. Our results also show that the high voltage-activated calcium channel subunit genes, alpha1A-D, alpha2, and beta1-4 are expressed in virtually all MNCs. However, the alpha1E subunit gene is not expressed at detectable levels in these cells. The expression of all of the beta-subunit genes in each MNC may account for the variations in physiological and pharmacological properties of the high voltage-activated channels found in these neurons. (Endocrinology 140: 5391-5401, 1999)

Identification and characterization of estrogen receptor alpha-containing neurons projecting to the vicinity of the gonadotropin-releasing hormone perikarya in the rostral preoptic area of the rat

Gonadal steroids exert a powerful regulatory influence upon the functioning of gonadotropin-releasing hormone (GnRH) neurons despite the apparent absence of gonadal steroid receptors in these cells. By using retrograde-tracing techniques combined with dual-labeling immunocytochemistry, we show here that distinct populations of estrogen receptor alpha (ERalpha)-containing neurons located in the hypothalamus and caudal brainstem project to the vicinity of the GnRH perikarya located in the rostral preoptic area (rPOA). The strongest estrogen-receptive afferent projection to this area originated from neurons located in the anteroventral periventricular and medial preoptic nuclei of the preoptic area. Approximately 50% of arcuate nucleus neurons projecting to the rPOA were demonstrated to synthesize either neuropeptide Y or beta-endorphin, but little evidence was found for ERalpha immunoreactivity in either of these specific subpopulations. Over 80% of all tyrosine hydroxylase-expressing neurons in the arcuate nucleus expressed ERalpha, but none projected to the rPOA. In the caudal brainstem, the A1 and A2 norepinephrine neurons comprised nearly all of the retrogradely labeled neurons. However, only the A2 afferents expressed ERalpha immunoreactivity, whereas the A1 afferents coexpressed neuropeptide Y. These observations, combined with the anterograde labeling data of others, provide neuroanatomical evidence for the existence of specific estrogen-receptive neuronal cell populations that project to the rPOA and may be involved in the estrogen-dependent transsynaptic regulation of GnRH neurons in the rat.

Neuroendocrine control of growth hormone secretion

The secretion of growth hormone (GH) is regulated through a complex neuroendocrine control system, especially by the functional interplay of two hypothalamic hypophysiotropic hormones, GH-releasing hormone (GHRH) and somatostatin (SS), exerting stimulatory and inhibitory influences, respectively, on the somatotrope. The two hypothalamic neurohormones are subject to modulation by a host of neurotransmitters, especially the noradrenergic and cholinergic ones and other hypothalamic neuropeptides, and are the final mediators of metabolic, endocrine, neural, and immune influences for the secretion of GH. Since the identification of the GHRH peptide, recombinant DNA procedures have been used to characterize the corresponding cDNA and to clone GHRH receptor isoforms in rodent and human pituitaries. Parallel to research into the effects of SS and its analogs on endocrine and exocrine secretions, investigations into their mechanism of action have led to the discovery of five separate SS receptor genes encoding a family of G protein-coupled SS receptors, which are widely expressed in the pituitary, brain, and the periphery, and to the synthesis of analogs with subtype specificity. Better understanding of the function of GHRH, SS, and their receptors and, hence, of neural regulation of GH secretion in health and disease has been achieved with the discovery of a new class of fairly specific, orally active, small peptides and their congeners, the GH-releasing peptides, acting on specific, ubiquitous seven-transmembrane domain receptors, whose natural ligands are not yet known.

Cell-specific gene expression in oxytocin and vasopressin magnocellular neurons

The oxytocin (OT) and vasopressin (VP) expressing magnocellular neurons in the hypothalamic-neurohypophysial system (HNS) have been the most studied of all the neuroendocrine cell-types. Despite this, our understanding of the mechanisms that underly the cell-specific expression of the peptide genes in these neurons has remained obscure. Part of the reason for this may be related to the close apposition of the OT and VP genes in the chromosomal locus, the genes being separated by as little as 3.5 kb in the mouse, and their interactions which are critical for cell-specific expression of the genes. Recent studies using intact rat OT and VP constructs in transgenic mice, and rat and mouse VP genes with CAT inserts in exon III as reporters in transgenic rats and mice, respectively, have suggested the presence of cell-specific enhancer elements in the 3' downstream (intergenic region, IGR) region of the VP gene. Evidence in favor of this view is presented from transgenic mouse studies on the expression of mouse OT- and VP-CAT gene constructs. Oxytocin and vasopressin phenotypes in the magnocellular neuronal population have traditionally been assessed by either immunocytochemical or in situ hybridization histochemical methods leading to the view that these genes are never coexpressed. However, more sensitive methods show that most OT cells also express some VP mRNA, and most VP cells contain some OT mRNA. A third phenotype containing equivalent levels of both OT and VP mRNA can also be found under some conditions, thereby complicating our analysis of cell-specificity. A continuing problem hindering studies of the regulation of OT and VP gene expression in neurons, is the absence of an appropriate cell line to examine these issues. We have found that stationary slice-explant cultures allow for excellent preservation of highly differentiated magnocellular neurons in long-term culture, and that these cultures can be used for physiological and pharmacological studies and analysis of gene expression.

POU domain factors in neural development

Transcription factors serve critical roles in the progressive development of general body plan, organ commitment, and finally, specific cell types. Comparison of the biological roles of a series of individual members within a family permits some generalizations to be made regarding the developmental events that are likely to be regulated by a particular class of transcription factors. Here, we evidence that the developmental functions of the family of transcription factors characterized by the POU DNA binding motif exerts roles in mammalian development. The POU domain family of transcription factors was defined following the observation that the products of three mammalian genes, Pit-1, Oct-1, and Oct-2, and the protein encoded by the C. elegans gene unc-86, shared a region of homology, known as the POU domain. The POU domain is a bipartite DNA binding domain, consisting of two highly conserved regions, tethered by a variable linker. The approximately 75 amino acid N-terminal region was called the POU-specific domain and the C-terminal 60 amino acid region, the POU-homeodomain. High-affinity site-specific DNA binding by POU domain transcription factors requires both the POU-specific and the POU-homeodomain. Resolution of the crystal structures of Oct-1 and Pit-1 POU domains bound to DNA as a monomer and homodimer, respectively, confirmed several of the in vitro findings regarding interactions of this bipartite DNA binding domain with DNA and has provided important information regarding the flexibility and versatility of POU domain proteins. Overall the crystal structure of a monomer of the Oct-1 POU domain bound to the octamer element was similar to that predicted by the NMR solution structures of the POU-specific domain and the POU-homeodomain in isolation, with the POU-specific domain consists of four alpha helices, with the second and third helices forming a structure similar to the helix-turn-helix motif of the lambda and 434 repressors; several of the DNA base contacts are also conserved. A homodimer of the Pit-1 POU domain was crystallized bound to a Pit-1 dimer DNA element that is closely related to a site in the proximal promoter of the prolactin gene. The structure of the Pit-1 POU domain on DNA is very similar to that of Oct-1, and the Pit-1 POU-homeodomain/DNA structure is strikingly similar to that of other homeodomains, including the Oct-1 POU-homeodomain. The DNA contacts made by the Pit-1 POU-specific domain are also similar to those of Oct-1 and conserved with many made by the prokaryotic repressors. In the Oct-1 crystal, the POU-specific domain recognizes a GCAT half-site, while the corresponding sequence recognized by the Pit-1 POU-specific domain, GTAT, is on the opposing strand. As a result, the orientation of the Pit-1 POU-specific domain relative to the POU-homeodomain is flipped, as compared to the Oct-1 crystal structure, indicating the remarkable flexibility of the POU-specific domain in adapting to variations in sequence within the site. Also in contrast to the Oct-1 monomer structure is the observation that the POU-specific and POU-homeodomain of each Pit-1 molecule make major groove contacts on the same face of the DNA, consistent with the constraints imposed by its 15 amino acid linker. As a result, the Pit-1 POU domain homodimer essentially surrounds its DNA binding site. In the Pit-1 POU domain homodimer the dimerization interface is formed between the C-terminal end of helix 3 of the POU-homeodomain of one Pit-1 molecule and the N-terminus of helix 1 and the loop between helices 3 and 4 of the POU-specific domain of the other Pit-1 molecule. In contrast to other homeodomain crystal structures, the C-terminus of helix 3 in the Pit-1 POU-homeo-domain has an extended structure. (ABSTRACT TRUNCATED)

Leptin receptor immunoreactivity in chemically defined target neurons of the hypothalamus

The adipose tissue-derived hormone leptin regulates body weight homeostasis by decreasing food intake and increasing energy expenditure. The weight-reducing action of leptin is thought to be mediated primarily by signal transduction through the leptin receptor (LR) in the hypothalamus. We have used immunohistochemistry to localize LR-immunoreactive (LR-IR) cells in the rat brain using an antiserum against a portion of the intracellular domain of LR that is common to all LR isoforms. The antiserum recognized the short and long isoforms of LR in transfected hematopoietic BaF3 cells. To examine the chemical nature of target cells for leptin, direct double-labeling immunofluorescence histochemistry was applied. The results show extensive distribution of LR-like immunoreactivity (LR-LI) in the brain with positively stained cells present, e.g., in the choroid plexus, cerebral cortex, hippocampus, thalamus, and hypothalamus. In the hypothalamus, strongly LR-IR neurons were present in the supraoptic nucleus (SON) and paraventricular nucleus (PVN), periventricular nucleus, arcuate nucleus, and lateral hypothalamus. Weaker LR-IR neurons were also demonstrated in the lateral and medial preoptic nuclei, suprachiasmatic nucleus, ventromedial and dorsomedial nuclei, and tuberomammillary nucleus. Confocal laser scanning microscopy showed LR-LI in the periphery of individual cells. In magnocellular neurons of the SON and PVN, LR-LI was demonstrated in vasopressin- and oxytocin-containing neurons. In parvocellular neurons of the PVN, LR-LI was demonstrated in many corticotropin-releasing hormone-containing neurons. LR-IR neurons were mainly seen in the ventromedial aspect of the arcuate nucleus, where LR-LI co-localized with neuropeptide Y. In the ventrolateral part of the arcuate nucleus, LR-LI was present in many large adrenocorticotropic hormone-IR proopiomelanocortin-containing neurons and in a few galanin-, neurotensin-, and growth hormone-releasing hormone-containing neurons. In the dorsomedial arcuate nucleus, few tyrosine hydroxylase (dopamine)-containing neurons were seen to have LR-LI. Melanin-concentrating hormone-containing neurons in the lateral hypothalamus had LR-LI. Based on the immunohistochemical results, possible interactions of leptin with brain mechanisms are discussed.

Corticotropin-releasing hormone (CRH)-containing neurons in the immature rat hippocampal formation: light and electron microscopic features and colocalization with glutamate decarboxylase and parvalbumin

Corticotropin-releasing hormone (CRH) excites hippocampal neurons and induces death of selected CA3 pyramidal cells in immature rats. These actions of CRH require activation of specific receptors that are abundant in CA3 during early postnatal development. Given the dramatic effects of CRH on hippocampal neurons and the absence of CRH-containing afferents to this region, we hypothesized that a significant population of CRHergic neurons exists in developing rat hippocampus. This study defined and characterized hippocampal CRH-containing cells by using immunocytochemistry, ultrastructural examination, and colocalization with gamma-aminobutyric acid (GABA)-synthesizing enzyme and calcium-binding proteins. Numerous, large CRH-immunoreactive (ir) neurons were demonstrated in CA3 strata pyramidale and oriens, fewer were observed in the corresponding layers of CA1, and smaller CRH-ir cells were found in stratum lacunosum-moleculare of Ammon's horn. In the dentate gyrus, CRH-ir somata resided in the granule cell layer and hilus. Ultrastructurally, CRH-ir neurons had aspiny dendrites and were postsynaptic to both asymmetric and symmetric synapses. CRH-ir axon terminals formed axosomatic and axodendritic symmetric synapses with pyramidal and granule cells. Other CRH-ir terminals synapsed on axon initial segments of principal neurons. Most CRH-ir neurons were coimmunolabeled for glutamate decarboxylase (GAD)-65 and GAD-67 and the majority also contained parvalbumin, but none were labeled for calbindin. These results confirm the identity of hippocampal CRH-ir cells as GABAergic interneurons. Further, a subpopulation of neurons immunoreactive for both CRH and parvalbumin and located within and adjacent to the principal cell layers consists of basket and chandelier cells. Thus, axon terminals of CRH-ir interneurons are strategically positioned to influence the excitability of the principal hippocampal neurons via release of both CRH and GABA.

Physiological regulation of hypothalamic neuropeptide Y release in lean and obese rats

The paraventricular nucleus (PVN) of the hypothalamus is an important site for the regulation of feeding behavior. Neuropeptide Y (NPY) injected into this nucleus strongly stimulates food intake. In the current study we measured NPY release in the PVN of unrestrained rats through the push-pull technique. The rats were placed in their habitual environment and conditions of life. NPY release was augmented by > 40% (P < 0.01) in Long-Evans rats deprived of food for 12 h. It returned to the baseline as measured in ad libitum-fed rats 90 min after food access. Its stimulation by 55 mM KCl in refed animals indicated that the whole stock of NPY was not used during a short fast. During the light-dark transition, when feeding behavior is initiated. NPY release in lean Zucker rats showed a peak 20 min after lights off and then declined. It corresponded well with the first feeding episodes. In the obese Zucker rats, this peak was absent. NPY release was totally anarchic but at a high level. The feeding behavior of the obese rats was not as time delimited as in the lean rats. This study performed in very physiological conditions therefore indicates that NPY release could drive feeding behavior in the normal life. Its dysregulation in obese rats could participate in overeating and absence of feeding rhythm measured in these rats and speed up the development of their obesity.

Molecular cloning of cyclic nucleotide-gated cation channel subunits from rat pineal gland

We have investigated the subunit composition and the molecular structure of the CNG channel expressed in rat pineal gland. Three types of subunits have been cloned: an alpha-subunit (CNG1), two beta-subunit splice variants (rCNG4.1 and rCNG4.2) and a hydrophilic glutamic acid-rich protein (rGARP). In situ hybridization with sections of rat brain revealed the co-expression of CNG1, CNG4 and GARP in pinealocytes. In addition, CNG4-specific transcripts were detected in the arcuate, periventricular and supraoptic nuclei of the hypothalamus.

Induction of c-fos messenger ribonucleic acid in neuropeptide Y and growth hormone (GH)-releasing factor neurons in the rat arcuate nucleus following systemic injection of the GH secretagogue, GH-releasing peptide-6

In this study we investigated the neurochemical identity of the arcuate cells activated following GH-releasing peptide-6 (GHRP-6) injection by comparing, on consecutive sections, the distribution c-fos messenger RNA (mRNA) with that of mRNAs for peptides synthesized in arcuate cells, including neuropeptide Y (NPY), GH-releasing factor (GRF), tyrosine hydroxylase, POMC, and somatostatin. Rats bearing chronically implanted jugular catheters were injected with either 50 micrograms GHRP-6 or vehicle. Thirty minutes later they were terminally anesthetized and perfused with fixative. Paraffin-embedded sections of 7 microns thickness were processed using in situ hybridization for either c-fos mRNA or mRNAs for the neurochemical markers. In GHRP-6-treated rats the mean (+/-SEM) number of cells expressing c-fos mRNA in the arcuate nucleus (23 +/- 2 cells/section per rat; n = 5) was significantly higher than for vehicle-treated controls (2 +/- 1 cells/section per rat; n = 5; P < 0.001, Mann-Whitney U test). Superimposed camera lucida maps indicated that, in GHRP-6-injected rats, neurochemically identifiable cells expressing c-fos mRNA also express NPY mRNA (51 +/- 4%), GRF mRNA (23 +/- 1%) tyrosine hydroxylase mRNA (11 +/- 3%), POMC mRNA (11 +/- 2%), or somatostatin mRNA (4 +/- 1%). Thus, the majority of cells expressing c-fos mRNA following GHRP-6 injection are NPY and GRF-containing cells.

Effect of oxytocin on growth hormone release in vitro

The effect of oxytocin (OT) on growth hormone (GH) secretion was investigated using dispersed rat anterior pituitary cells. OT dose-dependently inhibited GH secretion as well as GHRH-stimulated GH release. The inhibitory actions of OT on GH release were totally abolished by pretreatment with the OT-antagonist VAP 259. The peptides galanin and cholecystokinin did not affect the OT-induced inhibition on basal or GHRH-stimulated GH release. Several possible mechanisms by which OT may influence GH release are discussed.

Chronic intermittent salt loading enhances functional recovery from polydipsia and survival of vasopressinergic cells in the hypothalamic supraoptic nucleus following transection of the hypophysial stalk

Hypophysial stalk-transected (ST) and sham-operated animals were subjected to a chronic intermittent salt loading regimen (CISL) for 14 days beginning 1 day post surgery (dps). Animals were sacrificed at 15 and 36 dps. Three days after the termination of CISL, water consumption in ST + CISL animals decreased to the same level as that of sham-operated animals, while that of ST + water animals was maintained at a significantly higher level. The number of the surviving vasopressinergic neurons in the supraoptic nuclei of the ST + CISL group was significantly higher than that of ST + water group. CISL induced vasopressinergic axonal sprouting into the external zone of the median eminence, and formation of subependymal perivascular plexus. While CISL also enhanced regeneration of oxytocinergic axons into the external zone, it did not, however, have any effect on the number of oxytocinergic neurons surviving axotomy.

Sexually dimorphic expression of calcitonin gene-related peptide (CGRP) immunoreactivity by rat mediobasal hypothalamic neurons

Although the hypothalamic arcuate nucleus is a sexually dimorphic region of the rat brain, there are no reports of sex differences in the number of neurons containing specific neuropeptides within this structure. As cells synthesizing calcitonin gene-related peptide (CGRP) have been shown to exhibit sex differences in other steroid-receptive regions of the rat brain, we examined whether the CGRP-immunoreactive cells located in the mediobasal hypothalamus may also be sexually dimorphic. Immunostaining of sections from male and female colchicine-treated rats revealed a small population of CGRP-immunoreactive cells distributed throughout the arcuate nucleus. Immunoreactive cells were also detected in the lateral hypothalamic perifornical region, dorsomedial, posterior periventricular and ventral tuberomammillary nuclei, and zona incerta. Cell count analysis revealed approximately twice as many CGRP-immunoreactive cell profiles in the rostral (P < 0.01), middle (P < 0.001), and caudal (P < 0.01) thirds of the arcuate nucleus of male rats compared with females. A significant sex difference in immunoreactive cell numbers (male > female) was also detected within the caudal dorsomedial nucleus (P < 0.05) but not in the posterior periventricular nucleus, perifornical region and zona incerta. Although fibers immunoreactive for CGRP were identified in low density throughout the mediobasal hypothalamus, only female rats displayed prominent fiber staining in the periventricular region. Double-labelling immunofluorescence experiments revealed that the CGRP-immunoreactive cells within the zona incerta, but not the hypothalamus, were also immunoreactive for tyrosine hydroxylase; at least 60% of the A13 dopaminergic neurons co-express CGRP. These results provide evidence that sex differences exist in the number of specific neuropeptide-synthesizing cells within the hypothalamic arcuate nucleus and provide further examples of cell populations expressing CGRP immunoreactivity in a sexually dimorphic manner.

Expression of inducible cAMP early repressor (ICER) in hypothalamic magnocellular neurons

Cyclic AMP-responsive genes are regulated both positively and negatively by a number of constitutively expressed nuclear proteins. These proteins bind to cAMP-responsive DNA elements in their target genes and they are activated by protein kinase A-mediated phosphorylation. The cAMP response element modulator gene encodes for several constitutively expressed products. However, a second intronic promoter within the gene is inducible and produces another negatively acting transcription factor, inducible cAMP early repressor (ICER). ICER shows a diurnal pattern of expression in the pineal gland, but to date it has not been noted elsewhere in the brain. Here we show expression of ICER mRNA in hypothalamic magnocellular neurons following osmotic stimulation over a time course consistent with a modulatory effect on the expression of other immediate-early genes, such as c-fos. However, since ICER was not present in magnocellular neurons during parturition, its presence is not a prerequisite for the transient expression of c-fos.

Development and survival of the endocrine hypothalamus and posterior pituitary gland requires the neuronal POU domain factor Brn-2

Neurons comprising the endocrine hypothalamus are disposed in several nuclei that develop in tandem with their ultimate target the pituitary gland, and arise from a primordium in which three related class III POU domain factors, Brn-2, Brn-4, and Brn-1, are initially coexpressed. Subsequently, these factors exhibit stratified patterns of ontogenic expression, correlating with the appearance of distinct neuropeptides that define three major endocrine hypothalamic cell types. Strikingly, deletion of the Brn-2 genomic locus results in loss of endocrine hypothalamic nuclei and the posterior pituitary gland. Lack of Brn-2 does not affect initial hypothalamic developmental events, but instead results in a failure of differentiation to mature neurosecretory neurons of the paraventricular and supraoptic nuclei, characterized by an inability to activate genes encoding regulatory neuropeptides or to make correct axonal projections, with subsequent loss of these neurons. Thus, both neuronal and endocrine components of the hypothalamic-pituitary axis are critically dependent on the action of specific POU domain factors at a penultimate step in the sequential events that underlie the appearance of mature cellular phenotypes.