Increased expression of the N-methyl-D-aspartate receptor subunit, NR1, in immunohistochemically identified magnocellular hypothalamic neurons during dehydration

AMPA Receptor Function in Hypothalamic Synapses

AMPA receptors (AMPARs) are critical for mediating glutamatergic synaptic transmission and plasticity, thus playing a major role in the molecular machinery underlying cellular substrates of memory and learning. Their expression pattern, transport and regulatory mechanisms have been extensively studied in the hippocampus, but their functional properties in other brain regions remain poorly understood. Interestingly, electrophysiological and molecular evidence has confirmed a prominent role of AMPARs in the regulation of hypothalamic function. This review summarizes the existing evidence on AMPAR-mediated transmission in the hypothalamus, where they are believed to orchestrate the role of glutamatergic transmission in autonomous, neuroendocrine function, body homeostasis, and social behavior.

High Salt Intake Recruits Tonic Activation of NR2D Subunit-Containing Extrasynaptic NMDARs in Vasopressin Neurons

In addition to producing a classical excitatory postsynaptic current via activation of synaptic NMDA receptors (NMDARs), glutamate in the brain also induces a tonic NMDAR current (I _NMDA) via activation of extrasynaptic NMDARs (eNMDARs). However, since Mg²⁺ blocks NMDARs in nondepolarized neurons, the potential contribution of eNMDARs to the overall neuronal excitatory/inhibitory (E/I) balance remains unknown. Here, we demonstrate that chronic (7 d) salt loading (SL) recruited NR2D subunit-containing NMDARs to generate an Mg²⁺-resistant tonic I _NMDA in nondepolarized [V _h (holding potential) -70 mV] vasopressin (VP; but not oxytocin) supraoptic nucleus (SON) neurons in male rodents. Conversely, in euhydrated (EU) and 3 d SL mice, Mg²⁺-resistant tonic I _NMDA was not observed. Pharmacological and genetic intervention of NR2D subunits blocked the Mg²⁺-resistant tonic I _NMDA in VP neurons under SL conditions, while an NR2B antagonist unveiled Mg²⁺-sensitive tonic I _NMDA but not Mg²⁺-resistant tonic I _NMDA In the EU group VP neurons, an Mg²⁺-resistant tonic I _NMDA was not generated by increased ambient glutamate or treatment with coagonists (e.g., d-serine and glycine). Chronic SL significantly increased NR2D expression but not NR2B expression in the SON relative to the EU group or after 3 d under SL conditions. Finally, Mg²⁺-resistant tonic I _NMDA selectively upregulated neuronal excitability in VP neurons under SL conditions, independent of ionotropic GABAergic input. Our results indicate that the activation of NR2D-containing NMDARs constitutes a novel mechanism that generates an Mg²⁺-resistant tonic I _NMDA in nondepolarized VP neurons, thus causing an E/I balance shift in VP neurons to compensate for the hormonal demands imposed by a chronic osmotic challenge.SIGNIFICANCE STATEMENT The hypothalamic supraoptic nucleus (SON) consists of two different types of magnocellular neurosecretory cells (MNCs) that synthesize and release the following two peptide hormones: vasopressin (VP), which is necessary for regulation of fluid homeostasis; and oxytocin (OT), which plays a major role in lactation and parturition. NMDA receptors (NMDARs) play important roles in shaping neuronal firing patterns and hormone release from the SON MNCs in response to various physiological challenges. Our results show that prolonged (7 d) salt loading generated a Mg²⁺-resistant tonic NMDA current mediated by NR2D subunit-containing receptors, which efficiently activated nondepolarized VP (but not OT) neurons. Our findings support the hypothesis that NR2D subunit-containing NMDARs play an important adaptive role in adult brain in response to a sustained osmotic challenge.

P2X2 Receptor Expression and Function Is Upregulated in the Rat Supraoptic Nucleus Stimulated Through Refeeding After Fasting

Magnocellular neurons in the supraoptic nucleus (SON), which synthesize and release arginine vasopressin (AVP) and oxytocin (OT), express several subtypes of ATP-stimulated purinergic P2X receptors (P2XR) that modulate neuronal activity as well as neurotransmitter and hormone release. However, the physiological impact of this modulation is not well understood. Here, we tested a hypothesis that P2XRs play a role in the sustained release of hormones from SON neurons stimulated through fasting/refeeding. We studied the effect of 2 h of refeeding after 48 h of fasting on P2XR and P2YR mRNA expression and ATP-induced presynaptic and postsynaptic responses in the SON of 30-day-old rats. Quantitative real-time PCR revealed that the expression of P2X2R and AVP mRNA was upregulated, whereas P2X4R, P2X7R, P2Y2R, and OT mRNA levels were not significantly changed and P2Y1R mRNA expression was decreased. Whole-cell patch clamp recordings performed on isolated rat brain slices showed that the amplitude of the ATP-stimulated somatic current and the ATP-induced increases in the frequency of spontaneous GABAergic inhibitory postsynaptic currents were significantly higher in SON neurons from fasted/refed rats than in SON neurons from normally fed rats. No evidence was found for changes in the presynaptic effect of ATP in SON neurons not expressing somatic P2XRs. These results suggest that the increased activity of SON neurons synthesizing AVP is associated with enhanced expression of P2X2Rs on neuronal cell bodies and their GABAergic presynaptic nerve terminals.

Role of AMPA and NMDA receptors on vasopressin and oxytocin secretion induced by hypertonic extracellular volume expansion

Vasopressin (AVP) and oxytocin (OT) are essential for the control of extracellular fluid osmolality and volume. Secretion of these hormones is modulated by several mechanisms, including NMDA and AMPA L-glutamate receptors in magnocellular cells of paraventricular (PVN) and supraoptic (SON) hypothalamic nuclei. Thus, to better understand the participation of L-glutamate on the neuroendocrine control of AVP and OT, this work evaluated the effects of intracerebroventricular (icv) NMDA and AMPA receptor antagonists on plasma AVP and OT levels induced by extracellular volume expansion (EVE). Cannulated rats received icv NMDA (AP5) and AMPA (NBQX) antagonists in 10 and 30nmol/5μl/rat doses and were subjected to either isotonic (0.15 M NaCl, 2ml/100g) or hypertonic (0.30 M NaCl, 2ml/100g) EVE. Blood samples were collected for plasma AVP and OT determination. Isotonic EVE did not change plasma AVP and OT levels, but hypertonic EVE increased both AVP and OT plasma levels. AP5 reduced plasma AVP, but it did not change the OT level induced by hypertonic EVE. On the other hand, NBQX reduced plasma OT, but did not alter the AVP plasma level. Our data shows that L-glutamate controls the secretion of neurohypophyseal hormones through the NMDA receptor for AVP release, and through the AMPA receptor for OT release, both in response to hypertonic EVE. This article is protected by copyright. All rights reserved.

Brain-derived neurotrophic factor-tyrosine kinase B pathway mediates NMDA receptor NR2B subunit phosphorylation in the supraoptic nuclei following progressive dehydration

We studied the effects of water deprivation (WD) on the phosphorylation of tyrosine kinase B (TrkB) and NMDA receptor subunits in the supraoptic nucleus (SON) of the rat. Laser capture microdissection and quantitative reverse transcriptase polymerase chain reaction was used to demonstrate brain-derived neurotrophic factor (BDNF) and TrkB gene expression in vasopressin SON neurones. Immunohistochemistry confirmed BDNF staining in vasopressin neurones, whereas staining for phosphorylated TrkB was increased following WD. Western blot analysis of brain punches containing the SON revealed that tyrosine phosphorylation of TrkB (pTrkBY(515)), serine phosphorylation of NR1 (pNR1S(866) or pNR1) and tyrosine phosphorylation of NR2B subunits (pNR2BY(1472) or pNR2B) were significantly increased in WD animals compared to controls. Access to water for 2 h reduced pTrkBY(515) content to control levels without affecting pNR1 or pNR2B. Four hours of rehydration was needed to reduce pNR1 and pNR2B to control levels. To test whether increased phosphorylation of TrkB in the present study is mediated by BDNF, a group of animals were instrumented with right SON cannula coupled to mini-osmotic pumps filled with vehicle or TrkB-Fc fusion protein, which prevents BDNF binding to TrkB. In the left SON contralateral to the cannula, TrkB phosphorylation was significantly enhanced following WD. Separate analysis of the right SON, which received TrkB-Fc, showed that the TrkB receptor phosphorylation following WD was significantly attenuated. Although increased pNR1S(866) following WD was not affected by local infusion of TrkB-Fc, pNR2BY(1472) was significantly reduced. Co-immunoprecipitation revealed an increased physical interaction between Fyn kinase and NR2B and TrkB in the SON following WD. Thus, activation of TrkB in the SON following WD may affect cellular excitability through the phosphorylation of NR2B subunits.

NMDA receptor subunit expression in the supraoptic nucleus of adult rats: dominance of NR2B and NR2D

The supraoptic nucleus (SON) of the hypothalamus contains magnocellular neurosecretory neurons (MNC) which synthesize and release the peptide hormones vasopressin and oxytocin. Glutamate is a prominent excitatory neurotransmitter in the SON and regulates MNC excitability. NMDA receptors (NMDAR), a type of ionotropic glutamate receptor, mediate synaptic plasticity of MNCs and are necessary for characteristic burst firing patterns which serve to maximize hormone release. NMDARs are di- or tri-heteromeric complexes of NR1 and NR2 subunits. Receptor properties depend on NR2 subunit composition and variable splicing of NR1. We investigated the expression profile of NR1 and NR2 subunits in the SON at the mRNA and protein levels plus protein expression of NR1 splice variants in control and salt-loaded adult rats. There was robust mRNA expression of all subunits, with NR2D levels being the highest. At the protein level, NR1, NR2B, and NR2D were robustly expressed, while NR2A was weakly expressed. NR2C protein was not detected with either of the two antibodies tested. All four NR1 splice variant cassettes (N1, C1, C2, C2') were detected in the SON, although NR1 N1 expression was too low for accurate analysis. Three days of salt-loading did not alter mRNA, protein, or splice variant expression of NMDAR subunits in the SON. Robust NR2D protein expression has not been previously shown in MNCs and is uncommon in the adult brain. Although the functional significance of this unusual expression profile is unknown, it may contribute to important physiological characteristics of SON neurons, such as burst firing and resistance to excitotoxicity.

Cold-induced glutamate release in vivo from the magnocellular region of the paraventricular nucleus is involved in ovarian sympathetic activation

We previously reported that centrally-induced sympathetic activation in response to cold stress is associated with a polycystic ovarian condition in rats, and thyrotrophin-releasing hormone (TRH) released locally from the magnocellular region of the paraventricular nucleus (PVN) appears to be involved in this activation. Because TRH neurones express NMDA glutamate receptors, in the present study, we investigated the role of glutamate in the increased release of TRH from magnocellular neurones induced by cold stress and its relationship to ovarian neurotransmission. Animals with a push-pull cannula stereotaxically implanted into the magnocellular portion of the PVN were exposed to cold stress (4 degrees C for 64 h) and subjected to intracerebral perfusion. Perfusate fractions were obtained and analysed by high-performance liquid chromatography to measure glutamate and GABA levels. Glutamate, but not GABA, release increased significantly in animals perfused under cold exposure. In vivo administration of glutamate to the PVN increased TRH release. Injection of MK-801 into the magnocellular portion of the PVN reduced ovarian noradrenaline turnover and led to an increase in catecholamine concentration from the adrenal glands and celiac ganglia. Taken together, the results obtained in the present study strongly suggest that glutamate release from the magnocellular PVN is sensitive to cold stress and that glutamate acts through the NMDA receptor to mediate cold-induced TRH release. This in turn triggers hypothalamic-ovarian pathway activation, which might be responsible for the polycystic condition induced by cold stress and other ovarian pathologies characterised by increased sympathetic discharge.

Central mechanisms of osmosensation and systemic osmoregulation

Systemic osmoregulation is a vital process whereby changes in plasma osmolality, detected by osmoreceptors, modulate ingestive behaviour, sympathetic outflow and renal function to stabilize the tonicity and volume of the extracellular fluid. Furthermore, changes in the central processing of osmosensory signals are likely to affect the hydro-mineral balance and other related aspects of homeostasis, including thermoregulation and cardiovascular balance. Surprisingly little is known about how the brain orchestrates these responses. Here, recent advances in our understanding of the molecular, cellular and network mechanisms that mediate the central control of osmotic homeostasis in mammals are reviewed.

Influence of dehydration on the expression of neuropeptide Y Y1 receptors in hypothalamic magnocellular neurons

Regulation of vasopressin (VP) and oxytocin (OT) secretion involves integration of neural signals from hypothalamic osmoreceptors, ascending catecholaminergic and peptidergic cell groups in the brain stem, and local and autoregulatory afferents. Neuropeptide Y (NPY) is one factor that stimulates the release of VP and OT from the supraoptic (SON) and paraventricular nuclei of the hypothalamus via activation of Y1 receptors (Y1R). The current studies were designed to assess the regulation and distribution of NPY Y1R expression in the SON of male rats that were either given 2% NaCl drinking water (24-72 h) or water deprived (48 h). Subjecting male rats to these conditions resulted in significant increases in both the number of cells expressing Y1R immunoreactivity (ir) and the amount of Y1R protein per cell within the SON. Y1R immunoreactivity was increased in the magnocellular but not medial parvocellular paraventricular nuclei, and Y1R mRNA levels were increased in the SON of salt-loaded rats. Subpopulations of both VP and OT cells in the hypothalamus express Y1R immunoreactivity and a greater percentage of VP-ir cells express Y1R after salt loading. To control for potential effects of dehydration-induced anorexia, a group of euhydrate animals was pair fed with animals consuming 2% NaCl. No detectable change in Y1R expression was observed in the SON of pair-fed animals, even though body weights were significantly lower than controls. These data demonstrate that NPY Y1R gene and protein expression are increased in the SON of salt-loaded and water-deprived animals and provide a mechanism whereby NPY can support VP/OT release during prolonged challenges to fluid homeostasis.

Localization and osmotic regulation of vesicular glutamate transporter-2 in magnocellular neurons of the rat hypothalamus

In this report we present immunocytochemical and in situ hybridization evidence that magnocellular vasopressin and oxytocin neurons in the hypothalamic supraoptic and paraventricular nuclei express type-2 vesicular glutamate transporter, a marker for their glutamatergic neuronal phenotype. To address the issue of whether an increase in magnocellular neuron activity coincides with the altered synthesis of the endogenous glutamate marker, we have introduced a new dual-label in situ hybridization method which combines fluorescent and autoradiographic signal detection components for vasopressin and vesicular glutamate transporter-2 mRNAs, respectively. Application of this technique provided evidence that 2% sodium chloride in the drinking water for 7 days produced a robust and significant increase of vesicular glutamate transporter-2 mRNA in vasopressin neurons of the supraoptic nucleus. The immunocytochemical labeling of pituitary sections, followed by the densitometric analysis of vesicular glutamate transporter-2 immunoreactivity in the posterior pituitary, revealed a concomitant increase in vesicular glutamate transporter-2 protein levels at the major termination site of the magnocellular axons. These data demonstrate that magnocellular oxytocin as well as vasopressin cells contain the glutamatergic marker vesicular glutamate transporter-2, similarly to most of the parvicellular neurosecretory neurons examined so far. The robust increase in vesicular glutamate transporter-2 mRNA and immunoreactivity after salt loading suggests that the cellular levels of vesicular glutamate transporter-2 in vasopressin neurons are regulated by alterations in water-electrolyte balance. In addition to the known synaptic actions of excitatory amino acids in magnocellular nuclei, the new observations suggest novel mechanisms whereby glutamate of endogenous sources can regulate magnocellular neuronal functions.

Activity-dependent modulation of neurotransmitter innervation to vasopressin neurons of the supraoptic nucleus

Confocal microscopy was used to assess activity-dependent neuroplasticity in neurotransmitter innervation of vasopressin immunoreactive magnocellular neurons in the supraoptic nucleus (SON). Vesicular glutamate transporter 2, glutamic acid decarboxylase, and dopamine beta-hydroxylase (DBH) synaptic boutons were visualized in apposition to vasopressin neurons in the SON. A decrease in DBH synaptic boutons per cell was seen upon salt loading, indicating diminished noradrenergic/adrenergic innervation. Loss of DBH appositions to vasopressin neurons was associated with a general loss of DBH immunoreactivity in the SON. In contrast, the number of vesicular glutamate transporter 2 synaptic boutons per neuron increased with salt loading, consistent with increased glutamatergic drive of magnocellular SON neurons. Salt loading also caused an increase in the total number of glutamic acid decarboxylase synaptic boutons on vasopressinergic neurons, suggesting enhanced inhibitory innervation as well. These studies indicate that synaptic plasticity compensates for increased secretory demand and may indeed underlie increased secretion, perhaps via neurotransmitter-specific, activity-related changes in synaptic contacts on vasopressinergic magnocellular neurons in the SON.

Alteration of NMDA NR1 receptors within the paraventricular nucleus of hypothalamus in rats with heart failure

One of the pathophysiological characteristics of chronic heart failure (HF) is elevated sympathetic drive, which is a major factor contributing to the morbidity and mortality of HF. Resent evidence points to a central mechanism that contributes to the sympathetic abnormality in HF. The paraventricular nucleus (PVN) of the hypothalamus is an important site that integrates sympathetic nerve activity. Studies have shown that glutamate elicits excitatory effects on neurons in the PVN through the NMDA receptor. The goal of the present study was to examine the role of NMDA receptors in the altered sympathetic nerve activation during HF. The left coronary ligation-induced heart failure model in the rat was used. In alpha-chloralose and urethane anesthetized rats, microinjection of NMDA into the PVN (50 to 200 pmol) produced dose-dependent increases in renal sympathetic nerve discharge (RSND), arterial blood pressure (BP), and heart rate (HR). This response to NMDA was significantly potentiated (27+/-7%) in HF compared with sham rats. On the other hand, microinjection of the NMDA receptor antagonist AP-5 (4 to 16 nmol) into the PVN caused significant decreases in RSND, BP, and HR only in rats with HF but very slight changes in sham rats. Furthermore, using microdialysis and HPLC in combination with electrochemical detection techniques, we found that the glutamate level in the PVN was not increased significantly in HF compared with sham rats. However, using RT-PCR, Western blot, and immunofluorescence techniques, it was found that NMDA NR1 subunit mRNA expression and protein level in the PVN were significantly increased in HF compared with sham rats. These data suggest that the increased glutamatergic activity on sympathetic regulation, due to the upregulation of NMDA NR1 receptor subunits within the PVN may contribute to the elevated sympathoexcitation during HF.

Comparative distribution of neuropeptide Y Y1 and Y5 receptors in the rat brain by using immunohistochemistry

Neuropeptide Y (NPY) Y1 and Y5 receptor subtypes mediate many of NPY's diverse actions in the central nervous system. The present studies use polyclonal antibodies directed against the Y1 and Y5 receptors to map and compare the relative distribution of these NPY receptor subtypes within the rat brain. Antibody specificity was assessed by using Western analysis, preadsorption of the antibody with peptide, and preimmune serum controls. Immunostaining for the Y1 and Y5 receptor subtypes was present throughout the rostral-caudal aspect of the brain with many regions expressing both subtypes: cerebral cortex, hippocampus, hypothalamus, thalamus, amygdala, and brainstem. Further studies using double-label immunocytochemistry indicate that Y1R immunoreactivity (-ir) and Y5R-ir are colocalized in the cerebral cortex and caudate putamen. Y1 receptor ir was evident in the central amygdala, whereas both Y1- and Y5-immunoreactive cells and fibers were present in the basolateral amygdala. Corresponding with the physiology of NPY in the hypothalamus, both Y1R- and Y5R-ir was present within the paraventricular (PVN), supraoptic, arcuate nuclei, and lateral hypothalamus. In the PVN, Y5R-ir and Y1R-ir were detected in cells and fibers of the parvo- and magnocellular divisions. Intense immunostaining for these receptors was observed within the locus coeruleus, A1-5 and C1-3 nuclei, subnuclei of the trigeminal nerve and nucleus tractus solitarius. These data provide a detailed and comparative mapping of Y1 and Y5 receptor subtypes within cell bodies and nerve fibers in the brain which, together with physiological and electrophysiological studies, provide a better understanding of NPY neural circuitries.

Osmotic regulation of estrogen receptor-beta in rat vasopressin and oxytocin neurons

The vasopressin (VP) magnocellular neurosecretory cells (MNCs) in the supraoptic and paraventricular (PVN) nuclei are regulated by estrogen and exhibit robust expression of estrogen receptor (ER)-beta. In contrast, only approximately 7.5% of oxytocin (OT) MNCs express ER-beta. We examined the osmotic regulation of ER-beta mRNA expression in MNCs using quantitative in situ hybridization histochemistry. Hyper-osmolality induced via 2% hypertonic saline ingestion significantly decreased, whereas sustained hypo-osmolality induced via d-d-arginine VP and liquid diet increased ER-beta mRNA expression in MNCs (p < 0.05). Thus, the expression of ER-beta mRNA correlated inversely with changes in plasma osmolality. Because hyper-osmolality is a potent stimulus for VP and OT release, this suggests an inhibitory role for ER-beta in MNCs. Immunocytochemistry demonstrated that the decrease in ER-beta mRNA was translated into depletion of receptor protein content in hyper-osmotic animals. Numerous MNCs were positive for ER-beta in control animals, but they were virtually devoid of ER-beta-immunoreactivity (IR) in hyper-osmotic animals. The osmotically induced decrease in ER-beta expression was selective for MNCs because ER-beta-IR remained unaltered in PVN parvocellular neurons. Plasma estradiol and testosterone were not correlated with ER-beta mRNA expression after osmotic manipulation, suggesting that ER-beta expression was not driven by ligand availability. Expression of FOS-IR in MNCs with attenuated ER-beta-IR, and the absence of FOS-IR in parvocellular neurons that retain ER-beta-IR suggest a role for neuronal activation in the regulation of ER-beta expression in MNCs. Thus, osmotic modulation of ER-beta expression in MNCs may augment or attenuate an inhibitory effect of gonadal steroids on VP release.

The N-methyl-D-aspartate receptor subunit NR2B: localization, functional properties, regulation, and clinical implications

The N-methyl-D-aspartate (NMDA) receptor is an example of a heteromeric ligand-gated ion channel that interacts with multiple intracellular proteins by way of different subunits. NMDA receptors are composed of seven known subunits (NR1, NR2A-D, NR3A-B). The present review focuses on the NR2B subunit of the receptor. Over the last several years, an increasing number of reports have demonstrated the importance of the NR2B subunit in a variety of synaptic signaling events and protein-protein interactions. The NR2B subunit has been implicated in modulating functions such as learning, memory processing, pain perception, and feeding behaviors, as well as being involved in a number of human disorders. The following review provides a summary of recent findings regarding the structural features, localization, functional properties, and regulation of the NR2B subunit. The review concludes with a section discussing the role of NR2B in human diseases.

Functional synaptic plasticity in hypothalamic magnocellular neurons

The hypothalamic-neurohypophysial system undergoes dramatic morphological plasticity in response to physiological activation during parturition/lactation and dehydration, including somatic swelling, decreased glial coverage and increased synaptic innervation of the magnocellular neuroendocrine cells. Recent in-vitro electrophysiological studies in hypothalamic slices have demonstrated that coordinate changes in the synaptic physiology of the magnocellular neurons also occur under these conditions. Thus, the synaptic release of glutamate and GABA onto magnocellular neurons is increased during lactation and with chronic dehydration, and changes in postsynaptic glutamate and GABAA receptor expression lead to alterations of the functional properties of the glutamate and GABAA receptor channels. The presynaptic noradrenergic facilitation of glutamate release and inhibition of GABA release is also markedly enhanced following chronic dehydration. Additionally, both parturition and chronic dehydration are accompanied by an increase in the tonic activation of presynaptic metabotropic glutamate receptors due to the higher ambient glutamate concentration caused by decreased glial coverage and the resultant reduction in glutamate reuptake. Together, these electrophysiological studies reveal profound functional plasticity in the synaptic physiology of magnocellular neurons at parturition and following dehydration. The plastic changes support an increase in the excitability of magnocellular neuroendocrine cells by increasing glutamate inputs, decreasing GABA inputs, enhancing excitatory noradrenergic modulation, and reducing synaptic glutamatergic noise.

Contribution of N-methyl-d-aspartate receptors in the anteroventral third ventricular region to vasopressin secretion, but not to cardiovascular responses provoked by hyperosmolality and prostaglandin E2 in conscious rats

The aim of this study is to pursue roles of N-methyl-d-aspartate (NMDA) receptors in the anteroventral third ventricular region (AV3V; a pivotal area for autonomic functions) in controlling vasopressin (AVP) release and cardiovascular system. In conscious rats, we examined effects of AV3V infusion of MK-801 (a selective antagonist for NMDA receptor) on plasma AVP, osmolality, electrolytes, arterial pressure and heart rate, in the absence or presence of NMDA, hyperosmotic or prostaglandin (PG) E2 stimulus. The AV3V infusion of NMDA caused significant increases in plasma AVP, osmolality and sodium, hematocrit, arterial pressure and heart rate after 5 or 15min. When NMDA was administered into the cerebral ventricle, relatively smaller elevations were observed only in plasma AVP and arterial pressure. The effects of AV3V infusion of NMDA were nearly completely prevented by MK-801 applied to the same region before 15min. The application of MK-801 was also potent to block rises of plasma AVP elicited by AV3V injection of PGE2 or i.v. infusion of hypertonic saline. However, it inhibited neither increases of arterial pressure and heart rate due to the PGE2 treatment nor those of arterial pressure, plasma osmolality and sodium in response to the osmotic load. Histological analysis on the AV3V infusion sites of NMDA, MK-801 and PGE2 indicated that they had been located in the structures such as the median and medial preoptic nuclei, periventricular nucleus and medial preoptic area. These results suggest that stimulation of AV3V NMDA receptors in the basal state may facilitate AVP secretion and cause pressor and tachycardiac actions, and that these receptors may be involved in both the hyperosmolality- and PGE2-induced hormone release, but not in the cardiovascular responses to these stimuli.

Neuronal and glial response in the rat hypothalamus-neurohypophysis complex with streptozotocin-induced diabetes

This study was aimed to examine the neuronal and glial response in the hypothalamus and neurohypophysis of rats with streptozotocin-induced diabetes. At various time intervals after induction of diabetes the neurons in the paraventricular- (PVN) and supraoptic- (SON) nucleus showed upregulated arginine vasopressin (AVP) and oxytocin (OXT) immunoexpression, being most pronounced at 2 weeks. Concomitant to this was the hypertrophy of PVN and SON neurons. NMDAR1, which was constitutively and moderately expressed in normal rats, was markedly augmented, being most intense at 4 months. This coincided with the expression of neuronal nitric oxide synthase (nNOS). Contrary to this, the expression of GluR2/3 was progressively downregulated, so that it was hardly detected at 4 months. Both astrocytes and microglia marked by anti-GFAP and OX-42, respectively, appeared activated. In pars nervosa, the projection target of the axon terminals of PVN and SON neurons, massive axons and terminals (Herring bodies) laden with neurosecretions were observed in diabetic rats. Colocalization study showed that the neurosecretions were internalized by activated pituicytes and microglia associated with the axons. The present results suggest that the neurosecretion of PVN and SON neurons is enhanced in diabetes. This is coupled by upregulation of NMDAR1 and nNOS but downregulation of GluR2/3. It is speculated that the glutamate receptors and NO are linked to overactivation of PVN and SON neurons leading ultimately to cell death of some of them. The pituicytes and microglia in pars nervosa would help to modulate the release of neurosecretion.

Expression and plasticity of glutamate receptors in the supraoptic nucleus of the hypothalamus

Magnocellular neuroendocrine cells (MNCs) of the supraoptic nucleus of the hypothalamus (SON) produce and release the hormones vasopressin (VP) and oxytocin (OT) in response to a variety of stimuli to regulate body water and salt, parturition and lactation. Hormone release is influenced by the pattern of neuronal firing of these MNCs, which, in turn, is governed by intrinsic conductances and synaptic inputs, including those mediated by the neurotransmitter glutamate. Functional and molecular evidence has confirmed the expression of AMPA-, NMDA-, and metabotropic-type glutamate receptors in the SON, that together may orchestrate the effects of glutamatergic transmission on neuroendocrine function. However, the specific roles of the different subtypes of glutamate receptors is not yet clear. As with other central neurons, the subunit composition of glutamate receptors on MNCs will likely determine their properties and may potentially help define the differential properties of VP- and OT-producing MNCs. Possible functions of glutamate receptors on SON MNCs include altering excitatory synaptic transmission of osmotic information, neuronal firing, hormone production and release, and calcium signaling. Of interest are the anatomical, molecular, and functional changes at glutamatergic synapses in the SON that occur in response to pertinent physiological stimuli or development. These types of plasticity may include changes in glutamatergic synaptic density, glutamate receptor levels, or glutamate receptor subunit expression, all of which can affect the efficiency of synaptic transmission.

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.

Immunolabeling reveals cellular localization of the N-methyl-D-aspartate receptor subunit NR2B in neurosecretory cells but not astrocytes of the rat magnocellular nuclei

Previous studies suggest that activation of N-methyl-D-aspartate (NMDA) receptors facilitates phasic firing and spike clustering displayed by magnocellular neuroendocrine cells (MNCs) of the supraoptic (SON) and paraventricular nucleus of the hypothalamus (PVN). Osmotic stimulation produces similar activity patterns which, in turn, can lead to enhanced release of vasopressin and oxytocin from MNCs. Our laboratory has shown that dehydration regulates the expression of the NMDA receptor subunits, NR1 and NR2B, in the SON and PVN, suggesting their involvement in osmoregulation. In the present study, we examined the cellular localization of NR2B, one of the glutamate-binding subunits of the NMDA receptor, with an NR2B-specific antibody. Using double-label immunohistochemistry and three different detection methods with metallic, peroxidase, and fluorescence markers, it was found that both vasopressin and oxytocin-producing MNC populations synthesize NR2B. The incidence of NR2B colocalization with vasopressin-neurophysin in the SON and lateral magnocellular PVN (PVL) was 0.95 and 0.91, respectively. For oxytocin-neurophysin, the corresponding values were 0.97 and 0.95, respectively. Furthermore, the extent of colocalization in MNCs of the SON, PVL, retrochiasmatic SON, and accessory neurosecretory nuclei was similar. Astrocytes associated with the SON, and identified with antibodies targeting glial fibrillary acidic protein (GFAP) or vimentin, were not colabeled with NR2B. Our results demonstrate that NR2B protein is expressed by almost all MNCs and that it is equally prevalent in vasopressinergic and oxytocinergic populations of various magnocellular neuroendocrine nuclei supporting a role of NMDA receptors in MNC-mediated neurosecretory processes. Although NR2B may form part of functional NMDA receptors on MNCs, it is probably not present on astrocytes associated with nearby MNCs.

Osmotic activation of the hypothalamo-neurohypophysial system reversibly downregulates the NMDA receptor subunit, NR2B, in the supraoptic nucleus of the hypothalamus

NMDA receptor activation produces a characteristic pattern of neuronal firing in magnocellular neuroendocrine cells (MNCs) of the supraoptic nucleus of the hypothalamus (SON) which has been associated with greater hormone release in vivo and in vitro. In addition, i.c.v. administered NMDA receptor blockers suppress the dehydration-induced rise in plasma vasopressin and drinking. To investigate the role of NMDA receptor subunits in the neuroendocrine functions of the magnocellular neuroendocrine cells of the hypothalamus, we examined the effects of osmotic stimulation on the protein expression of the NMDA receptor subunits, NR1 and NR2B, important in binding glycine and glutamate, respectively. Homogenates of SON, paraventricular nucleus of the hypothalamus (PVN), cortex and lateral hypothalamus from control rats and rats given 2% saline water to drink for 4-10 days were subjected to SDS-PAGE and Western blot analysis. This saline water drinking regimen produced a significant rise in plasma osmolality levels. NR1 and NR2B immunoreactivity was detected in SON, PVN, lateral hypothalamus and cortex but not in liver homogenates using subunit-specific polyclonal antibodies and quantified using computer-assisted densitometry. Mean NR2B immunoreactivity was significantly lower in SON (29%) and PVN homogenates (23%) from saline-treated rats than in those from control rats. In addition, the effect of dehydration on NR2B was regionally specific since no significant changes in NR2B expression were observed in homogenates of cortex and lateral hypothalamus. Rehydration allowed recovery of plasma osmolality as well as NR2B protein levels in the SON. These results suggest that changes in NMDA receptor subunit expression contribute to the plasticity manifested by in magnocellular neuroendocrine cells in response to osmotic activation of the hypothalamo-neurohypophysial system. In addition, our results indicate that NMDA receptors on SON and PVN MNCs may contribute to neuroendocrinological functions associated with body fluid homeostasis.

Functional activation of punch-cultured magnocellular neuroendocrine cells by glutamate receptor subtypes

To provide a simple means to isolate and study the cellular functions of small groups of neurons, we developed a modified 'punch' culture procedure that facilitates acute and long-term in vitro physiological studies. Primary 'punch' cultures of magnocellular neuroendocrine cells (MNCs) from the supraoptic nucleus (SON) were established and the basic physiological effects of subtype-specific glutamate receptor agonists were characterized. MNCs from the punch cultures established a mature morphology in culture with extensive outgrowth of axons and varicosities. After 8 days, a single cultured SON punch produced an average of 10.0 +/- 2.1 pg AVP and contained an average of 222 +/- 53 vasopressin-neurophysin immunoreactive cells. Patch clamp recordings from MNCs demonstrated the presence of N-methyl-D-aspartate (NMDA)-sensitive and DL, alpha-amino-3-hydroxy-5-methylisoxazole propionic acid (AMPA)-receptors. Stimulation of metabotropic receptors with 1S,3R ACPD induced acute or gradual increases in intracellular calcium. NMDA, AMPA and metabotropic receptors all contributed to the secretion of vasopressin from the punch cultures with an agonist rank order potency of: NMDA (10 microM) > AMPA (1 microM) = 1S,3R ACPD (100 microM) > kainate (10 microM). This culture preparation should be useful for cellular studies of small groups of neuroendocrine and other cells.

Differences in the properties of ionotropic glutamate synaptic currents in oxytocin and vasopressin neuroendocrine neurons

Oxytocin (OT) and vasopressin (VP) hormone release from neurohypophysial terminals is controlled by the firing pattern of neurosecretory cells located in the hypothalamic supraoptic (SON) and paraventricular nuclei. Although glutamate is a key modulator of the electrical activity of both OT and VP neurons, a differential contribution of AMPA receptors (AMPARs) and NMDA receptors (NMDARs) has been proposed to mediate glutamatergic influences on these neurons. In the present study we examined the distribution and functional properties of synaptic currents mediated by AMPARs and NMDARs in immunoidentified SON neurons. Our results suggest that the properties of AMPA-mediated currents in SON neurons are controlled in a cell type-specific manner. OT neurons displayed AMPA-mediated miniature EPSCs (mEPSCs) with larger amplitude and faster decay kinetics than VP neurons. Furthermore, a peak-scaled nonstationary noise analysis of mEPSCs revealed a larger estimated single-channel conductance of AMPARs expressed in OT neurons. High-frequency summation of AMPA-mediated excitatory postsynaptic potentials was smaller in OT neurons. In both cell types, AMPA-mediated synaptic currents showed inward rectification, which was more pronounced in OT neurons, and displayed Ca2+ permeability. On the other hand, NMDA-mediated mEPSCs of both cell types had similar amplitude and kinetic properties. The cell type-specific expression of functionally different AMPARs can contribute to the adoption of different firing patterns by these neuroendocrine neurons in response to physiological stimuli.

Evidence for regulation of vasopressin gene transcription by the NMDA receptor

We investigated the effects of dizocilpine maleate (MK-801), an NMDA receptor antagonist, on arginine vasopressin heterogeneous nuclear RNA expression in the supraoptic nucleus in the rat hypothalamus. MK-801 treatment completely blocked the osmotic stimulus-induced increase in AVP hnRNA expression, but had no effect on basal AVP hnRNA expression in the SON. These observations indicate that the NMDA receptor is essential for regulation of AVP gene transcription in response to osmotic stimulation, but has no effect on steady-state AVP gene transcription.

Resistance of gonadotropin-releasing hormone neurons to glutamatergic neurotoxicity

Although many studies provide evidence that glutamatergic pathways regulate the secretion of gonadotropin-releasing hormone (GnRH) from the hypothalamus, it is controversial as to whether they act directly upon GnRH neurons. The aim of the current study was to determine whether GnRH neurons are susceptible to the neurotoxic actions of specific glutamate agonists (N-methyl-D-aspartate [NMDA] and kainic acid), the rationale being that neurotoxic loss of GnRH neurons would provide evidence that the perikarya possess specific classes of glutamate receptor. Unilateral 1-microl injections of NMDA (12-120 mM), kainic acid (0.5-2.5 mM), or vehicle were stereotaxically directed at the preoptic area (mPOA)/diagonal band of Broca (dbB) in the region of the organum vasculosum of the lamina terminalis (OVLT) of male adult hamsters (Phodopus sungorus). The number and appearance of GnRH neurons were determined by immunocytochemistry 3-8 days later. The morphology of GnRH neurons in the vicinity of the injection sites appeared normal after both kainic acid and NMDA treatment, and there was no significant decrease in the numbers of GnRH perikarya identified following these treatments. Both agonists caused massive cellular loss when injected directly into cortical areas and striatum. In the experimental studies, there was little neuronal loss within the mPOA or dbB after either toxin, despite clear neuronal loss in areas adjacent to the injection sites, including ventral striatum and olfactory cortex. In follow-up studies, immunocytochemical and in situ hybridisation analysis of the NMDAR1 and NMDAR2 glutamate receptor subunits confirmed their widespread distribution in regions containing GnRH perikarya, but no colocalization within GnRH neurons was observed. The susceptibility of neural areas to NMDA neurotoxicity did not correlate with any difference in the regional expression of these glutamate receptor subunits. The resistance of GnRH neurons to the neurotoxic actions of two different glutamate agonists and the failure to detect colocalisation of NMDAR1 or NMDAR2 subunits within GnRH perikarya are consistent with the notion that the effects of glutamate upon GnRH secretion are not exerted directly upon GnRH cell bodies.

Developmental plasticity of NR1 and NR2B subunit expression in the supraoptic nucleus of the rat hypothalamus

Vasopressin and oxytocin neuroendocrine cells within the supraoptic nucleus of the adult hypothalamus (SON) display mRNA expression for the NMDA receptor subunits, NR1 and NR2B, NR2C and NR2D. The NR2B subunit confers slow decay kinetics (relative to NR1/NR2A receptors) and high magnesium sensitivity to NMDA receptor responses--properties which may contribute to the NMDA receptor-mediated bursting manifested by these cells. Therefore, we examined NR2B protein expression and its developmental profile in the SON and compared it to that in the cortex and cerebellum--areas which have been studied previously. We performed Western blot analysis on SON homogenates from embryonic, postnatal (PN7, 14, 21), and adult rats using an NR2B-specific antibody. Adult NR2B levels in the SON and PVN were similar but low relative to those of cortex. SON NR2B protein levels rose in the first postnatal week, remained high through PN21, and later declined to significantly lower levels in the adult. A similar profile was observed in cerebellum, where NR2B expression displayed a sharp peak at PN14 and later declined to minimal or undetectable levels in the adult. In contrast, NR2B continued to be overexpressed through adulthood in the cortex. The ontogenic pattern for NR1 expression, which included unregulation during early postnatal life and adulthood, was similar in the SON and cortex. A different pattern was observed for the cerebellum, where NR1 levels increased gradually after ED17 to reach significantly greater adult levels. Of all three areas studied, the SON displayed the earliest developmental rise in NR1 levels. SON explant cultures proved to be a useful preparation, since they contained neurons which synthesized NR1 and NR2B subunits in quantities similar to those of ED17 SON. Our findings suggest that NMDA receptors on SON neuroendocrine cells are assembled using NR1 and NR2B subunits, and that their plastic expression in early postnatal life may play a role during development.

Characterization of N-methyl-d-aspartate receptors in the hyperammonemic sparse fur mouse

The N-methyl-d-aspartate (NMDA) receptor, a glutamate receptor subtype, is a ligand-gated ion channel. Overstimulation of NMDA receptors may increase intracellular Ca2+ concentrations to lethal levels in neurodegenerative disorders affecting the basal ganglia. Such excitotoxicity may also contribute to the loss of medium spiny neurons in the striata of the hyperammonemic sparse fur (spf/Y) mouse, a model of the X-linked disorder of the urea cycle, ornithine carbamoyltransferase deficiency (OCTD). Levels of quinolinic acid (QA), a potent NMDA agonist, are elevated in the brains of spf/Y mice. Further, direct injection of QA into the striatum produces selective degeneration of medium spiny neurons. Microglia, an endogenous source of QA in the brain, are abundant in spf/Y mice during the period of neuronal degeneration. The location and density of NMDA receptors was visualized by gold labelled immunocytochemistry with a polyclonal antibody to the NMDAR1 receptor subtype and their distribution quantified. A 58% reduction was found in the median density value in the layer V pyramidal neurons in fronto-parietal cortex (p<0.001), but no significant change was observed in the striatum. NMDA receptor binding was examined using [3H]dizocilpine ([3H]MK-801). Receptor density (Bmax) in the striata of clinically stable spf/Y mice and +/Y littermates was unchanged, but was decreased 15% (p<0.01) in the fronto-parietal cortices in clinically stable spf/Y mice compared with +/Y littermate controls.

Channel properties of NMDA receptors on magnocellular neuroendocrine cells cultured from the rat supraoptic nucleus

Application of N-methyl-D-aspartate (NMDA) to the supraoptic nucleus of the hypothalamus (SON) generates clustered firing that may be important in hormone release. However, synaptically evoked EPSPs recorded from SON neurons exhibit varying contributions from NMDA receptors. We used the high resolution of single-channel recording to examine the receptor and ion channel properties of NMDA receptors expressed by SON neurons in 'punch' culture. Biocytin introduced into individual neurons during patch clamp recording revealed large (32.1+/-3.3 micron), oblong somas and bipolar extensions typical of magnocellular neuroendocrine cells (MNCs). Rapid application of NMDA (100-300 microM) in the presence of 10 microM glycine to outside-out macropatches resulted in openings with an average conductance of 46. 9 pS and reversal potential of +3.9 mV. Increasing glycine from 0.03 to 30 microM increased the apparent frequency, duration and occurrence of overlapping NMDA-elicited openings. NMDA responses were inhibited by Mg2+ in a voltage-dependent manner and by the NMDA-site antagonist, D-(-)-2-amino-5-phosphonovaleric acid (D-APV). Application of saturating NMDA or glycine alone with the glycine-site antagonist, 5,7-dichlorokynurenate (DCK) or with D-APV, respectively, did not result in agonist-induced openings. NR1 immunoreactivity was observed in large neurons (>25 micron) with MNC-like morphology. These single-channel and immunocytochemical data confirm the presence of functional NR1-containing NMDA receptors in MNCs.

Intrinsic controls of intracellular calcium and intercellular communication in the regulation of neuroendocrine cell activity

1. The magnocellular hypothalamoneurohypophysial system, consisting chiefly of the supraoptic and paraventricular nuclei and their axonal projections to the pituitary neural lobe, has become a model for the study of neuroendocrine cell morphology, function, and plasticity. 2. Decades of research have produced a wealth of knowledge about the physiological conditions that activate this system, the peripheral target tissues affected by its outputs, and its capacity to undergo use-dependent, reversible reorganization. 3. Earlier research on the neural control of this system concentrated largely on the synaptic inputs that influence the activity of these magnocellular neurons and, while that task is still far from completed, methods have now been developed that permit insights to be gained into the control exercised by intrinsic cellular and molecular mechanisms. 4. This article reviews the current state of knowledge of roles played by these intrinsic mechanisms, including influences of intracellular calcium buffering, calcium release from internal stores and intercellular communication through gap junctions, in the control of neuroendocrine cell activity.