Ionic basis for the intrinsic activation of rat supraoptic neurones by hyperosmotic stimuli

Osmoregulation and the Hypothalamic Supraoptic Nucleus: From Genes to Functions

Due to the relatively high permeability to water of the plasma membrane, water tends to equilibrate its chemical potential gradient between the intra and extracellular compartments. Because of this, changes in osmolality of the extracellular fluid are accompanied by changes in the cell volume. Therefore, osmoregulatory mechanisms have evolved to keep the tonicity of the extracellular compartment within strict limits. This review focuses on the following aspects of osmoregulation: 1) the general problems in adjusting the "milieu interieur" to challenges imposed by water imbalance, with emphasis on conceptual aspects of osmosis and cell volume regulation; 2) osmosensation and the hypothalamic supraoptic nucleus (SON), starting with analysis of the electrophysiological responses of the magnocellular neurosecretory cells (MNCs) involved in the osmoreception phenomenon; 3) transcriptomic plasticity of SON during sustained hyperosmolality, to pinpoint the genes coding membrane channels and transporters already shown to participate in the osmosensation and new candidates that may have their role further investigated in this process, with emphasis on those expressed in the MNCs, discussing the relationships of hydration state, gene expression, and MNCs electrical activity; and 4) somatodendritic release of neuropeptides in relation to osmoregulation. Finally, we expect that by stressing the relationship between gene expression and the electrical activity of MNCs, studies about the newly discovered plastic-regulated genes that code channels and transporters in the SON may emerge.

Orexin-A Intensifies Mouse Pupillary Light Response by Modulating Intrinsically Photosensitive Retinal Ganglion Cells

We show for the first time that the neuropeptide orexin modulates pupillary light response, a non-image-forming visual function, in mice of either sex. Intravitreal injection of the orexin receptor (OXR) antagonist TCS1102 and orexin-A reduced and enhanced pupillary constriction in response to light, respectively. Orexin-A activated OX₁Rs on M2-type intrinsically photosensitive retinal ganglion cells (M2 cells), and caused membrane depolarization of these cells by modulating inward rectifier potassium channels and nonselective cation channels, thus resulting in an increase in intrinsic excitability. The increased intrinsic excitability could account for the orexin-A-evoked increase in spontaneous discharges and light-induced spiking rates of M2 cells, leading to an intensification of pupillary constriction. Orexin-A did not alter the light response of M1 cells, which could be because of no or weak expression of OX₁Rs on them, as revealed by RNAscope in situ hybridization. In sum, orexin-A is likely to decrease the pupil size of mice by influencing M2 cells, thereby improving visual performance in awake mice via enhancing the focal depth of the eye's refractive system.SIGNIFICANCE STATEMENT This study reveals the role of the neuropeptide orexin in mouse pupillary light response, a non-image-forming visual function. Intravitreal orexin-A administration intensifies light-induced pupillary constriction via increasing the excitability of M2 intrinsically photosensitive retinal ganglion cells by activating the orexin receptor subtype OX₁R. Modulation of inward rectifier potassium channels and nonselective cation channels were both involved in the ionic mechanisms underlying such intensification. Orexin could improve visual performance in awake mice by reducing the pupil size and thereby enhancing the focal depth of the eye's refractive system.

Co-expression of mouse TMEM63A, TMEM63B and TMEM63C confers hyperosmolarity activated ion currents in HEK293 cells

Osmoreception is essential for systemic osmoregulation, a process to stabilize the tonicity and volume of the extracellular fluid through regulating the ingestive behaviour, sympathetic outflow and renal function. The sensation of osmotic changes by osmoreceptor neurons is mediated by ion channels that detect the change of osmolarity in extracellular fluid. However, the molecular identity of these channels remains mysterious. AtCSC1and OSCA1,two closely related paralogues from Arabidopsis, have been demonstrated to form hyperosmolarity activated ion channels, which makes their mammalian orthologues-the members of TMEM63 proteins, possible candidates for osmoreceptor transduction channel. To test this possibility, we cloned the cDNAs of all the three members of the mouse TMEM63 family, TMEM63A, TMEM63B and TMEM63C from the mRNA from mouse brain. When all of the three subtypes of TMEM63 proteins were co-expressed in HEK293 cells, we recorded membrane currents evoked by hypertonic stimulation in these cells. However, the cells expressing the combinations of any two subtypes of TMEM63 proteins could not exhibit any hyperosmolarity evoked currents. Thus, all the three members of TMEM63 proteins are required to constitute a hyperosmolarity activated ion channel. We propose that the TMEM63 proteins may serve as an osmolarity sensitive ion channel for the osmoreception. Copyright © 2016 John Wiley & Sons, Ltd.

Neuronal-derived nitric oxide and somatodendritically released vasopressin regulate neurovascular coupling in the rat hypothalamic supraoptic nucleus

The classical model of neurovascular coupling (NVC) implies that activity-dependent axonal glutamate release at synapses evokes the production and release of vasoactive signals from both neurons and astrocytes, which dilate arterioles, increasing in turn cerebral blood flow (CBF) to areas with increased metabolic needs. However, whether this model is applicable to brain areas that also use less conventional neurotransmitters, such as neuropeptides, is currently unknown. To this end, we studied NVC in the rat hypothalamic magnocellular neurosecretory system (MNS) of the supraoptic nucleus (SON), in which dendritic release of neuropeptides, including vasopressin (VP), constitutes a key signaling modality influencing neuronal and network activity. Using a multidisciplinary approach, we investigated vasopressin-mediated vascular responses in SON arterioles of hypothalamic brain slices of Wistar or VP-eGFP Wistar rats. Bath-applied VP significantly constricted SON arterioles (Δ-41 ± 7%) via activation of the V1a receptor subtype. Vasoconstrictions were also observed in response to single VP neuronal stimulation (Δ-18 ± 2%), an effect prevented by V1a receptor blockade (V2255), supporting local dendritic VP release as the key signal mediating activity-dependent vasoconstrictions. Conversely, osmotically driven magnocellular neurosecretory neuronal population activity leads to a predominant nitric oxide-mediated vasodilation (Δ19 ± 2%). Activity-dependent vasodilations were followed by a VP-mediated vasoconstriction, which acted to limit the magnitude of the vasodilation and served to reset vascular tone following activity-dependent vasodilation. Together, our results unveiled a unique and complex form of NVC in the MNS, supporting a competitive balance between nitric oxide and activity-dependent dendritic released VP, in the generation of proper NVC responses.

Mechanical basis of osmosensory transduction in magnocellular neurosecretory neurones of the rat supraoptic nucleus

Rat magnocellular neurosecretory cells (MNCs) release vasopressin and oxytocin to promote antidiuresis and natriuresis at the kidney. The osmotic control of oxytocin and vasopressin release at the neurohypophysis is required for osmoregulation in these animals, and this release is mediated by a modulation of the action potential firing rate by the MNCs. Under basal (isotonic) conditions, MNCs fire action potentials at a slow rate, and this activity is inhibited by hypo-osmotic conditions and enhanced by hypertonicity. The effects of changes in osmolality on MNCs are mediated by a number of different factors, including the involvement of synaptic inputs, the release of taurine by local glial cells and regulation of ion channels expressed within the neurosecretory neurones themselves. We review recent findings that have clarified our understanding of how osmotic stimuli modulate the activity of nonselective cation channels in MNCs. Previous studies have shown that osmotically-evoked changes in membrane potential and action potential firing rate in acutely isolated MNCs are provoked mainly by a modulation of nonselective cation channels. Notably, the excitation of isolated MNCs during hypertonicity is mediated by the activation of a capsaicin-insensitive cation channel that MNCs express as an N-terminal variant of the transient receptor potential vanilloid 1 (Trpv1) channel. The activation of this channel during hypertonicity is a mechanical process associated with cell shrinking. The effectiveness of this mechanical process depends on the presence of a thin layer of actin filaments (F-actin) beneath the plasma membrane, as well as a densely interweaved network of microtubules (MTs) occupying the bulk of the cytoplasm of MNCs. Although the mechanism by which F-actin contributes to Trpv1 activation remains unknown, recent data have shown that MTs interact with Trpv1 channels via binding sites on the C-terminus, and that the force mediated through this complex is required for channel gating during osmosensory transduction. Indeed, displacement of this interaction prevents channel activation during shrinking, whereas increasing the density of these interaction sites potentiates shrinking-induced activation of Trpv1. Therefore, the gain of the osmosensory transduction process can be regulated bi-directionally through changes in the organisation of F-actin and MTs.

A RNA-Seq Analysis of the Rat Supraoptic Nucleus Transcriptome: Effects of Salt Loading on Gene Expression

Magnocellular neurons (MCNs) in the hypothalamo-neurohypophysial system (HNS) are highly specialized to release large amounts of arginine vasopressin (Avp) or oxytocin (Oxt) into the blood stream and play critical roles in the regulation of body fluid homeostasis. The MCNs are osmosensory neurons and are excited by exposure to hypertonic solutions and inhibited by hypotonic solutions. The MCNs respond to systemic hypertonic and hypotonic stimulation with large changes in the expression of their Avp and Oxt genes, and microarray studies have shown that these osmotic perturbations also cause large changes in global gene expression in the HNS. In this paper, we examine gene expression in the rat supraoptic nucleus (SON) under normosmotic and chronic salt-loading SL) conditions by the first time using "new-generation", RNA sequencing (RNA-Seq) methods. We reliably detect 9,709 genes as present in the SON by RNA-Seq, and 552 of these genes were changed in expression as a result of chronic SL. These genes reflect diverse functions, and 42 of these are involved in either transcriptional or translational processes. In addition, we compare the SON transcriptomes resolved by RNA-Seq methods with the SON transcriptomes determined by Affymetrix microarray methods in rats under the same osmotic conditions, and find that there are 6,466 genes present in the SON that are represented in both data sets, although 1,040 of the expressed genes were found only in the microarray data, and 2,762 of the expressed genes are selectively found in the RNA-Seq data and not the microarray data. These data provide the research community a comprehensive view of the transcriptome in the SON under normosmotic conditions and the changes in specific gene expression evoked by salt loading.

Unique interweaved microtubule scaffold mediates osmosensory transduction via physical interaction with TRPV1

The electrical activity of mammalian osmosensory neurons (ONs) is increased by plasma hypertonicity to command thirst, antidiuretic hormone release, and increased sympathetic tone during dehydration. Osmosensory transduction is a mechanical process whereby decreases in cell volume cause the activation of transient receptor potential vanilloid type-1 (TRPV1) channels to induce depolarization and increase spiking activity in ONs. However, it is not known how cell shrinking is mechanically coupled to channel activation. Using superresolution imaging, we found that ONs are endowed with a uniquely interweaved scaffold of microtubules throughout their somata. Microtubules physically interact with the C terminus of TRPV1 at the cell surface and provide a pushing force that drives channels activation during shrinking. Moreover, we found that changes in the density of these interactions can bidirectionally modulate osmosensory gain. Microtubules are thus an essential component of the vital neuronal mechanotransduction apparatus that allows the brain to monitor and correct body hydration.

Extracellular signal-regulated kinase phosphorylation in forebrain neurones contributes to osmoregulatory mechanisms

Vasopressin secretion from the magnocellular neurosecretory cells (MNCs) is crucial for body fluid homeostasis. Osmotic regulation of MNC activity involves the concerted modulation of intrinsic mechanosensitive ion channels, taurine release from local astrocytes as well as excitatory inputs derived from osmosensitive forebrain regions. Extracellular signal-regulated protein kinases (ERK) are mitogen-activated protein kinases that transduce extracellular stimuli into intracellular post-translational and transcriptional responses, leading to changes in intrinsic neuronal properties and synaptic function. Here, we investigated whether ERK activation (i.e. phosphorylation) plays a role in the functioning of forebrain osmoregulatory networks. We found that within 10 min after intraperitoneal injections of hypertonic saline (3 m, 6 m) in rats, many phosphoERK-immunopositive neurones were observed in osmosensitive forebrain regions, including the MNC containing supraoptic nuclei. The intensity of ERK labelling was dose-dependent. Reciprocally, slow intragastric infusions of water that lower osmolality reduced basal ERK phosphorylation. In the supraoptic nucleus, ERK phosphorylation predominated in vasopressin neurones vs. oxytocin neurones and was absent from astrocytes. Western blot experiments confirmed that phosphoERK expression in the supraoptic nucleus was dose dependent. Intracerebroventricular administration of the ERK phosphorylation inhibitor U 0126 before a hyperosmotic challenge reduced the number of both phosphoERK-immunopositive neurones and Fos expressing neurones in osmosensitive forebrain regions. Blockade of ERK phosphorylation also reduced hypertonically induced depolarization and an increase in firing of the supraoptic MNCs recorded in vitro. It finally reduced hypertonically induced vasopressin release in the bloodstream. Altogether, these findings identify ERK phosphorylation as a new element contributing to the osmoregulatory mechanisms of vasopressin release.

Hypertonicity increases NO production to modulate the firing rate of magnocellular neurons of the supraoptic nucleus of rats

Increases in plasma osmolality enhance nitric oxide (NO) levels in magnocellular neurosecretory cells (MNCs) of the supraoptic nucleus (SON) and modulate the secretion of both vasopressin (VP) and oxytocin (OT). In this paper, we describe the effects of hypertonicity on the electrical properties of MNCs by focusing on the nitrergic modulation of their activity in this condition. Membrane potentials were measured using the patch clamp technique, in the presence of both glutamatergic and GABAergic neurotransmission blockers, in coronal brain slices of male Wistar rats. The recordings were first made under a control condition (295 mosm/kg H2O), then in the presence of a hypertonic stimulus (330 mosm/kg H2O) and, finally, with a hypertonic stimulus plus 500 μM L-Arginine or 100 μM N-nitro-L-Arginine methyl ester hydrochloride (L-NAME). Hypertonicity per se increased the firing frequency of the neurons. L-Arginine prevented the increase in fire frequency induced by hypertonic stimulus, and L-NAME (inhibitor of nitric oxide synthase) induced an additional increase in frequency when applied together with the hypertonic solution. Moreover, L-Arginine hyperpolarizes the resting potential and decreases the peak value of the after-hyperpolarization; both effects were blocked by L-NAME and hypertonicity and/or L-NAME reduced the time constant of the rising phase of the after-depolarization. These results demonstrate that an intrinsic nitrergic system is part of the mechanisms controlling the excitability of MNCs of the SON when the internal fluid homeostasis is disturbed.

Effect of the interaction between food state and the action of estrogen on oxytocinergic system activity

Increased plasma osmolality by food intake evokes augmentation of plasma oxytocin (OT). Ovarian steroids may also influence the balance of body fluids by acting on OT neurones. Our aim was to determine if estrogen influences the activity of OT neurones in paraventricular nucleus (PVN) and supraoptic nucleus (SON) under different osmotic situations. Ovariectomized rats (OVX) were treated with either estradiol (E(2)) or vehicle and were divided into three groups: group I was fed ad libitum, group II underwent 48 h of fasting, and group III was refed after 48 h of fasting. On the day of the experiment, blood samples were collected to determine the plasma osmolality and OT. The animals were subsequently perfused, and OT/FOS immunofluorescence analysis was conducted on neurones in the PVN and the SON. When compared to animals which were fasted or fed ad libitum, the plasma osmolality of refed animals was higher, regardless of whether they were treated with vehicle or E(2). We observed neural activation of OT cells in vehicle- or E(2)-treated OVX rats refed after 48 h of fasting, but not in animals fed ad libitum or in animals that only underwent 48 h of fasting. Finally, the percentage of neurones that co-expressed OT and FOS was lower in both the PVN and the SON of animals treated with E(2) and refed, when compared to vehicle-treated animals. These results suggest that E(2) may have an inhibitory effect on OT neurones and may modulate the secretion of OT in response to the increase of osmolality induced by refeeding.

Osmosensory mechanisms in cellular and systemic volume regulation

Perturbations of cellular and systemic osmolarity severely challenge the function of all organisms and are consequently regulated very tightly. Here we outline current evidence on how cells sense volume perturbations, with particular focus on mechanisms relevant to the kidneys and to extracellular osmolarity and whole body volume homeostasis. There are a variety of molecular signals that respond to perturbations in cell volume and osmosensors or volume sensors responding to these signals. The early signals of volume perturbation include integrins, the cytoskeleton, receptor tyrosine kinases, and transient receptor potential channels. We also present current evidence on the localization and function of central and peripheral systemic osmosensors and conclude with a brief look at the still limited evidence on pathophysiological conditions associated with deranged sensing of cell volume.

Cell volume homeostatic mechanisms: effectors and signalling pathways

Cell volume homeostasis and its fine-tuning to the specific physiological context at any given moment are processes fundamental to normal cell function. The understanding of cell volume regulation owes much to August Krogh, yet has advanced greatly over the last decades. In this review, we outline the historical context of studies of cell volume regulation, focusing on the lineage started by Krogh, Bodil Schmidt-Nielsen, Hans-Henrik Ussing, and their students. The early work was focused on understanding the functional behaviour, kinetics and thermodynamics of the volume-regulatory ion transport mechanisms. Later work addressed the mechanisms through which cellular signalling pathways regulate the volume regulatory effectors or flux pathways. These studies were facilitated by the molecular identification of most of the relevant channels and transporters, and more recently also by the increased understanding of their structures. Finally, much current research in the field focuses on the most up- and downstream components of these paths: how cells sense changes in cell volume, and how cell volume changes in turn regulate cell function under physiological and pathophysiological conditions.

Glutamatergic inputs contribute to phasic activity in vasopressin neurons

Many neurons in the CNS display rhythmic patterns of activity to optimize excitation-secretion coupling. However, the mechanisms of rhythmogenesis are only partially understood. Magnocellular vasopressin (VP) neurons in the hypothalamus display a phasic activity that consists of alternative bursts of action potentials and silent periods. Previous observations from acute slices of adult hypothalamus suggested that VP cell rhythmicity depends on intrinsic membrane properties. However, such activity in vivo is nonregenerative. Here, we studied the mechanisms of VP neuron rhythmicity in organotypic slice cultures that, unlike acute slices, preserve functional synaptic connections. Comparative analysis of phasic firing of VP neurons in vivo, in acute slices, and in the cultures revealed that, in the latter, the activity was closely related to that observed in vivo. It was synaptically driven, essentially from glutamatergic inputs, and did not rely on intrinsic membrane properties. The glutamatergic synaptic activity was sensitive to osmotic challenges and kappa-opioid receptor activation, physiological stimuli known to affect phasic activity. Together, our data thus strongly suggest that phasic activity in magnocellular VP neurons is controlled by glutamatergic synaptic inputs rather than by intrinsic properties.

Acute hyperosmotic stimulus-induced Fos expression in neurons depends on activation of astrocytes in the supraoptic nucleus of rats

Acute hyperosmolarity induced a time-dependent expression of Fos protein in both neurons and astrocytes of the rat supraoptic nucleus, with peak Fos expression occurring at 45 min in astrocytes and at 90 min in neurons after hypertonic stimulation in vivo. To determine whether the two cell types were activated separately or in an integrated manner, animals were pretreated with fluorocitrate, a glial metabolic blocker or carbenoxolone, a gap junction blocker followed by an acute hypertonic stimulation similar to that of the controls. Antibodies against glial fibrillary acidic protein, connexin 43, vasopressin, and oxytocin were used in serial sections to identify the cellular elements of the supraoptic nucleus. It was found that interruption of astrocyte metabolism with fluorocitrate significantly reduced Fos protein expression in both astrocytes and neurons, whereas blockage of gap junctions with carbenoxolone clearly reduced Fos protein expression in neurons, but not in astrocytes. These results indicate that both neurons and astrocytes in the rat supraoptic nucleus are involved in regulating osmolarity. Astrocytes are activated first, whereas connexin 43 functional hemichannels in SON astrocytes are required for the subsequent activation of the neurons.

TRPV1 gene deficiency attenuates miniature EPSC potentiation induced by mannitol and angiotensin II in supraoptic magnocellular neurons

The release of arginine vasopressin (AVP) from the magnocellular neurosecretory cells (MNCs) in the supraoptic nucleus (SON) is crucial for body fluid homeostasis. The MNC activity is modulated by synaptic inputs and humoral factors. A recent study demonstrated that an N-terminal splice variant of the transient receptor potential vanilloid type 1 (TRPV1) is essential for osmosensory transduction in the SON. In the present study, we examined the effects of mannitol and angiotensin II on miniature EPSCs (mEPSCs) in the supraoptic MNCs using whole-cell patch-clamp recording in in vitro slice preparation. Mannitol (60 mm) and angiotensin II (0.1 microm) increased the frequency of mEPSCs without affecting the amplitude. These effects were attenuated by pre-exposure to a nonspecific TRPV channel blocker, ruthenium red (10 microm) and enhanced by pre-exposure to cannabinoid type1 receptor antagonist, AM251 (2 microm). Mannitol-induced potentiation of mEPSCs was not attenuated by angiotensin II receptor antagonist, losartan (10 microm), indicating independent pathways of mannitol and angiotensin II to the TRPV channels. The potentiation of mEPSCs by mannitol was not mimicked by a TRPV1 agonist, capsaicin, and also not attenuated by TRPV1 blockers, capsazepine (10 microm). PKC was involved in angiotensin II-induced potentiation of mEPSCs. The effects of mannitol and angiotensin II on the supraoptic MNCs in trpv1 knock-out mice were significantly attenuated compared with those in wild-type mice counterparts. The results suggest that hyperosmotic stimulation and angiotensin II independently modulate mEPSCs through capsaicin-insensitive TRPV1 channel in the presynaptic terminals of the SON.

Osmosensation in vasopressin neurons: changing actin density to optimize function

The proportional relation between circulating vasopressin concentration and plasma osmolality is fundamental for body fluid homeostasis. Although changes in the sensitivity of this relation are associated with pathophysiological conditions, central mechanisms modulating osmoregulatory gain are unknown. Here, we review recent data that sheds important light on this process. The cell autonomous osmosensitivity of vasopressin neurons depends on cation channels comprising a variant of the transient receptor potential vanilloid 1 (TRPV1) channel. Hyperosmotic activation is mediated by a mechanical process where sensitivity increases in proportion with actin filament density. Moreover, angiotensin II amplifies osmotic activation by a rapid stimulation of actin polymerization, suggesting that neurotransmitter-induced changes in cytoskeletal organization in osmosensory neurons can mediate central changes in osmoregulatory gain.

Neurotransmitter regulation of c-fos and vasopressin gene expression in the rat supraoptic nucleus

Acute increases in plasma osmotic pressure produced by intraperitoneal injection of hypertonic NaCl are sensed by osmoreceptors in the brain, which excite the magnocellular neurons (MCNs) in the supraoptic nucleus (SON) and the paraventricular nucleus (PVN) in the hypothalamus inducing the secretion of vasopressin (VP) into the general circulation. Such systemic osmotic stimulation also causes rapid and transient increases in the gene expression of c-fos and VP in the MCNs. In this study we evaluated potential signals that might be responsible for initiating these gene expression changes during acute hyperosmotic stimulation. We use an in vivo paradigm in which we stereotaxically deliver putative agonists and antagonists over the SON unilaterally, and use the contralateral SON in the same rat, exposed only to vehicle solutions, as the control SON. Quantitative real time-PCR was used to compare the levels of c-fos mRNA, and VP mRNA and VP heteronuclear (hn)RNA in the SON. We found that the ionotropic glutamate agonists (NMDA plus AMPA) caused an approximately 6-fold increase of c-fos gene expression in the SON, and some, but not all, G-coupled protein receptor agonists (e.g., phenylephrine, senktide, a NK-3-receptor agonist, and alpha-MSH) increased the c-fos gene expression in the SON from between 1.5 to 2-fold of the control SONs. However, none of these agonists were effective in increasing VP hnRNA as is seen with acute salt-loading. This indicates that the stimulus-transcription coupling mechanisms that underlie the c-fos and VP transcription increases during acute osmotic stimulation differ significantly from one another.

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.

MCP-1 enhances excitability of nociceptive neurons in chronically compressed dorsal root ganglia

Previous experimental results from our laboratory demonstrated that monocyte chemoattractant protein-1 (MCP-1) depolarizes or increases the excitability of nociceptive neurons in the intact dorsal root ganglion (DRG) after a chronic compression of the DRG (CCD), an injury that upregulates neuronal expression of both MCP-1 and mRNA for its receptor CCR2. We presently explore the ionic mechanisms underlying the excitatory effects of MCP-1. MCP-1 (100 nM) was applied, after CCD, to acutely dissociated small DRG neurons with nociceptive properties. Under current clamp, the proportion of neurons depolarized was similar to that previously observed for CCD-treated neurons in the intact ganglion, although the magnitude of depolarization was greater. MCP-1 induced a decrease in rheobase by 44 +/- 10% and some cells became spontaneously active at resting potential. Action potential width at a voltage equal to 10% of the peak height was increased from 4.94 +/- 0.23 to 5.90 +/- 0.47 ms. In voltage clamp, MCP-1 induced an inward current in 27 of 50 neurons held at -60 mV, which increased with concentration over the range of 3 to 300 nM (EC(50) = 45 nM). The MCP-1-induced current was not voltage dependent and had an estimated reversal potential of -27 mV. In addition, MCP-1 inhibited a voltage-dependent, noninactivating outward current, presumably a delayed rectifier type K(+) conductance. We conclude that MCP-1 enhances excitability in CCD neurons by, at least, two mechanisms: 1) activation of a nonvoltage-dependent depolarizing current with characteristics similar to a nonselective cation conductance and 2) inhibition of a voltage-dependent outward current.

Regional differences in the expression of Fos-like immunoreactivity after central salt loading in conscious rats: modulation by endogenous vasopressin and role of the area postrema

In this study, we examined the quantitative relationship between centrally administered hypertonic saline (HS) concentrations and the expression of Fos-like immunoreactivity (FLI) in brain regions involved in the homeostasis of body fluids. The regions examined were the organum vasculosum laminae terminalis (OVLT), the median preoptic nucleus (MnPO), the subfornical organ (SFO), the paraventricular nucleus (PVN), the supraoptic nucleus of the hypothalamus, the nucleus of the solitary tract (NTS), and the area postrema (AP). The experiments were performed in conscious rats with attention to the actual changes in central [Na(+)]. Hypertonic saline (0.3, 0.67, or 1.0 M) was delivered at 1 microl/min for 20 min. The changes in cerebrospinal fluid [Na(+)] during i.c.v. administration of 0.3 M hypertonic saline were compatible with those expected for thermal dehydration. FLI increased in a dose-dependent manner in the dorsomedial cap of the PVN and NTS. Although the pressor responses during central salt loading were not significantly affected by pretreatment with the peripheral vasopressin V(1) receptor antagonist OPC-21268, FLI expression in the PVN was significantly augmented. In addition, in AP-lesioned rats, FLI expression in the lateral magnocellular part of the PVN and NTS was significantly enhanced after central salt loading. These results suggest that the peripheral vasopressin system participates in negative feedback to modulate neuronal activities in the PVN, probably through the AP or direct action at the PVN in response to central osmotic and/or Na(+) stimulation.

Spike coding during osmotic stimulation of the rat supraoptic nucleus

Novel measures of coding based on interspike intervals were used to characterize the responses of supraoptic cells to osmotic stimulation. Infusion of hypertonic NaCl in vivo increased the firing rate of continuous (putative oxytocin) cells (Wilcoxon z= 3.84, P= 0.001) and phasic (putative vasopressin) cells (z= 2.14, P= 0.032). The irregularity of activity, quantified by the log interval entropy, was decreased for continuous (Student's t= 3.06, P= 0.003) but not phasic cells (t= 1.34, P= 0.181). For continuous cells, the increase in frequency and decrease in entropy was significantly greater (t= 2.61, P= 0.036 and t= 3.06, P= 0.007, respectively) than for phasic cells. Spike patterning, quantified using the mutual information between intervals, was decreased for phasic (z=-2.64, P= 0.008) but not continuous cells (z=-1.14, P= 0.256). Although continuous cells showed similar osmotic responses to mannitol infusion, phasic cells showed differences: spike frequency decreased (z=-3.70, P < 0.001) and entropy increased (t=-3.41, P < 0.001). Considering both cell types together, osmotic stimulation in vitro using 40 mm NaCl had little effect on firing rate (z=-0.319, P= 0.750), but increased both entropy (t= 2.75, P= 0.010) and mutual information (z=-2.73, P= 0.006) in contrast to the decreases (t= 2.92, P= 0.004 and z=-2.40, P= 0.017) seen in vivo. Responses to less severe osmotic stimulation with NaCl or mannitol were not significant. Potassium-induced depolarization in vitro increased firing rate (r= 0.195, P= 0.034), but the correlation with decreased entropy was not significant (r=-0.097, P= 0.412). Intracellular recordings showed a small depolarization and decrease in input resistance during osmotic stimulation with NaCl or mannitol, and membrane depolarization following addition of potassium. Differences in responses of oxytocin and vasopressin cells in vivo, suggest differences in the balance between the synaptic and membrane properties involved in coding their osmotic responses. The osmotic responses in vivo constrasted with those seen in vitro, which suggests that, in vivo, they depend on extrinsic circuitry. Differences in responses to osmolality and direct depolarization in vitro indicate that the mechanism of osmoresponsiveness within a physiological range is unlikely to be fully explained by depolarization.

A novel osmosensitive voltage gated cation current in rat supraoptic neurones

The magnocellular neurosecretory cells of the hypothalamus (MNCs) regulate water balance by releasing vasopressin and oxytocin as a function of plasma osmolality. Release is determined largely by the rate and pattern of action potentials generated in the MNC somata. Changes in firing are mediated in part by a stretch-inactivated non-selective cation current that causes the cells to depolarize when increased osmolality leads to cell shrinkage. We have obtained evidence for a new current that may regulate MNC firing during changes in external osmolality, using whole-cell patch clamp of acutely isolated rat MNC somata. In internal and external solutions lacking K+, with high concentrations of TEA, and with Na+ as the only likely permeant cation, the current appears as a slow inward current during depolarizations and yields a large tail current upon return to the holding potential of -80 mV. Approximately 60% of the MNCs tested (79 out of 134 cells) displayed a large increase in tail current density (from 5.2+/-0.9 to 10.5+/-1.4 pA pF-1; P<0.001) following an increase in external osmolality from 295 to 325 mosmol kg-1. The current is activated by depolarization to potentials above -60 mV and does not appear to depend on changes in internal Ca2+. The current is carried by Na+ under these conditions, but is blocked by Cs+ and Ba2+ and by internal K+, which suggests that the current could be a K+ current under physiological conditions. This current could play an important role in regulating the response of MNCs to osmolality.

Orexins cause depolarization via nonselective cationic and K+ channels in isolated locus coeruleus neurons

The locus coeruleus (LC) contains noradrenergic neurons that are innervated by orexin (ORX)-like immunoreactive axons and express both orexin receptor-1 and -2. We studied effects of ORX-A and -B (ORX-A/B) on dissociated LC neurons by using whole-cell patch clamp techniques. In current-clamp mode, LC neurons were depolarized by application of ORX-A (10(-7) M) [53% of neurons tested; 9.0+/-0.2 mV (n=5)], or ORX-B (10(-7) M) [38% of neurons tested; 4.0+/-0.1 mV (n=5)]. Firing frequencies of action potentials increased during application [1.1+/-0.2 Hz (n=5) in ORX-A; 0.8+/-0.2 Hz (n=5) in ORX-B] and returned to the control level [0.2+/-0.1 Hz (n=5)] after removal. The ORX-A/B-induced depolarization was well maintained in the presence of TTX (3x10(-7) M), CNQX (10(-6) M) and AP5 (10(-5) M). In voltage-clamp mode, removal of external Na+ suppressed both ORX-A/B-induced currents and shifted their reversal potentials from approximately -45 mV to -60 mV. In addition, ORX-A/B inhibited sustained K+ currents. These results suggest that ORX-A/B increase the firing frequency of LC neurons through the depolarization probably produced by both augmentation of the nonselective cationic conductance and inhibition of the sustained K+ conductance.

Secretin depolarizes nucleus tractus solitarius neurons through activation of a nonselective cationic conductance

The recent suggestion that secretin may be useful in treating autism and schizophrenia has begun to focus attention on the mechanisms underlying this gut-brain peptide's actions in the central nervous system (CNS). In vitro autoradiographic localization of125I-secretin binding sites in rat brain shows the highest binding density in the nucleus tractus solitarius (NTS). Recent evidence suggests that intravenous infusion of secretin causes fos activation in NTS, a relay station playing important roles in the central regulation of autonomic functions. In this study, whole cell patch-clamp recordings were obtained from 127 NTS neurons in rat medullary slices. The mean resting membrane potential of these neurons was -54.7 ± 0.3 mV, the mean input resistance was 3.7 ± 0.2 GΩ, and the action potential amplitude of these neurons was always >70 mV. Current-clamp studies showed that bath application of secretin depolarized the majority (80.8%; 42/52) of NTS neurons tested, whereas the remaining cells were either unaffected (17.3%; 9/52) or hyperpolarized (1.9%; 1/52). These depolarizing effects were maintained in the presence of 5 μM TTX and found to be concentration dependent from 10-12to 10-7M. Using voltage-clamp techniques, we also identified modulatory actions of secretin on specific ion channels. Our results demonstrate that while secretin is without effect on net whole cell potassium currents, it activates a nonselective cationic conductance (NSCC). These results show that NTS neurons are activated by secretin as a consequence of activation of a NSCC and support the emerging view that secretin can act as a neuropeptide within the CNS.

Deciphering the mechanisms of homeostatic plasticity in the hypothalamo-neurohypophyseal system—genomic and gene transfer strategies

The hypothalamo-neurohypophyseal system (HNS) is the specialised brain neurosecretory apparatus responsible for the production of a peptide hormone, vasopressin, that maintains water balance by promoting water conservation at the level of the kidney. Dehydration evokes a massive increase in the regulated release of hormone from the HNS, and this is accompanied by a plethora of changes in morphology, electrical properties and biosynthetic and secretory activity, all of which are thought to facilitate hormone production and delivery, and hence the survival of the organism. We have adopted a functional genomic strategy to understand the activity dependent plasticity of the HNS in terms of the co-ordinated action of cellular and genetic networks. Firstly, using microarray gene-profiling technologies, we are elucidating which genes are expressed in the HNS, and how the pattern of expression changes following physiological challenge. The next step is to use transgenic rats to probe the functions of these genes in the context of the physiological integrity of the whole organism.

Orexin-A depolarizes nucleus tractus solitarius neurons through effects on nonselective cationic and K+ conductances

The nucleus tractus solitarius (NTS) plays central roles in a number of autonomic functions including cardiovascular control. Orexin (ORX)-A is a 33-amino-acid peptide implicated in the central regulation of energy metabolism, sleep, and the cardiovascular system. Studies demonstrate the presence of ORX-immunoreactive axons and both OX(1)R (orexin receptor) and OX(2)R mRNA within NTS. In this study, whole cell patch-clamp recordings were obtained from NTS neurons in rat medullary slices. Current-clamp studies showed that bath application of various concentrations of ORX-A depolarized 90.7% (78 of 86) of neurons tested while the remaining cells were either unaffected or showed small hyperpolarizations in response to peptide administration. Depolarizing effects were maintained in the presence of 5 microM TTX, and were concentration dependent. Using voltage-clamp techniques, we also identified modulatory actions of ORX-A on specific ion channels. Our results demonstrate that not only does ORX-A inhibit a specific potassium conductance (the sustained K(+) current) in NTS neurons, but it also activates a nonselective cationic conductance (NSCC). These data suggest that ORX-A effects on central cardiovascular control may result from direct actions on NTS neurons and also highlight the ability of this peptide to influence neuronal excitability as a consequence of concurrent modulation of multiple ion channels.

Stretch-inactivated cation channels: cellular targets for modulation of osmosensitivity in supraoptic neurons

Rat magnocellular neurosecretory cells (MNCs) show an intrinsic sensitivity to acute changes in fluid osmolality. Experiments in acutely isolated supraoptic MNCs have shown that these responses are due to in part to the cell volume-dependent modulation of gadolinium-sensitive 33 pS stretch-inactivated cation (SIC) channels. Previous studies in vivo have shown that the slope (i.e. gain) of the 'osmosensory' relation between VP release and plasma osmolality can be increased or decreased under various physiological and pathological conditions. Here, we review recent work that shows how changes in external [Na] and excitatory neuropeptides such as angiotensin II (Ang II), cholecystokinin (CCK) and neurotensin (NT), may influence osmosensory gain in acutely isolated MNCs. Whole-cell and single-channel recording experiments have revealed that changes in external Na cause proportional changes in osmosensory gain as a result of modified SIC channel permeability and not by affecting mechanotransduction. In contrast, Ang II, CCK, or NT appear to convergently, and directly, stimulate the osmosensory cation conductance in MNCs. Preliminary analysis in current clamp further suggests that osmosensory gain may be increased upon exposure to these excitatory peptides. Whether such mechanisms contribute to the modulation of osmosensory gain in vivo remains to be established.

Cellular mechanisms of orexin actions on paraventricular nucleus neurones in rat hypothalamus

Using whole-cell patch clamp techniques we have examined the cellular mechanisms underlying the effects of orexin A (OX-A) on electrophysiologically identified magnocellular and parvocellular neurones in the rat hypothalamic paraventricular nucleus (PVN). The majority of magnocellular neurones (67 %) showed concentration-dependent, reversible depolarizations in response to OX-A. These effects were abolished in tetrodotoxin (TTX), suggesting them to be indirect effects on this population of neurones. OX-A also caused increases in excitatory postsynaptic current (EPSC) frequency and amplitude in magnocellular neurones. The former effects were again blocked in TTX while increases in mini-EPSC amplitude remained. Depolarizing effects of OX-A on magnocellular neurones were also found to be abolished by kynurenic acid, supporting the conclusion that these effects were the result of activation of a glutamate interneurone. Parvocellular neurones (73 % of those tested) also showed concentration-dependent, reversible depolarizations in response to OX-A. In contrast to magnocellular neurones, these effects were maintained in TTX, indicating direct effects of OX-A on this population of neurones. Voltage clamp analysis using slow voltage ramps demonstrated that OX-A enhanced a non-selective cationic conductance with a reversal potential of -40 mV in parvocellular neurones, effects which probably explain the depolarizing effects of this peptide in this subpopulation of PVN neurones. These studies have identified separate cellular mechanisms through which OX-A influences the excitability of magnocellular and parvocellular PVN neurones.

Adrenomedullin influences magnocellular and parvocellular neurons of paraventricular nucleus via separate mechanisms

We previously reported that adrenomedullin (AM) decreases blood pressure following microinjection into the paraventricular nucleus of the hypothalamus (PVN) of the rat. With the use of whole cell recordings in rat hypothalamic slice preparations, we characterized the effects of AM on electrophysiologically identified PVN neurons and described the membrane events underlying such actions. AM hyperpolarized magnocellular (type I) neurons in a dose-dependent manner, a response associated with an increase in the frequency and amplitude of inhibitory postsynaptic potentials. Blockade of action potentials with tetrodotoxin (TTX) abolished AM effects on membrane potential and synaptic activity in magnocellular neurons, suggesting direct actions on inhibitory interneurons. Furthermore, blockade of inhibitory synaptic transmission with the GABA(A) receptor antagonist bicuculline methiodide also abolished AM effects on membrane potential in magnocellular neurons. In contrast, parvocellular (type II) neurons depolarized following AM receptor activation. AM effects on parvocellular neurons were dose dependent and were maintained in the presence of TTX, indicating direct effects on this population of neurons. Voltage-clamp recordings from parvocellular neurons showed AM enhances a nonselective cationic conductance, suggesting a potential mechanism through which AM influences membrane potential. These observations show clear population-specific actions of AM on separate identified groups of PVN neurons. Such effects on magnocellular neurons likely contribute to the hypotensive actions of this peptide in PVN. Although the effects on parvocellular neurons may also contribute to such cardiovascular effects of AM, it is more likely that actions on this population of PVN neurons underlie the previously demonstrated activational effects of AM on the hypothalamic-pituitary-adrenal axis.

Orexin-A depolarizes dissociated rat area postrema neurons through activation of a nonselective cationic conductance

The area postrema (AP) is involved in the regulation of body fluid balance, feeding behavior, and cardiovascular function. Orexin (ORX)-A is a 33 aa peptide that regulates energy metabolism and sympathetic and cardiovascular actions. ORX immunoreactive axons and their varicose terminals have been found in AP. In this study, whole-cell, current- or voltage-clamp recordings were obtained from 108 dissociated rat AP neurons. The mean resting membrane potential of these neurons (n = 48) was -59.24 +/- 0.87 mV, the mean input resistance was 3.57 +/- 0.22 G(Omega), and the action potential amplitude of these cells was always >90 mV. Current-clamp studies showed bath application of ORX-A depolarized the majority of AP neurons tested (68.8%; 33 of 48), whereas small proportions of cells were either hyperpolarized (16.7%; 8 of 48) or unaffected (14.6%; 7 of 48). These depolarizing effects were found to be concentration dependent from 10(-8) to 10(-11) m. We then examined the contributions of specific ionic conductances to the ORX-A-induced excitation of AP neurons through whole-cell, voltage-clamp studies. Our results demonstrate that in contrast to previous studies on other neuronal populations, ORX-A did not affect net whole-cell potassium currents in AP neurons. Slow depolarizing voltage ramps, however, revealed that ORX-A enhanced a nonselective cationic conductance in AP neurons, effects which would explain the depolarizing effects of the peptide. These data demonstrate that AP neurons are directly influenced by ORX-A and suggest that ORX-A may exert its effects on the central control of feeding behavior and cardiovascular function through direct actions in AP.

Integration of sodium and osmosensory signals in vasopressin neurons

Vasopressin (antidiuretic hormone) release has been thought to be controlled by interacting osmoreceptors and Na(+)-detectors for > 20 years. Only recently, however, have molecular and cellular advances revealed how changes in the external concentration of Na+ and osmolality are detected during acute and chronic osmotic perturbations. In rat vasopressin-containing neurons, local osmosensitivity is conferred by intrinsic stretch-inactivated cation channels and by taurine release from surrounding glia. Na+ detection is accomplished by acute regulation of the permeability of stretch-inactivated channels and by changes in Na+ channel gene expression. These features provide a first glimpse of the integrative processes at work in a central osmoregulatory reflex.

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.

Responses of magnocellular neurons to osmotic stimulation involves coactivation of excitatory and inhibitory input: an experimental and theoretical analysis

How does a neuron, challenged by an increase in synaptic input, display a response that is independent of the initial level of activity? Here we show that both oxytocin and vasopressin cells in the supraoptic nucleus of normal rats respond to intravenous infusions of hypertonic saline with gradual, linear increases in discharge rate. In hyponatremic rats, oxytocin and vasopressin cells also responded linearly to intravenous infusions of hypertonic saline but with much lower slopes. The linearity of response was surprising, given both the expected nonlinearity of neuronal behavior and the nonlinearity of the oxytocin secretory response to such infusions. We show that a simple computational model can reproduce these responses well, but only if it is assumed that hypertonic infusions coactivate excitatory and inhibitory synaptic inputs. This hypothesis was tested first by applying the GABA(A) antagonist bicuculline to the dendritic zone of the supraoptic nucleus by microdialysis. During local blockade of GABA inputs, the response of oxytocin cells to hypertonic infusion was greatly enhanced. We then went on to directly measure GABA release in the supraoptic nucleus during hypertonic infusion, confirming the predicted rise. Together, the results suggest that hypertonic infusions lead to coactivation of excitatory and inhibitory inputs and that this coactivation may confer appropriate characteristics on the output behavior of oxytocin cells. The nonlinearity of oxytocin secretion that accompanies the linear increase in oxytocin cell firing rate reflects frequency-facilitation of stimulus-secretion coupling at the neurohypophysis.

Responses of magnocellular neurons to osmotic stimulation involves coactivation of excitatory and inhibitory input: an experimental and theoretical analysis

How does a neuron, challenged by an increase in synaptic input, display a response that is independent of the initial level of activity? Here we show that both oxytocin and vasopressin cells in the supraoptic nucleus of normal rats respond to intravenous infusions of hypertonic saline with gradual, linear increases in discharge rate. In hyponatremic rats, oxytocin and vasopressin cells also responded linearly to intravenous infusions of hypertonic saline but with much lower slopes. The linearity of response was surprising, given both the expected nonlinearity of neuronal behavior and the nonlinearity of the oxytocin secretory response to such infusions. We show that a simple computational model can reproduce these responses well, but only if it is assumed that hypertonic infusions coactivate excitatory and inhibitory synaptic inputs. This hypothesis was tested first by applying the GABA(A) antagonist bicuculline to the dendritic zone of the supraoptic nucleus by microdialysis. During local blockade of GABA inputs, the response of oxytocin cells to hypertonic infusion was greatly enhanced. We then went on to directly measure GABA release in the supraoptic nucleus during hypertonic infusion, confirming the predicted rise. Together, the results suggest that hypertonic infusions lead to coactivation of excitatory and inhibitory inputs and that this coactivation may confer appropriate characteristics on the output behavior of oxytocin cells. The nonlinearity of oxytocin secretion that accompanies the linear increase in oxytocin cell firing rate reflects frequency-facilitation of stimulus-secretion coupling at the neurohypophysis.

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.

Agonist activation of cytosolic Ca2+ in subfornical organ cells projecting to the supraoptic nucleus

The subfornical organ (SFO) is sensitive to both ANG II and ACh, and local application of these agents produces dipsogenic responses and vasopressin release. The present study examined the effects of cholinergic drugs, ANG II, and increased extracellular osmolarity on dissociated, cultured cells of the SFO that were retrogradely labeled from the supraoptic nucleus. The effects were measured as changes in cytosolic calcium in fura 2-loaded cells by using a calcium imaging system. Both ACh and carbachol increased intracellular ionic calcium concentration ([Ca2+]i). However, in contrast to the effects of muscarinic receptor agonists on SFO neurons, manipulation of the extracellular osmolality produced no effects, and application of ANG II produced only moderate effects on [Ca2+]i in a few retrogradely labeled cells. The cholinergic effects on [Ca2+]i could be blocked with the muscarinic receptor antagonist atropine and with the more selective muscarinic receptor antagonists pirenzepine and 4-diphenylacetoxy-N-methylpiperdine methiodide (4-DAMP). In addition, the calcium in the extracellular fluid was required for the cholinergic-induced increase in [Ca2+]i. These findings indicate that ACh acts to induce a functional cellular response in SFO neurons through action on a muscarinic receptor, probably of the M1 subtype and that the increase of [Ca2+]i, at least initially, requires the entry of extracellular Ca2+. Also, consistent with a functional role of M1 receptors in the SFO are the results of immunohistochemical preparations demonstrating M1 muscarinic receptor-like protein present within this forebrain circumventricular organ.

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 regulation of neuronal activity: a new role for taurine and glial cells in a hypothalamic neuroendocrine structure

Maintenance of osmotic pressure is a primary regulatory process essential for normal cell function. The osmolarity of extracellular fluids is regulated by modifying the intake and excretion of salts and water. A major component of this regulatory process is the neuroendocrine hypothalamo-neurohypophysial system, which consists of neurons located in the paraventricular and supraoptic nuclei. These neurons synthesize the neurohormones vasopressin and oxytocin and release them in the blood circulation. We here review the mechanisms responsible for the osmoregulation of the activity of these neurons. Notably, the osmosensitivity of the supraoptic nucleus is described including the recent data that suggests an important participation of taurine in the transmission of the osmotic information. Taurine is an amino acid mainly known for its involvement in cell volume regulation, as it is one of the major inorganic osmolytes used by cells to compensate for changes in extracellular osmolarity. In the supraoptic nucleus, taurine is highly concentrated in astrocytes, and released in an osmodependent manner through volume-sensitive anion channels. Via its agonist action on neuronal glycine receptors, taurine is likely to contribute to the inhibition of neuronal activity induced by hypotonic stimuli. This inhibitory influence would complement the intrinsic osmosensitivity of supraoptic neurons, mediated by excitatory mechanoreceptors activated under hypertonic conditions. These observations extend the role of taurine from the regulation of cell volume to that of the whole body fluid balance. They also point to a new role of supraoptic glial cells as active components in a neuroendocrine regulatory loop.

Hypo-osmolarity stimulates and high sodium concentration inhibits thyrotropin-releasing hormone secretion from rat hypothalamus

The hypothalamic paraventricular nucleus, representing cell bodies in which thyrotropin-releasing hormone is synthesized, and the median eminence, representing nerve terminals, were incubated in vitro. Various hypo- and hyperosmotic solutions were tested to determine osmotic sensitivity of thyrotropin-releasing hormone secretion. High KCl (56 mM) causing membrane depolarization was used as a non-specific control stimulus to induce thyrotropin-releasing hormone secretion. A 30% decrease of medium osmolarity (from 288 to 202 mOsmol/l) increased thyrotropin-releasing hormone secretion from both the paraventricular nucleus and median eminence. A 30% decrease of medium NaCl content by its replacement with choline chloride did not affect basal thyrotropin-releasing hormone secretion. Increasing medium osmolarity with biologically inactive L-glucose did not affect basal or KCl-induced thyrotropin-releasing hormone secretion from either structure. Medium made hyperosmotic (350-450 mOsmol/l) by increasing the NaCl concentration resulted in a dose-dependent decrease of basal thyrotropin-releasing hormone secretion and abolished KCl-induced thyrotropin-releasing hormone secretion. If an osmotically equivalent amount of choline chloride was substituted for NaCl, there was no effect on thyrotropin-releasing hormone secretion, indicating a specific action of Na+. This study indicates a specific sensitivity to high concentrations of Na+ ions of both thyrotropin-releasing hormone-producing parvocellular paraventricular neurons and thyrotropin-releasing hormone-containing nerve terminals in the median eminence.

Osmoregulation of vasopressin neurons: a synergy of intrinsic and synaptic processes

The release of vasopressin into the general circulation varies as a function of plasma osmolality and therefore plays a major role in systemic osmoregulation. In vivo, the secretion of this hormone in the neurohypophysis is primarily determined by the rate of action potential discharge of the magnocellular neurosecretory cells (MNCs) in the hypothalamus. Experiments done over the past 20 years have clarified much of the neurophysiological basis underlying this important osmoregulatory reflex. As discussed here, recent findings indicate that the regulation of the firing rate of MNCs during changes in systemic osmolality involves the concerted modulation of mechanosensitive ion channels in MNCs, as well as excitatory glutamatergic inputs derived from forebrain regions such as the organum vasculosum of the lamina terminalis.

Electrophysiological distinctions between oxytocin and vasopressin neurons in the supraoptic nucleus

Oxytocin and vasopressin neurons can be differentiated from one another, and from neurons in the immediately adjacent perinuclear zone, by their electrophysiological properties. In both sexes, oxytocin and vasopressin neurons are characterized by a prominent transient outward rectification which is conspicuously lacking in most perinuclear neurons. In addition, perinuclear neurons, some of which project to the supraoptic nucleus, exhibit a transient depolarization which underlies short bursts of spikes. Oxytocin neurons are characterized by: 1) the presence of a sustained outward rectifier above -50 mV, active below spike threshold; 2) a rebound depolarization following deactivation of the sustained rectification which can sustain short spike trains; and 3) a smaller transient outward rectification, probably associated with the potassium current, Ia. Vasopressin neurons show little of the sustained outward rectification and rebound depolarization, but have a stronger transient outward rectification. Although both cell types exhibit depolarizing afterpotentials, in vasopressin neurons these lead to plateau potentials underlying prolonged discharges. In oxytocin neurons, the depolarizing potential usually sustains a short spike discharge, but less often leads to prolonged bursts. These data suggest that the intrinsic properties of oxytocin and vasopressin neurons lead to quantitatively different forms of burst discharges, both of which may facilitate hormone release.

Effects of osmotic pressure and brain-derived neurotrophic factor on the survival of postnatal hypothalamic oxytocinergic and vasopressinergic neurons in dissociated cell culture

Neurons from hypothalamic paraventricular nuclei (PVN) and supraoptic nuclei (SON) from postnatal day 6-8 rats were enzymatically dissociated and separately maintained in monolayer cultures for 14 days. The osmotic pressure of the culture medium, based on Neurobasal medium (Life Technologies), was varied (255, 300 and 330 mOsm/l) by adjustment using mannitol. The survival of oxytocin (OT), vasopressin (VP) and oxytocin-vasopressin (OT/VP) coexpressing neurons were studied under these varied conditions, and the identification of the cell phenotypes in the cultures was carried out by using double-label immunofluorescence. Under control osmolar conditions (300 mOsm/l) equivalent numbers of OT and VP neurons were found in the SON (P = 0.8398) and PVN (P = 0.4721) cultures. The OT neurons' survival did not change in 255 or 330 mOsm media in the SON cultures, but the VP neurons in the SON cultures were significantly increased in 255 mOsm/l medium as compared to control (300 mOsm/l) medium (P = 0.0088). No significant changes were found in VP neuron survival in SON cultures between the 300-330 mOsm/l media (P = 0.2372). Similar data were obtained for the VP neurons in PVN-derived cultures, but the OT neurons in these cultures survived significantly better at 300 mOs/l than at 255 mOsm/l (P<0.0001), but were not significantly different at 330 mOsm/l (P = 0.1208). In general, the VP neurons were more vulnerable than OT neurons to increases of culture medium osmolarity with respect to their survival. The number of OT/VP coexpressing neurons was greater in SON-derived cell cultures as compared to PVN-derived cell cultures, and their numbers were higher in the lower osmolarity media. The effects of adding brain-derived neurotrophic factor (BDNF) to the culture medium on survival were determined. BDNF significantly increased the numbers of all three types of neurons in both PVN and SON cell cultures (P = 0.0001-0.0060). The phenotypically identified cells, cultured in the 300 mOsm/l medium, responded by depolarization or hyperpolarization when transferred to hypertonic or hypotonic perfusion salines, respectively.

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.