Characterization of vasopressin receptors in cultured cells derived from the region of rat brain circumventricular organs

Norepinephrine Inhibits Lipopolysaccharide-Stimulated TNF-α but Not Oxylipin Induction in n-3/n-6 PUFA-Enriched Cultures of Circumventricular Organs

Sensory circumventricular organs (sCVOs) are pivotal brain structures involved in immune-to-brain communication with a leaky blood-brain barrier that detect circulating mediators such as lipopolysaccharide (LPS). Here, we aimed to investigate the potential of sCVOs to produce n-3 and n-6 oxylipins after LPS-stimulation. Moreover, we investigated if norepinephrine (NE) co-treatment can alter cytokine- and oxylipin-release. Thus, we stimulated rat primary neuroglial sCVO cultures under n-3- or n-6-enriched conditions with LPS or saline combined with NE or vehicle. Supernatants were assessed for cytokines by bioassays and oxylipins by HPLC-MS/MS. Expression of signaling pathways and enzymes were analyzed by RT-PCR. Tumor necrosis factor (TNF)α bioactivity and signaling, IL-10 expression, and cyclooxygenase (COX)2 were increased, epoxide hydroxylase (Ephx)2 was reduced, and lipoxygenase 15-(LOX) was not changed by LPS stimulation. Moreover, LPS induced increased levels of several n-6-derived oxylipins, including the COX-2 metabolite 15d-prostaglandin-J2 or the Ephx2 metabolite 14,15-DHET. For n-3-derived oxylipins, some were down- and some were upregulated, including 15-LOX-derived neuroprotectin D1 and 18-HEPE, known for their anti-inflammatory potential. While the LPS-induced increase in TNFα levels was significantly reduced by NE, oxylipins were not significantly altered by NE or changes in TNFα levels. In conclusion, LPS-induced oxylipins may play an important functional role in sCVOs for immune-to-brain communication.

Sensory Circumventricular Organs, Neuroendocrine Control, and Metabolic Regulation

The central nervous system is critical in metabolic regulation, and accumulating evidence points to a distributed network of brain regions involved in energy homeostasis. This is accomplished, in part, by integrating peripheral and central metabolic information and subsequently modulating neuroendocrine outputs through the paraventricular and supraoptic nucleus of the hypothalamus. However, these hypothalamic nuclei are generally protected by a blood-brain-barrier limiting their ability to directly sense circulating metabolic signals—pointing to possible involvement of upstream brain nuclei. In this regard, sensory circumventricular organs (CVOs), brain sites traditionally recognized in thirst/fluid and cardiovascular regulation, are emerging as potential sites through which circulating metabolic substances influence neuroendocrine control. The sensory CVOs, including the subfornical organ, organum vasculosum of the lamina terminalis, and area postrema, are located outside the blood-brain-barrier, possess cellular machinery to sense the metabolic interior milieu, and establish complex neural networks to hypothalamic neuroendocrine nuclei. Here, evidence for a potential role of sensory CVO-hypothalamic neuroendocrine networks in energy homeostasis is presented.

Physiology of Astroglia

Astrocytes are principal cells responsible for maintaining the brain homeostasis. Additionally, these glial cells are also involved in homocellular (astrocyte-astrocyte) and heterocellular (astrocyte-other cell types) signalling and metabolism. These astroglial functions require an expression of the assortment of molecules, be that transporters or pumps, to maintain ion concentration gradients across the plasmalemma and the membrane of the endoplasmic reticulum. Astrocytes sense and balance their neurochemical environment via variety of transmitter receptors and transporters. As they are electrically non-excitable, astrocytes display intracellular calcium and sodium fluctuations, which are not only used for operative signalling but can also affect metabolism. In this chapter we discuss the molecules that achieve ionic gradients and underlie astrocyte signalling.

Persistent cytosolic Ca2+ increase induced by angiotensin II at nanomolar concentrations in acutely dissociated subfornical organ (SFO) neurons of rats

It is known that angiotensin II (AII) is sensed by subfornical organ (SFO) to induce drinking behaviors and autonomic changes. AII at picomolar concentrations have been shown to induce Ca²⁺ oscillations and increase in the amplitude and frequency of spontaneous Ca²⁺ oscillations in SFO neurons. The present study was conducted to examine effects of nanomolar concentrations of AII using the Fura-2 Ca²⁺-imaging technique in acutely dissociated SFO neurons. AII at nanomolar concentrations induced an initial [Ca²⁺]_i peak followed by a persistent [Ca²⁺]_i increase lasting for longer than 1 hour. By contrast, [Ca²⁺]_i responses to 50 mM K⁺, maximally effective concentrations of glutamate, carbachol, and vasopressin, and AII given at picomolar concentrations returned to the basal level within 20 min. The AII-induced [Ca²⁺]_i increase was blocked by the AT1 antagonist losartan. However, losartan had no effect when added during the persistent phase. The persistent phase was suppressed by extracellular Ca2+ removal, significantly inhibited by blockers of L and P/Q type Ca²⁺ channels , but unaffected by inhibition of Ca²⁺ store Ca²⁺ ATPase. The persistent phase was reversibly suppressed by GABA and inhibited by CaMK and PKC inhibitors. These results suggest that the persistent [Ca²⁺]_i increase evoked by nanomolar concentrations of AII is initiated by AT1 receptor activation and maintained by Ca²⁺ entry mechanisms in part through L and P/Q type Ca²⁺ channels, and that CaMK and PKC are involved in this process. The persistent [Ca²⁺]_i increase induced by AII at high pathophysiological levels may have a significant role in altering SFO neuronal functions.

Astrocytes in neuroendocrine systems: An overview

A class of glial cell, astrocytes, is highly abundant in the central nervous system (CNS). In addition to maintaining tissue homeostasis, astrocytes regulate neuronal communication and synaptic plasticity. There is an ever-increasing appreciation that astrocytes are involved in the regulation of physiology and behaviour in normal and pathological states, including within neuroendocrine systems. Indeed, astrocytes are direct targets of hormone action in the CNS, via receptors expressed on their surface, and are also a source of regulatory neuropeptides, neurotransmitters and gliotransmitters. Furthermore, as part of the neurovascular unit, astrocytes can regulate hormone entry into the CNS. This review is intended to provide an overview of how astrocytes are impacted by and contribute to the regulation of a diverse range of neuroendocrine systems: energy homeostasis and metabolism, reproduction, fluid homeostasis, the stress response and circadian rhythms.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

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

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

Haemodynamic and renal sympathetic responses to V1b vasopressin receptor activation within the paraventricular nucleus

NEW FINDINGS

What is the central question of this study? Does antagonism of V1b receptors prevent the haemodynamic and renal sympathetic nerve responses that occur with application of exogenous vasopressin into the paraventricular nucleus (PVN) of conscious, chronically instrumented rats? What is the main finding and its importance? Microinjection of vasopressin into the PVN increased mean arterial pressure, heart rate and renal sympathetic nerve activity, all of which were inhibited by pre-injection of the PVN with the V1b antagonist, nelivaptan. The administered vasopressin did not enter the peripheral circulation or increase plasma vasopressin. Ganglionic blockade prevented each of the responses, consistent with mediation by enhanced sympathetic output rather than an increase in circulating vasopressin. Vasopressin (VP) participates in regulation of haemodynamics and volume. Besides more classical actions as a circulating hormone, VP may act via release from axons and dendrites within the CNS. The paraventricular nucleus (PVN) possesses vasopressinergic neurons and a dense complement of VP receptors, including the V1b receptor, which has been implicated in several types of stress responses. We tested the hypothesis that antagonism of V1b receptors will prevent VP-induced increases in mean arterial pressure (MAP), heart rate (HR) and renal sympathetic nerve activity (RSNA). Studies were performed in conscious male Sprague-Dawley rats chronically instrumented with vascular catheters, renal nerve electrodes and a cannula stereotaxically directed into the PVN. Unilateral microinjection of VP into the PVN significantly increased MAP, HR and RSNA, peaking at 10 min. Pre-injection of the PVN with the selective V1b receptor antagonist, nelivaptan, did not alter baseline values but blocked the responses to VP. Ganglionic blockade with chlorisondamine decreased MAP and HR and abolished their increase in response to subsequent PVN application of VP. Injection of VP into the PVN did not alter plasma VP levels. Paraventricular nucleus injection with radiolabelled VP resulted in negligible radiolabelled VP in peripheral blood. These findings support the concept that, in basal conditions, PVN V1b receptor activation (rather than VP release into the periphery) may be implicated in the increases in MAP, HR and RSNA due to increased sympathetic outflow. While the role of V1a and oxytocin receptors cannot be excluded, these data suggest that further studies of the role of V1b receptor activation by endogenous VP during stress to effect neuroexcitation are warranted.

Is oxytocin a maternal-foetal signalling molecule at birth? Implications for development

The neuropeptide oxytocin was first noted for its capacity to promote uterine contractions and facilitate delivery in mammals. The study of oxytocin has grown to include awareness that this peptide is a neuromodulator with broad effects throughout the body. Accumulating evidence suggests that oxytocin is a powerful signal to the foetus, helping to prepare the offspring for the extrauterine environment. Concurrently, the use of exogenous oxytocin or other drugs to manipulate labour has become common practice. The use of oxytocin to expedite labour and minimise blood loss improves both infant and maternal survival under some conditions. However, further investigations are needed to assess the developmental consequences of changes in oxytocin, such as those associated with pre-eclampsia or obstetric manipulations associated with birth. This review focuses on the role of endogenous and exogenous oxytocin as a neurochemical signal to the foetal nervous system. We also examine the possible developmental consequences, including those associated with autism spectrum disorder, that arise from exogenous oxytocin supplementation during labour.

Neuropeptide transmission in brain circuits

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

Excitatory action of vasopressin in the brain of the rat: role of cAMP signaling

Brain vasopressin plays a role in behavioral and cognitive functions and in pathological conditions. Relevant examples are pair bonding, social recognition, fear responses, stress disorders, anxiety and depression. At the neuronal level, vasopressin exerts its effects by binding to V1a receptors. In the brainstem, vasopressin can excite facial motoneurons by generating a sustained inward current which is sodium-dependent, tetrodotoxin-insensitive and voltage-gated. This effect is independent of intracellular calcium mobilization and is unaffected by phospholipase Cβ (PLCβ) or protein kinase C (PKC) inhibitors. There are two major unsolved problems. (i) What is the intracellular signaling pathway activated by vasopressin? (ii) What is the exact nature of the vasopressin-sensitive cation channels? We performed recordings in brainstem slices. Facial motoneurons were voltage-clamped in the whole-cell configuration. We show that a major fraction, if not the totality, of the peptide effect was mediated by cAMP signaling and that the vasopressin-sensitive cation channels were directly gated by cAMP. These channels appear to exclude lithium, are suppressed by 2-aminoethoxydiphenylborane (2-APB) and flufenamic acid (FFA) but not by ruthenium red or amiloride. They are distinct from transient receptor channels and from cyclic nucleotide-regulated channels involved in visual and olfactory transduction. They present striking similarities with cation channels present in a variety of molluscan neurons. To our knowledge, the presence in mammalian neurons of channels having these properties has not been previously reported. Our data should contribute to a better knowledge of the neural mechanism of the central actions of vasopressin, and may be potentially significant in view of clinical applications.

Overview of cellular electrophysiological actions of vasopressin

The nonapeptide vasopressin acts both as a hormone and as a neurotransmitter/neuromodulator. As a hormone, its target organs include kidney, blood vessels, liver, platelets and anterior pituitary. As a neurotransmitter/neuromodulator, vasopressin plays a role in autonomic functions, such as cardiovascular regulation and temperature regulation and is involved in complex behavioral and cognitive functions, such as sexual behavior, pair-bond formation and social recognition. At the neuronal level, vasopressin acts by enhancing membrane excitability and by modulating synaptic transmission. The present review will focus on the electrophysiological effects of vasopressin at the cellular level. A large proportion of the experiments summarized here have been performed in in vitro systems, especially in brain and spinal cord slices of the rat. Vasopressin exerts a powerful excitatory action on motoneurons of young rats and mice. It acts by generating a cationic inward current and/or by reducing a potassium conductance. In addition, vasopressin enhances the inhibitory synaptic input to motoneurons. By virtue of these actions, vasopressin may regulate the functioning of neuronal networks involved in motor control. In the amygdala, vasopressin can directly excite a subpopulation of neurons, whereas oxytocin, a related neuropeptide, can indirectly inhibit these same neurons. In the lateral septum, vasopressin exerts a similar dual action: it excites directly a neuronal subpopulation, but causes indirect inhibition of virtually all lateral septal neurons. The actions of vasopressin in the amygdala and lateral septum may represent at least part of the neuronal substrate by which vasopressin influences fear and anxiety-related behavior and social recognition, respectively. Central vasopressin can modulate cardiovascular parameters by causing excitation of spinal sympathetic preganglionic neurons, by increasing the inhibitory input to cardiac parasympathetic neurons in the nucleus ambiguus, by depressing the excitatory input to parabrachial neurons, or by inhibiting glutamate release at solitary tract axon terminals. By acting in or near the hypothalamic supraoptic nucleus, vasopressin can influence magnocellular neuron activity, suggesting that the peptide may exert some control on its own release at neurohypophyseal axon terminals. The central actions of vasopressin are mainly mediated by receptors of the V(1A) type, although recent studies have also reported the presence of vasopressin V(1B) receptors in the brain. Major unsolved problems are: (i) what is the transduction pathway activated following stimulation of central vasopressin V(1A) receptors? (ii) What is the precise nature of the cation channels and/or potassium channels operated by vasopressin? (iii) Does vasopressin, by virtue of its second messenger(s), interfere with other neurotransmitter/neuromodulator systems? In recent years, information concerning the mechanism of action of vasopressin at the neuronal level and its possible role and function at the whole-animal level has been accumulating. Translation of peptide actions at the cellular level into autonomic, behavioral and cognitive effects requires an intermediate level of integration, i.e. the level of neuronal circuitry. Here, detailed information is lacking. Further progress will probably require the introduction of new techniques, such as targeted in vivo whole-cell recording, large-scale recordings from neuronal ensembles or in vivo imaging in small animals.

Vasopressin and angiotensin receptors of the medial septal area of the brain in the control of thirst and salt appetite induced by vasopressin in water-deprived and sodium-depleted rats

In this study we investigated the influence of d(CH(2))(5)-Tyr (Me)-AVP (A(1)AVP) and [Adamanteanacatyl(1),D-ET-D-Tyr(2), Val(4), aminobutyril(6),A(8,9)]-AVP (A(2)AVP), antagonists of V(1) and V(2) arginine(8)-vasopressin (AVP) receptors, respectively, as well as the effects of losartan and CGP42112A, antagonists of angiotensin II (ANGII) AT(1) and AT(2,) receptors, respectively, on water and 0.3 M sodium intake induced by water deprivation or sodium depletion (furosemide treatment) and enhanced by AVP injected into the medial septal area (MSA). A stainless steel cannula was implanted into the medial septal area (MSA) of male Holtzman rats AVP injection enhanced water and sodium intake in a dose-dependent manner. Pretreatment with V(1) antagonist injected into the MSA produced a dose-dependent reduction, whereas prior injection of V(2) antagonist increased, in a dose-dependent manner, the water and sodium responses elicited by the administration of AVP. Both AT(1) and AT(2) antagonists administered into the MSA elicited a concentration-dependent decrease in water and sodium intake induced by AVP, while simultaneous injection of the two antagonists was more effective in decreasing AVP responses. These results also indicate that the increase in water and sodium intake induced by AVP was mediated primarily by MSA AT(1) receptors.

The vasopressin-induced excitation of hypoglossal and facial motoneurons in young rats is mediated by V1a but not V1b receptors, and is independent of intracellular calcium signalling

As a hormone, vasopressin binds to three distinct receptors: V1a and V1b receptors, which induce phospholipase-Cbeta (PLCbeta) activation and Ca2+ mobilization; and V2 receptors, which are coupled to adenylyl cyclase. V1a and V1b receptors are also present in neurons. In particular, hypoglossal (XII) and facial (VII) motoneurons are excited following vasopressin-V1a receptor binding. The aim of the present study was double: (i) to determine whether V1b receptors contribute to the excitatory effect of vasopressin in XII and VII motoneurons; and (ii) to establish whether the action of vasopressin on motoneurons is mediated by Ca2+ signalling. Patch-clamp recordings were performed in brainstem slices of young rats. Vasopressin depolarized the membrane or generated an inward current. By contrast, [1-deamino-4-cyclohexylalanine] arginine vasopressin (d[Cha4]AVP), a V1b agonist, had no effect. The action of vasopressin was suppressed by Phaa-D-Tyr(Et)-Phe-Gln-Asn-Lys-Pro-Arg-NH2, a V1a antagonist, but not by SSR149415, a V1b antagonist. Thus, the vasopressin-induced excitation of brainstem motoneurons was exclusively mediated by V1a receptors. Light microscopic autoradiography failed to detect V1b binding sites in the facial nucleus. In motoneurons loaded with GTP-gamma-S, a non-hydrolysable analogue of GTP, the effect of vasopressin was suppressed, indicating that neuronal V1a receptors are G-protein-coupled. Intracellular Ca2+ chelation suppressed a Ca2+-activated potassium current, but did not affect the vasopressin-evoked current. H7 and GF109203, inhibitors of protein kinase C, were without effect on the vasopressin-induced excitation. U73122 and D609, PLCbeta inhibitors, were also without effect. Thus, excitation of brainstem motoneurons by V1a receptor activation is probably mediated by a second messenger distinct from that associated with peripheral V1a receptors.

Fear and power-dominance motivation: proposed contributions of peptide hormones present in cerebrospinal fluid and plasma

We propose that fear and power-dominance drive motivation are generated by the presence of elevated plasma and cerebrospinal fluid (CSF) levels of certain peptide hormones. For the fear drive, the controlling hormone is corticotropin releasing factor, and we argue that elevated CSF and plasma levels of this peptide which occur as a result of fear-evoking and other stressful experiences in the recent past are detected and transduced into neuronal activities by neurons in the vicinity of the third ventricle, primarily in the periventricular and arcuate hypothalamic nuclei. For the power-dominance drive, we propose that the primary signal is the CSF concentration of vasopressin, which is detected in two circumventricular organs, the subfornical organ and organum vasculosum of the lamina terminalis. We suggest that the peptide-generated signals detected in periventricular structures are transmitted to four areas in which neuronal activities represent fear and power-dominance: one in the medial hypothalamus, one in the dorsolateral quadrant of the periaqueductal gray matter, a third in the midline thalamic nuclei, and the fourth within medial prefrontal cortex. The probable purpose of this system is to maintain a state of fear or anger and consequent vigilant or aggressive behavior after the initial fear- or anger-inducing stimulus is no longer perceptible. We further propose that all the motivational drives, including thirst, hunger and sexual desire are generated in part by non-steroidal hormonal signals, and that the unstimulated motivational status of an individual is determined by the relative CSF and plasma levels of several peptide hormones.

Brain vasopressin and sodium appetite

Intraventricular injections of vasopressin (VP) and antagonists with varying degrees of specificity for the VP receptors were used to identify the action of endogenous brain VP on 0.3 M NaCl intake by sodium-deficient rats. Lateral ventricular injections of 100 ng and 1 microg VP caused barrel rotations and a dramatic decrease in NaCl intake by sodium-deficient rats and suppressed sucrose intake. Intraventricular injection of the V(1)/V(2) receptor antagonist [d(CH(2))(5)(1),O-Et-Tyr(2),Val(4), Arg(8)]VP and the V(1) receptor antagonist [d(CH(2))(5)(1),O-Me-Tyr(2),Arg(8)]VP (MeT-AVP) significantly suppressed NaCl intake by sodium-deficient rats without causing motor disturbances. MeT-AVP had no effect on sucrose intake (0.1 M). In contrast, the selective V(2) receptor antagonist had no significant effect on NaCl intake. Last, injections of 100 ng MeT-AVP decreased mean arterial blood pressure (MAP), whereas 100 ng VP elevated MAP and pretreatment with MeT-AVP blocked the pressor effect of VP. These results indicate that the effects produced by 100 ng MeT-AVP represent receptor antagonistic activity. These findings suggest that the effect of exogenous VP on salt intake is secondary to motor disruptions and that endogenous brain VP neurotransmission acting at V(1) receptors plays a role in the arousal of salt appetite.

The hypothalamus and hypertension

Most forms of hypertension are associated with a wide variety of functional changes in the hypothalamus. Alterations in the following substances are discussed: catecholamines, acetylcholine, angiotensin II, natriuretic peptides, vasopressin, nitric oxide, serotonin, GABA, ouabain, neuropeptide Y, opioids, bradykinin, thyrotropin-releasing factor, vasoactive intestinal polypeptide, tachykinins, histamine, and corticotropin-releasing factor. Functional changes in these substances occur throughout the hypothalamus but are particularly prominent rostrally; most lead to an increase in sympathetic nervous activity which is responsible for the rise in arterial pressure. A few appear to be depressor compensatory changes. The majority of the hypothalamic changes begin as the pressure rises and are particularly prominent in the young rat; subsequently they tend to fluctuate and overall to diminish with age. It is proposed that, with the possible exception of the Dahl salt-sensitive rat, the hypothalamic changes associated with hypertension are caused by renal and intrathoracic cardiopulmonary afferent stimulation. Renal afferent stimulation occurs as a result of renal ischemia and trauma as in the reduced renal mass rat. It is suggested that afferents from the chest arise, at least in part, from the observed increase in left auricular pressure which, it is submitted, is due to the associated documented impaired ability to excrete sodium. It is proposed, therefore, that the hypothalamic changes in hypertension are a link in an integrated compensatory natriuretic response to the kidney's impaired ability to excrete sodium.

Vasopressin- and oxytocin-induced activity in the central nervous system: electrophysiological studies using in-vitro systems

During the last two decades, it has become apparent that vasopressin and oxytocin, in addition to playing a role as peptide hormones, also act as neurotransmitters/neuromodulators. A number of arguments support this notion: (i) vasopressin and oxytocin are synthesized not only in hypothalamo-neurohypophysial cells, but also in other hypothalamic and extrahypothalamic cell bodies, whose axon projects to the limbic system, the brainstem and the spinal cord. (ii) Vasopressin and oxytocin can be shed from central axons as are classical neurotransmitters. (iii) Specific binding sites, i.e. membrane receptors having high affinity for vasopressin and oxytocin are present in the central nervous system. (iv) Vasopressin and oxytocin can alter the firing rate of selected neuronal populations. (v) In-situ injection of vasopressin and oxytocin receptor agonists and antagonists can interfere with behavior or physiological regulations. Morphological studies and electrophysiological recordings have evidenced a close anatomical correlation between the presence of vasopressin and oxytocin receptors in the brain and the neuronal responsiveness to vasopressin or oxytocin. These compounds have been found to affect membrane excitability in neurons located in the limbic system, hypothalamus, circumventricular organs, brainstem, and spinal cord. Sharp electrode intracellular recordings and whole-cell recordings, done in brainstem motoneurons or in spinal cord neurons, have revealed that vasopressin and oxytocin can directly affect neuronal excitability by opening non-specific cationic channels or by closing K(+) channels. These neuropeptides can also influence synaptic transmission, by acting either postsynaptically or upon presynaptic target neurons or axon terminals. Whereas, in cultured neurons, vasopressin and oxytocin appear to mobilize intracellular Ca(++), in brainstem slices, the action of oxytocin is mediated by a second messenger that is distinct from the second messenger activated in peripheral target cells. In this review, we will summarize studies carried out at the cellular level, i.e. we will concentrate on in-vitro approaches. Vasopressin and oxytocin will be treated together. Though acting via distinct receptors in distinct brain areas, these two neuropeptides appear to exert similar effects upon neuronal excitability.

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.

Involvement of non-neuronal brain cells in AVP-mediated regulation of water space at the cellular, organ, and whole-body level

Vasopressin (AVP) influences non-neuronal brain cells in cell-type specific manners: (1) it regulates water balance at the cellular level of brain parenchyma by adjusting astrocytic water permeability; (2) it contributes to the control of extracellular K(+) concentration ([K(+)](e)) in brain by stimulation of K(+) transfer from blood to brain, due to activation of an inwardly directed Na(+),K(+),Cl(-) cotransporter at the luminal membrane of capillary endothelial cells and opening of K(+) channels at their abluminal membrane; (3) it decreases formation of cerebrospinal fluid (CSF) by decreasing Cl(-) secretion into CSF by epithelial cells of the choroid plexus, probably by inhibition of Cl(-)/HCO(-)(3) exchange at their basolateral membrane; (4) it contributes to regulation of intracellular volume within the brain by regulation of water permeability in ependymal cells and subpial astrocytes; and (5) it exerts effects on specialized astrocytes in circumventricular organs, their adjacent glia limitans, and the neural pituitary, which regulate AVP release to the systemic circulation by altering the spatial relationship between neurons and their adjacent glial cells. A unified mechanism is proposed, which integrates most of the effects of AVP and may be of considerable importance for neuronal excitability and, thus, for behavior.

Evidence that atypical vasopressin V(2) receptor in inner medulla of kidney is V(1B) receptor

Vasopressin V(2) receptors at high-density and V(1B) receptors are candidates for the V(2)-like receptor, which evokes an increase in [Ca(2+)](i) when stimulated by the vasopressin V(2) receptor agonist 1-desamino-8-D-arginine vasopressin (dDAVP) in kidney inner medullary collecting duct. We compared the pharmacological characteristics of vasopressin V(2) and V(1B) receptors in Chinese hamster ovary (CHO) cells to those of vasopressin V(2)-like receptors in rat inner medullary collecting duct cells. The vasopressin V(1B) receptor-selective agonist [deamino-Cys(1), D-3-(Pyridyl)-Ala(2), Arg(8)]vasopressin (D3PVP) did not stimulate the [Ca(2+)](i) increase in high-density vasopressin V(2) receptor-expressing CHO cells, but did in inner medullary collecting duct cells. Moreover, the vasopressin V(1A)/V(2) receptor dual antagonist 4'-[(2-methyl-1,4,5,6-tetrahydroimidazo[4,5-d][1] benzazepin-6-yl)carbonyl] 2-phenylbenzanilide (YM087), which has no effect on vasopressin V(1B) receptors, did not block the [Ca(2+)](i) increase in inner medullary collecting duct cells when stimulated by dDAVP and D3PVP. On reverse transcription-polymerase chain reaction (RT-PCR) analysis of kidney, vasopressin V(1B) receptor mRNA was detected only in the medulla. We propose that the true nature of the vasopressin V(2)-like receptor in the inner medullary collecting duct is the vasopressin V(1B) receptor, rather than the vasopressin V(2) receptor expressed at high-density.

Astrocytes in sensory circumventricular organs of the rat brain express functional binding sites for endothelin

Sensory circumventricular organs bordering the anterior third cerebral ventricle, the subfornical organ and the organum vasculosum laminae terminalis, lack blood-brain barrier characteristics and are therefore accessible to circulating peptides like endothelins. Astrocytes of the rat subfornical organ and organum vasculosum laminae terminalis additionally showed immunocytochemical localization of endothelin-1/endothelin-3-like peptides, possibly acting as circumventricular organ-intrinsic modulators. Employing [125I]endothelin-1 as radioligand, quantitative autoradiography demonstrated specific binding sites throughout the rat organum vasculosum laminae terminalis and subfornical organ, and competitive displacement studies revealed expression of both ET(A) and ET(B) receptor subtypes for either circumventricular organ. ET(B) receptor binding prevailed for the whole brain and ET(A) receptors could be labelled in the peripheral vascular system. To characterize endothelin-specific receptors in astrocytes of both circumventricular organs, alterations in the intracellular calcium concentration due to endothelin-1/endothelin-3 stimulation were studied in primary culture of subfornical organ and organum vasculosum laminae terminalis cells obtained from early postnatal rat pups. Endothelin-1 and endothelin-3 induced Ca(2+) transients in 9-13% of either subfornical organ or organum vasculosum laminae terminalis astrocytes, respectively, and some glial cells (subfornical organ: 2%, organum vasculosum laminae terminalis: 5%) responded to both endothelin analogues. The antagonistic action of BQ123 specific for ET(A) receptors (74% of all astrocytes tested), and the pronounced responsiveness to the ET(B) receptor agonist [4Ala]ET-1 (subfornical organ: 27%, organum vasculosum laminae terminalis: 35%) demonstrated glial expression of both endothelin receptor subtypes. Agonist-induced elevations in the intracellular calcium concentration proved to be independent of extracellular Ca(2+). In summary, the results indicate that endothelin(s) interact(s) with circumventricular organ astrocytes. Competitive receptor binding techniques using brain tissue sections as well as a fura-2 loaded primary cell culture system of the subfornical organ and organum vasculosum laminae terminalis demonstrate glial expression of functional ET(A) and ET(B) receptors, with calcium as intracellular messenger emerging primarily from intracellular stores. Endothelin(s) of both circulating and circumventricular organ-intrinsic origin may afferently transfer information important for cardiovascular homeostasis to circumventricular organs serving as "windows to the brain".

Vasopressin increases [Ca(2+)](i) in differentiated astrocytes by activation of V1b/V3 receptors but has no effect in mature cortical neurons

Vasopressin (AVP) plays an important role in regulation of astrocytic, but not neuronal, water content and cell volume during hydro-osmotic challenge. To investigate the intracellular mechanism(s) signaling this response, [Ca(2+)](i) was measured fluorometrically in cultured cerebrocortical astrocytes and neurons, obtained from neonatal and fetal mouse brains, and matured during the culturing period. In astrocytes, [Ca(2+)](i) increased with an EC(50) of between 10(-10) and 10(-9) M AVP, the maximum increase was approximately 100 nM, and the response was independent of extracellular Ca(2+), identifying the receptor as being of the V1b/V3 subtype. In contrast, AVP had no effect on [Ca(2+)](i) in cortical neurons. This cellular difference is consistent with the ability of AVP to increase water permeability in astrocytes but not in neurons.

Intracellular calcium signalling in magnocellular neurones of the rat supraoptic nucleus: understanding the autoregulatory mechanisms

Oxytocin and vasopressin, released at the soma and dendrites of neurones, bind to specific autoreceptors and induce an increase in [Ca2+]i. In oxytocin cells, the increase results from a mobilisation of Ca2+ from intracellular stores, whereas in vasopressin cells, it results mainly from an influx of Ca2+ through voltage-dependent channels. The response to vasopressin is coupled to phospholipase C and adenylyl-cyclase pathways which are activated by V1 (V1a and V1b)- and V2-type receptors respectively. Measurements of [Ca2+]i in response to V1a and V2 agonists and antagonists suggest the functional expression of these two types of receptors in vasopressin neurones. The intracellular mechanisms involved are similar to those observed for the action of the pituitary adenylyl-cyclase-activating peptide (PACAP). Isolated vasopressin neurones exhibit spontaneous [Ca2+]i oscillations and these are synchronised with phasic bursts of electrical activity. Vasopressin modulates these spontaneous [Ca2+]i oscillations in a manner that depends on the initial state of the neurone, and such varied effects of vasopressin may be related to those observed on the electrical activity of vasopressin neurones in vivo.

V1a- and V2-type vasopressin receptors mediate vasopressin-induced Ca2+ responses in isolated rat supraoptic neurones

1. The pharmacological profile of receptors activated by vasopressin (AVP) in freshly dissociated supraoptic magnocellular neurones was investigated using specific V1a- and V2-type AVP receptor agonists and antagonists. 2. In 97 % of AVP-responding neurones (1-3000 nM) V1a or V2 receptor type agonists (F-180 and dDAVP, respectively) elicited dose-dependent [Ca2+]i transients that were suppressed by removal of external Ca2+. 3. The [Ca2+]i response induced by 1 microM F-180 or dDAVP was selectively blocked by 10 nM of V1a and V2 antagonists (SR 49059 and SR 121463A, respectively). The response to V1a agonist was maintained in the presence of the V2 antagonist, and the V2 agonist-induced response persisted in the presence of the V1a antagonist. 4. The [Ca2+]i response induced by 1 microM AVP was partially (61 %) blocked by 10 nM SR 121463A. This blockade was increased by a further 31 % with the addition of 10 nM SR 49059. Similarly, the AVP-induced response was partially (47 %) decreased by SR 49059, and a further inhibition of 33 % was achieved in the presence of SR 121463A. 5. We demonstrate that AVP acts on the magnocellular neurones via two distinct types of AVP receptors that exhibit the pharmacological profiles of V1a and V2 types. However, since V2 receptor mRNA is not expressed in the supraoptic nucleus (SON), and since V1b receptor transcripts are observed in the SON, we propose that the V2 receptor agonist and antagonist act on a 'V2-like' receptor or a new type of AVP receptor that remains to be elucidated. The possibility that V2 ligands act on the V1b receptor cannot be excluded.

Neuronal actions of oxytocin on the subfornical organ of male rats

The aim of this study was to investigate effects of oxytocin (OT) on electrical neuronal activities in rat subfornical organ (SFO) and compare its action with the well-described excitatory effects of blood-borne angiotensin II (ANG II) on the same SFO neurons. With the use of extracellular recordings from spontaneously active neurons in slice preparations of the SFO of male rats, 11.7% of tested neurons (n = 206) were excited and 9.7% were inhibited by superfusion with 10(-6) M OT. Both excitatory and inhibitory effects of OT were dose dependent with similar threshold concentrations and were blocked by a specific OT-receptor antagonist but not by a vasopressin receptor antagonist. Blocking synaptic transmission with low calcium medium suppressed only inhibitory effects of OT. All but one of the OT-sensitive neurons were also excited by superfusion with ANG II at a concentration much lower than required for OT, suggesting that synaptically released OT rather than blood-borne OT alters the activity of SFO neurons in vivo. The results support the hypothesis that neurally released OT may modulate SFO-mediated functions by acting on OT-sensitive neurons.

Vasopressin acting at V1-type receptors produces membrane depolarization in neonatal rat spinal lateral column neurons

Vasopressin-immunoreactive fibers have been visualized in the area of spinal lateral horn cells, including spinal sympathetic preganglionic neurons. The presence and nature of vasopressin receptors on neurons in this area were addressed using whole-cell patch-clamp techniques in transverse spinal cord slice preparations from neonatal rat. Bath applications of Arg8-vasopressin (VP) induced a slow-onset membrane depolarization accompanied by spike discharges and membrane oscillations. In voltage-clamp, applications of VP induced a reversible, tetrodotoxin-resistant and dose-dependent inward current in 90% of tested cells. This effect was blocked by a V1 receptor antagonist [D-(CH2)5 Tyr (Me)-VP], whereas a V2 receptor agonist [desamino-(D-Arg8)-vasopressin] was ineffective. Furthermore the applications of oxytocin produced significantly smaller depolarizations when compared with VP suggesting that, at least in the neonatal lateral horn cells, vasopressin rather than oxytocin is more effective ligand. Both the amplitude and duration of the VP effect were enhanced after intracellular dialysis with GTP-gamma-S, a non-hydrolyzable GTP analogue, whereas the inward current was significantly reduced after intracellular dialysis with GDP-beta-S, a stable analogue of GDP that competitively inhibits G-proteins. The observation that the VP-induced net inward current reversed at a potential close to the equilibrium for potassium ions and was associated with a decrease in membrane conductance in a majority of tested cells suggest mediation through closure of a leak potassium conductance. These data indicate that SPNs and other lateral horn cells possess functional G-protein-coupled V1-type vasopressin receptors that, in adult spinal cord, may contribute to CNS regulation of autonomic nervous system function.

Vasopressin and oxytocin action in the brain: cellular neurophysiological studies

During the last two decades it has become apparent that vasopressin (VP) and oxytocin (OT), in addition to playing a role as peptide hormones, also act as neurotransmitters. Morphological studies and electrophysiological recordings have shown a close anatomical correlation between the presence of these receptors and the neuronal responsiveness to VP or OT. These compounds have been found to affect membrane excitability in neurons located in the hippocampus, hypothalamus, lateral septum, brainstem, spinal cord and superior cervical ganglion. Sharp electrode intracellular and whole-cell recordings, done in brainstem motoneurons, have revealed that VP and OT can directly affect neuronal excitability by opening non-specific cationic channels. These neuropeptides can also influence synaptic transmission, by acting either postsynaptically or upon presynaptic target neurons or axon terminals. Whereas in some hypothalamic neurons OT appears to mobilize intracellular calcium, as revealed by calcium imaging techniques, in the brainstem the action of this neuropeptide is mediated by a second messenger which is distinct from the second messenger activated in peripheral target cells. Future studies should be aimed at elucidating the properties of the cationic channels responsible for the neuronal action of VP and OT, at identifying the brain-specific second messengers activated by these neuropeptides and at determining whether endogenous VP and OT can exert neuronal effects similar to those elicited by exogenous neuropeptides.

Vasopressin binding sites in the central nervous system: distribution and regulation

High affinity binding sites for vasopressin (VP) are widely distributed within the rat brain and spinal cord. Since their presence is associated with neuronal sensitivity to VP application, their anatomical distribution maps structures which could be activated by endogenous VP. Interestingly, marked species-related differences of the VP receptor distribution have been revealed. Some evidence has also been provided that mechanisms of receptor regulation may vary among species. In the rat, the expression of VP binding sites in some motor nuclei shows remarkable plasticity, in particular up-regulation after axotomy. These data suggest that VP may, in addition to affecting motoneuronal excitability, act as a trophic factor onto motoneurones.

Vasopressin and sensory circumventricular organs

The subfornical organ, the area postrema and the organum vasculosum of the lamina terminalis are considered to be sensory circumventricular organs as they contain neuronal somata which are located outside the blood-brain barrier and are thus capable of serving as 'sensors' for blood-borne humoral messengers. The endocrine hormone, vasopressin (VP), not only causes strong antidiuresis by acting on the kidney, but also exerts centrally mediated effects as a neuromodulator. Several lines of evidence suggest that VP can influence regulatory functions mediated by the sensory circumventricular organs, since vasopressinergic somata and terminals as well as VP receptors have been reposted to be present in these structures. These biochemical prerequisites offer the possibility that blood-borne VP might on the one hand act as a feedback signal from the periphery and, on the other hand, synaptically released or locally produced VP could modulate the known functions of sensory circumventricular organs, such as thirst, fever or cardiovascular regulation. This review focuses on the possible physiological relevance of VP acting on sensory circumventricular organs in view of recent evidence obtained from biochemical and electrophysiological studies at the cellular level.

Vasopressin's depolarizing action on neonatal rat spinal lateral horn neurons may involve multiple conductances

Vasopressin-immunoreactive fibers have been visualized in the area of spinal lateral horn cells, including spinal sympathetic preganglionic neurons (SPNs). The presence and nature of vasopressin receptors on 125 neurons in this area were addressed using whole-cell patch-clamp techniques in transverse spinal cord slice preparations from neonatal rat (11-21 days). Local pressure applications of Arg-vasopressin (AVP, 1 microM) induced a slow-onset membrane depolarization accompanied by spike discharges and membrane oscillations. In voltage-clamp, applications of AVP (10 nM-1 microM) induced a reversible, tetrodotoxin-resistant and dose-dependent inward current in 90% of tested cells. This effect was blocked by a V1 receptor antagonist [D-(CH2)5 Tyr (Me)-AVP], whereas a V2 receptor agonist [desamino-(D-Arg8)-vasopressin] was ineffective. Both the amplitude and duration of the AVP effect were significantly modified after intracellular dialysis of non-hydrolysable G-protein modulators. I-V relationships, examined in 75 cells, suggested two conductances. In 36 cells the net AVP current reversed approximately -102 mV, was associated with a decrease in membrane conductance and yielded linear I-V plots, suggesting mediation through closure of a resting potassium conductance. In a further 26 cells the I-V lines remained almost parallel in the voltage range used in this study (-130 to -40 mV), while the membrane conductance was decreased in a majority of these cells. In the remaining 13 cells the net AVP current was estimated to reverse approximately -30 mV and was associated with a small increase in membrane conductance, suggesting mediation through opening of a nonselective cationic conductance. These data indicate that the majority of SPNs and other lateral horn cells possess functional G-protein-coupled V1-type vasopressin receptors in the neonatal spinal cord. In the adult spinal cord, some of these receptors are likely to participate in CNS regulation of autonomic nervous system function.

Characterization of ionic currents of cells of the subfornical organ that project to the supraoptic nuclei

The subfornical organ (SFO) is a forebrain structure that converts peripheral blood-borne signals reflecting the hydrational state of the body to neural signals and then through efferent fibers conveys this information to several central nervous system structures. One of the forebrain areas receiving input from the SFO is the supraoptic nucleus (SON), a source of vasopressin synthesis and control of release from the posterior pituitary. Little is known of the transduction and transmission processes by which this conversion of systemic information to brain input occurs. As a step in elucidating these mechanisms, the present study characterized the ionic currents of dissociated cells of the SFO that were identified as neurons that send efferents to the SON. A retrograde tracer was injected into the SON area in eleven-day-old rats. After three days for retrograde transport of the label, the SFOs of these animals were dissociated and plated for tissue culture. The retrograde tracer was used to identify the soma of SFO cells projecting to the SON so that voltage-dependent ionic currents using whole-cell voltage clamp methods could be studied. The three types of currents in labeled SFO neurons were characterized as a 1) rapid, transient inward current that can be blocked by tetrodotoxin (TTX) characteristic of a sodium current; 2) slow-onset sustained outward current that can be blocked by tetraethylammonium (TEA) characteristic of a delayed rectifier potassium current; and 3) remaining outward current that has a rapid-onset and transient characteristic of a potassium A-type current.

Replication-deficient adenovirus vector transfer of gfp reporter gene into supraoptic nucleus and subfornical organ neurons

The present studies used defined cells of the subfornical organ (SFO) and supraoptic nuclei (SON) as model systems to demonstrate the efficacy of replication-deficient adenovirus (Ad) encoding green fluorescent protein (GFP) for gene transfer. The studies investigated the effects of both direct transfection of the SON and indirect transfection (i.e., via retrograde transport) of SFO neurons. The SON of rats were injected with Ad (2 x 10(6) pfu) and sacrificed 1-7 days later for cell culture of the SON and of the SFO. In the SON, GFP fluorescence was visualized in both neuronal and nonneuronal cells while only neurons in the SFO expressed GFP. Successful in vitro transfection of cultured cells from the SON and SFO was also achieved with Ad (2 x 10(6) to 2 x 10(8) pfu). The expression of GFP in in vitro transfected cells was higher in nonneuronal (approximately 28% in SON and SFO) than neuronal (approximately 4% in SON and 10% in SFO) cells. The expression of GFP was time and viral concentration related. No apparent alterations in cellular morphology of transfected cells were detected and electrophysiological characterization of transfected cells was similar between GFP-expressing and nonexpressing neurons. We conclude that (1) GFP is an effective marker for gene transfer in living SON and SFO cells, (2) Ad infects both neuronal and nonneuronal cells, (3) Ad is taken up by axonal projections from the SON and retrogradely transported to the SFO where it is expressed at detectable levels, and (4) Ad does not adversely affect neuronal viability. These results demonstrate the feasibility of using adenoviral vectors to deliver genes to the SFO-SON axis.

Angiotensin II-induced calcium signalling in neurons and astrocytes of rat circumventricular organs

The subfornical organ and organum vasculosum laminae terminalis represent neuroglial circumventricular organ structures bordering the anterior third cerebral ventricle. Owing to the absence of the blood-brain barrier, the cellular elements of the subfornical organ and the organum vasculosum laminae terminalis can be reached by circulating messenger molecules transferring afferent information. As demonstrated for the control of extracellular fluid composition, the circulating hormone angiotensin II acts on these sensory circumventricular organs to induce drinking, elevated peripheral resistance and neurohypophyseal hormone release via interaction with membrane-spanning receptor proteins. To characterize the cell-specific distribution of angiotensin II receptors within the circumventricular organs, primary cell cultures derived from the subfornical organ or organum vasculosum laminae terminalis of five- to six-day-old rat pups were used to measure alterations in intracellular calcium at the single cell level. Neurons and astrocytes were identified by immunocytochemical staining for specific marker proteins. Bath application of angiotensin II (10(-10)-10(-6) M) dose-dependently induced calcium transients in neurons (19.6%) and astrocytes (15.7%), and angiotensin II threshold concentrations to elicit intracellular calcium signalling proved to be one order of magnitude higher in astrocytes as compared to neurons (10(-9) M). At angiotensin II concentrations higher than 10(-7) M, pronounced desensitization of the angiotensin II receptor occurred. Employing the angiotensin II receptor antagonists losartan (DUP-753; AT1-receptor) and PD-123319 (AT2-receptor), exclusive expression of the AT1 receptor subtype coupled to intracellular calcium concentration signalling could be demonstrated for neurons and astrocytes. In all cells examined, the angiotensin II-evoked increase in intracellular calcium concentrations could be fully suppressed in the absence of extracellular calcium. Co-activation by angiotensin II and other agents (vasopressin, its fragment 8-arginine-vasopressin(4-9), oxytocin, endothelin) was indicated for subfornical organ neurons and organum vasculosum laminae terminalis astrocytes.

Glial calcium: homeostasis and signaling function

Glial cells respond to various electrical, mechanical, and chemical stimuli, including neurotransmitters, neuromodulators, and hormones, with an increase in intracellular Ca2+ concentration ([Ca2+]i). The increases exhibit a variety of temporal and spatial patterns. These [Ca2+]i responses result from the coordinated activity of a number of molecular cascades responsible for Ca2+ movement into or out of the cytoplasm either by way of the extracellular space or intracellular stores. Transplasmalemmal Ca2+ movements may be controlled by several types of voltage- and ligand-gated Ca(2+)-permeable channels as well as Ca2+ pumps and a Na+/Ca2+ exchanger. In addition, glial cells express various metabotropic receptors coupled to intracellular Ca2+ stores through the intracellular messenger inositol 1,4,5-triphosphate. The interplay of different molecular cascades enables the development of agonist-specific patterns of Ca2+ responses. Such agonist specificity may provide a means for intracellular and intercellular information coding. Calcium signals can traverse gap junctions between glial cells without decrement. These waves can serve as a substrate for integration of glial activity. By controlling gap junction conductance, Ca2+ waves may define the limits of functional glial networks. Neuronal activity can trigger [Ca2+]i signals in apposed glial cells, and moreover, there is some evidence that glial [Ca2+]i waves can affect neurons. Glial Ca2+ signaling can be regarded as a form of glial excitability.

Heterogeneous actions of vasopressin on ANG II-sensitive neurons in the subfornical organ of rats

The aim of this study was to investigate the effects of the antidiuretic hormone arginine vasopressin (AVP), which is released in vivo during dehydration and hypovolemia to prevent further water loss, on the activity of neurons in the subfornical organ (SFO). The SFO is a brain structure with an open blood-brain barrier and is critically involved in angiotensin II (ANG II)-dependent water intake. SFO neurons were recorded extracellularly in tissue slices of the rat brain and were tested for responsiveness to AVP and ANG II. About one-half of 159 neurons tested with an AVP concentration of 10(-6) M in the superfusion medium were responsive, and approximately equal proportions were excited and inhibited. Neurons exhibiting the different response types did not differ from each other with respect to spontaneous discharge rate, latency, and duration of the response. Excitatory and inhibitory responses to AVP were dose dependent and reversible, and their threshold concentrations (10(-8) to 10(-9) M) were similar. Superfusion with a medium low in Ca2+ and high in Mg2+ showed that the excitatory effect is most likely direct, whereas the inhibitory effect largely depends on inhibitory synaptic interaction. About one-half of the SFO neurons excited by ANG II (10(-7) M) were responsive to AVP (10(-6) M), and equal proportions were inhibited and excited. Both excitatory and inhibitory AVP actions were blocked by the V1-receptor antagonist, Manning compound, and neurons responsive to AVP did not respond to the V2-receptor agonist [deamino-Cys1,D-Arg8]vasopressin. It is concluded that AVP, probably released from synaptic terminals, may increase or decrease the activity of neurons in the SFO, many of which are activated by ANG II. In contrast to previous experiments on ducks, in which the exclusively excitatory effect of the avian antidiuretic hormone arginine vasotocin on ANG II-sensitive SFO neurons correlates well with the dipsogenic effect of both peptides, a greater functional heterogeneity exists among AVP-responsive neurons in the rat SFO.

The oxytocin-induced inward current in vagal neurons of the rat is mediated by G protein activation but not by an increase in the intracellular calcium concentration

The neuropeptide oxytocin can depolarize parasympathetic preganglionic neurons in the dorsal motor nucleus of the vagus nerve of the rat by generating a sustained inward current, which is sodium-dependent and tetrodotoxin-insensitive. The second messenger activated by oxytocin receptor binding is, however, not yet known. In the present study, we attempted to characterize it by using the whole-cell recording technique and brainstem slices. When loaded with GTP-gamma-S, a non-hydrolysable analogue of GTP, vagal neurons generated a persistent inward current in the absence of agonist and the oxytocin effect was suppressed, suggesting that the peptide-evoked current was mediated by G-protein activation. Loading vagal neurons with the calcium chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N',-tetraacetic acid (BAPTA) suppressed a calcium-dependent, slowly decaying potassium aftercurrent but did not affect the oxytocin response, suggesting that the latter was not mediated by an agonist-induced increase in the intracellular calcium concentration. Protein kinase C (PKC) activation was probably not involved, since the peptide-evoked current was not modified by loading neurons with the PKC inhibitor H7. Thus, the oxytocin-evoked current in vagal neurons was probably not mediated by phospholipase C-beta (PLC-beta) activation. Loading neurons with 8-Br-cAMP or with an adenylyl cyclase activator (forskolin) reduced the oxytocin-evoked current by about half. SQ 22536, an adenylyl cyclase inhibitor, reduced this current by a similar amount. However, the peptide-evoked current was unaffected by Rp-cAMPS and Sp-cAMPS, an inhibitor and an activator, respectively, of cAMP-dependent protein kinase (PKA). We suggest that oxytocin activates two distinct signalling pathways in vagal neurons: one which is cAMP-dependent, but PKA-independent, and one, unidentified, which is PLC-beta-and cAMP-independent. Each pathway accounts for about half of the peptide effect and both appear to involve G-protein activation.

1-desamino-8-D-arginine vasopressin (DDAVP) as an agonist on V1b vasopressin receptor

1-desamino-8-D-arginine vasopressin (DDAVP) is considered a standard vasopressin V2 receptor-selective agonist with a potent antidiuretic effect through V2 receptor without the induction of vasoconstriction through V1a receptor. Furthermore, DDAVP was reported to act as an agonist on non-V1a, non-V2 receptor to cause the accumulation of intracellular Ca2+ in several tissues. However, the agonistic activity of DDAVP against the other vasopressin receptor, V1b (or V3), which can accumulate intracellular Ca2+ and which we recently cloned, has not been clarified. Hence, we compared the characteristics of DDAVP on V1b receptor with those on the other vasopressin receptors. In binding experiments, DDAVP more strongly inhibited [3H]arginine vasopressin binding to V1b than to V2 receptor (Ki: 5.84 nM vs 65.9 nM). In addition, DDAVP dose-dependently stimulated inositol turnover in human V1b receptor-expressing COS-1 cells. DDAVP acted as a full agonist on human V1b receptor (EC50: 11.4 nM) as well as on human V2 receptor (EC50: 23.9 nM). However, DDAVP behaved as a partial agonist toward rat V1b receptor (intrinsic activity: 0.7, EC50: 43.5 nM), while there was no significant difference in the agonistic properties of arginine vasopressin on human and rat V1b receptor. In conclusion, DDAVP acts as an agonist on V1b receptor, as it does on V2 receptor. These findings will allow us to better understand the physiological role of V1b receptor in pancreatic beta cells and in the renal inner medullary collecting duct, and help us to identify as yet unknown vasopressin receptors through which DDAVP cause the accumulation of intracellular Ca2+ in other tissues.

Facilitation of passive avoidance response by newly synthesized cationized arginine vasopressin fragment 4-9 in rats

The effects of a newly synthesized cationized arginine vasopressin fragment 4-9 analogue (C-AVP-(4-9)) on learning and memory in rats were studied by the passive avoidance test. C-AVP-(4-9) and its parent peptide, arginine vasopressin fragment 4-9 (AVP-(4-9)), a well known potent neuropeptide, were subcutaneously injected 1.5 hr prior to the retention test. The most effective doses of C-AVP-(4-9) and AVP-(4-9) were 8.6 x 10(-2) and 1.3 nmol/kg, respectively. To evaluate the distribution of C-AVP-(4-9) in the control nervous system (CNS), apparent tissue-plasma concentration rations (Kp.app) of intravenously administered radioiodinated C-AVP-(4-9) (125I-C-AVP-(4-9)) in the CNS in mice were determined. At the apparent steady state of plasma concentration of 125I-C-AVP-(4-9), the Kp.app values of the 125I-C-AVP-(4-9) in the cerebrum, cerebellum and spinal cord were over 12 times higher than that of the vascular space marker which slightly penetrates the BBB. Moreover, the rat cerebral homogenate converted C-AVP-(4-9) into its parent peptide AVP-(4-9). These results suggest that the potent effects of C-AVP-(4-9) on learning and memory may be due to AVP-(4-9) generated as a result of distribution and metabolism of peripherally administered C-AVP-(4-9) in the CNS.

Interaction of vasopressin and angiotensin II in central control of blood pressure and thirst

It is now well recognized that systemically released angiotensin II (Ang II) and arginine vasopressin (AVP) act in concert in regulation of blood pressure and water-electrolyte balance. Numerous studies have also demonstrated that centrally applied Ang II and AVP cause significant alterations of the cardiovascular functions and body fluid balance. Moreover, it has been established that Ang II and AVP are released in the central nervous system during cardiovascular and osmotic disorders and that the cardiovascular regions of the brainstem and the osmoregulatory regions of the forebrain are extensively innervated by the angiotensinergic and vasopressinergic neurons. Some evidence indicates that the angiotensinergic and vasopressinergic system may interact in the central blood pressure control, although the significance of this interaction may differ in various species. Recently, attempts have been made to find out whether centrally released Ang II and AVP may play a role in the regulation of the cardiovascular system under physiological and pathophysiological conditions. With regard to this, the available evidence strongly suggests that the both systems may be involved in regulation of blood pressure under baseline conditions. In addition, the vasopressinergic system appears to be involved in the adjustment of cardiovascular functions to hypovolemia, whereas its role in regulation of blood pressure during the osmotic disorders is less clear. Regulation of blood pressure and heart rate by centrally released AVP under baseline conditions, during hypovolemia and in osmotic disorders is significantly altered in the spontaneously hypertensive rats. It is now well established that centrally applied Ang II and Ang III are potent dipsogenic compounds. There also is evidence that AVP may enhance the osmotic thirst. However, the physiological role of brain-derived AVP and Ang II in the control of water intake awaits further examination. The available evidence from rat studies does not give support to a significant cooperation between central angiotensinergic and vasopressinergic system in regulation of water intake.

AVP-fragment peptides induce Ca2+ transients in cells cultured from rat circumventricular organs

To investigate receptors for the naturally occurring fragments of [Arg8]vasopressin (AVP), Ca2+ measurements were performed in cells cultured from the subfornical organ (SFO) and the organum vasculosum of the lamina terminalis (OVLT). AVP(4-9) and AVP(4-8) applied in nanomolar concentrations triggered rises in the intracellular Ca2+ concentration in neurons and astrocytes cultured from both circumventricular organs (CVOs) which are located in the lamina terminalis of the rat brain. The determination of the investigated cell type was confirmed by immunocytochemistry with cell type-specific antibodies. The functional data from single cultured cells and receptor autoradiographic studies on tissue slices favour the existence of specific receptors for AVP fragments which are different from those for the parental AVP.