[Arg8]-vasopressin-induced increase in intracellular Ca2+ concentration in cultured rat hippocampal neurons

Alpha-1 antichymotrypsin is involved in astrocyte injury in concert with arginine-vasopressin during the development of acute hepatic encephalopathy

Background and aims

We developed a bio-artificial liver (BAL) using a radial-flow bioreactor and rescued mini-pig models with lethal acute liver failure (ALF). The point of the rescue is the recovery from hepatic encephalopathy (HE). HE on ALF has sometimes resulted in brain death following brain edema with astrocyte swelling. Several factors, including ammonia and glutamine, have been reported to induce astrocyte swelling and injury. However, many clinicians believe that there are any other factors involved in the development of HE. Therefore, the aim of this study was to identify novel HE-inducible factors, particularly those inducing astrocyte dysfunction.

Methods

Mini-pig plasma samples were collected at three time points: before the administration of toxins (α-amanitin and LPS), when HE occurred after the administration of toxins, and after treatment with extracorporeal circulation (EC) by the BAL. To identify the causative factors of HE, each plasma sample was subjected to a comparative proteome analysis with two-dimensional gel electrophoresis and mass spectrometry. To assess the direct effects of candidate factors on the astrocyte function and injury, in vitro experiments with human astrocytes were performed.

Results

Using a proteome analysis, we identified alpha-1 antichymotrypsin (ACT), which was increased in plasma samples from mini-pigs with HE and decreased in those after treatment with EC by BAL. In in vitro experiments with human astrocytes, ACT showed growth-inhibitory and cytotoxic effects on astrocytes. In addition, the expression of water channel protein aquaporin-4, which is induced in injured astrocytes, was increased following ACT treatment. Interestingly, these effects of ACT were additively enhanced by adding arginine-vasopressin (AVP) and were canceled by adding an AVP receptor antagonist.

Conclusions

These results suggest that ACT is involved in astrocyte injury and dysfunction in concert with AVP during the development of acute HE.

Selective V(1a) agonism attenuates vascular dysfunction and fluid accumulation in ovine severe sepsis

Vasopressin analogs are used as a supplement to norepinephrine in septic shock. The isolated effects of vasopressin agonists on sepsis-induced vascular dysfunction, however, remain controversial. Because V(2)-receptor stimulation induces vasodilation and procoagulant effects, a higher V(1a)- versus V(2)-receptor selectivity might be advantageous. We therefore hypothesized that a sole, titrated infusion of the selective V(1a)-agonist Phe(2)-Orn(8)-Vasotocin (POV) is more effective than the mixed V(1a)-/V(2)-agonist AVP for the treatment of vascular and cardiopulmonary dysfunction in methicillin resistant staphylococcus aureus pneumonia-induced, ovine sepsis. After the onset of hemodynamic instability, awake, chronically instrumented, mechanically ventilated, and fluid resuscitated sheep were randomly assigned to receive continuous infusions of either POV, AVP, or saline solution (control; each n = 6). AVP and POV were titrated to maintain mean arterial pressure above baseline - 10 mmHg. When compared with that of control animals, AVP and POV reduced neutrophil migration (myeloperoxidase activity, alveolar neutrophils) and plasma levels of nitric oxide, resulting in higher mean arterial pressures and a reduced vascular leakage (net fluid balance, chest and abdominal fluid, pulmonary bloodless wet-to-dry-weight ratio, alveolar and septal edema). Notably, POV stabilized hemodynamics at lower doses than AVP. In addition, POV, but not AVP, reduced myocardial and pulmonary tissue concentrations of 3-nitrotyrosine, VEGF, and angiopoietin-2, thereby leading to an abolishment of cumulative fluid accumulation (POV, 9 ± 15 ml/kg vs. AVP, 110 ± 13 ml/kg vs. control, 213 ± 16 ml/kg; P < 0.001 each) and an attenuated cardiopulmonary dysfunction (left ventricular stroke work index, PaO(2)-to-FiO(2) ratio) versus control animals. Highly selective V(1a)-agonism appears to be superior to unselective vasopressin analogs for the treatment of sepsis-induced vascular dysfunction.

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.

Dual modulation of inward rectifier potassium currents in olfactory neuronal cells by promiscuous G protein coupling of the oxytocin receptor

Oxytocin receptor is a seven transmembrane receptor widely expressed in the CNS that triggers G(i) or G(q) protein-mediated signaling cascades leading to the regulation of a variety of neuroendocrine and cognitive functions. We decided to investigate whether and how the promiscuous receptor/G protein coupling affects neuronal excitability. As an experimental model, we used the immortalized gonadotropin-releasing hormone-positive GN11 cell line displaying the features of immature, migrating olfactory neurons. Using RT-PCR analysis, we detected the presence of oxytocin receptors whose stimulation by oxytocin led to the accumulation of inositol phosphates and to the inhibition of cell proliferation, and the expression of several inward rectifier (IR) K+ channel subtypes. Moreover, electrophysiological and pharmacological inspections using whole-cell patch-clamp recordings evidenced that in GN11 cells, IR channel subtypes are responsive to oxytocin. In particular, we found that: (i) peptide activation of receptor either inhibited or stimulated IR conductances, and (ii) IR current inhibition was mediated by a pertussis toxin-resistant G protein presumably of the G(q/11) subtype, and by phospholipase C, whereas IR current activation was achieved via receptor coupling to a pertussis toxin-sensitive G(i/o) protein. The findings suggest that neuronal excitability might be tuned by a single peptide receptor that mediates opposing effects on distinct K+ channels through the promiscuous coupling to different G proteins.

Arginine vasopressin prevents against Abeta(25-35)-induced impairment of spatial learning and memory in rats

Amyloid beta protein (Abeta) is thought to be responsible for loss of memory in Alzheimer's disease (AD). A significant decrease in [Arg(8)]-vasopressin (AVP) has been found in the AD brain and in plasma; however, it is unclear whether this decrease in AVP is involved in Abeta-induced impairment of spatial cognition and whether AVP can protect against Abeta-induced deficits in cognitive function. The present study examined the effects of intracerebroventricular (i.c.v.) injection of AVP on spatial learning and memory in the Morris water maze test and investigated the potential protective function of AVP against Abeta-induced impairment in spatial cognition. The results were as follows: (1) i.c.v. injection of 25 nmol Abeta(25-35) resulted in a significant decline in spatial learning and memory; (2) 1 nmol and 10 nmol, but not 0.1 nmol, AVP injections markedly improved learning and memory; (3) pretreatment with 1 nmol or 10 nmol, but not 0.1 nmol, AVP effectively reversed the impairment in spatial learning and memory induced by Abeta(25-35); and (4) none of the drugs, including Abeta(25-35) and different concentrations of AVP, affected the vision or swimming speed of the rats. These results indicate that Abeta(25-35) could significantly impair spatial learning and memory in rats, and pretreatment with AVP centrally can enhance spatial learning and effectively prevent the behavioral impairment induced by neurotoxic Abeta(25-35). Thus, the present study provides further insight into the mechanisms by which Abeta impairs spatial learning and memory, suggesting that up-regulation of central AVP might be beneficial in the prevention and treatment of AD.

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.

Ca++ influx is essential for the hypotensive response to arginine vasopressin-induced neuron activation of the area postrema in the rat

We attempted to relate the signal pathway to the hypotension induced by arginine vasopressin (AVP) injection into the area postrema (AP) in urethane-anesthetized and ventilated rats with vagotomy. A femoral artery and vein were catheterized to measure the blood pressure (BP) and administer drugs, respectively. The rat was placed on a stereotaxic apparatus to expose the calamus sriptorius (CS) by craniostomy and maintained at normocapnia in hyperoxia. In protocol 1, hypotension evoked by AVP (3.0 x 10(-5) IU) microinjected into the AP 0.2 mm rostral to the CS of the midline was abolished by V(1A) antagonist, U73122 (phospholipase C blocker), and BAPTA-AM (Ca(++) chelator), suggesting that an increasing intracellular Ca(++) is essential for AVP-induced hypotension. In protocol 2, AVP-induced hypotension was abolished by EGTA (extracellular Ca(++) chelator) and Ca(++) blockers such as nifedipine, nimodipine (L-types), and omega-conotoxin MVIIC (P/Q-type), but not by omega-conotoxin GVIA (N-type). In protocol 3, AVP-induced hypotension was blocked by calphostin C (protein kinase C inhibitor) and mimicked by an increase in intracellular K(+) ions that was reversed by EGTA. Vehicle injections produced no changes in BP. In protocol 4, glutamate-induced hypotension was reversed by BAPTA-AM but not by EGTA or V(1A) antagonist. Our data suggest that AVP-induced hypotension depends on Ca(++) influx through a signal pathway from phospholipase C to protein kinase C which inactivates K(+) channels that may depolarize AP neurons to activate L- and P/Q-type Ca(++) channels. This may provide new insights into establishing a relationship between the signal pathway and physiological functions.

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.

Effects of vasopressin on isolated rat adrenal chromaffin cells

It has been demonstrated that arginine vasopressin (AVP) is synthesized not only in specific hypothalamic nuclei, but also in the adrenal medulla where it is thought to regulate adrenal functions by autocrine and paracrine mechanisms. In order to further characterise the effects of AVP on rat adrenal chromaffin cells, we examined: (a) the mRNA expression for V(1a) and V(1b) AVP receptors in these cells; (b) the effects of AVP on the membrane potential and membrane currents measured with the whole-cell patch-clamp technique; and (c) effect of AVP on catecholamine release from single adrenal chromaffin cells measured with carbon fibre microelectrodes. Reverse transcription-polymerase chain reaction (RT-PCR) on tissue punch samples obtained from the adrenal medulla demonstrated message for both the V(1a) and V(1b) receptors, while material obtained from the adrenal cortex showed expression of the V(1a) receptor only. Single-cell RT-PCR conducted on acutely isolated chromaffin cells showed message for the V(1a) receptor in 84% of cells, while 38% of cells also contained message for the V(1b) receptor (n=45). Under current-clamp recording, responses to AVP application (4-40 microM) were variable; 22/34 (65%) tested cells were depolarised, 29% hyperpolarised, and the remaining cells showed a biphasic response. Changes in membrane potential of either direction were dose-dependent and accompanied by a decrease in cell membrane resistance. Under voltage-clamp (V(hold)=-60 mV), AVP evoked inward current in 27/52 (52%) and outward current in 16/52 (31%) chromaffin cells. Both types of AVP-evoked responses were blocked by co-application of a nonselective V(1a)/V(1b) antagonist. Application of AVP evoked prolonged bursts of amperometric currents (indicative of catecholamine release) in 4/9 tested cells, but reduced the currents evoked by ACh application in all tested cells (n=7). These findings demonstrate a complex action of AVP on adrenal chromaffin cells, with individual adrenal chromaffin cells responding with either excitation or inhibition. This response pattern may be related to the expression of V(1) receptor subtypes.

Electrophysiological evidence for vasopressin V(1) receptors on neonatal motoneurons, premotor and other ventral horn neurons

Prominent arginine-vasopressin (AVP) binding and AVP V(1) type receptors are expressed early in the developing rat spinal cord. We sought to characterize their influence on neural excitability by using patch-clamp techniques to record AVP-induced responses from a population of motoneurons and interneurons in neonatal (5-18 days) rat spinal cord slices. Data were obtained from 58 thoracolumbar (T(7)-L(5)) motoneurons and 166 local interneurons. A majority (>90%) of neurons responded to bath applied AVP (10 nM to 3 microM) and (Phe(2), Orn(8))-vasotocin, a V(1) receptor agonist, but not V(2) or oxytocin receptor agonists. In voltage-clamp, postsynaptic responses in motoneurons were characterized by slowly rising, prolonged (7-10 min) and tetrodotoxin-resistant inward currents associated with a 25% reduction in a membrane potassium conductance that reversed near -100 mV. In interneurons, net AVP-induced inward currents displayed three patterns: decreasing membrane conductance with reversal near -100 mV, i.e., similar to that in motoneurons (24 cells); increasing conductance with reversal near -40 mV (21 cells); small reduction in conductance with no reversal within the current range tested (41 cells). A presynaptic component recorded in most neurons was evident as an increase in the frequency but not amplitude (in motoneurons) of inhibitory and excitatory postsynaptic currents (IPSCs and EPSCs), in large part due to AVP-induced firing in inhibitory (mainly glycinergic) and excitatory (glutamatergic) neurons synapsing on the recorded cells. An increase in frequency but not amplitude of miniature IPSCs and EPSCs also indicated an AVP enhancement of neurotransmitter release from axon terminals of inhibitory and excitatory interneurons. These observations provide support for a broad presynaptic and postsynaptic distribution of AVP V(1) type receptors and indicate that their activation can enhance the excitability of a majority of neurons in neonatal ventral spinal cord.

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.

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.

Effects of metabolic fragments of [Arg(8)]-vasopressin on nerve growth in cultured hippocampal neurons

The effects of metabolic fragments of [Arg(8)]-vasopressin (AVP), [pGlu(4), Cyt(6)]AVP (AVP(4-9)), and desglycinamide-[pGlu(4), Cyt(6)]AVP (AVP(4-8)) on the growth of hippocampal neurons in culture were investigated in comparison with those of AVP. AVP(4-9) caused a significant increase in filopodial length following 96 h of exposure at concentrations higher than 300 nM. AVP(4-9) was more potent than AVP. AVP(4-8) also induced an increase in filopodial length, but this effect was less than that of AVP. The selective V(1) agonist [Phe(2), Ile(3), Orn(8)]-vasopressin caused a significant increase in filopodial length, whereas the selective V(2) agonist [deamino-Cys(1), D-Arg(8)]-vasopressin showed no such effect. OPC-21268, a vasopressin V(1) antagonist, blocked AVP and AVP fragment-induced increases in filopodial length. However, the V(2) antagonist OPC-31260 showed no such effect. A23187, a representative Ca ionophore, also increased filopodial length, and the A23187-induced increase in filopodial length was potentiated by AVP and AVP fragments. These results indicated that AVP(4-9) and AVP(4-8) increased filopodial length in cultured hippocampal neurons by activating V(1) receptors. Both phenomena induced by AVP(4-9) and AVP(4-8) were associated with intracellular calcium mobilization.