Autoimmune inflammation triggers aberrant astrocytic calcium signaling to impair synaptic plasticity.

Cortical pathology involving inflammatory and neurodegenerative mechanisms is a hallmark of multiple sclerosis and a correlate of disease progression and cognitive decline. Astrocytes play a pivotal role in multiple sclerosis initiation and progression but astrocyte-neuronal network alterations contributing to gray matter pathology remain undefined. Here we unveil deregulation of astrocytic calcium signaling and astrocyte-to-neuron communication as key pathophysiological mechanisms of cortical dysfunction in the experimental autoimmune encephalomyelitis (EAE) model of multiple sclerosis. Using two-photon imaging ex vivo and fiber photometry in freely behaving mice, we found that acute EAE was associated with the emergence of spontaneously hyperactive cortical astrocytes exhibiting dysfunctional responses to cannabinoid, glutamate and purinoreceptor agonists. Abnormal astrocyte signaling by Gi and Gq protein coupled receptors was observed in the inflamed cortex. This was mirrored by treatments with pro-inflammatory factors both in vitro and ex vivo, suggesting cell-autonomous effects of the cortical neuroinflammatory environment. Finally, deregulated astrocyte calcium activity was associated with an enhancement of glutamatergic gliotransmission and a shift of astrocyte-mediated short-term and long-term plasticity mechanisms towards synaptic potentiation. Overall, our data identify astrocyte-neuronal network dysfunctions as key pathological features of gray matter inflammation in multiple sclerosis and potentially additional neuroimmunological disorders.

Tripartite synapses: glia, the unacknowledged partner.

According to the classical view of the nervous system, the numerically superior glial cells have inferior roles in that they provide an ideal environment for neuronal-cell function. However, there is a wave of new information suggesting that glia are intimately involved in the active control of neuronal activity and synaptic neurotransmission. Recent evidence shows that glia respond to neuronal activity with an elevation of their internal Ca2+ concentration, which triggers the release of chemical transmitters from glia themselves and, in turn, causes feedback regulation of neuronal activity and synaptic strength. In view of these new insights, this article suggests that perisynaptic Schwann cells and synaptically associated astrocytes should be viewed as integral modulatory elements of tripartite synapses.

Lateral regulation of synaptic transmission by astrocytes.

Fifteen years ago the concept of the "tripartite synapse" was proposed to conceptualize the functional view that astrocytes are integral elements of synapses. The signaling exchange between astrocytes and neurons within the tripartite synapse results in the synaptic regulation of synaptic transmission and plasticity through an autocrine form of communication. However, recent evidence indicates that the astrocyte synaptic regulation is not restricted to the active tripartite synapse but can be manifested through astrocyte signaling at synapses relatively distant from active synapses, a process termed lateral astrocyte synaptic regulation. This phenomenon resembles the classical heterosynaptic modulation but is mechanistically different because it involves astrocytes and its properties critically depend on the morphological and functional features of astrocytes. Therefore, the functional concept of the tripartite synapse as a fundamental unit must be expanded to include the interaction between tripartite synapses. Through lateral synaptic regulation, astrocytes serve as an active processing bridge for synaptic interaction and crosstalk between synapses with no direct neuronal connectivity, supporting the idea that neural network function results from the coordinated activity of astrocytes and neurons.

SNARE protein-dependent glutamate release from astrocytes.

We investigated the cellular mechanisms underlying the Ca(2+)-dependent release of glutamate from cultured astrocytes isolated from rat hippocampus. Using Ca(2+) imaging and electrophysiological techniques, we analyzed the effects of disrupting astrocytic vesicle proteins on the ability of astrocytes to release glutamate and to cause neuronal electrophysiological responses, i.e., a slow inward current (SIC) and/or an increase in the frequency of miniature synaptic currents. We found that the Ca(2+)-dependent glutamate release from astrocytes is not caused by the reverse operation of glutamate transporters, because the astrocyte-induced glutamate-mediated responses in neurons were affected neither by inhibitors of glutamate transporters (beta-threo-hydroxyaspartate, dihydrokainate, and L-trans-pyrrolidine-2,4-dicarboxylate) nor by replacement of extracellular sodium with lithium. We show that Ca(2+)-dependent glutamate release from astrocytes requires an electrochemical gradient necessary for glutamate uptake in vesicles, because bafilomycin A(1), a vacuolar-type H(+)-ATPase inhibitor, reduced glutamate release from astrocytes. Injection of astrocytes with the light chain of the neurotoxin Botulinum B that selectively cleaves the vesicle-associated SNARE protein synaptobrevin inhibited the astrocyte-induced glutamate response in neurons. Therefore, the Ca(2+)-dependent glutamate release from astrocytes is a SNARE protein-dependent process that requires the presence of functional vesicle-associated proteins, suggesting that astrocytes store glutamate in vesicles and that it is released through an exocytotic pathway.

Cd2+ regulation of the hyperpolarization-activated current IAB in crayfish muscle.

The effects of Cd2+ on the hyperpolarization-activated K(+)-mediated current called IAB (Araque, A., and W. Buño. 1994. Journal of Neuroscience. 14:399-408.) were studied under two-electrode voltage-clamp in opener muscle fibers of the crayfish Procambarus clarkii. IAB was reversibly reduced by extracellular Cd2+ in a concentration-dependent manner, obeying the Hill equation with IC50 = 0.452 +/- 0.045 mM and a Hill coefficient of 1 (determined from the maximal chord conductance of IAB). Cd2+ decreased the IAB conductance (GAB) and shifted its voltage dependence towards hyperpolarized potentials in a similar degree, without affecting the slope of the voltage dependence. The IAB activation time constant increased, whereas the IAB deactivation time constant was not modified by Cd2+. The IAB equilibrium potential (EAB) was unmodified by Cd2+, indicating that the selective permeability of IAB channels was not altered. IAB was unaffected by intracellular Cd2+. The Cd(2+)-regulation of IAB did not depend on [K+]o, and the effects of [K+]o on IAB were unchanged by Cd2+, indicating that Cd2+ did not compete with K+. Therefore, Cd2+ probably bound to a different site to that involved in the K+ permeability pathway. We conclude that Cd2+ affected the gating of IAB channels, interfering with their opening but not with their closing mechanism. The results can be explained by a kinetic model in which the binding of Cd2+ to the IAB channels would stabilize the gating apparatus at its resting position, increasing the energy barrier for the transition from the closed to the open channel states.

Angiotensinase activity is asymmetrically distributed in the amygdala, hippocampus and prefrontal cortex of the rat.

There are important asymmetries in brain functions such as emotional processing and stress response in humans and animals. Knowledge of the bilateral distribution of brain neurotransmitters is important to appropriately understand its functions. Some peptides such as those included in the renin-angiotensin system (RAS) and cholecystokinin (CCK) are related to modulation of behavior and stress. However, although angiotensin AT1 and CCK type 2 receptors were found in adult rat brain, there are no studies of their bilateral distribution in stress-related areas. The function of angiotensin peptides is depending on the action of several aminopeptidases (AP) called angiotensinases, some of them being also involved in the metabolism of CCK. We have studied the bilateral distribution of soluble (SOL) and membrane-bound (MEM) alanyl- (AlaAP), cystinyl- (CysAP), glutamyl- (GluAP) and aspartyl- (AspAP) AP activities in stress-related areas such as amygdala, hippocampus and medial prefrontal cortex of adult male rats in resting conditions. These enzymes are involved in the metabolism of angiotensins (AlaAP, CysAP, GluAP, AspAP) and CCK (GluAP, AspAP). In the amygdala, all the activities studied showed a right predominance with a significant difference ranging from 30% for SOL CysAP to 125% for SOL GluAP. In the hippocampus, there was a left predominance for SOL AlaAP, SOL and MEM CysAP and MEM AspAP activities (100, 80, 300 and 100% higher, respectively). In contrast, GluAP predominated remarkably in the right hippocampus (eight-fold for SOL and three-fold for MEM). In the prefrontal cortex, SOL and MEM CysAP and SOL AspAP predominated in the left hemisphere (40, 100 and 40% higher, respectively). These results demonstrated a heterogeneous bilateral pattern of angiotensinase activities in motivation and stress-related areas. This may reflect an uneven asymmetrical distribution of their endogenous substrates depending on the brain location and consequently, it would be also a reflect of the asymmetries in the functions they are involved in.

Circuit-specific signaling in astrocyte-neuron networks in basal ganglia pathways.

Astrocytes are important regulatory elements in brain function. They respond to neurotransmitters and release gliotransmitters that modulate synaptic transmission. However, the cell- and synapse-specificity of the functional relationship between astrocytes and neurons in certain brain circuits remains unknown. In the dorsal striatum, which mainly comprises two intermingled subtypes (striatonigral and striatopallidal) of medium spiny neurons (MSNs) and synapses belonging to two neural circuits (the direct and indirect pathways of the basal ganglia), subpopulations of astrocytes selectively responded to specific MSN subtype activity. These subpopulations of astrocytes released glutamate that selectively activated N-methyl-d-aspartate receptors in homotypic, but not heterotypic, MSNs. Likewise, astrocyte subpopulations selectively regulated homotypic synapses through metabotropic glutamate receptor activation. Therefore, bidirectional astrocyte-neuron signaling selectively occurs between specific subpopulations of astrocytes, neurons, and synapses.

Novel inward rectifier blocked by Cd2+ in crayfish muscle.

The characteristics of a voltage- and time-dependent inward rectifying current were examined with voltage clamp techniques in crayfish muscle. The inward current, carried by K+, was activated by hyperpolarization. Although this inward current increased with the extracellular K+ concentration [( K+]o), the voltage-dependence of the underlying conductance was independent of [K+]o. The current was unaffected by Cs+ and Ba2+, but was blocked by low concentrations of Cd2+. Therefore, this inward rectifier is different than previously described ones.

Glutamatergic postsynaptic block by Pamphobeteus spider venoms in crayfish.

The effects of toxins from venom glands of two south american spiders (Pamphobeteus platyomma and P. soracabae) on glutamatergic excitatory synaptic transmission were studied in the neuromuscular junction of the opener muscle of crayfish. The toxins selectively and reversibly blocked both excitatory postsynaptic currents and potentials in a dose-dependent manner. They also reversibly abolished glutamate-induced postsynaptic membrane depolarization. They had no effect on resting postsynaptic membrane conductance nor on postsynaptic voltage-gated currents. The synaptic facilitation and the frequency of miniature postsynaptic potentials were unaffected by the toxins, indicating that presynaptic events were not modified. Picrotoxin, a selective antagonist of the gamma-aminobutyric acid (GABA)A receptor, did not modify toxin effects. We conclude that both toxins specifically block the postsynaptic glutamate receptor-channel complex.

MOTOR NEURONES OF THE CRAYFISH WALKING SYSTEM POSSESS TEA+-REVEALED REGENERATIVE ELECTRICAL PROPERTIES

In crustaceans, some motor neurones (MNs) have been shown to be part of the central pattern generator in the stomatogastric system (Harris-Warrick et al. 1992; Moulins, 1990), the swimmeret system (Heitler, 1978) or the walking system (Chrachri and Clarac, 1990). These MNs induce changes in the central rhythm when depolarized and are conditional oscillators in the stomatogastric ganglion. Moreover, in the walking system, rhythmic activity can be triggered by muscarinic cholinergic agonists (Chrachri and Clarac, 1987). We have recently analyzed the role of muscarinic receptors in crayfish walking leg MNs (D. Cattaert and A. Araque, in preparation) and demonstrated that oxotremorine, a muscarinic agonist, evoked long-lasting depolarizing responses associated with an increased input resistance. The outward current blocked by oxotremorine is likely to be carried by K+, as is the case for the M current (IM) in vertebrates (Brown and Adams, 1980). In most neurones, K+ conductances play a principal role in maintaining the membrane potential at rest: for example, IM is active at the resting membrane potential, thus contributing to its maintenance, and the 'delayed-rectifier' (IK) assists the fast repolarization after an action potential. Some K+ conductances are Ca2+-dependent (IK,Ca) and are activated by an increase in internal Ca2+ concentration. In such cases, Ca2+ currents may result in hyperpolarization of the neurone through activation of IK,Ca. In opposition to these K+ currents, the direct effect of Na+ and Ca2+ conductances is to depolarize the neurone. For example, the persistant Na+ current (INap) that is responsible for the slow subthreshold depolarization termed slow pre-potentials (Gestrelius et al. 1983; Leung and Yim, 1991) participates in the formation of pacemaker depolarization (Barrio et al. 1991) and generates plateau-type responses in control conditions (Barrio et al. 1991; Llinas and Sugimori, 1980). Similarly Ca2+ or non-specific (Na+/Ca2+) conductances generate such events in Aplysia californica burster neurones (Adams and Benson, 1985), crustacean cardiac ganglion (Tazaki and Cooke, 1990), insect neurones (Hancox and Pitman, 1991) and crustacean stomatogastric ganglion (Kiehn and Harris-Warrick, 1992). Since crustacean MNs can participate in rhythm production, such depolarizing conductances may exist in most of them and may contribute to the long-lasting MN depolarizations and spike bursts present during locomotion.

Electrolyte excretion and sodium intake.

Established essential hypertension is characterized by normal equilibrium between the intake and renal excretion of sodium. Urinary sodium excretion is interrelated with that of other ions, such as potassium and calcium, and that the response of blood pressure to salt ingestion can be conditioned by the simultaneous intake of varying levels of those ions. The authors address three aspects: the correlations between urinary excretion of sodium and calcium and sodium and potassium in a population of untreated essential hypertensive persons, the response of blood pressure during the escape induced by exogenous mineralocorticoid administration in mild essential hypertension, and the effect of intravenous calcium gluconate infusion on sodium excretion and renal function. The first part shows that sodium excretion is closely correlated with that of other ions in essential hypertension, and the second part shows that, to escape from the sodium-retaining effect of a mineralocorticoid, mild hypertensive subjects must have increased blood pressure within or near the cutoff point that defines salt sensitivity. Of interest, the elevation in blood pressure takes place while sympathetic nervous activity is blunted. The third part provides evidence to explain one of the mechanisms by which calcium influences renal function and enhances renal sodium excretion. The intrarenal effects of low doses of calcium are dependent on the renal production of prostaglandins.

[Distribution of hepatitis C virus genotypes among the hemodialysis population in the province of Alicante]. / Distribución de los genotipos del virus C en la población de hemodiálisis de la provincia de Alicante.

UNLABELLED: Hepatitis C virus (HCV) genotypes are irregularly distributed among the different geographic area and groups at risk. OBJECTIVE: To study the different HCV genotypes and subtypes of hemodialyzed patients from Alicante. METHODS: We studied 640 patients on haemodialysis (HD) and we determined the RNA-HCV and the genotypes in the 120 patients with antibodies against HCV (HCV-Ab). We compared the results with the genotypes of 1,370 patients from other groups at risk in the same geographic area. RESULTS: RNA-HCV was not found in the serum in 15% (18/120) of the patients on HD who were HCV-Ab positive. Prevalence of the different genotypes in the 102 patients with positive viral RNA was the following: 1b: 56.8% (58/102), 1a: 19.6% (20/102), 3: 17% (17/102), 2a-2c: 1.9 (2/102), 2b: 0.9% (1/102) 4: 2.9 (3/102), 5: 0.9% (1/102). In conclusion, the genotype 1b was the most frequent in the patients studied in all these areas, and was the same as in the rest of the country. This genotype has been associated with the most severe hepatic disease and poor response to treatment, affecting the prognosis of these patients. The most frequent genotypes in HD in Alicante were 1b, 3 and 1a. HCV genotypes distribution among the HD units was not uniform in the different geographic areas. HCV genotypes distribution in the HD population is similar to other groups at risk from the same geographic area.

[New information pathways in the nervous system: communication between astrocytes and neurones]. / Nuevas vías de información en el sistema nervioso: comunicación entre astrocitos y neuronas.

INTRODUCTION AND METHOD: Astrocytes, a type of glial cell in the central nervous system (CNS), have been classically considered as trophic, structural and supportive cells for neurons. However, in recent years, accumulating evidence suggest a more active role of astrocytes in the physiology of neurons, being involved in the information processing of the CNS. Astrocytes exhibit both a form of excitability based on variations of the intracellular Ca2+ concentration, and a form of communication based on intercellular Ca2+ waves. Furthermore, synaptically released neurotransmitters mobilize Ca2+ from the astrocytic intracellular stores, i.e., the astrocytic cellular excitability can be triggered by the synaptic activity. Finally, astrocytes release the transmitter glutamate to the extracellular space through a Ca2+ dependent mechanism, modulating the neuronal electrical activity and the synaptic transmission. As a consequence of the demonstration of these new forms of cellular communication between astrocytes and neurons, the concept of tripartite synapse has been proposed, in which the synapse is functionally constituted by three elements, i.e., the pre and postsynaptic elements and the surrounding astrocytes. CONCLUSION: The novel results discussed in the present review support the presence of new and complex information pathways in the CNS, which are based on the existence of bidirectional communication between astrocytes and neurons, and which have relevant consequences on the cellular mechanisms responsible for the information processing of the CNS.

[Effect of pravastatin on hypercholesterolemia associated with proteinuria]. / Efecto de la pravastatina en la hipercolesterolemia asociada a proteinuria.

BASIS: Hyperlipidemia associated to nephrotic syndrome has been involved in the deterioration of the renal function in these patients. The reduction in the synthesis of cholesterol with pravastatin, an hydrophilic inhibitor of such synthesis, may improve both the dyslipemia, the renal function and the proteinuria. METHODS: We conducted a controlled open randomized study in 16 patients with proteinuria greater than 2 g/day, creatinine clearance greater than 0.5 ml/s (30 ml/min) and hypercholesterolemia with LDL cholesterol greater than 4.9 mmol/l (190 mg/dl) distributed in two groups. One of these groups received hypolipemiant dietetic treatment and 20-40 mg of pravastatin and the other group, only the dietetic treatment. RESULTS: The patients receiving pravastatin showed a 27% decrease in total plasmatic cholesterol, compared to a 6.7% in the control group (p < 0.01). This decrease was more evident in the LDL cholesterol fraction (44 vs 9%; p < 0.01). No significant modifications were observed in the HDL cholesterol fraction, triglycerides, renal function or proteinuria. Neither clinical nor enzymatiz adverse effects from hepatic or muscular origin were observed.

Synaptic regulation of the astrocyte calcium signal.

During the last years, a great amount of evidence demonstrates the existence of bidirectional communication between astrocytes and neurons, which has revealed an important active role of astrocytes in the physiology of the nervous system. As a consequence of this evidence, a new concept of the synaptic physiology--"the tripartite synapse"--has been proposed, in which the synapse is formed by three functional elements, i.e., the pre- and postsynaptic elements and the surrounding astrocytes. In this scenario astrocytes play an active role as dynamic regulatory elements in neurotransmission by reciprocally exchanging information with the pre- and postsynaptic elements. The control of the Ca2+ excitability in astrocytes is a key element in this loop of information exchange. In the present article we review and discuss our current knowledge of the properties of the astrocyte intracellular Ca2+ signal and its modulation by the synaptic activity.

Fast BK-type channel mediates the Ca(2+)-activated K(+) current in crayfish muscle.

The role of the Ca(2+)-activated K(+) current (I(K(Ca))) in crayfish opener muscle fibers is functionally important because it regulates the graded electrical activity that is characteristic of these fibers. Using the cell-attached and inside-out configurations of the patch-clamp technique, we found three different classes of channels with properties that matched those expected of the three different ionic channels mediating the depolarization-activated macroscopic currents previously described (Ca(2+), K(+), and Ca(2+)-dependent K(+) currents). We investigated the properties of the ionic channels mediating the extremely fast activating and persistent I(K(Ca)). These voltage- and Ca(2+)-activated channels had a mean single-channel conductance of approximately 70 pS and showed a very fast activation. Both the single-channel open probability and the speed of activation increased with depolarization. Both parameters also increased in inside-out patches, i.e., in high Ca(2+) concentration. Intracellular loading with the Ca(2+) chelator bis(2-aminophenoxy) ethane-N, N,N',N'-tetraacetic acid gradually reduced and eventually prevented channel openings. The channels opened at very brief delays after the pulse depolarization onset (<5 ms), and the time-dependent open probability was constant during sustained depolarization (< or =560 ms), matching both the extremely fast activation kinetics and the persistent nature of the macroscopic I(K(Ca)). However, the intrinsic properties of these single channels do not account for the partial apparent inactivation of the macroscopic I(K(Ca)), which probably reflects temporal Ca(2+) variations in the whole muscle fiber. We conclude that the channels mediating I(K(Ca)) in crayfish muscle are voltage- and Ca(2+)-gated BK channels with relatively small conductance. The intrinsic properties of these channels allow them to act as precise Ca(2+) sensors that supply the exact feedback current needed to control the graded electrical activity and therefore the contraction of opener muscle fibers.