Arachidonic acid blocks gap junctions between retinal horizontal cells

Arachidonic acid closes innexin/pannexin channels and thereby inhibits microglia cell movement to a nerve injury

Pannexons are membrane channels formed by pannexins and are permeable to ATP. They have been implicated in various physiological and pathophysiological processes. Innexins, the invertebrate homologues of the pannexins, form innexons. Nerve injury induces calcium waves in glial cells, releasing ATP through glial pannexon/innexon channels. The ATP then activates microglia. More slowly, injury releases arachidonic acid (ArA). The present experiments show that ArA itself reduced the macroscopic membrane currents of innexin- and of pannexin-injected oocytes; ArA also blocked K(+) -induced release of ATP. In leeches, whose large glial cells have been favorable for studying control of microglia migration, ArA blocked glial dye-release and, evidently, ATP-release. A physiological consequence in the leech was block of microglial migration to nerve injuries. Exogenous ATP (100 µM) reversed the effect, for ATP causes activation and movement of microglia after nerve injury, but nitric oxide directs microglia to the lesion. It was not excluded that metabolites of ArA may also inhibit the channels. But for all these effects, ArA and its non-metabolizable analog eicosatetraynoic acid (ETYA) were indistinguishable. Therefore, ArA itself is an endogenous regulator of pannexons and innexons. ArA thus blocks release of ATP from glia after nerve injury and thereby, at least in leeches, stops microglia at lesions.

Gap junctional coupling in the vertebrate retina: variations on one theme?

Gap junctions connect cells in the bodies of all multicellular organisms, forming either homologous or heterologous (i.e. established between identical or different cell types, respectively) cell-to-cell contacts by utilizing identical (homotypic) or different (heterotypic) connexin protein subunits. Gap junctions in the nervous system serve electrical signaling between neurons, thus they are also called electrical synapses. Such electrical synapses are particularly abundant in the vertebrate retina where they are specialized to form links between neurons as well as glial cells. In this article, we summarize recent findings on retinal cell-to-cell coupling in different vertebrates and identify general features in the light of the evergrowing body of data. In particular, we describe and discuss tracer coupling patterns, connexin proteins, junctional conductances and modulatory processes. This multispecies comparison serves to point out that most features are remarkably conserved across the vertebrate classes, including (i) the cell types connected via electrical synapses; (ii) the connexin makeup and the conductance of each cell-to-cell contact; (iii) the probable function of each gap junction in retinal circuitry; (iv) the fact that gap junctions underlie both electrical and/or tracer coupling between glial cells. These pan-vertebrate features thus demonstrate that retinal gap junctions have changed little during the over 500 million years of vertebrate evolution. Therefore, the fundamental architecture of electrically coupled retinal circuits seems as old as the retina itself, indicating that gap junctions deeply incorporated in retinal wiring from the very beginning of the eye formation of vertebrates. In addition to hard wiring provided by fast synaptic transmitter-releasing neurons and soft wiring contributed by peptidergic, aminergic and purinergic systems, electrical coupling may serve as the 'skeleton' of lateral processing, enabling important functions such as signal averaging and synchronization.

Myoendothelial gap junction frequency does not account for sex differences in EDHF responses in rat MCA

Previous findings from our laboratory have shown that dilations to endothelium-derived hyperpolarizing factor (EDHF) in rat middle cerebral artery (MCA) are less in females compared to males. Myoendothelial gap junctions (MEGJs) appear to mediate the transfer of hyperpolarization between endothelium and smooth muscle in males. In the present study, we hypothesized that MEGJs are the site along the EDHF pathway which is compromised in female rat MCA. Membrane potential in endothelium was measured using the voltage-sensitive dye di-8-ANEPPS and in smooth muscle using intracellular glass microelectrodes in the presence of l-NAME (3x10(-5 )M) and indomethacin (10(-5 )M). Electron microscopy was used to assess MEGJ characteristics. In endothelial cells, the di-8-ANEPPS fluorescence ratio change to 10(-5 )M UTP was similar in males (-2.9+/-0.5%) and females (-3.2+/-0.2%), indicating comparable degrees of endothelial cell hyperpolarization. However, smooth muscle cell hyperpolarization to 10(-5 )M UTP was significantly attenuated in females (0 mV hyperpolarization; -31+/-1.5 mV resting) compared to males (8 mV hyperpolarization; -28+/-1.7 mV resting). Ultrastructural evidence suggested that MEGJ frequency and area of contact were comparable between males and females. Taken together, our data suggest that in rat MCA, MEGJ frequency does not account for the reduced EDHF responses observed in females compared to males. We conclude that reduced myoendothelial coupling and/or homocellular coupling within the media may account for these differences.

Docosahexaenoic acid (22:6n-3) enrichment of membrane phospholipids increases gap junction coupling capacity in cultured astrocytes

Although it is agreed that n-3 polyunsaturated fatty acids (PUFAs) are important for brain function, it has yet to be demonstrated how they are involved in precise cellular mechanisms. We investigated the role of enhanced n-3 PUFA in astrocyte membranes on the gap junction capacity of these cells. Astrocytes isolated from newborn rat cortices were grown in medium supplemented with docosahexaenoic acid (DHA), the main n-3 PUFA in cell membranes, or arachidonic acid (AA), the main n-6 PUFA, plus an antioxidant (alpha-tocopherol or N-acetyl-cystein) to prevent peroxidation. The resulting three populations of astrocytes differed markedly in their n-3:n-6 PUFA ratios in phosphatidylethanolamine and phosphatidylcholine, the main phospholipids in membranes. DHA-supplemented cells had a physiological high n-3:n-6 ratio (1.58), unsupplemented cells had a low n-3:n-6 ratio (0.66) and AA-supplemented cells had a very low n-3:n-6 ratio (0.36), with excess n-6 PUFA. DHA-supplemented astrocytes had a greater gap junction capacity than unsupplemented cells or AA-supplemented cells. The enhanced gap junction coupling of DHA-enriched cells was associated with a more functional distribution of connexin 43 at cell interfaces (shown by immunocytochemistry) and more of the main phosphorylated isoform of connexin 43. These findings suggest that the high n-3:n-6 PUFA ratio that occurs naturally in astrocyte membranes is needed for optimal gap junction coupling in these cells.

A further study of a possible locus for schizophrenia on the X chromosome

Several studies suggest that the X chromosome may contain a gene for schizophrenia. In the present study, we recruited 142 male schizophrenic patients and their biological mothers from all parts of the United Kingdom to detect a genetic association for the SYP/CACNA1F locus in the Xp11 region and the FACL4 locus in the Xq22.3-Xq23 region. The haplotype-based haplotype relative risk (HHRR) analysis showed allelic association for rs2071316 (chi2=6.85, P=0.009) and rs5905724 (chi2=5.3, P=0.021) at the CACNA1F locus, but not for rs5943414 and rs1324805 at the FACL4 locus and rs3817678 at the SYP locus. The haplotype analysis showed a weak association for the rs3817678-rs2071316-rs5905724 haplotypes (chi2=12.19, df=4, P=0.016) but did not show such an association for the rs5943414-rs1324805 haplotypes (chi2=3.96, df=2, P=0.138). Because the linkage disequilibrium signal was detected only at the CACNA1F locus, this gene should perhaps be considered as being a candidate for schizophrenia although further work is needed to draw firm conclusions.

Electrical synapses: a dynamic signaling system that shapes the activity of neuronal networks

Gap junctions consist of intercellular channels dedicated to providing a direct pathway for ionic and biochemical communication between contacting cells. After an initial burst of publications describing electrical coupling in the brain, gap junctions progressively became less fashionable among neurobiologists, as the consensus was that this form of synaptic transmission would play a minimal role in shaping neuronal activity in higher vertebrates. Several new findings over the last decade (e.g. the implication of connexins in genetic diseases of the nervous system, in processing sensory information and in synchronizing the activity of neuronal networks) have brought gap junctions back into the spotlight. The appearance of gap junctional coupling in the nervous system is developmentally regulated, restricted to distinct cell types and persists after the establishment of chemical synapses, thus suggesting that this form of cell-cell signaling may be functionally interrelated with, rather than alternative to chemical transmission. This review focuses on gap junctions between neurons and summarizes the available data, derived from molecular, biological, electrophysiological, and genetic approaches, that are contributing to a new appreciation of their role in brain function.

Electrical synapses in the mammalian brain

Many neurons in the mammalian central nervous system communicate through electrical synapses, defined here as gap junction-mediated connections. Electrical synapses are reciprocal pathways for ionic current and small organic molecules. They are often strong enough to mediate close synchronization of subthreshold and spiking activity among clusters of neurons. The most thoroughly studied electrical synapses occur between excitatory projection neurons of the inferior olivary nucleus and between inhibitory interneurons of the neocortex, hippocampus, and thalamus. All these synapses require the gap junction protein connexin36 (Cx36) for robust electrical coupling. Cx36 appears to interconnect neurons exclusively, and it is expressed widely along the mammalian neuraxis, implying that there are undiscovered electrical synapses throughout the central nervous system. Some central neurons may be electrically coupled by other connexin types or by pannexins, a newly described family of gap junction proteins. Electrical synapses are a ubiquitous yet underappreciated feature of neural circuits in the mammalian brain.

Cell death in retinitis pigmentosa: gap junctions and the 'bystander' effect

Retinitis pigmentosa (RP) constitutes a group of genetically mediated, degenerative retinal diseases that display a broad range of phenotypes. There is appreciable heterogeneity in the pathogenetic mechanisms that underlie the various forms of RP, but a substantial percentage of the known cases arise as a consequence of mutations in rhodopsin or other rod-specific proteins. However, despite the fact that the genetic defect is expressed solely in the rod photoreceptors, otherwise healthy cone photoreceptors invariably die, resulting in severe visual impairment. In this paper, the author proposes a mechanism that may be responsible, at least in part, for this unfortunate circumstance. The basic premise of the hypothesis is that the spread of the disease from dying rods to genetically normal cones is a form of 'bystander' effect, mediated by the gap junctions that exist between these photoreceptor subtypes. On this view, agents that trigger the apoptotic process permeate the intercellular gap-junctional channels to carry the disease from rods to neighboring cones. If permeation of noxious substances through gap junctions is a significant factor in the non-cell-autonomous spread of photoreceptor degeneration, blocking transmission through these channels may provide a means for therapeutic intervention. Many substances are known to block gap-junctional communication, but if the rod-cone channel is to be targeted, it will be essential to identify the connexins that form the gap junctions between the two types of photoreceptor, and to develop drugs that selectively affect their junctional properties. Clearly, passage of toxic agents through gap junctions may not be the only form of cell-cell interaction by which dying rods could cause cone cell death, and in this brief account, the author considers other avenues that are currently being explored to explain this phenomenon.

Endothelium-derived relaxing factors: A focus on endothelium-derived hyperpolarizing factor(s)

Endothelium-derived hyperpolarizing factor (EDHF) is defined as the non-nitric oxide (NO) and non-prostacyclin (PGI2) substance that mediates endothelium-dependent hyperpolarization (EDH) of vascular smooth muscle cells (VSMC). Although both NO and PGI2 have been demonstrated to hyperpolarize VSMC by cGMP- and cAMP-dependent mechanisms, respectively, and in the case of NO by cGMP-independent mechanisms, a considerable body of evidence suggests that an additional cellular mechanism must exist that mediates EDH. Despite intensive investigation, there is no agreement as to the nature of the cellular processes that mediates the non-NO/PGI2 mediated hyperpolarization. Epoxyeicosatrienoic acids (EET), an endogenous anandamide, a small increase in the extracellular concentration of K+, and electronic coupling via myoendothelial cell gap junctions have all been hypothesized as contributors to EDH. An attractive hypothesis is that EDH is mediated via both chemical and electrical transmissions, however, the contribution from chemical mediators versus electrical transmission varies in a tissue- and species-dependent manner, suggesting vessel-specific specialization. If this hypothesis proves to be correct then the potential exists for the development of vessel and organ-selective vasodilators. Because endothelium-dependent vasodilatation is dysfunctional in disease states (i.e., atherosclerosis), selective vasodilators may prove to be important therapeutic agents.Key words: endothelium, nitric oxide, potassium channels, hyperpolarization, gap junctions.

Reactive oxygen species uncouple external horizontal cells in the carp retina and glutathione couples them again

We have investigated the effect of free radicals on the electrical gap junctions between horizontal cells in the carp retina. In our previous study, L-buthionine sulfoximine, an inhibitor of glutathione synthesis, caused uncoupling of horizontal cells four days after injection. In the present study, we have used paraquat, a generator of exogenous reactive oxygen species, to investigate whether it was the depletion of glutathione or an increase in the level of reactive oxygen species which resulted in horizontal cell uncoupling after L-buthionine sulfoximine injection. Intracellular recordings were made from L-type horizontal cells at various time-points after intravitreal injection of paraquat. Injection of 25nmol paraquat caused an increase in response amplitude to central spot light stimuli by two days after injection, which continued for a further two to three days and had almost disappeared by seven days after injection. There was also a sharp increase in reactive oxygen species production, peaking at four days and disappearing by seven days after injection, and an accompanying depletion and a restoration of glutathione levels with a similar time-course. Marking cells with Lucifer Yellow clearly illustrated uncoupling of horizontal cells after paraquat injection. If paraquat and L-buthionine sulfoximine were injected simultaneously, the increase in response to central spots was observed as early as one day after injection. This response amplitude was not more enhanced than that observed after L-buthionine sulfoximine injection alone, although a dramatic increase in the level of reactive oxygen species was observed. From these results, we suggest that reactive oxygen species are involved in uncoupling, while recovery from uncoupling is dependent on glutathione. Furthermore, we conclude that a balance between glutathione and reactive oxygen species levels is the most important factor controlling gap junctional intercellular communication of L-type horizontal cells in the carp retina.

Neurotransmitters and gap junctions in developing neural circuits

A growing body of evidence suggests that highly correlated, spontaneous neural activity plays an important role in shaping connections in the developing nervous system prior to the maturation of sensory afferents. In this article we discuss the mechanisms involved in the generation and the regulation of spontaneous activity patterns in the developing retina and the developing neocortex. Spontaneous activity in the developing retina propagates across the ganglion cell layer as waves of action potentials and drives rhythmic increases in intracellular calcium in retinal neurons. Retinal waves are mediated by a combination of chemical synaptic transmission and gap junctions, and the circuitry responsible for generating retinal waves changes with age and between species. In the developing cortex, spontaneous calcium elevations propagate across clusters of cortical neurons called domains. Cortical domains are generated by a regenerative mechanism involving second messenger diffusion through gap junctions and subsequent calcium release from internal stores. The neocortical gap junction system is regulated by glutamate-triggered second messenger systems as well as neuromodulatory transmitters, suggesting extensive interactions between synaptic transmission and information flow through gap junctions. The interaction between gap junctions and chemical synaptic transmission observed in these developing networks represent a powerful mechanism by which activity across large groups of neurons can be correlated.

Regulation of astrocyte gap junctions by hypoxia-reoxygenation

Confluent cultures of rat cortical astrocytes were subjected to 12-h hypoxia (<1% O(2)) followed by reoxygenation. Just after hypoxia, the cellular distribution, phosphorylation state and levels of connexin43 (Cx43), as well as the extent of dye coupling were as in control conditions. Nonetheless, 15-30 min after reoxygenation, dye coupling was transiently reduced by approximately 70%. The reduction in dye coupling occurred without changes in the state of phosphorylation or levels of Cx43. Nevertheless, it was correlated with a decrease in Cx43 reactivity found at membrane appositions and the appearance of intracellular Cx43-positive vesicle-like structures of variable size, suggesting internalization of gap junction channels. Reoxygenation-induced cellular uncoupling and redistribution of Cx43 were prevented by melatonin (500 microM), a potent-free radical scavenger, or indomethacin (50 microM), an inhibitor of the cyclooxygenase-dependent arachidonic acid metabolism. In astrocytes cultured under normoxia, the state of phosphorylation of Cx43 was not affected by antimycin A, a blocker of the mitochondrial oxidative metabolism, but phosphorylation was drastically reduced by iodoacetate, a blocker of anaerobic glycolysis. Thus, these results strongly suggest that reoxygenation-induced uncoupling is mediated by arachidonic acid byproducts that induce, at least, disorganization of Cx43 gap junction channels.

Arachidonic acid-induced dye uncoupling in rat cortical astrocytes is mediated by arachidonic acid byproducts

Arachidonic acid (AA) induced a concentration- and time-dependent reduction in gap junction-mediated dye coupling between cultured astrocytes. The effect was greatly diminished by inhibition of cyclooxygenases and lipoxygenases. The action of a low concentration of AA (5 microM) was also prevented by Ca2+-free extracellular solution or a high concentration of melatonin, a potent free radical scavenger, but not by Nomega-nitro-l-arginine, a nitric oxide (NO) synthase inhibitor. Thus, this effect may depend on Ca2+ influx and oxygen free radicals but not on NO generation. Cellular uncoupling induced by a high (100 microM), but not a low, AA concentration was rapidly reversed by washing with albumin containing solution. After reversal from 5 min but not 2.5 min inhibition with a high AA concentration dye coupling between astrocytes became refractory to a low concentration of AA, suggesting desensitization of the response elicited by a low concentration of the fatty acid. Dye uncoupling occurred without changes in levels and state of phosphorylation (immunoblotting and 32P-incorporation) of connexin43, the main astrocyte gap junctional protein. However, maximal cell uncoupling induced by a low (Slow action) but not by a high (Fast action) AA concentration was paralleled by a reduction in connexin43 (immunofluorescence) at cell-to-cell contacts. It is proposed that the AA-induced dye uncoupling is mediated by byproducts that induce rapid channel closure or slow removal of connexin43 gap junctions.

Regulation of gap junction coupling in the developing neocortex

In the developing mammalian, neocortex gap junctions represent a transient, metabolic, and electrical communication system. These gap junctions may play a crucial role during the formation and refinement of neocortical synaptic circuitries. This article focuses on two major points. First, the influence of gap junctions on electrotonic cell properties will be considered. Both the time-course and the amplitude of synaptic potentials depend, inter alia, on the integration capabilities of the postsynaptic neurons. These capabilities are, to a considerable extent, determined by the electrotonic characteristics of the postsynaptic cell. As a consequence, the efficacy of chemical synaptic inputs may be crucially affected by the presence of gap junctions. The second major topic is the regulation of gap junctional communication by neurotransmitters via second messenger pathways. The monoaminergic neuromodulators dopamine, noradrenaline, and serotonin reduce gap junction coupling via activation of two different intracellular signaling cascades--the cAMP/protein kinase A pathway and the IP3/Ca2+/protein kinase C pathway, respectively. In addition, gap junctional communication seems to be modulated by the nitric oxide (NO)/cGMP system. Since NO production can be stimulated by glutamate-induced calcium influx, the NO/cGMP-dependent modulation of gap junctions might represent a functional link between developing glutamatergic synaptic transmission and the gap junctional network. Thus, it might be of particular importance in view of a role of gap junctions during the process of circuit formation.

Intracellular acidification reduced gap junction coupling between immature rat neocortical pyramidal neurones

1. Developmental changes in electrophysiological properties of pyramidal neurones correlated with the developmental decline in gap junction-dependent dye coupling were investigated in coronal slices of rat prefrontal and sensorimotor cortex. Effects of intracellular acidification induced by application of weak organic acids on neuronal dye coupling, electrotonic parameters as well as synaptic potentials were examined using the patch clamp technique. Optical monitoring of intracellular pH revealed an acidic shift of 0.4-0.5 pH units following sodium propionate application. 2. Dye coupling between layer II-III neurones was prominent during the first two postnatal weeks. During this period, pre-incubation of slices with 30 mM of the sodium salts of weak organic acids reduced the number of cells coupled to the injected neurones by 64%. 3. Between postnatal days 1 and 18, the mean neuronal input resistance decreased significantly (by 81.0%). Both the membrane time constant (tau 0) and the first equalizing time constant (tau 1) also showed a significant developmental decline of 25.8 and 65.8%, respectively. Electrotonic length decreased by 34.9%. The electrophysiological properties of neurones displayed a pronounced intercellular variability which decreased with on-going development. 4. During the first two postnatal weeks, intracellular acidification led to a mean increase in neuronal input resistance of 55.9% and a mean decreae in electrotonic length of 22.2%. The membrane time constant was reduced by approximately 25% in the majority of neurones tested. Significant electrophysiological effects induced by intracellular acidification were not detected in uncoupled neurones from 18-day-old rats. 5. EPSP width at half-maximal amplitude showed a substantial reduction of approximately 50%, while rise times of the non-NMDA receptor-mediated EPSP components displayed no significant change during development. Both weak organic acids, as well as the gap junction blocker 1-octanol, reduced excitatory synaptic transmission independent of developmental age. 6. We conclude that gap junction permeability is regulated by intracellular pH in developing layer II-III pyramidal cells in the rat neocortex. The prominent correlation between pH-induced reduction in dye coupling and changes in electrophysiological cell properties suggests a significant influence of gap junctions on synaptic integration and information transfer in the immature neocortex.

Is cyclic AMP involved in the defibrillating effect of sotalol?

Ventricular fibrillation induced in animals pretreated with sotalol, a class III antiarrhythmic agent, would spontaneously terminate and revert into a sinus rhythm. This phenomenon has been attributed to the class III action of this drug, i.e., prolongation of myocardial action potential duration and effective refractory period. Since various observations suggested that these alone cannot explain the defibrillating phenomenon, we hypothesised that sotalol affected ventricular intercellular synchronization by increasing intercellular coupling. Our recent experimental studies have shown that sotalol antagonized the cellular decoupling to guinea pig ventricular muscle strip caused by perfusion with either a hypoxic normal Tyrode's solution or an oxygenated high Ca2+ Tyrode's solution. We assumed that the most likely mechanism for the restoration of intercellular coupling would be increasing intracellular cAMP concentration. In order to test this hypothesis, we studied the modification of this sotalol-induced recoupling by a cAMP dependent protein kinase inhibitor. The results clearly supported our assumption since the addition of Arg-Gly-Tyr-Ala-Leu- Gly (pure A- kinase inhibitor) prevented the aforementioned cellular recoupling action of sotalol in a dose-dependent manner. It can thus be concluded that changes in intracellular cAMP level are involved in the synchronizing / defibrillating effect of sotalol.