Contribution of connexin26 to electrical feedback inhibition in the turtle retina

Synaptic Scaffolds, Ion Channels and Polyamines in Mouse Photoreceptor Synapses: Anatomy of a Signaling Complex

Synaptic signaling complexes are held together by scaffold proteins, each of which is selectively capable of interacting with a number of other proteins. In previous studies of rabbit retina, we found Synapse-Associated Protein-102 (SAP102) and Channel Associated Protein of Synapse-110 (Chapsyn110) selectively localized in the tips of horizontal cell processes at contacts with rod and cone photoreceptors, along with several interacting ion channels. We have examined the equivalent suites of proteins in mouse retina and found similarities and differences. In the mouse retina we identified Chapsyn110 as the scaffold selectively localized in the tips of horizontal cells contacting photoreceptors, with Sap102 more diffusely present. As in rabbit, the inward rectifier potassium channel Kir2.1 was present with Chapsyn110 on the tips of horizontal cell dendrites within photoreceptor invaginations, where it could provide a hyperpolarization-activated current that could contribute to ephaptic signaling in the photoreceptor synapses. Pannexin 1 and Pannexin 2, thought to play a role in ephaptic and/or pH mediated signaling, were present in the outer plexiform layer, but likely not in the horizontal cells. Polyamines regulate many ion channels and control the degree of rectification of Kir2.1 by imposing a voltage-dependent block. During the day polyamine immunolabeling was unexpectedly high in photoreceptor terminals compared to other areas of the retina. This content was significantly lower at night, when polyamine content was predominantly in Müller glia, indicating daily rhythms of polyamine content. Both rod and cone terminals displayed the same rhythm. While polyamine content was not prominent in horizontal cells, if polyamines are released, they may regulate the activity of Kir2.1 channels located in the tips of HCs. The rhythmic change in polyamine content of photoreceptor terminals suggests that a daily rhythm tunes the behavior of suites of ion channels within the photoreceptor synapses.

The Potential Application of Pentacyclic Triterpenoids in the Prevention and Treatment of Retinal Diseases

Retinal diseases are a leading cause of impaired vision and blindness but some lack effective treatments. New therapies are required urgently to better manage retinal diseases. Natural pentacyclic triterpenoids and their derivatives have a wide range of activities, including antioxidative, anti-inflammatory, cytoprotective, neuroprotective, and antiangiogenic properties. Pentacyclic triterpenoids have great potential in preventing and/or treating retinal pathologies. The pharmacological effects of pentacyclic triterpenoids are often mediated through the modulation of signalling pathways, including nuclear factor erythroid-2 related factor 2, high-mobility group box protein 1, 11β-hydroxysteroid dehydrogenase type 1, and Src homology region 2 domain-containing phosphatase-1. This review summarizes recent in vitro and in vivo evidence for the pharmacological potential of pentacyclic triterpenoids in the prevention and treatment of retinal diseases. The present literature supports the further development of pentacyclic triterpenoids. Future research should now attempt to improve the efficacy and pharmacokinetic behaviour of the agents, possibly by the use of medicinal chemistry and targeted drug delivery strategies.

Horizontal Cell Feedback to Cone Photoreceptors in Mammalian Retina: Novel Insights From the GABA-pH Hybrid Model

How neurons in the eye feed signals back to photoreceptors to optimize sensitivity to patterns of light appears to be mediated by one or more unconventional mechanisms. Via these mechanisms, horizontal cells control photoreceptor synaptic gain and enhance key aspects of temporal and spatial center-surround receptive field antagonism. After the transduction of light energy into an electrical signal in photoreceptors, the next key task in visual processing is the transmission of an optimized signal to the follower neurons in the retina. For this to happen, the release of the excitatory neurotransmitter glutamate from photoreceptors is carefully regulated via horizontal cell feedback, which acts as a thermostat to keep the synaptic transmission in an optimal range during changes to light patterns and intensities. Novel findings of a recently described model that casts a classical neurotransmitter system together with ion transport mechanisms to adjust the alkaline milieu outside the synapse are reviewed. This novel inter-neuronal messaging system carries feedback signals using two separate, but interwoven regulated systems. The complex interplay between these two signaling modalities, creating synaptic modulation-at-a-distance, has obscured it’s being defined. The foundations of our understanding of the feedback mechanism from horizontal cells to photoreceptors have been long established: Horizontal cells have broad receptive fields, suitable for providing surround inhibition, their membrane potential, a function of stimulus intensity and size, regulates inhibition of photoreceptor voltage-gated Ca²⁺ channels, and strong artificial pH buffering eliminates this action. This review compares and contrasts models of how these foundations are linked, focusing on a recent report in mammals that shows tonic horizontal cell release of GABA activating Cl⁻ and HCO₃⁻ permeable GABA autoreceptors. The membrane potential of horizontal cells provides the driving force for GABAR-mediated HCO₃⁻ efflux, alkalinizing the cleft when horizontal cells are hyperpolarized by light or adding to their depolarization in darkness and contributing to cleft acidification via NHE-mediated H⁺ efflux. This model challenges interpretations of earlier studies that were considered to rule out a role for GABA in feedback to cones.

Evidence for functional GABAA but not GABAC receptors in mouse cone photoreceptors

At the first retinal synapse, horizontal cells (HCs) contact both photoreceptor terminals and bipolar cell dendrites, modulating information transfer between these two cell types to enhance spatial contrast and mediate color opponency. The synaptic mechanisms through which these modulations occur are still debated. The initial hypothesis of a GABAergic feedback from HCs to cones has been challenged by pharmacological inconsistencies. Surround antagonism has been demonstrated to occur via a modulation of cone calcium channels through ephaptic signaling and pH changes in the synaptic cleft. GABAergic transmission between HCs and cones has been reported in some lower vertebrates, like the turtle and tiger salamander. In these reports, it was revealed that GABA is released from HCs through reverse transport and target GABA receptors are located at the cone terminals. In mammalian retinas, there is growing evidence that HCs can release GABA through conventional vesicular transmission, acting both on autaptic GABA receptors and on receptors expressed at the dendritic tips of the bipolar cells. The presence of GABA receptors on mammalian cone terminals remains equivocal. Here, we looked specifically for functional GABA receptors in mouse photoreceptors by recording in the whole-cell or amphotericin/gramicidin-perforated patch clamp configurations. Cones could be differentiated from rods through morphological criteria. Local GABA applications evoked a Cl- current in cones but not in rods. It was blocked by the GABAA receptor antagonist bicuculline methiodide and unaffected by the GABAC receptor antagonist TPMPA [(1,2,5,6-tetrahydropyridin-4-yl)methylphosphinic acid]. The voltage dependency of the current amplitude was as expected from a direct action of GABA on cone pedicles but not from an indirect modulation of cone currents following the activation of the GABA receptors of HCs. This supports a direct role of GABA released from HCs in the control of cone activity in the mouse retina.

Connexins and Pannexins in cerebral ischemia

A common cause of mortality and long-term adult disability, cerebral ischemia or brain ischemia imposes a significant health and financial burden on communities worldwide. Cerebral ischemia is a condition that arises from a sudden loss of blood flow and consequent failure to meet the high metabolic demands of the brain. The lack of blood flow initiates a sequelae of cell death mechanisms, including the activation of the inflammatory pathway, which can ultimately result in irreversible brain tissue damage. In particular, Connexins and Pannexins are non-selective channels with a large pore that have shown to play time-dependent roles in the perpetuation of ischaemic injury. This review highlights the roles of Connexin and Pannexin channels in cell death mechanisms as a promising therapeutic target in cerebral ischemia, and in particular connexin hemichannels which may contribute most of the ATP release as a result of ischemia as well as during reperfusion. This article is part of a Special Issue entitled: Gap Junction Proteins edited by Jean Claude Herve.

Translating connexin biology into therapeutics

It is 45 years since gap junctions were first described. Universities face increasing commercial pressures and declining federal funding, with governments and funding foundations showing greater interest in gaining return on their investments. This review outlines approaches taken to translate gap junction research to clinical application and the challenges faced. The need for commercialisation is discussed and key concepts behind research patenting briefly described. Connexin channel roles in disease and injury are also discussed, as is identification of the connexin hemichannel as a therapeutic target which appears to play a role in both the start and perpetuation of the inflammasome pathway. Furthermore connexin hemichannel opening results in vascular dieback in acute injury and chronic disease. Translation to human indications is illustrated from the perspective of one connexin biotechnology company, CoDa Therapeutics, Inc.

Emerging functions of pannexin 1 in the eye

Pannexin 1 (Panx1) is a high-conductance, voltage-gated channel protein found in vertebrates. Panx1 is widely expressed in many organs and tissues, including sensory systems. In the eye, Panx1 is expressed in major divisions including the retina, lens and cornea. Panx1 is found in different neuronal and non-neuronal cell types. The channel is mechanosensitive and responds to changes in extracellular ATP, intracellular calcium, pH, or ROS/nitric oxide. Since Panx1 channels operate at the crossroad of major signaling pathways, physiological functions in important autocrine and paracrine feedback signaling mechanisms were hypothesized. This review starts with describing in depth the initial Panx1 expression and localization studies fostering functional studies that uncovered distinct roles in processing visual information in subsets of neurons in the rodent and fish retina. Panx1 is expressed along the entire anatomical axis from optical nerve to retina and cornea in glia, epithelial and endothelial cells as well as in neurons. The expression and diverse localizations throughout the eye points towards versatile functions of Panx1 in neuronal and non-neuronal cells, implicating Panx1 in the crosstalk between immune and neural cells, pressure related pathological conditions like glaucoma, wound repair or neuronal cell death caused by ischemia. Summarizing the literature on Panx1 in the eye highlights the diversity of emerging Panx1 channel functions in health and disease.

Extracellular ATP hydrolysis inhibits synaptic transmission by increasing ph buffering in the synaptic cleft

A slow mechanism of retinal synaptic inhibition involves hydrolysis of ATP released from pannexin 1 channels (from the tips of horizontal cell dendrites); the resulting protons and phosphates acidify the synaptic cleft, which inhibits neurotransmitter release.

Neuronal computations strongly depend on inhibitory interactions. One such example occurs at the first retinal synapse, where horizontal cells inhibit photoreceptors. This interaction generates the center/surround organization of bipolar cell receptive fields and is crucial for contrast enhancement. Despite its essential role in vision, the underlying synaptic mechanism has puzzled the neuroscience community for decades. Two competing hypotheses are currently considered: an ephaptic and a proton-mediated mechanism. Here we show that horizontal cells feed back to photoreceptors via an unexpected synthesis of the two. The first one is a very fast ephaptic mechanism that has no synaptic delay, making it one of the fastest inhibitory synapses known. The second one is a relatively slow (τ≈200 ms), highly intriguing mechanism. It depends on ATP release via Pannexin 1 channels located on horizontal cell dendrites invaginating the cone synaptic terminal. The ecto-ATPase NTPDase1 hydrolyses extracellular ATP to AMP, phosphate groups, and protons. The phosphate groups and protons form a pH buffer with a pKa of 7.2, which keeps the pH in the synaptic cleft relatively acidic. This inhibits the cone Ca²⁺ channels and consequently reduces the glutamate release by the cones. When horizontal cells hyperpolarize, the pannexin 1 channels decrease their conductance, the ATP release decreases, and the formation of the pH buffer reduces. The resulting alkalization in the synaptic cleft consequently increases cone glutamate release. Surprisingly, the hydrolysis of ATP instead of ATP itself mediates the synaptic modulation. Our results not only solve longstanding issues regarding horizontal cell to photoreceptor feedback, they also demonstrate a new form of synaptic modulation. Because pannexin 1 channels and ecto-ATPases are strongly expressed in the nervous system and pannexin 1 function is implicated in synaptic plasticity, we anticipate that this novel form of synaptic modulation may be a widespread phenomenon.

At the first retinal synapse, specific cells—horizontal cells (HCs)—inhibit photoreceptors and help to organize the receptive fields of another retinal cell type, bipolar cells. This synaptic interaction is crucial for visual contrast enhancement. Here we show that horizontal cells feed back to photoreceptors via a very fast ephaptic mechanism and a relatively slow mechanism. The slow mechanism requires ATP release via Pannexin 1 (Panx1) channels that are located on HC dendrites near the site where photoreceptors release the neurotransmitter glutamate to HCs and bipolar cells. The released ATP is hydrolyzed to produce AMP, phosphate groups, and protons; these phosphates and protons form a pH buffer, which acidifies the synaptic cleft. This slow acidification inhibits presynaptic calcium channels and consequently reduces the neurotransmitter release of photoreceptors. This demonstrates a new way in which ATP release can be involved in synaptic modulation. Surprisingly, the action of ATP is not purinergic but is mediated via changes in the pH buffer capacity in the synaptic cleft. Given the broad expression of Panx1 channels in the nervous system and the suggestion that Panx1 function underlies stabilization of synaptic plasticity and is needed for learning, we anticipate that this mechanism will be more widespread than just occurring at the first retinal synapse.

Expression of Pannexin1 in the outer plexiform layer of the mouse retina and physiological impact of its knockout

Pannexin1 (Panx1) belongs to a class of vertebrate proteins that exhibits sequence homology to innexins, the invertebrate gap junction proteins, and which also shares topological similarities with vertebrate gap junction proteins, the connexins. Unlike gap junctional channels, Panx1 forms single-membrane channels, whose functional role in neuronal circuits is still unsettled. We therefore investigated the subcellular distribution of Panx1 in the mouse retina of wildtype and Panx1-null mice by reverse-transcription polymerase chain reaction (RT-PCR), immunohistochemistry, and electron microscopy. Use of Panx1-deficient mice served as a model to assess the physiological role of Panx1 by electroretinographic recordings and also to ensure the specificity of the anti-Panx1 antibody labeling. Expression of Panx1 was found in type 3a OFF bipolar cells and in dendrites and axonal processes of horizontal cells. Panx1 was also found in horizontal cell dendrites representing the lateral elements of the triad synapse at cone and rod terminals. In vivo electroretinography of Panx1 knockout mice indicated an increased a- and b-wave compared to Panx1 wildtype mice under scotopic conditions. The effect on the b-wave was confirmed by in vitro electroretinograms from the inner retina. These results suggest that Panx1 channels serve as sinks for extracellular current flow making them possible candidates for the mediation of feedback from horizontal cells to photoreceptors.

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.

Lateral interactions in the outer retina

Lateral interactions in the outer retina, particularly negative feedback from horizontal cells to cones and direct feed-forward input from horizontal cells to bipolar cells, play a number of important roles in early visual processing, such as generating center-surround receptive fields that enhance spatial discrimination. These circuits may also contribute to post-receptoral light adaptation and the generation of color opponency. In this review, we examine the contributions of horizontal cell feedback and feed-forward pathways to early visual processing. We begin by reviewing the properties of bipolar cell receptive fields, especially with respect to modulation of the bipolar receptive field surround by the ambient light level and to the contribution of horizontal cells to the surround. We then review evidence for and against three proposed mechanisms for negative feedback from horizontal cells to cones: 1) GABA release by horizontal cells, 2) ephaptic modulation of the cone pedicle membrane potential generated by currents flowing through hemigap junctions in horizontal cell dendrites, and 3) modulation of cone calcium currents (I(Ca)) by changes in synaptic cleft proton levels. We also consider evidence for the presence of direct horizontal cell feed-forward input to bipolar cells and discuss a possible role for GABA at this synapse. We summarize proposed functions of horizontal cell feedback and feed-forward pathways. Finally, we examine the mechanisms and functions of two other forms of lateral interaction in the outer retina: negative feedback from horizontal cells to rods and positive feedback from horizontal cells to cones.

Connexin hemichannel mediated ephaptic inhibition in the retina

Connexins are the building blocks of gap-junctions; sign conserving electrical synapses. Recently it has been shown that connexins can also function as hemichannels and can mediate a sign inverting inhibitory synaptic signal from horizontal cells to cones via an ephaptic mechanism. In this review we will discuss the critical requirements for such an ephaptic interaction and relate these to the available experimental evidence. The highly conserved morphological structure of the cone synapse together with a number of specific connexin proteins and proteoglycans present in the synaptic complex of the cones creates a synaptic environment that allows ephaptic interactions. The connexins involved are members of a special group of connexins, encoded by the GJA9 and GJA10 genes. Surprisingly, in contrast to many other vertebrates, mouse and other rodents seem to lack a GJA9 encoded connexin. The specific combination of substances that block feedback and the highly specific modification of feedback in a zebrafish lacking Cx55.5 hemichannels all point to an ephaptic feedback mechanism from horizontal cells to cones. This article is part of a Special Issue entitled Electrical Synapses.

Mammalian retinal horizontal cells are unconventional GABAergic neurons

Lateral interactions at the first retinal synapse have been initially proposed to involve GABA by transporter-mediated release from horizontal cells, onto GABA(A) receptors expressed on cone photoreceptor terminals and/or bipolar cell dendrites. However, in the mammalian retina, horizontal cells do not seem to contain GABA systematically or to express membrane GABA transporters. We here report that mouse retinal horizontal cells express GAD65 and/or GAD67 mRNA, and were weakly but consistently immunostained for GAD65/67. While GABA was readily detected after intracardiac perfusion, it was lost during classical preparation for histology or electrophysiology. It could not be restored by incubation in a GABA-containing medium, confirming the absence of membrane GABA transporters in these cells. However, GABA was synthesized de novo from glutamate or glutamine, upon addition of pyridoxal 5'-phosphate, a cofactor of GAD65/67. Mouse horizontal cells are thus atypical GABAergic neurons, with no functional GABA uptake, but a glutamate and/or glutamine transport system allowing GABA synthesis, probably depending physiologically from glutamate released by photoreceptors. Our results suggest that the role of GABA in lateral inhibition may have been underestimated, at least in mammals, and that tissue pre-incubation with glutamine and pyridoxal 5'-phosphate should yield a more precise estimate of outer retinal processing.

The effect of aminosulfonate buffers on the light responses and intracellular pH of goldfish retinal horizontal cells

Retinal horizontal cell feedback acts as a gain control at the first synapse in the visual system and generates center-surround receptive fields in the outer retina. One model of feedback proposes that elevation of protons in the photoreceptor synaptic cleft produces feedback. Most evidence supporting the proton model has depended on the effect of proton buffers, in particular aminosulfonates, but these agents could potentially have effects other than external pH regulation. We therefore determined if the effects of aminosulfonates on horizontal cell rollback, an indicator of feedback, were consistent with external proton buffering. Intracellular recording from horizontal cells in isolated goldfish retina revealed that rollback was blocked only by aminosulfonates with an acid dissociation constant suited for buffering at the pH (7.5) of the Ringer's solution. In isolated goldfish horizontal cells, aminosulfonates, even those that did not block rollback, altered intracellular pH. This suggests that the effect of aminosulfonates on rollback is not because of changing intracellular pH. Measures of both intracellular and extracellular pH revealed that treatment with either glutamate or kainate resulted in acidification. As glutamate produced both internal and external acidification, intracellular and extracellular horizontal cell pH would be expected to increase in response to light, a change consistent with the proton model of feedback.

Gap-Junction Proteins in Retinal Development: New Roles for the “Nexus”

Gap-junction channels, the cytoplasmic proteins that associate with them, and the transcriptional networks that regulate them are increasingly being viewed as critical communications hubs for cell signaling in health and disease. As a result, the term “nexus,” which was the original structural name for these focal intercellular links, is coming back into use with new proteomic and transcriptomic meanings. The retina is better understood than any other part of the vertebrate central nervous system in respect of its developmental patterning, its diverse neuronal types and circuits, and the emergence of its definitive structure-function correlations. Thus, studies of the junctional and nonjunctional nexus roles of gap-junction proteins in coordinating retinal development should throw useful light on cell signaling in other developing nervous tissues.

Gating, permselectivity and pH-dependent modulation of channels formed by connexin57, a major connexin of horizontal cells in the mouse retina

Mouse connexin57 (Cx57) is expressed most abundantly in horizontal cells of the retina, and forms gap junction (GJ) channels, which constitute a structural basis for electrical and metabolic intercellular communication, and unapposed hemichannels (UHCs) that are involved in an exchange of ions and metabolites between the cytoplasm and extracellular milieu. By combining fluorescence imaging and dual whole-cell voltage clamp methods, we showed that HeLa cells expressing Cx57 and C-terminally fused with enhanced green fluorescent protein (Cx57-EGFP) form junctional plaques (JPs) and that only cell pairs exhibiting at least one JP demonstrate cell-to-cell electrical coupling and transfer of negatively and positively charged dyes with molecular mass up to approximately 400 Da. The permeability of the single Cx57 GJ channel to Alexa fluor-350 is approximately 90-fold smaller than the permeability of Cx43, while its single channel conductance (57 pS) is only 2-fold smaller than Cx43 (110 pS). Gating of Cx57-EGFP/Cx45 heterotypic GJ channels reveal that Cx57 exhibit a negative gating polarity, i.e. channels tend to close at negativity on the cytoplasmic side of Cx57. Alkalization of pH(i) from 7.2 to 7.8 increased gap junctional conductance (g(j)) of approximately 100-fold with pK(a) = 7.41. We show that this g(j) increase was caused by an increase of both the open channel probability and the number of functional channels. Function of Cx57 UHCs was evaluated based on the uptake of fluorescent dyes. We found that under control conditions, Cx57 UHCs are closed and open at [Ca(2+)](o) = approximately 0.3 mm or below, demonstrating that a moderate reduction of [Ca(2+)](o) can facilitate the opening of Cx57 UHCs. This was potentiated with intracellular alkalization. In summary, our data show that the open channel probability of Cx57 GJs can be modulated by pH(i) with very high efficiency in the physiologically relevant range and may explain pH-dependent regulation of cell-cell coupling in horizontal cell in the retina.

Spatial relationships of connexin36, connexin57 and zonula occludens-1 in the outer plexiform layer of mouse retina

Horizontal cells form gap junctions with each other in mammalian retina, and lacZ reporter analyses have recently indicated that these cells express the Cx57 gene, which codes for the corresponding gap junctional protein. Using anti-connexin57 antibodies, we detected connexin57 protein in immunoblots of mouse retina, and found punctate immunolabeling of this connexin co-distributed with calbindin-positive horizontal cells in the retinal outer plexiform layer. Double immunofluorescence labeling was conducted to determine the spatial relationships of connexin36, connexin57, the gap junction-associated protein zonula occludens-1 and the photoreceptor ribbon synapse-associated protein bassoon in the outer plexiform layer. Connexin36 was substantially co-localized with zonula occludens-1 in the outer plexiform layer, and both of these proteins were frequently located in close spatial proximity to bassoon-positive ribbon synapses. Connexin57 was often found adjacent to, but not overlapping with, connexin36-positive and zonula occludens-1-positive puncta, and was also located adjacent to bassoon-positive ribbon synapses at rod spherules, and intermingled with such synapses at cone pedicles. These results suggest zonula occludens-1 interaction with connexin36 but not with Cx57 in the outer plexiform layer, and an absence of connexin57/connexin36 heterotypic gap junctional coupling in mouse retina. Further, an arrangement of synaptic contacts within rod spherules is suggested whereby gap junctions between horizontal cell terminals containing connexin57 occur in very close proximity to ribbon synapses formed by rod photoreceptors, as well as in close proximity to Cx36-containing gap junctions between rods and cones.

Possible involvement of different connexin43 domains in plasma membrane permeabilization induced by ischemia-reperfusion

In vitro and in vivo studies support the involvement of connexin 43-based cell-cell channels and hemichannels in cell death propagation induced by ischemia-reperfusion. In this context, open connexin hemichannels in the plasma membrane have been proposed to act as accelerators of cell death. Progress on the mechanisms underlying the cell permeabilization induced by ischemia-reperfusion reveals the involvement of several factors leading to an augmented open probability and increased number of hemichannels on the cell surface. While open probability can be increased by a reduction in extracellular concentration of divalent cations and changes in covalent modifications of connexin 43 (oxidation and phosphorylation), increase in number of hemichannels requires an elevation of the intracellular free Ca(2+) concentration. Reversal of connexin 43 redox changes and membrane permeabilization can be induced by intracellular, but not extracellular, reducing agents, suggesting a cytoplasmic localization of the redox sensor(s). In agreement, hemichannels formed by connexin 45, which lacks cytoplasmic cysteines, or by connexin 43 with its C-terminal domain truncated to remove its cysteines are insensitive to reducing agents. Although further studies are required for a precise localization of the redox sensor of connexin 43 hemichannels, modulation of the redox potential is proposed as a target for the design of pharmacological tools to reduce cell death induced by ischemia-reperfusion in connexin 43-expressing cells.

The contribution of the outer retina to color constancy: a general model for color constancy synthesized from primate and fish data

Color constancy is one of the most impressive features of color vision systems. Although the phenomenon has been studied for decades, its underlying neuronal mechanism remains unresolved. Literature indicates an early, possibly retinal mechanism and a late, possibly cortical mechanism. The early mechanism seems to involve chromatic spatial integration and performs the critical calculations for color constancy. The late mechanism seems to make the color manifest. We briefly review the current evidence for each mechanism. We discuss in more detail a model for the early mechanism that is based on direct measurements of goldfish outer retinal processing and induces color constancy and color contrast. In this study we extrapolate this model to primate retina, illustrating that it is highly likely that a similar mechanism is also present in primates. The logical consequence of our experimental work in goldfish and our model is that the wiring of the cone/horizontal cell system sets the reference point for color vision (i.e., it sets the white point for that animal).

Horizontal cell receptive fields are reduced in connexin57-deficient mice

Horizontal cells are coupled by gap junctions; the extensive coupling of the horizontal cells is reflected in their large receptive fields, which extend far beyond the dendritic arbor of the individual cell. In the mouse retina, horizontal cells express connexin57 (Cx57). Tracer coupling of horizontal cells is impaired in Cx57-deficient mice, which suggests that the receptive fields of Cx57-deficient horizontal cells might be similarly reduced. To test this hypothesis we measured the receptive fields of horizontal cells from wildtype and Cx57-deficient mice. First, we examined the synaptic connections between horizontal cells and photoreceptors: no major morphological alterations were found. Moreover, horizontal cell spacing and dendritic field size were unaffected by Cx57 deletion. We used intracellular recordings to characterize horizontal cell receptive fields. Length constants were computed for each cell using the cell's responses to concentric light spots of increasing diameter. The length constant was dependent on the intensity of the stimulus: increasing stimulus intensity reduced the length constant. Deletion of Cx57 significantly reduced horizontal cell receptive field size. Dark resting potentials were strongly depolarized and response amplitudes reduced in Cx57-deficient horizontal cells compared to the wildtype, suggesting an altered input resistance. This was confirmed by patch-clamp recordings from dissociated horizontal cells; mean input resistance of Cx57-deficient horizontal cells was 27% lower than that of wildtype cells. These data thus provide the first quantification of mouse horizontal cell receptive field size and confirm the unique role of Cx57 in horizontal cell coupling and physiology.

Aminosulfonate Modulated pH-induced Conformational Changes in Connexin26 Hemichannels

Gap junction channels regulate cell-cell communication by passing metabolites, ions, and signaling molecules. Gap junction channel closure in cells by acidification is well documented; however, it is unknown whether acidification affects connexins or modulating proteins or compounds that in turn act on connexins. Protonated aminosulfonates directly inhibit connexin channel activity in an isoform-specific manner as shown in previously published studies. High-resolution atomic force microscopy of force-dissected connexin26 gap junctions revealed that in HEPES buffer, the pore was closed at pH < 6.5 and opened reversibly by increasing the pH to 7.6. This pH effect was not observed in non-aminosulfonate buffers. Increasing the protonated HEPES concentration did not close the pore, indicating that a saturation of the binding sites occurs at 10 mM HEPES. Analysis of the extracellular surface topographs reveals that the pore diameter increases gradually with pH. The outer connexon diameter remains unchanged, and there is a approximately 6.5 degrees rotation in connexon lobes. These observations suggest that the underlying mechanism closing the pore is different from an observed Ca2+-induced closure.

Retinal horizontal cell-specific promoter activity and protein expression of zebrafish connexin 52.6 and connexin 55.5

Connexins in retinal horizontal cells (HC) function in the processing of visual information. For example, gap junction-forming connexins may contribute to the spatial integration of visual stimuli. Additionally, connexin hemichannels have been hypothesized to participate in the feedback pathway from HCs to cones. To verify the identities of the zebrafish HC connexins, we performed promoter expression and immunohistochemical studies of connexin 52.6 (Cx52.6) and Cx55.5. Zebrafish embryos were microinjected with Cx52.6 or Cx55.5 promoter sequences and a green fluorescent protein reporter construct. Light and electron microscopic (EM) analysis showed green fluorescent protein expression exclusively in retinal HCs. Immunohistochemistry confirmed that HCs express Cx52.6 and Cx55.5 proteins. Light microscopy revealed Cx52.6 and Cx55.5 in the retinal inner nuclear and outer plexiform layers. Double labeling for Cx55.5 or Cx52.6 and cell-specific markers (tyrosine hydroxylase, protein kinase C-alpha, or GluR2) demonstrated that these connexins do not localize to interplexiform or ON bipolar cells, but most likely are present in HCs. Preembedding immuno-EM confirmed the HC-specific expression of Cx52.6 and Cx55.5 and illustrated the presence of these two connexins in gap junctions between HCs. The EM data also revealed robust labeling for Cx55.5 in hemichannels on HC dendrites in photoreceptor synaptic terminals. Voltage-clamp experiments in cultured cells demonstrated that Cx55.5-containing hemichannels can open at physiological membrane potentials. These results offer the first in vivo demonstration of the HC-specific activities of the Cx52.6 and Cx55.5 promoters. Furthermore, these data provide the first proof at the protein level for retinal HC-specific connexins in the zebrafish.

Species specificity of mammalian connexin-26 to form open voltage-gated hemichannels

Mutations of connexin-26 (Cx26) cause nonsyndromic hearing loss and other syndromes affecting ectoderm-derived tissues. While the exact mechanisms underlying these diseases remain elusive, Cx's are generally considered to mediate cell-to-cell communication by forming gap junction channels. We show here that unlike rat Cx26, human and sheep Cx26 form voltage-gated hemichannels when expressed in oocytes and Neuro2A cells. A single evolutionary amino acidic change at position 159 of the rodent protein, the replacement of aspartic acid with asparagine in the human and sheep proteins, accounts for this species specificity. At the resting potential and in normal millimolar extracellular calcium, open human Cx26 hemichannels can be detected both electrophysiologically and by dye uptake, although they did not affect cell viability. These hemichannels opened at approximately -50 mV and their activation increased by depolarization until they inactivate at positive membrane potentials. Single-channel analysis revealed that activation and inactivation involved two distinct voltage gating mechanisms and that the fully open hemichannel displays a conductance twice that of the intercellular channel. The existence of a hemichannel that opens under physiological control of the membrane potential may have important implications for the normal and pathological activity of Cx26 in humans, particularly with respect to hearing and the epidermis.

Electrical Feedback in the Cone Pedicle: A Computational Analysis

One of the fundamental principles of neuroscience is that direct electrical interactions between neurons are not possible without specialized electrical contacts, gap junctions, because the transmembrane resistance of neurons is typically much higher than the resistance of the adjacent extracellular space. However it has been proposed that in the retina direct electrical interactions between cones and second-order neurons occur due to the specific morphology of the cone synaptic terminal. This electrical mechanism could potentially explain the phenomenon of “negative feedback” from horizontal cells to cones and the recent finding that the tips of horizontal cell dendrites contain hemichannels has rekindled interest in the idea. We quantitatively evaluated the possibility that hemichannels and/or glutamate channels mediate electrical feedback from horizontal cells to cones. The calculations show that it is unlikely that an electrical mechanism plays a significant functional role because 1) the necessity of preserving adequate cone-to-horizontal-cell synaptic transmission limits the extracellular space resistance and the horizontal-cell dendritic transmembrane resistances to values at which the effectiveness of electrical feedback is very low and its electrical effect on the cone presynaptic membrane is negligible, 2) electrical feedback is most effective in the dark and weaker during light adaptation, which contradicts the experimental data, and 3) electrical negative feedback is associated with much stronger electrical positive feedback from horizontal cells to cones, a phenomenon that has never been reported. Therefore it is likely that negative feedback from horizontal cells to cones is chemical in nature.

Deletion of connexin45 in mouse retinal neurons disrupts the rod/cone signaling pathway between AII amacrine and ON cone bipolar cells and leads to impaired visual transmission

Connexin45 (Cx45) is known to be expressed in the retina, but its functional analysis was problematic because general deletion of Cx45 coding DNA resulted in cardiovascular defects and embryonic lethality at embryonic day 10.5. We generated mice with neuron-directed deletion of Cx45 and concomitant activation of the enhanced green fluorescent protein (EGFP). EGFP labeling was observed in bipolar, amacrine, and ganglion cell populations. Intracellular microinjection of fluorescent dyes in EGFP-labeled somata combined with immunohistological markers revealed Cx45 expression in both ON and OFF cone bipolar cells. The scotopic electroretinogram of mutant mice revealed a normal a-wave but a 40% reduction in the b-wave amplitude, similar to that found in Cx36-deficient animals, suggesting a possible defect in the rod pathway of visual transmission. Indeed, neurotransmitter coupling between AII amacrine cells and Cx45-expressing cone bipolar cells was disrupted in Cx45-deficient mice. These data suggest that both Cx45 and Cx36 participate in the formation of functional heterotypic electrical synapses between these two types of retinal neurons that make up the major rod pathway.

Expression and functions of neuronal gap junctions

Gap junctions are channel-forming structures in contacting plasma membranes that allow direct metabolic and electrical communication between almost all cell types in the mammalian brain. At least 20 connexin genes and 3 pannexin genes probably code for gap junction proteins in mice and humans. Gap junctions between murine neurons (also known as electrical synapses) can be composed of connexin 36, connexin 45 or connexin 57 proteins, depending on the type of neuron. Furthermore, pannexin 1 and 2 are likely to form electrical synapses. Here, we discuss the roles of connexin and pannexin genes in the formation of neuronal gap junctions, and evaluate recent functional analyses of electrical synapses that became possible through the characterization of mouse mutants that show targeted defects in connexin genes.

Role of connexin-based gap junction channels and hemichannels in ischemia-induced cell death in nervous tissue

Gap junction channels and hemichannels formed of connexin subunits are found in most cell types in vertebrates. Gap junctions connect cells via channels not open to the extracellular space and permit the passage of ions and molecules of approximately 1 kDa. Single connexin hemichannels, which are connexin hexamers, are present in the surface membrane before docking with a hemichannel in an apposed membrane. Because of their high conductance and permeability in cell-cell channels, it had been thought that connexin hemichannels remained closed until docking to form a cell-cell channel. Now it is clear that at least some hemichannels can open to allow passage of molecules between the cytoplasm and extracellular space. Here we review evidence that gap junction channels may allow intercellular diffusion of necrotic or apoptotic signals, but may also allow diffusion of ions and substances from healthy to injured cells, thereby contributing to cell survival. Moreover, opening of gap junction hemichannels may exacerbate cell injury or mediate paracrine or autocrine signaling. In addition to the cell specific features of an ischemic insult, propagation of cell damage and death within affected tissues may be affected by expression and regulation of gap junction channels and hemichannels formed by connexins.

Connexin-based gap junction hemichannels: gating mechanisms

Connexins (Cxs) form hemichannels and gap junction channels. Each gap junction channel is composed of two hemichannels, also termed connexons, one from each of the coupled cells. Hemichannels are hexamers assembled in the ER, the Golgi, or a post Golgi compartment. They are transported to the cell surface in vesicles and inserted by vesicle fusion, and then dock with a hemichannel in an apposed membrane to form a cell-cell channel. It was thought that hemichannels should remain closed until docking with another hemichannel because of the leak they would provide if their permeability and conductance were like those of their corresponding cell-cell channels. Now it is clear that hemichannels formed by a number of different connexins can open in at least some cells with a finite if low probability, and that their opening can be modulated under various physiological and pathological conditions. Hemichannels open in different kinds of cells in culture with conductance and permeability properties predictable from those of the corresponding gap junction channels. Cx43 hemichannels are preferentially closed in cultured cells under resting conditions, but their open probability can be increased by the application of positive voltages and by changes in protein phosphorylation and/or redox state. In addition, increased activity can result from the recruitment of hemichannels to the plasma membrane as seen in metabolically inhibited astrocytes. Mutations of connexins that increase hemichannel open probability may explain cellular degeneration in several hereditary diseases. Taken together, the data indicate that hemichannels are gated by multiple mechanisms that independently or cooperatively affect their open probability under physiological as well as pathological conditions.

Ocular manifestations of keratitis– ichthyosis–deafness (KID) syndrome

OBJECTIVE

Keratitis-ichthyosis-deafness (KID) syndrome is a rare congenital ectodermal dysplasia characterized by the association of hyperkeratotic skin lesions, moderate to profound sensorineural hearing loss and vascularizing keratitis. Mutations in the GJB2 gene coding for connexin 26, a component of gap junctions in epithelial cells, have been observed in several KID patients. Variable ocular manifestations of the disease in 3 patients with molecular genetically confirmed KID syndrome are reported.

DESIGN

Retrospective case series.

METHODS

Clinical examination and molecular genetic analysis for mutations in the GJB2 gene were performed in 3 patients with KID syndrome ages 5, 13, and 41 years.

RESULTS

Visual acuity ranged from normal to severe visual loss. The ocular signs included loss of eyebrows and lashes, thickened and keratinized lids, trichiasis, recurrent corneal epithelial defects, superficial and deep corneal stromal vascularization with scarring, keratoconjunctivitis sicca, and, in one patient, presumed limbal insufficiency. Whereas ocular surface integrity could be maintained with artificial tears in one patient, and an epithelial defect healed under conservative treatment in the second patient, multiple surgical procedures including superficial keratectomies, limbal allograft transplantation with systemic immunosuppression, amniotic membrane transplantation, lateral tarsorrhaphies, and lamellar keratoplasty could not preserve useful vision in the third patient.

CONCLUSIONS

KID syndrome may affect the ocular adnexae and surface with variable severity independent of the age of the patient. Lid abnormalities, corneal surface instability, limbal stem cell deficiency with resulting corneal complications, and dry eye are the main ocular manifestations.

Rhythmic expression of clock-controlled genes in retinal photoreceptors is sensitive to 18-beta-glycyrrhetnic acid and 18-alpha-glycyrrhetnic acid-3-hemisuccinate

Chicken retina contains circadian oscillators that drive rhythmic transcription of several genes expressed in photoreceptor cells. To determine if gap junctions assist in coordinating these transcript rhythms, we examined the effects of two compounds, 18alpha-glycyrrhetnic acid-3-hemisuccinate (ACO) and 18beta-glycyrrhetnic acid (18beta-GA), on photoreceptor iodopsin and arylalkylamine N-acetyltransferase (AANAT) transcript rhythms in embryonic chicken retinal explant cultures that were maintained under different lighting conditions. Both compounds, whose actions include reversibly block gap junction permeability, produced rapid and sustained reductions in iodopsin and AANAT mRNA levels, but did not alter the levels of guanylate cyclase activating protein-1 (GCAP1) mRNA, a noncircadian-regulated, photoreceptor-specific gene. The iodopsin and AANAT mRNA rhythms re-emerged in the cultured retinas within 24 h of removal of the compounds. These results show that the effects of ACO and 18beta-GA on iodopsin and AANAT mRNA levels were not due to generalized suppression of gene transcription. The dramatic reduction in the levels of iodopsin and AANAT mRNA induced by these compounds suggests a mechanism of action that directly affects the synthesis and/or degradation of these transcripts rather than the synchronization or function of the retinal oscillators that drive transcription of these genes.

The involvement of glutamate-gated channels in negative feedback from horizontal cells to cones

Photoreceptors are the light sensitive cells in the retina. They project to horizontal cells and bipolar cells via a glutamatergic feed forward pathway. Horizontal cells are strongly electrically coupled and integrate in that way the input from the photoreceptors. Horizontal cells feedback to cones negatively. The combined signal from the photoreceptors and the horizontal cells is sent to the bipolar cells. The feedback pathway from horizontal cells to cones is thought to form the basis for the center/surround organization of bipolar cells. The nature of the feedback pathway is an issue of intense debate. It was thought for a long time that this feedback pathway was GABAergic, because cones have GABA-receptors and horizontal cells release GABA via a GABA-transporter working in the reversed direction. However, recently we showed in goldfish that horizontal cells feed back to cones via an alternative mechanism. In goldfish, negative feedback from horizontal cells to cones shifts the calcium current of the cone to more negative potentials. This feedback pathway is independent of GABA, since feedback cannot be blocked by either saturating concentrations of PTX, the GABA-transporter blocker SKF89976A, or application of GABA. The mechanism of negative feedback from horizontal cells to cones involves hemichannels located at the tips of the invaginating horizontal cells, just opposite to the calcium channels of the cones. Current flowing through these hemichannels changes the extracellular potential deep in the synaptic cleft and in that way modulates the calcium current of the cones. Such a modulation of the extracellular potential is called ephaptic. If negative feedback from horizontal cells to cones is indeed ephaptic, other channels present in the synapse should also be able to act as a current source, i.e., should also be able to change the output of the cone. We showed that glutamate-gated channels present at the tips of the horizontal cell dendrites can also mediate feedback responses. Surprisingly, although the glutamate-gated conductance of the horizontal cells is eight times the hemichannel conductance, glutamate-gated channels are not the major current source in negative feedback from horizontal cells to cones. In this chapter we present evidence that this is due to the more focal localization of the hemichannels, compared to a diffuse and extrasynaptic localization of the glutamate-gated channels.

Ephaptic interactions within a chemical synapse: hemichannel-mediated ephaptic inhibition in the retina

The two best-known types of cell-cell communication are chemical synapses and electrical synapses, which are formed by gap junctions. A third, less well known, form of communication is ephaptic transmission, in which electric fields generated by a specific neuron alter the excitability of neighboring neurons as a result of their anatomical and electrical proximity. Ephaptic communication can be present in a variety of forms, each with their specific features and functional implications. One of these is ephaptic modulation within a chemical synapse. This type of communication has recently been proposed for the cone-horizontal cell synapse in the vertebrate retina. Evidence indicates that the extracellular potential in the synaptic terminal of photoreceptors is modulated by current flowing through connexin hemichannels at the tips of the horizontal cell dendrites, mediating negative feedback from horizontal cells to cones. This example can be added to the growing list of cases of ephaptic communication in the central nervous system.

Biophysical characterization of zebrafish connexin35 hemichannels

A subset of connexins can form unopposed hemichannels in expression systems, providing an opportunity for comparison of hemichannel gating properties with those of intact gap junction channels. Zebrafish connexin35 (Cx35) is a member of the Cx35/Cx36 subgroup of connexins highly expressed in the retina and brain. In the present study, we have shown that Cx35 expression in Xenopus oocytes and N2A cells produced large outward whole cell currents on cell depolarization. Using whole cell, cell-attached, and excised patch configurations, we obtained multichannel and single-channel current recordings attributable to the Cx35 hemichannels (I(hc)) that were activated and increased by stepwise depolarization of membrane potential (V(m)) and deactivated by hyperpolarization. The currents were not detected in untransfected N2A cells or in control oocytes injected with antisense Cx38. However, water-injected oocytes that were not treated with antisense showed activities attributable to Cx38 hemichannels that were easily distinguishable from Cx35 hemichannels by a significantly larger unitary conductance (gamma(hc): 250-320 pS). The gamma(hc) of Cx35 hemichannels exhibited a pronounced V(m) dependence; i.e., gamma(hc) increased/decreased with relative hyperpolarization/depolarization (gamma(hc) was 72 pS at V(m) = -100 mV and 35 pS at V(m) = 100 mV). Extrapolation to V(m) = 0 mV predicted a gamma(hc) of 48 pS, suggesting a unitary conductance of intact Cx35 gap junction channels of approximately 24 pS. Channel gating was also V(m) dependent: open time declined with negative V(m) and increased with positive V(m). The ability to break down the complex gating of intact intercellular channels into component hemichannels in vitro will help to evaluate putative physiological roles for hemichannels in vivo.

Functional expression of connexin57 in horizontal cells of the mouse retina

Horizontal cells are interneurons of the vertebrate retina that exhibit strong electrical and tracer coupling but the identity of the channel-forming connexins has remained elusive. Here we show that horizontal cells of the mouse retina express connexin57 (Cx57). We have generated Cx57-deficient mice by replacing the Cx57 coding region with a lacZ reporter gene, expressed under control of the endogenous Cx57 promoter. These mice were fertile and showed no obvious anatomical or behavioural abnormalities. Cx57 mRNA was expressed in the retina of wild-type littermates but was absent from the retina of Cx57-deficient mice. Previously reported results that the Cx57 gene was very weakly expressed in several other mouse tissues turned out to be unspecific. Cx57 mRNA is abundantly expressed in the retina and weakly in the thymus of adult mice but absent in all other adult tissues tested, including brain. Furthermore, Cx57 is expressed in embryonic kidney at E16.5 to E18.5 days post-conception, as indicated by the pattern of lacZ expression. Within the retina, lacZ signals were assigned exclusively to horizontal cells based on co-localization with cell-type-specific marker proteins. Microinjection of Neurobiotin into horizontal cells of isolated retinae revealed less than 1% of tracer coupling in Cx57-deficient retinae compared with wild-type controls. Cx57 is the first connexin identified in mammalian horizontal cells and the first connexin whose expression is apparently restricted to only one type of neuron.

Retinal bipolar cell input mechanisms in giant danio. I. Electroretinographic analysis

Electroretinograms (ERGs) were recorded from the giant danio (Danio aequipinnatus) to study glutamatergic input mechanisms onto bipolar cells. Glutamate analogs were applied to determine which receptor types mediate synaptic transmission from rods and cones to on and off bipolar cells. Picrotoxin, strychnine, and tetrodotoxin were used to isolate the effects of the glutamate analogs to the photoreceptor-bipolar cell synapse. Under photopic conditions, the group III metabotropic glutamate receptor (mGluR) antagonist (RS)-alpha-cyclopropyl-4-phosphonophenylglycine (CPPG) only slightly reduced the b-wave, whereas the excitatory amino acid transporter (EAAT) blocker dl-threo-beta-benzyl-oxyaspartate (TBOA) removed most of it. Complete elimination of the b-wave required both antagonists. The alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)/kainate receptor antagonist 2,3-dioxo-6-nitro-1,2,3,4-tetrahydrobenzo[f]quinoxaline-7-sulfonamide (NBQX) blocked the d-wave. Under scotopic conditions, rod and cone inputs onto on bipolar cells were studied by comparing the sensitivities of the b-wave to photopically matched green and red stimuli. The b-wave was >1 log unit more sensitive to the green than to the red stimulus under control conditions. In CPPG or l-AP4 (l-(+)-2-amino-4-phosphonobutyric acid, a group III mGluR agonist), the sensitivity of the b-wave to the green stimulus was dramatically reduced and the b-waves elicited by the 2 stimuli became nearly matched. The d-wave elicited by dim green stimuli, which presumably could be detected only by the rods, was eliminated by NBQX.

IN CONCLUSION

1) cone signals onto on bipolar cells involve mainly EAATs but also mGluRs (presumably mGluR6) to a lesser extent; 2) rods signal onto on bipolars by mainly mGluR6; 3) off bipolar cells receive signals from both photoreceptor types by AMPA/kainate receptors.

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.

Carbenoxolone inhibition of voltage-gated Ca channels and synaptic transmission in the retina

We show that carbenoxolone, a drug used to block hemichannels in the retina to test the ephaptic model of horizontal cell inhibitory feedback, has strong inhibitory effects on voltage-gated Ca channels. Carbenoxolone (100 microM) reduced photoreceptor-to-horizontal cell synaptic transmission by 92%. Applied to patch-clamped, isolated cone photoreceptors, carbenoxolone inhibited Ca channels with an EC(50) of 48 microM. At 100 microM, it reduced cone Ca channel current by 37%, reduced depolarization-evoked [Ca(2+)] signals in fluo-4 loaded retinal slices by 57% and inhibited Ca channels in Müller cells by 52%. A synaptic transfer model suggests that the degree of block of Ca channels accounts for the reduction in synaptic transmission. These results suggest broad inhibitory actions for carbenoxolone in the retina that must be considered when interpreting its effects on inhibitory feedback.