Immunogold evidence that neuronal gap junctions in adult rat brain and spinal cord contain connexin-36 but not connexin-32 or connexin-43

Network architecture of gap junction-coupled neuronal linkage in the striatum

Previous studies have revealed the existence of gap junctions between GABAergic interneurons of a particular type in the striatum. Because of the technical difficulties, however, there is no information about their positions within the striatal circuitry. We have developed a method to detect neuronal gap junctions reliably at the light microscopic level and thereby explored the network architecture of the gap junctional linkage. Gap junction-coupled networks among parvalbumin-containing GABAergic interneurons extended nonuniformly in the feline striatum. They were located predominantly in the methionine-enkephalin-poor matrix. Moreover, the density of gap junctional coupling showed a marked regional difference along the anterior-posterior axis of the striatum. The densest interconnectivity was found in the posterior part of both caudate nucleus and putamen that corresponds to the sensory-recipient area of the feline striatum. Electron microscopic observations provided clear evidence of internalization of neuronal gap junction, indicating the dynamic nature of gap junctional linkage between neurons in vivo. The nonuniform organization of gap junction networks suggests differential modes of information processing in heterogeneous subregions of the striatum.

Use of calcium imaging for analysis of neuronal gap junction coupling

We recently used Western blots for connexin 36 and neuronal dye coupling with neurobiotin to measure developmental decrease in neuronal gap junction coupling in cell cultures. To ask whether Ca2+ imaging also can be used to measure changes in the amount of neuronal gap junction coupling, we defined a Ca2+ coupling coefficient as the percentage of neurons with bicuculline-induced increases in intracellular Ca2+ that are suppressed by blocking gap junctions. We demonstrate in rat and mouse hypothalamic neuronal cultures that the Ca2+ coupling coefficient decreases during culture development, this decrease is prevented by manipulations that also prevent developmental decrease in neuronal gap junction coupling, and the coefficient is low in cultures lacking connexin 36. The results indicate that Ca2+ imaging is a useful tool to quantify the amount of neuronal gap junction coupling in cultures.

Regulation of neuronal connexin-36 channels by pH

Neurotransmission through electrical synapses plays an important role in the spike synchrony among neurons and oscillation of neuronal networks. Indeed, electrical transmission has been implicated in the hypersynchronous electrical activity of epilepsy. We have investigated the influence of intracellular pH on the strength of electrical coupling mediated by connexin36 (Cx36), the principal gap junction protein in the electrical synapses of vertebrates. In striking contrast to other connexin isoforms, the activity of Cx36 channels decreases following alkalosis rather than acidosis when it is expressed in Xenopus oocytes and N2A cells. This uncoupling of Cx36 channels upon alkalinization occurred in the vertebrate orthologues analyzed (human, mouse, chicken, perch, and skate). While intracellular acidification caused a mild or moderate increase in the junctional conductance of virtually all these channels, the coupling of the skate Cx35 channel was partially blocked by acidosis. The mutational analysis suggests that the Cx36 channels may contain two gating mechanisms operating with opposing sensitivity to pH. One gate, the dominant mechanism, closes for alkalosis and it probably involves an interaction between the C- and N-terminal domains, while a secondary acid sensing gate only causes minor, albeit saturating, changes in coupling following acidosis and alkalosis. Thus, we conclude that neuronal Cx36 channels undergo unique regulation by pH(i) since their activity is inhibited by alkalosis rather than acidosis. These data provide a novel basis to define the relevance and consequences of the pH-dependent modulation of Cx36 synapses under physiological and pathological conditions.

CNQX and AMPA inhibit electrical synaptic transmission: a potential interaction between electrical and glutamatergic synapses

Electrical synapses play an important role in signaling between neurons and the synaptic connections between many neurons possess both electrical and chemical components. Although modulation of electrical synapses is frequently observed, the cellular processes that mediate such changes have not been studied as thoroughly as plasticity in chemical synapses. In the leech (Hirudo sp), the competitive AMPA receptor antagonist CNQX inhibited transmission at the rectifying electrical synapse of a mixed glutamatergic/electrical synaptic connection. This CNQX-mediated inhibition of the electrical synapse was blocked by concanavalin A (Con A) and dynamin inhibitory peptide (DIP), both of which are known to inhibit endocytosis of neurotransmitter receptors. CNQX-mediated inhibition was also blocked by pep2-SVKI (SVKI), a synthetic peptide that prevents internalization of AMPA-type glutamate receptor. AMPA itself also inhibited electrical synaptic transmission and this AMPA-mediated inhibition was partially blocked by Con A, DIP and SVKI. Low frequency stimulation induced long-term depression (LTD) in both the electrical and glutamatergic components of these synapses and this LTD was blocked by SVKI. GYKI 52466, a selective non-competitive antagonist of AMPA receptors, did not affect the electrical EPSP, although it did block the glutamatergic component of these synapses. CNQX did not affect non-rectifying electrical synapses in two different pairs of neurons. These results suggest an interaction between AMPA-type glutamate receptors and the gap junction proteins that mediate electrical synaptic transmission. This putative interaction between glutamate receptors and gap junction proteins represents a novel mechanism for regulating the strength of synaptic transmission.

Noradrenergic modulation of electrical coupling in GABAergic networks of the hippocampus

Noradrenergic modulation of cortical circuits is involved in information processing, regulation of higher functions, and prevention of epileptic activity. Here, we studied the effects of noradrenaline on the functional connectivity of GABAergic networks of the hippocampus and show that electrical synapses between interneurons are a novel target of noradrenergic modulation in vitro. Application of noradrenaline or of the selective beta-adrenergic agonist isoproterenol decreased gap junction-based coupling in paired recordings from stratum lacunosum-moleculare interneurons by approximately 40%. Similar results were obtained after pharmacological stimulation of the adenylyl cyclase with forskolin. In contrast, the adenylyl cyclase antagonist MDL12330A [cis-N-(2-phenylcyclopentyl)azacyclotridec-1-en-2-amine] or the specific protein kinase A (PKA) inhibitor H89 (N-[2-(p-bromocinnamyl-amino)ethyl]-5-isoquinolinesulfonamide dihydrochloride) enhanced the basal strength of coupling by approximately 30%. In addition, PKA-mediated phosphorylation was critical for both isoproterenol- and forskolin-dependent regulation of coupling, because inclusion of the PKA antagonist KT5720 [(9S,10R,12R)-2,3,9,10,11,12-hexahydro-10-hydroxy-9-methyl-1-oxo-9,12-epoxy-1H-diindolo[1,2,3-fg:3',2',1'-kl]pyrrolo[3,4-i][1,6]benzodiazocine-10-carboxylicacid hexyl ester] in the recording pipettes prevented modulation. Lastly, we studied the effects of beta-adrenergic modulation on mixed polysynaptic transmission within the GABAergic network. Isoproterenol depressed propagation of GABA(A) receptor-mediated synaptic currents, but did not change significantly direct GABAergic input, indicating that regulation of electrical coupling adds flexibility to the information flow generated by chemical synapses. In conclusion, activation of beta-adrenergic receptors in stratum lacunosum-moleculare GABAergic networks reduces electrical synaptic transmission via a cAMP/PKA signaling cascade, and affects the degree of synaptic divergence within the circuit. We propose that this dynamic modulation and interplay between electrical and chemical synaptic transmission in GABAergic networks contributes to the tuning of memory processes in vivo, and prevents hypersynchronous activity.

Neuronal connexin expression in the cochlear nucleus of big brown bats

We present immunohistochemical data describing the presence and distribution of connexins, structural component of gap junctions, in the cochlear nuclei of adult big brown bats (Eptesicus fuscus). Echolocating big brown bats show microsecond scale echo-delay sensitivity that requires accurate synchronization of neuronal responses to the timing of echoes. Midbrain and auditory cortical neuronal response timing is similar to that observed in other non-echolocating mammals, suggesting that lower auditory processing nuclei may have specialized mechanisms for obtaining the required temporal hyperacuity. Our data shows that connexin 36, a gap junction protein specific to neurons, is most densely expressed in the bat's cochlear nuclear complex, the medullary region that receives and processes first-order afferents from the auditory nerve. Cx36 expression is absent in the cochlear nucleus of normal mice, which have high-frequency hearing sensitivity similar to big brown bats. Glial connexins, Cx26 and Cx43, expressed in astrocytes and several inner ear structures, are also found in the bat cochlear nucleus complex, associated with major fiber tracts in and around the cochlear nuclei. The extensive presence of neuronally-associated Cx36 in brainstem auditory structures of adult bats suggests a possible role for gap junctions in mediating echo-delay hyperacuity.

Immunohistochemical detection of connexin36 in sympathetic preganglionic and somatic motoneurons in the adult rat

Gap junctional communication in the adult CNS plays an important role in the synchronization of neuronal activities. In vitro studies have shown evidence of electrotonic coupling through gap junctions between sympathetic preganglionic motoneurons and between somatic motoneurons in the neonatal and adult rat spinal cord. Electrotonic transmission of membrane oscillations might be an important mechanism for recruitment of neurons and result in the generation of rhythmic sympathetic and somato-motor activity at the population level. Gap junctions in the adult spinal cord are constituted principally by connexin36 (Cx36). However, the distribution of Cx36 in specific neuronal populations of the spinal cord is unknown. Here, we identify Cx36-like immunoreactivity in sympathetic preganglionic and somatic motoneurons in thoracic spinal cord segments of the adult rat. For this purpose, double immunostaining against Cx36 and choline acetyltransferase (ChAT) was performed on transverse sections (20 μm) taken from spinal segments T6–T8. Cx36 punctate immunostaining was detected in the majority of ChAT-immunoreactive (-ir) neurons from lamina VII [intermediolateral cell column (IML) and intercalated cell group (IC)], lamina X [central autonomic nucleus (CA)] and in ventral horn neurons from laminae VIII and IX. Cx36 puncta were distributed in the neuronal somata and along dendritic processes. The presence of Cx36 in ChAT-ir neurons is consistent with electrical coupling between sympathetic preganglionic motoneurons and between somatic motoneurons through gap junctions in the adult spinal cord.

Expression of connexins in embryonic mouse neocortical development

During embryonic development, young neurons migrate from the ventricular zone to the cortical plate of the cerebral cortex. Disturbances in this neuronal migration have been associated with numerous diseases such as mental retardation, double cortex, Down syndrome, and epilepsy. One possible cause of these neuropathologies is an aberration in normal gap junctional communication. At least 20 connexin (Cx) genes encode gap junction proteins in mice and humans. A proper understanding of the role of specific connexins in the developing brain requires the characterization of their spatial and temporal pattern of expression. In the current study we performed all the experiments on mouse developing cortex at embryonic days (E) 14, 16, and 18, timepoints that are highly active with regard to cortical development. Using reverse transcription-polymerase chain reaction, Western blot analysis, and immunohistochemistry, we found that among the family of gap junction proteins, Cx26, Cx36, Cx37, Cx43, and Cx45 were expressed in the developing cortex of mice, Cx30 and Cx32 were absent, while Cx40 was expressed at a very low level. Our results demonstrate that Cx26 and Cx37 were evenly distributed in the cortical layers of developing brain, while Cx36 and Cx43 were more abundant in the ventricular zone and cortical plate. Cx45 distribution appeared to be more abundant at E18 compared to the other timepoints (E14 and E16). Thus, the present study provides identification and the distribution pattern for Cxs associated with cortical development during normal neuronal migration.

Gap junctions are required for NMDA receptor dependent cell death in developing neurons

A number of studies have indicated an important role for N-methyl-D-aspartate (NMDA) receptors in cell survival versus cell death decisions during neuronal development, trauma, and ischemia. Coupling of neurons by electrical synapses (gap junctions) is high or increases in neuronal networks during all three of these conditions. However, whether neuronal gap junctions contribute to NMDA receptor-regulated cell death is not known. Here we address the role of neuronal gap junction coupling in NMDA receptor-regulated cell death in developing neurons. We report that inactivation or hyperactivation of NMDA receptors induces neuronal cell death in primary hypothalamic cultures, specifically during the peak of developmental gap junction coupling. In contrast, increasing or decreasing NMDA receptor function when gap junction coupling is low has no or greatly reduced impact on cell survival. Pharmacological inactivation of gap junctions or knockout of neuronal connexin 36 prevents the cell death caused by NMDA receptor hypofunction or hyperfunction. The results indicate the critical role of neuronal gap junctions in cell death caused by increased or decreased NMDA receptor function in developing neurons. Based on these data, we propose the novel hypothesis that NMDA receptors and gap junctions work in concert to regulate neuronal survival.

Connexin36 vs. connexin32, "miniature" neuronal gap junctions, and limited electrotonic coupling in rodent suprachiasmatic nucleus

Suprachiasmatic nucleus (SCN) neurons generate circadian rhythms, and these neurons normally exhibit loosely-synchronized action potentials. Although electrotonic coupling has long been proposed to mediate this neuronal synchrony, ultrastructural studies have failed to detect gap junctions between SCN neurons. Nevertheless, it has been proposed that neuronal gap junctions exist in the SCN; that they consist of connexin32 or, alternatively, connexin36; and that connexin36 knockout eliminates neuronal coupling between SCN neurons and disrupts circadian rhythms. We used confocal immunofluorescence microscopy and freeze-fracture replica immunogold labeling to examine the distributions of connexin30, connexin32, connexin36, and connexin43 in rat and mouse SCN and used whole-cell recordings to re-assess electrotonic and tracer coupling. Connexin32-immunofluorescent puncta were essentially absent in SCN but connexin36 was relatively abundant. Fifteen neuronal gap junctions were identified ultrastructurally, all of which contained connexin36 but not connexin32, whereas nearby oligodendrocyte gap junctions contained connexin32. In adult SCN, one neuronal gap junction was >600 connexons, whereas 75% were smaller than 50 connexons, which may be below the limit of detectability by fluorescence microscopy and thin-section electron microscopy. Whole-cell recordings in hypothalamic slices revealed tracer coupling with neurobiotin in <5% of SCN neurons, and paired recordings (>40 pairs) did not reveal obvious electrotonic coupling or synchronized action potentials, consistent with few neurons possessing large gap junctions. However, most neurons had partial spikes or spikelets (often <1 mV), which remained after QX-314 [N-(2,6-dimethylphenylcarbamoylmethyl)triethylammonium bromide] had blocked sodium-mediated action potentials within the recorded neuron, consistent with spikelet transmission via small gap junctions. Thus, a few "miniature" gap junctions on most SCN neurons appear to mediate weak electrotonic coupling between limited numbers of neuron pairs, thus accounting for frequent detection of partial spikes and hypothetically providing the basis for "loose" electrical or metabolic synchronization of electrical activity commonly observed in SCN neuronal populations during circadian rhythms.

Gap junctional intercellular communication in hypoxia-ischemia-induced neuronal injury

Brain hypoxia-ischemia is a relatively common and serious problem in neonates and in adults. Its consequences include long-term histological and behavioral changes and reduction in seizure threshold. Gap junction intercellular communication is pivotal in the spread of hypoxia-ischemia related injury and in mediating its long-term effects. This review provides a comprehensive and critical review of hypoxia-ischemia and hypoxia in the brain and the potential role of gap junctions in the spread of the neuronal injury induced by these insults. It also presents the effects of hypoxia-ischemia and of hypoxia on the state of gap junctions in vitro and in vivo. Understanding the mechanisms involved in gap junction-mediated neuronal injury due to hypoxia will lead to the development of novel therapeutic strategies.

Heterogeneous electrotonic coupling and synchronization of rhythmic bursting activity in mouse Hb9 interneurons

The neurons and mechanisms involved in mammalian spinal cord networks that produce rhythmic locomotor activity remain largely undefined. Hb9 interneurons, a small population of discretely localized interneurons in the mouse spinal cord, are conditionally bursting neurons. Here we applied potassium channel blockers with the aim of increasing neuronal excitability and observed that under these conditions, postnatal Hb9 interneurons exhibited bursts of action potentials with underlying voltage-independent spikelets. The bursts were insensitive to antagonists to fast chemical synaptic transmission, and the bursting and spikelets were blocked by tetrodotoxin. Calcium imaging studies using 2-photon excitation in spinal cord slices revealed that clustered Hb9 interneurons exhibited synchronous and occasional asynchronous, calcium transients that were also insensitive to fast synaptic transmission blockade. All transients were blocked by the gap junction blocker carbenoxolone. Paired whole cell patch-clamp recordings of Hb9 interneurons in the late postnatal mouse revealed common chemical synaptic inputs but no evidence of current transfer (i.e., electrotonic coupling) between the neurons. However, Hb9 and a previously defined population of non-Hb9 interneurons were electrotonically coupled. In the absence of fast chemical transmission in the whole spinal cord preparation, 2-photon excitation calcium imaging revealed bursting activity of Hb9 interneurons synchronous with rhythmic ventral root output. Thus Hb9 interneurons are both endogenous bursters and rhythmically active within a heterogeneous electrotonically coupled network. A network with these properties could produce the wide range of stable rhythms necessary for locomotor activity.

Identification of connexin36 in gap junctions between neurons in rodent locus coeruleus

Locus coeruleus neurons are strongly coupled during early postnatal development, and it has been proposed that these neurons are linked by extraordinarily abundant gap junctions consisting of connexin32 (Cx32) and connexin26 (Cx26), and that those same connexins abundantly link neurons to astrocytes. Based on the controversial nature of those claims, immunofluorescence imaging and freeze-fracture replica immunogold labeling were used to re-investigate the abundance and connexin composition of neuronal and glial gap junctions in developing and adult rat and mouse locus coeruleus. In early postnatal development, connexin36 (Cx36) and connexin43 (Cx43) immunofluorescent puncta were densely distributed in the locus coeruleus, whereas Cx32 and Cx26 were not detected. By freeze-fracture replica immunogold labeling, Cx36 was found in ultrastructurally-defined neuronal gap junctions, whereas Cx32 and Cx26 were not detected in neurons and only rarely detected in glia. In 28-day postnatal (adult) rat locus coeruleus, immunofluorescence labeling for Cx26 was always co-localized with the glial gap junction marker Cx43; Cx32 was associated with the oligodendrocyte marker 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNPase); and Cx36 was never co-localized with Cx26, Cx32 or Cx43. Ultrastructurally, Cx36 was localized to gap junctions between neurons, whereas Cx32 was detected only in oligodendrocyte gap junctions; and Cx26 was found only rarely in astrocyte junctions but abundantly in pia mater. Thus, in developing and adult locus coeruleus, neuronal gap junctions contain Cx36 but do not contain detectable Cx32 or Cx26, suggesting that the locus coeruleus has the same cell-type specificity of connexin expression as observed ultrastructurally in other regions of the CNS. Moreover, in both developing and adult locus coeruleus, no evidence was found for gap junctions or connexins linking neurons with astrocytes or oligodendrocytes, indicating that neurons in this nucleus are not linked to the pan-glial syncytium by Cx32- or Cx26-containing gap junctions or by abundant free connexons composed of those connexins.

Gap junctions on hippocampal mossy fiber axons demonstrated by thin-section electron microscopy and freeze fracture replica immunogold labeling

Gap junctions have been postulated to exist between the axons of excitatory cortical neurons based on electrophysiological, modeling, and dye-coupling data. Here, we provide ultrastructural evidence for axoaxonic gap junctions in dentate granule cells. Using combined confocal laser scanning microscopy, thin-section transmission electron microscopy, and grid-mapped freeze-fracture replica immunogold labeling, 10 close appositions revealing axoaxonic gap junctions ( approximately 30-70 nm in diameter) were found between pairs of mossy fiber axons ( approximately 100-200 nm in diameter) in the stratum lucidum of the CA3b field of the rat ventral hippocampus, and one axonal gap junction ( approximately 100 connexons) was found on a mossy fiber axon in the CA3c field of the rat dorsal hippocampus. Immunogold labeling with two sizes of gold beads revealed that connexin36 was present in that axonal gap junction. These ultrastructural data support computer modeling and in vitro electrophysiological data suggesting that axoaxonic gap junctions play an important role in the generation of very fast (>70 Hz) network oscillations and in the hypersynchronous electrical activity of epilepsy.

Behavioral synergism between D(1) and D(2) dopamine receptors in mice does not depend on gap junctions

Activation of the D(1) and D(2) classes of dopamine receptor in the striatum synergistically promotes motor stereotypy. The mechanism of D(1)/D(2) receptor interaction remains unclear. To investigate the involvement of electrical synaptic transmission in this phenomenon, genetic inactivation of the neuronal gap junction (GJ) protein connexin 36 and pharmacological blockade of GJs were utilized. Stereotyped motor behavior was quantified after selective activation of D(1) receptors, D(2) receptors, or both receptors. These patterns of activation were produced by injection of the agonist apomorphine (3.0 mg/kg) 30 min after either the D(2) antagonist eticlopride (0.3 mg/kg), the D(1) antagonist SCH 23390 (0.1 mg/kg) or vehicle, respectively. Mixed background C57/BL6-129SvEv mice homozygous or heterozygous for the connexin 36 "knockout" allele displayed potent synergistic interaction between D(1) and D(2) receptor activation, and did not differ significantly from wild-type mice on any measure. All genotypes demonstrated long-lasting stereotypic sniffing, chewing, and/or licking after simultaneous activation of D(1) and D(2) receptors, effects that were absent following selective D(1) or D(2) activation. Swiss-Webster mice treated with the GJ blockers carbenoxolone (35 mg/kg), octanol (350 mg/kg) or mefloquine (50 mg/kg) also demonstrated the normal synergistic interaction between D(1) and D(2) receptors, although these drugs did block the grooming stimulated by selective D(1) receptor activation, independently of D(2) receptors. While D(1) receptor-stimulated grooming depends on GJs composed of connexins or possibly pannexins, the synergistic interaction of D(1) and D(2) receptors in control of stereotypy does not involve GJs.

Improved antigen retrieval in freeze-fracture cytochemistry by evaporation of carbon as first replication layer

The recently developed freeze-fracture replica immunolabeling technique uses sodium dodecyl sulfate to clean replicas obtained from chemically unfixed, rapidly frozen cells by evaporation of platinum as first and carbon as second replication layer. The detergent dissolves remains of cellular material with the exception of components which are in direct contact to the replica film. Membrane lipids and membrane protein complexes of the protoplasmic and the exoplasmic membrane halves remain attached to the replica film and are accessible for cytochemical localization. We immunolabeled the membrane proteins caveolin-1 and connexin 43 in mouse cell lines as well as the membrane attached protein tetrachloroethene reductive dehalogenase (PceA) in bacterial cells at freeze-fracture replicas generated by different evaporation parameters. The labeling experiments for caveolin-1 and the PceA showed that freeze-fracture replication of cellular membranes accomplished with thin platinum layers as well as replication with carbon as first evaporation layer lead in these cases to an improved antigen retrieval, whereas the labeling efficiency of connexin 43 was not affected by different evaporation conditions.

UNC-4 represses CEH-12/HB9 to specify synaptic inputs to VA motor neurons in C. elegans

In Caenorhabditis elegans, VA and VB motor neurons arise as lineal sisters but synapse with different interneurons to regulate locomotion. VA-specific inputs are defined by the UNC-4 homeoprotein and its transcriptional corepressor, UNC-37/Groucho, which function in the VAs to block the creation of chemical synapses and gap junctions with interneurons normally reserved for VBs. To reveal downstream genes that control this choice, we have employed a cell-specific microarray strategy that has now identified unc-4-regulated transcripts. One of these genes, ceh-12, a member of the HB9 family of homeoproteins, is normally restricted to VBs. We show that expression of CEH-12/HB9 in VA motor neurons in unc-4 mutants imposes VB-type inputs. Thus, this work reveals a developmental switch in which motor neuron input is defined by differential expression of transcription factors that select alternative presynaptic partners. The conservation of UNC-4, HB9, and Groucho expression in the vertebrate motor circuit argues that similar mechanisms may regulate synaptic specificity in the spinal cord.

Anticonvulsive effects of quinine on penicillin-induced epileptiform activity: an in vivo study

Epilepsy is an important problem in neurological disorders. The common features of all types of epilepsy are the synchronized and uncontrolled discharges of nerve cell assemblies. Recent studies claimed that gap junctions have a critical role in epileptic neuronal events. The aim of present study is to investigate the effects of connexin36 (Cx36) channel blocker quinine on penicillin-induced experimental epilepsy. For this purpose, 4 months old male Wistar rats were used in the present study. Permanent screw electrodes allowing EEG monitoring from conscious animals and permanent cannula providing the administration of the substances to the brain ventricle were placed into the cranium of rats under general anesthesia. At the end of the postoperative recovery period, epileptiform activity was generated by injecting 300 IU crystallized penicillin through the ventricular cannula. When the epileptiform activity, monitored from a digital recording system, reached maximal frequency and amplitude, quinine (200, 400 or 1000 nmol) was administered similar to penicillin. Effects of quinine on epileptiform activity were assessed by both electrophysiological and behavioral analysis. Quinine suppressed epileptiform activity by decreasing the amplitude and frequency of epileptiform spikes and by attenuating the epileptiform behavior. The outcomes of this study suggest that the blockade of Cx36 channels may contribute to the amelioration of epileptic activity.

Altered olivocerebellar activity patterns in the connexin36 knockout mouse

The inferior olive (IO) has among the highest densities of neuronal gap junctions in the nervous system. These gap junctions are proposed to be the underlying mechanism for generating synchronous Purkinje cell complex spike (CS) activity. Gap junctions between neurons are formed mostly by connexin36 proteins. Thus, the connexin36 knockout (Cx36KO) mouse provides an opportunity to test whether gap junction coupling between IO neurons is the basis of CS synchrony. Multiple electrode recordings of crus 2 CSs were obtained from wildtype (Wt) and Cx36KO mice. Wts showed statistically significant levels of CS synchrony, with the same spatial distribution as has been reported for other species: high CS synchrony levels occurred mostly among Purkinje cells within the same parasagittally-oriented cortical strip. In contrast, in Cx36KOs, synchrony was at chance levels and had no preferential spatial orientation, supporting the gap junction hypothesis. CS firing rates for Cx36KOs were significantly lower than for Wts, suggesting that electrical coupling is an important determinant of IO excitability. Rhythmic CS activity was present in both Wt and Cx36KOs, suggesting that individual IO cells can act as intrinsic oscillators. In addition, the climbing fiber reflex was absent in the Cx36KOs, validating its use as a tool for assessing electrical coupling of IO neurons. Zebrin II staining and anterograde tracing showed that cerebellar cortical organization and the topography of the olivocerebellar projection are normal in the Cx36KO. Thus, the differences in CS activity between Wts and Cx36KOs likely reflect the loss of electrical coupling of IO cells.

Carbenoxolone and mefloquine suppress tremor in the harmaline mouse model of essential tremor

Excessive olivo-cerebellar synchrony is implicated in essential tremor. Because synchrony in some networks is mediated by gap junctions, we examined whether the gap junction blockers heptanol, octanol, carbenoxolone, and mefloquine suppress tremor in the mouse harmaline model, and performed an open-treatment clinical study of mefloquine for essential tremor. Digitized motion was used to quantify tremor in mice administered harmaline, 20 mg/kg s.c. In mice the broad-spectrum gap junction blockers heptanol, octanol (350 mg/kg i.p. each), and carbenoxolone (20 mg/kg) suppressed harmaline tremor. Mefloquine (50 mg/kg), which blocks gap junctions containing connexin 36, robustly suppressed harmaline tremor. Glycyrrhizic acid (related to carbenoxolone) and chloroquine (related to mefloquine), which do not block gap junctions, failed to suppress harmaline tremor in mice. Clinically, tremor was assessed with standard rating scales, and subjects asked to take 62.5, 125, and 250 mg mefloquine weekly for 12 weeks at each dose. None of the four human subjects showed a meaningful tremor reduction with mefloquine, likely because clinical levels were below those required for efficacy. In view of recent genetic evidence, the anti-tremor mechanism of these compounds is uncertain but may represent a novel therapeutic target, possibly involving gap junctions other than those containing connexin 36.

Abundance and ultrastructural diversity of neuronal gap junctions in the OFF and ON sublaminae of the inner plexiform layer of rat and mouse retina

Neuronal gap junctions are abundant in both outer and inner plexiform layers of the mammalian retina. In the inner plexiform layer (IPL), ultrastructurally-identified gap junctions were reported primarily in the functionally-defined and anatomically-distinct ON sublamina, with few reported in the OFF sublamina. We used freeze-fracture replica immunogold labeling and confocal microscopy to quantitatively analyze the morphologies and distributions of neuronal gap junctions in the IPL of adult rat and mouse retina. Under "baseline" conditions (photopic illumination/general anesthesia), 649 neuronal gap junctions immunogold-labeled for connexin36 were identified in rat IPL, of which 375 were photomapped to OFF vs. ON sublaminae. In contrast to previous reports, the volume-density of gap junctions was equally abundant in both sublaminae. Five distinctive morphologies of gap junctions were identified: conventional crystalline and non-crystalline "plaques" (71% and 3%), plus unusual "string" (14%), "ribbon" (7%) and "reticular" (2%) forms. Plaque and reticular gap junctions were distributed throughout the IPL. However, string and ribbon gap junctions were restricted to the OFF sublamina, where they represented 48% of gap junctions in that layer. In string and ribbon junctions, curvilinear strands of connexons were dispersed over 5 to 20 times the area of conventional plaques having equal numbers of connexons. To define morphologies of gap junctions under different light-adaptation conditions, we examined an additional 1150 gap junctions from rats and mice prepared after 30 min of photopic, mesopic and scotopic illumination, with and without general anesthesia. Under these conditions, string and ribbon gap junctions remained abundant in the OFF sublamina and absent in the ON sublamina. Abundant gap junctions in the OFF sublamina of these two rodents with rod-dominant retinas revealed previously-undescribed but extensive pathways for inter-neuronal communication; and the wide dispersion of connexons in string and ribbon gap junctions suggests unique structural features of gap junctional coupling in the OFF vs. ON sublamina.

Association of connexin36 and zonula occludens-1 with zonula occludens-2 and the transcription factor zonula occludens-1-associated nucleic acid-binding protein at neuronal gap junctions in rodent retina

Most gap junctions between neurons in mammalian retina contain abundant connexin36, often in association with the scaffolding protein zonula occludens-1. We now investigate co-association of connexin36, zonula occludens-1, zonula occludens-2 and Y-box transcription factor 3 (zonula occludens-1-associated nucleic acid-binding protein) in mouse and rat retina. By immunoblotting, zonula occludens-1-associated nucleic acid-binding protein and zonula occludens-2 were both detected in retina, and zonula occludens-2 in retina was found to co-immunoprecipitate with connexin36. By immunofluorescence, the four proteins appeared as puncta distributed in the plexiform layers. In the inner plexiform layer, most connexin36-puncta were co-localized with zonula occludens-1, and many were co-localized with zonula occludens-1-associated nucleic acid-binding protein. Moreover, zonula occludens-1-associated nucleic acid-binding protein was often co-localized with zonula occludens-1. Nearly all zonula occludens-2-puncta were positive for connexin36, zonula occludens-1 and zonula occludens-1-associated nucleic acid-binding protein. In the outer plexiform layer, connexin36 was also often co-localized with zonula occludens-1-associated nucleic acid-binding protein. In connexin36 knockout mice, labeling of zonula occludens-1 was slightly reduced in the inner plexiform layer, zonula occludens-1-associated nucleic acid-binding protein was decreased in the outer plexiform layer, and both zonula occludens-1-associated nucleic acid-binding protein and zonula occludens-2 were markedly decreased in the inner sublamina of the inner plexiform layer, whereas zonula occludens-1, zonula occludens-2 and zonula occludens-1-associated nucleic acid-binding protein puncta persisted and remained co-localized in the outer sublamina of the inner plexiform layer. By freeze-fracture replica immunogold labeling, connexin36 was found to be co-localized with zonula occludens-2 within individual neuronal gap junctions. In addition, zonula occludens-1-associated nucleic acid-binding protein was abundant in a portion of ultrastructurally-defined gap junctions throughout the inner plexiform layer, and some of these junctions contained both connexin36 and zonula occludens-1-associated nucleic acid-binding protein. These distinct patterns of connexin36 association with zonula occludens-1, zonula occludens-2 and zonula occludens-1-associated nucleic acid-binding protein in different sublaminae of retina, and differential responses of these proteins to connexin36 gene deletion suggest differential regulatory and scaffolding roles of these gap junction accessory proteins. Further, the persistence of a subpopulation of zonula occludens-1/zonula occludens-2/zonula occludens-1-associated nucleic acid-binding protein co-localized puncta in the outer part of the inner plexiform layer of connexin36 knockout mice suggests close association of these proteins with other structures in retina, possibly including gap junctions composed of an as-yet-unidentified connexin.

Gap junctions among dendrites of cortical GABAergic neurons establish a dense and widespread intercolumnar network

Gap junctions are common between cortical GABAergic interneurons but little is known about their quantitative distribution along dendritic profiles. Here, we provide direct morphological evidence that parvalbumin-containing GABAergic neurons in layer 2/3 of the cat visual cortex form dense and far-ranging networks through dendritic gap junctions. Gap junction-coupled networks of parvalbumin neurons were visualized using connexin36 immunohistochemistry and confocal laser-scanning microscopy (CLSM). The direct correspondence of connexin36-immunopositve puncta and gap junctions was confirmed by examining the same structures in both CLSM and electron microscopy. Single parvalbumin neurons with large somata (> or =200 microm2) formed 60.3 +/- 12.2 (mean +/- SD) gap junctions with other cells whereby these contacts were not restricted to proximal dendrites but occurred at distances of up to 380 microm from the soma. In a Sholl analysis of large-type parvalbumin neurons, 21.9 +/- 7.9 gap junctions were within 50 microm of the soma, 21.7 +/- 7.6 gap junctions in a segment between 50 and 100 microm, 11.2 +/- 4.7 junctions between 100 and 150 microm, and 5.6 +/- 3.6 junctions were in more distal segments. Serially interconnected neurons could be traced laterally in a boundless manner through multiple gap junctions. Comparison to the orientation-preference columns revealed that parvalbumin-immunoreactive cells distribute randomly whereby their large dendritic fields overlap considerably and cover different orientation columns. It is proposed that this dense and homogeneous electrical coupling of interneurons supports the precise synchronization of neuronal populations with differing feature preferences thereby providing a temporal frame for the generation of distributed representations.

Electrical synapses--gap junctions in the brain

In the nervous system, interneuronal communication can occur via indirect or direct transmission. The mode of indirect communication involves chemical synapses, in which transmitters are released into the extracellular space to subsequently bind to the postsynaptic cell membrane. Direct communication is mediated by electrical synapses, and will be the focus of this review. The most prevalent group of electrical synapses are neuronal gap junctions (both terms are used interchangeably in this article), which directly connect the intracellular space of two cells by gap junction channels. The structural components of gap junction channels in the nervous system are connexin proteins, and, as recently identified, pannexin proteins. Connexin gap junction channels enable the intercellular, bidirectional transport of ions, metabolites, second messengers and other molecules smaller than 1 kD. More than 20 connexin genes have been found in the mouse and human genome. With the cloning of connexin36 (Cx36), a connexin protein with predominantly neuronal expression, the biochemical correlate of electrotonic transmission between neurons was identified. We outline the distribution of Cx36 as well as two other neuronal connexins (Cx57 and Cx45) in the nervous system, describing their spatial and temporal expression patterns. One focus in this review was the retina, as it shows many and diverse electrical synapses whose connexin components have been identified in fish and mammals. In view of the function of neuronal gap junctions, the network of inhibitory interneurons will be reviewed in detail, focussing on the hippocampus. Although in vivo data on pannexin proteins are still restricted to information on mRNA expression, electrophysiological data and the expression pattern in the nervous system have been included.

Regulation of gap junction coupling through the neuronal connexin Cx35 by nitric oxide and cGMP

Gap-junctional coupling among neurons is subject to regulation by a number of neurotransmitters including nitric oxide. We studied the mechanisms by which NO regulates coupling in cells expressing Cx35, a connexin expressed in neurons throughout the central nervous system. NO donors caused potent uncoupling of HeLa cells stably transfected with Cx35. This effect was mimicked by Bay 21-4272, an activator of guanylyl cyclase. A pharmacological analysis indicated that NO-induced uncoupling involved both PKG-dependent and PKG-independent pathways. PKA was involved in both pathways, suggesting that PKG-dependent uncoupling may be indirect. In vitro, PKG phosphorylated Cx35 at three sites: Ser110, Ser276, and Ser289. A mutational analysis indicated that phosphorylation on Ser110 and Ser276, sites previously shown also to be phosphorylated by PKA, had a significant influence on regulation. Ser289 phosphorylation had very limited effects. We conclude that NO can regulate coupling through Cx35 and that regulation is indirect in HeLa cells.

Expression and function of the neuronal gap junction protein connexin 36 in developing mammalian retina

With the advent of transgenic mice, much has been learned about the expression and function of gap junctions. Previously, we reported that retinal ganglion cells in mice lacking the neuronal gap junction protein connexin 36 (Cx36) have nearly normal firing patterns at postnatal day 4 (P4) but many more asynchronous action potentials than wild-type mice at P10 (Torborg et al. [2005] Nat. Neurosci. 8:72-78). With the goal of understanding the origin of this increased activity in Cx36-/- mice, we used a transgenic mouse (Deans et al. [2001] Neuron 31:477-485) to characterize the developmental expression of a Cx36 reporter in the retina. We found that Cx36 was first detected weakly at P2 and gradually increased in expression until it reached an adult pattern at P14. Although the onset of expression varied by cell type, we identified Cx36 in the glycinergic AII amacrine cell, glutamatergic cone bipolar cell, and retinal ganglion cells (RGCs). In addition, we used calcium imaging and multielectrode array recording to characterize further the firing patterns in Cx36-/- mice. Both correlated and asynchronous action potentials in P10 Cx36-/- RGCs were significantly inhibited by bath application of an ionotropic glutamate receptor antagonist, indicating that the increase in activity was synaptically mediated. Hence, both the expression patterns and the physiology suggest an increasing role for Cx36-containing gap junctions in suppressing RGC firing between waves during postnatal retinal development.

NMDA receptors regulate developmental gap junction uncoupling via CREB signaling

Signaling through gap junctions (electrical synapses) is important in the development of the mammalian central nervous system. Abundant between neurons during postnatal development, gap junction coupling subsequently decreases and remains low in the adult, confined to specific subsets of neurons. Here we report that developmental uncoupling of gap junctions in the rat hypothalamus in vivo and in vitro is associated with a decrease in connexin 36 (Cx36) protein expression. Both developmental gap junction uncoupling and Cx36 downregulation are prevented by the blockade of NMDA glutamate receptors, action potentials and the calcium-cyclic AMP response element binding protein (CREB), and are accelerated by CREB overexpression. Developmental gap junction uncoupling and Cx36 downregulation are not affected by blockade of non-NMDA glutamate receptors, and do not occur in hypothalamic neurons from NMDA receptor subunit 1 (NMDAR1) knockout mice. These results demonstrate that NMDA receptor activity contributes to the developmental uncoupling of gap junctions via CREB-dependent downregulation of Cx36.

Extended cocaine self-administration and deprivation produces region-specific and time-dependent changes in connexin36 expression in rat brain

Cocaine addiction is a disease that develops over time, and it is thought that drug-induced neuro-adaptations underlie the changes in behavior seen across the addictive process. While a number of alterations in synaptic transmission have been identified, little is currently known regarding cocaine's effects on gap junctional communication between neurons. Here we examine the effects of a cocaine self-administration regimen, previously shown to increase the reinforcing efficacy of cocaine, on the expression of the neuron-specific gap junction-forming protein connexin36 (C x 36). Using real-time RT-PCR and immunoblotting, we show that binge cocaine self-administration produces region-specific and time-dependent changes in C x 36 mRNA and protein expression in the nucleus accumbens, prefrontal cortex, and hippocampus. A number of changes in C x 36 were present 1 day and 7 days following self-administration, and C x 36 mRNA and protein appeared to be differentially regulated in a region-specific manner. C x 36 protein was significantly decreased in the prefrontal cortex 7 days following self-administration, a time point when behavioral sensitization to the reinforcing effects of cocaine is observed. These results suggest that changes in neuronal gap junction expression may be one mechanism by which cocaine self-administration produces enduring changes in behavior.

Ultrastructural localization of connexins (Cx36, Cx43, Cx45), glutamate receptors and aquaporin-4 in rodent olfactory mucosa, olfactory nerve and olfactory bulb

Odorant/receptor binding and initial olfactory information processing occurs in olfactory receptor neurons (ORNs) within the olfactory epithelium. Subsequent information coding involves high-frequency spike synchronization of paired mitral/tufted cell dendrites within olfactory bulb (OB) glomeruli via positive feedback between glutamate receptors and closely-associated gap junctions. With mRNA for connexins Cx36, Cx43 and Cx45 detected within ORN somata and Cx36 and Cx43 proteins reported in ORN somata and axons, abundant gap junctions were proposed to couple ORNs. We used freeze-fracture replica immunogold labeling (FRIL) and confocal immunofluorescence microscopy to examine Cx36, Cx43 and Cx45 protein in gap junctions in olfactory mucosa, olfactory nerve and OB in adult rats and mice and early postnatal rats. In olfactory mucosa, Cx43 was detected in gap junctions between virtually all intrinsic cell types except ORNs and basal cells; whereas Cx45 was restricted to gap junctions in sustentacular cells. ORN axons contained neither gap junctions nor any of the three connexins. In OB, Cx43 was detected in homologous gap junctions between almost all cell types except neurons and oligodendrocytes. Cx36 and, less abundantly, Cx45 were present in neuronal gap junctions, primarily at "mixed" glutamatergic/electrical synapses between presumptive mitral/tufted cell dendrites. Genomic analysis revealed multiple miRNA (micro interfering RNA) binding sequences in 3'-untranslated regions of Cx36, Cx43 and Cx45 genes, consistent with cell-type-specific post-transcriptional regulation of connexin synthesis. Our data confirm absence of gap junctions between ORNs, and support Cx36- and Cx45-containing gap junctions at glutamatergic mixed synapses between mitral/tufted cells as contributing to higher-order information coding within OB glomeruli.

Glial and neuronal connexin expression patterns in the rat spinal cord during development and following injury

Spinal cord injury induces a complex cascade of degenerative and remodeling events evolving over time. The possible roles of changed intercellular communication via gap junctions after spinal cord injury (SCI) have remained relatively unexplored. We investigated the temporospatial expression patterns of gap junctional genes and proteins, connexin 43 (Cx43), Cx36, and Cx32, by in situ hybridization and immunohistochemistry in the rat neonatal, adult normal, and adult injured spinal cord. Cx36 was strongly expressed in immature neurons, and levels declined markedly during development, whereas Cx43 and Cx32 persisted throughout adulthood. After a complete transection of the adult spinal cord, the levels of Cx43 mRNA and protein were up-regulated within hours, especially in gray matter rostral to the lesion, reaching over three times normal levels at 4 weeks postinjury. Cx43 immunoreactivity was seen primarily in astrocytes and rarely in microglia. In contrast, Cx36 and Cx32 mRNA and proteins were relatively sparse and unchanged after spinal cord injury along the entire axis of the spinal cord. Cx43 is the most abundant gap junctional protein in the adult CNS and has been shown to form channels between astrocytes as well as between astrocytes and oligodendrocytes. Long-term up-regulation of Cx43 in reactive astrocytes may be one critical component in the rearrangement of the local astroglial network following SCI.

Protein kinase A mediates regulation of gap junctions containing connexin35 through a complex pathway

Connexin 35 (Cx35) is a major component of electrical synapses in the central nervous system. Many gap junctions containing Cx35 are regulated by dopamine receptor pathways that involve protein kinase A (PKA). To study the mechanism of PKA regulation, we analyzed direct phosphorylation of Cx35 by PKA in vitro and studied the regulation of neurobiotin tracer coupling in HeLa cells expressing Cx35 or Cx35 mutants that lack phosphorylation sites. In Cx35-transfected cells, application of the PKA activator Sp-8-cpt-cAMPS caused a significant decline in coupling, while a PKA inhibitor, Rp-8-cpt-cAMPS, significantly increased tracer coupling. In vitro phosphorylation and mutagenic analysis showed that PKA phosphorylates Cx35 directly at two major sites, Ser110 in the intracellular loop and Ser276 in the carboxyl terminus. In addition, a minor phosphorylation site in the C-terminus was identified by truncation of the last 7 amino acids at Ser298. The mutations Ser110Ala or Ser276Ala significantly reduced regulation of coupling by the PKA activator while a combination of the two eliminated regulation. Truncation at Ser298 reversed the regulation such that the PKA activator significantly increased and the PKA inhibitor significantly decreased coupling. The activation was eliminated in the S110A, S276A, S298ter triple mutant. We conclude that PKA regulates Cx35 coupling in a complex manner that requires both major phosphorylation sites. Furthermore, the tip of the C-terminus acts as a "switch" that determines whether phosphorylation will inhibit or enhance coupling. Reliance on the combined states of three sites provides fine control over the degree of coupling through Cx35 gap junctions.

Connexin36 mediates spike synchrony in olfactory bulb glomeruli

Neuronal synchrony is important to network behavior in many brain regions. In the olfactory bulb, principal neurons (mitral cells) project apical dendrites to a common glomerulus where they receive a common input. Synchronized activity within a glomerulus depends on chemical transmission but mitral cells are also electrically coupled. We examined the role of connexin-mediated gap junctions in mitral cell coordinated activity. Electrical coupling as well as correlated spiking between mitral cells projecting to the same glomerulus was entirely absent in connexin36 (Cx36) knockout mice. Ultrastructural analysis of glomeruli confirmed that mitral-mitral cell gap junctions on distal apical dendrites contain Cx36. Coupled AMPA responses between mitral cell pairs were absent in the knockout, demonstrating that electrical coupling, not transmitter spillover, is responsible for synchronization. Our results indicate that Cx36-mediated gap junctions between mitral cells orchestrate rapid coordinated signaling via a novel form of electrochemical transmission.

Role of gap junctions in synchronized neuronal oscillations in the inferior olive

Inferior olivary (IO) neurons are electrically coupled through gap junctions and generate synchronous subthreshold oscillations of their membrane potential at a frequency of 1-10 Hz. Whereas the ionic mechanisms of these oscillatory responses are well understood, their origin and ensemble properties remain controversial. Here, the role of gap junctions in generating and synchronizing IO oscillations was examined by combining intracellular recordings with high-speed voltage-sensitive dye imaging in rat brain stem slices. Single cell responses and ensemble synchronized responses of IO neurons were compared in control conditions and in the presence of 18beta-glycyrrhetinic acid (18beta-GA), a pharmacological gap junction blocker. Under our experimental conditions, 18beta-GA had no adverse effects on intrinsic electroresponsive properties of IO neurons, other than the block of gap junction-dependent dye coupling and the resulting change in cells' passive properties. Application of 18beta-GA did not abolish single cell oscillations. Pharmacologically uncoupled IO neurons continued to oscillate with a frequency and amplitude that were similar to those recorded in control conditions. However, these oscillations were no longer synchronized across a population of IO neurons. Our optical recordings did not detect any clusters of synchronous oscillatory activity in the presence of the blocker. These results indicate that gap junctions are not necessary for generating subthreshold oscillations, rather, they are required for clustering of coherent oscillatory activity in the IO. The findings support the view that oscillatory properties of single IO neurons endow the system with important reset dynamics, while gap junctions are mainly required for synchronized neuronal ensemble activity.

Connexin36 mediates gap junctional coupling of alpha-ganglion cells in mouse retina

Alpha-ganglion cells are present in all vertebrate retinae and are subdivided into ON and OFF types according to their level of dendritic ramification within the inner plexiform layer. They have large dendritic fields and usually a good responsiveness to moving stimuli. They were the first ganglion cells in which tracer coupling was observed, suggesting the presence of gap junctions composed of unknown connexins. Here we show that ON-alpha-ganglion cells in the mouse retina are coupled to amacrine cells, whereas OFF-alpha-ganglion cells are coupled to other OFF-alpha-ganglion cells and to amacrine cells. These tracer coupling patterns were completely absent in mice deficient in connexin36 (Cx36). The expression of Cx36 protein in alpha-ganglion cells but not in coupled amacrine cells was confirmed in mice in which the Cx36 coding DNA was replaced by the lacZ reporter gene. The dendritic localization and the distribution pattern of Cx36 patches, analyzed in mice in which the enhanced green fluorescent protein (EGFP) was linked to the C-terminal region of the Cx36 protein, revealed a rather small number of fluorescent plaques and different patterns for ON- and OFF-alpha-ganglion cells. Furthermore, tracer coupling between OFF-alpha-ganglion cells could be inhibited by quinine, a gap junctional blocker with a slight preference for gap junctions formed by Cx36. These data strongly suggest that Cx36 gap junction channels are functional not only in interneurons but also in output neurons of the retina and are responsible for distinct coupling patterns of ganglion cells.

Biophysical evidence that connexin-36 forms functional gap junction channels between pancreatic mouse beta-cells

Connexin-36 (Cx36) is the only gap junction protein that has been unambiguously identified in rodent pancreatic beta-cells. However, properties of gap junction channel unitary currents between beta-cells remain unrevealed. To address whether Cx36 forms functional channels in beta-cells, we characterized biophysical properties of macro- and microscopic junctional currents recorded from dual whole cell voltage clamp isolated pairs of dispersed mouse beta-cells. Electrical coupling was recorded in 80% of cell pairs with a junctional conductance (g(j)) of 355 +/- 45 pS (n = 20). Transjunctional voltage dependence was identified in three of seven cell pairs with high-input membrane resistances. Normalized steady-state g(j) (Gj) and transjunctional-voltage relation were well described by a two-state Boltzmann equation [maximal conductance (Gmax) = 1.0, voltage-insensitive conductance (Gmin) = 0.3 and 0.28, voltage gating sensitivity (A) = 0.21 and 0.23, and voltage at which one-half of the initial voltage-dependent conductance was reached (Vo) = -85 and 87 mV for negative and positive potentials, respectively]. Halothane reversibly uncoupled beta-cell pairs, and, during recovery, unitary conductances of 5-10 pS were recorded while using patch pipettes containing mainly CsCl. Although these properties are similar to those previously described for Cx36 channels in mammalian cell systems, we found that beta-cell junctional currents were insensitive to quinine. Cx36 transcript and protein expression in islets and freshly dispersed cell preparations was confirmed by RT-PCR and immunofluorescence. In conclusion, biophysical properties of junctional channels between beta-cells are similar but not identical to those previously described for homomeric Cx36 channels. Cell type-specific mechanisms that may account for these differences are discussed.

Freeze-fracture and immunogold analysis of aquaporin-4 (AQP4) square arrays, with models of AQP4 lattice assembly

Each day, approximately 0.5-0.9 l of water diffuses through (primarily) aquaporin-1 (AQP1) channels in the human choroid plexus, into the cerebrospinal fluid of the brain ventricles and spinal cord central canal, through the ependymal cell lining, and into the parenchyma of the CNS. Additional water is also derived from metabolism of glucose within the CNS parenchyma. To maintain osmotic homeostasis, an equivalent amount of water exits the CNS parenchyma by diffusion into interstitial capillaries and into the subarachnoid space that surrounds the brain and spinal cord. Most of that efflux is through AQP4 water channels concentrated in astrocyte endfeet that surround capillaries and form the glia limitans. This report extends the ultrastructural and immunocytochemical characterizations of the crystalline aggregates of intramembrane proteins that comprise the AQP4 "square arrays" of astrocyte and ependymocyte plasma membranes. We elaborate on recent demonstrations in Chinese hamster ovary cells of the effects on AQP4 array assembly resulting from separate vs. combined expression of M1 and M23 AQP4, which are two alternatively spliced variants of the AQP4 gene. Using improved shadowing methods, we demonstrate sub-molecular cross-bridges that link the constituent intramembrane particles (IMPs) into regular square lattices of AQP4 arrays. We show that the AQP4 core particle is 4.5 nm in diameter, which appears to be too small to accommodate four monomeric proteins in a tetrameric IMP. Several structural models are considered that incorporate freeze-fracture data for submolecular "cross-bridges" linking IMPs into the classical square lattices that characterize, in particular, naturally occurring AQP4.

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.

Dynamics of electrical transmission at club endings on the Mauthner cells

Identifiable mixed electrical and chemical synapses on Mauthner cells, the club endings, have historically provided a window for the study of electrical transmission in vertebrates because of their accessibility for both physiological and ultrastructural characterization. Recent data show that electrical transmission at these terminals is mediated by connexin35 (Cx35), the fish ortholog of the mammalian neuronal gap junction protein, connexin36 (Cx36). While electrical synapses are still perceived by many as passive intercellular channels that lack modifiability, a wealth of experimental evidence shows that electrical synapses at club endings are very plastic and subject to dynamic regulatory control by several mechanisms. The widespread distribution of connexin35 and connexin36 and the ubiquity of some of the proposed regulatory elements suggest that other electrical synapses may be similarly regulated.

Identification and characterization of an aquaporin 1 immunoreactive amacrine-type cell of the mouse retina

Using immunocytochemistry, a type of amacrine cell that is immunoreactive for aquaporin 1 was identified in the mouse retina. AQP1 immunoreactivity was found in photoreceptor cells of the outer nuclear layer (ONL) and in a distinct type of amacrine cells of the inner nuclear layer (INL). AQP1-immunoreactive (IR) amacrine cell somata were located in the INL and their processes extended through strata 3 and 4 of the inner plexiform layer (IPL) with thin varicosities. The density of the AQP1-IR amacrine cells increased from 100/mm(2) in the peripheral retina to 350/mm(2) in the central retina. The AQP1-IR amacrine cells comprise 0.5% of the total amacrine cells. The AQP1-IR amacrine cell bodies formed a regular mosaic, which suggested that they represent a single type of amacrine cell. Double labeling with AQP1 and glycine, gamma-aminobutyric acid (GABA) or GAD(65) antiserum demonstrated that the AQP1-IR amacrine cells expressed GABA or GAD(65) but not glycine. Their synaptic input was primarily from other amacrine cell processes. They also received synaptic inputs from a few cone bipolar cells. The primary synaptic targets were ganglion cells, followed by other amacrine cells and cone bipolar cells. In addition, gap junctions between an AQP1-IR amacrine process and another amacrine process were rarely observed. In summary, a GABAergic amacrine cell type labeled by an antibody against AQP1 was identified in the mouse retina and was found to play a possible role in transferring a certain type of visual information from other amacrine or a few cone bipolar cells primarily to ganglion cells.

Alterations in metabolism and gap junction expression may determine the role of astrocytes as ?good samaritans? or executioners

Our knowledge of astroglia and their physiological and pathophysiological role(s) in the central nervous system (CNS) has grown during the past decade, revealing a complex picture. It is becoming increasingly clear that glia play a significant role in the homeostasis and function of the CNS and that neurons should no longer be considered the only cell type that responds, both rapidly and slowly, to electrochemical activity. We discuss recent advances in the field with an emphasis on the impact of hypoxia and ischemia on astrocytic metabolism and the functional relationship between glucose metabolism and gap junctions in astrocytes. We also address the controversy over whether astrocytic gap junctions mediate protection or killing of neurons during or after hypoxic or ischemic insults.

Perisomatic Feedback Inhibition Underlies Cholinergically Induced Fast Network Oscillations in the Rat Hippocampus In Vitro

Gamma frequency network oscillations are assumed to be important in cognitive processes, including hippocampal memory operations, but the precise functions of these oscillations remain unknown. Here, we examine the cellular and network mechanisms underlying carbachol-induced fast network oscillations in the hippocampus in vitro, which closely resemble hippocampal gamma oscillations in the behaving rat. Using a combination of planar multielectrode array recordings, imaging with voltage-sensitive dyes, and recordings from single hippocampal neurons within the CA3 gamma generator, active current sinks and sources were localized to the stratum pyramidale. These proximal currents were driven by phase-locked rhythmic inhibitory inputs to pyramidal cells from identified perisomatic-targeting interneurons. AMPA receptor-mediated recurrent excitation was necessary for the synchronization of interneuronal discharge, which strongly supports a synaptic feedback model for the generation of hippocampal gamma oscillations.

Mechanisms underlying gamma ('40 Hz') network oscillations in the hippocampus--a mini-review

Gamma-frequency oscillations (approximately 30-100 Hz) in cortical network activity have been proposed to provide a temporal structure for various forms of cognitive processing. This review provides an update on recent experiments addressing the mechanisms underlying gamma-frequency network oscillations in the rodent hippocampus. Particular emphasis is placed on the correlation between in vivo observations and in vitro models.

Cone photoreceptors in bass retina use two connexins to mediate electrical coupling

Electrical coupling via gap junctions is a common property of CNS neurons. In retinal photoreceptors, coupling plays important roles in noise filtering, intensity coding, and spatial processing. In many vertebrates, coupling is regulated during the course of light adaptation. To understand the mechanisms of this regulation, we studied photoreceptor gap junction proteins. We found that two connexins were expressed in bass cone photoreceptors. Connexin 35 (Cx35) mRNA was present in many cell types, including photoreceptors and amacrine, bipolar, and a few ganglion cells. Antibodies to Cx35 labeled abundant gap junctions in both the inner and outer plexiform layers. In the outer plexiform layer, numerous plaques colocalized with cone telodendria at crossing contacts and tip-to-tip contacts. Cx34.7 mRNA was found predominantly in the photoreceptor layer, primarily in cones. Cx34.7 immunolabeling was limited to small plaques immediately beneath cone pedicles and did not colocalize with Cx35. Cx34.7 plaques were associated with a dense complex of cone membrane beneath the pedicles, including apparent contacts between telodendria and cone pedicles. Tracer coupling studies of the connexins expressed in HeLa cells showed that coupling through Cx35 gap junctions was reduced by protein kinase A (PKA) activation and enhanced by PKA inhibition through a greater than fivefold activity range. Cx34.7 was too poorly expressed to study. PKA regulation suggests that coupling through Cx35 gap junctions can be controlled dynamically through dopamine receptor pathways during light adaptation. If Cx34.7 forms functional cell-cell channels between cones, it would provide a physically separate pathway for electrical coupling.

Gap junctions in developing neocortex: a review

Several independent lines of evidence indicate that gap junctional coupling is widespread and functionally important in early cortical development. The extensive expression of many connexins, the clusters of coupled neuroblasts or neurons revealed by tracer injections or the functional coactivation of neurons shown by calcium imaging can be seen as pieces of the same puzzle. At the same time, there is a lack of electrophysiological experiments at early developmental stages that directly demonstrate coupling between pairs of cells. The function of this gap junctional coupling is still mysterious, though it may be involved in controlling neurogenesis, differentiation or the formation of columnar microcircuits.

Update on connexins and gap junctions in neurons and glia in the mammalian nervous system

Among the 20 proposed members of the connexin family of proteins that form gap junctional intercellular communication (GJIC) channels in mammalian tissues, over half are reported to be expressed in the nervous system. There have been conflicting observations, however, concerning the particular connexins expressed by astrocytes, oligodendrocytes, Schwann cells and neurons. Identification of the several connexin proteins at gap junctions between each neuronal and glial cell type is essential for the rational design of investigations into the functions of GJIC between glial cells and into the functional contributions of electrical and "mixed" (chemical plus electrical) synapses to communication between neurons in the mammalian nervous system. In this report, we provide a summary of recent findings regarding the localization of connexins in gap junctions between glial cells and between neurons. Attention is drawn to technical considerations involved in connexin localization by light and electron microscope immunohistochemistry and to limitations of physiological methods and approaches currently used to analyze neuronal and glial coupling. Early physiological studies that provided evidence for the presence of gap junctions and electrical synapses in isolated regions of the mammalian brain and spinal cord are reexamined in light of recent evidence for widely expressed neuron-specific connexins and for the existence of several newly discovered types of gap junctions linking neurons.

Anticonvulsant effects of carbenoxolone in genetically epilepsy prone rats (GEPRs)

Carbenoxolone (CBX), the succinyl ester of glycyrrhetinic acid, is an inhibitor of gap junctional intercellular communication. Systemic administration of CBX was able to decrease the seizure severity score and to increase the latency time of seizure onset in genetically epilepsy prone rats (GEPRs). In particular, intravenous or intraperitoneal administration of carbenoxolone (5-30 mg/kg) produced a dose-dependent and significant reduction in the clonic and tonic phases of the audiogenic seizures in GEPRs. The anticonvulsant doses were not associated with an impairment of motor coordination. The bilateral microinjection of CBX (0.001-0.50 microg/0.5 microl) into the inferior colliculi, the substantia nigra (pars reticulata or compacta) and the inferior olivary complex was able to reduce the seizure severity score in a dose-dependent manner. The anticonvulsant effects were longer lasting after focal microinjection than after systemic administration. No anticonvulsant effects were observed following focal bilateral microinjections of glycyrrhizin into the same brain areas where CBX was shown to be effective.

Dendrodendritic electrical synapses between mammalian retinal ganglion cells

Electrical synapses between alpha-type ganglion cells were detected using combined techniques of dual patch-clamp recordings, intracellular labeling, electron microscopy, and channel subunit connexin immunocytochemistry in the albino rat retina. After intracellular injection of Neurobiotin into alpha-cells of inner (ON-center) and outer (OFF-center) ramifying types, measurement of tracer coupling resulted in a preferentially homologous occurrence among cells of the same morphological type (n = 19 of 24). In high-voltage as well as conventional electron microscopic analysis, direct dendrodendritic gap junctions (average size, 0.86 mum long) were present in contact sites between tracer-coupled alpha-cells. In simultaneous dual whole-cell recordings from pairs of neighboring alpha-cells, these cells generated TTX-sensitive sustained spiking against extrinsic current injection, and bidirectional electrical synapses (maximum coupling coefficient, 0.32) with symmetrical junction conductance (average, 1.35 nS) were observed in pairs with cells of the same morphological type. Precise temporal synchronization of spike activity (average time delay, 2.7 msec) was detected when depolarizing currents were simultaneously injected into the pairs. To address whether physiologically identified electrical synapses constitute gap junctional connectivity between cell pairs, identified neuronal connexin36 immunoreactivity was undertaken in Lucifer yellow-labeled cell pairs after patch-clamp recordings. All alpha-cells expressed connexin36, and confocal laser-scanning imaging demonstrated that connexin36 is primarily located at dendritic crossings between electrically coupled cells (seven sites in a pair, on average). These results give conclusive evidence for electrical synapses via dendrodendritic gap junctions involving connexin36 in alpha retinal ganglion cells of the same physiological type.

Hippocampal gamma-frequency oscillations: from interneurones to pyramidal cells, and back

GABAergic interneurones are necessary for the emergence of hippocampal gamma-frequency network oscillations, during which they play a key role in the synchronization of pyramidal cell firing. However, it remains to be resolved how distinct interneurone subtypes contribute to gamma-frequency oscillations, in what way the spatiotemporal pattern of interneuronal input affects principal cell activity, and by which mechanisms the interneurones themselves are synchronized. Here we summarize recent evidence from cholinergically induced gamma-frequency network oscillations in vitro, showing that perisomatic-targeting GABAergic interneurones provide prominent rhythmic inhibition in pyramidal cells, and that these interneurones are synchronized by recurrent excitation. We conclude by presenting a minimal integrate-and-fire network model which demonstrates that this excitatory-inhibitory feedback loop is sufficient to explain the generation of intrahippocampal gamma-frequency oscillations.

Oscillatory activity within rat substantia gelatinosa in vitro: a role for chemical and electrical neurotransmission

Although rhythmic behaviour of mammalian spinal ventral horn networks has been extensively studied little is known about oscillogenesis in the spinal dorsal horn. The aims of this in vitro study were to record and determine the underlying mechanisms of potassium-evoked network field oscillations in the substantia gelatinosa of the neonatal rat dorsal horn, a lamina involved in nociceptive processing. Transient pressure ejection of a potassium solution evoked reproducible rhythmic activity in discrete areas of the substantia gelatinosa which lasted for 5-15 s with a single prominent peak in the 4-12 Hz frequency band (7.7 +/- 0.1 Hz, n = 60). Oscillations of similar frequency and amplitude were also observed in isolated dorsal horn quadrants. Application of CNQX (10 microm) reduced peak power amplitude and integrated power area (from 4 to 12 Hz) of the power spectrum, whereas D-AP5 (50 microm) had no effect on the potassium-evoked rhythm. Bicuculline (30 microm) or strychnine (10 microm) reduced the power amplitude and area. On combination of bicuculline (30 microm) and strychnine (10 microm) the reductions in power amplitude and area were not significantly different (P > 0.05) when compared with application of either drug alone. The gap junction blockers carbenoxolone (100 microm) or octanol (1 mM) significantly reduced power amplitude and area. Although TTX (1 microm) or a calcium-free perfusate both caused reductions in the power amplitude and area, potassium-evoked rhythmic activity persisted. However, this persistent rhythm was further reduced on combination of calcium-free perfusate with octanol (1 mM) and was abolished using a cocktail of drugs. Blockade of the potassium delayed rectifier current by tetraethylammonium (5 mM) or the hyperpolarization-activated current (I(h)) by ZD7288 (10 microm) disrupted the synchronization of the potassium-induced oscillation. The frequency of potassium-induced rhythms was unaffected by any of the drugs tested. These novel findings demonstrate that transient pressure ejection of potassium evokes oscillatory activity in the substantia gelatinosa in vitro. This rhythm is partly dependent upon various receptors (AMPA/kainate, GABA(A) and glycine), ion channels (potassium delayed rectifier and I(h)) and gap junctions. Oscillatory behaviour in the substantia gelatinosa could potentially play a role in the processing of nociceptive signals.