Deformation of network connectivity in the inferior olive of connexin 36-deficient mice is compensated by morphological and electrophysiological changes at the single neuron level

Electrical Coupling and Passive Membrane Properties of AII Amacrine Cells

AII amacrine cells in the mammalian retina are connected via electrical synapses to on-cone bipolar cells and to other AII amacrine cells. To understand synaptic integration in these interneurons, we need information about the junctional conductance ( gj), the membrane resistance ( rm), the membrane capacitance ( Cm), and the cytoplasmic resistivity ( Ri). Due to the extensive electrical coupling, it is difficult to obtain estimates of rm, as well as the relative contribution of the junctional and nonjunctional conductances to the total input resistance of an AII amacrine cell. Here we used dual voltage-clamp recording of pairs of electrically coupled AII amacrine cells in an in vitro slice preparation from rat retina and applied meclofenamic acid (MFA) to block the electrical coupling and isolate single AII amacrines electrically. In the control condition, the input resistance ( Rin) was ∼620 MΩ and the apparent rm was ∼760 MΩ. After block of electrical coupling, determined by estimating gj in the dual recordings, Rin and rm were ∼4,400 MΩ, suggesting that the nongap junctional conductance of an AII amacrine cell is ∼16% of the total input conductance. Control experiments with nucleated patches from AII amacrine cells suggested that MFA had no effect on the nongap junctional membrane of these cells. From morphological reconstructions of AII amacrine cells filled with biocytin, we obtained a surface area of ∼900 μm2 which, with a standard value for Cm of 0.01 pF/μm2, corresponds to an average capacitance of ∼9 pF and a specific membrane resistance of ∼41 kΩ cm2. Together with information concerning synaptic connectivity, these data will be important for developing realistic compartmental models of the network of AII amacrine cells.

Oculopalatal tremor explained by a model of inferior olivary hypertrophy and cerebellar plasticity

The inferior olivary nuclei clearly play a role in creating oculopalatal tremor, but the exact mechanism is unknown. Oculopalatal tremor develops some time after a lesion in the brain that interrupts inhibition of the inferior olive by the deep cerebellar nuclei. Over time the inferior olive gradually becomes hypertrophic and its neurons enlarge developing abnormal soma-somatic gap junctions. However, results from several experimental studies have confounded the issue because they seem inconsistent with a role for the inferior olive in oculopalatal tremor, or because they ascribe the tremor to other brain areas. Here we look at 3D binocular eye movements in 15 oculopalatal tremor patients and compare their behaviour to the output of our recent mathematical model of oculopalatal tremor. This model has two mechanisms that interact to create oculopalatal tremor: an oscillator in the inferior olive and a modulator in the cerebellum. Here we show that this dual mechanism model can reproduce the basic features of oculopalatal tremor and plausibly refute the confounding experimental results. Oscillations in all patients and simulations were aperiodic, with a complicated frequency spectrum showing dominant components from 1 to 3 Hz. The model’s synchronized inferior olive output was too small to induce noticeable ocular oscillations, requiring amplification by the cerebellar cortex. Simulations show that reducing the influence of the cerebellar cortex on the oculomotor pathway reduces the amplitude of ocular tremor, makes it more periodic and pulse-like, but leaves its frequency unchanged. Reducing the coupling among cells in the inferior olive decreases the oscillation’s amplitude until they stop (at ∼20% of full coupling strength), but does not change their frequency. The dual-mechanism model accounts for many of the properties of oculopalatal tremor. Simulations suggest that drug therapies designed to reduce electrotonic coupling within the inferior olive or reduce the disinhibition of the cerebellar cortex on the deep cerebellar nuclei could treat oculopalatal tremor. We conclude that oculopalatal tremor oscillations originate in the hypertrophic inferior olive and are amplified by learning in the cerebellum.

Connexin-mediated communication controls cell proliferation and is essential in retinal histogenesis

Connexin (Cx) channels and hemichannels are involved in essential processes during nervous system development such as apoptosis, propagation of spontaneous activity and interkinetic nuclear movement. In the first part of this study, we extensively characterized Cx gene and protein expression during retinal histogenesis. We observed distinct spatio-temporal patterns among studied Cx and an overriding, ubiquitous presence of Cx45 in progenitor cells. The role of Cx-mediated communication was assessed by using broad-spectrum (carbenoxolone, CBX) and Cx36/Cx50 channel-specific (quinine) blockers. In vivo application of CBX, but not quinine, caused remarkable reduction in retinal thickness, suggesting changes in cell proliferation/apoptosis ratio. Indeed, we observed a decreased number of mitotic cells in CBX-injected retinas, with no significant changes in the expression of PCNA, a marker for cells in proliferative state. Taken together, our results pointed a pivotal role of Cx45 in the developing retina. Moreover, this study revealed that Cx-mediated communication is essential in retinal histogenesis, particularly in the control of cell proliferation.

Experience-dependent maturation of the glomerular microcircuit

Spontaneous and patterned activity, largely attributed to chemical transmission, shape the development of virtually all neural circuits. However, electrical transmission also has an important role in coordinated activity in the brain. In the olfactory bulb, gap junctions between apical dendrites of mitral cells increase excitability and synchronize firing within each glomerulus. We report here that the development of the glomerular microcircuit requires both sensory experience and connexin (Cx)36-mediated gap junctions. Coupling coefficients, which measure electrical coupling between mitral cell dendrites, were high in young mice, but decreased after postnatal day (P)10 because of a maturational increase in membrane conductance. Sensory deprivation, induced by unilateral naris occlusion at birth, slowed the morphological development of mitral cells and arrested the maturational changes in membrane conductance and coupling coefficients. As the coupling coefficients decreased in normal mice, a glutamate-mediated excitatory postsynaptic current (EPSC) between mitral cells emerged by P30. Although mitral-mitral EPSCs were generally unidirectional, they were not present in young adult Cx36(-/-) mice, suggesting that gap junctions are required for the development and/or function of the mature circuit. The experience-dependent transition from electrical transmission to combined chemical and electrical transmission provides a previously unappreciated mechanism that may tune the response properties of the glomerular microcircuit.

Excitatory effects of gap junction blockers on cerebral cortex seizure-like activity in rats and mice

PURPOSE

The role of gap junctions in seizures is an area of intense research. Many groups have reported anticonvulsant effects of gap junction blockade, strengthening the case for a role for gap junctions in ictogenesis. The cerebral cortex is underrepresented in this body of research. We have investigated the effect of gap junction blockade on seizure-like activity in rat and mouse cerebral cortex slices.

METHODS

Seizure-like activity was induced by perfusing with low-magnesium artificial cerebrospinal fluid. The effect of three gap junction blockers was investigated in rat cortical slices; quinine (200 and 400 microm), quinidine (100 and 200 microm), and carbenoxolone (100 and 200 microm). In addition, the effect of mefloquine was investigated in wild-type mice and connexin36 knockout mice. The data were analyzed for the effect on frequency and amplitude of seizure-like events.

RESULTS

Paradoxical excitatory effects on seizure-like activity were observed for all three agents in rat cortical slices. Quinine (200 microm) and carbenoxolone (100 microm) increased both the frequency and amplitude of seizure-like events. Quinidine (100 microm) increased the frequency of events. Higher doses of quinine (400 microm) and carbenoxolone (200 microm) had biphasic excitatory-inhibitory effects. Similar excitatory effects were observed in adult wild-type mouse cortical slices perfused with mefloquine (5 microm or 10 microm), but were absent in slices from connexin36-deficient mice.

DISCUSSION

In conclusion, we have shown a paradoxical proseizure effect of pharmacologic gap junction blockade in a cortical model of seizure-like activity. We suggest that this effect is probably due to a disruption of inhibitory interneuron coupling secondary to connexin36 blockade.

Properties of gap junction blockers and their behavioural, cognitive and electrophysiological effects: animal and human studies

Gap junctions play an important role in brain physiology. They synchronize neuronal activity and connect glial cells participating in the regulation of brain metabolism and homeostasis. Gap junction blockers (GJBs) include various chemicals that impair gap junction communication, disrupt oscillatory neuronal activity over a wide range of frequencies, and decrease epileptic discharges. The behavioural and clinical effects of GJBs suggest that gap junctions can be involved in the regulation of locomotor activity, arousal, memory, and breathing. Severe neuropsychiatric side effects suggest the involvement of gap junctions in mechanisms of consciousness. Unfortunately, the available GJBs are not selective and can bind to targets other than gap junctions. Other problems in behavioural studies include the possible adverse effects of GJBs, for example, retinal toxicity and hearing disturbances, changes in blood-brain transport, and the metabolism of other drugs. Therefore, it is necessary to design experiments properly to avoid false, misleading or uninterpretable results. We review the pharmacological properties and electrophysiological, behavioural and cognitive effects of the available gap junction blockers, such as carbenoxolone, glycyrrhetinic acid, quinine, quinidine, mefloquine, heptanol, octanol, anandamide, fenamates, 2-APB, several anaesthetics, retinoic acid, oleamide, spermine, aminosulfonates, and sodium propionate. It is concluded that despite a number of different problems, the currently used gap junction blockers could be useful tools in pharmacology and neuroscience.

Meclofenamic acid blocks electrical synapses of retinal AII amacrine and on-cone bipolar cells

Gap junction channels constitute specialized intercellular contacts that can serve as electrical synapses. In the rod pathway of the retina, electrical synapses between AII amacrine cells express connexin 36 (Cx36) and electrical synapses between AII amacrines and on-cone bipolar cells express Cx36 on the amacrine side and Cx36 or Cx45 on the bipolar side. For physiological investigations of the properties and functions of these electrical synapses, it is highly desirable to have access to potent pharmacological blockers with selective and reversible action. Here we use dual whole cell voltage-clamp recordings of pairs of AII amacrine cells and pairs of AII amacrine and on-cone bipolar cells in rat retinal slices to directly measure the junctional conductance (G(j)) between electrically coupled cells and to study the effect of the drug meclofenamic acid (MFA) on G(j). Consistent with previous tracer coupling studies, we found that MFA reversibly blocked the electrical synapse currents in a concentration-dependent manner, with complete block at 100 muM. Whereas MFA evoked a detectable decrease in G(j) within minutes of application, the time to complete block of G(j) was considerably longer, typically 20-40 min. After washout, G(j) recovered to 20-90% of the control level, but the time to maximum recovery was typically >1 h. These results suggest that MFA can be a useful drug to investigate the physiological functions of electrical synapses in the rod pathway, but that the slow kinetics of block and reversal might compromise interpretation of the results and that explicit monitoring of G(j) is desirable.

Electrical coupling mediates tunable low-frequency oscillations and resonance in the cerebellar Golgi cell network

Tonic motor control involves oscillatory synchronization of activity at low frequency (5-30 Hz) throughout the sensorimotor system, including cerebellar areas. We investigated the mechanisms underpinning cerebellar oscillations. We found that Golgi interneurons, which gate information transfer in the cerebellar cortex input layer, are extensively coupled through electrical synapses. When depolarized in vitro, these neurons displayed low-frequency oscillatory synchronization, imposing rhythmic inhibition onto granule cells. Combining experiments and modeling, we show that electrical transmission of the spike afterhyperpolarization is the essential component for oscillatory population synchronization. Rhythmic firing arises in spite of strong heterogeneities, is frequency tuned by the mean excitatory input to Golgi cells, and displays pronounced resonance when the modeled network is driven by oscillating inputs. In vivo, unitary Golgi cell activity was found to synchronize with low-frequency LFP oscillations occurring during quiet waking. These results suggest a major role for Golgi cells in coordinating cerebellar sensorimotor integration during oscillatory interactions.

Neuronal coupling via connexin36 contributes to spontaneous synaptic currents of striatal medium-sized spiny neurons

Gap junctions provide a means for electrotonic coupling between neurons, allowing for the generation of synchronous activity, an important contributor to learning and memory. Connexin36 (Cx36) is largely neuron specific and provides a target for genetic manipulation to determine the physiological relevance of neuronal coupling. Within the striatum, Cx36 is more specifically localized to the interneuronal population, which provides the main inhibitory input to the principal projection medium-sized spiny neurons. In the present study, we examined the impact of genetic ablation of Cx36 on striatal spontaneous synaptic activity. Patch-clamp recordings were performed from medium-sized spiny neurons, the primary target of interneurons. In Cx36 knockout mice, the frequencies of both excitatory and inhibitory spontaneous postsynaptic currents were reduced. We also showed that activation of dopamine receptors differentially modulated the frequency of GABAergic currents in Cx36 knockout mice compared with their wild-type littermates, suggesting that dopamine plays a role in altering the coupling of interneurons. Taken together, the present findings demonstrate that electrical coupling of neuronal populations is important for the maintenance of normal chemical synaptic interactions within the striatum.

Role of olivary electrical coupling in cerebellar motor learning

The level of electrotonic coupling in the inferior olive is extremely high, but its functional role in cerebellar motor control remains elusive. Here, we subjected mice that lack olivary coupling to paradigms that require learning-dependent timing. Cx36-deficient mice showed impaired timing of both locomotion and eye-blink responses that were conditioned to a tone. The latencies of their olivary spike activities in response to the unconditioned stimulus were significantly more variable than those in wild-types. Whole-cell recordings of olivary neurons in vivo showed that these differences in spike timing result at least in part from altered interactions with their subthreshold oscillations. These results, combined with analyses of olivary activities in computer simulations at both the cellular and systems level, suggest that electrotonic coupling among olivary neurons by gap junctions is essential for proper timing of their action potentials and thereby for learning-dependent timing in cerebellar motor control.

A new conditional mouse mutant reveals specific expression and functions of connexin36 in neurons and pancreatic beta-cells

Connexin36 (Cx36) is the main connexin isoform expressed in neurons of the central nervous system (CNS) and in pancreatic beta-cells, i.e. two types of excitable cells that share - in spite of their different origins - a number of common features. Previous studies on Cx36 deficient mice have documented that loss of Cx36 resulted in phenotypic abnormalities in both the CNS and the pancreas which, however, could not be attributed to specific cell types due to the general deletion nature of the animal model used. Attempts to address this limitation using cell type specific deletions generated by the Cre/loxP strategy have so far been complicated by the lack of Cx36 expression from the floxed allele. We have now generated a conditional Cx36 deficient mouse mutant in which the coding region of Cx36 is flanked by loxP sites, followed by a cyan fluorescent protein (CFP) reporter gene. Here we show that Cx36 was still expressed from the floxed allele in neurons and pancreatic beta-cells. In these cells, a 30-60% decrease of this protein, relative to the expression level of the wildtype allele, did not significantly perturb cell coupling. The deletion of Cx36 by ubiquitously and cell type specifically expressed Cre recombinases revealed that CFP functions as a reliable reporter for Cx36 expression in brain neurons and to some extent in retina neurons, but not in pancreas. Loss of Cx36 by Cre-mediated recombination was documented at transcript and protein levels. Cell type specific deletion of Cx36 in the endocrine pancreas revealed major alterations in the basal as well as the glucose-induced insulin secretion, hence specifically attributing to pancreatic Cx36 an important regulatory role in the control of beta-cell function. Cell type specific deletion of Cx36 in the CNS by suitable Cre recombinases should also help to elucidate the functional role of Cx36 in different neuronal subtypes.

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.

In vivo mouse inferior olive neurons exhibit heterogeneous subthreshold oscillations and spiking patterns

In vitro whole-cell recordings of the inferior olive have demonstrated that its neurons are electrotonically coupled and have a tendency to oscillate. However, it remains to be shown to what extent subthreshold oscillations do indeed occur in the inferior olive in vivo and whether its spatiotemporal firing pattern may be dynamically generated by including or excluding different types of oscillatory neurons. Here, we did whole-cell recordings of olivary neurons in vivo to investigate the relation between their subthreshold activities and their spiking behavior in an intact brain. The vast majority of neurons (85%) showed subthreshold oscillatory activities. The frequencies of these subthreshold oscillations were used to distinguish four main olivary subtypes by statistical means. Type I showed both sinusoidal subthreshold oscillations (SSTOs) and low-threshold Ca(2+) oscillations (LTOs) (16%); type II showed only sinusoidal subthreshold oscillations (13%); type III showed only low-threshold Ca(2+) oscillations (56%); and type IV did not reveal any subthreshold oscillations (15%). These subthreshold oscillation frequencies were strongly correlated with the frequencies of preferred spiking. The frequency characteristics of the subthreshold oscillations and spiking behavior of virtually all olivary neurons were stable throughout the recordings. However, the occurrence of spontaneous or evoked action potentials modified the subthreshold oscillation by resetting the phase of its peak toward 90 degrees . Together, these findings indicate that the inferior olive in intact mammals offers a rich repertoire of different neurons with relatively stable frequency settings, which can be used to generate and reset temporal firing patterns in a dynamically coupled ensemble.

Electrical synapses in basal ganglia

The basal ganglia (BG) provide a major integrative system of the forebrain involved in the organization of goal-directed behaviour. Pathological alteration of BG function leads to major motor and cognitive impairments such as observed in Parkinson's disease. Recent advances in BG research stress the role of neural oscillations and synchronization in the normal and pathological function of BG. As demonstrated in several brain structures, these patterns of neural activity can emerge from electrically coupled neuronal networks. This review aims at addressing the presence, functionality and putative role of electrical synapses in BG, with a particular emphasis on the striatum and the substantia nigra pars compacta (SNc), two main BG nuclei in which the existence and functional properties of neuronal coupling are best documented.

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.

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.

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.

Spatiotemporal distribution of Connexin45 in the olivocerebellar system

The olivocerebellar system is involved in the transmission of information to maintain sensory motor coordination. Gap junctions have been described in various types of neurons in this system, including the neurons in the inferior olive that provide the climbing fibers to Purkinje cells. While it is well established that Connexin36 is necessary for the formation of these neuronal gap junctions, it is not clear whether these electrical synapses can develop without Connexin45. Here we describe the development and spatiotemporal distribution of Connexin45 in relation to that of Connexin36 in the olivocerebellar system. During development Connexin45 is expressed in virtually all neurons of the inferior olive and cerebellar nuclei. During later postnatal development and adulthood there is a considerable overlap of expression of both connexins in subpopulations of all main olivary nuclei and cerebellar nuclei as well as in the stellate cells in the cerebellar cortex. Despite this prominent expression of Connexin45, ultrastructural analysis of neuronal gap junctions in null-mutants of Connexin45 showed that their formation appears normal in contrast to that in knockouts of Connexin36. These morphological data suggest that Connexin45 may play a modifying role in widely distributed, coupled neurons of the olivocerebellar system, but that it is not essential for the creation of its neuronal gap junctions.

Regulation of connexin expression

Gap junctions contain cell-cell communicating channels that consist of multimeric proteins called connexins and mediate the exchange of low-molecular-weight metabolites and ions between contacting cells. Gap junctional communication has long been hypothesized to play a crucial role in the maintenance of homeostasis, morphogenesis, cell differentiation, and growth control in multicellular organisms. The recent discovery that human genetic disorders are associated with mutations in connexin genes and experimental data on connexin knockout mice have provided direct evidence that gap junctional communication is essential for tissue functions and organ development. Thus far, 21 human genes and 20 mouse genes for connexins have been identified. Each connexin shows tissue- or cell-type-specific expression, and most organs and many cell types express more than one connexin. Cell coupling via gap junctions is dependent on the specific pattern of connexin gene expression. This pattern of gene expression is altered during development and in several pathological conditions resulting in changes of cell coupling. Connexin expression can be regulated at many of the steps in the pathway from DNA to RNA to protein. However, transcriptional control is one of the most important points. In this review, we summarize recent knowledge on transcriptional regulation of connexin genes by describing the structure of connexin genes and transcriptional factors that regulate connexin expression.

Normal motor learning during pharmacological prevention of Purkinje cell long-term depression

Systemic delivery of (1R-1-benzo thiophen-5-yl-2[2-diethylamino)-ethoxy] ethanol hydrochloride (T-588) prevented long-term depression (LTD) of the parallel fiber (PF)-Purkinje cell (PC) synapse induced by conjunctive climbing fiber and PF stimulation in vivo. However, similar concentrations of T-588 in the brains of behaving mice and rats affected neither motor learning in the rotorod test nor the learning of motor timing during classical conditioning of the eyeblink reflex. Rats given doses of T-588 that prevented PF-PC LTD were as proficient as controls in learning to adapt the timing of their conditioned eyeblink response to a 150- or 350-ms change in the timing of the paradigm. The experiment indicates that PF-PC LTD under control of the climbing fibers is not required for general motor adaptation or the learning of response timing in two common models of motor learning for which the cerebellum has been implicated. Alternative mechanisms for motor timing and possible functions for LTD in protection from excitotoxicity are discussed.

Rhythmicity, randomness and synchrony in climbing fiber signals

The role of the climbing fiber input to the cerebellum has been enigmatic, with recent studies focusing on its temporal and spatial firing patterns. Debate remains as to whether climbing fibers provide a periodic clock for coordinating movements or lead to long-term modification of Purkinje cell activity as the basis of motor learning. Rhythmic and synchronous activity of climbing fibers can cause movements at the same frequency in some preparations, suggesting a role in motor timing. However, in awake monkeys climbing fiber signals have been reported to occur at random, presenting a problem for clock theories. Yet synchronous patterns of discharge are consistently observed among several Purkinje cells within a narrow parasagittal longitudinal band. Here, we review recent experimental and theoretical studies and attempt to provide a coherent account of the interplay between rhythmicity, randomness and synchrony in climbing fiber activity, with a particular reference to studies in chaos.

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.

Loss of connexin36 channels alters beta-cell coupling, islet synchronization of glucose-induced Ca2+ and insulin oscillations, and basal insulin release

Normal insulin secretion requires the coordinated functioning of beta-cells within pancreatic islets. This coordination depends on a communications network that involves the interaction of beta-cells with extracellular signals and neighboring cells. In particular, adjacent beta-cells are coupled via channels made of connexin36 (Cx36). To assess the function of this protein, we investigated islets of transgenic mice in which the Cx36 gene was disrupted by homologous recombination. We observed that compared with wild-type and heterozygous littermates that expressed Cx36 and behaved as nontransgenic controls, mice homozygous for the Cx36 deletion (Cx36(-/-)) featured beta-cells devoid of gap junctions and failing to exchange microinjected Lucifer yellow. During glucose stimulation, islets of Cx36(-/-) mice did not display the regular oscillations of intracellular calcium concentrations ([Ca(2+)](i)) seen in controls due to the loss of cell-to-cell synchronization of [Ca(2+)](i) changes. The same islets did not release insulin in a pulsatile fashion, even though the overall output of the hormone in response to glucose stimulation was normal. However, under nonstimulatory conditions, islets lacking Cx36 showed increased basal release of insulin. These data show that Cx36-dependent signaling is essential for the proper functioning of beta-cells, particularly for the pulsatility of [Ca(2+)](i) and insulin secretion during glucose stimulation.

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.

Is autism due to brain desynchronization?

The hypothesis is presented that a disruption in brain synchronization contributes to autism by destroying the coherence of brain rhythms and slowing overall cognitive processing speed. Particular focus is on the inferior olive, a precerebellar structure that is reliably disrupted in autism and which normally generates a coherent 5-13 Hz rhythmic output. New electrophysiological data reveal that the continuity of the rhythmical oscillation in membrane potential generated by inferior olive neurons requires the formation of neuronal assemblies by the connexin36 protein that mediates electrical synapses and promotes neuronal synchrony. An experiment with classical eyeblink conditioning is presented to demonstrate that the inferior olive is necessary to learn about sequences of stimuli presented at intervals in the range of 250-500 ms, but not at 700 ms, revealing that a disruption of the inferior olive slows stimulus processing speed on the time scale that is lost in autistic children. A model is presented in which the voltage oscillation generated by populations of electrically synchronized inferior olivary neurons permits the utilization of sequences of stimuli given at, or faster than, 2 per second. It is expected that the disturbance in inferior olive structure in autism disrupts the ability of inferior olive neurons to become electrically synchronized and to generate coherent rhythmic output, thereby impairing the ability to use rapid sequences of cues for the development of normal language skill. Future directions to test the hypothesis are presented.

Expression of neural connexins and pannexin1 in the hippocampus and inferior olive: a quantitative approach

Electrical synapses (or neuronal gap junctions) are thought to be essential for the generation of synchronous oscillatory activities in various areas of the brain. In this study, we quantified the steady state mRNA expression levels of two neuronal gap junction proteins, connexin36 (Cx36) and connexin45 (Cx45), as well as of pannexin1, a member of a novel class of communicative junction forming proteins, and of connexin47 (Cx47) which is expressed in oligodendrocytes. The expression levels of these genes were compared in two regions known for oscillatory activity and which are equipped with electrically coupled neurons. Assessment of the levels of mRNA expression in the hippocampus and the nuclear complex of the inferior olive (IO) was achieved by means of laser microdissection (LMM) in combination with real time RT-PCR. Our results demonstrate the differential expression of Cx36, Cx45, pannexin1 and Cx47 in the hippocampus, with pannexin1 showing the highest level of expression followed by Cx36, Cx47, and Cx45. In the IO, pannexin1 showed a comparable expression level as in the hippocampus, but connexin expression levels were increased. Upon direct comparison, the combination of LMM and real time RT-PCR data generated specific, robust and reproducible results consistent with recent data reported about connexin expression in the nervous system. We conclude that the analytical strategy shown here provides a technological solution to overcome the less sensitive and notoriously less specific analysis of connexin expression by in situ hybridization.

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.

New insights into the expression and function of neural connexins with transgenic mouse mutants

Gap junctions represent direct intercellular conduits between contacting cells. The subunit proteins of these conduits are called connexins. To date, 20 and 21 connexin genes have been described in the mouse and human genome, respectively, many of them represent sequence-orthologous pairs. Targeted deletion of connexin genes in the mouse genome opened new insights into the biological function of these channel forming proteins, which, in some cases, could be correlated to phenotypic abnormalities in humans, suffering from inherited diseases caused by mutations in the corresponding orthologous connexin gene. Replacing the connexin coding DNA by an appropriate reporter gene has clarified in several cases its cell type specific expression in mouse brain. Various studies demonstrated that connexin36 is mainly expressed in interneurons of retina and brain. Targeted deletion of connexin36 evoked a loss of electrical signal transduction and interferes with synchrony which probably leads to defects in visual transmission and memory. Deletion of connexin43 in astrocytes of mouse brain resulted in increased spreading depression consistent with the notion of altered "spatial buffering" of K(+) ions and glutamate secreted by active neurons. General connexin30-deficiency led to hearing impairment and apoptosis of hair cells, similar to that observed in mice with cochlea specific deletion of connexin26. Reporter gene expression in connexin30-deficient mice indicated that astrocytes in certain brain regions and leptomeningeal as well as ependymal cells are labelled. Reporter gene expression in connexin45- and connexin47-deficient mice was used to reassign connexin45 expression to certain CNS neurons and connexin47 expression to oligodendrocytes.

Stimulus complexity dependent memory impairment and changes in motor performance after deletion of the neuronal gap junction protein connexin36 in mice

Gap junction channels, composed of connexin (Cx) proteins, are conduits for intercellular communication and metabolic exchange in the central nervous system. Connexin36 (Cx36) is expressed in distinct subpopulations of neurons throughout the mammalian brain. Deletion of the Cx36 gene in the mouse affected power and frequency of gamma and sharp wave-ripple oscillations, putative correlates of memory engram inscription. Here, we present a behavioral analysis of Cx36-deficient mice. Activity patterns, exploratory- and anxiety-related responses were largely unaffected by elimination of Cx36, while sensorimotor capacities and learning and memory processes were impaired. Repeated testing on the rotarod suggested that the Cx36-deficient mice showed slower motor-coordination learning. After a retention interval of 24 h the Cx36-deficient mice showed habituation to an open-field, but failed to habituate to a more complex spatial environment (Y-maze). A more pronounced memory impairment was found when Cx36 knockout mice had to remember recently explored objects. Cx36-deficient mice were unable to recognize objects after short delays of 15 and 45 min. These data suggest that lack of Cx36 induces memory impairments that vary in dependence of the complexity of the stimuli presented. Our results suggest that neuronal gap junctions incorporating Cx36 play a role in learning and memory.

Connexon connexions in the thalamocortical system

Electrical synapses are composed of gap junction channels that interconnect neurons. They occur throughout the mammalian brain, although this has been appreciated only recently. Gap junction channels, which are made of proteins called connexins, allow ionic current and small organic molecules to pass directly between cells, usually with symmetrical ease. Here we review evidence that electrical synapses are a major feature of the inhibitory circuitry in the thalamocortical system. In the neocortex, pairs of neighboring inhibitory interneurons are often electrically coupled, and these electrical connections are remarkably specific. To date, there is evidence that five distinct subtypes of inhibitory interneurons in the cortex make electrical interconnections selectively with interneurons of the same subtype. Excitatory neurons (i.e., pyramidal and spiny stellate cells) of the mature cortex do not appear to make electrical synapses. Within the thalamus, electrical coupling is observed in the reticular nucleus, which is composed entirely of GABAergic neurons. Some pairs of inhibitory neurons in the cortex and reticular thalamus have mixed synaptic connections: chemical (GABAergic) inhibitory synapses operating in parallel with electrical synapses. Inhibitory neurons of the thalamus and cortex express the gap junction protein connexin 36 (C x 36), and knocking out its gene abolishes nearly all of their electrical synapses. The electrical synapses of the thalamocortical system are strong enough to mediate robust interactions between inhibitory neurons. When pairs or groups of electrically coupled cells are excited by synaptic input, receptor agonists, or injected current, they typically display strong synchrony of both subthreshold voltage fluctuations and spikes. For example, activating metabotropic glutamate receptors on coupled pairs of cortical interneurons or on thalamic reticular neurons can induce rhythmic action potentials that are synchronized with millisecond precision. Electrical synapses offer a uniquely fast, bidirectional mechanism for coordinating local neural activity. Their widespread distribution in the thalamocortical system suggests that they serve myriad functions. We are far from a complete understanding of those functions, but recent experiments suggest that electrical synapses help to coordinate the temporal and spatial features of various forms of neural activity.

An expression atlas of connexin genes in the mouse

Connexin genes are involved in several human diseases such as hearing and dermatological and peripheral nerve disorders. Connexins are protein units of gap junctions and form homotypic, heterotypic, or heteromeric complexes known as connexons. Data on the expression patterns of members of this family are partial and fragmentary. We therefore cloned all the identifiable murine homologs of human CONNEXIN genes and analyzed their expression patterns in embryonic and neonatal mouse tissues. We found that connexins are preferentially expressed in tissues derived from ectoderm and/or endoderm. Our data provide a comprehensive and detailed atlas of expression of connexin genes and in some cases suggest possible interactions of proteins that are coexpressed in the same tissue. Knowledge of temporal and spatial distribution of connexins also allows the identification of candidate genes for human diseases and provides important insight into mechanisms that lead to human disorders due to mutations in CONNEXIN genes.

Strong coupling of nonlinear electronic and biological oscillators: reaching the "amplitude death" regime

Interaction between an electronic and a biological circuit has been investigated for a pair of electrically connected nonlinear oscillators, with a spontaneously oscillating olivary neuron as the single-cell biological element. By varying the coupling strength between the oscillators, we observe a range of behaviors predicted by model calculations, including a reversible low-energy dissipation "amplitude death" where the oscillations in the coupled system cease entirely.

Potent block of Cx36 and Cx50 gap junction channels by mefloquine

Recently, great interest has been shown in understanding the functional roles of specific gap junction proteins (connexins) in brain, lens, retina, and elsewhere. Some progress has been made by studying knockout mice with targeted connexin deletions. For example, such studies have implicated the gap junction protein Cx36 in synchronizing rhythmic activity of neurons in several brain regions. Although knockout strategies are informative, they can be problematic, because compensatory changes sometimes occur during development. Therefore, it would be extremely useful to have pharmacological agents that block specific connexins, without major effects on other gap junctions or membrane channels. We show that mefloquine, an antimalarial drug, is one such agent. It blocked Cx36 channels, expressed in transfected N2A neuroblastoma cells, at low concentrations (IC(50) approximately 300 nM). Mefloquine also blocked channels formed by the lens gap junction protein, Cx50 (IC(50) approximately 1.1 microM). However, other gap junctions (e.g., Cx43, Cx32, and Cx26) were only affected at concentrations 10- to 100-fold higher. To further examine the utility and specificity of this compound, we characterized its effects in acute brain slices. Mefloquine, at 25 microM, blocked gap junctional coupling between interneurons in neocortical slices, with minimal nonspecific actions. At this concentration, the only major side effect was an increase in spontaneous synaptic activity. Mefloquine (25 microM) caused no significant change in evoked excitatory or inhibitory postsynaptic potentials, and intrinsic cellular properties were also mostly unaffected. Thus, mefloquine is expected to be a useful tool to study the functional roles of Cx36 and Cx50.

Decrease of Hsp25 protein expression precedes degeneration of motoneurons in ALS-SOD1 mice

We have investigated the expression of Hsp25, a heat shock protein constitutively expressed in motoneurons, in amyotrophic lateral sclerosis (ALS) mice that express G93A mutant SOD1 (G93A mice). Immunocytochemistry and Western blotting showed that a decrease of Hsp25 protein expression occurred in motoneurons of G93A mice prior to the onset of motoneuron death and muscle weakness. This decrease in Hsp25 expression also preceded the appearance of SOD1 aggregates as identified by cellulose acetate filtration and Western blot analysis. In contrast to Hsp25 protein levels, Hsp25 mRNA as determined by in situ hybridization and RT-PCR, remained unchanged. This suggests that the decrease in Hsp25 protein levels occurs post-transcriptionally. In view of the cytoprotective properties of Hsp25 and the temporal relationship between decreased Hsp25 expression and the onset of motoneuron death, it is feasible that reduced Hsp25 concentration contributes to the degeneration of motoneurons in G93A mice. These data are consistent with the idea that mutant SOD1 may reduce the availability of the protein quality control machinery in motoneurons.

Electrotonic coupling between stratum oriens interneurones in the intact in vitro mouse juvenile hippocampus

Using the isolated juvenile (7-14 days) mouse whole hippocampus preparation, which contains intact complex local circuitry, 145 dual whole cell recordings were made from stratum oriens (s.o.) interneurones under infrared microscopy. In 11.7% of paired recordings, evidence for direct electrotonic coupling between the s.o. interneurones was obtained from the response of one interneurone to a long (400-600 ms) constant current pulse passed into the coupled interneurone. When specifically orienting the dual recordings in the transectional plane of the hippocampus, 18.5% of paired recordings showed electrotonic coupling. The coupling coefficient, estimated from averaged data, was 6.9 +/- 4.7%, ranging from 1.3 to 17.6%. The time constant of the electrotonically transmitted hyperpolarization was inversely related to the coupling coefficient between the two neurones. The electrotonic responses of one neurone to constant current pulses injected into the other coupled neurone were intermittent. Spikes in one of the coupled neurones were associated with small electrotonic EPSPs (spikelets) in the other coupled neurone, in those neuronal pairs with coupling coefficients greater than 10%. Failure of spikelet production following a spike in the coupled cell occurred 5-10% of the time. Electrotonic coupling and spikelets persisted in the presence of chemical synaptic transmission blockade by CNQX, APV and bicuculline, or in zero Ca(2+) perfusate, but were abolished by carbenoxolone (100 microm), a gap junctional blocker. These data confirm the existence of electrotonic coupling between s.o. interneurones, presumably via gap junctions located in dendrites.

Expression pattern of lacZ reporter gene representing connexin36 in transgenic mice

Targeted deletion of the connexin36 (Cx36) gene in the mouse genome leads to visual transmission defects, weakened synchrony of rhythmic inhibitory potentials in the neocortex, and disruption of gamma-frequency network oscillations. We have generated transgenic mice in which a reporter protein consisting of the exon1 coded N-terminal part of Cx36 fused to beta-galactosidase (N36-beta-gal) is expressed instead of Cx36. Here, we have used these mice for a detailed analysis of the reporter gene expression. By beta-gal staining of adult retina, we found expression of the lacZ reporter gene in the ganglion cell layer, in two rows of the inner nuclear layer, and in the photoreceptor layer. In the brain, beta-gal staining was present in gamma-aminobutyric acid (GABA)ergic neurons of the cerebellar nuclei, in non-GABAergic neurons of the inferior olive, in mitral cells of the olfactory bulb, and in parvalbumin-positive cells of the cerebral cortex. Outside the central nervous system, N36-beta-gal signals were detected in insulin producing beta-cells of the pancreas and in the medulla of the adrenal gland of adult Cx36(+/del[LacZ]) mice. This expression pattern suggests that Cx36 fulfills functional roles not only in several types of neurons in the retina and central nervous system but also in excitable cells of the pancreas and adrenal gland.

Fundamental role of inferior olive connexin 36 in muscle coherence during tremor

Inferior olive (IO) neurons are electrically coupled by cytosolic pores formed by the neuron-specific connexin 36 (Cx36). Electrical coupling in the IO figures prominently in current views about brain control of movement. However, a role for Cx36 in movement has been questioned and not definitively demonstrated. Previous reports have shown that embryonic deletion of the Cx36 gene resulted in almost complete loss of cytosolic and electrical coupling in the IO without an obvious deficit in movement, possibly due to developmental compensations in ionic conductances that can confound the approach of embryonic gene deletion. We used a replication-incompetent lentiviral vector to stably express a dominant-negative Cx36 mutant in the IO of adult rats. We show that interneuronal cytosolic coupling is severely reduced by the mutant Cx36, without effect on neuron morphology or electrical properties. Multisite electromyography revealed that blocking Cx36 in the IO impaired the coherence of muscle firing during harmaline tremor without affecting its rhythm. The data demonstrate that gap junction coupling within the IO mediated by Cx36 adds 10-20 ms of precision to the fine temporal coordination of muscle firing during movement.

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.

Fundamental role of inferior olive connexin 36 in muscle coherence during tremor

Inferior olive (IO) neurons are electrically coupled by cytosolic pores formed by the neuron-specific connexin 36 (Cx36). Electrical coupling in the IO figures prominently in current views about brain control of movement. However, a role for Cx36 in movement has been questioned and not definitively demonstrated. Previous reports have shown that embryonic deletion of the Cx36 gene resulted in almost complete loss of cytosolic and electrical coupling in the IO without an obvious deficit in movement, possibly due to developmental compensations in ionic conductances that can confound the approach of embryonic gene deletion. We used a replication-incompetent lentiviral vector to stably express a dominant-negative Cx36 mutant in the IO of adult rats. We show that interneuronal cytosolic coupling is severely reduced by the mutant Cx36, without effect on neuron morphology or electrical properties. Multisite electromyography revealed that blocking Cx36 in the IO impaired the coherence of muscle firing during harmaline tremor without affecting its rhythm. The data demonstrate that gap junction coupling within the IO mediated by Cx36 adds 10-20 ms of precision to the fine temporal coordination of muscle firing during movement.

Neuronal connexin36 association with zonula occludens-1 protein (ZO-1) in mouse brain and interaction with the first PDZ domain of ZO-1

Among the 20 members in the connexin family of gap junction proteins, only connexin36 (Cx36) is firmly established to be expressed in neurons and to form electrical synapses at widely distributed interneuronal gap junctions in mammalian brain. Several connexins have recently been reported to interact with the PDZ domain-containing protein zonula occludens-1 (ZO-1), which was originally considered to be associated only with tight junctions, but has recently been reported to associate with other structures including gap junctions in various cell types. Based on the presence of sequence corresponding to a putative PDZ binding motif in Cx36, we investigated anatomical relationships and molecular association of Cx36 with ZO-1. By immunofluorescence, punctate Cx36/ZO-1 colocalization was observed throughout the central nervous system of wild-type mice, whereas labelling for Cx36 was absent in Cx36 knockout mice, confirming the specificity of the anti-Cx36 antibodies employed. By freeze-fracture replica immunogold labelling, Cx36 and ZO-1 in brain were found colocalized within individual ultrastructurally identified gap junction plaques, although some plaques contained only Cx36 whereas others contained only ZO-1. Cx36 from mouse brain and Cx36-transfected HeLa cells was found to coimmunoprecipitate with ZO-1. Unlike other connexins that bind the second of the three PDZ domains in ZO-1, glutathione S-transferase-PDZ pull-down and mutational analyses indicated Cx36 interaction with the first PDZ domain of ZO-1, which required at most the presence of the four c-terminus amino acids of Cx36. These results demonstrating a Cx36/ZO-1 association suggest a regulatory and/or scaffolding role of ZO-1 at gap junctions that form electrical synapses between neurons in mammalian brain.

Neuronal connexin36 association with zonula occludens-1 protein (ZO-1) in mouse brain and interaction with the first PDZ domain of ZO-1

Among the 20 members in the connexin family of gap junction proteins, only connexin36 (Cx36) is firmly established to be expressed in neurons and to form electrical synapses at widely distributed interneuronal gap junctions in mammalian brain. Several connexins have recently been reported to interact with the PDZ domain-containing protein zonula occludens-1 (ZO-1), which was originally considered to be associated only with tight junctions, but has recently been reported to associate with other structures including gap junctions in various cell types. Based on the presence of sequence corresponding to a putative PDZ binding motif in Cx36, we investigated anatomical relationships and molecular association of Cx36 with ZO-1. By immunofluorescence, punctate Cx36/ZO-1 colocalization was observed throughout the central nervous system of wild-type mice, whereas labelling for Cx36 was absent in Cx36 knockout mice, confirming the specificity of the anti-Cx36 antibodies employed. By freeze-fracture replica immunogold labelling, Cx36 and ZO-1 in brain were found colocalized within individual ultrastructurally identified gap junction plaques, although some plaques contained only Cx36 whereas others contained only ZO-1. Cx36 from mouse brain and Cx36-transfected HeLa cells was found to coimmunoprecipitate with ZO-1. Unlike other connexins that bind the second of the three PDZ domains in ZO-1, glutathione S-transferase-PDZ pull-down and mutational analyses indicated Cx36 interaction with the first PDZ domain of ZO-1, which required at most the presence of the four c-terminus amino acids of Cx36. These results demonstrating a Cx36/ZO-1 association suggest a regulatory and/or scaffolding role of ZO-1 at gap junctions that form electrical synapses between neurons in mammalian brain.

Electrical coupling and neuronal synchronization in the Mammalian brain

Certain neurons in the mammalian brain have long been known to be joined by gap junctions, which are the most common type of electrical synapse. More recently, cloning of neuron-specific connexins, increased capability of visualizing cells within brain tissue, labeling of cell types by transgenic methods, and generation of connexin knockouts have spurred a rapid increase in our knowledge of the role of gap junctions in neural activity. This article reviews the many subtleties of transmission mediated by gap junctions and the mechanisms whereby these junctions contribute to synchronous firing.

Electrical synapses in the mammalian brain

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

Connexin36 distribution in putative CO2-chemosensitive brainstem regions in rat

Recent work from our laboratory has demonstrated that the gap junction proteins connexin26 (Cx26) and connexin32 (Cx32) are expressed in neurons in putative CO2-chemosensitive brainstem regions in both neonatal and adult rats. Whether the recently identified neuron-specific gap junction protein connexin36 (Cx36) is also present in these brainstem regions remains to be determined. Therefore, in the current experiments, immunoblot and immunohistochemical protocols were used to investigate the regional distribution and cellular localization of Cx36 in putative CO2-chemosensitive brainstem regions in both neonatal and adult rats. Immunoblot analyses revealed Cx36 expression in putative CO2-chemosensitive brainstem regions in each of the age groups examined, although both regional and developmental differences in the relative expression levels were detected. Immunohistochemical analyses confirmed Cx36 expression in neurons in each of the putative CO2-chemosensitive brainstem regions and revealed both somal and dendritic labeling patterns. These findings provide additional morphological evidence supporting the potential for gap junctional communication in these regions in both neonatal and adult rats. We propose that the gap junction protein Cx36 also contributes to the neuroanatomical substrate for gap junctional communication, which is hypothesized to play a role in central CO2 chemoreception.