Neuronal Gap Junctions: Expression, Function, And Implications For Behavior

NEURONAL GAP JUNCTIONS: EXPRESSION, FUNCTION, AND IMPLICATIONS FOR BEHAVIOR

Clinton B. McCracken and David C. S. Roberts Department of Physiology and Pharmacology, Wake Forest University School of Medicine Winston-Salem, North Carolina 27157, USA

I. II. III. IV. V. VI. VII. VIII. IX. X.

A Brief History of Gap Junctions Gap Junction Structure Gap Junctions in the Brain Electrical Coupling in the Brain Properties and Function of Electrical Synapses Modulation of Electrical Synapses and Gap-Junctional Coupling Use-Dependent Plasticity Local Factors: Voltage, pH, and Calcium Neurotransmitter and Second Messenger Modulation Concluding Remarks References

In this chapter, we will review what is currently known (and not known) about gap junction expression in neurons. We will discuss the composition of neuronal gap junctions, the functions of neuronal gap junctions acting as ‘‘electrical synapses,’’ and attempt to highlight some of the many controversies surrounding these issues. The latter portion of this chapter will be devoted to modulation and plasticity of junctional communication between neurons, with a particular emphasis on the potential consequences of alterations in neuronal coupling for neural function and behavior. This chapter is not directed at those who are currently studying gap junction neurobiology per se, rather, it is an attempt to convince neuroscientists less familiar with the subject of the importance of direct intracellular communication between neurons in brain function. Often assumed to be static, we know now that gap-junctional communication is plastic and subject to modulation, and this plasticity is likely to have meaningful consequences for neural activity. I. A Brief History of Gap Junctions

The concept that neurons interact through direct transfer of electrical current is not new. Indeed, this form of communication was championed by one faction of early neuroscientists as the primary mechanism of neural transmission (Eccles, INTERNATIONAL REVIEW OF NEUROBIOLOGY, VOL. 73 DOI: 10.1016/S0074-7742(06)73004-5

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1982), until pivotal studies by Loewi, Dale, and others produced incontrovertible evidence that neurons communicate using chemical messengers. This led to the dogma that neurons communicate by chemical neurotransmission only, and interest in electrical interactions between neurons rapidly faded. As such, the study of gap junction-mediated intracellular communication between neurons has only recently begun to attract attention from most contemporary neuroscientists. It has been known for some time that invertebrate neurons are electrically coupled (Furshpan and Potter, 1959; Watanabe, 1958), and this type of connection was also documented in vertebrates (Bennett et al., 1959). At the time, however, these direct neuronal interactions were thought to be a hallmark of lower organisms and of little significance in mammalian brain. Even with the demonstration of neuronal coupling in rodents (Baker and Llinas, 1971; Korn et al., 1973; Llinas et al., 1974), there remained very little study devoted to the subject, save for a small core of researchers. Development of more sophisticated microscopy techniques enabled identification of the morphological correlate of this coupling, from studies on heart, liver, and brain (Barr et al., 1965; Pappas et al., 1971; Revel et al., 1971). This structure was named the ‘‘gap junction,’’ from the studies of Revel and Karnovsky (1967). Gap junction-mediated communication has now been documented in virtually all cell types and tissues, and the bulk of our understanding of direct intracellular communication comes not from brain, where it was first characterized, but from examination of expression and function in other tissues. Studies of gap-junctional communication between neurons have generally remained on the periphery of neuroscience, even as breakthroughs were made in other disciplines regarding the molecular biology and biophysical properties of these channels. As shown in Fig. 1, studies of gap junctions in brain comprise only a small fraction of total gap junction studies. Major advances in the study of gap junctions included the cloning of a gap junction subunit and the determination of the gap junction channel crystal structure. These eVorts greatly facilitated the study of gap junction biology, and enabled comprehensive analysis of gap junctions in expression systems. Still, as indicated by the limited number of studies, these channels were thought to be of limited relevance to neuronal function. At the same time, critical roles for intercellular communication were being shown in development, cell growth, and diVerentiation; cardiovascular, hepatic, endocrine, and immune system functions; as well as in certain pathologies. The last 10 years or so have seen an explosion in studies of gap junctions in brain, largely due to advances in electrophysiology and molecular biology. As we will discuss below, a neuron-specific connexin has been identified, and has been shown to mediate current transfer between neurons in a number of brain areas using paired intracellular recordings. This has led to a reassessment of the functional significance of neuronal gap junctions in adult mammals.

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FIG. 1. Publications involving the structure and function of gap junctions published in the period 1971 through 2005, broken down into 5-year epochs. Bars represent the total number of publications during each interval as reported by PubMed. Search terms were ‘‘gap junction’’ and ‘‘gap junction AND brain.’’

Indeed, where in past years neuroscience textbooks either had no mention of gap junctions, they are now being accorded more attention. For example, Kandel et al. (2000) and Bear et al. (2001) give only passing reference to gap junctions; however the most recent edition of Fundamental Neuroscience (Squire et al., 2002) devotes a full chapter to the subject, reflecting the recent advances in the field. These developments have raised a plethora of new experimental questions. Whereas the main question was once ‘‘are they relevant?’’ the question has now become ‘‘how are they relevant?’’ Here we will outline some of the properties of gap junction channels, before discussing the modulation of junctional communication and its relevance for brain function.

II. Gap Junction Structure

Gap junctions were traditionally identified by their characteristic structural features when viewed with thin section electron microscopy. They appear as a close apposition of thickened plasma membranes separated by a small (2–3 nm) gap of extracellular space (Fig. 2A). Based on these observations, Revel and Karnovsky (1967) coined the term ‘‘gap junction’’ to describe these structures.

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FIG. 2. Gap junction ultrastructure. (A) Thin-section micrograph showing a gap junction between glial cells. Arrows indicate gap junction. Scale bar—200 nm. Adapted from Atlas of Ultrastructural Neurocytology. (B) Freeze-fracture micrograph of a gap-junction plaque. Each small dot on the plaque is a gap-junction channel (large black spots are gold beads). Scale bar—0.1 mm. Adapted from Rash et al. (2001b).

Freeze-fracture electron microscopy showed that the close membrane appositions were formed by ‘‘plaques’’ of hundreds to thousands of individual channels connecting the two cells (Fig. 2B). Full understanding of the gap junction structure was facilitated greatly by cloning of a gap-junction subunit, or connexin (Paul, 1986). This allowed detailed examination of gap junctions using the entire arsenal of molecular biological techniques. As mentioned, gap-junction channels are formed by connexin subunits, and more than 20 diVerent connexin genes have been identified (Evans and Martin, 2002; Willecke et al., 2002). These genes are named according to apparent molecular weight of the expressed protein (e.g., Cx43 is approximately 43 kDa). Individual connexin subunits have four membrane spanning domains that are highly conserved across the gene family (Saez et al., 2003), with two extracellular loops, and intracellular N- and C-termini. Six individual connexin subunits combine to form a hexameric hemichannel, or connexon, and two connexons from apposed membranes form the gap-junction channel (Fig. 3). Connexon hexamers can be homo- or heteromeric, and as well, gap-junction channels can be homo- or heterotypic. Only certain connexons can form functional heterotypic gap junctions, and compatibility is determined by individual connexin subtypes. It is believed that this may allow cells expressing multiple connexins to establish distinct functional connections. The gap-junction channel pore is approximately 1.2 nm in diameter, allowing transmission of ions and small molecules under 1 kD in size such as cAMP and IP3. Channel conductance, selective permeability, and gating influences all vary depending on connexin subtype composition (Kumar and Gilula, 1996), however, not all subtypes have been fully characterized in native tissue.

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FIG. 3. Schematic of a gap-junction plaque and connexin subunit hemichannels in apposed plasma membranes of neighbouring cells can dock to each other and form gap junction channels. Inset: Connexin protein subunits have four transmembrane domains. Adapted from Sohl et al. (2004).

III. Gap Junctions in the Brain

The study of gap junction expression and function in the brain has until recently proceeded along separate lines of research—physiology, anatomy, or molecular biology—that have only recently begun to converge. The relationship between electrical synapses and gap junctions was beginning to be clarified more than 30 years ago. At approximately the same time electrical synapses were discovered in mammals, ultrastructurally identified gap junctions between neurons were also reported in the mammalian brain (Sloper, 1972), and it has been known for some time that a number of connexin proteins are found in brain (Dermietzel et al., 1989; Shiosaka et al., 1989). Definitively establishing the connexin constituents of gap junctions between neurons remains a high priority. As mentioned, channels formed by diVerent connexins show diVerent properties such as channel conductance, selectivity of permeant molecules, and gating. Accordingly, gap junctions comprising diVerent connexins may aVect neuronal coupling in diVerent ways. Considerable progress has been made in determining the cellular localization of various connexins in the CNS, although a good deal of controversy remains.

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One confound is the fact that expression levels of many connexins show dramatic changes with development. Some conflicting reports of neuronal connexin expression could possibly be due to animals of diVerent ages being used. Intercellular coupling plays a significant role in neuronal development and development of functional neuronal architecture (Montoro and Yuste, 2004; Rozental et al., 2000), which is beyond the scope of this chapter. A number of other factors have impeded this line of work. Low expression levels, of both mRNA and protein, and protein expression at sites distal to the soma have impeded immunocytochemical studies. In situ hybridization and immunohistochemistry studies may be confounded by cross-reactions, and confirmation of antibody and probe specificity in knockout animals is essential (Meier et al., 2002). As well, studies performed using light microscopy lack the spatial resolution (i.e., less than 0.2 mm) to definitively assign connexin signals to particular cell types. The most definitive technique for identification of connexin constituents of gap junctions is freezefracture immunogold labeling (FRIL) (Nagy et al., 2004), which uses a combination of electron microscopy and immunolabeled antibodies that provides very high resolution. However, this technique is labor-intensive and has not been used in many brain areas, and gap-junction plaques formed by a small number of channels could conceivably escape detection. A relatively novel method for examining connexin expression involves replacing the coding region of the gene with a reporter gene such as
-galactosidase (e.g., Deans et al., 2001; Zhang et al., 2000). This technique has limitations as well, as ectopic expression may occasionally be observed due to interference with upstream regulatory elements (Sohl et al., 2004). As the field has progressed, it has become clear that all techniques have their advantages and disadvantages, and combinatorial approaches are necessary to achieve consensus. While approximately half of the currently known connexins are expressed in brain, only Cx36 has been unequivocally shown to be expressed in neurons. This connexin was first cloned in 1998 and shown to be highly expressed in neurons using in situ hybridization (Condorelli et al., 1998; Sohl et al., 1998). Cx36 expression is found in almost all brain areas, including neocortex, brainstem, basal ganglia, hippocampus, and cerebellum, and shows a developmentally regulated expression profile, with highest expression on postnatal day 7, declining to lower levels in adulthood. Subsequent studies combining in situ hybridization with immunolabeling for a neuronal marker confirmed that Cx36 was not only expressed in neurons, but also appeared to be neuron specific (Belluardo et al., 2000; Condorelli et al., 2000). Development of antibodies to Cx36 enabled direct study of protein localization to ultrastructurally-identified gap junctions between neurons in a number of brain regions using FRIL (Rash et al., 2000, 2001a,b), and, on a more macroscopic scale, using immunohistochemical techniques (Liu and Jones, 2003; Meier et al., 2002; Teubner et al., 2000). Data from these Cx36

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protein studies were in general concordance with the mRNA data. Further support for neuronal expression of this connexin was obtained using transgenic animals (generated by several groups) that expressed a reporter gene in the place of Cx36 (Deans et al., 2001; Degen et al., 2004; Landisman et al., 2002; Long et al., 2002). The bulk of these studies indicate that Cx36 is predominantly expressed in GABAergic neurons, usually interneurons, although expression has been observed elsewhere. In these Cx36-deficient animals, there is virtually no electrical coupling of interneurons, which adds considerably to the accumulated evidence for Cx36 contributing to at least one kind of electrical synapse. Evidence for other neuronal connexins is less robust. While Cx45 was originally characterized as an oligodendrocytic connexin (Dermietzel et al., 1997; Kunzelmann et al., 1997), recent reports have suggested that Cx45 may be expressed by restricted populations of neurons. In situ histochemistry indicated neuronal expression in most brain regions in young animals (Condorelli et al., 2003), although expression in nonneuronal cells was also observed. Using the same technique, another group also showed Cx45 expression in mature olfactory neurons (Zhang and Restrepo, 2002). Replacing the Cx45 coding region with a reporter gene showed that in adult animals, expression was confined to subregions of the hippocampus, thalamus, and cerebellum, with no signal detected in nonneuronal cells (Maxeiner et al., 2003). In addition to Cx45, several other connexins have been proposed to be in neurons. Before the discovery of Cx36, Cx32 was thought to be a major candidate for a neuronal connexin, although it is predominantly expressed in oligodendrocytes and Schwann cells (Scherer et al., 1995). Light microscopic studies have shown apparent neuronal expression of Cx32 mRNA and protein in mature animals (Dermietzel et al., 1989; Micevych and Abelson, 1991, 1996; Micevych et al., 1996; Nadarajah and Parnavelas, 1999; Nadarajah et al., 1996). As well, single-cell RT-PCR studies on electrically coupled neurons indicated the presence of Cx32 mRNA, although it was less common than Cx36 (Venance et al., 2000, 2004). However, the animals used in these studies were still juvenile, and the presence of Cx32 mRNA does not necessarily guarantee that the protein is expressed. Of interest is the fact that the Cx32 transcript found by Venance et al. (2000) in neurons is apparently a splice variant of the more common oligodendrocytic Cx32 transcript, and may represent a diYcult-to-detect neuronal subtype. Cx26 has also been reported in neurons (Honma et al., 2004; Solomon et al., 2001), although other studies suggest it is specific to astrocytes (Altevogt and Paul, 2004; Nagy et al., 2001). Further complicating matters, a reporter gene study showed Cx26 promoter activity in leptomeningeal cells only (Filippov et al., 2003). While Cx43 is generally acknowledged to be in astrocytes (Altevogt and Paul, 2004; Nagy et al., 2003; Ochalski et al., 1997), some studies have indicated this connexin may also be found in neurons (Nadarajah et al., 1996; Priest et al., 2001; Simburger et al.,

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1997), and particularly, in olfactory neurons (Theis et al., 2003; Zhang et al., 2000). These are selected examples of the kinds of discrepancies seen in the literature. It must be noted that of all the connexins mentioned, only Cx36 has been shown to participate in ultrastructurally defined neuronal gap junctions using FRIL, while Cx26, Cx32, and Cx43 have only been found in gap junctions connecting glial cells (Rash et al., 2000, 2001a,b). These studies remain somewhat limited due to the number of brain regions examined, and rely on antibody detection, which does not preclude the possibility of other neuronal connexins. Obviously, though, further studies are needed for these issues regarding cellular localization to be resolved. It will be necessary to determine whether connexin expression guarantees functional coupling, which may not always be the case. The cloning of the human, rat, and mouse genomes led to the proposition that new connexin genes are unlikely to be discovered (Willecke et al., 2002). However, recent developments indicate that connexins may not be the only gap junction-forming protein in mammals. Invertebrate gap junctions are formed from connexin homologs known as innexins (Phelan and Starich, 2001) that share structural but very little sequence homology with the connexin family. Recently, innexin-like genes were discovered in mammals (Panchin et al., 2000). These genes, referred to as pannexins, form functional gap-junction channels in expression systems, and one subtype shows expression in brain (Bruzzone et al., 2003; Weickert et al., 2005). Whether these proteins form functional gap junctions and/or electrical synapses between neurons has yet to be determined, but certainly presents intriguing possibilities for a novel substrate of direct intracellular communication between neurons.

IV. Electrical Coupling in the Brain

While electrical coupling between neurons in mammalian brain was discovered some time ago, there has been very little research on the subject until recently. The presence of electrical synapses was inferred from electron micrographs showing gap junctions between neurons in a number of brain regions (Kosaka, 1983; Kosaka and Hama, 1985; Sloper, 1972; Sloper and Powell, 1978; Sotelo et al., 1974). Technical limitations made it extremely diYcult to study electrical coupling between neurons directly using paired intracellular recordings. As such, a common functional assay to assess gap-junctional coupling was to examine transfer of dye from an injected cell to its neighbors, presumably through gap-junction channels (Stewart, 1978, 1981). This technique has been used to provide physiological evidence for gap-junctional communication in many brain areas (Andrew et al., 1981; Gutnick and Prince, 1981; MacVicar

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and Dudek, 1981a). Validation of dye coupling as a correlate of electrical coupling, through gap junctions was provided in a series of experiments where transfection of connexin mRNA into connexin-deficient cells induces both electrical and dye coupling (Eghbali et al., 1990, 1991; Moreno et al., 1991). However, dye coupling has limitations; unless great care is taken with slice preparation and dye loading, this technique is prone to artifact (Connors and Long, 2004; Gutnick et al., 1985), so caution is required. Moreover, while dye coupling implies electrical coupling, lack of dye coupling does not imply lack of electrical coupling (Gibson et al., 1999) due to any number of factors, including dye molecule size or size of the coupled network. The obstacles to paired intracellular recordings were largely overcome with the development of infrared diVerential interference contrast microscopy (IR-DIC) (Stuart et al., 1993). In 1999, using IR-DIC, several groups showed that pairs of interneurons in a number of brain regions were electrically coupled (Galarreta and Hestrin, 1999; Gibson et al., 1999; Koos and Tepper, 1999; Mann-Metzer and Yarom, 1999). Moreover, this coupling synchronized firing among coupled cells in the network. Here, we present a general overview of electrical synapses in brain (for in-depth review, see Bennett and Zukin, 2004; Connors and Long, 2004). Electrically coupled neurons have now been documented in many brain regions. Most of the electrically coupled neurons identified using dual recordings are GABAergic, and coupling seems to be generally restricted to neurons of the same class, although neither of these rules is absolute. In neocortex, four electrophysiologically identified subclasses of GABAergic interneurons—low threshold spiking cells (Gibson et al., 1999), fast spiking cells (Galarreta and Hestrin, 1999; Gibson et al., 1999), multipolar bursting cells (Blatow et al., 2003), and late spiking cells (Chu et al., 2003)—show extensive coupling among cells of the same type, but almost never to other classes of cells. This seems to be a general principle in electrical coupling. Supporting this notion, Galarreta et al. (2004) showed over 90% of tested cell pairs of an inhibitory interneuron subclass characterized by irregular spiking, and CB1 receptor expression was electrically coupled. In addition to cortical interneurons, paired recordings have revealed electrical synapses between hippocampal interneurons (Hormuzdi et al., 2001; Venance et al., 2000), inferior olivary (IO) neurons (Devor and Yarom, 2002; De Zeeuw et al., 2003; Long et al., 2002), cerebellar interneurons (Mann-Metzer and Yarom, 1999), thalamic reticular neurons (Landisman et al., 2002; Long et al., 2004), suprachiasmatic neurons (Long et al., 2005), and striatal interneurons (Koos and Tepper, 1999), all of which are GABAergic. A potential concern is that most paired recordings are done in juvenile animals when myelination is incomplete. This is because slices from adult animals are considerably more opaque, making visual identification of neurons using IR-DIC

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much more diYcult. Given the prominent developmental role for gap junctions and intercellular coupling, it is possible that these studies do not reflect coupling in the mature animal. These concerns were largely allayed in two studies where a reporter gene was used in the place of parvalbumin, a calcium-binding protein that is a marker gene for a subset of GABAergic interneurons (Galarreta and Hestrin, 2002; Meyer et al., 2002). Both groups showed that parvalbumin-containing interneurons showed similar incidences of coupling in juvenile and adult animals, although the strength of coupling appeared weaker and less common in adults. Of note, coupling between inhibitory and excitatory cells reported earlier in the literature (Venance et al., 2000), was seen in juvenile but not adult animals, lending credence to the notion that electrical synapses are found primarily between cells of the same type. Although there is a report showing gap junctions and coupling between neurons and glia in immature locus coeruleus (AlvarezMaubecin et al., 2000), these findings have been disputed as being artifactual (Nagy et al., 2004), and further studies are necessary for clarification. While studies on electrical synapses between neurons have generally shown coupling between similar interneurons and not between principal eVerent cells, this is not an absolute rule. A series of studies using electron microscopy, dye coupling, and dual intracellular recordings suggested hippocampal pyramidal cells were coupled via gap junctions (MacVicar and Dudek, 1980, 1981b; MacVicar et al., 1982), although subsequent reexamination of the electron micrographs led to the conclusion that cells connected by gap junctions were not actually neurons (Nagy et al., 2004). Support for the idea of electrical synapses between pyramidal cells came from both computational modeling studies, that suggested a role for this type of coupling in certain types of high-frequency oscillations (Draguhn et al., 1998; Traub et al., 2002), and experimental evidence, using dye coupling and antidromic stimulation in CA1 pyramidal cells (Schmitz et al., 2001). However, as pointed out by Connors and Long (2004), while there is abundant evidence for morphological gap junctions (Fukuda and Kosaka, 2000; Katsumaru et al., 1988; Kosaka, 1983; Kosaka and Hama, 1985) and electrical synapses (Hormuzdi et al., 2001; Venance et al., 2000) between hippocampal interneurons, none of these studies reported gap junctions or electrical coupling between pyramidal cells. A similar situation exists in the cortex. As mentioned above, there are numerous reports documenting gap junctions and electrical coupling between cortical interneurons, but very little evidence indicating the principal neurons of the cortex are coupled. While mature cortical pyramidal neurons have been reported to be dye coupled (Gutnick and Prince, 1981), subsequent studies indicated that dye coupling was present in immature animals and declined substantially in adulthood (Connors et al., 1983; Peinado et al., 1993; Rorig and Sutor, 1996b). Recent studies indicate principal output cells of other brain regions may also be functionally coupled. Medium spiny neurons, the GABAergic output neurons

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of the striatum, show dye transfer in mature animals (Cepeda et al., 1989; O’Donnell and Grace, 1993) and have recently been shown to be electrically coupled using paired intracellular recording (Venance et al., 2004). Dopaminergic neurons of the substantia nigra pars compacta have also been reported to be dye coupled (Grace and Bunney, 1983), although the opposite has also been reported (Lin et al., 2003). A very recent study used dye coupling and dual recordings to demonstrate that these neurons showed both electrical and tracer dye coupling (Vandecasteele et al., 2005). Interestingly, coupling disappeared between postnatal days 15–20 and reappeared between days 20–25. This may help explain the previous discrepant results. Another example of coupling of principal neurons is found in the inferior olivary nucleus, where climbing fibers, the main output neurons of this nucleus, are extensively coupled (Devor and Yarom, 2002).

V. Properties and Function of Electrical Synapses

What are the functional roles of electrical synapses? For in depth reviews on the electrophysiological properties of electrical synapses, see Bennett (1997); Bennett and Zukin (2004); and Galarreta and Hestrin (2001a). In invertebrates, electrical synapses allow very rapid transmission of electrical signals, however, at higher mammalian body temperatures electrical transmission is not significantly faster than chemical synaptic transmission (Bennett and Zukin, 2004). The unique aspect of electrical synapses is their reciprocity. Of the electrical synapses studied, almost all show equivalent coupling strength in both directions. As such, electrical synapses are neither inhibitory nor excitatory per se, but rather, synchronizing (Bennett and Zukin, 2004) as the eVect of a depolarizing current flowing from cell A to B will result in cell A becoming less depolarized. In eVect, electrical coupling normalizes the voltage diVerence between two coupled cells. This process is more eYcient for slow changes in membrane potential than fast, due to gap junctions acting like low-pass filters (Bennett, 1997; Galarreta and Hestrin, 2001a; Gibson et al., 2005). Low-pass filtering in eVect means that action potentials are not transmitted with 1:1 fidelity, but instead are greatly attenuated in the postjunctional cell, appearing as small amplitude ‘‘spikelets.’’ Slower and lower-amplitude changes in membrane potential are transmitted more eVectively. Across many of the paired recording studies described above, ‘‘coupling coeYcients’’—defined as the change in postjunctional voltage divided by the change in prejunctional (i.e., current-injected cell) voltage; expressed as a percentage—for small membrane potential changes range from 2 to 20%, with an average of approximately 8%. For action potentials the range of coupling coeYcients is from 0.5 to 2% (Galarreta and Hestrin, 2001a). Although action potentials are not faithfully transmitted through electrical synapses, rapid spikelet

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transmission allows submillisecond action potential coordination, and transmission of subthreshold membrane potential changes also enhances synchronous firing among the coupled cells (Galarreta and Hestrin, 1999). A detailed analysis of the biophysical properties of electrical synapses between cortical interneurons extended these findings, showing that electrical synapses synchronize interneuron firing at all firing frequencies (Gibson et al., 2005). Many electrically coupled cells are also coupled by reciprocal inhibitory chemical connections, which allows for very complex modulation of spiking. Though synchronous firing in inhibitory interneurons may be engendered by inhibitory chemical synapses alone, electrical coupling sharpens this synchrony considerably (Bartos et al., 2002; Placantonakis et al., 2004). What are the implications of gap junction-mediated synchrony for larger neuronal networks? Electrical coupling, by virtue of its properties of being fast, synchronizing, and bidirectional, is thought to be involved in the coordination of the synchronized, rhythmic firing oscillations of interneurons and principal cells seen in neocortex and hippocampus (Traub et al., 2003). Oscillations at diVerent discrete frequencies correlate with diVerent behavioral states (Buzsaki and Chrobak, 1995), and some oscillations, in particular those in the gammaband frequency (30–80 Hz), have been proposed to be involved in synchronizing neural activity across brain areas, and in emergent properties, such as consciousness (Buzsaki and Draguhn, 2004; Singer, 2001; Singer and Gray, 1995 ). Tamas et al. (2000), showed that the combination of inhibitory chemical synapses and electrical synapses was able to entrain gamma-frequency firing. As well, pharmacological gap-junction blockade reduced the synchrony of gamma oscillations in interneuron networks (Traub et al., 2001a). Further support for the role of electrical synapses between interneurons in these oscillations has come from studies of the Cx36 knockout (KO) mouse, which showed impaired gamma activity both in vitro (Hormuzdi et al., 2001) and in vivo (Buhl et al., 2003). Another type of synchronous network behavior, ultrafast oscillations (>200 Hz), has been proposed to depend on coupling between pyramidal cells based on modeling studies (Traub et al., 1999) and in vitro studies (Draguhn et al., 1998). These studies oVer the intriguing possibility that electrical coupling may modulate higher cognitive ability. Electrical coupling of neurons may also subserve other physiological functions. Computational studies (Marder, 1998) and experimental evidence (Galarreta and Hestrin, 2001b) suggest that electrical synapses may also act as coincidence detectors in interneuronal networks, where coincident inputs will promote cell firing, but noncoincident inputs will reduce network firing due to transmission of the afterhyperpolarization to the coupled cells. With regards to pathophysiology, gap junctions may also contribute to seizure activity. The ultrafast oscillations mentioned above may be involved in seizure initiation (Traub, 2003; Traub et al., 2001b), and gap-junctional communication has been proposed to contribute to

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the hypersynchronous firing seen in seizures (Gajda et al., 2003; Jahromi et al., 2002; Margineanu and Klitgaard, 2001; Uusisaari et al., 2002). The role of gap junctions in seizure activity has been reviewed elsewhere (Carlen et al., 2000; Perez Velazquez and Carlen, 2000) and will not be discussed further here. In addition to transfer of current, gap junctions between neurons allow passage of second messengers such as IP3 and cAMP (Kumar and Gilula, 1996). This neuronal biochemical coupling is not well defined in adults, but may aid in modulation of other aspects of the coupled network, coordinating metabolic eVects and ensuring neurons of the network act in concert. Movement of IP3 through gap junctions plays a pivotal role in shaping cortical architecture in development (Kandler and Katz, 1998); however, further studies are needed to determine the relevance of this metabolic coupling in adult animals. VI. Modulation of Electrical Synapses and Gap-Junctional Coupling

The preceding studies have contributed a wealth of information regarding the electrophysiological interaction of neurons via electrical synapses. However, none of the aforementioned studies eVectively addresses plasticity and modulation of gap-junctional communication, and other than in the most tangential sense, the potential impact on behavior. While it was long thought that gap junctions were merely static, selectively permeable membrane pores, a considerable evidence has accumulated indicating that junctional coupling is plastic and can be aVected in myriad ways by a number of factors. In this section, we will discuss various manipulations of gap-junctional communication, both short-term and long-term, and highlight developments especially relevant for behavior. VII. Use-Dependent Plasticity

While activity-dependent plasticity is a common feature of chemical synapses (e.g., long-term potentiation), there are currently no data to support this phenomenon in mammalian electrical synapses. However, activity-dependent potentiation and depression have been shown to occur at electrical synapses between the club endings of goldfish Mauthner cells (Pereda and Faber, 1996; Pereda et al., 1998; Yang et al., 1990). This potentiation is dependent on activation of closely associated NMDA receptors (found at the same nerve terminal; these terminals with both chemical and electrical transmission are known as ‘‘mixed synapses’’) and is thought to be mediated via phosphorylation. Interestingly, the connexin mediating this plasticity is Cx35, the fish homolog of Cx36. These proteins share several consensus sites for phosphorylation (Mitropoulou and

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Bruzzone, 2003), suggesting that Cx36 could be modulated in this way. In support of this notion, mixed synapses have been observed throughout mammalian brain and spinal cord (Fukuda and Kosaka, 2003; Rash et al., 1996, 2000; Sloper and Powell, 1978), and Cx36 and the NR1 subunit of the NMDA receptor has been observed in close proximity to Cx36-containing gap junctions (Rash et al., 2004). Although further evidence is needed, activity-dependent plasticity in electrical synapses in a manner analogous to chemical synapses could have implications for the molecular mechanisms of learning and memory.

VIII. Local Factors: Voltage, pH, and Calcium

Some of the most well-known and best-characterized modulators of gapjunctional coupling are transjunctional voltage, intracellular calcium levels, and intracellular pH. In vitro, acidification decreases junctional conductance, and alkalinization does the converse (Spray et al., 1981, 1984). Transjunctional voltage refers to the diVerence in internal voltage between the coupled cells, and current flow through the junctional channel is maximal when transjunctional voltage is zero (Kumar and Gilula, 1996). These factors enable rapid changes in channel conductance and permeability through gating mechanisms similar to those used by voltage-gated ion channels, and sensitivity to this modulation is generally determined by individual connexin subtypes (Harris, 2001). On the surface, it would seem that voltage gating might play a significant role in neuronal gap junctions, considering the large fluctuations in membrane potential exhibited by neurons. Ironically, the principal neuronal connexin shows the weakest voltage sensitivity of all connexins studied to date (Srinivas et al., 1999), and it is unlikely that this voltage-dependent gating is physiologically relevant (Connors and Long, 2004). Consistent with this notion, no voltage dependence was observed in electrically coupled cortical interneurons (Gibson et al., 1999). Calcium has long been known to inhibit gap-junctional coupling in vitro (Peracchia, 1978), possibly indirectly via calmodulin, which physically blocks the channel pore (Peracchia et al., 2000). This type of gating also oVers interesting possibilities regarding modulation of neuronal coupling, given importance of calcium in many other aspects of neuronal function. However, some reports indicate that the concentrations of intracellular calcium needed for inhibition of coupling are so high that they are not within the realm of normal physiology (Rozental et al., 2001). pH may play a more significant role at physiological conditions. Intracellular pH can fluctuate significantly as a function of neural activity (Chesler, 2003), and Cx36-mediated electrical coupling is eliminated by intracellular acidification (Teubner et al., 2000). pH manipulations have also been shown to eVect changes

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in dye coupling (Church and Baimbridge, 1991; Rorig et al., 1996) and spikelet amplitude in brain slices (Schmitz et al., 2001). The functional roles for this modulation of coupling are not known; one possibility might be that acidification produced by excessive neural activity inhibits coupling to prevent additional depolarization from electrically coupled neighbors.

IX. Neurotransmitter and Second Messenger Modulation

Perhaps the strongest argument for the relevance of neuronal coupling in behavior comes from studies of neurotransmitter eVects on dye coupling. Stimulation of various neurotransmitter receptors, especially by exogenous ligands, often has distinct behavioral eVects and allows correlation between observed behavioral eVects and specific modulation in dye coupling. Further research will hopefully begin to clarify the specific roles subserved by electrical and chemical synapses, respectively, in the actions of various neurotransmitters. Modulation of the dopamine (DA) system has potent eVects on junctional coupling. DA was first shown to modulate gap-junctional coupling in the retina, where both exogenously applied and endogenous DA decreased dye coupling in turtle horizontal cells (Piccolino et al., 1984; 1987). This finding was extended to mammalian retina shortly thereafter (Hampson et al., 1992, 1994). This modulation of coupling is thought to occur via a cAMP-dependent protein kinase resulting in phosphorylation of the gap-junctional channel and a subsequent decrease in probability of channel opening (Hampson et al., 1994; Lasater, 1987; McHahon et al., 1989; Mills and Massey, 1995). These studies provided the rationale for the examination of DAergic modulation of neuronal coupling in other brain areas, such as the striatum, which receives very dense DAergic innervation (Ungerstedt, 1971). As in the retina, manipulation of DAergic transmission alters neuronal coupling in the striatum. Electrolytic or 6-hydroxydopamine lesions of DA cell bodies significantly increased dye coupling between striatal output neurons (Cepeda et al., 1989; Onn and Grace, 1999). Studies using more subtle modulation of the DA system showed complex eVects on dye coupling. In vitro, activation of the D1 receptor decreased coupling in the nucleus accumbens (NAc), a ventral striatal area, while D2 stimulation enhanced coupling (O’Donnell and Grace, 1993). However, the eVects were slightly diVerent in diVerent subdivisions of the NAc (i.e., core versus shell), and eVects varied along a rostro-caudal gradient. In vivo, D2 stimulation (at doses suYcient to produce locomotor stimulation) enhanced dye coupling in the striatum, with no eVect of D1 modulation. Repeated treatment with the antipsychotic drugs haloperidol (a classical D2 antagonist) and clozapine (an atypical D2 antagonist) aVected dye coupling both in vivo and in vitro (O’Donnell and Grace,

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1995; Onn and Grace, 1996). Interestingly, the changes in dye coupling were only evident after extended treatment, and paralleled the time course of the delayed onset of therapeutic eVects seen with these drugs (Pickar, 1988). Also of note, clozapine that results in less motor impairment than haloperidol, did not produces changes in ‘‘motor-related’’ striatal regions. The above studies provide correlational evidence for the behavioral significance of DA modulation of neuronal coupling. While studies showing a causal relationship between DA, gap-junctional coupling, and behavior are exceedingly rare, it has been reported that pharmacological gap junction blockade inhibited the expression of certain DA-mediated stereotyped behaviors (Moore and Grace, 2002). There has been considerably less study of eVects other neurotransmitters have on gap-junctional communication. During development, junctional communication between neurons is aVected by norepinephrine (Rorig et al., 1995a), serotonin (Rorig and Sutor, 1996c), and nitric oxide (Rorig and Sutor, 1996a) in addition to DA (Rorig et al., 1995b). However, these neurotransmitters all reduced neuronal dye coupling, and this may be related specifically to developmental processes. The fact that some of these transmitters share eVector systems with DA opens the possibility that they mediate functionally significant eVects on neuronal coupling in mature animals. Many drugs of abuse, and particularly psychostimulants, have eVects mediated at least in part by DA. Repeated administration of psychostimulant drugs can produce enduring changes in both DA transmission and behavior (Vanderschuren and Kalivas, 2000), and evidence is beginning to accumulate that these drugs also produce persistent changes in gap-junctional communication. Withdrawal from repeated amphetamine administration has been shown to produce long-lasting changes in dye coupling between neurons (Onn and Grace, 2000) and has recently been reported to produce changes in Cx36 expression (McCracken et al., 2005a). Of note, the changes in both dye coupling and Cx36 expression parallel the behavioral changes induced by repeated amphetamine (Paulson and Robinson, 1995), in that a drug-free period is necessary for these alterations to manifest. A sensitizing regimen of cocaine self-administration also alters Cx36 expression in a similar manner to amphetamine (McCracken et al., 2005b). A withdrawal period is also necessary for cocaineinduced changes in Cx36, and these changes are present at a time point when behavioral sensitization is observed. While these studies are correlational, they suggest that alterations in gap-junctional communication between neurons may be a contributing mechanism to the lasting behavioral eVects produced by psychostimulants. There have been reports of other miscellaneous behaviorally active substances that may produce some of their eVects through actions on gap junctions. Ethanol has been reported to inhibit coupling in PC12 cells (Wentlandt et al., 2004), possibly due to eVects on the membrane, as is thought to be the

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mechanism for gap-junction blockade by longer chain alcohols (Bernardini et al., 1984) and volatile anesthetics (Burt and Spray, 1989). Ethanol has also been shown to aVect a measure of coupling in a population of ventral tegmental area GABAergic projection neurons (Stobbs et al., 2004). Oleamide, a sleep-inducing endogenous lipid, has also been reported to inhibit junctional coupling (Boger et al., 1998a,b; Guan et al., 1997) as has a related compound, the endogenous cannabinoid anandamide (Venance et al., 1995). A recent report demonstrated that the abused solvent toluene also inhibits gap-junction communciation in cultured cells (Del Re and Woodwald, 2005). The eVects of these compounds were observed on astrocytic gap junctions and whether neuronal coupling is aVected by these factors is not yet known. Perhaps the most-convincing demonstration of the functional importance of a particular gene comes from deficits engendered by the gene’s deletion. While the study of connexin-deficient transgenic animals is in its infancy, some reports do exist. The Cx36 KO mouse was initially thought to be rather normal in phenotype, save for impaired night vision due to lack of Cx36 in the retina (Guldenagel et al., 2001). However, detailed examination revealed impairments in complex memory tasks as well as motor behavior (Frisch et al., 2005). Moreover, a number of compensatory adaptations have been documented in the Cx36 KO mouse (De Zeeuw et al., 2003). These compensations involve changes in membrane electrical properties, which result in neurons from Cx36 KO animals behaving very similarly to wild-type neurons—suggesting the true degree of impairment due to Cx36 deletion may not yet be known. Surprisingly, deletion of the astrocytic connexins Cx30 (Dere et al., 2003) or Cx43 (Frisch et al., 2003) revealed altered behaviors and neurochemistry in these mice. This raises the very interesting possibility that manipulation of gap junctions between astrocytes, in addition to neurons, may have substantial implications for behavior.

X. Concluding Remarks

There is now considerable accumulated evidence regarding the roles of neuronal coupling via gap junctions in neural function. While a number of unresolved issues remain, it has become clear that this form of neuronal communication is both more prevalent and more significant than was once thought. The coming years will likely see major strides in this field, as the multidisciplinary approaches necessary for the study of the functional significance of neuronal coupling are becoming the norm in neuroscience. Understanding the eVects of neuronal coupling will greatly enhance our knowledge of basic mechanisms of brain function, and further our comprehension of the relationship between brain and behavior.

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References

Altevogt, B. M., and Paul, D. L. (2004). Four classes of intercellular channels between glial cells in the CNS. J. Neurosci. 24, 4313–4323. Alvarez-Maubecin, V., Garcia-Hernandez, F., Williams, J. T., and Van Bockstaele, E. J. (2000). Functional coupling between neurons and glia. J. Neurosci. 20, 4091–4098. Andrew, R. D., MacVicar, B. A., Dudek, F. E., and Hatton, G. I. (1981). Dye transfer through gap junctions between neuroendocrine cells of rat hypothalamus. Science 211, 1187–1189. Baker, R., and Llinas, R. (1971). Electrotonic coupling between neurones in the rat mesencephalic nucleus. J. Physiol. 212, 45–63. Barr, L., Dewey, M. M., and Berger, W. (1965). Propagation of action potentials and the structure of the nexus in cardiac muscle. J. Gen. Physiol. 48, 797–823. Bartos, M., Vida, I., Frotscher, M., Meyer, A., Monyer, H., Geiger, J. R., and Jonas, P. (2002). Fast synaptic inhibition promotes synchronized gamma oscillations in hippocampal interneuron networks. Proc. Natl. Acad. Sci. USA 99, 13222–13227. Bear, M. F., Connors, B. W., and Paradiso, M. A. (2001). ‘‘Neuroscience: Exploring the Brain,’’ 2nd ed. Lippincott Williams & Wilkins, Baltimore. Belluardo, N., Mudo, G., Trovato-Salinaro, A., Le Gurun, S., Charollais, A., Serre-Beinier, V., Amato, G., Haefliger, J. A., Meda, P., and Condorelli, D. F. (2000). Expression of connexin36 in the adult and developing rat brain. Brain Res. 865, 121–138. Bennett, M. V. L. (1997). Gap junctions as electrical synapses. J. Neurocytol. 26, 349–366. Bennett, M. V. L., Crain, S. M., and Grundfest, H. (1959). Electrophysiology of supramedullary neurons in Spheroides maculatus. I: Orthodromic and antidromic responses. J. Gen. Physiol. 43, 159–188. Bennett, M. V. L., and Zukin, R. S. (2004). Electrical coupling and neuronal synchronization in the mammalian brain. Neuron 41, 495–511. Bernardini, G., Peracchia, C., and Peracchia, L. L. (1984). Reversible eVects of heptanol on gap junction structure and cell-to-cell electrical coupling. Eur. J. Cell Biol. 34, 307–312. Blatow, M., Rozov, A., Katona, I., Hormuzdi, S. G., Meyer, A. H., Whittington, M. A., Caputi, A., and Monyer, H. (2003). A novel network of multipolar bursting interneurons generates theta frequency oscillations in neocortex. Neuron 38, 805–817. Boger, D. L., Henriksen, S. J., and Cravatt, B. F. (1998a). Oleamide: An endogenous sleep-inducing lipid and prototypical member of a new class of biological signaling molecules. Curr. Pharm. Des. 4, 303–314. Boger, D. L., Patterson, J. E., Guan, X., Cravatt, B. F., Lerner, R. A., and Gilula, N. B. (1998b). Chemical requirements for inhibition of gap junction communication by the biologically active lipid oleamide. Proc. Natl. Acad. Sci. USA 95, 4810–4815. Bruzzone, R., Hormuzdi, S. G., Barbe, M. T., Herb, A., and Monyer, H. (2003). Pannexins, a family of gap junction proteins expressed in brain. Proc. Natl. Acad. Sci. USA 100, 13644–13649. Buhl, D. L., Harris, K. D., Hormuzdi, S. G., Monyer, H., and Buzsaki, G. (2003). Selective impairment of hippocampal gamma oscillations in connexin-36 knock-out mouse in vivo. J. Neurosci. 23, 1013–1018. Burt, J. M., and Spray, D. C. (1989). Volatile anesthetics block intercellular communication between neonatal rat myocardial cells. Circ. Res. 65, 829–837. Buzsaki, G., and Chrobak, J. J. (1995). Temporal structure in spatially organized neuronal ensembles: A role for interneuronal networks. Curr. Opin. Neurobiol. 5, 504–510. Buzsaki, G., and Draguhn, A. (2004). Neuronal oscillations in cortical networks. Science 304, 1926–1929.

NEURONAL GAP JUNCTIONS

143

Carlen, P. L., Skinner, F., Zhang, L., Naus, C., Kushnir, M., and Perez Velazquez, J. L. (2000). The role of gap junctions in seizures. Brain Res. Rev. 32, 235–241. Cepeda, C., Walsh, J. P., Hull, C. D., Howard, S. G., Buchwald, N. A., and Levine, M. S. (1989). Dye-coupling in the neostriatum of the rat: I. Modulation by dopamine-depleting lesions. Synapse 4, 229–237. Chesler, M. (2003). Regulation and modulation of pH in the brain. Physiol Rev. 83, 1183–1221. Chu, Z., Galarreta, M., and Hestrin, S. (2003). Synaptic interactions of late-spiking neocortical neurons in layer 1. J. Neurosci. 23, 96–102. Church, J., and Baimbridge, K. G. (1991). Exposure to high-pH medium increases the incidence and extent of dye coupling between rat hippocampal CA1 pyramidal neurons in vitro. J. Neurosci. 11, 3289–3295. Condorelli, D. F., Belluardo, N., Trovato-Salinaro, A., and Mudo, G. (2000). Expression of Cx36 in mammalian neurons. Brain Res. Rev. 32, 72–85. Condorelli, D. F., Parenti, R., Spinella, F., Trovato Salinaro, A., Belluardo, N., Cardile, V., and Cicirata, F. (1998). Cloning of a new gap junction gene (Cx36) highly expressed in mammalian brain neurons. Eur. J. Neurosci. 10, 1202–1208. Condorelli, D. F., Trovato-Salinaro, A., Mudo, G., Mirone, M. B., and Belluardo, N. (2003). Cellular expression of connexins in the rat brain: Neuronal localization, eVects of kainate-induced seizures and expression in apoptotic neuronal cells. Eur. J. Neurosci. 18, 1807–1827. Connors, B. W., Benardo, L. S., and Prince, D. A. (1983). Coupling between neurons of the developing rat neocortex. J. Neurosci. 3, 773–782. Connors, B. W., and Long, M. A. (2004). Electrical synapses in the mammalian brain. Annu. Rev. Neurosci. 27, 393–418. De Zeeuw, C. I., Chorev, E., Devor, A., Manor, Y., Van Der Giessen, R. S., De Jeu, M. T., Hoogenraad, C. C., Bijman, J., Ruigrok, T. J., French, P., Jaarsma, D., Kistler, W. M., et al. (2003). 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. J. Neurosci. 23, 4700–4711. Deans, M. R., Gibson, J. R., Sellitto, C., Connors, B. W., and Paul, D. L. (2001). Synchronous activity of inhibitory networks in neocortex requires electrical synapses containing connexin36. Neuron 31, 477–485. Degen, J., Meier, C., Van Der Giessen, R. S., Sohl, G., Petrasch-Parwez, E., Urschel, S., Dermietzel, R., Schilling, K., De Zeeuw, C. I., and Willecke, K. (2004). Expression pattern of lacZ reporter gene representing connexin36 in transgenic mice. J. Comp. Neurol. 473, 511–525. Del Re, A. M., and Woodwald, J. J. (2005). Inhibition of gap junction currents by the abused solvent toluene. Drug Alcohol Depend. 78, 221–224. Dere, E., Souza-Silva, M. A., Frisch, C., Teubner, B., Sohl, G., Willecke, K., and Huston, J. P. (2003). Connexin30-deficient mice show increased emotionality and decreased rearing activity in the open-field along with neurochemical changes. Eur. J. Neurosci. 18, 629–638. Dermietzel, R., Farooq, M., Kessler, J. A., Althaus, H., Hertzberg, E. L., and Spray, D. C. (1997). Oligodendrocytes express gap junction proteins connexin32 and connexin45. Glia 20, 101–114. Dermietzel, R., Traub, O., Hwang, T. K., Beyer, E., Bennett, M. V. L., Spray, D. C., and Willecke, K. (1989). DiVerential expression of three gap junction proteins in developing and mature brain tissues. Proc. Natl. Acad. Sci. USA 86, 10148–10152. Devor, A., and Yarom, Y. (2002). Electrotonic coupling in the inferior olivary nucleus revealed by simultaneous double patch recordings. J. Neurophysiol. 87, 3048–3058. Draguhn, A., Traub, R. D., Schmitz, D., and JeVerys, J. G. (1998). Electrical coupling underlies highfrequency oscillations in the hippocampus in vitro. Nature 394, 189–192.

144

MCCRACKEN AND ROBERTS

Eccles, J. C. (1982). The synapse: From electrical to chemical transmission. Annu. Rev. Neurosci. 5, 325–339. Eghbali, B., Kessler, J. A., Reid, L. M., Roy, C., and Spray, D. C. (1991). Involvement of gap junctions in tumorigenesis: Transfection of tumor cells with connexin32 cDNA retards growth in vivo. Proc. Natl. Acad. Sci. USA 88, 10701–10705. Eghbali, B., Kessler, J. A., and Spray, D. C. (1990). Expression of gap junction channels in communication-incompetent cells after stable transfection with cDNA encoding connexin 32. Proc. Natl. Acad. Sci. USA 87, 1328–1331. Evans, W. H., and Martin, P. E. (2002). Gap junctions: Structure and function. Mol. Membr. Biol. 19, 121–136. Filippov, M. A., Hormuzdi, S. G., Fuchs, E. C., and Monyer, H. (2003). A reporter allele for investigating connexin 26 gene expression in the mouse brain. Eur. J. Neurosci. 18, 3183–3192. Frisch, C., Souza-Silva, M. A., Sohl, G., Guldenagel, M., Willecke, K., Huston, J. P., and Dere, E. (2005). Stimulus complexity dependent memory impairment and changes in motor performance after deletion of the neuronal gap junction protein connexin36 in mice. Behav. Brain Res. 157, 177–185. Frisch, C., Theis, M., Souza Silva, M. A., Dere, E., Sohl, G., Teubner, B., Namestkova, K., Willecke, K., and Huston, J. P. (2003). Mice with astrocyte-directed inactivation of connexin43 exhibit increased exploratory behaviour, impaired motor capacities, and changes in brain acetylcholine levels. Eur. J. Neurosci. 18, 2313–2318. Fukuda, T., and Kosaka, T. (2000). Gap junctions linking the dendritic network of GABAergic interneurons in the hippocampus. J. Neurosci. 20, 1519–1528. Fukuda, T., and Kosaka, T. (2003). Ultrastructural study of gap junctions between dendrites of parvalbumin-containing GABAergic neurons in various neocortical areas of the adult rat. Neuroscience 120, 5–20. Furshpan, E. J., and Potter, D. D. (1959). Transmission at the giant motor synapses of the crayfish. J. Physiol. 145, 289–325. Gajda, Z., Gyengesi, E., Hermesz, E., Ali, K. S., and Szente, M. (2003). Involvement of gap junctions in the manifestation and control of the duration of seizures in rats in vivo. Epilepsia 44, 1596–1600. Galarreta, M., Erdelyi, F., Szabo, G., and Hestrin, S. (2004). Electrical coupling among irregularspiking GABAergic interneurons expressing cannabinoid receptors. J. Neurosci. 24, 9770–9778. Galarreta, M., and Hestrin, S. (1999). A network of fast-spiking cells in the neocortex connected by electrical synapses. Nature 402, 72–75. Galarreta, M., and Hestrin, S. (2001a). Electrical synapses between GABA-releasing interneurons. Nat. Rev. Neurosci. 2, 425–433. Galarreta, M., and Hestrin, S. (2001b). Spike transmission and synchrony detection in networks of GABAergic interneurons. Science 292, 2295–2299. Galarreta, M., and Hestrin, S. (2002). Electrical and chemical synapses among parvalbumin fast-spiking GABAergic interneurons in adult mouse neocortex. Proc. Natl. Acad. Sci. USA 99, 12438–12443. Gibson, J. R., Beierlein, M., and Connors, B. W. (1999). Two networks of electrically coupled inhibitory neurons in neocortex. Nature 402, 75–79. Gibson, J. R., Beierlein, M., and Connors, B. W. (2005). Functional properties of electrical synapses between inhibitory interneurons of neocortical layer 4. J. Neurophysiol. 93, 467–480. Grace, A. A., and Bunney, B. S. (1983). Intracellular and extracellular electrophysiology of nigral dopaminergic neurons-3: Evidence for electrotonic coupling. Neuroscience 10, 333–348. Guan, X., Cravatt, B. F., Ehring, G. R., Hall, J. E., Boger, D. L., Lerner, R. A., and Gilula, N. B. (1997). The sleep-inducing lipid oleamide deconvolutes gap junction communication and calcium wave transmission in glial cells. J. Cell Biol. 139, 1785–1792.

NEURONAL GAP JUNCTIONS

145

Guldenagel, M., Ammermuller, J., Feigenspan, A., Teubner, B., Degen, J., Sohl, G., Willecke, K., and Weiler, R. (2001). Visual transmission deficits in mice with targeted disruption of the gap junction gene connexin36. J. Neurosci. 21, 6036–6044. Gutnick, M. J., Lobel-Yaakov, R., and Rimon, G. (1985). Incidence of neuronal dye-coupling in neocortical slices depends on the plane of section. Neuroscience 15, 659–666. Gutnick, M. J., and Prince, D. A. (1981). Dye coupling and possible electrotonic coupling in the guinea pig neocortical slice. Science 211, 67–70. Hampson, E. C., Vaney, D. I., and Weiler, R. (1992). Dopaminergic modulation of gap junction permeability between amacrine cells in mammalian retina. J. Neurosci. 12, 4911–4922. Hampson, E. C., Weiler, R., and Vaney, D. I. (1994). pH-gated dopaminergic modulation of horizontal cell gap junctions in mammalian retina. Proc. R. Soc. Lond. B Biol. Sci. 255, 67–72. Harris, A. L. (2001). Emerging issues of connexin channels: Biophysics fills the gap. Q. Rev. Biophys. 34, 325–472. Honma, S., De, S., Li, D., Shuler, C. F., and Turman, J. E., Jr. (2004). Developmental regulation of connexins 26, 32, 36, and 43 in trigeminal neurons. Synapse 52, 258–271. Hormuzdi, S. G., Pais, I., LeBeau, F. E., Towers, S. K., Rozov, A., Buhl, E. H., Whittington, M. A., and Monyer, H. (2001). Impaired electrical signaling disrupts gamma frequency oscillations in connexin 36-deficient mice. Neuron 31, 487–495. Jahromi, S. S., Wentlandt, K., Piran, S., and Carlen, P. L. (2002). Anticonvulsant actions of gap junctional blockers in an in vitro seizure model. J. Neurophysiol. 88, 1893–1902. Kandel, E. R., Schwartz, J. H., and Jessel, T. M. (2000). ‘‘Principles of Neural Science,’’ 4th ed. McGraw-Hill, New York. Kandler, K., and Katz, L. C. (1998). Coordination of neuronal activity in developing visual cortex by gap junction-mediated biochemical communication. J. Neurosci. 18, 1419–1427. Katsumaru, H., Kosaka, T., Heizmann, C. W., and Hama, K. (1988). Gap junctions on GABAergic neurons containing the calcium-binding protein parvalbumin in the rat hippocampus (CA1 region). Exp. Brain Res. 72, 363–370. Koos, T., and Tepper, J. M. (1999). Inhibitory control of neostriatal projection neurons by GABAergic interneurons. Nat. Neurosci. 2, 467–472. Korn, H., Sotelo, C., and Crepel, F. (1973). Electronic coupling between neurons in the rat lateral vestibular nucleus. Exp. Brain Res. 16, 255–275. Kosaka, T. (1983). Neuronal gap junctions in the polymorph layer of the rat dentate gyrus. Brain Res. 277, 347–351. Kosaka, T., and Hama, K. (1985). Gap junctions between non-pyramidal cell dendrites in the rat hippocampus (CA1 and CA3 regions): A combined Golgi-electron microscopy study. J. Comp. Neurol. 231, 150–161. Kumar, N. M., and Gilula, N. B. (1996). The gap junction communication channel. Cell 84, 381–388. Kunzelmann, P., Blumcke, I., Traub, O., Dermietzel, R., and Willecke, K. (1997). Coexpression of connexin45 and -32 in oligodendrocytes of rat brain. J. Neurocytol. 26, 17–22. Landisman, C. E., Long, M. A., Beierlein, M., Deans, M. R., Paul, D. L., and Connors, B. W. (2002). Electrical synapses in the thalamic reticular nucleus. J. Neurosci. 22, 1002–1009. Lasater, E. M. (1987). Retinal horizontal cell gap junctional conductance is modulated by dopamine through a cyclic AMP-dependent protein kinase. Proc. Natl. Acad. Sci. USA 84, 7319–7323. Lin, J. Y., van Wyk, M., Bowala, T. K., Teo, M. Y., and Lipski, J. (2003). Dendritic projections and dye-coupling in dopaminergic neurons of the substantia nigra examined in horizontal brain slices from young rats. J. Neurophysiol. 90, 2531–2535. Liu, X. B., and Jones, E. G. (2003). Fine structural localization of connexin-36 immunoreactivity in mouse cerebral cortex and thalamus. J. Comp. Neurol. 466, 457–467.

146

MCCRACKEN AND ROBERTS

Llinas, R., Baker, R., and Sotelo, C. (1974). Electrotonic coupling between neurons in cat inferior olive. J. Neurophysiol. 37, 560–571. Long, M. A., Deans, M. R., Paul, D. L., and Connors, B. W. (2002). Rhythmicity without synchrony in the electrically uncoupled inferior olive. J. Neurosci. 22, 10898–10905. Long, M. A., Jutras, M. J., Connors, B. W., and Burwell, R. D. (2005). Electrical synapses coordinate activity in the suprachiasmatic nucleus. Nat. Neurosci. 8, 61–66. Long, M. A., Landisman, C. E., and Connors, B. W. (2004). Small clusters of electrically coupled neurons generate synchronous rhythms in the thalamic reticular nucleus. J. Neurosci. 24, 341–349. MacVicar, B. A., and Dudek, F. E. (1980). Dye-coupling betwen CA3 pyramidal cells in slices of rat hippocampus. Brain Res. 196, 494–497. MacVicar, B. A., and Dudek, F. E. (1981a). Electrotonic coupling between pyramidal cells: A direct demonstration in rat hippocampal slices. Science 213, 782–784. MacVicar, B. A., and Dudek, F. E. (1981b). Electrotonic coupling between pyramidal cells: A direct demonstration in rat hippocampal slices. Science 213, 782–785. MacVicar, B. A., Ropert, N., and Krnjevic, K. (1982). Dye-coupling between pyramidal cells of rat hippocampus in vivo. Brain Res. 238, 239–244. Mann-Metzer, P., and Yarom, Y. (1999). Electrotonic coupling interacts with intrinsic properties to generate synchronized activity in cerebellar networks of inhibitory interneurons. J. Neurosci. 19, 3298–3306. Marder, E. (1998). Electrical synapses: Beyond speed and synchrony to computation. Curr. Biol. 8, R795–R797. Margineanu, D. G., and Klitgaard, H. (2001). Can gap-junction blockade preferentially inhibit neuronal hypersynchrony vs. excitability? Neuropharmacology 41, 377–383. Maxeiner, S., Kruger, O., Schilling, K., Traub, O., Urschel, S., and Willecke, K. (2003). Spatiotemporal transcription of connexin45 during brain development results in neuronal expression in adult mice. Neuroscience 119, 689–700. McCracken, C. B., Patel, K. M., Vrana, K. E., Paul, D. L., and Roberts, D. C. S. (2005a). Amphetamine withdrawal produces region-specific and time-dependent changes in connexin36 expression in rat brain. Synapse 56, 39–44. McCracken, C. B., Hamby, S. M., Patel, K. M., Morgan, D., Vrana, K. E., and Roberts, D. C. (2005b). Extended cocaine self-adminisration and deprivation produces region specific and timedependent changes in connexin 36 expression in rat brain. McHahon, D. G., Knapp, A. G., and Dowling, J. E. (1989). Horizontal cell gap junctions: Singlechannel conductance and modulation by dopamine. Proc. Natl. Acad. Sci. USA 86, 7639–7643. Meier, C., Petrasch-Parwez, E., Habbes, H. W., Teubner, B., Guldenagel, M., Degen, J., Sohl, G., Willecke, K., and Dermietzel, R. (2002). Immunohistochemical detection of the neuronal connexin36 in the mouse central nervous system in comparison to connexin36-deficient tissues. Histochem. Cell Biol. 117, 461–471. Meyer, A. H., Katona, I., Blatow, M., Rozov, A., and Monyer, H. (2002). In vivo labeling of parvalbumin-positive interneurons and analysis of electrical coupling in identified neurons. J. Neurosci. 22, 7055–7064. Micevych, P. E., and Abelson, L. (1991). Distribution of mRNAs coding for liver and heart gap junction proteins in the rat central nervous system. J. Comp. Neurol. 305, 96–118. Micevych, P. E., Popper, P., and Hatton, G. I. (1996). Connexin 32 mRNA levels in the rat supraoptic nucleus: Up-regulation prior to parturition and during lactation. Neuroendocrinology 63, 39–45. Mills, S. L., and Massey, S. C. (1995). DiVerential properties of two gap junctional pathways made by AII amacrine cells. Nature 377, 734–737.

NEURONAL GAP JUNCTIONS

147

Mitropoulou, G., and Bruzzone, R. (2003). Modulation of perch connexin35 hemi-channels by cyclic AMP requires a protein kinase A phosphorylation site. J. Neurosci. Res. 72, 147–157. Montoro, R. J., and Yuste, R. (2004). Gap junctions in developing neocortex: A review. Brain Res. Rev. 47, 216–226. Moore, H., and Grace, A. A. (2002). A role for electrotonic coupling in the striatum in the expression of dopamine receptor-mediated stereotypies. Neuropsychopharmacology 27, 980–992. Moreno, A. P., Eghbali, B., and Spray, D. C. (1991). Connexin32 gap junction channels in stably transfected cells: Unitary conductance. Biophys. J. 60, 1254–1266. Nadarajah, B., and Parnavelas, J. G. (1999). Gap junction-mediated communication in the developing and adult cerebral cortex. Novartis Found. Symp. 219, 157–170. Nadarajah, B., Thomaidou, D., Evans, W. H., and Parnavelas, J. G. (1996). Gap junctions in the adult cerebral cortex: Regional diVerences in their distribution and cellular expression of connexins. J. Comp. Neurol. 376, 326–342. Nagy, J. I., Dudek, F. E., and Rash, J. E. (2004). Update on connexins and gap junctions in neurons and glia in the mammalian nervous system. Brain Res. Rev. 47, 191–215. Nagy, J. I., Ionescu, A. V., Lynn, B. D., and Rash, J. E. (2003). Coupling of astrocyte connexins Cx26, Cx30, Cx43 to oligodendrocyte Cx29, Cx32, Cx47: Implications from normal and connexin32 knockout mice. Glia 44, 205–218. Nagy, J. I., Li, X., Rempel, J., Stelmack, G., Patel, D., Staines, W. A., Yasumura, T., and Rash, J. E. (2001). Connexin26 in adult rodent central nervous system: Demonstration at astrocytic gap junctions and colocalization with connexin30 and connexin43. J. Comp. Neurol. 441, 302–323. O’Donnell, P., and Grace, A. A. (1993). Dopaminergic modulation of dye coupling between neurons in the core and shell regions of the nucleus accumbens. J. Neurosci. 13, 3456–3471. O’Donnell, P., and Grace, A. A. (1995). DiVerent eVects of subchronic clozapine and haloperidol on dye coupling between neurons in the rat striatal complex. Neuroscience 66, 763–767. Ochalski, P. A., Frankenstein, U. N., Hertzberg, E. L., and Nagy, J. I. (1997). Connexin-43 in rat spinal cord: Localization in astrocytes and identification of heterotypic astro-oligodendrocytic gap junctions. Neuroscience 76, 931–945. Onn, S. P., and Grace, A. A. (1996). Repeated treatment with haloperidol and clozapine exerts diVerential eVects on dye coupling between neurons in subregions of striatum and nucleus accumbens. J. Neurosci. 15, 7024–7036. Onn, S. P., and Grace, A. A. (1999). Alterations in electrophysiological activity and dye coupling of striatal spiny and aspiny neurons in dopamine-denervated rat striatum recorded in vivo. Synapse 33, 1–15. Onn, S. P., and Grace, A. A. (2000). Amphetamine withdrawal alters bistable states and cellular coupling in rat prefrontal cortex and nucleus accumbens neurons recorded in vivo. J. Neurosci. 20, 2332–2345. Panchin, Y., Kelmanson, I., Matz, M., Lukyanov, K., Usman, N., and Lukyanov, S. (2000). A ubiquitous family of putative gap junction molecules. Curr. Biol. 10, 473–474. Pappas, G. D., Asada, Y., and Bennett, M. V. L. (1971). Morphological correlates of increased coupling resistance at an electrotonic synapse. J. Cell Biol. 49, 173–188. Paul, D. L. (1986). Molecular cloning of cDNA for rat liver gap junction protein. J. Cell Biol. 103, 123–134. Paulson, P. E., and Robinson, T. E. (1995). Amphetamine-induced time-dependent sensitization of dopamine neurotransmission in the dorsal and ventral striatum: A microdialysis study in behaving rats. Synapse 19, 56–65. Peinado, A., Yuste, R., and Katz, L. C. (1993). Extensive dye coupling between rat neocortical neurons during the period of circuit formation. Neuron 10, 103–114. Peracchia, C. (1978). Calcium eVects on gap junction structure and cell coupling. Nature 271, 669–671.

148

MCCRACKEN AND ROBERTS

Peracchia, C., Sotkis, A., Wang, X. G., Peracchia, L. L., and Persechini, A. (2000). Calmodulin directly gates gap junction channels. J. Biol. Chem. 275, 26220–26224. Pereda, A. E., Bell, T. D., Chang, B. H., Czernik, A. J., Nairn, A. C., Soderling, T. R., and Faber, D. S. (1998). Ca2þ/calmodulin-dependent kinase II mediates simultaneous enhancement of gapjunctional conductance and glutamatergic transmission. Proc. Natl. Acad. Sci. USA 95, 13272–13277. Pereda, A. E., and Faber, D. S. (1996). Activity-dependent short-term enhancement of intercellular coupling. J. Neurosci. 16, 983–992. Perez Velazquez, J. L., and Carlen, P. L. (2000). Gap junctions, synchrony and seizures. Trends Neurosci. 23, 68–74. Phelan, P., and Starich, T. A. (2001). Innexins get into the gap. Bioessays 23, 388–396. Piccolino, M., Neyton, J., and Gerschenfeld, H. M. (1984). Decrease of gap junction permeability induced by dopamine and cyclic adenosine 30 :50 -monophosphate in horizontal cells of turtle retina. J. Neurosci. 4, 2477–2488. Piccolino, M., Witkovsky, P., and Trimarchi, C. (1987). Dopaminergic mechanisms underlying the reduction of electrical coupling between horizontal cells of the turtle retina induced by d-amphetamine, bicuculline, and veratridine. J. Neurosci. 7, 2273–2284. Pickar, D. (1988). Perspectives on a time-dependent model of neuroleptic action. Schizophr. Bull. 14, 255–268. Placantonakis, D. G., Bukovsky, A. A., Zeng, X. H., Kiem, H. P., and Welsh, J. P. (2004). Fundamental role of inferior olive connexin 36 in muscle coherence during tremor. Proc. Natl. Acad. Sci. USA 101, 7164–7169. Priest, C. A., Thompson, A. J., and Keller, A. (2001). Gap junction proteins in inhibitory neurons of the adult barrel neocortex. Somatosens. Mot. Res. 18, 245–252. Rash, J. E., Dillman, R. K., Bilhartz, B. L., DuVy, H. S., Whalen, L. R., and Yasumura, T. (1996). Mixed synapses discovered and mapped throughout mammalian spinal cord. Proc. Natl. Acad. Sci. USA 93, 4235–4239. Rash, J. E., Pereda, A., Kamasawa, N., Furman, C. S., Yasumura, T., Davidson, K. G., Dudek, F. E., Olson, C., Li, X., and Nagy, J. I. (2004). High-resolution proteomic mapping in the vertebrate central nervous system: Close proximity of connexin35 to NMDA glutamate receptor clusters and co-localization of connexin36 with immunoreactivity for zonula occludens protein-1 (ZO-1). J. Neurocytol. 33, 131–151. Rash, J. E., Staines, W. A., Yasumura, T., Patel, D., Furman, C. S., Stelmack, G. L., and Nagy, J. I. (2000). Immunogold evidence that neuronal gap junctions in adult rat brain and spinal cord contain connexin-36 but not connexin-32 or connexin-43. Proc. Natl. Acad. Sci. USA 97, 7573–7578. Rash, J. E., Yasumura, T., Davidson, K. G., Furman, C. S., Dudek, F. E., and Nagy, J. I. (2001a). Identification of cells expressing Cx43, Cx30, Cx26, Cx32 and Cx36 in gap junctions of rat brain and spinal cord. Cell Commun. Adhes. 8, 315–320. Rash, J. E., Yasumura, T., Dudek, F. E., and Nagy, J. I. (2001b). Cell-specific expression of connexins and evidence of restricted gap junctional coupling between glial cells and between neurons. J. Neurosci. 21, 1983–2000. Revel, J. P., and Karnovsky, M. J. (1967). Hexagonal array of subunits in intercellular junctions of the mouse heart and liver. J. Cell Biol. 33, C7–C12. Revel, J. P., Yee, A. G., and Hudspeth, A. J. (1971). Gap junctions between electrotonically coupled cells in tissue culture and in brown fat. Proc. Natl. Acad. Sci. USA 68, 2924–2927. Rorig, B., Klausa, G., and Sutor, B. (1995a). Beta-adrenoreceptor activation reduces dye-coupling between immature rat neocortical neurones. Neuroreport 6, 1811–1815.

NEURONAL GAP JUNCTIONS

149

Rorig, B., Klausa, G., and Sutor, B. (1995b). Dye coupling between pyramidal neurons in developing rat prefrontal and frontal cortex is reduced by protein kinase A activation and dopamine. J. Neurosci. 15, 7386–7400. Rorig, B., Klausa, G., and Sutor, B. (1996). Intracellular acidification reduced gap junction coupling between immature rat neocortical pyramidal neurones. J. Physiol. 490(Pt 1), 31–49. Rorig, B., and Sutor, B. (1996a). Nitric oxide-stimulated increase in intracellular cGMP modulates gap junction coupling in rat neocortex. Neuroreport 7, 569–572. Rorig, B., and Sutor, B. (1996b). Regulation of gap junction coupling in the developing neocortex. Mol. Neurobiol. 12, 225–249. Rorig, B., and Sutor, B. (1996c). Serotonin regulates gap junction coupling in the developing rat somatosensory cortex. Eur. J. Neurosci. 8, 1685–1695. Rozental, R., Srinivas, M., Gokhan, S., Urban, M., Dermietzel, R., Kessler, J. A., Spray, D. C., and Mehler, M. F. (2000). Temporal expression of neuronal connexins during hippocampal ontogeny. Brain Res. Rev. 32, 57–71. Rozental, R., Srinivas, M., and Spray, D. C. (2001). How to close a gap junction channel. EYcacies and potencies of uncoupling agents. Methods Mol. Biol. 154, 447–476. Saez, J. C., Berthoud, V. M., Branes, M. C., Martinez, A. D., and Beyer, E. C. (2003). Plasma membrane channels formed by connexins: Their regulation and functions. Physiol. Rev. 83, 1359–1400. Scherer, S. S., Deschenes, S. M., Xu, Y. T., Grinspan, J. B., Fischbeck, K. H., and Paul, D. L. (1995). Connexin32 is a myelin-related protein in the PNS and CNS. J. Neurosci. 15, 8281–8294. Schmitz, D., Schuchmann, S., Fisahn, A., Draguhn, A., Buhl, E. H., Petrasch-Parwez, E., Dermietzel, R., Heinemann, U., and Traub, R. D. (2001). Axo-axonal coupling: A novel mechanism for ultrafast neuronal communication. Neuron 31, 831–840. Shiosaka, S., Yamamoto, T., Hertzberg, E. L., and Nagy, J. I. (1989). Gap junction protein in rat hippocampus: Correlative light and electron microscope immunohistochemical localization. J. Comp. Neurol. 281, 282–297. Simburger, E., Stang, A., Kremer, M., and Dermietzel, R. (1997). Expression of connexin43 mRNA in adult rodent brain. Histochem. Cell Biol. 107, 127–137. Singer, W. (2001). Consciousness and the binding problem. Ann. N. Y. Acad. Sci. 929, 123–146. Singer, W., and Gray, C. M. (1995). Visual feature integration and the temporal correlation hypothesis. Annu. Rev. Neurosci. 18, 555–586. Sloper, J. J. (1972). Gap junctions between dendrites in the primate neocortex. Brain Res. 44, 641–646. Sloper, J. J., and Powell, T. P. (1978). Gap junctions between dendrites and somata of neurons in the primate sensori-motor cortex. Proc. R. Soc. Lond. B Biol. Sci. 203, 39–47. Sohl, G., Degen, J., Teubner, B., and Willecke, K. (1998). The murine gap junction gene connexin36 is highly expressed in mouse retina and regulated during brain development. FEBS Lett. 428, 27–31. Sohl, G., Odermatt, B., Maxeiner, S., Degen, J., and Willecke, K. (2004). New insights into the expression and function of neural connexins with transgenic mouse mutants. Brain Res. Rev. 47, 245–259. Solomon, I. C., Halat, T. J., El-Maghrabi, M. R., and O’Neal, M. H. (2001). Localization of connexin26 and connexin32 in putative CO(2)-chemosensitive brainstem regions in rat. Respir. Physiol. 129, 101–121. Sotelo, C., Llinas, R., and Baker, R. (1974). Structural study of inferior olivary nucleus of the cat: Morphological correlates of electrotonic coupling. J. Neurophysiol. 37, 541–559. Spray, D. C., Harris, A. L., and Bennett, M. V. L. (1981). Gap junctional conductance is a simple and sensitive function of intracellular pH. Science 211, 712–715.

150

MCCRACKEN AND ROBERTS

Spray, D. C., White, R. L., de Carvalho, A. C., Harris, A. L., and Bennett, M. V. L. (1984). Gating of gap junction channels. Biophys. J. 45, 219–230. Squire, L. R., Bloom, F. E., McConnell, S., Roberts, J. L., Spitzer, N., and Zigmond, M. J. (2002). ‘‘Fundamental Neuroscience,’’ 2nd ed. Academic Press, San Diego. Srinivas, M., Rozental, R., Kojima, T., Dermietzel, R., Mehler, M., Condorelli, D. F., Kessler, J. A., and Spray, D. C. (1999). Functional properties of channels formed by the neuronal gap junction protein connexin36. J. Neurosci. 19, 9848–9855. Stewart, W. W. (1978). Functional connections between cells as revealed by dye-coupling with a highly fluorescent naphthalimide tracer. Cell 14, 741–759. Stewart, W. W. (1981). Lucifer dyes—highly fluorescent dyes for biological tracing. Nature 292, 17–21. Stobbs, S. H., Ohran, A. J., Lassen, M. B., Allison, D. W., Brown, J. E., and SteVensen, S. C. (2004). Ethanol suppression of ventral tegmental area GABA neuron electrical transmission involves N-methyl-D-aspartate receptors. J. Pharmacol. Exp. Ther. 311, 282–289. Stuart, G. J., Dodt, H. U., and Sakmann, B. (1993). Patch-clamp recordings from the soma and dendrites of neurons in brain slices using infrared video microscopy. Pflugers Arch. 423, 511–518. Tamas, G., Buhl, E. H., Lorincz, A., and Somogyi, P. (2000). Proximally targeted GABAergic synapses and gap junctions synchronize cortical interneurons. Nat. Neurosci. 3, 366–371. Teubner, B., Degen, J., Sohl, G., Guldenagel, M., Bukauskas, F. F., Trexler, E. B., Verselis, V. K., De Zeeuw, C. I., Lee, C. G., Kozak, C. A., Petrasch-Parwez, E., Dermietzel, R., and Willecke, K. (2000). Functional expression of the murine connexin 36 gene coding for a neuron-specific gap junctional protein. J. Membr. Biol. 176, 249–262. Theis, M., Sohl, G., Speidel, D., Kuhn, R., and Willecke, K. (2003). Connexin43 is not expressed in principal cells of mouse cortex and hippocampus. Eur. J. Neurosci. 18, 267–274. Traub, R. D. (2003). Fast oscillations and epilepsy. Epilepsy Curr. 3, 77–79. Traub, R. D., Draguhn, A., Whittington, M. A., Baldeweg, T., Bibbig, A., Buhl, E. H., and Schmitz, D. (2002). Axonal gap junctions between principal neurons: A novel source of network oscillations, and perhaps epileptogenesis. Rev. Neurosci. 13, 1–30. Traub, R. D., Kopell, N., Bibbig, A., Buhl, E. H., LeBeau, F. E., and Whittington, M. A. (2001a). Gap junctions between interneuron dendrites can enhance synchrony of gamma oscillations in distributed networks. J. Neurosci. 21, 9478–9486. Traub, R. D., Pais, I., Bibbig, A., LeBeau, F. E., Buhl, E. H., Hormuzdi, S. G., Monyer, H., and Whittington, M. A. (2003). Contrasting roles of axonal (pyramidal cell) and dendritic (interneuron) electrical coupling in the generation of neuronal network oscillations. Proc. Natl. Acad. Sci. USA 100, 1370–1374. Traub, R. D., Schmitz, D., JeVerys, J. G., and Draguhn, A. (1999). High-frequency population oscillations are predicted to occur in hippocampal pyramidal neuronal networks interconnected by axoaxonal gap junctions. Neuroscience 92, 407–426. Traub, R. D., Whittington, M. A., Buhl, E. H., LeBeau, F. E., Bibbig, A., Boyd, S., Cross, H., and Baldeweg, T. (2001b). A possible role for gap junctions in generation of very fast EEG oscillations preceding the onset of, and perhaps initiating, seizures. Epilepsia 42, 153–170. Ungerstedt, U. (1971). Stereotaxic mapping of the monamine pathways in the rat brain. Acta Physiol. Scand. Suppl. 367, 1–48. Uusisaari, M., Smirnov, S., Voipio, J., and Kaila, K. (2002). Spontaneous epileptiform activity mediated by GABA(A) receptors and gap junctions in the rat hippocampal slice following longterm exposure to GABA(B) antagonists. Neuropharmacology 43, 563–572. Vandecasteele, M., Glowinski, J., and Venance, L. (2005). Electrical synapses between dopaminergic neurons of the substantia nigra pars compacta. J. Neurosci. 25, 291–298. Vanderschuren, L. J., and Kalivas, P. W. (2000). Alterations in dopaminergic and glutamatergic transmission in the induction and expression of behavioral sensitization: A critical review of preclinical studies. Psychopharm 151, 99–120.

NEURONAL GAP JUNCTIONS

151

Venance, L., Glowinski, J., and Giaume, C. (2004). Electrical and chemical transmissions between striatal GABAergic output neurons in rat brain slices. J. Physiol. 559, 215–230. Venance, L., Piomelli, D., Glowinski, J., and Giaume, C. (1995). Inhibition by anandamide of gap junctions and intercellular calcium signalling in striatal astrocytes. Nature 376, 590–594. Venance, L., Rozov, A., Blatow, M., Burnashev, N., Feldmeyer, D., and Monyer, H. (2000). Connexin expression in electrically coupled postnatal rat brain neurons. Proc. Natl. Acad. Sci. USA 97, 10260–10265. Watanabe, A. (1958). The interaction of electrical activity among neurons of lobster cardiac ganglion. Jpn. J. Physiol. 8, 305–318. Weickert, S., Ray, A., Zoidl, G., and Dermietzel, R. (2005). Expression of neural connexins and pannexin1 in the hippocampus and inferior olive: A quantitative approach. Mol Brain Res. 133, 102–109. Wentlandt, K., Kushnir, M., Naus, C. C., and Carlen, P. L. (2004). Ethanol inhibits gap-junctional coupling between P19 cells. Alcohol. Clin. Exp. Res. 28, 1284–1290. Willecke, K., Eiberger, J., Degen, J., Eckardt, D., Romualdi, A., Guldenagel, M., Deutsch, U., and Sohl, G. (2002). Structural and functional diversity of connexin genes in the mouse and human genome. Biol. Chem. 383, 725–737. Yang, X. D., Korn, H., and Faber, D. S. (1990). Long-term potentiation of electronic coupling at mixed synapses. Nature 348, 542–545. Zhang, C., Finger, T. E., and Restrepo, D. (2000). Mature olfactory receptor neurons express connexin 43. J. Comp. Neurol. 426, 1–12. Zhang, C., and Restrepo, D. (2002). Expression of connexin 45 in the olfactory system. Brain Res. 929, 37–47.