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Electrical Synapses in the Mammalian Brain 24 May 2004 19:3 AR AR217-NE27-15.tex AR217-NE27-15.sgm LaTeX2e(2002/01/18) P1: IKH 10.1146/annurev.neuro.26.041002.131128 Annu. Rev. Neurosci. 2004. 27:393–418 doi: 10.1146/annurev.neuro.26.041002.131128 Copyright c 2004 by Annual Reviews. All rights reserved First published online as a Review in Advance on February 26, 2004 ELECTRICAL SYNAPSES IN THE MAMMALIAN BRAIN BarryW.Connors1 and Michael A. Long2 1Department of Neuroscience, Brown University, Providence, Rhode Island 02912; email: [email protected] 2Department of Brain and Cognitive Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139; email: [email protected] Key Words gap junction, electrotonic synapse, electrical coupling, connexin, connexin36 I Abstract Many neurons in the mammalian central nervous system communicate through electrical synapses, defined here as gap junction–mediated connections. Elec- trical 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 electri- cal 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 electri- cally 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. INTRODUCTION The mammalian brain excels at rapid information processing, thanks to its mas- Annu. Rev. Neurosci. 2004.27:393-418. Downloaded from arjournals.annualreviews.org by NEW YORK UNIVERSITY - BOBST LIBRARY on 01/10/10. For personal use only. sively parallel architecture and the high operational speed (by biological stan- dards) of its neuronal elements. Neurons generate rapid signals by controlling ionic current flow across their membranes. The most common mechanism for sig- naling between neurons is the neurotransmitter-releasing chemical synapse. Faster and simpler signaling can be achieved with electrical synapses, specialized junc- tions that allow ionic current to flow directly between neurons. The vast majority of electrical synapses are membrane-to-membrane appositions called gap junc- tions, which are clusters of transcellular channels composed of protein subunits termed connexins. This type of electrical synapse, also known as the electrotonic synapse, can mediate electrical coupling between cells, and it has functional prop- erties strikingly different from those of chemical synapses (Bennett 1977). Most 0147-006X/04/0721-0393$14.00 393 24 May 2004 19:3 AR AR217-NE27-15.tex AR217-NE27-15.sgm LaTeX2e(2002/01/18) P1: IKH 394 CONNORS LONG notably, vertebrate electrical synapses are bidirectional. With their speed, simplic- ity, and reciprocity, electrical synapses are a unique feature of neuronal circuits in the mammalian brain. Nevertheless, the prevalence of electrical synapses has been recognized only in the past few years. The notion that neurons communicate electrically is almost as old as the idea of bioelectricity itself (Eccles 1982, Cowan & Kandel 2001). Studies of crayfish and shrimp neurons offered the first compelling evidence for electrical synapses (Furshpan & Potter 1957, Watanabe 1958), and electrical synapses were revealed in the vertebrate central nervous system of teleost fish soon after (Bennett et al. 1959). Electrical synapses in mammalian brains proved much harder to find. Single-cell recordings provided the first strong evidence for mammalian electrical synapses in the mesencephalic nucleus of cranial nerve V (Hinrichsen 1970, Baker & Llinas´ 1971), the vestibular nucleus (Wylie 1973, Korn et al. 1973), and the inferior olivary nucleus (Llinas´ et al. 1974). The most convincing way to demonstrate electrotonic coupling is to record intracellularly from two neighboring cells simultaneously, a procedure that is exceptionally difficult to perform in the intact brain. Studies of electrical synapses in the mammalian brain languished for years, even as break- throughs were made in the general physiology of gap junctions (Spray et al. 1979, 1981) and the molecular biology of connexins (Willecke et al. 2002). Electrical synapses are now being intensively examined in mammals, thanks to recent technical developments in electrophysiology, isolated brain slice prepara- tions, cell labeling and imaging, molecular cloning, and transgenics. In this review, we examine the burgeoning evidence that electrical synapses function throughout the mammalian brain and describe pertinent studies of nonmammalian and non- neuronal gap junctions. We do not review the roles of gap junctions in early neural development and the functions of glia, and we do not discuss gap junction–free forms of electrical communication (Jefferys 1995). STRUCTURE AND MOLECULAR COMPOSITION OF ELECTRICAL SYNAPSES Gap Junctions and Connexins Annu. Rev. Neurosci. 2004.27:393-418. Downloaded from arjournals.annualreviews.org by NEW YORK UNIVERSITY - BOBST LIBRARY on 01/10/10. For personal use only. The most visible and common structural correlate of electrical synapses is the gap junction, which is seen most readily with electron microscopy using the freeze- fracture technique. Gap junctions are clusters of connexin-containing channels that are coextensive across regions of apposing membranes of coupled cells (Evans & Martin 2002). A gap of extracellular space separates the two membrane leaflets, usually by ∼2–3 nm. Gap junctions between neurons have been observed in the majority of cases where electrophysiological evidence for electrical synapses is well established. Neuronal gap junctions are often synonymous with electrical synapses, but there are apparent limitations to this assumption. Tissue preparation can both destroy 24 May 2004 19:3 AR AR217-NE27-15.tex AR217-NE27-15.sgm LaTeX2e(2002/01/18) P1: IKH ELECTRICAL SYNAPSES IN THE BRAIN 395 bona fide gap junctions and create spurious gap junction–like structures (Brightman & Reese 1969). Gap junctions are notoriously difficult to observe, and the absence of evidence for gap junctions cannot be construed as evidence for the absence of electrical coupling. Gap junction channels may be functional even when they are too widely dispersed to form conventional gap junctions (Williams & DeHaan 1981). Conversely, electrotonic coupling may be absent despite the presence of vir- tually normal-looking gap junctions (Raviola & Gilula 1973, De Zeeuw et al. 2003). Connexins are a family of proteins with ∼20 isoforms in humans and mice (Willecke et al. 2002). Across species there are ∼40 connexin orthologues. In the most commonly used nomenclature, connexins are named for their predicted molecular weights (e.g., Cx36 has a mass of ∼36 kDa). Each gap junction hemi- channel, also known as a connexon, is a connexin hexamer. Most cells can express multiple connexins, and connexons can be homomeric or heteromeric. Only some combinations of homomeric connexons can form functional heterotypic channels; Cx36 may function only homotypically (Al-Ubaidi et al. 2000, Teubner et al. 2000). Approximately half of the mammalian connexins are abundant in the central ner- vous system. Several are strongly expressed in astrocytes (Cx26, Cx30, Cx43) and oligodendrocytes (Cx29, Cx32, Cx47) (Nagy et al. 2001, Menichella et al. 2003, Odermatt et al. 2003), but most are not expressed in neurons (Rash et al. 2001a,b; Odermatt et al. 2003). Cx36, which is the mammalian orthologue of fish Cx35 (O’Brien et al., 1998), is expressed widely in the brain (Condorelli et al. 2000). Data from freeze-fracture replica immunolabeling indicate that Cx36 protein appears in neuron-neuron gap junctions but not in gap junctions between astrocytes and/or oligodendrocytes (Rash et al. 2001a,b). Single-cell reverse transcription (RT)-PCR shows that the Cx36 message is consistently present in neurons of the hippocampus and neocortex (Venance et al. 2000). When the gene for Cx36 is knocked out in mice, electrical coupling that normally occurs between certain neurons in the retina (Deans et al. 2002) and in the neocortex (Deans et al. 2001, Blatow et al. 2003), hip- pocampus (Hormuzdi et al. 2001), thalamic reticular nucleus (TRN) (Landisman et al. 2002), and inferior olivary nucleus (Long et al. 2002, DeZeeuw et al. 2003) is eliminated or profoundly reduced. If Cx36 is not the exclusive connexin involved in coupling these neurons, it is at least a necessary constituent. As prevalent as Cx36 seems to be, it is probably not the only connexin involved Annu. Rev. Neurosci. 2004.27:393-418. Downloaded from arjournals.annualreviews.org by NEW YORK UNIVERSITY - BOBST LIBRARY on 01/10/10. For personal use only. in mammalian electrical synapses. Neurons known to be electrically coupled, but which may not express Cx36, include those of the locus coeruleus (Christie et al. 1989, Alvarez et al. 2002), the horizontal cells of the retina (Deans & Paul 2001), and perhaps the pyramidal cells in the hippocampus (MacVicar & Dudek 1981, 1982; Draguhn et al. 1998; Schmitz et al. 2001). Cx45 is expressed in neurons of olfactory epithelium
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