The components of an electrical synapse as revealed by expansion microscopy of a single synaptic contact

Most nervous systems combine both transmitter-mediated and direct cell-cell communication, known as 'chemical' and 'electrical' synapses, respectively. Chemical synapses can be identified by their multiple structural components. Electrical synapses are, on the other hand, generally defined by the presence of a 'gap junction' (a cluster of intercellular channels) between two neuronal processes. However, while gap junctions provide the communicating mechanism, it is unknown whether electrical transmission requires the contribution of additional cellular structures. We investigated this question at identifiable single synaptic contacts on the zebrafish Mauthner cells, at which gap junctions coexist with specializations for neurotransmitter release and where the contact defines the anatomical limits of a synapse. Expansion microscopy of these contacts revealed a detailed map of the incidence and spatial distribution of proteins pertaining to various synaptic structures. Multiple gap junctions of variable size were identified by the presence of their molecular components. Remarkably, most of the synaptic contact's surface was occupied by interleaving gap junctions and components of adherens junctions, suggesting a close functional association between these two structures. In contrast, glutamate receptors were confined to small peripheral portions of the contact, indicating that most of the synaptic area works as an electrical synapse. Thus, our results revealed the overarching organization of an electrical synapse that operates with not one, but multiple gap junctions, in close association with structural and signaling molecules known to be components of AJs. The relationship between these intercellular structures will aid in establishing the boundaries of electrical synapses found throughout animal connectomes and provide insight into the structural organization and functional diversity of electrical synapses.

Neurobeachin controls the asymmetric subcellular distribution of electrical synapse proteins

The subcellular positioning of synapses and their specialized molecular compositions form the fundamental basis of neural circuits. Like chemical synapses, electrical synapses are constructed from an assortment of adhesion, scaffolding, and regulatory molecules, yet little is known about how these molecules localize to specific neuronal compartments. Here, we investigate the relationship between the autism- and epilepsy-associated gene Neurobeachin, the neuronal gap junction channel-forming Connexins, and the electrical synapse scaffold ZO1. Using the zebrafish Mauthner circuit, we find Neurobeachin localizes to the electrical synapse independently of ZO1 and Connexins. By contrast, we show Neurobeachin is required postsynaptically for the robust localization of ZO1 and Connexins. We demonstrate that Neurobeachin binds ZO1 but not Connexins. Finally, we find Neurobeachin is required to restrict electrical postsynaptic proteins to dendrites, but not electrical presynaptic proteins to axons. Together, the results reveal an expanded understanding of electrical synapse molecular complexity and the hierarchical interactions required to build neuronal gap junctions. Further, these findings provide novel insight into the mechanisms by which neurons compartmentalize the localization of electrical synapse proteins and provide a cell biological mechanism for the subcellular specificity of electrical synapse formation and function.

Trichloroacetic Acid Fixation and Antibody Staining of Zebrafish Larvae

Larval zebrafish have been established as an excellent model for examining vertebrate biology, with many researchers using the system for neuroscience. Controlling a fast escape response of the fish, the Mauthner cells and their associated network are an attractive model, given their experimental accessibility and fast development, driving ethologically relevant behavior in the first five days of development. Here, we describe methods for immunostaining electrical and chemical synapse proteins at 3-7 days post fertilization (dpf) in zebrafish using tricholoracetic acid fixation. The methods presented are ideally suited to easily visualize neural circuits and synapses within the fish.

On the location of electrical synapses

Location is of critical functional relevance for synapses, including electrical synapses, which are a form of neuronal communication mediated by cell-cell channels. In this issue of Developmental Cell, Palumbos et al. identify a mechanism that supports the localization and function of electrical synapses with subcellular specificity.

A forward genetic screen identifies Dolk as a regulator of startle magnitude through the potassium channel subunit Kv1.1

The acoustic startle response is an evolutionarily conserved avoidance behavior. Disruptions in startle behavior, particularly startle magnitude, are a hallmark of several human neurological disorders. While the neural circuitry underlying startle behavior has been studied extensively, the repertoire of genes and genetic pathways that regulate this locomotor behavior has not been explored using an unbiased genetic approach. To identify such genes, we took advantage of the stereotypic startle behavior in zebrafish larvae and performed a forward genetic screen coupled with whole genome analysis. We uncovered mutations in eight genes critical for startle behavior, including two genes encoding proteins associated with human neurological disorders, Dolichol kinase (Dolk), a broadly expressed regulator of the glycoprotein biosynthesis pathway, and the potassium Shaker-like channel subunit Kv1.1. We demonstrate that Kv1.1 and Dolk play critical roles in the spinal cord to regulate movement magnitude during the startle response and spontaneous swim movements. Moreover, we show that Kv1.1 protein is mislocalized in dolk mutants, suggesting they act in a common genetic pathway. Combined, our results identify a diverse set of eight genes, all associated with human disorders, that regulate zebrafish startle behavior and reveal a previously unappreciated role for Dolk and Kv1.1 in regulating movement magnitude via a common genetic pathway.

Electrical synaptic transmission requires a postsynaptic scaffolding protein

Electrical synaptic transmission relies on neuronal gap junctions containing channels constructed by Connexins. While at chemical synapses neurotransmitter-gated ion channels are critically supported by scaffolding proteins, it is unknown if channels at electrical synapses require similar scaffold support. Here, we investigated the functional relationship between neuronal Connexins and Zonula Occludens 1 (ZO1), an intracellular scaffolding protein localized to electrical synapses. Using model electrical synapses in zebrafish Mauthner cells, we demonstrated that ZO1 is required for robust synaptic Connexin localization, but Connexins are dispensable for ZO1 localization. Disrupting this hierarchical ZO1/Connexin relationship abolishes electrical transmission and disrupts Mauthner cell-initiated escape responses. We found that ZO1 is asymmetrically localized exclusively postsynaptically at neuronal contacts where it functions to assemble intercellular channels. Thus, forming functional neuronal gap junctions requires a postsynaptic scaffolding protein. The critical function of a scaffolding molecule reveals an unanticipated complexity of molecular and functional organization at electrical synapses.

Regulatory Roles of Metabotropic Glutamate Receptors on Synaptic Communication Mediated by Gap Junctions

Variations of synaptic strength are thought to underlie forms of learning and can functionally reshape neural circuits. Metabotropic glutamate receptors play key roles in regulating the strength of chemical synapses. However, information within neural circuits is also conveyed via a second modality of transmission: gap junction-mediated synapses. We review here evidence indicating that metabotropic glutamate receptors also play important roles in the regulation of synaptic communication mediated by neuronal gap junctions, also known as 'electrical synapses'. Activity-driven interactions between metabotropic glutamate receptors and neuronal gap junctions can lead to long-term changes in the strength of electrical synapses. Further, the regulatory action of metabotropic glutamate receptors on neuronal gap junctions is not restricted to adulthood but is also of critical relevance during brain development and contributes to the pathological mechanisms that follow brain injury.

Neuroscience: The Hidden Diversity of Electrical Synapses

The complete description of the expression of gap junction proteins in the nervous system of the worm reveals a great complexity of their distribution amongst different neuronal classes, opening an unprecedented opportunity to expose the functional diversity of electrical synapses.

A Cyfip2-Dependent Excitatory Interneuron Pathway Establishes the Innate Startle Threshold

Sensory experiences dynamically modify whether animals respond to a given stimulus, but it is unclear how innate behavioral thresholds are established. Here, we identify molecular and circuit-level mechanisms underlying the innate threshold of the zebrafish startle response. From a forward genetic screen, we isolated five mutant lines with reduced innate startle thresholds. Using whole-genome sequencing, we identify the causative mutation for one line to be in the fragile X mental retardation protein (FMRP)-interacting protein cyfip2. We show that cyfip2 acts independently of FMRP and that reactivation of cyfip2 restores the baseline threshold after phenotype onset. Finally, we show that cyfip2 regulates the innate startle threshold by reducing neural activity in a small group of excitatory hindbrain interneurons. Thus, we identify a selective set of genes critical to establishing an innate behavioral threshold and uncover a circuit-level role for cyfip2 in this process.

Using forward genetics, electrophysiology, and combined behavior and Ca²⁺ imaging in zebrafish, Marsden et al. show that cyfip2 regulates the acoustic startle threshold by controlling the activity of excitatory spiral fiber interneurons.

Science, art, society and Klimt’s University of Vienna paintings

At the turn of the 19th century the Austrian artist Gustav Klimt was commissioned to decorate the ceiling of the Great Hall of the University of Vienna. However the three paintings he produced – Philosophy, Medicine and Jurisprudence – were rejected by the university and later destroyed by retreating German troops during World War II. The story of these paintings, and another called Goldfish, illuminates common ground between art and science, and highlights ongoing tensions in the relationships between art, science and society.

Two Forms of Electrical Transmission Between Neurons

Electrical signaling is a cardinal feature of the nervous system and endows it with the capability of quickly reacting to changes in the environment. Although synaptic communication between nerve cells is perceived to be mainly chemically mediated, electrical synaptic interactions also occur. Two different strategies are responsible for electrical communication between neurons. One is the consequence of low resistance intercellular pathways, called "gap junctions", for the spread of electrical currents between the interior of two cells. The second occurs in the absence of cell-to-cell contacts and is a consequence of the extracellular electrical fields generated by the electrical activity of neurons. Here, we place present notions about electrical transmission in a historical perspective and contrast the contributions of the two different forms of electrical communication to brain function.

A Forward Genetic Screen in Zebrafish Identifies the G-Protein-Coupled Receptor CaSR as a Modulator of Sensorimotor Decision Making

Animals continuously integrate sensory information and select contextually appropriate responses. Here, we show that zebrafish larvae select a behavioral response to acoustic stimuli from a pre-existing choice repertoire in a context-dependent manner. We demonstrate that this sensorimotor choice is modulated by stimulus quality and history, as well as by neuromodulatory systems-all hallmarks of more complex decision making. Moreover, from a genetic screen coupled with whole-genome sequencing, we identified eight mutants with deficits in this sensorimotor choice, including mutants of the vertebrate-specific G-protein-coupled extracellular calcium-sensing receptor (CaSR), whose function in the nervous system is not well understood. We demonstrate that CaSR promotes sensorimotor decision making acutely through Gα_i/o and Gα_q/11 signaling, modulated by clathrin-mediated endocytosis. Combined, our results identify the first set of genes critical for behavioral choice modulation in a vertebrate and reveal an unexpected critical role for CaSR in sensorimotor decision making.

Electrical synapses in mammalian CNS: Past eras, present focus and future directions

Gap junctions provide the basis for electrical synapses between neurons. Early studies in well-defined circuits in lower vertebrates laid the foundation for understanding various properties conferred by electrical synaptic transmission. Knowledge surrounding electrical synapses in mammalian systems unfolded first with evidence indicating the presence of gap junctions between neurons in various brain regions, but with little appreciation of their functional roles. Beginning at about the turn of this century, new approaches were applied to scrutinize electrical synapses, revealing the prevalence of neuronal gap junctions, the connexin protein composition of many of those junctions, and the myriad diverse neural systems in which they occur in the mammalian CNS. Subsequent progress indicated that electrical synapses constitute key elements in synaptic circuitry, govern the collective activity of ensembles of electrically coupled neurons, and in part orchestrate the synchronized neuronal network activity and rhythmic oscillations that underlie fundamental integrative processes. This article is part of a Special Issue entitled: Gap Junction Proteins edited by Jean Claude Herve.

The Variable Strength of Electrical Synapses

In contrast to chemical synapses, less is known regarding the determinants of the strength of electrical synapses. New evidence from Szoboszlay et al. (2016) shows that the number of gap junctions is the dominant factor underlying the strength of electrical coupling between cerebellar interneurons.

Electrical transmission: Two structures, same functions?

Electrical synapses are finding increasing representation and importance in our understanding of signaling in the nervous system. In contrast to chemical synapses, at which molecules are evolutionary conserved, vertebrate and invertebrate electrical synapses represent molecularly different structures that share a common communicating strategy that allows them to serve very similar functions. A better understanding of differences and commonalities regarding the structure, function and regulation of vertebrate and invertebrate electrical synapses will lead to a better understanding of the properties and functional diversity of this modality of synaptic communication. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 517-521, 2017.

The electrical synapse: Molecular complexities at the gap and beyond

Gap junctions underlie electrical synaptic transmission between neurons. Generally perceived as simple intercellular channels, "electrical synapses" have demonstrated to be more functionally sophisticated and structurally complex than initially anticipated. Electrical synapses represent an assembly of multiple molecules, consisting of channels, adhesion complexes, scaffolds, regulatory machinery, and trafficking proteins, all required for their proper function and plasticity. Additionally, while electrical synapses are often viewed as strictly symmetric structures, emerging evidence has shown that some components forming electrical synapses can be differentially distributed at each side of the junction. We propose that the molecular complexity and asymmetric distribution of proteins at the electrical synapse provides rich potential for functional diversity. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 562-574, 2017.

Activity-dependent plasticity of electrical synapses: increasing evidence for its presence and functional roles in the mammalian brain

Gap junctions mediate electrical synaptic transmission between neurons. While the actions of neurotransmitter modulators on the conductance of gap junctions have been extensively documented, increasing evidence indicates they can also be influenced by the ongoing activity of neural networks, in most cases via local interactions with nearby glutamatergic synapses. We review here early evidence for the existence of activity-dependent regulatory mechanisms as well recent examples reported in mammalian brain. The ubiquitous distribution of both neuronal connexins and the molecules involved suggest this phenomenon is widespread and represents a property of electrical transmission in general.

Neurobiology: all synapses are created equal

There are two main modalities of communication between neurons, known as electrical and chemical synaptic transmission. Despite striking differences in their underlying mechanisms, new evidence suggests that the formation of electrically and chemically mediated synapses is under common regulatory processes.

Opioids potentiate electrical transmission at mixed synapses on the Mauthner cell

Opioid receptors were shown to modulate a variety of cellular processes in the vertebrate central nervous system, including synaptic transmission. While the effects of opioid receptors on chemically mediated transmission have been extensively investigated, little is known of their actions on gap junction-mediated electrical synapses. Here we report that pharmacological activation of mu-opioid receptors led to a long-term enhancement of electrical (and glutamatergic) transmission at identifiable mixed synapses on the goldfish Mauthner cells. The effect also required activation of both dopamine D1/5 receptors and postsynaptic cAMP-dependent protein kinase A, suggesting that opioid-evoked actions are mediated indirectly via the release of dopamine from varicosities known to be located in the vicinity of the synaptic contacts. Moreover, inhibitory inputs situated in the immediate vicinity of these excitatory synapses on the lateral dendrite of the Mauthner cell were not affected by activation of mu-opioid receptors, indicating that their actions are restricted to electrical and glutamatergic transmissions co-existing at mixed contacts. Thus, as their chemical counterparts, electrical synapses can be a target for the modulatory actions of the opioid system. Because gap junctions at these mixed synapses are formed by fish homologs of the neuronal connexin 36, which is widespread in mammalian brain, it is likely that this regulatory property applies to electrical synapses elsewhere as well.

Electrical synaptic transmission in developing zebrafish: properties and molecular composition of gap junctions at a central auditory synapse

In contrast to the knowledge of chemical synapses, little is known regarding the properties of gap junction-mediated electrical synapses in developing zebrafish, which provide a valuable model to study neural function at the systems level. Identifiable "mixed" (electrical and chemical) auditory synaptic contacts known as "club endings" on Mauthner cells (2 large reticulospinal neurons involved in tail-flip escape responses) allow exploration of electrical transmission in fish. Here, we show that paralleling the development of auditory responses, electrical synapses at these contacts become anatomically identifiable at day 3 postfertilization, reaching a number of ∼6 between days 4 and 9. Furthermore, each terminal contains ∼18 gap junctions, representing between 2,000 and 3,000 connexon channels formed by the teleost homologs of mammalian connexin 36. Electrophysiological recordings revealed that gap junctions at each of these contacts are functional and that synaptic transmission has properties that are comparable with those of adult fish. Thus a surprisingly small number of mixed synapses are responsible for the acquisition of auditory responses by the Mauthner cells, and these are likely sufficient to support escape behaviors at early developmental stages.

Hemichannel composition and electrical synaptic transmission: molecular diversity and its implications for electrical rectification

Unapposed hemichannels (HCs) formed by hexamers of gap junction proteins are now known to be involved in various cellular processes under both physiological and pathological conditions. On the other hand, less is known regarding how differences in the molecular composition of HCs impact electrical synaptic transmission between neurons when they form intercellular heterotypic gap junctions (GJs). Here we review data indicating that molecular differences between apposed HCs at electrical synapses are generally associated with rectification of electrical transmission. Furthermore, this association has been observed at both innexin and connexin (Cx) based electrical synapses. We discuss the possible molecular mechanisms underlying electrical rectification, as well as the potential contribution of intracellular soluble factors to this phenomenon. We conclude that asymmetries in molecular composition and sensitivity to cellular factors of each contributing hemichannel can profoundly influence the transmission of electrical signals, endowing electrical synapses with more complex functional properties.

Molecular and functional asymmetry at a vertebrate electrical synapse

Electrical synapses are abundant in the vertebrate brain, but their functional and molecular complexities are still poorly understood. We report here that electrical synapses between auditory afferents and goldfish Mauthner cells are constructed by apposition of hemichannels formed by two homologs of mammalian connexin 36 (Cx36) and that, while Cx35 is restricted to presynaptic hemiplaques, Cx34.7 is restricted to postsynaptic hemiplaques, forming heterotypic junctions. This molecular asymmetry is associated with rectification of electrical transmission that may act to promote cooperativity between auditory afferents. Our data suggest that, in similarity to pre- and postsynaptic sites at chemical synapses, one side in electrical synapses should not necessarily be considered the mirror image of the other. While asymmetry based on the presence of two Cx36 homologs is restricted to teleost fish, it might also be based on differences in posttranslational modifications of individual connexins or in the complement of gap junction-associated proteins.

On the training of future neuroscientists: insights from the Grass laboratory

The understanding of nature is a continuous process that requires the transference of current knowledge to future generations. In this NeuroView, we address the critical issue of training of future scientists, an essential aspect of scientific progress. As an example of the impact training programs can have on shaping future scientists, we focus on the experience of the Grass Laboratory, which provides early career investigators the opportunity to embark on independent research experiences. This uniquely designed program has contributed enormously to fostering the development of neuroscientists in the past 60 years and has left a recognizable mark on 20(th) and 21(st) century neuroscience research.

Intracellular magnesium-dependent modulation of gap junction channels formed by neuronal connexin36

Gap junction (GJ) channels composed of Connexin36 (Cx36) are widely expressed in the mammalian CNS and form electrical synapses between neurons. Here we describe a novel modulatory mechanism of Cx36 GJ channels dependent on intracellular free magnesium ([Mg(2+)]i). We examined junctional conductance (gj) and its dependence on transjunctional voltage (Vj) at different [Mg(2+)]i in cultures of HeLa or N2A cells expressing Cx36. We found that Cx36 GJs are partially inhibited at resting [Mg(2+)]i. Thus, gj can be augmented or reduced by lowering or increasing [Mg(2+)]i, respectively. Similar changes in gj and Vj-gating were observed using MgATP or K2ATP in pipette solutions, which increases or decreases [Mg(2+)]i, respectively. Changes in phosphorylation of Cx36 or in intracellular free calcium concentration were not involved in the observed Mg(2+)-dependent modulation of gj. Magnesium ions permeate the channel and transjunctional asymmetry in [Mg(2+)]i resulted in asymmetric Vj-gating. The gj of GJs formed of Cx26, Cx32, Cx43, Cx45, and Cx47 was also reduced by increasing [Mg(2+)]i, but was not increased by lowering [Mg(2+)]i; single-channel conductance did not change. We showed that [Mg(2+)]i affects both open probability and the number of functional channels, likely through binding in the channel lumen. Finally, we showed that Cx36-containing electrical synapses between neurons of the trigeminal mesencephalic nucleus in rat brain slices are similarly affected by changes in [Mg(2+)]i. Thus, this novel modulatory mechanism could underlie changes in neuronal synchronization under conditions in which ATP levels, and consequently [Mg(2+)]i, are modified.

Two independent forms of activity-dependent potentiation regulate electrical transmission at mixed synapses on the Mauthner cell

Mixed (electrical and chemical) synaptic contacts on the Mauthner cells, known as Club endings, constitute a valuable model for the study of vertebrate electrical transmission. While electrical synapses are still perceived by many as passive intercellular channels that lack modifiability, a wealth of experimental evidence shows that gap junctions at Club endings are subject to dynamic regulatory control by two independent activity-dependent mechanisms that lead to potentiation of electrical transmission. One of those mechanisms relies on activation of NMDA receptors and postsynaptic CaMKII. A second mechanism relies on mGluR activation and endocannabinoid production and is indirectly mediated via the release of dopamine from nearby varicosities, which in turn leads to potentiation of the synaptic response via a PKA-mediated postsynaptic mechanism. We review here these two forms of potentiation and their signaling mechanisms, which include the activation of two kinases with well-established roles as regulators of synaptic strength, as well as the functional implications of these two forms of potentiation. Special Issue entitled Electrical Synapses.

Electrical transmission between mammalian neurons is supported by a small fraction of gap junction channels

Electrical synapses formed by gap junctions between neurons create networks of electrically coupled neurons in the mammalian brain, where these networks have been found to play important functional roles. In most cases, interneuronal gap junctions occur at remote dendro-dendritic contacts, making difficult accurate characterization of their physiological properties and correlation of these properties with their anatomical and morphological features of the gap junctions. In the mesencephalic trigeminal (MesV) nucleus where neurons are readily accessible for paired electrophysiological recordings in brain stem slices, our recent data indicate that electrical transmission between MesV neurons is mediated by connexin36 (Cx36)-containing gap junctions located at somato-somatic contacts. We here review evidence indicating that electrical transmission between these neurons is supported by a very small fraction of the gap junction channels present at cell-cell contacts. Acquisition of this evidence was enabled by the unprecedented experimental access of electrical synapses between MesV neurons, which allowed estimation of the average number of open channels mediating electrical coupling in relation to the average number of gap junction channels present at these contacts. Our results indicate that only a small proportion of channels (~0.1 %) appear to be conductive. On the basis of similarities with other preparations, we postulate that this phenomenon might constitute a general property of vertebrate electrical synapses, reflecting essential aspects of gap junction function and maintenance.

Gap junction-mediated electrical transmission: regulatory mechanisms and plasticity

The term synapse applies to cellular specializations that articulate the processing of information within neural circuits by providing a mechanism for the transfer of information between two different neurons. There are two main modalities of synaptic transmission: chemical and electrical. While most efforts have been dedicated to the understanding of the properties and modifiability of chemical transmission, less is still known regarding the plastic properties of electrical synapses, whose structural correlate is the gap junction. A wealth of data indicates that, rather than passive intercellular channels, electrical synapses are more dynamic and modifiable than was generally perceived. This article will discuss the factors determining the strength of electrical transmission and review current evidence demonstrating its dynamic properties. Like their chemical counterparts, electrical synapses can also be plastic and modifiable. This article is part of a Special Issue entitled: The Communicating junctions, roles and dysfunctions.

Synergy between electrical coupling and membrane properties promotes strong synchronization of neurons of the mesencephalic trigeminal nucleus

Electrical synapses are known to form networks of extensively coupled neurons in various regions of the mammalian brain. The mesencephalic trigeminal (MesV) nucleus, formed by the somata of primary afferents originating in jaw-closing muscles, constitutes one of the first examples supporting the presence of electrical synapses in the mammalian CNS; however, the properties, functional organization, and developmental emergence of electrical coupling within this structure remain unknown. By combining electrophysiological, tracer coupling, and immunochemical analysis in brain slices of rat and mouse, we found that coupling is mostly restricted to pairs or small clusters of MesV neurons. Electrical transmission is supported by connexin36 (Cx36)-containing gap junctions at somato-somatic contacts where only a small proportion of channels appear to be open (∼0.1%). In marked contrast with most brain structures, coupling among MesV neurons increases with age, such that it is absent during early development and appears at postnatal day 8. Interestingly, the development of coupling parallels the development of intrinsic membrane properties responsible for repetitive firing in these neurons. We found that, acting together, sodium and potassium conductances enhance the transfer of signals with high-frequency content via electrical synapses, leading to strong spiking synchronization of the coupled neurons. Together, our data indicate that coupling in the MesV nucleus is restricted to mostly pairs of somata between which electrical transmission is supported by a surprisingly small fraction of the channels estimated to be present, and that coupling synergically interacts with specific membrane conductances to promote synchronization of these neurons.

Trafficking of gap junction channels at a vertebrate electrical synapse in vivo

Trafficking and turnover of transmitter receptors required to maintain and modify the strength of chemical synapses have been characterized extensively. In contrast, little is known regarding trafficking of gap junction components at electrical synapses. By combining ultrastructural and in vivo physiological analysis at identified mixed (electrical and chemical) synapses on the goldfish Mauthner cell, we show here that gap junction hemichannels are added at the edges of GJ plaques where they dock with hemichannels in the apposed membrane to form cell-cell channels and, simultaneously, that intact junctional regions are removed from centers of these plaques into either presynaptic axon or postsynaptic dendrite. Moreover, electrical coupling is readily modified by intradendritic application of peptides that interfere with endocytosis or exocytosis, suggesting that the strength of electrical synapses at these terminals is sustained, at least in part, by fast (in minutes) turnover of gap junction channels. A peptide corresponding to a region of the carboxy terminus that is conserved in Cx36 and its two teleost homologs appears to interfere with formation of new gap junction channels, presumably by reducing insertion of hemichannels on the dendritic side. Thus, our data indicate that electrical synapses are dynamic structures and that their channels are turned over actively, suggesting that regulated trafficking of connexons may contribute to the modification of gap junctional conductance.

The extent and strength of electrical coupling between inferior olivary neurons is heterogeneous

Gap junctions constitute the only form of synaptic communication between neurons in the inferior olive (IO), which gives rise to the climbing fibers innervating the cerebellar cortex. Although its exact functional role remains undetermined, electrical coupling was shown to be necessary for the transient formation of functional compartments of IO neurons and to underlie the precise timing of climbing fibers required for cerebellar learning. So far, most functional considerations assume the existence of a network of permanently and homogeneously coupled IO neurons. Contrasting this notion, our results indicate that coupling within the IO is highly variable. By combining tracer-coupling analysis and paired electrophysiological recordings, we found that individual IO neurons could be coupled to a highly variable number of neighboring neurons. Furthermore, a given neuron could be coupled at remarkably different strengths with each of its partners. Freeze-fracture analysis of IO glomeruli revealed the close proximity of glutamatergic postsynaptic densities to connexin 36-containing gap junctions, at distances comparable to separations between chemical transmitting domains and gap junctions in goldfish mixed contacts, where electrical coupling was shown to be modulated by the activity of glutamatergic synapses. On the basis of structural and molecular similarities with goldfish mixed synapses, we speculate that, rather than being hardwired, variations in coupling could result from glomerulus-specific long-term modulation of gap junctions. This striking heterogeneity of coupling might act to finely influence the synchronization of IO neurons, adding an unexpected degree of complexity to olivary networks.

Potentiation of electrical and chemical synaptic transmission mediated by endocannabinoids

Endocannabinoids are well established as inhibitors of chemical synaptic transmission via presynaptic activation of the cannabinoid type 1 receptor (CB1R). Contrasting this notion, we show that dendritic release of endocannabinoids mediates potentiation of synaptic transmission at mixed (electrical and chemical) synaptic contacts on the goldfish Mauthner cell. Remarkably, the observed enhancement was not restricted to the glutamatergic component of the synaptic response but also included a parallel increase in electrical transmission. This effect involved the activation of CB1 receptors and was indirectly mediated via the release of dopamine from nearby varicosities, which in turn led to potentiation of the synaptic response via a cAMP-dependent protein kinase-mediated postsynaptic mechanism. Thus, endocannabinoid release can potentiate synaptic transmission, and its functional roles include the regulation of gap junction-mediated electrical synapses. Similar interactions between endocannabinoid and dopaminergic systems may be widespread and potentially relevant for the motor and rewarding effects of cannabis derivatives.

Subthreshold sodium current underlies essential functional specializations at primary auditory afferents

Primary auditory afferents are generally perceived as passive, timing-preserving lines of communication. Contrasting this view, identifiable auditory afferents to the goldfish Mauthner cell undergo potentiation of their mixed--electrical and chemical--synapses in response to high-frequency bursts of activity. This property likely represents a mechanism of input sensitization because they provide the Mauthner cell with essential information for the initiation of an escape response. Consistent with this synaptic specialization, we show here that these afferents exhibit an intrinsic ability to respond with bursts of 200-600 Hz and this property critically relies on the activation of a persistent sodium current, which is counterbalanced by the delayed activation of an A-type potassium current. Furthermore, the interaction between these conductances with the membrane passive properties supports the presence of electrical resonance, whose frequency preference is consistent with both the effective range of hearing in goldfish and the firing frequencies required for synaptic facilitation, an obligatory requisite for the induction of activity-dependent changes. Thus our data show that the presence of a persistent sodium current is functionally essential and allows these afferents to translate behaviorally relevant auditory signals into patterns of activity that match the requirements of their fast and highly modifiable synapses. The functional specializations of these neurons suggest that auditory afferents might be capable of more sophisticated contributions to auditory processing than has been generally recognized.

Dynamics of electrical transmission at club endings on the Mauthner cells

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

Voltage-dependent enhancement of electrical coupling by a subthreshold sodium current

Voltage-dependent changes in electrical coupling are often attributed to a direct effect on the properties of gap junction channels. Identifiable auditory afferents terminate as mixed (electrical and chemical) synapses on the distal portion of the lateral dendrite of the goldfish Mauthner cells, a pair of large reticulospinal neurons involved in the organization of sensory-evoked escape responses. At these afferents, the amplitude of the coupling potential produced by the retrograde spread of signals from the postsynaptic Mauthner cell is dramatically enhanced by depolarization of the presynaptic terminal. We demonstrate here that this voltage-dependent enhancement of electrical coupling does not represent a property of the junctions themselves but the activation of a subthreshold sodium current present at presynaptic terminals that acts to amplify the synaptic response. We also provide evidence that this amplification operates under physiological conditions, enhancing synaptic communication from the Mauthner cells to the auditory afferents where electrical and geometrical properties of the coupled cells are unfavorable for retrograde transmission. Retrograde electrical communication at these afferents may play an important functional role by promoting cooperativity between afferents and enhancing transmitter release. Thus, the efficacy of an electrical synapse can be dynamically modulated in a voltage-dependent manner by properties of the nonjunctional membrane. Finally, asymmetric amplification of electrical coupling by intrinsic membrane properties, as at the synapses between auditory afferents and the Mauthner cell, may ensure efficient communication between neuronal processes of dissimilar size and shape, promoting neuronal synchronization.

Chemical synaptic activity modulates nearby electrical synapses

Most electrically coupled neurons also receive numerous chemical synaptic inputs. Whereas chemical synapses are known to be highly dynamic, gap junction-mediated electrical transmission often is considered to be less modifiable and variable. By using simultaneous pre- and postsynaptic recordings, we demonstrate at single mixed electrical and chemical synapses that fast chemical transmission interacts with gap junctions within the same ending to regulate their conductance. Such localized interaction is activity-dependent and could account for the large variation in strength of electrical coupling at auditory afferent synapses terminating on the Mauthner cell lateral dendrite. Thus, interactions between chemical and electrical synapses can regulate the degree of electrical coupling, making it possible for a given neuron to independently modify coupling at different electrical synapses with its neighbors.

Ca2+/calmodulin-dependent kinase II mediates simultaneous enhancement of gap-junctional conductance and glutamatergic transmission

While chemical synapses are very plastic and modifiable by defined activity patterns, gap junctions, which mediate electrical transmission, have been classically perceived as passive intercellular channels. Excitatory transmission between auditory afferents and the goldfish Mauthner cell is mediated by coexisting gap junctions and glutamatergic synapses. Although an increased intracellular Ca2+ concentration is expected to reduce gap junctional conductance, both components of the synaptic response were instead enhanced by postsynaptic increases in Ca2+ concentration, produced by patterned synaptic activity or intradendritic Ca2+ injections. The synaptically induced potentiations were blocked by intradendritic injection of KN-93, a Ca2+/calmodulin-dependent kinase (CaM-K) inhibitor, or CaM-KIINtide, a potent and specific peptide inhibitor of CaM-KII, whereas the responses were potentiated by injection of an activated form of CaM-KII. The striking similarities of the mechanisms reported here with those proposed for long-term potentiation of mammalian glutamatergic synapses suggest that gap junctions are also similarly regulated and indicate a primary role for CaM-KII in shaping and regulating interneuronal communication, regardless of its modality.

A fast synaptic potential mediated by NMDA and non-NMDA receptors

A fast synaptic potential mediated by NMDA and non-NMDA receptors. J. Neurophysiol. 78: 2693-2706, 1997. Excitatory synaptic transmission in the CNS often is mediated by two kinetically distinct glutamate receptor subtypes that frequently are colocalized, the N-methyl--aspartate (NMDA) and non-NMDA receptors. Their synaptic currents are typically very slow and very fast, respectively. We examined the pharmacological and physiological properties of chemical excitatory transmission at the mixed electrical and chemical synapses between auditory afferents and the goldfish Mauthner cell, in vivo. Previous physiological data have suggested the involvement of glutamate receptors in this fast excitatory postsynaptic potential (EPSP), the chemical component of which decays with a time constant of <2 ms. We demonstrate here that the pharmacological and voltage-dependent characteristics of the synaptic currents are consistent with glutamatergic transmission and that both NMDA and non-NMDA receptors are involved. The two components surprisingly exhibit quite similar kinetics even at resting potential, with the NMDA response being only slightly slower. Due to its fast kinetics and characteristic voltage dependence, NMDA receptor-mediated transmission at these first-order synapses contributes significantly to paired pulse and frequency-dependent facilitation of successive fast EPSPs during high-frequency repetitive firing, a presynaptic impulse pattern that induces activity-dependent homosynaptic changes in both electrical and chemical transmission. Thus NMDA receptor kinetics in this intact preparation are suited to its functional requirements, namely speed of information transmission and the ability to trigger changes in synaptic efficacy.

Nitric oxide synthase distribution in the goldfish Mauthner cell

NADPH-diaphorase histochemical staining was used to assess the distribution of the enzyme nitric oxide synthase (NOS) in the goldfish brain, with the emphasis on the Mauthner (M-) cell, a reticulospinal neuron, and its inputs. Labeling was specific for certain cell types, including the M-cell, which stained heavily. The reaction product in this neuron was uniformly distributed along its axon, soma, and ventral and lateral dendrites. Afferents which synapse with the M-cell were also NADPH-diaphorase positive, including an identified class of inhibitory interneurons and the large myelinated club endings (LMCE) of eighth nerve fibers. The presence of NADPH-diaphorase in a lower level brainstem circuit that undergoes activity-dependent long-term potentiation of both excitatory and inhibitory synapses and is accessible for morpho-functional correlations provides the opportunity to elucidate the mechanism and role of nitric oxide at the single cell level.

Activity-dependent short-term enhancement of intercellular coupling

It was reported previously that repeated brief tetanization of the posterior eight nerve can produce long-term homosynaptic potentiations of the electrotonic and chemical components of the mixed EPSP evoked in the Mauthner cell lateral dendrite by a single stimulus to the nerve. We show here that the same stimulus paradigm can lead, alternatively, to short-term enhancements of both excitatory responses. These transient modifications last for approximately 3 min, with a time course similar to post-tetanic potentiation at chemical synapses. However, a different stimulus pattern that transiently increases the presynaptic calcium concentration, paired-nerve stimuli, does not have any significant effect on electrotonic transmission, whereas it facilitates the chemically mediated EPSP. On the other hand, induction of the short-lasting potentiation of coupling, which depended on the discontinuous or burst-like property of the tetanizing paradigm, required NMDA-receptor activation and was blocked by postsynaptic intradendritic injections of the calcium chelator bis(2-aminophenoxy)ethane-N,N,N',N'-tetra-acetic acid. The ineffectiveness of presynaptic calcium in potentiating electrotonic coupling likely reflects the involvement of a calcium-dependent regulatory protein in the postsynaptic cell and suggests that hemichannels on the two sides of a gap junction plaque can be modified independently. NMDA-mediated modulation of gap junctions could be widespread, because both types of channels coexist during development and in several mammalian adult central nervous system structures such as hippocampus.

Retrograde synaptic communication via gap junctions coupling auditory afferents to the Mauthner cell

Large myelinated club endings of the goldfish eighth nerve arise in the sacculus and establish mixed electrotonic and chemical synapses with the distal part of the Mauthner (M-) cell's lateral dendrite. We show here, using paired pre- and postsynaptic recordings, that depolarizing currents generated postsynaptically (specifically, the mixed synaptic potential produced by activation of part of the afferent population) can in some cases excite the presynaptic fibers and cause them to backfire. Strikingly, while in some systems junctional properties prevent the antidromic spread of depolarizing currents, physiological properties of these afferents and the gap junctions promote backfiring: the amplitude of the coupling potential recorded from an afferent fiber is voltage dependent, increasing with depolarization and being reduced during hyperpolarization. Two mechanisms, with different kinetics, underlie this voltage dependence. One, a nonlinear membrane property of the afferent fiber itself, enhances the coupling potential as the afferent membrane depolarizes. The second mechanism, which is less sensitive to voltage and is symmetric about the resting potential, most likely represents voltage dependence of the junctional membrane. Additionally, we also show retrograde diffusion of low molecular weight substances, as the fluorescent dye Lucifer yellow and the tracer Neurobiotin were found in the terminals of afferent fibers after being injected postsynaptically into the M-cell. These results suggest that the gap junctions in these primary afferents are not only involved in fast anterograde synaptic transmission but also provide the substrate for a retrograde intercellular communication. The electrical coupling may modify the input-output relation between eighth nerve afferents and the lateral dendrite by synchronizing the population of already active fibers and by promoting the recruitment of new fibers via backfiring, such that weaker inputs produce relatively larger responses.

Postsynaptic modulation of synaptic efficacy at mixed synapses on the Mauthner cell

Extracellular application of dopamine in the synaptic bed of the lateral dendrite of the goldfish Mauthner (M-) cell enhances both the electrical and chemical components of the mixed excitatory postsynaptic potential (EPSP) evoked by ipsilateral eighth nerve stimulation (Pereda et. al., 1992). We describe here results of experiments designed to determine the locus of action of dopamine and the underlying cellular mechanisms. This amine acts independently on the two modes of transmission, since (1) the percentage increases in the two were not correlated, (2) the time courses of their modifications were independent, and (3) the observed increases in synaptic responses cannot be attributed to a generalized effect on M-cell input conductance, which was increased by dopamine, a change that would rather be expected to shunt the synaptic potentials. Also, dopamine does not produce presynaptic spike broadening and does not modify paired-pulse facilitation, two indications that it acts postsynaptically. The alterations in the mixed EPSP are presumably due to activation of a postsynaptic cAMP-dependent phosphorylation pathway. Specifically, they did not occur if the cAMP-dependent protein kinase inhibitor PKI5-24 was injected intradendritically prior to dopamine application, and they could, on the other hand, be mimicked by injections of the catalytic subunit of the cAMP-dependent protein kinase, PKACAT. In contrast, neither manipulation altered the M-cell input conductance directly or affected the dopamine-induced increase in conductance, suggesting this effect of dopamine is cAMP independent.(ABSTRACT TRUNCATED AT 250 WORDS)

Electrophysiological characteristics of the Mauthner cell of the weakly electric fish Gymnotus carapo

The Mauthner cell (M-cell) of the 'weakly electric fish' Gymnotus carapo was investigated with electrophysiological techniques. The antidromic action potential, the recurrent inhibitory input and the posterior VIIIth nerve excitatory input in this cell exhibited characteristics similar to those described in the goldfish. In addition, we found an excitatory input evoked by spinal stimulation at intensities subthreshold for M-cell axons.

Medullary control of lumbar motoneurons during carbachol-induced motor inhibition

The present study examined the effects of stimulation of the medullary nucleus reticularis gigantocellularis (NRGc) on the Ia-monosynaptic reflex and the membrane potential of lumbar motoneurons. Stimulation of the NRGc was carried out in acute decerebrate cats during motor suppression induced by the intrapontine microinjection of carbachol. During carbachol-induced motor suppression, compared with control conditions (prior to the administration of carbachol), NRGc stimulation resulted in a statistically significant reduction in the Ia-monosynaptic reflex. This effect was maximal at an interval of 45 ms following NRGc stimulation. NRGc stimulation also induced, in lumbar motoneurons, a large amplitude (3.17 mV +/- 0.36 [S.E.M.]), long duration (54.73 ms +/- 3.52 [S.E.M.]) inhibitory postsynaptic potential whose peak coincided with the interval of maximum reflex suppression. These results suggest that carbachol activates pontine neurons that excite cells of the medullary NRGc. We believe that these medullary neurons, in addition to those of the nucleus pontis oralis (NPO)7, participate in the modulation of the descending inhibitory pathway that is responsible for the phenomenon of response-reversal and generalized atonia during naturally occurring active (i.e. REM) sleep.

Renshaw cells are inactive during motor inhibition elicited by the pontine microinjection of carbachol

The present study was undertaken to determine whether the postsynaptic inhibition of motoneurons that occurs following the pontine microinjection of carbachol in the decerebrate cat is due to the activity of Renshaw cells. Thirty-two out of 37 Renshaw cells (86%) were spontaneously active prior to the administration of carbachol, whereas only 2 out of 13 Renshaw cells (15%) discharged during carbachol-induced motor inhibition. In addition, discrete inhibitory synaptic potentials were observed in 33% of the Renshaw cells from which intracellular recordings were obtained after carbachol administration, indicating that these cells were actively inhibited. The finding that a population of Renshaw cells, which inhibit motoneurons, were themselves inhibited during a period of profound motoneuron inhibition was quite unexpected. These results support the conclusion that Renshaw cells are not the inhibitory interneurons that are responsible for the powerful inhibition of motoneurons that occurs following the pontine microinjection of carbachol.

Motoneuron properties during motor inhibition produced by microinjection of carbachol into the pontine reticular formation of the decerebrate cat

It is well established that cholinergic agonists, when injected into the pontine reticular formation in cats, produce a generalized suppression of motor activity (1, 3, 6, 14, 18, 27, 33, 50). The responsible neuronal mechanisms were explored by measuring ventral root activity, the amplitude of the Ia-monosynaptic reflex, and the basic electrophysiological properties of hindlimb motoneurons before and after carbachol was microinjected into the pontine reticular formation of decerebrate cats. Intrapontine microinjections of carbachol (0.25-1.0 microliter, 16 mg/ml) resulted in the tonic suppression of ventral root activity and a decrease in the amplitude of the Ia-monosynaptic reflex. An analysis of intracellular records from lumbar motoneurons during the suppression of motor activity induced by carbachol revealed a considerable decrease in input resistance and membrane time constant as well as a reduction in motoneuron excitability, as evidenced by a nearly twofold increase in rheobase. Discrete inhibitory postsynaptic potentials were also observed following carbachol administration. The changes in motoneuron properties (rheobase, input resistance, and membrane time constant), as well as the development of discrete inhibitory postsynaptic potentials, indicate that spinal cord motoneurons were postsynaptically inhibited following the pontine administration of carbachol. In addition, the inhibitory processes that arose after carbachol administration in the decerebrate cat were remarkably similar to those that are present during active sleep in the chronic cat. These findings suggest that the microinjection of carbachol into the pontine reticular formation activates the same brain stem-spinal cord system that is responsible for the postsynaptic inhibition of alpha-motoneurons that occurs during active sleep.