NMDA receptor activation strengthens weak electrical coupling in mammalian brain

Association of connexin36 with adherens junctions at mixed synapses and distinguishing electrophysiological features of those at mossy fiber terminals in rat ventral hippocampus

BACKGROUND

Granule cells in the hippocampus project axons to hippocampal CA3 pyramidal cells where they form large mossy fiber terminals. We have reported that these terminals contain the gap junction protein connexin36 (Cx36) specifically in the stratum lucidum of rat ventral hippocampus, thus creating morphologically mixed synapses that have the potential for dual chemical/electrical transmission.

METHODOLOGY

Here, we used various approaches to characterize molecular and electrophysiological relationships between the Cx36-containing gap junctions at mossy fiber terminals and their postsynaptic elements and to examine molecular relationships at mixed synapses in the brainstem.

RESULTS

In rat and human ventral hippocampus, many of these terminals, identified by their selective expression of vesicular zinc transporter-3 (ZnT3), displayed multiple, immunofluorescent Cx36-puncta representing gap junctions, which were absent at mossy fiber terminals in the dorsal hippocampus. In rat, these were found in close proximity to the protein constituents of adherens junctions (i.e., N-cadherin and nectin-1) that are structural hallmarks of mossy fiber terminals, linking these terminals to the dendritic shafts of CA3 pyramidal cells, thus indicating the loci of gap junctions at these contacts. Cx36-puncta were also associated with adherens junctions at mixed synapses in the brainstem, supporting emerging views of the structural organization of the adherens junction-neuronal gap junction complex. Electrophysiologically induced long-term potentiation (LTP) of field responses evoked by mossy fiber stimulation was greater in the ventral than dorsal hippocampus.

CONCLUSIONS

The electrical component of transmission at mossy fiber terminals may contribute to enhanced LTP responses in the ventral hippocampus.

Sources of Calcium at Connexin 36 Gap Junctions in the Retina

Synaptic plasticity is a fundamental feature of the CNS that controls the magnitude of signal transmission between communicating cells. Many electrical synapses exhibit substantial plasticity that modulates the degree of coupling within groups of neurons, alters the fidelity of signal transmission, or even reconfigures functional circuits. In several known examples, such plasticity depends on calcium and is associated with neuronal activity. Calcium-driven signaling is known to promote potentiation of electrical synapses in fish Mauthner cells, mammalian retinal AII amacrine cells, and inferior olive neurons, and to promote depression in thalamic reticular neurons. To measure local calcium dynamics in situ, we developed a transgenic mouse expressing a GCaMP calcium biosensor fused to Connexin 36 (Cx36) at electrical synapses. We examined the sources of calcium for activity-dependent plasticity in retina slices using confocal or Super-Resolution Radial Fluctuations imaging. More than half of Cx36-GCaMP gap junctions responded to puffs of glutamate with transient increases in fluorescence. The responses were strongly dependent on NMDA receptors, in keeping with known activity-dependent signaling in some amacrine cells. We also found that some responses depended on the activity of voltage-gated calcium channels, representing a previously unrecognized source of calcium to control retinal electrical synaptic plasticity. The high prevalence of calcium signals at electrical synapses in response to glutamate application indicates that a large fraction of electrical synapses has the potential to be regulated by neuronal activity. This provides a means to tune circuit connectivity dynamically based on local activity.

Diversity of cellular physiology and morphology of Purkinje cells in the adult zebrafish cerebellum

This study was designed to explore the functional circuitry of the adult zebrafish cerebellum, focusing on its Purkinje cells and using whole-cell patch recordings and single cell labeling in slice preparations. Following physiological characterizations, the recorded single cells were labeled for morphological identification. It was found that the zebrafish Purkinje cells are surprisingly diverse. Based on their physiology and morphology, they can be classified into at least three subtypes: Type I, a narrow spike cell, which fires only narrow Na⁺ spikes (<3 ms in duration), and has a single primary dendrite with an arbor restricted to the distal molecular layer; Type II, a broad spike cell, which fires broad Ca²⁺ spikes (5-7 ms in duration) and has a primary dendrite with limited branching in the inner molecular layer and then further radiates throughout the molecular layer; and Type III, a very broad spike cell, which fires very broad Ca²⁺ spikes (≥10 ms in duration) and has a dense proximal dendritic arbor that is either restricted to the inner molecular layer (Type IIIa), or radiates throughout the entire molecular layer (Type IIIb). The graded paired-pulse facilitation of these Purkinje cells' responses to parallel fiber activations and the all-or-none, paired-pulse depression of climbing fiber activation are largely similar to those reported for mammals. The labeled axon terminals of these Purkinje cells end locally, as reported for larval zebrafish. The present study provides evidence that the corresponding functional circuitry and information processing differ from what has been well-established in the mammalian cerebellum.

A model of working memory for encoding multiple items and ordered sequences exploiting the theta-gamma code

Recent experimental evidence suggests that oscillatory activity plays a pivotal role in the maintenance of information in working memory, both in rodents and humans. In particular, cross-frequency coupling between theta and gamma oscillations has been suggested as a core mechanism for multi-item memory. The aim of this work is to present an original neural network model, based on oscillating neural masses, to investigate mechanisms at the basis of working memory in different conditions. We show that this model, with different synapse values, can be used to address different problems, such as the reconstruction of an item from partial information, the maintenance of multiple items simultaneously in memory, without any sequential order, and the reconstruction of an ordered sequence starting from an initial cue. The model consists of four interconnected layers; synapses are trained using Hebbian and anti-Hebbian mechanisms, in order to synchronize features in the same items, and desynchronize features in different items. Simulations show that the trained network is able to desynchronize up to nine items without a fixed order using the gamma rhythm. Moreover, the network can replicate a sequence of items using a gamma rhythm nested inside a theta rhythm. The reduction in some parameters, mainly concerning the strength of GABAergic synapses, induce memory alterations which mimic neurological deficits. Finally, the network, isolated from the external environment (“imagination phase”) and stimulated with high uniform noise, can randomly recover sequences previously learned, and link them together by exploiting the similarity among items.

NMDA Enhances and Glutamate Attenuates Synchrony of Spontaneous Phase-Locked Locus Coeruleus Network Rhythm in Newborn Rat Brain Slices

Locus coeruleus (LC) neurons are controlled by glutamatergic inputs. Here, we studied in brain slices of neonatal rats NMDA and glutamate effects on phase-locked LC neuron spiking at ~1 Hz summating to ~0.2 s-lasting bell-shaped local field potential (LFP). NMDA: 10 μM accelerated LFP 1.7-fold, whereas 25 and 50 μM, respectively, increased its rate 3.2- and 4.6-fold while merging discrete events into 43 and 56% shorter oscillations. After 4–6 min, LFP oscillations stopped every 6 s for 1 s, resulting in ‘oscillation trains’. A dose of 32 μM depolarized neurons by 8.4 mV to cause 7.2-fold accelerated spiking at reduced jitter and enhanced synchrony with the LFP, as evident from cross-correlation. Glutamate: 25–50 μM made rhythm more irregular and the LFP pattern could transform into 2.7-fold longer-lasting multipeak discharge. In 100 μM, LFP amplitude and duration declined. In 25–50 μM, neurons depolarized by 5 mV to cause 3.7-fold acceleration of spiking that was less synchronized with LFP. Both agents: evoked ‘post-agonist depression’ of LFP that correlated with the amplitude and kinetics of V_m hyperpolarization. The findings show that accelerated spiking during NMDA and glutamate is associated with enhanced or attenuated LC synchrony, respectively, causing distinct LFP pattern transformations. Shaping of LC population discharge dynamics by ionotropic glutamate receptors potentially fine-tunes its influence on brain functions.

Autocrine Neuromodulation and Network Activity Patterns in the Locus Coeruleus of Newborn Rat Slices

Already in newborns, the locus coeruleus (LC) controls multiple brain functions and may have a complex organization as in adults. Our findings in newborn rat brain slices indicate that LC neurons (i) generate at ~1 Hz a ~0.3 s-lasting local field potential (LFP) comprising summated phase-locked single spike discharge, (ii) express intrinsic ‘pacemaker’ or ‘burster’ properties and (iii) receive solely excitatory or initially excitatory−secondary inhibitory inputs. μ-opioid or ɑ2 noradrenaline receptor agonists block LFP rhythm at 100−250 nM whereas slightly lower doses transform its bell-shaped pattern into slower crescendo-shaped multipeak bursts. GABAA and glycine receptors hyperpolarize LC neurons to abolish rhythm which remains though unaffected by blocking them. Rhythm persists also during ionotropic glutamate receptor (iGluR) inhibition whereas <10 mV depolarization during iGluR agonists accelerates spiking to cause subtype-specific fast (spindle-shaped) LFP oscillations. Similar modest neuronal depolarization causing a cytosolic Ca2+ rise occurs (without effect on neighboring astrocytes) during LFP acceleration by CNQX activating a TARP-AMPA-type iGluR complex. In contrast, noradrenaline lowers neuronal Ca2+ baseline via ɑ2 receptors, but evokes an ɑ1 receptor-mediated ‘concentric’ astrocytic Ca2+ wave. In summary, the neonatal LC has a complex (possibly modular) organization to enable discharge pattern transformations that might facilitate discrete actions on target circuits.

Function and Plasticity of Electrical Synapses in the Mammalian Brain: Role of Non-Junctional Mechanisms

Simple Summary

Relevant brain functions, such as perception, organization of behavior, and cognitive processes, are the outcome of information processing by neural circuits. Within these circuits, communication between neurons mainly relies on two modalities of synaptic transmission: chemical and electrical. Moreover, changes in the strength of these connections, aka synaptic plasticity, are believed to underlie processes of learning and memory, and its dysfunction has been suggested to underlie a variety of neurological disorders. While the relevance of chemical transmission and its plastic changes are known in great detail, analogous mechanisms and functional impact of their electrical counterparts were only recently acknowledged. In this article, we review the basic physical principles behind electrical transmission between neurons, the plethora of functional operations supported by this modality of neuron-to-neuron communication, as well as the basic principles of plasticity at these synapses.

Abstract

Electrical transmission between neurons is largely mediated by gap junctions. These junctions allow the direct flow of electric current between neurons, and in mammals, they are mostly composed of the protein connexin36. Circuits of electrically coupled neurons are widespread in these animals. Plus, experimental and theoretical evidence supports the notion that, beyond synchronicity, these circuits are able to perform sophisticated operations such as lateral excitation and inhibition, noise reduction, as well as the ability to selectively respond upon coincident excitatory inputs. Although once considered stereotyped and unmodifiable, we now know that electrical synapses are subject to modulation and, by reconfiguring neural circuits, these modulations can alter relevant operations. The strength of electrical synapses depends on the gap junction resistance, as well as on its functional interaction with the electrophysiological properties of coupled neurons. In particular, voltage and ligand gated channels of the non-synaptic membrane critically determine the efficacy of transmission at these contacts. Consistently, modulatory actions on these channels have been shown to represent relevant mechanisms of plasticity of electrical synaptic transmission. Here, we review recent evidence on the regulation of electrical synapses of mammals, the underlying molecular mechanisms, and the possible ways in which they affect circuit function.

Is the inferior olive central to essential tremor? Yes

We consider the question whether the inferior olive (IO) is required for essential tremor (ET). Much evidence shows that the olivocerebellar system is the main system capable of generating the widespread synchronous oscillatory Purkinje cell (PC) complex spike (CS) activity across the cerebellar cortex that would be capable of generating the type of bursting cerebellar output from the deep cerebellar nuclei (DCN) that could underlie tremor. Normally, synchronous CS activity primarily reflects the effective electrical coupling of IO neurons by gap junctions, and traditionally, ET research has focused on the hypothesis of increased coupling of IO neurons as the cause of hypersynchronous CS activity underlying tremor. However, recent pathology studies of brains from humans with ET and evidence from mutant mice, particularly the hotfoot17 mouse, that largely replicate the pathology of ET, suggest that the abnormal innervation of multiple Purkinje cells (PCs) by climbing fibers (Cfs) is related to tremor. In addition, ET brains show partial PC loss and axon terminal sprouting by surviving PCs. This may provide another mechanism for tremor. It is proposed that in ET, these three mechanisms may promote tremor. They all involve hypersynchronous DCN activity and an intact IO, but the level at which excessive synchronization occurs may be at the IO level (from abnormal afferent activity to this nucleus), the PC level (via aberrant Cfs), or the DCN level (via terminal PC collateral innervation).

GABABR Modulation of Electrical Synapses and Plasticity in the Thalamic Reticular Nucleus

Two distinct types of neuronal activity result in long-term depression (LTD) of electrical synapses, with overlapping biochemical intracellular signaling pathways that link activity to synaptic strength, in electrically coupled neurons of the thalamic reticular nucleus (TRN). Because components of both signaling pathways can also be modulated by GABA_B receptor activity, here we examined the impact of GABA_B receptor activation on the two established inductors of LTD in electrical synapses. Recording from patched pairs of coupled rat neurons in vitro, we show that GABA_B receptor inactivation itself induces a modest depression of electrical synapses and occludes LTD induction by either paired bursting or metabotropic glutamate receptor (mGluR) activation. GABA_B activation also occludes LTD from either paired bursting or mGluR activation. Together, these results indicate that afferent sources of GABA, such as those from the forebrain or substantia nigra to the reticular nucleus, gate the induction of LTD from either neuronal activity or afferent glutamatergic receptor activation. These results add to a growing body of evidence that the regulation of thalamocortical transmission and sensory attention by TRN is modulated and controlled by other brain regions. Significance: We show that electrical synapse plasticity is gated by GABA_B receptors in the thalamic reticular nucleus. This effect is a novel way for afferent GABAergic input from the basal ganglia to modulate thalamocortical relay and is a possible mediator of intra-TRN inhibitory effects.

Convergent NMDA receptor-Pannexin1 signaling pathways regulate the interaction of CaMKII with Connexin-36

Ca2+/calmodulin-dependent protein kinase II (CaMKII) binding and phosphorylation of mammalian connexin-36 (Cx36) potentiate electrical coupling. To explain the molecular mechanism of how Cx36 modifies plasticity at gap junctions, we investigated the roles of ionotropic N-methyl-D-aspartate receptors and pannexin1 (Panx1) channels in regulating Cx36 binding to CaMKII. Pharmacological interference and site-directed mutagenesis of protein interaction sites shows that NMDA receptor activation opens Cx36 channels, causing the Cx36- CaMKII binding complex to adopt a compact conformation. Ectopic Panx1 expression in a Panx1 knock-down cell line is required to restore CaMKII mediated opening of Cx36. Furthermore, blocking of Src-family kinase activation of Panx1 is sufficient to prevent the opening of Cx36 channels. Our research demonstrates that the efficacy of Cx36 channels requires convergent calcium-dependent signaling processes in which activation of ionotropic N-methyl-D-aspartate receptor, Src-family kinase, and Pannexin1 open Cx36. Our results add to the best of our knowledge a new twist to mounting evidence for molecular communication between these core components of electrical and chemical synapses.

NMDA receptor inhibition increases, synchronizes, and stabilizes the collective pancreatic beta cell activity: Insights through multilayer network analysis

NMDA receptors promote repolarization in pancreatic beta cells and thereby reduce glucose-stimulated insulin secretion. Therefore, NMDA receptors are a potential therapeutic target for diabetes. While the mechanism of NMDA receptor inhibition in beta cells is rather well understood at the molecular level, its possible effects on the collective cellular activity have not been addressed to date, even though proper insulin secretion patterns result from well-synchronized beta cell behavior. The latter is enabled by strong intercellular connectivity, which governs propagating calcium waves across the islets and makes the heterogeneous beta cell population work in synchrony. Since a disrupted collective activity is an important and possibly early contributor to impaired insulin secretion and glucose intolerance, it is of utmost importance to understand possible effects of NMDA receptor inhibition on beta cell functional connectivity. To address this issue, we combined confocal functional multicellular calcium imaging in mouse tissue slices with network science approaches. Our results revealed that NMDA receptor inhibition increases, synchronizes, and stabilizes beta cell activity without affecting the velocity or size of calcium waves. To explore intercellular interactions more precisely, we made use of the multilayer network formalism by regarding each calcium wave as an individual network layer, with weighted directed connections portraying the intercellular propagation. NMDA receptor inhibition stabilized both the role of wave initiators and the course of waves. The findings obtained with the experimental antagonist of NMDA receptors, MK-801, were additionally validated with dextrorphan, the active metabolite of the approved drug dextromethorphan, as well as with experiments on NMDA receptor KO mice. In sum, our results provide additional and new evidence for a possible role of NMDA receptor inhibition in treatment of type 2 diabetes and introduce the multilayer network paradigm as a general strategy to examine effects of drugs on connectivity in multicellular systems.

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.

Synaptic Reshaping and Neuronal Outcomes in the Temporal Lobe Epilepsy

Temporal lobe epilepsy (TLE) is one of the most common types of focal epilepsy, characterized by recurrent spontaneous seizures originating in the temporal lobe(s), with mesial TLE (mTLE) as the worst form of TLE, often associated with hippocampal sclerosis. Abnormal epileptiform discharges are the result, among others, of altered cell-to-cell communication in both chemical and electrical transmissions. Current knowledge about the neurobiology of TLE in human patients emerges from pathological studies of biopsy specimens isolated from the epileptogenic zone or, in a few more recent investigations, from living subjects using positron emission tomography (PET). To overcome limitations related to the use of human tissue, animal models are of great help as they allow the selection of homogeneous samples still presenting a more various scenario of the epileptic syndrome, the presence of a comparable control group, and the availability of a greater amount of tissue for in vitro/ex vivo investigations. This review provides an overview of the structural and functional alterations of synaptic connections in the brain of TLE/mTLE patients and animal models.

Recent Insights into the Rhythmogenic Core of the Locomotor CPG

In order for locomotion to occur, a complex pattern of muscle activation is required. For more than a century, it has been known that the timing and pattern of stepping movements in mammals are generated by neural networks known as central pattern generators (CPGs), which comprise multiple interneuron cell types located entirely within the spinal cord. A genetic approach has recently been successful in identifying several populations of spinal neurons that make up this neural network, as well as the specific role they play during stepping. In spite of this progress, the identity of the neurons responsible for generating the locomotor rhythm and the manner in which they are interconnected have yet to be deciphered. In this review, we summarize key features considered to be expressed by locomotor rhythm-generating neurons and describe the different genetically defined classes of interneurons which have been proposed to be involved.

Brain Disorders and Chemical Pollutants: A Gap Junction Link?

The incidence of brain pathologies has increased during last decades. Better diagnosis (autism spectrum disorders) and longer life expectancy (Parkinson's disease, Alzheimer's disease) partly explain this increase, while emerging data suggest pollutant exposures as a possible but still underestimated cause of major brain disorders. Taking into account that the brain parenchyma is rich in gap junctions and that most pollutants inhibit their function; brain disorders might be the consequence of gap-junctional alterations due to long-term exposures to pollutants. In this article, this hypothesis is addressed through three complementary aspects: (1) the gap-junctional organization and connexin expression in brain parenchyma and their function; (2) the effect of major pollutants (pesticides, bisphenol A, phthalates, heavy metals, airborne particles, etc.) on gap-junctional and connexin functions; (3) a description of the major brain disorders categorized as neurodevelopmental (autism spectrum disorders, attention deficit hyperactivity disorders, epilepsy), neurobehavioral (migraines, major depressive disorders), neurodegenerative (Parkinson's and Alzheimer's diseases) and cancers (glioma), in which both connexin dysfunction and pollutant involvement have been described. Based on these different aspects, the possible involvement of pollutant-inhibited gap junctions in brain disorders is discussed for prenatal and postnatal exposures.

Novel Prefrontal Synthesis Intervention Improves Language in Children with Autism

Prefrontal synthesis (PFS) is defined as the ability to juxtapose mental visuospatial objects at will. Paralysis of PFS may be responsible for the lack of comprehension of spatial prepositions, semantically-reversible sentences, and recursive sentences observed in 30 to 40% of individuals with autism spectrum disorder (ASD). In this report we present data from a three-year-long clinical trial of 6454 ASD children age 2 to 12 years, which were administered a PFS-targeting intervention. Tablet-based verbal and nonverbal exercises emphasizing mental-juxtaposition-of-objects were organized into an application called Mental Imagery Therapy for Autism (MITA). The test group included participants who completed more than one thousand exercises and made no more than one error per exercise. The control group was selected from the rest of participants by a matching procedure. Each test group participant was matched to the control group participant by age, gender, expressive language, receptive language, sociability, cognitive awareness, and health score at first evaluation using propensity score analysis. The test group showed a 2.2-fold improvement in receptive language score vs. control group (p < 0.0001) and a 1.4-fold improvement in expressive language (p = 0.0144). No statistically significant change was detected in other subscales not targeted by the exercises. These findings show that language acquisition improves after training PFS and that a further investigation of the PFS-targeting intervention in a randomized controlled study is warranted.

Activity-dependent long-term potentiation of electrical synapses in the mammalian thalamus

Activity-dependent changes of synapse strength have been extensively characterized at chemical synapses, but the relationship between physiological forms of activity and strength at electrical synapses remains poorly characterized and understood. For mammalian electrical synapses comprising hexamers of connexin36, physiological forms of neuronal activity in coupled pairs have thus far only been linked to long-term depression; activity that results in strengthening of electrical synapses has not yet been identified. Here, we performed dual whole-cell current-clamp recordings in acute slices of P11-P15 Sprague-Dawley rats of electrically coupled neurons of the thalamic reticular nucleus (TRN), a central brain area that regulates cortical input from and attention to the sensory surround. Using TTA-A2 to limit bursting, we show that tonic spiking in one neuron of a pair results in long-term potentiation of electrical synapses. We use experiments and computational modeling to show that the magnitude of plasticity expressed alters the functionality of the synapse. Potentiation is expressed asymmetrically, indicating that regulation of connectivity depends on the direction of use. Furthermore, calcium pharmacology and imaging indicate that potentiation depends on calcium flux. We thus propose a calcium-based activity rule for bidirectional plasticity of electrical synapse strength. Because electrical synapses dominate intra-TRN connectivity, these synapses and their activity-dependent modifications are key dynamic regulators of thalamic attention circuitry. More broadly, we speculate that bidirectional modifications of electrical synapses may be a widespread and powerful principle for ongoing, dynamic reorganization of neuronal circuitry across the brain.NEW & NOTEWORTHY This work reveals a physiologically relevant form of activity pairing in coupled neurons that results in long-term potentiation of mammalian electrical synapses. These findings, in combination with previous work, allow the authors to propose a bidirectional calcium-based rule for plasticity of electrical synapses, similar to those demonstrated for chemical synapses. These new insights inform the field on how electrical synapse plasticity may modify the neural circuits that incorporate them.

Increased Purkinje Cell Complex Spike and Deep Cerebellar Nucleus Synchrony as a Potential Basis for Syndromic Essential Tremor. A Review and Synthesis of the Literature

We review advances in understanding Purkinje cell (PC) complex spike (CS) physiology that suggest increased CS synchrony underlies syndromic essential tremor (ET). We searched PubMed for papers describing factors that affect CS synchrony or cerebellar circuits potentially related to tremor. Inferior olivary (IO) neurons are electrically coupled, with the degree of coupling controlled by excitatory and GABAergic inputs. Clusters of coupled IO neurons synchronize CSs within parasagittal bands via climbing fibers (Cfs). When motor cortex is stimulated in rats at varying frequencies, whisker movement occurs at ~10 Hz, correlated with synchronous CSs, indicating that the IO/CS oscillatory rhythm gates movement frequency. Intra-IO injection of the GABA_A receptor antagonist picrotoxin increases CS synchrony, increases whisker movement amplitude, and induces tremor. Harmaline and 5-HT_2a receptor activation also increase IO coupling and CS synchrony and induce tremor. The hotfoot17 mouse displays features found in ET brains, including cerebellar GluRδ2 deficiency and abnormal PC Cf innervation, with IO- and PC-dependent cerebellar oscillations and tremor likely due to enhanced CS synchrony. Heightened coupling within the IO oscillator leads, through its dynamic control of CS synchrony, to increased movement amplitude and, when sufficiently intense, action tremor. Increased CS synchrony secondary to aberrant Cf innervation of multiple PCs likely also underlies hotfoot17 tremor. Deep cerebellar nucleus (DCN) hypersynchrony may occur secondary to increased CS synchrony but might also occur from PC axonal terminal sprouting during partial PC loss. Through these combined mechanisms, increased CS/DCN synchrony may plausibly underlie syndromic ET.

Calmodulin Binding to Connexin 35: Specializations to Function as an Electrical Synapse

Calmodulin binding is a nearly universal property of gap junction proteins, imparting a calcium-dependent uncoupling behavior that can serve in an emergency to decouple a stressed cell from its neighbors. However, gap junctions that function as electrical synapses within networks of neurons routinely encounter large fluctuations in local cytoplasmic calcium concentration; frequent uncoupling would be impractical and counterproductive. We have studied the properties and functional consequences of calmodulin binding to the electrical synapse protein Connexin 35 (Cx35 or gjd2b), homologous to mammalian Connexin 36 (Cx36 or gjd2). We find that specializations in Cx35 calmodulin binding sites make it relatively impervious to moderately high levels of cytoplasmic calcium. Calmodulin binding to a site in the C-terminus causes uncoupling when calcium reaches low micromolar concentrations, a behavior prevented by mutations that eliminate calmodulin binding. However, milder stimuli promote calcium/calmodulin-dependent protein kinase II activity that potentiates coupling without interference from calmodulin binding. A second calmodulin binding site in the end of the Cx35 cytoplasmic loop, homologous to a calmodulin binding site present in many connexins, binds calmodulin with very low affinity and stoichiometry. Together, the calmodulin binding sites cause Cx35 to uncouple only at extreme levels of intracellular calcium.

Electrical coupling controls dimensionality and chaotic firing of inferior olive neurons

We previously proposed, on theoretical grounds, that the cerebellum must regulate the dimensionality of its neuronal activity during motor learning and control to cope with the low firing frequency of inferior olive neurons, which form one of two major inputs to the cerebellar cortex. Such dimensionality regulation is possible via modulation of electrical coupling through the gap junctions between inferior olive neurons by inhibitory GABAergic synapses. In addition, we previously showed in simulations that intermediate coupling strengths induce chaotic firing of inferior olive neurons and increase their information carrying capacity. However, there is no in vivo experimental data supporting these two theoretical predictions. Here, we computed the levels of synchrony, dimensionality, and chaos of the inferior olive code by analyzing in vivo recordings of Purkinje cell complex spike activity in three different coupling conditions: carbenoxolone (gap junctions blocker), control, and picrotoxin (GABA-A receptor antagonist). To examine the effect of electrical coupling on dimensionality and chaotic dynamics, we first determined the physiological range of effective coupling strengths between inferior olive neurons in the three conditions using a combination of a biophysical network model of the inferior olive and a novel Bayesian model averaging approach. We found that effective coupling co-varied with synchrony and was inversely related to the dimensionality of inferior olive firing dynamics, as measured via a principal component analysis of the spike trains in each condition. Furthermore, for both the model and the data, we found an inverted U-shaped relationship between coupling strengths and complexity entropy, a measure of chaos for spiking neural data. These results are consistent with our hypothesis according to which electrical coupling regulates the dimensionality and the complexity in the inferior olive neurons in order to optimize both motor learning and control of high dimensional motor systems by the cerebellum.

Computational theory suggests that the cerebellum must decrease the dimensionality of its neuronal activity to learn and control high dimensional motor systems effectively, while being constrained by the low firing frequency of inferior olive neurons, one of the two major source of input signals to the cerebellum. We previously proposed that the cerebellum adaptively controls the dimensionality of inferior olive firing by adjusting the level of synchrony and that such control is made possible by modulating the electrical coupling strength between inferior olive neurons. Here, we developed a novel method that uses a biophysical model of the inferior olive to accurately estimate the effective coupling strengths between inferior olive neurons from in vivo recordings of spike activity in three different coupling conditions. We found that high coupling strengths induce synchronous firing and decrease the dimensionality of inferior olive firing dynamics. In contrast, intermediate coupling strengths lead to chaotic firing and increase the dimensionality of the firing dynamics. Thus, electrical coupling is a feasible mechanism to control dimensionality and chaotic firing of inferior olive neurons. In sum, our results provide insights into possible mechanisms underlying cerebellar function and, in general, a biologically plausible framework to control the dimensionality of neural coding.

NMDAR-mediated modulation of gap junction circuit regulates olfactory learning in C. elegans

Modulation of gap junction-mediated electrical synapses is a common form of neural plasticity. However, the behavioral consequence of the modulation and the underlying molecular cellular mechanisms are not understood. Here, using a C. elegans circuit of interneurons that are connected by gap junctions, we show that modulation of the gap junctions facilitates olfactory learning. Learning experience weakens the gap junctions and induces a repulsive sensory response to the training odorants, which together decouple the responses of the interneurons to the training odorants to generate learned olfactory behavior. The weakening of the gap junctions results from downregulation of the abundance of a gap junction molecule, which is regulated by cell-autonomous function of the worm homologs of a NMDAR subunit and CaMKII. Thus, our findings identify the function of a gap junction modulation in an in vivo model of learning and a conserved regulatory pathway underlying the modulation.

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.

Synergistic excitability plasticity in cerebellar functioning

The cerebellum, a universal processor for sensory acquisition and internal models, and its association with synaptic and nonsynaptic plasticity have been envisioned as the biological correlates of learning, perception, and even thought. Indeed, the cerebellum is no longer considered merely as the locus of motor coordination and its learning. Here, we introduce the mechanisms underlying the induction of multiple types of plasticity in cerebellar circuit and give an overview focusing on the plasticity of nonsynaptic intrinsic excitability. The discovery of long-term potentiation of synaptic responsiveness in hippocampal neurons led investigations into changes of their intrinsic excitability. This activity-dependent potentiation of neuronal excitability is distinct from that of synaptic efficacy. Systematic examination of excitability plasticity has indicated that the modulation of various types of Ca²⁺ - and voltage-dependent K⁺ channels underlies the phenomenon, which is also triggered by immune activity. Intrinsic plasticity is expressed specifically on dendrites and modifies the integrative processing and filtering effect. In Purkinje cells, modulation of the discordance of synaptic current on soma and dendrite suggested a novel type of cellular learning mechanism. This property enables a plausible synergy between synaptic efficacy and intrinsic excitability, by amplifying electrical conductivity and influencing the polarity of bidirectional synaptic plasticity. Furthermore, the induction of intrinsic plasticity in the cerebellum correlates with motor performance and cognitive processes, through functional connections from the cerebellar nuclei to neocortex and associated regions: for example, thalamus and midbrain. Taken together, recent advances in neuroscience have begun to shed light on the complex functioning of nonsynaptic excitability and the synergy.

Localized Calcium Signaling and the Control of Coupling at Cx36 Gap Junctions

A variety of electrical synapses are capable of activity-dependent plasticity, including both activity-dependent potentiation and activity-dependent depression. In several types of neurons, activity-dependent electrical synapse plasticity depends on changes in the local Ca2+ environment. To enable study of local Ca2+ signaling that regulates plasticity, we developed a GCaMP Ca2+ biosensor fused to the electrical synapse protein Connexin 36 (Cx36). Cx36-GCaMP transfected into mammalian cell cultures formed gap junctions at cell-cell boundaries and supported Neurobiotin tracer coupling that was regulated by protein kinase A signaling in the same way as Cx36. Cx36-GCaMP gap junctions robustly reported local Ca2+ increases in response to addition of a Ca2+ ionophore with increases in fluorescence that recovered during washout. Recovery was strongly dependent on Na+-Ca2+ exchange activity. In cells transfected with NMDA receptor subunits, Cx36-GCaMP revealed transient and concentration-dependent increases in local Ca2+ on brief application of glutamate. In HeLa cells, glutamate application increased Cx36-GCaMP tracer coupling through a mechanism that depended in part on Ca2+, calmodulin-dependent protein kinase II (CaMKII) activity. This potentiation of coupling did not require exogenous expression of glutamate receptors, but could be accomplished by endogenously expressed glutamate receptors with pharmacological characteristics reminiscent of NMDA and kainate receptors. Analysis of RNA Sequencing data from HeLa cells confirmed expression of NMDA receptor subunits NR1, NR2C, and NR3B. In summary, Cx36-GCaMP is an effective tool to measure changes in the Ca2+ microenvironment around Cx36 gap junctions. Furthermore, HeLa cells can serve as a model system to study glutamate receptor-driven potentiation of electrical synapses.

Using subthreshold events to characterize the functional architecture of the electrically coupled inferior olive network

The electrical connectivity in the inferior olive (IO) nucleus plays an important role in generating well-timed spiking activity. Here we combined electrophysiological and computational approaches to assess the functional organization of the IO nucleus in mice. Spontaneous fast and slow subthreshold events were commonly encountered during in vitro recordings. We show that whereas the fast events represent intrinsic regenerative activity, the slow events reflect the electrical connectivity between neurons (‘spikelets’). Recordings from cell pairs revealed the synchronized occurrence of distinct groups of spikelets; their rate and distribution enabled an accurate estimation of the number of connected cells and is suggestive of a clustered organization. This study thus provides a new perspective on the functional and structural organization of the olivary nucleus and a novel experimental and theoretical approach to study electrically coupled networks.

Connexins-Based Hemichannels/Channels and Their Relationship with Inflammation, Seizures and Epilepsy

Connexins (Cxs) are a family of 21 protein isoforms, eleven of which are expressed in the central nervous system, and they are found in neurons and glia. Cxs form hemichannels (connexons) and channels (gap junctions/electric synapses) that permit functional and metabolic coupling between neurons and astrocytes. Altered Cx expression and function is involved in inflammation and neurological diseases. Cxs-based hemichannels and channels have a relevance to seizures and epilepsy in two ways: First, this pathological condition increases the opening probability of hemichannels in glial cells to enable gliotransmitter release, sustaining the inflammatory process and exacerbating seizure generation and epileptogenesis, and second, the opening of channels favors excitability and synchronization through coupled neurons. These biological events highlight the global pathological mechanism of epilepsy, and the therapeutic potential of Cxs-based hemichannels and channels. Therefore, this review describes the role of Cxs in neuroinflammation and epilepsy and examines how the blocking of channels and hemichannels may be therapeutic targets of anti-convulsive and anti-epileptic treatments.

CaM Kinase: Still Inspiring at 40

The Ca²⁺/calmodulin (CaM)-dependent protein kinase II (CaMKII) was touted as a memory molecule, even before its involvement in long-term potentiation (LTP) was shown. The enzyme has not disappointed, with subsequent demonstrations of remarkable structural and regulatory properties. Its neuronal functions now extend to long-term depression (LTD), and last year saw the first direct evidence for memory storage by CaMKII. Although CaMKII may have taken the spotlight, it is a member of a large family of diverse and interesting CaM kinases. Our aim is to place CaMKII in context of the other CaM kinases and then review certain aspects of this kinase that are of current interest.

Localization of Retinal Ca2+/Calmodulin-Dependent Kinase II-β (CaMKII-β) at Bipolar Cell Gap Junctions and Cross-Reactivity of a Monoclonal Anti-CaMKII-β Antibody With Connexin36

Neuronal gap junctions formed by connexin36 (Cx36) and chemical synapses share striking similarities in terms of plasticity. Ca²⁺/calmodulin-dependent protein kinase II (CaMKII), an enzyme known to induce memory formation at chemical synapses, has recently been described to potentiate electrical coupling in the retina and several other brain areas via phosphorylation of Cx36. The contribution of individual CaMKII isoforms to this process, however, remains unknown. We recently identified CaMKII-β at electrical synapses in the mouse retina. Now, we set out to identify cell types containing Cx36 gap junctions that also express CaMKII-β. To ensure precise description, we first tested the specificity of two commercially available antibodies on CaMKII-β-deficient retinas. We found that a polyclonal antibody was highly specific for CaMKII-β. However, a monoclonal antibody (CB-β-1) recognized CaMKII-β but also cross-reacted with the C-terminal tail of Cx36, making localization analyses with this antibody inaccurate. Using the polyclonal antibody, we identified strong CaMKII-β expression in bipolar cell terminals that were secretagogin- and HCN1-positive and thus represent terminals of type 5 bipolar cells. In these terminals, a small fraction of CaMKII-β also colocalized with Cx36. A similar pattern was observed in putative type 6 bipolar cells although there, CaMKII expression seemed less pronounced. Next, we tested whether CaMKII-β influenced the Cx36 expression in bipolar cell terminals by quantifying the number and size of Cx36-immunoreactive puncta in CaMKII-β-deficient retinas. However, we found no significant differences between the genotypes, indicating that CaMKII-β is not necessary for the formation and maintenance of Cx36-containing gap junctions in the retina. In addition, in wild-type retinas, we observed frequent association of Cx36 and CaMKII-β with synaptic ribbons, i.e., chemical synapses, in bipolar cell terminals. This arrangement resembled the composition of mixed synapses found for example in Mauthner cells, in which electrical coupling is regulated by glutamatergic activity. Taken together, our data imply that CaMKII-β may fulfill several functions in bipolar cell terminals, regulating both Cx36-containing gap junctions and ribbon synapses and potentially also mediating cross-talk between these two types of bipolar cell outputs.

Quasiperiodic rhythms of the inferior olive

Inferior olivary activity causes both short-term and long-term changes in cerebellar output underlying motor performance and motor learning. Many of its neurons engage in coherent subthreshold oscillations and are extensively coupled via gap junctions. Studies in reduced preparations suggest that these properties promote rhythmic, synchronized output. However, the interaction of these properties with torrential synaptic inputs in awake behaving animals is not well understood. Here we combine electrophysiological recordings in awake mice with a realistic tissue-scale computational model of the inferior olive to study the relative impact of intrinsic and extrinsic mechanisms governing its activity. Our data and model suggest that if subthreshold oscillations are present in the awake state, the period of these oscillations will be transient and variable. Accordingly, by using different temporal patterns of sensory stimulation, we found that complex spike rhythmicity was readily evoked but limited to short intervals of no more than a few hundred milliseconds and that the periodicity of this rhythmic activity was not fixed but dynamically related to the synaptic input to the inferior olive as well as to motor output. In contrast, in the long-term, the average olivary spiking activity was not affected by the strength and duration of the sensory stimulation, while the level of gap junctional coupling determined the stiffness of the rhythmic activity in the olivary network during its dynamic response to sensory modulation. Thus, interactions between intrinsic properties and extrinsic inputs can explain the variations of spiking activity of olivary neurons, providing a temporal framework for the creation of both the short-term and long-term changes in cerebellar output.

Activity of the inferior olive, transmitted via climbing fibers to the cerebellum, regulates initiation and amplitude of movements, signals unexpected sensory feedback, and directs cerebellar learning. It is characterized by widespread subthreshold oscillations and synchronization promoted by strong electrotonic coupling. In brain slices, subthreshold oscillations gate which inputs can be transmitted by inferior olivary neurons and which will not—dependent on the phase of the oscillation. We tested whether the subthreshold oscillations had a measurable impact on temporal patterning of climbing fiber activity in intact, awake mice. We did so by recording neural activity of the postsynaptic Purkinje cells, in which complex spike firing faithfully represents climbing fiber activity. For short intervals (<300 ms) many Purkinje cells showed spontaneously rhythmic complex spike activity. However, our experiments designed to evoke conditional responses indicated that complex spikes are not predominantly predicated on stimulus history. Our realistic network model of the inferior olive explains the experimental observations via continuous phase modulations of the subthreshold oscillations under the influence of synaptic fluctuations. We conclude that complex spike activity emerges from a quasiperiodic rhythm that is stabilized by electrotonic coupling between its dendrites, yet dynamically influenced by the status of their synaptic inputs.

A Direct Comparison of Different Measures for the Strength of Electrical Synapses

During the last decades it became increasingly evident that electrical synapses are capable of activity-dependent plasticity. However, measuring the actual strength of electrical transmission remains difficult. Usually changes in coupling strength can only be inferred indirectly from measures such as the coupling coefficient and the coupling conductance. Because these are affected by both junctional and non-junctional conductance, plastic changes can potentially be due to both components. Furthermore, these techniques also require the blocking of chemical transmission, so that processes that involve crosstalk between chemical and electrical synapses will be suppressed. To directly examine the magnitude of errors that can occur, we use dual whole-cell current- and voltage-clamp recordings from the soma of the pair of easily accessible, electrically coupled Retzius cells in the leech to simultaneously determine coupling coefficients, coupling conductances and directly measured gap junctional currents. We present the first direct and comparative analysis of gap junction conductance using all three methods and analyze how each method would characterize the response of gap junctions to serotonin. The traditional coupling coefficients showed severe deficits in assessing the symmetry and strength of electrical synapses. These were reduced when coupling conductances were determined and were absent in the direct method. Additionally, both coupling coefficient and coupling conductance caused large and systematic errors in assessing the size and time course of the serotonin-induced changes of gap junctional currents. Most importantly, both measurements can easily be misinterpreted as implying long-term gap junctional plasticity, although the direct measurements confirm its absence. We thus show directly that coupling coefficients and coupling conductances can severely confound plastic changes in membrane and junctional conductance. Wherever possible, voltage clamp measurements should be chosen to accurately characterize the timing and strength of plasticity of electrical synapses. However, we also demonstrate that coupling coefficients can still yield a qualitatively correct picture when amended by independent measurements of the course of membrane resistance during the experiments.

Electrical synapses regulate both subthreshold integration and population activity of principal cells in response to transient inputs within canonical feedforward circuits

As information about the world traverses the brain, the signals exchanged between neurons are passed and modulated by synapses, or specialized contacts between neurons. While neurotransmitter-based synapses tend to exert either excitatory or inhibitory pulses of influence on the postsynaptic neuron, electrical synapses, composed of plaques of gap junction channels, continuously transmit signals that can either excite or inhibit a coupled neighbor. A growing body of evidence indicates that electrical synapses, similar to their chemical counterparts, are modified in strength during physiological neuronal activity. The synchronizing role of electrical synapses in neuronal oscillations has been well established, but their impact on transient signal processing in the brain is much less understood. Here we constructed computational models based on the canonical feedforward neuronal circuit and included electrical synapses between inhibitory interneurons. We provided discrete closely-timed inputs to the circuits, and characterize the influence of electrical synapse strength on both subthreshold summation and spike trains in the output neuron. Our simulations highlight the diverse and powerful roles that electrical synapses play even in simple circuits. Because these canonical circuits are represented widely throughout the brain, we expect that these are general principles for the influence of electrical synapses on transient signal processing across the brain.

The roles that electrical synapses play in neural oscillations, network synchronization and rhythmicity are well established, but their roles in neuronal processing of transient inputs are much less understood. Here, we used computational models of canonical feedforward circuits and networks to investigate how electrical synapses regulate the flow of transient signals passing through those circuits. We show that because the influence of electrical synapses on coupled neighbors can be either inhibitory or excitatory, their role in network information processing is heterogeneous, and powerful. Because electrical synapses between interneurons are widespread across the brain, and in addition to a growing body of evidence for their activity-dependent plasticity, we expect the effects we describe here to play a substantial role in how the brain processes incoming sensory inputs.

Targeting gap junction in epilepsy: Perspectives and challenges

Gap junctions (GJs) are multiple cellular intercellular connections that allow ions to pass directly into the cytoplasm of neighboring cells. Electrical coupling mediated by GJs plays a role in the generation of highly synchronous electrical activity. Accumulative investigations show that GJs in the brain are involved in the generation, synchronization and maintenance of seizure events. At the same time, GJ blockers exert potent curative potential on epilepsy in vivo or in vitro. This review aims to shed light on the role of GJs in epileptogenesis. Targeting GJs is likely to be served as a novel therapeutic approach on epileptic patients.

Long-term potentiation in an innexin-based electrical synapse

Electrical synapses are formed by two unrelated gap junction protein families, the primordial innexins (invertebrates) or the connexins (vertebrates). Although molecularly different, innexin- and connexin-based electrical synapses are strikingly similar in their membrane topology. However, it remains unclear if this similarity extends also to more sophisticated functions such as long-term potentiation which is only known in connexin-based synapses. Here we show that this capacity is not unique to connexin-based synapses. Using a method that allowed us to quantitatively measure gap-junction conductance we provide the first and unequivocal evidence of long-term potentiation in an innexin-based electrical synapse. Our findings suggest that long-term potentiation is a property that has likely existed already in ancestral gap junctions. They therefore could provide a highly potent system to dissect shared molecular mechanisms of electrical synapse plasticity.

Structural and Intermolecular Associations Between Connexin36 and Protein Components of the Adherens Junction-Neuronal Gap Junction Complex

Intimate structural and functional relationships between gap junctions and adherens junctions have been demonstrated in peripheral tissues, but have not been thoroughly examined in the central nervous system, where adherens junctions are often found in close proximity to neuronal gap junctions. Here, we used immunofluorescence approaches to document the localization of various protein components of adherens junctions in relation to those that we have previously reported to occur at electrical synapses formed by neuronal gap junctions composed of connexin36 (Cx36). The adherens junction constituents N-cadherin and nectin-1 were frequently found to localize near or overlap with Cx36-containing gap junctions in several brain regions examined. This was also true of the adherens junction-associated proteins α-catenin and β-catenin, as well as the proteins zonula occludens-1 and AF6 (aka, afadin) that were reported constituents of both adherens junctions and gap junctions. The deployment of the protein constituents of these junctions was especially striking at somatic contacts between primary afferent neurons in the mesencephalic trigeminal nucleus (MesV), where the structural components of adherens junctions appeared to be maintained in connexin36 null mice. These results support emerging views concerning the multi-molecular composition of electrical synapses and raise possibilities for various structural and functional protein-protein interactions at what now can be considered the adherens junction-neuronal gap junction complex. Further, the results point to intracellular signaling pathways that could potentially contribute to the assembly, maintenance and turnover of this complex, as well as to the dynamic nature of neuronal communication at electrical synapses.

Gap junction plasticity as a mechanism to regulate network-wide oscillations

Cortical oscillations are thought to be involved in many cognitive functions and processes. Several mechanisms have been proposed to regulate oscillations. One prominent but understudied mechanism is gap junction coupling. Gap junctions are ubiquitous in cortex between GABAergic interneurons. Moreover, recent experiments indicate their strength can be modified in an activity-dependent manner, similar to chemical synapses. We hypothesized that activity-dependent gap junction plasticity acts as a mechanism to regulate oscillations in the cortex. We developed a computational model of gap junction plasticity in a recurrent cortical network based on recent experimental findings. We showed that gap junction plasticity can serve as a homeostatic mechanism for oscillations by maintaining a tight balance between two network states: asynchronous irregular activity and synchronized oscillations. This homeostatic mechanism allows for robust communication between neuronal assemblies through two different mechanisms: transient oscillations and frequency modulation. This implies a direct functional role for gap junction plasticity in information transmission in cortex.

Inferior Olive HCN1 Channels Coordinate Synaptic Integration and Complex Spike Timing

Cerebellar climbing-fiber-mediated complex spikes originate from neurons in the inferior olive (IO), are critical for motor coordination, and are central to theories of cerebellar learning. Hyperpolarization-activated cyclic-nucleotide-gated (HCN) channels expressed by IO neurons have been considered as pacemaker currents important for oscillatory and resonant dynamics. Here, we demonstrate that in vitro, network actions of HCN1 channels enable bidirectional glutamatergic synaptic responses, while local actions of HCN1 channels determine the timing and waveform of synaptically driven action potentials. These roles are distinct from, and may complement, proposed pacemaker functions of HCN channels. We find that in behaving animals HCN1 channels reduce variability in the timing of cerebellar complex spikes, which serve as a readout of IO spiking. Our results suggest that spatially distributed actions of HCN1 channels enable the IO to implement network-wide rules for synaptic integration that modulate the timing of cerebellar climbing fiber signals.

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HCN1 channels in IO neurons control synaptic response and spiking activity

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Network actions of HCN1 channels enable bidirectional synaptic responses

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Local actions of HCN1 channels control spike timing and spikelet number

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In awake mice, HCN1 channels reduce timing variability of cerebellar complex spikes

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.

Differential Distribution of Retinal Ca2+/Calmodulin-Dependent Kinase II (CaMKII) Isoforms Indicates CaMKII-β and -δ as Specific Elements of Electrical Synapses Made of Connexin36 (Cx36)

AII amacrine cells are essential interneurons of the primary rod pathway and transmit rod-driven signals to ON cone bipolar cells to enable scotopic vision. Gap junctions made of connexin36 (Cx36) mediate electrical coupling among AII cells and between AII cells and ON cone bipolar cells. These gap junctions underlie a remarkable degree of plasticity and are modulated by different signaling cascades. In particular, Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) has been characterized as an important regulator of Cx36, capable of potentiating electrical coupling in AII cells. However, it is unclear which CaMKII isoform mediates this effect. To obtain a more detailed understanding of the isoform composition of CaMKII at retinal gap junctions, we analyzed the retinal distribution of all four CaMKII isoforms using confocal microscopy. These experiments revealed a differential distribution of CaMKII isoforms: CaMKII-α was strongly expressed in starburst amacrine cells, which are known to lack electrical coupling. CaMKII-β was abundant in OFF bipolar cells, which form electrical synapses in the outer and the inner retina. CaMKII-γ was diffusely distributed across the entire retina and could not be assigned to a specific cell type. CaMKII-δ labeling was evident in bipolar and AII amacrine cells, which contain the majority of Cx36-immunoreactive puncta in the inner retina. We double-labeled retinas for Cx36 and the four CaMKII isoforms and revealed that the composition of the CaMKII enzyme differs between gap junctions in the outer and the inner retina: in the outer retina, only CaMKII-β colocalized with Cx36-containing gap junctions, whereas in the inner retina, CaMKII-β and -δ colocalized with Cx36. This finding suggests that gap junctions in the inner and the outer retina may be regulated differently although they both contain the same connexin. Taken together, our study identifies CaMKII-β and -δ as Cx36-specific regulators in the mouse retina with CaMKII-δ regulating the primary rod pathway.

Modulation of Murine Olivary Connexin 36 Gap Junctions by PKA and CaMKII

The inferior olive (IO) is a nucleus located in the brainstem and it is part of the olivo-cerebellar loop. This circuit plays a fundamental role in generation and acquisition of coherent motor patterns and it relies on synchronous activation of groups of Purkinje cells (PC) in the cerebellar cortex. IO neurons integrate their intrinsic oscillatory activity with excitatory inputs coming from the somatosensory system and inhibitory feedback coming from the cerebellar nuclei. Alongside these chemical synaptic inputs, IO neurons are coupled to one another via connexin 36 (Cx36) containing gap junctions (GJs) that create a functional syncytium between neurons. Communication between olivary neurons is regulated by these GJs and their correct functioning contributes to coherent oscillations in the IO and proper motor learning. Here, we explore the cellular pathways that can regulate the coupling between olivary neurons. We combined in vitro electrophysiology and immunohistochemistry (IHC) on mouse acute brain slices to unravel the pathways that regulate olivary coupling. We found that enhancing the activity of the protein kinase A (PKA) pathway and blocking the Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) pathway can both down-regulate the size of the coupled network. However, these two kinases follow different mechanisms of action. Our results suggest that activation of the PKA pathway reduces the opening probability of the Cx36 GJs, whereas inhibition of the CaMKII pathway reduces the number of Cx36 GJs. The low densities of Cx36 proteins and electrical synapses in βCaMKII knock-out mice point towards an essential role for this protein kinase in regulating the density of GJs in the IO. Thus, the level of olivary coupling is a dynamic process and regulated by a variety of enzymes modulating GJs expression, docking and activity.

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.

The Potential Role of Gap Junctional Plasticity in the Regulation of State

Electrical synapses are the functional correlate of gap junctions and allow transmission of small molecules and electrical current between coupled neurons. Instead of static pores, electrical synapses are actually plastic, similar to chemical synapses. In the thalamocortical system, gap junctions couple inhibitory neurons that are similar in their biochemical profile, morphology, and electrophysiological properties. We postulate that electrical synaptic plasticity among inhibitory neurons directly interacts with the switching between different firing patterns in a state-dependent and type-dependent manner. In neuronal networks, electrical synapses may function as a modifiable resonance feedback system that enables stable oscillations. Furthermore, the plasticity of electrical synapses may play an important role in regulation of state, synchrony, and rhythmogenesis in the mammalian thalamocortical system, similar to chemical synaptic plasticity. Based on their plasticity, rich diversity, and specificity, electrical synapses are thus likely to participate in the control of consciousness and attention.

The Roles of the Olivocerebellar Pathway in Motor Learning and Motor Control. A Consensus Paper

For many decades, the predominant view in the cerebellar field has been that the olivocerebellar system's primary function is to induce plasticity in the cerebellar cortex, specifically, at the parallel fiber-Purkinje cell synapse. However, it has also long been proposed that the olivocerebellar system participates directly in motor control by helping to shape ongoing motor commands being issued by the cerebellum. Evidence consistent with both hypotheses exists; however, they are often investigated as mutually exclusive alternatives. In contrast, here, we take the perspective that the olivocerebellar system can contribute to both the motor learning and motor control functions of the cerebellum and might also play a role in development. We then consider the potential problems and benefits of it having multiple functions. Moreover, we discuss how its distinctive characteristics (e.g., low firing rates, synchronization, and variable complex spike waveforms) make it more or less suitable for one or the other of these functions, and why having multiple functions makes sense from an evolutionary perspective. We did not attempt to reach a consensus on the specific role(s) the olivocerebellar system plays in different types of movements, as that will ultimately be determined experimentally; however, collectively, the various contributions highlight the flexibility of the olivocerebellar system, and thereby suggest that it has the potential to act in both the motor learning and motor control functions of the cerebellum.

Design principles of electrical synaptic plasticity

Essentially all animals with nervous systems utilize electrical synapses as a core element of communication. Electrical synapses, formed by gap junctions between neurons, provide rapid, bidirectional communication that accomplishes tasks distinct from and complementary to chemical synapses. These include coordination of neuron activity, suppression of voltage noise, establishment of electrical pathways that define circuits, and modulation of high order network behavior. In keeping with the omnipresent demand to alter neural network function in order to respond to environmental cues and perform tasks, electrical synapses exhibit extensive plasticity. In some networks, this plasticity can have dramatic effects that completely remodel circuits or remove the influence of certain cell types from networks. Electrical synaptic plasticity occurs on three distinct time scales, ranging from milliseconds to days, with different mechanisms accounting for each. This essay highlights principles that dictate the properties of electrical coupling within networks and the plasticity of the electrical synapses, drawing examples extensively from retinal networks.

New insights into olivo-cerebellar circuits for learning from a small training sample

Artificial intelligence such as deep neural networks exhibited remarkable performance in simulated video games and 'Go'. In contrast, most humanoid robots in the DARPA Robotics Challenge fell down to ground. The dramatic contrast in performance is mainly due to differences in the amount of training data, which is huge and small, respectively. Animals are not allowed with millions of the failed trials, which lead to injury and death. Humans fall only several thousand times before they balance and walk. We hypothesize that a unique closed-loop neural circuit formed by the Purkinje cells, the cerebellar deep nucleus and the inferior olive in and around the cerebellum and the highest density of gap junctions, which regulate synchronous activities of the inferior olive nucleus, are computational machinery for learning from a small sample. We discuss recent experimental and computational advances associated with this hypothesis.

Electrical Coupling and Synchronized Subthreshold Oscillations in the Inferior Olive of the Rhesus Macaque

Inferior olive (IO) neurons are critical for motor coordination and exhibit oscillations in membrane potential that are subthreshold for spiking. The prevalence, coherence, and continuity of those subthreshold oscillations (STOs) depend upon resonant interactions between neighboring neurons supported by electrical coupling. Many studies of the olivocerebellar system in rodents, in which STOs were related to tremor, whisking, and licking, fueled a debate over whether IO STOs were relevant for primates whose repertoire of movement is generally less periodic. The debate was never well informed due to the lack of a direct examination of the physiological properties of primate IO neurons. Here, we obtained dual patch-clamp recordings of neighboring IO neurons from young adult macaques in brainstem slices and compared them to identical recordings from rats. Macaque IO neurons exhibited an equivalent prevalence of continuous STOs as rats (45 vs 54%, respectively). However, macaque STOs were slower (1-4 Hz) and did not overlap with the dominant 4-9 Hz frequency of rats. The slower STO frequency of macaques was at least partially due to a prolonged membrane time constant and increased membrane capacitance that could be attributed to stronger electrical coupling and greater total dendritic length. The presence of synchronized STOs in the IO of adult macaques, coincident with strong and prevalent electrical coupling, answers a fundamental outstanding question in cerebellar neuroscience and is consistent with a prominent role for synchronized oscillation in primate sensory-motor control.

SIGNIFICANCE STATEMENT

It was debated whether inferior olive (IO) neurons of primates behave as synchronized oscillators as was found for rodents using intracellular, optical, and multielectrode recordings. An inability to resolve this issue using single-Purkinje cell extracellular recordings in monkeys limited our understanding of timing mechanisms in the primate brain. Using dual whole-cell recordings from the IO of young adult rhesus macaques in acutely prepared brainstem slices, our work demonstrates that pairs of primate IO neurons show synchronized oscillations in membrane potential. The findings have strong mechanistic and translational relevance, as IO activation has been implicated in humans' perceptual timing of sensory events and motricity.

Connexins and pannexins: At the junction of neuro-glial homeostasis & disease

In the central nervous system (CNS), connexin (Cx)s and pannexin (Panx)s are an integral component of homeostatic neuronal excitability and synaptic plasticity. Neuronal Cx gap junctions form electrical synapses across biochemically similar GABAergic networks, allowing rapid and extensive inhibition in response to principle neuron excitation. Glial Cx gap junctions link astrocytes and oligodendrocytes in the pan-glial network that is responsible for removing excitotoxic ions and metabolites. In addition, glial gap junctions help constrain excessive excitatory activity in neurons and facilitate astrocyte Ca²⁺ slow wave propagation. Panxs do not form gap junctions in vivo, but Panx hemichannels participate in autocrine and paracrine gliotransmission, alongside Cx hemichannels. ATP and other gliotransmitters released by Cx and Panx hemichannels maintain physiologic glutamatergic tone by strengthening synapses and mitigating aberrant high frequency bursting. Under pathological depolarizing and inflammatory conditions, gap junctions and hemichannels become dysregulated, resulting in excessive neuronal firing and seizure. In this review, we present known contributions of Cxs and Panxs to physiologic neuronal excitation and explore how the disruption of gap junctions and hemichannels lead to abnormal glutamatergic transmission, purinergic signaling, and seizures.

A calcium-dependent pathway underlies activity-dependent plasticity of electrical synapses in the thalamic reticular nucleus

KEY POINTS

Electrical synapses are modified by various forms of activity, including paired activity in coupled neurons and tetanization of the input to coupled neurons. We show that plasticity of electrical synapses that results from paired spiking activity in coupled neurons depends on calcium influx and calcium-initiated signalling pathways. Plasticity that results from tetanization of input fibres does not depend on calcium influx or dynamics. These results imply that electrically coupled neurons have distinct sets of mechanisms for adjusting coupling according to the specific type of activity they experience.

ABSTRACT

Recent results have demonstrated modification of electrical synapse strength by varied forms of neuronal activity. However, the mechanisms underlying plasticity induction in central mammalian neurons are unclear. Here we show that the two established inductors of plasticity at electrical synapses in the thalamic reticular nucleus - paired burst spiking in coupled neurons, and mGluR-dependent tetanization of synaptic input - are separate pathways that converge at a common downstream endpoint. Using occlusion experiments and pharmacology in patched pairs of coupled neurons in vitro, we show that burst-induced depression depends on calcium entry via voltage-gated channels, is blocked by BAPTA chelation, and recruits intracellular calcium release on its way to activation of phosphatase activity. In contrast, mGluR-dependent plasticity is independent of calcium entry or calcium dynamics. Together, these results show that the spiking-initiated mechanisms underlying electrical synapse plasticity are similar to those that induce plasticity at chemical synapses, and offer the possibility that calcium-regulated mechanisms may also lead to alternate outcomes, such as potentiation. Because these mechanistic elements are widely found in mature neurons, we expect them to apply broadly to electrical synapses across the brain, acting as the crucial link between neuronal activity and electrical synapse strength.