Electrical coupling and neuronal synchronization in the Mammalian brain

Functional interaction of electrical coupling and H-current and its putative impact on inhibitory transmission

The flow of information within neural circuits depends on the communication between neurons, primarily taking place at chemical and electrical synapses. The coexistence of these two modalities of synaptic transmission and their dynamical interaction with voltage-gated membrane conductances enables a rich repertoire of complex functional operations. One such operation, coincidence detection, allows electrically coupled neurons to respond more strongly to simultaneous synaptic inputs than to temporally dispersed ones. Using the mesencephalic trigeminal (MesV) nucleus-a structure composed of large, somatically coupled neurons-as an experimental model, we first demonstrate that electrical coupling strength in the hyperpolarized voltage range is highly time-dependent due to the involvement of the IH current. We then show how this property influences the coincidence detection of hyperpolarizing signals. Specifically, simultaneous hyperpolarizing inputs induce larger membrane potential changes, resulting in stronger IH current activation. This, in turn, shortens the temporal window for coincidence detection. We propose that this phenomenon may be crucial for network dynamics in circuits of electrically coupled neurons that receive inhibitory synaptic inputs and express the IH current. In particular, molecular layer interneurons (MLIs) of the cerebellar cortex provide an ideal model for studying coincidence detection of inhibitory synaptic inputs, and how this operation is shaped by the voltage-dependent conductances like the IH current, potentially impacting on motor coordination and learning.

Ion-Electron Interactions in 2D Nanomaterials-Based Artificial Synapses for Neuromorphic Applications

With the increasing limitations of conventional computing techniques, particularly the von Neumann bottleneck, the brain's seamless integration of memory and processing through synapses offers a valuable model for technological innovation. Inspired by biological synapse facilitating adaptive, low-power computation by modulating signal transmission via ionic conduction, iontronic synaptic devices have emerged as one of the most promising candidates for neuromorphic computing. Meanwhile, the atomic-scale thickness and tunable electronic properties of van der Waals two-dimensional (2D) materials enable the possibility of designing highly integrated, energy-efficient devices that closely replicate synaptic plasticity. This review comprehensively analyzes advancements in iontronic synaptic devices based on 2D materials, focusing on electron-ion interactions in both iontronic transistors and memristors. The challenges of material stability, scalability, and device integration are evaluated, along with potential solutions and future research directions. By highlighting these developments, this review offers insights into the potential of 2D materials in advancing neuromorphic systems.

Transients versus network interactions give rise to multistability through trapping mechanism

In networked systems, the interplay between the dynamics of individual subsystems and their network interactions has been found to generate multistability in various contexts. Despite its ubiquity, the specific mechanisms and ingredients that give rise to multistability from such interplay remain poorly understood. In a network of coupled excitable units, we demonstrate that this interplay generating multistability occurs through a competition between the units' transient dynamics and their coupling. Specifically, the diffusive coupling between the units reinjects them into the excitability region of their individual state space, effectively trapping them there. We show that this trapping mechanism leads to the coexistence of multiple types of oscillations: periodic, quasi-periodic, and even chaotic, although the units separately do not oscillate. Interestingly, we find that the attractors emerge through different types of bifurcations-in particular, the periodic attractors emerge through either saddle-node of limit cycles bifurcations or homoclinic bifurcations-but in all cases, the reinjection mechanism is present.

Internalization of extracellular Tau oligomers in Alzheimer's disease

A key factor in the progression of Alzheimer's disease (AD) is internalization of extracellular Tau oligomers (ecTauOs) by neuroglial cells. Aberrant hyperphosphorylation of Tau results in their dissociation from microtubules and formation of toxic intracellular Tau oligomers (icTauOs). These are subsequently released to the extracellular space following neuronal dysfunction and death. Although receptor mediated internalization of these ecTauOs by other neurons, microglia and astrocytes can facilitate elimination, incomplete degradation thereof promotes inflammation, exacerbates pathologic spread and accelerates neurodegeneration. Targeting Tau oligomer degradation pathways, blocking internalization receptors, and mitigating neuroinflammation are proposed as therapeutic strategies to control Tau propagation and toxicity. This review highlights the urgent need for innovative approaches to prevent the spread of Tau pathology, emphasizing its implications for AD and related neurodegenerative diseases.

The mechanisms of electrical neuromodulation

The central and peripheral nervous systems are specialized to conduct electrical currents that underlie behaviour. When this multidimensional electrical system is disrupted by degeneration, damage, or disuse, externally applied electrical currents may act to modulate neural structures and provide therapeutic benefit. The administration of electrical stimulation can exert precise and multi-faceted effects at cellular, circuit and systems levels to restore or enhance the functionality of the central nervous system by providing an access route to target specific cells, fibres of passage, neurotransmitter systems, and/or afferent/efferent communication to enable positive changes in behaviour. Here we examine the neural mechanisms that are thought to underlie the therapeutic effects seen with current neuromodulation technologies. To gain further insights into the mechanisms associated with electrical stimulation, we summarize recent findings from genetic dissection studies conducted in animal models. KEY POINTS: Electricity is everywhere around us and is essential for how our nerves communicate within our bodies. When nerves are damaged or not working properly, using exogenous electricity can help improve their function at distinct levels - inside individual cells, within neural circuits, and across entire systems. This method can be tailored to target specific types of cells, nerve fibres, neurotransmitters and communication pathways, offering significant therapeutic potential. This overview explains how exogenous electricity affects nerve function and its potential benefits, based on research in animal studies. Understanding these effects is important because electrical neuromodulation plays a key role in medical treatments for neurological conditions.

Whole brain functional connectivity: Insights from next generation neural mass modelling incorporating electrical synapses

The ready availability of brain connectome data has both inspired and facilitated the modelling of whole brain activity using networks of phenomenological neural mass models that can incorporate both interaction strength and tract length between brain regions. Recently, a new class of neural mass model has been developed from an exact mean field reduction of a network of spiking cortical cell models with a biophysically realistic model of the chemical synapse. Moreover, this new population dynamics model can naturally incorporate electrical synapses. Here we demonstrate the ability of this new modelling framework, when combined with data from the Human Connectome Project, to generate patterns of functional connectivity (FC) of the type observed in both magnetoencephalography and functional magnetic resonance neuroimaging. Some limited explanatory power is obtained via an eigenmode description of frequency-specific FC patterns, obtained via a linear stability analysis of the network steady state in the neigbourhood of a Hopf bifurcation. However, direct numerical simulations show that empirical data is more faithfully recapitulated in the nonlinear regime, and exposes a key role of gap junction coupling strength in generating empirically-observed neural activity, and associated FC patterns and their evolution. Thereby, we emphasise the importance of maintaining known links with biological reality when developing multi-scale models of brain dynamics. As a tool for the study of dynamic whole brain models of the type presented here we further provide a suite of C++ codes for the efficient, and user friendly, simulation of neural mass networks with multiple delayed interactions.

A mixed electrical and chemical synapse in the thalamic reticular nucleus

The thalamic reticular nucleus (TRN) plays a major role in modulating the transfer of information from the thalamus to the cortex. GABAergic inhibition by the TRN is potentially synchronized by electrical synapses between TRN neurons, and TRN neurons are also sparsely connected to each other via chemical synapses. Paired recordings have shown that electrical coupling is abundant between TRN neurons, especially among those within close proximity, but combined electrical and chemical coupling has not yet been directly demonstrated in rats. Here, we report on a single pair of TRN neurons that were coupled both electrically and chemically. This is the only such example that we have found in hundreds of paired recordings of closely apposed neurons within the TRN.NEW & NOTEWORTHY Combined electrical and chemical coupling is demonstrated in a single couple of thalamic reticular nucleus (TRN) neurons. Single action potentials in one neuron resulted in a spikelet [electrical postsynaptic potential (ePSP)] followed by a longer lasting hyperpolarization [from an inhibitory postsynaptic potential (IPSP)] in the target neuron. The IPSPs were most prominent at depolarized potentials and all but disappeared when approaching the chloride equilibrium potential. This is the only such example that we have found in hundreds of paired recordings within the TRN.

Electrical synapse molecular diversity revealed by proximity-based proteomic discovery

Neuronal circuits are composed of synapses that are either chemical, where signals are transmitted via neurotransmitter release and reception, or electrical, where signals pass directly through interneuronal gap junction channels. While the molecular complexity that controls chemical synapse structure and function is well appreciated, the proteins of electrical synapses beyond the gap-junction-forming Connexins are not well defined. Yet, electrical synapses are expected to be molecularly complex beyond the gap junctions. Connexins are integral membrane proteins requiring vesicular transport and membrane insertion/retrieval to achieve function, homeostasis, and plasticity. Additionally, electron microscopy of neuronal gap junctions reveals neighboring electron dense regions termed the electrical synapse density (ESD). To reveal the molecular complexity of the electrical synapse proteome, we used proximity-dependent biotinylation (TurboID) linked to neural Connexins in zebrafish. Proteomic analysis of developing and mature nervous systems identifies hundreds of Connexin-associated proteins, with overlapping and distinct representation during development and adulthood. The identified protein classes span cell adhesion molecules, cytoplasmic scaffolds, vesicular trafficking, and proteins usually associated with the post synaptic density (PSD) of chemical synapses. Using circuits with stereotyped electrical and chemical synapses, we define molecular sub-synaptic compartments of ESD localizing proteins, we find molecular heterogeneity amongst electrical synapse populations, and we examine the synaptic intermingling of electrical and chemical synapse proteins. Taken together, these results reveal a new complexity of electrical synapse molecular diversity and highlight a novel overlap between chemical and electrical synapse proteomes. Moreover, human homologs of the electrical synapse proteins are associated with autism, epilepsy, and other neurological disorders, providing a novel framework towards understanding neuro-atypical states.

Synaptic communication within the microcircuits of pyramidal neurons and basket cells in the mouse prefrontal cortex

Basket cells are inhibitory interneurons in cortical structures with the potential to efficiently control the activity of their postsynaptic partners. Although their contribution to higher order cognitive functions associated with the medial prefrontal cortex (mPFC) relies on the characteristics of their synaptic connections, the way that they are embedded into local circuits is still not fully uncovered. Here, we determined the synaptic properties of excitatory and inhibitory connections between pyramidal neurons (PNs), cholecystokinin-containing basket cells (CCKBCs) and parvalbumin-containing basket cells (PVBCs) in the mouse mPFC. By performing paired recordings, we revealed that PVBCs receive larger unitary excitatory postsynaptic currents from PNs with shorter latency and faster kinetic properties compared to events evoked in CCKBCs. Also, unitary inhibitory postsynaptic currents in PNs were more reliably evoked by PVBCs than by CCKBCs, yet the former connections showed profound short-term depression. Moreover, we demonstrated that CCKBCs and PVBCs in the mPFC are connected with each other. Because alterations in PVBC function have been linked to neurological and psychiatric conditions such as Alzheimer's disease and schizophrenia and CCKBC vulnerability might play a role in mood disorders, a deeper understanding of the general features of basket cell synapses could serve as a reference point for future investigations with therapeutic objectives. KEY POINTS: Cholecystokinin- (CCKBCs) and parvalbumin-expressing basket cells (PVBCs) have distinct passive and active membrane properties. Pyramidal neurons give rise to larger unitary excitatory postsynaptic currents in PVBCs compared to events in CCKBCs. Unitary inhibitory postsynaptic currents in pyramidal neurons are more reliably evoked by PVBCs than by CCKBCs. Basket cells form chemical synapses and gap junctions with their own cell type. The two basket cell types are connected with each other.

Bilateral interactions of optic-flow sensitive neurons coordinate course control in flies

Animals rely on compensatory actions to maintain stability and navigate their environment efficiently. These actions depend on global visual motion cues known as optic-flow. While the optomotor response has been the traditional focus for studying optic-flow compensation in insects, its simplicity has been insufficient to determine the role of the intricate optic-flow processing network involved in visual course control. Here, we reveal a series of course control behaviours in Drosophila and link them to specific neural circuits. We show that bilateral electrical coupling of optic-flow-sensitive neurons in the fly's lobula plate are required for a proper course control. This electrical interaction works alongside chemical synapses within the HS-H2 network to control the dynamics and direction of turning behaviours. Our findings reveal how insects use bilateral motion cues for navigation, assigning a new functional significance to the HS-H2 network and suggesting a previously unknown role for gap junctions in non-linear operations.

Wirelessly steerable bioelectronic neuromuscular robots adapting neurocardiac junctions

Biological motions of native muscle tissues rely on the nervous system to interface movement with the surrounding environment. The neural innervation of muscles, crucial for regulating movement, is the fundamental infrastructure for swiftly responding to changes in body tissue requirements. This study introduces a bioelectronic neuromuscular robot integrated with the motor nervous system through electrical synapses to evoke cardiac muscle activities and steer robotic motion. Serving as an artificial brain and wirelessly regulating selective neural activation to initiate robot fin motion, a wireless frequency multiplexing bioelectronic device is used to control the robot. Frequency multiplexing bioelectronics enables the control of the robot locomotion speed and direction by modulating the flapping of the robot fins through the wireless motor innervation of cardiac muscles. The robots demonstrated an average locomotion speed of ~0.52 ± 0.22 millimeters per second, fin-flapping frequency up to 2.0 hertz, and turning locomotion path curvature of ~0.11 ± 0.04 radians per millimeter. These systems will contribute to the expansion of biohybrid machines into the brain-to-motor frontier for developing autonomous biohybrid systems capable of advanced adaptive motor control and learning.

Energy and synchronization between two neurons with nonlinear coupling

Consensus and synchronous firing in neural activities are relative to the physical properties of synaptic connections. For coupled neural circuits, the physical properties of coupling channels control the synchronization stability, and transient period for keeping energy diversity. Linear variable coupling results from voltage coupling via linear resistor by consuming certain Joule heat, and electric synapse coupling between neurons derives from gap junction connection under special electrophysiological condition. In this work, a voltage-controlled electric component with quadratic relation in the i-v (current-voltage) is used to connect two neural circuits composed of two variables. The energy function obtained by using Helmholtz theorem is consistent with the Hamilton energy function converted from the field energy in the neural circuit. Chaotic signals are encoded to approach a mixed signal within certain frequency band, and then its amplitude is adjusted to excite the neuron for detecting possible occurrence of nonlinear resonance. External stimuli are changed to trigger different firing modes, and nonlinear coupling activates changeable coupling intensity. It is confirmed that nonlinear coupling behaves functional regulation as hybrid synapse, and the synchronization transition between neurons can be controlled for reaching possible energy balance. The nonlinear coupling is helpful to keep energy diversity and prevent synchronous bursting because positive and negative feedback is switched with time. As a result, complete synchronization is suppressed and phase lock is controlled between neurons with energy diversity.

Thermodynamic model for memory

A thermodynamic model for memory formation is proposed. Key points include: 1) Any thought or consciousness corresponds to a thermodynamic system of nerve cells. 2) The system concept of nerve cells can only be described by thermodynamics of condensed matter. 3) The memory structure is logically associated with the system structure or the normal structure of biology. 4) The development of our thoughts is processed irreversibly, and numerous states or thoughts can be generated. 5) Memory formation results from the reorganization and change of cellular structures (or memory structures), which are related to nerve cell skeleton and membrane. Their alteration can change the excitability of nerve cells and the pathway of neural impulse conduction. 6) Amnesia results from the loss of thermodynamic stability of the memory structure, which can be achieved by different ways. Some related phenomena and facts are discussed. The analysis shows that thermodynamics can account for the basic properties of memory.

Prefrontal cortical network dysfunction from acute neurotoxicant exposure

Prefrontal cortical (PFC) dysfunction has been linked to disorders exhibiting deficits in cognitive performance, attention, motivation, and impulse control. Neurons of the PFC are susceptible to glutamatergic excitotoxicity, an effect associated with cortical degeneration in frontotemporal disorders (FTDs). PFC susceptibility to environmental toxicant exposure, one possible contributor to sporadic FTD, has not been systematically studied. Here, we tested the ability of a well-known environmental neurotoxicant, methylmercury (MeHg), to induce hyperexcitability in medial prefrontal cortex (mPFC) excitatory pyramidal neurons, using whole cell patch-clamp recording. Acute MeHg exposure (20 μM) produced significant mPFC dysfunction, with a shift in the excitatory to inhibitory (E-I) balance toward increased excitability. Both excitatory postsynaptic current (EPSC) and inhibitory postsynaptic current (IPSC) charges were significantly increased after MeHg exposure. MeHg increased EPSC frequency, but there was no observable effect on IPSC frequency, EPSC amplitude or IPSC amplitude. Neither evoked AMPA receptor- nor NMDA receptor-mediated EPSC amplitudes were affected by MeHg. However, excitatory synapses experienced a significant reduction in paired-pulse depression and probability of release. In addition, MeHg induced temporal synchrony in spontaneous IPSCs, reflecting mPFC inhibitory network dysfunction. MeHg exposure also produced increased intrinsic excitability in mPFC neurons, with an increase in action potential firing rate. The observed effects of MeHg on mPFC reflect key potential mechanisms for neuropsychological symptoms from MeHg poisoning. Therefore, MeHg has a significant effect on mPFC circuits known to contribute to cognitive and emotional function and might contribute to etiology of neurodegenerative diseases, such as FTD.NEW & NOTEWORTHY Prefrontal cortical neurons are highly susceptible to glutamatergic excitotoxicity associated with neuronal degeneration in frontal dementia and to environmental toxicant exposure, one potential contributor to FTD. However, this has not been systematically studied. Our results demonstrate that methylmercury exposure leads to hyperexcitability of prefrontal cortical neurons by shifting excitatory to inhibitory (E-I) balance and raising sensitivity for spiking. Our results provide a mechanism by which environmental neurotoxicants may contribute to pathogenesis of diseases such as FTD.

Retinoic acid reduces the formation of, and acutely modulates, invertebrate electrical synapses

Communication between cells in the nervous system is dependent on both chemical and electrical synapses. Factors that can affect chemical synapses have been well studied, but less is known about factors that influence electrical synapses. Retinoic acid, the vitamin A metabolite, is a known regulator of chemical synapses, but few studies have examined its capacity to regulate electrical synapses. In this study, we determine that retinoic acid is capable of rapidly altering the strength of electrical synapses in an isomer- and cell-dependent manner. Furthermore, we provide evidence that this acute effect might be independent of either the retinoid receptors or the activation of a protein kinase. In addition to the rapid modulatory effects of retinoic acid, we provide data to suggest that retinoic acid is also capable of regulating the formation of electrical synapses. Long-term exposure to both all-trans-retinoic acid or 9-cis-retinoic acid reduced the proportion of cell pairs forming electrical synapses, as well as reduced the strength of electrical synapses that did form. In summary, this study provides insights into the role that retinoids might play in both the formation and modulation of electrical synapses in the central nervous system.NEW & NOTEWORTHY Retinoids are known modulators of chemical synapses and mediate synaptic plasticity in the nervous system, but little is known of their effects on electrical synapses. Here, we show that retinoids selectively reduce electrical synapses in a cell- and isomer-dependent manner. This modulatory action on existing electrical synapses was rapid and nongenomic in nature. We also showed for the first time that longer retinoid exposures inhibit the formation of electrical synapses.

A hyperpolarizing neuron recruits undocked innexin hemichannels to transmit neural information in Caenorhabditis elegans

While depolarization of the neuronal membrane is known to evoke the neurotransmitter release from synaptic vesicles, hyperpolarization is regarded as a resting state of chemical neurotransmission. Here, we report that hyperpolarizing neurons can actively signal neural information by employing undocked hemichannels. We show that UNC-7, a member of the innexin family in Caenorhabditis elegans, functions as a hemichannel in thermosensory neurons and transmits temperature information from the thermosensory neurons to their postsynaptic interneurons. By monitoring neural activities in freely behaving animals, we find that hyperpolarizing thermosensory neurons inhibit the activity of the interneurons and that UNC-7 hemichannels regulate this process. UNC-7 is required to control thermotaxis behavior and functions independently of synaptic vesicle exocytosis. Our findings suggest that innexin hemichannels mediate neurotransmission from hyperpolarizing neurons in a manner that is distinct from the synaptic transmission, expanding the way of neural circuitry operations.

Synchronization dynamics of phase oscillators on power grid models

We investigate topological and spectral properties of models of European and US-American power grids and of paradigmatic network models as well as their implications for the synchronization dynamics of phase oscillators with heterogeneous natural frequencies. We employ the complex-valued order parameter-a widely used indicator for phase ordering-to assess the synchronization dynamics and observe the order parameter to exhibit either constant or periodic or non-periodic, possibly chaotic temporal evolutions for a given coupling strength but depending on initial conditions and the systems' disorder. Interestingly, both topological and spectral characteristics of the power grids point to a diminished capability of these networks to support a temporarily stable synchronization dynamics. We find non-trivial commonalities between the synchronization dynamics of oscillators on seemingly opposing topologies.

Assembly mechanisms of the neuronal gap junction channel connexin 36 elucidated by Cryo-EM

Electrical synapses are essential components of neural circuits. Neuronal signal transduction across electrical synapses is primarily mediated by gap junction channels composed of Connexin36 (Cx36), the lack of which causes impaired electrical coupling between certain neurons including cortical interneurons and thalamic reticular nucleus (TRN) neurons. However, the structural basis underlying Cx36 function and assembly remains elusive. Recently, Lee et al. reported cryo-EM structures of Cx36, thus provided first insights of its gating mechanism. Here, we report a consistent cryo-EM structure of Cx36 determined in parallel, and describe unique interactions underpinning its assembly mechanism in complementary to the competing work. In particular, we found non-canonical electrostatic interactions between protomers from opposing hemichannels and a steric complementary site between adjacent protomers within a hemichannel, which together provide a structural explanation for the assembly specificity in homomeric and heteromeric gap junction channels.

Dynamic electrical synapses rewire brain networks for persistent oscillations and epileptogenesis

One of the very fundamental attributes for telencephalic neural computation in mammals involves network activities oscillating beyond the initial trigger. The continuing and automated processing of transient inputs shall constitute the basis of cognition and intelligence but may lead to neuropsychiatric disorders such as epileptic seizures if carried so far as to engross part of or the whole telencephalic system. From a conventional view of the basic design of the telencephalic local circuitry, the GABAergic interneurons (INs) and glutamatergic pyramidal neurons (PNs) make negative feedback loops which would regulate the neural activities back to the original state. The drive for the most intriguing self-perpetuating telencephalic activities, then, has not been posed and characterized. We found activity-dependent deployment and delineated functional consequences of the electrical synapses directly linking INs and PNs in the amygdala, a prototypical telencephalic circuitry. These electrical synapses endow INs dual (a faster excitatory and a slower inhibitory) actions on PNs, providing a network-intrinsic excitatory drive that fuels the IN-PN interconnected circuitries and enables persistent oscillations with preservation of GABAergic negative feedback. Moreover, the entities of electrical synapses between INs and PNs are engaged in and disengaged from functioning in a highly dynamic way according to neural activities, which then determine the spatiotemporal scale of recruited oscillating networks. This study uncovers a special wide-range and context-dependent plasticity for wiring/rewiring of brain networks. Epileptogenesis or a wide spectrum of clinical disorders may ensue, however, from different scales of pathological extension of this unique form of telencephalic plasticity.

Calcium-Sensitive Subthreshold Oscillations and Electrical Coupling in Principal Cells of Mouse Dorsal Cochlear Nucleus

In higher sensory brain regions, slow oscillations (0.5-5 Hz) associated with quiet wakefulness and attention modulate multisensory integration, predictive coding, and perception. Although often assumed to originate via thalamocortical mechanisms, the extent to which subcortical sensory pathways are independently capable of slow oscillatory activity is unclear. We find that in the first station for auditory processing, the cochlear nucleus, fusiform cells from juvenile mice (of either sex) generate robust 1-2 Hz oscillations in membrane potential and exhibit electrical resonance. Such oscillations were absent prior to the onset of hearing, intrinsically generated by hyperpolarization-activated cyclic nucleotide-gated (HCN) and persistent Na+ conductances (NaP) interacting with passive membrane properties, and reflected the intrinsic resonance properties of fusiform cells. Cx36-containing gap junctions facilitated oscillation strength and promoted pairwise synchrony of oscillations between neighboring neurons. The strength of oscillations were strikingly sensitive to external Ca2+, disappearing at concentrations >1.7 mM, due in part to the shunting effect of small-conductance calcium-activated potassium (SK) channels. This effect explains their apparent absence in previous in vitro studies of cochlear nucleus which routinely employed high-Ca2+ extracellular solution. In contrast, oscillations were amplified in reduced Ca2+ solutions, due to relief of suppression by Ca2+ of Na+ channel gating. Our results thus reveal mechanisms for synchronous oscillatory activity in auditory brainstem, suggesting that slow oscillations, and by extension their perceptual effects, may originate at the earliest stages of sensory processing.

D-type K+ current rules the function of electrically coupled neurons in a species-specific fashion

Electrical synapses supported by gap junctions are known to form networks of electrically coupled neurons in many regions of the mammalian brain, where they play relevant functional roles. Yet, how electrical coupling supports sophisticated network operations and the contribution of the intrinsic electrophysiological properties of neurons to these operations remain incompletely understood. Here, a comparative analysis of electrically coupled mesencephalic trigeminal (MesV) neurons uncovered remarkable difference in the operation of these networks in highly related species. While spiking of MesV neurons might support the recruitment of coupled cells in rats, this rarely occurs in mice. Using whole-cell recordings, we determined that the higher efficacy in postsynaptic recruitment in rat's MesV neurons does not result from coupling strength of larger magnitude, but instead from the higher excitability of coupled neurons. Consistently, MesV neurons from rats present a lower rheobase, more hyperpolarized threshold, as well as a higher ability to generate repetitive discharges, in comparison to their counterparts from mice. This difference in neuronal excitability results from a significantly higher magnitude of the D-type K+ current (ID) in MesV neurons from mice, indicating that the magnitude of this current gates the recruitment of postsynaptic-coupled neurons. Since MesV neurons are primary afferents critically involved in the organization of orofacial behaviors, activation of a coupled partner could support lateral excitation, which by amplifying sensory inputs may significantly contribute to information processing and the organization of motor outputs.

Different species, different gap junctions?

By describing the analogies and differences of a simple homologous circuit involving electrical synapses—the simplest form of synaptic communication—in two closely related species, this article demonstrates how we must be wary of cross-species generalizations.

Connexin36 RNA Expression in the Cochlear Nucleus of the Echolocating Bat, Eptesicus fuscus

PURPOSE

The echolocating bat is used as a model for studying the auditory nervous system because its specialized sensory capabilities arise from general mammalian auditory percepts such as pitch and sound source localization. These percepts are mediated by precise timing within neurons and networks of the lower auditory brainstem, where the gap junction protein Connexin36 (CX36) is expressed. Gap junctions and electrical synapses in the central nervous system are associated with fast transmission and synchronous patterns of firing within neuronal networks. The purpose of this study was to identify areas where CX36 was expressed in the bat cochlear nucleus to shed light on auditory brainstem networks in a hearing specialist animal model.

METHODS

We investigated the distribution of CX36 RNA throughout the cochlear nucleus complex of the echolocating big brown bat, Eptesicus fuscus, using in situ hybridization. As a qualitative comparison, we visualized Gjd2 gene expression in the cochlear nucleus of transgenic CX36 reporter mice, species that hear ultrasound but do not echolocate.

RESULTS

In both the bat and the mouse, CX36 is expressed in the anteroventral and in the dorsal cochlear nucleus, with more limited expression in the posteroventral cochlear nucleus. These results are generally consistent with previous work based on immunohistochemistry.

CONCLUSION

Our data suggest that the anatomical substrate for CX36-mediated electrical neurotransmission is conserved in the mammalian CN across echolocating bats and non-echolocating mice.

Circadian clock organization in the retina: From clock components to rod and cone pathways and visual function

Circadian (24-h) clocks are cell-autonomous biological oscillators that orchestrate many aspects of our physiology on a daily basis. Numerous circadian rhythms in mammalian and non-mammalian retinas have been observed and the presence of an endogenous circadian clock has been demonstrated. However, how the clock and associated rhythms assemble into pathways that support and control retina function remains largely unknown. Our goal here is to review the current status of our knowledge and evaluate recent advances. We describe many previously-observed retinal rhythms, including circadian rhythms of morphology, biochemistry, physiology, and gene expression. We evaluate evidence concerning the location and molecular machinery of the retinal circadian clock, as well as consider findings that suggest the presence of multiple clocks. Our primary focus though is to describe in depth circadian rhythms in the light responses of retinal neurons with an emphasis on clock control of rod and cone pathways. We examine evidence that specific biochemical mechanisms produce these daily light response changes. We also discuss evidence for the presence of multiple circadian retinal pathways involving rhythms in neurotransmitter activity, transmitter receptors, metabolism, and pH. We focus on distinct actions of two dopamine receptor systems in the outer retina, a dopamine D4 receptor system that mediates circadian control of rod/cone gap junction coupling and a dopamine D1 receptor system that mediates non-circadian, light/dark adaptive regulation of gap junction coupling between horizontal cells. Finally, we evaluate the role of circadian rhythmicity in retinal degeneration and suggest future directions for the field of retinal circadian biology.

Synchronization of inspiratory burst onset along the ventral respiratory column in the neonate mouse is mediated by electrotonic coupling

Breathing is a singularly robust behavior, yet this motor pattern is continuously modulated at slow and fast timescales to maintain blood-gas homeostasis, while intercalating orofacial behaviors. This functional multiplexing goes beyond the rhythmogenic function that is typically ascribed to medullary respiration-modulated networks and may explain lack of progress in identifying the mechanism and constituents of the respiratory rhythm generator. By recording optically along the ventral respiratory column in medulla, we found convergent evidence that rhythmogenic function is distributed over a dispersed and heterogeneous network that is synchronized by electrotonic coupling across a neuronal syncytium. First, high-speed recordings revealed that inspiratory onset occurred synchronously along the column and did not emanate from a rhythmogenic core. Second, following synaptic isolation, synchronized stationary rhythmic activity was detected along the column. This activity was attenuated following gap junction blockade and was silenced by tetrodotoxin. The layering of syncytial and synaptic coupling complicates identification of rhythmogenic mechanism, while enabling functional multiplexing.

The circadian regulation of extracellular ATP

Extracellular ATP is a potent signaling molecule released from various cells throughout the body and is intimately involved in the pathophysiological functions of the nervous system and immune system by activating P2 purinergic receptors. Recent increasingly studies showed that extracellular ATP exhibits circadian oscillation with an approximately 24-h periodicity, which participates in regulatory pathways of central oscillator suprachiasmatic nucleus and peripheral oscillator bladder, respectively. Oscillators modulate the protein expression of ATP release channels and ectonucleotidase activity through clock genes; indeed, real-time alterations of ATP release and degradation determine outcomes of temporal character on extracellular ATP rhythm. The regulatory pathways on extracellular ATP rhythm are different in central and peripheral systems. In this review, we summarize the circadian rhythm of extracellular ATP and discuss several circadian regulatory pathways in different organs via ATP release and degradation, to provide a new understanding for purinergic signaling in the regulatory mechanism of circadian rhythm and a potential target to research the circadian regulation of extracellular ATP in other circadian oscillators.

Drug repurposing for cardiovascular diseases: New targets and indications for probenecid

The available pharmacological options in the management of cardiovascular diseases such as ischaemic heart disease and subsequent heart failure are effective in slowing the progression of this condition. However, the long-term prognosis is still poor, raising the demand for new therapeutic strategies. Drug repurposing is a time- and cost-effective drug development strategy that offers approved and abandoned drugs a new chance for new indications. Recently, drugs used for the management of gout-related inflammation such as canakinumab or colchicine have been considered for drug repurposing in cardiovascular indications. The old uricosuric drug, probenecid, has been identified as a novel therapeutic option in the management of specific cardiac diseases as well. Probenecid can modulate myocardial contractility and vascular tone and exerts anti-inflammatory properties. The mechanisms behind these beneficial effects might be related inhibition of inflammasomes, and to modulation purinergic-pannexin-1 signalling and TRPV2 channels, which are recently identified molecular targets of probenecid. In this review, we provide an overview on repurposing probenecid for ischaemic heart disease and subsequent heart failure by summarizing the related experimental and clinical data and propose its potential repurposing to treat cardiovascular diseases.

Recent progress in three-terminal artificial synapses based on 2D materials: from mechanisms to applications

Synapses are essential for the transmission of neural signals. Synaptic plasticity allows for changes in synaptic strength, enabling the brain to learn from experience. With the rapid development of neuromorphic electronics, tremendous efforts have been devoted to designing and fabricating electronic devices that can mimic synapse operating modes. This growing interest in the field will provide unprecedented opportunities for new hardware architectures for artificial intelligence. In this review, we focus on research of three-terminal artificial synapses based on two-dimensional (2D) materials regulated by electrical, optical and mechanical stimulation. In addition, we systematically summarize artificial synapse applications in various sensory systems, including bioplastic bionics, logical transformation, associative learning, image recognition, and multimodal pattern recognition. Finally, the current challenges and future perspectives involving integration, power consumption and functionality are outlined.

Cellular composition and circuit organization of the locus coeruleus of adult mice

The locus coeruleus (LC) houses the vast majority of noradrenergic neurons in the brain and regulates many fundamental functions, including fight and flight response, attention control, and sleep/wake cycles. While efferent projections of the LC have been extensively investigated, little is known about its local circuit organization. Here, we performed large-scale multipatch recordings of noradrenergic neurons in adult mouse LC to profile their morpho-electric properties while simultaneously examining their interactions. LC noradrenergic neurons are diverse and could be classified into two major morpho-electric types. While fast excitatory synaptic transmission among LC noradrenergic neurons was not observed in our preparation, these mature LC neurons connected via gap junction at a rate similar to their early developmental stage and comparable to other brain regions. Most electrical connections form between dendrites and are restricted to narrowly spaced pairs or small clusters of neurons of the same type. In addition, more than two electrically coupled cell pairs were often identified across a cohort of neurons from individual multicell recording sets that followed a chain-like organizational pattern. The assembly of LC noradrenergic neurons thus follows a spatial and cell-type-specific wiring principle that may be imposed by a unique chain-like rule.

Dynamical response of Autaptic Izhikevich Neuron disturbed by Gaussian white noise

Using the improved memristive Izhikevich neuron model, the effects of autaptic connection as well as electromagnetic induction are studied on the dynamical behavior of neuronal spiking. Using bifurcation analysis for membrane potentials, the effects of autaptic and electromagnetic parameters on the mode transition in electrical activities of the neuron model are investigated. Furthermore, white Gaussian noise is considered in the neuron model, to evaluate the effect of electromagnetic disturbance on the firing pattern of the neuron using the coefficient of variation. The bifurcation diagram versus autaptic conductance and time delay has been extensively studied. The results show that the effects of autaptic connection as well as electromagnetic induction on the spiking behavior of neurons can be well demonstrated by using the Izhikevich model. The electrical activities of the Izhikevich neuron model become more complex when the effects of autaptic connection and electromagnetic induction are considered in the neuron model. Using the Izhikevich neuron model, the high variety of spiking/bursting patterns is represented in the bifurcation diagram of inter-spike interval versus autaptic or electromagnetic parameters. Noise can have distinct effects on the spiking activity of the neuron, for the subthreshold input current, increasing the intensity of the electromagnetic noise increases the regularity of the neuron spiking, but for the suprathreshold input current, the regularity of spiking decreases with noise.

Gap Junctions and Ageing

Gap junctions, comprising connexin proteins, create conduits directly coupling the cytoplasms of adjacent cells. Expressed in essentially all tissues, dynamic gap junction structures enable the exchange of small molecules including ions and second messengers, and are central to maintenance of homeostasis and synchronized excitability. With such diverse and critical roles throughout the body, it is unsurprising that alterations to gap junction and/or connexin expression and function underlie a broad array of age-related pathologies. From neurological dysfunction to cardiac arrhythmia and bone loss, it is hard to identify a human disease state that does not involve reduced, or in some cases inappropriate, intercellular communication to affect organ function. With a complex life cycle encompassing several key regulatory steps, pathological gap junction remodeling during ageing can arise from alterations in gene expression, translation, intracellular trafficking, and posttranslational modification of connexins. Connexin proteins are now known to "moonlight" and perform a variety of non-junctional functions in the cell, independent of gap junctions. Furthermore, connexin "hemichannels" on the cell surface can communicate with the extracellular space without ever coupling to an adjacent cell to form a gap junction channel. This chapter will focus primarily on gap junctions in ageing, but such non-junctional connexin functions will be referred to where appropriate and the full spectrum of connexin biology should be noted as potentially causative/contributing to some findings in connexin knockout animals, for example.

Interneuronal network model of theta-nested fast oscillations predicts differential effects of heterogeneity, gap junctions and short term depression for hyperpolarizing versus shunting inhibition

Theta and gamma oscillations in the hippocampus have been hypothesized to play a role in the encoding and retrieval of memories. Recently, it was shown that an intrinsic fast gamma mechanism in medial entorhinal cortex can be recruited by optogenetic stimulation at theta frequencies, which can persist with fast excitatory synaptic transmission blocked, suggesting a contribution of interneuronal network gamma (ING). We calibrated the passive and active properties of a 100-neuron model network to capture the range of passive properties and frequency/current relationships of experimentally recorded PV+ neurons in the medial entorhinal cortex (mEC). The strength and probabilities of chemical and electrical synapses were also calibrated using paired recordings, as were the kinetics and short-term depression (STD) of the chemical synapses. Gap junctions that contribute a noticeable fraction of the input resistance were required for synchrony with hyperpolarizing inhibition; these networks exhibited theta-nested high frequency oscillations similar to the putative ING observed experimentally in the optogenetically-driven PV-ChR2 mice. With STD included in the model, the network desynchronized at frequencies above ~200 Hz, so for sufficiently strong drive, fast oscillations were only observed before the peak of the theta. Because hyperpolarizing synapses provide a synchronizing drive that contributes to robustness in the presence of heterogeneity, synchronization decreases as the hyperpolarizing inhibition becomes weaker. In contrast, networks with shunting inhibition required non-physiological levels of gap junctions to synchronize using conduction delays within the measured range.

Contributions from both the brain and the vascular network guide behavior in the colonial tunicate Botryllus schlosseri

We studied the function, development and aging of the adult nervous system in the colonial tunicate Botryllus schlosseri. Adults, termed zooids, are filter-feeding individuals. Sister zooids group together to form modules, and modules, in turn, are linked by a shared vascular network to form a well-integrated colony. Zooids undergo a weekly cycle of regression and renewal during which mature zooids are replaced by developing buds. The zooid brain matures and degenerates on this 7-day cycle. We used focal extracellular recording and video imaging to explore brain activity in the context of development and degeneration and to examine the contributions of the nervous system and vascular network to behavior. Recordings from the brain revealed complex firing patterns arising both spontaneously and in response to stimulation. Neural activity increases as the brain matures and declines thereafter. Motor behavior follows the identical time course. The behavior of each zooid is guided predominantly by its individual brain, but sister zooids can also exhibit synchronous motor behavior. The vascular network also generates action potentials that are largely independent of neural activity. In addition, the entire vascular network undergoes slow rhythmic contractions that appear to arise from processes endogenous to vascular epithelial cells. We found that neurons in the brain and cells of the vascular network both express multiple genes for voltage-gated Na+ and Ca2+ ion channels homologous (based on sequence) to mammalian ion channel genes.

Neural synchrony in cortical networks: mechanisms and implications for neural information processing and coding

Synchronization of neuronal discharges on the millisecond scale has long been recognized as a prevalent and functionally important attribute of neural activity. In this article, I review classical concepts and corresponding evidence of the mechanisms that govern the synchronization of distributed discharges in cortical networks and relate those mechanisms to their possible roles in coding and cognitive functions. To accommodate the need for a selective, directed synchronization of cells, I propose that synchronous firing of distributed neurons is a natural consequence of spike-timing-dependent plasticity (STDP) that associates cells repetitively receiving temporally coherent input: the “synchrony through synaptic plasticity” hypothesis. Neurons that are excited by a repeated sequence of synaptic inputs may learn to selectively respond to the onset of this sequence through synaptic plasticity. Multiple neurons receiving coherent input could thus actively synchronize their firing by learning to selectively respond at corresponding temporal positions. The hypothesis makes several predictions: first, the position of the cells in the network, as well as the source of their input signals, would be irrelevant as long as their input signals arrive simultaneously; second, repeating discharge patterns should get compressed until all or some part of the signals are synchronized; and third, this compression should be accompanied by a sparsening of signals. In this way, selective groups of cells could emerge that would respond to some recurring event with synchronous firing. Such a learned response pattern could further be modulated by synchronous network oscillations that provide a dynamic, flexible context for the synaptic integration of distributed signals. I conclude by suggesting experimental approaches to further test this new hypothesis.

A new path to mental disorders: Through gap junction channels and hemichannels

Behavioral disturbances related to emotional regulation, reward processing, cognition, sleep-wake regulation and activity/movement represent core symptoms of most common mental disorders. Increasing empirical and theoretical evidence suggests that normal functioning of these behavioral domains relies on fine graded coordination of neural and glial networks which are maintained and modulated by intercellular gap junction channels and unapposed pannexin or connexin hemichannels. Dysfunctions in these networks might contribute to the development and maintenance of psychopathological and neurobiological features associated with mental disorders. Here we review and discuss the evidence indicating a prominent role of gap junction channel and hemichannel dysfunction in core symptoms of mental disorders. We further discuss how the increasing knowledge on intercellular gap junction channels and unapposed pannexin or connexin hemichannels in the brain might lead to deeper mechanistic insight in common mental disorders and to the development of novel treatment approaches. We further attempt to exemplify what type of future research on this topic could be integrated into multidimensional approaches to understand and cure mental disorders.

3D Neural Network Composed of Neurospheroid and Bionanohybrid on Microelectrode Array to Realize the Spatial Input Signal Recognition in Neurospheroid

There have been several studies for demonstration of 2D neural network using living cells or organic/inorganic molecules, but to date, there is no report of development of a 3D neural network in vitro. Based on developed bionanohybrid composed of protein, DNA, molybdenum disulfide nanoparticles, and peptides for controlling electrophysiological states of living cells, here, the in vitro 3D neural network composed of the bionanohybrid, 3D neurospheroid and the microelectrode array (MEA) is developed. After production of the 3D neurospheroid derived from human neural stem cells, the bionanohybrid developed on the MEA successfully semi-penetrates the neurites of the 3D neurospheroid and forms the 3D neural network. The developed 3D neural network successfully exhibited the electrophysiological output signals of the 3D neurospheroid by transmitting the input signal applied by the bionanohybrid. Moreover, by using the selectively immobilized bionanohybrid on the MEA, the spatial input signal recognition in the neurospheroid of 3D neural network is realized for the first time. This newly developed in vitro 3D neural network provides a promising strategy to be applied in brain-on-a-chip, brain disease-related drug efficacy evaluation, bioelectronics, and bioelectronic medicine.

On the Diverse Functions of Electrical Synapses

Electrical synapses are the neurophysiological product of gap junctional pores between neurons that allow bidirectional flow of current between neurons. They are expressed throughout the mammalian nervous system, including cortex, hippocampus, thalamus, retina, cerebellum, and inferior olive. Classically, the function of electrical synapses has been associated with synchrony, logically following that continuous conductance provided by gap junctions facilitates the reduction of voltage differences between coupled neurons. Indeed, electrical synapses promote synchrony at many anatomical and frequency ranges across the brain. However, a growing body of literature shows there is greater complexity to the computational function of electrical synapses. The paired membranes that embed electrical synapses act as low-pass filters, and as such, electrical synapses can preferentially transfer spike after hyperpolarizations, effectively providing spike-dependent inhibition. Other functions include driving asynchronous firing, improving signal to noise ratio, aiding in discrimination of dissimilar inputs, or dampening signals by shunting current. The diverse ways by which electrical synapses contribute to neuronal integration merits furthers study. Here we review how functions of electrical synapses vary across circuits and brain regions and depend critically on the context of the neurons and brain circuits involved. Computational modeling of electrical synapses embedded in multi-cellular models and experiments utilizing optical control and measurement of cellular activity will be essential in determining the specific roles performed by electrical synapses in varying contexts.

Mechanisms of SARS-CoV-2 and Male Infertility: Could Connexin and Pannexin Play a Role?

The impact of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) on male infertility has lately received significant attention. SARS-CoV-2, the virus that causes coronavirus disease (COVID-19) in humans, has been shown to impose adverse effects on both the structural components and function of the testis, which potentially impact spermatogenesis. These adverse effects are partially explained by fever, systemic inflammation, oxidative stress, and an increased immune response leading to impaired blood-testis barrier. It has been well established that efficient cellular communication via gap junctions or functional channels is required for tissue homeostasis. Connexins and pannexins are two protein families that mediate autocrine and paracrine signaling between the cells and the extracellular environment. These channel-forming proteins have been shown to play a role in coordinating cellular communication in the testis and epididymis. Despite their role in maintaining a proper male reproductive milieu, their function is disrupted under pathological conditions. The involvement of these channels has been well documented in several physiological and pathological conditions and their designated function in infectious diseases. However, their role in COVID-19 and their meaningful contribution to male infertility remains to be elucidated. Therefore, this review highlights the multivariate pathophysiological mechanisms of SARS-CoV-2 involvement in male reproduction. It also aims to shed light on the role of connexin and pannexin channels in disease progression, emphasizing their unexplored role and regulation of SARS-CoV-2 pathophysiology. Finally, we hypothesize the possible involvement of connexins and pannexins in SARS-CoV-2 inducing male infertility to assist future research ideas targeting therapeutic approaches.

Intrinsic Sources and Functional Impacts of Asymmetry at Electrical Synapses

Electrical synapses couple inhibitory neurons across the brain, underlying a variety of functions that are modifiable by activity. Despite recent advances, many functions and contributions of electrical synapses within neural circuitry remain underappreciated. Among these are the sources and impacts of electrical synapse asymmetry. Using multi-compartmental models of neurons coupled through dendritic electrical synapses, we investigated intrinsic factors that contribute to effective synaptic asymmetry and that result in modulation of spike timing and synchrony between coupled cells. We show that electrical synapse location along a dendrite, input resistance, internal dendritic resistance, or directional conduction of the electrical synapse itself each alter asymmetry as measured by coupling between cell somas. Conversely, we note that asymmetrical gap junction (GJ) conductance can be masked by each of these properties. Furthermore, we show that asymmetry modulates spike timing and latency of coupled cells by up to tens of milliseconds, depending on direction of conduction or dendritic location of the electrical synapse. Coordination of rhythmic activity between two cells also depends on asymmetry. These simulations illustrate that causes of asymmetry are diverse, may not be apparent in somatic measurements of electrical coupling, influence dendritic processing, and produce a variety of outcomes on spiking and synchrony of coupled cells. Our findings highlight aspects of electrical synapses that should always be included in experimental demonstrations of coupling, and when assembling simulated networks containing electrical synapses.

On the Organization of Connexin36 Expression in Electrically Coupled Cholinergic V0c Neurons (Partition Cells) in the Spinal Cord and Their C-terminal Innervation of Motoneurons

Large cholinergic neurons (V0c neurons; aka, partition cells) in the spinal cord project profusely to motoneurons on which they form C-terminal contacts distinguished by their specialized postsynaptic subsurface cisterns (SSCs). The V0c neurons are known to be rhythmically active during locomotion and release of acetylcholine (ACh) from their terminals is known to modulate the excitability of motoneurons in what appears to be a task-dependent manner. Here, we present evidence that a subpopulation of V0c neurons express the gap junction forming protein connexin36 (Cx36), indicating that they are coupled by electrical synapses. Based on immunofluorescence imaging and the use of Cx36BAC-enhanced green fluorescent protein (eGFP) mice in which C-terminals immunolabelled for their marker vesicular acetylcholine transporter (vAChT) are also labelled for eGFP, we found a heterogeneous distribution of eGFP+ C-terminals on motoneurons at cervical, thoracic and lumber spinal levels. The density of C-terminals on motoneurons varied as did the proportion of those that were eGFP+ vs. eGFP-. We present evidence that fast vs. slow motoneurons have a greater abundance of these terminals and fast motoneurons also have the highest density that were eGFP+. Thus, our results indicate that a subpopulation of V0c neurons projects preferentially to fast motoneurons, suggesting that the capacity for synchronous activity conferred by electrical synapses among networks of coupled V0c neurons enhances their dynamic capabilities for synchronous regulation of motoneuron excitability during high muscle force generation. The eGFP+ vs. eGFP- V0c neurons were more richly innervated by serotonergic terminals, suggesting their greater propensity for regulation by descending serotonergic systems.

The Amino Terminal Domain and Modulation of Connexin36 Gap Junction Channels by Intracellular Magnesium Ions

Electrical synapses between neurons in the mammalian CNS are predominantly formed of the connexin36 (Cx36) gap junction (GJ) channel protein. Unique among GJs formed of a number of other members of the Cx gene family, Cx36 GJs possess a high sensitivity to intracellular Mg²⁺ that can robustly act to modulate the strength of electrical synaptic transmission. Although a putative Mg²⁺ binding site was previously identified to reside in the aqueous pore in the first extracellular (E1) loop domain, the involvement of the N-terminal (NT) domain in the atypical response of Cx36 GJs to pH was shown to depend on intracellular levels of Mg²⁺. In this study, we examined the impact of amino acid substitutions in the NT domain on Mg²⁺ modulation of Cx36 GJs, focusing on positions predicted to line the pore funnel, which constitutes the cytoplasmic entrance of the channel pore. We find that charge substitutions at the 8th, 13th, and 18th positions had pronounced effects on Mg²⁺ sensitivity, particularly at position 13 at which an A13K substitution completely abolished sensitivity to Mg²⁺. To assess potential mechanisms of Mg²⁺ action, we constructed and tested a series of mathematical models that took into account gating of the component hemichannels in a Cx36 GJ channel as well as Mg²⁺ binding to each hemichannel in open and/or closed states. Simultaneous model fitting of measurements of junctional conductance, g_j, and transjunctional Mg²⁺ fluxes using a fluorescent Mg²⁺ indicator suggested that the most viable mechanism for Cx36 regulation by Mg²⁺ entails the binding of Mg²⁺ to and subsequent stabilization of the closed state in each hemichannel. Reduced permeability to Mg²⁺ was also evident, particularly for the A13K substitution, but homology modeling of all charge-substituted NT variants showed only a moderate correlation between a reduction in the negative electrostatic potential and a reduction in the permeability to Mg²⁺ ions. Given the reported role of the E1 domain in Mg²⁺ binding together with the impact of NT substitutions on gating and the apparent state-dependence of Mg²⁺ binding, this study suggests that the NT domain can be an integral part of Mg²⁺ modulation of Cx36 GJs likely through the coupling of conformational changes between NT and E1 domains.

Recurrence-Based Synchronization Analysis of Weakly Coupled Bursting Neurons Under External ELF Fields

We investigate the response characteristics of a two-dimensional neuron model exposed to an externally applied extremely low frequency (ELF) sinusoidal electric field and the synchronization of neurons weakly coupled with gap junction. We find, by numerical simulations, that neurons can exhibit different spiking patterns, which are well observed in the structure of the recurrence plot (RP). We further study the synchronization between weakly coupled neurons in chaotic regimes under the influence of a weak ELF electric field. In general, detecting the phases of chaotic spiky signals is not easy by using standard methods. Recurrence analysis provides a reliable tool for defining phases even for noncoherent regimes or spiky signals. Recurrence-based synchronization analysis reveals that, even in the range of weak coupling, phase synchronization of the coupled neurons occurs and, by adding an ELF electric field, this synchronization increases depending on the amplitude of the externally applied ELF electric field. We further suggest a novel measure for RP-based phase synchronization analysis, which better takes into account the probabilities of recurrences.

Bionanohybrid composed of metalloprotein/DNA/MoS2/peptides to control the intracellular redox states of living cells and its applicability as a cell-based biomemory device

The development of cell-based bioelectronic devices largely depends on the direct control of intracellular redox states. However, most related studies have focused on the accurate measurement of electrical signals from living cells, whereas direct intracellular state control remains largely unexplored. Here, we developed a biocompatible transmembranal bionanohybrid structure composed of a recombinant metalloprotein, DNA, molybdenum disulfide nanoparticles (MoS₂), and peptides to control intracellular redox states, which can be used as a cell-based biomemory device. Using the capacitance of MoS₂ located inside the cell, the bionanohybrid controled the intracellular redox states of living cells by recording and extracting intracellular charges, which inturn was achieved by activating (writing) and deactivating (erasing) the cells. As a proof of concept, cell-based biomemory functions including writing, reading, and erasing were successfully demonstrated and confirmed via electrochemical methods and patch-clamp analyses, resulting in the development of the first in vitro cell-based biomemory device. This newly developed bionanohybrid provides a novel approach to control cellular redox states for cell-based bioelectronic applications, and can be applicable in a wide range of biological fields including bioelectronic medicine and intracellular redox status regulation.

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.

An electrically coupled pioneer circuit enables motor development via proprioceptive feedback in Drosophila embryos

Precocious movements are widely seen in embryos of various animal species. Whether such movements via proprioceptive feedback play instructive roles in motor development or are a mere reflection of activities in immature motor circuits is a long-standing question. Here we image the emerging motor activities in Drosophila embryos that lack proprioceptive feedback and show that proprioceptive experience is essential for the development of locomotor central pattern generators (CPGs). Downstream of proprioceptive inputs, we identify a pioneer premotor circuit composed of two pairs of segmental interneurons, whose gap-junctional transmission requires proprioceptive experience and plays a crucial role in CPG formation. The circuit autonomously generates rhythmic plateau potentials via IP₃-mediated Ca²⁺ release from internal stores, which contribute to muscle contractions and hence produce proprioceptive feedback. Our findings demonstrate the importance of self-generated movements in instructing motor development and identify the cells, circuit, and physiology at the core of this proprioceptive feedback.

Kuramoto Model-Based Analysis Reveals Oxytocin Effects on Brain Network Dynamics

The oxytocin effects on large-scale brain networks such as Default Mode Network (DMN) and Frontoparietal Network (FPN) have been largely studied using fMRI data. However, these studies are mainly based on the statistical correlation or Bayesian causality inference, lacking interpretability at the physical and neuroscience level. Here, we propose a physics-based framework of the Kuramoto model to investigate oxytocin effects on the phase dynamic neural coupling in DMN and FPN. Testing on fMRI data of 59 participants administrated with either oxytocin or placebo, we demonstrate that oxytocin changes the topology of brain communities in DMN and FPN, leading to higher synchronization in the FPN and lower synchronization in the DMN, as well as a higher variance of the coupling strength within the DMN and more flexible coupling patterns at group level. These results together indicate that oxytocin may increase the ability to overcome the corresponding internal oscillation dispersion and support the flexibility in neural synchrony in various social contexts, providing new evidence for explaining the oxytocin modulated social behaviors. Our proposed Kuramoto model-based framework can be a potential tool in network neuroscience and offers physical and neural insights into phase dynamics of the brain.

Connexin43 hemichannels contribute to working memory and excitatory synaptic transmission of pyramidal neurons in the prefrontal cortex of rats

The gap junction is essential for the communication between astrocytes and neurons by various connexins. Connexin43 hemichannels (Cx43 HCs), one of important subunits of gap junction protein, is highly expressed in astrocytes. It has been demonstrated that Cx43 HCs is involved in synaptic plasticity and learning and memory. However, whether the role of Cx43 HCs in the prefrontal cortex (PFC), a key brain region mediating cognitive and executive functions including working memory, still remains unclear. Here, we investigate that the role of Cx43 HCs in working memory through pharmacological inhibition of Cx43 HCs in the PFC. Gap26, a specific hemichannels blocker for Cx43 HCs, was bilaterally infused into the prelimbic (PrL) area of the PFC and then spatial working memory was examined in delayed alternation task in T-maze. Furthermore, the effect of Gap26 on synaptic transmission of prefrontal pyramidal neurons was examined using whole-cell patch recording in slice containing PFC. The demonstrate that inhibition of prefrontal cortex Cx43 HCs impairs the working memory and excitatory synaptic transmission of PFC neurons, suggesting that Cx43 HCs in the PFC contributes to working memory and excitatory synaptic transmission of neurons in rats.

New Insights on the Role of Connexins and Gap Junctions Channels in Adipose Tissue and Obesity

Due to the inability to curb the excessive increase in the prevalence of obesity and overweight, it is necessary to comprehend in more detail the factors involved in the pathophysiology and to appreciate more clearly the biochemical and molecular mechanisms of obesity. Thus, understanding the biological regulation of adipose tissue is of fundamental relevance. Connexin, a protein that forms intercellular membrane channels of gap junctions and unopposed hemichannels, plays a key role in adipogenesis and in the maintenance of adipose tissue homeostasis. The expression and function of Connexin 43 (Cx43) during the different stages of the adipogenesis are differentially regulated. Moreover, it has been shown that cell–cell communication decreases dramatically upon differentiation into adipocytes. Furthermore, inhibition of Cx43 degradation or constitutive overexpression of Cx43 blocks adipocyte differentiation. In the first events of adipogenesis, the connexin is highly phosphorylated, which is likely associated with enhanced Gap Junction (GJ) communication. In an intermediate state of adipocyte differentiation, Cx43 phosphorylation decreases, as it is displaced from the membrane and degraded through the proteasome; thus, Cx43 total protein is reduced. Cx is involved in cardiac disease as well as in obesity-related cardiovascular diseases. Different studies suggest that obesity together with a high-fat diet are related to the production of remodeling factors associated with expression and distribution of Cx43 in the atrium.

A gap-junction-mediated, calcium-signaling network controls blood progenitor fate decisions in hematopoiesis

Stem cell homeostasis requires coordinated fate decisions among stem cells that are often widely distributed within a tissue at varying distances from their stem cell niche. This requires a mechanism to ensure robust fate decisions within a population of stem cells. Here, we show that, in the Drosophila hematopoietic organ, the lymph gland (LG), gap junctions form a network that coordinates fate decisions between blood progenitors. Using live imaging of calcium signaling in intact LGs, we find that blood progenitors are connected through a signaling network. Blocking gap junction function disrupts this network, alters the pattern of encoded calcium signals, and leads to loss of progenitors and precocious blood cell differentiation. Ectopic and uniform activation of the calcium-signaling mediator CaMKII restores progenitor homeostasis when gap junctions are disrupted. Overall, these data show that gap junctions equilibrate cell signals between blood progenitors to coordinate fate decisions and maintain hematopoietic homeostasis.