Biology subjects

Thirumalai, V.

Publications and source records attributed to Thirumalai, V..

10 recordsLinked to original sources

bioRxiv2025

Molecular Configuration, Regulation and Function of Heterochannel Electrical Synapses

Neuron-specific expression of particular gap junction channel components defines the configuration and functional properties of electrical synapses. However, how a neuron utilises multiple, simultaneously expressed channel proteins - connexins or innexins -to make meaningful connections with distinct synaptic partners remains largely unknown. Using the posterior mechanosensory circuit in C. elegans, we discovered that individual electrical synapses can be formed by clustering together molecularly distinct gap junction channel-types made of three different innexin proteins, INX-1, UNC-7, and UNC-9. In this previously unknown configuration, which we term as heterochannel synapses, molecularly distinct gap junction channel types functionally collaborate to regulate posterior touch sensory behaviour, enhancing functional robustness. We show that the synaptic trafficking of the molecularly different channel types within a heterochannel synapse is independently regulated by discrete and conserved kinesin motor proteins, while distinct molecular pathways involving channel-specific retrograde kinesins regulate their turnover. These independent, channel-specific regulations also make individual synapse-level alterations in the composition of heterochannel synapses possible under altered environmental conditions, providing a novel mechanism for electrical synapse plasticity. Finally, we present evidence of heterochannel electrical synapses in C. elegans locomotory circuits and in the cerebellar Purkinje neurons of zebrafish larvae. Altogether, we demonstrate a novel heterochannel organization of electrical synapses, their regulation, and functional importance, which may be a conserved feature of metazoan nervous system.

neuroscience↗

bioRxiv2025

Developmental synaptic pruning in the olivo-cerebellar circuit sculpts predictive processing

Developmental synaptic pruning is thought to refine neural circuits by eliminating excess connectivity, but whether this process changes the computations performed by developing circuits remains poorly understood. Here, we show that synaptic pruning sharpens neural representations in the developing zebrafish cerebellum. Climbing fiber (CF) inputs to Purkinje neurons (PNs) progressively transition from poly- to predominantly mono-innervation during the first two weeks of life. In vivo whole-cell recordings reveal that individual CFs convey distinct sensorimotor information, and that convergence of differently tuned CFs degrades the precision of PN representations. As this convergence is reduced during development, PN responses become more selective and temporally precise. Later-stage PNs also exhibit stronger predictive responses to expected sensory events and enhanced responses to violations of those expectations, accompanied by improved behavioral adaptation. These findings suggest that developmental synaptic pruning is not simply a process of reducing excess connectivity, but can contribute to the functional specialization of neural circuits by sharpening neuronal representations and enabling increasingly precise predictive sensorimotor computations.

neuroscience↗

bioRxiv2024

Highly synchronized inhibition from Purkinje cells entrains cerebellar output in zebrafish

Cerebellar function, known to be important for motor learning and motor coordination, is mediated by efferent neurons that project to diverse motor areas. To understand cerebellar function, it is imperative to study how these efferent neurons integrate inputs from the principal neurons of the cerebellar cortex, the inhibitory Purkinje neurons (PNs). In zebrafish, PNs are bistable and we show here that bistability influences spike synchrony among PNs. Bistability also alters spike correlation with motor bouts. We asked how PN population synchrony influences Eurydendroid cells (ECs), which are postsynaptic targets of PNs and are the cerebellar efferent cells in zebrafish. Using optogenetics, we artificially modulated population synchrony of PNs over millisecond time scales and showed that under conditions of high synchrony, EC firing is briefly suppressed and entrained by PN spiking. However, the magnitude of such modulation is relatively small and indicates a strong combined influence of other synaptic inputs on EC spiking. Key PointsO_LICerebellar Purkinje neurons (PN) in larval zebrafish alter simple spike correlations with each other based on cellular state. C_LIO_LIThey also alter simple spike correlations with motor bouts as a function of state. C_LIO_LIWe altered PN population synchrony in a graded manner using optogenetics. C_LIO_LIPN targets are cerebellar efferent neurons, which in teleosts are called eurydendroid cells. C_LIO_LIWhen PN population is firing with high synchrony, eurydendroid cells are entrained better than when the PN input is asynchronous. C_LIO_LIThis can explain how PNs use bistability to modulate their influence on cerebellar output and ultimately, motor behavior. C_LI

neuroscience↗

bioRxiv2023

Ionic conductances driving tonic firing in Purkinje neurons of larval zebrafish

Purkinje neurons are critical for the functioning of the cerebellum, which is among the oldest and most conserved regions of the vertebrate brain. In mammals and in larval zebrafish, Purkinje neurons can generate tonic firing even when isolated from the network. Here we investigated the ionic basis of tonic firing in Purkinje neurons of larval zebrafish using voltage clamp for isolation of membrane currents along with pharmacology. We discovered that these neurons express L-type and P/Q-type high voltage-gated calcium currents, T-type low voltage-gated calcium currents and SK and BK-type calcium dependent potassium currents. Among these, L-type calcium currents and SK-type calcium-dependent potassium currents were indispensable for tonic firing, while blocking T-type, P/Q-type and BK currents had little effect in comparison. We observed that action potentials were broadened when either L-type or SK channels were blocked. Based on these results, we propose that calcium entry via L-type calcium channels activates SK potassium channels leading to faster action potential repolarization, in turn aiding the removal of inactivation of sodium channels. This allows larval zebrafish Purkinje neurons to continue to fire tonically for sustained periods. In mammals also, tonic firing in Purkinje neurons is driven by calcium channels coupling to calcium-dependent potassium channels, yet the specific types of channels involved are different. We therefore suggest that coupling of calcium channels and calcium-dependent potassium channels could be a conserved mechanism for sustaining long bouts of high frequency firing. Key pointsO_LITonic firing is an intrinsic property of Purkinje neurons in mammals and fish. C_LIO_LIThese neurons express multiple types of voltage-gated conductances including L-type, T-type, and P/Q-type calcium currents and SK- and BK-type calcium-dependent potassium currents. C_LIO_LIBlocking L-type calcium channels and SK-type calcium dependent potassium channels resulted in spike broadening and reduced tonic firing. C_LIO_LIL-type calcium currents were activated during the repolarization of the spike. C_LIO_LIBased on this we conclude that calcium entry via L-type channels activates SK-channels causing faster repolarization of the spike and therefore sustained tonic firing. C_LI Graphical abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=194 SRC="FIGDIR/small/569172v3_ufig1.gif" ALT="Figure 1"> View larger version (27K): org.highwire.dtl.DTLVardef@4ac7a5org.highwire.dtl.DTLVardef@1792e18org.highwire.dtl.DTLVardef@c1557forg.highwire.dtl.DTLVardef@d4084f_HPS_FORMAT_FIGEXP M_FIG C_FIG Legend: Top: The activation of voltage gated sodium channels (INa, pink), voltage gated calcium channels (ICa, dotted green), and SK-type channels (ISK, teal) during the action potential (Vm, black). Bottom: Under control conditions, sodium entry via voltage-gated sodium channels leads to depolarisation resulting in the activation of calcium channels. The elevation of intracellular calcium levels activates SK-type calcium-dependent potassium channels which repolarise the membrane, removing sodium channel inactivation. With L-type voltage-gated calcium channels blocked, this process is affected causing cessation of sustained tonic spiking.

neuroscience↗

bioRxiv2022

A machine-learning tool to identify bistable states from calcium imaging data

Mapping neuronal activation using calcium imaging in vivo during behavioral tasks has advanced our understanding of nervous system function. In almost all of these studies, calcium imaging is used to infer spike probabilities since action potentials activate voltage-gated calcium channels and increase intracellular calcium levels. However, neurons not only fire action potentials, but also convey information via intrinsic dynamics such as by generating bistable membrane potential states. While a number of tools for spike inference have been developed and are currently being used, no tool exists for converting calcium imaging signals to maps of cellular state in bistable neurons. Purkinje neurons (PNs) in the larval zebrafish cerebellum exhibit membrane potential bistability, firing either tonically or in bursts. Several studies have implicated the role of a population code in cerebellar function, with bistability adding an extra layer of complexity to this code. In this manuscript we develop a tool, CaMLSort which uses convolutional recurrent neural networks to classify calcium imaging traces as arising from either tonic or bursting cells. We validate this classifier using a number of different methods and find that it performs well on simulated event rasters as well as real biological data that it had not previously seen. Moreover, we find that CaMLsort generalizes to other bistable neurons, such as dopaminergic neurons in the ventral tegmental area of mice. Thus, this tool offers a new way of analyzing calcium imaging data from bistable neurons to understand how they participate in network computation and natural behaviors. Key Points SummaryCalcium imaging - the gold standard of inferring neuronal activity - does not report cellular state in neurons that are bistable, such as Purkinje neurons in the cerebellum of larval zebrafish. We model the relationship between Purkinje neuron electrical activity and its corresponding calcium signal to compile a dataset of state-labelled simulated calcium signals. We apply machine-learning methods to this dataset to develop a tool that can classify the state of a Purkinje neuron using only its calcium signal, which works well on real data even though it was trained only on simulated data. CaMLsort also generalizes well to bistable neurons in a different brain region (ventral tegmental area) in a different model organism (mouse). This tool offers a new way of analyzing calcium imaging data from populations of bistable neurons, thereby facilitating our understanding of how these neurons carry out their functions in a circuit.

neuroscience↗

bioRxiv2021

Adaptable internal representations drive cerebellum-mediated predictive control of an innate behavior

The cerebellum is known to optimize behavioral responses via error minimization, yet how past sensory experience shapes cerebellar output is still poorly understood. We imaged cerebellar Purkinje cells (PCs) in larval zebrafish during a behavioral assay in which they learnt to expect the direction of optic flow stimulus. PCs encoded signals of expectation and a graded error signal when a deviant stimulus was encountered. The amplitude of the error signal scaled based on stimulus history over minute-timescales. Granule cells (GC) did not encode error, however, some GC types encoded putative expectation signals. When PC population-wide readouts of expectation and error signals were strong, swim latency was low and vice versa, indicating participation of these signals in motor planning. Consistent with this, lesioning the cerebellum abolished expectation-dependent modulation of swim latency. Based on these results we propose that PCs use expectation and error signals to acquire and update internal models of the world. HighlightsO_LILarval zebrafish learn to expect repeatedly encountered optic flow stimuli C_LIO_LIStimulus history over minutes contributes to an internal model of the sensory world C_LIO_LIPurkinje neuron activity encodes deviations from expected direction of optic flow C_LIO_LIThe cerebellum uses learnt expectations to generate lower latency swim responses C_LI

neuroscience↗

bioRxiv2020

High behavioural variability mediated by altered neuronal excitability in auts2 mutant zebrafish

Autism spectrum disorders (ASDs) are characterized by abnormal behavioral traits arising from neural circuit dysfunction. While a number of genes have been implicated in ASDs, in most cases, a clear understanding of how mutations in these genes lead to circuit dysfunction and behavioral abnormality is absent. The autism susceptibility candidate 2 (AUTS2) gene is one such gene, associated with ASDs, intellectual disability and a range of other neurodevelopmental conditions. Yet, the function of AUTS2 in neural development and circuit function is not at all known. Here, we undertook functional analysis of Auts2a, the main homolog of AUTS2 in zebrafish, in the context of the escape behavior. Escape behavior in wild type zebrafish is critical for survival and is therefore, reliable, rapid, and has well-defined kinematic properties. Auts2a-/- zebrafish are viable, have normal gross morphology and can generate escape behavior with normal kinematics. However, the behavior is unreliable and delayed, with high trial-to-trial variability in the latency. We demonstrate that this is due to the reduced excitability of Mauthner neurons resulting in unreliable firing with stimuli that normally elicit the escape response. Combined with previous studies that show Auts2-regulation of the transcription of ion channel proteins, our results suggest that Auts2 sets the excitability of neurons by activating a set transcriptional program. Significance statementAUTS2 is one among recently identified autism susceptibility candidate genes, whose function in neuronal circuits is unclear. Using zebrafish as a model organism, we probe the function of Auts2a (homolog of mammalian AUTS2) at the cellular, network and behavioral levels. The escape behavior of Auts2a mutant zebrafish is highly variable with normal short latency escapes, long latency escapes and total failures across trials in the same fish. This occurs because neuronal excitability is inappropriately set in the Mauthner neurons of mutants leading to the large trial-to-trial variability in responses. The behavioral variability is fully explained by variability in firing action potentials in the Mauthner neuron, providing an integrative understanding of how behavioral variability arises from mutations at the genetic level.

neuroscience↗

bioRxiv2020

Gjd2b-mediated gap junctions promote glutamatergic synapse formation and dendritic elaboration in Purkinje neurons

Gap junctions between neurons serve as electrical synapses, in addition to conducting metabolites and signaling molecules. These functions of gap junctions have led to the idea that during development, gap junctions could prefigure chemical synapses. We present evidence for this idea at a central, glutamatergic synapse and provide some mechanistic insights. Here, we show that reduction or loss of Gjd2b-containing gap junctions led to a decrease in glutamatergic synapse density in cerebellar Purkinje neurons (PNs) in larval zebrafish. Gjd2b-/- larvae exhibited faster mEPSCs and a consistent decrease in dendritic arbor size. These PNs also showed decreased branch elongations but normal rate of branch retractions. Further, the dendritic growth deficits in gjd2b-/- mutants were rescued by expressing full length Gjd2b in single PNs. This suggests that Gjd2b may form heterotypic channels with other connexins in gjd2b-/- larvae, though it is not clear if PNs in wild type animals make homotypic or heterotypic gap junction channels. Dendritic growth deficits were not rescued by expressing a deletion mutant of Gjd2b unable to form functional channels. Finally, the expression levels of five isoforms of camkii were increased in gjd2b-/- larvae and inhibition of CaMKII restored dendritic arbor lengths of mutant larvae to wild type levels. These results suggest a link between signaling via Gjd2b-containing gap junctions, CaMKII function and dendritic growth. In sum, our results demonstrate that Gjd2b-mediated gap junctions are key regulators of glutamatergic synapse formation and dendritic elaboration in PNs.

neuroscience↗

bioRxiv2019

Neuromodulatory selection of motor neuron recruitment patterns in a visuomotor behavior increases speed

Animals generate locomotion at different speeds to suit their behavioral needs. Spinal circuits generate locomotion at these varying speeds by sequential activation of different spinal interneurons and motor neurons. Larval zebrafish can generate slow swims for prey capture and exploration by activation of secondary motor neurons and much faster and vigorous swims during escapes and struggles via the additional activation of primary motor neurons. Neuromodulators are known to alter motor output of spinal circuits yet their precise role in speed regulation is not understood well. Here, in the context of optomotor response (OMR), an innate, evoked locomotor behavior, we show that dopamine (DA) provides an additional layer to regulation of swim speed in larval zebrafish. Activation of D1-like receptors increases swim speed during OMR in free-swimming larvae. By analysing tail bend kinematics in head-restrained larvae, we show that the increase in speed is actuated by larger tail bends. Whole cell patch clamp recordings from motor neurons reveal that during OMR, typically only secondary motor neurons are active while primary motor neurons are quiescent. Activation of D1-like receptors increases motor drive from secondary motor neurons by decreasing spike threshold and latency. In addition, D1-like receptor activation enhances excitability and recruits quiescent primary motor neurons. Our findings provide an example of neuromodulatory reconfiguration of spinal motor neuron speed modules such that members are selectively recruited and motor drive is increased to effect changes in locomotor speed.\n\nHighlightsO_LIZebrafish larvae generate swims of increased speed during optomotor response when D1-like receptors are activated.\nC_LIO_LID1-like receptor activation increases the extent of tail bending during forward swims and turns resulting in increased swim speed.\nC_LIO_LINeuromodulation via D1-like receptors increases motor drive by enhancing excitability of slow motor neurons. In addition, D1-like receptor activation recruits quiescent fast motor neurons to increase swim speed.\nC_LIO_LIThis demonstrates neuromodulatory selection of motor neurons belonging to different speed modules to alter swimming behavior.\nC_LI\n\nGraphical abstract\n\nO_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=180 SRC=\"FIGDIR/small/683649v2_ufig1.gif\" ALT=\"Figure 1\">\nView larger version (34K):\norg.highwire.dtl.DTLVardef@17e90c4org.highwire.dtl.DTLVardef@524e45org.highwire.dtl.DTLVardef@190b1ecorg.highwire.dtl.DTLVardef@913733_HPS_FORMAT_FIGEXP M_FIG C_FIG

neuroscience↗

bioRxiv2016

Loss of the habenula neuromodulator Kisspeptin1 disrupts learning

Learning how to actively avoid a predictable aversive stimulus involves two steps: recognizing the cue that predicts upcoming punishment, and learning a behavioral response that will lead to avoidance. In zebrafish, ventral habenula (vHb) neurons have been proposed to participate in both steps by encoding the expected aversiveness of a stimulus. vHb neurons increase their firing rate as expectation of punishment grows, but reduce their activity as avoidance learning occurs. How the change in vHb activity occurs is not known. Here, we ask whether the neuromodulator kisspeptin1, which is expressed in the ventral habenula, could be involved. Kiss1 mutants were generated with Cas9 using guide RNAs targeted to the signal sequence. Mutants, which have a stop codon upstream of the kisspeptin1 peptide, have a deficiency in learning to avoid a shock that is predicted by light. Electrophysiology indicates that kisspeptin1 has a concentration-dependent effect on vHb neurons: depolarizing at low concentrations and hyperpolarizing at high concentrations. These data suggest that as the fish learns to cope with a threat, kisspeptin1 may differentially modulate vHb neurons. This implies that learning a behavioral strategy to overcome a stressor is accompanied by physiological change in habenula neurons.\n\nSignificance statementLearning to deal with adversity can positively affect ones ability to cope with challenges in the immediate future. Control thus causes short-term change in the brain. Here, we show that the neuromodulator kisspeptin1 is required to learn to avoid a punishment. Expression and electrophysiological recordings suggest that this molecule functions by controlling the ventral habenula, a region of the brain that mediates fear by regulating serotonin release. Kisspeptin1 may be a potential player in resilience developed as a result of control, extending previous findings that it can reduce fear.

animal behavior and cognition↗