Layer-specific properties of the persistent sodium current in sensorimotor cortex

Involvement of GABAergic Interneuron Subtypes in 4-Aminopyridine-Induced Seizure-Like Events in Mouse Entorhinal Cortex in Vitro

Single-unit recordings performed in temporal lobe epilepsy patients and in models of temporal lobe seizures have shown that interneurons are active at focal seizure onset. We performed simultaneous patch-clamp and field potential recordings in entorhinal cortex slices of GAD65 and GAD67 C57BL/6J male mice that express green fluorescent protein in GABAergic neurons to analyze the activity of specific interneuron (IN) subpopulations during acute seizure-like events (SLEs) induced by 4-aminopyridine (4-AP; 100 μm). IN subtypes were identified as parvalbuminergic (INPV, n = 17), cholecystokinergic (INCCK), n = 13], and somatostatinergic (INSOM, n = 15), according to neurophysiological features and single-cell digital PCR. INPV and INCCK discharged at the start of 4-AP-induced SLEs characterized by either low-voltage fast or hyper-synchronous onset pattern. In both SLE onset types, INSOM fired earliest before SLEs, followed by INPV and INCCK discharges. Pyramidal neurons became active with variable delays after SLE onset. Depolarizing block was observed in ∼50% of cells in each INs subgroup, and it was longer in IN (∼4 s) than in pyramidal neurons (<1 s). As SLE evolved, all IN subtypes generated action potential bursts synchronous with the field potential events leading to SLE termination. High-frequency firing throughout the SLE occurred in one-third of INPV and INSOM We conclude that entorhinal cortex INs are very active at the onset and during the progression of SLEs induced by 4-AP. These results support earlier in vivo and in vivo evidence and suggest that INs have a preferential role in focal seizure initiation and development.SIGNIFICANCE STATEMENT Focal seizures are believed to result from enhanced excitation. Nevertheless, we and others demonstrated that cortical GABAergic networks may initiate focal seizures. Here, we analyzed for the first time the role of different IN subtypes in seizures generated by 4-aminopyridine in the mouse entorhinal cortex slices. We found that in this in vitro focal seizure model, all IN types contribute to seizure initiation and that INs precede firing of principal cells. This evidence is in agreement with the active role of GABAergic networks in seizure generation.

Membrane Resonance in Pyramidal and GABAergic Neurons of the Mouse Perirhinal Cortex

The perirhinal cortex (PRC) is a polymodal associative region of the temporal lobe that works as a gateway between cortical areas and hippocampus. In recent years, an increasing interest arose in the role played by the PRC in learning and memory processes, such as object recognition memory, in contrast with certain forms of hippocampus-dependent spatial and episodic memory. The integrative properties of the PRC should provide all necessary resources to select and enhance the information to be propagated to and from the hippocampus. Among these properties, we explore in this paper the ability of the PRC neurons to amplify the output voltage to current input at selected frequencies, known as membrane resonance. Within cerebral circuits the resonance of a neuron operates as a filter toward inputs signals at certain frequencies to coordinate network activity in the brain by affecting the rate of neuronal firing and the precision of spike timing. Furthermore, the ability of the PRC neurons to resonate could have a fundamental role in generating subthreshold oscillations and in the selection of cortical inputs directed to the hippocampus. Here, performing whole-cell patch-clamp recordings from perirhinal pyramidal neurons and GABAergic interneurons of GAD67-GFP⁺ mice, we found, for the first time, that the majority of PRC neurons are resonant at their resting potential, with a resonance frequency of 0.5–1.5 Hz at 23°C and of 1.5–2.8 Hz at 36°C. In the presence of ZD7288 (blocker of HCN channels) resonance was abolished in both pyramidal neurons and interneurons, suggesting that I_h current is critically involved in resonance generation. Otherwise, application of TTx (voltage-dependent Na⁺ channel blocker) attenuates the resonance in pyramidal neurons but not in interneurons, suggesting that only in pyramidal neurons the persistent sodium current has an amplifying effect. These experimental results have also been confirmed by a computational model. From a functional point of view, the resonance in the PRC would affect the reverberating activity between neocortex and hippocampus, especially during slow wave sleep, and could be involved in the redistribution and strengthening of memory representation in cortical regions.

Kinetic properties of persistent Na+ current orchestrate oscillatory bursting in respiratory neurons

The rhythmic pattern of breathing depends on the pre-Bötzinger complex (preBötC) in the brainstem, a vital circuit that contains a population of neurons with intrinsic oscillatory bursting behavior. Here, we investigate the specific kinetic properties that enable voltage-gated sodium channels to establish oscillatory bursting in preBötC inspiratory neurons, which exhibit an unusually large persistent Na⁺ current (I_NaP). We first characterize the kinetics of I_NaP in neonatal rat brainstem slices in vitro, using whole-cell patch-clamp and computational modeling, and then test the contribution of I_NaP to rhythmic bursting in live neurons, using the dynamic clamp technique. We provide evidence that subthreshold activation, persistence at suprathreshold potentials, slow inactivation, and slow recovery from inactivation are kinetic features of I_NaP that regulate all aspects of intrinsic rhythmic bursting in preBötC neurons. The slow and cumulative inactivation of I_NaP during the burst active phase controls burst duration and termination, while the slow recovery from inactivation controls the duration of the interburst interval. To demonstrate this mechanism, we develop a Markov state model of I_NaP that explains a comprehensive set of voltage clamp data. By adding or subtracting a computer-generated I_NaP from a live neuron via dynamic clamp, we are able to convert nonbursters into intrinsic bursters, and vice versa. As a control, we test a model with inactivation features removed. Adding noninactivating I_NaP into nonbursters results in a pattern of random transitions between sustained firing and quiescence. The relative amplitude of I_NaP is the key factor that separates intrinsic bursters from nonbursters and can change the fraction of intrinsic bursters in the preBötC. I_NaP could thus be an important target for regulating network rhythmogenic properties.

Mutations in SCN3A cause early infantile epileptic encephalopathy

OBJECTIVE

Voltage-gated sodium (Na⁺ ) channels underlie action potential generation and propagation and hence are central to the regulation of excitability in the nervous system. Mutations in the genes SCN1A, SCN2A, and SCN8A, encoding the Na⁺ channel pore-forming (α) subunits Nav1.1, 1.2, and 1.6, respectively, and SCN1B, encoding the accessory subunit β₁ , are established causes of genetic epilepsies. SCN3A, encoding Nav1.3, is known to be highly expressed in brain, but has not previously been linked to early infantile epileptic encephalopathy. Here, we describe a cohort of 4 patients with epileptic encephalopathy and heterozygous de novo missense variants in SCN3A (p.Ile875Thr in 2 cases, p.Pro1333Leu, and p.Val1769Ala).

METHODS

All patients presented with treatment-resistant epilepsy in the first year of life, severe to profound intellectual disability, and in 2 cases (both with the variant p.Ile875Thr), diffuse polymicrogyria.

RESULTS

Electrophysiological recordings of mutant channels revealed prominent gain of channel function, with a markedly increased amplitude of the slowly inactivating current component, and for 2 of 3 mutants (p.Ile875Thr and p.Pro1333Leu), a leftward shift in the voltage dependence of activation to more hyperpolarized potentials. Gain of function was not observed for Nav1.3 variants known or presumed to be inherited (p.Arg1642Cys and p.Lys1799Gln). The antiseizure medications phenytoin and lacosamide selectively blocked slowly inactivating over transient current in wild-type and mutant Nav1.3 channels.

INTERPRETATION

These findings establish SCN3A as a new gene for infantile epileptic encephalopathy and suggest a potential pharmacologic intervention. These findings also reinforce the role of Nav1.3 as an important regulator of neuronal excitability in the developing brain, while providing additional insight into mechanisms of slow inactivation of Nav1.3. Ann Neurol 2018;83:703-717.

Thrombin modulates persistent sodium current in CA1 pyramidal neurons of young and adult rat hippocampus

Serine protease thrombin, a key factor of blood coagulation, participates in many neuronal processes important for normal brain functioning and during pathological conditions involving abnormal neuronal synchronization, neurodegeneration and inflammation. Our previous study on CA3 pyramidal neurons showed that application ofthrombin through the activation of specific protease-activated receptor 1 (PAR1) produces a significant hyperpolarizing shift of the activation of the TTX-sensitive persistent voltage-gated Na+ current (I(Nap)) thereby affecting membrane potential and seizure threshold at the network level. It was shown that PAR1 is also expressed in CA1 area of hippocampus and can be implicated in neuronal damage in this area after status epilepticus. The aim of the present study was to evaluate the effect of thrombin on I(NaP) in CA1 pyramidal neurons from adult and young rats. Using whole cell patch-clamp technique we demonstrate that thrombin application results in the hyperpolarization shift of I(NaP) activation as well as increase in the I(NaP) amplitude in both age groups. We have found that I(NaP) in pyramidal neurons of hippocampal CA 1 region is more vulnerable to the thrombin action than I(NaP) in pyramidal neurons of hippocampal CA3 region. We have also found that the immature hippocampus is more sensitive to thrombin action which emphasizes the contribution of thrombin-dependent pathway to the regulation of neuronal activity in immature brain.

Contribution of intrinsic motoneuron properties to discharge hysteresis and its estimation based on paired motor unit recordings: a simulation study

Motoneuron activity is strongly influenced by the activation of persistent inward currents (PICs) mediated by voltage-gated sodium and calcium channels. However, the amount of PIC contribution to the activation of human motoneurons can only be estimated indirectly. Simultaneous recordings of pairs of motor units have been used to provide an estimate of the PIC contribution by using the firing rate of the lower threshold unit to provide an estimate of the common synaptic drive to both units, and the difference in firing rate (ΔF) of this lower threshold unit at recruitment and de-recruitment of the higher threshold unit to estimate the PIC contribution to activation of the higher threshold unit. It has recently been suggested that a number of factors other than PIC can contribute to ΔF values, including mechanisms underlying spike frequency adaptation and spike threshold accommodation. In the present study, we used a set of compartmental models representing a sample of 20 motoneurons with a range of thresholds to investigate how several different intrinsic motoneuron properties can potentially contribute to variations in ΔF values. We drove the models with linearly increasing and decreasing noisy conductance commands of different rate of rise and duration and determined the influence of different intrinsic mechanisms on discharge hysteresis (the difference in excitatory drive at recruitment and de-recruitment) and ΔF. Our results indicate that, although other factors can contribute, variations in discharge hysteresis and ΔF values primarily reflect the contribution of dendritic PICs to motoneuron activation.

Fast-onset long-term open-state block of sodium channels by A-type FHFs mediates classical spike accommodation in hippocampal pyramidal neurons

Classical accommodation is a form of spike frequency adaptation in neurons whereby excitatory drive results in action potential output of gradually decreasing frequency. Here we describe an essential molecular component underlying classical accommodation in juvenile mouse hippocampal CA1 pyramidal neurons. A-type isoforms of fibroblast growth factor homologous factors (FHFs) bound to axosomatic voltage-gated sodium channels bear an N-terminal blocking particle that drives some associated channels into a fast-onset, long-term inactivated state. Use-dependent accumulating channel blockade progressively elevates spike voltage threshold and lengthens interspike intervals. The FHF particle only blocks sodium channels from the open state, and mutagenesis studies demonstrate that this particle uses multiple aliphatic and cationic residues to both induce and maintain the long-term inactivated state. The broad expression of A-type FHFs in neurons throughout the vertebrate CNS suggests a widespread role of these sodium channel modulators in the control of neural firing.

Attention-dependent reductions in burstiness and action-potential height in macaque area V4

Attention improves the encoding of visual stimuli. One mechanism that is implicated in facilitating sensory encoding is the firing of action potentials in bursts. We tested the hypothesis that when spatial attention is directed to a stimulus, this causes an increase in burst firing to the attended stimulus. To the contrary, we found an attention-dependent reduction in burstiness among putative pyramidal neurons in macaque area V4. We accounted for this using a conductance-based Hodgkin-Huxley style model in which attentional modulation stems from scaling excitation and inhibition. The model exhibited attention-dependent increases in firing rate and made the surprising and correct prediction that when attention is directed into a neuron’s receptive field, this reduces action potential height. The model thus provided a unified explanation for three distinct forms of attentional modulation, two of them novel, and implicates scaling of the responses of excitatory and inhibitory input populations in mediating attention.

Hippocampal hyperexcitability and specific epileptiform activity in a mouse model of Dravet syndrome

PURPOSE

Dravet syndrome (DS) is caused by dominant mutations of the SCN1A gene, encoding the NaV 1.1 sodium channel α subunit. Gene targeted mouse models of DS mutations replicate patients' phenotype and show reduced γ-aminobutyric acid (GABA)ergic inhibition. However, little is known on the properties of network hyperexcitability and on properties of seizure generation in these models. In fact, seizures have been studied thus far with surface electroencephalography (EEG), which did not show if specific brain regions are particularly involved. We have investigated hyperexcitability and epileptiform activities generated in neuronal networks of a mouse model of DS.

METHODS

We have studied heterozygous NaV 1.1 knock-out mice performing field potential recordings in combined hippocampal/cortical slices in vitro and video/depth electrode intracerebral recordings in vivo during hyperthermia-induced seizures.

KEY FINDINGS

In slices, we have disclosed specific signs of hyperexcitability of hippocampal circuits in both the pre-epileptic and epileptic periods, and a specific epileptiform activity was generated in the hippocampus upon application of the convulsant 4-aminopyridine in the epileptic period. During in vivo hyperthermia-induced seizures, we have observed selective hippocampal activity in early preictal phases and pronounced hippocampal activity in the ictal phase.

SIGNIFICANCE

We have identified specific epileptiform activities and signs of network hyperexcitability, and disclosed the important role of the hippocampus in seizure generation in this model. These activities may be potentially used as targets for screenings of antiepileptic approaches.

Phenytoin inhibits the persistent sodium current in neocortical neurons by modifying its inactivation properties

The persistent Na⁺ current (I_NaP) is important for neuronal functions and can play a role in several pathologies, although it is small compared to the transient Na⁺ current (I_NaT). Notably, I_NaP is not a real persistent current because it undergoes inactivation with kinetics in the order of tens of seconds, but this property has often been overlooked. Na⁺ channel blockers, drugs used for treating epilepsy and other diseases, can inhibit I_NaP, but the mechanism of this action and the conditions in which I_NaP can be actually inhibited have not been completely clarified yet. We evaluated the action of phenytoin (PHT), a prototype anti-epileptic Na⁺ channel blocker, on I_NaP inactivation in pyramidal neurons of rat sensorimotor cortical slices at different concentrations, from 5 to 100 µM. PHT did not modify I_NaP evoked with depolarizing voltage ramps of 50 or 100 mVs⁻¹, but decreased I_NaP evoked by slower voltage ramps (10 mVs⁻¹). However, at all of the tested concentrations, PHT decreased I_NaP evoked by faster ramps when they were preceded by inactivating pre-pulses. Moreover, PHT shifted towards negative potentials the voltage-dependence of I_NaP inactivation and accelerated its kinetics of development also at depolarized potentials (+40 mV), not consistently with a simple inactivated state stabilizer. Therefore, our study shows a prominent PHT effect on I_NaP inactivation rather than an open channel block, which is instead often implied. I_NaP is inhibited by PHT only in conditions that induce major I_NaP inactivation. These results highlight the importance of I_NaP inactivation not only for physiological functions but also as drug target, which could be shared by other therapeutic drugs. Through this action PHT can reduce I_NaP-induced long-lasting pathological depolarisations and intracellular sodium overload, whereas shorter I_NaP actions should not be modified. These properties set the conditions of efficacy and the limits of PHT as I_NaP inhibitor.

Determinants of synaptic integration and heterogeneity in rebound firing explored with data-driven models of deep cerebellar nucleus cells

Significant inroads have been made to understand cerebellar cortical processing but neural coding at the output stage of the cerebellum in the deep cerebellar nuclei (DCN) remains poorly understood. The DCN are unlikely to just present a relay nucleus because Purkinje cell inhibition has to be turned into an excitatory output signal, and DCN neurons exhibit complex intrinsic properties. In particular, DCN neurons exhibit a range of rebound spiking properties following hyperpolarizing current injection, raising the question how this could contribute to signal processing in behaving animals. Computer modeling presents an ideal tool to investigate how intrinsic voltage-gated conductances in DCN neurons could generate the heterogeneous firing behavior observed, and what input conditions could result in rebound responses. To enable such an investigation we built a compartmental DCN neuron model with a full dendritic morphology and appropriate active conductances. We generated a good match of our simulations with DCN current clamp data we recorded in acute slices, including the heterogeneity in the rebound responses. We then examined how inhibitory and excitatory synaptic input interacted with these intrinsic conductances to control DCN firing. We found that the output spiking of the model reflected the ongoing balance of excitatory and inhibitory input rates and that changing the level of inhibition performed an additive operation. Rebound firing following strong Purkinje cell input bursts was also possible, but only if the chloride reversal potential was more negative than −70 mV to allow de-inactivation of rebound currents. Fast rebound bursts due to T-type calcium current and slow rebounds due to persistent sodium current could be differentially regulated by synaptic input, and the pattern of these rebounds was further influenced by HCN current. Our findings suggest that active properties of DCN neurons could play a crucial role for signal processing in the cerebellum.

Persistent sodium current and its role in epilepsy

Sodium currents are essential for the initiation and propagation of neuronal firing. Alterations of sodium currents can lead to abnormal neuronal activity, such as occurs in epilepsy. The transient voltage-gated sodium current mediates the upstroke of the action potential. A small fraction of sodium current, termed the persistent sodium current (I(NaP)), fails to inactivate significantly, even with prolonged depolarization. I(NaP) is activated in the subthreshold voltage range and is capable of amplifying a neuron's response to synaptic input and enhancing its repetitive firing capability. A burgeoning literature is documenting mutations in sodium channels that underlie human disease, including epilepsy. Some of these mutations lead to altered neuronal excitability by increasing I(NaP). This review focuses on the pathophysiological effects of I(NaP) in epilepsy.

Transient oxidative stress evokes early changes in the functional properties of neonatal rat hypoglossal motoneurons in vitro

Oxidative stress of motoneurons is believed to be an important contributor to neurodegeneration underlying the familial (and perhaps even the sporadic) form of amyotrophic lateral sclerosis (ALS). This concept has generated numerous rodent genetic models with inborn oxidative stress to mimic the clinical condition. ALS is, however, a predominantly sporadic disorder probably triggered by environmental causes. Thus, it is interesting to understand how wild-type motoneurons react to strong oxidative stress as this response might cast light on the presymptomatic disease stage. The present study used, as a model, hypoglossal motoneurons from the rat brainstem slice to investigate how hydrogen peroxide could affect synaptic transmission and intrinsic motoneuron excitability in relation to their survival. Hydrogen peroxide (1 mm; 30 min) induced inward current or membrane depolarization accompanied by an increase in input resistance, enhanced firing and depressed spontaneous synaptic events. Despite enhanced intracellular oxidative processes, there was no death of motoneurons, although most cells were immunopositive for activating transcription factor 3, a stress-related transcription factor. Voltage-clamp experiments indicated increased frequency of excitatory or inhibitory miniature events, and reduced voltage-gated persistent currents of motoneurons. The global effect of this transient oxidative challenge was to depress the input flow from the premotor interneurons to motoneurons that became more excitable due to a combination of enhanced input resistance and impaired spike afterhyperpolarization. Our data show previously unreported changes in motoneuron activity associated with cell distress caused by a transient oxidative insult.

Modulation of intrinsic spiking in spinal cord neurons

The vertebrate spinal cord is equipped with a number of neuronal networks that underlie repetitive patterns of behavior as locomotion. Activity in such networks is mediated not only by intrinsic cellular properties but also by synaptic coupling. In this study, we focused on the modulation of the intrinsic activity by 5-hydroxytryptamine (5-HT, serotonin) and the cholinergic agonist muscarine in spinal cord cultures (embryonic age 14 rats). We investigated theses cultures (slices and dissociated cells) at the network level using multielectrode arrays (MEAs) and at the cellular level using whole cell patch clamp. All cultures showed bursting network activity and intrinsic activity when gamma-aminobutyric acid, glycine, and glutamate transmission was blocked. Using MEAs, we observed an increase of the intrinsic activity in the ventral part of the slices with 5-HT and muscarine. In single-cell recordings we found that 43 and 35% of the cells that were silent in the absence of fast synaptic activity were transformed into intrinsically spiking cells by 5-HT and muscarine, respectively. We tested the hypothesis that these neuromodulators act via modulation of the persistent sodium currents (I(NaP)) in these neurons. We found that 5-HT increased threefold the amplitude of I(NaP), specifically in the nonintrinsically spiking cells, and thus switched these cells into intrinsically spiking cells via activation of 5-HT(2) receptor and the phospholipase C pathway. In contrast, the effect of muscarine on nonintrinsically spiking neurons seems to be independent of I(NaP). We conclude from these findings that serotoninergic and cholinergic modulation can turn silent into spontaneously spiking neurons and thus initiate new sources of activity for rhythm generation in spinal networks.

Regulation of synaptic transmission by ambient extracellular glutamate

Many neuroscientists assume that ambient extracellular glutamate concentrations in the nervous system are biologically negligible under nonpathological conditions. This assumption is false. Hundreds of studies over several decades suggest that ambient extracellular glutamate levels in the intact mammalian brain are approximately 0.5 to approximately 5 microM. This has important implications. Glutamate receptors are desensitized by glutamate concentrations significantly lower than needed for receptor activation; 0.5 to 5 microM of glutamate is high enough to cause constitutive desensitization of most glutamate receptors. Therefore, most glutamate receptors in vivo may be constitutively desensitized, and ambient extracellular glutamate and receptor desensitization may be potent but generally unrecognized regulators of synaptic transmission. Unfortunately, the mechanisms regulating ambient extracellular glutamate and glutamate receptor desensitization remain poorly understood and understudied.

Differential Expression of Intrinsic Membrane Currents in Defined Cell Types of the Anterolateral Bed Nucleus of the Stria Terminalis

The anterolateral group of the bed nucleus of the stria terminalis (BNSTALG) plays a critical role in a diverse array of behaviors, although little is known of the physiological properties of neurons in this region. Using whole cell patch-clamp recordings from rat BNSTALG slices in vitro, we describe three distinct physiological cell types. Type I neurons were characterized by the presence of a depolarizing sag in response to hyperpolarizing current injection that resembled activation of the hyperpolarization-activated cation current Ih and a regular firing pattern in response to depolarizing current injection. Type II neurons exhibited the same depolarizing sag in response to hyperpolarizing current injection, but burst-fired in response to depolarizing current injection, which was indicative of the activation of the low-threshold calcium current IT. Type III neurons did not exhibit a depolarizing sag in response to hyperpolarizing current injection, but instead exhibited a fast time-independent rectification that became more pronounced with increased amplitude of hyperpolarizing current injection, and was indicative of activation of the inwardly rectifying potassium current IK(IR). Type III neurons also exhibited a regular firing pattern in response to depolarizing current. Using voltage-clamp analysis we further characterized the primary active currents that shaped the physiological properties of these distinct cell types, including Ih, IT, IK(IR), the voltage-dependent potassium current IA, and the persistent sodium current INaP. The functional relevance of each cell type is discussed in relation to prior anatomical studies, as well as how these currents may interact to modulate neuronal activity within the BNSTALG.

Activity-Dependent Initiation of a Prolonged Depolarization in Aplysia Bag Cell Neurons: Role for a Cation Channel

The translation of prior activity into changes in excitability is essential for memory and the initiation of behavior. After brief synaptic input, the bag cell neurons of Aplysia californica undergo a nearly 30-min afterdischarge to release egg-laying hormone. The present study examines a prolonged depolarization in cultured bag cell neurons. A 5-Hz, 10-s action potential train elicited a depolarization of about 10 mV, which lasted ≤30 min and was reduced by calmodulin kinase inhibition. Very broad action potentials (resulting from TEA application) decreased prolonged depolarization amplitude, indicating that strong Ca2+ influx did not necessarily promote the response. The prolonged depolarization current ( IPD) was recorded after 5-Hz, 10-s trains of square voltage pulses of varying duration (10–150 ms). Despite Ca2+ influx increasing steadily with pulse duration, IPD was most reliably initiated at 100 ms, suggesting a Ca2+ window or limit exists for triggering IPD. Consistent with this, modestly broader action potentials, evoked by lengthening the train current-pulse duration, resulted in smaller prolonged depolarizations. With respect to the properties of IPD, it displayed a linear current–voltage relationship with a reversal potential of about −45 mV that was shifted to approximately −25 mV by lowering internal K+ or about −56 mV by lowering external Na+ and Ca2+. IPD was blocked by Gd3+, but was not antagonized by MDL-123302A, SKF-96365, 2-APB, tetrodotoxin, or flufenamic acid. Optimal Ca2+ influx may activate calmodulin kinase and a voltage-independent, nonselective cation channel to initiate the prolonged depolarization, thereby contributing to the afterdischarge and reproduction.