Nav1.6 sodium channels are critical to pacemaking and fast spiking in globus pallidus neurons

Sodium channel SCN8A (Nav1.6): properties and de novo mutations in epileptic encephalopathy and intellectual disability

The sodium channel Nav1.6, encoded by the gene SCN8A, is one of the major voltage-gated channels in human brain. The sequences of sodium channels have been highly conserved during evolution, and minor changes in biophysical properties can have a major impact in vivo. Insight into the role of Nav1.6 has come from analysis of spontaneous and induced mutations of mouse Scn8a during the past 18 years. Only within the past year has the role of SCN8A in human disease become apparent from whole exome and genome sequences of patients with sporadic disease. Unique features of Nav1.6 include its contribution to persistent current, resurgent current, repetitive neuronal firing, and subcellular localization at the axon initial segment (AIS) and nodes of Ranvier. Loss of Nav1.6 activity results in reduced neuronal excitability, while gain-of-function mutations can increase neuronal excitability. Mouse Scn8a (med) mutants exhibit movement disorders including ataxia, tremor and dystonia. Thus far, more than ten human de novo mutations have been identified in patients with two types of disorders, epileptic encephalopathy and intellectual disability. We review these human mutations as well as the unique features of Nav1.6 that contribute to its role in determining neuronal excitability in vivo. A supplemental figure illustrating the positions of amino acid residues within the four domains and 24 transmembrane segments of Nav1.6 is provided to facilitate the location of novel mutations within the channel protein.

Synchrony in Parkinson's disease: importance of intrinsic properties of the external globus pallidus

The mechanisms for the emergence and transmission of synchronized oscillations in Parkinson's disease, which are potentially causal to motor deficits, remain debated. Aside from the motor cortex and the subthalamic nucleus, the external globus pallidus (GPe) has been shown to be essential for the maintenance of these oscillations and plays a major role in sculpting neural network activity in the basal ganglia (BG). While neural activity of the healthy GPe shows almost no correlations between pairs of neurons, prominent synchronization in the β frequency band arises after dopamine depletion. Several studies have proposed that this shift is due to network interactions between the different BG nuclei, including the GPe. However, recent studies demonstrate an important role for the properties of neurons within the GPe. In this review, we will discuss these intrinsic GPe properties and review proposed mechanisms for activity decorrelation within the dopamine-intact GPe. Failure of the GPe to desynchronize correlated inputs can be a possible explanation for synchronization in the whole BG. Potential triggers of synchronization involve the enhancement of GPe-GPe inhibition and changes in ion channel function in GPe neurons.

Spectral reconstruction of phase response curves reveals the synchronization properties of mouse globus pallidus neurons

The propensity of a neuron to synchronize is captured by its infinitesimal phase response curve (iPRC). Determining whether an iPRC is biphasic, meaning that small depolarizing perturbations can actually delay the next spike, if delivered at appropriate phases, is a daunting experimental task because negative lobes in the iPRC (unlike positive ones) tend to be small and may be occluded by the normal discharge variability of a neuron. To circumvent this problem, iPRCs are commonly derived from numerical models of neurons. Here, we propose a novel and natural method to estimate the iPRC by direct estimation of its spectral modes. First, we show analytically that the spectral modes of the iPRC of an arbitrary oscillator are readily measured by applying weak harmonic perturbations. Next, applying this methodology to biophysical neuronal models, we show that a low-dimensional spectral reconstruction is sufficient to capture the structure of the iPRC. This structure was preserved reasonably well even with added physiological scale jitter in the neuronal models. To validate the methodology empirically, we applied it first to a low-noise electronic oscillator with a known design and then to cortical pyramidal neurons, recorded in whole cell configuration, that are known to possess a monophasic iPRC. Finally, using the methodology in conjunction with perforated-patch recordings from pallidal neurons, we show, in contrast to recent modeling studies, that these neurons have biphasic somatic iPRCs. Biphasic iPRCs would cause lateral somatically targeted pallidal inhibition to desynchronize pallidal neurons, providing a plausible explanation for their lack of synchrony in vivo.

Physiological phenotype and vulnerability in Parkinson's disease

This review will focus on the principles underlying the hypothesis that neuronal physiological phenotype-how a neuron generates and regulates action potentials-makes a significant contribution to its vulnerability in Parkinson's disease (PD) and aging. A cornerstone of this hypothesis is that the maintenance of ionic gradients underlying excitability can pose a significant energetic burden for neurons, particularly those that have sustained residence times at depolarized membrane potentials, broad action potentials, prominent Ca(2+) entry, and modest intrinsic Ca(2+) buffering capacity. This energetic burden is shouldered in neurons primarily by mitochondria, the sites of cellular respiration. Mitochondrial respiration increases the production of damaging superoxide and other reactive oxygen species (ROS) that have widely been postulated to contribute to cellular aging and PD. Many of the genetic mutations and toxins associated with PD compromise mitochondrial function, providing a mechanistic linkage between known risk factors and cellular physiology that could explain the pattern of pathology in PD. Because much of the mitochondrial burden created by this at-risk phenotype is created by Ca(2+) entry through L-type voltage-dependent channels for which there are antagonists approved for human use, a neuroprotective strategy to reduce this burden is feasible.

Active decorrelation in the basal ganglia

The cytoarchitecturally-homogeneous appearance of the globus pallidus, subthalamic nucleus and substantia nigra has long been said to imply a high degree of afferent convergence and sharing of inputs by nearby neurons. Moreover, axon collaterals of neurons in the external segment of the globus pallidus and the substantia nigra pars reticulata arborize locally and make inhibitory synapses on other cells of the same type. These features suggest that the connectivity of the basal ganglia may impose spike-time correlations among the cells, and it has been puzzling that experimental studies have failed to demonstrate such correlations. One possible solution arises from studies of firing patterns in basal ganglia cells, which reveal that they are nearly all pacemaker cells. Their high rate of firing does not depend on synaptic excitation, but they fire irregularly because a dense barrage of synaptic inputs normally perturbs the timing of their autonomous activity. Theoretical and computational studies show that the responses of repetitively-firing neurons to shared input or mutual synaptic coupling often defy classical intuitions about temporal synaptic integration. The patterns of spike-timing among such neurons depend on the ionic mechanism of pacemaking, the level of background uncorrelated cellular and synaptic noise, and the firing rates of the neurons, as well as the properties of their synaptic connections. Application of these concepts to the basal ganglia circuitry suggests that the connectivity and physiology of these nuclei may be configured to prevent the establishment of permanent spike-timing relationships between neurons. The development of highly synchronous oscillatory patterns of activity in Parkinson's disease may result from the loss of pacemaking by some basal ganglia neurons, and accompanying breakdown of the mechanisms responsible for active decorrelation.

Short-term depression of external globus pallidus-subthalamic nucleus synaptic transmission and implications for patterning subthalamic activity

The frequency and pattern of activity in the reciprocally connected GABAergic external globus pallidus (GPe) and glutamatergic subthalamic nucleus (STN) are closely related to motor function. Although phasic, unitary GPe-STN inputs powerfully pattern STN activity ex vivo, correlated GPe-STN activity is not normally observed in vivo. To test the hypothesis that the GPe's influence is constrained by short-term synaptic depression, unitary GPe-STN inputs were stimulated in rat and mouse brain slices at rates and in patterns that mimicked GPe activity in vivo. Together with connectivity estimates these data were then used to simulate GPe-STN transmission. Unitary GPe-STN synaptic connections initially generated large conductances and transmitted reliably. However, the amplitude and reliability of transmission declined rapidly (τ = 0.6 ± 0.5 s) to <10% of their initial values when connections were stimulated at the mean rate of GPe activity in vivo (33 Hz). Recovery from depression (τ = 17.3 ± 18.9 s) was also longer than pauses in tonic GPe activity in vivo. Depression was the result of the limited supply of release-ready vesicles and was in sharp contrast to Calyx of Held transmission, which exhibited 100% reliability. Injection of simulated GPe-STN conductances revealed that synaptic depression caused tonic, nonsynchronized GPe-STN activity to disrupt rather than abolish autonomous STN activity. Furthermore, synchronous inhibition of tonically active GPe-STN neurons or phasic activity of GPe-STN neurons reliably patterned STN activity through disinhibition and inhibition, respectively. Together, these data argue that the frequency and pattern of GPe activity profoundly influence its transmission to the STN.

Antagonism of lidocaine inhibition by open-channel blockers that generate resurgent Na current

Na channels that generate resurgent current express an intracellular endogenous open-channel blocking protein, whose rapid binding upon depolarization and unbinding upon repolarization minimizes fast and slow inactivation. Na channels also bind exogenous compounds, such as lidocaine, which functionally stabilize inactivation. Like the endogenous blocking protein, these use-dependent inhibitors bind most effectively at depolarized potentials, raising the question of how lidocaine-like compounds affect neurons with resurgent Na current. We therefore recorded lidocaine inhibition of voltage-clamped, tetrodotoxin-sensitive Na currents in mouse Purkinje neurons, which express a native blocking protein, and in mouse hippocampal CA3 pyramidal neurons with and without a peptide from the cytoplasmic tail of NaVβ4 (the β4 peptide), which mimics endogenous open-channel block. To control channel states during drug exposure, lidocaine was applied with rapid-solution exchange techniques during steps to specific voltages. Inhibition of Na currents by lidocaine was diminished by either the β4 peptide or the native blocking protein. In peptide-free CA3 cells, prolonging channel opening with a site-3 toxin, anemone toxin II, reduced lidocaine inhibition; this effect was largely occluded by open-channel blockers, suggesting that lidocaine binding is favored by inactivation but prevented by open-channel block. In constant 100 μm lidocaine, current-clamped Purkinje cells continued to fire spontaneously. Similarly, the β4 peptide reduced lidocaine-dependent suppression of spiking in CA3 neurons in slices. Thus, the open-channel blocking protein responsible for resurgent current acts as a natural antagonist of lidocaine. Neurons with resurgent current may therefore be less susceptible to use-dependent Na channel inhibitors used as local anesthetic, antiarrhythmic, and anticonvulsant drugs.

The energy cost of action potential propagation in dopamine neurons: clues to susceptibility in Parkinson's disease

Dopamine neurons of the substantia nigra pars compacta (SNc) are uniquely sensitive to degeneration in Parkinson's disease (PD) and its models. Although a variety of molecular characteristics have been proposed to underlie this sensitivity, one possible contributory factor is their massive, unmyelinated axonal arbor that is orders of magnitude larger than other neuronal types. We suggest that this puts them under such a high energy demand that any stressor that perturbs energy production leads to energy demand exceeding supply and subsequent cell death. One prediction of this hypothesis is that those dopamine neurons that are selectively vulnerable in PD will have a higher energy cost than those that are less vulnerable. We show here, through the use of a biology-based computational model of the axons of individual dopamine neurons, that the energy cost of axon potential propagation and recovery of the membrane potential increases with the size and complexity of the axonal arbor according to a power law. Thus SNc dopamine neurons, particularly in humans, whose axons we estimate to give rise to more than 1 million synapses and have a total length exceeding 4 m, are at a distinct disadvantage with respect to energy balance which may be a factor in their selective vulnerability in PD.

Ca(V)1.3-driven SK channel activation regulates pacemaking and spike frequency adaptation in mouse chromaffin cells

Mouse chromaffin cells (MCCs) fire spontaneous action potentials (APs) at rest. Ca(v)1.3 L-type calcium channels sustain the pacemaker current, and their loss results in depolarized resting potentials (V(rest)), spike broadening, and remarkable switches into depolarization block after BayK 8644 application. A functional coupling between Ca(v)1.3 and BK channels has been reported but cannot fully account for the aforementioned observations. Here, using Ca(v)1.3(-/-) mice, we investigated the role of Ca(v)1.3 on SK channel activation and how this functional coupling affects the firing patterns induced by sustained current injections. MCCs express SK1-3 channels whose tonic currents are responsible for the slow irregular firing observed at rest. Percentage of frequency increase induced by apamin was found inversely correlated to basal firing frequency. Upon stimulation, MCCs build-up Ca(v)1.3-dependent SK currents during the interspike intervals that lead to a notable degree of spike frequency adaptation (SFA). The major contribution of Ca(v)1.3 to the subthreshold Ca(2+) charge during an AP-train rather than a specific molecular coupling to SK channels accounts for the reduced SFA of Ca(v)1.3(-/-) MCCs. Low adaptation ratios due to reduced SK activation associated with Ca(v)1.3 deficiency prevent the efficient recovery of Na(V) channels from inactivation. This promotes a rapid decline of AP amplitudes and facilitates early onset of depolarization block following prolonged stimulation. Thus, besides serving as pacemaker, Ca(v)1.3 slows down MCC firing by activating SK channels that maintain Na(V) channel availability high enough to preserve stable AP waveforms, even upon high-frequency stimulation of chromaffin cells during stress responses.

The Hodgkin-Huxley heritage: from channels to circuits

The Hodgkin-Huxley studies of the action potential, published 60 years ago, are a central pillar of modern neuroscience research, ranging from molecular investigations of the structural basis of ion channel function to the computational implications at circuit level. In this Symposium Review, we aim to demonstrate the ongoing impact of Hodgkin's and Huxley's ideas. The Hodgkin-Huxley model established a framework in which to describe the structural and functional properties of ion channels, including the mechanisms of ion permeation, selectivity, and gating. At a cellular level, the model is used to understand the conditions that control both the rate and timing of action potentials, essential for neural encoding of information. Finally, the Hodgkin-Huxley formalism is central to computational neuroscience to understand both neuronal integration and circuit level information processing, and how these mechanisms might have evolved to minimize energy cost.

Firing rate and pattern heterogeneity in the globus pallidus arise from a single neuronal population

Intrinsic heterogeneity in networks of interconnected cells has profound effects on synchrony and spike-time reliability of network responses. Projection neurons of the globus pallidus (GPe) are interconnected by GABAergic inhibitory synapses and in vivo fire continuously but display significant rate and firing pattern heterogeneity. Despite being deprived of most of their synaptic inputs, GPe neurons in slices also fire continuously and vary greatly in their firing rate (1-70 spikes/s) and in regularity of their firing. We asked if this rate and pattern heterogeneity arises from separate cell types differing in rate, local synaptic interconnections, or variability of intrinsic properties. We recorded the resting discharge of GPe neurons using extracellular methods both in vivo and in vitro. Spike-to-spike variability (jitter) was measured as the standard deviation of interspike intervals. Firing rate and jitter covaried continuously, with slow firing being associated with higher variability than faster firing, as would be expected from heterogeneity arising from a single physiologically distinct cell type. The relationship between rate and jitter was unaffected by blockade of GABA and glutamate receptors. When the firing rate of individual neurons was altered with constant current, jitter changed to maintain the rate-jitter relationship seen across neurons. Long duration (30-60 min) recordings showed slow and spontaneous bidirectional drift in rate similar to the across-cell heterogeneity. Paired recordings in vivo and in vitro showed that individual cells wandered in rate independently of each other. Input conductance and rate wandered together, in a manner suggestive that both were due to fluctuations of an inward current.

Resurgent-like currents in mouse vas deferens myocytes are mediated by NaV1.6 voltage-gated sodium channels

Patch-clamp experiments were performed to investigate the molecular properties of resurgent-like currents in single smooth muscle cells dispersed from mouse vas deferens, utilizing both Na(V)1.6-null mice (Na(V)1.6(-/-)), lacking the expression of the Scn8a Na(+) channel gene, and their wild-type littermates (Na(V)1.6(+/+)). Na(V)1.6 immunoreactivity was clearly visible in dispersed smooth muscle cells obtained from Na(V)1.6(+/+), but not Na(V)1.6(-/-), vas deferens. Following a depolarization to +30 mV from a holding potential of -70 mV (to produce maximal inactivation of the Na(+) current), repolarization to voltages between -60 and +20 mV elicited a tetrodotoxin (TTX)-sensitive inward current in Na(V)1.6(+/+), but not Na(V)1.6(-/-), vas deferens myocytes. The resurgent-like current in Na(V)1.6(+/+) vas deferens myocytes peaked at approximately -20 mV in the current-voltage relationship. The peak amplitude of the resurgent-like current remained at a constant level when the membrane potential was repolarized to -20 mV following the application of depolarizing rectangular pulses to more positive potentials than +20 mV. 4,9-Anhydrotetrodotoxin (4,9-anhydroTTX), a selective Na(V)1.6 blocking toxin, purified from a crude mixture of TTX analogues by LC-FLD techniques, reversibly suppressed the resurgent-like currents. β-Pompilidotoxin, a voltage-gated Na(+) channel activator, evoked sustained resurgent-like currents in Na(V)1.6(+/+) but not Na(V)1.6(-/-) murine vas deferens myocytes. These results strongly indicate that, primarily, resurgent-like currents are generated as a result of Na(V)1.6 channel activity.

Striatal input- and rate-dependent effects of muscarinic receptors on pallidal firing

The globus pallidus (GP) plays a key role in the overall basal ganglia (BG) activity. Despite evidence of cholinergic inputs to GP, their role in the spiking activity of GP neurons has not received attention. We examine the effect of local activation and blockade of muscarinic receptors (MRs) in the spontaneous firing of GP neurons both in normal and ipsilateral striatum-lesioned rats. We found that activation of MRs produces heterogeneous responses in both normal and ipsilateral striatum-lesioned rats: in normal rats the response evoked by MRs depends on the predrug basal firing rate; the inhibition evoked by MRs is higher in normal rats than in striatum-lesioned rats; the number of neurons that undergo inhibition is lower in striatum-lesioned rats than in normal rats. Our data suggest that modulation of MRs in the GP depends on the firing rate before their activation and on the integrity of the striato-pallidal pathway.

Voltage-gated sodium channel-associated proteins and alternative mechanisms of inactivation and block

Voltage-gated sodium channels mediate inward current of action potentials upon membrane depolarization of excitable cells. The initial transient sodium current is restricted to milliseconds through three distinct channel-inactivating and blocking mechanisms. All pore-forming alpha subunits of sodium channels possess structural elements mediating fast inactivation upon depolarization and recovery within milliseconds upon membrane repolarization. Accessory subunits modulate fast inactivation dynamics, but these proteins can also limit current by contributing distinct inactivation and blocking particles. A-type isoforms of fibroblast growth factor homologous factors (FHFs) bear a particle that induces long-term channel inactivation, while sodium channel subunit Navβ4 employs a blocking particle that rapidly dissociates upon membrane repolarization to generate resurgent current. Despite their different physiological functions, the FHF and Navβ4 particles have similarity in amino acid composition and mechanisms for docking within sodium channels. The three competing channel-inactivating and blocking processes functionally interact to regulate a neuron's intrinsic excitability.

Intrinsic and integrative properties of substantia nigra pars reticulata neurons

The GABA projection neurons of the substantia nigra pars reticulata (SNr) are output neurons for the basal ganglia and thus critical for movement control. Their most striking neurophysiological feature is sustained, spontaneous high frequency spike firing. A fundamental question is: what are the key ion channels supporting the remarkable firing capability in these neurons? Recent studies indicate that these neurons express tonically active type 3 transient receptor potential (TRPC3) channels that conduct a Na-dependent inward current even at hyperpolarized membrane potentials. When the membrane potential reaches -60 mV, a voltage-gated persistent sodium current (I(NaP)) starts to activate, further depolarizing the membrane potential. At or slightly below -50 mV, the large transient voltage-activated sodium current (I(NaT)) starts to activate and eventually triggers the rapid rising phase of action potentials. SNr GABA neurons have a higher density of I(NaT), contributing to the faster rise and larger amplitude of action potentials, compared with the slow-spiking dopamine neurons. I(NaT) also recovers from inactivation more quickly in SNr GABA neurons than in nigral dopamine neurons. In SNr GABA neurons, the rising phase of the action potential triggers the activation of high-threshold, inactivation-resistant Kv3-like channels that can rapidly repolarize the membrane. These intrinsic ion channels provide SNr GABA neurons with the ability to fire spontaneous and sustained high frequency spikes. Additionally, robust GABA inputs from direct pathway medium spiny neurons in the striatum and GABA neurons in the globus pallidus may inhibit and silence SNr GABA neurons, whereas glutamate synaptic input from the subthalamic nucleus may induce burst firing in SNr GABA neurons. Thus, afferent GABA and glutamate synaptic inputs sculpt the tonic high frequency firing of SNr GABA neurons and the consequent inhibition of their targets into an integrated motor control signal that is further fine-tuned by neuromodulators including dopamine, serotonin, endocannabinoids, and H₂O₂.

Rbfox proteins regulate alternative splicing of neuronal sodium channel SCN8A

The SCN8A gene encodes the voltage-gated sodium channel Na(v)1.6, a major channel in neurons of the CNS and PNS. SCN8A contains two alternative exons,18N and 18A, that exhibit tissue specific splicing. In brain, the major SCN8A transcript contains exon 18A and encodes the full-length sodium channel. In other tissues, the major transcript contains exon 18N and encodes a truncated protein, due to the presence of an in-frame stop codon. Selection of exon 18A is therefore essential for generation of a functional channel protein, but the proteins involved in this selection have not been identified. Using a 2.6 kb Scn8a minigene containing exons 18N and 18A, we demonstrate that co-transfection with Fox-1 or Fox-2 initiates inclusion of exon 18A. This effect is dependent on the consensus Fox binding site located 28 bp downstream of exon 18A. We examined the alternative splicing of human SCN8A and found that the postnatal switch to exon 18A is completed later than 10 months of age. In purified cell populations, transcripts containing exon 18A predominate in neurons but are not present in oligodendrocytes or astrocytes. Transcripts containing exon 18N appear to be degraded by nonsense-mediated decay in HEK cells. Our data indicate that RBFOX proteins contribute to the cell-specific expression of Na(v)1.6 channels in mature neurons.

Cross-species conservation of open-channel block by Na channel β4 peptides reveals structural features required for resurgent Na current

Voltage-gated Na channels in many neurons, including several in the cerebellum and brainstem, are specialized to allow rapid firing of action potentials. Repetitive firing is facilitated by resurgent Na current, which flows upon repolarization as Na channels recover through open states from block by an endogenous protein. The best candidate blocking protein to date is Na(V)β4. The sequence of this protein diverges among species, however, while high-frequency firing is maintained, raising the question of whether the proposed blocking action of the Na(V)β4 cytoplasmic tail has been conserved. Here, we find that, despite differences in the Na(V)β4 sequence, Purkinje cells isolated from embryonic chick have resurgent currents with kinetics and amplitudes indistinguishable from those in mouse Purkinje cells. Furthermore, synthetic peptides derived from the divergent Na(V)β4 cytoplasmic tails from five species have the capacity to induce resurgent current in mouse hippocampal neurons, which lack a functional endogenous blocking protein. These data further support a blocking role for Na(V)β4 and also indicate the relative importance of different residues in inducing open-channel block. To investigate the contribution of the few highly conserved residues to open-channel block, we synthesized several mutant peptides in which the identities and relative orientations of a phenylalanine and two lysines were disrupted. These mutant peptides produced currents with vastly different kinetics than did the species-derived peptides, suggesting that these residues are required for an open-channel block that approximates physiological resurgent Na current. Thus, if other blocking proteins exist, they may share these structural elements with the Na(V)β4 cytoplasmic tail.

Molecular and functional differences in voltage-activated sodium currents between GABA projection neurons and dopamine neurons in the substantia nigra

GABA projection neurons (GABA neurons) in the substantia nigra pars reticulata (SNr) and dopamine projection neurons (DA neurons) in substantia nigra pars compacta (SNc) have strikingly different firing properties. SNc DA neurons fire low-frequency, long-duration spikes, whereas SNr GABA neurons fire high-frequency, short-duration spikes. Since voltage-activated sodium (Na(V)) channels are critical to spike generation, the different firing properties raise the possibility that, compared with DA neurons, Na(V) channels in SNr GABA neurons have higher density, faster kinetics, and less cumulative inactivation. Our quantitative RT-PCR analysis on immunohistochemically identified nigral neurons indicated that mRNAs for pore-forming Na(V)1.1 and Na(V)1.6 subunits and regulatory Na(V)β1 and Na(v)β4 subunits are more abundant in SNr GABA neurons than SNc DA neurons. These α-subunits and β-subunits are key subunits for forming Na(V) channels conducting the transient Na(V) current (I(NaT)), persistent Na current (I(NaP)), and resurgent Na current (I(NaR)). Nucleated patch-clamp recordings showed that I(NaT) had a higher density, a steeper voltage-dependent activation, and a faster deactivation in SNr GABA neurons than in SNc DA neurons. I(NaT) also recovered more quickly from inactivation and had less cumulative inactivation in SNr GABA neurons than in SNc DA neurons. Furthermore, compared with nigral DA neurons, SNr GABA neurons had a larger I(NaR) and I(NaP). Blockade of I(NaP) induced a larger hyperpolarization in SNr GABA neurons than in SNc DA neurons. Taken together, these results indicate that Na(V) channels expressed in fast-spiking SNr GABA neurons and slow-spiking SNc DA neurons are tailored to support their different spiking capabilities.

Functional connectivity and integrative properties of globus pallidus neurons

The globus pallidus consists of the external (GPe) and the internal (GPi) segments. The GPe and GPi have different functional roles. The GPe is located centrally within multiple basal ganglia feedforward and feedback connections. The GPi is an output nucleus of the basal ganglia. A complex interplay between intrinsic pacemaking conductances and the balance of glutamatergic and GABAergic input largely determines the rate and pattern of firing of pallidal neurons. The initial part of this article introduces recent findings made in vivo that are related to the roles of glutamatergic and GABAergic inputs in the control of pallidal activity. The latter part describes the roles of intrinsic mechanisms of GPe neurons in the integration of the synaptic inputs. The presence of dendritic voltage-gated sodium channels may allow the initiation of dendritic spikes, giving distal inputs on the long and thin GPe dendrites an opportunity to strongly shape spiking activity. Basal ganglia disorders including Parkinson's disease, hemiballismus, and dystonias are accompanied by increased irregularity and synchronized bursts of pallidal activity. These changes may be in part due to changes in the GABA release in the GPe and GPi, but also involve intrinsic cellular changes in pallidal neurons.

Resurgent Na+ current: a new avenue to neuronal excitability control

Integrative and firing properties are important characteristics of neuronal circuits and these responses are determined in large part by the repertoire of ion channels they express, which can vary considerably between cell types. Recently, a new mode of operation of voltage dependent sodium channels has been described that generates a so-called resurgent Na+ current. Accumulating evidence suggests resurgent Na current participates in the generation of sub-threshold inward Na+ current causing membrane depolarization which provides the necessary drive to fire high-frequency action potentials. Recent studies indicate that resurgent Na+ current could be a more widespread feature than previously thought.

Impact of fast sodium channel inactivation on spike threshold dynamics and synaptic integration

Neurons spike when their membrane potential exceeds a threshold value. In central neurons, the spike threshold is not constant but depends on the stimulation. Thus, input-output properties of neurons depend both on the effect of presynaptic spikes on the membrane potential and on the dynamics of the spike threshold. Among the possible mechanisms that may modulate the threshold, one strong candidate is Na channel inactivation, because it specifically impacts spike initiation without affecting the membrane potential. We collected voltage-clamp data from the literature and we found, based on a theoretical criterion, that the properties of Na inactivation could indeed cause substantial threshold variability by itself. By analyzing simple neuron models with fast Na inactivation (one channel subtype), we found that the spike threshold is correlated with the mean membrane potential and negatively correlated with the preceding depolarization slope, consistent with experiments. We then analyzed the impact of threshold dynamics on synaptic integration. The difference between the postsynaptic potential (PSP) and the dynamic threshold in response to a presynaptic spike defines an effective PSP. When the neuron is sufficiently depolarized, this effective PSP is briefer than the PSP. This mechanism regulates the temporal window of synaptic integration in an adaptive way. Finally, we discuss the role of other potential mechanisms. Distal spike initiation, channel noise and Na activation dynamics cannot account for the observed negative slope-threshold relationship, while adaptive conductances (e.g. K+) and Na inactivation can. We conclude that Na inactivation is a metabolically efficient mechanism to control the temporal resolution of synaptic integration.

Neurons spike when their combined inputs exceed a threshold value, but recent experimental findings have shown that this value also depends on the inputs. Thus, to understand how neurons respond to input spikes, it is important to know how inputs modify the spike threshold. Spikes are generated by sodium channels, which inactivate when the neuron is depolarized, raising the threshold for spike initiation. We found that inactivation properties of sodium channels could indeed cause substantial threshold variability in central neurons. We then analyzed in models the implications of this form of threshold modulation on neuronal function. We found that this mechanism makes neurons more sensitive to coincident spikes and provides them with an energetically efficient form of gain control.

The use of automated parameter searches to improve ion channel kinetics for neural modeling

The voltage and time dependence of ion channels can be regulated, notably by phosphorylation, interaction with phospholipids, and binding to auxiliary subunits. Many parameter variation studies have set conductance densities free while leaving kinetic channel properties fixed as the experimental constraints on the latter are usually better than on the former. Because individual cells can tightly regulate their ion channel properties, we suggest that kinetic parameters may be profitably set free during model optimization in order to both improve matches to data and refine kinetic parameters. To this end, we analyzed the parameter optimization of reduced models of three electrophysiologically characterized and morphologically reconstructed globus pallidus neurons. We performed two automated searches with different types of free parameters. First, conductance density parameters were set free. Even the best resulting models exhibited unavoidable problems which were due to limitations in our channel kinetics. We next set channel kinetics free for the optimized density matches and obtained significantly improved model performance. Some kinetic parameters consistently shifted to similar new values in multiple runs across three models, suggesting the possibility for tailored improvements to channel models. These results suggest that optimized channel kinetics can improve model matches to experimental voltage traces, particularly for channels characterized under different experimental conditions than recorded data to be matched by a model. The resulting shifts in channel kinetics from the original template provide valuable guidance for future experimental efforts to determine the detailed kinetics of channel isoforms and possible modulated states in particular types of neurons.

HCN channelopathy in external globus pallidus neurons in models of Parkinson's disease

Parkinson's disease is a common neurodegenerative disorder characterized by a profound motor disability that is traceable to the emergence of synchronous, rhythmic spiking in neurons of the external segment of the globus pallidus (GPe). The origins of this pathophysiology are poorly defined for the generation of pacemaking. After the induction of a parkinsonian state in mice, there was a progressive decline in autonomous GPe pacemaking, which normally serves to desynchronize activity. The loss was attributable to the downregulation of an ion channel that is essential in pacemaking, the hyperpolarization and cyclic nucleotide-gated (HCN) channel. Viral delivery of HCN2 subunits restored pacemaking and reduced burst spiking in GPe neurons. However, the motor disability induced by dopamine (DA) depletion was not reversed, suggesting that the loss of pacemaking was a consequence, rather than a cause, of key network pathophysiology, a conclusion that is consistent with the ability of L-type channel antagonists to attenuate silencing after DA depletion.

Dendritic sodium channels regulate network integration in globus pallidus neurons: a modeling study

The globus pallidus (GP) predominantly contains GABAergic projection neurons that occupy a central position in the indirect pathway of the basal ganglia. They have long dendrites that can extend through one-half the diameter of the GP in rats, potentially enabling convergence and interaction between segregated basal ganglia circuits. Because of the length and fine diameter of GP dendrites, however, it is unclear how much influence distal synapses have on spiking activity. Dendritic expression of fast voltage-dependent Na(+) channels (NaF channels) can enhance the importance of distal excitatory synapses by allowing for dendritic spike initiation and by subthreshold boosting of EPSPs. Antibody labeling has demonstrated the presence of NaF channel proteins in GP dendrites, but the quantitative expression density of the channels remains unknown. We built a series of nine GP neuron models that differed only in their dendritic NaF channel expression level to assess the functional impact of this parameter. The models were all similar in their basic electrophysiological features; however, higher expression levels of dendritic NaF channels increased the relative effectiveness of distal inputs for both excitatory and inhibitory synapses, broadening the effective extent of the dendritic tree. Higher dendritic NaF channel expression also made the neurons more resistant to tonic inhibition and highly sensitive to clustered synchronous excitation. The dendritic NaF channel expression pattern may therefore be a critical determinant of convergence for both the striatopallidal and subthalamopallidal projections, while also dictating which spatiotemporal input patterns are most effective at driving GP neuron output.

Mechanisms of sustained high firing rates in two classes of vestibular nucleus neurons: differential contributions of resurgent Na, Kv3, and BK currents

To fire at high rates, neurons express ionic currents that work together to minimize refractory periods by ensuring that sodium channels are available for activation shortly after each action potential. Vestibular nucleus neurons operate around high baseline firing rates and encode information with bidirectional modulation of firing rates up to several hundred Hz. To determine the mechanisms that enable these neurons to sustain firing at high rates, ionic currents were measured during firing by using the action potential clamp technique in vestibular nucleus neurons acutely dissociated from transgenic mice. Although neurons from the YFP-16 line fire at rates higher than those from the GIN line, both classes of neurons express Kv3 and BK currents as well as both transient and resurgent Na currents. In the fastest firing neurons, Kv3 currents dominated repolarization at all firing rates and minimized Na channel inactivation by rapidly transitioning Na channels from the open to the closed state. In slower firing neurons, BK currents dominated repolarization at the highest firing rates and sodium channel availability was protected by a resurgent blocking mechanism. Quantitative differences in Kv3 current density across neurons and qualitative differences in immunohistochemically detected expression of Kv3 subunits could account for the difference in firing range within and across cell classes. These results demonstrate how divergent firing properties of two neuronal populations arise through the interplay of at least three ionic currents.

Control of transient, resurgent, and persistent current by open-channel block by Na channel beta4 in cultured cerebellar granule neurons

Voltage-gated Na channels in several classes of neurons, including cells of the cerebellum, are subject to an open-channel block and unblock by an endogenous protein. The Na(V)beta4 (Scn4b) subunit is a candidate blocking protein because a free peptide from its cytoplasmic tail, the beta4 peptide, can block open Na channels and induce resurgent current as channels unblock upon repolarization. In heterologous expression systems, however, Na(V)beta4 fails to produce resurgent current. We therefore tested the necessity of this subunit in generating resurgent current, as well as its influence on Na channel gating and action potential firing, by studying cultured cerebellar granule neurons treated with siRNA targeted against Scn4b. Knockdown of Scn4b, confirmed with quantitative RT-PCR, led to five electrophysiological phenotypes: a loss of resurgent current, a reduction of persistent current, a hyperpolarized half-inactivation voltage of transient current, a higher rheobase, and a decrease in repetitive firing. All disruptions of Na currents and firing were rescued by the beta4 peptide. The simplest interpretation is that Na(V)beta4 itself blocks Na channels of granule cells, making this subunit the first blocking protein that is responsible for resurgent current. The results also demonstrate that a known open-channel blocking peptide not only permits a rapid recovery from nonconducting states upon repolarization from positive voltages but also increases Na channel availability at negative potentials by antagonizing fast inactivation. Thus, Na(V)beta4 expression determines multiple aspects of Na channel gating, thereby regulating excitability in cultured cerebellar granule cells.

Persistent Nav1.6 current at axon initial segments tunes spike timing of cerebellar granule cells

Cerebellar granule (CG) cells generate high-frequency action potentials that have been proposed to depend on the unique properties of their voltage-gated ion channels. To address the in vivo function of Nav1.6 channels in developing and mature CG cells, we combined the study of the developmental expression of Nav subunits with recording of acute cerebellar slices from young and adult granule-specific Scn8a KO mice. Nav1.2 accumulated rapidly at early-formed axon initial segments (AISs). In contrast, Nav1.6 was absent at early postnatal stages but accumulated at AISs of CG cells from P21 to P40. By P40-P65, both Nav1.6 and Nav1.2 co-localized at CG cell AISs. By comparing Na(+) currents in mature CG cells (P66-P74) from wild-type and CG-specific Scn8a KO mice, we found that transient and resurgent Na(+) currents were not modified in the absence of Nav1.6 whereas persistent Na(+) current was strongly reduced. Action potentials in conditional Scn8a KO CG cells showed no alteration in threshold and overshoot, but had a faster repolarization phase and larger post-spike hyperpolarization. In addition, although Scn8a KO CG cells kept their ability to fire action potentials at very high frequency, they displayed increased interspike-interval variability and firing irregularity in response to sustained depolarization. We conclude that Nav1.6 channels at axon initial segments contribute to persistent Na(+) current and ensure a high degree of temporal precision in repetitive firing of CG cells.

Phase response curve analysis of a full morphological globus pallidus neuron model reveals distinct perisomatic and dendritic modes of synaptic integration

Synchronization of globus pallidus (GP) neurons and cortically entrained oscillations between GP and other basal ganglia nuclei are key features of the pathophysiology of Parkinson's disease. Phase response curves (PRCs), which tabulate the effects of phasic inputs within a neuron's spike cycle on output spike timing, are efficient tools for predicting the emergence of synchronization in neuronal networks and entrainment to periodic input. In this study we apply physiologically realistic synaptic conductance inputs to a full morphological GP neuron model to determine the phase response properties of the soma and different regions of the dendritic tree. We find that perisomatic excitatory inputs delivered throughout the interspike interval advance the phase of the spontaneous spike cycle yielding a type I PRC. In contrast, we demonstrate that distal dendritic excitatory inputs can either delay or advance the next spike depending on whether they occur early or late in the spike cycle. We find this latter pattern of responses, summarized by a biphasic (type II) PRC, was a consequence of dendritic activation of the small conductance calcium-activated potassium current, SK. We also evaluate the spike-frequency dependence of somatic and dendritic PRC shapes, and we demonstrate the robustness of our results to variations of conductance densities, distributions, and kinetic parameters. We conclude that the distal dendrite of GP neurons embodies a distinct dynamical subsystem that could promote synchronization of pallidal networks to excitatory inputs. These results highlight the need to consider different effects of perisomatic and dendritic inputs in the control of network behavior.

Robust pacemaking in substantia nigra dopaminergic neurons

Dopaminergic neurons of the substantia nigra pars compacta are autonomous pacemakers. This activity is responsible for the sustained release of dopamine necessary for the proper functioning of target structures, such as the striatum. Somatodendritic L-type Ca2+ channels have long been viewed as important, if not necessary, for this activity. The studies reported here challenge this viewpoint. Using a combination of optical and electrophysiological approaches in brain slices, it was found that antagonism of L-type Ca2+ channel effectively stopped dendritic Ca2+ oscillations but left autonomous pacemaking unchanged. Moreover, damping intracellular Ca2+ oscillations with exogenous buffer had little effect on pacemaking rate. Although not necessary for pacemaking, L-type channels helped support pacemaking when challenged with cationic channel blockers. Simulations suggested that the insensitivity to antagonism of L-type channels reflected the multichannel nature of the pacemaking process. The robustness of pacemaking underscores its biological importance and provides a framework for understanding how therapeutics targeting L-type Ca2+ channels might protect dopaminergic neurons in Parkinson's disease without compromising their function.

Calcium-activated SK channels influence voltage-gated ion channels to determine the precision of firing in globus pallidus neurons

Globus pallidus (GP) neurons fire rhythmically in the absence of synaptic input, suggesting that they may encode their inputs as changes in the phase of their rhythmic firing. Action potential afterhyperpolarization (AHP) enhances precision of firing by ensuring that the ion channels recover from inactivation by the same amount on each cycle. Voltage-clamp experiments in slices showed that the longest component of the GP neuron's AHP is blocked by apamin, a selective antagonist of calcium-activated SK channels. Application of 100 nm apamin also disrupted the precision of firing in perforated-patch and cell-attached recordings. SK channel blockade caused a small depolarization in spike threshold and made it more variable, but there was no reduction in the maximal rate of rise during an action potential. Thus, the firing irregularity was not caused solely by a reduction in voltage-gated Na(+) channel availability. Subthreshold voltage ramps triggered a large outward current that was sensitive to the initial holding potential and had properties similar to the A-type K(+) current in GP neurons. In numerical simulations, the availability of both Na(+) and A-type K(+) channels during autonomous firing were reduced when SK channels were removed, and a nearly equal reduction in Na(+) and K(+) subthreshold-activated ion channel availability produced a large decrease in the neuron's slope conductance near threshold. This change made the neuron more sensitive to intrinsically generated noise. In vivo, this change would also enhance the sensitivity of GP neurons to small synaptic inputs.

Autonomous initiation and propagation of action potentials in neurons of the subthalamic nucleus

The activity of the subthalamic nucleus (STN) is intimately related to movement and is generated, in part, by voltage-dependent Na(+) (Na(v)) channels that drive autonomous firing. In order to determine the principles underlying the initiation and propagation of action potentials in STN neurons, 2-photon laser scanning microscopy was used to guide tight-seal whole-cell somatic and loose-seal cell-attached axonal/dendritic patch-clamp recordings and compartment-selective ion channel manipulation in rat brain slices. Action potentials were first detected in a region that corresponded most closely to the unmyelinated axon initial segment, as defined by Golgi and ankyrin G labelling. Following initiation, action potentials propagated reliably into axonal and somatodendritic compartments with conduction velocities of approximately 5 m s(-1) and approximately 0.7 m s(-1), respectively. Action potentials generated by neurons with axons truncated within or beyond the axon initial segment were not significantly different. However, axon initial segment and somatic but not dendritic or more distal axonal application of low [Na(+)] ACSF or the selective Na(v) channel blocker tetrodotoxin consistently depolarized action potential threshold. Finally, somatodendritic but not axonal application of GABA evoked large, rapid inhibitory currents in concordance with electron microscopic analyses, which revealed that the somatodendritic compartment was the principal target of putative inhibitory inputs. Together the data are consistent with the conclusions that in STN neurons the axon initial segment and soma express an excess of Na(v) channels for the generation of autonomous activity, while synaptic activation of somatodendritic GABA(A) receptors regulates the axonal initiation of action potentials.

Role of Axonal NaV1.6 Sodium Channels in Action Potential Initiation of CA1 Pyramidal Neurons

In many neuron types, the axon initial segment (AIS) has the lowest threshold for action potential generation. Its active properties are determined by the targeted expression of specific voltage-gated channel subunits. We show that the Na+ channel NaV1.6 displays a striking aggregation at the AIS of cortical neurons. To assess the functional role of this subunit, we used Scn8a med mice that are deficient for NaV1.6 subunits but still display prominent Na+ channel aggregation at the AIS. In CA1 pyramidal cells from Scn8a med mice, we found a depolarizing shift in the voltage dependence of activation of the transient Na+ current ( INaT), indicating that NaV1.6 subunits activate at more negative voltages than other NaV subunits. Additionally, persistent and resurgent Na+ currents were significantly reduced. Current-clamp recordings revealed a significant elevation of spike threshold in Scn8a med mice as well as a shortening of the estimated delay between spike initiation at the AIS and its arrival at the soma. In combination with simulations using a realistic computer model of a CA1 pyramidal cell, our results imply that a hyperpolarized voltage dependence of activation of AIS NaV1.6 channels is important both in determining spike threshold and localizing spike initiation to the AIS. In addition to altered spike initiation, Scn8a med mice also showed a strongly reduced spike gain as expected with combined changes in persistent and resurgent currents and spike threshold. These results suggest that NaV1.6 subunits at the AIS contribute significantly to its role as spike trigger zone and shape repetitive discharge properties of CA1 neurons.

Channel density distributions explain spiking variability in the globus pallidus: a combined physiology and computer simulation database approach

Globus pallidus (GP) neurons recorded in brain slices show significant variability in intrinsic electrophysiological properties. To investigate how this variability arises, we manipulated the biophysical properties of GP neurons using computer simulations. Specifically, we created a GP neuron model database with 100,602 models that had varying densities of nine membrane conductances centered on a hand-tuned model that replicated typical physiological data. To test the hypothesis that the experimentally observed variability can be attributed to variations in conductance densities, we compared our model database results to a physiology database of 146 slice recordings. The electrophysiological properties of generated models and recordings were assessed with identical current injection protocols and analyzed with a uniform set of measures, allowing a systematic analysis of the effects of varying voltage-gated and calcium-gated conductance densities on the measured properties and a detailed comparison between models and recordings. Our results indicated that most of the experimental variability could be matched by varying conductance densities, which we confirmed with additional partial block experiments. Further analysis resulted in two key observations: (1) each voltage-gated conductance had effects on multiple measures such as action potential waveform and spontaneous or stimulated spike rates; and (2) the effect of each conductance was highly dependent on the background context of other conductances present. In some cases, such interactions could reverse the effect of the density of one conductance on important excitability measures. This context dependence of conductance density effects is important to understand drug and neuromodulator effects that work by affecting ion channels.

Functional properties and differential neuromodulation of Na(v)1.6 channels

The voltage-gated sodium channel Na(v)1.6 plays unique roles in the nervous system, but its functional properties and neuromodulation are not as well established as for Na(V)1.2 channels. We found no significant differences in voltage-dependent activation or fast inactivation between Na(V)1.6 and Na(V)1.2 channels expressed in non-excitable cells. In contrast, the voltage dependence of slow inactivation was more positive for Na(v)1.6 channels, they conducted substantially larger persistent sodium currents than Na(v)1.2 channels, and they were much less sensitive to inhibition by phosphorylation by cAMP-dependent protein kinase and protein kinase C. Resurgent sodium current, a hallmark of Na(v)1.6 channels in neurons, was not observed for Na(V)1.6 expressed alone or with the auxiliary beta(4) subunit. The unique properties of Na(V)1.6 channels, together with the resurgent currents that they conduct in neurons, make these channels well-suited to provide the driving force for sustained repetitive firing, a crucial property of neurons.

Similar properties of transient, persistent, and resurgent Na currents in GABAergic and non-GABAergic vestibular nucleus neurons

Sodium currents in fast firing neurons are tuned to support sustained firing rates >50-60 Hz. This is typically accomplished with fast channel kinetics and the ability to minimize the accumulation of Na channels into inactivated states. Neurons in the medial vestibular nuclei (MVN) can fire at exceptionally high rates, but their Na currents have never been characterized. In this study, Na current kinetics and voltage-dependent properties were compared in two classes of MVN neurons with distinct firing properties. Non-GABAergic neurons (fluorescently labeled in YFP-16 transgenic mice) have action potentials with faster rise and fall kinetics and sustain higher firing rates than GABAergic neurons (fluorescently labeled in GIN transgenic mice). A previous study showed that these neurons express a differential balance of K currents. To determine whether the Na currents in these two populations were different, their kinetics and voltage-dependent properties were measured in acutely dissociated neurons from 24- to 40-day-old mice. All neurons expressed persistent Na currents and large transient Na currents with resurgent kinetics tuned for fast firing. No differences were found between the Na currents expressed in GABAergic and non-GABAergic MVN neurons, suggesting that differences in properties of these neurons are tuned by their K currents.