Impaired Motor Function in Mice With Cell-Specific Knockout of Sodium ChannelScn8a(NaV1.6) in Cerebellar Purkinje Neurons and Granule Cells

Translating cerebellar Purkinje neuron physiology to progress in dominantly inherited ataxia

The cerebellum is an important structure for accurate control and timing of movement, and Purkinje neurons in the cerebellar cortex are key players in cerebellar motor control. Cerebellar dysfunction can result in ataxia, a disorder characterized by postural instability, gait disturbances and motor incoordination. Cerebellar ataxia is a symptom of a number of conditions, and the emerging evidence that Purkinje neuron dysfunction, in particular, abnormal Purkinje neuron repetitive firing, is a major driver of motor dysfunction in a subset of dominantly inherited ataxias is dicussed. Abnormalities in Purkinje neuron excitability that are observed in mouse models of each of these disorders, and where appropriate describe studies linking particular ion channels to aberrant excitability are also discussed. Common mechanisms of dysfunction and speculate about potential therapeutic targets, suggesting that Purkinje neuron firing abnormalities are a novel target for improving motor dysfunction in patients with some forms of dominantly inherited ataxia are proposed.

Toluene decreases Purkinje cell output by enhancing inhibitory synaptic transmission in the cerebellar cortex

Toluene belongs to a class of psychoactive drugs known as inhalants. Found in common household products such as adhesives, paint products, and aerosols, toluene is inhaled for its intoxicating and euphoric properties. Additionally, exposure to toluene disrupts motor behaviors in a manner consistent with impairments to cerebellar function. Previous work has suggested a role of GABA in mediating toluene's neurobehavioral effects, but how this manifests in the cerebellar cortex is not yet understood. In the present study, we examined the effects of toluene on cerebellar Purkinje cell action potential output and inhibitory synaptic transmission onto Purkinje cells using patch clamp electrophysiology in acute rat cerebellar slices. Toluene (1mM) reduced the frequency of Purkinje cell action potential output without affecting input resistance. Furthermore, toluene dose-dependently enhanced inhibitory synaptic transmission onto Purkinje cells, increasing the amplitude and frequency of inhibitory postsynaptic currents; no change in the frequency of action potentials from molecular layer interneurons was noted. The observed decreases in Purkinje cell action potential output could contribute to toluene-evoked impairments in cerebellar and motor functions.

HCN1 channels in cerebellar Purkinje cells promote late stages of learning and constrain synaptic inhibition

Neural computations rely on ion channels that modify neuronal responses to synaptic inputs. While single cell recordings suggest diverse and neurone type-specific computational functions for HCN1 channels, their behavioural roles in any single neurone type are not clear. Using a battery of behavioural assays, including analysis of motor learning in vestibulo-ocular reflex and rotarod tests, we find that deletion of HCN1 channels from cerebellar Purkinje cells selectively impairs late stages of motor learning. Because deletion of HCN1 modifies only a subset of behaviours involving Purkinje cells, we asked whether the channel also has functional specificity at a cellular level. We find that HCN1 channels in cerebellar Purkinje cells reduce the duration of inhibitory synaptic responses but, in the absence of membrane hyperpolarization, do not affect responses to excitatory inputs. Our results indicate that manipulation of subthreshold computation in a single neurone type causes specific modifications to behaviour.

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.

Convergence of genetic and environmental factors on parvalbumin-positive interneurons in schizophrenia

Schizophrenia etiology is thought to involve an interaction between genetic and environmental factors during postnatal brain development. However, there is a fundamental gap in our understanding of the molecular mechanisms by which environmental factors interact with genetic susceptibility to trigger symptom onset and disease progression. In this review, we summarize the most recent findings implicating oxidative stress as one mechanism by which environmental insults, especially early life social stress, impact the development of schizophrenia. Based on a review of the literature and the results of our own animal model, we suggest that environmental stressors such as social isolation render parvalbumin-positive interneurons (PVIs) vulnerable to oxidative stress. We previously reported that social isolation stress exacerbates many of the schizophrenia-like phenotypes seen in a conditional genetic mouse model in which NMDA receptors (NMDARs) are selectively ablated in half of cortical and hippocampal interneurons during early postnatal development (Belforte et al., 2010). We have since revealed that this social isolation-induced effect is caused by impairments in the antioxidant defense capacity in the PVIs in which NMDARs are ablated. We propose that this effect is mediated by the down-regulation of PGC-1α, a master regulator of mitochondrial energy metabolism and anti-oxidant defense, following the deletion of NMDARs (Jiang et al., 2013). Other potential molecular mechanisms underlying redox dysfunction upon gene and environmental interaction will be discussed, with a focus on the unique properties of PVIs.

Strength and timing of motor responses mediated by rebound firing in the cerebellar nuclei after Purkinje cell activation

The cerebellum refines the accuracy and timing of motor performance. How it encodes information to perform these functions is a major topic of interest. We performed whole cell and extracellular recordings of Purkinje cells (PCs) and cerebellar nuclei neurons (CNs) in vivo, while activating PCs with light in transgenic mice. We show for the first time that graded activation of PCs translates into proportional CN inhibition and induces rebound activity in CNs, which is followed by graded motor contractions timed to the cessation of the stimulus. Moreover, activation of PC ensembles led to disinhibition of climbing fiber activity, which coincided with rebound activity in CNs. Our data indicate that cessation of concerted activity in ensembles of PCs can regulate both timing and strength of movements via control of rebound activity in CNs.

Iberiotoxin-sensitive and -insensitive BK currents in Purkinje neuron somata

Purkinje cells have specialized intrinsic ionic conductances that generate high-frequency action potentials. Disruptions of their Ca or Ca-activated K (KCa) currents correlate with altered firing patterns in vitro and impaired motor behavior in vivo. To examine the properties of somatic KCa currents, we recorded voltage-clamped KCa currents in Purkinje cell bodies isolated from postnatal day 17-21 mouse cerebellum. Currents were evoked by endogenous Ca influx with approximately physiological Ca buffering. Purkinje somata expressed voltage-activated, Cd-sensitive KCa currents with iberiotoxin (IBTX)-sensitive (>100 nS) and IBTX-insensitive (>75 nS) components. IBTX-sensitive currents activated and partially inactivated within milliseconds. Rapid, incomplete macroscopic inactivation was also evident during 50- or 100-Hz trains of 1-ms depolarizations. In contrast, IBTX-insensitive currents activated more slowly and did not inactivate. These currents were insensitive to the small- and intermediate-conductance KCa channel blockers apamin, scyllatoxin, UCL1684, bicuculline methiodide, and TRAM-34, but were largely blocked by 1 mM tetraethylammonium. The underlying channels had single-channel conductances of ∼150 pS, suggesting that the currents are carried by IBTX-resistant (β4-containing) large-conductance KCa (BK) channels. IBTX-insensitive currents were nevertheless increased by small-conductance KCa channel agonists EBIO, chlorzoxazone, and CyPPA. During trains of brief depolarizations, IBTX-insensitive currents flowed during interstep intervals, and the accumulation of interstep outward current was enhanced by EBIO. In current clamp, EBIO slowed spiking, especially during depolarizing current injections. The two components of BK current in Purkinje somata likely contribute differently to spike repolarization and firing rate. Moreover, augmentation of BK current may partially underlie the action of EBIO and chlorzoxazone to alleviate disrupted Purkinje cell firing associated with genetic ataxias.

Synchrony and neural coding in cerebellar circuits

The cerebellum regulates complex movements and is also implicated in cognitive tasks, and cerebellar dysfunction is consequently associated not only with movement disorders, but also with conditions like autism and dyslexia. How information is encoded by specific cerebellar firing patterns remains debated, however. A central question is how the cerebellar cortex transmits its integrated output to the cerebellar nuclei via GABAergic synapses from Purkinje neurons. Possible answers come from accumulating evidence that subsets of Purkinje cells synchronize their firing during behaviors that require the cerebellum. Consistent with models predicting that coherent activity of inhibitory networks has the capacity to dictate firing patterns of target neurons, recent experimental work supports the idea that inhibitory synchrony may regulate the response of cerebellar nuclear cells to Purkinje inputs, owing to the interplay between unusually fast inhibitory synaptic responses and high rates of intrinsic activity. Data from multiple laboratories lead to a working hypothesis that synchronous inhibitory input from Purkinje cells can set the timing and rate of action potentials produced by cerebellar nuclear cells, thereby relaying information out of the cerebellum. If so, then changing spatiotemporal patterns of Purkinje activity would allow different subsets of inhibitory neurons to control cerebellar output at different times. Here we explore the evidence for and against the idea that a synchrony code defines, at least in part, the input–output function between the cerebellar cortex and nuclei. We consider the literature on the existence of simple spike synchrony, convergence of Purkinje neurons onto nuclear neurons, and intrinsic properties of nuclear neurons that contribute to responses to inhibition. Finally, we discuss factors that may disrupt or modulate a synchrony code and describe the potential contributions of inhibitory synchrony to other motor circuits.

Neurological perspectives on voltage-gated sodium channels

The activity of voltage-gated sodium channels has long been linked to disorders of neuronal excitability such as epilepsy and chronic pain. Recent genetic studies have now expanded the role of sodium channels in health and disease, to include autism, migraine, multiple sclerosis, cancer as well as muscle and immune system disorders. Transgenic mouse models have proved useful in understanding the physiological role of individual sodium channels, and there has been significant progress in the development of subtype selective inhibitors of sodium channels. This review will outline the functions and roles of specific sodium channels in electrical signalling and disease, focusing on neurological aspects. We also discuss recent advances in the development of selective sodium channel inhibitors.

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.

Motor dysfunction and cerebellar Purkinje cell firing impairment in Ebf2 null mice

Mice deficient for the transcription factor Ebf2 lose a subset of Purkinje cells during development and have a hypotrophic cerebellar cortex. Related motor symptoms and the function of Purkinje cells surviving in the adult have not been studied so far. Ebf2 null mice presented locomotor impairment and a deficiency of motor coordination and motor learning. Ebf2 null Purkinje cells of the anterior lobe, relative to wild-type controls, were patch-clamp recorded in acutely prepared slices. While immature Purkinje cells (10-20 postnatal days) of Ebf2 null mice showed no significant difference relative to wild-types, in the adult they featured a higher input resistance, increased anomalous rectification, decreased first spike latency, higher initial firing frequency, lower voltage threshold and reduced afterhyperpolarizations and post-burst hyperpolarizations. These parameters indicate a difference in the response to both hyperpolarizing and depolarizing stimuli, corresponding to an altered cerebellar cortical output signaling. In contrast, adult climbing fibers attained a normal monoinnervation pattern and the parallel fiber-Purkinje cell synapse showed evoked postsynaptic currents and paired-pulse facilitation functionally indistinguishable from wild-type PCs. These results suggest that the motor deficits exhibited by Ebf2 null mice could be due, at least in part, to an impairment of the firing properties of surviving Purkinje cells. These findings indicate that Ebf2 is important for the development and maintenance of normal Purkinje cell discharge properties.

Voltage-gated sodium channels: biophysics, pharmacology, and related channelopathies

Voltage-gated sodium channels (VGSC) are multi-molecular protein complexes expressed in both excitable and non-excitable cells. They are primarily formed by a pore-forming multi-spanning integral membrane glycoprotein (α-subunit) that can be associated with one or more regulatory β-subunits. The latter are single-span integral membrane proteins that modulate the sodium current (I_Na) and can also function as cell adhesion molecules. In vitro some of the cell-adhesive functions of the β-subunits may play important physiological roles independently of the α-subunits. Other endogenous regulatory proteins named “channel partners” or “channel interacting proteins” (ChiPs) like caveolin-3 and calmodulin/calmodulin kinase II (CaMKII) can also interact and modulate the expression and/or function of VGSC. In addition to their physiological roles in cell excitability and cell adhesion, VGSC are the site of action of toxins (like tetrodotoxin and saxitoxin), and pharmacologic agents (like antiarrhythmic drugs, local anesthetics, antiepileptic drugs, and newly developed analgesics). Mutations in genes that encode α- and/or β-subunits as well as the ChiPs can affect the structure and biophysical properties of VGSC, leading to the development of diseases termed sodium “channelopathies”.  This review will outline the structure, function, and biophysical properties of VGSC as well as their pharmacology and associated channelopathies and highlight some of the recent advances in this field.

Pinceau organization in the cerebellum requires distinct functions of neurofascin in Purkinje and basket neurons during postnatal development

Basket axon collaterals synapse onto the Purkinje soma/axon initial segment (AIS) area to form specialized structures, the pinceau, which are critical for normal cerebellar function. Mechanistic details of how the pinceau become organized during cerebellar development are poorly understood. Loss of cytoskeletal adaptor protein Ankyrin G (AnkG) results in mislocalization of the cell adhesion molecule Neurofascin (Nfasc) at the Purkinje AIS and abnormal organization of the pinceau. Loss of Nfasc in adult Purkinje neurons leads to slow disorganization of the Purkinje AIS and pinceau morphology. Here, we used mouse conditional knock-out techniques to show that selective loss of Nfasc, specifically in Purkinje neurons during early development, prevented maturation of the AIS and resulted in loss of Purkinje neuron spontaneous activity and pinceau disorganization. Loss of Nfasc in both Purkinje and basket neurons caused abnormal basket axon collateral branching and targeting to Purkinje soma/AIS, leading to extensive pinceau disorganization, Purkinje neuron degeneration, and severe ataxia. Our studies reveal that the Purkinje Nfasc is required for AIS maturation and for maintaining stable contacts between basket axon terminals and the Purkinje AIS during pinceau organization, while the basket neuron Nfasc in combination with Purkinje Nfasc is required for proper basket axon collateral outgrowth and targeting to Purkinje soma/AIS. Thus, cerebellar pinceau organization requires coordinated mechanisms involving specific Nfasc functions in both Purkinje and basket neurons.

Neural precursors (NPCs) from adult L967Q mice display early commitment to "in vitro" neuronal differentiation and hyperexcitability

The pathogenic factors leading to selective degeneration of motoneurons in ALS are not yet understood. However, altered functionality of voltage-dependent Na(+) channels may play a role since cortical hyperexcitability was described in ALS patients and riluzole, the only drug approved to treat ALS, seems to decrease glutamate release via blockade or inactivation of voltage-dependent Na(+) channels. The wobbler mouse, a murine model of motoneuron degeneration, shares some of the clinical features of human ALS. At early stages of the wobbler disease, increased cortical hyperexcitability was observed. Moreover, riluzole reduced motoneuron loss and muscular atrophy in treated wobbler mice. Here, we focussed our attention on specific electrophysiological properties, like voltage-activated Na(+) currents and underlying regenerative electrical activity, as read-outs of the neuronal maturation process of neural stem/progenitor cells (NPCs) isolated from the subventricular zone (SVZ) of adult early symptomatic wobbler mice. In self-renewal conditions, the rate of wobbler NPC proliferation "in vitro" was 30% lower than that of healthy mice. Conversely, the number of wobbler NPCs displaying early neuronal commitment and action potentials was significantly higher. Upon switching from proliferative to differentiative conditions, NPCs underwent significant changes in the key properties of voltage gated Na(+) currents. The most notable finding, in cells with neuronal morphology, was an increase in Na(+) current density that strictly correlated with an increased probability to generate action potentials. This feature was remarkably more pronounced in neurons differentiated from wobbler NPCs that upon sustained stimulation, displayed short trains of pathological facilitation. In agreement with this result, an increase in the number of c-Fos positive cells, a surrogate marker of neuronal network activation, was observed in the mesial cortex of the wobbler mice "in situ". Thus these findings, all together, suggest that a state of early neuronal hyperexcitability may be a major contributor of motoneuron vulnerability.

Interaction of voltage-gated sodium channel Nav1.6 (SCN8A) with microtubule-associated protein Map1b

The mechanism by which voltage-gated sodium channels are trafficked to the surface of neurons is not well understood. Our previous work implicated the cytoplasmic N terminus of the sodium channel Na(v)1.6 in this process. We report that the N terminus plus the first transmembrane segment (residues 1-153) is sufficient to direct a reporter to the cell surface. To identify proteins that interact with the 117-residue N-terminal domain, we carried out a yeast two-hybrid screen of a mouse brain cDNA library. Three clones containing overlapping portions of the light chain of microtubule-associated protein Map1b (Mtap1b) were recovered from the screen. Interaction between endogenous Na(v)1.6 channels and Map1b in mouse brain was confirmed by co-immunoprecipitation. Map1b did not interact with the N terminus of the related channel Na(v)1.1. Alanine-scanning mutagenesis of the Na(v)1.6 N terminus demonstrated that residues 77-80 (VAVP) contribute to interaction with Map1b. Co-expression of Na(v)1.6 with Map1b in neuronal cell line ND7/23 resulted in a 50% increase in current density, demonstrating a functional role for this interaction. Mutation of the Map1b binding site of Na(v)1.6 prevented generation of sodium current in transfected cells. The data indicate that Map1b facilitates trafficking of Na(v)1.6 to the neuronal cell surface.

The splicing regulator Rbfox2 is required for both cerebellar development and mature motor function

The Rbfox proteins (Rbfox1, Rbfox2, and Rbfox3) regulate the alternative splicing of many important neuronal transcripts and have been implicated in a variety of neurological disorders. However, their roles in brain development and function are not well understood, in part due to redundancy in their activities. Here we show that, unlike Rbfox1 deletion, the CNS-specific deletion of Rbfox2 disrupts cerebellar development. Genome-wide analysis of Rbfox2(-/-) brain RNA identifies numerous splicing changes altering proteins important both for brain development and mature neuronal function. To separate developmental defects from alterations in the physiology of mature cells, Rbfox1 and Rbfox2 were deleted from mature Purkinje cells, resulting in highly irregular firing. Notably, the Scn8a mRNA encoding the Na(v)1.6 sodium channel, a key mediator of Purkinje cell pacemaking, is improperly spliced in RbFox2 and Rbfox1 mutant brains, leading to highly reduced protein expression. Thus, Rbfox2 protein controls a post-transcriptional program required for proper brain development. Rbfox2 is subsequently required with Rbfox1 to maintain mature neuronal physiology, specifically Purkinje cell pacemaking, through their shared control of sodium channel transcript splicing.

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.

Delayed postnatal loss of P/Q-type calcium channels recapitulates the absence epilepsy, dyskinesia, and ataxia phenotypes of genomic Cacna1a mutations

Inherited loss of P/Q-type calcium channel function causes human absence epilepsy, episodic dyskinesia, and ataxia, but the molecular "birthdate" of the neurological syndrome and its dependence on prenatal pathophysiology is unknown. Since these channels mediate transmitter release at synapses throughout the brain and are expressed early in embryonic development, delineating the critical circuitry and onset underlying each of the emergent phenotypes requires targeted control of gene expression. To visualize P/Q-type Ca(2+) channels and dissect their role in neuronal networks at distinct developmental stages, we created a novel conditional Cacna1a knock-in mouse by inserting the floxed green fluorescent protein derivative Citrine into the first exon of Cacna1a and then crossed it with a postnatally expressing PCP2-Cre line for delayed Purkinje cell (PC) gene deletion within the cerebellum and sparsely in forebrain (purky). PCs in purky mice lacked P/Q-type calcium channel protein and currents within the first month after birth, displayed altered spontaneous firing, and showed impaired neurotransmission. Unexpectedly, adult purky mice exhibited the full spectrum of neurological deficits seen in mice with genomic Cacna1a ablation. Our results show that the ataxia, dyskinesia, and absence epilepsy caused by inherited disorders of the P/Q-type channel arise from signaling defects beginning in late infancy, revealing an early window of opportunity for therapeutic intervention.

Molecular signaling how do axons die?

Axons depend critically on axonal transport both for supplying materials and for communicating with cell bodies. This chapter looks at each activity, asking what aspects are essential for axon survival. Axonal transport declines in neurodegenerative disorders, such as Alzheimer's disease, amyotrophic lateral sclerosis, and multiple sclerosis, and in normal ageing, but whether all cargoes are equally affected and what limits axon survival remains unclear. Cargoes can be differentially blocked in some disorders, either individually or in groups. Each missing protein cargo results in localized loss-of-function that can be partially modeled by disrupting the corresponding gene, sometimes with surprising results. The axonal response to losing specific proteins also depends on the rates of protein turnover and on whether the protein can be locally synthesized. Among cargoes with important axonal roles are components of the PI3 kinase, Mek/Erk, and Jnk signaling pathways, which help to communicate with cell bodies and to regulate axonal transport itself. Bidirectional trafficking of Bdnf, NT-3, and other neurotrophic factors contribute to intra- and intercellular signaling, affecting the axon's cellular environment and survival. Finally, several adhesion molecules and gangliosides are key determinants of axon survival, probably by mediating axon-glia interactions. Thus, failure of long-distance intracellular transport can deprive axons of one, few, or many cargoes. This can lead to axon degeneration either directly, through the absence of essential axonal proteins, or indirectly, through failures in communication with cell bodies and nonneuronal cells.

Alcohol withdrawal and brain injuries: beyond classical mechanisms

Unmanaged sudden withdrawal from the excessive consumption of alcohol (ethanol) adversely alters neuronal integrity in vulnerable brain regions such as the cerebellum, hippocampus, or cortex. In addition to well known hyperexcitatory neurotransmissions, ethanol withdrawal (EW) provokes the intense generation of reactive oxygen species (ROS) and the activation of stress-responding protein kinases, which are the focus of this review article. EW also inflicts mitochondrial membranes/membrane potential, perturbs redox balance, and suppresses mitochondrial enzymes, all of which impair a fundamental function of mitochondria. Moreover, EW acts as an age-provoking stressor. The vulnerable age to EW stress is not necessarily the oldest age and varies depending upon the target molecule of EW. A major female sex steroid, 17β-estradiol (E2), interferes with the EW-induced alteration of oxidative signaling pathways and thereby protects neurons, mitochondria, and behaviors. The current review attempts to provide integrated information at the levels of oxidative signaling mechanisms by which EW provokes brain injuries and E2 protects against it.

Disruption of the olivo-cerebellar circuit by Purkinje neuron-specific ablation of BK channels

The large-conductance voltage- and calcium-activated potassium (BK) channels are ubiquitously expressed in the brain and play an important role in the regulation of neuronal excitation. Previous work has shown that the total deletion of these channels causes an impaired motor behavior, consistent with a cerebellar dysfunction. Cellular analyses showed that a decrease in spike firing rate occurred in at least two types of cerebellar neurons, namely in Purkinje neurons (PNs) and in Golgi cells. To determine the relative role of PNs, we developed a cell-selective mouse mutant, which lacked functional BK channels exclusively in PNs. The behavioral analysis of these mice revealed clear symptoms of ataxia, indicating that the BK channels of PNs are of major importance for normal motor coordination. By using combined two-photon imaging and patch-clamp recordings in these mutant mice, we observed a unique type of synaptic dysfunction in vivo, namely a severe silencing of the climbing fiber-evoked complex spike activity. By performing targeted pharmacological manipulations combined with simultaneous patch-clamp recordings in PNs, we obtained direct evidence that this silencing of climbing fiber activity is due to a malfunction of the tripartite olivo-cerebellar feedback loop, consisting of the inhibitory synaptic connection of PNs to the deep cerebellar nuclei (DCN), followed by a projection of inhibitory DCN afferents to the inferior olive, the origin of climbing fibers. Taken together, our results establish an essential role of BK channels of PNs for both cerebellar motor coordination and feedback regulation in the olivo-cerebellar loop.

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.

Loss of beta-III spectrin leads to Purkinje cell dysfunction recapitulating the behavior and neuropathology of spinocerebellar ataxia type 5 in humans

Mutations in SPTBN2, the gene encoding beta-III spectrin, cause spinocerebellar ataxia type 5 in humans (SCA5), a neurodegenerative disorder resulting in loss of motor coordination. How these mutations give rise to progressive ataxia and what the precise role beta-III spectrin plays in normal cerebellar physiology are unknown. We developed a mouse lacking full-length beta-III spectrin and found that homozygous mice reproduced features of SCA5 including gait abnormalities, tremor, deteriorating motor coordination, Purkinje cell loss, and cerebellar atrophy (molecular layer thinning). In vivo analysis reveals an age-related reduction in simple spike firing rate in surviving beta-III(-/-) Purkinje cells, whereas in vitro studies show these neurons to have reduced spontaneous firing, smaller sodium currents, and dysregulation of glutamatergic neurotransmission. Our data suggest an early loss of EAAT4- (protein interactor of beta-III spectrin) and a subsequent loss of GLAST-mediated uptake may play a role in neuronal pathology. These findings implicate a loss of beta-III spectrin function in SCA5 pathogenesis and indicate that there are at least two physiological effects of beta-III spectrin loss that underpin a progressive loss of inhibitory cerebellar output, namely an intrinsic Purkinje cell membrane defect due to reduced sodium currents and alterations in glutamate signaling.

Ethanol withdrawal acts as an age-specific stressor to activate cerebellar p38 kinase

We investigated whether protein kinase p38 plays a role in the brain-aging changes associated with repeated ethanol withdrawal (EW). Ovariectomized young, middle-age and older rats, with or without 17β-estradiol (E2) implantation, received a 90-day ethanol with repeated withdrawal. They were tested for active pP38 expression in cerebellar Purkinje neurons and whole-cerebellar lysates using immunohistochemistry and enzyme-linked immunosorbent assay, respectively. They were also tested for the Rotarod task to determine the behavioral manifestation of cerebellar neuronal stress and for reactive oxygen species (ROS) and mitochondrial protein carbonyls to determine oxidative mechanisms. Middle-age EW rats showed higher levels of pP38-positive Purkinje neurons/cerebellar lysates, which coincided with increased mitochondrial protein oxidation than other diet/age groups. Exacerbated motor deficit due to age-EW combination also began at the middle-age. In comparison, ROS contents peaked in older EW rats. E2 treatment mitigated each of the EW effects to a different extent. Collectively, pP38 may mediate the brain-aging changes associated with pro-oxidant EW at vulnerable ages and in vulnerable neurons in a manner protected by estrogen.

Expression profile of PTPIP51 in mouse brain

This study demonstrates the expression of the novel protein protein tyrosine phophatase-interacting protein 51 (PTPIP51) in mammalian brain tissue. Serial sections of the whole adult mouse brain were analyzed for PTPIP51 protein and mRNA by immunohistochemistry, immunoblotting, RT-PCR, and in situ hybridization. Recent investigations by Yu et al. (2008) describe PTPIP51 as being capable of activating Raf-1, thereby modulating the MAPK pathway. The role of Raf-1, as well as of 14-3-3, in neurological disorders is well established. PTPIP51 expression was confined to neurons in the following structures: the piriform cortex and their connections to the anterior commissure, nucleus accumbens, paraventricular and supraoptical nuclei, neurohypophysis, superior colliculus, genu of facialis nerve, spinal trigeminal tract, inferior cerebellar peduncle, and cerebellum. In the cerebellum, a subpopulation of Purkinje cells and their dendrites was strongly PTPIP51 positive. Moreover, PTPIP51 was found to be colocalized with vasopressin and its transport protein neurophysin II in the neuroendocrine nuclei and their connections to the neurohypophysis. The data presented here suggest a role of PTPIP51 in neuronal homeostasis, axonal growth, and transport.

Stabilization of Ca current in Purkinje neurons during high-frequency firing by a balance of Ca-dependent facilitation and inactivation

Purkinje neurons fire spontaneous action potentials at ∼50 spikes/sec and generate more than 100 spikes/sec during cerebellum-mediated behaviors. Many voltage-gated channels, including Ca channels, can inactivate and/or facilitate with repeated stimulation, raising the question of how these channels respond to regular, rapid trains of depolarizations. To test whether Ca currents are modulated during firing, we recorded voltage-clamped Ca currents, predominantly carried by P-type Ca channels, from acutely dissociated mouse Purkinje neurons at 30-33°C (1 mM Ca). With 0.5 mM intracellular EGTA, 1-second trains of either spontaneous action potential waveforms or brief depolarizing steps at 50 Hz evoked Ca tail currents that were stable, remaining within 5% of the first tail current throughout the train. Higher frequency trains (100 and 200 Hz) elicited a maximal inactivation of <10%. To test whether this stability of Ca currents resulted from a lack of modulation or from an equilibrium between facilitation and inactivation, we manipulated the permeant ion (Ca vs. Ba) and Ca buffering (0.5 vs. 10 mM EGTA). With low buffering, Ba accelerated the initial inactivation evoked by 1-second trains, but reduced its extent at 200 Hz, consistent with an early calcium-dependent facilitation (CDF) and late calcium-dependent inactivation (CDI) at high frequencies. Increasing the Ca buffer favored CDF. These data suggest that stable Ca current amplitudes result from a balance of CDF, CDI, and voltage-dependent inactivation. This modest net Ca-dependent modulation may contribute to the ability of Purkinje neurons to sustain long periods of regular firing and synaptic transmission.

Transcriptional and electrophysiological maturation of neocortical fast-spiking GABAergic interneurons

Fast-spiking (FS) interneurons are important elements of neocortical circuitry that constitute the primary source of synaptic inhibition in adult cortex and impart temporal organization on ongoing cortical activity. The highly specialized intrinsic membrane and firing properties that allow cortical FS interneurons to perform these functions are attributable to equally specialized gene expression, which is ultimately coordinated by cell-type-specific transcriptional regulation. Although embryonic transcriptional events govern the initial steps of cell-type specification in most cortical interneurons, including FS cells, the electrophysiological properties that distinguish adult cortical cell types emerge relatively late in postnatal development, and the transcriptional events that drive this maturational process are not known. To address this, we used mouse whole-genome microarrays and whole-cell patch clamp to characterize the transcriptional and electrophysiological maturation of cortical FS interneurons between postnatal day 7 (P7) and P40. We found that the intrinsic and synaptic physiology of FS cells undergoes profound regulation over the first 4 postnatal weeks and that these changes are correlated with primarily monotonic but bidirectional transcriptional regulation of thousands of genes belonging to multiple functional classes. Using our microarray screen as a guide, we discovered that upregulation of two-pore K(+) leak channels between P10 and P25 contributes to one of the major differences between the intrinsic membrane properties of immature and adult FS cells and found a number of other candidate genes that likely confer cell-type specificity on mature FS cells.

FGF14 N-terminal splice variants differentially modulate Nav1.2 and Nav1.6-encoded sodium channels

The Intracellular Fibroblast Growth Factor (iFGF) subfamily includes four members (FGFs 11-14) of the structurally related FGF superfamily. Previous studies showed that the iFGFs interact directly with the pore-forming (alpha) subunits of voltage-gated sodium (Nav) channels and regulate the functional properties of sodium channel currents. Sequence heterogeneity among the iFGFs is thought to confer specificity to this regulation. Here, we demonstrate that the two N-terminal alternatively spliced FGF14 variants, FGF14-1a and FGF14-1b, differentially regulate currents produced by Nav1.2 and Nav1.6 channels. FGF14-1b, but not FGF14-1a, attenuates both Nav1.2 and Nav1.6 current densities. In contrast, co-expression of an FGF14 mutant, lacking the N-terminus, increased Nav1.6 current densities. In neurons, both FGF14-1a and FGF14-1b localized at the axonal initial segment, and deletion of the N-terminus abolished this localization. Thus, the FGF14 N-terminus is required for targeting and functional regulation of Nav channels, suggesting an important function for FGF14 alternative splicing in regulating neuronal excitability.

Cerebellum-related characteristics of Scn8a-mutant mice

Among ten sodium channel alpha-subunit genes mapped in human and mouse genomes, the SCN8A gene is primarily expressed in neurons and glia. Mice with two types of Scn8a null mutations--Scn8a ( med ) and Scn8a ( medTg )--live for only 21-24 days, but those with incomplete mutations-Scn8a ( medJ ) and Scn8a ( medJo )--and those with knockout of Scn8a only in cerebellar Purkinje cells live to adult age. We review here previous work on cerebellum and related regions of Scn8a mutant mice and include some newer immunohistochemical and microchemical results. The resurgent sodium current that underlies the repeated firing of Purkinje cells is reduced in Scn8a mutant and knockout mice. Purkinje cells of mutant mice have greatly reduced spontaneous activity, as do the analogous cartwheel cells of the dorsal cochlear nucleus. Up-regulation of GABA(A) receptors in regions to which Purkinje cells project may partially compensate for their decreased activity in the mutant mice. The somata of cerebellar Purkinje cells of Scn8a ( medJ ) and Scn8a ( medJo ) mice, as revealed by PEP-19 immunoreaction, are slightly smaller than normal, and their axons, especially in Scn8a ( medJo ) mice, sometimes show enlargements similar to those in other types of mutant mice. Density of GABA-like immunoreactivity is decreased in Purkinje somata and regions of termination in deep cerebellar and vestibular nuclei of Scn8a ( medJ ) mice, but measured GABA concentration is not significantly reduced in microdissected samples of these regions. The concentrations of taurine and glutamine are significantly increased in cerebellar-related regions of Scn8a ( medJ ) mice, possibly suggesting up-regulation of glial amino acid metabolism.

The ataxia3 mutation in the N-terminal cytoplasmic domain of sodium channel Na(v)1.6 disrupts intracellular trafficking

The ENU-induced neurological mutant ataxia3 was mapped to distal mouse chromosome 15. Sequencing of the positional candidate gene Scn8a encoding the sodium channel Na(v)1.6 identified a T>C transition in exon 1 resulting in the amino acid substitution p.S21P near the N terminus of the channel. The cytoplasmic N-terminal region is evolutionarily conserved but its function has not been well characterized. ataxia3 homozygotes exhibit a severe disorder that includes ataxia, tremor, and juvenile lethality. Unlike Scn8a null mice, they retain partial hindlimb function. The mutant transcript is stable but protein abundance is reduced and the mutant channel is not detected in its usual site of concentration at nodes of Ranvier. In whole-cell patch-clamp studies of transfected ND7/23 cells that were maintained at 37 degrees C, the mutant channel did not produce sodium current, and function was not restored by coexpression of beta1 and beta2 subunits. However, when transfected cells were maintained at 30 degrees C, the mutant channel generated voltage-dependent inward sodium currents with an average peak current density comparable with wild type, demonstrating recovery of channel activity. Immunohistochemistry of primary cerebellar granule cells from ataxia3 mice demonstrated that the mutant protein is retained in the cis-Golgi. This trafficking defect can account for the low level of Na(v)1.6-S21P at nodes of Ranvier in vivo and at the surface of transfected cells. The data demonstrate that the cytoplasmic N-terminal domain of the sodium channel is required for anterograde transport from the Golgi complex to the plasma membrane.

Heterozygous mutations of the voltage-gated sodium channel SCN8A are associated with spike-wave discharges and absence epilepsy in mice

In a chemical mutagenesis screen, we identified the novel Scn8a^8J allele of the gene encoding the neuronal voltage-gated sodium channel Na_v1.6. The missense mutation V929F in this allele alters an evolutionarily conserved residue in the pore loop of domain 2 of Na_v1.6. Electroencephalography (EEG) revealed well-defined spike-wave discharges (SWD), the hallmark of absence epilepsy, in Scn8a^8J heterozygotes and in heterozygotes for two classical Scn8a alleles, Scn8a^med (null) and Scn8a^med-jo (missense). Mouse strain background had a significant effect on SWD, with mutants on the C3HeB/FeJ strain showing a higher incidence than on C57BL/6J. The abnormal EEG patterns in heterozygous mutant mice and the influence of genetic background on SWD make SCN8A an attractive candidate gene for common human absence epilepsy, a genetically complex disorder.

Nothing can be coincidence: synaptic inhibition and plasticity in the cerebellar nuclei

Many cerebellar neurons fire spontaneously, generating 10-100 action potentials per second even without synaptic input. This high basal activity correlates with information-coding mechanisms that differ from those of cells that are quiescent until excited synaptically. For example, in the deep cerebellar nuclei, Hebbian patterns of coincident synaptic excitation and postsynaptic firing fail to induce long-term increases in the strength of excitatory inputs. Instead, excitatory synaptic currents are potentiated by combinations of inhibition and excitation that resemble the activity of Purkinje and mossy fiber afferents that is predicted to occur during cerebellar associative learning tasks. Such results indicate that circuits with intrinsically active neurons have rules for information transfer and storage that distinguish them from other brain regions.

Multiple regulatory elements mediating neuronal-specific expression of zebrafish sodium channel gene, scn8aa

Zebrafish scn8aa sodium channels mediate the majority of sodium conductance, which is essential for the embryonic locomotor activities. Here, we investigated the transcriptional regulation of scn8aa in developing zebrafish embryos by constructing a GFP reporter driven by a 15-kb fragment of scn8aa gene designed as scn8aa:GFP. GFP expression patterns of scn8aa:GFP temporally and spatially recapitulated the expression of endogenous scn8aa mRNA during zebrafish embryonic development, with one exception in the inner nuclear layer of the retina. Three novel elements, along with an evolutionarily conserved element shared with mouse SCN8A, modulated neuronal-specific expression of scn8aa. The deletion of each positive element reduced the expression levels in neurons without inducing ectopic GFP expression in non-neuronal cells. Our results demonstrate that these four regulatory elements function cooperatively to enhance scn8aa expression in the zebrafish nervous system.

Axonal Na+ channels ensure fast spike activation and back-propagation in cerebellar granule cells

In most neurons, Na+ channels in the axon are complemented by others localized in the soma and dendrites to ensure spike back-propagation. However, cerebellar granule cells are neurons with simplified architecture in which the dendrites are short and unbranched and a single thin ascending axon travels toward the molecular layer before bifurcating into parallel fibers. Here we show that in cerebellar granule cells, Na+ channels are enriched in the axon, especially in the hillock, but almost absent from soma and dendrites. The impact of this channel distribution on neuronal electroresponsiveness was investigated by multi-compartmental modeling. Numerical simulations indicated that granule cells have a compact electrotonic structure allowing excitatory postsynaptic potentials to diffuse with little attenuation from dendrites to axon. The spike arose almost simultaneously along the whole axonal ascending branch and invaded the hillock the activation of which promoted spike back-propagation with marginal delay (<200 micros) and attenuation (<20 mV) into the somato-dendritic compartment. These properties allow granule cells to perform sub-millisecond coincidence detection of pre- and postsynaptic activity and to rapidly activate Purkinje cells contacted by the axonal ascending branch.

Inherited neuronal ion channelopathies: new windows on complex neurological diseases

Studies of genetic forms of epilepsy, chronic pain, and migraine caused by mutations in ion channels have given crucial insights into molecular mechanisms, pathogenesis, and therapeutic approaches to complex neurological disorders. Gain-of-function missense mutations in the brain type-I sodium channel Na(V)1.1 are a primary cause of generalized epilepsy with febrile seizures plus. Loss-of-function mutations in Na(V)1.1 channels cause severe myoclonic epilepsy of infancy, an intractable childhood epilepsy. Studies of a mouse model show that this disease is caused by selective loss of sodium current and excitability of GABAergic inhibitory interneurons, which leads to hyperexcitability, epilepsy, and ataxia. Mutations in the peripheral sodium channel Na(V)1.7 cause familial pain syndromes. Gain-of-function mutations cause erythromelalgia and paroxysmal extreme pain disorder as a result of hyperexcitability of sensory neurons, whereas loss-of-function mutations cause congenital indifference to pain because of attenuation of action potential firing. These experiments have defined correlations between genotype and phenotype in chronic pain diseases and focused attention on Na(V)1.7 as a therapeutic target. Familial hemiplegic migraine is caused by mutations in the calcium channel, Ca(V)2.1, which conducts P/Q-type calcium currents that initiate neurotransmitter release. These mutations increase activation at negative membrane potentials and increase evoked neurotransmitter release at cortical glutamatergic synapses. Studies of a mouse genetic model show that these gain-of-function effects lead to cortical spreading depression, aura, and potentially migraine. Overall, these experiments indicate that imbalance in the activity of excitatory and inhibitory neurons is an important underlying cause of these diseases.

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.

FGF14 regulates the intrinsic excitability of cerebellar Purkinje neurons

A missense mutation in the fibroblast growth factor 14 (FGF14) gene underlies SCA27, an autosomal dominant spinocerebellar ataxia in humans. Mice with a targeted disruption of the Fgf14 locus (Fgf14(-/-)) develop ataxia resembling human SCA27. We tested the hypothesis that loss of FGF14 affects the firing properties of Purkinje neurons, which play an important role in motor control and coordination. Current clamp recordings from Purkinje neurons in cerebellar slices revealed attenuated spontaneous firing in Fgf14(-/-) neurons. Unlike in the wild type animals, more than 80% of Fgf14(-/-) Purkinje neurons were quiescent and failed to fire repetitively in response to depolarizing current injections. Immunohistochemical examination revealed reduced expression of Nav1.6 protein in Fgf14(-/-) Purkinje neurons. Together, these observations suggest that FGF14 is required for normal Nav1.6 expression in Purkinje neurons, and that the loss of FGF14 impairs spontaneous and repetitive firing in Purkinje neurons by altering the expression of Nav1.6 channels.

Purkinje-cell-restricted restoration of Kv3.3 function restores complex spikes and rescues motor coordination in Kcnc3 mutants

The fast-activating/deactivating voltage-gated potassium channel Kv3.3 (Kcnc3) is expressed in various neuronal cell types involved in motor function, including cerebellar Purkinje cells. Spinocerebellar ataxia type 13 (SCA13) patients carrying dominant-negative mutations in Kcnc3 and Kcnc3-null mutant mice both display motor incoordination, suggested in mice by increased lateral deviation while ambulating and slips on a narrow beam. Motor skill learning, however, is spared. Mice lacking Kcnc3 also exhibit muscle twitches. In addition to broadened spikes, recordings of Kcnc3-null Purkinje cells revealed fewer spikelets in complex spikes and a lower intraburst frequency. Targeted reexpression of Kv3.3 channels exclusively in Purkinje cells in Kcnc3-null mice as well as in mice also heterozygous for Kv3.1 sufficed to restore simple spike brevity along with normal complex spikes and to rescue specifically coordination. Therefore, spike parameters requiring Kv3.3 function in Purkinje cells are involved in the ataxic null phenotype and motor coordination, but not motor learning.

Postnatal Changes in the Inactivation Properties of Voltage-Gated Sodium Channels Contribute to the Mature Firing Pattern of Spinal Motoneurons

Most mammals are born with the necessary spinal circuitry to produce a locomotor-like pattern of neural activity. However, rodents seldom demonstrate weight-supported locomotor behavior until the second or third postnatal week, possibly due to the inability of the neuromuscular system to produce sufficient force during this early postnatal period. As spinal motoneurons mature they are seen to fire an increasing number of action potentials at an increasing rate, which is a necessary component of greater force production. The mechanisms responsible for this enhanced ability of motoneurons are not completely defined. In the present study we assessed the biophysical properties of the developing voltage-gated sodium current to determine their role in the maturing firing pattern. Using dissociated postnatal lumbar motoneurons in short-term culture (18–24 h) we demonstrate that currents recorded from the most mature postnatal age group (P10–P12) were significantly better able to maintain channels in an available state during repetitive stimulation than were the younger age groups (P1–P3, P4–P6, P7–P9). This ability correlated with the ability of channels to recover more quickly and more completely from an inactivated state. These age-related differences were seen in the absence of changes in the voltage dependence of channel gating. Differences in both closed-state inactivation and slow inactivation were also noted between the age groups. The results indicate that changes in the inactivation properties of voltage-gated sodium channels are important for the development of a mature firing pattern in spinal motoneurons.

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.

Exaggerated emotional behavior in mice heterozygous null for the sodium channel Scn8a (Nav1.6)

The Scn8a gene encodes the alpha-subunit of Na(v)1.6, a neuronal voltage-gated sodium channel. Mice homozygous for mutations in the Scn8a gene exhibit motor impairments. Recently, we described a human family with a heterozygous protein truncation mutation in SCN8A. Rather than motor impairment, neuropsychological abnormalities were more common, suggesting a role for Scn8a in a more diverse range of behaviors. Here, we characterize mice heterozygous for a null mutation of Scn8a (Scn8a(+/-)mice) in a number of behavioral paradigms. We show that Scn8a(+/-)mice exhibit greater conditioned freezing in the Pavlovian fear conditioning paradigm but no apparent abnormalities in other learning and memory paradigms including the Morris water maze and conditioned taste avoidance paradigm. Furthermore, we find that Scn8a(+/-)mice exhibit more pronounced avoidance of well-lit, open environments as well as more stress-induced coping behavior. Together, these data suggest that Scn8a plays a critical role in emotional behavior in mice. Although the behavioral phenotype observed in the Scn8a(+/-)mice only partially models the abnormalities in the human family, we anticipate that the Scn8a(+/-)mice will serve as a valuable tool for understanding the biological basis of emotion and the human diseases in which abnormal emotional behavior is a primary component.

Expression of NR2B in cerebellar granule cells specifically facilitates effect of motor training on motor learning

It is believed that gene/environment interaction (GEI) plays a pivotal role in the development of motor skills, which are acquired via practicing or motor training. However, the underlying molecular/neuronal mechanisms are still unclear. Here, we reported that the expression of NR2B, a subunit of NMDA receptors, in cerebellar granule cells specifically enhanced the effect of voluntary motor training on motor learning in the mouse. Moreover, this effect was characterized as motor learning-specific and developmental stage-dependent, because neither emotional/spatial memory was affected nor was the enhanced motor learning observed when the motor training was conducted starting at the age of 3 months old in these transgenic mice. These results indicate that changes in the expression of gene(s) that are involved in regulating synaptic plasticity in cerebellar granule cells may constitute a molecular basis for the cerebellum to be involved in the GEI by facilitating motor skill learning.

Cortical glutamatergic neurons mediate the motor sedative action of diazepam

The neuronal circuits mediating the sedative action of diazepam are unknown. Although the motor-depressant action of diazepam is suppressed in alpha1(H101R) homozygous knockin mice expressing diazepam-insensitive alpha1-GABA(A) receptors, global alpha1-knockout mice show greater motor sedation with diazepam. To clarify this paradox, attributed to compensatory up-regulation of the alpha2 and alpha3 subunits, and to further identify the neuronal circuits supporting diazepam-induced sedation, we generated Emx1-cre-recombinase-mediated conditional mutant mice, selectively lacking the alpha1 subunit (forebrain-specific alpha1(-/-)) or expressing either a single wild-type (H) or a single point-mutated (R) alpha1 allele (forebrain-specific alpha1(-/H) and alpha1(-/R) mice, respectively) in forebrain glutamatergic neurons. In the rest of the brain, alpha1(-/R) mutants are heterozygous alpha1(H101R) mice. Forebrain-specific alpha1(-/-) mice showed enhanced diazepam-induced motor depression and increased expression of the alpha2 and alpha3 subunits in the neocortex and hippocampus, in comparison with their pseudo-wild-type littermates. Forebrain-specific alpha1(-/R) mice were less sensitive than alpha1(-/H) mice to the motor-depressing action of diazepam, but each of these conditional mutants had a similar behavioral response as their corresponding control littermates. Unexpectedly, expression of the alpha1 subunit was reduced in forebrain, notably in alpha1(-/R) mice, and the alpha3 subunit was up-regulated in neocortex, indicating that proper alpha1 subunit expression requires both alleles. In conclusion, conditional manipulation of GABA(A) receptor alpha1 subunit expression can induce compensatory changes in the affected areas. Specifically, alterations in GABA(A) receptor expression restricted to forebrain glutamatergic neurons reproduce the behavioral effects seen after a global alteration, thereby implicating these neurons in the motor-sedative effect of diazepam.

Where is the trace in trace conditioning?

Intensive mapping of the essential cerebellar brain circuits for Pavlovian eyeblink conditioning appeared relatively complete by 2000, but new data indicate the need for additional differentiation of cerebellar regions and mechanisms coding delay and trace conditioning. This is especially important, as trace conditioning is an experimentally tractable model of declarative learning. The temporal gap in trace eyeblink conditioning may be bridged by forebrain regions through pontine-cerebellar nuclear connections that can bypass cerebellar cortex, whereas a cerebellar cortical long-term-depression-like process appears to be required to support normal delay conditioning. Experiments focusing on the role of cerebellar cortex and deep nuclei in delay versus trace conditioning add perspective on brain substrates of these seemingly similar paradigms, which differ only by a brief stimulus-free time gap between conditioned and unconditioned stimuli. This temporal gap appears to impose forebrain dependencies and differentially engage different cerebellar circuitry during acquisition of conditioned responses.