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 β4 (Scn4b) subunit is a candidate blocking protein because a free peptide from its cytoplasmic tail, the β4 peptide, can block open Na channels and induce resurgent current as channels unblock upon repolarization. In heterologous expression systems, however, Na V β4 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 β4 peptide. The simplest interpretation is that Na V β4 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 β4 expression determines multiple aspects of Na channel gating, thereby regulating excitability in cultured cerebellar granule cells., one of four β subunits of voltage-gated Na channels (1), is implicated in several pathologies: Na V β4 is down-regulated in Huntington's disease (2), cleaved by enzymes activated in Alzheimer's Disease (3), and mutated in some long-QT syndromes (4), raising the question of how it modulates Na currents in neurons and other cells. Among the proposed roles for Na V β4, based on studies of a peptide fragment of its cytoplasmic tail, is that it may act as an open-channel blocker of Na channels in neurons that produce resurgent current, i.e., reopening of Na channels upon repolarization from positive voltages (5).Resurgent current is present in several neuronal classes, including cell types in the cerebellum, brainstem, subthalamic nuclei, and dorsal root ganglia (6-11). As in other cells, voltage-gated Na channels in these neurons are closed at negative voltages and open upon depolarization. After opening, however, channels are blocked rapidly by an endogenous protein that prevents the fast inactivation gate from binding. Upon repolarization, this blocker is expelled, and resurgent current flows as channels reopen before either inactivating or deactivating, depending on the voltage (12). In Purkinje cells, modeling studies (13,14) and experiments on Na V 1.6 mutant mice, in which resurgent currents are reduced (14-16), have led to the proposal that Na channels t...

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SummaryAsparagine chemotaxis in Bacillus subtilis appears to involve two partially redundant adaptation mechanisms: a receptor methylation-independent process that operates at low attractant concentrations and a receptor methylation-dependent process that is required for optimal responses to high concentrations. In order to elucidate these processes, chemotactic responses were assessed for strains expressing methylation-defective mutations in the asparagine receptor, McpB, in which all 10 putative receptors (10del), five receptors (5del) or only the native copy of mcpB were deleted. This was done in both the presence and the absence of the methylesterase CheB. We found that: (i) only responses to high concentrations of asparagine were impaired; (ii) the presence of all heterologous receptors fully compensated for this defect, whereas responses progressively worsened as more receptors were taken away; (iii) methyl-group turnover occurred on heterologous receptors after the addition of asparagine, and these methylation changes were required for the restoration of normal swimming behaviour; (iv) in the absence of the methylesterase, the presence of heterologous receptors in some cases caused impaired chemotaxis; and (v) either a certain threshold number of receptors must be present to promote basal CheA activity, or one or more of the receptors missing in the 10del background (but present in the 5del background) is required for establishing basal CheA activity. Taken together, these findings suggest that many or all chemoreceptors work as an ensemble that constitutes a robust chemotaxis system. We propose that

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.