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Nav1.6 channels encode I NaP in motoneurons
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Persistent sodium current (INaP) in the spinal locomotor network promotes two distinct nonlinear firing patterns: a self-sustained spiking triggered by a brief excitation in bistable motoneurons and bursting oscillations in interneurons of the central pattern generator (CPG). Here, we identify the NaV channels responsible for INaP and their role in...
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... no significance; **p < 0.01; ***p < 0.001 (two-tailed Fisher test for D; one-way ANOVA with multiple comparisons for E, G, and H; comparison of the fits for L). See also Figure S4. ...
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... the Nav1.6 blocker diminished the capacity to spike at high frequencies and led to a frank failure of the repetitive firing, not observed with ICA. The genetic deletion of Nav1.1 or Nav1.6 broadly recapitulated the distinct contributions of the two channels in firing properties of motoneurons, with only one-third of Nav1.6 À/À motoneurons remaining bistable (Fig- ure S4). In Nav1.6 À/À motoneurons, the expression of Nav1.1 was not restrained to the proximal part of the AIS like in wild type but was spread along the full length of the AIS (Figures S2C and S2D). ...
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... response to a slow ramp voltage, motoneurons and interneurons typically displayed a large inward voltage-dependent current attributable to I NaP (Figures 4A and 4E; see also Tazerart et al. 8 ). In motoneurons, the Nav1.1 blocker ICA did not significantly alter I NaP ( Figures 4A-4D). By contrast, the Nav1.6 blocker 4,9-ahTTX strongly decreased I NaP in amplitude and shifted toward more depolarized values its voltage activation threshold without modifying the half-activation potential ( Figures 4A-4D). ...
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... motoneurons, the Nav1.1 blocker ICA did not significantly alter I NaP ( Figures 4A-4D). By contrast, the Nav1.6 blocker 4,9-ahTTX strongly decreased I NaP in amplitude and shifted toward more depolarized values its voltage activation threshold without modifying the half-activation potential ( Figures 4A-4D). The coapplication of 4,9-ahTTX and ICA abolished I NaP ( Figure 4B). ...
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... contrast, the Nav1.6 blocker 4,9-ahTTX strongly decreased I NaP in amplitude and shifted toward more depolarized values its voltage activation threshold without modifying the half-activation potential ( Figures 4A-4D). The coapplication of 4,9-ahTTX and ICA abolished I NaP ( Figure 4B). Data were recapitulated in Nav1.1 À/À and Nav1.6 À/À motoneurons ( Figures S6A-S6D), and the residual I NaP was abolished by 4,9-ahTTX and ICA, respectively ( Figures S6E-S6H). ...
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... were recapitulated in Nav1.1 À/À and Nav1.6 À/À motoneurons ( Figures S6A-S6D), and the residual I NaP was abolished by 4,9-ahTTX and ICA, respectively ( Figures S6E-S6H). In interneurons, in the presence of either ICA or 4,9-ahTTX, the peak amplitude of I NaP was mostly halved without affecting its activation threshold or the half-activation potential ( Figures 4E-4H). Likewise, the coapplication of 4,9-ahTTX and ICA abolished I NaP ( Figure 4F). ...
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... interneurons, in the presence of either ICA or 4,9-ahTTX, the peak amplitude of I NaP was mostly halved without affecting its activation threshold or the half-activation potential ( Figures 4E-4H). Likewise, the coapplication of 4,9-ahTTX and ICA abolished I NaP ( Figure 4F). Astonishingly, biophysical properties of I NaP in Nav1.1 À/À and Nav1.6 À/À interneurons were similar to wild-type littermates ( Figures S6I-S6L), but it was almost inhibited by 4,9-ahTTX and ICA, respectively ( Figure S6M-S6P). ...
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... with motoneurons transduced with the scramble short hairpin RNA (shRNA), the proportion of bistable motoneurons markedly decreased to $23% in Nav1.6-shRNA mice (Figures 6G-6I) with a significant decrease of I NaP amplitude without modifying the activation threshold ( Figure 6J-6L). The amplitude of the spike and firing rates also decreased without affecting the spike threshold or the excitability of motoneurons ( Figures S4M-S4S). ...
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Citations
... Profound tonic spastic seizure without or minimum clonus is a robust clinical phenotype in Scn8a D/+ mice. Scn8a is expressed in murine spinal motoneurons, 72 and the GOF mutation may directly increase the susceptibility to neurogenic spasms. 73 The frequent appearance of tonic spasms during and after seizure-SD complex suggests that SD might create an excitatory brain state favoring this paroxysmal motor event. ...
Spreading depolarization (SD) is a slowly propagating wave of massive cellular depolarization that transiently impairs the function of affected brain regions. While SD typically arises as an isolated hemispheric event, we previously reported that reducing M-type potassium current (IKM) by ablation of Kcnq2 in forebrain excitatory neurons results in tightly coupled spontaneous bilateral seizure-SD complexes in the awake mouse cortex. Here we find that enhanced persistent Na+ current due to gain-of-function (GOF) mutations in Scn8a (N1768D/+, hereafter D/+) produces a similar compound cortical excitability phenotype. Chronic DC-band EEG recording detected spontaneous bilateral seizure-SD complexes accompanied by seizures with a profound tonic component, which occurs predominantly during the light phase and were detected in the mutant mice across ages between P40-100. Laser speckle contrast imaging of cerebral blood flow dynamics resolved SD as bilateral wave of hypoperfusion and subsequent hour-lasting hypoperfusion in Scn8a D/+ cortex in awake head-restrained mice subjected to a subconvulsive PTZ. Subcortical recordings in freely moving mice revealed that approximately half of the spontaneous cortical seizure-SD complexes arose with concurrent SD-like depolarization in the thalamus and delayed depolarization in the striatum. In contrast, SD-like DC potential shifts were rarely detected in the hippocampus or upper pons. Consistent with the high spontaneous incidence in vivo , cortical slices from Scn8a D/+ mice showed a raised SD susceptibility, and pharmacological inhibition of persistent Na ⁺ current (INaP), which is enhanced in Scn8a D/+ neurons, inhibited SD generation in cortical slices ex vivo , indicating that INaP contributes to SD susceptibility. Ex vivo Ca ²⁺ imaging studies using acute brain slices expressing genetic Ca ²⁺ sensor (Thy1-GCAMP6s) demonstrated that pharmacological activation of IKM suppressed Ca ²⁺ spikes and SD, whereas IKM inhibitor drastically increased the frequency of Ca ²⁺ spikes in the hippocampus of Scn8a D/+ mice, but not in WT, suggesting that IKM restrains the hyperexcitability resulting from Scn8a GOF mutation. Together, our study identifies a cortical SD phenotype in Scn8a GOF mice shared with the Kcnq2 -cKO model of developmental epileptic encephalopathy and reveals that an imbalance of non-inactivating inward and outward membrane currents bidirectionally modulates spatiotemporal SD susceptibility.
... In line with this, an interesting potential tool for future studies is the recombinant rAAV2-retro, whose single i.m. injection in neonatal mice enabled the extensive transduction of lower motoneurons in the spinal cord. 61,62 If the rAAV2-retro also proves to be efficient in adult animals, then its peculiar features of retrograde axonal transduction would permit the selective performance of injections in spastic muscles of the limbs and the transduction of limited motoneuron pools. ...
Animals requiring purposeful movement for survival are endowed with mechanoreceptors, called proprioceptors, that provide essential sensory feedback from muscles and joints to spinal cord circuits, which modulates motor output. Despite the essential nature of proprioceptive signaling in daily life, the mechanisms governing proprioceptor activity are poorly understood. Here, we identified nonredundant roles for two voltage-gated sodium channels (Na V s), Na V 1.1 and Na V 1.6, in mammalian proprioception. Deletion of Na V 1.6 in somatosensory neurons (Na V 1.6 cKO mice) causes severe motor deficits accompanied by loss of proprioceptive transmission, which contrasts with our previous findings using similar mouse models to target Na V 1.1 (Na V 1.1 cKO ). In Na V 1.6 cKO animals, we observed impairments in proprioceptor end-organ structure and a marked reduction in skeletal muscle myofiber size that were absent in Na V 1.1 cKO mice. We attribute the differential contributions of Na V 1.1 and Na V 1.6 to distinct cellular localization patterns. Collectively, we provide evidence that Na V s uniquely shape neural signaling within a somatosensory modality.
Animals that require purposeful movement for survival are endowed with mechanosensory neurons called proprioceptors that provide essential sensory feedback from muscles and joints to spinal cord circuits, which modulates motor output. Despite the essential nature of proprioceptive signaling in daily life, the mechanisms governing proprioceptor activity are poorly understood. Here, we have identified distinct and nonredundant roles for two voltage-gated sodium channels (NaVs), NaV1.1 and NaV1.6, in mammalian proprioception. Deletion of NaV1.6 in somatosensory neurons (NaV1.6 cKO mice) causes severe motor deficits accompanied by complete loss of proprioceptive transmission, which contrasts with our previous findings using similar mouse models to target NaV1.1 (NaV1.1 cKO ). In NaV1.6 cKO animals, loss of proprioceptive feedback caused non-cell- autonomous impairments in proprioceptor end-organs and skeletal muscle that were absent in NaV1.1 cKO mice. We attribute the differential contribution of NaV1.1 and NaV1.6 in proprioceptor function to distinct cellular localization patterns. Collectively, these data provide the first evidence that NaV subtypes uniquely shape neurotransmission within a somatosensory modality.
Teaser
Voltage gated sodium channels differentially encode mammalian proprioception via distinct cellular localization patterns.
Bistability in spinal motoneurons supports tonic spike activity in the absence of excitatory drive. Earlier work in adult preparations suggested that smaller motoneurons innervating slow antigravity muscle fibers are more likely to generate bistability for postural maintenance. However, whether large motoneurons innervating fast-fatigable muscle fibers display bistability is still controversial. To address this, we examined the relationship between soma size and bistability in lumbar (L4-L5) ventrolateral a-motoneurons of ChAT-GFP and Hb9-GFP mice during the first four weeks of life. We found that as neuron size increases, the prevalence of bistability rises. Smaller a-motoneurons lack bistability, while larger fast a-motoneurons (MMP-9 ⁺ /Hb9 ⁺ ) with a soma area ≥ 400µm ² exhibit significantly higher bistability. Ionic currents associated with bistability, including the persistent Nav1.6 current, thermosensitive Trpm5 Ca ²⁺ -activated Na ⁺ current and the slowly inactivating Kv1.2 current, also scale with cell size. Serotonin evokes full bistability in large motoneurons with partial bistable properties, but not in small motoneurons. Our study provides important insights into the neural mechanisms underlying bistability and how motoneuron size correlates with bistability in mice.
