Sodium currents and action potentials were characterized in Purkinje neurons from ataxic mice lacking expression of the sodium channel Scn8a. Peak transient sodium current was approximately 60% of that in normal mice, but subthreshold sodium current was affected much more. Steady-state current elicited by voltage ramps was reduced to approximately 30%, and resurgent sodium current, an unusual transient current elicited on repolarization following strong depolarizations, was reduced to 8%-18%. In jolting mice, with a missense mutation in Scn8a, steady-state and resurgent current were also reduced, with altered voltage dependence and kinetics. Both spontaneous firing and evoked bursts of spikes were diminished in cells from null and jolting mice. Evidently Scn8a channels carry most subthreshold sodium current and are crucial for repetitive firing.

Dopamine (DA) is a well established modulator of prefrontal cortex (PFC) function, yet the cellular mechanisms by which DA exerts its effects in this region are controversial. A major point of contention is the consequence of D 1 DA receptor activation. Several studies have argued that D 1 receptors enhance the excitability of PFC pyramidal neurons by augmenting voltagedependent Na ϩ currents, particularly persistent Na ϩ currents. However, this conjecture is based on indirect evidence. To provide a direct test of this hypothesis, we combined voltageclamp studies of acutely isolated layer V-VI prefrontal pyramidal neurons with single-cell RT-PCR profiling. Contrary to prediction, the activation of D 1 or D 5 DA receptors consistently suppressed rapidly inactivating Na ϩ currents in identified corticostriatal pyramidal neurons. This modulation was attenuated by a D 1 /D 5 receptor antagonist, mimicked by a cAMP analog, and blocked by a protein kinase A (PKA) inhibitor. In the same cells the persistent component of the Na ϩ current was unaffected by D 1 /D 5 receptor activation-suggesting that rapidly inactivating and persistent Na ϩ currents arise in part from different channels. Single-cell RT-PCR profiling showed that pyramidal neurons coexpressed three ␣-subunit mRNAs (Nav1.1, 1.2, and 1.6) that code for the Na ϩ channel pore. In neurons from Nav1.6 null mice the persistent Na ϩ currents were significantly smaller than in wild-type neurons. Moreover, the residual persistent currents in these mutant neuronswhich are attributable to Nav1.1/1.2 channels-were reduced significantly by PKA activation. These results argue that D 1 /D 5 DA receptor activation reduces the rapidly inactivating component of Na ϩ current in PFC pyramidal neurons arising from Nav1.1/1.2 Na ϩ channels but does not modulate effectively the persistent component of the Na ϩ current that is attributable to Nav1.6 Na ϩ channels.

Marshall syndrome is a rare, autosomal dominant skeletal dysplasia that is phenotypically similar to the more common disorder Stickler syndrome. For a large kindred with Marshall syndrome, we demonstrate a splice-donor-site mutation in the COL11A1 gene that cosegregates with the phenotype. The G+1-->A transition causes in-frame skipping of a 54-bp exon and deletes amino acids 726-743 from the major triple-helical domain of the alpha1(XI) collagen polypeptide. The data support the hypothesis that the alpha1(XI) collagen polypeptide has an important role in skeletal morphogenesis that extends beyond its contribution to structural integrity of the cartilage extracellular matrix. Our results also demonstrate allelism of Marshall syndrome with the subset of Stickler syndrome families associated with COL11A1 mutations.

Scn8a encodes an abundant, widely distributed voltage-gated sodium channel found throughout the central and peripheral nervous systems. Mice with different mutant alleles of Scn8a provide models of the movement disorders ataxia, dystonia, tremor and progressive paralysis. We previously reported that the phenotype of the hypomorphic allele of Scn8a, medJ, is dependent upon an unlinked modifier locus, Scnm1. Strain C57BL/6J carries a sensitive allele of the modifier locus that results in juvenile lethality. We now provide evidence that the modifier acts on the splicing efficiency of the mutant splice donor site. Mutant mice display either 90% or 95% reduction in the proportion of correctly spliced mRNA, depending on modifier genotype. The abundance of the channel protein, Na(v)1.6, is also reduced by an order of magnitude in medJ mice, resulting in delayed maturation of nodes of Ranvier, slowed nerve conduction velocity, reduced muscle mass and reduction of brain metabolic activity. medJ mice provide a model for the physiological effects of sodium channel deficiency and the molecular mechanism of bigenic disease.

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