Sodium current density correlates with expression of specific alternatively spliced sodium channel mRNAs in single neurons.

Elements within the first cytoplasmic loop of voltage-gated sodium channels have been implicated in regulating channel function. We have examined the role of alternative splicing within the first cytoplasmic loop of the Drosophila sodium channel gene para in regulating sodium current expression, using single-cell RT-PCR. In addition to a previously described exon (a), we identified a second exon in this region, designated exon i. Alternative splicing of exons a and i results in the expression of four para transcripts that are present individually or in combination within single neurons. Analysis of sodium current density and the pattern of para mRNA expression suggested that the presence of exon a was necessary though not sufficient for expression of sodium currents in cultured embryonic neurons. A similar pattern of alternative splicing of para mRNA was also evident in RNA isolated from whole embryos. Combined with our observation that the patterns of alternative splicing of para mRNA change during development, these findings suggest that neuronal sodium current expression in vivo, is also modulated by alternative splicing.