Individual variation and hormonal modulation of sodium channel alpha and beta1 subunits in the electric organ correlate with variation in a social signal
The electric fish Sternopygus macrurus emits an electric organ discharge (EOD) composed of a series of pulses. EOD frequency and duration are individually unique, sexually dimorphic and regulated by steroid hormones. Previous studies have shown the EOD pulse is partially shaped by a sodium current, whose rate and voltage dependence of inactivation correlate with EOD frequency and pulse duration, and are modulated by androgens. In this study I tested whether the gradient in sodium current inactivation across EOD frequency might be due to regulation on sodium channel α and β1 subunits. Full-length sequences of the two sodium channel α subunits in the electric organ of Sternopygus macrurus, smNav1.4a and smNav1.4b were cloned. Furthermore, two smNav1.4b mRNA transcripts (smNav1.4bL and smNav1.4bS), with alternative first exons and translated into proteins with and without an extended N terminus respectively, were identified. Electric organ expresses smNav1.4a and smNav1.4b at comparable levels and preferentially expresses smNav1.4bL. The mRNA level of smNav1.4bL but not smNav1.4a, correlates with EOD frequency. I also cloned the sodium channel β1 subunit in Sternopygus and found two splice forms of this gene (β1L and β1S). They exhibit a distinct pattern of differential expression in different tissues. In the electric organ, the mRNA levels of β1 and the splicing preference for β1S correlate with EOD frequency. An androgen implant lowered EOD frequency. It also lowered the mRNA levels of smNav1.4bL, smNav1.4bS and β1, but did not affect smNav1.4a or the splicing preference of β1. Expression of smNav1.4bL or smNav1.4bS alone, or together with β1L or β1S in Xenopus oocytes revealed the kinetic properties of these subunits. Importantly, smNav1.4bL and β1S, whose expressions correlate with EOD frequency, show faster inactivation rates and negative shifts of voltage dependence, consistent with the natural phenotype of high EOD frequency fish. Furthermore, two mutagenesis studies addressed the functions of the novel regions in smNav1.4bL and β1S. These results suggest multiple levels of mRNA control on sodium channel α and β1 subunits underlie the cellular excitability in the electric organ and correlate with the variation in an important social signal, EOD, in Sternopygus.