Channel activators reduce the expression of sodium channel α-subunit mRNA in developing neurons
Abstract The expression of rat brain sodium channel α-subunit (Na+I, Na+II and Na+III) and β1-subunit mRNAs was examined in rat fetal brain neurons in culture. A combined technique of reverse transcription and polymerase chain reaction (RT-PCR) was used. Two different PCR primer sets were designed to obtain simultaneous amplification of the three α-subunit mRNAs. All three molecules were detected in fetal neurons but the expression pattern (Na+III > Na+II ⪢ Na+I) was different than that observed in adult tissue (Na+II > Na+I > Na+III). Expression of the β1-subunit mRNA was detected using a specific PCR primer set. Doublet bands were amplified, from fetal cells and adult brain mRNA. To get further insight into the molecular mechanism that underlie activity dependent plasticity of sodium channels, we studied the effect on the expression of sodium channel subunits mRNA of a 60 h incubation of cells in the presence of a scorpion neurotoxin that blocks channel inactivation. An overall decrease in the expression of all three α-subunit mRNAs was observed whereas the β1-subunit mRNA was unaffected by the same treatment. When cells were incubated with the scorpion neurotoxin together with tetrodotoxin, to block Na+ influx through channels, the decrease in mRNA expression was not observed. Finally, a 60 h continuous depolarization of cells induced by application of a high concentration KCl solution did not mimic the effect of the scorpion toxin. These observations suggest that a persistent activation of the sodium channels is able to down-regulate mRNA expression for α-subunits but not for the β1-subunit.