CHANNEL ACTIVATORS REDUCE THE EXPRESSION OF SODIUM-CHANNEL ALPHA-SUBUNIT MESSENGER-RNA IN DEVELOPING NEURONS

The expression of rat brain sodium channel alpha-subunit(Na+I, Na+II a nd N-a+III) and beta(1)-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 a lpha-subunit mRNAs. All three molecules were detected in fetal neurons but the expression pattern (Na+III > Na+II much greater than Na+I) wa s different than that observed in adult tissue (Na+II > Na+I > Na+III) . Expression of the beta(1)-subunit mRNA was detected using a specific PCR primer set. Doublet bands were amplified, from fetal cells and ad ult brain mRNA. To get further insight into the molecular mechanism th at underlie activity dependent plasticity of sodium channels, we studi ed the effect on the expression of sodium channel subunits mRNA of a 6 0 h incubation of cells in the presence of a scorpion neurotoxin that blocks channel inactivation. An overall decrease in the expression of all three alpha-subunit mRNAs was observed whereas the beta(1)-subunit mRNA was unaffected by the same treatment. When cells were incubated with the scorpion neurotoxin together with tetrodotoxin, to block Nainflux through channels, the decrease in mRNA expression was not obser ved. Finally, a 60 h continuous depolarization of cells induced by app lication of a high concentration KCl solution did not mimic the effect of the scorpion toxin. These observations suggest that a persistent a ctivation of the sodium channels is able to down-regulate mRNA express ion for alpha-subunits but not for the beta(1)-subunit.