Brain and heart sodium channel subtype mRNA expression in rat cerebral cortex

The expression of mRNAs coding for the alpha subunit of rat brain and rat heart sodium channels has been studied in adult and neonatal rat cerebral cortex using the reverse transcription-polymerase chain reaction (RT-PCR). Rat brain sodium channel subtype I, II, IIA, and III sequences were simultaneously amplified in the same PCR using a single oligonucleotide primer pair matched to all four subtype sequences. Identification of each subtype-specific product was inferred from the appearance of unique fragments when the product was digested with specific restriction enzymes. By using this RT-PCR method, products arising from mRNAs for all four brain sodium channel subtypes were identified in RNA extracted from adult rat cerebral cortex. The predominant component was type IIA with lesser levels of types I, II, and III. In contrast, the type II and IIA sequences were the predominant RT-PCR products in neonatal rat cortex, with slightly lower levels of type III and undetectable levels of type I. Thus, from neonate to adult, type II mRNA levels decrease relative to type IIA levels. Using a similar approach, we detected mRNA coding for the rat heart sodium channel in neonatal and adult rat cerebral cortex and in adult rat heart. These results reveal that mRNAs coding for the heart sodium channel and all four previously sequenced rat brain sodium channel subtypes are expressed in cerebral cortex and that type II and IIA channels may be differentially regulated during development.