The expression of rat brain voltage-sensitive Na+ channel mRNAs in astrocytes

Astrocytes from various regions of CNS have been shown to express voltage-activated Na+ currents. To date, three distinct subtypes (I, II and III) of Na+ channels have been cloned from rat brain. We have applied a combined technique of reverse transcription and polymerase chain reaction (RT-PCR) to examine the expression of rat brain Na+ channels in rat astrocytes in vivo and in vitro. Five PCR primer sets were used to amplify coding or 3' non-coding regions of subtype I, II, and III Na+ channels. We were able to amplify all three of these rat brain Na+ channel subtypes from rat optic nerve, which does not have neuronal cell bodies but does contain astrocytes known to express voltage-sensitive Na+ channels. In studies on cultured spinal cord astrocytes, we were also able to amplify all three subtypes of rat brain Na+ channel mRNAs. In control experiments, RT-PCR was performed on RNAs prepared from several rat tissues, including brain, skeletal muscle, and liver. Rat brain was shown to express the three Na+ channel subtypes as expected. In rat skeletal muscle, subtype I and III Na+ channel mRNAs, but not subtype II, were amplified. In rat liver, Na+ channel messages were not detectable. The present study provides the first direct evidence that astrocytes in vivo and in vitro express rat brain voltage-sensitive Na+ channel mRNAs, which have been considered as mainly neuronal-type Na+ channel messages.