INCREASE IN MESSENGER-RNA ENCODING NEONATAL-II AND NEONATAL-III SODIUM-CHANNEL ALPHA-ISOFORMS DURING KAINATE-INDUCED SEIZURES IN ADULT-RAT HIPPOCAMPUS

Subtypes I, II and III of sodium channel alpha-subunit mRNAs were anal yzed in adult rat brain areas after kainate-induced seizures. Tissue s amples were microdissected from occipital neocortex, CA1 and CA3 hippo campus areas and dentate gyrus. Three reverse transcriptase-polymerase chain reaction (RT-PCR) protocols were undertaken to amplify these mR NAs. Amplification products were then distinguished after digestion by restriction enzymes, electrophoresis separation and densitometric ana lysis of gel profiles. PCR 1 evidenced the relative percentage of mRNA s I, II and III as well as neonatal II and LII subtype isoforms, which resulted from an alternative splicing. PCR 2 and 3 were performed to focus on the neonatal vs. adult ratio in II and III subtypes, respecti vely. Seizures were shown to induce an increase in both neonatal subty pes, which suggested an alteration at the splicing level. These change s exhibited a peculiar brain regional distribution, the maximal effect being observed in dentate gyrus and hippocampus CA1 area. In situ hyb ridization experiments, using a digoxigenin-labeled oligonucleotide pr obe-specific for neonatal II and III mRNAs, confirmed this increase in neonatal mRNA subtypes. These changes were transient, reaching a maxi mum 6 h after drug injection, then disappearing between 12 and 48 h. T hey were prevented by a pre-treatment of animals by MK-801, a non-comp etitive antagonist of NMDA receptors. This work, thus, suggested that KA-induced seizures can be accompanied by transient alteration in the splicing pattern of sodium channel alpha-subunit mRNAs which resulted in an increase in expression of their neonatal isoforms within localiz ed areas of adult rat brain.