ALTERNATIVE SPLICING AT THE C-TERMINAL BUT NOT AT THE N-TERMINAL DOMAIN OF THE NMDA RECEPTOR NR1 IS ALTERED IN THE KINDLED HIPPOCAMPUS

Several lines of evidence suggest that N-methyl-D-aspartate (NMDA) rec eptors significantly contribute to the development of kindling. In add ition, a lasting enhancement of the NMDA receptor function has been su ggested to play a significant role in the chronic hyperexcitability oc curring in the hippocampus after kindling epileptogenesis. We have inv estigated whether hippocampal kindling induces changes in the NMDA rec eptor at the molecular level by assessing the expression of mRNAs of t he different spliced variants at the N-terminal (exon 5) and C-termina l (exon 21) position of the NMDA receptor 1 (NR1) gene by means of the reverse transcription-polymerase chain reaction. Alternative splicing at exon 5 confers different sensitivity of the NMDA receptor to polya mines while exon 21 encodes a 37-amino acid insert containing the majo r phosphorylation sites for protein kinase C. One week after the acqui sition of stage 5 of kindling in rats (generalized tonic-clonic seizur es), the relative abundance of the two alternatively spliced forms at the C-terminal domain, respectively containing (+) or lacking (-) exon 21, was reversed compared to controls (implanted with electrodes but not stimulated) in the dorsal hippocampus ipsilateral and contralatera l to the electrical stimulation. The exon 21(+)/exon 21(-) mRNA ratio for controls was 1.3 +/- 0.04 (mean +/- SE); for ipsilaterally kindled rats it was 0.64 +/- 0.05 (P < 0.05), and for contralaterally kindled rats it was 0.48 +/- 0.07 (P < 0.01). Similar bilateral effects were observed in the ventral hippocampus (temporal pole). No changes were f ound 4 weeks after stage 5 seizures and 1 week after the induction of a single afterdischarge. No significant alterations were induced by ki ndling in the relative abundance of the spliced variants containing or lacking exon 5. Our findings show selective changes in alternative sp licing of the NR1 gene after repeated application of an epileptogenic stimulus. This may generate receptors with different functional proper ties, which may contribute to the increased sensitivity for the induct ion of generalized seizures during kindling.