ALTERNATIVE RNA SPLICING OF THE NMDA RECEPTOR NR1 MESSENGER-RNA IN THE NEURONS OF THE TELEOST ELECTROSENSORY SYSTEM

The sequence for cDNA encoding the NMDA receptor subunit 1 (aptNR1) of the weakly electric fish Apteronotus leptorhynchus has been determine d. The deduced amino acid sequence is similar to 88% identical to othe r vertebrate NR1 proteins, with sequence homology extending to the alt ernatively spliced cassettes N1 and C1. The fish and mammalian N1 and C1 splice cassettes are identical at 20 of 21 and 30 of 37 amino acid positions, respectively. We did not detect a C2 splice cassette in apt NR1 mRNA, but we did find two novel C-terminal alternative splice cass ettes labeled C1' and C1''. The relative levels of NR1 transcripts con taining the N1 and C1 splice cassettes were determined by using RNase protection and in situ hybridization analysis. N1-containing mRNAs are more abundant in cauda[ brain regions, similar to the pat terns repor ted for mammalian brain. In contrast, the relative levels of transcrip ts containing the C1 splice cassette are much lower in fish than in ma mmals, averaging only 9% for the whole brain. The levels of C1 splicin g increased in more rostra[ brain regions. In situ hybridizations with N1- and C1-specific probes demonstrated that N1 cassette splicing occ urs in most neurons but that C1 splicing is heterogeneous and is restr icted to a subset of neuronal types in the electrosensory system.