Molecular and functional studies of tilapia (Oreochromis mossambicus) NMDA receptor NR1 subunits

NMDA (N-methyl-d-aspartate) receptor, a subclass of the ionotropic glutamate receptors, participates in synaptic transmission and plays important roles in various higher brain functions in the vertebrate central nervous system. Here, we report the cloning of two NR1 subunits of tilapia (Oreochromis mossambicus). Phylogenetic analysis strongly supports that the two tilapia NR1 genes are paralogous, resulting from a gene duplication event in the teleost lineage. The electrophysiological and pharmacological properties of the tilapia NR1.2 subunit coexpressed with rat NR2B in the Xenopus oocytes are similar to that of the recombinant rat NR1/NR2B. Both tilapia NR1 transcripts are alternatively spliced at the N and C terminal coding regions. The C terminal exons, C1' and C1", originally discovered in the knifefish NR1 gene, are present in the tNR1.1 but not in the tNR1.2. Majorities of the NR1 transcripts expressed in the tilapia and zebrafish brains do not include these alternative splice exons. The splicing patterns of NR1 transcripts differ in various brain subregions. The regional expression patterns of splice variants are not fully preserved between tilapia and zebrafish. Nevertheless, tectum opticum regions of teleost and rat express high levels of NR1 splicing variant with N1 cassette.