Cloning and localization of exon 5-containing isoforms of the NMDAR1 subunit in human and rat brains.

Nine isoforms of the rat NMDAR1 receptor subunit have been previously identified, of which several have an alternatively spliced N-terminal insert believed to be important in proton sensitivity of the receptor. The cloning of the human homologues of NMDAR1-3b (hNMDA1-1) and NMDAR1-4b (hNMDA1-2), both bearing the insert, is reported here. A monoclonal antibody generated against the N-terminal region of these isoforms showed reactivity with at least two distinct human brain proteins of approximately 115 kDa. This antibody was further characterized by using a series of truncated fusion proteins and splice variants of NMDAR1 demonstrating its specific recognition of an epitope within the 21-amino acid N-terminal insert, encoded by exon 5. Western blot and immunocytochemical studies were performed to examine the expression of the exon 5-containing isoforms of the NMDAR1 subunit in both rat and human brain.