Molecular and functional characterization of Xenopus laevis N-methyl-d-aspartate receptors.

N-methyl-d-aspartate (NMDA) receptors of many different vertebrates have been characterized in the past. However, little information is available about amphibian NMDA receptors. Here, we investigated the South African clawed frog Xenopus laevis NR1 subunit at the molecular and functional level. In this subunit, which is obligatory for functional NMDA receptor complexes, we found three exons, the N1, C1, and C3 cassettes, being alternatively spliced. Combinations of these cassettes generated six different splice variants, which were functionally characterized in oocytes. The Xenopus NR1 isoforms generally showed the same functional properties as their mammalian homologs when coexpressed with rat NR2B. In coexpression with Xenopus NR2B, however, some properties changed significantly. This included a Zn(2+)-mediated potentiation of current amplitudes for some subunit combinations which lasted for several minutes. This mechanism presents a novel form of Xenopus NMDA receptor modulation, possibly mediating a form of short-term potentiation in the Xenopus central nervous system.