CA2-KINASE-C POTENTIATION OF RECOMBINANT NMDA RECEPTORS( INFLUX AMPLIFIES PROTEIN)

Protein kinase C (PKC) potentiates NMDA receptors in hippocampal, trig eminal, and spinal neurons. Although PKC phosphorylates the NMDA recep tor subunit NR1 at four residues within the C terminal splice cassette C1, the molecular mechanisms underlying PKC potentiation of NMDA resp onses are not yet known. The present study examined the role of Ca2+ i n PKC potentiation of recombinant NMDA receptors expressed in Xenopus oocytes. We found that Ca2+ influx through PKC-potentiated NMDA recept ors can further increase the NMDA response (''Ca2+ amplification''). C a2+ amplification required a rise in intracellular Ca2+ concentration at or near the intracellular end of the channel and was independent of Ca2+-activated Cl- current. Ca2+ amplification depended on extracellu lar Ca2+ concentration during NMDA application and not during PKC acti vation. Ca2+ amplification was reduced by the membrane-permeant Ca2+-c helating agent BAPTA-AM. Mutant receptors with greatly reduced Ca2+ pe rmeability did not exhibit Ca2+ amplification. Receptors containing th e NR1 N-terminal splice cassette showed more Ca2+ amplification, possi bly because of their larger basal current and therefore greater Ca2+ i nflux. Contrary to expectation, splicing out the two C-terminal splice cassettes of NR1 enhanced PKC potentiation in a manner independent of extracellular Ca2+. This observation indicates that PKC potentiation does not require phosphorylation of the C1 cassette of the NR1 subunit . PKC potentiation of NMDA receptors in vivo is likely to be affected by Ca2+ amplification of the potentiated signal; the degree of amplifi cation will depend in part on alternative splicing of the NR1 subunit, which is regulated developmentally and in a cell-specific manner.