Differential modulation of NR2A and NR2B containing NMDA receptors by protein kinase A

Abstract

Ca2+ influx through NMDARs (N-methyl-D-aspartate receptors) triggers long-lasting changes in synaptic efficacy, such as long-term potentiation, by activating a network of kinases and phosphatases. Recent findings in our laboratories indicate that Ca2+ permeability of neuronal NMDARs is under the control of the cAMP/PKA signaling cascade. Direct blockade of PKA by H-89 or PKI(14-22) or depletion of cAMP by inhibition of adenylyl cyclase by SQ22536 reduced the peak phase of NMDA-elicited currents and markedly decreased Ca2+-dependent desensitization in hippocampal neurons. After block of adenylyl cyclase with SQ22536 (and depletion of cAMP), the constitutively active form of PKA (introduced via the recording pipette) or the membrane-permeable analog of cyclic AMP, 8-bromo-cAMP, potentiated NMDA currents and restored desensitization. Simultaneous Ca2+ imaging with Oregon Green BAPTA-1 and recording of NMDA currents demonstrated a greater degree of inhibition of Ca2+ influx than of NMDA current by the peptide inhibitor PKI(14-22). In adult hippocampal slices, PKA inhibition reduced Ca2+ influx through synaptic NMDA receptors with little change in amplitude of NMDA EPSCs. At birth, NMDARs are primarily NR2B-containing, and there is a progressive inclusion of NR2A with maturation. We expressed NR1/NR2A and NR1/NR2B receptors in HEK-293 cells. The PKA inhibitor H-89 markedly decreased NMDA-induced Ca2+ permeation through NR1/NR2A receptors with little change in current, whereas H-89 significantly depressed current responses and Ca2+ influx through NR1/NR2B receptors. We further showed that PKA more profoundly modulated NMDA currents in younger vs. older neurons. We propose that PKA modulation of NMDA currents is developmentally regulated at least in part as a consequence of the NR2A/NR2B switch during the critical period.