N-METHYL-D-ASPARTATE RECEPTOR ACTIVATION INCREASES CAMP LEVELS AND VOLTAGE-GATED CA2+ CHANNEL ACTIVITY IN AREA CA1 OF HIPPOCAMPUS

Tetanic stimulation of the Schaffer collateral inputs into area CA1 of the hippocampus causes N-methyl-D-aspartate (NMDA) receptor activation, an effect that contributes to the induction of long-term potentiation (LTP) in this region. The present studies demonstrate that LTP-inducing tetanic stimulation in rat hippocampal area CA1 elicited increased levels of cAMP. The elevation of cAMP was blocked by the NMDA receptor antagonist DL-2-amino-5-phosphonovaleric acid (APV). Bath application of NMDA also resulted in an increase in cAMP in CA1, an effect that was blocked by both APV and removal of extracellular Ca2+. These findings suggest that activation of NMDA receptors elicits a Ca2+-dependent increase in cAMP, and taken together with the data from tetanic stimulation, suggest that NMDA-receptor-mediated increases in cAMP could play a role in the induction of LTP in area CA1. One role for cAMP may be to increase Ca2+ influx through voltage-gated Ca2+ channels, as it was observed that application of either 8-bromo-cAMP or NMDA increased the fractional open time of high-threshold Ca2+ channels in CA1 pyramidal cells. Our results raise the possibility that a positive-feedback loop for Ca2+ influx in area CA1 exists. In this model, NMDA receptor-mediated Ca2+ influx leads to an enhancement of further Ca2+ influx via intermediate steps of increased cAMP and subsequent increased voltage-gated Ca2+ channel activity.