A DUAL MECHANISM FOR IMPAIRMENT OF GABA(A) RECEPTOR ACTIVITY BY NMDA RECEPTOR ACTIVATION IN RAT CEREBELLUM GRANULE CELLS

The function of the GABA(A) receptor has been studied using the whole cell voltage clamp recording technique in rat cerebellum granule cells in culture. Activation of NMDA-type glutamate receptors causes a redu ction in the effect of GABA. Full GABA(A) receptor activity was recove red after washing out NMDA and NMDA action was prevented in a Mg++ con taining medium. The NMDA effect was also absent when extracellular CA( ++) was replaced by Ba++ and when 10 mM Bapta was present in the intra cellular solution. Charge accumulations via voltage activated Ca++ cha nnels greater than the ones via NMDA receptors do not cause any reduct ion in GABA(A) receptor function, suggesting that Ca++ influx through NMDA receptor channels is critical for the effect. The NMDA effect was reduced by including adenosine-5'-O-3-thiophosphate (ATP-gamma-S) in the internal solution and there was a reduction in the NMDA effect cau sed by deltamethrin, a calcineurin inhibitor. Part of the NMDA induced GAB(A) receptor impairment was prevented by prior treatment with L-ar ginine. Analogously, part of the NMDA effect was prevented by blockage of NO-synthase activity by N-omega-nitro-L-arginine. A combination of NO-synthase and calcineurin inhibitors completely eliminated the NMDA action. An analogous result was obtained by combining the NO-synthase inhibitor with the addition of ATP-gamma-S to the pipette medium. The additivity of the prevention of the NMDA impairment of GABA(A) recept or by blocking the L-arginine/NO pathway and inhibiting calcineurin ac tivity suggests an independent involvement of these two pathways in th e interaction between NMDA and the GABA(A) receptor. On the one hand C a++ influx across NMDA channels activates calcineurin and dephosphoryl ates the GABA(A) receptor complex directly or dephosphorylates protein s critical for the function of the receptor. On the other hand, Ca++ i nflux activates NO-synthase and induces nitric oxide production, which regulates such receptors via protein kinase G activity.