Résumé
In primary cultures of cerebellar granule cells, the Ca
2+ influx resulting from K
2+ depolarization (35 mM) was equal to one-third of that observed with 100 μM N-methyl-D-aspartate (NMDA) and was reduced in a major part (90%) by NMDA receptor antagonists. The rank order of potency of these competitive and non-competitive NMDA receptor antagonists was very close to their affinity for the NMDA and phencyclidine sites respectively. Granular cell depolarization with 35 mM K
+ also induced a large increase in the extracellular glutamate concentration. Repeated washes of the culture wells, addition of glutamate pyruvate transaminase ( + 2 mM pyruvate), or pretreatment of the cells with tetanus toxin resulted in a parallel reduction of the extracellular glutamate concentration and
45Ca
2+ uptake measured after a 35 mM K
+ stimulation. Dihydropyridine (BAY K-8644) stimulated the release of glutamate in a nifedipine-sensitive manner in the presence of 15 mM K
+. However, nifedipine (1 μM), which decreased by 60% the K
+-induced
45Ca
2+ uptake, did not reduce the 35 mM K
+-evoked glutamate release. Taken together, these results demonstrated that in cerebellar granule cell cultures, 90% of the 35 mM K
+-stimulated
45Ca
2+ influx resulted from the release of glutamate and the consecutive activation of NMDA receptors. Activation of these glutamate receptors then allows Ca
2+ influx to occur through L-type voltage-operated Ca
2+ channels.