The interactions between plasma membrane depolarization and glutamate receptor activation in the regulation of cytoplasmic free calcium in cultured cerebellar granule cells

The complex modulation of cytoplasmic free calcium concentration ([Ca²⁺]_c) in primary cultures of cerebellar granule cells in response to glutamate receptor agonists has been the subject of several contradictory reports. We here show that 3 components of the [Ca²⁺]_c response can be distinguished: (1) Ca²⁺ entry through voltage-dependent Ca²⁺ channels, following KCl^- or receptor-evoked depolarization, (2) Ca²⁺ entry through NMDA receptor channels, and (3) liberation of internal Ca²⁺ via a metabolotropic receptor. Depolarization with KCl induced a transient [Ca²⁺]_c response (subject to voltage inactivation) decaying to a sustained plateau (largely inhibited by nifedipine). The NMDA response was potentiated by glycine, totally inhibited by (+)5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate (MK-801), and blocked by Mg²⁺ in a voltage-sensitive manner. Polarized cells displayed small responses to quisqualate (QA) and α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA). Depolarization enhanced a transient response to QA, but not to AMPA. Trans-1-amino-1,3-cyclopentanedicarboxylic acid (trans-ACPD), a selective agonist for the metabolotropic glutamate receptor, caused a transient elevation of [Ca²⁺]_c, which was blocked by prior exposure to QA but not AMPA. The prolonged [Ca²⁺]_c response to kainate (KA) can be resolved into 2 major components: an indirect NMDA receptor-mediated response due to released glutamate and a nifedipine-sensitive component consistent with depolarization-mediated entry via Ca²⁺ channels. 6-Cyano-7-nitroquinoxaline-2,3-dione (CNQX), QA at >10 MM, and AMPA (but not trans-ACPD) reversed the KA response, consistent with an inactivation of the KA receptor. Our results show that the [Ca²⁺]_c responses to individual agonists involve complex interactions, suggesting that great care is required in the interpretation of agonist-stimulated cellular responses.