Potentiation of [formula omitted] elevation of intracellular Ca 2+ concentrations by exogenous glycine in cerebellar granule cells

Abstract

The effect of glycine on the intracellular free Ca 2+ concentration ([Ca 2+]) i response to N- methyl- d-aspartate (NMDA) was examined in small groups of cerebellar granule cells loaded with fura 2. NMDA alone evoked a long-lasting monophasic [Ca 2+] i plateau, which was abolished by removal of extracellular Ca 2+, or addition of the NMDA channel antagonist dizocilpine or the glycine site antagonist 5,7-dichlorokynurenic acid, virtually unaffected by the L-type Ca 2+ channel antagonist (-)-PN 202 791, and greatly, though variably, potentiated by addition of glycine. In the presence of glycine the response to NMDA was clearly biphasic. However, there was no consistent relationship between the magnitudes of the peak and plateau phases of the response, and their temporal relationship was also highly variable. The potentiation seen with exogenous glycine was highly dependent on plating density, which may be the result of higher levels of endogenous glycine in more dense cultures. Our results provide an explanation of the inconsistent findings previously reported by different groups on the potentiation of the [Ca 2+] i response to NMDA by exogenous glycine.