THE INVOLVEMENT OF MULTIPLE CALCIUM-CHANNEL SUBTYPES IN GLUTAMATE RELEASE FROM CEREBELLAR GRANULE CELLS AND ITS MODULATION BY GABA(B) RECEPTOR ACTIVATION

In this study, we have examined both the ability of various Ca2+ channel sub-types to support the release of [H-3]glutamate from cerebellar granule neurons and the mechanism of action involved in the modulation of glutamate release by the GABA(B) receptor agonist, (-)-baclofen. Cerebellar granule neurons were stimulated to release newly synthesized [H-3]glutamate by K+-evoked depolarization. Stimulated release was entirely calcium-dependent and abolished by the presence of 200 mu M cadmium. Release of glutamate was not affected by either tetrodotoxin or 5-aminophosphonovaleric acid but was potentiated by dihydrokainate and inhibited by 6-cyano-7-nitroquinoxaline-2,3-dione. Stimulated glutamate release was partially inhibited by both the L-type calcium channel blocker, nicardipine, and the N-type calcium channel blocker, omega-conotoxin GVIA; however, the P/Q-type calcium channel blocker omega-agatoxin IVA inhibited release of glutamate only after pre-incubation of cells with omega-conotoxin GVIA. K+-stimulated release of glutamate was observed when stimulated either in the presence of Ca2+ or of Ba2+ and similar inhibition of release by (-)-baclofen was seen under both conditions. In contrast to these results, ionomycin-evoked glutamate release was greatly reduced as compared to K+-evoked release and was not modulated by (-)-baclofen. In the presence of omega-conotoxin GVIA alone, inhibition of release by (-)-baclofen was attenuated but not abolished. Following block of nicardipine-sensitive channels, inhibition of release by (-)-baclofen was still present, and after prior block of omega-conotoxin GVIA-sensitive channels the presence of nicardipine restored the ability of (-)-baclofen to inhibit residual release of glutamate. Modulation of glutamate release by (-)-baclofen was unaffected by the presence of omega-agatoxin IVA alone; however, after block of both omega-conotoxin GVIA- and omega-agatoxin IVA-sensitive channels, inhibition of release by (-)-baclofen was completely abolished. These results indicate that multiple sub-types of voltage-dependent calcium channels are present on the presynaptic terminals of cerebellar granule neurons and support K+-stimulated release of [H-3]glutamate. Modulation of release by GABA(B) receptor activation appears to be dependent upon interaction of this receptor with a number of voltage-sensitive calcium channels, including omega-conotoxin GVIA-sensitive and omega-agatoxin IVA-sensitive channels.