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+ chann el 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. C erebellar granule neurons were stimulated to release newly synthesized [H-3]glutamate by K+-evoked depolarization. Stimulated release was en tirely calcium-dependent and abolished by the presence of 200 mu M cad mium. 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 glutama te release was partially inhibited by both the L-type calcium channel blocker, nicardipine, and the N-type calcium channel blocker, omega-co notoxin GVIA; however, the P/Q-type calcium channel blocker omega-agat oxin IVA inhibited release of glutamate only after pre-incubation of c ells with omega-conotoxin GVIA. K+-stimulated release of glutamate was observed when stimulated either in the presence of Ca2+ or of Ba2+ an d similar inhibition of release by (-)-baclofen was seen under both co nditions. In contrast to these results, ionomycin-evoked glutamate rel ease was greatly reduced as compared to K+-evoked release and was not modulated by (-)-baclofen. In the presence of omega-conotoxin GVIA alo ne, inhibition of release by (-)-baclofen was attenuated but not aboli shed. Following block of nicardipine-sensitive channels, inhibition of release by (-)-baclofen was still present, and after prior block of o mega-conotoxin GVIA-sensitive channels the presence of nicardipine res tored the ability of (-)-baclofen to inhibit residual release of gluta mate. Modulation of glutamate release by (-)-baclofen was unaffected b y the presence of omega-agatoxin IVA alone; however, after block of bo th omega-conotoxin GVIA- and omega-agatoxin IVA-sensitive channels, in hibition of release by (-)-baclofen was completely abolished. These re sults indicate that multiple sub-types of voltage-dependent calcium ch annels are present on the presynaptic terminals of cerebellar granule neurons and support K+-stimulated release of [H-3]glutamate. Modulatio n of release by GABA(B) receptor activation appears to be dependent up on interaction of this receptor with a number of voltage-sensitive cal cium channels, including omega-conotoxin GVIA-sensitive and omega-agat oxin IVA-sensitive channels.