The involvement of multiple calcium channel sub-types in glutamate release from cerebellar granule cells and its modulation by GABAB receptor activation

Abstract In this study, we have examined both the ability of various Ca 2+ channel sub-types to support the release of [ 3 H]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 [ 3 H]glutamate by K + -evoked depolarization. Stimulated release was entirely calcium-dependent and abolished by the presence of 200 μM 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, ω-conotoxin GVIA; however, the P/Q-type calcium channel blocker ω-agatoxin IVA inhibited release of glutamate only after pre-incubation of cells with ω-conotoxin GVIA. K + -stimulated release of glutamate was observed when stimulated either in the presence of Ca 2+ or of Ba 2+ 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 ω-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 ω-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 ω-agatoxin IVA alone; however, after block of both ω-conotoxin GVIA- and ω-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 [ 3 H]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 ω-conotoxin GVIA-sensitive and ω-agatoxin IVA-sensitive channels.