MODULATION OF CALCIUM-DEPENDENT AND CALCIUM-INDEPENDENT COMPONENTS OF VERATRIDINE-EVOKED RELEASE OF GLUTAMATE FROM RAT CEREBELLUM

The entry of Ca2+ into the presynaptic neuronal terminal is considered to be a prerequisite for exocytosis. However, reports suggest that a Ca2+-independent component of release can exist for some neurotransmitters. In this study we have used veratridine-stimulated release of glutamate from rat cerebellar slices to investigate Ca2+-dependent and -independent release. A 1-min pulse of veratridine (10 muM) induced release of glutamate in both Ca2+-replete and Ca2+-free ACSF. Both modes of release, however, could be elicited in a sequential manner following a single application of veratridine in Ca2+-free ACSF, with return to Ca2+-replete conditions 5 min post-pulse. This separation permitted the modulation of either, or both, phases of release. Apamin and dihydrokainate had little effect on Ca2+-independent release but produced enhancement of the Ca2+-dependent phase. Tetrodotoxin abolished both phases of release when applied with the veratridine pulse, but had no effect on the Ca2+-dependent phase alone. The Ca2+-dependent phase was partially sensitive to Co2+, although the Ca2+ channel blockers verapamil, amiloride, omega-conotoxin and ruthenium red were ineffective, suggesting a lack of involvement of L-, N- or T-type channels. The possible mechanisms mediating the Ca2+-dependent and -independent components of endogenous glutamate release from cerebellar slices are discussed.