CHARACTERISTICS OF CALCIUM CHANNELS RESPONSIBLE FOR VOLTAGE-ACTIVATEDCALCIUM-ENTRY IN RAT CEREBELLAR GRANULE CELLS

The properties and characteristics of calcium channel openings in cere bellar granule cells were analysed by the cell-attached patch-clamp te chnique. At depolarized potentials, with 110 mM Ba2+ as the divalent c harge carrier, 36% of the patches displayed activity that consisted of elementary events whose amplitude ranged from - 0.3 to - 1.75 pA at 0 mV, giving rise to a high threshold current. In this population of ev ents at least four different types of channel openings were identified by their distinct biophysical and pharmacological properties. Two typ es of channel openings, with conductances around 24 and 7 pS, had simi lar characteristics in that both opened following two modes of gating characterized by brief (approximately 2 ms) and longer openings (appro ximately 8 ms) and both were sensitive to dihydropyridines. A further type of channel opening, with a conductance around 11 pS gated mainly with brief openings (approximately 1 ms), was shown to be insensitive to dihydropyridines but was undetectable in recordings from the cells that had been treated with omega-conotoxin. The last type of event was revealed after treatment of the cell with nicardipine or nifedipine a nd omega-conotoxin. The corresponding channel had a conductance of 19 pS and opened in one dominant mode characterized by brief openings (ap proximately 1 ms). The data obtained on single-channel activity of cer ebellar granule cells are compared with the properties of the total cu rrent recorded in whole-cell conditions.