FUNCTIONAL DIVERSITY OF P-TYPE AND R-TYPE CALCIUM CHANNELS IN RAT CEREBELLAR NEURONS

By combining single-channel and whole-cell patch-clamp recordings, we have established the sensitivity to omega-agatoxin IVA and omega-conot oxin MVIIC (SNX-230) of G1, G2, and G3, the three novel non-L-, non-N- type Ca2- channels characterized previously in rat cerebellar granule cells. G1 channels were blocked irreversibly by both omega-conotoxin M VIIC and low doses of omega-agatoxin IVA (saturation at 50 nM). Thus, according to pharmacological criteria, G1 channels must be classified as P-type Ca2+ channels. Being slowly inactivating during depolarizing pulses and completely inactivated at voltages in which steady-state i nactivation of P-type channels in Purkinje cells is negligible, G1 rep resents a novel P subtype. Neither G2 nor G3 was blocked irreversibly by omega-conotoxin MVIIC, and therefore both are R-type Ca2+ channels. G2 and G3 have some biophysical properties similar to those of low-vo ltage-activated (LVA) Ca2- channels (e.g., voltage range for steady-st ate inactivation. V-1/2 = -90 mV), some properties similar to those of high-voltage-activated (HVA) Ca2+ channels (e.g., high sensitivity to Cd2+ block), and other properties intermediate between those of LVA a nd HVA Ca2+ channels, with LVA properties prevailing in G2 and HVA pro perties prevailing in G3. The R-type whole-cell current was inhibited by Ni2+ with a biphasic dose-response curve (IC50: 4 and 153 mu M), su ggesting that G2 and G3 may have a different sensitivity to Ni2+ block . Our results uncover functional diversity of both native P-type and R -type Ca2+ channels and show that R subtypes with distinct biophysical properties are coexpressed in rat cerebellar granule cells.