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 ω-agatoxin IVA and ω-conotoxin 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 ω-conotoxin MVIIC and low doses of ω-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 inactivation of P-type channels in Purkinje cells is negligible, G1 represents a novel P subtype. Neither G2 nor G3 was blocked irreversibly by ω-conotoxin MVIIC, and therefore both are R-type Ca2+channels. G2 and G3 have some biophysical properties similar to those of low-voltage-activated (LVA) Ca2+channels (e.g., voltage range for steady-state inactivation,V1/2 = −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 and HVA Ca2+channels, with LVA properties prevailing in G2 and HVA properties prevailing in G3. The R-type whole-cell current was inhibited by Ni2+with a biphasic dose–response curve (IC50: 4 and 153 μm), suggesting 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.