Abstract

High voltage-activated (HVA) Ca²⁺ current (I_Ca) was recorded from neonatal rat hippocampal and adult rat dorsal root ganglion neurons. In both cell types, (+)-3α-hydroxy-5α-androstane-17β-carbonitrile [(+)-ACN], a neuroactive steroid, had no effect on nifedipine- (L-type) or ω-agatoxin IVA- (P-type) sensitive I_Ca. Selective blockade of N-type current with ω-conotoxin GVIA and of Q-type current with ω-conotoxin MVIIC indicated that (+)-ACN inhibits both N- and Q-type current components in both cell types. Current persisting after blockade of all other current components (R-type) was also sensitive to (+)-ACN. Half-blockade of (+)-ACN-sensitive HVA current occurred in the range of 3–25 μm, with N-type current somewhat more sensitive than Q- or R-type. The (+)-ACN enantiomer, (−)-ACN, and pregnanolone were somewhat less effective at inhibiting total HVA current than (+)-ACN, whereas several steroid analogs, including alfaxalone, were relatively ineffective at inhibiting total HVA current. Neither guanosine-5′-O-(2-thio)diphosphate nor guanosine-5′-O-(3-thio)triphosphate altered the ability of (+)-ACN to inhibit HVA current in dorsal root ganglion neurons, indicating that (+)-ACN acts directly on Ca²⁺channels. The partial selectivity exhibited by (+)-ACN among different HVA current components suggests that manipulations of steroid analogues may be a useful strategy in the generation of more selective, more potent, small-molecular-weight HVA channel blockers.