Enantioselective Blockade of T-type Ca2+ Current in Adult Rat Sensory Neurons by a Steroid That Lacks γ-Aminobutyric Acid-Modulatory Activity

A number of steroids seem to have anesthetic effects resulting primarily from their ability to potentiate currents gated by γ-aminobutyric acidA (GABAA) receptor activation. One such compound is (3α,5α,17β)-3-hydroxyandrostane-17-carbonitrile [(+)-ACN]. We were interested in whether carbonitrile substitution at other ring positions might result in other pharmacological consequences. Here we examine effects of (3β,5α,17β)-17-hydroxyestrane-3-carbonitrile [(+)-ECN] on GABAA receptors and Ca2+channels. In contrast to (+)-ACN, (+)-ECN does not potentiate GABAA-receptor activated currents, nor does it directly gate GABAA-receptor mediated currents. However, both steroids produce an enantioselective reduction of T-type current. (+)-ECN blocked T current with an IC50 value of 0.3 μm with a maximal block of 41%. (+)-ACN produced a partial block of T current (44% maximal block) with an IC50 value of 0.4 μm. Block of T current showed mild use- and voltage-dependence. The (−)-ECN enantiomer was about 33 times less potent than (+)-ECN, with an IC50 value of 10 μm and an amount of maximal block comparable to (+)-ECN. (+)-ECN was less effective at blocking high-voltage-activated Ca2+ current in DRG neurons (IC50 value of 9.3 μm with maximal block of about 27%) and hippocampal neurons. (+)-ECN (10 μm) had minimal effects on voltage-gated sodium and potassium currents in rat chromaffin cells. The results identify a steroid with no effects on GABAAreceptors that produces a partial inhibition of T-type Ca2+current with reasonably high affinity and selectivity. Further study of steroid actions on T currents may lead to even more selective and potent agents.