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 acid(A) (GABA(A)) 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 GABA(A) receptors and Ca²⁺ channels. In contrast to (+)-ACN, (+)-ECN does not potentiate GABA(A)- receptor activated currents, nor does it directly gate GABA(A)-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 μU 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 μU 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 μU 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 asteroid with no effects on GABA(A) receptors 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.