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

  1. Slobodan M. Todorovic1,
  2. Murali Prakriya1,
  3. Yasunori M. Nakashima1,1,
  4. Kent R. Nilsson2,
  5. Mingchen Han2,
  6. Charles F. Zorumski3,
  7. Douglas F. Covey2 and
  8. Christopher J. Lingle1
  1. 1Department of Anesthesiology (S.M.T., M.P., Y.M.N.,C.J.L.),2Department of Molecular Biology and Pharmacology (K.R.N., M.H., D.F.C.), and 3Department of Psychiatry (C.F.Z.), Washington University School of Medicine, St. Louis, Missouri 63110

    Abstract

    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.

    Footnotes

    • Send reprint requests to: Dr. Christopher Lingle, Box 8054, Department of Anesthesiology, Washington University School of Medicine, St. Louis, MO 63110. E-mail:clingle{at}morpheus.wustl.edu

    • ↵1 Current affiliation: Surgical Operating Theatre, Kyushu University Hospital, Fukuoka, 812-8582, Japan

    • This work was supported by National Institutes of Health Grants GM47969 (C.F.Z., D.F.C., and C.J.L.) and MH00964 (C.F.Z.).

    • Abbreviations:
      HVA
      high-voltage-activated
      (+)-ACN
      (3α, 5α, 17β)-3-hydroxyandrostane-17-carbonitrile
      (+)-ECN
      (3β, 5α, 17β)-17-hydroxyestrane-3-carbonitrile
      GVIA
      ω-conotoxin GVIA
      MVIIC
      ω-conotoxin MVIIC
      GDPβS
      guanosine 5′-O-(2-thiodiphosphate)
      GTPγS
      guanosine 5′-(γ-thio)triphosphate
      GABAA
      γ-aminobutyric acidA
      DRG
      dorsal root ganglion
      Rs
      series resistance
      Cm
      whole-cell capacitance
      EGTA
      ethylene glycol bis(β-aminoethyl ether)-N,N,N′,N′-tetraacetic acid
      HEPES
      4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid
      DMSO
      dimethyl sulfoxide
      HEDTA
      N-hydroxyethylethylenediaminetriacetic acid
      • Received June 18, 1998.
      • Accepted August 14, 1998.
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    1. Molecular Pharmacology November 1, 1998 vol. 54 no. 5 918-927
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