The influence of neuroactive steroid lipophilicity on GABAA receptor modulation: Evidence for a low-affinity interaction

Mariangela Chisari, Lawrence N. Eisenman, Kathiresan Krishnan, Achintya K. Bandyopadhyaya, Cunde Wang, Amanda Taylor, Ann Benz, Douglas F. Covey, Charles F. Zorumski, Steven Mennerick

Research output: Contribution to journalArticlepeer-review

51 Scopus citations

Abstract

Anesthetic steroids with actions at γ-aminobutyric acid type A receptors (GABAARs) may access transmembrane domain binding site(s) directly from the plasma cell membrane. Accordingly, the effective concentration in lipid phase and the ability of the steroid to meet pharmacophore requirements for activity will both contribute to observed steady-state potency. Furthermore, onset and offset of receptor effects may be rate limited by lipid partitioning. Here we show that several GABA-active steroids, including naturally occurring neurosteroids, of different lipophilicity differ in kinetics and potency at GABAARs. The hydrophobicity ranking predicted relative potency of GABAAR potentiation and predicted current offset kinetics. Kinetic offset differences among steroids were largely eliminated by γ-cyclodextrin, a scavenger of unbound steroid, suggesting that affinity differences among the analogues are dwarfed by the contributions of nonspecific accumulation. A 7-nitrobenz-2-oxa-1,3-diazole (NBD)-tagged fluorescent analogue of the low-lipophilicity alphaxalone (C17-NBD-alphaxalone) exhibited faster nonspecific accumulation and departitioning than those of a fluorescent analogue of the high-lipophilicity (3α,5α)-3-hydroxypregnan-20-one (C17-NBD-3α5αA). These differences were paralleled by differences in potentiation of GABAAR function. The enantiomer of C17-NBD-3α5αA, which does not satisfy pharmacophore requirements for steroid potentiation, exhibited identical fluorescence kinetics and distribution to C17-NBD-3α5αA, but was inactive at GABA ARs. Simple simulations supported our major findings, which suggest that neurosteroid binding affinity is low. Therefore both specific (e.g., fulfilling pharmacophore requirements) and nonspecific (e.g., lipid solubility) properties contribute to the potency and longevity of anesthetic steroid action.

Original languageEnglish
Pages (from-to)1254-1264
Number of pages11
JournalJournal of neurophysiology
Volume102
Issue number2
DOIs
StatePublished - Aug 2009

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