title: Biphasic modulation of GABA(A) receptor binding by steroids suggests functional correlates.
abstract: Neuroactive steroids and other positive modulators of GABA(A) receptors showed regional variation in both the efficacy and potency for modulation of [35S]TBPS binding to rat brain membrane homogenates, with biphasic concentration-dependence. GABA present in the binding assays prevented the enhancement phase of the steroid concentration-dependence plot while the antagonists bicuculline and RU5135 prevented the inhibition phase. Using recombinant GABA(A) receptors, expressed in insect cell line Sf9 using baculovirus, enhancement by steroids of [35S]TBPS binding was sensitive to the presence of the gamma2 subunit and the nature of the alpha subunit (alpha1 beta2 gamma2S > alpha1 beta2, alpha6 beta2, alpha6 beta2 gamma2S, and alpha6 beta2 delta). As in cerebellum, addition of RU5135 reduced the inhibitory phase and revealed a small enhancement of TBPS binding by neuroactive steroids. The subunit-dependent interactions of steroid and GABA site ligands are consistent with a three-state model in which the receptor mono-liganded by GABA or steroid has a different affinity for TBPS than the resting state, and the receptor biliganded by GABA, steroid, or both has little affinity for TBPS.
authors.affiliation: Department of Molecular & Medical Pharmacology, UCLA School of Medicine, Los Angeles, CA 90095-1735, USA.
authors.forename: S
authors.initials: S
authors.lastname: Srinivasan
authors.forename: D W
authors.initials: DW
authors.lastname: Sapp
authors.forename: A J
authors.initials: AJ
authors.lastname: Tobin
authors.forename: R W
authors.initials: RW
authors.lastname: Olsen
mesh_terms: D000731:Androstanes / Q000494:pharmacology; D000777:Anesthetics / Q000494:pharmacology; D000818:Animals; D001378:Azasteroids / Q000494:pharmacology; D016367:Baculoviridae / Q000235:genetics; D001640:Bicuculline / Q000494:pharmacology; D001921:Brain / Q000378:metabolism*; D019086:Bridged Bicyclo Compounds, Heterocyclic / Q000378:metabolism*; D002460:Cell Line; D002462:Cell Membrane / Q000378:metabolism; D018756:GABA Antagonists / Q000494:pharmacology; D015870:Gene Expression; D011277:Pregnanediones / Q000494:pharmacology; D051381:Rats; D011963:Receptors, GABA-A / Q000187:drug effects / Q000235:genetics / Q000378:metabolism*; D011994:Recombinant Proteins / Q000378:metabolism; D018411:Spodoptera; D013256:Steroids / Q000494:pharmacology*; D013462:Sulfur Radioisotopes; D005680:gamma-Aminobutyric Acid / Q000494:pharmacology
chemical_list: D000731:Androstanes; D000777:Anesthetics; D001378:Azasteroids; D019086:Bridged Bicyclo Compounds, Heterocyclic; D018756:GABA Antagonists; D011277:Pregnanediones; D011963:Receptors, GABA-A; D011994:Recombinant Proteins; D013256:Steroids; D013462:Sulfur Radioisotopes; D005680:gamma-Aminobutyric Acid; C037476:tert-butylbicyclophosphorothionate; C006477:alphaxalone; C032061:RU 5135; D001640:Bicuculline