- Allopregnanolone (3α5α-P), pregnanolone, and their synthetic derivatives are potent positive allosteric modulators (PAMs) of GABAA receptors (GABAARs) with in vivo anesthetic, anxiolytic, and anti-convulsant effects. Mutational analysis, photoaffinity labeling, and structural studies have provided evidence for intersubunit and intrasubunit steroid-binding sites in the GABAAR transmembrane domain, but revealed only little definition of their binding properties. Here, we identified steroid-binding sites in purified human α1β3 and α1β3γ2 GABAARs by photoaffinity labeling with [3H]21-[4-(3-(trifluoromethyl)-3H-diazirine-3-yl)benzoxy]allopregnanolone ([3H]21-pTFDBzox-AP), a potent GABAAR PAM.
- Many neuroactive steroids potently and allosterically modulate pentameric ligand-gated ion channels, including GABAA receptors (GABAAR) and nicotinic acetylcholine receptors (nAChRs). Allopregnanolone and its synthetic analog alphaxalone are GABAAR-positive allosteric modulators (PAMs), whereas alphaxalone and most neuroactive steroids are nAChR inhibitors. In this report, we used 11β-(p-azidotetrafluorobenzoyloxy)allopregnanolone (F4N3Bzoxy-AP), a general anesthetic and photoreactive allopregnanolone analog that is a potent GABAAR PAM, to characterize steroid-binding sites in the Torpedo α2βγδ nAChR in its native membrane environment.
- Extrasynaptic γ-aminobutyric acid type A receptors (GABAARs),which contribute generalized inhibitory tone to the mammalian brain, are major targets for general anesthetics. To identify anesthetic binding sites in an extrasynaptic GABAAR, we photolabeled human α4β3δ GABAARs purified in detergent with [3H]azietomidate and a barbiturate, [3H]R-mTFD-MPAB, photoreactive anesthetics that bind with high selectivity to distinct but homologous intersubunit binding sites in the transmembrane domain of synaptic α1β3γ2 GABAARs.
- Background: For some chiral barbiturates, one isomer potentiates and the other inhibits GABA responses by binding to unknown sites.Results: A photoreactive convulsant barbiturate identifies a transmembrane intersubunit-binding site between the γ and β subunits.Conclusion: Positive and negative allosteric modulators can bind to a common intersubunit site.Significance: This study defines a novel mode of regulation of GABAAR responses.