title: Lactone modulation of the gamma-aminobutyric acid A receptor: evidence for a positive modulatory site.
abstract: The gamma-aminobutyric acid-A (GABA(A)) receptor complex is allosterically modulated by a variety of substances, some of clinical importance. Barbiturates and neurosteroids augment GABA-currents and also directly gate the channel. A variety of gamma-butyrolactone analogues also modulate GABA-induced currents, with some potentiating and others inhibiting. Because several gamma-thiobutyrolactone analogues have biphasic effects on GABA currents, experiments with wild-type and picrotoxinin-insensitive GABA(A) receptors were performed to analyze whether some gamma-thiobutyrolactones interact with two distinguishable sites on the GABA(A) receptor. beta-Ethyl-beta-methyl-gamma-thiobutyrolactone inhibited GABA-induced currents at low concentrations (0.001-1 mM), but potentiated GABA-induced currents at higher concentrations (3-10 mM) in wild-type alpha1beta2gamma2-subunit containing ionophores. The related alpha-ethyl-alpha-methyl-gamma-thiobutyrolactone potentiated submaximal GABA currents in wild-type receptors at both low and high concentrations (0.1-10 mM). Mutations in the second transmembrane domain of alpha1, beta2, or gamma2 conferred picrotoxinin-insensitivity onto GABA(A) receptor complexes. When these mutated alpha1, beta2, or gamma2 subunits were incorporated into the receptor complex, beta-ethyl-beta-methyl-gamma-thiobutyrolactone potentiated GABA currents over the entire concentration range (0.1-10 mM). Neither the potentiating activity nor the EC50 of alpha-ethyl-alpha-methyl-gamma-thiobutyrolactone changed in the mutant receptors. Further studies demonstrated that the mutations did not affect the EC50 of chlordiazepoxide or phenobarbital. These and our earlier results identify a modulatory site on the GABA(A) receptor distinct from that interacting with barbiturates, benzodiazepines, and steroids. Additionally, they show that the gamma-butyrolactones probably interact at two different sites on the ionophore to produce opposite effects on GABA-mediated current.
authors.affiliation: Department of Anatomy and Neurobiology, Washington University School of Medicine, Saint Louis, Missouri 63110, USA.
authors.forename: K L
authors.initials: KL
authors.lastname: Williams
authors.forename: J B
authors.initials: JB
authors.lastname: Tucker
authors.forename: G
authors.initials: G
authors.lastname: White
authors.forename: D S
authors.initials: DS
authors.lastname: Weiss
authors.forename: J A
authors.initials: JA
authors.lastname: Ferrendelli
authors.forename: D F
authors.initials: DF
authors.lastname: Covey
authors.forename: J E
authors.initials: JE
authors.lastname: Krause
authors.forename: S M
authors.initials: SM
authors.lastname: Rothman
mesh_terms: D015107:4-Butyrolactone / Q000031:analogs & derivatives / Q000494:pharmacology; D000818:Animals; D002707:Chlordiazepoxide / Q000494:pharmacology; D005260:Female; D005442:Flumazenil / Q000494:pharmacology; D007783:Lactones / Q000494:pharmacology*; D010634:Phenobarbital / Q000494:pharmacology; D010852:Picrotoxin / Q000494:pharmacology; D011963:Receptors, GABA-A / Q000187:drug effects*; D014982:Xenopus laevis; D005680:gamma-Aminobutyric Acid / Q000494:pharmacology
chemical_list: D015107:4-Butyrolactone; D002707:Chlordiazepoxide; D005442:Flumazenil; D007783:Lactones; D010634:Phenobarbital; D010852:Picrotoxin; D011963:Receptors, GABA-A; D005680:gamma-Aminobutyric Acid; C051894:alpha-ethyl, alpha-methyl-thiobutyrolactone