Lactone modulation of the gamma-aminobutyric acid A receptor: evidence for a positive modulatory site.
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.
Williams
K L
KL
Department of Anatomy and Neurobiology, Washington University School of Medicine, Saint Louis, Missouri 63110, USA.
Tucker
J B
JB
White
G
G
Weiss
D S
DS
Ferrendelli
J A
JA
Covey
D F
DF
Krause
J E
JE
Rothman
S M
SM
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
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