GABA_A Receptor Populations Bind Agonists and Antagonists Differentially and with Opposite Affinities

Abstract

Pretreatment of synaptosomal membranes with a diazo-coupling reagent and the presence of Cl- ions were used to differentiate high- and low-affinity populations of postsynaptic gamma-aminobutyric acid (GABAA) receptors. The super-low-affinity GABAA receptors were characterized by the enhancing effect of GABA on [3H]diazepam binding. The GABA antagonists 2-(3-carboxypropyl)-3-amino-4-methyl-6-phenylpyridazinium chloride (SR 95103) and 3-alpha-hydroxy-16-imino-5 beta-17-aza-androstan-11-one (R 5135) shifted and suppressed the dose-response curve of GABA on diazepam binding. SR 95103 displaced the lower affinity [3H]GABA binding with higher potency. Dissociation of the binding of the antagonist 2-(3-carboxypropyl)-3-amino-6-p-methoxyphenylpyridazinium bromide ([3H]SR 95531) was polyphasic. Displacing potencies of SR 95531 and GABA were examined on the major (85%) rapid and minor slower phases of dissociation separated kinetically. The slower phase corresponded to higher affinity binding of SR 95531 which was displaced by GABA with about 10 times less potency. Photoaffinity labeling with muscimol decreased the number of [3H]muscimol binding sites by 27%. It decreased the displacing potency of GABA by 72%, but not that of bicuculline methiodide. These findings can be explained by a preferential binding of antagonists to hydrophobic accessory sites around low-affinity GABAA receptors.

Characterization of antagonistic activity and binding properties of SR 95531, a pyridazinl‐GABA derivative, in rat brain and cultured cerebellar neuronal cells

“…It has also been demonstrated that other compounds that have detergent activity, such as lysophosphatidylcholine and digitonin, increase [3H]muscimol binding (Ito and Kuriyama, 1982). Thus the decrease in r3H]SR 95531 binding as well as the increase in the binding of [3Hlmuscimol induced by lysophosphatidylcholine treatment possibly antagonist-preferring binding sites (Maksay, 1988;Olsen and Snowman, 1983). Our results support this hypothesis, since the low-affinity binding site of r3H1SR 95531 seems to be responsible for the antagonistic action of SR 95531 in GABA receptor-gated C1-flux.…”

“…It has been suggested that antagonists interact not only with GABA recognition sites, but also hydrophobic accessory sites (Maksay and Ticku, 1984). Earlier reports also suggested that the antagonists interacted competitively with high-affinity GABA sites, and not competitively with lower-affinity sites due to preferential interaction with hydrophobic sites (Maksay, 1988). As a consequence of PLase A, cleavage of membrane phospholipids, unsaturated free fatty acids and lysosphospholipids are released.…”

Steroid control of uterine motility via γ-aminobutyric acidA receptors in the rabbit: a novel mechanism?

“…findings of others that bicuculline acts as a noncompetitive antagonist of low affinity (functional) GABAA receptors in the brain (for review see Maksay, 1988), although Riesz & Erdö (1985) reported occasional reversal of bicuculline-induced contractions by muscimol. In contrast to direct effects on uterine motility mediated via GABAA receptors, the GABAB receptor-mediated uterine excitation may be indirect or may ensue from metabolic factors, as it occurs after a delay of about 1 -8 min both for GABA and baclofen.…”

Dissociation of Muscimol, SR 95531, and Strychnine from GABAA and Glycine Receptors, Respectively, Suggests Similar Cooperative Interactions

“…The high-affinity GABAA agonist [3H]muscimol has been successfully used in dissociation studies, even at physiological temperature (Yang and Olsen, 1987). Dissociation studies were extended here to the new GABAA antagonist [ 3H]2-( 3-carboxypropyl)-3-amino-6-p-methoxyphenylpyndazinium bromide ([3H]SR 95531) (Heaulme et al, 1987) which has preferential affinity to the lower affinity GABAA binding sites (Maksay, 1988;McCabe et al, 1988).…”