Positive allosteric modulation of native and recombinant,gamma-aminobutyric acid(B) receptors by 2,6-Di-tert-butyl-4-(3-hydroxy-2,2-dimethyl-propyl)-phenol (CGP7930) and its aldehyde analog CGP13501

The compounds CGP7930 [2,6-Di-tert-butyl-4-(3-hydroxy-2,2-dimethyl-propyl)- phenol] and its close analog CGP13501 were identified as positive modulator s of gamma -aminobutyric acid, (GABA(B)) receptor function. They potentiate GABA-stimulated guanosine 5'-O-(3-[S-35]thiotriphosphate) (GTP gamma[S-35] ) binding to membranes from a GABA(B(1b/2)) expressing Chinese hamster ovar y (CHO) cell line at low micromolar concentrations and are ineffective in t he absence of GABA. The structurally related compounds propofol and malonob en are inactive. Similar effects of CGP7930 are seen in a GTP gamma[S-35] b inding assay using a native GABA(B) receptor preparation (rat brain membran es). Receptor selectivity is demonstrated because no modulation of glutamat e-induced GTP gamma[S-35] binding is seen in a CHO cell line expressing the metabotropic glutamate receptor subtype 2. Dose-response curves with GABA in the presence of different fixed concentrations of CGP7930 reveal an incr ease of both the potency and maximal efficacy of GABA at the GABA(B(1b/2)) heteromer. Radioligand binding studies show that CGP7930 increases the affi nity of agonists but acts at a site different from the agonist binding site . Agonist affinity is not modulated by CGP7930 at homomeric GABA(B(1b)) rec eptors. In addition to GTP gamma[S-35] binding, we show that CGP7930 also h as modulatory effects in cellular assays such as GABA(B) receptor-mediated activation of inwardly rectifying potassium channels in Xenopus laevis oocy tes and Ca2+ signaling in human embryonic kidney 293 cells. Furthermore, we show that CGP7930 enhances the inhibitory effect of L-baclofen on the osci llatory activity of cultured cortical neurons. This first demonstration of positive allosteric modulation at GABA(B) receptors may represent a novel m eans of therapeutic interference with the GABA-ergic system.