TY - JOUR T1 - <em>N</em>,<em>N</em>′-Dicyclopentyl-2-methylsulfanyl-5-nitro-pyrimidine-4,6-diamine (GS39783) and Structurally Related Compounds: Novel Allosteric Enhancers of γ-Aminobutyric Acid<sub>B</sub> Receptor Function JF - Journal of Pharmacology and Experimental Therapeutics JO - J Pharmacol Exp Ther SP - 322 LP - 330 DO - 10.1124/jpet.103.053074 VL - 307 IS - 1 AU - Stephan Urwyler AU - Mario F. Pozza AU - Kurt Lingenhoehl AU - Johannes Mosbacher AU - Christina Lampert AU - Wolfgang Froestl AU - Manuel Koller AU - Klemens Kaupmann Y1 - 2003/10/01 UR - http://jpet.aspetjournals.org/content/307/1/322.abstract N2 - N,N′-Dicyclopentyl-2-methylsulfanyl-5-nitro-pyrimidine-4,6-diamine (GS39783) and structurally related compounds are described as novel allosteric enhancers of GABAB receptor function. They potentiate GABA-stimulated guanosine 5′-O-(3-[35S]thio)triphosphate ([35S]GTPγS) binding to membranes from a GABAB(1b/2)-expressing Chinese hamster ovary cell line at low micromolar concentrations, but do not stimulate [35S]GTPγS binding by themselves. Similar effects of GS39783 are seen on native GABAB receptors in rat brain membranes. Concentration-response curves with GABA in the presence of different fixed concentrations of GS39783 reveal an increase of both the potency and maximal efficacy of GABA at the GABAB(1b/2) heterodimer. In radioligand binding experiments, GS39783 reduces the kinetic rate constants of the association and dissociation of [3H]3-aminopropylphosphinic acid, resulting in a net increase in affinity for the agonist radioligand. In equilibrium binding experiments (displacement of the antagonist ligand [3H]CGP62349), GS39783 increases agonist affinities. Agonist displacement curves are biphasic, probably reflecting the G protein-coupled and uncoupled states of the receptor. The proportion of the high-affinity component is increased by GS39783, suggesting that the G protein coupling of the receptor is also promoted by the positive modulator. We also show that GS39783 has modulatory effects in cellular assays such as GABAB receptor-mediated activation of inwardly rectifying potassium channels in Xenopus oocytes and Ca2+ signaling in human embryonic kidney 293 cells. In a more physiological context, GS39783 is shown to suppress paired pulse inhibition in rat hippocampal slices. This effect is reversed by the competitive GABAB receptor antagonist CGP55845A and is produced most likely by enhancing the effect of synaptically released GABA at presynaptic GABAB receptors. The American Society for Pharmacology and Experimental Therapeutics ER -