diamine (GS39783) and Structurally Related Compounds: Novel Allosteric Enhancers of -Aminobutyric Acid B Receptor Function

N,N-Dicyclopentyl-2-methylsulfanyl-5-nitro-pyrimidine-4,6- diamine (GS39783) and structurally related compounds are de- scribed as novel allosteric enhancers of GABAB receptor function. They potentiate GABA-stimulated guanosine 5-O-(3-( 35 S)thio)- triphosphate (( 35 S)GTPS) binding to membranes from a GABAB(1b/2)-expressing Chinese hamster ovary cell line at low micromolar concentrations, but do not stimulate ( 35 S)GTPS 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 ( 3 H)3- aminopropylphosphinic acid, resulting in a net increase in affinity for the agonist radioligand. In equilibrium binding experiments (displacement of the antagonist ligand ( 3 H)CGP62349), GS39783 increases agonist affinities. Agonist displacement curves are bi- phasic, probably reflecting the G protein-coupled and uncoupled states of the receptor. The proportion of the high-affinity compo- nent is increased by GS39783, suggesting that the G protein coupling of the receptor is also promoted by the positive modu- lator. 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 Ca 2 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 re- leased GABA at presynaptic GABAB receptors.