RT Journal Article SR Electronic T1 Reversible modulation of opioid receptor binding in intact neural cells by endogenous guanosine triphosphate. JF Molecular Pharmacology JO Mol Pharmacol FD American Society for Pharmacology and Experimental Therapeutics SP 690 OP 695 VO 48 IS 4 A1 N Yabaluri A1 F Medzihradsky YR 1995 UL http://molpharm.aspetjournals.org/content/48/4/690.abstract AB Incubation of SH-SY5Y neural cells with mycophenolic acid (MPA), an inhibitor of inosine monophosphate dehydrogenase (the key enzyme in purine nucleotide biosynthesis), reduced the cellular content of GTP by 94% relative to its concentration in control cells (43 nmol/mg protein) without altering the level of GDP. Although in GTP-depleted intact cells the receptor binding parameters (Kd and Bmax) of the opioid antagonist [3H]naltrexone were unchanged from those in untreated cells, the binding affinity of the mu-selective opioid agonist [3H]Tyr-D-Ala-Gly-(Me)- Phe-Gly-ol ([3H]DAMGO) was enhanced 2-fold. Furthermore, the kinetics of ligand/receptor interaction revealed that in the nucleotide-depleted cells, the dissociation rate constant for [3H]DAMGO was reduced by 44%. Initial exposure of SH-SY5Y cells to pertussis toxin reduced high-affinity ligand binding by 95% and abolished the effect of MPA treatment. Renewed incubation of the GTP-depleted cells with guanosine restored the original GTP levels and agonist binding. Neither MPA nor guanosine treatment changed the Bmax of [3H]DAMGO binding. Forskolin- and prostaglandin E1-stimulated adenylyl cyclase activities were decreased significantly in GTP-depleted cells. DAMGO and [D-Pen2,D-Pen5]enkephalin inhibitions of adenylyl cyclase were also affected with MPA treatment. Maximal inhibition of forskolin-stimulated adenylyl cyclase activity by both of the agonists was reduced, whereas MPA caused a 2-fold reduction in potency for DAMGO. The results show that reduction in endogenous GTP levels leads to noticeable changes in agonist, receptor, and G protein interactions, as measured by agonist binding, and to subsequent diminution of the signal transduction, as reflected by the cAMP levels.