Distribution of nociceptin and Ro64-6198 activated [35S]-GTPgammaS binding in the rat brain

The peptide, nociceptin, was discovered as the endogenous ligand for the opioid-like receptor, ORL1. Since its discovery, this peptide has been shown to modulate the perception of pain, modulate feeding and produce behavioral effects in rodent models of mood disorders. Recently, the non-peptide agonist {(1S,3aS)-8-(2,3,3a,4,5,6-hexahydro-1H-phenalen-1-yl)-1-phenyl-1,3,8-triaza-spiro[4,5]decan-4-one} (Ro64-6198) of the ORL1 receptor has been reported in the literature. In the present study, we compared the distribution and potency of Ro64-6198 with nociceptin for their ability to stimulate [(35)S]-GTPgammaS binding to sections of rat brain. In initial studies, Ro64-6198 inhibited (125)I-nociceptin binding to the hORL1 receptors with a K(i) of 1.75 nM compared with 0.20 nM for nociceptin. To assess agonist potency in a whole cell assay, a cell line expressing the hORL1 receptor and G(alpha15) was created and used for calcium mobilization studies. In this assay system, Ro64-6198 increased intracellular calcium with an EC(50) of 52nM compared with 24 nM for nociceptin. Having verified the agonist properties of Ro64-6198, we then assessed the potency and distribution of ORL1 receptor activation in rat brain sections. In dose-response studies, Ro64-6198 increased [(35)S]-GTPgammaS binding to a variety of brain regions with EC(50) values ranging from 84.9 to 2,143 nM depending on the brain regions evaluated. These potencies were similar to that seen for nociceptin, but substantially lower than values established using [(125)I] nociceptin binding to the cloned human ORL1 receptor. In general, the brain distribution of agonist stimulated [(35)S]-GTPgammaS binding was similar when either Ro64-6198 or nociceptin were used. Using these techniques, we have demonstrated, for the first time that Ro64-6198 activates [(35)S]-GTPgammaS binding to rat brain sections and this compound stimulates a similar population of receptors as nociceptin.