Non-peptidergic OP4 receptor agonist inhibits morphine antinociception but does not influence morphine dependence

The non-peptidergic opioid receptor-like 1 (ORL1, OP4) receptor ligand, Ro 64-6198 [(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], is a full agonist of the OP4 receptor. The aim of this study was to evaluate whether this compound influences morphine antinociception and dependence in mice. Ro 64-6198 inhibits the acute analgesic effect of morphine in the tail-immersion test, however, when given chronically during the acquisition of morphine dependence, development of this dependence is not prevented. The acute injection of Ro 64-6198 suppresses withdrawal escape jumps in morphine dependent mice, though this effect may be a result of the loss of locomotor activity induced by this compound and/or its myorelaxant action. The study provides evidence that stimulation of the OP4 receptor suppresses acute morphine antinociception, but is not sufficient to inhibit the development of morphine dependence in mice.