The effects of kappa-opioid receptor agonists trans-3,4-dichloro-N-(2-(1-pyrollidinyl)-cyclohexyl) benzeneacetamide ((-)-U50,488H) and 17-cyclopropylmethyl-3,14beta-dihydroxy-4,5alpha-epoxy-6beta-[N-methyl-trans-3-(3-furyl)acrylamido]morphinan hydrochloride (TRK-820) on the G-protein activation and antinociception induced by the selective mu-opioid receptor agonist, [D-Ala(2),N-MePhe(4),Gly-ol(5)]enkephalin (DAMGO), were determined in mice. G-protein activation was measured by monitoring the guanosine-5'-O-(3-[35S]thio)triphosphate ([35S]GTPgammaS) binding in the mouse pons/medulla. DAMGO (10 microM) produced a marked increase of [35S]GTPgammaS binding to the mouse pons/medulla membrane. On the other hand, both TRK-820 and (-)-U50,488H produced small but significant increases of [35S]GTPgammaS binding to the mouse pons/medulla membrane. These increases by both TRK-820 and (-)-U50,488H were completely reversed by the selective kappa-opioid receptor antagonist, norbinaltorphimine. Under these same conditions, the DAMGO-induced increase of [35S]GTPgammaS binding was significantly attenuated by TRK-820 in a concentration-dependent manner, but not by (-)-U50,488H. In the tail-flick test, DAMGO (16 ng) given intracerebroventricularly (i.c.v.), produced a marked antinociception. The antinociception induced by DAMGO was dose-dependently blocked by co-treatment with TRK-820, but not (-)-U50,488H, in mice pretreated with norbinaltorphimine (5 microg, i.c.v.). The present results provide direct evidence for the antagonistic property of TRK-820 for mu-opioid receptors, in addition to the full agonistic property for kappa-opioid receptors.