In-vivo studies with the opioid antagonist, 16-methylcyprenorphine.

The effect of the opioid antagonist, 16-methylcyprenorphine (RX8008M), on the antinociceptive action of the mu-selective agonist, morphine, and the kappa-selective agonist, U50488H, has been investigated in the mouse abdominal constriction test. RX8008M produced a dose-dependent antagonism of the antinociceptive effect of morphine, but did not antagonize the response to U50488H. RX8008M should prove a useful probe for the in-vivo characterization of the receptor selectivity of opioid drugs.