Modification of Antinociceptive Action of Morphine by Dopamine Agonist and Antagonist in Mice

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Abstract
A marked potentiation of the antinociceptive action of morphine was produced by haloperidol, but apomorphine had no effect on the antinociceptive action of morphine in acute experiments. Following chronic treatment with haloperidol, the antinociceptive action of morphine was significantly suppressed by apomorphine, and apomorphine shifted the dose-response curve of morphine to the right and increased the ED50 value of morphine by 2.3-fold. These results suggest that the suppressive action of apomorphine on the antinociceptive effect of morphine in chronic haloperidol-treated mice may be due to an increased sensitivity of postsynaptic dopaminergic receptors to apomorphine.