Involvement of μ-opioid receptors in potentiation of apomorphine-induced climbing behavior by morphine: studies using μ-opioid receptor gene knockout mice

The present study examined the hypothesis that μ-opioid receptors contribute to a behavioral stimulation produced by stimulation of dopamine receptors by comparing responses in μ-opioid receptor knockout and wild type mice. Apomorphine-induced climbing behavior was augmented by 65%, in wild type mice, but not in μ-knockout, following subcutaneous administration of morphine (15 mg/kg). Moreover, pretreatment with either naloxone (an opioid receptor antagonist) or haloperidol (a mixed D₁/D₂ receptor antagonist) eliminated the enhancement by morphine of climbing behavior in wild type mice. These results indicate that expression of μ-opioid receptors plays an important role in the enhancement of climbing behavior induced by the dopamine receptor agonist, apomorphine. Furthermore, this augmentation is mediated by interaction between dopamine and μ-opioid receptors. Theme: Neurotransmitters, modulators, transporters, and receptors. Topic: Opioid and opiate receptors. Copyright (C) 2000 Elsevier Science B.V.