Differential effects of morphine, DPDPE, and U-50488 on apomorphine-induced climbing behavior in μ-opioid receptor knockout mice

The present study examined the hypothesis whether the opioid receptors (μ, δ, and κ) contribute to a behavioral dopaminergic activation produced by dopamine receptor agonist, apomorphine, by comparing responses in wild type and μ-opioid receptor knockout mice. The data suggest that expression of μ-opioid receptors plays an important role in the enhancement of climbing behavior induced by apomorphine. Compared to wild type mice, a response in the dopaminergic behavior by treatment with δ-receptor agonist, DPDPE, is more sensitive to the mice lacking μ-opioid receptor. Treatment with κ-receptor agonist, U-50488, is potentiated the apomorphine-induced climbing behavior in wild type and μ-opioid receptor knockout mice. These responses may be independent of that through μ-opioid receptors. Therefore, the our results show that dopaminergic activation measured by climbing behavior in μ-opioid receptors knockout mice are differently regulated by μ-, δ-, and κ-opioid receptor agonists.