Differential effects of micro-opioid, delta-opioid and kappa-opioid receptor agonists on dopamine receptor agonist-induced climbing behavior in mice.

Interactions between the dopaminergic system and opioids have not been adequately clarified. The present study was designed to investigate the effects of micro-opioid (morphine), delta-opioid (SNC80) and kappa-opioid (U50 488H) receptor agonists on dopamine receptor agonist-induced climbing behavior in mice. Apomorphine (dopamine-receptor agonist) increased stereotyped climbing behavior, unlike methamphetamine, morphine, U-50 488H and (+/-)7-hydroxy-N,N-di-n-propyl-2-aminotetralin hydrobromide (D2-like receptor agonist). Furthermore, SKF81297 (D1 receptor agonist) and SNC80 caused climbing behavior. In addition, while morphine (20 mg/kg), but not U50 488H or SNC80, significantly attenuated high-dose apomorphine (2.0 mg/kg)-induced climbing behavior, it significantly potentiated low-dose apomorphine (0.5 mg/kg)-induced climbing behavior. These results suggest that morphine may have dual effects on the behavioral effects induced by apomorphine. Furthermore, we interestingly showed that the combination of apomorphine or SKF81297 and SNC80 enhanced frequent nonstereotypic climbing behavior, suggesting that delta/D1 interactions may play a prominent role in the expression of certain types of behavior in mice. Thus, micro-opioid, delta-opioid and kappa-opioid receptor agonists induce possible differential effects on the dopaminergic system in mice.