Differential effects of μ-, δ- and κ-opioid receptor agonists on the discriminative stimulus properties of cocaine in rats

Abstract

The effects of selective μ-, δ- and κ-opioid receptor agonists on the discriminative stimulus properties of cocaine were examined in rats trained to discriminate between cocaine (10 mg/kg) and saline. Cocaine produced a dose-related increase in cocaine-appropriate responses in all of the rats. In generalization tests, neither morphine (μ-opioid receptor agonist) nor N-methyl- N-7-(1-pyrrolidinyl)-1-oxaspiro[4,5]dec-8-11-4-benzofuranacetamide (U50,488H: κ-opioid receptor agonist) generalized to the discriminative stimulus properties of cocaine. On the other hand, the newly synthesized non-peptide selective δ-opioid receptor agonist 2-methyl-4 aα-(3-hydroxyphenyl)-1,2,3,4,4 a,5,12,12 aα-octahydro-quinolino(2,3,3,– g)isoquinoline (TAN-67) partially generalized (56.7% cocaine-appropriate responses) to the discriminative stimulus properties of cocaine. Intracerebroventricular (i.c.v.) administration of [ d-Ala 2]deltorphin II (peptide δ 2-opioid receptor agonist) completely generalized, while neither [ d-Ala 2,MePhe 4,Gly-ol 5]enkephalin (DAMGO; μ-opioid receptor agonist) nor [ d-Pen 2, d-Pen 5]enkephalin (DPDPE; δ 1-opioid receptor agonist) generalized to the discriminative stimulus properties of cocaine. These results suggest that the discriminative stimulus properties of cocaine may be partially mediated by δ-opioid (especially δ 2-opioid) receptors. In combination tests, pretreatment with morphine (3.0 mg/kg) and TAN-67 (3.0 and 10 mg/kg) significantly potentiated the discriminative stimulus properties cocaine. In contrast, pretreatment with U50,488H (2.0 and 4.0 mg/kg) scarcely shifted the discriminative stimulus properties of cocaine. Furthermore, the potentiating effect of 3.0 mg/kg morphine on the discriminative stimulus properties of cocaine was attenuated by 2.0 mg/kg U50,488H. In contrast, the potentiating effect of 10 mg/kg TAN-67 on the discriminative stimulus properties of cocaine was not reversed by either 2.0 or 4.0 mg/kg U50,488H. These results suggest that μ-, δ- and κ-opioid receptor agonists modulate the discriminative stimulus properties of cocaine through different mechanisms, perhaps through different effects on the dopaminergic system. © 1997 Elsevier Science B.V.