μ- and κ-opioid receptor-meiated opioid effects on social play in juvenile rats

Abstract

Previously, morphine has been shown to influence social play behavior in rats on two levels. An increasing effect on social play was interpreted as an effect on the rewarding aspects of social play. A lower dose of morphine abolished the effects of an unfamiliar environment on social play, supposedly by affecting the integration of environmental stimuli. In the present study the effects of receptor-specific opioid drugs on social play and measures of social behavior unrelated to play were investigated. Fentanyl, a μ-opioid receptor agonist, seemingly mimicked both effects of morphine. The μ-opioid receptor antagonist, β-funaltrexamine, decreased social play, although a low dose of this drug increased it. BUBUC (Tyr- d-Cys(StBu)-Gly-Phe-Leu-Thr(OtBu)) and naltrindole, a δ-opioid receptor agonist and δ-opioid receptor antagonist, respectively, had no effects on social behavior. The κ-opioid receptor agonist, U50,488H ( trans-3,4-dichloro- N-methyl- N-[2-(1-pyrrolidinyl)-cyclohexyl]-benzenea-cetamide), dose dependently suppressed all measures of social behavior. The κ-opioid receptor antagonist, nor-binaltorphimine, abolished the effects of an unfamiliar environment on social play. These studies suggest that the opioidergic effect on social play is mediated through μ- and κ-opioid receptor systems.