Abstract
To elucidate the role of endogenous opioid systems in social play, we investigated the effects of morphine on playful and non-playful social behaviors in juvenile rats. These effects were studied under various circumstances, being 1, 3.5 or 24 hours of social isolation before testing, known to yield a minimal, half-maximal and maximal induction of social play (1), and different levels of illumination during testing. It appeared that morphine increased playful rather than nonplayful social behaviors, in a dose-dependent manner. This increasing effect was apparent under dim light circumstances, independent of the duration of previous isolation and with a maximal effect at a dose of 1.0 mg/kg of morphine. This effect can be interpreted as an effect on the rewarding aspects of play
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