Morphine withdrawal aggression: modification with D1 and D2 receptor agonists.

Morphine withdrawal increases aggressive behaviors, induces explosive motor behaviors, and disrupts homeostatic functions in mice and rats. While many of these effects appear to result from altered dopaminergic activity during morphine withdrawal, the relative contributions of the D1 and D2 receptor subtypes remain unclear. In the present experiments, the D1 agonist SKF 38393 and the D2 agonist quinpirole were administered to male "resident" Swiss-Webster mice 5 h after the removal of a subcutaneously-implanted morphine or placebo pellet. These mice were then observed alone to determine changes in various motor activities and in confrontation with a group-housed male "intruder" to assess changes in aggressive behaviors. SKF 38393 decreased the display of aggressive behaviors by placebo and morphine-withdrawn mice without consistently altering walking or rearing. Quinpirole greatly decreased the display of aggressive behaviors by placebo mice and decreased aggressive behaviors in morphine-withdrawn mice to a lesser degree. The inhibitory effects of quinpirole were not specific to aggressive behaviors; low quinpirole doses also decreased the display of walking and rearing. In mice which received a low dose of SKF 38393 preceding quinpirole injection, pretreatment with the D1 agonist did not alter the effects of the D2 agonist quinpirole on motor activities but maintained high levels of aggression in morphine-withdrawn mice. The differential modification of aggressive and motor behaviors by selective dopaminergic agonists during morphine withdrawal further supports the suggestion that aggressive and motor behaviors are controlled independently; furthermore, D1 receptor stimulation appears to have particular relevance for the display of aggressive behaviors during morphine withdrawal.