Involvement of endogenous opioids with forced swimming-induced immobility in mice.

The present study investigated the involvement of endogenous opioid mechanisms with the immobility response induced in mice by forced swimming. Pretreatment with the narcotic antagonist naloxone (0.625--40.0 mg/kg) caused a dose-dependent decrease in the duration of immobility in mice subjected to a 10 min swim test. This effect was more pronounced in C57BL/6J mice than in BALB/C mice. A low dose of morphine (0.15 mg/kg) potentiated immobility whereas higher doses (0.625/10.0 mg/kg) had no demonstrable effect on immobility in these strains. The results suggest that release of endogenous opioids may be a physiological event promoting natural cataleptic-like behaviors in mice.