CHRONIC VARIABLE STRESS OR CHRONIC MORPHINE FACILITATES IMMOBILITY INA FORCED SWIM TEST - REVERSAL BY NALOXONE

The behaviors displayed in a forced swim test were investigated in rat s previously exposed to a chronic variable stress treatment or chronic administration of morphine. In addition, to further explore the parti cipation of an endogenous opiate mechanism in these behavioral effects , naloxone was either administered during the chronic treatment (prior to each stress or morphine exposure) or immediately prior to the forc ed swim test. Animals were submitted daily to a different stressor for 1 week or injected with morphine (10 mg/kg, IP) for 6 days, whereas c ontrols were unmanipulated except for the injection process. On the da y following the last stressor, control and stressed animals were admin istered saline or naloxone (2 mg/kg, IP) 15 min prior to the forced sw im test. Morphine treated animals were similarly tested on the third d ay following the last morphine injection. In a separate group of rats, naloxone (2 mg/kg, IP) was administered daily 10 min prior to each st resser of the chronic stress regime or each daily morphine injection. A significant increase in the time spent in immobility was observed in stressed animals as well as in rats chronically treated with morphine . In both groups, this potentiated immobility was attenuated by naloxo ne pretreatment prior to the forced swim test or when given before eac h daily stresser or morphine injection. In addition, the concurrent ex posure to stress or morphine along with naloxone administration enhanc ed struggling in the first 5 min of the forced swim test. Taken togeth er, the results of these experiments support the conclusion that the i ncrease in immobility seen following chronic variable stress or repeat ed morphine exposure is modulated by the activation of an endogenous o piate mechanism, given that this effect is attenuated by naloxone admi nistration.