PRELIMINARY NOTES CHRONIC N~T~XONE ADMINIST~TION REVERSES THE SUPPRESSIVE EFFECT OF CROWDING ON BODY WEIGHT GAIN IN RATS

Summary- Rats kept under crowding condition exhibited a slower rate of body weight gain compared to single housed rats. Chronic naltrexone administration reversed the suppressive effect of crowding on body weight gain but it had no effect on body weight gain in single housed rats. The results suggest that endogenous opioids may mediate the effect of crowding stress on appetitive behavior in rats. Acute injections of the opiate antagonist naloxone has been reported to suppress appetitive behavior in previously deprived rats and in genetically obese mice and rats (Rogers, Frenk, Taylor & Liebeskind, 1978; Margules, Moisset, Lewis, Shibuya & Pert, 1978; HoLtzman, 1979). By contrast, no such effect was noted in animals that were expose chronically to naloxone (Schulz, Wuster & Herr, 1979) or to the long acting opiate antagonist naltrexone (Paul, Diaz & Baily, 1978). Recently we noticed that chronic naltrexone administrat