Abstract
Male and female rats of Sprague-Dawley and Long-Evans strains were given daily intraperitoneal iniections of morphine sulfate in increasing dosage over a 15-day period to a terminal dose of 405 mg/kg/day. During withdrawal, fighting behavior occurred equally among older male rats (60 days at start of experiment) of both strains, housed in groups of six, but not at all in younger ones (30 days). A strain difference found earlier in 45-day-old rats was confirmed. Females of either strain or age showed no aggressive behavior during withdrawal. Decreasing available cage space increased the number of aggressive encounters, despite a reduction of group size to four. Saline-injected males of either strain rarely displayed aggression. A circadian rhythmicity of such behavior was not found. There was evidence for a seasonal variation in frequency of morphine-withdrawal-induced aggression, levels being higher in summer than in winter.
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