Reduction of morphine-withdrawal aggression by conditional social stimuli.

Sixty male hooded rats were made physically dependent on morphine by steadily increasing doses of morphine sulphate. A maintenance dose of 400 mg/kg/day was reached in 10 days and was continued for 5 additional days. At the end of the 15-day period all rats were withdrawn for 72 h and aggressive responses (attacks, rearing, and vocalization) were recorded for a 60-min period. One treatment group, in which a social experience had been paired with each morphine injection, showed significantly less morphine-withdrawal aggression than rats in two other groups which either remained socially isolated throughout the addiction period, or were grouped both at the time of morphine injection and during between-injection intervals.