Early housing experience modifies morphine self-administration and physical dependence in adult rats

Male Sprague-Dawley rats were raised from weaning in one of the three housing conditions: one, two or four per housing unit. At 60 days of age, animals were moved to individual cages and tested in the open field. Following a baseline period in which animals were allowed to adapt to their new housing condition, animals had their water replaced with a 0.8 mg/ml morphine sulfate solution. Following 12 days of access to the drug, animals were injected with naloxone and abstinence precipitated. While no differences were found in body weight among the three groups of animals at 60 days of age, significant differences in open field behavior were noted. Animals that were raised in groups were found to be more active in the open field than animals raised in isolation. Early housing experience was also found to modify later morphine consumption and physical dependence. Animals raised in isolation exhibited a trend to start drinking morphine sooner and experienced less severe withdrawal symptoms following naloxone administration than group-raised animals.