Spontaneous withdrawal in opiate-dependent Fischer 344, Lewis and Sprague-Dawley rats

The Lewis (LEW) and Fischer 344 (F344) inbred rat strains react differentially to acute morphine administration for a variety of behavioral and neurochemical measures. Investigations into effects of chronic morphine are less common, and investigations assessing dependence have been limited to those utilizing antagonist-precipitated withdrawal. The present experiment extended these assessments by examining spontaneous withdrawal in the LEW and F344 strains. In this preparation, males of the LEW, F344 and the outbred Sprague-Dawley (SD) strain were made dependent on morphine. Following this, opiate administration was terminated and animals were examined for spontaneous withdrawal by the acquisition of a withdrawal-associated taste aversion, changes in body weight loss and the display of several behaviors characteristic of opiate withdrawal. Although all morphine-treated subjects decreased body weight and avoided consumption of the withdrawal-associated solution, indicating successful induction of dependence, no difference between the strains emerged in these indices of withdrawal severity. The only strain difference to appear in the behavioral indicators of withdrawal was with diarrhea (LEW>F344). That the strains differ in acute reactivity to opioids, but not in the overall severity of withdrawal, was discussed in relation to the need to examine the relationship between neurochemical and behavioral data in a variety of neural systems and behavioral endpoints.