CHANGES IN FOOD-MAINTAINED PROGRESSIVE-RATIO RESPONDING OF RATS FOLLOWING CHRONIC BUPRENORPHINE OR METHADONE ADMINISTRATION

Seven rats lever pressed under a progressive ratio 3 (PR 3) schedule o f food presentation; the number of responses per reinforcer systematic ally increased during each session. Break point (i.e., the number of r esponses in the last completed ratio before session termination) was m easured under daily methadone (4.5 mg/kg and 3.0 mg/kg) or buprenorphi ne (0.3 mg/kg) administered prior to experimental sessions. Both drugs initially eliminated rats' food-maintained progressive-ratio respondi ng. Break points during chronic methadone did not return to baseline l evels after 80 drug sessions and a dose reduction. In contrast, break points during chronic buprenorphine administration were considerably a bove baseline control levels for two rats and returned to baseline lev els for the third.