Sensitization to the morphine-like discriminative stimulants effects of buprenorphine in rats

An experiment was performed to determine whether chronic non-contingent adm inistration of morphine would produce cross-sensitization to the cueing pro perties of buprenorphine or D-amphetamine. To this end the sensitivity to t he discriminative stimulus effects of morphine, buprenorphine and D-ampheta mine was determined in rats trained to discriminate 10 mg kg(-1) morphine f rom saline in a food-reinforced operant task. Seven rats were given repeate d noncontingent treatments with morphine (20 mg kg(-1) on saline or no-test days and 10 mg kg(-1) on drug days) starting 20 days before the beginning of discrimination training; another six animals received injections of sali ne. Chronic administration of morphine resulted in sensitization to the dis criminative stimulus effect of this drug and in cross-sensitization to the discriminative stimulus effect of buprenorphine. D-Amphetamine produced onl y saline lever selection in all rats. In conclusion, the present results co nfirm that the stimulus properties of opioid drugs may be enhanced, rather than decreased, in animals with a history of repeated non-contingent treatm ent with morphine. Sensitization to central-acting drugs is thought to play a role in the psychopathology of drug abuse. Hence, the present results po int out the necessity of considering the effects of drugs which show tolera nce, and those which show sensitization, under any particular drug regimen. (C) 2000 Academic press.