Latent inhibition in the aversion to oral methadone

Fourteen adult Sprague-Dawley rats received daily 3 mg/kg naltrexone (Group One, n = 7) or saline (Group Two, n = 7) injections for 24 days. During this time they underwent forced choice testing with 0.125 mg/ml methadone (the unconditioned stimulus, UCS) versus taste-balanced 0.04 mg/ml quinine placebo solutions. The handling, injection ritual, and taste cues served as a conditioned stimulus (CS)-complex. While Group Two (CS-UCS paired) animals showed pronounced pharmacological methadone aversions, those in Group One (CS pre-exposed rats in which the effects of methadone were blocked by the naltrexone) maintained a moderate intake of the opiate solution. When the injection conditions were reversed for 10 days, no change in percent methadone solution occurred for either group; thus, Group One displayed a latent inhibition effect after the CS pre-exposure, while Group Two maintained its previously acquired aversion. Testing after a 3-month drug free period, however, revealed the acquisition of a comparable methadone aversion by Group One (hence, recovery from the latent inhibition observed in the first reversal phase). Parallels with latent inhibition and retention in conditioned taste aversion studies were drawn, and further support for generality in the laws of learning, suggested.