The discriminative stimulus properties of naloxone in a conditioned taste aversion procedure

Animals injected with the opiate antagonist naloxone hydrochloride prior to a saccharin-LiCl pairing and with its vehicle prior to saccharin alone rapidly acquired a drug discrimination. That is, they avoided saccharin following the administration of naloxone and consumed saccharin following its vehicle, after only three conditioning trials. Control subjects for which naloxone never signaled a saccharin-LiCl pairing consumed saccharin following both naloxone and vehicle administration. During dose-substitution sessions, animals displayed greater drug-appropriate responding as the dose of naloxone increased. When a range of doses of naltrexone was given in place of naloxone prior to saccharin access, subjects displayed dose-dependent naloxone-appropriate responding. When a range of doses of morphine was substituted for naloxone, subjects displayed vehicle-appropriate responding at all doses. The fact that naloxone was an effective stimulus at low doses and with few conditioning trials suggests that the conditioned taste aversion procedure is a sensitive index of drug discrimination learning.