Post-trial induction of conditioned apomorphine stimulant and inhibitory response effects: evidence for potent trace conditioning of drug effects.

The Pavlovian conditioning of drug effects has frequently been demonstrated using protocols that are variants of Pavlovian delay conditioning. We undertook to determine if drug conditioning could be induced using a Pavlovian trace conditioning procedure. Rats were tested in a novel open-field environment for 5 min and in post-trial phase were injected either with vehicle, 2.0 mg/kg or 0.05 mg/kg apomorphine immediately or after a delay of 15 min. The procedure was repeated three times and subsequently a 30 min non-drug test was given. The vehicle and 15 min post-trial apomorphine groups did not differ and in the 30 min test their locomotion scores were equivalent to another vehicle group tested for the first time. The group that received 2.0 mg/kg apomorphine immediately post-trial had a progressive increase in activity over the three sessions and also initially in the 30 min test. The results for the 0.05 mg/kg immediate post-test group were a mirror image of the 2.0 mg/kg apomorphine group. Post-trial apomorphine treatments can induce potent conditioned effects indicative of the efficacy of trace conditioning of drug effects. These finding suggest that trace conditioning may be an important contributor to the potency of conditioned-drug effects in the development of drug addiction.