Research articleThe acquisition, extinction and spontaneous recovery of Pavlovian drug conditioning induced by post-trial dopaminergic stimulation/inhibition

- Post-trial apomorphine induces rapid stimulant/inhibitory conditioned drug effects.
- Post-trial conditioned drug effects exhibit extinction and spontaneous recovery.
- Dopamine trace conditioning provides a new approach to psycho-stimulant conditioning.

In contextual drug conditioning, the onset of the drug treatment is contiguous with the contextual cues. Evidence suggests that drug conditioning also can occur if there is a discontinuity between the onset of the drug effect and offset of the contextual cues. Here we examine whether post-trial contextual drug conditioning conforms to several Pavlovian conditioning tenets namely: acquisition, extinction and spontaneous recovery. Six groups of rats received apomorphine (0.05 or 2.0 mg/kg) and vehicle immediately or after a 15 min delay following a 5 min non-drug exposure to an open-field during three successive days (conditioning phase). The extinction phase occurred on days 4-8, in which all post-trial treatments were vehicle injections. After 2 days of non-testing, the final test was performed. The results showed that on the first test day, the activity levels of the 6 groups were statistically equivalent. On test day 2, there were marked differences in activity levels selectively between the two immediate post-trial apomorphine treatment groups. The immediate low dose apomorphine group displayed a reduction in activity and the immediate high dose group an increase in activity relative to their day 1 levels. The activity levels of both vehicle groups and both apomorphine delay groups remained equivalent to their day 1 activity levels. On test day 3, the differences in activity levels between the two immediate post-trial apomorphine groups increased but the activity levels of the vehicle groups and the 15 min delay post-trial apomorphine groups remained unchanged. In the extinction phase, the conditioned activity differences between the two immediate post-trial apomorphine groups were gradually eliminated. During the final test, the activity differences between the immediate post-trial apomorphine groups were partially restored, indicative of spontaneous recovery. These findings are consistent with several basic elements of Pavlovian conditioning and are supportive of drug induced trace conditioning.