Susceptibility to morphine place conditioning: relationship with stress-induced locomotion and novelty-seeking behavior in juvenile and adult rats.

Previous studies demonstrated that the rewarding effect of psychostimulants, such as amphetamine and cocaine, can be predicted by locomotor activity toward novelty in a free-choice situation but not motor response developed in inescapable environment. However, whether this relationship also exists with narcotic morphine remains unclear. In the present study, the relationship between morphine place conditioning and open field as well as novelty-seeking behavior was examined in both juvenile and adult rats. By using arena open field and the same arena containing novel object, we investigated the initial open-field activity and novelty-seeking behavior after familiarization process, respectively, in juvenile and adult rats. Subsequently, the relationship between morphine (2 mg/kg) place conditioning and the above two behaviors was examined. Our results demonstrated that morphine place conditioning effect was readily acquired in both groups. The magnitude of this effect positively correlated with novelty-seeking intensity but not with open-field activity. This is the case whether juvenile or adult group was examined separately or across ages. However, only rats with high response to novelty (NHR) from their respective group expressed significant duration increase in drug-paired compartment. Rats with low response to novelty (NLR) showed no sign of this effect after the same drug training, suggesting slow acquisition of this effect in NLRs. These results also indicated that novelty-seeking actions and the rewarding effect of morphine possessed a common pathway and that neural and hormonal substrates activated in a mild stress environment like in the open field may not be critically involved in this process. The ontogenetic specificity and nonspecificity between different-aged rats as with the above relationship were discussed in this paper.