Social crowding sensitizes high-responding rats to psychomotor-stimulant effects of morphine

Large individual differences have been identified toward varied addictive effects as evidenced in self-administration, place conditioning, and psychomotor stimulation paradigms, which have been primarily attributed to the role of congenital factors. However, it remains unknown whether environmental factors, like extraneous social stress events, could distinctively modulate animals with differentiated biobehavioral traits, such as rats with higher motor activity (high responder, HR) developed in a novel environment and their counterparts, LR (low responder) rats. In the present study, the influence of social crowding procedure upon morphine psychomotor effect was investigated. Moreover, the roles social stress played, respectively, on HRs and LRs were explored based on previous observation that HRs not only responded more to drugs but also to stress. Our results revealed that social crowding procedure could sensitize morphine psychomotor effect as a whole, and this effect was only evident for HR but not LR rats. The individual differences toward morphine psychomotor effects was indiscernible in rats housed in normal social conditions and only turned out to be significant under stress conditions. Given the fact that the occurrence of human addictive behavior usually happens within social environment permeated with various stress factors, the genetic and environmental elements may collaboratively contribute to the ultimate susceptibility of drug-prone individuals.