Differences in amphetamine and morphine sensitivity in lateralized and non-lateralized rats: locomotor activity and drug self-administration.

Morphine and d-amphetamine were tested for their effects on locomotor activity and for their propensities to be intravenously self-administered in rats that had been screened for their tendencies to rotate (turn in circles) spontaneously at night; noctural rotation was used as a behavioral index of asymmetry in the dopaminergic nigrostriatal system. Lateralized (rotating) rats were more sensitive to the locomotor stimulant effects of d-amphetamine than non-lateralized (non-rotating) rats. The stimulant effects of low doses of morphine were also greater in lateralized rats, whereas the depressant effects of high doses of morphine were greater in non-lateralized rats. Lateralized rats self-administered more d-amphetamine than non-lateralized rats whereas non-lateralized rats self-administered more morphine than lateralized rats. The data indicate that the degree of lateralization in some brain pathways is a source of interindividual variation in drug sensitivity--this may in part be responsible for the individual tendencies of humans to selectively abuse particular types of drugs.