Effect of social isolation on the reinforcing properties of morphine in the conditioned place preference test.

It is widely accepted that early environmental influences may affect the behaviour of the adult animal and the responsivity to psychotropic drugs. Isolation rearing is an important variable in this regard. The present experiments compared the effects of isolation and social rearing of rats on the reinforcing properties of morphine. Male Lister hooded rats were raised from weaning either alone (isolation reared rats) or in groups of four rats per cage (socially reared rats). Four weeks later, the rats were tested for their sensitivity to morphine using a conditioned place preference test. Comparisons were made between socially and isolation-reared rats with respect to preconditioning and postconditioning following either saline or morphine (1 and 5 mg/kg SC) pretreatment. The results from the preconditioning phase demonstrated that rats reared either socially or in isolation had least preference for one quadrant that was then selected as the treatment quadrant. After saline conditioning, the socially reared rats still showed a significant (p < 0.05) less preference for the treatment quadrant relative to the opposite quadrant. Following morphine (1 and 5 mg/kg) conditioning, socially reared rats spent significantly more time (p < 0.05) in the treatment quadrant relative to the opposite quadrant, whereas isolation-reared rats failed to display morphine-induced place preference. These results demonstrate that social isolation decreases the reinforcing properties of morphine.