Isolation decreases physical and motivational aspects of morphine withdrawal

Environmental manipulations such as social housing conditions of animals may play a role in the expression of individual differences in response to drugs. This study aimed to evaluate whether isolated and grouped mice develop different degrees of morphine dependence. Isolated and grouped mice were rendered morphine dependent employing two different methods of induction: a fast or slow protocol, both reaching the same maximum daily dose (100 mg/kg). Naloxone-induced morphine withdrawal was assessed using a modified Gellert-Holtzman scale and a conditioned place aversion (CPA) procedure. Isolated animals manifested fewer signs of physical dependence than grouped mice and only those receiving two daily morphine doses presented significantly higher scores on the Gellert-Holtzman scale than controls. Similarly, in CPA, although all morphine-treated animals developed aversion, its intensity was only significantly higher than in controls in grouped animals receiving two daily doses. Analgesic response, measured with the hot-plate test, showed that isolated mice presented longer latencies to lick their paws (even without drug administration), suggesting that they had a higher level of endogenous opiates. It can be argued that isolated animals may be less sensitive to morphine than the non-isolated and therefore tolerate greater quantities or require more drug to produce the same effects. The results suggest that variability in the response to opiates could be affected by environmental manipulations.