Individual differences in the induction of sensitisation or tolerance to the motor effects of morphine in mice

Acute morphine administration produces hyperactivity in most strains of mice and some studies indicate that repeated treatment can induce a potentiation of this effect (sensitisation). With the object of determining whether the hyperactivity induced by morphine in OF1 mice shows sensitisation after repeated administration, we tested the effect of the pre-exposure to six daily injections of morphine on the subsequent hyperactivity induced by the same dose of this drug. Animals were treated with 40 mg/kg of morphine for seven (lays. After the last injection they were tested in an actimeter at different points at time (15, 30, 45 and 60 min). This repeated schedule of treatment produced sensitisation in some animals but tolerance in others. These results suggest the existence of great individual differences in the motor effects of repeated morphine administration and that the development of sensitisation and tolerance in outbred mice may be subjected to genetic variation.