Postnatal social environment affects morphine analgesia in male mice

Seventy-eight male mice of the Swiss CD-1 strain were maintained from birth to weaning either in litters containing only male pups (MM), or in litters containing both male and female pups (MF). Body weight gain and neurobehavioral development were assessed, and no differences between MM and MF males were found. On postnatal day 40 one male from each litter (N = 13 per group) was injected either with vehicle (saline solution, 8%) or with morphine (5 or 10 mg/kg). Fifteen min after the injection, forepaw licking latencies were measured in a hot plate test (55 +/- 1 degree C). MF males showed longer latencies than MM males, and this difference was statistically significant after both morphine doses. These results indicate that the function of systems mediating response to painful stimulation and opiate analgesia can be affected by variations in early social conditions.