Brief social isolation, pain responsiveness, and morphine analgesia in young rats.

Vocalizations induced by brief tail-shock were measured in young rats that were either socially or individually housed. A single day of isolation increased tail-shock-induced vocalizations, and in animals isolated 3--4 days, morphine (4 mg/kg) was less effective in reducing vocalizations than in socially housed animals. The data suggest that social contact can sustain endogenous analgesic processes in the body.