Social isolation: effects on pain threshold and stress-induced analgesia.

Individually housed DBA/2 mice showed higher pain thresholds than grouped mice. Stress-induced analgesia was evident in grouped but not in isolated mice. Since also morphine injections did not result in analgesic effects in isolated mice, it is suggested that social isolation results in an increased release of opioids which may produce a decreased sensitivity at the opiate receptor level. The role of endogenous opioids in relation to social isolation is discussed.