Stress-induced analgesia in pigs.

In order to assess pain sensitivity in pigs, the tail-withdrawal reflex induced by immersing the tail into warm water was used. Reaction times were found to be a negative function of water temperature. They were not modified by 1 mg/kg morphine or naloxone. Higher doses of morphine (5 mg/kg) induced excitation which interfered with the assessment of the tail-withdrawal reflex. Low doses of xylazine (0.1-0.2 mg/kg) decreased reaction times while clearly sedative doses (10 mg/kg) had no effect. Various stressors (handling, exposure to a new environment, shock delivery, social isolation) enhanced reaction times and this increase was not blocked by naloxone.