Opioid receptors in the ventral tegmental area contribute to stress-induced analgesia in the formalin test for tonic pain.

Exposure to stress can induce analgesia in the formalin test for tonic pain. Here, we report that blockade of opioid receptors by bilateral infusions of naltrexone methylbromide (NMB) (0.1 microgram/0.5 microliter/side) in the ventral tegmental area (VTA) in the midbrain can reduce stress-induced analgesia in this test. These findings indicate that endogeneous opioids act in the VTA to mediate stress-induced analgesia in tonic pain.