Exposure to novelty induces naltrexone-reversible analgesia in rats.

The exposure of rats for 2 min to an open field, to a small box, or to inhibitory avoidance training in the small box was followed by a mild analgesia measured by the tail-flick method. The analgesia was observed as soon as 10 s after the exposure and lasted between 10 and 30 min. It was not observed in animals previously made familiar with the test situation, and it was reversed by the administration of naltrexone (0.1 mg/kg). The data suggest that novelty per se is a sufficient stimulus to activate an opioid-mediated analgesic stimulus.