Characterization of socio-psychological stress-induced antinociception in the formalin test in mice

The antinociceptive effect induced by exposure to socio-psychological (PSY) stress using a communication box was assessed by the formalin test in mice, compared with those by exposure to footshock (FS) stress and forced swimming (SW) stress. After the termination of stress exposure, whereas exposure to FS- and SW-stress resulted in the attenuation of the formalin-induced biphasic pain response over 15 min, no appreciable antinociceptive effect was found in the case of PSY stress. When exposure to PSY stress was started during the period of early or late phase of pain after the formalin injection, the antinociceptive effect was maintained for 5-15 min; however, further exposure to PSY stress was not effective for producing antinociception. In the tail-pinch test, likewise, exposure to PSY stress longer than 5 min rather decreased the intensity of antinociception. We conclude that PSY stress in this tonic pain paradigm produces antinociception, but further continuous exposure to the emotional stress caused mice to become recuperative even in such a fear-inducing situation.