The involvement of opioid delta receptor in restraint-induced antinociception in mice

Restraint for 1 h induced significant antinociceptive activity in both male and female mice. The restraint animals all showed an increase in response time to the hot plate test at 55 degrees C. The antinociceptive activity decreased with time after restraint but was still apparent at 1 h post restraint. In the male animals, prior administration of opioid delta receptor antagonist naltrindole (0.5-2.0 mg/kg s.c.) 15 min before restraint for 1 h did not affect the degree of antinociceptive activity induced by restraint. In addition, the same doses of naltrindole administered s.c. immediately after restraint for 1 h also did not affect the degree of antinociceptive activity in male mice. However, for female mice the same doses of naltrindole administered s.c. before restraint dose-dependently suppressed the antinociceptive activity induced by restraint. When administered immediately after restraint for 1 h, naltrindole also reversed the antinociceptive activity in female mice. The response times measured 10 min after restraint were significantly reduced when compared with the saline restraint controls. It is concluded that restraint can induce antinociceptive activity in mice; however, different mechanisms may be involved in the antinociception observed. In male mice the endogenous opioid systems did not seem to play a significant role in restraint-induced antinociception, whereas for female animals blockade of opioid delta receptors greatly diminished the antinociception observed after restraint.