Inhibitory effect of intrathecally administered nociceptin on the expression of Fos-like immunoreactivity in the rat formalin test.

Nociceptin, an opioid receptor-like 1 receptor agonist, has been reported to produce an analgesic effect in the rat formalin test. The authors examined the effect of 17 nmol of nociceptin on the expression of Fos-like immunoreactivity (Fos-LI) in the spinal dorsal horn induced by paw formalin injection and compared the effect of 17 nmol of nociceptin with that of equiantinociceptive dose (3 nmol) of morphine. Nociceptin and morphine were administered intrathecally 10 min before the formalin injection and the expression of Fos-LI was examined 2.5 h after the formalin injection. Both 17 nmol of nociceptin and 3 nmol of morphine suppressed the expression of Fos-LI in laminae I-II, but not in laminae III-V, to the same extent. These data suggest that intratheally administered nociceptin and morphine suppress the nociceptive input into the laminae I-II and produce antinociceptive effect.