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 t o produce an analgesic effect in the rat formalin test. The authors examine d the effect of 17 nmol of nociceptin on the expression of Fos-like immunor eactivity (Fos-LI) in the spinal dorsal horn induced by paw formalin inject ion and compared the effect of 17 nmol of nociceptin with that of equiantin ociceptive dose (3 nmol) of morphine. Nociceptin and morphine were administ ered 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 la minae I-II, but not in laminae III-V, to the same extent. These data sugges t that intratheally administered nociceptin and morphine suppress the nocic eptive input into the laminae I-II and produce antinociceptive effect. (C) 2000 Elsevier Science ireland Ltd. All rights reserved.