Effects of intrathecal orphanin FQ on a flexor reflex in the rat after inflammation or peripheral nerve section

We examined the effects of intrathecal orphanin FQ, the endogenous ligand f or the orphan opioid-like receptor, on the hamstring nociceptive flexor ref lex in decerebrate, spinalized, unanesthetized rats after carrageenan-induc ed inflammation or unilateral sciatic nerve transection. As described previ ously [Xu, X.-J., Hao, J.-X., Wiesenfeld-Hallin, Z., 1996. Orphanin FQ or a ntiorphanin FQ: potent spinal antinociceptive effect of orphanin FQ/orphani n FQ in the rat. NeuroReport 7, 2092-2094.], intrathecal orphanin FQ induce d a dose-dependent depression of the flexor reflex with a ED50 of 965 ng. I nitial reflex facilitation was noted in some experiments at lower doses (10 or 100 ng). A similar bi-phasic response pattern to intrathecal orphanin F Q was observed in experiments conducted in inflamed or axotomized rats. How ever, the magnitude of the initial reflex facilitation was significantly in creased in inflamed rats compared to normals whereas the duration of reflex depression was significantly shortened. The ED50 for reflex depression was 2.4 mu g for inflamed rats. In contrast, axotomy did not significantly alt er the facilitatory and depressive effect of orphanin FQ with ED50 for refl ex depression being 374 ng. These results confirmed an inhibitory action of orphanin FQ on spinal nociception in rats. It is suggested that the effect of orphanin FQ may be modulated by inflammation and nerve injury. In parti cular, unlike morphine, there seems to be no reduction in the effect of spi nal orphanin FQ in inducing antinociception after peripheral nerve axotomy. (C) 1999 Elsevier Science B.V. All rights reserved.