Background: Since the anti-inflammatory effects of usual doses of morphine (10mg/kg) is proved, and also, regarding to the role of C-fibers in the release of substance P and CGRP and progression of inflammation were found, and the other hand, the excitatory effects of very low doses of morphine (1µg/kg) on C-fibers have also been reported, present study has examined the role of C-fibers, usual and very low doses of morphine in presence or reduced C-fibers on paw edema induced by sub plantar injection of formalin.

Materials & Methods: In this study, male Wistar rats (180-200g) were Categorized into two groups. One as vehicle (which received neonatal capsaicin vehicle, C-normal) and experimental (which received neonatal capsaicin, C-lesion). Each category contains three groups which were treated by saline, 10 mg/kg and 1µg/kg of morphine (i.p). In each group, paw edema volume induced by injection of 0.05 cc of 2.5% formalin, was acquired by plethysmometric test.

Results: In C-normal groups, morphine showed an anti-inflammatory effect at the dose of 10mg/kg (p<0.01) and pro-inflammatory effect at the dose of 1µg/kg (p<0.01). Reduction of C-fibers in C-lesion groups diminished the inflammation induced by formalin (p<0.001). Also in these groups, 10 mg/kg dose of morphine showed a more potent anti-inflammatory effects (p<0.001) and the pro-inflammatory effects of 1µg/kg dose of morphine reduced (p<0.001).

Conclusion: C-fibers by releasing substance P and CGRP can play an important role in inflammation induction in injured tissue. The results showed that, the reduction of these fibers can lead to inflammation reduction. So, the presences of C-fibers are so important in inflammation induction and edema.