Synergism between tramadol and meloxicam in the formalin test involves both opioidergic and serotonergic pathways

This study was designed to evaluate the antinociceptive interaction of the tramadol-meloxicam combination in different proportions (tramadol 1 meloxicam in 1:1, 1:3, and 3:1 ratios), as well as the role of nitric oxide, opioidergic, and serotonergic pathways in the antinociceptive effect of the combination. The effects of individual drugs and fixed-ratio combinations were assayed using the 3% formalin test in mice. Isobolographic analysis was employed to characterize the synergism produced by the combinations. Tramadol (3.16-10 mg/kg, i.m.), meloxicam (3.16-17.8 mg/kg, i.m.), and tramadol-meloxicam combinations produced a dose-dependent antinociceptive effect. ED ₃₀ values were estimated for the individual drugs, and isobolograms were constructed. The tramadol 1 meloxicam 1:1 and 1:3 ratio combinations showed synergistic interactions while the 3:1 ratio produced additive effects. Naloxone (1 mg/kg, i.m.) or methiothepin (0.1 mg/kg, i.m.), but not L-NAME (3 mg/kg, i.m.), prevented the antinociceptive effects of the combination. These data suggest that (1) the tramadol-meloxicam combination produces a functional synergistic interaction that involves both opioid and serotonin receptors, and (2) this combination may be a promising tool in pain management.