Dipyrone sparing effect of L-tryptophan in nociceptive tests in mice.

NFM Rocha, AMR Carvalho, ERV Rios, LF Vasconcelos, ML Dias, DS Macedo, FCF Sousa. Universidade Federal do Ceará, Fisiologia e Farmacologia, Brazil

Dipyrone (metamizol) and L-tryptophan are two substances historically associated to pain relief. Dipyrone has a controversial association with serious toxic effects. We utilized the abdominal writhing test induced by acetic acid (modified of Koster, 1959) and hot plate test (Ed & Leimbach, 1953) to evaluate the antinociceptive effect of the Dipyrone, L-tryptophan and the association of both. Were used swiss mice (male and female) weighing 22-32g divided into groups containing 6-12 animals. The Dipyrone (25, 50, 100 or 200 mg/kg) and L-tryptophan (12.5, 25, 50 or 100 mg/kg) were administered by intraperitoneal route 30 minutes before the injection of 0.6% acetic acid. The dipyrone exhibited an antinociceptive effect at the higher doses 100 and 200 mg/kg (6.55 ± 1.57 and 5.33 ± 0.843, respectively, p< 0.05) compared to mean number of abdominal writhes of control group (24.31 ± 2.88). By the other hand; the L-tryptophan did not exhibit the antinociceptive effect being no different of control group [L-tryptophan 12.5 mg/kg (23.00 ± 3.00), 25 mg/kg (25.50 ± 2.87), 50 mg/kg (22.29 ± 4.04) or 100 mg/kg (18.43 ± 3.84)]. Several combination of treatment were realized demonstrating that L-tryptophan is capable to diminish the doses needed to dipyrone shows antinociceptive effect [Dipyrone 50 mg/kg (28.20 ± 2.75), L-tryptophan 25 mg/kg + Dipyrone 50 mg/kg (4.43 ± 2.51, p<0.05 vs control group) Dipyrone 25 mg/kg (24.40 ± 2.71), L-tryptophan 50 mg/kg + Dipyrone 25 mg/kg (10.00 ± 2.65, p< 0.05 vs control group)]; the association with L-tryptophan is able to reduce the antinociceptive doses of dipyrone. In the hot plate test the association showed the same capacity to L-tryptophan reduces the antinociceptive dose of dipyrone, thus we show for the first time the dipyrone sparing effect of L-tryptophan. L-tryptophan is a precursor of 5-HT synthesis and it is metabolized to others active compounds, in order to verified the participation of 5-HT synthesis on the effect of the dipyrone and the synergic effect of L-tryptophan, groups of animals were depleted of 5-HT by p-clorophenylanine (inhibitor of 5-HT synthesis, 100 mg/kg) before the abdominal writhing induction. The 5-HT depletion by PCPA blocks partially the dipyrone 100 mg/kg antinociceptive action [PCPA (17.83 ± 3.59, p< 0.05 vs control and dipyrone 100 mg/kg) Dipyrone 100 mg/kg (6.55 ± 1.59), and antagonize the synergic effect of L-tryptophan + Dipyrone (22.75 ± 6.18, p<0.05 vs L-tryptophan 50 + Dipyrone 50 mg/kg)]. 5-HT influence on the antinociceptive effect of dipyrone was detected by the increase of amount of 5-HT in the brains of animals (quantified in homogenates of whole brain) treated with antinociceptive dose of dipyrone (100 mg/Kg)[Control (0.028 ± 0.003 μM/g), dipyrone 100 mg/Kg (0.077 ± 0.022 μM/g, p<0.05 vs control) . Non antinociceptive doses of dipyrone were not able to increase central 5-HT amount [25 mg/kg(0.028 ± 0.00581 μM/g) 50 mg/kg(0,012 ± 0,00299 μM/g)]. The treatment with L-tryptophan (25 to 100 mg/kg, p< 0.05 vs control) increases the central 5-HT amount [Control (0.0277 ± 0.00296 μM/g), L-tryptophan 25 mg/kg (0.0722 ± 0.00457 μM/g) and 100 mg/kg (0.0612 ± 0.00738 μM/g). We assessed the antinociceptive activity of Kynurenic acid, L-tryptophan metabolite, and this substance exhibited a powerful antinociceptive activity at the doses of 200, 50 and 25 mg/kg (12.20 ± 1.46; 7.000 ± 2.74; 6.400 ± 1.57, respectively, p< 0.05 vs control); at the dose of 18.75 the effect was intermediary and was not verified the antinociceptive activity when administrated doses of 12.5 or 6.25 mg/kg. However, these lower doses exhibited a synergic effect with dipyrone in reduction of abdominal writhes number (1.40 ± 0.40 or 1.50 ± 0.645, respectively, p< 0.05 vs control). Previous administration of luzindol (melatonin receptor antagonist, 10 mg/kg) blocks partially the effect of dipyrone sparing effect of L-tryptophan in abdominal writhing test Control. Conclusion: L-tryptophan exhibited dipyrone sparing effect in nociceptive tests in mice and this effect looks to be involved with L-tryptophan biotransformation to 5-HT, Kynurenic acid and Melatonin.