Percutaneous sciatic nerve block with tramadol induces analgesia and motor blockade in two animal pain models

Abstract

Local anesthetic efficacy of tramadol has been reported following intradermal application. Our aim was to investigate the effect of perineural tramadol as the sole analgesic in two pain models. Male Wistar rats (280-380 g; N = 5/group) were used in these experiments. A neurostimulation-guided sciatic nerve block was performed and 2% lidocaine or tramadol (1.25 and 5 mg) was perineurally injected in two different animal pain models. In the flinching behavior test, the number of flinches was evaluated and in the plantar incision model, mechanical and heat thresholds were measured. Motor effects of lidocaine and tramadol were quantified and a motor block score elaborated. Tramadol, 1.25 mg, completely blocked the first and reduced the second phase of the flinching behavior test. In the plantar incision model, tramadol (1.25 mg) increased both paw withdrawal latency in response to radiant heat (8.3 ± 1.1, 12.7 ± 1.8, 8.4 ± 0.8, and 11.1 ± 3.3 s) and mechanical threshold in response to von Frey filaments (459 ± 82.8, 447.5 ± 91.7, 320.1 ± 120, 126.43 ± 92.8 mN) at 5, 15, 30, and 60 min, respectively. Sham block or contralateral sciatic nerve block did not differ from perineural saline injection throughout the study in either model. The effect of tramadol was not antagonized by intraperitoneal naloxone. High dose tramadol (5 mg) blocked motor function as well as 2% lidocaine. In conclusion, tramadol blocks nociception and motor function in vivo similar to local anesthetics.

Figure 1.. Effect of perineural sciatic tramadol (1.25 mg) in the presence or absence of intraperitoneal naloxone (1 mg/kg) on the flinching behavior induced by 1% formalin in the hindpaw. No difference was found between these groups, but they differed from the perineural saline injection and Sham block groups (^*P < 0.05, Kruskal-Wallis test followed by the Mann-Whitney test). Data are reported as means ± SEM for 5 animals per group.

Figure 2.. Effect of perineural sciatic tramadol (1.25 mg) in the presence or absence of intraperitoneal naloxone (1 mg/kg) on mechanical withdrawal threshold. No difference was found between these groups, but they differed from the perineural saline injection and Sham block groups at all times (^*P < 0.05, Kruskal-Wallis test followed by the Mann-Whitney test). The arrow shows the time of surgery. Data are reported as means ± SEM for 5 animals per group.

Figure 3.. Effect of perineural sciatic tramadol (1.25 mg) in the presence or absence of intraperitoneal naloxone (1 mg/kg) on heat withdrawal threshold. The two groups were similar at 15, 30, and 60 min but differed significantly from the perineural saline and Sham block groups (^*P < 0.05, Kruskal-Wallis test followed by the Mann-Whitney test). ⁺P < 0.05 for perineural tramadol compared to naloxone ip/tramadol (Kruskal-Wallis test followed by the Mann-Whitney test). The arrow indicates the time of surgery. Data are reported as means ± SEM for 5 animals per group.