Comparison of the action potential, block quality and neurotoxic effects of capsaicin and bupivacaine applied to the rat sciatic nerve

Introduction: Comparison of the effects of local anesthetics applied in peripheral nerve blocks and adjuvant drugs added to avoid dose-dependent toxic effects of local anesthetics on the block durations are only available in clinical studies. In our study, our aim is to evaluate the nerve conduction velocity, block quality and neurotoxic effects of capsaicin at different doses added to bupivacaine on a rat experimental model. Thus, we aimed to assess capsaicin as an adjuvant agent. Material and methods: In our study, 36 rats were used; Saline (n=4) and Control (n=4) groups, as study groups Group DK: Capsaicin low dose (0,01 % capsaicin, n=7), Group YK: Capsaicin high dose (0,1 % capsaicin, n =7), Group DKB: Bupivacine+low-dose capsaicin (0,01 % capsaicin + 0,25 % bupivacaine, n=7), Group YKB: Bupivacaine+high-dose capsaicin (0,1 % capsaicin + 0.25 % bupivacaine), n=7), a total of 6 groups were formed. After the rats were anesthetized by administering ketamine hydrochloride 90 mg/kg and xylazine 3 mg/kg intraperitoneally, the left sciatic nerves were opened by making a longitudinal incision. Control recording was taken by placing two recording electrodes distal and proximal to the explored part. Study drugs were prepared in 0.2 ml volume and dripped onto the rat sciatic nerve. Compound action potentials and nerve conduction velocities were recorded electronically by giving 10 currents of 10 mA at 5-minute intervals with a bioamplifier device (0.min, 5.min, 10.min, 15.min, 20.min, 25.min, 30.min, 35.min, 40.min, 45.min). In order to evaluate the motor and sensory block caused by the drugs independently of the pain due to surgery, 18 hours after the procedure, after the same drug was infiltrated into the right sciatic nerve region, which is the intact side, motor and sensory tests of the rats were performed with 0.min, 30.min, 1, 2, 6, 12 and 24 hours with Hot-Plate and Paw-Withdrawal tests in both extremities. Afterwards, sciatic nerve samples taken from the sacrificed rats were evaluated histopathologically. Results: It was observed that 0,1 % and 0,01 % concentrations of capsaicin applied to the rat sciatic nerve significantly reduced nerve conduction velocities. The effect on the compound action potential was found that both 0,1 % concentration and 0,01 % concentration containing bupivacaine had similar results with the 0,1 % concentration without bupivacaine, but was significantly different from the control and v 0,01 % concentration without bupivacaine. When behavioral tests were evaluated, it was observed that capsaicin at a concentration of 0,1 % was the group that suppressed the motor and sensory response the most compared to all groups. On the contrary, although capsaicin at a concentration of 0.01% decreased nerve conduction velocity and made a significant change in the compound action potential, it was observed that it caused significant changes both in itself and in the control group without suppressing the motor and sensory response. Neurodegeneration was not detected in any group in histopathological examination. When the mast cell count was examined, it was observed that 0,01 % capsaicin combined with 0,25 % bupivacaine had similar histopathological features to the control and saline groups, while the mast cell count was higher in the other study groups than in the control and saline groups. Conclusion: Although capsaicin at a concentration of 0,1% with or without bupivacaine effectively creates nerve block, although neurodegeneration is not observed, we think that it may cause neuroinflammation. Keywords: Capsaicin, bupivacaine, action potential, nociception, rat