Contralateral effect of amitriptyline and bupivacaine for sciatic nerve block in an animal model of inflammation

Background. Using a carrageenan inflammation rat model, we evaluatedtwo experimental approaches to prolong sciatic nerve block oncontralateral hyperalgesia. Method. We performed ipsilateral sciatic nerve block on theinflamed hind paw with bupivacaine-loaded microspheres suspendedin dexamethasone (bupivacaine 12.5 mg) and with amitriptyline(6.25 and 12.5 mg) as ultralong-acting local anaesthetics. Bupivacaine(1.25 mg) was used as long-acting local anaesthetic and salinewas used as a control. The sixth group received amitriptyline6.25 mg intraperitoneally (n=10 for each group). Results. The duration of ipsilateral nerve block was 2 h forbupivacaine, 7 h for amitriptyline 6.25 mg, 11 h for amitriptyline12.5 mg and 21 h for bupivacaine-loaded microspheres in suspensionwith dexamethasone. Whereas contralateral hyperalgesia was notobserved during block produced by bupivacaine-loaded microspheres,contralateral hyperalgesia was observed with sciatic nerve blockusing amitriptyline. Conclusions. Because of the differential effect observed onthe contralateral side, the mechanism underlying the prolongationof ipsilateral block with amitriptyline may not result onlyfrom a prolonged Na⁺ channel blockade but might be explainedby a local toxic effect or lack of systemic actions.