Hyperalgesia caused by nerve transection: Long-lasting block prevents early hyperalgesia in the receptive field of the surviving nerve

The aim of our study was to test the hypothesis that a long-lasting N-butyl tetracaine nerve block (>2 wk) would be much more effective in the prevent ion of hyperalgesia caused by nerve transection than the shortlasting lidoc aine block. The study was performed with the use of the saphenous nerve sec tion model in rats. The saphenous nerve was exposed and injected with salin e, lidocaine (37 mM), or N-butyl tetracaine (37 mM). Ten minutes later, the nerve was transected in some of the rats. The development of mechanical hy peralgesia (pressure threshold) of the hindpaw was assessed during a 5-wk p eriod. In rats with saphenous nerve transection without nerve block (saline injection), 3 h after the transection, the pressure threshold decreased by approximately 30% (from 175 +/- 11 g to 122 +/- 23 g, P < 0.0001); the thr eshold increased somewhat the next day, then it remained stable for 2 wk, w ith a slow process of recovery afterward. N-butyl tetracaine block without nerve transection caused a slow-developing decrease in the pressure thresho ld with the first statistically significant change at the sixth day. The co mparison of the preventive effects of lidocaine and N-butyl tetracaine bloc ks on early hyperalgesia caused by nerve transection demonstrated that both lidocaine and N-butyl tetracaine prevented hyperalgesia 3 h after the tran section. However, the protective effect of lidocaine disappeared the next d ay. In contrast, N-butyl tetracaine prevented early hyperalgesia for almost a week. The slow-developing late hyperalgesia caused by longlasting nerve block makes it impossible to study the protective effect of such a block on late hyperalgesia caused by axotomy. As far as early hyperalgesia is conce rned, the preventive effect of the N-butyl tetracaine was much longer than that of lidocaine and continued for approximately 1 wk. Implications: A lon g-lasting nerve block can prevent early hyperalgesia caused by nerve transe ction.