Reduction in inflammation-induced sensitization of dorsal horn neurons by transcutaneous electrical nerve stimulation in anesthetized rats

Transcutaneous electrical nerve stimulation (TENS) is utilized to treat a v ariety of painful conditions. Inflamed animals present with an increased re sponse to noxious stimuli, i.e., hyperalgesia, at the site of injury (prima ry hyperalgesia) and outside the site of injury (secondary hyperalgesia). F urther, following acute inflammation, dorsal horn neurons show an increased responsiveness to peripherally applied stimuli, which has been termed sens itization. Previous studies demonstrate a reduction in dorsal horn neuron a ctivity following TENS treatment in normal animals and a reduction in prima ry and secondary hyperalgesia in acutely inflamed animals. The purpose of t his study was to examine the effects of TENS on dorsal horn neurons sensiti zed by acute inflammation. Extracellular recordings from wide dynamic range (WDR), high threshold (HT) and low threshold (LT) dorsal horn neurons in a nesthetized rats were assessed for spontaneous activity, responses to innoc uous and noxious mechanical stimulation and receptive field size. Responses were measured before and 3 h after induction of inflammation, and immediat ely and 1 h after application of either high (100 Hz) or low (4 Hz) frequen cy TENS (motor intensity, pulse duration 100 ys). TENS was applied to the i nflamed paw to encompass the receptive field of the neuron for 20 min. WDR and HT dorsal horn neurons sensitized to mechanical stimulation after induc tion of inflammation. Application of either high or low frequency TENS to t he inflamed paw reduced both innocuous and noxious evoked responses of WDR and HT dorsal horn neurons immediately and 1 h after treatment with TENS. C omparison of responses after TENS with baseline responses showed that the e voked responses in the majority of WDR and HT cells returned to or fell bel ow baseline responses. TENS had no effect on responses of LT neurons. In su mmary, central neuron sensitization is reduced by TENS and may underlie the reduction in hyperalgesia observed after treatment with TENS.