Transcutaneous electrical nerve stimulation (
TENS) is utilized to treat a variety of painful conditions. Inflamed animals present with an increased response to noxious stimuli, i.e.,
hyperalgesia, at the site of injury (primary
hyperalgesia) and outside the site of injury (secondary
hyperalgesia). Further, following acute
inflammation, dorsal horn neurons show an increased responsiveness to peripherally applied stimuli, which has been termed sensitization. Previous studies demonstrate a reduction in dorsal horn neuron activity following
TENS treatment in normal animals and a reduction in primary and secondary
hyperalgesia in acutely inflamed animals. The purpose of this study was to examine the effects of
TENS on dorsal horn neurons sensitized by acute
inflammation. Extracellular recordings from wide dynamic range (WDR), high threshold (HT) and low threshold (LT) dorsal horn neurons in anesthetized rats were assessed for spontaneous activity, responses to innocuous and noxious mechanical stimulation and receptive field size. Responses were measured before and 3 h after induction of
inflammation, and immediately and 1 h after application of either high (100 Hz) or low (4 Hz) frequency
TENS (motor intensity, pulse duration = 100 microseconds).
TENS was applied to the inflamed paw to encompass the receptive field of the neuron for 20 min. WDR and HT dorsal horn neurons sensitized to mechanical stimulation after induction of
inflammation. Application of either high or low frequency
TENS to the inflamed paw reduced both innocuous and noxious evoked responses of WDR and HT dorsal horn neurons immediately and 1 h
after treatment with
TENS. Comparison of responses after
TENS with baseline responses showed that the evoked responses in the majority of WDR and HT cells returned to or fell below baseline responses.
TENS had no effect on responses of LT neurons. In summary, central neuron sensitization is reduced by
TENS and may underlie the reduction in
hyperalgesia observed
after treatment with
TENS.