Repeated 100 Hz TENS for the Treatment of Chronic Inflammatory Hyperalgesia and Suppression of Spinal Release of Substance P in Monoarthritic Rats

Transcutaneous electrical nerve stimulation (TENS) has been shown to be an effective measure for pain relief. The aim of the present study was to determine the optimal intensity and interval of repeated 100 Hz TENS for the treatment of chronic inflammatory hyperalgesia in a monoarthritic pain model of the rat, and to assess the changes of the spinal substance P (SP) release in response to TENS treatment. A reliable, reproducible chronic monoarthritic pain model was produced by intra-articular injection of complete Freund's adjuvant (CFA) at single ankle joint. The efficacy of 100 Hz TENS treatments with different frequencies and intensities was compared. In the acute period (within 3 weeks) of monoarthritis, twice-a-week schedule of TENS reduced the swelling of the inflamed ankle significantly. In the stable period (4–9 weeks), however, once-a-week schedule produced a significantly better therapeutic effect on both inflammation and arthritic hyperalgesia than that of twice- or five-times-a-week schedule. Using three levels of intensity of TENS, we found that the weaker (1-1-2 mA) stimulation produced significantly better therapeutic effects. Repeated TENS produced a reduction of SP content in spinal perfusate in parallel with the progressive reduction of the arthritic pain scores. Our results suggest that (i) consecutive TENS treatments produced cumulative effect for chronic hyperalgesia, (ii) for chronic inflammatory hyperalgesia, a weaker intensity and more sparsely arranged treatment schedule may produce better therapeutic effect and (iii) a decrease in SP release may serve as one of the possible neurochemical mechanisms underlying the therapeutic effects of multiple TENS treatments on chronic inflammatory hyperalgesia.