Transcutaneous electrical nerve stimulation (TENS) is a commonly utilized non-pharmacological, non-invasive treatment for pain. GABA is a neurotransmitter in the dorsal horn of the spinal cord that mediates analgesia locally, and also through activation of supraspinal sites. TENS reduces hyperalgesia through activation of receptor-mediated pathways at the level of the spinal cord, and supraspinally. The current study tested the hypothesis that either high or low frequency TENS applied to the inflamed knee joint increases GABA in the spinal cord dorsal horn and activates GABA receptors spinally. We utilized microdialysis to sample the extracellular fluid before, during and after TENS and analyzed GABA in dialysates with high performance liquid chromatography. We analyzed the extracellular GABA concentrations in animals with and without knee joint inflammation induced by intra-articular injection of kaolin and carrageenan. We further tested if spinal blockade of GABA receptors prevents the antihyperalgesia produced by TENS in rats with joint inflammation. We show that high frequency TENS increases extracellular GABA concentrations in the spinal cord in animals with and without joint inflammation. The increases in GABA do not occur in response to low frequency TENS, and there are no increases in glycine in response to low or high frequency TENS. However, the reduction in primary hyperalgesia by both high and low frequency TENS is prevented by spinal blockade of GABA A receptors with bicuculline. Thus, high frequency TENS increases release of GABA in the deep dorsal horn of the spinal cord, and both high and low frequency TENS reduce primary hyperalgesia by activation of GABA A receptors spinally.

Chronic muscle pain is a major clinical problem that is often associated with fatigue. Conversely, chronic fatigue conditions are commonly associated with muscle pain. We tested the hypothesis that muscle fatigue enhances hyperalgesia associated with injection of acidic saline into muscle. We evaluated mechanical sensitivity of the paw (von Frey) in mice after 2 intramuscular injections of saline (20 µL; pH 4, pH 5, pH 6, pH 7.2) in a fatigue and a control group. To induce fatigue, mice were run for 2 h/day for 2 days prior to the first injection and 2 h/day for 2 days prior to the second injection. Muscle lactate, pCO 2 , pO 2 , creatinine kinase, phosphate, and histology were examined after the fatigue task and compared to a control group. Grip force was significantly decreased after 2 h of running indicating fatigue. The fatigue task did not induce muscle damage as there was no difference in muscle lactate, pCO 2 , pO 2 , creatinine kinase, phosphate, or histology. The fatigue task altered the dose-response relationship to intramuscular acidic saline injections. Mechanical hyperalgesia was observed in both fatigue and control groups after intramuscular injection of pH 4.0, but only the fatigue group after injection of pH 5. Neither the fatigue nor the control group developed hyperalgesia in response to intramuscular injection of pH 6 or pH 7.2. In conclusion, fatigue modified the susceptibility of mice to acid injection of pH 5.0 to result in mechanical hyperalgesia after 2 injections of pH 5.0. The fatigue task did not produce measurable changes in the muscle tissue suggesting a central mechanism mediating the enhancement of hyperalgesia.Perspective-These data therefore show that muscle fatigue can enhance the likelihood that one develops pain to a mild insult. Clinically, this could relate to the development of pain from such conditions as repetitive strain injury, and may relate to the interrelationship between chronic pain and fatigue. KeywordsFatigue; exercise; acid; protons; low-pH saline Pain arising from musculoskeletal disorders is a major clinical problem globally, 58 and fibromyalgia affects 2% of population in the United States. 57 As high as 76% of people with chronic widespread musculoskeletal pain conditions, such as fibromyalgia and arthritis, also report fatigue. 26 Conversely, as high as 94% of people with chronic fatigue syndrome report musculoskeletal pain and approximately half of the population has chronic widespread musculoskeletal pain. 56,60 Pain is increased during and after exercise in people with fibromyalgia and chronic fatigue syndrome. 20,48,54,55 Similarly, muscle fatigue is increased in people with chronic fatigue syndrome. 3,10,23 To assess mechanisms of chronic widespread pain, we developed an animal model that mimics the clinical symptoms. Two injections of acidic saline (pH 4.0), 5 days apart, into 1 muscle results in bilateral long-lasting hyperalgesia of the paw, 41 muscle,59 and the viscera28 without muscle tissue damage.41 The intramuscular pH decreases to a p...

Transcutaneous electrical nerve stimulation (TENS) is a commonly utilized non-pharmacological treatment for pain. Studies show that low-and high-frequency TENS utilize opioid, serotonin and/or muscarinic receptors in the spinal cord to reduce hyperalgesia induced by joint inflammation in rats. As there is an increase in glutamate and aspartate levels in the spinal cord after joint inflammation, and opioids reduce glutamate and aspartate release, we hypothesized that TENS reduces release of glutamate and aspartate in animals with joint inflammation by activation of opioid receptors. Using microdialysis and HPLC with fluorescence detection, we examined the release pattern of glutamate and aspartate in the dorsal horn in response to either low-frequency (4 Hz) or high-frequency (100 Hz) TENS. We examined the effects of TENS on glutamate and aspartate release in animals with and without joint inflammation. Highfrequency, but not low-frequency, TENS significantly reduced spinal glutamate and aspartate in animals with joint inflammation compared with levels in those without joint inflammation. The reduced release of glutamate and aspartate by highfrequency TENS was prevented by spinal blockade of deltaopioid receptors with naltrindole. Thus, we conclude that high-frequency TENS activates delta-opioid receptors consequently reducing the increased release of glutamate and aspartate in the spinal cord.

Objective-To determine the release pattern of serotonin and noradrenaline in the spinal cord in response to transcutaneous electric nerve stimulation (TENS) delivered at low or high frequency.Design-Prospective randomized allocation of 3 treatments. Setting-Research laboratory. Animals-Male Sprague-Dawley rats (weight range, 250-350g).Intervention-Knee joints of rats were inflamed with a mixture of 3% carrageenan and 3% kaolin for 24 hours prior to placement of push-pull cannulae into the dorsal horn of the spinal cord. Pushpull samples were collected in 10-minute intervals before, during, and after treatment with lowfrequency TENS (4Hz), high-frequency TENS (100Hz), or sham TENS. TENS was applied to the inflamed knee joint for 20 minutes at sensory intensity and 100-μs pulse duration. Push-pull samples were analyzed for serotonin and noradrenaline by high performance liquid chromatography with coulemetric detection. Main Outcome Measures-Spinal concentrations of serotonin and noradrenaline.Results-Low-frequency TENS significantly increased serotonin concentrations during and immediately after treatment. There was no change in serotonin with high-frequency TENS, nor was there a change in noradrenaline with low-or high-frequency TENS.Conclusions-Low-frequency TENS releases serotonin in the spinal cord to produce antihyperalgesia by activation of serotonin receptors. KeywordsHyperalgesia; Pain; Rehabilitation; Serotonin; Transcutaneous electric nerve stimulation TRANSCUTANEOUS ELECTRIC NERVE stimulation (TENS) is a common yet important nonpharmacologic treatment used as an adjunct treatment for a variety of painful conditions by health care practitioners. TENS is the application of surface electrodes to the skin for pain

Chronic muscle pain is common and often difficult to treat. In this study, we further characterize a model of chronic muscle pain induced by repeated intramuscular injection of acidic saline. Two injections of acid into muscle separated by 5 days result in secondary mechanical hyperalgesia that lasts for up to 4 weeks. Blockade of spinal NMDA receptors prior to the second injection intramuscular acid injection delays the onset of hyperalgesia, where as the maintenance phase of hyperalgesia, evaluated 1 week after the second intramuscular injection, is dependent on activation of spinal AMPA/kainate and NMDA receptors. In order to determine if behavioral hyperalgesia and glutamate receptor involvement are associated with increased concentrations of excitatory amino acids (EAA), we utilized microdialysis to evaluate extracellular glutamate and aspartate concentrations in the spinal dorsal horn during the first and second intramuscular acid injections, and 1 week after the development of mechanical hyperalgesia. The second intramuscular injection evoked a calcium-dependent increase in both spinal glutamate and aspartate concentrations. Glutamate concentrations within the dorsal horn were also increased 1 week after the second acid injection. Our data suggest increased release of spinal EAAs in the dorsal horn contributes to the development and maintenance of hyperalgesia.