Increased release of serotonin in the spinal cord during low, but not high, frequency transcutaneous electric nerve stimulation in rats with joint inflammation.

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, 250-350 g). 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. Push-pull samples were collected in 10-minute intervals before, during, and after treatment with low-frequency TENS (4 Hz), high-frequency TENS (100 Hz), or sham TENS. TENS was applied to the inflamed knee joint for 20 minutes at sensory intensity and 100-mus 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.