In this study we investigated the involvement of cutaneous versus knee joint afferents in the antihyperalgesia produced by
transcutaneous electrical nerve stimulation (
TENS) by differentially blocking primary afferents with
local anesthetics.
Hyperalgesia was induced in rats by inflaming one knee joint with 3%
kaolin-
carrageenan and assessed by measuring paw withdrawal latency to heat before and 4 hours after injection. Skin surrounding the inflamed knee joint was anesthetized using an
anesthetic cream (EMLA). Low (4 Hz) or high (100 Hz) frequency
TENS was then applied to the anesthetized skin. In another group, 2%
lidocaine gel was injected into the inflamed knee joint, and low or high frequency
TENS was applied. Control experiments were done using vehicles. In control and EMLA groups, both low and high frequency
TENS completely reversed
hyperalgesia. However, injection of
lidocaine into the knee joint prevented antihyperalgesia produced by both low and high frequency
TENS. Recordings of cord dorsum potentials showed that both low and high frequency
TENS at sensory intensity activates only large diameter afferent fibers. Increasing intensity to twice the motor threshold recruits Adelta afferent fibers. Furthermore, application of
EMLA cream to the skin reduces the amplitude of the cord dorsum potential by 40% to 70% for both high and low frequency
TENS, confirming a loss of large diameter primary afferent input after EMLA is applied to the skin. Thus, inactivation of joint afferents, but not cutaneous afferents, prevents the antihyperalgesia effects of
TENS. We conclude that large diameter primary afferent fibers from deep tissue are required and that activation of cutaneous afferents is not sufficient for
TENS-induced antihyperalgesia.
PERSPECTIVE:
Transcutaneous electrical nerve stimulation (
TENS) is an accepted clinical modality used for
pain relief. It is generally believed that
TENS analgesia is caused mainly by cutaneous afferent activation. In this study by differentially blocking cutaneous and deep tissue primary afferents, we show that the activation of large diameter primary afferents from deep somatic tissues, and not cutaneous afferents, are pivotal in causing
TENS analgesia.