Release of GABA and activation of GABA(A) in the spinal cord mediates the effects of TENS in rats

Abstract

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.

Fig. 1

Schematic diagram representing potential pathways involved in low (A) or high (B) frequency TENS antihyperalgesia. RVM=rostra ventral medulla; Endo=endomorphin-2; Glu=glutamate; STT=spinothalamic tract; SRT=spinoreticular path; 5-HT=serotonin.

Fig. 2

Time course of changes in extracellular concentrations of GABA during (10–20 min) and after (30–60 min) treatment with low frequency (middle panel), high frequency (bottom panel), or no TENS (top panel) for animals with (open circles) and without (closed circles) knee joint inflammation. Increases in GABA in the extracellular fluid were delayed beginning immediately after removal of TENS and continuing through 60 min. The increases were similar between the group with joint inflammation and the group without joint inflammation. No changes in GABA were observed in the group that received low frequency TENS, or the group that did not receive TENS. Data are expressed as a percent of baseline with baseline set at 100% as the mean±S.E.M. *, P <0.05, significantly different than no TENS.

Fig. 3

Summary of the responses during and after TENS for changes in GABA and glycine. The average increase after treatment with high frequency TENS was signicantly greater than the group that did not receive TENS or the group that received low frequency TENS. There was no change in GABA during TENS, and no change in glycine either during or after TENS. Data are the mean±S.E.M. *, P <0.05.

Fig. 4

Extracellular concentrations for glycine during and after high frequency (bottom panel), low frequency (middle panel) or no TENS (top panel) treatment in animals with (closed circles) and without (open symbols) inflammation. No changes in glycine were observed in the group that received low frequency TENS, high frequency TENS or the group that did not receive TENS. Data are expressed as a percent of baseline with baseline set at 100% as the mean±S.E.M.

Fig. 5

Graphs represent the compression withdrawal threshold for groups of rats that received high frequency TENS or low frequency TENS with ACSF (open squares) or with bicuculline (closed circles). Twenty-four hours after inflammation there was a decrease in the compression withdrawal threshold when compared to baseline. In the groups treated with ACSF as a control, TENS increased the compression withdrawal threshold back toward baseline. In contrast the groups treated with bicuculline TENS did not increase the compression withdrawal threshold. The insets show the percent inhibition of hyperalgesia such that 0% is no change, 100% is complete reversal, and >100% is analgesia. *, P <0.05, significantly different from ACSF group.