Spinal 5-HT(2) and 5-HT(3) receptors mediate low, but not high, frequency TENS-induced antihyperalgesia in rats

Abstract

Transcutaneous electrical nerve stimulation (TENS) is a form of non-pharmacological treatment for pain. Involvement of descending inhibitory systems is implicated in TENS-induced analgesia. In the present study, the roles of spinal 5-HT and alpha(2)-adrenoceptors in TENS analgesia were investigated in rats. Hyperalgesia was induced by inflaming the knee joint with 3% kaolin-carrageenan mixture and assessed by measuring paw withdrawal latency (PWL) to heat before and 4 h after injection. The (1). alpha(2)-adrenergic antagonist yohimbine (30 microg), (2). 5-HT antagonist methysergide (5-HT(1). and 5-HT(2). 30 microg), one of the 5-HT receptor subtype antagonists, (3). NAN-190 (5-HT(1A), 15 microg), (4). ketanserin (5-HT(2A), 30 microg), (5). MDL-72222 (5-HT(3), 12 microg), or (6). vehicle was administered intrathecally prior to TENS treatment. Low (4 Hz) or high (100 Hz) frequency TENS at sensory intensity was then applied to the inflamed knee for 20 min and PWL was determined. Selectivity of the antagonists used was confirmed using respective agonists administered intrathecally. Yohimbine had no effect on the antihyperalgesia produced by low or high frequency TENS. Methysergide and MDL-72222 prevented the antihyperalgesia produced by low, but not high, frequency TENS. Ketanserin attenuated the antihyperalgesic effects of low frequency TENS whereas NAN-190 had no effect. The results from the present study show that spinal 5-HT receptors mediate low, but not high, frequency TENS-induced antihyperalgesia through activation of 5-HT(2A) and 5-HT(3) receptors in rats. Furthermore, spinal noradrenergic receptors are not involved in either low or high frequency TENS antihyperalgesia.

Fig. 1

Bar graphs show the PWLs before (open bars), after joint inflammation (solid bars) and after treatment with drug and TENS/no TENS (hatched bars) for the ipsilateral and contralateral paws. Effects of intrathecal yohimbine and methysergide on carrageenan-induced hyper-algesia compared to saline control are shown in the graphs on the left panel (no TENS; graphs A, D, G). Effects of intrathecal saline, yohimbine or methysergide on low frequency TENS antihyperalgesia are shown in the middle panel (low TENS; graphs B, E, H), and effects of saline, yohimbine (30 μg) or methysergide (30 μg) on high frequency TENS are shown in the right panel (high TENS; graphs C, F, I). *, Significantly different from vehicle control group and also from its own baseline. K/C, kaolin and carrageenan; Contra, contralateral; Ipsi, ipsilateral.

Fig. 2

Bar graphs show the PWLs before (open bars), after joint inflammation (solid bars) and after treatment with drug and TENS/no TENS (hatched bars) for the ipsilateral and contralateral paws. Effects of intrathecal 20% DMSO, NAN-190 (15 μg, 5-HT_1A antagonist), ketanserin (30 μg, 5-HT₂ antagonist) and MDL-72222 (12 μg, 5-HT₃ antagonist) on the carrageenan-induced decrease in PWL are shown in the left panels (no TENS; graphs A, C, E, G). Effects of intrathecal 20% DMSO, NAN-190, ketanserin and MDL-72222, on low frequency TENS antihyperalgesia are shown in the right panels (low TENS; graphs B, D, F, H). *, Significantly different from vehicle control group and also from their own baseline. K/C, kaolin and carrageenan; Contra, contralateral; Ipsi, ipsilateral.

Fig. 3

The left cluster of bars in each graph shows ipsilateral PWLs before (open bar), 4 h after injection of kaolin—carrageenan injection into the knee joint (solid bar), and after intrathecal vehicle + agonists (hatched bar). Right cluster shows ipsilateral PWLs before (open), 4 h after kaolin—carrageenan injection (solid), and after intrathecal antagonist + agonist (hatched). *, PWLs were significantly increased compared to PWLs after kaolin—carrageenan. #, PWLs for antagonist + agonist were significantly less than PWLs from vehicle + agonist.