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

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.     

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.     

Transcutaneous electrical nerve stimulation activates peripherally located alpha-2A adrenergic receptors

The alpha2A and alpha2C adrenergic receptor (AR) subtypes mediate antinociception when activated by the endogenous ligand norepinephrine. These receptors also produce antinociceptive synergy when activated concurrently with opioid receptor activation. The involvement of the opioid receptors in the mechanisms governing transcutaneous electrical nerve stimulation (TENS) has been well described. While spinal alpha-2 ARs do not appear to be involved in TENS antihyperalgesia in rats, the noradrenergic analgesic system also involves supraspinal and peripheral sites. Thus, a broader evaluation of the potential contribution of alpha-2 AR to TENS is warranted. The current study compared the antihyperalgesic efficacy of high (100 Hz) and low (4 Hz) frequency TENS in mutant mice lacking a functional alpha2A AR against their respective wildtype counterparts. The degree of secondary heat hyperalgesia induced by intra-articular injection of carrageenan/kaolin (3%) mixture did not differ among the experimental groups. However, the antihyperalgesia induced by both low and high frequency TENS was significantly diminished in alpha2A mutant mice compared to controls. The alpha2 adrenergic receptor selective antagonist, SK&F 86466, reversed TENS-mediated antihyperalgesia when delivered intra-articularly, but not when delivered intrathecally or intracerebroventricularly. These data suggest that peripheral alpha2 ARs contribute, in part, to TENS antihyperalgesia. This pharmacodynamic response is consistent with previous anatomical observations that alpha2A ARs are expressed on primary afferent neurons and macrophages near injured tissue.     

The neurokinin-1 receptor antagonist RP 67580 reduces the sensitization of primary afferents by substance P in the rat.

The inflammatory mediator substance P (SP) produces a variety of biological effects in several tissues by binding to the tachykinin receptor neurokinin 1 (NK1) and, to a lesser extent, by binding to the neurokinin 2 receptor (NK2). To assess the sensitizing effect of SP on articular afferent fibres the NK1receptor antagonist RP 67580 was applied in normal and acutely inflamed rat knee joints.Altogether 38 fine afferent nerve fibres from the rat knee with conduction velocities of 0.71-13.5 m/s were recorded as single units, during non-noxious and noxious joint rotations. SP, injected i.a. as a bolus close to the knee joint, was able to sensitize 45.5% (10 of 22) of the units recorded from normal joints and 33.3% (five of 15) of afferents from inflamed joints. The following i.a. application of RP 67580 in a range of 20-200 nmol antagonized in a dose-dependent manner the sensitizing effect of SP in a large proportion of slowly conducting articular afferents from normal (66.7%) and inflamed (46.2%) knee joints. Subsequent SP application enhanced the afferent sensitivity further. The electrophysiological results presented here further support the suggestion that the sensitization of afferents by SP in the rat knee joint is mediated mainly by the NK1 receptor, which is probably located on the primary afferents.     

Blockade of opioid receptors in rostral ventral medulla prevents antihyperalgesia produced by transcutaneous electrical nerve stimulation (TENS).

Although transcutaneous electrical nerve stimulation (TENS) is used extensively in inflammatory joint conditions such as arthritis, the underlying mechanisms are unclear. This study aims to demonstrate an opiate-mediated activation of descending inhibitory pathways from the rostral ventral medulla (RVM) in the antihyperalgesia produced by low- (4 Hz) or high-frequency (100 Hz) TENS. Paw withdrawal latency to radiant heat, as an index of secondary hyperalgesia, was recorded before and after knee joint inflammation (induced by intra-articular injection of 3% kaolin and carrageenan) and after TENS/no TENS coadministered with naloxone (20 microg/1 microl), naltrindole (5 microg/1 microl), or vehicle (1 microl) microinjected into the RVM. The selectivity of naloxone and naltrindole doses was tested against the mu-opioid receptor agonist [D-Ala2,N-Me-Phe4,Gly-ol5]-enkephalin (DAMGO) (20 ng, 1 microl) and the delta2-opioid receptor agonist deltorphin (5 microg, 1 microl) in the RVM. Naloxone microinjection into the RVM blocks the antihyperalgesia produced by low frequency (p < 0.001), but not that produced by high-frequency TENS (p > 0.05). In contrast, naltrindole injection into the RVM blocks the antihyperalgesia produced by high-frequency (p < 0.05), but not low-frequency (p > 0.05) TENS. The analgesia produced by DAMGO and deltorphin is selectively blocked by naloxone (p < 0.05) and naltrindole (p < 0.05), respectively. Thus, the dose of naloxone and naltrindole used in the current study blocks mu- and delta-opioid receptors, respectively. Hence, low-frequency and high-frequency TENS produces antihyperalgesia by activation of mu- and delta-opioid receptors, respectively, in the RVM.     

Effect of varying frequency, intensity, and pulse duration of transcutaneous electrical nerve stimulation on primary hyperalgesia in inflamed rats

OBJECTIVES: To examine the effect of varying frequency, intensity, and pulse duration of transcutaneous electrical nerve stimulation (TENS) on primary hyperalgesia (increased response to a noxious stimuli) to heat and mechanical stimuli induced by carrageenan paw inflammation in rats. DESIGN: Inflammation was induced by injection of 3% carrageenan into the hindpaw. Two frequencies (high, 100 Hz; low, 4 Hz), 2 intensities (high, motor; low, sensory), and 2 pulse durations (100 microsec, 250 microsec) were applied for 20 minutes to the inflamed paw. The paw withdrawal latency (PWL) to radiant heat, threshold to mechanical stimuli, and spontaneous pain-related behaviors were measured before and 4 hours after induction of inflammation, after TENS, and at 8, 12, and 24 hours after inflammation. A 3-factor (frequency, intensity, pulse duration) repeated-measures (time) design was used to analyze the changes in PWL. Mechanical threshold and spontaneous pain-related behaviors were compared for frequency, intensity, and pulse duration with a Kruskal-Wallis analysis of variance. RESULTS: For changes in PWL to heat, there was an effect for time (p = .0001) and frequency (p =.0001), but not for intensity (p = .45) or pulse duration (p = .21). For changes in mechanical threshold, there was also an effect for frequency (p = .007), but not for intensity (p = .055) or pulse duration (p = .058), after treatment with TENS. High-frequency TENS significantly reduced the primary hyperalgesia to heat and mechanical stimuli when compared with controls receiving no TENS or treatment with low-frequency TENS. High-frequency motor TENS also reduced spontaneous pain-related behaviors for 1 day after treatment. CONCLUSION: High-frequency TENS reduces primary hyperalgesia to heat and mechanical stimuli for up to 1 day after treatment. In contrast, low-frequency TENS is ineffective in reducing primary hyperalgesia. Varying intensity or pulse duration had no effect on the degree of antihyperalgesia produced by high-frequency TENS.     

Release of endogenous opioids following transcutaneous electric nerve stimulation in an experimental model of acute inflammatory pain.

Transcutaneous electric nerve stimulation (TENS) is a noninvasive treatment used in physiotherapy practice to promote analgesia in acute and chronic inflammatory conditions. The aim of the present study was to investigate the action mechanism of TENS at high (HF: 130 Hz) and low (LF: 10 Hz) frequencies in an inflammation model produced by the injection of carrageenan in rat paws (Cg; 250 microg). After carrageenan administration (0 time), either HF or LF TENS was applied to the inflamed paw of rats for 20 minutes, and hyperalgesia was assessed hourly using the modified Randall-Selitto method (1957). HF and LF TENS inhibited the carrageenan-induced hyperalgesia by 100%. Pretreatment of animals with intraplantar naltrexone (Nx; 50 microg) reversed the analgesic effect of the LF TENS but did not alter the effect of HF TENS. The application of HF and LF TENS to the contralateral paw reversed the hyperalgesia of the inflamed paw similar to that observed when TENS was applied to the inflamed paw. However, LF TENS presented a longer-lasting analgesic effect than HF TENS. Our data demonstrate that HF and LF TENS induced antihyperalgesia. We also report that the antihyperalgesia provoked by LF TENS is partially due to the local release of endogenous opioids. PERSPECTIVE: This study offers important information about physiotherapy practices aimed at pain relieving. TENS is a noninvasive treatment that promotes analgesia in acute and chronic inflammatory conditions. Scientists, patients, and the general population may benefit from this knowledge.     

Nociceptive nerve activity in an experimental model of knee joint osteoarthritis of the guinea pig: effect of intra-articular hyaluronan application

Nociceptive impulse activity was recorded extracellularly from single A delta and C primary afferents of the guinea pig's medial articular nerve after induction of an experimental osteoarthritis in the knee joint by partial medial menisectomy and transection of the anterior cruciate ligament (PMM+TACL). Also, the analgesic effects of intra-articular hyaluronan solutions were evaluated. Healthy, PMM+TACL operated, sham-operated (opening of the joint capsule without PMM and TACL surgery) and acutely inflamed (intra-articular kaolin-carrageenan, K-C) animals were used. The stimulus protocol consisted of torque meter-controlled, standardized innocuous and noxious inward and outward rotations of the joint. This stimulus protocol of 50 s duration was repeated every 5 min for 70 min. One day, one week and three weeks after PMM+TACL, the movement-evoked discharges of A delta articular afferents were increased significantly over values found in sham-operated animals. The discharges of C fibers were significantly augmented only one week after PMM+TACL surgery. Filling of the joint cavity with a high viscosity hyaluronan solution (hylan G-F 20, Synvisc) immediately and three days after surgery reduced significantly the enhanced nerve activity observed in joint afferent fibers one day and one week after surgery. Augmentation of movement-evoked discharges in K-C acutely inflamed knee joints was similar to that observed one week after PMM+TACL. Our results indicate that in the PMM+TACL model of osteoarthritis in guinea pigs, enhancement of nociceptive responses to joint movement was primarily associated to post-surgical inflammation. Intra-articular injection of an elastoviscous hyaluronan solution reduced the augmented nerve activity.     

Keratinocytes acting on injured afferents induce extreme neuronal hyperexcitability and chronic pain

Keratinocytes play an important role in the dialog between skin and cutaneous sensory neurons. They are an essential source of cutaneous nerve growth factor (NGF), a neurotrophin that contributes to persistent pain in inflammation and neuropathy. We studied the interaction of human keratinocytes (hKTs) and regenerating afferent nerve fibers by transplanting hKTs into a ligated and transected peripheral nerve. The hKTs self-assembled into a multi-laminar spheroid cellular structure resembling the stratum spinosum of epidermis. Axonal sprouts surrounded the structure although they were excluded from entry. Levels of NGF were elevated at the transplant site. Whole cell patch–clamp recordings from primary afferent neurons whose cut axons were present near the transplanted hKTs displayed extreme hyperexcitability. These neurons generated high frequency trains of action potentials during step depolarization stimuli, and they sometimes showed afterdischarge and fired spontaneously at resting membrane potential. This spontaneous firing originated from subthreshold membrane potential oscillations. The animals with the hKT transplants exhibited spontaneous pain behavior manifest as autotomy. The results demonstrate that an interaction between injured/regenerating nerve fibers and keratinocytes such as may occur during wound healing, results in afferent hyperexcitability and pain. These results have implications for persistent pain associated with burn and traumatic skin injuries.     

Sensitization of primary afferents to mechanical and heat stimuli after incision in a novel in vitro mouse glabrous skin-nerve preparation

In this study, we recorded activity from afferent fibers innervating the mouse plantar skin, the same region evaluated in pain behavior experiments. We compared responses of afferents from incised and unincised hind paw skin. The plantar skin together with attached medial and lateral plantar nerves was dissected until they could be completely removed intact and placed in an organ bath chamber continuously perfused with oxygenated Kreb’s solution with the temperature maintained at 32 °C. Afferent nerve activities to feedback-controlled mechanical and heat stimuli and cooling were recorded. Eighty-five single Aδ- and C-fiber afferents were recorded, 42 from control and the remainder from incised animals. A greater proportion of C-fibers (11/34) from incised skin had spontaneous activity than in the unincised preparation (2/32). The mechanical thresholds of both Aδ- and C-fiber units were not different between control and incised groups but the responses to suprathreshold mechanical stimulation were increased in low threshold Aδ- and C-fibers. The greatest change in heat sensitivity was apparent when multi-fiber total activity was measured; threshold was reduced, total spikes were greater and the peak discharge frequency was increased. In summary, feedback-controlled stimulation identified mechanical sensitization after incision in an in vitro preparation. Few fibers were excited by cooling. Heat sensitization of primary afferents was more prominent when activities of unclassified afferents are included. The preparation allows us to study afferent function of the same tissue that is examined for in vivo pain behavior assays in mice.     

Role of ASIC3 in the primary and secondary hyperalgesia produced by joint inflammation in mice

The acid sensing ion channel 3 (ASIC3) is critical for the development of secondary hyperalgesia as measured by mechanical stimulation of the paw following muscle insult. We designed experiments to test whether ASIC3 was necessary for the development of both primary and secondary mechanical hyperalgesia that develops after joint inflammation. We used ASIC3 −/− mice and examined the primary (response to tweezers) and secondary hyperalgesia (von-Frey filaments) that develops after joint inflammation comparing to ASIC3 +/+ mice. We also examined the localization of ASIC3 to the knee joint afferents innervating the synovium using immunohistochemical techniques before and after joint inflammation. We show that secondary mechanical hyperalgesia does not develop in ASIC3 −/− mice. However, the primary mechanical hyperalgesia of the inflamed knee joint still develops in ASIC3 −/− mice and is similar to ASIC3 +/+ mice. In knee joint synovium from ASIC3 +/+ mice without joint inflammation, ASIC3 was not localized to joint afferents that were stained with an antibody to protein gene product (PGP) 9.5 or calcitonin gene-related peptide (CGRP). ASIC3 was found, however, in synoviocytes of the knee joint of uninflamed mice. In ASIC3 +/+ mice with joint inflammation, ASIC3 co-localized with PGP 9.5 or CGRP in joint afferents innervating the synovium. We conclude that the decreased pH that occurs after inflammation would activate ASIC3 on primary afferent fibers innervating the knee joint, increasing the input to the spinal cord resulting in central sensitization manifested behaviorally as secondary hyperalgesia of the paw.     

Convergence of cutaneous, tooth pulp, visceral, neck and muscle afferents onto nociceptive and non-nociceptive neurones in trigeminal subnucleus caudalis (medullary dorsal horn) and its implications for referred pain

Because of the likely involvement of central convergence of afferent inputs in mechanisms underlying referred pain, the activity of single neurones was recorded in the cat's trigeminal (V) subnucleus caudalis (medullary dorsal horn) to test for the presence and extent of convergent inputs to the neurones. In chloralose-anaesthetized or decerebrate unanaesthetized cats, electrical stimuli were applied to afferents supplying facial skin, oral mucosa, canine and premolar tooth pulp, laryngeal mucosa, cervical skin and muscle, and jaw and tongue muscles, and tactile and noxious mechanical and thermal stimuli were applied to skin and mucosa. Considerable proportions of caudalis neurones which could be functionally classified on the basis of their cutaneous receptive field properties as low-threshold mechanoreceptive (LTM), wide-dynamic-range (WDR), or nociceptive-specific (NS) neurones, could be excited by electrical stimulation of several of these afferent inputs. Extensive convergence of afferent inputs, including inputs from skin or mucosal areas outside the neuronal oral-facial receptive field delineated by natural stimuli, was a particular feature of the units classified as cutaneous nociceptive neurones (i.e., WDR and NS). On the basis of antidromic activation, 15% of these WDR and NS neurones were shown to have a direct projection to the contralateral thalamus. The findings question the use of terminology and classifications of somatosensory neurones based only on the cutaneous receptive field properties of the neurones since distinctions between the different neuronal populations become less obvious when properties other than those related to cutaneous afferent inputs are taken into account. Moreover, the observations of extensive convergence of different types of afferents, which was especially apparent in cutaneous nociceptive neurones, also suggest a role for these neurones in mediating deep pain and in spread and referral of pain.     

Low frequency TENS is less effective than high frequency TENS at reducing inflammation-induced hyperalgesia in morphine-tolerant rats.

Both transcutaneous electrical nerve stimulation (TENS) and morphine are commonly used for relief of pain. Extensive research has been done on the effectiveness of each of these two methods for pain relief when given independently. However, very little literature exists examining the effectiveness of their combined use. Systemically administered morphine activates mu opioid receptors and when administered for prolonged periods results in analgesic tolerance. Low (4 Hz) and high (100 Hz) frequency TENS activate mu- and delta-opioid receptors, respectively, It is thus possible that TENS would be less effective in morphine-tolerant subjects. The current study investigated the effectiveness of high- and low-frequency TENS in the reversal of hyperalgesia in inflamed rats that were morphine-tolerant. Morphine tolerance was induced by subcutaneous implantation of morphine pellets over 10 days. Knee joint inflammation was induced by injection of kaolin and carrageenan into the knee joint cavity. Secondary heat hyperalgesia was tested by measuring the paw withdrawal latency to radiant heat (1) before pellet implantation (either morphine or placebo), (2) after pellet implantation and before inflammation, (3) after inflammation and (4) after TENS. Both high (100 Hz) and low (4 Hz) frequency TENS caused nearly 100% inhibition of secondary hyperalgesia in animals receiving placebo pellets. In contrast, the hyperalgesia in morphine-tolerant animals with knee joint inflammation was unaffected by low frequency TENS but fully reversed by high frequency TENS. These results suggest that patients who are tolerant to morphine may respond better to high frequency TENS than to low frequency TENS.     

Spinal blockade of opioid receptors prevents the analgesia produced by TENS in arthritic rats

Transcutaneous electrical nerve stimulation (TENS) is commonly used for relief of pain. The literature on the clinical application of TENS is extensive. However, surprisingly few reports have addressed the neurophysiological basis for the actions of TENS. The gate control theory of pain is typically used to explain the actions of high-frequency TENS, whereas, low-frequency TENS is typically explained by release of endogenous opioids. The current study investigated the role of mu, delta, and kappa opioid receptors in antihyperalgesia produced by low- and high-frequency TENS by using an animal model of inflammation. Antagonists to mu (naloxone), delta (naltrinodole), or kappa (nor-binaltorphimine) opioid receptors were delivered to the spinal cord by microdialysis. Joint inflammation was induced by injection of kaolin and carrageenan into the knee-joint cavity. Withdrawal latency to heat was assessed before inflammation, during inflammation, after drug (or artificial cerebral spinal fluid as a control) administration, and after drug (or artificial cerebral spinal fluid) administration + TENS. Either high- (100 Hz) or low- frequency (4 Hz) TENS produced approximately 100% inhibition of hyperalgesia. Low doses of naloxone, selective for mu opioid receptors, blocked the antihyperalgesia produced by low-frequency TENS. High doses of naloxone, which also block delta and kappa opioid receptors, prevented the antihyperalgesia produced by high-frequency TENS. Spinal blockade of delta opioid receptors dose-dependently prevented the antihyperalgesia produced by high-frequency TENS. In contrast, blockade of kappa opioid receptors had no effect on the antihyperalgesia produced by either low- or high-frequency TENS. Thus, low-frequency TENS produces antihyperalgesia through mu opioid receptors and high-frequency TENS produces antihyperalgesia through delta opioid receptors in the spinal cord.     

Development of opioid tolerance with repeated transcutaneous electrical nerve stimulation administration

The analgesia produced by low and high frequency transcutaneous electrical nerve stimulation (TENS) is mediated by the release of mu- or delta-opioids, respectively in the central nervous system. Repeated administration of either mu- or delta-opioid agonists induce opioid analgesic tolerance. Thus, we tested if repeated administration of TENS (either low or high frequency) in rats leads to a development of tolerance to its antihyperalgesic effects with a corresponding cross-tolerance to mu- and delta-opioid agonists. Unilateral knee joint inflammation (3% carrageenan) was induced in adult Sprague-Dawley rats. Either low (4 Hz) or high frequency (100 Hz) TENS was administered for 6 days (20 min daily) to the inflamed knee joint under halothane anesthesia. The no TENS controls were administered anesthesia only for the same period. Withdrawal threshold to mechanical stimuli was measured before and after administration of TENS on each day and also on the sixth day. A separate group of animals was tested for tolerance to either the mu-opioid agonist, morphine (1.32, 3.95, 13.2 nmol/10 ml, intrathecal (i.t.)) or the delta-opioid agonist, SNC-80 (6, 20, 60, 120 nmol/10 ml, i.t.) 30 min after i.t. administration. The reduced mechanical withdrawal threshold following the induction of inflammation was reversed by the application of TENS. However, repeatedly administering either low or high frequency TENS for 6 days, lead to a diminution in its effectiveness in reversing the ipsilateral secondary mechanical hyperalgesia by the fourth day. The effects of morphine in the low and SNC-80 in the high frequency TENS groups were significantly less than the group that did not receive TENS. On the other hand, morphine and SNC-80 were similar to the no TENS control in the high and low frequency TENS groups, respectively. Thus, repeated administration of low and high frequency TENS leads to a development of opioid tolerance with a corresponding cross-tolerance to i.t. administered mu- and delta-opioid agonists, respectively. Clinically, it can be inferred that a treatment schedule of repeated daily TENS administration should be avoided to possibly obviate the induction of tolerance.     

Cholecystokinin receptors mediate tolerance to the analgesic effect of TENS in arthritic rats

Transcutaneous electrical nerve stimulation (TENS) is a treatment for pain that involves placement of electrical stimulation through the skin for pain relief. Previous work from our laboratory shows that repeated application of TENS produces analgesic tolerance by the fourth day and a concomitant cross-tolerance at spinal opioid receptors. Prior pharmacological studies show that blockade of cholecystokinin (CCK) receptors systemically and spinally prevents the development of analgesic tolerance to repeated doses of opioid agonists. We therefore hypothesized that systemic and intrathecal blockade of CCK receptors would prevent the development of analgesic tolerance to TENS, and cross-tolerance at spinal opioid receptors. In animals with knee joint inflammation (3% kaolin/carrageenan), high (100 Hz) or low frequency (4 Hz) TENS was applied daily and the mechanical withdrawal thresholds of the muscle and paw were examined. We tested thresholds before and after inflammation, and before and after TENS. Animals treated systemically, prior to TENS, with the CCK antagonist, proglumide, did not develop tolerance to repeated application of TENS on the fourth day. Spinal blockade of CCK-A or CCK-B receptors blocked the development of tolerance to high and low frequency TENS, respectively. In the same animals we show that spinal blockade of CCK-A receptors prevents cross-tolerance at spinal delta-opioid receptors that normally occurs with high frequency TENS; and blockade of CCK-B receptors prevents cross-tolerance at spinal mu-opioid receptors that normally occurs with low frequency TENS. Thus, we conclude that blockade of CCK receptors prevents the development of analgesic tolerance to repeated application of TENS in a frequency-dependent manner.     

The effect of varying frequency and intensity of transcutaneous electrical nerve stimulation on secondary mechanical hyperalgesia in an animal model of inflammation.

For years, transcutaneous electrical nerve stimulation (TENS) has been used clinically for the treatment of many types of pain. Although there have been many studies conducted on the efficacy of TENS in the clinical setting, the results are conflicting. The purpose of our investigation was to determine the effect of varying frequency and intensity of TENS on secondary mechanical hyperalgesia induced by acute joint inflammation. Male Sprague-Dawley rats were injected with a mixture of 3% carrageenan and 3% kaolin (100 microL in 0.9% sterile saline) into the joint cavity of one knee. The response threshold to mechanical stimuli was determined before inflammation of the knee joint; 4 hours after inflammation; immediately after the administration of TENS (approximately 5 hours after inflammation); and at 8, 12, and 24 hours after inflammation. TENS was applied to the inflamed knee joint at either high (100 Hz) or low (4 Hz) frequency and at either sensory or motor intensity. Sensory intensity was just below the threshold for motor contraction, and motor intensity was 2 x threshold for motor contraction. Either low- or high-frequency TENS is equally successful in reducing secondary mechanical hyperalgesia. Similarly, either sensory- or motor-intensity TENS equally reduces secondary mechanical hyperalgesia. Thus, selection of TENS should be based on patient comfort and symptoms for relief of secondary mechanical hyperalgesia.     

The effect of transcutaneous electrical nerve stimulation on local and distal cutaneous blood flow following a prolonged heat stimulus in healthy subjects

The aim of this study was to investigate the effects of transcutaneous electrical nerve stimulation (TENS) on blood flow and skin temperature following an elevation of baseline blood flow using infrared preheating. A randomized controlled approach was used whereby 66 healthy human subjects (33 male, 33 female) were allocated to one of three intervention groups (n=22 per group, equal male and female): Control, Low frequency TENS (4 Hz/200 micros), or High frequency TENS (110 Hz/200 micros). TENS was applied just below motor threshold over the median nerve of the right forearm for 15 min immediately following an infrared preheating. Cutaneous blood flow and skin temperature were recorded at 3-min intervals from the forearm and fingertips during TENS and for 15 min following TENS. Analysis of data revealed no significant differences between High and Low frequency TENS for cutaneous blood flow or skin temperature at the forearm. A small and short lived increase in cutaneous blood flow at the index finger was observed on TENS groups compared with control when TENS was switched off. TENS reduced skin temperature when compared to control during the first 9 min of the 15-min stimulation period at the middle finger but not at the index finger. It was concluded that the effects of high and low frequency TENS when applied below motor threshold produced changes in blood flow and skin temperature that were transient and small.     

体表镇痛剂对深部炎症痛缓解作用机制探讨

目的:本研究通过行为学和电生理学等方法 ,探索表面镇痛剂缓解机体深部组织疼痛的作用机制。方法:成年SD大鼠(雌雄各半),用完全佐剂(complete Freund’s adjuvant,CFA)构建左侧胫骨前肌炎症痛模型。随机分为2组:实验组(镇痛剂贴敷)和对照组(普通透气胶布贴敷)。分别观察大鼠的痛行为学;用伊文氏蓝的渗出测定血管渗透性;用电生理方法记录皮和肌肉的神经纤维放电情况,并观察去神经后的纤维放电情况。结果:表面镇痛剂有缓解肌肉炎症痛的作用,且具有药物特异的潜伏期(从10分钟到1~2小时)和作用时程。镇痛剂作用的皮肤区表现为血管渗出通透性增加。神经的纤维活动记录发现镇痛剂对A-纤维的兴奋性没有影响,也不能诱发其自发电活动;但却可以提高皮肤C-纤维的兴奋性,诱发C-纤维的自发电活动。电生理实验中潜伏期和作用时程与行为学实验中药物起效时间的对比证实药物对肌肉的镇痛作用与皮肤C-纤维的自发电活动相关。来自炎症肌肉的C-纤维传入电活动记录证明,表面镇痛剂可有效的抑制肌肉C-纤维的自发传入电活动;若去除药物作用的皮肤区的神经支配,则药物对深部组织的炎症痛的缓解作用消失。结论:表面镇痛剂可能通过激活皮肤的痛觉感受器,反射性减弱深部炎症肌肉的损害性传入活动。     

The effect of high‐ and low‐frequency transcutaneous electrical nerve stimulation upon cutaneous blood flow and skin temperature in healthy subjects

The reported non‐analgesic effects of transcutaneous electrical nerve stimulation (TENS) include alterations to the local circulation; however, research in this area has produced equivocal findings. In the present study, the effect of low‐ (4 Hz) and high‐frequency (110 Hz) TENS on forearm skin blood perfusion was assessed using laser Doppler flowmetry. The effect on skin temperature was also assessed using a skin thermistor. Thirty healthy human volunteers were recruited and randomly assigned to a control or one of the two treatment groups. TENS was applied to the skin overlying the median nerve under double‐blind conditions for 15 min. Blood flow and skin temperature readings were recorded pre‐TENS, during TENS application and continued for 15 min post‐TENS application. Analysis of results showed significant increases in blood perfusion during the treatment period in the low‐frequency group when compared to the other two groups (P = 0·0106; ANOVA). No significant changes in skin temperature were observed. The results of this study demonstrate that low‐frequency TENS produces a local increase in cutaneous blood flow.