The organization and function of endogenous antinociceptive systems

Evidence that tactile stimulation inhibits nociceptive sensations produced by innocuous contact cooling

It was recently shown that stinging, pricking or burning is reliably perceived by some individuals when the skin is cooled to temperatures as mild as 25-30 degrees C. These seemingly paradoxical sensations, which have been termed innocuous-cold nociception (ICN), were significant only when cooling was produced by a thermode resting statically on the skin (static contact); touching an already cooled thermode to the skin (dynamic contact) produced reports of only coolness and cold. The present study investigated the hypothesis that ICN is inhibited by tactile stimulation produced when a thermode contacts the skin. Experiment 1 pitted the tactile hypothesis against an alternative explanation that inhibition results from higher rates of skin cooling during dynamic contact. ICN was measured at three different cooling rates (-1.0, -2.5, -5.0 degrees C/s) when the thermode was resting on the skin or was touched to the skin at the moment cooling began. The results supported the tactile hypothesis: faster cooling rates during static contact led to stronger rather than weaker nociceptive sensations, and ICN was suppressed even when dynamic contact was coincident with the onset of cooling, and thus could not affect cooling rate. Experiment 2 confirmed the latter result and showed that suppression was greatest at 28 degrees C, less at 24 degrees C, and not significant at 18 degrees C. We conclude that dynamic tactile stimulation produced by contact with a surface inhibits the nociceptive component of innocuous but not noxious cooling. The implications of this conclusion for the role of cold perception in behavioral thermoregulation versus haptic perception, and for theories of cold perception in general, are discussed.

Understanding neuropathic pain

Neuropathic pain is defined as a chronic pain condition that occurs or persists after a primary lesion or dysfunction of the peripheral or central nervous system. Traumatic injury of peripheral nerves also increases the excitability of nociceptors in and around nerve trunks and involves components released from nerve terminals (neurogenic inflammation) and immunological and vascular components from cells resident within or recruited into the affected area. Action potentials generated in nociceptors and injured nerve fibers release excitatory neurotransmitters at their synaptic terminals such as L-glutamate and substance P and trigger cellular events in the central nervous system that extend over different time frames. Short-term alterations of neuronal excitability, reflected for example in rapid changes of neuronal discharge activity, are sensitive to conventional analgesics, and do not commonly involve alterations in activity-dependent gene expression. Novel compounds and new regimens for drug treatment to influence activity-dependent long-term changes in pain transducing and suppressive systems (pain matrix) are emerging.

[Acupuncture in the treatment of pain--hypothesis to adaptive effects]

A basic principle in conventional pain therapy is that the treatment should be tailored to the pathological mechanism of the disease. This is based on the knowledge of the effector mechanisms of the applied treatment modalities. Although for acupuncture the mode of action still remains elusive in many parts, evidence about its mechanisms in pain treatment is growing. A better understanding of the hypalgesic effects of acupuncture might lead to a more differentiated and mechanism guided application. The aim of this article is to evaluate the scientific data about the neurobiological mechanisms of acupuncture in the treatment of pain. Data are critically evaluated regarding their relevance for clinical practice. Possible mechanisms are differentiated in local and systemic effects and the question of point specificity is discussed. Additionally a comprehensive hypothesis is set up for the long-term effects of acupuncture in the treatment of chronic pain. In this context acupuncture is considered as a mode of repetitive, nociceptive stimulation, which induces adaptive processes on different physiological levels leading to an improved ability of the nociceptive system to cope with painful stimuli.

Electrical stimulation of the anterior cingulate cortex reduces responses of rat dorsal horn neurons to mechanical stimuli

The anterior cingulate cortex (ACC) is involved in the affective and motivational aspect of pain perception. Behavioral studies show a decreased avoidance behavior to noxious stimuli without change in mechanical threshold after stimulation of the ACC. However, as part of the neural circuitry of behavioral reflexes, there is no evidence showing that ACC stimulation alters dorsal horn neuronal responses. We hypothesize that ACC stimulation has two phases: a short-term phase in which stimulation elicits antinociception and a long-term phase that follows stimulation to change the affective response to noxious input. To begin testing this hypothesis, the purpose of this study was to examine the response of spinal cord dorsal horn neurons during stimulation of the ACC. Fifty-eight wide dynamic range spinal cord dorsal horn neurons from adult Sprague-Dawley rats were recorded in response to graded mechanical stimuli (brush, pressure, and pinch) at their respective receptive fields, while simultaneous stepwise electrical stimulations (300 Hz, 0.1 ms, at 10, 20, and 30 V) were applied in the ACC. The responses to brush at control, 10, 20, and 30 V, and recovery were 14.2 +/- 1.4, 12.3 +/- 1.2, 10.9 +/- 1.2, 10.3 +/- 1.1, and 14.1 +/- 1.4 spikes/s, respectively. The responses to pressure at control, 10, 20, and 30 V, and recovery were 39.8 +/- 4.7, 25.6 +/- 3.0, 25.0 +/- 3.0, 21.6 +/- 2.4, and 34.2 +/- 3.7 spikes/s, respectively. The responses to pinch at control, 10, 20, and 30 V, and recovery were 40.7 +/- 3.8, 30.6 +/- 3.1, 27.8 +/- 2.8, 27.2 +/- 3.2, and 37.4 +/- 3.9 spikes/s, respectively. We conclude that electrical stimulation of the ACC induces significant inhibition of the responses of spinal cord dorsal horn neurons to noxious mechanical stimuli. The stimulation-induced inhibition begins to recover as soon as the stimulation is terminated. These results suggest differential short-term and long-term modulatory effects of the ACC stimulation on nociceptive circuits.

Neonatal hind paw injury alters processing of visceral and somatic nociceptive stimuli in the adult rat

Tissue damage during the first few weeks after birth can have profound effects on sensory processing in the adult. We have recently reported that a short-lasting inflammation of the neonatal rat hind paw produces baseline hypoalgesia and exacerbated hyperalgesia after reinflammation of that hind paw in the adult. Because the contralateral hind paw and forepaws also displayed hypoalgesia, we speculated that effects of the initial injury were not somatotopically restricted and would alter visceral sensory processing as well. In the present study we tested this hypothesis by examining the effects of neonatal hind paw injury at P3 or P14 on visceral and somatic sensitivity in the adult rat. In P3 rats, the visceromotor response evoked by colorectal distention in the absence of colonic inflammation was attenuated in carrageenan-treated neonatal rats compared to naive rats. Colonic inflammation in the adult reversed this hypoalgesia and evoked a level of visceral hyperalgesia similar to naive rats. There were no consequences of the P14 injury observed in the adult. In a second experiment, colonic inflammation in naive rats induced viscerosomatic inhibition to thermal stimulation of the forepaw and hind paw. This inhibition was reversed, and the paw withdrawal latency was slightly decreased in neonatal (P3) carrageenan-treated rats. Rats treated on P14 appeared similar to naive rats. These data support the hypothesis that neonatal hind paw injury during a critical period permanently alters sensory processing of multiple sensory modalities in the adult. Animals develop with greater inhibitory processing of somatic and visceral stimuli throughout the neuraxis. However, inflammation in the adult in previously uninjured tissue reverses the hypoalgesia and evokes development of normal hyperexcitability associated with tissue injury.

PERSPECTIVE

Trauma experienced by premature infants can lead to alterations in sensory processing throughout life. This study shows that short-term somatic tissue injury to neonatal rats during a well-defined critical period alters several aspects of viscerosensory processing in the adult, demonstrating that injury to one tissue affects sensory processing throughout the body.

Interactions of galanin and opioids in nociceptive modulation in the arcuate nucleus of hypothalamus in rats

The fact that galanin, beta-endorphin and their receptors are present in the arcuate nucleus of hypothalamus (ARC), coupled with our previous observation that both beta-endorphin and galanin play antinociceptive roles in pain modulation in the ARC, made it of interest to study their interactions. The hindpaw withdrawal latency (HWL) in response to noxious thermal and mechanical stimulation was assessed by the hot-plate test and the Randall Selitto Test. We showed that the antinociceptive effect induced by intra-ARC injection of galanin was dose-dependently attenuated by the following intra-ARC injection of naloxone. Furthermore, intra-ARC administration of the selective mu-opioid receptor antagonist beta-funaltrexamine (beta-FNA) attenuated the increased HWL induced by intra-ARC injection of galanin in a dose-dependent manner, while the delta-opioid receptor antagonist naltrindole or the kappa-opioid receptor antagonist nor-binaltorphimine (nor-BNI) did not. Moreover, intra-ARC injection of a galanin receptor antagonist galantide attenuated intraperitoneal morphine-induced increases in HWLs. These results demonstrate that the antinociceptive effect of galanin was related to the opioid system, especially mu-opioid receptor was involved in, and that systemic morphine induced antinociception involves galanin in the ARC.

Detection and assessment of xenobiotic-induced sensory neuropathy

Amongst the adverse effects of xenobiotics used for industrial or therapeutic purposes is the tendency of some compounds to cause painful sensory neuropathies. Chemotherapeutic agents, antiretrovirals, metals, and many other classes of compounds have been reported to cause such neuropathies. These conditions can be assessed in rodent models during short- or long-term exposure. In this unit, two methods for the detection of xenobiotic-induced neuropathies are discussed. The first assay uses noxious thermal stimulation of the rat hind paw to follow changes in sensitivity as might occur with a progressive peripheral neuropathy. The second method involves the application of nylon von Frey filaments to the hind paw of the rodent to assess the development of decreasing nociceptive thresholds as might occur in the setting of a painful peripheral neuropathy.

Supraspinal and systemic administration of the nicotinic-cholinergic agonist (±)-epibatidine has inhibitory effects on C-fiber reflexes in the rat

This study assessed the effects of the nicotinic agonist (+/-)-epibatidine (EPIB) on the C-fiber flexor reflex in the anesthetized rat. Electrical stimulation of the hindpaw produces a long latency (> 150 ms) C-fiber mediated electromyographic (EMG) burst in hindlimb flexor muscles. EPIB (0.01, 0.03 micromol/kg, i.p.) significantly reduced (p < 0.05) C-fiber -related EMG activity by 46 and 64%, respectively. This effect was similar to that produced by the opioid morphine (21.0 micromol/kg, i.p.) and the NMDA receptor antagonist MK-801 (3.0 micromol/kg, i.p.). Nicotinic receptor blockade with the antagonists mecamylamine (5.0 micromol/kg, i.p.) and chlorisondamine (23.0 nmol/rat, intracerebroventricular) attenuated the effects of systemic EPIB on the C-fiber reflex. EPIB injection (0.04 nmol/rat) into the nucleus raphe magnus significantly decreased C-fiber EMG activity by 67%, suggesting a supraspinal site of action. In contrast, EPIB (0.6 nmol/rat) administered into the lumbar spinal cord significantly increased the C-fiber reflex by 117%. In summary, systemic and supraspinal EPIB exerted an inhibitory effect on central pain transmitting pathways, while a stimulatory effect is elicited in the spinal cord. The inhibitory effects are consistent with the reported analgesic properties of EPIB. The excitatory effect may be related to the reported algogenic responses when EPIB is administered intrathecally.

Functional imaging and the neural systems of chronic pain

Pain remains a serious health care problem affecting millions of individuals, costing billions of dollars, and causing an immeasurable amount of human suffering. In designing improved therapies, there is still much to learn about peripheral nociceptor, nerves, and the spinal cord, and brain stem modulatory systems. Nevertheless, it is the brain that presents us with an incredible opportunity to understand the experience we call pain. Functional neuroimaging is helping to unlock the secrets of the sensory and emotional components of pain and its autonomic responses. These techniques are helping us to understand that pain is not a static disease with the pathologic findings localized to the periphery but is instead a highly plastic condition affecting multiple central neural systems. Functional neuroimaging is transforming our understanding of the neurobiology of pain and will be instrumental in helping us to design more rational treatments ultimately aimed at reducing the impact of pain on our patients. It is opening windows into the function of the brain that were previously closed.

Role for Raphe Magnus Neuronal Responses in the Behavioral Reactions to Colorectal Distension

The brain stem is necessary for the expression of behavioral reactions to noxious visceral inputs. Neurons in raphe magnus (RM) and the adjacent nucleus reticularis magnocellularis (NRMC) respond to visceral stimuli and can facilitate the behavioral reaction to visceral stimulation. To determine which RM and NRMC cells could play a role in generating the reaction to colorectal distension (CRD), the responses of RM and NRMC cells to multiple intensities of CRD were compared with simultaneously evoked cardiovascular and visceromotor reactions in halothane-anesthetized rats. Most neurons (89%) responded to CRD with one of three basic response patterns. For cells with a graded response pattern, the response magnitude increased with increasing stimulation intensity. For flat responding cells, the response magnitude was not different across suprathreshold stimulation intensities. Finally, neurons with a switch response pattern responded to low- and high-intensity CRD in opposing directions. Cells were either inhibited or excited by CRD in each of these categories. Responses of cells with both graded and switch response patterns were significantly correlated with CRD-evoked tachycardia, pressor reaction, and hunching. The activity of graded-responding cells have the greatest predictive value for CRD-evoked reactions. Flat-responding cells have nonlinear responses that may augment reactions to stimuli above the noxious threshold. Cells with switch type response patterns may contribute to differential reactions evoked by CRD stimuli within the noxious range. In sum, RM and NRMC neurons respond to CRD with a variety of patterns, each of which may contribute to the sculpting of CRD reactions in different ways.

Serotoninergic-mediated inhibition of substance P sensitive deep dorsal horn neurons: a combined electrophysiological and morphological study in vitro

Dorsal horn neurons that express the neurokinin 1 receptor (NK-1R) play an important role in nociceptive processing. The targetting of NK-1R neurons by serotoninergic (5-hydroxytryptamine, 5-HT) axons would provide a straightforward means to exert an inhibitory analgesic effect at spinal level. This study used single cell electrophysiology to analyse and correlate the responses of rat deep DH neurons in vitro to both 5-HT and the NK-1R agonist [Sar9,Met(O2)11]-substance P (SP). Subsequently a combination of immunocytochemistry and confocal imaging was applied to biocytin-filled laminae III-VI neurons to reveal putative 5-HT innervation in this neuronal sample. A population of neurons was identified in which 5-HT (50 microM) significantly attenuated the dorsal root-evoked excitatory postsynaptic potential and [Sar9,Met(O2)11]-SP (2 microM) induced a direct tetrodotoxin-resistant depolarisation. Immunolabelling revealed that all of these neurons were inhibited by 5-HT, including those that were excited by [Sar9,Met(O2)11]-SP, were overlaid by a plexus of 5-HT immunoreactive fibres and in some instances, closely apposed putative contacts with somata and proximal dendrites identified although their incidence was low. Inhibition by 5-HT of deep DH neurons directly responsive to SP may account at least in part for monoamine-induced modulation of nociceptive processing in the spinal cord.

Substance P microinjected into the periaqueductal gray matter induces antinociception and is released following morphine administration

The aims of the present study were to investigate, in rats, the behavioral effects of substance P (SP) microinjected into the ventrolateral periaqueductal gray (PAG) and the effects of the neurokinin 1 (NK-1) receptor antagonist [d-Arg1, d-Trp7, 9, Leu11]-substance P (Spantide). The effect of morphine administration on the release of SP in the ventrolateral PAG was also investigated using microdialysis in awake rats. SP microinjected into the ventrolateral part of the PAG induced significant increases in the hindpaw withdrawal latencies (HWLs) to thermal and mechanical stimulation as an antinociceptive response. The NK-1 receptor antagonist blocked these effects but exhibited no antinociceptive effect alone. Subcutaneous administration of morphine increased basal SP-like immunoreactivity (SP-LI) release in the microdialysate obtained from the ventrolateral PAG of freely moving rats. Our results demonstrate that SP injected into the ventrolateral PAG induces an antinociceptive effect via activation of NK-1 receptors. Morphine administered systemically induces the release of SP in the ventrolateral PAG. We suggest that an increased release of SP in the PAG may contribute to opioid antinociception.

The short- and long-term benefit in chronic low back pain through adjuvant electrical versus manual auricular acupuncture

Acupuncture is an established adjuvant analgesic modality for the treatment of chronic pain. Electrical stimulation of acupuncture points is considered to increase acupuncture analgesia. In this prospective, randomized, double-blind, controlled study we tested the hypothesis that auricular electroacupuncture (EA) relieves pain more effectively than conventional manual auricular acupuncture (CO) in chronic low back pain patients with insufficient pain relief (visual analogue scale [VAS] > or = 5) treated with standardized analgesic therapy. Disposable acupuncture needles were inserted in the auricular acupuncture points 29, 40, and 55 of the dominant side and connected to a newly developed battery-powered miniaturized stimulator worn behind the ear. Patients were randomized into group EA (n = 31) with continuous low-frequency auricular EA (1 Hz biphasic constant current of 2 mA) and group CO (n = 30) without electrical stimulation (sham-electroacupuncture). Treatment was performed once weekly for 6 wk, and in each group needles were withdrawn 48 h after insertion. During the study period and a 3-mo follow-up, patients were asked to complete the McGill questionnaire. Psychological well being, activity level, quality of sleep, and pain intensity were assessed by means of VAS; moreover, analgesic drug consumption was documented. Pain relief was significantly better in group EA during the study and the follow-up period as compared with group CO. Similarly, psychological well-being, activity, and sleep were significantly improved in group EA versus group CO, the consumption of analgesic rescue medication was less, and more patients returned to full-time employment. Neuropathic pain in particular improved in patients treated with EA. There were no adverse side effects. These results are the first to demonstrate that continuous EA stimulation of auricular acupuncture points improves the treatment of chronic low back pain in an outpatient population.

IMPLICATIONS

Continuous electrical stimulation of auricular acupuncture points using the new point stimulation device P-stim significantly decreases pain intensity and improves psychological well-being, activity, and sleep in chronic low back pain patients.

Comparison Between Electro-Acupuncture and Hydrotherapy, Both in Combination With Patient Education and Patient Education Alone, on the Symptomatic Treatment of Osteoarthritis of the Hip

OBJECTIVES

The aim of the study was to evaluate the therapeutic effect of electro-acupuncture (EA) and hydrotherapy, both in combination with patient education or with patient education alone, in the treatment of osteoarthritis in the hip.

METHODS

Forty-five patients, aged 42-86 years, with radiographic changes consistent with osteoarthritis in the hip, pain related to motion, pain on load, and ache were chosen. They were randomly allocated to EA, hydrotherapy, both in combination with patient education, or patient education alone. Outcome measures were the disability rating index (DRI), global self-rating index (GSI), and visual analogue scale (VAS). Assessments were done before the intervention and immediately after the last treatment and 1, 3, and 6 months after the last treatment.

RESULTS

Pain related to motion and pain on load was reduced up to 3 months after last the treatment in the hydrotherapy group and up to 6 months in the EA group. Ache during the day was significantly improved in both the EA and hydrotherapy group up to 3 months after the last treatment. Ache during the night was reduced in the hydrotherapy group up to 3 months after the last treatment and in the EA group up to 6 months after. Disability in functional activities was improved in EA and hydrotherapy groups up to 6 months after the last treatment. Quality of life was also improved in EA and hydrotherapy groups up to 3 months after the last treatment. There were no changes in the education group alone.

DISCUSSION

In conclusion, EA and hydrotherapy, both in combination with patient education, induce long-lasting effects, shown by reduced pain and ache and by increased functional activity and quality of life, as demonstrated by differences in the pre- and post-treatment assessments.

Muscle stretch-induced modulation of noxiously activated dorsal horn neurons of feline spinal cord

The present work was designed to check for the possibility of interactions between mechanical innocuous and chemically induced noxious muscle afferent inputs on discharge behavior of nociceptive superficial dorsal horn neurons (SDHNs) of the spinal cord in decerebrated cats. The innocuous and noxious stimuli were applied separately and in combination, so that the effects of the innocuous stimulus on nociceptive processing could be evaluated. The innocuous stimulus consisted of ramp-and-hold stretches of the gastrocnemius muscles, whereas the noxious stimulus consisted of i.a. injections of bradykinin (BK; 0.5-1 ml, 50 microg/ml) into the arterial circulation of same muscles. Only neurons up to approximately 1mm depth and those that responded to noxious pinch of the gastrocnemius muscles were selected for further analysis. The activity of 16 dorsal horn neurons was recorded extracellularly with high-impedance glass microelectrodes, out of which seven responded to stretch, while 12 neurons responded to bradykinin injections. The bradykinin injections induced three types of responses: excitatory, inhibitory and mixed. The majority of the neurons that showed excitatory and mixed responses to bradykinin were also influenced by stretches applied directly after the bradykinin injection. In these neurons, the stretch usually counteracted the bradykinin-induced response, i.e. shortening and reducing bradykinin-induced excitation and re-exciting the cells after bradykinin-induced inhibition. The mechanism of the stretch modulation is proposed to reside in a segmental spinal control of the nociceptive transmission.

Long-lasting changes in rostral ventral medulla neuronal activity after inflammation

Activity-dependent synaptic plasticity occurs in many regions of the central nervous system. It is known that spinal sensory transmission undergoes long-lasting changes after tissue injury and inflammation, but much less is known about descending modulation. In the present study, we demonstrate that tissue injury causes long-lasting changes within the rostral ventral medulla, a region critical for descending modulation of spinal sensory transmission. Subcutaneous formalin injections induced changes in the activity of rostral ventral medulla neurons, particularly during phase 2 (10 to 55 minutes after injection). The activity of neutral cells, which showed no response to acute noxious stimuli, was significantly decreased after formalin injection. Furthermore, 2 "silent" cells became active after the formalin injection. To demonstrate directly that descending biphasic modulation from the rostral ventral medulla was affected after the inflammation, we investigated descending modulation of a spinal nociceptive reflex produced by focal electrical stimulation in the rostral ventral medulla and found that both facilitation and inhibition were significantly decreased. These data suggest that rostral ventral medulla circuits modulating spinal sensory transmission undergo profound and long-lasting changes after tissue injury and inflammation. This may contribute to the pathological modification of nociceptive processing in chronic pain states.

Effects of corticotropin-releasing factor on plasticity of optically recorded neuronal activity in the substantia gelatinosa of rat spinal cord slices

We examined the effects of corticotropin-releasing factor (CRF) on plasticity of optically recorded neuronal activity in the substantia gelatinosa (lamina II) of 12-18-day-old rat spinal cord slices stained with a voltage-sensitive dye. Single-pulse test stimulation to the dorsal root that activated A and C fibres evoked prolonged (>100 ms) light-absorption change in the lamina II. This response represents the gross membrane potential change of all elements along the slice depth. After conditioning high-frequency stimulation of A-fibre-activating strength, test stimulus elicited less neuronal activity [-27+/-1% (7), (average+/-SE (n)), P<0.01 (*) at 45-60 min after conditioning]. When CRF (1 microM, 10 min) was applied during conditioning, the neuronal activity was facilitated rather than suppressed [+20+/-3% (5), P<0.05]. CRF alone exhibited insignificant effect [-5+/-1% (4), P=0.2]. In the presence of the inhibitory amino acid antagonists bicuculline (1 microM) and strychnine (0.3 microM) in the perfusate, in contrast, the conditioning facilitated it [+27+/-1% (12)*], and CRF treatment during conditioning inhibited the facilitation dose-dependently [0.1 microM: +18+/-2% (5)*, 1 microM: +13+/-1% (7)*]. Although interneuronal actions might contribute, these results suggest that CRF may have dual effects on excitatory synaptic transmission within the lamina II depending upon cellular conditions: a conversion from the induction of long-term depression to long-term potentiation (LTP), and inhibition of LTP induction. Since the LTP is thought to be responsible at least in part for the persistent pain, CRF could regulate the induction.

An antinociceptive role of galanin in the arcuate nucleus of hypothalamus in intact rats and rats with inflammation

In the arcuate nucleus of hypothalamus (ARC), galaninergic fibers form synaptic contacts with proopiomelanocortin neurons, which are involved in pain modulation. The present study assessed the role of exogenous and endogenous galanin in the modulation of nociception in the ARC of rats. The hindpaw withdrawal latency (HWL) to thermal and mechanical stimulation was assessed by the hot-plate test and the Randall Selitto Test. Intra-ARC injection of galanin dose-dependently increased the HWLs in intact rats, indicating an antinociceptive role of exogenous galanin in the ARC. The antinociceptive effect of galanin was blocked by following intra-ARC injection of galantide, a putative galanin receptor antagonist, suggesting that the antinociceptive effect of galanin is mediated by galanin receptors. Moreover, intra-ARC injection of galanin increased the HWL in rats with inflammation. Intra-ARC administration of galantide alone reduced the HWLs in rats with inflammation, while there were no influences of galantide on the HWL in intact rats. Taken together, the results show that galanin has an antinociceptive role in the ARC of intact rats and rats with inflammation.

Electrical Stimulation of Auricular Acupuncture Points Is More Effective Than Conventional Manual Auricular Acupuncture in Chronic Cervical Pain: A Pilot Study

In this prospective, randomized, double-blinded, controlled study, we tested the hypothesis that auricular electroacupuncture relieves pain more effectively than conventional manual auricular acupuncture. We studied 21 chronic cervical pain patients without radicular symptoms with insufficient pain relief (visual analogue scale >5) treated with standardized analgesic therapy. All patients received disposable acupuncture needles on the dominant side on the following acupuncture points: cervical spine, shen men, and cushion. In 10 patients, needles were continuously stimulated (2-mA constant current, 1 Hz monophasic) by using the electrical point stimulation device P-STIM. In 11 control patients, no electrical stimulation was administered. All needles were withdrawn 48 h after insertion. Acupuncture was performed once a week for 6 wk. Patients had to complete a questionnaire assessing pain intensity, psychological well-being, activity, sleep, and demand for rescue medication (lornoxicam and tramadol). The reduction in pain scores was significant in the electrical acupuncture group. Similarly, psychological well-being, activity, and sleep were significantly improved in patients receiving electrical acupuncture, and consumption of rescue medication was significantly less. These results demonstrate that continuous electrical stimulation of auricular acupuncture points by using the new point stimulation device P-STIM improves the treatment of chronic cervical pain in an outpatient population.

IMPLICATIONS

Continuous electrical stimulation of auricular acupuncture points by using the new point stimulation device P-STIM significantly decreases pain intensity and significantly improves psychological well-being, activity, and sleep in chronic cervical pain patients.

Involvement of intracellular calcium and protein phosphatases in long-term depression of A-fiber-mediated primary afferent neurotransmission

Long-term depression (LTD) of monosynaptic and polysynaptic excitatory postsynaptic potentials (EPSPs) in substantia gelatinosa (SG) neurons can be induced by brief high-frequency electrical stimulation (HFS, 300 pulses at 100 Hz) of primary afferent fibers in dorsal roots. Here we examined the possible cellular mechanism underlying spinal LTD. Conventional intracellular recordings were made from SG neurons in a transverse slice-dorsal root preparation of the young rat lumbar spinal cord. LTD of both monosynaptic and polysynaptic EPSPs was induced in 16 of 24 SG neurons by HFS of dorsal root in either the presence or absence of the GABA(A) receptor antagonist bicuculline and the glycine receptor antagonist strychnine. Loading the postsynaptic cell with BAPTA, an intracellular Ca(2+) chelator, almost completely blocked the induction of LTD. Induction of LTD was abolished by bath application of calyculin A (100 nM), a potent inhibitor of protein phosphatases 1 and 2A. These results indicate that: (i) a rise in postsynaptic Ca(2+) is necessary for LTD induction, (ii) synaptic activation of protein phosphatases 1 and 2A plays an important role in the induction of LTD of primary afferent A-fiber neurotransmission in the young rat spinal cord, and (iii) the effect of LTD may be physiologically relevant for transmission and integration of sensory information, including nociception.

Involvement of endogenous beta-endorphin in antinociception in the arcuate nucleus of hypothalamus in rats with inflammation

Although exogenous administration of beta-endorphin to the arcuate nucleus of hypothalamus (ARC) had been shown to produce antinociception, the role of endogenous beta-endorphin of the ARC in nociceptive processing has not been studied directly. The aim of the present study was to investigate the effect of endogenous beta-endorphin in the ARC on nociception in rats with carrageenan-induced inflammation. The hindpaw withdrawal latency (HWL) to noxious thermal and mechanical stimulation was assessed by the hot-plate test and the Randall Selitto Test. Intra-ARC injection of naloxone had no significant influence on the HWL to thermal and mechanical stimulation in intact rats. The HWL decreased significantly after intra-ARC injection of 1 or 10 microg of naloxone in rats with inflammation, but not with 0.1 microg of naloxone. Furthermore, intra-ARC administration of the selective mu-opioid receptor antagonist beta-funaltrexamine (beta-FNA) decreased the nociceptive response latencies to both stimulation in a dose-dependent manner in rats with inflammation, while intra-ARC administration of the selective delta-opioid receptor antagonist naltrindole or the selective kappa-opioid receptor antagonist nor-binaltorphimine (nor-BNI) showed no influences on the nociceptive response latency. The antiserum against beta-endorphin, administered to the ARC, also dose-dependently reduced the HWL in rats with inflammation. The results indicate that endogenous beta-endorphin in the ARC plays an important role in the endogenous antinociceptive system in rats with inflammation, and that its effect is predominantly mediated by the mu-opioid receptor.

Differential recruitment of endogenous pain inhibitory systems in neuropathic pain patients

Neuronal hyperexcitability is a key finding in patients with neuropathic pain. Contributing to hyperexcitability may be decreased activity in the endogenous pain inhibitory systems. The present study aimed at recruiting descending inhibition, by the use of painful heterotopic stimulation (HTS), in 16 patients with peripheral chronic neuropathic pain and associated brush-evoked allodynia. Two experiments were performed: one examined the effect of HTS on ongoing pain and intensity of brush-evoked allodynia and the other tested the effect of HTS on ongoing pain and area of brush-evoked allodynia. Both experiments consisted of two sessions, one with painful cold HTS (1 degrees C water bath) another with non-painful neutral HTS (32 degrees C water bath). The area of brush-evoked allodynia was significantly reduced (P=0.003) during painful HTS, as compared to non-painful HTS. In contrast, neither the intensity of brush-evoked allodynia nor the ongoing pain was significantly changed. The results indicate that endogenous pain modulating systems can alter some aspects of chronic neuropathic brush-evoked allodynia. The differential effect of painful HTS on ongoing pain and area of brush-evoked allodynia suggest that separate mechanisms are involved.

Raphe magnus neurons respond to noxious colorectal distension

Physiological studies of neurons in raphe magnus (RM) and the adjacent nucleus reticularis magnocellularis (NRMC) have demonstrated that the response to noxious cutaneous stimulation predicts the response to opioid administration and therefore a cell's functional role in nociceptive modulation. Although visceral stimulation, like opioids, elicits antinociception, little is known about how RM and NRMC cells respond to visceral stimulation. Therefore RM and NRMC cells were tested for their responses to both colorectal distension (CRD) and noxious cutaneous heat in halothane-anesthetized rats. Less than a third of serotonergic cells responded to CRD with small increases or decreases in discharge rate. In contrast, almost two-thirds of nonserotonergic cells responded to CRD stimulation with either excitatory (35%) or inhibitory (30%) responses to CRD. The response to heat did not predict the response to CRD with nearly equal proportions of heat-excited, -inhibited, and -unaffected cells being excited, inhibited, or unaffected by CRD. The dissociation between the responses to cutaneous heat and CRD demonstrates that cell classes based on the response to noxious heat are not homogeneous and may play multiple functional roles.

Discharge of raphe magnus ON and OFF cells is predictive of the motor facilitation evoked by repeated laser stimulation

Medullary raphe magnus (RM) on and off cells are thought to modulate spinal nociception by gating withdrawals evoked by noxious stimulation. To test whether withdrawal initiation is the target of RM modulation, we examined the relationship between on and off cell discharge and motor withdrawal evoked by noxious laser heat in halothane-anesthetized rats. The cellular responses of both cell types began during the 50 msec after onset of the tail flick, peaked within 200 msec, and outlasted the duration of the motor reaction. Thus, it is unlikely that the target of on and off cell modulation is withdrawal initiation; instead, on and off cells may modulate reactions to repeated noxious stimulation. We therefore tested whether laser heat-evoked changes in RM cell discharge were predictive of the modulatory effects of one noxious stimulus on the reaction to a subsequent noxious stimulus. Two pulses of laser heat were presented at interpulse intervals of 0.8, 2.0, or 10.0 sec. The motor withdrawal evoked by the second pulse was significantly enhanced relative to that evoked by the first pulse. The observed motor enhancement depended on supraspinal input because it was not present in spinalized rats. Comparison of the relative changes in motor and cellular activity preceding double laser heat stimulation revealed parallel changes between motor facilitation, decreases in off cell discharge, and increases in on cell discharge. This finding suggests a preparatory role for RM on and off cells in enhancing reactions to a noxious stimulus that closely follows another noxious stimulus.

Modulation of noxious and non-noxious spinal mechanical transmission from the rostral medial medulla in the rat

Modulatory influences on spinal mechanical transmission from the rostral medial medulla (RMM) were studied. Noxious stimulation, produced by von Frey-like monofilaments, and non-noxious stimulation, produced by a soft brush, was applied to the glabrous skin of the hind foot. At 28 sites in RMM, electrical stimulation facilitated responses to noxious mechanical stimulation at low intensities (5-25 microA) and inhibited responses of the same neurons at greater intensities (50-100 microA) of stimulation. At 24 and 9 other sites in RMM, stimulation at all intensities only inhibited or only facilitated, respectively, responses to noxious mechanical stimulation of the hind foot. Stimulus-response functions to mechanical stimulation were shifted leftward by low intensities and decreased by high intensities of stimulation. Inhibitory influences were found to descend in the dorsolateral funiculi; facilitatory effects were contained in the ventral spinal cord. Descending modulation of non-noxious brush stimulation revealed biphasic facilitatory-inhibitory effects (9 sites in RMM), only inhibitory effects (14 sites) and only facilitatory effects (8 sites). The effects of electrical stimulation were replicated by intra-RMM administration of glutamate; a low concentration (0.25 nmol) facilitated and a greater concentration (2.5 nmol) inhibited spinal mechanical transmission, providing evidence that cells in RMM are sufficient to engage descending influences. Descending modulatory effects were specific for the site of stimulation, not for the spinal neuron, because modulation of the same neuron was different from different sites in RMM. These results show that spinal mechanical transmission, both noxious and non-noxious, is subject to descending influences, including facilitatory influences that may contribute to exaggerated responses to peripheral stimuli in some chronic pain states.

Immediate effects of dry needling and acupuncture at distant points in chronic neck pain: results of a randomized, double-blind, sham-controlled crossover trial

To evaluate immediate effects of two different modes of acupuncture on motion-related pain and cervical spine mobility in chronic neck pain patients compared to a sham procedure. Thirty-six patients with chronic neck pain and limited cervical spine mobility participated in a prospective, randomized, double-blind, sham-controlled crossover trial. Every patient was treated once with needle acupuncture at distant points, dry needling (DN) of local myofascial trigger points and sham laser acupuncture (Sham). Outcome measures were motion-related pain intensity (visual analogue scale, 0-100 mm) and range of motion (ROM). In addition, patients scored changes of general complaints using an 11-point verbal rating scale. Patients were assessed immediately before and after each treatment by an independent (blinded) investigator. Multivariate analysis was used to assess the effects of true acupuncture and needle site independently. For motion-related pain, use of acupuncture at non-local points reduced pain scores by about a third (11.2 mm; 95% CI 5.7, 16.7; P = 0.00006) compared to DN and sham. DN led to an estimated reduction in pain of 1.0 mm (95% CI -4.5, 6.5; P = 0.7). Use of DN slightly improved ROM by 1.7 degrees (95% CI 0.2, 3.2; P = 0.032) with use of non-local points improving ROM by an additional 1.9 degrees (95% CI 0.3, 3.4; P = 0.016). For patient assessment of change, non-local acupuncture was significantly superior both to Sham (1.7 points; 95% CI 1.0, 2.5; P = 0.0001) and DN (1.5 points; 95% CI 0.4, 2.6; P = 0.008) but there was no difference between DN and Sham (0.1 point; 95% CI -1.0, 1.2; P = 0.8). Acupuncture is superior to Sham in improving motion-related pain and ROM following a single session of treatment in chronic neck pain patients. Acupuncture at distant points improves ROM more than DN; DN was ineffective for motion-related pain.

Acupuncture mechanisms for clinically relevant long-term effects--reconsideration and a hypothesis

From the author's direct involvement in clinical research, the conclusion has been drawn that clinically relevant long-term pain relieving effects of acupuncture (>6 months) can be seen in a proportion of patients with nociceptive pain. The mechanisms behind such effects are considered in this paper. From the existing experimental data some important conclusions can be drawn: 1. Much of the animal research only represents short-term hypoalgesia probably induced by the mechanisms behind stress-induced analgesia (SIA) and the activation of diffuse noxious inhibitory control (DNIC). 2. Almost all experimental acupuncture research has been performed with electro-acupuncture (EA) even though therapeutic acupuncture is mostly gentle manual acupuncture (MA). 3. Most of the experimental human acupuncture pain threshold (PT) research shows only fast and very short-term hypoalgesia, and, importantly, PT elevation in humans does not predict the clinical outcome. 4. The effects of acupuncture may be divided into two main components--acupuncture analgesia and therapeutic acupuncture. A hypothesis on the mechanisms of therapeutic acupuncture will include: 1. Peripheral events that might improve tissue healing and give rise to local pain relief through axon reflexes, the release of neuropeptides with trophic effects, dichotomising nerve fibres and local endorphins. 2. Spinal mechanisms, for example, gate-control, long-term depression, propriospinal inhibition and the balance between long-term depression and long-term potentiation. 3. Supraspinal mechanisms through the descending pain inhibitory system, DNIC, the sympathetic nervous system and the HPA-axis. Is oxytocin also involved in the long-term effects? 4. Cortical, psychological, "placebo" mechanisms from counselling, reassurance and anxiety reduction.

Biphasic modulation of spinal visceral nociceptive transmission from the rostroventral medial medulla in the rat

Descending inhibitory and facilitatory influences from the rostroventral medulla (RVM) on responses of lumbosacral spinal neurons to noxious colorectal distension (CRD, 80 mmHg, 20 s) were studied. At 25 sites in the RVM, electrical stimulation produced biphasic effects, facilitating responses of spinal neurons to CRD at lesser intensities of stimulation (5-25 microA) and inhibiting responses of the same neurons at greater intensities of stimulation (50-100 microA). At 38 other sites in the RVM, electrical stimulation produced only intensity-dependent inhibition of neuron responses to CRD. At another 13 sites in the RVM, electrical stimulation (5-100 microA) produced only facilitatory effects on responses to CRD. Descending modulatory effects were selective for distension-evoked activity; spontaneous activities of the same spinal neurons were not significantly affected by electrical stimulation that either facilitated or inhibited neuron responses to CRD. Neuron responses to graded CRD (20-100 mmHg) were positively accelerating functions that were shifted leftward or rightward, respectively, by lesser, facilitatory intensities or greater, inhibitory intensities of RVM stimulation. L-glutamate microinjection into the RVM replicated the effects of electrical stimulation, producing similar biphasic modulatory effects as produced by electrical stimulation. Microinjection of glutamate into the RVM at a low dose (5 nmoles) facilitated responses of spinal neurons to CRD and inhibited responses of the same neurons at a greater dose (50 nmoles). In some experiments, microinjection of lidocaine (0.5 microl of 4% solution) or the neurotoxin ibotenic acid (0.5 microl, 10 microg) into the RVM produced reversible or long-lasting, respectively, decreases in spontaneous activity and responses of spinal neurons to CRD. These results reveal that spinal visceral nociceptive transmission is subject to a tonic descending excitatory influence from the RVM and that descending modulatory effects from the RVM on visceral nociceptive transmission are qualitatively similar to modulation of cutaneous nociceptive transmission.

Glutamate receptors and persistent pain: targeting forebrain NR2B subunits

Glutamate is the fast excitatory transmitter in mammalian brains. It binds to two major classes of glutamate receptors: ionotropic and metabotropic receptors. Ionotropic receptors contain three subtype receptors, including N-methyl-d-aspartate (NMDA) receptors. Activation of NMDA receptors is important for initiating long-lasting changes in synapses. In the forebrain structures that are known to contribute to the formation and storage of information, NMDA receptors have an important role in persistent inflammatory pain by reinforcing glutamate sensory transmission. Mice with enhanced forebrain NMDA receptor function demonstrate selective enhancement of persistent pain and allodynia. Drugs targeting NMDA NR2B subunits in the forebrain could serve as a new class of medicine for controlling persistent pain in humans.

Role of calcitonin gene-related peptide and its antagonist on the evoked discharge frequency of wide dynamic range neurons in the dorsal horn of the spinal cord in rats

The present study was performed to explore the effect of calcitonin gene-related peptide (CGRP) and its antagonist CGRP8-37 on the evoked discharge frequency of wide dynamic range (WDR) neurons in the dorsal horn of the spinal cord in rats. Recording was performed with a multibarrelled glass micropipette and the chemicals were delivered by iontophoresis. The discharge of WDR neurons was evoked by transdermic electrical stimulation applied on the ipsilateral hindpaw. (1) Iontophoretic application of CGRP at an ejection current of 100 nA increased the discharge frequency of WDR neurons significantly. (2) Iontophoretic application of CGRP8-37 at an ejection current of 80 or 160 nA induced significant decreases in the discharge frequency of WDR neurons, but not at 40 nA. (3) Iontophoretic application of CGRP8-37 not only antagonized the CGRP-induced increase in the evoked discharge frequency of WDR neurons but also induced a significant decrease in the evoked discharge frequency of WDR neurons compared to basal levels. The results indicate that CGRP and its receptors play a facilitary role on the transmission and/or modulation of nociceptive information in the dorsal horn of the spinal cord in rats.

Serotonin alters multi-segmental convergence patterns in spinal cord deep dorsal horn and intermediate laminae neurons in an in vitro young rat preparation

Each spinal neuron has a receptive field that corresponds to stimulation of a specific area of skin or subcutaneous tissue. Receptive fields are plastic and can be altered during development and injury but the actions of neuromodulators, such as serotonin (5-hydroxytryptamine, 5-HT) on receptive field properties are not well known. We used stimulation of multiple adjacent dorsal root spinal segments as a measure of "receptive field size" to determine the effects of 5-HT on multi-segmental convergent input onto neurons in laminae IV-VII. Whole-cell patch-clamp recordings were undertaken in the in vitro hemisected thoracolumbar spinal cord of rats aged 8-10 days old. Based on synaptic responses, neurons could be divided into two predominant groups and 5-HT exerted different effects on these groups. The first group received excitatory post-synaptic potentials (EPSPs) from the homonymous dorsal root but inhibitory post-synaptic potentials (IPSPs) with increasing amplitude from more distant dorsal roots. In this group, 5-HT preferentially depressed the IPSPs from adjacent nerve roots while leaving the EPSP intact. The second group received short-latency EPSPs from all segments stimulated and 5-HT potently depressed all synaptic input. In both populations the depressant actions of 5-HT increased with dose (0.1-10.0 microM). Bicuculline and strychnine did not affect the 5-HT induced short-latency synaptic depression. These results suggest that descending serotonergic systems depress spinal sensory convergence in a graded and differentiated manner. The findings are discussed in relation to the modulation of nociceptive signaling.

Physiological and anatomic evidence for functional subclasses of serotonergic raphe magnus cells

Serotonergic cells in the medullary nucleus raphe magnus (RM) and adjacent nucleus reticularis magnocellularis (NRMC) project to the spinal cord where they are likely to modulate nociceptive transmission. Previous studies have suggested that these cells are physiologically and anatomically heterogeneous. In the present investigation, we examined whether subclasses of serotonergic RM and NRMC cells can be delineated based on their response to a visceral stimulus, and whether any such subclasses are morphologically distinct. Most RM and NRMC serotonergic cells tested (81 of 116) responded to retraction of the descending aorta into a polyethylene tube (the snare stimulus) with 57% of all cells tested excited and 13% inhibited. Responses of serotonergic cells to the snare outlasted the stimulus, were not reflective of evoked cardiovascular changes, and were observed in sino-aortic deafferented rats, evidence that the snare stimulus does not influence serotonergic cell discharge through activation of baroreceptors. Because serotonergic cells responsive to the snare were also responsive to mechanical brushing within the retroperitoneum, the snare is likely to change serotonergic cell discharge by means of the activation of mechanosensitive visceral afferents. Intracellular labeling of physiologically characterized serotonergic RM and NRMC cells showed that cells that were responsive to the snare stimulus had simpler axonal collateralization patterns than cells that were unresponsive to the snare stimulus. This association between morphological and physiological properties provides additional evidence that subpopulations of serotonergic cells exist and serve varied physiological functions.

Acupuncture produces central activations in pain regions

Acupuncture is largely used for pain control in several pathological conditions. Its effects on the central nervous system are not well defined. We investigated the effect of the application of acupuncture to 13 normal subjects (males, 21-32 years). H2(15)O bolus PET scans were read before the application of the needles (Rest, R) and after 25 min of needle insertion. Data were acquired by scanning in 3-D mode. The acupuncture application, true acupuncture (TA), was alternated to a placebo needle application (PA) in two different sequences (seven and six subjects, respectively), either R,PA,R, TA or R,TA,R,PA, a period of 15 min being left after every first TA or PA to allow for the recovery of basal conditions. Here we show that classic acupuncture activates the left Anterior Cingulus, the Insulae bilaterally, the Cerebellum bilaterally, the left Superior Frontal Gyrus, and the right Medial and Inferior Frontal Gyri. Most of the activated areas are shared with areas activated in acute and chronic pain states as described in the literature. Thus acupuncture appears to act by activating areas also involved in pain. This indicates that acupuncture could relief pain by unbalancing the equilibrium of distributed pain-related central networks.

The NK1 receptor is essential for the full expression of noxious inhibitory controls in the mouse

Behavioral analysis of the NK1 receptor gene knock-out (NK1-/-) mouse indicated that substance P was closely involved in orchestrating the physiological and behavioral response of the animal to major environmental stressors. In particular, endogenous pain control mechanisms, such as stress-induced analgesia were substantially impaired in mutant mice, suggesting a reduction in descending inhibitory controls to the spinal cord from the brainstem. To directly test the integrity of descending controls in NK1-/- mice, we have analyzed c-Fos expression in laminae I-II of the lumbar and cervical cord and in the rostral ventromedial medulla in an experimental paradigm known to require recruitment of descending inhibitory controls. Anesthetized mice were stimulated with water at 50 degrees C either on their forepaw, hindpaw, or on both the hindpaw plus forepaw concurrently. Wild-type mice, naive or treated with an NK1 antagonist (RP67580) or its inactive isomer (RP68651), were compared with NK1-/- mice. C-Fos expression at the lumbar laminae I-II level was significantly reduced, whereas it was significantly greater in the raphe magnus and pallidus nuclei in the double stimulation situation in wild-type compared with NK1-/- mice. Blocking the NK1 receptor pharmacologically reproduced, in an enantiomere-selective manner, the data from NK1-/- mice, with no evidence for recruitment of descending inhibition at the lumbar cord level after forepaw stimulation. The present study demonstrates that the NK1 receptor is essential for the full development of noxiously evoked descending inhibition.

Vagal afferent signaling of a gastric mucosal acid insult to medullary, pontine, thalamic, hypothalamic and limbic, but not cortical, nuclei of the rat brain

Although gastric acid is a factor in upper abdominal pain, the signaling and processing of a gastric mucosal acid insult within the brain are not known. This study examined which nuclei in the rat brain respond to challenge of the gastric mucosa by a noxious concentration of hydrochloric acid (HCl) and whether the central input is carried by vagal afferent neurons. Activation of neurons in the brain was mapped by in situ hybridization autoradiography of messenger ribonucleic acid (mRNA) for the immediate early gene c-fos 45 min after intragastric administration of saline or HCl. Following intragastric HCl (0.5 M) challenge, many neurons in the nucleus tractus solitarii, lateral parabrachial nucleus, thalamic and hypothalamic paraventricular nucleus, supraoptic nucleus, central amygdala and medial/lateral habenula expressed c-fos mRNA as compared to intragastric treatment with saline (0.15 M). However, c-fos transcription in the insular cortex was not enhanced by the gastric acid insult. Hypertonic saline (0.5 M) caused only a minor expression of c-fos mRNA in the hypothalamus and amygdala. The acid-evoked c-fos induction in subcortical nuclei was depressed by at least 80% five days after bilateral subdiaphragmatic vagotomy. Collectively, these observations indicate that vagal afferent input from the acid-threatened gastric mucosa does not reach the insular cortex but leads to activation of subcortical brain nuclei that are involved in emotional, behavioral, neuroendocrine, autonomic and antinociceptive reactions to a noxious stimulus.

Cholinergic, noradrenergic, and serotonergic inhibition of fast synaptic transmission in spinal lumbar dorsal horn of rat

It is known that spinal nociceptive sensory transmission receives descending inhibitory and facilitatory modulation from supraspinal structures. Glutamate is the major fast excitatory transmitter between primary afferent fibers and spinal dorsal horn neurons. In whole-cell patch clamp recordings from dorsal horn neurons in spinal slices, we investigated synaptic mechanisms for inhibitory modulation at the lumbar level of the spinal cord. Application of the cholinergic receptor agonist carbachol produced a dose-dependent inhibition of glutamate-mediated excitatory postsynaptic currents (EPSCs) (IC(50) 13 microM). Postsynaptic injection of two different types of G-protein inhibitors, guanosine 5'-O-2-thiophosphate or guanosine 5'-O-3-thiotriphosphate, blocked the inhibition produced by carbachol. Clonidine, a selective alpha-adrenergic receptor agonist, also produced a dose-dependent inhibition of EPSCs (IC(50) 7 microM) that was reduced by postsynaptic inhibition of G-proteins. The inhibitory effect of serotonin was likewise mediated by postsynaptic G-proteins. Our results suggest that activation of postsynaptic neurotransmitter receptors plays a critical role in inhibition of glutamate mediated sensory responses by acetylcholine, norepinephrine, and serotonin. Our results support the hypothesis that descending sensory modulation may be mediated by multiple neurotransmitter receptors in the spinal cord.

Factors that influence the persistence of stimulation-induced aversion

Brain stimulation reward in certain regions has been shown to produce analgesia to externally applied painful stimuli. In the present experiments, we studied how electrical self-stimulation of the dorsal raphe (DR) nucleus modifies the aversive effects of electrical stimulation of the nucleus reticularis gigantocellularis (Gi) or of the dorsal tegmentum (DTg). In the first study, the threshold for latency to escape aversive Gi stimulation was tracked before and after exposure to rewarding DR stimulation. Only a few sessions of DR self-stimulation were required to produce a complete and long-lasting inhibition of Gi aversion. In the second study, the aversion induced by DTg stimulation rapidly disappeared following a few test sessions at that site. Unlike our previous experience with Gi aversion that required either pairing with rewarding lateral hypothalamic (LH) or ventral tegmental area (VTA) pulses in order to increase the threshold for latency to escape Gi aversion, in this study, simply brief experience with rewarding DR stimulation in unpaired trials was sufficient to entirely suppress Gi-induced aversion. Even more surprising was the finding that unlike the Gi, aversion obtained from activation of the DTg does not persist, its threshold for escape quickly increases, and within a few sessions is no longer evident. One interpretation of these findings is that the aversion mechanisms associated with the Gi and DTg are differentially susceptible to analgesic processes.

Striatal modulation of the jaw opening reflex

The effect of striatal electrical and chemical conditioning stimulation (L-glutamate 80-160 nmoles/0.5 microl) on the jaw opening reflex (JOR) was studied in Sprague-Dawley male rats anesthetized with urethane. The JOR was evoked by stimulation of the tooth pulp of lower incisors. This response was suppressed by transection of the dental root, which indicates according with the bibliography, a specific activation of the pulp nerves. Three type of responses were obtained on the evoked JOR by conditioning stimulation of the striatum; being the main one the suppression of the reflex elicited by tooth pulp activation. A second type of response was an increase of the tooth-JOR amplitude. This effect was observed more frequently with glutamate stimulation rather than with electrical activation of the striatum. A third response was observed with chemical stimulation but not by electrical stimulation of the striatum. This was a triphasic response which consisted in an increase followed by an inhibition and a late increase of the tooth-JOR amplitude. A biphasic effect, an increase prior to a decrease of the JOR amplitude, was also recorded with a minor frequency. The distribution of effective sites for electrical and chemical stimulation within the striatum are mainly similar located in the rostral aspect of the nucleus, with the inhibitory sites in the middle of the nucleus and intermingled with the excitatory ones. The complex responses (tri/biphasic) were observed ventrally and caudally in the nucleus. On the basis of the results mentioned above, one could assume that the striatum is related to the modulation of the JOR evoked probably by nociceptive stimulation. However, activation of other type of fibers could not be ruled out.

Visceral pain in humans: lessons from animals

Acute and persistent neuropathic and inflammatory injuries of healthy animals have contributed importantly to our current understanding of nociception and pain. Studies have differentiated somatic from visceral nociceptive input, and elucidated the pathways of transduction, transmission, perception, and modulation of the input. Other animal studies have identified important genetic and environmental influences on responses to nociception. Studies of naturally occurring visceral pain syndromes in animals also have added to our understanding of comparable syndromes in humans. Because of the aversive nature of pain, use of healthy animals to study pain in the service of other animals and humans is a decision to be taken carefully, and carries with it the responsibility of treating the animals as humanely as possible.

Absence of spinal response to extracorporeal shock waves on the endogenous opioid systems in the rat

Extracorporeal shock wave therapy (ESWT) seems to be a new therapeutic strategy for chronic pain due to tendopathies. Neurophysiological mechanisms of action for pain relief following ESWT are still unknown. The aim of this study was to investigate if the analgesic effect of ESWT is caused by modulation of the endogenous spinal opioid system. Rats were treated with two different energy flux densities (0.04 and 0.11mJ/mm(2)) and immunohistochemical analysis of met-enkephalin (MRGL) and dynorphin (Dyn) was performed at 4 or 72 h after ESWT. ESWT had no modulatory influence on the expression of the spinal opioid systems. Different energy doses or repetitive treatment did not alter MRGL or Dyn immunoreactivity in the spinal cord. Furthermore, a delayed effect of ESWT at 72 h after treatment was not detectable. We conclude from these findings that the analgesic effects of ESWT treatment are not supported by endogenous opioids.

Expression of metabotropic glutamate receptors mRNA in the thalamus and brainstem of monoarthritic rats

Evidence for the involvement of metabotropic glutamate receptors (mGluR) in sensory processing has been emerging. Additionally, the differential distribution of distinct mGluR subtypes mRNA in particular thalamic nuclei of normal rats suggests that they could be involved in the processing of somatosensory information. In the present study, mGluR1, 3, 4 and 7 mRNAs expression was investigated by in situ hybridisation in selected brainstem and thalamic nuclei of adult monoarthritic rats at different time points of the disease (2, 4 and 14 days). Monoarthritic rats displayed behavioural and physical signs of painful arthritis at all time points. At 2 days of monoarthritis the mGluR1 mRNA expression was decreased mainly in the ventrobasal complex (VB) and in the posterior thalamic nuclei (Po) contralateral to the inflamed joint. The mGluR4 mRNA expression was also reduced, but minimum values were found at 4 days of monoarthritis, when no changes could be found in mGluR1 mRNA expression. At 14 days, mGluR4 mRNA expression was similar to controls, while mGluR1 mRNA was again reduced. Similar decreases of mGluR7 mRNA expression in the VB and Po were found at all time points, while mGluR3 mRNA expression was bilaterally increased in the reticular thalamic nucleus (Rt). In the brainstem no changes could be found in the expression of any mGluR subtype mRNA. The reduced expression of mGluR1, 4 and 7 transcripts in VB and Po, and the increases of mGluR3 mRNA in the Rt may contribute to counteract the increased noxious input arising from the periphery.

Non-serotonergic midbrain neurons are involved in picrotoxin-induced analgesia. An immunohistochemical study in the rat

Intravenous picrotoxin inhibits spinal nociceptive neurons through disinhibitory activation of neurons in the periaqueductal gray (PAG) and nucleus raphe dorsalis (NRD), where the descending antinociceptive system arises. We found Fos-like immunoreactivity in PAG/NRD neurons after intravenous injection of picrotoxin. This distribution of c-Fos expression is consistent with a role of PAG/NRD for antinociception; neurons with intense Fos-like immunoreactivity was also clustered in the Edinger-Westphal nucleus (EW). Double fluorescence immunohistochemistry for c-Fos and serotonin revealed that PAG/NRD/EW neurons expressing c-Fos were non-serotonergic. These data suggest that non-serotonergic PAG/NRD/EW neurons are involved in the picrotoxin-induced analgesia.

Activation of group I metabotropic glutamate receptors induces long-term depression at sensory synapses in superficial spinal dorsal horn

Low-frequency stimulation of primary afferent Adelta-fibers can induce long-term depression of synaptic transmission in rat superficial spinal dorsal horn. Here, we have identified another form of long-term depression in superficial spinal dorsal horn neurons that is induced by specific group I but not group II metabotropic glutamate receptor (mGluR) agonists. Synaptic strength between Adelta-fibers and dorsal horn neurons was examined by intracellular recordings in a spinal cord-dorsal root slice preparation from young rat. In the presence of bicuculline and strychnine, bath application of (1S,3R)-1-aminocyclopentane-1, 3-dicarboxylic acid ((1S,3R)-ACPD) or the specific group I mGluR agonist (S)-3,5-dihydroxyphenylglycine ((S)-3,5-DHPG) but not the specific group II mGluR agonist (2S,2'R,3'R)-2-(2', 3'-dicarboxycyclopropyl)glycine (DCG-IV) for 20 min produced an acute and a long-term depression of synaptic strength. Bath application of the N-methyl-D-aspartate receptor antagonist D-2-amino-5-phosphonovaleric acid did not affect these depressions by (1S,3R)-ACPD. After pre-incubation of slices with pertussis toxin, a G-protein inhibitor, (1S,3R)-ACPD still induced acute and long-term depressions. The phospholipase C inhibitor U73122 stereoselectively blocked the induction of long-term depression without affecting acute synaptic inhibition. This study demonstrates that, in the spinal cord, direct activation of group I mGluRs that are coupled to phospholipase C through pertussis toxin-insensitive G-proteins induces a long-term depression of synaptic strength. This may be relevant to the processing of sensory information in the spinal cord, including nociception.

Descending facilitatory modulation of a behavioral nociceptive response by stimulation in the adult rat anterior cingulate cortex

It is well documented that the descending endogenous analgesia system, including the periaqueductal gray (PAG) and the rostral ventral medulla (RVM), play an important role in modulation of nociceptive transmission and morphine- and cannabinoid-produced analgesia. Neurons in the PAG receive inputs from different nuclei of higher structures, including the anterior cingulate cortex (ACC). However, it is unclear if stimulation of neurons in the ACC modulates spinal nociceptive transmission. The present study has examined the effects of electrical stimulation and chemical activation of metabotropic glutamate receptors (mGluRs) in the ACC on a spinal nociceptive tail-flick (TF) reflex induced by noxious heating. Activation of the ACC at high intensities (up to 500 microA) of electrical stimulation did not produce any antinociceptive effect. Instead, at most sites within the ACC (n = 36 of 41 sites), electrical stimulation produced significant facilitation of the TF reflex (i.e. decreases in TF latency). Chemical activation of mGluRs within the ACC also produced a facilitatory effect. Descending facilitation from the ACC apparently relays at the RVM. Electrical stimulation in the RVM produces a biphasic modulatory effect, showing facilitation at low intensities and inhibition at higher intensities. The present study provides evidence that activation of mGluRs within the ACC can facilitate spinal nociception.

Emerging cell and molecular strategies for the study and treatment of painful peripheral neuropathies

Pharmacologic treatment for the symptoms of painful neuropathy has been problematic, because there has been limited understanding of the underlying etiologies and systemic levels that an effective dose can have on multiple side effects. The use of molecular methods, such as gene deletion from knockout mice and cellular minipumps for delivery of biologic antinociceptive molecules, has led to a better understanding of the underlying mechanisms of the induction of intractable neuropathic pain. The initiation of an excitatory cascade after injury or disease leads to the induction of various second messenger systems, loss or down-regulation of the endogenous inhibitory spinal GABA system and central sensitization, causing such pain. The development and use of cellular minipumps, immortalized cell lines bioengineered to secrete various antinociceptive molecules for the reversal of neuropathic pain, makes cellular therapy a strategy for clinical use in the next few years. The development of molecular "disimmortalization" technologies will make the use of such engineered cell lines safe for human use. Direct somatic gene transfer for neuropathic pain will eventually overcome the problems associated with transplantation of non-autologous and xenogenic cells. These virus-mediated methods, although at the early stages of evolution and use, offer large-scale production of biologic agents that can be conveniently and confidently used for the long-term relief of chronic neuropathic pain in a clinical setting, without systemic effects or surgical interventions.

The effect of periaqueductal gray lesions on responses to age-specific threats in infant rats

During early ontogeny infant rats show specific responses to a variety of age-dependent threatening situations. When isolated from nest and dam, they emit ultrasonic vocalizations and show decreased reactivity to noxious stimulation, or analgesia. When exposed to an unfamiliar adult male, they become immobile and analgesic. The midbrain periaqueductal gray (PAG) is an important area within the circuitry that controls responses to threatening stimuli in the adult. Little is known about the functions of the PAG in early life. It was hypothesized that the PAG mediates the responses to the age-specific threats social isolation and male exposure in the infant rat. Rat pups were lesioned electrolytically either in the lateral or the ventrolateral PAG on postnatal day 7, tested in social isolation on day 10, and exposed to a male on day 14. On day 10 during isolation, ultrasonic vocalizations and isolation-induced analgesia were decreased in both lesion groups. On day 14, male-induced immobility and analgesia were decreased in ventrally lesioned animals. In conclusion, the PAG seems to play a developmentally continuous role in age-specific responses to threat such as ultrasonic vocalization, analgesia, and immobility.