Thermographic observations on rats with experimental neuropathic pain

Orofacial anti-hypernociceptive effect of citral in acute and persistent inflammatory models in rats

Orofacial pain has significant psychological and physiological effects. Citral (3,7-dimethyl-2,6-octadienal) is the main component of Cymbopogon citratus (DC) Stapf, an herb with analgesic properties. Although citral has been considered a potent analgesic, its putative effects on orofacial pain are still unknown.

OBJECTIVE

The objective of this study is to test the hypothesis that citral modulates orofacial pain using two experimental models: formalin-induced hyperalgesia in the vibrissae area and during persistent temporomandibular hypernociception using Complete Freund's Adjuvant - CFA test.

METHODS

For the formalin test, citral (100 and 300 mg/kg, oral gavage) or its vehicle (Tween 80, 1 %) were given 1 h before the formalin injection subcutaneously (sc) into the vibrissae area. For the CFA model, we analyzed the prophylactic (100 mg/kg of citral by oral gavage, 1 h before CFA injection) and the chronic therapeutic (citral treatment 1-hour post-CFA injection and daily post-CFA injection) effect of citral or its vehicle in animals treated with CFA for 8 days.

RESULTS

Citral caused a decrease in formalin-induced local inflammation and the time spent performing nociceptive behavior in a dose-dependent fashion. Similarly, prophylactic and therapeutic citral treatment decreased the CFA-induced persistent mechanical hypernociception in the temporomandibular area.

CONCLUSION

Our data strengthen the notion that citral plays a powerful antinociceptive role by decreasing orofacial hypernociception in formalin and CFA models.

Learning pain in context: Response-conditioned placebo analgesia and nocebo hyperalgesia in male rats with chronic neuropathic pain

BACKGROUND

Animal models of placebo analgesia and nocebo hyperalgesia have great potential to assist in the development of novel treatments for chronic pain that exploit or inhibit these phenomena. This study sought to elicit both conditioned placebo analgesia and conditioned nocebo hyperalgesia in rats with chronic neuropathic pain using non-pharmacological, contextual conditioning approaches, similar to those most often used in humans.

METHODS

Sciatic nerve-injured male Sprague-Dawley rats (n = 80), and sham controls (n = 16), underwent a conditioning procedure in which three different thermal stimulus intensities (4 °C, 20 °C or 30 °C) were paired with contextual cues. Injured hind paw withdrawal behaviours were used to determine pain sensitivity, and either conditioned analgesia or conditioned hyperalgesia was evoked by re-exposing the rats to the same context with either an increased or decreased thermal stimulus, respectively.

RESULTS

Stronger conditioned analgesia and conditioned hyperalgesia were seen when rats were conditioned in a more complex environment, highlighting the importance of context in these processes. Rats that did not undergo conditioning procedures showed fewer hind paw withdrawals, indicating a learned component to these pain behaviours.

CONCLUSIONS

Our data call attention to context and learning as two critical factors in the development of placebo and nocebo effects in male rodents with a neuropathic injury. Additionally, the response-conditioning model we present in this study affords better comparisons between human and animal studies, in particular for those seeking to identify commonalities in the neurobiological mechanisms of placebo and nocebo responses.

Application of digital infrared thermography for carpal tunnel syndrome evaluation

We investigated the thermographic findings of carpal tunnel syndrome (CTS). We enrolled 304 hands with electrodiagnostically identified CTS and 88 control hands. CTS hands were assigned to duration groups (D1, < 3 months; D2, 3‒6 months; D3, 6‒12 months; D4, ≥ 12 months) and severity groups (S1, very mild; S2, mild; S3, moderate; S4, severe). The temperature difference between the median and ulnar nerve territories (ΔM-U territories) decreased as CTS duration and severity increased. Significant differences in ΔM-U territories between the D1 and D3, D1 and D4, D2 and D4, and S1 and S4 groups (P = 0.003, 0.001, 0.001, and < 0.001, respectively) were observed. Thermal anisometry increased as CTS duration and severity increased. Significant differences in thermal anisometry between the D1 and D4 as well as the D2 and D4 groups (P = 0.005 and 0.04, respectively) were noted. Thermal anisometry was higher in the S4 group than in the S1, S2, and S3 groups (P = 0.009, < 0.001, and 0.003, respectively). As CTS progresses, skin temperature tends to decrease and thermal variation tends to increase in the median nerve-innervated area. Thermographic findings reflect the physiological changes of the entrapped median nerve.

Effects of photobiomodulation therapy on neuropathic pain in rats: evaluation of nociceptive mediators and infrared thermography

Nerve injury induces release of peptides and upregulation of receptors such as substance P and transient receptor potential receptor V1 (TRPV1), which contribute to the development and maintenance of chronic pain. Photobiomodulation therapy (PBMT) is a nonpharmacological strategy that promotes tissue repair and reduces pain and inflammation. However, the molecular basis for PBMT effects on neuropathic pain is still unclear. We investigated the effects of PBMT on substance P, TRPV1, and superficial temperature change in a rodent model of neuropathic pain. We evaluated substance P and TRPV1 in dorsal root ganglia (DRG L4 to L6) at baseline, 14 days after chronic constriction injury (CCI) and after PBMT. We also assessed the superficial temperature of tarsal, metatarsal, tibia, and fibula regions before and after PBMT using infrared thermography. Substance P and TRPV1 levels increased in DRG of CCI rats compared to naive and sham rats and decreased after PBMT. Infrared thermography showed increased temperature of tarsal, metatarsal, tibia, and fibula regions in CCI rats, which was decreased after PBMT. There were no statistical differences between CCI rats with PBMT, sham, and naive rats in any assay. PBMT reduces nociceptive mediators and hind paw and leg's temperature in a rodent model of neuropathic pain, suggesting that PBMT may play a modulatory role in thermoregulation, neurogenic inflammation, and thermal sensitivity in peripheral nerve injuries. Therefore, PBMT appears to be a valuable strategy for neuropathic pain treatment in clinical settings.

Infrared thermal imaging associated with pain in laboratory animals

The science of animal welfare has evolved over the years, and recent scientific advances have enhanced our comprehension of the neurological, physiological, and ethological mechanisms of diverse animal species. Currently, the study of the affective states (emotions) of nonhuman animals is attracting great scientific interest focused primarily on negative experiences such as pain, fear, and suffering, which animals experience in different stages of their lives or during scientific research. Studies underway today seek to establish methods of evaluation that can accurately measure pain and then develop effective treatments for it, because the techniques available up to now are not sufficiently precise. One innovative technology that has recently been incorporated into veterinary medicine for the specific purpose of studying pain in animals is called infrared thermography (IRT), a technique that works by detecting and measuring levels of thermal radiation at different points on the body’s surface with high sensitivity. Changes in IRT images are associated mainly with blood perfusion, which is modulated by the mechanisms of vasodilatation and vasoconstriction. IRT is an efficient, noninvasive method for evaluating and controlling pain, two critical aspects of animal welfare in biomedical research. The aim of the present review is to compile and analyze studies of infrared thermographic changes associated with pain in laboratory research involving animals.

Skin temperature contribution to the decrease in withdrawal latency following chronic constriction injury

BACKGROUND

Chronic constriction injury (CCI) is widely used as an animal neuropathic pain model. Neuropathic pain is considered to exist when withdrawal latency to thermal stimulation is decreased after inducing a CCI to the sciatic nerve. However, it is known that CCI leads to changes in skin temperature and that skin temperature can affect withdrawal latency. Aim of this study was to compare withdrawal latencies of constricted and contralateral hind limbs, to thermal stimulation, at the same artificially-induced skin temperatures.

METHODS

Neuropathic pain was induced by four ligatures on the left sciatic nerve in adult male Wistar rats. Withdrawal latencies were measured from the 11th to 14th day after ligation, in different ambient temperatures, using the plantar test (Hargreaves method). By changing ambient we produced different hind limb skin temperatures.

RESULTS

Our results show that (1) CCI cause an increase in skin temperature; (2) the withdrawal latency was inversely related to ambient and skin temperature in the same manner for both the ligated and contralateral hind limbs; and (3) withdrawal latencies did not differ significantly for the ligated and contralateral hind limbs when the temperature of the hind limbs was artificially made the same (i.e., by changing the ambient temperature).

CONCLUSIONS

Withdrawal latencies to thermal stimulation did not differ on ligated and contralateral hind limb after CCI to the sciatic nerve if the temperature of the hind limbs was artificially or mathematically made the same. This finding may have significant impact on the interpretation results of neuropathic pain research.

Reconstruction of a 10-mm-long median nerve gap in an ischemic environment using autologous conduits with different patterns of blood supply: A comparative study in the rat

The aim of this study was to evaluate in the Wistar rat the efficacy of various autologous nerve conduits with various forms of blood supply in reconstructing a 10-mm-long gap in the median nerve (MN) under conditions of local ischemia. A 10-mm-long median nerve defect was created in the right arm. A loose silicone tube was placed around the nerve gap zone, in order to simulate a local ischemic environment. Rats were divided in the following experimental groups (each with 20 rats): the nerve Graft (NG) group, in which the excised MN segment was reattached; the conventional nerve flap (CNF) and the arterialized neurovenous flap (ANVF) groups in which the gap was bridged with homonymous median nerve flaps; the prefabricated nerve flap (PNF) group in which the gap was reconstructed with a fabricated flap created by leaving an arteriovenous fistula in contact with the sciatic nerve for 5 weeks; and the two control groups, Sham and Excision groups. In the latter group, the proximal stump of the MN nerve was ligated and no repair was performed. The rats were followed for 100 days. During this time, they did physiotherapy. Functional, electroneuromyographic and histological studies were performed. The CNF and ANVF groups presented better results than the NG group in the following assessments: grasping test, nociception, motor stimulation threshold, muscle weight, and histomorphometric evaluation. Radial deviation of the operated forepaw was more common in rats that presented worse results in the other outcome variables. Overall, CNFs and ANVFs produced a faster and more complete recovery than NGs in the reconstruction of a 10-mm-long median nerve gap in an ischemic environment in the Wistar rat. Although, results obtained with CNFs were in most cases were better than ANVFs, these differences were not statistically significant for most of the outcome variables.

Skin temperature changes following sciatic nerve injury in rats

In the clinical setting, skin temperature is both easily evaluated and useful in assessments of sympathetic dysfunction. The present study purposed to observe the serial skin temperature changes of both hindlimbs following several types of sciatic nerve injury (complete transection and ligation model [CTL], crush injury model [CRI], and chronic constriction injury model [CCI]) in Sprague-Dawley rats and, further, to delineate the possible mechanisms through various evaluation methods. The temperature differences between the intact and injured areas (ΔT) on the plantar surface and toes varied among the CTL, CRI, and CCI injury models during the acute stage (7 days post-injury). During the subacute to chronic stages (7-28 days post-injury), ΔT on the plantar area and toes of the CCI model were higher than those of the CTL and CRI models. The sciatic functional index was gradually restored in the CRI and CCI models, but was unchanged in the CTL model. The CTL model showed constant hypoesthesia; the CRI model, contrastingly, was restored to normal, and the CCI model showed gradual hyperesthesia until 28 days post-injury. The latency and amplitude of the compound muscle action potential (CMAP) in the involved plantar muscle was not found in the CTL group 4 weeks post-injury, but showed gradual restoration in the CRI and CCI models. Regression analysis revealed that the ΔT in the plantar area and toes were affected only by the CMAP amplitude in the involved plantar muscle; therefore, it can be said that the skin temperature on the injured area after sciatic nerve injury was influenced by the functional status of the involved muscle. Measurement of skin temperature can differentiate mild peripheral nerve injury from moderate-to-severe injuries, although its clinical significance might be limited.

Thermographic evaluation of hind paw skin temperature and functional recovery of locomotion after sciatic nerve crush in rats

INTRODUCTION

Peripheral nerves are often damaged by direct mechanical injury, diseases, and tumors. The peripheral nerve injuries that result from these conditions can lead to a partial or complete loss of motor, sensory, and autonomic functions, which in turn are related to changes in skin temperature, in the involved segments of the body. The aim of this study was to evaluate the changes in hind paw skin temperature after sciatic nerve crush in rats in an attempt to determine whether changes in skin temperature correlate with the functional recovery of locomotion.

METHODS

Wistar rats were divided into three groups: control (n = 7), sham (n = 25), and crush (n = 25). All groups were subjected to thermographic, functional, and histological assessments.

RESULTS

ΔT in the crush group was different from the control and sham groups at the 1st, 3rd and 7rd postoperative days (p<0.05). The functional recovery from the crush group returned to normal values between the 3rd and 4th week post-injury, and morphological analysis of the nerve revealed incomplete regeneration at the 4th week after injury.

DISCUSSION

This study is the first demonstration that sciatic nerve crush in rats induces an increase in hind paw skin temperature and that skin temperature changes do not correlate closely with functional recovery.

Painful peripheral neuropathies

Peripheral neuropathies are a heterogeneous group of diseases affecting peripheral nerves. The causes are multiple: hereditary, metabolic, infectious, inflammatory, toxic, traumatic. The temporal profile includes acute, subacute and chronic conditions. The majority of peripheral neuropathies cause mainly muscle weakness and sensory loss, positive sensory symptoms and sometimes pain. When pain is present, however, it is usually extremely intense and among the most disabling symptoms for the patients. In addition, the neurological origin of the pain is often missed and patients receive inadequate or delayed specific treatment. Independently of the disease causing the peripheral nerve injury, pain originating from axonal pathology or ganglionopathy privileges neuropathies affecting smaller fibres, a clinical observation that points towards abnormal activity within nociceptive afferents as a main generator of pain. Natural activation of blood vessels or perineurial nociceptive network by pathology also causes intense pain. Pain of this kind, i.e. nerve trunk pain, is among the heralding symptoms of inflammatory or ischemic mononeuropathy and for its intensity represents itself a medical emergency. Neuropathic pain quality rekindles the psychophysical experience of peripheral nerves intraneural microstimulation i.e. a combination of large and small fibres sensation temporally distorted compared to physiological perception evoked by natural stimuli. Pins and needles, burning, cramping mixed with numbness, and tingling are the wording most used by patients. Nociceptive pain instead is most often described as aching, deep and dull. Good command of peripheral nerve anatomy and pathophysiology allows timely recognition of the different pain components and targeted treatment, selected according to intensity, type and temporal profile of the pain.

The assessment of cold hyperalgesia after an incision

BACKGROUND

Although cold hypersensitivity is a well-documented phenomenon in animals and humans with inflammatory and neuropathic pain, little is known about the presence of cold hyperalgesia after surgery. Therefore, we studied primary cold hyperalgesia after a surgical incision in mice.

METHODS

Before and after plantar incision, inflammation with complete Freund adjuvant, and spared nerve ligation, unrestrained male animals were placed on a Peltier-cooled cold plate with a surface temperature of 0 degrees C and withdrawal latencies were measured. Additionally, incision-induced cold hyperalgesia was also assessed in female animals. Furthermore, skin temperature before and after plantar incision and inflammation were assessed by using infrared thermography (Varioscan LW 3011; Infratec, Dresden, Germany).

RESULTS

Cold hyperalgesia to a noxious cold stimulus was observed after inflammation and nerve injury but not after a surgical incision. Similar results were demonstrated for female animals after incision. Furthermore, a significant increase in skin temperature was recorded after inflammation but not after incision, indicating that a surgery evokes only minor inflammatory effects.

CONCLUSION

The present data give strong evidence that a surgical incision does not cause cold hyperalgesia. Furthermore, a lack of cold hyperalgesia in unrestrained male and female mice after incision was not due to increased skin temperature after incision. Finally, we demonstrated that in contrast to a surgical incision, inflammation and nerve injury generate intense cold hyperalgesia and an increase in skin temperature, suggesting that different mechanisms are involved in surgical and inflammatory or neuropathic pain.

Changes in cardiovascular parameters and plasma norepinephrine level in rats after chronic constriction injury on the sciatic nerve

To evaluate whether neuropathic pain affects autonomic nervous activities, we investigated daily change in cardiovascular parameters and plasma norepinephrine (NE) in free-moving rats after chronic constriction injury (CCI) on the sciatic nerve. Arterial blood pressure (BP), heart rate (HR), and the power spectrum of pulse interval variability were analyzed. Daily change in motor activity and nociceptive behavior was also measured from some CCI rats. In others, NE from daily blood samples was quantified and spontaneous pain was evaluated by daily monitoring of foot guarding behavior. We identified three stages in the daily change of cardiovascular parameters and plasma NE level over 3 weeks following CCI. The first stage (up to 3 days after the surgery) was characterized by increased MAP and HR, especially in the daytime, even though plasma NE was unchanged and motor activity decreased. The second stage (mid first to mid second postoperative weeks) was characterized by increased daytime MAP and HR, and the animals developed punctate hyperalgesia in the affected hindpaw. An NE surge that may have been related to spontaneous pain was present 3-5 days after CCI. The third stage, which appeared after the second postoperative week, was characterized by normalized MAP and decreased HR, and increased high-frequency (0.8-3.0Hz) power in pulse interval variability, which is an index of cardiac parasympathetic tone. These results demonstrated that cardiovascular function was kept high through sympathetic and non-sympathetic activity for 2 weeks after CCI, followed by a predominance of parasympathetic tone.

Efficacy of Stellate Ganglion Blockade for the Management of Type 1 Complex Regional Pain Syndrome

INTRODUCTION

The purpose of this study was to examine the efficacy of stellate ganglion blockade (SGB) in patients with complex regional pain syndromes (CRPS I) of their hands.

METHODS

After IRB approval and patient informed consent, 25 subjects, with a clinical diagnosis of CRPS I of one hand as defined by the International Association for the Study of Pain (IASP) criteria, had three SGB's performed at weekly intervals. Laser Doppler fluxmetric hand perfusion studies were performed on the normal and CRPS I hands pre- and post-SGB therapy. No patient was included in this study if they used tobacco products or any medication or substance that could affect sympathetic function. The appropriate parametric and nonparametric data analyses were performed and a p value <0.05 was used to reject the null hypothesis.

RESULTS

Symptom onset of CRPS I until the initiation of SGB therapy ranged between 3 to 34 weeks. Following the SGB series, patient pain relief was as follows: group I, 10/25 (40%) had complete symptom relief; group II, 9/25 (36%) had partial relief and group III, 6/25 (24%) had no relief. The duration of symptoms until SGB therapy was: group I, 4.6 +/- 1.8 weeks, group II, 11.9 +/- 1.6 weeks and group III, 35.8 +/- 27 weeks. Compared with the normal control hand, the skin perfusion in the CRPS I affected hand was greater in group I and decreased in groups II and III.

DISCUSSION

The results of our study demonstrate that an inverse relationship exists between hand perfusion and the duration of symptoms of CRPS I. On the other hand, a positive correlation exists between SGB efficacy and how soon SGB therapy is initiated. A duration of symptoms greater than 16 weeks before the initial SGB and/or a decrease in skin perfusion of 22% between the normal and affected hands adversely affects the efficacy of SGB therapy.

Segmental regulation pattern of body surface temperature in the rat hindlimb

Body surface zones or 'thermatomes', whose temperature is regulated by a single spinal segment, were investigated by thermography in the rat hindlimb. First, the spatial relation between the dermatome delineated by dye extravasation and the corresponding thermatome was investigated in rats pretreated with intravenous application of Evans blue. Electrical stimulation of the spinal nerves and sympathetic trunk segments at L3 and L5 induced a distal dominant temperature decrease. In contrast, Evans blue extravasation appeared in the medial (in L3) and lateral (in L5) paw only by electrical stimulation of the spinal nerves. Second, thermatomes L1-L5 were determined in other rats. Electrical stimulation of the sympathetic trunk segments L1-L6 produced a temperature decrease in the abdomen, hindlimb, and tail. However, the hindlimb temperature was regulated mainly by L2-L5 levels, particularly by L4 and L5. The abdomen was regulated uniformly by L1-L6, and the tail by L6. It was demonstrated that thermatomes are manifested differently from the corresponding dermatomes in the rat hindlimb.

Neuropathic pain: clinical characteristics and diagnostic workup

Neuropathic pain is part of the neurological disease spectrum and may be an expression of severe medical pathology. Painful neuropathies have multiple disguises and may to a certain extent be mimicked by non-neurological pain conditions. Painful neuropathic conditions express themselves with spontaneous and/or abnormal stimulus-evoked pain. The diagnosis of peripheral or central neuropathic pain should be made only when the history and signs are indicative of neuropathy in conjunction with neuroanatomically correlated pain distribution and sensory abnormalities within the area of pain. A future mechanism-based classification of pain has recently been suggested to facilitate the development of mechanism-tailored treatment strategies. This is a sound approach and should be pursued. It is mandatory, however, to retain the traditional organ-based diagnostic workup, which should precede further in-depth characterization of specific pain mechanisms. Extensive preparatory work is needed on how to link certain symptoms and signs to specific mechanisms, as elucidated from animal studies, before we can introduce mechanism-coupled treatment strategies.

Effects of lowering barometric pressure on guarding behavior, heart rate and blood pressure in a rat model of neuropathic pain

We investigated whether lowering barometric pressure by 20 mmHg (LP) aggravates the guarding behavior suggestive of spontaneous pain following sciatic nerve chronic constriction injury (CCI) in rats. Systemic blood pressure (BP) and heart rate (HR) of unrestrained rats were recorded telemetrically during LP both before and after the CCI surgery. CCI rats showed guarding posture in normopressure conditions, and LP increased the cumulative time of this behavior. Baseline BP but not HR was increased following CCI. LP increased BP and HR of the rats only before the CCI surgery. Animals after CCI surgery showed variable (BP, HR) and transient (HR) responses to LP. These results indicate that (1) LP aggravated spontaneous pain and increased BP and HR in the CCI rats, and (2) CCI surgery influenced BP and HR of rats.

Application of nucleus pulposus to the nerve root simultaneously reduces blood flow in dorsal root ganglion and corresponding hindpaw in the rat

STUDY DESIGN

An experimental study to clarify the effects of nucleus pulposus on blood flow in the dorsal root ganglion and hindpaws.

OBJECTIVES

To investigate the effects of application of nucleus pulposus to nerve root on blood flow in the dorsal root ganglion and the corresponding hindpaw.

SUMMARY OF BACKGROUND DATA

It has been reported experimentally that application of nucleus pulposus into the epidural space induces morphologic and functional changes in the nerve roots and induces compartment syndrome in the dorsal root ganglia. However, it has not been clarified which of these changes induces symptoms in the lower limbs.

METHODS

Sixteen adult, female Sprague-Dawley rats had the left L5 nerve root and associated dorsal root ganglions exposed. Autologous nucleus pulposus was applied to the L5 nerve root, just proximal to the dorsal root ganglion (NP group). For control, the same volume of muscle tissue was applied similarly to the neural tissue (control group). Blood flow in the dorsal root ganglion, corresponding hindpaw, and the contralateral hindpaw was continuously monitored by two-channel laser Doppler flowmeter for 3 hours. After measurement of blood flow, the nerve root and dorsal root ganglion were processed for histology and evaluated by light microscope.

RESULTS

Blood flow in the NP group was reduced, not only in the dorsal root ganglion, but also in the corresponding hindpaw. These reductions were statistically significant compared with the control group (P < 0.01). Edema was the principal pathologic finding seen consistently in the nerve roots and in many of the associated dorsal root ganglia from nucleus pulposus-treated animals.

CONCLUSION

Application of nucleus pulposus to nerve root decreased blood flow in the dorsal root ganglion and corresponding hindpaw. These basic pathophysiologic changes are associated with compression injuries caused by herniated discs and are accepted neuropathologic mechanisms of injury associated with painful neuropathies. These acute observations in the dorsal root ganglion and the hindpaw may be important initial factors in the pathogenesis of radicular leg pain (sciatica) due to disc herniation.

Reflex sympathetic dystrophy: facts and hypotheses

Reflex sympathetic dystrophy (RSD) syndrome has been recognized clinically for many years. It is most often initiated by trauma to a nerve, neural plexus, or soft tissue. Diagnostic criteria are the presence of regional pain and other sensory changes following a noxious event. The pain is associated with changes in skin colour, skin temperature, abnormal sweating, oedema, and sometimes motor abnormalities. The clinical course is commonly divided into three stages: first (acute or hyperaemic), second (dystrophic or ischaemic), and third (atrophic) stage. The diagnosis is primarily clinical, but roentgenography, scintigraphy, thermography, electromyography and assessment of nerve conduction velocity can help to confirm the diagnosis. Although a wide variety of treatments have been recommended, the only therapies found to be effective in large studies aim at interfering with the activity of the sympathetic nervous system. To this end, efferent sympathetic nerve activity can be interrupted surgically or chemically. Alternatively, adrenoceptor blockers may be used to relieve pain. Numerous theories have been proposed to explain the pathophysiology. Sympathetic dysfunction, which often has been purported to play a pivotal role in RSD, has been suggested to consist of an increased rate of efferent sympathetic nerve impulses towards the involved extremity induced by increased afferent activity. However, the results of several experimental studies suggest that sympathetic dysfunction consists of supersensitivity to catecholamines induced by (partial) autonomic denervation. Besides, it has been suggested that excitation of sensory nerve fibres at axonal level causes release of neuropeptides at the peripheral endings of these fibres. These neuropeptides may induce vasodilation, increase vascular permeability, and excite surrounding sensory nerve fibres -- a phenomenon referred to as neurogenic inflammation. At the level of the central nervous system, it has been suggested that the increased input from peripheral nociceptors alters the central processing mechanisms.

Impairment level sumscore in reflex sympathetic dystrophy of one upper extremity

OBJECTIVES

To construct a set of instruments to measure the level of impairment in reflex sympathetic dystrophy (RSD), to convert the scores for each instrument into one score and to derive a compounded impairment level sumscore (ISS), and to evaluate reliability, validity, and responsiveness of the ISS.

DATA SOURCES AND EXTRACTION

Literature search in Medline (1985 to 1995), non-Medline journals, and references from these reports for measurement parameters and suitable, reliable, responsive, and validated instruments.

DATA SYNTHESIS

Four measurement parameters (pain, active range of motion, temperature, and volume) and five measurement instruments were chosen to construct the ISS. All outcomes were converted into one score, resulting in an addable score.

RESULTS

Forty-five patients with RSD of one upper extremity were evaluated. As expected for this heterogeneous index, the internal consistency of the ISS was poor to moderate. The ISS seemed to have content and concurrent validity. The responsiveness of the ISS was adequate.

CONCLUSIONS

A multicomponent test was constructed to map alterations in the level of impairment in RSD patients. Each of the parameter scores could be translated into a uniform score. Adding the scores yielded the ISS.

The cold plate as a test of nociceptive behaviors: description and application to the study of chronic neuropathic and inflammatory pain models

A cold plate apparatus was designed to test the responses of unrestrained rats to low temperature stimulation of the plantar aspect of the paw. At plate temperatures of 10 degrees C and 5 degrees C, rats with either chronic constriction injury (CCI) of the sciatic nerve or complete Freund's adjuvant (CFA) induced inflammation of the hindpaw displayed a stereotyped behavior. Brisk lifts of the treated hindpaw were recorded, while no evidence of other nociceptive behaviors could be discerned. The most consistent responses were obtained with a plate temperature of 5 degrees C in three 5-min testing periods, separated by 10-min intervals during which the animals were returned to a normal environment. Concomitantly to cold testing, the rats were evaluated for their response to heat (plantar test) and mechanical (von Frey hairs) stimuli. In both injury models, while responses to heat stimuli had normalized at 60 days post-injury, a clear lateralization of responses to cold was observed throughout the entire study period. Systemic lidocaine, clonidine, and morphine suppressed responses to cold in a dose-related fashion. At doses that did not affect motor or sensory behavior, both lidocaine and its quaternary derivative QX-314 similarly reduced paw lifts, suggesting that cold hyperalgesia is in part due to peripheral altered nociceptive processing. Clonidine was more potent in CCI then in CFA rats in reducing the response to cold. Paradoxically, clonidine increased the withdrawal latencies to heat in the CCI hindpaw at 40 days and thereafter, at a time when both hindpaws had the same withdrawal latencies in control animals. Morphine was also more potent on CCI than CFA cold responses, indicating that, chronically, CFA-induced hyperalgesia might be opiate resistant. Evidence for tonic endogenous inhibition of cold hyperalgesia was obtained for CFA rats, when systemic naltrexone significantly increased the number of paw lifts; this was not found in rats with CCI. At 60 days, neither morphine nor naltrexone affected cold-induced paw lifting in CFA rats, suggesting that the neuronal circuit mediating cold hyperalgesia in these animals had become opiate insensitive. In conclusion, the cold plate was found to be a reliable method for detecting abnormal nociceptive behavior even at long intervals after nerve or inflammatory injuries, when responses to other nociceptive stimuli have returned to near normal. The results of pharmacological studies suggest that cold hyperalgesia is in part a consequence of altered sensory processing in the periphery, and that it can be independently modulated by opiate and adrenergic systems.

Afferent activity from myelinated inferior alveolar nerve fibers in ferrets after constriction or section and regeneration

To investigate possible peripheral mechanisms for post-injury sensory disorders in the trigeminal system, we have made electrophysiological recordings from myelinated fibres in the inferior alveolar nerve (IAN) which have previously sustained an injury. In earlier experiments we have shown that axons in ligature-induced neuromas of the IAN develop spontaneous activity and mechanical sensitivity. The present study has investigated these responses after two different types of injury. In 24 anaesthetised adult male ferrets the left IAN was either chronically constricted by four loose chromic gut ligatures (12 animals) or sectioned and regeneration permitted (12 animals). After recovery periods of 3 days, 1, 3, 6, 12 or 24 weeks, single unit recordings were made from the nerve proximal to the injury site. The proportion of units which were spontaneously active ranged from 0% to 19% after constriction injury and from 0% to 10% after nerve section and regeneration. Both groups revealed a marked variability between individual animals at similar time periods. Mechanical sensitivity was found in 0-42% of units after constriction and 0-25% of units after nerve section; both groups showed a significant negative correlation between mechanical sensitivity and recovery period. None of the fibres which had regained peripheral receptive fields was either spontaneously active or mechanically sensitive. There was no significant difference between the levels of spontaneous activity or mechanical sensitivity in the two groups or that previously found in ligature-induced neuromas. Thus we conclude that widely differing types of peripheral nerve injury are capable of initiating similar raised levels of afferent activity in myelinated inferior alveolar nerve fibres.

Skin blood flow abnormalities in a rat model of neuropathic pain: result of decreased sympathetic vasoconstrictor outflow?

Loose ligation of a sciatic nerve in rats provokes signs and symptoms like those observed in human conditions of neuropathic pain. Some of these have been associated with sympathetic dysfunction. Since the skin microcirculation in the rat is strongly influenced by sympathetic tone, abnormalities in skin blood flow may be used as an indirect measure of sympathetic dysfunction. We measured, by means of laser Doppler flowmetry, skin blood flow at the plantar surface of the rat hind paw before and after ipsilateral loose sciatic nerve ligation. We assessed basal skin blood flow as well as the vasoconstrictor response which follows cooling of the rat abdomen. The effectiveness of this response may be used as a measure of sympathetic vasoconstrictor outflow. As compared to the values obtained before ligation (= 100%): (1) the vasoconstrictor response was impaired (65%, P < 0.01) from day 1 onwards, whereas (2) basal skin blood flow was increased (171%; P < 0.01) from day 3 until day 5, and decreased (51%, P < 0.0001) from day 7 until day 28. At day 28, blockade of impulse propagation in the loosely ligated sciatic nerve (by means of lidocaine) did not increase the lowered level of skin blood flow. These findings suggest that in the chronic construction injury model loose ligation of a sciatic nerve reduces sympathetic vasoconstrictor outflow, which, in turn may induce supersensitivity of skin microvessels to catecholamines.

Effect of intravenous sodium amytal on cutaneous limb temperatures and sympathetic skin responses in normal subjects and pain patients with and without Complex Regional Pain Syndromes (type I and II). I

This study examined the effects of intravenous administration of sodium amytal (SA), a medium action barbiturate, on cutaneous limb temperatures and sympathetic skin responses (SSR) to electrical stimulation. Eight normal volunteers and 13 patients with musculoskeletal pain, somatoform pain disorders or nerve/root injury (with findings strictly limited to the distribution of the distribution of the involved nerve) were compared to 15 patients with Complex Regional Pain syndromes (one of whom had documented nerve injury). The Complex Regional Pain Syndromes (CRPS) patients were characterized by the presence of severe diffuse limb pain and extraterritorial sensory, sudomotor and vasomotor abnormalities (i.e., not confined to the site of injury or the distribution of the injured nerve). The CRPS patients were different from the normal controls and the non-CRPS patients in their tendency to warm significantly many of their limbs (not just the symptomatic ones). SSR were reduced or lost in a few limbs only in all three groups, irrespective of the increase or decrease of limb temperature and the side of symptoms. We argue that the enhanced thermogenic effect of SA in CRPS patients is due to generalized central changes of thermoregulatory control specifically in this group.

Influence of partial nerve injury in the rat on efferent function of sympathetic and antidromically acting sensory nerve fibers

OBJECTIVE

To investigate how partial injury of a large peripheral nerve affects efferent (vasomotor) function of sympathetic and antidromically acting sensory nerve fibers.

DESIGN

Randomized animal study.

MATERIALS AND METHODS

We assessed, by laser Doppler flowmetry, skin blood flow (SBF) in the hindpaw of male Lewis rats before partial injury of the ipsilateral sciatic nerve (through loose ligation) as well as at an early stage (day 4) and at a later stage (day 21) after this procedure. This procedure has been reported to induce signs and symptoms like those observed in patients with causalgia. At the two time points after nerve injury, SBF was assessed before and after (chemical) blockade of sensory and nonsensory (sympathetic) sciatic nerve fibers. Furthermore, at day 21 we measured the density of sympathetic nerve fibers in footpad arteries.

MEASUREMENTS AND MAIN RESULTS

At day 4, compared with preligation values, we observed an increase in SBF that was reduced by blockade of sensory nerve fibers. Subsequent blockade of nonsensory nerve fibers further reduced SBF. At day 21, SBF was decreased compared with preligation values. Blockade of sensory nerve fibers further reduced SBF, and subsequent blockade of nonsensory nerve fibers did so as well. The density of sympathetic nerve fibers was lower on the ligated side than on the nonligated side.

CONCLUSIONS

Partial injury of the rat sciatic nerve causes an ipsilateral increase in SBF at an early stage, which is followed by a decrease at a later stage. At both stages, antidromically acting sensory and orthodromically acting nonsensory (sympathetic) nerve fibers are involved in the vasodilator response. At a later stage, however, neurogenic vasodilator mechanisms are overruled by a nonneurogenic vasoconstrictor mechanism. The latter may consist of supersensitivity of skin microvessels to catecholamines consequent to reduced neurogenic disposition of catecholamines.

Changes in CM and CAP with sedation and temperature in the guinea pig: facts and interpretation

The influence of xylazine on the amplitude, latency and waveform of VIIIth nerve compound action potential (CAP) and cochlear microphonic (CM) in response to clicks at 95 dB SPL in normal awake preimplanted guinea pigs was investigated. The animals' temperature was monitored but no thermoregulation was exerted, except in one control experiment. Following a 0.2 ml injection of xylazine, CM showed minor variations while CAP audiograms for tone pips between 0.5 and 25 kHz remained normal. However, a progressive decrease in temperature and a strongly correlated increase in CAP amplitude and in N1 and N2 latencies were noticed. For peak N1 the changes were equivalent to linear amplitude and time expansions, and could be reproduced through CAP synthesis with convolution methods using time expanded unit response model and firing density functions. All changes were maximal after 2 h of sedation and recovered within approximately another 2 h. Whereas xylazine is known to induce hypothermia, all the changes disappeared if the animal was thermoregulated. Therefore the changes are interpreted as a result of hypothermia. The mechanism of N1 latency lengthening and increase in amplitude during hypothermia can be understood as a simultaneous increase in spike duration, hair cell/nerve synaptic delay and postsynaptic time constant. This hypothesis yielded a theoretical temperature coefficient for N1 latency (-52 microseconds/degree C) matching that measured experimentally (-55 microseconds/degree C). When compared with peak N1, peak N2 appeared relatively more expanded. Arguments about the origin of N2 are discussed.

Venlafaxine hydrochloride (Effexor) relieves thermal hyperalgesia in rats with an experimental mononeuropathy

Venlafaxine hydrochloride (Effexor) is a structurally novel antidepressant that inhibits reuptake of 5-hydroxytryptamine and noradrenaline, but unlike the older antidepressants, has few side-effects. The objective of this study was to determine whether venlafaxine relieves thermal hyperalgesia in rats with neuropathic pain due to chronic constriction injury (CCI) of the sciatic nerve. Paw withdrawal latency (PWL) to heat was tested for each hind paw. A painful neuropathy was induced in 24 male Sprague-Dawley rats (Group 1) as described by Bennett and Xie. Rats randomly received either oral venlafaxine (22 mg/kg) or placebo via gavage feeding beginning the day after surgery. Postoperative PWL testing began 3 days after CCI (Time 0). A second group of 12 rats (Group 2) was used to confirm that venlafaxine reverses hyperalgesia in rats with a fully developed neuropathic lesion. These animals began to receive oral venlafaxine (22 mg/kg) starting on the third postoperative day, after the presence of thermal hyperalgesia was verified through PWL testing. Testing was continued for 5 days, during venlafaxine administration. A third group of 12 rats (Group 3) had activity measured before and after treatment with venlafaxine (22 mg/kg). Rats in the placebo group developed thermal hyperalgesia while those that received venlafaxine did not. Venlafaxine also appeared to have a mild non-specific analgesic effect that increased PWL in the sham limb. In Group 2, thermal hyperalgesia was present on day 3, but following treatment with venlafaxine, thermal hyperalgesia resolved. Activity measurements confirmed that venlafaxine was not sedating in this rat model.

Skin blood flow disturbances in the contralateral limb in a peripheral mononeuropathy in the rat

Electrical excitation of nociceptive afferents in an extremity has been demonstrated to increase skin blood flow in the contralateral extremity. Hence, one would expect that loose sciatic nerve ligation, which induces an experimental painful peripheral neuropathy, may also provoke a vasodilator response in the contralateral hindpaw. On the non-ligated side, such a response may involve inhibited skin vasoconstrictor activity as well as neurogenically mediated active vasodilation. We studied skin blood flow changes in the rat hindpaw consequent to contralateral loose sciatic nerve ligation. After ligation, we also investigated whether blockade of afferent input from the ligated sciatic nerve to the spinal cord, by means of lidocaine, overrules the vasodilator response in the non-ligated paw. On the non-ligated side, we assessed the vasoconstrictor response of skin microvessels to cooling of the rat abdomen as a measure of skin vasoconstrictor activity in this paw. In order to investigate the involvement of sensory and/or non-sensory nerve fibers in the non-ligated sciatic nerve on skin blood flow abnormalities in the non-ligated paw, we studied the influence of blockade of these fibers through successive capsaicin and lidocaine application. We show that loose ligation of the sciatic nerve induces a vasodilator response in the contralateral hindpaw, which is completely abolished by blockade of afferent input from the ligated sciatic nerve. From day 1 after ligation, skin vasoconstrictor activity in the non-ligated paw was reduced, as indicated by an impaired vasoconstrictor response to cooling of the rat abdomen. Besides, blockade of sensory but not of non-sensory nerve fibers on the non-ligated side attenuated the vasodilator response in this paw. The data presented here indicate that loose ligation of the rat sciatic nerve induces a vasodilator response in the contralateral hindpaw. On the non-ligated side, this vasodilator response may involve inhibition of skin vasoconstrictor activity, as well as antidromically acting sensory nerve fibers.

Reflex sympathetic dystrophy: skin blood flow, sympathetic vasoconstrictor reflexes and pain before and after surgical sympathectomy

To examine the pathophysiological mechanisms of vascular disturbances and to assess the role of the sympathetic nervous system, 12 patients with reflex sympathetic dystrophy (RSD) of the hand were studied using laser Doppler flowmetry. Cutaneous blood flow, skin resistance and skin temperature were measured at the affected and contralateral hands. Sympathetic vasoconstrictor reflexes were induced bilaterally by deep inspiration. Four patients were treated with unilateral surgical sympathectomy and pain and vascular changes were documented in follow-up investigations. (1) After acclimatization in cold environment (< or = 18 degrees C) blood flow and skin temperature were considerably lower on the affected side in 10 patients. No additional vasoconstrictor reflexes could be elicited. (2) After acclimatization in warm environment (22-24 degrees C) blood flow and skin temperature demonstrated no side differences in all cases. Vasoconstrictor responses were the same on both sides. (3) After sympathectomy vasoconstrictor reflexes were absent. Skin resistance was considerably higher on the affected side. In the first 4 weeks the affected hand was warmer and blood flow was higher compared with the healthy side. Thereafter, skin temperature and perfusion slowly decreased and the affected hand turned from warm to cold. Very regular high amplitude vasomotion waves occurred unilaterally. There were no signs of reinnervation. Two patients had long-term pain relief. We conclude as follows. (1) Side differences in skin temperature and blood flow are no static descriptors in RSD. They are dynamic values depending critically on environmental temperature. Therefore, they have to be interpreted with care when defining reliable diagnostic criteria. (2) Vascular disturbances in RSD are not due to constant overactivity of sympathetic vasoconstrictor neurons. Changes in vascular sensitivity to cold temperature and circulating catecholamines may be responsible for vascular abnormalities. Alternatively, RSD may be associated with an abnormal (side different) reflex pattern of sympathetic vasoconstrictor neurons due to thermoregulatory and emotional stimuli generated in the central nervous system. (3) After sympathectomy, denervation supersensitivity of blood vessels and intense vasomotion may be associated with recurrence of pain in some patients.

Asymmetry and time-course of cutaneous sympathetic reflex responses following sustained excitation of chemosensitive nociceptors in humans

Sympathetic reflex responses were elicited in human volunteers by sustained selective excitation of nociceptors by noxious chemicals, namely topical application of mustard oil which elicited burning pain, or histamine which induced itching in a skin area of 5 cm2 on the volar aspect of one forearm. Stimulus-related sympathetic reflex responses were studied by means of computer-assisted infrared thermography of the palmar aspects of both hands. Nociceptive stimulation induced a decrease of skin surface temperature in both hands interpreted as vasoconstriction. The magnitude of the reflex cooling was correlated with the magnitude of the sensation (r = 0.49), but independent of the quality of sensation (itch or pain). The temperature reduction was maintained for more than 30 min and its time-course matched the time-courses of pain or itch sensations. It is concluded that the sustained and selective excitation of nociceptors elicits a sustained sympathetic reflex response, which adapts very slowly. The time-course of the reflexes suggests that these are not arousal responses, but may be indicators of nociceptive processing in conscious humans. Contralateral temperature decreases were consistently smaller than ipsilateral ones. Thus, sustained nociceptive-specific vasoconstrictor reflexes may be somatotopically organised with an emphasis on areas close to the painful stimulus (homotopic), which has so far only been shown in animals. The study thus demonstrates for the first time in humans the presence of a sympathetic reflex asymmetry, which is specific for nociceptive afferent input.

Influence of the sympathetic nervous system in the development of abnormal pain-related behaviours in a rat model of neuropathic pain

This study evaluated the effect of surgical sympathectomy on pain-related behaviours in a well established model of peripheral mononeuropathy produced by loose ligatures around the common sciatic nerve in the rat. Behavioural abnormalities include spontaneous abnormal position of the hindpaw after the nerve constriction, indicative of "spontaneous pain", and changes in responses to mechanical or thermal stimuli applied to this paw. These changes are usually maximal at week 2 after the surgery, stable until weeks 3-4, and disappear between weeks 8 and 12. To assess the role of the sympathetic nervous system in the development and persistence of these abnormalities, four groups of rats were behaviourally tested: (i) rats receiving a complete sham surgery, (ii) rats with a sciatic nerve constriction produced by loose ligatures around the common nerve trunk plus a sham sympathectomy, (iii) rats receiving a lumbar sympathectomy with a sham nerve ligature, and (iv) rats receiving a simultaneous surgical lumbar sympathectomy and a sciatic nerve constriction. The efficacy of the sympathectomy was assessed by the measure of the noradrenaline level in the sciatic nerve. Sympathectomy reduced selectively or even prevented the abnormal reaction to cold temperature and to heat (45 degrees C) in rats with a peripheral mononeuropathy. In contrast, the abnormal reaction to mechanical pressure was not influenced, and the behavioural abnormalities indicating spontaneous pain were still present. Sympathectomy alone resulted in a reduction of the vocalization threshold to pressure on both hindpaws, but also a short-lasting increased tolerance to cold immersion. This study confirms the selective role of the sympathetic nervous system in affecting the development and maintenance of some abnormal pain-related behaviours to thermal stimuli in rats with a moderate, but persistent, constriction of one sciatic nerve.

Pre-injury lidocaine treatment prevents thermal hyperalgesia and cutaneous thermal abnormalities in a rat model of peripheral neuropathy

The effect of lidocaine pretreatment on thermal hyperalgesia and thermal skin asymmetries provoked by experimental mononeuropathy was investigated in rats. Forty anesthetized rats were given sciatic nerve ligatures according to the technique of Bennett and Xie. Rats were divided into 3 groups: 16 were ligated without lidocaine, 16 were ligated after lidocaine bathing of the nerve, and 8 were ligated after systemic lidocaine (6-8 mg/kg). Six sham-operated rats for each group were also prepared. From the first postoperative day the responses to the hot-plate test were assessed daily for 4 weeks by tracking the paw-licking latency (PLL) for both hindpaws. Shorter or longer latencies on the operated side were respectively considered sign of hyperalgesia and hypoalgesia. Infrared thermographic images of plantar hindpaws were taken in 22 operated rats in the 2nd postoperative week. Thermographic images of 8 non-operated rats were used as control. Animals operated without lidocaine exhibited shorter PLL (P < 0.001) and a decreased skin temperature on the operated side (P < 0.001). In the lidocaine-pretreated rats, no paw-licking reflex was present for a variable postoperative period (1 week or more) and afterwards there was a trend toward recovery of normal PLL values at the 4th week; the hindpaw skin temperature was symmetrical and normal. Sham-operated rats had normal tests. It is postulated here that lidocaine prevents behavioral and thermal manifestation of mononeuropathy by blocking early afferent injury barrage.

Systemic clonidine differentially modulates the abnormal reactions to mechanical and thermal stimuli in rats with peripheral mononeuropathy

The antinociceptive action of the systemically administered alpha 2-adrenoceptor agonist clonidine was evaluated in a rat model of peripheral unilateral mononeuropathy produced by loose ligatures around the common sciatic nerve, using nociceptive tests based on mechanical (vocalization threshold to paw pressure) or thermal (struggle latency to paw immersion in a cold (10 degrees C) or hot (44 degrees C) water bath) stimuli. Experiments were performed 2-3 weeks after surgery when pain-related behavior was fully developed. We demonstrated a dissociative action depending on the test used: clonidine (30-100 micrograms/kg i.v.) had a moderate effect on the abnormal reactions to the mechanical stimulus. By contrast it dramatically increased the struggle latency to hot or cold stimuli. These latter effects were completely prevented by prior administration of the alpha 2-adrenoceptor antagonist idazoxan (0.5 mg/kg i.v.).

Sensory and sympathetic contributions to nerve injury-induced sensory abnormalities in the rat

Peripheral neuropathy can be associated with a variety of symptoms, including spontaneous unpleasant sensations and pain, as well as increased sensitivity to sensory stimuli. A peripheral neuropathy model involving an L5 spinal nerve lesion in male rats has been used to gain insight into the mechanisms that underlie symptoms that develop after nerve injury. This model was used to study the involvement of sensory fibres, the sympathetic postganglionic neuron and the role of nerve growth factor in the induction and maintenance of altered sensory function in the nerve territory of the intact L4 spinal nerve. Sensory testing was done with calibrated von Frey filaments and a radiant heat apparatus [Hargreaves K. et al. (1988) Pain 32, 77-88] and the occurrence of abnormal spontaneous behaviour was recorded. L5 spinal nerve resection produced increased mechanical and heat sensitivity as well as abnormal spontaneous behaviours. Surgical sympathectomy at the L5 but not at the L4 spinal nerve level alleviated all sensory abnormalities. However, a lesion of preganglionic fibres to the L5 level had no significant effect on sensory abnormalities. Thus, sympathetic postganglionic neurons at the level of spinal nerve injury can contribute to neuropathy symptoms independent of input from preganglionic neurons. Postganglionic sympathetic nerve crush alone led to increased mechanical sensitivity but not to increased heat sensitivity nor to abnormal spontaneous behaviour, further emphasizing the role of sympathetic postganglionic neuron changes for the development of increased mechanical sensitivity. An L5 spinal nerve resection in rats treated neonatally with capsaicin induced increased mechanical sensitivity which was slower in onset and lower in magnitude than that in untreated littermates and was abolished by postganglionic sympathectomy. Nerve growth factor perfused onto the cut L5 spinal nerve also markedly delayed the onset of increased mechanical sensitivity. Two pathophysiological mechanisms leading to central changes may be necessary to produce altered sensations in this model: (i) ongoing activity in C-fibres, independent of sympathetic postganglionic neuron activity and (ii) activity in sensory fibres modulated by a sensory-sympathetic interaction in the injured spinal nerve or dorsal root ganglion. The sympathetic postganglionic neuron contribution is independent of preganglionic sympathetic outflow from the central nervous system, suggesting a novel mechanism by which sympathetic efferent terminals can regulate sensory fibre activity. A contribution of a loss of neurotrophic factors to the sympathetic postganglionic neuron following nerve lesion is also suggested to contribute to the symptoms induced by the spinal nerve lesion.

Alleviation of neuropathic pain symptoms by xenogeneic chromaffin cell grafts in the spinal subarachnoid space

Recent data have suggested that adrenal medullary tissue allografts in the spinal cord subarachnoid space, by releasing catecholamines and opioid peptides, attenuate responses to various acute noxious stimuli and chronic pain-related behaviors. However, the application of this approach is limited by the low availability of allogeneic donor material. Alternatively, chromaffin cells from xenogeneic sources such as the bovine adrenal medulla are plentiful and simple to extract. The goal of this study was to evaluate the potential for bovine chromaffin cell xenografts in the rat spinal subarachnoid space to alleviate chronic pain. This was assessed in an animal model of neuropathic pain induced by loose ligation of the sciatic nerve, which resulted in allodynia, hyperalgesia, and skin temperature abnormalities. Two weeks after nerve injury, animals were implanted with either isolated bovine chromaffin cells or control bovine adrenal fibroblasts in the spinal subarachnoid space at the level of lumbar enlargement and immunosuppressed with cyclosporine A. In animals with chromaffin cell implants, but not fibroblast implants, both cold allodynia and thermal hyperalgesia were markedly reduced or eliminated as early as 1 week following implantation and hind paw skin temperature asymmetry was also normalized. These beneficial effects were maintained without decrement or apparent tolerance for the 9 week course of the symptomology. The analgesic effects of chromaffin cell grafts were partially attenuated following i.t. injection of naloxone and phentolamine separately and in combination, suggesting involvement of spinal opioid and alpha-adrenergic receptors. Following termination of behavioral studies, immunocytochemical analysis revealed robust survival of chromaffin cells in the implants. These results demonstrate that chromaffin cell xenografts may be effective in alleviating pain of neurogenic origin.

Effects of sympathectomy on the cutaneous temperature abnormalities in rats with chronic constriction injury of the sciatic nerve

Effects of surgical sympathectomy on the cutaneous temperature abnormalities of plantar surface evoked by the chronic constriction injury (CCI) of the sciatic nerve were investigated in the rat. In normal animals, there were very small temperature differences between both plantar surfaces. There were also very small temperature differences in plantar surfaces following the sympathectomy prior to CCI. In rats with CCI, the cutaneous temperature of the nerve-injured plantar surface was significantly higher (warmer) than that of the contralateral plantar surface during the first week following CCI, and then became lower (cooler). Surgical sympathectomy prior to and just after CCI significant suppressed the temperature abnormalities during the first week, but no effect was observed after 2 weeks following CCI. These observations indicate that sympathetic vasoconstriction may contribute to the cutaneous temperature abnormalities evoked by CCI during the early stage, but does not affect the abnormalities at later stages.

Behavioral manifestations of an experimental model for peripheral neuropathy produced by spinal nerve ligation in the primate

A goal of the present study was to document the behavioral changes observed in a model of painful neuropathy in the primate (Macaca fascicularis). A neuropathic state was induced by tight ligation of the L7 spinal nerve, just distal to the L7 dorsal root ganglion. Sensory testing was done on the ventral surface of the foot, a region that includes the L7 dermatome. Within 1 week following surgery, all monkeys (n = 3) developed a marked sensitivity to mechanical stimulation (with a camel hair brush and von Frey hairs), indicating the presence of mechanical allodynia. In 2 animals, the increased sensitivity to mechanical stimulation was also observed on the contralateral side. The threshold for withdrawal to a heat stimulus decreased, indicating the presence of heat hyperalgesia. Presentation of various cooling stimuli, such as acetone and cold water baths, suggested that cold allodynia had also developed. These behavioral phenomena are similar to those seen in humans diagnosed with peripheral neuropathic pain. The behavioral abnormalities are discussed in relation to the responses of spinothalamic tract cells recorded from primates with the same peripheral nerve injury (Palecek et al. 1992).

Use and misuse of conventional electrodiagnosis, quantitative sensory testing, thermography, and nerve blocks in the evaluation of painful neuropathic syndromes

A number of laboratory tests are critically important in the quest to diagnose presence or absence of organic neuropathic dysfunction and to establish the relevance of such to the subjective pain complaints. However, none of these tests has absolute diagnostic value and their results must be interpreted in the light of the clinical picture. Conventional electrophysiology evaluates function of large caliber afferent and motor fibers leaving the function of small caliber afferent fibers unexplored, and cannot explore the basis for positive sensory phenomena. The quantitative somatosensory thermotest is the best test available to explore function of small caliber afferents. It allows documentation of positive sensory phenomena in the form of thermal hyperalgesia. Because it is a psychophysical test, it lacks localizing value. Thermography sensitively detects and precisely delineates areas of cutaneous thermal change of neural origin. Three types of diagnostic neurologic blocks are used in the clinic: compression-ischemia, local anesthetic and sympathetic blocks. Although they may provide important information about the pathophysiology of pain and hyperalgesias, adequate placebo control is of the essence because chronic neuropathic pain patients may express a high incidence of placebo response.

An animal model of neuropathic pain: a review

Recent work has succeeded in producing models of painful peripheral neuropathies in laboratory animals. There is evidence that the animals experience both abnormal spontaneous pain and abnormal evoked pains (allodynia and hyperalgesia). Experimental analyses of these models have demonstrated potential pathophysiologic mechanisms in both the peripheral and central nervous systems; it is likely that the model neuropathic pain syndromes are due to several different mechanisms. One line of evidence suggests that these pain states gradually become centralized due to an excitotoxic effect on spinal cord dorsal horn inhibitory interneurons. The role of the sympathetic nervous system appears to vary, depending on the type of nerve injury and the temporal evolution of the syndrome. There is evidence indicating that the abnormality of cutaneous temperature regulation that often accompanies painful peripheral neuropathy is not necessarily due to the activity of sympathetic vasomotor efferents.

Interactions between sympathetic vasoconstrictor outflow and C nociceptor-induced antidromic vasodilatation

The interaction between orthodromic sympathetic vasoconstrictor and antidromic vasodilator effects of C-fiber stimulation was studied in normal human volunteers. Excitation of C fibers was achieved through administration of intermittent painful intraneural microstimulation to upper limb nerves, while recording cutaneous thermal emission profiles in the projected fields of cutaneous sensation. During initial stimulation, when both systems are co-activated, skin temperature decreased. Temperature increased after termination of the stimulus. Eventually, temperature decreased again upon renewed stimulation. Thus, sympathetic vasoconstriction was found to override the antidromic vasodilator effect induced by stimulation of C nociceptors. When interpreting abnormal deviations of skin temperature in patients with peripheral nerve disorders, the dynamic interplay between opposing vasomotor effects driven by sympathetic and somatic systems must be taken into consideration.

A system to measure thermal nociception

Hyperalgesia is commonly demonstrated by measuring the difference in latency to thermal nociception between a control and injured condition. Historically these tests have been conducted using either a hot plate or radiant heat source to stimulate thermal receptors in the foot. The design of a simple and inexpensive circuit to measure thermal nociception based on a method originally introduced by Hargreaves et al. (1988) is described. The design uses a movable, high-intensity light source to heat either the right or left footpad of the test animal. The advantage of this method is that it permits each animal to serve as its own control since the experimental and control footpads can be tested independently. The design possesses greater sensitivity than mechanical hyperalgesia test procedures and provides an automated endpoint that electronically records the latency to removal of the foot from the heat source. Additional features of this design which reduce variability include a constant temperature floor to reduce the effect of altered footpad temperature that may be caused by autonomic dysfunction in the experimental limb and a method of reproducibly locating the heat source prior to testing. The response to a unilateral lesion is demonstrated with this device.

Streptozocin-induced diabetic rats: behavioural evidence for a model of chronic pain

Painful diabetic neuropathy is one of the most common complications of insulin-dependent diabetes in man. Conflicting results have been obtained in experimentally diabetic animals subjected to pain stimuli. This work aimed to systematically study the response of rats made diabetic (hyperglycemia > or = 14 mM) by injection of streptozocin (STZ) (75 mg/kg, i.p.), to various pain stimuli: mechanical, thermal (warm and cold) and chemical. The time course of the scores was followed for 4 weeks simultaneously with the clinical symptoms (weight, body and skin temperature, motility) and hyperglycemia. A decrease in reaction thresholds to noxious heat stimuli (44 degrees C and 46 degrees C) and to non-painful thermal (cold: 10 degrees C, and warm: 38-42 degrees C) and mechanical stimulation (paw pressure) was observed. This can be considered as evidence for hyperalgesia and allodynia, respectively. These troubles appeared gradually and required at least 2 weeks of diabetes to reach statistical significance. Four weeks after the induction of diabetes, the scores obtained in diabetic rats injected with formalin were greater than those in normal rats, indicating hyperalgesia. Variation in sensitivity to pain occurred at the same time as arrested weight increase, fall in skin temperature, some amyotrophy measured in terms of hind-paw volume, and the usual polyuria-polydipsia syndrome. Spontaneous motor activity of the rats was lowered. This model is thus of interest as the observed reactions to noxious and non-noxious stimuli correspond to hyperalgesia and allodynia, symptoms encountered in painful diabetic neuropathy in man. Operating conditions for this model are discussed.

Reduced pain-related behavior by adrenal medullary transplants in rats with experimental painful peripheral neuropathy

Adrenal medullary transplants in the spinal subarachnoid space, by providing a continual source of opioid peptides and catecholamines, offer a potentially important adjunct in the management of chronic pain. While previous studies have shown that this approach is effective against high-intensity phasic stimuli, adrenal medullary implants need to be evaluated against long-term and abnormal pain syndromes before transplantation can be used for human chronic pain. Using a recently developed model of painful peripheral neuropathy, the effects of adrenal medullary chromaffin cells transplanted into the subarachnoid space was evaluated. Peripheral mononeuropathy was induced by loosely tying 4 ligatures (4-0 chromic gut) around the right sciatic nerve. This procedure produces various pain symptoms including allodynia, hyperalgesia and dysesthesia. Rats were given either adrenal medullary tissue or control striated muscle transplants. Animals with adrenal medullary tissue transplants showed markedly decreased allodynia to innocuous cold as early as 1 week post-transplantation. In addition, hyperalgesia to a noxious thermal stimulus was eliminated by adrenal medullary, but not control, transplants. Touch-evoked allodynia was only slightly reduced by adrenal medullary transplants. In addition, indicators of spontaneous pain appeared reduced in animals with adrenal medullary transplants. These findings indicate that adrenal medullary transplants may be effective in reducing neuropathic pain.