Effects of lowering ambient temperature on pain-related behaviors in a rat model of neuropathic pain

Effect of ambient temperature on outpatient admission for osteoarthritis and rheumatoid arthritis in a subtropical Chinese city

Background

Current findings on the impact of weather conditions on osteoarthritis (OA) and rheumatoid arthritis (RA) are sparse and not conclusive. This study aimed to investigate the relationship between temperature change and OA/RA admission.

Methods

Daily OA/RA admission, meteorological data and pollutants from 1 January 2014 to 31 December 2017 in Hefei, China, were collected. We quantified the relationship between ambient temperature and OA/RA admission using a distributed lag nonlinear model (DLNM). Stratified analyses by gender and age were also examined.

Results

Temperature decrease was significantly associated with RA admission (25th percentile of temperature versus 50th percentile of temperature), with the acute and largest effect at current days lag (RR: 1.057, 95%CI: 1.005–1.111). However, no significant association between temperature and OA admission was observed. When conducting subgroup analyses by individual characteristics, we found that females and patients aged 41–65 years were more vulnerable to temperature decrease than males, patients aged 0–40 and ≧66 years, respectively.

Conclusions

This study suggested that temperature decrease was a risk factor for increases in RA admission. Females and patients aged 41–65 years were particularly vulnerable to the effect of temperature decrease.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12889-021-11994-0.

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.

Exploring acute-to-chronic neuropathic pain in rats after contusion spinal cord injury

Spinal cord injury (SCI) causes chronic pain in 65% of individuals. Unfortunately, current pain management is inadequate for many SCI patients. Rodent models could help identify how SCI pain develops, explore new treatment strategies, and reveal whether acute post-SCI morphine worsens chronic pain. However, few studies explore or compare SCI-elicited neuropathic pain in rats. Here, we sought to determine how different clinically relevant contusion SCIs in male and female rats affect neuropathic pain, and whether acute morphine worsens later chronic SCI pain. First, female rats received sham surgery, or 150kDyn or 200kDyn midline T9 contusion SCI. These rats displayed modest mechanical allodynia and long-lasting thermal hyperalgesia. Next, a 150kDyn (1s dwell) midline contusion SCI was performed in male and female rats. Interestingly, males, but not females showed SCI-elicited mechanical allodynia; rats of both sexes had thermal hyperalgesia. In this model, acute morphine treatment had no significant effect on chronic neuropathic pain symptoms. Unilateral SCIs can also elicit neuropathic pain that could be exacerbated by morphine, so male rats received unilateral T13 contusion SCI (100kDyn). These rats exhibited significant, transient mechanical allodynia, but not thermal hyperalgesia. Acute morphine did not exacerbate chronic pain. Our data show that specific rat contusion SCI models cause neuropathic pain. Further, chronic neuropathic pain elicited by these contusion SCIs was not amplified by our course of early post-trauma morphine. Using clinically relevant rat models of SCI could help identify novel pain management strategies.

Baclofen reversed thermal place preference in rats with chronic constriction injury

Chronic constriction injury to the sciatic nerve was used as an animal model of neuropathic pain. Instead of frequently used reflex-based tests we used an operant thermal place preference test to evaluate signs of neuropathic pain and the effect of baclofen administration in rats with neuropathy. Chronic constriction injury was induced by four loose ligations of the sciatic nerve. Thermal place preference (45 °C vs. 22 °C and 45 °C vs. 11 °C) was measured after the ligation and after the administration of baclofen in sham and experimental rats. Rats with the chronic constriction injury spent significantly less time on the colder plate compared to sham operated animals at the combination 45 °C vs. 11 °C. After administration of baclofen (10 mg/kg s.c.), the aversion to the colder plate in rats with chronic constriction injury disappeared. At the combination 45 °C vs. 22 °C, no difference in time spent on colder and/or warmer plate was found between sham and experimental animals. These findings show the importance of cold allodynia evaluation in rats with chronic constriction injury and the effectiveness of baclofen in this neuropathic pain model.

Inverse association between air pressure and rheumatoid arthritis synovitis

Rheumatoid arthritis (RA) is a bone destructive autoimmune disease. Many patients with RA recognize fluctuations of their joint synovitis according to changes of air pressure, but the correlations between them have never been addressed in large-scale association studies. To address this point we recruited large-scale assessments of RA activity in a Japanese population, and performed an association analysis. Here, a total of 23,064 assessments of RA activity from 2,131 patients were obtained from the KURAMA (Kyoto University Rheumatoid Arthritis Management Alliance) database. Detailed correlations between air pressure and joint swelling or tenderness were analyzed separately for each of the 326 patients with more than 20 assessments to regulate intra-patient correlations. Association studies were also performed for seven consecutive days to identify the strongest correlations. Standardized multiple linear regression analysis was performed to evaluate independent influences from other meteorological factors. As a result, components of composite measures for RA disease activity revealed suggestive negative associations with air pressure. The 326 patients displayed significant negative mean correlations between air pressure and swellings or the sum of swellings and tenderness (p = 0.00068 and 0.00011, respectively). Among the seven consecutive days, the most significant mean negative correlations were observed for air pressure three days before evaluations of RA synovitis (p = 1.7×10⁻⁷, 0.00027, and 8.3×10⁻⁸, for swellings, tenderness and the sum of them, respectively). Standardized multiple linear regression analysis revealed these associations were independent from humidity and temperature. Our findings suggest that air pressure is inversely associated with synovitis in patients with RA.

Temperature sensing by thermal TRP channels: thermodynamic basis and molecular insights

All organisms need to sense temperature in order to survive and adapt. But how they detect and perceive temperature remains poorly understood. Recent discoveries of thermal Transient Receptor Potential (TRP) ion channels have shed light on the problem and unravel molecular entities for temperature detection and transduction in mammals. Thermal TRP channels belong to the large family of transient receptor potential channels. They are directly activated by heat or cold in physiologically relevant temperature ranges, and the activation is exquisitely sensitive to temperature changes. Thermodynamically, this strong temperature dependence of thermal channels occurs due to large enthalpy and entropy changes associated with channel opening. Thus understanding how the channel proteins obtain their exceptionally large energetics is central toward determining functional mechanisms of thermal TRP channels. The purpose of this chapter is to provide a comprehensive review on critical issues and challenges facing the problem, with emphases on underlying biophysical and molecular mechanisms.

Transcutaneous electrical nerve stimulation as prehospital emergency interventional care: treating acute pelvic pain in young women

Objectives.  In Europe, patients with acute pelvic pain are transported to the hospital in an ambulance without an emergency physician. We hypothesized that transcutaneous electrical nerve stimulation (TENS) would be an effective noninvasive procedure for pain treatment. Methods.  We conducted a prospective, randomized, blinded study where 100 women were randomly assigned into a real- or a sham-TENS group. TENS began before the transport to the ambulance and was left in place until the arrival at the hospital. Each patient rated her pain on paper using a visual analog scale. Results.  Compared to sham TENS, patients with active TENS felt that their pain was reduced by half after treatment (p < 0.01), anxiety scores significantly decreased (p < 0.01), heart rate and arteriolar vasoconstriction decreased significantly (p < 0.01), and nausea (p < 0.01) was lessened. Overall satisfaction with the received care was significantly higher (p < 0.01). Conclusion.  TENS is a safe, rapid, and effective analgesic treatment for acute pelvic pain.

The rate and magnitude of atmospheric pressure change that aggravate pain-related behavior of nerve injured rats

Complaints of patients with chronic pain may increase when the weather changes. The exact mechanism for weather change-induced pain has not been clarified. We have previously demonstrated that artificially lowering barometric pressure (LP) intensifies pain-related behaviors in rats with neuropathic pain [chronic constriction injury (CCI) and spinal nerve ligation (SNL)]. In the present study, we examined the rate and magnitude of LP that aggravates neuropathic pain. We measured pain-related behaviors [number of paw lifts to von Frey hair (VFH) stimulation] in awake rats after SNL or CCI surgery, and found that rates of decompression ≥5 hPa/h and ≥10 hPa/h and magnitudes of decompression ≥5 hPa and ≥10 hPa augmented pain-related behaviors in SNL and CCI rats, respectively. These results indicate that LP within the range of natural weather patterns augments neuropathic pain in rats, and that SNL rats are more sensitive to LP than CCI rats.

The inner ear is involved in the aggravation of nociceptive behavior induced by lowering barometric pressure of nerve injured rats

Patients suffering from neuropathic pain often complain of pain aggravation when the weather is changing. The exact mechanism for weather change-induced pain has not been clarified. We have previously demonstrated that experimentally lowering barometric pressure (LP) intensifies pain-related behaviors in rats with chronic constriction injury (CCI). In the present experiment we examined whether this pain aggravating effect of LP exposure in nerve injured rats is still present after lesioning of the inner ear. We used both CCI and spinal nerve ligation (SNL) models for this study. We injected into the middle ear sodium arsanilate solution (100mg/ml, 50microl/ear), which is known to degenerate vestibular hair cells, under anesthesia the day before surgery. Rats were exposed to LP (27hPa decrease over 8min) 7-9 days after CCI or 5-8 days after SNL surgery, and pain-related behavior (number of paw lifts induced by von Frey hair stimuli) was measured. When the inner ear lesioned SNL or CCI rats were exposed to LP, they showed no augmentation of pain-related behavior. On the other hand, the pain aggravating effect of a temperature decrease (from 24 to 17 degrees C) was maintained in both SNL and CCI rats. These results suggest that the barometric sensor/sensing system influencing nociceptive behavior during LP in rats is located in the inner ear.

Transcutaneous electrical nerve stimulation: an effective treatment for pain caused by renal colic in emergency care

PURPOSE

Acute renal colic is one of the most anguishing forms of pain in humans. We hypothesized that TENS is an effective pain treatment in patients with acute renal colic.

MATERIALS AND METHODS

A total of 100 patients with acute flank pain and suspected renal colic consented to participate in our study. Paramedic 1 recorded baseline parameters at the emergency site and at the end of transportation. Paramedic 2 performed TENS in patients randomly assigned to G1 with actual TENS or to G2 with sham TENS. Pain and anxiety were measured using paper based visual analog scales on a scale of 0 to 100 mm.

RESULTS

Of 100 screened patients 73 had renal colic, including 39 in G1 and 34 in G2. There was no significant difference with regard to potentially influencing factors, such as patient age, sex, weight, height, blood pressure and heart rate, pain, nausea and anxiety between the groups before treatment. G1 showed a significant mean pain decrease +/- SD of more than 50% (85.7 +/- 10.5 to 33.3 +/- 16.0 mm, p <0.01). G2 showed no variation in mean pain scores (85.8 +/- 18.0 to 82.6 +/- 14.3 mm). G1 showed changes in the mean anxiety score (69.0 +/- 8.4 to 37.7 +/- 15.1 mm, p <0.01), nausea score (90.7 +/- 9.2 to 44.9 +/- 22.0 mm) and heart rate (92 +/- 10 to 64 +/- 8 bpm), while G2 showed nonsignificant changes.

CONCLUSIONS

This trial shows that local TENS is a rapid and effective treatment for renal colic pain. We found TENS to be a good nondrug therapy under the difficult circumstances of out of hospital rescue.

Bilateral chronic constriction of the sciatic nerve: a model of long-term cold hyperalgesia

Effects of chronic constriction injury (CCI) and sham surgery of both sciatic nerves were evaluated for reflex lick/guard (L/G) and operant escape responses to thermal stimulation of rats. Experiment 1 compared L/G and escape responses to 0.3 degrees C, 43 degrees C, and 47 degrees C stimulation during a period of 60 days after CCI. Experiment 2 evaluated escape from 44 degrees C, 47 degrees C, and 10 degrees C for 100 days after CCI. The rats escaped from heat or cold stimulation of the paws in a dark compartment by climbing on a thermally neutral platform in a brightly lit compartment. For reflex testing, a single compartment provided no escape option. There was no significant effect of bilateral CCI on reflex or escape responses to nociceptive heat. However, there were long-term increases in the duration of L/G responding during trials of 0.3 degrees C stimulation and in the duration of escape responding to 10 degrees C. Hyperalgesia for cold was confirmed by a preference test, with a 2-compartment shuttle box with one floor heated (45 degrees C) and the other floor cooled (10 degrees C). Occupancy of the heated compartment was significantly increased by CCI (indicating a relative aversion for cold).

PERSPECTIVE

For preclinical testing of treatments for allodynia/hyperalgesia after nerve injury, it is crucial to use methods of testing that are sensitive to effects on nociception throughout the neuraxis. Operant escape testing satisfies this criterion and is sensitive to bilateral CCI of rats, which avoids asymmetric postural/motor influences of unilateral CCI.

Antihyperalgesic, but not antiallodynic, effect of melatonin in nerve-injured neuropathic mice: Possible involvements of the L-arginine-NO pathway and opioid system

The present study was undertaken to determine the effects of intracerebroventricular (i.c.v.) and intraperitoneal (i.p.) melatonin on mechanical allodynia and thermal hyperalgesia in mice with partial tight ligation of the sciatic nerve, and how the nitric oxide (NO) precursor l-arginine and the opiate antagonist naloxone influence this effect. A plantar analgesic meter was used to assess thermal hyperalgesia, and nerve injury-induced mechanical hyperalgesia was assessed with von Frey filaments. 1-5 weeks following the surgery, marked mechanical allodynia and thermal hyperalgesia developed in neuropathic mice. Intracerebroventricular and intraperitoneal melatonin, with its higher doses, produced a blockade of thermal hyperalgesia, but not mechanical allodynia. Administration of both l-arginine and naloxone, at doses which produced no effect on their own, partially reversed antihyperalgesic effect of melatonin. These results suggest that although it has different effects on neuropathic pain-related behaviors, melatonin may have clinical utility in neuropathic pain therapy in the future. It is also concluded that l-arginine-NO pathway and opioidergic system are involved in the antihyperalgesic effect of melatonin in nerve-injured mice.

Weather conditions and spinal patients

STUDY DESIGN

A retrospective study.

OBJECTIVES

To evaluate the effects of various weather conditions on reported health status.

SUMMARY OF BACKGROUND DATA

Existing literature shows no definite conclusions regarding the effect of weather on patients with spinal pathology.

METHODS

Initial visit data from 23 American centers participating in the National Spine Network included demographic information and SF-36-based health status. Weather conditions when and where patients were seen were obtained from the National Climatic Data Center and U.S. Naval Observatory. SF-36 outcomes were predicted using multiple regression techniques from weather parameters, which included high and low temperature, average dew point, wet bulb, barometric pressure, total precipitation, phase of the moon, and length of sunlight.

RESULTS

A total of 26,862 of 54,062 patients were identified. Two models were compared: model 1 included the above weather predictors and model 2 added age and gender. For each SF-36 scale and subscale and the Physical and Mental composite Scores, both models were statistically significant, although only model 2 for physical function produced an R value greater than 1%. Barometric pressure was the only weather predictor that was consistently significant. Increased pressure was associated with worse outcomes. Although age and gender were significant additions to the prediction equation, overall, the practical contribution was minimal.

CONCLUSION

A statistically significant relationship between weather factors and SF-36-based health status exists but has minimal clinical significance. These factors had minimal if any effect on mental health-related scores bringing into question either the relationship between weather and psychological status, at least in patients with spinal problems or the usefulness of the instrument used.

Artificially produced meteorological changes aggravate pain in adjuvant-induced arthritic rats

To examine the effects of change in meteorological parameters on pain-related behaviors in a simulated arthritic condition, rats with an injection of complete Freund's adjuvant into the tibio-tarsal joint were exposed to low barometric pressure (20 mmHg below the natural atmospheric pressure) and low ambient temperature (7 degrees C lower than 22 degrees C) in a climate-controlled room. When the arthritic rats were exposed to these environments, the already increased number of hindpaw withdrawals in response to noxious mechanical stimulation (hyperalgesia) was further increased, and a hindpaw withdrawal response to innocuous mechanical stimulation (allodynia) began to occur. Such exposures did not influence any of the pain-related behaviors of the control rats. These results show that lowering barometric pressure and ambient temperature within the range of natural environmental fluctuation intensify pain in arthritic rats.

Responses of C-fiber low threshold mechanoreceptors and nociceptors to cold were facilitated in rats persistently inflamed and hypersensitive to cold

Cold allodynia is an annoying symptom in conditions of chronic inflammation such as rheumatoid arthritis. To examine whether primary afferent nerve activities are changed in association with hypersensitivity to cold, we recorded single nerve activities from the sural nerve in persistently inflamed rats in vivo. Inflammation was induced by an injection of complete Freund's adjuvant (CFA) solution into the tibio-tarsal joint. Inflamed rats showed an increased number of paw shakes to paw immersion in 25 degrees C water (pre-inflammation: 1.15+/-0.58, 2-week inflammation: 4.70+/-1.15). We also recorded cutaneous C-fiber activities under pentobarbital anesthesia and studied their responses to thermal and mechanical stimuli. The response of C-low threshold mechanoreceptors to cooling (total discharges between 27 and 23 degrees C) increased 1.8-fold (control group: 5.17+/-1.04 impulses, inflamed group: 9.38+/-1.47 impulses). In addition, the proportion of C-nociceptor units responding to cold down to 2 degrees C was significantly greater in the inflamed group (9 out of 18 units; threshold: 10.0+/-2.6 degrees C) than in the intact group (1 out of 14 units; threshold: 4.0 degrees C). These results suggest that the facilitated responses of these primary afferents are associated with cold hypersensitivity in chronically inflamed conditions.

Local active warming: an effective treatment for pain, anxiety and nausea caused by renal colic

PURPOSE

Based on previous studies showing that warming decreases trauma pain in emergency care we hypothesized that local active warming of the abdomen and lower back region could decrease pain in acute renal colic cases during emergency transport.

MATERIALS AND METHODS

After obtaining informed consent 100 patients were divided into 2 groups, including those who received active warming of the abdomen and lower back region (42C) and those who received no warming. Pain, nausea and anxiety were rated by the patients using visual analog scales. Statistical evaluation was performed using the t test with p <0.05 considered significant.

RESULTS

In group 1 a significant pain decrease was recorded in all cases using a visual analog score (VAS) (82.7 +/- 9.5 to 36.3 +/- 16.0 mm VAS, p <0.01). In group 2 patient pain scores remained comparable (81.8 +/- 13.0 to 80.6 +/- 12.3 mm VAS). In group 1 anxiety significantly decreased (79.0 +/- 8.9 and 30.7 +/- 14.1 mm VAS before and after treatment, respectively, p <0.01). In group 2 a nonsignificant change in score was noted (79.7 +/- 20.5 to 75.2 +/- 19.7 mm VAS). In group 1 a significant decrease in nausea was recorded in all cases (85.7 +/- 11.2 to 40.6 +/- 23.0 mm VAS, p <0.01). In group 2 patient nausea scores remained comparable (79.2 +/- 22.0 to 80.3 +/- 22.4 mm VAS, respectively).

CONCLUSIONS

Local active warming is an effective and easy to learn pain treatment for patients with acute renal colic in emergency care.

The influence of local active warming on pain relief of patients with cholelithiasis during rescue transport

Upper abdominal pain, a frequent symptom of the presence of gallstone disease, is the cause of 6% of the emergency calls of the Austrian emergency system. Pain resulting from cholelithiasis is characteristically severe. Recent data show that active warming during emergency transport of trauma victims is effective in reducing pain. Therefore, we hypothesized that local active warming of the abdomen would be an effective pain treatment for patients with acute cholelithiasis and could be provided by paramedics. Sixty patients (>19 yr) consented to participate in this study. They were divided into two groups: Group 1, who received active warming of the upper abdomen with a carbon-fiber warming blanket (42 degrees C), and Group 2, who received no warming of the abdomen. Neither group received any drug-based pain care. Patients were asked to rate their pain and anxiety by using visual analog scales (VAS). Statistical evaluation was performed with Student's t-test; P < 0.05 was considered significant. In Group 1, a significant (P < 0.01) pain reduction was recorded in all cases on a visual analog scale (VAS), from 86.8 +/- 5.5 mm to 41.2 +/- 16.2 mm. In Group 2, the patients' pain scores remained comparable, from 88.3 +/- 9.9 mm to 88.1 +/- 10.0 mm on a VAS. In comparing Group 1 with Group 2 on arrival at the hospital, pain scores showed a significant difference (P < 0.01). In Group 1, the VAS score changes for anxiety were significantly reduced (P < 0.01), from 82.7 +/- 10.8 mm before treatment to 39.0 +/- 14.0 mm after treatment. In Group 2, a nonsignificant change of this score was noted, from 84.5 +/- 14.6 mm to 83.5 +/- 8.4 mm. Comparing Group 1 with Group 2 on arrival at the hospital showed a significant difference in anxiety scores (P < 0.01). We conclude that local active warming is an effective and easy-to-learn treatment for pain resulting from acute cholelithiasis in emergency care.

IMPLICATIONS

Active local warming of the upper abdomen is an effective treatment for patients with cholelithiasis being transported to the hospital by paramedics who are not permitted to provide any drug-based pain care. We observed no negative side effects of this treatment.

Pathobiology of neuropathic pain

This review deals with physiological and biological mechanisms of neuropathic pain, that is, pain induced by injury or disease of the nervous system. Animal models of neuropathic pain mostly use injury to a peripheral nerve, therefore, our focus is on results from nerve injury models. To make sure that the nerve injury models are related to pain, the behavior was assessed of animals following nerve injury, i.e. partial/total nerve transection/ligation or chronic nerve constriction. The following behaviors observed in such animals are considered to indicate pain: (a) autotomy, i.e. self-attack, assessed by counting the number of wounds implied, (b) hyperalgesia, i.e. strong withdrawal responses to a moderate heat stimulus, (c) allodynia, i.e. withdrawal in response to non-noxious tactile or cold stimuli. These behavioral parameters have been exploited to study the pharmacology and modulation of neuropathic pain. Nerve fibers develop abnormal ectopic excitability at or near the site of nerve injury. The mechanisms include unusual distributions of Na(+) channels, as well as abnormal responses to endogenous pain producing substances and cytokines such as tumor necrosis factor alpha (TNF-alpha). Persistent abnormal excitability of sensory nerve endings in a neuroma is considered a mechanism of stump pain after amputation. Any local nerve injury tends to spread to distant parts of the peripheral and central nervous system. This includes erratic mechano-sensitivity along the injured nerve including the cell bodies in the dorsal root ganglion (DRG) as well as ongoing activity in the dorsal horn. The spread of pathophysiology includes upregulation of nitric oxide synthase (NOS) in axotomized neurons, deafferentation hypersensitivity of spinal neurons following afferent cell death, long-term potentiation (LTP) of spinal synaptic transmission and attenuation of central pain inhibitory mechanisms. In particular, the efficacy of opioids at the spinal level is much decreased following nerve injury. Repeated or prolonged noxious stimulation and the persistent abnormal input following nerve injury activate a number of intracellular second messenger systems, implying phosphorylation by protein kinases, particularly protein kinase C (PKC). Intracellular signal cascades result in immediate early gene (IEG) induction which is considered as the overture of a widespread change in protein synthesis, a general basis for nervous system plasticity. Although these processes of increasing nervous system excitability may be considered as a strategy to compensate functional deficits following nerve injury, its by-product is widespread nervous system sensitization resulting in pain and hyperalgesia. An important sequela of nerve injury and other nervous system diseases such as virus attack is apoptosis of neurons in the peripheral and central nervous system. Apoptosis seems to induce neuronal sensitization and loss of inhibitory systems, and these irreversible processes might be in common to nervous system damage by brain trauma or ischemia as well as neuropathic pain. The cellular pathobiology including apoptosis suggests future strategies against neuropathic pain that emphasize preventive aspects.