Swim stress reduces chronic pain in mice through an opioid mechanism

Mechanism of exercise-induced analgesia: what we can learn from physically active animals

Physical activity has become a first-line treatment in rehabilitation settings for individuals with chronic pain. However, research has only recently begun to elucidate the mechanisms of exercise-induced analgesia. Through the study of animal models, exercise has been shown to induce changes in the brain, spinal cord, immune system, and at the site of injury to prevent and reduce pain. Animal models have also explored beneficial effects of exercise through different modes of exercise including running, swimming, and resistance training. This review will discuss the central and peripheral mechanisms of exercise-induced analgesia through different modes, intensity, and duration of exercise as well as clinical applications of exercise with suggestions for future research directions.

Voluntary Exercise Suppresses Choroidal Neovascularization in Mice

Purpose

To determine the effect of voluntary exercise on choroidal neovascularization (CNV) in mice.

Methods

Age-matched wild-type C57BL/6J mice were housed in cages equipped with or without running wheels. After four weeks of voluntary running or sedentariness, mice were subjected to laser injury to induce CNV. After surgical recovery, mice were placed back in cages with or without exercise wheels for seven days. CNV lesion volumes were measured by confocal microscopy. The effect of wheel running only in the seven days after injury was also evaluated. Macrophage abundance and cytokine expression were quantified.

Results

In the first study, exercise-trained mice exhibited a 45% reduction in CNV volume compared to sedentary mice. In the replication study, a 32% reduction in CNV volume in exercise-trained mice was observed (P = 0.029). Combining these two studies, voluntary exercise was found to reduce CNV by 41% (P = 0.0005). Exercise-trained male and female mice had similar CNV volumes (P = 0.99). The daily running distance did not correlate with CNV lesion size. Exercise only after the laser injury without a preconditioning period did not reduce CNV size (P = 0.41). CNV lesions of exercise-trained mice also exhibited significantly lower F4/80+ macrophage staining and Vegfa and Ccl2 mRNA expression.

Conclusions

These findings provide the first experimental evidence that voluntary exercise improves CNV outcomes. These studies indicate that exercise before laser treatment is required to improve CNV outcomes.

Voluntary exercise reduces both chemotherapy-induced neuropathic nociception and deficits in hippocampal cellular proliferation in a mouse model of paclitaxel-induced peripheral neuropathy

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Paclitaxel treatment did not alter voluntary running activity.

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Voluntary running reduced mechanical and cold allodynia induced by paclitaxel.

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Voluntary running reduced paclitaxel-induced deficits in hippocampal cellular proliferation.

Chemotherapy-induced peripheral neuropathy (CIPN) is a common dose-limiting side-effect of all major chemotherapeutic agents. Here, we explored efficacy of voluntary exercise as a nonpharmacological strategy for suppressing two distinct adverse side effects of chemotherapy treatment. We evaluated whether voluntary running would suppress both neuropathic pain and deficits in hippocampal cell proliferation in a mouse model of CIPN induced by the taxane chemotherapeutic agent paclitaxel. Mice were given free access to running wheels or were housed without running wheels during one of three different intervention phases: 1) during the onset (i.e. development phase) of paclitaxel-induced neuropathy, 2) prior to dosing with paclitaxel or its vehicle, or 3) following the establishment (i.e. maintenance phase) of paclitaxel-induced neuropathy. Paclitaxel treatment did not alter running wheel behavior relative to vehicle-treated animals in any study. Animals that engaged in voluntary running during the development phase of paclitaxel-induced neuropathy failed to display mechanical or cold hypersensitivities relative to sedentary control animals that did not have access to running wheels. A prior history of voluntary running delayed the onset of, but did not fully prevent, development of paclitaxel-induced neuropathic pain behavior. Voluntary running reduced already established mechanical and cold allodynia induced by paclitaxel. Importantly, voluntary running did not alter mechanical or cold responsivity in vehicle-treated animals, suggesting that the observed antinociceptive effect of exercise was dependent upon the presence of the pathological pain state. In the same animals evaluated for nociceptive responding, paclitaxel also reduced cellular proliferation but not cellular survival in the dentate gyrus of the hippocampus, as measured by immunohistochemistry for Ki67 and BrdU expression, respectively. Voluntary running abrogated paclitaxel-induced reductions in cellular proliferation to levels observed in vehicle-treated mice and also increased BrdU expression levels irrespective of chemotherapy treatment. Our studies support the hypothesis that voluntary exercise may be beneficial in suppressing both neuropathic pain and markers of hippocampal cellular function that are impacted by toxic challenge with chemotherapeutic agents.

Effects of voluntary exercise on antiretroviral therapy-induced neuropathic pain in mice

Antiretroviral therapy (ART) often results in painful peripheral neuropathy. Given that voluntary exercise has been shown to be beneficial in terms of modulating pain-like behaviors in various animal models of peripheral neuropathy, we have investigated the effects of voluntary wheel running on neuropathic pain induced by chronic ART. We first established an animal model of peripheral neuropathy induced by chronic 2',3'-dideoxycytidine (ddC) treatment. We showed that mice receiving ddC (3 mg/kg/day) had increased mechanical and thermal sensitivity at 9 weeks after the onset of the treatment. We also found that voluntary wheel running attenuated or delayed the onset of ddC-induced peripheral neuropathy. This phenomenon was associated with the attenuation of dorsal root ganglion nociceptive neuron membrane excitability and reduction in the expression of the transient receptor potential cation channel subfamily V member 1 (TRPV1). Taken together, these results suggest that voluntary exercise is an effective strategy by which ART-induced peripheral neuropathy can be alleviated.

Does exercise increase or decrease pain? Central mechanisms underlying these two phenomena

Exercise is an integral part of the rehabilitation of patients suffering a variety of chronic musculoskeletal conditions, such as fibromyalgia, chronic low back pain and myofascial pain. Regular physical activity is recommended for treatment of chronic pain and its effectiveness has been established in clinical trials for people with a variety of pain conditions. However, exercise can also increase pain making participation in rehabilitation challenging for the person with pain. Animal models of exercise-induced pain have been developed and point to central mechanisms underlying this phenomena, such as increased activation of NMDA receptors in pain-modulating areas. Meanwhile, a variety of basic science studies testing different exercise protocols, show exercise-induced analgesia involves activation of central inhibitory pathways. Opioid, serotonin and NMDA mechanisms acting in rostral ventromedial medulla promote analgesia associated with exercise. This review explores and discusses current evidence on central mechanisms underlying exercised-induced pain and analgesia.

Pain perception and cardiovascular system response among athletes playing contact sports

The aim of this study was to determine whether the contact sports change the perception of pain as assessed by the cold pressor test (CPT), and if the test induces the same reaction of the cardiovascular system in contact athletes and non-athletes. The study involved 321 healthy men; 140 contact athletes and 181 students of the University of Szczecin (control). Pain threshold and pain tolerance were evaluated using CPT. Cardiovascular measurements were made during CPT. The contact athletes showed a much higher tolerance to pain than the control group (median time 120 vs. 94 s, respectively, p = 0.0002). The thresholds of pain in both groups did not differ significantly between the groups. Systolic blood pressure measured before and during the test in all three measurements was statistically significantly higher in athletes compared with the control group. Heart rate and diastolic blood pressure did not differ significantly between the studied groups.

Neuroprotective Effects of Exercise Treatments After Injury: The Dual Role of Neurotrophic Factors

BACKGROUND

Shared connections between physical activity and neuroprotection have been studied for decades, but the mechanisms underlying this effect of specific exercise were only recently brought to light. Several evidences suggest that physical activity may be a reasonable and beneficial method to improve functional recovery in both peripheral and central nerve injuries and to delay functional decay in neurodegenerative diseases. In addition to improving cardiac and immune functions, physical activity may represent a multifunctional approach not only to improve cardiocirculatory and immune functions, but potentially modulating trophic factors signaling and, in turn, neuronal function and structure at times that may be critical for neurodegeneration and regeneration.

METHODS

Research content related to the effects of physical activity and specific exercise programs in normal and injured nervous system have been reviewed.

RESULTS

Sustained exercise, particularly if applied at moderate intensity and early after injury, exerts anti-inflammatory and pro-regenerative effects, and may boost cognitive and motor functions in aging and neurological disorders. However, newest studies show that exercise modalities can differently affect the production and function of brain-derived neurotrophic factor and other neurotrophins involved in the generation of neuropathic conditions. These findings suggest the possibility that new exercise strategies can be directed to nerve injuries with therapeutical benefits.

CONCLUSION

Considering the growing burden of illness worldwide, understanding of how modulation of neurotrophic factors contributes to exercise-induced neuroprotection and regeneration after peripheral nerve and spinal cord injuries is a relevant topic for research, and represents the beginning of a new non-pharmacological therapeutic approach for better rehabilitation of neural disorders.

Effect of environment on the long-term consequences of chronic pain

Much evidence from pain patients and animal models shows that chronic pain does not exist in a vacuum but has varied comorbidities and far-reaching consequences. Patients with long-term pain often develop anxiety and depression and can manifest changes in cognitive functioning, particularly with working memory. Longitudinal studies in rodent models also show the development of anxiety-like behavior and cognitive changes weeks to months after an injury causing long-term pain. Brain imaging studies in pain patients and rodent models find that chronic pain is associated with anatomical and functional alterations in the brain. Nevertheless, studies in humans reveal that lifestyle choices, such as the practice of meditation or yoga, can reduce pain perception and have the opposite effect on the brain as does chronic pain. In rodent models, studies show that physical activity and a socially enriched environment reduce pain behavior and normalize brain function. Together, these studies suggest that the burden of chronic pain can be reduced by nonpharmacological interventions.

Voluntary Exercise Training: Analysis of Mice in Uninjured, Inflammatory, and Nerve-Injured Pain States

Both clinical and animal studies suggest that exercise may be an effective way to manage inflammatory and neuropathic pain conditions. However, existing animal studies commonly use forced exercise paradigms that incorporate varying degrees of stress, which itself can elicit analgesia, and thus may complicate the interpretation of the effects of exercise on pain. We investigated the analgesic potential of voluntary wheel running in the formalin model of acute inflammatory pain and the spared nerve injury model of neuropathic pain in mice. In uninjured, adult C57BL/6J mice, 1 to 4 weeks of exercise training did not alter nociceptive thresholds, lumbar dorsal root ganglia neuronal excitability, or hindpaw intraepidermal innervation. Further, exercise training failed to attenuate formalin-induced spontaneous pain. Lastly, 2 weeks of exercise training was ineffective in reversing spared nerve injury-induced mechanical hypersensitivity or in improving muscle wasting or hindpaw denervation. These findings indicate that in contrast to rodent forced exercise paradigms, short durations of voluntary wheel running do not improve pain-like symptoms in mouse models of acute inflammation and peripheral nerve injury.

Swim therapy reduces mechanical allodynia and thermal hyperalgesia induced by chronic constriction nerve injury in rats

OBJECTIVE

Neuropathic pain is common and often difficult to treat because it generally does not respond well to the currently available pain medications or nerve blocks. Recent studies in both humans and animals have suggested that exercise may induce a transient analgesia and reduce acute pain in normal healthy individuals. We examined whether swim therapy could alleviate neuropathic pain in rats.

DESIGN

Rats were trained to swim over a 2-week period in warm water. After the rats were trained, neuropathic pain was induced by constricting the right sciatic nerve, and regular swimming was resumed. The sensitivity of each hind paw was monitored using the Hargreaves test and von Frey test to evaluate the withdrawal response thresholds to heat and touch.

RESULTS

The paw ipsilateral to the nerve ligation expressed pain-like behaviors including thermal hyperalgesia and mechanical allodynia. Regular swim therapy sessions significantly reduced the mechanical allodynia and thermal hyperalgesia. Swim therapy had little effect on the withdrawal thresholds for the contralateral paw. In addition, swim therapy alone did not alter the thermal or mechanical thresholds of normal rats.

CONCLUSIONS

The results suggest that regular exercise, including swim therapy, may be an effective treatment for neuropathic pain caused by nerve injuries. This study, showing that swim therapy reduces neuropathic pain behavior in rats, provides a scientific rationale for clinicians to test the efficacy of exercise in the management of neuropathic pain. It may prove to be a safe and cost-effective therapy in a variety of neuropathic pain states.

Association of liking and reinforcing value with children's physical activity

This study determined whether liking and relative reinforcing value (RRV) of physical activity were associated with time youth spend in moderate-to-vigorous physical activity (MVPA). Boys (n=21) and girls (n=15) age 8 to 12 years were measured for height, weight, aerobic fitness, liking and RRV of physical activity, and minutes in MVPA using accelerometers. Independence of liking and RRV of physical activity was established by a low correlation (r=0.08, p=0.64). Using MVPA as the dependent variable and hierarchical regression to control for individual differences in age, aerobic fitness, and time the accelerometer was worn in step 1 (R(2) of 0.60 for step 1), addition of liking and RRV of physical activity in step 2 produced an incremental increase in R(2) of 0.12 (p<0.01). When using median splits of the RRV and liking data to form subject groups, children with both a high liking and RRV of physical activity participated in greater (p< or = 0.02) MVPA (1340+/-70 min/week) than children with high RRV-low liking (1040+/-95 min/week), low RRV-high liking (978+/-88 min/week), or low RRV-low liking (1007+/-68 min/week) of physical activity. Thus, liking and RRV of physical activity are independently associated with MVPA. The combination of a high reinforcing value and liking of physical activity is associated with 33% greater participation in MVPA.

Extended Swimming Exercise Reduces Inflammatory and Peripheral Neuropathic Pain in Rodents

Physical exercise is often recommended to patients who have chronic pain. However, only a small number of studies report exercise-induced analgesia in the setting of inflammatory pain, and even fewer relate long-term exercise to reductions in neuropathic pain. To address these questions, we evaluated the effect of extended swimming exercise in animal models of inflammatory (intraplantar injection of dilute formalin) and neuropathic (partial peripheral nerve injury) pain. We found that 9 days of swimming exercise in 37 degrees C water for 90 min/d decreased licking and flinching responses to formalin, as compared with nonexercised control animals. In addition, 18 to 25 days of swimming decreased nerve injury-induced cold allodynia and thermal hyperalgesia in rats, and 7 days of swimming decreased nerve injury-induced thermal hyperalgesia in mice. Our data indicate that swimming exercise reduces behavioral hypersensitivity in formalin- and nerve injury-induced animal models of persistent pain.

PERSPECTIVE

Surprisingly, few animal studies have investigated the effects of extended exercise on chronic pain. Our results support the use of exercise as a nonpharmacological approach for the management of peripheral neuropathic pain.

Pain perception after running a 100-mile ultramarathon

OBJECTIVE

To determine if pain perception is affected by an extreme bout of exercise that causes ongoing exercise-related pain.

DESIGN

Repeated-measures design.

SETTING

Pre-race registration area and finish area of an endurance race.

PARTICIPANTS

Twenty-one competitors in the 2005 Western States 100 Mile Endurance Run and 11 control subjects who were assisting at the race but not running.

INTERVENTIONS

Not applicable.

MAIN OUTCOME MEASURES

Overall pain and pain ratings on a pressure pain test before and after the event.

RESULTS

Mean overall pain +/- standard deviation on a 100-mm scale increased (P<.05) from 3+/-6mm before the run to 39+/-28mm after the run among the runners. The faster runners showed a mean reduction (P<.05) in pain ratings after the race of 15+/-20mm (on a 100-mm scale), whereas there was no change for the slower runners and controls. Findings were confirmed by model-based analysis.

CONCLUSIONS

The faster runners in a 100-mile (161-km) running race experience a modest temporary reduction in pressure pain perception that does not appear to be augmented by ongoing pain related to the exercise. The lack of a reduction in pain perception among the slower runners may be because an extreme bout of exercise of this nature can "exhaust" the systems responsible for exercise-induced analgesia in all but the most well-trained of runners, or that these systems were not activated because the slower runners were unable to maintain a high enough exercise intensity during the later stages of the race.

Effect of paroxetine on enhanced contextual fear induced by single prolonged stress in rats

RATIONALE

Single prolonged stress (SPS) is an animal model of posttraumatic stress disorder (PTSD) that can reproduce enhanced hypothalamo-pituitary-adrenal negative feedback.

OBJECTIVES

We examined whether SPS can produce an enhanced psychophysiological reactivity to laboratory stressors unrelated to trauma and whether paroxetine (PRX) can alleviate the enhanced anxiety and fear response in rats subjected to SPS. Furthermore, the effect of PRX on pain sensitivity was examined in rats with and without SPS.

METHODS

Rats were subjected to SPS (restraint for 2 h, forced swim for 20 min, and ether anesthesia) and then kept undisturbed for 14 days. After that, contextual fear response was assessed. Twenty-four hours after foot shock conditioning, freezing behavior was measured during reexposure to the shock environment for 3 min. Pain sensitivity was assessed by the flinch-jump test. PRX (0.01, 0.03, or 0.1 mg/mL) was chronically administered orally in drinking water.

RESULTS

Rats subjected to SPS showed a significant increase in contextual freezing compared to rats without SPS. Chronic administration of PRX at concentrations of 0.03 and 0.1 mg/mL (which produced serum concentrations similar to those that are clinically relevant) caused significant suppression of the enhanced contextual freezing. Acute administration of PRX at a dose producing clinically relevant serum concentrations did not affect the enhanced freezing.

CONCLUSIONS

Our results suggest that SPS can reproduce behavioral alteration similar to that observed in patients with PTSD, and this elevated fear response can be alleviated by the chronic administration of PRX at doses producing clinically relevant serum concentrations.

Low-intensity exercise reverses chronic muscle pain in the rat in a naloxone-dependent manner

OBJECTIVE

To determine the effects of low-intensity exercise on chronic muscle pain and potential activation of the endogenous opioid system.

DESIGN

Randomized placebo-controlled trial.

SETTING

Animal laboratory.

ANIMALS

Sixty-three male Sprague-Dawley rats.

INTERVENTIONS

Rats performed a low-intensity exercise protocol for 5 consecutive days after the induction of chronic muscle pain. In a separate experiment, naloxone or saline was administered systemically before 5 low-intensity exercise sessions.

MAIN OUTCOME MEASURE

Mechanical hyperalgesia was measured using von Frey filaments to determine the mechanical withdrawal threshold.

RESULTS

Low-intensity exercise increased mechanical withdrawal threshold in the chronic muscle pain model. Naloxone attenuated the antihyperalgesic effects of low-intensity exercise.

CONCLUSIONS

Low-intensity exercise reversed mechanical hyperalgesia in the chronic muscle pain model through activation of the endogenous opioid system.

The effects of restraint stress on nociceptive responses induced by formalin injected in rat's TMJ

It has been reported that stress can alter nociception from superficial tissues, such as skin and subcutaneous region. However, the influence of stress on an experimental deep nociception model is not understood. In this study, the temporomandibular joint (TMJ) formalin test was used to evaluate the effects of acute and chronic restraint stress on nociceptive responses in rats. Animals were initially submitted to one session of acute restraint stress (1 h) or exposed to chronic stress (40 days-1 h/day). Then, animals were killed immediately to collect blood for hormonal determinations by radioimmunoassay, or submitted to the TMJ formalin test to evaluate nociception. Rats submitted to acute restraint presented a performance similar to unstressed controls in the TMJ formalin test, whereas chronically stressed rats showed an increase in nociceptive responses. After 40 days of restraint, morphine was injected i.p. (1, 5 mg/kg or saline). The stressed rats displayed decreased morphine effects on nociception compared to unstressed controls. These findings suggest that repeated stress can produce hyperalgesia, which is, at least in part, due to alterations in the activity of opioid systems. This model may help elucidate the underlying neural mechanisms that mediate the effects of repeated stress on orofacial pain.

Synergistic interaction between mazindol, an anorectic drug, and swim-stress on analgesic responses in the formalin test in mice

The present study examined the interaction between mazindol (MZ), an anorectic drug extensively used in Brazil and opioid/non-opioid endogenous analgesic systems activated by swim-stress. Further, the role of opioid, dopamine and N-methyl-D-aspartate (NMDA) receptors in mediating the analgesic effect was evaluated. The stress-induced analgesia of a 3-min swimming at 32 degrees C (opioid/non-opioid) and 20 degrees C (non-opioid) were assessed using the formalin test. Male Swiss mice were intraperitoneally injected with naloxone (1.0 mg/kg), sulpiride (3.0 mg/kg), MK-801 (0.075 mg/kg) or saline/vehicle 15 min prior, and with MZ (0.5 mg/kg) or saline/vehicle 5 min prior to swimming. The dose of MZ (0.5 mg/kg) did not cause analgesic effect, however, the association of MZ and swim-stress at both temperatures displayed synergistic interaction on analgesia that was blocked by sulpiride and MK-801 but not by naloxone. The present results suggest that MZ and swim-stress acted synergistically on analgesic responses, involving mainly the non-opioid component and possibly mediated by dopamine D2 receptors and NMDA receptors.

A randomized clinical trial of the treatment effects of massage compared to relaxation tape recordings on diffuse long-term pain

BACKGROUND

Long-term musculoskeletal pain is a common problem in primary health care settings that is difficult to treat. Two common treatments are mental relaxation and massage. Scientific studies show contradictory results. Furthermore, many studies lack long-term follow-up even though it is a chronic disorder. The purpose of this randomized clinical trial was to assess possible effects of massage as compared to listening to relaxation tapes in conditions of 'diffuse' and long-term musculoskeletal pain.

METHODS

129 patients from primary health care suffering from long-term musculoskeletal pain were randomized to either a massage or mental relaxation group, and assessed before, during and after treatment.

RESULTS

During treatment there was a significant improvement in the three main outcome measures: self-rated health, mental energy, and muscle pain only in the massage group as compared to the relaxation group. However, at the 3-month post-treatment follow-up, there was a significant worsening in the outcome measures (time x group effect p < 0.05) back to initial rating levels in the massage group as compared to no changes in the relaxation group.

CONCLUSION

Massage, but not mental relaxation, is beneficial in attenuating diffuse musculoskeletal symptoms. Beneficial effects were registered only during treatment. This lack of long-term benefits could be due to the short treatment period or treatments such as these do not address the underlying causes of pain. Future studies of long-term pain should include longer treatment periods and post-treatment follow-up. It might also be worthwhile assessing the long-term benefits from booster treatment after the initial intense treatment period.

Sensitivity to the effects of opioids in rats with free access to exercise wheels: mu-opioid tolerance and physical dependence

RATIONALE

Exercise stimulates the release of endogenous opioid peptides and increases nociceptive (i.e. pain) thresholds in both human and animal subjects. During chronic, long-term exercise, sensitivity to the effects of morphine and other mu opioids decreases, leading some investigators to propose that exercise may lead to the development of cross tolerance to exogenously administered opioid agonists.

OBJECTIVE

The purpose of the present investigation was to examine the effects of chronic exercise on sensitivity to mu opioids, and to determine whether these effects can be attributed to the development of opioid tolerance and dependence.

METHODS

Rats were obtained at weaning and housed singly in standard polycarbonate cages (sedentary) or modified cages equipped with exercise wheels (exercise). After 6 weeks under these conditions, opioids possessing a range of relative efficacy at the mu receptor (morphine, levorphanol, buprenorphine, butorphanol, nalbuphine) were examined in a warm-water tail-withdrawal procedure.

RESULTS

Morphine, levorphanol and buprenorphine produced maximal levels of antinociception in both groups of rats, but all were more potent in sedentary rats than in exercising rats. Butorphanol and nalbuphine produced maximal levels of antinociception in sedentary rats under some conditions in which they failed to produce antinociception in exercising rats. Sensitivity to the effects of buprenorphine was decreased in sedentary rats that were transferred to cages equipped with exercise wheels, and increased in exercising rats that were transferred to sedentary housing conditions. In the latter group, exercise output prior to housing reassignment was positively correlated with increases in sensitivity to buprenorphine following housing reassignment. Naloxone administration precipitated a mild withdrawal syndrome in exercising rats that was not readily apparent in sedentary rats.

CONCLUSIONS

These data suggest that chronic exercise leads to the development of mu-opioid tolerance and physical dependence, and that these effects are similar to those produced by chronic opioid administration.

Analgesia following exercise: a review

Over the past 20 years a number of studies have examined whether analgesia occurs following exercise. Exercise involving running and cycling have been examined most often in human research, with swimming examined most often in animal research. Pain thresholds and pain tolerances have been found to increase following exercise. In addition, the intensity of a given pain stimulus has been rated lower following exercise. There have been a number of different noxious stimuli used in the laboratory to produce pain, and it appears that analgesia following exercise is found more consistently for studies that used electrical or pressure stimuli to produce pain, and less consistently in studies that used temperature to produce pain. There is also limited research indicating that analgesia can occur following resistance exercise and isometric exercise. Currently, the mechanism(s) responsible for exercise-induced analgesia are poorly understood. Although involvement of the endogenous opioid system has received mixed support in human research, results from animal research seem to indicate that there are multiple analgesia systems, including opioid and non-opioid systems. It appears from animal research that properties of the exercise stressor are important in determining which analgesic system is activated during exercise.

Reduction of stress-induced analgesia following ethanol exposure in mice

In the present study, we examined the effects of ethanol treatment on the subsequent expression of opioid and nonopioid forms of swim stress-induced analgesia (SSIA). In Experiment 1, mice were injected with ethanol (2.5 g/kg, i.p.) or an equal volume of saline once a day for two days. Animals received no treatment on day 3. On day 4, the animals were tested for opioid (3-min swim in water maintained at 32 degrees C) or nonopioid (3-min swim in water maintained at 20 degrees C) SSIA in the hotplate test (52 degrees C). Mice pretreated with ethanol injections showed a decrease in nonopioid SSIA, but not in opioid SSIA. In Experiment 2, mice were given an ethanol solution (10%) or tap water to drink for 15 days. On day 16, all animals were given tap water to drink. On day 17, the animals were tested for opioid or nonopioid SSIA. Neither form of SSIA was modified in mice that drank the ethanol solution. These results show that ethanol pretreatment can modify nonopioid endogenous analgesic responses in mice. Further, the route of administration influences the effects of ethanol pretreatment on SSIA.

Behavioural and hormonal effects of restraint stress and formalin test in male and female rats

The formalin test was used to measure the analgesia induced by restraint in male and female rats. Animals were restrained for 30 min or left undisturbed in their cage and then (1) killed immediately to collect blood for hormonal determinations; or (2) subcutaneously injected with formalin in the hind paw (or sham-injected), introduced to an open field for recording of behaviour, and killed at the end of this procedure. In both experiments, corticosterone was found to be higher in females. In Experiment 1, the ability of restraint to be stressful was confirmed by the increase in corticosterone in both sexes and by the decrease of testosterone in males. In Experiment 2, restraint-treatment induced a reduction in licking and flexing that was limited to the second phase. The reduction occurred in different periods and to a different degree in the two sexes; it was greater in females. Spontaneous behaviours showed sex differences in restraint-treated but not in formalin-treated animals. The results show that the hormonal effects observed after restraint are not present after the formalin test and that the marked analgesia observed with phasic painful stimuli does not occur with a longer-lasting one such as that induced by formalin, after which only partial and short-lasting effects were observed.

Delta-opiod receptor-mediated forced swimming stress-induced antinociception in the formalin test

Forced swimming stress-induced antinociception (FSSIA) was assessed using the formalin test. Male ICR mice, weighing about 30 g, were forced to swim in water at 20 degrees C for 3 min. In unstressed mice, SC injection of formalin (0.5%) to the hindpaw caused a biphasic response: an immediate nociceptive response (first phase) followed by a tonic response (second phase). Although forced swimming stress (FSS) had no effect on the duration of the first-phase response, FSS significantly reduced the duration of the second-phase response. The effect of FSSIA on the second-phase response was blocked by naltrindole (1 mg/kg, SC), a selective delta-opioid receptor antagonist, but not by beta-funaltrexamine (20 mg/kg, SC), a selective mu-opioid receptor antagonist. These results indicate that FSS may selectively reduce the second phase of the formalin-induced nociceptive response, primarily through delta-opioid receptors.

The formalin test: an evaluation of the method

The formalin test for nociception, which is predominantly used with rats and mice, involves moderate, continuous pain generated by injured tissue. In this way it differs from most traditional tests of nociception which rely upon brief stimuli of threshold intensity. In this article we describe the main features of the formalin test, including the characteristics of the stimulus and how changes in nociceptive behaviour may be measured and interpreted. The response to formalin shows an early and a late phase. The early phase seems to be caused predominantly by C-fibre activation due to the peripheral stimulus, while the late phase appears to be dependent on the combination of an inflammatory reaction in the peripheral tissue and functional changes in the dorsal horn of the spinal cord. These functional changes seem to be initiated by the C-fibre barrage during the early phase. In mice, the behavioural response in the late phase depends on the ambient temperature. We argue that the peripheral tissue temperature as well as other factors influencing the peripheral inflammation may affect the response, possibly confounding the results obtained with the test. Furthermore, we discuss the methods of recording the response and the value of observing more than one aspect of behaviour. Scoring of several behavioural variables provides a means of assessing motor or sensorimotor function as possible causes for changes in behaviour. In conclusion, the formalin test is a valuable addition to the battery of methods available to study nociception.

NMDA receptor antagonist MK-801 blocks non-opioid stress-induced analgesia in the formalin test

The analgesic effect of a 3-min swim stress was assessed using the formalin test. Male Swiss mice were injected i.p. with naloxone (0.1 or 1.0 mg/kg), MK-801 (0.075 mg/kg) or saline 15 min prior to swimming in water maintained at 20 degrees C or 32 degrees C. The mice were then injected with 20 microliters of 5% formalin into the plantar surface of 1 hind paw and pain behaviour (time spent licking the injected paw) was continuously monitored during the subsequent 10 min. Swim stress produced a significant reduction in pain behaviour at both 20 degrees C and 32 degrees C. MK-801 completely blocked the analgesia produced by both the 20 degrees C and 32 degrees C swim. At a dose of 0.1 mg/kg, naloxone partially antagonized the analgesia produced by the 32 degrees C swim but did not affect the analgesia produced by the 20 degrees C swim. Naloxone at a dose of 1.0 mg/kg had no effect on swim stress-induced analgesia. Neither MK-801 nor 0.1 mg/kg naloxone altered baseline pain behaviour, although 1.0 mg/kg naloxone did significantly reduce it. It is unlikely that the effect of MK-801 on swim stress-induced analgesia is due to an interaction with an opioid mechanism, as MK-801 had no effect on morphine analgesia. These results suggest that the analgesia produced by the 20 degrees C swim stress in the formalin test is non-opioid in nature and mediated via the NMDA receptor, whereas the 32 degrees C swim stress-induced analgesia has both an opioid and non-opioid component.

In vivo occupation of mouse brain opioid receptors by endogenous enkephalins: blockade of enkephalin degrading enzymes by RB 101 inhibits [3H]diprenorphine binding

With the aim of possibly studying the local activity of brain enkephalinergic pathways by autoradiography and positron emission tomography, preliminary competition experiments of [3H]diprenorphine binding in mouse brain were carried out after i.v. administration of the first systemically-active mixed inhibitor of enkephalin degrading enzymes RB 101 (N(R,S)-2-benzyl-3[(S)-(2-amino-4-methylthiobutyldithio]-1-oxoprop yl]- L-phenylalanine benzyl ester). Although devoid of affinity for the opioid binding sites, RB 101 inhibited the [3H]diprenorphine binding to the opioid receptors in a dose-dependent manner. This effect, very likely due to an RB 101-induced increase in extracellular levels of enkephalins, reached a plateau at a dose of 10 mg/kg, where almost 30% displacement was observed. Intravenous administration of either 5 or 20 mg/kg of RB 101 in mice submitted to warm-swim stress led to an additional [3H]diprenorphine displacement, which reached 45% compared to unstressed controls. This ceiling effect could account for the reported minimal morphine-like side effects induced by mixed inhibitors. A large increase in endogenous enkephalin levels induced by RB 101, associated or not with stress, was also indirectly demonstrated by the analgesic responses elicited by i.v. injection of the mixed inhibitor. This effect was blocked by naloxone but not by the delta antagonist naltrindole (NTI), supporting a preferential implication of mu receptors in supraspinal analgesia. Taken together, these results suggest that RB 101 could be used to determine the precise in vivo localization of enkephalinergic pathways recruited by various stimuli.

Stress analgesia: the opioid analgesia of long swims suppresses the non-opioid analgesia induced by short swims in mice

In mice, room temperature swimming for as short a period as 15 sec has been found to induce a non-opioid analgesia with a time course of 10-12 min. As the duration of the swim is increased, an opioid analgesia develops with a longer persistence (25-30 min); the development of the opioid analgesia appears to suppress the expression of the non-opioid analgesia so that none of the latter is evident after 3 min swims. The characteristics of the tail-flick nociceptive test are also described.

Neurobiological background of pain and analgesia: the attempt at revaluation according to position of the organism's adaptive activity

The most adequate and successful way to understand the essence of any complex psychophysiological phenomenon, including pain, is obviously the study of its origin, its genesis, i.e., its biological background. Based on critical analysis of recent literature and our own electrophysiological, biochemical and pharmacological data we tried to overcome the difficulties and contradictions derived from the traditional reflex approach and analytical orientation in understanding the experimental investigation of pain-related problems and to determine the neurobiological background of pain and analgesia through the notion of the organism's adaptive activity. Interrelations between the notion of pain and other biological and psychological ideas, the place and functional significance of pain and endogenous analgesic mechanisms in the organization, maintenance and regulation of the organism's adaptive activity, characterization of the involvement of endogenous opioid peptides and monoamines in central processes associated with pain and analgesia, the essence and mechanisms of pain-depressing activity of the opiates are the main stages in our neurobiological consideration of the phenomenon of pain and its natural and pharmacological regulation.

Nociceptive sensitivity/behavioral reactivity regulation in rats during aversive states of different nature: its mediation by opioid peptides

To study the regulation of nociceptive sensitivity/behavioral reactivity in animals during aversive states of different nature, the changes of vocalization thresholds and tail-withdrawal latencies were investigated in rats in free behavior, during restraint stress, after acute trauma to an extremity and under intraperitoneal acetic acid administration. To understand opioid peptide involvement in mediation of the changes obtained, this analysis was also done during opiate receptor blockade by naloxone. The data on the modification of vocalization and movement reactivity as well as on the changes of suprarenal weight and gastric ulceration, produced in normal and naloxone-treated rats by innoxious stressogenic, noxious somatic and visceral stimulation are discussed in relation with: 1. the peculiarities of sensitivity and responsivity of animals to external stimuli in aversive environment; 2. the role of these changes in maintenance of an animal's adaptive activity produced by environmental threat and their mediation by endogenous opioids; 3. the functional significance of the activation of endogenous opioidergic neurotransmission in organization, realization and modification of an animal's adaptive activity, directed on behavioral escape from aversive environment as well as on satisfaction of actual biological and zoosocial needs, in regulation of precise conformity among homeostasis, behavior and variable environment.