N-methyl-D-aspartic acid (NMDA) receptor antagonist MK-801 blocks non-opioid stress-induced analgesia. II. Comparison across three swim-stress paradigms in selectively bred mice

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.

Low intensity blood flow restriction exercise: Rationale for a hypoalgesia effect

Exercise-induced hypoalgesia is characterised by a reduction in pain sensitivity following exercise. Recently, low intensity exercise performed with blood flow restriction has been shown to induce hypoalgesia. The purpose of this manuscript is to discuss the mechanisms of exercise-induced hypoalgesia and provide rationale as to why low intensity exercise performed with blood flow restriction may induce hypoalgesia. Research into exercise-induced hypoalgesia has identified several potential mechanisms, including opioid and endocannabinoid-mediated pain inhibition, conditioned pain modulation, recruitment of high threshold motor units, exercise-induced metabolite production and an interaction between cardiovascular and pain regulatory systems. We hypothesise that several mechanisms consistent with prolonged high intensity exercise may drive the hypoalgesia effect observed with blood flow restriction exercise. These are likely triggered by the high level of intramuscular stress in the exercising muscle generated by blood flow restriction including hypoxia, accumulation of metabolites, accelerated fatigue onset and ischemic pain. Therefore, blood flow restriction exercise may induce hypoalgesia through similar mechanisms to prolonged higher intensity exercise, but at lower intensities, by changing local tissue physiology, highlighting the importance of the blood flow restriction stimulus. The potential to use blood flow restriction exercise as a pain modulation tool has important implications following acute injury and surgery, and for several load compromised populations with chronic pain.

Mechanisms of placebo analgesia: A dual-process model informed by insights from cross-species comparisons

Placebo treatments are pharmacologically inert, but are known to alleviate symptoms across a variety of clinical conditions. Associative learning and cognitive expectations both play important roles in placebo responses, however we are just beginning to understand how interactions between these processes lead to powerful effects. Here, we review the psychological principles underlying placebo effects and our current understanding of their brain bases, focusing on studies demonstrating both the importance of cognitive expectations and those that demonstrate expectancy-independent associative learning. To account for both forms of placebo analgesia, we propose a dual-process model in which flexible, contextually driven cognitive schemas and attributions guide associative learning processes that produce stable, long-term placebo effects. According to this model, the placebo-induction paradigms with the most powerful effects are those that combine reinforcement (e.g., the experience of reduced pain after placebo treatment) with suggestions and context cues that disambiguate learning by attributing perceived benefit to the placebo. Using this model as a conceptual scaffold, we review and compare neurobiological systems identified in both human studies of placebo analgesia and behavioral pain modulation in rodents. We identify substantial overlap between the circuits involved in human placebo analgesia and those that mediate multiple forms of context-based modulation of pain behavior in rodents, including forebrain-brainstem pathways and opioid and cannabinoid systems in particular. This overlap suggests that placebo effects are part of a set of adaptive mechanisms for shaping nociceptive signaling based on its information value and anticipated optimal response in a given behavioral context.

Naloxone exacerbates memory impairments and depressive-like behavior after mild traumatic brain injury (mTBI) in mice with upregulated opioid system activity

The neuroprotective role of the endogenous opioid system in the pathophysiological sequelae of brain injury remains largely ambiguous. Noteworthy, almost no data is available on how its genetically determined activity influences the outcome of mild traumatic brain injury. Thus, the aim of our study was to examine the effect of opioid receptor blockage on cognitive impairments produced by mild traumatic brain injury in mice selectively bred for high (HA) and low (LA) swim-stress induced analgesia that show innate divergence in opioid system activity. Mild traumatic brain injury was induced with a weight-drop device on anaesthetized mice. Naloxone (5mg/kg) was intraperitoneally delivered twice a day for 7days to non-selectively block opioid receptors. Spatial memory performance and manifestations of depressive-like behavior were assessed using the Morris Water Maze and tail suspension tests, respectively. Mild traumatic brain injury resulted in a significant deterioration of spatial memory performance and severity of depressive-like behavior in the LA mouse line as opposed to HA mice. Opioid receptor blockage with naloxone unmasked cognitive deficits in HA mice but was without effect in the LA line. The results suggest a protective role of genetically predetermined enhanced opioid system activity in suppression of mild brain trauma-induced cognitive impairments. Mice selected for high and low swim stress-induced analgesia might therefore be a useful model to study the involvement of the opioid system in the pathophysiology and neurological outcome of traumatic brain injury.

Fluoxetine reverses the behavioral despair induced by neurogenic stress in mice: role of N-methyl-d-aspartate and opioid receptors

Opioid and N-methyl-d-aspartate (NMDA) receptors mediate different effects of fluoxetine. We investigated whether opioid and NMDA receptors are involved in the protective effect of fluoxetine against the behavioral despair induced by acute physical stress in male mice. We used the forced swimming test (FST), tail suspension test (TST), and open-field test (OFT) for behavioral evaluation. We used fluoxetine, naltrexone (opioid receptor antagonist), MK-801 (NMDA receptor antagonist), morphine (opioid receptor agonist), and NMDA (NMDA receptor agonist). Acute foot-shock stress (FSS) significantly induced behavioral despair (depressive-like) and anxiety-like behaviors in tests. Fluoxetine (5 mg/kg) reversed the depressant-like effect of FSS, but it did not alter the locomotion and anxiety-like behavior in animals. Acute administration of subeffective doses of naltrexone (0.3 mg/kg) or MK-801 (0.01 mg/kg) potentiated the antidepressant-like effect of fluoxetine, while subeffective doses of morphine (1 mg/kg) and NMDA (75 mg/kg) abolished this effect of fluoxetine. Also, co-administration of subeffective doses of naltrexone (0.05 mg/kg) and MK-801 (0.003 mg/kg) with fluoxetine (1 mg/kg) induced a significant decrease in the immobility time in FST and TST. Our results showed that opioid and NMDA receptors (alone or in combination) are involved in the antidepressant-like effect of fluoxetine against physical stress.

Mechanisms of exercise-induced hypoalgesia

The purpose of this study was to examine opioid and endocannabinoid mechanisms of exercise-induced hypoalgesia (EIH). Fifty-eight men and women (mean age = 21 years) completed 3 sessions. During the first session, participants were familiarized with the temporal summation of heat pain and pressure pain protocols. In the exercise sessions, following double-blind administration of either an opioid antagonist (50 mg naltrexone) or placebo, participants rated the intensity of heat pulses and indicated their pressure pain thresholds and pressure pain ratings before and after 3 minutes of submaximal isometric exercise. Blood was drawn before and after exercise. Results indicated that circulating concentrations of 2 endocannabinoids, N-arachidonylethanolamine and 2-arachidonoylglycerol, as well as related lipids oleoylethanolamide, palmitoylethanolamide, N-docosahexaenoylethanolamine, and 2-oleoylglycerol, increased significantly (P < .05) following exercise. Pressure pain thresholds increased significantly (P < .05), whereas pressure pain ratings decreased significantly (P < .05) following exercise. Also, temporal summation ratings were significantly lower (P < .05) following exercise. These changes in pain responses did not differ between the placebo and naltrexone conditions (P > .05). A significant association was found between EIH and docosahexaenoylethanolamine. These results suggest involvement of a nonopioid mechanism in EIH following isometric exercise.

PERSPECTIVE

Currently, the mechanisms responsible for EIH are unknown. This study provides support for a potential endocannabinoid mechanism of EIH following isometric exercise.

Significance of neuronal cytochrome P450 activity in opioid-mediated stress-induced analgesia

Stressful environmental changes can suppress nociceptive transmission, a phenomenon known as "stress-induced analgesia". Depending on the stressor and the subject, opioid or non-opioid mechanisms are activated. Brain μ opioid receptors mediate analgesia evoked either by exogenous agents (e.g. morphine), or by the release of endogenous opioids following stressful procedures. Recent work with morphine and neuronal cytochrome P450 (P450)-deficient mice proposed a signal transduction role for P450 enzymes in µ analgesia. Since µ opioid receptors also mediate some forms of stress-induced analgesia, the present studies assessed the significance of brain P450 activity in opioid-mediated stress-induced analgesia. Two widely-used models of opioid stress-induced analgesia (restraint and warm water swim) were studied in both sexes of wild-type control and P450-deficient (Null) mice. In control mice, both stressors evoked moderate analgesic responses which were blocked by pretreatment with the opioid antagonist naltrexone, confirming the opioid nature of these responses. Consistent with literature, sex differences (control female>control male) were seen in swim-induced, but not restraint-induced, analgesia. Null mice showed differential responses to the two stress paradigms. As compared with control subjects, Null mice showed highly attenuated restraint-induced analgesia, showing a critical role for neuronal P450s in this response. However, warm water swim-induced analgesia was unchanged in Null vs. control mice. Additional control experiments confirmed the absence of morphine analgesia in Null mice. These results are the first to show that some forms of opioid-mediated stress-induced analgesia require brain neuronal P450 activity.

Association between the A107V substitution in the δ-opioid receptors and ethanol drinking in mice selected for high and low analgesia

Experimental evidence suggests that endogenous opioids play an important role in the development of ethanol addiction. In this study, we employed two mouse lines divergently bred for opioid-mediated stress-induced analgesia. In comparison with HA (high analgesia line) mice, LA (low analgesia line) mice, having lower opioid receptor system activity, manifest enhanced basal as well as stress-induced ethanol drinking. Here, we found that recently discovered C320T transition in exon 2 of the δ-opioid receptor gene (EU446125.1), which results in an A107V substitution (ACA23171.1), leads to higher ethanol preference in CT mice compared with CC homozygotes. This genetic association is particularly evident under chronic mild stress (CMS) conditions. The interaction between stress and ethanol intake was significantly stronger in HA than in LA mice. Ethanol almost completely attenuated the pro-depressive effect of CMS (assessed with the tail suspension test) in both the CC and CT genotypes in the HA line. In the LA mice, a lack of response to ethanol was observed in the CC genotype, whereas ethanol consumption strengthened depressive-like behaviours in CT individuals. Our results suggest that constitutively active A107V substitution in δ-opioid receptors may be involved in stress-enhanced vulnerability to ethanol abuse and in the risk of ethanol dependence.

Chronic mild stress facilitates melanoma tumor growth in mouse lines selected for high and low stress-induced analgesia

Both chronic stress conditions and hyperergic reaction to environmental stress are known to enhance cancer susceptibility. We described two mouse lines that displayed high (HA) and low (LA) swim stress-induced analgesia (SSIA) to investigate the relationship between inherited differences in sensitivity to stress and proneness to an increased growth rate of subcutaneously inoculated melanoma. These lines display several genetic and physiological differences, among which distinct sensitivity to mutagens and susceptibility to cancer are especially noticeable. High analgesic mice display high proneness both to stress and a rapid local spread of B16F0 melanoma. However, stress-resistant LA mice do not develop melanoma tumors after inoculation, or if so, tumors regress spontaneously. We found that the chronic mild stress (CMS) procedure leads to enhanced interlinear differences in melanoma susceptibility. Tumors developed faster in stress conditions in both lines. However, LA mice still displayed a tendency for spontaneous regression, and 50% of LA mice did not develop a tumor, even under stressed conditions. Moreover, we showed that chronic stress, but not tumor progression, induces depressive behavior, which may be an important clue in cancer therapy. Our results clearly indicate how the interaction between genetic susceptibility to stress and environmental stress determine the risk and progression of melanoma. To our knowledge, HA/LA mouse lines are the first animal models of distinct melanoma progression mediated by inherited differences in stress reactivity.

Influence of preferred versus prescribed exercise on pain in fibromyalgia

PURPOSE

The purpose of this study was to examine the influence of a preferred- versus a prescribed-intensity exercise session on pain in women with fibromyalgia (FM).

METHODS

Twenty-one women with FM (mean age = 44 yr) completed two randomly assigned exercise sessions consisting of 20 min of cycle ergometry at a self-selected intensity and a prescribed intensity. Experimental pain perception was assessed before and after aerobic exercise. During exercise, HR, watts, RPE, and muscle pain were assessed every 5 min. Clinical pain was assessed with the Short-Form McGill Pain Questionnaire (SF-MPQ) immediately and 24, 48, 72, and 96 h after exercise. Data were analyzed with repeated-measures ANOVA.

RESULTS

Women with FM preferred a lower intensity of exercise than what was prescribed as indicated by significantly lower HR, watts, and RPE responses (P < 0.05). Muscle pain in the legs, however, was similar in the two conditions and significantly increased during exercise (P < 0.05). Pain thresholds and pain tolerances increased significantly after exercise, whereas peak pain ratings decreased after exercise (P < 0.05). Furthermore, pain (SF-MPQ) in the follow-up period was found to be lower than baseline (P < 0.05).

CONCLUSIONS

It is concluded that the women with FM who participated in this study experienced significant improvements in pain after exercise. The results from this study are novel and indicate that recommendations for exercise prescription for individuals with FM should consider the preferred-intensity exercise model as a strategy to reduce pain.

Stress-induced analgesia and endogenous opioid peptides: the importance of stress duration

Stress is known to elicit pain relief, a phenomenon referred to as stress-induced analgesia. Based on stress parameters, opioid and non-opioid intrinsic pain inhibitory systems can be activated. In the present study, we assessed whether changing the duration of stress would affect the involvement of endogenous opioids in antinociception elicited by swim in warm water (32 °C), known to be opioid-mediated. Using mice lacking beta-endorphin, enkephalins or dynorphins and their respective wild-type littermates, we assessed the role of each opioid peptide in antinociception induced by a short (3 min) vs. long (15 min) swim. Mice were tested for baseline hot plate latency, exposed to swim (3 or 15 min) in warm water (32 °C) and then tested for antinociception at 5, 15 and 30 min. Our results revealed that both swim paradigms induced significant antinociception in wild-type mice. However, the short swim failed to induce antinociception in beta-endorphin-deficient mice, illustrating that beta-endorphin is important in this form of stress-induced antinociception. On the other hand, antinociception elicited by the long swim was only slightly reduced in beta-endorphin-deficient mice despite pretreatment with naloxone, a non-selective opioid receptor antagonist, significantly attenuated the antinociception elicited by the long swim. Nevertheless, a delayed hyperalgesic response developed in mice lacking beta-endorphin following exposure to either swim paradigm. On the other hand, mice lacking enkephalins or dynorphins and their respective wild-type littermates expressed a comparable antinociceptive response and did not exhibit the delayed hyperalgesic response. Together, our results suggest that the endogenous opioid peptide beta-endorphin not only mediates antinociception induced by the short swim but also prevents the delayed hyperalgesic response elicited by either swim paradigm.

Examination of the dose-response relationship between pain perception and blood pressure elevations induced by isometric exercise in men and women

The purpose of this study was to examine exercise-induced hypoalgesia (EIH) in men and women, and whether different magnitudes of BP elevations induced by isometric exercise systematically influenced pain perception. Twenty-five men and 25 women performed isometric exercise at 25% MVC for 1-min, 3-min, and 5-min while BP and pain perception were assessed. Results indicated that BP was significantly elevated (p<0.05) by isometric exercise in a dose-response manner. Pain thresholds were found to be elevated while pain ratings were lower (p<0.05) immediately following isometric exercise but not in a dose-response manner. It was concluded that isometric exercise produced EIH in men and women, and there was not a dose-response relationship between BP and EIH.

Involvement of the nitric oxide/(C)GMP/K(ATP) pathway in antinociception induced by exercise in rats

AIMS

Physical exercise is responsible for increasing the nociceptive threshold. The present study aimed to investigate the involvement of the nitric oxide/(C)GMP/K(ATP) pathway in antinociception induced by acute aerobic exercise (AAc) in rats.

MAIN METHODS

Wistar rats performed exercise in a rodent treadmill, according to an AAc protocol. The nociceptive threshold was measured by mechanical and thermal nociceptive tests (paw-withdrawal, tail-flick and face-flick). To investigate the involvement of the NO/(C)GMP/K(ATP) pathway the following nitric oxide synthase (NOS) unspecific and specific inhibitors were used: N-nitro-l-arginine (NOArg), Aminoguanidine, N(5)-(1-Iminoethyl)-l-ornithine dihydrocloride (L-NIO), N(omega)-Propyl-l-arginine (L-NPA); guanylyl cyclase inhibitor, 1H-[1,2,4]oxidiazolo[4,3-a]quinoxalin-1-one (ODQ); and K(ATP) channel blocker, Glybenclamide; all administered subcutaneously at a dose of 2mg/kg 10min before exercise started. Plasma and cerebrospinal fluid (CSF) nitrite levels were determined by spectrophotometry.

KEY FINDINGS

In the paw-withdrawal, tail-flick and face-flick tests, the AAc protocol produced antinociception, which lasted for more than 15min. This effect was significantly reversed (P<0.05) by NOS specific and unspecific inhibitors, guanylyl cyclase inhibitor (ODQ) and K(ATP) channel blocker (Glybenclamide). Acute exercise was also responsible for increasing nitrite levels in both plasma and cerebrospinal fluid.

SIGNIFICANCE

Taken together, these results suggest that the NO/(C)GMP/K(ATP) pathway participates in antinociception induced by exercise.

Involvement of NTS2 receptors in stress-induced analgesia

Stress activates multiple neural systems that suppress pain sensation. This adaptive phenomenon referred as stress-induced analgesia (SIA) is mediated by the activation of endogenous pain inhibitory systems. Both opioid and non-opioid forms of SIA have been elicited in rodents according to stressor parameters and duration. There is accumulating evidence that the endogenous neurotensin (NT) system plays an important role in SIA. Especially, NT-deficient mice were shown to exhibit reduced SIA following water avoidance or restraint stress. Since central NT produces naloxone-insensitive analgesic effects by acting on spinal and supraspinal NTS2 receptors, we hypothesized that NT might mediate non-opioid SIA through NTS2 activation. Here, we evaluated the influence of an opioid-independent severe stress produced by a cold-water swim for 3 min at 15 degrees C on rodent offspring's pain perception. Our results demonstrated that mice lacking NTS2 exhibit significantly reduced SIA following cold-water swim stress. Indeed, NTS2 knockout mice submitted to both acute (plantar test) and tonic (formalin test) pain stimuli show a greater sensitivity to pain in comparison to wild-type littermates. Accordingly, pretreatment with the NT receptor antagonist SR142948A results in a hyperalgesic response to stress induced by cold-water swim. Endogenous NT regulates hypothalamic-pituitary-adrenal axis activity in stress condition by increasing corticosterone plasma levels. Accordingly, the plasma levels of corticosterone measured by radioimmunoassay are significantly reduced in non-stressed and stressed NTS2-deficient mice in comparison with wild-type mice. To further investigate the site of action of NT in mediating SIA, we microinjected NTS2 agonists in lumbar spinal cord and quantified post-stress sensitivity to pain in rats using the plantar test. Exogenously administered NTS2 analogs, JMV-431, beta-lactotensin and NT69L markedly enhance the magnitude and duration of stress antinociception in both 25- and 60-day-old rats. In sum, by using genetic and pharmacological approaches, we demonstrated here that NTS2 receptors mediate non-opioid SIA. Our results also revealed that the release of endogenous NT in response to stress requires the presence of NTS2 to stimulate corticotropin-releasing factor (CRF)-induced elevation of plasma corticosterone, and that NTS2 receptors localized at the lumbar spinal cord participate to the disinhibition of descending pain control pathways. Therefore, these data highlight the significance of NTS2 as a novel target for the treatment of pain and stress-related disorders.

Contralateral Attenuation of Pain After Short-Duration Submaximal Isometric Exercise

Only a small amount of research has been conducted examining whether exercise-induced hypoalgesia (EIH) occurs after isometric exercise. Thus, the purpose of this investigation was to examine whether EIH occurred in women after short-duration submaximal isometric exercise and whether the responses were restricted to the exercised hand (ipsilateral) or also occurred in the nonexercised (contralateral) hand. Fourteen healthy women (mean age = 19.5 years) completed 2 sets of submaximal (40% to 50% of max) isometric exercise consisting of squeezing a dynamometer for 2 minutes with the dominant hand. A pressure stimulus was applied to the forefinger on the dominant and nondominant hands for 2 minutes before and after isometric exercise. Participants pressed a button when the stimulus became painful, indicating pain threshold (PT), and also rated the intensity of the stimulus every 15 seconds, using a pain rating scale (PR). Results indicated that there were significant trials effects (P < .05) for PT and PR, but the main effect for hands was not significant (P > .05). PTs were found to be elevated, whereas PRs were reduced for both hands after isometric exercise. It is concluded that submaximal isometric exercise performed for 2 minutes resulted in ipsilateral and contralateral hypoalgesic responses.

PERSPECTIVE

The findings from the present study demonstrated that short-duration nonexhaustive isometric exercise was associated with hypoalgesic responses in the exercised and nonexercised hands. It appears that short-duration submaximal isometric exercise resulted in generalized (ie, ipsilateral and contralateral) pain-inhibitory responses in healthy young women.

Repeated stress alters the ability of nicotine to activate the hypothalamic-pituitary-adrenal axis

Acute nicotine administration has been shown to activate the hypothalamic-pituitary-adrenal (HPA) axis and stimulate secretion of adrenocorticotrophic hormone (ACTH), corticosterone/cortisol and beta-endorphin (beta-END) in both rodents and humans, raising the possibility that activation of the HPA axis by nicotine may mediate some of the effects of nicotine. Since stress can increase the risk of drug use and abuse, we hypothesized that repeated stress would increase the ability of nicotine to stimulate the secretion of HPA hormones. To test our hypothesis, mice were exposed to repeated stress (swimming in 15 degrees C water for 3 min/day for 5 days) and killed 15 min after injection of saline or nicotine (0.1 mg/kg, s.c.). Repeated exposure to stress increased the ability of nicotine to stimulate plasma ACTH (p<0.05) and beta-END (p<0.05), but not corticosterone secretion. In contrast, repeated exposure to stress increased the post-saline injection levels of corticosterone (p<0.05), but not ACTH and beta-END. The present results suggest that chronic stress leads to an enhanced sensitivity of some components of the HPA axis to a subsequent nicotine challenge.

Increased stress-induced analgesia in mice lacking the proneuropeptide convertase PC2

Many neuropeptides involved in pain perception are generated by endoproteolytic cleavages of their precursor proteins by the proprotein convertases PC1 and PC2. To investigate the role of PC2 in nociception and analgesia, we tested wild-type and PC2-null mice for their responses to mechanical and thermal nociceptive stimuli, before and after a short swim in cold or warm water. Basal responses and responses after a cold swim were similar between the two groups. However, after a short forced swim in warm water, PC2-null mice were significantly less responsive to the stimuli than wild-type mice, an indication of increased opioid-mediated stress-induced analgesia. The enhanced analgesia in PC2-null mice may be caused by an accumulation of opioid precursor processing intermediates with potent analgesic effects, or by loss of anti-opioid peptides.

Dissociation of analgesic and hormonal responses to forced swim stress using opioid receptor knockout mice

Exposure to stress triggers hormonal and behavioral responses. It has been shown that the endogenous opioid system plays a role in some physiological reactions to stress. The opioid system was described to mediate analgesia induced by mild stressors and to modulate the activation of the hypothalamic-pituitary-adrenal axis. Our study assessed the contribution of opioid receptors in stress-induced analgesia and adrenocorticotropic hormone (ACTH) and corticosterone release by a genetic approach. We performed a parallel analysis of mice deficient in mu, delta, or kappa opioid receptors, as well as of triple opioid receptor knockout mice, following exposure to a mild stress (3-min swim at 32 degrees C). In wild-type mice, stress elicited an increase in jumping latency on the hot plate, which was influenced by gender and genetic background. This analgesic response was reversed both by naloxone and by the triple mutation, and decreased in mu and delta opioid receptor knockout females. In wild-type females, stress also delayed front- and hindpaw behaviors in the hot plate test and increased tail-flick latency in the tail immersion test. Opioid receptor deletion however did not affect these stress responses. In addition, stress produced an increase in ACTH and corticosterone plasma levels. This endocrine response remained unchanged in all mutant strains. Therefore our data indicate that, under our stress conditions, the endogenous opioid system is recruited to produce some analgesia whereas it does not influence hypothalamic-pituitary-adrenal axis activity. This implies that brain circuits mediating analgesic and hormonal responses to stress can be dissociated.

Dopamine receptor mechanism(s) and antinociception and tolerance induced by swim stress in formalin test

In the present study, involvement of D1 and D2 dopamine receptors in the antinociception and tolerance induced by water swim stress in the formalin test has been investigated. Water swim stress at 20 degrees C temperature induced antinociception in both phases of the formalin test. Intraperitoneal administration of the D2 dopamine receptor antagonist, sulpiride (25 and 50 mg/kg) reduced swim stress-induced antinociception in the second phase of the formalin test. A higher dose of the D1 dopamine receptor antagonist, SCH23390 (0.1 mg/kg, intraperitoneal) also reduced swim stress-induced antinociception in both phases of the test. Exposure to 3 min water swimming stress, once daily for 3 days, induced tolerance to swim stress-induced antinociception in the second phase of the formalin test. Administration of sulpiride (12.5, 25 and 50 mg/kg), during exposure to water swimming stress (once daily for 3 days), decreased tolerance in the second phase, whereas the antagonist (12.5 and 50 mg/kg) increased pain scores in the first phase of the formalin test. Sulpiride (25 mg/kg) treatment however, once daily for 3 days with no water swimming stress, did not alter swim stress-induced antinociception (0.5, 1 and 3 min tests). Similarly, repeated treatment with SCH23390 (0.05 mg/kg) and water swimming stress did not alter tolerance induced by water swimming stress. Repeated administration of the antagonist in the absence of water swimming stress also did not change swim stress-induced antinociception. The results may indicate a possible involvement of both dopamine D1 and D2 receptors in the antinociception induced by swim stress and D2 receptor mechanism in the tolerance induced by repeated swim stress.

Cross-tolerance between antinociception induced by swim-stress and morphine in formalin test

The present study investigated cross-tolerance between antinociception induced by water swim-stress and morphine in the formalin test. Intraperitoneal administration of morphine (3, 6 and 9 mg/kg) induced dose-dependent antinociception in both phases of the formalin test. Mice treated with a lower dose of morphine (25 mg/kg), once daily for 3 days, showed tolerance to antinociception induced by a lower test dose of morphine (3 mg/kg). Similar repeated treatments with a higher dose of morphine (50 mg/kg) produced tolerance to antinociception induced by different test doses of morphine (3, 6 and 9 mg/kg). Exposure to water swim-stress, once daily for 2 or 3 days in order to induce tolerance, also decreased morphine-induced antinociception. Swim-stress exposure for 2 or 3 days also tends to potentiate tolerance induced by a lower dose of morphine. Acute swim-stress of different durations (0.5, 1 and 3 min) induced antinociception in both phases of the formalin test, which was not reduced by naloxone, but showed even more antinociception in the second phase. The response to swim stress was decreased in mice treated with higher doses of morphine, but not those animals that received swimming stress (3 min) once daily for 2-3 days, in order to induce habituation to swim-stress-induced antinociception. The results may indicate a possible cross-tolerance between antinociception induced by morphine and by swim stress.

Sex differences in pain and analgesia: the role of gonadal hormones

There is now strong evidence for sex differences in pain and analgesia. These differences imply that gonadal steroid hormones such as estradiol and testosterone modulate sensitivity to pain and analgesia. The goal of this review is to present an overview of gonadal steroid modulation of pain and analgesia in animals and humans, and to describe mechanisms by which males' and females' biology may differentially predispose them to pain and to analgesic effects of drugs and stress. Evidence is presented to demonstrate that sex differences in pain and analgesia may be both quantitative and qualitative in nature. Current research suggests that sex-specific management of clinical pain will be a reality in the not-so-distant future.

Acute progesterone can recruit sex-specific neurochemical mechanisms mediating swim stress-induced and kappa-opioid analgesia in mice

There is a qualitative sex difference in the neurochemical mediation of stress-induced and kappa-opioid analgesia; these phenomena are dependent on N-methyl-d-aspartic acid (NMDA) receptors in males but not females. Progesterone modulation of this sex difference was examined in mice. Analgesia against thermal nociception was produced by forced cold water swim or by systemic administration of the kappa-opioid agonist, U50,488. As seen previously, the NMDA receptor antagonist MK-801 blocked both forms of analgesia in male but not female mice. Also as in previous studies, this sex difference was found to be dependent on ovarian hormones such that ovariectomy induced female mice to "switch" to the male-like, NMDAergic system. We now demonstrate that a single injection of progesterone (50 microg), systemically administered 30 min before analgesia assessment, is sufficient to restore female-specific mediation of analgesia (i.e., insensitivity to MK-801 blockade) in ovariectomized female mice. The rapidity of this neurochemical "switching" action of progesterone suggests mediation via cell surface receptors or the action of neuroactive steroid metabolites of progesterone.

Differential modulation of nociceptive neural responses in medial and lateral pain pathways by peripheral electrical stimulation: a multichannel recording study

It is well accepted that peripheral electrical stimulation (PES) can produce an analgesic effect in patients with acute and chronic pain. However, the neural basis underlying stimulation-induced analgesia remains unclear. In the present study, we examined the pain-related neural activity modified by peripheral stimulation in rats. The stimulation frequency of pulses applied to needle electrodes in the hindlimb was 2 Hz alternating with 100 Hz, with 0.6 ms pulse width for 2 Hz and 0.2 ms for 100 Hz. The intensity of the stimulation was increased stepwise from 1 to 3 mA with each 1-mA step lasting for 10 min. The nociceptive neural and behavioral responses were examined immediately after the termination of stimulation. Using a multiple-channel recording technique, we simultaneously recorded the activity of many single neurons located in the primary somatosensory and anterior cingulate cortex (ACC), as well as the ventral posterior and medial dorsal thalamus in behaving rats. Our results showed that peripheral electrical stimulation significantly reduced the nociceptive responses in ventroposterior thalamus and somatosensory cortex, indicating an inhibition of nociceptive processing. In contrast, the analgesic stimulation produced a significant increase in mediodorsal thalamus while a less significant decrease in cingulate cortex, reflecting a complicated effect associated with combined antinociceptive activation and nociceptive suppression. These results support the idea that peripheral electrical stimulation can ultimately alter the pain perception by specifically inhibiting the nociceptive transmission in the sensory pathway while mobilizing the antinociceptive action in the affective pathway, thus to produce pain relief.

Internal and External Factors Affecting the Development of Neuropathic Pain in Rodents. Is It All About Pain?

It is important to know the factors that will influence animal models of neuropathic pain. A good reproducibility and predictability in different strains of animals for a given test increases the clinical relevance and possible targeting. An obligatory requirement for enabling comparisons of results of different origin is a meticulous definition of the specific sensitivities of a model for neuropathic pain and a description of the test conditions. Factors influencing neuropathic pain behavior can be subdivided in external and internal factors. The most important external factors are; timing of the measurement of pain after induction of neuropathy, circadian rhythms, seasonal influences, air humidity, influence of order of testing, diet, social variables, housing and manipulation, cage density, sexual activity, external stress factors, and influences of the experimenter. The internal factors are related to the type of animal, its genetic background, gender, age, and the presence of homeostatic adaptation mechanisms to specific situations or stress. In practice, the behavioral presentations to pain depend on the combination of genetic and environmental factors such as accepted social behavior. It also depends on the use of genetic manipulation of the animals such as in transgenic animals. These make the interpretation of data even more difficult. Differences of pain behavior between in- and outbred animals will be better understood by using modern analysis techniques. Substrains of animals with a high likelihood for developing neuropathic pain make the unraveling of specific pathophysiological mechanisms possible. Concerning the effect of stress on pain, it is important to differentiate between external and internal stress such as social coping behavior. The individual dealing with this stress is species sensitive, and depends on the genotype and the social learning. In the future, histo-immunological and genetic analysis will highlight similarities of the different pathophysiological mechanisms of pain between different species and human subjects. The final objective for the study of pain is to describe the genetics of the eliciting pain mechanisms in humans and to look for correlations with the knowledge from basic research. Therefore, it is necessary to know the genetic evolution of the different mechanisms in chronic pain. In order to be able to control the clinical predictability of a putative treatment the evolutionary pharmacogenomic structure of specific transmitters and receptors must be clarified.

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.

Effects of gestational stress and neonatal handling on pain, analgesia, and stress behavior of adult mice

Stressors presented during the late prenatal and early postnatal periods can have long-term effects on offspring behavior, due to the sensitive periods in the formation of brain circuitry associated with early development. This study investigated the long-term effects of prenatal (restraint during the last week of gestation) and postnatal (daily handling for 14 days postnatal) stress, alone and in combination, on adulthood pain behavior, analgesic responses to stress and morphine, and on behavioral indices of stress reactivity. We found that all of the adult responses measured were altered by perinatal manipulations. Nociceptive thresholds were increased by prenatal or by postnatal stress in males and females; application of both stressors in combination negated these effects. Elevations in morphine analgesia were also observed in animals undergoing either perinatal stressor, but not in those who received both stressors. Behavioral and analgesic responses to stress were consistent with previous observations of reduced stress responsiveness following neonatal handling, with some sex-specific findings. Male and female handled subjects exhibited decreases in stress behavior, and both groups of female handled subjects (regardless of prenatal stress [PS] condition) exhibited decreases in stress-induced analgesia (SIA). Males, on the other hand, exhibited decreases in SIA only if they were prenatally stressed (regardless of handling condition). Thus, prenatal and postnatal stressors have differing effects on the neural circuitry underlying pain, pain inhibition, and stress behavior.

Effect of cold acclimation and repeated swimming on opioid and nonopioid swim stress-induced analgesia in selectively bred mice

Swiss-Webster mice selectively bred for high swim stress-induced analgesia (SSIA) were exposed to continuous ambient cold (5 degrees C) for 6 weeks or to daily 3-min swims for 14 consecutive days either in 20 or 32 degrees C water. Thereafter, mice subjected to the particular procedure were injected intraperitoneally with 10 mg/kg of naltrexone HCl and were tested for modification of the opioid and nonopioid component of SSIA. SSIA was reduced following swims at either water temperature and was antagonized by naltrexone to greater extent than in nonswimming mice. Thus, the nonopioid (i.e. naltrexone-resistant) portion of the overall SSIA was significantly reduced, whereas the opioid (naltrexone-sensitive) portion became relatively augmented. In contrast, SSIA differed neither in magnitude nor in sensitivity to naltrexone between cold-acclimated and unacclimated mice. Swim hypothermia as well as the nonopioid component of SSIA were decreased after repeated swimming at 32 and 20 degrees C, but remained unchanged after cold acclimation. This argues for the essential role of an extrathermal, probably emotional in nature, factor not only in the elicitation of nonopioid SSIA, but also in the modulation of thermoregulatory processes during swimming. We suggest that the emergency component of swim stress, together with initial moderate hypothermic challenge, first produces the opioid form of SSIA, and subsequently, as the swim continues, also affects the thermoregulatory processes maintaining thermal homeostasis. This causes further increase in swim hypothermia and raises its stressing property to induce the nonopioid form of SSIA.

Antidepressant-type effect of the NK3 tachykinin receptor agonist aminosenktide in mouse lines differing in endogenous opioid system activity

The influence of the tachykinin NK3 receptor agonist, aminosenktide on the immobility in the forced swimming test was studied in mouse lines selectively bred for divergent magnitudes of stress-induced analgesia. The high analgesia (HA) line is known to display enhanced, and the low analgesia (LA) line displays reduced activity of the opioid system. Aminosenktide at doses of 125 microg/kg or 250 microg/kg intraperitoneally (IP) reduced, in naltrexone-reversible manner, the immobility more of opioid receptor-dense HA than of unselected mice, but was ineffective in the opioid receptor-deficient LA line. The effect of aminosenktide was quite similar to the antiimmobility action of desipramine (10 mg/kg IP), a prototypic antidepressant agent. None of the compounds increased animals' locomotion as found with an open field test; therefore their antiimmobility effect cannot be attributed to a change in general motility. The results claim that aminosenktide causes an antidepressant effect, and endogenous opioids are involved in this process.

Reduced antinociception and plasma extravasation in mice lacking a neuropeptide Y receptor

Neuropeptide Y (NPY) is believed to exert antinociceptive actions by inhibiting the release of substance P and other 'pain neurotransmitters' in the spinal cord dorsal horn. However, the physiological significance and potential therapeutic value of NPY remain obscure. It is also unclear which receptor subtype(s) are involved. To identify a possible physiological role for the NPY Y1 receptor in pain transmission, we generated NPY Y1 receptor null mutant (Y1-/-) mice by homologous recombination techniques. Here we show that Y1-/- mice develop hyperalgesia to acute thermal, cutaneous and visceral chemical pain, and exhibit mechanical hypersensitivity. Neuropathic pain is increased, and the mice show a complete absence of the pharmacological analgesic effects of NPY. In the periphery, Y1 receptor activation is sufficient and required for substance P release and the subsequent development of neurogenic inflammation and plasma leakage. We conclude that the Y1 receptor is required for central physiological and pharmacological NPY-induced analgesia and that its activation is both sufficient and required for the release of substance P and initiation of neurogenic inflammation.

Arthritic calcitonin/alpha calcitonin gene-related peptide knockout mice have reduced nociceptive hypersensitivity

Peripheral inflammation induced with a knee joint injection of a mixture of kaolin/carrageenan (k/c) produces primary and secondary hyperalgesia. Inflammatory pain is thought to involve a variety of transmitters released from nerve terminals, including amino acids, substance P (SP) and calcitonin gene-related peptide (CGRP). In the present study, mice deficient in the calcitonin/alpha CGRP gene (CGRP(-/-)) displayed normal responses to noxious stimuli. However, the CGRP knockout mice failed to demonstrate development of secondary hyperalgesia after induction of knee joint inflammation in two tests that assess central sensitization, through testing at sites remote from the primary insult. Nociceptive behavioral responses were assessed using the hot-plate test and paw withdrawal latency (PWL) to radiant heat applied to the hindpaw. The CGRP(-/-) mice showed no signs of secondary hyperalgesia after development of knee joint inflammation, while the expected significant decrease in the PWL was observed in the CGRP(+/+) mice as control. The CGRP(-/-) mice also had a prolonged rather than a shortened response latency in the hot-plate test 4 h after knee joint injection of k/c. Immunohistological study showed that CGRP-like immunoreactivity (CGRP-LI) was absent in the spinal cord and dorsal root ganglia taken from the CGRP(-/-) mice. These results indicate that endogenous CGRP plays an important role in the plastic neurogenic changes occurring in response to peripheral inflammatory events including the development of nociceptive behaviors.

Acoustic startle and open-field behavior in mice bred for magnitude of swim analgesia

Acoustic startle response (ASR) and open-field activity was examined in the 46th generation of mice that have been selectively bred for high analgesia (HA) and for low analgesia (LA) induced by 3-min swimming in 20 degrees C water. These lines were earlier found to differ in brain opioid receptor density and in the expression of opioid-mediated phenomena, as analgesic sensitivity to opiates and reversibility of swim stress-induced analgesia (SSIA) by naloxone. For comparison, a randomly bred control (C) line was used. To measure the amplitude of ASR, the mice were exposed to 110-dB acoustic stimuli in a Coulbourn apparatus. In saline-injected mice, the ASR force was found significantly lower in the LA than in the HA, as well in the C line, but did not differ between the two last lines. Naltrexone hydrochloride (10 mg/kg IP 30 min before ASR testing) augmented the startle in the opioid receptor-dense HA line, but had no effect in the opioid receptor-deficient LA line, as well in the C line; therefore, the ASR magnitude in naltrexone-injected HA mice was significantly higher compared to the C line. HA mice displayed less activity in an open-field test; that is, they remained immobile longer in the center of the field, and thereafter performed less ambulation and less rearing against the wall compared to the LA line. Naltrexone failed to modify the open-field activity in any line. The results confirm that the pattern of ASR depends on the genetic makeup of the animals. The higher amplitude of ASR, taken together with the lower open-field activity of HA mice, can be interpreted in terms of higher anxiety level, compared to the LA line. It is suggested that the higher ASR in HA mice relies on a nonopioid mechanism, which is tonically inhibited by the opioid system.

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.

Pain perception and response: central nervous system mechanisms

Although several decades of studies have detailed peripheral and ascending nociceptive pathways to the thalamus and cerebral cortex, pain is a symptom that has remained difficult to characterize anatomically and physiologically. Positron emission tomography (PET) and functional magnetic imaging (fMRI) have recently demonstrated a number of cerebral and brain stem loci responding to cutaneous noxious stimuli. However, intersubject variability, both in the frequency and increased or decreased intensity of the responses, has caused uncertainty as to their significance. Nevertheless, the large number of available imaging studies have shown that many areas with recognized functions are frequently affected by painful stimuli. With this evidence and recent developments in tracing central nervous system connections between areas responding to noxious stimuli, it is possible to identify nociceptive pathways that are within, or contribute to, afferent spino-thalamo-cortical sensory and efferent skeletomotor and autonomic motor systems. In this study it is proposed that cortical and nuclear mechanisms for pain perception and response are hierarchically arranged with the prefrontal cortex at its highest level. Nevertheless, all components make particular contributions without which certain nociceptive failures can occur, as in pathological pain arising in some cases of nervous system injury.

Reduction of stress-induced analgesia but not of exogenous opioid effects in mice lacking CB1 receptors

CB1 cannabinoid receptors are widely distributed in the central nervous system where they mediate most of the cannabinoid-induced responses. Here we have evaluated the interactions between the CB1 cannabinoid receptors and the endogenous opioid system by assaying a number of well-characterized opioid responses, e.g. antinociception and stress-mediated effects, on mutant mice in which the CB1 receptor gene was invalidated. The spontaneous responses to various nociceptive stimuli (thermal, mechanical and visceral pain) were not changed in mutant CB1 mice. Furthermore, the absence of the CB1 cannabinoid receptor did not modify the antinociceptive effects induced by different opioid agonists: morphine (preferential mu opioid agonist), D-Pen2-D-Pen5-enkephalin (DPDPE) and deltorphin II (selective delta opioid agonists), and U-50,488H (selective kappa opioid agonist) in the hot-plate and tail-immersion tests. In contrast, the stress-induced opioid mediated responses were modified in CB1 mutants. Indeed, these mutants did not exhibit antinociception following a forced swim in water at 34 degrees C and presented a decrease in the immobility induced by the previous exposure to electric footshock. However, the antinociception induced by a forced swim in water at 10 degrees C was preserved in CB1 mutants. These results indicate that CB1 receptors are not involved in the antinociceptive responses to exogenous opioids, but that a physiological interaction between the opioid and cannabinoid systems is necessary to allow the development of opioid-mediated responses to stress.

Analgesia in selectively bred mice exposed to cold in helium/oxygen atmosphere

In order to evaluate the stressing role of swim hypothermia in producing swim stress-induced analgesia (SSIA), we examined whether a mere decrease in the animals' core temperature without swimming would be sufficient to elicit analgesia. The subjects were Swiss-Webster mice selectively bred for 37 and 40 generations for divergent magnitudes of SSIA. High (HA) and low analgesia (LA) mice were exposed for 15 min to temperatures in the range between -5 and +20 degrees C in 79% He/21% O2 (Heliox) atmosphere. The Heliox exposure produced ambient temperature-dependent hypothermia and analgesia, as assessed with a hot-plate test (56 degrees C). The post-Heliox analgesia was of much higher magnitude in HA than in LA mice. The steeper slope of regression of the magnitude of analgesia upon hypothermia in HA mice indicates that these mice are far more sensitive to the analgesic effect of hypothermia than LA mice. Naltrexone HCl (10 mg/kg i.p.) attenuated analgesia in ambient temperature-dependent manner in HA, but not in LA mice. In view of the apparent similarity of Heliox-induced analgesia and SSIA we suggest that hypothermia is a powerful component of swim stress to induce SSIA in the mouse.

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.

Alterations in swim stress-induced analgesia and hypothermia following serotonergic or NMDA antagonists in the rostral ventromedial medulla of rats

Serotonergic, NMDA, or opioid antagonists in the rostral ventromedial medulla (RVM) reduce morphine analgesia elicited from the periaqueductal gray (PAG). Continuous (CCWS) and intermittent (ICWS) cold-water swims elicit respective naltrexone-insensitive and naltrexone-sensitive analgesic responses. CCWS analgesia is reduced by systemic NMDA receptor antagonism and by systemic, but not intrathecal serotonergic antagonism. ICWS analgesia is reduced by both systemic and intrathecal serotonergic antagonism, but unaffected by systemic NMDA antagonism. The present study evaluated whether serotonergic (methysergide: 5-10 microg) or competitive [AP7 (2-amino-7-phosphonoheptanoic acid): 0.01-0.1 microg] or non-competitive [MK-801 (dizocilipine maleate): 0.3-3 microg] NMDA antagonists in the RVM altered CCWS and ICWS analgesia and hypothermia as well as basal nociceptive latencies. Methysergide in the RVM significantly potentiated CCWS, but not ICWS analgesia. In contrast, AP7 in the RVM significantly potentiated ICWS analgesia. Antagonist-induced changes in either hypothermia or basal nociception failed to account for any alterations in stress-induced analgesia. These data suggest that serotonergic, but not NMDA, receptors in the RVM may mediate collateral inhibition between mesencephalic morphine analgesia and naltrexone-insensitive CCWS analgesia.

The effect of genotype on sensitivity to inflammatory nociception: characterization of resistant (A/J) and sensitive (C57BL/6J) inbred mouse strains

The important role of genetic factors in the mediation of sensitivity to pain and pain inhibition is being increasingly appreciated. In an attempt to systematically study the genotypic influences on inflammatory nociception, we conducted a survey of the nociceptive responsivity of three common outbred mouse strains and 11 inbred mouse strains on the formalin test. The formalin test is known to display a biphasic temporal pattern of behavioral and electrophysiological activity, defined by an acute/early phase and a tonic/late phase. Nociceptive sensitivity (licking/biting of the affected area) to a subcutaneous injection of 5% formalin (25 microl volume) into the plantar surface of the right hindpaw displayed moderate heritability in both phases (0.38 and 0.46, respectively). One strain, A/J, was identified as extremely resistant to formalin nociception, displaying total licking in the acute and tonic phases that was 60% and 87% lower, respectively, than the grand mean of all strains. A subsequent series of experiments were performed to characterize the difference between A/J and C57BL/6J mice. The findings establish this inbred strain comparison as a useful genetic model of nociceptive sensitivity.

Altered nociception, analgesia and aggression in mice lacking the receptor for substance P

The peptide neurotransmitter substance P modulates sensitivity to pain by activating the neurokinin-1 (NK-1) receptor, which is expressed by discrete populations of neurons throughout the central nervous system. Substance P is synthesized by small-diameter sensory 'pain' fibres, and release of the peptide into the dorsal horn of the spinal cord following intense peripheral stimulation promotes central hyperexcitability and increased sensitivity to pain. However, despite the availability of specific NK-1 antagonists, the function of substance P in the perception of pain remains unclear. Here we investigate the effect of disrupting the gene encoding the NK-1 receptor in mice. We found that the mutant mice were healthy and fertile, but the characteristic amplification ('wind up') and intensity coding of nociceptive reflexes was absent. Although substance P did not mediate the signalling of acute pain or hyperalgesia, it was essential for the full development of stress-induced analgesia and for an aggressive response to territorial challenge, demonstrating that the peptide plays an unexpected role in the adaptive response to stress.

Sensory changes and pain after abdominal hysterectomy: a comparison of anesthetic supplementation with fentanyl versus magnesium or ketamine

Drugs interacting with opioid or N-methyl-D-aspartate (NMDA) receptors may have differing effects on post-surgical sensory changes, such as central inhibition or spinal excitation. We compared the effect of supplementing isoflurane/N2O/O2 anesthesia with an opioid agonist (fentanyl [n = 15]) or two drugs inhibiting the NMDA system differently (magnesium, ketamine [n = 15 in each group]) on sensory changes after abdominal hysterectomy. Electric sensation, pain detection, and pain tolerance thresholds were determined (preoperatively and 1, 4, 24 h, and 5 days postoperatively) in arm, thoracic, incision, and leg dermatomes together with pain scores and cumulative morphine consumption. Thresholds relative to the arm were derived to unmask segmental sensory changes hidden by generalized changes. Absolute thresholds were increased 1-24 h, returning to baseline on Day 5, without overall differences among drugs. Fentanyl thresholds were lower 1 h and higher 5 days postoperatively compared with magnesium and ketamine; thresholds were lower at 24 h for magnesium versus ketamine. Relative thresholds increased compared with baseline only with fentanyl (1-4 h); none decreased. Pain scores and morphine consumption were similar. Thus, all adjuvants suppressed spinal sensitization after surgery. Fentanyl showed the most, and magnesium the least, central sensory inhibition up to 5 days postoperatively, with different patterns of inhibition directly postsurgery versus later. Differences in sensory processing were not reflected in clinical measures.

IMPLICATIONS

We studied the effects on postsurgical sensory processing of general anesthesia supplemented by drugs affecting opioid or N-methyl-D-aspartate receptors using sensory thresholds. Generalized central sensory inhibition, differently affected by the drugs, predominated after surgery. All drugs suppressed spinal excitation. Clinical pain measures did not reflect sensory change.

Identification of a sex-specific quantitative trait locus mediating nonopioid stress-induced analgesia in female mice

It is increasingly appreciated that the sexes differ in their perception of noxious stimuli and in their responsivity to exogenous and endogenous analgesic manipulations. We previously reported the existence of qualitative sex differences in the neurochemical mediation of nonopioid (i.e., naloxone-insensitive) stress-induced analgesia (SIA) produced by forced swims and suggested that female mice possess a sex-specific SIA mechanism. This female-specific system is now known to be estrogen-dependent, to be ontogenetically organized, and to vary with reproductive status; however, its neurochemical identity remains obscure. In an attempt to identify candidate genes underlying SIA in both sexes, we performed a two-phase quantitative trait locus (QTL) mapping experiment using the BXD/Ty recombinant inbred (RI) set derived from DBA/2J (D2) and C57BL/6J (B6) inbred mouse strains and (B6xD2)F2 hybrid mice derived from these same progenitors. All mice were subjected to 3 min forced swims in 15 degrees C water; nociceptive sensitivity on the 54 degrees C hot-plate assay was assessed immediately before and 2 min after cessation of the swim. We report the localization of a QTL statistically associated with SIA magnitude [p = 0.00000012; logarithm of the odds (LOD) = 6.1] in female mice only. This female-specific QTL, which we name Fsia1, is located on chromosome 8 at 52-84 cM from the centromere and accounts for 17-26% of the overall trait variance in this sex. The present data provide further evidence of the existence of a female-specific SIA mechanism and highlight the important role of both genetic background and gender in the inhibition of pain.

Sex differences in N-methyl-D-aspartate involvement in kappa opioid and non-opioid predator-induced analgesia in mice

There are suggestions of sex differences in N-methyl-D-aspartate (NMDA) receptor system involvement in the mediation of analgesia. The present study examined the effects of the specific, competitive NMDA antagonist, NPC 12626, on the nociceptive (50 degrees C hot plate) responses of reproductive male and female laboratory mice exposed to (i) an ethologically relevant aversive stimulus, the odor of a predator and (ii) administration of the kappa opiate agonist, U69,593. A 30-s exposure to 2-propylithietane, the major component of weasel odor, elicited a 'non-opioid' analgesia that was in both sexes insensitive to naloxone and the kappa opiate antagonist nor-binaltorphimine. In male mice this non-opioid analgesia was antagonized by NPC 1262, while in reproductive females the predator-induced analgesia was insensitive to NPC 12626. Similarly, NPC 12626 attenuated the analgesic effects of the kappa opiate agonist, U69,593, in male mice while having no significant effects on the equivalent levels of kappa opiate analgesia in females. These results show that there are sex differences in NMDA involvement in the expression and, or mediation of both non-opioid stress-induced and kappa opiate-mediated analgesia.

Sex and genotype determine the selective activation of neurochemically-distinct mechanisms of swim stress-induced analgesia

A growing literature documents the important influence of organismic factors such as sex and genotype on pain sensitivity and pain modulation. We recently determined that 3-min forced swims in 15 degrees C water produce non-opioid (i.e., naloxone-insensitive) analgesia in outbred Swiss-Webster mice of both sexes; this form of stress-induced analgesia (SIA) is significantly attenuated by the N-methyl-D-aspartate (NMDA) antagonist, dizocilpine (MK-801) in males, but not females. A pilot study designed to confirm the non-opioid and (in male mice) NMDAergic nature of 15 degrees C swim SIA in the C57BL/6J and DBA/2J inbred strains used widely in gene mapping was conducted, using the hot-plate (54 degrees C) assay of nociception. In female mice of both strains, 15 degrees C swim SIA was insensitive to antagonism by either naloxone (10 mg/kg, i.p.) or dizocilpine (0.1 mg/kg. i.p.). In male C57BL/ 6J mice, the observed SIA was naloxone-insensitive, but was attenuated by dizocilpine. This pattern of results is virtually identical to that obtained using Swiss-Webster mice in this and previous studies. However, male DBA/2J mice displayed SIA that was significantly attenuated by naloxone, but insensitive to dizocilpine antagonism. These findings support the hypothesis that genetic factors and sex, in addition to stressor parameters, can determine the selective recruitment of alternative central mechanisms of pain inhibition.

Pain responses, anxiety and aggression in mice deficient in pre-proenkephalin

Enkephalins are endogenous opioid peptides that are derived from a pre-proenkephalin precursor protein. They are thought to be vital in regulating many physiological functions, including pain perception and analgesia, responses to stress, aggression and dominance. Here we have used a genetic approach to study the role of the mammalian opioid system. We disrupted the pre-proenkephalin gene using homologous recombination in embryonic stem cells to generate enkephalin-deficient mice. Mutant enk-/- animals are healthy, fertile, and care for their offspring, but display significant behavioural abnormalities. Mice with the enk-/- genotype are more anxious and males display increased offensive aggressiveness. Mutant animals show marked differences from controls in supraspinal, but not in spinal, responses to painful stimuli. Unexpectedly, enk-/- mice exhibit normal stress-induced analgesia. Our results show that enkephalins modulate responses to painful stimuli. Thus, genetic factors may contribute significantly to the experience of pain.