Hyperalgesia induced by emotional stress in the rat: an experimental animal model of human anxiogenic hyperalgesia

Mating-induced analgesia is dependent of copulatory male pattern in high- and low- yawning male rats

Mating behavior in rodents can modulate pain sensations in both sexes. In males, the execution of mounts, intromissions, and ejaculations induced a progressive increase in their vocalization thresholds induced by tail shocks and other types of noxious stimuli. We selectively inbred two sublines from Sprague-Dawley (SD) rats that differed in their spontaneous yawning frequency. The high-yawning (HY) subline had a mean of 20 yawns/h and a different pattern of sexual behavior characterized by longer interintromission intervals and more sexual bouts that delayed ejaculation. The low-yawning (LY) subline and SD rats yawned as a mean 2 and 1 yawns/h, respectively. So, we determine mating-induced analgesia in HY, LY, and SD male rats by measuring vocalization thresholds in response to noxious electric tail shocks. Our results showed that the magnitude of mating-induced analgesia was lower in HY and LY rats with respect to SD rats. When the rats performed different components of male sexual pattern, both sublines exhibited a significantly lower increase in their vocalization thresholds with respect to SD rats-being sublines less responsive regarding mating-induced analgesia. Pain modulation mechanisms depend on responses to stress, so the low levels of analgesia obtained in the yawning sublines may be due either to differences in their response to stress in other paradigms, or to atypical performance of male sexual behavior during mating, an event which as a stressful event in rats. Therefore, the yawning sublines are a suitable model for analyzing how a different temporal pattern in the display of male sexual behavior affects analgesia mechanisms. Our results concur with Wistar rats with different endophenotypes that could apply to humans as well.

α9-nAChR knockout mice exhibit dysregulation of stress responses, affect and reward-related behaviour

The α9α10-subtype of nicotinic acetylcholine receptor (nAChR) has recently garnered interest in biomedicine and is being pursued as an analgesic target. However, the receptor exhibits diverse tissue distribution, the function of which is known to varying degrees, and targeting this receptor for clinical treatments without a broad understanding of its function may have adverse consequences. The α9α10-nAChR is expressed in the adrenal and pituitary glands, suggesting a potential role in the stress response, but little is known about its function in this tissue. Here we determined a role for the α9α10-nAChR in behavioural and physiological stress responses, by comparing the stress- and affect-related phenotypes of wildtype and α9-nAChR knockout mice. Naïve knockout mice exhibited largely normal behaviour on standard tests of affective behaviour. However, after sub-chronic restraint stress knockout mice showed significantly decreased stress-induced arousal and increased anxiety-like behaviour when compared to wildtype animals. Physiologically, corticosterone responses were muted in knockout mice after an acute stressor, but exaggerated in response to the same stressor after undergoing sub-chronic stress. Behavioural profiling of the α9-nAChR knockout mice in the home-cage revealed that circadian patterns of activity were altered when compared to wildtype controls. Furthermore, knockout mice showed altered responses to a period of reward discounting, resulting in anhedonia-like behaviour in a sucrose preference test where WT mice continued to seek reward. These experiments uncover a novel role for the α9α10-nAChR in mounting a normal stress response and in the regulation of affective- and reward-related behaviour, and suggest that pursuing the receptor for clinical treatments may not be as straightforward as has been suggested.

Increasing Pain Sensation Eliminates the Inhibitory Effect of Depression on Evoked Pain in Rats

Although previous studies have suggested that depression may be associated with inhibition of evoked pain but facilitation of spontaneous pain, the mechanisms underlying these relationships are unclear. The present study investigated whether the difference between evoked and spontaneous pain on sensory (descending inhibition) and affective (avoidance motivation) components contributes to the divergent effects of depression on them. Depressive-like behavior was produced in male Wistar rats by unpredictable chronic mild stress (UCMS). Tone-laser conditioning and formalin-induced conditioned place avoidance (F-CPA) were used to explore avoidance motivation in evoked and spontaneous pain, respectively. Behavioral pharmacology experiments were conducted to examine descending inhibition of both evoked (thermal stimulation) and spontaneous pain behavior (formalin pain). The results revealed that the inhibitory effect of depression on evoked pain was eliminated following repeated thermal stimuli. Avoidance behavior in the tone-laser conditioning task was reduced in UCMS rats, relative to controls. However, avoidance motivation for formalin pain in the UCMS group was similar to controls. 5-HT_1A receptor antagonism interfered with inhibition of pain responses over time. The present study demonstrated that the inhibitory effect of depression on evoked pain dissipates with increased nociception and that the sensory-discriminative and affective-motivational components of pain are jointly involved in the divergent effects of depression on pain.

Neurobiology of stress-induced hyperalgesia

The intensity and severity of perceived pain does not correlate consistently with the degree of peripheral or central nervous system tissue damage or with the intensity of primary afferent or spinal nociceptive neurone activity. In this respect, the modulation of pain by emotion and context is now widely recognized. In particular, stress, fear and anxiety exert potent, but complex, modulatory influences on pain. Stress can either suppress pain (stress-induced analgesia) or exacerbate it (stress-induced hyperalgesia; SIH) depending on the nature, duration and intensity of the stressor. Herein, we review the methods and models used to study the phenomenon of SIH in rodents and humans and then present a detailed discussion of our current understanding of neural substrates and neurobiological mechanisms. The review provides perspectives and challenges for the current and future treatment of pain and the co-morbidity of pain with stress-related psychiatric disorders including anxiety and depression.

Predator odor stress alters corticotropin-releasing factor-1 receptor (CRF1R)-dependent behaviors in rats

Humans with stress-related anxiety disorders exhibit increases in arousal and alcohol drinking, as well as altered pain processing. Our lab has developed a predator odor stress model that produces reliable and lasting increases in alcohol drinking. Here, we utilize this predator odor stress model to examine stress-induced increases in arousal, nociceptive processing, and alcohol self-administration by rats, and also to determine the effects of corticotropin-releasing factor-1 receptors (CRF1Rs) in mediating these behavioral changes. In a series of separate experiments, rats were exposed to predator odor stress, then tested over subsequent days for thermal nociception in the Hargreaves test, acoustic startle reactivity, or operant alcohol self-administration. In each experiment, rats were systemically injected with R121919, a CRF1R antagonist, and/or vehicle. Predator odor stress increased thermal nociception (i.e., hyperalgesia) and acoustic startle reactivity. Systemic administration of R121919 reduced thermal nociception and hyperarousal in stressed rats but not unstressed controls, and reduced operant alcohol responding over days. Stressed rats exhibited increased sensitivity to the behavioral effects of R121919 in all three tests, suggesting up-regulation of brain CRF1Rs number and/or function in stressed rats. These results suggest that post-stress alcohol drinking may be driven by a high-nociception high-arousal state, and that brain CRF1R signaling mediates these stress effects.

Forced swim-induced musculoskeletal hyperalgesia is mediated by CRF2 receptors but not by TRPV1 receptors

The exacerbation of musculoskeletal pain by stress in humans is modeled by the musculoskeletal hyperalgesia in rodents following a forced swim. We hypothesized that stress-sensitive corticotropin releasing factor (CRF) receptors and transient receptor vanilloid 1 (TRPV1) receptors are responsible for the swim stress-induced musculoskeletal hyperalgesia. We confirmed that a cold swim (26 °C) caused a transient, morphine-sensitive decrease in grip force responses reflecting musculoskeletal hyperalgesia in mice. Pretreatment with the CRF2 receptor antagonist astressin 2B, but not the CRF1 receptor antagonist NBI-35965, attenuated this hyperalgesia. Desensitizing the TRPV1 receptor centrally or peripherally using desensitizing doses of resiniferatoxin (RTX) failed to prevent the musculoskeletal hyperalgesia produced by cold swim. SB-366791, a TRPV1 antagonist, also failed to influence swim-induced hyperalgesia. Together these data indicate that swim stress-induced musculoskeletal hyperalgesia is mediated, in part, by CRF2 receptors but is independent of the TRPV1 receptor.

Ethnic differences in physical pain sensitivity: role of acculturation

Although research suggests that Asian Americans are more reactive to physical pain than European Americans, some evidence suggests that the observed differences in ethnicity may actually reflect Asian Americans' differing levels of acculturation. Two studies were conducted to test this hypothesis. In Study 1, first- and second-generation Asian Americans and European Americans took part in a cold pressor task. Evidence of heightened pain responses was found only among first-generation Asian Americans. Study 2 further controlled for ethnicity and replicated this pattern in finding heightened pain reactions among mainland Chinese students in Hong Kong relative to Hong Kong Chinese students. These findings suggest a role for acculturation in accounting for ethnic differences in physical pain sensitivity.

Environmentally induced antinociception and hyperalgesia in rats and mice

Stress can enhance and inhibit nociception depending on the situation. Thus, simply shifting the context from the elevated plus maze (EPM) which has been shown to produce stress-induced antinociception to a different environment could produce drastic and rapid changes in nociception. The present experiment tested this hypothesis by assessing nociception in rats and mice during and immediately after removal from the maze. Experiment 1 found hyperalgesia in female and male rats tested on the hot plate immediately after exposure to the elevated plus maze. This hyperalgesia occurred with or without the added stress of a hind paw formalin injection and regardless of whether rats were exposed to an EPM with open (oEPM) or enclosed (eEPM) arms despite a clear antinociceptive effect while on the oEPM. Experiment 2 showed a similar shift from antinociception to nociception on the formalin test in mice immediately after removing them from the EPM. These data demonstrate that a mild stressor such as the EPM can produce both antinociception and hyperalgesia depending on the context. This shift from antinociception to hyperalgesia occurs rapidly and is evident in mice, male and female rats, and with the hot plate and formalin tests.

Selective ablation of mu-opioid receptor expressing neurons in the rostral ventromedial medulla attenuates stress-induced mechanical hypersensitivity

AIMS

Chronic stress-related conditions are often associated with stress-induced hyperalgesia. However, the neural circuitry responsible for producing stress-induced hyperalgesia is not well characterized. The aim of this study was to determine the contribution of mu-opioid expressing brainstem neurons to the expression of stress-induced hyperalgesia.

MAIN METHODS

The present study utilized a model of stress-induced mechanical hypersensitivity that involved application of repeated, light tactile whisker pad stimulation (WPS) in rats. Repeated WPS (10 applications/session, 4 sessions/h in 1 day, sessions on days 1-5 and 8-12) increased defensive-aggressive and hypervigilant behaviors, and produced hypersensitivity to tactile stimulation of the hind paw. In order to test the possible involvement of mu-opioid receptor expressing neurons in the rostral ventral medulla (RVM) to this response, rats received RVM microinjections of the toxin conjugate dermorphin-saporin or its control, saporin. Fourteen days later rats underwent either WPS or sham conditioning.

KEY FINDINGS

Repeated WPS produced defensive-aggressive behaviors directed towards the stimulus and mechanical hypersensitivity of the hind paw that persisted for up to 2 weeks after the final WPS session. Dermorphin-saporin, but not saporin, microinjections prevented the development of hind paw mechanical hypersensitivity, but did not affect the defensive-aggressive behaviors.

SIGNIFICANCE

The finding that chronic stress produces mechanical hypersensitivity through circuitry that involves the RVM provides a potential neurobiological basis for the complex interaction between chronic stress and pain.

Do psychological and physiological stressors alter the acute pain response to castration and tail docking in lambs?

OBJECTIVE

To investigate whether events that may be stressful to young lambs, including simulated infection or social isolation, modulate pain experienced by lambs following castration and tail docking (C/D).

STUDY DESIGN

Randomised, controlled, prospective study.

ANIMALS

Fifty male lambs born to 46 second-parity Mule ewes.

METHODS

Lambs were allocated randomly to one of four groups, experiencing either a potential stressor or handling on day 2 after birth, followed by C/D or handling only on day 3. Quantitative sensory testing (QST) data [mechanical nociceptive thresholds (MNT), Semmes Weinstein filaments (SW), response to cold] and serum cortisol concentration were measured at time points after application of treatments to lambs on days 2 and 3 after birth. The treatment groups were LPS, injection of bacterial lipopolysaccharide IV on day 2, C/D on day 3; ISOL, isolation from the dam for 10 minutes on day 2, C/D on day 3; CAST, handling only on day 2, C/D on day 3; CONT, handled only on days 2 and 3.

RESULTS

Castration and tail docking caused transient hypoalgesia as measured by MNT and SW. Simulated infection and isolation caused hyperalgesia 3 hours after application, indicated by a reduction in MNT, however they did not alter the pain response to C/D compared to lambs in the CAST group. Injection of LPS and C/D caused increased serum cortisol concentration. The magnitude of the cortisol response to C/D was not altered by prior exposure to either LPS or isolation.

CONCLUSIONS AND CLINICAL RELEVANCE

LPS and isolation did not modulate the response to C/D but did cause hyperalgesia. This highlights the importance of flock health management and husbandry techniques to reduce the incidence of either systemic infection or psychological stressors in young lambs.

A reassessment of stress-induced "analgesia" in the rat using an unbiased method

An increased tail-flick latency to noxious heat during or after stress in the rodent is usually interpreted as a stress-induced reduction in pain sensitivity and often described as a form of stress-induced "analgesia." However, this measure is an indirect and flawed measure of the change in nociceptive threshold to noxious heat. A major confound of the latency measure is the initial temperature of the tail, which can drop down to room temperature during stress, the consequence of a marked sympathetically mediated vasoconstriction in the skin of the extremities. We addressed this issue with tail-flick tests during contextual fear using infrared thermography to monitor temperature changes and a CO2 laser to deliver the heat stimulus. The experiment revealed a 4.2°C increase of the nociceptive threshold, confirming a true antinociceptive effect. However, its contribution to the increased withdrawal latency was less than two-thirds (63.2%). Nearly one-third (32.2%) was due to the drop in tail temperature (4.4°C), which also slowed conduction along sensory fibers (2.2%, included in the 32.2%). The remaining 4.6% was due to an increase in decisional/motor latency. This new unbiased method establishes beyond doubt that a conditioned stress response is associated with true antinociception to noxious heat. It also confirms that stress-induced changes in skin temperature can be a major confound in tail-flick tests. The present study shows, for the first time, the exact contribution of these two components of the tail-flick latency for a stress response. Less than two-thirds of the increase in tail-flick latency to noxious heat, evoked by conditioned fear, reflects true antinociception. The remaining is due to skin vasoconstriction.

Psychological stress induces temporary masticatory muscle mechanical sensitivity in rats

To explore the relationship between psychological stress and masticatory muscle pain, we created a communication stress animal model to determine whether psychological stress could induce increased mechanical sensitivity in masticatory muscles and to study the changes of mechanical nociceptive thresholds after stress removal. Forty-eight male Sprague-Dawley rats were divided into a control group (CON), a foot-shocked group (FS, including 3 subgroups recorded as FS-1, FS-2, and FS-3), a psychological stress group (PS), and a drug treatment group (DT). PS and DT rats were confined in a communication box for one hour a day to observe the psychological responses of neighboring FS rats.Measurements of the mechanical nociceptive thresholds of the bilateral temporal and masseter muscles showed a stimulus-response relationship between psychological stress and muscle mechanical sensitivity. The DT rats, who received a diazepam injection, showed almost the same mechanical sensitivity of the masticatory muscles to that of the control in response to psychological stress. Fourteen days after the psychological stressor was removed, the mechanical nociceptive thresholds returned to normal. These findings suggest that psychological stress is directly related to masticatory muscle pain. Removal of the stressor could be a useful method for relieving mechanical sensitivity increase induced by psychological stress.

Stress and tension-type headache mechanisms

Stress is widely demonstrated as a contributing factor in tension-type headache (TTH). The mechanisms underlying this remain unclear at present. Recent research indicates the importance of central pain processes in tension-type headache (TTH) pathophysiology. Concurrently, research with animals and healthy humans has begun to elucidate the relationship between stress and pain processing in the central nervous system, including central pain processes putatively dysfunctional in TTH. Combined, these two fields of research present new insights and hypotheses into possible mechanisms by which stress may contribute to TTH. To date, however, there has been no comprehensive review of this literature. The present paper provides such a review, which may be valuable in facilitating a broader understanding of the central mechanisms by which stress may contribute to TTH.

Differential regulation of morphine antinociceptive effects by endogenous enkephalinergic system in the forebrain of mice

BACKGROUND

Mice lacking the preproenkephalin (ppENK) gene are hyperalgesic and show more anxiety and aggression than wild-type (WT) mice. The marked behavioral changes in ppENK knock-out (KO) mice appeared to occur in supraspinal response to painful stimuli. However the functional role of enkephalins in the supraspinal nociceptive processing and their underlying mechanism is not clear. The aim of present study was to compare supraspinal nociceptive and morphine antinociceptive responses between WT and ppENK KO mice.

RESULTS

The genotypes of bred KO mice were confirmed by PCR. Met-enkephalin immunoreactive neurons were labeled in the caudate-putamen, intermediated part of lateral septum, lateral globus pallidus, intermediated part of lateral septum, hypothalamus, and amygdala of WT mice. Met-enkephalin immunoreactive neurons were not found in the same brain areas in KO mice. Tail withdrawal and von Frey test results did not differ between WT and KO mice. KO mice had shorter latency to start paw licking than WT mice in the hot plate test. The maximal percent effect of morphine treatments (5 mg/kg and 10 mg/kg, i.p.) differed between WT and KO mice in hot plate test. The current source density (CSD) profiles evoked by peripheral noxious stimuli in the primary somatosenstory cortex (S1) and anterior cingulate cortex (ACC) were similar in WT and KO mice. After morphine injection, the amplitude of the laser-evoked sink currents was decreased in S1 while the amplitude of electrical-evoked sink currents was increased in the ACC. These differential morphine effects in S1 and ACC were enhanced in KO mice. Facilitation of synaptic currents in the ACC is mediated by GABA inhibitory interneurons in the local circuitry. Percent increases in opioid receptor binding in S1 and ACC were 5.1% and 5.8%, respectively.

CONCLUSION

The present results indicate that the endogenous enkephalin system is not involved in acute nociceptive transmission in the spinal cord, S1, and ACC. However, morphine preferentially suppressed supraspinal related nociceptive behavior in KO mice. This effect was reflected in the potentiated differential effects of morphine in the S1 and ACC in KO mice. This potentiation may be due to an up-regulation of opioid receptors. Thus these findings strongly suggest an antagonistic interaction between the endogenous enkephalinergic system and exogenous opioid analgesic actions in the supraspinal brain structures.

Analgesia and hyperalgesia from CRF receptor modulation in the central nervous system of Fischer and Lewis rats

This study examines the contribution of central corticotropin-releasing factor (CRF) to pain behavior. CRF is the principal modulator of the hypothalamo-pituitary-adrenal (HPA) axis, in addition to acting on many other areas of the central nervous system. We compared nociceptive thresholds (heat and mechanical) and pain behavior in response to a sustained stimulus (formalin test) between Fischer and Lewis rats that have different HPA axis activity. Intracerebroventricular (i.c.v.) administration of CRF produced dose-dependent antinociception at a lower dose in Lewis (40 ng, paw pinch 71+/-0 g) compared to Fischer rats (200 ng, 112+/-3 g). The antinociceptive effect of CRF was mostly preserved in adrenalectomized Fischer rats. The i.c.v. administration of the CRF receptor antagonist, astressin, had a hyperalgesic effect, suggesting that CRF is tonically active. Lewis rats required higher doses of astressin (5 ng, paw pinch 51+/-1 g) to show nociceptive effects compared to Fischer rats (1 ng, 79+/-1 g). Only Lewis rats vocalized during mechanical stimulus, and this behavior was prevented by diazepam or morphine but was worsened by CRF, despite its antinociceptive property. In the formalin test, CRF and astressin had the largest effect on the interphase suggesting that they act on the endogenous pain inhibitory system. CRF also increased anxiety/fear-like behaviors in the forced swim and predator odor tests. Our results establish that central CRF is a key modulator of pain behavior and indicates that CRF effects on nociception are largely independent of its mood modulating effect as well as its control of the HPA axis.

Repeated sound stress enhances inflammatory pain in the rat

While it is well established that acute stress can produce antinociception, a phenomenon referred to as stress-induced analgesia, repeated exposure to stress can have the opposite effect. Since, chronic pain syndromes, such as fibromyalgia and rheumatoid arthritis, may be triggered and/or exacerbated by chronic stress, we have evaluated the effect of repeated stress on mechanical nociceptive threshold and inflammatory hyperalgesia. Using the Randall-Selitto paw pressure test to quantify nociceptive threshold in the rat, we found that repeated non-habituating sound stress enhanced the mechanical hyperalgesia induced by the potent inflammatory mediator, bradykinin, which, in normal rats, produces hyperalgesia indirectly by stimulating the release of prostaglandin E2 from sympathetic nerve terminals. Hyperalgesia induced by the direct-acting inflammatory mediator, prostaglandin E2 as well as the baseline nociceptive threshold, were not affected. Adrenal medullectomy or denervation, reversed the effect of sound stress. In sound stressed animals, bradykinin-hyperalgesia had a more rapid latency to onset and was no longer inhibited by sympathectomy, compatible with a direct effect of bradykinin on primary afferent nociceptors. In addition, implants of epinephrine restored bradykinin-hyperalgesia in sympathectomized non-stressed rats, lending further support to the suggestion that increased plasma levels of epinephrine can sensitize primary afferents to bradykinin. These results suggest that stress-induced enhancement of inflammatory hyperalgesia is associated with a change in mechanism by which bradykinin induces hyperalgesia, from being sympathetically mediated to being sympathetically independent. This sympathetic-independent enhancement of mechanical hyperalgesia is mediated by the stress-induced release of epinephrine from the adrenal medulla.

Effects of magnetic field exposure on open field behaviour and nociceptive responses in mice

Results of previous studies have shown that nociceptive sensitivity in male C57 mice is enhanced by exposure to a regular 37 Hz or an irregularly varying (<1 Hz) electromagnetic field. In order to test whether these fields affect more generally mouse behaviour, we placed Swiss CD-1 mice in a novel environment (open field test) and exposed them for 2 h to these two different magnetic field conditions. Hence, we analysed how duration and time course of various behavioural patterns (i.e. exploration, rear, edge chew, self-groom, sit, walk and sleep) and nociceptive sensitivity had been affected by such exposure. Nociceptive sensitivity was significantly greater in magnetically treated mice than in controls. The overall time spent in exploratory activities was significantly shorter in both magnetically treated groups (< 1 Hz, 33% and 37 Hz, 29% of total time), than in controls (42%). Conversely, the time spent in sleeping was markedly longer in the treated groups (both 27% of total time) than in controls (11%). These results suggest that exposure to altered magnetic fields induce a more rapid habituation to a novel environment.

Delayed stress-induced antinociceptive effect of nitric oxide synthase inhibition in the dentate gyrus of rats

Stimulation of the hippocampal formation can modulate nociceptive mechanisms, whereas painful stimuli can activate this structure. Stress exposure can produce plastic changes in the hippocampus. Nitric oxide (NO) is an important neuroregulatory agent present in the hippocampus. The objective of the present study was to investigate the effects of intrahippocampal administration of N(omega)-nitro-L-arginine methyl ester hydrochloride (L-NAME), an inhibitor of NO synthase (NOS), on nociceptive processes in stressed and nonstressed rats. Male Wistar rats (n=6-11/group) received unilateral microinjection of L-NAME (50-300 nmol/0.2 microl) into the dentate gyrus (DG) of the dorsal hippocampus. Immediately after the injection tail-flick reflex latency was measured. Stressed animals were submitted to 2 h of restraint and tested immediately or 1, 2, 5 or 10 days later. L-NAME failed to modify nociception in nonstressed rats. However, 5 days after, restraint L-NAME, at all doses tested, produced an antinociceptive effect (ANOVA, P<.05). The dose-response curve had an inverted U shape. L-NAME antinociceptive effect was antagonized by previous treatment with L-arginine (150 nmol/0.2 microl, P<.05). The results suggest that the modulation of nociceptive processes by NO in the dorsal hippocampus is dependent on previous stress exposure and on poststress interval.

The effects of age and isolation period on two phases of behavioral response to foot shock in isolation-reared rats

Using rats as subjects, the effects of a period of isolation and the subjects' age during isolation on the response to foot shock were systematically examined in three experiments. Both the thresholds of shock that evoked a jumping response and that evoked a thrashing response were measured. The results suggest that the threshold for jumping response decreased when rats were isolated during the postweaning stage of development. On the other hand, the threshold for thrashing response decreased when the subjects were isolated for more than 39 days, independently of their age during isolation. Possible causes of the differential effects of these two factors are discussed in relation to the developmental process of these two behavioral indices.

[Critical analysis of animal models of acute pain. II]

OBJECTIVE

To analyse models of acute pain in experimental animals.

DATA SOURCES

References were obtained from computerised bibliographic data banks (Medline and others) and the authors' personal documents.

DATA SYNTHESIS

The majority of tests permit only a measurement of threshold, whereas clinical pain is almost always prolonged. The relationships between tests of acute pain and motor activity are reviewed from a number of standpoints; in particular we consider the influence, which postural adjustments of the animal may exert on motor responses in the limbs and the significance of flexor and extensor reflexes. In analysing the problem of the sensitivity of tests, we raise the following questions: 1) what type(s) of fibres underlie the observed responses and might these be different depending on whether one is stimulating a healthy or an inflamed tissue; 2) what significance do measurements of "latency" have when a stimulus is increasing; 3) how valid are the methods of analysing the results? The influence of species and the genetic line used in tests and the specificity and predictivity of tests are considered. Finally, we review those factors, which may distort behavioural measurements in animals, notably--pharmacokinetics, interactions between heterotopic stimuli, environmental factors and related psychophysiological/psychological considerations (subjective "undesirable" phenomena, learning phenomena). We pay particular attention to related physiological functions (thermoregulation, vasomotricity, blood pressure). These considerations lead us to re-position nociception within a much larger homeostatic framework which in addition to pain, includes phenomena such as anxiety and vegetative functions. They also suggest that we should define an "effective stimulus" as one, which activates nociceptive nerve terminals after a physical stimulus, has passed through a "peripheral lens" which regulates its intensity for reasons, which are physical, albeit of biological origin. Finally they remind us that the "system of pain" forms part of a whole set of subsystems--sensory, motor, vegetative, emotional, motivational--which scientific method, being reductionist by nature, cannot study in its entirety. However one must consider results of nociceptive tests within this general context.

CONCLUSION

It is only by taking the approach described in this review, that fundamental and clinical research can interact usefully.

Epinephrine produces a beta-adrenergic receptor-mediated mechanical hyperalgesia and in vitro sensitization of rat nociceptors

Hyperalgesic and nociceptor sensitizing effects mediated by the beta-adrenergic receptor were evaluated in the rat. Intradermal injection of epinephrine, the major endogenous ligand for the beta-adrenergic receptor, into the dorsum of the hindpaw of the rat produced a dose-dependent mechanical hyperalgesia, quantified by the Randall-Selitto paw-withdrawal test. Epinephrine-induced hyperalgesia was attenuated significantly by intradermal pretreatment with propranolol, a beta-adrenergic receptor antagonist, but not by phentolamine, an alpha-adrenergic receptor antagonist. Epinephrine-induced hyperalgesia developed rapidly; it was statistically significant by 2 min after injection, reached a maximum effect within 5 min, and lasted 2 h. Injection of a more beta-adrenergic receptor-selective agonist, isoproterenol, also produced dose-dependent hyperalgesia, which was attenuated by propranolol but not phentolamine. Epinephrine-induced hyperalgesia was not affected by indomethacin, an inhibitor of cyclo-oxygenase, or by surgical sympathectomy. It was attenuated significantly by inhibitors of the adenosine 3',5'-cyclic monophosphate signaling pathway (the adenylyl cyclase inhibitor, SQ 22536, and the protein kinase A inhibitors, Rp-adenosine 3',5'-cyclic monophosphate and WIPTIDE), inhibitors of the protein kinase C signaling pathway (chelerythrine and bisindolylmaleimide) and a mu-opioid receptor agonist DAMGO ([D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin). Consistent with the hypothesis that epinephrine produces hyperalgesia by a direct action on primary afferent nociceptors, it was found to sensitize small-diameter dorsal root ganglion neurons in culture, i. e., to produce an increase in number of spikes and a decrease in latency to firing during a ramped depolarizing stimulus. These effects were blocked by propranolol. Furthermore epinephrine, like several other direct-acting hyperalgesic agents, caused a potentiation of tetrodotoxin-resistant sodium current, an effect that was abolished by Rp-adenosine 3',5'-cyclic monophosphate and significantly attenuated by bisindolylmaleimide. Isoproterenol also potentiated tetrodotoxin-resistant sodium current. In conclusion, epinephrine produces cutaneous mechanical hyperalgesia and sensitizes cultured dorsal root ganglion neurons in the absence of nerve injury via an action at a beta-adrenergic receptor. These effects of epinephrine are mediated by both the protein kinase A and protein kinase C second-messenger pathways.

Sensibility threshold in patients with masticatory muscle pain

Sensibility and pain thresholds were measured in 27 patients with myofascial pain and craniomandibular disorders and in 19 controls. A monopolar electric stimulator with increasing direct current values was used to record pain and sensibility thresholds in the lower incisors. Patients with myofascial pain had a tendency towards lower sensibility and lower pain thresholds than the controls, although the differences were not statistically significant.

Administration of AP5, a glutamate antagonist, unmasks glycine analgesic actions in the rat

The effect of intrathecal (IT) injection of glycine alone or in combination with 2-amino-5-phosphonopentanoate (AP5) on two nociceptive tests--the vocalization threshold to tail-shock (VTTS) and the tail-flick latency (TFL)--was studied in ovariectomized Sprague-Dawley rats. IT injection of 400 micrograms glycine induced a nonsignificant decrease, that is, in comparison with saline, in both nociceptive thresholds. IT AP5 (10 micrograms) provoked a slight but significant increase in both nociceptive thresholds within the first 15 min postinjection. Combination of both glycine (400 micrograms) and AP5 (10 micrograms) produced marked and prolonged analgesia in both tests, which was significantly different from that obtained with AP5 alone. The results suggest that IT glycine acting through the strychnine-sensitive Gly1 receptor produces analgesia provided its effect on the Gly2 receptor linked to the NMDA receptor is prevented by an antagonist.

Effects of intrathecal antibodies to substance P, calcitonin gene-related peptide and galanin on repeated cold stress-induced hyperalgesia: comparison with carrageenan-induced hyperalgesia

Rats exposed to a cold environment (4 degrees C) for 30 min every 1 h during the day and at night show a gradual decrease in the nociceptive threshold for pressure stimulation. Such hyperalgesia, referred to as repeated cold stress (RCS)-induced hyperalgesia, is stable for at least 4 h and maintained for 3 days only by exposing to cold overnight; thus, no adaptation to RCS is apparent. Hyperalgesia gradually returns over 4 days after cold exposure ceases. To determine whether three neuropeptides, substance P (SP), calcitonin gene-related peptide (CGRP) and galanin (GAL), which are present in the superficial dorsal horn including primary afferent terminals, would be responsible for RCS-induced hyperalgesia, we examined the effects of intrathecal injections of their antibodies (used as inhibitors of neuropeptide-mediated synaptic transmission) on the nociceptive threshold of RCS rats, and compared this with the antibody effect on carrageenan-induced hyperalgesia. An intrathecal injection of anti-SP antibody significantly inhibited the hyperalgesia of RCS rats as well as carrageenan-induced hyperalgesia, and slightly increased the nociceptive threshold of non-RCS rats. Anti-CGRP antibody produced an improvement in the hyperalgesia of RCS rats as well as carrageenan-induced hyperalgesia without having an effect on the nociceptive threshold of non-RCS rats. Although anti-GAL antibody significantly inhibited carrageenan-induced hyperalgesia, it did not affect the nociceptive threshold of RCS and non-RCS rats. The present results suggest that enhancement of synaptic transmission mediated by SP and CGRP, but not GAL, in the spinal dorsal horn is, at least in part, involved in RCS-induced hyperalgesia.

Copulatory analgesia in male rats ensues from arousal, motor activity, and genital stimulation: blockage by manipulation and restraint

The effect of copulation on the vocalization threshold to tail shock (VTTS) was assessed in freely-moving, sexually experienced, Wistar male rats. Mean VTTS during the first copulation was 40% above the baseline values and slightly decreased during the first postejaculatory interval (PEI; 28% above baseline). VTTS mean values further increased during the second copulatory series (93% above baseline and 63% during the PEI). Testing at the same intervals in noncopulating rats did not induce analgesia. VTTS values also increased after the display of either one ejaculation (E), five intromissions (I), or five mounts (M), the analgesia persisting for at least 20 min. Analgesia following M was smaller and shorter than that observed after either I or E. Analgesia developed gradually during copulation since VTTS values after five I were significantly higher than those after one I. The incidence of vocalizations to suprathreshold shocks (STS, 20% above the VTTS) occurring during various phases of copulation was also studied to determine the onset and short-term fluctuations of copulatory analgesia. The proportion of STS inducing vocalizations decreased from 91% (mean of individual proportions) before copulation, to 24% during copulation, and to 25% during the PEI. A maximal reduction in the proportion of vocalizations to STS was found during the last third of copulation. Nearly all vocalizations to STS during copulation occurred when subjects were quiescent, while few or no vocalizations occurred when rats were engaged in sexual activity, i.e., during M, I, or penile grooming. In contrast to the above-mentioned data, no significant analgesia was observed using the tail-flick latency test (TFL) following either E, five I, or five M.(ABSTRACT TRUNCATED AT 250 WORDS)

Clinical and pathophysiological observations in migraine and tension-type headache explained by integration of vascular, supraspinal and myofascial inputs

A vascular-supraspinal-myogenic (VSM) model for pain in migraine based on our previous clinical and pathophysiological observations is proposed. According to the model, perceived pain (headache) intensity is determined by the sum of nociception from cephalic arteries and pericranial myofascial tissues converging upon the same neurons and integrated with supraspinal effects (usually facilitating). Vascular input predominates over myofascial input in migraine, whereas significance of supraspinal facilitation is difficult to estimate. The importance of these 3 effects may vary between patients and in the same individual with time. The model is in accordance with recent experimental studies showing convergence of somatovisceral afferents upon n. caudalis neurons. Also, long term potentiation due to nociceptive activation and sensitization of neurons to input from wider areas and non-nociceptive stimuli are relevant to our model. In tension-type headache, nociception is primarily myofascial, but vascular input cannot be disregarded. Supraspinal facilitation probably plays a large, sometimes dominant role (the MSV model). The model explains much of the complexity of the clinical picture of these disorders as well as their tendency to overlap and to change into one another. Also, a number of pathophysiological observations such as why muscles are tender during migraine, why trigger-point injection may cure migraine attacks and why chronic tension-type headache is often associated with episodes of pulsating pain, can be explained. The model gives a rational explanation of empirically developed, internationally accepted, multimodal treatment strategies for migraine and tension-type headache. It may thus serve a useful purpose in explaining the disorder to patients. Finally, the model points to several avenues of future research in animals and man.

Buspirone effect on the development of antinociceptive reactions

The anxiolytic agents, buspirone and diazepam, increase the paw lick latency of rats in the hot plate test, the effect being dose-dependent and exceeding that of morphine. The action of buspirone was not accompanied by ataxic and sedative effects which were observed in rats on diazepam. Buspirone (up to 25 mg/kg) and diazepam (up to 5 mg/kg) neither change the tail flick latency nor potentiate the action of morphine on the test. A buspirone dose of 2 mg/kg administered to animals before foot shock, or the dose of 1.5 mg/kg before cold swimming stress, led to a significant increase in hot plate latency 1 min after stress as compared to the control. The effect of buspirone on the paw lick reaction in rats may be related to the activation of antinociceptive mechanisms and inhibition of an emotional-motivational component of the pain reaction.

Effect of semiconductor GaAs laser irradiation on pain perception in mice

The influence of subacute exposure (11 exposures within 16 days) of mice to the low power (GaAs) semiconductive laser-stimulated irradiation on pain perception was investigated. The pain perception was determined by the latency of foot-licking or jumping from the surface of a 53 degrees C hot plate. Repeated hot-plate testing resulted in shortening of latencies in both sham- and laser-irradiated mice. Laser treatment (wavelength, 905 nm; frequency, 256 Hz; irradiation time, 50 sec; pulse duration, 100 nsec; distance, 3 cm; peak irradiance, 50 W/cm2 in irradiated area; and total exposure, 0.41 mJ/cm2) induced further shortening of latencies, suggesting its stimulatory influence on pain perception. Administration of morphine (20 mg/kg) prolonged the latency of response to the hot plate in both sham- and laser-irradiated mice. This prolongation tended to be lesser in laser-irradiated animals. Further investigations are required to elucidate the mechanism of the observed effect of laser.

Analgesia or hyperalgesia following stress correlates with emotional behavior in rats

Analgesia following exposure to a variety of noxious and non-noxious stressors is well documented and is commonly referred to as stress-induced analgesia. Hyperalgesia following stress has also been reported. The present study shows that a mild stressor (15 min of vibration) produced increased tail-flick latencies (TFL) in some rats, but decreased latencies in other rats. The results of the individual subjects were reproduced in a later session: the rats showing increased TFL on day 1, responded with increased TFL on day 2. Rats showing decreased TFL on day 1 responded with decreased TFL on day 2. Whichever reaction occurred, analgesia or hyperalgesia, this correlated with the animal's behavior during the stress procedure. Analgesia was produced in quiet rats and hyperalgesia in hyperemotional ones. Various peripheral nerve stimulation procedures producing hyperemotional reactions also resulted in lowering of the pain threshold. The results of the present study show behavioral modulation of pain mechanisms.

Noradrenergic mechanisms in mediation of stress-induced hyperalgesia in rats

Exposure to a mild stressor (15 min of vibration) produced analgesia in some rats and hyperalgesia in other rats from the same batch treated in the same way. Rats which responded with decreased tail-flick latencies (TFL) showed signs of hyperemotionality during the stress procedure. Stress-induced hyperalgesia was abolished by the administration of diazepam (2.5 mg/kg i.p.) and clonidine (25 micrograms/kg i.p.). It is suggested that the reversal of hyperalgesia was due to the anxiolytic properties of the drugs. Yohimbine, an alpha 2-adrenoceptor antagonist (5 mg/kg i.p.), antagonized the effect of clonidine. The influence of clonidine on stress-induced hyperalgesia may be mediated by alpha 2-adrenoceptors.