Sleep disorders and their association with laboratory pain sensitivity in temporomandibular joint disorder

STUDY OBJECTIVES

We characterized sleep disorder rates in temporomandibular joint disorder (TMD) and evaluated possible associations between sleep disorders and laboratory measures of pain sensitivity.

DESIGN

Research diagnostic examinations were conducted, followed by two consecutive overnight polysomnographic studies with morning and evening assessments of pain threshold.

SETTING

Orofacial pain clinic and inpatient sleep research facility.

PARTICIPANTS

Fifty-three patients meeting research diagnostic criteria for myofascial TMD.

INTERVENTIONS

N/A.

MEASUREMENTS AND RESULTS

We determined sleep disorder diagnostic rates and conducted algometric measures of pressure pain threshold on the masseter and forearm. Heat pain threshold was measured on the forearm; 75% met self-report criteria for sleep bruxism, but only 17% met PSG criteria for active sleep bruxism. Two or more sleep disorders were diagnosed in 43% of patients. Insomnia disorder (36%) and sleep apnea (28.4%) demonstrated the highest frequencies. Primary insomnia (PI) (26%) comprised the largest subcategory of insomnia. Even after controlling for multiple potential confounds, PI was associated with reduced mechanical and thermal pain thresholds at all sites (P < 0.05). Conversely, the respiratory disturbance index was associated with increased mechanical pain thresholds on the forearm (P < 0.05).

CONCLUSIONS

High rates of PI and sleep apnea highlight the need to refer TMD patients complaining of sleep disturbance for polysomnographic evaluation. The association of PI and hyperalgesia at a nonorofacial site suggests that PI may be linked with central sensitivity and could play an etiologic role in idiopathic pain disorders. The association between sleep disordered breathing and hypoalgesia requires further study and may provide novel insight into the complex interactions between sleep and pain-regulatory processes.

Antinociceptive effects of nociceptin/orphanin FQ administered intrathecally in monkeys

Nociceptin/orphanin FQ (N/OFQ) is the endogenous peptide for the NOP receptors. Depending on the doses, intrathecal administration of N/OFQ has dual actions (ie, hyperalgesia and antinociception) in rodents. However, the pharmacological profile of intrathecal N/OFQ is not fully known in primates. The aim of this study was to investigate behavioral effects of intrathecal N/OFQ over a wide dose range and to compare its effects with ligands known to produce hyperalgesia or antinociception in monkeys. Intrathecal N/OFQ from 1 fmol to 1 nmol did not produce any hyperalgesic or scratching responses. In contrast, intrathecal substance P 100 nmol produced hyperalgesia, and intrathecal DAMGO 10 nmol produced antinociception. At the dose range between 10 nmol and 1 micromol, intrathecal N/OFQ dose-dependently produced thermal antinociception against a noxious stimulus in 2 intensities. More importantly, N/OFQ in combined with intrathecal morphine dose-dependently potentiated morphine-induced antinociception without inhibiting morphine-induced itch/scratching. Taken together, this study is the first to provide a unique functional profile of intrathecal N/OFQ over a wide dose range in primates. Intrathecal N/OFQ produces thermal antinociception without anti-morphine actions or scratching responses, indicating that N/OFQ or NOP receptor agonists represent a promising target as spinal analgesics.

PERSPECTIVE

Intrathecal administration of N/OFQ only produced thermal antinociception, not hyperalgesia, in monkeys. In addition, intrathecal N/OFQ does not have anti-morphine actions or itch/scratching responses. This study strongly supports the therapeutic potential of N/OFQ or NOP receptor agonists as spinal analgesics for clinical trials.

Reciprocal changes in vanilloid (TRPV1) and endocannabinoid (CB1) receptors contribute to visceral hyperalgesia in the water avoidance stressed rat

BACKGROUND

Increasing evidence suggests that chronic stress plays an important role in the pathophysiology of several functional gastrointestinal disorders. We investigated whether cannabinoid receptor 1 (CB1) and vanilloid receptor 1 (TRPV1; transient receptor potential vanilloid 1) are involved in stress-induced visceral hyperalgesia.

METHODS

Male rats were exposed to 1 h water avoidance (WA) stress daily for 10 consecutive days. The visceromotor response (VMR) to colorectal distension (CRD) was measured. Immunofluorescence and western blot analysis were used to assess the expression of CB1 and TRPV1 receptors in dorsal root ganglion (DRG) neurons.

RESULTS

WA stressed rats demonstrated a significant increase in the serum corticosterone levels and faecal pellet output compared to controls supporting stimulation of the hypothalamic-pituitary-adrenal (HPA) axis. The VMR increased significantly at pressures of 40 and 60 mm Hg in WA stress rats compared with controls, respectively, and was associated with hyperalgesia. The endogenous CB1 agonist anandamide was increased significantly in DRGs from stressed rats. Immunofluorescence and western blot analysis showed a significant decrease in CB1 and a reciprocal increase in TRPV1 expression and phosphorylation in DRG neurons from stressed rats. These reciprocal changes in CB1 and TRPV1 were reproduced by treatment of control DRGs with anandamide in vitro. In contrast, treatment of control DRGs in vitro with the CB1 receptor agonist WIN 55,212-2 decreased the levels of TRPV1 and TRPV1 phosphorylation. Treatment of WA stress rats in situ with WIN 55,212-2 or the TRPV1 antagonist capsazepine prevented the development of visceral hyperalgesia and blocked the upregulation of TRPV1.

CONCLUSIONS

These results suggest that the endocannabinoid (CB1) and TRP (TRPV1) pathways may play a potentially important role in stress-induced visceral hyperalgesia.

Persistent pain after groin hernia surgery: a qualitative analysis of pain and its consequences for quality of life

BACKGROUND

Despite a high prevalence of persistent groin pain after hernia repair, the specific nature of the pain and its clinical manifestation are poorly known. The aim of this study was to determine the type of post-herniorrhaphy pain and its influence on daily life.

METHODS

In order to assess long-term pain qualitatively and to explore how it affects quality of life, 100 individuals with persisting pain, identified in a cohort study of patients operated for groin hernia, were neurologically examined, along with 100 pain-free controls matched for age, gender and type of operation. The patients were asked to answer the SF-36 questionnaire, the hospital anxiety and depression scale, the Swedish Scales of Personality (SSP) and a standardised questionnaire for assessing everyday life coping. The patients were approached approximately 4.9 years after surgery.

RESULTS

Twenty-two patients from the pain group had become pain free by the time of examination, whereas 76 patients still had pain, of whom 47 (68%) suffered from neuropathic pain and 11 from nociceptive pain. The remaining patients suffered from mixed pain, neuropathic and nociceptive, or were found to have another reason for pain. All dimensions of SF-36 were poorer for the pain group than the control group.

CONCLUSION

Persistent post-herniorrhaphy pain is mainly neuropathic and has a substantial impact on health-related quality of life.

Quantitative somatic sensory testing and functional imaging of the response to painful stimuli before and after cingulotomy for obsessive-compulsive disorder (OCD)

The middle cingulate cortex (MCC) has been implicated in pain processing by studies of cingulotomy for chronic pain and imaging studies documenting increased MCC blood flow in response to acute pain. The only previous report of quantitative sensory testing following cingulotomy described increased intensity and unpleasantness ratings of painful hot and cold stimuli in a single patient with psychiatric disease. We now report a case in which perception of pain and temperature was assessed before and after cingulotomy for obsessive-compulsive disorder (OCD). Positron emission tomographic (PET) studies of the blood flow response to acute pain were carried out using a single subject design which allowed for statistical evaluation of postoperative blood flow changes in this case. Postoperatively, the patient demonstrated increased intensity and unpleasantness ratings of painful thermal waterbath stimuli. The PET studies demonstrated preoperative contact heat pain-evoked activation of the bilateral MCC/SMA (supplementary motor area) and the left (contralateral) fronto-parietal operculum. Postoperative pain-evoked activation was demonstrated in the right (ipsilateral) parasylvian cortex but not of the MCC/SMA. Prior studies of forebrain lesions, and of cortical synchrony during the application of painful stimuli suggest the presence of functional connectivity between components of the MCC/SMA and the fronto-parietal opercula. Therefore present results suggest that cingulate lesions disinhibit ipsilateral parasylvian cortex and so are independent evidence of functional connectivity between these cortical areas, the defining characteristic of modules in a pain network.

Effects of cytidine 5'-diphosphocholine (CDP-choline) on the thermal nociceptive threshold in streptozotocin-induced diabetic mice

Neuropathy accompanied by abnormal sensory perception is the most common complication in insulin-dependent and -independent diabetes mellitus. Since there are very few effective therapeutic regimens for sensory abnormalities in diabetes, we examined the effect of cytidine 5'-diphosphocholine (CDP)-choline on the thermal nociceptive threshold in streptozotocin-induced diabetic mice using the tail-flick test. Diabetic mice showed a shorter tail-flick latency at 1-4 weeks after streptozotocin treatment and a longer tail-flick latency after 8-12 weeks. This hyper- and hypoalgesia in diabetic mice was almost completely inhibited by daily treatment with CDP-choline (100 mg/kg/day, p.o.) beginning on the day of streptozotocin treatment. Daily treatment with CDP-choline beginning 5 weeks after streptozotocin treatment attenuated the development of hypoalgesia. Diabetic mice showed a significant increase in Na(+)-K(+)-ATPase activity at 3 weeks after streptozotocin treatment, whereas Na(+)-K(+)-ATPase activity was decreased at 12 weeks after treatment. These alterations were normalized by daily treatment with CDP-choline (100 mg/kg/day, p.o.) beginning the day of streptozotocin treatment. These results provide evidence to support the therapeutic potency of CDP-choline on the development of thermal hyper- and hypoalgesia and the progression of thermal hypoalgesia in diabetic mice. Moreover, these effects of CDP-choline may result from the normalization of Na(+)-K(+)-ATPase activity.

Effect of cyclooxygenase and nitric oxide synthase inhibitors on vincristine induced hyperalgesia in rats

The purpose of this study was to investigate the effect of cyclooxygenase (COX) inhibitors and nitric oxide synthase (NOS) inhibitors on the development of vincristine (VIN)-induced hyperalgesia. Indomethacin (IND) and celecoxib (CEX) were used as relatively selective inhibitors of COX-1 and COX-2, respectively. NOS inhibitors included the nonspecific inhibitor NG-nitro-Larginine (L-NOArg) and L-N6-(1-iminoethyl)lysine (L-NIL), which preferentially acts on inducible NOS, as well as 7-nitroindazole (7-NI), which is a relatively specific neuronal NO synthase inhibitor. Both IND and CEX markedly suppressed hyperalgesia, whereas all three NOS inhibitors prevented the development of hyperalgesia due to VIN administration. The results of this study suggest participation of COX-1 and COX-2 as well as iNOS and nNOS in the transmission of pain stimuli in VIN-induced hyperalgesia.

Nocebo hyperalgesia: how anxiety is turned into pain

PURPOSE OF REVIEW

Nocebo hyperalgesia is a phenomenon that is opposite to placebo analgesia and whereby expectation of pain increase plays a crucial role. In recent times, both the neuroanatomical and the neurochemical bases of the nocebo effect and of nocebo-related effects have begun to be explored. Here, we highlight recent advances in our understanding of the neurobiology of the nocebo hyperalgesic effect.

RECENT FINDINGS

A typical nocebo hyperalgesic response occurs following the administration of an inert substance which the subject believes to be a hyperalgesic agent (negative placebo or nocebo). It has been shown that the subject's negative expectations of pain worsening induce anticipatory anxiety about the impending pain increase and this triggers the activation of cholecystokinin that, in turn, facilitates pain transmission. Accordingly, cholecystokinin antagonists have been found to prevent this anxiety-induced hyperalgesia. Brain-imaging studies have shown that the perceived intensity of a painful stimulus following negative expectations of pain increase is higher than in the absence of negative expectations and this is associated with changes in activation of specific brain regions.

SUMMARY

Since pain appears to be amplified by anxiety through the activation of cholecystokininergic systems, new therapeutic strategies, such as new cholecystokinin antagonists, can be envisaged whenever pain has an important anxiety component.

Contribution of calcium channel subtypes to the intracellular calcium signal in sensory neurons: the effect of injury

BACKGROUND

Although the activation-induced intracellular Ca signal is disrupted by sensory neuron injury, the contribution of specific Ca channel subtypes is unknown.

METHODS

Transients in dissociated rat dorsal root ganglion neurons were recorded using fura-2 microfluorometry. Neurons from control rats and from neuropathic animals after spinal nerve ligation were activated either by elevated bath K or by field stimulation. Transients were compared before and after application of selective blockers of voltage-activated Ca channel subtypes.

RESULTS

Transient amplitude and area were decreased by blockade of the L-type channel, particularly during sustained K stimulation. Significant contributions to the Ca transient are attributable to the N-, P/Q-, and R-type channels, especially in small neurons. Results for T-type blockade varied widely between cells. After injury, transients lost sensitivity to N-type and R-type blockers in axotomized small neurons, whereas adjacent small neurons showed decreased responses to blockers of R-type channels. Axotomized large neurons were less sensitive to blockade of N- and P/Q-type channels. After injury, neurons adjacent to axotomy show decreased sensitivity of K-induced transients to L-type blockade but increased sensitivity during field stimulation.

CONCLUSIONS

All high-voltage-activated Ca current subtypes contribute to Ca transients in sensory neurons, although the L-type channel contributes predominantly during prolonged activation. Injury shifts the relative contribution of various Ca channel subtypes to the intracellular Ca transient induced by neuronal activation. Because this effect is cell-size specific, selective therapies might potentially be devised to differentially alter excitability of nociceptive and low-threshold sensory neurons.

Antihyperalgesic effects induced by the IL-1 receptor antagonist anakinra and increased IL-1β levels in inflamed and osteosarcoma-bearing mice

Based on the well established involvement of IL-1beta in inflammatory hyperalgesia, we have assessed the possible role played by IL-1beta in a murine model of bone cancer-induced pain. With this aim, we measured IL-1beta levels at the region of the tibia and the spinal cord in mice bearing a tibial osteosarcoma induced by the inoculation of NCTC 2472 cells, and we tested whether the IL-1 receptor antagonist, anakinra, inhibits some hypernociceptive reactions evoked by the neoplastic injury. Parallel experiments were performed in mice with a chronic inflammatory process (intraplantar injection of complete Freund's adjuvant, CFA). IL-1beta levels were increased in the tibial region of osteosarcoma-bearing mice and in the paws of inflamed mice. To a lesser extent, the content of IL-1beta in the spinal cord was also augmented in both situations. Osteosarcoma-induced thermal hyperalgesia was inhibited by 30 and 100 mg/kg of systemic anakinra, but only 300 mg/kg prevented inflammatory thermal hyperalgesia. Mechanical hyperalgesia induced by the osteosarcoma was blocked by 100 and 300 mg/kg of anakinra, whereas a partial reversion of inflammatory mechanical hyperalgesia was induced by 300 mg/kg. Anakinra, intrathecally administered (1 and 10 microg) did not modify hyperalgesia of either origin. Besides, both tumoral and inflammatory mechanical allodynia remained unaltered after the administration of anakinra. In conclusion, some hyperalgesic symptoms observed in this model of bone cancer are mediated by the peripheral release of IL-1beta and may be inhibited by antagonists of type I IL-1 receptors with a similar or greater potency than symptoms produced by inflammation.

Changes in Pain Perception and Descending Inhibitory Controls Start at Middle Age in Healthy Adults

OBJECTIVES

Previous studies have shown a reduction of diffuse noxious inhibitory controls (DNICs) in elderly adults compared with younger adults. Unfortunately, little is known regarding the developmental course of DNIC deficits and so it is still unclear whether middle-aged adults also show a DNIC deficit. The aims of the present study were to better characterize the developmental time course of the change in DNIC response by adding a middle-aged group. The role of expectations was also investigated.

METHODS

The pain thresholds (PTs) of 20 young, 20 middle-aged, and 20 healthy elderly volunteers were assessed before and during a cold pressor task (water at 7 degrees C). Acute nociceptive stimuli were administered using a thermode and consisted of a range of painless and painful heat pulses.

RESULTS

Analyses showed that thermal PTs increase by middle age but that the DNIC-induced increase in PT dampens progressively with advancing age. DNIC response was negatively correlated with advancing age, however, expectations regarding DNIC efficacy did not vary with age. This suggests that age-related changes in the size of the DNIC response are not best explained by an age-related change in expectation.

DISCUSSION

The findings tell us that changes in pain perception and endogenous pain modulation arrive earlier than previously suggested. Studies on aging and pain should include a middle-aged group when comparing pain measures across the adult lifespan.

The Influence of Conditioned Fear on Human Pain Thresholds: Does Preparedness Play a Role?

Emotionally charged facial expressions (happy, fear) served as conditioned stimuli in a differential fear conditioning procedure. Expressions were presented in pseudo-random order on a computer monitor. For half of the participants, the fear expression was paired with an aversive electric stimulation (UCS), whereas the happy expression was unpaired. The other participants had the opposite pairing. To assess the influence of conditioned fear on pain, expressions were shown again in the absence of the UCS and pain threshold was assessed during each expression. The latency of finger withdrawal from a radiant heat device was used to index pain threshold. Skin conductance response (SCR) and self-reported emotion were measured to assess fear conditioning. Consistent with preparedness theory, differential fear conditioning was only present when the fear expression was paired with the UCS. Moreover, pain threshold was only influenced by fear conditioning in persons for whom the fear expression was paired with the UCS. Specifically, finger withdrawal latencies were lower (suggesting hyperalgesia) during the fear expression than during the happy expression; an effect that was not present before CS-UCS pairing. This work suggests that some stimuli are more readily associated with an aversive event and can lead to pain enhancement.

PERSPECTIVE

Although preliminary, these results suggest that fear-relevant environmental stimuli (including facial expressions) may provide important environmental cues during aversive events that influence the level of pain experienced.

Analysis of the antinociceptive effect of systemic administration of tramadol and dexmedetomidine combination on rat models of acute and neuropathic pain

The aim of the present study was to investigate the possible antinociceptive effect of systemic administration of tramadol and dexmedetomidine either alone or in combination on acute and neuropathic pain models in rats. The antinociceptive effects of intraperitoneal (i.p.) tramadol (5-20 mg/kg) and dexmedetomidine (5-20 microg/kg) and three different combinations of tramadol+dexmedetomidine (5+5, 5+10 and 10+5, mg/kg+microg/kg, respectively) were measured by tail-flick and hot-plate methods in acute pain. The effects on the sciatic nerve ligation-induced neuropathic pain was tested by i.p. administration of tramadol (5 mg/kg), dexmedetomidine (5 microg/kg) and tramadol+dexmedetomidine combination (5+5) using a thermal plantar test. Sedation/motor-incoordination was assessed on rotarod. Tramadol and dexmedetomidine produced dose-related antinociception in tail-flick and hot-plate tests. In both tests, combination of these drugs produced an antinociceptive effect that is greater than that produced by tramadol or dexmedetomidine alone at several time points. In hot-plate test, tramadol+dexmedetomidine combination (5+10) exerted the strongest antinociceptive effect, while tramadol+dexmedetomidine combination (10+5) was significantly most effective in tail-flick test. In the neuropathic pain, the antinociceptive effect exerted by tramadol+dexmedetomidine combination (5+5) was also significantly greater than their applications alone. In rotarod test, tramadol (30 and 40 mg/kg), dexmedetomidine (30 and 40 microg/kg), tramadol+dexmedetomidine combination (10+10, 20+20) produced sedation/motor-incoordination, whereas tramadol (5-20 mg/kg), dexmedetomidine (5-20 microg/kg) and tramadol+dexmedetomidine combination (5+5, 5+10 and 10+5) did not produce any effect on sedation/motor-incoordination. The combination of tramadol and dexmedetomidine was more effective in increasing the pain threshold in acute and neuropathic pain when compared with the administration of either of these drugs alone.

Psychophysics, Flare, and Neurosecretory Function in Human Pain Models: Capsaicin Versus Electrically Evoked Pain

Intradermal capsaicin injection (CAP) and electrical current stimulation (ES) are analyzed in respect to patterns and test-retest reliability of pain as well as sensory and neurosecretory changes. In 10 healthy subjects, 2x CAP (50 microg) and 2x ES (5 to 30 mA) were applied to the volar forearm. The time period between 2 identical stimulations was about 4 months. Pain ratings, areas of mechanical hyperalgesia, and allodynia were assessed. The intensity of sensory changes was quantified by using quantitative sensory testing. Neurogenic flare was assessed by using laser Doppler imaging. Calcitonin gene-related peptide (CGRP) release was quantified by dermal microdialysis in combination with an enzyme immunoassay. Time course and peak pain ratings were different between CAP and ES. Test-retest correlation was high (r > or = 0.73). Both models induced primary heat hyperalgesia and primary plus secondary pin-prick hyperalgesia. Allodynia occurred in about half of the subjects. Maximum flare sizes did not differ between CAP and ES, but flare intensities were higher for ES. Test-retest correlation was higher for flare sizes than for flare intensity. A significant CGRP release could only be measured after CAP. The different time courses of pain stimulation (CAP: rapidly decaying pain versus ES: pain plateau) led to different peripheral neurosecretory effects but induced similar central plasticity and hyperalgesia.

PERSPECTIVE

The present study gives a detailed overview of psychophysical and neurosecretory characteristics induced by noxious stimulation with capsaicin and electrical current. We describe differences, similarities, and reproducibility of these human pain models. These data might help to interpret past and future results of human pain studies using experimental pain.

When words are painful: Unraveling the mechanisms of the nocebo effect

The nocebo effect is a phenomenon that is opposite to the placebo effect, whereby expectation of a negative outcome may lead to the worsening of a symptom. Thus far, its study has been limited by ethical constraints, particularly in patients, as a nocebo procedure is per se stressful and anxiogenic. It basically consists in delivering verbal suggestions of negative outcomes so that the subject expects clinical worsening. Although some natural nocebo situations do exist, such as the impact of negative diagnoses upon the patient and the patient's distrust in a therapy, the neurobiological mechanisms have been understood in the experimental setting under strictly controlled conditions. As for the placebo counterpart, the study of pain has been fruitful in recent years to understand both the neuroanatomical and the neurochemical bases of the nocebo effect. Recent experimental evidence indicates that negative verbal suggestions induce anticipatory anxiety about the impending pain increase, and this verbally-induced anxiety triggers the activation of cholecystokinin (CCK) which, in turn, facilitates pain transmission. CCK-antagonists have been found to block this anxiety-induced hyperalgesia, thus opening up the possibility of new therapeutic strategies whenever pain has an important anxiety component. Other conditions, such as Parkinson's disease, although less studied, have been found to be affected by nocebo suggestions as well. All these findings underscore the important role of cognition in the therapeutic outcome, and suggest that nocebo and nocebo-related effects might represent a point of vulnerability both in the course of a disease and in the response to a therapy.

Corticosteroid receptor-mediated mechanisms in the amygdala regulate anxiety and colonic sensitivity

Our previous studies have shown that stereotaxic implantation of corticosterone (Cort) onto the central amygdaloid nucleus increases both anxiety and colonic sensitivity. The goal of this study was to examine the relative importance of amygdaloid glucocorticoid (GR) and mineralocorticoid receptor (MR)-mediated mechanisms in the induction of anxiety and colonic hypersensitivity. In male Fischer 344 rats, Cort or cholesterol micropellets were stereotaxically implanted bilaterally at the dorsal boundary of the central amygdaloid nucleus either alone or in combination with a GR antagonist, mifepristone, or a MR antagonist, spironolactone. Anxiety was assessed on the elevated plus maze and quantified as the percentage of time spent in open arm exploration. Colonic sensitivity was measured by recording a visceromotor response, the number of abdominal muscle contractions in response to colorectal distension. In Cort-implanted rats there was a significant reduction in the percentage of time spent in the open arms of the elevated plus maze compared with cholesterol controls, indicating increased anxiety. Furthermore, colonic hypersensitivity was observed in response to colorectal distension compared with rats with cholesterol implants. In rats with Cort implants combined with either a GR or MR antagonist, there was a significant inhibition of anxiety and colonic hypersensitivity. Our data suggest that both GR and MR play a critical role in Cort-induced anxiety and colonic hypersensitivity.

Static magnetic field-induced anti-nociceptive effect and the involvement of capsaicin-sensitive sensory nerves in this mechanism

Data concerning the effect of static magnetic field (SMF) on nociceptive processes are contradictory in the literature probably due to differences in species, characteristics of the magnetic fields, and duration of the exposure. The aim of the present series of experiments was to elucidate the action of acute full-body exposure of mice to a special SMF developed and validated by us on acute visceral and somatic chemonociception and inflammatory mechanical hyperalgesia. SMF exposure significantly diminished the number of acetic acid- or MgSO4-induced abdominal contractions (acute visceral nociception), formalin-evoked paw lickings and liftings in both phase I (acute somatic nociception) and phase II (acute inflammatory nociception) and mechanical hyperalgesia evoked by i.pl. injection of carrageenan as well as the TRPV1 capsaicin receptor agonist resiniferatoxin. Selective inactivation of capsaicin-sensitive sensory fibres by high dose resiniferatoxin pretreatment decreased nocifensive behaviours in phase II of the formalin test to a similar extent suggesting that pro-inflammatory neuropeptides such as substance P and calcitonin gene-related peptide released from these fibres are involved in this inflammatory reaction. Significant inhibitory effects of SMF on formalin-induced nociception and carrageenan-evoked hyperalgesia were absent in resiniferatoxin-pretreated mice, which also points out that capsaicin-sensitive nerves are involved in the SMF-induced anti-nociceptive action.

Mechanical and cold hypersensitivity in nerve-injured C57BL/6J mice is not associated with fear-avoidance- and depression-related behaviour

BACKGROUND

Neuropathic pain is associated with significant co-morbidity, including anxiety and depression, which impact considerably on the overall patient experience. However, pain co-morbidity symptoms are rarely assessed in animal models of neuropathic pain. To improve the clinical validity of a widely used rodent model of traumatic peripheral neuropathy, we have investigated fear-avoidance- and depression-related behaviours in nerve-injured and sham-operated mice over a 4 week period.

METHODS

Male C57BL/6J mice were subjected to partial sciatic nerve ligation (PSNL) or sham surgery and were assessed on days 7, 14, and 28 after operation. Withdrawal thresholds to punctate mechanical and cooling stimuli were measured. Mice were tested on the novel open-field and elevated plus-maze tests for fear-avoidance behaviour, and on the tail suspension test for depression-related behaviour.

RESULTS

Hypersensitivity to punctate mechanical and cool stimuli was evident up to day 28 after PSNL. However, there was no change in fear-avoidance- or depression-related behaviours regardless of interval after-surgery.

CONCLUSION

These data demonstrate that pain behaviour in nerve-injured C57BL/6J mice was not associated with alterations in emotion-related behaviours.

Behavioral and immunohistological assessment of painful neuropathy induced by a single oxaliplatin injection in the rat

In clinical use, a single infusion of oxaliplatin, widely used to treat metastatic colorectal cancer, induces specific sensory neurotoxicity signs triggered or aggravated by exposure to cold. To study the pathophysiology of these symptoms, we developed and characterized an animal model that reproduces the effects of a single intraperitoneal oxaliplatin administration (3, 6 and 12 mg/kg). Significant allodynia and hyperalgesia to cold stimuli were rapidly observed from 24 h to day 5 with a maximum lowering of 76% at t+30 h versus control. Other behavioral assessments revealed rapid persistent mechanical allodynia, but no thermal hyperalgesia or allodynia to heat and no hyperalgesia to mechanical stimuli. An immunohistochemical study in the superficial layers of the spinal dorsal horn revealed a marked increase in substance P immunoreactivity versus controls (12% versus 4%), whereas calcitonin gene-related peptide (CGRP) immunoreactivity was unchanged. This new animal model for the first time closely mimics the effects observed in humans after a single oxaliplatin infusion, especially onset and highly intense sensory disturbances, hypersensitivity to cold with allodynia and hyperalgesia signs. This model may help to elucidate the mechanisms of this thermal hypersensitivity, especially the possible involvement of small-diameter A-fibers in cold allodynia symptoms. These selective effects may clue up the mechanistic basis for the acute oxaliplatin neuropathy leading to a better understanding of the clinical condition and to optimize its treatment.

Intravenous mononuclear marrow cells reverse neuropathic pain from experimental mononeuropathy

BACKGROUND

Stem cells mediate neuroprotection in a variety of nervous system injury models. In this study, we evaluated a potential role for stem cells in pain therapies. Marrow mononuclear cells containing mixed stem cell populations were used because of wide experience with these cells in experimental and clinical transplantation.

METHODS

After sciatic nerve chronic constriction injury (CCI), adult male Sprague Dawley rats were treated with freshly isolated marrow mononuclear cells (10(7) cells in 0.5 mL IV) from the same strain, or with carrier. The major end points of analysis were thermal and mechanical hypersensitivity using paw withdrawal latency (PWL) to a calibrated heat source and paw withdrawal response to von Frey filaments, evaluated by a blinded investigator.

RESULTS

Marrow transplantation did not prevent pain, and 5 days after CCI all animals were equivalently lesioned. However, 10 days after CCI, rats that received marrow transplants demonstrated paw withdrawal response and PWL patterns indicating recovery from pain, whereas untreated rats continued to have significant pain behavior patterns. For example, PWL values for marrow-treated animals were similar to baseline pre-CCI values (P = 0.54) but significantly shorter latency to withdrawal indicative of continuing pain was seen in untreated rats compared with pre-CCI values (P < 0.001).

CONCLUSIONS

These studies suggest that stem or progenitor cell-mediated therapies may be useful for the treatment of pain after nerve injury, and deserve further study to elucidate the mechanisms of analgesia.

Increased Tumor Necrosis Factor-α and Prostaglandin E2 Concentrations in the Cerebrospinal Fluid of Rats with Inflammatory Hyperalgesia: The Effects of Analgesic Drugs

BACKGROUND

We examined the changes in cerebrospinal fluid (CSF) concentrations of prostaglandin E2 (PGE2) and tumor necrosis factor-alpha (TNF-alpha) after intraplantar administration of complete Freund's adjuvant (CFA) in rats. In addition, we investigated whether different analgesic drugs orally administered at antihyperalgesic doses were able to prevent the changes in PGE2 and TNF-alpha spinal levels associated with hindpaw inflammation.

METHODS

The Randall-Selitto paw-withdrawal test was used to measure inflammatory hyperalgesia. Tramadol (7.5 mg/kg), paracetamol (65 mg/kg), tramadol plus paracetamol and nimesulide (5 mg/kg) were administered orally twice a day, starting from the first day after the CFA injection. PGE2 in the CSF was measured by enzyme immunoassay, and TNF-alpha by ELISA. Behavioral and biochemical parameters were measured on Day 7 after intraplantar injection of CFA or saline.

RESULTS

Withdrawal thresholds to mechanical stimuli decreased markedly in the CFA-treated paw. In these animals the quantification of proinflammatory mediators in the CSF revealed a significant increase in both PGE2 and TNF-alpha concentrations. All the pharmacological treatments prevented the development of the hyperalgesia as well as the PGE2 increase in the CSF. Conversely, a prevention of the increase in TNF-alpha levels was observed only in rats treated with nimesulide or tramadol and paracetamol in combination.

CONCLUSIONS

Our results demonstrate that peripheral inflammatory hyperalgesia is associated with significant changes of proinflammatory mediators in the CSF. It is important to note, however, that spinal PGE2 and TNF-alpha proved to be differently affected by pharmacological treatments able to fully abolish the hyperalgesia.

Muscle tenderness in different types of facial pain and its relation to anxiety and depression: A cross-sectional study on 649 patients

To evaluate in patients with different types of facial pain the association between muscle tenderness and a set of characteristics, 649 consecutive outpatients with facial myogenous pain (MP), TMJ disorder, neuropathic pain (NP) and facial pain disorder (FPD) (DSM-IV) were enrolled. For each patient a psychological assessment on the Axis 1 of the DSM-IV and standardized palpation of pericranial and cervical muscles were carried out. A pericranial muscle tenderness score (PTS), a cervical muscle tenderness score (CTS) and a cumulative tenderness score (CUM, range 0-6) were calculated. Univariate analyses (one-way analysis of variance or chi(2) test) indicated that both age- and sex-distribution, tenderness scores and prevalence of psychiatric disorders markedly differed between groups. The prevalence of depression was highest in FPD patients (44.9%). Both muscle tenderness scores (either PTS or CTS) and prevalence of anxiety were higher in patients with MP than in those with TMJ or NP. To assess associations between CUM score and patients' demographic and clinical characteristics an ordered logit model was fit and interactions between psychiatric disorders and diagnostic groups were tested. The analysis showed that, regardless of the diagnostic group, anxiety and depression independently increase the likelihood of having one point higher muscle tenderness score (OR=1.55, 95% CI: 1.13-2.12 and OR=1.56, 95% CI: 1.10-2.21, respectively). A careful screening for the presence of an underlying psychiatric disorder, either anxiety or depression, should be part of the clinical evaluation in patients suffering from facial pain.

Descending analgesia--when the spine echoes what the brain expects

Changes in pain produced by psychological factors (e.g., placebo analgesia) are thought to result from the activity of specific cortical regions. However, subcortical nuclei, including the periaqueductal gray and the rostroventral medulla, also show selective activation when subjects expect pain relief. These brainstem regions send inhibitory projections to the spine and produce diffuse analgesic responses. Regrettably the precise contribution of spinal mechanisms in predicting the strength of placebo analgesia is unknown. Here, we show that expectations regarding pain radically change the strength of spinal nociceptive responses in humans. We found that contrary to expectations of analgesia, expectations of hyperalgesia completely blocked the analgesic effects of descending inhibition on spinal nociceptive reflexes. Somatosensory-evoked brain potentials and pain ratings further confirmed changes in spino-thalamo-cortical responses consistent with expectations and with changes in the spinal response. These findings provide direct evidence that the modulation of pain by expectations is mediated by endogenous pain modulatory systems affecting nociceptive signal processing at the earliest stage of the central nervous system. Expectation effects, therefore, depend as much about what takes place in the spine as they do about what takes place in the brain. Furthermore, complete suppression of the analgesic response normally produced by descending inhibition suggests that anti-analgesic expectations can block the efficacy of pharmacologically valid treatments which has important implications for clinical practice.