Social transfer of pain in mice

A complex relationship exists between the psychosocial environment and the perception and experience of pain, and the mechanisms of the social communication of pain have yet to be elucidated. The present study examined the social communication of pain and demonstrates that "bystander" mice housed and tested in the same room as mice subjected to inflammatory pain or withdrawal from morphine or alcohol develop corresponding hyperalgesia. Olfactory cues mediate the transfer of hyperalgesia to the bystander mice, which can be measured using mechanical, thermal, and chemical tests. Hyperalgesia in bystanders does not co-occur with anxiety or changes in corticosterone and cannot be explained by visually dependent emotional contagion or stress-induced hyperalgesia. These experiments reveal the multifaceted relationship between the social environment and pain behavior and support the use of mice as a model system for investigating these factors. In addition, these experiments highlight the need for proper consideration of how experimental animals are housed and tested.

Behavioral profile assessment in offspring of Swiss mice treated during pregnancy and lactation with caffeine

The association between caffeine consumption and various psychiatric manifestations has long been observed. The objective was to assess the behavioral profile in offspring of Swiss mice treated during pregnancy and lactation with caffeine. For this purpose, two groups (n = 6 each and BW ~ 35 g) of female mice were treated during pregnancy and lactation by: tap water and caffeine solution at a concentration of 0.3 mg/mL through oral route. The offspring obtained, by completing 70 days of life, was underwent a behavioral battery test. Statistical analysis was performed by student t test and the different significance adopted was p < 0.05. According to our results, it was not found any significant differences in tail suspension and forced swimming tests. In anxiety related responses however, the mice of caffeine group had greater number of fecal pellets (178 %, p = 0.001) in the open field test, higher number of attempts (51 %, p = 0.03) in light-dark box and decreased percentage of entries in open arms (41 %, p = 0.01) in elevated plus maze test. Moreover, in the marble burying test, there was a significant decrease in the number of buried marbles compared with controls (110 %, p = 0,002). In the meantime, in the von Frey test, it was observed an exacerbation of mechanical allodynia both in basal conditions and after the carrageenan administration (p < 0.001). Furthermore, caffeine treatment during pregnancy and lactation causes long-term behavioral changes in the mice offspring that manifest later in life.

Involvement of the dynorphin/KOR system on the nociceptive, emotional and cognitive manifestations of joint pain in mice

Joint pain is a major clinical problem mainly associated to osteoarthritis, and characterized by articular cartilage degradation resulting in a complex chronic pain state that includes nociceptive, emotional and cognitive manifestations. Memory impairment, depressive- and anxiety-like symptoms have been reported to be associated with chronic pain, leading to a decrease of life quality. In this study, we evaluated the involvement of the endogenous dynorphin/kappa opioid receptor (KOR) system on the nociceptive, emotional, cognitive, neurochemical and epigenetic manifestations of joint pain. The murine model of monosodium iodoacetate (MIA) was used to induce joint pain in knockout mice for KOR (KOR-KO), prodynorphin (PDYN-KO) and their wild-type (WT) littermates. KOR-KO and PDYN-KO mice developed mechanical allodynia after intra-articular injection of MIA. This allodynia was significantly increased in both KOR-KO and PDYN-KO when compared to WT mice. Accordingly, both mutants showed increased microglial activation on the lumbar section of the spinal cord after MIA. The emotional responses were evaluated by measuring anxiety-like behaviour in the elevated plus maze and anhedonia as depressive-like behaviour, and cognitive alterations in the object recognition paradigm. Emotional and cognitive impairments after joint pain were differently modified in KOR-KO and PDYN-KO mice. Alterations of corticotropin-releasing factor (CRF) on the amygdala and hippocampus and down regulation of histone 3 acetylation on the amygdala suggest a possible mechanism to explain these emotional and cognitive manifestations. Our results reveal a specific involvement of the dynorphin/KOR system on joint pain manifestations that are usually associated to osteoarthritis.

Irritable bowel syndrome and visceral hypersensitivity : risk factors and pathophysiological mechanisms

Irritable bowel syndrome (IBS) is a common functional gastro-intestinal disorder, characterized by abdominal pain and altered intestinal motility. Visceral hypersensitivity is an important hallmark feature of IBS and is believed to underlie abdominal pain in patients with IBS. The two main risk factors associated with the development of IBS are gastrointestinal inflammation and psychological distress. On a peripheral level, visceral sensitivity seems to be modulated by several mechanisms. Immune cells in the mucosal wall, such as mast cells, and enterochromaffin cells may sensitize afferent nerves by release of their mediators. Furthermore, increased mucosal permeability, altered intestinal microflora and dietary habits may contribute to this feature. On a central level, an increased prevalence of psychiatric comorbidities is demonstrated in IBS patients, alongside alterations in the hormonal brain-gut axis, increased vigilance towards intestinal stimuli and functional and structural changes in the brain. The pathogenesis of IBS is complicated and multifactorial and the treatment remains clinically challenging. Dietary measures and symptomatic control are the cornerstones for IBS treatment and may be sufficient for patients experiencing mild symptoms, alongside education, reassurance and an effective therapeutic physician-patient relationship. New pharmacological therapies are aimed at interfering with mediator release and/or blockade of the relevant receptors within the gut wall, while modulation of the intestinal flora and diet may also be of therapeutic benefit. Tricyclic anti-depressants and serotonin reuptake inhibitors act both on a central and peripheral level by modulating pain signalling pathways.

Mechanical Conflict System: A Novel Operant Method for the Assessment of Nociceptive Behavior

A new operant test for preclinical pain research, termed the Mechanical Conflict System (MCS), is presented. Rats were given a choice either to remain in a brightly lit compartment or to escape to a dark compartment by crossing an array of height-adjustable nociceptive probes. Latency to escape the light compartment was evaluated with varying probe heights (0, .5, 1, 2, 3, and 4 mm above compartment floor) in rats with neuropathic pain induced by constriction nerve injury (CCI) and in naive control rats. Escape responses in CCI rats were assessed following intraperitoneal administration of pregabalin (10 and 30 mg/kg), morphine (2.5 and 5 mg/kg), and the tachykinin NK1 receptor antagonist, RP 67580 (1 and 10 mg/kg). Results indicate that escape latency increased as a function of probe height in both naive and CCI rats. Pregabalin (10 and 30 mg/kg) and morphine (5 mg/kg), but not RP 67580, decreased latency to escape in CCI rats suggesting an antinociceptive effect. In contrast, morphine (10 mg/kg) but not pregabalin (30 mg/kg) increased escape latency in naive rats suggesting a possible anxiolytic action of morphine in response to light-induced fear. No order effects following multiple test sessions were observed. We conclude that the MCS is a valid method to assess behavioral signs of affective pain in rodents.

[EXPERIMENTAL MODELS OF ALCOHOLIC NEUROPATHY IN RATS.]

The aim of this work was to study the behavioral and histopathomorphological signs of peripheral neuropathy development in male Wistar rats on the model of alcoholic neuropathy. Chronic consumption of ethanol solution with concentration increasing from 7.47 to 26.2% (w/w) resulted in neuropathy (allodynia) de- velopment after 8 weeks of chronic alcohol administration. The behavioral signs of allodynia became significant on the 8th week and were retained up to the end of experiment (15 weeks of ethanol administration). The reference drug gabapentin effectively reduced the manifestation of allodinia. Histological exami- nation of sciatic nerve preparations from animals killed after ethanol consumption for 5, 10 and 15 weeks revealed the development of histopathomorphological pattern with increasing duration of chronic alcoholization. At the initial stage, the morphological basis of observed behavioral manifestations was provided by excess lipid deposition in peri/epineurium of nerve specimens). The further increase in treatment duration (up to 10 and 15 weeks) was associated with demye- lination and development of inflammation of the sciatic nerve. This experimental model allows one to investigate the efficacy of new neuroprotective and ana- lgesic substances - potential drugs for both prevention and management of neuropathy.

Endomorphin analog analgesics with reduced abuse liability, respiratory depression, motor impairment, tolerance, and glial activation relative to morphine

Opioids acting at the mu opioid receptor (MOR) are the most effective analgesics, however adverse side effects severely limit their use. Of particular importance, abuse liability results in major medical, societal, and economic problems, respiratory depression is the cause of fatal overdoses, and tolerance complicates treatment and increases the risk of side effects. Motor and cognitive impairment are especially problematic for older adults. Despite the host of negative side effects, opioids such as morphine are commonly used for acute and chronic pain conditions. Separation of analgesia from unwanted effects has long been an unmet goal of opioid research. Novel MOR agonist structures may prove critical for greater success. Here we tested metabolically stable analogs of the endomorphins, endogenous opioids highly selective for the MOR. Compared to morphine, the analogs showed dramatically improved analgesia-to-side-effect ratios. At doses providing equal or greater antinociception than morphine in the rat, the analogs showed reduced a) respiratory depression, b) impairment of motor coordination, c) tolerance and hyperalgesia, d) glial p38/CGRP/P2X7 receptor signaling, and e) reward/abuse potential in both conditioned place preference and self-administration tests. Differential effects on glial activation indicate a mechanism for the relative lack of side effects by the analogs compared to morphine. The results suggest that endomorphin analogs described here could provide gold standard pain relief mediated by selective MOR activation, but with remarkably safer side effect profiles compared to opioids like morphine.

The affective dimension of pain as a risk factor for drug and alcohol addiction

Addiction, or substance use disorder (SUD), is a devastating psychiatric disease composed of multiple elemental features. As a biobehavioral disorder, escalation of drug and/or alcohol intake is both a cause and consequence of molecular neuroadaptations in central brain reinforcement circuitry. Multiple mesolimbic areas mediate a host of negative affective and motivational symptoms that appear to be central to the addiction process. Brain stress- and reinforcement-related regions such as the central amygdala (CeA), prefrontal cortex (PFC), and nucleus accumbens (NAc) also serve as central processors of ascending nociceptive input. We hypothesize that a sensitization of brain mechanisms underlying the processing of persistent and maladaptive pain contributes to a composite negative affective state to drive the enduring, relapsing nature of addiction, particularly in the case of alcohol and opioid use disorder. At the neurochemical level, pain activates central stress-related neuropeptide signaling, including the dynorphin and corticotropin-releasing factor (CRF) systems, and by this process may facilitate negative affect and escalated drug and alcohol use over time. Importantly, the widespread prevalence of unresolved pain and associated affective dysregulation in clinical populations highlights the need for more effective analgesic medications with reduced potential for tolerance and dependence. The burgeoning epidemic of prescription opioid abuse also demands a closer investigation into the neurobiological mechanisms of how pain treatment could potentially represent a significant risk factor for addiction in vulnerable populations. Finally, the continuing convergence of sensory and affective neuroscience fields is expected to generate insight into the critical balance between pain relief and addiction liability, as well as provide more effective therapeutic strategies for chronic pain and addiction.

Chronic inflammatory pain upregulates expression of P2Y2 receptor in small-diameter sensory neurons

Roles of ionotropic purinergic (P2X) receptors in chronic pain have been intensively investigated. However, the contribution of metabotropic purinergic (P2Y) receptors to pathological pain is controversial. In the present study, using single cell RT-PCR (reverse transcription-polymerase chain reaction) and single cell nested-PCR techniques, we examined the expression of P2X(2), P2X(3), P2Y(1) and P2Y(2) mRNA transcripts in retrogradely labeled cutaneous sensory neurons from mouse lumber dorsal root ganglia (DRGs) following peripheral inflammation. The percentage of cutaneous sensory neurons expressing P2Y(2) mRNA transcripts increased after complete Freund's adjuvant (CFA) treatment. Particularly, the P2Y(2) mRNA transcripts were more frequently detected in small-diameter cutaneous neurons from CFA-treated mice than those from control mice. Coexpression of P2Y(2) and P2X (P2X(2) or P2X(3)) mRNAs was more frequently observed in cutaneous sensory neurons from CFA-treated mice relative to controls. Pain behavioral tests showed that the blockade of P2Y receptors by suramin attenuated mechanical allodynia evoked either by CFA or uridine triphosphate (UTP), an endogenous P2Y(2) and P2Y(4) agonist. These results suggest that chronic inflammatory pain enhances expression of P2Y(2) receptor in peripheral sensory neurons that innervate the injured tissue and the activation of P2Y receptors contributes to mechanical allodynia following inflammation.

Co-Administration of Pioglitazone Improves Fluoxetine's Antinociceptive, Neuroprotective, and Antidepressant Effects in Chronic Constriction Injury in Rats

BACKGROUND

Chronic pain may be associated with diabetes mellitus and/or depression. Use of therapies that target both comorbidities is encouraged.

OBJECTIVE

This study was designed to investigate the potential antinociceptive, neuroprotective, and antidepressant effects of combinations of pioglitazone or metformin with fluoxetine in chronic constriction injury (CCI) in rats.

STUDY DESIGN

Experimental trial in rats.

SETTING

University lab in in Saudi Arabia.

METHODS

Two sets of experiments were performed. In each one, 9 groups of rats (n = 8) were used: sham, CCI, and 7 CCI-treated groups. Treatments were given orally starting on day 7 post-surgery as follows (mg/kg/day): fluoxetine (10, 20, and 40), pioglitazone (20), metformin (50), fluoxetine (20) +' pioglitazone, and fluoxetine (20) +' metformin. In the first set, on day 14 post-surgery mechanical allodynia, thermal hyperalgesia, and serum cytokines were measured. Moreover, immunoreactivity of glial fibrillary acidic protein (GFAP, a marker for astrocytic activation) in the spinal cord was assessed and histopathological changes in the ipsilateral sciatic nerve were examined. In the second set, on days 14 and 21 post-surgery the forced swimming test was done.

RESULTS

In the first set, all treatments significantly decreased mechanical allodynia while all treatments except F10 and F20 significantly decreased thermal hyperalgesia compared to the CCI group. The F20+'M group showed the highest decreases, however still significantly lower than those of the sham group. The treatments didn't impair motor function in the rotarod test. All treatments significantly decreased serum levels of tumor necrosis factor- α, interleukin-6, and monocyte chemoattractant protein-1 while increasing the level of interleukin-10. The CCI-induced marked increase of GFAP immunoexpression has been reduced to moderate with fluoxetine (40) and pioglitazone, and to mild with metformin and the combination groups. The CCI-induced changes in sciatic nerve were less in fluoxetine (40), pioglitazone, and metformin groups, and least in the combination groups. In the second set, the immobility duration was significantly reduced by F20, F40, P, F20+'P, and F20+'M compared to the CCI group. The F20+'P group showed the highest decrease, however still significantly lower than that of the sham group. The treatments didn't affect locomotor activity in the open field test.

LIMITATIONS

Measuring the cytokines levels only in blood and not in the spinal cord and sciatic nerve and measuring the outcome measures in the first set of experiments at only one time-point.

CONCLUSIONS

Co-administration of pioglitazone or metformin with low-dose fluoxetine improved mechanical allodynia, thermal hyperalgesia, and neurohistopathological changes while co-administration of pioglitazone, but not metformin, improved the depressive-like behavior in the peripheral nerve injury model of neuropathic pain in rats. Extrapolation of the current results to clinical reality could be beneficial for pain patients with diabetes and/or depression, however this needs further confirmatory studies.

Altered pain sensitivity in elderly women with chronic neck pain

Background

Age-related changes occur in both the peripheral and central nervous system, yet little is known about the influence of chronic pain on pain sensitivity in older persons. The aim of this study was to investigate pain sensitivity in elders with chronic neck pain compared to healthy elders.

Methods

Thirty elderly women with chronic neck pain and 30 controls were recruited. Measures of pain sensitivity included pressure pain thresholds, heat/cold pain thresholds and suprathreshold heat pain responses. The pain measures were assessed over the cervical spine and at a remote site, the tibialis anterior muscle.

Results

Elders with chronic neck pain had lower pressure pain threshold over the articular pillar of C5-C6 and decreased cold pain thresholds over the cervical spine and tibialis anterior muscle when compared with controls (p < 0.05). There were no between group differences in heat pain thresholds and suprathreshold heat pain responses (p > 0.05).

Conclusion

The presence of pain hypersensitivity in elderly women with chronic neck pain appears to be dependent on types of painful stimuli. This may reflect changes in the peripheral and central nervous system with age.

Enhanced short-term sensitization of facial compared with limb heat pain

Habituation and sensitization are important features of individual sensitivity to repetitive noxious stimulation and have been investigated in numerous studies. However, it is unclear whether these phenomena vary depending on the site of stimulation. Here we compared short-term and long-term effects of painful heat stimulation on the forehead and limb using an established longitudinal heat pain paradigm performed over 8 consecutive days in 36 healthy volunteers. Participants were randomized into 2 groups; participants received repetitive heat pain stimulation either on the left volar forearm or on the left side of the forehead. Our data show a comparable degree of habituation over the course of 8 days in both groups. However, participants in the trigeminal stimulation group exhibited stronger within-session sensitization (indexed by a higher within-session increase in pain intensity ratings) than those who received the forearm stimulation. Furthermore, over the course of the experiment we found a correlation between habituation and anxiety, showing less habituation in participants with higher trait anxiety scores. Our findings are in line with somatotopic differences in response to painful stimulation and a higher proneness of trigeminal pain to sensitization processes, which might be explained by the biological relevance of the head and facial area for vital functions. The contribution of this sensitivity to the development and maintenance of clinical facial pain and headache disorders warrants further investigation.

PERSPECTIVE

This study uses psychophysical methods to evaluate the differences in long-term habituation and short-term sensitization to heat pain between the trigeminal and spinal systems. We found stronger sensitization for trigeminal compared with nociceptive stimuli on the forearm. The contribution of this sensitivity to clinical pain states warrants further investigation.

Inflammatory pain memory facilitates occlusal interference-induced masticatory muscle hyperalgesia in rats

BACKGROUND

Patients with an orofacial pain history appear to be more susceptible to occlusal interference pain in dental practice for unknown reasons. Pain memory has a critical function in subsequent pain perception. This study aims to explore whether orofacial pain memory could affect the masticatory muscle pain perception for occlusal interference.

METHODS

Cross-injection of 2% carrageenan into bilateral masseters in male rats was carried out to establish the inflammatory pain memory model. The effects of pain memory on masseter muscle nociception were tested by applying crowns with heights beyond the occlusal plane by 0.2 or 0.4 mm onto a maxillary molar 2 weeks after inflammation in the right masseter. The 0.4-mm crowns were removed on day 2 or day 4 after application to further confirm the effects of pain memory. Moreover, memory impairment was established using ibotenic acid (IBO) infusion into the bilateral hippocampus, followed by behaviour tests, including the Morris water maze test and the locomotor activity test. The relationship between pain memory and occlusal interference-induced masseter muscle pain perception was subsequently re-examined. The head withdrawal thresholds of masseters on both sides were measured to reflect the perception.

RESULTS

Inflammatory pain memory aggravated the 0.2-mm crown-induced mechanical hyperalgesia of the masseters, but not in the 0.4-mm crown group. However, the recovery of the 0.4-mm crown-induced mechanical hyperalgesia was postponed. The effects of pain memory were reversed in rats with impaired mnemonic function of the hippocampus.

CONCLUSIONS

Inflammatory pain memory facilitated occlusal interference-induced masseter muscle pain.

Upregulation of α₂δ-1 Calcium Channel Subunit in the Spinal Cord Contributes to Pelvic Organ Cross-Sensitization in a Rat Model of Experimentally-Induced Endometriosis

Pelvic organ cross-sensitization, also termed as viscero-visceral referred hyperalgesia, is a major contributor to painful endometriosis. Its underlying mechanism is poorly understood. Clinical and basic studies have shown that gabapentin, a drug that binds to the α2δ-1 subunit of voltage-dependent calcium channels (Cavα2δ-1), is effective in treating chronic visceral pain. Accordingly, we hypothesized that pelvic organ cross-sensitization in painful endometriosis is mediated by an upregulation of Cavα2δ-1 in the spinal cord. We examined if the dysregulation of spinal Cavα2δ-1 subunit may play an important role in the development of ectopic growths-to-colon cross-sensitization in a rat model of experimentally-induced endometriosis. Our findings suggest that there was an increased Cavα2δ-1 expression in the dorsal horn and an ectopic growths-to-colon cross-sensitization in female rats with established endometriosis. Intrathecal administration of gabapentin (300 μg) remarkably reduced the ectopic growths-to-colon cross-sensitization in rats with established endometriosis. Furthermore, intrathecal injection of Cavα2δ-1 antisense oligodeoxynucleotides reversed the ectopic growths-to-colon cross-sensitization and also normalized the upregulation of spinal Cavα2δ-1 expression in endometriosis rats. The current study suggests that the upregulation of Cavα2δ-1 in the spinal cord may contribute to pelvic organ cross-sensitization in painful endometriosis. Our study may provide a biological basis for selectively targeting this pathway to relieve viscero-visceral referred hyperalgesia in patients with painful endometriosis.

Hyperexcitability in pain matrices in patients with fibromyalgia

OBJECTIVES

Emerging evidence associates fibromyalgia (FM) with pain system dysfunction. In this study, using laser evoked potentials (LEPs) and paired laser stimuli, we tested excitability in the pain matrices and sought possible changes in patients with FM.

METHODS

In 20 patients with FM and 15 healthy subjects, after recording control nociceptive system-mediated Aδ- and C-fibre-related LEPs, we measured excitability in the pain matrices by testing the Aδ-LEP conditioned by a preceding C-LEP.

RESULTS

No difference was found in control LEP amplitudes for Aδ- or C-fibres between patients and healthy subjects. Conversely, the Aδ-LEP amplitude, conditioned by a preceding C-LEP, was significantly higher in patients than in healthy subjects (p<0.001).

CONCLUSIONS

Objective evidence from increased conditioned Aδ-LEP amplitudes reflecting hyperexcitability in the pain matrices in FM, provides diagnostically useful information and might help in developing new therapeutic approaches.

Development of a bedside pain assessment kit for the classification of patients with osteoarthritis

There are no standardized bedside assessments for subtyping patients with osteoarthritis (OA) based on pain mechanisms. Thus, we developed a bedside sensory testing kit (BSTK) to classify OA patients based on sensory profiles potentially indicative of pain mechanism. After usability and informal reliability testing (n = 22), the kit was tested in a formal reliability study (n = 20). Patients completed questionnaires and sensory testing: pressure algometry to detect hyperalgesia; repeat algometry after heterotopic noxious conditioning stimulation to measure diffuse noxious inhibitory control (DNIC); light touch using Von Frey filaments; and cold allodynia using a brass rod. The procedure was brief and well tolerated. Algometry and filament testing were highly reliable [intra-class correlation coefficients (ICCs) 0.71-0.91]; DNIC was acceptably reliable (ICCs 0.53-0.91); brass rod reliability was inconclusive. Patients were classified empirically into four groups: "All abnormal findings" (primary and secondary hyperalgesia and dysfunctional DNIC); "all normal findings"; and two intermediate groups. The "all abnormal findings" group had more neuropathic pain symptoms, and lower WOMAC total, stiffness, and activity scores than the "all normal findings" group. Simple BSTK procedures, consolidated in a kit, reliably classified OA patients into subgroups based on sensory profile, suggesting that OA patients differ in underlying pain mechanisms. Further research is needed to confirm these subgroups and determine their validity in predicting response to treatment.

The effect of chemically induced colitis, psychological stress and their combination on visceral pain in female Wistar rats

Visceral sensitivity is of pathophysiological importance in abdominal pain disorders and can be modulated by inflammation and stress. However, it is unclear whether inflammation and stress alter visceral perception independently of each other or in conjunction through neuroendocrine interactions. Therefore, we compared the short- and long-term effects of experimental colitis and water avoidance stress (WAS), alone or in combination, on visceral sensitivity in female Wistar rats. Colitis was induced by trinitrobenzene sulfonic acid (TNBS) and colonoscopically confirmed. During WAS, rats were placed on a platform surrounded by water for 1 h. Visceral sensitivity was assessed by quantifying the visceromotor responses (VMRs) to colorectal distension. Activation of the hypothalamic-pituitary-adrenal axis was determined by measuring serum corticosterone in a separate protocol. TNBS instillation resulted in overt colitis, associated with significant visceral hypersensitivity during the acute inflammatory phase (3 days post-TNBS; n = 8/group); after colitis had subsided (28 days post-TNBS), hypersensitivity was resolved (n = 4-8/group). Single WAS was associated with increased VMRs of a magnitude comparable to acute TNBS-induced hypersensitivity (n = 8/group). However, after repetitive WAS no significant hypersensitivity was present (n = 8/group). No additive effect of colitis and stress was seen on visceral pain perception (n = 6-8/group). Corticosterone levels were only increased in acute TNBS-colitis, acute WAS and their combination. To conclude, both colitis and stress successfully induced short-term visceral hypersensitivity and activated the hypothalamic-pituitary-adrenal axis, but long-term effects were absent. In addition, our current findings do not support an additive effect of colitis and stress on visceral sensitivity in female Wistar rats.

[Behavior and functional state of the dopaminergic brain system in pups of depressive WAG/Rij rats]

In the present work, it has been studied for the first time behavior and functional state of the dopaminergic brain system in pups of "depressive" WAG/Rij rats. Offspring of "depressive" WAG/Rij rats at age of 6-16 days compared with offspring of "normal" (non-depressed) outbred rats of the same age exhibited reduced rate of pshychomotor development, lower body weight, attenuation in integration of coordinated reflexes and vestibular function (greater latency of righting reflex, abnormal negative geotaxis), hyper-reactivity to tactile stimulation, reduced motivation to contact with mother (reduced infant-mother attachment). Differences in a nest seeking response induced by olfactory stimuli (olfactory discrimination test) and in locomotor activity (tests "gait reflex" and "small open field") have not been revealed. Acute injection of the antagonist of D2-like dopamine receptors clebopride 20 min before testing aggravated mother-oriented behavior in 15-days-old pups of both "depressive" and "non-depressive" rats. However this effect was greater in pups of "depressive" WAG/Rij rats compared with pups of "normal" rats that may indicate reduced functional activity of the dopaminergic brain system in offspring of "depressive" rats. It is proposed that reduced attachment behavior in pups of "depressive" WAG/Rij rats might be a consequence of maternal depression and associated with it reduced maternal care. Moreover, reduced attachment behavior in pups of "depressive" rats might be an early precursor (a marker) of depressive-like pathology which become apparent later in life (approximately at age of 3 months).

Ovariectomy results in variable changes in nociception, mood and depression in adult female rats

Decline in the ovarian hormones with menopause may influence somatosensory, cognitive, and affective processing. The present study investigated whether hormonal depletion alters the nociceptive, depressive-like and learning behaviors in experimental rats after ovariectomy (OVX), a common method to deplete animals of their gonadal hormones. OVX rats developed thermal hyperalgesia in proximal and distal tail that was established 2 weeks after OVX and lasted the 7 weeks of the experiment. A robust mechanical allodynia was also occurred at 5 weeks after OVX. In the 5^th week after OVX, dilute formalin (5%)-induced nociceptive responses (such as elevating and licking or biting) during the second phase were significantly increased as compared to intact and sham-OVX females. However, chronic constriction injury (CCI) of the sciatic nerve-induced mechanical allodynia did not differ as hormonal status (e.g. OVX and ovarian intact). Using formalin-induced conditioned place avoidance (F-CPA), which is believed to reflect the pain-related negative emotion, we further found that OVX significantly attenuated F-CPA scores but did not alter electric foot-shock-induced CPA (S-CPA). In the open field and forced swimming test, there was an increase in depressive-like behaviors in OVX rats. There was no detectable impairment of spatial performance by Morris water maze task in OVX rats up to 5 weeks after surgery. Estrogen replacement retrieved OVX-induced nociceptive hypersensitivity and depressive-like behaviors. This is the first study to investigate the impacts of ovarian removal on nociceptive perception, negative emotion, depressive-like behaviors and spatial learning in adult female rats in a uniform and standard way.

Characteristics of sensitization associated with chronic pain conditions

OBJECTIVES

To describe and understand varieties and characteristics of sensitization contributing to hyperalgesia in participants with chronic pain conditions.

METHODS

Thermal stimulation was delivered to the face, forearm, and calf of pain-free participants and individuals with irritable bowel syndrome, temporomandibular pain disorder (TMD), and fibromyalgia syndrome (FM). Three-second contacts by a preheated thermode occurred at 30-second intervals in ascending and then in descending series (0.7°C steps).

RESULTS

Thermal pain ratings during ascending series were greater at each site in individuals diagnosed with chronic pain. Intense pain at the time of testing further enhanced the ratings at all sites, but mild or moderate clinical pain did not have this effect. Thermal pain in all participants was greater during descending series compared with the ascending series of arm and leg stimulation. The hypersensitivity during the descending series was comparable in pain-free, FM and TMD participants but was increased in duration for arm or leg stimulation of FM participants.

DISCUSSION

The widespread sensitization for irritable bowel syndrome and TMD participants does not rely on mechanisms of spatial and temporal summation often invoked to explain widespread hyperalgesia associated with chronic pain. Increased sensitivity during descending series of stimulation of an arm or leg but not the face indicates a propensity for sensitization of nociceptive input to the spinal cord. Abnormally prolonged sensitization for FM participants reveals a unique influence of widespread chronic pain referred to deep somatic tissues.

Role of CXCR2 and TRPV1 in functional, inflammatory and behavioural changes in the rat model of cyclophosphamide-induced haemorrhagic cystitis

BACKGROUND AND PURPOSE

Cyclophosphamide induces urotoxicity characterized by the development of cystitis, which involves bladder overactivity and inflammation. Here, we investigated the roles of chemokine receptor 2 (CXCR2) and transient receptor potential vanilloid 1 (TRPV1) channels in a rat model of cyclophosphamide-induced cystitis.

EXPERIMENTAL APPROACH

Cystitis induced by cyclophosphamide in rats was assessed by gross morphology, histology and immunohistochemistry of bladder tissue. mRNA for CXCR2 and TRPV1 channels were measured by RT-PCR. Nociceptive responses in paw and abdomen, along with cystometric measures were recorded.

KEY RESULTS

Cyclophosphamide, i.p., induced pain behaviour, bladder inflammation and voiding dysfunction. The CXCR2 antagonist, SB225002, the TRPV1 channel antagonist, SB366791 or their combination reduced the mechanical hypersensitivity of paw and abdominal area and nociceptive behaviour after cyclophosphamide. Cyclophosphamide-induced cystitis was characterized by haemorrhage, oedema, neutrophil infiltration and other inflammatory changes, which were markedly decreased by the antagonists. Up-regulation of CXCR2 and TRPV1 mRNA in the bladder after cyclophosphamide was inhibited by SB225002, SB366791 or their combination. Expression of CXCR2 and TRPV1 channels was increased in the urothelium after cyclophosphamide. Bladder dysfunction was shown by increased number of non-voiding contractions (NVCs) and bladder pressures and a reduction in bladder capacity (BC), voided volume (VV) and voiding efficiency (VE). SB225002 or its combination with SB366791 reduced bladder pressures, whereas SB225002, SB366791 or their combination increased BC, VV and VE, and also reduced the number of NVCs.

CONCLUSIONS AND IMPLICATIONS

CXCR2 and TRPV1 channels play important roles in cyclophosphamide-induced cystitis in rats and could provide potential therapeutic targets for cystitis.

Classical conditioning and pain: conditioned analgesia and hyperalgesia

This article reviews situations in which stimuli produce an increase or a decrease in nociceptive responses through basic associative processes and provides an associative account of such changes. Specifically, the literature suggests that cues associated with stress can produce conditioned analgesia or conditioned hyperalgesia, depending on the properties of the conditioned stimulus (e.g., contextual cues and audiovisual cues vs. gustatory and olfactory cues, respectively) and the proprieties of the unconditioned stimulus (e.g., appetitive, aversive, or analgesic, respectively). When such cues are associated with reducers of exogenous pain (e.g., opiates), they typically increase sensitivity to pain. Overall, the evidence concerning conditioned stress-induced analgesia, conditioned hyperalagesia, conditioned tolerance to morphine, and conditioned reduction of morphine analgesia suggests that selective associations between stimuli underlie changes in pain sensitivity.

Specificity and sensitivity of objective tests to detect possible malingering in fibromyalgia: a case-control study in 211 Spanish patients

OBJECTIVES

To characterise patients diagnosed with fibromyalgia (FM) who present a clinical profile suggestive of simulation.

METHODS

Observational case-control study of 218 patients who met the classification criteria for FM. The profile supporting simulation was based on the proposed criteria for evaluating disability related to the simulation of pain.

RESULTS

Compared with controls (n=105), patients with suspected simulation of FM (n=106) had a higher mean age (52.5 vs. 49.2 years, p=0.003), a higher frequency of primary education (88.7% vs. 58.1%; p<0.001), a higher percentage of separated/widowed persons (33.9% vs. 8.6%, p<0.001), a higher frequency of psychiatric disorders (100% vs. 67.6%, p<0.001), a higher mean number of positive 'control' tender points (4.5 vs. 1.3, p<0.001), a higher mean FIQ questionnaire score (89.8 vs. 68.8, p<0.001) and a lower mean LHS questionnaire score (41.0 vs. 59.9, p<0.001). Patients with suspected simulation were able to walk a shorter distance in the 6-minute walk test than controls (231.0 vs. 356.3 metres, p<0.001), while the appearance of allodynia was achieved with a significantly lower mmHg pressure (159.8 vs. 229.9 mm Hg, p<0.001).

CONCLUSIONS

Some physical/functional tests, together with the administration of specific questionnaires, may identify a subgroup of patients with FM with a profile consistent with simulation or malingering; these patients have a differentiated demographic and psychiatric profile in comparison with FM patients without a profile of simulation.

[Analgesic effect of ferulic acid on CCI mice: behavior and neurobiological analysis]

To study the analgesic effect of chronic administration with ferulic acid, and preliminarily discuss its mechanism. Thermal hyperalgesia and mechanical allodynia tests were conducted to observe the analgesic effect of chronic administration with ferulic acid on CCI mice. The neurochemical detection method was applied to observe the effect chronic administration with ferulic acid on monoamine neurotransmitter and monoamine oxidase activity. Compared with the normal group, CCI mice showed notable reduction in heat sensation and nociceptive threshold in and mechanical allodynia. Ferulic acid (10, 20, 40 and 80 mg x kg(-1), po) could significantly reverse the situations. In an in-depth study, we found that the reason for these results was that ferulic acid was dose-dependent in increasing 5-HT and NE levels in hippocampus, frontal cortex and amygdale and could inhibit MAO-A activity in mouse brains. These results showed that ferulic acid has the analgesic effect. Its mechanism may be related to the inhibition of monoamine oxidase activity and the increase in monoamine neurotransmitter in mouse brains.