Behavioral, hormonal, and neural alterations induced by social contagion for pain in mice.

Neurobiology of social contagion/empathy aims to collaborate with the development of treatments for human disorders characterized by the absence of this response - autism spectrum disorder, schizophrenia, and antisocial personality disorder. Previous studies using sustained aversive stimuli (e.g., neuropathic pain or stress) to induce social contagion behaviors in rodents have demonstrated that these conditions may increase hypernociception, anxiogenic-like effects, and defensive behaviors in cagemates. To amplify the knowledge about behavioral, hormonal, and neural alterations induced by cohabitation with a pair in neuropathic pain, we investigated the effects of this protocol on (i) pain (writhing, formalin, hot plate tests) and depression (sucrose splash test) responses, (ii) the serum levels of corticosterone, testosterone, and oxytocin, (iii) noradrenalin, dopamine and its metabolite (DOPAC and HVA) levels in the amygdaloid complex and insular cortex, (iv) neuronal activation pattern (FosB labeling) in the ventral tegmental area (VTA), paraventricular nucleus of the hypothalamus (PVN) and supraoptic nucleus (SO). One day after weaning, male Swiss mice were housed in pairs for 14 days. Then, they were divided into two groups: sciatic nerve constricted cagemate [CNC; i.e., one animal of each pair was subjected to sciatic nerve constriction (NC)], and cagemate sham (CS; a similar procedure but with no nerve constriction), and housed for further 14 days. After 28 days of cohabiting, four independent groups were subjected to (a) behavioral analyses (Exp. 1) and (b) blood samples collected for Elisa assays of corticosterone, testosterone, and oxytocin (Exp. 2), remotion of brains for the (c) HPLC in the noradrenaline dopamine and metabolites quantification (Exp. 3) or (d) immunoassays analyses for FosB labeling (Exp. 4). Results showed that cohabitation with a conspecific in chronic pain induces hypernociception and antinociception in the writhing and formalin tests, respectively, and anhedonic-like effects in the sucrose splash test. Hormonal results indicated a decrease in plasma corticosterone only in nerve constricted mice, in testosterone (CNC and NC animals), and an increase in oxytocin serum levels. The neurochemical analyses demonstrated that the social contagion for pain protocol increases in dopamine turnover in the amygdala and insula. This assay also revealed an increase in noradrenaline levels and dopamine turnover within the insula of NC mice. In the FosB labeling measure, we observed a rise in the VTA, PVN and SO in the CNC group whereas for the NC group an increase of this activation pattern occurred only in the VTA. Present results suggest the role of hormones (testosterone and oxytocin) and neurotransmitters (dopamine) in the modulation of behavioral changes induced by social contagion in animals cohabitating with a conspecific in pain.

Evidence that increased cholecystokinin (CCK) in the periaqueductal gray (PAG) facilitates changes in Resident-Intruder social interactions triggered by peripheral nerve injury.

Individual differences in the effects of a chronic neuropathic injury on social behaviours characterize both the human experience and pre-clinical animal models. The impacts of these changes to the well-being of the individual are often underappreciated. Earlier work from our laboratory using GeneChip® microarrays identified increased cholecystokinin (CCK) gene expression in the periaqueductal gray (PAG) of rats that showed persistent changes in social interactions during a Resident-Intruder encounter following sciatic nerve chronic constriction injury (CCI). In this study, we confirmed these gene regulation patterns using RT-PCR and identified the anatomical location of the CCK-mRNA as well as the translated CCK peptides in the midbrains of rats with a CCI. We found that rats with persistent CCI-induced changes in social behaviours had increased CCK-mRNA in neurons of the ventrolateral PAG and dorsal raphe nuclei, as well as increased CCK-8 peptide expression in terminal boutons located in the lateral and ventrolateral PAG. The functional significance of these changes was explored by microinjecting small volumes of CCK-8 into the PAG of uninjured rats and observing their Resident-Intruder social interactions. Disturbances to social interactions identical to those observed in CCI rats were evoked when injection sites were located in the rostral lateral and ventrolateral PAG. We suggest that CCI-induced changes in CCK expression in these PAG regions contributes to the disruptions to social behaviours experienced by a subset of individuals with neuropathic injury.

Dynamic Change of Endocannabinoid Signaling in the Medial Prefrontal Cortex Controls the Development of Depression After Neuropathic Pain.

Many patients with chronic pain conditions suffer from depression. The mechanisms underlying pain-induced depression are still unclear. There are critical links of medial prefrontal cortex (mPFC) synaptic function to depression, with signaling through the endocannabinoid (eCB) system as an important contributor. We hypothesized that afferent noxious inputs after injury compromise activity-dependent eCB signaling in the mPFC, resulting in depression. Depression-like behaviors were tested in male and female rats with traumatic neuropathy [spared nerve injury (SNI)], and neuronal activity in the mPFC was monitored using the immediate early gene c-fos and in vivo electrophysiological recordings. mPFC eCB Concentrations were determined using mass spectrometry, and behavioral and electrophysiological experiments were used to evaluate the role of alterations in eCB signaling in depression after pain. SNI-induced pain induced the development of depression phenotypes in both male and female rats. Pyramidal neurons in mPFC showed increased excitability followed by reduced excitability in the onset and prolonged phases of pain, respectively. Concentrations of the eCBs, 2-arachidonoylglycerol (2-AG) in the mPFC, were elevated initially after SNI, and our results indicate that this resulted in a loss of CB1R function on GABAergic interneurons in the mPFC. These data suggest that excessive release of 2-AG as a result of noxious stimuli triggers use-dependent loss of function of eCB signaling leading to excessive GABA release in the mPFC, with the final result being behavioral depression.SIGNIFICANCE STATEMENT Pain has both somatosensory and affective components, so the complexity of mechanisms underlying chronic pain is best represented by a biopsychosocial model that includes widespread CNS dysfunction. Many patients with chronic pain conditions develop depression. The mechanism by which pain causes depression is unclear. Although manipulation of the eCB signaling system as an avenue for providing analgesia per se has not shown much promise in previous studies. An important limitation of past research has been inadequate consideration of the dynamic nature of the connection between pain and depression as they develop. Here, we show that activity-dependent synthesis of eCBs during the initial onset of persistent pain is the critical link leading to depression when pain is persistent.

Oral efficacy of Δ(9)-tetrahydrocannabinol and cannabidiol in a mouse neuropathic pain model.

The psychoactive and non-psychoactive constituents of cannabis, Δ9-tetrahydrocannabinol (THC) and cannabidiol (CBD), have synergistic analgesic efficacy in animal models of neuropathic pain when injected systemically. However, the relevance of this preclinical synergy to clinical neuropathic pain studies is unclear because many of the latter use oral administration. We therefore examined the oral effectiveness of these phytocannabinoids and their interactions in a mouse chronic constriction injury (CCI) model of neuropathic pain. THC produced a dose-dependent reduction in mechanical and cold allodynia, but also induced side-effects with similar potency. CBD also reduced allodynia, albeit with lower potency than THC, but did not produce cannabinoid-like side-effects at any dose tested. Combination THC:CBD produced a dose-dependent reduction in allodynia, however, it displayed little to no synergy. Combination THC:CBD produced substantial, synergistic side-effects which increased with the proportion of CBD. These findings demonstrate that oral THC and CBD, alone and in combination, have analgesic efficacy in an animal neuropathic pain model. Unlike prior systemic injection studies, combination THC:CBD lacks analgesic synergy when delivered orally. Furthermore, both THC and combination THC:CBD display a relatively poor therapeutic window when delivered orally. This suggests that CBD provides a safer, albeit lower efficacy, oral treatment for nerve injury induced neuropathic pain than THC-containing preparations. This article is part of the special issue on 'Cannabinoids'.

Differential modulation of the anterior cingulate and insular cortices on anxiogenic-like responses induced by empathy for pain.

Mice cohabiting with a conspecific in chronic pain display anxiogenesis in the elevated plus-maze (EPM). Given that the anterior cingulate (ACC) and insular (InC) cortices play a role in the modulation of anxiety, pain, and emotional contagion, we investigated (a) the FosB activation in both brain areas and (b) the effects of intra-ACC or -InC injection of cobalt chloride (CoCl2, a synaptic blocker), on the anxiety of mice cohabiting with a cagemate suffering pain. Twenty-one days after birth, male Swiss mice were housed in pairs for 14 days to establish familiarity. On the 14th day, mice were divided into two groups: cagemate sciatic nerve constriction (CNC; i.e., one animal of each pair was subjected to sciatic nerve constriction), and cagemate sham (CS; i.e., a similar procedure but without suffering nerve constriction). After that, both groups were housed again with the same pairs for the other 14 days. On the 28th day, mice had their brains removed for the immunoassays analyses (Exp. 1). For experiments 2 and 3, on the 23rd day, the cagemates received guide cannula implantation bilaterally in the ACC or InC and, on the 28th day, they received local injections of saline or CoCl2, and then were exposed to the EPM. Results showed that cohabitation with a conspecific with chronic pain decreases and increases neuronal activation (FosB) within the ACC and InC, respectively. Intra-ACC or InC injection of CoCl2 reversed the anxiogenic effect in those animals that cohabited with a conspecific in chronic pain. ACC and InC seem to modulate anxiety induced by emotional contagion in animals cohabitating with a conspecific suffering pain.

Potassium Chloride Cotransporter 2 Inhibits Neuropathic Pain and Future Development of Neurodegeneration.

Persistent neuropathic pain (NP) causes future development of neurodegenerative diseases, e.g., Alzheimer' disease, and thus needs to be optimally treated. Surgically-induced neuropathic pain (SNPP) is a persistent pain that occurs in nearly half of the individuals after common operations. Here, we showed that specific activation of 5-hydroxytryptamine (5-HT) type 2A receptors by systemic administration of TCB-2 [(4-bromo-3,6-dimethoxybenzocyclobuten-1-yl) methylamine hydrobromide] improved the function of potassium chloride cotransporter 2 (KCC2), resulting in reduction in neuropathic pain after chronic constriction injury (CCI), a rat model that mimics SNPP. Moreover, TCB-2 administration attenuated both mechanical and thermal hyperalgesia, likely through augmentation of dorsal horn KCC2 levels, since this effect was abolished by intrathecal provision of dihydroindenyl oxy alkanoic acid (DIOA), which blocked the effects of KCC2. Furthermore, TCB-2-mediated re-activation of KCC2 likely reduces future development of neurodegeneration in rats. Together, our data support further studies on the possibility of using this strategy to reduce postoperative pain and future neurodegenerative disorders in clinic.

Rat ultrasonic vocalizations as a measure of the emotional component of chronic pain.

In humans, chronic pain is often expressed as a spontaneous emotional response which can lead to fragmented sleep. Rat 50-kHz and 20-kHz ultrasonic vocalizations are well-established measures of positive and negative emotional states, respectively. The rat chronic constriction injury model was used to induce chronic pain, and ultrasonic vocalizations were measured in both the heterospecific rough-and-tumble play (i.e. tickling) test as well as during 24-hour home cage recordings. Rates of hedonic 50-kHz ultrasonic vocalizations during the non-stimulus periods of the tickling test, as well as the rewarding value of tickling, were reduced in chronic constriction injury rats compared to sham controls. In the 24-hour home cage recording study, chronic constriction injury animals showed a reduced amplitude in circadian activity, as well as reduced hedonic 50-kHz ultrasonic vocalizations and increased evoked and spontaneous aversive 20-kHz ultrasonic vocalizations. These data demonstrate that rat ultrasonic vocalizations can be used to capture core symptoms of chronic pain and may be useful in the elucidation of the neuronal mechanisms that underlie the affective component of pain.

Behavioral effects of combined morphine and MK-801 administration to the locus coeruleus of a rat neuropathic pain model.

The persistent activation of N-methyl-d-aspartate acid receptors (NMDARs) seems to be responsible for a series of changes in neurons associated with neuropathic pain, including the failure of opioids that act through mu-opioid receptors (MORs) to provide efficacious pain relief. As the noradrenergic locus coeruleus (LC) forms part of the endogenous analgesic system, we explored how intra-LC administration of morphine, a MORs agonist, alone or in combination with MK-801, a NMDARs antagonist, affects the sensorial and affective dimension of pain in a rat model of neuropathic pain; chronic constriction injury (CCI). Intra-LC microinjection of morphine induced analgesia in CCI rats, as evident in the von Frey and cold plate test 7 and 30 days after surgery, although it was not able to reverse pain-related aversion when evaluated using the place escape/avoidance test. However, the thermal anti-nociception produced by morphine was enhanced when it was administered to the LC of CCI animals in combination with MK-801, without altering its effects on the mechanical thresholds. Furthermore, pain-related aversion was reduced by co-administration of these agents, yet only in the short-term CCI (7 day) rats. Overall the data indicate that administration of morphine to the LC produces analgesia in nerve injured animals and that this effect is potentiated in specific pain modalities by the co-administration of MK-801. While a combination of morphine and MK-801 could reduce pain-related aversion in short-term neuropathic animals, it was ineffective in the long-term, suggesting that its sensorial effects and its influence on the affective component of pain are regulated by different mechanisms.

Chronic Sciatic Neuropathy in Rat Reduces Voluntary Wheel-Running Activity With Concurrent Chronic Mechanical Allodynia.

BACKGROUND: Animal models of peripheral neuropathy produced by a number of manipulations are assessed for the presence of pathologic pain states such as allodynia. Although stimulus-induced behavioral assays are frequently used and important to examine allodynia (ie, sensitivity to light mechanical touch; von Frey fiber test), other measures of behavior that reflect overall function are not only complementary to stimulus-induced responsive measures, but are also critical to gain a complete understanding of the effects of the pain model on quality of life, a clinically relevant aspect of pain on general function. Voluntary wheel-running activity in rodent models of inflammatory and muscle pain is emerging as a reliable index of general function that extends beyond stimulus-induced behavioral assays. Clinically, reports of increased pain intensity occur at night, a period typically characterized with reduced activity during the diurnal cycle. We therefore examined in rats whether alterations in wheel-running activity were more robust during the inactive phase compared with the active phase of their diurnal cycle in a widely used rodent model of chronic peripheral neuropathic pain, the sciatic nerve chronic constriction injury (CCI) model. METHODS: In adult male Sprague Dawley rats, baseline (BL) hindpaw threshold responses to light mechanical touch were assessed using the von Frey test before measuring BL activity levels using freely accessible running wheels (1 hour/day for 7 sequential days) to quantify the distance traveled. Running wheel activity BL values are expressed as total distance traveled (m). The overall experimental design was after BL measures, rats underwent either sham or CCI surgery followed by repeated behavioral reassessment of hindpaw thresholds and wheel-running activity levels for up to 18 days after surgery. Specifically, separate groups of rats were assessed for wheel-running activity levels (1 hour total/trial) during the onset (within first 2 hours) of either the (1) inactive (n = 8/group) or (2) active (n = 8/group) phase of the diurnal cycle. An additional group of CCI-treated rats (n = 8/group) was exposed to a locked running wheel to control for the potential effects of wheel-running exercise on allodynia. The 1-hour running wheel trial period was further examined at discrete 20-minute intervals to identify possible pattern differences in activity during the first, middle, and last portions of the 1-hour trial. The effect of neuropathy on activity levels was assessed by measuring the change from their respective BLs to distance traveled in the running wheels. RESULTS: Although wheel-running distances between groups were not different at BL from rats examined during either the inactive phase of the diurnal cycle or active phase of the diurnal cycle, sciatic nerve CCI reduced running wheel activity levels compared with sham-operated controls during the inactive phase. In addition, compared with sham controls, bilateral low-threshold mechanical allodynia was observed at all time points after surgical induction of neuropathy in rats with free-wheel and locked-wheel access. Allodynia in CCI compared with shams was replicated in rats whose running wheel activity was examined during the active phase of the diurnal cycle. Conversely, no significant reduction in wheel-running activity was observed in CCI-treated rats compared with sham controls at any time point when activity levels were examined during the active diurnal phase. Finally, running wheel activity patterns within the 1-hour trial period during the inactive phase of the diurnal cycle were relatively consistent throughout each 20-minute phase. CONCLUSIONS: Compared with nonneuropathic sham controls, a profound and stable reduction of running wheel activity was observed in CCI rats during the inactive phase of the diurnal cycle. A concurrent robust allodynia persisted in all rats regardless of when wheel-running activity was examined or whether they ran on wheels, suggesting that acute wheel-running activity does not alter chronic low-intensity mechanical allodynia as measured using the von Frey fiber test. Overall, these data support that acute wheel-running exercise with limited repeated exposures does not itself alter allodynia and offers a behavioral assay complementary to stimulus-induced measures of neuropathic pain.

Anti-nociceptive, anti-hyperalgesic and anti-arthritic activity of amides and extract obtained from Piper amalago in rodents.

ETHNOPHARMACOLOGICAL RELEVANCE: Piper amalago (Piperaceae) has been used in folk medicine as an analgesic. This study aimed to evaluate the pharmacological effects of extract and pure amides obtained from P. amalago on pain to provide a pharmacological basis for their use in traditional medicine. AIM OF THE STUDY: This study evaluated the anti-nociceptive, anti-hyperalgesic, anti-arthritic and anti-depressive activities of the ethanolic extract of P. amalago (EEPA) and the amides N-[7-(3',4'-methylenedioxyphenyl)-2(Z),4(Z)-heptadienoyl] pyrrolidine (1) and N-[7-(3',4'-methylenedioxyphenyl)-2(E),4(E)-heptadienoyl] pyrrolidine (2) obtained from P. amalago in animal models. MATERIALS AND METHODS: Mice treated daily with EEPA (100mg/kg, p.o.) were assayed for 20 days for knee edema (micrometer measurement), mechanical hyperalgesia (analgesiometer analysis), heat sensitivity and immobility (forced swim test) in the Complete Freund's Adjuvant (CFA) model. Cold (acetone test) and mechanical hyperalgesia (electronic von Frey analysis) responses were evaluated for 15 days in rats treated with oral EEPA (100mg/kg) in the spared nerve injury (SNI) model. Meanwhile, mice were evaluated for carrageenan-induced edema and mechanical hyperalgesia and for nociception using the formalin model after a single administration of EEPA (100mg/kg) or amides 1 and 2 (1mg/kg). RESULTS: Amides (1) and (2) were detected and isolated from the EEPA. The EEPA inhibited mechanical hyperalgesia, knee edema, and heat hyperalgesia, but not depressive-like behavior, induced by the intraplantar injection of CFA. When evaluated in the SNI model, the EEPA inhibited mechanical and cold hyperalgesia. The EEPA, 1 and 2 prevented the mechanical hyperalgesia induced by carrageenan and the anti-nociceptive effects in both phases of formalin nociception. The EEPA did not induce alterations in the open field test. CONCLUSION: The EEPA was effective for inhibition of pain and arthritic parameters but was not effective against depressive-like behavior; additionally, it did not alter locomotor activity. The amides obtained seemed to be the active component(s) present in the EEPA because they proved to be anti-nociceptive and anti-hyperalgesic in models of acute pain. Considering that few drugs are currently available for the treatment of chronic pain, especially neuropathic pain, the present results may have clinical relevance and open new possibilities for the development of new anti-hyperalgesic and anti-arthritic agents from P. amalago.

Baclofen reversed thermal place preference in rats with chronic constriction injury.

Chronic constriction injury to the sciatic nerve was used as an animal model of neuropathic pain. Instead of frequently used reflex-based tests we used an operant thermal place preference test to evaluate signs of neuropathic pain and the effect of baclofen administration in rats with neuropathy. Chronic constriction injury was induced by four loose ligations of the sciatic nerve. Thermal place preference (45 °C vs. 22 °C and 45 °C vs. 11 °C) was measured after the ligation and after the administration of baclofen in sham and experimental rats. Rats with the chronic constriction injury spent significantly less time on the colder plate compared to sham operated animals at the combination 45 °C vs. 11 °C. After administration of baclofen (10 mg/kg s.c.), the aversion to the colder plate in rats with chronic constriction injury disappeared. At the combination 45 °C vs. 22 °C, no difference in time spent on colder and/or warmer plate was found between sham and experimental animals. These findings show the importance of cold allodynia evaluation in rats with chronic constriction injury and the effectiveness of baclofen in this neuropathic pain model.

Antinociceptive and hypnotic activities of pregabalin in a neuropathic pain-like model in mice.

To evaluate the antinociceptive and hypnotic effects of pregabalin, we established a neuropathic pain-like model in mice using partial sciatic nerve ligation (PSNL), and examined thermal hyperalgesia, mechanical allodynia, electroencephalogram, rota-rod testing, and c-Fos expression in the anterior cingulate cortex. Gabapentin was used as a reference drug in the study. Pregabalin administered i.g. at 12.5 and 25mg/kg prolonged the duration of thermal latencies by 1.4- and 1.6-fold and increased the mechanical threshold by 2.2- and 3.1-fold 3h after administration, respectively, but did not affect motor coordination in PSNL mice, compared with vehicle control. Pregabalin (12.5 and 25mg/kg) given at 6:30 increased the amount of non-rapid eye movement sleep in a 4-h period by 1.3- and 1.4-fold, respectively, in PSNL mice. However, pregabalin (25mg/kg) given at 20:30 did not alter the sleep pattern in normal mice. Immunohistochemical study showed that PSNL increased c-Fos expression in the neurons of anterior cingulate cortex by 2.1-fold, which could be reversed by pregabalin. These results indicate that pregabalin is an effective treatment for both neuropathic pain and sleep disturbance in PSNL mice.

Novel use of perineural pregabalin infusion for analgesia in a rat neuropathic pain model.

BACKGROUND: The anticonvulsant drugs pregabalin and gabapentin are often used systemically to treat some forms of chronic neuropathic pain. However, many patients report side effects serious enough to cause discontinuation of the drug. Here we present evidence that pregabalin may block neuropathic pain when applied to the site of nerve injury in a rat neuropathic pain model. METHODS: Forty male Sprague Dawley rats were randomized into 4 groups: sciatic nerve crush injury with perineural pregabalin treatment (treatment), crush injury with perineural saline treatment (saline control), crush injury with subcutaneous pregabalin treatment (systemic drug control), and sham surgery (sham surgery control). Animals received either continuous infusions of 1% pregabalin for 7 days (treatment and systemic control) or saline (saline control) and were tested for pain behaviors using incapacitance meter, guarding scores, and radiant heat withdrawal latency (Hargreaves method). Nerves were studied using histology and immunohistochemistry for α(2)δ-1 receptors thought to mediate the central analgesic action of pregabalin. RESULTS: Treatment rats had significantly better guarding scores than systemic drug controls or saline controls (P < 0.0001) and had significantly better incapacitance scores than systemic drug controls and saline controls (P ≤ 0.001). Hargreaves method data showed hypoalgesia in all injured animals with no difference among injured groups (P = 0.80). Qualitatively, immunohistochemistry likely showed equivalent expression of the α(2)δ-1 calcium channel at the injured nerve site in all nerve-injured animals. CONCLUSIONS: Perineural pregabalin administration produced superior analgesia compared with that of systemic pregabalin in this neuropathic pain model. Perineural pregabalin treatment may provide a useful alternative to systemic pregabalin treatment for neuropathic pain.

[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.

Assessment of oxidative parameters in rat spinal cord after chronic constriction of the sciatic nerve.

Although reactive oxygen species (ROS) are involved in neuropathic pain, the direct relationship between these species and chronic constriction of sciatic nerve (CCI) has not been studied in spinal cord. Thus, this study induced CCI in rats and these animals were sacrificed 3 and 10 days after the surgical procedure to determine the superoxide dismutase (SOD) and catalase activities, as well as ascorbic acid, hydrogen peroxide (H(2)O(2)) and lipid hydroperoxide levels in lumbosacral spinal cord. Von Frey Hair and hot plate tests were performed to assess the degree of mechanical and thermal hyperalgesia at days 0, 3 and 10. The results showed that CCI significantly induced mechanical and thermal hyperalgesia at days 3 and 10. Parallel there was increase in spinal cord lipid hydroperoxide at days 3 and 10 in rats submitted to CCI. In Sham rats a significant increase in this parameter occurred at day 10. H(2)O(2) decreased at day 10 only in CCI group. SOD activity was decreased in Sham and CCI groups at day 3, while catalase activity was increased in CCI rats at days 3 and 10. Ascorbic acid levels were reduced only in CCI rats at day 3. Although the role of such changes is unclear, many were not specific to neuropathic pain and the differences could be related to different degrees of central sensitization in Sham and CCI rats.

Learned avoidance from noxious mechanical simulation but not threshold semmes weinstein filament stimulation after nerve injury in rats.

UNLABELLED: Noxious mechanical stimulation evokes a complex and sustained hyperalgesic motor response after peripheral nerve injury that contrasts with a brief and simple withdrawal seen after noxious stimulation in control animals or after threshold punctate mechanical stimulation by the von Frey technique. To test which of these behaviors indicate pain, the aversiveness of the experience associated with each was determined using a passive avoidance test in rats after sciatic nerve ligation (SNL) or skin incision alone. After 18 days, step-down latency was measured during 9 sequential trials at 10-minute intervals. At each trial, rats received either no stimulus, needle stimuli, or threshold Semmes Weinstein (SW) filament stimuli after stepping down. Reactions were either a hyperalgesic response or a brief reflexive withdrawal. In SNL animals, needle stimulation produced substantial learned avoidance when animals showed hyperalgesic responses but produced minimal prolonged latency in SNL animals that showed only simple withdrawal responses. No learned avoidance developed using threshold SW testing in SNL animals. These findings show that needle stimulation is aversive in rats responding with hyperalgesic behavior. In contrast, SW stimulation, as well as needle stimulation that produced mere withdrawal, is minimally aversive. PERSPECTIVE: The validity of measures of pain in animals is open to question. We demonstrated that needle stimulation is aversive in rats that respond with hyperalgesic-type behavior and is therefore a valid indicator of pain. Stimulation by SW is minimally aversive and is a problematic indicator of pain.

Increased susceptibility to development of anhedonia in rats with chronic peripheral nerve injury: involvement of sleep deprivation?

The main purpose of the present study was to evaluate whether REM sleep deprivation (RSD) influences the development of anhedonia in rats in a peripheral neuropathy model induced by sciatic nerve constriction injury (CCI). Anhedonia was measured by assessing daily water/sucrose intake. Four groups were assessed: control (CTRL), CCI, RSD, and CCI+RSD (n=8/group). Intake data were collected at baseline (mean of 3 days), on the 1st and 2nd days after a CCI or SHAM procedure, during 4 days of RSD, and during an additional 10 days (rebound period or equivalent in home-cage rats). Control rats spontaneously and progressively increased sucrose intake, reaching final daily volumes significantly greater than respective initial baseline amounts. RSD promoted an additional and immediate significant increase in sucrose intake during sleep deprivation days. The CCI group did not display a spontaneous, progressive increase in sucrose intake. When CCI was combined with RSD, the increase in sucrose intake induced by RSD was significantly lower than in animals submitted to RSD alone; the (CCI+RSD) group also failed to show a spontaneous and progressive increase in sucrose intake. The present findings indicate that animal model of chronic neuropathy exhibits reduced sucrose ingestion. Accordingly, this anhedonic condition that constitutes to the core manifestation of depressive states did not occur in response to a single episode of total RSD.

Depression-like behaviour in rats with mononeuropathy is reduced by the CB2-selective agonist GW405833.

The current study assessed whether the chronic constriction injury (CCI) model of neuropathic pain causes depression-like behaviour in animals, and if this depression-like behaviour can be reversed by anti-nociceptive and/or antidepressant drugs. CCI of the sciatic nerve in rats was selected as a neuropathic pain model, mechanical hypersensitivity was assessed by punctuate mechanical stimuli, and depression-like behaviour was evaluated in the forced swimming test (FST) measuring the time of immobility, climbing and swimming. The CCI rats displayed a significant mechanical hypersensitivity (sham 27+/-2g, CCI 12+/-2g; P<0.001) and a significant increase in time of immobility (sham 133+/-14s, CCI 201+/-9s; P<0.001). As time of swimming was unchanged, immobility was increased at the expense of climbing behaviour (sham 105+/-17s, CCI 63+/-9s; P<0.05). There was no difference in ambulation between sham and CCI animals. In sham and CCI animals, desipramine (20mg/kg) significantly reduced immobility (sham+vehicle 134+/-19s, sham+desipramine 79+/-13s; P<0.01, CCI+vehicle 195+/-8s, CCI+desipramine 140+/-11s; P<0.05) and increased climbing behaviour (sham+vehicle 118+/-21s, sham+desipramine 182+/-16s; P<0.05, CCI+vehicle 59+/-8s, CCI+desipramine 112+/-14s; P<0.05) with little effect on mechanical hypersensitivity. In contrast in CCI animals the cannabinoid CB2-selective agonist GW405833 (2,3-dichloro-phenyl)-[5-methoxy-2-methyl-3-(2-morpholin-4-yl-ethyl)-indol-1-yl]-methanone) (30 mg/kg) significantly attenuated immobility (CCI+vehicle 191+/-7s, GW405833 145+/-14s; P<0.01) and mechanical hypersensitivity (CCI+vehicle 15+/-1g, CCI+GW405833 24+/-1g; P<0.001). Moreover, differently from desipramine, GW405833 did not change the climbing behaviour. These data suggest that rats subjected to the CCI model of neuropathic pain develop depression-like behaviour, which can be reversed by appropriate anti-nociceptive treatment.

Neuropathic pain is associated with depressive behaviour and induces neuroplasticity in the amygdala of the rat.

Chronic pain is associated with the development of affective disorders but the underlying mechanisms are not fully understood. Changes in brain centres implicated in both emotional and pain processing are likely to be critical in the interplay of pain control and affective emotional behaviour. In the present study, we assessed emotional behaviour and performed a structural analysis of the amygdala (AMY) in neuropathic rats after two months of hyperalgesia and allodynia, induced by the spared nerve injury model (SNI). When compared with Sham-controls, SNI animals displayed signs of depressive-like behaviour. In addition, we found an increased amygdalar volume in SNI rats. No alterations were found in the dendritic arborizations of AMY neurons but, surprisingly, the amygdalar hypertrophy was associated with an increased cell proliferation [bromodeoxyuridine (BrdU)-positive cells] in the central (CeA) and basolateral (BLA) amygdaloid nuclei. The phenotypic analysis of the newly-acquired cells revealed that they co-label for neuronal markers (BrdU+NeuN and BrdU+Calbindin), but not for differentiated glial cells (BrdU+glial fibrillary acidic protein). We demonstrate that neuropathic pain promotes generation of new neurons in the AMY. Given the established role of the AMY in emotional behaviour, we propose that these neuroplastic changes might contribute for the development of depressive-like symptoms that are usually present in prolonged pain syndromes in humans.

Usefulness of antidepressants for improving the neuropathic pain-like state and pain-induced anxiety through actions at different brain sites.

Clinically, it is well known that chronic pain induces depression, anxiety, and a reduced quality of life. There have been many reports on the relationship between pain and emotion. We previously reported that chronic pain induced anxiety with changes in opioidergic function in the central nervous system. In this study, we evaluated the anxiolytic-like effects of several types of antidepressants under a chronic neuropathic pain-like state and searched for the brain site of action where antidepressants show anxiolytic or antinociceptive effects. Sciatic nerve-ligated mice exhibited thermal hyperalgesia and tactile allodynia from days 7 to 28 after nerve ligation. At 4 weeks after ligation, these mice showed a significant anxiety-related behavior in the light-dark test and the elevated plus-maze test. Under these conditions, repeated administration of antidepressants, including the tricyclic antidepressant (TCA) imipramine, the serotonin noradrenaline reuptake inhibitor (SNRI) milnacipran, and the selective serotonin reuptake inhibitor (SSRI) paroxetine, significantly prevented the anxiety-related behaviors induced by chronic neuropathic pain. These antidepressants also produced a significant reduction in thermal hyperalgesia and tactile allodynia. Moreover, the microinjection of paroxetine into the basolateral amygdala or cingulate cortex reduced anxiety-related behavior, and microinjection into the primary somatosensory cortex significantly attenuated thermal hyperalgesia. These findings suggest that serotonergic antidepressants are effective for treating anxiety associated with chronic neuropathic pain and may be useful for treating neuropathic pain with emotional dysfunction such as anxiety. Furthermore, SSRIs show anxiolytic and antinociceptive effects by acting on different brain regions.