Role of Nociceptive Arcuate Nucleus Neurons in Chloroquine-induced Pruritic Behaviors in Mice

Despite its clinical importance, the underlying central mechanisms of pruritic behaviors are poorly understood. To investigate the role of nociceptive arcuate nucleus neurons in chloro-quine-induced pruritic behaviors in mice, we tested the effect of arcuate nucleus neurons and interscapular brown adipose tissue (IBAT) on itch produced by intradermal injection of chloroquine in the nape of the neck. Our results provide several lines of evidence for an important role of nociceptive arcuate nucleus neurons in chloroquine-induced pruritic behavior: (1) Intradermal microinjection of chloro-quine resulted in a dramatic increase in itch behaviors accompanied by the activation of c-Fos positive neurons in arcuate nucleus; (2) Microinjection of chloroquine significantly increased IBAT temperature in the mice. These findings suggested that chloroquine-induced pruritic behaviors were associated with the activity of nociceptive arcuate nucleus neurons.

Involvement of group Ⅲ metabotropic glutamate receptors in the modulation of spinal nociceptive signals

BACKGROUND： Previous morphological studies have demonstrated that group Ⅲ metabotropic glutamate receptors （mGluRs） are commonly found in nociceptive pathways, particularly in the terminals of primary afferent fibers in the spinal dorsal horn. OBJECTIVE： To investigate the role of group Ⅲ mGluRs in a rat model of spinal nociception by intrathecal administration of a selective agonist, L-Serine-O-phosphate （L-SOP）. DESIGN, TIME AND SETTING： A randomized, controlled, animal experiment. The study was performed at the Department of Physiology and Neurobiology, Shanxi Medical University, between March 2007 and May 2008. MATERIALS： L-SOP of group Ⅲ mGluRs （Tocris Cookson Ltd, UK）, formalin （Sigma, USA）, rabbit anti-c-Fos polyclonal antibody and biotin-labeled goat anti-rabbit IgG （Cell Signaling Technology, USA） were used in this study. METHODS： A total of 26 healthy Wistar rats, aged 1 month and weighing 100-120 g, were subjected to intrathecal catheter implantation. After 5-8 days, 10 rats were selected according to experimental requirements. L-SOP 250 nmol in 10 μL, or the equivalent volume of normal saline, was administered by intrathecal injection into the L3-5 region of the spinal cord in the experimental and control groups, respectively. After 15 minutes, formalin （5%, 50 μL） was subcutaneously injected into the plantar of the left hindpaw of each rat to establish formalin-induced pain models. MAIN OUTCOME MEASURES： Nociceptive behavioral responses and immunohistochemical examination of Fos expression. RESULTS： Intrathecal injection of L-SOP significantly attenuated the second phase nociceptive response compared with the control group （P 〈 0.05）, and Fos expression in the spinal dorsal horn was significantly decreased along with the number of Fos-like immunoreactive neurons （P 〈 0.05）. CONCLUSION： Group Ⅲ mGluRs are involved in the modulation of nociceptive signals, and their activation suppresses the transmission of nociceptive signals.

Effect of Microiontophoretically Applied Cyproheptadine on the Nociceptive Unit Discharge in the Nucleus Parafascicularis of Thalamus

Experiments were performed on 25 rats. Putative serotonergic antagonist, Cyproheptadine(Cyp), was applied microiontophoretically on the nucleus parafascicularis of the thalamus (Pf) andthe effect of Cyp on Pf nociceptive neuronal discharge was observed. The result suggested that Cypcan increase the spontaneous and evoked discharge rates of Pf nociceptive neuron, and that Cypmay antagonize the inhibitory effect of endogenous serotonin on the spontaneous and evokeddischarges of Pf nociceptive neurons.

Nociceptive and Neuropathic Pain Qualities in Men and Women with Acute Coronary Syndromes: A Complex Pain Presentation

Background: Cardiac pain arising from acute coronary syndrome (ACS) is a multi-factorial phenomenon. Historically, episodes of cardiac pain have been captured using a one-dimensional numeric pain rating scale. Lacking in clinical practice are acute pain assessments that employ a comprehensive evaluation of an emergent ACS episode. Aim: To examine the sensory-discriminative, motivational-affective and cognitive-evaluative dimensions of ACS-related pain. Methods: A descriptive-correlational, repeated-measure design was used to collect data on 121 ACS patients of their cardiac pain intensity. The (numeric rating scale-NRS 0-10 scale) measured chest pain “Now” and “Worst pain in the previous 2 hours over 8 hours” and the McGill Pain Questionnaire Short-Form (MPQ-SF) measured pain at 4 hours. Results: Mean age was 67.6 ± 13, 50% were male, 60% had unstable angina and 40% had Non-ST-elevation myocardial infarction. Cardiac pain intensity scores remained in the mild range from 1.1 ± 2.2 to 2.4 ± 2.7. MPQ-SF: 66% described pain as distressing and 26% reported pain was horrible or excruciating. Participants described ACS pain quality as acute injury (nociceptive pain: heavy, cramping, stabbing), as nerve damage (neuropathic: gnawing, hot-burning, shooting) and as a mixture of acute and chronic pain qualities (aching, tender and throbbing). Conclusions: Patients reported both nociceptive and neuropathic cardiac pain. It is unclear if pain perceptions are due to: i) pathophysiology of clot formation, ii) occurrence of a first or repeated ACS episode, or iii) complex co-morbidities. Pain arising from ACS requires an understanding of the interplay of ischemic, metabolic and neuropathophysiological mechanisms that contribute to complex cardiac pain experiences.

The central nucleus of amygdala is involved in tolerance to the antinociceptive effect of NSAIDs

Aim: Repeated microinjections of non-opioid an-algesics into the midbrain periaqueductal gray matter and rostral ventro-medial medulla induce antinociception with development of tolerance. Antinociception following systemic administra-tion of non-steroidal anti-inflammatory drugs (N SAIDs) also exhibit tolerance. Presently our aim was to investigate the development of tolerance to the antinociceptive effects of NSAIDs analgine, ketorolac, and xefocam microinjected into cen-tral nucleus of amygdala (Ce) in rats. Methods: Under anesthesia with thiopental a stainless steel guide cannula was stereotaxically implanted uni- laterally or bilaterally into the Ce using stereo-taxic atlas coordinates, and anchored to the cra- nium by dental cement. Five days after surgery, 3 μl of these NSAIDs were injected via the injec-tion cannula while the rat was gently restrained. Twenty min post microinjection, i.e. 10-min be-fore the peak of the drugs’ effect is normally rea- ched, animals were tested with tail flick (TF) and hot plate (HP) tests. On the 5th experimental day all animals received a Ce microinjection of mor-phine. Results: Daily microinjection of NSAIDs into the Ce uni- or bilaterally, produced antino-ciception with development of complete toler-ance over a 5-day period. Following the treat-ment period, morphine microinjection into the Ce failed to elicit antinociception, indicating cro- ss-tolerance to the antinociceptive effect of N SAIDs. In other words, the “non-opioid tolerant” rats showed cross-tolerance to morphine. Con-clusions: Our data confirmed the suggestion that NSAIDs interact with endogenous opioid systems, which likely play a key role in the development of tolerance to the antinociceptive effects of NSA IDs.

Involvement of A5/A7 noradrenergic neurons and B2 serotonergic neurons in nociceptive processing:a fiber photometry study

In the central nervous system,the A6 noradrenaline(NA)and the B3 serotonin(5-HT)cell groups are well-recognized players in the descending antinociceptive system,while other NA/5-HT cell groups are not well characterized.A5/A7 NA and B25-HT cells project to the spinal horn and form descending pathways.We recorded G-Ca MP6 green fluorescence signal intensities in the A5/A7 NA and the B25-HT cell groups of awake mice in response to acute tail pinch stimuli,acute heat stimuli,and in the context of a non-noxious control test,using fiber photometry with a calcium imaging system.We first introduced G-Ca MP6 in the A5/A7 NA or B25-HT neuronal soma,using transgenic mice carrying the tetracycline-controlled transactivator transgene under the control of either a dopamineβ-hydroxylase or a tryptophan hydroxylase-2 promoters and by the site-specific injection of adeno-associated virus(AAV-Tet O(3 G)-G-Ca MP6).After confirming the specific expression patterns of G-Ca MP6,we recorded G-Ca MP6 green fluorescence signals in these sites in awake mice in response to acute nociceptive stimuli.G-Ca MP6 fluorescence intensity in the A5,A7,and B2 cell groups was rapidly increased in response to acute nociceptive stimuli and soon after,it returned to baseline fluorescence intensity.This was not observed in the non-noxious control test.The results indicate that acute nociceptive stimuli rapidly increase the activities of A5/A7 NA or B25-HT neurons but the non-noxious stimuli do not.The present study suggests that A5/A7 NA or B25-HT neurons play important roles in nociceptive processing in the central nervous system.We suggest that A5/A7/B2 neurons may be new therapeutic targets.All performed procedures were approved by the Institutional Animal Use Committee of Kagoshima University(MD17105)on February 22,2018.

Sex Differences in Antinociceptive Effects Induced by Gravity Stress in Rats

A number of studies have demonstrated that sex differentially affects responses to stress and pain. In this study, sex-related differences in pain responding were investigated in a gravity-induced analgesia model, where the effects of stressful high-gravity loading (1.5G or 2.0G for 10 min) on nociceptive behavior in male and female rats were investigated. In each rat, eight sites (nose, both forepaws, upper and lower back, both hind paws and tail) were selected to apply noxious stimuli using a von Frey-type needle stimulator. The threshold values of the withdrawal responses were measured. In order to confirm the involvement of endogenous opioids in gravity-induced antinociceptive effects, naloxone-HCl (an opioid antagonist) was used. Effective analgesic effects could be induced by strong (2.0G) gravity loading, and clear sex differences were observed. Gravity-induced analgesic effects were more effective in males than in females, indicating that males are more sensitive to stress than females judging from nociceptive modulation. Naloxone-HCl produced a more pronounced suppression of nociceptive behavior in male rats, suggesting that gravity loading may activate endogenous opioids more readily in males than in females.

An investigation of the antinociceptive effects of Riluzole in hyperal gesia models of mice

Objective： To investigate the antinociceptive effects of Riluzole administered intraperitoneally in three hyperalgesia model of mice. Methods： Antinociceptive tests in C57BL mice were investigated with formalin test,acetic acid induced writhing test and tail-immersion test. The effects of intraperitoneally Riluzole 2 mg/kg ,4 mg/kg and 8 mg/kg on the pain threshold were observed. Result： We found that i.p. treatment with Riluzole （4 mg/kg and 8 mg/kg） blocked the second phase flinching behavior compared with vehicle （P 〈 0.05）, but not during the first phase in the formalin test. In addition to the formalin test, Riluzole at different dose （from 2 to 8 mg/kg） attenuated acetic acid induced writhing response when compared to vehicle group （P 〈 0.05）. In the tail-immersion test, Riluzole at the highest dose （8 mg/kg） caused significant increase in tail flick response latency as compared to vehicle animals or compared with Baseline （P 〈 0.05）. Conclusion： Our results suggest that glutamate release inhibitor Riluzole can attenuate nociceptive behavior and has differrent antinociceptive characteristic according to the various pain models.

Ventral Hippocampal CA1 Pyramidal Neurons Encode Nociceptive Information

As a main structure of the limbic system,the hippocampus plays a critical role in pain perception and chronicity.The ventral hippocampal CA1(vCA1)is closely associated with negative emotions such as anxiety,stress,and fear,yet how vCA1 neurons encode nociceptive information remains unclear.Using in vivo electrophysiological recording,we characterized vCA1 pyramidal neuron subpopulations that exhibited inhibitory or excitatory responses to plantar stimuli and were implicated in encoding stimuli modalities in naïve rats.Functional heterogeneity of the vCA1 pyramidal neurons was further identified in neuropathic pain conditions:the proportion and magnitude of the inhibitory response neurons paralleled mechanical allodynia and contributed to the confounded encoding of innocuous and noxious stimuli,whereas the excitatory response neurons were still instrumental in the discrimination of stimulus properties.Increased theta power and theta-spike coupling in vCA1 correlated with nociceptive behaviors.Optogenetic inhibition of vCA1 pyramidal neurons induced mechanical allodynia in naïve rats,whereas chemogenetic reversal of the overall suppressed vCA1 activity had analgesic effects in rats with neuropathic pain.These results provide direct evidence for the representations of nociceptive information in vCA1.

Changes in the Bispectral Index in Response to Loss of Consciousness and No Somatic Movement to Nociceptive Stimuli in Elderly Patients

Background： Bispectral index （BIS） is considered very useful to guide anesthesia care in elderly patients, but its use is controversial for the evaluation of the adequacy of analgesia.This study compared the BIS changes in response to loss of consciousness （LOC） and loss of somatic response （LOS） to nociceptive stimuli between elderly and young patients receiving intravenous target-controlled infusion （TCI） of propofol and remifentanil.Methods： This study was performed on 52 elderly patients （aged 65-78 years） and 52 young patients （aged 25-58 years）, American Society of Anesthesiologists physical status Ⅰ or Ⅱ.Anesthesia was induced with propofol administered by TCI.A standardized noxious electrical stimulus （transcutaneous electrical nerve stimulation, [TENS]） was applied （50 Hz, 80 mA, 0.25 ms pulses for 4 s） to the ulnar nerve at increasing remifentanil predicted effective-site concentration （Ce） until patients lost somatic response to TENS.Changes in awake, prestimulus, poststimulus BIS, heart rate, mean arterial pressure, pulse oxygen saturation, predicted plasma concentration, Ce of propofol, and remifentanil at both LOC and LOS clinical points were investigated.Results： BISLOC in elderly group was higher than that in young patient group （65.4 ＆#177; 9.7 vs.57.6 ＆#177; 12.3） （t =21.58, P 〈 0.0001） after TCI propofol, and the propofol Ce at LOC was 1.6 ＆#177; 0.3 μg/ml in elderly patients, which was significantly lower than that in young patients （2.3 ＆#177; 0.5 μg/ml） （t =7.474, P 〈 0.0001）.As nociceptive stimulation induced BIS to increase, the mean of BIS maximum values after TENS was significantly higher than that before TENS in both age groups （t =8.902 and t =8.019, P 〈 0.0001）.With increasing Ce of remifentanil until patients lost somatic response to TENS, BISLOS was the same as the BISLOC in elderly patients （65.6 ＆#177; 10.7 vs.65.4 ＆#177; 9.7）, and there were no marked differences between elderly and young patient groups in BISawake, BISLOS, and Ce of remifentanil required for LOS.Conclusion： In elderly patients, BIS can be used as an indicator for hypnotic-analgesic balance and be helpful to guide the optimal administration of propofol and remifentanil individually.

Interaction and regulatory functions of μ- and δ-opioid receptors in nociceptive afferent neurons

μ-opioid receptor （MOR） agonists such as morphine are powerful analgesics used for pain therapy. However, the use of these drugs is limited by their side-effects, which include antinociceptive tolerance and dependence. Earlier studies reported that MOR analgesic tolerance is reduced by blockade of 5-opioid receptors （DORs） that interact with MORs. Recent studies show that the MOR/DOR interaction in nociceptive afferent neurons in the dorsal root ganglion may contribute to morphine analgesic tolerance. Further analysis of the mechanisms for regulating the trafficking of receptors, ion channels and signaling molecules in nociceptive afferent neurons would help to understand the nociceptive mechanisms and improve pain therapy.

Effects of Tualang honey in modulating nociceptive responses at the spinal cord in offspring of prenatally stressed rats

Objective: This study was done to determine whether Tualang honey could prevent the altered nociceptive behaviour, with its associated changes of oxidative stress markers and morphology of the spinal cord,among the offspring of prenatally stressed rats.Methods: Pregnant rats were divided into three groups: control, stress, and stress treated with Tualang honey. The stress and stress treated with Tualang honey groups were subjected to restraint stress from day 11 of pregnancy until delivery. Ten week old male offspring(n = 9 from each group) were given formalin injection and their nociceptive behaviours were recorded. After 2 h, the rats were sacrificed, and their spinal cords were removed to assess oxidative stress activity and morphology. Nociceptive behaviour was analysed using repeated measures analysis of variance(ANOVA), while the levels of oxidative stress parameters and number of Nissl-stained neurons were analysed using a one-way ANOVA.Results: This study demonstrated that prenatal stress was associated with increased nociceptive behaviour, changes in the oxidative stress parameters and morphology of the spinal cord of offspring exposed to prenatal stress; administration of Tualang honey reduced the alteration of these parameters.Conclusion: This study provides a preliminary understanding of the beneficial effects of Tualang honey against the changes in oxidative stress and neuronal damage in the spinal cord of the offspring of prenatally stressed rats.

Fibroblast growth factor 7 is a nociceptive modulator secreted via large dense-core vesicles

Fibroblast growth factor(FGF)7,a member of FGF family,is initially found to be secreted from mesenchymal cells to repair epithelial tissues.However,its functions in the nervous system are largely unknown.The present study showed that FGF7 was a neuromodulator localized in the large dense-core vesicles(LDCVs)in nociceptive neurons.FGF7 was mainly expressed in small-diameter neurons of the dorsal root ganglion and could be transported to the dorsal spinal cord.Interestingly,FGF7 was mostly stored in LDCVs that did not contain neuropeptide substance P.Electrophysiological recordings in the spinal cord slice showed that buffer-applied FGF7 increased the amplitude of excitatory post-synaptic current evoked by stimulating the sensory afferent fibers.Behavior tests showed that intrathecally applied FGF7 potentiated the formalin-induced acute nociceptive response.Moreover,both acute and inflammatory nociceptive responses were significantly reduced in Fgf7-deficient mice.These results suggest that FGF7 exerts an excitatory modulation of nociceptive afferent transmission.

Ionic Mechanisms for the Acute Nociceptive Signals Induced by Bradykinin

Bradykinin is an inflammatory mediator and one of the most potent endogenous pain-inducing substances. When released at the site of tissue damage or inflammation

Evaluation of Antinociceptive and Anti-inflammatory Effects of Aqueous Extract of Armadillidium vulgare Latreille

Objective： To assess the antinociceptive and anti-inflammatory properties of the aqueous extract of Armadillidium vulgare（AV）. Methods： The antinociceptive effect of AV（400, 600 and 800 mg/kg） was investigated in mice using the acetic acid-induced writhing, formalin-induced nociceptive, and hot plate tests. Phlogogen-induced paw edema using carrageenan, dextran, or compound 48/80 as phlogogen was used as inflammatory models to evaluate AV＇s anti-inflammatory effect. Additionally, the bioactive substances glucosamine（GLc N） and taurine in AV were determined using high-performance liquid chromatography. Results： Oral treatment of the mice with AV（600 and 800 mg/kg） significantly reduced the number of writhes in the acetic acid-induced writhing test（P〈0.01） but not the hot plate test（P〉0.05）. All doses tested significantly inhibited paw-withdrawal during the second phase of the formalin-induced nociceptive model（P〈0.01）. AV demonstrated a strong anti-inflammatory effect in all those inflammatory models（P〈0.05）. Conclusions： AV has antinociceptive and anti-inflammatory effects, providing scientific evidence of the efficacy of its traditional use in pain treatment. Furthermore, GLc N and taurine contribute, at least in part, to the anti-inflammatory activity of AV.

Antinociceptive and anti-inflammatory activities of leaf extracts from Annona tomentosa R.E.Fr

OBJECTIVE：Annona tomentosa R.E.Fr is a species not endemic to Brazil that belongs to the phytogeographic areas of the Amazon,Cerrado and Pantanal.Popularly known as“araticum rasteiro”or“araticum de moita”,A.tomentosa is edible and tea made from the leaves has been used as an antiinflammatory by native communities.There is no scientific evidence for these uses of A.tomentosa,especially those related to the control of pain and inflammation.For this reason,in the present study we evaluated the antinociceptive and anti-inflammatory activities of partitions from the methanolic extract of A.tomentosa leaves（A.tomentosa leaf methanolic extract（ATFM）in hexane partition：ATFM-H;ATFM in dichloromethane partition：ATFM-D;ATFM in ethyl acetate partition：ATFM-Ac;ATFM in butanol partition：ATFM-B）in mice.METHODS：The antinociceptive effects of leaf extracts from A.tomentosa were evaluated by abdominal writhing and tail-flick tests,while the anti-inflammatory effects were evaluated by paw oedema and air-pouch tests.The locomotor activity was evaluated with the open-field test.Furthermore,we evaluated the possible action mechanism of A.tomentosa,using naloxone,nitro-L-arginine methyl ester,glibenclamide,atropine,naltrindole and norbinaltorphimine in tail-flick tests.The productions of tumor necrosis factorα（TNF-α）and interleukin（IL）-1βwere also evaluated.RESULTS：The chromatographic fractionation of the partitions of the methanolic extract from the leaves of A.tomentosa revealed the presence of diterpenes,flavonoids,and steroids compounds.From the analysis of the hexane partition kaurenoic acid was identified as the major component.ATFM-H and ATFM-D had a significant antinociceptive effect in acute pain models in mice.The ATFM-H showed central antinociceptive effect from the involvement of theδopioid receptors,without causing alterations in the locomotor activity of the mice,while ATFM-D was effective in decreasing paw oedema and TNF-α and IL-1β production.CONCLUSION：These results demonstrate that leaf extracts from A.tomentosa present antinociceptive and anti-inflammatory effects that can to be used in relieving algesic and inflammatory conditions.

Stem cells and pain

ain can be defined as an unpleasant sensory and emotional experience caused by either actual or potential tissue damage or even resemble that unpleasant experience.For years,science has sought to find treatment alternatives,with minimal side effects,to relieve pain.However,the currently available pharmacological options on the market show significant adverse events.Therefore,the search for a safer and highly efficient analgesic treatment has become a priority.Stem cells(SCs)are non-specialized cells with a high capacity for replication,self-renewal,and a wide range of differentiation possibilities.In this review,we provide evidence that the immune and neuromodulatory properties of SCs can be a valuable tool in the search for ideal treatment strategies for different types of pain.With the advantage of multiple administration routes and dosages,therapies based on SCs for pain relief have demonstrated meaningful results with few downsides.Nonetheless,there are still more questions than answers when it comes to the mechanisms and pathways of pain targeted by SCs.Thus,this is an evolving field that merits further investigation towards the development of SCbased analgesic therapies,and this review will approach all of these aspects.

Emerging roles of GPR109A in regulation of neuroinflammation in neurological diseases and pain

Neuroinflammation plays a critical role in the pathological process of multiple neurological disorders and pathological pain conditions.GPR109A,a Gi protein-coupled receptor,has emerged as an important therapeutic target for controlling inflammation in various tissues and organs.In this review,we summarized current data about the role of GPR109A in neuroinflammation.Specifically,we focused on the pharmacological features of GPR109A and signaling pathways used by GPR109A to ameliorate neuroinflammation and symptoms in Alzheimer’s disease,Parkinson’s disease,multiple sclerosis,stroke,and pathological pain conditions.

Efficacy of nanofibrous conduits in repair of longsegment sciatic nerve defects

Our previous studies have histomorphologically confirmed that nanofibrous poly（3-hydroxybutyrate- co-3-hydroxyvalerate） conduit can be used to repair 30-mm-long sciatic nerve defects. However, the repair effects on rat behaviors remain poorly understood. In this study, we used nanofibrous poly（3-hydroxybutyrate-co-3-hydroxyvalerate） conduit and autologous sciatic nerve to bridge 30-ram-long rat sciatic nerve gaps. Within 4 months after surgery, rat sciatic nerve functional re- covery was evaluated per month by behavioral analyses, including toe out angle, toe spread anal- ysis, walking track analysis, extensor postural thrust, swimming test, open-field analysis and no- ciceptive function. Results showed that rat sciatic nerve functional recovery was similar after nanofibrous poly（3-hydroxybutyrate-co-3-hydroxyvalerate） conduit and autologous nerve grafting. These findings suggest that nanofibrous poly（3-hydroxybutyrate-co-3-hydroxyvalerate） conduit is suitable in use for repair of long-segment sciatic nerve defects.

Intrathecal MK-801 inhibits formalin-induced activation of spinal p38-MAPK in rats

BACKGROUND： p38 mitogen-activated protein kinase （MAPK） plays an instrumental role in signal transduction from the cell surface to the nucleus, while subcutaneous injection of formalin can induce increased activation of spinal p38 MAPK. However, the mechanisms underlying the formalin-induced activation of spinal p38 MAPK in rats are unclear. OBJECTIVE： To observe the effects of N-methyl-D-aspartic acid （NMDA） receptor antagonist MK-801 on the formalin-induced activation of spinal p38 MAPK in rats. DESIGN, TIME AND SETTING： This randomized grouping, controlled animal experiment was performed at the Department of Physiology and Neurobiology, Shanxi Medical University between May and November 2007. MATERIALS： Forty eight healthy, adult Wistar rats were randomly divided into two groups： formalin ＋ normal saline （n = 12） and formalin ＋ MK-801 （n = 36）. The formalin ＋ MK-801 group was further divided into three subgroups according to the dosage of MK-801 （10, 50, and 100 nmol/L, 12 rats for each subgroup） METHODS： Following anesthesia, polyethylene tubing filled with sterile normal saline was implanted into the subarachnoid cavity. On postoperative days 5-8, rats received a 15 minute perfusion of normal saline or MK-801 （10, 50, and 100 nmol/L） in the formalin ＋ normal saline and formalin ＋ MK-801 groups, respectively, followed by formalin injection for the induction of nociceptive behavior. MAIN OUTCOME MEASURES： Detection of total p38 MAPK and of phosphorylated p38 MAPK by western Blot analysis; observation of nociceptive behaviors in the 1 hour after formalin injection. RESULTS： Western Blot analysis revealed that injection of formalin had no effect on total p38 MAPK expression but resulted in increased phosphorylation of p38 MAPK in the spinal cord. This increase was apparent after 5 minutes, peaked at 20 minutes, and thereafter descended and reached control levels after 45 minutes. Pretreatment with MK-801 （10, 50, 100 nmol/L） resulted in a dose-dependent reduction of p38 MAPK phosphorylation in the spinal cord, 20 minutes after formalin injection. Injection of 50 and 100 nmol/L MK-801 produced a suppression of the first phase of nociceptive behaviors, and all three doses of MK-801 resulted in dose-dependent inhibition of the second phase of nociceptive behaviors. CONCLUSION： The NMDA receptor participates in formalin-induced activation of p38 MAPK in the rat spinal cord.