Activation of Anterior Cingulate Cortex Extracellular Signal-Regulated Kinase-1 and -2 (ERK1/2) Regulates Acetic Acid-Induced, Pain-Related Anxiety in Adult Female Mice

Sex differences in visceral sensitivity and brain activity in a rat model of comorbid pain: a longitudinal study

ABSTRACT

Temporomandibular disorder (TMD) and irritable bowel syndrome (IBS) are 2 chronic overlapping pain conditions (COPCs) that present with significant comorbidity. Both conditions are more prevalent in women and are exacerbated by stress. While peripheral mechanisms might contribute to pain hypersensitivity for each individual condition, mechanisms underlying the comorbidity are poorly understood, complicating pain management when multiple conditions are involved. In this study, longitudinal behavioral and functional MRI-based brain changes have been identified in an animal model of TMD-like pain (masseter muscle inflammation followed by stress) that induces de novo IBS-like comorbid visceral pain hypersensitivity in rats. In particular, data indicate that increased activity in the insula and regions of the reward and limbic systems are associated with more pronounced and longer-lasting visceral pain behaviors in female rats, while the faster pain resolution in male rats may be due to increased activity in descending pain inhibitory pathways. These findings suggest the critical role of brain mechanisms in chronic pain conditions and that sex may be a risk factor of developing COPCs.

Modulation of visceral pain by brain nuclei and brain circuits and the role of acupuncture: a narrative review

Visceral pain is a complex and heterogeneous pain condition that is often associated with pain-related negative emotional states, including anxiety and depression, and can exert serious effects on a patient's physical and mental health. According to modeling stimulation protocols, the current animal models of visceral pain mainly include the mechanical dilatation model, the ischemic model, and the inflammatory model. Acupuncture can exert analgesic effects by integrating and interacting input signals from acupuncture points and the sites of pain in the central nervous system. The brain nuclei involved in regulating visceral pain mainly include the nucleus of the solitary tract, parabrachial nucleus (PBN), locus coeruleus (LC), rostral ventromedial medulla (RVM), anterior cingulate cortex (ACC), paraventricular nucleus (PVN), and the amygdala. The neural circuits involved are PBN-amygdala, LC-RVM, amygdala-insula, ACC-amygdala, claustrum-ACC, bed nucleus of the stria terminalis-PVN and the PVN-ventral lateral septum circuit. Signals generated by acupuncture can modulate the central structures and interconnected neural circuits of multiple brain regions, including the medulla oblongata, cerebral cortex, thalamus, and hypothalamus. This analgesic process also involves the participation of various neurotransmitters and/or receptors, such as 5-hydroxytryptamine, glutamate, and enkephalin. In addition, acupuncture can regulate visceral pain by influencing functional connections between different brain regions and regulating glucose metabolism. However, there are still some limitations in the research efforts focusing on the specific brain mechanisms associated with the effects of acupuncture on the alleviation of visceral pain. Further animal experiments and clinical studies are now needed to improve our understanding of this area.

Licochalcone Mediates the Pain Relief by Targeting the Voltage-Gated Sodium Channel

Licorice is a traditional Chinese medicine and recorded to have pain relief effects in national pharmacopoeia, but the mechanisms behind these effects have not been fully explored. Among the hundreds of compounds in licorice, licochalcone A (LCA) and licochalcone B (LCB) are two important components belonging to the chalcone family. In this study, we compared the analgesic effects of these two licochalcones and the molecular mechanisms. LCA and LCB were applied in cultured dorsal root ganglion (DRG) neurons, and the voltage-gated sodium (NaV) currents and action potentials were recorded. The electrophysiological experiments showed that LCA can inhibit NaV currents and dampen excitabilities of DRG neurons, whereas LCB did not show inhibition effect on NaV currents. Because the NaV1.7 channel can modulate Subthreshold membrane potential oscillations in DRG neuron, which can palliate neuropathic pain, HEK293T cells were transfected with NaV1.7 channel and recorded with whole-cell patch clamp. LCA can also inhibit NaV1.7 channels exogenously expressed in HEK293T cells. We further explored the analgesic effects of LCA and LCB on formalin-induced pain animal models. The animal behavior tests revealed that LCA can inhibit the pain responses during phase 1 and phase 2 of formalin test, and LCB can inhibit the pain responses during phase 2. The differences of the effects on NaV currents between LCA and LCB provide us with the basis for developing NaV channel inhibitors, and the novel findings of analgesic effects indicate that licochalcones can be developed into effective analgesic medicines. SIGNIFICANCE STATEMENT: This study found that licochalcone A (LCA) can inhibit voltage-gated sodium (NaV) currents, dampen excitabilities of dorsal root ganglion neurons, and inhibit the NaV1.7 channels exogenously expressed in HEK293T cells. Animal behavior tests showed that LCA can inhibit the pain responses during phase 1 and phase 2 of formalin test, whereas licochalcone B can inhibit the pain responses during phase 2. These findings indicate that licochalcones could be the leading compounds for developing NaV channel inhibitors and effective analgesic medicines.

Janus effect of the anterior cingulate cortex: Pain and emotion

Over the past 20 years, clinical and preclinical studies point to the anterior cingulate cortex (ACC) as a site of interest for several neurological and psychiatric conditions. The ACC plays a critical role in emotion, autonomic regulation, pain processing, attention, memory and decision making. An increasing number of studies have demonstrated the involvement of the ACC in the emotional component of pain and its comorbidity with emotional disorders such as anxiety and depression. Thanks to the development of animal models combined with state-of-the-art technologies, we now have a better mechanistic understanding of the functions of the ACC. Hence, the primary aim of this review is to compile the most recent preclinical studies on the role of ACC in the emotional component and consequences of chronic pain. Herein, we thus thoroughly describe the pain-induced electrophysiological, molecular and anatomical alterations in the ACC and in its related circuits. Finally, we discuss the next steps that are needed to strengthen our understanding of the involvement of the ACC in emotional and pain processing.

Inhibition of ERK1/2 regulates cognitive function by decreasing expression levels of PSD-95 in the Hippocampus of CIH Rats

Obstructive sleep apnea syndrome (OSAS) is a potentially severe sleep disorder characterized by intermittent hypoxia, and there is growing evidence that OSAS can lead to cognitive decline. Extracellular signal-regulated protein kinase 1/2 (ERK1/2) plays a key role in synaptic plasticity. We established CIH model in male SD rats and examined their expression of p-ERK1/2 and PSD-95, as well as in CIH group, the effect of SL327 on the expression of p-ERK1/2 and PSD-95 in hippocampus of CIH model rats was observed by pretreating the experimental rats with SL327 during peak time of p-ERK1/2 expression. Mean oxygen saturation in the tail artery was lower in the CIH group. In the CIH groups exhibited increased escape latencies in the navigation test and decreased numbers of platform crossings in the space exploration test. Reduced volume, irregular structure, deepened cytoplasmic eosinophilic staining in the cytoplasm, and decreased nuclear size were found in hippocampal neurons in the 28-d CIH 、28-d CIH + SL327 group. The hippocampus of CIH rats^, p-ERK expressions gradually increased with prolonged CIH exposure but decreased after SL327 treatment. Moreover, PSD-95 expressions gradually reduced in the 14-d CIH, 21-d CIH, and 28-d CIH groups but increased in the SL327-treated group. The SL327 intervention decreased p-ERK1/2 expression, increased PSD-95 expression, and improved cognitive function in CIH rats. The present findings provide some insights into the mechanisms underlying OSAS-associated cognitive impairment.

Diazepam and SL-327 synergistically attenuate anxiety-like behaviours in mice - Possible hippocampal MAPKs specificity

Intracellular signalling pathways have been extensively studied as therapeutic targets for the treatment of mental diseases. Our attention has been caught by two kinases potentially involved in anxiety, ERK1/2 and CaMKII. The study aimed to examine changes in the activation of ERK1/2 and CaMKII concerning anxiolytic-like behaviours in mice. To evaluate anxiety-related response in mice, we used the open field test and the elevated plus maze test. Behavioural studies were complemented with the immunoblotting analysis to identify proteins of interest in the cortex, hippocampus, and striatum. We analysed the phosphorylation status of ERK1/2 and CaMKII in mice treated with a well-known anxiolytic drug - diazepam. Next, the blockade of ERK1/2 pathway by SL-327, a selective MEK1/2 inhibitor, was checked for anxiolytic action. Finally, the co-administration of subeffective doses of diazepam and SL-327 was investigated for a potential synergistic anxiolytic effect. Anxiolytic effects of acute diazepam are accompanied by decreased p-ERK1/2 and upregulation of p-CaMKII. Subchronic treatment with SL-327 leads to the manifestation of anxiolytic-like behaviours and changes in the phosphorylation status of both kinases in a diazepam-like manner. Co-administration of subeffective doses of SL-327 and diazepam induces anxiolysis, which is CaMKII-independent and correlates to selectively decreased phosphoactive ERK1/2 in the hippocampus. The MEK-ERK pathway is significantly involved in anxiolytic action of diazepam and its prolonged inhibition produces anxiolytic-like phenotype in mice. ERK inhibition could be used to manage anxiety symptoms in a benzodiazepine-sparing regimen for treatment of anxiety.

Antinociceptive Effects of the GPR55 Antagonist CID16020046 Injected into the Rat Anterior Cingulate Cortex

The G-protein coupled receptor, GPR55, modulates nociceptive processing. Given the expression of GPR55 in the anterior cingulate cortex (ACC), a key brain region involved in the cognitive and affective dimensions of pain, the present study tested the hypothesis that GPR55 signalling in the ACC facilitates inflammatory pain behaviour in rats. The expression of GPR55 in the ACC was confirmed by both western blotting and immunostaining, with evidence for neuronal localisation. Microinjection of the selective GPR55 antagonist CID16020046 into the ACC of adult male Sprague-Dawley rats significantly reduced second phase formalin-evoked nociceptive behaviour compared with vehicle-treated controls. CID16020046 administration was associated with a reduction in phosphorylation of extracellular signal-regulated kinase (ERK), a downstream target of GPR55 activation, in the ACC. Intra-ACC administration of CID16020046 prevented the formalin-induced increases in expression of mRNA coding for the immediate early gene and marker of neuronal activity, c-Fos, in the ipsilateral dorsal horn of the spinal cord. Intra-plantar injection of formalin reduced tissue levels of the endogenous GPR55 ligand 2-arachidonoyl-sn-glycero-3-phosphoinositol (2-AGPI) in the ACC, likely reflecting its increased release/utilisation. These data suggest that endogenous activation of GPR55 signalling and increased ERK phosphorylation in the ACC facilitates inflammatory pain via top-down modulation of descending pain control.

The rostral ventromedial medulla orexin 1 receptors and extracellular signal-regulated kinase in hippocampus are involved in modulation of anxiety behavior induced by dental pulp nociception in adult male rats

OBJECTIVES

To explore the role of rostral ventromedial medulla (RVM) orexin 1 receptors (OX1R) on orofacial nociception -induced anxiety and locomotion in rats.

DESIGN

Forty two adult male Wistar rats (220-270 gr) were randomly divided into 7 groups (n = 6) as follows: untreated control, capsaicin, capsaicin vehicle-treated group (sham operation), capsaicin groups pretreated by intra-RVM administration orexin 1 receptor (OX1R) agonist (orexin A) or antagonist (SB-334867) and the capsaicin groups treated by drugs vehicles (DMSO or aCSF). Orofacial nociception was induced by intradental application of capsaicin (100 μg) into the incisors of rats. Anxiety level and locomotor activity were measured by the elevated plus maze (EPM) and open field (OF) tests, respectively. Hippocampal levels of phosphorylated extracellular signal regulated Kinase (p-ERK) was also assessed by western blotting.

RESULTS

Intradental application of capsaicin significantly increased anxiety and decreased locomotion behaviors. Intra-RVM microinjection of orexin-A significantly prevented capsaicin-induced anxiety-like behavior and increased locomotor activity in the EPM and OF tests. These effects were inhibited by SB-334867. Furthermore, orexin-A significantly increased p-ERK levels in capsaicin-treated rats. This effect was inhibited by pretreatment of the rats with SB-334867.

CONCLUSIONS

The results suggest that both OX1R signaling in the RVM and hippocampal p-ERK signaling are involved in orofacial nociception-induced anxiety as well as locomotor activity.

Electroacupuncture Alleviates Pain-Related Emotion by Upregulating the Expression of NPS and Its Receptor NPSR in the Anterior Cingulate Cortex and Hypothalamus

Objective

Electroacupuncture (EA) is reported effective in alleviating pain-related emotion; however, the underlying mechanism of its effects still needs to be elucidated. The NPS-NPSR system has been validated for the involvement in the modulation of analgesia and emotional behavior. Here, we aimed to investigate the role of the NPS-NPSR system in the anterior cingulate cortex (ACC), hypothalamus, and central amygdala (CeA) in the use of EA to relieve affective pain modeled by complete Freund's adjuvant- (CFA-) evoked conditioned place aversion (C-CPA). Materials and Methods. CFA injection combined with a CPA paradigm was introduced to establish the C-CPA model, and the elevated O-maze (EOM) was used to test the behavioral changes after model establishment. We further explored the expression of NPS and NPSR at the protein and gene levels in the brain regions of interest by immunofluorescence staining and quantitative real-time PCR.

Results

We observed that EA stimulation delivered to the bilateral Zusanli (ST36) and Kunlun (BL60) acupoints remarkably inhibited sensory pain, pain-evoked place aversion, and anxiety-like behavior. The current study showed that EA significantly enhanced the protein expression of this peptide system in the ACC and hypothalamus, while the elevated expression of NPSR protein alone was just confined to the affected side in the CeA. Moreover, EA remarkably upregulated the mRNA expression of NPS in CeA, ACC, and hypothalamus and NPSR mRNA in the hypothalamus and CeA.

Conclusions

These data suggest the effectiveness of EA in alleviating affective pain, and these benefits may at least partially be attributable to the upregulation of the NPS-NPSR system in the ACC and hypothalamus.

The Effects of Enriched Environmental Intervention on the Parturition Outcome and Filial Health of Pregnant Mice With Chronic Pain

Chronic pain (CP) adversely impacts people's health. However, the influence of CP on pregnant women and their fetuses remains unclear. Few reasonable interventions have been offered to these women. This study aimed to examine the effects of enriched environmental intervention (EEI) on the delivery outcome and offspring health of pregnant mice with CP. Forty pregnant mice were equally and randomly divided into four groups (control, sham, CP, and CP + EEI). Monoarthritis was induced by subcutaneous injection of the complete Freund's adjuvant (100 μg/100 μl) into the left hind paw of mice on Day 0 of gestation. Sham mice received 100 µl of sterile phosphate-buffered saline subcutaneously. General health status and delivery outcomes of pregnant mice and general physical status, growth, and development of offspring were observed and compared between groups. CP mice exhibited less weight gain, higher rate of premature delivery, smaller litter size, lower live birth rate, lower filial birth weight, shorter filial tail length, and lower filial rates of achieving righting reflex and crawling compared to mice in the control/sham/CP + EEI groups. There were no significant differences between mice in the CP + EEI group and those in the control and sham groups. CP during pregnancy led to serious adverse impacts on both the mother and fetus mice in this study, and EEI was an effective treatment for these effects.

Activation of Extracellular Signal-Regulated Kinases (ERK 1/2) in the Locus Coeruleus Contributes to Pain-Related Anxiety in Arthritic Male Rats

BACKGROUND

There is increasing evidence suggesting that the Locus Coeruleus plays a role in pain-related anxiety. Indeed, we previously found that prolonged arthritis produces anxiety-like behavior in rats, along with enhanced expression of phosphorylated extracellular signal-regulated kinase 1/2 (a marker of plasticity) in the Locus Coeruleus. However, it is unknown how this effect correlates with the electrophysiological activity of Locus Coeruleus neurons or pain-related anxiety.

METHODS

Using the complete Freund's adjuvant model of monoarthritis in male Sprague-Dawley rats, we studied the behavioral attributes of pain and anxiety as well as Locus Coeruleus electrophysiology in vivo 1 (MA1W) and 4 weeks (MA4W) after disease induction.

RESULTS

The manifestation of anxiety in MA4W was accompanied by dampened tonic Locus Coeruleus activity, which was coupled to an exacerbated evoked Locus Coeruleus response to noxious stimulation of the inflamed and healthy paw. When a mitogen-activating extracellular kinase inhibitor was administered to the contralateral Locus Coeruleus of MA4W, the phosphorylated extracellular signal-regulated kinase 1/2 levels in the Locus Coeruleus were restored and the exaggerated evoked response was blocked, reversing the anxiogenic-like behavior while pain hypersensitivity remained unaltered.

CONCLUSION

As phosphorylated extracellular signal-regulated kinase 1/2 blockade in the Locus Coeruleus relieved anxiety and counteracted altered LC function, we propose that phosphorylated extracellular signal-regulated kinase 1/2 activation in the Locus Coeruleus plays a crucial role in pain-related anxiety.

The Effects of Inflammatory Tooth Pain on Anxiety in Adult Male Rats

Introduction:

This study aimed to examine the effects of induced inflammatory tooth pain on anxiety level in adult male rats.

Methods:

The mandibular incisors of 56 adult male rats were cut off and prefabricated crowns were fixed on the teeth. Formalin and capsaicin were injected intradentally to induce inflammatory tooth pain. Diazepam treated group received diazepam 30 minutes before intradental injection. The anxiety-related behavior was evaluated with elevated plus maze test.

Results:

Intradental application of chemical noxious stimuli, capsaicin and formalin, significantly affected nociceptive behaviors (P<0.001). Capsaicin (P<0.001) and formalin (P<0.01) significantly increased the anxiety levels in rats by decrease in the duration of time spent in open arm and increase in the duration of time spent in closed arm. Rats that received capsaicin made fewer open arm entries compared to the control animals (P<0.05). Capsaicin (P<0.001) and formalin (P<0.01) treated rats showed more stretch attend postures compared to the control and sham operated animals. In diazepampretreated rats, capsaicin induced algesic effect was prevented (P<0.001).

Conclusion:

Inflammatory pulpal pain has anxiogenic effect on rats, whereas diazepam premedication showed both anxiolytic and pain reducing effects.

Peripheral Brain Derived Neurotrophic Factor Precursor Regulates Pain as an Inflammatory Mediator

The precursor of brain derived neurotrophic factor (proBDNF), the unprocessed BDNF gene product, binds to its receptors and exerts the opposing biologic functions of mature BDNF. proBDNF is expressed in the peripheral tissues but the functions of peripheral proBDNF remain elusive. Here we showed that proBDNF and its predominant receptor, p75 pan-neurotrophin receptor were upregulated in the nerve fibers and inflammatory cells in the local tissue in inflammatory pain. Neutralization of proBDNF by polyclonal antibody attenuated pain in different models of inflammatory pain. Unilateral intra-plantar supplementation of proBDNF by injecting exogenous proBDNF or ectopic overexpression resulted in pain hypersensitivity and induced spinal phosphorylated extracellular signal-regulated kinase activation. Exogenous proBDNF injection induced the infiltration of inflammatory cells and the activation of proinflammatory cytokines, suggesting that inflammatory reaction contributed to the pro-algesic effect of proBDNF. Finally, we generated monoclonal anti-proBDNF antibody that could biologically block proBDNF. Administration of monoclonal Ab-proBDNF attenuated various types of inflammatory pain and surgical pain. Thus, peripheral proBDNF is a potential pain mediator and anti-proBDNF pretreatment may alleviate the development of inflammatory pain.

Strong Manual Acupuncture Stimulation of "Huantiao" (GB 30) Reduces Pain-Induced Anxiety and p-ERK in the Anterior Cingulate Cortex in a Rat Model of Neuropathic Pain

Persistent neuropathic pain is associated with anxiety. The phosphorylation of extracellular signal-regulated kinase (p-ERK) in the anterior cingulate cortex (ACC) plays an important role in pain-induced anxiety. Acupuncture is widely used for pain and anxiety. However, little is known about which acupuncture technique is optimal on pain-induced anxiety and the relationship between acupuncture effect and p-ERK. The rat model was induced by L5 spinal nerve ligation (SNL). Male adult SD rats were randomly divided into control, SNL, strong manual acupuncture (sMA), mild manual acupuncture (mMA), and electroacupuncture (EA) group. Bilateral "Huantiao" (GB 30) were stimulated by sMA, mMA, and EA, respectively. The pain withdrawal thresholds (PWTs) and anxiety behavior were measured, and p-ERK protein expression and immunoreactivity cells in ACC were detected. PWTs increased significantly in both sMA and EA groups. Meanwhile, anxiety-like behavior was improved significantly in the sMA and mMA groups. Furthermore, the overexpression of p-ERK induced by SNL was downregulated by strong and mild manual acupuncture. Therefore, strong manual acupuncture on bilateral "Huantiao" (GB 30) could be a proper therapy relieving both pain and pain-induced anxiety. The effect of different acupuncture techniques on pain-induced anxiety may arise from the regulation of p-ERK in ACC.

Long-term upregulation of cortical glutamatergic AMPA receptors in a mouse model of chronic visceral pain

Background

Irritable bowel syndrome (IBS) is one of the most common functional gastrointestinal disorders and it causes long-lasting visceral pain and discomfort. AMPA receptor mediated long-term potentiation (LTP) has been shown to play a critical role in animal models of neuropathic and inflammatory pain. No report is available for central changes in the ACC of mice with chronic visceral pain.

Results

In this study, we used integrative methods to investigate potential central plastic changes in the anterior cingulate cortex (ACC) of a visceral pain mouse model induced by intracolonic injection of zymosan. We found that visceral pain induced an increased expression of AMPA receptors (at the post synapses) in the ACC via an enhanced trafficking of the AMPA receptors to the membrane. Both GluA1 and GluA2/3 subunits were significantly increased. Supporting biochemical changes, excitatory synaptic transmission in the ACC were also significantly enhanced. Microinjection of AMPA receptor inhibitor IEM1460 into the ACC inhibited visceral and spontaneous pain behaviors. Furthermore, we found that the phosphorylation of GluA1 at the Ser845 site was increased, suggesting that GluA1 phosphorylation may contribute to AMPA receptor trafficking. Using genetically knockout mice lacking calcium-calmodulin stimulated adenylyl cyclase subtype 1 (AC1), we found that AMPA receptor phosphorylation and its membrane trafficking induced by zymosan injection were completely blocked.

Conclusions

Our results provide direct evidence for cortical AMPA receptors to contribute to zymosan-induced visceral and spontaneous pain and inhibition of AC1 activity may help to reduce chronic visceral pain.

ERK1/2: Function, signaling and implication in pain and pain-related anxio-depressive disorders

Despite the increasing knowledge regarding pain modulation, the understanding of the mechanisms behind a complex and pathologic chronic pain condition is still insufficient. These knowledge gaps might result in ineffective therapeutic approaches to relieve painful sensations. As a result, severe untreated chronic pain frequently triggers the onset of new disorders such as depression and/or anxiety, and therefore, both the diagnosis and treatment of patients suffering from chronic pain become seriously compromised, prompting a self-perpetuating cycle of symptomatology. The extracellular signal-regulated kinases 1 and 2 (ERK1/2) are molecules strongly implicated in the somatic component of pain at the spinal cord level and have been emerging as mediators of the emotional-affective component as well. Although these molecules might represent good biomarkers, their use as pharmacological targets is still open to discussion as paradoxical information has been obtained. Here we review the current scientific literature regarding ERK1/2 signaling in the modulation of pain, depression and anxiety, including the emotional-affective spheres of the pain experience.

Differential roles of hippocampal glutamatergic receptors in neuropathic anxiety-like behavior after partial sciatic nerve ligation in rats

BACKGROUND

Neuropathic pain evoked by nerve injury is frequently accompanied by deterioration of emotional behaviors, but the underlying signaling mechanisms remain elusive. Glutamate (Glu) is the major mediator of excitatory synaptic transmission throughout the brain, and abnormal activity of the glutamatergic system has been implicated in the pathophysiology of pain and associated emotional comorbidities. In this study we used the partial sciatic nerve ligation (PSNL) model of neuropathic pain in rats to characterize the development of anxiety-like behavior, the expression of glutamatergic receptors, and the phosphorylation of extracellular signal-regulated kinase (ERK) in the hippocampus, the region that encodes memories related to emotions.

RESULTS

We found that the mechanical withdrawal threshold was significantly reduced and an anxiety-like behavior was increased as determined via open field tests and elevated plus-maze tests at 28 days after injury. No significant differences were found in the ratio of sucrose preference and immobility time detected by sucrose preference tests and forced swimming tests respectively, possibly due to the timing factor. The expression of N-methyl-D-aspartate (NMDA) receptor subtypes NR1 and NR2B, but not NR2A, GluR1, or GluR2 (the main subtype of the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid [AMPA] receptor) in the hippocampus of injured rats was significantly reduced. Moreover, PSNL resulted in decreased phosphorylation of ERK1/2 in the hippocampus. Intriguingly, treatment with D-serine (a co-agonist of NMDA receptor, 1 g/kg intraperitoneally) reduced the anxiety-like behavior but not the mechanical hypersensitivity induced by PSNL.

CONCLUSIONS

PSNL can induce significant anxiety-like but not depression-like behavior, and trigger down-regulation of NMDA but not AMPA receptors in the hippocampus at 28 days after injury.

Reversal of monoarthritis-induced affective disorders by diclofenac in rats

BACKGROUND

Nonsteroidal anti-inflammatory drugs are effective for arthritic pain, but it is unknown whether they also benefit anxiety and depression that frequently coexist with pain. Using the monoarthritis model, the authors evaluated the activation of extracellular signal-regulated kinases 1 and 2 (ERK1/2) in structures implicated in both sensorial and emotional pain spheres, and it was verified whether analgesia can reverse monoarthritis-mediated affective responses.

METHODS

Monoarthritis was induced in male rats by complete Freund's adjuvant injection. Allodynia (ankle-bend test), mechanical hyperalgesia (paw-pinch test), anxiety- and depression-like behaviors (elevated zero maze and forced swimming tests, respectively), and ERK1/2 phosphorylation (Western blot) in the spinal cord, paragigantocellularis nucleus, locus coeruleus, and prefrontal cortex were evaluated at 4, 14, and 28 days postinoculation (n = 6 per group). Changes in these parameters were evaluated after induction of analgesia by topical diclofenac (n = 5 to 6 per group).

RESULTS

Despite the pain hypersensitivity and inflammation throughout the testing period, chronic monoarthritis (28 days) also resulted in depressive- (control [mean ± SEM]: 38.3 ± 3.7 vs. monoarthritis: 51.3 ± 2.0; P < 0.05) and anxiogenic-like behaviors (control: 36.8 ± 3.7 vs. monoarthritis: 13.2 ± 2.9; P < 0.001). These changes coincided with increased ERK1/2 activation in the spinal cord, paragigantocellularis, locus coeruleus, and prefrontal cortex (control vs. monoarthritis: 1.0 ± 0.0 vs. 5.1 ± 20.8, P < 0.001; 0.9 ± 0.0 vs. 1.9 ± 0.4, P < 0.05; 1.0 ± 0.3 vs. 2.9 ± 0.6, P < 0.01; and 1.0 ± 0.0 vs. 1.8 ± 0.1, P < 0.05, respectively). Diclofenac decreased the pain threshold of the inflamed paw and reversed the anxio-depressive state, restoring ERK1/2 activation levels in the regions analyzed.

CONCLUSION

Chronic monoarthritis induces affective disorders associated with ERK1/2 phosphorylation in paragigantocellularis, locus coeruleus, and prefrontal cortex which are reversed by diclofenac analgesia. (Anesthesiology 2014; 120:1476-90).

"Bedside-to-Bench" Behavioral Outcomes in Animal Models of Pain: Beyond the Evaluation of Reflexes

Despite the myriad promising new targets and candidate analgesics recently identified in preclinical pain studies, little translation to novel pain medications has been generated. The pain phenotype in humans involves complex behavioral alterations, including changes in daily living activities and psychological disturbances. These behavioral changes are not reflected by the outcome measures traditionally used in rodents for preclinical pain testing, which are based on reflexes evoked by sensory stimuli of different types (mechanical, thermal or chemical). These measures do not evaluate the impact of the pain experience on the global behavior or disability of the animals, and therefore only consider a limited aspect of the pain phenotype. The development of relevant new outcomes indicative of pain to increase the validity of animal models of pain has been increasingly pursued over the past few years. The aim has been to translate “bedside-to-bench” outcomes from the human pain phenotype to rodents, in order to complement traditional pain outcomes by providing a closer and more realistic measure of clinical pain in rodents. This review summarizes and discusses the most important nonstandard outcomes for pain assessment in preclinical studies. The advantages and drawbacks of these techniques are considered, and their potential impact on the validation of potential analgesics is evaluated.

Different TRPV1-mediated brain responses to intragastric infusion of capsaicin and capsiate

Capsaicin and capsiate, which is an analogue of capsaicin, are agonists of capsaicin-binding transient potential vanilloid 1 (TRPV1) receptors. However, their physiological effects are different. Capsaicin induces thermogenesis and nociception, while the different kinetics of capsiate result in thermogenesis without nociception in the oral cavity. In the present study, using functional magnetic resonance imaging, we compared the brain activation after intragastric infusion of non-nociceptive levels of capsaicin and capsiate in wild-type and TRPV1-knockout (KO) mice. Capsaicin activated several brain regions, such as the periaqueductal grey (PAG), thalamic nuclei and hypothalamus, including the medial preoptic area (mPOA) and ventromedial hypothalamus (VMH). Most of these areas were not activated in TRPV1-KO mice. Capsiate activated several regions, including the thalamic nuclei, mPOA and VMH but not PAG in wild-type mice. Most of the activated areas were not activated by intragastric capsiate infusion in TRPV1-KO mice. These results demonstrate that TRPV1 is critical for the induction of activation in the hypothalamus by capsaicin and capsiate, and these distinct brain activations could help to explain the individual physiological reactions of capsaicin and capsiate.