Chronic pain induces anxiety with concomitant changes in opioidergic function in the amygdala

Neuroplasticity of Supraspinal Structures Associated with Pathological Pain

Peripheral nerve and spinal cord injuries, along with other painful syndromes such as fibromyalgia, diabetic neuropathy, chemotherapeutic neuropathy, trigeminal neuralgia, complex regional pain syndrome, and/or irritable bowel syndrome, cause several neuroplasticity changes in the nervous system along its entire axis affecting the different neuronal nuclei. This paper reviews these changes, focusing on the supraspinal structures that are involved in the modulation and processing of pain, including the periaqueductal gray matter, red nucleus, locus coeruleus, rostral ventromedial medulla, thalamus, hypothalamus, basal ganglia, cerebellum, habenula, primary, and secondary somatosensory cortex, motor cortex, mammillary bodies, hippocampus, septum, amygdala, cingulated, and prefrontal cortex. Hyperexcitability caused by the modification of postsynaptic receptor expression, central sensitization, and potentiation of presynaptic delivery of neurotransmitters, as well as the reduction of inhibitory inputs, changes in dendritic spine, neural circuit remodeling, alteration of gray matter, and upregulation of proinflammatory mediators (e.g., cytokines) by reactivation of astrocytes and microglial cells are the main functional, structural, and molecular neuroplasticity changes observed in the above supraspinal structures, associated with pathological pain. Studying these changes in greater depth may lead to the implementation and improvement of new therapeutic strategies against pathological pain. Anat Rec, 300:1481-1501, 2017. © 2017 Wiley Periodicals, Inc.

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.

Effects of paclitaxel on mechanical sensitivity and morphine reward in male and female C57Bl6 mice

This study evaluated the hypothesis that a paclitaxel treatment regimen sufficient to produce mechanical allodynia would alter sensitivities of male and female mice to the conditioned rewarding and reinforcing effects of morphine. Saline or paclitaxel were administered on Days 1, 3, 5, and 7 in male and female C57Bl/6 mice to induce morphine-reversible mechanical allodynia as measured by the Von Frey filament test. Paclitaxel treatment did not change sensitivity to morphine conditioned place preference (CPP) relative to saline treatment in either male or female mice. Morphine produced peak self-administration under a fixed ratio-1 (FR1) schedule of reinforcement for 0.03 mg/kg morphine per infusion in female mice and 0.1 mg/kg morphine per infusion in male mice. During the progressive ratio experiments, saline treatment in male mice decreased the number of morphine infusions for 12 days whereas the paclitaxel-treated male mice maintained responding for morphine similar to baseline levels during the same time period. However, paclitaxel did not have an overall effect on the reinforcing efficacy of morphine assessed over a limited dose range during the course of the repeated self-administration. These results suggest that the reward-related behavioral effects of morphine are overall not robustly altered by the presence of paclitaxel treatment under the current dosing regimen, with the exception of maintaining a small yet significant higher baseline than saline treatment during the development of allodynia in male mice. (PsycINFO Database Record

κ Opioid receptor activation in different brain regions differentially modulates anxiety-related behaviors in mice

κ Opioid receptor system is widely implicated in the regulation of emotion. However, the findings about the role on anxiety in rodents are highly controversial, since both anxiogenic- and anxiolytic-like effects have been reported with κ opioid receptor activation. The mechanism and the underlying neuroanatomical substrates are unexplored. In the present study, we first investigated the effects of κ agonist U50,488H on anxiety-related behaviors over a wide range of doses, and we found that U50,488H produced dual effects in anxiety, with low dose being anxiogenic and high dose being anxiolytic. To assess the potential neuroanatomical substrates, we used phosphorylation of extracellular signal-related kinase1/2 (pERK1/2) to map the underlying neural circuits. We found that the anxiogenic effect of U50,488H was paralleled by an increase of pERK1/2 in the nucleus accumbens, whereas the anxiolytic effect was paralleled by an increase of pERK1/2 in the lateral septal nucleus. We then examined the behavioral consequences with locally microinjection of U50,488H, and we found that microinjection of U50,488H into the nucleus accumbens exerted anxiogenic-like effects, whereas microinjection of U50,488H into the lateral septal nucleus. Both effects can be abolished by κ antagonist nor-BNI pretreatment. To the best of our knowledge, the present work firstly provides the neuroanatomical sites that mediating the dual anxiogenic- and anxiolytic-like effects of U50,488H in mice. This study may help to explain current controversial role of κ receptor activation in anxiety-related behaviors in rodents, and may open new perspectives in the areas of anxiety disorders and κ receptor function.

Chronic pain causes a persistent anxiety state leading to increased ethanol intake in CD1 mice

Mood disorders and chronic pain are closely linked, but limited progress has been made in understanding the role of chronic and neuropathic pain in the aetiopathogenesis of depression. To explore the pathological mechanisms that mediate the association between pain and depressive-like behaviours, we studied the time-dependent effect of neuropathic pain on the development of anxiety-like and despair behaviours in CD1 mice. We analysed behavioural data, neuroinflammation reactions and changes in neurotransmitter (glutamate and serotonin) levels in the mouse prefrontal cortex. Sciatic-operated mice displayed long-lasting anxiety-like and despair behaviours, starting 5 and 20 days after partial sciatic nerve ligation, respectively. Glutamatergic neurotransmission and IL-1β cytokine expression were enhanced in the prefrontal cortex of mice with neuropathic pain. We found no change in serotonin metabolism, cytokine IL-6 or brain-derived neurotrophic factor levels. While sciatic-operated mice exposed to intermittent ethanol intake (20% v/v) using the drinking in the dark procedure consumed higher amounts of ethanol than sham-operated mice, thermal allodynia and despair behaviour were not attenuated by ethanol consumption. Our findings reveal an association between glutamatergic neurotransmission and pain-induced mood disorders, and indicate that moderate ethanol consumption does not relieve nociceptive and depressive behaviours associated with chronic pain in mice.

Sex Differences in Kappa Opioid Receptor Function and Their Potential Impact on Addiction

Behavioral, biological, and social sequelae that lead to drug addiction differ between men and women. Our efforts to understand addiction on a mechanistic level must include studies in both males and females. Stress, anxiety, and depression are tightly linked to addiction, and whether they precede or result from compulsive drug use depends on many factors, including biological sex. The neuropeptide dynorphin (DYN), an endogenous ligand at kappa opioid receptors (KORs), is necessary for stress-induced aversive states and is upregulated in the brain after chronic exposure to drugs of abuse. KOR agonists produce signs of anxiety, fear, and depression in laboratory animals and humans, findings that have led to the hypothesis that drug withdrawal-induced DYN release is instrumental in negative reinforcement processes that drive addiction. However, these studies were almost exclusively conducted in males. Only recently is evidence available that there are sex differences in the effects of KOR activation on affective state. This review focuses on sex differences in DYN and KOR systems and how these might contribute to sex differences in addictive behavior. Much of what is known about how biological sex influences KOR systems is from research on pain systems. The basic molecular and genetic mechanisms that have been discovered to underlie sex differences in KOR function in pain systems may apply to sex differences in KOR function in reward systems. Our goals are to discuss the current state of knowledge on how biological sex contributes to KOR function in the context of pain, mood, and addiction and to explore potential mechanisms for sex differences in KOR function. We will highlight evidence that the function of DYN-KOR systems is influenced in a sex-dependent manner by: polymorphisms in the prodynorphin (pDYN) gene, genetic linkage with the melanocortin-1 receptor (MC1R), heterodimerization of KORs and mu opioid receptors (MORs), and gonadal hormones. Finally, we identify several gaps in our understanding of “if” and “how” DYN and KORs modulate addictive behavior in a sex-dependent manner. Future work may address these gaps by building on the mechanistic studies outlined in this review. Ultimately this will enable the development of novel and effective addiction treatments tailored to either males or females.

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

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

Effect of environment on the long-term consequences of chronic pain

Much evidence from pain patients and animal models shows that chronic pain does not exist in a vacuum but has varied comorbidities and far-reaching consequences. Patients with long-term pain often develop anxiety and depression and can manifest changes in cognitive functioning, particularly with working memory. Longitudinal studies in rodent models also show the development of anxiety-like behavior and cognitive changes weeks to months after an injury causing long-term pain. Brain imaging studies in pain patients and rodent models find that chronic pain is associated with anatomical and functional alterations in the brain. Nevertheless, studies in humans reveal that lifestyle choices, such as the practice of meditation or yoga, can reduce pain perception and have the opposite effect on the brain as does chronic pain. In rodent models, studies show that physical activity and a socially enriched environment reduce pain behavior and normalize brain function. Together, these studies suggest that the burden of chronic pain can be reduced by nonpharmacological interventions.

High Times for Painful Blues: The Endocannabinoid System in Pain-Depression Comorbidity

Depression and pain are two of the most debilitating disorders worldwide and have an estimated cooccurrence of up to 80%. Comorbidity of these disorders is more difficult to treat, associated with significant disability and impaired health-related quality of life than either condition alone, resulting in enormous social and economic cost. Several neural substrates have been identified as potential mediators in the association between depression and pain, including neuroanatomical reorganization, monoamine and neurotrophin depletion, dysregulation of the hypothalamo-pituitary-adrenal axis, and neuroinflammation. However, the past decade has seen mounting evidence supporting a role for the endogenous cannabinoid (endocannabinoid) system in affective and nociceptive processing, and thus, alterations in this system may play a key role in reciprocal interactions between depression and pain. This review will provide an overview of the preclinical evidence supporting an interaction between depression and pain and the evidence supporting a role for the endocannabinoid system in this interaction.

Pituitary adenylate cyclase activating polypeptide in stress-related disorders: data convergence from animal and human studies

The maladaptive expression and function of several stress-associated hormones have been implicated in pathological stress and anxiety-related disorders. Among these, recent evidence has suggested that pituitary adenylate cyclase activating polypeptide (PACAP) has critical roles in central neurocircuits mediating stress-related emotional behaviors. We describe the PACAPergic systems, the data implicating PACAP in stress biology, and how altered PACAP expression and signaling may result in psychopathologies. We include our work implicating PACAP signaling within the bed nucleus of the stria terminalis in mediating the consequences of stressor exposure and relatedly, describe more recent studies suggesting that PACAP in the central nucleus of the amygdala may impact the emotional aspects of chronic pain states. In aggregate, these results are consistent with data suggesting that PACAP dysregulation is associated with posttraumatic stress disorder in humans.

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.

Combined administration of buprenorphine and naltrexone produces antidepressant-like effects in mice

Opiates have been used historically for the treatment of depression. Renewed interest in the use of opiates as antidepressants has focused on the development of kappa opioid receptor (κ-receptor) antagonists. Buprenorphine acts as a partial µ-opioid receptor agonist and a κ-receptor antagonist. By combining buprenorphine with the opioid antagonist naltrexone, the activation of µ-opioid receptors will be reduced and the κ-antagonist properties enhanced. We have established that a combination dose of buprenorphine (1 mg/kg) with naltrexone (1 mg/kg) functions as a short-acting κ-antagonist in the mouse tail withdrawal test. Furthermore, this dose combination is neither rewarding nor aversive in the conditioned place preference paradigm, and is without significant locomotor effects. We have shown for the first time that systemic co-administration of buprenorphine (1 mg/kg) with naltrexone (1 mg/kg) in CD-1 mice produced an antidepressant-like response in behaviours in both the forced swim test and novelty induced hypophagia task. Behaviours in the elevated plus maze and light dark box were not significantly altered by treatment with buprenorphine alone, or in combination with naltrexone. We propose that the combination of buprenorphine with naltrexone represents a novel, and potentially a readily translatable approach, to the treatment of depression.

The role of the dynorphin/κ opioid receptor system in anxiety

Anxiety disorders are the most common and prevalent forms of psychiatric disease, although the biological basis of anxiety is not well understood. The dynorphin/κ opioid receptor system is widely distributed in the central nervous system and has been shown to play a critical role in modulating mood and emotional behaviors. In the present review, we summarize current literature relating to the role played by the dynorphin/κ opioid receptor system in anxiety and κ opioid receptor antagonists as potential therapeutic agents for the treatment of anxiety disorders.

Temporal dynamics of anxiety phenotypes in a dental pulp injury model

Background

Accumulating clinical and preclinical evidence indicates that chronic pain is often comorbid with persistent low mood and anxiety. However, the mechanisms underlying pain-induced anxiety, such as its causality, temporal progression, and relevant neural networks are poorly understood, impeding the development of efficacious therapeutic approaches.

Results

Here, we have identified the sequential emergence of anxiety phenotypes in mice subjected to dental pulp injury (DPI), a prototypical model of orofacial pain that correlates with human toothache. Compared with sham controls, mice subjected to DPI by mechanically exposing the pulp to the oral environment exhibited significant signs of anxiogenic effects, specifically, altered behaviors on the elevated plus maze (EPM), novelty-suppressed feeding (NSF) tests at 1 but not 3 days after the surgery. Notably, at 7 and 14 days, the DPI mice again avoided the open arm, center area, and novelty environment in the EPM, open field, and NSF tests, respectively. In particular, DPI-induced social phobia and increased repetitive grooming did not occur until 14 days after surgery, suggesting that DPI-induced social anxiety requires a long time. Moreover, oral administration of an anti-inflammatory drug, ibuprofen, or an analgesic agent, ProTx-II, which is a selective inhibitor of Na_V1.7 sodium channels, both significantly alleviated DPI-induced avoidance in mice. Finally, to investigate the underlying central mechanisms, we pharmacologically blocked a popular form of synaptic plasticity with a GluA2-derived peptide, long-term depression, as that treatment significantly prevented the development of anxiety phenotype upon DPI.

Conclusions

Together, these results suggest a temporally progressive causal relationship between orofacial pain and anxiety, calling for more in-depth mechanistic studies on concomitant pain and anxiety disorders.

Anxiolytic-like effects of translocator protein (TSPO) ligand ZBD-2 in an animal model of chronic pain

The activation of Translocator protein (18 kDa) (TSPO) has been demonstrated to mediate rapid anxiolytic efficacy in stress response and stress-related disorders. This protein is involved in the synthesis of endogenous neurosteroids that promote γ-aminobutyric acid (GABA)-mediated neurotransmission in the central neural system. However, little is known about the functions and the underlying mechanisms of TSPO in chronic pain-induced anxiety-like behaviors. The novel TSPO ligand N-benzyl-N-ethyl-2-(7,8-dihydro-7-benzyl-8-oxo-2-phenyl-9H-purin-9-yl) acetamide (ZBD-2) was used in the present study. We found that ZBD-2 (0.15 or 1.5 mg/kg) significantly attenuated anxiety-like behaviors in mice with chronic inflammatory pain induced by hindpaw injection of complete Freund’s adjuvant (CFA). However, the treatment did not alter the nociceptive threshold or inflammation in the hindpaw. Hindpaw injection of CFA induced the upregulation of TSPO, GluR1-containing α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, and NR2B-containing N-methyl-d-aspartate (NMDA) receptors in the basolateral amygdala (BLA). ZBD-2 administration reversed the alterations of the abovementioned proteins in the BLA of the CFA-injected mice. Electrophysiological recording revealed that ZBD-2 could prevent an imbalance between excitatory and inhibitory transmissions in the BLA synapses of CFA-injected mice. Therefore, as the novel ligand of TSPO, ZBD-2 induced anxiolytic effects, but did not affect the nociceptive threshold of mice under chronic pain. The anxiolytic effects of ZBD-2 were related to the regulation of the balance between excitatory and inhibitory transmissions in the BLA.

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.

Sex differences in depression-like behavior after nerve injury are associated with differential changes in brain-derived neurotrophic factor levels in mice subjected to early life stress

We recently demonstrated that exposure to early life stress exacerbates nerve injury-induced thermal and mechanical hypersensitivity in adult male and female mice. Accumulating evidence suggests that chronic pain causes emotional dysfunction, such as anxiety and depression. In the present study, we investigated the impact of early life stress on depression-like behavior after nerve injury in mice. In addition, we examined the expression of brain-derived neurotrophic factor (BDNF), which is known to be involved in the pathogenesis of depression. Early life stress was induced by maternal separation between 2 and 3 weeks of age combined with social isolation after weaning (MSSI). At 9 weeks of age, the sciatic nerve was partially ligated to elicit neuropathic pain. Depression-like behavior was evaluated using the forced swim test at 12 weeks of age. Tissue samples from different regions of the brain were collected at the end of maternal separation (3 weeks of age) or after the forced swim test (12 weeks of age). At 12 weeks of age, immobility time in the forced swim test was increased only in MSSI-stressed female mice with nerve injury. BDNF expression was increased in male, but not female, MSSI-stressed mice at 3 weeks of age. However, MSSI stress did not impact BDNF expression in male or female mice at 12 weeks of age. Our findings suggest that exposure to early life stress exacerbates emotional dysfunction induced by neuropathic pain in a sex-dependent manner. Changes in BDNF expression after early life stress may be associated with neuropathic pain-induced depression-like behavior in adulthood. Furthermore, sex differences in BDNF expression after exposure to early life stress may contribute to sex-specific susceptibility to neuropathic pain-induced emotional dysfunction.

A novel anxiogenic role for the delta opioid receptor expressed in GABAergic forebrain neurons

BACKGROUND

The delta opioid receptor (DOR) is broadly expressed throughout the nervous system; it regulates chronic pain, emotional responses, motivation, and memory. Neural circuits underlying DOR activities have been poorly explored by genetic approaches. We used conditional mouse mutagenesis to elucidate receptor function in GABAergic neurons of the forebrain.

METHODS

We characterized DOR distribution in the brain of Dlx5/6-CreXOprd1(fl/fl) (Dlx-DOR) mice and tested main central DOR functions through behavioral testing.

RESULTS

The DOR proteins were strongly deleted in olfactory bulb and striatum and remained intact in cortex and basolateral amygdala. Olfactory perception, circadian activity, and despair-like behaviors were unchanged. In contrast, locomotor stimulant effects of SNC80 (DOR agonist) and SKF81297 (D1 agonist) were abolished and increased, respectively. The Dlx-DOR mice showed lower levels of anxiety in the elevated plus maze, opposing the known high anxiety in constitutive DOR knockout animals. Also, Dlx-DOR mice reached the food more rapidly in a novelty suppressed feeding task, despite their lower motivation for food reward observed in an operant paradigm. Finally, c-fos protein staining after novelty suppressed feeding was strongly reduced in amygdala, concordant with the low anxiety phenotype of Dlx-DOR mice.

CONCLUSIONS

We demonstrate that DORs expressed in the forebrain mediate the described locomotor effect of SNC80 and inhibit D1-stimulated hyperactivity. Our data also reveal an unanticipated anxiogenic role for this particular DOR subpopulation, with a potential novel adaptive role. In emotional responses, DORs exert dual anxiolytic and anxiogenic roles, both of which may have implications in the area of anxiety disorders.

Pain affects depression through anxiety, fatigue, and sleep in multiple sclerosis

OBJECTIVE

Over a quarter million individuals in the United States have multiple sclerosis (MS). Chronic pain and depression are disproportionately high in this population. The purpose of this study was to examine the relationship between chronic pain and depression in MS and to examine potentially meditational effects of anxiety, fatigue, and sleep.

METHOD

We used cross-sectional data from self-reported instruments measuring multiple symptoms and quality of life indicators in this study. We used structural equation modeling to model direct and indirect effects of pain on depression in a sample of 1,245 community-dwelling individuals with MS. Pain interference, depression, fatigue, and sleep disturbance were modeled as latent variables with 2 to 3 indicators each. The model controlled for age, sex, disability status (Expanded Disability Status Scale), and social support.

RESULTS

A model with indirect effects of pain on depression had adequate fit and accounted for nearly 80% of the variance in depression. The effects of chronic pain on depression were almost completely mediated by fatigue, anxiety, and sleep disturbance. Higher pain was associated with greater fatigue, anxiety, and sleep disturbance, which in turn were associated with higher levels of depression. The largest mediating effect was through fatigue. Additional analyses excluded items with common content and suggested that the meditational effects observed were not attributable to content overlap across scales.

CONCLUSION

Individuals living with MS who report high levels of chronic pain and depressive symptoms may benefit from treatment approaches that can address sleep, fatigue, and anxiety.

The anterior cingulate cortex is a critical hub for pain-induced depression

BACKGROUND

Besides chronic stress, chronic pain is a prevalent determinant for depression. Changes induced in specific brain regions by sustained pain may alter the processing of affective information, thus resulting in anxiodepressive disorders. Here, we compared the role of the anterior cingulate cortex (ACC) and the posterior insular cortex in the anxiodepressive, sensory, and affective aspects of chronic pain.

METHODS

Neuropathic pain was induced by cuffing the right sciatic nerve of C57BL/6J mice. Lesions were performed by local injection of ibotenic acid and chronic activation of the ACC by optogenetic stimulation. Anxiodepressive-related behaviors were evaluated through the novelty suppressed feeding, marble burying, splash, and forced swimming tests. Mechanical thresholds were determined using von Frey filaments, and the relief of spontaneous pain was determined by using place conditioning.

RESULTS

The ACC lesion prevented the anxiodepressive consequences of chronic pain without affecting the sensory mechanical allodynia. Conversely, the tonic or spontaneous pain and the anxiodepressive consequences of pain remained present after posterior insular cortex lesion, even though the mechanical allodynia was suppressed. Furthermore, optogenetic stimulation of the ACC was sufficient to induce anxiety and depressive-like behaviors in naïve animals.

CONCLUSIONS

Our results show that, at cortical level, the sensory component of chronic pain remains functionally segregated from its affective and anxiodepressive components. Spontaneous tonic pain and evoked allodynia can be experimentally dissociated. Furthermore, the ACC appears as a critical hub for mood disorders, including for the anxiodepressive consequences of chronic pain, and thus constitutes an important target for divulging the underlying mechanism.

Anxiety- and depression-like behavior and impaired neurogenesis evoked by peripheral neuropathy persist following resolution of prolonged tactile hypersensitivity

Pain and depression are frequently associated with and often persist after resolution of an initial injury. Identifying the extent to which depression remains causally associated with ongoing physical discomfort during chronic pain, or becomes independent of it, is an important problem for basic neuroscience and psychiatry. Difficulty in distinguishing between effects of ongoing aversive sensory input and its long-term consequences is a significant roadblock, especially in animal models. To address this relationship between localized physical discomfort and its more global consequences, we investigated cellular and behavioral changes during and after reversing a mouse model of neuropathic pain. Tactile allodynia produced by placing a plastic cuff around the sciatic nerve resolved within several days when the cuff was removed. In contrast, the changes in elevated O-maze, forced-swim, Y-maze spontaneous alternation and novel-object recognition test performance that developed after nerve cuff placement remained for at least 3 weeks after the nerve cuffs were removed, or 10-15 d following complete normalization of mechanical sensitivity. Hippocampal neurogenesis, measured by doublecortin and proliferating cell nuclear antigen expression, was also suppressed after nerve cuff placement and remained suppressed 3 weeks after cuff removal. FosB expression was elevated in the central nucleus of the amygdala and spinal cord dorsal horn only in mice with ongoing allodynia. In contrast, FosB remained elevated in the basolateral amygdala of mice with resolved nociception and persisting behavioral effects. These observations suggest that different processes control tactile hypersensitivity and the behavioral changes and impaired neurogenesis that are associated with neuropathic allodynia.

The influence of μ-opioid and noradrenaline reuptake inhibition in the modulation of pain responsive neurones in the central amygdala by tapentadol in rats with neuropathy

Treatments for neuropathic pain are either not fully effective or have problematic side effects. Combinations of drugs are often used. Tapentadol is a newer molecule that produces analgesia in various pain models through two inhibitory mechanisms, namely central μ-opioid receptor (MOR) agonism and noradrenaline reuptake inhibition. These two components interact synergistically, resulting in levels of analgesia similar to opioid analgesics such as oxycodone and morphine, but with more tolerable side effects. The right central nucleus of the amygdala (CeA) is critical for the lateral spinal ascending pain pathway, regulates descending pain pathways and is key in the emotional-affective components of pain. Few studies have investigated the pharmacology of limbic brain areas in pain models. Here we determined the actions of systemic tapentadol on right CeA neurones of animals with neuropathy and which component of tapentadol contributes to its effect. Neuronal responses to multimodal peripheral stimulation of animals with spinal nerve ligation or sham surgery were recorded before and after two doses of tapentadol. After the higher dose of tapentadol either naloxone or yohimbine were administered. Systemic tapentadol resulted in dose-dependent decrease in right CeA neuronal activity only in neuropathy. Both naloxone and yohimbine reversed this effect to an extent that was modality selective.

The interactions of the components of tapentadol are not limited to the synergy between the MOR and α₂-adrenoceptors seen at spinal levels, but are seen at this supraspinal site where suppression of responses may relate to the ability of the drug to alter affective components of pain.

Does the kappa opioid receptor system contribute to pain aversion?

The kappa opioid receptor (KOR) and the endogenous peptide-ligand dynorphin have received significant attention due the involvement in mediating a variety of behavioral and neurophysiological responses, including opposing the rewarding properties of drugs of abuse including opioids. Accumulating evidence indicates this system is involved in regulating states of motivation and emotion. Acute activation of the KOR produces an increase in motivational behavior to escape a threat, however, KOR activation associated with chronic stress leads to the expression of symptoms indicative of mood disorders. It is well accepted that KOR can produce analgesia and is engaged in chronic pain states including neuropathic pain. Spinal studies have revealed KOR-induced analgesia in reversing pain hypersensitivities associated with peripheral nerve injury. While systemic administration of KOR agonists attenuates nociceptive sensory transmission, this effect appears to be a stress-induced effect as anxiolytic agents, including delta opioid receptor agonists, mitigate KOR agonist-induced analgesia. Additionally, while the role of KOR and dynorphin in driving the dysphoric and aversive components of stress and drug withdrawal has been well characterized, how this system mediates the negative emotional states associated with chronic pain is relatively unexplored. This review provides evidence that dynorphin and the KOR system contribute to the negative affective component of pain and that this receptor system likely contributes to the high comorbidity of mood disorders associated with chronic neuropathic pain.

RGS9-2 modulates sensory and mood related symptoms of neuropathic pain

The signal transduction modulator Rgs9-2 (Regulator of G protein signaling 9-2) plays a key role in dopaminergic and opioidergic transmission in the striatum. Rgs9-2 is a potent modulator of opiate reward and analgesia, but its role in chronic pain remains unknown. Here, we use the spared nerve injury model (SNI), to evaluate the influence of Rgs9-2 in sensory symptoms, as well as in anxiety and depression-like behaviors observed under neuropathic pain conditions. Our data demonstrate that knockout of the Rgs9 gene reduces the intensity of thermal hyperalgesia and mechanical allodynia the first few days after nerve injury. This small, but significant effect is only observed at early time points after nerve injury, whereas after the first week of SNI, Rgs9 knockout (Rgs9KO) and Rgs9 wildtype (Rgs9WT) mice show similar levels of mechanical allodynia and thermal hyperalgesia. Furthermore, Rgs9-2 deletion exacerbates anxiety and depression like behaviors several weeks after the emergence of the neuropathic pain symptoms. Our findings also reveal a temporal and regional regulation of Rgs9-2 protein expression by neuropathic pain, as Rgs9-2 levels are reduced in the spinal cord a few days after nerve injury, whereas decreased Rgs9-2 levels in the Nucleus Accumbens (NAc) are only observed several weeks after nerve injury. Thus, adaptations in Rgs9-2 activity in the spinal cord and in the NAc may contribute to sensory and affective components of neuropathic pain.

Emotional consequences of neuropathic pain: insight from preclinical studies

Mood disorders such as depression and anxiety are frequently observed in patients suffering from chronic pain, including neuropathic pain. While this comorbidity is clinically well established, the underlying mechanism(s) remained unclear. The recent development of animal models now allows addressing the consequences of neuropathic pain. In this review, we report the preclinical evidences from anatomical, neuroimaging, behavioral, pharmacological and biochemical studies that address the anxiodepressive consequences of neuropathic pain. We present an overview of rodent models of these consequences and we discuss the challenges and parameters to consider for generating these models. We then discuss the possible mechanism(s) underlying anxiodepressive consequences by describing morphological and functional changes. Information is provided concerning neuroanatomical changes and plasticity, including LTP and LTD, in the anterior cingulate cortex, the insula, the hippocampus, the amygdala and the mesolimbic system, neuroendocrine parameters concerning the hypothalamo-pituitary-adrenal axis, neuroimmune response including the role of glial cells and cytokines, monoamine systems and changes in locus coeruleus noradrenergic system, and neurotrophic factors such as BDNF.

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).

Suppression of bone marrow-derived microglia in the amygdala improves anxiety-like behavior induced by chronic partial sciatic nerve ligation in mice

Chronic neuropathic pain causes abnormal sensitivities such as hyperalgesia and allodynia, and emotional abnormalities such as anxiety and depression. Although spinal cord microglia are involved in abnormal sensitivity to neuropathic pain, no previous studies have examined the mechanism of neuropathic pain-induced anxiety. Here, we examined the involvement of bone marrow (BM)-derived microglia aggregated in the amygdalae of mice with chronic neuropathic pain in the development of anxiety-like behavior. We prepared partial sciatic nerve ligations (PSNL) in mice that received bone marrow transplantation from green fluorescent protein (GFP)-Tg mice after irradiation with head protection, and examined GFP-positive microglia in the central nuclei of the amygdalae (CeA). On day 28 after PSNL, BM-derived microglia aggregated in the CeA concurrent with anxiety-like behavior. BM-derived microglia in the CeA highly expressed interleukin (IL)-1β and C-C chemokine receptor type 2 (CCR2). In addition, neurons in the CeA highly expressed monocyte chemotactic protein-1 (MCP-1), a ligand for CCR2, in PSNL-treated mice compared to sham-operated mice, suggesting that the MCP-1/CCR2 axis is involved in the recruitment of BM-derived microglia. Oral administration of a CCR2 antagonist decreased the number of BM-derived microglia in the CeA, and successfully reversed the anxiety-like behavior and hypersensitivity to mechanical stimuli in PSNL-treated mice. Microinjections of an IL-1β receptor antagonist directly into the CeA successfully reversed the anxiety-like behavior in the PSNL-treated mice even though the neuropathic pain persisted. These results suggest that the recruitment of BM-derived microglia to the CeA via the MCP-1/CCR2 axis and neuron-microglia interactions might be important in the pathogenesis of neuropathic pain-induced anxiety.

Monoarthritis-induced emotional and cognitive impairments in rats are sensitive to low systemic doses or intra-amygdala injections of morphine

Chronic pain is a multidimensional experience that not only includes changes in nociception but also impairments in emotional and cognitive functions, not often taken into account in preclinical research. The present study investigated emotional and cognitive impairments in an animal model of persistent inflammatory pain as well as the involvement of the basolateral complex (BLC) of the amygdala in these components. Monoarthritis was induced by intra-articular injection of complete Freund׳s adjuvant. Mechanical hypersensitivity, anxiety and depressive-like behaviours as well as cognitive capacities were assessed using several tests, such as von Frey, social interaction, open field, saccharin preference, spatial and social recognition memory tests. The effects of morphine administered systemically or into the BLC of the amygdala were also studied. Monoarthritic rats exhibited mechanical hypersensitivity, anxiety and depressive-like behaviours as well as cognitive impairments. Whereas low systemic doses and intra-BLC infusion of morphine failed to reduce mechanical hypersensitivity, they reversed monoarthritis-induced anxiety-like behaviours and cognitive impairments. Our findings further support a crucial role of amygdala in the effect of morphine on emotional/cognitive components of pain and not on mechanical hypersensitivity. Finally, our study highlights the interest of a multi-behavioural approach in the assessment of pain and the analgesic effect of drugs.

Behavioral changes following experimentally-induced acute myocardial infarction in rats

Rats with experimentally-induced acute myocardial infarction (AMI) have proven to be a clinically relevant model for visceral pain. As there are no behavioral data available on rats in the postinfarction period, we aimed to identify specific pain-related behavioral changes following AMI to increase the validity of the model. AMI was induced by left coronary artery ligation and pain-related behavior was analyzed using the open field test (OFT) and elevated plus maze (EPM). Morphine was applied following AMI induction to differentiate pain-related changes from those related to nonspecific global changes in responsiveness. AMI was histologically confirmed. Hypolocomotion was consistently evident in all behavioral tests for both the infarcted group and sham group. In the OFT, both AMI and sham rats exhibited less exploratory behavior and less activity. A similar pattern of behavior was observed in EPM, where both surgical groups showed fewer entries to the open arms and spent less time in the open arms. The sham group with an intact pericardium showed the same pattern of activity as control rats. The reduction in activity and rearing observed following AMI was successfully reversed following morphine injection. This effect was abolished after naloxone application allowing us to attribute observed changes specifically to pain.This study demonstrates that pain-related behavior in the acute postinfarction period is generally characterized by reduced mobility and explorative behavior. Our results showed that cardiac ischemia as a consequence of experimentally-induced infarction is a less important source of pain behavior than manipulation of the pericardium.

Effects of curcumin on pain threshold and on the expression of nuclear factor κ B and CX3C receptor 1 after sciatic nerve chronic constrictive injury in rats

OBJECTIVE

To investigate the effects of curcumin on pain threshold and the expressions of nuclear factor κ B (NF-κ B) and CX3C chemokine receptor 1 (CX3CR1) in spinal cord and dorsal root ganglion (DRG) of the rats with sciatic nerve chronic constrictive injury.

METHODS

One hundred and twenty male Sprague Dawley rats, weighing 220-250 g, were randomly divided into 4 groups. Sham surgery (sham) group: the sciatic nerves of rats were only made apart but not ligated; chronic constrictive injury (CCI) group: the sciatic nerves of rats were only ligated without any drug treatment; curcumin treated injury (Cur) model group: the rats were administrated with curcumin 100 mg/(kg·d) by intraperitoneal injection for 14 days after CCI; solvent control (SC) group: the rats were administrated with the solvent at the same dose for 14 days after CCI. Thermal withdrawal latency (TWL) and mechanical withdrawal threshold (MWT) of rats were respectively measured on pre-operative day 2 and postoperative day 1, 3, 5, 7, 10 and 14. The lumbar segment L4-5 of the spinal cord and the L4, L5 DRG was removed at post-operative day 3, 7 and 14. The change of nuclear factor κ B (NF-κ B) p65 expression was detected by Western blotting while the expression of CX3CR1 was determined by immunohistochemical staining.

RESULTS

Compared with the sham group, the TWL and MWT of rats in the CCI group were significantly decreased on each post-operative day (P<0.01), which reached a nadir on the 3rd day after CCI, and the expressions of NF-κ B p65 and CX3CR1 were markedly increased in spinal cord dorsal horn and DRG. In the Cur group, the TWL of rats were significantly increased than those in the CCI group on post-operative day 7, 10 and 14 (P<0.05) and MWT increased than those in the CCI group on post-operative day 10 and 14 (P<0.05). In addition, the administration of curcumin significantly decreased the positive expressions of NF-κ B p65 and CX3CR1 in spinal cord and DRG (P<0.05).

CONCLUSION

Our study suggests that curcumin could ameliorate the CCI-induced neuropathic pain, probably through inhibiting CX3CR1 expression by the activation of NF-κ B p65 in spinal cord and DRG.

The effect of opiodergic system and testosterone on anxiety behavior in gonadectomized rats

BACKGROUND AND AIM

Removal of the testes (gonadectomy; GDX), the primary source of androgens, increases anxiety behavior in several tasks. Opioids are known to play a role in mediating the effects of androgen. In the present study, the effect of testosterone and opioidergic system on anxiety behavior was investigated.

METHODS

Adult male Wistar rats were bilaterally castrated. The elevated plus maze which is a useful test to investigate the effects of anxiogenic or anxiolytic drugs in rodents was used.

RESULTS

The data indicated that there is a decrease, 10 days after castration, in the percentage of OAT (the ratio of time spent in the open arms to total times spent in any arms × 100) and OAE (the ratio of entries into open arms to total entries × 100) but not locomotor activity, showing anxiogenic-like effects of gonadectomy. Intraperitoneal injection of testosterone (200, 300 and 450 mg/kg) and morphine (2.5, 5 and 7.5mg/kg), before testing 10 days after castration, showed an increase in OAT and OAE. Furthermore, injection of naloxone (5 and 7.5mg/kg, i.p.), 5 min before testing 10 days after castration, decreased OAT and OAE. Also, injection of a significant dose of testosterone (300 mg/kg, i.p.), 1h before the injection of different doses of morphine (1, 2.5, 5 and 7.5mg/kg, i.p.), 10 days after castration, did not significantly alter OAT, OAE and locomotor activity. While, administration of a significant dose of testosterone (300 mg/kg, i.p.), 1h before the infusion of different doses of naloxone (1, 2.5, 5 and 7.5mg/kg, i.p.), 10 days after castration, decreased OAT and OAE.

CONCLUSION

The results show the involvement of testosterone and opioidergic system in anxiogenic-like behaviors induced by gonadectomy.

"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.

Effect of chronic pain on fentanyl self-administration in mice

The development of opioid addiction in subjects with established chronic pain is an area that is poorly understood. It is critically important to clearly understand the neurobiology associated with propensity toward conversion to addiction under conditions of chronic pain. To pose the question whether the presence of chronic pain influences motivation to self-administer opioids for reward, we applied a combination of rodent models of chronic mechanical hyperalgesia and opioid self-administration. We studied fentanyl self-administration in mice under three conditions that induce chronic mechanical hyperalgesia: inflammation, peripheral nerve injury, and repeated chemotherapeutic injections. Responding for fentanyl was compared among these conditions and their respective standard controls (naïve condition, vehicle injection or sham surgery). Acquisition of fentanyl self-administration behavior was reduced or absent in all three conditions of chronic hyperalgesia relative to control mice with normal sensory thresholds. To control for potential impairment in ability to learn the lever-pressing behavior or perform the associated motor tasks, all three groups were evaluated for acquisition of food-maintained responding. In contrast to the opioid, chronic hyperalgesia did not interfere with the reinforcing effect of food. These studies indicate that the establishment of chronic hyperalgesia is associated with reduced or ablated motivation to seek opioid reward in mice.

Synergistic effects of celecoxib and bupropion in a model of chronic inflammation-related depression in mice

This study was aimed to characterize the depression-like behaviour in the classical model of chronic inflammation induced by Complete Freund's Adjuvant (CFA). Male Swiss mice received an intraplantar (i.pl.) injection of CFA (50 µl/paw) or vehicle. Behavioural and inflammatory responses were measured at different time-points (1 to 4 weeks), and different pharmacological tools were tested. The brain levels of IL-1β and BDNF, or COX-2 expression were also determined. CFA elicited a time-dependent edema formation and mechanical allodynia, which was accompanied by a significant increase in the immobility time in the tail suspension (TST) or forced-swimming (FST) depression tests. Repeated administration of the antidepressants imipramine (10 mg/kg), fluoxetine (20 mg/kg) and bupropion (30 mg/kg) significantly reversed depression-like behaviour induced by CFA. Predictably, the anti-inflammatory drugs dexamethasone (0.5 mg/kg), indomethacin (10 mg/kg) and celecoxib (30 mg/kg) markedly reduced CFA-induced edema. The oral treatment with the analgesic drugs dipyrone (30 and 300 mg/kg) or pregabalin (30 mg/kg) significantly reversed the mechanical allodyinia induced by CFA. Otherwise, either dipyrone or pregabalin (both 30 mg/kg) did not significantly affect the paw edema or the depressive-like behaviour induced by CFA, whereas the oral treatment with dipyrone (300 mg/kg) was able to reduce the immobility time in TST. Noteworthy, CFA-induced edema was reduced by bupropion (30 mg/kg), and depression behaviour was prevented by celecoxib (30 mg/kg). The co-treatment with bupropion and celecoxib (3 mg/kg each) significantly inhibited both inflammation and depression elicited by CFA. The same combined treatment reduced the brain levels of IL-1β, as well as COX-2 immunopositivity, whilst it failed to affect the reduction of BDNF levels. We provide novel evidence on the relationship between chronic inflammation and depression, suggesting that combination of antidepressant and anti-inflammatory agents bupropion and celecoxib might represent an attractive therapeutic strategy for depression.

Asymmetry of the endogenous opioid system in the human anterior cingulate: a putative molecular basis for lateralization of emotions and pain

Lateralization of the processing of positive and negative emotions and pain suggests an asymmetric distribution of the neurotransmitter systems regulating these functions between the left and right brain hemispheres. By virtue of their ability to selectively mediate euphoria, dysphoria, and pain, the μ-, δ-, and κ-opioid receptors and their endogenous ligands may subserve these lateralized functions. We addressed this hypothesis by comparing the levels of the opioid receptors and peptides in the left and right anterior cingulate cortex (ACC), a key area for emotion and pain processing. Opioid mRNAs and peptides and 5 "classical" neurotransmitters were analyzed in postmortem tissues from 20 human subjects. Leu-enkephalin-Arg (LER) and Met-enkephalin-Arg-Phe, preferential δ-/μ- and κ-/μ-opioid agonists, demonstrated marked lateralization to the left and right ACC, respectively. Dynorphin B (Dyn B) strongly correlated with LER in the left, but not in the right ACC suggesting different mechanisms of the conversion of this κ-opioid agonist to δ-/μ-opioid ligand in the 2 hemispheres; in the right ACC, Dyn B may be cleaved by PACE4, a proprotein convertase regulating left-right asymmetry formation. These findings suggest that region-specific lateralization of neuronal networks expressing opioid peptides underlies in part lateralization of higher functions, including positive and negative emotions and pain in the human brain.

Maternal deprivation is associated with sex-dependent alterations in nociceptive behavior and neuroinflammatory mediators in the rat following peripheral nerve injury

Early-life stress is associated with an increased risk of developing affective disorders and chronic pain conditions. This study examined the effect of maternal deprivation (MD) on nociceptive responding prior to and following peripheral nerve injury (L5-L6 spinal nerve ligation [SNL]). Because neuroimmune signaling plays an important role in pain and affective disorders, associated alterations in glial and cytokine expression were assessed in key brain regions associated with emotional and nociceptive responding, the hippocampus and prefrontal cortex. MD female, but not male, rats exhibited thermal hypoalgesia and mechanical allodynia compared with control (non-MD) counterparts. SNL resulted in mechanical and cold allodynia in MD and control rats of both sexes. However, MD females exhibited enhanced SNL-induced allodynic responding compared with non-MD counterparts. Interleukin 6 (IL-6) expression was reduced in the prefrontal cortex of MD-SNL males when compared with non-SNL counterparts. Glial fibrillary acidic protein and IL-1β expression in the hippocampus of MD-SNL males was increased compared with non-MD controls. MD-SNL females exhibited reduced tumor necrosis factor alpha in the prefrontal cortex with a concomitant increase in IL-6 and tumor necrosis factor alpha expression in the hippocampus, compared with either MD or SNL alone. In conclusion, MD female, but not male, rats exhibit enhanced nociceptive responding following peripheral nerve injury, effects that may relate to the distinct neuroinflammatory profile observed in female versus male rats.

PERSPECTIVE

This study demonstrates that females rats exposed to early-life stress exhibit enhanced neuropathic pain responding, effects that are associated with alterations in neuroinflammatory mediators. Increased understanding of the interactions among early-life stress, gender, and pain may lead to the identification of novel therapeutic targets for the treatment of chronic pain disorders.

Delta opioid receptors in brain function and diseases

Evidence that the delta opioid receptor (DOR) is an attractive target for the treatment of brain disorders has strengthened in recent years. This receptor is broadly expressed in the brain, binds endogenous opioid peptides, and shows as functional profile highly distinct from those of mu and kappa opioid receptors. Our knowledge of DOR function has enormously progressed from in vivo studies using pharmacological tools and genetic approaches. The important role of this receptor in reducing chronic pain has been extensively overviewed; therefore this review focuses on facets of delta receptor activity relevant to psychiatric and other neurological disorders. Beneficial effects of DOR agonists are now well established in the context of emotional responses and mood disorders. DOR activation also regulates drug reward, inhibitory controls and learning processes, but whether delta compounds may represent useful drugs in the treatment of drug abuse remains open. Epileptogenic and locomotor-stimulating effects of delta agonists appear drug-dependent, and the possibility of biased agonism at DOR for these effects is worthwhile further investigations to increase benefit/risk ratio of delta therapies. Neuroprotective effects of DOR activity represent a forthcoming research area. Future developments in DOR research will benefit from in-depth investigations of DOR function at cellular and circuit levels.

The amygdala between sensation and affect: a role in pain

The amygdala is a structure of the temporal lobe thought to be involved in assigning emotional significance to environmental information and triggering adapted physiological, behavioral and affective responses. A large body of literature in animals and human implicates the amygdala in fear. Pain having a strong affective and emotional dimension, the amygdala, especially its central nucleus (CeA), has also emerged in the last twenty years as key element of the pain matrix. The CeA receives multiple nociceptive information from the brainstem, as well as highly processed polymodal information from the thalamus and the cerebral cortex. It also possesses the connections that allow influencing most of the descending pain control systems as well as higher centers involved in emotional, affective and cognitive functions. Preclinical studies indicate that the integration of nociceptive inputs in the CeA only marginally contributes to sensory-discriminative components of pain, but rather contributes to associated behavior and affective responses. The CeA doesn’t have a major influence on responses to acute nociception in basal condition, but it induces hypoalgesia during aversive situation, such as stress or fear. On the contrary, during persistent pain states (inflammatory, visceral, neuropathic), a long-lasting functional plasticity of CeA activity contributes to an enhancement of the pain experience, including hyperalgesia, aversive behavioral reactions and affective anxiety-like states.

Cognitive and emotional control of pain and its disruption in chronic pain

Chronic pain is one of the most prevalent health problems in our modern world, with millions of people debilitated by conditions such as back pain, headache and arthritis. To address this growing problem, many people are turning to mind-body therapies, including meditation, yoga and cognitive behavioural therapy. This article will review the neural mechanisms underlying the modulation of pain by cognitive and emotional states - important components of mind-body therapies. It will also examine the accumulating evidence that chronic pain itself alters brain circuitry, including that involved in endogenous pain control, suggesting that controlling pain becomes increasingly difficult as pain becomes chronic.