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

Ferulic acid via attenuation of oxidative stress and neuro-immune response utilizes antinociceptive effect in mouse model of formalin test

Introduction

Plenty evidences suggests that neuroinflammation and oxidative stress augmented the neural sensitivity specifying that neuro-immune response is involved in the pathophysiology of pain. Ferulic acid (FA), a natural antioxidant found in various fruits, has various pharmacological properties. The purpose of the current study was to assess the antinociceptive effect of FA in a mouse model of formalin test with focus on its anti-neuroinflammatory and antioxidative stress effects.

Methods

The injection of FA (40 mg/kg), piroxicam (2 mg/kg), and saline (0.9% NaCl) (1 ml/kg) was done intraperitoneally and after one hour, formalin injected into the plantar surface of the hind paw of mice. Then pain behavior was documented during 60 min. Then mice were euthanized and prefrontal cortex (PFC) samples were taken. Malondialdehyde (MDA) level, antioxidant capacity and expression of inflammatory genes, counting tumor necrosis factor (TNF-) and interleukine 1 (IL-1) evaluated in the PFC.

Results

exhibited that FA declined the pain behavior following injection of formalin. Besides, FA significantly diminished the MDA level and increased the antioxidant capacity in the PFC. We revealed that FA diminished the expression of TNF-α and IL-1β genes in the PFC.

Conclusion

We conclude that FA exerted antinociceptive effects in the formalin test in mice, at least partially, by reducing oxidative stress and neuroimmune response in the PFC.

Surgical incision pain induced an increase in alcohol consumption in mice

INTRODUCTION

Large population-based studies have suggested a link between increased alcohol use and reduced pain. In addition, these studies suggest that higher levels of pain intensity are associated with an increase in alcohol consumption and rates of hazardous drinking which potentiates the risk of developing alcohol use disorders (AUD). The mechanisms and determinants of the alcohol-pain interaction can be studied in preclinical studies.

METHODS

The overall goal of this study is to use animal models to explore the impact of acute postoperative pain on alcohol intake. To achieve this, we characterized the timeline and levels of alcohol intake and preference in mice after laparotomy in the 2-bottle choice paradigm.

RESULTS

Our results show that laparotomy surgery increased alcohol intake and preference in male mice but not females in the 2-bottle choice and 3-bottle choice assays. In addition, ketoprofen administration blocked the increase in alcohol consumption in male mice after laparotomy. We also found that changes in alcohol initial sensitivity and acute functional tolerance, using loss of righting reflex (LORR) response, occur after surgery in mice.

CONCLUSION

Taken together, these findings suggests that sex, pain and alcohol sensitivity-related factors may modulate the relationship between alcohol consumption and pain.

Amelioration of pain and anxiety in sleep-deprived rats by intra-amygdala injection of cinnamaldehyde

Background

Sleep disorders are accompanied by increased anxiety and somatic pain. In addition, it has been observed that anxiety and pain have a boosting effect on each other, resulting in continued sleep disturbances. Amygdala's (CeA) central nucleus plays a crucial role in these processes. Cinnamaldehyde (Cinn) is an aromatic compound with anti-anxiety, antioxidant, and sleep-promoting properties. The present study uses sleep-deprived rats to examine the effects of an intra-CeA injection of Cinn on pain and anxiety.

Methods

Sleep deprivation (SD) was induced using the platform technique. 35 male Wistar rats were divided into five groups. Anxiety state and nociception were evaluated among groups using formalin test (F.T.), open field test (OFT), and elevated plus maze (EPM). Anxiety tests (OFT and EPM) were conducted in all groups. The first group was undergone FT without induction of SD (SD-FT+). The second group received SD without FT(SD+FT-). The third group received both SD and FT(SD+FT+). The treatment and vehicle groups have undergone both SD and FT in addition to the respectively intra-CeA injection of Cinn (SD+FT+ Cinn) and Cinn vehicle (SD+FT+ VC). The recorded behaviors were analyzed between groups using IBM SPSS 24th version.

Results

SD did not lead to any significant difference in nociceptive behaviors in FT between groups SD-FT+ and SD+FT+ (P ≥ 0.05). At the same time, there was a considerable discrepancy in rearing behaviors (P < 0.006) and the number of fecal boli (P < 0.004) recorded in OFM between these groups. Treatment with Cinn led to decreased nociception (P < 0.038), decreased rearing behaviors (P < 0.01), and reduced defecation (P < 0.004) in group SD + FT+ Cinn in comparison to the group SD+FT+. There were no differences in anxiety test results between the first and second groups (P ≥ 0.05).

Conclusion

SD can lead to elevated anxiety, while intra-CeA injection of Cinn ameliorated both perceptions of acute pain and anxiety. Besides, the conduction of FT before the anxiety test led to no disturbance in the results of anxiety tests.

Inflammatory pain affects alcohol intake in a dose-dependent manner in male rats in the intermittent access model

Introduction

Epidemiological studies have shown that there is a relation between pain and alcohol use disorder (AUD). Persistent pain is directly correlated with an increment in alcohol consumption and an increased risk of developing an AUD. Greater levels of pain intensity and unpleasantness are associated with higher levels of relapse, an increase in alcohol consumption, rates of hazardous drinking, and delay to seek for treatment. However, this interaction has not been deeply studied in the preclinical setting.

Methods

Here, we aim to evaluate how inflammatory pain affects levels of alcohol drinking in male and female rats with a history of alcohol. For that, we used an intermittent access 2-bottle choice paradigm combined with the complete Freund Adjuvant (CFA) model of inflammatory pain.

Results

Our results show that CFA-induced inflammatory pain does not alter total intake of 20% alcohol in male or female rats. Interestingly, in males, the presence of CFA-induced inflammatory pain blunts the decrease of alcohol intake when higher concentrations of alcohol are available, whereas it does not have an effect on intake at any concentration in female rats.

Conclusion

Altogether, this study provides relevant data and constitutes an important contribution to the study of pain and AUD and it highlights the necessity to design better behavioral paradigms in animal models that are more translational and reflect current epidemiological findings.

Foramen lacerum impingement of trigeminal nerve root as a rodent model for trigeminal neuralgia

Trigeminal neuralgia (TN) is a classic neuralgic pain condition with distinct clinical characteristics. Modeling TN in rodents is challenging. Recently, we found that a foramen in the rodent skull base, the foramen lacerum, provides direct access to the trigeminal nerve root. Using this access, we developed a foramen lacerum impingement of trigeminal nerve root (FLIT) model and observed distinct pain-like behaviors in rodents, including paroxysmal asymmetric facial grimaces, head tilt when eating, avoidance of solid chow, and lack of wood chewing. The FLIT model recapitulated key clinical features of TN, including lancinating pain-like behavior and dental pain-like behavior. Importantly, when compared with a trigeminal neuropathic pain model (infraorbital nerve chronic constriction injury [IoN-CCI]), the FLIT model was associated with significantly higher numbers of c-Fos-positive cells in the primary somatosensory cortex (S1), unraveling robust cortical activation in the FLIT model. On intravital 2-photon calcium imaging, synchronized S1 neural dynamics were present in the FLIT but not the IoN-CCI model, revealing differential implication of cortical activation in different pain models. Taken together, our results indicate that FLIT is a clinically relevant rodent model of TN that could facilitate pain research and therapeutics development.

Sensory-motor response elicited by first time intraoral administered ethanol after trigeminal neuropathic injury

Recent findings have shown a relationship between alcohol use disorders (AUD) and chronic pain. Preclinical models have demonstrated that chronic pain, including trigeminal nerve injury, increases ethanol consumption throughout extended administration periods. Nevertheless, it remains unclear whether chronic pain induces a greater susceptibility to developing AUD by altering motor control consumption regardless of the symptomatology of neuropathic pain and if sex influences this susceptibility. We used a former prolonged pain experience model induced by a constriction of the mental nerve (mNC) to answer this question. We analyzed ethanol consumption in a short access protocol to reduce the post-ingestional effects and compared licking microstructure between groups. The constriction of the mental nerve induced evoked and spontaneous pain and reduction in the hedonic value of sucrose. The differences in alcohol consumption were not reflective of the former prolonged pain experience. Female mice showed a more efficient dynamic of consumption of alcohol reflected in a long burst of licking and a less variable licking rate within a cluster.

Role of IL-6 in the regulation of neuronal development, survival and function

The pleiotropic cytokine interleukin-6 (IL-6) is emerging as a molecule with both beneficial and destructive potentials. It can exert opposing actions triggering either neuron survival after injury or causing neurodegeneration and cell death in neurodegenerative or neuropathic disorders. Importantly, neurons respond differently to IL-6 and this critically depends on their environment and whether they are located in the peripheral or the central nervous system. In addition to its hub regulator role in inflammation, IL-6 is recently emerging as an important regulator of neuron function in health and disease, offering exciting possibilities for more mechanistic insight into the pathogenesis of mental, neurodegenerative and pain disorders and for developing novel therapies for diseases with neuroimmune and neurogenic pathogenic components.

Drug Addiction: Hyperkatifeia/Negative Reinforcement as a Framework for Medications Development

Compulsive drug seeking that is associated with addiction is hypothesized to follow a heuristic framework that involves three stages (binge/intoxication, withdrawal/negative affect, and preoccupation/anticipation) and three domains of dysfunction (incentive salience/pathologic habits, negative emotional states, and executive function, respectively) via changes in the basal ganglia, extended amygdala/habenula, and frontal cortex, respectively. This review focuses on neurochemical/neurocircuitry dysregulations that contribute to hyperkatifeia, defined as a greater intensity of negative emotional/motivational signs and symptoms during withdrawal from drugs of abuse in the withdrawal/negative affect stage of the addiction cycle. Hyperkatifeia provides an additional source of motivation for compulsive drug seeking via negative reinforcement. Negative reinforcement reflects an increase in the probability of a response to remove an aversive stimulus or drug seeking to remove hyperkatifeia that is augmented by genetic/epigenetic vulnerability, environmental trauma, and psychiatric comorbidity. Neurobiological targets for hyperkatifeia in addiction involve neurocircuitry of the extended amygdala and its connections via within-system neuroadaptations in dopamine, enkephalin/endorphin opioid peptide, and γ-aminobutyric acid/glutamate systems and between-system neuroadaptations in prostress corticotropin-releasing factor, norepinephrine, glucocorticoid, dynorphin, hypocretin, and neuroimmune systems and antistress neuropeptide Y, nociceptin, endocannabinoid, and oxytocin systems. Such neurochemical/neurocircuitry dysregulations are hypothesized to mediate a negative hedonic set point that gradually gains allostatic load and shifts from a homeostatic hedonic state to an allostatic hedonic state. Based on preclinical studies and translational studies to date, medications and behavioral therapies that reset brain stress, antistress, and emotional pain systems and return them to homeostasis would be promising new targets for medication development. SIGNIFICANCE STATEMENT: The focus of this review is on neurochemical/neurocircuitry dysregulations that contribute to hyperkatifeia, defined as a greater intensity of negative emotional/motivational signs and symptoms during withdrawal from drugs of abuse in the withdrawal/negative affect stage of the drug addiction cycle and a driving force for negative reinforcement in addiction. Medications and behavioral therapies that reverse hyperkatifeia by resetting brain stress, antistress, and emotional pain systems and returning them to homeostasis would be promising new targets for medication development.

Pain-induced alterations in the dynorphinergic system within the mesocorticolimbic pathway: Implication for alcohol addiction

Latest studies have revealed that pain negatively impacts on reward processing and motivation leading to negative affective states and stress. These states not only reduce quality of life of patients by increasing the appearance of psychiatric comorbidities, but also have an important impact on vulnerability to drug abuse, including alcohol. In fact, clinical, epidemiological but also preclinical studies have revealed that the presence of pain is closely related to alcohol use disorders (AUDs). All this evidence suggests that pain is a factor that increases the risk of suffering AUD, predicting heavy drinking behavior and relapse drinking in those patients with a previous history of AUD. The negative consequences of chronic pain and its impact on stress and AUD are likely mediated by alterations in the central nervous system, especially in the stress and reward systems. Therefore, pain and stress impact on dopaminergic mesolimbic pathway can lead to an increase in drug abuse liability. In this mini review we analyze the interaction between pain, stress, and alcohol addiction, and how dynamic changes in the kappa opioid system might play a crucial role in the development of compulsive alcohol drinking in chronic pain patients.

Chronic inflammatory pain alters alcohol-regulated frontocortical signaling and associations between alcohol drinking and thermal sensitivity

Alcohol use disorder (AUD) is a chronic, relapsing psychiatric disorder that is characterized by the emergence of negative affective states. The transition from recreational, limited intake to uncontrolled, escalated intake is proposed to involve a transition from positive to negative reinforcement mechanisms for seeking alcohol. Past work has identified the emergence of significant hyperalgesia/allodynia in alcohol-dependent animals, which may serve as a key negative reinforcement mechanism. Chronic pain has been associated with enhanced extracellular signal-regulated kinase (ERK) activity in cortical and subcortical nociceptive areas. Additionally, both pain and AUD have been associated with increased activity of the glucocorticoid receptor (GR), a key mediator of stress responsiveness. The objectives of the current study were to first determine relationships between thermal nociceptive sensitivity and alcohol drinking in male Wistar rats. While inflammatory pain induced by complete Freund's adjuvant (CFA) administration did not modify escalation of home cage drinking in animals over four weeks, the relationship between drinking levels and hyperalgesia symptoms reversed between acute (1 week) and chronic (3-4 week) periods post-CFA administration, suggesting that either the motivational or analgesic effects of alcohol may be altered over the time course of chronic pain. We next examined ERK and GR phosphorylation in pain-related brain areas (including the central amygdala and prefrontal cortex subregions) in animals experiencing acute withdrawal from binge alcohol administration (2 g/kg, 6 h withdrawal) and CFA administration (four weeks) to model the neurobiological consequences of binge alcohol exposure in the context of pain. We observed a significant interaction between alcohol and pain state, whereby alcohol withdrawal increased ERK phosphorylation across all four frontocortical areas examined, although this effect was absent in animals experiencing chronic inflammatory pain. Alcohol withdrawal also increased GR phosphorylation across all four frontocortical areas, but these changes were not altered by CFA. Interestingly, we observed significant inter-brain regional correlations in GR phosphorylation between the insula and other regions investigated only in animals exposed to both alcohol and CFA, suggesting coordinated activity in insula circuitry and glucocorticoid signaling in this context. The results of these studies provide a greater understanding of the neurobiology of AUD and will contribute to the development of effective treatment strategies for comorbid AUD and pain.

Metabolomic analysis of animal models of depression

BACKGROUND

Our understanding of the molecular mechanisms of depression remains largely unclear. Previous studies have shown that the prefrontal cortex (PFC) is among most important brain regions that exhibits metabolic changes in depression. A comprehensive analysis based on candidate metabolites in the PFC of animal models of depression will provide valuable information for understanding the pathogenic mechanism underlying depression.

METHODS

Candidate metabolites that are potentially involved in the metabolic changes of the PFC in animal models of depression were retrieved from the Metabolite Network of Depression Database. The significantly altered metabolic pathways were revealed by canonical pathway analysis, and the relationships among altered pathways were explored by pathway crosstalk analysis. Additionally, drug-associated pathways were investigated using drug-associated metabolite set enrichment analysis. The interrelationships among metabolites, proteins, and other molecules were analyzed by molecular network analysis.

RESULTS

Among 88 candidate metabolites, 87 altered canonical pathways were identified, and the top five ranked pathways were tRNA charging, the endocannabinoid neuronal synapse pathway, (S)-reticuline biosynthesis II, catecholamine biosynthesis, and GABA receptor signaling. Pathway crosstalk analysis revealed that these altered pathways were grouped into three interlinked modules involved in amino acid metabolism, nervous system signaling/neurotransmitters, and nucleotide metabolism. In the drug-associated metabolite set enrichment analysis, the main enriched drug pathways were opioid-related and antibiotic-related action pathways. Furthermore, the most significantly altered molecular network was involved in amino acid metabolism, molecular transport, and small molecule biochemistry.

CONCLUSIONS

This study provides important clues for the metabolic characteristics of the PFC in depression.

Neuroinflammation, Pain and Depression: An Overview of the Main Findings

Chronic pain is a serious public health problem with a strong affective-motivational component that makes it difficult to treat. Most patients with chronic pain suffer from severe depression; hence, both conditions coexist and exacerbate one another. Brain inflammatory mediators are critical for maintaining depression-pain syndrome and could be substrates for it. The goal of our paper was to review clinical and preclinical findings to identify the neuroinflammatory profile associated with the cooccurrence of pain and depression. In addition, we aimed to explore the regulatory effect of neuronal reorganization on the inflammatory response in pain and depression. We conducted a quantitative review supplemented by manual screening. Our results revealed inflammatory signatures in different preclinical models and clinical articles regarding depression-pain syndrome. We also identified that improvements in depressive symptoms and amelioration of pain can be modulated through direct targeting of inflammatory mediators, such as cytokines and molecular inhibitors of the inflammatory cascade. Additionally, therapeutic targets that improve and regulate the synaptic environment and its neurotransmitters may act as anti-inflammatory compounds, reducing local damage-associated molecular patterns and inhibiting the activation of immune and glial cells. Taken together, our data will help to better elucidate the neuroinflammatory profile in pain and depression and may help to identify pharmacological targets for effective management of depression-pain syndrome.

From Pleasure to Pain, and Back Again: The Intricate Relationship Between Alcohol and Nociception

AIMS

A close and bidirectional relationship between alcohol consumption and pain has been previously reported and discussed in influential reviews. The goal of the present narrative review is to provide an update on the developments in this field in order to guide future research objectives.

METHODS

We evaluated both epidemiological and neurobiological literature interrogating the relationship between alcohol use and pain for the presence of significant effects. We outlined studies on interactions between alcohol use and pain using both self-reports and objective experimental measures and discussed potential underlying mechanisms of these interactions.

RESULTS

Epidemiological, preclinical and clinical literature point to three major interactions between alcohol use and pain: (a) alcohol use leading to hyperalgesia, (b) alcohol use moderating pain and hyperalgesia and (c) chronic pain as a risk factor predisposing to alcohol relapse. Neurobiological studies using animal models to assess these interactions have transitioned from mostly involuntary modes of experimenter-controlled alcohol administration to self-administration procedures, and increasingly indicate that neuronal circuits implicated in both withdrawal and anticipation stages of alcohol use disorder also have a role in chronic pain. Mechanistically, alterations in GABA, glutamate, the corticotropin-releasing factor system, endogenous opioids and protein kinase C appear to play crucial roles in this maladaptive overlap.

CONCLUSIONS

Many of the principles explaining the interactions between alcohol and pain remain on a strong foundation, but continuing progress in modeling these interactions and underlying systems will provide a clearer basis for understanding, and ultimately treating, the damaging aspects of this interaction.

Alcohol and Pain: A Translational Review of Preclinical and Clinical Findings to Inform Future Treatment Strategies

Alcohol use disorder (AUD) and chronic pain are enduring and devastating conditions that share an intersecting epidemiology and neurobiology. Chronic alcohol use itself can produce a characteristic painful neuropathy, while the regular analgesic use of alcohol in the context of nociceptive sensitization and heightened affective pain sensitivity may promote negative reinforcement mechanisms that underlie AUD maintenance and progression. The goal of this review was to provide a broad translational framework that communicates research findings spanning preclinical and clinical studies, including a review of genetic, molecular, behavioral, and social mechanisms that facilitate interactions between persistent pain and alcohol use. We also consider recent evidence that will shape future investigations into novel treatment mechanisms for pain in individuals suffering from AUD.

Reduced sensitivity to ethanol and excessive drinking in a mouse model of neuropathic pain

The co-occurrence of chronic pain and alcohol use disorders (AUDs) involves complex interactions between genetic and neurophysiological aspects, and the research has reported mixed findings when they both co-occur. There is also an indication of a gender-dependent effect; males are more likely to use alcohol to cope with chronic pain problems than females. Recently, a new conceptualization has emerged, proposing that the negative affective component of pain drives and maintains alcohol-related behaviors. We studied in a longitudinal fashion alterations in alcohol drinking patterns and pain thresholds in a mouse model of chronic neuropathic pain in a sex-dependent manner. Following partial denervation (spared nerve injury [SNI]), stimulus-evoked pain responses were measured before chronic alcohol consumption, during drinking, during a deprivation phase, and following an episode of excessive drinking. During the course of alcohol drinking, we observed pronounced sex differences in pain thresholds. Male mice showed a strong increase in pain thresholds, suggesting an analgesic effect induced by alcohol over time, an effect that was not observed in female mice. SNI mice did not differ from sham-operated controls in baseline alcohol consumption. However, following a deprivation phase and the reintroduction of ethanol, male SNI mice but not female mice showed more pronounced excessive drinking than controls. Finally, we observed decreased central ethanol sensitivity in male SNI mice but not in females. Together with our finding, that ethanol is able to decrease a pain-induced negative affective memory we come to following conclusion. We propose that a lower sensitivity to the intoxicating effects of alcohol together with the ability of alcohol to reduce the negative affective component of pain may explain the higher co-occurrence of AUD in male chronic pain patients.

The molecular neurobiology of chronic pain-induced depression

The increasing number of individuals with comorbidities poses an urgent need to improve the management of patients with multiple co-existing diseases. Among these comorbidities, chronic pain and mood disorders, two long-lasting disabling conditions that significantly reduce the quality of life, could be cited first. The recent development of animal models accelerated the studies focusing on the underlying mechanisms of the chronic pain and depression/anxiety comorbidity. This review provides an overview of clinical and pre-clinical studies performed over the past two decades addressing the molecular aspects of the comorbid relationship of chronic pain and depression. We thus focused on the studies that investigated the molecular characteristics of the comorbid relationship between chronic pain and mood disorders, especially major depressive disorders, from the genetic and epigenetic point of view to key neuromodulators which have been shown to play an important role in this comorbidity.

Influence of neuropathic pain on nicotinic acetylcholine receptor plasticity and behavioral responses to nicotine in rats

Tobacco smoking is particularly evident in individuals experiencing chronic pain. This complex relationship is poorly understood at both molecular and behavioral levels. Here, we describe experiments aimed at understanding whether a chronic pain state induces neuroadaptations into the brain or peripheral nerves that involve nicotinic acetylcholine receptors (nAChRs) and whether these neuroadaptations directly lead to increased vulnerability to nicotine addiction or to the development of coping strategies to relieve pain symptoms. We found that ligation of the rat L5 spinal nerve led to a dramatic downregulation in the mRNA expression levels of all nAChR subunits examined in dorsal root ganglia and a time-dependent downregulation of discrete subunits, particularly in the cingulate cortex and the amygdala. Spinal nerve ligation and sham-operated rats showed minor or no changes in patterns of acquisition and motivation for nicotine taking. Spinal nerve ligation rats also showed similar vulnerability to nicotine seeking as sham animals when reinstatement was induced by nicotine-associated cues, but failed to reinstate lever pressing when relapse was induced by nicotine priming. Spinal nerve ligation and sham rats were equally sensitive to nicotine-induced anxiety-like behavior and antinociception; however, nicotine produced a potent and long-lasting antiallodynic effect in spinal nerve ligation rats. These results demonstrate that chronic pain leads to plasticity of nAChRs that do not directly facilitate nicotine addictive behaviors. Instead, nicotine potently decreases allodynia, an effect that could lead to increased nicotine consumption in chronic pain subjects.

Mental nerve injury induces novelty seeking behaviour leading to increasing ethanol intake in Wistar rats

OBJECTIVE

Dental treatment and orofacial surgeries may induce chronic neuropathic orofacial pain (CNOP). This kind of pain affects adaptability to environmental changes in both model animals and humans. Part of the adaptation process depends on the ability to distinguish between familiar and novel stimuli. CNOP induces novelty seeking behaviour as a deficit in environmental adaptation. Alternatively, novelty seeking is a sign for susceptibility to the development of substance abuse. Evidence shows that CNOP leads to alcoholism in animal models. The behavioural relationship between CNOP, novelty seeking behaviour and substance abuse is unknown. In this article, we investigate if CNOP produces an increase in novelty seeking and leads to increasing ethanol intake.

DESIGN

Firstly, we used mental nerve injury as a neuropathic orofacial pain model to evaluate both thermal and mechanical allodynia. We used the novel recognition task to determine novelty seeking behaviour and the drink in darkness protocol to assess ethanol intake.

RESULTS

Our results show that mental nerve constriction increases novelty seeking behaviour (p = 0.01) and correlates with ethanol binge consumption (r² = 0.68, p = 0.0008).

CONCLUSIONS

The present study demonstrates, for the first time, that trigeminal nerve injury, which induces CNOP, is enough to provide novelty seeking behaviour and lead to increasing ethanol intake. The increase of novelty seeking behaviour can serve as a predictor of risk of developing substance abuse. The treatment of CNOP involves a high risk of producing addiction. The level of novelty seeking evaluation in patients with neuropathic pain before treatment is critical.

Role of the Prefrontal Cortex in Pain Processing

The prefrontal cortex (PFC) is not only important in executive functions, but also pain processing. The latter is dependent on its connections to other areas of the cerebral neocortex, hippocampus, periaqueductal gray (PAG), thalamus, amygdala, and basal nuclei. Changes in neurotransmitters, gene expression, glial cells, and neuroinflammation occur in the PFC during acute and chronic pain, that result in alterations to its structure, activity, and connectivity. The medial PFC (mPFC) could serve dual, opposing roles in pain: (1) it mediates antinociceptive effects, due to its connections with other cortical areas, and as the main source of cortical afferents to the PAG for modulation of pain. This is a ‘loop’ where, on one side, a sensory stimulus is transformed into a perceptual signal through high brain processing activity, and perceptual activity is then utilized to control the flow of afferent sensory stimuli at their entrance (dorsal horn) to the CNS. (2) It could induce pain chronification via its corticostriatal projection, possibly depending on the level of dopamine receptor activation (or lack of) in the ventral tegmental area-nucleus accumbens reward pathway. The PFC is involved in biopsychosocial pain management. This includes repetitive transcranial magnetic stimulation, transcranial direct current stimulation, antidepressants, acupuncture, cognitive behavioral therapy, mindfulness, music, exercise, partner support, empathy, meditation, and prayer. Studies demonstrate the role of the PFC during placebo analgesia, and in establishing links between pain and depression, anxiety, and loss of cognition. In particular, losses in PFC grey matter are often reversible after successful treatment of chronic pain.

Altered Brain Excitability and Increased Anxiety in Mice With Experimental Colitis: Consideration of Hyperalgesia and Sex Differences

Crohn’s disease (CD) and ulcerative colitis (UC) are incurable lifelong inflammatory bowel diseases (IBD) with a rising worldwide incidence. IBD is characterized by diarrhea, rectal bleeding, severe cramping and weight loss. However, there is a growing evidence that IBD is also associated with anxiety- and depression-related disorders, which further increase the societal burden of these diseases. Given the limited knowledge of central nervous system (CNS) changes in IBD, we investigated CNS-related comorbidities in a mouse model of experimental colitis induced by dextran sulfate sodium (DSS) administration in drinking water for 5 days. In male and female C57BL6J mice, DSS treatment caused increased brain excitability, revealed by a decrease in seizure onset times after intraperitoneal administration of kainic acid. Moreover, both sexes showed increased anxiety-related behavior in the elevated plus-maze (EPM) and open field (OF) paradigms. We assessed somatic pain levels, because they may influence behavioral responses. Only male mice were hyperalgesic when tested with calibrated von Frey hairs and on the hotplate for mechanical and thermal pain sensitivity respectively. Administration of diazepam (DZP; ip, 1 mg/kg) 30 min before EPM rescued the anxious phenotype and improved locomotion, even though it significantly increased thermal sensitivity in both sexes. This indicates that the altered behavioral response is unlikely attributable to an interference with movement due to somatic pain in females. We show that experimental colitis increases CNS excitability in response to administration of kainic acid, and increases anxiety-related behavior as revealed using the EPM and OF tests.

Dynorphinergic system alterations in the corticostriatal circuitry of neuropathic mice support its role in the negative affective component of pain

The dynorphinergic system is involved in pain transmission at spinal level, where dynorphin exerts antinociceptive or pronociceptive effects, based on its opioid or non‐opioid actions. Surprisingly, little evidence is currently available concerning the supraspinal role of the dynorphinergic system in pain conditions. The present study aimed to investigate whether neuropathic pain is accompanied by prodynorphin (Pdyn) and κ‐opioid receptor (Oprk1) gene expression alterations in selected mouse brain areas. To this end, mice were subjected to chronic constriction injury of the right sciatic nerve and neuropathic pain behavioral signs were ascertained after 14 days. At this interval, a marked increase in Pdyn mRNA in the anterior cingulate cortex (ACC) and prefrontal cortex (PFC) was observed. Oprk1 gene expression was increased in the PFC, and decreased in the ACC and nucleus accumbens (NAc). No changes were observed in the other investigated regions. Because of the relationship between dynorphin and the brain‐derived neurotrophic factor, and the role of this neurotrophin in chronic pain‐related neuroplasticity, we investigated brain‐derived neurotrophic factor gene (Bdnf) expression in the areas showing Pdyn or Oprk1 mRNAs changes. Bdnf mRNA levels were increased in both the ACC and PFC, whereas no changes were assessed in the NAc. Present data indicate that the dynorphinergic system undergoes quite selective alterations involving the corticostriatal circuitry during neuropathic pain, suggesting a contribution to the negative affective component of pain. Moreover, parallel increases in Pdyn and Bdnf mRNA at cortical level suggest the occurrence of likely interactions between these systems in neuropathic pain maladaptive neuroplasticity.

A mouse model for chronic pain-induced increase in ethanol consumption

Chronic pain conditions are often comorbid with alcohol abuse. "Self-medication" with alcohol introduces a host of problems associated with the abuse of alcohol which over time has the potential of exacerbating the painful condition. Despite the prevalence of chronic pain being associated with alcohol abuse, rodent models which mimic the comorbid conditions are lacking. In this study, we model osteoarthritis (OA) in C57BL/6J mice by surgically destabilizing the medial meniscus (DMM). Sham-operated mice served as controls. Thirteen weeks after surgery, DMM but not sham-operated mice exhibited pronounced incapacitance of the surgically manipulated hind limb compared with the nonsurgically manipulated hind limb. At this time, the mice were exposed to the 2-bottle ethanol choice, beginning with 2.5% with a gradual increasing to 20%. Compared with sham controls, DMM mice consumed more EtOH and preferred EtOH over water at the 20% EtOH concentration. Histological analysis verified that the DMM mice exhibited significant damage to the articular cartilage and osteophyte growth compared with sham controls and these measures of the severity of OA correlated with the amount of ethanol intake. Thus, the combination of the DMM model of OA with the enhanced two-bottle ethanol choice is a potential preclinical approach in mice by which the basis of the comorbid association of alcohol abuse and chronic pain conditions can be explored.