Chronic Pain States Research Articles

The effects of chronic neuropathic pain states on the discriminative stimulus effect of fentanyl and other MOR agonists.

Pleasant subjective effects of drugs (e.g., euphoria) have been demonstrated to contribute to their abuse potential. In humans, there is some evidence that acute pain states may decrease the positive subjective effects of opioids; however, no studies have directly tested the impact of a long-lasting pain state. Therefore, the goal of this study was to directly evaluate the discriminative stimulus of mu opioid receptor (MOR) agonist, fentanyl, or the non-opioid drug of abuse, cocaine, in the presence or absence of spared nerve injury (SNI) induced chronic neuropathic pain. Prior to surgery, MOR agonists (fentanyl, morphine, nalbuphine) dose-dependently increased % fentanyl-like responding, as expected; surprisingly, after surgery, we saw small, significant rightward shifts in the fentanyl and morphine dose response curves in both sham and SNI groups suggesting that the observed shifts were not due to chronic pain. In both sham and SNI groups, there was an increase in the generalization of nalbuphine to the fentanyl-discriminative stimulus. There was no change in the discriminative stimulus of cocaine (or amphetamine substitutions) over 4 months of SNI-induced chronic neuropathic pain or sham states, suggesting that the SNI model failed to alter the discriminative stimuli of fentanyl and cocaine. Following induction of chronic neuropathic pain, there was an observed increase in quinpirole-induced generalization to the cocaine discriminative stimulus. In the future, studies should directly examine the abuse potential of low efficacy MOR agonists and dopaminergic agonists in the presence and absence of chronic pain states.

Interactions of pain and opioids on conditioned place preference in rodents.

Opioid analgesics are the most effective medications used for the treatment of pain, however there are significant risks associated with repeated opioid use including opioid misuse and opioid use disorder development. Chronic pain affects millions of adults in the United States, and opioid misuse is often comorbid with pain conditions in individuals who are repeatedly treated with opioids. In addition to providing pain relief, opioids produce rewarding effects, but in chronic pain states, reward processing can become dysregulated. The conditioned place preference task is commonly used to measure the rewarding properties of opioids in rodents. During this task, opioid administration is paired with a distinct environment through repeated conditioning and the change in an animal's preference for the paired environment indicates whether the opioid is rewarding or not. Rodent pain models can be combined with conditioned place preference to examine the effects of pain on opioid reward. The existing preclinical literature on pain effects on conditioned place preference is conflicting, where pain conditions have been reported to enhance, suppress, or have no effect on opioid reward. This review will discuss several factors that may contribute to these discordant findings including conditioning session duration and number, rodent strain differences in opioid sensitivity, analgesic properties of opioids at tested doses, locomotor effects at tested doses, and diurnal variation in pain sensitivity. Future studies should consider how these factors contribute to opioid conditioned place preference in both pain and pain-free animals to have a better understanding of the interactions between pain and opioid reward.

How timing and preexisting pain affect outcomes of TMR: a systematic review

Introduction: Targeted muscle reinnervation (TMR) is increasingly common in the care of major limb amputation to limit amputation-related pain. This review aims to elucidate how chronic pain states and length of delay prior to TMR affect its success and outcomes. Methods: Manuscripts were collected from three databases. Articles were first screened and excluded based on exclusion criteria. The remaining manuscripts were independently reviewed to determine inclusion. Article and patient demographics, as well as pain outcomes, were extracted. Data were analyzed based on pain condition, amputation vs. neuroma, and time from amputation/injury to surgery. Results: The literature search yielded 723 articles, with 41 meeting the inclusion criteria. Twenty-one articles included patients with residual limb pain (RLP) and phantom limb pain (PLP), including 14 on amputation and 6 on neuroma excision. Five articles included cancer-related amputation. Complex Regional Pain Syndrome (CRPS) was discussed in 3 articles, ischemia or infection in 2 articles, and neurofibromatosis 1 in 1 article. Twenty-two articles described TMR at the time of amputation. Conclusions: TMR is effective at preventing neuroma formation and limiting pain when performed at the time of amputation. Delayed patients had a greater improvement in RLP but less of an improvement in PLP, when assessed against immediate TMR patients who were compared to non-TMR standard amputees. In the presence of chronic pain states, such as CRPS, there is also improved analgesia. However, current clinical data are limited, indicating a need for further research into the use of TMR for chronic pain management.

A Computational Framework for Understanding the Impact of Prior Experiences on Pain Perception and Neuropathic Pain.

Pain perception is influenced not only by sensory input from afferent neurons but also by cognitive factors such as prior expectations. It has been suggested that overly precise priors may be a key contributing factor to chronic pain states such as neuropathic pain. However, it remains an open question how overly precise priors in favor of pain might arise. Here, we first verify that a Bayesian approach can describe how statistical integration of prior expectations and sensory input results in pain phenomena such as placebo hypoalgesia, nocebo hyperalgesia, chronic pain, and spontaneous neuropathic pain. Our results indicate that the value of the prior, which is determined by the internal model parameters, may be a key contributor to these phenomena. Next, we apply a hierarchical Bayesian approach to update the parameters of the internal model based on the difference between the predicted and the perceived pain, to reflect that people integrate prior experiences in their future expectations. In contrast with simpler approaches, this hierarchical model structure is able to show for placebo hypoalgesia and nocebo hyperalgesia how these phenomena can arise from prior experiences in the form of a classical conditioning procedure. We also demonstrate the phenomenon of offset analgesia, in which a disproportionally large pain decrease is obtained following a minor reduction in noxious stimulus intensity. Finally, we turn to simulations of neuropathic pain, where our hierarchical model corroborates that persistent non-neuropathic pain is a risk factor for developing neuropathic pain following denervation, and additionally offers an interesting prediction that complete absence of informative painful experiences could be a similar risk factor. Taken together, these results provide insight to how prior experiences may contribute to pain perception, in both experimental and neuropathic pain, which in turn might be informative for improving strategies of pain prevention and relief.

The Mechanism and Inflammatory Markers Involved in the Potential Use of N-acetylcysteine in Chronic Pain Management.

N-acetylcysteine (NAC) has established use as an antidote for acetaminophen overdose and treatment for pulmonary conditions and nephropathy. It plays a role in regulating oxidative stress and interacting with various cytokines including IL-1β, TNFα, IL-8, IL-6, IL-10, and NF-κB p65. The overexpression of reactive oxygen species (ROS) is believed to contribute to chronic pain states by inducing inflammation and accelerating disease progression, favoring pain persistence in neuropathic and musculoskeletal pain conditions. Through a comprehensive review, we aim to explore the mechanisms and inflammatory pathways through which NAC may manage neuropathic and musculoskeletal pain. Evidence suggests NAC can attenuate neuropathic and musculoskeletal pain through mechanisms such as inhibiting matrix metalloproteinases (MMPs), reducing reactive oxygen species (ROS), and enhancing glutamate transport. Additionally, NAC may synergize with opioids and other pain medications, potentially reducing opioid consumption and enhancing overall pain management. Further research is needed to fully elucidate its therapeutic potential and optimize its use in pain management. As an adjuvant therapy, NAC shows potential for chronic pain management, offering significant benefits for public health.

A Missing Puzzle in Preclinical Studies-Are CCR2, CCR5, and Their Ligands' Roles Similar in Obesity-Induced Hypersensitivity and Diabetic Neuropathy?-Evidence from Rodent Models and Clinical Studies.

Research has shown that obesity is a low-grade inflammatory disease that is often associated with comorbidities, such as diabetes and chronic pain. Recent data have indicated that chemokines may play a role in these conditions due to their pronociceptive and chemotactic properties, which promote hypersensitivity and inflammation. Accumulating evidence suggests that CCR2, CCR5, and their ligands (CCL2, CCL3, CCL4, CCL5, CCL7, CCL8, CCL11 CCL12, and/or CCL13) play a role in rodent models of pain and obesity, as well as in patients with diabetes and obesity. It was proven that the blockade of CCR2 and CCR5, including the simultaneous blockade of both receptors by dual antagonists, effectively reduces hypersensitivity to thermal and mechanical stimuli in chronic pain states, including diabetic neuropathy. The present review discusses these chemokine receptors and the role of their ligands in diabetes and obesity, as well as their involvement in diabetic neuropathy and obesity-induced hypersensitivity.

Sex differences in the orofacial antinociceptive effect of metformin and the role of transient receptor potential channels.

Metformin is classified as a biguanide and is used in the treatment of type 2 diabetes. It is used worldwide and has been investigated in drug repositioning. The present study aims to investigate whether there is sexual dimorphism in the orofacial antinociceptive effect of metformin and the participation of TRP channels. Acute nociceptive behavior was induced by administering cinnamaldehyde or capsaicin to the upper lip. Nociceptive behavior was assessed through orofacial rubbing, and the effects of pre-treatment with metformin (125 or 250mg/Kg) or vehicle (control) were tested on the behavior. Nociceptive behavior was also induced by formalin injected into the temporomandibular joint. The chronic pain model involved infraorbital nerve transection (IONX) was evaluated using Von Frey electronic filaments. Trpv1 gene expression was analyzed in the nerve ganglion. Docking experiments were performed. Metformin, but not the vehicle, produced antinociception (p < 0.0001) in all acute nociceptive behaviors in both sexes, and these effects were attenuated by the TRPV1 antagonist capsazepine and the TRPA1 antagonist HC-030031. In IONX with better (**p < 0.01, ****p < 0.0001 vs. control) results in females. TRPV1 gene expression was observed in the metformin treated group (*p < 0.05 vs. control). Docking experiments revealed that metformin may interact with TRPV1 and TRPA1 channels. Metformin promotes orofacial antinociception in both sexes in acute pain and is more effective in chronic pain in females than in males, through the modulation of TRPV1 and TRPA1 channels. These preclinical findings suggest a potential repositioning of metformin as an analgesic agent in acute and chronic orofacial pain states.

Prospective Study of Preoperative Negative Affect and Postoperative Pain in Patients Undergoing Thoracic Surgery: The Moderating Role of Sex.

Objective: Preoperative negative affect is a risk factor for worse postoperative pain, but research investigating this association among patients undergoing thoracic surgery is inconsistent. Additionally, female patients often report greater negative affect and postoperative pain than males. This prospective observational study investigated the association between preoperative negative affect and postoperative pain after thoracic surgery and whether this association differed by sex. Methods: Patients (n = 105) undergoing thoracic surgery completed preoperative assessments of pain and negative affect (PROMIS anxiety and depression short forms). Patients reported their daily worst pain over the first 7 postoperative days, and an index score of acute postoperative pain was created. Six months after surgery, a subsample of patients (n = 60) reported their worst pain. Results: Higher levels of preoperative anxiety (r = 0.25, p = 0.011) and depression (r = 0.20, p = 0.042) were associated with greater acute postoperative pain, but preoperative negative affect was not related to chronic postsurgical pain (anxiety: r = 0.19, p = 0.16; depression: r = -0.01, p = 0.94). Moderation analyses revealed that the associations between both preoperative anxiety (b = 0.12, 95% CI [0.04, 0.21], p = 0.004) and depression (b = 0.15, 95% CI [0.04, 0.26], p = 0.008) with acute postoperative pain were stronger among females than males. Similarly, the association between preoperative anxiety and chronic postsurgical pain was stronger among females (b = 0.11, 95% CI [0.02, 0.20], p = 0.022), but the association between preoperative depression and chronic pain did not differ based on sex (b = 0.13, 95% CI [-0.07, 0.34], p = 0.201]). Conclusions: Our findings suggest that negative affect may be especially important to the experience of pain following thoracic surgery among female patients, whose degree of preoperative anxiety may indicate vulnerability to progress to a chronic pain state. Preoperative interventions aimed at reducing negative affect and pain may be particularly useful among females with high negative affect before thoracic surgery.

Cryoablation in Chronic Pain Management: A Narrative Review

Abstract Cryoneurolysis is a technique of applying extreme cold temperatures to specific sensory nerves, causing a prolonged period of loss of conduction through that nerve. This technique has found widespread application in the field of interventional pain medicine in a variety of conditions including chronic postsurgical pain, phantom limb pain (PLP), and cancer pain. Its use in chronic pain states has been aided further by the advent of handheld cryoneurolysis devices and the widespread use of ultrasonographic imaging to guide and target nerves accurately. Chronic pain states – chronic knee pain from osteoarthritis, refractory PLP, temporomandibular disorders, cervicogenic headaches, low back pain, different peripheral neuropathies, and metastatic bone tumor pain – are some of the conditions where cryoneurolysis has been proven to benefit patients in cases where the symptoms were refractory to other conventionally used modalities. Chronic pain is a complex and multifaceted condition and hence requires a variety of individualized treatment approaches. Current research on this treatment method for chronic pain indicates that it carries low risk and, when used correctly, offers extended pain relief without causing motor function loss. This narrative review includes all types of scientific studies, published between 2010 and 2023, which were selected using a specific set of keywords through two different literature search engines. In conclusion, it explains how cryoneurolysis works to provide long-term pain relief and summarizes the latest evidence supporting its use in chronic pain management.

Randomized, placebo-controlled study on the effects of intravenous GSK3858279 (anti-CCL17) on a battery of evoked pain tests in healthy participants.

C-C Motif Chemokine Ligand 17 (CCL17) is a chemokine that binds and signals through the G-protein coupled CC-chemokine receptor 4 and has been implicated in the development of inflammatory and arthritic pain. GSK3858279 is a high-affinity, first-in-class, monoclonal antibody, binding specifically to CCL17 and inhibiting downstream signaling. In this phase I, randomized, single-center, double-blind, placebo-controlled, three-period, incomplete-block crossover study (NCT04114656), the analgesic effects and safety of intravenous GSK3858279 were assessed in a battery of evoked acute pain assessments on healthy, adult (aged ≥18 years), male participants. Participants were randomized 1:1 to receive either one placebo (0.9% w/v NaCl) dose followed by two GSK3858279 doses (PAA treatment sequence), or one GSK3858279 dose followed by two placebo doses (APP treatment sequence). The co-primary end points were ultraviolet B heat pain detection threshold (°C), cold pressor time to pain tolerance threshold (PTT, sec), and electrical PTT (mA, single stimulus). Twenty-one participants were enrolled (PAA = 11; APP = 10). Mean age (standard deviation) was 29.3 (7.9) years for PAA, 31.1 (7.7) years for APP. No significant differences were observed in the analgesic effect between GSK3858279 and placebo for any end point. Exposure to GSK3858279 was similar between Period 1 (APP sequence), and Periods 2 and 3 (PAA sequence), with some GSK3858279 carry-over. Changes in serum CCL17 levels were consistent with the expected GSK3858279 activity. All drug-related adverse events were mild in intensity and caused no discontinuations. The absence of an efficacy signal in this acute pain model does not preclude efficacy in chronic pain states.

Evolving role of immunology in chronic pain medicine: tissue necrosis factor and interleukin modulatory treatments

Our immune system acts to protect us in times of stress and traumatic injury. As part of the immune response, the body produces various cytokines, which mediate or modulate immune functions. Such cytokines include tumor necrosis factor (TNF) and interleukin 6 (IL-6) and IL-17. These cytokines can also act on the nervous system to influence pain perception. TNF-α triggers an inflammatory response and two forms of programmed cell death, apoptosis and necroptosis, depending on the pathological state. For individuals with chronic conditions relating to immune deficiency, the actions of these cytokines can present as chronic pain states, significantly altering quality of life. One attractive potential solution for treating this immune linked pain is by altering signaling pathways of pain-enhancing cytokines. Infliximab and etanercept are TNF inhibitors that are currently on the market for use in the treatment of chronic pain. Secukinumab and tocilizumab serve as IL inhibitors, utilized for a similar purpose. These novel immunotherapies have shown efficacy in numerous clinical studies with acceptable side effect profiles. In this review, we summarize the pharmacological profiles of these drugs and discuss their usage in treating chronic pain.

Nucleus accumbens myocyte enhancer factor 2C mediates the maintenance of peripheral nerve injury-induced physiological and behavioral maladaptations.

Preclinical and clinical work has demonstrated altered plasticity and activity in the nucleus accumbens (NAc) under chronic pain states, highlighting critical therapeutic avenues for the management of chronic pain conditions. In this study, we demonstrate that myocyte enhancer factor 2C (MEF2C), a master regulator of neuronal activity and plasticity, is repressed in NAc neurons after prolonged spared nerve injury (SNI). Viral-mediated overexpression of Mef2c in NAc neurons partially ameliorated sensory hypersensitivity and emotional behaviors in mice with SNI, while also altering transcriptional pathways associated with synaptic signaling. Mef2c overexpression also reversed SNI-induced potentiation of phasic dopamine release and neuronal hyperexcitability in the NAc. Transcriptional changes induced by Mef2c overexpression were different than those observed after desipramine treatment, suggesting a mechanism of action different from antidepressants. Overall, we show that interventions in MEF2C-regulated mechanisms in the NAc are sufficient to disrupt the maintenance of chronic pain states, providing potential new treatment avenues for neuropathic pain.

Peripheral Nerve Stimulation in Postoperative Analgesia: A Narrative Review.

Recent research has shown the effectiveness of peripheral nerve stimulators (PNS) in managing chronic pain conditions. Ongoing studies aim to explore its potential application in treating acute postoperative pain states. The purpose of this systematic review is to assess the role of PNS in providing relief for postoperative pain. Clinical studies investigating the use of peripheral nerve stimulators (PNS) for analgesia following various surgeries, such as total knee arthroplasty, anterior cruciate ligament repair, ankle arthroplasty, rotator cuff repair, hallux valgus correction, and extremity amputation, have shown promising results. Lead placement locations include the brachial plexus, sciatic, femoral, tibial, genicular, perineal, sural, radial, median, and ulnar nerves. These studies consistently report clinically significant reductions in pain scores, and some even indicate a decrease in opioid consumption following PNS for postoperative pain. PNS involves the subcutaneous placement of electrode leads to target peripheral nerve(s) followed by delivery of an electric current via an external pulse generator. While the precise mechanism is not fully understood, the theory posits that PNS modulates electrical stimulation, hindering the signaling of nociceptive pain. PNS presents itself as an alternative to opioid therapy, holding promise to address the opioid epidemic by offering a nonpharmacologic approach for both acute and chronic pain states.

Probing white matter microstructure in youth with chronic pain and its relation to catastrophizing using neurite orientation dispersion and density imaging.

Chronic pain is common in young people and can have a major life impact. Despite the burden of chronic pain, mechanisms underlying chronic pain development and persistence are still poorly understood. Specifically, white matter (WM) connectivity has remained largely unexplored in pediatric chronic pain. Using diffusion-weighted imaging, this study examined WM microstructure in adolescents (age M = 15.8 years, SD = 2.8 years) with chronic pain (n = 44) compared with healthy controls (n = 24). Neurite orientation dispersion and density imaging modeling was applied, and voxel-based whole-white-matter analyses were used to obtain an overview of potential alterations in youth with chronic pain and tract-specific profile analyses to evaluate microstructural profiles of tracts of interest more closely. Our main findings are that (1) youth with chronic pain showed widespread elevated orientation dispersion compared with controls in several tracts, indicative of less coherence; (2) signs of neurite density tract-profile alterations were observed in several tracts of interest, with mainly higher density levels in patients; and (3) several WM microstructural alterations were associated with pain catastrophizing in the patient group. Implicated tracts include both those connecting cortical and limbic structures (uncinate fasciculus, cingulum, anterior thalamic radiation), which were associated with pain catastrophizing, as well as sensorimotor tracts (corticospinal tract). By identifying alterations in the biologically informative WM microstructural metrics orientation dispersion and neurite density, our findings provide important and novel mechanistic insights for understanding the pathophysiology underlying chronic pain. Taken together, the data support alterations in fiber organization as a meaningful characteristic, contributing process to the chronic pain state.

Opiate-reduction protocol for common outpatient spinal procedures: a long-term feasibility study and single-center experience.

The opioid epidemic continues to be at the forefront of public health. As a response to this crisis, many statewide and national medical groups have sought to develop opioid-prescribing guidelines for both acute and chronic pain states. Given the lack of evidence in the neurosurgical landscape, the authors' institution implemented opioid-prescribing guidelines for common outpatient spinal procedures in 2017, subsequently demonstrating a significant reduction in the narcotics prescribed. However, the ability to maintain the results garnered from such guidelines long term has not been described. The objective of this study was to evaluate postoperative opioid utilization at a high-volume quaternary referral center 5 years after the initial implementation of an opioid-reduction protocol for common outpatient spinal procedures. From the electronic medical records, authors collected data on the number of tablets and total morphine equivalent dose (MED) prescribed, acute postoperative readmissions for pain concerns, refill requests, and conversion to long-term opiate use in the 5 years following implementation of an opioid-reduction protocol for common outpatient spinal procedures. These procedures, undertaken in opiate-naive patients, included 1- or 2-level anterior cervical discectomy and fusion, 1- or 2-level cervical disc replacement, and 1- or 2-level laminectomy, laminotomy, or foraminotomy (cervical or lumbar). The total quantity of narcotics was reduced by 37.0 tablets per patient after protocol implementation and over the 5-year period thereafter. Generally, patients were discharged with an average of 23.3 tablets, concurrent with the initial goal of 24 tablets, set forth in 2017. These results confirm an ongoing reduction in opiate quantities prescribed and overall morphine equivalent totals at the time of discharge over the course of 5 years after initial protocol implementation. A standardized discharge protocol for postoperative outpatient spinal procedures can lead to long-term reductions in opioid discharge quantity, without compromising patient safety or increasing the utilization of hospital resources through readmissions, refill requests, or clinic phone calls. This study provides an example of a feasible and effective discharge prescription regimen that may be generalizable to common outpatient neurosurgical procedures with long-term evidence that a small intervention can lead to ongoing reduced quantities of postoperative opioids at the time of discharge.

Does pain tolerance mediate the effect of physical activity on chronic pain in the general population? The Tromsø Study.

Knowledge is needed regarding mechanisms acting between physical activity (PA) and chronic pain. We investigated whether cold pain tolerance mediates an effect of leisure-time physical activity on the risk of chronic pain 7 to 8 years later using consecutive surveys of the population-based Tromsø Study. We included participants with information on baseline leisure-time PA (LTPA) and the level of cold pressor-assessed cold pain tolerance, who reported chronic pain status at follow-up as any of the following: chronic pain for ≥3 months, widespread chronic pain, moderate-to-severe chronic pain, or widespread moderate-to-severe chronic pain. We included 6834 participants (52% women; mean age, 55 years) in counterfactual mediation analyses. Prevalence decreased with severity, for example, 60% for chronic pain vs 5% for widespread moderate-to-severe chronic pain. People with one level higher LTPA rating (light to moderate or moderate to vigorous) at baseline had lower relative risk (RR) of 4 chronic pain states 7 to 8 years later. Total RR effect of a 1-level LTPA increase was 0.95 (0.91-1.00), that is, -5% decreased risk. Total effect RR for widespread chronic pain was 0.84 (0.73-0.97). Indirect effect for moderate-to-severe chronic pain was statistically significant at RR 0.993 (0.988-0.999); total effect RR was 0.91 (0.83-0.98). Statistically significantly mediated RR for widespread moderate-to-severe chronic pain was 0.988 (0.977-0.999); total effect RR was 0.77 (0.64-0.94). This shows small mediation of the effect of LTPA through pain tolerance on 2 moderate-to-severe chronic pain types. This suggests pain tolerance to be one possible mechanism through which PA modifies the risk of moderate-to-severe chronic pain types with and without widespread pain.

Chronic Inflammatory Pain Alters Expression of Limbic MAPK Phosphatases.

Brain mechanisms involved in comorbidity between chronic pain conditions and clinical depression are still largely unknown. Our previous studies demonstrated that expression of mitogen-activated protein kinase phosphatase-1 (MKP-1) is both necessary and sufficient for the development of enhanced behavioral emotionality (i.e., depressive- like behaviors) in rodents. Here, we investigated the role of the dual specificity phosphatase (DUSP) gene family, specifically MKP-1, MKP-2 and MKP-3, in limbic brain areas involved in affective pain processing and stress responses. Male rats exposed to 21 days of peripheral inflammatory pain exhibited a robust increase in MKP-1 gene expression within the hippocampus, prefrontal cortex (PFC) and anterior cingulate cortex (ACC). Similar upregulation of hippocampal MKP-1 was also observed in female animals exposed to the same 21-day paradigm. However, the overall pattern of MKP-1 expression across various limbic areas differed in females exposed to chronic pain, as significant downregulation of MKP-1 was observed in the ACC, while no changes were detected within the PFC. Furthermore, similar limbic region-specific variances in pain-related dysregulation were also observed for MKP-2 and MKP-3. Finally, pain-induced upregulation of limbic MKP-1 was blocked by low-dose ketamine treatment (10 mg/kg) previously shown to produce rapid antidepressant effects in rodents. Overall, the results of this study suggest that chronic pain activates specific MKPs/DUSPs within limbic brain regions, which may underlie previously reported pain-related decreases in MAPK signaling. Thus, dysregulation of MKP-1 and other DUSP genes may play an important role in the development of mood disorders associated with chronic pain state.

The anterior cingulate cortex controls the hyperactivity in subthalamic neurons in male mice with comorbid chronic pain and depression.

Neurons in the subthalamic nucleus (STN) become hyperactive following nerve injury and promote pain-related responses in mice. Considering that the anterior cingulate cortex (ACC) is involved in pain and emotion processing and projects to the STN, we hypothesize that ACC neurons may contribute to hyperactivity in STN neurons in chronic pain. In the present study, we showed that ACC neurons enhanced activity in response to noxious stimuli and to alterations in emotional states and became hyperactive in chronic pain state established by spared nerve injury of the sciatic nerve (SNI) in mice. In naïve mice, STN neurons were activated by noxious stimuli, but not by alterations in emotional states. Pain responses in STN neurons were attenuated in both naïve and SNI mice when ACC neurons were inhibited. Furthermore, optogenetic activation of the ACC-STN pathway induced bilateral hyperalgesia and depression-like behaviors in naive mice; conversely, inhibition of this pathway is sufficient to attenuate hyperalgesia and depression-like behaviors in SNI mice and naïve mice subjected to stimulation of STN neurons. Finally, mitigation of pain-like and depression-like behaviors in SNI mice by inhibition of the ACC-STN projection was eliminated by activation of STN neurons. Our results demonstrate that hyperactivity in the ACC-STN pathway may be an important pathophysiology in comorbid chronic pain and depression. Thus, the ACC-STN pathway may be an intervention target for the treatment of the comorbid chronic pain and depression.

The Gut Microbiota and Chronic Pain.

To examine the effects and interactions between gut microbia and chronic pain. The gut microbiome has been an area of interest in both the scientific and general audience due to a growing body of evidence suggesting its influence in a variety of health and disease states. Communication between the central nervous system (CNS) and gut microbiome is said to be bidirectional, in what is referred to as the gut-brain axis. Chronic pain is a prevalent costly personal and public health burden and so, there is a vested interest in devising safe and efficacious treatments. Numerous studies, many of which are animal studies, have been conducted to examine the gut microbiome's role in the pathophysiology of chronic pain states, such as neuropathy, inflammation, visceral pain, etc. As the understanding of this relationship grows, so does the potential for therapeutic targeting of the gut microbiome in chronic pain.

Effects of (2R,6R)-hydroxynorketamine in assays of acute pain-stimulated and pain-depressed behaviors in mice.

Ketamine has been shown to produce analgesia in various acute and chronic pain states; however, abuse liability concerns have limited its utility. The ketamine metabolite (2R,6R)-hydroxynorketamine (HNK) has been shown to produce antidepressant-like effects similar to ketamine without abuse liability concerns. (2R,6R)-HNK produces sustained analgesia in models of chronic pain, but has yet to be evaluated in models of acute pain. The present study evaluated the efficacy of acute (2R,6R)-HNK administration (one injection) in assays of pain-stimulated (52- and 56-degree hot plate test and acetic acid writhing) and pain-depressed behavior (locomotor activity and rearing) in male and female C57BL/6 mice. In assays of pain-stimulated behaviors, (2R,6R)-HNK (1-32 mg/kg) failed to produce antinociception in the 52- and 56-degree hot plate and acetic acid writhing assays. In assays of pain-depressed behaviors, 0.56% acetic acid produced a robust depression of locomotor activity and rearing that was not blocked by pretreatment of (2R,6R)-HNK (3.2-32 mg/kg). The positive controls morphine (hot plate test) and ketoprofen (acetic acid writhing, locomotor activity, and rearing) blocked pain-stimulated and pain-depressed behaviors. Finally, the effects of intermittent (2R,6R)-HNK administration were evaluated in 52-degree hot plate and pain-depressed locomotor activity and rearing. Intermittent administration of (2R,6R)-HNK also did not produce antinociceptive effects in the hot plate or pain-depressed locomotor activity assays. These results suggest that (2R,6R)-HNK is unlikely to have efficacy in treating acute pain; however, the efficacy of (2R,6R)-HNK in chronic pain states should continue to be evaluated.