Striatal opioid receptor availability is related to acute and chronic pain perception in arthritis: does opioid adaptation increase resilience to chronic pain?

Self-regulated analgesia in males but not females is mediated by endogenous opioids

Converging lines of preclinical and clinical research indicate that females, in stark contrast to males, display an increased prevalence of chronic pain. Females also demonstrate weaker analgesic efficacy in response to opioid therapies when compared with males. These sex-specific differences may be driven by dimorphic endogenous opioidergic responses. In rodent models, analgesia exhibited in males but not females was reversed by inhibiting endogenous opioidergic reception. In humans, the sex-specific endogenous system(s) supporting the direct attenuation of evoked pain has not been identified. To determine whether opioidergic blockade reverses self-regulated analgesia in males as compared to females, the present study combined two operationally analogous clinical trials (n = 98; 51 females and 47 males). In a double-blinded, counterbalanced study involving healthy (n = 39) and chronic low back pain (n = 59) populations, a high-dose naloxone (μ-, κ-, δ-opioid antagonist) vs. placebo-saline cross-over design (15 mg/kg bolus +0.1 mg/kg/h) tested the hypothesis that endogenous opioids mediate analgesia in males but not females. An 11-point visual analog scale (VAS) (0 = no pain; 10 = worst pain imaginable) evaluated pain ratings in response to noxious heat stimulation (49 °C; calf). After baseline pain testing, participants were randomized to a validated four-session mindfulness meditation or sham mindfulness meditation training intervention. Participants practiced their respective meditation during noxious heat, intravenous high-dose naloxone, and placebo saline, respectively. In males and females, meditation significantly lowered evoked pain during saline infusion. Intravenous naloxone inhibited analgesia in males, but pain relief was well preserved in females. The present findings indicate that endogenous opioids mediate self-regulated analgesia in males but not females and underscore the need to establish sex-specific pain therapeutics.

Cerebral Mechanism of Tuina on the Descending Pain Inhibitory System in Knee Osteoarthritis: Protocol for a Randomized Controlled Trial

BACKGROUND

Knee osteoarthritis (KOA) is reputedly the most common musculoskeletal disease of the lower limbs and the main cause of pain and disability among older individuals. Pain is the most significant and widespread symptom of KOA. The descending pain inhibitory system has a cardinal role in normal pain consciousness, and its malfunction may be one of the pathophysiological mechanisms in KOA. Crucially, the rostral ventromedial medulla (RVM) and periaqueductal gray (PAG), as important components of the descending pain inhibitory system, directly modulate the activity of the spinal neurons involved in pain transmission. Tuina, a manual therapy, is effective and safe for reducing clinical symptoms of KOA; however, the mechanism that influences pain through the descending pain inhibitory system in KOA is unclear.

OBJECTIVE

This study aims to investigate the modulatory implications of Tuina on the RVM and PAG, which have critical roles in the descending pain inhibitory system in patients with KOA.

METHODS

This randomized controlled parallel trial will be conducted at the Tuina Clinic of the Third Affiliated Hospital of Henan University of Chinese Medicine (Zhengzhou, China). Patients with KOA will be randomly assigned (1:1) to 6 weeks of health education or Tuina. All patients in both groups will accept a resting-state functional magnetic resonance scan at the beginning and end of the experiment, and the resting-state functional connectivity and the voxel-based morphometry analysis will be performed to detect the RVM and PAG function and structure changes. The clinical outcome assessments will be (1) the pressure pain thresholds, (2) the Numerical Rating Scale, (3) the Hamilton Depression Scale (HAMD), and (4) the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC). Considering that this trial is a study of resting-state functional magnetic resonance imaging technology, resting-state functional connectivity and voxel-based morphometry are the primary outcomes, and clinical outcome assessments are secondary outcomes. Adverse events will be documented and assessed throughout. All main analyses will be carried out on the basis of the intention-to-treat principle. The outcome evaluators and data statisticians will be masked to the treatment group assignment to reduce the risk of bias.

RESULTS

This trial was approved by the ethics committee of the Third Affiliated Hospital of Henan University of Chinese Medicine. Enrollment began in December 2023, and the results of this trial are expected to be submitted for publication in May 2025.

CONCLUSIONS

This trial will identify a possible relationship between function and structure changes of RVM and PAG and the improvement of clinical variables, elucidating the effect of Tuina on the descending pain inhibitory system of patients with KOA. This trial will provide much-needed knowledge for Tuina for patients with KOA.

TRIAL REGISTRATION

Chinese Clinical Trial Registry ChiCTR2300070289; https://www.chictr.org.cn/showproj.html?proj=182570.

INTERNATIONAL REGISTERED REPORT IDENTIFIER (IRRID)

PRR1-10.2196/52820.

Right posterior insular epidural stimulation in rats with neuropathic pain induces a frequency-dependent and opioid system-mediated reduction of pain and its comorbid anxiety and depression

Neuropathic pain (NP) is a sensory, emotional, and persistent disturbing experience caused by a lesion or disease of the somatosensory system which can lead when chronic to comorbidities such as anxiety and depression. Available treatments (pharmacotherapy, neurostimulation) have partial and unpredictable response; therefore, it seems necessary to find a new therapeutical approach that could alleviate most related symptoms and improve patients 'emotional state'. Posterior Insula seems to be a potential target of neurostimulation for pain relief. However, its effects on pain-related anxiety and depression remain unknown. Using rats with spared nerve injury (SNI), this study aims to elucidate the correlation between NP and anxio-depressive disorders, evaluate potential analgesic, anxiolytic, and antidepressant effects of right posterior insula stimulation (IS) using low (LF-IS, 50 Hz) or high (HF-IS, 150 Hz) frequency and assess endogenous opioid involvement in these effects. Results showed positive correlation between NP, anxiety, and depression. LF-IS reversed anhedonia and despair-like behavior through pain alleviation, whereas HF-IS only reduced anhedonia, all effects involving endogenous opioids. These findings support the link between NP and anxio-depressive disorders. Moreover, IS appears to have analgesic, anxiolytic and antidepressant effects mediated by the endogenous opioid system, making it a promising target for neurostimulation.

The analgesic effects of botulinum neurotoxin by modulating pain-related receptors; A literature review

Botulinum neurotoxins (BoNTs), produced by Clostridium botulinum, have been used for the treatment of various central and peripheral neurological conditions. Recent studies have suggested that BoNTs may also have a beneficial effect on pain conditions. It has been hypothesized that one of the mechanisms underlying BoNTs' analgesic effects is the inhibition of pain-related receptors' transmission to the neuronal cell membrane. BoNT application disrupts the integration of synaptic vesicles with the cellular membrane, which is responsible for transporting various receptors, including pain receptors such as TRP channels, calcium channels, sodium channels, purinergic receptors, neurokinin-1 receptors, and glutamate receptors. BoNT also modulates the opioidergic system and the GABAergic system, both of which are involved in the pain process. Understanding the cellular and molecular mechanisms underlying these effects can provide valuable insights for the development of novel therapeutic approaches for pain management. This review aims to summarize the experimental evidence of the analgesic functions of BoNTs and discuss the cellular and molecular mechanisms by which they can act on pain conditions by inhibiting the transmission of pain-related receptors.

Psychological and neurological predictors of acupuncture effect in patients with chronic pain: a randomized controlled neuroimaging trial

ABSTRACT

Chronic pain has been one of the leading causes of disability. Acupuncture is globally used in chronic pain management. However, the efficacy of acupuncture treatment varies across patients. Identifying individual factors and developing approaches that predict medical benefits may promise important scientific and clinical applications. Here, we investigated the psychological and neurological factors collected before treatment that would determine acupuncture efficacy in knee osteoarthritis. In this neuroimaging-based randomized controlled trial, 52 patients completed a baseline assessment, 4-week acupuncture or sham-acupuncture treatment, and an assessment after treatment. The patients, magnetic resonance imaging operators, and outcome evaluators were blinded to treatment group assignment. First, we found that patients receiving acupuncture treatment showed larger pain intensity improvements compared with patients in the sham-acupuncture arm. Second, positive expectation, extraversion, and emotional attention were correlated with the magnitude of clinical improvements in the acupuncture group. Third, the identified neurological metrics encompassed striatal volumes, posterior cingulate cortex (PCC) cortical thickness, PCC/precuneus fractional amplitude of low-frequency fluctuation (fALFF), striatal fALFF, and graph-based small-worldness of the default mode network and striatum. Specifically, functional metrics predisposing patients to acupuncture improvement changed as a consequence of acupuncture treatment, whereas structural metrics remained stable. Furthermore, support vector machine models applied to the questionnaire and brain features could jointly predict acupuncture improvement with an accuracy of 81.48%. Besides, the correlations and models were not significant in the sham-acupuncture group. These results demonstrate the specific psychological, brain functional, and structural predictors of acupuncture improvement and may offer opportunities to aid clinical practices.

Pain-related opioidergic and dopaminergic neurotransmission: Dual meta-Analyses of PET radioligand studies

Molecular mechanisms of the interaction between opioidergic and dopaminergic processing during pain-related experiences in the human brain are still incompletely understood. This is partially due to the invasive nature of the available techniques to visualize and measure metabolic activity. Positron Emission Tomography (PET) radioligand studies using radioactive substances are still the only available modality to date that allows for the investigation of the molecular mechanisms in the human brain. The most commonly studied PET radiotracers are [11C]-carfentanil (CFN) and [11C]- or [18F]-diprenorphine (DPN), which bind to opioid receptors, and [11C]-raclopride (RAC) and [18F]-fallypride (FAL) tracers, which bind to dopamine receptors. The current meta-analysis examines pain-related studies that used aforementioned opioid and dopamine radioligands in an effort to consolidate the available data into the most likely activated regions. Our primary goal was to identify regions of shared opioid/dopamine neurotransmission during pain-related experiences using within-subject approach. Seed-based d Mapping (SDM) analysis of previously published voxel coordinate data showed that opioidergic activations were strongest in the bilateral caudate, thalamus, right putamen, cingulate gyrus, midbrain, inferior frontal gyrus, and left superior temporal gyrus. The dopaminergic studies showed that the bilateral caudate, thalamus, right putamen, cingulate gyrus, and left putamen had the highest activations. We were able to see a clear overlap between opioid and dopamine activations in a majority of the regions during pain-related experiences, though there were some unique areas of dopaminergic activation such as the left putamen. Regions unique to opioidergic activation included the midbrain, inferior frontal gyrus, and left superior temporal gyrus. Here we provide initial evidence for the functional overlap between opioidergic and dopaminergic processing during aversive states in humans.

A nigra-subthalamic circuit is involved in acute and chronic pain states

ABSTRACT

The basal ganglia modulate somatosensory pain pathways but it is unclear whether a common circuit exists to mitigate hyperalgesia in pain states induced by peripheral nociceptive stimuli. As a key output nucleus of the basal ganglia, the substantia nigra pars reticulata (SNr) may be a candidate for this role. To test this possibility, we optogenetically modulated SNr GABAergic neurons and examined pain thresholds in freely behaving male mice in inflammatory and neuropathic pain states as well as comorbid depression in chronic pain. We observed that stimulation of either SNr GABAergic neurons or their projections to the subthalamic nucleus (STN) significantly alleviated nociceptive responses in all pain states on the contralateral side and comorbid depression in chronic pain, and that this analgesic effect was eliminated when SNr-STN GABAergic projection was blocked. However, SNr modulation did not affect baseline pain thresholds. We also found that SNr-STN GABAergic projection was attenuated in pain states, resulting in disinhibition of STN neurons. Thus, impairment of the SNr-STN GABAergic circuit may be a common pathophysiology for the maintenance of hyperalgesia in both inflammatory and neuropathic pain states and the comorbid depression in chronic pain; compensating this circuit has potential to effectively treat related pain conditions.

Risk for opioid misuse in chronic pain patients is associated with endogenous opioid system dysregulation

µ-Opioid receptors (MOR) are a major target of endogenous and exogenous opioids, including opioid pain medications. The µ-opioid neurotransmitter system is heavily implicated in the pathophysiology of chronic pain and opioid use disorder and, as such, central measures of µ-opioid system functioning are increasingly being considered as putative biomarkers for risk to misuse opioids. To explore the relationship between MOR system function and risk for opioid misuse, 28 subjects with chronic nonspecific back pain completed a clinically validated measure of opioid misuse risk, the Pain Medication Questionnaire (PMQ), and were subsequently separated into high (PMQ > 21) and low (PMQ ≤ 21) opioid misuse risk groups. Chronic pain patients along with 15 control participants underwent two separate [¹¹C]-carfentanil positron emission tomography scans to explore MOR functional measures: one at baseline and one during a sustained pain-stress challenge, with the difference between the two providing an indirect measure of stress-induced endogenous opioid release. We found that chronic pain participants at high risk for opioid misuse displayed higher baseline MOR availability within the right amygdala relative to those at low risk. By contrast, patients at low risk for opioid misuse showed less pain-induced activation of MOR-mediated, endogenous opioid neurotransmission in the nucleus accumbens. This study links human in vivo MOR system functional measures to the development of addictive disorders and provides novel evidence that MORs and µ-opioid system responsivity may underlie risk to misuse opioids among chronic pain patients.

Disulfiram Abrogates Morphine Tolerance-A Possible Role of µ-Opioid Receptor-Related G-Protein Activation in the Striatum

One of the key strategies for effective pain management involves delaying analgesic tolerance. Early clinical reports indicate an extraordinary effectiveness of off-label disulfiram-an agent designed for alcohol use disorder-in potentiating opioid analgesia and abrogation of tolerance. Our study aimed to determine whether sustained µ-opioid signaling upon disulfiram exposure contributes to these phenomena. Wistar rats were exposed to acute and chronic disulfiram and morphine cotreatment. Nociceptive thresholds were assessed with the mechanical Randal-Selitto and thermal tail-flick tests. µ-opioid receptor activation in brain structures important for pain processing was carried out with the [35S]GTPγS assay. The results suggest that disulfiram (12.5-50 mg/kg i.g.) augmented morphine antinociception and diminished morphine (25 mg/kg, i.g.) tolerance in a supraspinal, opioid-dependent manner. Disulfiram (25 mg/kg, i.g.) induced a transient enhancement of µ-opioid receptor activation in the periaqueductal gray matter (PAG), rostral ventromedial medulla (RVM), hypothalamus, prefrontal cortex and the dorsal striatum at day 1 of morphine treatment. Disulfiram rescued µ-opioid receptor signaling in the nucleus accumbens and caudate-putamen 14 days following morphine and disulfiram cotreatment. The results of this study suggest that striatal µ-opioid receptors may contribute to the abolition of morphine tolerance following concomitant treatment with disulfiram.

A somatosensory cortex input to the caudal dorsolateral striatum controls comorbid anxiety in persistent pain

Chronic pain and anxiety symptoms are frequently encountered clinically, but the neural circuit mechanisms underlying the comorbid anxiety symptoms in pain (CASP) in context of chronic pain remain unclear. Using viral neuronal tracing in mice, we identified a previously unknown pathway whereby glutamatergic neurons from layer 5 of the hindlimb primary somatosensory cortex (S1) (Glu), a well-known brain region involved in pain processing, project to GABAergic neurons in the caudal dorsolateral striatum (GABA). In a persistent inflammatory pain model induced by complete Freund's adjuvant injection, enhanced excitation of the Glu→GABA pathway was found in mice exhibiting CASP. Reversing this pathway using chemogenetic or optogenetic approaches alleviated CASP. In addition, the optical activation of Glu terminals in the cDLS produced anxiety-like behaviors in naive mice. Overall, the current study demonstrates the putative importance of a novel Glu→GABA pathway in controlling at least some aspects of CASP.

A highly reproducible method for the measurement of [6-O-methyl-11 C]diprenorphine and its radio-metabolites based on solid-phase extraction and radio-high-pressure liquid chromatography

Described here is a method for the measurement of the radio-metabolites of the positron emission tomography radiotracer [6-O-methyl-11 C]diprenorphine ([11 C]diprenorphine) using in-line solid-phase extraction (SPE) combined with radio-high-pressure liquid chromatography analysis. We believe that this method offers a reliable and reproducible approach to [11 C]diprenorphine metabolite analysis. In addition, different SPE stationary phases are assessed for their efficiency for loading, retention and elution of the parent molecule and its metabolites. Having assessed C4, phenyl and C18 stationary phase, we concluded that a C18 SPE was optimal for our method. Finally, in silico predictions of diprenorphine metabolism were compared with in vivo metabolism of [11 C]diprenorphine induced by hepatic microsomal digestion and analysed by matrix-assisted laser desorption/ionisation mass spectrometry. It was found that there was a high degree of agreement between the two methods and in particular the formation of the diprenorphine-3-glucuronide metabolite.

µ-Opioid Activity in Chronic TMD Pain Is Associated with COMT Polymorphism

Clinicians have the dilemma of prescribing opioid or nonopioid analgesics to chronic pain patients; however, the impact of pain on our endogenous µ-opioid system and how our genetic profile (specifically catechol-O-methyltransferase [COMT] polymorphisms) impacts its activation are currently unknown. Twelve chronic temporomandibular disorder (TMD) patients and 12 healthy controls (HCs) were scanned using positron emission tomography (PET) with [11C]carfentanil, a selective radioligand for µ-opioid receptors (µORs). The first 45 min of each PET measured the µOR nondisplaceable binding potential (BPND) at resting state, and the last 45 min consisted of a 20-min masseteric pain challenge with an injection of 5% hypertonic saline. Participants were also genotyped for different COMT alleles. There were no group differences in µOR BPND at resting state (early phase). However, during the masseteric pain challenge (late phase), TMD patients exhibited significant reductions in µOR BPND (decreased [11C]carfentanil binding) in the contralateral parahippocampus (P = 0.002) compared to HCs. The µOR BPND was also significantly lower in TMD patients with longer pain chronicity (P < 0.001). When considering COMT genotype and chronic pain suffering, TMD patients with the COMT158Met substitution had higher pain sensitivity and longer pain chronicity with a 5-y threshold for µOR BPND changes to occur in the parahippocampus. Together, the TMD diagnosis, COMT158Met substitution, and pain chronicity explained 52% of µOR BPND variance in the parahippocampus (cumulative R2 = 52%, P < 0.003, and HC vs. TMD Cohen's effect size d = 1.33 SD). There is strong evidence of dysregulation of our main analgesic and limbic systems in chronic TMD pain. The data also support precision medicine by helping identify TMD patients who may be more susceptible to chronic pain sensitivity and opioid dysfunction based on their genetic profile.

Modulatory effects of different exercise modalities on the functional connectivity of the periaqueductal grey and ventral tegmental area in patients with knee osteoarthritis: a randomised multimodal magnetic resonance imaging study

BACKGROUND

Knee osteoarthritis is a prevalent disorder with unsatisfactory treatment options. Both physical and mindful exercises may be able to relieve its pain symptoms. We compared the modulatory effects of different exercise modalities on the periaqueductal grey (PAG) and ventral tegmental area (VTA), which play important roles in descending opioidergic pathways and reward/motivation systems in patients with knee osteoarthritis.

METHODS

We recruited and randomised 140 patients into Tai Chi, Baduanjin, stationary cycling, and health education control groups for 12 weeks. Knee injury and Osteoarthritis Outcome Score (KOOS), functional and structural MRI, and blood biomarkers were measured at the beginning and end of the experiment. We used the PAG and VTA as seeds in resting-state functional connectivity (rsFC) analysis.

RESULTS

Compared with the control group: (i) all exercises significantly increased KOOS pain sub-scores (pain reduction) and serum programmed death 1 (PD-1) concentrations; (ii) all exercises decreased right PAG rsFC with the medial orbital prefrontal cortex, and the decreased rsFC was associated with improvements in knee pain; and (iii) grey matter volume in the medial orbital prefrontal cortex was significantly increased in all exercise groups. There was also significantly decreased rsFC between the left VTA and the medial orbital prefrontal cortex in the Tai Chi and Baduanjin groups.

CONCLUSIONS

Exercise can simultaneously modulate the rsFC of the descending opioidergic pathway and reward/motivation system and blood inflammation markers. Elucidating the shared and unique mechanisms of different exercise modalities may facilitate the development of exercise-based interventions for chronic pain.

CLINICAL TRIAL REGISTRATION

ChiCTR-IOR-16009308.

Positron emission tomography imaging of endogenous mu-opioid mechanisms during pain and migraine

The enormous advancements in the medical imaging methods witnessed in the past decades have allowed clinical researchers to study the function of the human brain in vivo, both in health and disease. In addition, a better understanding of brain responses to different modalities of stimuli such as pain, reward, or the administration of active or placebo interventions has been achieved through neuroimaging methods. Although magnetic resonance imaging has provided important information regarding structural, hemodynamic, and metabolic changes in the central nervous system related to pain, magnetic resonance imaging does not address modulatory pain systems at the molecular level (eg, endogenous opioid). Such important information has been obtained through positron emission tomography, bringing insights into the neuroplastic changes that occur in the context of the pain experience. Positron emission tomography studies have not only confirmed the brain structures involved in pain processing and modulation but also have helped elucidate the neural mechanisms that underlie healthy and pathological pain regulation. These data have shown some of the biological basis of the interindividual variability in pain perception and regulation. In addition, they provide crucial information to the mechanisms that drive placebo and nocebo effects, as well as represent an important source of variability in clinical trials. Positron emission tomography studies have also permitted exploration of the dynamic interaction between behavior and genetic factors and between different pain modulatory systems. This narrative review will present a summary of the main findings of the positron emission tomography studies that evaluated the functioning of the opioidergic system in the context of pain.

Chronic Pain: Structural and Functional Changes in Brain Structures and Associated Negative Affective States

Chronic pain is a condition in which pain progresses from an acute to chronic state and persists beyond the healing process. Chronic pain impairs function and decreases patients' quality of life. In recent years, efforts have been made to deepen our understanding of chronic pain and to develop better treatments to alleviate chronic pain. In this review, we summarize the results of previous studies, focusing on the mechanisms underlying chronic pain development and the identification of neural areas related to chronic pain. We review the association between chronic pain and negative affective states. Further, we describe the structural and functional changes in brain structures that accompany the chronification of pain and discuss various neurotransmitter families involved. Our review aims to provide guidance for the development of future therapeutic approaches that could be used in the management of chronic pain.

Impact of early adverse life events and sex on functional brain networks in patients with urological chronic pelvic pain syndrome (UCPPS): A MAPP Research Network study

Pain is a highly complex and individualized experience with biopsychosocial components. Neuroimaging research has shown evidence of the involvement of the central nervous system in the development and maintenance of chronic pain conditions, including urological chronic pelvic pain syndrome (UCPPS). Furthermore, a history of early adverse life events (EALs) has been shown to adversely impact symptoms throughout childhood and into adulthood. However, to date, the role of EAL's in the central processes of chronic pain have not been adequately investigated. We studied 85 patients (56 females) with UCPPS along with 86 healthy controls (HCs) who had resting-state magnetic resonance imaging scans (59 females), and data on EALs as a part of the Multidisciplinary Approach to the Study of Chronic Pelvic Pain (MAPP) Research Network Study. We used graph theory methods in order to investigate the impact of EALs on measures of centrality, which characterize information flow, communication, influence, and integration in a priori selected regions of interest. Patients with UCPPS exhibited lower centrality in the right anterior insula compared to HCs, a key node in the salience network. Males with UCPPS exhibited lower centrality in the right anterior insula compared the HC males. Females with UCPPS exhibited greater centrality in the right caudate nucleus and left angular gyrus compared to HC females. Males with UCPPS exhibited lower centrality in the left posterior cingulate, angular gyrus, middle temporal gyrus, and superior temporal sulcus, but greater centrality in the precuneus and anterior mid-cingulate cortex (aMCC) compared to females with UCPPS. Higher reports of EALs was associated with greater centrality in the left precuneus and left aMCC in females with UCPPS. This study provides evidence for disease and sex-related alterations in the default mode, salience, and basal ganglia networks in patients with UCPPS, which are moderated by EALs, and associated with clinical symptoms and quality of life (QoL).

Chronic neuropathic pain reduces opioid receptor availability with associated anhedonia in rat

Supplemental Digital Content is Available in the Text.

Chronic pain reduces opioid receptor expression in the rat striatum, where the correlation between receptor expression and anhedonia may represent a molecular substrate for comorbid depression.

The opioid system plays a critical role in both the experience and management of pain. Although acute activation of the opioid system can lead to pain relief, the effects of chronic pain on the opioid system remain opaque. Cross-sectional positron emission tomography (PET) studies show reduced availability of brain opioid receptors in patients with chronic pain but are unable to (1) determine whether these changes are due to the chronic pain itself or due to preexisting or medication-induced differences in the endogenous opioid system, and (2) identify the neurobiological substrate of reduced opioid receptor availability. We investigated these possibilities using a well-controlled longitudinal study design in rat. Using [¹⁸F]-FDPN-PET in either sham rats (n = 17) or spared nerve injury rats (n = 17), we confirmed reduced opioid receptor availability in the insula, caudate–putamen, and motor cortex of nerve injured rats 3 months after surgery, indicating that painful neuropathy altered the endogenous opioid system. Immunohistochemistry showed reduced expression of the mu-opioid receptor, MOR1, in the caudate–putamen and insula. Neither the opioid peptide enkephalin nor the neuronal marker NeuN differed between groups. In nerve-injured animals, sucrose preference, a measure of anhedonia/depression-like behavior, positively correlated with PET opioid receptor availability and MOR1-immunoreactivity in the caudate–putamen. These findings provide new evidence that the altered supraspinal opioid receptor availability observed in human patients with chronic pain may be a direct result of chronic pain. Moreover, reduced opioid receptor availability seems to reflect decreased receptor expression, which may contribute to pain-induced depression.

The Contribution of Endogenous Modulatory Systems to TMS- and tDCS-Induced Analgesia: Evidence from PET Studies

Chronic pain is an important public health issue. Moreover, its adequate management is still considered a major clinical problem, mainly due to its incredible complexity and still poorly understood pathophysiology. Recent scientific evidence coming from neuroimaging research, particularly functional magnetic resonance (fMRI) and positron emission tomography (PET) studies, indicates that chronic pain is associated with structural and functional changes in several brain structures that integrate antinociceptive pathways and endogenous modulatory systems. Furthermore, the last two decades have witnessed a huge increase in the number of studies evaluating the clinical effects of noninvasive neuromodulatory methods, especially transcranial magnetic stimulation (TMS) and transcranial direct current stimulation (tDCS), which have been proved to effectively modulate the cortical excitability, resulting in satisfactory analgesic effects with minimal adverse events. Nevertheless, the precise neuromechanisms whereby such methods provide pain control are still largely unexplored. Recent studies have brought valuable information regarding the recruitment of different modulatory systems and related neurotransmitters, including glutamate, dopamine, and endogenous opioids. However, the specific neurocircuits involved in the analgesia produced by those therapies have not been fully elucidated. This review focuses on the current literature correlating the clinical effects of noninvasive methods of brain stimulation to the changes in the activity of endogenous modulatory systems.

Analgesia by Deletion of Spinal Neurokinin 1 Receptor Expressing Neurons Using a Bioengineered Substance P-Pseudomonas Exotoxin Conjugate

Cell deletion approaches to pain directed at either the primary nociceptive afferents or second-order neurons are highly effective analgesic manipulations. Second-order spinal neurons expressing the neurokinin 1 (NK1) receptor are required for the perception of many types of pain. To delete NK1+ neurons for the purpose of pain control, we generated a toxin–peptide conjugate using DTNB-derivatized (Cys₀) substance P (SP) and a N-terminally truncated Pseudomonas exotoxin (PE35) that retains the endosome-release and ADP-ribosylation enzymatic domains but with only one free sulfhydryl side chain for conjugation. This allowed generation of a one-to-one product linked by a disulfide bond (SP-PE35). In vitro, Chinese hamster ovary cells stably transfected with the NK1 receptor exhibited specific cytotoxicity when exposed to SP-PE35 (IC₅₀ = 5 × 10⁻¹¹M), whereas the conjugate was nontoxic to NK2 and NK3 receptor-bearing cell lines. In vivo studies showed that, after infusion into the spinal subarachnoid space, the toxin was extremely effective in deleting NK1 receptor-expressing cells from the dorsal horn of the spinal cord. The specific cell deletion robustly attenuated thermal and mechanical pain sensations and inflammatory hyperalgesia but did not affect motoric capabilities. NK1 receptor cell deletion and antinociception occurred without obvious lesion of non–receptor-expressing cells or apparent reorganization of primary afferent innervation. These data demonstrate the extraordinary selectivity and broad-spectrum antinociceptive efficacy of this ligand-directed protein therapeutic acting via receptor-mediated endocytosis. The loss of multiple pain modalities including heat and mechanical pinch, transduced by different populations of primary afferents, shows that spinal NK1 receptor-expressing neurons are critical points of convergence in the nociceptive transmission circuit. We further suggest that therapeutic end points can be effectively and safely achieved when SP-PE35 is locally infused, thereby producing a regionally defined analgesia.

Enhancing treatment of osteoarthritis knee pain by boosting expectancy: A functional neuroimaging study

Objectives

Expectation can significantly modulate pain and treatment effects. This study aims to investigate if boosting patients' expectancy can enhance the treatment of knee osteoarthritis (KOA), and its underlying brain mechanism.

Methods

Seventy-four KOA patients were recruited and randomized to three groups: boosted acupuncture (with a manipulation to enhance expectation), standard acupuncture, or treatment as usual (TAU). Each patient underwent six treatments before being debriefed, and four additional treatments after being debriefed. The fMRI scans were applied during the first and sixth treatment sessions.

Results

We found significantly decreased knee pain in the boosted acupuncture group compared to the standard acupuncture or TAU groups after both six and ten treatments. Resting state functional connectivity (rsFC) analyses using the nucleus accumbens (NAc) as the seed showed rsFC increases between the NAc and the medial prefrontal cortex (MPFC)/rostral anterior cingulate cortex (rACC) and dorsolateral prefrontal cortex in the boosted group as compared to the standard acupuncture group after multiple treatments. Expectancy scores after the first treatment were significantly associated with increased NAc-rACC/MPFC rsFC and decreased knee pain following treatment.

Conclusions

Our study provides a novel method and mechanism for boosting the treatment of pain in patients with KOA. Our findings may shed light on enhancing outcomes of pharmacological and integrative medicines in clinical settings.

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Acupuncture with enhanced expectancy produced greater pain relief in KOA patients.

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NAc – ACC/MPFC rsFC increased after acupuncture with enhanced expectancy.

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NAc – ACC/MPFC rsFC increases are associated with clinical improvements.

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Our findings provide a novel method for boosting the treatment of chronic pain.

Basic/Translational Development of Forthcoming Opioid- and Nonopioid-Targeted Pain Therapeutics

Opioids represent an efficacious therapeutic modality for some, but not all pain states. Singular reliance on opioid therapy for pain management has limitations, and abuse potential has deleterious consequences for patient and society. Our understanding of pain biology has yielded insights and opportunities for alternatives to conventional opioid agonists. The aim is to have efficacious therapies, with acceptable side effect profiles and minimal abuse potential, which is to say an absence of reinforcing activity in the absence of a pain state. The present work provides a nonexclusive overview of current drug targets and potential future directions of research and development. We discuss channel activators and blockers, including sodium channel blockers, potassium channel activators, and calcium channel blockers; glutamate receptor-targeted agents, including N-methyl-D-aspartate, α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid, and metabotropic receptors. Furthermore, we discuss therapeutics targeted at γ-aminobutyric acid, α2-adrenergic, and opioid receptors. We also considered antagonists of angiotensin 2 and Toll receptors and agonists/antagonists of adenosine, purine receptors, and cannabinoids. Novel targets considered are those focusing on lipid mediators and anti-inflammatory cytokines. Of interest is development of novel targeting strategies, which produce long-term alterations in pain signaling, including viral transfection and toxins. We consider issues in the development of druggable molecules, including preclinical screening. While there are examples of successful translation, mechanistically promising preclinical candidates may unexpectedly fail during clinical trials because the preclinical models may not recapitulate the particular human pain condition being addressed. Molecular target characterization can diminish the disconnect between preclinical and humans' targets, which should assist in developing nonaddictive analgesics.

Predictive mechanisms linking brain opioids to chronic pain vulnerability and resilience

Chronic pain is a major global healthcare problem that is currently inadequately treated. In addition, the current use of opioids for treatment has reached far beyond the paucity of evidence for long-term benefits relative to risks. Benefit-risk models for opioid and non-opioid treatments would benefit from a rational, mechanism-based understanding of neuroplastic and neurochemical contributions to chronic pain. Here, we evaluate the findings and limitations of representative research investigating brain neuroplasticity and neurochemistry in chronic pain. In sum, the mechanisms of pain-related neuroplasticity in the brain remain poorly understood because neuroimaging studies have been largely descriptive. We argue that definition is needed of optimal (pain-resilient) and suboptimal (pain-vulnerable) functioning of the endogenous opioid system in order to identify neurochemical contributions to aberrant neuroplasticity in chronic pain. We outline the potential benefits of computational approaches that utilize evolutionary and statistical optimality principles, illustrating this approach with mechanistic hypotheses on opioid function. In particular, we discuss the role of predictive mechanisms in perceptual and associative plasticity and evidence for their modulation by endogenous opioids. Future research should attempt to utilize formal computational models to provide evidence for the clinical validity of this approach, thereby providing a rational basis for future treatment and, ideally, prevention.

LINKED ARTICLES

This article is part of a themed section on Emerging Areas of Opioid Pharmacology. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.14/issuetoc.

Endogenous Opiates and Behavior: 2015

This paper is the thirty-eighth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2015 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia, stress and social status, tolerance and dependence, learning and memory, eating and drinking, drug abuse and alcohol, sexual activity and hormones, pregnancy, development and endocrinology, mental illness and mood, seizures and neurologic disorders, electrical-related activity and neurophysiology, general activity and locomotion, gastrointestinal, renal and hepatic functions, cardiovascular responses, respiration and thermoregulation, and immunological responses.

Brain imaging of pain: state of the art

Pain is a complex sensory and emotional experience that is heavily influenced by prior experience and expectations of pain. Before the development of noninvasive human brain imaging, our grasp of the brain's role in pain processing was limited to data from postmortem studies, direct recording of brain activity, patient experience and stimulation during neurosurgical procedures, and animal models of pain. Advances made in neuroimaging have bridged the gap between brain activity and the subjective experience of pain and allowed us to better understand the changes in the brain that are associated with both acute and chronic pain. Additionally, cognitive influences on pain such as attention, anticipation, and fear can now be directly observed, allowing for the interpretation of the neural basis of the psychological modulation of pain. The use of functional brain imaging to measure changes in endogenous neurochemistry has increased our understanding of how states of increased resilience and vulnerability to pain are maintained.

Experimental pain phenotyping in community-dwelling individuals with knee osteoarthritis

Pain among individuals with knee osteoarthritis (OA) is associated with significant disability in older adults, and recent evidence demonstrates enhanced experimental pain sensitivity. Although previous research showed considerable heterogeneity in the OA clinical pain presentation, less is known regarding the variability in responses to experimental pain. The present study included individuals with knee OA (n = 292) who participated in the Understanding Pain and Limitations in Osteoarthritic Disease study and completed demographic and psychological questionnaires followed by a multimodal quantitative sensory testing (QST) session. Quantitative sensory testing measures were subjected to variable reduction procedures to derive pain sensitivity index scores, which in turn were entered into a cluster analysis. Five clusters were significantly different across all pain sensitivity index variables (P < 0.001) and were characterized by: (1) low pain sensitivity to pressure pain (N = 39); (2) average pain sensitivity across most modalities (N = 88); (3) high temporal summation of punctate pain (N = 38); (4) high cold pain sensitivity (N = 80); and (5) high sensitivity to heat pain and temporal summation of heat pain (N = 41). Clusters differed significantly by race, gender, somatic reactivity, and catastrophizing (P < 0.05). Our findings support the notion that there are distinct subgroups or phenotypes based on experimental pain sensitivity in community-dwelling older adults with knee OA, expanding previous findings of similar cluster characterizations in healthy adults. Future research is needed to further understand the pathophysiological mechanisms underlying pain within these subgroups, which may be of added value in tailoring effective treatments for people with OA.

The effects of intra-articular resiniferatoxin on monosodium iodoacetate-induced osteoarthritic pain in rats

This study was performed to investigate whether an intra-articular injection of transient receptor potential vanilloid 1 (TRPV1) receptor agonist, resiniferatoxin (RTX) would alleviate behavioral signs of arthritic pain in a rat model of osteoarthritis (OA). We also sought to determine the effect of RTX treatment on calcitonin gene-related peptide (CGRP) expression in the spinal cord. Knee joint inflammation was induced by intra-articular injection of monosodium iodoacetate (MIA, 8 mg/50 µl) and weight bearing percentage on right and left hindpaws during walking, paw withdrawal threshold to mechanical stimulation, and paw withdrawal latency to heat were measured to evaluate pain behavior. Intra-articular administration of RTX (0.03, 0.003 and 0.0003%) at 2 weeks after the induction of knee joint inflammation significantly improved reduction of weight bearing on the ipsilateral hindlimb and increased paw withdrawal sensitivity to mechanical and heat stimuli. The reduction of pain behavior persisted for 3~10 days according to each behavioral test. The MIA-induced increase in CGRP immunoreactivity in the spinal cord was decreased by RTX treatment in a dose-dependent manner. The present study demonstrated that a single intra-articular administration of RTX reduced pain behaviors for a relatively long time in an experimental model of OA and could normalize OA-associated changes in peptide expression in the spinal cord.