Corticolimbic circuitry in the modulation of chronic pain and substance abuse

Intrinsic brain connectivity alterations despite intact pain inhibition in subjects with neuropathic pain after spinal cord injury: a pilot study

Endogenous pain modulation in humans is frequently investigated with conditioned pain modulation (CPM). Deficient pain inhibition is a proposed mechanism that contributes to neuropathic pain (NP) after spinal cord injury (SCI). Recent studies have combined CPM testing and neuroimaging to reveal neural correlates of CPM efficiency in chronic pain. This study investigated differences in CPM efficiency in relation to resting-state functional connectivity (rsFC) between 12 SCI-NP subjects and 13 age- and sex-matched healthy controls (HC). Twelve and 11 SCI-NP subjects were included in psychophysical and rsFC analyses, respectively. All HC were included in the final analyses. Psychophysical readouts were analysed to determine CPM efficiency within and between cohorts. Group differences of rsFC, in relation to CPM efficiency, were explored with seed-to-voxel rsFC analyses with pain modulatory regions, e.g. ventrolateral periaqueductal gray (vlPAG) and amygdala. Overall, pain inhibition was not deficient in SCI-NP subjects and was greater in those with more intense NP. Greater pain inhibition was associated with weaker rsFC between the vlPAG and amygdala with the visual and frontal cortex, respectively, in SCI-NP subjects but with stronger rsFC in HC. Taken together, SCI-NP subjects present with intact pain inhibition, but can be differentiated from HC by an inverse relationship between CPM efficiency and intrinsic connectivity of supraspinal regions. Future studies with larger cohorts are necessary to consolidate the findings in this study.

An ethogram analysis of cutaneous thermal pain sensitivity and oxycodone reward-related behaviors in rats

Inter-relationships between pain sensitivity, drug reward, and drug misuse are of considerable interest given that many analgesics exhibit misuse potential. Here we studied rats as they underwent a series of pain- and reward-related tests: cutaneous thermal reflex pain, induction and extinction of conditioned place preference to oxycodone (0.56 mg/kg), and finally the impact of neuropathic pain on reflex pain and reinstatement of conditioned place preference. Oxycodone induced a significant conditioned place preference that extinguished throughout repeated testing. Correlations identified of particular interest included an association between reflex pain and oxycodone-induced behavioral sensitization, and between rates of behavioral sensitization and extinction of conditioned place preference. Multidimensional scaling analysis followed by k-clustering identified three clusters: (1) reflex pain, rate of behavioral sensitization and rate of extinction of conditioned place preference (2) basal locomotion, locomotor habituation, acute oxycodone-stimulated locomotion and rate of change in reflex pain during repeated testing, and (3) magnitude of conditioned place preference. Nerve constriction injury markedly enhanced reflex pain but did not reinstate conditioned place preference. These results suggest that high rates of behavioral sensitization predicts faster rates of extinction of oxycodone seeking/reward, and suggest that cutaneous thermal reflex pain may be predictive of both.

Pain-related cortico-limbic plasticity and opioid signaling

Neuroplasticity in cortico-limbic circuits has been implicated in pain persistence and pain modulation in clinical and preclinical studies. The amygdala has emerged as a key player in the emotional-affective dimension of pain and pain modulation. Reciprocal interactions with medial prefrontal cortical regions undergo changes in pain conditions. Other limbic and paralimbic regions have been implicated in pain modulation as well. The cortico-limbic system is rich in opioids and opioid receptors. Preclinical evidence for their pain modulatory effects in different regions of this highly interactive system, potentially opposing functions of different opioid receptors, and knowledge gaps will be described here. There is little information about cell type- and circuit-specific functions of opioid receptor subtypes related to pain processing and pain-related plasticity in the cortico-limbic system. The important role of anterior cingulate cortex (ACC) and amygdala in MOR-dependent analgesia is most well-established, and MOR actions in the mesolimbic system appear to be similar but remain to be determined in mPFC regions other than ACC. Evidence also suggests that KOR signaling generally serves opposing functions whereas DOR signaling in the ACC has similar, if not synergistic effects, to MOR. A unifying picture of pain-related neuronal mechanisms of opioid signaling in different elements of the cortico-limbic circuitry has yet to emerge. This article is part of the Special Issue on "Opioid-induced changes in addiction and pain circuits".

Insights into the mechanisms underlying opioid use disorder and potential treatment strategies

Opioid use disorder is a worldwide societal problem and public health burden. Strategies for treating opioid use disorder can be divided into those that target the opioid receptor system and those that target non-opioid receptor systems, including the dopamine and glutamate receptor systems. Currently, the clinical drugs used to treat opioid use disorder include the opioid receptor agonists methadone and buprenorphine, which are limited by their abuse liability, and the opioid receptor antagonist naltrexone, which is limited by poor compliance. Therefore, the development of effective medications with lower abuse liability and better potential for compliance is urgently needed. Based on recent advances in the understanding of the neurobiological mechanisms underlying opioid use disorder, potential treatment strategies and targets have emerged. This review focuses on the progress made in identifying potential targets and developing medications to treat opioid use disorder, including progress made by our laboratory, and provides insights for future medication development. LINKED ARTICLES: This article is part of a themed issue on Advances in Opioid Pharmacology at the Time of the Opioid Epidemic. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v180.7/issuetoc.

Neurophysiology of hypnosis in chronic pain: A review of recent literature

Chronic pain is a complex phenomenon which includes biological, psychological, and socio-professional factors that undermine patients' everyday life. Currently, only few patients significantly benefit from pharmacological treatments and many have to stop them because of negative side effects. Moreover, no medication or treatment addresses all aspects of chronic pain at once (i.e., sensations, emotions, behaviors, and cognitions), positioning chronic pain as an important public health issue and thus contributing to high health-care costs. Consequently, patients and health-care providers are increasingly turning to complementary non-pharmacological techniques such as hypnosis. Clinical research has demonstrated a decrease of pain perception, pain interference, depression and anxiety, and an increase in global quality of life when patients with chronic pain have benefited from hypnosis learning. Neuroimaging studies offer a possible explanation of these results by focusing on neural processes of pain modulation in chronic pain patients' brain. Studies conducted with chronic pain patients showed a modulation of pain matrix activity during hypnosis with a specific involvement of the anterior cingulate cortex (related to emotional and cognitive processing of pain). Therefore, hypnosis seems to act upon regions underlying emotion and cognition, with an influence on pain perception and emotional regulation. In this review, we propose to carry out a review of the recent literature on hypnosis in chronic pain management. A better understanding of the beneficial effects of hypnosis on chronic pain and its neurophysiology should enable more systematic use of this technique in the management of this complex health problem.

The Influence of Dietary Supplementations on Neuropathic Pain

Neuropathic pain is defined as pain caused by a lesion or disease of the somatosensory nervous system and affects 7–10% of the worldwide population. Neuropathic pain can be induced by the use of drugs, including taxanes, thus triggering chemotherapy-induced neuropathic pain or as consequence of metabolic disorders such as diabetes. Neuropathic pain is most often a chronic condition, and can be associated with anxiety and depression; thus, it negatively impacts quality of life. Several pharmacologic approaches exist; however, they can lead numerous adverse effects. From this perspective, the use of nutraceuticals and diet supplements can be helpful in relieve neuropathic pain and related symptoms. In this review, we discuss how diet can radically affect peripheral neuropathy, and we focus on the potential approaches to ameliorate this condition, such as the use of numerous nutritional supplements or probiotics.

Contribution of AMPA Receptor-Mediated LTD in LA/BLA-CeA Pathway to Comorbid Aversive and Depressive Symptoms in Neuropathic Pain

Comorbid anxiety and depressive symptoms in chronic pain are a common health problem, but the underlying mechanisms remain unclear. Previously, we have demonstrated that sensitization of the CeA neurons via decreased GABAergic inhibition contributes to anxiety-like behaviors in neuropathic pain rats. In this study, by using male Sprague Dawley rats, we reported that the CeA plays a key role in processing both sensory and negative emotional-affective components of neuropathic pain. Bilateral electrolytic lesions of CeA, but not lateral/basolateral nucleus of the amygdala (LA/BLA), abrogated both pain hypersensitivity and aversive and depressive symptoms of neuropathic rats induced by spinal nerve ligation (SNL). Moreover, SNL rats showed structural and functional neuroplasticity manifested as reduced dendritic spines on the CeA neurons and enhanced LTD at the LA/BLA-CeA synapse. Disruption of GluA2-containing AMPAR trafficking and endocytosis from synapses using synthetic peptides, either pep2-EVKI or Tat-GluA2(3Y), restored the enhanced LTD at the LA/BLA-CeA synapse, and alleviated the mechanical allodynia and comorbid aversive and depressive symptoms in neuropathic rats, indicating that the endocytosis of GluA2-containing AMPARs from synapses is probably involved in the LTD at the LA/BLA-CeA synapse and the comorbid aversive and depressive symptoms in neuropathic pain in SNL-operated rats. These data provide a novel mechanism for elucidating comorbid aversive and depressive symptoms in neuropathic pain and highlight that structural and functional neuroplasticity in the amygdala may be important as a promising therapeutic target for comorbid negative emotional-affective disorders in chronic pain.SIGNIFICANCE STATEMENT Several studies have demonstrated the high comorbidity of negative affective disorders in patients with chronic pain. Understanding the affective aspects related to chronic pain may facilitate the development of novel therapies for more effective management. Here, we unravel that the CeA plays a key role in processing both sensory and negative emotional-affective components of neuropathic pain, and LTD at the amygdaloid LA/BLA-CeA synapse mediated by GluA2-containing AMPAR endocytosis underlies the comorbid aversive and depressive symptoms in neuropathic pain. This study provides a novel mechanism for elucidating comorbid aversive and depressive symptoms in neuropathic pain and highlights that structural and functional neuroplasticity in the amygdala may be important as a promising therapeutic target for comorbid negative emotional-affective disorders in chronic pain.

Determinants of Treatment Response in Painful Diabetic Peripheral Neuropathy: A Combined Deep Sensory Phenotyping and Multimodal Brain MRI Study

Painful diabetic peripheral neuropathy (DPN) is difficult to manage, as treatment response is often varied. The primary aim of this study was to examine differences in pain phenotypes between responders and nonresponders to intravenous lidocaine treatment using quantitative sensory testing. The secondary aim was to explore differences in brain structure and functional connectivity with treatment response. Forty-five consecutive patients who received intravenous lidocaine treatment for painful DPN were screened. Twenty-nine patients who met the eligibility criteria (responders, n = 14, and nonresponders, n = 15) and 26 healthy control subjects underwent detailed sensory profiling. Subjects also underwent multimodal brain MRI. A greater proportion of patients with the irritable (IR) nociceptor phenotype were responders to intravenous lidocaine treatment compared with nonresponders. The odds ratio of responding to intravenous lidocaine was 8.67 times greater (95% CI 1.4-53.8) for the IR nociceptor phenotype. Responders to intravenous lidocaine also had significantly greater mean primary somatosensory cortex cortical volume and functional connectivity between the insula cortex and the corticolimbic circuitry. This study provides preliminary evidence for a mechanism-based approach for individualizing therapy in patients with painful DPN.

Impaired frontostriatal functional connectivity among chronic opioid using pain patients is associated with dysregulated affect

Preclinical studies have shown effects of chronic exposure to addictive drugs on glutamatergic-mediated neuroplasticity in frontostriatal circuitry. These initial findings have been paralleled by human functional magnetic resonance imaging (fMRI) research demonstrating weaker frontostriatal resting-state functional connectivity (rsFC) among individuals with psychostimulant use disorders. However, there is a dearth of human imaging literature describing associations between long-term prescription opioid use, frontostriatal rsFC, and brain morphology among chronic pain patients. We hypothesized that prescription opioid users with chronic pain, as compared with healthy control subjects, would evidence weaker frontostriatal rsFC coupled with less frontostriatal gray matter volume (GMV). Further, those opioid use-related deficits in frontostriatal circuitry would be associated with negative affect and drug misuse. Prescription opioid users with chronic pain (n = 31) and drug-free healthy controls (n = 30) underwent a high-resolution anatomical and an eyes-closed resting-state functional scan. The opioid group, relative to controls, exhibited weaker frontostriatal rsFC, and less frontostriatal GMV in both L.NAc and L.vmPFC. Frontostriatal rsFC partially mediated group differences in negative affect. Within opioid users, L.NAc GMV predicted opioid misuse severity. The current study revealed that prescription opioid use in the context of chronic pain is associated with functional and structural abnormalities in frontostriatal circuitry. These results suggest that opioid use-related abnormalities in frontostriatal circuitry may undergird disturbances in affect that may contribute to the ongoing maintenance of opioid use and misuse. These findings warrant further examination of interventions to treat opioid pathophysiology in frontostriatal circuitry over the course of treatment.

Cannabidiol increases the nociceptive threshold in a preclinical model of Parkinson's disease

Medications that improve pain threshold can be useful in the pharmacotherapy of Parkinson's disease (PD). Pain is a prevalent PD's non-motor symptom with a higher prevalence of analgesic drugs prescription for patients. However, specific therapy for PD-related pain are not available. Since the endocannabinoid system is expressed extensively in different levels of pain pathway, drugs designed to target this system have promising therapeutic potential in the modulation of pain. Thus, we examined the effects of the 6-hydroxydopamine- induced PD on nociceptive responses of mice and the influence of cannabidiol (CBD) on 6-hydroxydopamine-induced nociception. Further, we investigated the pathway involved in the analgesic effect of the CBD through the co-administration with a fatty acid amide hydrolase (FAAH) inhibitor, increasing the endogenous anandamide levels, and possible targets from anandamide, i.e., the cannabinoid receptors subtype 1 and 2 (CB1 and CB2) and the transient receptor potential vanilloid type 1 (TRPV1). We report that 6-hydroxydopamine- induced parkinsonism decreases the thermal and mechanical nociceptive threshold, whereas CBD (acute and chronic treatment) reduces this hyperalgesia and allodynia evoked by 6-hydroxydopamine. Moreover, ineffective doses of either FAAH inhibitor or TRPV1 receptor antagonist potentialized the CBD-evoked antinociception while an inverse agonist of the CB1 and CB2 receptor prevented the antinociceptive effect of the CBD. Altogether, these results indicate that CBD can be a useful drug to prevent the parkinsonism-induced nociceptive threshold reduction. They also suggest that CB1 and TRPV1 receptors are important for CBD-induced analgesia and that CBD could produce these analgesic effects increasing endogenous anandamide levels.

The Science of Addiction Through the Lens of Opioid Treatment for Chronic Noncancer Pain

Addiction is a chronic, relapsing brain disease. It is not the same as physical dependence (i.e., withdrawal) and tolerance, but is characterized by loss of control over the use of the substance, continued use despite consequences, compulsive use, and cravings. Addiction involves functional changes to brain pathways involved in reward, stress and learning, and these changes can last a long time after the addictive substance is no longer used. Repeated interruption of normal brain function from repetitive use of addictive substances can hijack normal reward mechanisms resulting in fundamental alterations in brain structure and function. Over time, addictive substances can bring about a false fixed prediction error that cannot be rectified during learning. And, for individuals who develop an addiction, initial impulsive drug use progresses to compulsive drug use and this progression also has neurobiological underpinnings. Drug addiction is partly heritable, although there is no single gene coding specifically for the disease of addiction. Genetic factors contribute to a vulnerability to develop both addiction and addiction comorbidities, and play an appreciable role in responses to and metabolism of addictive substances, and most likely, the experience of rewarding effects. Patients with pain who are addicted to opioids have, in addition to the pain disorder, a chronic relapsing brain disease that can be life-threatening. Addiction can be treated and controlled, but not cured. Treatment of pain in individuals with addiction is a complex clinical challenge. Holistic assessment, interprofessional approaches, use of established guidelines, and non-pharmacological complementary modalities are needed.

Pain-related nucleus accumbens function: modulation by reward and sleep disruption

The nucleus accumbens (NAc) has been implicated in sleep, reward, and pain modulation, but the relationship between these functional roles is unclear. This study aimed to determine whether NAc function at the onset and offset of a noxious thermal stimulus is enhanced by rewarding music, and whether that effect is reversed by experimental sleep disruption. Twenty-one healthy subjects underwent functional magnetic resonance imaging scans on 2 separate days after both uninterrupted sleep and experimental sleep disruption. During functional magnetic resonance imaging scans, participants experienced noxious stimulation while listening to individualized rewarding or neutral music. Behavioral results revealed that rewarding music significantly reduced pain intensity compared with neutral music, and disrupted sleep was associated with decreased pain intensity in the context of listening to music. In whole-brain family-wise error cluster-corrected analysis, the NAc was activated at pain onset, but not during tonic pain or at pain offset. Sleep disruption attenuated NAc activation at pain onset and during tonic pain. Rewarding music altered NAc connectivity with key nodes of the corticostriatal circuits during pain onset. Sleep disruption increased reward-related connectivity between the NAc and the anterior midcingulate cortex at pain onset. This study thus indicates that experimental sleep disruption modulates NAc function during the onset of pain in a manner that may be conditional on the presence of competing reward-related stimuli. These findings point to potential mechanisms for the interaction between sleep, reward, and pain, and suggest that sleep disruption affects both the detection and processing of aversive stimuli that may have important implications for chronic pain.

Inhibitory Plasticity of Mesocorticolimbic Circuits in Addiction and Mental Illness

Behavioral adaptations occur through remodeling of brain circuits, as arising, for instance, from experience-dependent synaptic plasticity. Drugs of abuse and aversive stimuli, such as stress, act on the mesocorticolimbic system, dysregulating adaptive mechanisms and leading to a variety of aberrant behaviors associated with neuropsychiatric disorders. Until recently, research in the field has commonly focused on experience-dependent synaptic plasticity at excitatory synapses. However, there is growing evidence that synaptic plasticity within inhibitory circuits is an important contributor to maladaptive behaviors. We speculate that restoring normal inhibitory synaptic transmission is a promising therapeutic target for correcting some of the circuit abnormalities underlying neuropsychiatric disorders.