Predicting value of pain and analgesia: nucleus accumbens response to noxious stimuli changes in the presence of chronic pain

Altered cerebral blood flow in the anterior cingulate cortex is associated with neuropathic pain

OBJECTIVE

To assess the cerebral blood flow (CBF) in patients with diabetic neuropathic pain, and its changes after duloxetine therapy.

METHODS

Using iodine-123-N-isopropyl-p-iodoamphetamine single-photon emission computed tomography (IMP-SPECT), we performed a cross-sectional study of 44 patients with diabetes, and compared CBF in those with (n = 24) and without neuropathic pain (n = 20). In patients with neuropathic pain, we also longitudinally assessed changes in CBF 3 months after treatment with duloxetine.

RESULTS

IMP-SPECT with voxel-based analyses showed a significant increase in cerebral blood flow in the right anterior cingulate cortex and a decrease in the left ventral striatum in patients with neuropathic pain, compared with those without pain. After duloxetine treatment, volume of interest analyses revealed a decrease in cerebral blood flow in the anterior cingulate cortex in patients with significant pain relief but not in non-responders. Furthermore, voxel-based whole brain correlation analyses demonstrated that greater baseline CBF in the anterior cingulate cortex was associated with better pain relief on the numerical rating scale.

CONCLUSIONS

Our results suggest that the development of neuropathic pain is associated with increased activity in the anterior cingulate cortex, and greater baseline activation of this region may predict treatment responsiveness to pharmacological intervention.

TRIAL REGISTRATION NUMBER

UMIN000017130;Results.

Preliminary report: parasympathetic tone links to functional brain networks during the anticipation and experience of visceral pain

The mechanisms that underpin the anti-nociceptive effect of the parasympathetic nervous system (PNS) on visceral pain remain incompletely understood. We sought to describe the effect of resting parasympathetic tone on functional brain networks during the anticipation and experience of oesophageal pain. 21 healthy participants had their resting cardiac vagal tone (CVT), a validated measure of the PNS, quantified, and underwent functional magnetic resonance imaging during the anticipation and experience of painful oesophageal distention. The relationship between resting CVT and functional brain networks was examined using 11 hypothesis-driven nodes and network-based statistics. A network comprising all nodes was apparent in individuals with high resting CVT, compared to those with low CVT, during oesophageal pain (family wise error rate (FWER)-corrected p < 0.048). Functional connections included the thalamus-amygdala, thalamus-hypothalamus, hypothalamus-nucleus accumbens, amygdala-pallidum, pallidum-nucleus accumbens and insula-pallidum. A smaller network was seen during pain anticipation, comprising the amygdala, pallidum and anterior insula (FWER-corrected p < 0.049). These findings suggest that PNS tone is associated with functional brain networks during the anticipation and experience of visceral pain. Given the role of these subcortical regions in the descending inhibitory modulation of pain, these networks may represent a potential neurobiological explanation for the anti-nociceptive effect of the PNS.

Inhibition of the Prefrontal Projection to the Nucleus Accumbens Enhances Pain Sensitivity and Affect

Cortical mechanisms that regulate acute or chronic pain remain poorly understood. The prefrontal cortex (PFC) exerts crucial control of sensory and affective behaviors. Recent studies show that activation of the projections from the PFC to the nucleus accumbens (NAc), an important pathway in the brain's reward circuitry, can produce inhibition of both sensory and affective components of pain. However, it is unclear whether this circuit is endogenously engaged in pain regulation. To answer this question, we disrupted this circuit using an optogenetic strategy. We expressed halorhodopsin in pyramidal neurons from the PFC, and then selectively inhibited the axonal projection from these neurons to neurons in the NAc core. Our results reveal that inhibition of the PFC or its projection to the NAc, heightens both sensory and affective symptoms of acute pain in naïve rats. Inhibition of this corticostriatal pathway also increased nociceptive sensitivity and the aversive response in a chronic neuropathic pain model. Finally, corticostriatal inhibition resulted in a similar aversive phenotype as chronic pain. These results strongly suggest that the projection from the PFC to the NAc plays an important role in endogenous pain regulation, and its impairment contributes to the pathology of chronic pain.

Emotional and Motivational Pain Processing: Current State of Knowledge and Perspectives in Translational Research

Pain elicits fear and anxiety and promotes escape, avoidance, and adaptive behaviors that are essential for survival. When pain persists, motivational priority and attention shift to pain-related information. Such a shift often results in impaired functionality, leading to maladaptive pain-related fear and anxiety and escape and avoidance behaviors. Neuroimaging studies in chronic pain patients have established that brain activity, especially in cortical and mesolimbic regions, is different from activity observed during acute pain in control subjects. In this review, we discuss the psychophysiological and neuronal factors that may be associated with the transition to chronic pain. We review information from human studies on neural circuits involved in emotional and motivational pain processing and how these circuits are altered in chronic pain conditions. We then highlight findings from animal research that can increase our understanding of the molecular and cellular mechanisms underlying emotional-motivational pain processing in the brain. Finally, we discuss how translational approaches incorporating results from both human and animal investigations may aid in accelerating the discovery of therapies.

Acute Aversive Stimuli Rapidly Increase the Activity of Ventral Tegmental Area Dopamine Neurons in Awake Mice

The ventral tegmental area (VTA) is one of the origins of the brain dopaminergic system and is involved in regulating various physiological functions such as pain processing and motivation. In this study, we utilized a fiber photometry system to specifically investigate the activity of dopamine neurons in the VTA using dopamine transporter promoter-driven Cre recombinase-expressing mice and site-specific infection of adeno-associated virus carrying the FLEX G-CaMP6 gene. As expected, expression of G-CaMP6 was restricted to VTA dopamine neurons. We recorded G-CaMP6 green fluorescent signal, which reflected dopaminergic neuronal activity, in awake mice exposed to tail pinch, ultrasonic sound, predator odor, and a male intruder mouse. These stimuli resulted in a rapid and short-lasting increase in the activity of VTA dopamine neurons while the control stimuli of a gentle tail touch and appearance of empty box did not induce any changes. In addition, fluorescence intensity was not changed by any of these stimuli in the control animals expressing hrGFP instead of G-CaMP6 in VTA dopamine neurons. Our data clearly show that acute aversive stimuli rapidly increase the activity of VTA dopamine neurons and thus suggest a salience-processing role.

Social defeat stress induces hyperalgesia and increases truncated BDNF isoforms in the nucleus accumbens regardless of the depressive-like behavior induction in mice

Epidemiological studies have shown a close association between pain and depression. There is evidence showing this association as patients with depression show a high chronic pain prevalence and vice versa. Considering that social stress is critical for the development of depression in humans, we used a social defeat stress (SDS) model which induces depressive-like behavior in mice. In this model, mice are exposed to an aggressor mouse for ten days, suffering brief periods of agonistic contact and long periods of sensory contact. Some mice display social avoidance, a depressive-like behavior, and are considered susceptible, while some mice do not, and are considered resilient. Thus, we investigated the nociceptive behavior of mice submitted to SDS and the neuroplastic changes in dopaminergic mesolimbic system. Our results showed that the stressed mice (resilient and susceptible) presented a higher sensitivity to pain than the control mice in chemical and mechanical tests. We also verified that susceptible mice have higher Bdnf mRNA in the VTA compared to the resilient and control mice. The stressed mice had less mature BDNF and more truncated BDNF protein in the NAc compared with control mice. Although social stress may trigger the development of depression and hyperalgesia, these two conditions may manifest independently as social stress induced hyperalgesia even in mice that did not display depressive-like behavior. Also, increased Bdnf in the VTA seems to be associated with depressive-like behavior, whereas high levels of truncated BDNF and low mature BDNF appear to be associated with hyperalgesia induced by social defeat stress.

A medial prefrontal cortex-nucleus acumens corticotropin-releasing factor circuitry for neuropathic pain-increased susceptibility to opioid reward

Recent studies have shown that persistent pain facilitates the response to morphine reward. However, the circuit mechanism underlying this process remains ambiguous. In this study, using chronic constriction injury (CCI) of the sciatic nerve in mice, we found that persistent neuropathic pain reduced the minimum number of morphine conditioning sessions required to induce conditioned place preference (CPP) behavior. This dose of morphine had no effect on the pain threshold. In the medial prefrontal cortex (mPFC), which is involved in both pain and emotion processing, corticotropin-releasing factor (CRF) expressing neuronal activity was increased in CCI mice. Chemogenetic inhibition of mPFC CRF neurons reversed CCI-induced morphine CPP facilitation. Furthermore, the nucleus acumens (NAc) received mPFC CRF functional projections that exerted excitatory effects on NAc neurons. Optogenetic inhibition of mPCF neuronal terminals or local infusion of the CRF receptor 1 (CRFR1) antagonist in the NAc restored the effects of neuropathic pain on morphine-induced CPP behavior, but not in normal mice. On a molecular level, in CCI mice, CRFR1 protein expression was increased in the NAc by a histone dimethyltransferase G9a-mediated epigenetic mechanism. Local G9a knockdown increased the expression of CRFR1 and mimicked CCI-induced hypersensitivity to acquiring morphine CPP. Taken together, these findings demonstrate a previously unknown and specific mPFC CRF engagement of NAc neuronal circuits, the sensitization of which facilitates behavioral responses to morphine reward in neuropathic pain states via CRFR1s.

Chronic inflammatory pain decreases the glutamate vesicles in presynaptic terminals of the nucleus accumbens

Reward system has been proved to be important to nociceptive behavior, and the nucleus accumbens (NAc) is a key node in reward circuitry. It has been further revealed that dopamine system modulates the NAc to influence the pain sensation, whereas the role of glutamatergic projection in the NAc in the modulation of chronic pain is still elusive. In this study, we used a complete Freund’s adjuvant-induced chronic inflammatory pain model to explore the changes of the glutamatergic terminals in the NAc, and we found that following the chronic inflammation, the protein level of vesicular glutamate transporter1 (VGLUT1) was significantly decreased in the NAc. Immunofluorescence staining further showed a reduced expression of VGLUT1-positive terminals in the dopamine receptor 2 (D2R) spiny projection neurons of NAc after chronic inflammatory pain. Furthermore, using a whole-cell recording in double transgenic mice, in which dopamine receptor 1- and D2R-expressing neurons can be visualized, we found that the frequency of spontaneous excitatory postsynaptic currents was significantly decreased and paired-pulse ratio of evoked excitatory postsynaptic currents was increased in D2R neurons, but not in dopamine receptor 1 neurons in NAc of complete Freund’s adjuvant group. Moreover, the abnormal expression of soluble N-ethylmaleimide-sensitive factor attachment protein receptor complex contributed to the reduced formation of glutamate vesicles. Hence, our results demonstrated that decreased glutamate release in the indirect pathway of the NAc may be a critical mechanism for chronic pain and provided a novel evidence for the presynaptic mechanisms in chronic pain regulation.

Diffuse noxious inhibitory controls and brain networks are modulated in a testosterone-dependent manner in Sprague Dawley rats

Diffuse noxious inhibitory control (DNIC), which involves endogenous pain modulation, has been investigated as a potential mechanism for the differences in pain modulation observed between men and women, though the literature shows contradictory findings. We used a capsaicin-induced DNIC behavioral assay and resting state functional magnetic resonance imaging (rsfMRI) to assess the effect of testosterone on pain modulation and related brain circuitry in rats. We hypothesized that testosterone is required for DNIC that leads to efficient pain inhibition by increasing descending pain modulation. Male, female, and orchidectomized (GDX) male rats had a capsaicin injection into the forepaw to induce DNIC and mechanical thresholds were observed on the hindpaw. rsfMRI scans were acquired before and after capsaicin injection to analyze the effects of DNIC on periaqueductal gray (PAG), anterior cingulate cortex (ACC) and nucleus accumbens (NAc) connectivity to the whole brain. The strength of DNIC was higher in males compared to females and GDX males. PAG connectivity with prelimbic cortex (PrL), ACC and insula was stronger in males compared to females and GDX males, whereas females and GDX males had increased connectivity between the right ACC, hippocampus and thalamus. GDX males also showed a stronger connectivity between right ACC and NAc, and right NAc with PrL, ACC, insula and thalamus. Our findings suggest that testosterone plays a key role in reinforcing the endogenous pain inhibitory system, while circuitries related to reward and emotion are more strongly recruited in the absence of testosterone.

Learned expectations and uncertainty facilitate pain during classical conditioning

Pain spontaneously activates adaptive and dynamic learning processes affecting the anticipation of, and the responses to, future pain. Computational models of associative learning effectively capture the production and ongoing changes in conditioned anticipatory responses (eg, skin conductance response), but the impact of this dynamic process on unconditional pain responses remains poorly understood. Here, we investigated the dynamic modulation of pain and the nociceptive flexion reflex by fear learning in healthy human adult participants undergoing a classical conditioning procedure involving an acquisition, reversal and extinction phase. Conditioned visual stimuli (CS+) coterminated with a noxious transcutaneous stimulation applied to the sural nerve on 50% of trials (unconditioned stimuli). Expected pain probabilities and cue associability were estimated using computational modeling by fitting a hybrid learning model to skin conductance response elicited by the CS+. Multilevel linear regression analyses confirmed that trial-by-trial changes in expected pain and associability positively predict ongoing fluctuations in pain outcomes. Mediation analysis further demonstrated that both expected probability and associability affect pain perception through a direct effect and an indirect effect mediated by descending modulatory mechanisms affecting spinal nociceptive activity. Moderation analyses further showed that hyperalgesic effects of associability were larger in individuals reporting more harm vigilance and less emotional detachment. Higher harm vigilance was also associated with a stronger mediation of hyperalgesic effects by spinal processes. These results demonstrate how dynamic changes in pain can be explained by associative learning theory and that resilient attitudes towards fear/pain can attenuate the adverse impact of adaptive aversive learning processes on pain.

Longitudinal Structural and Functional Brain Network Alterations in a Mouse Model of Neuropathic Pain

Neuropathic pain affects multiple brain functions, including motivational processing. However, little is known about the structural and functional brain changes involved in the transition from an acute to a chronic pain state. Here we combined behavioral phenotyping of pain thresholds with multimodal neuroimaging to longitudinally monitor changes in brain metabolism, structure and connectivity using the spared nerve injury (SNI) mouse model of chronic neuropathic pain. We investigated stimulus-evoked pain responses prior to SNI surgery, and one and twelve weeks following surgery. A progressive development and potentiation of stimulus-evoked pain responses (cold and mechanical allodynia) were detected during the course of pain chronification. Voxel-based morphometry demonstrated striking decreases in volume following pain induction in all brain sites assessed - an effect that reversed over time. Similarly, all global and local network changes that occurred following pain induction disappeared over time, with two notable exceptions: the nucleus accumbens, which played a more dominant role in the global network in a chronic pain state and the prefrontal cortex and hippocampus, which showed lower connectivity. These changes in connectivity were accompanied by enhanced glutamate levels in the hippocampus, but not in the prefrontal cortex. We suggest that hippocampal hyperexcitability may contribute to alterations in synaptic plasticity within the nucleus accumbens, and to pain chronification.

The Role of Ventral Tegmental Area Gamma-Aminobutyric Acid in Chronic Neuropathic Pain after Spinal Cord Injury in Rats

Spinal cord injury (SCI) frequently results in chronic neuropathic pain (CNP). However, the understanding of brain neural circuits in CNP modulation is unclear. The present study examined the changes of ventral tegmental area (VTA) putative GABAergic and dopaminergic neuronal activity with CNP attenuation in rats. SCI was established by T10 clip compression injury (35 g, 1 min) in rats, and neuropathic pain behaviors, in vivo extracellular single-cell recording of putative VTA gamma-aminobutyric acid (GABA)/dopamine neurons, extracellular GABA level, glutamic acid decarboxylase (GAD), and vesicular GABA transporters (VGATs) were measured in the VTA, respectively. The results revealed that extracellular GABA level was significantly increased in the CNP group (50.5 ± 18.9 nM) compared to the sham control group (10.2 ± 1.7 nM). In addition, expression of GAD_65/67, c-Fos, and VGAT exhibited significant increases in the SCI groups compared to the sham control group. With regard to neuropathic pain behaviors, spontaneous pain measured by ultrasound vocalizations (USVs) and evoked pain measured by paw withdrawal thresholds showed significant alteration, which was reversed by intravenous (i.v.) administration of morphine (0.5-5.0 mg/kg). With regard to in vivo electrophysiology, VTA putative GABAergic neuronal activity (13.6 ± 1.7 spikes/sec) and putative dopaminergic neuronal activity (2.4 ± 0.8 spikes/sec) were increased and decreased, respectively, in the SCI group compared to the sham control group. These neuronal activities were reversed by i.v. administration of morphine. The present study suggests that chronic increase of GABAergic neuronal activity suppresses dopaminergic neuronal activity in the VTA and is responsible for negative emotion and motivation for attenuation of SCI-induced CNP.

Pain Neuroimaging in Humans: A Primer for Beginners and Non-Imagers

Human pain neuroimaging has exploded in the past 2 decades. During this time, the broader neuroimaging community has continued to investigate and refine methods. Another key to progress is exchange with clinicians and pain scientists working with other model systems and approaches. These collaborative efforts require that non-imagers be able to evaluate and assess the evidence provided in these reports. Likewise, new trainees must design rigorous and reliable pain imaging experiments. In this article we provide a guideline for designing, reading, evaluating, analyzing, and reporting results of a pain neuroimaging experiment, with a focus on functional and structural magnetic resonance imaging. We focus in particular on considerations that are unique to neuroimaging studies of pain in humans, including study design and analysis, inferences that can be drawn from these studies, and the strengths and limitations of the approach.

Anterior insular volume decrease is associated with dysfunction of the reward system in patients with chronic pain

BACKGROUND

Chronification of pain is associated with both anatomical and functional alterations of the brain. Alteration in regional grey matter volume might potentially be associated with modified activity of specific brain networks. In this cross-sectional, observational study, we sought to identify brain regions with grey matter volume changes in patients with chronic pain and to reveal its significance by analysing alteration in functional connectivity from those regions. We further explored relevance of such alterations with psychometrics of chronic pain.

METHODS

We recruited 23 patients with chronic pain and 17 age-, gender-matched healthy control subjects. After completing multiple psychophysical questionnaires, each subject underwent resting-state functional magnetic resonance imaging and three-dimensional anatomical imaging on a 3 Tesla magnetic resonance imaging scanner.

RESULTS

Patients with chronic pain showed significant volume decrease at the right anterior insular cortex (p < 0.001) and the left middle cingulate cortex (p < 0.001) compared with healthy controls. They also showed decreased connectivity between the right anterior insular cortex and the left nucleus accumbens in negative association with the Pain Catastrophizing Scale (R²  = 0.20, p = 0.046) and the Beck's Depression Inventory scores (R²  = 0.24, p = 0.017).

CONCLUSIONS

Decreased grey matter volumes of those core regions for affective processing of pain might be a common cerebral feature shared by, at least some of, different aetiologies of chronic pain. Dysfunctional network between the anterior insular cortex and the nucleus accumbens might reflect affective and motivational disability involved in chronic pain. Such anatomical and functional profiles could potentially be part of a cerebral signature for chronification of pain.

SIGNIFICANCE

This article illustrates decreased network activity of the reward system in association with insular cortical volume decrease in patients with chronic pain, and its close relationships with affective and cognitive morbidity of pain. Attenuation of brain's reward system involving cortical plastic changes might have a key role in chronification of pain.

The control of tonic pain by active relief learning

Tonic pain after injury characterises a behavioural state that prioritises recovery. Although generally suppressing cognition and attention, tonic pain needs to allow effective relief learning to reduce the cause of the pain. Here, we describe a central learning circuit that supports learning of relief and concurrently suppresses the level of ongoing pain. We used computational modelling of behavioural, physiological and neuroimaging data in two experiments in which subjects learned to terminate tonic pain in static and dynamic escape-learning paradigms. In both studies, we show that active relief-seeking involves a reinforcement learning process manifest by error signals observed in the dorsal putamen. Critically, this system uses an uncertainty ('associability') signal detected in pregenual anterior cingulate cortex that both controls the relief learning rate, and endogenously and parametrically modulates the level of tonic pain. The results define a self-organising learning circuit that reduces ongoing pain when learning about potential relief.

Effectiveness of myofascial trigger point therapy in chronic back pain patients is considerably increased when combined with a new, integrated, low-frequency shock wave vibrotherapy (Cellconnect Impulse): A two-armed, measurement repeated, randomized, controlled pragmatic trial

BACKGROUND

The prevalence of chronic back pain poses major challenges for all health care systems and patients worldwide. Myofascial trigger therapy (MT), although a very popular standard non-pharmaceutical form of treatment, only shows small to medium effectiveness.

OBJECTIVE

To test a new vibrotreatment (Cellconnect Impulse) transmitting low-frequency, vertical shock waves in a routine clinical practice.

METHODS

Eligible patients were adults seeking physiotherapeutic treatment. They were randomly allocated to either six treatments of MT or to six treatments of combined MT and vibrotreatment. Outcome parameters were pain intensity, pain days, pain duration, and quality of life.

RESULTS

The pain relieving effects of the combined treatment were very large (d= 1.6). It clearly outperformed MT and considerably improved patients' health related quality of life.

CONCLUSIONS

Combining MT with Cellconnect Impulse enhances the physiotherapeutic effectiveness of treating chronic back pain.

Extracellular N-acetylaspartylglutamate released in the nucleus accumbens modulates the pain sensation: Analysis using a microdialysis/mass spectrometry integrated system

Various small molecules act as neurotransmitters and orchestrate neural communication. Growing evidence suggests that not only classical neurotransmitters but also several small molecules, including amino acid derivatives, modulate synaptic transmission. As conditions of acute and chronic pain alter neuronal excitability in the nucleus accumbens, we hypothesized that small molecules released in the nucleus accumbens might play important roles in modulating the pain sensation. However, it is not easy to identify possible pain modulators owing to the absence of a method for comprehensively measuring extracellular small molecules in the brain. In this study, through the use of an emerging metabolomics technique, namely ion chromatography coupled with high-resolution mass spectrometry, we simultaneously analyzed the dynamics of more than 60 small molecules in brain fluids collected by microdialysis, under both the application of pain stimuli and the administration of analgesics. We identified N-acetylaspartylglutamate as a potential pain modulator that is endogenously released in the nucleus accumbens. Infusion of N-acetylaspartylglutamate into the nucleus accumbens significantly attenuated the pain induced by the activation of sensory nerves through optical stimulation. These findings suggest that N-acetylaspartylglutamate released in the nucleus accumbens could modulate pain sensation.

Association between brain and low back pain

Most chronic low back pain includes elements of nociceptive pain, neuropathic pain, and nonorganic pain. We conducted screening for nonorganic pain with use of the Brief Scale for Psychiatric Problems in Orthopaedic Patients (BS-POP), which is simple and can be used for multidimensional assessment. Research on pain areas using functional magnetic resonance imaging (fMRI) and positron emission tomography has shown that the dopamine system contributes to the pathology of chronic low back pain. Chronic low back pain patients show decreased activation of the anterior cingulate cortex, prefrontal cortex, and nucleus accumbens. Given that both the anterior cingulate cortex and prefrontal cortex belong to the descending inhibitory system, and that the nucleus accumbens, which is involved in the dopamine system, releases μ-opioids that act to relieve pain, decreased activation in these three brain regions may be related to decreased function of the descending inhibitory system. A pathological condition that can be explained at the molecular biological level clearly exists between chronic low back pain and psychosocial factors, and investigations of a pathological condition of chronic low back pain including brain function are needed.

Parallels and Overlap: The Integration of Homeostatic Signals by Mesolimbic Dopamine Neurons

Motivated behaviors are often initiated in response to perturbations of homeostasis. Indeed, animals and humans have fundamental drives to procure (appetitive behaviors) and eventually ingest (consummatory behaviors) substances based on deficits in body fluid (e.g., thirst) and energy balance (e.g., hunger). Consumption, in turn, reinforces motivated behavior and is therefore considered rewarding. Over the years, the constructs of homeostatic (within the purview of the hypothalamus) and reward (within the purview of mesolimbic circuitry) have been used to describe need-based vs. need-free consumption. However, many experiments have demonstrated that mesolimbic circuits and "higher-order" brain regions are also profoundly influenced by changes to physiological state, which in turn generate behaviors that are poised to maintain homeostasis. Mesolimbic pathways, particularly dopamine neurons of the ventral tegmental area (VTA) and their projections to nucleus accumbens (NAc), can be robustly modulated by a variety of energy balance signals, including post-ingestive feedback relaying nutrient content and hormonal signals reflecting hunger and satiety. Moreover, physiological states can also impact VTA-NAc responses to non-nutritive rewards, such as drugs of abuse. Coupled with recent evidence showing hypothalamic structures are modulated in anticipation of replenished need, classic boundaries between circuits that convey perturbations in homeostasis and those that drive motivated behavior are being questioned. In the current review, we examine data that have revealed the importance of mesolimbic dopamine neurons and their downstream pathways as a dynamic neurobiological mechanism that provides an interface between physiological state, perturbations to homeostasis, and reward-seeking behaviors.

"Luteal Analgesia": Progesterone Dissociates Pain Intensity and Unpleasantness by Influencing Emotion Regulation Networks

Background: Pregnancy-induced analgesia is known to occur in association with the very high levels of estradiol and progesterone circulating during pregnancy. In women with natural ovulatory menstrual cycles, more modest rises in these hormones occur on a monthly basis. We therefore hypothesized that the high estradiol high progesterone state indicative of ovulation would be associated with a reduction in the pain experience. Methods: We used fMRI and a noxious thermal stimulus to explore the relationship between sex steroid hormones and the pain experience. Specifically, we assessed the relationship with stimulus-related activity in key regions of networks involved in emotion regulation, and functional connectivity between these regions. Results: We demonstrate that physiologically high progesterone levels are associated with a reduction in the affective component of the pain experience and a dissociation between pain intensity and unpleasantness. This dissociation is related to decreased functional connectivity between the inferior frontal gyrus and amygdala. Moreover, we have shown that in the pre-ovulatory state, the traditionally "male" sex hormone, testosterone, is the strongest hormonal regulator of pain-related activity and connectivity within the emotional regulation network. However, following ovulation the traditionally "female" sex hormones, estradiol and progesterone, appear to dominate. Conclusions: We propose that a phenomenon of "luteal analgesia" exists with potential reproductive advantages.

Neural Mechanisms of Offset Analgesia

One feels a disproportionately large decrease of pain sensation on a slight decrease of thermal pain stimulus. Such phenomenon is termed offset analgesia and considered mediated by endogenous analgesic mechanisms. Offset analgesia was found attenuated in patients with neuropathic pain. We further found that such attenuation occurred in a more heterogeneous population of patients with chronic pain. By functional magnetic resonance imaging, we also found negative blood oxygenation level-dependent signals at those areas concerned with descending pain modulatory and reward systems during offset analgesia in the same cohort of patients. We propose that dysfunction of those systems, as revealed by attenuation of offset analgesia, might well be part of neural mechanisms of pain chronification.

Activation of ventral tegmental area dopaminergic neurons reverses pathological allodynia resulting from nerve injury or bone cancer

Chronic pain induced by nerve damage due to trauma or invasion of cancer to the bone elicits severe ongoing pain as well as hyperalgesia and allodynia likely reflecting adaptive changes within central circuits that amplify nociceptive signals. The present study explored the possible contribution of the mesolimbic dopaminergic circuit in promoting allodynia related to neuropathic and cancer pain. Mice with ligation of the sciatic nerve or treated with intrafemoral osteosarcoma cells showed allodynia to a thermal stimulus applied to the paw on the injured side. Patch clamp electrophysiology revealed that the intrinsic neuronal excitability of ventral tegmental area (VTA) dopamine neurons projecting to the nucleus accumbens (N.Acc.) was significantly reduced in those mice. We used tyrosine hydroxylase (TH)-cre mice that were microinjected with adeno-associated virus (AAV) to express channelrhodopsin-2 (ChR2) to allow optogenetic stimulation of VTA dopaminergic neurons in the VTA or in their N.Acc. terminals. Optogenetic activation of these cells produced a significant but transient anti-allodynic effect in nerve injured or tumor-bearing mice without increasing response thresholds to thermal stimulation in sham-operated animals. Suppressed activity of mesolimbic dopaminergic neurons is likely to contribute to decreased inhibition of N.Acc. output neurons and to neuropathic or cancer pain-induced allodynia suggesting strategies for modulation of pathological pain states.

Threat Response System: Parallel Brain Processes in Pain vis-à-vis Fear and Anxiety

Pain is essential for avoidance of tissue damage and for promotion of healing. Notwithstanding the survival value, pain brings about emotional suffering reflected in fear and anxiety, which in turn augment pain thus giving rise to a self-sustaining feedforward loop. Given such reciprocal relationships, the present article uses neuroscientific conceptualizations of fear and anxiety as a theoretical framework for hitherto insufficiently understood pathophysiological mechanisms underlying chronic pain. To that end, searches of PubMed-indexed journals were performed using the following Medical Subject Headings' terms: pain and nociception plus amygdala, anxiety, cognitive, fear, sensory, and unconscious. Recursive sets of scientific and clinical evidence extracted from this literature review were summarized within the following key areas: (1) parallelism between acute pain and fear and between chronic pain and anxiety; (2) all are related to the evasion of sensory-perceived threats and are subserved by subcortical circuits mediating automatic threat-induced physiologic responses and defensive actions in conjunction with higher order corticolimbic networks (e.g., thalamocortical, thalamo-striato-cortical and amygdalo-cortical) generating conscious representations and valuation-based adaptive behaviors; (3) some instances of chronic pain and anxiety conditions are driven by the failure to diminish or block respective nociceptive information or unconscious treats from reaching conscious awareness; and (4) the neural correlates of pain-related conscious states and cognitions may become autonomous (i.e., dissociated) from the subcortical activity/function leading to the eventual chronicity. Identifying relative contributions of the diverse neuroanatomical sources, thus, offers prospects for the development of novel preventive, diagnostic, and therapeutic strategies in chronic pain patients.

Evaluation of Chronic Pain Using Magnetic Resonance (MR) Neuroimaging Approaches: What the Clinician Needs to Know

OBJECTIVES

Numerous neuroimaging techniques have been recently used to investigate central mechanisms involved in pain perception and to examine morphological and functional brain alterations associated with chronic pain. Compared to self-reporting approaches, objective imaging techniques are expected to potentially lead to better pain assessment and guide management. This comprehensive scoping review aims to identify recent magnetic resonance imaging (MRI) approaches that have been used to characterize the brain of chronic pain subjects, using structural, chemical and functional MRI techniques.

METHODS

A systematic search and review of the literature was conducted and the resultant studies were critically examined for relevance.

RESULTS

MRI neuroimaging of various chronic pain conditions were summarized. We classified the collected studies into: structural brain alterations, VBM (voxel based morphology) examination of structural changes, DTI, changes in brain chemistry, functional and blood flow brain alterations.

DISCUSSION

From our clinical experience, we have noted that most clinicians are not aware of the capabilities of advanced MRI methods in assessing cortical manifestations of chronic pain. In addition, many clinicians are not aware of the cortical alterations present in individuals with chronic pain. This comprehensive scoping review thus sets out to first summarize MRI neuroimaging techniques that are available in the current literature to examine chronic pain. We then identify cortical MR approaches that have been able to reliably predict transition from acute to chronic pain. Finally, we summarize MRI neuroimaging techniques that have been used to track treatment response of individuals with chronic pain.

Ghrelin attenuated hyperalgesia induced by chronic nitroglycerin: CGRP and TRPV1 as targets for migraine management

Background

According to the neurovascular theory of migraine, activation of the trigeminovascular system contributes to the development of migraine. This study examined the effects of chronic intraperitoneal ghrelin (150 µg/kg) treatment on the development of chronic migraine induced by intermittent injection of nitroglycerin 10 mg/kg.

Methods

Baseline and post-drug (2 h following nitroglycerin injection) mechanical and thermal sensitivity were assessed by von Frey hair and tail immersion tests, respectively on days 1, 3, 5, 7, 9 and 11. Moreover, we investigated the effect of ghrelin treatment on nitroglycerin-induced aversive behavior by using a two-chamber conditioned place aversion paradigm. At the end of behavioral testing, on day 11, animals were sacrificed and plasma concentration of calcitonin gene-related peptide was measured using a rat-specific enzyme-linked immunosorbent assay kit. Also, real time polymerase chain reaction was used to quantify mRNA expression of calcitonin gene-related peptide and transient receptor potential vanilloid 1 in the trigeminal ganglion.

Results

Our results indicated that nitroglycerin activated the trigeminovascular system, which was reflected by mechanical and thermal hypersensitivity and elevation of mRNA expression of calcitonin gene-related peptide and transient receptor potential vanilloid-1, as migraine markers, and plasma calcitonin gene-related peptide levels. Moreover, chronic nitroglycerin injection induced conditioned place aversion and body weight loss. Nevertheless, ghrelin modulated nitroglycerin-triggered changes in transient receptor potential vanilloid-1 and calcitonin gene-related peptide expression, and mitigated nitroglycerin-induced hyperalgesia.

Conclusion

These results provide the first convincing evidence that ghrelin has a modulating effect on central sensitization induced by chronic intermittent nitroglycerin, and its antinociceptive effect may be related to a reduction of these factors in the trigeminal ganglion.

Rationale for and approach to preoperative opioid weaning: a preoperative optimization protocol

The practice of chronic opioid prescription for chronic non-cancer pain has come under considerable scrutiny within the past several years as mounting evidence reveals a generally unfavorable risk to benefit ratio and the nation reels from the grim mortality statistics associated with the opioid epidemic. Patients struggling with chronic pain tend to use opioids and also seek out operative intervention for their complaints, which combination may be leading to increased postoperative "acute-on-chronic" pain and fueling worsened chronic pain and opioid dependence. Besides worsened postoperative pain, a growing body of literature, reviewed herein, indicates that preoperative opioid use is associated with significantly worsened surgical outcomes, and severely increased financial drain on an already severely overburdened healthcare budget. Conversely, there is evidence that preoperative opioid reduction may result in substantial improvements in outcome. In the era of accountable care, efforts such as the Enhanced Recovery After Surgery (ERAS) protocol have been introduced in an attempt to standardize and facilitate evidence-based perioperative interventions to optimize surgical outcomes. We propose that addressing preoperative opioid reduction as part of a targeted optimization approach for chronic pain patients seeking surgery is not only logical but mandatory given the stakes involved. Simple opioid reduction/abstinence however is not likely to occur in the absence of provision of viable and palatable alternatives to managing pain, which will require a strong focus upon reducing pain catastrophization and bolstering self-efficacy and resilience. In response to a call from our surgical community toward that end, we have developed a simple and easy-to-implement outpatient preoperative optimization program focusing on gentle opioid weaning/elimination as well as a few other high-yield areas of intervention, requiring a minimum of resources.

Reward Circuitry Plasticity in Pain Perception and Modulation

Although pain is a widely known phenomenon and an important clinical symptom that occurs in numerous diseases, its mechanisms are still barely understood. Owing to the scarce information concerning its pathophysiology, particularly what is involved in the transition from an acute state to a chronic condition, pain treatment is frequently unsatisfactory, therefore contributing to the amplification of the chronic pain burden. In fact, pain is an extremely complex experience that demands the recruitment of an intricate set of central nervous system components. This includes cortical and subcortical areas involved in interpretation of the general characteristics of noxious stimuli. It also comprises neural circuits that process the motivational-affective dimension of pain. Hence, the reward circuitry represents a vital element for pain experience and modulation. This review article focuses on the interpretation of the extensive data available connecting the major components of the reward circuitry to pain suffering, including the nucleus accumbens, ventral tegmental area, and the medial prefrontal cortex; with especial attention dedicated to the evaluation of neuroplastic changes affecting these structures found in chronic pain syndromes, such as migraine, trigeminal neuropathic pain, chronic back pain, and fibromyalgia.

Brain activity for tactile allodynia: a longitudinal awake rat functional magnetic resonance imaging study tracking emergence of neuropathic pain

Tactile allodynia, a condition in which innocuous mechanical stimuli are perceived as painful, is a common feature of chronic pain. However, how the brain reorganizes in relation to the emergence of tactile allodynia is still largely unknown. This may stem from the fact that experiments in humans are cross-sectional in nature, whereas animal brain imaging studies typically require anaesthesia rendering the brain incapable of consciously sensing or responding to pain. In this longitudinal functional magnetic resonance imaging study in awake rats, we tracked brain activity with the development of tactile allodynia. Before injury, innocuous air-puff stimuli evoked a distributed sensory network of activations, including contralateral somatosensory cortices, thalamus, insula, and cingulate cortex. Moreover, the primary somatosensory cortex displayed a graded response tracking air-puff stimulus intensities. After neuropathic injury, and for stimuli in which the intensity exceeded the paw withdrawal threshold (evoking tactile allodynia), the blood oxygenation level-dependent response in the primary somatosensory cortex was equivalent to that evoked by the identical stimulus before injury. In contrast, nucleus accumbens and prefrontal brain areas displayed abnormal activity to normally innocuous stimuli when such stimuli induced tactile allodynia at 28 days after peripheral nerve injury, which had not been the case at 5 days after injury. Our data indicate that tactile allodynia-related nociceptive inputs are not observable in the primary somatosensory cortex BOLD response. Instead, our data suggest that, in time, tactile allodynia differentially engages neural circuits that regulate the affective and motivational components of pain.

Hedonic and motivational responses to food reward are unchanged in rats with neuropathic pain

Rewards influence responses to acute painful stimuli, but the relationship of chronic pain to hedonic or motivational aspects of reward is not well understood. We independently evaluated hedonic qualities of sweet or bitter tastants and motivation to seek food reward in rats with experimental neuropathic pain induced by L5/6 spinal nerve ligation. Hedonic response was measured by implantation of intraoral catheters to allow passive delivery of liquid solutions, and "liking/disliking" responses were scored according to a facial reactivity scale. Spinal nerve ligation rats did not differ from controls in either "liking" or "disliking" reactions to intraoral sucrose or quinine, respectively, at postsurgery day 21, suggesting no differences in perceived hedonic value of sweet or bitter tastants. To assess possible motivational deficits during acute and chronic pain, we used fixed- and progressive-ratio response paradigms of sucrose pellet presentation in rats with transient inflammatory or chronic neuropathic pain. Assessment of response acquisition and break points under the progressive ratio schedule revealed no differences between sham and spinal nerve ligation rats for up to 120 days after injury. However, rats with inflammation showed decrements in lever pressing and break points on days 1 and 2 after complete Freund adjuvant injection that normalized by day 4, consistent with transient ongoing pain. Thus, although acute ongoing inflammatory pain may transiently reduce reward motivation, we did not detect influences of chronic neuropathic pain on hedonic or motivational responses to food rewards. Adaptations that allow normal reward responding to food regardless of chronic pain may be of evolutionary benefit to promote survival.

Brain-Derived Neurotrophic Factor in the Mesolimbic Reward Circuitry Mediates Nociception in Chronic Neuropathic Pain

BACKGROUND

The mesolimbic reward system plays a critical role in modulating nociception; however, its underlying molecular, cellular, and neural circuitry mechanisms remain unknown.

METHODS

Chronic constrictive injury (CCI) of the sciatic nerve was used to model neuropathic pain. Projection-specific in vitro recordings in mouse brain slices and in vivo recordings from anesthetized animals were used to measure firing of dopaminergic neurons in the ventral tegmental area (VTA). The role of VTA-nucleus accumbens (NAc) circuitry in nociceptive regulation was assessed using optogenetic and pharmacological manipulations, and the underlying molecular mechanisms were investigated by Western blotting, enzyme-linked immunosorbent assays, and conditional knockdown techniques.

RESULTS

c-Fos expression in and firing of contralateral VTA-NAc dopaminergic neurons were elevated in CCI mice, and optogenetic inhibition of these neurons reversed CCI-induced thermal hyperalgesia. CCI increased the expression of brain-derived neurotrophic factor (BDNF) protein but not messenger RNA in the contralateral NAc. This increase was reversed by pharmacological inhibition of VTA dopaminergic neuron activity, which induced an antinociceptive effect that was neutralized by injecting exogenous BDNF into the NAc. Moreover, inhibition of BDNF synthesis in the VTA with anisomycin or selective knockdown of BDNF in the VTA-NAc pathway was antinociceptive in CCI mice.

CONCLUSIONS

These results reveal a novel mechanism of nociceptive modulation in the mesolimbic reward circuitry and provide new insight into the neural circuits involved in the processing of nociceptive information.

HIV Distal Neuropathic Pain Is Associated with Smaller Ventral Posterior Cingulate Cortex

Objective

. Despite modern antiretroviral therapy, HIV-associated neuropathy is one of the most prevalent, disabling and treatment-resistant complications of HIV disease. The presence and intensity of distal neuropathic pain is not fully explained by the degree of peripheral nerve damage. A better understanding of brain structure in HIV distal neuropathic pain may help explain why some patients with HIV neuropathy report pain while the majority does not. Previously, we reported that more intense distal neuropathic pain was associated with smaller total cerebral cortical gray matter volumes. The objective of this study was to determine which parts of the cortex are smaller.

Methods

. HIV positive individuals with and without distal neuropathic pain enrolled in the multisite (N = 233) CNS HIV Antiretroviral Treatment Effects (CHARTER) study underwent structural brain magnetic resonance imaging. Voxel-based morphometry was used to investigate regional brain volumes in these structural brain images.

Results

. Left ventral posterior cingulate cortex was smaller for HIV positive individuals with versus without distal neuropathic pain (peak P  = 0.017; peak t = 5.15; MNI coordinates x = -6, y = -54, z = 20). Regional brain volumes within cortical gray matter structures typically associated with pain processing were also smaller for HIV positive individuals having higher intensity ratings of distal neuropathic pain.

Conclusions

. The posterior cingulate is thought to be involved in inhibiting the perception of painful stimuli. Mechanistically a smaller posterior cingulate cortex structure may be related to reduced anti-nociception contributing to increased distal neuropathic pain.

Altered right anterior insular connectivity and loss of associated functions in adolescent chronic fatigue syndrome

Impairments in cognition, pain intolerance, and physical inactivity characterize adolescent chronic fatigue syndrome (CFS), yet little is known about its neurobiology. The right dorsal anterior insular (dAI) connectivity of the salience network provides a motivational context to stimuli. In this study, we examined regional functional connectivity (FC) patterns of the right dAI in adolescent CFS patients and healthy participants. Eighteen adolescent patients with CFS and 18 aged-matched healthy adolescent control participants underwent resting-state functional magnetic resonance imaging. The right dAI region of interest was examined in a seed-to-voxel resting-state FC analysis using SPM and CONN toolbox. Relative to healthy adolescents, CFS patients demonstrated reduced FC of the right dAI to the right posterior parietal cortex (PPC) node of the central executive network. The decreased FC of the right dAI–PPC might indicate impaired cognitive control development in adolescent CFS. Immature FC of the right dAI–PPC in patients also lacked associations with three known functional domains: cognition, pain and physical activity, which were observed in the healthy group. These results suggest a distinct biological signature of adolescent CFS and might represent a fundamental role of the dAI in motivated behavior.

Affective Circuitry Alterations in Patients with Trigeminal Neuralgia

Trigeminal neuralgia (TN) is a severe chronic neuropathic facial pain disorder. Affect-related behavioral and structural brain changes have been noted across chronic pain disorders, but have not been well-studied in TN. We examined the potential impact of TN (37 patients: 23 with right-sided TN, 14 with left-sided TN), compared to age- and sex-matched healthy controls, on three major white matter tracts responsible for carrying affect-related signals—i.e., cingulum, fornix, and medial forebrain bundle. Diffusion magnetic resonance imaging (dMRI), deterministic multi-tensor tractography for tract modeling, and a model-driven region-of-interest approach was used. We also used volumetric gray matter analysis on key targets of these pathways (i.e., hippocampus, cingulate cortex subregions, nucleus accumbens, and ventral diencephalon). Hypotheses included: (1) successful modeling of tracts; (2) altered white matter microstructure of the cingulum and medial forebrain bundle (via changes in dMRI metrics such as fractional anisotropy, and mean, axial, and radial diffusivities) compared to controls; (3) no alterations in the control region of the fornix; (4) corresponding decreases in gray matter volumes. Results showed (1) all 325 tracts were successfully modeled, although 11 were partially complete; (2) The cingulum and medial forebrain bundle (MFB) were altered in those with TN, with dMRI metric changes in the middle (p = 0.001) and posterior cingulum (p < 0.0001), and the MFB near the ventral tegmental area (MFB-VTA) (p = 0.001). The posterior cingulum and MFB-VTA also showed unilateral differences between right- and left-sided TN patients; (3) No differences were noted at any fornix subdivision; (4) decreased volumes were noted for the hippocampus, posterior cingulate, nucleus accumbens, and ventral diencephalon. Together, these results support the notion of selectively altered affective circuits in patients with TN, which may be related to the experience of negative affect and the increased comorbidity of mood and anxiety disorders in this population.

A comparative study of cognitive behavioural therapy and shared reading for chronic pain

The case for psychosocial interventions in relation to chronic pain, one of the most common health issues in contemporary healthcare, is well-established as a means of managing the emotional and psychological difficulties experienced by sufferers. Using mixed methods, this study compared a standard therapy for chronic pain, cognitive behavioural therapy (CBT), with a specific literature-based intervention, shared reading (SR) developed by national charity, The Reader. A 5-week CBT group and a 22-week SR group for patients with chronic pain ran in parallel, with CBT group members joining the SR group after the completion of CBT. In addition to self-report measures of positive and negative affect before and after each experience of the intervention, the 10 participants kept twice-daily (12-hourly) pain and emotion diaries. Qualitative data were gathered via literary-linguistic analysis of audio/video-recordings and transcriptions of the CBT and SR sessions and video-assisted individual qualitative interviews with participants. Qualitative evidence indicates SR's potential as an alternative or long-term follow-up or adjunct to CBT in bringing into conscious awareness areas of emotional pain otherwise passively suffered by patients with chronic pain. In addition, quantitative analysis, albeit of limited pilot data, indicated possible improvements in mood/pain for up to 2 days following SR. Both findings lay the basis for future research involving a larger sample size.

AMPAkines Target the Nucleus Accumbens to Relieve Postoperative Pain

BACKGROUND

AMPAkines augment the function of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors in the brain to increase excitatory outputs. These drugs are known to relieve persistent pain. However, their role in acute pain is unknown. Furthermore, a specific molecular and anatomic target for these novel analgesics remains elusive.

METHODS

The authors studied the analgesic role of an AMPAkine, CX546, in a rat paw incision (PI) model of acute postoperative pain. The authors measured the effect of AMPAkines on sensory and depressive symptoms of pain using mechanical hypersensitivity and forced swim tests. The authors asked whether AMPA receptors in the nucleus accumbens (NAc), a key node in the brain's reward and pain circuitry, can be a target for AMPAkine analgesia.

RESULTS

Systemic administration of CX546 (n = 13), compared with control (n = 13), reduced mechanical hypersensitivity (50% withdrawal threshold of 6.05 ± 1.30 g [mean ± SEM] vs. 0.62 ± 0.13 g), and it reduced depressive features of pain by decreasing immobility on the forced swim test in PI-treated rats (89.0 ± 15.5 vs. 156.7 ± 18.5 s). Meanwhile, CX546 delivered locally into the NAc provided pain-relieving effects in both PI (50% withdrawal threshold of 6.81 ± 1.91 vs. 0.50 ± 0.03 g; control, n = 6; CX546, n = 8) and persistent postoperative pain (spared nerve injury) models (50% withdrawal threshold of 3.85 ± 1.23 vs. 0.45 ± 0.00 g; control, n = 7; CX546, n = 11). Blocking AMPA receptors in the NAc with 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo[f]quinoxaline-2,3-dione inhibited these pain-relieving effects (50% withdrawal threshold of 7.18 ± 1.52 vs. 1.59 ± 0.66 g; n = 8 for PI groups; 10.70 ± 3.45 vs. 1.39 ± 0.88 g; n = 4 for spared nerve injury groups).

CONCLUSIONS

AMPAkines relieve postoperative pain by acting through AMPA receptors in the NAc.

Dysfunction of Nucleus Accumbens Is Associated With Psychiatric Problems in Patients With Chronic Low Back Pain: A Functional Magnetic Resonance Imaging Study

STUDY DESIGN

A cross-sectional study.

OBJECTIVE

The aim of this study was to evaluate activity of the nucleus accumbens (NAc) in response to lumbar mechanical stimulation in patients with chronic low back pain (cLBP) using functional magnetic resonance imaging (fMRI).

SUMMARY OF BACKGROUND DATA

Although a modified activity of the NAc was characterized in cLBP patients, its pathological significance has yet to be determined. We hypothesized that NAc activation in response to pain might differ depending on the extent of psychiatric problems, which might be associated with the affective/motivational background of chronic pain.

METHODS

Twenty-one patients with cLBP (four men, 17 women) were recruited. Subjects were divided into two groups on the basis of scores on the patient version of the Brief Scale for Psychiatric problems in Orthopaedic Patients (BS-POP) scores: ≥17 (High Score, HiS group) and <17 (non-High Score, non-HiS group). Each subject was placed in the prone position on a 3-Tesla magnetic resonance imaging (MRI) scanner and stimulated by mechanical stimulation on the left lower back. Three blocks of 30-second pain stimulus calibrated at either 3 or 5 on an 11-grade numerical rating scale (NRS) were applied with intervening 30-second rest conditions during whole-brain echo-planar imaging. Functional images were analyzed using a multisubject general linear model with Bonferroni multiple comparisons.

RESULTS

Subjects in the HiS group had more intense daily pain and lower quality of life than those in the non-HiS group (P < 0.05). Catastrophic thinking in relation to pain experience did not differ between the groups. Activation at the NAc was smaller in the HiS group than in the non-HiS group (P < 0.001).

CONCLUSION

The presence of psychiatric problems was associated with attenuated activity of the NAc in cLBP patients. Dysfunction of the NAc might potentially be involved in the affective/motivational factors in the chronification of LBP.

LEVEL OF EVIDENCE

N/A.

Corticostriatal Regulation of Acute Pain

The mechanisms for acute pain regulation in the brain are not well understood. The prefrontal cortex (PFC) provides top-down control of emotional processes, and it projects to the nucleus accumbens (NAc). This corticostriatal projection forms an important regulatory pathway within the brain’s reward system. Recently, this projection has been suggested to control both sensory and affective phenotypes specifically associated with chronic pain. As this projection is also known to play a role in the transition from acute to chronic pain, we hypothesized that this corticostriatal circuit can also exert a modulatory function in the acute pain state. Here, we used optogenetics to specifically target the projection from the PFC to the NAc. We tested sensory pain behaviors with Hargreaves’ test and mechanical allodynia, and aversive pain behaviors with conditioned place preference (CPP) test. We found that the activation of this corticostriatal circuit gave rise to bilateral relief from peripheral nociceptive inputs. Activation of this circuit also provided important control for the aversive response to transient noxious stimulations. Hence, our results support a novel role for corticostriatal circuitry in acute pain regulation.

Relief of Pain-Depressed Behavior in Rats by Activation of D1-Like Dopamine Receptors

Clinically significant pain often includes a decrease in both behavior and mesolimbic dopamine signaling. Indirect and/or direct dopamine receptor agonists may alleviate pain-related behavioral depression. To test this hypothesis, the present study compared effects of indirect and direct dopamine agonists in a preclinical assay of pain-depressed operant responding. Male Sprague-Dawley rats with chronic indwelling microelectrodes in the medial forebrain bundle were trained in an intracranial self-stimulation (ICSS) procedure to press a lever for pulses of electrical brain stimulation. Intraperitoneal injection of dilute lactic acid served as an acute noxious stimulus to depress ICSS. Intraperitoneal lactic acid-induced depression of ICSS was dose-dependently blocked by the dopamine transporter inhibitor methylphenidate and the D1-selective agonist SKF82958, but not by the D2/3-selective agonists quinpirole, pramipexole, or sumanirole. The antinociceptive effects of methylphenidate and SKF82958 were blocked by the D1-selective antagonist SCH39166. Acid-induced stimulation of a stretching response was evaluated in separate groups of rats, but all agonists decreased acid-stimulated stretching, and antagonism experiments were inconclusive due to direct effects of the antagonists when administered alone. Taken together, these results suggest that D1-receptor stimulation is both sufficient to block acid-induced depression of ICSS and necessary for methylphenidate antinociception in this procedure. Conversely, D2/3-receptor stimulation is not sufficient to relieve pain-depressed behavior. These results support the hypothesis that pain-related depression of dopamine D1 receptor signaling contributes to pain-related depression of behavior in rats. Additionally, these results support further consideration of indirect dopamine agonists and direct D1 receptor agonists as candidate treatments for pain-related behavioral depression.

Neuroplasticity of Supraspinal Structures Associated with Pathological Pain

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

Neuropathic pain promotes adaptive changes in gene expression in brain networks involved in stress and depression

Neuropathic pain is a complex chronic condition characterized by various sensory, cognitive, and affective symptoms. A large percentage of patients with neuropathic pain are also afflicted with depression and anxiety disorders, a pattern that is also seen in animal models. Furthermore, clinical and preclinical studies indicate that chronic pain corresponds with adaptations in several brain networks involved in mood, motivation, and reward. Chronic stress is also a major risk factor for depression. We investigated whether chronic pain and stress affect similar molecular mechanisms and whether chronic pain can affect gene expression patterns that are involved in depression. Using two mouse models of neuropathic pain and depression [spared nerve injury (SNI) and chronic unpredictable stress (CUS)], we performed next-generation RNA sequencing and pathway analysis to monitor changes in gene expression in the nucleus accumbens (NAc), the medial prefrontal cortex (mPFC), and the periaqueductal gray (PAG). In addition to finding unique transcriptome profiles across these regions, we identified a substantial number of signaling pathway-associated genes with similar changes in expression in both SNI and CUS mice. Many of these genes have been implicated in depression, anxiety, and chronic pain in patients. Our study provides a resource of the changes in gene expression induced by long-term neuropathic pain in three distinct brain regions and reveals molecular connections between pain and chronic stress.

Multiple faces of pain: effects of chronic pain on the brain regulation of facial expression

Pain behaviors are shaped by social demands and learning processes, and chronic pain has been previously suggested to affect their meaning. In this study, we combined functional magnetic resonance imaging with in-scanner video recording during thermal pain stimulations and use multilevel mediation analyses to study the brain mediators of pain facial expressions and the perception of pain intensity (self-reports) in healthy individuals and patients with chronic back pain (CBP). Behavioral data showed that the relation between pain expression and pain report was disrupted in CBP. In both patients with CBP and healthy controls, brain activity varying on a trial-by-trial basis with pain facial expressions was mainly located in the primary motor cortex and completely dissociated from the pattern of brain activity varying with pain intensity ratings. Stronger activity was observed in CBP specifically during pain facial expressions in several nonmotor brain regions such as the medial prefrontal cortex, the precuneus, and the medial temporal lobe. In sharp contrast, no moderating effect of chronic pain was observed on brain activity associated with pain intensity ratings. Our results demonstrate that pain facial expressions and pain intensity ratings reflect different aspects of pain processing and support psychosocial models of pain suggesting that distinctive mechanisms are involved in the regulation of pain behaviors in chronic pain.

Increased fronto-hippocampal connectivity in the Prrxl1 knockout mouse model of congenital hypoalgesia

Despite the large number of studies addressing how prolonged painful stimulation affects brain functioning, there are only a handful of studies aimed at uncovering if persistent conditions of reduced pain perception would also result in brain plasticity. Permanent hypoalgesia induced by neonatal injection of capsaicin or carrageenan has already been shown to affect learning and memory and to induce alterations in brain gene expression. In this study, we used the Prrxl1 model of congenital mild hypoalgesia to conduct a detailed study of the neurophysiological and behavioral consequences of reduced pain experience. Prrxl1 knockout animals are characterized by selective depletion of small diameter primary afferents and abnormal development of the superficial dorsal laminae of the spinal cord, resulting in diminished pain perception but normal tactile and motor behaviour. Behavioral testing of Prrxl1 mice revealed that these animals have reduced anxiety levels, enhanced memory performance, and improved fear extinction. Neurophysiological recordings from awake behaving Prrxl1 mice show enhanced altered fronto-hippocampal connectivity in the theta- and gamma-bands. Importantly, although inflammatory pain by Complete Freund Adjuvant injection caused a decrease in fronto-hippocampal connectivity in the wild-type animals, Prrxl1 mice maintained the baseline levels. The onset of inflammatory pain also reverted the differences in forebrain expression of stress- and monoamine-related genes in Prrxl1 mice. Altogether our results suggest that congenital hypoalgesia may have an effect on brain plasticity that is the inverse of what is usually observed in animal models of chronic pain.