Functional interaction between medial thalamus and rostral anterior cingulate cortex in the suppression of pain affect

The translocator protein gene is associated with endogenous pain modulation and the balance between glutamate and γ-aminobutyric acid in fibromyalgia and healthy subjects: a multimodal neuroimaging study

ABSTRACT

A cerebral upregulation of the translocator protein (TSPO), a biomarker of glial activation, has been reported in fibromyalgia subjects (FMS). The TSPO binding affinity is genetically regulated by the Ala147Thr polymorphism in the TSPO gene (rs6971) and allows for a subject classification into high affinity binders (HABs) and mixed/low affinity binders (MLABs). The aim of the present multimodal neuroimaging study was to examine the associations of the TSPO polymorphism with: (1) conditioned pain modulation, (2) expectancy-modulated pain processing assessed during functional magnetic resonance imaging, and (3) the concentration and balance of glutamate and γ-aminobutyric acid in the rostral anterior cingulate cortex and thalamus using proton magnetic resonance spectroscopy in FMS (n = 83) and healthy controls (n = 43). The influence of TSPO on endogenous pain modulation presented in the form of TSPO HABs, as opposed to MLABs, displaying less efficient descending pain inhibition and expectancy-induced reduction of pain. Translocator protein HABs in both groups (FM and healthy controls) were found to have higher thalamic glutamate concentrations and exhibit a pattern of positive correlations between glutamate and γ-aminobutyric acid in the rostral anterior cingulate cortex, not seen in MLABs. Altogether, our findings point to TSPO-related mechanisms being HAB-dependent, brain region-specific, and non-FM-specific, although in FMS the disadvantage of an aberrant pain regulation combined with an HAB genetic set-up might hamper pain modulation more strongly. Our results provide evidence for an important role of TSPO in pain regulation and brain metabolism, thereby supporting the ongoing drug development targeting TSPO-associated mechanisms for pain relief.

Affective empathy and prosocial behavior in rodents

Empathy is an essential function for humans as social animals. Emotional contagion, the basic form of afffective empathy, comprises the cognitive process of perceiving and sharing the affective state of others. The observational fear assay, an animal model of emotional contagion, has enabled researchers to undertake molecular, cellular, and circuit mechanism of this behavior. Such studies have revealed that observational fear is mediated through neural circuits involved in processing the affective dimension of direct pain experiences. A mouse can also respond to milder social stimuli induced by either positive or negative emotional changes in another mouse, which seems not dependent on the affective pain circuits. Further studies should explore how different neural circuits contribute to integrating different dimensions of affective empathy.

The Effect of Optogenetic Inhibition of the Anterior Cingulate Cortex in Neuropathic Pain Following Sciatic Nerve Injury

Cortical disinhibition is the underlying pathological alteration contributing to neuropathic pain associated with peripheral nerve injury. Nerve injury resulting in disinhibition of the anterior cingulate cortex has been reported. However, the effect of optogenetic inhibition of the anterior cingulate cortex (ACC) on the sensory component of nerve injury-induced neuropathic pain has not been well studied. To investigate the feasibility of optogenetic ACC modulation, we injected an optogenetic virus or a null virus into the ACC of a nerve injury-induced neuropathic pain model. The unilateral ACC was modulated, and the optogenetic effect was measured by mechanical and thermal sensitivity tests. The assessment was performed in "pre-light off," "stimulation-yellow light on," and "post-light off" states. Optogenetic inhibition of the ACC in injury models revealed improved mechanical and thermal latencies with profound pain-relieving effects against nerve injury-induced neuropathic pain. The sensory thalamic discharge in electrophysiological in vivo recordings was also altered during laser stimulation. This finding indicates that hyperactivity of the ACC in nerve injury increases output to the spinothalamic tract through direct or indirect pathways. The direct photoinhibition of ACC neurons could play a vital role in restoring equilibrium and provide novel insight into techniques that can assuage peripheral nerve injury-induced neuropathic pain.

Collateral Projections from the Medullary Dorsal Horn to the Ventral Posteromedial Thalamic Nucleus and the Parafascicular Thalamic Nucleus in the Rat

Medullary dorsal horn (MDH), the homolog of spinal dorsal horn, plays essential roles in processing of nociceptive signals from orofacial region toward higher centers, such as the ventral posteromedial thalamic nucleus (VPM) and parafascicular thalamic nucleus (Pf), which belong to the sensory-discriminative and affective aspects of pain transmission systems at the thalamic level, respectively. In the present study, in order to provide morphological evidence for whether neurons in the MDH send collateral projections to the VPM and Pf, a retrograde double tracing method combined with immunofluorescence staining for substance P (SP), SP receptor (SPR) and Fos protein was used. Fluoro-gold (FG) was injected into the VPM and the tetramethylrhodamine-dextran (TMR) was injected into the Pf. The result revealed that both FG- and TMR-labeled projection neurons were observed throughout the entire extent of the MDH, while the FG/TMR double-labeled neurons were mainly located in laminae I and III. It was also found that some of the FG/TMR double-labeled neurons within lamina I expressed SPR and were in close contact with SP-immunoreactive (SP-ir) terminals. After formalin injection into the orofacial region, 41.4% and 34.3% of the FG/TMR double-labeled neurons expressed Fos protein in laminae I and III, respectively. The present results provided morphological evidence for that some SPR-expressing neurons within the MDH send collateral projections to both VPM and Pf and might be involved in sensory-discriminative and affective aspects of acute orofacial nociceptive information transmission.

Regional Differences Within the Anterior Cingulate Cortex in the Generation Versus Suppression of Pain Affect in Rats

The anterior cingulate cortex (ACC) modulates emotional responses to pain. Whereas, the caudal ACC (cACC) promotes expression of pain affect, the rostral ACC (rACC) contributes to its suppression. Both subdivisions receive glutamatergic innervation, and the present study evaluated the contribution of N-methyl-d-aspartic acid (NMDA) receptors within these subdivisions to rats' expression of pain affect. Vocalizations that follow a brief noxious tail shock (vocalization afterdischarges, VAD) are a validated rodent model of pain affect. The threshold current for eliciting VAD was increased in a dose-dependent manner by injecting NMDA into the rACC, but performance (latency, amplitude, and duration) at threshold was not altered. Alternately, the threshold current for eliciting VAD was not altered following injection of NMDA into the cACC, but its amplitude and duration at threshold were increased in a dose-dependent manner. These effects were limited to Cg1 of the rACC and cACC, and blocked by pretreatment of the ACC with the NMDA receptor antagonist d-2-amino-5-phosphonovalerate. These findings demonstrate that NMDA receptor agonism within the cACC and rACC either increases or decreases emotional responses to noxious stimulation, respectively. PERSPECTIVE: NMDA receptor activation of the rostral and caudal ACC respectively inhibited or enhanced rats' emotional response to pain. These findings mirror those obtained from human neuroimaging studies; thereby, supporting the use of this model system in evaluating the contribution of ACC to pain affect.

Synapse-specific opioid modulation of thalamo-cortico-striatal circuits

The medial thalamus (MThal), anterior cingulate cortex (ACC) and striatum play important roles in affective-motivational pain processing and reward learning. Opioids affect both pain and reward through uncharacterized modulation of this circuitry. This study examined opioid actions on glutamate transmission between these brain regions in mouse. Mu-opioid receptor (MOR) agonists potently inhibited MThal inputs without affecting ACC inputs to individual striatal medium spiny neurons (MSNs). MOR activation also inhibited MThal inputs to the pyramidal neurons in the ACC. In contrast, delta-opioid receptor (DOR) agonists disinhibited ACC pyramidal neuron responses to MThal inputs by suppressing local feed-forward GABA signaling from parvalbumin-positive interneurons. As a result, DOR activation in the ACC facilitated poly-synaptic (thalamo-cortico-striatal) excitation of MSNs by MThal inputs. These results suggest that opioid effects on pain and reward may be shaped by the relative selectivity of opioid drugs to the specific circuit components.

Endogenous opioidergic dysregulation of pain in fibromyalgia: a PET and fMRI study

Endogenous opioid system dysfunction potentially contributes to chronic pain in fibromyalgia (FM), but it is unknown if this dysfunction is related to established neurobiological markers of hyperalgesia. We previously reported that µ-opioid receptor (MOR) availability was reduced in patients with FM as compared with healthy controls in several pain-processing brain regions. In the present study, we compared pain-evoked functional magnetic resonance imaging with endogenous MOR binding and clinical pain ratings in female opioid-naive patients with FM (n = 18) using whole-brain analyses and regions of interest from our previous research. Within antinociceptive brain regions, including the dorsolateral prefrontal cortex (r = 0.81, P < 0.001) and multiple regions of the anterior cingulate cortex (all r > 0.67; all P < 0.02), reduced MOR availability was associated with decreased pain-evoked neural activity. Additionally, reduced MOR availability was associated with lower brain activation in the nucleus accumbens (r = 0.47, P = 0.050). In many of these regions, pain-evoked activity and MOR binding potential were also associated with lower clinical affective pain ratings. These findings are the first to link endogenous opioid system tone to regional pain-evoked brain activity in a clinical pain population. Our data suggest that dysregulation of the endogenous opioid system in FM could lead to less excitation in antinociceptive brain regions by incoming noxious stimulation, resulting in the hyperalgesia and allodynia commonly observed in this population. We propose a conceptual model of affective pain dysregulation in FM.

Inflammatory pain by carrageenan recruits low-frequency local field potential changes in the anterior cingulate cortex

The anterior cingulate cortex (ACC) has been extensively cited as a key area for processing pain affect. While local field potential (LFP) studies in other fields have yielded a great deal of information about neural oscillations, there is a poverty in the pain literature about the neural LFP profile related to pain, particularly in freely moving animals. In this study, we revealed the LFP profile in the ACC in freely moving rats during carrageenan inflammation. Mechanical allodynia was recorded before and after unilateral injection of carrageenan/saline in the left hindpaw. LFP activity in the ACC was recorded at baseline, after injection, and after injection with mechanical stimulation to the paw using a von Frey filament. This study uniquely reveals that carrageenan injection significantly recruited ACC LFP activity in delta, theta, and alpha bands (0-13Hz). Application of von Frey mechanical stimulation to the carrageenan-injected paw resulted in a significant increase in delta, theta, and alpha bands over and above what was recruited by carrageenan alone and further expanded the LFP range to additionally include beta activity (13-30Hz). Taken together, these data reveal significant changes in the lowest-frequency activities in the LFP range during painful inflammation, which merit attention. LFP is a powerful window to reveal wide-range, integrated synaptic processing by low-frequency cellular events during behavior. Information about LFP during pain broadens the scope of our understanding of pain mechanisms, our greatest resource for designing management approaches.

Altered fMRI resting-state connectivity in individuals with fibromyalgia on acute pain stimulation

BACKGROUND

Fibromyalgia is a chronic widespread pain condition, with patients commonly reporting other symptoms such as sleep difficulties, memory complaints and fatigue. The use of magnetic resonance imaging (MRI) in fibromyalgia has allowed for the detection of neural abnormalities, with alterations in brain activation elicited by experimental pain and alterations in resting state connectivity related to clinical pain.

METHODS

In this study, we sought to monitor state changes in resting brain connectivity following experimental pressure pain in fibromyalgia patients and healthy controls. Twelve fibromyalgia patients and 15 healthy controls were studied by applying discrete pressure stimuli to the thumbnail bed during MRI. Resting-state functional MRI scanning was performed before and immediately following experimental pressure pain. We investigated changes in functional connectivity to the thalamus and the insular cortex.

RESULTS

Acute pressure pain increased insula connectivity to the anterior cingulate and the hippocampus. Additionally, we observed increased thalamic connectivity to the precuneus/posterior cingulate cortex, a known part of the default mode network, in patients but not in controls. This connectivity was correlated with changes in clinical pain.

CONCLUSIONS

These data reporting changes in resting-state brain activity following a noxious stimulus suggest that the acute painful stimuli may contribute to the alteration of the neural signature of chronic pain. WHAT DOES THIS STUDY/ADD?: In this study acute pain application shows an echo in functional connectivity and clinical pain changes in chronic pain.

Mice Lacking Serotonin 2C Receptors Have increased Affective Responses to Aversive Stimuli

Although central serotonergic systems are known to influence responses to noxious stimuli, mechanisms underlying serotonergic modulation of pain responses are unclear. We proposed that serotonin 2C receptors (5-HT2CRs), which are expressed within brain regions implicated in sensory and affective responses to pain, contribute to the serotonergic modulation of pain responses. In mice constitutively lacking 5-HT2CRs (2CKO mice) we found normal baseline sensory responses to noxious thermal, mechanical and chemical stimuli. In contrast, 2CKO mice exhibited a selective enhancement of affect-related ultrasonic afterdischarge vocalizations in response to footshock. Enhanced affect-related responses to noxious stimuli were also exhibited by 2CKO mice in a fear-sensitized startle assay. The extent to which a brief series of unconditioned footshocks produced enhancement of acoustic startle responses was markedly increased in 2CKO mice. As mesolimbic dopamine pathways influence affective responses to noxious stimuli, and these pathways are disinhibited in 2CKO mice, we examined the sensitivity of footshock-induced enhancement of startle to dopamine receptor blockade. Systemic administration of the dopamine D2/D3 receptor antagonist raclopride selectively reduced footshock-induced enhancement of startle without influencing baseline acoustic startle responses. We propose that 5-HT2CRs regulate affective behavioral responses to unconditioned aversive stimuli through mechanisms involving the disinhibition of ascending dopaminergic pathways.

N-methyl-D-aspartate receptor agonism and antagonism within the amygdaloid central nucleus suppresses pain affect: differential contribution of the ventrolateral periaqueductal gray

The amygdala contributes to the generation of pain affect, and the amygdaloid central nucleus (CeA) receives nociceptive input that is mediated by glutamatergic neurotransmission. The present study compared the contribution of N-methyl-d-aspartate (NMDA) receptor agonism and antagonism in the CeA to generation of the affective response of rats to an acute noxious stimulus. Vocalizations that occur following a brief tail shock (vocalization afterdischarges) are a validated rodent model of pain affect and were preferentially suppressed, in a dose-dependent manner, by bilateral injection into the CeA of NMDA (.1, .25, .5, or 1 μg/side) or the NMDA receptor antagonist d-(-)-2-amino-5-phosphopentanoic acid (AP5; 1, 2, or 4 μg/side). Vocalizations that occur during tail shock were suppressed to a lesser degree, whereas spinal motor reflexes (tail flick and hind limb movements) were unaffected by injection of NMDA or AP5 into the CeA. Injection of NMDA, but not AP5, into the CeA increased c-Fos immunoreactivity in the ventrolateral periaqueductal gray, and unilateral injection of the μ-opiate receptor antagonist H-d-Phe-Cys-Tyr-d-Trp-Arg-Thr-Pen-Thr-NH2 (CTAP; .25 μg) into ventrolateral periaqueductal gray prevented the antinociception generated by injection of NMDA into the CeA. These findings demonstrate that although NMDA receptor agonism and antagonism in the CeA produce similar suppression of pain behaviors, they do so via different neurobiologic mechanisms.

PERSPECTIVE

The amygdala contributes to production of the emotional dimension of pain. NMDA receptor agonism and antagonism within the CeA suppressed rats' emotional response to acute painful stimulation. Understanding the neurobiology underlying emotional responses to pain will provide insights into new treatments for pain and its associated affective disorders.

Preclinical assessment of pain: improving models in discovery research

To date, animal models have not sufficiently "filtered" targets for new analgesics, increasing the failure rate and cost of drug development. Preclinical assessment of "pain" has historically relied on measures of evoked behavioral responses to sensory stimuli in animals. Such measures can often be observed in decerebrated animals and therefore may not sufficiently capture affective and motivational aspects of pain, potentially diminishing translation from preclinical studies to the clinical setting. Further, evidence indicates that there are important mechanistic differences between evoked behavioral responses of hypersensitivity and ongoing pain, limiting evaluation of mechanisms that could mediate aspects of clinically relevant pain. The mechanisms underlying ongoing pain in preclinical models are currently being explored and may serve to inform decisions towards the transition from drug discovery to drug development for a given target.

Thalamic metabolic alterations with cognitive dysfunction in idiopathic trigeminal neuralgia: a multivoxel spectroscopy study

INTRODUCTION

Although abnormalities in metabolite compositions in the thalamus are well described in patients with idiopathic trigeminal neuralgia (ITN), differences in distinct thalamic subregions have not been measured with proton magnetic resonance spectroscopy ((1)H-MRS), and whether there are correlations between thalamic metabolites and cognitive function still remain unknown.

METHODS

Multivoxel MRS was recorded to investigate the metabolic alterations in the thalamic subregions of patients with ITN. The regions of interest were localized in the anterior thalamus (A-Th), intralaminar portion of the thalamus (IL-Th), posterior lateral thalamus (PL-Th), posterior medial thalamus (PM-Th), and medial and lateral pulvinar of the thalamus (PuM-Th and PuL-Th). The N-acetylaspartate to creatine (NAA/Cr) and choline to creatine (Cho/Cr) ratios were measured in the ITN and control groups. Scores of the visual analogue scale (VAS) and the Montreal Cognitive Assessment (MoCA) were analyzed to correlate with the neuroradiological findings.

RESULTS

The NAA/Cr ratio in the affected side of PM-Th and PL-Th in ITN patients was statistically lower than that in the corresponding regions of the thalamus in controls. The NAA/Cr ratio in the affected PM-Th was negatively associated with VAS and disease duration. Furthermore, decreases of NAA/Cr and Cho/Cr were detected in the affected side of IL-Th, and lower Cho/Cr was positively correlated with MoCA values in the ITN group.

CONCLUSIONS

Our result of low level of NAA/Cr in the affected PM-Th probably serves as a marker of the pain-rating index, and decreased Cho/Cr in IL-Th may be an indicator of cognitive disorder in patients with ITN.

Neuroinflammation in Patients with Chronic Fatigue Syndrome/Myalgic Encephalomyelitis: An ¹¹C-(R)-PK11195 PET Study

Chronic fatigue syndrome/myalgic encephalomyelitis (CFS/ME) is a disease characterized by chronic, profound, disabling, and unexplained fatigue. Although it is hypothesized that brain inflammation is involved in the pathophysiology of CFS/ME, there is no direct evidence of neuroinflammation in patients with CFS/ME. Activation of microglia or astrocytes is related to neuroinflammation. (11)C-(R)-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl)-3-isoquinoline-carboxamide ((11)C-(R)-PK11195) is a ligand of PET for a translocator protein that is expressed by activated microglia or astrocytes. We used (11)C-(R)-PK11195 and PET to investigate the existence of neuroinflammation in CFS/ME patients.

METHODS

Nine CFS/ME patients and 10 healthy controls underwent (11)C-(R)-PK11195 PET and completed questionnaires about fatigue, fatigue sensation, cognitive impairments, pain, and depression. To measure the density of translocator protein, nondisplaceable binding potential (BP(ND)) values were determined using linear graphical analysis with the cerebellum as a reference region.

RESULTS

The BP(ND) values of (11)C-(R)-PK11195 in the cingulate cortex, hippocampus, amygdala, thalamus, midbrain, and pons were 45%-199% higher in CFS/ME patients than in healthy controls. In CFS/ME patients, the BP(ND) values of (11)C-(R)-PK11195 in the amygdala, thalamus, and midbrain positively correlated with cognitive impairment score, the BP(ND) values in the cingulate cortex and thalamus positively correlated with pain score, and the BP(ND) value in the hippocampus positively correlated with depression score.

CONCLUSION

Neuroinflammation is present in widespread brain areas in CFS/ME patients and was associated with the severity of neuropsychologic symptoms. Evaluation of neuroinflammation in CFS/ME patients may be essential for understanding the core pathophysiology and for developing objective diagnostic criteria and effective medical treatments.

Separating analgesia from reward within the ventral tegmental area

Activation of the dopaminergic mesolimbic reward circuit that originates in the ventral tegmental area (VTA) is postulated to preferentially suppress emotional responses to noxious stimuli, and presumably contributes to the addictive liability of strong analgesics. VTA dopamine neurons are activated via cholinergic afferents and microinjection of carbachol (cholinergic agonist) into VTA is rewarding. Here, we evaluated regional differences within VTA in the capacity of carbachol to suppress rats' affective response to pain (vocalization afterdischarges, VADs) and to support conditioned place preference (CPP) learning. As carbachol is a non-specific agonist, muscarinic and nicotinic receptor involvement was assessed by administering atropine (muscarinic antagonist) and mecamylamine (nicotinic antagonist) into VTA prior to carbachol treatment. Unilateral injections of carbachol (4μg) into anterior VTA (aVTA) and posterior VTA (pVTA) suppressed VADs and supported CPP; whereas, injections into midVTA failed to effect either VADs or CPP. These findings corroborate the hypothesis that the neural substrates underlying affective analgesia and reward overlap. However, the extent of the overlap was only partial. Whereas both nicotinic and muscarinic receptors contributed to carbachol-induced affective analgesia in aVTA, only muscarinic receptors mediated the analgesic action of carbachol in pVTA. The rewarding effects of carbachol are mediated by the activation of both nicotinic and muscarinic receptors in both aVTA and pVTA. The results indicate that analgesia and reward are mediated by separate cholinergic mechanisms within pVTA. Nicotinic receptor antagonism within pVTA failed to attenuate carbachol-induced analgesia, but prevented carbachol-induced reward. As addictive liability of analgesics stem from their rewarding properties, the present findings suggest that these processes can be neuropharmacologically separated within pVTA.

Endogenous opiates and behavior: 2011

This paper is the thirty-fourth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2011 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration (Section 16); and immunological responses (Section 17).

NMDA or non-NMDA receptor antagonism within the amygdaloid central nucleus suppresses the affective dimension of pain in rats: evidence for hemispheric synergy

The amygdala contributes to generation of affective behaviors to threats. The prototypical threat to an individual is exposure to a noxious stimulus and the amygdaloid central nucleus (CeA) receives nociceptive input that is mediated by glutamatergic neurotransmission. The present study evaluated the contribution of glutamate receptors in CeA to generation of the affective response to acute pain in rats. Vocalizations that occur following a brief noxious tail shock (vocalization afterdischarges) are a validated rodent model of pain affect, and were preferentially suppressed by bilateral injection into CeA of the NMDA receptor antagonist D-2-amino-5-phosphonovalerate (AP5, 1 μg, 2 μg, or 4 μg) or the non-NMDA receptor antagonist 6-Cyano-7-nitroquinoxaline-2,3-dione disodium (CNQX, .25 μg, .5 μg, 1 μg, or 2 μg). Vocalizations that occur during tail shock were suppressed to a lesser degree, whereas spinal motor reflexes (tail flick and hind limb movements) were unaffected by injection of AP5 or CNQX into CeA. Unilateral administration of AP5 or CNQX into CeA of either hemisphere also selectively elevated vocalization thresholds. Bilateral administration of AP5 or CNQX produced greater increases in vocalization thresholds than the same doses of antagonists administered unilaterality into either hemisphere indicating synergistic hemispheric interactions.

PERSPECTIVE

The amygdala contributes to production of emotional responses to environmental threats. Blocking glutamate neurotransmission within the central nucleus of the amygdala suppressed rats' emotional response to acute painful stimulation. Understanding the neurobiology underlying emotional responses to pain will provide insights into new treatments for pain and its associated affective disorders.

Erasing injury-related cortical synaptic potentiation as a new treatment for chronic pain

Synaptic plasticity in the spinal cord and the cortex is believed to be important for the amplification of painful information in chronic pain conditions. The investigation of molecular mechanism responsible for maintaining injury-related plastic changes, such as through the study of long-term potentiation in these structures, provides potential novel targets for designing new medicine for chronic pain. Recent studies using integrative neurobiological approaches demonstrate that protein kinase M zeta (PKMζ) maintains pain-induced persistent changes in the anterior cingulate cortex (ACC), and inhibiting PKMζ by ζ-pseudosubstrate inhibitory peptide produces analgesic effects in animal models of chronic pain. We propose that targeting PKMζ, or its up- or downstream signaling proteins, in the ACC may provide novel clinical treatment for chronic pain.