A 'double-edged' role for type-5 metabotropic glutamate receptors in pain disclosed by light-sensitive drugs.

We used light-sensitive drugs to identify the brain region-specific role of mGlu5 metabotropic glutamate receptors in the control of pain. Optical activation of systemic JF-NP-26, a caged, normally inactive, negative allosteric modulator (NAM) of mGlu5 receptors, in cingulate, prelimbic, and infralimbic cortices and thalamus inhibited neuropathic pain hypersensitivity. Systemic treatment of alloswitch-1, an intrinsically active mGlu5 receptor NAM, caused analgesia, and the effect was reversed by light-induced drug inactivation in the prelimbic and infralimbic cortices, and thalamus. This demonstrates that mGlu5 receptor blockade in the medial prefrontal cortex and thalamus is both sufficient and necessary for the analgesic activity of mGlu5 receptor antagonists. Surprisingly, when the light was delivered in the basolateral amygdala, local activation of systemic JF-NP-26 reduced pain thresholds, whereas inactivation of alloswitch-1 enhanced analgesia. Electrophysiological analysis showed that alloswitch-1 increased excitatory synaptic responses in prelimbic pyramidal neurons evoked by stimulation of presumed BLA input, and decreased BLA-driven feedforward inhibition of amygdala output neurons. Both effects were reversed by optical silencing and reinstated by optical reactivation of alloswitch-1. These findings demonstrate for the first time that the action of mGlu5 receptors in the pain neuraxis is not homogenous, and suggest that blockade of mGlu5 receptors in the BLA may limit the overall analgesic activity of mGlu5 receptor antagonists. This could explain the suboptimal effect of mGlu5 NAMs on pain in human studies and validate photopharmacology as an important tool to determine ideal target sites for systemic drugs.

Cells and circuits for amygdala neuroplasticity in the transition to chronic pain.

Maladaptive plasticity is linked to the chronification of diseases such as pain, but the transition from acute to chronic pain is not well understood mechanistically. Neuroplasticity in the central nucleus of the amygdala (CeA) has emerged as a mechanism for sensory and emotional-affective aspects of injury-induced pain, although evidence comes from studies conducted almost exclusively in acute pain conditions and agnostic to cell type specificity. Here, we report time-dependent changes in genetically distinct and projection-specific CeA neurons in neuropathic pain. Hyperexcitability of CRF projection neurons and synaptic plasticity of parabrachial (PB) input at the acute stage shifted to hyperexcitability without synaptic plasticity in non-CRF neurons at the chronic phase. Accordingly, chemogenetic inhibition of the PB→CeA pathway mitigated pain-related behaviors in acute, but not chronic, neuropathic pain. Cell-type-specific temporal changes in neuroplasticity provide neurobiological evidence for the clinical observation that chronic pain is not simply the prolonged persistence of acute pain.

Dysfunction of Small-Conductance Ca2+-Activated Potassium (SK) Channels Drives Amygdala Hyperexcitability and Neuropathic Pain Behaviors: Involvement of Epigenetic Mechanisms.

Neuroplasticity in the amygdala and its central nucleus (CeA) is linked to pain modulation and pain behaviors, but cellular mechanisms are not well understood. Here, we addressed the role of small-conductance Ca2+-activated potassium (SK) channels in pain-related amygdala plasticity. The facilitatory effects of the intra-CeA application of an SK channel blocker (apamin) on the pain behaviors of control rats were lost in a neuropathic pain model, whereas an SK channel activator (NS309) inhibited pain behaviors in neuropathic rats but not in sham controls, suggesting the loss of the inhibitory behavioral effects of amygdala SK channels. Brain slice electrophysiology found hyperexcitability of CeA neurons in the neuropathic pain condition due to the loss of SK channel-mediated medium afterhyperpolarization (mAHP), which was accompanied by decreased SK2 channel protein and mRNA expression, consistent with a pretranscriptional mechanisms. The underlying mechanisms involved the epigenetic silencing of the SK2 gene due to the increased DNA methylation of the CpG island of the SK2 promoter region and the change in methylated CpG sites in the CeA in neuropathic pain. This study identified the epigenetic dysregulation of SK channels in the amygdala (CeA) as a novel mechanism of neuropathic pain-related plasticity and behavior that could be targeted to control abnormally enhanced amygdala activity and chronic neuropathic pain.

Ginger Polyphenols Reverse Molecular Signature of Amygdala Neuroimmune Signaling and Modulate Microbiome in Male Rats with Neuropathic Pain: Evidence for Microbiota-Gut-Brain Axis.

Emerging evidence shows that the gut microbiota plays an important role in neuropathic pain (NP) via the gut-brain axis. Male rats were divided into sham, spinal nerve ligation (SNL), SNL + 200 mg GEG/kg BW (GEG200), and SNL + 600 mg GEG/kg BW (GEG600) for 5 weeks. The dosages of 200 and 600 mg GEG/kg BW for rats correspond to 45 g and 135 g raw ginger for human daily consumption, respectively. Both GEG groups mitigated SNL-induced NP behavior. GEG-supplemented animals had a decreased abundance of Rikenella, Muribaculaceae, Clostridia UCG-014, Mucispirillum schaedleri, RF39, Acetatifactor, and Clostridia UCG-009, while they had an increased abundance of Flavonifactor, Hungatella, Anaerofustis stercorihominis, and Clostridium innocuum group. Relative to sham rats, Fos and Gadd45g genes were upregulated, while Igf1, Ccl2, Hadc2, Rtn4rl1, Nfkb2, Gpr84, Pik3cg, and Abcc8 genes were downregulated in SNL rats. Compared to the SNL group, the GEG200 group and GEG600 group had increases/decreases in 16 (10/6) genes and 11 (1/10) genes, respectively. GEG downregulated Fos and Gadd45g genes and upregulated Hdac2 genes in the amygdala. In summary, GEG alleviates NP by modulating the gut microbiome and reversing a molecular neuroimmune signature.

Impaired amygdala astrocytic signaling worsens neuropathic pain-associated neuronal functions and behaviors.

Introduction: Pain is a clinically relevant health care issue with limited therapeutic options, creating the need for new and improved analgesic strategies. The amygdala is a limbic brain region critically involved in the regulation of emotional-affective components of pain and in pain modulation. The central nucleus of amygdala (CeA) serves major output functions and receives nociceptive information via the external lateral parabrachial nucleus (PB). While amygdala neuroplasticity has been linked causally to pain behaviors, non-neuronal pain mechanisms in this region remain to be explored. As an essential part of the neuroimmune system, astrocytes that represent about 40-50% of glia cells within the central nervous system, are required for physiological neuronal functions, but their role in the amygdala remains to be determined for pain conditions. In this study, we measured time-specific astrocyte activation in the CeA in a neuropathic pain model (spinal nerve ligation, SNL) and assessed the effects of astrocyte inhibition on amygdala neuroplasticity and pain-like behaviors in the pain condition. Methods and Results: Glial fibrillary acidic protein (GFAP, astrocytic marker) immunoreactivity and mRNA expression were increased at the chronic (4 weeks post-SNL), but not acute (1 week post-SNL), stage of neuropathic pain. In order to determine the contribution of astrocytes to amygdala pain-mechanisms, we used fluorocitric acid (FCA), a selective inhibitor of astrocyte metabolism. Whole-cell patch-clamp recordings were performed from neurons in the laterocapsular division of the CeA (CeLC) obtained from chronic neuropathic rats. Pre-incubation of brain slices with FCA (100 µM, 1 h), increased excitability through altered hyperpolarization-activated current (Ih) functions, without significantly affecting synaptic responses at the PB-CeLC synapse. Intra-CeA injection of FCA (100 µM) had facilitatory effects on mechanical withdrawal thresholds (von Frey and paw pressure tests) and emotional-affective behaviors (evoked vocalizations), but not on facial grimace score and anxiety-like behaviors (open field test), in chronic neuropathic rats. Selective inhibition of astrocytes by FCA was confirmed with immunohistochemical analyses showing decreased astrocytic GFAP, but not NeuN, signal in the CeA. Discussion: Overall, these results suggest a complex modulation of amygdala pain functions by astrocytes and provide evidence for beneficial functions of astrocytes in CeA in chronic neuropathic pain.

Chemogenetic Manipulation of Amygdala Kappa Opioid Receptor Neurons Modulates Amygdala Neuronal Activity and Neuropathic Pain Behaviors.

Neuroplasticity in the central nucleus of the amygdala (CeA) plays a key role in the modulation of pain and its aversive component. The dynorphin/kappa opioid receptor (KOR) system in the amygdala is critical for averse-affective behaviors in pain conditions, but its mechanisms are not well understood. Here, we used chemogenetic manipulations of amygdala KOR-expressing neurons to analyze the behavioral consequences in a chronic neuropathic pain model. For the chemogenetic inhibition or activation of KOR neurons in the CeA, a Cre-inducible viral vector encoding Gi-DREADD (hM4Di) or Gq-DREADD (hM3Dq) was injected stereotaxically into the right CeA of transgenic KOR-Cre mice. The chemogenetic inhibition of KOR neurons expressing hM4Di with a selective DREADD actuator (deschloroclozapine, DCZ) in sham control mice significantly decreased inhibitory transmission, resulting in a shift of inhibition/excitation balance to promote excitation and induced pain behaviors. The chemogenetic activation of KOR neurons expressing hM3Dq with DCZ in neuropathic mice significantly increased inhibitory transmission, decreased excitability, and decreased neuropathic pain behaviors. These data suggest that amygdala KOR neurons modulate pain behaviors by exerting an inhibitory tone on downstream CeA neurons. Therefore, activation of these interneurons or blockade of inhibitory KOR signaling in these neurons could restore control of amygdala output and mitigate pain.

Associations Between Vitamin D Deficiency/Insufficiency and Depression Expose Health Disparities in Older Rural West Texans: A Project FRONTIER Study.

OBJECTIVE: To determine associations between Vitamin D (VD) levels and clinical depression through the Geriatric Depression Scale (GDS) and its questions and subdomains, stratified by demographics and Hispanic/Latino ethnicity (HLE). DESIGN, SETTING, AND PARTICIPANTS: A cohort of 299 Project FRONTIER participants aged 62.6 ± 11.7 years old, 70.9% female, and 40.5% HLE were used. Standard correlation and regression analyses were employed. MEASUREMENTS: The main outcome measures were VD (serum 25(OH)-VD) level, GDS-30 (30-item questionnaire), GDS-30 subfactors and questions, and HLE status. VD categories were defined as VD deficiency (VDD; ≤20 ng/mL), VD insufficiency (VDI; 21-29 ng/mL), VD sufficiency (30-38 ng/mL) and high VD sufficiency (>38 ng/mL). RESULTS: The majority (61.5%) of samples fell into VDD/VDI categories. A significant negative association was found between VD level and GDS-30 total score. VD level was negatively correlated with Dysphoria and Meaninglessness GDS-30 subfactors. Although GDS subfactors were similar between HLE and non-HLE groups, VD levels were significantly lower in HLE samples. Finally, HLE/non-HLE groups were differentially stratified across VD categories. Only 4% of HLEs fell into the high VD sufficient category, suggesting low VD supplementation. CONCLUSION: A significant negative association between VD level and depressive symptoms was revealed in our aging Project FRONTIER participants. HLE individuals were overrepresented in VDD/VDI samples, and VDD/VDI was associated primarily with the Dysphoria GDS subdomain. Regression analysis predicted high VD sufficiency (95.5 ng/mL) to be associated with no depressive symptoms (GDS=0). Our results underscore troubling disparities in VD-related depressive symptoms between HLE and non-HLE populations.

Ginger alleviates mechanical hypersensitivity and anxio-depressive behavior in rats with diabetic neuropathy through beneficial actions on gut microbiome composition, mitochondria, and neuroimmune cells of colon and spinal cord.

The relationship among gut microbiota, mitochondrial dysfunction/neuroinflammation, and diabetic neuropathic pain (DNP) has received increased attention. Ginger has antidiabetic and analgesic effects because of its anti-inflammatory property. We examined the effects of gingerols-enriched ginger (GEG) supplementation on pain-associated behaviors, gut microbiome composition, and mitochondrial function and neuroinflammation of colon and spinal cord in DNP rats. Thirty-three male rats were randomly divided into 3 groups: control group, DNP group (high-fat diet plus single dose of streptozotocin at 35 mg/kg body weight, and GEG group (DNP+GEG at 0.75% in the diet for 8 weeks). Von Frey and open field tests were used to assess pain sensitivity and anxio-depressive behaviors, respectively. Colon and spinal cord were collected for gene expression analysis. 16S rRNA gene sequencing was done from cecal samples and microbiome data analysis was performed using QIIME 2. GEG supplementation mitigated mechanical hypersensitivity and anxio-depressive behavior in DNP animals. GEG supplementation suppressed the dynamin-related protein 1 protein expression (colon) and gene expression (spinal cord), astrocytic marker GFAP gene expression (colon and spinal cord), and tumor necrosis factor-α gene expression (colon, P < .05; spinal cord, P = .0974) in DNP rats. GEG supplementation increased microglia/macrophage marker CD11b gene expression in colon and spinal cord of DNP rats. GEG treatment increased abundance of Acinetobacter, Azospirillum, Colidextribacter, and Fournierella but decreased abundance of Muribaculum intestinale in cecal feces of rats. This study demonstrates that GEG supplementation decreased pain, anxio-depression, and neuroimmune cells, and improved the composition of gut microbiomes and mitochondrial function in rats with diabetic neuropathy.

A "double-edged" role for type-5 metabotropic glutamate receptors in pain disclosed by light-sensitive drugs.

Knowing the site of drug action is important to optimize effectiveness and address any side effects. We used light-sensitive drugs to identify the brain region-specific role of mGlu5 metabotropic glutamate receptors in the control of pain. Optical activation of systemic JF-NP-26, a caged, normally inactive, negative allosteric modulator (NAM) of mGlu5 receptors, in cingulate, prelimbic and infralimbic cortices and thalamus inhibited neuropathic pain hypersensitivity. Systemic treatment of alloswitch-1, an intrinsically active mGlu5 receptor NAM, caused analgesia, and the effect was reversed by light-induced drug inactivation in in the prelimbic and infralimbic cortices, and thalamus. This demonstrates that mGlu5 receptor blockade in the medial prefrontal cortex and thalamus is both sufficient and necessary for the analgesic activity of mGlu5 receptor antagonists. Surprisingly, when light was delivered in the basolateral amygdala, local activation of systemic JF-NP-26 reduced pain thresholds, whereas inactivation of alloswitch-1 enhanced analgesia. Electrophysiological analysis showed that alloswitch-1 increased excitatory synaptic responses in prelimbic pyramidal neurons evoked by stimulation of BLA input, and decreased feedforward inhibition of amygdala output neurons by BLA. Both effects were reversed by optical silencing and reinstated by optical reactivation of alloswitch-1. These findings demonstrate for the first time that the action of mGlu5 receptors in the pain neuraxis is not homogenous, and suggest that blockade of mGlu5 receptors in the BLA may limit the overall analgesic activity of mGlu5 receptor antagonists. This could explain the suboptimal effect of mGlu5 NAMs on pain in human studies and validate photopharmacology as an important tool to determine ideal target sites for systemic drugs.

Alcohol withdrawal and pain: Peripheral mechanisms join central circuits.

Negative affective aspects of alcohol withdrawal and pain involve converging brain circuits. In this issue of Neuron, Son et al.1 identify a peripheral mechanism of an alcohol-withdrawal-induced headache-like condition, which is centered on mast-cell-specific receptor MrgprB2 activated by corticotropin-releasing factor (CRF) in dura mater to drive nociception.

Investigating the Relationship Between Marital Status and Ethnicity on Neurocognitive Functioning in a Rural Older Population: A Project FRONTIER Study.

OBJECTIVES: Research indicates being married is related to better physical and psychological health. Little is known regarding the relationship between marital status and neurocognitive functioning and whether it differs based on ethnicity (Hispanic vs non-Hispanic). This is the first study to examine this relationship in a sample of aging adults in rural Texas. METHODS: Data from 1,864 participants (Mage = 59.68, standard deviation [SD]age = 12.21), who were mostly Hispanic (n = 1,053), women (n = 1,295), and married (n = 1,125) from Project Facing Rural Obstacles to Healthcare Now Through Intervention, Education, & Research were analyzed. Neuropsychological testing comprised Repeatable Battery for the Assessment of Neuropsychological Status, Trails Making Test, and Clock Drawing. Participants were dichotomized, married, and unmarried. RESULTS: There was a significant interaction between Hispanic identity and marital status on overall neurocognitive functioning (F(1, 1,480) = 4.79, p < .05, ηp2 = 0.003). For non-Hispanic individuals, married individuals had higher overall neurocognitive functioning compared to unmarried individuals, whereas neurocognitive functioning for Hispanic individuals did not significantly differ between married and unmarried individuals. There were significant main effects as married individuals (M = 84.95, SD = 15.56) had greater overall neurocognitive functioning than unmarried individuals (M = 83.47, SD = 15.86; F(1, 1,480) = 14.67, p < .001, ηp2 = 0.01), Hispanic individuals (M = 78.02, SD = 14.25) had lower overall neurocognitive functioning than non-Hispanic individuals (M = 91.43, SD = 15.07; F(1, 1,480) = 284.99, p < .001, ηp2 = 0.16). DISCUSSION: Hispanics living in rural areas experience additional stressors that could lead to worse neurocognitive functioning, which is supported by the Lifespan Biopsychosocial Model of Cumulative Vulnerability and Minority Health, which postulates that race/ethnicity/socioeconomic-status-related stressors exacerbate the impact of other life stressors. Reduction of stress on rural Hispanics should be a priority as it could positively affect their neurocognitive functioning.

Opposing Effects on Descending Control of Nociception by µ and κ Opioid Receptors in the Anterior Cingulate Cortex.

BACKGROUND: The efficiency of descending pain modulation, commonly assessed with the conditioned pain modulation procedure, is diminished in patients with chronic pain. The authors hypothesized that the efficiency of pain modulation is controlled by cortical opioid circuits. METHODS: This study evaluated the effects of µ opioid receptor activation in the anterior cingulate cortex on descending control of nociception, a preclinical correlate of conditioned pain modulation, in male Sprague-Dawley rats with spinal nerve ligation-induced chronic pain or in sham-operated controls. Additionally, the study explored the consequences of respective activation or inhibition of κ opioid receptor in the anterior cingulate cortex of naive rats or animals with neuropathic pain. Descending control of nociception was measured as the hind paw withdrawal response to noxious pressure (test stimulus) in the absence or presence of capsaicin injection in the forepaw (conditioning stimulus). RESULTS: Descending control of nociception was diminished in the ipsilateral, but not contralateral, hind paw of rats with spinal nerve ligation. Bilateral administration of morphine in the anterior cingulate cortex had no effect in shams but restored diminished descending control of nociception without altering hypersensitivity in rats with neuropathic pain. Bilateral anterior cingulate cortex microinjection of κ opioid receptor antagonists, including nor-binaltorphimine and navacaprant, also re-established descending control of nociception in rats with neuropathic pain without altering hypersensitivity and with no effect in shams. Conversely, bilateral injection of a κ opioid receptor agonist, U69,593, in the anterior cingulate cortex of naive rats inhibited descending control of nociception without altering withdrawal thresholds. CONCLUSIONS: Anterior cingulate cortex κ opioid receptor activation therefore diminishes descending control of nociception both in naive animals and as an adaptive response to chronic pain, likely by enhancing net descending facilitation. Descending control of nociception can be restored by activation of µ opioid receptors in the anterior cingulate cortex, but also by κ opioid receptor antagonists, providing a nonaddictive alternative to opioid analgesics. Navacaprant is now in advanced clinical trials.

Turmeric Bioactive Compounds Alleviate Spinal Nerve Ligation-Induced Neuropathic Pain by Suppressing Glial Activation and Improving Mitochondrial Function in Spinal Cord and Amygdala.

This study examined the effects of turmeric bioactive compounds, curcumin C3 complex® (CUR) and bisdemethoxycurcumin (BDMC), on mechanical hypersensitivity and the gene expression of markers for glial activation, mitochondrial function, and oxidative stress in the spinal cord and amygdala of rats with neuropathic pain (NP). Twenty-four animals were randomly assigned to four groups: sham, spinal nerve ligation (SNL, an NP model), SNL+100 mg CUR/kg BW p.o., and SNL+50 mg BDMC/kg BW p.o. for 4 weeks. Mechanical hypersensitivity was assessed by the von Frey test (VFT) weekly. The lumbosacral section of the spinal cord and the right amygdala (central nucleus) were collected to determine the mRNA expression of genes (IBA-1, CD11b, GFAP, MFN1, DRP1, FIS1, PGC1α, PINK, Complex I, TLR4, and SOD1) utilizing qRT-PCR. Increased mechanical hypersensitivity and increased gene expression of markers for microglial activation (IBA-1 in the amygdala and CD11b in the spinal cord), astrocyte activation (GFAP in the spinal cord), mitochondrial dysfunction (PGC1α in the amygdala), and oxidative stress (TLR4 in the spinal cord and amygdala) were found in untreated SNL rats. Oral administration of CUR and BDMC significantly decreased mechanical hypersensitivity. CUR decreased CD11b and GFAP gene expression in the spinal cord. BDMC decreased IBA-1 in the spinal cord and amygdala as well as CD11b and GFAP in the spinal cord. Both CUR and BDMC reduced PGC1α gene expression in the amygdala, PINK1 gene expression in the spinal cord, and TLR4 in the spinal cord and amygdala, while they increased Complex I and SOD1 gene expression in the spinal cord. CUR and BDMC administration decreased mechanical hypersensitivity in NP by mitigating glial activation, oxidative stress, and mitochondrial dysfunction.

Pain and aging: A unique challenge in neuroinflammation and behavior.

Chronic pain is one of the most common, costly, and potentially debilitating health issues facing older adults, with attributable costs exceeding $600 billion annually. The prevalence of pain in humans increases with advancing age. Yet, the contributions of sex differences, age-related chronic inflammation, and changes in neuroplasticity to the overall experience of pain are less clear, given that opposing processes in aging interact. This review article examines and summarizes pre-clinical research and clinical data on chronic pain among older adults to identify knowledge gaps and provide the base for future research and clinical practice. We provide evidence to suggest that neurodegenerative conditions engender a loss of neural plasticity involved in pain response, whereas low-grade inflammation in aging increases CNS sensitization but decreases PNS sensitivity. Insights from preclinical studies are needed to answer mechanistic questions. However, the selection of appropriate aging models presents a challenge that has resulted in conflicting data regarding pain processing and behavioral outcomes that are difficult to translate to humans.

Tai Chi exercise reduces circulating levels of inflammatory oxylipins in postmenopausal women with knee osteoarthritis: results from a pilot study.

Background: Tai Chi (TC) controls pain through mind-body exercise and appears to alter inflammatory mediators. TC actions on lipid biomarkers associated with inflammation and brain neural networks in women with knee osteoarthritic pain were investigated. Methods: A single-center, pre- and post-TC group (baseline and 8 wk) exercise pilot study in postmenopausal women with knee osteoarthritic pain was performed. 12 eligible women participated in TC group exercise. The primary outcome was liquid chromatography tandem mass spectrometry determination of circulating endocannabinoids (eCB) and oxylipins (OxL). Secondary outcomes were correlations between eCB and OxL levels and clinical pain/limitation assessments, and brain resting-state function magnetic resonance imaging (rs-fMRI). Results: Differences in circulating quantitative levels (nM) of pro-inflammatory OxL after TC were found in women. TC exercise resulted in lower OxL PGE1 and PGE2 and higher 12-HETE, LTB4, and 12-HEPE compared to baseline. Pain assessment and eCB and OxL levels suggest crucial relationships between TC exercise, inflammatory markers, and pain. Higher plasma levels of eCB AEA, and 1, 2-AG were found in subjects with increased pain. Several eCB and OxL levels were positively correlated with left and right brain amygdala-medial prefrontal cortex functional connectivity. Conclusion: TC exercise lowers pro-inflammatory OxL in women with knee osteoarthritic pain. Correlations between subject pain, functional limitations, and brain connectivity with levels of OxL and eCB showed significance. Findings indicate potential mechanisms for OxL and eCB and their biosynthetic endogenous PUFA precursors that alter brain connectivity, neuroinflammation, and pain. Clinical Trial Registration: ClinicalTrials.gov, identifier: NCT04046003.

Hmgb1 Silencing in the Amygdala Inhibits Pain-Related Behaviors in a Rat Model of Neuropathic Pain.

Chronic pain presents a therapeutic challenge due to the highly complex interplay of sensory, emotional-affective and cognitive factors. The mechanisms of the transition from acute to chronic pain are not well understood. We hypothesized that neuroimmune mechanisms in the amygdala, a brain region involved in the emotional-affective component of pain and pain modulation, play an important role through high motility group box 1 (Hmgb1), a pro-inflammatory molecule that has been linked to neuroimmune signaling in spinal nociception. Transcriptomic analysis revealed an upregulation of Hmgb1 mRNA in the right but not left central nucleus of the amygdala (CeA) at the chronic stage of a spinal nerve ligation (SNL) rat model of neuropathic pain. Hmgb1 silencing with a stereotaxic injection of siRNA for Hmgb1 into the right CeA of adult male and female rats 1 week after (post-treatment), but not 2 weeks before (pre-treatment) SNL induction decreased mechanical hypersensitivity and emotional-affective responses, but not anxiety-like behaviors, measured 4 weeks after SNL. Immunohistochemical data suggest that neurons are a major source of Hmgb1 in the CeA. Therefore, Hmgb1 in the amygdala may contribute to the transition from acute to chronic neuropathic pain, and the inhibition of Hmgb1 at a subacute time point can mitigate neuropathic pain.

Studies on diketopiperazine and dipeptide analogs as opioid receptor ligands.

Using the structure of gliotoxin as a starting point, we have prepared two different chemotypes with selective affinity to the kappa opioid receptor (KOR). Using medicinal chemistry approaches and structure-activity relationship (SAR) studies, structural features required for the observed affinity were identified, and advanced molecules with favorable Multiparameter Optimization (MPO) and Ligand Lipophilicity (LLE) profiles were prepared. Using the Thermal Place Preference Test (TPPT), we have shown that compound2 blocks the antinociceptive effect of U50488, a known KOR agonist. Multiple reports suggest that modulation of KOR signaling is a promising therapeutic strategy in treating neuropathic pain (NP). As a proof-of-concept study, we tested compound 2 in a rat model of NP and recorded its ability to modulate sensory and emotional pain-related behaviors. Observed in vitro and in vivo results suggest that these ligands can be used to develop compounds with potential application as pain therapeutics.

Pain-related cortico-limbic plasticity and opioid signaling.

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

Kappa opioid receptor agonists produce sexually dimorphic and prolactin-dependent hyperalgesic priming.

ABSTRACT: Repeated stress produces hyperalgesic priming in preclinical models, but underlying mechanisms remain uncertain. As stress engages kappa opioid receptors (KORs), we hypothesized that repeated administration of KOR agonists might mimic, in part, stress-induced hyperalgesic priming. The potential contribution of circulating prolactin (PRL) and dysregulation of the expression of PRL receptor (PRLR) isoforms in sensory neurons after KOR agonist administration was also investigated. Mice received 3 daily doses of U-69593 or nalfurafine as a "first-hit" stimulus followed by assessment of periorbital tactile allodynia. Sixteen days after the first KOR agonist administration, animals received a subthreshold dose of inhalational umbellulone, a TRPA1 agonist, as the second-hit stimulus and periorbital allodynia was assessed. Cabergoline, a dopamine D2 receptor agonist, was used to inhibit circulating PRL in additional cohorts. Prolactin receptor isoforms were quantified in the V1 region of the trigeminal ganglion after repeated doses of U-69593. In both sexes, KOR agonists increased circulating PRL and produced allodynia that resolved within 14 days. Hyperalgesic priming, revealed by umbellulone-induced allodynia in animals previously treated with the KOR agonists, also occurred in both sexes. However, repeated U-69593 downregulated the PRLR long isoform in trigeminal neurons only in female mice. Umbellulone-induced allodynia was prevented by cabergoline co-treatment during priming with KOR agonists in female, but not male, mice. Hyperalgesic priming therefore occurs in both sexes after either biased or nonbiased KOR agonists. However, a PRL/PRLR-dependence is observed only in female nociceptors possibly contributing to pain in stress-related pain disorders in females.