Tumor necrosis factor receptor type-1 in sensory neurons contributes to induction of chronic enhancement of inflammatory hyperalgesia in rat

T cells at the interface of neuroimmune communication

The immune system protects the host from infection and works to heal damaged tissue after infection or injury. There is increasing evidence that the immune system and the nervous system work in concert to achieve these goals. The sensory nervous system senses injury, infection, and inflammation, which results in a direct pain signal. Direct activation of peripheral sensory nerves can drive an inflammatory response in the skin. Immune cells express receptors for numerous transmitters released from sensory and autonomic nerves, which allows the nervous system to communicate directly with the immune system. This communication is bidirectional because immune cells can also produce neurotransmitters. Both innate and adaptive immune cells respond to neuronal signaling, but T cells appear to be at the helm of neuroimmune communication.

Mast cell-sensory neuron crosstalk in allergic diseases

Mast cells (MCs) are tissue-resident immune cells, well-positioned at the host-environment interface for detecting external antigens and playing a critical role in mobilizing innate and adaptive immune responses. Sensory neurons are afferent neurons innervating most areas of the body but especially in the periphery, where they sense external and internal signals and relay information to the brain. The significance of MC-sensory neuron communication is now increasingly becoming recognized, especially because both cell types are in close physical proximity at the host-environment interface and around major organs of the body and produce specific mediators that can activate each other. In this review, we explore the roles of MC-sensory neuron crosstalk in allergic diseases, shedding light on how activated MCs trigger sensory neurons to initiate signaling in pruritus, shock, and potentially abdominal pain in allergy, and how activated sensory neurons regulate MCs in homeostasis and atopic dermatitis associated with contact hypersensitivity and type 2 inflammation. Throughout the review, we also discuss how these 2 sentinel cell types signal each other, potentially resulting in a positive feedback loop that can sustain inflammation. Unraveling the mysteries of MC-sensory neuron crosstalk is likely to unveil their critical roles in various disease conditions and enable the development of new therapeutic approaches to combat these maladies.

Nerve-myeloid cell interactions in persistent human pain: a reappraisal using updated cell subset classifications

ABSTRACT

The past 20 years have seen a dramatic shift in our understanding of the role of the immune system in initiating and maintaining pain. Myeloid cells, including macrophages, dendritic cells, Langerhans cells, and mast cells, are increasingly implicated in bidirectional interactions with nerve fibres in rodent pain models. However, our understanding of the human setting is still poor. High-dimensional functional analyses have substantially changed myeloid cell classifications, with recently described subsets such as epidermal dendritic cells and DC3s unveiling new insight into how myeloid cells interact with nerve fibres. However, it is unclear whether this new understanding has informed the study of human chronic pain. In this article, we perform a scoping review investigating neuroimmune interactions between myeloid cells and peripheral nerve fibres in human chronic pain conditions. We found 37 papers from multiple pain states addressing this aim in skin, cornea, peripheral nerve, endometrium, and tumour, with macrophages, Langerhans cells, and mast cells the most investigated. The directionality of results between studies was inconsistent, although the clearest pattern was an increase in macrophage frequency across conditions, phases, and tissues. Myeloid cell definitions were often outdated and lacked correspondence with the stated cell types of interest; overreliance on morphology and traditional structural markers gave limited insight into the functional characteristics of investigated cells. We therefore critically reappraise the existing literature considering contemporary myeloid cell biology and advocate for the application of established and emerging high-dimensional proteomic and transcriptomic single-cell technologies to clarify the role of specific neuroimmune interactions in chronic pain.

NAAA-regulated lipid signaling in monocytes controls the induction of hyperalgesic priming in mice

Circulating monocytes participate in pain chronification but the molecular events that cause their deployment are unclear. Using a mouse model of hyperalgesic priming (HP), we show that monocytes enable progression to pain chronicity through a mechanism that requires transient activation of the hydrolase, N-acylethanolamine acid amidase (NAAA), and the consequent suppression of NAAA-regulated lipid signaling at peroxisome proliferator-activated receptor-α (PPAR-α). Inhibiting NAAA in the 72 hours following administration of a priming stimulus prevented HP. This effect was phenocopied by NAAA deletion and depended on PPAR-α recruitment. Mice lacking NAAA in CD11b+ cells - monocytes, macrophages, and neutrophils - were resistant to HP induction. Conversely, mice overexpressing NAAA or lacking PPAR-α in the same cells were constitutively primed. Depletion of monocytes, but not resident macrophages, generated mice that were refractory to HP. The results identify NAAA-regulated signaling in monocytes as a control node in the induction of HP and, potentially, the transition to pain chronicity.

The role of neuroinflammation in the transition of acute to chronic pain and the opioid-induced hyperalgesia and tolerance

Current evidence suggests that activation of glial and immune cells leads to increased production of proinflammatory mediators, creating a neuroinflammatory state. Neuroinflammation has been proven to be a fundamental mechanism in the genesis of acute pain and its transition to neuropathic and chronic pain. A noxious event that stimulates peripheral afferent nerve fibers may also activate pronociceptive receptors situated at the dorsal root ganglion and dorsal horn of the spinal cord, as well as peripheral glial cells, setting off the so-called peripheral sensitization and spreading neuroinflammation to the brain. Once activated, microglia produce cytokines, chemokines, and neuropeptides that can increase the sensitivity and firing properties of second-order neurons, upregulating the signaling of nociceptive information to the cerebral cortex. This process, known as central sensitization, is crucial for chronification of acute pain. Immune-neuronal interactions are also implicated in the lesser-known complex regulatory relationship between pain and opioids. Current evidence suggests that activated immune and glial cells can alter neuronal function, induce, and maintain pathological pain, and disrupt the analgesic effects of opioid drugs by contributing to the development of tolerance and dependence, even causing paradoxical hyperalgesia. Such alterations may occur when the neuronal environment is impacted by trauma, inflammation, and immune-derived molecules, or when opioids induce proinflammatory glial activation. Hence, understanding these intricate interactions may help in managing pain signaling and opioid efficacy beyond the classical pharmacological approach.

Prevalence and characteristics of pain in moderate-to-severe obstructive sleep apnea patients and effect of CPAP treatment

Pain problems are common in patients with obstructive sleep apnea (OSA), but few studies have thoroughly evaluated pain in these patients. The objective of this study was to examine the prevalence and characteristics of pain in moderate-to-severe OSA patients and the effect of continuous positive airway pressure (CPAP) treatment. Moderate-to-severe OSA patients and healthy controls (HC) completed the Short Form McGill Pain Questionnaire (SF-MPQ) and a portion of the Brief Pain Inventory (BPI) Short Form to assess pain characteristics. The Epworth Sleepiness Scale (ESS), the Short Form-36 (SF-36), and the Hospital Anxiety and Depression Scale (HADS) were used to measure daytime sleepiness, health-related quality of life (HRQoL), and psychological status, respectively. The OSA patients with pain were divided into a CPAP-treated group and a CPAP-untreated group based on their adherence to CPAP. The subjects' pain intensity was reassessed after 3 months. The prevalence of pain was 57.5% in OSA versus 27.1% in HC (p < 0.001). Head (39.0%) accounted for the highest proportion of overall pain locations in subjects with OSA, with 28.8% of OSA patients experiencing headaches. Pain in OSA was associated with impaired HRQoL and psychological problems. Patients with very severe OSA had an increased risk for pain problems (OR: 7.000, p = 0.041). Associated factors for pain intensity in OSA included age, ESS ≥ 9.0, and lowest pulse oximetry (LSpO2) < 80.0%. Pain intensity in OSA decreased significantly after CPAP treatment (p < 0.001). Pain was prevalent among patients with moderate-to-severe OSA and was associated with depression, anxiety, and a lower HRQoL. Patients with very severe OSA had an increased risk for pain problems. The intensity of pain in OSA can be predicted by age, ESS ≥ 9.0, and LSpO2 < 80.0%, and it can be alleviated through CPAP treatment.

Neuroendocrine mechanisms in oxaliplatin-induced hyperalgesic priming

ABSTRACT

Stress plays a major role in the symptom burden of oncology patients and can exacerbate cancer chemotherapy-induced peripheral neuropathy (CIPN), a major adverse effect of many classes of chemotherapy. We explored the role of stress in the persistent phase of the pain induced by oxaliplatin. Oxaliplatin induced hyperalgesic priming, a model of the transition to chronic pain, as indicated by prolongation of hyperalgesia produced by prostaglandin E 2 , in male rats, which was markedly attenuated in adrenalectomized rats. A neonatal handling protocol that induces stress resilience in adult rats prevented oxaliplatin-induced hyperalgesic priming. To elucidate the role of the hypothalamic-pituitary-adrenal and sympathoadrenal neuroendocrine stress axes in oxaliplatin CIPN, we used intrathecally administered antisense oligodeoxynucleotides (ODNs) directed against mRNA for receptors mediating the effects of catecholamines and glucocorticoids, and their second messengers, to reduce their expression in nociceptors. Although oxaliplatin-induced hyperalgesic priming was attenuated by intrathecal administration of β 2 -adrenergic and glucocorticoid receptor antisense ODNs, oxaliplatin-induced hyperalgesia was only attenuated by β 2 -adrenergic receptor antisense. Administration of pertussis toxin, a nonselective inhibitor of Gα i/o proteins, attenuated hyperalgesic priming. Antisense ODNs for Gα i 1 and Gα o also attenuated hyperalgesic priming. Furthermore, antisense for protein kinase C epsilon, a second messenger involved in type I hyperalgesic priming, also attenuated oxaliplatin-induced hyperalgesic priming. Inhibitors of second messengers involved in the maintenance of type I (cordycepin) and type II (SSU6656 and U0126) hyperalgesic priming both attenuated hyperalgesic priming. These experiments support a role for neuroendocrine stress axes in hyperalgesic priming, in male rats with oxaliplatin CIPN.

Ketoprofen, lysine and gabapentin co-crystal magnifies synergistic efficacy and tolerability of the constituent drugs: Pre-clinical evidences towards an innovative therapeutic approach for neuroinflammatory pain

Chronic pain is an enormous public health concern, and its treatment is still an unmet medical need. Starting from data highlighting the promising effects of some nonsteroidal anti-inflammatory drugs in combination with gabapentin in pain treatment, we sought to combine ketoprofen lysine salt (KLS) and gabapentin to obtain an effective multimodal therapeutic approach for chronic pain. Using relevant in vitro models, we first demonstrated that KLS and gabapentin have supra-additive effects in modulating key pathways in neuropathic pain and gastric mucosal damage. To leverage these supra-additive effects, we then chemically combined the two drugs via co-crystallization to yield a new compound, a ternary drug-drug co-crystal of ketoprofen, lysine and gabapentin (KLS-GABA co-crystal). Physicochemical, biodistribution and pharmacokinetic studies showed that within the co-crystal, ketoprofen reaches an increased gastrointestinal solubility and permeability, as well as a higher systemic exposure in vivo compared to KLS alone or in combination with gabapentin, while both the constituent drugs have increased central nervous system permeation. These unique characteristics led to striking, synergistic anti-nociceptive and anti-inflammatory effects of KLS-GABA co-crystal, as well as significantly reduced spinal neuroinflammation, in translational inflammatory and neuropathic pain rat models, suggesting that the synergistic therapeutic effects of the constituent drugs are further boosted by the co-crystallization. Notably, while strengthening the therapeutic effects of ketoprofen, KLS-GABA co-crystal showed remarkable gastrointestinal tolerability in both inflammatory and chronic neuropathic pain rat models. In conclusion, these results allow us to propose KLS-GABA co-crystal as a new drug candidate with high potential clinical benefit-to-risk ratio for chronic pain treatment.

Gonadal hormone-dependent nociceptor sensitization maintains nociplastic pain state in female mice

ABSTRACT

Nociplastic pain conditions develop predominantly in women. We recently established a murine nociplastic pain model by applying postinjury thermal (40°C) stimulation to an injured (capsaicin-injected) area, triggering a transition to a nociplastic pain state manifesting as persistent mechanical hypersensitivity outside of the previously injured area. The nociplastic pain state was centrally maintained by spinal microglia in males but peripherally by ongoing afferent activity at the previously injured area in females. Here, we investigated whether gonadal hormones are critical for the development of this peripherally maintained nociplastic pain state in females. Although the transition to a nociplastic pain state still occurred in ovariectomized females, the pain state was maintained neither by ongoing afferent activity at the previously injured area nor by spinal microglia. Estradiol reconstitution a week before the injury plus postinjury stimulation, but not after the transition had already occurred, restored the development of peripherally maintained nociplastic mechanical hypersensitivity in ovariectomized females. G protein-coupled estrogen receptor antagonism during the transition phase mimicked ovariectomy in gonad-intact females, whereas the receptor antagonism after the transition gradually alleviated the nociplastic mechanical hypersensitivity. At the previously injured area, afferents responsive to allyl isothiocyanate (AITC), a TRPA1 agonist, contributed to the maintenance of nociplastic mechanical hypersensitivity in gonad-intact females. In ex vivo skin-nerve preparations, only AITC-responsive afferents from the nociplastic pain model in gonad-intact females showed ongoing activities greater than control. These results suggest that gonadal hormones are critical for peripherally maintained nociplastic pain state in females by sensitizing AITC-responsive afferents to be persistently active.

Toxicity of micro(nano)plastics with different size and surface charge on human nasal epithelial cells and rats via intranasal exposure

Micro (nano)plastics (MNPs) have become emerging environmental contaminants, yet their toxicity and systemic effects via intranasal exposure remain unclear. This study investigated the in vitro toxicity of thirteen polystyrene MNPs with different surface functionalization (carboxylic (C-PS), amino (A-PS), and bare (PS)) and sizes (20-2000 nm) on human nasal epithelial cells (HNEpCs) at 10-1250 μg/mL as well as their in vivo toxicity to rats via intranasal administration at 125 μg/mL. The in vitro study showed that PS20, PS50, A-PS50, PS500, and A-PS500 significantly inhibited cell viability, which was dependent on particle concentration. A-PS induced higher cytotoxicity than C-PS and PS, and most MNPs inhibited cell proliferation after 24-h. Flow cytometry analysis suggested that PS induced cell apoptosis, while A-PS caused cell necrosis. MNPs were phagocytosed by HNEpCs and entered nucleus. The in vivo study showed that MNPs inhibited dietary behaviors of rats. Histological analysis indicated that PS20, PS200, and A-PS50 thinned out nasal mucosa. Immunohistochemical analysis revealed that exposure to PS20, PS200, and A-PS50 enhanced expression of transient receptor potential cation channel subfamily M (melastatin) member 8 (TRPM8). Systemic effects including hepatocyte cytoplasmic vacuolation and renal tubule dilatation were observed. The results suggested that nasal inhalation of MNPs may disturb energy metabolism and damage upper respiratory tract, liver, and kidneys.

The Role of Inflammation, Hypoxia, and Opioid Receptor Expression in Pain Modulation in Patients Suffering from Obstructive Sleep Apnea

Obstructive sleep apnea (OSA) is a relatively common disease in the general population. Besides its interaction with many comorbidities, it can also interact with potentially painful conditions and modulate its course. The association between OSA and pain modulation has recently been a topic of concern for many scientists. The mechanism underlying OSA-related pain connection has been linked with different pathophysiological changes in OSA and various pain mechanisms. Furthermore, it may cause both chronic and acute pain aggravation as well as potentially influencing the antinociceptive mechanism. Characteristic changes in OSA such as nocturnal hypoxemia, sleep fragmentation, and systemic inflammation are considered to have a curtailing impact on pain perception. Hypoxemia in OSA has been proven to have a significant impact on increased expression of proinflammatory cytokines influencing the hyperalgesic priming of nociceptors. Moreover, hypoxia markers by themselves are hypothesized to modulate intracellular signal transduction in neurons and have an impact on nociceptive sensitization. Pain management in patients with OSA may create problems arousing from alterations in neuropeptide systems and overexpression of opioid receptors in hypoxia conditions, leading to intensification of side effects, e.g., respiratory depression and increased opioid sensitivity for analgesic effects. In this paper, we summarize the current knowledge regarding pain and pain treatment in OSA with a focus on molecular mechanisms leading to nociceptive modulation.

Postinjury stimulation triggers a transition to nociplastic pain in mice

ABSTRACT

Acute injury-induced pain can transition to chronic nociplastic pain, which predominantly affects women. To facilitate studies on the underlying mechanisms of nociplastic pain, we developed a mouse model in which postinjury thermal stimulation (intermittent 40°C water immersion for 10 minutes at 2 hours postcapsaicin) prolongs capsaicin (ie, experimental injury)-induced transient mechanical hypersensitivity outside of the injury area. Although capsaicin injection alone induced mechanical and thermal hypersensitivity that resolved in ∼7 days (slower recovery in females), the postinjury stimulation prolonged capsaicin-induced mechanical, but not thermal, hypersensitivity up to 3 weeks in both sexes. When postinjury stimulation was given at a lower intensity (30°C) or at later time points (40°C at 1-3 days postcapsaicin), chronification of mechanical hypersensitivity occurred only in females. Similar chronification could be induced by a different postinjury stimulation modality (vibration of paw) or with a different injury model (plantar incision). Notably, the 40°C postinjury stimulation did not prolong capsaicin-induced inflammation in the hind paw, indicating that the prolonged mechanical hypersensitivity in these mice arises without clear evidence of ongoing injury, reflecting nociplastic pain. Although morphine and gabapentin effectively alleviated this persistent mechanical hypersensitivity in both sexes, sexually dimorphic mechanisms mediated the hypersensitivity. Specifically, ongoing afferent activity at the previously capsaicin-injected area was critical in females, whereas activated spinal microglia were crucial in males. These results demonstrate that postinjury stimulation of the injured area can trigger the transition from transient pain to nociplastic pain more readily in females, and sex-dependent mechanisms maintain the nociplastic pain state.

Behavioral characterization, potential clinical relevance and mechanisms of latent pain sensitization

Chronic pain is a debilitating disorder that can occur as painful episodes that alternates with bouts of remission and occurs despite healing of the primary insult. Those episodes are often triggered by stressful events. In the last decades, a similar situation has been evidenced in a wide variety of rodent models (including inflammatory pain, neuropathy and opioid-induced hyperalgesia) where animals develop a chronic latent hyperalgesia that silently persists after behavioral signs of pain resolution. This state, referred as latent pain sensitization, is due to the compensatory activation of antinociceptive systems, such as the opioid system or NPY and its receptors. A transitory phase of hyperalgesia can then be reinstated by pharmacological or genetic blockade of these antinociceptive systems or by submitting animals to acute stress. Those observations reveal that there is a constant endogenous analgesia responsible for chronic pain inhibition that might paradoxically contribute to maintain this maladaptive state and could then participate to the transition from acute to chronic pain. Thus, demonstration of the existence of this phenomenon in humans and a better understanding of the mechanisms by which latent pain sensitization develops and maintains over long periods of time will be of particular interest to help identifying new therapeutic strategies and targets for chronic pain treatment. The present review aims to recapitulate behavioral expression, potential clinical relevance, cellular mechanisms and intracellular signaling pathways involved so far in latent pain sensitization.

β2-and β3-Adrenergic Receptors Contribute to Cancer-Evoked Pain in a Mouse Model of Osteosarcoma via Modulation of Neural Macrophages

The mechanisms involved in the development and maintenance of cancer pain remain largely unidentified. Recently, it has been reported that β-adrenergic receptors (β-ARs), mainly β2-and β3-ARs, contribute to tumor proliferation and progression and may favor cancer-associated pain and neuroinflammation. However, the mechanism underlying β-ARs in cancer pain is still unknown. Here, we investigated the role of β1-, β2-and β3-ARs in a mouse model of cancer pain generated by the para-tibial injection of K7M2 osteosarcoma cells. Results showed a rapid tumor growth in the soft tissue associated with the development of mechanical allodynia in the hind paw ipsilateral to the injected site. In addition to reduce tumor growth, both propranolol and SR59230A, β1-/β2-and β3-AR antagonists, respectively, attenuated mechanical allodynia, the number of macrophages and an oxidative stress by-product accumulated in the ipsilateral tibial nerve. The selective β1-AR antagonist atenolol was able to slightly reduce the tumor growth but showed no effect in reducing the development of mechanical allodynia. Results suggest that the development of the mechanical allodynia in K7M2 osteosarcoma-bearing mice is mediated by oxidative stress associated with the recruitment of neural macrophages, and that antagonism of β2-and β3-ARs contribute not solely to the reduction of tumor growth, but also in cancer pain. Thus, the targeting of the β2-and β3-ARs signaling may be a promising therapeutic strategy against both tumor progression and the development of cancer-evoke pain in osteosarcoma.

Acute P38-Mediated Enhancement of P2X3 Receptor Currents by TNF-α in Rat Dorsal Root Ganglion Neurons

Purpose

Tumor necrosis factor-α (TNF-α) is a pro-inflammatory cytokine and involves in a variety of pain conditions. Some findings suggest that TNF-α may act directly on primary afferent neurons to induce acute pain hypersensitivity through non-transcriptional regulation. This study investigated whether TNF-α had an effect on functional activity of P2X3 receptors in primary sensory neurons. Herein, we report that a brief (5 min) application of TNF-α rapidly enhanced the electrophysiological activity of P2X3 receptors in rat dorsal root ganglia (DRG) neurons.

Methods

Electrophysiological recordings were carried out on rat DRG neurons, and nociceptive behavior was quantified in rats.

Results

A brief (5 min) exposure of TNF-α rapidly increased P2X3 receptor-mediated and α,β-methylene-ATP (α,β-meATP)-evoked inward currents in a dose-dependent manner. The potentiation of P2X3 receptor-mediated ATP currents by TNF-α was voltage-independent. TNF-α shifted the concentration-response curve for α,β-meATP upwards, with an increase of 31.57 ± 6.81% in the maximal current response to α,β-meATP. This acute potentiation of ATP currents by TNF-α was blocked by p38 mitogen-activated protein kinase (MAPK) inhibitor SB202190, but not by non-selective cyclooxygenase inhibitor indomethacin, suggesting involvement of p38 MAPK, but not cyclooxygenase. Moreover, intraplantar injection of TNF-α and α,β-meATP produced a synergistic effect on mechanical allodynia in rats. TNF-α-induced mechanical allodynia was also alleviated after local P2X3 receptors were blocked.

Conclusion

These results suggested that TNF-α rapidly sensitized P2X3 receptors in primary sensory neurons via a p38 MAPK dependent pathway, which revealed a novel peripheral mechanism underlying acute mechanical hypersensitivity by peripheral administration of TNF-α.

Role of GABAAR in the Transition From Acute to Chronic Pain and the Analgesic Effect of Electroacupuncture on Hyperalgesic Priming Model Rats

Chronic pain is a costly health problem that impairs health-related quality of life when not effectively treated. Regulating the transition from acute to chronic pain is a new therapeutic strategy for chronic pain that presents a major clinical challenge. The underlying mechanisms of pain transition are not entirely understood, and strategies for preventing this transition are lacking. Here, a hyperalgesic priming model was used to study the potential mechanism by which γ-aminobutyric acid receptor type A (GABAAR) in the dorsal root ganglion (DRG) contributes to pain transition. Furthermore, electroacupuncture (EA), a modern method of acupuncture, was administered to regulate pain transition, and the mechanism underlying EA’s regulatory effect was investigated. Hyperalgesic priming was induced by intraplanar injection of carrageenan (Car)/prostaglandin E₂ (PGE₂). The decrease in mechanical withdrawal threshold (MWT) induced by PGE₂ returned to baseline 4 h after injection in NS + PGE₂ group, and still persisted 24 h after injection in Car + PGE₂ group. Lower expression of GABAAR in the lumbar DRG was observed in the model rats. Furthermore, activating or blocking GABAAR could reversed the long-lasting hyperalgesia induced by Car/PGE₂ injection or produced a persistent hyperalgesia. In addition, GABAAR may be involved in Protein Kinase C epsilon (PKCε) activation in the DRG, a mark molecular of pain transition. EA considerably increased the mechanical pain thresholds of hyperalgesic priming model mammals in both the acute and chronic phases. Furthermore, EA upregulated the expression of GABAAR and inhibited the activation of PKCε in the DRG. In addition, peripheral administration of picrotoxin blocked the analgesic effect of EA on the model rats and abolished the regulatory effect of EA on PKCε activation. These findings suggested that GABAAR plays a key role in both the transition from acute to chronic pain and the analgesic effect of EA on hyperalgesic priming.

Change in prostaglandin signaling during sickness syndrome hyperalgesia after ovariectomy in female rats

The present study investigated hyperalgesia during sickness syndrome in female rats. Hyperalgesia was induced by an intraperitoneal injection of lipopolysaccharide (LPS) or an intracerebroventricular injection of prostaglandin E₂ (PGE₂). No differences were found in basal mechanical and thermal thresholds or in LPS-induced hyperalgesia in sham-operated animals in the diestrus or proestrus phase or in ovariectomized (OVX) animals. However, higher levels of PGE₂ where found in the cerebrospinal fluid of OVX animals compared to sham-operated females. Intracerebroventricular injection of PGE₂ produced rapid mechanical hyperalgesia in sham-operated rats while these responses were observed at later times in OVX animals. The protein kinase A (PKA) inhibitor H-89 reduced mechanical PGE₂-induced hyperalgesia in OVX female rats, whereas no effect was observed in sham-operated animals. In contrast, the exchange protein activated by cyclic adenosine monophosphate (cAMP; Epac) inhibitor ESI-09 reduced mechanical PGE₂-induced hyperalgesia, whereas no effect was observed in OVX animals. PGE₂ also induced thermal hyperalgesia in sham-operated and OVX female rats and a similar effect of ESI-09 was observed. These results suggest that PGE₂-induced hyperalgesia that is observed during sickness syndrome has different signaling mechanisms in cycling and OVX female rats involving the activation of the cAMP-Epac or cAMP-PKA pathways, respectively.

T Cells as Guardians of Pain Resolution

Despite successful research efforts aimed at understanding pain mechanisms, there is still no adequate treatment for many patients suffering from chronic pain. The contribution of neuroinflammation to chronic pain is widely acknowledged. Here, we summarize findings indicating that T cells play a key role in the suppression of pain. An active contribution of the immune system to resolution of pain may explain why immunosuppressive drugs are often not sufficient to control pain. This would also imply that dysregulation of certain immune functions promote transition to chronic pain. Conversely, stimulating the endogenous immune-mediated resolution pathways may provide a potent approach to treat chronic pain.

Inflammatory pain in peripheral tissue depends on the activation of the TNF-α type 1 receptor in the primary afferent neuron

The mechanism underlying the role of tumor necrosis factor alpha (TNF-α) in the development of inflammatory hyperalgesia has been extensively studied, mainly the role of TNF-α in the release of pro-inflammatory cytokines. The current concept relies in the fact that TNF-α stimulates the cascade release of other pro-inflammatory cytokines, such as IL-1β, IL-6, and IL-8 (CINC-1 in rats), triggering the release of the final inflammatory mediator prostaglandin E₂ (PGE₂ ) and sympathetic amines that directly sensitize the nociceptors. However, this may not be the sole mechanism involved as the blockade of TNF-α synthesis by thalidomide prevents hyperalgesia without interrupting the synthesis of IL-1β, IL-6, and CINC-1. Therefore, we hypothesized that activation of TNF-α receptor type 1 (TNFR1) by TNF-α increases nociceptors' susceptibility to the action of PGE₂ and dopamine. We have found out that intrathecal administration of oligodeoxynucleotide-antisense (ODN-AS) against TNFR1 or thalidomide prevented carrageenan-induced hyperalgesia. The co-administration of TNF-α with a subthreshold dose of PGE₂ or dopamine that does not induce hyperalgesia by itself in the hind paw of Wistar rats pretreated with dexamethasone (to prevent the endogenous release of cytokines) induced a robust hyperalgesia that was prevented by intrathecal treatment with ODN-AS against TNFR1. We consider that the activation of neuronal TNFR1 by TNF-α decisively increases the susceptibility of the peripheral afferent neuron to the action of final inflammatory mediators - PGE₂ and dopamine - that ultimately induce hyperalgesia. This mechanism may also underlie the analgesic action of thalidomide.

Oncostatin M induces hyperalgesic priming and amplifies signaling of cAMP to ERK by RapGEF2 and PKA

Hyperalgesic priming is characterized by enhanced nociceptor sensitization by pronociceptive mediators, prototypically PGE₂ . Priming has gained interest as a mechanism underlying the transition to chronic pain. Which stimuli induce priming and what cellular mechanisms are employed remains incompletely understood. In adult male rats, we present the cytokine Oncostatin M (OSM), a member of the IL-6 family, as an inducer of priming by a novel mechanism. We used a high content microscopy based approach to quantify the activation of endogenous PKA-II and ERK of thousands sensory neurons in culture. Incubation with OSM increased and prolonged ERK activation by agents that increase cAMP production such as PGE₂ , forskolin, and cAMP analogs. These changes were specific to IB4/CaMKIIα positive neurons, required protein translation, and increased cAMP-to-ERK signaling. In both, control and OSM-treated neurons, cAMP/ERK signaling involved RapGEF2 and PKA but not Epac. Similar enhancement of cAMP-to-ERK signaling could be induced by GDNF, which acts mostly on IB4/CaMKIIα-positive neurons, but not by NGF, which acts mostly on IB4/CaMKIIα-negative neurons. In vitro, OSM pretreatment rendered baseline TTX-R currents ERK-dependent and switched forskolin-increased currents from partial to full ERK-dependence in small/medium sized neurons. In summary, priming induced by OSM uses a novel mechanism to enhance and prolong coupling of cAMP/PKA to ERK1/2 signaling without changing the overall pathway structure.

Novel Analgesics with Peripheral Targets

A limited number of peripheral targets generate pain. Inflammatory mediators can sensitize these. The review addresses targets acting exclusively or predominantly on sensory neurons, mediators involved in inflammation targeting sensory neurons, and mediators involved in a more general inflammatory process, of which an analgesic effect secondary to an anti-inflammatory effect can be expected. Different approaches to address these systems are discussed, including scavenging proinflammatory mediators, applying anti-inflammatory mediators, and inhibiting proinflammatory or facilitating anti-inflammatory receptors. New approaches are contrasted to established ones; the current stage of progress is mentioned, in particular considering whether there is data from a molecular and cellular level, from animals, or from human trials, including an early stage after a market release. An overview of publication activity is presented, considering a IuPhar/BPS-curated list of targets with restriction to pain-related publications, which was also used to identify topics.

Peripheral nitric oxide signaling directly blocks inflammatory pain

Pain is a classical sign of inflammation, and sensitization of primary sensory neurons (PSN) is the most important mediating mechanism. This mechanism involves direct action of inflammatory mediators such as prostaglandins and sympathetic amines. Pharmacologic control of inflammatory pain is based on two principal strategies: (i) non-steroidal anti-inflammatory drugs targeting inhibition of prostaglandin production by cyclooxygenases and preventing nociceptor sensitization in humans and animals; (ii) opioids and dipyrone that directly block nociceptor sensitization via activation of the NO signaling pathway. This review summarizes basic concepts of inflammatory pain that are necessary to understand the mechanisms of peripheral NO signaling that promote peripheral analgesia; we also discuss therapeutic perspectives based on the modulation of the NO pathway.

Neuraxial Cytokines in Pain States

A high-intensity potentially tissue-injuring stimulus generates a homotopic response to escape the stimulus and is associated with an affective phenotype considered to represent pain. In the face of tissue or nerve injury, the afferent encoding systems display robust changes in the input–output function, leading to an ongoing sensation reported as painful and sensitization of the nociceptors such that an enhanced pain state is reported for a given somatic or visceral stimulus. Our understanding of the mechanisms underlying this non-linear processing of nociceptive stimuli has led to our appreciation of the role played by the functional interactions of neural and immune signaling systems in pain phenotypes. In pathological states, neural systems interact with the immune system through the actions of a variety of soluble mediators, including cytokines. Cytokines are recognized as important mediators of inflammatory and neuropathic pain, supporting system sensitization and the development of a persistent pathologic pain. Cytokines can induce a facilitation of nociceptive processing at all levels of the neuraxis including supraspinal centers where nociceptive input evokes an affective component of the pain state. We review here several key proinflammatory and anti-inflammatory cytokines/chemokines and explore their underlying actions at four levels of neuronal organization: (1) peripheral nociceptor termini; (2) dorsal root ganglia; (3) spinal cord; and (4) supraspinal areas. Thus, current thinking suggests that cytokines by this action throughout the neuraxis play key roles in the induction of pain and the maintenance of the facilitated states of pain behavior generated by tissue injury/inflammation and nerve injury.

Immunopotentiator thymosin alpha-1 attenuates inflammatory pain by modulating the Wnt3a/β-catenin pathway in spinal cord

The mechanism of inflammatory pain involves the central nervous system (CNS) and the immune system. It is reported that immunopotentiator thymosin alpha-1 (Tα1) can reduce inflammation, protect neurons and strengthen the immune function. However, the roles of Tα1 in inflammatory pain still remain unclear. In this study, we found Tα1 can attenuate the complete Freund's adjuvant (CFA)-induced mechanical allodynia and heat hyperalgesia. Meanwhile, it reduced the upregulation of CFA-induced inflammatory mediators (interferon (IFN)-γ, tumor necrosis factor-α and brain-derived neurotrophic factor). In addition, we found the Wnt3a/β-catenin pathway was activated in spinal cord after the injection of CFA, paralleling with pain hypersensitivity. However, Tα1 reversed this status. In summary, Tα1 could attenuate inflammatory pain by modulating the Wnt3a/β-catenin pathway. It might be related to the downregulation of inflammatory mediators.

Oxyntomodulin attenuates TNF‑α induced neuropathic pain by inhibiting the activation of the NF‑κB pathway

Neuropathic pain is rarely diagnosed. Oxyntomodulin is peripherally and centrally distributed; however, the potential mechanisms underlying the effects of oxyntomodulin in attenuating nociception remain unclear; thus, we aimed to explore them in the present study. A neuropathic pain model in male C57BL/6 mice was induced by intrathecal injection of tumor necrosis factor‑α (TNF‑α), and the duration of nociceptive behavioral responses was measured with a stop‑watch timer within 30 min. Western blotting was used to explore the protein levels of ionized calcium binding adaptor molecule‑1 (IBA1), nuclear factor‑κB (NF‑κB) phosphorylated‑p65, interleukin (IL)‑6 and IL‑1β. We performed reverse transcription‑quantitative polymerase chain reaction and ELISA were performed to determine the mRNA and protein expression levels of IL‑6 and IL‑1β, respectively. An MTT assay was conducted to detect BV2 cell viability. Oxyntomodulin was observed to attenuate TNF‑α‑induced pain hypersensitivity in mice, as well as the expression of IBA1, NF‑κB p‑p65, IL‑6 and IL‑1β in the spinal cord. Oxyntomodulin exhibited no cytotoxicity on BV2 cells, and attenuated TNF‑α‑induced IL‑6 and IL‑1β production and release in BV2 cells and culture medium, respectively. Collectively, we proposed oxyntomodulin to attenuate TNF‑α induced neuropathic pain associated with the release of glial cytokines IL‑6 and IL‑1β via inhibiting the activation of the NF‑κB pathway.

In Vitro Nociceptor Neuroplasticity Associated with In Vivo Opioid-Induced Hyperalgesia

Opioid-induced hyperalgesia (OIH) is a serious adverse event produced by opioid analgesics. Lack of an in vitro model has hindered study of its underlying mechanisms. Recent evidence has implicated a role of nociceptors in OIH. To investigate the cellular and molecular mechanisms of OIH in nociceptors, in vitro, subcutaneous administration of an analgesic dose of fentanyl (30 μg/kg, s.c.) was performed in vivo in male rats. Two days later, when fentanyl was administered intradermally (1 μg, i.d.), in the vicinity of peripheral nociceptor terminals, it produced mechanical hyperalgesia (OIH). Additionally, 2 d after systemic fentanyl, rats had also developed hyperalgesic priming (opioid-primed rats), long-lasting nociceptor neuroplasticity manifested as prolongation of prostaglandin E₂ (PGE₂) hyperalgesia. OIH was reversed, in vivo, by intrathecal administration of cordycepin, a protein translation inhibitor that reverses priming. When fentanyl (0.5 nm) was applied to dorsal root ganglion (DRG) neurons, cultured from opioid-primed rats, it induced a μ-opioid receptor (MOR)-dependent increase in [Ca²⁺]_i in 26% of small-diameter neurons and significantly sensitized (decreased action potential rheobase) weakly IB4⁺ and IB4^- neurons. This sensitizing effect of fentanyl was reversed in weakly IB4⁺ DRG neurons cultured from opioid-primed rats after in vivo treatment with cordycepin, to reverse of OIH. Thus, in vivo administration of fentanyl induces nociceptor neuroplasticity, which persists in culture, providing evidence for the role of nociceptor MOR-mediated calcium signaling and peripheral protein translation, in the weakly IB4-binding population of nociceptors, in OIH.SIGNIFICANCE STATEMENT Clinically used μ-opioid receptor agonists such as fentanyl can produce hyperalgesia and hyperalgesic priming. We report on an in vitro model of nociceptor neuroplasticity mediating this opioid-induced hyperalgesia (OIH) and priming induced by fentanyl. Using this model, we have found qualitative and quantitative differences between cultured nociceptors from opioid-naive and opioid-primed animals, and provide evidence for the important role of nociceptor μ-opioid receptor-mediated calcium signaling and peripheral protein translation in the weakly IB4-binding population of nociceptors in OIH. These findings provide information useful for the design of therapeutic strategies to alleviate OIH, a serious adverse event of opioid analgesics.

Contribution of synovial macrophages to rat advanced osteoarthritis pain resistant to cyclooxygenase inhibitors

Most advanced knee osteoarthritis (OA) patients experience chronic pain resistant to cyclooxygenase (COX) inhibitors. However, the cells and molecules involved in this advanced OA pain remain poorly understood. In this study, we developed a rat model of advanced knee OA by modification of the monoiodoacetate-induced OA pain model and examined involvement of synovial macrophages in advanced OA pain. Cyclooxygenase inhibitors, such as celecoxib and naproxen, and a steroid were ineffective, but an opioid and anti-nerve growth factor (NGF) antibody was effective for pain management in the advanced OA model. Similar to advanced OA patients, histological analysis indicated severe bone marrow damages, synovitis, and cartilage damage and an increase of macrophages with high expression of interleukin-1β, NGF, nitric oxide synthase (NOS) 1, NOS2, and COX-2 in the knee joint of the advanced OA model. Intravenous injection of clodronate liposomes depleted synovial macrophages, which decreased the level of not only proinflammatory mediator interleukin-1β but also NGF in the knee joint, leading to pain suppression in the advanced OA model. These data suggest the involvement of synovial macrophages in advanced knee OA pain resistant to COX inhibitors by increasing proinflammatory mediators, and that drugs targeting synovial macrophages might have potent analgesic effects.

Role of Nociceptor Toll-like Receptor 4 (TLR4) in Opioid-Induced Hyperalgesia and Hyperalgesic Priming

In addition to analgesia, opioids produce opioid-induced hyperalgesia (OIH) and neuroplasticity characterized by prolongation of inflammatory-mediator-induced hyperalgesia (hyperalgesic priming). We evaluated the hypothesis that hyperalgesia and priming induced by opioids are mediated by similar nociceptor mechanisms. In male rats, we first evaluated the role of nociceptor Toll-like receptor 4 (TLR4) in OIH and priming induced by systemic low-dose morphine (LDM, 0.03 mg/kg). Intrathecal oligodeoxynucleotide antisense to TLR4 mRNA (TLR4 AS-ODN) prevented OIH and prolongation of prostaglandin E₂ hyperalgesia (priming) induced by LDM. In contrast, high-dose morphine (HDM, 3 mg/kg) increased nociceptive threshold (analgesia) and induced priming, neither of which was attenuated by TLR4 AS-ODN. Protein kinase C ε (PKCε) AS-ODN also prevented LDM-induced hyperalgesia and priming, whereas analgesia and priming induced by HDM were unaffected. Treatment with isolectin B4 (IB4)-saporin or SSP-saporin (which deplete IB4⁺ and peptidergic nociceptors, respectively), or their combination, prevented systemic LDM-induced hyperalgesia, but not priming. HDM-induced priming, but not analgesia, was markedly attenuated in both saporin-treated groups. In conclusion, whereas OIH and priming induced by LDM share receptor and second messenger mechanisms in common, action at TLR4 and signaling via PKCε, HDM-induced analgesia, and priming are neither TLR4 nor PKCε dependent. OIH produced by LDM is mediated by both IB4⁺ and peptidergic nociceptors, whereas priming is not dependent on the same population. In contrast, priming induced by HDM is mediated by both IB4⁺ and peptidergic nociceptors. Implications for the use of low-dose opioids combined with nonopioid analgesics and in the treatment of opioid use disorder are discussed.SIGNIFICANCE STATEMENT Opioid-induced hyperalgesia (OIH) and priming are common side effects of opioid agonists such as morphine, which acts at μ-opioid receptors. We demonstrate that OIH and priming induced by systemic low-dose morphine (LDM) share action at Toll-like receptor 4 (TLR4) and signaling via protein kinase C ε (PKCε) in common, whereas systemic high-dose morphine (HDM)-induced analgesia and priming are neither TLR4 nor PKCε dependent. OIH produced by systemic LDM is mediated by isolectin B4-positive (IB4⁺) and peptidergic nociceptors, whereas priming is dependent on a different class of nociceptors. Priming induced by systemic HDM is, however, mediated by both IB4⁺ and peptidergic nociceptors. Our findings may provide useful information for the use of low-dose opioids combined with nonopioid analgesics to treat pain and opioid use disorders.

Intact mast cell content during mild head injury is required for development of latent pain sensitization: implications for mechanisms underlying post-traumatic headache

Post-traumatic headache (PTH) is one of the most common, debilitating, and difficult symptoms to manage after a traumatic head injury. Although the mechanisms underlying PTH remain elusive, recent studies in rodent models suggest the potential involvement of calcitonin gene-related peptide (CGRP), a mediator of neurogenic inflammation, and the ensuing activation of meningeal mast cells (MCs), proalgesic resident immune cells that can lead to the activation of the headache pain pathway. Here, we investigated the relative contribution of MCs to the development of PTH-like pain behaviors in a model of mild closed-head injury (mCHI) in male rats. We initially tested the relative contribution of peripheral CGRP signaling to the activation of meningeal MCs after mCHI using a blocking anti-CGRP monoclonal antibody. We then used a prophylactic MC granule depletion approach to address the hypotheses that intact meningeal MC granule content is necessary for the development of PTH-related pain-like behaviors. The data suggest that after mCHI, ongoing activation of meningeal MCs is not mediated by peripheral CGRP signaling and does not contribute to the development of the mCHI-evoked cephalic mechanical pain hypersensitivity. Our data, however, also reveal that the development of latent sensitization, manifested as persistent hypersensitivity upon the recovery from mCHI-evoked acute cranial hyperalgesia to the headache trigger glyceryl trinitrate requires intact MC content during and immediately after mCHI. Collectively, our data implicate the acute activation of meningeal MCs as mediator of chronic pain hypersensitivity after a concussion or mCHI. Targeting MCs may be explored for early prophylactic treatment of PTH.

Thymosin Alpha-1 Inhibits Complete Freund's Adjuvant-Induced Pain and Production of Microglia-Mediated Pro-inflammatory Cytokines in Spinal Cord

Activation of inflammatory responses regulates the transmission of pain pathways through an integrated network in the peripheral and central nervous systems. The immunopotentiator thymosin alpha-1 (Tα1) has recently been reported to have anti-inflammatory and neuroprotective functions in rodents. However, how Tα1 affects inflammatory pain remains unclear. In the present study, intraperitoneal injection of Tα1 attenuated complete Freund's adjuvant (CFA)-induced pain hypersensitivity, and decreased the up-regulation of pro-inflammatory cytokines (TNF-α, IL-1β, and IL-6) in inflamed skin and the spinal cord. We found that CFA-induced peripheral inflammation evoked strong microglial activation, but the effect was reversed by Tα1. Notably, Tα1 reversed the CFA-induced up-regulation of vesicular glutamate transporter (VGLUT) and down-regulated the vesicular γ-aminobutyric acid transporter (VGAT) in the spinal cord. Taken together, these results suggest that Tα1 plays a therapeutic role in inflammatory pain and in the modulation of microglia-induced pro-inflammatory cytokine production in addition to mediation of VGLUT and VGAT expression in the spinal cord.

Systemic Morphine Produces Dose-dependent Nociceptor-mediated Biphasic Changes in Nociceptive Threshold and Neuroplasticity

We investigated the dose dependence of the role of nociceptors in opioid-induced side-effects, hyperalgesia and pain chronification, in the rat. Systemic morphine produced a dose-dependent biphasic change in mechanical nociceptive threshold. At lower doses (0.003-0.03 mg/kg, s.c.) morphine induced mechanical hyperalgesia, while higher doses (1-10 mg/kg, s.c.) induced analgesia. Intrathecal (i.t.) oligodeoxynucleotide (ODN) antisense to mu-opioid receptor (MOR) mRNA, attenuated both hyperalgesia and analgesia. 5 days after systemic morphine (0.03-10 mg/kg s.c.), mechanical hyperalgesia produced by intradermal (i.d.) prostaglandin E₂ (PGE₂) was prolonged, indicating hyperalgesic priming at the peripheral terminal of the nociceptor. The hyperalgesia induced by i.t. PGE₂ (400 ng/10 µl), in groups that received 0.03 (that induced hyperalgesia) or 3 mg/kg (that induced analgesia) morphine, was also prolonged, indicating priming at the central terminal of the nociceptor. The prolongation of the hyperalgesia induced by i.d. or i.t. PGE₂, in rats previously treated with either a hyperalgesic (0.03 mg/kg, s.c.) or analgesic (3 mg/kg, s.c.) dose, was reversed by i.d. or i.t. injection of the protein translation inhibitor cordycepin (1 µg), indicative of Type I priming at both terminals. Although pretreatment with MOR antisense had no effect on priming induced by 0.03 mg/kg morphine, it completely prevented priming by 3 mg/kg morphine, in both terminals. Thus, the induction of hyperalgesia, but not priming, by low-dose morphine, is MOR-dependent. In contrast, induction of both hyperalgesia and priming by high-dose morphine is MOR-dependent. The receptor at which low-dose morphine acts to produce priming remains to be established.

Monoamines as Drug Targets in Chronic Pain: Focusing on Neuropathic Pain

Monoamines are involved in regulating the endogenous pain system and indeed, peripheral and central monoaminergic dysfunction has been demonstrated in certain types of pain, particularly in neuropathic pain. Accordingly, drugs that modulate the monaminergic system and that were originally designed to treat depression are now considered to be first line treatments for certain types of neuropathic pain (e.g., serotonin and noradrenaline (and also dopamine) reuptake inhibitors). The analgesia induced by these drugs seems to be mediated by inhibiting the reuptake of these monoamines, thereby reinforcing the descending inhibitory pain pathways. Hence, it is of particular interest to study the monoaminergic mechanisms involved in the development and maintenance of chronic pain. Other analgesic drugs may also be used in combination with monoamines to facilitate descending pain inhibition (e.g., gabapentinoids and opioids) and such combinations are often also used to alleviate certain types of chronic pain. By contrast, while NSAIDs are thought to influence the monoaminergic system, they just produce consistent analgesia in inflammatory pain. Thus, in this review we will provide preclinical and clinical evidence of the role of monoamines in the modulation of chronic pain, reviewing how this system is implicated in the analgesic mechanism of action of antidepressants, gabapentinoids, atypical opioids, NSAIDs and histaminergic drugs.

Role of TNF-α in Regulating the Exercise Pressor Reflex in Rats With Femoral Artery Occlusion

Responses of sympathetic nerve activity and arterial blood pressure are augmented during activation of the exercise pressor reflex in rats with femoral artery occlusion. The present study examined the role played by proinflammatory tumor necrosis factor-α (TNF-α) in regulating augmented sympathetic responsiveness induced by stimulation of muscle metabolic receptors and static muscle contraction following 72 h of femoral artery occlusion. We first observed that the levels of TNF-α and protein expression of TNF-α receptor type 1 (TNFR1) were increased in the dorsal root ganglion (DRG) of hindlimbs with femoral artery occlusion. Note that TNF-α was observed within DRG neurons of C-fiber afferent nerves. Capsaicin (TRPV1 agonist) and AITC (TRPA1 agonist) were injected into arterial blood supply of the hindlimbs to stimulate metabolically sensitive thin-fiber muscle afferents. The effects of these injections on the sympathetic and pressor responses were further examined in control rats and rats with femoral artery occlusion. As TNF-α synthesis suppressor pentoxifylline (PTX) was previously administered into the hindlimb with femoral artery occlusion, sympathetic, and pressor responses induced by capsaicin and AITC were attenuated. In occluded rats, PTX also attenuated the exaggeration of blood pressure response induced by muscle contraction, but not by passive tendon stretch. Overall, the results suggest that TNF-α plays a role in modulating exaggerated sympathetic nervous activity via the metabolic component of the exercise pressor reflex when the hindlimb muscles are ischemic in peripheral arterial disease.

Role of Exercise Activity in Alleviating Neuropathic Pain in Diabetes via Inhibition of the Pro-Inflammatory Signal Pathway

Hyperalgesia and allodynia are commonly observed in patients with diabetic neuropathy. The treatment and management of painful peripheral neuropathy is important in these patients. The purpose of this study was to examine the role of exercise in modulating neuropathic pain induced by diabetes. Diabetes was induced in rats by an intraperitoneal injection of streptozotocin (STZ). Control rats received saline injections. Groups included control rats without exercise (NT-control, n = 12), control rats with exercise (EX-control, n = 16), STZ rats without exercise (NT-STZ, n = 18), and STZ rats with exercise (EX-STZ, n = 22). Rats in EX groups ran on a treadmill 4 days/week for 5 weeks beginning from the week of STZ administration. Mechanical hypersensitivity (mechanical paw withdrawal thresholds [PWTs]) and glucose levels were tested weekly. Then, enzyme-linked immunoassay and Western blot analysis were used to determine the levels of pro-inflammatory cytokines (PICs) and their receptors in sensory nerves. PWTs were significantly increased after 4–5 weeks of exercise in STZ rats ( p < .05 vs. NT-STZ rats). Inhibition of neuropathic pain by exercise in STZ rats was accompanied by decreases in interleukin (IL)-1β, IL-6, and tumor necrosis factor (TNF)-α levels and downregulated expression of their receptors. Furthermore, blocking individual PIC receptors elevated PWTs to a greater degree in STZ rats ( p < .05 vs. control rats). Overall, our data suggest that exercise can play a role in improving neuropathic pain induced by STZ and that PIC signaling is a part of the mechanism involved in this effect.

Epac and Nociceptor Sensitization

Abstract

Primary sensory neurons are responsible for transmitting sensory information from the peripheral to the central nervous system. Their responses to incoming stimulation become greatly enhanced and prolonged following inflammation, giving rise to exaggerated nociceptive responses and chronic pain. The inflammatory mediator, prostaglandin E2 (PGE2), released from the inflamed tissue surrounding the terminals of sensory neurons contributes to the abnormal pain responses. PGE2 acts on G protein-coupled EP receptors to activate adenylyl cyclase, which catalyzes the conversion of adenosine triphosphate to cyclic adenosine 3′,5′-monophosphate (cAMP). Under normal conditions, cAMP activates primarily protein kinase A. After inflammation, cAMP also activates the exchange proteins activated by cAMP (Epacs) to produce exaggerated PGE2-mediated hyperalgesia. The role of cAMP-Epac signaling in the generation of hypersensitivity is the topic of this review.

CD44 Signaling Mediates High Molecular Weight Hyaluronan-Induced Antihyperalgesia

We studied, in male Sprague Dawley rats, the role of the cognate hyaluronan receptor, CD44 signaling in the antihyperalgesia induced by high molecular weight hyaluronan (HMWH). Low molecular weight hyaluronan (LMWH) acts at both peptidergic and nonpeptidergic nociceptors to induce mechanical hyperalgesia that is prevented by intrathecal oligodeoxynucleotide antisense to CD44 mRNA, which also prevents hyperalgesia induced by a CD44 receptor agonist, A6. Ongoing LMWH and A6 hyperalgesia are reversed by HMWH. HMWH also reverses the hyperalgesia induced by diverse pronociceptive mediators, prostaglandin E₂, epinephrine, TNFα, and interleukin-6, and the neuropathic pain induced by the cancer chemotherapy paclitaxel. Although CD44 antisense has no effect on the hyperalgesia induced by inflammatory mediators or paclitaxel, it eliminates the antihyperalgesic effect of HMWH. HMWH also reverses the hyperalgesia induced by activation of intracellular second messengers, PKA and PKC_ε, indicating that HMWH-induced antihyperalgesia, although dependent on CD44, is mediated by an intracellular signaling pathway rather than as a competitive receptor antagonist. Sensitization of cultured small-diameter DRG neurons by prostaglandin E₂ is also prevented and reversed by HMWH. These results demonstrate the central role of CD44 signaling in HMWH-induced antihyperalgesia, and establish it as a therapeutic target against inflammatory and neuropathic pain.SIGNIFICANCE STATEMENT We demonstrate that hyaluronan (HA) with different molecular weights produces opposing nociceptive effects. While low molecular weight HA increases sensitivity to mechanical stimulation, high molecular weight HA reduces sensitization, attenuating inflammatory and neuropathic hyperalgesia. Both pronociceptive and antinociceptive effects of HA are mediated by activation of signaling pathways downstream CD44, the cognate HA receptor, in nociceptors. These results contribute to our understanding of the role of the extracellular matrix in pain, and indicate CD44 as a potential therapeutic target to alleviate inflammatory and neuropathic pain.

The Interface of Mechanics and Nociception in Joint Pathophysiology: Insights From the Facet and Temporomandibular Joints

Chronic joint pain is a widespread problem that frequently occurs with aging and trauma. Pain occurs most often in synovial joints, the body's load bearing joints. The mechanical and molecular mechanisms contributing to synovial joint pain are reviewed using two examples, the cervical spinal facet joints and the temporomandibular joint (TMJ). Although much work has focused on the macroscale mechanics of joints in health and disease, the combined influence of tissue mechanics, molecular processes, and nociception in joint pain has only recently become a focus. Trauma and repeated loading can induce structural and biochemical changes in joints, altering their microenvironment and modifying the biomechanics of their constitutive tissues, which themselves are innervated. Peripheral pain sensors can become activated in response to changes in the joint microenvironment and relay pain signals to the spinal cord and brain where pain is processed and perceived. In some cases, pain circuitry is permanently changed, which may be a potential mechanism for sustained joint pain. However, it is most likely that alterations in both the joint microenvironment and the central nervous system (CNS) contribute to chronic pain. As such, the challenge of treating joint pain and degeneration is temporally and spatially complicated. This review summarizes anatomy, physiology, and pathophysiology of these joints and the sensory pain relays. Pain pathways are postulated to be sensitized by many factors, including degeneration and biochemical priming, with effects on thresholds for mechanical injury and/or dysfunction. Initiators of joint pain are discussed in the context of clinical challenges including the diagnosis and treatment of pain.

Age-Dependent Sexual Dimorphism in Susceptibility to Develop Chronic Pain in the Rat

Neonatal pain has been suggested to contribute to the development and/or persistence of adult pain. Observations from animal models have shown that neonatal inflammation produces long-term changes in sensory neuron function, which can affect the susceptibility of adults to develop persistent pain. We used a preclinical model of transition to chronic pain, hyperalgesic priming, in which a previous inflammatory stimulus triggers a long-lasting increase in responsiveness to pro-algesic mediators, prototypically prostaglandin E₂ (PGE₂), to investigate if post-natal age influences susceptibility of adult rats to develop chronic pain. Priming was induced by tumor necrosis factor alpha (TNFα), in male and female rats, 1, 2, 3, 4, 5 or 7weeks after birth. When adults (8weeks after birth), to evaluate for the presence of priming, PGE₂ was injected at the same site as TNFα. In males that had received TNFα at post-natal weeks 1, 2 or 3, priming was attenuated compared to the 4-, 5- and 7-week-old treated groups, in which robust priming developed. In contrast, in females treated with TNFα at post-natal week 1, 2, 3, or 4, but not at 5 or 7, priming was present. This age and sex difference in the susceptibility to priming was estrogen-dependent, since injection of TNFα in 3-week-old males and 5-week-old females, in the presence of the estrogen receptor antagonist ICI 182,780, did produce priming. These results suggest that estrogen levels, which vary differently in males and females over the post-natal period, until they stabilize after puberty, impact pain as an adult.

Inhibition of TNF reduces mechanical orofacial hyperalgesia induced by Complete Freund's Adjuvant by a TRPV1-dependent mechanism in mice

BACKGROUND

Inflammation in the orofacial region results in pain and is associated with many pathological states, including migraine, neuralgias and temporomandibular disorder. Although extensively studied, the mechanisms responsible for these conditions are not known and effective treatments are lacking. We reported earlier that the proinflammatory cytokine tumor necrosis factor (TNF) plays an important role in regulation of trigeminal ganglion (TG) neuron function in vitro. In the present study we investigated the role of TNF in mechanical hypersensitivity in mice.

METHODS

We employed the Complete Freund's Adjuvant (CFA)-induced model of orofacial pain and evaluated the effect of blocking of soluble TNF activity by peripheral administration of the novel dominant negative TNF biologic, XPro1595.

RESULTS

We show that CFA administration into the lower lip causes hyperalgesia and an increase in both expression of transient receptor potential vanilloid subfamily member 1 (TRPV1) mRNA and in the average intensity of TRPV1 protein immunoreactivity in TG neurons. We also show that intraperitoneal administration of XPro1595 prevents both CFA-induced mechanical hypersensitivity and, as shown in immunohistochemical staining - upregulation of TRPV1 protein expression in TG neurons.

CONCLUSIONS

We conclude that one of the possible regulatory mechanisms of TNF in pain involves upregulation of the nociceptor TRPV1, and that peripheral treatment with a selective anti-soluble TNF biologic can prevent hyperalgesia caused by inflammation in the orofacial region. Therefore, these new findings suggest that XPro1595 may serve as a novel treatment for orofacial pain disorders.

Epac-protein kinase C alpha signaling in purinergic P2X3R-mediated hyperalgesia after inflammation

Protein kinase C alpha plays a major role in mediating Epac-dependent enhancement of purinergic P2X3R activity in dorsal root ganglion neurons after inflammation.

Sensitization of purinergic P2X3 receptors (P2X3Rs) is a major mechanism contributing to injury-induced exaggerated pain responses. We showed in a previous study that cyclic adenosine monophosphate (cAMP)–dependent guanine nucleotide exchange factor 1 (Epac1) in rat sensory dorsal root ganglia (DRGs) is upregulated after inflammatory injury, and it plays a critical role in P2X3R sensitization by activating protein kinase C epsilon (PKCε) inside the cells. protein kinase C epsilon has been established as the major PKC isoform mediating injury-induced hyperalgesic responses. On the other hand, the role of PKCα in receptor sensitization was seldom considered. Here, we studied the participation of PKCα in Epac signaling in P2X3R-mediated hyperalgesia. The expression of both Epac1 and Epac2 and the level of cAMP in DRGs are greatly enhanced after complete Freund adjuvant (CFA)–induced inflammation. The expression of phosphorylated PKCα is also upregulated. Complete Freund adjuvant (CFA)–induced P2X3R-mediated hyperalgesia is not only blocked by Epac antagonists but also by the classical PKC isoform inhibitors, Go6976, and PKCα-siRNA. These CFA effects are mimicked by the application of the Epac agonist, 8-(4-chlorophenylthio)-2 -O-methyl-cAMP (CPT), in control rats, further confirming the involvement of Epacs. Because the application of Go6976 prior to CPT still reduces CPT-induced hyperalgesia, PKCα is downstream of Epacs to mediate the enhancement of P2X3R responses in DRGs. The pattern of translocation of PKCα inside DRG neurons in response to CPT or CFA stimulation is distinct from that of PKCε. Thus, in contrast to prevalent view, PKCα also plays an essential role in producing complex inflammation-induced receptor-mediated hyperalgesia.

Gi-protein-coupled 5-HT1B/D receptor agonist sumatriptan induces type I hyperalgesic priming

We have recently described a novel form of hyperalgesic priming (type II) induced by agonists at two clinically important Gi-protein-coupled receptors (Gi-GPCRs), mu-opioid and A1-adenosine. Like mu-opioids, the antimigraine triptans, which act at 5-HT1B/D Gi-GPCRs, have been implicated in pain chronification. We determined whether sumatriptan, a prototypical 5-HT1B/D agonist, produces type II priming. Characteristic of hyperalgesic priming, intradermal injection of sumatriptan (10 ng) induced a change in nociceptor function such that a subsequent injection of prostaglandin-E2 (PGE2) induces prolonged mechanical hyperalgesia. However, onset to priming was delayed 3 days, characteristic of type I priming. Also characteristic of type I priming, a protein kinase Cε, but not a protein kinase A inhibitor attenuated the prolongation phase of PGE2 hyperalgesia. The prolongation of PGE2 hyperalgesia was also permanently reversed by intradermal injection of cordycepin, a protein translation inhibitor. Also, hyperalgesic priming did not occur in animals pretreated with pertussis toxin or isolectin B4-positive nociceptor toxin, IB4-saporin. Finally, as observed for other agonists that induce type I priming, sumatriptan did not induce priming in female rats. The prolongation of PGE2 hyperalgesia induced by sumatriptan was partially prevented by coinjection of antagonists for the 5-HT1B and 5-HT1D, but not 5-HT7, serotonin receptors and completely prevented by coadministration of a combination of the 5-HT1B and 5-HT1D antagonists. Moreover, the injection of selective agonists, for 5-HT1B and 5-HT1D receptors, also induced hyperalgesic priming. Our results suggest that sumatriptan, which signals through Gi-GPCRs, induces type I hyperalgesic priming, unlike agonists at other Gi-GPCRs, which induce type II priming.

Sexual Dimorphism in a Reciprocal Interaction of Ryanodine and IP3 Receptors in the Induction of Hyperalgesic Priming

Hyperalgesic priming, a model of pain chronification in the rat, is mediated by ryanodine receptor-dependent calcium release. Although ryanodine induces priming in both sexes, females are 5 orders of magnitude more sensitive, by an estrogen receptor α (EsRα)-dependent mechanism. An inositol 1,4,5-triphosphate (IP₃) receptor inhibitor prevented the induction of priming by ryanodine. For IP₃ induced priming, females were also more sensitive. IP₃-induced priming was prevented by pretreatment with inhibitors of the sarcoendoplasmic reticulum calcium ATPase and ryanodine receptor. Antisense to EsRα prevented the induction of priming by low-dose IP₃ in females. The induction of priming by an EsRα agonist was ryanodine receptor-dependent and prevented by the IP₃ antagonist. Thus, an EsRα-dependent bidirectional interaction between endoplasmic reticulum IP₃ and ryanodine receptor-mediated calcium signaling is present in the induction of hyperalgesic priming, in females. In cultured male DRG neurons, IP₃ (100 μm) potentiated depolarization-induced transients produced by extracellular application of high-potassium solution (20 mm, K20), in nociceptors incubated with β-estradiol. This potentiation of depolarization-induced calcium transients was blocked by the IP₃ antagonist, and not observed in the absence of IP₃ IP₃ potentiation was also blocked by ryanodine receptor antagonist. The application of ryanodine (2 nm), instead of IP₃, also potentiated K20-induced calcium transients in the presence of β-estradiol, in an IP₃ receptor-dependent manner. Our results point to an EsRα-dependent, reciprocal interaction between IP₃ and ryanodine receptors that contributes to sex differences in hyperalgesic priming.SIGNIFICANCE STATEMENT The present study demonstrates a mechanism that plays a role in the marked sexual dimorphism observed in a model of the transition to chronic pain, hyperalgesic priming. This mechanism involves a reciprocal interaction between the endoplasmic reticulum receptors, IP₃ and ryanodine, in the induction of priming, regulated by estrogen receptor α in the nociceptor of female rats. The presence of this signaling pathway modulating the susceptibility of nociceptors to develop plasticity may contribute to our understanding of sex differences observed clinically in chronic pain syndromes.

Regulation of Expression of Hyperalgesic Priming by Estrogen Receptor α in the Rat

Hyperalgesic priming, a sexually dimorphic model of transition to chronic pain, is expressed as prolongation of prostaglandin E2-induced hyperalgesia by the activation of an additional pathway including an autocrine mechanism at the plasma membrane. The autocrine mechanism involves the transport of cyclic adenosine monophosphate (AMP) to the extracellular space, and its conversion to AMP and adenosine, by ecto-5'phosphodiesterase and ecto-5'nucleotidase, respectively. The end product, adenosine, activates A1 receptors, producing delayed onset prolongation of prostaglandin E2 hyperalgesia. We tested the hypothesis that the previously reported, estrogen-dependent, sexual dimorphism observed in the induction of priming is present in the mechanisms involved in its expression, as a regulatory effect on ecto-5'nucleotidase by estrogen receptor α (EsRα), in female rats. In the primed paw AMP hyperalgesia was dependent on conversion to adenosine, being prevented by ecto-5'nucleotidase inhibitor α,β-methyleneadenosine 5'-diphosphate sodium salt and A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine. To investigate an interaction between EsRα and ecto-5'nucleotidase, we treated primed female rats with oligodeoxynucleotide antisense or mismatch against EsRα messenger RNA. Whereas in rats treated with antisense AMP-induced hyperalgesia was abolished, the A1 receptor agonist N⁶-cyclopentiladenosine still produced hyperalgesia. Thus, EsRα interacts with this autocrine pathway at the level of ecto-5'nucleotidase. These results demonstrate a sexually dimorphic mechanism for the expression of priming.

PERSPECTIVE

This study presents evidence of an estrogen-dependent mechanism of expression of chronic pain in female rats, supporting the suggestion that differential targets must be considered when establishing protocols for the treatment of painful conditions in men and women.

Adenosine-A1 receptor agonist induced hyperalgesic priming type II

We have recently shown that repeated exposure of the peripheral terminal of the primary afferent nociceptor to the mu-opioid receptor (MOR) agonist DAMGO ([D-Ala, N-Me-Phe, Gly-ol]-enkephalin acetate salt) induces a model of transition to chronic pain that we have termed type II hyperalgesic priming. Similar to type I hyperalgesic priming, there is a markedly prolonged response to subsequent administration of proalgesic cytokines, prototypically prostaglandin E2 (PGE2). However, type II hyperalgesic priming differs from type I in being rapidly induced, protein kinase A (PKA), rather than PKCε dependent, not reversed by a protein translation inhibitor, occurring in female as well as in male rats, and isolectin B4-negative neuron dependent. We report that, as with the repeated injection of a MOR agonist, the repeated administration of an agonist at the A1-adenosine receptor, also a Gi-protein coupled receptor, N-cyclopentyladenosine (CPA), also produces priming similar to DAMGO-induced type II hyperalgesic priming. In this study, we demonstrate that priming induced by repeated exposure to this A1-adenosine receptor agonist shares the same mechanisms, as MOR-agonist induced priming. However, the prolongation of PGE2 hyperalgesia induced by repeated administration of CPA depends on G-protein αi subunit activation, differently from DAMGO-induced type II priming, in which it depends on the β/γ subunit. These data implicate a novel form of Gi-protein signaling pathway in the type II hyperalgesic priming induced by repeated administration of an agonist at A1-adenosine receptor to the peripheral terminal of the nociceptor.

Neurovascular contributions to migraine: Moving beyond vasodilation

Migraine is the third most common disease worldwide, the most common neurological disorder, and one of the most common pain conditions. Despite its prevalence, the basic physiology and underlying mechanisms contributing to the development of migraine are still poorly understood and development of new therapeutic targets is long overdue. Until recently, the major contributing pathophysiological event thought to initiate migraine was cerebral and meningeal arterial vasodilation. However, the role of vasodilation in migraine is unclear and recent findings challenge its necessity. While vasodilation itself may not contribute to migraine, it remains possible that vessels play a role in migraine pathophysiology in the absence of vasodilation. Blood vessels consist of a variety of cell types that both release and respond to numerous mediators including growth factors, cytokines, adenosine triphosphate (ATP), and nitric oxide (NO). Many of these mediators have actions on neurons that can contribute to migraine. Conversely, neurons release factors such as norepinephrine and calcitonin gene-related peptide (CGRP) that act on cells native to blood vessels. Both normal and pathological events occurring within and between vascular cells could thus mediate bi-directional communication between vessels and the nervous system, without the need for changes in vascular tone. This review will discuss the potential contribution of the vasculature, specifically endothelial cells, to current neuronal mechanisms hypothesized to play a role in migraine. Hypothalamic activity, cortical spreading depression (CSD), and dural afferent input from the cranial meninges will be reviewed with a focus on how these mechanisms can influence or be impacted by blood vessels. Together, the data discussed will provide a framework by which vessels can be viewed as important potential contributors to migraine pathophysiology, even in light of the current uncertainty over the role of vasodilation in this disorder.

Consistent sex-dependent effects of PKMζ gene ablation and pharmacological inhibition on the maintenance of referred pain

BACKGROUND

Persistently active PKMζ has been implicated in maintaining spinal nociceptive sensitization that underlies pain hypersensitivity. However, evidence for PKMζ in the maintenance of pain hypersensitivity comes exclusively from short-term studies in males using pharmacological agents of questionable selectivity. The present study examines the contribution of PKMζ to long-lasting allodynia associated with neuropathic, inflammatory, or referred visceral and muscle pain in males and females using pharmacological inhibition or genetic ablation.

RESULTS

Pharmacological inhibition or genetic ablation of PKMζ reduced mild formalin pain and slowly developing contralateral allodynia in nerve-injured rats, but not moderate formalin pain or ipsilateral allodynia in models of neuropathic and inflammatory pain. Pharmacological inhibition or genetic ablation of PKMζ also effectively reduced referred visceral and muscle pain in male, but not in female mice and rats.

CONCLUSION

We show pharmacological inhibition and genetic ablation of PKMζ consistently attenuate long-lasting pain hypersensitivity. However, differential effects in models of referred versus inflammatory and neuropathic pain, and in males versus females, highlight the roles of afferent input-dependent masking and sex differences in the maintenance of pain hypersensitivity.

Marked Sexual Dimorphism in the Role of the Ryanodine Receptor in a Model of Pain Chronification in the Rat

Hyperalgesic priming, an estrogen dependent model of the transition to chronic pain, produced by agonists at receptors that activate protein kinase C epsilon (PKCε), occurs in male but not in female rats. However, activation of second messengers downstream of PKCε, such as the ryanodine receptor, induces priming in both sexes. Since estrogen regulates intracellular calcium, we investigated the interaction between estrogen and ryanodine in the susceptibility to develop priming in females. The lowest dose of ryanodine able to induce priming in females (1 pg) is 1/100,000^th that needed in males (100 ng), an effect dependent on the activation of ryanodine receptors. Treatment of female rats with antisense to estrogen receptor alpha (ERα), but not beta (ERβ), mRNA, prevented the induction of priming by low dose ryanodine, and the ERα agonist, PPT, induced ryanodine receptor-dependent priming. In vitro application of ryanodine in low concentration (2 nM) to small DRG neurons cultured from females, significantly potentiated calcium release via ryanodine receptors induced by caffeine. This effect was only observed in IB4+ neurons, cultured in the presence of β-estradiol or PPT. Our results demonstrate a profound regulatory role of ERα in ryanodine receptor-dependent transition to chronic pain.

Regulation of neuronal excitability by release of proteins from glial cells

Effects of glial cells on electrical isolation and shaping of synaptic transmission between neurons have been extensively studied. Here we present evidence that the release of proteins from astrocytes as well as microglia may regulate voltage-activated Na(+) currents in neurons, thereby increasing excitability and speed of transmission in neurons kept at distance from each other by specialized glial cells. As a first example, we show that basic fibroblast growth factor and neurotrophin-3, which are released from astrocytes by exposure to thyroid hormone, influence each other to enhance Na(+) current density in cultured hippocampal neurons. As a second example, we show that the presence of microglia in hippocampal cultures can upregulate Na(+) current density. The effect can be boosted by lipopolysaccharides, bacterial membrane-derived stimulators of microglial activation. Comparable effects are induced by the exposure of neuron-enriched hippocampal cultures to tumour necrosis factor-α, which is released from stimulated microglia. Taken together, our findings suggest that release of proteins from various types of glial cells can alter neuronal excitability over a time course of several days. This explains changes in neuronal excitability occurring in states of thyroid hormone imbalance and possibly also in seizures triggered by infectious diseases.

The Role of Store-operated Calcium Channels in Pain

Store-operated calcium channels (SOCCs) are calcium-selective cation channels. Recently, there has been explosive growth in establishing the molecular mechanisms that mediate store-operated Ca(2+) entry (SOCE) and the role of this process in normal cellular function and disease states. SOCCs and its components appear to play an important role in many Ca(2+)-dependent processes in nonexcitable cells and are implicated in several possible disorders including allergies, multiple sclerosis, cancer, and inflammatory bowel disease. Recent studies have shown that SOCCs are expressed in the central nervous system (CNS) and involved in neuronal functions and pathological conditions, including chronic pain. In this chapter, we discuss SOCE and its physiological and pathological roles in the CNS. More specifically, we discuss the expression and function of SOCCs and their downstream signaling mechanisms under chronic pain conditions.