Lysophosphatidic acid receptor type-1 mediates brain activation in micro-positron emission tomography analysis in a fibromyalgia-like mouse model.

The intermittent cold stress-induced generalized pain response mimics the pathophysiological and pharmacotherapeutic features reported for fibromyalgia patients, including the presence of chronic generalized pain and female dominance. In addition, the intermittent cold stress-induced generalized pain is abolished in lysophosphatidic acid receptor type-1 knockout mice, as reported in many cases of neuropathic pain models. This study aimed to identify the brain loci involved in the intermittent cold stress generalized pain response and test their dependence on the lysophosphatidic acid receptor type-1. Positron emission tomography analyses using 2-deoxy-2-[18 F]fluoro-d-glucose in the presence of a pain stimulus showed that intermittent cold stress causes a significant increase in uptake in the ipsilateral regions, including the salience networking-related anterior cingulate cortex and insular cortex and the cognition-related hippocampus. A significant decrease was observed in the default mode network-related posterior cingulate cortex. Almost these intermittent cold stress-induced changes were abolished in lysophosphatidic acid receptor type-1 knockout mice. There results suggest that the intermittent cold stress-induced generalized pain response is mediated by the lysophosphatidic acid receptor type-1 in specific brain loci related to salience networking and cognition, which may lead to further developments in the treatment of fibromyalgia.

Secreted PLA2-III is a possible therapeutic target to treat neuropathic pain.

Lysophosphatidic acid (LPA) plays a critical role in developing and maintaining chronic pain in various animal models. Previous studies have reported that cytosolic and calcium-independent phospholipase A2 (PLA2) is involved in the LPA receptor-mediated amplification of LPA production in the spinal dorsal horn (SDH) after nerve injury, while the involvement of secreted PLA2 (sPLA2) remains unclear. The present study revealed that only sPLA2 -III among 11 species of PLA2 showed a significant upregulation of gene expression in the SDH. Intraspinal injection of adeno-associated virus-miRNA targeting sPLA2-III prevented hyperalgesia and unique hypoalgesia in mice treated with partial sciatic nerve ligation. In addition, intrathecal treatment with antisense oligodeoxynucleotide or siRNA targeting sPLA2-III significantly reversed the established thermal hyperalgesia. In the high-throughput screening of sPLA2-III inhibitors from the chemical library, we identified two hit compounds. Through in vitro characterization of PLA2 inhibitor profiles and in vivo assessment of the anti-hyperalgesic effects of known PLA2 inhibitors as well as hit compounds, sPLA2-III was found to be a novel therapeutic target molecule for the treatment of Neuropathic pain.

Alleviation of the chronic stress response attributed to the antioxidant and anti-inflammatory effects of electrolyzed hydrogen water.

Reactive oxygen species (ROS) are highly reactive and directly attack surrounding biomolecules to deteriorate cellular and tissue functions. Meanwhile, ROS also serve as signaling mediators to upregulate pro-inflammatory cytokine expression via activation of the nuclear factor kappa B signaling pathway, and the increased pro-inflammatory cytokines trigger respiratory burst of inflammatory cells that further accelerates ROS production in the inflamed tissue. Such crosstalk between ROS and inflammatory responses leads to a chain reaction of negativity, and cause progression of several chronic pathologies. Since molecular hydrogen is known to preferentially remove cytotoxic hydroxyl radicals and peroxynitrites, and to prevent cell and tissue damage, we here examined whether electrolyzed hydrogen water (EHW) enriched with molecular hydrogen and reactive hydrogen storing platinum nanoparticles dissolved from an electrode could alleviate oxidative stress and inflammation induced by continuous stress challenges. Five-day continuous stress loading to rats elevated reactive oxygen metabolites-derived compounds (d-ROMs), interleukin (IL)-1ß, and adrenocorticotropic hormone (ACTH) levels and decreased the biological antioxidant potential (BAP) level. Drinking EHW during 5-day continuous stress loading significantly alleviated all of these changes. The results suggest that EHW could suppress stress-response-associated oxidative stress and IL-1ß level elevation in vivo, and that drinking of EHW is effective for controlling stress responses via its antioxidant potential.

Mirtazapine, an α2 Antagonist-Type Antidepressant, Reverses Pain and Lack of Morphine Analgesia in Fibromyalgia-Like Mouse Models.

Treatment of fibromyalgia is an unmet medical need; however, its pathogenesis is still poorly understood. In a series of studies, we have demonstrated that some pharmacological treatments reverse generalized chronic pain but do not affect the lack of morphine analgesia in the intermittent cold stress (ICS)-induced fibromyalgia-like pain model in mice. Here we report that repeated intraperitoneal treatments with mirtazapine, which is presumed to disinhibit 5-hydroxytriptamine (5-HT) release and activate 5-HT1 receptor through mechanisms of blocking presynaptic adrenergic α2 and postsynaptic 5-HT2 and 5-HT3 receptors, completely reversed the chronic pain for more than 4 to 5 days after the cessation of treatments. The repeated mirtazapine treatments also recovered the morphine analgesia after the return of nociceptive threshold to the normal level. The microinjection of small interfering RNA (siRNA) adrenergic α2a receptor (ADRA2A) into the habenula, which showed a selective upregulation of α2 receptor gene expression after ICS, reversed the hyperalgesia but did not recover the morphine analgesia. However, both reversal of hyperalgesia and recovery of morphine analgesia were observed when siRNA ADRA2A was administered intracerebroventricularly. As the habenular is reported to be involved in the emotion/reward-related pain and hypoalgesia, these results suggest that mirtazapine could attenuate pain and/or augment hypoalgesia by blocking the habenular α2 receptor after ICS. The recovery of morphine analgesia in the ICS model, on the other hand, seems to be mediated through a blockade of α2 receptor in unidentified brain regions. SIGNIFICANCE STATEMENT: This study reports possible mechanisms underlying the complete reversal of hyperalgesia and recovery of morphine analgesia by mirtazapine, a unique antidepressant with adrenergic α2 and serotonergic receptor antagonist properties, in a type of intermittently repeated stress (ICS)-induced fibromyalgia-like pain model. Habenula, a brain region which is related to the control of emotional pain, was found to play key roles in the antihyperalgesia, whereas other brain regions appeared to be involved in the recovery of morphine analgesia in the ICS model.

NR2A-NMDA Receptor Blockade Reverses the Lack of Morphine Analgesia Without Affecting Chronic Pain Status in a Fibromyalgia-Like Mouse Model.

We have developed an experimental fibromyalgia-like mouse model using intermittent cold stress (ICS), where chronic pain is generalized, female predominant, and abolished in type 1 lysophosphatidic acid receptor-knockout (LPA1 -/-) mice but is not reversed by systemic or brain treatment with morphine. We investigated two issues in the present study: (1) whether chronic pain mechanisms and lack of brain morphine analgesia are associated in the ICS model and (2) what mechanisms are involved in the lack of morphine analgesia. ICS-induced hyperalgesia was not affected in µ-opioid receptor-knockout (MOPr -/-) mice, whereas the lack of brain morphine analgesia remained unchanged in LPA1 -/- mice, which completely abolished the hyperalgesia in the ICS model. In contrast, the lack of morphine analgesia was abolished in NR2A-NMDA receptor-knockout (NR2A -/- ) mice and blocked by intracerebroventricular (i.c.v.) injection of (R)-CPP, an NR2A antagonist, or by microinjection of siRNA NR2A into the periaqueductal gray matter region, whereas no change was observed with Ro 04-5595, an NMDA receptor subtype 2B antagonist (i.c.v.). The lack of morphine analgesia was also reversed by concomitant treatment with 1 mg/kg intraperitoneal (i.p.) of dextromethorphan, which possesses NMDA receptor antagonist activity but no analgesic activity. Finally, the hyperalgesia was completely reversed by methadone, which possesses both MOPr agonist and NMDA receptor antagonist activity. Indeed, methadone analgesia was abolished in MOPr -/- mice. These results suggest that chronic pain status and lack of morphine analgesia are independent of each other, and that lack of morphine analgesia is mediated by activation of the NR2A-NMDA receptor system. SIGNIFICANCE STATEMENT: This study reports that a type of intermittently repeated stress causes widespread pain that does not respond to morphine. Because this lack of morphine analgesia is not affected in mice, in which chronic pain is abolished, the mechanisms underlying chronic pain and lack of morphine analgesia are independent of each other. Through speculation that a lack of morphine analgesia may be a secondary event to endogenous opioid analgesic tolerance, the authors demonstrate that an antiopioid N-methyl-D-aspartate receptor system counterbalances the µ-opioid receptor-mediated analgesic mechanisms in the intermittent cold stress model.

LPA5 signaling is involved in multiple sclerosis-mediated neuropathic pain in the cuprizone mouse model.

Lysophosphatidic acid (LPA) and LPA1 receptor signaling play a crucial role in the initiation of peripheral nerve injury-induced neuropathic pain through the alternation of pain-related genes/proteins expression and demyelination. However, LPA and its signaling in the brain are still poorly understood. In the present study, we revealed that the LPA5 receptor expression in corpus callosum elevated after the initiation of demyelination, and the hyperalgesia through Aδ-fibers following cuprizone-induced demyelination was mediated by LPA5 signaling. These data suggest that LPA5 signaling may play a key role in the mechanisms underlying neuropathic pain following demyelination in the brain.

A mimetic of the mSin3-binding helix of NRSF/REST ameliorates abnormal pain behavior in chronic pain models.

The neuron-restrictive silencing factor NRSF/REST binds to neuron-restrictive silencing elements in neuronal genes and recruits corepressors such as mSin3 to inhibit epigenetically neuronal gene expression. Because dysregulation of NRSF/REST is related to neuropathic pain, here, we have designed compounds to target neuropathic pain based on the mSin3-binding helix structure of NRSF/REST and examined their ability to bind to mSin3 by NMR. One compound, mS-11, binds strongly to mSin3 with a binding mode similar to that of NRSF/REST. In a mouse model of neuropathic pain, mS-11 was found to ameliorate abnormal pain behavior and to reverse lost peripheral morphine analgesia. Furthermore, even in the less well epigenetically defined case of fibromyalgia, mS-11 ameliorated symptoms in a mouse model, suggesting that fibromyalgia is related to the dysfunction of NRSF/REST. Taken together, these findings show that the chemically optimized mimetic mS-11 can inhibit mSin3-NRSF/REST binding and successfully reverse lost peripheral and central morphine analgesia in mouse models of pain.

Modulation of Nicotine-Associated Behaviour in Rats By µ-Opioid Signals from the Medial Prefrontal Cortex to the Nucleus Accumbens Shell.

Nicotine addiction is a concern worldwide. Most mechanistic investigations are on nicotine substance dependence properties based on its pharmacological effects. However, no effective therapeutic treatment has been established. Nicotine addiction is reinforced by environments or habits. We demonstrate the neurobiological basis of the behavioural aspect of nicotine addiction. We utilized the conditioned place preference to establish nicotine-associated behavioural preferences (NABP) in rats. Brain-wide neuroimaging analysis revealed that the medial prefrontal cortex (mPFC) was activated and contributed to NABP. Chemogenetic manipulation of µ-opioid receptor positive (MOR+) neurons in the mPFC or the excitatory outflow to the nucleus accumbens shell (NAcShell) modulated the NABP. Electrophysiological recording confirmed that the MOR+ neurons directly regulate the mPFC-NAcShell circuit via GABAA receptors. Thus, the MOR+ neurons in the mPFC modulate the formation of behavioural aspects of nicotine addiction via direct excitatory innervation to the NAcShell, which may provide new insight for the development of effective therapeutic strategies.

Fibromyalgia Animal Models Using Intermittent Cold and Psychological Stress.

Fibromyalgia (FM) is a chronic pain condition characterized by widespread musculoskeletal pain and other frequent symptoms such as fatigue, sleep disturbance, cognitive impairment, and mood disorder. Based on the view that intermittent stress would be the most probable etiology for FM, intermittent cold- and intermittent psychological stress-induced generalized pain (ICGP and IPGP) models in mice have been developed and validated as FM-like pain models in terms of the patho-physiological and pharmacotherapeutic features that are shared with clinical versions. Both models show long-lasting and generalized pain and female-predominant sex differences after gonadectomy. Like many other neuropathic pain models, ICGP and IPGP were abolished in lysophosphatidic acid receptor 1 (LPAR1) knock-out mice or by LPAR1 antagonist treatments, although deciding the clinical importance of this mechanism depends on waiting for the development of a clinically available LPAR1 antagonist. On the other hand, the nonsteroidal anti-inflammatory drug diclofenac with morphine did not suppress hyperalgesia in these models, and this is consistent with the clinical findings. Pharmacological studies suggest that the lack of morphine analgesia is associated with opioid tolerance upon the stress-induced release of endorphins and subsequent counterbalance through anti-opioid NMDA receptor mechanisms. Regarding pharmacotherapy, hyperalgesia in both models was suppressed by pregabalin and duloxetine, which have been approved for FM treatment in clinic. Notably, repeated treatments with mirtazapine, an α2 adrenergic receptor antagonist-type antidepressant, and donepezil, a drug for treating Alzheimer's disease, showed potent therapeutic actions in these models. However, the pharmacotherapeutic treatment should be carried out 3 months after stress, which is stated in the FM guideline, and many preclinical studies, such as those analyzing molecular and cellular mechanisms, as well as additional evidence using different animal models, are required. Thus, the ICGP and IPGP models have the potential to help discover and characterize new therapeutic medicines that might be used for the radical treatment of FM, although there are several limitations to be overcome.

Regional neuroinflammation induced by peripheral infection contributes to fatigue-like symptoms: a [18F]DPA-714 positron emission tomography study in rats.

Introduction: A series of symptoms, including fever, widespread pain, fatigue, and even ageusia, have frequently been reported in the context of various infections, such as COVID-19. Although the pathogenic mechanisms underlying an infection causing fever and pain have been well established, the mechanisms of fatigue induced by infection in specific brain regions remain unclear. Methods: To elucidate whether and how the peripheral infection cause fatigue via regional neuroinflammation, we performed a brain-wide investigation of neuroinflammation in a peripheral pseudoinfection rat model using [18F]DPA-714 positron emission tomography (PET) imaging analysis, in which the polyriboinosinic: polyribocytidylic acid (poly I:C) was intraperitoneally injected. Results: Transient fever lasting for several hours and subsequent suppression of spontaneous activity lasting a few days were induced by poly I:C treatment. Significant increase in plasma interleukin (IL)-1ß, IL-6 and tumour necrosis factor (TNF)-α were observed at 2 and 4 h following poly I:C treatment. PET imaging analysis revealed that the brain uptake of [18F]DPA-714 was significantly increased in several brain regions one day after poly I:C treatment, such as the dorsal raphe (DR), parvicellular part of red nucleus (RPC), A5 and A7 noradrenergic nucleus, compared with the control group. The accumulation of [18F]DPA-714 in the DR, RPC and A5 was positively correlated with subsequent fatigue-like behavior, and that in the A7 tended to positively correlate with fever. Discussion: These findings suggest that peripheral infection may trigger regional neuroinflammation, which may cause specific symptoms such as fatigue. A similar mechanism might be involved in COVID-19.

FDG PET Imaging of the Pain Matrix in Neuropathic Pain Model Rats.

Pain is an unpleasant subjective experience that is usually modified by complex multidimensional neuropsychological processes. Increasing numbers of neuroimaging studies in humans have characterized the hierarchical brain areas forming a pain matrix, which is involved in the different dimensions of pain components. Although mechanistic investigations have been performed extensively in rodents, the homologous brain regions involved in the multidimensional pain components have not been fully understood in the rodent brain. Herein, we successfully identified several brain regions activated in response to mechanical allodynia in neuropathic pain rat models using an alternative neuroimaging method based on 2-deoxy-2-[18F]fluoro-d-glucose positron emission tomography (FDG PET) scanning. Regions such as the medial prefrontal cortex, primary somatosensory cortex hindlimb region, and the centrolateral thalamic nucleus were identified. Moreover, brain activity in these regions was positively correlated with mechanical allodynia-related behavioral changes. These results suggest that FDG PET imaging in neuropathic pain model rats enables the evaluation of regional brain activity encoding the multidimensional pain aspect. It could thus be a fascinating tool to bridge the gap between preclinical and clinical investigations.

Lysophosphatidic Acid Receptor 1- and 3-Mediated Hyperalgesia and Hypoalgesia in Diabetic Neuropathic Pain Models in Mice.

Lysophosphatidic acid (LPA) signaling is known to play key roles in the initiation and maintenance of various chronic pain models. Here we examined whether LPA signaling is also involved in diabetes-induced abnormal pain behaviors. The high-fat diet (HFD) showing elevation of blood glucose levels and body weight caused thermal, mechanical hyperalgesia, hypersensitivity to 2000 or 250 Hz electrical-stimulation and hyposensitivity to 5 Hz stimulation to the paw in wild-type (WT) mice. These HFD-induced abnormal pain behaviors and body weight increase, but not elevated glucose levels were abolished in LPA1-/- and LPA3-/- mice. Repeated daily intrathecal (i.t.) treatments with LPA1/3 antagonist AM966 reversed these abnormal pain behaviors. Similar abnormal pain behaviors and their blockade by daily AM966 (i.t.) or twice daily Ki16425, another LPA1/3 antagonist was also observed in db/db mice which show high glucose levels and body weight. Furthermore, streptozotocin-induced similar abnormal pain behaviors, but not elevated glucose levels or body weight loss were abolished in LPA1-/- and LPA3-/- mice. These results suggest that LPA1 and LPA3 play key roles in the development of both type I and type II diabetic neuropathic pain.

Allodynia by Splenocytes From Mice With Acid-Induced Fibromyalgia-Like Generalized Pain and Its Sexual Dimorphic Regulation by Brain Microglia.

Fibromyalgia (FM), a disease of unknown etiology characterized by chronic generalized pain, is partly recapitulated in an animal model induced by repeated acid saline injections into the gastrocnemius muscle. Here, we attempted to investigate the sex difference in pain hypersensitivity (mechanical allodynia and hypersensitivity to electrical stimulation) in the repeated acid saline-induced FM-like generalized pain (AcGP) model. The first unilateral acid injection into gastrocnemius muscle at day 0/D0 and second injection at D5 (post day 0, P0) induced transient and long-lasting mechanical allodynia, respectively, on both sides of male and female mice. The pretreatment with gonadectomy did not affect the first injection-induced allodynia in both sexes, but gradually reversed the second injection-induced allodynia in male but not female mice. Moreover, the AcGP in male mice was abolished by intracerebroventricular minocycline treatments during D4-P4 or P5-P11, but not by early treatments during D0-D5 in male but not female mice, suggesting that brain microglia are required for AcGP in late-onset and sex-dependent manners. We also found that the intravenous treatments of splenocytes derived from male but not female mice treated with AcGP caused allodynia in naive mice. In addition, the purified CD4+ T cells derived from splenocytes of acid-treated male mice retained the ability to cause allodynia in naive mice. These findings suggest that FM-like AcGP has multiple sexual dimorphic mechanisms.

Lysophosphatidic acid LPA1 and LPA3 receptors play roles in the maintenance of late tissue plasminogen activator-induced central poststroke pain in mice.

We developed a mouse model for central post-stroke pain (CPSP), a centrally-originated neuropathic pain (NeuP). In this mode, mice were first injected with Rose Bengal, followed by photo-irradiation of left middle cerebral artery (MCA) to generate thrombosis. Although the MCA thrombosis was soon dissolved, the reduced blood flow remained for more than 24 h due to subsequent occlusion of microvessels. This photochemically induced thrombosis (PIT) model showed a hypersensitivity to the electrical stimulation of both sides of paw, but did not show any abnormal pain in popular thermal or mechanical nociception tests. When tissue-type plasminogen activator (tPA) was injected 6 h after the PIT stress, tPA-dependent hypersensitivity to the electrical paw stimulation and stable thermal and mechanical hyperalgesia on both sides for more than 17 or 18 days after the PIT treatment. These hyperalgesic effects were abolished in lysophosphatidic acid receptor 1 (LPA1)- and lysophosphatidic acid receptor 3 (LPA3)-deficient mice. When Ki-16425, an LPA1 and LPA3 antagonist was treated twice daily for 6 days consecutively, the thermal and mechanical hyperalgesia at day 17 and 18 were significantly reversed. The liquid chromatography-mass spectrometry (LC-MS/MS) analysis revealed that there is a significant increase in several species of LPA molecules in somatosensory S-I and medial dorsal thalamus (MD), but not in striatum or ventroposterior thalamus. All these results suggest that LPA1 and LPA3 signaling play key roles in the development and maintenance of CPSP.

Tyrosyl-tRNA synthetase: A potential kyotorphin synthetase in mammals.

Kyotorphin (KTP; L-tyrosyl-l-arginine), an opioid-like analgesic discovered in the bovine brain, is potentially a neuromodulator because of its localization in synaptosomes, the existence of a specific KTP receptor, and the presence of its biosynthetic enzyme in the brain. KTP is formed in the brain from its constituent amino acids, L-tyrosine and L-arginine, by an enzyme termed KTP synthetase. However, the latter has never been identified. We aimed to test the hypothesis that tyrosyl-tRNA synthetase (TyrRS) is also KTP synthetase. We found that recombinant hTyrRS synthesizes KTP from tyrosine, arginine, and ATP, with Km = 1400 µM and 200 µM for arginine and tyrosine, respectively. TyrRS knockdown of PC12 cells with a small interfering RNA (siRNA) in the presence of 1.6 mM tyrosine, arginine, proline, or tryptophan significantly reduced the level of KTP, but not those of tyrosine-tyrosine, tyrosine-proline, or tyrosine-tryptophan. siRNA treatment did not affect cell survival or proliferation. In mice, TyrRS levels were found to be greater in the midbrain and medulla oblongata than in other brain regions. When arginine was administered 2 h prior to brain dissection, the KTP levels in these regions plus olfactory bulb significantly increased, although basal brain KTP levels remained relatively even. Our conclusion is further supported by a positive correlation across brain regions between TyrRS expression and arginine-accelerated KTP production.

Blockade of analgesic effects following systemic administration of N-methyl-kyotorphin, NMYR and arginine in mice deficient of preproenkephalin or proopiomelanocortin gene.

Kyotorphin is a unique biologically active neuropeptide (l-tyrosine-l-arginine), which is reported to have opioid-like analgesic actions through a release of Met-enkephalin from the brain slices. N-methyl-l-tyrosine-l-arginine (NMYR), an enzymatically stable mimetic of kyotorphin, successfully caused potent analgesic effects in thermal and mechanical nociception tests in mice when it was given through systemic routes. NMYR analgesia was abolished in µ-opioid receptor-deficient (MOP-KO) mice, and by intracerebroventricular (i.c.v.) injection of naloxone and of N-methyl l-leucine-l-arginine (NMLR), a kyotorphin receptor antagonist. In the Ca2+-mobilization assay using CHO cells expressing Gαqi5 and hMOPr or hDOPr, however, the addition of kyotorphin neither activated MOPr-mechanisms, nor affected the concentration-dependent activation of DAMGO- or Met-Enkephalin-induced MOPr activation, and Met-enkephalin-induced DOPr activation. NMYR-analgesia was significantly attenuated in preproenkephalin (PENK)- or proopioimelanocortin (POMC)-KO mice. The systemic administration of arginine, which is reported to elevate the level of endogenous kyotorphin selectively in midbrain and medulla oblongata, pain-related brain regions, caused significant analgesia, and the analgesia was reversed by i.c.v. injection of NMLR or naloxone. In addition, PENK- and POMC-KO mice also attenuated the arginine-induced analgesia. All these findings suggest that NMYR and arginine activate brain kyotorphin receptor in direct and indirect manner, respectively and both compounds indirectly cause the opioid-like analgesia through the action of endogenous opioid peptides.

Involvement of lysophosphatidic acid-induced astrocyte activation underlying the maintenance of partial sciatic nerve injury-induced neuropathic pain.

We have previously demonstrated that lysophosphatidic acid (LPA) plays key roles in the initial mechanisms for neuropathic pain (NeuP) development. Here, we examined whether LPA receptor mechanisms and LPA production are related to the glial activation at a late stage after partial sciatic nerve ligation (pSNL) by use of microglial inhibitor, Mac1-saporin or astrocyte inhibitor, and L-α-aminoadipate (L-AA). Although single intrathecal injection of LPA1/3 antagonist, Ki-16425 did not affect the pain threshold at day 7 after the spinal cord injury, repeated treatments of each compound gradually reversed the basal pain threshold to the control level. The intrathecal administration of a microglia inhibitor, Mac-1-saporin reversed the late hyperalgesia and LPA production at day 14 after the pSNL, whereas L-AA inhibited the hyperalgesia, but had no effect on LPA production. The involvement of LPA receptors in astrocyte activation in vivo was evidenced by the findings that Ki-16425 treatments abolished the upregulation of CXCL1 in activated astrocytes in the spinal dorsal horn of mice at day 14 after the pSNL, and that Ki-16425 reversed the LPA-induced upregulation of several chemokine gene expressions in primary cultured astrocytes. Finally, we found that significant hyperalgesia was observed with intrathecal administration of primary cultured astrocytes, which had been stimulated by LPA in a Ki-16425-reversible manner. All these findings suggest that LPA production and LPA1/3 receptor activation through differential glial mechanisms play key roles in the maintenance as well as initiation mechanisms in NeuP.

LPA1 receptor involvement in fibromyalgia-like pain induced by intermittent psychological stress, empathy.

Treatment for fibromyalgia is an unmet medical need and its pathogenesis is still poorly understood. The present study demonstrated that intermittent psychological stress (IPS), or empathy causes generalized chronic abnormal pain with female predominance. The persistence of the pain phenotype was dependent on the unpredictability of the stressor. The pain was reversed by pregabalin (PGB), duloxetine (DLX) or mirtazapine (Mir), but not by diclofenac or morphine. Differential administration of these existing medicines revealed that the sites of PGB and Mir actions exist in the brain, but not in the spinal cord, while that of DLX is preferentially in the spinal cord. It is interesting to note that the intracerebroventricular injection of PGB or Mir showed potent analgesia for 24 h or longer, though systemic injection of these medicines shows anti-hyperalgesia just for several hours. These results indicate that initial intense actions in the target brain may prevent the forthcoming development of pain memory. IPS-induced abnormal pain was prevented in mice deficient of lysophosphatidic acid receptor 1 (LPA1) gene, and completely cured by the repeated intrathecal treatments with LPA1 antagonist, AM966, which did not show acute action. All these results suggest that IPS model is an experimental animal model, which mimics the pathophysiology and pharmacotherapy in fibromyalgia in clinic, and LPA1 signaling plays crucial roles in the IPS-induced fibromyalgia-like abnormal pain.