Impaired Lactate Release in Dorsal CA1 Astrocytes Contributed to Nociceptive Sensitization and Comorbid Memory Deficits in Rodents.

BACKGROUND: Memory deficits are a common comorbid disorder in patients suffering from neuropathic pain. The mechanisms underlying the comorbidities remain elusive. The hypothesis of this study was that impaired lactate release from dysfunctional astrocytes in dorsal hippocampal CA1 contributed to memory deficits. METHODS: A spared nerve injury model was established to induce both pain and memory deficits in rats and mice of both sexes. von Frey tests, novel object recognition, and conditioned place preference tests were applied to evaluate the behaviors. Whole-cell recording, fiber photometry, Western blotting, and immunohistochemistry combined with intracranial injections were used to explore the underlying mechanisms. RESULTS: Animals with spared sciatic nerve injury that had displayed nociception sensitization or memory deficit comorbidities demonstrated a reduction in the intrinsic excitability of pyramidal neurons, accompanied by reduced Ca2+ activation in astrocytes (ΔF/F, sham: 6 ± 2%; comorbidity: 2 ± 0.4%) and a decrease in the expression of glial fibrillary acidic protein and lactate levels in the dorsal CA1. Exogenous lactate supply or increasing endogenous lactate release by chemogenetic activation of astrocytes alleviated this comorbidity by enhancing the cell excitability (129 ± 4 vs. 88 ± 10 for 3.5 mM lactate) and potentiating N-methyl-d-aspartate receptor-mediated excitatory postsynaptic potentials of pyramidal neurons. In contrast, inhibition of lactate synthesis, blocking lactate transporters, or chemogenetic inhibition of astrocytes resulted in comorbidity-like behaviors in naive animals. Notably, ß2-adrenergic receptors in astrocytes but not neurons were downregulated in dorsal CA1 after spared nerve injury. Microinjection of a ß2 receptor agonist into dorsal CA1 or activation of the noradrenergic projections onto the hippocampus from the locus coeruleus alleviated the comorbidity, possibly by increasing lactate release. CONCLUSIONS: Impaired lactate release from dysfunctional astrocytes, which could be rescued by activation of the locus coeruleus, led to nociception and memory deficits after peripheral nerve injury.

Physiology education in China: the current situation and changes over the past 3 decades.

OBJECTIVE: As an experimental biological science, physiology has been taught as an integral component of medical curricula for a long time in China. The teaching effectiveness of physiology courses will directly affect students' learning of other medical disciplines. The purpose of this study is to investigate the current situation and changes in physiology teaching over 30 years in Chinese medical schools. METHODS: National survey was conducted online on the platform SoJump via WeChat and the web. The head of the physiology department in medical school was asked to indicate the information of physiology education from three periods: 1991-2000, 2001-2010, and 2011-2020. The responses of 80 leaders of the Department of Physiology from mainland Chinese medical schools were included in the study for analysis. RESULTS: The survey showed that the class hours, both of theory and practice, had been decreased. During the past 20 years, the total number of physiology teachers, the number of physiology teachers who had been educated in medical schools, and the number of technicians had been reduced, whereas teachers with doctor's degrees had been increased. In addition to traditional didactic teaching, new teaching approaches, including problem-based learning/case-based learning/team-based learning, integrated curriculum and formative evaluation systems, had been employed, mostly for more than 5 years, in some medical schools. CONCLUSION: The present study has provided historical data regarding the current status of physiology education in China and that in the past thirty years by showing that physiology education in China has developed quickly,even it faces many challenges.

Liver X Receptor α in Sciatic Nerve Exerts an Alleviating Effect on Neuropathic Pain Behaviors Induced by Crush Injury.

Peripheral nerve injury often leads to neuropathic pain. In the present study, we assessed the role of liver x receptor alpha (LXRα), an oxysterol regulated nuclear transcription factor that promotes reverse cholesterol transport and alternative (M2) macrophage activation, in the development of neuropathic pain. We found that compared to WT mice, in LXRα knockout mice the development of mechanical allodynia following sciatic nerve crush was accelerated and the duration was prolonged. Furthermore, the expression of M1-like macrophage marker iNOS and M1-like macrophages inducer hydrogen peroxide (H2O2) was increased, whereas expression of M2 macrophage marker arginase-1 (Arg-1) and interleukin-10 (IL-10) was reduced in the sciatic nerve of LXRα knockout mice. Moreover, peri-sciatic administration of LXRs agonist GW3965, immediately after the nerve crush, into wild type mice, suppressed the mechanical allodynia induced by crush injury. GW3965 also suppressed the expression of iNOS and production of H2O2 in the injured nerve and enhanced the expression of IL-10 and Arg-1. Importantly, peri-sciatic administration of IL-10 neutralization antibody prevented the alleviating effect of GW3965 on mechanical allodynia. Altogether, these results indicates that the lack of LXRα in the sciatic nerve results in an augmented inflammatory profile of macrophages, which ultimately speed up the development of neuropathic pain and dampen its recovery following nerve injury. Activation of LXRα by its agonist might rebalance the neuroprotective and neurotoxic macrophage phenotypes, and thus alleviate the neuropathic pain behavior.

Normalization of magnesium deficiency attenuated mechanical allodynia, depressive-like behaviors, and memory deficits associated with cyclophosphamide-induced cystitis by inhibiting TNF-α/NF-κB signaling in female rats.

BACKGROUND: Bladder-related pain symptoms in patients with bladder pain syndrome/interstitial cystitis (BPS/IC) are often accompanied by depression and memory deficits. Magnesium deficiency contributes to neuroinflammation and is associated with pain, depression, and memory deficits. Neuroinflammation is involved in the mechanical allodynia of cyclophosphamide (CYP)-induced cystitis. Magnesium-L-Threonate (L-TAMS) supplementation can attenuate neuroinflammation. This study aimed to determine whether and how L-TAMS influences mechanical allodynia and accompanying depressive symptoms and memory deficits in CYP-induced cystitis. METHODS: Injection of CYP (50 mg/kg, intraperitoneally, every 3 days for 3 doses) was used to establish a rat model of BPS/IC. L-TAMS was administered in drinking water (604 mg·kg-1·day-1). Mechanical allodynia in the lower abdomen was assessed with von Frey filaments using the up-down method. Forced swim test (FST) and sucrose preference test (SPT) were used to measure depressive-like behaviors. Novel object recognition test (NORT) was used to detect short-term memory function. Concentrations of Mg2+ in serum and cerebrospinal fluid (CSF) were measured by calmagite chronometry. Western blot and immunofluorescence staining measured the expression of tumor necrosis factor-α/nuclear factor-κB (TNF-α/NF-κB), interleukin-1ß (IL-1ß), and N-methyl-D-aspartate receptor type 2B subunit (NR2B) of the N-methyl-D-aspartate receptor in the L6-S1 spinal dorsal horn (SDH) and hippocampus. RESULTS: Free Mg2+ was reduced in the serum and CSF of the CYP-induced cystitis rats on days 8, 12, and 20 after the first CYP injection. Magnesium deficiency in the serum and CSF correlated with the mechanical withdrawal threshold, depressive-like behaviors, and short-term memory deficits (STMD). Oral application of L-TAMS prevented magnesium deficiency and attenuated mechanical allodynia (n = 14) and normalized depressive-like behaviors (n = 10) and STMD (n = 10). The upregulation of TNF-α/NF-κB signaling and IL-1ß in the L6-S1 SDH or hippocampus was reversed by L-TAMS. The change in NR2B expression in the SDH and hippocampus in the cystitis model was normalized by L-TAMS. CONCLUSIONS: Normalization of magnesium deficiency by L-TAMS attenuated mechanical allodynia, depressive-like behaviors, and STMD in the CYP-induced cystitis model via inhibition of TNF-α/NF-κВ signaling and normalization of NR2B expression. Our study provides evidence that L-TAMS may have therapeutic value for treating pain and comorbid depression or memory deficits in BPS/IC patients.

Rostral ventromedial medulla-mediated descending facilitation following P2X7 receptor activation is involved in the development of chronic post-operative pain.

Chronic postsurgical pain (CPSP) remains a medical problem. Whether the descending modulation of nociceptive transmission from the rostral ventromedial medulla (RVM) plays a role in CPSP induced by skin/muscle incision and retraction (SMIR) in the thigh is still unknown. In this study, we found that SMIR surgery, which induced either bilateral or unilateral mechanical allodynia, activated microglia, and up-regulated interleukin-1ß (IL-1ß), an important cytokine, and 8-hydroxyguanine, an oxidative stress marker in the RVM. In addition, the release of 5-hydroxytryptamine (5-HT) was increased in the ipsilateral and contralateral RVM in rats with either bilateral or unilateral pain following SMIR. The 5-HT level increase, 5-HT3 receptor (5-HT3R) up-regulation, and microglia activation were found bilaterally in SMIR rats with bilateral pain, but only ipsilaterally in SMIR rats with unilateral pain. The intrathecal injection of the 5-HT3R antagonist Y25130 prevented the development of CPSP and the activation of spinal microglia induced by SMIR. Furthermore, P2X7 receptor (P2X7R) was up-regulated in microglia in the RVM. The microinjection of the P2X7R antagonist brilliant blue G (BBG, a non-competitive P2X7R antagonist) into the RVM prevented the development of mechanical allodynia, inhibited the activation of microglia, and decreased the expression of IL-1ß and 8-hydroxyguanine in the RVM following SMIR. Importantly, BBG injected into the RVM also decreased the activation of microglia and the level of 5-HT in the lumbar 3 (L3) spinal cord. The microinjection of the P2X7R agonist BzATP, the NADPH oxidase activator phorbol-12-myristate-13-acetate, or IL-1ß into the RVM induced bilateral mechanical allodynia, microglia activation, and 5-HT release in the L3 spinal dorsal horn. Taken together, P2X7R activation in microglia in the RVM following SMIR might be responsible for the development of CPSP via activating descending serotonergic pathway. OPEN SCIENCE BADGES: This article has received a badge for *Open Materials* because it provided all relevant information to reproduce the study in the manuscript. The complete Open Science Disclosure form for this article can be found at the end of the article. More information about the Open Practices badges can be found at https://cos.io/our-services/open-science-badges/.

The role of P2X7R/ERK signaling in dorsal root ganglia satellite glial cells in the development of chronic postsurgical pain induced by skin/muscle incision and retraction (SMIR).

The mechanisms of chronic postsurgical pain remain to be elucidated. We reported here that skin/muscle incision and retraction (SMIR), a rat model of postsurgical pain, phosphorylated the extracellular regulated protein kinases (ERK) signaling components c-Raf, MEK (ERK kinase) and ERK1/2 in lumbar 3 dorsal root ganglion (L3 DRG) in rats. Intrathecal injection of ERK specific inhibitor SCH772984 suppressed the mechanical allodynia induced by SMIR. Furthermore, SMIR upregulated tumor necrosis factor alpha (TNFα) in L3 DRG, which could be inhibited by SCH772984. Intrathecal injection of TNF antagonist Etanercept could also inhibit the mechanical allodynia and the increased ERK phosphorylation in L3 DRG induced by SMIR. In addition, immunofluorescent data showed that P2X7R was located exclusively in GFAP labeled satellite glial cells and was highly colocalized with p-ERK1/2 following SMIR. Pretreatment with P2X7R antagonist Brilliant Blue G (BBG) could also block the mechanical allodynia, inhibited the phosphorylation of c-Raf, MEK, ERK1/2, and decrease the expression of TNF-α. Finally, intrathecal injection of BzATP produced mechanical allodynia and induced ERK phosphorylation in satellite glial cells in L3 DRG. Thus, P2X7R activation in satellite glial cells in L3 DRG, leading to a positive feedback between ERK pathway activation and TNF-α production, is suggested to be involved in the induction of chronic postsurgical pain following SMIR.

Withdrawal from spinal application of remifentanil induces long-term potentiation of c-fiber-evoked field potentials by activation of Src family kinases in spinal microglia.

It is well known that remifentanil, a widely used intravenous anesthesia drug, can paradoxically induce hyperalgesia. The underlying mechanisms are still not clear despite the wide investigations. The present study demonstrated that withdrawal from spinal application of remifentanil could dose-dependently induce long term potentiation (LTP) of C-fiber evoked field potentials. Remifentanil withdrawal could activate Src family kinases (SFKs) in microglia, and upregulate the expression of tumor necrosis factor alpha (TNFα) in spinal dorsal horn. Furthermore, pretreatment with either microglia inhibitor Minocycline, SFKs inhibitor PP2 or TNF αneutralization antibody could block remifentanil withdrawal induced spinal LTP, whereas supplement of recombinant rat TNFα to the spinal cord could reverse the inhibitory effect of Minocycline or PP2 on remifentanil withdrawal induced LTP. Our results suggested that TNFαrelease following SFKs activation in microglia is involved in the induction of LTP induced by remifentanil withdrawal.

Liver X Receptor α Is Involved in Counteracting Mechanical Allodynia by Inhibiting Neuroinflammation in the Spinal Dorsal Horn.

BACKGROUND: Liver X receptors, including α and ß isoforms, are ligand-activated transcription factors. Whether liver X receptor α plays a role in neuropathic pain is unknown. METHODS: A spared nerve injury model was established in adult male rats and mice. Von Frey tests were performed to evaluate the neuropathic pain behavior; Western blot and immunohistochemistry were performed to understand the underlying mechanisms. RESULTS: Intrathecal injection of a specific liver X receptor agonist T0901317 or GW3965 could either prevent the development of mechanical allodynia or alleviate the established mechanical allodynia, both in rats and wild-type mice. GW3965 could inhibit the activation of glial cells and the expression of tumor necrosis factor-α (mean ± SD: 196 ± 48 vs. 119 ± 57; n = 6; P < 0.01) and interleukin 1ß (mean ± SD: 215 ± 69 vs. 158 ± 74; n = 6; P < 0.01) and increase the expression of interleukin 10 in the spinal dorsal horn. All of the above effects of GW3965 could be abolished by liver X receptor α mutation. Moreover, more glial cells were activated, and more interleukin 1ß was released in the spinal dorsal horn in liver X receptor α knockout mice than in wild-type mice after spared nerve injury. Aminoglutethimide, a neurosteroid synthesis inhibitor, blocked the inhibitory effect of T0901317 on mechanical allodynia, on the activation of glial cells, and on the expression of cytokines. CONCLUSIONS: Activation of liver X receptor α inhibits mechanical allodynia by inhibiting the activation of glial cells and rebalancing cytokines in the spinal dorsal horn via neurosteroids.

Oral Application of Magnesium-L-Threonate Attenuates Vincristine-induced Allodynia and Hyperalgesia by Normalization of Tumor Necrosis Factor-α/Nuclear Factor-κB Signaling.

BACKGROUND: Antineoplastic agents, including vincristine, often induce neuropathic pain and magnesium deficiency clinically, but the causal link between them has not been determined. No drug is available for treating this form of neuropathic pain. METHODS: Injection of vincristine (0.1 mg · kg · day, intraperitoneally, for 10 days) was used to induce nociceptive sensitization, which was accessed with von Frey hairs and the plantar tester in adult male Sprague-Dawley rats. Magnesium-L- threonate was administered through drinking water (604 mg · kg · day). Extracellular and intracellular free Mg were measured by Calmagite chromometry and flow cytometry. Molecular biologic and electrophysiologic experiments were performed to expose the underlying mechanisms. RESULTS: Vincristine injection induced allodynia and hyperalgesia (n = 12), activated tumor necrosis factor-α/nuclear factor-κB signaling, and reduced free Mg in cerebrospinal fluid by 21.7 ± 6.3% (mean ± SD; n = 13) and in dorsal root ganglion neurons by 27 ± 6% (n = 11). Reducing Mg activated tumor necrosis factor-α/nuclear factor-κB signaling in cultured dorsal root ganglion neurons. Oral application of magnesium-L-threonate prevented magnesium deficiency and attenuated both activation of tumor necrosis factor-α/nuclear factor-κB signaling and nociceptive sensitization (n = 12). Mechanistically, vincristine induced long-term potentiation at C-fiber synapses, up-regulated N-methyl-D-aspartate receptor type 2B subunit of N-methyl-D-aspartate receptor, and led to peptidergic C-fiber sprouting in spinal dorsal horn (n = 6 each). The vincristine-induced pathologic plasticity was blocked by intrathecal injection of nuclear factor-κB inhibitor (n = 6), mimicked by tumor necrosis factor-α, and substantially prevented by oral magnesium-L-threonate (n = 5). CONCLUSIONS: Vincristine may activate tumor necrosis factor-α/nuclear factor-κB pathway by reduction of intracellular magnesium, leading to spinal pathologic plasticity and nociceptive sensitization. Oral magnesium-L-threonate that prevents the magnesium deficiency is a novel approach to prevent neuropathic pain induced by chemotherapy.

EXPRESS: Methylcobalamin ameliorates neuropathic pain induced by vincristine in rats: Effect on loss of peripheral nerve fibers and imbalance of cytokines in the spinal dorsal horn.

BACKGROUND: Vincristine, a widely used chemotherapeutic agent, often induces painful peripheral neuropathy and there are currently no effective drugs to prevent or treat this side effect. Previous studies have shown that methylcobalamin has potential analgesic effect in diabetic and chronic compression of dorsal root ganglion model; however, whether methylcobalamin has effect on vincristine-induced painful peripheral neuropathy is still unknown. RESULTS: We found that vincristine-induced mechanical allodynia and thermal hyperalgesia, accompanied by a significant loss of intraepidermal nerve fibers in the plantar hind paw skin and an increase in the incidence of atypical mitochondria in the sciatic nerve. Moreover, in the spinal dorsal horn, the activity of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase and the protein expression of p-p65 as well as tumor necrosis factor a was increased, whereas the protein expression of IL-10 was decreased following vincristine treatment. Furthermore, intraperitoneal injection of methylcobalamin could dose dependently attenuate vincristine-induced mechanical allodynia and thermal hyperalgesia, which was associated with intraepidermal nerve fibers rescue, and atypical mitochondria prevalence decrease in the sciatic nerve. Moreover, methylcobalamin inhibited the activation of NADPH oxidase and the downstream NF-kB pathway. Production of tumor necrosis factor a was also decreased and production of IL-10 was increased in the spinal dorsal horn following methylcobalamin treatment. Intrathecal injection of Phorbol-12-Myristate-13-Acetate, a NADPH oxidase activator, could completely block the analgesic effect of methylcobalamin. CONCLUSIONS: Methylcobalamin attenuated vincrinstine-induced neuropathic pain, which was accompanied by inhibition of intraepidermal nerve fibers loss and mitochondria impairment. Inhibiting the activation of NADPH oxidase and the downstream NF-kB pathway, resulting in the rebalancing of proinflammatory and anti-inflammatory cytokines in the spinal dorsal horn might also be involved. These findings might provide potential target for preventing vincristine-induced neuropathic pain.

Neuropathic Pain: Sensory Nerve Injury or Motor Nerve Injury?

Peripheral nerve injury often induces chronic neuropathic pain. Peripheral nerve is consisted of sensory fibers and motor fibers, it is questioned injury to which type of fibers is responsible for generation of neuropathic pain? Because neuropathic pain is sensory disorder, it is generally believed that the disease should be induced by injury to sensory fibers. In recent years, however, emergent evidence shows that motor fiber injury but not sensory fiber injury is necessary and sufficient for induction of neuropathic pain. Motor fiber injury leads to neuropathic pain by upregulating pro-inflammatory cytokines and brain-derived neurotrophic factor in pain pathway.

Inhibition of Neuropathic Pain by a Single Intraperitoneal Injection of Diazepam in the Rat: Possible Role of Neurosteroids.

Diazepam binds with the same high affinity to the central benzodiazepine receptor (CBR) and the peripheral benzodiazepine receptor, which has been renamed translocator protein (TSPO). Both receptors could promote neurosteroid synthesis. In the present study, we investigated whether a single dose of diazepam could inhibit neuropathic pain induced by L5 spinal nerve ligation (L5 SNL), and whether CBR and TSPO mediated this effect. We found that a single intraperitoneal injection of diazepam 9 d after L5 SNL significantly depressed the established mechanical allodynia and thermal hyperalgesia, which persisted until the end of the experiments. Furthermore, the effects were mimicked by a single intraperitoneal injection of Ro5-4864, a specific TSPO agonist and pregnenolone, a neurosteroid precursor. In addition, we found that the inhibitory effect of diazepam was also completely blocked by pretreatment with a specific CBR antagonist, flumazenil. The effects of diazepam or Ro5-4864 on neuropathic pain were completely blocked by pretreatment with a neurosteroid synthesis inhibitor, aminoglutethimide (AMG). Finally, any one of the three drugs, diazepam, Ro5-4864 and pregnenolone, could reduce the activation of astrocytes and the production of interleukin-1beta (IL-1ß) in the L5 spinal dorsal horn 14 d after L5 SNL. These results suggest that in addition to exerting effects on CBR, diazepam may inhibit neuropathic pain via TSPO, which promotes neurosteroid formation, subsequently reducing the activation of astrocytes and production of cytokines.

Calpain-2 contributes to neuropathic pain following motor nerve injury via up-regulating interleukin-6 in DRG neurons.

Motor nerve injury by L5 ventral root transection (L5-VRT) initiates interleukin-6 (IL-6) up-regulation in primary afferent system contributing to neuropathic pain. However, the early upstream regulatory mechanisms of IL-6 after L5-VRT are still unknown. Here, we monitored both the activity of calpain, a calcium-dependent protease suggested as one of the earliest mediators for cytokine regulation, and the expression of IL-6 in bilateral L4-L6 dorsal root ganglias (DRGs) soon after L5-VRT. We found that the protein level of calpain-2 in DRGs, but not calpain-1 was increased transiently in the first 10 min(-1)h ipsilaterally and 20 min(-1)h contralaterally after L5-VRT, long before mechanical allodynia was initiated (5-15 h ipsilaterally and 15 h(-1)d contralaterally). The early activation of calpain evaluated by the generation of spectrin breakdown products (SBDP) correlated well with IL-6 up-regulation in bilateral DRGs. Double immunofluorescence staining revealed that almost all the calpain-2 positive neurons expressed IL-6, indicating an association between calpain-2 and IL-6. Inhibition of calpain by pre-treatment with MDL28170 (25mg/kg, i.p.) attenuated the rat mechanical allodynia and prevented the early up-regulation of IL-6 following L5-VRT. Addition of exogenous calpain-2 onto the surface of left L5 DRG triggered a temporal allodynia and increased IL-6 in bilateral DRGs simultaneously. Taken together, the early increase of calpain-2 in L5-VRT rats might be responsible for the induction of allodynia via up-regulating IL-6 in DRG neurons.

The upregulation of translocator protein (18 kDa) promotes recovery from neuropathic pain in rats.

At present, effective drug for treatment of neuropathic pain is still lacking. Recent studies have shown that the ligands of translocator protein (TSPO, 18 kDa), a peripheral receptor for benzodiazepine, modulate inflammatory pain. Here, we report that TSPO was upregulated in astrocytes and microglia in the ipsilateral spinal dorsal horn of rats following L5 spinal nerve ligation (L5 SNL), lasting until the vanishing of the behavioral signs of neuropathic pain (∼50 d). Importantly, a single intrathecal injection of specific TSPO agonists Ro5-4864 or FGIN-1-27 at 7 and 21 d after L5 SNL depressed the established mechanical allodynia and thermal hyperalgesia dramatically, and the effect was abolished by pretreatment with AMG, a neurosteroid synthesis inhibitor. Mechanically, Ro5-4864 substantially inhibited spinal astrocytes but not microglia, and reduced the production of tumor necrosis factor-α (TNF-α) in vivo and in vitro. The anti-neuroinflammatory effect was also prevented by AMG. Interestingly, TSPO expression returned to control levels or decreased substantially, when neuropathic pain healed naturally or was reversed by Ro5-4864, suggesting that the role of TSPO upregulation might be to promote recovery from the neurological disorder. Finally, the neuropathic pain and the upregulation of TSPO by L5 SNL were prevented by pharmacological blockage of Toll-like receptor 4 (TLR4). These data suggested that TSPO might be a novel therapeutic target for the treatment of neuropathic pain.

Upregulation of tumor necrosis factor-alpha in nucleus accumbens attenuates morphine-induced rewarding in a neuropathic pain model.

Treatment of neuropathic pain with opioid analgesics remains controversial and a major concern is the risk of addiction. Here, we investigated this issue with spared nerve injury (SNI) model of neuropathic pain in rats and mice. SNI prevented conditioned place preference (CPP) induced by low dose (3.5mg/kg) of morphine (MOR), which was effective for anti-allodynia, but not by high dose (⩾5.0 mg/kg) of MOR. Tumor necrosis factor-alpha (TNF-α) was upregulated in nucleus accumbens (NAcc) following SNI. The inhibitory effect of SNI on MOR-induced CPP was blocked by either genetic deletion of TNF receptor 1 (TNFR1) or microinjection of anti-TNF-α into the NAcc and was mimicked by intra-NAcc injection of TNF-α in sham rats. Furthermore, SNI reduced dopamine (DA) level and upregulated dopamine transporter (DAT) in the NAcc, but did not affect total tyrosine hydroxylase (TH) or phospho-TH (p-TH), a rate-limiting enzyme of catecholamine biosynthesis, in ventral tegmental area (VTA). Accordingly, the increase in DA reuptake but not decrease in its synthesis may lead to the reduction of DA level. Finally, the upregulation of DAT in the NAcc of SNI animals was again blocked by either genetic deletion of TNFR1 or NAcc injection of anti-TNF-α, and was mimicked by NAcc injection of TNF-α in sham animals. Thus, our data provided novel evidence that upregulation of TNF-α in NAcc may attenuate MOR-induced rewarding by upregulation of DAT in NAcc under neuropathic pain condition.

Deactivation of dorsal CA1 pyramidal neurons projecting to medial prefrontal cortex contributes to neuropathic pain and short-term memory impairment.

ABSTRACT: Neuropathic pain after peripheral nerve injury is a multidimensional experience that includes sensory, affective, and cognitive components that interact with one another. Hypoexcitation of the medial prefrontal cortex (mPFC) was observed in mice with peripheral nerve injury, but the changes in neural inputs onto the mPFC have not been completely explored. Here, we report that the neural terminals from the dorsal hippocampus CA1 (dCA1) form excitatory connection with layer 5 pyramidal neurons in the prelimbic area (PrL) of the mPFC. Spared nerve injury (SNI) induced a reduction in the intrinsic excitability of dCA1 pyramidal neurons innervating the PrL and impairment in excitatory synaptic transmission onto dCA1 pyramidal cells. Specifically, activating the neural circuit from dCA1 to mPFC alleviated neuropathic pain behaviors and improved novel object recognition ability in SNI mice, whereas deactivating this pathway in naïve animals recapitulated tactile allodynia and memory deficits. These results indicated that hypoactivity in dCA1 pyramidal cells after SNI in turn deactivated layer 5 pyramidal neurons in PrL and ultimately caused pain hypersensitivity and memory deficits.

MiR-184-5p represses neuropathic pain by regulating CCL1/CCR8 signaling interplay in the spinal cord in diabetic mice.

BACKGROUND: Diabetic neuropathic pain (DNP) is a serious complication for diabetic patients involving nervous system. MicroRNAs (miRNAs) are small-noncoding RNAs which are dysregulated in neuropathic pain, and might be critical molecules for pain treatment. Our previous study has shown miR-184-5p was significantly downregulated in DNP. Therefore, the mechanism of miR-184-5p in DNP was investigated in this study. METHODS: A DNP model was established through streptozotocin (STZ). The pharmacological tools were injected intrathecally, and pain behavior was evaluated by paw withdrawal mechanical thresholds (PWMTs). Bioinformatics analysis, Dual-luciferase reporter assay and fluorescence-in-situ-hybridization (FISH) were used to seek and confirm the potential target genes of miR-184-5p. The expression of relative genes and proteins was analyzed by quantitative reverse transcriptase real-time PCR (qPCR) and western blotting. RESULTS: MiR-184-5p expression was down-regulated in spinal dorsal on days 7 and 14 after STZ, while intrathecal administration of miR-184-5p agomir attenuates neuropathic pain induced by DNP and intrathecal miR-184-5p antagomir induces pain behaviors in naïve mice. Chemokine CC motif ligand 1 (CCL1) was found to be a potential target of miR-184-5p and the protein expression of CCL1 and the mRNA expression of CCR8 were up-regulated in spinal dorsal on days 7 and 14 after STZ. The luciferase reporter assay and FISH demonstrated that CCL1 is a direct target of miR-184-5p. MiR-184-5p overexpression attenuated the expression of CCL1/CCR8 in DNP; intrathecal miR-184-5p antagomir increased the expression of CCL1/CCR8 in spinal dorsal of naïve mice. CONCLUSION: This research illustrates that miR-184-5p alleviates DNP through the inhibition of CCL1/CCR8 signaling expression.

Activation of Beta-adrenergic Receptors Upregulates the Signal-to-Noise Ratio of Auditory Input in the Medial Prefrontal Cortex and Mediates Auditory Fear Conditioning.

Norepinephrine (NE) is involved in auditory fear conditioning (AFC) in posttraumatic stress disorder (PTSD). However, it is still unclear how it acts on neurons. We aimed to investigate whether the activation of the ß-adrenergic receptor (ß-AR) improves AFC by sensitization of the prelimbic (PL) cortex at the animal, cellular, and molecular levels. In vivo single-cell electrophysiological recording was used to characterize the changes in neurons in the PL cortex after AFC. Then, PL neurons were locally administrated by the ß-AR agonist isoproterenol (ISO), the GABAaR agonist muscimol, or intervened by optogenetic method, respectively. Western blotting and immunohistochemistry were finally used to assess molecular changes. Noise and low-frequency tones induced similar AFC. The expression of ß-ARs in PL cortex neurons was upregulated after fear conditioning. Microinjection of muscimol into the PL cortex blocked the conformation of AFC, whereas ISO injection facilitated AFC. Moreover, PL neurons can be distinguished into two types, with type I but not type II neurons responding to conditioned sound and being regulated by ß-ARs. Our results showed that ß-ARs in the PL cortex regulate conditional fear learning by activating type I PL neurons.

Activation of the bile acid receptors TGR5 and FXR in the spinal dorsal horn alleviates neuropathic pain.

AIMS: Beyond digestion, bile acids have been recognized as signaling molecules with broad paracrine and endocrine functions by activating plasma membrane receptor (Takeda G protein-coupled receptor 5, TGR5) and the nuclear farnesoid X receptor (FXR). The present study investigated the role of bile acids in alleviating neuropathic pain by activating TGR5 and FXR. METHOD: Neuropathic pain was induced by spared nerve injury (SNI) of the sciatic nerve. TGR5 or FXR agonist was injected intrathecally. Pain hypersensitivity was measured with Von Frey test. The amount of bile acids was detected using a bile acid assay kit. Western blotting and immunohistochemistry were used to assess molecular changes. RESULTS: We found that bile acids were downregulated, whereas the expression of cytochrome P450 cholesterol 7ahydroxylase (CYP7A1), a rate-limiting enzyme for bile acid synthesis, was upregulated exclusively in microglia in the spinal dorsal horn after SNI. Furthermore, the expression of the bile acid receptors TGR5 and FXR was increased in glial cells and GABAergic neurons in the spinal dorsal horn on day 7 after SNI. Intrathecal injection of either TGR5 or FXR agonist on day 7 after SNI alleviated the established mechanical allodynia in mice, and the effects were blocked by TGR5 or FXR antagonist. Bile acid receptor agonists inhibited the activation of glial cells and ERK pathway in the spinal dorsal horn. All of the above effects of TGR5 or FXR agonists on mechanical allodynia, on the activation of glial cells, and on ERK pathway were abolished by intrathecal injection of the GABAA receptor antagonist bicuculline. CONCLUSION: These results suggest that activation of TGR5 or FXR counteracts mechanical allodynia. The effect was mediated by potentiating function of GABAA receptors, which then inhibited the activation of glial cells and neuronal sensitization in the spinal dorsal horn.