Spinal alarmin HMGB1 and the activation of TLR4 lead to chronic stress-induced nociceptive hypersensitivity in rodents.

Chronic stress affects millions of people around the world, and it can trigger different behavioral disorders like nociceptive hypersensitivity and anxiety, among others. However, the mechanisms underlaying these chronic stress-induced behavioral disorders have not been yet elucidated. This study was designed to understand the role of high-mobility group box-1 (HMGB1) and toll-like receptor 4 (TLR4) in chronic stress-induced nociceptive hypersensitivity. Chronic restraint stress induced bilateral tactile allodynia, anxiety-like behaviors, phosphorylation of extracellular signal-regulated kinase (ERK) and p38 mitogen-activated protein kinase (p38MAPK) and activation of spinal microglia. Moreover, chronic stress enhanced HMGB1 and TLR4 protein expression at the dorsal root ganglion, but not at the spinal cord. Intrathecal injection of HMGB1 or TLR4 antagonists reduced tactile allodynia and anxiety-like behaviors induced by chronic stress. Additionally, deletion of TLR4 diminished the establishment of chronic stress-induced tactile allodynia in male and female mice. Lastly, the antiallodynic effect of HMGB1 and TLR4 antagonists were similar in stressed male and female rats and mice. Our results suggest that chronic restraint stress induces nociceptive hypersensitivity, anxiety-like behaviors, and up-regulation of spinal HMGB1 and TLR4 expression. Blockade of HMGB1 and TLR4 reverses chronic restraint stress-induced nociceptive hypersensitivity and anxiety-like behaviors and restores altered HMGB1 and TLR4 expression. The antiallodynic effects of HMGB1 and TLR4 blockers in this model are sex independent. TLR4 could be a potential pharmacological target for the treatment of the nociceptive hypersensitivity associated with widespread chronic pain.

ATF2, but not ATF3, participates in the maintenance of nerve injury-induced tactile allodynia and thermal hyperalgesia.

Transcription factors are proteins that modulate the transcriptional rate of target genes in the nucleus in response to extracellular or cytoplasmic signals. Activating transcription factors 2 (ATF2) and 3 (ATF3) respond to environmental signals and maintain cellular homeostasis. There is evidence that inflammation and nerve injury modulate ATF2 and ATF3 expression. However, the function of these transcription factors in pain is unknown. The purpose of this study was to investigate the contribution of ATF2 and ATF3 to nerve injury-induced neuropathic pain. L5/6 spinal nerve ligation induced tactile allodynia and thermal hyperalgesia. Moreover, nerve damage enhanced ATF2 and ATF3 protein expression in injured L5/6 dorsal root ganglia and spinal cord but not in uninjured L4 dorsal root ganglia. Nerve damage also enhanced ATF2 immunoreactivity in dorsal root ganglia and spinal cord 7 to 21 days post-injury. Repeated intrathecal post-treatment with a small-interfering RNA targeted against ATF2 (ATF2 siRNA) or anti-ATF2 antibody partially reversed tactile allodynia and thermal hyperalgesia. In contrast, ATF3 siRNA or anti-ATF3 antibody did not modify nociceptive behaviors. ATF2 immunoreactivity was found in dorsal root ganglia and spinal cord co-labeling with NeuN mainly in non-peptidergic (IB4+) but also in peptidergic (CGRP+) neurons. ATF2 was found mainly in small- and medium-sized neurons. These results suggest that ATF2, but not ATF3, is found in strategic sites related to spinal nociceptive processing and participates in the maintenance of neuropathic pain in rats.

Evaluation of the neonatal streptozotocin model of diabetes in rats: Evidence for a model of neuropathic pain.

BACKGROUND: The purpose of this study was to evaluate the participation of satellite glial cells (SGC), microglia and astrocytes in a model of streptozotocin-induced diabetes initiated in neonatal rats (nSTZ) and to determine the pharmacological profile for pain relief. METHODS: nSTZ was used to induce experimental diabetes. Von Frey filaments were used to assess tactile allodynia. Drugs were given by systemic administration. Western blotting and immunohistochemistry were used to determine protein expression and cellular localization. RESULTS: nSTZ produced mild hyperglycemia, weight loss, glucose intolerance, and reduction of nerve conduction velocity of C fibers. Moreover, nSTZ enhanced activating transcription factor 3 (ATF3) immunoreactivity in dorsal root ganglia (DRG) and sciatic nerve of adult rats. ATF3 was found in SGC (GFAP+ cells) surrounding DRG at week 16. Late changes in ATF3 immunoreactivity in DRG correlated with up-regulation of ATF3 and GFAP protein expression. nSTZ increased GFAP and OX-42 immunoreactivity and percentage of hypertrophied and ameboid microglia in the spinal dorsal horn. These changes correlated with the presence of mechanical hypersensitivity (tactile allodynia). Administration of gabapentin (30-100mg/kg, po) and metformin (200mg/kg/day, po for 2 weeks) alleviated tactile allodynia, whereas morphine (1-3mg/kg, ip) had a modest effect. CONCLUSIONS: Results suggest that nSTZ leads to activation of SGC, microglia and astrocytes in DRG and spinal cord. Pharmacological profile in the nSTZ model resembles diabetic neuropathic pain in humans. Our findings support the conclusion that the nSTZ rat model has utility for the study of a long-lasting diabetic neuropathic pain.

Anion exchanger 3 in dorsal root ganglion contributes to nerve injury-induced chronic mechanical allodynia and thermal hyperalgesia.

OBJECTIVE: To determine the role of anion exchanger 3 (AE3) in dorsal root ganglion (DRG) in nerve injury-induced chronic nociception in the rat. METHODS: Spared nerve injury (SNI) was used to induce neuropathic pain. Von Frey filaments and Hargreaves test were used to assess tactile allodynia and thermal hyperalgesia, respectively. Drugs were given by intrathecal administration. Western blotting was used to determine AE3 expression in DRG. KEY FINDINGS: SNI produced long-lasting mechanical allodynia and thermal hyperalgesia. AE3 was found in DRG of sham-operated rats. SNI enhanced baseline AE3 expression in L4 and L5 DRGs at days 7 and 14, respectively. In contrast, SNI did not affect AE3 expression in L6 DRG. AE3 expression returned to baseline levels 21 days after SNI. Intrathecal 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) (5-50 µg) pretreatment prevented SNI-induced allodynia and, at a lesser extent, hyperalgesia. Moreover, DIDS (50 µg) reduced SNI-induced AE3 upregulation in L4, but not L5, DRGs. Intrathecal DIDS (5-50 µg) or anti-AE3 antibody (1 µg), but not vehicle, post-treatment (6 days) partially reversed SNI-induced allodynia and hyperalgesia. DIDS or anti-AE3 antibody post-treatment diminished SNI-induced AE3 upregulation in L4 and L5 DRGs. CONCLUSIONS: Data suggest that AE3 is present in DRG and contributes to mechanical allodynia and thermal hyperalgesia in neuropathic rats.

Role of spinal 5-HT5A, and 5-HT1A/1B/1D, receptors in neuropathic pain induced by spinal nerve ligation in rats.

Serotonin (5-HT) participates in pain modulation by interacting with different 5-HT receptors. The role of 5-HT5A receptor in neuropathic pain has not previously studied. The purpose of this study was to investigate: A) the role of 5-HT5A receptors in rats subjected to spinal nerve injury; B) the expression of 5-HT5A receptors in dorsal spinal cord and dorsal root ganglia (DRG). Neuropathic pain was induced by L5/L6 spinal nerve ligation. Tactile allodynia in neuropathic rats was assessed with von Frey filaments. Western blot methodology was used to determine 5-HT5A receptor protein expression. Intrathecal administration (on day 14th) of 5-HT (10-100 nmol) or 5-carboxamidotryptamine (5-CT, 0.03-0.3 nmol) reversed nerve injury-induced tactile allodynia. Intrathecal non-selective (methiothepin, 0.1-0.8 nmol) and selective (SB-699551, 1-10 nmol) 5-HT5A receptor antagonists reduced, by ~60% and ~25%, respectively, the antiallodynic effect of 5-HT (100 nmol) or 5-CT (0.3 nmol). Moreover, both selective 5-HT1A and 5-HT1B/1D receptor antagonists, WAY-100635 (0.3-1 nmol) and GR-127935 (0.3-1 nmol), respectively, partially diminished the antiallodynic effect of 5-HT or 5-CT by about 30%. Injection of antagonists, by themselves, did not affect allodynia. 5-HT5A receptors were expressed in the ipsilateral dorsal lumbar spinal cord and DRG and L5/L6 spinal nerve ligation did not modify 5-HT5A receptor protein expression in those sites. Results suggest that 5-HT5A receptors reduce pain processing in the spinal cord and that 5-HT and 5-CT reduce neuropathic pain through activation of 5-HT5A and 5-HT1A/1B/1D receptors. These receptors could be an important part of the descending pain inhibitory system.

5-HT2B Receptor Antagonists Reduce Nerve Injury-Induced Tactile Allodynia and Expression of 5-HT2B Receptors.

Preclinical Research This work was performed to assess the effects of intrathecal serotonin 2B (5-HT2B ) receptor antagonists in rats with neuropathic pain. With RS-127445, its effect was also determined on 5-HT2B receptor expression. Neuropathic pain was induced by L5/L6 spinal nerve ligation. Western blotting was used to determine 5-HT2B receptor expression. Dose-response curves with the 5-HT2B receptor antagonists 2-amino-4-(4-fluoronaphth-1-yl)-6-isopropylpyridine (RS-127445, 1-100 nmol) and 1-[(2-chloro-3,4-dimethoxyphenyl)methyl]-2,3,4,9-tetrahydro-6-methyl-1H-pyrido[3,4-b]indole hydrochloride (LY-266097, 1-100 nmol) were performed in rats. Tactile allodynia of the left hind paw (ipsilateral) was assessed for 8 h after compound administration. Intrathecal injection of the 5-HT2B receptor antagonists RS-127445 and LY-266097 diminished spinal nerve ligation-induced allodynia. In contrast, intrathecal injection of the 5-HT2 receptor agonist (±)-2,5-dimethoxy-4-iodoamphetamine hydrochloride (DOI, 10 nmol) did not modify tactile allodynia induced by nerve ligation. L5/L6 nerve ligation increased expression of the 5-HT2B receptors in the ipsilateral, but not contralateral, dorsal root ganglia. Furthermore, nerve injury also enhanced 5-HT2B receptor expression in the ipsilateral dorsal part of the spinal cord. Intrathecal treatment with RS-127445 (100 nmol) diminished spinal nerve injury-induced increased expression of 5-HT2B receptors in dorsal root ganglia and spinal cord. Our results imply that spinal 5-HT2B receptors are present on sites related to nociception and participate in neuropathic pain. © 2014 Wiley Periodicals, Inc.