A hyperalgesic effect of intracerebroventricular cytokine-induced neutrophil chemoattractant-1 in the rat paw pressure test

Participation of CXCL1 in the glial cells during neuropathic pain

Neuropathic pain is a chronic pain characterized by injury to the central or peripheral nervous system and that most often causes disability in individuals. Among the mechanisms involved in central sensitization during neuropathic pain are cytokines and chemokines released by spinal glial cells; however, these mechanisms are not well elucidated. Thus, the present study aimed to investigate the involvement of Chemokine (C-X-C motif) ligand 1 (CXCL1) and glial cells in this process. Male Wistar rats weighing 220-240 g were used and underwent a neuropathic pain model induced by chronic constriction injury (CCI). To investigate the involvement of CXCL1, chemokine receptor type 2 (CXCR2), mitogen-activated protein kinases (MAPK) p38, and microglia and astrocytes, the following drugs were used: SB225002, an CXCR2 antagonist; SML0543, a MAPK p38 inhibitor; minocycline, a microglia inhibitor; fluorocitrate, an astrocytes inhibitor; and recombinant CXCL1. The microglia, astrocytes, CXCL1, and MAPK p38 protein levels was evaluated by a Western blot assay. Furthermore, an immunofluorescence assay was performed to localize microglia and astrocytes immunoreactivity in the spinal cord. The results demonstrated that both CCI and CXCL1 induced nociception, and this effect was reversed by SB225002. In addition, minocycline, fluorocitrate, and SML0543 reversed the mechanical allodynia induced by CCI. Furthermore, there was an increase of spinal CXCL1 and microglial marker Iba1 protein levels , which was reversed by SB225002. This antagonist also reduced the Iba1 immunoreactivity in spinal cord. Thus, the present study suggests that the CXCL1 chemokine participates in neuropathic pain through CXCR2 activation in spinal microglia.

Macrophage migration inhibitory factor modulates formalin induced behaviors in rats

Cytokine proteins are involved in different signaling pathways throughout the central nervous system. To study the efficacy of an inflammatory cytokine, the macrophage migration inhibitory factor (MIF), which acts via several receptor molecules including the receptor CXCR2, male rats’ behaviors were determined after intracerebroventricular (ICV) administration of MIF. There were three treatments: One group received only the cytokine, a second group received MIF and an CXCR2 antagonist (SB265610), and a third, control group received only the carrier medium saline. All rats were subjected to a subcutaneous injection of formalin in the hind paw after the ICV administration. Pain behaviors induced after formalin injection showed increased values in the MIF group of licking in the first phase and increased values of flexing, licking and paw-jerk in the second phase. On the contrary, spontaneous behaviors induced by formalin injection changed alternatively between the two groups compared with saline. These results suggest a possible effect of cytokine MIF on central nervous processes implicated in pain modulation mediated by the receptor CXCR2.

Evidence of different mediators of central inflammation in dysfunctional and inflammatory pain--interleukin-8 in fibromyalgia and interleukin-1 β in rheumatoid arthritis

The purpose of this study was to relate central inflammation to autonomic activity (heart rate variability (HRV)) in patients with rheumatoid arthritis (RA) and fibromyalgia (FM). RA patients had reduced parasympathetic activity and FM patients had increased sympathetic activity compared to healthy controls. Comparisons between RA and FM showed higher cerebrospinal fluid (CSF) interleukin (IL)-1β inversely correlated to parasympathetic activity in RA. The FM patients had higher concentrations of CSF IL-8, IL-1Ra, IL-4 and IL-10, but none of these cytokines correlated with HRV. In conclusion, we found different profiles of central cytokines, i.e., elevated IL-1β in inflammatory pain (RA) and elevated IL-8 in dysfunctional pain (FM).

Evidence of central inflammation in fibromyalgia-increased cerebrospinal fluid interleukin-8 levels

Activation of glia cells resulting in intrathecal elevation of cytokines and chemokines has been hypothesized in chronic pain syndromes such as fibromyalgia. To our knowledge, this is the first study assessing intrathecal concentrations of pro-inflammatory substances in fibromyalgia. We report elevated cerebrospinal fluid and serum concentrations of interleukin-8, but not interleukin-1beta, in FM patients. This profile is in accordance with FM symptoms being mediated by sympathetic activity rather than dependent on prostaglandin associated mechanisms and supports the hypothesis of glia cell activation in response to pain mechanisms.

Spinal CCL2 pronociceptive action is no longer effective in CCR2 receptor antagonist-treated rats

A better understanding of the mechanisms linked to chemokine pronociceptive effects is essential for the development of new strategies to better prevent and treat chronic pain. Among chemokines, MCP-1/CCL2 involvement in neuropathic pain processing is now established. However, the mechanisms by which MCP-1/CCL2 exerts its pronociceptive effects are still poorly understood. In the present study, we demonstrate that MCP-1/CCL2 can alter pain neurotransmission in healthy rats. Using immunohistochemical studies, we first show that CCL2 is constitutively expressed by primary afferent neurons and their processes in the dorsal horn of the spinal cord. We also observe that CCL2 is co-localized with pain-related peptides (SP and CGRP) and capsaicin receptor (VR1). Accordingly, using in vitro superfusion system of lumbar dorsal root ganglion and spinal cord explants of healthy rats, we show that potassium or capsaicin evoke calcium-dependent release of CCL2. In vivo, we demonstrate that intrathecal administration of CCL2 to healthy rats produces both thermal hyperalgesia and sustained mechanical allodynia (up to four consecutive days). These pronociceptive effects of CCL2 are completely prevented by the selective CCR2 antagonist (INCB3344), indicating that CCL2-induced pain facilitation is elicited via direct spinal activation of CCR2 receptor. Therefore, preventing the activation of CCR2 might provide a fruitful strategy for treating pain.

Behavioural, histological and cytokine responses during hyperalgesia induced by carrageenan injection in the rat tail

We produced experimental inflammatory hyperalgesia by injecting carrageenan into the tail of Sprague-Dawley rats. We compared the rats' voluntary running wheel activity following carrageenan injection into the tail to that after carrageenan injection into the hind paw, the conventional site of inflammation, to identify whether the site of inflammatory-induced hyperalgesia altered voluntary activity. We also measured voluntary running before and after injection of carrageenan or saline into the tail or hind paw, and in separate groups of rats we measured the nociceptive response and the associated pro-inflammatory cytokine profiles following a carrageenan injection into the tail. Female rats were injected intradermally with either 2 mg carrageenan or saline into the dorsal surface of the tail. Withdrawal responses to noxious heat (49 degrees C water), and punctate mechanical (electronic anaesthesiometer) challenges were recorded in 12 rats for 3 days before and 1 h to 48 h after injection. In a separate group of rats, interleukin (IL)-1beta, IL-6, tumour necrosis factor-alpha (TNF-alpha) and cytokine-induced neutrophil chemoattractant (CINC-1) concentrations were measured in plasma and tail tissue samples taken at the site of injection, 3 h, 6 h and 24 h after injections. Voluntary wheel running was reduced significantly following carrageenan injection into the hind paw compared to that after saline injection into the hind paw. Carrageenan injection into the tail did not result in significant reduction in wheel running compared to that after saline injection into the tail. Both thermal and mechanical hyperalgesia were present after carrageenan injection into the tail (P<0.01, ANOVA). The hyperalgesia at the site coincided with significant increases in TNF-alpha, IL-1beta, IL-6 and CINC-1 tissue concentrations, peaking 6 h after carrageenan injection (P<0.01, ANOVA). We conclude that carrageenan injection into the tail produces inflammatory hyperalgesia with underlying pro-inflammatory cytokine release, but does not affect voluntary running wheel activity in rats.

Intracisternal administration of chemokines facilitated formalin-induced behavioral responses in the orofacial area of freely moving rats

The present study investigated the effects of intracisternal administration of MCP-1, Rantes or IL-8 on pain transmission in the orofacial area. We also investigated mechanisms of hyperalgesic responses produced by intracisternal administration of IL-8. An orofacial formalin test was employed to assess the effects of chemokines on nociceptive processing. For each animal, the number of behavioral responses and the time spent grooming, rubbing and/or scratching the facial region proximal to the formalin injection site was recorded for nine successive 5-min intervals. Intracisternal administration of MCP-1, Rantes or IL-8 significantly increased formalin-induced scratching behavioral responses in the orofacial area. Intracisternal pretreatment with indomethacin, a non-selective cyclooxygenase inhibitor, did not block IL-8-induced hyperalgesia. Pretreatment with 100 microg propranolol, a non-selective beta-adrenergic receptor antagonist and 50 microg atenolol, a selective beta(1)-adrenergic receptor antagonist, inhibited the number of scratches and the duration of scratching produced by 1 ng of IL-8 injected intracisternally. These results indicate that intracisternal administration of chemokines produce a hyperalgesic response with an orofacial inflammatory pain model and that the IL-8-induced hyperalgesia is mediated by central beta(1)-adrenergic receptor.

Antibody array analysis of peripheral and blood cytokine levels in rats after masseter inflammation

This study was undertaken to evaluate the changes in cytokine levels in response to orofacial deep tissue inflammation. Inflammation was induced by injecting complete Freund's adjuvant (CFA, 0.05 ml 1:1 oil/saline suspension) into the masseter of the male Sprague-Dawley rat under brief halothane anesthesia. At 30 min, 5 h and 24 h after CFA injection (n = 3-4/time point), tissues were dissected from masseter and total proteins isolated. Rat Cytokine Antibody Array 1.1 (RayBiotech) coated with 19 specific cytokine antibodies were probed with protein samples and the relative cytokine levels were compared. Compared to saline-injected rats, there were significant increases (p < 0.05-0.01) in the levels of seven cytokines in the masseter tissue after CFA, including interleukin (IL)-1beta (5 h), IL-6 (5 h), tumor necrosis factor-alpha (5 h), monocyte chemoattractant protein-1 (5 h, 24 h), cytokine-induced neutrophil chemoattractant-2 and -3 (5 h, 24 h), and tissue inhibitor of metalloproteinase-1 (5 h, 24 h). All 19 cytokines were detected in the blood samples, but they did not show significant changes after inflammation. Masseter hyperalgesia and allodynia occurred at 30 min and persisted at 5-24 h after inflammation, as assessed by probing the skin above the masseter with von Frey filaments. The present results indicate selective localized cytokine responses to masseter inflammation. Although different cytokines exist in the blood, their levels did not mirror, nor did not appear to depend on, the local cytokine levels. The findings provide specific targets for further studying the involvement of cytokines in orofacial inflammation and hyperalgesia.

CINC-1 is an acute-phase protein induced by focal brain injury causing leukocyte mobilization and liver injury

Following injury or infection, the liver releases acute-phase proteins (APP). After a severe focal injury, this systemic response can be excessive and may lead to multiorgan dysfunction (MODS). CINC-1 is a neutrophil chemoattractant, and we have now established that it also functions as an early APP after injury to the brain or to peripheral tissues. After induction of a focal inflammatory lesion in the brain, there is rapid hepatic and serum CINC-1 induction, which is associated with increases in neutrophil numbers within the liver and within the circulation. CINC-1-mediated recruitment of neutrophils to organs distant from the primary injury site may contribute to MODS. Indeed, we found that enzyme markers of liver tissue injury are increased in the serum following generation of a focal inflammatory lesion in the brain. Neutralization of CINC-1 in the periphery reversed brain-injury-induced neutrophil mobilization and inhibited recruitment of neutrophils to the brain and to the liver. Thus, a significant component of the hepatic acute-phase response is the release of chemokines by the liver, which act to amplify the inflammatory response and modulate the subsequent leukocytosis and secondary tissue damage. Hepatic CINC-1 synthesis following injury presents a novel focus for treatment of inflammation.

Changing the chemokine gradient: CINC1 crosses the blood-brain barrier

Chemokines are a large family of small, inducible, secreted, chemoattractant cytokines that are involved in inflammatory processes. It is well known that systemic and CNS infections cause disruption of the blood-brain barrier (BBB); however, it is not clear how chemokines are involved in this process. We studied the pharmacokinetics of the passage of the chemokine cytokine-induced neutrophil chemoattractant-1 (CINC1) from blood to brain after i.v. bolus injection and its efflux out of the brain after i.c.v. injection. Radiolabeled CINC1 was injected i.v. into mice, and the results were determined by multiple-time regression analysis. Using HPLC, we detected intact CINC1 in brain homogenate and blood after i.v. administration. CINC1 accumulated in the cerebral vasculature but also crossed the BBB completely and rapidly. No saturation of the influx was found, suggesting that either CINC1 crossed the BBB by simple diffusion or the dynamic interactions of binding and internalization precluded the self-inhibition typical of a transport system. Furthermore, there was no efflux system, with CINC1 exiting the brain at the same rate as reabsorption of CSF. The CINC1 injected into blood or CSF did not cause any breakdown of the BBB during the course of the experiments. Thus, the influx of CINC1 may alter the "chemokine gradient" across the BBB and therefore affect inflammatory reactions involving the CNS.