Resiniferatoxin reversibly blocks adjuvant-induced thermal hyperalgesia in the rat

Analgesic and Anti-Inflammatory Effects of the Synthetic Neurosteroid Analogue BNN27 during CFA-Induced Hyperalgesia

Dehydroepiandrosterone (DHEA), an adrenal and neurosteroid hormone with strong neuroprotective and immunomodulatory properties, and ligand for all high-affinity neurotrophin tyrosine kinase receptors (Trk), also exerts important effects on hyperalgesia. Its synthetic, 17-spiro-epoxy analogue, BNN27, cannot be converted to estrogen or androgen as DHEA; it is a specific agonist of TrkA, the receptor of pain regulator Nerve Growth Factor (NGF), and it conserves the immunomodulatory properties of DHEA. Our study aimed to evaluate the anti-nociceptive and anti-inflammatory properties of BNN27 during Complete Freund’s Adjuvant (CFA)-induced inflammatory hyperalgesia in mice. Hyperalgesia was evaluated using the Hargreaves test. Inflammatory markers such as cytokines, NGF and opioids were measured, additionally to corticosterone and the protein kinase B (AKT) signaling pathway. We showed for the first time that treatment with BNN27 reversed hyperalgesia produced by CFA. The effect of BNN27 involved the inhibition of NGF in the dorsal root ganglia (DRG) and the increased synthesis of opioid peptides and their receptors in the inflamed paw. We also found alterations in the cytokine levels as well as in the phosphorylation of AKT2. Our findings strongly support that BNN27 represents a lead molecule for the development of analgesic and anti-inflammatory compounds with potential therapeutic applications in inflammatory hyperalgesia.

Targeting Pain-evoking Transient Receptor Potential Channels for the Treatment of Pain

Chronic pain affects billions of lives globally and is a major public health problem in the United States. However, pain management is still a challenging task due to a lack of understanding of the fundamental mechanisms of pain. In the past decades transient receptor potential (TRP) channels have been identified as molecular sensors of tissue damage and inflammation. Activation/sensitization of TRP channels in peripheral nociceptors produces neurogenic inflammation and contributes to both somatic and visceral pain. Pharmacological and genetic studies have affirmed the role of TRP channels in multiple forms of inflammatory and neuropathic pain. Thus pain-evoking TRP channels emerge as promising therapeutic targets for a wide variety of pain and inflammatory conditions.

Regulation of avoidant behaviors and pain by the anti-inflammatory tyrosine phosphatase SHP-1

The protein tyrosine phosphatase SHP-1 is a critical regulator of cytokine signaling and inflammation. Mice homozygous for a null allele at the SHP-1 locus have a phenotype of severe inflammation and are hyper-responsive to the TLR4 ligand LPS. TLR4 stimulation in the CNS has been linked to both neuropathic pain and sickness behaviors. To determine if reduction in SHP-1 expression affects LPS-induced behaviors, responses of heterozygous SHP-1-deficient (me/+) and wild-type (+/+) mice to LPS were measured. Chronic (4-week) treatment with LPS induced avoidant behaviors indicative of fear/anxiety in me/+, but not +/+, mice. These behaviors were correlated with a LPS-induced type 2 cytokine, cytokine receptor, and immune effector arginase profile in the brains of me/+ mice not found in +/+ mice. Me/+ mice also had a constitutively greater level of TLR4 in the CNS than +/+ mice. Additionally, me/+ mice displayed constitutively increased thermal sensitivity compared to +/+ mice, measured by the tail-flick test. Moreover, me/+ glial cultures were more responsive to LPS than +/+ glia. Therefore, the reduced expression of SHP-1 in me/+ imparts haploinsufficiency with respect to the control of CNS TLR4 and pain signaling. Furthermore, type 2 cytokines become prevalent during chronic TLR4 hyperstimulation in the CNS and are associated positively with behaviors that are usually linked to type 1 pro-inflammatory cytokines. These findings question the notion that type 2 immunity is solely anti-inflammatory in the CNS and indicate that type 2 immunity induces/potentiates CNS inflammatory processes.

Co-induction of cyclooxygenase-2 [correction of cyclooxyenase-2] and early growth response gene (Egr-1) in spinal cord in a clinical model of persistent inflammation and hyperalgesia

BACKGROUND

This study characterised the effects of persistent peripheral inflammation of the foot on pain and spinal cord expression of cyclooxygenase-1 and -2 (COX-1 and COX-2) and early growth response gene 1 (Egr-1), known markers of neuronal plasticity, in a clinical model of naturally-occurring inflammatory disease and hyperalgesia in sheep ('footrot'), before and after routine treatment (parenteral treatment with antibiotics and antiseptic footbathing). The temporal pattern of expression of COX-1, COX-2 and Egr-1 mRNA and protein were analysed using real-time PCR and Western blotting.

RESULTS

Animals affected with persistent peripheral inflammation displayed significant hyperalgesia and lameness (a proxy indicator of spontaneous pain) restricted to the inflamed limb. Hyperalgesia and lameness were significantly attenuated 1 day after treatment, and resolved further by day 7 and day 3, respectively. COX-2 but not COX-1, protein expression was up-regulated in spinal cord from lame animals on day 0, before treatment. Following treatment and attenuation of pain behaviours, levels of COX-2 returned to control levels. Significant induction of Egr-1 mRNA and protein were observed in spinal cord from lame animals. Three days after treatment, levels of Egr-1 mRNA returned to control levels, however, Egr-1 protein remained elevated.

CONCLUSION

Elevated levels of spinal COX-2 and Egr-1 protein correlate with the presence of pain and hyperalgesia, and may underlie persistent pain, although a direct causal link has still to be established. Understanding the temporal pattern of expression of key mediators in clinical pain states may lead to better strategies to manage pain.

Metallopeptidase inhibition potentiates bradykinin-induced hyperalgesia

The neuropeptide bradykinin (BK) sensitizes nociceptor activation following its release in response to inflammatory injury. Thereafter, the bioactivity of bradykinin is controlled by the enzymatic activities of circulating peptidases. One such enzyme, the metalloendopeptidase EC3.4.24.15 (EP24.15), is co-expressed with bradykinin receptors in primary afferent neurons. In this study, using approaches encompassing pharmacology, biochemistry, cell biology, and behavioral animal models, we identified a crucial role for EP24.15 and the closely related EP24.16 in modulating bradykinin-mediated hyperalgesia. Pharmacological analyses indicated that EP24.15 and EP24.16 inhibition significantly enhances bradykinin type-2 receptor activation by bradykinin in primary trigeminal ganglia cultures. In addition, bradykinin-induced sensitization of TRPV1 activation was increased in the presence of the EP24.15/16 inhibitor JA-2. Furthermore, behavioral analyses illustrated a significant dose-response relationship between JA-2 and bradykinin-mediated thermal hyperalgesia. These results indicate an important physiological role for the metallopeptidases EP24.15 and EP24.16 in regulating bradykinin-mediated sensitization of primary afferent nociceptors.

Spinal astrocyte and microglial activation contributes to rat pain-related behaviors induced by the venom of scorpion Buthus martensi Karch

The present study investigated whether spinal astrocyte and microglia were activated in Buthus martensi Karch (BmK) venom-induced rat pain-related behaviors. The results showed that glial fibrillary acidic protein (GFAP) immunoreactivity indicative astrocyte activation in bilateral spinal cord started to increase by day 3, peaked at day 7 and gradually reversed at day 14 following intraplantar injection of BmK venom. Western blotting analysis confirmed GFAP expression was up-regulated by BmK venom. In contrast, bilateral spinal increase of OX-42 immunoreactivity indicative of microglial activation began at 4h peaked at day 1 and gradually reversed by days 3 to 7 after the administration of BmK venom. Pretreatment with either intrathecal injection of fluorocitrate or intraperitonial injection of minocycline, and two glial activation inhibitors, suppressed the spontaneous nociceptive responses, and prevented the primary thermal and bilateral mechanical hyperalgesia induced by BmK venom. The post-treatment with fluorocitrate or minocycline could not affect the mechanical hyperalgesia. Moreover, minocycline partially inhibited BmK venom-induced spinal c-Fos expression but lack of effects on BmK venom-induced paw edema. Taken together, the current study demonstrated that spinal astrocyte and microglial activation may contribute to BmK venom-induced rat pain-related behaviors. Thus, spinal glia may represent novel targets for effective treatment of pain syndrome associated with scorpion envenomation.

Corticofugal outputs facilitate acute, but inhibit chronic pain in rats

It has been widely accepted that the primary somatosensory cortex (SI) plays an essential role in the sensory-discriminative aspect of pain perception. However, it remains unclear whether the SI has a role in the descending modulation of pain. Although there are abundant fibers projecting back from sensory cortex to thalamic nuclei, and the influence of cortical modulation from SI on the thalamic nociceptive relay neurons has been addressed, little is known about how the cortical outputs modulate the nociceptive behaviors resulting from tissue injury or evoked by painful stimulation. The present study was designed to test whether the cortical outputs influenced the nociceptive behaviors using rat models of noxious thermal-induced acute pain, formalin-induced acute and CFA-evoked chronic inflammatory pain. The results showed that intracortical microinjection of GABAA agonist muscimol significantly reduced the first and second phase behaviors in formalin tests and elevated the nociceptive thresholds in the thermal stimulus-elicited acute pain, suggesting a facilitatory influence of SI on the acute pain sensation. By contrast, microinjection of GABAA antagonist bicuculline remarkably reduced the thermal hyperalgesia of the CFA-inflamed hindpaws, indicating an inhibitory effect of SI output in the chronic pain state. The opposite modulatory effects in acute and chronic pain states suggest that there exists a functional switch for the SI cortex at different stages of pain disease, which is of great significance for the biological adaptation.

Functional depletion of capsaicin-sensitive primary afferent fibers attenuates rat pain-related behaviors and paw edema induced by the venom of scorpion Buthus martensi Karch

The role of capsaicin-sensitive primary afferent fibers in rat pain-related behaviors and paw edema induced by scorpion Buthus martensi Karch (BmK) venom was investigated in this study. It was found that functional depletion of capsaicin-sensitive primary afferent fibers with a single systemic injection of resiniferatoxin (RTX) dramatically decreased spontaneous nociceptive behaviors, prevented the development of primary mechanical and thermal hyperalgesia as well as mirror-image mechanical hyperalgesia. RTX treatment significantly attenuated BmK venom-induced c-Fos expression in all laminaes of bilateral L4-L5 lumbar spinal cord, especially in superficial laminaes. Moreover, RTX treatment markedly reduced the early paw edema induced by BmK venom. Thus, the results indicate that capsaicin-sensitive primary afferent fibers play a critical role in various pain-related behaviors and paw edema induced by BmK venom in rats.

Degranulation of mast cells and histamine release involved in rat pain-related behaviors and edema induced by scorpion Buthus martensi Karch venom

In the present study, it was investigated whether the degranulation of mast cells and histamine release were involved in rat pain-related behaviors and edema induced by the venom of scorpion Buthus martensi Karch (BmK) or not. It was found that the obvious degranulation of mast cells could be triggered in rat hindpaw skin by BmK venom. The chronic degranulation of mast cells using compound 48/80 relieved the spontaneous nociceptive responses, the primary thermal and bilateral mechanical hyperalgesia and the rat paw edema, as well as partially reduced c-Fos expression in superficial layers (laminae I-II) of bilateral spinal cord induced by BmK venom. In addition, individual peripheral co-administration of either 100 nmol chlorpheniramine or 100 nmol pyrilamine (histamine H(1) receptor antagonist) or 500 nmol cimetidine (histamine H(2) receptor antagonist) and BmK venom suppressed the spontaneous nociceptive responses, partially the primary thermal and bilateral mechanical hyperalgesia and rat paw edema induced by BmK venom. Thus, these results suggest that the peripheral cellular incidents of mast cells degranulation and histamine release are involved in BmK venom-induced pain-related behaviors and inflammation.

Neurokinin-1 receptor in peripheral nerve terminals mediates thermal hyperalgesia

Neurokinin-1 receptor (NK-1) plays an important role in nociception. The present study was to explore whether activation of peripheral NK-1 receptor, especially expressed on primary sensory afferents, could induce hyperalgesia and sensitize C-type sensory afferents. (1) Intraplantar administration of NK-1 agonist [Sar9, Met(O2)11]SP (Sar-SP, 0.2, 1 nmol, 20 microl) produced significant thermal hyperalgesia and edema, which was blocked by co-injection of NK-1 antagonist WIN51,708 (10 nmol). But in the rats with compound 48/80 treatment for mast cell depletion, the Sar-SP-induced edema, but not hyperalgesia, was attenuated. (2) Close-arterial injection of Sar-SP (1 nmol, 0.1 ml) excited and sensitized sensory C afferents of the sural nerve to heat stimuli. The results suggest involvement of NK-1 receptors expressed on the peripheral afferent terminals in thermal hyperalgesia mediated by directly sensitizing C-type sensory afferents.

Analgesic effects of capsazepine and resiniferatoxin on bone cancer pain in mice

In the present paper, we describe the analgesic effects induced by the transient receptor potential vanilloid type 1 (TRPV1) antagonist, capsazepine, and the TRPV1 agonist, resiniferatoxin, on the thermal hyperalgesia induced by the presence of a tibial osteosarcoma or an inflammatory process in mice. The administration of capsazepine abolished the osteosarcoma-induced hyperalgesia at a dose range (3-10 mg/kg; s.c.) ineffective to inhibit the hyperalgesia elicited by the intraplantar administration of complete Freund's adjuvant (CFA). In contrast, the administration of resiniferatoxin (0.01-0.1 mg/kg; s.c.) inhibited both the osteosarcoma- and the CFA-induced hyperalgesia. Remarkably, a single dose of resiniferatoxin abolished the osteosarcoma-induced hyperalgesia for several days and completely prevented the instauration of thermal hyperalgesia when administered at the initial stages of osteosarcoma development. The potential of drugs acting through TRPV1 for the management of some types of bone cancer pain is proposed.

Changes in P2X3 receptor expression in the trigeminal ganglion following monoarthritis of the temporomandibular joint in rats

The pathophysiological mechanisms of orofacial deep-tissue pain is still unclear. Previously, P2X receptors (P2XR) in sensory neurons have been shown to play a role in the signal transduction of cutaneous pain. We investigated the functional significance of P2X3R in relation to orofacial deep-tissue pain caused by monoarthritis of the temporomandibular joint (TMJ). Monoarthritis was induced by the injection of complete Freund's adjuvant (CFA) into the unilateral TMJ of the rat. The pain associated with monoarthritis was assessed by the pressure pain threshold (PPT), which was defined as the amount of pressure required to induce vocalization. Fifteen days after CFA-treatment, changes in PPT were examined after injection of P2XR agonists or antagonists into the TMJ. The number of cells expressing P2X3R in trigeminal ganglia (TG) was investigated by immunohistochemistry. Inflamed TMJ showed a continuous decline in PPT during the experimental period (P<0.001). Injection of alpha,beta-meATP, an agonist of P2X1,3,2/3R, dramatically reduced the bilateral PPTs of both inflamed and non-inflamed TMJs (P<0.01) although beta,gamma-me-l-ATP, a selective agonist of P2X1R, did not. The decreased PPTs of inflamed TMJ were reversed either by PPADS, an antagonist of P2X1,2,3,5,1/5,4/5R, or by TNP-ATP, an antagonist of P2X1,3,2/3,1/5R. Immunohistochemically, the number of P2X3R-positive cells increased in the small cell group in TG (P<0.01), whereas there was no change in medium or large cell groups after the CFA-injection. Retrograde tracing confirmed that TMJ neurons in the TG exhibited P2X3R immunoreactivity. Our results suggested that P2X3R plays an important role in orofacial pressure pain caused by monoarthritis of TMJ.

TRPV1 desensitisation and endogenous vanilloid involvement in the enhanced analgesia induced by capsaicin in inflamed tissues

The intra-plantar acute administration of 10 microg of capsaicin to mice which had received complete Freund's adjuvant (CFA) 1 week before inhibits the thermal inflammatory hyperalgesia it induces and even produces a long-lasting analgesia for at least 2 weeks. In this study, we show that the administration of capsaicin (10 microg) also reduces the immediate licking behavior evoked by the intra-plantar administration of a lower dose of capsaicin (0.1 microg), the duration of this inhibitory effect being greater in CFA-inflamed mice (at least 2 weeks) than in non-inflamed animals (less than 4 days). Since this reduction of capsaicin-induced licking behavior may be interpreted as a consequence of the transient receptor potential vanilloid 1 receptor (TRPV1) unresponsiveness, we conclude that the administration of 10 microg of capsaicin into inflamed tissues can render the TRPV1 desensitised. We next explored whether endogenous vanilloids released during inflammation contribute to maintain the analgesia triggered by exogenous capsaicin. The acute administration of capsazepine (10 microg; intra-plantarly (i.pl.)) abolished the analgesic effect induced by the injection of capsaicin 1 week before in inflamed mice. From these results, it may be proposed that the maintenance by endovanilloids of the TRPV1 desensitisation induced by capsaicin could contribute to prolonging the analgesic effect induced by this agonist in inflamed tissues.