Amelia A. Staniland

Inhibition of spinal microglial cathepsin S for the reversal of neuropathic pain

A recent major conceptual advance has been the recognition of the importance of immune system–neuronal interactions in the modulation of brain function, one example of which is spinal pain processing in neuropathic states. Here, we report that in peripheral nerve-injured rats, the lysosomal cysteine protease cathepsin S (CatS) is critical for the maintenance of neuropathic pain and spinal microglia activation. After injury, CatS was exclusively expressed by activated microglia in the ipsilateral dorsal horn, where expression peaked at day 7, remaining high on day 14. Intrathecal delivery of an irreversible CatS inhibitor, morpholinurea-leucine-homophenylalanine-vinyl phenyl sulfone (LHVS), was antihyperalgesic and antiallodynic in neuropathic rats and attenuated spinal microglia activation. Consistent with a pronociceptive role of endogenous CatS, spinal intrathecal delivery of rat recombinant CatS (rrCatS) induced hyperalgesia and allodynia in naïve rats and activated p38 mitogen-activated protein kinase (MAPK) in spinal cord microglia. A bioinformatics approach revealed that the transmembrane chemokine fractalkine (FKN) is a potential substrate for CatS cleavage. We show that rrCatS incubation reduced the levels of cell-associated FKN in cultured sensory neurons and that a neutralizing antibody against FKN prevented both FKN- and CatS-induced allodynia, hyperalgesia, and p38 MAPK activation. Furthermore, rrCatS induced allodynia in wild-type but not CX3CR1-knockout mice. We suggest that under conditions of increased nociception, microglial CatS is responsible for the liberation of neuronal FKN, which stimulates p38 MAPK phosphorylation in microglia, thereby activating neurons via the release of pronociceptive mediators.

P2X7-Dependent Release of Interleukin-1β and Nociception in the Spinal Cord following Lipopolysaccharide

The cytokine interleukin-1β (IL-1β) released by spinal microglia in enhanced response states contributes significantly to neuronal mechanisms of chronic pain. Here we examine the involvement of the purinergic P2X7 receptor in the release of IL-1β following activation of Toll-like receptor-4 (TLR4) in the dorsal horn, which is associated with nociceptive behavior and microglial activation. We observed that lipopolysaccharide (LPS)-induced release of IL-1β was prevented by pharmacological inhibition of the P2X7 receptor with A-438079, and was absent in spinal cord slices taken from P2X7 knock-out mice. Application of ATP did not evoke release of IL-1β from the dorsal horn unless preceded by an LPS priming stimulus, and this release was dependent on P2X7 receptor activation. Extensive phosphorylation of p38 MAPK in microglial cells in the dorsal horn was found to correlate with IL-1β secretion following both LPS and ATP. In behavioral studies, intrathecal injection of LPS in the lumbar spinal cord produced mechanical hyperalgesia in rat hindpaws, which was attenuated by concomitant injections of either a nonspecific (oxidized ATP) or a specific (A-438079) P2X7 antagonist. In addition, LPS-induced hypersensitivity was observed in wild-type but not P2X7 knock-out mice. These data suggest a critical role for the P2X7 receptor in the enhanced nociceptive transmission associated with microglial activation and secretion of IL-1β in the dorsal horn. We suggest that CNS-penetrant P2X7 receptor antagonists, by targeting microglia in pain-enhanced response states, may be beneficial for the treatment of persistent pain.

Assessment and treatment of pain in people with dementia

Many elderly people experience pain and regularly take analgesic medication. Pain is also frequent in people with dementia, particularly those with severe disease. As no robust clinical guidelines are available for the treatment of pain in the context of dementia, the risk of inadequate treatment in individuals with this condition is high. Furthermore, our understanding of the aetiology of pain and the potential role of dementia-associated neuropathology in pain is limited. These issues are important in the clinical management of individuals with dementia, as untreated pain is a major contributor to reduced quality of life and disability, and can lead to increased behavioural and psychological symptoms. Assessment scales to identify pain in people with dementia have been highlighted in recent studies, but there is little evidence for consistency between these tools. Numerous studies have evaluated various approaches for the treatment of pain, including stepped-care protocols and/or administration of paracetamol and opioid medications. In this Review, we summarize the best-available evidence regarding the aetiology, assessment and treatment of pain in people with dementia. Further validation of assessment tools and large-scale trials of treatment approaches in people with dementia are needed to improve clinical guidance for the treatment of pain in these individuals.

A distinct role for transient receptor potential ankyrin 1, in addition to transient receptor potential vanilloid 1, in tumor necrosis factor α–induced inflammatory hyperalgesia and Freund's complete adjuvant–induced monarthritis

OBJECTIVE To investigate the involvement of transient receptor potential ankyrin 1 (TRPA1) in inflammatory hyperalgesia mediated by tumor necrosis factor α(TNFα) and joint inflammation. METHODS Mechanical hyperalgesia was assessed in CD1 mice, mice lacking functional TRP vanilloid 1 (TRPV1-/-) or TRPA1 (TRPA1-/-), or respective wildtype (WT) mice. An automated von Frey system was used, following unilateral intraplantar injection of TNFα or intraarticular injection of Freunds complete adjuvant (CFA). Knee swelling and histologic changes were determined in mice treated with intraarticular injections of CFA. RESULTS TNFα induced cyclooxygenase-independent bilateral mechanical hyperalgesia in CD1 mice. The selective TRPV1 receptor antagonist SB-366791 had no effect on mechanical hyperalgesia when it was coinjected with TNFα, but intrathecally administered SB- 366791 attenuated bilateral hyperalgesia, indicating the central but not peripheral involvement of TRPV1 receptors. A decrease in pain sensitivity was also observed in TRPV1-/- mice. Intraplantar coadministration of the TRPA1 receptor antagonist AP-18 with TNFα inhibited bilateral hyperalgesia. Intrathecal treatment with AP-18 also reduced TNFα-induced hyperalgesia. CFA-induced mechanical hyperalgesia in CD1 mice was attenuated by AP-18 (administered by intraarticular injection 22 hours after the administration of CFA). Furthermore, intraarticular CFA–induced ipsilateral mechanical hyperalgesia was maintained for 3 weeks in TRPA1 WT mice. In contrast, TRPA1-/- mice exhibited mechanical hyperalgesia for only 24 hours after receiving CFA. CONCLUSION Evidence suggests that endogenous activation of peripheral TRPA1 receptors plays a critical role in the development of TNFα-induced mechanical hyperalgesia and in sustaining the mechanical hyperalgesia observed after intraaarticular injection of CFA. These results suggest that blockade of TRPA1 receptors may be beneficial in reducing the chronic pain associated with arthritis.

Reduced inflammatory and neuropathic pain and decreased spinal microglial response in fractalkine receptor (CX3CR1) knockout mice

J. Neurochem. (2010) 114, 1143–1157.

Fractalkine/CX3CR1 Signalling in Chronic Pain and Inflammation

The development of new therapeutic approaches to the treatment of painful neuropathies requires a better understanding of the mechanisms that underlie chronic pain syndromes. There is increasing evidence that immune competent cells such as microglia contribute to the development of chronic pain states. Chemokines play a pivotal role in mediating neuronal-microglial communication which leads to increased nociception. Fractalkine (FKN) is structurally unique amongst the family of chemokines and their receptors and expressed both in the central nervous system and peripheral nerves, as well as in endothelial cells and lymphocytes. Signalling via the CX3CR1 receptor, FKN is able to mediate critical physiological functions necessary for immune regulation. In its soluble forms FKN mediates chemotaxis of immune cells whilst membrane bound FKN acts as an adhesion molecule mediating leukocyte capture and infiltration. As FKN/CX3CR1 is such a key signalling pair for homeostatic functions it is not surprising that it is implicated in a large number of diseases in which imbalance of the immune system is implied. Here we review the evidence that FKN/CX3CR1 mediates neuron-microglial communication in chronic pain states and is therefore key in the development of neuropathic pain. In addition, the contribution of FKN/CX3CR1 signalling to the pathogenesis and progression of two chronic inflammatory conditions, atherosclerosis and rheumatoid arthritis, are discussed.

Mice lacking acid‐sensing ion channels (ASIC) 1 or 2, but not ASIC3, show increased pain behaviour in the formalin test

Extracellular acidification is a component of the inflammatory process and may be a factor driving the pain accompanying it. Acid‐sensing ion channels (ASICs) are neuronal proton sensors and evidence suggests they are involved in signalling inflammatory pain. The aims of this study were to (1) clarify the role of ASICs in nociception and (2) confirm their involvement in inflammatory pain and determine whether this was subunit specific.

181 ABSENCE OF THERMAL HYPERALGESIA AFTER PERIPHERAL INFLAMMATION IN MICE LACKING THE FRACTALKINE RECEPTOR CX3CR1

by a model of central sensitization (5 min/46oC). Heat pain (HP) was reported continuously on a 10 cm VAS. Area of secondary hyperalgesia (SHA) was assessed with a Von Frey filament. Results: Clinical NSP was significantly different between sessions: 1.9±1.4 cm on days with low NSP vs. 4.6±1.5 cm on days with high NSP (p < 0.001). SHA was not different between sessions: 52.1±13.5 cm vs. 52.2±14.3 cm (p =0.98). There was a significant correlation between perceived HP and development of SHA (p < 0.05). Conclusion: The difference in NSP indicate that we succeeded in recruiting subjects at different levels of clinical pain. However, the different levels of clinical pain did not predict development of cutaneous hyperalgesia. The results indicate that, with the present model, there is no increase in central sensibility during periods with high levels of deep-tissue pain.