Calcitonin gene-related peptide (CGRP) is a 37-amino acid neuropeptide. Discovered 30 years ago, it is produced as a consequence of alternative RNA processing of the calcitonin gene. CGRP has two major forms (α and β). It belongs to a group of peptides that all act on an unusual receptor family. These receptors consist of calcitonin receptor-like receptor (CLR) linked to an essential receptor activity modifying protein (RAMP) that is necessary for full functionality. CGRP is a highly potent vasodilator and, partly as a consequence, possesses protective mechanisms that are important for physiological and pathological conditions involving the cardiovascular system and wound healing. CGRP is primarily released from sensory nerves and thus is implicated in pain pathways. The proven ability of CGRP antagonists to alleviate migraine has been of most interest in terms of drug development, and knowledge to date concerning this potential therapeutic area is discussed. Other areas covered, where there is less information known on CGRP, include arthritis, skin conditions, diabetes, and obesity. It is concluded that CGRP is an important peptide in mammalian biology, but it is too early at present to know if new medicines for disease treatment will emerge from our knowledge concerning this molecule.

Objective. To investigate the involvement of transient receptor potential ankyrin 1 (TRPA1) in inflammatory hyperalgesia mediated by tumor necrosis factor ␣ (TNF␣) 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 TNF␣ or intraarticular injection of Freund's complete adjuvant (CFA). Knee swelling and histologic changes were determined in mice treated with intraarticular injections of CFA.Results. TNF␣ 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 TNF␣, 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 TNF␣ inhibited bilateral hyperalgesia. Intrathecal treatment with AP-18 also reduced TNF␣-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 TNF␣-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.Sensory nerves consisting of C and A␦ nerve fibers innervate joints and skin and are often located in close association with blood vessels (1-3). Nerve stimulation in inflamed joints is considered to play a primary role in arthritis-related pain, and sensory afferent nerves have been located in joints, where they terminate in subsynovial connective tissue (4). Certain members of the transient receptor potential (TRP) receptor family, which are expressed on sensory nerves, are involved as key molecular integrators in the initiation and maintenance of joint pain, although the precise mechanisms involved are unclear (5,6).

Objective. To investigate the endogenous involvement of transient receptor potential vanilloid 1 (TRPV1) in a model of knee joint inflammation in the mouse.Methods. Following characterization of wild-type (WT) and TRPV1-knockout mice, inflammation was induced via intraarticular (IA) injection of Freund's complete adjuvant (CFA). Knee swelling was assessed as diameter, and inflammatory heat hyperalgesia was determined using the Hargreaves technique, for up to 3 weeks. At 18 hours, acute hyperpermeability was measured with 125 I-albumin, and cytokines and myeloperoxidase activity, a marker of neutrophils, were assayed in synovial fluid extracts. The possibility that exogenous tumor necrosis factor ␣ (TNF␣) was involved in influencing TRPV1 activation was investigated in separate experiments.Results. Increased levels of knee swelling, hyperpermeability, leukocyte accumulation, and TNF␣ were found in WT mice 18 hours after IA CFA treatment compared with saline treatment. Significantly less knee swelling and hyperpermeability were found in TRPV1 ؊/؊ mice, but leukocyte accumulation and TNF␣ levels were similar in WT and TRPV1 ؊/؊ mice. Knee swelling in response to CFA remained significantly higher for a longer period in WT mice compared with TRPV1 ؊/؊ mice, with thermal hyperalgesic sensitivity observed at 24 hours and at 1 week in WT, but not TRPV1 ؊/؊ , mice. Knee swelling was attenuated (P < 0.05) in TRPV1 ؊/؊ compared with WT mice 4 hours after IA administration of TNF␣.Conclusion. Our findings indicate that TRPV1 has a role in acute and chronic inflammation in the mouse knee joint. Thus, selective antagonism of TRPV1 should be considered as a potential target for treatment of acute and chronic joint inflammation.

Inflammatory diseases associated with pain are often difficult to treat in the clinic due to insufficient understanding of the nociceptive pathways involved. Recently, there has been considerable interest in the role of reactive oxygen species (ROS) in inflammatory disease, but little is known of the role of hydrogen peroxide (H(2)O(2)) in hyperalgesia. In the present study, intraplantar injection of H(2)O(2)-induced a significant dose- and time-dependent mechanical and thermal hyperalgesia in the mouse hind paw, with increased c-fos activity observed in the dorsal horn of the spinal cord. H(2)O(2) also induced significant nociceptive behavior such as increased paw licking and decreased body liftings. H(2)O(2) levels were significantly raised in the carrageenan-induced hind paw inflammation model, showing that this ROS is produced endogenously in a model of inflammation. Moreover, superoxide dismutase and catalase significantly reduced carrageenan-induced mechanical and thermal hyperalgesia, providing evidence of a functionally significant endogenous role. Thermal, but not mechanical, hyperalgesia in response to H(2)O(2) (i.pl.) was longer lasting in TRPV1 wild type mice compared to TRPV1 knockouts. It is unlikely that downstream lipid peroxidation was increased by H(2)O(2). In conclusion, we demonstrate a notable effect of H(2)O(2) in mediating inflammatory hyperalgesia, thus highlighting H(2)O(2) removal as a novel therapeutic target for anti-hyperalgesic drugs in the clinic.

TNFalpha plays a pivotal role in rheumatoid arthritis (RA) but little is known of the mechanisms that link the inflammatory and nociceptive effects of TNFalpha. We have established a murine model of TNFalpha-induced TRPV1-dependent bilateral thermal hyperalgesia that then allowed us to identify distinct peripheral mechanisms involved in mediating TNFalpha-induced ipsilateral and contralateral hyperalgesia. Thermal hyperalgesia and inflammation were assessed in both hindpaws following unilateral intraplantar (i.pl.) TNFalpha. The hyperalgesic mechanisms were analysed through pharmacogenetic approaches involving TRPV1(-/-) mice and TRPV1 antagonists. To study the mediators downstream of TNFalpha, cyclooxygenase (COX) and PKC inhibitors were utilised and cytokine and prostaglandin levels assessed. The role of neutrophils was determined through use of the selectin inhibitor, fucoidan. We show that TNFalpha (10pmol) causes thermal hyperalgesia (1-4h) in the ipsilateral inflamed and contralateral uninjured hindpaws, which is TRPV1-dependent. GF109203X, a PKC inhibitor, suppressed the hyperalgesia indicating that PKC is involved in TRPV1 sensitisation. Ipsilateral COX-2-derived prostaglandins were also crucial to the development of the bilateral hyperalgesia. The prevention of neutrophil accumulation with fucoidan attenuated hyperalgesia at 4 but not at 1h, indicating a role in the maintenance but not in the induction of bilateral hyperalgesia. However, TNFalpha-induced IL-1beta generation in both paws and the presence of local IL-1beta in the contralateral paw were essential for the development of bilateral hyperalgesia. These results identify a series of peripheral events through which TNFalpha triggers and maintains bilateral inflammatory pain. This potentially allows a better understanding of mechanisms involved in TNFalpha-dependent pain pathways in symmetrical diseases such as arthritis.