TRPA1 Mediates Mechanical Sensitization in Nociceptors during Inflammation

Abstract

Inflammation is a part of the body’s natural response to tissue injury which initiates the healing process. Unfortunately, inflammation is frequently painful and leads to hypersensitivity to mechanical stimuli, which is difficult to treat clinically. While it is well established that altered sensory processing in the spinal cord contributes to mechanical hypersensitivity (central sensitization), it is still debated whether primary afferent neurons become sensitized to mechanical stimuli after tissue inflammation. We induced inflammation in C57BL/6 mice via intraplantar injection of Complete Freund’s Adjuvant. Cutaneous C fibers exhibited increased action potential firing to suprathreshold mechanical stimuli. We found that abnormal responses to intense mechanical stimuli were completely suppressed by acute incubation of the receptive terminals with the TRPA1 inhibitor, HC-030031. Further, elevated responses were predominantly exhibited by a specific subgroup of C fibers, which we determined to be C-Mechano Cold sensitive fibers. Thus, in the presence of HC-030031, C fiber mechanical responses in inflamed mice were not different than responses in saline-injected controls. We also demonstrate that injection of the HC-030031 compound into the hind paw of inflamed mice alleviates behavioral mechanical hyperalgesia without affecting heat hyperalgesia. Further, we pharmacologically anesthetized the TRPA1-expressing fibers in vivo by co-injecting the membrane-impermeable sodium channel inhibitor QX-314 and the TRPA1 agonist cinnamaldehyde into the hind paw. This approach also alleviated behavioral mechanical hyperalgesia in inflamed mice but left heat hypersensitivity intact. Our findings indicate that C-Mechano Cold sensitive fibers exhibit enhanced firing to suprathreshold mechanical stimuli in a TRPA1-dependent manner during inflammation, and that input from these fibers drives mechanical hyperalgesia in inflamed mice.

Permeation and block of TRPV1 channels by the cationic lidocaine derivative QX-314

“…These experiments were motivated by the concept of using TRPV1 channels as portals for delivering charged drug molecules into primary pain-sensing neurons to inhibit pain signaling in vivo selectively Kim et al 2010;Liu et al 2011;Roberson et al 2011), an approach recently extended to using light-controlled QX-314 derivatives (Mourot et al 2012) and to using TRPA1 channels as portals (Lennertz et al 2012). Our finding that QX-314 can enter through the standard pore and does not require pore dilation is likely advantageous for in vivo and potential clinical use because pore dilation typically requires large concentrations of agonist (Banke et al 2010;Chung et al 2008), which may be difficult to deliver and maintain in vivo (e.g., using perineural infusion to produce nerve block).…”

Transient Receptor Potential Channels as Drug Targets: From the Science of Basic Research to the Art of Medicine

“…Cinnamaldehyde (a TRPA1 agonist) activates TRPA1 and renders it permeable to QX-314 (Lennertz et al, 2012). This approach is very similar to (although probably less effective than) that previously described to silence TRPV1 + afferents by a combination of capsaicin and QX-314 (Binshtok et al, 2007;Ries et al, 2009) since TRPA1 is less permeable to QX-314 than TRPV1 (Nakagawa and Hiura, 2013).…”

Nerve Growth Factor and Nociception: From Experimental Embryology to New Analgesic Therapy

“…The observation that elevated NGF in the skin does not appear to provoke secondary mechanical hyperalgesia suggests that nociceptor sensitization plays a prominent role in this model. In general, it has been remarkably difficult to convincingly demonstrate nociceptor sensitization to mechanical stimuli in a variety of inflammatory models as conflicting results have been published (Andrew and Greenspan, 1999;Lewin and Moshourab, 2004;Milenkovic et al, 2008;Lennertz et al, 2012), Indeed, initial studies failed to detect prominent mechanical sensitization of nociceptors after acute or long term NGF exposure Obreja et al, 2011b). The UV-B sunburn model is an interesting system to study peripheral mechanisms of mechanical hyperalgesia, as there is convincing evidence that central mechanisms do not play a prominent role in this model (Bishop et al, 2009(Bishop et al, , 2010.…”

“…Similarly to ASIC proteins there is some evidence that TRPA1 channels are regulated by NGF availability (Malin et al, 2011), and deletion of the TRPA1 gene leads to complex changes in the mechanosensitivity of identified C-fiber afferents innervating the hairy skin (Kwan et al, 2009). There is solid pharmacological evidence that TRPA1 blockade can prevent the moderate sensitization of C-fibers to supra-threshold mechanical stimulation following complete Freund's adjuvant inflammation (Lennertz et al, 2012).…”