Selective activation of nociceptors by P2X receptor agonists in normal and inflamed rat skin

Abstract

ATP can elicit pain in humans and, together with other P2X channel agonists, can produce nocifensive responses in rodents. We used the rat in vitro skin‐nerve preparation to quantify primary afferent responses to ATP and its stable analogue α,β‐methylene ATP in normal and carrageenan‐inflamed skin. Both ATP and α,β‐methylene ATP were found to specifically activate the peripheral terminals of Aδ and C‐fibre nociceptors in the skin. Thirty‐nine per cent of the nociceptors tested responded to the maximal dose of α,β‐methylene ATP (5 mm). In contrast, non‐nociceptive, low‐threshold mechano‐sensitive fibres were never activated by the same agonist concentrations. Amongst the nociceptor population, C‐mechanoheat fibres (C‐MH or polymodal nociceptors) were markedly more responsive to P2X agonists than mechanonociceptors (C‐M nociceptors) with Aδ‐ or C‐fibre axons. Both C‐mechanoheat and C‐mechanonociceptors were activated by α,β‐methylene ATP doses as low as 50 μm. In skin inflamed with carrageenan 3‐4 h before recording both the number of responsive C‐fibre nociceptors and their response magnitude increased. The increased neural response under inflammatory conditions was largely observed in C‐mechanoheat or polymodal nociceptors. After low doses of P2X agonists C‐MH fibres but not C‐M fibres developed elevated ongoing activity and this effect was only seen after carrageenan inflammation. The time course of α,β‐methylene ATP‐evoked discharges in nociceptors was found to correlate well with the time course of behavioural nocifensive responses in rats to the same agonist described in a previous study (Hamilton et al. 1999). We conclude that the rapid increase in the number of α,β‐methylene ATP responsive nociceptors and the increased magnitude of the neural response following carrageenan inflammation explains why very low concentrations of such agonists can cause pain in inflammatory states.

Effects of A‐317491, a novel and selective P2X3/P2X2/3 receptor antagonist, on neuropathic, inflammatory and chemogenic nociception following intrathecal and intraplantar administration

“…Intraplantar administration of various P2 receptor agonists results in short-lasting licking, biting and lifting of the injured paw (Jarvis et al, 2001;Wismer et al, 2003), and can lead to mechanical allodynia (Tsuda et al, 2000). Peripheral injections of P2 receptor agonists can also enhance nociception in animal models of chemical (formalin) and inflammatory (carrageenan) pain (Sawynok & Reid, 1997;Hamilton et al, 2001). Intrathecal delivery of P2 receptor agonists increases sensitivity to mechanical stimulation in naive animals and induces thermal hyperalgesia (Tsuda et al, 1999a;Okada et al, 2002).…”

Ca 2+ /calmodulin-dependent protein kinase II potentiates ATP responses by promoting trafficking of P2X receptors

“…There is good evidence that participation of P2X receptors in nociception is potentiated with enhanced nerve stimulation as the result of inf lammation or nerve injury (7,10). Furthermore, CaMKII participates in decoding temporal information of neuronal activity into intracellular signaling by its frequency-dependent autonomous activation (30).…”

“…Inflammation and tissue injuries cause a large increase in the expression of P2X3 receptors (7) and the release of cytosolic ATP from damaged cells (8). These changes greatly enhance P2X receptor activity, thus contributing to sensitization of DRG neurons (7,8) and exaggerated responses to nonnoxious and noxious stimuli (allodynia and hyperalgesia) (9)(10)(11)(12). It has been shown that protein kinases are activated after tissue injuries (13,14).…”

Pathophysiology and Therapeutic Potential of Purinergic Signaling

“…There is an increase of P2X 3 and P2X 2/3 nociceptive receptors on sensory nerve endings in inflamed skin (Hamilton et al, 2001), and antagonists are being developed as analgesics (see section II.B.). Data have been presented to support a pathogenic role for keratinocytederived ATP in irritant dermatitis (Mizumoto et al, 2003).…”