ATP decreases mechanical sensitivity of muscle thin-fiber afferents in rats. (August 2015) - Matsuda, Teru
Highlights: ATP (1 μM–1 mM) excited 20–60% of rat muscular mechanosensitive C fibers recorded in vitro. ATP, with or without excitation, suppressed the mechanical responses of these fibers. ATP-induced suppression was reversed by P2X antagonists, suramin and PPADS. Intramuscular injection of ATP did not induce apparent nocifensive behavior. Abstract: ATP is an energy rich substance contained in cells in the order of mM. It is released when cells are damaged and when muscle is compressed or contracted. Subcutaneous injection of ATP induces pain-related behavior and hyperalgesia to mechanical and heat stimulation in rats. However, the effects of ATP in muscle have not been fully studied. In the present study we examined the effects of ATP on muscle C-fiber afferent activities using single fiber recordings, and on nociceptive behavior. Muscle C-fiber activities were recorded in vitro using extensor digitorum longus muscle–common peroneal nerve preparations excised from rats deeply anesthetized with pentobarbital. ATP (100 μM and 1 mM, but not 1 μM) superfused for 5 min before the mechanical stimulation suppressed the mechanical responses of muscle thin fibers irrespective of whether they excited the fiber. This suppressive effect was reversed by P2X receptor antagonists PPADS (100 μM) and suramin (300 μM). We also found that subcutaneous injection of ATP (10 mM) induced nociceptive behavior, whereas intramuscular injection had no effect. These findings showed that effects of ATP on muscle afferents differ from those on cutaneous afferents.