ATP decreases mechanical sensitivity of muscle thin-fiber afferents in rats.

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.

Change with age in muscular mechanical hyperalgesia after lengthening contraction in rats.

To determine whether there is any change by aging in mechanical hyperalgesia (delayed onset muscle soreness) after lengthening contraction (LC, also termed as eccentric contraction), we applied LC to the dorsi-flexors of the hind legs in young (7-week-old) and aged (130-week-old) rats and examined the change in mechanical withdrawal threshold of the exercised muscle with a Randall-Selitto apparatus and by c-Fos expression in the dorsal horn. The baseline mechanical withdrawal threshold did not differ among two age groups. One day after LC the withdrawal threshold started to decrease in both age groups, however, the duration of decreased withdrawal threshold was different: young rats had their withdrawal threshold lowered only for 3 days after LC while that of aged rats remained lowered two more days, showing delayed recovery in aged rats. Induction of c-Fos expression in the spinal dorsal horn by compression of the muscle was examined in aged animals 3 days after LC. Significantly larger numbers of c-Fos positive neurons was observed in the superficial dorsal horn than the control animals (no treatment). This increase was observed not only in L4 but also in L5, a wider distribution than in young animals (L4 only) in our previous report [Taguchi, T., Matsuda, T., Tamura, R., Sato, J., Mizumura, K., 2005a. Muscular mechanical hyperalgesia revealed by behavioural pain test and c-Fos expression in the spinal dorsal horn after eccentric contraction in rats.

Muscular mechanical hyperalgesia revealed by behavioural pain test and c-Fos expression in the spinal dorsal horn after eccentric contraction in rats.

Delayed onset muscle soreness (DOMS) is quite common, but the mechanism for this phenomenon is still not understood; even the existence of muscle tenderness (mechanical hyperalgesia) has not been demonstrated in experimental models. We developed an animal model of DOMS by inducing eccentric contraction (lengthening contraction, ECC) to the extensor digitorum longus muscle (EDL), and investigated the existence of mechanical hyperalgesia in the EDL by means of behavioural pain tests (Randall-Selitto test and von Frey hair test, applied to/through the skin on the EDL muscle) and c-Fos expression in the spinal dorsal horn. We found that the mechanical withdrawal threshold measured with the Randall-Selitto apparatus decreased significantly between 1 and 3 days after ECC, while that measured by von Frey hairs did not. The group that underwent stretching of the muscle only (SHAM group) showed no change in mechanical pain threshold in either test. These results demonstrated that the pain threshold of deep tissues (possibly of the muscle) decreased after ECC. c-Fos immunoreactivity in the dorsal horn (examined 2 days after ECC/SHAM exercise) was not changed by either ECC or compression (1568 mN) to the EDL muscle by itself, but it was significantly increased by applying compression to the EDL muscle 2 days after ECC. This increase was observed in the superficial dorsal horn of the L4 segment of the ipsilateral side, and was clearly suppressed by morphine treatment (10 mg kg(-1), i.p.). These results demonstrated the existence of mechanical hyperalgesia in the muscle subjected to ECC. This model could be used for future study of the neural mechanism of muscle soreness.