[Local injection of alpha, beta-methylene ATP induces excitation of primary afferent fibers in rats].

Abstract

Objective: To observe the excitation and sensitization of dorsal cutaneous primary afferent fibers induced by P2X agonist alpha, beta-methylene ATP (alphabetame-ATP) in rats.

Methods: By means of single fiber electrophysiological recordings on the nerve filaments isolated from the dorsal cutaneous branches of the T(9)-T(13) spinal nerves, the effects of alphabetame-ATP (100 micromol/L, 10 microl) injection into the cutaneous receptive field on the mechanical threshold and spontaneous discharge of rat primary sensory afferent units were observed.

Results: The mean mechanical threshold of Adelta and C fibers prior to alphabetame-ATP injection were 0.384+/-0.018 and 0.943+/-0.102 mN, and lowered to 0.304+/-0.013 and 0.659+/-0.071 mN after the injection, respectively (P<0.05, P<0.01). The mechanical threshold of the Abeta fibers before alphabetame-ATP injection was 0.301+/-0.019 mN, and slightly lowered to 0.288+/-0.018 mN after the injection (P>0.05). Injection of alphabetame-ATP (10 microl) into the receptive fields evoked spontaneous discharge in 7.7% of the Abeta fibers, 66.7% of Adelta fibers and 75.0% of C fibres, respectively, and the proportions of Adelta and C fibers with spontaneous discharge evoked by alphabetame-ATP were significantly greater than that of Abeta fibers (P<0.05). In the control experiments, injection of saline did not significantly affect spontaneous discharge or excite the nerve fibers. The mean discharge frequency of Adelta and C fibers increased from 0.73+/-0.24 and 0.54+/-0.21 impulses/min before injection to 3.05+/-0.65 and 8.53+/-2.04 impulses/min during the injection, with subsequent reduction to 2.40+/-0.60 and 6.68+/-1.68 impulses/min in the following 5 min (P<0.05). In contrast to Adelta and C fibers, Abeta fibers exhibited no significant changes in the mean discharge frequency in response to alphabetame-ATP (0.23+/-0.09 impulses/min before injection, 0.28+/-0.09 impulses/min during injection and 0.22+/-0.14 impulses/min after injection, P>0.05). The excitatory effects of alphabetame-ATP on the discharge rate in Adelta and C primary afferent terminal could be observed in the entire course of experiment.

Conclusion: Peripheral application of alphabetame-ATP, an ATP analogue, excites and sensitizes a subpopulation of Adelta and C fibers but not Abeta fibers of rat dorsal cutaneous primary afferent fibers.