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

Objective To observe the excitation and sensitization of dorsal cutaneous primary afferent fibers induced by P2X agonist alpha, beta-methylene ATP (αβme-ATP) in rats. Methods By means of single fiber electrophysiological recordings on the nerve filaments isolated from the dorsal cutaneous branches of the T9-T13 spinal nerves, the effects of αβme-ATP (100 μmol/L, 10 μl) 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 Aδ and C fibers prior to αβme-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 (P0.05, P0.01). The mechanical threshold of the Aβ fibers before αβme-ATP injection was 0.301±0.019 mN, and slightly lowered to 0.288±0.018 mN after the injection (P0.05). Injection of αβme-ATP (10 μl) into the receptive fields evoked spontaneous discharge in 7.7% of the Aβ fibers, 66.7% of Aδ fibers and 75.0% of C fibres, respectively, and the proportions of Aδ and C fibers with spontaneous discharge evoked by αβme-ATP were significantly greater than that of Aβ fibers (P0.05). In the control experiments, injection of saline did not significantly affect spontaneous discharge or excite the nerve fibers. The mean discharge frequency of Aδ 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 (P0.05). In contrast to Aδ and C fibers, Aβ fibers exhibited no significant changes in the mean discharge frequency in response to αβme-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, P0.05). The excitatory effects of αβme-ATP on the discharge rate in Aδ and C primary afferent terminal could be observed in the entire course of experiment. Conclusion Peripheral application of αβme-ATP, an ATP analogue, excites and sensitizes a subpopulation of Aδ and C fibers but not Aβ fibers of rat dorsal cutaneous primary afferent fibers.