Role of Piezo channels in group IV muscle afferent fiber mechanosensation in rats

Abstract

During exercise, skeletal muscle afferentnerves are activated by both mechanical and chemical stimuli within contracting muscle, subsequently contributing to an increase in blood pressure (termed the exercise pressor reflex; EPR). Although metabolically-sensitive receptors mediating the EPR have been described, the mechano-gated channels responsible for mechanosensation in thin muscle afferents (i.e. group III and IV fibers) remain to be elucidated. Piezo channels are known to be mechanically-activated ion channels that play crucial roles in several mechanotransduction processes in various organs. Moreover, Piezo channels are expressed in sensory tissues including dorsal root ganglion associated with skeletal muscle group III and IV afferents. Evidence suggests that antagonism of Piezo channels reduces the pressor response to activation of the EPR in rats. However, whether antagonism of Piezo channels affects muscle afferent discharge during mechanical stimulation remains unknown. PURPOSE The purpose of this study was to determine the impact of GsMTx4, a selective Piezo channel inhibitor, on neuronal responses to mechanical stimulation in group IV muscle afferents. It was hypothesized that GsMTx4 reduces the neural response to mechanical stimulation in sensory afferent fibers innervating skeletal muscle. METHODS Sprague-Dawley rats were used in this experiment (8-12 wks old). Using the ex vivo extensor digitorum longus muscle-nerve preparation, the responsiveness of mechanically activated group IV fibers was assessed by obtaining single-fiber recordings. Action potentials elicited by a mechanical stimulus (0-196 mN ramped stimulus) were recorded before and 20-min after injection of either Krebs-buffer solution (control), gadolinium (a non-specific mechano-gated channel antagonist, 10 mM) or GsMTx4 (250 μM) near the receptive field. RESULTS Conduction velocities of the group IV fibers assessed ranged between 0.32 and 0.77 m/s (average 0.56±0.10 m/s, n=22). The percentage of group IV fibers unable to generate an action potential during mechanical stimulation after injection of Krebs-buffer, Gadolinium and GsMTx4 were 0% (0 out of 6 fibers), 29% (2 out of 7 fibers) and 22% (2 out of 9 fibers), respectively. Fibers whose action potentials were partially attenuated by Krebs-buffer, Gadolinium and GsMTx4 were 50% (3 out of 6 fibers), 29% (2 out of 7 fibers) and 56% (5 out of 9 fibers), respectively. Compared to control, the reduction in neural responsiveness to mechanical stimulation tended to be greater in GsMTx4 (-1±49 vs. -48±38 %, P=0.08) albeit similar to Gadolinium (-71±33%, P=0.32). CONCLUSIONS The data demonstrate, for the first time, that GsMTx4 reduces the neuronal response to mechanical stimulation in group IV muscle afferent fibers of rats. These findings suggest that Piezo channels may significantly contribute to mechanosensation in sensory afferent fibers innervating skeletal muscle.