Contribution of nerve growth factor to upregulation of P2X₃ expression in DRG neurons of rats with femoral artery occlusion

Femoral artery occlusion augments the sympathetic nerve and pressorresponses to muscle contraction and muscle metabolites injected intothe arterial blood supply of the hindlimb muscles in rats. The underlyingmechanism by which these reflex responses are enhanced aftermuscle vascular insufficiency is unclear. Purinergic P2X₃ receptor hasbeen reported to contribute to the metabolic component of the exercisepressor reflex. Thus the purpose of this study was to examine ifchronic femoral occlusion would alter the expression of P2X₃ indorsal root ganglion (DRG) neurons of rats. Also, P2X₃-mediatedsympathetic responsiveness was examined after femoral occlusion. Inaddition, the role played by nerve growth factor (NGF) in regulatingthe expression and response of P2X₃ was examined. Western blotanalysis showed that 24 h of femoral ligation increased the levels ofP2X₃ (optical density: 0.93 ± 0.07 in control and 1.37 ± 0.10 afterocclusion; P < 0.05 vs. control). The fluorescence immunohistochemistryfurther demonstrated that the occlusion elevated P2X₃ expressionin DRG neurons (percentage of P2X₃-positive cells: 33 ± 3% incontrol and 51 ± 3% in occlusion; P < 0.05 vs. control). Furthermore,the results showed that responses of renal sympathetic nerve activityand blood pressure to stimulation of P2X were greater in occluded ratsthan responses in control rats by injection of α,β-methylene ATP intothe arterial blood supply of the hindlimb muscle. Finally, infusion ofNGF in the hindlimb muscles of healthy rats increased P2X₃ (opticaldensity: 0.98 ± 0.12 in control and 1.37 ± 0.16 with NGF; P < 0.05vs. control). The pressor response to injection of α,β-methylene ATPwas increased in the rats with NGF infusion. Likewise, blocking NGFattenuated exaggeration of the reflex response induced by α,β-methyleneATP in occluded rats. The findings of this study suggest that thelevels of P2X₃ in primary afferent neurons are upregulated as theblood supply to the hindlimb is deficient under ischemic conditions,leading to augmentation of the muscle reflex. NGF is closely relatedto increases in P2X₃ receptor expression and response.