The Role Played by Adenosine in Modulating Reflex Sympathetic and Pressor Responses Evoked by Stimulation of TRPV1 in Muscle Afferents

Abstract

Background/aims: Activation of metabolite-sensitive transient receptor potential vanilloid type 1 (TRPV1) receptors (capsaicin receptors) in afferent nerves of the hindlimb muscles of rats increases renal sympathetic nerve activity (RSNA) and blood pressure (BP) via a reflex mechanism. The purpose of this study was to examine the role of adenosine in modulating the reflex RSNA and BP responses to stimulation of TRPV1.

Methods: RSNA and BP responses were recorded in rats. Immunofluorescence and patch-clamp methods were employed to examine the receptor mechanisms responsible for the effects of adenosine.

Results: Adenosine, in the concentration of 100 µM, injected into the femoral artery had an inhibitory effect on the reflex RSNA and BP responses induced by capsaicin. Likewise, arterial injection of adenosine analogue CGS21680 (A2A subtype receptor agonist, 10 µM and100 µM) also attenuated the reflex responses. In addition, co-existence of A2A and TRPV1 was observed in the dorsal root ganglion neurons. The prior application of adenosine or CGS21680 inhibited the magnitude of capsaicin-induced currents in muscle sensory neurons.

Conclusion: Adenosine contributes to muscle afferent TRPV1-engaged reflex sympathetic and pressor responses. It is likely that TRPV1 response is impaired as the levels of adenosine are increased in the hindlimb muscles under diseased conditions.

Fig. 1

Top panel: Effects of adenosine on responses of the renal sympathetic nervous activity (RSNA), mean arterial pressure (MAP) and heart rate (HR) induced by capsaicin (1.0 µg/kg body weight) injected into the femoral artery. P < 0.05 vs. control and recovery. Number of animals = 12. There were no significant differences in baseline MAP and HR among groups (P > 0.05). Bottom panel: Effects of adenosine analogue CGS21680 on responses of the RSNA, MAP and HR induced by capsaicin (1.0 µg/kg body weight) injected into the femoral artery. P < 0.05 vs. control and recovery. Number of animals = 10.

Fig. 2

Immunofluorescence was employed to examine double-labeling for A_2A and TRPV1. Representative photomicrographs show that A_2A and TRPV1 staining were presented in the same DRG neurons. Arrows indicate representative cells positive for both A_2A and TRPV1 after they were merged. Scale bar = 50 µm.

Fig. 3

(A): Effects of adenosine on capsaicin-induced currents in DRG neurons. Top panel: Original traces of DRG neuron response to 1 µM capsaicin. Bottom panel: Average data show that adenosine (1 µM) attenuated peak amplitude of inward currents in DRG neurons as compared with only capsaicin application. Number of neurons = 19. *P < 0.05 vs. capsaicin. (B): Effects of adenosine analogue CGS21680 on capsaicin-induced currents in DRG neurons. Top panel: Original traces of DRG neuron response to 0.1–10 µM CGS21680. Bottom panel: Average data show that CGS21680 attenuated peak amplitude of inward currents in DRG neurons as compared with only capsaicin application. Number of neurons = 26. *P < 0.05 vs. capsaicin. (C): The prior application of 10 µM of MSX-3 attenuated the effects of adenosine (1 µM) on capsaicin-currents in 9 DRG neurons (P > 0.05 vs. capsaicin alone).