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NEUROHUMORAL CONTROL OF CARDIOVASCULAR FUNCTION

Responses of thoracic spinal neurons to activation and desensitization of cardiac TRPV1-containing afferents in rats

¹Department of Physiology, University of Oklahoma Health Sciences Center, Oklahoma City, Oklahoma; and ²Department of Anatomy and Cell Biology, Oklahoma State University Center for Health Sciences, Tulsa, Oklahoma

Submitted 31 March 2006 ; accepted in final form 2 August 2006

The purpose of this study was to examine how upper thoracic spinal neurons responded to activation and desensitization of cardiac transient receptor potential vanilloid-1 (TRPV1)-containing afferent fibers. Extracellular potentials of single T3 spinal neurons were recorded in pentobarbital-anesthetized, paralyzed, and ventilated male rats. To activate cardiac nociceptive receptors, a catheter was placed in the pericardial sac to administer various chemicals: bradykinin (BK; 10 µg/ml, 0.2 ml), capsaicin (CAP, 10 µg/ml, 0.2 ml), or a mixture of algesic chemicals (AC; 0.2 ml) containing adenosine 10^–3 M, BK, serotonin, histamine, and PGE₂, 10^–5 M for each. Spinal neurons that responded to intrapericardial BK and/or CAP were used in this study. Results showed that 81% (35/43) of the neurons had excitatory responses to both intrapericardial BK and CAP, and the remainder responded to either BK or CAP. Intrapericardial resiniferatoxin (RTX) (0.2 µg/ml, 0.2 ml, 1 min), which desensitizes TRPV1-containing nerve endings, abolished excitatory responses to both BK (n = 8) and CAP (n = 7), and to AC (n = 5) but not to somatic stimuli. Intrapericardial capsazepine (1 mg/ml, 0.2 ml, 3 min), a specific antagonist of TRPV1, sharply attenuated excitatory responses to CAP in 5/5 neurons, but responses to BK in 5/5 neurons was maintained. Additionally, intrapericardial capsazepine had no significant effect on excitatory responses to AC in 3/3 neurons. These data indicated that intrapericardial BK-initiated spinal neuronal responses were linked to cardiac TRPV1-containing afferent fibers, but were not dependent on TRPV1. Intraspinal signaling for cardiac nociception was mediated through CAP-sensitive afferent fibers innervating the heart.

bradykinin; capsaicin; resiniferatoxin; sympathetic afferent; vagal afferent

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