Role of capsaicin-sensitive afferent neurons in receptive relaxation induced by gastric distension in rats.

We investigated the role of capsaicin-sensitive afferent neurons in receptive relaxation of the rat stomach in response to distension. Under urethane anesthesia, a balloon with barostat was inserted through the pylorus and placed in the forestomach. Isobaric distension was performed in a stepwise increment of 2 mmHg, each lasting for 2 min, while the corresponding intragastric volume was recorded. Gastric distension produced the intraballoon volume in a progressive manner with saturation, suggesting the occurrence of receptive relaxation of the stomach during distension. Intragastric application of capsaicin significantly enhanced the degree of receptive relaxation. The capsaicin-induced enhancement of receptive relaxation was totally abolished by bilateral vagotomy as well as chemical ablation of capsaicin-sensitive afferent neurons. Likewise, the enhanced receptive relaxation in response to stomach distension was also significantly attenuated by pretreatment of the animals with NG-nitro-L-arginine methyl ester (L-NAME, an inhibitor of nitric oxide (NO) synthase), calcitonin gene related peptide (CGRP)8-37 (CGRP antagonist), indomethacin and ONO-8711 (EP1 receptor antagonist). These results suggest that capsaicin significantly enhanced the receptive relaxation induced by gastric distention through both vagal nerves and capsaicin-sensitive afferent neurons. This process is intervened by endogenous NO and CGRP in addition to prostaglandins (PGs), and the effect of PGs may be mediated by EP1 receptors.