Capsaicin effects on muscularis mucosa of opossum esophagus: substance P release from afferent nerves?

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Abstract
The effects of capsaicin on the function and structure of muscularis mucosa of opossum esophagus were studied. In this tissue there are numerous nerves containing a substance P-like immunoreactive substance (SPLS), and electrical field stimulation (EFS) leads to a phasic response that appears to be due to release of acetylcholine, followed by a tonic response at higher frequencies of stimulation that appears to be due to release of a SPLS. The acetylcholine released by EFS and exogenous muscarinic agonists inhibits release of this SPLS (8). In the present study capsaicin (5 X 10(-5) M) was shown to cause a tonic submaximal contraction in most cases. This was prevented by substance P tachyphylaxis and by pretreatment with the partial agonist [D-Pro2, D-Trp7,9]substance P, the antagonist [D-Arg1, D-Trp7, Leu11]substance P, and tetrodotoxin. This response to capsaicin could not be repeated even after 2 h. Capsaicin also abolished the tonic response to high-frequency EFS without affecting phasic responses and reduced markedly the enhanced tonic response after atropine had abolished the phasic response. This occurred with or without a preliminary contraction to capsaicin. This tonic response to high-frequency EFS recovered completely 1-2 h after washing out capsaicin. Then, a further administration of capsaicin had no direct effect but again abolished tonic responses to EFS. Capsaicin reduced responses to exogenous substance P or carbachol only partially. When tissues were fixed and studied after capsaicin had abolished tonic responses to EFS, specific damage to nerve varicosities or synaptic vesicles in nerve varicosities could not be demonstrated.(ABSTRACT TRUNCATED AT 250 WORDS)