Receptors for neurotransmitters in opossum oesophagus muscularis mucosa

Muscularis mucosa of the distal oesophagus of the opossum contains nerves which release acetylcholine and substance P(SP)-like material on field stimulation. The release of SP-like material appeared to be inhibited by the presence of exogenous muscarinic agonists and potentiated by muscarinic antagonists. Analysis of the postjunctional receptors involved using carbachol, McNeil A-343 (McN A-343), atropine and pirenzepine suggested that the receptors were not typical M2-muscarinic receptors. The potency of agonists and antagonists were consistent with some receptor properties resembling M1-muscarinic receptors. Prejunctional receptors to opiates, adenosine, agonists at alpha 2-adrenoceptors and prostaglandins were not detected. Receptors for tachykinins were present on the muscle in this tissue, but did not resemble clearly either SP-E or SP-P type receptors. They appear to be undifferentiated since most tachykinins were of similar potency. These results suggest that not all postjunctional muscarinic receptors in intestinal smooth muscle are M2 in type. There may be a gradation of types between M1 and M2.