Distribution of substance P in the enteric plexuses of the small intestine of the platypus, Ornithorhynchus anatinus.

A study was made of substance P-like-immunoreactive nerves within the small intestine of platypus, an Australian prototherian mammal. Both immunoreactive nerve cell bodies and fibers were present. No immunoreactive fibers were found in mesenteric nerves or on intramural blood vessels, suggesting that extrinsic sensory neurons containing substance P do not innervate platypus ileum. This was further supported by the result of in vitro experiments. Although applied substance P (10(-9) M-3 x 10(-8) M) caused contraction of the longitudinal muscle, neither mesenteric nerve stimulation nor application of capsaicin caused contractions.