ACTIVATION OF NEUROKININ-1 RECEPTORS ON INTERSTITIAL-CELLS OF CAJAL OF THE GUINEA-PIG SMALL-INTESTINE BY SUBSTANCE-P

The aims of this work were to determine whether cells that are similar to the interstitial cells of Cajal (ICC) and have immunoreactivity fo r the neurokinin 1 (NK1) receptor are indeed ICC; to determine whether the agonist, substance P, binds to and activates the receptor on pres umptive ICC; and to investigate the relationship between substance P-i mmunoreactive nerve fibres and ICC. ICC at the level of the myenteric plexus and in the deep muscular plexus in the duodenum and ileum of th e guinea-pig were investigated. Immunoreactivities for the ICC marker, Kit, and the NK1 receptor were colocalised in ICC of the myenteric an d deep muscular plexuses. In tissue fixed immediately after its remova l from the animal, NK1 receptor-immunoreactive ICC were found at the l evel of the myenteric plexus in the duodenum, but not in the ileum, an d in the deep muscular plexus in the duodenum and ileum. The majority of receptor immunoreactivity was on the cell surface. ICC were exposed to substance P (10(-7) M), initially at 4 degrees C for 1 h to allow the agonist to bind, followed by incubation at 37 degrees C to allow r eceptor internalisation to proceed. Exposure to substance P caused the NK1 receptor immunoreactivity to aggregate in clumps in the cytoplasm of ICC of the myenteric and deep muscular plexuses, including the ICC of the myenteric plexus of the ileum, where NK1 receptor immunoreacti vity was not seen if tissue was not exposed to substance P Substance P , to which the fluorescent label, cyanine 3.18 (Cy-3), was coupled, bo und to the ICC. The Cy-3-substance P was internalised with the recepto r following warming to 37 degrees C. Many, but not all, ICC were close ly apposed by nerve fibres with immunoreactivity for substance P. It i s concluded that the NK1 receptor immunoreactivity on ICC represents r eceptor that is functional in the sense that it binds the natural agon ist substance P and undergoes agonist-induced internalisation. ICC are likely to receive excitatory innervation from the close approaches of tachykinin-containing nerve fibres.