Substance P and Neurokinin 1 receptor - expression is affected in the ileum of mice with mutation in the W locus

The tachykinin substance P (SP) acts on the gut muscle coat via its preferred receptor, neurokinin 1 (NK1r). In the mouse ileum, NK1r-immunoreactivity (NK1r-IR) was detected in neurons, in the interstitial cells of Cajal at the deep muscular plexus (ICC-DMP) and the myoid cells of the villi. SP-IR was detected in neurons and varicose nerve fibers, which were especially numerous at the DMP and closely associated with the ICC-DMP. In mice with a mutation in the W locus (ckit mutant animals), innervation is suggested to be normal although few studies have actually tested this hypothesis. Indeed, studies demonstrating ICC-DMP integrity are lacking and whether SP- and NK1r-IR are normal in these animals has not been investigated. Our aim was to perform an immunohistochemical study on the ileum of a strain of heterozygous mice with a mutation in the W locus, the W(e/+) mice, to test this hypothesis. SP-IR nerve fibers were significantly more numerous than in wild type mice; NK1r-IR was clustered on the plasma membrane and also intracytoplasmatic in the neurons, but absent in the ICC-DMP. The richness in SP-IR nerve fibers and the NK1r-IR distribution in the neurons, similar to that of activated cells, might be attempts to compensate for the SP preferred receptor absence at the ICC-DMP. In conclusion, SP content and NK1r expression are noticeably different in c-kit mutants with respect to wild type mice, and probably causing an anomalous tachykininergic control of intestinal motility. Physiological studies on Wmutant mice have to take into account that innervation in this animal model is affected by the c-kit mutation.