Neurokinin-1 Receptor Agonists Are Involved in Mediating Neutrophil Accumulation in the Inflamed, But Not Normal, Cutaneous Microvasculature: An In Vivo Study Using Neurokinin-1 Receptor Knockout Mice

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Abstract
We have used tachykinin neurokinin-1 receptor (NK1 receptor) knockout mice to learn of the link between NK1 receptors and neutrophil accumulation in normal naive skin, as compared with inflamed skin. Intradermal substance P (300 pmol) induced edema formation in wild-type mice, but not in NK1 knockout mice, as expected. However, in contrast to IL-1β (0.3 pmol), substance P did not induce neutrophil accumulation in wild-type mice. IL-1β-induced neutrophil accumulation was similar in wild-type and knockout mice, but a significant (p < 0.05) contributory effect of added NK1 agonists, which by themselves have no effect on neutrophil accumulation in normal skin, was observed. The results support the concept that NK1 agonists such as substance P cannot act on their own to mediate neutrophil accumulation in naive skin and provide direct evidence that in inflamed skin, under certain circumstances, the NK1 receptor can play a pivotal role in modulating neutrophil accumulation during the ongoing inflammatory process. We investigated responses to two inflammatory stimuli (carrageenin and zymosan). Neutrophil accumulation was significantly attenuated (p < 0.001) in carrageenin- but not zymosan-induced inflammation in NK1 knockout mice. The carrageenin (500 μg)-induced response was inhibited (p < 0.05) by a NK1 receptor antagonist, SR140333 (480 nmol/kg i.v. at −5 min), in the wild-type group. The bradykinin B1 and B2 receptor antagonists (desArg9[Leu8]bradykinin and HOE 140) each reduced neutrophil accumulation to carrageenin in wild-type animals (p < 0.05), but did not cause further reduction of the suppressed response of knockout mice. The results provide evidence that kinin receptors participate in NK1 receptor-dependent neutrophil accumulation in inflamed mouse skin.