Substance P-induced inflammatory responses in guinea-pig skin: the effect of specific NK1 receptor antagonists and the role of endogenous mediators.

1. The sensory neuropeptide substance P (SP), when released from sensory nerves, has been implicated in the development of neurogenic inflammation. In the present study, using an in vivo model system, we have characterized and investigated the mechanisms underlying SP-induced leukocyte accumulation and oedema formation in the guinea-pig. 2. Intradermally injected SP (i.d., 10(-13) - 10(-9) mol per site), induced a dose- and time-dependent accumulation of 111In-neutrophils, 111In-eosinophils and oedema formation as measured by the local accumulation of i.v. injected 125I-albumin. The leukocyte accumulation evoked by SP was significant at 10(-10) and 10(-9) mol per site, whereas oedema formation was significant at the lowest dose tested (10(-13) mol per site). 3. The NK1 receptor antagonists, CP-96,345 (1 mg kg-1, i.v.) and RP-67,580 (10 micrograms per site, i.d.), significantly attenuated the oedema formation induced by the lower doses of SP. Oedema formation and leukocyte accumulation induced by 10(-9) mol per site SP were unaffected by either antagonist. 4. SP-elicited responses were not significantly affected by the platelet activating factor (PAF) receptor antagonist, UK-74,505 (2.5 mg kg-1, i.v.) or the H1 histamine receptor antagonist, chlorpheniramine (10(-8) mol per site, i.d.). However, the 111In-eosinophil accumulation, but not the 111In-neutrophil accumulation or oedema formation, induced by SP was significantly inhibited by the specific 5-lipoxygenase (5-LO) inhibitor, ZM-230,487 (10(-8) mol per site, i.d.). 5. The accumulation of both 111 In-neutrophils and 111 In-eosinophils induced by SP was abolished in guinea-pigs treated i.v. with an anti-CD18 monoclonal antibody 6.5E F(ab')2 (2.5 mg kg-1). The oedema response was unaffected in these animals.6. These results suggest that SP-induced inflammatory events may be mediated via two mechanisms involving NK1 receptor-dependent and independent pathways. Oedema formation induced by the lower doses of SP may be mediated via the direct activation of NK1 receptors whilst, at higher doses, oedema formation and leukocyte accumulation may be mediated via the release of secondary mediators, possibly mast cell derived, with 5-LO products playing an important role in the leukocyte infiltration. The leukocyte accumulation, but not the oedema induced by SP, is dependent on the expression of the CD18antigen on leukocytes.