Pivotal role of endogenous tachykinins and the NK1 receptor in mediating leukocyte accumulation, in the absence of oedema formation, in response to TNFα in the cutaneous microvasculature

Tachykinins including substance P (SP) are well known to play a role in influencing oedema formation and leukocyte accumulation during tissue insult and inflammation. Cutaneous inflammatory models to characterize a TNFα-dependent mechanism where endogenous SP act via the NK1 receptor to promote leukocyte accumulation in the absence of oedema formation were used. We found that TNFα induced dose-dependent leukocyte accumulation at 4 h, which returned towards basal levels at 8 h in NK1+/+ mice. This response was absent in both the NK1+/+ mice treated with an NK1 receptor antagonist and NK1−/− mice. At the highest dose IL-6 induced a significant accumulation in NK1+/+ and NK1−/− mice but IL-12 was ineffective. SP induced skin oedema but none of the cytokines did. Either co-injection of SP with low dose of TNFα (0.3 pmol/site) or SP previously injected (30 min) to TNFα evoked a significant increase in MPO activity when compared with that induced by the cytokine alone. In contrast, SP injected i.d. 3.5 h after TNFα failed to produce additive response. Control, but not capsaicin-pretreated rats (to deplete sensory nerves), exhibited a marked increase in MPO activity in response to TNFα. Histological analysis showed that TNFα caused tissue infiltrate of leukocytes in NK1+/+ mice, whilst leukocytes accumulated at intravascular sites in NK1−/− mice, but did not appear to emigrate, suggesting a defect in trans-endothelial migration. Interestingly, monocytes in addition to neutrophils accumulated 4 h post TNFα injection. In conclusion, the NK1 receptor plays a functional role in mediating leukocyte accumulation independently of the historically important NK1 mediated oedema formation. It seems that TNFα directly activates sensory nerve in addition to its chemoattractant activity. The NK1 receptor agonist influences the accumulation of monocytes in addition to that of PMN by 4 h, thus revealing an important influence of the NK1 receptor on TNFα mediated events in mouse skin.