ABSTRACT
Figure 1 Possible mechanisms of substance P-induced itch and scratch. Substance P (SP) may act directly on primary afferents. SP acts on mast cells to release histamine, in which NK j tachykinin receptors (NK]R) are not involved. SP acts on NKjR of keratinocytes to release leukotriene B4 (LTB4), which elicits scratching. Keratinocytes may produce various mediators, such as SP itself, nitric oxide (NO), nerve growth factor (NGF), platelet-activating factor (PAF), vasoactive intestinal polypeptide (VIP), enkephalin (ENK), and cytokines, some of which may also be involved in SP-induced scratching. NK j R is also present in Langerhans cells, macrophages, and endothelial cells, but the role of these cells in SP-induced itch and/or scratch remains unclear.
