Physiology of tachykinins:
The tachykinins are a group of plasma kinins that play a major role in inflammation and neurotransmission hence most of them are referred to as neuropeptides.13,14,15,16 The two human tachykinin genes are called TAC1 and TAC3.17
The major source of tachykinins is in the gut are enteric neurons, followed by nerve fibres from dorsal root and vagal ganglia. Tachykinin-containing fibres surround enteric ganglia, ramify through muscle, form a perivascular mesh around submucosal arteries, and supply the mucosa. This explains the fact of isolation of substance P from Gut.2 The important roles of tachykinins include Neuro-neuronal transmission, Protective secretory responses to infection.18,19,20,21,22 Tachykinins also participate in the inflammatory responses to infection, including formation of granulomas, sites of chronic inflammation that prevent spread of infectious agents.23 Tachykinins stimulate smooth muscle contraction of the human ureter, mostly by activating the NK2R.24 Tac1Tac3 , and Tac4  are expressed by human sperm, and tachykinins increase sperm motility by NK1R- and NK2R-dependent mechanisms.25 Mouse and human keratinocytes express NK1R and NK2R. The consensus of multiple studies is that SP and NKA control the capacity of keratinocytes to serve as cytokine factories by regulating production of proinflammatory cytokines.26 Thus tachykinins are involved in a vast array of physiological functions that makes them a potential target for drug development.