Principles of tachykininergic co-transmission in the peripheral and enteric nervous system.

The tachykinins substance P (SP) and neurokinin A (NKA) are synthesized and released from nerves in the peripheral and enteric nervous system (PNS and ENS). They act as nonadrenergic noncholinergic (NANC) excitatory transmitters in mammalian airways, and the genitourinary and gastrointestinal tract. At the postjunctional level, both NK(1) (SP-preferring) and NK(2) (NKA-preferring) receptors are often co-expressed by target cells innervated by TKergic nerves. Thus an issue of duplication seems to exists with regard to peripheral tachykininergic co-transmission, the duplication involving both messengers (the peptides) and effectors (the receptors). By using receptor selective antagonists it has been possible to dissect the relative contribution of different receptors to TKergic co-transmission: the available results indicate that multiple arrangements exist involving both summation, cooperation and specialization of different messengers/effectors in producing the overall response.