The study of tachykinin receptors.
1. Three high affinity receptors (NK1, NK2 and NK3) recognizing the C-terminal end of tachykinins have been described, with probable endogenous ligands substance P (SP-), neurokinin A and neurokinin B, respectively. 2. Other receptors recognizing the N-terminal regions of SP exist in adrenal medulla and central nervous system (CNS) with a low affinity receptor on mast cells. 3. NK1 receptors are associated with blood vessels, all types of smooth muscle, enteric ganglia and glands. NK2 receptors are found on gastrointestinal and urinary smooth muscle, but few peripheral NK3 receptors have been described. In contrast, NK1, NK3 but not NK2 receptors are widely distributed in the CNS. 4. Problems associated with the study of tachykinin receptors include low selectivity of endogenous tachykinins, poor antagonists and susceptibility of peptides to enzymatic degradation.