This chapter discusses tachykinin receptors in the spinal cord. In 1931, von Euler and Gaddum isolated a hypotensive agent from equine brain that they named “substance P” (SP), because of its powdered form. 50 years later, two similar peptides were isolated in mammals, and are now known as “neurokinin A” (NKA) and “neurokinin B” (NKB). All three of these peptides cause a rapid contraction of the smooth muscle of the gut; hence, this family of peptides is referred to as “tachykinins”. They are found extensively in the periphery, where they function as vasodilators and potent constrictors of many smooth muscles, and in the brain, where they are involved in neurotransmission. The focus of this chapter is on tachykinin receptors in the spinal cord, however, when relevant information is unavailable from studies of spinal cord; studies using other tissues are discussed. Erspamer et al. noted different potencies for SP and its analogues in different assays. This observation led to the discovery of three tachykinin receptors—namely, NK1,NK2, and NK3. The existence of these receptor subtypes has been confirmed by molecular biology studies, in which the genes for these receptors have been isolated. SP binds preferentially to the NK1 receptor, NKA to the NK2 receptor and NKB to the NK3 receptor. Binding characteristics for these receptors have been extensively characterized in tissues containing a single population of tachykinin receptors.
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