Leu-Met-NHz) is an undecapeptide that acts as a neurotransmitter/modulator in the mammalian peripheral and central nervous systems.',* Substance P is part of a family of peptides known as tachykinins that share the common C-terminal sequence Phe-x-Gl~-Leu- Met-NH,.~ This family includes the peptides eledoisin, physalaemin, and kassinin, which are all of nonmammalian origin? Recently, two additional mammalian tachykinins were ...