Abstract
Receptor binding characteristics of labelled substance P (SP) and substance K (SK) were investigated in guinea pig membrane preparations and by in vitro receptor autoradiography. The two ligands bind to single class of sites with K d of 2.1 nM and B max of 37.0 fmol/mg protein for SP and 10.6 nM and 50.3 fmol/mg protein for SK. Ligand selectivity patterns markedly differ for the two ligands. On SP binding sites, SP > physalaemin > eledoisin > SK > kassinin > Neuromedin K. On the other hand, SK > kassinin > Neuromedin K > eledoisin > SP > physalaemin on SK binding sites. The autoradiographic distribution of SP and SK binding sites is also different. While the cerebral cortex, substantia nigra and cerebellum contain very low to low densities of SP sites, they are all rich in SK binding sites. These data clearly demonstrate the existence of at least two classes of tachykinin binding sites in guinea pig brain. Moreover, the high density of SK binding sites present in the substantia nigra suggests possible physiological roles for SK in this region.
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