Species differences between [3H] substance P binding in rat and guinea-pig shown by the use of peptide agonists and antagonists.

The affinities of various substance P agonists and antagonists for NK1 receptors in rat and guinea-pig tissues were compared. Striking species differences were observed. Both septide and [D-Pro4,D-Trp7,9]SP-(4-11) possessed much higher affinity for sites in the guinea-pig (brain and ileum) than for sites in the rat brain. These results could be explained by differences in the structure of the NK1 receptor according to the species, although the existence of various subtypes of NK1 binding sites in the two species cannot be excluded.