Stereospecificity of SP1 and SP2 substance P receptors.

Previous studies with N-terminal fragments of substance P (SP) have suggested the existence of two separate SP receptor populations. SP1 receptors are found in guinea pig ilea and rat colons. SP2 receptors are found in mouse spinal cords and rat salivary glands. We have now found that substitution of Gly9 in substance P's C-terminal hexapeptide leads to an analog (L-Pro9 SP6-11) which selectively and potently stimulates SP2 receptors. In contrast, substitution of the same residue with D-Proline results in a potent and selective agonist for SP1 receptors. The data dramatically confirm the distinction between SP1 and SP2 receptors and demonstrate that the two receptors have distinct stereochemical architectures.

Use of substance P fragments to differentiate substance P receptors of different tissues.

The C- and N-terminal fragments of substance P were compared to the parent molecule with respect to their ability to: (a) contract the isolated guinea pig ileum, (b) induce salivation in the rat, (c) excite single cat dorsal horn neurones, and (d) induce scratching by intracranial injections in mice. C-terminal fragments as small as the heptapeptide were potent SP agonists on all assay systems. C-terminal fragments containing five amino acids or less were, at most, only weakly active. The C-terminal hexapeptide was a potent SP receptor stimulant on the isolated guinea pig ileum and, when directly applied by microiontophoresis, on cat dorsal horn neurons. However, the same compound was only 2-5% as potent as substance P in eliciting salivation and scratching in vivo, an indication that this fragment may be especially labile to enzymatic degradation. N-terminal fragments were totally inactive on the isolated guinea pig ileum. On the rat salivation and central nervous system assays, however, N-terminal fragments were capable of weak SP-like activity. It is concluded that SP receptors exist in multiple forms which we have labelled SP1 and SP2 receptors for those insensitive or sensitive to N-terminal fragments, respectively.