Intrathecal injection of mice with substance P or its C-terminal fragments evokes a well
documented behavioral syndrome characterized by caudally-directed biting and scratching.
We have previously shown that repeated injections of substance P result in naloxone-
sensitive desensitization to this substance P-induced behavior, possibly through interactions
of N-terminal fragments of substance P with mu opiate binding sites. The present
investigation tests the hypothesis that substance P metabolites play a role in the …