Correlation of substance P-induced desensitization with substance P amino terminal metabolites in the mouse spinal cord

Intrathecal injection of mice with substance P (SP) or its C-terminal fragments results in a behavioral syndrome characterized by reciprocal caudally directed biting and scratching. Repeated injection of SP, but not SP C-terminal fragments, results in a decrease in the intensity of, or desensitization to, these SP-induced behaviors. Peptidase inhibitors, phosphoramidon (PH), bacitracin (BAC), diprotin A (DPA) and angiotensin converting enzyme inhibitor (ACEI OR SQ20881), together with [³H]SP, were used to investigate the possible accumulation of tritiated N-terminal metabolites in the mouse spinal cord in vivo during the development of desensitization to SP. SP N-terminal metabolites in the spinal cord were quantified by reverse-phase HPLC. The magnitude of SP-induced desensitization correlated well (r=.95) with total SP N-terminal metabolites recovered from the spinal cords of the same mice studied in vivo. The magnitude of SP-induced desensitization was also found to be negatively correlated (r=.95) with total recovered intact [³H]SP. The rank order of potency of the peptidase inhibitors in decreasing the magnitude of SP-induced desensitization was BAC = PH >> ACEI > DPA. The order of potency for in vitro inhibition of SP metabolism using synaptic membrane-derived peptidases was BAC > PH >> ACEI. These results support the hypothesis that desensitization to SP-induced behaviors depends, at least in part, on the concentration of SP N-terminal metabolites in the spinal cord.