The role of enzymatic processing in the biological actions of substance P

There is considerable evidence that substance P (SP) is a neurotransmitter in the CNS. Current findings suggest that the effects of synaptically released SP are terminated by enzymatic breakdown, primarily by endopeptidase 3.4.24.11 (endo 24.11). The products of cleavage by endo 24.11 include the amino-terminal fragment SP(1-7). Evidence suggests that SP is involved in mediating baroreceptor reflex activity in the nucleus of the solitary tract (NTS). Microinjection of SP into the NTS lowered blood pressure and heart rate. Microinjection of SP(1-7) into the NTS reproduced the effects of SP on both heart rate and blood pressure. Intra-NTS injection of phosphoramidon, an inhibitor of endo 24.11 activity, completely blocked the effects of a subsequent injection of SP. This blocking effect of phosphoramidon was unaltered by pretreatment with the opiate inhibitor naloxone. In contrast, phosphoramidon failed to block the depressor and bradycardic effects of SP(1-7). The implications of these findings regarding the role of endo 24.11 in the metabolism of SP are discussed.