An N-terminal fragment of substance P, substance P(1–7), down-regulates neurokinin-1 binding in the mouse spinal cord

Abstract

Injected intrathecally, substance P (SP) down-regulates neurokinin-1 (NK-1) binding in the spinal cord and desensitizes rats to the behavioral effect of SP. N-terminal fragments of SP, such as SP(1–7), induce antinociception and play a role in desensitization to SP in mice. The goal of this study was to assess the abilities of N- and C-terminal fragments of SP to down-regulate NK-1 binding. Binding of [ 3H]SP to mouse spinal cord membranes was inhibited by SP, CP-96,345, and to a lesser extent by SP(5–11), but not SP(1–7), consistent with these binding sites being NK-1 receptors. Injection of SP(5–11) intrathecally did not affect the affinity ( K d) or concentration ( B max) of [ 3H]SP binding. However, injection of 1 nmol of SP(1–7) decreased the B max of [ 3H]SP binding in the spinal cord at 6 h after its injection just as this dose of SP decreased the B max at 24 h. These data suggest that the N-terminus of SP is responsible for down-regulation of NK-1 binding. As SP(5–11) did not down-regulate NK-1 binding, activation of NK-1 sites does not appear necessary or sufficient for down-regulation of SP binding. In contrast, SP(1–7), in spite of its inability to interact with NK-1 sites, did down-regulate SP binding, suggesting an indirect mechanism dissociated from NK-1 receptors.