Multiple tachykinin binding sites in peripheral tissues and in brain

Tachykinin binding sites in guinea pig urinary bladder (GPUB), rat salivary gland (RSG), hamster urinary bladder (HUB), rat vas deferens (RVD) and rat cerebral cortex (RCC) were compared using ¹²⁵I-Bolton Hunter conjugates of substance P (¹²⁵I-BHSP), eledoisin (¹²⁵I-BHE) and neurokinin A (¹²⁵I-BHNKA). In typical SP-P tissues (GPUB, RSG) and in RCC, SP was the most potent displacer of ¹²⁵I-BHSP and [Glp⁶, D-Pro⁹]-SP(6–11) was 90 times less active than [Glp⁶, L-Pro⁹]-SP(6–11) while SP methyl ester (SPOMe) was 5–10 times more active than the Bolton Hunter conjugate of SPOMe (I-BHSPOMe). On the other hand, in typical SP-E tissues (HUB, RVD), neurokinin A was most potent in displacing ¹²⁵I-BHE and [Glp⁶,D-Pro⁹]-SP(6–11) was over 300 times more active than [Glp⁶,L-Pro⁹]-SP(6–11) while SPOMe was 160 times less active than I-BHSPOMe. In rat cerebral cortex, the rank order of potency of tachykinins and related analogues in displacing ¹²⁵I-BHE was distinct from that of peripheral SP-E sites, with neurokinin B being the most potent displacer, and SPOMe was over 1 000 times more active than I-BHSPOMe; ¹²⁵I-BHE binding sites in CNS may represent a third category of tachykinin receptor, designated SP-N.