Pharmacological characterisation of two tachykinin binding sites in the rat cerebral cortex

The pharmacological properties of two types of tachykinin receptor were characterised on rat cortical synaptosomes using 125I-Bolton Hunter substance P (125I-BHSP) or with 125I-Bolton Hunter eledoisin (125I-BHE). Shorter SP C-terminal fragments, such as SP (6-11) or (pGlu)-SP (6-11), were more potent than SP itself or longer SP C-terminal fragments in competing for 125I-BHE binding; their efficacy was comparable to that of eledoisin. In contrast, longer SP C-terminal fragments exhibited a higher affinity than shorter ones for the 125I-BHSP binding sites as previously reported. SP N-terminal fragments were devoid of activity on either type of binding sites. SP methyl ester inhibited 125I-BHSP binding but was without effect on 125I-BHE binding whilst, DiMe-C7, a metabolically stable tachykinin analog, had the opposite selectivity. Eledoisin related peptide (ERP) was less effective than either SP or eledoisin on 125I-BHSP and 125I-BHE binding sites respectively. Finally, the undecapeptide or octapeptide SP antagonists, which are weak inhibitors of 125I-BHSP binding, had negligable activity on 125I-BHE binding sites.