Specific Binding of a ¹²⁵I‐Substance P Derivative to Rat Brain Synaptosomes

Binding characteristics of 125 I‐labeled Bolton‐Hunter substance P ( 125 I‐BHSP), a radioactive analogue of substance P, were studied with a crude syn‐aptosomal fraction (P 2 ) of the rat brain. A specific binding of 125 I‐BHSP that was saturable, reversible, and temperature dependent was demonstrated. Under the conditions of the binding assay (20°C, 5 min), nonspecific binding represented no more than 25% of total binding, and in contrast to [ 3 H]dopamine 125 I‐BHSP was not taken up within synaptosomes by a ouabain‐sensitive process. Scatchard analysis indicated the existence of a single population of noninteracting sites with a high affinity ( K D , 470 pM) and a low‐density ( B max , 13 fmol/mg protein). Substance P and different substance P analogues were tested for their competitive potencies in inhibiting 125 I‐BHSP binding to synaptosomes. (Tyr 8 ) substance P, substance P, and BHSP strongly inhibited 125 I‐BHSP specific binding. Among various tachykinins, physalaemin was the most potent (one fourth the potency of substance P). When substance P C‐terminal fragments were tested for their ability to compete with 125 I‐BHSP binding, a good relationship was found between competitive activity and peptide length. N‐terminal fragments of substance P were ineffective. However, (nor‐Leu 11 )substance P and the methyl ester of substance P 1‐11 slightly reduced 125 I‐BHSP binding. (D‐Pro 2 , D‐Phe 7 , D‐Trp 9 ) substance P and (D‐Pro 2 , D‐Trp 7,9 )substance P, two substance P antagonists, were also tested. (D‐Pro 2 , D‐Trp 7,9 ) substance P was the only one to inhibit 125 I‐BHSP binding even slightly. Results were compared with those previously obtained concerning 125 I‐BHSP binding to mesencephalic embryonic cells in primary cultures using similar conditions for the binding assay.