PHARMACOLOGICAL ANALYSIS OF [H-3] SENKTIDE BINDING TO NK3 TACHYKININ RECEPTORS IN GUINEA-PIG ILEUM LONGITUDINAL MUSCLE-MYENTERIC PLEXUS AND CEREBRAL-CORTEX MEMBRANES

1. The binding properties and pharmacological specificity of the selective NK3 tachykinin receptor agonist [3H]-senktide ([3H]-succinyl[Asp6,MePhe8] substance P (6-11)) have been examined in homogenates of guinea-pig ileum longitudinal muscle-myenteric plexus (LM/MP) and cerebral cortex. 2. Scatchard analysis of saturation binding studies in guinea-pig ileum LM/MP and cerebral cortex membranes indicated that [3H]-senktide bound to a single site with apparent high affinity, K(D) = 2.21 ± 0.65 nM; B(max) = 13.49 ± 0.04 fmol mg-1 protein in ileum and K(D) = 8.52 ± 0.45 nM; B(max) = 76.3 ± 1.6 fmol mg-1 protein in cortex (values are means ± ranges; n = 2). 3. The pharmacological profile for tachykinins and analogues in displacing [3H]-senktide from ileum membranes was: [MePhe7] neurokinin B > neurokinin B (NKB) ≃ senktide > eledoisin > substance P (SP) > neurokinin A(NKA) > physalaemin > [Sar9,Met(O2)11]SP > [Nle10]NKA(4-10) = [Glp6,L-Pro9]-SP(6-11) > substance P methyl ester, consistent with [3H]-senktide binding to an NK3 subtype of tachykinin receptor. A similar rank order of affinity was obtained for these peptides in displacing [3H]-senktide from cortex membranes. 4. Several tachykinin receptor agonists were tested for their ability for their ability to displace [3H]-senktide from ileal and cortical NK3 binding sites and were found to be either weak displacers (pIC50 < 5.00) or inactive. 5. The binding of [3H]-senktide to cortex membranes was inhibited by GTP (pIC50 = 6.49) and GTP-γ-S (pIC50 = 6.67) with ATP being at least three orders of magnitude less potent (pIC50 = 3.55). 6. These results indicate that both central and peripheral NK3 receptors share a similar pharmacological specificity and that they may be labelled selectively with the NK3 receptor agonist [3H]-senktide.