[H-3][beta-Ala(8)]neurokinin A-(4-10): A novel, selective radioligand for the tachykinin NK2 receptor

The binding characteristics of the novel radioligand [H-3][beta-Ala(8)]neurokinin A-(4-10) were assessed in hamster urinary bladder membranes. This labelled compound bound in a reversible, highly specific and concentration-dependent manner to a single class of high affinity binding sites with a K-d of 1.8 +/- 0.2 nM and a B-max of 139 +/- 21 fmol/mg of protein. Specific binding of [H-3][beta-Ala(8)]neurokinin A-(4-10) was displaced only by NK2, but not by NK1 or NK3, tachykinin receptor agonists and antagonists. Neurokinin A, [beta-Ala(8)]neurokinin A-(4-10), L 659877 [cyclo(Leu-Met-Gln-Trp-Phe-Gly)], MEN 10376 (H-Asp-Tyr-D-Trp-Val-D-Trp-D-Trp-Lys-NH2), MEN 10627 [cyclo(Met-Asp-Trp-Phe-Dap-Leu)cyclo(2 beta-5 beta)] and SR 48968 [(S)-N-methyl-N-[4-(4-acetylamino-4-phenylpiperidino)-2- (3,4-dichlorophenyl)butyl]benzamide] displaced the binding with K-i values of 0.4 +/- 0.1 nM, 1.9 +/- 0.36 nM, 3.05 +/- 0.1 nM, 7.9 +/- 0.4 mu M, 0.36 +/- 0.02 nM and 2.5 +/- 0.9 nM, respectively. Functional data, obtained in isolated hamster urinary bladder strips with the newly developed tachykinin NK, receptor antagonists (MEN 10627 and SR 48968), showed a good agreement with binding data. This novel radioligand could represent a new useful tool for the assessment of tachykinin NK2 receptor antagonists.