Differential modulation by GTPγS of agonist and inverse agonist binding to h5-HT1A receptors revealed by [3H]-WAY100,635

1 The interaction of serotonergic ligands at human (h) 5-HT1A receptors expressed in Chinese hamster ovary cells was examined with the selective 'neutral' 5-HT1A antagonist [H-3]-WAY100,635. Its binding was saturable (K-D=0.056 nM) with a B-max (3.65 pmol mg(-1)) significantly higher than that of two other selective 5-HT1A radioligands: the partial agonist, [H-3]-S15535 (2.77 pmol mg(-1)) and the agonist, [H-3]-8-OH-DPAT (2.02 pmol mg(-1)). 2 The influence of GTP gammaS (100 muM) on the binding affinity of 15 serotonergic agonists, partial agonists, antagonists and inverse agonists was investigated in competition binding experiments with [H-3]-WAY100,635. 3 Agonists. including 5-HT, 8-OH-DPAT and buspirone, displayed biphasic isotherms which shifted to the right in the presence of GTP gammaS. In contrast, isotherms of the inverse agonists, methiothepin, (+)butaclamol and spiperone, were shifted to the left in the presence of GTP gammaS. Unlabelled WAY100,635 was the only ligand that was unaffected by GTP gammaS, consistent with 'neutral' antagonist properties. 4 The magnitude of affinity changes induced by GTP gammaS for 13 ligands was highly correlated (r = 0.98) with their efficacy (positive and negative) previously determined by [S-35]GTP gammaS binding. 5 In contrast, the napthylpiperazine derivative and high efficacy agonist, S14506. displayed only a modest GTP gammaS shift, in accordance with previous indications of 'atypical' binding properties of this ligand. A further full agonist, S14671, which is chemically closely-related to S14506, also displayed a minimal GTP gammaS shift, underpinning this observation. 6 In conclusion, [H-3]-WAY100,635 constitutes a useful neutral antagonist radioligand for the characterization of drug actions at h5-HT1A receptors. GTP gammaS-induced affinity changes of agonist and inverse agonist competition isotherms generally correlate well with ligand efficacy, with the notable exception of two chemically-similar agents, S14506 and S14671, which are efficacious agonists, yet relatively insensitive to h5-HT1A receptor/G-protein coupling changes.