Structural changes induced by the binding of agonists, antagonists and inverse agonists to the cloned δ-opioid receptor from human brain immobilized in a solid-supported lipid bilayer were monitored using plasmon-waveguide resonance (PWR) spectroscopy. Agonist (e.g. deltorphin II) binding causes an increase in membrane thickness because of receptor elongation, a mass density increase due to an influx of lipid molecules into the bilayer, and an increase in refractive index anisotropy due to transmembrane helix and fatty acyl chain ordering. In contrast, antagonist (e.g. TIPPpsi) binding produces no measurable change in either membrane thickness or mass density, and a significantly larger increase in refractive index anisotropy, the latter thought to be due to a greater extent of helix and acyl chain ordering within the membrane interior. These results are closely similar to those reported earlier for another agonist (DPDPE) and antagonist (naltrindol) [Salamon et al. (2000) Biophys. J. 79, 2463-2474]. In addition, we now find that an inverse agonist (TMT-Tic) produces membrane thickness, mass density and refractive index anisotropy increases which are similar to, but considerably smaller than, those generated by agonists. Thus, a third conformational state is produced by this ligand, different from those formed by agonists and antagonists. These results shed new light on the mechanisms of ligand-induced G-protein-coupled receptor functioning. The potential utilization of this new biophysical method to examine structural changes both parallel and perpendicular to the membrane normal for GPCRs is emphasized.
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