Binding of GTPγ[ 35 S] is regulated by GDP and receptor activation. Studies with the nociceptin/orphanin FQ receptor

Background and purpose:  We have examined the effects of ligand efficacy and receptor density on the binding of guanosine 5′‐[γ‐thio]triphosphate (GTPγS) and GDP to the nociceptin/orphanin FQ (N/OFQ) peptide receptor (NOP)‐coupled G‐proteins. Experimental approach:  In GTPγ[ 35 S] binding experiments, using stable (CHO hNOP ) and inducible (CHO INDhNOP ) recombinant human and rat NOP we have measured: (i) ligand‐specific GDP requirements; (ii) the effects of receptor density on guanine nucleotide affinity/capacity; and (iii) the effect of ligand efficacy on GTPγS association kinetics. Key results:  GTPγS competition curves were shallow and modelled by high‐ and low‐affinity components that were relatively consistent between cell types and tissue preparations. In the presence of 1 µM N/OFQ a high‐affinity GDP binding site was also present, but the fraction of total binding was reduced. In an efficacy‐dependent manner, the partial agonists [F/G]N/OFQ(1‐13)NH 2 ([Phe 1 ψ(CH 2 ‐NH)Gly 2 ]‐nociceptin(1‐13)NH 2 ) and naloxone benzoylhydrazone both reduced the fraction of high‐affinity sites for GDP (relative to basal). While the pIC 50 for high‐affinity GDP binding site did not decrease in the presence of 1 µM N/OFQ, N/OFQ produced a significant reduction in pIC 50 for the low‐affinity site. Agonist‐mediated decrease in affinity for GDP binding was efficacy‐dependent. GDP displayed three affinities: high, conserved in the presence and absence of ligand; intermediate, present as a low fraction under basal conditions; low (efficacy‐dependent), present during receptor activation representing the majority of binding. Conclusions and implications:  The affinity of GTPγ[ 35 S] was regulated by GDP and receptor activation caused increased binding of GTPγ[ 35 S] through a reduction in GDP affinity.