κ-Opioid receptors are differentially labeled by arylacetamides and benzomorphans

Abstract

Using Chinese Hamster Ovary cell membranes that stably expressed the human κ-opioid receptor, we investigated the hypothesis that κ 1- and κ 2-opioid receptors, historically defined by their phrmacological selectivity for either arylacetamides or benzomorphans are, in fact, different affinity states or binding sites on the same κ-opioid receptors. Receptor binding studies showed that GTPγS potently inhibited [ 3H](5α,7α,8β)-(+)- N-methyl- N-(7-[1-pyrrolidinyl]-1-oxaspiro [4.5]dec-8-yl)-benzeneacetamide (U69,593) binding, compared to virtually no inhibition of [ 3H]bremazocine binding. Saturation binding experiments showed a three-fold decrease in [ 3H]U69,593 affinity in the presence of GTPγS, but GTPγS had no effect on [ 3H]bremazocine affinity. The κ-opioid receptor antagonist nor-binaltorphimine had a four-fold higher affinity for [ 3H]U69,593-labeled receptors than for [ 3H]bremazocine-labeled receptors. Functional selectivity studies, measuring the stimulation of [ 35S]GTPγS agonist-induced binding, showed a significantly higher U69,593-induced G protein-receptor activation in comparison to the stimulation observed with bremazocine. These results suggest that pharmacologically defined 1κ-opioid receptor subtypes may be different affinity states of the same receptor.