Received for publication August 30, 2006.
Revised October 20, 2006.
Accepted for publication October 20, 2006.

SALVINORIN A: ALLOSTERIC INTERACTIONS AT THE MU OPIOID RECEPTOR

Abstract

Salvinorin A is a hallucinogenic kappa opioid receptor agonist that lacks the usual basic nitrogen atom present in other known opioid ligands. Our first published studies indicated that Salvinorin A weakly inhibited mu receptor binding and subsequent experiments revealed that Salvinorin A partially inhibited mu receptor binding. We therefore hypothesized that Salvinorin A allosterically modulates mu receptor binding. To test this hypothesis, we used CHO cells expressing the cloned human opioid receptor. Salvinorin A partially inhibited [³H]DAMGO (0.5, 2.0 and 8.0 nM) binding with E_MAX values of 78.6%, 72.1% and 45.7%, respectively and EC₅₀ values of 955, 1124 and 4527 nM, respectively. Salvinorin A also partially inhibited [³H]diprenorphine (0.02, 0.1 and 0.5 nM) binding with E_MAX values of 86.2%, 64%, and 33.6%, respectively and EC₅₀ values of 1231, 866, 3078 nM, respectively. Saturation binding studies with [³H]DAMGO showed that Salvinorin A (10 and 30 µM) decreased the mu receptor Bmax and increased the Kd in a dose-dependent, rather than a linear, manner. Saturation binding studies with [³H]diprenorphine showed that Salvinorin A (10 and 40 µM) decreased the mu receptor Bmax and increased the Kd in a dose-dependent, rather than a linear, manner. Similar findings were observed in rat brain with [³H]DAMGO. Kinetic experiments demonstrated that Salvinorin A altered the dissociation kinetics of both [³H]DAMGO and [³H]diprenorphine binding to mu receptors. Additionally, Salvinorin A acted as an uncompetitive inhibitor of DAMGO-stimulated [³⁵S]-GTP--S binding. Viewed collectively, these data support the hypothesis that Salvinorin A allosterically modulates the mu opioid receptor.

Key words: allosteric, kappa receptor, mu receptor, opioid, receptor binding, salvinorin a

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