Salvinorin A: a potent naturally occurring nonnitrogenous kappa opioid selective agonist

Abstract

Salvia divinorum, whose main active ingredient is the neoclerodane diterpene Salvinorin A, is a hallucinogenic plant in the mint family that has been used in traditional spiritual practices for its psychoactive properties by the Mazatecs of Oaxaca, Mexico. More recently, S. divinorum extracts and Salvinorin A have become more widely used in the U.S. as legal hallucinogens. We discovered that Salvinorin A potently and selectively inhibited (3)H-bremazocine binding to cloned kappa opioid receptors. Salvinorin A had no significant activity against a battery of 50 receptors, transporters, and ion channels and showed a distinctive profile compared with the prototypic hallucinogen lysergic acid diethylamide. Functional studies demonstrated that Salvinorin A is a potent kappa opioid agonist at cloned kappa opioid receptors expressed in human embryonic kidney-293 cells and at native kappa opioid receptors expressed in guinea pig brain. Importantly, Salvinorin A had no actions at the 5-HT(2A) serotonin receptor, the principal molecular target responsible for the actions of classical hallucinogens. Salvinorin A thus represents, to our knowledge, the first naturally occurring nonnitrogenous opioid-receptor subtype-selective agonist. Because Salvinorin A is a psychotomimetic selective for kappa opioid receptors, kappa opioid-selective antagonists may represent novel psychotherapeutic compounds for diseases manifested by perceptual distortions (e.g., schizophrenia, dementia, and bipolar disorders). Additionally, these results suggest that kappa opioid receptors play a prominent role in the modulation of human perception.

Figure 1

Molecular modeling predicts Salvinorin A is a structurally novel κ opioid ligand. A shows the structure of Salvinorin A, enadoline, U69593 and LSD whereas B shows a superimposition of the structures of Salvinorin A and U69593. C shows potential residues on the KOR identified by molecular modeling, which might interact with Salvinorin A, and D shows a model of Salvinorin A's interactions with the KOR (see supporting information for further details).

Figure 2

Large-scale screening of human cloned GPCRs reveals Salvinorin A is selective for KOR. Shown is the mean percent inhibition of radioligand binding or functional activity (metabotropic glutamate receptors only) to 50 receptors and transporters for LSD (yellow bars) and Salvinorin A (red bars) tested at 10 μM. With the exception of the rat β1 and β2 adrenergic and bovine dopamine transporter (DAT) all of the assays were performed with cloned human receptors heterologously expressed (see Materials and Methods and supporting information on the PNAS web site for details). As can be seen (arrow), Salvinorin A inhibited only KOR binding at 10 μM. See Table 5 for details. SERT, serotonin transporter; NET, norepinephrine transporter; DAT, dopamine transporter; rGABAA, rat GABA-A receptor.

Figure 3

Salvinorin A is a potent KOR agonist. A shows that Salvinorin A potently inhibits 3H-bremazocine binding to cloned KORs, whereas B shows the ability of Salvinorin A to inhibit forskolin-stimulated adenylate cyclase in KOR-393 cells. Data represent the mean ± SD of triplicate determinations from a representative experiment that has been replicated three times. For the inhibition of forskolin-stimulated cyclase activity, an EC₅₀ value of 1 ± 0.5 nM was calculated for Salvinorin A, compared with an EC₅₀ value of 1.2 ± 0.6 nM for U69593.