Ignavine: a novel allosteric modulator of the μ opioid receptor

Abstract

Processed Aconiti tuber (PAT) is used to treat pain associated with various disorders. Although it has been demonstrated that the κ opioid receptor (KOR) signaling pathway is a mediator of the analgesic effect of PAT, active components affecting opioid signaling have not yet been identified. In this study, we explored candidate components of PAT by pharmacokinetic analysis and identified ignavine, which is a different structure from aconitine alkaloids. A receptor binding assay of opioid receptors showed that ignavine specifically binds the μ opioid receptor (MOR), not the KOR. Receptor internalization assay in MOR-expressing cell lines revealed that ignavine augmented the responses produced by D-Ala(2)-N-Me-Phe(4)-Gly-ol(5)-enkephalin (DAMGO), a representative MOR agonist, at a low concentration and inhibited it at a higher concentration. Ignavine also exerted positive modulatory activity for DAMGO, endomorphin-1 and morphine in cAMP assay. Additionally, ignavine alone showed an analgesic effect in vivo. In silico simulation analysis suggested that ignavine would induce a unique structural change distinguished from those induced by a representative MOR agonist and antagonist. These data collectively suggest the possibility that ignavine could be a novel allosteric modulator of the MOR. The present results may open the way for the development of a novel pain management strategy.

Allostery at opioid receptors: modulation with small molecule ligands

“…Ignavine (Figure ) is a diterpene alkaloid isolated from the plant Aconitum japonica (Saito et al , ; Ohbuchi et al , ). There is evidence that the ‘processed aconite tuber’ has analgesic activity mediated by κ‐receptors, although the specific κ‐receptor agonist has not been isolated (Ohbuchi et al , ). Ignavine itself gives a biphasic antinociceptive dose–response curve in the mouse tail‐flick and tail pressure tests.…”

“…Ignavine itself gives a biphasic antinociceptive dose–response curve in the mouse tail‐flick and tail pressure tests. The title of a recent publication (Ohbuchi et al , ) states ignavine is a ‘novel allosteric modulator of the μ‐receptor’. This claim is based on the finding that the compound both enhances and inhibits the activity of the μ‐receptor orthosteric agonist DAMGO to inhibit cAMP accumulation and to cause internalization of μ‐receptors in HEK 293 cells depending on the ignavine concentration (1 or 10 μM respectively).…”

Insights from molecular dynamics simulations to exploit new trends for the development of improved opioid drugs

“…Figure 1 shows two-dimensional (2D) structures of confirmed PAMs at the μand/or δ-opioid receptors (MOR and DOR, respectively), specifically: the MOR-PAMs BMS-986121 [8], BMS-986122 [8], and MS1 [9], and the DOR-PAM BMS-986187 [10], which shows 100-fold weaker PAM activity at MOR. A few natural products such as cannabinoid type-1 receptor (CB1) agonists cannabidiol and Δ 9 -tetrahydrocannabinol [11], the potent κ-opioid receptor (KOR) orthosteric agonist salvinorin A [12], and ignavine [13] have also been suggested to exhibit allosteric properties at opioid receptors, but not without reservation [7]. Although SCH202676 has also been suggested to act as an allosteric modulator at DOR, MOR, and KOR [14], there are reports that suggest that this compound binds covalently to the receptor, and is therefore not an allosteric modulator [15,16].…”

Recent Advances in the Realm of Allosteric Modulators for Opioid Receptors for Future Therapeutics

“…It was recently discovered that the compound ignavine (Figure 3), one of the main alkaloids in Aconite root, interacts with the MOR, exhibiting an IC 50 of 2.0 μ M. 43 A receptor internalization assay determined that ignavine can modulate MOR activity in such a way that is dependent upon its concentration. GFP-tagged MOR expressing HEK-293 cells that were treated with 1 μ M DAMGO showed vesicle internalization at 20 min.…”