Christopher R. McCurdy

Opioid receptors and legal highs: Salvia divinorum and Kratom

Salvia divinorum and Mitragyna speciosa (“Kratom”), two unscheduled dietary supplements whose active agents are opioid receptor agonists, have discrete psychoactive effects that have contributed to their increasing popularity. Salvia divinorum contains the highly selective kappa- opioid receptor agonist salvinorin A; this compound produces visual hallucinations and synesthesia. Mitragynine, the major alkaloid identified from Kratom, has been reported as a partial opioid agonist producing similar effects to morphine. An interesting minor alkaloid of Kratom, 7-hydroxymitragynine, has been reported to be more potent than morphine. Both Kratom alkaloids are reported to activate supraspinal mu- and delta- opioid receptors, explaining their use by chronic narcotics users to ameliorate opioid withdrawal symptoms. Despite their widespread Internet availability, use of Salvia divinorum and Kratom represents an emerging trend that escapes traditional methods of toxicologic monitoring. The purpose of this article is to familiarize toxicologists and poison control specialists with these emerging psychoactive dietary supplements.

Self-treatment of opioid withdrawal using kratom (Mitragynia speciosa korth)

BACKGROUND Kratom (Mitragynia speciosa korth) is recognized increasingly as a remedy for opioid withdrawal by individuals who self-treat chronic pain. CASE DESCRIPTION A patient who had abruptly ceased injection hydromorphone abuse self-managed opioid withdrawal and chronic pain using kratom. After co-administering the herb with modafinil he experienced a tonic-clonic seizure, but he reported only modest abstinence once kratom administration stopped. We confirmed the identity of the plant matter he ingested as kratom and identified no contaminants or adulterants. We also conducted high-throughput molecular screening and the binding affinity at mu, delta and kappa receptors of mitragynine. CONCLUSION We report the self-treatment of chronic pain and opioid withdrawal with kratom. The predominant alkaloid of kratom, mitragynine, binds mu- and kappa-opioid receptors, but has additional receptor affinities that might augment its effectiveness at mitigating opioid withdrawal. The natural history of kratom use, including its clinical pharmacology and toxicology, are poorly understood.

Antinociceptive profile of salvinorin A, a structurally unique kappa opioid receptor agonist

Salvinorin A, is a structurally unique, non-nitrogenous, kappa opioid receptor (KOP) agonist. Given the role of KOPs in analgesic processes, we set out to determine whether salvinorin A has antinociceptive activity in thermal and chemo-nociceptive assays. The tail-flick assay was employed to investigate 1) salvinorin As (0.5, 1.0, 2.0, and 4.0 mg/kg) dose-response and time-course (10, 20, and 30 min) effects in a thermal nociceptive assay, and 2) the ability for the KOP antagonist norBNI (10.0 mg/kg) to prevent salvinorin A antinociception. The hotplate assay was utilized as a second thermal nociceptive measure to test salvinorin As dose-response effects. The acetic acid abdominal constriction assay was used to study salvinorin As dose-response and time-course (over 30 min) effects in a chemo-nociceptive assay. Together, these studies revealed that salvinorin A produces a dose-dependent antinociception that peaked at 10 min post-injection but rapidly returned to baseline. Additionally, pretreatment with the KOP antagonist norbinaltorphimine (norBNI) reversed salvinorin A-induced antinociception. These findings demonstrate that salvinorin A produces a KOP mediated antinociceptive effect with a short duration of action.

Targeting the Methyl Erythritol Phosphate (MEP) Pathway for Novel Antimalarial, Antibacterial and Herbicidal Drug Discovery: Inhibition of 1-Deoxy-D-Xylulose-5-Phosphate Reductoisomerase (DXR) Enzyme

The 2-C-methyl-D-erythritol-4-phosphate (MEP) pathway for isoprenoid biosynthesis has come under increased scrutiny as a target for novel antimalarial, antibacterial and herbicidal agents. 1-Deoxy-D-xylulose 5-phosphate reductoisomerase (DXR) is a key enzyme of the pathway that catalyzes the rearrangement and nicotinamide adenine dinucleotide phosphate (NADPH)-dependent reduction of 1-deoxy-D-xylulose 5-phosphate (DXP) to MEP. The unique properties of DXR make it a remarkable and rational target for drug design. First, it is a vital enzyme for synthesis of isoprenoids in algae, plants, several eubacteria including the pathogenic bacteria like Bacillus anthracis, Helicobacter pylori, Yersinia pestis, Mycobacterium tuberculosis and the malarial parasite, Plasmodium falciparum. Second, there are no functional equivalents to DXR in humans, making it an attractive target for therapeutic intervention. Third, DXR appears to be a valid target and the results from fosmidomycin (1), the only available DXR inhibitor under clinical trials, suggests synergistic effects with the lincosamide antibiotics, lincomycin and clindamycin. Despite drug design efforts in this area, no successful drug specifically designed to inhibit DXR has emerged yet. This review summarizes the recent and promising developments with respect to the current knowledge of the MEP pathway with emphasis on the understanding of the structure and the catalytic mechanism of the DXR enzyme and the global quest for therapeutically useful inhibitors of DXR.

Mitragyna speciosa, A Psychoactive Tree from Southeast Asia with Opioid Activity

Mitragyna speciosa Korth. (Rubiaceae) is a tree that is commonly found in Southeast Asia. Leaves from this tree have been traditionally been used for both their stimulant properties as well as an opium substitute. The tree/leaves are currently illegal in four countries, but is currently legal and widely available in the United States. To date over 40 compounds have been isolated from the leaves. The major alkaloid found within the crude extract, mitragynine, has been the subject of many pharmacological studies. In addition to the pharmacological studies, two total syntheses of mitragynine have been published as well as general structure-activity relationships (SARs) with respect to opioid activity.

Sigma receptor agonists: Receptor binding and effects on mesolimbic dopamine neurotransmission assessed by microdialysis

BACKGROUND Subtypes of sigma (σ) receptors, σ₁ and σ₂, can be pharmacologically distinguished, and each may be involved in substance-abuse disorders. σ-Receptor antagonists block cocaine place conditioning and σ-receptor agonists are self-administered in rats that previously self-administered cocaine. Self-administration of abused drugs has been related to increased dopamine (DA) neurotransmission, however, σ-receptor agonist effects on mesolimbic DA are not fully characterized. METHODS Receptor-binding studies assessed affinities of σ-receptor ligands for σ-receptor subtypes and the DA transporter; effects on DA transmission in the rat nucleus accumbens shell were assessed using in vivo microdialysis. RESULTS Cocaine (.1-1.0 mg/kg intravenous [IV]), the nonselective σ(½)-receptor agonist DTG (1.0-5.6 mg/kg IV), and the selective σ₁-receptor agonist PRE-084 (.32-10 mg/kg IV) dose-dependently increased DA to ∼275%, ∼150%, and ∼160% maxima, respectively. DTG-induced stimulation of DA was antagonized by the nonselective σ(½)-receptor antagonist BD 1008 (10 mg/kg intraperitoneal [IP]) and the preferential σ₂-receptor antagonist SN 79 (1-3 mg/kg IP), but not by the preferential σ₁-receptor antagonist, BD 1063 (10-30 mg/kg IP). Neither PRE-084 nor cocaine was antagonized by BD 1063 or BD 1008. CONCLUSIONS σ-Receptor agonists stimulated DA in a brain area critical for reinforcing effects of cocaine. DTG effects on DA appear to be mediated by σ₂-receptors rather than σ₁-receptors. However, DA stimulation by cocaine or PRE-084 does not likely involve σ-receptors. The relatively low potency on DA transmission of the selective σ₁-receptor agonist, PRE-084, and its previously reported potent reinforcing effects, suggest a dopamine-independent reinforcing pathway that may contribute to substance-abuse disorders.

Herbal medicines for the management of opioid addiction: safe and effective alternatives to conventional pharmacotherapy?

Striking increases in the abuse of opioids have expanded the need for pharmacotherapeutic interventions. The obstacles that confront effective treatment of opioid addiction — shortage of treatment professionals, stigma associated with treatment and the ability to maintain abstinence — have led to increased interest in alternative treatment strategies among both treatment providers and patients alike. Herbal products for opioid addiction and withdrawal, such as kratom and specific Chinese herbal medications such as WeiniCom, can complement existing treatments. Unfortunately, herbal treatments, while offering some advantages over existing evidence-based pharmacotherapies, have poorly described pharmacokinetics, a lack of supportive data derived from well controlled clinical trials, and severe toxicity, the cause for which remains poorly defined. Herbal products, therefore, require greater additional testing in rigorous clinical trials before they can expect widespread acceptance in the management of opioid addiction.

The hallucinogen derived from Salvia divinorum, salvinorin A, has κ-opioid agonist discriminative stimulus effects in rats

Data from clinical and preclinical studies converge implicating the plant-derived hallucinogen salvinorin A as an important pharmacologic tool; this psychoactive compound may expand scientific understandings on mammalian kappa-opioid receptor systems. Human salvinorin A effects, consistent with kappa-opioid receptor agonism, include antinociception, sedation, dysphoria and distorted perceptions. The experiments reported here measured salvinorin A (1-3mg/kg, i.p.) discriminative stimulus properties in male Sprague-Dawley rats conditioned to recognize the discriminative stimulus cue generated by the well characterized kappa-opioid agonist U-69593 (0.56 mg/kg, i.p.). At three distinct active doses, salvinorin A fully substituted for U-69593 without altering response rates. The lever choice pattern in U-69593 trained animals reverted to vehicle lever responding when a kappa selective antagonist compound, nor-BNI (4.5 nM, i.c.v.) was administered 1h prior to salvinorin A, yet nor-BNI alone failed to impact the rate or pattern of subject responses. These findings confirm and extend results published after similar drug discrimination tests were performed in rhesus monkeys. The discussion section of this article highlights public concern over salvinorin A misuse and emphasizes several potential pharmacotherapeutic applications for salvinorin A or analogue compounds.

The Sigma-2 Receptor and Progesterone Receptor Membrane Component 1 are Different Binding Sites Derived From Independent Genes

The sigma-2 receptor (S2R) is a potential therapeutic target for cancer and neuronal diseases. However, the identity of the S2R has remained a matter of debate. Historically, the S2R has been defined as (1) a binding site with high affinity to 1,3-di-o-tolylguanidine (DTG) and haloperidol but not to the selective sigma-1 receptor ligand (+)-pentazocine, and (2) a protein of 18–21 kDa, as shown by specific photolabeling with [3H]-Azido-DTG and [125I]-iodoazido-fenpropimorph ([125I]-IAF). Recently, the progesterone receptor membrane component 1 (PGRMC1), a 25 kDa protein, was reported to be the S2R (Nature Communications, 2011, 2:380). To confirm this identification, we created PGRMC1 knockout NSC34 cell lines using the CRISPR/Cas9 technology. We found that in NSC34 cells devoid of or overexpressing PGRMC1, the maximum [3H]-DTG binding to the S2R (Bmax) as well as the DTG-protectable [125I]-IAF photolabeling of the S2R were similar to those of wild-type control cells. Furthermore, the affinities of DTG and haloperidol for PGRMC1 (KI = 472 μM and 350 μM, respectively), as determined in competition with [3H]-progesterone, were more than 3 orders of magnitude lower than those reported for the S2R (20–80 nM). These results clarify that PGRMC1 and the S2R are distinct binding sites expressed by different genes.

A unique binding epitope for salvinorin A, a non-nitrogenous kappa opioid receptor agonist

Salvinorin A is a potent kappa opioid receptor (KOP) agonist with unique structural and pharmacological properties. This non‐nitrogenous ligand lacks nearly all the structural features commonly associated with opioid ligand binding and selectivity. This study explores the structural basis to salvinorin A binding and selectivity using a combination of chimeric and single‐point mutant opioid receptors. The experiments were designed based on previous models of salvinorin A that locate the ligand within a pocket formed by transmembrane (TM) II, VI, and VII. More traditional sites of opioid recognition were also explored, including the highly conserved aspartate in TM III (D138) and the KOP selectivity site E297, to determine the role, if any, that these residues play in binding and selectivity. The results indicate that salvinorin A recognizes a cluster of residues in TM II and VII, including Q115, Y119, Y312, Y313, and Y320. Based on the position of these residues within the receptor, and prior study on salvinorin A, a model is proposed that aligns the ligand vertically, between TM II and VII. In this orientation, the ligand spans residues that are spaced one to two turns down the face of the helices within the receptor cavity. The ligand is also in close proximity to EL‐2 which, based on chimeric data, is proposed to play an indirect role in salvinorin A binding and selectivity.