Effect of Iboga alkaloids on µ-opioid receptor-coupled G protein activation.

OBJECTIVE: The iboga alkaloids are a class of small molecules defined structurally on the basis of a common ibogamine skeleton, some of which modify opioid withdrawal and drug self-administration in humans and preclinical models. These compounds may represent an innovative approach to neurobiological investigation and development of addiction pharmacotherapy. In particular, the use of the prototypic iboga alkaloid ibogaine for opioid detoxification in humans raises the question of whether its effect is mediated by an opioid agonist action, or if it represents alternative and possibly novel mechanism of action. The aim of this study was to independently replicate and extend evidence regarding the activation of µ-opioid receptor (MOR)-related G proteins by iboga alkaloids. METHODS: Ibogaine, its major metabolite noribogaine, and 18-methoxycoronaridine (18-MC), a synthetic congener, were evaluated by agonist-stimulated guanosine-5´-O-(γ-thio)-triphosphate ([(35)S]GTPγS) binding in cells overexpressing the recombinant MOR, in rat thalamic membranes, and autoradiography in rat brain slices. RESULTS AND SIGNIFICANCE: In rat thalamic membranes ibogaine, noribogaine and 18-MC were MOR antagonists with functional Ke values ranging from 3 uM (ibogaine) to 13 uM (noribogaine and 18MC). Noribogaine and 18-MC did not stimulate [(35)S]GTPγS binding in Chinese hamster ovary cells expressing human or rat MORs, and had only limited partial agonist effects in human embryonic kidney cells expressing mouse MORs. Ibogaine did not did not stimulate [(35)S]GTPγS binding in any MOR expressing cells. Noribogaine did not stimulate [(35)S]GTPγS binding in brain slices using autoradiography. An MOR agonist action does not appear to account for the effect of these iboga alkaloids on opioid withdrawal. Taken together with existing evidence that their mechanism of action also differs from that of other non-opioids with clinical effects on opioid tolerance and withdrawal, these findings suggest a novel mechanism of action, and further justify the search for alternative targets of iboga alkaloids.

Neuropharmacology of the naturally occurring kappa-opioid hallucinogen salvinorin A.

Salvia divinorum is a perennial sage native to Oaxaca, Mexico, that has been used traditionally in divination rituals and as a treatment for the "semimagical" disease panzón de borrego. Because of the intense "out-of-body" experiences reported after inhalation of the pyrolized smoke, S. divinorum has been gaining popularity as a recreational hallucinogen, and the United States and several other countries have regulated its use. Early studies isolated the neoclerodane diterpene salvinorin A as the principal psychoactive constituent responsible for these hallucinogenic effects. Since the finding that salvinorin A exerts its potent psychotropic actions through the activation of KOP receptors, there has been much interest in elucidating the underlying mechanisms behind its effects. These effects are particularly remarkable, because 1) salvinorin A is the first reported non-nitrogenous opioid receptor agonist, and 2) its effects are not mediated by the 5-HT(2A) receptor, the classic target of hallucinogens such as lysergic acid diethylamide and mescaline. Rigorous investigation into the structural features of salvinorin A responsible for opioid receptor affinity and selectivity has produced numerous receptor probes, affinity labels, and tools for evaluating the biological processes responsible for its observed psychological effects. Salvinorin A has therapeutic potential as a treatment for pain, mood and personality disorders, substance abuse, and gastrointestinal disturbances, and suggests that nonalkaloids are potential scaffolds for drug development for aminergic G-protein coupled receptors.

N-desalkylquetiapine, a potent norepinephrine reuptake inhibitor and partial 5-HT1A agonist, as a putative mediator of quetiapine's antidepressant activity.

Quetiapine is an atypical antipsychotic drug that is also US FDA approved for treating bipolar depression, albeit by an unknown mechanism. To discover the potential mechanism for this apparently unique action, we screened quetiapine, its metabolite N-Desalkylquetiapine, and dibenzo[b,f][1,4]thiazepine-11(10-H)-one (DBTO) against a large panel of G-protein-coupled receptors, ion channels, and neurotransmitter transporters. DBTO was inactive at all tested molecular targets. N-Desalkylquetiapine had a high affinity (3.4 nM) for the histamine H(1) receptor and moderate affinities (10-100 nM) for the norepinephrine reuptake transporter (NET), the serotonin 5-HT(1A), 5-HT(1E), 5-HT(2A), 5-HT(2B), 5-HT(7) receptors, the alpha(1B)-adrenergic receptor, and the M(1), M(3), and M(5) muscarinic receptors. The compound had low affinities (100-1000 nM) for the 5-HT(1D), 5-HT(2C), 5-HT(3), 5-HT(5), 5-HT(6), alpha(1A), alpha(2A), alpha(2B), alpha(2C), H(2), M(2), M(4), and dopamine D(1), D(2), D(3), and D(4) receptors. N-Desalkylquetiapine potently inhibited human NE transporter with a K(i) of 12 nM, about 100-fold more potent than quetiapine itself. N-Desalkylquetiapine was also 10-fold more potent and more efficacious than quetiapine at the 5-HT(1A) receptor. N-Desalkylquetiapine was an antagonist at 5-HT(2A), 5-HT(2B), 5-HT(2C), alpha(1A), alpha(1D), alpha(2A), alpha(2C), H(1), M(1), M(3), and M(5) receptors. In the mouse tail suspension test, N-Desalkylquetiapine displayed potent antidepressant-like activity in VMAT2 heterozygous mice at doses as low as 0.1 mg/kg. These data strongly suggest that the antidepressant activity of quetiapine is mediated, at least in part, by its metabolite N-Desalkylquetiapine through NET inhibition and partial 5-HT(1A) agonism. Possible contributions of this metabolite to the side effects of quetiapine are discussed.

Synthetic studies of neoclerodane diterpenes from Salvia divinorum: exploration of the 1-position.

Modification of the C-1 ketone of salvinorin A (2a) produces analogues with opioid antagonist properties. Of particular significance is the finding that 1-deoxo-1,10-dehydrosalvinorin A (11a) is a moderately potent antagonist at all three opioid receptor subtypes, and that herkinorin (2b), a mu agonist, is converted to a weak antagonist by removal of the C-1 ketone (3b and 11b). These observations suggest that the ketone of 2b is a key structural feature responsible for mu agonist activity.

Synthesis of salvinorin A analogues as opioid receptor probes.

Several neoclerodanes, such as salvinorin A (1) and herkinorin (3), have recently been shown to possess opioid receptor activity in vitro and in vivo. To explore the structure-affinity relationships of this interesting class of compounds, we have synthesized a series of analogues from 1 isolated from Salvia divinorum. Here, we report the semisynthesis of neoclerodane diterpenes and their structure-affinity relationships at opioid receptors. This work will allow the further development of novel opioid receptor ligands.

Design and synthesis of noncompetitive metabotropic glutamate receptor subtype 5 antagonists.

A series of diaryl amides was designed and synthesized as novel nonethynyl mGluR5 antagonists. The systematic variation of the pharmacophoric groups led to the identification of a lead compound that demonstrated micromolar affinity for the mGluR5. Further optimization resulted in compounds with improved binding affinities and antagonist profiles, in vitro.

Synthetic studies of neoclerodane diterpenes from Salvia divinorum: semisynthesis of salvinicins A and B and other chemical transformations of salvinorin A.

Salvinorin A (1) is a hallucinogenic neoclerodane diterpene isolated from the widely available psychoactive plant Salvia divinorum and is the first example of a non-nitrogenous opioid receptor ligand. At present, there is little information available as to why this compound is selective for kappa opioid receptors. One approach to better understanding the mode of binding of 1 at kappa receptors is to systematically alter the structure of 1 and examine the effects on opioid receptor affinity and activity. Currently, there is a paucity of methods described for the preparation of analogues derived from 1. Here, we report the investigation of several chemical transformations of 1 isolated from S. divinorum. In particular, this work provides a semisynthesis of salvinicins A (2) and B (3) and has identified 10a as the first neoclerodane diterpene with delta opioid antagonist activity.

Targeted screening for biogenic amine transporters: potential applications for natural products.

The biogenic amine transporters (BATs) are integral membrane proteins that terminate the actions of dopamine (DA), serotonin (5-HT) and norepinephrine (NE) by pumping these substrates from the extracellular space back into the nerve terminal. Numerous drugs and medications target BATs, acting as inhibitors or substrates. This paper will review some of the methods used to measure the activity of test drugs at the BATs. These methods include traditional uptake inhibition assays and transporter binding assays, as well as methods developed in our lab to determine if a test agent is a BAT substrate or inhibitor. Newer methods, developed in our lab, are used to determine the potency of test drugs as BAT substrates in a relatively high throughput manner. The potential application of these methods to characterizing natural products will be discussed in reference to results obtained with "purified" natural products, such as ephedrine stereoisomers.

Salvinicins A and B, new neoclerodane diterpenes from Salvia divinorum.

[reaction: see text] Two new neoclerodane diterpenes, salvinicins A (4) and B (5), were isolated from the dried leaves of Salvia divinorum. The structures of these compounds were elucidated by spectroscopic techniques, including (1)H and (13)C NMR, NOESY, HMQC, and HMBC. The absolute stereochemistry of these compounds was assigned on the basis of single-crystal X-ray crystallographic analysis of salvinicin A (4) and a 3,4-dichlorobenzoate derivative of salvinorin B.