Comparison of the behavioral effects of ibogaine from three sources: mediation of discriminative activity

Microdosing psychedelics: More questions than answers? An overview and suggestions for future research

BACKGROUND

In the past few years, the issue of 'microdosing' psychedelics has been openly discussed in the public arena where claims have been made about their positive effect on mood state and cognitive processes such as concentration. However, there are very few scientific studies that have specifically addressed this issue, and there is no agreed scientific consensus on what microdosing is.

AIM

This critique paper is designed to address questions that need to be answered by future scientific studies and to offer guidelines for these studies.

APPROACH

Owing to its proximity for a possible approval in clinical use and short-lasting pharmacokinetics, our focus is predominantly on psilocybin. Psilocybin is allegedly, next to lysergic acid diethylamide (LSD), one of the two most frequently used psychedelics to microdose. Where relevant and available, data for other psychedelic drugs are also mentioned.

CONCLUSION

It is concluded that while most anecdotal reports focus on the positive experiences with microdosing, future research should also focus on potential risks of (multiple) administrations of a psychedelic in low doses. To that end, (pre)clinical studies including biological (e.g. heart rate, receptor turnover and occupancy) as well as cognitive (e.g. memory, attention) parameters have to be conducted and will shed light on the potential negative consequences microdosing could have.

The developing utility of zebrafish models for cognitive enhancers research

Whereas cognitive impairment is a common symptom in multiple brain disorders, predictive and high-throughput animal models of cognition and behavior are becoming increasingly important in the field of translational neuroscience research. In particular, reliable models of the cognitive deficits characteristic of numerous neurobehavioral disorders such as Alzheimer’s disease and schizophrenia have become a significant focus of investigation. While rodents have traditionally been used to study cognitive phenotypes, zebrafish (Danio rerio) are gaining popularity as an excellent model to complement current translational neuroscience research. Here we discuss recent advances in pharmacological and genetic approaches using zebrafish models to study cognitive impairments and to discover novel cognitive enhancers and neuroprotective mechanisms.

Serotonergic hallucinogens and emerging targets for addiction pharmacotherapies

Only time will tell if serotonergic hallucinogen-assisted psychotherapy treatment paradigms for SUDs will prove to be safe and effective in double-blind, placebo-controlled clinical trials. If they are, they would truly constitute a novel psychopharmacologic-psychosocial treatment paradigm to treat addictive disorders, although the risk of adverse psychological events would have to be controlled through a careful screening process and the risk of misuse of the substances or developing use syndromes would have to be considered, although the overall risk would be low because, as mentioned, SHs are unlike all other drugs of abuse in that they do not appear to produce dependence syndromes. There effects on the NA and DA range from inhibition to slight activation, all this without producing addiction. The ability of these medicinal tools to treat a range of addictive, psychiatric, and existential disorders is remarkable in scope and possibility. They truly represent a potential paradigmatic shift within the field of psychiatry, too interesting to not explore further.

Ibogaine, an anti-addictive drug: pharmacology and time to go further in development. A narrative review

Ibogaine is an indole alkaloid derived from the bark of the root of the African shrub Tabernanthe iboga. Psychoactive properties of ibogaine have been known for decades. More recently, based on experimental data from animals and anectodal reports in human, it has been found that this drug has anti-addictive effects. Several patents were published between 1969 and 1995. The pharmacology of ibogaine is quite complex, affecting many different neurotransmitter systems simultaneously. However, the pharmacological targets underlying the physiological and psychological actions of ibogaine are not completely understood. Ibogaine is rapidly metabolized in the body in noribogaine. The purpose of this article was to review data from the literature concerning physicochemical properties, bio-analytical methods, and pharmacology of ibogaine; this article will be focused on the use of this drug as anti-addictive agent.

Mechanisms of Antiaddictive Actions of Ibogainea

Ibogaine, an alkaloid extracted from Tabemanthe iboga, is being studied as a potential long-acting treatment for oploid and stimulant abuse as well as for alcoholism and smoking. Studies in this laboratory have used animal models to characterize ibogaine's interactions with drugs of abuse, and to investigate the mechanisms responsible. Ibogaine, as well as its metabolite, noribogaine, can decrease both morphine and cocaine self-administration for several days in some rats; shorter-lasting effects appear to occur on ethanol and nicotine intake. Acutely, both ibogaine and noribogaine decrease extracellular levels of dopamine in the nucleus accumbens of the rat brain. Ibogaine pretreatment (19 hours beforehand) blocks morphine-induced dopamine release and morphine-induced locomotor hyperactivity while, in contrast, it enhances similar effects of stimulants (cocaine and amphetamine). Ibogaine pretreatment also blocks nicotine-induced dopamine release. Both ibogaine and noribogaine bind to kappa opioid and N-methyl-d-aspartate (NMDA) receptors and to serotonin uptake sites; ibogaine also binds to sigma-2 and nicotinic receptors. The relative contributions of these actions are being assessed. Our ongoing studies in rats suggest that kappa agonist and NMDA antagonist actions contribute to ibogaine's effects on opioid and stimulant self-administration, while the serotonergic actions may be more important for ibogaine-induced decreases in alcohol intake. A nicotinic antagonist action may mediate ibogaine-induced reduction of nicotine preferences in rats. A sigma-2 action of ibogaine appears to mediate its neurotoxicity. Some effects of ibogaine (e.g., on morphine and cocaine self-administration, morphine-induced hyperactivity, cocaine-induced increases in nucleus accumbens dopamine) are mimicked by a kappa agonist (U50,488) and/or a NMDA antagonist (MK-801). Moreover, a combination of a kappa antagonist and a NMDA agonist will partially reverse several of ibogaine's effects. Ibogaine's long-term effects may be mediated by slow release from fat tissue (where ibogaine is sequestered) and conversion to noribogaine. Different receptors, or combinations of receptors, may mediate interactions of ibogaine with different drugs of abuse.

Medication Development of Ibogaine as a Pharmacotherapy for Drug Dependencea

The potential for deriving new psychotherapeutic medications from natural sources has led to renewed interest in rain forest plants as a source of lead compounds for the development of antiaddiction medications. Ibogaine is an indole alkaloid found in the roots of Tabernanthe iboga (Apocynaceae family), a rain forest shrub that is native to equatorial Africa. Ibogaine is used by indigenous peoples in low doses to combat fatigue, hunger and in higher doses as a sacrament in religious rituals. Members of American and European addict self-help groups have claimed that ibogaine promotes long-term drug abstinence from addictive substances, including psychostimulants and cocaine. Anecdotal reports attest that a single dose of ibogaine eliminates withdrawal symptoms and reduces drug cravings for extended periods of time. The purported antiaddictive properties of ibogaine require rigorous validation in humans. We have initiated a rising tolerance study using single administration to assess the safety of ibogaine for the treatment of cocaine dependency. The primary objectives of the study are to determine safety, pharmacokinetics and dose effects, and to identify relevant parameters of efficacy in cocaine-dependent patients. Pharmacokinetic and pharmacodynamic characteristics of ibogaine in humans are assessed by analyzing the concentration-time data of ibogaine and its desmethyl metabolite (noribogaine) from the Phase I trial, and by conducting in vitro experiments to elucidate the specific disposition processes involved in the metabolism of both parent drug and metabolite. The development of clinical safety studies of ibogaine in humans will help to determine whether there is a rationale for conducting efficacy trials in the future.

Drug discrimination studies with ibogaine

The results of the studies described here support the hypothesis that ibogaine produces its effects via selective interactions with multiple receptors. It appears that 5-HT2A, 5-HT2C, and sigma 2 receptors are involved in mediating the stimulus effects of ibogaine. In addition, opiate receptors may also be involved. In contrast, sigma 1, PCP/MK-801, 5-HT3, and 5-HT1A receptors do not appear to play a major role. Ibogaine's hallucinogenic effects may be explained by its interactions with 5-HT2A and 5-HT2C receptors, while its putative antiaddictive properties may result from its interactions with sigma 2 and opiate receptors. Alternatively, the possibility that ibogaine's hallucinogenic properties underlie its antiaddictive effects, as previously suggested (34), would support a role for 5-HT2 receptors in mediating the reported therapeutic effects of ibogaine. Certainly many questions remain regarding ibogaine's mechanism of action. Although drug discrimination will be useful for answering some of those questions, the true potential of this technique is realized whin it is combined with other techniques. The next few years promise to be fruitful with respect to our understanding of this agent. Reasons supporting this belief include advances in the study of sigma receptors, interest in ibogaine's effects on second messenger systems, and the development of ibogaine congeners such as 18-methoxycoronaridine (35). In conclusion, the aforementioned studies should serve to guide further endeavors. Pertinent questions have been generated: What is the role of sigma receptors in the effects of ibogaine, especially with regard to addiction? How does ibogaine affect opiate neurotransmission? What effects, if any, do the Harmala alkaloids have on addiction phenomena? What is the mechanism of action of harmaline? Can 10-hydroxyibogamine serve as a discriminative stimulus and, if so, what receptor interactions mediate its stimulus effects? Does the ibogaine-trained stimulus generalize to novel agents, including 18-methoxycoronaridine?

Noribogaine (12-hydroxyibogamine): a biologically active metabolite of the antiaddictive drug ibogaine

Ibogaine (IBO) is a plant-derived alkaloid that is being evaluated as a possible medication for substance use disorders. When administered peripherally to monkeys and humans, IBO is rapidly converted to an o-demethylated metabolite, 12-hydroxyibogamine (NORIBO). We have found in rats that peak blood levels of NORIBO can exceed those of the parent compound, and NORIBO persists in the bloodstream for at least 24 h. Surprisingly few studies have examined the in vivo biological activity of NORIBO. In the present series of experiments, we compared the effects of intravenous (i.v.) administration of IBO and NORIBO (1 and 10 mg/kg) on unconditioned behaviors, circulating stress hormones, and extracellular levels of dopamine (DA) and serotonin (5-HT) in the nucleus accumbens of male rats. IBO caused dose-related increases in tremors and forepaw treading, whereas NORIBO did not. Both IBO and NORIBO produced significant elevations in plasma corticosterone and prolactin, but IBO was more potent as a stimulator of corticosterone secretion. Neither drug affected extracellular DA levels in the nucleus accumbens. However, both IBO and NORIBO increased extracellular 5-HT levels, and NORIBO was more potent in this regard. The present data demonstrate that NORIBO is biologically active and undoubtedly contributes to the in vivo pharmacological profile of IBO in rats. Most importantly, NORIBO appears less likely to produce the adverse effects associated with IBO (i.e., tremors and stress-axis activation), suggesting that the metabolite may be a safer alternative for medication development.

Comparative neurobiological effects of ibogaine and MK-801 in rats

Ibogaine is a plant-derived alkaloid with putative 'anti-addictive' properties. Although ibogaine binds to multiple targets in the brain, recent evidence suggests the drug acts as an N-methyl-D-aspartate (NMDA) antagonist similar to MK-801. The purpose of the present study was to compare neurochemical and neuroendocrine effects of ibogaine and MK-801 in vivo. Male rats received either i.p. saline, ibogaine (10 and 100 mg/kg), or MK-801 (0.1 and 1 mg/kg). Groups of rats (N=6-8/group) were decapitated 30 or 60 min after injection. Brains were harvested for analysis of dopamine (DA) and its metabolites, while trunk blood was collected for analysis of plasma corticosterone and prolactin. Ibogaine produced marked dose-dependent reductions in tissue DA with concurrent increases in the metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA). This profile of ibogaine-induced effects on DA metabolism was consistently observed in the cortex, striatum, olfactory tubercle, and hypothalamus. MK-801, on the other hand, did not reduce DA levels in any brain region but did cause modest region-specific elevations in DA metabolites. Ibogaine and MK-801 caused comparable elevations in circulating corticosterone, but only ibogaine increased prolactin. The present findings show that the effects of ibogaine on DA neurotransmission and neuroendocrine secretion are not fully mimicked by MK-801. Thus, the wide spectrum of in vivo actions of ibogaine can probably not be explained simply on the basis of antagonism at NMDA receptors.

The acute effects of monoamine reuptake inhibitors on the stimulus effects of hallucinogens

In a previous study it was observed that fluoxetine potentiates the stimulus effects of lysergic acid diethylamide (LSD). In the present investigation, stimulus control was established in groups of rats using as training drugs the hallucinogens lysergic acid diethylamide (LSD); 0.1 mg/kg), (-)-2,5-dimethoxy-4-methylamphetamine [(-)-DOM; 0.56 mg/kg], ibogaine (10 mg/kg), and 5-methoxy-N,N-dimethyltryptamine (5-MeO-DMT; 3 mg/kg). A two-lever, fixed-ratio 10, positively reinforced task with saline controls was employed. The hypotheses tested were that (a) monoamine uptake inhibitors other than fluoxetine potentiate the discriminative effects of LSD, and (b) hallucinogens other than LSD are potentiated by acute pretreatment with monoamine uptake inhibitors. The effects of a range of doses of each of the training drugs were determined both alone and following pretreatment with the monoamine reuptake inhibitors fluoxetine, fluvoxamine, and venlafaxine. In LSD-trained subjects, all three reuptake inhibitors caused a significant increase in LSD-appropriate responding. Similar results were observed in rats trained with (-)-DOM and with ibogaine. In 5-MeO-DMT-trained subjects, only fluoxetine resulted in an enhancement of drug-appropriate responding. The reuptake inhibitors given alone elicited varying degrees of responses appropriate for the respective training drugs. For fluoxetine in rats trained with LSD and ibogaine, for venlafaxine in LSD trained, and for fluvoxamine in (-)-DOM trained, the degree of responding met our criterion for intermediate responding, i.e., significantly different from both training conditions. Subsequent experiments in (-)-DOM-trained subjects examined a range of doses of each of the reuptake inhibitors in combination with a fixed dose of (-)-DOM (0.1 mg/kg), which alone yielded about 50% (-)-DOM-appropriate responding. With the exception of the point obtained with the highest dose of venlafaxine, all data were compatible with additivity of effects rather than true potentiation. In summary, the present data extend our previous observation of the augmentation of the stimulus effects of LSD by fluoxetine to include other hallucinogens. The mechanisms by which these interactions arise and possible differential effects of acute and chronic treatment remain to be established.

Differential responses by neurotensin systems in extrapyramidal and limbic structures to ibogaine and cocaine

Ibogaine (Endabuse) is a psychoactive indole alkaloid found in the West African shrub, Tabernanthe iboga. This drug interrupts cocaine and amphetamine abuse and has been proposed for treatment of addiction to these stimulants. However, the mechanism of action that explains its pharmacological properties is unclear. Since previous studies demonstrated differential effects of psychotomimetic drugs (cocaine and methamphetamine) on neuropeptides such as neurotensin (NT), the present study was designed to determine: (1) the effects of ibogaine on striatal, nigral, cortical, and accumbens neurotensin-like immunoreactivity (NTLI); (2) the effects of selective dopamine antagonists on ibogaine-induced changes in NT concentrations in these brain areas; and (3) the effects of ibogaine pretreatment on cocaine-induced changes in striatal, nigral, cortical and accumbens NTLI content. Ibogaine treatments profoundly affected NT systems by increasing striatal, nigral, and accumbens NTLI content 12 h after the last drug administration. In contrast, NTLI concentrations were not significantly increased in the frontal cortex after ibogaine treatment. The ibogaine-induced increases in NTLI in striatum, nucleus accumbens and substantia nigra were blocked by coadministration of the selective D1 receptor antagonist, SCH 23390. The D2 receptor antagonist, eticlopride, blocked the ibogaine-induced increase in nigral NTLI, but not in striatum and nucleus accumbens. Ibogaine pretreatment significantly blocked the striatal and nigral increases of NTLI resulting from a single cocaine administration. Whereas many of the responses by NT systems to ibogaine resembled those which occur after cocaine, there were also some important differences. These data suggest that NT may contribute to an interaction between ibogaine and the DA system and may participate in the pharmacological actions of this drug.

Further investigations of the serotonergic properties of the ibogaine-induced discriminative stimulus

1. 5-HT3, 5-HT2C, and 5-HT1A receptor ligands were assessed in rats trained to discriminate ibogaine from water. 2. Significant ibogaine-appropriate responding was observed following treatment with the 5-HT2C agonists MK-212 (79.6%) and mCPP (76.4%). This substitution was completely antagonized by metergoline, an agent with 5-HT2C antagonist properties. However, metergoline was ineffective against ibogaine itself. This suggests that although ibogaine may act as an agonist at 5-HT2C receptors, this interaction is not essential to its discriminative cue. 3. Neither the 5-HT3 agonist, mCPBG (44.3%), nor the 5-HT3 antagonist, ondansetron (48.9%) substituted for ibogaine. Likewise, the 5-HT1A agonist 8-OH-DPAT (34.7%) and the 5-HT1A antagonist WAY-100635 (30.1%) failed to substitute. Furthermore, WAY-100635 failed to antagonize the ibogaine cue. 4. Unlike 5-HT2C receptors, 5-HT1A and 5-HT3 receptors do not appear to be involved in the ibogaine stimulus.

Acute iboga alkaloid effects on extracellular serotonin (5-HT) levels in nucleus accumbens and striatum in rats

The iboga alkaloid, ibogaine, its metabolite, noribogaine, and the congener, 18-methoxycoronaridine (18-MC) have all been claimed to have anti-addictive properties in animal models, but the mechanisms underlying these effects are unclear. Ibogaine and noribogaine were shown to have affinity for the serotonin transporter, and inhibition of serotonin reuptake has been proposed to be involved in their anti-addictive actions. It is not known yet if 18-MC also has this property. In vivo microdialysis and HPLC (microbore) were used to determine acute changes in extracellular serotonin levels in nucleus accumbens (NAC) and striatum (STR) after both i.p. (40 mg/kg for all drugs) and i.v. (1-10 mg/kg for ibogaine and noribogaine) drug administration in awake freely moving female Sprague-Dawley rats (250-275 g). After i.p. administration, ibogaine, noribogaine and 18-MC had very different effects on extracellular serotonin levels in both NAC and STR: ibogaine elicited large increases (up to 25-fold in NAC and 10- fold in STR), noribogaine produced moderate increases (up to 8-fold in NAC and 5-fold in STR), and 18-MC had no effect in either brain region. These and other data suggest that (1) the serotonergic system may not be an essential factor in the anti-addictive actions of these drugs; (2) ibogaine (or an unidentified metabolite) may release serotonin as well as inhibit its reuptake; (3) stimulation of the ascending serotonergic system may mediate ibogaine's hallucinogenic effect; and (4) 18-MC probably has no affinity for the serotonin transporter, and is unlikely to be a hallucinogen.

Effects of chronic ibogaine treatment on cerebellar Purkinje cells in the rat

The present investigation assessed the chronic toxicity of ibogaine on cerebellar Purkinje cells in male Fischer 344 rats. A behaviorally active dose of ibogaine (10 mg/kg, i.p.) was administered to a group of six subjects every other day for 60 days while the control group received an equivalent volume of saline (1 ml/kg). Estimates of Purkinje cell number were determined using the optical dissector/fractionator technique. No significant differences in Purkinje cell number were observed between the ibogaine (243764[+/-32766]) and control groups (230813[+/-16670]).

The effects of noribogaine and harmaline in rats trained with ibogaine as a discriminative stimulus

In the present investigation, Fischer-344 rats were trained to discriminate 10.0 mg/kg of ibogaine from water using a pretreatment time of 60 minutes. Analysis of dose response data generated an ED50 of 4.6 mg/kg. The time course of the ibogaine (10.0 mg/kg) cue was also determined. The stimulus reached a maximum level of 94% ibogaine-appropriate responding at the 60-min pretreatment time. This was followed by a time-dependent decrease in ibogaine-appropriate responding. At a pretreatment time of 8 hrs only 6.4% drug-appropriate responding was observed. In substitution experiments, intermediate generalization was observed with a metabolite of ibogaine, 12-hydroxyibogamine [noribogaine] (71.6%) whereas complete generalization was seen with harmaline (83.5%).

Neuroendocrine and neurochemical effects of acute ibogaine administration: a time course evaluation

Ibogaine (IBO) is an indole alkaloid that is reported to facilitate drug abstinence in substance abusers. Despite considerable investigation, the mechanism of IBO action in vivo and its suitability as a treatment for drug addiction remains unclear. The present study was designed to evaluate the time-course effects of acute IBO on neuroendocrine and neurochemical indices. Adult male rats were treated with i.p. saline or 50 mg/kg IBO and sacrificed 15, 30, 60, 120 min and 24 h later. Trunk blood was collected for hormone measures and brains were dissected for neurochemical analyses. IBO produced a rapid elevation in plasma prolactin that declined to control levels by 60 min. Corticosterone levels increased 15 min after drug administration, continued to increase for 120 min, but returned to control levels 24 h after dosing. IBO decreased dopamine (DA) concentrations in the striatum and frontal cortex at 30, 60 and 120 min after injection while DA metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), were elevated over the same time period. 24 h after IBO, DOPAC concentrations in striatum and HVA levels in the frontal cortex were below control values. Serotonin (5-HT) and its metabolite 5-hydroxyindole acetic acid (5-HIAA) were decreased at 60 min after IBO administration only in the striatum. These data indicate that a single injection of IBO produces a spectrum of effects that includes: (1) elevation of plasma prolactin and corticosterone, (2) short- and long-term effects on DA neurotransmission, and (3) modest, transient effects of 5-HT neurotransmission. The effects of IBO reported herein may have relevance to the anti-addictive properties of this drug, and this proposal warrants further investigation.

Identification of a primary metabolite of ibogaine that targets serotonin transporters and elevates serotonin

Ibogaine is a hallucinogenic indole with putative efficacy for the treatment of cocaine, stimulant and opiate abuse. The purported efficacy of ibogaine following single dose administrations has led to the suggestion that a long-acting metabolite of ibogaine may explain in part how the drug reduces craving for psychostimulants and opiates. We report here that 12-hydroxyibogamine, a primary metabolite of ibogaine, displays high affinity for the 5-HT transporter and elevates extracellular 5-HT. In radioligand binding assays, 12-hydroxyibogamine was 50-fold more potent at displacing radioligand binding at the 5-HT transporter than at the DA transporter. Ibogaine and 12-hydroxyibogamine were equipotent at the dopamine transporter. In vivo microdialysis was used to evaluate the acute actions of ibogaine and 12-hydroxyibogamine on the levels of DA and 5-HT. Administration of 12-hydroxyibogamine produced a marked dose-related elevation of extracellular 5-HT. Ibogaine and 12-hydroxyibogamine failed to elevate DA levels in the nucleus accumbens over the dose range tested. The elevation in synaptic levels of 5-HT by 12-hydroxyibogamine may heighten mood and attenuate drug craving. The effects of the active metabolite on 5-HT transmission may account in part for the potential of ibogaine to interrupt drug-seeking behavior in humans.

Identification and quantification of the indole alkaloid ibogaine in biological samples by gas chromatography-mass spectrometry

A sensitive and highly selective analytical chemical method for measuring the indole alkaloid ibogaine in biological samples has been developed. The method utilizes organic extraction, derivatization with trifluoroacetic anhydride, and detection by combined gas chromatography-mass spectrometry. The deuterated analog of ibogaine, O-[Cd3]-ibogaine, was synthesized and used as an internal standard for the method. Standard curves, constructed from variable amounts of ibogaine (50-400 ng) and a fixed amount of internal standard (250 ng) were linear. The method has an approximate detection limit of at least 20 ng/mL of tissue extract (180 ng/g tissue), with a coefficient of variation of 8 to 12.5%. Chemical stability studies with the method found that aqueous ibogaine solutions (1-10 mg/mL) could be stored at 10 degrees for up to 7 months with no more than 10% loss. The method was also used to measure brain ibogaine levels in rats 1 and 19 hr after a single dose of drug (40 mg/kg, i.p.); the results suggest a rapid disappearance of the drug after i.p. dosing. The method will help reveal the pharmacokinetic properties of this putative anti-addictive agent in animals and humans.