Hallucinogenic agents as discriminative stimuli: a correlation with serotonin receptor affinities

1-(2,5-Dimethoxy-4-iodophenyl)-2-aminopropane (DOI): From an Obscure to Pivotal Member of the DOX Family of Serotonergic Psychedelic Agents - A Review

1-(2,5-Dimethoxy-4-iodophenyl)-2-aminopropane (DOI, or DOX where X = -I) was first synthesized in 1973 in a structure-activity study to explore the effect of various aryl substituents on the then newly identified, and subsequently controlled, hallucinogenic agent 1-(2,5-dimethoxy-4-methylphenyl)-2-aminopropane (DOM, or DOX where X = -CH3). Over time, DOI was found to be a serotonin (5-HT) receptor agonist using various peripheral 5-HT receptor tissue assays and later, following the identification of multiple families of central 5-HT receptors, an agonist at 5-HT2 serotonin receptors in rat and, then, human brain. Today, classical hallucinogens, currently referred to as serotonergic psychedelic agents, are receiving considerable attention for their potential therapeutic application in various neuropsychiatric disorders including treatment-resistant depression. Here, we review, for the first time, the historical and current developments that led to DOI becoming a unique, perhaps a landmark, agent in 5-HT2 receptor research.

Safety, tolerability, pharmacodynamic and wellbeing effects of SPL026 (dimethyltryptamine fumarate) in healthy participants: a randomized, placebo-controlled phase 1 trial

Background

Due to their potential impact on mood and wellbeing there has been increasing interest in the potential of serotonergic psychedelics such as N,N-dimethyltryptamine (DMT) in the treatment of major depressive disorder (MDD).

Aim

The aim of Part A of this study was to evaluate the safety, tolerability, pharmacokinetics (PK) and pharmacodynamic (PD) profile of escalating doses of SPL026 (DMT fumarate) in psychedelic-naïve healthy participants to determine a dose for administration to patients with MDD in the subsequent Phase 2a part of the trial (Part B: not presented in this manuscript).

Methods

In the Phase 1, randomized, double-blind, placebo-controlled, parallel-group, single dose-escalation trial, psychedelic-naïve participants were randomized to placebo (n = 8) or four different escalating doses [9, 12, 17 and 21.5 mg intravenously (IV)] of SPL026 (n = 6 for each dose) together with psychological support from 2 therapy team members. PK and acute (immediately following dosing experience) psychometric measures [including mystical experience questionnaire (MEQ), ego dissolution inventory (EDI), and intensity rating visual analogue scale (IRVAS)] were determined. Additional endpoints were measured as longer-term change from baseline to days 8, 15, 30 and 90. These measures included the Warwick and Edinburgh mental wellbeing scale and Spielberger's state-trait anxiety inventory.

Results

SPL026 was well tolerated, with an acceptable safety profile, with no serious adverse events. There was some evidence of a correlation between maximum plasma concentration and increased IRVAS, MEQ, and EDI scores. These trends are likely to require confirmation in a larger sample size. Using the analysis of the safety, tolerability, PD, PK results, doses of 21.5 mg SPL026 were the most likely to provide an intense, tolerated experience.

Conclusion

Based on the data obtained from this part of the trial, a dose of 21.5 mg SPL026 given as a 2-phase IV infusion over 10 min (6 mg/5 min and 15.5 mg/5 min) was selected as the dose to be taken into patients in Part B (to be presented in a future manuscript).Clinical trial registration:www.clinicaltrials.gov, identifier NCT04673383; https://www.clinicaltrialsregister.eu, identifier 2020-000251-13; https://www.isrctn.com/, identifier ISRCTN63465876.

A narrative synthesis of research with 5-MeO-DMT

BACKGROUND

5-Methoxy-N,N-dimethyltryptamine (5-MeO-DMT) is a naturally occurring, short-acting psychedelic tryptamine, produced by a variety of plant and animal species. Plants containing 5-MeO-DMT have been used throughout history for ritual and spiritual purposes. The aim of this article is to review the available literature about 5-MeO-DMT and inform subsequent clinical development.

METHODS

We searched PubMed database for articles about 5-MeO-DMT. Search results were cross-checked against earlier reviews and reference lists were hand searched. Findings were synthesised using a narrative synthesis approach. This review covers the pharmacology, chemistry and metabolism of 5-MeO-DMT, as well epidemiological studies, and reported adverse and beneficial effects.

RESULTS

5-MeO-DMT is serotonergic agonist, with highest affinity for 5-HT1A receptors. It was studied in a variety of animal models, but clinical studies with humans are lacking. Epidemiological studies indicate that, like other psychedelics, 5-MeO-DMT induces profound alterations in consciousness (including mystical experiences), with potential beneficial long-term effects on mental health and well-being.

CONCLUSION

5-MeO-DMT is a potentially useful addition to the psychedelic pharmacopoeia because of its short duration of action, relative lack of visual effects and putatively higher rates of ego-dissolution and mystical experiences. We conclude that further clinical exploration is warranted, using similar precautions as with other classic psychedelics.

How to account for hallucinations in the interpretation of the antidepressant effects of psychedelics: a translational framework

RATIONALE

Recent trials with psychedelics in major depressive disorder and treatment-resistant depression showed remarkable improvements in depressive symptoms that can last for up to several months after even a single administration. The lack of an appropriate placebo control group-as patients are often able to discriminate the subjective effects of the drug-and an incomplete understanding of the role of the hallucinogenic and mystical experience, hampers the interpretation of these therapeutic effects.

OBJECTIVES

To control for these factors, we developed a translational framework based on establishing pharmacokinetic/pharmacodynamic (PK/PD) relationships in rodents and humans for hallucinogenic (i.e., discriminative stimulus effects in rodents and humans; head twitch responses in rodents; questionnaires in humans) and therapeutic effects. For the latter, we selected the pattern separation and attentional set-shifting tasks as measures for cognitive flexibility because of their high translational value. We predict that these PK/PD analyses will lead to a more objective evaluation of improvements in patients compared to relying only on the currently used self-reported questionnaires. We hypothesize that-if the role of the hallucinogenic experience is not central in the antidepressant effects of psychedelics-the ED₅₀'s for the therapeutic effects will be significantly lower than for the hallucinogenic and mystical effects.

CONCLUSION

Our framework will help to inform future studies that aim at the elucidation of the mechanism(s) of action of psychedelics in depression, and the role of the acute subjective and/or hallucinogenic experience in their effects.

Chemoenzymatic Synthesis of 5-Methylpsilocybin: A Tryptamine with Potential Psychedelic Activity

A novel analogue of psilocybin was produced by hybrid chemoenzymatic synthesis in sufficient quantity to enable bioassay. Utilizing purified 4-hydroxytryptamine kinase from Psilocybe cubensis, chemically synthesized 5-methylpsilocin (2) was enzymatically phosphorylated to provide 5-methylpsilocybin (1). The zwitterionic product was isolated from the enzymatic step with high purity utilizing a solvent-antisolvent precipitation approach. Subsequently, 1 was tested for psychedelic-like activity using the mouse head-twitch response assay, which indicated activity that was more potent than the psychedelic dimethyltryptamine, but less potent than that of psilocybin.

[Ayahuasca - religion, life-style or drug?]

Ayahuasca is a psychoactive drug which has been used by indigenous cultures in the amazonas basin for hundreds of years for medical and religious purpose. Backpackers who came in contact with ayahuasca exported its use in the western world and increased its popularity. By presenting a case report of a patient seeking medical help due to psychotic symptoms after having attended an ayahuasca ritual we give an short overview of pharmacology, legal status, use and side effects of the substance.

Evaluation of SYA16263 as a new potential antipsychotic agent without catalepsy

SYA16263 exhibited moderate radioligand binding affinity at the D₂ receptor and produced inhibition of apomorphine-induced climbing behavior in mice with an ED₅₀ value of 3.88 mg/kg IP, predicting potential antipsychotic effects in humans. Analysis of plasma and brains from rats injected IP with SYA16263 over the course of 24 h revealed a log [brain]/[plasma] (log BB) at Cmax observed equal to 1.08, indicating that SYA16263 enters the brain and is predicted to cross the blood brain barrier (BBB) readily. When tested in animal behavior tests for catalepsy, SYA16263 did not produce catalepsy at doses up to 19 times the apomorphine ED₅₀ value predicting little or no extra-pyramidal (EPS) side effects in humans. This is similar to aripiprazole, which is associated with a low incidence of EPS in humans, but unlike haloperidol which is known to cause severe EPS in humans. Functional activities for SYA16263 show that it acts as a D₂ agonist at both the Gi and β-arrestin pathways, similar to, but better than aripiprazole, which could account for the absence of the catalepsy observed. Taken together, the receptor binding profile, the functional status, the animal behavioral tests and the log BB value, all provide evidence for further pre-clinical testing of SYA16263 as a potential antipsychotic agent with an interesting profile and a unique mechanism of action resulting in no EPS even up to 19 times the ED₅₀ value.

Dark Classics in Chemical Neuroscience: N, N-Dimethyltryptamine (DMT)

Though relatively obscure, N, N-dimethyltryptamine (DMT) is an important molecule in psychopharmacology as it is the archetype for all indole-containing serotonergic psychedelics. Its structure can be found embedded within those of better-known molecules such as lysergic acid diethylamide (LSD) and psilocybin. Unlike the latter two compounds, DMT is ubiquitous, being produced by a wide variety of plant and animal species. It is one of the principal psychoactive components of ayahuasca, a tisane made from various plant sources that has been used for centuries. Furthermore, DMT is one of the few psychedelic compounds produced endogenously by mammals, and its biological function in human physiology remains a mystery. In this review, we cover the synthesis of DMT as well as its pharmacology, metabolism, adverse effects, and potential use in medicine. Finally, we discuss the history of DMT in chemical neuroscience and why this underappreciated molecule is so important to the field of psychedelic science.

Effects of N, N-Dimethyltryptamine on Rat Behaviors Relevant to Anxiety and Depression

Depression and anxiety disorders are debilitating diseases resulting in substantial economic costs to society. Traditional antidepressants often take weeks to months to positively affect mood and are ineffective for about 30% of the population. Alternatives, such as ketamine, a dissociative anesthetic capable of producing hallucinations, and the psychoactive tisane ayahuasca, have shown great promise due to their fast-acting nature and effectiveness in treatment-resistant populations. Here, we investigate the effects of N, N-dimethyltryptamine (DMT), the principle hallucinogenic component of ayahuasca, in rodent behavioral assays relevant to anxiety and depression using adult, male, Sprague-Dawley rats. We find that while DMT elicits initial anxiogenic responses in several of these paradigms, its long-lasting effects tend to reduce anxiety by facilitating the extinction of cued fear memory. Furthermore, DMT reduces immobility in the forced swim test, which is a characteristic behavioral response induced by many antidepressants. Our results demonstrate that DMT produces antidepressant and anxiolytic behavioral effects in rodents, warranting further investigation of ayahuasca and classical psychedelics as treatments for depression and post-traumatic stress disorder.

Psychedelics Promote Structural and Functional Neural Plasticity

Atrophy of neurons in the prefrontal cortex (PFC) plays a key role in the pathophysiology of depression and related disorders. The ability to promote both structural and functional plasticity in the PFC has been hypothesized to underlie the fast-acting antidepressant properties of the dissociative anesthetic ketamine. Here, we report that, like ketamine, serotonergic psychedelics are capable of robustly increasing neuritogenesis and/or spinogenesis both in vitro and in vivo. These changes in neuronal structure are accompanied by increased synapse number and function, as measured by fluorescence microscopy and electrophysiology. The structural changes induced by psychedelics appear to result from stimulation of the TrkB, mTOR, and 5-HT2A signaling pathways and could possibly explain the clinical effectiveness of these compounds. Our results underscore the therapeutic potential of psychedelics and, importantly, identify several lead scaffolds for medicinal chemistry efforts focused on developing plasticity-promoting compounds as safe, effective, and fast-acting treatments for depression and related disorders.

Receptor binding profiles and behavioral pharmacology of ring-substituted N,N-diallyltryptamine analogs

Substantial effort has been devoted toward understanding the psychopharmacological effects of tryptamine hallucinogens, which are thought to be mediated by activation of 5-HT_2A and 5-HT_1A receptors. Recently, several psychoactive tryptamines based on the N,N-diallyltryptamine (DALT) scaffold have been encountered as recreational drugs. Despite the apparent widespread use of DALT derivatives in humans, little is known about their pharmacological properties. We compared the binding affinities of DALT and its 2-phenyl-, 4-acetoxy-, 4-hydroxy-, 5-methoxy-, 5-methoxy-2-methyl-, 5-fluoro-, 5-fluoro-2-methyl-, 5-bromo-, and 7-ethyl-derivatives at 45 receptor and transporter binding sites. Additionally, studies in C57BL/6 J mice examined whether these substances induce the head twitch response (HTR), a 5-HT_2A receptor-mediated response that is widely used as a behavioral proxy for hallucinogen effects in humans. Most of the test drugs bound to serotonin receptors, σ sites, α₂-adrenoceptors, dopaminergic D₃ receptors, histaminergic H₁ receptors, and the serotonin transporter. DALT and several of the ring-substituted derivatives were active in the HTR assay with the following rank order of potency: 4-acetoxy-DALT > 5-fluoro-DALT > 5-methoxy-DALT > 4-hydroxy-DALT > DALT > 5-bromo-DALT. 2-Phenyl-DALT, 5-methoxy-2-methyl-DALT, 5-fluoro-2-methyl-DALT, and 7-ethyl-DALT did not induce the HTR. HTR potency was not correlated with either 5-HT_1A or 5-HT_2A receptor binding affinity, but a multiple regression analysis indicated that 5-HT_2A and 5-HT_1A receptors make positive and negative contributions, respectively, to HTR potency (R² = 0.8729). In addition to supporting the established role of 5-HT_2A receptors in the HTR, these findings are consistent with evidence that 5-HT_1A activation by tryptamine hallucinogens buffers their effects on HTR. This article is part of the Special Issue entitled 'Psychedelics: New Doors, Altered Perceptions'.

Comparison of the discriminative stimulus effects of dimethyltryptamine with different classes of psychoactive compounds in rats

RATIONALE

There has been increased recreational use of dimethyltryptamine (DMT), but little is known of its discriminative stimulus effects.

OBJECTIVES

The present study assessed the similarity of the discriminative stimulus effects of DMT to other types of hallucinogens and to psychostimulants.

METHODS

Rats were trained to discriminate DMT from saline. To test the similarity of DMT to known hallucinogens, the ability of (+)-lysergic acid diethylamide (LSD), (-)-2,5-dimethoxy-4-methylamphetamine (DOM), (+)-methamphetamine, or (+/-)3,4-methylenedioxymethyl amphetamine (MDMA) to substitute in DMT-trained rats was tested. The ability of DMT to substitute in rats trained to discriminate each of these compounds was also tested. To assess the degree of similarity in discriminative stimulus effects, each of the compounds was tested for substitution in all of the other training groups.

RESULTS

LSD, DOM, and MDMA all fully substituted in DMT-trained rats, whereas DMT fully substituted only in DOM-trained rats. Full cross-substitution occurred between DMT and DOM, LSD and DOM, and (+)-methamphetamine and MDMA. MDMA fully substituted for (+)-methamphetamine, DOM, and DMT, but only partially for LSD. In MDMA-trained rats, LSD and (+)-methamphetamine fully substituted, whereas DMT and DOM did not fully substitute. No cross-substitution was evident between (+)-methamphetamine and DMT, LSD, or DOM.

CONCLUSIONS

DMT produces discriminative stimulus effects most similar to those of DOM, with some similarity to the discriminative stimulus effects of LSD and MDMA. Like DOM and LSD, DMT seems to produce predominately hallucinogenic-like discriminative stimulus effects and minimal psychostimulant effects, in contrast to MDMA which produced hallucinogen- and psychostimulant-like effects.

The roles of 5-HT1A and 5-HT2 receptors in the effects of 5-MeO-DMT on locomotor activity and prepulse inhibition in rats

RATIONALE

The hallucinogen 5-methoxy-N,N-dimethyltryptamine (5-MeO-DMT) is structurally similar to other indoleamine hallucinogens such as LSD. The present study examined the effects of 5-MeO-DMT in rats using the Behavioral Pattern Monitor (BPM), which enables analyses of patterns of locomotor activity and exploration, and the prepulse inhibition of startle (PPI) paradigm.

OBJECTIVES

A series of interaction studies using the serotonin (5-HT)(1A) antagonist WAY-100635 (1.0 mg/kg), the 5-HT(2A) antagonist M100907 (1.0 mg/kg), and the 5-HT(2C) antagonist SER-082 (0.5 mg/kg) were performed to assess the respective contributions of these receptors to the behavioral effects of 5-MeO-DMT (0.01, 0.1, and 1.0 mg/kg) in the BPM and PPI paradigms.

RESULTS

5-MeO-DMT decreased locomotor activity, investigatory behavior, the time spent in the center of the BPM chamber, and disrupted PPI. All of these effects were antagonized by WAY-100635 pretreatment. M100907 pretreatment failed to attenuate any of these effects, while SER-082 pretreatment only antagonized the PPI disruption produced by 5-MeO-DMT.

CONCLUSIONS

While the prevailing view was that the activation of 5-HT(2) receptors is solely responsible for hallucinogenic drug effects, these results support a role for 5-HT(1A) receptors in the effects of the indoleamine hallucinogen 5-MeO-DMT on locomotor activity and PPI in rats.

Serotonin receptor subtype mediation of the interoceptive discriminative stimuli induced by 5-methoxy-N,N-dimethyltryptamine

Male Wistar rats were trained to discriminate the interoceptive effects of 5-methoxy-N,N-dimethyltryptamine (5-OMe-DMT; 1.25 mg/kg, IP) from saline in a two-lever operant chamber. Following discrimination learning, the following drugs (with ED50 dose in mg/kg IP) dose-dependently generalized: lysergic acid diethylamide (LSD, 0.04), 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT, 0.11), 6-methoxy-4-(dipropyl-amino)-1,3,4,5-tetrahydrobenz(c,d)indole hydrochloride (BAY R 1531, 0.15), 5-OMe-DMT itself (0.63), ipsapirone (TVX Q 7821, 2.7), and buspirone (3.8). The potencies of these drugs in generalization tests were best correlated with their binding affinities for the 5-HT1A serotonin receptor subtype (as measured by displacement of 3H-ipsapirone in the hippocampus). Drugs not, or only partially generalizing included quipazine, bufotenin, m-trifluoromethylphenylpiperazine (TFMPP), 5-methoxy-3(1,2,3,6-tetrahydropyridine-4-yl)-1H-indole succinate (RU 24969), citalopram, clomipramine, 1,4-dihydro-2,6-dimethyl-3-nitro-4(2-trifluoromethylphenyl)-pyridine-5- carboxylate (BAY K 8644), the buspirone metabolite 1-pyrimidinyl-piperazine (1-PP), methysergide, metergoline, and metitepine. Of the last three compounds with antagonistic activity at 5-HT receptors, as well as ketanserin, pizotifen, and ritanserin, only metitepine and pindolol could fully block the 5-OMe-DMT stimulus. Pizotifen blocked the generalization of quipazine fully, that of 5-OMe-DMT only partially, and that of ipsapirone not at all. These data indicate that the 5-HT1A receptor subtype is strongly involved in the transduction of the interoceptive discriminative stimuli induced by 5-OMe-DMT, with 5-HT2 agonism also playing a possible role.

Drug discrimination in neurobiology

Areas of neurobiological interest are identified towards which drug discrimination (DD) studies have made important contributions. DD allows ligand actions to be analyzed at the whole organism level, with a neurobiological specificity that is exquisite and often unrivalled. DD analyses have thus been made of a vast array of CNS agents acting on receptors, enzymes, or ion channels, including most drugs of abuse. DD uniquely offers access to the study of subjective drug effects in animals, using a methodology that also is transposable to humans and has generated unprecedented models of pathology (e.g., chronic pain, opiate addiction). Parametric studies of such independent variables as training dose and reinforcement provide refined insights into the dynamic psychophysiological mechanisms of both drug effects and behavior. Three different mechanisms have been identified by which discriminative, and perhaps other behaviors, can come about. DD also is superbly sensitive to small, partial activation of molecular substrates; this has enabled DD analyses to pioneer the unravelling of molecular mechanisms of drug action (attributing, f.ex., LSD's particular subjective effects to an unusual, partial activation of 5-HT, and perhaps other receptors). DD has both oriented and served as a tool to conduct drug discovery research (e.g., pirenperone-risperidone, loperamide). The DD response arguably constitutes a quantal, rather than graded, variable, and as such allows a comprehension of molecular, pharmacological, and behavioral mechanisms that would have been otherwise inaccessible. Perhaps most important are the following further contributions. One is the notion that particular, different levels of receptor activation are associated with qualities of neurobiological actions that also differ and are unique, this notion arguably constituting the most significant addition to affinity and intrinsic activity since the earliest theoretical conceptions of molecular pharmacology. Another contribution consists of studies that render redundant the notion of tolerance and identify fundamental mechanisms of signal transduction; these mechanisms account for apparent tolerance, dependence, addiction, and sensitization, and appear to operate ubiquitously in a bewildering array of biological systems.

Dihydrobenzofuran analogues of hallucinogens. 4. Mescaline derivatives

Dihydrobenzofuran and tetrahydrobenzodifuran functionalities were employed as conformationally restricted bioisosteres of the aromatic methoxy groups in the prototypical hallucinogen, mescaline (1). Thus, 4-(2-aminoethyl)-6,7-dimethoxy-2,3-dihydrobenzofuran hydrochloride (8) and 1-(8-methoxy-2,3,5,6-tetrahydrobenzo[1,2-b:5,4-b']difuran-4-yl)-2- aminoethane hydrochloride (9) were prepared and evaluated along with 1 for activity in the two-lever drug discrimination (DD) paradigm in rats trained to discriminate saline from LSD tartrate (0.08 mg/kg). Also, 1, 8, and 9 were assayed for their ability to displace [3H]ketanserin from rat cortical homogenate 5-HT2A receptors and [3H]8-OH-DPAT from rat hippocampal homogenate 5-HT1A receptors. In addition, these compounds were evaluated for their ability to compete for agonist and antagonist binding to cells expressing cloned human 5-HT2A, 5-HT2B, and 5-HT2C receptors. Finally, agonist efficacy was assessed by measurement of phosphoinositide hydrolysis in NIH 3T3 cells expressing the rat 5-HT2A or 5-HT2C receptors. Although 1 fully substituted for LSD in the DD assays (ED50 = 33.5 mumol/kg), neither 8 nor 9 substituted for LSD, with just 50% of the rats administered 8 selecting the drug lever, and only 29% of the rats administered 9 selecting the drug lever. All of the test compounds had micromolar affinity for the 5-HT1A and 5-HT2A receptors in rat brain homogenate. Curiously, the rank order of affinities of the compounds at 5-HT2A sites was opposite their order of potency in the behavioral assay. An evaluation for ability to stimulate phosphoinositide turnover as a measure of functional efficacy revealed that all the compounds were of approximately equal efficacy to serotonin in 5-HT2C receptors. At 5-HT2A receptors, however, 8 and 9 were significantly less efficacious, eliciting only 61 and 45%, respectively, of the maximal response. These results are consistent with the proposed mechanism of action for phenethylamine hallucinogens, that such compounds must be full agonists at the 5-HT2A receptor subtype. In contrast to the 2,5-dimethoxy-substituted phenethylamines, where rigidification of the methoxy groups had no deleterious effect on activity, the loss of activity in the 3,4,5-trioxygenated mescaline analogues may suggest that the 3 and 5 methoxy groups must remain conformationally mobile to enable receptor activation.

Effects of ketanserin on the discrimination of electrical stimulation of the dorsal raphé nucleus in rats

The electrical stimulation of the dorsal raphé nucleus was used as a training cue in a discrimination paradigm. Sprague-Dawley rats were trained to discriminate between electrical stimulation (ES; 200 microA) of the dorsal raphé nucleus (DRN) and non-stimulation. This was accomplished by associating ES with intraperitoneal (i.p.) injection of lithium chloride (LiCl), following the session with electrical stimulation. This made the drinking of saccharin during ES aversive by conditioned taste aversion. Following training, rats decreased saccharin consumption in ES sessions. This discrimination was learned within three pairings of the ES with LiCl. Lowering the ES current to 50-100 microA resulted in levels of saccharin consumption similar to non-stimulation levels, whereas 150 microA showed a response intermediate between the stimulation response at 200 microA and non-stimulation. The discrimination of ES of the DRN (200 microA) was not affected by prior administration of the 5-HT2 antagonist ketanserin (1 or 2 mg/kg, i.p.), suggesting that activation of 5-HT2 receptors is not the primary discriminative cue generated by ES. However, the 5-HT2A/2C agonist DOI (0.25-0.5 mg/kg, i.p.), substituted for ES of the DRN, i.e. animals reduced saccharin consumption following DOI administration. This substitution of DOI for ES was antagonized by the administration of ketanserin (1 mg/kg, i.p.). These results suggest that ES of the DRN has properties that are similar to the activation of 5-HT2A/2C receptors by DOI. However, other stimuli such as activation of other 5-HT receptors are also involved, given that the discriminative cues of ES are not blocked by the 5-HT2A/2C antagonist ketanserin.

Dihydrobenzofuran analogues of hallucinogens. 3. Models of 4-substituted (2,5-dimethoxyphenyl)alkylamine derivatives with rigidified methoxy groups

Tetrahydrobenzodifuran functionalities were employed as conformationally restricted bioisosteres of the aromatic methoxy groups in prototypical hallucinogenic phenylalkylamines 1 and 2. Thus, a series of 8-substituted 1-(2,3,6,7-tetrahydrobenzo[1,2-b:4,5-b']difuran-4-yl)-2-aminoal kanes (7a-e) were prepared and evaluated for activity in the two-lever drug discrimination paradigm in rats trained to discriminate saline from LSD tartrate (0.08 mg/kg) and for the ability to displace [3H]ketanserin from rat cortical homogenate 5-HT2A receptors and [3H]-8-OH-DPAT from rat hippocampal homogenate 5-HT1A receptors. In addition, 1-(8-bromo-2,3,6,7-tetrahydrobenzo[1,2-b:4,5-b']difuran-4-yl)-2-am inopropane (7b), which was found to be extremely potent in the rat in vivo assays, was evaluated for its ability to compete with [125I]DOI and [3H]ketanserin binding to cells expressing cloned human 5-HT2A, 5-HT2B, and 5-HT2C receptors. All of the dihydrofuranyl compounds having a hydrophobic substituent para to the alkylamine side chain had activities in both the in vitro and in vivo assays that equaled or surpassed the activity of the analogous conformationally flexible parent compounds. For example, 7b substituted for LSD in the drug discrimination assay with an ED50 of 61 nmol/kg and had Kj values in the nanomolar to subnanomolar range for the displacement of radioligand from rat and human 5-HT2 receptors, making it one of the most potent hallucinogen-like phenylalkylamine derivatives reported to date. The results suggest that the dihydrofuran rings in these new analogues effectively model the active binding conformations of the methoxy groups of the parent compounds 1 and 2. In addition, the results provide information about the topography and relative orientation of residues involved in agonist binding in the serotonin 5-HT2 receptors.

Electrical stimulation of the dorsal raphe nucleus as a discriminative stimulus: generalization to (+/-)-DOI

Electrical stimulation of the dorsal raphe nucleus of Sprague-Dawley rats was used as the cue for discrimination using a taste aversion paradigm. Rats were trained to associate saccharin drinking during electrical stimulation of the dorsal raphe nucleus with LiCl injection after the session as the aversive unconditioned stimulus. In sessions without stimulation, rats were allowed to consume saccharin and received a saline injection after the session. Suppression of saccharin consumption during electrical stimulation was learned within 12 trials. Rats trained in the reverse discrimination, i.e., sessions with no electrical stimulation paired with LiCl injection, showed a similar learning curve. Animals injected prior to the session with the hallucinogenic 5-HT2 agonist (+/-)-DOI associated DOI with electrical stimulation of the dorsal raphe nucleus. Thus, animals may be trained to discriminate electrical stimulation of the dorsal raphe nucleus. Furthermore, animals generalize from activation of 5-HT2 receptors to electrical stimulation of the dorsal raphe nucleus.

Serotonin (5-HT) induces IPSPs in pyramidal layer cells of rat piriform cortex: evidence for the involvement of a 5-HT2-activated interneuron

In a slice preparation of rat piriform cortex, both intracellular and extracellular techniques were used to examine the pharmacological and electrophysiological actions of serotonin (5-HT). Bath application of 5-HT resulted in either depolarization (57%), hyperpolarization (34%) or no change (9%) in membrane potential of cells in the pyramidal cell layer (layer II) of piriform cortex. Additionally, when KCl-containing electrodes were used, 5-HT induced an increase in depolarizing synaptic potentials in 41% of these cells. It was concluded that these potentials were reverse inhibitory post-synaptic potentials (IPSPs) because they were blocked by bicuculline and tetrodotoxin. The induction of IPSPs by 5-HT was blocked by the 5-HT2-selective antagonist ritanserin. By recording extracellularly in the presence of 5-HT, a group of 5-HT-activated, putative interneurons was found at the border of layers II and III of piriform cortex, 5-HT but not norepinephrine activation was blocked by ritanserin. The actions of 5-HT were mimicked by the 5-HT2 agonist alpha-methyl-5-HT; the 5-HT2 partial agonist, 2,5-dimethoxy-4-methyl-amphetamine had a small agonist action of its own and blunted the effect of 5-HT. Activation of a larger group of putative interneurons by the more universal excitant N-methyl-D-aspartate showed that the 5-HT-activated interneurons represented 23% of the interneurons located on the border between layers II and III. We conclude that 5-HT induces IPSPs in layer II pyramidal cells by activating a subpopulation of interneurons at the border of layers II and III of piriform cortex.

Further studies on the dose-dependent stimulus properties of 5-methoxy-N,N-dimethyltryptamine

Twenty-two rats were trained to discriminate either 1.5 mg/kg of 5-methoxy-N,N-dimethyltryptamine (5-OMe DMT) from saline in a standard two-lever operant procedure. Once responding was stable, various doses of several serotonin (5-HT) antagonists, i.e., cyproheptadine (CYP), methysergide (UML), cinanserin (CIN), and methergoline (MCE), were administered in combination with 5-OMe DMT, to assess the ability of each antagonist to attenuate each 5-OMe DMT-stimulus. The 5-OMe DMT-stimulus at 1.5 mg/kg was completely antagonized by CYP, and was partially attenuated by CIN and MCE. UML had negligible effects on 5-OMe DMT-appropriate responding. In the 3.0 mg/kg 5-OMe DMT-trained rats, UML and MCE partially blocked the 5-OMe DMT-stimulus; CYP and CIN had no significant effect on 5-OMe DMT-appropriate responding. The results suggest that until the in vivo effects and mechanism of action of 5-OMe DMT and certain 5-HT antagonists are better understood, caution is advised when conclusions are drawn from studies employing these agents.

Justicia pectoralis: a study of the basis for its use as a hallucinogenic snuff ingredient

The use of Justicia pectoralis var. stenophylla as a Virola snuff admixture and also as the sole ingredient of a snuff was investigated. Extracts of the plant did not contain alkaloids, although the ubiquitous compound, betaine, was isolated because of its reaction with alkaloid reagents. Nor did extracts have any significant effect upon the gross behavioral effects, or increased spontaneous motor activity, elicited in mice by 5-methoxy-N,N-dimethyl-tryptamine (5-MeODMT), the primary psychotropic constituent of the Virola resin snuff. Coumarin and umbelliferone were identified because they are major constituents of the plant and because of their ability to relax smooth muscle.

Behavioral effects of 5-methoxy-N,N-dimethyltryptamine and dose-dependent antagonism by BC-105

The discriminative effects of 5-methoxy-N,N-dimethyltryptamine (5-OMeDMT) were studied in rats trained to discriminate 1.5 mg/kg or 3.0 mg/kg 5-OMeDMT from saline. A series of antagonist and generalization tests revealed that (1) antagonism of the 5-OMeDMT stimulus response by the presumed serotonin antagonist BC-105 depended on the dose of 5-OMeDMT, (2) the 5-OMeDMT stimulus generalized to LSD, and (3) like 5-OMeDMT, antagonism of the LSD generalization response by BC-105 depended on the dose of LSD. In a second study, with rats responding under a variable-interval (VI) 15-s schedule of reinforcement, doses of 1.0-3.0 mg/kg 5-OMeDMT significantly decreased response rate. Furthermore, the decrease in responding produced by the administration of 1.5 mg/kg (but not by 3.0 mg/kg) 5-OMeDMT was blocked by BC-105. This dose-dependent antagonism was of particular interest since the 1.5 mg/kg and 3.0 mg/kg dose of 5-O-MeDMT had essentially the same effect on responding when given alone. The results of both studies emphasize the importance of 5-OMeDMT dose in antagonism experiments.

Indolealkylamine and phenalkylamine hallucinogens: a brief overview

Various indolealkylamine and phenalkylamine derivatives are hallucinogenic in man and/or are behaviorally active in animals. This overview is divided into two parts. The first part attempts to bring together information concerning the activity of indolealkylamines (i.e., tryptamines, alpha-methyltryptamines, N,N-dimethyltryptamines, N-alkyltryptamines, lysergic acid derivatives and beta-carbolines) and phenalkylamines (i.e., phenethylamines, phenylisopropylamines) along with major key references, and with emphasis on those agents not recently reviewed. The latter portion of this overview describes some of the work being conducted in our laboratories in an effort to elucidate the role of the neurotransmitter serotonin in the mechanism of action of various indolealkylamine and phenalkylamine hallucinogens.

Opposite effects of N,N-dimethyltryptamine (DMT) and 5-methoxy-n,n-dimethyltryptamine (5-MeODMT) on acoustic startle: spinal vs brain sites of action

The present studies examined the role of the spinal cord and the brain in mediating the effects of the hallucinogens N,N-dimethyltryptamine (DMT) and 5-methoxy-N,N-dimethyltryptamine (5-MeODMT) on the acoustic startle response in the rat. Systemic administration of these agents, which distributes to both the brain and spinal cord, produced opposite effects, as DMT depressed and 5-MeODMT increased acoustic startle. However, when administered directly into the lateral ventricle in the forebrain (intraventricular administration) 5-hydroxytryptamine (5-HT), DMT and 5-MeODMT all depressed acoustic startle, DMT and 5-MeODMT being about equipotent in this regard. In contrast, when administered directly into the spinal cord subarachnoid space (intrathecal administration), 5-HT and 5-MeODMT increased startle, whereas DMT was without effect. In another series of studies, the effects of systemically-administered DMT and 5-MeODMT on the "startle" elicited by electrical stimulation of the nucleus reticularis pontis caudalis (RPC) were determined. Since the RPC is the last nucleus of the primary startle circuit before the spinal cord, agents which act downstream from the RPC (i.e., in the lower brainstem and spinal cord) would be expected to alter RPC-elicited "startle," while agents which act upstream from the RPC would be without effect. Given systemically, 5-MeODMT markedly increased RPC-elicited "startle" while DMT was without effect. These data indicate that DMT and 5-MeODMT are equipotent in depressing startle through actions in the brain. In contrast, the difference in the effects of DMT and 5-MeODMT on acoustic startle is related to the spinal excitatory effects of 5-MeODMT which DMT does not possess. From the present results it is suggested that the relative potencies of DMT and 5-MeODMT in other behavioral measures may relate to the role of brain (equipotent) or spinal (5-MeODMT more potent than DMT) sites of action for the various behaviors.

Comparative discriminative stimulus effects of 5-methoxy-N,N-dimethyltryptamine and LSD

Rats were trained to discriminate injections of either 5-OMe DMT (1.5 mg/kg) or LSD (0.096 mg/kg) from saline in a two-lever drug discrimination task. After stable discrimination performances were attained (greater than 85%) in each group, dose-response generalizations between the two groups of animals were examined. The results revealed that the 5-OMe DMT-stimulus response generalized to LSD and that the LSD-stimulus response generalized to 5-OMe DMT. Furthermore, both the 5-OMe DMT-stimulus and the LSD-stimulus could be significantly attenuated by the serotonin antagonist BC-105. However, the pattern of the dose-related antagonism by BC-105 was different between the drug stimuli. It was concluded that while the discriminative stimulus effects of 5-OMe DMT and LSD may be mediated via a common serotonergic system, the receptor interaction of these agents within that pharmacological system may be somewhat different.

Discriminative stimulus properties of DOM and several molecular modifications

Rats trained to discriminate racemic 2,5-dimethoxy-4-methylphenylisopropylamine, (+/-)-DOM (1.0 mg/kg), from saline in a two-lever drug discrimination task were challenged with the optimal isomers of DOM as well as with several related agents which represent minor molecular modifications of the DOM structure. Generalization of the (+/-)-DOM stimulus was found to occur to R(-)-DOM, S(+)-DOM, (+/-)-2,5-dimethoxyphenylisopropylamine (2,5-DMA), R(-)-2,-5-DMA, and the 2-demethyl derivative of (+/-)-DOM. The 3-methyl positional isomer of (+/-)-DOM was found to produce only 34% DOM-appropriate responding at the highest dose tested while administration of S(+)-2,5-DMA and the 5-demethyl derivative of (+/-)-DOM resulted in disruption of behavior.

Hallucinogens as discriminative stimuli: a comparison of 4-OMe DMT and 5-OMe DMT with their methythio counterparts

Rats, trained to discriminate 1.5 mg/kg of the hallucinogenic agent 5-methoxy-N,N-dimethyltryptamine (5-OMe DMT) from saline in a two-lever drug discrimination task, were challenged with various doses of the 4-methoxy, 4-methylthio and 5-methylthio derivatives of DMT. The 5-OMe DMT cue was found to generalize to all three of these agents; the order of potency is 5-OMe greater than 5-SMe greater than 4-OMe greater than 4-SMe DMT.

Psilocybin as a discriminative stimulus: lack of specificity in an animal behavior model for 'hallucinogens'

Fifteen rats were trained to discriminate between the tryptamine hallucinogen psilocybin (4-phosphoryloxy-N,N-dimethyltryptamine; 1.0 mg/kg) and saline in a two-lever choice task. Dose-response and time-response curves were obtained. The psilocybin cue generalized to psilocin (the dephosphorylated congener of psilocybin) and to the prototypical indoleamine hallucinogen LSD, but not to the phenylethylamine hallucinogen mescaline. These results indicate that the hallucinogenic effects of these drugs in humans may not be identical with their discriminative stimulus functions in animals, and that these four compounds may not be members of a single drug class. The term 'hallucinogen' may thus be a misnomer in the context of drug discrimination studies in nonhumans.

Behavioral properties of psychoactive phenylisopropylamines in rats

Rats were trained to discriminate injections of 5-methoxy-N,N-dimethyltryptamine (5-OMe DMT, 3.0 mg/kg) a hallucinogenic agent for which a serotonergic mechanism has been implicated, from saline in a two-lever drug discrimination task. After reliable levels of accuracy (greater than or equal to 85%) were attained, the ability of the 5-OMe DMT cue to generalize to 36 substituted phenylisopropylamines (or their optical isomers) was assessed. The results reveal that, in general, the challenge compounds could be differentiated into three broad categories: Those that produced 5-OMe DMT-appropriate responding (generalization), those that produced partial 5-OMe DMT-appropriate responding (partial generalization) and those that produced negligible 5-OMe DMT-appropriate responding. It is concluded that certain of the substituted phenylisopropylamines, unlike amphetamine itself, can produce effects in rats similar to those produced by the training dose of 5-OMe DMT, and that a serotonergic mechanism might be involved.

Speculations on the mechanism of action of hallucinogenic indolealkylamines

In this review we attempt to develop a fluid theoretical model which is being used as a strategy-base for future experimentation. The first two sections (A and B) describe how we have conducted our research, and present the perspective value of each. This is important because the research strategies developed in these laboratories over the last 5 years combine in vitro and in vivo pharmacological techniques as a means of understanding mechanisms of drug action. Sections C and D attempt to describe how we interpret our data and how we have utilized these data to formulate hypotheses concerning drug mechanisms. The last section of this review sets forth our own ideas on how we believe hallucinogenic agents produce their effects and presents some original data, which we feel, allows us to develop the overall hypotheses presented.

DOM and related 2,5-dimethoxy-4-alkylphenylisopropylamines: behavioral and serotonin receptor properties

Using an isolated rat fundus preparation, the 4-methyl (DOM), ethyl (DOET), propyl (DOPR) butyl (DOBU), tertiary butyl (DOTB) and amyl (DOAM) derivatives of 2,5-dimethoxy-phenylisopropylamine (2,5-DMA) were found to possess quite similar serotonin receptor affinities (pA2 - 7.02-7.22). The fundus preparation could not be used to determine pD2 values because all of the compounds were found to interact in an agonistic manner both with serotonin and PRT (phenoxybenzamine resistant tryptamine) receptors. Administration of DOET, DOPR, DOBU, DOTB and DOAM to animals (rats) trained to discriminate 5-OMe DMT from saline resulted only in partial generalization. While each of these agents possesses a high 5-HT receptor affinity, and while their behavioral effects might, therefore, involve a serotonergic component, the stimulus properties of these compounds are qualitatively dissimilar to those produced by the training dose of 5-OMe DMT.

Discriminative stimulus properties of arecoline: a new approach for studying central muscarinic receptors

Rats were trained to discriminate arecoline (1.74 mg/kg) from saline in a milk-reinforced (variable interval 12s) two-lever operant paradigm. The discriminative stimulus (DS) effects of arecoline were antagonized by atropine sulfate, but not by atropine methylnitrate or mecamylamine. In contrast to the effects on discrimination, atropine did not antagonize the response rate suppressant effects of arecoline. The DS effect of arecoline completely generalized to oxotremorine, partially generalized to pilocarpine, and did not generalize to nicotine. These data demonstrate that the DS effect of arecoline depends on central muscarinic receptors.