Structural correlation between apomorphine and LSD: involvement of dopamine as well as serotonin in the actions of hallucinogens

The Varieties of the Psychedelic Experience: A Preliminary Study of the Association Between the Reported Subjective Effects and the Binding Affinity Profiles of Substituted Phenethylamines and Tryptamines

Classic psychedelics are substances of paramount cultural and neuroscientific importance. A distinctive feature of psychedelic drugs is the wide range of potential subjective effects they can elicit, known to be deeply influenced by the internal state of the user ("set") and the surroundings ("setting"). The observation of cross-tolerance and a series of empirical studies in humans and animal models support agonism at the serotonin (5-HT)2A receptor as a common mechanism for the action of psychedelics. The diversity of subjective effects elicited by different compounds has been attributed to the variables of "set" and "setting," to the binding affinities for other 5-HT receptor subtypes, and to the heterogeneity of transduction pathways initiated by conformational receptor states as they interact with different ligands ("functional selectivity"). Here we investigate the complementary (i.e., not mutually exclusive) possibility that such variety is also related to the binding affinity for a range of neurotransmitters and monoamine transporters including (but not limited to) 5-HT receptors. Building on two independent binding affinity datasets (compared to "in silico" estimates) in combination with natural language processing tools applied to a large repository of reports of psychedelic experiences (Erowid's Experience Vaults), we obtained preliminary evidence supporting that the similarity between the binding affinity profiles of psychoactive substituted phenethylamines and tryptamines is correlated with the semantic similarity of the associated reports. We also showed that the highest correlation was achieved by considering the combined binding affinity for the 5-HT, dopamine (DA), glutamate, muscarinic and opioid receptors and for the Ca+ channel. Applying dimensionality reduction techniques to the reports, we linked the compounds, receptors, transporters and the Ca+ channel to distinct fingerprints of the reported subjective effects. To the extent that the existing binding affinity data is based on a low number of displacement curves that requires further replication, our analysis produced preliminary evidence consistent with the involvement of different binding sites in the reported subjective effects elicited by psychedelics. Beyond the study of this particular class of drugs, we provide a methodological framework to explore the relationship between the binding affinity profiles and the reported subjective effects of other psychoactive compounds.

Psychedelics

Psychedelics (serotonergic hallucinogens) are powerful psychoactive substances that alter perception and mood and affect numerous cognitive processes. They are generally considered physiologically safe and do not lead to dependence or addiction. Their origin predates written history, and they were employed by early cultures in many sociocultural and ritual contexts. After the virtually contemporaneous discovery of (5R,8R)-(+)-lysergic acid-N,N-diethylamide (LSD)-25 and the identification of serotonin in the brain, early research focused intensively on the possibility that LSD and other psychedelics had a serotonergic basis for their action. Today there is a consensus that psychedelics are agonists or partial agonists at brain serotonin 5-hydroxytryptamine 2A receptors, with particular importance on those expressed on apical dendrites of neocortical pyramidal cells in layer V. Several useful rodent models have been developed over the years to help unravel the neurochemical correlates of serotonin 5-hydroxytryptamine 2A receptor activation in the brain, and a variety of imaging techniques have been employed to identify key brain areas that are directly affected by psychedelics. Recent and exciting developments in the field have occurred in clinical research, where several double-blind placebo-controlled phase 2 studies of psilocybin-assisted psychotherapy in patients with cancer-related psychosocial distress have demonstrated unprecedented positive relief of anxiety and depression. Two small pilot studies of psilocybin-assisted psychotherapy also have shown positive benefit in treating both alcohol and nicotine addiction. Recently, blood oxygen level-dependent functional magnetic resonance imaging and magnetoencephalography have been employed for in vivo brain imaging in humans after administration of a psychedelic, and results indicate that intravenously administered psilocybin and LSD produce decreases in oscillatory power in areas of the brain's default mode network.

Hallucinogens

Hallucinogens (psychedelics) are psychoactive substances that powerfully alter perception, mood, and a host of cognitive processes. They are considered physiologically safe and do not produce dependence or addiction. Their origin predates written history, and they were employed by early cultures in a variety of sociocultural and ritual contexts. In the 1950s, after the virtually contemporaneous discovery of both serotonin (5-HT) and lysergic acid diethylamide (LSD-25), early brain research focused intensely on the possibility that LSD or other hallucinogens had a serotonergic basis of action and reinforced the idea that 5-HT was an important neurotransmitter in brain. These ideas were eventually proven, and today it is believed that hallucinogens stimulate 5-HT(2A) receptors, especially those expressed on neocortical pyramidal cells. Activation of 5-HT(2A) receptors also leads to increased cortical glutamate levels presumably by a presynaptic receptor-mediated release from thalamic afferents. These findings have led to comparisons of the effects of classical hallucinogens with certain aspects of acute psychosis and to a focus on thalamocortical interactions as key to understanding both the action of these substances and the neuroanatomical sites involved in altered states of consciousness (ASC). In vivo brain imaging in humans using [(18)F]fluorodeoxyglucose has shown that hallucinogens increase prefrontal cortical metabolism, and correlations have been developed between activity in specific brain areas and psychological elements of the ASC produced by hallucinogens. The 5-HT(2A) receptor clearly plays an essential role in cognitive processing, including working memory, and ligands for this receptor may be extremely useful tools for future cognitive neuroscience research. In addition, it appears entirely possible that utility may still emerge for the use of hallucinogens in treating alcoholism, substance abuse, and certain psychiatric disorders.

Drug discrimination and receptor binding studies of N-isopropyl lysergamide derivatives

Isopropyl (IPLA), N-methyl-N-isopropyl (MIPLA), N-ethyl-N-isopropyl (EIPLA), and N,N-diisopropyl (DIPLA) lysergamides were evaluated for lysergic acid diethylamide (LSD)-like activity. In rats trained to discriminate 0.08 mg/kg LSD tartrate from saline, each of the subject compounds completely substituted, with an ED50 two to three times larger than that of LSD except for DIPLA, which had an ED50 about eightfold greater. Similarly, all the compounds displaced [125I](R)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane ([125I]DOI) from rat cortical homogenates and displaced [3H]8-hydroxy-2-(di-n-propylamino)tetralin ([3H]8-OH-DPAT) from rat hippocampal homogenates with KI values similar to those of LSD, again with the exception of DIPLA, which had about nine- and fourfold lower affinities, respectively. Interestingly, all the compounds had four- to fivefold lower affinities than LSD in displacing [3H]ketanserin from 5-HT2 binding sites. Molecular modeling studies found that all the compounds had low energy conformations similar to LSD. No correlation between the activity of the compounds and the preferred conformation of the amide substituents was apparent. In summary, N-alkyl-N-isopropyl analogs of LSD retain LSD-like activity in drug discrimination and 5-HT1A and 5-HT2 agonist binding assays only until the N-alkyl substitution is as large as ethyl; LSD-like activity dramatically drops when the second alkyl substituent is N-isopropyl.

A three dimensional receptor model of the dopamine D2 receptor from computer graphic analyses of D2 agonists

Four potent D2 agonists were employed to define a primary pharmacophore for the D2 receptor. Hypothetical receptor points, representing interaction points on a receptor were built on to each molecule. These points and the nitrogen atom were averaged to give the coordinates (A) of the primary pharmacophore: R1 (0.00, 3.50, 0.00), R2 (0.00, -3.50, 0.00), R3 (5.79, 2.06, 0.00), and nitrogen (5.13, -0.63, 0.37). Eight structural classes of D2 agonists were then superimposed on to the primary pharmacophore to aid in the location of secondary binding sites. The secondary sites include two lipophilic clefts, an area of steric bulk, a region to hydrogen bond 'meta' hydroxy groups and a 'critical region' accepting methoxy and halogen substituents but not hydroxy substituents. The model has the potential to design and predict activity of novel D2 agonist compounds.

MENTHOR, a database system for the storage and retrieval of three-dimensional molecular structures and associated data searchable by substructural, biologic, physical, or geometric properties

MENTHOR is a database system for the storage and retrieval of three-dimensional coordinate and charge information on molecules as well as of traditional biological and physical properties. Our molecular graphics system retrieves from MENTHOR structural information in individual molecules and receptor map/macromolecular binding site hypotheses. Substructural searches of MENTHOR are used to find starting coordinates for molecular modeling and traditional database searches of MENTHOR identify compounds for which modeling is needed. It also forms the data to be searched with ALLADDIN, our substructure/geometric search program. MENTHOR expedites molecular modeling by organizing previous work and facilitating transmission of information between individuals. Examples from modeling of D-2 receptor agonists are shown.

A stereochemical and conformational model of dopaminergic agonist and antagonist activity: further evaluation

Conformational energy calculations using the Molecular Mechanics II (MM2) program have been performed on 2-aminotetrahydronaphthalene (ATN) and 2-aminoindan derivatives which are active or inactive at dopamine receptors. The results were used to test a stereochemical and conformational model previously proposed for dopaminergic activity. The conformer predicted to be optimal for agonist activity was found to have relatively low energy (less than 1.5 kcal/mol) for all of the agonists examined. The model successfully: (1) explained the relative activity or inactivity of compounds such as cis- and trans-1-methyl-5-hydroxyl ATN derivatives and the corresponding cis- and trans-octohydrobenzo[f]quinolines; (2) predicted the more potent antipode of 2-aminoindan dopaminergic agonists; and (3) explained the structure--activity peculiarities of 3-(3-hydroxyphenyl)-N-alkylpiperidines in which the potency is increased for (3S)-isomers and decreased for (3R)-isomers when the N-alkyl group is greater than propyl. Predictions of postsynaptic dopaminergic antagonism were also made for some of the compounds. In agreement with previous conclusions, the inactivity of ATN derivatives with a 2-methyl or 5-propyl group was attributed to steric interference at the receptor since those groups did not have a significant conformational effect on the receptor ligand.

The biologically active conformation of ergot alkaloids

Molecular mechanics and NMR studies of the D ring conformation of ergot alkaloids demonstrate that both D1 and D2 forms may exist in solution. The comparison of the geometric parameters defining the spatial relations between the aromatic moieties and the basic nitrogen of conformationally restricted dopamine analogs, and that of ergolene, shows the D1 conformation to be the bioactive one.

Patterns of exploration in rats distinguish lisuride from lysergic acid diethylamide

In order to further validate a previously proposed animal model of the effects of LSD in humans, doses of 5, 15, 30 and 60 micrograms/kg lisuride (a non-hallucinogenic congener of LSD) were studied using a behavioral pattern monitor (BPM). The BPM provided both quantitative measures of crossovers, rearings, and holepokes and qualitative measures of spatial patterns of locomotion. A holeboard chamber connected to a homecage provided two test situations. Rats were tested either with (free exploration) or without access to the homecage (forced exploration). In both situations, lisuride exhibited a biphasic dose-response curve for horizontal locomotion (low dose suppression and high dose enhancement), while rearing was significantly reduced at all doses. Lisuride also produced a dose-dependent increase in the perseverative quality of locomotor patterns. A comparison of these results with our previous studies with lysergic acid diethalmide (LSD) indicate that, with the exception of rearings, lisuride fails to mimic LSD's characteristic effects on exploratory activity. Rather, lisuride exhibited many similarities to the dopamine angonist apomorphine.

The role of hallucinogenic drugs and sensory stimuli in Peruvian ritual healing

On Peru's north coast, curanderos perform night-long curing ceremonies in which the mescaline-bearing cactus San Pedro is used. The concept of autonomic nervous system tuning is presented as the best explanation for the overall physiological effect of the ritual. An assessment of the possible therapeutic impact of this condition is provided. Organic response and symbolic message are seen to resonate in the purificatory connotations of the ritual.

Amphetamine and LSD as discriminative stimuli: alterations following neonatal monoamine reductions

Male adult Sprague-Dawley rats (70 days of age), neonatally depleted of either 5-hydroxytryptamine (5HT) via 5,7-dihydroxytryptamine (5,7-DHT; ICS) + desmethylimipramine (DMI; IP) at 3 days of age or dopamine (DA) via 6-hydroxydopamine (6-OHDA; ICS) + DMI at 14 days of age, were trained to discriminate either d-LSD-tartrate (80 micrograms/kg; IP) or d-amphetamine (d-AMPH) sulfate (0.90 mg/kg; IP) from saline utilizing a two lever drug discrimination paradigm. A neurochemical analysis at the termination of these studies revealed the following in terms of %DA or %5HT (presented in that order) depleted with respect to the appropriate vehicle control group: telencephalon; 96 and 96%, diencephalon; 51 and 31%, and brain stem; 76 and 80%. Rats learned to discriminate either d-AMPH or LSD regardless of amine depleted. In addition, the depletion of 5HT had little effects on dose or drug generalizations, or the ability of known antagonists to antagonize the discrimination stimulus (DS) effects of either LSD or d-AMPH. The effect of DA depletion, on the other hand, was to increase the sensitivity of the LSD DS at low doses, while decreasing the sensitivity of the d-amphetamine DS. DA depletion also had the effect of reducing the effectiveness of the LSD-antagonists, pizotifen maleate (BC105), while the opposite was observed for the d-AMPH antagonist, trifluoperazine HCI. These data suggest that: (1) LSD and d-amphetamine discrimination stimuli are not mediated and/or influenced via the compromised aspects of the 5HT systems (other central mechanisms may have compromised for these 5HT deficits); (2) the LSD DS is mediated or influenced both by serotonergic and dopaminergic mechanisms; and (3) the d-amphetamine DS is mediated by certain aspects of the dopaminergic system with little evidence for the involvement of 5HT systems.

Indolamine hallucinogens as MAO inhibitor agents--theoretical approach

A new theoretical hypothesis on the mode of action of LSD and other hallucinogens of the indolamine family is proposed. In view of the suggestions that LSD acts both on the 5-HT system as an antagonist and on the DA system as an agonist, we suggest that in addition to 5-HT blocking capacity, LSD and the other indolamine hallucinogens act as MAO inhibitors.

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.

Lack of specificity of an animal behavior model for hallucinogenic drug action

It has been proposed recently that the occurrence of drug-induced limb-flicking (LF) and abortive grooming (AG) in cats can serve as a viable animal behavior model for the actions of hallucinogens in humans. If this is the case, such behaviors should occur reliably following the administration of drugs that produce hallucinations in humans and should not occur after administration of other, non-hallucinogenic drugs--a hypothesis that was examined in the present experiment. The frequency of LF and AG were observed in 12 cats which were given a wide range of doses of the potent hallucinogen, d-LSD (0.01-0.16 mg/kg), as well as several other compounds. The results showed that three non-hallucinogenic agents which are related to LSD in various ways, the ergot derivative lisuride, the serotonin agonist, quipazine, and the dopamine agonist, apomorphine, significantly increased LF frequency. Lisuride and quipazine also caused AG. Cocaine did not elicit either behavior. Thus, it was concluded that the proposed model cannot be regarded as specific to hallucinogenic drugs. In addition, the frequency of these behaviors, as well as their reliability and robustness, were shown to be party dependent on the environment in which observation occurs.

A characteristic effect of hallucinogens on investigatory responding in rats

The disruption of the temporal distribution of investigatory responses by rats in a novel hole-board following lysergic acid diethylamide-25 (LSD), as described in a companion paper (Geyer and Light, 1979), was found to be a characteristic effect of a variety of hallucinogens. Similar effects were produced by indoleamine hallucinogens, such as LSD, N,N-dimethyltryptamine, and psilocin, and by phenylethylamine hallucinogens, such as mescaline or 2,5-dimethoxy-4-methylamphetamine (DOM). Congeners of DOM that are inactive in humans had no significant effects. Furthermore, of a variety of other psychoactive drugs tested, only apomorphine produced an effect similar to that of the hallucinogens. These results suggest that a simple behavioral measure of exploration in a hole-board may provide a useful animal model with which to examine the common effects of hallucinogens.

The molecular structure of adrenergic and dopaminergic substances

At the end of the last century it was established that the different nerve cells along a neuronal path do not come into direct physical contact with one another, but that there are narrow gaps between them, called synapses (Sherrington, 1897; Ramón y Cajal, 1906). Elliot (1905) made the basic experimental observation that the propagation of nerve impulses across a synapse might be mediated by specific chemical agents (see Fig. i). Such substances are now called neurotransmitters, and some 20 different compounds putatively responsible for synaptic transmission in different parts of the nervous system are known at present, e.g. a few recently isolated polypeptides. The most extensively studied transmitters are acetylcholine and the catecholamine group, consisting of dopamine (a), noradrenaline (b), and adrenaline (c).

Effect of alpha-methyltryptamine on spontaneous activity in mice

A standard dose of 10 mg/kg (48 mu mole/kg) of (+/-)-alpha-methyltryptamine (AMT) induced a significant and long lasting increase in spontaneous activity in mice. Pretreatment of mice with either pimozide or alpha-methyl-para-tyrosine methyl ester HCl (AMPT) prevented the activity increase induced by AMT. In similar trials, methysergide or para-chlorophenylalanine (PCPA) were also found to antagonize the development of hyperactivity following a standard dose of AMT. The results suggest that both endogenous dopamine and serotonin many participate in the hyperactivity induced by AMT.

Lysergic acid diethylamide antagonizes shaking induced in rats by five chemically different compounds

Thyrotropin-releasing hormone (TRH), sodium valproate, AF-3-5 (1-[2-hydroxyphenyl]-4-[3-nitrophenyl]-1,2,3,6-tetrahydropyrimidine-2-one), RX336-M (7,8-dihydro-5',6'-dimethylcyclohex-5'-eno-1',2',8',14 codeinone), and Sgd 8473 (alpha-[4-chlorobenzylideneamino)-oxy]-isobutyric acid) each induced repetitive shaking of the body of rats after intraperitoneal injection. This action of the five diverse chemicals appears to be subserved by a common pharmacological component, because pretreatment with d-lysergic acid diethylamide (0.03--1.0 mg kg-1, s.c.) attenuated the shaking behavior in a dose-related manner, and cross tolerance was found between RX336-M and TRH, sodium valproate, and AG-3-5.

A new view of the structural relationship between LSD and mescaline

An examination of the effects of S-(-) and R-(+) 2-amino-1,2,3,4-tetrahydronaphthalene (2-AT) on mouse spontaneous activity and rabbit EEG has shown that the S-(-) enantiomer shows selective central effects similar to those of hallucinogens like mescaline. A stereochemical analysis of these results indicates that the structural relationship between mescaline, or other phenethylamine type hallucinogens, may involve correspondence between the aromatic ring of the phenethylamines and the pyrrole portion of the indole nucleus in LSD.