Indoleamine and the phenethylamine hallucinogens: mechanisms of psychotomimetic action (original) (raw)
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Neuropsychopharmacology, 1999
This brief review traces the serotonin (5-HT) hypothesis of the action of hallucinogenic drugs from the early 1950s to the present day. There is now converging evidence from biochemical, electrophysiological, and behavioral studies that the two major classes of psychedelic hallucinogens, the indoleamines (e.g., LSD) and the phenethylamines (e.g., mescaline), have a common site of action as partial agonists at 5-HT 2A and other 5-HT 2 receptors in the central nervous system. The noradrenergic locus coeruleus and the cerebral cortex are among the regions where hallucinogens have prominent effects through their actions upon a 5-HT 2A receptors. Recently, we have observed a novel effect of hallucinogens-a 5-HT 2A receptor-mediated enhancement of nonsynchronous, late components of glutamatergic excitatory postsynaptic potentials at apical dendrites of layer V cortical pyramidal cells. We propose that an effect of hallucinogens upon glutamatergic transmission in the cerebral cortex may be responsible for the higher-level cognitive, perceptual, and affective distortions produced by these drugs. [Neuropsychopharmacology 21:16S-23S, 1999] © 1999 American College of Neuropsychopharmacology. Published by Elsevier Science Inc.
Psychopharmacology, 1988
Alterations in brain serotonergic function have been implicated in the mechanism of action of LSD, mescaline, and other similarly acting hallucinogenic drugs of abuse such as STP (2,5-dimethoxyphenylisopropylamine; DOM). In order to test the hypothesis that the mechanism of action of LSD and phenylisopropylamine hallucinogens is through stimulation of a specific brain serotonin receptor sub-type, the affinities of these compounds for radiolabelled 5-HT2, 5-HT1A, 5-HT1B, and 5-HTIc receptors have been determined using recently developed in vitro radioligand binding methodologies. The 5-HT2 receptor was labelled with the agonist/hallucinogen radioligand 3H-DOB (4bromo-2,5-dimethoxyphenylisopropylamine). The 5-HT1A, 5-HT1B, and 5-HTlc receptors were labelled with 3H-OH-DPAT, 3H-5-HT, and 3H-mesulergine, respectively. In general, the phenylisopropylamines displayed 10-100fold higher affinities for the 5-HT/receptor than for the 5-HTlc receptor and 100-1000 fold higher affinities for the 5-HT2 receptor than for the 5-HT~A or 5-HT~B receptor. There was a strong correlation between hallucinogenic potencies and 5-HT2 receptor affinities of the phenylisopropylamines (r=0.90); the correlation coefficients for the 5-HT~A, 5-HTzB, and 5-HTzc were 0.73, 0.85, and 0.78, respectively. Because there is no evidence that 5-HT1A-selective or 5-HTtB-selective agonists are hallucinogenic and because the phenylisopropylamines are potent hallucinogens, a 5-HT2 receptor interaction is implicated and supports our previous suggestions to this effect. A secondary role for 5-HT~c receptors cannot be discounted at this time. These results, when integrated with other receptor pharmacological information, indicate that an important component of the mechanism of action of LSD and the phenylisopropylamine hallucinogens is through stimulation of brain 5-HT 2 receptors.
Pharmacology Biochemistry and Behavior, 1981
The hypothesis that 5-hydroxytryptamine (5-HT) neurons and/or receptors are involved in the mechanism of action of hallucinogens is supported by the fact that intraventricular administration of the neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) selectively destroys central 5-HT neurons in the brain and potentiates the behavioral effects of lysergic acid diethylamide (LSD), 2,5-dimethoxy-4-methylamphetamine (DOM) and mescaline. The locus in the brain where this potentiation might occur is not known. In the present experiment, the medial forebrain bundle (MFB) was studied because it is the primary tract containing fibers from the cell bodies in the raphe nuclei to forebrain structures receiving 5-HT input. Male rats received 5,7-DHT () or vehicle injections bilaterally into the MFB; this procedure caused a significant reduction of 5-HT in the cortex, hippocampus and hypothalamus of lesioned rats, but not in the striatum. Regional dopamine and norepinephrine concentrations were not affected by this treatment. The behavioral effects of the hallucinogens were tested in a situation in which the animals pressed a bar under a fixed ratio-40 (FR-40) schedule of food reinforcement. The disruptive effects of LSD responding were enhanced in the 5,7-DHT-treated animals, while the effects of DOM were diminished; there was no change in the response to mescaline. These data suggest that, while 5-HT neurons are involved in the behavioral effects of hallucinogens, the precise sites and/or mechanisms of action of LSD, DOM and mescaline may differ.
Multiple receptors contribute to the behavioral effects of indoleamine hallucinogens
Neuropharmacology, 2011
Serotonergic hallucinogens produce profound changes in perception, mood, and cognition. These drugs include phenylalkylamines such as mescaline and 2,5-dimethoxy-4-methylamphetamine (DOM), and indoleamines such as (+)-lysergic acid diethylamide (LSD) and psilocybin. Despite their differences in chemical structure, the two classes of hallucinogens produce remarkably similar subjective effects in humans, and induce cross-tolerance. The phenylalkylamine hallucinogens are selective 5-HT 2 receptor agonists, whereas the indoleamines are relatively nonselective for serotonin (5-HT) receptors. There is extensive evidence, from both animal and human studies, that the characteristic effects of hallucinogens are mediated by interactions with the 5-HT 2A receptor. Nevertheless, there is also evidence that interactions with other receptor sites contribute to the psychopharmacological and behavioral effects of the indoleamine hallucinogens. This article reviews the evidence demonstrating that the effects of indoleamine hallucinogens in a variety of animal behavioral paradigms are mediated by both 5-HT 2 and non-5-HT 2 receptors.
Altered states of consciousness induced by hallucinogens (H-ASC) is still a vaguely understood phenomenon. Taken the diverse psychological effects they exert, the main mechanism of action of hallucinogens; LSD, ibogaine, THC, PCP, MDMA, methamphetamine, mescaline, psilocybin and DMT, of which psychological effects are discussed in the article, are not properly understood and explained by the modern methods of neuroscience due to the lack of vigorous research. The involvement of some receptors, such as, 5-HT 2 (and probably other 5-HT receptors), glutamate and dopamine receptors, adrenergic and cannabinoid receptors, is one of the mechanisms, however it is not easy to explain such incongruent psychological effects by only receptor and neurotransmitter systems alone, since H-ASCs have, sometimes, their own unity and gestalt, unfolding the subconscious, in the "voyages" they induce, although the perception may, or may not, be distorted depending on the person, and "set and setting". They induce visual, tactile and auditory hallucinations; synesthesia; perception of fractals, geometrical and kaleidoscopic images with vivid colors; perception of two dimensional pictures as three dimensional, animated and moving; distortions and alterations in the body perception; alterations in the perceptions of temporal-spatial continuum and time; changes in the perception of the ego and the self; feelings of unification with nature and universe, peak experiencesmimicking satori or nirvana-, ecstasy, rapture, extreme euphoria, excitement and happiness, oceanic bliss, self-fulfillment, referred as "good trip", as well as, dysphoria, anxiety, mania, delirium, psychosis, acute schizophrenia, collapse of the self, known as "bad trip"; depending upon the mood, affection and psychology of the person, and "set and setting". Mysticomimetic effects of H-ASCs, imitate the consciousness states of ancient mystics, probably, by means of activating prefrontal cortex, limbic system and the right temporal lobe. A hypothetical "holographic brain theory" may give some extra insights about the explanation of some of the effects of H-ASCs. It should be taken into account that H-ASCs, can be accepted as a good tool to investigate the nature of consciousness, brain and the human psyche, as well as some of the H-ASCs are good models of psychosis, too. More detailed scientific research should be performed to understand the basic and real mechanisms of H-ASCs, to comprehend and unravel the mystery of human mind and consciousness, since scientific medical research on hallucinogens has been legalized since 1992.
The behavioral pharmacology of hallucinogens
Biochemical Pharmacology, 2008
Until very recently, comparatively few scientists were studying hallucinogenic drugs. Nevertheless, selective antagonists are available for relevant serotonergic receptors, the majority of which have now been cloned, allowing for reasonably thorough pharmacological investigation. Animal models sensitive to the behavioral effects of the hallucinogens have been established and exploited. Sophisticated genetic techniques have enabled the development of mutant mice, which have proven useful in the study of hallucinogens. The capacity to study post-receptor signaling events has lead to the proposal of a plausible mechanism of action for these compounds. The tools currently available to study the hallucinogens are thus more plentiful and scientifically advanced than were those accessible to earlier researchers studying the opioids, benzodiazepines, cholinergics, or other centrally active compounds. The behavioral pharmacology of phenethylamine, tryptamine, and ergoline hallucinogens are described in this review, paying particular attention to important structure activity relationships which have emerged, receptors involved in their various actions, effects on conditioned and unconditioned behaviors, and in some cases, human psychopharmacology. As clinical interest in the therapeutic potential of these compounds is once again beginning to emerge, it is important to recognize the wealth of data derived from controlled preclinical studies on these compounds.
Journal of Psychopharmacology, 2011
Psilocin (4-hydroxy-N,N-dimethyltryptamine) is a hallucinogen that acts as an agonist at 5-HT 1A , 5-HT 2A , and 5-HT 2C receptors. Psilocin is the active metabolite of psilocybin, a hallucinogen that is currently being investigated clinically as a potential therapeutic agent. In the present investigation, we used a combination of genetic and pharmacological approaches to identify the serotonin (5-HT) receptor subtypes responsible for mediating the effects of psilocin on head twitch response (HTR) and the behavioral pattern monitor (BPM) in C57BL/6J mice. We also compared the effects of psilocin with those of the putative 5-HT 2C receptor-selective agonist 1-methylpsilocin and the hallucinogen and non-selective serotonin receptor agonist 5-methoxy-N,N-dimethyltryptamine (5-MeO-DMT). Psilocin, 1-methylpsilocin, and 5-MeO-DMT induced the HTR, effects that were absent in mice lacking the 5-HT 2A receptor gene. When tested in the BPM, psilocin decreased locomotor activity, holepoking, and time spent in the center of the chamber, effects that were blocked by the selective 5-HT 1A antagonist WAY-100635 but were not altered by the selective 5-HT 2C antagonist SB 242,084 or by 5-HT 2A receptor gene deletion. 5-MeO-DMT produced similar effects when tested in the BPM, and the action of 5-MeO-DMT was significantly attenuated by WAY-100635. Psilocin and 5-MeO-DMT also decreased the linearity of locomotor paths, effects that were mediated by 5-HT 2C and 5-HT 1A receptors, respectively. In contrast to psilocin and 5-MeO-DMT, 1-methylpsilocin (0.6-9.6 mg/kg) was completely inactive in the BPM. These findings confirm that psilocin acts as an agonist at 5-HT 1A , 5-HT 2A , and 5-HT 2C receptors in mice, whereas the behavioral effects of 1-methylpsilocin indicate that this compound is acting at 5-HT 2A sites but is inactive at the 5-HT 1A receptor. The fact that 1-methylpsilocin displays greater pharmacological selectivity than psilocin indicates that 1-methylpsilocin represents a potentially useful alternative to psilocybin for development as a potential therapeutic agent.
The serotonin 5-HT2C receptor and the non-addictive nature of classic hallucinogens
Journal of psychopharmacology (Oxford, England), 2017
Classic hallucinogens share pharmacology as serotonin 5-HT2A, 5-HT2B, and 5-HT2C receptor agonists. Unique among most other Schedule 1 drugs, they are generally non-addictive and can be effective tools in the treatment of addiction. Mechanisms underlying these attributes are largely unknown. However, many preclinical studies show that 5-HT2C agonists counteract the addictive effects of drugs from several classes, suggesting this pharmacological property of classic hallucinogens may be significant. Drawing from a comprehensive analysis of preclinical behavior, neuroanatomy, and neurochemistry studies, this review builds rationale for this hypothesis, and also proposes a testable, neurobiological framework. 5-HT2C agonists work, in part, by modulating dopamine neuron activity in the ventral tegmental area-nucleus accumbens (NAc) reward pathway. We argue that activation of 5-HT2C receptors on NAc shell, GABAergic, medium spiny neurons inhibits potassium Kv1.x channels, thereby enhancin...
Serotonergic Hallucinogen-Induced Visual Perceptual Alterations
Current topics in behavioral neurosciences, 2016
Serotonergic hallucinogens, such as lysergic acid diethylamide (LSD), psilocybin, and N,N-dimethyltryptamine (DMT), are famous for their capacity to temporally and profoundly alter an individual's visual experiences. These visual alterations show consistent attributes despite large inter-and intra-individual variances. Many reports document a common perception of colors as more saturated, with increased brightness and contrast in the environment ("Visual Intensifications"). Environmental objects might be altered in size ("Visual illusions") or take on a modified and special meaning for the subject ("Altered self-reference"). Subjects may perceive light flashes or geometrical figures containing recurrent patterns ("Elementary imagery and hallucinations") influenced by auditory stimuli ("Audiovisual synesthesia"), or they may envision images of people, animals, or landscapes ("Complex imagery and hallucinations") without any physical stimuli supporting their percepts. This wide assortment of visual phenomena suggests that one single neuropsychopharmacological mechanism is unlikely to explain such vast phenomenological diversity. Starting with mechanisms that act at the cellular level, the key role of 5-HT2A receptor activation and the subsequent increased cortical excitation will be considered. Next, it will be shown that area specific anatomical and dynamical features link increased excitation to the specific visual contents of hallucinations. The decrease of alpha oscillations by hallucinogens will then be introduced as a systemic mechanism for amplifying internal-driven excitation that overwhelms stimulus-induced excitations. Finally, the hallucinogen-induced parallel decrease of the N170 visual evoked potential and increased medial P1 potential will be discussed as key mechanisms for inducing a dysbalance between global