In 1943, the chemist Albert Hoffman accidentally discovered the remarkably potent hallucinogenic properties of LSD (d-lysergic acid diethylamide). Within the next decade, serotonin (5-hydroxytryptamine; 5-HT), an endogenous indoleamine compound structurally related to LSD (Figure 1), was found in various tissues of the body including the brain. Based on this structural similarity and the finding that LSD coula antagonize 5-HT in peripheral tissues, it was proposed independently by Gaddum and Woolley that the hallucinogenic effects of LSD might result from an antagonism of 5-HT in the brain. This hypothesis was soon expanded to include the possibility that LSD could mimic as well as antagonize the actions of 5-HT.1
The first direct demonstration that LSD interacts with brain 5-HT came in 1961 from the biochemical studies of Freedman who showed that this drug could produce a small, dose-dependent increase in the concentration of endogenous 5-HT.2 However, the significance of this change was unclear since the cellular location and function of 5-HT in the brain was unknown at the time. Nevertheless, by the early 1960s the 5-HT hypothesis of the action of LSD had become firmly established in the field.
THE ACTION OF LSD ON SEROTONERGIC NEURONS
LSD Inhibits Serotonergic Neurons of the Raphe Nuclei
The modern era of research on the actions of LSD in the central nervous system began in the mid-1960s following the discovery and mapping by histochemical fluorescence methods of serotonergic and other monoaminergic neuronal pathways in the brain.3 The maps revealed that serotonergic neuronal cell bodies were clustered in the raphe nuclei of the brain stem. In 1968, based on this new information, the first single-cell electrophysiological recordings from identified serotonergic neurons were performed.4
These experiments, conducted in anesthetized rats, showed that systemically administered LSD has a potent inhibitory effect on the tonically firing serotonergic neurons of the dorsal raphe nucleus. The local application of LSD by microiontophoresis indicated that the inhibition was through a direct action on the somatodendritic region of serotonergic neurons.5,6 The resulting reduction in impulse flow provided an explanation for the previously observed increase in brain 5-HT levels that had been detected by biochemical methods.
Other indoleamine hallucinogens such as DMT (?,?-dimethyltryptamine and psilo* cybin) were also shown to inhibit serotonergic neurons in the raphe nucleus. However, mescaline (Figure 1) and various other substituted phenethylamine hallucinogens did not share with the indoleamines the ability to inhibit raphe neurons.7 Thus, inhibition of serotonergic neurons did not appear to represent a unitary mechanism for the action of indoleamine and phenethylamine hallucinogens.
Figure 1. Structural formulae for serotonin (5-HT), LSD, mescaline, and the simple indoleamine hallucinogen DMT fN.N-tíimefbyftryptamine). The chemical structures are drawn so as to emphasize common structural features such as the indolethylamine nucleus of 5-HT, LSD, and DMT and the phenethylamine nucleus shared by LSD and mescaline.
LSD Inhibits Raphe Neurons Through 5-HT1 Receptors
The recent delineation of multiple 5-HT receptor subtypes by radiolabeled ligand binding and molecular methods (eg, 5-HT1, 5-HT2, 5-HT3, 5-HT4)8 now provides a basis for explaining the difference between the effects of the indoleamine and phenethylamine hallucinogens on serotonergic neurons. Serotonergic raphe neurons have a high density of 5-HT^sub 1A^ but not other subtypes of 5-HT receptors. Since they mediate responses of serotonergic neurons to their own transmitter, these receptors have been termed somatodendritic autoreceptors.
LSD is a potent agonist at 5-HT^sub 1A^ autoreceptors, thus accounting for its direct inhibitory effect on raphe neurons. On the other hand, mescaline and other phenethylamines have negligible affinity for 5-HT^sub 1A^ receptors, explaining their inability to inhibit serotonergic raphe neurons. The action of LSD at 5-HT^sub 1A^ autoreceptors is shared by a number of selective 5-HT^sub 1A^ agonists such as buspirone, which are known from clinical studies to have anxiolytic rather than hallucinogenic effects. Thus, no correlation exists between the activity of various drugs at 5-HT^sub 1A^ receptors and the presence or absence of hallucinogenic properties.
ACTIONS OF LSD AT POSTSYNAPTIC SITES
Affinity for 5-HT2 Receptors Correlates with Hallucinogenic Potency
In contrast to their disparate affinities for 5-HT^sub 1A^ receptors, there is a very good correlation between the affinity of both indoleamine and phenethylamine hallucinogens for 5-HT2 receptors and hallucinogenic potency in humans.9,10 Unlike 5-HT^sub 1A^ receptors, 5-HT2 receptors are not located presynaptically on serotonergic cell bodies but rather are found on subpopulations of neurons in postsynaptic regions. Quantitative autoradiographic studies show high concentrations of 5-HT2 receptors in selected regions of the brain including the neocortex (layers TVN), piriform cortex, claustrum, nucleus accumbens, olfactory tubercle, facial nucleus, and the n. tractus solitarius.11 A high density of 5-HT2 receptor mRNA has been demonstrated by in situ hybridization in the same locations.12 Immunocytochemical studies also show the presence of 5-HT2 receptors in some of the same regions.13
Por purposes of illustration, this review will focus on two brain regions, the locus coeruleus and the cerebral cortex, where the physiological actions of both LSD and the phenethylamine hallucinogens have been shown to be mediated by 5-HT2 receptors.
Hallucinogens Enhance Sensory Responses in the Locus Coeruleus
The locus coeruleus (LC) consists of two dense clusters of noradrenergic neurons located bilaterally in the upper pons at the lateral border of the fourth ventricle. The LC, which projects diffusely to virtually all regions of the neuraxis, receives an extraordinary convergence of somatic, visceral, and other sensory inputs from all regions of the body and has been likened to a novelty detector.14,16 Thus, the LC represents a nodal point for the detection of significant changes in both the internal and external environment and for relaying this information to the remainder of the central nervous system.
The systemic administration of LSD, mescaline, and other psychedelic hallucinogens in anesthetized rats results in a decrease in spontaneous activity and, paradoxically, a facilitation of the activation of LC neurons by sensory stimuli.16 The effects of hallucinogens on LC neurons can be reversed by low intravenous doses of selective 5-HT2 antagonists such as ritanserin.17 Antipsychotic drugs are also able to reverse the actions of hallucinogens in the locus coeruleus at doses correlating with their affinity for 5-HT2, but not dopamine and adrenergic receptors.1^ Of particular note is the fact that spiperone, which has almost a 1,000-fold greater affinity for 5-HT2 than for the closely related 5-HT^sub 1C^ receptors, completely blocks the effects of the hallucinogens at extremely low doses. Thus, the effects of hallucinogens on the LC appear to mediated by 5-HT2 rather than 5-HT^sub 1C^ receptors.
The effects of systemically administered hallucinogens are not through a local action on LC cell bodies as they are not mimicked by the microiontophoretic application of the drugs. Moreover, the excitation of LC neurons by the local application of acetylcholine, glutamate, or substance P is not enhanced by the systemic administration of mescaline or LSD.16 These results imply that hallucinogens are acting indirectly, presumably via afferents to the LC Consequently, the LC itself cannot be used as a model for studying the direct cellular actions of hallucinogens. Nevertheless, the effects of the hallucinogens on the LC are of interest because this nucleus receives such an extraordinarily widespread convergence of sensory information, both somatosensory and visceral, relaying this information to virtually all other parts of the neuraxis.
Hallucinogens Excite a Subpopulation of lnterneurons in the Cerebral Cortex
The majority of 5-HT2 receptors in the brain are located in the cerebral cortex. Accordingly, the effects of hallucinogens on perceptual and cognitive functions are likely to be mediated predominantly within this structure. In brain slices, pyramidal cells in various regions of the cerebral cortex have been found to respond to 5-HT by either a small hyperpolarization, depolarization, or no change in potential.1921 It has been suggested that the depolarizations are mediated by 5-HT2 receptors since they can be blocked by 5-HT2 antagonists. However, the interpretation of this data is somewhat unclear since 5-HT2 receptors do not seem to be expressed by pyramidal cells in the cortex.12,13
Recently, we have found a novel effect of 5-HT in piriform cortex: the enhancement of spontaneous inhibitory postsynaptic potentials (IPSPs), which can be observed in pyramidal cells.21 A similar induction of IPSPs by 5-HT has also been seen in prefrontal cortex (Sheldon and Aghajanian, unpublished observation). The IPSPs in piriform cortex are blocked by the GABA antagonist bicuculline, suggesting that it is an excitation of GABAergic interneurons by 5-HT that gives rise to IPSPs in the pyramidal cells. In accord with this expectation, a subpopulation of cortical interneurons has been found that are excited directly by 5-HT. The 5-HT2Zo-HT^sub 1C^ antagonist, ritanserin, blocks the 5-HT-induced activation of these interneurons. The hallucinogens LSD and DOM behave as partial agonists in this system, producing a modest activation by themselves but occluding the full effect of 5-HT.
The effects of antipsychotic drugs on 5-HT activations of cortical interneurons have also been examined. Two atypical antipsychotic drugs, risperidone and clozapine, in a concentration range corresponding to clinically efficacious blood levels, are able to block 5-HT2 responses of interneurons in rat piriform cortex.22 In contrast, the conventional antipsychotic drugs chlorpromazine and haloperidol do not block 5-HT2 responses, at least in a clinically relevant concentration range. These results indicate that certain atypical antipsychotic drugs could exert their antipsychotic effects, at least in part, by antagonizing 5-HT2 receptors on cortical interneurons.
SUMIVIARY OF BASIC STUDIES
5-HT2 Receptors Mediate the Physiological Actions of Hallucinogens
Physiological actions shared by LSD and the phenethylamine hallucinogens have been described in detail for two brain regions, the LC and the cerebral cortex. In these and other regions, there is pharmacological evidence that the neuronal effects of both classes of hallucinogens are mediated through 5-HT2 receptors. Thus, a unitary mechanism - a shared action at 5-HT2 but not 5-HT^sub 1A^ receptors - holds for the two major classes of psychedelic hallucinogens (Figure 2). Thus, 50 years after the discovery of LSD, the 5-HT hypothesis can be reformulated in terms of a specific 5-HT receptor subtype, the 5-HT2 receptor.
5-HT2 Receptors Also Mediate the Behavioral Effects of Hallucinogens
The shared behavioral effects of indoleamine and phenethylamine hallucinogens are also mediated through 5-HT2 receptors.9,23,25 What then is the relationship between the behavioral effects of hallucinogens and their actions on single neurons? We can now begin to provide neuronally and regionally specific answers to this question. For example, the enhancement of sensory responsivity of LC neurons may contribute to the characteristic intensification of certain kinds of perceptual experience produced by hallucinogens in humans. In the cerebral cortex, the persistent (and presumably inappropriate) activation of a subpopulation of interneurons that express 5-HT2 receptors may underlie some of the cognitive and perceptual distortions produced by these drugs. Of course, 5-HT2 receptors also are expressed in neurons elsewhere in the central nervous system and it will be important to see how these also contribute to the overall response to hallucinogens.
Figure 2. Schematic representation of the shared action of indoleamine (eg, LSD and DMT) and phenethylamine hallucinogens (eg, mescaline and DOM) on 5-HT2 receptors on postsynaptic neurons and the nonshared action at somatodendritic 5-HT,A autoreceptors located on 5-HT neurons of the raphe nuclei where only the indoleamine hallucinogens have direct activity. Postsynaptic neurons with 5-HT2 receptors include a subpopulation of cortical interneurons, facial motoneurons, and unidentified afferents to the locus coeruleus (LC), The arrow/dotted line connotes the low efficacy of indoleamine hallucinogens at postsynaptic 5-HT^sub 1A^ receptors (5-HT^sub 1A^) located on hippocampal and cortical pyramidal cells. 5-HT itself is depicted as having equivalent activity at postsynaptic 5-HT^sub 1A^ and 5-HT2 receptors.
Do Presynaptic 5-HT^sub 1A^ Receptors Play a Contributory Role?
In contrast to the phenethylamines, LSD and the other indoleamine hallucinogens re, semble 5-HT itself in having a broad spectrum of activity at multiple 5-HT receptors. This raises questions as to how the indoleamines can be hallucinogens if they simply act in the same way as the endogenous transmitter. A possible answer may lie in the fact that LSD and the other indoleamine hallucinogens are much more efficacious as agonists at somatodendritic 5-HT^sub 1A^ receptors than they are at postsynaptic 5-HT^sub 1A^ receptors.6,26
By inhibiting the firing of serotonergic neurons via somatodendritic 5-HT^sub 1A^ receptors, the indoleamine hallucinogens would reduce the tonic release of 5-HT onto postsynaptic neurons. Since they are poor agonists at postsynaptic 5-HT^sub 1A^ receptors, their primary action postsynaptically would be on 5-HT2 (or 5-HT^sub 1C^) receptors. Thus, the net effect on postsynaptic neurons would be similar for the indoleamine and phenethylamine hallucinogens, namely an imbalance between the activation of 5-HT2 receptors relative to other 5-HT receptor subtypes (Figure 2).
5-HT2 Receptors, Atypical Antipsychotic Drugs, and Schizophrenia
Investigations into the mechanism of action of hallucinogenic drugs have been heuristic in promoting interest in the functional significance of 5-HT2 receptors. Clinically, 5HT2 receptors have been implicated in the action of atypical antipsychotic drugs and the pathogenesis of schizophrenia.
As described above, atypical antipsychotic drugs such as clozapine readily block 5-HT2 receptor-mediated activations of cortical interneurons. In view of these pharmacological findings, it is interesting that there may be decreased 5-HT2 receptor binding in frontal cortex of postmortem tissue obtained from schizophrenic patients.27 Other postmortem studies show a selective loss of interneurons but not pyramidal cells in anterior cingulate and prefrontal cortices in brains of schizophrenic patients.28
As yet, it is not known whether 5-HT2 receptor-expressing interneurons are among those that are lost. However, since there is a decrease in 5-HT2 receptor binding, it is possible that the interneuronal abnormality in schizophrenia may involve 5-HT2-expressing cells. Thus, studies on the anatomy, physiology, and pharmacology of cortical interneurons that express 5-HT2 receptors are of interest in relation to a growing number of postmortem studies on the possible involvement of these receptors in the pathophysiology of schizophrenia.
Implications for the Treatment of "Bad Trips"
There is now ample evidence that 5-HT2 antagonists are effective in blocking both the electrophysiological and behavioral effects of hallucinogenic drugs in animal model systems. Thus, 5-HT2 antagonists offer great promise as potential treatments for the acute adverse reactions ("bad trips") that are experienced by some individuals with the use of LSD and other psychedelic hallucinogens.
As yet there are no FDA-approved selective 5-HT2 antagonists available for use clinically. There are a few classical 5-HT antagonists (eg, cyproheptadine), approved for other indications that may have effectiveness against the hallucinogens since they have antagonist activity at 5-HT2 receptors. However, side effects produced by actions at other receptors may limit their clinical utility.
Based on preclinical studies, clozapine should also be effective as a treatment, but its use is currently restricted to drugresistant schizophrenia. Other prospective antidotes include recently developed "atypical" antipsychotic drugs such as risperidone which have a very high affinity for 5-HT2 receptors. Obviously, as such drugs become available on the US market it would be of interest to test them as possible specific treatments for adverse reactions to the psychedelic hallucinogens.
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