Inhibition of return in the human 5HT2A agonist and NMDA antagonist model of psychosis

Neurobiological research on N,N-dimethyltryptamine (DMT) and its potentiation by monoamine oxidase (MAO) inhibition: from ayahuasca to synthetic combinations of DMT and MAO inhibitors

The potent hallucinogen N,N-dimethyltryptamine (DMT) has garnered significant interest in recent years due to its profound effects on consciousness and its therapeutic psychopotential. DMT is an integral (but not exclusive) psychoactive alkaloid in the Amazonian plant-based brew ayahuasca, in which admixture of several β-carboline monoamine oxidase A (MAO-A) inhibitors potentiate the activity of oral DMT, while possibly contributing in other respects to the complex psychopharmacology of ayahuasca. Irrespective of the route of administration, DMT alters perception, mood, and cognition, presumably through agonism at serotonin (5-HT) 1A/2A/2C receptors in brain, with additional actions at other receptor types possibly contributing to its overall psychoactive effects. Due to rapid first pass metabolism, DMT is nearly inactive orally, but co-administration with β-carbolines or synthetic MAO-A inhibitors (MAOIs) greatly increase its bioavailability and duration of action. The synergistic effects of DMT and MAOIs in ayahuasca or synthetic formulations may promote neuroplasticity, which presumably underlies their promising therapeutic efficacy in clinical trials for neuropsychiatric disorders, including depression, addiction, and post-traumatic stress disorder. Advances in neuroimaging techniques are elucidating the neural correlates of DMT-induced altered states of consciousness, revealing alterations in brain activity, functional connectivity, and network dynamics. In this comprehensive narrative review, we present a synthesis of current knowledge on the pharmacology and neuroscience of DMT, β-carbolines, and ayahuasca, which should inform future research aiming to harness their full therapeutic potential.

Cognitive functioning associated with acute and subacute effects of classic psychedelics and MDMA - a systematic review and meta-analysis

Classic psychedelics and MDMA have a colorful history of recreational use, and both have recently been re-evaluated as tools for the treatment of psychiatric disorders. Several studies have been carried out to assess potential long-term effects of a regular use on cognition, delivering distinct results for psychedelics and MDMA. However, to date knowledge is scarce on cognitive performance during acute effects of those substances. In this systematic review and meta-analysis, we investigate how cognitive functioning is affected by psychedelics and MDMA during the acute drug effects and the sub-acute ("afterglow") window. Our quantitative analyses suggest that acute cognitive performance is differentially affected by psychedelics when compared to MDMA: psychedelics impair attention and executive function, whereas MDMA primarily affects memory, leaving executive functions and attention unaffected. Our qualitative analyses reveal that executive functioning and creativity may be increased during a window of at least 24 h after the acute effects of psychedelics have subsided, whereas no such results have been observed for MDMA. Our findings may contribute to inform recommendations on harm reduction for recreational settings and to help fostering differential approaches for the use of psychedelics and MDMA within a therapeutic framework.

Pharmacological, neural, and psychological mechanisms underlying psychedelics: A critical review

This paper provides a critical review of several possible mechanisms at different levels of analysis underlying the effects and therapeutic potential of psychedelics. At the (1) biochemical level, psychedelics primarily affect the 5-HT_2A receptor, increase neuroplasticity, offer a critical period for social reward learning, and have anti-inflammatory properties. At the (2) neural level, psychedelics have been associated with reduced efficacy of thalamo-cortical filtering, the loosening of top-down predictive signaling and an increased sensitivity to bottom-up prediction errors, and activation of the claustro-cortical-circuit. At the (3) psychological level, psychedelics have been shown to induce altered and affective states, they affect cognition, induce belief change, exert social effects, and can result in lasting changes in behavior. We outline the potential for a unifying account of the mechanisms underlying psychedelics and contrast this with a model of pluralistic causation. Ultimately, a better understanding of the specific mechanisms underlying the effects of psychedelics could allow for a more targeted therapeutic approach. We highlight current challenges for psychedelic research and provide a research agenda to foster insight in the causal-mechanistic pathways underlying the efficacy of psychedelic research and therapy.

Models of psychedelic drug action: modulation of cortical-subcortical circuits

Classic psychedelic drugs such as psilocybin and lysergic acid diethylamide (LSD) have recaptured the imagination of both science and popular culture, and may have efficacy in treating a wide range of psychiatric disorders. Human and animal studies of psychedelic drug action in the brain have demonstrated the involvement of the serotonin 2A (5-HT2A) receptor and the cerebral cortex in acute psychedelic drug action, but different models have evolved to try to explain the impact of 5-HT2A activation on neural systems. Two prominent models of psychedelic drug action (the cortico-striatal thalamo-cortical, or CSTC, model and relaxed beliefs under psychedelics, or REBUS, model) have emphasized the role of different subcortical structures as crucial in mediating psychedelic drug effects. We describe these models and discuss gaps in knowledge, inconsistencies in the literature and extensions of both models. We then introduce a third circuit-level model involving the claustrum, a thin strip of grey matter between the insula and the external capsule that densely expresses 5-HT2A receptors (the cortico-claustro-cortical, or CCC, model). In this model, we propose that the claustrum entrains canonical cortical network states, and that psychedelic drugs disrupt 5-HT2A-mediated network coupling between the claustrum and the cortex, leading to attenuation of canonical cortical networks during psychedelic drug effects. Together, these three models may explain many phenomena of the psychedelic experience, and using this framework, future research may help to delineate the functional specificity of each circuit to the action of both serotonergic and non-serotonergic hallucinogens.

Dissecting the shared genetic basis of migraine and mental disorders using novel statistical tools

Migraine is three times more prevalent in people with bipolar disorder or depression. The relationship between schizophrenia and migraine is less certain although glutamatergic and serotonergic neurotransmission are implicated in both. A shared genetic basis to migraine and mental disorders has been suggested but previous studies have reported weak or non-significant genetic correlations and five shared risk loci. Using the largest samples to date and novel statistical tools, we aimed to determine the extent to which migraine's polygenic architecture overlaps with bipolar disorder, depression and schizophrenia beyond genetic correlation, and to identify shared genetic loci. Summary statistics from genome-wide association studies were acquired from large-scale consortia for migraine (n cases = 59 674; n controls = 316 078), bipolar disorder (n cases = 20 352; n controls = 31 358), depression (n cases = 170 756; n controls = 328 443) and schizophrenia (n cases = 40 675, n controls = 64 643). We applied the bivariate causal mixture model to estimate the number of disorder-influencing variants shared between migraine and each mental disorder, and the conditional/conjunctional false discovery rate method to identify shared loci. Loci were functionally characterized to provide biological insights. Univariate MiXeR analysis revealed that migraine was substantially less polygenic (2.8 K disorder-influencing variants) compared to mental disorders (8100-12 300 disorder-influencing variants). Bivariate analysis estimated that 800 (SD = 300), 2100 (SD = 100) and 2300 (SD = 300) variants were shared between bipolar disorder, depression and schizophrenia, respectively. There was also extensive overlap with intelligence (1800, SD = 300) and educational attainment (2100, SD = 300) but not height (1000, SD = 100). We next identified 14 loci jointly associated with migraine and depression and 36 loci jointly associated with migraine and schizophrenia, with evidence of consistent genetic effects in independent samples. No loci were associated with migraine and bipolar disorder. Functional annotation mapped 37 and 298 genes to migraine and each of depression and schizophrenia, respectively, including several novel putative migraine genes such as L3MBTL2, CACNB2 and SLC9B1. Gene-set analysis identified several putative gene sets enriched with mapped genes including transmembrane transport in migraine and schizophrenia. Most migraine-influencing variants were predicted to influence depression and schizophrenia, although a minority of mental disorder-influencing variants were shared with migraine due to the difference in polygenicity. Similar overlap with other brain-related phenotypes suggests this represents a pool of 'pleiotropic' variants that influence vulnerability to diverse brain-related disorders and traits. We also identified specific loci shared between migraine and each of depression and schizophrenia, implicating shared molecular mechanisms and highlighting candidate migraine genes for experimental validation.

Psychedelic Cognition—The Unreached Frontier of Psychedelic Science

Psychedelic compounds hold the promise of changing the face of neuroscience and psychiatry as we know it. There have been numerous proposals to use them to treat a range of neuropsychiatric conditions such as depression, anxiety, addiction and PTSD; and trials to date have delivered positive results in favor of the novel therapeutics. Further to the medical use, the wider healthy population is gaining interest in these compounds. We see a surge in personal use of psychedelic drugs for reasons not limited to spiritual enhancement, improved productivity, aiding the management of non-pathological anxiety and depression, and recreational interests. Notably, microdosing—the practice of taking subacute doses of psychedelic compounds—is on the rise. Our knowledge about the effects of psychedelic compounds, however, especially in naturalistic settings, is still fairly limited. In particular, one of the largest gaps concerns the acute effects on cognition caused by psychedelics. Studies carried out to date are riddled with limitations such as having disparate paradigms, small sample sizes, and insufficient breadth of testing on both unhealthy and healthy volunteers. Moreover, the studies are majoritarily limited to laboratory settings and do not assess the effects at multiple dosages within the same paradigm nor at various points throughout the psychedelic experience. This review aims to summarize the studies to date in relation to how psychedelics acutely affect different domains of cognition. In the pursuit of illuminating the current limitations and offering long-term, forward-thinking solutions, this review compares and contrasts findings related to how psychedelics impact memory, attention, reasoning, social cognition, and creativity.

Neuropsychological Functioning in Users of Serotonergic Psychedelics - A Systematic Review and Meta-Analysis

Background: Serotonergic psychedelics (SPs) like LSD, psilocybin, DMT, and mescaline are a heterogeneous group of substances that share agonism at 5-HT2a receptors. Besides the ability of these substances to facilitate profoundly altered states of consciousness, persisting psychological effects have been reported after single administrations, which outlast the acute psychedelic effects. In this review and meta-analysis, we investigated if repeated SP use associates with a characteristic neuropsychological profile indicating persisting effects on neuropsychological function. Methods: We conducted a systematic review of studies investigating the neuropsychological performance in SP users, searching studies in Medline, Web of Science, embase, ClinicalTrials.gov, and EudraCT. Studies were included if they reported at least one neuropsychological measurement in users of SPs. Studies comparing SP users and non-users that reported mean scores and standard deviations were included in an exploratory meta-analysis. Results: 13 studies (N = 539) published between 1969 and 2020 were included in this systematic review. Overall, we found that only three SPs were specifically investigated: ayahuasca (6 studies, n = 343), LSD (5 studies, n = 135), and peyote (1 study, n = 61). However, heterogeneity of the methodological quality was high across studies, with matching problems representing the most important limitation. Across all SPs, no uniform pattern of neuropsychological impairment was identified. Rather, the individual SPs seemed to be associated with distinct neuropsychological profiles. For instance, one study (n = 42) found LSD users to perform worse in trials A and B of the Trail-Making task, whereas meta-analytic assessment (5 studies, n = 352) of eleven individual neuropsychological measures indicated a better performance of ayahuasca users in the Stroop incongruent task (p = 0.03) and no differences in the others (all p > 0.05). Conclusion: The majority of the included studies were not completely successful in controlling for confounders such as differences in non-psychedelic substance use between SP-users and non-users. Our analysis suggests that LSD, ayahuasca and peyote may have different neuropsychological consequences associated with their use. While LSD users showed reduced executive functioning and peyote users showed no differences across domains, there is some evidence that ayahuasca use is associated with increased executive functioning.

Impaired inhibition of return during free-viewing behaviour in patients with schizophrenia

Schizophrenia affects various aspects of cognitive and behavioural functioning. Eye movement abnormalities are commonly observed in patients with schizophrenia (SZs). Here we examined whether such abnormalities reflect an anomaly in inhibition of return (IOR), the mechanism that inhibits orienting to previously fixated or attended locations. We analyzed spatiotemporal patterns of eye movement during free-viewing of visual images including natural scenes, geometrical patterns, and pseudorandom noise in SZs and healthy control participants (HCs). SZs made saccades to previously fixated locations more frequently than HCs. The time lapse from the preceding saccade was longer for return saccades than for forward saccades in both SZs and HCs, but the difference was smaller in SZs. SZs explored a smaller area than HCs. Generalized linear mixed-effect model analysis indicated that the frequent return saccades served to confine SZs' visual exploration to localized regions. The higher probability of return saccades in SZs was related to cognitive decline after disease onset but not to the dose of prescribed antipsychotics. We conclude that SZs exhibited attenuated IOR under free-viewing conditions, which led to restricted scene scanning. IOR attenuation will be a useful clue for detecting impairment in attention/orienting control and accompanying cognitive decline in schizophrenia.

Double-blind comparison of the two hallucinogens psilocybin and dextromethorphan: effects on cognition

OBJECTIVES

Classic psychedelics (serotonin 2A receptor agonists) and dissociative hallucinogens (NMDA receptor antagonists), though differing in pharmacology, may share neuropsychological effects. These drugs, however, have undergone limited direct comparison. This report presents data from a double-blind, placebo-controlled within-subjects study comparing the neuropsychological effects of multiple doses of the classic psychedelic psilocybin with the effects of a single high dose of the dissociative hallucinogen dextromethorphan (DXM).

METHODS

Twenty hallucinogen users (11 females) completed neurocognitive assessments during five blinded drug administration sessions (10, 20, and 30 mg/70 kg psilocybin; 400 mg/70 kg DXM; and placebo) in which participants and study staff were informed that a large range of possible drug conditions may have been administered.

RESULTS

Global cognitive impairment, assessed using the Mini-Mental State Examination during peak drug effects, was not observed with psilocybin or DXM. Orderly and dose-dependent effects of psilocybin were observed on psychomotor performance, working memory, episodic memory, associative learning, and visual perception. Effects of DXM on psychomotor performance, visual perception, and associative learning were in the range of effects of a moderate to high dose (20 to 30 mg/70 kg) of psilocybin.

CONCLUSIONS

This was the first study of the dose effects of psilocybin on a large battery of neurocognitive assessments. Evidence of delirium or global cognitive impairment was not observed with either psilocybin or DXM. Psilocybin had greater effects than DXM on working memory. DXM had greater effects than all psilocybin doses on balance, episodic memory, response inhibition, and executive control.

A Polygenic Risk Score of glutamatergic SNPs associated with schizophrenia predicts attentional behavior and related brain activity in healthy humans

Multiple genetic variations impact on risk for schizophrenia. Recent analyses by the Psychiatric Genomics Consortium (PGC2) identified 128 SNPs genome-wide associated with the disorder. Furthermore, attention and working memory deficits are core features of schizophrenia, are heritable and have been associated with variation in glutamatergic neurotransmission. Based on this evidence, in a sample of healthy volunteers, we used SNPs associated with schizophrenia in PGC2 to construct a Polygenic-Risk-Score (PRS) reflecting the cumulative risk for schizophrenia, along with a Polygenic-Risk-Score including only SNPs related to genes implicated in glutamatergic signaling (Glu-PRS). We performed Factor Analysis for dimension reduction of indices of cognitive performance. Furthermore, both PRS and Glu-PRS were used as predictors of cognitive functioning in the domains of Attention, Speed of Processing and Working Memory. The association of the Glu-PRS on brain activity during the Variable Attention Control (VAC) task was also explored. Finally, in a second independent sample of healthy volunteers we sought to confirm the association between the Glu-PRS and both performance in the domain of Attention and brain activity during the VAC.We found that performance in Speed of Processing and Working Memory was not associated with any of the Polygenic-Risk-Scores. The Glu-PRS, but not the PRS was associated with Attention and brain activity during the VAC. The specific effects of Glu-PRS on Attention and brain activity during the VAC were also confirmed in the replication sample.Our results suggest a pathway specificity in the relationship between genetic risk for schizophrenia, the associated cognitive dysfunction and related brain processing.

Neuropharmacology of N,N-dimethyltryptamine

N,N-dimethyltryptamine (DMT) is an indole alkaloid widely found in plants and animals. It is best known for producing brief and intense psychedelic effects when ingested. Increasing evidence suggests that endogenous DMT plays important roles for a number of processes in the periphery and central nervous system, and may act as a neurotransmitter. This paper reviews the current literature of both the recreational use of DMT and its potential roles as an endogenous neurotransmitter. Pharmacokinetics, mechanisms of action in the periphery and central nervous system, clinical uses and adverse effects are also reviewed. DMT appears to have limited neurotoxicity and other adverse effects except for intense cardiovascular effects when administered intravenously in large doses. Because of its role in nervous system signaling, DMT may be a useful experimental tool in exploring how the brain works, and may also be a useful clinical tool for treatment of anxiety and psychosis.

S-ketamine influences strategic allocation of attention but not exogenous capture of attention

We investigated whether s-ketamine differentially affects strategic allocation of attention. In Experiment 1, (1) a less visible cue was weakly masked by the onsets of competing placeholders or (2) a better visible cue was not masked because it was presented in isolation. Both types of cue appeared more often opposite of the target (75%) than at target position (25%). With this setup, we tested for strategic attention shifts to the opposite side of the cues and for exogenous attentional capture toward the cue's side in a short cue-target interval, as well as for (reverse) cueing effects in a long cue-target interval after s-ketamine and after placebo treatment in a double-blind within-participant design. We found reduced strategic attention shifts after cues presented without placeholders for the s-ketamine compared to the placebo treatment in the short interval, indicating an early effect on the strategic allocation of attention. No differences between the two treatments were found for exogenous attentional capture by less visible cues, suggesting that s-ketamine does not affect exogenous attentional capture in the presence of competing distractors. Experiment 2 confirmed that the competing onsets of the placeholders prevented the strategic cueing effect. Taken together, the results indicate that s-ketamine affects strategic attentional capture, but not exogenous attentional capture. The findings point to a more prominent role of s-ketamine during top-down controlled forms of attention that require suppression of automatic capture than during automatic capture itself.

Pharmacology of hallucinations: several mechanisms for one single symptom?

Hallucinations are complex misperceptions, that principally occur in schizophrenia or after intoxication induced by three main classes of drugs: psychostimulants, psychedelics, and dissociative anesthetics. There are at least three different pharmacological ways to induce hallucinations: (1) activation of dopamine D2 receptors (D2Rs) with psychostimulants, (2) activation of serotonin 5HT2A receptors (HT2ARs) with psychedelics, and (3) blockage of glutamate NMDA receptors (NMDARs) with dissociative anesthetics. In schizophrenia, the relative importance of NMDAR and D2R in the occurrence of hallucinations is still debated. Slight clinical differences are observed for each etiology. Thus, we investigated whether the concept of hallucination is homogenous, both clinically and neurobiologically. A narrative review of the literature is proposed to synthesize how the main contributors in the field have approached and tried to solve these outstanding questions. While some authors prefer one explanatory mechanism, others have proposed more integrated theories based on the different pharmacological psychosis models. In this review, such theories are discussed and faced with the clinical data. In addition, the nosological aspects of hallucinations and psychosis are addressed. We suggest that if there may be common neurobiological pathways between the different pharmacological systems that are responsible for the hallucinations, there may also be unique properties of each system, which explains the clinical differences observed.

Acute effects of ayahuasca on neuropsychological performance: differences in executive function between experienced and occasional users

BACKGROUND

Ayahuasca, a South American psychotropic plant tea containing the psychedelic 5-HT2A receptor agonist N,N-dimethyltryptamine, has been shown to increase regional cerebral blood flow in prefrontal brain regions after acute administration to humans. Despite interactions at this level, neuropsychological studies have not found cognitive deficits in abstinent long-term users.

OBJECTIVES

Here, we wished to investigate the effects of acute ayahuasca intake on neuropsychological performance, specifically on working memory and executive function.

METHODS

Twenty-four ayahuasca users (11 long-term experienced users and 13 occasional users) were assessed in their habitual setting using the Stroop, Sternberg, and Tower of London tasks prior to and following ayahuasca intake.

RESULTS

Errors in the Sternberg task increased, whereas reaction times in the Stroop task decreased and accuracy was maintained for the whole sample following ayahuasca intake. Interestingly, results in the Tower of London showed significantly increased execution and resolution times and number of movements for the occasional but not the experienced users. Additionally, a correlation analysis including all subjects showed that impaired performance in the Tower of London was inversely correlated with lifetime ayahuasca use.

CONCLUSIONS

Acute ayahuasca administration impaired working memory but decreased stimulus-response interference. Interestingly, detrimental effects on higher cognition were only observed in the less experienced group. Rather than leading to increased impairment, greater prior exposure to ayahuasca was associated with reduced incapacitation. Compensatory or neuromodulatory effects associated with long-term ayahuasca intake could underlie preserved executive function in experienced users.

Persistent effects of chronic clozapine on the cellular and behavioral responses to LSD in mice

RATIONALE

In schizophrenia patients, optimal treatment with antipsychotics requires weeks to months of sustained drug therapy. However, single administration of antipsychotic drugs can reverse schizophrenia-like behavioral alterations in rodent models of psychosis. This raises questions about the physiological relevance of such antipsychotic-like activity.

OBJECTIVE

This study evaluates the effects of chronic treatment with clozapine on the cellular and behavioral responses induced by the hallucinogenic serotonin 5-HT(2A) receptor agonist lysergic acid diethylamide (LSD) as a mouse model of psychosis.

METHOD

Mice were treated chronically (21 days) with 25 mg/kg/day clozapine. Experiments were conducted 1, 7, 14, and 21 days after the last clozapine administration. [(3)H]Ketanserin binding and 5-HT ( 2A ) mRNA expression were determined in mouse somatosensory cortex. Head-twitch behavior, expression of c-fos, which is induced by all 5-HT(2A) agonists, and expression of egr-1 and egr-2, which are LSD-like specific, were assayed.

RESULTS

Head-twitch response was decreased and [(3)H]ketanserin binding was downregulated in 1, 7, and 14 days after chronic clozapine. 5-HT ( 2A ) mRNA was reduced 1 day after chronic clozapine. Induction of c-fos, but not egr-1 and egr-2, was rescued 7 days after chronic clozapine. These effects were not observed after short treatment (2 days) with clozapine or chronic haloperidol (1 mg/kg/day).

CONCLUSION

Our findings provide a murine model of chronic atypical antipsychotic drug action and suggest downregulation of the 5-HT(2A) receptor as a potential mechanism involved in these persistent therapeutic-like effects.

Use of cannabis enhances attentional inhibition

OBJECTIVE

Orienting attention to an irrelevant location hampers the response to subsequent targets presented at that location in relation to novel, not previously attended, locations. This inhibitory effect has been named inhibition of return. We conducted an experiment to study the temporal course of inhibition of return in users of cannabis.

METHOD

Twenty-five cannabis users who self-reported a regular frequency of cannabis use in joints per month, and 26 drug-free controls participated in the study. We employed a typical inhibition of return task with a single cue and manipulated the time interval between the onset of the cue and the target (150, 350, 550, 1500, and 2550 ms). Participants were asked to detect the onset of the target regardless of its location.

RESULTS

The group of cannabis users showed a significantly greater overall inhibition relative to the group of nonusers. Furthermore, inhibition of return appeared earlier (at the 350 ms cue-target interval) in the user group.

CONCLUSIONS

This is the first study to show that attentional inhibition is enhanced in cannabis users. More research is needed to determine whether greater inhibition represents an advantage or disadvantage for visual search performance of cannabis users.

On the measurement of the effects of alcohol and illicit substances on inhibition of return

RATIONALE

Inhibition of return (IOR) refers to the delayed orienting of attention to previously inspected locations in favour of novel locations. Given its implications for visual attention and search, researchers have begun to investigate whether IOR may be impaired by the use of alcohol or illicit substances (e.g. d-amphetamine).

OBJECTIVES

The present paper reviews the existing literature exploring the impact of alcohol and other drugs on IOR through the use of the model spatial cueing task developed by Posner.

RESULTS

Studies were located that investigated IOR paradigm with respect to either (a) acute effects of alcohol or other psychoactive substances and (b) hallucinogenic drug states as models for psychosis. Findings suggest that alcohol, d-amphetamine and some hallucinogens may alter the timecourse of IOR. This review also yields a critical qualitative analysis of the methodology of studies in this field of research and the implications of particular methodological features for interpreting previous findings.

CONCLUSIONS

The importance of using multiple stimulus onset asynchronies, employing a cue-back to centre paradigm and distinguishing between acute and chronic substance use are emphasized. Furthermore, questions are raised as to whether findings suggest an impact of psychoactive substances on the subcortical mechanisms that play a critical role in the generation of IOR or are an indirect effect resulting from impairment of the cortical mechanisms responsible for voluntary disengagement of attention. Directions for future research and particular methodological approaches are highlighted.

Auditory orienting and inhibition of return in schizophrenia: a functional magnetic resonance imaging study

Patients with schizophrenia (SP) exhibit deficits in both attentional reorienting and inhibition of return (IOR) during visual tasks. However, it is currently unknown whether these deficits are supramodal in nature and how these deficits relate to other domains of cognitive dysfunction. In addition, the neuronal correlates of this pathological orienting response have not been investigated in either the visual or auditory modality. Therefore, 30 SP and 30 healthy controls (HC) were evaluated with an extensive clinical protocol and functional magnetic resonance imaging (fMRI) during an auditory cuing paradigm. SP exhibited both increased costs and delayed IOR during auditory orienting, suggesting a prolonged interval for attentional disengagement from cued locations. Moreover, a delay in the development of IOR was associated with cognitive deficits on formal neuropsychological testing in the domains of attention/inhibition and working memory. Event-related fMRI showed the characteristic activation of a frontoparietal network (invalid trials>valid trials), but there were no differences in functional activation between patients and HC during either attentional reorienting or IOR. Current results suggest that orienting deficits are supramodal in nature in SP, and are related to higher-order cognitive deficits that directly interfere with day-to-day functioning.

Psilocybin-induced deficits in automatic and controlled inhibition are attenuated by ketanserin in healthy human volunteers

The serotonin-2A receptor (5-HT(2A)R) has been implicated in the pathogenesis of schizophrenia and related inhibitory gating and behavioral inhibition deficits of schizophrenia patients. The hallucinogen psilocybin disrupts automatic forms of sensorimotor gating and response inhibition in humans, but it is unclear so far whether the 5-HT(2A)R or 5-HT(1A)R agonist properties of its bioactive metabolite psilocin account for these effects. Thus, we investigated whether psilocybin-induced deficits in automatic and controlled inhibition in healthy humans could be attenuated by the 5-HT(2A/2C)R antagonist ketanserin. A total of 16 healthy participants received placebo, ketanserin (40 mg p.o.), psilocybin (260 μg/kg p.o.), or psilocybin plus ketanserin in a double-blind, randomized, and counterbalanced order. Sensorimotor gating was measured by prepulse inhibition (PPI) of the acoustic startle response. The effects on psychopathological core dimensions and behavioral inhibition were assessed by the altered states of consciousness questionnaire (5D-ASC), and the Color-Word Stroop Test. Psilocybin decreased PPI at short lead intervals (30 ms), increased all 5D-ASC scores, and selectively increased errors in the interference condition of the Stroop Test. Stroop interference and Stroop effect of the response latencies were increased under psilocybin as well. Psilocybin-induced alterations were attenuated by ketanserin pretreatment, whereas ketanserin alone had no significant effects. These findings suggest that the disrupting effects of psilocybin on automatic and controlled inhibition processes are attributable to 5-HT(2A)R stimulation. Sensorimotor gating and attentional control deficits of schizophrenia patients might be due to changes within the 5-HT(2A)R system.

Association between Serotonin-Related Polymorphisms in 5HT2A, TPH1, TPH2 Genes and Bipolar Disorder in Korean Population

OBJECTIVE

The aim of the present study was to examine the association between serotonin-related gene polymorphisms and bipolar disorder in the Korean population. In addition, we sought to explore the relationship between the clinical characteristics of bipolar patients and serotonin-related gene polymorphisms.

METHODS

Inpatients with bipolar disorder (n=103) and control subjects (n=86) were genotyped for 5HT2A 1438A/G, tryptophan hydroxylase 1 (TPH1) 218 A/C, and TPH2 703G/T. We divided patients with bipolar disorder into two groups according to the presence of psychotic symptoms. The severity of their symptoms was measured using the Young Mania Rating Scale (YMRS) and the Brief Psychiatric Rating Scale (BPRS).

RESULTS

There were no significant differences in the genotype distributions or allelic frequencies in the three serotonergic polymorphisms between patients with bipolar disorder and normal controls. There were significant differences in genotype distributions and allele frequencies of the 5-HT2A -1438A/G polymorphism between the psychotic mania group and the non-psychotic mania group (genotype: chi(2)=7.50, p=0.024; allele: chi(2)=5.92, p=0.015). However, after Bonferroni correction this signifact difference disappeared. We did not find significant differences in the genotype distributions or allelic frequencies in the TPH1 218 A/C and TPH2 703G/T polymorphisms between the psychotic mania group and non-psychotic mania group.

CONCLUSION

We failed to found the statistically significant association between three polymorphisms and bipolar disorder. However, there was a trend towards association between 5-HT2A -1438A/G polymorphism and psychotic symptom in bipolar disorder. Future research should seek to clarify this association.

Hallucinogens as discriminative stimuli in animals: LSD, phenethylamines, and tryptamines

BACKGROUND

Although man's first encounters with hallucinogens predate written history, it was not until the rise of the sister disciplines of organic chemistry and pharmacology in the nineteenth century that scientific studies became possible. Mescaline was the first to be isolated and its chemical structure determined. Since then, additional drugs have been recovered from their natural sources and synthetic chemists have contributed many more. Given their profound effects upon human behavior and the need for verbal communication to access many of these effects, some see humans as ideal subjects for study of hallucinogens. However, if we are to determine the mechanisms of action of these agents, establish hypotheses testable in human subjects, and explore the mechanistic links between hallucinogens and such apparently disparate topics as idiopathic psychosis, transcendental states, drug abuse, stress disorders, and cognitive dysfunction, studies in animals are essential. Stimulus control by hallucinogens has provided an intuitively attractive approach to the study of these agents in nonverbal species.

OBJECTIVE

The intent of this review is to provide a brief account of events from the time of the first demonstration of hallucinogen-induced stimulus control to the present. In general, the review is limited to lysergic acid diethylamide (LSD) and the hallucinogenic derivatives of phenethylamine and tryptamine.

RESULTS

The pharmacological basis for stimulus control by LSD and hallucinogenic phenethylamines and tryptamines is serotonergic in nature. The 5-HT(2A) receptor appears to be the primary site of action with significant modulation by other serotonergic sites including 5-HT(2C) and 5-HT(1A) receptors. Interactions with other neurotransmitters, especially glutamate and dopamine, are under active investigation. Most studies to date have been conducted in the rat but transgenic mice offer interesting possibilities.

CONCLUSIONS

Hallucinogen-induced stimulus control provides a unique behavioral tool for the prediction of subjective effects in man and for the elucidation of the pharmacological mechanisms of the action of these agents.

Pharmacological modulation of the neural basis underlying inhibition of return (IOR) in the human 5-HT2A agonist and NMDA antagonist model of psychosis

RATIONALE

Attentional deficits are common symptoms in schizophrenia. Recent evidence suggests that schizophrenic patients show abnormalities in spatial orienting of attention, particularly a deficit of inhibition of return (IOR). IOR is mostly thought to reflect an automatic, inhibitory mechanism protecting the organism from redirecting attention to previously scanned, insignificant locations. Pharmacologic challenges with hallucinogens have been used as models for psychosis.

OBJECTIVES

The aim of this study was to investigate the neural correlates underlying orienting of attention in the human N-methyl-D-aspartic acid antagonist and 5-HT2A agonist models of psychosis.

MATERIALS AND METHODS

Fourteen healthy volunteers participated in a randomized, double-blind, cross-over event-related functional magnetic resonance imaging (fMRI) study with dimethyltryptamine (DMT) and S-ketamine. We administered a covert orienting of attention task with nonpredictive peripheral cues, and we scanned the subjects on two separate days at least 14 days apart with a placebo and a verum condition on each day.

RESULTS

DMT, but not S-ketamine, slowed down reaction times significantly. IOR was blunted after DMT, but not after S-ketamine. Relative to placebo, S-ketamine increased activation in the IOR condition in the right superior frontal gyrus, left superior temporal gyrus, and right midfrontal frontal gyrus.

CONCLUSIONS

The discrepancy between the behavioral and functional imaging outcome indicates that pharmacological fMRI might be a sensitive tool to detect drug-modulated blood oxygenation level-dependent signal changes in the absence of behavioral abnormalities. Our findings might help to further clarify the contradictory findings of IOR in schizophrenic patients and might, thus, shed more light on possible differential pathomechanisms of schizophrenic symptoms.

S-ketamine and GABA-A-receptor interaction in humans: an exploratory study with I-123-iomazenil SPECT

OBJECTIVE

We aimed to probe for regional cerebral effects of S-ketamine on in vivo GABA-A-receptor binding in healthy human subjects.

METHODS

We investigated I-123-iomazenil SPECT before, during and after administration of S-ketamine in a blinded placebo-controlled study design (n = 12 in both groups). Analyses of SPECT were performed with voxel-based statistical parametric mapping (SPM), and statistical comparisons were made between the groups. We also assessed biochemical and behavioural changes during S-ketamine infusion.

RESULTS

S-ketamine induced positive and negative symptoms measured by the Brief Psychiatric Rating scale (BPRS). It increased the cortisol and prolactin levels. Image analysis revealed significantly decreased I-123-iomazenil binding in bilateral dorsomedial prefrontal cortex during S-ketamine administration when compared to placebo.

CONCLUSION

Our study delivers preliminary evidence for an in vivo interaction of S-ketamine with GABA-A-receptors in human dorsomedial prefrontal cortex.

Human hallucinogen research: guidelines for safety

There has recently been a renewal of human research with classical hallucinogens (psychedelics). This paper first briefly discusses the unique history of human hallucinogen research, and then reviews the risks of hallucinogen administration and safeguards for minimizing these risks. Although hallucinogens are relatively safe physiologically and are not considered drugs of dependence, their administration involves unique psychological risks. The most likely risk is overwhelming distress during drug action ('bad trip'), which could lead to potentially dangerous behaviour such as leaving the study site. Less common are prolonged psychoses triggered by hallucinogens. Safeguards against these risks include the exclusion of volunteers with personal or family history of psychotic disorders or other severe psychiatric disorders, establishing trust and rapport between session monitors and volunteer before the session, careful volunteer preparation, a safe physical session environment and interpersonal support from at least two study monitors during the session. Investigators should probe for the relatively rare hallucinogen persisting perception disorder in follow-up contact. Persisting adverse reactions are rare when research is conducted along these guidelines. Incautious research may jeopardize participant safety and future research. However, carefully conducted research may inform the treatment of psychiatric disorders, and may lead to advances in basic science.

Mismatch negativity generation in the human 5HT2A agonist and NMDA antagonist model of psychosis

RATIONALE

Many studies have reported deficits of mismatch negativity (MMN) in schizophrenic patients. Pharmacological challenges with hallucinogens in healthy humans are used as models for psychotic states. Previous studies reported a significant reduction of MMN after ketamine (N-methyl-D-aspartate acid [NMDA] antagonist model) but not after psilocybin (5HT2A agonist model).

OBJECTIVES

The aim of the present study was to directly compare the two models of psychosis using an intraindividual crossover design.

MATERIALS AND METHODS

Fifteen healthy subjects participated in a randomized, double-blind, crossover study with a low and a high dose of the 5HT2A agonist dimethyltryptamine (DMT) and the NMDA antagonist S-ketamine. During electroencephalographic recording, the subjects were performing the AX-version of a continuous performance test (AX-CPT). A source analysis of MMN was performed on the basis of a four-source model of MMN generation.

RESULTS

Nine subjects completed both experimental days with the two doses of both drugs. Overall, we found blunted MMN and performance deficits in the AX-CPT after both drugs. However, the reduction in MMN activity was overall more pronounced after S-ketamine intake, and only S-ketamine had a significant impact on the frontal source of MMN.

CONCLUSIONS

The NDMA antagonist model and the 5HT2A agonist model of psychosis display distinct neurocognitive profiles. These findings are in line with the view of the two classes of hallucinogens modeling different aspects of psychosis.

Identification of a serotonin/glutamate receptor complex implicated in psychosis

The psychosis associated with schizophrenia is characterized by alterations in sensory processing and perception. Some antipsychotic drugs were identified by their high affinity for serotonin 5-HT2A receptors (2AR). Drugs that interact with metabotropic glutamate receptors (mGluR) also have potential for the treatment of schizophrenia. The effects of hallucinogenic drugs, such as psilocybin and lysergic acid diethylamide, require the 2AR and resemble some of the core symptoms of schizophrenia. Here we show that the mGluR2 interacts through specific transmembrane helix domains with the 2AR, a member of an unrelated G-protein-coupled receptor family, to form functional complexes in brain cortex. The 2AR-mGluR2 complex triggers unique cellular responses when targeted by hallucinogenic drugs, and activation of mGluR2 abolishes hallucinogen-specific signalling and behavioural responses. In post-mortem human brain from untreated schizophrenic subjects, the 2AR is upregulated and the mGluR2 is downregulated, a pattern that could predispose to psychosis. These regulatory changes indicate that the 2AR-mGluR2 complex may be involved in the altered cortical processes of schizophrenia, and this complex is therefore a promising new target for the treatment of psychosis.

Cajal and brain plasticity: insights relevant to emerging concepts of mind

The main legacies of Cajal are his drawings of brain structure and their connections, and his ideas of brain plasticity, not only in the mature brain but also during development and after brain injury. As the 21st century begins, many scientists are asking an old question: "how does the brain express the mind?" Although most models of mind incorporate the brain connections produced by Cajal, his ideas of plasticity are largely ignored. The purpose of this chapter is to review how some of Cajal's ideas can be useful in understanding the expression of the mind. I have also introduced several concepts and facts not available during Cajal's life. I cover the concept of homeostasis, the global projections of the monoamine neurons, and the actions of "mind-expanding" drugs. The global projecting neurons, because their monoamine transmitters have such a long history, are considered 1st order systems. The point-to-point connections are considered 2nd order systems. Their importance in theories of functional localization studies is briefly reviewed. Finally, a new model is presented called "Plastic Homeostasis," which incorporates the plastic interactions between 1st and 2nd order neurons. It is hoped that this review will encourage others to study the ideas presented by Cajal when considering functions of the brain. The emerging models of the mind would be well served by a review of the theoretical writing of Cajal.

[Inhibition of return in schizophrenia: a review]

INTRODUCTION

Most visual environments contain more information than the human brain can process in real time. To overcome this limitation, the attention system acts as a filter by selectively orienting attention to specific regions of the visual field. This ability to orient attention can be reflected in covert shift processes of attention.

LITERATURE FINDINGS

In a typical covert orienting task, subjects have to maintain fixation on a central cross and respond as quickly as possible to a target, which appears in a peripheral box following a cue that summons attention to the direction where the target is going to appear (valid cueing) or to the contralateral direction (invalid cueing). When the cues are nonpredictive, the response characteristics critically depend on stimulus-onset asynchrony (SOA). With short SOAs (<300ms), valid cues result in a reaction time advantage over invalid trials, which is due to a reflexive shift of attention towards the source of stimulation. In contrast, with longer SOAs, valid cues result in longer reaction times to the subsequent target.

DISCUSSION

This phenomenon is known as the inhibition of return and is mostly thought to reflect an inhibitory mechanism protecting the organism from redirecting attention to previously scanned insignificant locations. Many studies have reported blunted or delayed inhibition of return in patients with schizophrenia. However, some authors reported normal amounts of inhibition of return. This can be partly explained by the use of manipulations of the covert orienting of the attention paradigm that is known to enhance the course of inhibition of return.

CONCLUSION

The deficit of inhibition of return seems to be time-stable and to be unrelated to psychopathology or length of illness. The contribution of neuroleptic medication to this deficit cannot be determined. Recent data suggest a deficit of inhibition of return in two human models of psychosis (dimethyltryptamine and ketamine). Further studies should clarify whether blunted inhibition of return might represent a trait marker of schizophrenia.

Prepulse inhibition of the startle reflex and its attentional modulation in the human S-ketamine and N,N-dimethyltryptamine (DMT) models of psychosis

Patients with schizophrenia exhibit diminished prepulse inhibition (PPI) of the acoustic startle reflex and deficits in the attentional modulation of PPI. Pharmacological challenges with hallucinogens are used as models for psychosis in both humans and animals. Remarkably, in contrast to the findings in schizophrenic patients and in animal hallucinogen models of psychosis, previous studies with healthy volunteers demonstrated increased levels of PPI after administration of low to moderate doses of either the antiglutamatergic hallucinogen ketamine or the serotonergic hallucinogen psilocybin. The aim of the present study was to investigate the influence of moderate and high doses of the serotonergic hallucinogen N,N-dimethyltryptamine (DMT) and the N-methyl-D-aspartate antagonist S-ketamine on PPI and its attentional modulation in humans. Fifteen healthy volunteers were included in a double-blind cross-over study with two doses of DMT and S-ketamine. Effects on PPI and its attentional modulation were investigated. Nine subjects completed both experimental days with the two doses of both drugs. S-ketamine increased PPI in both dosages, whereas DMT had no significant effects on PPI. S-ketamine decreased and DMT tended to decrease startle magnitude. There were no significant effects of either drug on the attentional modulation of PPI. In human experimental hallucinogen psychoses, and even with high, clearly psychotogenic doses of DMT or S-ketamine, healthy subjects failed to exhibit the predicted attenuation of PPI. In contrast, PPI was augmented and the startle magnitude was decreased after S-ketamine. These data point to important differences between human hallucinogen models and both animal hallucinogen models of psychosis and naturally occurring schizophrenia.