A primate model for the study of hallucinogens

Preclinical and translational models for delirium: Recommendations for future research from the NIDUS delirium network

Delirium is a common, morbid, and costly syndrome that is closely linked to Alzheimer's disease (AD) and AD-related dementias (ADRD) as a risk factor and outcome. Human studies of delirium have advanced our knowledge of delirium incidence and prevalence, risk factors, biomarkers, outcomes, prevention, and management. However, understanding of delirium neurobiology remains limited. Preclinical and translational models for delirium, while challenging to develop, could advance our knowledge of delirium neurobiology and inform the development of new prevention and treatment approaches. We discuss the use of preclinical and translational animal models in delirium, focusing on (1) a review of current animal models, (2) challenges and strategies for replicating elements of human delirium in animals, and (3) the utility of biofluid, neurophysiology, and neuroimaging translational markers in animals. We conclude with recommendations for the development and validation of preclinical and translational models for delirium, with the goal of advancing awareness in this important field.

A narrative synthesis of research with 5-MeO-DMT

BACKGROUND

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

METHODS

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

RESULTS

5-MeO-DMT is serotonergic agonist, with highest affinity for 5-HT_1A receptors. It was studied in a variety of animal models, but clinical studies with humans are lacking. Epidemiological studies indicate that, like other psychedelics, 5-MeO-DMT induces profound alterations in consciousness (including mystical experiences), with potential beneficial long-term effects on mental health and well-being.

CONCLUSION

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

Effect of Hallucinogens on Unconditioned Behavior

Because of the ethical and regulatory hurdles associated with human studies, much of what is known about the psychopharmacology of hallucinogens has been derived from animal models. However, developing reliable animal models has proven to be a challenging task due to the complexity and variability of hallucinogen effects in humans. This chapter focuses on three animal models that are frequently used to test the effects of hallucinogens on unconditioned behavior: head twitch response (HTR), prepulse inhibition of startle (PPI), and exploratory behavior. The HTR has demonstrated considerable utility in the neurochemical actions of hallucinogens. However, the latter two models have clearer conceptual bridges to human phenomenology. Consistent with the known mechanism of action of hallucinogens in humans, the behavioral effects of hallucinogens in rodents are mediated primarily by activation of 5-HT2A receptors. There is evidence, however, that other receptors may play secondary roles. The structure-activity relationships (SAR) of hallucinogens are reviewed in relation to each model, with a focus on the HTR in rats and mice.

Recent advances in the neuropsychopharmacology of serotonergic hallucinogens

Serotonergic hallucinogens, such as (+)-lysergic acid diethylamide, psilocybin, and mescaline, are somewhat enigmatic substances. Although these drugs are derived from multiple chemical families, they all produce remarkably similar effects in animals and humans, and they show cross-tolerance. This article reviews the evidence demonstrating the serotonin 5-HT2A receptor is the primary site of hallucinogen action. The 5-HT2A receptor is responsible for mediating the effects of hallucinogens in human subjects, as well as in animal behavioral paradigms such as drug discrimination, head twitch response, prepulse inhibition of startle, exploratory behavior, and interval timing. Many recent clinical trials have yielded important new findings regarding the psychopharmacology of these substances. Furthermore, the use of modern imaging and electrophysiological techniques is beginning to help unravel how hallucinogens work in the brain. Evidence is also emerging that hallucinogens may possess therapeutic efficacy.

Anxiolytic drug discovery: what are the novel approaches and how can we improve them?

INTRODUCTION

Contemporary biological psychiatry uses experimental (animal) models to increase our understanding of affective disorder pathogenesis. Despite the well-recognized spectrum nature of affective disorders, modern anxiolytic drug discovery mainly targets specific pathways and molecular determinants within a single phenotypic domain. However, greater understanding of the integrative mechanisms and pathogenesis is essential in order to develop new effective therapies.

AREAS COVERED

In this review, the authors emphasize the importance of a 'domain interplay-oriented' approach to experimental affective research. They also highlight the need to expand the scope of anxiolytic drug targets to better understand the pathogenesis of anxiety-spectrum disorders.

EXPERT OPINION

There is the potential to markedly improve the utility of animal models for affective disorders. First, the authors suggest that one such way would be by analyzing the systems of several domains and their interplay to better understand disease pathogenesis. Further, it could also be improved by expanding the range of model species and by extending the spectrum of anxiolytic drug targets; this would help to focus on emerging and unconventional systems to better develop new therapies.

Perspectives on zebrafish models of hallucinogenic drugs and related psychotropic compounds

Among different classes of psychotropic drugs, hallucinogenic agents exert one of the most prominent effects on human and animal behaviors, markedly altering sensory, motor, affective, and cognitive responses. The growing clinical and preclinical interest in psychedelic, dissociative, and deliriant hallucinogens necessitates novel translational, sensitive, and high-throughput in vivo models and screens. Primate and rodent models have been traditionally used to study cellular mechanisms and neural circuits of hallucinogenic drugs' action. The utility of zebrafish ( Danio rerio ) in neuroscience research is rapidly growing due to their high physiological and genetic homology to humans, ease of genetic manipulation, robust behaviors, and cost effectiveness. Possessing a fully characterized genome, both adult and larval zebrafish are currently widely used for in vivo screening of various psychotropic compounds, including hallucinogens and related drugs. Recognizing the growing importance of hallucinogens in biological psychiatry, here we discuss hallucinogenic-induced phenotypes in zebrafish and evaluate their potential as efficient preclinical models of drug-induced states in humans.

Non-human primates: model animals for developmental psychopathology

Non-human primates have been used to model psychiatric disease for several decades. The success of this paradigm has issued from comparable cognitive skills, brain morphology, and social complexity in adult monkeys and humans. Recently, interest in biological psychiatry has focused on similar brain, social, and emotional developmental processes in monkeys. In part, this is related to evidence that early postnatal experiences in human development may have profound implications for subsequent mental health. Non-human primate studies of postnatal phenomenon have generally fallen into three basic categories: experiential manipulation (largely manipulations of rearing), pharmacological manipulation (eg drug-induced psychosis), and anatomical localization (defined by strategic surgical damage). Although these efforts have been very informative each of them has certain limitations. In this review we highlight general findings from the non-human primate postnatal developmental literature and their implications for primate models in psychiatry. We argue that primates are uniquely capable of uncovering interactions between genes, environmental challenges, and development resulting in altered risk for psychopathology.

The behavioral pharmacology of hallucinogens

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.

Serotonin and Dopamine Interactions in Rodents and Primates: Implications for Psychosis and Antipsychotic Drug Development

Since the late 1950s, appreciation of dopamine receptor blockade has played a primary role in understanding the mechanism underlying the therapeutic effects of antipsychotic drugs in schizophrenic patients in treating the positive symptoms of schizophrenia (e.g., delusions and hallucinations). Development of the second generation of antipsychotic drugs, otherwise known as atypical antipsychotic drugs, has resulted in treatments with improved subjective tolerability but relatively modest improvements in the negative symptoms of schizophrenia such as avolition, flat affect, and anhedonia. The major current challenge is to develop medications which can further improve negative symptoms treatment and also tackle the intractable clinical problems of cognitive impairment associated with schizophrenia. Further advances along these lines with respect to the dopaminergic and serotonergic neurostransmitter systems will be aided by an appreciation of the interaction between dopamine and serotonin receptor subtypes in a range of key brain structures, such as the prefrontal cortex, thalamus, striatum, amygdala, hippocampus, and the brain stem nuclei, from which the cell bodies of monoaminergic-containing neurons originate. Increasing emphasis on the use of animal models which are homologous to critical aspects of the pathophysiology in the brains of schizophrenic patients will also be required, especially as negative symptoms and cognitive impairment become an important focus for generating novel therapeutics.

Differential effects of 5-HT agonists and antagonists on the repeated acquisition and performance of response sequences in monkeys

As a means of characterizing the role of 5-HT1A and 5-HT2A receptors in learning, 5-hydroxytryptamine (5-HT) agonists and antagonists with selective affinities for each receptor subtype (i.e. 8-hydroxy-dipropylaminotetralin (8-OH-DPAT), (-)-4-(dipropylamino)-1,3,4,5-tetrahydrobenz-(c,d,)indole-6-carboxamide (LY228729), (+/-)-1-(4-iodo-2,5-dimeth-oxyphenyl)-2-aminopropane hydrochloride (DOI), 4-iodo-N-[2- [4-(methoxyphenyl)-1-piperazinyl] ethyl]-N-2-pyridinyl-benzamide hydrochloride (p-MPPI), N-[2- [4- (2-methoxyphenyl)-1-piperazinyl] ethyl] -N-2-pyridinyl-cyclohexanecarboxamide maleate (WAY-100635), 1-(2-methoxyphenyl)-4-[4-(2-phthalimido)butyllpiperazine hydrobromide (NAN-190) and ritanserin) were administered to monkeys responding under a multiple schedule of repeated acquisition and performance. In addition, a selective 5-HT1A receptor agonist (8-OH-DPAT) was administered in combination with a 5-HT2A receptor antagonist (ritanserin) to examine any potential interactions between the two 5-HT receptor subtypes. When administered alone, 8-OH-DPAT (0.1-3.2mg/kg), LY228729 (0.32-3.2 mg/kg) and DOI (0.018-3.2 mg/kg) dose-dependently decreased overall response rate in both schedule components, and generally increased the percentage of errors in the acquisition components at doses lower than those that increased the percentage of errors in the performance components. At the doses of each drug tested (i.e. 0.1 or 0.32 mg/kg), both p-MPPI and WAY-100635 antagonized the disruptive effects of 8-OH-DPAT, by shifting the dose-effect curves for overall response rate and the percentage of errors to the right. In contrast, ritanserin (0.32 or 1mg/kg) had little or no effect on the disruptions produced by 8-OH-DPAT, but it effectively antagonized the rate-decreasing and error-increasing effects produced by the 5-HT2A agonist DOI. Administration of the 5-HT1A antagonists WAY-100635 and NAN-190 alone produced dose-dependent rate-decreasing effects, but the effects on accuracy of responding in the acquisition components differed from those of the 5-HT1A agonists (8-OH-DPAT and LY228729), in that they did not produce an increase in the percentage of errors. Together, these results suggest that 5-HT is capable of disrupting learning in monkeys through actions at both the 5-HT1A and 5-HT2A receptors, and that 5-HT2A receptor antagonism does not unilaterally modify the effects produced by 5-HTA1A receptor activation.

Differential tolerance to biological and subjective effects of four closely spaced doses of N,N-dimethyltryptamine in humans

Tolerance of the behavioral effects of the short-acting, endogenous hallucinogen, N,N-dimethyltryptamine (DMT) is seen inconsistently in animals, and has not been produced in humans. The nature and time course of responses to repetitive, closely spaced administrations of an hallucinogenic dose of DMT were characterized. Thirteen experienced hallucinogen users received intravenous 0.3 mg/kg DMT fumarate, or saline placebo, four times, at 30 min intervals, on 2 separate days, in a randomized, double-blind, design. Tolerance to "psychedelic" subjective effects did not occur according to either clinical interview or Hallucinogen Rating Scale scores. Adrenocorticotropic hormone (ACTH), prolactin, cortisol, and heart rate responses decreased with repeated DMT administration, although blood pressure did not. These data demonstrate the unique properties of DMT relative to other hallucinogens and underscore the differential regulation of the multiple processes mediating the effects of DMT.

Behavioral effects of (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane, (DOI) in the elevated plus-maze test

The serotonin (5-hydroxytryptamine, 5-HT) system has consistently been implicated in the actions of (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) and other hallucinogens. Recent evidence suggest that the 5-HT2A/2C receptor subtypes may be major targets for such drugs in the CNS. DOI-treated hooded rats (0.1-5.0 mg/kg) and DOI treated ICR mice (0.1-2.0 mg/kg), displayed aversions at lower doses and anti-aversions at higher doses to the open arms of the plus-maze. Mianserin (0.5 mg/kg) and ketanserin (0.1 mg/kg) blocked the anti-aversive behavior, but only mianserin was effective at reversing the aversions produced by the higher doses of DOI in the ICR mice. DOI produced an intense aversion in the DBA/2 and anti-aversion in the C57/BL6 mice to the open arms of the plus-maze. These opposing actions of DOI in the plus-maze may be exploited in studying the neurobehavioral effects of hallucinogens. Since flumazenil was ineffective at blocking the DOI induced changes, it was concluded that the mechanism of DOI induced anxiolysis or anxiogenesis may not involve an action at the benzodiazepine receptors.

Alpha-adrenergic and 5-HT2-serotonergic effects of some beta-phenylethylamines on isolated rat thoracic aorta

1. 2C-H [2-(2,5-dimethoxyphenyl)ethylamine] (pD2 = 6.74), TMPEA [2,(2,4,5-trimethoxyphenyl)ethylamine] (pD2 = 5.83), 2C-D [2-(2,5-dimethoxy-4-methylphenyl)ethylamine] (pD2 = 5.06), homoveratrylamine [DMPEA, 2-(4,5-dimethoxyphenyl)ethylamine] (pD2 = 4.46) and homopiperonylamine [MDPEA, 2-(3,4-methylenedioxyphenyl)ethylamine] (pD2 = 4.19), elicit concentration-dependent contraction of the isolated rat thoracic aorta. 2. At 9.9 x 10(-6) M, 2C-N [2-(2,5-dimethoxy-4-nitrophenyl)ethylamine] behaves as a competitive antagonist to serotonin in this preparation. 3. Considering previous results with the structurally related 2C-B [2-(4-bromo-2,5-dimethoxyphenyl)ethylamine], weak or partial agonistic activity or antagonism of aortic contraction appears to be related to psychedelic properties reported in humans for phenylethylamines.

Comparative effects of LSD and lisuride: clues to specific hallucinogenic drug actions

This review compares the effects of LSD and its nonhallucinogenic congener lisuride hydrogen maleate (LHM) on various biochemical, behavioral and electrophysiological indices of neuronal function. The underlying rationale is that any differences between the effects of LSD and LHM might be relevant to neuronal actions which are unique and specific to hallucinogenic drugs and thereby provide clues to the neurobiological substrates of hallucinogenesis. In biochemical studies, LHM appears to be very similar to LSD with respect to its actions on monoaminergic (5-HT, DA, NE) systems. The major difference between the two ergots appears quantitative in nature since LHM is more potent than LSD, especially on DA neurochemistry. Needed at the present time are additional comparative studies of LSD and LHM with respect to other biochemical measures, for example on the release of 5-HT and DA and comparisons at more molecular levels such as subcellular compartmentation. Also necessary are more intensive regional analyses on specific subpopulations of 5-HT and DA systems (mesolimbic, mesostriatal and mesocortical). Behavioral studies are relatively uniform in their characterization of the greater DA-ergic activity of LHM as compared to LSD. In particular, the drug discrimination (DD) procedure has indicated a more specific interaction of LSD with 5-HT neuronal systems as compared to LHM and has successfully differentiated the relative roles of 5-HT and DA systems in the behavioral effects of LSD and LHM. Electrophysiological studies have been consistent with both biochemical and behavioral findings with respect to the much greater effect of LHM on DA receptors. In fact, the effects of LSD on DA-containing neurons are both weak and heterogeneous, again indicating a need for more detailed analyses of specific DA projection systems. The greater potency of LHM than LSD on 5-HT containing dorsal raphe neurons has lessened the attractiveness of the once popular theory that hallucinogenic efficacy is related to diminution of impulse flow in 5-HT systems but has also spawned greater interest in the possible role of postsynaptic 5-HT receptors in hallucinogenic drug action. Thus far, the most interesting finding is the ability of LSD and other hallucinogens, but not LHM, to potentiate an excitomodulatory effect of 5-HT in the facial motor nucleus. If such a phenomenon occurs more generally in the CNS, the importance of this finding will be greatly enhanced. Preliminary data is presented which suggests that LSD may also induce such an effect in a limbic forebrain structure, the nucleus accumbens.(ABSTRACT TRUNCATED AT 400 WORDS)