Characterisation of human 5-hydroxytryptamine2A and 5-hydroxytryptamine2C receptors expressed in the human neuroblastoma cell line SH-SY5Y: comparative stimulation by hallucinogenic drugs

G protein-coupled receptors as targets for transformative neuropsychiatric therapeutics

G protein-coupled receptors (GPCRs) constitute the largest family of druggable genes in the human genome. Even though perhaps 30% of approved medications target GPCRs, they interact with only a small number of them. Here, we consider whether there might be new opportunities for transformative therapeutics for neuropsychiatric disorders by specifically targeting both known and understudied GPCRs. Using psychedelic drugs that target serotonin receptors as an example, we show how recent insights into the structure, function, signaling, and cell biology of these receptors have led to potentially novel therapeutics. We next focus on the possibility that nonpsychedelic 5-HT2A receptor agonists might prove to be safe and rapidly acting antidepressants. Finally, we examine understudied and orphan GPCRs using the MRGPR family of receptors as an example.

Experimental strategies to discover and develop the next generation of psychedelics and entactogens as medicines

Research on classical psychedelics (psilocybin, LSD and DMT) and entactogen, MDMA, has produced a renaissance in the search for more effective drugs to treat psychiatric, neurological and various peripheral disorders. Psychedelics and entactogens act though interaction with 5-HT_2A and other serotonergic receptors and/or monoamine reuptake transporters. 5-HT, which serves as a neurotransmitter and hormone, is ubiquitously distributed in the brain and peripheral organs, tissues and cells where it has vasoconstrictor, pro-inflammatory and pro-nociceptive actions. Serotonergic psychedelics and entactogens have known safety and toxicity risks. For these drugs, the risks been extensively researched and empirically assessed through human experience. However, novel drug-candidates require thorough non-clinical testing not only to predict clinical efficacy, but also to address the risks they pose during clinical development and later after approval as prescription medicines. We have defined the challenges researchers will encounter when developing novel serotonergic psychedelics and entactogens. We describe screening techniques to predict clinical efficacy and address the safety/toxicity risks emerging from our knowledge of the existing drugs: 1) An early-stage, non-clinical screening cascade to pharmacologically characterise novel drug-candidates. 2) Models to detect hallucinogenic activity. 3) Models to differentiate hallucinogens from entactogens. 4) Non-clinical preclinical lead optimisation technology (PLOT) screening to select drug-candidates. 5) Modified animal models to evaluate the abuse and dependence risks of novel psychedelics in Safety Pharmacology testing. Our intention has been to design non-clinical screening strategies that will reset the balance between benefits and harms to deliver more effective and safer novel psychedelics for clinical use. This article is part of the Special Issue on 'National Institutes of Health Psilocybin Research Speaker Series'.

In vitro assays for the functional characterization of (psychedelic) substances at the serotonin receptor 5-HT2A R

Serotonergic psychedelics are substances that induce alterations in mood, perception, and thought, and have the activation of serotonin (5-HT) 2A receptors (5-HT_2A Rs) as a main pharmacological mechanism. Besides their appearance on the (illicit) drug market, e.g. as new psychoactive substances, their potential therapeutic application is increasingly explored. This group of substances demonstrates a broad structural variety, leading to insufficiently described structure-activity relationships, hence illustrating the need for better functional characterization. This review therefore elaborates on the in vitro molecular techniques that have been used the most abundantly for the characterization of (psychedelic) 5-HT_2A R agonists. More specifically, this review covers assays to monitor the canonical G protein signaling pathway (e.g. measuring G protein recruitment/activation, inositol phosphate accumulation, or Ca²⁺ mobilization), assays to monitor non-canonical G protein signaling (such as arachidonic acid release), assays to monitor β-arrestin recruitment or signaling, and assays to monitor receptor conformational changes. In particular, focus lies on the mechanism behind the techniques, and the specific advantages and challenges that are associated with these. Additionally, several variables are discussed that one should consider when attempting to compare functional outcomes from different studies, both linked to the specific assay mechanism and linked to its specific execution, as these may heavily impact the assay outcome.

3,4-Methylenedioxy methamphetamine, synthetic cathinones and psychedelics: From recreational to novel psychotherapeutic drugs

The utility of classical drugs used to treat psychiatric disorders (e.g., antidepressants, anxiolytics) is often limited by issues of lack of efficacy, delayed onset of action or side effects. Psychoactive substances have a long history of being used as tools to alter consciousness and as a gateway to approach the unknown and the divinities. These substances were initially obtained from plants and animals and more recently by chemical synthesis, and its consumption evolved toward a more recreational use, leading to drug abuse-related disorders, trafficking, and subsequent banning by the authorities. However, these substances, by modulation of certain neurochemical pathways, have been proven to have a beneficial effect on some psychiatric disorders. This evidence obtained under medically controlled conditions and often associated with psychotherapy, makes these substances an alternative to conventional medicines, to which in many cases the patient does not respond properly. Such disorders include post-traumatic stress disease and treatment-resistant depression, for which classical drugs such as MDMA, ketamine, psilocybin and LSD, among others, have already been clinically tested, reporting successful outcomes. The irruption of new psychoactive substances (NPS), especially during the last decade and despite their recreational and illicit uses, has enlarged the library of substances with potential utility on these disorders. In fact, many of them were synthetized with therapeutic purposes and were withdrawn for concrete reasons (e.g., adverse effects, improper pharmacological profile). In this review we focus on the basis, existing evidence and possible use of synthetic cathinones and psychedelics (specially tryptamines) for the treatment of mental illnesses and the properties that should be found in NPS to obtain new therapeutic compounds.

Molecular insights into psychedelic drug action

A confluence of factors has renewed interest in the scientific understanding and translational potential of psychedelic drugs such as lysergic acid diethylamide (LSD), mescaline, and psilocybin: the desire for additional approaches to mental health care, incremental progress in basic and clinical research, and the reconsideration and relaxation of existing drug policies. With the United States Food and Drug Administration's designation of psilocybin as a "Breakthrough Therapy" for treatment-resistant depression, a new path has been forged for the conveyance of psychedelics to the clinic. Essential to the further development of such applications, however, is a clearer understanding of how these drugs exert their effects at the molecular level. Here we review the current knowledge regarding the molecular details of psychedelic drug actions and suggest that these discoveries can facilitate new insights into their hallucinogenic and therapeutic mechanisms.

International Union of Basic and Clinical Pharmacology. CX. Classification of Receptors for 5-hydroxytryptamine; Pharmacology and Function

5-HT receptors expressed throughout the human body are targets for established therapeutics and various drugs in development. Their diversity of structure and function reflects the important role 5-HT receptors play in physiologic and pathophysiological processes. The present review offers a framework for the official receptor nomenclature and a detailed understanding of each of the 14 5-HT receptor subtypes, their roles in the systems of the body, and, where appropriate, the (potential) utility of therapeutics targeting these receptors. SIGNIFICANCE STATEMENT: This review provides a comprehensive account of the classification and function of 5-hydroxytryptamine receptors, including how they are targeted for therapeutic benefit.

Behavioral and pharmacokinetic interactions between monoamine oxidase inhibitors and the hallucinogen 5-methoxy-N,N-dimethyltryptamine

Monoamine oxidase inhibitors (MAOIs) are often ingested together with tryptamine hallucinogens, but relatively little is known about the consequences of their combined use. We have shown previously that monoamine oxidase-A (MAO-A) inhibitors alter the locomotor profile of the hallucinogen 5-methoxy-N,N-dimethyltryptamine (5-MeO-DMT) in rats, and enhance its interaction with 5-HT2A receptors. The goal of the present studies was to investigate the mechanism for the interaction between 5-MeO-DMT and MAOIs, and to determine whether other behavioral responses to 5-MeO-DMT are similarly affected. Hallucinogens disrupt prepulse inhibition (PPI) in rats, an effect typically mediated by 5-HT2A activation. 5-MeO-DMT also disrupts PPI but the effect is primarily attributable to 5-HT1A activation. The present studies examined whether an MAOI can alter the respective contributions of 5-HT1A and 5-HT2A receptors to the effects of 5-MeO-DMT on PPI. A series of interaction studies using the 5-HT1A antagonist WAY-100,635 and the 5-HT2A antagonist MDL 11,939 were performed to assess the respective contributions of these receptors to the behavioral effects of 5-MeO-DMT in rats pretreated with an MAOI. The effects of MAO-A inhibition on the pharmacokinetics of 5-MeO-DMT and its metabolism to bufotenine were assessed using liquid chromatography-electrospray ionization-selective reaction monitoring-tandem mass spectrometry (LC-ESI-SRM-MS/MS). 5-MeO-DMT (1mg/kg) had no effect on PPI when tested 45-min post-injection but disrupted PPI in animals pretreated with the MAO-A inhibitor clorgyline or the MAO-A/B inhibitor pargyline. The combined effect of 5-MeO-DMT and pargyline on PPI was antagonized by pretreatment with either WAY-100,635 or MDL 11,939. Inhibition of MAO-A increased the level of 5-MeO-DMT in plasma and whole brain, but had no effect on the conversion of 5-MeO-DMT to bufotenine, which was found to be negligible. The present results confirm that 5-MeO-DMT can disrupt PPI by activating 5-HT2A, and indicate that MAOIs alter 5-MeO-DMT pharmacodynamics by increasing its accumulation in the central nervous system.

5-hydroxytryptamine- and dopamine-releasing effects of ring-substituted amphetamines on rat brain: a comparative study using in vivo microdialysis

Using in vivo microdialysis, a comparative study was conducted to examine the effects of amphetamine-related compounds (methamphetamine, MAP; 3,4-methylenedioxymethamphetamine, MDMA; p-methoxyamphetamine, PMA; p-methoxymethamphetamine, PMMA; 4-methylthioamphetamine, 4-MTA; 3,4,5-trimethoxyamphetamine, TMA; 2,5-dimethoxy-4-iodoamphetamine, DOI) on extracellular levels of serotonin (5-HT) and dopamine (DA). Dialysates were assayed using HPLC equipped with electrochemical detector following i.p. administration with each drug at a dose of 5 mg/kg. MAP was found to drastically and rapidly increase 5-HT and DA levels (870% and 1460%, respectively). PMA, PMMA, and 4-MTA slightly increased DA levels (150-290%) but remarkably increased 5-HT levels (540-900%). In contrast, TMA and DOI caused no detectable changes in levels of both monoamines. We observed that the potent DA-releasing action of MAP was remarkably decreased by introduction of methoxy or methylthio group at the para position (MAP vs. PMMA or 4-MTA), but introduction of two additional adjacent methoxy groups into PMA totally abolished its 5-HT-/DA-releasing action (PMA vs. TMA). In addition, para-mono-substituted compounds inhibited both monoamine oxidase (MAO) enzymes more strongly than other compounds; PMA and 4-MTA exhibited submicromolar IC50 values for MAO-A. On the other hand, TMA scarcely affected the activity of both MAO enzymes as well as extracellular levels of 5-HT and DA. In this comparative study, MDMA, PMA, and 4-MTA functioned similar to PMMA, a typical empathogen; these findings therefore could be helpful in clarifying the psychopharmacological properties of amphetamine-related, empathogenic designer drugs.

From pharmacogenetics to pharmacogenomics: the way toward the personalization of antidepressant treatment

OBJECTIVE

Major depressive disorder is the most common psychiatric disorder, worldwide, yet response and remission rates are still unsatisfactory. The identification of genetic predictors of antidepressant (AD) response could provide a promising opportunity to improve current AD efficacy through the personalization of treatment. The major steps and findings along this path are reviewed together with their clinical implications and limitations.

METHOD

We systematically reviewed the literature through MEDLINE and Embase database searches, using any word combination of "antidepressant," "gene," "polymorphism," "pharmacogenetics," "genome-wide association study," "GWAS," "response," and "adverse drug reactions." Experimental works and reviews published until March 2012 were collected and compared.

RESULTS

Numerous genes pertaining to several functional systems were associated with AD response. The more robust findings were found for the following genes: solute carrier family 6 (neurotransmitter transporter), member 4; serotonin receptor 1A and 2A; brain-derived neurotrophic factor; and catechol-O-methyltransferase. Genome-wide association studies (GWASs) provided many top markers, even if none of them reached genome-wide significance.

CONCLUSIONS

AD pharmacogenetics have not produced any knowledge applicable to routine clinical practice yet, as results were mainly inconsistent across studies. Despite this, the rising awareness about methodological deficits of past studies could allow for the identication of more suitable strategies, such as the integration of the GWAS approach with the candidate gene approach, and innovative methodologies, such as pathway analysis and study of depressive endophenotypes.

Multiple receptors contribute to the behavioral effects of indoleamine hallucinogens

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 non-selective 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.

Agonist-directed trafficking of signalling at serotonin 5-HT2A, 5-HT2B and 5-HT2C-VSV receptors mediated Gq/11 activation and calcium mobilisation in CHO cells

Several examples of agonist-directed trafficking of receptor signalling at 5-HT2A and 5-HT2C receptors have been reported that involve independent downstream transduction pathways. We now report the functional selectivity of a series of chemically diverse agonists at human (h)5-HT2A, h5-HT2B and h5-HT2C-VSV by examining two related responses, the upstream activation of Gq/11 proteins in comparison with its associated cascade of calcium mobilisation. At the h5-HT2A receptor, d-lysergic acid diethylamide (LSD) and the antiparkinsonian agents lisuride, bromocriptine and pergolide exhibit a higher potency for Gq/11 activation than calcium release in contrast with all the other tested ligands such as 5-HT, mCPP and BW723C86, that show an opposite preference of signalling pathway. Comparable observations are made at h5-HT2B and h5-HT2C-VSV receptors, suggesting a similar mechanism of functional selectivity for the three serotonin receptors. Interestingly, the non-hallucinogenic compound lisuride behaves as a partial agonist for both Gq/11 activation and calcium release at the three 5-HT2 receptors, in contrast with DOI, LSD, pergolide and bromocriptine, which are known to provoke hallucinations, and behave as more efficacious agonists. Hence, a functional selectivity for Gq/11 activation together with a threshold of efficacy at h5-HT2A (and possibly h5-HT2B and/or h5-HT2C-VSV) may contribute to hallucinogenic liability. Thus, our results extend the notion of agonist-directed trafficking of receptor signalling to all the 5-HT2-receptor family and indicate that measures of Gq/11 activation versus calcium release may be useful to identify more effective therapeutic drugs with limited side effects.

Complex discriminative stimulus properties of (+)lysergic acid diethylamide (LSD) in C57Bl/6J mice

RATIONALE

The drug discrimination procedure is the most frequently used in vivo model of hallucinogen activity. Historically, most drug discrimination studies have been conducted in the rat. With the development of genetically modified mice, a powerful new tool has become available for investigating the mechanisms of drug-induced behavior. The current paper is part of an ongoing effort to determine the utility of the drug discrimination technique for evaluating hallucinogenic drugs in mice.

OBJECTIVE

To establish the training procedures and characterize the stimulus properties of (+)lysergic acid diethylamide (LSD) in mice.

METHODS

Using a two-lever drug discrimination procedure, C57Bl/6J mice were trained to discriminate 0.45 mg/kg LSD vs saline on a VI30 sec schedule of reinforcement, with vanilla-flavored Ensure serving as the reinforcer.

RESULTS

As in rats, acquisition was orderly, but the training dose was nearly five-fold higher for mice than rats. LSD lever selection was dose-dependent. Time-course studies revealed a rapid loss of the LSD stimulus effects. The 5-HT(2A/2C) receptor agonist, 2,5-dimethoxy-4-bromoamphetamine [(-)DOB] (1.0 mg/kg), substituted fully for LSD and the 5-HT(1A) receptor agonist, 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT) (1.6 mg/kg), substituted partially for LSD. Pretreatment with the 5-HT(2A) receptor-selective antagonist, MDL 100907, or the 5-HT(1A)-selective antagonist WAY 100635, showed that each antagonist only partially blocked LSD discrimination. Substitution of 1.0 mg/kg (-)DOB for LSD was fully blocked by pretreatment with MDL 100907 but unaltered by WAY 100635 pretreatment.

CONCLUSIONS

These data suggest that in mice the stimulus effects of LSD have both a 5-HT(2A) receptor and a 5-HT(1A) receptor component.

The role of 5ht2c receptors in affective disorders

5-HT(2C) receptors are predominantly localised in the brain and their dysregulation may contribute to particular symptoms of anxiety and depression. The marked affinity of several clinically established psychotropic agents sites (e.g., tricyclic antidepressants, clozapine, fluoxetine) for 5-HT(2C) receptor has generated interest in the therapeutic potential of selective, high affinity 5-HT(2C) receptor ligands. Like the selective serotonin re-uptake inhibitor (SSRI) fluoxetine, high affinity selective agonists such as Ro 60-0175 and Ro 60-0332 have potent in vivo activity in animal models suggestive of therapeutic action against depression, obsessive-compulsive disorder (OCD) and panic disorders. In contrast, 5-HT(2C) receptor antagonists such as SB-200646A or SB-221284 show signs of anxiolytic-like activity in tests for conditioned and phobic-like anxiety in rodents whereas they are inactive in tests indicative of antidepressant, antiOCD and antipanic activity. These results are consistent with an important hypothesis proposing that 5-HT has a complex, dual action on the neural mechanism of anxiety by either facilitating or inhibiting different kinds of anxiety in different brain regions. They also suggest that 5-HT(2C) receptor subtypes play an important role in the therapeutic properties of SSRIs. Certain 5-HT(2C) receptor antagonists may possess negative efficacy at 5-HT(2C) receptors and, as inverse agonists, may control constitutive receptor activity possibly characterising some psychopathological states. Receptor variants exist in the human population and indicate possible associations between somatic mutations in the 5-HT(2C) receptor and psychopathology or response to drug treatment. Selective 5-HT(2C) receptor ligands may offer innovative and improved therapeutic opportunities for the biological treatment of specific aspects of psychiatric syndromes.

The hallucinogen 1-[2,5-dimethoxy-4-iodophenyl]-2-aminopropane (DOI) increases cortical extracellular glutamate levels in rats

Activation of the cerebral cortex is seen during hallucinations. The 5-HT(2A/C) agonist 1-[2,5-dimethoxy-4-iodophenyl]-2-aminopropane (DOI) is a potent hallucinogen that has been proposed to act by targeting 5-HT(2A) heteroceptors on thalamocortical neurons and eliciting release of glutamate from these cells, which in turn drives cortical neurons. We used in vivo microdialysis to determine if DOI increases extracellular glutamate levels. Systemic administration of DOI significantly increased extracellular glutamate levels in the somatosensory cortex of the freely-moving rat. Similarly, intracortical administration of DOI by reverse dialysis increased cortical extracellular glutamate levels. No consistent changes in either extracellular GABA or glycine levels were observed in response to DOI. The increase in glutamate levels elicited by intracortical DOI was blocked by treatment with the selective 5-HT(2A) antagonist MDL 100,907. These data are consistent with the hypothesis that 5-HT(2A) receptor-mediated regulation of glutamate release is the mechanism through which hallucinogens activate the cerebral cortex.

Activation of 5-HT2 receptors induces glycogenolysis in the rat brain

The effect of 5-HT(2) receptor activation on brain glycogen and the extracellular concentration of glucose was investigated in the present study. An injection of 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) (2 mg/kg, i.p.) or mescaline (10 mg/kg, i.p.) at an ambient temperature of 29 degrees C produced a 35-45% decrease in brain glycogen that persisted for at least 2 h. DOI also increased the extracellular concentration of glucose in the striatum by 60%. Maintenance of rats at 22 degrees C significantly attenuated DOI-induced glycogenolysis, as well as DOI-induced hyperthermia, and the increase in the extracellular concentration of glucose in the striatum. DOI-induced hyperthermia, glycogenolysis and increase in the extracellular concentration of glucose also were attenuated in rats treated with the 5-HT(2) receptor antagonist, 6-methyl-1-(methylethyl)-ergoline-8beta-carboxylic acid 2-hydroxy-1-methylpropyl ester maleate (LY-53,857) (3 mg/kg, ip). These results support the conclusion that 5-HT(2) receptor activation promotes glycogenolysis and that hyperthermia exerts a prominent role in this process.

A unique phenotype of 5-HT2C, agonist-induced GTPgamma35S binding, transferable to 5-HT2A and 5-HT2B, upon swapping intracellular regions

1 The human 5-HT(2C) receptor, when expressed heterologously in various mammalian cell lines (HEK293, SH-EP and NIH-3T3) at various receptor densities (6 to 45 pmol mg(-1) protein), mediates robust agonist-induced GTPgamma(35)S binding from coupling to G(i) subtypes of G proteins, in addition to G(q/11). Such a phenotype, however, was not seen with the human 5-HT(2A) and 5-HT(2B) receptors, indicating their common pathway with 5-HT(2C) limited to G(q/11), not including G(i). 2 Because intracellular regions are largely responsible for signalling pathways, we prepared the chimeras of the 5-HT(2A) and 5-HT(2B) receptors where the second and third intracellular loops, and the C-terminal region were replaced with the 5-HT(2C) counterparts. 3 The chimeras showed robust agonist-induced GTPgamma(35)S binding. Relative intrinsic efficacies of agonists from the GTPgamma(35)S binding were nearly identical to the reported values for their parent receptors as measured with Ca(2+) or [(3)H]-inositol phosphate accumulation. Also the chimeras displayed the same ligand-binding properties as the parent receptors. 4 We conclude that the phenotype of agonist-induced GTPgamma(35)S binding is unique to 5-HT(2C) among the 5-HT(2) receptor family, and is transferable to 5-HT(2A) and 5-HT(2B), upon swapping intracellular sequences, without altering their receptor pharmacology.

The mRNA expression of serotonin 2C subtype receptors uncoupled with inositol hydrolysis in NG108-15 cells

Cell culture systems seem to be useful for clarifying the cellular physiological mechanisms of serotonin 2C subtype receptors (5-HT2CR) and related drug action mechanisms. However, there are still few reports about cells that contain intrinsic 5-HT2CR. This report demonstrates by using RT/PCR that 5-HT2CR mRNA exists in splicing variant forms in NGI08-15 cells. The PCR results using a pair of primers that recognized sequences near the third intracellular loop site showed two neighboring bands at about 500 bp upon electrophoresis in acrylamide gels. The sequence analysis demonstrated that one band was the rat 5-HT2CR sequence and the other one was that of the mouse. Serotonin, however, did not enhance the inositol phosphates formation in NG108-15 cells. It has been reported that post-translational modifications of RNA, splicing and editing, occur at the site of the second intracellular loop domain in 5-HT2CR mRNA. Accordingly, a pair of primers that recognized this site were designed. The molecular size of the PCR product was shorter than that expected based on the sequence of the native 5-HT2CR. The fragment lacked the 95 nucleotides of native 5-HT2CR mRNA. This seems to be the reason why serotonin did not enhance inositol phosphates formation in NG108-15 cells.

Metachlorophenylpiperazine (m-CPP) induced intracavernous pressure responses in anaesthetized rats

Here we have recorded the effects of metachlorophenylpiperazine (m-CPP) on intracavernous pressure (ICP) in anesthetized rats pretreated with various pharmacological agents in an attempt to determine the mechanism and relevance of the m-CPP induced ICP response to other models of erection. m-CPP elicited consistent and significantly greater increases in ICP (71.5+/-6.6 mmHg) compared with the mixed 5-HT(2a/2c) agonists trifluoromethylphenylpiperazine (3.4+/-1.3 mmHg) and quipazine (10.9+/-1.8 mmHg). Blockade of 5-HT(2a) receptors with ketanserin failed to unmask any stimulatory effect of quipazine (7.2+/-1.0 mmHg). m-CPP induced ICP responses (71+/-7.0 mmHg) were unaffected in the presence of mianserin (63+/-5 mmHg) and ketanserin (51+/-12 mmHg). Spiperone significantly reduced the m-CPP induced increase in ICP (8.0+/-1.0 mmHg). Naloxone, yohimbine and 8-hydroxy-2-(di-n-propylamino)tetralin (8-OHDPAT) failed to elicit increases in ICP on their own. All three drugs significantly reduced the latency to the first m-CPP induced ICP response compared to saline. Yohimbine increased the duration of m-CPP induced ICP responses whereas 8-OHDPAT increased the mean number of m-CPP induced ICP responses compared to saline. The effects of m-CPP on ICP in anesthetized rats may not be mediated by 5-HT(2c) receptors and appears to be similar to erection in copula, but not erection elicited by other drugs or penile sheath retraction.

Differences in potency and efficacy of a series of phenylisopropylamine/phenylethylamine pairs at 5-HT(2A) and 5-HT(2C) receptors

The pharmacological profile of a series of (+/-)-2,5-dimethoxy-4-(X)-phenylisopropylamines (X=I, Br, NO(2), CH(3), or H) and corresponding phenylethylamines, was determined in Xenopus laevis oocytes injected with cRNA coding for rat 5-HT(2A) or 5-HT(2C) receptors. The efficacy and relative potency of these drugs were determined and compared to classical 5-HT(2) receptor agonists and antagonists. The rank order of agonist potency at the 5-HT(2A) receptor was: alpha-methyl-5-HT=5-HT>m-CPP>MK-212; at the 5-HT(2C) receptor the order was: 5-HT>alpha-methyl-5-HT>MK-212>m-CPP. All these compounds were full agonists at the 5-HT(2C) receptor, but alpha-methyl-5-HT and m-CPP showed lower efficacy at the 5-HT(2A) receptor. 4-(4-Fluorobenzoyl)-1-(4-phenylbutyl)piperidine (4F 4PP) was 200 times more potent as a 5-HT(2A) antagonist than at 5-HT(2C) receptors. Conversely, RS 102221 was 100 times more potent as a 5-HT(2C) antagonist, confirming their relative receptor selectivities. The phenylisopropylamines were partial agonists at the 5-HT(2A) receptor, with I(max) relative to 5-HT in the 22+/-7 to 58+/-15% range; the corresponding phenylethylamines had lower or undetectable efficacies. All these drugs had higher efficacies at 5-HT(2C) receptors; DOI was a full 5-HT(2C) agonist. 2C-I and the other phenylethylamines examined showed relative efficacies at the 5-HT(2C) receptor ranging from 44+/-10% to 76+/-16%. 2C-N was a 5-HT(2) receptor antagonist; the mechanism was competitive at the 5-HT(2A), but non-competitive at the 5-HT(2C) receptor. The antagonism was time-dependent at the 5-HT(2C) receptor but independent of pre-incubation time at the 5-HT(2A) receptor subtype. The alpha-methyl group determines the efficacy of these phenylalkylamines at the 5-HT(2A) and 5-HT(2C) receptors.

Serotonin antagonist profiling on 5HT2A and 5HT2C receptors by nonequilibrium intracellular calcium response using an automated flow-through fluorescence analysis system, HT-PS 100

Characterization of the potencies of agonists and antagonists in cell-based assays can be complicated by nonequilibrium conditions of functional response. We assessed the potencies of a series of serotonin (5HT) antagonists by inhibition of intracellular calcium response in HEK 293 cells expressing 5HT(2A) or 5HT(2C) receptors. An automated system, HT-PS 100, was used to profile the antagonists in two experimental setups: coadministration of agonist and antagonist to cells and preincubation of the cells with antagonist prior to agonist administration. We showed that the antagonist potencies (pIC(50) values) determined in the preincubation configuration were close to or exceeded those measured in the coadministration configuration. Closeness of the potencies determined in the two configurations supposedly reflected a rapid antagonist-receptor equilibration, whereas a significantly higher preincubation potency implied slow antagonist dissociation from the receptor. Schild analysis of the inhibition of serotonin-induced cell response by a competitive 5HT(2A) antagonist, spiperone, showed a typical competitive inhibition pattern when both the agonist and antagonist were applied simultaneously. Contrary to this, an insurmountable diminishing of the maximal cell response to serotonin was observed when the cells were preincubated with spiperone. We conclude that a combination of the coadministration and preincubation experimental setups is necessary for appropriate mechanistic interpretation and quantitative assessment of the antagonist activity when using transient functional readouts.

5-HT2A receptor-stimulated phosphoinositide hydrolysis in the stimulus effects of hallucinogens

The role of 5-HT2A-mediated stimulation of phosphoinositide hydrolysis in the discriminative effects of hallucinogens was investigated in PC12 cells stably expressing the rat 5-HT2A receptor (PC12-5-HT2A cells). The hallucinogenic compounds, D-lysergic acid diethylamide (LSD), (-)2,5-dimethoxy-4-methylamphetamine (DOM), psilocybin, N,N-dimethyltryptamine (DMT), 5-methoxy-N,N-dimethyltryptamine (MDMT) and N,N-diethyltryptamine (DET), all caused a concentration-dependent increase in the generation of [3H]inositol phosphates. The nonhallucinogenic compounds, 6-fluoro-N,N-diethyltryptamine (6-F-DET), lisuride and quipazine, also displayed significant efficacy in stimulating phosphoinositide hydrolysis, while 2-bromo-lysergic acid diethylamide (BOL), which is not a hallucinogen, did not alter inositol phosphate generation. The beta-carbolines, harmaline and harmane, also did not alter phosphoinositide hydrolysis. Comparison of these results with previous drug discrimination studies indicated the apparent lack of correlation between the degree of substitution in LSD- and DOM-trained animals and efficacy in stimulating phosphoinositide hydrolysis. The present study indicates that 5-HT2A-mediated stimulation of phosphoinositide hydrolysis does not appear to be the sole critical signaling mechanism involved in the discriminative effects of hallucinogens.

The serotonergic 5-HT(2C) agonist m-chlorophenylpiperazine increases weight-supported locomotion without development of tolerance in rats with spinal transections

The direct serotonergic agonist, m-chlorophenylpiperazine (m-CPP), displays high efficacy at 5-HT(2C) receptors. Systemic administration of m-CPP increased dramatically the percentage of weight-supported steps made on a treadmill by rats with complete midthoracic spinal transections. The improvement in motor function occurred in rats with grafts of fetal spinal cord into the site of transection (transplant rats) and in spinal rats without grafts (spinal rats). m-CPP produced a therapeutic action with its first administration and after 14 single daily injections. In contrast, the serotonin and norepinephrine reuptake inhibitor, chlorimipramine (CMI), failed to enhance weight support during 21 days of treatment. The results imply that stimulating directly 5-HT(2C) receptors restores postural support after spinal injury. Thus, 5-HT(2C) agonists are candidates for treating spinal patients chronically without the development of tolerance.

Pharmacological characterisation of human 5-HT2 receptor subtypes

Prompted by conflicting literature, this study compared the pharmacology of human 5-hydroxytryptamine2 (5-HT2) receptors expressed in SH-SY5Y cells using a fluorometric imaging plate reader (FLIPR) based Ca2+ assay. 5-Hydroxytryptamine (5-HT) increased intracellular calcium concentration ([Ca2+]i) at 5-HT2A, 5-HT2B and 5-HT2C receptors (pEC(50)=7.73+/-0.03, 8.86+/-0.04 and 7.99+/-0.04, respectively) and these responses were inhibited by mesulergine (pKB=7.42+/-0.06, 8.77+/-0.10 and 9.52+/-0.11). A range of selective agonists and antagonists displayed the expected pharmacology at each receptor subtype. Sodium butyrate pretreatment increased receptor expression in SH-SY5Y/5-HT2B (15-fold) and SH-SY5Y/5-HT2C cells (7-fold) and increased agonist potencies and relative efficacies. In contrast, sodium butyrate pretreatment of SH-SY5Y/5-HT(2A) cells did not affect receptor expression. The present study provides a direct comparison of agonist and antagonist pharmacology at 5-HT(2) receptor subtypes in a homogenous system and confirms that agonist potency and efficacy varies with the level of receptor expression.

A high-throughput glow-type aequorin assay for measuring receptor-mediated changes in intracellular calcium levels

A glow-type aequorin luminescence assay for measuring receptor-mediated stimulation of intracellular calcium levels is described and characterized. The human 5-hydroxytryptamine(2A) receptor stably coexpressed in human embryonic kidney cells with apoaequorin was used to characterize the system and showed that following the flash reaction, a stable luminescence signal could be measured using a microplate scintillation counter for between 3 and 7 h after the addition of receptor agonist. Furthermore, this luminescence was dependent on the concentration of agonist used and gave potency values that were stable over this time period. Testing a range of 5-hydroxytryptamine(2A) receptor agonists gave the expected rank order of potency for this receptor. The glow luminescence could also be inhibited by 5-hydroxytryptamine(2A) receptor antagonists, generating affinity values that directly correlated with those determined for inhibition of the flash reaction carried out under the same buffer conditions. The assay therefore gave pharmacologically relevant data and allows a significant improvement of throughput over the traditional flash-type measurements made using an injecting luminometer.

Antagonist binding at 5-HT(2A) and 5-HT(2C) receptors in the rabbit: high correlation with the profile for the human receptors

This study examined the binding of serotonin receptor antagonists at the 5-HT(2A) and 5-HT(2C) receptors of the rabbit's cerebral cortex. The 5-HT(2A) receptor was characterized by the binding of [3H]MDL 100,907 (R(+)-alpha-(2, 3-dimethoxyphenyl)-1-[2-(4-fluorophenylethyl)]-4-piperidine-methan ol) to cortical membranes and the 5-HT(2C) receptor by the binding of [3H]mesulergine in the presence of the selective 5-HT(2A) receptor ligand spiperone. Both [3H]MDL 100,907 and [3H]mesulergine demonstrated high affinity binding to single sites in rabbit membranes. Based on Scatchard plots of [3H]MDL 100,907 binding, the mean B(max) was 8.5+/-0.7 fmol/mg tissue and the mean K(d) was 33. 1+/-3.5 pM. For [3H]mesulergine binding the mean B(max) was 3.70+/-0. 58 fmol/mg tissue and the mean K(d) was 0.35+/-0.05 nM. Binding of [3H]MDL 100,907 to the 5-HT(2A) receptor and of [3H]mesulergine to the 5-HT(2C) receptor was confirmed by displacement studies with highly selective 5-HT(2A) and 5-HT(2C) receptor ligands. The pharmacological profile of these ligands in rabbits correlated highly with published values for 5-HT(2A) (r=0.91, P<0.001) and 5-HT(2C) (r=0.94, P<0.001) receptors in humans. There was also a high correlation between the profiles for human and rat 5-HT(2C) receptor (r=0.92, P<0.001), but not for 5-HT(2A) receptors (r=0.53, P>0.10). It was concluded that the rabbit provides an appropriate animal model for studies attempting to predict the pharmacology of human 5-HT(2A) and 5-HT(2C) receptors.

Bufotenine: toward an understanding of possible psychoactive mechanisms

A review of the neuropharmacology of the alleged hallucinogen bufotenine is presented, including recent experimental results showing activity similar to LSD and other known hallucinogens (psilocin and 5-MeO-DMT) at the purported hallucinogenic serotonin (5-HT) receptors, 5-HT2A and 5-HT2C. In addition, current reports of computer modeling of the receptors and ligand binding sites give evidence of bufotenine's ability to bind and activate these receptors. While binding and activation of the purported hallucinogenic receptors are not the full extent of the hallucinogenic signature, this evidence shows support for the rationale that the reported lack of the drug's classic hallucinogenic response in human experiments is due to poor ability to cross the blood brain barrier (BBB), not lack of activation of the appropriate brain receptors. Further evidence is reviewed that in some physiological states, some drugs with characteristics similar to bufotenine which do not normally cross the BBB, cross it and enter the brain. While direct human experimental evidence of bufotenine's hallucinogenic activity seems lacking, the above combined factors are considered, and possible explanations of bufotenine's reported psychoactivity are suggested. Additionally, updated experimental models testing the possible nature of bufotenine's hallucinogenic potential are proposed.

Functional characterization of agonists at recombinant human 5-HT2A, 5-HT2B and 5-HT2C receptors in CHO-K1 cells

1. The goal of this study was to characterize the agonist pharmacology of human 5-HT2A, 5-HT2B and 5-HT2C (VSV) receptors expressed in CHO-K1 (Chinese hamster ovary) cells. 2. We used a fluorometric imaging plate reader (FLIPR) which allows rapid detection of rises in intracellular calcium levels upon the addition of agonists. 3. Stimulation of all three receptors by 5-HT caused a robust concentration dependent increase in intracellular calcium levels. No such effect was observed from non-transfected control CHO-K1 cells. 4. The rank order of potency of agonists at the different receptor subtypes varied. Tryptamines, BW-723C86, d-norfenfluramine, Ro 60-0175 and LSD exhibited the following rank order of potency; 5-HT2B>5-HT2C>5-HT2A. Piperazines such as m-Chlorophenylpiperazine (mCPP), ORG-12962, MK-212 and also ORG-37684 exhibited a rank order of potency of 5-HT2C>5-HT2B>5-HT2A. The phenylisopropylamines DOI and DOB had a rank order of 5-HT2A>5-HT2B>5-HT2C. 5. Many agonists tested had partial agonist actions when compared to 5-HT, and a wide range of relative efficacies were exhibited, which was cell line dependent. For example, mCPP had a relative efficacy of 65% at 5-HT2C receptors but <25% at either 5-HT2A or 5-HT2B receptors. 6. Interpretation of literature values of functional assays using different cell lines, different receptor expression levels and different receptor isoforms, is complex. Species differences and the previous use of antagonist radioligands to characterize agonist potency in binding assays emphasizes the importance of studying agonists in the same experiment using the same assay conditions and parental cell lines.

High-affinity agonist binding correlates with efficacy (intrinsic activity) at the human serotonin 5-HT2A and 5-HT2C receptors: evidence favoring the ternary complex and two-state models of agonist action

Many modern models of receptor-G protein function assume that there is a direct relationship between high-affinity agonist binding and efficacy. The validity of this assumption has been recently questioned for the serotonin 5-HT2A receptor. We examined the intrinsic activities of various ligands in activating phosphoinositide hydrolysis and measured their respective binding affinities to the high- and low-affinity states of the 5-HT2C (VNV isoform) and 5-HT(2A) receptors. Ligand binding affinities for the high-affinity state of the receptors were determined using 1-(4-[125I]iodo-2,5-dimethoxyphenyl)2-aminopropane, whereas [3H]mesulergine and N-[3H]methylspiperone were used, in the presence of excess guanine nucleotide [guanosine 5'-O-(3-thiotriphosphate)], to define binding to the low-affinity state of the 5-HT2C and 5-HT2A receptors, respectively. Antagonists labeled the high- and low-affinity states of each receptor with comparable affinities. Previously identified inverse agonists of the 5-HT2C receptor behaved as silent antagonists in our systems even when the receptor was overexpressed at a relatively high density. In contrast, the ability of agonists to bind differentially to the high- and low-affinity states of the 5-HT2A and 5-HT2C receptors was highly correlated (r2 = 0.86 and 0.96, respectively) with their intrinsic activities. These data suggest that high-affinity agonist states can account for agonist efficacy at human 5-HT2A or 5-HT2C receptors without the need for considering additional transition or active states of the receptor-ligand complex. The procedure described herein may expedite drug discovery efforts by predicting intrinsic activities of ligands solely from ligand binding assays.

Endogenous serotonin-2A and -2C receptors in Balb/c-3T3 cells revealed in serotonin-free medium: desensitization and down-regulation by serotonin

We studied the endogenous expression of the serotonin-2A (5-hydroxytryptamine2A, 5-HT2A) 5-HT2C, and a splice-variant of the 5-HT2C receptor in murine Balb/c-3T3 fibroblast cells that is revealed when these cells are maintained in medium containing 5-HT-free serum. RNA editing of the 5-HT2C receptor was exclusively at a single brain-specific site. Addition of 5-HT (EC50 = 23 +/- 2.9 nM) induced an immediate release of calcium from an ionomycin-sensitive intracellular store by coupling to a pertussis toxin-insensitive pathway. The 5-HT-induced calcium mobilization displayed a 5-HT-2-like pharmacology, and ligand binding analyses indicated the presence of specific binding sites (27.5 +/- 2 fmol/mg protein) with a 5-HT2A-like pharmacology. Although the 5-HT2A receptor site was predominant, the smaller component of 5-HT2C receptors alone was sufficient to mediate a maximal calcium response. The 5-HT-induced increase in [Ca2+]i was reversibly inhibited by >75% following a 12-hr pretreatment (T1/2 = 2 hr) with 5-HT (EC50 = 400 nM). Extended treatment (24-96 hr) with 5-HT induced a complete functional desensitization that was associated with a partial (60%) reduction in 5-HT2 receptor number, implicating both receptor down-regulation and post-receptor mechanisms in 5-HT-induced desensitization. Long-term (hours to days) treatment with 5-HT did not modulate DNA synthesis, cell proliferation, or transformation in Balb/c-3T3 cells. These results demonstrate that Balb/c-3T3 cells express endogenous 5-HT2 receptors that are desensitized by the 5-HT present in normal serum, illustrating the importance of growth conditions in the identification of receptor responsiveness. The lack of proliferative response to 5-HT in Balb/c-3T3 suggests a putative role of desensitization as a "safety valve" to prevent abnormal cell growth during sustained 5-HT2 receptor activation.

Comparative desensitization of the human 5-HT2A and 5-HT2C receptors expressed in the human neuroblastoma cell line SH-SY5Y

1. We have used previously characterized clones of the human neuroblastoma cell line, SH-SY5Y. constitutively expressing either the human 5-HT2A or 5-HT2C receptor to compare their desensitization profiles after exposure to 5-HT. 2. 5-HT stimulated [3H]-inositol phosphate ([3H]-IPx) production at both the 5-HT2C (pEC50=8.03+/-0.15) and 5-HT2A receptors (pEC50=7.15+/-0.08), with maximal responses occurring after exposure to 1 microM and 10 microM 5-HT, respectively. 3. Exposure of cells to maximally effective concentrations of 5-HT caused time- and concentration-dependent desensitization of [3H]-IPx formation. The 5-HT2A response desensitized slower (t1/2 = 110 min) and with lower sensitivity than that of the 5-HT2C receptor (t1/2 = 12.5 min). In each case, desensitization was blocked by co-administration of a specific receptor antagonist. Following exposure to 10 microM 5-HT for 2 h, both receptors exhibited extensive desensitization, with subsequent responses to 5-HT reduced by more than 80%. 4. 5-HT stimulated Ins(1,4,5)P3 production with a potency similar to that for [3H]-IPx production at each receptor. In both cases Ins(1,4,5)P3 levels peaked rapidly then returned to basal level within a short time. This peak consistently occurred earlier for the 5-HT2C receptor (5 s) than for the 5-HT2A receptor (20 s). 5. Prior exposure of SH-SY5Y/5-HT2C cells to 5-HT (1 microM/15 min) caused a significant decrease in the 5-HT-stimulated peak in Ins(1,4,5)P3 levels whereas no such change occurred in SH-SY5Y/5-HT2A cells following exposure to 10 microM 5-HT for 15 min. 6 These results indicate that the human 5-HT2A and 5-HT2C receptors both exhibit desensitization at the level of inositol phosphate formation when expressed in the same cellular environment, with the 5-HT2C receptor being more sensitive to 5-HT-mediated desensitization than the 5-HT2A receptor.

Effects of serotonergic drugs on lateralized aggression and aggressive displays in Anolis carolinensis

Previous work demonstrated that the brains of many reptiles, including the American chameleon Anolis carolinensis (A. carolinensis), are functionally 'split'. Because the left eye in this species projects predominantly to the right hemisphere, and vice versa, inferences about lateralized brain functioning can be made in A. carolinensis by observation of eye use during behavioral encounters. Using this model, past work suggested that territorial aggression in Anolis is under the preferential control of the right hemisphere, and that acute stress or chronic alcohol exposure selectively reduces right hemisphere mediated territorial aggression. In addition, drugs which increase serotonin (5-HT) in the synaptic cleft inhibit aggressive responding in anoles in both hemispheres. The current experiment examined whether or not the administration of the serotonin agonists 8-hydroxy-2-(di-n-propylamine) tetralin (8-OHDPAT), quipazine, or meta-chlorophenylbiguanide (mCPBG) alter territorial aggression in Anolis. Nine adult socially isolated male A. carolinensis underwent a series of behavioral trials during which an antagonistic male was introduced into the cage. Once stable responding was initiated, all subjects were injected in a semi-randomized crossover manner with the following agents, (1) lactated Ringer's, (2) the 5-HT2 agonist quipazine (1.5 mg/kg and 3.0 mg/kg), (3) the 5-HT1 agonist 8-OHDPAT (83 mg/kg), and (4) the 5-HT3 agonist mCPBG (3.0 mg/kg and 9 mg/kg). Twenty minutes post injections, the male intruder was reintroduced into the subject's cage. Several behaviors were recorded, including: (1) the time to the first aggressive response, (2) the number of aggressive episodes mediated by the left eye or right eye, and (3) changes in skin color and posture. Aggressive responding was virtually eliminated in all subjects injected with 8-OHDPAT. On the other hand, one-way ANOVA found that both the 9 mg/kg dose of mCPBG (P=0.007), and the 3.0 mg/kg dose of quipazine (P=0.035), selectively decreased territorial aggression mediated by the left eye/right hemisphere compared to lactated Ringer's controls, but had no effect on aggression mediated by the right eye/left hemisphere. Although 8-OHDPAT inhibited aggression, injected subjects developed phenotypic displays of aggressive coloring/posturing, such as blackening of the eye spot and a raising of the neck crest. These results suggest that aggressive action can be differentiated from phenotypic displays that accompany aggression by a 5-HT1 agonist. They also indicate that there is an asymmetrical effect of 5-HT2/5-HT3 serotonin agonists on hemispheric mediation of aggression in this species.

SB 242084, a selective and brain penetrant 5-HT2C receptor antagonist

SB 242084 has a high affinity (pKi 9.0) for the cloned human 5-HT2C receptor and 100- and 158-fold selectivity over the closely related cloned human 5-HT2B and 5-HT2A subtypes respectively. SB 242084 had over 100-fold selectivity over a range of other 5-HT, dopamine and adrenergic receptors. In studies of 5-HT-stimulated phosphatidylinositol hydrolysis using SH-SY5Y cells stably expressing the cloned human 5-HT2C receptor, SB 242084 acted as an antagonist with a pKb of 9.3, which closely resembled its corresponding receptor binding affinity. SB 242084 potently inhibited m-chlorophenylpiperazine (mCPP, 7 mgkg i.p. 20 min pre-test)-induced hypolocomotion in rats, a model of in vivo central 5-HT2C receptor function, with an ID50 of 0.11 mg/kg i.p., and 2.0 mg/kg p.o. SB 242084 (0.1-1 mg/kg i.p.) exhibited an anxiolytic-like profile in the rat social interaction test, increasing time spent in social interaction, but having no effect on locomotion. SB 242084 (0.1-1 mg/kg i.p.) also markedly increased punished responding in a rat Geller-Seifter conflict test of anxiety, but had no consistent effect on unpunished responding. A large acute dose of SB 242084 (30 mg/kg p.o.) had no effect on seizure susceptibility in the rat maximal electroshock seizure threshold test. Also, while SB 242084 (2 and 6 mg/kg p.o. 1 hr pre-test) antagonized the hypophagic response to mCPP, neither acute nor subchronic administration of the drug, for 5 days at 2 or 6 mg/kg p.o. twice daily, affected food intake or weight gain. The results suggest that SB 242084 is the first reported selective potent and brain penetrant 5-HT2C receptor antagonist and has anxiolytic-like activity, but does not possess either proconvulsant or hyperphagic properties which are characteristic of mutant mice lacking the 5-HT2C receptor.