Atheir I. Abbas

Predicting new molecular targets for known drugs

Although drugs are intended to be selective, at least some bind to several physiological targets, explaining side effects and efficacy. Because many drug–target combinations exist, it would be useful to explore possible interactions computationally. Here we compared 3,665 US Food and Drug Administration (FDA)-approved and investigational drugs against hundreds of targets, defining each target by its ligands. Chemical similarities between drugs and ligand sets predicted thousands of unanticipated associations. Thirty were tested experimentally, including the antagonism of the β1 receptor by the transporter inhibitor Prozac, the inhibition of the 5-hydroxytryptamine (5-HT) transporter by the ion channel drug Vadilex, and antagonism of the histamine H4 receptor by the enzyme inhibitor Rescriptor. Overall, 23 new drug–target associations were confirmed, five of which were potent (<100 nM). The physiological relevance of one, the drug N,N-dimethyltryptamine (DMT) on serotonergic receptors, was confirmed in a knockout mouse. The chemical similarity approach is systematic and comprehensive, and may suggest side-effects and new indications for many drugs.

Remote Control of Neuronal Activity in Transgenic Mice Expressing Evolved G Protein-Coupled Receptors

Examining the behavioral consequences of selective CNS neuronal activation is a powerful tool for elucidating mammalian brain function in health and disease. Newly developed genetic, pharmacological, and optical tools allow activation of neurons with exquisite spatiotemporal resolution; however, the inaccessibility to light of widely distributed neuronal populations and the invasiveness required for activation by light or infused ligands limit the utility of these methods. To overcome these barriers, we created transgenic mice expressing an evolved G protein-coupled receptor (hM3Dq) selectively activated by the pharmacologically inert, orally bioavailable drug clozapine-N-oxide (CNO). Here, we expressed hM3Dq in forebrain principal neurons. Local field potential and single-neuron recordings revealed that peripheral administration of CNO activated hippocampal neurons selectively in hM3Dq-expressing mice. Behavioral correlates of neuronal activation included increased locomotion, stereotypy, and limbic seizures. These results demonstrate a powerful chemical-genetic tool for remotely controlling the activity of discrete populations of neurons in vivo.

Amisulpride is a potent 5-HT7 antagonist: relevance for antidepressant actions in vivo

RationaleAmisulpride is approved for clinical use in treating schizophrenia in a number of European countries and also for treating dysthymia, a mild form of depression, in Italy. Amisulpride has also been demonstrated to be an antidepressant for patients with major depression in many clinical trials. In part because of the selective D2/D3 receptor antagonist properties of amisulpride, it has long been widely assumed that dopaminergic modulation is the proximal event responsible for mediating its antidepressant and antipsychotic properties.ObjectivesThe purpose of these studies was to determine if amisulpride’s antidepressant actions are mediated by off-target interactions with other receptors.Materials and MethodsWe performed experiments that: (1) examined the pharmacological profile of amisulpride at a large number of central nervous system (CNS) molecular targets and, (2) after finding high potency antagonist affinity for human 5-HT7a serotonin receptors, characterized the actions of amisulpride as an antidepressant in wild-type and 5-HT7 receptor knockout mice.ResultsWe discovered that amisulpride was a potent competitive antagonist at 5-HT7a receptors and that interactions with no other molecular target investigated in this paper could explain its antidepressant actions in vivo. Significantly, and in contrast to their wild-type littermates, 5-HT7 receptor knockout mice did not respond to amisulpride in two widely used rodent models of depression, the tail suspension test and the forced swim test.ConclusionsThese results indicate that 5-HT7a receptor antagonism, and not D2/D3 receptor antagonism, likely underlies the antidepressant actions of amisulpride.

Inhibition of Mediodorsal Thalamus Disrupts Thalamofrontal Connectivity and Cognition

Cognitive deficits are central to schizophrenia, but the underlying mechanisms still remain unclear. Imaging studies performed in patients point to decreased activity in the mediodorsal thalamus (MD) and reduced functional connectivity between the MD and prefrontal cortex (PFC) as candidate mechanisms. However, a causal link is still missing. We used a pharmacogenetic approach in mice to diminish MD neuron activity and examined the behavioral and physiological consequences. We found that a subtle decrease in MD activity is sufficient to trigger selective impairments in prefrontal-dependent cognitive tasks. In vivo recordings in behaving animals revealed that MD-PFC beta-range synchrony is enhanced during acquisition and performance of a working memory task. Decreasing MD activity interfered with this task-dependent modulation of MD-PFC synchrony, which correlated with impaired working memory. These findings suggest that altered MD activity is sufficient to disrupt prefrontal-dependent cognitive behaviors and could contribute to the cognitive symptoms observed in schizophrenia.

Mice with altered serotonin 2C receptor RNA editing display characteristics of Prader-Willi Syndrome

RNA transcripts encoding the 2C-subtype of serotonin (5HT(2C)) receptor undergo up to five adenosine-to-inosine editing events to encode twenty-four protein isoforms. To examine the effects of altered 5HT(2C) editing in vivo, we generated mutant mice solely expressing the fully-edited (VGV) isoform of the receptor. Mutant animals present phenotypic characteristics of Prader-Willi syndrome (PWS) including a failure to thrive, decreased somatic growth, neonatal muscular hypotonia, and reduced food consumption followed by post-weaning hyperphagia. Though previous studies have identified alterations in both 5HT(2C) receptor expression and 5HT(2C)-mediated behaviors in both PWS patients and mouse models of this disorder, to our knowledge the 5HT(2C) gene is the first locus outside the PWS imprinted region in which mutations can phenocopy numerous aspects of this syndrome. These results not only strengthen the link between the molecular etiology of PWS and altered 5HT(2C) expression, but also demonstrate the importance of normal patterns of 5HT(2C) RNA editing in vivo.

PSD-95 is essential for hallucinogen and atypical antipsychotic drug actions at serotonin receptors.

Here, we report that postsynaptic density protein of 95 kDa (PSD-95), a postsynaptic density scaffolding protein, classically conceptualized as being essential for the regulation of ionotropic glutamatergic signaling at the postsynaptic membrane, plays an unanticipated and essential role in mediating the actions of hallucinogens and atypical antipsychotic drugs at 5-HT2A and 5-HT2C serotonergic G-protein-coupled receptors. We show that PSD-95 is crucial for normal 5-HT2A and 5-HT2C expression in vivo and that PSD-95 maintains normal receptor expression by promoting apical dendritic targeting and stabilizing receptor turnover in vivo. Significantly, 5-HT2A- and 5-HT2C-mediated downstream signaling is impaired in PSD-95null mice, and the 5-HT2A-mediated head-twitch response is abnormal. Furthermore, the ability of 5-HT2A inverse agonists to normalize behavioral changes induced by glutamate receptor antagonists is abolished in the absence of PSD-95 in vivo. These results demonstrate that PSD-95, in addition to the well known role it plays in scaffolding macromolecular glutamatergic signaling complexes, profoundly modulates metabotropic 5-HT2A and 5-HT2C receptor function.

Thalamic projections sustain prefrontal activity during working memory maintenance

The mediodorsal thalamus (MD) shares reciprocal connectivity with the prefrontal cortex (PFC), and decreased MD–PFC connectivity is observed in schizophrenia patients. Patients also display cognitive deficits including impairments in working memory, but a mechanistic link between thalamo–prefrontal circuit function and working memory is missing. Using pathway-specific inhibition, we found directional interactions between mouse MD and medial PFC (mPFC), with MD-to-mPFC supporting working memory maintenance and mPFC-to-MD supporting subsequent choice. We further identify mPFC neurons that display elevated spiking during the delay, a feature that was absent on error trials and required MD inputs for sustained maintenance. Strikingly, delay-tuned neurons had minimal overlap with spatially tuned neurons, and each mPFC population exhibited mutually exclusive dependence on MD and hippocampal inputs. These findings indicate a role for MD in sustaining prefrontal activity during working memory maintenance. Consistent with this idea, we found that enhancing MD excitability was sufficient to enhance task performance.

Pimavanserin tartrate: a 5-HT2A inverse agonist with potential for treating various neuropsychiatric disorders

Background: Pimavanserin tartrate is the first 5-HT2A inverse agonist to enter clinical trials as a treatment for L-dopa-induced psychosis in Parkinsons disease and for augmentation of low-dose risperidone treatment in schizophrenia. Pimavanserin is also being evaluated as a possible anti-insomnia drug. Objective: To discuss the potential of pimavanserin to fill multiple therapeutic needs. Methods: The problems with currently approved antipsychotics and sleep agents are explored to highlight how pimavanserin might address some longstanding issues in the treatment of psychosis and insomnia. Results/conclusions: In Phase II clinical trials, pimavanserin seemed to be safe, well-tolerated and efficacious in treating L-dopa-induced psychosis without worsening motor symptoms. Pimavanserin also potentiated the therapeutic effects of low-dose risperidone, reduced haloperidol-induced akathisia, and increased slow-wave sleep in older individuals.

The presynaptic component of the serotonergic system is required for clozapine's efficacy.

Clozapine, by virtue of its absence of extrapyramidal side effects and greater efficacy, revolutionized the treatment of schizophrenia, although the mechanisms underlying this exceptional activity remain controversial. Combining an unbiased cheminformatics and physical screening approach, we evaluated clozapines activity at >2350 distinct molecular targets. Clozapine, and the closely related atypical antipsychotic drug olanzapine, interacted potently with a unique spectrum of molecular targets. This distinct pattern, which was not shared with the typical antipsychotic drug haloperidol, suggested that the serotonergic neuronal system was a key determinant of clozapines actions. To test this hypothesis, we used pet1−/− mice, which are deficient in serotonergic presynaptic markers. We discovered that the antipsychotic-like properties of the atypical antipsychotic drugs clozapine and olanzapine were abolished in a pharmacological model that mimics NMDA-receptor hypofunction in pet1−/− mice, whereas haloperidols efficacy was unaffected. These results show that clozapines ability to normalize NMDA-receptor hypofunction, which is characteristic of schizophrenia, depends on an intact presynaptic serotonergic neuronal system.

Arresting serotonin

After decades of relative quiescence, G protein coupled receptor theory (GPCR) is undergoing a conceptual revolution driven in large part by findings like those of Schmid et al. as reported in this issue of PNAS (1). Since 1966, hypotheses of drug action at GPCRs have been driven by the quaint notion of “intrinsic efficacy” (2), which proposes that drugs which completely activate GPCRs (e.g., full agonists) remain full agonists regardless of the cellular milieu (see ref. 3 for a recent review). In other words, to paraphrase Gertrude Stein, “an agonist is an agonist is an agonist.” Over the past several years, based mainly on in vitro findings with synthetic agonists, it has become evident that the notion of “intrinsic efficacy” is a myth and that the cellular milieu is a critical determinant of drug action. As Schmid et al. elegantly demonstrate, the actions in vivo of the naturally occurring agonist serotonin (5-hydroxytryptamine; 5-HT) are profoundly altered by the complement of arrestins expressed in neurons (1). These findings will force neuropharmacologists to fundamentally alter their notions of drug actions at neuronal receptors and will have a major impact on central nervous system (CNS) drug discovery efforts.