The GABAergic Hypothesis for Cognitive Disabilities in Down Syndrome.

Down syndrome (DS) is a genetic disorder caused by the presence of a third copy of chromosome 21. DS affects multiple organs, but it invariably results in altered brain development and diverse degrees of intellectual disability. A large body of evidence has shown that synaptic deficits and memory impairment are largely determined by altered GABAergic signaling in trisomic mouse models of DS. These alterations arise during brain development while extending into adulthood, and include genesis of GABAergic neurons, variation of the inhibitory drive and modifications in the control of neural-network excitability. Accordingly, different pharmacological interventions targeting GABAergic signaling have proven promising preclinical approaches to rescue cognitive impairment in DS mouse models. In this review, we will discuss recent data regarding the complex scenario of GABAergic dysfunctions in the trisomic brain of DS mice and patients, and we will evaluate the state of current clinical research targeting GABAergic signaling in individuals with DS.

Altered explorative strategies and reactive coping style in the FSL rat model of depression.

Modeling depression in animals is based on the observation of behaviors interpreted as analog to human symptoms. Typical tests used in experimental depression research are designed to evoke an either-or outcome. It is known that explorative and coping strategies are relevant for depression, however these aspects are generally not considered in animal behavioral testing. Here we investigate the Flinders Sensitive Line (FSL), a rat model of depression, compared to the Sprague-Dawley (SD) rat in three independent tests where the animals are allowed to express a more extensive behavioral repertoire. The multivariate concentric square field™ (MCSF) and the novel cage tests evoke exploratory behaviors in a novel environment and the home cage change test evokes social behaviors in the re-establishment of a social hierarchy. In the MCSF test, FSL rats exhibited less exploratory drive and more risk-assessment behavior compared to SD rats. When re-exposed to the arena, FSL, but not SD rats, increased their exploratory behavior compared to the first trial and displayed risk-assessment behavior to the same extent as SD rats. Thus, the behavior of FSL rats was more similar to that of SDs when the rats were familiar with the arena. In the novel cage test FSL rats exhibited a reactive coping style, consistent with the reduced exploration observed in the MCSF. Reactive coping is associated with less aggressive behavior. Accordingly, FSL rats displayed less aggressive behavior in the home cage change test. Taken together, our data show that FSL rats express altered exploratory behavior and reactive coping style. Reduced interest is a core symptom of depression, and individuals with a reactive coping style are more vulnerable to the disease. Our results support the use of FSL rats as an animal model of depression and increase our understanding of the FSL rat beyond the behavioral dimensions targeted by the traditional depression-related tests.

Hippocampal-Dependent Antidepressant Action of the H3 Receptor Antagonist Clobenpropit in a Rat Model of Depression.

BACKGROUND: Histamine is a modulatory neurotransmitter regulating neuronal activity. Antidepressant drugs target modulatory neurotransmitters, thus ultimately regulating glutamatergic transmission and plasticity. Histamine H3 receptor (H3R) antagonists have both pro-cognitive and antidepressant effects; however, the mechanism by which they modulate glutamate transmission is not clear. We measured the effects of the H3R antagonist clobenpropit in the Flinders Sensitive Line (FSL), a rat model of depression with impaired memory and altered glutamatergic transmission. METHODS: Behavioral tests included the forced swim test, memory tasks (passive avoidance, novel object recognition tests), and anxiety-related paradigms (novelty suppressed feeding, social interaction, light/dark box tests). Hippocampal protein levels were detected by Western blot. Hippocampal plasticity was studied by in slice field recording of CA3-CA1 long-term synaptic potentiation (LTP), and glutamatergic transmission by whole-cell patch clamp recording of excitatory postsynaptic currents (EPSCs) in CA1 pyramidal neurons. RESULTS: Clobenpropit, administered systemically or directly into the hippocampus, decreased immobility during the forced swim test; systemic injections reversed memory deficits and increased hippocampal GluN2A protein levels. FSL rats displayed anxiety-related behaviors not affected by clobenpropit treatment. Clobenpropit enhanced hippocampal plasticity, but did not affect EPSCs. H1R and H2R antagonists prevented the clobenpropit-induced increase in LTP and, injected locally into the hippocampus, blocked clobenpropit's effect in the forced swim test. CONCLUSIONS: Clobenpropit's antidepressant effects and the enhanced synaptic plasticity require hippocampal H1R and H2R activation, suggesting that clobenpropit acts through disinhibition of histamine release. Clobenpropit reverses memory deficits and increases hippocampal GluN2A expression without modifying anxiety-related phenotypes or EPSCs in CA1 pyramidal neurons.

Dysfunctional hippocampal activity affects emotion and cognition in mood disorders.

Mood disorders, such as major depressive disorder (MDD), bipolar disorder and generalized anxiety disorder usually comprise mood related as well as cognitive symptoms and the interaction between these symptoms is still not clear. Most antidepressant drugs have a positive effect on mood but do not treat the cognitive dysfunctions or even aggravate the symptoms. In this review we will evaluate the association between mood and cognition in the context of mood disorders. In the first section we will summarize the brain circuits at the intersection between cognition and emotion, highlighting the role of the hippocampus. In the second section, we will survey the contribution of the glutamate and GABA systems in the pathophysiology of mood disorders, making an effort to understand the link between emotions and cognition and how novel therapeutic approaches deal with them. In the third section we will explore the monoamine involvement in the emotion/cognition duality in the context of mood disorders. Finally we will underline the role of synaptic plasticity and neurogenesis in depression. We consider that a broader knowledge about the integrative mechanisms involved in specific aspects of mood disorders is crucial in the development of more powerful and effective antidepressant drugs. This article is part of a Special Issue entitled: Brain Integration.

Targeting glutamate system for novel antipsychotic approaches: relevance for residual psychotic symptoms and treatment resistant schizophrenia.

Antipsychotics are the mainstay of schizophrenia treatment. However, approximately one third of schizophrenic patients do not respond or respond poorly to antipsychotics. Therefore, there is a need for new approaches that can improve schizophrenia treatment significantly. Promising strategies arise from the modulation of glutamatergic system, according to its proposed involvement in schizophrenia pathogenesis. In this review, we critically updated preclinical and clinical data on the modulation of glutamate N-methyl-D-aspartate (NMDA) receptor activity by NMDA-Rs co-agonists, glycine transporters inhibitors, AMPAkines, mGluR5 agonists, NMDA-Rs partial agonists. We focused on: 1) preclinical results in animal models mimicking the pathophysiology of psychosis, mainly believed to be responsible of negative and cognitive symptoms, and predicting antipsychotic-like activity of these compounds; and 2) clinical efficacy in open-label and double-blind trials. Albeit promising preclinical findings for virtually all compounds, clinical efficacy has not been confirmed for D-cycloserine. Contrasting evidence has been reported for glycine and D-serine, that may however have a role as add-on agents. More promising results in humans have been found for glycine transporter inhibitors. AMPAkines appear to be beneficial as pro-cognitive agents, while positive allosteric modulators of mGluR5 have not been tested in humans. Memantine has been proposed in early stages of schizophrenia, as it may counteract the effects of glutamate excitotoxicity correlated to high glutamate levels, slowing the progression of negative symptoms associated to more advanced stages of the illness.

Role of aberrant striatal dopamine D1 receptor/cAMP/protein kinase A/DARPP32 signaling in the paradoxical calming effect of amphetamine.

Attention deficit/hyperactivity disorder (ADHD) is characterized by inattention, impulsivity, and motor hyperactivity. Several lines of research support a crucial role for the dopamine transporter (DAT) gene in this psychiatric disease. Consistently, the most commonly prescribed medications in ADHD treatment are stimulant drugs, known to preferentially act on DAT. Recently, a knock-in mouse [DAT-cocaine insensitive (DAT-CI)] has been generated carrying a cocaine-insensitive DAT that is functional but with reduced dopamine uptake function. DAT-CI mutants display enhanced striatal extracellular dopamine levels and basal motor hyperactivity. Herein, we showed that DAT-CI animals present higher striatal dopamine turnover, altered basal phosphorylation state of dopamine and cAMP-regulated phosphoprotein 32 kDa (DARPP32) at Thr75 residue, but preserved D(2) receptor (D(2)R) function. However, although we demonstrated that striatal D(1) receptor (D(1)R) is physiologically responsive under basal conditions, its stimulus-induced activation strikingly resulted in paradoxical electrophysiological, behavioral, and biochemical responses. Indeed, in DAT-CI animals, (1) striatal LTP was completely disrupted, (2) R-(+)-6-chloro-7,8-dihydroxy-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrobromide (SKF 81297) treatment induced paradoxical motor calming effects, and (3) SKF 81297 administration failed to increase cAMP/protein kinase A (PKA)/DARPP32 signaling. Such biochemical alteration selectively affected dopamine D(1)Rs since haloperidol, by blocking the tonic inhibition of D(2)R, unmasked a normal activation of striatal adenosine A(2A) receptor-mediated cAMP/PKA/DARPP32 cascade in mutants. Most importantly, our studies highlighted that amphetamine, nomifensine, and bupropion, through increased striatal dopaminergic transmission, are able to revert motor hyperactivity of DAT-CI animals. Overall, our results suggest that the paradoxical motor calming effect induced by these drugs in DAT-CI mutants depends on selective aberrant phasic activation of D(1)R/cAMP/PKA/DARPP32 signaling in response to increased striatal extracellular dopamine levels.