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A cardinal feature of post-traumatic stress disorder (PTSD) is a long-lasting paradoxical alteration of memory with hypermnesia for salient traumatic cues and amnesia for peri-traumatic contextual cues. So far, pharmacological therapeutic approach of this stress-related disorder is poorly developed mainly because of the lack of animal model for this paradoxical memory alteration. Using a model that precisely recapitulates the two memory components of PTSD in mice, we tested if brexpiprazole, a new antipsychotic drug with pro-cognitive effects in rodents, may persistently prevent the expression of PTSD-like memory induced by injection of corticosterone immediately after fear conditioning. Acute administration of brexpiprazole (0.3 mg/kg) 7 days’ post-trauma first blocks the expression of the maladaptive fear memory for a salient but irrelevant trauma-related cue. In addition, it enhances (with superior efficacy when compared to diazepam, prazosin, and escitalopram) memory for the traumatic context, correct predictor of the threat. This beneficial effect of brexpiprazole is overall maintained 1 week after treatment. In contrast brexpiprazole fully spares normal/adaptive cued fear memory, showing that the effect of this drug is specific to an abnormal/maladaptive (PTSD-like) fear memory of a salient cue. Finally, this treatment not only promotes the switch from PTSD-like to normal fear memory, but also normalizes most of the alterations in the hippocampal–amygdalar network activation associated with PTSD-like memory, as measured by C-Fos expression. Altogether, these preclinical data indicate that brexpiprazole could represent a new pharmacological treatment of PTSD promoting the normalization of traumatic memory.

Vacuolar H(+)-ATPase–dependent (V-ATPase–dependent) functions are critical for neural proteostasis and are involved in neurodegeneration and brain tumorigenesis. We identified a patient with fulminant neurodegeneration of the developing brain carrying a de novo splice site variant in ATP6AP2 encoding an accessory protein of the V-ATPase. Functional studies of induced pluripotent stem cell–derived (iPSC-derived) neurons from this patient revealed reduced spontaneous activity and severe deficiency in lysosomal acidification and protein degradation leading to neuronal cell death. These deficiencies could be rescued by expression of full-length ATP6AP2. Conditional deletion of Atp6ap2 in developing mouse brain impaired V-ATPase–dependent functions, causing impaired neural stem cell self-renewal, premature neuronal differentiation, and apoptosis resulting in degeneration of nearly the entire cortex. In vitro studies revealed that ATP6AP2 deficiency decreases V-ATPase membrane assembly and increases endosomal-lysosomal fusion. We conclude that ATP6AP2 is a key mediator of V-ATPase–dependent signaling and protein degradation in the developing human central nervous system.

Previous studies have shown that partial and full 5-HT1A receptor agonists reduce antipsychotic-induced catalepsy. Consequently, some antipsychotics combining balanced efficacy between dopamine (DA) D2 antagonism or partial agonism and 5-HT1A receptor agonism have a low propensity to induce extrapyramidal side effects (EPS), as reflected by low cataleptogenic activity in rodents. In the present experiments, we attempted to explore the importance of pre- and postsynaptic 5-HT1A agonistic properties of brexpiprazole and aripiprazole in the context of neurological side-effect liabilities. Additional measures of prefrontal cortical serotonin (5-HT) and DA levels using microdialysis were used to support that brexpiprazole has a preferential agonist effect on presynaptic 5-HT1A receptors. Brexpiprazole (3.0 and 10mg/kg, p.o.) as well as aripiprazole (8.0 and 30mg/kg, p.o.) failed to induce catalepsy in rats. Brexpiprazole (10mg/kg, p.o.) significantly reduced the cataleptic response induced by haloperidol (0.63mg/kg, s.c.), while aripiprazole (1.0-100mg/kg, p.o.) failed to reverse the effect of haloperidol and only showed a numeric decrease at 10mg/kg, (p.o.). When 5-HT1A receptors were blocked by the selective antagonist, WAY100635 (1.0mg/kg, s.c.), cataleptogenic properties of brexpiprazole (10mg/kg; p.o), but not aripiprazole (8.0 and 30mg/kg, p.o.) were unmasked. The ("biased") 5-HT1A receptor agonists F15599 (postsynaptic preference) and F13714 (presynaptic preference) had differential effects on haloperidol-induced catalepsy: F13714 (0.16mg/kg, s.c.) counteracted catalepsy, whereas F15599 (0.040mg/kg, s.c.) had no significant effect at regionally-selective doses. These data support a role of presynaptic 5-HT1A receptors in the anticataleptic effect of brexpiprazole. The selective 5-HT2A antagonist M100907 (0.10mg/kg, s.c.) had no effect on haloperidol-induced catalepsy, arguing against a major role of 5-HT2A receptors in the cataleptogenic profile of brexpiprazole. The findings with brexpiprazole were supported using microdialysis studies: Brexpiprazole (3.0 and 10mg/kg, p.o.) decreased extracellular 5-HT levels in the medial prefrontal cortex (mPFC), while it failed to affect extracellular DA in the same samples, suggesting that the 5-HT1A agonist properties of brexpiprazole may be preferentially presynaptic. In conclusion, these results confirm that brexpiprazole and aripiprazole have low propensities to induce EPS. However, the low EPS risk of brexpiprazole is more likely dependent on its agonist properties on presynaptic 5-HT1A receptors, while that of aripiprazole is less sensitive to 5-HT1A receptor antagonism.

International audience; Genetic disruptions of the forkhead box transcription factor FOXP2 in humans cause an autosomal-dominant speech and language disorder. While FOXP2 expression pattern are highly conserved, its role in specific brain areas for mammalian social behaviors remains largely unknown. Here we studied mice carrying a homozygous cortical Foxp2 deletion. The postnatal development and gross morphological architecture of mutant mice was indistinguishable from wildtype (WT) littermates. Unbiased behavioral profiling of adult mice revealed abnormalities in approach behavior towards conspecifics as well as in the reciprocal responses of WT interaction partners. Furthermore mutant mice showed alterations in acoustical parameters of ultrasonic vocalizations (USV), which also differed in function of the social context. Cell type-specific gene expression profiling of cortical pyramidal neurons revealed aberrant regulation of genes involved in social behavior. In particular Foxp2 mutants showed the downregulation of Mint2 (Apba2), a gene involved in approach behavior in mice and autism spectrum disorder in humans. Taken together these data demonstrate that cortical Foxp2 is required for normal social behaviors in mice.

Defeat, Distress, and Glucocorticoids Understanding how individuals control emotions and cope with stressful events is a major clinical concern and of importance for the treatment of psychiatric illnesses (see the Perspective by McEwen ). Barik et al. (p. 332 ) discovered that aggressive defeat stress in mice caused glucocortioid release and increased activity in the dopamine system. Deleting the glucocorticoid receptors in dopaminoceptive neurons completely prevented the social avoidance that usually follows aggressive defeat. How the combination of genetic factors and environmental stressors during adolescence determines adult behavior and how their disturbance results in neuropsychiatric disorders is poorly understood. Niwa et al. (p. 335 ) found that isolation stress during adolescence, which does not cause any long-lasting changes in wild-type mice, induced significant neurochemical and behavioral alterations in mutant mice expressing a dominant-negative variant of the disrupted in schizophrenia 1 gene under the control of the prion protein promoter. These deficits could be reversed by a glucocorticoid receptor antagonist.

Previous studies have shown that partial and full 5-HT

Serotonin 2C receptors (5-HT(2C)R) have been shown to undergo post-transcriptional RNA editing. This modification affects the affinity, coupling and constitutive activity of the receptor. In vivo, manipulations such as stress or antidepressant administration dramatically modify the pattern of 5-HT(2C)R mRNA editing, suggesting that this phenomenon might be involved in the pathophysiology of stress-related disorders. Indeed, alterations of 5-HT(2C)R mRNA editing have been observed in depressed patients. Thus, the recent development of mice expressing either the non-edited (5-HT(2C)R-INI) or the fully-edited form of 5-HT(2C)R (5-HT(2C)R-VGV) provides a novel opportunity to investigate the relevance of this phenomenon in the context of stress-related disorders. We observed that both 5-HT(2C)R-INI and 5-HT(2C)R-VGV mice exhibit exaggerated anxiety-like behaviors in the elevated-plus maze paradigm. This phenotype was observed when the INI or VGV mutations were present in mice on a BALB/c background, as well as non-significant trends in the same direction in mice on a C57BL/6J background. In animal models of antidepressant-like activity, the absence of editing of 5-HT(2C)R mRNA (5-HT(2C)R-INI) induced an increase in the time spent immobile in the forced-swim test (FST) and tail suspension test (TST). Complete editing of 5-HT(2C) receptor mRNA (5-HT(2C)R-VGV) induced antidepressant-like behavior in the FST and TST, as reflected by a significant decrease in time spent immobile. These phenotypes were unrelated to alterations in locomotor activity in both 5-HT(2C)R-INI and 5-HT(2C)R-VGV. In the TST, these phenotypes were accompanied by a decrease and an increase in response to desipramine in 5-HT(2C)R-INI and 5-HT(2C)R-VGV, respectively. These data constitute the first in vivo demonstration of a role for 5-HT(2C)R mRNA editing in anxiety- and depression-related behaviors.

There is an increasing demand for a novel non-nicotinic, nondopaminergic therapeutic approach to nicotine addiction. GABAergic mechanisms have been implicated in drug dependence. Recently, a novel GABAB receptor allosteric-positive modulator, GS39783, was characterized. There are no investigations to date on the effects of GABAB receptor-positive modulators in animal models of nicotine reinforcement. Conditioned place preference (CPP) paradigms are based on the principle that animals, like humans, would learn to seek environmental stimuli that have been previously associated with rewarding events. Here we show that nicotine (0.06 mg/kg s.c.) induced a robust CPP response. Furthermore, GS39783 (30-100 mg/kg p.o.) during the conditioning phase blocked the rewarding effects of nicotine in the CPP paradigm in rats. However, GS39783 did not significantly alter the CPP effects of nicotine when given only immediately before the CPP test. A growing body of evidence suggests that repeated administration of drugs of abuse induced long-term molecular changes in brain plasticity, most notably an accumulation of DeltaFosB, in the striatal complex that contribute to the manifestation of dependence. There was a significant accumulation of DeltaFosB in the nucleus accumbens, but not in the dorsal striatum, of rats treated daily for 5 days with nicotine (0.06 mg/kg i.p.). GS39783 completely (30-100 mg/kg p.o.) counteracted these nicotine-induced molecular adaptations when given before the CPP acquisition phase but not when administered immediately before the test phase. Taken together, the behavioral and molecular changes induced by nicotine occur in concert and are concomitantly amenable to reversal by GABAB receptor-positive modulators.

Metabotropic GABAB receptors predominantly function as heterodimers of GABAB(1) and GABAB(2) subunits, but GABAB(1) can also form functional receptors in the absence of GABAB(2). Mice lacking the GABAB(1) subunit have altered behavioural responses in tests for anxiety and depression. In these studies, we investigated anxiety and depression in GABAB(2)-deficient mice. We compared the effects directly with that of genetic deletion of the GABAB(1) receptor subunit. Both GABAB(1) and GABAB(2)-deficient mice were found to be more anxious than wild type in the light-dark box paradigm. In contrast, these mice exhibited an antidepressant-like behaviour in the forced swim test. Taken together, these data suggest that heterodimeric GABAB(1,2) receptors are required for the normal regulation of emotional behaviour.

Since its introduction almost 20 years ago, the tail suspension test has become one of the most widely used models for assessing antidepressant-like activity in mice. The test is based on the fact that animals subjected to the short-term, inescapable stress of being suspended by their tail, will develop an immobile posture. Various antidepressant medications reverse the immobility and promote the occurrence of escape-related behaviour. This review focuses on the utility this test as part of a research program aimed at understanding the mechanism of action of antidepressants. We discuss the inherent difficulties in modeling depression in rodents. We describe how the tail suspension differs from the closely related forced swim test. Further, we address some key issues associated with using the TST as a model of antidepressant action. We discuss issues regarding whether it satisfies criteria to be a valid model for assessing depression-related behavioural traits. We elaborate on the tests' ease of use, strain differences observed in the test and gender effects in the test. We focus on the utility of the test for genetic analysis. Furthermore, we discuss the concept of whether immobility maybe a behavioural trait relevant to depression. All of the available pharmacological data using the test in genetically modified mice is collated. Special attention is given to selective breeding programs such as the Rouen 'depressed' mice which have been bred for high and low immobility in the tail suspension test. We provide an extensive pooling of the pharmacological studies published to date using the test. Finally, we provide novel pharmacological validation of an automated system (Bioseb) for assessing immobility. Taken together, we conclude that the tail suspension test is a useful test for assessing the behavioural effects of antidepressant compounds and other pharmacological and genetic manipulations relevant to depression.

GABABreceptors mediate slow synaptic inhibition in the nervous system. In transfected cells, functional GABABreceptors are usually only observed after coexpression of GABAB(1)and GABAB(2)subunits, which established the concept of heteromerization for G-protein-coupled receptors. In the heteromeric receptor, GABAB(1)is responsible for binding of GABA, whereas GABAB(2)is necessary for surface trafficking and G-protein coupling. Consistent with thesein vitroobservations, the GABAB(1)subunit is also essential for all GABABsignalingin vivo. Mice lacking the GABAB(1)subunit do not exhibit detectable electrophysiological, biochemical, or behavioral responses to GABABagonists. However, GABAB(1)exhibits a broader cellular expression pattern than GABAB(2), suggesting that GABAB(1)could be functional in the absence of GABAB(2). We now generated GABAB(2)-deficient mice to analyze whether GABAB(1)has the potential to signal without GABAB(2)in neurons. We show that GABAB(2)-/-mice suffer from spontaneous seizures, hyperalgesia, hyperlocomotor activity, and severe memory impairment, analogous to GABAB(1)-/-mice. This clearly demonstrates that the lack of heteromeric GABAB(1,2)receptors underlies these phenotypes. To our surprise and in contrast to GABAB(1)-/-mice, we still detect atypical electrophysiological GABABresponses in hippocampal slices of GABAB(2)-/-mice. Furthermore, in the absence of GABAB(2), the GABAB(1)protein relocates from distal neuronal sites to the soma and proximal dendrites. Our data suggest that association of GABAB(2)with GABAB(1)is essential for receptor localization in distal processes but is not absolutely necessary for signaling. It is therefore possible that functional GABABreceptors exist in neurons that naturally lack GABAB(2)subunits.

The role of GABAB receptors in various behavioral processes has been largely defined using the prototypical GABAB receptor agonist baclofen. However, baclofen induces sedation, hypothermia and muscle relaxation, which may interfere with its use in behavioral paradigms. Although there is much evidence for a role of the inhibitory neurotransmitter GABA in the pathophysiology of anxiety, the role of GABAB receptors in these disorders is largely unclear. We recently identified GS39783 (N,N'-dicyclopentyl-2-methylsulfanyl-5-nitro-pyrimidine-4,6-diamine) as a selective allosteric positive modulator at GABAB receptors. The aim of the present study was to broadly characterize the effects of GS39783 in well-validated rodent models for motor activity, cognition, and anxiety. The following tests were included: locomotor activity in rats and mice, rotarod and traction tests (including determinations of core temperature) in mice, passive avoidance in mice and rats, elevated plus maze in rats, elevated zero maze in mice and rats, stress-induced hyperthermia in mice, and pentobarbital- and ethanol-induced sleep in mice. Unlike baclofen and/or the benzodiazepine chlordiazepoxide, GS39783 had no effect in any of the tests for locomotion, cognition, temperature, or narcosis. Most interestingly, GS39783 had anxiolytic-like effects in all the tests used. Overall, the data obtained here suggest that positive modulation of GABAB receptors may serve as a novel therapeutic strategy for the development of anxiolytics, with a superior side effect profile to both baclofen and benzodiazepines.

Forkhead-box protein P2 is a transcription factor that has been associated with intriguing aspects of cognitive function in humans, non-human mammals, and song-learning birds. Heterozygous mutations of the human FOXP2 gene cause a monogenic speech and language disorder. Reduced functional dosage of the mouse version (Foxp2) causes deficient cortico-striatal synaptic plasticity and impairs motor-skill learning. Moreover, the songbird orthologue appears critically important for vocal learning. Across diverse vertebrate species, this well-conserved transcription factor is highly expressed in the developing and adult central nervous system. Very little is known about the mechanisms regulated by Foxp2 during brain development. We used an integrated functional genomics strategy to robustly define Foxp2-dependent pathways, both direct and indirect targets, in the embryonic brain. Specifically, we performed genome-wide in vivo ChIP–chip screens for Foxp2-binding and thereby identified a set of 264 high-confidence neural targets under strict, empirically derived significance thresholds. The findings, coupled to expression profiling and in situ hybridization of brain tissue from wild-type and mutant mouse embryos, strongly highlighted gene networks linked to neurite development. We followed up our genomics data with functional experiments, showing that Foxp2 impacts on neurite outgrowth in primary neurons and in neuronal cell models. Our data indicate that Foxp2 modulates neuronal network formation, by directly and indirectly regulating mRNAs involved in the development and plasticity of neuronal connections., Author Summary Foxp2 codes for an intriguing regulatory protein that provides a window into unusual aspects of brain function in multiple species. For example, the gene is implicated in speech and language disorders in humans, song learning in songbirds, and learning of rapid movement sequences in mice. Foxp2 acts by tuning the expression levels of other genes (its downstream targets). In this study we used genome-wide techniques to comprehensively identify the major targets of Foxp2 in the embryonic brain, in order to understand its roles in fundamental biological pathways during neurodevelopment, which we followed up through functional analyses of neurons. Most notably, we found that Foxp2 directly and indirectly regulates networks of genes that alter the length and branching of neuronal projections, an important route for modulating the wiring of neural connections in the developing brain. Overall, our findings shed light on how Foxp2 directs particular features of nervous system development, helping us to build bridges between genes and complex aspects of brain function.

RNA editing that converts adenosine to inosine replaces the gene-encoded Ile, Asn, and Ile (INI) of serotonin [5-hydroxytryptamine (5-HT)] receptor 2C (5-HT2CR) with Val, Gly, and Val (VGV). Up to 24 different 5-HT2CR isoforms are detected in different brain regions (Burns et al., 1997; Fitzgerald et al., 1999; Wang et al., 2000). To elucidate the physiological significance of5-HT2CRmRNA editing, we derived mutant mouse lines harboring a knock-inINIorVGVallele, resulting in sole expression of one of two extremely different editing isoforms 5-HT2CR-INI (editing blocked) or -VGV (fully edited). AlthoughINImice grew normally,VGVmice had a severely reduced fat mass, despite compensatory hyperphagia, as a result of constitutive activation of the sympathetic nervous system and increased energy expenditure. Furthermore, serotonergic neurotransmission was oversensitized inVGVmice, most likely because of the increased cell surface expression of VGV receptors. Melanocortin 4 receptor (MC4R) regulates energy homeostasis (Balthasar et al., 2005; Heisler et al., 2006; Lam et al., 2008), andMc4r−/−mice are obese because of hyperphagia and reduced energy expenditure (Huszar et al., 1997). However, the elevated energy expenditure ofVGVmice could not be rescued in theMc4r−/−background, indicating the presence of a distinct signaling pathway mediated via 5-HT2CR-VGV that dominates the MC4R-dependent pathway in control of energy expenditure. Our results highlight the importance of regulated5-HT2CRmRNA editing, because dysregulation could result in the pathological consequences such as growth retardation seen inVGVmice.

Exposure to cocaine induces selective behavioral and molecular adaptations. In rodents, acute cocaine induces increased locomotor activity, whereas prolonged drug exposure results in behavioral locomotor sensitization, which is thought to be a consequence of drug-induced neuroadaptive changes. Recent attention has been given to compounds activating GABA(B) receptors as potential antiaddictive therapies. In particular, the principle of allosteric positive GABA(B) receptor modulators is very promising in this respect, as positive modulators lack the sedative and muscle relaxant properties of full GABA(B) receptor agonists such as baclofen. Here, we investigated the effects of systemic application of the GABA(B) receptor-positive modulator GS39783 (N,N'-dicyclopentyl-2-methylsulfanyl-5-nitro-pyrimidine-4, 6-diamine) in animals treated with acute and chronic cocaine administration. Both GS39783 and baclofen dose dependently attenuated acute cocaine-induced hyperlocomotion. Furthermore, both compounds also efficiently blocked cocaine-induced Fos induction in the striatal complex. In chronic studies, GS39783 induced a modest attenuation of cocaine-induced locomotor sensitization. Chronic cocaine induces the accumulation of the transcription factor deltaFosB and upregulates cAMP-response-element-binding protein (CREB) and dopamine- and cAMP-regulated phosphoprotein of 32 kDa (DARPP-32). GS39783 blocked the induction/activation of DARPP-32 and CREB in the nucleus accumbens and dorsal striatum and partially inhibited deltaFosB accumulation in the dorsal striatum. In summary, our data provide evidence that GS39783 attenuates the acute behavioral effects of cocaine exposure in rodents and in addition prevents the induction of selective long-term adaptive changes in dopaminergic signaling pathways. Further investigation of GABA(B) receptor-positive modulation as a novel therapeutic strategy for the treatment of cocaine dependence and possibly other drugs of abuse is therefore warranted.

Metabotropic glutamate receptor (mGluR) subtypes (mGluR1 to mGluR8) act as important pre- and postsynaptic regulators of neurotransmission in the CNS. These receptors consist of two domains, an extracellular region containing the orthosteric agonist site and a transmembrane heptahelical domain involved in G protein activation and recognition of several recently synthesized pharmacological modulators. The presynaptic receptor mGluR7 shows the highest evolutionary conservation within the family, but no selective pharmacological tool was known. Here we characterize an mGluR7-selective agonist, N , N ′-dibenzhydrylethane-1,2-diamine dihydrochloride (AMN082), which directly activates receptor signaling via an allosteric site in the transmembrane domain. At transfected mammalian cells expressing mGluR7, AMN082 potently inhibits cAMP accumulation and stimulates GTPγS binding (EC 50 -values, 64-290 nM) with agonist efficacies comparable with those of l -2-amino-4-phosphonobutyrate (L-AP 4 ) and superior to those of l -glutamate. AMN082 (≤10 μM) failed to show appreciable activating or inhibitory effects at other mGluR subtypes and selected ionotropic GluRs. Chimeric receptor studies position the binding site of AMN082 in the transmembrane region of mGluR7, and we demonstrate that this allosteric agonist has little, if any, effect on the potency of orthosteric ligands. Here we provide evidence for full agonist activity mediated by the heptahelical domain of family 3 G protein-coupled receptors (which have mGluR-like structure) that may lead to drug development opportunities. Further, AMN082 is orally active, penetrates the blood-brain barrier, and elevates the plasma stress hormones corticosterone and corticotropin in an mGluR7-dependent fashion. Therefore, AMN082 is a valuable tool for unraveling the role of mGluR7 in stress-related CNS disorders.

Regulation of neurotransmission via group-III metabotropic glutamate receptors (mGluR4, -6, -7, and -8) has recently been implicated in the pathophysiology of affective disorders, such as major depression and anxiety. For instance, mice with a targeted deletion of the gene for mGluR7 (mGluR7-/-) showed antidepressant and anxiolytic-like effects in a variety of stress-related paradigms, including the forced swim stress and the stress-induced hyperthermia tests. Deletion of mGluR7 reduces also amygdala- and hippocampus-dependent conditioned fear and aversion responses. Since the hypothalamic-pituitary-adrenal (HPA) axis regulates the stress response we investigate whether parameters of the HPA axis at the levels of selected mRNA transcripts and endocrine hormones are altered in mGluR7-deficient mice. Over all, mGluR7-/- mice showed only moderately lower serum levels of corticosterone and ACTH compared with mGluR7+/+ mice. More strikingly however, we found strong evidence for upregulated glucocorticoid receptor (GR)-dependent feedback suppression of the HPA axis in mice with mGluR7 deficiency: (i) mRNA transcripts of GR were significantly upregulated in the hippocampus of mGluR7-/- animals, (ii) similar increases were seen with 5-HT1A receptor transcripts, which are thought to be directly controlled by the transcription factor GR and finally (iii) mGluR7-/- mice showed elevated sensitivity to dexamethasone-induced suppression of serum corticosterone when compared with mGluR7+/+ animals. These results indicate that mGluR7 deficiency causes dysregulation of HPA axis parameters, which may account, at least in part, for the phenotype of mGluR7-/- mice in animal models for anxiety and depression. In addition, we present evidence that protein levels of brain-derived neurotrophic factor are also elevated in the hippocampus of mGluR7-/- mice, which we discuss in the context of the antidepressant-like phenotype found in those animals. We conclude that genetic ablation of mGluR7 in mice interferes at multiple sites in the neuronal circuitry and molecular pathways implicated in affective disorders.

Although there is much evidence for a role of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) in the pathophysiology of anxiety and depression, the role of GABA(B) receptors in behavioral processes related to these disorders has not yet been fully established. GABA(B) receptors are G-protein-coupled receptors, which act as functional heterodimers made up of GABA(B(1)) and GABA(B(2)) subunits. Using recently generated GABA(B(1)) -/- mice, which lack functional GABA(B) receptors, and pharmacological tools we assessed the role of GABA(B) receptors in anxiety- and antidepressant-related behaviors. In the light-dark box, GABA(B(1)) -/- mice were more anxious than their wild-type littermates (less time spent in the light; reduced number of transitions). GABA(B(1)) -/- mice were also more anxious in the staircase test. Conversely, acute and chronic treatment with GS39783, a novel GABA(B) receptor positive modulator, decreased anxiety in the light-dark box and elevated zero maze tests for anxiety. On the other hand, GABA(B(1)) -/- mice had decreased immobility (antidepressant-like behavior) in the forced swim test (FST). These behavioral effects are unrelated to alterations in locomotor activity. In confirmation of the genetic data, acute and chronic treatment with CGP56433A, a selective GABA(B) receptor antagonist, also decreased immobility in the FST, whereas GS39783 did not alter this behavior. Taken together, these data suggest that positive modulation of the GABA(B) receptor may serve as a novel therapeutic strategy for the development of anxiolytics, whereas GABA(B) receptor antagonism may serve as a basis for the generation of novel antidepressants.

The ability to modify mice genetically has been one of the major breakthroughs in modern medical science affecting every discipline including psychiatry. It is hoped that the application of such technologies will result in the identification of novel targets for the treatment of diseases such as depression and to gain a better understanding of the molecular pathophysiological mechanisms that are regulated by current clinically effective antidepressant medications. The advent of these tools has resulted in the need to adopt, refine and develop mouse-specific models for analyses of depression-like behavior or behavioral patterns modulated by antidepressants. In this review, we will focus on the utility of current models (eg forced swim test, tail suspension test, olfactory bulbectomy, learned helplessness, chronic mild stress, drug-withdrawal-induced anhedonia) and research strategies aimed at investigating novel targets relevant to depression in the mouse. We will focus on key questions that are considered relevant for examining the utility of such models. Further, we describe other avenues of research that may give clues as to whether indeed a genetically modified animal has alterations relevant to clinical depression. We suggest that it is prudent and most appropriate to use convergent tests that draw on different antidepressant-related endophenotypes, and complimentary physiological analyses in order to provide a program of information concerning whether a given phenotype is functionally relevant to depression-related pathology.

gamma-Hydroxybutyrate (GHB), a metabolite of gamma-aminobutyric acid (GABA), is proposed to function as a neurotransmitter or neuromodulator. gamma-Hydroxybutyrate and its prodrug, gamma-butyrolactone (GBL), recently received increased public attention as they emerged as popular drugs of abuse. The actions of GHB/GBL are believed to be mediated by GABAB and/or specific GHB receptors, the latter corresponding to high-affinity [3H]GHB-binding sites coupled to G-proteins. To investigate the contribution of GABAB receptors to GHB actions we studied the effects of GHB in GABAB(1)-/- mice, which lack functional GABAB receptors. Autoradiography reveals a similar spatial distribution of [3H]GHB-binding sites in brains of GABAB(1)-/- and wild-type mice. The maximal number of binding sites and the KD values for the putative GHB antagonist [3H]6,7,8,9-tetrahydro-5-hydroxy-5H-benzocyclohept-6-ylidene acetic acid (NCS-382) appear unchanged in GABAB(1)-/- compared with wild-type mice, demonstrating that GHB- are distinct from GABAB-binding sites. In the presence of the GABAB receptor positive modulator 2,6-di-tert-butyl-4-(3-hydroxy-2,2-dimethyl-propyl)-phenol GHB induced functional GTPgamma[35S] responses in brain membrane preparations from wild-type but not GABAB(1)-/- mice. The GTPgamma[35S] responses in wild-type mice were blocked by the GABAB antagonist [3-[[1-(S)-(3,4dichlorophenyl)ethyl]amino]-2-(S)-hydroxy-propyl]-cyclohexylmethyl phosphinic acid hydrochloride (CGP54626) but not by NCS-382. Altogether, these findings suggest that the GHB-induced GTPgamma[35S] responses are mediated by GABAB receptors. Following GHB or GBL application, GABAB(1)-/- mice showed neither the hypolocomotion, hypothermia, increase in striatal dopamine synthesis nor electroencephalogram delta-wave induction seen in wild-type mice. It, therefore, appears that all studied GHB effects are GABAB receptor dependent. The molecular nature and the signalling properties of the specific [3H]GHB-binding sites remain elusive.

Glutamate signalling through the N-methyl-D-aspartate (NMDA) receptor is of critical importance for normal central nervous system (CNS) function, as indicated by the marked behavioural disturbances produced by non-subtype selective NMDA antagonists such as dizocilpine (MK-801).The present studies were designed to investigate the involvement of the two major NMDA receptor subunits in the central nervous system, i.e. NR2A and NR2B, on sensorimotor gating in mice.These experiments utilised the non-subtype-selective NMDA antagonist dizocilpine, a line of NR2A-KO mice and the selective NR2B antagonist Ro 63-1908, in the study of pre-pulse inhibition of the startle response (PPI).The non-selective NMDA receptor antagonist dizocilpine (0.1-1 mg/kg, IP) robustly disrupted PPI in wild-type mice. Conversely, selective genetic or pharmacological inhibition of either the NMDA NR2A or NR2B receptor subunit containing receptors, respectively, had no effect on PPI. Thus, NR2A KO mice showed normal PPI compared with wild-type littermate controls, and administration of Ro 63-1908 (1-10 mg/kg IP) to wild-type mice did not affect PPI. However, selective inhibition of NR2A and NR2B by administration of Ro 63-1908 to NR2A KO mice significantly disrupted PPI.These data imply that concomitant inhibition of both NR2A and NR2B subunit-containing NMDA receptors is necessary to disrupt PPI, suggesting that inhibition of NR2A and NR2B-containing NMDA receptors is required to elicit behaviours suggestive of psychomimetic effects in man.

Recently, we demonstrated that mice lacking the GABA(B(1)) subunit were more anxious than wild-type animals in several behavioural paradigms, most notably in the light-dark test. In an attempt to assess the effects of classical benzodiazepine anxiolytics on anxiety-like behaviour observed in these mice, animals were administered either chlordiazepoxide (10 mg/kg, p.o.) or diazepam (7.5 mg/kg, p.o.) prior to testing in the light-dark box. Surprisingly, in contrast with the wild-type mice, neither benzodiazepines decreased anxiety-like behaviour in GABA(B(1))(-/-) mice. These data suggest that targeted deletion of GABA(B(1)) subunit alters GABA(A) receptor function in vivo.

Clinically, SSRIs are widely prescribed in the treatment of several anxiety disorders, although very few pre-clinical studies have observed a beneficial effect of this class of drugs in animal models of anxiety. Furthermore, the biphasic pattern observed clinically, an exacerbation of anxiety followed by beneficial effects, is rarely observed in animal studies. In the present study we document this clinical phenomenon in several behavioural paradigms. While a single injection of citalopram induced anxiogenic effects, three administrations of citalopram were sufficient to elicit anxiolytic effects. Congruent with these data, we observed that short-term repeated administration of citalopram was accompanied by increased activation of cAMP response element-binding protein (CREB) in the hippocampus and desensitization of 5-HT1A receptors, two phenomena well associated with chronic rather than acute actions of antidepressants. Moreover, effects of citalopram were abolished in CREBalphaDelta mutant animals in the elevated zero maze (EZM) and tail suspension test (TST), but not in novelty-induced hypophagia (NIH). Further, the desensitization of 5-HT1A receptors elicited by citalopram was not affected by CREB deficiency. The significance of the EZM and TST paradigms in predicting therapeutic efficacy is well known while effects in NIH and 5-HT1A sensitization are less well-established. These data demonstrate that behavioural responses to citalopram are dependent on the frequency of its administration, and that these responses are differentially dependent on CREB function.