Loss of TREM2 rescues hyperactivation of microglia, but not lysosomal deficits and neurotoxicity in models of progranulin deficiency

Haploinsufficiency of the progranulin (PGRN)-encoding gene (GRN) causes frontotemporal lobar degeneration (GRN-FTLD) and results in microglial hyperactivation, TREM2 activation, lysosomal dysfunction, and TDP-43 deposition. To understand the contribution of microglial hyperactivation to pathology, we used genetic and pharmacological approaches to suppress TREM2-dependent transition of microglia from a homeostatic to a disease-associated state. Trem2 deficiency in Grn KO mice reduced microglia hyperactivation. To explore antibody-mediated pharmacological modulation of TREM2-dependent microglial states, we identified antagonistic TREM2 antibodies. Treatment of macrophages from GRN-FTLD patients with these antibodies led to reduced TREM2 signaling due to its enhanced shedding. Furthermore, TREM2 antibody-treated PGRN-deficient microglia derived from human-induced pluripotent stem cells showed reduced microglial hyperactivation, TREM2 signaling, and phagocytic activity, but lysosomal dysfunction was not rescued. Similarly, lysosomal dysfunction, lipid dysregulation, and glucose hypometabolism of Grn KO mice were not rescued by TREM2 ablation. Synaptic loss and neurofilament light-chain (NfL) levels, a biomarker for neurodegeneration, were further elevated in the Grn/Trem2 KO cerebrospinal fluid (CSF). These findings suggest that TREM2-dependent microglia hyperactivation in models of GRN deficiency does not promote neurotoxicity, but rather neuroprotection.

Cre-Activation in ErbB4-Positive Neurons of Floxed Grin1/NMDA Receptor Mice Is Not Associated With Major Behavioral Impairment

Extensive evidence suggests a dysfunction of the glutamate NMDA receptor (NMDAR) in schizophrenia, a severe psychiatric disorder with putative early neurodevelopmental origins, but clinical onset mainly during late adolescence. On the other hand, pharmacological models using NMDAR antagonists and the clinical manifestation of anti-NMDAR encephalitis indicate that NMDAR blockade/hypofunction can trigger psychosis also at adult stages, without any early developmental dysfunction. Previous genetic models of NMDAR hypofunction restricted to parvalbumin-positive interneurons indicate the necessity of an early postnatal impairment to trigger schizophrenia-like abnormalities, whereas the cellular substrates of NMDAR-mediated psychosis at adolescent/adult stages are unknown. Neuregulin 1 (NRG1) and its receptor ErbB4 represent schizophrenia-associated susceptibility factors that closely interact with NMDAR. To determine the neuronal populations implicated in "late" NMDAR-driven psychosis, we analyzed the effect of the inducible ablation of NMDARs in ErbB4-expressing cells in mice during late adolescence using a pharmacogenetic approach. Interestingly, the tamoxifen-inducible NMDAR deletion during this late developmental stage did not induce behavioral alterations resembling depression, schizophrenia or anxiety. Our data indicate that post-adolescent NMDAR deletion, even in a wider cell population than parvalbumin-positive interneurons, is also not sufficient to generate behavioral abnormalities resembling psychiatric disorders. Other neuronal substrates that have to be revealed by future studies, may underlie post-adolescent NMDAR-driven psychosis.

Systematic analysis of severity in a widely used cognitive depression model for mice

Animal models in psychiatric research are indispensable for insights into mechanisms of behaviour and mental disorders. Distress is an important aetiological factor in psychiatric diseases, especially depression, and is often used to mimic the human condition. Modern bioethics requires balancing scientific progress with animal welfare concerns. Therefore, scientifically based severity assessment of procedures is a prerequisite for choosing the least compromising paradigm according to the 3Rs principle. Evidence-based severity assessment in psychiatric animal models is scarce, particularly in depression research. Here, we assessed severity in a cognitive depression model by analysing indicators of stress and well-being, including physiological (body weight and corticosterone metabolite concentrations) and behavioural (nesting and burrowing behaviour) parameters. Additionally, a novel approach for objective individualised severity grading was employed using clustering of voluntary wheel running (VWR) behaviour. Exposure to the paradigm evoked a transient elevation of corticosterone, but neither affected body weight, nesting or burrowing behaviour. However, the performance in VWR was impaired after recurrent stress exposure, and the individual severity level increased, indicating that this method is more sensitive in detecting compromised welfare. Interestingly, the direct comparison to a somatic, chemically induced colitis model indicates less distress in the depression model. Further objective severity assessment studies are needed to classify the severity of psychiatric animal models in order to balance validity and welfare, reduce the stress load and thus promote refinement.

Effect of three different forms of handling on the variation of aggression-associated parameters in individually and group-housed male C57BL/6NCrl mice

Mice are social animals hence group-housing of mice is preferred over individual housing. However, aggression in group-housed male mice under laboratory housing conditions is a well-known problem leading to serious health issues, including injury or death. Therefore, group-housed mice are frequently separated for welfare reasons. In this study, we investigated the effect of 3 different handling methods (tail, forceps, tube) in 2 different housing conditions (single vs. group) on the variance of aggression-associated parameters in male C57BL/6NCrl mice over 8 weeks. Blood glucose concentration, body weight, body temperature, plus number and severity of bite wounds and barbering intensity in group-housed mice were recorded. An assessment of nest complexity was also performed weekly. Feces were collected in week 3 and 7 for analysis of corticosterone metabolites. We also monitored the level of aggression by recording the behavior of group-housed animals after weekly cage cleaning. An open field test followed by a social novel object test, a light/dark box test, a hotplate and a resident-intruder test were performed at the end of the 8-week handling period. Post-mortem, we assessed organ weights. We found that forceps-handled mice, independent of the housing condition, had significantly higher levels of stress-induced-hyperthermia and enhanced aggression after cage cleaning, and they performed worse in the nest complexity test. In addition, handling male mice by the tail seems to be most effective to reduce aggressiveness after transferring animals into new cages, thereby representing an appropriate refinement.

TDP-43 induces p53-mediated cell death of cortical progenitors and immature neurons

TAR DNA-binding protein 43 (TDP-43) is a key player in neurodegenerative diseases including frontotemporal lobar degeneration (FTLD) and amyotrophic lateral sclerosis (ALS). Accumulation of TDP-43 is associated with neuronal death in the brain. How increased and disease-causing mutant forms of TDP-43 induce cell death remains unclear. Here we addressed the role of TDP-43 during neural development and show that reduced TDP-43 causes defects in neural stem/progenitor cell proliferation but not cell death. However, overexpression of wild type and TDP-43^A315T proteins induce p53-dependent apoptosis of neural stem/progenitors and human induced pluripotent cell (iPS)-derived immature cortical neurons. We show that TDP-43 induces expression of the proapoptotic BH3-only genes Bbc3 and Bax, and that p53 inhibition rescues TDP-43 induced cell death of embryonic mouse, and human cortical neurons, including those derived from TDP-43^G298S ALS patient iPS cells. Hence, an increase in wild type and mutant TDP-43 induces p53-dependent cell death in neural progenitors developing neurons and this can be rescued. These findings may have important implications for accumulated or mutant TDP-43 induced neurodegenerative diseases.

May the use of different background strains 'strain' the stress-related phenotype of GR+/- mice?

Genetically altered mice are available on different background strains. While respective backcrosses are often performed for pragmatic reasons, e.g. references, comparability, or existing protocols, the interaction between the mutations per se and the background strain often remains a neglected factor. The heterozygous mutation of the glucocorticoid receptor gene (GR) represents a well-examined model for depressive-like behavior in mice. To address the question in how far a robust depressive-like phenotype on a distinct background strain may allow a generalized conclusion, we analyzed respective phenotypes in two commonly used inbred strains: i.) C57BL/6N and ii.) BALB/c. Beside the use of different genetic models, we also extended our approach by applying two alternative paradigms to induce a depressive-like phenotype. Our study therefore comprised the model of 'unpredictable chronic mild stress' (UCMS) for four weeks and 'learned helplessness' (LH), which were used to study the role of GR, a key player in the development of depression. In the course of the experiment two cohorts of male GR^+/- mice on either C57BL/6N or BALB/c background strain underwent a behavioral test battery to assess basal and depressive-like features. While both stress paradigms were functional in inducing depressive-like changes, the results were strictly strain-dependent. The genetic consequences became even more obvious under non-stress conditions with significant effects detected in BALB/c mice, which indicates a different basal stress predisposition due to differences in the genetic background.

Effects of p75NTR deficiency on cholinergic innervation of the amygdala and anxiety-like behavior

The p75 neurotrophin receptor (p75NTR) is a low-affinity receptor that is capable of binding neurotrophins. Two different p75NTR knockout mouse lines are available either with a deletion in Exon III (p75NTRExIII-/- ) or in Exon IV (p75NTRExIV-/- ). In p75NTRExIII knockout mice, only the full-length p75NTR is deleted, whereas in p75NTRExIV knockout mice, the full-length as well as the truncated isoform of the receptor is deleted. Deletion of p75NTR has been shown to affect, among others, the septohippocampal cholinergic innervation pattern and neuronal plasticity within the hippocampus. We hypothesize that deletion of p75NTR also alters the morphology and physiology of a further key structure of the limbic system, the amygdala. Our results indicate that deletion of p75NTR also increases cholinergic innervation in the basolateral amygdala in adult as well as aged p75NTRExIII-/- and p75NTRExIV-/- mice. The p75NTRExIV-/- mice did not display altered long-term potentiation (LTP) in the basolateral amygdala as compared to age-matched control littermates. However, p75NTRExIII-/- mice display stronger LTP in the basolateral amygdala compared to age-matched controls. Bath-application of K252a (a trk antagonist) did not inhibit the induction of LTP in the basolateral amygdala, but reduced the level of LTP in p75NTRExIII-/- mice to levels seen in respective controls. Moreover, p75NTRExIII-/- mice display altered behavior in the dark/light box. Thus, deletion of p75NTR in mice leads to physiological and morphological changes in the amygdala and altered behavior that is linked to the limbic system.

Minocycline exacerbates apoptotic neurodegeneration induced by the NMDA receptor antagonist MK-801 in the early postnatal mouse brain

NMDA receptor (NMDAR) antagonists induce in perinatal rodent cortical apoptosis and protracted schizophrenia-like alterations ameliorated by antipsychotic treatment. The broad-spectrum antibiotic minocycline elicits antipsychotic and neuroprotective effects. Here we tested, if minocycline protects also against apoptosis triggered by the NMDAR antagonist MK-801 at postnatal day 7. Surprisingly, minocycline induced widespread cortical apoptosis and exacerbated MK-801-triggered cell death. In some areas such as the subiculum, the pro-apoptotic effect of minocycline was even more pronounced than that elicited by MK-801. These data reveal among antipsychotics unique pro-apoptotic properties of minocycline, raising concerns regarding consequences for brain development and the use in children.

Assessing the Cognitive Translational Potential of a Mouse Model of the 22q11.2 Microdeletion Syndrome

A chromosomal microdeletion at the 22q11.2 locus is associated with extensive cognitive impairments, schizophrenia and other psychopathology in humans. Previous reports indicate that mouse models of the 22q11.2 microdeletion syndrome (22q11.2DS) may model the genetic basis of cognitive deficits relevant for neuropsychiatric disorders such as schizophrenia. To assess the models usefulness for drug discovery, a novel mouse (Df(h22q11)/+) was assessed in an extensive battery of cognitive assays by partners within the NEWMEDS collaboration (Innovative Medicines Initiative Grant Agreement No. 115008). This battery included classic and touchscreen-based paradigms with recognized sensitivity and multiple attempts at reproducing previously published findings in 22q11.2DS mouse models. This work represents one of the most comprehensive reports of cognitive functioning in a transgenic animal model. In accordance with previous reports, there were non-significant trends or marginal impairment in some tasks. However, the Df(h22q11)/+ mouse did not show comprehensive deficits; no robust impairment was observed following more than 17 experiments and 14 behavioral paradigms. Thus - within the current protocols - the 22q11.2DS mouse model fails to mimic the cognitive alterations observed in human 22q11.2 deletion carriers. We suggest that the 22q11.2DS model may induce liability for cognitive dysfunction with additional "hits" being required for phenotypic expression.

FGF-2 deficiency causes dysregulation of Arhgef6 and downstream targets in the cerebral cortex accompanied by altered neurite outgrowth and dendritic spine morphology

Fibroblast growth factor 2 (FGF-2) is an abundant growth factor in the brain and exerts multiple functions on neural cells ranging from cell division, cell fate determination to differentiation. However, many details of the molecular mechanisms underlying the diverse functions of FGF-2 are poorly understood. In a comparative microarray analysis of motor sensory cortex (MSC) tissue of adult knockout (FGF-2(-/-)) and control (FGF-2(+/+)) mice, we found a substantial number of regulated genes, which are implicated in cytoskeletal machinery dynamics. Specifically, we found a prominent downregulation of Arhgef6. Arhgef6 mRNA was significantly reduced in the FGF-2(-/-) cortex, and Arhgef6 protein virtually absent, while RhoA protein levels were massively increased and Cdc42 protein levels were reduced. Since Arhgef6 is localized to dendritic spines, we next analyzed dendritic spines of adult FGF2(-/-) and control mouse cortices. Spine densities were significantly increased, whereas mean length of spines on dendrites of layer V of MSC neurons in adult FGF-2(-/-) mice was significantly decreased as compared to respective controls. Furthermore, neurite length in dissociated cortical cultures from E18 FGF-2(-/-) mice was significantly reduced at DIV7 as compared to wildtype neurons. Despite the fact that altered neuronal morphology and alterations in dendritic spines were observed, FGF-2(-/-) mice behave relatively unsuspicious in several behavioral tasks. However, FGF-2(-/-) mice exhibited decreased thermal pain sensitivity in the hotplate-test.

Adult AMPA GLUA1 receptor subunit loss in 5-HT neurons results in a specific anxiety-phenotype with evidence for dysregulation of 5-HT neuronal activity

Both the glutamatergic and serotonergic (5-HT) systems are implicated in the modulation of mood and anxiety. Descending cortical glutamatergic neurons regulate 5-HT neuronal activity in the midbrain raphe nuclei through α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and N-methyl-D-aspartate (NMDA) receptors. To analyze the functional role of GLUA1-containing AMPA receptors in serotonergic neurons, we used the Cre-ERT2/loxP-system for the conditional inactivation of the GLUA1-encoding Gria1 gene selectively in 5-HT neurons of adult mice. These Gria1(5-HT-/-) mice exhibited a distinct anxiety phenotype but showed no alterations in locomotion, depression-like behavior, or learning and memory. Increased anxiety-related behavior was associated with significant decreases in tryptophan hydroxylase 2 (TPH2) expression and activity, and subsequent reductions in tissue levels of 5-HT, its metabolite 5-hydroxyindoleacetic acid (5-HIAA), and norepinephrine in the raphe nuclei. However, TPH2 expression and activity as well as monoamine levels were unchanged in the projection areas of 5-HT neurons. Extracellular electrophysiological recordings of 5-HT neurons revealed that, while α1-adrenoceptor-mediated excitation was unchanged, excitatory responses to AMPA were enhanced and the 5-HT1A autoreceptor-mediated inhibitory response to 5-HT was attenuated in Gria1(5-HT-/-) mice. Our data show that a loss of GLUA1 protein in 5-HT neurons enhances AMPA receptor function and leads to multiple local molecular and neurochemical changes in the raphe nuclei that dysregulate 5-HT neuronal activity and induce anxiety-like behavior.

Immobilization stress sensitizes rat dorsal horn neurons having input from the low back

BACKGROUND

Stress is known to promote several forms of muscle pain including non-specific low back pain. However, the question if stress alone activates nociceptive central neurons has not been studied systematically. Here, we investigated the influence of repeated immobilization stress on dorsal horn neurons and behaviour in the rat.

METHODS

The stress consisted of immobilization in a narrow tube for 1 h on 12 days. Single dorsal horn neurons were recorded with microelectrodes introduced into the spinal segment L2. In this segment, about 14% of the neurons responded to mechanical stimulation of the subcutaneous soft tissues of the low back in naïve rats. The neurons often behaved like wide dynamic range cells in that they had a low mechanical threshold and showed graded responses to noxious stimuli.

RESULTS

The stress-induced changes in neuronal response behaviour were (1) appearance of new receptive fields in the deep tissues of the hindlimb, (2) increased input from deep soft tissues, but unchanged input from the skin and (3) significant increase in resting activity. Surprisingly, the pressure-pain threshold of the low back remained unchanged, although dorsal horn neurons were sensitized. In the open field test, the rats showed signs of increased anxiety.

CONCLUSIONS

This study shows that stress alone is sufficient to sensitize dorsal horn neurons. The data may explain the enhanced pain low back patients report when they are under stress. The increased resting discharge may lead to spontaneous pain.

Inducible forebrain-specific ablation of the transcription factor Creb during adulthood induces anxiety but no spatial/contextual learning deficits

The cyclic AMP (cAMP)-response element binding protein (CREB) is an activity-dependent transcription factor playing a role in synaptic plasticity, learning and memory, and emotional behavior. However, the impact of Creb ablation on rodent behavior is vague as e.g., memory performance of different Creb mutant mice depends on the specific type of mutation per se but additionally on the background and learning protocol differences. Here we present the first targeted ablation of CREB induced during adulthood selectively in principal forebrain neurons in a pure background strain of C57BL/6 mice. All hippocampal principal neurons exhibited lack of CREB expression. Mutant mice showed a severe anxiety phenotype in the openfield and novel object exploration test as well as in the Dark-Light Box Test, but unaltered hippocampus-dependent long-term memory in the Morris water maze and in context dependent fear conditioning. On the molecular level, CREB ablation led to CREM up regulation in the hippocampus and frontal cortex which may at least in part compensate for the loss of CREB. BDNF, a postulated CREB target gene, was down regulated in the frontal lobe but not in the hippocampus; neurogenesis remained unaltered. Our data indicate that in the adult mouse forebrain the late onset of CREB ablation can, in case of memory functionality, be compensated for and is not essential for memory consolidation and retrieval during adulthood. In contrast, the presence of CREB protein during adulthood seems to be pivotal for the regulation of emotional behavior.

Transcription factors Foxa1 and Foxa2 are required for adult dopamine neurons maintenance

The proteins Foxa1 and Foxa2 belong to the forkhead family of transcription factors and are involved in the development of several tissues, including liver, pancreas, lung, prostate, and the neural system. Both Foxa1 and Foxa2 are also crucial for the specification and differentiation of dopamine (DA) neurons during embryonic development, while about 30% of mice with an embryonic deletion of a single allele of the Foxa2 gene exhibit an age-related asymmetric loss of DA neurons and develop locomotor symptoms resembling Parkinson's disease (PD). Notably, both Foxa1 and Foxa2 factors continue to be expressed in the adult dopamine system. To directly assess their functions selectively in adult DA neurons, we induced genetic deletions of Foxa1/2 transcription factors in mice using a tamoxifen inducible tissue-specific CreERT2 recombinase expressed under control of the dopamine transporter (DAT) promoter (DATCreERT2). The conditional DA neurons-specific ablation of both genes, but not of Foxa2 alone, in early adulthood, caused a decline of striatal dopamine and its metabolites, along with locomotor deficits. At early pre-symptomatic stages, we observed a decline in aldehyde dehydrogenase family 1, subfamily A1 (Aldh1a1) protein expression in DA neurons. Further analyses revealed a decline of aromatic amino acid decarboxylase (AADC) and a complete loss of DAT expression in these neurons. These molecular changes ultimately led to a reduction of DA neuron numbers in the substantia nigra pars compacta (SNpc) of aged cFoxa1/2^−/− mice, resembling the progressive course of PD in humans. Altogether, in this study, we address the molecular, cellular, and functional role of both Foxa1 and Foxa2 factors in the maintenance of the adult dopamine system which may help to find better approaches for PD treatment.

Touchscreen-paradigm for mice reveals cross-species evidence for an antagonistic relationship of cognitive flexibility and stability

The abilities to either flexibly adjust behavior according to changing demands (cognitive flexibility) or to maintain it in the face of potential distractors (cognitive stability) are critical for adaptive behavior in many situations. Recently, a novel human paradigm has found individual differences of cognitive flexibility and stability to be related to common prefrontal networks. The aims of the present study were, first, to translate this paradigm from humans to mice and, second, to test conceptual predictions of a computational model of prefrontal working memory mechanisms, the Dual State Theory, which assumes an antagonistic relation between cognitive flexibility and stability. Mice were trained in a touchscreen-paradigm to discriminate visual cues. The task involved “ongoing” and cued “switch” trials. In addition distractor cues were interspersed to test the ability to resist distraction, and an ambiguous condition assessed the spontaneous switching between two possible responses without explicit cues. While response times did not differ substantially between conditions, error rates (ER) increased from the “ongoing” baseline condition to the most complex condition, where subjects were required to switch between two responses in the presence of a distracting cue. Importantly, subjects switching more often spontaneously were found to be more distractible by task irrelevant cues, but also more flexible in situations, where switching was required. These results support a dichotomy of cognitive flexibility and stability as predicted by the Dual State Theory. Furthermore, they replicate critical aspects of the human paradigm, which indicates the translational potential of the testing procedure and supports the use of touchscreen procedures in preclinical animal research.

Implications of p75NTR for dentate gyrus morphology and hippocampus-related behavior revisited

The pan-neurotrophin receptor p75NTR is expressed in the adult brain in a discrete pattern. Although numerous studies have addressed its implications for hippocampal functions, the generated sets of data are surprisingly conflicting. We have therefore set out to re-investigate the impact of a deletion of the full-length p75NTR receptor on several parameters of the dentate gyrus (DG), including neurogenesis and hippocampus-related behavior by using p75NTR(ExIII) knockout mice. Moreover, we investigated further parameters of the DG (cholinergic innervation, dendritic spines). In addition, we analyzed on the morphological level the impact of aging by comparing adult and aged p75NTR(ExIII) mice and their age-matched littermates. Adult (4-6 months old), but not aged (20 months old), p75NTR(ExIII) knockout mice display an enhanced volume of the DG. However, adult neurogenesis within the adult DG was unaffected in both adult and aged p75NTR(ExIII) knockout mice. We could further demonstrate that the change in the volume of the DG was accompanied by an increased cholinergic innervation and increased spine densities of granule cells in adult, but not aged p75NTR deficient mice. These morphological changes in the adult p75NTR deficient mice were accompanied by specific alterations in their behavior, including altered behavior in the Morris water maze test, indicating impairments in spatial memory retention.

Phenotype of mice with inducible ablation of GluA1 AMPA receptors during late adolescence: relevance for mental disorders

Adolescence is characterized by important molecular and anatomical changes with relevance for the maturation of brain circuitry and cognitive function. This time period is of critical importance in the emergence of several neuropsychiatric disorders accompanied by cognitive impairment, such as affective disorders and schizophrenia. The molecular mechanisms underlying these changes at neuronal level during this specific developmental stage remains however poorly understood. GluA1-containing AMPA receptors, which are located predominantly on hippocampal neurons, are the primary molecular determinants of synaptic plasticity. We investigated here the consequences of the inducible deletion of GluA1 AMPA receptors in glutamatergic neurons during late adolescence. We generated mutant mice with a tamoxifen-inducible deletion of GluA1 under the control of the CamKII promoter for temporally and spatially restricted gene manipulation. GluA1 ablation during late adolescence induced cognitive impairments, but also marked hyperlocomotion and sensorimotor gating deficits. Unlike the global genetic deletion of GluA1, inducible GluA1 ablation during late adolescence resulted in normal sociability. Deletion of GluA1 induced redistribution of GluA2 subunits, suggesting AMPA receptor trafficking deficits. Mutant animals showed increased hippocampal NMDA receptor expression and no change in striatal dopamine concentration. Our data provide new insight into the role of deficient AMPA receptors specifically during late adolescence in inducing several cognitive and behavioral alterations with possible relevance for neuropsychiatric disorders.

Pharmacological blockade of GluN2B-containing NMDA receptors induces antidepressant-like effects lacking psychotomimetic action and neurotoxicity in the perinatal and adult rodent brain

NMDA receptor (NMDAR) antagonists like ketamine and MK-801 possess remarkable antidepressant effects with fast onset. However, they over-stimulate the retrosplenial cortex, evoking psychosis-like effects and neuronal injury, revealed by de novo induction of the heat shock protein 70 (Hsp70). Moreover, early in the development MK-801 triggers widespread cortical apoptosis, inducing extensive caspase-3 expression. Altogether these data raise strong concerns on the clinical applicability of NMDAR antagonist therapies. Therefore, the development of novel therapeutics targeting more specifically NMDAR to avoid psychotomimetic effects is necessary. Here we investigated a GluN2B (NR2B) antagonist in behavioral and neurotoxicity paradigms in rats to assess its potential as possible alternative to unspecific NMDA receptor antagonists. We found that treatment with the GluN2B specific antagonist Ro 25-6981 evoked robust antidepressant-like effects. Moreover, Ro 25-6981 did not cause hyperactivity as displayed after treatment with unspecific NMDAR antagonists, a correlate of psychosis-like effects in rodents. Additionally, Ro 25-6981, unlike MK-801, did not induce caspase-3 and HSP70 expression, markers of neurotoxicity in the perinatal and adult brain, respectively. Moreover, unexpectedly, in the adult retrosplenial cortex Ro 25-6981 pretreatment significantly reduced MK-801-triggered neurotoxicity. Our results suggest that GluN2B antagonists may represent valuable alternatives to unspecific NMDAR antagonists with robust antidepressant efficacy and a more favorable side-effect profile.

Voluntary exercise does not ameliorate context memory and hyperarousal in a mouse model for post-traumatic stress disorder (PTSD)

OBJECTIVES

We investigated the effects of voluntary wheel running as model for intervention on the development of contextual fear and hyperarousal in a mouse model of post-traumatic stress disorder (PTSD). Physical exercise in general has been associated with improved hippocampus-dependent memory performance both in animals and humans. However, studies that have tried to link physical exercise and contextual conditioning in an animal model of PTSD, revealed mixed findings.

METHODS

Here we tested contextual fear conditioning, generalized fear response, acoustic startle response and emotionality in C57BL/6NCrl mice which had free access to a running wheel for 28 days, compared with control animals which did not run and mice which did not receive a shock during the conditioning phase.

RESULTS

We found no significant effects of voluntary running on the above-mentioned variables, except for enhanced anxiety levels in the Dark-Light-Box and O-Maze tests of running mice.

CONCLUSIONS

Our results suggest that running as a model for intervention does not ameliorate contextual aversive learning but has the potency to change emotional behaviours.

Hippocampal serotonin-1A receptor function in a mouse model of anxiety induced by long-term voluntary wheel running

We have recently demonstrated that, in C57/Bl6 mice, long-term voluntary wheel running is anxiogenic, and focal hippocampal irradiation prevents the increase in anxiety-like behaviors and neurobiological changes in the hippocampus induced by wheel running. Evidence supports a role of hippocampal 5-HT1A receptors in anxiety. Therefore, we investigated hippocampal binding and function of 5-HT1A receptors in this mouse model of anxiety. Four weeks of voluntary wheel running resulted in hippocampal subregion-specific changes in 5-HT1A receptor binding sites and function, as measured by autoradiography of [(3) H] 8-hydroxy-2-(di-n-propylamino)tetralin binding and agonist-stimulated binding of [(35) S]GTPγS to G proteins, respectively. In the dorsal CA1 region, 5-HT1A receptor binding and function were not altered by wheel running or irradiation. In the dorsal dentate gyrus and CA2/3 region, 5-HT1A receptor function was decreased by not only running but also irradiation. In the ventral pyramidal layer, wheel running resulted in a decrease of 5-HT1A receptor function, which was prevented by irradiation. Neither irradiation nor wheel running affected 5-HT1A receptors in medial prefrontal cortex or in the dorsal or median raphe nuclei. Our data indicate that downregulation of 5-HT1A receptor function in ventral pyramidal layer may play a role in anxiety-like behavior induced by wheel running.

Significant increase in anxiety during aging in mGlu5 receptor knockout mice

Glutamatergic mechanisms regulate neuronal circuits implicated in mood and anxiety. Emotional disorders as anxiety and depression are particularly difficult to treat during aging and mechanisms underlying emotional disturbances in the brain of the elderly are poorly understood. This may result from the small number of studies investigating these disorders in aged animals. Among glutamate receptors, metabotropic mGlu5 receptors are thought to play an important role, since their pharmacological blockade induces strong anxiolytic effects. However, the implication of mGlu5 in regulating anxiety is not yet completely understood. Here we analyzed both young adult and aged mice lacking mGlu5 receptors, to clarify, if genetic deletion of the receptor induces similar to pharmacological blockade anxiolytic effects. Unexpectedly, mGlu5 receptor knockout (KO) mice showed increased anxiety accentuating with aging. In contrast, young adult mice displayed an anti-depressive-like phenotype that was no longer detectable in aged animals. Our data support important distinct roles of mGlu5 receptors in modulating anxiety and depression during aging.

A functional Tph2 C1473G polymorphism causes an anxiety phenotype via compensatory changes in the serotonergic system

The association of single-nucleotide polymorphisms (SNPs) in the human tryptophan hydroxylase 2 (TPH2) gene with anxiety traits and depression has been inconclusive. Observed inconsistencies might result from the fact that TPH2 polymorphisms have been studied in a genetically heterogeneous human population. A defined genetic background, control over environmental factors, and the ability to analyze the molecular and neurochemical consequences of introduced genetic alterations constitute major advantages of investigating SNPs in inbred laboratory mouse strains. To investigate the behavioral and neurochemical consequences of a functional C1473G SNP in the mouse Tph2 gene, we generated congenic C57BL/6N mice homozygous for the Tph2 1473G allele. The Arg(447) substitution in the TPH2 enzyme resulted in a significant reduction of the brain serotonin (5-HT) in vivo synthesis rate. Despite decreased 5-HT synthesis, we could detect neither a reduction of brain region-specific 5-HT concentrations nor changes in baseline and stress-induced 5-HT release using a microdialysis approach. However, using a [(35)S]GTP-γ-S binding assay and 5-HT(1A) receptor autoradiography, a functional desensitization of 5-HT(1A) autoreceptors could be identified. Furthermore, behavioral analysis revealed a distinct anxiety phenotype in homozygous Tph2 1473G mice, which could be reversed with chronic escitalopram treatment. Alterations in depressive-like behavior could not be detected under baseline conditions or after chronic mild stress. These findings provide evidence for an involvement of functional Tph2 polymorphisms in anxiety-related behaviors, which are likely not caused directly by alterations in 5-HT content or release but are rather due to compensatory changes during development involving functional desensitization of 5-HT(1A) autoreceptors.

Drosha regulates neurogenesis by controlling neurogenin 2 expression independent of microRNAs

Temporal regulation of embryonic neurogenesis is controlled by hypostable transcription factors. The mechanism of the process is unclear. Here we show that the RNase III Drosha and DGCR8 (also known as Pasha), key components of the microRNA (miRNA) microprocessor, have important functions in mouse neurogenesis. Loss of microprocessor in forebrain neural progenitors resulted in a loss of stem cell character and precocious differentiation whereas Dicer deficiency did not. Drosha negatively regulated expression of the transcription factors Neurogenin 2 (Ngn2) and NeuroD1 whereas forced Ngn2 expression phenocopied the loss of Drosha. Neurog2 mRNA contains evolutionarily conserved hairpins with similarities to pri-miRNAs, and associates with the microprocessor in neural progenitors. We uncovered a Drosha-dependent destabilization of Neurog2 mRNAs consistent with microprocessor cleavage at hairpins. Our findings implicate direct and miRNA-independent destabilization of proneural mRNAs by the microprocessor, which facilitates neural stem cell (NSC) maintenance by blocking accumulation of differentiation and determination factors.

Pramipexole is active in depression tests and modulates monoaminergic transmission, but not brain levels of BDNF in mice

The dopamine D(2)/D(3) receptor agonist pramipexole exerts antidepressive capacities in patients with Parkinson's disease with little evidence for patients with affective diseases only. Little is known about the neurobiological basis of these antidepressive effects. In this study, C57BL/6N mice received acute or chronic (3 weeks) treatment with pramipexole in different dosages (0.1, 0.3, 1, and 3mg/kg b.w.) and imipramine or saline serving as positive and negative controls. To characterize antidepressant-like effects mice underwent behavioral characterization. In a second experiment dosages of pramipexole shown to be effective were used and candidate brain regions including hippocampus, frontal cortex and striatum were analyzed for levels of 5-hydroxytryptamine (5-HT), noradrenaline and dopamine and their metabolites as well as brain-derived neurotrophic factor (BDNF) to investigate possible neurochemical correlates of behavioral changes. Whereas acute treatment with pramipexole resulted in antidepressive-like effects in the Porsolt Forced Swim Test, Novel Cage Test, Openfield Test and Dark-light-Box Test and a tendency but insignificant effect in the Tail Suspension Test, chronic treatment did not show significant effects in any of the behavioral analyses. Neurochemical analyses revealed a highly significant effect on dopaminergic metabolites in the striatum as well as a moderate transient modulation of the serotonergic system in the hippocampus. BDNF levels were not affected by any dosage and treatment regime in any brain region investigated. In conclusion, the present data substantiate antidepressive effects of pramipexole and indicate a contribution of the dopaminergic and serotonergic metabolism in these effects, but argue against an eminent role of BDNF.

Differences in mouse maternal care behavior - is there a genetic impact of the glucocorticoid receptor?

Depressive episodes are frequently preceded by stressful life events. Evidence from genetic association studies suggests a role for the glucocorticoid receptor (GR), an essential element in the regulation of stress responses, in the pathophysiology of the disorder. Since the stress response system is affected by pregnancy and postpartum-associated changes, it has also been implicated in the pathophysiology of postpartum depression. Using a 2×2 factorial design, we investigated whether a heterozygous deletion of GR would influence maternal care behavior in C57BL/6 and Balb/c mice, two inbred strains known to display qualitative differences in this behavior. Behavioral observation was carried out between postnatal days 1 and 7, followed by a pup retrieval test on postnatal days 7 or 8. While previously noted inter-strain differences were confirmed for different manifestations of caring behavior, self-maintenance and neglecting behaviors as well as the pup retrieval test, no strain-independent effect of the GR mutation was noted. However, an interaction between GR genotype and licking/grooming behavior was observed: it was down-regulated in heterozygous C57BL/6 mice to the level recorded for Balb/c mice. Home cage observation poses minimal disturbance of the dam and her litter as compared to more invasive assessments of dams' emotional behavior. This might be a reason for the absence of any overall effects of the GR mutation, particularly since GR heterozygous animals display a depressive-like phenotype under stressful conditions only. Still, the subtle effect we observed may point towards a role of GR in postpartum affective disorders.

APP and APLP2 are essential at PNS and CNS synapses for transmission, spatial learning and LTP

Despite its key role in Alzheimer pathogenesis, the physiological function(s) of the amyloid precursor protein (APP) and its proteolytic fragments are still poorly understood. Previously, we generated APPsα knock-in (KI) mice expressing solely the secreted ectodomain APPsα. Here, we generated double mutants (APPsα-DM) by crossing APPsα-KI mice onto an APLP2-deficient background and show that APPsα rescues the postnatal lethality of the majority of APP/APLP2 double knockout mice. Surviving APPsα-DM mice exhibited impaired neuromuscular transmission, with reductions in quantal content, readily releasable pool, and ability to sustain vesicle release that resulted in muscular weakness. We show that these defects may be due to loss of an APP/Mint2/Munc18 complex. Moreover, APPsα-DM muscle showed fragmented post-synaptic specializations, suggesting impaired postnatal synaptic maturation and/or maintenance. Despite normal CNS morphology and unaltered basal synaptic transmission, young APPsα-DM mice already showed pronounced hippocampal dysfunction, impaired spatial learning and a deficit in LTP that could be rescued by GABA(A) receptor inhibition. Collectively, our data show that APLP2 and APP are synergistically required to mediate neuromuscular transmission, spatial learning and synaptic plasticity.

AMPA GluR-A receptor subunit mediates hippocampal responsiveness in mice exposed to stress

Because stress represents a major precipitating event for psychiatric disorders, it is important to identify molecular mechanisms that may be altered in vulnerable individuals when exposed to stress. Here, we studied GluR-A(-/-) mice, animals with compromised AMPA receptor signaling, and characterized by a schizophrenic as well as depressive phenotype to investigate changes occurring in response to an acute stress. Wild-type and GluR-A(-/-) mice were exposed to a single immobilization stress and sacrificed immediately after the end of the stress for the analysis of activity regulated genes and of glutamatergic synapse responsiveness. The acute stress produced a marked increase in the hippocampal expression of Arc (activity-regulated cytoskeletal-associated protein) in GluR-A(-/-) , but not in wild-type mice, which was associated with a similar increase of phospho-CaMKII, a partner in the action of Arc. When looking at the glutamatergic response to stress in wild-type animals, we found that stress increased GluR-A phosphorylation on serine831, an effect that was paralleled by a significant increase of the phosphorylation of the main NMDA receptor subunits, that is, NR-1 and NR-2B. Conversely, the stress-induced modulation of NMDA receptor subunits was not observed in GluR-A(-/-) mice. We suggest that enhanced stress responsiveness in GluR-A(-/-) mice may be due, at least in part, to their inability to activate NMDA-mediated glutamatergic neurotransmission, suggesting that the integrity of AMPA/NMDA receptor function may be important for successful coping under stressful conditions.

Voluntary exercise induces anxiety-like behavior in adult C57BL/6J mice correlating with hippocampal neurogenesis

Several studies investigated the effect of physical exercise on emotional behaviors in rodents; resulting findings however remain controversial. Despite the accepted notion that voluntary exercise alters behavior in the same manners as antidepressant drugs, several studies reported opposite or no effects at all. In an attempt to evaluate the effect of physical exercise on emotional behaviors and brain plasticity, we individually housed C57BL/6J male mice in cages equipped with a running wheel. Three weeks after continuous voluntary running we assessed their anxiety- and depression-like behaviors. Tests included openfield, dark-light-box, elevated O-maze, learned helplessness, and forced swim test. We measured corticosterone metabolite levels in feces collected over a 24-h period and brain-derived neurotrophic factor (BDNF) in several brain regions. Furthermore, cell proliferation and adult hippocampal neurogenesis were assessed using Ki67 and Doublecortin. Voluntary wheel running induced increased anxiety in the openfield, elevated O-maze, and dark-light-box and higher levels of excreted corticosterone metabolites. We did not observe any antidepressant effect of running despite a significant increase of hippocampal neurogenesis and BDNF. These data are thus far the first to indicate that the effect of physical exercise in mice may be ambiguous. On one hand, the running-induced increase of neurogenesis and BDNF seems to be irrelevant in tests for depression-like behavior, at least in the present model where running activity exceeded previous reports. On the other hand, exercising mice display a more anxious phenotype and are exposed to higher levels of stress hormones such as corticosterone. Intriguingly, numbers of differentiating neurons correlate significantly with anxiety parameters in the openfield and dark-light-box. We therefore conclude that adult hippocampal neurogenesis is a crucial player in the genesis of anxiety.

Regulation of cortical and hippocampal 5-HT(1A) receptor function by corticosterone in GR+/- mice

Our objective in the present study was to examine 5-HT(1A) receptor function in prefrontal cortex and hippocampus of GR+/- mice, which appear to be an appropriate murine model of depression. 5-HT(1A) receptor function was determined by measuring [(35)S]GTPgammaS binding stimulated by the 5-HT(1A) receptor agonist 8-OH-DPAT (1 microM), an indication of the capacity of the receptor to activate G proteins. 5-HT(1A) receptor expression was determined by measuring the binding of [(3)H]8-OH-DPAT (2 nM). We observed no effect of the constitutive reduction in GR on 5-HT(1A) receptor-stimulated [(35)S]GTPgammaS binding or 5-HT(1A) receptor binding sites. Corticosterone treatment (10mg/kg, sc once daily for 21 days) of wild-type mice resulted in a decrease in 5-HT(1A) receptor function in prefrontal cortex [8-OH-DPAT-stimulated [(35)S]GTPgammaS binding (% above basal), vehicle-treated: 39+/-4.9; corticosterone-treated: 17+/-2.8], but not in hippocampus. The constitutive reduction in GR expression prevented the down-regulation of 5-HT(1A) receptor function in frontal cortex by chronic corticosterone administration. In contrast, corticosterone treatment of GR+/- mice resulted in an increase in 5-HT(1A) receptor function in hippocampus which reached statistical significance in CA2/3 region [8-OH-DPAT-stimulated [(35)S]GTPgammaS binding (% above basal), vehicle-treated: 41+/-9.7; corticosterone-treated: 94+/-23]. These changes seem to be evoked by a combined effect of high corticosterone levels and GR deficiency. Although GR+/- mice do not exhibit changes in baseline corticosterone, the constitutive deficiency in GR appears to have unmasked regulatory effects of elevated corticosterone in the maintenance of 5-HT(1A) receptor function in prefrontal cortex and hippocampus.

AMPA receptor subunit 1 (GluR-A) knockout mice model the glutamate hypothesis of depression

Recent evidence indicates that glutamate homeostasis and neurotransmission are altered in major depressive disorder, but the nature of the disruption and the mechanisms by which it contributes to the syndrome are unclear. Glutamate can act via AMPA, NMDA, or metabotropic receptors. Using targeted mutagenesis, we demonstrate here that mice with deletion of the main AMPA receptor subunit GluR-A represent a depression model with good face and construct validity, showing behavioral and neurochemical features of depression also postulated for human patients. GluR-A(-/-) mice display increased learned helplessness, decreased serotonin and norepinephrine levels, and disturbed glutamate homeostasis with increased glutamate levels and increased NMDA receptor expression. These results correspond well with current concepts regarding the role of AMPA and NMDA receptors in depression, postulating that compounds that augment AMPA receptor signaling or decrease NMDA receptor functions have antidepressant effects. GluR-A(-/-) mice represent a model to investigate the pathophysiology underlying the depressive phenotype and to identify changes in neural plasticity and resilience evoked by the genetic alterations in glutamatergic function. Furthermore, GluR-A(-/-) mice may be a valuable tool to study biological mechanisms of AMPA receptor modulators and the efficacy of NMDA antagonists in reducing behavioral or biochemical changes that correlate with increased helplessness.

Evaluation of effects of previous exposure to an acute stressor before testing for depression-like behaviours in mice

Test batteries are an essential and broadly used tool for behavioural phenotyping, especially with regard to mouse models of particular diseases, such as depression. Facing the problem of an often limited number of mutant animals, it therefore seems crucial to develop and optimise such test batteries in terms of an ideal throughput of subjects. This study aimed to characterize several common stressors, which are used for the investigation of depressive-like features with regard to their capability of each of them to affect performance in a subsequent behavioural test. Here we investigated swim-, restraint- and footshock-stress in male C57/BL6 mice, focusing on post-stress corticosterone elevations as well as potential effects on the behavioural level. The stressors increased circulating corticosterone levels when assessed 1 h after exposure. On the behavioural level, no test interactions could be detected, which suggests, that in general, combining these test conditions in experiments with a restricted availability of animals seems to be rather unproblematic.