Suitability of tamoxifen-induced mutagenesis for behavioral phenotyping

Consequences of NMDA receptor deficiency can be rescued in the adult brain

N-methyl-D-aspartate receptors (NMDARs) are required to shape activity-dependent connections in the developing and adult brain. Impaired NMDAR signalling through genetic or environmental insults causes a constellation of neurodevelopmental disorders that manifest as intellectual disability, epilepsy, autism, or schizophrenia. It is not clear whether the developmental impacts of NMDAR dysfunction can be overcome by interventions in adulthood. This question is paramount for neurodevelopmental disorders arising from mutations that occur in the GRIN genes, which encode NMDAR subunits, and the broader set of mutations that disrupt NMDAR function. We developed a mouse model where a congenital loss-of-function allele of Grin1 can be restored to wild type by gene editing with Cre recombinase. Rescue of NMDARs in adult mice yields surprisingly robust improvements in cognitive functions, including those that are refractory to treatment with current medications. These results suggest that neurodevelopmental disorders arising from NMDAR deficiency can be effectively treated in adults.

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.

Single-cell RNA-seq analysis revealed long-lasting adverse effects of tamoxifen on neurogenesis in prenatal and adult brains

The CreER/LoxP system is widely accepted to track neural lineages and study gene functions upon tamoxifen (TAM) administration. We have observed that prenatal TAM treatment caused high rates of delayed delivery and fetal mortality. This substance could produce undesired results, leading to data misinterpretation. Here, we report that administration of TAM during early stages of cortical neurogenesis promoted precocious neural differentiation, while it inhibited neural progenitor cell (NPC) proliferation. The TAM-induced inhibition of NPC proliferation led to deficits in cortical neurogenesis, dendritic morphogenesis, synaptic formation, and cortical patterning in neonatal and postnatal offspring. Mechanistically, by employing single-cell RNA-sequencing (scRNA-seq) analysis combined with in vivo and in vitro assays, we show TAM could exert these drastic effects mainly through dysregulating the Wnt-Dmrta2 signaling pathway. In adult mice, administration of TAM significantly attenuated NPC proliferation in both the subventricular zone and the dentate gyrus. This study revealed the cellular and molecular mechanisms for the adverse effects of TAM on corticogenesis, suggesting that care must be taken when using the TAM-induced CreER/LoxP system for neural lineage tracing and genetic manipulation studies in both embryonic and adult brains.

The effects of tamoxifen on mouse behavior

The CreER^T2 recombinase system is an advanced method to temporally control site-specific mutagenesis in adult rodents. In this process, tamoxifen is injected to induce Cre recombinase expression, and then, Cre recombinase can excise LoxP-flanked DNA. However, tamoxifen is a nonselective estrogen receptor antagonist that may influence behavioral alterations. Therefore, we designed five different protocols (acute effects, chronic effects, chronic effects after social defeat model, chronic effects after learned helplessness model, chronic effects after isolation models) to explore whether tamoxifen affects mouse behavior. Researching the acute/chronic effects of tamoxifen, we found that tamoxifen could influence locomotor activity, anxiety and immobility time in the forced swimming test. Researching the chronic effects of tamoxifen after social defeat/learned helplessness/isolation models, we found that tamoxifen could also influence locomotor activity, social interaction and anxiety. Therefore, the effects of tamoxifen are more complex than previously reported. Our results show, for the first time, that tamoxifen affects behavior in mouse models. Meanwhile, we compare the effects of tamoxifen in different protocols. These results will provide important information when designing similar experiments.

Tamoxifen suppresses pancreatic β-cell proliferation in mice

Tamoxifen is a mixed agonist/antagonist estrogen analogue that is frequently used to induce conditional gene deletion in mice using Cre-loxP mediated gene recombination. Tamoxifen is routinely employed in extremely high-doses relative to typical human doses to induce efficient gene deletion in mice. Although tamoxifen has been widely assumed to have no influence upon β-cells, the acute developmental and functional consequences of high-dose tamoxifen upon glucose homeostasis and adult β-cells are largely unknown. We tested if tamoxifen influences glucose homeostasis in male mice of various genetic backgrounds. We then carried out detailed histomorphometry studies of mouse pancreata. We also performed gene expression studies with islets of tamoxifen-treated mice and controls. Tamoxifen had modest effects upon glucose homeostasis of mixed genetic background (F1 B6129SF1/J) mice, with fasting hyperglycemia and improved glucose tolerance but without overt effects on fed glucose levels or insulin sensitivity. Tamoxifen inhibited proliferation of β-cells in a dose-dependent manner, with dramatic reductions in β-cell turnover at the highest dose (decreased by 66%). In sharp contrast, tamoxifen did not reduce proliferation of pancreatic acinar cells. β-cell proliferation was unchanged by tamoxifen in 129S2 mice but was reduced in C57Bl6 genetic background mice (decreased by 59%). Gene expression studies revealed suppression of RNA for cyclins D1 and D2 within islets of tamoxifen-treated mice. Tamoxifen has a cytostatic effect on β-cells, independent of changes in glucose homeostasis, in mixed genetic background and also in C57Bl6 mice. Tamoxifen should be used judiciously to inducibly inactivate genes in studies of glucose homeostasis.

CAMK2-Dependent Signaling in Neurons Is Essential for Survival

Ca²⁺/calmodulin-dependent protein kinase II (CAMK2) is a key player in synaptic plasticity and memory formation. Mutations in Camk2a or Camk2b cause intellectual disability in humans, and severe plasticity and learning deficits in mice, indicating unique functions for each isoform. However, considering the high homology between CAMK2A and CAMK2B, it is conceivable that for critical functions, one isoform compensates for the absence of the other, and that the full functional spectrum of neuronal CAMK2 remains to be revealed.Here we show that germline as well as adult deletion of both CAMK2 isoforms in male or female mice is lethal. Moreover, Ca²⁺-dependent activity as well as autonomous activity of CAMK2 is essential for survival. Loss of both CAMK2 isoforms abolished LTP, whereas synaptic transmission remained intact. The double-mutants showed no gross morphological changes of the brain, and in contrast to the long-considered role for CAMK2 in the structural organization of the postsynaptic density (PSD), deletion of both CAMK2 isoforms did not affect the biochemical composition of the PSD. Together, these results reveal an essential role for CAMK2 signaling in early postnatal development as well as the mature brain, and indicate that the full spectrum of CAMK2 requirements cannot be revealed in the single mutants because of partial overlapping functions of CAMK2A and CAMK2B.SIGNIFICANCE STATEMENT CAMK2A and CAMK2B have been studied for over 30 years for their role in neuronal functioning. However, most studies were performed using single knock-out mice. Because the two isoforms show high homology with respect to structure and function, it is likely that some redundancy exists between the two isoforms, meaning that for critical functions CAMK2B compensates for the absence of CAMK2A and vice versa, leaving these functions to uncover. In this study, we generated Camk2a/Camk2b double-mutant mice, and observed that loss of CAMK2, as well as the loss of Ca²⁺-dependent and Ca²⁺-independent activity of CAMK2 is lethal. These results indicate that despite 30 years of research the full spectrum of CAMK2 functioning in neurons remains to be unraveled.

Cav1.2 L-type calcium channels regulate stress coping behavior via serotonin neurons

Human genetic variation in the gene CACNA1C, which codes for the alpha-1c subunit of Cav1.2 L-type calcium channels (LTCCs), has been broadly associated with enhanced risk for neuropsychiatric disorders including major depression, bipolar and schizophrenia. Little is known about the specific neural circuits through which CACNA1C and Cav1.2 LTCCs impact disease etiology. However, serotonin (5-HT) neurotransmission has been consistently implicated in these neuropsychiatric disorders and Cav1.2 LTCCs may influence 5-HT neuron activity during relevant behavioral states such as stress. We utilized a temporally controlled and 5-HT neuron specific Cacna1c knockout mouse model to assess stress-coping behavior using the forced swim test and dorsal raphe (DR) 5-HT neuron Fos activation. Furthermore, we assessed 5-HT1A receptor function and feedback inhibition of the DR following administration of the 5-HT1A antagonist WAY-100635. We find that 5-HT neuron Cacna1c knockout disrupts active-coping behavior in the forced swim test and that this behavioral effect is rescued by blocking 5-HT1A receptors. Moreover, Cacna1c knockout mice display enhanced Fos expression in caudal DR 5-HT neurons and an enhanced response to a 5-HT1A receptor antagonist in rostral DR 5-HT neurons, indicating that loss of Cacna1c disrupts both 5-HT neuron activation and 5-HT1A dependent feedback inhibition across the caudal to rostral DR. Collectively, these results reveal an important role for 5-HT neuron Cav1.2 LTCCs in stress-coping behavior and 5-HT1A receptor function. This suggests that alterations in CACNA1C function or expression could influence the development or treatment of neuropsychiatric disorder through serotonergic mechanisms.

Analysis of hippocampal-dependent learning and memory behaviour in mice lacking Nfix from adult neural stem cells

Objective

The active place avoidance task (APA) is a behavioural task used to assess learning and memory in rodents. This task relies on the hippocampus, a region of the cerebral cortex capable of generating new neurons from neural stem cells. In this study, to gain further insight into the behavioural phenotype of mice deficient in the transcription factor Nfix, a gene expressed by adult neural stem cells, we examined learning and memory parameters from the APA task that were not published in our original investigation. We analysed time to first and second shock, maximum path and time of shock avoidance, number of entries into the shock zone and time spent in the shock zone. We also assessed performance in the APA task based on sex.

Results

We found mice deficient in Nfix displayed decreased latency to second shock compared to the control mice. Nfix deficient mice entered the shock zone more frequently and also spent more time in the shock zone. Our data provides further insights into the memory deficits evident in Nfix mutant mice, indicating these mice have a memory retrieval problem and may employ a different navigation strategy in the APA task.

Electronic supplementary material

The online version of this article (10.1186/s13104-018-3652-7) contains supplementary material, which is available to authorized users.

Generation and Analysis of Striated Muscle Selective LINC Complex Protein Mutant Mice

The linker of nucleoskeleton and cytoskeleton (LINC) complex mediates intracellular cross talk between the nucleus and the cytoplasm. In striated muscle, the LINC complex provides structural support to the myocyte nucleus and plays an essential role in regulating gene expression and mechanotransduction. A wide range of cardiac and skeletal myopathies have been linked to mutations in LINC complex proteins. Studies utilizing tissue-specific knockout and mutant mouse models have revealed important insights into the roles of the LINC complex in striated muscle. In this chapter, we describe several feasible approaches for generating striated muscle-specific gene knockout and mutant mouse models to study LINC complex protein function in cardiac and skeletal muscle. The experimental procedures used for phenotyping and analysis of LINC complex knockout mice are also described.

Forebrain-specific, conditional silencing of Staufen2 alters synaptic plasticity, learning, and memory in rats

Background

Dendritic messenger RNA (mRNA) localization and subsequent local translation in dendrites critically contributes to synaptic plasticity and learning and memory. Little is known, however, about the contribution of RNA-binding proteins (RBPs) to these processes in vivo.

Results

To delineate the role of the double-stranded RBP Staufen2 (Stau2), we generate a transgenic rat model, in which Stau2 expression is conditionally silenced by Cre-inducible expression of a microRNA (miRNA) targeting Stau2 mRNA in adult forebrain neurons. Known physiological mRNA targets for Stau2, such as RhoA, Complexin 1, and Rgs4 mRNAs, are found to be dysregulated in brains of Stau2-deficient rats. In vivo electrophysiological recordings reveal synaptic strengthening upon stimulation, showing a shift in the frequency-response function of hippocampal synaptic plasticity to favor long-term potentiation and impair long-term depression in Stau2-deficient rats. These observations are accompanied by deficits in hippocampal spatial working memory, spatial novelty detection, and in tasks investigating associative learning and memory.

Conclusions

Together, these experiments reveal a critical contribution of Stau2 to various forms of synaptic plasticity including spatial working memory and cognitive management of new environmental information. These findings might contribute to the development of treatments for conditions associated with learning and memory deficits.

Electronic supplementary material

The online version of this article (doi:10.1186/s13059-017-1350-8) contains supplementary material, which is available to authorized users.

Tamoxifen Activation of Cre-Recombinase Has No Persisting Effects on Adult Neurogenesis or Learning and Anxiety

Adult neurogenesis is a tightly regulated process continuously taking place in the central nervous system of most mammalian species. In neuroscience research, transgenic animals bearing the tamoxifen-inducible CreER^T2-Lox system are widely used. In this study, we made use of a Nestin-CreER^T2/R26R-YFP transgenic mouse model in which the CreER^T2 activates the expression of YFP in multipotent neural stem cells upon tamoxifen application. Humoral factors, such as the levels of estrogens, have been reported to affect the hippocampal neurogenesis. The application of tamoxifen, a mixed agonist/antagonist of the estrogen receptor that permeates the blood-brain-barrier, could thus influence adult neurogenesis. Although the functions of adult neurogenesis are yet to be fully deciphered, a reciprocal interaction between rates of neurogenesis on the one hand and learning and mood regulation on the other hand, has been suggested. The impact of tamoxifen on neurogenesis and behavior was therefore addressed following five daily applications according to the open field test, the elevated plus maze, and Morris water maze. In addition, the impact of short-term tamoxifen application on progenitor cell proliferation, morphology, and fate in the neurogenic niche of the dentate gyrus were investigated. Finally, the influence of the route of administration (oral vs. intra-peritoneal) and gender-specific response were scrutinized. The sub-acute analysis did neither reveal significant differences in behavior, such as voluntary motor activity, anxiety behavior, and spatial learning, nor in cell proliferation, cell survival, dendritic arborization or maturation rate within the dentate gyrus between saline solution-, corn oil-, and tamoxifen-treated groups. Finally, neither the route of application, nor the gender of treated mice influenced the response to tamoxifen. We conclude that short tamoxifen treatments used to activate the CreER^T2 system in transgenic mouse models does not have a measurable impact on adult neurogenesis or the here tested behavior, and is therefore appropriate for most studies in the field.

Evidence for an expanded time-window to mitigate a reactivated fear memory by tamoxifen

The mechanisms underpinning the persistence of emotional memories are inaccurately understood. Advancing the current level of understanding with regards to this aspect is of potential translational value for the treatment of post-traumatic stress disorder (PTSD), which stems from an abnormal aversive memory formation. Tamoxifen (TMX) is a drug used in chemotherapy for breast cancer and associated with poor cognitive performances. The present study investigated whether the systemic administration of TMX (1.0-50mg/kg) during and/or beyond the reconsolidation time-window could attenuate a reactivated contextual fear memory in laboratory animals. When administered 0, 6 or 9h (but not 12h) post-memory retrieval and reactivation, TMX (50mg/kg) reduced the freezing behavior in male rats re-exposed to the paired context on day 7, but not on day 1, suggesting a specific impairing effect on memory persistence. Importantly, this effect lasts up to 21 days, but it is prevented by omitting the memory retrieval or memory reactivation. When female rats in the diestrous or proestrous phase were used, the administration of TMX 6h after retrieving and reactivating the fear memory also impaired its persistence. Altogether, regardless of the gender, the present results indicate that the TMX is able to disrupt the persistence of reactivated fear memories in an expanded time-window, which could shed light on a new promising therapeutic strategy for PTSD.

The molecular, temporal and region-specific requirements of the beta isoform of Calcium/Calmodulin-dependent protein kinase type 2 (CAMK2B) in mouse locomotion

Genetic approaches using temporal and brain region-specific restricted gene deletions have provided a wealth of insight in the brain regions and temporal aspects underlying spatial and associative learning. However, for locomotion such extensive studies are still scarce. Previous studies demonstrated that Camk2b^–/– mice, which lack the β isoform of Calcium/Calmodulin-dependent protein kinase 2 (CAMK2B), show very severe locomotion deficits. However, where these locomotion deficits originate is unknown. Here we made use of novel Camk2b mutants (Camk2b^f/f and Camk2b^T287A), to explore the molecular, temporal and brain region-specific requirements of CAMK2B for locomotion. At the molecular level we found that normal locomotion requires Calcium/Calmodulin mediated activation of CAMK2B, but CAMK2B autonomous activity is largely dispensable. At a systems level, we found that global deletion of Camk2b in the adult mouse causes only mild locomotion deficits, suggesting that the severe locomotion deficits of Camk2b^–/– mice are largely of developmental origin. However, early onset deletion of Camk2b in cerebellum, striatum or forebrain did not recapitulate the locomotion deficits, suggesting that these deficits cannot be attributed to a single brain area. Taken together, these results provide the first insights into the molecular, temporal and region-specific role of CAMK2B in locomotion.

Morphological and behavioral characterization of adult mice deficient for SrGAP3

SrGAP3 belongs to the family of Rho GTPase proteins. These proteins are thought to play essential roles in development and in the plasticity of the nervous system. SrGAP3-deficient mice have recently been created and approximately 10 % of these mice developed a hydrocephalus and died shortly after birth. The others survived into adulthood, but displayed neuroanatomical alteration, including increased ventricular size. We now show that SrGAP3-deficient mice display increased brain weight together with increased hippocampal volume. This increase was accompanied by an increase of the thickness of the stratum oriens of area CA1 as well as of the thickness of the molecular layer of the dentate gyrus (DG). Concerning hippocampal adult neurogenesis, we observed no significant change in the number of proliferating cells. The density of doublecortin-positive cells also did not vary between SrGAP3-deficient mice and controls. By analyzing Golgi-impregnated material, we found that, in SrGAP3-deficient mice, the morphology and number of dendritic spines was not altered in the DG. Likewise, a Sholl-analysis revealed no significant changes concerning dendritic complexity as compared to controls. Despite the distinct morphological alterations in the hippocampus, SrGAP3-deficient mice were relatively inconspicuous in their behavior, not only in the open-field, nest building but also in the Morris water-maze. However, the SrGAP3-deficient mice showed little to no interest in burying marbles; a behavior that is seen in some animal models related to autism, supporting the view that SrGAP3 plays a role in neurodevelopmental disorders.

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.

Genetic Pharmacotherapy as an Early CNS Drug Development Strategy: Testing Glutaminase Inhibition for Schizophrenia Treatment in Adult Mice

Genetic pharmacotherapy is an early drug development strategy for the identification of novel CNS targets in mouse models prior to the development of specific ligands. Here for the first time, we have implemented this strategy to address the potential therapeutic value of a glutamate-based pharmacotherapy for schizophrenia involving inhibition of the glutamate recycling enzyme phosphate-activated glutaminase. Mice constitutively heterozygous for GLS1, the gene encoding glutaminase, manifest a schizophrenia resilience phenotype, a key dimension of which is an attenuated locomotor response to propsychotic amphetamine challenge. If resilience is due to glutaminase deficiency in adulthood, then glutaminase inhibitors should have therapeutic potential. However, this has been difficult to test given the dearth of neuroactive glutaminase inhibitors. So, we used genetic pharmacotherapy to ask whether adult induction of GLS1 heterozygosity would attenuate amphetamine responsiveness. We generated conditional floxGLS1 mice and crossed them with global CAG^ERT2cre∕+ mice to produce GLS1 iHET mice, susceptible to tamoxifen induction of GLS1 heterozygosity. One month after tamoxifen treatment of adult GLS1 iHET mice, we found a 50% reduction in GLS1 allelic abundance and glutaminase mRNA levels in the brain. While GLS1 iHET mice showed some recombination prior to tamoxifen, there was no impact on mRNA levels. We then asked whether induction of GLS heterozygosity would attenuate the locomotor response to propsychotic amphetamine challenge. Before tamoxifen, control and GLS1 iHET mice did not differ in their response to amphetamine. One month after tamoxifen treatment, amphetamine-induced hyperlocomotion was blocked in GLS1 iHET mice. The block was largely maintained after 5 months. Thus, a genetically induced glutaminase reduction—mimicking pharmacological inhibition—strongly attenuated the response to a propsychotic challenge, suggesting that glutaminase may be a novel target for the pharmacotherapy of schizophrenia. These results demonstrate how genetic pharmacotherapy can be implemented to test a CNS target in advance of the development of specific neuroactive inhibitors. We discuss further the advantages, limitations, and feasibility of the wider application of genetic pharmacotherapy for neuropsychiatric drug development.

Cannabinoid type-1 receptor signaling in central serotonergic neurons regulates anxiety-like behavior and sociability

The endocannabinoid (eCB) system possesses neuromodulatory functions by influencing the release of various neurotransmitters, including γ-aminobutyric acid (GABA) and glutamate. A functional interaction between eCBs and the serotonergic system has already been suggested. Previously, we showed that cannabinoid type-1 (CB1) receptor mRNA and protein are localized in serotonergic neurons of the raphe nuclei, implying that the eCB system can modulate serotonergic functions. In order to substantiate the physiological role of the CB1 receptor in serotonergic neurons of the raphe nuclei, we generated serotonergic 5-hydroxytryptamine (5-HT) neuron-specific CB 1 receptor-deficient mice, using the Cre/loxP system with a tamoxifen-inducible Cre recombinase under the control of the regulatory sequences of the tryptophan hydroxylase 2 gene (TPH2-CreER (T2)), thus, restricting the recombination to 5-HT neurons of the central nervous system (CNS). Applying several different behavioral paradigms, we revealed that mice lacking the CB1 receptor in serotonergic neurons are more anxious and less sociable than control littermates. Thus, we were able to show that functional CB1 receptor signaling in central serotonergic neurons modulates distinct behaviors in mice.

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.

Temporal and region-specific requirements of αCaMKII in spatial and contextual learning

The α isoform of the calcium/calmodulin-dependent protein kinase II (αCaMKII) has been implicated extensively in molecular and cellular mechanisms underlying spatial and contextual learning in a wide variety of species. Germline deletion of Camk2a leads to severe deficits in spatial and contextual learning in mice. However, the temporal and region-specific requirements for αCaMKII have remained largely unexplored. Here, we generated conditional Camk2a mutants to examine the influence of spatially restricted and temporally controlled expression of αCaMKII. Forebrain-specific deletion of the Camk2a gene resulted in severe deficits in water maze and contextual fear learning, whereas mice with deletion restricted to the cerebellum learned normally. Furthermore, we found that temporally controlled deletion of the Camk2a gene in adult mice is as detrimental as germline deletion for learning and synaptic plasticity. Together, we confirm the requirement for αCaMKII in the forebrain, but not the cerebellum, in spatial and contextual learning. Moreover, we highlight the absolute requirement for intact αCaMKII expression at the time of learning.

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.

The effects of tamoxifen on learning, memory and brain tissues oxidative damage in ovariectomized and naïve female rats

Background:

Regarding the modulatory effects of tamoxifen (TAM) on the actions of estrogen in the present study, the effects of TAM on learning, memory and brain tissues oxidative damage in ovariectomized (OVX) and naοve female rats was investigated.

Materials and Methods:

The animals were divided into: (1) Sham, (2) OVX, (3) Sham-tamoxifen (Sham-TAM) and (4) ovariectomized-tamoxifen (OVX-TAM). The animals of the Sham-TAM and OVX-TAM groups were treated by TAM (1 mg/kg; 4 weeks).

Results:

In Morris water maze, the escape latency in the OVX group was higher than in the Sham group (P < 0.01). The time latency in the animals of OVX-TAM group was lower than that of OVX group (P < 0.01); however, there were no significant differences between the Sham-TAM and Sham groups. In the probe trial, the time spent in target quadrant (Q₁) by the animals of OVX group was lower than that of Sham group (P < 0.01). Interestingly, the animals of OVX-TAM group spent more times in target quadrant (Q₁) compared with OVX group (P < 0.01). In passive avoidance test, the animals of OVX group had lower latencies to enter the dark compartment compared with the Sham group (P < 0.05). The time latency to enter the dark compartment by animals of OVX-TAM group was higher than in OVX group (P < 0.01). In OVX-TAM group, the total thiol concentration was significantly higher (P < 0.05) and malondialdehyde concentration was lower (P < 0.01) than OVX group.

Conclusions:

These results allow us to propose that TAM enhances learning and memory of OVX rats. The possible mechanism may be due to the protective effects against brain tissues oxidative damage.

Impact of adolescent GluA1 AMPA receptor ablation in forebrain excitatory neurons on behavioural correlates of mood disorders

Glutamatergic dysfunctions have recently been postulated to play a considerable role in mood disorders. However, molecular mechanisms underlying these effects have been poorly deciphered. Previous work demonstrated the contribution of GluA1-containing AMPA receptors (AMPAR) to a depression-like and anxiety-like phenotype. Here we investigated the effect of temporally and spatially restricted gene manipulation of GluA1 on behavioural correlates of mood disorders in mice. Here we show that tamoxifen-induced GluA1 deletion restricted to forebrain glutamatergic neurons of post-adolescent mice does not induce depression- and anxiety-like changes. This differs from the phenotype of mice with global AMPAR deletion suggesting that for mood regulation AMPAR may be particularly important on inhibitory interneurons or already early in development.

Lack of protracted behavioral abnormalities following intermittent or continuous chronic mild hypoxia in perinatal C57BL/6 mice

Several prospective studies indicated perinatal hypoxia as risk factor for psychiatric disorders like schizophrenia. It is thought that hypoxia prior to or during birth may contribute to alterations leading to the protracted clinical manifestation during young adulthood. However, only a small fraction of children with a history of perinatal hypoxia develop later psychotic symptoms, therefore it is not known if hypoxia alone is sufficient to trigger long-term behavioral changes. Here we exposed C57BL/6 mice from postnatal day 3-7 (P3-P7) to two established paradigms of chronic mild hypoxia (10% ambient O2), intermittent and continuous. Subsequently, mice were analysed during young adult stages using several basic behavioral tests. Previous studies demonstrated severe, but only transient, cortical damage in these paradigms; it is not clear, if these reversible morphological changes are accompanied by long-term behavioral effects. We found that neither intermittent nor continuous perinatal hypoxia induced long-term behavioral alterations. This may be due to the high regenerative capacity of the perinatal brain. Other possibilities include a potential resistance to perinatal hypoxia of the mouse strain used here or a level of hypoxia that was insufficient to trigger significant behavioral changes. Therefore, our data do not exclude a role of perinatal hypoxia as risk factor for psychiatric disorders. They rather suggest that either other, more severe hypoxic conditions like anoxia, or the presence of additional factors (as genetic risk factors) are necessary for generating long-term behavioral abnormalities.

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.

Transient downregulation of Dab1 protein levels during development leads to behavioral and structural deficits: relevance for psychiatric disorders

Psychiatric disorders have been hypothesized to originate during development, with genetic and environmental factors interacting in the etiology of disease. Therefore, developmentally regulated genes have received attention as risk modulators in psychiatric diseases. Reelin is an extracellular protein essential for neuronal migration and maturation during development, and its expression levels are reduced in psychiatric disorders. Interestingly, several perinatal insults that increase the risk of behavioral deficits alter Reelin signaling. However, it is not known whether a dysfunction in Reelin signaling during perinatal stages increases the risk of psychiatric disorders. Here we used a floxed dab1 allele to study whether a transient decrease in Dab1, a key component of the Reelin pathway, is sufficient to induce behavioral deficits related to psychiatric disorders. We found that transient Dab1 downregulation during perinatal stages leads to permanent abnormalities of structural layering in the neocortex and hippocampus. In contrast, conditional inactivation of the dab1 gene in the adult brain does not result in additional layering abnormalities. Furthermore, perinatal Dab1 downregulation causes behavior impairments in adult mice, such as deficits in memory, maternal care, pre-pulse inhibition, and response to cocaine. Some of these deficits were also found to be present in adolescence. We also show that D-cycloserine rescues the cognitive deficits observed in floxed dab1 mice with layering alterations in the hippocampus and neocortex. Our results indicate a causal relation between the downregulation of Dab1 protein levels during development and the structural and behavioral deficits associated with psychiatric diseases in the adult.

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.

Targeted mutagenesis tools for modelling psychiatric disorders

In the 1980s, the basic principles of gene targeting were discovered and forged into sharp tools for efficient and precise engineering of the mouse genome. Since then, genetic mouse models have substantially contributed to our understanding of major neurobiological concepts and are of utmost importance for our comprehension of neuropsychiatric disorders. The "domestication" of site-specific recombinases and the continuous creative technological developments involving the implementation of previously identified biological principles such as transcriptional and posttranslational control now enable conditional mutagenesis with high spatial and temporal resolution. The initiation and successful accomplishment of large-scale efforts to annotate functionally the entire mouse genome and to build strategic resources for the research community have significantly accelerated the rapid proliferation and broad propagation of mouse genetic tools. Addressing neurobiological processes with the assistance of genetic mouse models is a routine procedure in psychiatric research and will be further extended in order to improve our understanding of disease mechanisms. In light of the highly complex nature of psychiatric disorders and the current lack of strong causal genetic variants, a major future challenge is to model of psychiatric disorders more appropriately. Humanized mice, and the recently developed toolbox of site-specific nucleases for more efficient and simplified tailoring of the genome, offer the perspective of significantly improved models. Ultimately, these tools will push the limits of gene targeting beyond the mouse to allow genome engineering in any model organism of interest.

Genetic models of schizophrenia and related psychotic disorders: progress and pitfalls across the methodological "minefield"

The challenge of modelling a complex and multifaceted disorder such as schizophrenia is epitomised by the considerable degree of phenotypic variability described in patients and by the absence of specific and consistent neuropathological biomarkers. The pattern and severity of a range of clinical features, including florid psychotic symptoms such as hallucinations and delusions, negative symptoms and cognitive dysfunction, together with age at onset, course of illness and other indices, can vary greatly between individual patients. The undefined nature of the relationship between diagnosis and underlying aetiology has complicated research in the field of clinical and preclinical neuroscience, thereby making it difficult to generate or evaluate appropriate disease models of schizophrenia. In the present review, we explore those conceptual and practical issues that relate specifically to the genetic modelling of schizophrenia and related disorders in rodents. Practical issues that impact on the robustness of endophenotypic findings and their translational relevance are discussed with reference to evidence from selective genetic models of candidate risk genes and copy number variants implicated in schizophrenia.

Addictive drugs and plasticity of glutamatergic synapses on dopaminergic neurons: what have we learned from genetic mouse models?

Drug-induced changes in the functional properties of neurons in the mesolimbic dopaminergic system are attractive candidates for the molecular underpinnings of addiction. A central question in this context has been how drugs of abuse affect synaptic plasticity on dopaminergic cells in the ventral tegmental area. We now know that the intake of addictive drugs is accompanied by a complex sequence of alterations in the properties of excitatory synapses on dopaminergic neurons, mainly driven by signaling and redistribution of NMDA- and AMPA-receptors. It has, however, been unclear how these molecular changes are related to the behavioral effects of addictive drugs. Recently, new genetic tools have permitted researchers to perform genetic intervention with plasticity-related molecules selectively in dopaminergic cells and to subsequently study the behaviors of genetically modified mice. These studies have started to reveal how plasticity and drug-induced behavior are connected as well as what role plasticity in dopaminergic cells may have in general reward learning. The findings thus far show that there is not a one-to-one relation between plastic events and specific behaviors and that the early responses to drugs of abuse are to a large extent independent of the types of synaptic plasticity so far targeted. In contrast, plasticity in dopaminergic cells indeed is an important regulator of the persistence of behaviors driven by drug associations, making synaptic plasticity in dopaminergic cells an important field of study for understanding the mechanisms behind relapse.

Cardiac-specific inducible and conditional gene targeting in mice

Mouse genetic engineering has revolutionized our understanding of the molecular and genetic basis of heart development and disease. This technology involves conditional tissue-specific and temporal transgenic and gene targeting approaches, as well as introduction of polymorphisms into the mouse genome. These approaches are increasingly used to elucidate the genetic pathways underlying tissue homeostasis, physiology, and pathophysiology of adult heart. They have also led to the development of clinically relevant models of human cardiac diseases. Here, we review the technologies and their limitations in general and the cardiovascular research community in particular.

Tetracycline inducible gene manipulation in serotonergic neurons

The serotonergic (5-HT) neuronal system has important and diverse physiological functions throughout development and adulthood. Its dysregulation during development or later in adulthood has been implicated in many neuropsychiatric disorders. Transgenic animal models designed to study the contribution of serotonergic susceptibility genes to a pathological phenotype should ideally allow to study candidate gene overexpression or gene knockout selectively in serotonergic neurons at any desired time during life. For this purpose, conditional expression systems such as the tet-system are preferable. Here, we generated a transactivator (tTA) mouse line (TPH2-tTA) that allows temporal and spatial control of tetracycline (Ptet) controlled transgene expression as well as gene deletion in 5-HT neurons. The tTA cDNA was inserted into a 196 kb PAC containing a genomic mouse Tph2 fragment (177 kb) by homologous recombination in E. coli. For functional analysis of Ptet-controlled transgene expression, TPH2-tTA mice were crossed to a Ptet-regulated lacZ reporter line (Ptet-nLacZ). In adult double-transgenic TPH2-tTA/Ptet-nLacZ mice, TPH2-tTA founder line L62-20 showed strong serotonergic β-galactosidase expression which could be completely suppressed with doxycycline (Dox). Furthermore, Ptet-regulated gene expression could be reversibly activated or inactivated when Dox was either withdrawn or added to the system. For functional analysis of Ptet-controlled, Cre-mediated gene deletion, TPH2-tTA mice (L62-20) were crossed to double transgenic Ptet-Cre/R26R reporter mice to generate TPH2-tTA/Ptet-Cre/R26R mice. Without Dox, 5-HT specific recombination started at E12.5. With permanent Dox administration, Ptet-controlled Cre-mediated recombination was absent. Dox withdrawal either postnatally or during adulthood induced efficient recombination in serotonergic neurons of all raphe nuclei, respectively. In the enteric nervous system, recombination could not be detected. We generated a transgenic mouse tTA line (TPH2-tTA) which allows both inducible and reversible transgene expression and inducible Cre-mediated gene deletion selectively in 5-HT neurons throughout life. This will allow precise delineation of serotonergic gene functions during development and adulthood.

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.

Genetic approaches to study glial cells in the rodent brain

The development, function, and pathology of the brain depend on interactions of neurons and different types of glial cells, namely astrocytes, oligodendrocytes, microglia, and ependymal cells. Understanding neuron-glia interactions in vivo requires dedicated experimental approaches to manipulate each cell type independently. In this review, we first summarize techniques that allow for cell-specific gene modification including targeted mutagenesis and viral transduction. In the second part, we describe the genetic models that allow to target the main glial cell types in the central nervous system. The existing arsenal of approaches to study glial cells in vivo and its expansion in the future are key to understand neuron-glia interactions under normal and pathologic conditions.

In vivo imaging and noninvasive ablation of pyramidal neurons in adult NEX-CreERT2 mice

To study the function of individual neurons that are embedded in a complex neural network is difficult in mice. Conditional mutagenesis permits the spatiotemporal control of gene expression including the ablation of cells by toxins. To direct expression of a tamoxifen-inducible variant of Cre recombinase (CreERT2) selectively to cortical neurons, we replaced the coding region of the murine Nex1 gene by CreERT2 cDNA via homologous recombination in embryonic stem cells. When injected with tamoxifen, adult NEX-CreERT2 mice induced reporter gene expression exclusively in projection neurons of the neocortex and hippocampus. By titrating the tamoxifen dosage, we achieved recombination in single cells, which allowed multiphoton imaging of neocortical neurons in live mice. When hippocampal projection neurons were genetically ablated by induced expression of diphteria toxin, within 20 days the inflammatory response included the infiltration of CD3+ T cells. This marks a striking difference from similar studies, in which dying oligodendrocytes failed to recruit cells of the adaptive immune system.

Generating conditional knockout mice

Gene targeting in ES cells is extensively used to generate designed mouse mutants and to study gene function in vivo. Knockout mice that harbor a null allele in their germline provide appropriate genetic models of inherited diseases and often exhibit embryonic or early postnatal lethality. To study gene function in adult mice and in selected cell types, a refined strategy for conditional gene inactivation has been developed that relies on the DNA recombinase Cre and its recognition (loxP) sites. For conditional mutagenesis, a target gene is modified by the insertion of two loxP sites that enable to excise the flanked (floxed) gene segment through Cre-mediated recombination. Conditional mutant mice are obtained by crossing the floxed strain with a Cre transgenic line such that the target gene becomes inactivated in vivo within the expression domain of Cre. A large collection of Cre transgenic lines has been generated over time and can be used in a combinatorial manner to achieve gene inactivation in many different cell types. A growing number of CreER(T2) transgenic mice further allows for inducible inactivation of floxed alleles in adult mice upon administration of tamoxifen. This chapter covers the design and construction of loxP flanked alleles and refers to the vectors, ES cells, and mice generated by the European conditional mouse mutagenesis (EUCOMM) project. We further describe the design and use of Cre and CreER(T2) transgenic mice and a convenient breeding strategy to raise conditional mutants and controls for phenotype analysis.

Pet-1 is required across different stages of life to regulate serotonergic function

Transcriptional cascades are required for specification of 5-HT neurons and 5-HT modulated behaviors. Expression of several cascade factors extends across lifespan suggesting their control of behavior may not be temporally restricted to programming normal numbers of 5-HT neurons. We applied new mouse conditional targeting approaches to investigate ongoing requirements for Pet-1, a cascade factor required for the initiation of 5-HT synthesis but whose expression persists into adulthood. We found that Pet-1 was required after 5-HT neuron generation, for multiple steps in 5-HT neuron maturation including axonal innervation to the somatosensory cortex, firing properties, and 5-HT_1A and 5-HT_1B autoreceptor expression. Targeting Pet-1 in adult 5-HT neurons showed that it was still needed to preserve normal anxiety-related behaviors through direct autoregulated control of serotonergic gene expression. These findings show that Pet-1 is required across lifespan and therefore behavioral pathogenesis can result from both developmental and adult-onset alterations in serotonergic transcription.

Behavioral and neurochemical characterization of mice deficient in the N-type Ca2+ channel alpha1B subunit

N-type voltage-dependent calcium channels (VDCCs) play an important role in neurotransmission, synaptic plasticity, and brain development. They are composed of several subunits named alpha(1), alpha(2), delta, beta and gamma. The alpha(1) subunit is essential for channel functions and determines fundamental channel properties. Since N-type VDCC are critically involved in the release of neurotransmitters and clinical relevance, we predicted that alpha(1) subunit KO mice would show several alterations in behavior. In the present study, we investigated neuronal functions in mice lacking the alpha(1B) (Ca(V)2.2) subunit of the N-type calcium channels. Ca(V)2.2(-/-) mice exhibited a significant increase in locomotion on an activity wheel during the dark phase. Furthermore, when challenged with apomorphine, mutant mice showed enhanced locomotor activity. Cognitive functions were examined using a Y-maze task for short-term memory and a passive avoidance task for long-term memory. The Y-maze revealed no differences in spontaneous alternation behavior between mutant and wild-type mice. The passive avoidance test revealed that the latency time in mutant mice was significantly decreased. The mutant mice showed prepulse inhibition deficits reminiscent of the sensorimotor gating deficits observed in a large majority of schizophrenic patients. Decreases in baseline levels of dopamine and serotonin within the striata and frontal cortices of mutant mice were also observed. These results suggest that Ca(2+) in the central nervous system modulates various neurophysiological functions, such as locomotor activity, long-term memory, and sensorimotor gating through the alpha(1B) subunit of the N-type calcium channels.

Inducible gene manipulations in serotonergic neurons

An impairment of the serotonergic (5-HT) system has been implicated in the etiology of many neuropsychiatric disorders. Despite the considerable genetic evidence, the exact molecular and pathophysiological mechanisms underlying this dysfunction remain largely unknown. To address the lack of instruments for the molecular dissection of gene function in serotonergic neurons we have developed a new mouse transgenic tool that allows inducible Cre-mediated recombination of genes selectively in 5-HT neurons of all raphe nuclei. In this transgenic mouse line, the tamoxifen-inducible CreERT2 recombinase is expressed under the regulatory control of the mouse tryptophan hydroxylase 2 (Tph2) gene locus (177 kb). Tamoxifen treatment efficiently induced recombination selectively in serotonergic neurons with minimal background activity in vehicle-treated mice. These genetic manipulations can be initiated at any desired time during embryonic development, neonatal stage or adulthood. To illustrate the versatility of this new tool, we show that Brainbow-1.0L(TPH2-CreERT2) mice display highly efficient recombination in serotonergic neurons with individual 5-HT neurons labeling with multiple distinct fluorescent colors. This labeling is well suited for visualization and tracing of serotonergic neurons and their network architecture. Finally, the applicability of TPH2-CreERT2 for loxP-flanked candidate gene manipulation is evidenced by our successful knockout induction of the ubiquitously expressed glucocorticoid-receptor exclusively in 5-HT neurons of adult mice. The TPH2-CreERT2 line will allow detailed analysis of gene function in both developing and adult serotonergic neurons.

Glutamate receptors on dopamine neurons control the persistence of cocaine seeking

Cocaine strengthens excitatory synapses onto midbrain dopamine neurons through the synaptic delivery of GluR1-containing AMPA receptors. This cocaine-evoked plasticity depends on NMDA receptor activation, but its behavioral significance in the context of addiction remains elusive. Here, we generated mice lacking the GluR1, GluR2, or NR1 receptor subunits selectively in dopamine neurons. We report that in midbrain slices of cocaine-treated mice, synaptic transmission was no longer strengthened when GluR1 or NR1 was abolished, while in the respective mice the drug still induced normal conditioned place preference and locomotor sensitization. In contrast, extinction of drug-seeking behavior was absent in mice lacking GluR1, while in the NR1 mutant mice reinstatement was abolished. In conclusion, cocaine-evoked synaptic plasticity does not mediate concurrent short-term behavioral effects of the drug but may initiate adaptive changes eventually leading to the persistence of drug-seeking behavior.