Decreased locomotor activity in mice expressing tTA under control of the CaMKII alpha promoter

Acquired Expression of Mutant Mitofusin 2 Causes Progressive Neurodegeneration and Abnormal Behavior

Neurons have high plasticity in developmental and juvenile stages that decreases in adulthood. Mitochondrial dynamics are highly important in neurons to maintain normal function. To compare dependency on mitochondrial dynamics in juvenile and adult stages, we generated a mouse model capable of selective timing of the expression of a mutant of the mitochondrial fusion factor Mitofusin 2 (MFN2). Mutant expression in the juvenile stage had lethal effects. Contrastingly, abnormalities did not manifest until 150 d after mutant expression during adulthood. After this silent 150 d period, progressive neurodegeneration, abnormal behaviors, and learning and memory deficits similar to those seen in human neurodegenerative diseases were observed. This indicates that abnormal neuronal mitochondrial dynamics seriously affect survival during early life stages and can also significantly damage brain function after maturation. Our findings highlight the need to consider the timing of disease onset in mimicking human neurodegenerative diseases.SIGNIFICANCE STATEMENT To compare the dependency on mitochondrial dynamics in neurons in juvenile and adult stages, we generated a mouse model expressing a mutant of the mitochondrial fusion factor MFN2 in an arbitrary timing. Juvenile expression of the mutant showed acute and severe phenotypes and had lethal effects; however, post-adult expression induced delayed but progressive phenotypes resembling those found in human neurodegenerative diseases. Our results indicate that abnormal neuronal mitochondrial dynamics seriously affect survival during early life stages and can also significantly damage brain function after maturation. This strongly suggests that the timing of expression should be considered when establishing an animal model that closely resembles human neurodegenerative diseases.

A Novel hAPP/htau Mouse Model of Alzheimer's Disease: Inclusion of APP With Tau Exacerbates Behavioral Deficits and Zinc Administration Heightens Tangle Pathology

The brains of those with Alzheimer's disease have amyloid and tau pathology; thus, mice modeling AD should have both markers. In this study, we characterize offspring from the cross of the J20 (hAPP) and rTg4510 (htau) strains (referred to as dual Tg). Behavior was assessed at both 3.5 and 7 months, and biochemical differences were assessed at 8 months. Additionally, mice were placed on zinc (Zn) water or standard lab water in order to determine the role of this essential biometal. Behavioral measures examined cognition, emotion, and aspects of daily living. Transgenic mice (dual Tg and htau) showed significant deficits in spatial memory in the Barnes Maze at both 3.5 and 7 months compared to controls. At 7 months, dual Tg mice performed significantly worse than htau mice (p < 0.01). Open field and elevated zero maze (EZM) data indicated that dual Tg and htau mice displayed behavioral disinhibition compared to control mice at both 3.5 and 7 months (p < 0.001). Transgenic mice showed significant deficits in activities of daily living, including burrowing and nesting, at both 3.5 and 7 months compared to control mice (p < 0.01). Dual Tg mice built very poor nests, indicating that non-cognitive tasks are also impacted by AD. Overall, dual Tg mice demonstrated behavioral deficits earlier than those shown by the htau mice. In the brain, dual Tg mice had significantly less free Zn compared to control mice in both the dentate gyrus and the CA3 of the hippocampus (p < 0.01). Dual Tg mice had increased tangles and plaques in the hippocampus compared to htau mice and the dual Tg mice given Zn water displayed increased tangle pathology in the hippocampus compared to htau mice on Zn water (p < 0.05). The dual Tg mouse described here displays pathology reminiscent of the human AD condition and is impaired early on in both cognitive and non-cognitive behaviors. This new mouse model allows researchers to assess how both amyloid and tau in combination impact behavior and brain pathology.

Presymptomatic change in microRNAs modulates Tau pathology

MicroRNAs (miRs) are 18~23 nucleotides long non-coding RNAs that regulate gene expression. To explore whether miR alterations in tauopathy contribute to pathological conditions, we first determined which hippocampal miRs are altered at the presymptomatic and symptomatic stages of tauopathy using rTg4510 mice (Tau mice), a well-characterized tauopathy model. miR-RNA pairing analysis using QIAGEN Ingenuity Pathway Analysis (IPA) revealed 401 genes that can be regulated by 71 miRs altered in Tau hippocampi at the presymptomatic stage. Among several miRs confirmed with real-time qPCR, miR142 (−3p and −5p) in Tau hippocampi were significantly upregulated by two-weeks of age and onward. Transcriptome studies by RNAseq and IPA revealed several overlapping biological and disease associated pathways affected by either Tau or miR142 overexpression, including Signal Transducer and Activator of Transcription 3 (Stat3) and Tumor Necrosis Factor Receptor 2 (Tnfr2) signaling pathways. Similar to what was observed in Tau brains, overexpressing miR142 in wildtype cortical neurons augments mRNA levels of Glial Fibrillary Acidic Protein (Gfap) and Colony Stimulating Factor 1 (Csf1), accompanied by a significant increase in microglia and reactive astrocyte numbers. Taken together, our study suggests that miR alterations by Tau overexpression may contribute to the neuroinflammation observed in Tau brains.

Postnatal separation prevents the development of prenatal stress-induced anxiety in association with changes in oestrogen receptor and oxytocin immunoreactivity in female mandarin vole (Microtus mandarinus) offspring

Oestrogen has both anxiogenic and anxiolytic effects because of variation in opposing action on alpha (ERα) and beta (ERβ) estrogen receptors in the medial preoptic area (mPOA), bed nucleus of the stria terminalis (BNST) and medial amygdala (MeA). Oxytocin (OT) reverses some of the anxiogenic effects of oestrogen in the hypothalamic paraventricular nucleus (PVN) and supraoptic nucleus (SON). Because anxiety disorders are twice as common in women as in men, and oestrogen and OT are more important in females, we examined interactions between prenatal restraint stress (GS) and postnatal early short-term maternal separation (MS) and female mandarin vole behaviour, estrogen receptors and OT. The results show that adult female offspring from GS/noMS mothers showed increased anxiety in open-field and elevated plus-maze tests and had lower serum 17-beta-oestradiol (E₂ ) levels than female offspring from GS/MS, noGS/MS and noGS/noMS mothers. GS/noMS females had more immunoreactive neurons for ERα in several brain regions and less ERβ- and OT-immunoreactive neurons in brain areas compared to GS/MS, noGS/MS and noGS/noMS offspring. Interestingly, noGS/MS and GS/MS offspring were similar to noGS/noMS offspring in that they did not develop anxiety as adults. We propose that MS alters the serum concentration of E₂ and that the ERβ/ERα ratio and OT level in the brain may be responsible for the decrease in anxiety-like behaviour in adult female offspring initially exposed to anxiety-inducing conditions via an adverse foetal environment.

The L-type voltage-gated calcium channel CaV1.2 mediates fear extinction and modulates synaptic tone in the lateral amygdala

L-type voltage-gated calcium channels (LVGCCs) have been implicated in both the formation and the reduction of fear through Pavlovian fear conditioning and extinction. Despite the implication of LVGCCs in fear learning and extinction, studies of the individual LVGCC subtypes, Ca_V1.2 and Ca_V1.3, using transgenic mice have failed to find a role of either subtype in fear extinction. This discontinuity between the pharmacological studies of LVGCCs and the studies investigating individual subtype contributions could be due to the limited neuronal deletion pattern of the Ca_V1.2 conditional knockout mice previously studied to excitatory neurons in the forebrain. To investigate the effects of deletion of Ca_V1.2 in all neuronal populations, we generated Ca_V1.2 conditional knockout mice using the synapsin1 promoter to drive Cre recombinase expression. Pan-neuronal deletion of Ca_V1.2 did not alter basal anxiety or fear learning. However, pan-neuronal deletion of Ca_V1.2 resulted in a significant deficit in extinction of contextual fear, implicating LVGCCs, specifically Ca_V1.2, in extinction learning. Further exploration on the effects of deletion of Ca_V1.2 on inhibitory and excitatory input onto the principle neurons of the lateral amygdala revealed a significant shift in inhibitory/excitatory balance. Together these data illustrate an important role of Ca_V1.2 in fear extinction and the synaptic regulation of activity within the amygdala.

Tracking progressive pathological and functional decline in the rTg4510 mouse model of tauopathy

BACKGROUND

The choice and appropriate use of animal models in drug discovery for Alzheimer's disease (AD) is pivotal to successful clinical translation of novel therapeutics, yet true alignment of research is challenging. Current models do not fully recapitulate the human disease, and even exhibit various degrees of regional pathological burden and diverse functional alterations. Given this, relevant pathological and functional endpoints must be determined on a model-by-model basis. The present work explores the rTg4510 mouse model of tauopathy as a case study to define best practices for the selection and validation of cognitive and functional endpoints for the purposes of pre-clinical AD drug discovery.

METHODS

Male rTg4510 mice were first tested at an advanced age, 12 months, in multiple behavioural assays (step 1). Severe tau pathology and neurodegeneration was associated with profound locomotor hyperactivity and spatial memory deficits. Four of these assays were then selected for longitudinal assessment, from 4 to 12 months, to investigate whether behavioural performance changes as a function of accumulation of tau pathology (step 2). Experimental suppression of tau pathology-via doxycycline administration-was also investigated for its effect on functional performance.

RESULTS

Progressive behavioural changes were detected where locomotor activity and rewarded alternation were found to most closely correlate with tau burden and neurodegeneration. Doxycycline initiated at 4 months led to a 50% suppression of transgene expression, which was sufficient to prevent subsequent increases in tau pathology and arrest related functional decline.

CONCLUSIONS

This two-step approach demonstrates the importance of selecting assays most sensitive to the phenotype of the model. A robust relationship was observed between pathological progression, development of phenotype, and their experimental manipulation-three crucial factors for assessing the translational relevance of future pre-clinical findings.

Deletion of fibroblast growth factor 22 (FGF22) causes a depression-like phenotype in adult mice

Specific growth factors induce formation and differentiation of excitatory and inhibitory synapses, and are essential for brain development and function. Fibroblast growth factor 22 (FGF22) is important for specifying excitatory synapses during development, including in the hippocampus. Mice with a genetic deletion of FGF22 (FGF22KO) during development subsequently have fewer hippocampal excitatory synapses in adulthood. As a result, FGF22KO mice are resistant to epileptic seizure induction. In addition to playing a key role in learning, the hippocampus is known to mediate mood and anxiety. Here, we explored whether loss of FGF22 alters affective, anxiety or social cognitive behaviors in mice. We found that relative to control mice, FGF22KO mice display longer duration of floating and decreased latency to float in the forced swim test, increased immobility in the tail suspension test, and decreased preference for sucrose in the sucrose preference test, which are all suggestive of a depressive-like phenotype. No differences were observed between control and FGF22KO mice in other behavioral assays, including motor, anxiety, or social cognitive tests. These results suggest a novel role for FGF22 specifically in affective behaviors.

Tau overexpression impacts a neuroinflammation gene expression network perturbed in Alzheimer's disease

Filamentous inclusions of the microtubule-associated protein, tau, define a variety of neurodegenerative diseases known as tauopathies, including Alzheimer's disease (AD). To better understand the role of tau-mediated effects on pathophysiology and global central nervous system function, we extensively characterized gene expression, pathology and behavior of the rTg4510 mouse model, which overexpresses a mutant form of human tau that causes Frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17). We found that the most predominantly altered gene expression pathways in rTg4510 mice were in inflammatory processes. These results closely matched the causal immune function and microglial gene-regulatory network recently identified in AD. We identified additional gene expression changes by laser microdissecting specific regions of the hippocampus, which highlighted alterations in neuronal network activity. Expression of inflammatory genes and markers of neuronal activity changed as a function of age in rTg4510 mice and coincided with behavioral deficits. Inflammatory changes were tau-dependent, as they were reversed by suppression of the tau transgene. Our results suggest that the alterations in microglial phenotypes that appear to contribute to the pathogenesis of Alzheimer's disease may be driven by tau dysfunction, in addition to the direct effects of beta-amyloid.

Effects of the C57BL/6 strain background on tauopathy progression in the rTg4510 mouse model

Background

Cross-breeding of transgenic mice is commonly used to assess gene-gene interactions, particularly in the context of disease. Strain background changes can influence the phenotype of mouse models and can confound crossbreeding studies. We sought to determine if changing the strain background of a commonly used mouse model of tauopathy (rTg4510) would significantly impact the originally reported phenotype. On the original F1 FVB/N x 129S6 background, rTg4510 mice present with progressive cognitive decline, increased insoluble tau, robust tau pathology and age-dependent neurodegeneration. One of the most common strains in mouse modeling is C57BL/6. We and others have previously reported that this strain background alters the phenotypes of various models, including the JNPL3 model of tauopathy. To determine if the phenotype of rTg4510 mice was similarly affected by the introduction of the C57BL/6 background, we compared rTg4510 mice on the original F1 FVB/N x 129S6 background to rTg4510 mice on an F1 FVB/N x C57BL/6NTac (B6/NTac) background, herein termed rTg4510_B6.

Results

Despite a small, but significant increase in soluble human tau levels, young rTg4510_B6 mice had equivalent levels of tau phosphorylation, aggregation and cognitive impairments as age-matched rTg4510 mice. At 6.5 months of age, rTg4510_B6 mice displayed hyperphosphorylated insoluble tau and robust cortical tau neuropathology that was equivalent to age-matched rTg4510 mice; however, 10.5-month-old rTg4510_B6 mice had greater amounts of phospho-tau in the cortex and hippocampus when compared to age-matched rTg4510 mice. Non-transgenic (NT) littermates of rTg4510_B6 (NT_B6) mice also had greater amounts of cortical and hippocampal phospho-tau at 10.5 months of age when compared to NT littermates of rTg4510 mice. Additionally, older rTg4510_B6 mice had gross forebrain neurodegeneration that was equivalent to age-matched rTg4510 mice.

Conclusions

Overall, our data shows that introduction of the C57BL/6 strain into the rTg4510 mouse background modestly alters the tau pathology that was originally reported in rTg4510 on the F1 FVB/129 background. In contrast, behavioral and neurodegenerative outcomes were not altered. These studies support the use of the rTg4510 mouse model on a partial C57BL/6 strain background without losing fidelity of the phenotype and suggest that the C57BL/6 background does not inherently protect against tauopathy.

Strain background influences neurotoxicity and behavioral abnormalities in mice expressing the tetracycline transactivator

The tet-off system has been widely used to create transgenic models of neurological disorders including Alzheimer's, Parkinson's, Huntington's, and prion disease. The utility of this system lies in the assumption that the tetracycline transactivator (TTA) acts as an inert control element and does not contribute to phenotypes under study. Here we report that neuronal expression of TTA can affect hippocampal cytoarchitecture and behavior in a strain-dependent manner. While studying neurodegeneration in two tet-off Alzheimer's disease models, we unexpectedly discovered neuronal loss within the dentate gyrus of single transgenic TTA controls. Granule neurons appeared most sensitive to TTA exposure during postnatal development, and doxycycline treatment during this period was neuroprotective. TTA-induced degeneration could be rescued by moving the transgene onto a congenic C57BL/6J background and recurred on reintroduction of either CBA or C3H/He backgrounds. Quantitative trait analysis of B6C3 F2 TTA mice identified a region on Chromosome 14 that contains a major modifier of the neurodegenerative phenotype. Although B6 mice were resistant to degeneration, they were not ideal for cognitive testing. F1 offspring of TTA C57BL/6J and 129X1/SvJ, FVB/NJ, or DBA/1J showed improved spatial learning, but TTA expression caused subtle differences in contextual fear conditioning on two of these backgrounds, indicating that strain and genotype can interact independently under different behavioral settings. All model systems have limitations that should be recognized and mitigated where possible; our findings stress the importance of mapping the effects caused by TTA alone when working with tet-off models.

On the occurrence of hypomyelination in a transgenic mouse model: a consequence of the myelin basic protein promoter?

Central nervous system hypomyelination is a feature common to a number of transgenic (Tg) mouse lines that express a variety of unrelated exogenous (i.e. non-central nervous system) transgenes. In this report, we document hypomyelination structurally by immunocytochemistry and functionally in the Tg line MBP-JE, which over expresses the chemokine CCL2 (MCP-1) within oligodendrocytes targeted by a myelin basic protein (MBP) promoter. Analysis of hypomyelinated optic nerves of Tg mice revealed progressive decrease in oligodendrocyte numbers with age (p < 0.01). Although molecular mechanisms underlying hypomyelination in this and other Tg models remain largely unknown, we present preliminary findings on oligodendrocyte progenitor cell (OPC) cultures in which, although OPC expressed CCR2, the receptor for CCL2, treatment with CCL2 had no significant effect on OPC proliferation, differentiation, or apoptosis. We suggest that hypomyelination in the MBP-JE model might not be due to CCL2 expression but rather the result of transcriptional dysfunction related to random insertion of the MBP promoter that disrupts myelinogenesis and leads to oligodendrocyte demise. Because an MBP promoter is a common denominator in most Tg lines displaying hypomyelination, we hypothesize that use of myelin gene sequences in the regulator region of Tg constructs might underlie this perturbation of myelination in such models.

Early-life forebrain glucocorticoid receptor overexpression increases anxiety behavior and cocaine sensitization

BACKGROUND

Genetic factors and early-life adversity are critical in the etiology of mood disorders and substance abuse. Because of their role in the transduction of stress responses, glucocorticoid hormones and their receptors could serve as both genetic factors and mediators of environmental influences. We have shown that constitutive overexpression of the glucocorticoid receptor (GR) in forebrain results in increased emotional reactivity and lability in mice. Here, we asked whether there was a critical period for the emergence of this phenotype.

METHODS

We generated a mouse line with inducible GR overexpression specifically in forebrain. Anxiety-like behaviors and cocaine-induced sensitization were assessed in adult mice following GR overexpression during different periods in development. The molecular basis of the behavioral phenotype was examined using microarray analyses of dentate gyrus and nucleus accumbens.

RESULTS

Transient overexpression of GR during early life led to increased anxiety and cocaine sensitization, paralleling the phenotype of lifelong GR overexpression. This increased emotional reactivity was not observed when GR overexpression was induced after weaning. Glucocorticoid receptor overexpression in early life is sufficient to alter gene expression patterns for the rest of the animal's life, with dentate gyrus being more responsive than nucleus accumbens. The altered transcripts are implicated in GR and axonal guidance signaling in dentate gyrus and dopamine receptor signaling in nucleus accumbens.

CONCLUSIONS

Transient overexpression of GR early in life is both necessary and sufficient for inducing transcriptome-wide changes in the brain and producing a lifelong increase in vulnerability to anxiety and drugs of abuse.

Intersectin 1 contributes to phenotypes in vivo: implications for Down's syndrome

Intersectin 1 (ITSN1) is a human chromosome 21 (HSA21) gene product encoding a multidomain scaffold protein that functions in endocytosis, signal transduction, and is implicated in Down's syndrome, Alzheimer's Disease, and potentially other neurodegenerative diseases through activation of c-Jun N-terminal kinase. We report for the first time that ITSN1 proteins are elevated in individuals with Down's syndrome of varying ages. However, ITSN1 levels decreased in aged cases with Down's syndrome with Alzheimer's disease-like neuropathology. Analysis of a novel ITSN1 transgenic mouse reveals that ITSN1 overexpression results in a sex-dependent decrease in locomotor activity. This study reveals a link between overexpression of specific ITSN1 isoforms and behavioral phenotypes and has implications for human neurodegenerative diseases such as Down's syndrome and Alzheimer's disease.

NR2B-deficient mice are more sensitive to the locomotor stimulant and depressant effects of ethanol

The NR2B subunit of N-methyl d-aspartate glutamate receptors influences pharmacological properties and confers greater sensitivity to the modulatory effects of ethanol. This study examined behavioral responses to acute ethanol in a conditional knockout mouse model that allowed for a delayed genetic deletion of the NR2B subunit to avoid mouse lethality. Mice lacking the NR2B gene (knockout) were produced by mating NR2B[f/f] mice with CAMKIIa-driven tTA transgenic mice and the tetO-CRE transgenic mice. Adult male and female offspring representing each of the resultant genotypes (knockout, CAM, CRE and wildtype mice) were tested for open-field locomotor activity following acute low- and high-dose ethanol challenge as well as loss of righting reflex. Findings indicate that male and female mice lacking the NR2B subunit exhibited greater overall activity in comparison to other genotypes during the baseline locomotor activity test. NR2B knockout mice exhibited an exaggerated stimulant response to 1.5 g/kg (i.p.) and an exaggerated depressant response to 3.0 g/kg (i.p.) ethanol challenge. In addition, NR2B knockout mice slept longer following a high dose of ethanol (4.0 g/kg, i.p.). To evaluate pharmacokinetics, clearance rates of ethanol (1.5, 4.0 g/kg, i.p.) were measured and showed that female NR2B knockouts had a faster rate of metabolism only at the higher ethanol dose. Western blot analyses confirmed significant reduction in NR2B expression in the forebrain of knockout mice. Collectively, these data indicate that the NR2B subunit of the N-methyl d-aspartate glutamate receptor is involved in regulating low-dose stimulant effects of ethanol and the depressant/hypnotic effects of ethanol.

Deletion of the mouse homolog of KCNAB2, a gene linked to monosomy 1p36, results in associative memory impairments and amygdala hyperexcitability

Ablation of the distal end of the short arm of chromosome 1 [1p36 deletion syndrome (1p36DS)] is one of the most commonly occurring terminal deletion syndromes in humans, occurring in ∼1 in 5000 newborns. Subjects with 1p36DS manifest a wide range of clinical features including growth delay, congenital heart defects, and craniofacial dysmorphism. In addition, individuals with 1p36DS often exhibit some form of neurological abnormality and are typically cognitively impaired. Although there is significant variability with regard to the extent of the deletion, several genes have been mapped to region 1p36 that are known to regulate neuronal function. One such gene--KCNAB2--encodes the potassium channel auxiliary subunit Kvβ2, which has been previously shown to modulate voltage-gated potassium currents in heterologous expression systems. Here, we present experiments characterizing mice in which the ortholog of KCNAB2 was deleted. We find that deletion of Kcnab2 in mice leads to deficits in associative learning and memory. In addition, using whole-cell current-clamp, we find that deletion of Kcnab2 leads to a reduction in the slow afterhyperpolarization following a burst of action potentials and a concomitant increase in neuronal excitability in projection neurons in the lateral nucleus of the amygdala. Our results suggest that loss of Kvβ2 likely contributes to the cognitive and neurological impairments observed in 1p36DS patients.

Inducible and conditional transgenic mouse models of schizophrenia

Schizophrenia is a devastating disorder. Despite the advance in research techniques in the last couple of decades, the pathogenesis of the disorder still remains poorly understood. Given the lack of pathognomonic feature of the disease and difficulty to analyze molecular pathways in patients, animal models have been instrumental in advancing our understanding of the disease. Recent progress in genetics has identified candidate susceptibility genes for schizophrenia, and generation of new genetic animal models has begun to provide valuable insights into the disease development. However, the complex neurodevelopmental and heterogeneous nature of schizophrenia still poses tremendous challenges for creating credible mouse models. In this review, we will discuss how current genetic systems of temporal and conditional regulation of gene expression have shed lights on the functions of the candidate genes in mouse models of schizophrenia. We also consider the strength and weaknesses of each model. We will argue that further development of more sophisticated genetic animal models is crucial for clarifying the unknowns of schizophrenia.

X11-like protein deficiency is associated with impaired conflict resolution in mice

Understanding how emotion is generated, how conflicting emotions are regulated, and how emotional states relate to sophisticated behaviors is a crucial challenge in brain research. Model animals showing selective emotion-related phenotypes are highly useful for examining these issues. Here, we describe a novel mouse model that withdraws in approach-avoidance conflicts. X11-like (X11L)/Mint2 is a neuronal adapter protein with multiple protein-protein interaction domains that interacts with several proteins involved in modulating neuronal activity. X11L-knock-out (KO) mice were subordinate under competitive feeding conditions. X11L-KO mice lost significantly more weight than cohoused wild-type mice without signs of decreased motivation to eat or physical weakness. In a resident-intruder test, X11L-KO mice showed decreased intruder exploration behavior. Moreover, X11L-KO mice displayed decreased marble-burying, digging and burrowing behaviors, indicating aberrant ethological responses to attractive stimuli. In contrast, X11L-KO mice were indistinguishable from wild-type mice in the open field, elevated plus maze, and light/dark transition tests, which are often used to assess anxiety-like behavior. Neurochemical analysis revealed a monoamine imbalance in several forebrain regions. The defective ethological responses and social behaviors in X11L-KO mice were rescued by the expression of X11L under a Camk2a promoter using the Tet-OFF system during development. These findings suggest that X11L is involved in the development of neuronal circuits that contribute to conflict resolution.

L-type voltage-gated calcium channels in conditioned fear: a genetic and pharmacological analysis

Using pharmacological approaches, others have suggested that L-type voltage-gated calcium channels (L-VGCCs) mediate both consolidation and extinction of conditioned fear. In the absence of L-VGCC isoform-specific antagonists, we have begun to investigate the subtype-specific role of LVGCCs in consolidation and extinction of conditioned fear using a molecular genetics approach. Previously, we used this approach to demonstrate that the Ca(v)1.3 isoform mediates consolidation, but not extinction, of contextually conditioned fear. Here, we used mice in which the gene for the L-VGCC pore-forming subunit Ca(v)1.2 was conditionally deleted in forebrain excitatory neurons (Ca(v)1.2(cKO) mice) to address the role of Ca(v)1.2 in consolidation and extinction of conditioned fear. We demonstrate that Ca(v)1.2(cKO) mice consolidate and extinguish conditioned fear as well as control littermates. These data suggest that Ca(v)1.2 is not critical for these processes and together with our previous data argue against a role for either of the brain-expressed L-VGCCs (Ca(v)1.2 or Ca(v)1.3) in extinction of conditioned fear. Additionally, we present data demonstrating that the L-VGCC antagonist nifedipine, which has been used in previous conditioned fear extinction studies, impairs locomotion, and induces an aversive state. We further demonstrate that this aversive state can enter into associations with conditioned stimuli that are present at the time that it is experienced, suggesting that previous studies using nifedipine were likely confounded by drug toxicity. Taken together, our genetic and pharmacological data argue against a role for Ca(v)1.2 in consolidation of conditioned fear as well as a role for L-VGCCs in extinction of conditioned fear.

Exaggerated emotional behavior in mice heterozygous null for the sodium channel Scn8a (Nav1.6)

The Scn8a gene encodes the alpha-subunit of Na(v)1.6, a neuronal voltage-gated sodium channel. Mice homozygous for mutations in the Scn8a gene exhibit motor impairments. Recently, we described a human family with a heterozygous protein truncation mutation in SCN8A. Rather than motor impairment, neuropsychological abnormalities were more common, suggesting a role for Scn8a in a more diverse range of behaviors. Here, we characterize mice heterozygous for a null mutation of Scn8a (Scn8a(+/-)mice) in a number of behavioral paradigms. We show that Scn8a(+/-)mice exhibit greater conditioned freezing in the Pavlovian fear conditioning paradigm but no apparent abnormalities in other learning and memory paradigms including the Morris water maze and conditioned taste avoidance paradigm. Furthermore, we find that Scn8a(+/-)mice exhibit more pronounced avoidance of well-lit, open environments as well as more stress-induced coping behavior. Together, these data suggest that Scn8a plays a critical role in emotional behavior in mice. Although the behavioral phenotype observed in the Scn8a(+/-)mice only partially models the abnormalities in the human family, we anticipate that the Scn8a(+/-)mice will serve as a valuable tool for understanding the biological basis of emotion and the human diseases in which abnormal emotional behavior is a primary component.