Reduction of ß-amyloid accumulation by reticulon 3 in transgenic mice.

Inhibition of the ß-secretase, BACE1, which cleaves amyloid precursor protein (APP) to produce ß-amyloid protein (Aß), is thought to be a feasible therapeutic strategy for Alzheimer's disease. Reticulon (RTN) proteins such as RTN3 have been identified as membrane proteins that interact with BACE1 and inhibit its Aß-generating activity. In this study, we investigated whether RTN3 can regulate Aß production in vivo, using transgenic (Tg) mice expressing APP with Swedish and London mutations (APP Tg mice) and those expressing RTN3; the latter mice showed ~1.4-fold higher expression levels of RTN3 protein in the cerebral cortex than non-Tg controls. We analyzed the brains of single APP Tg and double APP/RTN3 Tg mice at the age of approximately 15 months. The levels of secreted APP-ß, a direct BACE1 cleavage product of APP, in Tris-soluble fraction were considerably reduced in the hippocampus and cerebral cortex of APP/RTN3 Tg mice relative to those in APP Tg mice. Immunohistochemical analyses demonstrated that Aß burden and plaques were significantly (by approximately 50%) decreased in both the hippocampus and cerebral cortex of double Tg mice compared to APP Tg mice. Furthermore, the levels of guanidine-soluble Aß40 and Aß42 in these brain regions of APP/RTN3 Tg mice were relatively lower than those in APP Tg mice. These findings indicate that even a small increase in RTN3 expression exerts suppressive effects on amyloidogenic processing of APP and Aß accumulation through modulation of BACE1 activity in vivo, and suggest that induction of RTN3 might be an effective therapeutic strategy against Alzheimer's disease.

Temporal expression and mitochondrial localization of a Foxp2 isoform lacking the forkhead domain in developing Purkinje cells.

FOXP2, a forkhead box-containing transcription factor, forms homo- or hetero-dimers with FOXP family members and localizes to the nucleus, while FOXP2(R553H), which contains a mutation related to speech/language disorders, features reduced DNA binding activity and both cytoplasmic and nuclear localization. In addition to being a loss-of-function mutation, it is possible that FOXP2(R553H) also may act as a gain-of-function mutation to inhibit the functions of FOXP2 isoforms including FOXP2Ex10+ lacking forkhead domain. Foxp2(R552H) knock-in mouse pups exhibit impaired ultrasonic vocalization and poor dendritic development in Purkinje cells. However, expressions of Foxp2 isoforms in the developing Purkinje are unclear. The appearance of 'apical cytoplasmic swelling' (mitochondria-rich regions that are the source of budding processes) correlates with dendritic development of Purkinje cells. In the present study, we focused on Foxp2 isoforms localizing to the apical cytoplasmic swelling and identified two isoforms lacking forkhead domain: Foxp2Ex12+ and Foxp2Ex15. They partly localized to the membrane fraction that includes mitochondria. Foxp2Ex12+ mainly localized to the apical cytoplasmic swelling in early developing Purkinje cells at the stellate stage (P2-P4). Mitochondrial localization of Foxp2Ex12+ in Purkinje cells was confirmed by immune-electron microscopic analysis. Foxp2Ex12+ may play a role in dendritic development in Purkinje cells.

Behavioral and neuromorphological characterization of a novel Tuba1 mutant mouse.

As part of the RIKEN large-scale N-ethyl-N-nitrosourea (ENU) mutagenesis project, we screened mice with a dominant mutation that exhibited abnormal behavior using an open-field test and a home-cage activity test. We tested 495 male progeny of C57BL/6J males treated with ENU and untreated C3H/HeJ females using the open-field test and isolated behavioral mutant M101736, which exhibited a significant increase in spontaneous locomotor activity. We identified a missense mutation in the Tuba1 gene, which encodes the TUBA1 protein, and designated the mutant gene Tuba1(Rgsc1736). This mutation results in an aspartic acid to glycine substitution in the TUBA1 protein. Detailed analyses revealed that Tuba1(Rgsc1736) heterozygotes exhibited inattention to novel objects and aberrant patterns of home-cage activity. The results of a behavioral pharmacological analysis using methylphenidate and morphological analyses of embryonic and adult brains suggested that Tuba1(Rgsc1736) is a novel animal model for neurodevelopmental disorders.

Demyelination can proceed independently of axonal degradation during Wallerian degeneration in wlds mice.

Peripheral nerve injury induces axonal degeneration and demyelination, which are collectively referred to as Wallerian degeneration. It is generally assumed that axonal degeneration is a trigger for the subsequent demyelination processes such as myelin destruction and de-differentiation of Schwann cells, but the detailed sequence of events that occurs during this initial phase of demyelination following axonal degeneration remains unclear. Here we performed a morphological analysis of injured sciatic nerves of wlds mice, a naturally occurring mutant mouse in which Wallerian degeneration shows a significant delay. The slow Wallerian degerenation phenotype of the wlds mutant mice would enable us to dissect the events that take place during the initial phase of demyelination. Ultrastrucural analysis using electron microscopy showed that the initial process of myelin destruction was activated in injured nerves of wlds mice even though they exhibit morphologically complete protection of axons against nerve injury. We also found that some intact axons were completely demyelinated in degenerating nerves of wlds mice. Furthermore, we observed that de-differentiation of myelinating Schwann cells gradually proceeded even though the axons remained morphologically intact. These data suggest that initiation and progression of demyelination in injured peripheral nerves is, at least in part, independent of axonal degeneration.

Appearance of phagocytic microglia adjacent to motoneurons in spinal cord tissue from a presymptomatic transgenic rat model of amyotrophic lateral sclerosis.

Microglial activation occurs early during the pathogenesis of amyotrophic lateral sclerosis (ALS). Recent evidence indicates that the expression of mutant Cu(2+)/Zn(2+) superoxide dismutase 1 (SOD1) in microglia contributes to the late disease progression of ALS. However, the mechanism by which microglia influence the neurodegenerative process and disease progression in ALS remains unclear. In this study, we revealed that activated microglia aggregated in the lumbar spinal cord of presymptomatic mutant SOD1(H46R) transgenic rats, an animal model of familial ALS. The aggregated microglia expressed a marker of proliferating cell, Ki67, and phagocytic marker proteins ED1 and major histocompatibility complex (MHC) class II. The motoneurons near the microglial aggregates showed weak choline acetyltransferase (ChAT) immunoreactivity and contained reduced granular endoplasmic reticulum and altered nucleus electron microscopically. Furthermore, immunopositive signals for tumor necrosis factor-alpha (TNFalpha) and monocyte chemoattractant protein-1 (MCP-1) were localized in the aggregated microglia. These results suggest that the activated and aggregated microglia represent phagocytic features in response to early changes in motoneurons and possibly play an important role in ALS disease progression during the presymptomatic stage.

Hippocampal SH2-containing protein-tyrosine phosphatases are involved in extinction of contextual fear.

Contextual fear memory is attenuated by re-exposure of animals to a context alone without pairing it with an unconditioned stimulus, and this phenomenon is referred to as fear extinction. In this study we demonstrated that stereotaxic injection of an inhibitor of Src homology 2-containing protein-tyrosine phosphatases 1 and 2 (SHP1/2), NSC87877, into the hippocampus significantly suppressed extinction of contextual fear in the mouse. Intra-hippocampal injection of NSC87877, however, had no effect on the initial memory formation in contextual fear conditioning. These findings suggest that SHP1/2 activity in the hippocampus is involved in the control of contextual fear extinction.

Efficient gene transfer into neurons in monkey brain by adeno-associated virus 8.

Although the adeno-associated virus (AAV) vector is a promising tool for gene transfer into neurons, especially for therapeutic purposes, neurotropism in primate brains is not fully elucidated for specific AAV serotypes. Here, we injected AAV serotype 8 (AAV8) vector carrying the enhanced green fluorescent protein (EGFP) gene under a ubiquitous promoter into the cerebral cortex, striatum and substantia nigra of common marmosets. Robust neuronal EGFP expression was observed at all injected sites. Cell typing with immunohistochemistry confirmed efficient AAV8-mediated gene transfer into the pyramidal neurons in the cortex, calbindin-positive medium spiny neurons in the striatum and dopaminergic neurons in the substantia nigra. The results indicate a preferential tropism of AAV8 for subsets of neurons, but not for glia, in monkey brains.

Phenotypic characterization of a new Grin1 mutant mouse generated by ENU mutagenesis.

In the RIKEN large-scale N-ethyl-N-nitrosourea (ENU) mutagenesis project we screened mice with a dominant mutation that exhibited abnormal behavior in the open-field test, passive avoidance test and home-cage activity test. We tested 2045 progeny of C57BL/6J males treated with ENU and untreated DBA/2J females in the open-field test and isolated behavioral mutant M100174, which exhibited a significant increase in spontaneous locomotor activity. We identified a missense mutation in the Grin1 gene, which encodes NMDA receptor subunit 1, and designated the mutant gene Grin1(Rgsc174). This mutation results in an arginine to cysteine substitution in the C0 domain of the protein. Detailed analyses revealed that Grin1(Rgsc174) heterozygote exhibited increased novelty-seeking behavior and slight social isolation in comparison with the wild type. In contrast to other Grin1 mutant mice, this mutant exhibited no evidence of heightened anxiety. These results indicate that this is a unique behavioral Grin1 gene mutant mouse that differs from the known Grin1 mutant mice. The results of immunohistochemical and biochemical analyses suggested that impaired interaction between the glutamatergic pathway and dopaminergic pathway may underlie the behavioral phenotypes of the Grin1(Rgsc174) mutant.

[Somatotopy in the emotional expression by the amygdala].

Sensory information of various modalities is integrated in the amygdala, where the emotional information is subsequently generated. The resultant emotional information is then sent to the widespread cortical and subcortical areas. This output to the cortex is directed to the prefrontal cortex and the medial temporal lobe, a memory system, and modulates the higher cortical functions as well as the learning and memory. The output to the subcortical structures is directed to the hypothalamus and brainstem, and the autonomic, hormonal and behavioral responses are elicited as the expression of the emotional response. Taking into account the above mentioned emotional system, the examination of somatotopic representation in the amygdala and related structures is very difficult. However, recent studies have elucidated the somatotopic representation of the amygdalar output system in the brain of rhesus monkey. The lateral basal nucleus of the monkey amygdala was proved to project to the cingulate motor cortex M3, and the target cortical neurons project somatotopically to the facial nucleus in the brain stem. Accordingly, the amygdala-cingulate cortex-facial nucleus system is involved in the emotional expression by the facial movement. This system might be involved in the pathophysiology of temporal lobe epilepsy and the facial expression of patients with Parkinson disease. Furthermore, the amygdala-cingulate cortex projection develops from adolescence to adulthood and reflects the emotional maturation and the development of social adaptation.

Hippocampal Fyn activity regulates extinction of contextual fear.

Contextual fear memory is attenuated by reexposure of animals to a context alone without pairing it with an unconditioned stimulus, a phenomenon referred to as fear extinction. Here, we report that Fyn tyrosine kinase in the hippocampus is involved in the extinction of contextual fear. We inhibited Src-family tyrosine kinases in the dorsal hippocampus by stereotaxic injection of an inhibitor, PP2, and observed facilitation of extinction. We then biochemically analyzed dorsal hippocampal tissue during extinction training, and found that activated Fyn was significantly downregulated among the Src-family tyrosine kinases examined. These findings suggest that downregulation of Fyn activity facilitates the extinction of contextual fear.

Decreased expression of Fyn protein and disbalanced alternative splicing patterns in platelets from patients with schizophrenia.

Fyn, a Src-family kinase, is highly expressed in brain tissue and blood cells. In the mouse brain, Fyn participates in brain development, synaptic transmission through the phosphorylation of N-methyl-d-aspartate (NMDA) receptor subunits, and the regulation of emotional behavior. Recently, we found that Fyn is required for the signal transduction in striatal neurons that is initiated by haloperidol, an antipsychotic drug. To determine whether Fyn abnormalities are present in patients with schizophrenia, we analyzed Fyn expression in platelet samples from 110 patients with schizophrenia, 75 of the patients' first-degree relatives, and 130 control subjects. A Western blot analysis revealed significantly lower levels of Fyn protein among the patients with schizophrenia and their relatives, compared with the level in the control group. At the mRNA level, the splicing patterns of fyn were altered in the patients and their relatives; specifically, the ratio of fynDelta7, in which exon 7 is absent, was elevated. An expression study in HEK293T cells revealed that FynDelta7 had a dominant-negative effect on the phosphorylation of Fyn's substrate. These results suggest novel deficits in Fyn function, manifested as the downregulation of Fyn protein or the altered transcription of the fyn gene, in patients with schizophrenia.

Nicotinamide mononucleotide adenylyltransferase expression in mitochondrial matrix delays Wallerian degeneration.

Studies of naturally occurring mutant mice, wld(s), showing delayed Wallerian degeneration phenotype, suggest that axonal degeneration is an active process. We previously showed that increased nicotinamide adenine dinucleotide (NAD)-synthesizing activity by overexpression of nicotinamide mononucleotide adenylyltransferase (NMNAT) is the essential component of the Wld(s) protein, the expression of which is responsible for the delayed Wallerian degeneration phenotype in wld(s) mice. Indeed, NMNAT overexpression in cultured neurons provides robust protection to neurites, as well. To examine the effect of NMNAT overexpression in vivo and to analyze the mechanism that causes axonal protection, we generated transgenic mice (Tg) overexpressing NMNAT1 (nuclear isoform), NMNAT3 (mitochondrial isoform), or the Wld(s) protein bearing a W258A mutation, which disrupts NAD-synthesizing activity of the Wld(s) protein. Wallerian degeneration delay in NMNAT3-Tg was similar to that in wld(s) mice, whereas axonal protection in NMNAT1-Tg or Wld(s)(W258A)-Tg was not detectable. Detailed analysis of subcellular localization of the overexpressed proteins revealed that the axonal protection phenotype was correlated with localization of NMNAT enzymatic activity to mitochondrial matrix. Furthermore, we found that isolated mitochondria from mice showing axonal protection expressed unchanged levels of respiratory chain components, but were capable of increased ATP production. These results suggest that axonal protection by NMNAT expression in neurons is provided by modifying mitochondrial function. Alteration of mitochondrial function may constitute a novel tool for axonal protection, as well as a possible treatment of diseases involving axonopathy.

NMDA receptor antagonist memantine promotes cell proliferation and production of mature granule neurons in the adult hippocampus.

Memantine, which is used clinically for the treatment of Alzheimer's disease (AD), is classified as an N-methyl-d-aspartate (NMDA) receptor antagonist. Since previous studies have shown that NMDA receptor antagonists promote neurogenesis in the adult brain, we examined the effect of memantine on neurogenesis in the adult mouse hippocampus. We intraperitoneally injected 3-month-old mice with memantine (at 10 or 50 mg/kg body weight) followed by 5-bromo-2-deoxyuridine (BrdU) injections (3x) after 3 days. We then examined the number of BrdU+ cells in the dentate gyrus (DG) of the hippocampus at different time points. The number of BrdU+ cells in the 50 mg/kg memantine-injected group increased by 2.1-fold (1 day after BrdU-injection), 3.4-fold (after 7 days), and 6.8-fold (after 28 days), whereas the 10 mg/kg dose of memantine had little effect on labeling compared to the control group. Immunohistochemical staining at 28 days after BrdU-injection revealed that the newly generated cells in the 50 mg/kg memantine-group had differentiated into mature granule neurons. Moreover, when 12-month-old mice were injected with memantine, cell proliferation was promoted in the DG (3.7-fold). These findings demonstrate that memantine promotes the proliferation of neural progenitor cells and the production of mature granule neurons in the adult hippocampus.

Reduced proliferative activity of primary POMGnT1-null myoblasts in vitro.

Protein O-linked mannose beta1,2-N-acetylglucosaminyltransferase 1 (POMGnT1) is an enzyme that transfers N-acetylglucosamine to O-mannose of glycoproteins. Mutations of the POMGnT1 gene cause muscle-eye-brain (MEB) disease. To obtain a better understanding of the pathogenesis of MEB disease, we mutated the POMGnT1 gene in mice using a targeting technique. The mutant muscle showed aberrant glycosylation of alpha-DG, and alpha-DG from mutant muscle failed to bind laminin in a binding assay. POMGnT1(-/-) muscle showed minimal pathological changes with very low-serum creatine kinase levels, and had normally formed muscle basal lamina, but showed reduced muscle mass, reduced numbers of muscle fibers, and impaired muscle regeneration. Importantly, POMGnT1(-/-) satellite cells proliferated slowly, but efficiently differentiated into multinuclear myotubes in vitro. Transfer of a retrovirus vector-mediated POMGnT1 gene into POMGnT1(-/-) myoblasts completely restored the glycosylation of alpha-DG, but proliferation of the cells was not improved. Our results suggest that proper glycosylation of alpha-DG is important for maintenance of the proliferative activity of satellite cells in vivo.

The Alzheimer's disease drug memantine increases the number of radial glia-like progenitor cells in adult hippocampus.

New neurons are continuously generated in the hippocampus of the adult mammalian brain, and N-methyl-D-aspartate receptor (NMDA-R) antagonists have been found to increase the number of newly generated neurons in the dentate gyrus (DG) of the adult hippocampus. In this study, we examined the effect of memantine, an NMDA-R antagonist that is clinically used for the treatment of Alzheimer's disease, on primary progenitor cells exhibiting a radial glia-like (RGL) morphology in the DG. We injected 3-month-old mice with memantine (50 mg/kg body weight, intraperitoneally [i.p.]); 3 days later, we injected the mice with 5-bromo-2-deoxyuridine (BrdU; 75 mg/kg body weight, i.p.). We then counted the number of BrdU-labeled RGL progenitor cells in the DG 1 or 7 days after the BrdU-injection. The number of BrdU-labeled RGL progenitor cells had increased significantly by 5.1-fold on day 1 and by 13.7-fold on day 7 after BrdU-injection. Immunohistochemical staining revealed that the BrdU-labeled RGL progenitor cells expressed two primary progenitor cell marker proteins, nestin and Sox2. These results clearly demonstrated that memantine promotes the proliferation of RGL progenitor cells. We also found that memantine increased the ratio of horizontally aligned RGL progenitor cells, which are probably produced by symmetric division. These findings suggest that memantine increases the proliferation of primary progenitor cells and expands the primary progenitor cell pool in the adult hippocampus by stimulating symmetric division.

Development of the mouse amygdala as revealed by enhanced green fluorescent protein gene transfer by means of in utero electroporation.

The amygdala is located in the caudal part of the ventral telencephalon. It is composed of many subdivisions and is involved in the control of emotion. It is important to know the mechanisms of amygdalar development in order to analyze the pathogenesis of emotional disorders, but they are still not adequately understood. In the present study the migration, differentiation, and distribution of amygdalar neurons in the mouse embryo were investigated by means of in utero electroporation. Ventricular zone cells in restricted regions, that is, the caudal ganglionic eminence (CGE), the ventral pallium, the lateral pallium, and the diencephalon, were labeled with an expression vector of the enhanced green fluorescent protein (EGFP) gene. Labeling at embryonic day (E)10 revealed that the central nucleus originates from the neuroepithelium in the ganglionic eminence and the labeling at E11 and E12 revealed that the basolateral complex originates from the neuroepithelium of the ventral and lateral pallia. The introduction of the EGFP gene into the neuroepithelium of the third ventricle at E11 showed that the medial nucleus originates, at least in part, from the neuroepithelium of the diencephalon and migrates over the diencephalo-telencephalic boundary. The radial glial arrangement corresponded well with the initial migration of amygdalar neurons, and the radial processes later formed the boundary demarcating the basolateral complex. These findings indicate that the neurons originating from the temporally and spatially restricted neuroepithelium in both the telencephalon and diencephalon migrate and differentiate to form the mosaic of amygdalar subdivisions.

A deficit of brain dystrophin impairs specific amygdala GABAergic transmission and enhances defensive behaviour in mice.

Duchenne muscular dystrophy (DMD) is accompanied by cognitive deficits and psychiatric symptoms. In the brain, dystrophin, the protein responsible for DMD, is localized to a subset of GABAergic synapses, but its role in brain function has not fully been addressed. Here, we report that defensive behaviour, a response to danger or a threat, is enhanced in dystrophin-deficient mdx mice. Mdx mice consistently showed potent defensive freezing responses to a brief restraint that never induced such responses in wild-type mice. Unconditioned and conditioned defensive responses to electrical footshock were also enhanced in mdx mice. No outstanding abnormality was evident in the performances of mdx mice in the elevated plus maze test, suggesting that the anxiety state is not altered in mdx mice. We found that, in mdx mice, dystrophin is expressed in the amygdala, and that, in the basolateral nucleus (BLA), the numbers of GABA(A) receptor alpha2 subunit clusters are reduced. In BLA pyramidal neurons, the frequency of norepinephrine-induced GABAergic inhibitory synaptic currents was reduced markedly in mdx mice. Morpholino oligonucleotide-induced expression of truncated dystrophin in the brains of mdx mice, but not in the muscle, ameliorated the abnormal freezing response to restraint. These results suggest that a deficit of brain dystrophin induces an alteration of amygdala local inhibitory neuronal circuits and enhancement of fear-motivated defensive behaviours in mice.

Alpha-CaMKII deficiency causes immature dentate gyrus, a novel candidate endophenotype of psychiatric disorders.

Elucidating the neural and genetic factors underlying psychiatric illness is hampered by current methods of clinical diagnosis. The identification and investigation of clinical endophenotypes may be one solution, but represents a considerable challenge in human subjects. Here we report that mice heterozygous for a null mutation of the alpha-isoform of calcium/calmodulin-dependent protein kinase II (alpha-CaMKII+/-) have profoundly dysregulated behaviours and impaired neuronal development in the dentate gyrus (DG). The behavioral abnormalities include a severe working memory deficit and an exaggerated infradian rhythm, which are similar to symptoms seen in schizophrenia, bipolar mood disorder and other psychiatric disorders. Transcriptome analysis of the hippocampus of these mutants revealed that the expression levels of more than 2000 genes were significantly changed. Strikingly, among the 20 most downregulated genes, 5 had highly selective expression in the DG. Whereas BrdU incorporated cells in the mutant mouse DG was increased by more than 50 percent, the number of mature neurons in the DG was dramatically decreased. Morphological and physiological features of the DG neurons in the mutants were strikingly similar to those of immature DG neurons in normal rodents. Moreover, c-Fos expression in the DG after electric footshock was almost completely and selectively abolished in the mutants. Statistical clustering of human post-mortem brains using 10 genes differentially-expressed in the mutant mice were used to classify individuals into two clusters, one of which contained 16 of 18 schizophrenic patients. Nearly half of the differentially-expressed probes in the schizophrenia-enriched cluster encoded genes that are involved in neurogenesis or in neuronal migration/maturation, including calbindin, a marker for mature DG neurons. Based on these results, we propose that an "immature DG" in adulthood might induce alterations in behavior and serve as a promising candidate endophenotype of schizophrenia and other human psychiatric disorders.

Alteration of inflammatory cytokine production in the injured central nervous system of tenascin-deficient mice.

Although tenascin-C (TN) is highly up-regulated during the proliferation of reactive astrocytes, little is known about the function of TN at injury sites in the central nervous system (CNS). Here, the function of TN-expressing astrocytes in the injured brain was investigated by analyzing TN-deficient mice with stab-wound injuries of the cerebral cortex. Glial fibrillary acid protein expression after injury was down-regulated earlier in TN-deficient mice than in wild-type (WT) mice. To evaluate immune responses in the injured CNS in the absence of TN, inflammatory cytokine production was examined after unilateral stab injuries of the cerebral cortex in TN-deficient and WT mice. The expression of interleukin (IL)-1beta, tumor necrosis factor-a and IL-6 was higher in TN-deficient mice, whereas levels of IL-4 and granulocyte colony-stimulating factor were lower in TN-deficient mice than WT mice. Our findings suggest that TN helps to regulate production of inflammatory cytokines in the injured brain.

Activation of Fyn tyrosine kinase in the mouse dorsal hippocampus is essential for contextual fear conditioning.

Fyn-tyrosine-kinase-deficient mice exhibit defects in the Morris water maze test and long-term potentiation (LTP) induction in the hippocampus, and given that LTP has been postulated as the neural basis for memory formation, Fyn may be required for hippocampus-dependent memory formation. However, how Fyn is involved in the process of memory formation is unclear. To investigate the role of Fyn in hippocampal memory formation, we first tested the behavior of Fyn-deficient mice by contextual fear conditioning. A mouse was placed in a context and a foot shock was delivered, so that the mouse associated the context with the shock. We found that the freezing response of Fyn-deficient mice to the context was impaired at 24 h after conditioning. We then measured freezing at 1 h after conditioning, and found that their short-term contextual fear memory was also impaired. We used Western blotting to examine the mode of Fyn activation in dorsal hippocampal tissue following contextual fear conditioning. Fyn activation peaked as early as 5-10 min after contextual fear conditioning and persisted for at least 40 min. Concomitant increases in tyrosine phosphorylation of several proteins, including NR2B, were also observed, but no increases in tyrosine phosphorylation were observed in Fyn-deficient mice. Thus, both short-term and long-term (24-h) contextual fear memory were impaired in Fyn-deficient mice, and Fyn activation in the dorsal hippocampus transiently increased after contextual fear conditioning. These findings strongly suggest that activation of the Fyn signaling pathway is involved in hippocampus-dependent formation of contextual fear memory.