Differential expression of EGR-1 mRNA in the amygdala following diazepam in contextual fear conditioning

Resilience and Vulnerability to Trauma: Early Life Interventions Modulate Aversive Memory Reconsolidation in the Dorsal Hippocampus

Early life experiences program lifelong responses to stress. In agreement, resilience and vulnerability to psychopathologies, such as posttraumatic stress disorder (PTSD), have been suggested to depend on the early background. New therapies have targeted memory reconsolidation as a strategy to modify the emotional valence of traumatic memories. Here, we used animal models to study the molecular mechanism through which early experiences may later affect aversive memory reconsolidation. Handling (H)—separation of pups from dams for 10 min—or maternal separation (MS) — 3-h separation—were performed from PDN1–10, using non-handled (NH) litters as controls. Adult males were trained in a contextual fear conditioning (CFC) task; 24 h later, a short reactivation session was conducted in the conditioned or in a novel context, followed by administration of midazolam 3 mg/kg i.p. (mdz), known to disturb reconsolidation, or vehicle; a test session was performed 24 h after. The immunocontent of relevant proteins was studied 15 and 60 min after memory reactivation in the dorsal hippocampus (dHc) and basolateral amygdala complex (BLA). Mdz-treated controls (NH) showed decreased freezing to the conditioned context, consistent with reconsolidation impairment, but H and MS were resistant to labilization. Additionally, MS males showed increased freezing to the novel context, suggesting fear generalization; H rats showed lower freezing than the other groups, in accordance with previous suggestions of reduced emotionality facing adversities. Increased levels of Zif268, GluN2B, β-actin and polyubiquitination found in the BLA of all groups suggest that memory reconsolidation was triggered. In the dHc, only NH showed increased Zif268 levels after memory retrieval; also, a delay in ERK1/2 activation was found in H and MS animals. We showed here that reconsolidation of a contextual fear memory is insensitive to interference by a GABAergic drug in adult male rats exposed to different neonatal experiences; surprisingly, we found no differences in the reconsolidation process in the BLA, but the dHc appears to suffer temporal desynchronization in the engagement of reconsolidation. Our results support a hippocampal-dependent mechanism for reconsolidation resistance in models of early experiences, which aligns with current hypotheses for the etiology of PTSD.

Conditioned stimulus presentations alter anxiety level in fear-conditioned mice

It is generally believed that fear is rapidly triggered by a distinct cue while anxiety onset is less precise and not associated with a distinct cue. Although it has been claimed that both processes can be measured with certain independence of each other, it is unclear how exactly they differ. In this study, we measured anxiety in mice that received discriminative fear conditioning using behavioral, heart rate and calcium (Ca²⁺) responses in the ventral hippocampal CA1 (vCA1) neurons. We found that the occurrence of fear significantly interfered with anxiety measurements under various conditions. Diazepam reduced basal anxiety level but had no effect during the presentation of conditioned stimulus (CS). Injection of an inhibitory peptide of PKMzeta (ZIP) into the basolateral amygdala almost entirely abolished CS-triggered fear expression and reduced anxiety to basal level. Heart rate measures suggested a small reduction in anxiety during CS-. Calcium responses in the lateral hypothalamus-projecting vCA1 neurons showed a steady decay during CS suggesting a reduced anxiety. Thus, under our experimental conditions, CS presentations likely reduce anxiety level in the fear-conditioned mice.

Nucleus incertus ablation disrupted conspecific recognition and modified immediate early gene expression patterns in 'social brain' circuits of rats

Social interaction involves neural activity in prefrontal cortex, septum, hippocampus, amygdala and hypothalamus. Notably, these areas all receive projections from the nucleus incertus (NI) in the pontine tegmentum. Therefore, we investigated the effect of excitotoxic lesions of NI neurons in adult male, Wistar rats on performance in a social discrimination test, and associated changes in immediate-early gene protein levels. NI was lesioned with quinolinic acid, and after recovery, rats underwent two trials in the 3-chamber test. In the first trial, NI-lesioned and sham-lesioned rats spent longer exploring a conspecific than an inanimate object. By contrast, in the second trial, NI-lesioned rats visited the familiar and novel conspecific chambers equally, whereas sham-lesioned rats spent longer engaging with the novel rat. Quantification of Fos- and Egr-1-immunoreactivity (IR) levels in brain areas implicated in social behaviour, revealed that social encounter and NI lesion produced complex, differential changes. For example, Egr-1-IR was broadly decreased in several amygdala nuclei in NI-lesioned rats relative to sham, but Fos-IR levels were unaltered. In hippocampus, NI-lesioned rats displayed decreased Fos-IR in CA2 and CA3, while Egr-1-IR was increased in the polymorphic dentate gyrus, CA1, CA2 and subiculum of NI-lesioned rats, relative to sham. Social encounter-related Egr-1-IR was also decreased in septum and anterior and lateral hypothalamus of NI-lesioned rats. Overall, these data suggest NI networks can modulate the activity of sensory, emotional and executive brain areas involved in social recognition, with a likely involvement of neuronal Egr-1 activation in amygdala, septum and hypothalamus, and Erg-1 inhibition in hippocampus.

Amygdaloid zif268 participated in the D-cycloserine facilitation effect on the extinction of conditioned fear

RATIONALE

The involvement of glutamate in fear extinction is perhaps the most promising in terms of facilitating clinical interventions for posttraumatic stress disorder (PTSD). This study was aimed at elucidating the possible role of zif268 on the D-cycloserine (DCS) facilitation effect on extinction.

OBJECTIVE

We investigated the association between zif268 and the extinction of conditioned fear by using antisense oligodeoxynucleotide (ODN) of zif268 and the fear-potentiated startle paradigm.

METHODS

Male adult Wistar rats were injected DCS (15 mg/kg, IP) 15 min prior to the extinction training, administered with antisense or sense ODN (800 pmol) of zif268 and subjected for fear-potentiated startle paradigm (FPS) and Western blot.

RESULTS

Either context exposure or cue exposure elevated the expression of zif268 in the basolateral nucleus of the amygdala (BLA) (p < 0.05 and p < 0.01, respectively) compared to the control group. Additionally, zif268 expression in BLA was further elevated by the glutamate NMDA receptor agonist DCS administration. Intra-amygdaloid injection of the antisense ODN of zif268 blocked the facilitation effect of DCS on the extinction of conditioned fear. Subsequent control experiments indicated that administration of vehicle or zif268 sense ODN did not alter the facilitation of DCS and that the blockage effect of zif268 antisense ODN was not due to lasting damage to the amygdala.

CONCLUSIONS

Our results suggest that zif268 within the amygdala participates in the DCS facilitation effect on the extinction of conditioned fear.

Alterations in brain-derived neurotrophic factor in the mouse hippocampus following acute but not repeated benzodiazepine treatment

Benzodiazepines (BZs) are safe drugs for treating anxiety, sleep, and seizure disorders, but their use also results in unwanted effects including memory impairment, abuse, and dependence. The present study aimed to reveal the molecular mechanisms that may contribute to the effects of BZs in the hippocampus (HIP), an area involved in drug-related plasticity, by investigating the regulation of immediate early genes following BZ administration. Previous studies have demonstrated that both brain derived neurotrophic factor (BDNF) and c-Fos contribute to memory- and abuse-related processes that occur within the HIP, and their expression is altered in response to BZ exposure. In the current study, mice received acute or repeated administration of BZs and HIP tissue was analyzed for alterations in BDNF and c-Fos expression. Although no significant changes in BDNF or c-Fos were observed in response to twice-daily intraperitoneal (i.p.) injections of diazepam (10 mg/kg + 5 mg/kg) or zolpidem (ZP; 2.5 mg/kg + 2.5 mg/kg), acute i.p. administration of both triazolam (0.03 mg/kg) and ZP (1.0 mg/kg) decreased BDNF protein levels within the HIP relative to vehicle, without any effect on c-Fos. ZP specifically reduced exon IV-containing BDNF transcripts with a concomitant increase in the association of methyl-CpG binding protein 2 (MeCP2) with BDNF promoter IV, suggesting that MeCP2 activity at this promoter may represent a ZP-specific mechanism for reducing BDNF expression. ZP also increased the association of phosphorylated cAMP response element binding protein (pCREB) with BDNF promoter I. Future work should examine the interaction between ZP and DNA as the cause for altered gene expression in the HIP, given that BZs can enter the nucleus and intercalate into DNA directly.

Reversal of novelty-induced hyperlocomotion and hippocampal c-Fos expression in GluA1 knockout male mice by the mGluR2/3 agonist LY354740

Dysfunctional glutamatergic neurotransmission has been implicated in schizophrenia and mood disorders. As a putative model for these disorders, a mouse line lacking the GluA1 subunit (GluA1-KO) of the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) glutamate receptor displays a robust novelty-induced hyperlocomotion associated with excessive neuronal activation in the hippocampus. Agonists of metabotropic glutamate 2/3 receptors (mGluR2/3) inhibit glutamate release in various brain regions and they have been shown to inhibit neuronal activation in the hippocampus. Here, we tested a hypothesis that novelty-induced hyperlocomotion in the GluA1-KO mice is mediated via excessive hippocampal neuronal activation by analyzing whether an mGluR2/3 agonist inhibits this phenotypic feature. GluA1-KO mice and littermate wildtype (WT) controls were administered with (1S,2S,5R,6S)-2-aminobicyclo[3.1.0]hexane-2,6-dicarboxylic acid (LY354740) (15 mg/kg, i.p.) 30 min before a 2-h exposure to novel arenas after which c-Fos immunopositive cells were analyzed in the hippocampus. LY354740 (15 mg/kg) decreased hyperactivity in male GluA1-KO mice, with only a minimal effect in WT controls. This was observed in two cohorts of animals, one naïve to handling and injections, another pre-handled and accustomed to injections. LY354740 (15 mg/kg) also reduced the excessive c-Fos expression in the dorsal hippocampal CA1 pyramidal cell layer in maleGluA1-KO mice, while not affecting c-Fos levels in WT mice. In female mice, no significant effect for LY354740 (15 mg/kg) on hyperactive behavior or hippocampal c-Fos was observed in either genotype or treatment cohort. A higher dose of LY354740 (30 mg/kg) alleviated hyperlocomotion of GluA1-KO males, but not that of GluA1-KO females. In conclusion, the excessive behavioral hyperactivity of GluA1-KO mice can be partly prevented by reducing neuronal excitability in the hippocampus with the mGluR2/3 agonist suggesting that the hippocampal reactivity is strongly involved in the behavioral phenotype of GluA1-KO mice.

Artificial theta stimulation impairs encoding of contextual fear memory

Several experiments have demonstrated an intimate relationship between hippocampal theta rhythm (4-12 Hz) and memory. Lesioning the medial septum or fimbria-fornix, a fiber track connecting the hippocampus and the medial septum, abolishes the theta rhythm and results in a severe impairment in declarative memory. To assess whether there is a causal relationship between hippocampal theta and memory formation we investigated whether restoration of hippocampal theta by electrical stimulation during the encoding phase also restores fimbria-fornix lesion induced memory deficit in rats in the fear conditioning paradigm. Male Wistar rats underwent sham or fimbria-fornix lesion operation. Stimulation electrodes were implanted in the ventral hippocampal commissure and recording electrodes in the septal hippocampus. Artificial theta stimulation of 8 Hz was delivered during 3-min free exploration of the test cage in half of the rats before aversive conditioning with three foot shocks during 2 min. Memory was assessed by total freezing time in the same environment 24 h and 28 h after fear conditioning, and in an intervening test session in a different context. As expected, fimbria-fornix lesion impaired fear memory and dramatically attenuated hippocampal theta power. Artificial theta stimulation produced continuous theta oscillations that were almost similar to endogenous theta rhythm in amplitude and frequency. However, contrary to our predictions, artificial theta stimulation impaired conditioned fear response in both sham and fimbria-fornix lesioned animals. These data suggest that restoration of theta oscillation per se is not sufficient to support memory encoding after fimbria-fornix lesion and that universal theta oscillation in the hippocampus with a fixed frequency may actually impair memory.

Neuromodulatory property of standardized extract Ginkgo biloba L. (EGb 761) on memory: behavioral and molecular evidence

Although it has been suggested that the standardized Ginkgo biloba leaf extract (Egb 761) may have a beneficial effect on memory, the cellular and molecular changes that underlie this process are not yet well defined. The present study evaluated the effects of acute (one dose) or subacute treatments (one daily dose/seven days) with EGb 761 (0.5 g kg(-1) and 1.0 g kg(-1)) on rats submitted to a conditioned emotional response (CER) in comparison with positive (4 mg kg(-1) Diazepam) and negative (12%Tween 80) control groups. To this end, eighty (n=10/group) adult, male, Wistar rats (+/-250-300 g) were used in an off-baseline CER procedure. We here observed that the rats submitted to an acute and subacute EGb 761 treatments had acquisition of fear conditioning. Additionally, we investigate if the expression of genes previously associated with classical conditioning (CREB-1 and GAP-43) and new candidate genes (GFAP) are modulated following EGb 761 acute treatment. CREB-1, GAP-43 and GFAP mRNA and protein expressions were evaluated using both quantitative PCR (qPCR) and immunohistochemical analysis, respectively. We here show, for the first time, that EGb 761 modulated GAP-43, CREB-1 and GFAP expression in the prefrontal cortex, amygdala and hippocampus. We observed an underexpression of GAP-43 in all structures evaluated and over-expression of GFAP in the amygdala and hippocampus following acute G. biloba treatment when compared to control group (Tween; p<0.01). GAP-43 expression was decreased in prefrontal cortex and hippocampus in the subacute treatment with EGb 761. Subacute treatment with EGb 761 lead to a decreased CREB-1 in mPFC (p<0.001) and increased in the hippocampus to 1.0 g kg(-1)G. biloba group (p<0.001). The results obtained from immunohistochemical analysis support our aforementioned findings and revealed that the changes in expression occurred within specific regions in the areas evaluated. All together, our findings not only provide new evidence for a role of EGb 761 on memory but also identify molecular changes that underlie the fear memory consolidation.

Genetic approaches to modeling anxiety in animals

Anxiety disorders are a growing health problem world-wide. However, the causative factors, etiology, and underlying mechanisms of anxiety disorders, as for most psychiatric disorders, remain relatively poorly understood. The current status of clinical research indicates that anxiety traits and anxiety disorder in man have a genetic component, and therefore genetic modeling in animals is a logical approach to gain a greater insight into their neurobiology. However, it is also clear that the nature of these genetic contributions is highly complex. Moreover, the success of this approach is largely contingent upon the utility of available behavioral paradigms for modeling anxiety-related behaviors in mice. Animal genetic models provide a unique and comprehensive methodological tool to aid discovery into the etiology, neurobiology, and ultimately, the therapy of human anxiety disorders. The approach, however, is challenged with a number of complexities. In particular, the heterogeneous nature of anxiety disorders in man coupled with the associated multifaceted and descriptive diagnostic criteria, create challenges in both animal modeling and in clinical research. In this article, we describe some of the powerful modem genetic techniques that are uniquely amenable to the laboratory mouse and thus provide a strategy for approaching some of these challenges. Moreover, we focus on recent advances which have highlighted the relative contribution of genetic modeling in animals to the understanding of underlying neurobiology and genetic basis of anxiety disorders.

Conditioned fear inhibits c-fos mRNA expression in the central extended amygdala

We have shown previously that unconditioned stressors inhibit neurons of the lateral/capsular division of the central nucleus of the amygdala (CEAl/c) and oval division of the bed nucleus of the stria terminalis (BSTov), which form part of the central extended amygdala. The current study investigated whether conditioned fear inhibits c-fos mRNA expression in these regions. Male rats were trained either to associate a visual stimulus (light) with footshock or were exposed to the light alone. After training, animals were replaced in the apparatus, and 2 h later injected remotely, via a catheter, with amphetamine (2 mg/kg i.p.), to induce c-fos mRNA and allow inhibition of expression to be measured. The rats were then presented with 15 visual stimuli over a 30 minute period. As expected, fear conditioned animals that were not injected with amphetamine, had extremely low levels of c-fos mRNA in the central extended amygdala. In contrast, animals that were trained with the light alone (no fear conditioning) and were injected with amphetamine had high levels of c-fos mRNA in the CEAl/c and BSTov. Animals that underwent fear conditioning, and were re-exposed to the conditioned stimulus after amphetamine injection had significantly reduced levels of c-fos mRNA in both the BSTov and CEAl/c, compared to the non-conditioned animals. These data suggest that conditioned fear can inhibit neurons of the central extended amygdala. Because these neurons are GABAergic, and project to the medial CEA (an amygdaloid output region), this may be a novel mechanism whereby conditioned fear potentiates amygdaloid output.

Influence of massed and distributed context preexposure on contextual fear and Egr-1 expression in the basolateral amygdala

Preexposure to the conditioning context can influence the expression of context-conditioned fear. We used behavioral and early growth response gene (egr-1) assays in rats to study the effects of massed and distributed context preexposure on context-conditioned fear. The results demonstrated that massed context preexposure impaired acquisition of contextual fear, an effect here referred to as delayed shock deficit. Spaced context preexposure produced similar inhibitory effects. Significantly, the introduction of a brief change of context prior to conditioning completely reversed the deficit induced by massed, but not by distributed, context preexposure. This reversibility was inversely related to the duration of the context shift. The acquisition of context-conditioned fear was associated with enhanced Egr-1 expression in the basolateral amygdala (BLA). No such increase was evident in animals undergoing distributed context preexposure or in those experiencing massed preexposure without change of context. Remarkably, a brief change of context prior to conditioning not only facilitated learning following massed preexposure but also elicited a significant elevation of Egr-1 protein levels in the BLA. The findings shown demonstrated that the inhibitory effects of massed and distributed context preexposure on conditioning could be dissociable both behaviorally and physiologically. We suggest that the delayed shock deficit associated with massed preexposure derives from perceptual fade-out or inattention and its reversal by a brief change of context from attentional recovery.

Functional internal complexity of amygdala: focus on gene activity mapping after behavioral training and drugs of abuse

The amygdala is a heterogeneous brain structure implicated in processing of emotions and storing the emotional aspects of memories. Gene activity markers such as c-Fos have been shown to reflect both neuronal activation and neuronal plasticity. Herein, we analyze the expression patterns of gene activity markers in the amygdala in response to either behavioral training or treatment with drugs of abuse and then we confront the results with data on other approaches to internal complexity of the amygdala. c-Fos has been the most often studied in the amygdala, showing specific expression patterns in response to various treatments, most probably reflecting functional specializations among amygdala subdivisions. In the basolateral amygdala, c-Fos expression appears to be consistent with the proposed role of this nucleus in a plasticity of the current stimulus-value associations. Within the medial part of the central amygdala, c-Fos correlates with acquisition of alimentary/gustatory behaviors. On the other hand, in the lateral subdivision of the central amygdala, c-Fos expression relates to attention and vigilance. In the medial amygdala, c-Fos appears to be evoked by emotional novelty of the experimental situation. The data on the other major subdivisions of the amygdala are scarce. In conclusion, the studies on the gene activity markers, confronted with other approaches involving neuroanatomy, physiology, and the lesion method, have revealed novel aspects of the amygdala, especially pointing to functional heterogeneity of this brain region that does not fit very well into contemporarily active debate on serial versus parallel information processing within the amygdala.

Altered brain activity processing in high-anxiety rodents revealed by challenge paradigms and functional mapping

Pathological anxiety involves aberrant processing of emotional information that is hypothesized to reflect perturbations in fear/anxiety pathways. The affected neurobiological substrates in patients with different anxiety disorders are just beginning to be revealed. Important leads for this research can be derived from findings obtained in psychopathologically relevant rodent models of enhanced anxiety, by revealing where in the brain neuronal processing in response to diverse challenges is different to that in animals with lower anxiety levels. Different functional mapping methods in various rodent models, including psychogenetically selected lines or genetically modified animals, have been used for this purpose. These studies show that the divergent anxiety-related behavioral response of high-anxiety- vs. normal and/or low-anxiety rodents to emotional challenges is associated with differential neuronal activation in restricted parts of proposed fear/anxiety circuitries including brain areas thought to be important in stress, emotion and memory. The identification of neuronal populations showing differential activation depends in part on the applied emotional challenge, indicating that specific facets of elicited fear or anxiety preferentially engage particular parts of the fear/anxiety circuitry. Hence, only the use of an array of different challenges will reveal most affected brain areas. A number of the neuronal substrates identified are suggested as candidate mediators of dysfunctional brain activation in pathological anxiety. Indeed, key findings revealed in these rodent models show parallels to observations in human symptom provocation studies comparing anxiety disorder patients with healthy volunteers. Work to investigate exactly which of the changed neuronal activation patterns in high-anxiety rodents has to be modulated by therapeutic drugs to achieve effective anxiolysis and via which neurochemical pathways this can be accomplished is at its early stages but has identified a small number of promising candidates. Extending these approaches should help to provide further insight into these mechanisms, revealing new leads for therapeutic targets and strategies.

Cognitive enhancement with central thalamic electrical stimulation

Central thalamic electrical stimulation has been proposed as a method for remediation of acquired cognitive disability. Long-standing experimental and clinical observations indicate a key role for neurons within the central thalamus in maintaining the alert waking state and facilitating attended behaviors. Here, we show that continuous high frequency (100 Hz) electrical stimulation of the central thalamus generates widespread cortical activation of c-fos across all cortical layers and a selective pattern of regulation of zif268 within the supragranular, granular, and infragranular cortical laminae. Significant elevation of both immediate early genes also is seen in the dentate gyrus of the hippocampus. Use of the same stimulation parameters is shown to facilitate untrained goal-directed seeking behavior and object recognition memory in rodents. An overall increase of exploratory motor behaviors and grooming activity also is observed, consistent with a global increase in arousal. Taken together, these studies indicate that electrical stimulation of the central thalamus may enhance cognitive performance through neocortical and hippocampal neuronal activation and specific regulation of gene expression.

Expression of egr-1 (zif268) mRNA in select fear-related brain regions following exposure to a predator

Research has demonstrated that immediate-early genes/inducible transcriptional factors (e.g., c-fos, egr-1) are increased in amygdala nuclei (lateral, basal and central nuclei) known to be involved in fear conditioning, footshock stress and novelty. Although these data suggest that expression of inducible transcriptional factors are involved in fear, other non-shock ethologically based paradigms (predator or predator odor exposure) do not appear to increase c-fos in the lateral and basal nuclei. While the lack of c-fos expression may indicate that predator stress does not engage the lateral and basal amygdala nuclei, it may be that c-fos in the amygdala is not responsive to predator exposure. Therefore, egr-1, which increases in the lateral nucleus following fear conditioning, footshock and novelty, was assessed to determine if its expression is induced in rats exposed to a cat. Five minutes of cat exposure did not increase expression of egr-1 mRNA in the lateral nucleus of the amygdala. egr-1 was increased in the paraventricular nucleus of the hypothalamus, indicating cat-induced stress, and visual cortex compared to rats that were either confined for 5 min or handled. In the lateral periaqueductal gray, handled rats displayed a left hemisphere dominance, which disappeared in both the cat-exposed and confined group, suggesting that immobility, induced by either cat-induced stress or unstressed confinement, increased right hemisphere egr-1 expression. The results are discussed in a context of differences and similarities in neural circuitry for conditioned and unconditioned fear.

Experience-dependent regulation of zif268 gene expression and spatial learning

Environmental enrichment (EE) is known to enhance the cognitive ability of rodents. To translate EE to the human condition, it is important to understand the parameters of its efficacy. In this study, we examine if the cognitive enhancement associated with EE is permanent and whether a developmental window exists for its efficacy. Rats were housed in continuous isolation (ISO), continuous enrichment (EE), enrichment from postnatal day (PN) 21-50, and then isolation from PN50-79 (PM), or isolation from PN21-50 and then enriched from PN50-79 (CW). Spatial learning ability and basal expression of the immediate-early genes zif268 and Arc as well as the NR1 subunit of the NMDA receptor were assessed. Rats housed in an enriched environment at the time of testing (EE and CW) performed significantly better in the spatial learning task than rats housed in an isolated environment at the time of testing (ISO and PM). Enhanced performance in the spatial learning task was associated with a higher expression of zif268 only in the CA3/CA4 region of the hippocampus. Our study further defines parameters that make environmental enrichment effective in enhancing learning performance and the findings may be helpful in the translation of this intervention to the human condition.

Combined application of behavior genetics and microarray analysis to identify regional expression themes and gene-behavior associations

In this report we link candidate genes to complex behavioral phenotypes by using a behavior genetics approach. Gene expression signatures were generated for the prefrontal cortex, ventral striatum, temporal lobe, periaqueductal gray, and cerebellum in eight inbred strains from priority group A of the Mouse Phenome Project. Bioinformatic analysis of regionally enriched genes that were conserved across all strains revealed both functional and structural specialization of particular brain regions. For example, genes encoding proteins with demonstrated anti-apoptotic function were over-represented in the cerebellum, whereas genes coding for proteins associated with learning and memory were enriched in the ventral striatum, as defined by the Expression Analysis Systematic Explorer (EASE) application. Association of regional gene expression with behavioral phenotypes was exploited to identify candidate behavioral genes. Phenotypes that were investigated included anxiety, drug-naive and ethanol-induced distance traveled across a grid floor, and seizure susceptibility. Several genes within the glutamatergic signaling pathway (i.e., NMDA/glutamate receptor subunit 2C, calmodulin, solute carrier family 1 member 2, and glutamine synthetase) were identified in a phenotype-dependent and region-specific manner. In addition to supporting evidence in the literature, many of the genes that were identified could be mapped in silico to surrogate behavior-related quantitative trait loci. The approaches and data set described herein serve as a valuable resource to investigate the genetic underpinning of complex behaviors.

Genes and neurons: molecular insights to fear and anxiety

Experimental animal models provide an important tool for the identification of inheritable components of fear and anxiety. 'Pavlovian' fear conditioning has been tremendously successful to characterize the neuronal circuitry and cellular mechanisms of the formation, consolidation and extinction of fear memories. Here we summarize recent progress that has led to the identification of gene products contributing to such experience-dependent changes in fear and anxiety and may guide the search for genetic factors involved in the development and treatment of human anxiety disorders.

Altered gene expression in the subdivisions of the amygdala of Fyn-deficient mice as revealed by laser capture microdissection and mKIAA cDNA array analysis

Fyn-tyrosine-kinase-deficient mice exhibit increased fearfulness and display enhanced excitability in the amygdala. To gain insight into the molecular changes associated with the increased excitability of the amygdala, we used a newly developed cDNA array system comprising mouse KIAA cDNA clones to identify novel genes differentially expressed in the amygdala of fyn(-/-) and fyn(+/-) mice following administration of N-methyl-D-aspartate (NMDA). Laser capture microdissection in combination with PCR-based cDNA amplification allowed us to analyze gene expression in each amygdalar subdivision. The statistical significance of the differential expressions was tested by one-way analysis of variance (ANOVA) by the false discovery rate controlling approach. Among the 805 mKIAA cDNA clones tested, only the expression level of mKIAA1577 (Zinc finger SWIM domain containing protein 6; gene name, Zswim6) showed statistically significant change in regard to the genotype and amygdalar subdivision. Namely, only the lowered expression of mKIAA1577 in the central nucleus of fyn(-/-) mice 1 h after NMDA administration (2.1-fold lower relative to fyn(+/-) mice) was statistically significant. In situ hybridization analysis confirmed the downregulation of the mRNA in the central nucleus of the fyn(-/-) mice 1 h after NMDA administration (3.2-fold lower relative to fyn(+/-) mice). The NMDA-induced change in gene expression was partially blocked by the NMDA antagonist D-AP-5. These results suggest that Fyn deficiency was responsible for the NMDA-induced downregulation of a specific gene in the amygdalar central nucleus.

Immediate-early gene expression in the central nucleus of the amygdala is not specific for anxiolytic or anxiogenic drugs

The lateral, basal, and central nuclei of the amygdala are part of a circuitry that instantiates many fear and anxious behaviors. One line of support indicates that immediate-early gene (IEG) expression (e.g., c-fos and egr-1 (zif268)) is increased in these nuclei following fear conditioning. Other research finds that anxiogenic drugs working through various mechanisms induce IEG expression in the central nucleus of the amygdala (CeA) suggesting that expression is a neural marker for fear and anxiety. However, several studies have also found that anxiolytic drugs induce IEG expression in the CeA. Expression of egr-1 in the CeA and lateral nucleus of the amygdala following administration of anxiolytic and anxiogenic benzodiazepine and serotonin agonists and antagonists was investigated. The first experiment determined behaviorally active anxiolytic and anxiogenic doses for two anxiogenic drugs (FG 7142 and mCPP) and two anxiolytic drugs (diazepam and buspirone). The effects of anxiogenic and anxiolytic doses of these drugs on egr-1 expression in the amygdala were then tested in a second experiment. All four drugs increased egr-1 in the CeA indicating that increased egr-1 mRNA expression in the CeA is not specific to anxiolytic or anxiogenic effects of the drugs. We suggest that IEG expression in the CeA may be due to activation of circuits that are associated with systemic physiological homeostasis perturbed by a number of drugs including anxiogenic and anxiolytic compounds.

Comparison of alterations in c-fos and Egr-1 (zif268) expression throughout the rat brain following acute administration of different classes of antidepressant compounds

The majority of immediate-early gene (IEG) studies focus on a few key brain regions associated with the class of psychoactive compound being studied. Recently, using a meta-analysis of the c-fos literature, we demonstrated the utility of c-fos profiling to classify such compounds. The present study examined acute delivery of a range of antidepressant classes; fluoxetine, imipramine, LiCl, and mirtazapine. The dual aims were to study the IEG profiles of these varying classes of antidepressants throughout the rat brain and to compare the utility of c-fos or Egr-1 as IEGs to classify clinically efficacious antidepressants. All antidepressants increased c-fos mRNA in the central amygdala, as previously shown, while c-fos was also increased in the anterior insular cortex and significantly decreased within the septum. Although acute antidepressant administration altered c-fos expression in a number of brain regions, Egr-1 expression was only significantly altered in the central amygdala, suggesting that Egr-1 may not be as useful a marker to investigate acute antidepressant treatment. The fact that these drugs, including the previously unclassified antidepressant mirtazapine, share a number of common loci of activation, which are implicated by human and animal studies in depression, adds further support to the use of IEG mapping to classify psychoactive compounds.

Diazepam affects the nuclear thyroid hormone receptor density and their expression levels in adult rat brain

Thyroid hormones (THs) are involved in the occurrence of anxiety and affective disorders; however, the effects following an anxiolytic benzodiazepine treatment, such as diazepam administration, on the mechanism of action of thyroid hormones has not yet been investigated. The effect of diazepam on the in vitro nuclear T3 binding, on the relative expression of the TH receptors (TRs) and on the synaptosomal TH availability were examined in adult rat cerebral hemispheres 24 h after a single intraperitoneal dose (5 mg/kg BW) of this tranquillizer. Although, diazepam did not affect the availability of TH either in blood circulation or in the synaptosomal fraction, it decreased (33%) the nuclear T3 maximal binding density (B(max)). No differences were observed in the equilibrium dissociation constant (K(d)). The TRalpha2 variant (non-T3-binding) mRNA levels were increased by 33%, whereas no changes in the relative expression of the T3-binding isoforms of TRs (TRalpha1, TRbeta1) were observed. This study shows that a single intraperitoneal injection of diazepam affects within 24 h, the density of the nuclear TRs and their expression pattern. The latest effect occurs in an isoform-specific manner involving specifically the TRalpha2 mRNA levels in adult rat brain.

Inhibition of the central extended amygdala by loud noise and restraint stress

It is well established that the central nucleus of the amygdala (CEA) is involved in responses to stress, fear and anxiety. Many studies have used c-fos expression to map the brain's response to processive stress, but curiously the CEA generally is not highly activated. We have previously shown that exposure to a novel vs. home environment reduces amphetamine-induced activation of the lateral CEA (CEAl) and the oval nucleus of the bed nucleus of the stria terminalis (BSTov). This is consistent with the idea that processive stress inhibits neurons in these nuclei. We have tested this hypothesis by exposing rats to noise, at a range of intensities from non-stressful to stressful, or to restraint conditions, immediately after a remote injection of amphetamine, 2 mg/kg i.p., or interleukin-1beta (IL-1beta) 0.5 microg/kg i.p. (used to obtain a level of c-fos mRNA against which to measure inhibition). In keeping with our hypothesis, amphetamine- or IL-1beta-induced c-fos and zif-268 mRNA were significantly decreased in the CEAl and BSTov under conditions of loud noise or restraint stress compared with control conditions. This inhibition does not require a stress-induced rise in corticosterone because data were similar in animals that had been adrenalectomized with a low-dose corticosterone replacement. As both the CEAl and BSTov are highly gamma-aminobutyric acid (GABA) -ergic and project to the medial CEA (CEAm), their inhibition potentially causes an increased input to the CEAm. As the CEAm is a major output nucleus of the amygdala, this could have important consequences within the neural circuitry controlling responses to processive stress.

A gene for neuronal plasticity in the mammalian brain: Zif268/Egr-1/NGFI-A/Krox-24/TIS8/ZENK?

Zif268 is a transcription regulatory protein, the product of an immediate early gene. Zif268 was originally described as inducible in cell cultures; however, it was later shown to be activated by a variety of stimuli, including ongoing synaptic activity in the adult brain. Recently, mice with experimentally mutated zif268 gene have been obtained and employed in neurobiological research. In this review we present a critical overview of Zif268 expression patterns in the naive brain and following neuronal stimulation as well as functional data with Zif268 mutants. In conclusion, we suggest that Zif268 expression and function should be considered in a context of neuronal activity that is tightly linked to neuronal plasticity.

Learning-induced activation of transcription factors among multiple memory systems

Experimental evidence for multiple memory systems grew initially from reports that integrity of the medial temporal lobes is necessary for some, but not all, types of memory formation. A primary inference from many studies of multiple memory systems is that they operate independently during encoding, storage, and retrieval of information. An accumulation of recent evidence, however, suggests that multiple memory systems may interact under some conditions. At the cellular level of analysis, it is accepted widely that protein synthesis is necessary for the formation of long-term memory and recent efforts have focused on the mechanisms by which learning-induced gene transcription and translation are regulated. The present review examines learning-induced activation of transcription factors among multiple memory systems. The results indicate that studies of transcriptional regulation, in conjunction with other experimental approaches, can provide complementary lines of evidence to further understanding of the extent to which multiple memory systems are independent or interactive.

Mild hypoxia preconditioning prevents impairment of passive avoidance learning and suppression of brain NGFI-A expression induced by severe hypoxia

The aim of this work was to study effects of mild preconditioning hypobaric hypoxia (380 Torr for 2 h, repeated 3 or 6 times spaced at 24 h) on brain NGFI-A immunoreactivity and passive avoidance (PA) behavior in rats exposed to severe hypoxia (160 Torr for 3 h). Severe hypobaric hypoxia produced extensive neuronal loss in hippocampal CA1, while the preceding hypoxic preconditioning had clear protective effect on neuronal viability of vulnerable hippocampal cells. Besides, the hypoxic preconditioning prevented impairment of acquisition and retention of PA caused by severe hypoxia. The six-trial hypobaric preconditioning was more effective in protection against PA learning deficits in severe hypoxia exposed rats than the three-trial preconditioning. The preconditioning up-regulated severe hypoxia-suppressed neocortical and hippocampal expression of NGFI-A, suggesting a possible role for NGFI-A in the neuroprotective mechanisms activated by hypoxic preconditioning.

An egr-1 (zif268) antisense oligodeoxynucleotide infused into the amygdala disrupts fear conditioning

Studies of gene expression following fear conditioning have demonstrated that the inducible transcription factor, egr-1, is increased in the lateral nucleus of the amygdala shortly following fear conditioning. These studies suggest that egr-1 and its protein product Egr-1 in the amygdala are important for learning and memory of fear. To directly test this hypothesis, an egr-1 antisense oligodeoxynucleotide (antisense-ODN) was injected bilaterally into the amygdala prior to contextual fear conditioning. The antisense-ODN reduced Egr-1 protein in the amygdala and interfered with fear conditioning. A 250-pmole dose produced an 11% decrease in Egr-1 protein and reduced long-term memory of fear as measured by freezing in a retention test 24 h after conditioning, but left shock-induced freezing intact. A larger 500-pmole dose produced a 25% reduction in Egr-1 protein and significantly decreased both freezing immediately following conditioning and freezing in the retention test. A nonsense-ODN had no effect on postshock or retention test freezing. In addition, 500 pmole of antisense-ODN infused prior to the retention test in previously trained rats did not reduce freezing, indicating that antisense-ODN did not suppress conditioned fear behavior. Finally, rats infused with 500 pmole of antisense-ODN displayed unconditioned fear to a predator odor, demonstrating that unconditioned freezing was unaffected by the antisense-ODN. The data indicate that the egr-1 antisense-ODN interferes with learning and memory processes of fear without affecting freezing behavior and suggests that the inducible transcription factor Egr-1 within the amygdala plays important functions in long-term learning and memory of fear.

cDNA microarray analysis of gene expression in anxious PVG and SD rats after cat-freezing test

To identify genes involved in the development of anxiety or fear, we analyzed the gene expression profiles of the cortex of anxious hooded PVG and Sprague-Dawley (SD) rats after exposure to the cat-freezing test apparatus. These two rat strains showed a marked difference in the extent of anxious behavior on the cat-freezing test; the hooded PVG rats showed highly anxious behavior while a low anxiety state was observed in SD rats. A cDNA microarray consisting of 5,931 genes was employed to investigate the global mRNA expression profiles of anxiety-related genes. According to the assumption that an abundance ratio of > or =1.5 is indicative of a change in gene expression, we detected 16 upregulated and 38 downregulated genes in PVG hooded and SD rats. Some of these genes have not yet been associated with anxiety (e.g. FGF), while other genes were recently found to be expressed in an anxious state (e.g., rat nerve growth factor-induced gene, NGFI-A). Our study also focused on the expression of some neurotransmitter receptors that have already been proven to be relevant to anxiety or fear, e.g., gamma-aminobutyric acid (GABA), cholecystokinin (CCK) and 5-HT(3) receptors. To further confirm the microarray data, the mRNA expressions of three genes: rat activity-regulated cytoskeleton-associated gene (Arc), rat NGFI-A gene and rat 5-HT(3) receptor (5-HT(3)R) mRNA, were studied by reverse transcription-polymerase chain reaction (RT-PCR). The results of RT-PCR were basically consistent with those from cDNA microarray. Our study therefore demonstrated that the microarray technique is an efficient tool for analyzing global expression profiles of anxiety-related genes, which may also provide further insight into the molecular mechanisms underlying the states of anxiety and fear.

Effects of the phytoestrogen coumestrol on locomotor and fear-related behaviors in female mice

Effects of the estrogenic plant isoflavonoid coumestrol (COUM) on locomotor activity and fear-related behaviors in both the absence and the presence of estradiol benzoate (EB) were examined in adult ovariectomized Swiss-Webster mice. In a running wheel paradigm conducted over 10 days, with treatment beginning 12 days prior to testing, daily subcutaneous (sc) injections of 10 microg of COUM did not influence locomotor activity, whereas even the low dose of EB supplied by sc Silastic implant (2.5 microg lasting 5 weeks) produced a significant increase in locomotor activity over animals receiving vehicle alone. In animals receiving both COUM and EB, locomotor activity was significantly diminished compared to the activity observed in animals receiving EB alone. The same animals were also tested in three behavioral paradigms having known activity and fear components (open field, dark/light transition, elevated plus-maze). COUM did not produce significant effects in these assays. However, in an associative fear learning paradigm (fear conditioning), COUM produced significantly less freezing ("fearfulness") in the conditioned fear task than with animals receiving vehicle or a low dose of EB, which did not differ from each other. Unlike EB, which has comparable affinities for estrogen receptor (ER) alpha and ER beta, COUM has a higher affinity for ER beta than for ER alpha. COUM might disrupt estrogen-enhanced locomotor activity by altering the dynamic by which EB acts on the brain through these two ER isoforms. The conditioned fear result suggests that COUM may produce additional effects through alternative pathways.

N-Methyl-D-aspartate receptor antagonism blocks contextual fear conditioning and differentially regulates early growth response-1 messenger RNA expression in the amygdala: implications for a functional amygdaloid circuit of fear

N-Methyl-D-aspartate receptors in the amygdala are known to be crucial for the learning of conditioned fear, although the molecular cascades that N-methyl-D-aspartate receptors regulate are not well understood. Recent experiments from our laboratory have shown that messenger RNA expression of the immediate-early messenger gene, early growth response gene 1, increases in the lateral nucleus of the amygdala following contextual fear conditioning. However, the regulation of the increase in early growth response gene 1 expression is not known. To determine if N-methyl-D-aspartate receptors regulate both fear conditioning and the increase in early growth response gene 1 expression in the lateral nucleus of the amygdala, rats were infused i.c.v. with 2.5microg of the N-methyl-D-aspartate antagonist, DL-2-amino-5-phosphonovalerate. Most rats were killed 30min following one-trial contextual fear conditioning and their brains were processed for in situ hybridization detection of early growth response gene 1 messenger RNA expression. The remainder of the rats was tested for retention of fear conditioning 24h later. In DL-2-amino-5-phosphonovalerate-treated rats, post-shock freezing remained intact, whereas fear-conditioned freezing during the retention test was abolished. Image analysis of early growth response gene 1 messenger RNA revealed that DL-2-amino-5-phosphonovalerate blocked the fear-conditioning-associated increase in early growth response gene 1 expression in the lateral nucleus of the amygdala. In addition, DL-2-amino-5-phosphonovalerate significantly increased early growth response gene 1 expression in the central nucleus of the amygdala. The results reveal differential regulation of early growth response gene 1 messenger RNA in the amygdala by N-methyl-D-aspartate receptors and argue for a functional role of early growth response gene 1 in the formation of long-term memory for contextual fear. Furthermore, the results indicate a functional neuroanatomical circuit within the amygdala that includes dampening of excitatory and activation of inhibitory processes in distinct amygdala nuclei, resulting in the reduction of fear conditioning.

Induction of NGFI-B mRNA following contextual fear conditioning and its blockade by diazepam

Expression of the immediate-early gene, NGFI-B (nerve growth factor inducible gene B), was examined in the amygdala, hippocampus, and neocortex following contextual fear conditioning. Rats were either handled, placed within the testing context without receiving the footshock, received a footshock immediately upon placement within the context, or received a footshock after a 3-min delay (delayed-shock). Only the delayed-shock group displayed a fear response (freezing) in the post-shock period and in a retention test 24 h after fear conditioning. Expression of NGFI-B mRNA was increased in the dorsolateral part of the lateral nucleus of the amygdala (LaDL) and the neocortex 30 min following conditioning in the delayed-shock group compared to the other three groups. In addition, following a retention test conducted 24 h after fear conditioning, NGFI-B mRNA expression was increased in the neocortex of the delayed-shock group compared to the handled group. In a subsequent experiment, the effects of pretreatment with the anxiolytic drug, diazepam, on fear conditioning and the concomitant increases in NGFI-B mRNA were investigated. Rats administered a 2.5 mg/kg, i.p. dose of diazepam before fear conditioning did not acquire contextual fear as demonstrated by a lack of freezing in a retention test. Although diazepam blocked fear conditioning while the 40% propylene glycol, 10% ethanol vehicle solution did not, both diazepam and the vehicle reduced the conditioning-induced increase in NGFI-B expression in the LaDL. In contrast, the fear-conditioning-induced NGFI-B increase in the neocortex was blocked by diazepam, but not by the vehicle. The data suggest that the transcriptional factor NGFI-B in the LaDL and neocortex may play a functional role in learning and memory of contextual fear, but blocking the increase in NGFI-B expression in the LaDL is not essential for diazepam to interfere with fear conditioning.

Specific induction of early growth response gene 1 in the lateral nucleus of the amygdala following contextual fear conditioning in rats

Although the amygdala is known to be crucial for fear conditioning, little is known about the molecular and cellular mechanisms in the amygdala that are important for fear conditioning. One possible mechanism may be the activation of immediate-early genes, which function as regulatory factors of transcriptional processes. To investigate whether one of the major immediate-early gene families is involved in the learning and memory of fear, we examined the effects of fear conditioning on the expression of the four members of the early growth response (EGR) gene family, EGR-1, EGR-2, EGR-3, and EGR-4. Image analysis of in situ hybridization of messenger RNA of the four family members was performed in the amygdala, hippocampus, and neocortex 15, 30 and 60min following one-trial contextual fear conditioning. Rats were either handled, placed within the testing context without receiving the footshock, and received a footshock immediately upon placement within the context, or received a footshock after a 3-min delay (delayed-shock). Of the four groups, only the delayed-shock group exhibited a fear response (freezing). EGR-1 messenger RNA expression in the dorsolateral part of the lateral amygdaloid nucleus was significantly greater in the delayed-shock group compared with the other groups 15 and 30min following the conditioning. The increased expression of EGR-1 was specifically localized to the lateral nucleus of the amygdala; expression in the hippocampus and cortex was not increased by fear conditioning. In contrast, the expression of EGR-2, EGR-3, and EGR-4 messenger RNA was not increased in the amygdala, hippocampus or cortex following fear conditioning. In addition, following a retention test conducted 24h after fear conditioning, no increases were found in the expression of EGR-1 messenger RNA expression in the amygdala, hippocampus or cortex. The results demonstrate that of the four genes of the EGR family of transcription-regulatory factors, only EGR-1 messenger RNA in the dorsolateral portion of the lateral nucleus of the amygdala was specifically increased with contextual fear conditioning. It is suggested that EGR-1 plays a functional role during learning, but not retrieval, of contextual fear within the lateral nucleus of the amygdala.