Animals learn that a tone can predict the occurrence of an electric shock through classical conditioning. Mice or rats trained in this manner display fear responses, such as freezing behaviour, when they hear the conditioned tone. Studies using amygdalectomized rats have shown that the amygdala is required for both the acquisition and expression of learned fear responses. Freezing to a conditioned tone is enhanced following damage to the dorsal part of the medial prefrontal cortex, indicating that this area may be involved in fear reduction. Here we show that prefrontal neurons reduce their spontaneous activity in the presence of a conditioned aversive tone as a function of the degree of fear. The depression in prefrontal spontaneous activity is related to amygdala activity but not to the freezing response itself. These data indicate that, in the presence of threatening stimuli, the amygdala controls both fear expression and prefrontal neuronal activity. They suggest that abnormal amygdala-induced modulation of prefrontal neuronal activity may be involved in the pathophysiology of certain forms of anxiety disorder.

Vitamin A and its derivatives, the retinoids, have been implicated recently in the synaptic plasticity of the hippocampus and might therefore play a role in associated cognitive functions. Acting via transcription factors, retinoids can regulate gene expression via their nuclear receptors [retinoic acid receptors (RARs) and retinoid X receptors]. In a series of experiments, the present study investigated the possible role of age-related downregulation of retinoid-mediated transcription events in the cognitive decline seen in aged mice. We observed that the brain (and hippocampal) levels of retinoid receptors and the expression of specific associated target genes were restored to presenescent (adult) levels in aged mice after acute administration (150 g/kg, s.c.) of retinoic acid (RA). These effects of RA, however, could be abolished by the coadministration of an RAR antagonist. RA was also demonstrated to alleviate the agerelated deficit in the CA1 long-term potentiation efficacy of aged mice in vivo. Moreover, RA was found to alleviate completely the performance deficit of aged mice to the control level in a two-stage spatial discrimination paradigm designed to assess relational memory. This promnesic effect of RA was again susceptible to abolition by RAR antagonist treatment. The parallel molecular, cellular, and behavioral correlates associated with the decrease of retinoid receptor expression and its normalization demonstrated here suggest that the fine regulation of retinoid-mediated gene expression is fundamentally important to optimal brain functioning and higher cognition. Specifically, a naturally occurring dysregulation of retinoidmediated molecular events might be a potential etiological factor for cognitive deterioration during senescence.

Hippocampal synaptic structure and function exhibit marked variations during the estrus cycle of female rats. Estradiol activates the mitogen-activated protein (MAP) kinase pathway in numerous cell types, and MAP kinase has been shown to play a critical role in the mechanisms underlying synaptic plasticity. Here, we report that endogenous estrogen produces a tonic phosphorylation͞activa-tion of extracellular signal-regulated kinase 2 (ERK2)͞MAP kinase throughout the female rat brain and an increase in tyrosine phosphorylation of NR2 subunits of N-methyl-D-aspartate (NMDA) receptors. Moreover, cyclic changes in estrogen levels during the estrus cycle of female rats are associated with corresponding changes in the levels of activation of ERK2, the state of tyrosine phosphorylation of NR2 subunits of NMDA receptors, and the magnitude of long-term potentiation in hippocampus. Thus, cyclic changes in female sexual hormones result in marked variations in the state of activation of a major cellular signaling pathway critical for learning and memory and in a cellular model of learning and memory.E strogens have profound effects on hippocampal structure and physiology (1-4) and on hippocampal-dependent learning and memory (5, 6). In particular, estrogens have been shown to increase the density of dendritic spines on CA1 pyramidal neurons (7). In hippocampal slices, 17␤-estradiol increases electrophysiological responses elicited by activation of both ␣-amino-3-hydroxy-5-methylisoxazole propionic acid and N-methyl-Daspartate (NMDA) receptors and the magnitude of long-term potentiation (LTP) in field CA1 (8, 9). At the cellular level, numerous laboratories have shown that, in addition to direct genomic effects, 17␤-estradiol activates the extracellular regulated kinase͞mitogen-activated protein (ERK͞MAP) kinase pathway (10-12), an effect associated with the neurotrophic͞ neuroprotective actions of estrogen (13,14). We recently reported that estrogen-mediated activation of the ERK͞MAP kinase pathway in hippocampus was involved in the rapid effects of estrogen on NMDA receptors and LTP through tyrosine phosphorylation of NR2 subunits of NMDA receptors (15).The ERK͞MAP kinase pathway is a central cellular signaling pathway linking numerous extracellular signals to membrane receptors, transcription factors, and gene regulation (16) and is critically involved in synaptic plasticity, learning, and memory (17)(18)(19)(20). Pharmacological manipulations directed at blocking this pathway have consistently produced impairment in synaptic plasticity, learning, and memory, and this pathway is activated with LTP-inducing tetanus or in different learning paradigms. The present study analyzed whether endogenous estrogen regulates the state of activation of the ERK͞MAP kinase pathway and whether cyclic variations in estrogen levels during the female estrus cycle suffice to modify this pathway in the brain. Moreover, we determined the state of tyrosine phosphorylation of NMDA-receptor subunits, as well as the magnitude of LTP in field CA1 o...

Stress impairs hippocampal long-term potentiation (LTP), a model of synaptic plasticity that is assumed to underlie memory formation. In the amygdala, little is known about the effects of stress on LTP, or about its longevity. Here we assessed the ability of entorhinal cortex (EC) stimulation to induce LTP simultaneously in the basal amygdaloid nucleus (B) and in the dentate gyrus (DG) of freely behaving Wistar rats. We also tested whether LTP persists over days. Once established, we investigated the effects of acute vs. repeated inescapable stressful experiences on LTP in both structures. Results show that B, like DG, sustained LTP for 7 days. Furthermore, a single exposure to moderate stress facilitated LTP in B but did not affect DG LTP. Stress re-exposure inhibited LTP in DG but only long-lasting LTP (>3 days) in B. Behaviourally, animals exhibited a higher immobility when re-exposed to the stressor than with a single/first exposure. These data support a role for B in memory storage. Furthermore, they support a differential involvement of the amygdala and hippocampus in memory formation and storage depending on the emotional characteristics of the experience.

We studied changes in thalamo-prefrontal cortical transmission in behaving mice following both low-frequency stimulation of the mediodorsal thalamus (MD) and during extinction of a conditioned fear response. Electrical stimulation of the MD induces a field potential in the medial prefrontal cortex (mPFC) characterized by two initial negative-positive complexes (N1-P1 and N2-P2) followed by two positive-negative complexes (P2-N3 and P3-N4). The N1-P1 and N2-P2 complexes were identified as resulting from orthodromic and antidromic prefrontal activation, respectively. Because the two complexes were not often easily dissociated, plasticity in the prefrontal synaptic transmission was considered to result from changes in N1-P2 amplitude. Low-frequency thalamic stimulation (1, 200 pulses at 2 Hz) produced either long-term (at least 32 min) depression or potentiation of the N1-P2 amplitude. Mice submitted to fear conditioning (tone-shock association), displayed on the first day of extinction (tone-alone presentations) a strong freezing behavior, which decreased progressively, but was still high the following day. Extinction of conditioned fear was accompanied the first day by a depression of prefrontal transmission, which was converted into potentiation the following day. Potentiation of prefrontal transmission lasted at least 24 h following the second day of the fear extinction procedure. In conclusion, low-frequency thalamic stimulation can produce, in behaving mice, either depression or potentiation of prefrontal synaptic transmission. Decrease in prefrontal synaptic transmission observed during the first day of extinction may reflect processing of the high degree of predictiveness of danger (unconditioned stimulus: US) by the aversive conditioned stimulus (CS). However, the subsequent potentiation of transmission in the mPFC may be related to processing of cognitive information such as the CS will no longer be followed by the US, even if emotional response (freezing) to the CS is still high.

In addition to metabolic and cardiovascular disorders, obesity is associated with adverse cognitive and emotional outcomes. Its growing prevalence during adolescence is particularly alarming since recent evidence indicates that obesity can affect hippocampal function during this developmental period. Adolescence is a decisive period for maturation of the amygdala and the hypothalamic-pituitaryadrenal (HPA) stress axis, both required for lifelong cognitive and emotional processing. However, little data are available on the impact of obesity during adolescence on amygdala function. Herein, we therefore evaluate in rats whether juvenile high-fat diet (HFD)-induced obesity alters amygdala-dependent emotional memory and whether it depends on HPA axis deregulation. Exposure to HFD from weaning to adulthood, i.e., covering adolescence, enhances long-term emotional memories as assessed by odor-malaise and tone-shock associations. Juvenile HFD also enhances emotion-induced neuronal activation of the basolateral complex of the amygdala (BLA), which correlates with protracted plasma corticosterone release. HFD exposure restricted to adulthood does not modify all these parameters, indicating adolescence is a vulnerable period to the effects of HFD-induced obesity. Finally, exaggerated emotional memory and BLA synaptic plasticity after juvenile HFD are alleviated by a glucocorticoid receptor antagonist. Altogether, our results demonstrate that juvenile HFD alters HPA axis reactivity leading to an enhancement of amygdala-dependent synaptic and memory processes. Adolescence represents a period of increased susceptibility to the effects of diet-induced obesity on amygdala function.

The level of controllability has been shown to modulate the effects of stress on physiology and behavior. In the present study, we investigated the effects of controllable vs. uncontrollable stressors on plasticity in hippocampal CA1, the dentate gyrus (DG), and basal amygdala nucleus (B) in the rat, using the electrophysiological procedure of long-term potentiation (LTP). A naive group was left undisturbed until the electrophysiological recording commenced. Rats of the two controllable stress groups were trained in the Morris water maze to locate an invisible underwater platform (the first group), or visible platform (the second group), thus escaping from the water, before the recording. The uncontrollable stress group underwent the same procedure (exposure time to water was adjusted to the averaged exposure time of the first controllable group) without the escape platform. We first assessed the effects of stress and controllability on LTP in CA1. Both controllable stressors and the uncontrollable stress impaired CA1 LTP, with a more robust effect induced by the uncontrollable stress. We further assessed the effects of the same procedures on LTP in DG and B. The uncontrollable stress enhanced LTP in DG and increased baseline responses (suggesting uncontrollable stress-induced plasticity) in the amygdala. All the stressors decreased amygdalar LTP. An assessment of plasma levels of corticosterone (CORT), following the behavioral procedures, revealed an enhancement in CORT release following the uncontrollable, but not controllable stress, indicating the uncontrollable condition as the most stressful. These findings provide insight into the differential effects of stress and stress controllability on different hippocampal subregions and the amygdala.

The neural circuit linking the medial prefrontal cortex (mPFC) and the basolateral amygdala (BLA) has crucial roles in both the acquisition and the extinction of fear. However, the mechanism by which this circuit encodes fear and extinction remains unknown. In this study, we monitored changes in the magnitude of evoked field potentials (EFPs) in the mPFC-BLA and BLA-mPFC pathways following auditory fear conditioning and extinction, in freely moving rats. We report that extinction of fear is mediated by depression of the EFPs in the mPFC-BLA and by potentiation in the reciprocal pathway of BLA-mPFC. Interestingly, reinstatement of fear was associated with recovery of freezing and with reversal of the changes in EFPs that were observed following extinction in both pathways. The findings indicate that the mPFC-BLA circuit expresses differential changes following fear and extinction and point to dynamic and plastic changes underlying fear, extinction, and reinstatement. Manipulations targeting these different types of plasticity could constitute a therapeutic tool for the treatment of anxiety disorders.