Obesogenic diet induces sex-specific alterations of contextual fear memory and associated hippocampal activity in mice.

In addition to metabolic and cardiovascular disorders, obesity is associated with cognitive deficits in humans and animal models. We have previously shown that obesogenic high-fat and sugar diet intake during adolescence (adoHFSD) impairs hippocampus (HPC)-dependent memory in rodents. These results were obtained in males only and it remains to evaluate whether adoHFSD has similar effect in females. Therefore, here, we investigated the effects of adoHFSD consumption on HPC-dependent contextual fear memory and associated brain activation in male and female mice. Exposure to adoHFSD increased fat mass accumulation and glucose levels in both males and females but impaired contextual fear memory only in males. Compared with females, contextual fear conditioning induced higher neuronal activation in the dorsal and ventral HPC (CA1 and CA3 subfields) as well as in the medial prefrontal cortex in males. Also, adoHFSD-fed males showed enhanced c-Fos expression in the dorsal HPC, particularly in the dentate gyrus, and in the basolateral amygdala compared with the other groups. Finally, chemogenetic inactivation of the dorsal HPC rescued adoHFSD-induced memory deficits in males. Our results suggest that males are more vulnerable to the effects of adoHFSD on HPC-dependent aversive memory than females, due to overactivation of the dorsal HPC.

Chemogenetic activation or inhibition of histaminergic neurons bidirectionally modulates recognition memory formation and retrieval in male and female mice.

Several lines of evidence demonstrate that the brain histaminergic system is fundamental for cognitive processes and the expression of memories. Here, we investigated the effect of acute silencing or activation of histaminergic neurons in the hypothalamic tuberomamillary nucleus (TMNHA neurons) in vivo in both sexes in an attempt to provide direct and causal evidence of the necessary role of these neurons in recognition memory formation and retrieval. To this end, we compared the performance of mice in two non-aversive and non-rewarded memory tests, the social and object recognition memory tasks, which are known to recruit different brain circuitries. To directly establish the impact of inactivation or activation of TMNHA neurons, we examined the effect of specific chemogenetic manipulations during the formation (acquisition/consolidation) or retrieval of recognition memories. We consistently found that acute chemogenetic silencing of TMNHA neurons disrupts the formation or retrieval of both social and object recognition memory in males and females. Conversely, acute chemogenetic activation of TMNHA neurons during training or retrieval extended social memory in both sexes and object memory in a sex-specific fashion. These results suggest that the formation or retrieval of recognition memory requires the tonic activity of histaminergic neurons and strengthen the concept that boosting the brain histaminergic system can promote the retrieval of apparently lost memories.

Nucleus accumbens D1- and D2-expressing neurons control the balance between feeding and activity-mediated energy expenditure.

Accumulating evidence points to dysregulations of the Nucleus Accumbens (NAc) in eating disorders (ED), however its precise contribution to ED symptomatic dimensions remains unclear. Using chemogenetic manipulations in male mice, we found that activity of dopamine D1 receptor-expressing neurons of the NAc core subregion facilitated effort for a food reward as well as voluntary exercise, but decreased food intake, while D2-expressing neurons have opposite effects. These effects are congruent with D2-neurons being more active than D1-neurons during feeding while it is the opposite during running. Chronic manipulations of each subpopulations had limited effects on energy balance. However, repeated activation of D1-neurons combined with inhibition of D2-neurons biased behavior toward activity-related energy expenditure, whilst the opposite manipulations favored energy intake. Strikingly, concomitant activation of D1-neurons and inhibition of D2-neurons precipitated weight loss in anorexia models. These results suggest that dysregulations of NAc dopaminoceptive neurons might be at the core of EDs.

Obesogenic diet induces circuit-specific memory deficits in mice.

Obesity is associated with neurocognitive dysfunction, including memory deficits. This is particularly worrisome when obesity occurs during adolescence, a maturational period for brain structures critical for cognition. In rodent models, we recently reported that memory impairments induced by obesogenic high-fat diet (HFD) intake during the periadolescent period can be reversed by chemogenetic manipulation of the ventral hippocampus (vHPC). Here, we used an intersectional viral approach in HFD-fed male mice to chemogenetically inactivate specific vHPC efferent pathways to nucleus accumbens (NAc) or medial prefrontal cortex (mPFC) during memory tasks. We first demonstrated that HFD enhanced activation of both pathways after training and that our chemogenetic approach was effective in normalizing this activation. Inactivation of the vHPC-NAc pathway rescued HFD-induced deficits in recognition but not location memory. Conversely, inactivation of the vHPC-mPFC pathway restored location but not recognition memory impairments produced by HFD. Either pathway manipulation did not affect exploration or anxiety-like behaviour. These findings suggest that HFD intake throughout adolescence impairs different types of memory through overactivation of specific hippocampal efferent pathways and that targeting these overactive pathways has therapeutic potential.

Endogenous cannabinoids in the piriform cortex tune olfactory perception.

Sensory perception depends on interactions between external inputs transduced by peripheral sensory organs and internal network dynamics generated by central neuronal circuits. In the sensory cortex, desynchronized network states associate with high signal-to-noise ratio stimulus-evoked responses and heightened perception. Cannabinoid-type-1-receptors (CB1Rs) - which influence network coordination in the hippocampus - are present in anterior piriform cortex (aPC), a sensory paleocortex supporting olfactory perception. Yet, how CB1Rs shape aPC network activity and affect odor perception is unknown. Using pharmacological manipulations coupled with multi-electrode recordings or fiber photometry in the aPC of freely moving male mice, we show that systemic CB1R blockade as well as local drug infusion increases the amplitude of gamma oscillations in aPC, while simultaneously reducing the occurrence of synchronized population events involving aPC excitatory neurons. In animals exposed to odor sources, blockade of CB1Rs reduces correlation among aPC excitatory units and lowers behavioral olfactory detection thresholds. These results suggest that endogenous endocannabinoid signaling promotes synchronized population events and dampen gamma oscillations in the aPC which results in a reduced sensitivity to external sensory inputs.

Central activation of the fatty acid sensor GPR120 suppresses microglia reactivity and alleviates sickness- and anxiety-like behaviors.

G protein-coupled receptor 120 (GPR120, Ffar4) is a sensor for long-chain fatty acids including omega-3 polyunsaturated fatty acids (n-3 PUFAs) known for beneficial effects on inflammation, metabolism, and mood. GPR120 mediates the anti-inflammatory and insulin-sensitizing effects of n-3 PUFAs in peripheral tissues. The aim of this study was to determine the impact of GPR120 stimulation on microglial reactivity, neuroinflammation and sickness- and anxiety-like behaviors by acute proinflammatory insults. We found GPR120 mRNA to be enriched in  both murine and human microglia, and in situ hybridization revealed GPR120 expression in microglia of the nucleus accumbens (NAc) in mice. In a manner similar to or exceeding n-3 PUFAs, GPR120 agonism (Compound A, CpdA) strongly attenuated lipopolysaccharide (LPS)-induced proinflammatory marker expression in primary mouse microglia, including tumor necrosis factor-α (TNF-α) and interleukin-1ß (IL-1ß), and inhibited nuclear factor-ĸB translocation to the nucleus. Central administration of CpdA to adult mice blunted LPS-induced hypolocomotion and anxiety-like behavior and reduced TNF-α, IL-1ß and IBA-1 (microglia marker) mRNA in the NAc, a brain region modulating anxiety and motivation and implicated in neuroinflammation-induced mood deficits. GPR120 agonist pre-treatment attenuated NAc microglia reactivity and alleviated sickness-like behaviors elicited by central injection TNF-α and IL-1ß. These findings suggest that microglial GPR120 contributes to neuroimmune regulation and behavioral changes in response to acute infection and elevated brain cytokines. GPR120 may participate in the protective action of n-3 PUFAs at the neural and behavioral level and offers potential as treatment target for neuroinflammatory conditions.

Memory deficits in a juvenile rat model of type 1 diabetes are due to excess 11ß-HSD1 activity, which is upregulated by high glucose concentrations rather than insulin deficiency.

AIMS/HYPOTHESIS: Children with diabetes may display cognitive alterations although vascular disorders have not yet appeared. Variations in glucose levels together with relative insulin deficiency in treated type 1 diabetes have been reported to impact brain function indirectly through dysregulation of the hypothalamus-pituitary-adrenal axis. We have recently shown that enhancement of glucocorticoid levels in children with type 1 diabetes is dependent not only on glucocorticoid secretion but also on glucocorticoid tissue concentrations, which is linked to 11ß-hydroxysteroid dehydrogenase type 1 (11ß-HSD1) activity. Hypothalamus-pituitary-adrenal axis dysfunction and memory alteration were further dissected in a juvenile rat model of diabetes showing that excess 11ß-HSD1 activity within the hippocampus is associated with hippocampal-dependent memory deficits. Here, to investigate the causal relationships between diabetes, 11ß-HSD1 activity and hippocampus-dependent memory deficits, we evaluated the beneficial effect of 11ß-HSD1 inhibition on hippocampal-related memory in juvenile diabetic rats. We also examined whether diabetes-associated enhancement of hippocampal 11ß-HSD1 activity is due to an increase in brain glucose concentrations and/or a decrease in insulin signalling. METHODS: Diabetes was induced in juvenile rats by daily i.p. injection of streptozotocin for 2 consecutive days. Inhibition of 11ß-HSD1 was obtained by administrating the compound UE2316 twice daily by gavage for 3 weeks, after which hippocampal-dependent object location memory was assessed. Hippocampal 11ß-HSD1 activity was estimated by the ratio of corticosterone/dehydrocorticosterone measured by LC/MS. Regulation of 11ß-HSD1 activity in response to changes in glucose or insulin levels was determined ex vivo on acute brain hippocampal slices. The insulin regulation of 11ß-HSD1 was further examined in vivo using virally mediated knockdown of insulin receptor expression specifically in the hippocampus. RESULTS: Our data show that inhibiting 11ß-HSD1 activity prevents hippocampal-related memory deficits in diabetic juvenile rats. A significant increase (53.0±9.9%) in hippocampal 11ß-HSD1 activity was found in hippocampal slices incubated in high glucose conditions (13.9 mmol/l) vs normal glucose conditions (2.8 mmol/l) without insulin. However, 11ß-HSD1 activity was not affected by variations in insulin concentration either in the hippocampal slices or after a decrease in hippocampal insulin receptor expression. CONCLUSIONS/INTERPRETATION: Together, these data demonstrate that an increase in 11ß-HSD1 activity contributes to memory deficits observed in juvenile diabetic rats and that an excess of hippocampal 11ß-HSD1 activity stems from high glucose levels rather than insulin deficiency. 11ß-HSD1 might be a therapeutic target for treating cognitive impairments associated with diabetes.

Dynamic developmental changes in neurotransmitters supporting infant attachment learning.

Infant survival relies on rapid identification, remembering and behavioral responsiveness to caregivers' sensory cues. While neural circuits supporting infant attachment learning have largely remained elusive in children, use of invasive techniques has uncovered some of its features in rodents. During a 10-day sensitive period from birth, newborn rodents associate maternal odors with maternal pleasant or noxious thermo-tactile stimulation, which gives rise to a preference and approach behavior towards these odors, and blockade of avoidance learning. Here we review the neural circuitry supporting this neonatal odor learning, unique compared to adults, focusing specifically on the early roles of neurotransmitters such as glutamate, GABA (Gamma-AminoButyric Acid), serotonin, dopamine and norepinephrine, in the olfactory bulb, the anterior piriform cortex and amygdala. The review highlights the importance of deepening our knowledge of age-specific infant brain neurotransmitters and behavioral functioning that can be translated to improve the well-being of children during typical development and aid in treatment during atypical development in childhood clinical practice, and the care during rearing of domestic animals.

Pathway specific interventions reveal the multiple roles of ventral hippocampus projections in cognitive functions.

Since the 1950s study of Scoville and Milner on the case H.M., the hippocampus has attracted neuroscientists' attention. The hippocampus has been traditionally divided into dorsal and ventral parts, each of which projects to different brain structures and mediates various functions. Despite a predominant interest in its dorsal part in animal models, especially regarding episodic-like and spatial cognition, recent data highlight the role of the ventral hippocampus (vHPC), as the main hippocampal output, in cognitive processes. Here, we review recent studies conducted in rodents that have used advanced in vivo functional techniques to specifically monitor and manipulate vHPC efferent pathways and delineate the roles of these specific projections in learning and memory processes. Results highlight that vHPC projections to basal amygdala are implicated in emotional memory, to nucleus accumbens in social memory and instrumental actions and to prefrontal cortex in all the above as well as in object-based memory. Some of these hippocampal projections also modulate feeding and anxiety-like behaviours providing further evidence that the "one pathway-one function" view is outdated and future directions are proposed to better understand the role of hippocampal pathways and shed further light on its connectivity and function.

Hippocampal oxytocin is involved in spatial memory and synaptic plasticity deficits following acute high-fat diet intake in juvenile rats.

The hippocampus undergoes maturation during juvenility, a period of increased vulnerability to environmental challenges. We recently found that acute high-fat diet (HFD) impaired hippocampal long-term potentiation (LTP) and hippocampal-dependent spatial memory. We also recently reported that similar HFD exposure affected prefrontal plasticity and social memory through decreased oxytocin levels in the prefrontal cortex. In the present study, we therefore evaluated whether hippocampal oxytocin levels are also affected by juvenile HFD and could mediate deficits of hippocampal LTP and spatial memory. We found that postweaning HFD decreased oxytocin levels in the CA1 of the dorsal hippocampus. Interestingly, systemic injection of high, but not low, dose of oxytocin rescued HFD-induced LTP impairment in CA1. Moreover, deficits in long-term object location memory (OLM) were prevented by systemic injection of both high and low dose of oxytocin as well as by intra-CA1 infusion of oxytocin receptor agonist. Finally, we found that blocking oxytocin receptors in CA1 impaired long-term OLM in control-fed juvenile rats. These results suggest that acute HFD intake lowers oxytocin levels in the CA1 that lead to CA1 plasticity impairment and spatial memory deficits in juveniles. Further, these results provide the first evidence for the regulatory role of oxytocin in spatial memory.

Chemogenetic stimulation of adult neurogenesis, and not neonatal neurogenesis, is sufficient to improve long-term memory accuracy.

Hippocampal adult neurogenesis is involved in many memory processes from learning, to remembering and forgetting. However, whether or not the stimulation of adult neurogenesis is a sufficient condition to improve memory performance remains unclear. Here, we developed and validated, using ex-vivo electrophysiology, a chemogenetic approach that combines selective tagging and activation of discrete adult-born neuron populations. Then we demonstrated that, in rats, this activation can improve accuracy and strength of remote memory. These results show that stimulation of adult-born neuron activity can counteract the natural fading of memory traces that occurs with the passage of time. This opens up new avenues for treating memory problems that may arise over time.

Higher-order trace conditioning in newborn rabbits.

Temporal contingency is a key factor in associative learning but remains weakly investigated early in life. Few data suggest simultaneous presentation is required for young to associate different stimuli, whereas adults can learn them sequentially. Here, we investigated the ability of newborn rabbits to perform sensory preconditioning and second-order conditioning using trace intervals between odor presentations. Strikingly, pups are able to associate odor stimuli with 10- and 30-sec intervals in sensory preconditioning and second-order conditioning, respectively. The effectiveness of higher-order trace conditioning in newborn rabbits reveals that very young animals can display complex learning despite their relative immaturity.

Voluntary physical activity improves spatial and recognition memory deficits induced by post-weaning chronic exposure to a high-fat diet.

Childhood and adolescent exposure to obesogenic environments has contributed to the development of several health disorders, including neurocognitive impairment. Adolescence is a critical neurodevelopmental window highly influenced by environmental factors that affect brain function until adulthood. Post-weaning chronic exposure to a high-fat diet (HFD) adversely affects memory performance; physical activity is one approach to coping with these dysfunctions. Previous studies indicate that voluntary exercise prevents HFD's detrimental effects on memory; however, it remains to evaluate whether it has a remedial/therapeutical effect when introduced after a long-term HFD exposure. This study was conducted on a diet-induced obesity mice model over six months. After three months of HFD exposure (without interrupting the diet) access to voluntary physical activity was provided. HFD produced weight gain, increased adiposity, and impaired glucose tolerance. Voluntary physical exercise ameliorated glucose tolerance and halted weight gain and fat accumulation. Additionally, physical activity mitigated HFD-induced spatial and recognition memory impairments. Our data indicate that voluntary physical exercise starting after several months of periadolescent HFD exposure reverses metabolic and cognitive alterations demonstrating that voluntary exercise, in addition to its known preventive effect, also has a restorative impact on metabolism and cognition dysfunctions associated with obesity.

Differential Recruitment of the Infralimbic Cortex in Recent and Remote Retrieval and Extinction of Aversive Memory in Post-Weanling Rats.

BACKGROUND: We previously showed that the infralimbic medial prefrontal cortex (IL-mPFC) plays an important role in recent and remote memory retrieval and extinction of conditioned odor aversion (COA) and contextual fear conditioning (CFC) in adult rats. Because the mPFC undergoes maturation during post-weaning, here, we aimed to explore (1) whether post-weanling rats can form recent and remote COA and CFC memory, and (2) the role of the IL-mPFC in mediating these processes. METHODS: To investigate the retrieval process, we transiently inactivated the IL-mPFC with lidocaine prior to the retrieval test at either recent or remote time points. To target the consolidation process, we applied the protein synthesis inhibitor after the retrieval at recent or remote time points. RESULTS: Our results show that the post-weanling animals were able to develop both recent and remote memory of both COA and CFC. IL-mPFC manipulations had no effect on retrieval or extinction of recent and remote COA memory, suggesting that the IL has no effect in COA at this developmental stage. In contrast, the IL-mPFC played a role in (1) the extinction of recent, but not remote, CFC memory, and (2) the retrieval of remote, but not recent, CFC memory. Moreover, remote, but not recent, CFC retrieval enhanced c-Fos protein expression in the IL-mPFC. CONCLUSIONS: Altogether, these results point to a differential role of the IL-mPFC in recent and remote CFC memory retrieval and extinction and further confirm the differences in the role of IL-mPFC in these processes in post-weanling and adult animals.

N-3 PUFA deficiency disrupts oligodendrocyte maturation and myelin integrity during brain development.

Westernization of dietary habits has led to a progressive reduction in dietary intake of n-3 polyunsaturated fatty acids (n-3 PUFAs). Low maternal intake of n-3 PUFAs has been linked to neurodevelopmental disorders, conditions in which myelination processes are abnormal, leading to defects in brain functional connectivity. Only little is known about the role of n-3 PUFAs in oligodendrocyte physiology and white matter development. Here, we show that lifelong n-3 PUFA deficiency disrupts oligodendrocytes maturation and myelination processes during the postnatal period in mice. This has long-term deleterious consequences on white matter organization and hippocampus-prefrontal functional connectivity in adults, associated with cognitive and emotional disorders. Promoting developmental myelination with clemastine, a first-generation histamine antagonist and enhancer of oligodendrocyte precursor cell differentiation, rescues memory deficits in n-3 PUFA deficient animals. Our findings identify a novel mechanism through which n-3 PUFA deficiency alters brain functions by disrupting oligodendrocyte maturation and brain myelination during the neurodevelopmental period.

Neural Substrates of Incidental Associations and Mediated Learning: The Role of Cannabinoid Receptors.

The ability to form associations between different stimuli in the environment to guide adaptive behavior is a central element of learning processes, from perceptual learning in humans to Pavlovian conditioning in animals. Like so, classical conditioning paradigms that test direct associations between low salience sensory stimuli and high salience motivational reinforcers are extremely informative. However, a large part of everyday learning cannot be solely explained by direct conditioning mechanisms - this includes to a great extent associations between individual sensory stimuli, carrying low or null immediate motivational value. This type of associative learning is often described as incidental learning and can be captured in animal models through sensory preconditioning procedures. Here we summarize the evolution of research on incidental and mediated learning, overview the brain systems involved and describe evidence for the role of cannabinoid receptors in such higher-order learning tasks. This evidence favors a number of contemporary hypotheses concerning the participation of the endocannabinoid system in psychosis and psychotic experiences and provides a conceptual framework for understanding how the use of cannabinoid drugs can lead to altered perceptive states.

Adult-born neurons immature during learning are necessary for remote memory reconsolidation in rats.

Memory reconsolidation, the process by which memories are again stabilized after being reactivated, has strengthened the idea that memory stabilization is a highly plastic process. To date, the molecular and cellular bases of reconsolidation have been extensively investigated particularly within the hippocampus. However, the role of adult neurogenesis in memory reconsolidation is unclear. Here, we combined functional imaging, retroviral and chemogenetic approaches in rats to tag and manipulate different populations of rat adult-born neurons. We find that both mature and immature adult-born neurons are activated by remote memory retrieval. However, only specific silencing of the adult-born neurons immature during learning impairs remote memory retrieval-induced reconsolidation. Hence, our findings show that adult-born neurons immature during learning are required for the maintenance and update of remote memory reconsolidation.

Chemogenetic silencing of hippocampus and amygdala reveals a double dissociation in periadolescent obesogenic diet-induced memory alterations.

In addition to numerous metabolic comorbidities, obesity is associated with several adverse neurobiological outcomes, especially learning and memory alterations. Obesity prevalence is rising dramatically in youth and is persisting in adulthood. This is especially worrying since adolescence is a crucial period for the maturation of certain brain regions playing a central role in memory processes such as the hippocampus and the amygdala. We previously showed that periadolescent, but not adult, exposure to obesogenic high-fat diet (HFD) had opposite effects on hippocampus- and amygdala-dependent memory, impairing the former and enhancing the latter. However, the causal role of these two brain regions in periadolescent HFD-induced memory alterations remains unclear. Here, we first showed that periadolescent HFD induced long-term, but not short-term, object recognition memory deficits, specifically when rats were exposed to a novel context. Using chemogenetic approaches to inhibit targeted brain regions, we then demonstrated that recognition memory deficits are dependent on the activity of the ventral hippocampus, but not the basolateral amygdala. On the contrary, the HFD- induced enhancement of conditioned odor aversion specifically requires amygdala activity. Taken together, these findings suggest that HFD consumption throughout adolescence impairs long-term object recognition memory through alterations of ventral hippocampal activity during memory acquisition. Moreover, these results further highlight the bidirectional effects of adolescent HFD on hippocampal and amygdala functions.

Dietary vitamin A supplementation prevents early obesogenic diet-induced microbiota, neuronal and cognitive alterations.

BACKGROUND: Early consumption of obesogenic diets, rich in saturated fat and added sugar, is associated with a plethora of biological dysfunctions, at both peripheral and brain levels. Obesity is also linked to decreased vitamin A bioavailability, an essential molecule for brain plasticity and memory function. METHODS: Here we investigated in mice whether dietary vitamin A supplementation (VAS) could prevent some of the metabolic, microbiota, neuronal and cognitive alterations induced by obesogenic, high-fat and high-sugar diet (HFSD) exposure from weaning to adulthood, i.e. covering periadolescent period. RESULTS: As expected, VAS was effective in enhancing peripheral vitamin A levels as well as hippocampal retinoic acid levels, the active metabolite of vitamin A, regardless of the diet. VAS attenuated HFSD-induced excessive weight gain, without affecting metabolic changes, and prevented alterations of gut microbiota α-diversity. In HFSD-fed mice, VAS prevented recognition memory deficits but had no effect on aversive memory enhancement. Interestingly, VAS alleviated both HFSD-induced higher neuronal activation and lower glucocorticoid receptor phosphorylation in the hippocampus after training. CONCLUSION: Dietary VAS was protective against the deleterious effects of early obesogenic diet consumption on hippocampal function, possibly through modulation of the gut-brain axis.

Bidirectional modulation of hippocampal and amygdala synaptic plasticity by post-weaning obesogenic diet intake in male rats: Influence of the duration of diet exposure.

Obesity is a chronic condition associated with adverse memory and emotional outcomes in humans and animal models. We have recently demonstrated that post-weaning (i.e., periadolescent) high-fat diet (HFD)-induced obesity has opposite effect on hippocampal and amygdala-dependent memory in rodents: while HFD consumption impairs spatial and relational memory, it enhances cue-dependent emotional memory. However, it is still not clear whether this bidirectional HFD effect on memory is related to bidirectional alterations of hippocampal and amygdala synaptic plasticity and if it is influenced by the duration of diet intake. In the current study, we compared in male rats the impact of 2-3 and 6-7 months of HFD intake starting at weaning, thus covering adolescence, on in vivo long-term potentiation (LTP) recorded simultaneously in the hippocampal area CA1 and the basolateral amygdala (BLA). As expected, 6-7 months of HFD intake abolished LTP in the CA1 and enhanced LTP in the BLA. However, 2-3 months of of HFD exposure enhanced LTP in both CA1 and BLA suggesting a transient compensatory mechanism in hippocampus. These results indicate that post-weaning HFD intake progressively leads to bidirectional modulation of hippocampal and amygdala synaptic plasticity, as we previously demonstrated for related memory processes, yet with a different temporal dynamic.