Retrieval Of Contextual Fear Memory Research Articles

Memory encoding and retrieval by retrosplenial parvalbumin interneurons are impaired in Alzheimer’s disease model mice

Memory deficits in Alzheimer's disease (AD) show a strong link with GABAergic interneuron dysfunctions.1,2,3,4,5,6,7 The ensemble dynamics of GABAergic interneurons represent memory encoding and retrieval,8,9,10,11,12 but how GABAergic interneuron dysfunction affects inhibitory ensemble dynamics in AD is unknown. As the retrosplenial cortex (RSC) is critical for episodic memory13,14,15,16 and is affected by β-amyloid accumulation in early AD,17,18,19,20,21 we address this question by performing Ca2+ imaging in RSC parvalbumin (PV)-expressing interneurons during a contextual fear memory task in healthy control mice and the 5XFAD mouse model of AD. We found that populations of PV interneurons responsive to aversive electric foot shocks during contextual fear conditioning (shock-responsive) significantly decreased in the 5XFAD mice, indicating dysfunctions in the recruitment of memory-encoding PV interneurons. In the control mice, ensemble activities of shock-responsive PV interneurons were selectively upregulated during the freezing epoch of the contextual fear memory retrieval, manifested by synaptic potentiation of PV interneuron-mediated inhibition. However, such changes in ensemble dynamics during memory retrieval and synaptic plasticity were both absent in the 5XFAD mice. Optogenetic silencing of PV interneurons during contextual fear conditioning in the control mice mimicked the memory deficits in the 5XFAD mice, while optogenetic activation of PV interneurons in the 5XFAD mice restored memory retrieval. These results demonstrate the critical roles of contextual fear memory-encoding PV interneurons for memory retrieval. Furthermore, synaptic dysfunction of PV interneurons may disrupt the recruitment of PV interneurons and their ensemble dynamics underlying contextual fear memory retrieval, subsequently leading to memory deficits in AD.

A thalamic-hippocampal CA1 signal for contextual fear memory suppression, extinction, and discrimination

The adaptive regulation of fear memories is a crucial neural function that prevents inappropriate fear expression. Fear memories can be acquired through contextual fear conditioning (CFC) which relies on the hippocampus. The thalamic nucleus reuniens (NR) is necessary to extinguish contextual fear and innervates hippocampal CA1. However, the role of the NR-CA1 pathway in contextual fear is unknown. We developed a head-restrained virtual reality CFC paradigm, and demonstrate that mice can acquire and extinguish context-dependent fear responses. We found that inhibiting the NR-CA1 pathway following CFC lengthens the duration of fearful freezing epochs, increases fear generalization, and delays fear extinction. Using in vivo imaging, we recorded NR-axons innervating CA1 and found that NR-axons become tuned to fearful freezing following CFC. We conclude that the NR-CA1 pathway actively suppresses fear by disrupting contextual fear memory retrieval in CA1 during fearful freezing behavior, a process that also reduces fear generalization and accelerates extinction.

Postsynaptic histamine H3 receptors in ventral basal forebrain cholinergic neurons modulate contextual fear memory

Overly strong fear memories can cause pathological conditions. Histamine H3 receptor (H3R) has been viewed as an optimal drug target for CNS disorders, but its role in fear memory remains elusive. We find that a selective deficit of H3R in cholinergic neurons, but not in glutamatergic neurons, enhances freezing level during contextual fear memory retrieval without affecting cued memory. Consistently, genetically knocking down H3R or chemogenetically activating cholinergic neurons in the ventral basal forebrain (vBF) mimics this enhanced fear memory, whereas the freezing augmentation is rescued by re-expressing H3R or chemogenetic inhibition of vBF cholinergic neurons. Spatiotemporal regulation of H3R by a light-sensitive rhodopsin-H3R fusion protein suggests that postsynaptic H3Rs in vBF cholinergic neurons, but not presynaptic H3Rs of cholinergic projections in the dorsal hippocampus, are responsible for modulating contextual fear memory. Therefore, precise modulation of H3R in a cell-type- and subcellular-location-specific manner should be explored for pathological fear memory.

Consolidation, reconsolidation, and extinction of contextual fear memory depend on de novo protein synthesis in the locus coeruleus

Memory consolidation is the process underlying the stabilization of labile short-term memory and the generation of long-term memory for persistent memory storage. The retrieval of contextual fear memory induces two distinct and opposite memory processes: reconsolidation and extinction. Reconsolidation re-stabilizes retrieved memory for re-storage, whereas memory extinction weakens fear memory and generates a new inhibitory memory. Importantly, the requirement for new gene expression is a critical biochemical feature of the consolidation, reconsolidation, and long-term extinction of memory. The locus coeruleus (LC) is a small nucleus in the brain stem that is composed predominantly of noradrenergic neurons that project to many brain regions. Recent studies have shown that the LC plays modulatory roles in the consolidation and extinction of auditory fear memory through its projections to brain regions contributing to memory storage. Here, we show that the LC is required for the consolidation, reconsolidation, and long-term extinction of contextual fear memory. We first observed that c-fos expression was induced in the LC following contextual fear conditioning to induce consolidation and following short and long re-exposure to the conditioning context to induce reconsolidation and long-term extinction, respectively. More importantly, inhibition of protein synthesis in the LC by a micro-infusion of anisomycin blocked the consolidation, reconsolidation, and long-term extinction of contextual fear memory. Our findings suggest that consolidation, reconsolidation, and long-term extinction occur in the LC and that the LC plays an essential role in memory storage and maintenance.

Influence of Sex and Muscarinic Activity on Memory Retrieval in Mouse Model of Traumatic Brain Injury.

Traumatic brain injury (TBI) is a serious global risk factor leading to the onset of cognitive impairment and neurodegenerative diseases. Cognitive and memory impairment following a TBI is associated with the dysregulation of cholinergic neurotransmission in the brains of subjects. The extent of memory impairment following a TBI is linked with the sex of the subject. This study aimed to identify the sex-dimorphic role of muscarinic cholinergic modulation in neurological functioning and episodic memory retrieval in a mouse model of TBI. Balb/c mice were divided into four groups of males and four groups of females (i.e., Sham, TBI, TBI + Scopolamine 1 mg/kg, and TBI + Donepezil 1 mg/kg). After training with the Morris water maze test and fear conditioning, all groups were subjected to brain injury (7.84 × 10-5 J impact force) except for the Sham mice. Following brain injury, scopolamine or donepezil was administered to the respective groups for 5 days. Acute scopolamine immediately after brain trauma showed a neuroprotective effect in the males only, while subchronic donepezil significantly impaired neurological functioning in both sexes. Subchronic scopolamine and donepezil treatment reversed the TBI-induced retrograde amnesia for spatial memory in male mice. Contextual fear memory retrieval was not affected by the TBI and treatments in both sexes. Thus, we concluded that the sex-dimorphic response of the muscarinic receptors in TBI-induced memory impairment depends on the type of memory. This study highlights the potential for therapeutic modalities in TBI subjects.

TRPV1 modulation of contextual fear memory depends on stimulus intensity and endocannabinoid signalling in the dorsal hippocampus

The transient receptor potential vanilloid type-1 (TRPV1) channels have been implicated in the modulation of aversive responses. The endocannabinoid anandamide acts as an endogenous TRPV1 agonist, exerting opposite functions at TRPV1 and type-1 cannabinoid receptors (CB1R). Here we tested the hypothesis that hippocampal TRPV1 modulates contextual fear memory retrieval and investigated the influence of the aversive stimulus intensity as well as the role of endocannabinoid signaling. Male C57BL/6J mice were tested for contextual fear memory after low-, moderate-, or high-intensity shock protocols. The selective TRPV1 blockers SB366791 (1-10 nmol) and 6-I-NC (2 nmol) were infused via intra-dorsal hippocampus before the retrieval test session. The local levels of endocannabinoids and Arc and Zif268 mRNAs, involved in synaptic plasticity and memory, were quantified. First, both TRPV1 blockers reduced memory retrieval in animals exposed to moderate or high (but not low) intensity training protocols. In the second series of results, the magnitude of the freezing responses positively correlated with the hippocampal anandamide levels; TRPV1 and CB1R were found co-localized in this brain region; and the CB1R antagonist, AM251, prevented the effects of SB366791. Thus, endocannabinoid signaling possibly mediates the effects of TRPV1 blockers. Finally, inhibition of memory retrieval by TRPV1 blockers increased Arc and Zif268 mRNAs and impaired fear memory reinstatement. In conclusion, the modulation of fear memories by dorsal hippocampal TRPV1 channels may depend on the aversive stimulus intensity and occur via anandamide/CB1 signaling. Moreover, TRPV1 blockers promote Arc and Zif268 transcription, with subsequent attenuation of aversive memory reinstatement.

Myt1l haploinsufficiency leads to obesity and multifaceted behavioral alterations in mice

BackgroundThe zinc finger domain containing transcription factor Myt1l is tightly associated with neuronal identity and is the only transcription factor known that is both neuron-specific and expressed in all neuronal subtypes. We identified Myt1l as a powerful reprogramming factor that, in combination with the proneural bHLH factor Ascl1, could induce neuronal fate in fibroblasts. Molecularly, we found it to repress many non-neuronal gene programs, explaining its supportive role to induce and safeguard neuronal identity in combination with proneural bHLH transcriptional activators. Moreover, human genetics studies found MYT1L mutations to cause intellectual disability and autism spectrum disorder often coupled with obesity.MethodsHere, we generated and characterized Myt1l-deficient mice. A comprehensive, longitudinal behavioral phenotyping approach was applied.ResultsMyt1l was necessary for survival beyond 24 h but not for overall histological brain organization. Myt1l heterozygous mice became increasingly overweight and exhibited multifaceted behavioral alterations. In mouse pups, Myt1l haploinsufficiency caused mild alterations in early socio-affective communication through ultrasonic vocalizations. In adulthood, Myt1l heterozygous mice displayed hyperactivity due to impaired habituation learning. Motor performance was reduced in Myt1l heterozygous mice despite intact motor learning, possibly due to muscular hypotonia. While anxiety-related behavior was reduced, acoustic startle reactivity was enhanced, in line with higher sensitivity to loud sound. Finally, Myt1l haploinsufficiency had a negative impact on contextual fear memory retrieval, while cued fear memory retrieval appeared to be intact.LimitationsIn future studies, additional phenotypes might be identified and a detailed characterization of direct reciprocal social interaction behavior might help to reveal effects of Myt1l haploinsufficiency on social behavior in juvenile and adult mice.ConclusionsBehavioral alterations in Myt1l haploinsufficient mice recapitulate several clinical phenotypes observed in humans carrying heterozygous MYT1L mutations and thus serve as an informative model of the human MYT1L syndrome.

Distinct Contribution of Granular and Agranular Subdivisions of the Retrosplenial Cortex to Remote Contextual Fear Memory Retrieval.

The retrieval of recent and remote memories are thought to rely on distinct brain circuits and mechanisms. The retrosplenial cortex (RSC) is robustly activated during the retrieval of remotely acquired contextual fear memories (CFMs), but the contribution of particular subdivisions [granular (RSG) vs agranular retrosplenial area (RSA)] and the circuit mechanisms through which they interact to retrieve remote memories remain unexplored. In this study, using both anterograde and retrograde viral tracing approaches, we identified excitatory projections from layer 5 pyramidal neurons of the RSG to the CA1 stratum radiatum/lacunosum-moleculare of the dorsal hippocampus and the superficial layers of the RSA in male mice. We found that chemogenetic or optogenetic inhibition of the RSG-to-CA1, but not the RSG-to-RSA, pathway selectively impairs the retrieval of remote CFMs. Collectively, our results uncover a specific role for the RSG in remote CFM recall and provide circuit evidence that RSG-mediated remote CFM retrieval relies on direct RSG-to-CA1 connectivity. The present study provides a better understanding of brain circuit mechanisms underlying the retrieval of remote CFMs and may help guide the development of therapeutic strategies to attenuate remote traumatic memories that lead to mental health issues such as post-traumatic stress disorder.SIGNIFICANCE STATEMENT The RSC is implicated in contextual information processing and remote recall. However, how different subdivisions of the RSC and circuit mechanisms through which they interact to underlie remote memory recall remain unexplored. This study shows that granular subdivision of the RSC and its input to hippocampal area CA1 contributes to the retrieval of remote contextual fear memories. Our results support the hypothesis that the RSC and hippocampus require each other to preserve fear memories and may provide a novel therapeutic avenue to attenuate remote traumatic memories in patients with post-traumatic stress disorder.

High Salt Intake Increases Contextual Fear Retrieval and Synaptic Inhibition of Medial Entorhinal Cortex‐Projecting Neurons of the Basolateral Amygdala

Fear-related neuropsychiatric disorders are becoming increasingly prevalent. Although the impact of psychological stress on fear disorders is well studied, less attention has been paid to the impact of physiological stressors. High salt (NaCl) intake is a common physiological stressor, well known to increase the prevalence and severity of cardiovascular disease (CVD), but effects on fear-related mental health disorders, which are often co-morbid with CVD, have only recently been investigated. Our lab reported that mice consuming excess salt exhibit anxiety-like active coping behaviors and heightened basolateral amygdala (BLA) neuronal activation following swim stress. BLA principal neurons (PN) are critical in processing fear memory and their projections to medial entorhinal cortex (BLA-mEC) are implicated specifically in contextual fear memory. Thus, we formulated and tested the hypothesis that high salt intake increases contextual fear memory retrieval linked to hyper-activation of BLA PNs following retrieval testing. Because BLA PNs are normally under strong control by local interneurons (INs), we further hypothesized that high salt intake reduces ongoing synaptic inhibition of BLA-mEC PNs. To increase salt intake, normal drinking water was replaced with a solution of 4% NaCl for 7 consecutive days. Immediately thereafter, contextual fear conditioning was performed and 24 h later mice were re-exposed to the same context for memory retrieval with fear quantified as percentage of time spent posturally immobile (i.e., freezing behavior). Ninety min after fear retrieval testing, mice underwent perfusion fixation and Fos immunostaining of BLA. In a separate treatment group, mice received rhodamine retrobead injections in the mEC to identify BLA-mEC neurons in brain slices prepared 3-5 days later. Results indicate that high salt intake (HSI) increased fear memory retrieval compared to normal salt intake (NSI) (HSI: 69 ± 5% (n=9), NSI: 44 ± 6% (n=9), unpaired t-test P=0.0038) without changing the mean intensity of Fos immunofluorescence in BLA (HSI: 220 ± 20 F/F0 (n=6), NSI: 240 ± 15 F/FBKGD (n=6), unpaired t-test P=0.4412). Compared to NSI, HSI increased the amplitude (HSI: 32 ± 0.6 pA (n=7), NSI: 22 ± 0.4 pA (n=9), unpaired t-test P<0.0001) and frequency (HSI: 5.3 ± 1.4 Hz (n=7), NSI: 2.5 ± 0.8 Hz (n=9), unpaired t-test P=0.0415) of spontaneous inhibitory postsynaptic currents recorded from BLA-mEC neurons. Thus, HSI enhances retrieval of contextual fear memory despite an apparent increase of inhibitory synaptic control of BLA-mEC PNs and lack of evidence of greater overall BLA neuronal transcriptional activation. Whether greater synaptic inhibition is maintained in vivo to buffer HSI-induced excitatory drive remains to be determined. Presently, our findings are consistent either with HSI acting downstream of BLA to enhance contextual fear memory retrieval or that Fos immunoreactivity does not accurately index strengthening of excitatory input to BLA caused by HSI. Whichever the case might be, findings warrant further investigation into the underlying mechanisms of HSI-induced enhancement of fear memory retrieval.

Hypertension and Fear‐Related Neuropsychiatric Disorder Co‐Morbidity: Is Body Fluid Hyperosmolality a Required Link?

High salt (NaCl) intake (HSI) is a physiological stressor that can increase the prevalence and severity of arterial hypertension (HTN), which is often comorbid with fear-related psychiatric disorders. Here, we sought to determine the contribution of HSI in HTN and fear disorder co-morbidity. Preliminary studies in male C57Bl6 mice established that salt loading (SL) introduced by replacing drinking water with a 4% NaCl solution induced significant HTN (+ 21 ± 5 mmHg, n=3) (P=0.0437) while consumption of high salt diet (HSD) by provision of chow containing 4% NaCl did not (+2 ± 5 mmHg, n=3) (P=0.06042). We formulated the hypothesis that HSI will enhance fear memory retrieval when accompanied by HTN but will not do so when HSI fails to raise mean arterial pressure (MAP). To test this, we subjected male C57Bl6 mice to 5 weeks of HSD or a normal salt diet (NSD, 0.4% NaCl). Treated mice then underwent contextual fear conditioning and retrieval testing 24 h later. Fear behavior was quantified as the percentage of time spent freezing (i.e., posturally immobile). A third cohort of HSD mice underwent surgical implantation of an osmotic mini-pump to deliver the pro-hypertensive hormone angiotensin II (Ang II, 600 ng/kg/min) or vehicle (Veh) for 14 d prior to fear conditioning. A subset of mice were instrumented with a radio telemetry transmitter to monitor MAP during AngII/Veh treatment. Blood was collected upon completion of experiments to measure serum osmolality. Analysis by unpaired t-test reveal that MAP was significantly increased by treatment with AngII (+28 ± 3 mmHg, n=5) compared to Veh (-1 ± 1 mmHg, n=2) (P=0.0014). During contextual fear memory retrieval, freezing behavior was not different between normotensive HSD mice and normotensive NSD mice (HSD: 45± 6%, n=10, NSD: 44± 7%. n=9) (P=0.9194). Furthermore, treatment of HSD mice with AngII did not significantly increase freezing behavior compared to Veh (AngII: 58 ± 3%, n=9, Veh: 62 ± 16%, n=3) (P=0.7273). These results suggest that unlike SL, which induces HTN and promotes contextual fear memory, HSD does not, even when HTN is induced by co-treatment with AngII. Overall, findings suggest that SL promotes HTN and fear memory through parallel, independent mechanisms, and that consumption of high salt-containing food does not increase fear memory regardless of whether accompanied by HTN or not. Published evidence indicates that unlike HSD alone, or in combination with AngII, SL significantly increases body fluid osmolality. This suggests that body fluid hyperosmolality, rather than salt intake per se, contributes to HTN and fear-related disorder co-morbidity. Consistent with this possibility, serum osmolality in the present study was unaltered relative to NSD (316 ± 1 mOsm, n=27) both by HSD alone (313 ± 2 mOsm, n=19) (P=0.1401) and by HSD with AngII (320 ± 2 mOsm, n=10) (P=0.0801).

An inhibitory hippocampal-thalamic pathway modulates remote memory retrieval.

Memories are supported by distributed hippocampal-thalamic-cortical networks, but the brain regions that contribute to network activity may vary with memory age. This process of reorganization is referred to as systems consolidation, and previous studies have examined the relationship between the activation of different hippocampal, thalamic, and cortical brain regions and memory age at the time of recall. While the activation of some brain regions increases with memory age, other regions become less active. In mice, here we show that the active disengagement of one such brain region, the anterodorsal thalamic nucleus, is necessary for recall at remote time-points and, in addition, which projection(s) mediate such inhibition. Specifically, we identified a sparse inhibitory projection from CA3 to the anterodorsal thalamic nucleus that becomes more active during systems consolidation, such that it is necessary for contextual fear memory retrieval at remote, but not recent, time-points post-learning.

Tumor necrosis factor α negatively regulates the retrieval and reconsolidation of hippocampus-dependent memory

Neural inflammation is associated with cognitive decline, especially learning and memory. Tumor necrosis factor α (TNFα) is a major cytokine generated during neuroinflammation. Previous studies indicated that TNFα impairs hippocampus-dependent memory including contextual fear and spatial memories. However, it is unknown which memory processes are impaired by TNFα. Here, we show that TNFα blocked the retrieval and reconsolidation of contextual fear and spatial memories. Micro-infusion of TNFα into the dorsal hippocampus at 6–18 h before retrieval impaired the retrieval of contextual fear memory, although micro-infusion before contextual fear conditioning had no effect on memory formation. Interestingly, hippocampal TNFα micro-infusion before memory retrieval decreased freezing responses, even at 24 h after retrieval, suggesting that TNFα impairs the reconsolidation of contextual fear memory. Similarly, hippocampal TNFα micro-infusion impaired the retrieval and reconsolidation of spatial memory in the Morris water maze. Consistent with these observations, hippocampal TNFα micro-infusion before retrieval blocked the induction of c-fos expression in the hippocampus, which is a marker of neural activation, in response to the retrieval of contextual fear memory. Collectively, our findings indicate that TNFα negatively regulates the retrieval and reconsolidation of hippocampus-dependent memory.

A coral‐derived compound suppresses hippocampal inflammation and improved memory retrieval in triple transgenic Alzheimer’s mice

AbstractBackgroundThe relationship between neuronal inflammation and impairment of memory retrieval is not well established, partially because of the lack of appropriate compound to test this hypothesis. Here we investigated effects of a novel coral‐derived compound on hippocampal inflammation markers and contextual fear memory retrieval using a triple‐transgenic AD (3xTg‐AD) mouse modelMethodThe coral‐derived compound was administered to mice with intraperitoneal injection before subjected to trace fear conditioning. Expression of inflammation markers were measured by western blot analysis or flourescent immunihistochemijcal staining.ResultResults indicate that coral‐derived compound is effective on reducing expression levels of inflammatory markers including TNF‐α, COX‐2, and iNOS in the hippocamapal region, and increased expression of postsynaptic density protein‐95 (PSD‐95) in the hippocampus of 3xTg‐AD mice. It also increased dendritic spine density in hippocampus. Phenotypically, this compound enhanced hippocampadependent retrieval of contextual memory after trace fear conditioning.ConclusionOur results suggest the strong linkage between hippocampal inflammation and memory retrieval in AD, and that the coral‐derived compound may serve as a drug leading for a hippocampal anti‐inflammatory treatment to improve memory retrieval in AD patients.

DNA Double-Strand Breaks Are a Critical Regulator of Fear Memory Reconsolidation.

Numerous studies have shown that following retrieval, a previously consolidated memory requires increased transcriptional regulation in order to be reconsolidated. Previously, it was reported that histone H3 lysine-4 trimethylation (H3K4me3), a marker of active transcription, is increased in the hippocampus after the retrieval of contextual fear memory. However, it is currently unknown how this epigenetic mark is regulated during the reconsolidation process. Furthermore, though recent evidence suggests that neuronal activity triggers DNA double-strand breaks (DSBs) in some early-response genes, it is currently unknown if DSBs contribute to the reconsolidation of a memory following retrieval. Here, using chromatin immunoprecipitation (ChIP) analyses, we report a significant overlap between DSBs and H3K4me3 in area CA1 of the hippocampus during the reconsolidation process. We found an increase in phosphorylation of histone H2A.X at serine 139 (H2A.XpS139), a marker of DSB, in the Npas4, but not c-fos, promoter region 5 min after retrieval, which correlated with increased H3K4me3 levels, suggesting that the two epigenetic marks may work in concert during the reconsolidation process. Consistent with this, in vivo siRNA-mediated knockdown of topoisomerase II β, the enzyme responsible for DSB, prior to retrieval, reduced Npas4 promoter-specific H2A.XpS139 and H3K4me3 levels and impaired long-term memory, indicating an indispensable role of DSBs in the memory reconsolidation process. Collectively, our data propose a novel mechanism for memory reconsolidation through increases in epigenetic-mediated transcriptional control via DNA double-strand breaks.

Contextual fear memory retrieval by correlated ensembles of ventral CA1 neurons

Ventral hippocampal CA1 (vCA1) projections to the amygdala are necessary for contextual fear memory. Here we used in vivo Ca2+ imaging in mice to assess the temporal dynamics by which ensembles of vCA1 neurons mediate encoding and retrieval of contextual fear memories. We found that a subset of vCA1 neurons were responsive to the aversive shock during context conditioning, their activity was necessary for memory encoding, and these shock-responsive neurons were enriched in the vCA1 projection to the amygdala. During memory retrieval, a population of vCA1 neurons became correlated with shock-encoding neurons, and the magnitude of synchronized activity within this population was proportional to memory strength. The emergence of these correlated networks was disrupted by inhibiting vCA1 shock responses during memory encoding. Thus, our findings suggest that networks of cells that become correlated with shock-responsive neurons in vCA1 are essential components of contextual fear memory ensembles.

Contextual processing elicits sex differences in dorsal hippocampus activation following footshock and context fear retrieval

Establishing a contextual representation of an environment places specific spatial-temporal processing demands on the mammalian hippocampus, a region showing sex-differences in processing capabilities. However, evidence for sex differences in these processing demands during contextual fear learning remains limited. Here, we examined the relationship among contextual processing, timing of footshock, and activation of the dorsal hippocampus and basolateral amygdalar nuclei (BLA) in male and female mice (C57Bl/6 J). We modified the initial exposure time to the conditioning context prior to administration, or not, of a single footshock. We then quantified Fos- ir neurons activated by acquisition or retrieval of contextual fear memories in the rostral half of the dorsal CA1 (proximal – distal regions), CA3, Dentate Gyrus and basolateral amygdalar nuclei corresponding to atlas levels of the Allen Reference Atlas. In experiment 1, we found that sex differences in context elicited freezing were evident at the longest context placement-to-shock interval and that context fear retrieval with increasing contextual exposure periods increased CA1 Fos-ir in males, but not females. In experiment 2, we observed that an aversive footshock in males potentiated CA1 activation, while it downregulated CA1 activation in females. We also found that an aversive footshock independent of sex moderated Dentate Gyrus activation that normally showed increased activation with greater context exposure periods. Lastly, we identified a heightened responsiveness in BLA neurons to both footshock and length of context exposure in females compared to males. Overall, our findings suggest that sex differences in contextual fear conditioning may arise from marked sex differences in the contextual processing demands of the dorsal hippocampus subfields largely modulated by aversive outcomes.

Associative and plastic thalamic signaling to the lateral amygdala controls fear behavior.

Decades of research support the idea that associations between a conditioned stimulus (CS) and an unconditioned stimulus (US) are encoded in the lateral amygdala (LA) during fear learning. However, direct proof for the sources of CS and US information is lacking. Definitive evidence of the LA as the primary site for cue association is also missing. Here, we show that calretinin (Calr)-expressing neurons of the lateral thalamus (Calr+LT neurons) convey the association of fast CS (tone) and US (foot shock) signals upstream from the LA in mice. Calr+LT input shapes a short-latency sensory-evoked activation pattern of the amygdala via both feedforward excitation and inhibition. Optogenetic silencing of Calr+LT input to the LA prevents auditory fear conditioning. Notably, fear conditioning drives plasticity in Calr+LT neurons, which is required for appropriate cue and contextual fear memory retrieval. Collectively, our results demonstrate that Calr+LT neurons provide integrated CS-US representations to the LA that support the formation of aversive memories.

Encoding of contextual fear memory in hippocampal\u2013amygdala circuit

In contextual fear conditioning, experimental subjects learn to associate a neutral context with an aversive stimulus and display fear responses to a context that predicts danger. Although the hippocampal–amygdala pathway has been implicated in the retrieval of contextual fear memory, the mechanism by which fear memory is encoded in this circuit has not been investigated. Here, we show that activity in the ventral CA1 (vCA1) hippocampal projections to the basal amygdala (BA), paired with aversive stimuli, contributes to encoding conditioned fear memory. Contextual fear conditioning induced selective strengthening of a subset of vCA1–BA synapses, which was prevented under anisomycin-induced retrograde amnesia. Moreover, a subpopulation of BA neurons receives stronger monosynaptic inputs from context-responding vCA1 neurons, whose activity was required for contextual fear learning and synaptic potentiation in the vCA1–BA pathway. Our study suggests that synaptic strengthening of vCA1 inputs conveying contextual information to a subset of BA neurons contributes to encoding adaptive fear memory for the threat-predictive context.

Inhibition of steroid receptor coactivator-1 in the hippocampus impairs the consolidation and reconsolidation of contextual fear memory in mice

AimsSteroid receptor coactivator-1 (SRC-1) is a key coactivator for the efficient transcriptional activity of steroids in the regulation of hippocampal functions. However, the effect of SRC-1 on hippocampal memory processes remains unknown. Our aim was to investigate the roles of hippocampal SRC-1 in the consolidation and reconsolidation of contextual fear memory in mice. Main methodsContextual fear conditioning paradigm was constructed in adult male C57BL/6 mice to examine the fear learning and memory processes. Adeno-associated virus (AAV) vector-mediated RNA interference (RNAi) was infused into hippocampus to block hippocampal SRC-1 level. Immunofluorescent staining was used to detect the efficiency of transfection. High plus maze and open field test were used to determine anxiety and locomotor activity. Western blot analyses were used to detect the expression of SRC-1 and synaptic proteins in the hippocampus. Key findingsWe first showed that the expression of SRC-1 was regulated by fear conditioning training in a time-dependent manner, and knockdown of SRC-1 impaired contextual fear memory consolidation without affecting innate anxiety or locomotor activity. In addition, hippocampal SRC-1 was also regulated by the retrieval of contextual fear memory, and downregulation of SRC-1 disrupted fear memory reconsolidation. Moreover, knockdown of SRC-1 reversed the increased GluR1 and PSD-95 levels induced by contextual fear memory retrieval. SignificanceOur data indicate that hippocampal SRC-1 is required for the consolidation and reconsolidation of contextual fear memory, and SRC-1 may be a potential therapeutic target for mental disorders that are involved in hippocampal memory dysfunction.

Limited replicability of drug-induced amnesia after contextual fear memory retrieval in rats.

With the ultimate goal of investigating boundary conditions for post-reactivation amnesia, we set out to replicate studies in which systemic, post-reactivation administration of midazolam, propranolol, or cycloheximide resulted in amnesia for contextual fear memories. Our experiments involved conceptual as well as exact replications of previously published studies. In most of our experiments, we adopted a procedure that conformed to the standard 3-day protocol typically used in the literature, with contextual fear conditioning on day 1, unreinforced re-exposure to the conditioning context followed by systemic injection of the amnestic drug on day 2, and a memory retention test on day 3. Given the plethora of successful studies with large effects sizes and the absence of any failed replications in the literature, we were surprised to find that we were generally unable to replicate those findings. Our results suggest that post-reactivation amnesia by systemic drug administration in rats is more difficult to obtain than what would be expected based on published empirical reports. At present, it remains unclear which conditions determine the success of this procedure.