Successful In vivo Calcium Imaging with a Head-Mount Miniaturized Microscope in the Amygdala of Freely Behaving Mouse

In vivo real-time monitoring of neuronal activities in freely moving animals is one of key approaches to link neuronal activity to behavior. For this ...

Korea Advanced Institute of Science and Technology (KAIST)

In Pavlovian fear conditioning, the lateral amygdala (LA) has been highlighted as a key brain site for association between sensory cues and aversive stimuli. However, learning-related changes are also found in upstream sensory regions such as thalamus and cortex. To isolate the essential neural circuit components for fear memory association, we tested whether direct activation of presynaptic sensory inputs in LA, without the participation of upstream activity, is sufficient to form fear memory in mice. Photostimulation of axonal projections from the two main auditory brain regions, the medial geniculate nucleus of the thalamus and the secondary auditory cortex, was paired with aversive footshock. Twenty-four hours later the same photostimulation induced robust conditioned freezing and this fear memory formation was disrupted when glutamatergic synaptic transmission was locally blocked in the LA. Therefore, our results prove for the first time that synapses between sensory input areas and the LA, previously implicated as a crucial brain site for fear memory formation, actually are sufficient to serve as a conditioned stimulus. Our results strongly support the idea that the LA may be sufficient to encode and store associations between neutral cue and aversive stimuli during natural fear conditioning as a critical part of a broad fear memory engram.

Optogenetic activation of presynaptic inputs in lateral amygdala forms associative fear memory.

Although epigenetic mechanisms of gene expression regulation have recently been implicated in memory consolidation and persistence, the role of nucleosome-remodeling is largely unexplored. Recent studies show that the functional loss of BAF53b, a postmitotic neuron-specific subunit of the BAF nucleosome-remodeling complex, results in the deficit of consolidation of hippocampus-dependent memory and cocaine-associated memory in the rodent brain. However, it is unclear whether BAF53b expression is regulated during memory formation and how BAF53b regulates fear memory in the amygdala, a key brain site for fear memory encoding and storage. To address these questions, we used viral vector approaches to either decrease or increase BAF53b function specifically in the lateral amygdala of adult mice in auditory fear conditioning paradigm. Knockdown of  before training disrupted long-term memory formation with no effect on short-term memory, basal synaptic transmission, and spine structures. We observed in our qPCR analysis that BAF53b was induced in the lateral amygdala neurons at the late consolidation phase after fear conditioning. Moreover, transient BAF53b overexpression led to persistently enhanced memory formation, which was accompanied by increase in thin-type spine density. Together, our results provide the evidence that BAF53b is induced after learning, and show that such increase of BAF53b level facilitates memory consolidation likely by regulating learning-related spine structural plasticity. Recent works in the rodent brain begin to link nucleosome remodeling-dependent epigenetic mechanism to memory consolidation. Here we show that BAF53b, an epigenetic factor involved in nucleosome remodeling, is induced in the lateral amygdala neurons at the late phase of consolidation after fear conditioning. Using specific gene knockdown or overexpression approaches, we identify the critical role of BAF53b in the lateral amygdala neurons for memory consolidation during long-term memory formation. Our results thus provide an idea about how nucleosome remodeling can be regulated during long-term memory formation and contributes to the permanent storage of associative fear memory in the lateral amygdala, which is relevant to fear and anxiety-related mental disorders.

BAF53b, a Neuron-Specific Nucleosome Remodeling Factor, Is Induced after Learning and Facilitates Long-Term Memory Consolidation.

The dentate gyrus (DG) of the hippocampus is essential for contextual and spatial memory processing. While lesion or silencing of the DG impairs contextual memory encoding and recall, overly activated DG also prevents proper memory retrieval. Abnormally elevated activity in the DG is repeatedly reported in amnesic mild cognitive impairment (aMCI) patients or aged adults. Although the correlation between memory failure and abnormally active hippocampus is clear, their causal relationship or the underlying nature of such interfering activity is not well understood. Using optogenetics aided by a carefully controlled adeno-associated virus infection system, we were able to examine the differential effects of abnormally activated hippocampus on mice motor behavior and memory function, depending on the extent of the stimulation. Optogenetic stimulation of massive proportion of dorsal DG cells resulted in memory retrieval impairment, but also induced increase in general locomotion. Random additional activity in a sparse population of dorsal DG neurons, however, interfered with contextual memory recall without inducing hyperactivity. Our findings thus establish the causal role of elevated DG activity on memory recall failure, suggesting such aberrant DG activity may contribute to amnesic symptoms in aMCI patients and aged adults.

Specific disruption of contextual memory recall by sparse additional activity in the dentate gyrus.

Prefrontal brain areas are implicated in the control of fear behavior. However, how prefrontal circuits control fear response to innate threat is poorly understood. Here, we show that the anterior cingulate cortex (ACC) and its input to the basolateral nucleus of amygdala (BLA) contribute to innate fear response to a predator odor in mice. Optogenetic inactivation of the ACC enhances freezing response to fox urine without affecting conditioned freezing. Conversely, ACC stimulation robustly inhibits both innate and conditioned freezing. Circuit tracing and slice patch recordings demonstrate a monosynaptic glutamatergic connectivity of ACC-BLA but no or very sparse ACC input to the central amygdala. Finally, our optogenetic manipulations of the ACC-BLA projection suggest its inhibitory control of innate freezing response to predator odors. Together, our results reveal the role of the ACC and its projection to BLA in innate fear response to olfactory threat stimulus.

Anterior cingulate cortex and its input to the basolateral amygdala control innate fear response.

Ginseng has been shown to produce a cognitive improvement effect. The key molecular components in ginseng that produce pharmacological effects are ginsenosides. Previous studies reported a memory improvement effect of a few major ginsenosides. However, the identity of specific minor ginsenosides mediating such function remains unknown. Here, we report that a minor ginsenoside F1 improves memory function in APPswe/PSEN1dE9 (APP/PS1) double-transgenic Alzheimer's disease (AD) model mice. After 8-wk oral administration of F1 jelly, we observed that spatial working memory, but not context-dependent fear memory, was restored in AD mice. To search for a possible underlying molecular and cellular mechanism, we investigated the effect of F1 on Aβ plaque. We observed F1 administration reduced the Aβ plaque area and density in the cortex, but not in the hippocampus of AD mice. Next, we tested for the effect of F1 on the expression level of key molecules involved in learning and memory. Results from Western blot assay revealed that an abnormally reduced level of a phosphorylated form of CREB in the hippocampus of AD mice was restored to a normal level by F1 administration. Moreover, in the same animals, BDNF level was augmented in the cortex. Our results, therefore, suggest that minor ginsenoside F1 constitutes a promising target to develop therapeutic agents for AD.

Minor ginsenoside F1 improves memory in APP/PS1 mice.

A unique feature of fear memory is its persistence that is highly relevant to fear and anxiety-related mental disorders. Recurrent reactivation of neural representations acquired from a traumatic event is thought to contribute to the indelibility of fear memory. Given a well-established role of hippocampus for memory reactivation, hippocampus is likely involved in consolidation process of fear memory. However, evidence suggests that formation of fear memory to a discrete sensory cue is hippocampus-independent. Here, using a pharmacological reversible inactivation of dorsal hippocampus in auditory cued fear conditioning by local infusion of muscimol, we demonstrate in mice that hippocampus is critical for remote memory formation of learned fear to the discrete sensory cue. Muscimol infusion before conditioning did not affect formation of recent auditory fear memory as previously reported. Same muscimol infusion, however, impaired remote auditory fear memory. Muscimol infusion before remote test of auditory fear memory did not affect memory retrieval, indicating hippocampus is not a brain site for storage of remote cued fear memory. Moreover, memory reactivation enforced by re-exposure to the conditioned tone could compensate for hippocampal inactivation, as memory-reactivated mice showed normal remote auditory fear memory despite hippocampal inactivation. Our findings support that hippocampus may have a general role for consolidation of remote associative memory through reactivation of memory trace, giving an insight into how learned fear persists over time.

A critical role of hippocampus for formation of remote cued fear memory.

Endurance represents a highly adaptive function of fear memory and a major cause of maladaptive fear and anxiety-related mental disorders. However, less is known about the mechanisms underlying the persistence of fear memory. The epigenetic mechanism of gene expression regulation recently emerged as an important mechanism for memory persistence. In the previous study, we found that BAF53b, a neuron-specific subunit of BAF chromatin remodeling complex, is induced after auditory cued fear conditioning in the lateral amygdala (LA) and crucial for recent fear memory formation. In this study using mice of both sexes, we report a delayed induction of BAF53b in the LA 48 hours after auditory fear conditioning and its critical role for the persistence of established fear memory. In order to specifically block the delayed but not early induced BAF53b function, we employed a post-learning knockdown method based on RNAi. The transient knockdown of BAF53b using siRNA in the lateral amygdala 24 hours after cued fear conditioning led to specific impairment of remote but not recent memory retrieval. RNA-seq analyses identified fibroblast growth factor 1 (FGF1) as a candidate downstream effector. Consistently, post-learning administration of FGF1 peptide rescued memory persistence in Baf53b knockdown mice. These results demonstrate the crucial role of BAF53b and FGF1 in persistent retention of fear memory, giving insights into how fear memory persistently stored through consolidation processes and suggests candidate target for treating mental disorders related to traumatic memory.It is still unclear how once consolidated memory persists over time. In this study, we report the delayed induction of nucleosome remodeling factor BAF53b in the lateral nucleus of amygdala after fear learning and its crucial role for persistence of established memory beyond 24 hours after learning. Our data link the regulation of BAF53b and fibroblast growth factor1 (FGF1) expression in the amygdala to fear memory persistence. Results from this study open a new direction to understand the time dependent continuous consolidation processes potentially by nucleosome-remodeling mechanism enabling long-lasting memory formation and give insights into how to treat mental disorders caused by enduring traumatic memory.

Jin-Hee Han

Department of Biological Sciences,
KAIST Institute for BioCentury (KIB)

Korea Advanced Institute of Science and Technology (KAIST)

Successful In vivo Calcium Imaging with a Head-Mount Miniaturized Microscope in the Amygdala of Freely Behaving Mouse

Jin-Hee Han

.css-o250i3{font-size:18px;font-family:Open Sans,sans-serif;line-height:21.6px;}Department of Biological Sciences,KAIST Institute for BioCentury (KIB)

Korea Advanced Institute of Science and Technology (KAIST)

Successful In vivo Calcium Imaging with a Head-Mount Miniaturized Microscope in the Amygdala of Freely Behaving Mouse

Department of Biological Sciences,
KAIST Institute for BioCentury (KIB)