Synaptic plasticity associated with a memory engram in the basolateral amygdala.

Synaptic plasticity is a cellular mechanism putatively underlying learning and memory. However, it is unclear whether learning induces synaptic modification globally or only in a subset of neurons in associated brain regions. In this study, we genetically identified neurons activated during contextual fear learning and separately recorded synaptic efficacy from recruited and nonrecruited neurons in the mouse basolateral amygdala (BLA). We found that the fear learning induces presynaptic potentiation, which was reflected by an increase in the miniature EPSC frequency and by a decrease in the paired-pulse ratio. Changes occurred only in the cortical synapses targeting the BLA neurons that were recruited into the fear memory trace. Furthermore, we found that fear learning reorganizes the neuronal ensemble responsive to the conditioning context in conjunction with the synaptic plasticity. In particular, the neuronal activity during learning was associated with the neuronal recruitment into the context-responsive ensemble. These findings suggest that synaptic plasticity in a subset of BLA neurons contributes to fear memory expression through ensemble reorganization.

Impairment of fear memory consolidation and expression by antihistamines.

Antihistamines are widely used to treat allergy symptoms. First-generation antihistamines have adverse effects on the central nervous system (CNS), such as hypnotic and amnesic effects, whereas second-generation antihistamines have poor brain penetration, and therefore, have fewer CNS-related adverse effects. Memory consists of several phases, including acquisition, consolidation, expression, and extinction. It remains unclear whether these phases are affected by antihistamines. We investigated the effects of diphenhydramine, a first-generation antihistamine, and levocetirizine and olopatadine, second-generation antihistamines, on memory phases. Mice were subjected to fear conditioning on day 1 and tested on day 2. Antihistamines were administered before conditioning, immediately after conditioning, or before the test session. Diphenhydramine (30mg/kg) decreased freezing time when administered immediately after conditioning or before the test session. These effects were not attributable to a change in locomotor activity. Levocetirizine (0.1, 1, 10mg/kg) and olopatadine (1, 10, 20mg/kg) had no effects on conditioned fear. We also examined the effect of diphenhydramine and levocetirizine on the expression of an activity-dependent gene associated with the test session. Diphenhydramine, but not levocetirizine, increased Arc transcription in the central nucleus of the amygdala. These data indicate that diphenhydramine, but not levocetirizine or olopatadine, impairs the consolidation and expression of conditioned fear.

Memory coding in plastic neuronal subpopulations within the amygdala.

Specific neuronal subpopulations within specific brain areas are responsible for learning and memory. A fear memory engages a subset of lateral amygdala neurons, but whether multiple contextual fear memories engage the same or different subsets of lateral amygdala neurons remains unclear. Here, we demonstrate the representation of multiple contextual fear memories in the amygdala with cellular and temporal resolution using a large-scale imaging method. Mice were conditioned with a footshock in 2 separate chambers. They were then re-exposed to either the same conditioning chamber twice or 2 different conditioning chambers. The activities of individual neurons related to the re-exposures were determined by the subcellular distribution of Arc/Arg3.1 RNA. Reactivation of different memories activated partially (about 50%) overlapping neurons, whereas reactivation of the same memory activated more overlapping (about 65%) neurons. These findings indicate that lateral amygdala neurons related to different fear memories are partly common, and that a small but significant neuronal population (2.7% of total lateral amygdala neurons) encodes differences in individual fear memories. Moreover, memory retrieval increased the size of the neuronal subpopulation activated during subsequent retrieval. Taken together, our findings indicate that small plastic subsets of neurons encode fear memories from individual contexts.

Experience-dependent Homer1a expression in excitatory and inhibitory neurons.

Homer1a, an activity-dependent induced member of the scaffold protein family Homer, plays an essential role in synaptic reorganization and is widely used as a neuronal activity marker. However, the cell type transcribing Homer1a remains unidentified. Here, we investigated the main cell types of the amygdala, hippocampus, primary somatosensory cortex, and dorsal striatum that express Homer1a. Homer1a expression relevant to the baseline neural activity as well as exposure to unfamiliar and conditioned contexts was preferentially detected in the excitatory neurons within the basolateral amygdala, hippocampus, and neocortex, and in inhibitory neurons within the central amygdala and dorsal striatum. These findings indicate that Homer1a is expressed in the principal neurons of each region regardless of whether they are excitatory or inhibitory neurons.

Persistent neural activity regulates Arc/Arg3.1 transcription in the dentate gyrus.

The activity-regulated cytoskeleton-associated gene (Arc, also known as Arg3.1) is an effector immediate-early gene rapidly induced by strong neural activity. Although a number of studies have revealed significant functions of Arc and Arc has come into widespread use as a neural activity marker in behavioral studies, the mechanisms regulating Arc transcription remain unclear. Here, we examined the conditions of Arc transcription in acute slices of dentate gyrus. Surprisingly, kainic acid (1 µM to 10 mM) application to slices did not induce Arc transcription, although intraperitoneal injection of kainic acid (20 mg/kg) induced robust Arc transcription. No types of high-frequency stimulation examined induced Arc transcription in acute slices. These findings indicate that Arc transcription is dramatically suppressed in acute slices of the dentate gyrus, in which background neural activity is markedly reduced. Burst stimulation increased the number of Arc-expressing cells in the presence of picrotoxin, in which excitation was maintained even after the end of stimulation. Moreover, the involvement of background neural activity in Arc transcription was tested by application of carbachol, a muscarinic receptor agonist. Carbachol also increased the number of Arc-expressing cells, which was blocked by atropine, a muscarinic receptor antagonist. Taken together, these findings suggest that persistent background activity is critical for Arc transcription.