【正文】 es, hypothesizing that extinction training in conjunction with CI994 would allow for increased histone acetylationmediated neuroplasticity. We focused on the spaced extinction paradigm , as this paradigm was the only one in which VEH and CI994treated animals showed different fear responses immediately following the extinction first measured hippocampal histone acetylation using immunohistochemistry 1 hr after the end of the extinction found that, pared to the histone acetylation of VEHtreated animals and that of 1 hr after remote memory recall, CI994treated animals showed significantly higher acetylation levels. This was consistent with reduced hippocampal HDAC2 activity upon HDACitreatment. In the prefrontal cortex, where remote memories are primarily stored, histone acetylation following extinction training in bination with CI994 was also elevated relative to VEHtreated animals and was parable to levels 1 hr after remote memory recall, which may indicate persistent engagement of cortical areas in HDACitreated animals. For the following analyses, we nevertheless focused exclusively on the hippocampus, as this brain area is critical for the incorporation of new information into a given memory trace during reconsolidation of contextual fear memories. Alongside histone acetylation, another critical aspect of neuroplasticity is the transcription of genes related to synaptic plasticity, learning, and memory. To investigate those, we performed RNA sequencing of hippocampal extracts of both VEH and CI994treated animals 1 hr after pletion of remote memory extinction. At an expression fold change cutoff of , we found 475 differentially expressed genes (DEGs) between CI994 and VEHtreated animals, among which 199 genes showed higher expression in CI994treated animals. Upon generation of pathway and gene ontology analyses, we noticed these DEGs to be implicated in biological processes related to neuronal plasticity, such as learning or memory, regulation of transmission of nerve impulse, cell morphogenesis and projection, cation transport, and synaptic transmission. These findings suggest that, by using CI994 in bination with extinction training, neuroplasticity at the level of gene expression was enhanced. Moreover, we detected several key regulators of neuronal plasticity to be significantly upregulated in CI994treated animals. These genes include the immediateearly genes Arc and cFos, which are both critically involved in synaptic plasticity and memoryrelated processes, Adcy6, an adenylate cyclase regulating neurite extension, Npas4, a transcription factor regulating contextual memory formation, and Igf2, which facilitates memory retention and extinction of recent fear analysis of independent CI994 and VEHtreated samples confirmed the expression changes of these genes. Importantly, no such expression changes were detected when the HDACi was administered without extinction training, in line with the observation that the HDACi alone had no behavioral effect. The genes with higher expression also showed higher acetylation in their promoter region. Intriguingly, such increased histone acetylation occurred despite HDAC2 being bound to their promoter. These findings suggest that the HDACi treatment following remote memory recall can pensate for the absence of HDAC2 nitrosylation and its persistent binding to the chromatin, thereby increasing histone acetylationmediated neuroplasticity at the level of gene expression.HDAC Inhibitors Lead to Increased Neuroplasticity during Memory Extinction HDAC Lastly, we conducted a series of experiments to assess whether such increased expression of neuroplasticityrelated genes would also result in facilitated synaptic plasticity. We first measured hippocampal glucose utilization 1 hr after pletion of remote memory extinction by means of radiolabeled [3 H]2 deoxyglucose ([3 H]2DG) uptake, which reflects neuronal activity. We found that CI994treated animals showed higher hippocampal [3 H]2DG uptake pared to VEHtreated animals, depicting increased metabolic engagement of this brain area following memory contrast, [3 H]2DG measurements did not differ between VEH and CI994treated animals in the piriform cortex, a brain region implicated in olfactory memories and thus unrelated to the formation of contextual memories.Next, to test whether such increased metabolic activity was indicative of increased neuronal plasticity, we assessed longterm potentiation (LTP) at Schaffer collaterals by electrophysiological recordings at the same time point. Using triple thetaburst stimulation (3 3 TBS), we observed that CI994treated animals displayed facilitated LTP pared to VEHtreated ones. In contrast, we found no difference in basal synaptic transmission, as the slope of field excitatory postsynaptic potential (fEPSP) elicited by a given presynaptic fiber volley did not differ between the two groups. Likewise, we detected no significant difference in pairedpulse facilitation. These results suggest that a bination of extinction training and CI994 administration leads to increased synaptic plasticity. Finally, we investigated whether such increased synaptic plasticity might also be acpanied by increased structural plasticity. For this, we measured the number of dendrites by microtubuleassociated protein 2 (MAP2) immunohistochemistry, dendritic branching and the number of dendritic spines by Golgi staining, and the density of synapses by transmission electron microscopy and synaptophysin immunohistochemistry, a presynaptic marker, 1 hr after pletion of memory extinction. These measurements reflect the degree of neuronal connectivity and thereby synaptic strength. We found that CI994treated animals displayed an elevated number of MAP2stained dendrites pared to VEHtreated ones in the stratum radiatum layer of the , we detected a significant enhancement of dendritic branching between 120 and 170 mm fro