Molecular Brain

REV-ERB{beta} is a transcriptional repressor of nuclear receptors that regulates the circadian rhythm and plays an important role in regulation of the proliferation, differentiation, and maturation of neurons. Dysregulation of the circadian rhythm has been associated with neuropsychiatric disorders, and activation of REV-ERBs can induce anxiolytic behavior in mice. Furthermore, hippocampal neurogenesis is important in the effects of antidepressants. However, the role of REV-ERB{beta} in adult hippocampal neurogenesis in vivo at the single-cell level is not known. In this study, protein localization of REV-ERB{beta} in the subgranular zone of the hippocampal dentate gyrus (DG) was mainly shown in NeuN-positive neurons, and the effect of expressing a dominant negative form of REV-ERB{beta} lacking the C-terminal region on newborn neurons in the hippocampal DG of adult mice was examined to investigate the role of REV-ERB{beta} in neurogenesis. A retroviral vector containing the dominant negative REV-ERB{beta} or a control vector was injected into the mouse DG. At 4 weeks after injection, the morphology of dendrites and dendritic spines of newborn neurons labeled by the virus was examined. Expression of the dominant negative form of REV-ERB{beta} inhibited dendrite outgrowth and branching and decreased dendritic spine formation in newborn neurons in the adult mouse hippocampal DG. This study revealed a new role for REV-ERB&{beta} in adult hippocampal neurogenesis at the single cell level, and the results will provide insight into neurogenesis in the adult brain and its relationship with psychiatric disorders.

The functioning of neuronal networks critically depends on the coordinated interaction between excitation and inhibition. The properties of glutamatergic and GABAergic synapses are finely tuned by network activity, and the heterosynaptic nature of inhibitory plasticity further emphasizes their functional coupling. Although the dependence of glutamatergic plasticity on protein synthesis is well established, the role of translation in inhibitory plasticity remains unclear. Herein, we investigated how protein synthesis supports the concurrent plasticity at inhibitory and excitatory inputs onto CA1 pyramidal neurons. To this end, we applied a high-frequency stimulation protocol of CA3-CA1 input combined with prolonged postsynaptic depolarization of principal cells. This induced both glutamatergic long-term potentiation (LTP) and inhibitory LTP (iLTP) at synapses formed by somatostatin-expressing interneurons onto pyramidal neurons. Although LTP was observed in slices from both juvenile ([≤]45 days) and adult (>45 days) mice, iLTP was only present in the adult group, revealing an unusual developmental profile. Moreover, in adult animals, excitatory LTP was required for the induction of iLTP but did not determine its magnitude. We found that bath application of translation inhibitor cycloheximide (CHX) strongly attenuated LTP and converted co-occurring iLTP into iLTD. However, when CHX was restricted to the recorded postsynaptic neuron, the initial phase of iLTP proceeded normally, but GABAergic currents gradually returned to baseline levels, indicating intact induction but impaired maintenance. In contrast, postsynaptic CHX left excitatory LTP unaffected. Together, these findings identify two distinct translational mechanisms: a postsynaptic one necessary for strengthening inhibition and a non-postsynaptic process required to potentiate excitation.

The small GTPase Rem2 is a key signaling molecule involved in synaptic formation, dendritic complexity, spine formation, and regulation of CaMKII-dependent long-term potentiation (LTP). However, it remains unclear how Rem2 explicitly impacts learning and memory. To address this issue, we deleted Rem2 specifically in dorsal CA1 neurons of male and female mice and assessed spatial learning using a spatial object recognition (SOR) task. We found that males outperform females in this behavioral assay and that deleting Rem2 did not impact spatial learning in males. In contrast, Rem2 deletion significantly improved spatial learning in females enabling them to perform as well as males in the SOR task. Using an automated Western blot system to assess all known AMPA- and NMDA-mediated glutamate receptor (AMPAR and NMDAR) subunits in each mouse, we found that the sex-dependent change in SOR we observed was likely due to Rem2 increasing GluN2D expression in the interneurons of female mice only. Furthermore, using S-statistics to compare the overall AMPAR and NMDAR composition among groups, we found that males and females normally have divergent covariance structures but this divergence is eliminated when Rem2 is deleted from CA1 neurons. These results reveal an unexpected role of Rem2 in maintaining sex differences in glutamate receptor composition and spatial learning abilities. To our knowledge, this is the first demonstration of a signaling molecule that confers sexual dimorphism to excitatory synapses. As such, Rem2 may play a critical role in understanding how sex-dependent symptoms of neurodevelopmental and neurodegenerative disorders arise.

Tackling developmental changes in long-term potentiation (LTP) expression during adolescence is crucial to understand disease susceptibility in neurodevelopmental disorders, epileptogenesis or drug abuse. This paper investigated the alterations in the resting phosphorylation state of synaptic proteins essential for synaptic transmission and hippocampal LTP expression from weaning (3 weeks) to adulthood (12 weeks). Synaptic AMPA GluA1 and GluA2 subunit levels increased, yet to different extents, as the GluA1/GluA2 ratio also increased. Conversely, GluA1 phosphorylation in both Ser831 and Ser845 progressively decreased from weaning to adulthood, suggesting an enhanced availability of these sites for activity-dependent phosphorylation. In accordance, LTP induced by different theta-burst stimulation (TBS) intensities in the CA1 area of rat hippocampal slices enhanced gradually in this developmental period. In contrast, basal phosphorylation of GluN1, GluN2B and CaMKII increased during adolescence. Altogether, these findings suggest that alterations in AMPA receptor subunit composition and basal phosphorylation are crucial for the maturation of hippocampal LTP during adolescence, while a mild enhancement in synaptic CaMKII levels may further provide the necessary structural support to LTP expression and stability. Given the reported involvement of GluA1 phosphorylation changes in epileptogenesis and neurodevelopmental disorders, these findings provide important insights into hippocampal synaptic plasticity in normal and altered brain development and epileptogenesis.

RNA granules are biomolecular condensates that are reversibly formed through multivalent interactions among mRNAs and RNA-binding proteins (RBPs). RNA granules include stress granules and neuronal transport granules, which share common components and functions in translational regulation. Fragile X messenger ribonucleoprotein (FMRP) is an RBP encoded by FMR1, whose loss of function causes fragile X syndrome (FXS), a neurodevelopmental disorder. FMRP is a component of both stress granules and neuronal transport granules. Although several FXS-associated missense mutations in FMRP have been identified, it remains unclear whether these mutations affect the assembly of FMRP into RNA granules. Here, we found that the KH1 domain mutation G266E markedly reduced the accumulation of FMRP in stress granules in HEK293T cells, whereas the KH2 domain mutation I304N had little effect. In addition, both G266E and I304N reduced the accumulation of FMRP within neuronal transport granules in FMRP-deficient mouse cortical neurons. Because G266E and I304N have been reported to impair the RNA-binding and/or RNA-associated functions of FMRP, our findings suggest that the KH1 and KH2 domains are important for the assembly of FMRP into RNA granules. These findings further raise the possibility that impaired RNA granule assembly may contribute to the pathogenesis of FXS and other neurodevelopmental disorders.

Schema memory refers to generalized knowledge extracted from multiple related episodes. According to systems consolidation theory, such representations emerge during sleep through the transformation of hippocampus-dependent episodic memories into neocortical schema representations. Whether this process requires mature episodic memory expression during early development remains unclear. Here, we examined schema formation in infant rats with an immature hippocampus. At postnatal day (PD) 25, pups (n=24) were tested in an adapted object-place paradigm enabling abstraction of a spatial regularity across eight consecutive episodes. Following encoding, pups either slept or remained awake for two hours, and schema memory was tested 22 hours later. Control groups were exposed to pseudorandomized object configurations. Schema memory was expressed only after exposure to spatial regularities and only when sleep followed encoding. To directly assess episodic memory under comparable conditions, an additional group of pups (n=12) was tested in a standard single-episode object-place recognition (OPR) task with post-encoding sleep. At a 4-hours test, these pups did not show episodic object-place memory. These findings indicate that sleep supports schema memory formation during early life under conditions in which the expression of robust single-episode memory is not yet evident, suggesting that schema abstraction in infancy does not depend on fully developed hippocampal episodic representations.

Sleep disorders and wakefulness constitute a pronounced and pervasive health risk that contributes to numerous cerebral pathologies such as stroke, depression, and neurodegenerative disorders. The hippocampus (HC) plays vital roles in learning and memory and is especially susceptible to the pathological insults imposed by sleep deprivation (SD). While SD disrupts neuronal network signaling and triggers inflammatory processes, the impact of SD on metabolism within the HC presently remains poorly understood. In this study, we assessed the progressive and cumulative effects of partial SD in the live mouse HC CA1 region using two-photon microscopy. Using a custom-designed implantable cannula enabling direct HC visualization and administration of dyes and pharmacological agents directly to HC tissue, we longitudinally monitored spontaneous neuronal activity and partial pressure of oxygen (pO2) within HC CA1 tissue before and during a 7-day period of partial SD and after a 7- day period of recovery. Our results revealed a progressive decline in spontaneous neural activity, alongside sustained elevation in tissue pO2; during the 7-day partial SD period. Notably, after 7 days of recovery, neuronal activity did not return to baseline, and tissue oxygenation continued to increase. Our findings emphasize chronic SD's cumulative and enduring impairments to neuronal function and metabolism in the HC and reveal uncoupling between metabolic and functional recovery that outlasts the sleep restriction period.

Aim: ROR{gamma}t-transgenic mice provide a model of sustained T helper 17-cell bias, but the associated hippocampal transcriptional profile is incompletely characterized. We investigated gene-level and gene-set expression differences in this model. Methods: Bilateral hippocampal samples from 10-week-old male ROR{gamma}t-transgenic mice and wild-type littermates (n = 3 per genotype) underwent bulk RNA sequencing. Gene-level differential expression was evaluated using the Empirical Analysis of DGE tool in CLC Genomics Workbench. Preranked gene set enrichment analysis examined coordinated expression differences after expression filtering and exclusion of Rorc (13,813 genes). Results: Sixteen genes met a false discovery rate < 0.05 (nine higher and seven lower in transgenic mice), including Rorc, C4b, Xbp1, Nptx2 and Mt2. Of 212 candidates selected using unadjusted P < 0.05 and a twofold-change threshold, 155 (73.1%) had a mean expression below 0.5 reads per kilobase of transcript per million mapped reads. Hallmark interferon--response, interferon-{gamma}-response, and unfolded-protein-response gene sets showed positive enrichment. Thirteen Gene Ontology biological-process sets met a false discovery rate < 0.05, including negatively enriched sets related to excitatory postsynaptic potential, glutamatergic transmission, and calcineurin-mediated signaling. Il17a and Il17f had zero reported counts, and none of the displayed leukocyte or central nervous system cell-marker genes met false discovery rate < 0.05. Conclusion: This exploratory dataset identifies interferon-response, unfolded-protein-response and synaptic transcriptional signatures associated with the ROR{gamma}t-transgenic genotype. The cellular sources, causal mediators and functional consequences remain unresolved.

The cerebellar nuclei (CN) has demonstrated its influence on cognitive behavior via the cerebello-cortico circuit, yet the role of CN critical period mechanisms and how they may influence cognitive behavior, such as parvalbumin (PV) expressing interneurons enwrapped by perineuronal nets (PNNs), is still unclear. Therefore, we investigated the role of the lateral CN (LCN) PV cell calcium activity while animals performed a visual discrimination touchscreen cognitive task. All animals received the PV cell calcium indicator GCaMP6f at postnatal day 21 (P21). We targeted the LCN critical period by manipulating neural activity in male mice using the inhibitory Designer Receptors Exclusively Activated by Designer Drugs (DREADDs) from postnatal day 21 to 35 or by injecting an Hapln1-AAV vector to selectively target LCN PNN development. After animals completed the visual discrimination task, cerebellar tissue was collected for viral recovery and antibody staining for PNN components, Hapln1 and aggrecan. Results revealed DREADD animals showed improved reversal learning, an increase in calcium response to learning-related activity and altered PNN expression (Hapln1 and aggrecan). Hapln1 treated animals displayed a decrease in final day acquisition performance, lower reversal performance compared to DREADD groups, a decrease in reversal calcium learning-related activity, and an increase in PNN expression (Hapln1). Together, these data provide further evidence of LCN mechanisms associated with learning as well as the importance of understanding region-specific critical periods of plasticity.

Dietary magnesium (Mg2+) supplement enhances memory in mammals and Drosophila, but whether and how Mg2+ changes molecular states of neurons, remains unknown. Here we used single-cell RNA sequencing to map cell-type specific transcriptional responses to memory-enhancing dietary Mg2+. These analyses revealed changes in Hippo-pathway gene expression in memory-relevant {beta} Kenyon Cells (KCs) of the mushroom bodies. Targeted knock-down of Merlin, hippo (hpo) and warts demonstrated requirement in {beta} KCs for Mg2+-enhanced, but not baseline memory. Further study of Hippo signaling identified Yorkie (yki) transcriptional coactivator and DNA-binding partner Scalloped (sd) to be required for baseline and Mg2+-enhanced memory. Moreover, releasing Yki::Sd of inhibition from Tondu-domain containing growth inhibitor (Tgi) specifically in {beta} KCs enhanced memory, independent of diet. Analyses of Yki::Sd target genes identified several required in {beta} KCs for baseline memory, including crb and Glut1 which are also Mg2+-induced in a Hippo-dependent manner in {beta} KCs. These genes suggest that aiding the metabolic demands of long-term memory and facilitating structural neuronal plasticity are key mediators of Mg2+-induced memory enhancement. We propose pharmacological enhancement of Sd-dependent transcription, and activity of particular Sd-target genes may have therapeutic potential for memory disorders.

Primary cilia, which are present in most brain cells, are essential for brain development and function. During early brain development, dysfunction of the primary cilia can lead to a broad spectrum of disorders, collectively termed ciliopathies, that include brain malformations and intellectual disability. Although the role of primary cilia in brain development is well-established, cilia-mediated signaling in mature neurons and the contribution of cilia to neuronal circuit function remain controversial. Using mouse genetic and behavioral studies, single-cell electrophysiology, and 2-photon imaging, we show that deletion of primary cilia from adult hippocampal neurons is not required for hippocampal circuit function or behavior. Chronic genetic deletion or acute laser ablation of primary cilia from mature pyramidal neurons in the CA1 or CA3 regions of the hippocampus did not affect neuronal excitability, basal synaptic transmission, or long-term synaptic plasticity at excitatory CA3-CA1 hippocampal synapses. Moreover, the loss of primary cilia did not affect hippocampal-dependent learning and memory or anxiety-like behaviors. These results challenge the prevailing view of cilia function in mature hippocampal neurons and suggest that neuronal cilia in the adult hippocampus do not serve as major signaling hubs for pathways essential for neuronal function or behavior.

The 2{delta} family of auxiliary voltage-gated calcium channel (VGCC) subunits have critical but incompletely understood roles in brain function. Parvalbumin-positive (PV+) interneurons in the hippocampus highly express the 2{delta}-2 isoform, and mice lacking 2{delta}-2 exhibit spontaneous seizures. Thus, we examined PV+ neuron-mediated synaptic inhibition in acutely prepared brain slices from 2{delta}-2 knockout (KO) mice. In the inner molecular layer of the dentate gyrus, 2{delta}-2 KO mice demonstrated an increase in the excitation/inhibition ratio of synaptic inputs onto granule cells. We then used optogenetics to activate PV+ interneurons, which produced dramatically smaller inhibitory synaptic currents in granule cells from 2{delta}-2 KO mice. There was a reduction in PV+ inputs onto granule cells as determined by immunostaining. Functionally, these inputs had a lower probability of GABA release and a decreased readily releasable pool of vesicles compared to littermate controls. VGCC coupling to presynaptic vesicle release was also reduced in dentate gyrus PV+ cells in 2{delta}-2 KO mice, based on manipulations of intracellular and extracellular calcium. Together, our data indicate that 2{delta}-2 plays a critical role in PV+ interneuron-mediated synaptic inhibition, which may contribute to seizures in 2{delta}-2 mutant mice.

The regulation of presynaptic mechanisms by the fragile X messenger ribonucleoprotein (FMRP) remains incompletely understood despite their potential importance to synaptic dysfunction reported in Fragile X syndrome. We combined post-developmental, cell-specific deletion of FMRP in dentate granule cells of the hippocampus with optogenetics, whole-cell patch-clamp electrophysiology and super resolution imaging. The selective loss of FMRP in DG cells did not alter their intrinsic excitability. The amplitude of synaptic currents was reduced at the output mossy fiber synapses in the absence of presynaptic FMRP, whereas short-term facilitation remained intact. We demonstrate that basal synaptic activity is regulated by PKA at mature mossy fiber synapses, and our results indicate alterations in cAMP-PKA signaling and a potential reduction in the readily releasable pool size in mossy fiber boutons lacking FMRP. Presynaptic FMRP is therefore required for efficient basal neurotransmission at mature mossy fiber synapses and likely acts through cAMP-PKA-dependent signaling pathway. Significance statementThe fragile X messenger ribonucleoprotein (FMRP) has been extensively studied as a regulator of postsynaptic mechanisms. However, its physiological functions in presynaptic terminals remain incompletely understood. Using post-developmental, cell-specific deletion of FMRP, we show that presynaptic FMRP is required to maintain efficient basal neurotransmission at hippocampal mossy fiber synapses and identify cAMP-PKA signaling as a potential mechanism for this effect. Taken together, these results highlight a physiological role for presynaptic FMRP during basal synaptic activity and demonstrate how time-controlled gene-editing approaches via viral infection can distinguish primary molecular functions from developmental compensations, thereby shedding new light on both the presynaptic function of FMRP and the pathophysiology of Fragile X syndrome. HighlightsO_LIPresynaptic loss of FMRP reduces basal synaptic activity at hippocampal Mf synapses. C_LIO_LIBasal synaptic activity is regulated by PKA in adult Mf synapses. C_LIO_LIAC-cAMP-PKA pathway is impaired during basal synaptic activity in the absence of presynaptic FMRP. C_LIO_LIRRP size is very likely reduced in AZs at Mf terminals. C_LI

Axonal projections and precise synapse formation are essential for the establishment of functional neural circuits, and their dysregulation contributes to various psychiatric and neurodegenerative disorders. Although numerous extracellular cues secreted from surrounding tissues regulate axon guidance and synapse formation, the mechanisms by which neurons locally regulate terminal patterning within their projection fields remain poorly understood. Bone morphogenetic protein 4 (BMP4) is a classical morphogen that is involved in early neural tube patterning during development. However, its postnatal function remains unclear. Previous in vitro studies using hippocampal glutamatergic neurons demonstrated that BMP4 can act locally on axons to suppress synapse formation, raising the possibility that BMP4 regulates terminal differentiation after target innervation. Here, we investigated the role of BMP4 in postnatal cholinergic circuit development using cholinergic neuron-specific Bmp4 conditional knockout (Bmp4 cKO) mice, with adult analyses performed in males. Basal forebrain cholinergic neurons project extensively throughout the neocortex and are essential for higher brain functions, including arousal and attention. Bmp4 cKO mice exhibited increased cholinergic axon terminals in the neocortex, hippocampus, and amygdala, as well as increased axonal ramification in the neocortex without affecting cytoarchitectonic organization or cholinergic neuron numbers in the basal forebrain. These findings demonstrate that BMP4 functions as a negative regulator of cholinergic terminal formation and axonal arborization, suggesting that BMP4 may act locally to restrict cholinergic axonal expansion during neural circuit maturation.

BackgroundsEarly-life environmental insults cause persistent neurodevelopmental abnormalities accompanied by transcriptional and epigenetic dysregulation despite removal of the original insult or postnatal intervention. However, transcriptomic and epigenomic responses to developmental insults and subsequent treatment during active neurodevelopment remain insufficiently characterized. Developmental iron deficiency (ID) provides a unique model for investigating this question because iron is an essential cofactor for TET DNA dioxygenases and developmental ID causes persistent behavioral and molecular alterations despite iron repletion. ResultsWe integrated the hippocampal transcriptome, DNA methylome (5mC), and hydroxymethylome (5hmC) in male rats at postnatal day 15 following developmental ID and postnatal iron treatment, using Oxford Nanopore sequencing for native DNA modification profiling. Developmental ID induced substantial transcriptional and epigenetic alterations associated with synaptic function, neurodevelopment, and neuroinflammation. Postnatal iron treatment induced a hierarchical response across molecular layers: while transcriptomic alterations largely normalized, 5mC showed only partial recovery, and 5hmC showed extensive de novo modifications. Recovered, persistent, and newly emerged epigenetic marks were associated with increasingly specialized biological functions, from broad neurodevelopmental processes to specific pathways. Furthermore, while 5mC enrichment was associated with transcriptionally suppressed pathways, 5hmC enrichment showed weaker coupling with concurrent transcriptomic activity, suggesting epigenetic poising rather than immediate transcriptional output. MergeOmics integration identified key driver genes showing post-treatment epigenetic regulation despite transcriptional recovery. ConclusionsMolecular recovery following developmental ID extends beyond transcriptomic normalization, involving persistent and extensive epigenetic remodeling. This study provides a framework for understanding molecular responses following early-life environmental insults and highlights the importance of delineating persistent regulatory reprogramming.

The processes by which epileptic insults precipitate the molecular, cellular, and network alterations in the brain that lead to epilepsy are poorly understood. We previously discovered that after status epilepticus (SE - an epilepsy inducing severe bout of seizures) the H3K27 methylase Enhancer of Zeste Homolog 2 (EZH2) is robustly induced and drives repression of genes. Both systemic pharmacological inhibition of EZH2 and deletion of EZH2 in neurons exacerbates epilepsy progression, suggesting that acute EZH2 induction may exert a net protective effect against disease progression chronically. However, the mechanisms underlying EZH2-mediated control of the putative protective and pathological pathways in disease progression are still unknown. To interrogate the mechanisms of EZH2 function post-SE, we used bulk CUT&RUN- sequencing against H3K27me3 in tandem with bulk RNA-sequencing in hippocampi of naive and 4d. post-SE mice to profile epigenomic and transcriptomic changes. Differential peak analysis showed that H3K27me3 was enriched both at loci pre-marked by H3K27me3 in the naive hippocampus as well as in loci that were de novo methylated after SE, consistent with the SE-dependent induction of EZH2 protein levels. Multi-omic integration of CUT&RUN and RNA-seq data revealed a module of genes that were coordinately H3K27me3 enriched, transcriptionally repressed, and annotated to ontological terms involved in neuronal signaling and network excitability. This result suggests that EZH2 induction may function in part to control network excitability after injury. To test this, we treated mice acutely post-SE with the EZH2 inhibitor UNC1999 and found that EZH2 inhibition attenuated H3K27me3 induction and dampened repression of target network excitability genes in response to SE. Functionally, UNC1999 treatment significantly increased seizure probability acutely and exacerbated disease severity in the chronic period. Taken together, these results suggest that EZH2 induction after injury may function to maintain network excitability to lower seizure probability acutely to protect against disease progression.

Long-term depression (LTD) is a form of synaptic plasticity implicated in tasks involving the modification or elimination of previously learned information. While glial glutamate transporters can control the strength of synaptic plasticity, much less is known about the contribution of the neuronal glutamate transporter EAAT3 in controlling hippocampal LTD and learning processes. Here, we report that overexpression of EAAT3 in principal neurons, but not in GABAergic interneurons, impairs heterosynaptic GABAergic synaptic plasticity (iLTD) and homosynaptic excitatory LTD in the hippocampus. LTD impairments can be reversed by inhibiting EAAT3 or by a brief exogenous activation of mGluR during LTD induction, suggesting that, by limiting glutamate spillover between neighboring synapses, EAAT3 contributes to setting the strength of different forms of hippocampal LTD. Moreover, mice overexpressing EAAT3 in principal neurons, but not in GABAergic interneurons, display impaired reversal learning, a phenotype that can be rescued by blocking EAAT3 in vivo. Together, these findings reveal that, by controlling the strength of hippocampal LTD, EAAT3 contributes to cognitive flexibility required for processing new information.

Traumatic brain injury (TBI) is one of the leading causes of acquired temporal lobe epilepsy. TBI drives hippocampal circuit rearrangements that may contribute to increased seizure risk, such as altered inhibitory circuit function and aberrant post-traumatic neurogenesis. In the hippocampal dentate gyrus, adult-born dentate granule cells (DGCs) acquire inhibitory synaptic inputs from parvalbumin-expressing (PV) interneurons early in their maturation. These inputs are important for circuit integration and feedforward inhibition of these neurons. To test whether DGCs born after TBI have functionally altered PV-mediated innervation, we used genetically modified mice, retroviral vectors, and optogenetics to study adult-born and mature DGCs after TBI. Although DGCs born after TBI acquired inhibitory synaptic inputs during their maturation, PV-mediated inhibition of adult-born DGCs was persistently reduced following TBI. This was not observed in mature granule cells and was not due to TBI-induced changes in PV cell density. This deficit in PV-mediated functional innervation was associated with a transient reduction in release probability at these synapses, which normalized as DGCs matured despite ongoing reduction of functional PV input. Surprisingly, although spontaneous inhibitory postsynaptic currents were reduced for mature granule cells after TBI, these were unchanged in adult-born DGCs. Taken together, these data demonstrate distinct differences in the de novo development and maintenance of PV+ synapses in the dentate gyrus after TBI. The addition of neurons with reduced PV+ interneuron-mediated feed-forward inhibition to the dentate gyrus could contribute to hippocampal hyperexcitability after severe brain injury.

The formation of long-term memory during sleep depends on the reactivation and redistribution of recently acquired mnemonic information during non-rapid eye movement (NREM) sleep. Animal studies suggest that hippocampal memory replay during slow-wave sleep is coordinated through the interaction of sharp-wave ripples, thalamocortical sleep spindles, and neocortical slow oscillations (SOs). However, direct evidence for the contribution of hippocampal network dynamics to sleep-dependent memory consolidation in humans remains limited. Here, we investigated sleep-dependent memory consolidation in patients (n=13) with transient global amnesia (TGA), a clinical syndrome associated with focal and transient lesions of the hippocampal CA1 region. Patients completed a verbal paired-associative learning task followed by nocturnal polysomnography and subsequent memory retrieval during the acute phase of TGA (acute condition) and again after clinical recovery (follow-up condition). Overnight memory consolidation was significantly impaired during the acute phase compared with the follow-up session. NREM EEG theta power (4-8 Hz) was reduced during the acute phase of TGA. Importantly, increases in theta power from the acute to the follow-up session predicted corresponding improvements in memory consolidation within individuals. In contrast, established NREM markers of sleep-dependent memory consolidation, including sleep spindle density, SO density, and SO-spindle coupling, did not differ between the acute and follow-up conditions. These findings suggest that transient hippocampal CA1 dysfunction disrupts sleep-related hippocampal network dynamics reflected in reduced NREM theta activity, which in turn is associated with impaired memory consolidation. Sleep-related theta oscillations may therefore represent a functional marker of hippocampal network integrity during sleep-dependent memory consolidation in humans.

The cerebellum supports a wide range of motor and non-motor behaviors, yet the specific cerebellar regions and neuronal populations underlying this functional diversity remain poorly understood. Here, we mapped patterns of immediate early gene activation in the cerebellar cortex during the accelerating rotarod task, an established form of cerebellum-dependent motor learning. Using the immediate early gene c-Fos to identify cells active during the task, we found significantly greater activation throughout the vermis and in crus I of the hemispheres, specifically within the molecular and granule cell layers, compared to naive controls. However, a second control group, which was exposed to the rotarod apparatus but not placed on the rotating rod, revealed similar patterns of c-Fos expression, suggesting that novelty contributed substantially to the observed activation. To separate this potential novelty signal from the motor component of the task, mice were habituated to the apparatus prior to testing. This habituation resulted in an overall decrease in c-Fos expression across cerebellar lobules, with significant motor activity-related differences found in the anterior vermis. Finally, to determine how learning influences c-Fos expression patterns, mice were subjected to a multi-day motor learning paradigm in which genetic tagging of c-Fos-expressing neurons was performed on day 1, followed by c-Fos immunostaining on day 5. We found that, in mice that had learned the task, there was no significant increase in c-Fos signal above naive controls, indicating that rotarod task performance no longer recruited the same level of cerebellar cortical activity, consistent with a potentially reduced role for the cerebellar cortex once the task is well learned. Overall, these findings reveal distinct patterns of activation associated with novelty, motor behaviour, and learning, providing a framework for linking behavior to regional cerebellar function.