Impaired cognitive discrimination and discoordination of coupled theta-gamma oscillations in Fmr1 knockout mice

Sex differences in cholinergic signaling affect functional outcomes for theta-gamma coordination in hippocampal subcircuits following experimental febrile status epilepticus

OBJECTIVE

Sex determines cognitive outcome in animal models of early life seizure, where males exhibit impaired hippocampal-dependent learning and memory compared with females. The physiological underpinnings of this sex effect are unclear. Cholinergic signaling is essential for the generation of hippocampal oscillations, and supplementation of cholinergic precursors prior to status epilepticus in immature male rats prevents subsequent memory deficits. We hypothesized that there are sex differences in acetylcholine circuits and their response to experimental febrile status epilepticus (eFSE).

METHODS

eFSE was induced in male and female rat pups. We transversed the hippocampus of postnatal day >60 control (CTL) and eFSE rats with a 64-channel laminar silicon probe to assay cholinergic-dependent theta oscillations under urethane anesthesia. Local field potential properties were compared during (1) baseline sensory stimulation, (2) pharmacological stimulation via acetylcholine reuptake blockade, and (3) sensory stimulation after muscarinic acetylcholine receptor block (atropine).

RESULTS

In all groups, a baseline tail pinch could elicit theta oscillations via corticohippocampal synaptic input. Following atropine, a tail pinch response could no longer be elicited in CTL male, CTL female, or eFSE female rats. In contrast, induced slow theta power in eFSE males after atropine was not decreased to spontaneous levels. Analysis of oscillation bandwidths revealed sex differences in acetylcholine modulation of theta frequency and slow gamma frequency and power. This study also identified significant effects of both sex and eFSE on baseline theta-gamma comodulation, indicating a loss of coupling in eFSE males and a potential gain of function in eFSE females.

SIGNIFICANCE

There are differences in cholinergic modulation of theta and gamma signal coordination between male and female rats. These differences may underlie worse cognitive outcomes in males following eFSE. Promoting the efficacy of muscarinic acetylcholine signaling prior to or following early life seizures could elucidate a mechanism for the temporal discoordination of neural signals within and between hippocampus and neocortex and provide a novel therapeutic approach for improving cognitive outcomes.

Adult Inception of Ketogenic Diet Therapy Increases Sleep during the Dark Cycle in C57BL/6J Wild Type and Fragile X Mice

Sleep problems are a significant phenotype in children with fragile X syndrome. Our prior work assessed sleep-wake cycles in Fmr1KO male mice and wild type (WT) littermate controls in response to ketogenic diet therapy where mice were treated from weaning (postnatal day 18) through study completion (5-6 months of age). A potentially confounding issue with commencing treatment during an active period of growth is the significant reduction in weight gain in response to the ketogenic diet. The aim here was to employ sleep electroencephalography (EEG) to assess sleep-wake cycles in mice in response to the Fmr1 genotype and a ketogenic diet, with treatment starting at postnatal day 95. EEG results were compared with prior sleep outcomes to determine if the later intervention was efficacious, as well as with published rest-activity patterns to determine if actigraphy is a viable surrogate for sleep EEG. The data replicated findings that Fmr1KO mice exhibit sleep-wake patterns similar to wild type littermates during the dark cycle when maintained on a control purified-ingredient diet but revealed a genotype-specific difference during hours 4-6 of the light cycle of the increased wake (decreased sleep and NREM) state in Fmr1KO mice. Treatment with a high-fat, low-carbohydrate ketogenic diet increased the percentage of NREM sleep in both wild type and Fmr1KO mice during the dark cycle. Differences in sleep microstructure (length of wake bouts) supported the altered sleep states in response to ketogenic diet. Commencing ketogenic diet treatment in adulthood resulted in a 15% (WT) and 8.6% (Fmr1KO) decrease in body weight after 28 days of treatment, but not the severe reduction in body weight associated with starting treatment at weaning. We conclude that the lack of evidence for improved sleep during the light cycle (mouse sleep time) in Fmr1KO mice in response to ketogenic diet therapy in two studies suggests that ketogenic diet may not be beneficial in treating sleep problems associated with fragile X and that actigraphy is not a reliable surrogate for sleep EEG in mice.

From synapses to circuits: What mouse models have taught us about how autism spectrum disorder impacts hippocampal function

Autism spectrum disorder (ASD) is a complex neurodevelopmental disorder that impacts a variety of cognitive and behavioral domains. While a genetic component of ASD has been well-established, none of the numerous syndromic genes identified in humans accounts for more than 1% of the clinical patients. Due to this large number of target genes, numerous mouse models of the disorder have been generated. However, the focus on distinct brain circuits, behavioral phenotypes and diverse experimental approaches has made it difficult to synthesize the overwhelming number of model animal studies into concrete throughlines that connect the data across levels of investigation. Here we chose to focus on one circuit, the hippocampus, and one hypothesis, a shift in excitatory/inhibitory balance, to examine, from the level of the tripartite synapse up to the level of in vivo circuit activity, the key commonalities across disparate models that can illustrate a path towards a better mechanistic understanding of ASD's impact on hippocampal circuit function.

Rescue of sharp wave-ripples and prevention of network hyperexcitability in the ventral but not the dorsal hippocampus of a rat model of fragile X syndrome

Fragile X syndrome (FXS) is a genetic neurodevelopmental disorder characterized by intellectual disability and is related to autism. FXS is caused by mutations of the fragile X messenger ribonucleoprotein 1 gene (Fmr1) and is associated with alterations in neuronal network excitability in several brain areas including hippocampus. The loss of fragile X protein affects brain oscillations, however, the effects of FXS on hippocampal sharp wave-ripples (SWRs), an endogenous hippocampal pattern contributing to memory consolidation have not been sufficiently clarified. In addition, it is still not known whether dorsal and ventral hippocampus are similarly affected by FXS. We used a Fmr1 knock-out (KO) rat model of FXS and electrophysiological recordings from the CA1 area of adult rat hippocampal slices to assess spontaneous and evoked neural activity. We find that SWRs and associated multiunit activity are affected in the dorsal but not the ventral KO hippocampus, while complex spike bursts remain normal in both segments of the KO hippocampus. Local network excitability increases in the dorsal KO hippocampus. Furthermore, specifically in the ventral hippocampus of KO rats we found an increased effectiveness of inhibition in suppressing excitation and an upregulation of α1GABAA receptor subtype. These changes in the ventral KO hippocampus are accompanied by a striking reduction in its susceptibility to induced epileptiform activity. We propose that the neuronal network specifically in the ventral segment of the hippocampus is reorganized in adult Fmr1-KO rats by means of balanced changes between excitability and inhibition to ensure normal generation of SWRs and preventing at the same time derailment of the neural activity toward hyperexcitability.

Relaxed Alert Electroencephalography Screening for Mild Traumatic Brain Injury in Athletes

Due to the mildness of initial injury, many athletes with recurrent mild traumatic brain injury (mTBI) are misdiagnosed with other neuropsychiatric illnesses. This study was designed as a proof-of-principle feasibility trial for athletic trainers at a sports facility to generate electroencephalograms (EEGs) from student athletes for discriminating (mTBI) associated EEGs from uninjured ones. A total of 47 EEGs were generated, with 30 athletes recruited at baseline (BL) pre-season, after a concussive injury (IN), and post-season (PS). Outcomes included: 1) visual analyses of EEGs by a neurologist; 2) support vector machine (SVM) classification for inferences about whether particular groups belonged to the three subgroups of BL, IN, or PS; and 3) analyses of EEG synchronies including phase locking value (PLV) computed between pairs of distinct electrodes. All EEGs were visually interpreted as normal. SVM classification showed that BL and IN could be discriminated with 81% accuracy using features of EEG synchronies combined. Frontal inter-hemispheric phase synchronization measured by PLV was significantly lower in the IN group. It is feasible for athletic trainers to record high quality EEGs from student athletes. Also, spatially localized metrics of EEG synchrony can discriminate mTBI associated EEGs from control EEGs.

Ketogenic Diet Affects Sleep Architecture in C57BL/6J Wild Type and Fragile X Mice

Nearly half of children with fragile X syndrome experience sleep problems including trouble falling asleep and frequent nighttime awakenings. The goals here were to assess sleep-wake cycles in mice in response to Fmr1 genotype and a dietary intervention that reduces hyperactivity. Electroencephalography (EEG) results were compared with published rest-activity patterns to determine if actigraphy is a viable surrogate for sleep EEG. Specifically, sleep-wake patterns in adult wild type and Fmr1KO littermate mice were recorded after EEG electrode implantation and the recordings manually scored for vigilance states. The data indicated that Fmr1KO mice exhibited sleep-wake patterns similar to wild type littermates when maintained on a control purified ingredient diet. Treatment with a high-fat, low-carbohydrate ketogenic diet increased the percentage of non-rapid eye movement (NREM) sleep in both wild type and Fmr1KO mice during the dark cycle, which corresponded to decreased activity levels. Treatment with a ketogenic diet flattened diurnal sleep periodicity in both wild type and Fmr1KO mice. Differences in several sleep microstructure outcomes (number and length of sleep and wake bouts) supported the altered sleep states in response to a ketogenic diet and were correlated with altered rest-activity cycles. While actigraphy may be a less expensive, reduced labor surrogate for sleep EEG during the dark cycle, daytime resting in mice did not correlate with EEG sleep states.

Fmr1-KO mice failure to detect object novelty associates with a post-test decrease of structural and synaptic plasticity upstream of the hippocampus

Mice with deletion of the FMR1 gene show episodic memory impairments and exhibit dendritic spines and synaptic plasticity defects prevalently identified in non-training conditions. Based on evidence that synaptic changes associated with normal or abnormal memory emerge when mice are cognitively challenged, here we examine whether, and how, fragile entorhinal and hippocampal synapses are remodeled when mice succeed or fail to learn. We trained Fmr1 knockout (KO) and wild-type C57BL/6J (WT) mice in the novel object recognition (NOR) paradigm with 1 h or 24 h training-to-test intervals and then assessed whether varying the time between the presentation of similar and different objects modulates NOR performance and plasticity along the entorhinal cortex-hippocampus axis. At the 1 h-interval, KO mice failed to discriminate the novel object, showed a collapse of spines in the lateral entorhinal cortex (LEC), and of long-term potentiation (LTP) in the lateral perforant path (LPP), but a normal increase in hippocampal spines. At the 24 h, they exhibited intact NOR performance, typical LEC and hippocampal spines, and exaggerated LPP-LTP. Our findings reveal that the inability of mice to detect object novelty primarily stands in their impediment to elaborate, and convey to the hippocampus, sensory/perceptive object representations.

Experience-dependent changes in hippocampal spatial activity and hippocampal circuit function are disrupted in a rat model of Fragile X Syndrome

BACKGROUND

Fragile X syndrome (FXS) is a common single gene cause of intellectual disability and autism spectrum disorder. Cognitive inflexibility is one of the hallmarks of FXS with affected individuals showing extreme difficulty adapting to novel or complex situations. To explore the neural correlates of this cognitive inflexibility, we used a rat model of FXS (Fmr1^-/y).

METHODS

We recorded from the CA1 in Fmr1^-/y and WT littermates over six 10-min exploration sessions in a novel environment-three sessions per day (ITI 10 min). Our recordings yielded 288 and 246 putative pyramidal cells from 7 WT and 7 Fmr1^-/y rats, respectively.

RESULTS

On the first day of exploration of a novel environment, the firing rate and spatial tuning of CA1 pyramidal neurons was similar between wild-type (WT) and Fmr1^-/y rats. However, while CA1 pyramidal neurons from WT rats showed experience-dependent changes in firing and spatial tuning between the first and second day of exposure to the environment, these changes were decreased or absent in CA1 neurons of Fmr1^-/y rats. These findings were consistent with increased excitability of Fmr1^-/y CA1 neurons in ex vivo hippocampal slices, which correlated with reduced synaptic inputs from the medial entorhinal cortex. Lastly, activity patterns of CA1 pyramidal neurons were dis-coordinated with respect to hippocampal oscillatory activity in Fmr1^-/y rats.

LIMITATIONS

It is still unclear how the observed circuit function abnormalities give rise to behavioural deficits in Fmr1^-/y rats. Future experiments will focus on this connection as well as the contribution of other neuronal cell types in the hippocampal circuit pathophysiology associated with the loss of FMRP. It would also be interesting to see if hippocampal circuit deficits converge with those seen in other rodent models of intellectual disability.

CONCLUSIONS

In conclusion, we found that hippocampal place cells from Fmr1^-/y rats show similar spatial firing properties as those from WT rats but do not show the same experience-dependent increase in spatial specificity or the experience-dependent changes in network coordination. Our findings offer support to a network-level origin of cognitive deficits in FXS.

Homeostatic plasticity and excitation-inhibition balance: The good, the bad, and the ugly

In this review, we discuss the significance of the synaptic excitation/inhibition (E/I) balance in the context of homeostatic plasticity, whose primary goal is thought to maintain neuronal firing rates at a set point. We first provide an overview of the processes through which patterned input activity drives synaptic E/I tuning and maturation of circuits during development. Next, we emphasize the importance of the E/I balance at the synaptic level (homeostatic control of message reception) as a means to achieve the goal (homeostatic control of information transmission) at the network level and consider how compromised homeostatic plasticity associated with neurological diseases leads to hyperactivity, network instability, and ultimately improper information processing. Lastly, we highlight several pathological conditions related to sensory deafferentation and describe how, in some cases, homeostatic compensation without appropriate sensory inputs can result in phantom perceptions.

Cortical interneurons in autism

The mechanistic underpinnings of autism remain a subject of debate and controversy. Why do individuals with autism share an overlapping set of atypical behaviors and symptoms, despite having different genetic and environmental risk factors? A major challenge in developing new therapies for autism has been the inability to identify convergent neural phenotypes that could explain the common set of symptoms that result in the diagnosis. Although no striking macroscopic neuropathological changes have been identified in autism, there is growing evidence that inhibitory interneurons (INs) play an important role in its neural basis. In this Review, we evaluate and interpret this evidence, focusing on recent findings showing reduced density and activity of the parvalbumin class of INs. We discuss the need for additional studies that investigate how genes and the environment interact to change the developmental trajectory of INs, permanently altering their numbers, connectivity and circuit engagement.

Increased 2-arachidonoyl-sn-glycerol levels normalize cortical responses to sound and improve behaviors in Fmr1 KO mice

Background

Individuals with Fragile X syndrome (FXS) and autism spectrum disorder (ASD) exhibit an array of symptoms, including sociability deficits, increased anxiety, hyperactivity, and sensory hyperexcitability. It is unclear how endocannabinoid (eCB) modulation can be targeted to alleviate neurophysiological abnormalities in FXS as behavioral research reveals benefits to inhibiting cannabinoid (CB) receptor activation and increasing endocannabinoid ligand levels. Here, we hypothesize that enhancement of 2-arachidonoyl-sn-glycerol (2-AG) in Fragile X mental retardation 1 gene knock-out (Fmr1 KO) mice may reduce cortical hyperexcitability and behavioral abnormalities observed in FXS.

Methods

To test whether an increase in 2-AG levels normalized cortical responses in a mouse model of FXS, animals were subjected to electroencephalography (EEG) recording and behavioral assessment following treatment with JZL-184, an irreversible inhibitor of monoacylglycerol lipase (MAGL). Assessment of 2-AG was performed using lipidomic analysis in conjunction with various doses and time points post-administration of JZL-184. Baseline electrocortical activity and evoked responses to sound stimuli were measured using a 30-channel multielectrode array (MEA) in adult male mice before, 4 h, and 1 day post-intraperitoneal injection of JZL-184 or vehicle. Behavior assessment was done using the open field and elevated plus maze 4 h post-treatment.

Results

Lipidomic analysis showed that 8 mg/kg JZL-184 significantly increased the levels of 2-AG in the auditory cortex of both Fmr1 KO and WT mice 4 h post-treatment compared to vehicle controls. EEG recordings revealed a reduction in the abnormally enhanced baseline gamma-band power in Fmr1 KO mice and significantly improved evoked synchronization to auditory stimuli in the gamma-band range post-JZL-184 treatment. JZL-184 treatment also ameliorated anxiety-like and hyperactivity phenotypes in Fmr1 KO mice.

Conclusions

Overall, these results indicate that increasing 2-AG levels may serve as a potential therapeutic approach to normalize cortical responses and improve behavioral outcomes in FXS and possibly other ASDs.

Supplementary Information

The online version contains supplementary material available at 10.1186/s11689-021-09394-x.

Recurrent febrile seizures alter intrahippocampal temporal coordination but do not cause spatial learning impairments

OBJECTIVE

Febrile seizures (FSs) are the most common form of seizures in children. Single short FSs are benign, but FSs lasting longer than 30 min, termed febrile status epilepticus, may result in neurological sequelae. However, there is little information about an intermediary condition, brief recurrent FSs (RFSs). The goal of this study was to determine the role of RFSs on spatial learning and memory and the properties of spontaneous hippocampal signals.

METHODS

A hippocampus-dependent active avoidance task was used to assess spatial learning and memory in adult rats that underwent experimental RFSs (eRFSs) in early life compared with their littermate controls. Following completion of the task, we utilized high-density laminar probes to measure spontaneous hippocampal CA1 circuit activity under urethane anesthesia, which allowed for the simultaneous recording of input regions in CA1 associated with both CA3 and entorhinal cortex.

RESULTS

RFSs did not result in deficits in the active avoidance spatial test, a hippocampus-dependent test of spatial learning and memory. However, in vivo high-density laminar electrode recordings from eRFS rats had significantly altered power and frequency expression of theta and gamma bandwidths as well as signaling efficacy along the CA1 somatodendritic axis. Thus, although eRFS modified CA1 neuronal input/output dynamics, these alterations were not sufficient to impair active avoidance spatial behavior.

SIGNIFICANCE

These findings indicate that although eRFSs do not result in spatial cognitive deficits in the active avoidance task, recurrent seizures do alter the brain and result in longstanding changes in the temporal organization of the hippocampus.

Deletion of Fmr1 from Forebrain Excitatory Neurons Triggers Abnormal Cellular, EEG, and Behavioral Phenotypes in the Auditory Cortex of a Mouse Model of Fragile X Syndrome

Fragile X syndrome (FXS) is a leading genetic cause of autism with symptoms that include sensory processing deficits. In both humans with FXS and a mouse model [Fmr1 knockout (KO) mouse], electroencephalographic (EEG) recordings show enhanced resting state gamma power and reduced sound-evoked gamma synchrony. We previously showed that elevated levels of matrix metalloproteinase-9 (MMP-9) may contribute to these phenotypes by affecting perineuronal nets (PNNs) around parvalbumin (PV) interneurons in the auditory cortex of Fmr1 KO mice. However, how different cell types within local cortical circuits contribute to these deficits is not known. Here, we examined whether Fmr1 deletion in forebrain excitatory neurons affects neural oscillations, MMP-9 activity, and PV/PNN expression in the auditory cortex. We found that cortical MMP-9 gelatinase activity, mTOR/Akt phosphorylation, and resting EEG gamma power were enhanced in CreNex1/Fmr1Flox/y conditional KO (cKO) mice, whereas the density of PV/PNN cells was reduced. The CreNex1/Fmr1Flox/y cKO mice also show increased locomotor activity, but not the anxiety-like behaviors. These results indicate that fragile X mental retardation protein changes in excitatory neurons in the cortex are sufficient to elicit cellular, electrophysiological, and behavioral phenotypes in Fmr1 KO mice. More broadly, these results indicate that local cortical circuit abnormalities contribute to sensory processing deficits in autism spectrum disorders.

Shifted phase of EEG cross-frequency coupling in individuals with Phelan-McDermid syndrome

BACKGROUND

Phelan-McDermid Syndrome (PMS) is a rare condition caused by deletion or mutation of the SHANK3 gene. Individuals with PMS frequently present with intellectual disability, autism spectrum disorder, and other neurodevelopmental challenges. Electroencephalography (EEG) can provide a window into network-level function in PMS.

METHODS

Here, we analyze EEG data collected across multiple sites in individuals with PMS (n = 26) and typically developing individuals (n = 15). We quantify oscillatory power, alpha-gamma phase-amplitude coupling strength, and phase bias, a measure of the phase of cross frequency coupling thought to reflect the balance of feedforward (bottom-up) and feedback (top-down) activity.

RESULTS

We find individuals with PMS display increased alpha-gamma phase bias (U = 3.841, p < 0.0005), predominantly over posterior electrodes. Most individuals with PMS demonstrate positive overall phase bias while most typically developing individuals demonstrate negative overall phase bias. Among individuals with PMS, strength of alpha-gamma phase-amplitude coupling was associated with Sameness, Ritualistic, and Compulsive behaviors as measured by the Repetitive Behavior Scales-Revised (Beta = 0.545, p = 0.011).

CONCLUSIONS

Increased phase bias suggests potential circuit-level mechanisms underlying phenotype in PMS, offering opportunities for back-translation of findings into animal models and targeting in clinical trials.

Chronic intermittent hypoxia alters main olfactory bulb activity and olfaction

Olfactory dysfunction is commonly observed in patients with obstructive sleep apnea (OSA), which is related to chronic intermittent hypoxia (CIH). OSA patients exhibit alterations in discrimination, identification and odor detection threshold. These olfactory functions strongly rely on neuronal processing within the main olfactory bulb (MOB). However, a direct evaluation of the effects of controlled CIH on olfaction and MOB network activity has not been performed. Here, we used electrophysiological field recordings in vivo to evaluate the effects of 21-day-long CIH on MOB network activity and its response to odors. In addition, we assessed animals´ olfaction with the buried food and habituation/dishabituation tests. We found that mice exposed to CIH show alterations in MOB spontaneous activity in vivo, consisting of a reduction in beta and gamma frequency bands power along with an increase in the theta band power. Likewise, the MOB was less responsive to odor stimulation, since the proportional increase of the power of its population activity in response to four different odorants was smaller than the one observed in control animals. These CIH-induced MOB functional alterations correlate with a reduction in the ability to detect, habituate and discriminate olfactory stimuli. Our findings indicate that CIH generates alterations in the MOB neural network, which could be involved in the olfactory deterioration in patients with OSA.

Acute pharmacological inhibition of matrix metalloproteinase-9 activity during development restores perineuronal net formation and normalizes auditory processing in Fmr1 KO mice

Individuals with Fragile X Syndrome (FXS) and autism spectrum disorder (ASD) exhibit cognitive impairments, social deficits, increased anxiety, and sensory hyperexcitability. Previously, we showed that elevated levels of matrix metalloproteinase-9 (MMP-9) may contribute to abnormal development of parvalbumin (PV) interneurons and perineuronal nets (PNNs) in the developing auditory cortex (AC) of Fmr1 knock-out (KO) mice, which likely underlie auditory hypersensitivity. Thus, MMP-9 may serve as a potential target for treatment of auditory hypersensitivity in FXS. Here, we used the MMP-2/9 inhibitor, SB-3CT, to pharmacologically inhibit MMP-9 activity during a specific developmental period and to test whether inhibition of MMP-9 activity reverses neural oscillation deficits and behavioral impairments by enhancing PNN formation around PV cells in Fmr1 KO mice. Electroencephalography (EEG) was used to measure resting state and sound-evoked electrocortical activity in auditory and frontal cortices of postnatal day (P)22-23 male mice before and one-day after treatment with SB-3CT (25 mg/kg) or vehicle. At P27-28, animal behaviors were tested to measure the effects of the treatment on anxiety and hyperactivity. Results show that acute inhibition of MMP-9 activity improved evoked synchronization to auditory stimuli and ameliorated mouse behavioral deficits. MMP-9 inhibition enhanced PNN formation, increased PV levels and TrkB phosphorylation yet reduced Akt phosphorylation in the AC of Fmr1 KO mice. Our results show that MMP-9 inhibition during early postnatal development is beneficial in reducing some auditory processing deficits in the FXS mouse model and may serve as a candidate therapeutic for reversing sensory hypersensitivity in FXS and possibly other ASDs.

Intact Auditory Cortical Cross-Frequency Coupling in Early and Chronic Schizophrenia

BACKGROUND

Previous work has identified a hierarchical organization of neural oscillations that supports performance of complex cognitive and perceptual tasks, and can be indexed with phase-amplitude coupling (PAC) between low- and high-frequency oscillations. Our aim was to employ enhanced source localization afforded by magnetoencephalography (MEG) to expand on earlier reports of intact auditory cortical PAC in schizophrenia and to investigate how PAC may evolve over the early and chronic phases of the illness.

METHODS

Individuals with early schizophrenia (n=12) (≤5 years of illness duration), chronic schizophrenia (n=16) (>5 years of illness duration) and healthy comparators (n = 17) performed the auditory steady state response (ASSR) to 40, 30, and 20 Hz stimuli during MEG recordings. We estimated amplitude and PAC on the MEG ASSR source localized to the auditory cortices.

RESULTS

Gamma amplitude during 40-Hz ASSR exhibited a significant group by hemisphere interaction, with both patient groups showing reduced right hemisphere amplitude and no overall lateralization in contrast to the right hemisphere lateralization demonstrated in controls. We found significant PAC in the right auditory cortex during the 40-Hz entrainment condition relative to baseline, however, PAC did not differ significantly between groups.

CONCLUSIONS

In the current study, we demonstrated an apparent sparing of ASSR-related PAC across phases of the illness, in contrast with impaired cortical gamma oscillation amplitudes. The distinction between our PAC and evoked ASSR findings supports the notion of separate but interacting circuits for the generation and maintenance of sensory gamma oscillations. The apparent sparing of PAC in both early and chronic schizophrenia patients could imply that the neuropathology of schizophrenia differentially affects these mechanisms across different stages of the disease. Future studies should investigate the distinction between PAC during passive tasks and more cognitively demanding task such as working memory so that we can begin to understand the influence of schizophrenia neuropathology on the larger framework for modulating neurocomputational capacity.

Impaired Reliability and Precision of Spiking in Adults But Not Juveniles in a Mouse Model of Fragile X Syndrome

Fragile X syndrome (FXS) is the most common source of intellectual disability and autism. Extensive studies have been performed on the network and behavioral correlates of the syndrome, but our knowledge about intrinsic conductance changes is still limited. In this study, we show a differential effect of FMRP knockout in different subsections of hippocampus using whole-cell patch clamp in mouse hippocampal slices. We observed no significant change in spike numbers in the CA1 region of hippocampus, but a significant increase in CA3, in juvenile mice. However, in adult mice we see a reduction in spike number in the CA1 with no significant difference in CA3. In addition, we see increased variability in spike numbers in CA1 cells following a variety of steady and modulated current step protocols. This effect emerges in adult mice (8 weeks) but not juvenile mice (4 weeks). This increased spiking variability was correlated with reduced spike number and with elevated AHP. The increased AHP arose from elevated SK currents (small conductance calcium-activated potassium channels), but other currents involved in medium AHP, such as I_h and M, were not significantly different. We obtained a partial rescue of the cellular variability phenotype when we blocked SK current using the specific blocker apamin. Our observations provide a single-cell correlate of the network observations of response variability and loss of synchronization, and suggest that the elevation of SK currents in FXS may provide a partial mechanistic explanation for this difference.

Long-Lasting Input-Specific Experience-Dependent Changes of Hippocampus Synaptic Function Measured in the Anesthetized Rat

How experience causes long-lasting changes in the brain is a central question in neuroscience. The common view is that synaptic function is altered by experience to change brain circuit functions that underlie conditioned behavior. We examined hippocampus synaptic circuit function in vivo, in three groups of animals, to assess the impact of experience on hippocampus function in rats. The “conditioned” group acquired a shock-conditioned place response during a cognitively-challenging, hippocampus synaptic plasticity-dependent task. The no-shock group had similar exposure to the environmental conditions but no conditioning. The home-cage group was experimentally naive. After the one-week retention test, under anesthesia, we stimulated the perforant path inputs to CA1, which terminate in stratum lacunosum moleculare (slm), and to the dentate gyrus (DG), which terminate in the molecular layer. We find synaptic compartment specific changes that differ amongst the groups. The evoked field EPSP (fEPSP) and pre-spike field response are enhanced only at the DG input layer and only in conditioned animals. The DG responses, measured by the population spiking activity and post-spike responses, are enhanced in both the conditioned and no-shock groups compared to home-cage animals. These changes are pathway specific because no differences are observed in slm of CA1. These findings demonstrate long-term, experience-dependent, pathway-specific alterations to synaptic circuit function of the hippocampus.

Synaptic plasticity/dysplasticity, process memory and item memory in rodent models of mental dysfunction

Activity-dependent changes in the effective connection strength of synapses are a fundamental feature of a nervous system. This so-called synaptic plasticity is thought to underlie storage of information in memory and has been hypothesized to be crucial for the effects of cognitive behavioral therapy. Synaptic plasticity stores information in a neural network, creating a trace of neural activity from past experience. The plasticity can also change the behavior of the network so the network can differentially transform/compute information in future activations. We discuss these two related but separable functions of synaptic plasticity; one we call "item memory" as it represents and stores items of information in memory, the other we call "process memory" as it encodes and stores functions such as computations to modify network information processing capabilities. We review evidence of item and process memory operations in behavior and evidence that experience modifies the brain's functional networks. We discuss neurodevelopmental rodent models relevant for understanding mental illness and compare two models in which one model, neonatal ventral hippocampal lesion (NVHL) has beneficial adult outcomes after being exposed to an adolescent cognitive experience that is potentially similar to cognitive behavioral therapy. The other model, gestational day 17 methylazoxymethanol acetate (GD17-MAM), does not benefit from the same adolescent cognitive experience. We propose that process memory is altered by early cognitive experience in NVHL rats but not in GD17-MAM rats, and discuss how dysplasticity factors may contribute to the differential adult outcomes after early cognitive experience in the NVHL and MAM models.

Cognitive Behavior Classification From Scalp EEG Signals

Electroencephalography (EEG) has become increasingly valuable outside of its traditional use in neurology. EEG is now used for neuropsychiatric diagnosis, neurological evaluation of traumatic brain injury, neurotherapy, gaming, neurofeedback, mindfulness, and cognitive enhancement training. The trend to increase the number of EEG electrodes, the development of novel analytical methods, and the availability of large data sets has created a data analysis challenge to find the "signal of interest" that conveys the most information about ongoing cognitive effort. Accordingly, we compare three common types of neural synchrony measures that are applied to EEG-power analysis, phase locking, and phase-amplitude coupling to assess which analytical measure provides the best separation between EEG signals that were recorded, while healthy subjects performed eight cognitive tasks-Hopkins Verbal Learning Test and its delayed version, Stroop Test, Symbol Digit Modality Test, Controlled Oral Word Association Test, Trail Marking Test, Digit Span Test, and Benton Visual Retention Test. We find that of the three analytical methods, phase-amplitude coupling, specifically theta (4-7 Hz)-high gamma (70-90 Hz) obtained from frontal and parietal EEG electrodes provides both the largest separation between the EEG during cognitive tasks and also the highest classification accuracy between pairs of tasks. We also find that phase-locking analysis provides the most distinct clustering of tasks based on their utilization of long-term memory. Finally, we show that phase-amplitude coupling is the least sensitive to contamination by intense jaw-clenching muscle artifact.

Parvalbumin Cell Ablation of NMDA-R1 Leads to Altered Phase, But Not Amplitude, of Gamma-Band Cross-Frequency Coupling

Altered gamma-band electrophysiological activity in individuals with autism spectrum disorder (ASD) is well documented, and analogous gamma-band alterations are recapitulated in several preclinical murine models relevant to ASD. Such gamma-band activity is hypothesized to underlie local circuit processes. Gamma-band cross-frequency coupling (CFC), a related though distinct metric, interrogates local neural circuit signal integration. Several recent studies have observed perturbed gamma-band CFC in individuals with ASD, although the direction of change remains unresolved. It also remains unclear whether murine models relevant to ASD recapitulate this altered gamma-band CFC. As such, this study examined whether mice with parvalbumin (PV) cell-specific ablation of NMDA-R1 (PVcre/NR1fl/fl) demonstrated altered gamma-band CFC as compared with their control littermates (PVcre/NR1+/+-mice that do not have the PV cell-specific ablation of NMDA-R1). Ten mice of each genotype had 4 min of "resting" electroencephalography recorded and analyzed. First, resting electrophysiological power was parsed into the canonical frequency bands and genotype-related differences were subsequently explored so as to provide context for the subsequent CFC analyses. PVcre/NR1fl/fl mice exhibited an increase in resting power specific to the high gamma-band, but not other frequency bands, as compared with PVcre/NR1+/+. CFC analyses then examined both the standard magnitude (strength) of CFC and the novel metric PhaseMax-which denotes the phase of the lower frequency signal at which the peak higher frequency signal power occurred. PVcre/NR1fl/fl mice exhibited altered PhaseMax, but not strength, of gamma-band CFC as compared with PVcre/NR1+/+ mice. As such, this study suggests a potential novel metric to explore when studying neuropsychiatric disorders.

Maternal High Fat Diet-Induced Obesity Modifies Histone Binding and Expression of Oxtr in Offspring Hippocampus in a Sex-Specific Manner

Maternal obesity during pregnancy increases risk for neurodevelopmental disorders in offspring, although the underlying mechanisms remain unclear. Epigenetic deregulation associates with many neurodevelopmental disorders, and recent evidence indicates that maternal nutritional status can alter chromatin marks in the offspring brain. Thus, maternal obesity may disrupt epigenetic regulation of gene expression during offspring neurodevelopment. Using a C57BL/6 mouse model, we investigated whether maternal high fat diet (mHFD)-induced obesity alters the expression of genes previously implicated in the etiology of neurodevelopmental disorders within the Gestational Day 17.5 (GD 17.5) offspring hippocampus. We found significant two-fold upregulation of oxytocin receptor (Oxtr) mRNA in the hippocampus of male, but not female, GD 17.5 offspring from mHFD-induced obese dams (p < 0.05). To determine whether altered histone binding at the Oxtr gene promoter may underpin these transcriptional changes, we then performed chromatin immunoprecipitation (ChIP). Consistent with the Oxtr transcriptional changes, we observed increased binding of active histone mark H3K9Ac at the Oxtr transcriptional start site (TSS) in the hippocampus of mHFD male (p < 0.05), but not female, offspring. Together, these data indicate an increased vulnerability of male offspring to maternal obesity-induced changes in chromatin remodeling processes that regulate gene expression in the developing hippocampus, and contributes to our understanding of how early life nutrition affects the offspring brain epigenome.

Developmental Changes in EEG Phenotypes in a Mouse Model of Fragile X Syndrome

Fragile X Syndrome (FXS) is a leading genetic cause of autism and intellectual disabilities. Sensory-processing deficits are common in humans with FXS and an animal model, the Fmr1 knockout (KO) mouse, manifesting in the auditory system as debilitating hypersensitivity and abnormal electroencephalographic (EEG) and event-related potential (ERP) phenotypes. FXS is a neurodevelopmental disorder, but how EEG/ERP phenotypes change during development is unclear. Therefore, we characterized baseline and stimulus-evoked EEG in auditory and frontal cortex of developing (postnatal day (P) 21 and P30) and adult (P60) wildtype (WT) and Fmr1 KO mice with the FVB genetic background. We found that baseline gamma-band power and N1 amplitude of auditory ERP were increased in frontal cortex of Fmr1 KO mice during development and in adults. Baseline gamma power was increased in auditory cortex at P30. Genotype differences in stimulus-evoked gamma power were present in both cortical regions, but the direction and strength of the changes were age-dependent. These findings suggest that cortical deficits are present during early development and may contribute to sensory-processing deficits in FXS, which in turn may lead to anxiety and delayed language. Developmental changes in EEG measures indicate that observations at a single time-point during development are not reflective of FXS disease progression and highlight the need to identify developmental trajectories and optimal windows for treatment.

Toward the Language Oscillogenome

Language has been argued to arise, both ontogenetically and phylogenetically, from specific patterns of brain wiring. We argue that it can further be shown that core features of language processing emerge from particular phasal and cross-frequency coupling properties of neural oscillations; what has been referred to as the language ‘oscillome.’ It is expected that basic aspects of the language oscillome result from genetic guidance, what we will here call the language ‘oscillogenome,’ for which we will put forward a list of candidate genes. We have considered genes for altered brain rhythmicity in conditions involving language deficits: autism spectrum disorders, schizophrenia, specific language impairment and dyslexia. These selected genes map on to aspects of brain function, particularly on to neurotransmitter function. We stress that caution should be adopted in the construction of any oscillogenome, given the range of potential roles particular localized frequency bands have in cognition. Our aim is to propose a set of genome-to-language linking hypotheses that, given testing, would grant explanatory power to brain rhythms with respect to language processing and evolution.

Inhibitory Interneurons Regulate Temporal Precision and Correlations in Cortical Circuits

GABAergic interneurons, which are highly diverse, have long been thought to contribute to the timing of neural activity as well as to the generation and shaping of brain rhythms. GABAergic activity is crucial not only for entrainment of oscillatory activity across a neural population, but also for precise regulation of the timing of action potentials and the suppression of slow-timescale correlations. The diversity of inhibition provides the potential for flexible regulation of patterned activity, but also poses a challenge to identifying the elements of excitatory-inhibitory interactions underlying network engagement. This review highlights the key roles of inhibitory interneurons in spike correlations and brain rhythms, describes several scales on which GABAergic inhibition regulates timing in neural networks, and identifies potential consequences of inhibitory dysfunction.

Gamma oscillations in cognitive disorders

Gamma oscillations (∼25-100 Hz) are believed to play a role in cognition. Accordingly, aberrant gamma oscillations have been observed in several cognitive disorders, including Alzheimer's disease and Fragile X syndrome. Here, we review how recent results showing abnormal gamma rhythms in Alzheimer's disease and Fragile X syndrome help reveal links between cellular disturbances and cognitive impairments. We also discuss how gamma results from rodent models of Alzheimer's disease and Fragile X syndrome may provide insights about unique functions of distinct slow (∼25-50 Hz) and fast gamma (∼55-100 Hz) subtypes. Finally, we consider studies employing brain stimulation paradigms in Alzheimer's disease and discuss how such studies may reveal causal relationships between gamma impairments and memory disturbances.

Translation-relevant EEG phenotypes in a mouse model of Fragile X Syndrome

Identification of comparable biomarkers in humans and validated animal models will facilitate pre-clinical to clinical therapeutic pipelines to treat neurodevelopmental disorders. Fragile X Syndrome (FXS) is a leading known genetic cause of intellectual disability with symptoms that include increased anxiety, social and sensory processing deficits. Recent EEG studies in humans with FXS have identified neural oscillation deficits that include enhanced resting state gamma power and reduced inter-trial coherence of sound evoked gamma oscillations. To determine if analogous phenotypes are present in an animal model of FXS, we recorded EEGs in awake, freely moving Fmr1 knock out (KO) mice using similar stimuli as in the human studies. We report remarkably similar neural oscillation phenotypes in the Fmr1 KO mouse including enhanced resting state gamma power and reduced evoked gamma synchronization. The gamma band inter-trial coherence of neural response was reduced in both auditory and frontal cortex of Fmr1 KO mice stimulated with a sound whose envelope was modulated from 1 to 100 Hz, similar to that seen in humans with FXS. These deficits suggest a form of enhanced 'resting state noise' that interferes with the ability of the circuit to mount a synchronized response to sensory input, predicting specific sensory and cognitive deficits in FXS. The abnormal gamma oscillations are consistent with parvalbumin neuron and perineuronal net deficits seen in the Fmr1 KO mouse auditory cortex indicating that the EEG biomarkers are not only clinically relevant, but could also be used to probe cellular and circuit mechanisms of sensory hypersensitivity in FXS.

Impaired hippocampal representation of place in the Fmr1-knockout mouse model of fragile X syndrome

Fragile X syndrome (FXS) is an X-chromosome linked intellectual disability and the most common known inherited single gene cause of autism spectrum disorder (ASD). Building upon demonstrated deficits in neuronal plasticity and spatial memory in FXS, we investigated how spatial information processing is affected in vivo in an FXS mouse model (Fmr1-KO). Healthy hippocampal neurons (so-called place cells) exhibit place-related activity during spatial exploration, and their firing fields tend to remain stable over time. In contrast, we find impaired stability and reduced specificity of Fmr1-KO spatial representations. This is a potential biomarker for the cognitive dysfunction observed in FXS, informative on the ability to integrate sensory information into an abstract representation and successfully retain this conceptual memory. Our results provide key insight into the biological mechanisms underlying cognitive disabilities in FXS and ASD, paving the way for a targeted approach to remedy these.

Abnormal Sleep Architecture and Hippocampal Circuit Dysfunction in a Mouse Model of Fragile X Syndrome

Fragile X syndrome (FXS) is the most common heritable cause of intellectual disability and single-gene cause of autism spectrum disorder. The Fmr1 null mouse models much of the human disease including hyperarousal, sensory hypersensitivity, seizure activity, and hippocampus-dependent cognitive impairment. Sleep architecture is disorganized in FXS patients, but has not been examined in Fmr1 knockout (Fmr1-KO) mice. Hippocampal neural activity during sleep, which is implicated in memory processing, also remains uninvestigated in Fmr1-KO mice. We performed in vivo electrophysiological studies of freely behaving Fmr1-KO mice to assess neural activity, in the form of single-unit spiking and local field potential (LFP), within the hippocampal CA1 region during multiple differentiated sleep and wake states. Here, we demonstrate that Fmr1-KO mice exhibited a deficit in rapid eye movement sleep (REM) due to a reduction in the frequency of bouts of REM, consistent with sleep architecture abnormalities of FXS patients. Fmr1-KO CA1 pyramidal cells (CA1-PCs) were hyperactive in all sleep and wake states. Increased low gamma power in CA1 suggests that this hyperactivity was related to increased input to CA1 from CA3. By contrast, slower sharp-wave ripple events (SWRs) in Fmr1-KO mice exhibited longer event duration, slower oscillation frequency, with reduced CA1-PC firing rates during SWRs, yet the incidence rate of SWRs remained intact. These results suggest abnormal neuronal activity in the Fmr1-KO mouse during SWRs, and hyperactivity during other wake and sleep states, with likely adverse consequences for memory processes.

Of Men and Mice: Modeling the Fragile X Syndrome

The Fragile X Syndrome (FXS) is one of the most common forms of inherited intellectual disability in all human societies. Caused by the transcriptional silencing of a single gene, the fragile x mental retardation gene FMR1, FXS is characterized by a variety of symptoms, which range from mental disabilities to autism and epilepsy. More than 20 years ago, a first animal model was described, the Fmr1 knock-out mouse. Several other models have been developed since then, including conditional knock-out mice, knock-out rats, a zebrafish and a drosophila model. Using these model systems, various targets for potential pharmaceutical treatments have been identified and many treatments have been shown to be efficient in preclinical studies. However, all attempts to turn these findings into a therapy for patients have failed thus far. In this review, I will discuss underlying difficulties and address potential alternatives for our future research.

Abnormal hippocampal theta and gamma hypersynchrony produces network and spike timing disturbances in the Fmr1-KO mouse model of Fragile X syndrome

Neuronal networks can synchronize their activity through excitatory and inhibitory connections, which is conducive to synaptic plasticity. This synchronization is reflected in rhythmic fluctuations of the extracellular field. In the hippocampus, theta and gamma band LFP oscillations are a hallmark of the processing of spatial information and memory. Fragile X syndrome (FXS) is an intellectual disability and the most common genetic cause of autism spectrum disorder (Belmonte and Bourgeron, 2006). Here, we investigated how neuronal network synchronization in the mouse hippocampus is compromised by the Fmr1 mutation that causes FXS (Santos et al., 2014), relating recently observed single-cell level impairments (Arbab et al., 2017) to neuronal network aberrations. We implanted tetrodes in hippocampus of freely moving Fmr1-KO and littermate wildtype (WT) mice (Mientjes et al., 2006), to record spike trains from multiple, isolated neurons as well as LFPs in a spatial exploration paradigm. Compared to wild type mice, Fmr1-KO mice displayed greater power of hippocampal theta oscillations, and higher coherence in the slow gamma band. Additionally, spike trains of Fmr1-KO interneurons show decreased spike-count correlations and they are hypersynchronized with theta and slow gamma oscillations. The hypersynchronization of Fmr1-KO oscillations and spike timing reflects functional deficits in local networks. This network hypersynchronization pathologically decreases the heterogeneity of spike-LFP phase coupling, compromising information processing within the hippocampal circuit. These findings may reflect a pathophysiological mechanism explaining cognitive impairments in FXS and autism, in which there is anomalous processing of social and environmental cues and associated deficits in memory and cognition.

Enhanced Operant Extinction and Prefrontal Excitability in a Mouse Model of Angelman Syndrome

Angelman syndrome (AS), a neurodevelopmental disorder associated with intellectual disability, is caused by loss of maternal allele expression of UBE3A in neurons. Mouse models of AS faithfully recapitulate disease phenotypes across multiple domains, including behavior. Yet in AS, there has been only limited study of behaviors encoded by the prefrontal cortex, a region broadly involved in executive function and cognition. Because cognitive impairment is a core feature of AS, it is critical to develop behavioral readouts of prefrontal circuit function in AS mouse models. One such readout is behavioral extinction, which has been well described mechanistically and relies upon prefrontal circuits in rodents. Here we report exaggerated operant extinction in male AS model mice, concomitant with enhanced excitability in medial prefrontal neurons from male and female AS model mice. Abnormal behavior was specific to operant extinction, as two other prefrontally dependent tasks (cued fear extinction and visuospatial discrimination) were largely normal in AS model mice. Inducible deletion of Ube3a during adulthood was not sufficient to drive abnormal extinction, supporting the hypothesis that there is an early critical period for development of cognitive phenotypes in AS. This work represents the first formal experimental analysis of prefrontal circuit function in AS, and identifies operant extinction as a useful experimental paradigm for modeling cognitive aspects of AS in mice.SIGNIFICANCE STATEMENT Prefrontal cortex encodes "high-level" cognitive processes. Thus, understanding prefrontal function is critical in neurodevelopmental disorders where cognitive impairment is highly penetrant. Angelman syndrome is a neurodevelopmental disorder associated with speech and motor impairments, an outwardly happy demeanor, and intellectual disability. We describe a behavioral phenotype in a mouse model of Angelman syndrome and related abnormalities in prefrontal cortex function. We hypothesize that robust and reliable prefrontally encoded behavior may be used to model cognitive impairments in Angelman syndrome.

Normal CA1 Place Fields but Discoordinated Network Discharge in a Fmr1-Null Mouse Model of Fragile X Syndrome

Silence of FMR1 causes loss of fragile X mental retardation protein (FMRP) and dysregulated translation at synapses, resulting in the intellectual disability and autistic symptoms of fragile X syndrome (FXS). Synaptic dysfunction hypotheses for how intellectual disabilities like cognitive inflexibility arise in FXS predict impaired neural coding in the absence of FMRP. We tested the prediction by comparing hippocampus place cells in wild-type and FXS-model mice. Experience-driven CA1 synaptic function and synaptic plasticity changes are excessive in Fmr1-null mice, but CA1 place fields are normal. However, Fmr1-null discharge relationships to local field potential oscillations are abnormally weak, stereotyped, and homogeneous; also, discharge coordination within Fmr1-null place cell networks is weaker and less reliable than wild-type. Rather than disruption of single-cell neural codes, these findings point to invariant tuning of single-cell responses and inadequate discharge coordination within neural ensembles as a pathophysiological basis of cognitive inflexibility in FXS. VIDEO ABSTRACT.

Control of recollection by slow gamma dominating mid-frequency gamma in hippocampus CA1

Behavior is used to assess memory and cognitive deficits in animals like Fmr1-null mice that model Fragile X Syndrome, but behavior is a proxy for unknown neural events that define cognitive variables like recollection. We identified an electrophysiological signature of recollection in mouse dorsal Cornu Ammonis 1 (CA1) hippocampus. During a shocked-place avoidance task, slow gamma (SG) (30–50 Hz) dominates mid-frequency gamma (MG) (70–90 Hz) oscillations 2–3 s before successful avoidance, but not failures. Wild-type (WT) but not Fmr1-null mice rapidly adapt to relocating the shock; concurrently, SG/MG maxima (SG_dom) decrease in WT but not in cognitively inflexible Fmr1-null mice. During SG_dom, putative pyramidal cell ensembles represent distant locations; during place avoidance, these are avoided places. During shock relocation, WT ensembles represent distant locations near the currently correct shock zone, but Fmr1-null ensembles represent the formerly correct zone. These findings indicate that recollection occurs when CA1 SG dominates MG and that accurate recollection of inappropriate memories explains Fmr1-null cognitive inflexibility.

Behavior is often used as proxy to study memory and cognitive deficits in animals like Fmr1-KO mice that model Fragile X Syndrome, the most prevalent single-gene cause of intellectual disability and autism. However, it is unclear what neural events define cognitive variables like recollection of memory and cognitive inflexibility. We identified a signature of recollection in the local field potentials of mouse dorsal CA1 hippocampus. When mice on a rotating platform avoided an invisible, fixed shock zone, slow gamma (30–50 Hz) oscillations dominated mid-frequency gamma (70–90 Hz) oscillations (SG_dom) 2–3 s before mice successfully avoided the shock zone. Wild-type but not Fmr1-KO mice adapt to relocating the shock zone; concurrently, SG_dom decreases in wild-type but not in cognitively inflexible Fmr1-KO mice. During SG_dom, principal cell ensembles represent distant locations; during place avoidance, these are avoided places in the shock zone vicinity. During shock relocation, wild-type ensembles encode distant locations near the currently correct shock zone, but Fmr1-KO ensembles manifest representational inflexibility, encoding the formerly correct zone. These findings suggest evidence for competition amongst CA1 inputs for CA1 information-processing modes and indicate that recollection occurs when CA1 slow gamma dominates mid-frequency gamma and that accurate recollection of inappropriate memories explains Fmr1-KO cognitive inflexibility.

Drug development for neurodevelopmental disorders: lessons learned from fragile X syndrome

Neurodevelopmental disorders such as fragile X syndrome (FXS) result in lifelong cognitive and behavioural deficits and represent a major public health burden. FXS is the most frequent monogenic form of intellectual disability and autism, and the underlying pathophysiology linked to its causal gene, FMR1, has been the focus of intense research. Key alterations in synaptic function thought to underlie this neurodevelopmental disorder have been characterized and rescued in animal models of FXS using genetic and pharmacological approaches. These robust preclinical findings have led to the implementation of the most comprehensive drug development programme undertaken thus far for a genetically defined neurodevelopmental disorder, including phase IIb trials of metabotropic glutamate receptor 5 (mGluR5) antagonists and a phase III trial of a GABA_B receptor agonist. However, none of the trials has been able to unambiguously demonstrate efficacy, and they have also highlighted the extent of the knowledge gaps in drug development for FXS and other neurodevelopmental disorders. In this Review, we examine potential issues in the previous studies and future directions for preclinical and clinical trials. FXS is at the forefront of efforts to develop drugs for neurodevelopmental disorders, and lessons learned in the process will also be important for such disorders.

Effects of regulatory BC1 RNA deletion on synaptic plasticity, learning, and memory

Nonprotein coding dendritic BC1 RNA regulates translation of mRNAs in neurons. We examined two lines of BC1 knockout mice and report that loss of BC1 RNA exaggerates group I mGluR-stimulated LTD of the Schaffer collateral synapse, with one of the lines showing a much more enhanced DHPG-induced LTD than the other. When the animals were given the hippocampus-synaptic plasticity-dependent active place avoidance task, learning and memory were impaired in the BC1-KO line with the more severely altered DHPG-induced LTD. These findings indicate a role for BC1 RNA control of mGluR-dependent synaptic function in hippocampus and associated cognitive ability.

Phencyclidine Discoordinates Hippocampal Network Activity But Not Place Fields

We used the psychotomimetic phencyclidine (PCP) to investigate the relationships among cognitive behavior, coordinated neural network function, and information processing within the hippocampus place cell system. We report in rats that PCP (5 mg/kg, i.p.) impairs a well learned, hippocampus-dependent place avoidance behavior in rats that requires cognitive control even when PCP is injected directly into dorsal hippocampus. PCP increases 60-100 Hz medium-freguency gamma oscillations in hippocampus CA1 and these increases correlate with the cognitive impairment caused by systemic PCP administration. PCP discoordinates theta-modulated medium-frequency and slow gamma oscillations in CA1 LFPs such that medium-frequency gamma oscillations become more theta-organized than slow gamma oscillations. CA1 place cell firing fields are preserved under PCP, but the drug discoordinates the subsecond temporal organization of discharge among place cells. This discoordination causes place cell ensemble representations of a familiar space to cease resembling pre-PCP representations despite preserved place fields. These findings point to the cognitive impairments caused by PCP arising from neural discoordination. PCP disrupts the timing of discharge with respect to the subsecond timescales of theta and gamma oscillations in the LFP. Because these oscillations arise from local inhibitory synaptic activity, these findings point to excitation-inhibition discoordination as the root of PCP-induced cognitive impairment.SIGNIFICANCE STATEMENT Hippocampal neural discharge is temporally coordinated on timescales of theta and gamma oscillations in the LFP and the discharge of a subset of pyramidal neurons called "place cells" is spatially organized such that discharge is restricted to locations called a cell's "place field." Because this temporal coordination and spatial discharge organization is thought to represent spatial knowledge, we used the psychotomimetic phencyclidine (PCP) to disrupt cognitive behavior and assess the importance of neural coordination and place fields for spatial cognition. PCP impaired the judicious use of spatial information and discoordinated hippocampal discharge without disrupting firing fields. These findings dissociate place fields from spatial cognitive behavior and suggest that hippocampus discharge coordination is crucial to spatial cognition.

Cell Type-Specific mRNA Dysregulation in Hippocampal CA1 Pyramidal Neurons of the Fragile X Syndrome Mouse Model

Fragile X syndrome (FXS) is a genetic disorder due to the silencing of the Fmr1 gene, causing intellectual disability, seizures, hyperactivity, and social anxiety. All these symptoms result from the loss of expression of the RNA binding protein fragile X mental retardation protein (FMRP), which alters the neurodevelopmental program to abnormal wiring of specific circuits. Aberrant mRNAs translation associated with the loss of Fmr1 product is widely suspected to be in part the cause of FXS. However, precise gene expression changes involved in this disorder have yet to be defined. The objective of this study was to identify the set of mistranslated mRNAs that could contribute to neurological deficits in FXS. We used the RiboTag approach and RNA sequencing to provide an exhaustive listing of genes whose mRNAs are differentially translated in hippocampal CA1 pyramidal neurons as the integrative result of FMRP loss and subsequent neurodevelopmental adaptations. Among genes differentially regulated between adult WT and Fmr1^-/y mice, we found enrichment in FMRP-binders but also a majority of non-FMRP-binders. Interestingly, both up- and down-regulation of specific gene expression is relevant to fully understand the molecular deficiencies triggering FXS. More importantly, functional genomic analysis highlighted the importance of genes involved in neuronal connectivity. Among them, we show that Klk8 altered expression participates in the abnormal hippocampal dendritic spine maturation observed in a mouse model of FXS.

Measuring Mental Workload with EEG+fNIRS

We studied the capability of a Hybrid functional neuroimaging technique to quantify human mental workload (MWL). We have used electroencephalography (EEG) and functional near-infrared spectroscopy (fNIRS) as imaging modalities with 17 healthy subjects performing the letter n-back task, a standard experimental paradigm related to working memory (WM). The level of MWL was parametrically changed by variation of n from 0 to 3. Nineteen EEG channels were covering the whole-head and 19 fNIRS channels were located on the forehead to cover the most dominant brain region involved in WM. Grand block averaging of recorded signals revealed specific behaviors of oxygenated-hemoglobin level during changes in the level of MWL. A machine learning approach has been utilized for detection of the level of MWL. We extracted different features from EEG, fNIRS, and EEG+fNIRS signals as the biomarkers of MWL and fed them to a linear support vector machine (SVM) as train and test sets. These features were selected based on their sensitivity to the changes in the level of MWL according to the literature. We introduced a new category of features within fNIRS and EEG+fNIRS systems. In addition, the performance level of each feature category was systematically assessed. We also assessed the effect of number of features and window size in classification performance. SVM classifier used in order to discriminate between different combinations of cognitive states from binary- and multi-class states. In addition to the cross-validated performance level of the classifier other metrics such as sensitivity, specificity, and predictive values were calculated for a comprehensive assessment of the classification system. The Hybrid (EEG+fNIRS) system had an accuracy that was significantly higher than that of either EEG or fNIRS. Our results suggest that EEG+fNIRS features combined with a classifier are capable of robustly discriminating among various levels of MWL. Results suggest that EEG+fNIRS should be preferred to only EEG or fNIRS, in developing passive BCIs and other applications which need to monitor users' MWL.

Neural synchronization deficits linked to cortical hyper-excitability and auditory hypersensitivity in fragile X syndrome

Background

Studies in the fmr1 KO mouse demonstrate hyper-excitability and increased high-frequency neuronal activity in sensory cortex. These abnormalities may contribute to prominent and distressing sensory hypersensitivities in patients with fragile X syndrome (FXS). The current study investigated functional properties of auditory cortex using a sensory entrainment task in FXS.

Methods

EEG recordings were obtained from 17 adolescents and adults with FXS and 17 age- and sex-matched healthy controls. Participants heard an auditory chirp stimulus generated using a 1000-Hz tone that was amplitude modulated by a sinusoid linearly increasing in frequency from 0–100 Hz over 2 s.

Results

Single trial time-frequency analyses revealed decreased gamma band phase-locking to the chirp stimulus in FXS, which was strongly coupled with broadband increases in gamma power. Abnormalities in gamma phase-locking and power were also associated with theta-gamma amplitude-amplitude coupling during the pre-stimulus period and with parent reports of heightened sensory sensitivities and social communication deficits.

Conclusions

This represents the first demonstration of neural entrainment alterations in FXS patients and suggests that fast-spiking interneurons regulating synchronous high-frequency neural activity have reduced functionality. This reduced ability to synchronize high-frequency neural activity was related to the total power of background gamma band activity. These observations extend findings from fmr1 KO models of FXS, characterize a core pathophysiological aspect of FXS, and may provide a translational biomarker strategy for evaluating promising therapeutics.

Electronic supplementary material

The online version of this article (doi:10.1186/s13229-017-0140-1) contains supplementary material, which is available to authorized users.

Aberrant cognitive phenotypes and altered hippocampal BDNF expression related to epigenetic modifications in mice lacking the post-synaptic scaffolding protein SHANK1: Implications for autism spectrum disorder

Autism spectrum disorder (ASD) is a class of neurodevelopmental disorders characterized by persistent deficits in social communication/interaction, together with restricted/repetitive patterns of behavior. ASD is among the most heritable neuropsychiatric conditions, and while available evidence points to a complex set of genetic factors, the SHANK gene family has emerged as one of the most promising candidates. Here, we assessed ASD-related phenotypes with particular emphasis on social behavior and cognition in Shank1 mouse mutants in comparison to heterozygous and wildtype littermate controls across development in both sexes. While social approach behavior was evident in all experimental conditions and social recognition was only mildly affected by genotype, Shank1^-/- null mutant mice were severely impaired in object recognition memory. This effect was particularly prominent in juveniles, not due to impairments in object discrimination, and replicated in independent mouse cohorts. At the neurobiological level, object recognition deficits were paralleled by increased brain-derived neurotrophic factor (BDNF) protein expression in the hippocampus of Shank1^-/- mice; yet BDNF levels did not differ under baseline conditions. We therefore investigated changes in the epigenetic regulation of hippocampal BDNF expression and detected an enrichment of histone H3 acetylation at the Bdnf promoter1 in Shank1^-/- mice, consistent with increased learning-associated BDNF. Together, our findings indicate that Shank1 deletions lead to an aberrant cognitive phenotype characterized by severe impairments in object recognition memory and increased hippocampal BDNF levels, possibly due to epigenetic modifications. This result supports the link between ASD and intellectual disability, and suggests epigenetic regulation as a potential therapeutic target.

A resting EEG study of neocortical hyperexcitability and altered functional connectivity in fragile X syndrome

BACKGROUND

Cortical hyperexcitability due to abnormal fast-spiking inhibitory interneuron function has been documented in fmr1 KO mice, a mouse model of the fragile X syndrome which is the most common single gene cause of autism and intellectual disability.

METHODS

We collected resting state dense-array electroencephalography data from 21 fragile X syndrome (FXS) patients and 21 age-matched healthy participants.

RESULTS

FXS patients exhibited greater gamma frequency band power, which was correlated with social and sensory processing difficulties. Second, FXS patients showed increased spatial spreading of phase-synchronized high frequency neural activity in the gamma band. Third, we observed increased negative theta-to-gamma but decreased alpha-to-gamma band amplitude coupling, and the level of increased theta power was inversely related to the level of resting gamma power in FXS.

CONCLUSIONS

Increased theta band power and coupling from frontal sources may represent a mechanism providing compensatory inhibition of high-frequency gamma band activity, potentially contributing to the widely varying level of neurophysiological and behavioral abnormalities and treatment response seen in full-mutation FXS patients. These findings extend preclinical observations and provide new mechanistic insights into brain alterations and their variability across FXS patients. Electrophysiological measures may provide useful translational biomarkers for advancing drug development and individualizing treatments for neurodevelopmental disorders with associated neuronal hyperexcitability.

GABA-B Agonist Baclofen Normalizes Auditory-Evoked Neural Oscillations and Behavioral Deficits in the Fmr1 Knockout Mouse Model of Fragile X Syndrome

Fragile X syndrome is a genetic condition resulting from FMR1 gene mutation that leads to intellectual disability, autism-like symptoms, and sensory hypersensitivity. Arbaclofen, a GABA-B agonist, has shown efficacy in some individuals with FXS but has become unavailable after unsuccessful clinical trials, prompting interest in publicly available, racemic baclofen. The present study investigated whether racemic baclofen can remediate abnormalities of neural circuit function, sensory processing, and behavior in Fmr1 knockout mice, a rodent model of fragile X syndrome. Fmr1 knockout mice showed increased baseline and auditory-evoked high-frequency gamma (30-80 Hz) power relative to C57BL/6 controls, as measured by electroencephalography. These deficits were accompanied by decreased T maze spontaneous alternation, decreased social interactions, and increased open field center time, suggestive of diminished working memory, sociability, and anxiety-like behavior, respectively. Abnormal auditory-evoked gamma oscillations, working memory, and anxiety-related behavior were normalized by treatment with baclofen, but impaired sociability was not. Improvements in working memory were evident predominantly in mice whose auditory-evoked gamma oscillations were dampened by baclofen. These findings suggest that racemic baclofen may be useful for targeting sensory and cognitive disturbances in fragile X syndrome.

Modeling fast and slow gamma oscillations with interneurons of different subtype

Experimental and theoretical studies demonstrate that neuronal gamma oscillations crucially depend on interneurons, but current models do not consider the diversity of known interneuron subtypes. Moreover, in CA1 of the hippocampus, experimental evidence indicates the presence of multiple gamma oscillators, two of which may be coordinated by differing interneuron populations. In this article, we show that models of networks with competing interneuron populations with different postsynaptic effects are sufficient to generate, within CA1, distinct oscillatory regimes. We find that strong mutual inhibition between the interneuron populations permits distinct fast and slow gamma states, whereas weak mutual inhibition generates mixed gamma states. We develop idealized firing rate models to illuminate dynamic properties of these competitive gamma networks, and reinforce these concepts with basic spiking models. The models make several explicit predictions about gamma oscillators in CA1. Specifically, interneurons of different subtype phase-lock to different gamma states, and one population of interneurons is silenced and the other active during fast and slow gamma events. Finally, mutual inhibition between interneuron populations is necessary to generate distinct gamma states. Previous experimental studies indicate that fast and slow gamma oscillations reflect different information processing modes, although it is unclear whether these rhythms are intrinsic or imposed. The models outlined demonstrate that basic architectures can locally generate these oscillations, as well as capture other features of fast and slow gamma, including theta-phase preference and spontaneous transitions between gamma states. These models may extend to describe general dynamics in networks with diverse interneuron populations.NEW & NOTEWORTHY The oscillatory coordination of neural signals is crucial to healthy brain function. We have developed an idealized neuronal model that generates distinct fast and slow gamma oscillations, a known feature of the rodent hippocampus. Our work provides a mechanism of this phenomenon, as well as a theoretical framework for future experiments concerning hippocampal gamma. It moreover offers a tractable model of competitive gamma oscillations that is generalizable across the nervous system.

Active place avoidance is no more stressful than unreinforced exploration of a familiar environment

Training in the active place avoidance task changes hippocampus synaptic function, the dynamics of hippocampus local field potentials, place cell discharge, and active place avoidance memory is maintained by persistent PKMζ activity. The extent to which these changes reflect memory processes and/or stress responses is unknown. We designed a study to assess stress within the active place avoidance task by measuring serum corticosterone (CORT) at different stages of training. CORT levels did not differ between trained mice that learned to avoid the location of the mild foot shock, and untrained no-shock controls exposed to the same environment for the same amount of time. Yoked mice, that received unavoidable shocks in the same time sequence as the trained mice, had significantly higher CORT levels than mice in the trained and no-shock groups after the first trial. This increase in CORT disappeared by the fourth trial the following day, and levels of CORT for all groups matched that of home cage controls. The data demonstrate that place avoidance training is no more stressful than experiencing a familiar environment. We conclude that changes in neural function as a result of active place avoidance training are likely to reflect learning and memory processes rather than stress. © 2016 Wiley Periodicals, Inc.

Memory deficits with intact cognitive control in the methylazoxymethanol acetate (MAM) exposure model of neurodevelopmental insult

Cognitive impairments are amongst the most debilitating deficits of schizophrenia and the best predictor of functional outcome. Schizophrenia is hypothesized to have a neurodevelopmental origin, making animal models of neurodevelopmental insult important for testing predictions that early insults will impair cognitive function. Rats exposed to methylazoxymethanol acetate (MAM) at gestational day 17 display morphological, physiological and behavioral abnormalities relevant to schizophrenia. Here we investigate the cognitive abilities of adult MAM rats. We examined brain activity in MAM rats by histochemically assessing cytochrome oxidase enzyme activity, a metabolic marker of neuronal activity. To assess cognition, we used a hippocampus-dependent two-frame active place avoidance paradigm to examine learning and spatial memory, as well as cognitive control and flexibility using the same environment and evaluating the same set of behaviors. We confirmed that adult MAM rats have altered hippocampal morphology and brain function, and that they are hyperactive in an open field. The latter likely indicates MAM rats have a sensorimotor gating deficit that is common to many animal models used for schizophrenia research. On first inspection, cognitive control seems impaired in MAM rats, indicated by more errors during the two-frame active place avoidance task. Because MAM rats are hyperactive throughout place avoidance training, we considered the possibility that the hyperlocomotion may account for the apparent cognitive deficits. These deficits were reduced on the basis of measures of cognitive performance that account for motor activity differences. However, though other aspects of memory are intact, the ability of MAM rats to express trial-to-trial memory is delayed compared to control rats. These findings suggest that spatial learning and cognitive abilities are largely intact, that the most prominent cognitive deficit is specific to acquiring memory in the MAM neurodevelopmental model, and that hyperactivity can confound assessments of cognition in animal models of mental dysfunction.