Spontaneous Activity Patterns Are Altered in the Developing Visual Cortex of the Fmr1 Knockout Mouse

Network state transitions during cortical development

Mammalian cortical networks are active before synaptogenesis begins in earnest, before neuronal migration is complete, and well before an animal opens its eyes and begins to actively explore its surroundings. This early activity undergoes several transformations during development. The most important of these is a transition from episodic synchronous network events, which are necessary for patterning the neocortex into functionally related modules, to desynchronized activity that is computationally more powerful and efficient. Network desynchronization is perhaps the most dramatic and abrupt developmental event in an otherwise slow and gradual process of brain maturation. In this Review, we summarize what is known about the phenomenology of developmental synchronous activity in the rodent neocortex and speculate on the mechanisms that drive its eventual desynchronization. We argue that desynchronization of network activity is a fundamental step through which the cortex transitions from passive, bottom-up detection of sensory stimuli to active sensory processing with top-down modulation.

The NKCC1 Inhibitor Bumetanide Restores Cortical Feedforward Inhibition and Lessens Sensory Hypersensitivity in Early Postnatal Fragile X Mice

BACKGROUND

Exaggerated responses to sensory stimuli, a hallmark of fragile X syndrome, contribute to anxiety and learning challenges. Sensory hypersensitivity is recapitulated in the Fmr1 knockout (KO) mouse model of fragile X syndrome. Recent studies in Fmr1 KO mice have demonstrated differences in the activity of cortical interneurons and a delayed switch in the polarity of GABA (gamma-aminobutyric acid) signaling during development. Previously, we reported that blocking the chloride transporter NKCC1 with the diuretic bumetanide could rescue synaptic circuit phenotypes in the primary somatosensory cortex (S1) of Fmr1 KO mice. However, it remains unknown whether bumetanide can rescue earlier circuit phenotypes or sensory hypersensitivity in Fmr1 KO mice.

METHODS

We used acute and chronic systemic administration of bumetanide in Fmr1 KO mice and performed in vivo 2-photon calcium imaging to record neuronal activity, while tracking mouse behavior with high-resolution videos.

RESULTS

We demonstrated that layer 2/3 pyramidal neurons in the S1 of Fmr1 KO mice showed a higher frequency of synchronous events on postnatal day 6 than wild-type controls. This was reversed by acute administration of bumetanide. Furthermore, chronic bumetanide treatment (postnatal days 5-14) restored S1 circuit differences in Fmr1 KO mice, including reduced neuronal adaptation to repetitive whisker stimulation, and ameliorated tactile defensiveness. Bumetanide treatment also rectified the reduced feedforward inhibition of layer 2/3 neurons in the S1 and boosted the circuit participation of parvalbumin interneurons.

CONCLUSIONS

This further supports the notion that synaptic, circuit, and sensory behavioral phenotypes in Fmr1 KO can be mitigated by inhibitors of NKCC1, such as the Food and Drug Administration-approved diuretic bumetanide.

Drosophila Contributions towards Understanding Neurofibromatosis 1

Neurofibromatosis 1 (NF1) is a multisymptomatic disorder with highly variable presentations, which include short stature, susceptibility to formation of the characteristic benign tumors known as neurofibromas, intense freckling and skin discoloration, and cognitive deficits, which characterize most children with the condition. Attention deficits and Autism Spectrum manifestations augment the compromised learning presented by most patients, leading to behavioral problems and school failure, while fragmented sleep contributes to chronic fatigue and poor quality of life. Neurofibromin (Nf1) is present ubiquitously during human development and postnatally in most neuronal, oligodendrocyte, and Schwann cells. Evidence largely from animal models including Drosophila suggests that the symptomatic variability may reflect distinct cell-type-specific functions of the protein, which emerge upon its loss, or mutations affecting the different functional domains of the protein. This review summarizes the contributions of Drosophila in modeling multiple NF1 manifestations, addressing hypotheses regarding the cell-type-specific functions of the protein and exploring the molecular pathways affected upon loss of the highly conserved fly homolog dNf1. Collectively, work in this model not only has efficiently and expediently modelled multiple aspects of the condition and increased understanding of its behavioral manifestations, but also has led to pharmaceutical strategies towards their amelioration.

Improvement of sensory deficits in fragile X mice by increasing cortical interneuron activity after the critical period

Changes in the function of inhibitory interneurons (INs) during cortical development could contribute to the pathophysiology of neurodevelopmental disorders. Using all-optical in vivo approaches, we find that parvalbumin (PV) INs and their immature precursors are hypoactive and transiently decoupled from excitatory neurons in postnatal mouse somatosensory cortex (S1) of Fmr1 KO mice, a model of fragile X syndrome (FXS). This leads to a loss of parvalbumin INs (PV-INs) in both mice and humans with FXS. Increasing the activity of future PV-INs in neonatal Fmr1 KO mice restores PV-IN density and ameliorates transcriptional dysregulation in S1, but not circuit dysfunction. Critically, administering an allosteric modulator of Kv3.1 channels after the S1 critical period does rescue circuit dynamics and tactile defensiveness. Symptoms in FXS and related disorders could be mitigated by targeting PV-INs.

Circuit-level theories for sensory dysfunction in autism: convergence across mouse models

Individuals with autism spectrum disorder (ASD) exhibit a diverse range of behavioral features and genetic backgrounds, but whether different genetic forms of autism involve convergent pathophysiology of brain function is unknown. Here, we analyze evidence for convergent deficits in neural circuit function across multiple transgenic mouse models of ASD. We focus on sensory areas of neocortex, where circuit differences may underlie atypical sensory processing, a central feature of autism. Many distinct circuit-level theories for ASD have been proposed, including increased excitation-inhibition (E-I) ratio and hyperexcitability, hypofunction of parvalbumin (PV) interneuron circuits, impaired homeostatic plasticity, degraded sensory coding, and others. We review these theories and assess the degree of convergence across ASD mouse models for each. Behaviorally, our analysis reveals that innate sensory detection behavior is heightened and sensory discrimination behavior is impaired across many ASD models. Neurophysiologically, PV hypofunction and increased E-I ratio are prevalent but only rarely generate hyperexcitability and excess spiking. Instead, sensory tuning and other aspects of neural coding are commonly degraded and may explain impaired discrimination behavior. Two distinct phenotypic clusters with opposing neural circuit signatures are evident across mouse models. Such clustering could suggest physiological subtypes of autism, which may facilitate the development of tailored therapeutic approaches.

Delaying the GABA Shift Indirectly Affects Membrane Properties in the Developing Hippocampus

During the first two postnatal weeks, intraneuronal chloride concentrations in rodents gradually decrease, causing a shift from depolarizing to hyperpolarizing GABA responses. The postnatal GABA shift is delayed in rodent models for neurodevelopmental disorders and in human patients, but the impact of a delayed GABA shift on the developing brain remains obscure. Here we examine the direct and indirect consequences of a delayed postnatal GABA shift on network development in organotypic hippocampal cultures made from 6- to 7-d-old mice by treating the cultures for 1 week with VU0463271, a specific inhibitor of the chloride exporter KCC2. We verified that VU treatment delayed the GABA shift and kept GABA signaling depolarizing until DIV9. We found that the structural and functional development of excitatory and inhibitory synapses at DIV9 was not affected after VU treatment. In line with previous studies, we observed that GABA signaling was already inhibitory in control and VU-treated postnatal slices. Surprisingly, 14 d after the VU treatment had ended (DIV21), we observed an increased frequency of spontaneous inhibitory postsynaptic currents in CA1 pyramidal cells, while excitatory currents were not changed. Synapse numbers and release probability were unaffected. We found that dendrite-targeting interneurons in the stratum radiatum had an elevated resting membrane potential, while pyramidal cells were less excitable compared with control slices. Our results show that depolarizing GABA signaling does not promote synapse formation after P7, and suggest that postnatal intracellular chloride levels indirectly affect membrane properties in a cell-specific manner.SIGNIFICANCE STATEMENT During brain development, the action of neurotransmitter GABA shifts from depolarizing to hyperpolarizing. This shift is a thought to play a critical role in synapse formation. A delayed shift is common in rodent models for neurodevelopmental disorders and in human patients, but its consequences for synaptic development remain obscure. Here, we delayed the GABA shift by 1 week in organotypic hippocampal cultures and carefully examined the consequences for circuit development. We find that delaying the shift has no direct effects on synaptic development, but instead leads to indirect, cell type-specific changes in membrane properties. Our data call for careful assessment of alterations in cellular excitability in neurodevelopmental disorders.

The effect of single-cell knockout of Fragile X Messenger Ribonucleoprotein on synaptic structural plasticity

Fragile X Syndrome (FXS) is the best-known form of inherited intellectual disability caused by the loss-of-function mutation in a single gene. The FMR1 gene mutation abolishes the expression of Fragile X Messenger Ribonucleoprotein (FMRP), which regulates the expression of many synaptic proteins. Cortical pyramidal neurons in postmortem FXS patient brains show abnormally high density and immature morphology of dendritic spines; this phenotype is replicated in the Fmr1 knockout (KO) mouse. While FMRP is well-positioned in the dendrite to regulate synaptic plasticity, intriguing in vitro and in vivo data show that wild type neurons embedded in a network of Fmr1 KO neurons or glia exhibit spine abnormalities just as neurons in Fmr1 global KO mice. This raises the question: does FMRP regulate synaptic morphology and dynamics in a cell-autonomous manner, or do the synaptic phenotypes arise from abnormal pre-synaptic inputs? To address this question, we combined viral and mouse genetic approaches to delete FMRP from a very sparse subset of cortical layer 5 pyramidal neurons (L5 PyrNs) either during early postnatal development or in adulthood. We then followed the structural dynamics of dendritic spines on these Fmr1 KO neurons by in vivo two-photon microscopy. We found that, while L5 PyrNs in adult Fmr1 global KO mice have abnormally high density of thin spines, single-cell Fmr1 KO in adulthood does not affect spine density, morphology, or dynamics. On the contrary, neurons with neonatal FMRP deletion have normal spine density but elevated spine formation at 1 month of age, replicating the phenotype in Fmr1 global KO mice. Interestingly, these neurons exhibit elevated thin spine density, but normal total spine density, by adulthood. Together, our data reveal cell-autonomous FMRP regulation of cortical synaptic dynamics during adolescence, but spine defects in adulthood also implicate non-cell-autonomous factors.

fmr1 Mutation Alters the Early Development of Sensory Coding and Hunting and Social Behaviors in Larval Zebrafish

Autism spectrum disorders (ASDs) are developmental in origin; however, little is known about how they affect the early development of behavior and sensory coding. The most common inherited form of autism is Fragile X syndrome (FXS), caused by a mutation in FMR1 Mutation of fmr1 in zebrafish causes anxiety-like behavior, hyperactivity, and hypersensitivity in auditory and visual processing. Here, we show that zebrafish fmr1-/- mutant larvae of either sex also display changes in hunting behavior, tectal coding, and social interaction. During hunting, they were less successful at catching prey and displayed altered behavioral sequences. In the tectum, representations of prey-like stimuli were more diffuse and had higher dimensionality. In a social behavioral assay, they spent more time observing a conspecific but responded more slowly to social cues. However, when given a choice of rearing environment fmr1-/- larvae preferred one with reduced visual stimulation, and rearing them in this environment reduced genotype-specific effects on tectal excitability. Together, these results shed new light on how fmr1-/- changes the early development of neural systems and behavior in a vertebrate.SIGNIFICANCE STATEMENT Autism spectrum disorders (ASDs) are caused by changes in early neural development. Animal models of ASDs offer the opportunity to study these developmental processes in greater detail than in humans. Here, we found that a zebrafish mutant for a gene which in humans causes one type of ASD showed early alterations in hunting behavior, social behavior, and how visual stimuli are represented in the brain. However, we also found that mutant fish preferred reduced visual stimulation, and rearing them in this environment reduced alterations in neural activity patterns. These results suggest interesting new directions for using zebrafish as a model to study the development of brain and behavior in ASDs, and how the impact of ASDs could potentially be reduced.

A Novel Mechanism for T1R-Independent Taste Responses to Concentrated Sugars

Recent findings from our laboratory demonstrated that the rostral nucleus of the solitary tract (rNST) retains some responsiveness to sugars in double-knock-out mice lacking either the T1R1+T1R3 (KO1+3) or T1R2+T1R3 (KO2+3) taste receptor heterodimers. Here, we extended these findings in the parabrachial nucleus (PBN) of male and female KO1+3 mice using warm stimuli to optimize sugar responses and employing additional concentrations and pharmacological agents to probe mechanisms. PBN T1R-independent sugar responses, including those to concentrated glucose, were more evident than in rNST. Similar to the NST, there were no "sugar-best" neurons in KO1+3 mice. Nevertheless, 1000 mm glucose activated nearly 55% of PBN neurons, with responses usually occurring in neurons that also displayed acid and amiloride-insensitive NaCl responses. In wild-type (WT) mice, concentrated sugars activated the same electrolyte-sensitive neurons but also "sugar-best" cells. Regardless of genotype, phlorizin, an inhibitor of the sodium-glucose co-transporter (SGLT), a component of a hypothesized alternate glucose-sensing mechanism, did not diminish responses to 1000 mm glucose. The efficacy of concentrated sugars for driving neurons broadly responsive to electrolytes implied an origin from Type III taste bud cells. To test this, we used the carbonic anhydrase (CA) inhibitor dorzolamide (DRZ), previously shown to inhibit amiloride-insensitive sodium responses arising from Type III taste bud cells. Dorzolamide had no effect on sugar-elicited responses in WT sugar-best PBN neurons but strongly suppressed them in WT and KO1+3 electrolyte-generalist neurons. These findings suggest a novel T1R-independent mechanism for hyperosmotic sugars, involving a CA-dependent mechanism in Type III taste bud cells.SIGNIFICANCE STATEMENT Since the discovery of Tas1r receptors for sugars and artificial sweeteners, evidence has accrued that mice lacking these receptors maintain some behavioral, physiological, and neural responsiveness to sugars. But the substrate(s) has remained elusive. Here, we recorded from parabrachial nucleus (PBN) taste neurons and identified T1R-independent responses to hyperosmotic sugars dependent on carbonic anhydrase (CA) and occurring primarily in neurons broadly responsive to NaCl and acid, implying an origin from Type III taste bud cells. The effectiveness of different sugars in driving these T1R-independent responses did not correlate with their efficacy in driving licking, suggesting they evoke a nonsweet sensation. Nevertheless, these salient responses are likely to comprise an adequate cue for learned preferences that occur in the absence of T1R receptors.

Developmental deficits of MGE-derived interneurons in the Cntnap2 knockout mouse model of autism spectrum disorder

Interneurons are fundamental cells for maintaining the excitation-inhibition balance in the brain in health and disease. While interneurons have been shown to play a key role in the pathophysiology of autism spectrum disorder (ASD) in adult mice, little is known about how their maturation is altered in the developing striatum in ASD. Here, we aimed to track striatal developing interneurons and elucidate the molecular and physiological alterations in the Cntnap2 knockout mouse model. Using Stereo-seq and single-cell RNA sequencing data, we first characterized the pattern of expression of Cntnap2 in the adult brain and at embryonic stages in the medial ganglionic eminence (MGE), a transitory structure producing most cortical and striatal interneurons. We found that Cntnap2 is enriched in the striatum, compared to the cortex, particularly in the developing striatal cholinergic interneurons. We then revealed enhanced MGE-derived cell proliferation, followed by increased cell loss during the canonical window of developmental cell death in the Cntnap2 knockout mice. We uncovered specific cellular and molecular alterations in the developing Lhx6-expressing cholinergic interneurons of the striatum, which impacts interneuron firing properties during the first postnatal week. Overall, our work unveils some of the mechanisms underlying the shift in the developmental trajectory of striatal interneurons which greatly contribute to the ASD pathogenesis.

Gabrb3 is required for the functional integration of pyramidal neuron subtypes in the somatosensory cortex

Dysfunction of gamma-aminobutyric acid (GABA)ergic circuits is strongly associated with neurodevelopmental disorders. However, it is unclear how genetic predispositions impact circuit assembly. Using in vivo two-photon and widefield calcium imaging in developing mice, we show that Gabrb3, a gene strongly associated with autism spectrum disorder (ASD) and Angelman syndrome (AS), is enriched in contralaterally projecting pyramidal neurons and is required for inhibitory function. We report that Gabrb3 ablation leads to a developmental decrease in GABAergic synapses, increased local network synchrony, and long-lasting enhancement in functional connectivity of contralateral-but not ipsilateral-pyramidal neuron subtypes. In addition, Gabrb3 deletion leads to increased cortical response to tactile stimulation at neonatal stages. Using human transcriptomics and neuroimaging datasets from ASD subjects, we show that the spatial distribution of GABRB3 expression correlates with atypical connectivity in these subjects. Our studies reveal a requirement for Gabrb3 during the emergence of interhemispheric circuits for sensory processing.

Tonotopic differentiation of presynaptic neurotransmitter-releasing machinery in the auditory brainstem during the prehearing period and its selective deficits in Fmr1 knockout mice

Tonotopic organization is a fundamental feature of the auditory system. In the developing auditory brainstem, the ontogeny and maturation of neurotransmission progress from high to low frequencies along the tonotopic axis. To explore the underlying mechanism of this tonotopic development, we aim to determine whether the presynaptic machinery responsible for neurotransmitter release is tonotopically differentiated during development. In the current study, we examined vesicular neurotransmitter transporters and calcium sensors, two central players responsible for loading neurotransmitter into synaptic vesicles and for triggering neurotransmitter release in a calcium-dependent manner, respectively. Using immunocytochemistry, we characterized the distribution patterns of vesicular glutamate transporters (VGLUTs) 1 and 2, vesicular gamma-aminobutyric acid transporter (VGAT), and calcium sensor synaptotagmin (Syt) 1 and 2 in the developing mouse medial nucleus of the trapezoid body (MNTB). We identified tonotopic gradients of VGLUT1, VGAT, Syt1, and Syt2 in the first postnatal week, with higher protein densities in the more medial (high-frequency) portion of the MNTB. These gradients gradually flattened before the onset of hearing. In contrast, VGLUT2 was distributed relatively uniformly along the tonotopic axis during this prehearing period. In mice lacking Fragile X mental retardation protein, an mRNA-binding protein that regulates synaptic development and plasticity, progress to achieve the mature-like organization was altered for VGLUT1, Syt1, and Syt2, but not for VGAT. Together, our results identified novel organization patterns of selective presynaptic proteins in immature auditory synapses, providing a potential mechanism that may contribute to tonotopic differentiation of neurotransmission during normal and abnormal development.

Developmental stage-specific spontaneous activity contributes to callosal axon projections

The developing neocortex exhibits spontaneous network activity with various synchrony levels, which has been implicated in the formation of cortical circuits. We previously reported that the development of callosal axon projections, one of the major long-range axonal projections in the brain, is activity dependent. However, what sort of activity and when activity is indispensable are not known. Here, using a genetic method to manipulate network activity in a stage-specific manner, we demonstrated that network activity contributes to callosal axon projections in the mouse visual cortex during a ‘critical period’: restoring neuronal activity during that period resumed the projections, whereas restoration after the period failed. Furthermore, in vivo Ca²⁺ imaging revealed that the projections could be established even without fully restoring highly synchronous activity. Overall, our findings suggest that spontaneous network activity is selectively required during a critical developmental time window for the formation of long-range axonal projections in the cortex.

Epigenetic and Transcriptional Regulation of Spontaneous and Sensory Activity Dependent Programs During Neuronal Circuit Development

Spontaneous activity generated before the onset of sensory transduction has a key role in wiring developing sensory circuits. From axonal targeting, to synapse formation and elimination, to the balanced integration of neurons into developing circuits, this type of activity is implicated in a variety of cellular processes. However, little is known about its molecular mechanisms of action, especially at the level of genome regulation. Conversely, sensory experience-dependent activity implements well-characterized transcriptional and epigenetic chromatin programs that underlie heterogeneous but specific genomic responses that shape both postnatal circuit development and neuroplasticity in the adult. In this review, we focus on our knowledge of the developmental processes regulated by spontaneous activity and the underlying transcriptional mechanisms. We also review novel findings on how chromatin regulates the specificity and developmental induction of the experience-dependent program, and speculate their relevance for our understanding of how spontaneous activity may act at the genomic level to instruct circuit assembly and prepare developing neurons for sensory-dependent connectivity refinement and processing.

Brain-wide visual habituation networks in wild type and fmr1 zebrafish

Habituation is a form of learning during which animals stop responding to repetitive stimuli, and deficits in habituation are characteristic of several psychiatric disorders. Due to technical challenges, the brain-wide networks mediating habituation are poorly understood. Here we report brain-wide calcium imaging during larval zebrafish habituation to repeated visual looming stimuli. We show that different functional categories of loom-sensitive neurons are located in characteristic locations throughout the brain, and that both the functional properties of their networks and the resulting behavior can be modulated by stimulus saliency and timing. Using graph theory, we identify a visual circuit that habituates minimally, a moderately habituating midbrain population proposed to mediate the sensorimotor transformation, and downstream circuit elements responsible for higher order representations and the delivery of behavior. Zebrafish larvae carrying a mutation in the fmr1 gene have a systematic shift toward sustained premotor activity in this network, and show slower behavioral habituation.

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.

Impact of In Utero Exposure to Antiepileptic Drugs on Neonatal Brain Function

In utero brain development underpins brain health across the lifespan but is vulnerable to physiological and pharmacological perturbation. Here, we show that antiepileptic medication during pregnancy impacts on cortical activity during neonatal sleep, a potent indicator of newborn brain health. These effects are evident in frequency-specific functional brain networks and carry prognostic information for later neurodevelopment. Notably, such effects differ between different antiepileptic drugs that suggest neurodevelopmental adversity from exposure to antiepileptic drugs and not maternal epilepsy per se. This work provides translatable bedside metrics of brain health that are sensitive to the effects of antiepileptic drugs on postnatal neurodevelopment and carry direct prognostic value.

Oxytocin Shapes Spontaneous Activity Patterns in the Developing Visual Cortex by Activating Somatostatin Interneurons

Spontaneous network activity shapes emerging neuronal circuits during early brain development prior to sensory perception. However, how neuromodulation influences this activity is not fully understood. Here, we report that the neuromodulator oxytocin differentially shapes spontaneous activity patterns across sensory cortices. In vivo, oxytocin strongly decreased the frequency and pairwise correlations of spontaneous activity events in the primary visual cortex (V1), but it did not affect the frequency of spontaneous network events in the somatosensory cortex (S1). Patch-clamp recordings in slices and RNAscope showed that oxytocin affects S1 excitatory and inhibitory neurons similarly, whereas in V1, oxytocin targets only inhibitory neurons. Somatostatin-positive (SST+) interneurons expressed the oxytocin receptor and were activated by oxytocin in V1. Accordingly, pharmacogenetic silencing of V1 SST+ interneurons fully blocked oxytocin's effect on inhibition in vitro as well its effect on spontaneous activity patterns in vivo. Thus, oxytocin decreases the excitatory/inhibitory (E/I) ratio by recruiting SST+ interneurons and modulates specific features of V1 spontaneous activity patterns that are crucial for the wiring and refining of developing sensory circuits.

Altered brain-wide auditory networks in a zebrafish model of fragile X syndrome

BACKGROUND

Loss or disrupted expression of the FMR1 gene causes fragile X syndrome (FXS), the most common monogenetic form of autism in humans. Although disruptions in sensory processing are core traits of FXS and autism, the neural underpinnings of these phenotypes are poorly understood. Using calcium imaging to record from the entire brain at cellular resolution, we investigated neuronal responses to visual and auditory stimuli in larval zebrafish, using fmr1 mutants to model FXS. The purpose of this study was to model the alterations of sensory networks, brain-wide and at cellular resolution, that underlie the sensory aspects of FXS and autism.

RESULTS

Combining functional analyses with the neurons' anatomical positions, we found that fmr1-/- animals have normal responses to visual motion. However, there were several alterations in the auditory processing of fmr1-/- animals. Auditory responses were more plentiful in hindbrain structures and in the thalamus. The thalamus, torus semicircularis, and tegmentum had clusters of neurons that responded more strongly to auditory stimuli in fmr1-/- animals. Functional connectivity networks showed more inter-regional connectivity at lower sound intensities (a - 3 to - 6 dB shift) in fmr1-/- larvae compared to wild type. Finally, the decoding capacities of specific components of the ascending auditory pathway were altered: the octavolateralis nucleus within the hindbrain had significantly stronger decoding of auditory amplitude while the telencephalon had weaker decoding in fmr1-/- mutants.

CONCLUSIONS

We demonstrated that fmr1-/- larvae are hypersensitive to sound, with a 3-6 dB shift in sensitivity, and identified four sub-cortical brain regions with more plentiful responses and/or greater response strengths to auditory stimuli. We also constructed an experimentally supported model of how auditory information may be processed brain-wide in fmr1-/- larvae. Our model suggests that the early ascending auditory pathway transmits more auditory information, with less filtering and modulation, in this model of FXS.

Shifting Developmental Trajectories During Critical Periods of Brain Formation

Critical periods of brain development are epochs of heightened plasticity driven by environmental influence necessary for normal brain function. Recent studies are beginning to shed light on the possibility that timely interventions during critical periods hold potential to reorient abnormal developmental trajectories in animal models of neurological and neuropsychiatric disorders. In this review, we re-examine the criteria defining critical periods, highlighting the recently discovered mechanisms of developmental plasticity in health and disease. In addition, we touch upon technological improvements for modeling critical periods in human-derived neural networks in vitro. These scientific advances associated with the use of developmental manipulations in the immature brain of animal models are the basic preclinical systems that will allow the future translatability of timely interventions into clinical applications for neurodevelopmental disorders such as intellectual disability, autism spectrum disorders (ASD) and schizophrenia.

Disruption of Critical Period Plasticity in a Mouse Model of Neurofibromatosis Type 1

Neurofibromatosis type 1 (NF1) is a common monogenic neurodevelopmental disorder associated with physical and cognitive problems. The cognitive issues are thought to arise from increased release of the neurotransmitter GABA. Modulating the signaling pathways causing increased GABA release in a mouse model of NF1 reverts deficits in hippocampal learning. However, clinical trials based on these approaches have so far been unsuccessful. We therefore used a combination of slice electrophysiology, in vivo two-photon calcium imaging, and optical imaging of intrinsic signal in a mouse model of NF1 to investigate whether cortical development is affected in NF1, possibly causing lifelong consequences that cannot be rescued by reducing inhibition later in life. We find that, in NF1 mice of both sexes, inhibition increases strongly during the development of the visual cortex and remains high. While this increase in cortical inhibition does not affect spontaneous cortical activity patterns during early cortical development, the critical period for ocular dominance plasticity is shortened in NF1 mice due to its early closure but unaltered onset. Notably, after environmental enrichment, differences in inhibitory innervation and ocular dominance plasticity between NF1 mice and WT littermates disappear. These results provide the first evidence for critical period dysregulation in NF1 and suggest that treatments aimed at normalizing levels of inhibition will need to start at early stages of development.SIGNIFICANCE STATEMENT Neurofibromatosis type 1 is associated with cognitive problems for which no treatment is currently available. This study shows that, in a mouse model of neurofibromatosis type 1, cortical inhibition is increased during development and critical period regulation is disturbed. Rearing the mice in an environment that stimulates cognitive function overcomes these deficits. These results uncover critical period dysregulation as a novel mechanism in the pathogenesis of neurofibromatosis type 1. This suggests that targeting the affected signaling pathways in neurofibromatosis type 1 for the treatment of cognitive disabilities may have to start at a much younger age than has so far been tested in clinical trials.