Rapid critical period induction by tonic inhibition in visual cortex

Astrocyte regulation of critical period plasticity across neural circuits

Critical periods are brief windows of heightened neural circuit plasticity that allow circuits to permanently reset their structure and function to facilitate robust organismal behavior. Understanding the cellular and molecular mechanisms that instruct critical period timing is of broad clinical interest, as altered developmental plasticity is linked to multiple neurodevelopmental disorders. While intrinsic, neuronal mechanisms shape both neural circuit remodeling and critical period timing, recent data indicate that signaling from astrocytes and surrounding glia can both promote and limit critical period plasticity. In this short review, we discuss recent breakthroughs in our understanding of astrocytes in critical period plasticity and highlight pioneering work in Drosophila.

Pramipexole, a D3 receptor agonist, increases cortical gamma power and biochemical correlates of cortical excitation; implications for mood disorders

Major depressive disorder (MDD) has been associated with deficits in working memory as well as underlying gamma oscillation power. Consistent with this, overall reductions in cortical excitation have also been described with MDD. In previous work, we have demonstrated that the monoamine reuptake inhibitor venlafaxine increases gamma oscillation power in ex vivo hippocampal slices and that this is associated with concomitant increases in pyramidal arbour and reduced levels of plasticity-restricting perineuronal nets (PNNs). In the present study, we have examined the effects of chronic treatment with pramipexole (PPX), a D3 dopamine receptor agonist, for its effects on gamma oscillation power as measured by in vivo electroencephalography (EEG) recordings in female BALB/c and C57Bl6 mice. We observe a modest but significant increase in 20-50 Hz gamma power with PPX in both strains. Additionally, biochemical analysis of prefrontal cortex lysates from PPX-treated BALB/c mice shows a number of changes that could contribute to, or follow from, increased pyramidal excitability and/or gamma power. PPX-associated changes include reduced levels of specific PNN components as well as tissue inhibitor of matrix metalloproteases-1 (TIMP-1), which inhibits long-term potentiation of synaptic transmission. Consistent with its effects on gamma power, PNN proteins and TIMP-1, chronic PPX treatment also improves working memory and reduces anhedonia. Together these results add to an emerging literature linking extracellular matrix and/or gamma oscillation power to both mood and cognition.

Leveraging neural plasticity for the treatment of amblyopia

Amblyopia is a form of visual cortical impairment that arises from abnormal visual experience early in life. Most often, amblyopia is a unilateral visual impairment that can develop as a result of strabismus, anisometropia, or a combination of these conditions that result in discordant binocular experience. Characterized by reduced visual acuity and impaired binocular function, amblyopia places a substantial burden on the developing child. Although frontline treatment with glasses and patching can improve visual acuity, residual amblyopia remains for most children. Newer binocular-based therapies can elicit rapid recovery of visual acuity and may also improve stereoacuity in some children. Nevertheless, for both treatment modalities full recovery is elusive, recurrence of amblyopia is common, and improvements are negligible when treatment is administered at older ages. Insights derived from animal models about the factors that govern neural plasticity have been leveraged to develop innovative treatments for amblyopia. These novel therapies exhibit efficacy to promote recovery, and some are effective even at ages when conventional treatments fail to yield benefit. Approaches for enhancing visual system plasticity and promoting recovery from amblyopia include altering the balance between excitatory and inhibitory mechanisms, reversing the accumulation of proteins that inhibit plasticity, and harnessing the principles of metaplasticity. Although these therapies have exhibited promising results in animal models, their safety and ability to remediate amblyopia need to be evaluated in humans.

Layer specific regulation of critical period timing and maturation of mouse visual cortex by endocannabinoids

Plasticity during the critical period is important for the functional maturation of cortical neurons. While characteristics of plasticity are diverse among cortical layers, it is unknown whether critical period timing is controlled by a common or unique molecular mechanism among them. We here clarified layer-specific regulation of the critical period timing of ocular dominance plasticity in the primary visual cortex. Mice lacking the endocannabinoid synthesis enzyme diacylglycerol lipase-α exhibited precocious critical period timing, earlier maturation of inhibitory synaptic function in layers 2/3 and 4, and impaired development of the binocular matching of orientation selectivity exclusively in layer 2/3. Activation of cannabinoid receptor restored ocular dominance plasticity at the normal critical period in layer 2/3. Suppression of GABAA receptor rescued precocious ocular dominance plasticity in layer 4. Therefore, endocannabinoids regulate critical period timing and maturation of visual function partly through the development of inhibitory synaptic functions in a layer-dependent manner.

Developmental hearing loss-induced perceptual deficits are rescued by genetic restoration of cortical inhibition

Even a transient period of hearing loss during the developmental critical period can induce long-lasting deficits in temporal and spectral perception. These perceptual deficits correlate with speech perception in humans. In gerbils, these hearing loss-induced perceptual deficits are correlated with a reduction of both ionotropic GABAA and metabotropic GABAB receptor-mediated synaptic inhibition in auditory cortex, but most research on critical period plasticity has focused on GABAA receptors. Therefore, we developed viral vectors to express proteins that would upregulate gerbil postsynaptic inhibitory receptor subunits (GABAA, Gabra1; GABAB, Gabbr1b) in pyramidal neurons, and an enzyme that mediates GABA synthesis (GAD65) presynaptically in parvalbumin-expressing interneurons. A transient period of developmental hearing loss during the auditory critical period significantly impaired perceptual performance on two auditory tasks: amplitude modulation depth detection and spectral modulation depth detection. We then tested the capacity of each vector to restore perceptual performance on these auditory tasks. While both GABA receptor vectors increased the amplitude of cortical inhibitory postsynaptic potentials, only viral expression of postsynaptic GABAB receptors improved perceptual thresholds to control levels. Similarly, presynaptic GAD65 expression improved perceptual performance on spectral modulation detection. These findings suggest that recovering performance on auditory perceptual tasks depends on GABAB receptor-dependent transmission at the auditory cortex parvalbumin to pyramidal synapse and point to potential therapeutic targets for developmental sensory disorders.

In vivo whole-cortex marker of excitation-inhibition ratio indexes cortical maturation and cognitive ability in youth

A balanced excitation-inhibition ratio (E/I ratio) is critical for healthy brain function. Normative development of cortex-wide E/I ratio remains unknown. Here, we noninvasively estimate a putative marker of whole-cortex E/I ratio by fitting a large-scale biophysically plausible circuit model to resting-state functional MRI (fMRI) data. We first confirm that our model generates realistic brain dynamics in the Human Connectome Project. Next, we show that the estimated E/I ratio marker is sensitive to the gamma-aminobutyric acid (GABA) agonist benzodiazepine alprazolam during fMRI. Alprazolam-induced E/I changes are spatially consistent with positron emission tomography measurement of benzodiazepine receptor density. We then investigate the relationship between the E/I ratio marker and neurodevelopment. We find that the E/I ratio marker declines heterogeneously across the cerebral cortex during youth, with the greatest reduction occurring in sensorimotor systems relative to association systems. Importantly, among children with the same chronological age, a lower E/I ratio marker (especially in the association cortex) is linked to better cognitive performance. This result is replicated across North American (8.2 to 23.0 y old) and Asian (7.2 to 7.9 y old) cohorts, suggesting that a more mature E/I ratio indexes improved cognition during normative development. Overall, our findings open the door to studying how disrupted E/I trajectories may lead to cognitive dysfunction in psychopathology that emerges during youth.

In-vivo whole-cortex marker of excitation-inhibition ratio indexes cortical maturation and cognitive ability in youth

A balanced excitation-inhibition ratio (E/I ratio) is critical for healthy brain function. Normative development of cortex-wide E/I ratio remains unknown. Here we non-invasively estimate a putative marker of whole-cortex E/I ratio by fitting a large-scale biophysically-plausible circuit model to resting-state functional MRI (fMRI) data. We first confirm that our model generates realistic brain dynamics in the Human Connectome Project. Next, we show that the estimated E/I ratio marker is sensitive to the GABA-agonist benzodiazepine alprazolam during fMRI. Alprazolam-induced E/I changes are spatially consistent with positron emission tomography measurement of benzodiazepine receptor density. We then investigate the relationship between the E/I ratio marker and neurodevelopment. We find that the E/I ratio marker declines heterogeneously across the cerebral cortex during youth, with the greatest reduction occurring in sensorimotor systems relative to association systems. Importantly, among children with the same chronological age, a lower E/I ratio marker (especially in association cortex) is linked to better cognitive performance. This result is replicated across North American (8.2 to 23.0 years old) and Asian (7.2 to 7.9 years old) cohorts, suggesting that a more mature E/I ratio indexes improved cognition during normative development. Overall, our findings open the door to studying how disrupted E/I trajectories may lead to cognitive dysfunction in psychopathology that emerges during youth.

Glial regulation of critical period plasticity

Animal behavior, from simple to complex, is dependent on the faithful wiring of neurons into functional neural circuits. Neural circuits undergo dramatic experience-dependent remodeling during brief developmental windows called critical periods. Environmental experience during critical periods of plasticity produces sustained changes to circuit function and behavior. Precocious critical period closure is linked to autism spectrum disorders, whereas extended synaptic remodeling is thought to underlie circuit dysfunction in schizophrenia. Thus, resolving the mechanisms that instruct critical period timing is important to our understanding of neurodevelopmental disorders. Control of critical period timing is modulated by neuron-intrinsic cues, yet recent data suggest that some determinants are derived from neighboring glial cells (astrocytes, microglia, and oligodendrocytes). As glia make up 50% of the human brain, understanding how these diverse cells communicate with neurons and with each other to sculpt neural plasticity, especially during specialized critical periods, is essential to our fundamental understanding of circuit development and maintenance.

Extensive topographic remapping and functional sharpening in the adult rat visual pathway upon first visual experience

Understanding the dynamics of stability/plasticity balances during adulthood is pivotal for learning, disease, and recovery from injury. However, the brain-wide topography of sensory remapping remains unknown. Here, using a first-of-its-kind setup for delivering patterned visual stimuli in a rodent magnetic resonance imaging (MRI) scanner, coupled with biologically inspired computational models, we noninvasively mapped brain-wide properties-receptive fields (RFs) and spatial frequency (SF) tuning curves-that were insofar only available from invasive electrophysiology or optical imaging. We then tracked the RF dynamics in the chronic visual deprivation model (VDM) of plasticity and found that light exposure progressively promoted a large-scale topographic remapping in adult rats. Upon light exposure, the initially unspecialized visual pathway progressively evidenced sharpened RFs (smaller and more spatially selective) and enhanced SF tuning curves. Our findings reveal that visual experience following VDM reshapes both structure and function of the visual system and shifts the stability/plasticity balance in adults.

Thalamic regulation of a visual critical period and motor behavior

During the visual critical period (CP), sensory experience refines the structure and function of visual circuits. The basis of this plasticity was long thought to be limited to cortical circuits, but recently described thalamic plasticity challenges this dogma and demonstrates greater complexity underlying visual plasticity. Yet how visual experience modulates thalamic neurons or how the thalamus modulates CP timing is incompletely understood. Using a larval zebrafish, thalamus-centric ocular dominance model, we show functional changes in the thalamus and a role of inhibitory signaling to establish CP timing using a combination of functional imaging, optogenetics, and pharmacology. Hemisphere-specific changes in genetically defined thalamic neurons correlate with changes in visuomotor behavior, establishing a role of thalamic plasticity in modulating motor performance. Our work demonstrates that visual plasticity is broadly conserved and that visual experience leads to neuron-level functional changes in the thalamus that require inhibitory signaling to establish critical period timing.

From circuits to behavior: Amygdala dysfunction in fragile X syndrome

Fragile X syndrome (FXS) is a neurodevelopmental disorder caused by a repeat expansion mutation in the promotor region of the FMR1 gene resulting in transcriptional silencing and loss of function of fragile X messenger ribonucleoprotein 1 protein (FMRP). FMRP has a well-defined role in the early development of the brain. Thus, loss of the FMRP has well-known consequences for normal cellular and synaptic development leading to a variety of neuropsychiatric disorders including an increased prevalence of amygdala-based disorders. Despite our detailed understanding of the pathophysiology of FXS, the precise cellular and circuit-level underpinnings of amygdala-based disorders is incompletely understood. In this review, we discuss the development of the amygdala, the role of neuromodulation in the critical period plasticity, and recent advances in our understanding of how synaptic and circuit-level changes in the basolateral amygdala contribute to the behavioral manifestations seen in FXS.

Developmental hearing loss-induced perceptual deficits are rescued by cortical expression of GABAB receptors

Even transient periods of developmental hearing loss during the developmental critical period have been linked to long-lasting deficits in auditory perception, including temporal and spectral processing, which correlate with speech perception and educational attainment. In gerbils, hearing loss-induced perceptual deficits are correlated with a reduction of both ionotropic GABAA and metabotropic GABAB receptor-mediated synaptic inhibition in auditory cortex, but most research on critical period plasticity has focused on GABAA receptors. We developed viral vectors to express both endogenous GABAA or GABAB receptor subunits in auditory cortex and tested their capacity to restore perception of temporal and spectral auditory cues following critical period hearing loss in the Mongolian gerbil. HL significantly impaired perception of both temporal and spectral auditory cues. While both vectors similarly increased IPSCs in auditory cortex, only overexpression of GABAB receptors improved perceptual thresholds after HL to be similar to those of animals without developmental hearing loss. These findings identify the GABAB receptor as an important regulator of sensory perception in cortex and point to potential therapeutic targets for developmental sensory disorders.

Rapid synaptic and gamma rhythm signature of mouse critical period plasticity

Early-life experience enduringly sculpts thalamocortical (TC) axons and sensory processing. Here, we identify the very first synaptic targets that initiate critical period plasticity, heralded by altered cortical oscillations. Monocular deprivation (MD) acutely induced a transient (<3 h) peak in EEG γ-power (~40 Hz) specifically within the visual cortex, but only when the critical period was open (juvenile mice or adults after dark-rearing, Lynx1-deletion, or diazepam-rescued GAD65-deficiency). Rapid TC input loss onto parvalbumin-expressing (PV) inhibitory interneurons (but not onto nearby pyramidal cells) was observed within hours of MD in a TC slice preserving the visual pathway - again once critical periods opened. Computational TC modeling of the emergent γ-rhythm in response to MD delineated a cortical interneuronal gamma (ING) rhythm in networks of PV-cells bearing gap junctions at the start of the critical period. The ING rhythm effectively dissociated thalamic input from cortical spiking, leading to rapid loss of previously strong TC-to-PV connections through standard spike-timing-dependent plasticity rules. As a consequence, previously silent TC-to-PV connections could strengthen on a slower timescale, capturing the gradually increasing γ-frequency and eventual fade-out over time. Thus, ING enables cortical dynamics to transition from being dominated by the strongest TC input to one that senses the statistics of population TC input after MD. Taken together, our findings reveal the initial synaptic events underlying critical period plasticity and suggest that the fleeting ING accompanying a brief sensory perturbation may serve as a robust readout of TC network state with which to probe developmental trajectories.

Hippocampal parvalbumin interneurons play a critical role in memory development

Episodic memories formed in early childhood rapidly decay, but their latent traces remain stored long term. These memories require the dorsal hippocampus (dHPC) and seem to undergo a developmental critical period. It remains to be determined whether the maturation of parvalbumin interneurons (PVIs), a major mechanism of critical periods, contributes to memory development. Here, we show that episodic infantile learning significantly increases the levels of parvalbumin in the dHPC 48 h after training. Chemogenetic inhibition of PVIs before learning indicated that these neurons are required for infantile memory formation. A bilateral dHPC injection of the γ-aminobutyric acid type A receptor agonist diazepam after training elicited long-term memory expression in infant rats, although direct PVI chemogenetic activation had no effect. Finally, PVI activity was required for brain-derived neurotrophic factor (BDNF)-dependent maturation of memory competence, i.e., adult-like long-term memory expression. Thus, dHPC PVIs are critical for the formation of infantile memories and for memory development.

Basolateral Amygdala Hyperexcitability Is Associated with Precocious Developmental Emergence of Fear-Learning in Fragile X Syndrome

Fragile X Syndrome is a neurodevelopmental disorder and the most common monogenic cause of intellectual disability, autism spectrum disorders, and anxiety disorders. Loss of fragile x mental retardation protein results in disruptions of synaptic development during a critical period of circuit formation in the BLA. However, it is unknown how these alterations impact microcircuit development and function. Using a combination of electrophysiologic and behavioral approaches in both male (Fmr1-/y) and female (Fmr1-/-) mice, we demonstrate that principal neurons in the Fmr1KO BLA exhibit hyperexcitability during a sensitive period in amygdala development. This hyperexcitability contributes to increased excitatory gain in fear-learning circuits. Further, synaptic plasticity is enhanced in the BLA of Fmr1KO mice. Behavioral correlation demonstrates that fear-learning emerges precociously in the Fmr1KO mouse. Early life 4,5,6,7-tetrahydroisoxazolo [5,4-c]pyridin-3ol intervention ameliorates fear-learning in Fmr1KO mice. These results suggest that critical period plasticity in the amygdala of the Fmr1KO mouse may be shifted to earlier developmental time points.SIGNIFICANCE STATEMENT In these studies, we identify early developmental alterations in principal neurons in the Fragile X syndrome BLA. We show that, as early as P14, excitability and feedforward excitation, and synaptic plasticity are enhanced in Fmr1KO lateral amygdala. This correlates with precocious emergence of fear-learning in the Fmr1KO mouse. Early life 4,5,6,7-tetrahydroisoxazolo [5,4-c]pyridin-3ol intervention restores critical period plasticity in WT mice and ameliorates fear-learning in the Fmr1KO mouse.

Development of visual response selectivity in cortical GABAergic interneurons

The visual cortex of carnivores and primates displays a high degree of modular network organization characterized by local clustering and structured long-range correlations of activity and functional properties. Excitatory networks display modular organization before the onset of sensory experience, but the developmental timeline for modular networks of GABAergic interneurons remains under-explored. Using in vivo calcium imaging of the ferret visual cortex, we find evidence that before visual experience, interneurons display weak orientation tuning and widespread, correlated activity in response to visual stimuli. Robust modular organization and orientation tuning are evident with as little as one week of visual experience. Furthermore, we find that the maturation of orientation tuning requires visual experience, while the reduction in widespread, correlated network activity does not. Thus, the maturation of inhibitory cortical networks occurs in a delayed, parallel process relative to excitatory neurons.

Excitatory neurons in the carnivore and primate visual cortex display orientation selectivity arranged in a modular fashion before the onset of visual experience, but the developmental timeline for visual response selectivity of inhibitory neurons is unknown. Using in vivo calcium imaging in ferret visual cortex, the authors show that experience-dependent emergence of orientation selectivity develops in a modular fashion for inhibitory interneurons over the first week of visual experience.

Parvalbumin-Positive Interneurons Regulate Cortical Sensory Plasticity in Adulthood and Development Through Shared Mechanisms

Parvalbumin-positive neurons are the largest class of GABAergic, inhibitory neurons in the central nervous system. In the cortex, these fast-spiking cells provide feedforward and feedback synaptic inhibition onto a diverse set of cell types, including pyramidal cells, other inhibitory interneurons, and themselves. Cortical inhibitory networks broadly, and cortical parvalbumin-expressing interneurons (cPVins) specifically, are crucial for regulating sensory plasticity during both development and adulthood. Here we review the functional properties of cPVins that enable plasticity in the cortex of adult mammals and the influence of cPVins on sensory activity at four spatiotemporal scales. First, cPVins regulate developmental critical periods and adult plasticity through molecular and structural interactions with the extracellular matrix. Second, they activate in precise sequence following feedforward excitation to enforce strict temporal limits in response to the presentation of sensory stimuli. Third, they implement gain control to normalize sensory inputs and compress the dynamic range of output. Fourth, they synchronize broad network activity patterns in response to behavioral events and state changes. Much of the evidence for the contribution of cPVins to plasticity comes from classic models that rely on sensory deprivation methods to probe experience-dependent changes in the brain. We support investigating naturally occurring, adaptive cortical plasticity to study cPVin circuits in an ethologically relevant framework, and discuss recent insights from our work on maternal experience-induced auditory cortical plasticity.

A Practical Guide to Sparse k-Means Clustering for Studying Molecular Development of the Human Brain

Studying the molecular development of the human brain presents unique challenges for selecting a data analysis approach. The rare and valuable nature of human postmortem brain tissue, especially for developmental studies, means the sample sizes are small (n), but the use of high throughput genomic and proteomic methods measure the expression levels for hundreds or thousands of variables [e.g., genes or proteins (p)] for each sample. This leads to a data structure that is high dimensional (p ≫ n) and introduces the curse of dimensionality, which poses a challenge for traditional statistical approaches. In contrast, high dimensional analyses, especially cluster analyses developed for sparse data, have worked well for analyzing genomic datasets where p ≫ n. Here we explore applying a lasso-based clustering method developed for high dimensional genomic data with small sample sizes. Using protein and gene data from the developing human visual cortex, we compared clustering methods. We identified an application of sparse k-means clustering [robust sparse k-means clustering (RSKC)] that partitioned samples into age-related clusters that reflect lifespan stages from birth to aging. RSKC adaptively selects a subset of the genes or proteins contributing to partitioning samples into age-related clusters that progress across the lifespan. This approach addresses a problem in current studies that could not identify multiple postnatal clusters. Moreover, clusters encompassed a range of ages like a series of overlapping waves illustrating that chronological- and brain-age have a complex relationship. In addition, a recently developed workflow to create plasticity phenotypes (Balsor et al., 2020) was applied to the clusters and revealed neurobiologically relevant features that identified how the human visual cortex changes across the lifespan. These methods can help address the growing demand for multimodal integration, from molecular machinery to brain imaging signals, to understand the human brain’s development.

Vision is required for the formation of binocular neurons prior to the classical critical period

Depth perception emerges from the development of binocular neurons in primary visual cortex. Vision is required for these neurons to acquire their mature responses to visual stimuli. The prevailing view is that vision does not influence binocular circuitry until the onset of the critical period, about a week after eye opening, and that plasticity of visual responses is triggered by increased inhibition. Here, we show that vision is required to form binocular neurons and to improve binocular tuning and matching from eye opening until critical period closure. Enhancing inhibition does not accelerate this process. Vision soon after eye opening improves the tuning properties of binocular neurons by strengthening and sharpening ipsilateral eye cortical responses. This progressively changes the population of neurons in the binocular pool, and this plasticity is sensitive to interocular differences prior to critical period onset. Thus, vision establishes binocular circuitry and guides binocular plasticity from eye opening.

Exposure to Sevoflurane, But Not Ketamine, During Early-life Brain Development has Long-Lasting Effects on GABAA Receptor Mediated Inhibitory Neurotransmission

Understanding the different mechanisms associated with different anesthetic targeted receptors is critical towards identifying accurate long-term outcome measures as a result of early-life anesthetic exposure. We examined changes in GABA_A receptor mediated neurotransmission by a predominately GABA_A receptor targeted anesthetic, sevoflurane or a predominately NMDA receptor targeted anesthetic, ketamine. Postnatal day 7 male mice were exposed to sevoflurane or ketamine and examined as adults for changes in inhibitory neurotransmission and its associated change in induced seizure activity. Paired pulse stimulation experiment showed that early-life sevoflurane treated mice had significantly less hippocampal CA1 inhibition later in life. There was significantly increased CA1 excitatory output in the sevoflurane treated group compared to the no sevoflurane treated group after the GABA agonist muscimol. Similar to our previously established data for early-life sevoflurane, here we established early-life ketamine administration resulted in neurodevelopmental behavioral changes later in life. However, muscimol did not produce a significant difference on the excitatory CA1 output between early-life ketamine group and saline group. While sevoflurane treated mice showed significantly higher induced seizure intensities and shorter latency periods to reach seizure intensity stage 5 (Racine score) compared with no sevoflurane treated mice, this phenomenon was not observed in the ketamine vs. saline treated groups. Early-life sevoflurane, but not ketamine, exposure reduced GABAergic inhibition and enhanced seizure activity later in life. The results indicate that early-life exposure to different anesthetics lead to distinct long-term effects and their unique pathways require mechanistic studies to understand induced long-lasting changes in the brain.

The role of GABAergic signalling in neurodevelopmental disorders

GABAergic inhibition shapes the connectivity, activity and plasticity of the brain. A series of exciting new discoveries provides compelling evidence that disruptions in a number of key facets of GABAergic inhibition have critical roles in the aetiology of neurodevelopmental disorders (NDDs). These facets include the generation, migration and survival of GABAergic neurons, the formation of GABAergic synapses and circuit connectivity, and the dynamic regulation of the efficacy of GABAergic signalling through neuronal chloride transporters. In this Review, we discuss recent work that elucidates the functions and dysfunctions of GABAergic signalling in health and disease, that uncovers the contribution of GABAergic neural circuit dysfunction to NDD aetiology and that leverages such mechanistic insights to advance precision medicine for the treatment of NDDs.

Kv3.1 channels regulate the rate of critical period plasticity

Experience-dependent plasticity within visual cortex is controlled by postnatal maturation of inhibitory circuits, which are both morphologically diverse and precisely connected. Gene-targeted disruption of the voltage-dependent potassium channel K_v3.1 broadens action potentials and reduces net inhibitory function of parvalbumin (PV)-positive GABA subtypes within the neocortex. In mice lacking K_v3.1, the rate of input loss from an eye deprived of vision was slowed two-fold, despite otherwise normal critical period timecourse and receptive field properties. Rapid ocular dominance plasticity was restored by local or systemic enhancement of GABAergic transmission with acute benzodiazepine infusion. Diazepam instead exacerbated a global suppression of slow-wave oscillations during sleep described previously in these mutant mice, which therefore did not account for the rescued plasticity. Rapid ocular dominance shifts closely reflected K_v3.1 gene dosage that prevented prolonged spike discharge of their target pyramidal cells in vivo or the spike amplitude decrement of fast-spiking cells during bouts of high-frequency firing in vitro. Late postnatal expression of this unique channel in fast-spiking interneurons thus subtly regulates the speed of critical period plasticity with implications for mental illnesses.

A sex-dependent delayed maturation of visual plasticity induced by adverse experiences in early childhood

Adverse experiences in early life have a long-term impact on the development of brain, which in turn increases the susceptibility to mental illness during adulthood, especially in female subjects. However, whether and how the visual cortex is affected by these adverse experiences as well as the mechanisms underlying the sex difference are largely unknown. Here, we established a new mouse model of early-life chronic mild stress (ECMS) without anxiety or depression-like behavior in adulthood. ECMS mice showed normal maturation of visual acuity and orientation/direction selectivity, whereas their visual cortical neurons preferred lower spatial frequency (SF) and higher temporal frequency (TF) than control mice. Meanwhile the development of ocular dominance (OD) plasticity was delayed. Specifically, compared with control mice, ECMS mice in the early stage of the critical period (CP) showed a reduction in GABA synthesis enzyme expression as well as lower OD plasticity which could be occluded by diazepam. In contrast, ECMS mice in the late stage of CP showed stronger OD plasticity, accompanied by higher expression of N-methyl-D-aspartate (NMDA) receptor NR2B subunit. Interestingly, only female ECMS mice at adulthood maintained juvenile-like OD plasticity as well as high NR2B expressions. Artificial increase in estradiol level in ECMS males via estradiol supplementary diminished this sex difference. Lastly, OD plasticity was abolished in adult ECMS females either performed with the bilateral ovariectomy in prepuberty, or directly infused with NR2B antagonist Ro 25–6981 into the visual cortex. Overall, our study demonstrates that early adverse experiences have a lasting effect on visual development of mice in a sex-dependent manner, which is mediated by the estradiol-NR2B pathway.

Ocular dominance plasticity: Molecular mechanisms revisited

Ocular dominance plasticity (ODP) is a type of cortical plasticity operating in visual cortex of mammals that are endowed with binocular vision based on the competition-driven disparity. Earlier, a molecular mechanism was proposed that catecholamines play an important role in the maintenance of ODP in kittens. Having survived the initial test, the hypothesis was further advanced to identify noradrenaline (NA) as a key factor that regulates ODP in the immature cortex. Later, the ODP-promoting effect of NA is extended to the adult with age-related limitations. Following the enhanced NA availability, the chain events downstream lead to the β-adrenoreceptor-induced cAMP accumulation, which in turn activates the protein kinase A. Eventually, the protein kinase translocates to the cell nucleus to activate cAMP responsive element binding protein (CREB). CREB is a cellular transcription factor that controls the transcription of various genes, underpinning neuronal plasticity and long-term memory. In the advent of molecular genetics in that various types of new tools have become available with relative ease, ODP research has lightly adopted in the rodent model the original concepts and methodologies. Here, after briefly tracing the strategic maturation of our quest, the review moves to the later development of the field, with the emphasis placed around the following issues: (a) Are we testing ODP per se? (b) What does monocular deprivation deprive of the immature cortex? (c) The critical importance of binocular competition, (d) What is the adult plasticity? (e) Excitation-Inhibition balance in local circuits, and (f) Species differences in the animal models.

Rats deficient in the GAD65 isoform exhibit epilepsy and premature lethality

GABA is synthesized by glutamate decarboxylase (GAD), which has two isoforms, namely, GAD65 and GAD67, encoded by the Gad2 and Gad1 genes, respectively. GAD65-deficient (Gad2^-/- ) mice exhibit a reduction in brain GABA content after 1 month of age and show spontaneous seizures in adulthood. Approximately 25% of Gad2^-/- mice died by 6 months of age. Our Western blot analysis demonstrated that the protein expression ratio of GAD65 to GAD67 in the brain was greater in rats than in mice during postnatal development, suggesting that the contribution of each GAD isoform to GABA functions differs between these two species. To evaluate whether GAD65 deficiency causes different phenotypes between rats and mice, we generated Gad2^-/- rats using TALEN genome editing technology. Western blot and immunohistochemical analyses with new antibodies demonstrated that the GAD65 protein was undetectable in the Gad2^-/- rat brain. Gad2^-/- pups exhibited spontaneous seizures and paroxysmal discharge in EEG at postnatal weeks 3-4. More than 80% of the Gad2^-/- rats died at postnatal days (PNDs) 17-23. GABA content in Gad2^-/- brains was significantly lower than those in Gad2^+/- and Gad2^+/+ brains at PND17-19. These results suggest that the low levels of brain GABA content in Gad2^-/- rats may lead to epilepsy followed by premature death, and that Gad2^-/- rats are more severely affected than Gad2^-/- mice. Considering that the GAD65/GAD67 ratio in human brains is more similar to that in rat brains than in mouse brains, Gad2^-/- rats would be useful for further investigating the roles of GAD65 in vivo.

Warped rhythms: Epileptic activity during critical periods disrupts the development of neural networks for human communication

It is well established that temporal lobe epilepsy-the most common and well-studied form of epilepsy-can impair communication by disrupting social-emotional and language functions. In pediatric epilepsy, where seizures co-occur with the development of critical brain networks, age of onset matters: The earlier in life seizures begin, the worse the disruption in network establishment, resulting in academic hardship and social isolation. Yet, little is known about the processes by which epileptic activity disrupts developing human brain networks. Here we take a synthetic perspective-reviewing a range of research spanning studies on molecular and oscillatory processes to those on the development of large-scale functional networks-in support of a novel model of how such networks can be disrupted by epilepsy. We seek to bridge the gap between research on molecular processes, on the development of human brain circuitry, and on clinical outcomes to propose a model of how epileptic activity disrupts brain development.

Neuroplasticity of the visual cortex: in sickness and in health

Brain plasticity refers to the ability of synaptic connections to adapt their function and structure in response to experience, including environmental changes, sensory deprivation and injuries. Plasticity is a distinctive, but not exclusive, property of the developing nervous system. This review introduces the concept of neuroplasticity and describes classic paradigms to illustrate cellular and molecular mechanisms underlying synapse modifiability. Then, we summarize a growing number of studies showing that the adult cerebral cortex retains a significant degree of plasticity highlighting how the identification of strategies to enhance the plastic potential of the adult brain could pave the way for the development of novel therapeutic approaches aimed at treating amblyopia and other neurodevelopmental disorders. Finally, we analyze how the visual system adjusts to neurodegenerative conditions leading to blindness and we discuss the crucial role of spared plasticity in the visual system for sight recovery.

Silent Synapse-Based Mechanisms of Critical Period Plasticity

Critical periods are postnatal, restricted time windows of heightened plasticity in cortical neural networks, during which experience refines principal neuron wiring configurations. Here, we propose a model with two distinct types of synapses, innate synapses that establish rudimentary networks with innate function, and gestalt synapses that govern the experience-dependent refinement process. Nascent gestalt synapses are constantly formed as AMPA receptor-silent synapses which are the substrates for critical period plasticity. Experience drives the unsilencing and stabilization of gestalt synapses, as well as synapse pruning. This maturation process changes synapse patterning and consequently the functional architecture of cortical excitatory networks. Ocular dominance plasticity (ODP) in the primary visual cortex (V1) is an established experimental model for cortical plasticity. While converging evidence indicates that the start of the critical period for ODP is marked by the maturation of local inhibitory circuits, recent results support our model that critical periods end through the progressive maturation of gestalt synapses. The cooperative yet opposing function of two postsynaptic signaling scaffolds of excitatory synapses, PSD-93 and PSD-95, governs the maturation of gestalt synapses. Without those proteins, networks do not progress far beyond their innate functionality, resulting in rather impaired perception. While cortical networks remain malleable throughout life, the cellular mechanisms and the scope of critical period and adult plasticity differ. Critical period ODP is initiated with the depression of deprived eye responses in V1, whereas adult ODP is characterized by an initial increase in non-deprived eye responses. Our model proposes the gestalt synapse-based mechanism for critical period ODP, and also predicts a different mechanism for adult ODP based on the sparsity of nascent gestalt synapses at that age. Under our model, early life experience shapes the boundaries (the gestalt) for network function, both for its optimal performance as well as for its pathological state. Thus, reintroducing nascent gestalt synapses as plasticity substrates into adults may improve the network gestalt to facilitate functional recovery.

Does physical exercise and congruent visual stimulation enhance perceptual learning?

PURPOSE

There is currently great interest in methods that can modulate brain plasticity, both in terms of understanding the basic mechanisms, and in the remedial application to situations of sensory loss. Recent work has focussed on how different manipulations might be combined to produce new settings that reveal synergistic actions. Here we ask whether a prominent example of adult visual plasticity, called perceptual learning, is modified by other environmental factors, such as visual stimulation and physical exercise.

METHODS

We quantified the magnitude, rate and transfer of perceptual learning using a peripheral Vernier alignment task, in two groups of subjects matched for a range of baseline factors (e.g. age, starting Vernier threshold, baseline fitness). We trained subjects for 5 days on a Vernier alignment task. In one group, we introduced an exercise protocol with congruent visual stimulation. The control group received the same visual stimulation, but did not exercise prior to measurement of Vernier thresholds.

RESULTS

Although the task generated large amounts of learning (~40%) and some transfer to untrained conditions in both groups, there were no specific benefits associated with either the addition of an exercise schedule or congruent visual stimulation.

CONCLUSION

In adults, short periods of physical exercise and visual stimulation do not enhance perceptual learning.

Early Adversity and Critical Periods: Neurodevelopmental Consequences of Violating the Expectable Environment

It is now widely recognized that children exposed to adverse life events in the first years of life are at increased risk for a variety of neural, behavioral, and psychological sequelae. As we discuss in this paper, adverse events represent a violation of the expectable environment. If such violations occur during a critical period of brain development, the detrimental effects of early adversity are likely to be long lasting. Here we discuss the various ways adversity becomes neurobiologically embedded, and how the timing of such adversity plays an important role in determining outcomes. We conclude our paper by offering recommendations for how to elucidate the neural mechanisms responsible for the behavioral sequelae and how best to model the effects of early adversity.

Critical periods of brain development

Brain plasticity is maximal at specific time windows during early development known as critical periods (CPs), during which sensory experience is necessary to establish optimal cortical representations of the surrounding environment. After CP closure, a range of functional and structural elements prevent passive experience from eliciting significant plastic changes in the brain. The transition from a plastic to a more fixed state is advantageous as it allows for the sequential consolidation and retention of new and more complex perceptual, motor, and cognitive functions. However, the formation of stable neural representations may pose limitations on future revisions to the circuitry. If sensory experience is abnormal or absent during this time, it can have profound effects on sensory representations in adulthood, resulting in quasi-permanent adaptations that can make it nearly impossible to learn certain skills or process certain stimulus properties later on in life. This chapter begins with a brief introduction to experience-dependent plasticity throughout the lifespan (Section Introduction). Next, we define what constitutes a CP (Section What Are Critical Periods?) and review some of the key CPs in the visual and auditory systems (Section Key Critical Periods of Sensory Systems). We then discuss the mechanisms whereby cortical plasticity is regulated both locally and through neuromodulatory systems (Section How Are Critical Periods Regulated?). Finally, we highlight studies showing that CPs can be extended beyond their normal epochs, closed prematurely, or reopened during adult life by merely altering sensory inputs (Section Timing of Critical Periods: Can CP Plasticity Be Extended, Limited, or Reactivated?).

Neuregulin and ErbB expression is regulated by development and sensory experience in mouse visual cortex

Neuregulins (NRGs) are protein ligands that impact neural development and circuit function. NRGs signal through the ErbB receptor tyrosine kinase family. NRG1/ErbB4 signaling in parvalbumin-expressing (PV) inhibitory interneurons is critical for visual cortical plasticity. There are multiple types of NRGs and ErbBs that can potentially contribute to visual cortical plasticity at different developmental stages. Thus, it is important to understand the normal developmental expression profiles of NRGs and ErbBs in specific neuron types in the visual cortex, and to study whether and how their expression changes in PV inhibitory neurons and excitatory neurons track with sensory perturbation. Cell type-specific translating ribosome affinity purification and qPCR was used to compare mRNA expression of nrg1,2,3,4 and erbB1,2,3,4 in PV and excitatory neurons in mouse visual cortex. We show that the expression of nrg1 and nrg3 decreases in PV neurons at the critical period peak, postnatal day 28 (P28) after monocular deprivation and dark rearing, and in the adult cortex (at P104) after 2-week long dark exposure. In contrast, nrg1 expression by excitatory neurons is unchanged at P28 and P104 following sensory deprivation, whereas nrg3 expression by excitatory neurons shows changes depending on the age and the mode of sensory deprivation. ErbB4 expression in PV neurons remains consistently high and does not appear to change in response to sensory deprivation. These data provide new important details of cell type-specific NRG/ErbB expression in the visual cortex and support that NRG1/ErbB4 signaling is implicated in both critical period and adult visual cortical plasticity.

Emerging Roles of Synapse Organizers in the Regulation of Critical Periods

Experience remodels cortical connectivity during developmental windows called critical periods. Experience-dependent regulation of synaptic strength during these periods establishes circuit functions that are stabilized as critical period plasticity wanes. These processes have been extensively studied in the developing visual cortex, where critical period opening and closure are orchestrated by the assembly, maturation, and strengthening of distinct synapse types. The synaptic specificity of these processes points towards the involvement of distinct molecular pathways. Attractive candidates are pre- and postsynaptic transmembrane proteins that form adhesive complexes across the synaptic cleft. These synapse-organizing proteins control synapse development and maintenance and modulate structural and functional properties of synapses. Recent evidence suggests that they have pivotal roles in the onset and closure of the critical period for vision. In this review, we describe roles of synapse-organizing adhesion molecules in the regulation of visual critical period plasticity and we discuss the potential they offer to restore circuit functions in amblyopia and other neurodevelopmental disorders.

Critical Period Regulation by Thyroid Hormones: Potential Mechanisms and Sex-Specific Aspects

Adequate perinatal levels of thyroid hormones (THs) are required for normal brain function and development. Studies in non-mammalian species suggest that TH might be involved in the regulation of critical periods (CPs) of heightened plasticity. Yet, it is largely unknown what mechanisms controlling such CPs might be under TH regulation. Here, we briefly review the influence of TH in early life across evolution. We discuss possible links between TH and known circuit and/or molecular mechanisms determining the timing of CPs of heightened brain plasticity. We focus on the role of parvalbumin-positive (PV) interneurons since their maturation defines CP onset and closure. Specifically, abnormal PV circuits are associated with low perinatal levels of TH, possibly because thyroid hypofunction may increase oxidative stress and/or dysregulate Otx2-mediated maturation of neuroprotective perineuronal nets. In addition, the level of cholinergic transmission is important for CP plasticity. Potentially, TH levels could affect gain changes in cholinergic transmission that can alter brain development. We believe that understanding how TH impacts CPs of circuit refinement will shed light onto the principles underlying normal developmental trajectories. Given that the thyroid gland expresses estrogen and androgen receptors, its activity can potentially be regulated differently between the sexes, contributing to sexually dimorphic behaviors.

5-HTR2A and 5-HTR3A but not 5-HTR1A antagonism impairs the cross-modal reactivation of deprived visual cortex in adulthood

Visual cortical areas show enhanced tactile responses in blind individuals, resulting in improved behavioral performance. Induction of unilateral vision loss in adult mice, by monocular enucleation (ME), is a validated model for such cross-modal brain plasticity. A delayed whisker-driven take-over of the medial monocular zone of the visual cortex is preceded by so-called unimodal plasticity, involving the potentiation of the spared-eye inputs in the binocular cortical territory. Full reactivation of the sensory-deprived contralateral visual cortex is accomplished by 7 weeks post-injury. Serotonin (5-HT) is known to modulate sensory information processing and integration, but its impact on cortical reorganization after sensory loss, remains largely unexplored. To address this issue, we assessed the involvement of 5-HT in ME-induced cross-modal plasticity and the 5-HT receptor (5-HTR) subtype used. We first focused on establishing the impact of ME on the total 5-HT concentration measured in the visual cortex and in the somatosensory barrel field. Next, the changes in expression as a function of post-ME recovery time of the monoamine transporter 2 (vMAT2), which loads 5-HT into presynaptic vesicles, and of the 5-HTR_1A and 5-HTR_3A were assessed, in order to link these temporal expression profiles to the different types of cortical plasticity induced by ME. In order to accurately pinpoint which 5-HTR exactly mediates ME-induced cross-modal plasticity, we pharmacologically antagonized the 5-HTR_1A, 5-HTR_2A and 5-HTR_3A subtypes. This study reveals brain region-specific alterations in total 5-HT concentration, time-dependent modulations in vMAT2, 5-HTR_1A and 5-HTR_3A protein expression and 5-HTR antagonist-specific effects on the post-ME plasticity phenomena. Together, our results confirm a role for 5-HTR_1A in the early phase of binocular visual cortex plasticity and suggest an involvement of 5-HTR_2A and 5-HTR_3A but not 5-HTR_1A during the late cross-modal recruitment of the medial monocular visual cortex. These insights contribute to the general understanding of 5-HT function in cortical plasticity and may encourage the search for improved rehabilitation strategies to compensate for sensory loss.

Excitatory and inhibitory synapse reorganization immediately after critical sensory experience in a vocal learner

Excitatory and inhibitory synapses are the brain’s most abundant synapse types. However, little is known about their formation during critical periods of motor skill learning, when sensory experience defines a motor target that animals strive to imitate. In songbirds, we find that exposure to tutor song leads to elimination of excitatory synapses in HVC (used here as a proper name), a key song generating brain area. A similar pruning is associated with song maturation, because juvenile birds have fewer excitatory synapses, the better their song imitations. In contrast, tutoring is associated with rapid insertion of inhibitory synapses, but the tutoring-induced structural imbalance between excitation and inhibition is eliminated during subsequent song maturation. Our work suggests that sensory exposure triggers the developmental onset of goal-specific motor circuits by increasing the relative strength of inhibition and it suggests a synapse-elimination model of song memorization.

A wide range of species use complex sounds to communicate, including humans and songbirds like zebra finches. During a critical period of learning, infants and young animals learn how to remember and discriminate this ‘language’ from other sounds. However, the changes that happen in the brain during this learning period are not well understood.

The process of learning forms new connections between neurons in the brain and prunes away old connections. These connections, known as synapses, come in different types. Signals sent across excitatory synapses increase the activity of the receiving neuron, while signals sent across inhibitory synapses reduce neuron activity.

What happens to the synapses in the brain during the critical period? To find out, Huang et al. used electron microscopy to examine the brains of young zebra finches that either had never heard birdsong, or had just heard birdsong for the first time. A single day of hearing song dramatically shifted the balance of excitatory and inhibitory synapses in the main vocal control area of the young birds’ brains. The number of excitatory synapses decreased, and the number of inhibitory synapses increased.

The balance between excitation and inhibition is important for the brain to work correctly. Therefore, as well as helping us to understand how infants learn their first language, the results presented by Huang et al. could also help us to improve treatments for conditions where this balance goes wrong, such as mood disorders. For example, tailoring the time point of medication intake in combination with sensory exposure therapies could improve how effectively either one works.

iPlasticity: Induced juvenile-like plasticity in the adult brain as a mechanism of antidepressants

The network hypothesis of depression proposes that mood disorders reflect problems in information processing within particular neural networks. Antidepressants (AD), including selective serotonin reuptake inhibitors (SSRI), function by gradually improving information processing within these networks. AD have been shown to induce a state of juvenile-like plasticity comparable to that observed during developmental critical periods: Such critical-period-like plasticity allows brain networks to better adapt to extrinsic and intrinsic signals. We have coined this drug-induced state of juvenile-like plasticity 'iPlasticity.' A combination of iPlasticity induced by chronic SSRI treatment together with training, rehabilitation, or psychotherapy improves symptoms of neuropsychiatric disorders and issues underlying the developmentally or genetically malfunctioning networks. We have proposed that iPlasticity might be a critical component of AD action. We have demonstrated that iPlasticity occurs in the visual cortex, fear erasure network, extinction of aggression caused by social isolation, and spatial reversal memory in rodent models. Chronic SSRI treatment is known to promote neurogenesis and to cause dematuration of granule cells in the dentate gyrus and of interneurons, especially parvalbumin interneurons enwrapped by perineuronal nets in the prefrontal cortex, visual cortex, and amygdala. Brain-derived neurotrophic factor (BDNF), via its receptor tropomyosin kinase receptor B, is involved in the processes of synaptic plasticity, including neurogenesis, neuronal differentiation, weight of synapses, and gene regulation of synaptic formation. BDNF can be activated by both chronic SSRI treatment and neuronal activity. Accordingly, the BDNF/tropomyosin kinase receptor B pathway is critical for iPlasticity, but further analyses will be needed to provide mechanical insight into the processes of iPlasticity.

Stimulation of Acetylcholine Release and Pharmacological Potentiation of Cholinergic Transmission Affect Cholinergic Receptor Expression Differently during Visual Conditioning

Cholinergic stimulation coupled with visual conditioning enhances the visual acuity and cortical responses in the primary visual cortex. To determine which cholinergic receptors are involved in these processes, qRT-PCR was used. Two modes of cholinergic enhancement were tested: a phasic increase of acetylcholine release by an electrical stimulation of the basal forebrain cholinergic nucleus projecting to the visual cortex, or a tonic pharmacological potentiation of the cholinergic transmission by the acetylcholine esterase inhibitor, donepezil. A daily visual exposure to sine-wave gratings (training) was paired with the cholinergic enhancement, up to 14 days. qRT-PCR was performed at rest, 10 min, one week or two weeks of visual/cholinergic training with samples of the visual and somatosensory cortices, and the BF for determining mRNA expression of muscarinic receptor subtypes (m1, m2, m3, m4, m5), nicotinic receptor subunits (α3, α4, α7, β2, β4), and NMDA receptors, GAD65 and ChAT, as indexes of cortical plasticity. A Kruskal-Wallis test showed a modulation of the expression in the visual cortex of m2, m3, m4, m5, α7, β4, NMDA and GAD65, but only β4 within the basal forebrain and none of these mRNA within the somatosensory cortex. The two modes of cholinergic enhancement induced different effects on mRNA expression, related to the number of visual conditioning sessions and receptor specificity. This study suggests that the combination of cholinergic enhancement and visual conditioning is specific to the visual cortex and varies between phasic or tonic manipulation of acetylcholine levels.

The Impact of Perineuronal Net Digestion Using Chondroitinase ABC on the Intrinsic Physiology of Cortical Neurons

Perineuronal nets (PNNs) are a form of aggregate Extracellular Matrix (ECM) in the brain. Recent evidence suggests that the postnatal deposition of PNNs may play an active role in regulating neuroplasticity and, potentially, neurological disorders. Observations of high levels of PNN expression around somas, proximal dendrites, and axon initial segments of a subtype of neurons have also led to proposals that PNNs may modulate the intrinsic properties of the neurons they ensheathe. While high levels of PNNs are postnatally expressed throughout the neocortex, it is still unclear how they impact the neuronal physiology of the many classes and subtypes of neurons that exist. In this study, we demonstrate that Chondroitinase ABC digestion of PNNs from acute cortical slices from juvenile mice (P28-35) resulted in neuron-specific impacts on intrinsic physiology. Fast spiking (FS) interneurons showed decreased input resistance, resting membrane potential (RMP), reduced action potential (AP) peaks and altered spontaneous synaptic inputs. Low-Threshold Spiking interneurons showed altered rebound depolarizations and decreased frequency of spontaneous synaptic inputs. Putative excitatory neurons; regular spiking, bursting, and doublet phenotypes did not demonstrate any alterations. Our data indicate that chABC-sensitive PNNs may specifically regulate the intrinsic and synaptic physiology of inhibitory interneurons.

Critical periods in amblyopia

The shift in ocular dominance (OD) of binocular neurons induced by monocular deprivation is the canonical model of synaptic plasticity confined to a postnatal critical period. Developmental constraints on this plasticity not only lend stability to the mature visual cortical circuitry but also impede the ability to recover from amblyopia beyond an early window. Advances with mouse models utilizing the power of molecular, genetic, and imaging tools are beginning to unravel the circuit, cellular, and molecular mechanisms controlling the onset and closure of the critical periods of plasticity in the primary visual cortex (V1). Emerging evidence suggests that mechanisms enabling plasticity in juveniles are not simply lost with age but rather that plasticity is actively constrained by the developmental up-regulation of molecular 'brakes'. Lifting these brakes enhances plasticity in the adult visual cortex, and can be harnessed to promote recovery from amblyopia. The reactivation of plasticity by experimental manipulations has revised the idea that robust OD plasticity is limited to early postnatal development. Here, we discuss recent insights into the neurobiology of the initiation and termination of critical periods and how our increasingly mechanistic understanding of these processes can be leveraged toward improved clinical treatment of adult amblyopia.

Direct intertectal inputs are an integral component of the bilateral sensorimotor circuit for behavior in Xenopus tadpoles

The circuit controlling visually guided behavior in nonmammalian vertebrates, such as Xenopus tadpoles, includes retinal projections to the contralateral optic tectum, where visual information is processed, and tectal motor outputs projecting ipsilaterally to hindbrain and spinal cord. Tadpoles have an intertectal commissure whose function is unknown, but it might transfer information between the tectal lobes. Differences in visual experience between the two eyes have profound effects on the development and function of visual circuits in animals with binocular vision, but the effects on animals with fully crossed retinal projections are not clear. We tested the effect of monocular visual experience on the visuomotor circuit in Xenopus tadpoles. We show that cutting the intertectal commissure or providing visual experience to one eye (monocular visual experience) is sufficient to disrupt tectally mediated visual avoidance behavior. Monocular visual experience induces asymmetry in tectal circuit activity across the midline. Repeated exposure to monocular visual experience drives maturation of the stimulated retinotectal synapses, seen as increased AMPA-to-NMDA ratios, induces synaptic plasticity in intertectal synaptic connections, and induces bilaterally asymmetric changes in the tectal excitation-to-inhibition ratio (E/I). We show that unilateral expression of peptides that interfere with AMPA or GABA_A receptor trafficking alters E/I in the transfected tectum and is sufficient to degrade visuomotor behavior. Our study demonstrates that monocular visual experience in animals with fully crossed visual systems produces asymmetric circuit function across the midline and degrades visuomotor behavior. The data further suggest that intertectal inputs are an integral component of a bilateral visuomotor circuit critical for behavior. NEW & NOTEWORTHY The developing optic tectum of Xenopus tadpoles represents a unique circuit in which laterally positioned eyes provide sensory input to a circuit that is transiently monocular, but which will be binocular in the animal's adulthood. We challenge the idea that the two lobes of tadpole optic tectum function independently by testing the requirement of interhemispheric communication and demonstrate that unbalanced sensory input can induce structural and functional plasticity in the tectum sufficient to disrupt function.

Enhancement of visual cortex plasticity by dark exposure

Dark rearing is known to delay the time course of the critical period for ocular dominance plasticity in the visual cortex. Recent evidence suggests that a period of dark exposure (DE) may enhance or reinstate plasticity even after closure of the critical period, mediated through modification of the excitatory-inhibitory balance and/or removal of structural brakes on plasticity. Here, we investigated the effects of a week of DE on the recovery from a month of monocular deprivation (MD) in the primary visual cortex (V1) of juvenile mice. Optical imaging of intrinsic signals revealed that ocular dominance in V1 of mice that had received DE recovered slightly more quickly than of mice that had not, but the level of recovery after three weeks was similar in both groups. Two-photon calcium imaging showed no significant difference in the recovery of orientation selectivity of excitatory neurons between the two groups. Parvalbumin-positive (PV+) interneurons exhibited a smaller ocular dominance shift during MD but again no differences in subsequent recovery. The percentage of PV+ cells surrounded by perineuronal nets, a structural brake on plasticity, was lower in mice with than those without DE. Overall, DE causes a modest enhancement of mouse visual cortex plasticity.This article is part of the themed issue 'Integrating Hebbian and homeostatic plasticity'.

Chondroitin Sulfate Is Required for Onset and Offset of Critical Period Plasticity in Visual Cortex

Ocular dominance plasticity is easily observed during the critical period in early postnatal life. Chondroitin sulfate (CS) is the most abundant component in extracellular structures called perineuronal nets (PNNs), which surround parvalbumin-expressing interneurons (PV-cells). CS accumulates in PNNs at the critical period, but its function in earlier life is unclear. Here, we show that initiation of ocular dominance plasticity was impaired with reduced CS, using mice lacking a key CS-synthesizing enzyme, CSGalNAcT1. Two-photon in vivo imaging showed a weaker visual response of PV-cells with reduced CS compared to wild-type mice. Plasticity onset was restored by a homeoprotein Otx2, which binds the major CS-proteoglycan aggrecan and promotes its further expression. Continuous CS accumulation together with Otx2 contributed bidirectionally to both onset and offset of plasticity, and was substituted by diazepam, which enhances GABA function. Therefore, CS and Otx2 may act as common inducers of both onset and offset of the critical period by promoting PV-cell function throughout the lifetime.

The Impact of Ecological Niche on Adaptive Flexibility of Sensory Circuitry

Evolution and development are interdependent, particularly with regard to the construction of the nervous system and its position as the machine that produces behavior. On the one hand, the processes directing development and plasticity of the brain provide avenues through which natural selection can sculpt neural cell fate and connectivity, and on the other hand, they are themselves subject to selection pressure. For example, mutations that produce heritable perturbations in neuronal birth and death rates, transcription factor expression, or availability of axon guidance factors within sensory pathways can markedly affect the development of form and thus the function of stimulus decoding circuitry. This evolvability of flexible circuits makes them more adaptable to environmental variation. Although there is general agreement on this point, whether the sensitivity of circuits to environmental influence and the mechanisms underlying development and plasticity of sensory pathways are similar across species from different ecological niches has received almost no attention. Neural circuits are generally more sensitive to environmental influences during an early critical period, but not all niches afford the same access to stimuli in early life. Furthermore, depending on predictability of the habitat and ecological niche, sensory coding circuits might be more susceptible to sensory experience in some species than in others. Despite decades of work on understanding the mechanisms underlying critical period plasticity, the importance of ecological niche in visual pathway development has received little attention. Here, I will explore the relationship between critical period plasticity and ecological niche in mammalian sensory pathways.

Ovarian Hormones Organize the Maturation of Inhibitory Neurotransmission in the Frontal Cortex at Puberty Onset in Female Mice

The frontal cortex matures late in development, showing dramatic changes after puberty onset, yet few experiments have directly tested the role of pubertal hormones in cortical maturation. One mechanism thought to play a primary role in regulating the maturation of the neocortex is an increase in inhibitory neurotransmission, which alters the balance of excitation and inhibition. We hypothesized that pubertal hormones could regulate maturation of the frontal cortex by this mechanism. Here, we report that manipulations of gonadal hormones do significantly alter the maturation of inhibitory neurotransmission in the cingulate region of the mouse medial frontal cortex, an associative region that matures during the pubertal transition and is implicated in decision making, learning, and psychopathology. We find that inhibitory neurotransmission, but not excitatory neurotransmission, increases onto cingulate pyramidal neurons during peri-pubertal development and that this increase can be blocked by pre-pubertal, but not post-pubertal, gonadectomy. We next used pre-pubertal hormone treatment to model early puberty onset, a phenomenon increasingly observed in girls living in developed nations. We find that pre-pubertal hormone treatment drives an early increase in inhibitory neurotransmission in the frontal cortex, but not the somatosensory cortex, suggesting that earlier puberty can advance cortical maturation in a regionally specific manner. Pre-pubertal hormone treatment also accelerates maturation of tonic inhibition and performance in a frontal-cortex-dependent reversal-learning task. These data provide rare evidence of enduring, organizational effects of ovarian hormones at puberty and provide a potential mechanism by which gonadal hormones could regulate the maturation of the associative neocortex.

Transplantation of GABAergic interneurons for cell-based therapy

Many neurological disorders stem from defects in or the loss of specific neurons. Neuron transplantation has tremendous clinical potential for central nervous system therapy as it may allow for the targeted replacement of those cells that are lost in diseases. Normally, most neurons are added during restricted periods of embryonic and fetal development. The permissive milieu of the developing brain promotes neuronal migration, neuronal differentiation, and synaptogenesis. Once this active period of neurogenesis ends, the chemical and physical environment of the brain changes dramatically. The brain parenchyma becomes highly packed with neuronal and glial processes, extracellular matrix, myelin, and synapses. The migration of grafted cells to allow them to home into target regions and become functionally integrated is a key challenge to neuronal transplantation. Interestingly, transplanted young telencephalic inhibitory interneurons are able to migrate, differentiate, and integrate widely throughout the postnatal brain. These grafted interneurons can also functionally modify local circuit activity. These features have facilitated the use of interneuron transplantation to study fundamental neurodevelopmental processes including cell migration, cell specification, and programmed neuronal cell death. Additionally, these cells provide a unique opportunity to develop interneuron-based strategies for the treatment of diseases linked to interneuron dysfunction and neurological disorders associated to circuit hyperexcitability.

Role of α4-containing GABAA receptors in limiting synaptic plasticity and spatial learning of female mice during the pubertal period

Expression of α4βδ GABA_A receptors (GABARs) increases at the onset of puberty on dendritic spines of CA1 hippocampal pyramidal cells. These receptors reduce activation of NMDA receptors (NMDARs), impair induction of long-term potentiation (LTP) and reduce hippocampal-dependent spatial learning. These effects are not seen in the δ-/- mouse, implicating α4βδ GABARs. Here we show that knock-out of α4 also restores synaptic plasticity and spatial learning in female mice at the onset of puberty (verified by vaginal opening). To this end, field excitatory post-synaptic potentials (fEPSPs) were recorded from the stratum radiatum of CA1 hippocampus in the slice from +/+ and α4-/- pubertal mice (PND 35-44). Induction of LTP, in response to stimulation of the Schaffer collaterals with theta burst stimulation (TBS), was unsuccessful in the +/+ hippocampus, but reinstated by α4 knock-out (~65% potentiation) but not by blockade of α5-GABARs with L-655,708 (50nM). In order to compare spatial learning in the two groups of mice, animals were trained in an active place avoidance task where the latency to first enter a shock zone is a measure of learning. α4-/- mice had significantly longer latencies by the third learning trial, suggesting better spatial learning, compared to +/+ animals, who did not reach the criterion for learning (120s latency). These findings suggest that knock-out of the GABAR α4 subunit restores synaptic plasticity and spatial learning at puberty and is consistent with the concept that the dendritic α4βδ GABARs which emerge at puberty selectively impair CNS plasticity. This article is part of a Special Issue entitled SI: Adolescent plasticity.