Rapid critical period induction by tonic inhibition in visual cortex

Distinct effects of perinatal exposure to fluoxetine or methylmercury on parvalbumin and perineuronal nets, the markers of critical periods in brain development

The in utero exposure to common chemical stressors, environmental pollutant methylmercury and antidepressant fluoxetine, results in behavioral impairments persistent into adulthood. Modulation of critical periods in brain development may alter proper network formation and lastingly impair brain function. To investigate whether early-life stressors can modulate critical periods, we analyzed the development of parvalbumin (PV) and perineuronal nets (PNNs) in the dentate gyrus and CA1 area of the hippocampus and the basolateral amygdala in mice perinatally exposed to either fluoxetine or methylmercury. The number of PV and PNN neurons, and PV intensity, were analyzed by fluorescent immunohistochemistry at the postnatal ages P17 (ongoing critical period) and P24 (closing critical period). The exposure to fluoxetine did not affect the number of PV cells and PV intensity but decreased PNN formation around the cells at P17 and P24 in all tissues. In contrast, perinatal methylmercury inhibited the development of PV interneurons and PV expression at P17 only, but at P24 these parameters were restored. Methylmercury strongly increased PNN formation from P17 to P24 in the amygdala only. We suggest that perinatal fluoxetine and methylmercury might delay the closure and the onset, respectively, of the critical periods in the amygdala and hippocampus.

Interneuron epigenomes during the critical period of cortical plasticity: Implications for schizophrenia

Schizophrenia, a major psychiatric disorder defined by delusions and hallucinations, among other symptoms, often with onset in early adulthood, is potentially associated with molecular and cellular alterations in parvalbumin-expressing fast spiking interneurons and other constituents of the cortical inhibitory GABAergic circuitry. The underlying mechanisms, including the role of disease-associated risk factors operating in adolescence such as drug abuse and social stressors, remain incompletely understood. Here, we summarize emerging findings from animal models, highlighting the ability of parvalbuminergic interneurons (PVI) to induce, during the juvenile period, long-term plastic changes in prefrontal and visual cortex, thereby altering perception, cognition and behavior in the adult. Of note, molecular alterations in PVI from subjects with schizophrenia, including downregulated expression of a subset of GABAergic genes, have also been found in juvenile stress models of the disorder. Some of the transcriptional alterations observed in schizophrenia postmortem brain could be linked to changes in the epigenetic architecture of GABAergic gene promoters, including dysregulated DNA methylation, histone modification patterns and disruption of promoter-enhancer interactions at site of chromosomal loop formations. Therefore, we predict that, in the not-to-distant future, PVI- and other cell-type specific epigenomic mappings in the animal model and human brain will provide novel insights into the pathophysiology of schizophrenia and related psychotic diseases, including the role of cortical GABAergic circuitry in shaping long-term plasticity and cognitive function of the cerebral cortex.

GABA receptors in brain development, function, and injury

This review presents a brief overview of the γ-aminobutyric acid (GABA) system in the developing and mature central nervous system (CNS) and its potential connections to pathologies of the CNS. γ-aminobutyric acid (GABA) is a major neurotransmitter expressed from the embryonic stage and throughout life. At an early developmental stage, GABA acts in an excitatory manner and is implicated in many processes of neurogenesis, including neuronal proliferation, migration, differentiation, and preliminary circuit-building, as well as the development of critical periods. In the mature CNS, GABA acts in an inhibitory manner, a switch mediated by chloride/cation transporter expression and summarized in this review. GABA also plays a role in the development of interstitial neurons of the white matter, as well as in oligodendrocyte development. Although the underlying cellular mechanisms are not yet well understood, we present current findings for the role of GABA in neurological diseases with characteristic white matter abnormalities, including anoxic-ischemic injury, periventricular leukomalacia, and schizophrenia. Development abnormalities of the GABAergic system appear particularly relevant in the etiology of schizophrenia. This review also covers the potential role of GABA in mature brain injury, namely transient ischemia, stroke, and traumatic brain injury/post-traumatic epilepsy.

Cortical parvalbumin and somatostatin GABA neurons express distinct endogenous modulators of nicotinic acetylcholine receptors

BACKGROUND

Inhibition from GABAergic interneurons in brain circuits is a critical component of cognitive function. This inhibition is regulated through a diverse network of neuromodulation. A number of recent studies suggest that one of the major regulators of interneuron function is nicotinic acetylcholinergic transmission and dysregulation of both systems is common in psychiatric conditions. However, how nicotinic modulation impacts specific subpopulations of diverse GABAergic interneurons remains in question. One potential way of conferring specificity to the convergence of GABAergic and nicotinic signaling is through the expression of a unique family of nicotinic acetycholine receptor modulators, the Lynx family. The present study sought to identify members of the Lynx family enriched in cortical interneurons and to elucidate subpopulations of GABAergic neurons that express unique nicotinic modulators.

RESULTS

We utilize double fluorescence in situ hybridization to examine the interneuronal expression of the Lynx family in adult mouse visual cortex. We find that two of the Lynx family members, Lynx1 and Lypd6, are enriched in interneuron populations in cortex. Nearly all parvalbumin interneurons express Lynx1 but we did not detect Lypd6 in this population. Conversely, in somatostatin interneurons Lypd6 was found in a subset localized to deep cortical layers but no somatostatin neurons show detectable levels of Lynx1. Using a combination of genetic and viral manipulations we further show that a subpopulation of deep-layer cortico-cortical long-range somatostatin neurons also express Lypd6.

CONCLUSIONS

This work shows that distinct subpopulations of GABAergic interneurons express unique Lynx family members. The pattern of expression of Lynx family members within interneurons places them in a unique position to potentially regulate the convergence of GABAergic and nicotinic systems, dysfunction of which are characteristic of psychiatric disorders.

Conditional deletion of Mecp2 in parvalbumin-expressing GABAergic cells results in the absence of critical period plasticity

Mutations in the X-linked gene encoding the transcriptional modulator methyl-CpG-binding protein 2 (MeCP2) impair postnatal development of the brain. Here we use neuronal-type specific gene deletion in mice to show that conditional Mecp2 deletion in GABAergic parvalbumin-expressing (PV) cells (PV-Mecp2(-/y)) does not cause most Rett-syndrome-like behaviours, but completely abolishes experience-dependent critical period plasticity of primary visual cortex (V1) that develops normal visual functions. However, selective loss of Mecp2 in GABAergic somatostatin-expressing cells or glutamatergic pyramidal cells does not affect the critical period plasticity. MeCP2-deficient PV cells exhibit high intrinsic excitability, selectively reduced efficacy of recurrent excitatory synapses in V1 layer 4 circuits, and decreased evoked visual responses in vivo. Enhancing cortical gamma-aminobutyric acid (GABA) inhibition with diazepam infusion can restore critical period plasticity in both young and adult PV-Mecp2(-/y) mice. Thus, MeCP2 expression in inhibitory PV cells during the critical period is essential for local circuit functions underlying experience-dependent cortical plasticity.

Mouse vision as a gateway for understanding how experience shapes neural circuits

Genetic programs controlling ontogeny drive many of the essential connectivity patterns within the brain. Yet it is activity, derived from the experience of interacting with the world, that sculpts the precise circuitry of the central nervous system. Such experience-dependent plasticity has been observed throughout the brain but has been most extensively studied in the neocortex. A prime example of this refinement of neural circuitry is found in primary visual cortex (V1), where functional connectivity changes have been observed both during development and in adulthood. The mouse visual system has become a predominant model for investigating the principles that underlie experience-dependent plasticity, given the general conservation of visual neural circuitry across mammals as well as the powerful tools and techniques recently developed for use in rodent. The genetic tractability of mice has permitted the identification of signaling pathways that translate experience-driven activity patterns into changes in circuitry. Further, the accessibility of visual cortex has allowed neural activity to be manipulated with optogenetics and observed with genetically-encoded calcium sensors. Consequently, mouse visual cortex has become one of the dominant platforms to study experience-dependent plasticity.

Experience-dependent specialization of receptive field surround for selective coding of natural scenes

At eye opening, neurons in primary visual cortex (V1) are selective for stimulus features, but circuits continue to refine in an experience-dependent manner for some weeks thereafter. How these changes contribute to the coding of visual features embedded in complex natural scenes remains unknown. Here we show that normal visual experience after eye opening is required for V1 neurons to develop a sensitivity for the statistical structure of natural stimuli extending beyond the boundaries of their receptive fields (RFs), which leads to improvements in coding efficiency for full-field natural scenes (increased selectivity and information rate). These improvements are mediated by an experience-dependent increase in the effectiveness of natural surround stimuli to hyperpolarize the membrane potential specifically during RF-stimulus epochs triggering action potentials. We suggest that neural circuits underlying surround modulation are shaped by the statistical structure of visual input, which leads to more selective coding of features in natural scenes.

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V1 firing is more selective to natural than phase-scrambled surround stimuli

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Improved selectivity is caused by Vm hyperpolarisation prior to RF-spiking events

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Natural surround sensitivity is experience dependent, absent in immature/deprived V1

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Vision shapes circuits to improve V1 coding of natural scenes across RF and surround

Balancing plasticity/stability across brain development

The potency of the environment to shape brain function changes dramatically across the lifespan. Neural circuits exhibit profound plasticity during early life and are later stabilized. A focus on the cellular and molecular bases of these developmental trajectories has begun to unravel mechanisms, which control the onset and closure of such critical periods. Two important concepts have emerged from the study of critical periods in the visual cortex: (1) excitatory-inhibitory circuit balance is a trigger; and (2) molecular "brakes" limit adult plasticity. The onset of the critical period is determined by the maturation of specific GABA circuits. Targeting these circuits using pharmacological or genetic approaches can trigger premature onset or induce a delay. These manipulations are so powerful that animals of identical chronological age may be at the peak, before, or past their plastic window. Thus, critical period timing per se is plastic. Conversely, one of the outcomes of normal development is to stabilize the neural networks initially sculpted by experience. Rather than being passively lost, the brain's intrinsic potential for plasticity is actively dampened. This is demonstrated by the late expression of brake-like factors, which reversibly limit excessive circuit rewiring beyond a critical period. Interestingly, many of these plasticity regulators are found in the extracellular milieu. Understanding why so many regulators exist, how they interact and, ultimately, how to lift them in noninvasive ways may hold the key to novel therapies and lifelong learning.

Boosting visual cortex function and plasticity with acetylcholine to enhance visual perception

The cholinergic system is a potent neuromodulatory system that plays critical roles in cortical plasticity, attention and learning. In this review, we propose that the cellular effects of acetylcholine (ACh) in the primary visual cortex during the processing of visual inputs might induce perceptual learning; i.e., long-term changes in visual perception. Specifically, the pairing of cholinergic activation with visual stimulation increases the signal-to-noise ratio, cue detection ability and long-term facilitation in the primary visual cortex. This cholinergic enhancement would increase the strength of thalamocortical afferents to facilitate the treatment of a novel stimulus while decreasing the cortico-cortical signaling to reduce recurrent or top-down modulation. This balance would be mediated by different cholinergic receptor subtypes that are located on both glutamatergic and GABAergic neurons of the different cortical layers. The mechanisms of cholinergic enhancement are closely linked to attentional processes, long-term potentiation (LTP) and modulation of the excitatory/inhibitory balance. Recently, it was found that boosting the cholinergic system during visual training robustly enhances sensory perception in a long-term manner. Our hypothesis is that repetitive pairing of cholinergic and sensory stimulation over a long period of time induces long-term changes in the processing of trained stimuli that might improve perceptual ability. Various non-invasive approaches to the activation of the cholinergic neurons have strong potential to improve visual perception.

Sensory integration in mouse insular cortex reflects GABA circuit maturation

Insular cortex (IC) contributes to a variety of complex brain functions, such as communication, social behavior, and self-awareness through the integration of sensory, emotional, and cognitive content. How the IC acquires its integrative properties remains unexplored. We compared the emergence of multisensory integration (MSI) in the IC of behaviorally distinct mouse strains. While adult C57BL/6 mice exhibited robust MSI, this capacity was impaired in the inbred BTBR T+tf/J mouse model of idiopathic autism. The deficit reflected weakened γ-aminobutyric acid (GABA) circuits and compromised postnatal pruning of cross-modal input. Transient pharmacological enhancement by diazepam in BTBR mice during an early sensitive period rescued inhibition and integration in the adult IC. Moreover, impaired MSI was common across three other monogenic models (GAD65, Shank3, and Mecp2 knockout mice) displaying behavioral phenotypes and parvalbumin-circuit abnormalities. Our findings offer developmental insight into a key neural circuit relevant to neuropsychiatric conditions like schizophrenia and autism.

Distinct roles of homeoproteins in brain topographic mapping and in neural circuit formation

The construction of the brain is a highly regulated process, requiring coordination of various cellular and molecular mechanisms that together ensure the stability of the cerebrum architecture and functions. The mature brain is an organ that performs complex computational operations using specific sensory information from the outside world and this requires precise organization within sensory networks and a separation of sensory modalities during development. We review here the role of homeoproteins in the arealization of the brain according to sensorimotor functions, the micropartition of its cytoarchitecture, and the maturation of its sensory circuitry. One of the most interesting observation about homeoproteins in recent years concerns their ability to act both in a cell-autonomous and non-cell-autonomous manner. The highlights in the present review collectively show how these two modes of action of homeoproteins confer various functions in shaping cortical maps.

Treatment of amblyopia in the adult: insights from a new rodent model of visual perceptual learning

Amblyopia is the most common form of impairment of visual function affecting one eye, with a prevalence of about 1–5% of the total world population. Amblyopia usually derives from conditions of early functional imbalance between the two eyes, owing to anisometropia, strabismus, or congenital cataract, and results in a pronounced reduction of visual acuity and severe deficits in contrast sensitivity and stereopsis. It is widely accepted that, due to a lack of sufficient plasticity in the adult brain, amblyopia becomes untreatable after the closure of the critical period in the primary visual cortex. However, recent results obtained both in animal models and in clinical trials have challenged this view, unmasking a previously unsuspected potential for promoting recovery even in adulthood. In this context, non invasive procedures based on visual perceptual learning, i.e., the improvement in visual performance on a variety of simple visual tasks following practice, emerge as particularly promising to rescue discrimination abilities in adult amblyopic subjects. This review will survey recent work regarding the impact of visual perceptual learning on amblyopia, with a special focus on a new experimental model of perceptual learning in the amblyopic rat.

Adult cortical plasticity following injury: Recapitulation of critical period mechanisms?

A primary goal of research on developmental critical periods (CPs) is the recapitulation of a juvenile-like state of malleability in the adult brain that might enable recovery from injury. These ambitions are often framed in terms of the simple reinstatement of enhanced plasticity in the growth-restricted milieu of an injured adult brain. Here, we provide an analysis of the similarities and differences between deprivation-induced and injury-induced cortical plasticity, to provide for a nuanced comparison of these remarkably similar processes. As a first step, we review the factors that drive ocular dominance plasticity in the primary visual cortex of the uninjured brain during the CP and in adults, to highlight processes that might confer adaptive advantage. In addition, we directly compare deprivation-induced cortical plasticity during the CP and plasticity following acute injury or ischemia in mature brain. We find that these two processes display a biphasic response profile following deprivation or injury: an initial decrease in GABAergic inhibition and synapse loss transitions into a period of neurite expansion and synaptic gain. This biphasic response profile emphasizes the transition from a period of cortical healing to one of reconnection and recovery of function. Yet while injury-induced plasticity in adult shares several salient characteristics with deprivation-induced plasticity during the CP, the degree to which the adult injured brain is able to functionally rewire, and the time required to do so, present major limitations for recovery. Attempts to recapitulate a measure of CP plasticity in an adult injury context will need to carefully dissect the circuit alterations and plasticity mechanisms involved while measuring functional behavioral output to assess their ultimate success.

Loudness perception affected by early age hearing loss

Tinnitus and hyperacusis, commonly seen in adults, are also reported in children. Although clinical studies found children with tinnitus and hyperacusis often suffered from recurrent otitis media, there is no direct study on how temporary hearing loss in the early age affects the sound loudness perception. In this study, sound loudness changes in rats affected by perforation of the tympanic membranes (TM) have been studied using an operant conditioning based behavioral task. We detected significant increases of sound loudness and susceptibility to audiogenic seizures (AGS) in rats with bilateral TM damage at postnatal 16 days. As increase to sound sensitivity is commonly seen in hyperacusis and tinnitus patients, these results suggest that early age hearing loss is a high risk factor to induce tinnitus and hyperacusis in children. In the TM damaged rats, we also detected a reduced expression of GABA receptor δ and α6 subunits in the inferior colliculus (IC) compared to the controls. Treatment of vigabatrin (60 mg/kg/day, 7-14 days), an anti-seizure drug that inhibits the catabolism of GABA, not only blocked AGS, but also significantly attenuated the loudness response. Administration of vigabatrin following the early age TM damage could even prevent rats from developing AGS. These results suggest that TM damage at an early age may cause a permanent reduction of GABA tonic inhibition which is critical towards the maintenance of normal loudness processing of the IC. Increasing GABA concentration during the critical period may alleviate the impairment in the brain induced by early age hearing loss.

Sublinear binocular integration preserves orientation selectivity in mouse visual cortex

Inputs from the two eyes are first combined in simple cells in the primary visual cortex. Consequently, visual cortical neurons need to have the flexibility to encode visual features under both monocular and binocular situations. Here we show that binocular orientation selectivity of mouse simple cells is nearly identical to monocular orientation selectivity in both anaesthetized and awake conditions. In vivo whole-cell recordings reveal that the binocular integration of membrane potential responses is sublinear. The sublinear integration keeps binocularly evoked depolarizations below threshold at non-preferred orientations, thus preserving orientation selectivity. Computational simulations based on measured synaptic conductances indicate that inhibition promotes sublinear binocular integration, which are further confirmed by experiments using genetic and pharmacological manipulations. Our findings therefore reveal a cellular mechanism for how visual system can switch effortlessly between monocular and binocular conditions. The same mechanism may apply to other sensory systems that also integrate multiple channels of inputs.

Long-term inhibitory plasticity in visual cortical layer 4 switches sign at the opening of the critical period

Sensory microcircuits are refined by experience during windows of heightened plasticity termed "critical periods" (CPs). In visual cortex the effects of visual deprivation change dramatically at the transition from the pre-CP to the CP, but the cellular plasticity mechanisms that underlie this change are poorly understood. Here we show that plasticity at unitary connections between GABAergic Fast Spiking (FS) cells and Star Pyramidal (SP) neurons within layer 4 flips sign at the transition between the pre-CP and the CP. During the pre-CP, coupling FS firing with SP depolarization induces long-term depression of inhibition at this synapse, whereas the same protocol induces long-term potentiation of inhibition at the opening of the CP. Despite being of opposite sign, both forms of plasticity share expression characteristics--a change in coefficient of variation with no change in paired-pulse ratio--and depend on GABAB receptor signaling. Finally, we show that the reciprocal SP → FS synapse also acquires the ability to undergo long-term potentiation at the pre-CP to CP transition. Thus, at the opening of the CP, there are coordinated changes in plasticity that allow specific patterns of activity within layer 4 to potentiate feedback inhibition by boosting the strength of FS ↔ SP connections.

A theory of the transition to critical period plasticity: inhibition selectively suppresses spontaneous activity

What causes critical periods (CPs) to open? For the best-studied case, ocular dominance plasticity in primary visual cortex in response to monocular deprivation (MD), the maturation of inhibition is necessary and sufficient. How does inhibition open the CP? We present a theory: the transition from pre-CP to CP plasticity arises because inhibition preferentially suppresses responses to spontaneous relative to visually driven input activity, switching learning cues from internal to external sources. This differs from previous proposals in (1) arguing that the CP can open without changes in plasticity mechanisms when activity patterns become more sensitive to sensory experience through circuit development, and (2) explaining not simply a transition from no plasticity to plasticity, but a change in outcome of MD-induced plasticity from pre-CP to CP. More broadly, hierarchical organization of sensory-motor pathways may develop through a cascade of CPs induced as circuit maturation progresses from "lower" to "higher" cortical areas.

Environmental enrichment rescues binocular matching of orientation preference in mice that have a precocious critical period

Experience shapes neural circuits during critical periods in early life. The timing of critical periods is regulated by both genetics and the environment. Here we study the functional significance of such temporal regulations in the mouse primary visual cortex, where critical period plasticity drives binocular matching of orientation preference. We find that the binocular matching is permanently disrupted in mice that have a precocious critical period due to genetically enhanced inhibition. The disruption is specific to one type of neuron, the complex cells, which, as we reveal, normally match after the simple cells. Early environmental enrichment completely rescues the deficit by inducing histone acetylation and consequently advancing the matching process to coincide with the precocious plasticity. Our experiments thus demonstrate that the proper timing of the critical period is essential for establishing normal binocularity and the detrimental impact of its genetic misregulation can be ameliorated by environmental manipulations via epigenetic mechanisms.

Neurochemical changes within human early blind occipital cortex

Early blindness results in occipital cortex neurons responding to a wide range of auditory and tactile stimuli. These changes in tuning properties are accompanied by an extensive reorganization of the occipital cortex that includes alterations in anatomical structure, neurochemical and metabolic pathways. Although it has been established in animal models that neurochemical pathways are heavily affected by early visual deprivation, the effects of blindness on these pathways in humans is still not well characterized. Here, using (1)H magnetic resonance spectroscopy in nine early blind and normally sighted subjects, we find that early blindness is associated with higher levels of creatine, choline and myo-Inositol and indications of lower levels of GABA within the occipital cortex. These results suggest that the cross-modal responses associated with early blindness may, at least in part, be driven by changes within occipital biochemical pathways.

Rescue of inhibitory synapse strength following developmental hearing loss

Inhibitory synapse dysfunction may contribute to many developmental brain disorders, including the secondary consequences of sensory deprivation. In fact, developmental hearing loss leads to a profound reduction in the strength of inhibitory postsynaptic currents (IPSCs) in the auditory cortex, and this deficit persists into adulthood. This finding is consistent with the general theory that the emergence of mature synaptic properties requires activity during development. Therefore, we tested the prediction that inhibitory strength can be restored following developmental hearing loss by boosting GABAergic transmission in vivo. Conductive or sensorineural hearing loss was induced surgically in gerbils prior to hearing onset and GABA agonists were then administered for one week. IPSCs were subsequently recorded from pyramidal neurons in a thalamocortical brain slice preparation. Administration of either a GABA_A receptor a1 subunit specific agonist (zolpidem), or a selective GABA reuptake inhibitor (SGRI), rescued IPSC amplitude in hearing loss animals. Furthermore, this restoration persisted in adults, long after drug treatment ended. In contrast, a GABA_B receptor agonist baclofen did not restore inhibitory strength. IPSCs could also be restored when SGRI administration began 3 weeks after sensory deprivation. Together, these results demonstrate long-lasting restoration of cortical inhibitory strength in the absence of normal experience. This suggests that in vivo GABA_A receptor activation is sufficient to promote maturation, and this principle may extend to other developmental disorders associated with diminished inhibitory function.

Development and plasticity of the primary visual cortex

Hubel and Wiesel began the modern study of development and plasticity of primary visual cortex (V1), discovering response properties of cortical neurons that distinguished them from their inputs and that were arranged in a functional architecture. Their findings revealed an early innate period of development and a later critical period of dramatic experience-dependent plasticity. Recent studies have used rodents to benefit from biochemistry and genetics. The roles of spontaneous neural activity and molecular signaling in innate, experience-independent development have been clarified, as have the later roles of visual experience. Plasticity produced by monocular visual deprivation (MD) has been dissected into stages governed by distinct signaling mechanisms, some of whose molecular players are known. Many crucial questions remain, but new tools for perturbing cortical cells and measuring plasticity at the level of changes in connections among identified neurons now exist. The future for the study of V1 to illuminate cortical development and plasticity is bright.

Experience-dependent regulation of functional maps and synaptic protein expression in the cat visual cortex

Although the basis of our knowledge of experience-dependent plasticity comes from studies on carnivores and primates, studies examining the physiological and molecular mechanisms that underlie development and plasticity have increasingly employed mice. We have used several common rearing paradigms, such as dark-rearing and monocular deprivation (MD), to examine the timing of the physiological and molecular changes to altered experience in the cat primary visual cortex. Dark-rearing from birth or for 1 week starting at 4 weeks of age produced a similar reduction in the amplitude of responses measured through intrinsic signal imaging and a reduction in orientation selectivity. One week of visual experience following dark-rearing until 4 weeks of age yielded normal responses in both amplitude and orientation selectivity. The depression of deprived-eye responses was similar in magnitude after 2 and 7 days of MD. In contrast, non-deprived-eye responses almost doubled in magnitude after 7 days compared with 2 days of MD. These changes in the functional properties of primary visual cortex neurons were mirrored by specific changes in synaptic protein expression. Changes in proteins such as the NR2A and NR2B subunits of the N-methyl-D-aspartate receptor, postsynaptic density protein 95, alpha-CA(2+) /calmodulin-dependent protein kinase II (αCaMKII), and GABA(A) α1a indicated that the levels of sensory activity regulated mechanisms associated with both excitatory (NR2A and NR2B) and inhibitory (GABA(A) α1a) transmission so as to maintain response homeostasis. Additionally, we found that MD regulated the AMPA receptor glutamate (GluR1) subunit as well as signalling molecules (αCaMKII and synaptic Ras GTPase activating protein, SynGAP) downstream of N-methyl-D-aspartate receptors. Proteins in a common signalling pathway appeared to have similar developmental expression profiles that were broadly similar between cats and rodents.

Amblyopia: background to the special issue on stroke recovery

In this introductory article, we summarize the evidence from humans and animal models on the shaping of postnatal visual development by focused binocular input. When balanced input is missing during a sensitive period, deficits emerge, including seemingly permanent impairments in visual acuity that are labeled amblyopia. Rodent models have identified neurochemical changes that control the onset of such sensitive periods and molecular and structural brakes that lead to the diminution of the plasticity thereafter. Both animal and human studies of amblyopia have recently identified exciting ways to remediate vision in adulthood that bear some similarity to the interventions that have proved successful in promoting recovery from stroke.

GABAergic inhibition regulates developmental synapse elimination in the cerebellum

Functional neural circuit formation during development involves massive elimination of redundant synapses. In the cerebellum, one-to-one connection from excitatory climbing fiber (CF) to Purkinje cell (PC) is established by elimination of early-formed surplus CFs. This process depends on glutamatergic excitatory inputs, but contribution of GABAergic transmission remains unclear. Here, we demonstrate impaired CF synapse elimination in mouse models with diminished GABAergic transmission by mutation of a single allele for the GABA synthesizing enzyme GAD67, by conditional deletion of GAD67 from PCs and GABAergic interneurons or by pharmacological inhibition of cerebellar GAD activity. The impaired CF synapse elimination was rescued by enhancing GABA(A) receptor sensitivity in the cerebellum by locally applied diazepam. Our electrophysiological and Ca2+ imaging data suggest that GABA(A) receptor-mediated inhibition onto the PC soma from molecular layer interneurons influences CF-induced Ca2+ transients in the soma and regulates CF synapse elimination from postnatal day 10 (P10) to around P16.

Cortical GABAergic interneurons in cross-modal plasticity following early blindness

Early loss of a given sensory input in mammals causes anatomical and functional modifications in the brain via a process called cross-modal plasticity. In the past four decades, several animal models have illuminated our understanding of the biological substrates involved in cross-modal plasticity. Progressively, studies are now starting to emphasise on cell-specific mechanisms that may be responsible for this intermodal sensory plasticity. Inhibitory interneurons expressing γ-aminobutyric acid (GABA) play an important role in maintaining the appropriate dynamic range of cortical excitation, in critical periods of developmental plasticity, in receptive field refinement, and in treatment of sensory information reaching the cerebral cortex. The diverse interneuron population is very sensitive to sensory experience during development. GABAergic neurons are therefore well suited to act as a gate for mediating cross-modal plasticity. This paper attempts to highlight the links between early sensory deprivation, cortical GABAergic interneuron alterations, and cross-modal plasticity, discuss its implications, and further provide insights for future research in the field.

The perineuronal net and the control of CNS plasticity

Perineuronal nets (PNNs) are reticular structures that surround the cell body of many neurones, and extend along their dendrites. They are considered to be a specialized extracellular matrix in the central nervous system (CNS). PNN formation is first detected relatively late in development, as the mature synaptic circuitry of the CNS is established and stabilized. Its unique distribution in different CNS regions, the timing of its establishment, and the changes it undergoes after injury all point toward diverse and important functions that it may be performing. The involvement of PNNs in neuronal plasticity has been extensively studied over recent years, with developmental, behavioural, and functional correlations. In this review, we will first briefly detail the structure and organization of PNNs, before focusing our discussion on their unique roles in neuronal development and plasticity. The PNN is an important regulator of CNS plasticity, both during development and into adulthood. Production of critical PNN components is often triggered by appropriate sensory experiences during early postnatal development. PNN deposition around neurones helps to stabilize the established neuronal connections, and to restrict the plastic changes due to future experiences within the CNS. Disruption of PNNs can reactivate plasticity in many CNSs, allowing activity-dependent changes to once again modify neuronal connections. The mechanisms through which PNNs restrict CNS plasticity remain unclear, although recent advances promise to shed additional light on this important subject.

Development of disease-modifying treatment of schizophrenia

Development of disease-modifying therapies requires an innovative approach to drug development where novel drugs are designed to target mechanisms of interest rather than to produce preclinical effects similar to those of currently used antipsychotics. Application of such novel strategy will undoubtedly require a very deep understanding of the disease biology that is just starting to emerge. Alternatively, one may let environmental experiences of the diseased individual guide the repair process and use drugs only to facilitate the effects of experience. Such an approach would bring together functional experience that is age-, environment- and disease-dependent with the plasticity resources that may otherwise not be available. There are currently no preclinical drug-environment interaction models that can be claimed to have significant degrees of validity. Therefore, from a drug development perspective, principles that combine acute symptomatic and disease-modifying properties are clearly preferred. The question arises then how such treatments can be differentiated from those that have only symptomatic effects (i.e., most currently used antipsychotic medications). One expectation is that the former will show superior and broader efficacy (especially with longer treatment duration). Another possibility is that disease-modifying drugs will be particularly useful at the very earliest stages of the disease. Society and medical communities may not be ready yet to initiate the treatment as early as during the prodromal phase, but the situation may change by the time the science advances enough to bring a convincing case of a drug with disease-modification potential.

Age-dependent effect of hearing loss on cortical inhibitory synapse function

The developmental plasticity of excitatory synapses is well established, particularly as a function of age. If similar principles apply to inhibitory synapses, then we would expect manipulations during juvenile development to produce a greater effect and experience-dependent changes to persist into adulthood. In this study, we first characterized the maturation of cortical inhibitory synapse function from just before the onset of hearing through adulthood. We then examined the long-term effects of developmental conductive hearing loss (CHL). Whole cell recordings from gerbil thalamocortical brain slices revealed a significant decrease in the decay time of inhibitory currents during the first 3 mo of normal development. When assessed in adults, developmental CHL led to an enduring decrease of inhibitory synaptic strength, whereas the maturation of synaptic decay time was only delayed. Early CHL also depressed the maximum discharge rate of fast-spiking, but not low-threshold-spiking, inhibitory interneurons. We then asked whether adult onset CHL had a similar effect, but neither inhibitory current amplitude nor decay time was altered. Thus inhibitory synapse function displays a protracted development during which deficits can be induced by juvenile, but not adult, hearing loss. These long-lasting changes to inhibitory function may contribute to the auditory processing deficits associated with early hearing loss.

Inhibitory plasticity underlies visual deprivation-induced loss of receptive field refinement in the adult superior colliculus

Increasing evidence shows that sensory experience is not necessary for initial patterning of neural circuitry but is essential for maintenance and plasticity. We have investigated the role of visual experience in development and plasticity of inhibitory synapses in the retinocollicular pathway of an altricial rodent, the Syrian hamster. We reported previously that visual receptive field (RF) refinement in superior colliculus (SC) occurs with the same time course in long-term dark-reared (LTDR) as in normally-reared hamsters, but RFs in LTDR animals become unrefined in adulthood. Here we provide support for the hypothesis that this failure to maintain refined RFs into adulthood results from inhibitory plasticity at both pre- and postsynaptic levels. Iontophoretic application of gabazine, a GABA(A) receptor antagonist, or muscimol, a GABA(A) receptor agonist, had less of an effect on RF size and excitability of adult LTDR animals than in short-term DR animals or normal animals. Consistent with these physiological observations, the percentage of GABA-immunoreactive neurons was significantly decreased in the SC of LTDR animals compared to normal animals and to animals exposed to a normal light cycle early in development, before LTDR. Thus GABAergic inhibition in the SC of LTDR animals is reduced, weakening the inhibitory surround and contributing significantly to the visual deprivation-induced enlargement of RFs seen. Our results argue that early visually-driven activity is necessary to maintain the inhibitory circuitry intrinsic to the adult SC and to protect against the consequences of visual deprivation.

Structure and function relationships during ocular dominance plasticity in the visual cortex

Our ability to learn relies on the potential of the neocortex to change its neuronal circuits through experience. This change is mediated by the loss or formation of synaptic contacts or the adjustment of their synaptic strength. In recent decades, the primary visual cortex has proven an excellent system for studying structure/function relationships during plasticity in the neocortex. Here we describe current knowledge about the structural changes in inhibitory or excitatory synapses that accompany experience dependent plasticity in the visual cortex. We discuss unresolved issues and technical developments that will help to provide answers in the near future.

Maturation of GABAergic inhibition promotes strengthening of temporally coherent inputs among convergent pathways

Spike-timing-dependent plasticity (STDP), a form of Hebbian plasticity, is inherently stabilizing. Whether and how GABAergic inhibition influences STDP is not well understood. Using a model neuron driven by converging inputs modifiable by STDP, we determined that a sufficient level of inhibition was critical to ensure that temporal coherence (correlation among presynaptic spike times) of synaptic inputs, rather than initial strength or number of inputs within a pathway, controlled postsynaptic spike timing. Inhibition exerted this effect by preferentially reducing synaptic efficacy, the ability of inputs to evoke postsynaptic action potentials, of the less coherent inputs. In visual cortical slices, inhibition potently reduced synaptic efficacy at ages during but not before the critical period of ocular dominance (OD) plasticity. Whole-cell recordings revealed that the amplitude of unitary IPSCs from parvalbumin positive (Pv+) interneurons to pyramidal neurons increased during the critical period, while the synaptic decay time-constant decreased. In addition, intrinsic properties of Pv+ interneurons matured, resulting in an increase in instantaneous firing rate. Our results suggest that maturation of inhibition in visual cortex ensures that the temporally coherent inputs (e.g. those from the open eye during monocular deprivation) control postsynaptic spike times of binocular neurons, a prerequisite for Hebbian mechanisms to induce OD plasticity.

Evidence suggests that maturation of inhibition is required for the development of plasticity to proceed in the visual cortex. However, the mechanisms by which increased inhibition promotes plasticity are not clear. Here we characterized the maturation of synaptic and intrinsic ionic properties of parvalbumin-positive interneurons, a prominent subtype of inhibitory neuron in the cortex. We used a simple integrate-and-fire model to simulate the influence of maturation of inhibition on associative plasticity rules. We simulated two input pathways that converged onto a single postsynaptic neuron. The temporal pattern of activity was constructed differently for the two pathways: one pathway represented visually-driven activity, while the other pathway represented sensory-deprived activity. In mature circuits it is established that postsynaptic cells can select for sensory-driven inputs over deprived inputs, even in the case that deprived inputs have an initial advantage in synaptic size or number. We demonstrated that maturation of inhibition was required for postsynaptic cells to appropriately select sensory-driven patterns of activity when challenged with an opponent pathway of greater size. These results outline a mechanism by which maturation of inhibition can promote plasticity in the young, a period of development that is characterized by heightened learning.

Critical period plasticity matches binocular orientation preference in the visual cortex

Changes of ocular dominance in the visual cortex can be induced by visual manipulations during a critical period in early life. However, the role of critical period plasticity in normal development is unknown. Here we show that at the onset of this time window, the preferred orientations of individual cortical cells in the mouse are mismatched through the two eyes and the mismatch decreases and reaches adult levels by the end of the period. Deprivation of visual experience during this period irreversibly blocks the binocular matching of orientation preference, but has no effect in adulthood. The critical period of binocular matching can be delayed by long-term visual deprivation from birth, like that of ocular dominance plasticity. These results demonstrate that activity-dependent changes induced by normal visual experience during the well-studied critical period serve to match eye-specific inputs in the cortex, thus revealing a physiological role for critical period plasticity during normal development.

Parvalbumin-containing neurons, perineuronal nets and experience-dependent plasticity in murine barrel cortex

The ability to undergo experience-dependent plasticity in the neocortex is often limited to early development, but also to particular cortical loci and specific experience. In layers II-IV of the barrel cortex, plasticity evoked by removing all but one vibrissae (univibrissa rearing) does not have a time limit except for layer IV barrels, where it can only be induced during the first postnatal week. In contrast, deprivation of every second vibrissa (chessboard deprivation) removes time limits for plasticity. The mechanism permitting plasticity in response to chessboard deprivation and halting it in reply to univibrissa rearing is unknown. Condensation of chondroitin sulfate proteoglycans into perineuronal nets and an increase in intracortical inhibition mediated by parvalbumin-containing interneurons are implicated in closing the critical period for ocular dominance plasticity. These factors could also be involved in setting up the critical period in barrels in a way that depends on a particular sensory experience. We therefore examined changes in density of parvalbumin-containing cells and perineuronal nets during development of mouse barrel cortex and after brief univibrissa and chessboard experience in adolescence. We observed a progressive increase in the density of the two markers across cortical layers between postnatal day 10 and 20, which was especially pronounced in the barrels. Univibrissa rearing, but not chessboard deprivation, increased the density of perineuronal nets and parvalbumin-containing cells in the deprived barrels, but only those that immediately neighbour the undeprived barrel. These data suggest the involvement of both tested factors in closing the critical period in barrels in an experience-dependent manner.

Common circuit defect of excitatory-inhibitory balance in mouse models of autism

One unifying explanation for the complexity of Autism Spectrum Disorders (ASD) may lie in the disruption of excitatory/inhibitory (E/I) circuit balance during critical periods of development. We examined whether Parvalbumin (PV)-positive inhibitory neurons, which normally drive experience-dependent circuit refinement (Hensch Nat Rev Neurosci 6:877–888, 1), are disrupted across heterogeneous ASD mouse models. We performed a meta-analysis of PV expression in previously published ASD mouse models and analyzed two additional models, reflecting an embryonic chemical insult (prenatal valproate, VPA) or single-gene mutation identified in human patients (Neuroligin-3, NL-3 R451C). PV-cells were reduced in the neocortex across multiple ASD mouse models. In striking contrast to controls, both VPA and NL-3 mouse models exhibited an asymmetric PV-cell reduction across hemispheres in parietal and occipital cortices (but not the underlying area CA1). ASD mouse models may share a PV-circuit disruption, providing new insight into circuit development and potential prevention by treatment of autism.

Visual deprivation decreases somatic GAD65 puncta number on layer 2/3 pyramidal neurons in mouse visual cortex

Proper functioning of the visual system depends on maturation of both excitatory and inhibitory synapses within the visual cortex. Considering that perisomatic inhibition is one of the key factors that control the critical period in visual cortex, it is pertinent to understand its regulation by visual experience. To do this, we developed an immunohistochemical method that allows three-dimensional (3D) analysis of the glutamic acid decarboxylase (GAD) 65-positive inhibitory terminals in the visual cortex. Using this method on transgenic mice expressing yellow fluorescence protein (YFP) in a subset of neurons, we found that the number of somatic GAD65-puncta on individual layer 2/3 pyramidal neurons is reduced when mice are dark-reared from birth and reverted to normal levels by re-exposure to light. There was no change in GAD65-puncta volume or intensity. These results support the reorganization of inhibitory circuitry within layer 2/3 of visual cortex in response to changes in visual experience.

Fast homeostatic plasticity of inhibition via activity-dependent vesicular filling

Synaptic activity in the central nervous system undergoes rapid state-dependent changes, requiring constant adaptation of the homeostasis between excitation and inhibition. The underlying mechanisms are, however, largely unclear. Chronic changes in network activity result in enhanced production of the inhibitory transmitter GABA, indicating that presynaptic GABA content is a variable parameter for homeostatic plasticity. Here we tested whether such changes in inhibitory transmitter content do also occur at the fast time scale required to ensure inhibition-excitation-homeostasis in dynamic cortical networks. We found that intense stimulation of afferent fibers in the CA1 region of mouse hippocampal slices yielded a rapid and lasting increase in quantal size of miniature inhibitory postsynaptic currents. This potentiation was mediated by the uptake of GABA and glutamate into presynaptic endings of inhibitory interneurons (the latter serving as precursor for the synthesis of GABA). Thus, enhanced release of inhibitory and excitatory transmitters from active networks leads to enhanced presynaptic GABA content. Thereby, inhibitory efficacy follows local neuronal activity, constituting a negative feedback loop and providing a mechanism for rapid homeostatic scaling in cortical circuits.

Gene expression patterns in visual cortex during the critical period: synaptic stabilization and reversal by visual deprivation

The mapping of eye-specific, geniculocortical inputs to primary visual cortex (V1) is highly sensitive to the balance of correlated activity between the two eyes during a restricted postnatal critical period for ocular dominance plasticity. This critical period is likely to have amplified expression of genes and proteins that mediate synaptic plasticity. DNA microarray analysis of transcription in mouse V1 before, during, and after the critical period identified 31 genes that were up-regulated and 22 that were down-regulated during the critical period. The highest-ranked up-regulated gene, cardiac troponin C, codes for a neuronal calcium-binding protein that regulates actin binding and whose expression is activity-dependent and relatively selective for layer-4 star pyramidal neurons. The highest-ranked down-regulated gene, synCAM, also has actin-based function. Actin-binding function, G protein signaling, transcription, and myelination are prominently represented in the critical period transcriptome. Monocular deprivation during the critical period reverses the expression of nearly all critical period genes. The profile of regulated genes suggests that synaptic stability is a principle driver of critical period gene expression and that alteration in visual activity drives homeostatic restoration of stability.

Influences of un-modulated acoustic inputs on functional maturation and critical-period plasticity of the primary auditory cortex

Sensory experiences contribute to the development and specialization of signal processing capacities in the mammalian auditory system during a "critical period" of postnatal development. Earlier studies have shown that passive exposure to tonal stimuli during this postnatal epoch induces a large-scale expansion of the representations of those stimuli within the primary auditory cortex (A1) [Zhang LI, Bao S, Merzenich MM (2001) Persistent and specific influences of early acoustic environments on primary auditory cortex. Nat Neurosci 4:1123-1130]. Here, we show that exposing rat pups through the normal critical period epoch and beyond to continuous, un-modulated, moderate-level tones induces no such representational distortion, and in fact disrupts the normal development of frequency response selectivity and tonotopicity all across area A1. The area of cortex responding selectively to continuously exposed sound frequencies was actually reduced, when compared with rats reared in normal environments. Strong exposure-driven plasticity characteristic of the critical period could be induced well beyond the normal end of the critical period, by simply modulating the tonal stimulus. Thus, continuous tone exposure, like continuous noise exposure [Chang EF, Merzenich MM (2003) Environmental noise retards auditory cortical development. Science 300:498-502], ineffectively induces critical period plasticity, and indefinitely blocks the closure of a normally-brief critical period window. These findings again demonstrate the crucial role of temporally structured inputs for inducing the progressive cortical maturational changes that result in the closure of the critical period window.

The missing piece in the 'use it or lose it' puzzle: is inhibition regulated by activity or does it act on its own accord?

We have gained enormous insight into the mechanisms underlying both activity-dependent and (to a lesser degree) -independent plasticity of excitatory synapses. Recently, cortical inhibition has been shown to play a vital role in the formation of critical periods for sensory plasticity. As such, sculpting of neuronal circuits by inhibition may be a common mechanism by which activity organizes or reorganizes brain circuits. Disturbances in the balance of excitation and inhibition in the neocortex provoke abnormal activities, such as epileptic seizures and abnormal cortical development. However, both the process of experience-dependent postnatal maturation of neocortical inhibitory networks and its underlying mechanisms remain elusive. Mechanisms that match excitation and inhibition are central to achieving balanced function at the level of individual circuits. The goal of this review is to reinforce our understanding of the mechanisms by which developing inhibitory networks are able to adapt to sensory inputs, and to maintain their balance with developing excitatory networks. Discussion is centered on the following questions related to experience-dependent plasticity of neocortical inhibitory networks: 1) What are the roles of GABAergic inhibition in the postnatal maturation of neocortical circuits? 2) Does the maturation of neocortical inhibitory circuits proceed in an activity-dependent manner or do they develop independently of sensory inputs? 3) Does activity regulate inhibitory networks in the same way it regulates excitatory networks? 4) What are the molecular and cellular mechanisms that underlie the activity-dependent maturation of inhibitory networks? 5) What are the functional advantages of experience-dependent plasticity of inhibitory networks to network processing in sensory cortices?

Activity-dependent reorganization of local circuitry in the developing visceral sensory system

Neural activity during critical periods could fine-tune functional synaptic connections. N-methyl-d-aspartate (NMDA) receptor activation is critically implicated in this process and blockade leads to disruption of normal circuit formation. This phenomenon has been well investigated in several neural systems including the somatosensory system, but not yet evidenced in the visceral sensory system. Ultrastructural analysis of GABAergic synapses and electrophysiological analysis of inhibitory and excitatory postsynaptic currents of the rat caudal nucleus tractus solitarii (NTS) cells revealed that developmental changes in the synaptic organizations were blocked by MK-801, an NMDA receptor antagonist, when administered at postnatal days 5-8, a presumed critical period for the visceral sensory system. Normal synapse reorganization during postnatal development dictates undifferentiated neonatal caudal NTS neurons in terms of synaptic input patterns measured by electron microscopy and electrophysiology into two cell groups: small and large cells under far stronger excitatory and inhibitory influence, respectively. Blockade by MK-801 during the critical period might leave adult neurons wired in the undifferentiated synaptic networks, possibly preventing synapse elimination and subsequent stabilization of the proper wiring.

Bidirectional regulation of Munc13-3 protein expression by age and dark rearing during the critical period in mouse visual cortex

Rearing in darkness slows the time course of the visual cortical critical period, such that at 5 weeks of age normal cats are more plastic than dark-reared cats, while at 20 weeks dark-reared cats are more plastic [Mower GD (1991) The effect of dark rearing on the time course of the critical period in cat visual cortex. Dev Brain Res 58:151-158]. Thus, genes that are important for visual cortical plasticity should show differences in expression between normal and dark-reared visual cortex that are of opposite direction in young versus older animals. Previously, we showed by differential display polymerase chain reaction and northern blotting that mRNA for Munc13-3, a mammalian homologue of the C. elegans uncoordinated (unc) gene, shows such bidirectional regulation in cat visual cortex [Yang CB, Zheng YT, Li GY, Mower GD (2002) Identification of Munc13-3 as a candidate gene for critical period neuroplasticity in visual cortex. J Neurosci 22:8614-8618]. Here, the analysis is extended to Munc13-3 protein in mouse visual cortex, which will provide the basis for gene manipulation analysis. In mice, Munc13-3 protein was elevated 2.3-fold in dark-reared compared with normal visual cortex at 3.5 weeks and 2.0-fold in normal compared with dark-reared visual cortex at 9.5 weeks. Analysis of variance of protein levels showed a significant interaction, indicating that the effect of dark rearing depended on age. This bidirectional regulation was restricted to visual cortex and did not occur in frontal cortex. Bidirectional regulation was also specific to Munc13-3 and was not found for other Munc13 family members. Munc13 proteins serve a central priming function in synaptic vesicle exocytosis at glutamatergic and GABAergic synapses and this work contributes to the growing evidence indicating a role of Munc13 genes in synaptic plasticity.