Suppression of cortical NMDA receptor function prevents development of orientation selectivity in the primary visual cortex

Adaptation-induced sharpening of orientation tuning curves in the mouse visual cortex

OBJECTIVE

Orientation selectivity is an emergent property of visual neurons across species with columnar and noncolumnar organization of the visual cortex. The emergence of orientation selectivity is more established in columnar cortical areas than in noncolumnar ones. Thus, how does orientation selectivity emerge in noncolumnar cortical areas after an adaptation protocol? Adaptation refers to the constant presentation of a nonoptimal stimulus (adapter) to a neuron under observation for a specific time. Previously, it had been shown that adaptation has varying effects on the tuning properties of neurons, such as orientation, spatial frequency, motion and so on.

BASIC METHODS

We recorded the mouse primary visual neurons (V1) at different orientations in the control (preadaptation) condition. This was followed by adapting neurons uninterruptedly for 12 min and then recording the same neurons postadaptation. An orientation selectivity index (OSI) for neurons was computed to compare them pre- and post-adaptation.

MAIN RESULTS

We show that 12-min adaptation increases the OSI of visual neurons ( n  = 113), that is, sharpens their tuning. Moreover, the OSI postadaptation increases linearly as a function of the OSI preadaptation.

CONCLUSION

The increased OSI postadaptation may result from a specific dendritic neural mechanism, potentially facilitating the rapid learning of novel features.

The effects of the NMDAR co-agonist D-serine on the structure and function of optic tectal neurons in the developing visual system

The N-methyl-D-aspartate type glutamate receptor (NMDAR) is a molecular coincidence detector which converts correlated patterns of neuronal activity into cues for the structural and functional refinement of developing circuits in the brain. D-serine is an endogenous co-agonist of the NMDAR. We investigated the effects of potent enhancement of NMDAR-mediated currents by chronic administration of saturating levels of D-serine on the developing Xenopus retinotectal circuit. Chronic exposure to the NMDAR co-agonist D-serine resulted in structural and functional changes in the optic tectum. In immature tectal neurons, D-serine administration led to more compact and less dynamic tectal dendritic arbors, and increased synapse density. Calcium imaging to examine retinotopy of tectal neurons revealed that animals raised in D-serine had more compact visual receptive fields. These findings provide insight into how the availability of endogenous NMDAR co-agonists like D-serine at glutamatergic synapses can regulate the refinement of circuits in the developing brain.

Structure, Function, and Pharmacology of Glutamate Receptor Ion Channels

Many physiologic effects of l-glutamate, the major excitatory neurotransmitter in the mammalian central nervous system, are mediated via signaling by ionotropic glutamate receptors (iGluRs). These ligand-gated ion channels are critical to brain function and are centrally implicated in numerous psychiatric and neurologic disorders. There are different classes of iGluRs with a variety of receptor subtypes in each class that play distinct roles in neuronal functions. The diversity in iGluR subtypes, with their unique functional properties and physiologic roles, has motivated a large number of studies. Our understanding of receptor subtypes has advanced considerably since the first iGluR subunit gene was cloned in 1989, and the research focus has expanded to encompass facets of biology that have been recently discovered and to exploit experimental paradigms made possible by technological advances. Here, we review insights from more than 3 decades of iGluR studies with an emphasis on the progress that has occurred in the past decade. We cover structure, function, pharmacology, roles in neurophysiology, and therapeutic implications for all classes of receptors assembled from the subunits encoded by the 18 ionotropic glutamate receptor genes. SIGNIFICANCE STATEMENT: Glutamate receptors play important roles in virtually all aspects of brain function and are either involved in mediating some clinical features of neurological disease or represent a therapeutic target for treatment. Therefore, understanding the structure, function, and pharmacology of this class of receptors will advance our understanding of many aspects of brain function at molecular, cellular, and system levels and provide new opportunities to treat patients.

Effects of ketamine on orientation selectivity and variability of neuronal responses in primary visual cortex

The plasticity of the adult brain is one of the most highly evolving areas of recent neuroscience research. It has been acknowledged that the visual cortex in adulthood can adapt and restructure the neuronal connections in response to a sensory experience or to an imposed input such as in adaptation or ocular deprivation protocols. In order to understand the basic cellular mechanisms of plasticity in the primary visual cortex (V1), we examined the effects of ketamine, a non-competitive, glutamatergic NMDAR (N-methyl-D-aspartate receptor) antagonist, on the orientation of cortical cells by measuring their response variability and the Gaussian tuning curves in adult anesthetised mouse and cat. Neurons were recorded extracellularly using glass electrodes. The ketamine was applied locally by placing a custom-cut filter paper (1x1mm) soaked in ketamine solution (10 mg/ml) on the cortical surface next the site of the recording tip, in both species. Our results show that the local and acute exposure of ketamine on V1 changes the preferred orientation of the visual neurons established during the critical period of development. Furthermore, ketamine also leads to a decrease in the orientation selectivity (measured by orientation selectivity index, OSI) and the variability of neuronal evoked responses (measured by Fano factor), but does not affect spontaneous activity. These results suggest that ketamine induces plasticity in V1 neurons that might be operated by a different pathway than that of NMDARs.

Sound Induces Change in Orientation Preference of V1 Neurons: Audio-Visual Cross-Influence

In the cortex, demarcated unimodal sensory regions often respond to unforeseen sensory stimuli and exhibit plasticity. The goal of the current investigation was to test evoked responses of primary visual cortex (V1) neurons when an adapting auditory stimulus is applied in isolation. Using extracellular recordings in anesthetized cats, we demonstrate that, unlike the prevailing observation of only slight modulations in the firing rates of the neurons, sound imposition in isolation entirely shifted the peaks of orientation tuning curves of neurons in both supra- and infragranular layers of V1. Our results suggest that neurons specific to either layer dynamically integrate features of sound and modify the organization of the orientation map of V1. Intriguingly, these experiments present novel findings that the mere presentation of a prolonged auditory stimulus may drastically recalibrate the tuning properties of the visual neurons and highlight the phenomenal neuroplasticity of V1 neurons.

Comparative effects of adaptation on layers II-III and V-VI neurons in cat V1

V1 is fundamentally grouped into columns that descend from layers II-III to V-VI. Neurons inherent to visual cortex are capable of adapting to changes in the incoming stimuli that drive the cortical plasticity. A principle feature called orientation selectivity can be altered by the presentation of non-optimal stimulus called 'adapter'. When triggered, LGN cells impinge upon layer IV and further relay the information to deeper layers via layers II-III. Using different adaptation protocols, neuronal plasticity can be investigated. Superficial neurons in area V1 are well acknowledged to exhibit attraction and repulsion by shifting their tuning peaks when challenged by a non-optimal stimulus called 'adapter'. Layers V-VI neurons in spite of partnering layers II-III neurons in cortical computation have not been explored simultaneously toward adaptation. We believe that adaptation not only affects cells specific to a layer but modifies the entire column. In this study, through simultaneous multiunit recordings in anesthetized cats using a multichannel depth electrode, we show for the first time how layers V-VI neurons (1000-1200 μm) along with layers II-III neurons (300-500 μm) exhibit plasticity in response to adaptation. Our results demonstrate that superficial and deeper layer neurons react synonymously toward adapter by exhibiting similar behavioral properties. The neurons displayed similar amplitude of shift and maintained equivalent sharpness of Gaussian tuning peaks before and the following adaptation. It appears that a similar mechanism, belonging to all layers, is responsible for the analog outcome of the neurons' experience with adapter.

Kynurenine pathway metabolism following prenatal KMO inhibition and in Mecp2+/- mice, using liquid chromatography-tandem mass spectrometry

To quantify the full range of tryptophan metabolites along the kynurenine pathway, a liquid chromatography – tandem mass spectrometry method was developed and used to analyse brain extracts of rodents treated with the kynurenine-3-mono-oxygenase (KMO) inhibitor Ro61-8048 during pregnancy. There were significant increases in the levels of kynurenine, kynurenic acid, anthranilic acid and 3-hydroxy-kynurenine (3-HK) in the maternal brain after 5 h but not 24 h, while the embryos exhibited high levels of kynurenine, kynurenic acid and anthranilic acid after 5 h which were maintained at 24 h post-treatment. At 24 h there was also a strong trend to an increase in quinolinic acid levels (P = 0.055). No significant changes were observed in any of the other kynurenine metabolites. The results confirm the marked increase in the accumulation of some neuroactive kynurenines when KMO is inhibited, and re-emphasise the potential importance of changes in anthranilic acid. The prolonged duration of metabolite accumulation in the embryo brains indicates a trapping of compounds within the embryonic CNS independently of maternal levels. When brains were examined from young mice heterozygous for the meCP2 gene – a potential model for Rett syndrome - no differences were noted from control mice, suggesting that the proposed roles for kynurenines in autism spectrum disorder are not relevant to Rett syndrome, supporting its recognition as a distinct, independent, condition.

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Pregnant rats were treated with an inhibitor of kynurenine-3-monoxygenase.

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Levels of several kynurenine metabolites increased in the maternal and foetal brains.

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The maternal changes at 5 h disappeared by 24 h, but were maintained in embryos.

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No changes were noted in the brains of Mecp2^+/− mice.

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KMO inhibition but not Mecp2^+/− suppression alters kynurenine metabolism.

Interplay of orientation selectivity and the power of low- and high-gamma bands in the cat primary visual cortex

Gamma oscillations are ubiquitous in brain and are believed to be inevitable for information processing in brain. Here, we report that distinct bands (low, 30-40Hz and high gamma, 60-80Hz) of stimulus-triggered gamma oscillations are systematically linked to the orientation selectivity index (OSI) of neurons in the cat primary visual cortex. The gamma-power is high for the highly selective neurons in the low-gamma band, whereas it is high for the broadly selective neurons in the high-gamma band. We suggest that the low-gamma band is principally implicated in feed-forward excitatory flow, whereas the high-gamma band governs the flow of this excitation.

Reduced Oblique Effect in Children with Autism Spectrum Disorders (ASD)

People are very precise in the discrimination of a line orientation relative to the cardinal (vertical and horizontal) axes, while their orientation discrimination sensitivity along the oblique axes is less refined. This difference in discrimination sensitivity along cardinal and oblique axes is called the "oblique effect." Given that the oblique effect is a basic feature of visual processing with an early developmental origin, its investigation in children with Autism Spectrum Disorder (ASD) may shed light on the nature of visual sensory abnormalities frequently reported in this population. We examined line orientation sensitivity along oblique and vertical axes in a sample of 26 boys with ASD (IQ > 68) and 38 typically developing (TD) boys aged 7-15 years, as well as in a subsample of carefully IQ-matched ASD and TD participants. Children were asked to detect the direction of tilt of a high-contrast black-and-white grating relative to vertical (90°) or oblique (45°) templates. The oblique effect was reduced in children with ASD as compared to TD participants, irrespective of their IQ. This reduction was due to poor orientation sensitivity along the vertical axis in ASD children, while their ability to discriminate line orientation along the oblique axis was unaffected. We speculate that this deficit in sensitivity to vertical orientation may reflect disrupted mechanisms of early experience-dependent learning that takes place during the critical period for orientation selectivity.

On the Role of Glutamate in Presynaptic Development: Possible Contributions of Presynaptic NMDA Receptors

Proper formation and maturation of synapses during development is a crucial step in building the functional neural circuits that underlie perception and behavior. It is well established that experience modifies circuit development. Therefore, understanding how synapse formation is controlled by synaptic activity is a key question in neuroscience. In this review, we focus on the regulation of excitatory presynaptic terminal development by glutamate, the predominant excitatory neurotransmitter in the brain. We discuss the evidence that NMDA receptor activation mediates these effects of glutamate and present the hypothesis that local activation of presynaptic NMDA receptors (preNMDARs) contributes to glutamate-dependent control of presynaptic development. Abnormal glutamate signaling and aberrant synapse development are both thought to contribute to the pathogenesis of a variety of neurodevelopmental disorders, including autism spectrum disorders, intellectual disability, epilepsy, anxiety, depression, and schizophrenia. Therefore, understanding how glutamate signaling and synapse development are linked is important for understanding the etiology of these diseases.

Altered hippocampal plasticity by prenatal kynurenine administration, kynurenine-3-monoxygenase (KMO) deletion or galantamine

Glutamate receptors sensitive to N-methyl-D-aspartate (NMDA) are involved in embryonic brain development but their activity may be modulated by the kynurenine pathway of tryptophan metabolism which includes an agonist (quinolinic acid) and an antagonist (kynurenic acid) at these receptors. Our previous work has shown that prenatal inhibition of the pathway produces abnormalities of brain development. In the present study kynurenine and probenecid (both 100mg/kg, doses known to increase kynurenic acid levels in the brain) were administered to female Wistar rats on embryonic days E14, E16 and E18 of gestation and the litter was allowed to develop to post-natal day P60. Western blotting revealed no changes in hippocampal expression of several proteins previously found to be altered by inhibition of the kynurenine pathway including the NMDA receptor subunits GluN1, GluN2A and GluN2B, as well as doublecortin, Proliferating Cell Nuclear Antigen (PCNA), sonic hedgehog and unco-ordinated (unc)-5H1 and 5H3. Mice lacking the enzyme kynurenine-3-monoxygenase (KMO) also showed no changes in hippocampal expression of several of these proteins or the 70-kDa and 100-kDa variants of Disrupted in Schizophrenia-1 (DISC1). Electrical excitability of pyramidal neurons in the CA1 region of hippocampal slices was unchanged, as was paired-pulse facilitation and inhibition. Long-term potentiation was decreased in the kynurenine-treated rats and in the KMO(-/-) mice, but galantamine reversed this effect in the presence of nicotinic receptor antagonists, consistent with evidence that it can potentiate glutamate at NMDA receptors. It is concluded that interference with the kynurenine pathway in utero can have lasting effects on brain function of the offspring, implying that the kynurenine pathway is involved in the regulation of early brain development.

Altered modulation of gamma oscillation frequency by speed of visual motion in children with autism spectrum disorders

BACKGROUND

Recent studies link autism spectrum disorders (ASD) with an altered balance between excitation and inhibition (E/I balance) in cortical networks. The brain oscillations in high gamma-band (50-120 Hz) are sensitive to the E/I balance and may appear useful biomarkers of certain ASD subtypes. The frequency of gamma oscillations is mediated by level of excitation of the fast-spiking inhibitory basket cells recruited by increasing strength of excitatory input. Therefore, the experimental manipulations affecting gamma frequency may throw light on inhibitory networks dysfunction in ASD.

METHODS

Here, we used magnetoencephalography (MEG) to investigate modulation of visual gamma oscillation frequency by speed of drifting annular gratings (1.2, 3.6, 6.0 °/s) in 21 boys with ASD and 26 typically developing boys aged 7-15 years. Multitaper method was used for analysis of spectra of gamma power change upon stimulus presentation and permutation test was applied for statistical comparisons. We also assessed in our participants visual orientation discrimination thresholds, which are thought to depend on excitability of inhibitory networks in the visual cortex.

RESULTS

Although frequency of the oscillatory gamma response increased with increasing velocity of visual motion in both groups of participants, the velocity effect was reduced in a substantial proportion of children with ASD. The range of velocity-related gamma frequency modulation correlated inversely with the ability to discriminate oblique line orientation in the ASD group, while no such correlation has been observed in the group of typically developing participants.

CONCLUSIONS

Our findings suggest that abnormal velocity-related gamma frequency modulation in ASD may constitute a potential biomarker for reduced excitability of fast-spiking inhibitory neurons in a subset of children with ASD.

Modulation of functional connectivity following visual adaptation: homeostasis in V1

Sensory neurons exhibit remarkable adaptability in acquiring new optimal selectivity to unfamiliar features when a new stimulus becomes prevalent in the environment. In conventionally prepared adult anesthetized cats, we used visual adaptation to change the preferred orientation selectivity in V1 neurons. Cortical circuits are dominated by complex and intricate connections between neurons. Cross-correlation of cellular spike-trains discloses the putative functional connection between two neurons. We sought to investigate changes in these links following a 12 min uninterrupted application of a specific, usually non-preferred, orientation. We report that visual adaptation, mimicking training, modulates the magnitude of crosscorrelograms suggesting that the strength of inter-neuronal relationships is modified. While individual cell-pairs exhibit changes in their response correlation strength, the average correlation of the recorded cell cluster remains unchanged. Hence, visual adaptation induces plastic changes that impact the connectivity between neurons.

Prenatal inhibition of the kynurenine pathway leads to structural changes in the hippocampus of adult rat offspring

Glutamate receptors for N-methyl-d-aspartate (NMDA) are involved in early brain development. The kynurenine pathway of tryptophan metabolism includes the NMDA receptor agonist quinolinic acid and the antagonist kynurenic acid. We now report that prenatal inhibition of the pathway in rats with 3,4-dimethoxy-N-[4-(3-nitrophenyl)thiazol-2-yl]benzenesulphonamide (Ro61-8048) produces marked changes in hippocampal neuron morphology, spine density and the immunocytochemical localisation of developmental proteins in the offspring at postnatal day 60. Golgi–Cox silver staining revealed decreased overall numbers and lengths of CA1 basal dendrites and secondary basal dendrites, together with fewer basal dendritic spines and less overall dendritic complexity in the basal arbour. Fewer dendrites and less complexity were also noted in the dentate gyrus granule cells. More neurons containing the nuclear marker NeuN and the developmental protein sonic hedgehog were detected in the CA1 region and dentate gyrus. Staining for doublecortin revealed fewer newly generated granule cells bearing extended dendritic processes. The number of neuron terminals staining for vesicular glutamate transporter (VGLUT)-1 and VGLUT-2 was increased by Ro61-8048, with no change in expression of vesicular GABA transporter or its co-localisation with vesicle-associated membrane protein-1. These data support the view that constitutive kynurenine metabolism normally plays a role in early embryonic brain development, and that interfering with it has profound consequences for neuronal structure and morphology, lasting into adulthood.

Adaptation shifts preferred orientation of tuning curve in the mouse visual cortex

In frontalized mammals it has been demonstrated that adaptation produces shift of the peak of the orientation tuning curve of neuron following frequent or lengthier presentation of a non-preferred stimulus. Depending on the duration of adaptation the shift is attractive (toward the adapter) or repulsive (away from the adapter). Mouse exhibits a salt-and-pepper cortical organization of orientation maps, hence this species may respond differently to adaptation. To examine this question, we determined the effect of twelve minutes of adaptation to one particular orientation on neuronal orientation tuning curves in V1 of anesthetized mice. Multi-unit activity of neurons in V1 was recorded in a conventional fashion. Cells were stimulated with sine-wave drifting gratings whose orientation tilted in steps. Results revealed that similarly to cats and monkeys, majority of cells shifted their optimal orientation in the direction of the adapter while a small proportion exhibited a repulsive shift. Moreover, initially untuned cells showing poor tuning curves reacted to adaptation by displaying sharp orientation selectivity. It seems that modification of the cellular property following adaptation is a general phenomenon observed in all mammals in spite of the different organization pattern of the visual cortex. This study is of pertinence to comprehend the mechanistic pathways of brain plasticity.

Prenatal activation of Toll-like receptors-3 by administration of the viral mimetic poly(I:C) changes synaptic proteins, N-methyl-D-aspartate receptors and neurogenesis markers in offspring

Background

There is mounting evidence for a neurodevelopmental basis for disorders such as autism and schizophrenia, in which prenatal or early postnatal events may influence brain development and predispose the young to develop these and related disorders. We have now investigated the effect of a prenatal immune challenge on brain development in the offspring. Pregnant rats were treated with the double-stranded RNA polyinosinic:polycytidylic acid (poly(I:C); 10 mg/kg) which mimics immune activation occurring after activation of Toll-like receptors-3 (TLR3) by viral infection. Injections were made in late gestation (embryonic days E14, E16 and E18), after which parturition proceeded naturally and the young were allowed to develop up to the time of weaning at postnatal day 21 (P21). The brains of these animals were then removed to assess the expression of 13 different neurodevelopmental molecules by immunoblotting.

Results

Measurement of cytokine levels in the maternal blood 5 hours after an injection of poly(I:C) showed significantly increased levels of monocyte chemoattractant protein-1 (MCP-1), confirming immune activation. In the P21 offspring, significant changes were detected in the expression of GluN1 subunits of NMDA receptors, with no difference in GluN2A or GluN2B subunits or the postsynaptic density protein PSD-95 and no change in the levels of the related small GTPases RhoA or RhoB, or the NMDA receptor modulator EphA4. Among presynaptic molecules, a significant increase in Vesicle Associated Membrane Protein-1 (VAMP-1; synaptobrevin) was seen, with no change in synaptophysin or synaptotagmin. Proliferating Cell Nuclear Antigen (PCNA), as well as the neurogenesis marker doublecortin were unchanged, although Sox-2 levels were increased, suggesting possible changes in the rate of new cell differentiation.

Conclusions

The results reveal the induction by prenatal poly(I:C) of selective molecular changes in the brains of P21 offspring, affecting primarily molecules associated with neuronal development and synaptic transmission. These changes may contribute to the behavioural abnormalities that have been reported in adult animals after exposure to poly(I:C) and which resemble symptoms seen in schizophrenia and related disorders.

Facilitation of neocortical presynaptic terminal development by NMDA receptor activation

Background

Neocortical circuits are established through the formation of synapses between cortical neurons, but the molecular mechanisms of synapse formation are only beginning to be understood. The mechanisms that control synaptic vesicle (SV) and active zone (AZ) protein assembly at developing presynaptic terminals have not yet been defined. Similarly, the role of glutamate receptor activation in control of presynaptic development remains unclear.

Results

Here, we use confocal imaging to demonstrate that NMDA receptor (NMDAR) activation regulates accumulation of multiple SV and AZ proteins at nascent presynaptic terminals of visual cortical neurons. NMDAR-dependent regulation of presynaptic assembly occurs even at synapses that lack postsynaptic NMDARs. We also provide evidence that this control of presynaptic terminal development is independent of glia.

Conclusions

Based on these data, we propose a novel NMDAR-dependent mechanism for control of presynaptic terminal development in excitatory neocortical neurons. Control of presynaptic development by NMDARs could ultimately contribute to activity-dependent development of cortical receptive fields.

A reaction-diffusion model to capture disparity selectivity in primary visual cortex

Decades of experimental studies are available on disparity selective cells in visual cortex of macaque and cat. Recently, local disparity map for iso-orientation sites for near-vertical edge preference is reported in area 18 of cat visual cortex. No experiment is yet reported on complete disparity map in V1. Disparity map for layer IV in V1 can provide insight into how disparity selective complex cell receptive field is organized from simple cell subunits. Though substantial amounts of experimental data on disparity selective cells is available, no model on receptive field development of such cells or disparity map development exists in literature. We model disparity selectivity in layer IV of cat V1 using a reaction-diffusion two-eye paradigm. In this model, the wiring between LGN and cortical layer IV is determined by resource an LGN cell has for supporting connections to cortical cells and competition for target space in layer IV. While competing for target space, the same type of LGN cells, irrespective of whether it belongs to left-eye-specific or right-eye-specific LGN layer, cooperate with each other while trying to push off the other type. Our model captures realistic 2D disparity selective simple cell receptive fields, their response properties and disparity map along with orientation and ocular dominance maps. There is lack of correlation between ocular dominance and disparity selectivity at the cell population level. At the map level, disparity selectivity topography is not random but weakly clustered for similar preferred disparities. This is similar to the experimental result reported for macaque. The details of weakly clustered disparity selectivity map in V1 indicate two types of complex cell receptive field organization.

Repetitive adaptation induces plasticity of spatial frequency tuning in cat primary visual cortex

Sensory neurons display transient changes in their response properties following prolonged exposure to an appropriate stimulus (adaptation). In adult cat primary visual cortex, spatial frequency-selective neurons shift their preferred spatial frequency (SF) after being adapted to a non-preferred SF. In anesthetized cats prepared for electrophysiological recordings in the visual cortex, we applied a non-preferred spatial frequency for two successive periods of adaptation (a recovery and interval of ∼90 min separated both phases of adaptation) in order to determine if a first adaptation retained an influence on a second adaptation. The first application of a non-preferred SF shifted the tuning curve of the cell mainly in the direction of the imposed SF. The results showed that attractive shifts occurred more frequently (68%) than repulsive (12%) changes in cortical cells. The increase of responsivity was band-limited and occurred around the imposed SF, while flanked responses remained unmodified in all conditions. After a recovery period allowing neurons to restore their original SF tuning curves, we carried out a second adaptation which produced four major results: (1) a higher proportion of repulsive shifts (31%) compared to attractive shifts (49%), (2) an increase of the magnitude of the attractive shifts, (3) an additional enhancement of the evoked firing rate for the newly acquired SF, and (4) for the acquired SF the variability coefficient decreased following the second adaptation. The supplementary response changes suggest that neurons in area 17 keep a "memory" trace of the previous stimulus properties. It also highlights the dynamic nature of basic neuronal properties in adult cortex since repeated adaptations modified both the spatial frequency tuning selectivity and the response strength to the preferred spatial frequency. These enhanced neuronal responses suggest that the range of adaptation-induced plasticity available to the visual system is broader than anticipated.

Early valproic acid exposure alters functional organization in the primary visual cortex

Epilepsy is one of the most common neurologic disorders and affects 0.5 to 1% of pregnant women. The use of antiepileptic drugs, which is usually continued throughout pregnancy, can cause in offspring mild to severe sensory deficits. Neuronal selectivity to stimulus orientation is a basic functional property of the visual cortex that is crucial for perception of shapes and borders. Here we investigate the effects of early exposure to valproic acid (Val) and levetiracetam (Lev), commonly used antiepileptic drugs, on the development of cortical neuron orientation selectivity and organization of cortical orientation columns. Ferrets pups were exposed to Val (200mg/kg), Lev (100mg/kg) or saline every other day between postnatal day (P) 10 and P30, a period roughly equivalent to the third trimester of human gestation. Optical imaging of intrinsic signals or single-unit recordings were examined at P42-P84, when orientation selectivity in the ferret cortex has reached a mature state. Optical imaging of intrinsic signals revealed decreased contrast of orientation maps in Val- but not Lev- or saline-treated animals. Moreover, single-unit recordings revealed that early Val treatment also reduced orientation selectivity at the cellular level. These findings indicate that Val exposure during a brief period of development disrupts cortical processing of sensory information at a later age and suggest a neurobiological substrate for some types of sensory deficits in fetal anticonvulsant syndrome.

Adaptive behavior of neighboring neurons during adaptation-induced plasticity of orientation tuning in VI

Background

Sensory neurons display transient changes of their response properties following prolonged exposure to an appropriate stimulus (adaptation). In adult cat primary visual cortex, orientation-selective neurons shift their preferred orientation after being adapted to a non-preferred orientation. The direction of those shifts, towards (attractive) or away (repulsive) from the adapter depends mostly on adaptation duration. How the adaptive behavior of a neuron is related to that of its neighbors remains unclear.

Results

Here we show that in most cases (75%), cells shift their preferred orientation in the same direction as their neighbors. We also found that cells shifting preferred orientation differently from their neighbors (25%) display three interesting properties: (i) larger variance of absolute shift amplitude, (ii) wider tuning bandwidth and (iii) larger range of preferred orientations among the cluster of cells. Several response properties of V1 neurons depend on their location within the cortical orientation map. Our results suggest that recording sites with both attractive and repulsive shifts following adaptation may be located in close proximity to iso-orientation domain boundaries or pinwheel centers. Indeed, those regions have a more diverse orientation distribution of local inputs that could account for the three properties above. On the other hand, sites with all cells shifting their preferred orientation in the same direction could be located within iso-orientation domains.

Conclusions

Our results suggest that the direction and amplitude of orientation preference shifts in V1 depend on location within the orientation map. This anisotropy of adaptation-induced plasticity, comparable to that of the visual cortex itself, could have important implications for our understanding of visual adaptation at the psychophysical level.

Phosphodiesterase inhibition increases CREB phosphorylation and restores orientation selectivity in a model of fetal alcohol spectrum disorders

Background

Fetal alcohol spectrum disorders (FASD) are the leading cause of mental retardation in the western world and children with FASD present altered somatosensory, auditory and visual processing. There is growing evidence that some of these sensory processing problems may be related to altered cortical maps caused by impaired developmental neuronal plasticity.

Methodology/Principal Findings

Here we show that the primary visual cortex of ferrets exposed to alcohol during the third trimester equivalent of human gestation have decreased CREB phosphorylation and poor orientation selectivity revealed by western blotting, optical imaging of intrinsic signals and single-unit extracellular recording techniques. Treating animals several days after the period of alcohol exposure with a phosphodiesterase type 1 inhibitor (Vinpocetine) increased CREB phosphorylation and restored orientation selectivity columns and neuronal orientation tuning.

Conclusions/Significance

These findings suggest that CREB function is important for the maturation of orientation selectivity and that plasticity enhancement by vinpocetine may play a role in the treatment of sensory problems in FASD.

Sensory activity differentially modulates N-methyl-D-aspartate receptor subunits 2A and 2B in cortical layers

Activity-dependent modulation of N-methyl-d-aspartate (NMDA) receptors containing selective NR2 subunits has been implicated in plastic processes in developing and adult sensory cortex. Aiming to reveal differential sensitivity of NR2 subunits to sustained changes in sensory activity, we utilized four paradigms that blocked, reinstated, or initiated sensory visual activity. Laminar prevalence of N-methyl-d-aspartate receptor subunit 2A- (NR2A)- and N-methyl-d-aspartate receptor subunit 2B- (NR2B)-containing synapses in visual cortex of postnatal and adult ferrets was assessed using quantitative electron microscopy. Light-deprivation at all ages resulted in a downregulation of NR2A, while recovery from deprivation resulted in an upregulation. Furthermore, premature eyelid opening caused a precocious increase of NR2A. Thus, transitions between periods of dark and light rapidly and bidirectionally regulate NR2A, regardless of cortical layer or age. In contrast, NR2B regulation is layer- and age-dependent. Only in layer IV, NR2B prevalence displays a one-time decline about 3 weeks after the initiation of sensory activity upon normal or premature eyelid opening, or upon termination of dark-rearing. Incongruity in patterns of NR2A and NR2B modulation by activity is consistent with involvement of these subunits in two distinct, yet partially co-occurring processes: developmental plasticity with a critical period, and lifelong plasticity that is established in early developmental ages.

Visual cells remember earlier applied target: plasticity of orientation selectivity

BACKGROUND

A canonical proposition states that, in mature brain, neurons responsive to sensory stimuli are tuned to specific properties installed shortly after birth. It is amply demonstrated that that neurons in adult visual cortex of cats are orientation-selective that is they respond with the highest firing rates to preferred oriented stimuli.

METHODOLOGY/PRINCIPAL FINDINGS

In anesthetized cats, prepared in a conventional fashion for single cell recordings, the present investigation shows that presenting a stimulus uninterruptedly at a non-preferred orientation for twelve minutes induces changes in orientation preference. Across all conditions orientation tuning curves were investigated using a trial by trial method. Contrary to what has been previously reported with shorter adaptation duration, twelve minutes of adaptation induces mostly attractive shifts, i.e. toward the adapter. After a recovery period allowing neurons to restore their original orientation tuning curves, we carried out a second adaptation which produced three major results: (1) more frequent attractive shifts, (2) an increase of their magnitude, and (3) an additional enhancement of responses at the new or acquired preferred orientation. Additionally, we also show that the direction of shifts depends on the duration of the adaptation: shorter adaptation in most cases produces repulsive shifts, whereas adaptation exceeding nine minutes results in attractive shifts, in the same unit. Consequently, shifts in preferred orientation depend on the duration of adaptation.

CONCLUSION/SIGNIFICANCE

The supplementary response improvements indicate that neurons in area 17 keep a memory trace of the previous stimulus properties, thereby upgrading cellular performance. It also highlights the dynamic nature of basic neuronal properties in adult cortex since repeated adaptations modified both the orientation tuning selectivity and the response strength to the preferred orientation. These enhanced neuronal responses suggest that the range of neuronal plasticity available to the visual system is broader than anticipated.

Mechanisms underlying development of visual maps and receptive fields

Patterns of synaptic connections in the visual system are remarkably precise. These connections dictate the receptive field properties of individual visual neurons and ultimately determine the quality of visual perception. Spontaneous neural activity is necessary for the development of various receptive field properties and visual feature maps. In recent years, attention has shifted to understanding the mechanisms by which spontaneous activity in the developing retina, lateral geniculate nucleus, and visual cortex instruct the axonal and dendritic refinements that give rise to orderly connections in the visual system. Axon guidance cues and a growing list of other molecules, including immune system factors, have also recently been implicated in visual circuit wiring. A major goal now is to determine how these molecules cooperate with spontaneous and visually evoked activity to give rise to the circuits underlying precise receptive field tuning and orderly visual maps.

NMDA receptor antagonists reveal age-dependent differences in the properties of visual cortical plasticity

The suggestion that NMDA receptor (NMDAR)-dependent plasticity is subunit specific, with NR2B-types required for long-term depression (LTD) and NR2A-types critical for the induction of long-term potentiation (LTP), has generated much attention and considerable debate. By investigating the suggested subunit-specific roles of NMDARs in the mouse primary visual cortex over development, we report several important findings that clarify the roles of NMDAR subtypes in synaptic plasticity. We observed that LTD was not attenuated by application of ifenprodil, an NR2B-type antagonist, or NVP-AAM007, a less selective NR2A-type antagonist. However, we were surprised that NVP-AAM007 completely blocked adult LTP (postnatal day (P) 45-90), while only modestly affecting juvenile LTP (P21-28). To assess whether this developmental transition reflected an increasing role for NR2A-type receptors with maturity, we characterized the specificity of NVP-AAM007. We found not only that NVP-AAM007 lacks discernable subunit specificity but also that the effects of NVP-AAM077 on LTP could be mimicked using subsaturating concentrations of APV, a global NMDAR antagonist. These results indicate that the effects of NVP-AAM077 on synaptic plasticity are largely explained by nonspecific blockade of NMDARs. Moreover our findings are the first to reveal a developmental increase in the sensitivity of LTP to NMDAR antagonism. We suggest that discrepant reports describing the effect of NVP-AAM077 on LTP may be partially explained by this developmental shift in the properties of LTP. These results indicate that the degree of NMDAR activation required for LTP increases with development, providing insight into a novel underlying mechanism governing the properties of synaptic plasticity.

Synchrony between orientation-selective neurons is modulated during adaptation-induced plasticity in cat visual cortex

BACKGROUND

Visual neurons respond essentially to luminance variations occurring within their receptive fields. In primary visual cortex, each neuron is a filter for stimulus features such as orientation, motion direction and velocity, with the appropriate combination of features eliciting maximal firing rate. Temporal correlation of spike trains was proposed as a potential code for linking the neuronal responses evoked by various features of a same object. In the present study, synchrony strength was measured between cells following an adaptation protocol (prolonged exposure to a non-preferred stimulus) which induce plasticity of neurons' orientation preference.

RESULTS

Multi-unit activity from area 17 of anesthetized adult cats was recorded. Single cells were sorted out and (1) orientation tuning curves were measured before and following 12 min adaptation and 60 min after adaptation (2) pairwise synchrony was measured by an index that was normalized in relation to the cells' firing rate. We first observed that the prolonged presentation of a non-preferred stimulus produces attractive (58%) and repulsive (42%) shifts of cell's tuning curves. It follows that the adaptation-induced plasticity leads to changes in preferred orientation difference, i.e. increase or decrease in tuning properties between neurons. We report here that, after adaptation, the neuron pairs that shared closer tuning properties display a significant increase of synchronization. Recovery from adaptation was accompanied by a return to the initial synchrony level.

CONCLUSION

We conclude that synchrony reflects the similarity in neurons' response properties, and varies accordingly when these properties change.

Pioneers of cortical plasticity: six classic papers by Wiesel and Hubel

This essay looks at six APS classic papers published by D. H. Hubel and T. N. Wiesel that first identified a developmental critical period for environment influenced receptive field plasticity in the visual pathway. These classic papers are freely available online. These are listed here, in chronological order.

Wiesel TN, Hubel DH.

Effects of visual deprivation on morphology and physiology of cells in the cat's lateral geniculate body. J Neurophysiol 26: 978–993, 1963 ( http://jn.physiology.org/cgi/reprint/26/6/978 ).

Hubel DH, Wiesel TN.

Receptive fields of cells in striate cortex of very young, visually inexperienced kittens. J Neurophysiol 26: 994–1002, 1963 ( http://jn.physiology.org/cgi/reprint/26/6/994 ).

Wiesel TN, Hubel DH.

Single-cell responses in striate cortex of kittens deprived of vision in one eye. J Neurophysiol 26: 1003–1017, 1963 ( http://jn.physiology.org/cgi/reprint/26/6/1003 ).

Wiesel TN, Hubel DH.

Comparison of the effects of unilateral and bilateral eye closure on cortical unit responses in kittens. J Neurophysiol 28: 1029–1040, 1965 ( http://jn.physiology.org/cgi/reprint/28/6/1029 ).

Hubel DH, Wiesel TN.

Binocular interaction in striate cortex of kittens reared with artificial squint. J Neurophysiol 28: 1041–1059, 1965 ( http://jn.physiology.org/cgi/reprint/28/6/1041 ).

Wiesel TN, Hubel DH.

Extent of recovery from the effects of visual deprivation in kittens. J Neurophysiol 28: 1060–1072, 1965 ( http://jn.physiology.org/cgi/reprint/28/6/1060 ).

Neocortical plasticity deficits in fetal alcohol spectrum disorders: lessons from barrel and visual cortex

Fetal Alcohol Spectrum Disorder (FASD) is characterized by a constellation of behavioral and physiological abnormalities, including learning and sensory deficits. There is growing evidence that abnormalities of neuronal plasticity underlie these deficits. However, the cellular and molecular mechanisms by which prenatal alcohol exposure disrupts neuronal plasticity remain elusive. Recently, studies with the barrel and the visual cortex as models to study the effects of early alcohol exposure on neuronal plasticity shed light on this subject. In this Mini-Review, we discuss the effects of ethanol exposure during development on neuronal plasticity and suggest environmental and pharmacological approaches to ameliorate these problems.

Postnatal development of excitatory postsynaptic currents in nucleus accumbens medium spiny neurons

We have recorded excitatory postsynaptic currents (EPSCs) evoked by local electrical stimulation in 243 nucleus accumbens (nAcb) neurons in vitro during postnatal development from the day of birth (postnatal day 0; P0) to P27 and in young adults rats (P59-P71). An EPSC sensitive to glutamatergic antagonists was found in all neurons. In the majority of cases (189/243), the EPSC had two distinct components: an early one sensitive to 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and a late one that was sensitive to D-2-amino-5-phosphonovaleric acid (APV) showing that early and late components of the EPSC were mediated by AMPA/kainate (KA) and N-methyl-D-aspartate (NMDA) receptors respectively. During the first four postnatal days, the amplitudes of both the AMPA/KA and NMDA components of the EPSC were relatively small and then began to increase until the end of the second postnatal week. Whereas the amplitude of the early component appeared to stabilize from that point on, the late component began to decrease and became virtually undetectable in preparations from animals older than 3 weeks unless the AMPA/KA response was blocked with CNQX. In addition, the ratio between the amplitude of the NMDA and AMPA/KA receptor-mediated components of the EPSC followed a developmental pattern parallel to that of the NMDA receptor component showing an increase during the first two postnatal weeks followed by a decrease. Together, these results show that, during postnatal development, there is a period when NMDA receptor-mediated EPSC are preeminent and that time frame might represent a period during which the development of the nAcb might be sensitive to environmental manipulation.

Protein synthesis-independent plasticity mediates rapid and precise recovery of deprived eye responses

Monocular deprivation (MD) for a few days during a critical period of development leads to loss of cortical responses to stimulation of the deprived eye. Despite the profound effects of MD on cortical function, optical imaging of intrinsic signals and single-unit recordings revealed that deprived eye responses and orientation selectivity recovered a few hours after restoration of normal binocular vision. Moreover, recovery of deprived eye responses was not dependent upon mRNA translation, but required cortical activity. Interestingly, this fast recovery and protein synthesis independence was restricted to the hemisphere contralateral to the previously deprived eye. Collectively, these results implicate a relatively simple mechanistic process in the reactivation of a latent set of connections following restoration of binocular vision and provide new insight into how recovery of cortical function can rapidly occur in response to changes in sensory experience.

RNA interference of Xenopus NMDAR NR1 in vitro and in vivo

Here, we present a short interfering RNA (siRNA) application that reduces the expression of NR1 the obligate subunit of the NMDA receptor (NMDAR) and virtually eliminates NMDAR function in a small subset of neurons within otherwise normally developing Xenopus laevis tadpoles. We designed two plasmids each containing a CMV promoter driving a dsRed "reporter" cDNA and DNA coding for one short hairpin RNA (shRNA) under the control of the U6 promoter. The shRNA was cleaved to produce a siRNA against NR1 transcript (iNR1). NR1 transcript and protein, differentiation and survival of NR1 knockdown neurons, were assayed in vivo. iNR1 effects on NMDAR function used Xenopus tectal neuron cultures, Ca2+ imaging and patch-clamp electrophysiology. NR1 transcript, protein and NMDAR function was significantly reduced or eliminated in iNR1-expressing neurons. Protein, transcript and function for the closely related AMPA receptor (AMPAR) subunit GluR1 was unperturbed. In vivo imaging of small groups of iNR1 neurons at two different ages revealed that siRNA knockdown of the NMDAR does not affect differentiation or survival of young neurons. Thus, with siRNA, molecular hypotheses about synaptic mechanisms of circuit formation can be rapidly tested in intact developing vertebrates using normal neurons in the same animals as controls.

Development of human visual cortex: a balance between excitatory and inhibitory plasticity mechanisms

Formation of neural circuitry in the developing visual cortex is shaped by experience during the critical period. A number of mechanisms, including N-methyl-D-aspartate (NMDA) receptor activation and gamma-aminobutyric acid (GABA)-mediated inhibition, are crucial in determining onset and closure of the critical period for visual plasticity. Animal models have shown that a threshold level of tonic inhibition must be reached for critical period plasticity to occur and that NMDA receptors contribute to Hebbian synaptic plasticity in the developing visual cortex. There are a number of developmental changes in these glutamatergic and GABAergic mechanisms that have been linked to plasticity; however, those changes have been shown only in animal models, and their development in the human visual cortex is not known. We have addressed this question by studying the expression of the major glutamatergic receptors, GABA(A) receptors, and glutamic acid decarboxylase (GAD) isoforms during the first 6 years of postnatal development of human visual cortex. There are significant changes in the expression of these proteins during postnatal development of human visual cortex. The time course of the changes is quite prolonged and suggests that it may set the pace for the prolonged critical period in human visual development. The changes also affect the nature of spatial and temporal integration in visual cortical neurons and thereby contribute to the maturation of visual functions.

NR2 subunit-dependence of NMDA receptor channel block by external Mg2+

The vital roles played by NMDA receptors in CNS physiology depend critically on powerful voltage-dependent channel block by external Mg(2+) (Mg(2+)(o)). NMDA receptor channel block by Mg(2+)(o) depends on receptor subunit composition: NR1/2A receptors (receptors composed of NR1 and NR2A subunits) and NR1/2B receptors are more strongly inhibited by Mg(2+)(o) than are NR1/2C or NR1/2D receptors. We investigated the effects of Mg(2+)(o) on single-channel and whole-cell currents recorded from recombinant NR1/2D and NR1/2A receptors expressed in HEK293 and 293T cells. The main conclusions are as follows: (1) Voltage-dependent inhibition by Mg(2+)(o) of whole-cell NR1/2D receptor responses was at least 4-fold weaker than inhibition of NR1/2A receptor responses at all voltages tested. (2) Channel block by Mg(2+)(o) reduced the duration of NR1/2D receptor single-channel openings; this reduction was used to estimate the apparent blocking rate of Mg(2+)(o) (k(+,app)). The k(+,app) for NR1/2D receptors was similar to but moderately slower than the k(+,app) obtained from cortical NMDA receptors composed of NR1, NR2A and NR2B subunits at all voltages tested. (3) Mg(2+)(o) blocking events induced an additional component in the closed-duration distribution; this component was used to estimate the apparent unblocking rate of Mg(2+)(o) (k(-,app)). The k(-,app) for NR1/2D receptors was much faster than the k(-,app) for cortical receptors at all voltages tested. The voltage-dependence of the k(-,app) of NR1/2D and cortical receptors differed in a manner that suggested that Mg(2+)(o) may permeate NR1/2D receptors more easily than cortical receptors. (4) Mg(2+)(o) inhibits NR1/2D receptors less effectively than cortical receptors chiefly because Mg(2+)(o) unbinds much more rapidly from NR1/2D receptors.

Early alcohol exposure induces persistent alteration of cortical columnar organization and reduced orientation selectivity in the visual cortex

Fetal alcohol syndrome (FAS) is a major cause of learning and sensory deficits in children. The visual system in particular is markedly affected, with an elevated prevalence of poor visual perceptual skills. Developmental problems involving the neocortex are likely to make a major contribution to some of these abnormalities. Neuronal selectivity to stimulus orientation, a functional property thought to be crucial for normal vision, may be especially vulnerable to alcohol exposure because it starts developing even before eye opening. To address this issue, we examined the effects of early alcohol exposure on development of cortical neuron orientation selectivity and organization of cortical orientation columns. Ferrets were exposed to ethanol starting at postnatal day (P) 10, when the functional properties and connectivity of neocortical neurons start to develop. Alcohol exposure ended at P30, just before eye opening at P32. Following a prolonged alcohol-free period (15-35 days), long-term effects of early alcohol exposure on cortical orientation selectivity were examined at P48-P65, when orientation selectivity in normal ferret cortex has reached a mature state. Optical imaging of intrinsic signals revealed decreased contrast of orientation maps in alcohol- but not saline-treated animals. Moreover, single-unit recordings revealed that early alcohol treatment weakened neuronal orientation selectivity while preserving robust visual responses. These findings indicate that alcohol exposure during a brief period of development disrupts cortical processing of sensory information at a later age and suggest a neurobiological substrate for some types of sensory deficits in FAS.

Enriched environment and acceleration of visual system development

Rearing mice from birth in an enriched environment leads to a conspicuous acceleration of visual system development appreciable at behavioral, electrophysiological and molecular level. Little is known about the possible mechanisms of action through which enriched environment affects visual system development. It has been suggested that differences in maternal behavior between enriched and non-enriched conditions could contribute to the earliest effects of enriched environment on visual development and that neurotrophins, BDNF in particular, might be involved. Here, we examined Brain Derived Neurotrophic Factor (BDNF) levels in the visual cortex during development and showed that an increase occurs in the first week of life in enriched pups compared to standard reared pups; BDNF levels at birth were equal in the two groups. This suggests a postnatal rather than a prenatal effect of environment on BDNF. A detailed analysis of maternal care behavior showed that pups raised in a condition of social and physical enrichment experienced higher levels of licking behavior and physical contact compared to standard reared pups and that enhanced levels of licking were also provided to pups in an enriched environment where no adult females other than the mother were present. Thus, different levels of maternal care in different environmental conditions could act as indirect mediator for the earliest effects of enrichment on visual system development. Some of the effects of different levels of maternal care on the offspring behavior are long lasting. We measured the visual acuity of differentially reared mice at the end of the period of visual acuity development (postnatal day 45) and at 12 months of age, using a behavioral discrimination task. We found better learning abilities and higher visual acuity in enriched compared to standard reared mice at both ages.

Developmental patterns of NMDAR expression within the song system do not recur during adult vocal plasticity in zebra finches

All songbirds learn to sing during postnatal development but then display species differences in the capacity to learn song in adulthood. While the mechanisms that regulate avian vocal plasticity are not well characterized, one contributing factor may be the composition of N-methyl-D-aspartate receptors (NMDAR). Previous studies of an anterior forebrain pathway implicated in vocal plasticity revealed significant regulation of NMDAR subunit expression during the developmental sensitive period for song learning. Much less is known about the developmental regulation of NMDAR subunit expression in regions that participate more directly in motor aspects of song behavior. We show here that an increase in NR2A subunit mRNA and a decrease in NR2B subunit mRNA within the vocal motor pathway accompany song learning in zebra finches; however, manipulations that can alter the timing of song learning did not alter the course of these developmental changes. We also tested whether adult deafening, a treatment that provokes vocal change in songbirds that normally sing a stable song throughout adulthood, would render NMDAR subunit expression more similar to that observed developmentally. We report that NR2A and NR2B mRNA levels did not change within the anterior forebrain or vocal motor pathways after adult deafening, even after substantial changes in song structure. These results indicate that vocal plasticity does not require "juvenile patterns" of NMDAR gene expression in the avian song system.

Neonatal alcohol exposure induces long-lasting impairment of visual cortical plasticity in ferrets

Fetal alcohol syndrome is a major cause of learning and sensory deficits. These disabilities may result from disruption of neocortex development and plasticity. Alcohol exposure during the third trimester equivalent of human gestation may have especially severe and long-lasting consequences on learning and sensory processing, because this is when the functional properties and connectivity of neocortical neurons start to develop. To address this issue, we used the monocular deprivation model of neural plasticity, which shares many common mechanisms with learning. Ferrets were exposed to ethanol (3.5 mg/kg, i.p.) on alternate days for 3 weeks starting on postnatal day (P) 10. Animals were then monocularly deprived at the peak of ocular dominance plasticity after a prolonged alcohol-free period (15-20 d). Quantitative single-unit electrophysiology revealed that alcohol exposure disrupted ocular dominance plasticity while preserving robust visual responses. Moreover, optical imaging of intrinsic signals revealed that the reduction in visual cortex area driven by the deprived eye was much less pronounced in ethanol-treated than in control animals. Alcohol exposure starting at a later age (P20) did not disrupt ocular dominance plasticity, indicating that timing of exposure is crucial for the effects on visual plasticity. In conclusion, alcohol exposure during a brief period of development impairs ocular dominance plasticity at a later age. This model provides a novel approach to investigate the consequences of fetal alcohol exposure and should contribute to elucidate how alcohol disrupts neural plasticity.

Chronic neonatal N-methyl-D-aspartate receptor blockade induces learning deficits and transient hypoactivity in young rats

A blockade of N-methyl-D-aspartate (NMDA)-type of glutamate receptor in rodents is believed to provide a pharmacological model of schizophrenia-related psychosis. Since neurodevelopmental abnormality, at least partly, could contribute to the pathogenesis of schizophrenia, the aim of this study was to recapitulate cognitive impairments accompanying this disorder in rats by a chronic neonatal treatment with a noncompetitive NMDA antagonist MK-801. Rat pups were treated with a low dose of MK-801 (0.05 mg/kg s.c.) chronically from early postnatal period (PD 7-49) known to be critical for glutamatergic system maturation. Locomotor activity in the "open-field" test, anxiety level in the elevated plus-maze test, and learning capacity in food rewarded spatial task were examined in young animals. Chronic MK-801 treatment produced a decrease of spontaneous motor and exploratory activity in 16- to 28-day-old rats. At the same time, a hyperlocomotion in response to acute administration of MK-801 was observed as well. Spatial learning of MK-801-treated rats was found to be negatively affected. Treated rats were able to respond to stress stimuli in the adequate manner but their anxiety level was found to be lower than in controls. Behavioral disturbances appeared to be temporary, and no such abnormalities could be detected at the age of 16 weeks. Thus, even mild chronic neonatal blockade of NMDA receptors may lead to a specific pattern of cognitive abnormalities presumably resulting from impairments of sensory information processing at the cortical-basal ganglia level.

Separable features of visual cortical plasticity revealed by N-methyl-D-aspartate receptor 2A signaling

How individual receptive field properties are formed in the maturing sensory neocortex remains largely unknown. The shortening of N-methyl-d-aspartate (NMDA) receptor currents by 2A subunit (NR2A) insertion has been proposed to delimit the critical period for experience-dependent refinement of circuits in visual cortex. In mice engineered to maintain prolonged NMDA responses by targeted deletion of NR2A, the sensitivity to monocular deprivation was surprisingly weakened but restricted to the typical critical period and delayed normally by dark rearing from birth. Orientation preference instead failed to mature, occluding further effects of dark rearing. Interestingly, a full ocular dominance plasticity (but not orientation bias) was selectively restored by enhanced inhibition, reflecting an imbalanced excitation in the absence of NR2A. Many of the downstream pathways involved in NMDA signaling are coupled to the receptor through a variety of protein-protein interactions and adaptor molecules. To further investigate a mechanistic dissociation of receptive field properties in the developing visual system, mice carrying a targeted disruption of the NR2A-associated 95-kDa postsynaptic density (PSD95) scaffolding protein were analyzed. Although the development and plasticity of ocular dominance was unaffected, orientation preference again failed to mature in these mice. Taken together, our results demonstrate that the cellular basis generating individual sensory response properties is separable in the developing neocortex.

Orientation selectivity is reduced by monocular deprivation in combination with PKA inhibitors

We have previously shown that the protein kinase A (PKA) inhibitor, 8-chloroadenosine-3',5'-monophosphorothioate (Rp-8-Cl-cAMPS), abolishes ocular dominance plasticity in the cat visual cortex. Here we investigate the effect of this inhibitor on orientation selectivity. The inhibitor reduces orientation selectivity in monocularly deprived animals but not in normal animals. In other words, PKA inhibitors by themselves do not affect orientation selectivity, nor does monocular deprivation by itself, but monocular deprivation in combination with a PKA inhibitor does affect orientation selectivity. This result is found for the receptive fields in both deprived and nondeprived eyes. Although there is a tendency for the orientation selectivity in the nondeprived eye to be higher than the orientation selectivity in the deprived eye, the orientation selectivity in both eyes is considerably less than normal. The result is striking in animals at 4 wk of age. The effect of the monocular deprivation on orientation selectivity is reduced at 6 wk of age and absent at 9 wk of age, while the effect on ocular dominance shifts is less changed in agreement with previous results showing that the critical period for orientation/direction selectivity ends earlier than the critical period for ocular dominance. We conclude that closure of one eye in combination with inhibition of PKA reduces orientation selectivity during the period that orientation selectivity is still mutable and that the reduction in orientation selectivity is transferred to the nondeprived eye.