The effect of dark rearing on the time course of the critical period in cat visual cortex

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.

Binaural interactions develop in the auditory brainstem of children who are deaf: effects of place and level of bilateral electrical stimulation

Bilateral cochlear implants (CIs) might promote development of binaural hearing required to localize sound sources and hear speech in noise for children who are deaf. These hearing skills improve in children implanted bilaterally but remain poorer than normal. We thus questioned whether the deaf and immature human auditory system is able to integrate input delivered from bilateral CIs. Using electrophysiological measures of brainstem activity that include the Binaural Difference (BD), a measure of binaural processing, we showed that a period of unilateral deprivation before bilateral CI use prolonged response latencies but that amplitudes were not significantly affected. Tonotopic organization was retained to some extent as evidenced by an elimination of the BD with large mismatches in place of stimulation between the two CIs. Smaller place mismatches did not affect BD latency or amplitude, indicating that the tonotopic organization of the auditory brainstem is underdeveloped and/or not well used by CI stimulation. Finally, BD amplitudes decreased when the intensity of bilateral stimulation became weighted to one side and this corresponded to a perceptual shift of sound away from midline toward the side of increased intensity. In summary, bilateral CI stimulation is processed by the developing human auditory brainstem leading to perceptual changes in sound location and potentially improving hearing for children who are deaf.

The development and activity-dependent expression of aggrecan in the cat visual cortex

The Cat-301 monoclonal antibody identifies aggrecan, a chondroitin sulfate proteoglycan in the cat visual cortex and dorsal lateral geniculate nucleus (dLGN). During development, aggrecan expression increases in the dLGN with a time course that matches the decline in plasticity. Moreover, examination of tissue from selectively visually deprived cats shows that expression is activity dependent, suggesting a role for aggrecan in the termination of the sensitive period. Here, we demonstrate for the first time that the onset of aggrecan expression in area 17 also correlates with the decline in experience-dependent plasticity in visual cortex and that this expression is experience dependent. Dark rearing until 15 weeks of age dramatically reduced the density of aggrecan-positive neurons in the extragranular layers, but not in layer IV. This effect was reversible as dark-reared animals that were subsequently exposed to light showed normal numbers of Cat-301-positive cells. The reduction in aggrecan following certain early deprivation regimens is the first biochemical correlate of the functional changes to the γ-aminobutyric acidergic system that have been reported following early deprivation in cats.

Identification of α-Chimaerin as a Candidate Gene for Critical Period Neuronal Plasticity in Cat and Mouse Visual Cortex

Background

In cat visual cortex, critical period neuronal plasticity is minimal until approximately 3 postnatal weeks, peaks at 5 weeks, gradually declines to low levels at 20 weeks, and disappears by 1 year of age. Dark rearing slows the entire time course of this critical period, such that at 5 weeks of age, normal cats are more plastic than dark reared cats, whereas at 20 weeks, dark reared cats are more plastic. Thus, a stringent criterion for identifying genes that are important for plasticity in visual cortex is that they show differences in expression between normal and dark reared that are of opposite direction in young versus older animals.

Results

The present study reports the identification by differential display PCR of a novel gene, α-chimaerin, as a candidate visual cortex critical period plasticity gene that showed bidirectional regulation of expression due to age and dark rearing. Northern blotting confirmed the bidirectional expression and 5'RACE sequencing identified the gene. There are two alternatively-spliced α-chimaerin isoforms: α1 and α2. Western blotting extended the evidence for bidirectional regulation of visual cortex α-chimaerin isoform expression to protein in cats and mice. α1- and α2-Chimaerin were elevated in dark reared compared to normal visual cortex at the peak of the normal critical period and in normal compared to dark reared visual cortex at the nadir of the normal critical period. Analysis of variance showed a significant interaction in both cats and mice for both α-chimaerin isoforms, indicating that the effect of dark rearing depended on age. This differential expression was not found in frontal cortex.

Conclusions

Chimaerins are RhoGTPase-activating proteins that are EphA4 effectors and have been implicated in a number of processes including growth cone collapse, axon guidance, dendritic spine development and the formation of corticospinal motor circuits. The present results identify α-chimaerin as a candidate molecule for a role in the postnatal critical period of visual cortical plasticity.

Somatosensory and visual crossmodal plasticity in the anterior auditory field of early-deaf cats

It is well known that the post-natal loss of sensory input in one modality can result in crossmodal reorganization of the deprived cortical areas, but deafness fails to induce crossmodal effects in cat primary auditory cortex (A1). Because the core auditory regions (A1, and anterior auditory field AAF) are arranged as separate, parallel processors, it cannot be assumed that early-deafness affects one in the same manner as the other. The present experiments were conducted to determine if crossmodal effects occur in the anterior auditory field (AAF). Using mature cats (n = 3), ototoxically deafened postnatally, single-unit recordings were made in the gyral and sulcal portions of the AAF. In contrast to the auditory responsivity found in the hearing controls, none of the neurons in early-deafened AAF were activated by auditory stimulation. Instead, the majority (78%) were activated by somatosensory cues, while fewer were driven by visual stimulation (44%; values include unisensory and bimodal neurons). Somatosensory responses could be activated from all locations on the body surface but most often occurred on the head, were often bilateral (e.g., occupied portions of both sides of the body), and were primarily excited by low-threshold hair receptors. Visual receptive fields were large, collectively represented the contralateral visual field, and exhibited conventional response properties such as movement direction and velocity preferences. These results indicate that, following post-natal deafness, both somatosensory and visual modalities participate in crossmodal reinnervation of the AAF, consistent with the growing literature that documents deafness-induced crossmodal plasticity outside A1.

The Newborn Individualized Developmental Care and Assessment Program (NIDCAP) with Kangaroo Mother Care (KMC): Comprehensive Care for Preterm Infants

State-of-the-art Newborn Intensive Care Units (NICUs), instrumental in the survival of high-risk and ever-earlier-born preterm infants, often have costly human repercussions. The developmental sequelae of newborn intensive care are largely misunderstood. Developed countries eager to export their technologies must also transfer the knowledge-base that encompasses all high-risk and preterm infants' personhood as well as the neuro-essential importance of their parents. Without such understanding, the best medical care, while assuring survival jeopardizes infants' long-term potential and deprives parents of their critical role. Exchanging the womb for the NICU environment at a time of rapid brain growth compromises preterm infants' early development, which results in long-term physical and mental health problems and developmental disabilities. The Newborn Individualized Developmental Care and Assessment Program (NIDCAP) aims to prevent the iatrogenic sequelae of intensive care and to maintain the intimate connection between parent and infant, one expression of which is Kangaroo Mother Care. NIDCAP embeds the infant in the natural parent niche, avoids over-stimulation, stress, pain, and isolation while it supports self-regulation, competence, and goal orientation. Research demonstrates that NIDCAP improves brain development, functional competence, health, and life quality. It is cost effective, humane, and ethical, and promises to become the standard for all NICU care.

Activity-regulated genes as mediators of neural circuit plasticity

Modifications of neuronal circuits allow the brain to adapt and change with experience. This plasticity manifests during development and throughout life, and can be remarkably long lasting. Evidence has linked activity-regulated gene expression to the long-term structural and electrophysiological adaptations that take place during developmental critical periods, learning and memory, and alterations to sensory map representations in the adult. In all these cases, the cellular response to neuronal activity integrates multiple tightly coordinated mechanisms to precisely orchestrate long-lasting, functional and structural changes in brain circuits. Experience-dependent plasticity is triggered when neuronal excitation activates cellular signaling pathways from the synapse to the nucleus that initiate new programs of gene expression. The protein products of activity-regulated genes then work via a diverse array of cellular mechanisms to modify neuronal functional properties. Synaptic strengthening or weakening can reweight existing circuit connections, while structural changes including synapse addition and elimination create new connections. Posttranscriptional regulatory mechanisms, often also dependent on activity, further modulate activity-regulated gene transcript and protein function. Thus, activity-regulated genes implement varied forms of structural and functional plasticity to fine-tune brain circuit wiring.

Use it or lose it? Lessons learned from the developing brains of children who are deaf and use cochlear implants to hear

In the present paper, we review what is currently known about the effects of deafness on the developing human auditory system and ask: Without use, does the immature auditory system lose the ability to normally function and mature? Any change to the structure or function of the auditory pathways resulting from a lack of activity will have important implications for future use through an auditory prosthesis such as a cochlear implant. Data to date show that deafness in children arrests and disrupts normal auditory development. Multiple changes to the auditory pathways occur during the period of deafness with the extent and type of change being dependent upon the age and stage of auditory development at onset of deafness, the cause or type of deafness, and the length of time the immature auditory pathways are left without significant input. Structural changes to the auditory nerve, brainstem, and cortex have been described in animal models of deafness as well in humans who are deaf. Functional changes in deaf auditory pathways have been evaluated by using a cochlear implant to stimulate the auditory nerve with electrical pulses. Studies of electrically evoked activity in the immature deaf auditory system have demonstrated that auditory brainstem development is arrested and that thalamo-cortical areas are vulnerable to being taken over by other competitive inputs (cross-modal plasticity). Indeed, enhanced peripheral sight and detection of visual movement in congenitally deaf cats and adults have been linked to activity in specific areas of what would normally be auditory cortex. Cochlear implants can stimulate developmental plasticity in the auditory brainstem even after many years of deafness in childhood but changes in the auditory cortex are limited, at least in part, by the degree of reorganization which occurred during the period of deafness. Consequently, we must identify hearing loss rapidly (i.e., at birth for congenital deficits) and provide cochlear implants to appropriate candidates as soon as possible. Doing so has facilitated auditory development in the thalamo-cortex and allowed children who are deaf to perceive and use spoken language.

Cellular and laminar expression of Dab-1 during the postnatal critical period in cat visual cortex and the effects of dark rearing

This study describes postnatal critical period changes in cellular and laminar expression of Dab-1, a gene shown to play a role in controlling neuronal positioning during embryonic brain development, in cat visual cortex and the effects of dark rearing (DR). At 1week, there is dense cellular staining which is uniform across cortical layers and very light neuropil staining. At the peak of the critical period (5weeks), dense cell staining is largely restricted to large pyramidal cells of deep layer III and layer V, there is faint cell body staining throughout all cortical layers, neuropil staining is markedly increased and uniform in layers III to VI. This dramatic change in laminar and cellular labeling is independent of visual input, since immunostaining is similar in 5-week DR cats. By 10weeks, the mature laminar and cellular staining pattern is established and the major subsequent change is a further reduction in the density of cellular staining in all cortical layers. Neuropil staining is pronounced and uniform across cortical layers. These developmental changes are altered by DR. Quantification by cell counts indicated that age and DR interact such that differences in cellular expression are opposite in direction between 5- and 20-week-old cats. This bidirectional regulation of cellular expression is the same in all cortical laminae. The bidirectional regulation of cellular expression matches the effects of age and DR on physiological plasticity during the critical period as assessed by ocular dominance shifts in response to monocular deprivation.

Relative contribution of feedforward excitatory connections to expression of ocular dominance plasticity in layer 4 of visual cortex

Brief monocular deprivation (MD) shifts ocular dominance (OD) in primary visual cortex by causing depression of responses to the deprived eye. Here we address the extent to which the shift is expressed by a modification of excitatory synaptic transmission. An OD shift was first induced with 3 days of MD, and then the influences of intracortical polysynaptic inhibitory and excitatory synapses were pharmacologically removed, leaving only "feedforward" thalamocortical synaptic currents. The results show that the rapid OD shift following MD is strongly expressed at the level of thalamocortical synaptic transmission.

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.

Reducing intracortical inhibition in the adult visual cortex promotes ocular dominance plasticity

Experience-dependent plasticity in the cortex is often higher during short critical periods in postnatal development. The mechanisms limiting adult cortical plasticity are still unclear. Maturation of intracortical GABAergic inhibition is suggested to be crucial for the closure of the critical period for ocular dominance (OD) plasticity in the visual cortex. We find that reduction of GABAergic transmission in the adult rat visual cortex partially reactivates OD plasticity in response to monocular deprivation (MD). This is accompanied by an enhancement of activity-dependent potentiation of synaptic efficacy but not of activity-dependent depression. We also found a decrease in the expression of chondroitin sulfate proteoglycans in the visual cortex of MD animals with reduced inhibition, after the reactivation of OD plasticity. Thus, intracortical inhibition is a crucial limiting factor for the induction of experience-dependent plasticity in the adult visual cortex.

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.

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.

The foundations of development and deprivation in the visual system

The pioneering work of Torsten Wiesel and David Hubel on the development and deprivation of the visual system will be summarised, together with some comments on their influence, and some personal reminiscences by the author.

Regulation of NMDA receptor subunit expression and its implications for LTD, LTP, and metaplasticity

NMDA-type glutamate receptors (NMDARs) mediate many forms of synaptic plasticity. These tetrameric receptors consist of two obligatory NR1 subunits and two regulatory subunits, usually a combination of NR2A and NR2B. In the neonatal neocortex NR2B-containing NMDARs predominate, and sensory experience facilitates a developmental switch in which NR2A levels increase relative to NR2B. In this review, we clarify the roles of NR2 subunits in synaptic plasticity, and argue that a primary role of this shift is to control the threshold, rather than determining the direction, for modifying synaptic strength. We also discuss recent studies that illuminate the mechanisms regulating NR2 subunits, and suggest that the NR2A/NR2B ratio is regulated by multiple means, which may control the ratio both locally at individual synapses and globally in a cell-wide manner. Finally, we use the visual cortex as a model system to illustrate how activity-dependent modifications in the NR2A/NR2B ratio may contribute to the development of cortical functions.

Termination of lesion-induced plasticity in the mouse barrel cortex in the absence of oligodendrocytes

Termination of developmental plasticity occurs at specific points in development, and the mechanisms responsible for it are not well understood. One hypothesis that has been proposed is that oligodendrocytes (OLs) play an important role. Consistent with this, we found that OLs appeared in the mouse somatosensory cortex at the end of the critical period for whisker lesion-induced barrel structural plasticity. To test this hypothesis, we used two mouse lines with defective OL differentiation: Olig1-deficient and jimpy. In Olig1-deficient mice, although OLs were totally absent, the termination of lesion-induced plasticity was not delayed. The timing was normal even when the cytoarchitectonic barrel formation was temporarily blocked by pharmacological treatment in Olig1-deficient mice. Furthermore, the termination was not delayed in jimpy mice. These results demonstrate that, even though OLs appear at the end of the critical period, OLs are not intrinsically necessary for the termination of lesion-induced plasticity. Our findings underscore a mechanistic distinction between the termination of thalamocortical axonal plasticity in the barrel cortex and that in the visual cortex, in which OL-derived Nogo-A/B was recently suggested to be essential.

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.

Face perception in monkeys reared with no exposure to faces

Infant monkeys were reared with no exposure to any faces for 6-24 months. Before being allowed to see a face, the monkeys showed a preference for human and monkey faces in photographs, and they discriminated human faces as well as monkey faces. After the deprivation period, the monkeys were exposed first to either human or monkey faces for a month. Soon after, the monkeys selectively discriminated the exposed species of face and showed a marked difficulty in regaining the ability to discriminate the other nonexposed species of face. These results indicate the existence of an experience-independent ability for face processing as well as an apparent sensitive period during which a broad but flexible face prototype develops into a concrete one for efficient processing of familiar faces.

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.

Factors that are critical for plasticity in the visual cortex

Factors that may be critical for plasticity in the visual cortex are evaluated according to three criteria. (1) Do antagonists to the factor abolish plasticity? (2) Does the concentration or activity of the factor peak with the critical period for plasticity? (3) Does rearing in the dark, which postpones the critical period, affect the factor in a similar fashion? N-methyl-D-aspartate receptors fulfil all three criteria. Metabotropic glutamate receptors fulfil two of them. Most other putative factors do not fulfil more than one.

Learning to hear: plasticity of auditory cortical processing

Sensory experience and auditory cortex plasticity are intimately related. This relationship is most striking during infancy when changes in sensory input can have profound effects on the functional organization of the developing cortex. But a considerable degree of plasticity is retained throughout life, as demonstrated by the cortical reorganization that follows damage to the sensory periphery or by the more controversial changes in response properties that are thought to accompany perceptual learning. Recent studies in the auditory system have revealed the remarkably adaptive nature of sensory processing and provided important insights into the way in which cortical circuits are shaped by experience and learning.

Optimization of Somatic Inhibition at Critical Period Onset in Mouse Visual Cortex

Local GABAergic circuits trigger visual cortical plasticity in early postnatal life. How these diverse connections contribute to critical period onset was investigated by nonstationary fluctuation analysis following laser photo-uncaging of GABA onto discrete sites upon individual pyramidal cells in slices of mouse visual cortex. The GABA(A) receptor number decreased on the soma-proximal dendrite (SPD), but not at the axon initial segment, with age and sensory deprivation. Benzodiazepine sensitivity was also higher on the immature SPD. Too many or too few SPD receptors in immature or dark-reared mice, respectively, were adjusted to critical period levels by benzodiazepine treatment in vivo, which engages ocular dominance plasticity in these animal models. Combining GAD65 deletion with dark rearing from birth confirmed that an intermediate number of SPD receptors enable plasticity. Site-specific optimization of perisomatic GABA response may thus trigger experience-dependent development in visual cortex.

Early visual experience prevents but cannot reverse deprivation-induced loss of refinement in adult superior colliculus

The role of sensory experience in the development and plasticity of the visual system has been widely studied. It has generally been reported that once animals reach adulthood, experience-dependent visual plasticity is reduced. We have found that visual experience is not needed for the refinement of receptive fields (RFs) in the superior colliculus (SC) but instead is necessary to maintain them in adulthood (Carrasco et al., 2005). Without light exposure, RFs in SC of hamsters refine by postnatal day 60 as usual but then enlarge, presumably reducing visual acuity. In this study we examine whether a brief period of light exposure during early postnatal development would be sufficient to prevent RF enlargement in adulthood, and whether prolonged light exposure in adulthood could reverse the deprivation-induced increase in RF size. We found that an early postnatal period of at least 30 days of visual experience was sufficient to maintain refined RFs in the adult SC. Prolonged visual experience in adulthood could not reverse the RF enlargement resulting from long-term dark rearing, reflecting a loss of plasticity at this age. Our results suggest that, unlike in visual cortex, dark rearing does not indefinitely extend the critical period of plasticity in SC. Rather, there is a limited time window when early experience can protect RFs from the detrimental effects of visual deprivation in adulthood. These results contribute to understanding adult brain plasticity and argue for the importance of early visual experience in protecting the adult visual system.

Critical period window for spectral tuning defined in the primary auditory cortex (A1) in the rat

Experience-dependent plasticity during development results in the emergence of highly adapted representations of the external world in the adult brain. Previous studies have convincingly shown that the primary auditory cortex (A1) of the rat possesses a postnatal period of sensory input-driven plasticity but its precise timing (onset, duration, end) has not been defined. In the present study, we examined the effects of pure-tone exposure on the auditory cortex of developing rat pups at different postnatal ages with a high temporal resolution. We found that pure-tone exposure resulted in profound, persistent alterations in sound representations in A1 only if the exposure occurred during a brief period extending from postnatal day 11 (P11) to P13. We also found that postnatal sound exposure in this epoch led to striking alterations in the cortical representation of sound intensity.

Identification of disabled-1 as a candidate gene for critical period neuroplasticity in cat and mouse visual cortex

Rearing in darkness slows the time course of the critical period in visual cortex, such that at 5 weeks of age normal cats are more plastic than dark-reared cats, whereas at 20 weeks dark-reared cats are more plastic [G. D. Mower (1991)Dev. Brain Res., 58, 151-158]. Thus, a stringent criterion is that genes that are important for plasticity in visual cortex will show differences in expression between normal and dark-reared visual cortex that are of opposite direction in young vs. older animals. The present study reports the identification by differential display PCR of Dab-1, the mammalian homolog of the drosophila disabled-1 gene, as a candidate gene for critical period neuronal plasticity, expression of which is regulated according to this criterion in cat visual cortex. Evidence for this bidirectional direction regulation is extended to Dab-1 protein in cat and mouse visual cortex and shown to be specific to visual cortex, not occurring in frontal cortex. The Reelin/Dab-1 pathway has well-documented functions in cell migration during prenatal life and increasing evidence indicates that in postnatal brain the pathway plays a role in synaptic plasticity. The present results extend this evidence by directly implicating Dab-1 in postnatal critical period plasticity of visual cortex.

Lifelong learning: ocular dominance plasticity in mouse visual cortex

Ocular dominance plasticity has long served as a successful model for examining how cortical circuits are shaped by experience. In this paradigm, altered retinal activity caused by unilateral eye-lid closure leads to dramatic shifts in the binocular response properties of neurons in the visual cortex. Much of the recent progress in identifying the cellular and molecular mechanisms underlying ocular dominance plasticity has been achieved by using the mouse as a model system. In this species, monocular deprivation initiated in adulthood also causes robust ocular dominance shifts. Research on ocular dominance plasticity in the mouse is starting to provide insight into which factors mediate and influence cortical plasticity in juvenile and adult animals.

Stimulus for rapid ocular dominance plasticity in visual cortex

Although it has been known for decades that monocular deprivation shifts ocular dominance in kitten striate cortex, uncertainty persists about the adequate stimulus for deprivation-induced losses of cortical responsiveness. In the current study we compared the effects of 2 days of lid closure and 2 days of monocular blur using an overcorrecting contact lens. Our finding of comparable ocular dominance shifts in visual cortex indicates that deprived-eye response depression is not a result of reduced retinal illumination. The quality rather than the quantity of retinal illumination is the key factor for ocular dominance plasticity. These data have implications for both the mechanism and treatment of amblyopia.

Nitric Oxide and Synaptic Dynamics in the Adult Brain: Physiopathological Aspects

The adult brain retains the capacity to rewire mature neural circuits in response to environmental changes, brain damage or sensory and motor experiences. Two plastic processes, synaptic remodeling and neurogenesis, have been the subject of numerous studies due to their involvement in the maturation of the nervous system, their prevalence and re-activation in adulthood, and therapeutic relevance. However, most of the research looking for the mechanistic and molecular events underlying synaptogenic phenomena has been focused on the extensive synaptic reorganization occurring in the developing brain. In this stage, a vast number of synapses are initially established, which subsequently undergo a process of activity-dependent refinement guided by target-derived signals that act as synaptotoxins or synaptotrophins, promoting either loss or consolidation of pre-existing synaptic contacts, respectively. Nitric oxide (NO), an autocrine and/or paracrine-acting gaseous molecule synthesized in an activity-dependent manner, has ambivalent actions. It can act by mediating synapse formation, segregation of afferent inputs, or growth cone collapse and retraction in immature neural systems. Nevertheless, little information exists about the role of this ambiguous molecule in synaptic plasticity processes occurring in the adult brain. Suitable conditions for elucidating the role of NO in adult synaptic rearrangement include physiopathological conditions, such as peripheral nerve injury. We have recently developed a crush lesion model of the XIIth nerve that induces a pronounced stripping of excitatory synaptic boutons from the cell bodies of hypoglossal motoneurons. The decline in synaptic coverage was concomitant with de novo expression of the neuronal isoform of NO synthase in motoneurons. We have demonstrated a synaptotoxic action of NO mediating synaptic withdrawal and preventing synapse formation by cyclic GMP (cGMP)-dependent and, probably, S-nitrosylation-mediated mechanisms, respectively. This action possibly involves the participation of other signaling molecules working together with NO. Brain-derived neurotrophic factor (BDNF), a target-derived synaptotrophin synthesized and released postsynaptically in an activity-dependent form, is a potential candidate for effecting such a concerted action. Several items of evidence support an interrelationship between NO and BDNF in the regulation of synaptic remodeling processes in adulthood: i) BDNF and its receptor TrkB are expressed by motoneurons and upregulated by axonal injury; ii) they promote axon arborization and synaptic formation, and modulate the structural dynamics of excitatory synapses; iii) NO and BDNF each control the production and activity of the other at the level of individual synapses; iv) the NO/cGMP pathway inhibits BDNF secretion; and finally, v) BDNF protects F-actin from depolymerization by NO, thus preventing the collapsing and retracting effects of NO on growth cones. Therefore, we propose a mechanism of action in which the NO/BDNF ratio regulates synapse dynamics after peripheral nerve lesion. This hypothesis also raises the possibility that variations in this NO/BDNF balance constitute a common hallmark leading to synapse loss in the progression of diverse neurodegenerative diseases such as amyotrophic lateral sclerosis, Alzheimer's and Parkinson's diseases.

Critical period plasticity in local cortical circuits

Neuronal circuits in the brain are shaped by experience during 'critical periods' in early postnatal life. In the primary visual cortex, this activity-dependent development is triggered by the functional maturation of local inhibitory connections and driven by a specific, late-developing subset of interneurons. Ultimately, the structural consolidation of competing sensory inputs is mediated by a proteolytic reorganization of the extracellular matrix that occurs only during the critical period. The reactivation of this process, and subsequent recovery of function in conditions such as amblyopia, can now be studied with realistic circuit models that might generalize across systems.

Experience-driven plasticity of visual cortex limited by myelin and Nogo receptor

Monocular deprivation normally alters ocular dominance in the visual cortex only during a postnatal critical period (20 to 32 days postnatal in mice). We find that mutations in the Nogo-66 receptor (NgR) affect cessation of ocular dominance plasticity. In NgR-/- mice, plasticity during the critical period is normal, but it continues abnormally such that ocular dominance at 45 or 120 days postnatal is subject to the same plasticity as at juvenile ages. Thus, physiological NgR signaling from myelin-derived Nogo, MAG, and OMgp consolidates the neural circuitry established during experience-dependent plasticity. After pathological trauma, similar NgR signaling limits functional recovery and axonal regeneration.

Ganglion cell densities in normal and dark-reared turtle retinas

In dark-reared, neonatal turtle retinas, ganglion cell receptive fields and dendritic trees grow faster than normal. As a result, their areas may become, on average, up to twice as large as in control retinas. This raises the question of whether the coverage factor of dark-reared ganglion cells is larger than normal. Alternatively, dark rearing may lead to smaller-than-normal cell densities by accelerating apoptosis. To test these alternatives, we investigated the effect of light deprivation on densities and soma sizes of turtle retinal ganglion cells. For this purpose, we marked these cells using retrograde labeling of fixed turtle retinas with DiI (1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate). Control turtles were maintained in a regular 12-h light/dark cycle from hatching until 4 weeks of age, whereas dark-reared turtles were maintained in total darkness for the same period. Ganglion cells in the control and dark-reared retinas were found to be similar in density and soma sizes. These results show that the mean coverage factor of turtle dark-reared ganglion cells is larger than normal.

Rapid eye movement sleep deprivation revives a form of developmentally regulated synaptic plasticity in the visual cortex of post-critical period rats

The critical period for observing a developmentally regulated form of synaptic plasticity in the visual cortex of young rats normally ends at about postnatal day 30. This developmentally regulated form of in vitro long-term potentiation (LTP) can be reliably induced in layers II-III by aiming high frequency, theta burst stimulation (TBS) at the white matter situated directly below visual cortex (LTPWM-III). Previous work has demonstrated that suppression of sensory activation of visual cortex, achieved by rearing young rats in total darkness from birth, delays termination of the critical period for inducing LTPWM-III. Subsequent data also demonstrated that when rapid eye movement sleep (REMS) is suppressed, thereby reducing REMS cortical activation, just prior to the end of the critical period, termination of this developmental phase is delayed, and LTPWM-III can still be reliably produced in the usual post-critical period. Here, we report that for approximately 3 weeks immediately following the usual end of the critical period, suppression of REMS disrupts the maturational processes that close the critical period, and LTPWM-III is readily induced in brain slices taken from these somewhat older animals. Insofar as in vitro LTP is a model for the cellular and molecular changes that underlie developmental synaptic plasticity, these results suggest that mechanisms of synaptic plasticity, which participate in brain development and perhaps also in learning and memory processes, remain susceptible to the effects of REMS deprivation during the general period of adolescence in the rat.

Postnatal expression profile of OBCAM implies its involvement in visual cortex development and plasticity

This study examined the expression of a neuron-specific cell adhesion molecule, OBCAM (opioid-binding cell adhesion molecule), at both the mRNA and protein levels in the cat primary visual cortex at various postnatal ages, using cDNA array analysis and immunocytochemistry. Results obtained using both methods showed that the expression level of OBCAM was high in young and low in older and adult visual cortex. OBCAM-immunoreactivities were associated predominantly with perikarya and dendrites of pyramidal neurons, and OBCAM-immunopositive neurons were present in all cortical layers. Immunostaining of OBCAM in adult visual cortex showed a reduced number of immunopositive neurons and neurites and relatively lower staining intensities as compared with younger animals. In addition, the number of OBCAM-immunopositive neurons was significantly higher in the visual cortex of 4-month-old animals dark-reared from birth than those in age-matched normally reared animals. These results suggest that OBCAM may play an important role in visual cortex development and plasticity.

Visual experience and maturation of retinal synaptic pathways

The retinal synaptic network continues its maturational refinement after eye opening in mammals. This synaptic refinement is reflected in changes of retinal neuron synaptic activity and connectivity. In mature retina, the dendrites of retinal ganglion cells (RGCs) in the inner plexiform layer (IPL) of retina are separated into ON or OFF sublamina. At early developmental stage, however, the dendrites of most RGCs are ramified throughout the IPL. Recently we found that the postnatal maturational processes converting bistratified ON-OFF responsive RGCs to monostratified ON and OFF responsive RGCs depend upon visual stimulation after eye opening.

Critical Period Mechanisms in Developing Visual Cortex

Binocular vision is shaped by experience during a critical period of early postnatal life. Loss of visual acuity following monocular deprivation is mediated by a shift of spiking output from the primary visual cortex. Both synaptic and network explanations have been offered for this heightened brain plasticity. Direct experimental control over its timing, duration, and closure has now been achieved through a consideration of balanced local circuit excitation-inhibition. Notably, canonical models of homosynaptic plasticity at excitatory synapses alone (LTP/LTD) fail to produce predictable manipulations of the critical period in vivo. Instead, a late functional maturation of intracortical inhibition is the driving force, with one subtype in particular standing out. Parvalbumin-positive large basket cells that innervate target cell bodies with synapses containing the alpha1-subunit of GABA(A) receptors appear to be critical. With age, these cells are preferentially enwrapped in peri-neuronal nets of extracellular matrix molecules, whose disruption by chondroitinase treatment reactivates ocular dominance plasticity in adulthood. In fact, critical period plasticity is best viewed as a continuum of local circuit computations ending in structural consolidation of inputs. Monocular deprivation induces an increase of endogenous proteolytic (tPA-plasmin) activity and consequently motility of spines followed by their pruning, then re-growth. These early morphological events faithfully reflect competition only during the critical period and lie downstream of excitatory-inhibitory balance on a timescale (of days) consistent with the physiological loss of deprived-eye responses in vivo. Ultimately, thalamic afferents retract or expand accordingly to hardwire the rapid functional changes in connectivity. Competition detected by local inhibitory circuits then implemented at an extracellular locus by proteases represents a novel, cellular understanding of the critical period mechanism. It is hoped that this paradigm shift will lead to novel therapies and training strategies for rehabilitation, recovery from injury, and lifelong learning in adulthood.

The relationship between relative eye usage and ocular dominance shifts in cat visual cortex

A novel modification of the alternate monocular deprivation paradigm was used to quantitatively define the relationship between relative eye usage and the shift in visual cortical ocular dominance toward the advantaged eye. Both eyes of cats were alternately occluded by contact lenses during daily visual exposure sessions with varying ratios of relative eye usage: 1:1, 1.7:1, 3:1, 7:1, 50:1, 100:0. Only 100:0 and 50:1 ratios produced an ocular dominance shift in favor of the more experienced eye. The ocular dominance shift in 100:0 cats occurred in all cortical layers but only in extragranular layers of 50:1 cats. A steep power function described the data, indicating that an extreme imbalance in relative eye usage (>90%) is required for an ocular dominance shift.

Critical period regulation

Neuronal circuits are shaped by experience during critical periods of early postnatal life. The ability to control the timing, duration, and closure of these heightened levels of brain plasticity has recently become experimentally accessible, especially in the developing visual system. This review summarizes our current understanding of known critical periods across several systems and species. It delineates a number of emerging principles: functional competition between inputs, role for electrical activity, structural consolidation, regulation by experience (not simply age), special role for inhibition in the CNS, potent influence of attention and motivation, unique timing and duration, as well as use of distinct molecular mechanisms across brain regions and the potential for reactivation in adulthood. A deeper understanding of critical periods will open new avenues to "nurture the brain"-from international efforts to link brain science and education to improving recovery from injury and devising new strategies for therapy and lifelong learning.

The unexpected consequences of a noisy environment

A recent study found that the functional organization of auditory cortex was disrupted when rats were exposed to a moderate level of continuous noise during early development. However, this detrimental effect on auditory cortex could be remedied later by stimulating the noise-reared rats with structured sounds. These findings suggest that the endpoint of the "critical period" could be extended well into adult life, which has significant implications for our understanding of cortical plasticity.

Synaptic and Molecular Mechanisms Regulating Plasticity during Early Learning

Many behaviors are learned most easily during a discrete developmental period, and it is generally agreed that these "sensitive periods" for learning reflect the developmental regulation of molecular or synaptic properties that underlie experience-dependent changes in neural organization and function. Avian song learning provides one example of such temporally restricted learning, and several features of this behavior and its underlying neural circuitry make it a powerful model for studying how early experience sculpts neural and behavioral organization. Here we describe evidence that within the basal ganglia-thalamocortical loop implicated in vocal learning, song acquisition engages N-methyl-d-aspartate receptors (NMDARs), as well as signal transduction cascades strongly implicated in other instances of learning. Furthermore, NMDAR phenotype changes in parallel with developmental and seasonal periods for vocal plasticity. We also review recent studies in the avian song system that challenge the popular notion that sensitive periods for learning reflect developmental changes in the NMDAR that alter thresholds for synaptic plasticity.

Visual experience promotes the isotropic representation of orientation preference

Within the visual cortex of several mammalian species, more circuitry is devoted to the representation of vertical and horizontal orientations than oblique orientations. The sensitivity of this representation of orientation preference to visual experience during cortical maturation and the overabundance of cardinal contours in the environment suggest that vision promotes the development of this cortical anisotropy. We tested this idea by measuring the distribution of cortical orientation preference and the degree of orientation selectivity in developing normal and dark-reared ferrets using intrinsic signal optical imaging. The area of the angle map of orientation preference representing cardinal and oblique orientations was determined; in addition, orientation selectivity indices were computed separately for cardinal and oblique difference images. In normal juvenile animals, we confirm a small, but statistically significant overrepresentation of near horizontal orientations in the cortical angle map. However, the degree of anisotropy did not increase in the weeks that followed eye opening when orientation selectivity matured; rather, it decreased. In dark-reared ferrets, an even greater cortical anisotropy emerged, but angle maps in these animals developed an apparently anomalous overrepresentation of near vertical orientations. Thus, the overrepresentation of cardinal orientations in the visual cortex does not require experience with an anisotropic visual environment; indeed, cortical anisotropy can develop in the complete absence of vision. These observations suggest that the role of visual experience in cortical maturation is to promote the isotropic representation of orientation preference.

Light deprivation suppresses the light response of inner retina in both young and adult mouse

The retinal synaptic network continues its development after birth in mammals. Recent studies show that postnatal development of retinal circuitry depends on visual stimulation. We sought to determine whether there is a time period during which the retina shows evidence of increased plasticity. We examined the effects of light deprivation on the retinal light response of mouse retina using electroretinogram (ERG) measurements. Our results showed that dark rearing mice from birth to postnatal day (P) 30, 60, and 90 suppressed the amplitudes of oscillatory potentials (OPs) and the magnitudes of suppression were age independent. In addition, dark-rearing-produced suppression of OP amplitudes can be completely reversed in both young and adult mice by returning them to cyclic light/dark conditions for 1 to 2 weeks. However, the recovery time course was age dependent with younger animals needing a longer time to achieve a full recovery. Furthermore, dark rearing of P60 mice raised under cyclic light/dark conditions for 30 days resulted in a similar magnitude of suppression of OP amplitudes as in age-matched mice dark reared from birth. These findings demonstrate that both the normal developmental changes and the maintenance of mature inner retinal light response in adult animals require visual stimulation. These results indicate a degree of activity-dependent plasticity in mouse retina that has not been previously described.