A semi-persistent adult ocular dominance plasticity in visual cortex is stabilized by activated CREB

The synaptic proteome during development and plasticity of the mouse visual cortex

During brain development, the neocortex shows periods of enhanced plasticity, which enables the acquisition of knowledge and skills that we use and build on in adult life. Key to persistent modifications of neuronal connectivity and plasticity of the neocortex are molecular changes occurring at the synapse. Here we used isobaric tag for relative and absolute quantification to measure levels of 467 synaptic proteins in a well-established model of plasticity in the mouse visual cortex and the regulation of its critical period. We found that inducing visual cortex plasticity by monocular deprivation during the critical period increased levels of kinases and proteins regulating the actin-cytoskeleton and endocytosis. Upon closure of the critical period with age, proteins associated with transmitter vesicle release and the tubulin- and septin-cytoskeletons increased, whereas actin-regulators decreased in line with augmented synapse stability and efficacy. Maintaining the visual cortex in a plastic state by dark rearing mice into adulthood only partially prevented these changes and increased levels of G-proteins and protein kinase A subunits. This suggests that in contrast to the general belief, dark rearing does not simply delay cortical development but may activate signaling pathways that specifically maintain or increase the plasticity potential of the visual cortex. Altogether, this study identified many novel candidate plasticity proteins and signaling pathways that mediate synaptic plasticity during critical developmental periods or restrict it in adulthood.

Evidence for cross-modal plasticity in adult mouse visual cortex following monocular enucleation

The goal of this study was to assess cortical reorganization in the visual system of adult mice in detail. A combination of deprivation of one eye and stimulation of the remaining eye previously led to the identification of input-specific subdivisions in mouse visual cortex. Using this information as a reference map, we established to what extent each of these functional subdivisions take part in cortical reactivation and reorganization upon unilateral enucleation. A recovery experiment revealed a differential laminar and temporal reactivation profile. Initiation of infragranular recovery of molecular activity near the border with nonvisual cortex and simultaneous hyperactivation of this adjacent cortex implied a partial nonvisual contribution to this plasticity. The strong effect of somatosensory deprivation as well as stimulation on infragranular visual cortex activation in long-term enucleated animals support this view. Furthermore, targeted tracer injections in visual cortex of control and enucleated animals revealed preexisting connections between the visual and somatosensory cortices of adult mice as possible mediators. In conclusion, this study supports an important cross-modal component in reorganization of adult mouse visual cortex upon monocular enucleation.

A refractory period for rejuvenating GABAergic synaptic transmission and ocular dominance plasticity with dark exposure

Dark exposure initiated in adulthood reactivates robust ocular dominance plasticity in the visual cortex. Here, we show that a critical component of the response to dark exposure is the rejuvenation of inhibitory synaptic transmission, resulting in a decrease in functional inhibitory synaptic density, a decrease in paired-pulse depression, and a reexpression of endocannabinoid-dependent inhibitory long-term depression (iLTD). Importantly, pharmacological acceleration of the maturation of inhibition in dark-exposed adults inhibits the reexpression of iLTD and the reactivation of ocular dominance plasticity. Surprisingly, dark exposure initiated earlier in postnatal development does not rejuvenate inhibitory synaptic transmission or facilitate rapid ocular dominance plasticity, demonstrating the presence of a refractory period for the regulation of synaptic plasticity by visual deprivation.

Laminar-specific maturation of GABAergic transmission and susceptibility to visual deprivation are related to endocannabinoid sensitivity in mouse visual cortex

The developmental period when neuronal responses are modified by visual experience is reported to start and end earlier in layer 4 than in layer 2/3 of the visual cortex, and the maturation of GABAergic inhibitory circuits is suggested to determine the timing of this period. Here, we examine whether the laminar difference in such timing corresponds to a difference in the time course of the functional maturation of GABAergic synaptic transmission to star pyramidal and pyramidal cells in layers 4 and 2/3, respectively, of the mouse visual cortex and whether the development of the strength of GABAergic transmission is affected by visual deprivation in a laminar-specific manner. Our analysis of developmental changes in inhibitory postsynaptic currents of star pyramidal and pyramidal cells evoked by electrical stimulation of afferents or action potentials of fast-spiking GABAergic neurons revealed that there was a sequential maturation of GABAergic function from layers 4 to 2/3. The maturation of inhibition in layer 4 occurred at postnatal week 3, which preceded by 1 week that of layer 2/3. Visual deprivation by dark rearing arrested the functional development of GABAergic transmission in layer 2/3, whereas dark rearing was not so effective in layer 4. GABAergic synapses in layer 2/3 were sensitive to an agonist for cannabinoid type 1 receptors and not normally matured in receptor knock-out mice, whereas those in layer 4 were not so. These results suggest laminar-specific maturation of inhibition and susceptibility to visual deprivation, which may be related to the laminar difference in sensitivity to endocannabinoids.

Experience-dependent reactivation of ocular dominance plasticity in the adult visual cortex

A crucial issue in neurobiology is to understand the main mechanisms restricting neural plasticity to brief windows of early postnatal life. The visual system is one of the paradigmatic models for studying experience-dependent plasticity. The closure of one eye (monocular deprivation, MD) causes a marked ocular dominance (OD) shift of neurons in the primary visual cortex only during the critical period. Here, we report that environmental enrichment (EE), a condition of increased sensory-motor stimulation, reactivates OD plasticity in the adult visual cortex, as assessed with both visual evoked potentials and single-unit recordings. This effect is accompanied by a marked increase in cerebral serotonin (5-HT) levels. Blocking 5-HT enhancement in the visual cortex of EE rats completely prevents the OD shift induced by MD. We also found that EE leads to a reduced intracortical GABAergic inhibition and an increased BDNF expression and that the modulation of these molecular factors is neutralized by cortical infusion of the 5-HT synthesis inhibitor pCPA. Our results show that EE rejuvenates the adult visual cortex and that 5-HT is a crucial factor in this process, triggering a cascade of molecular events that allow the reinstatement of neural plasticity. The non-invasive nature of EE makes this paradigm particularly eligible for clinical application.

Difference in binocularity and ocular dominance plasticity between GABAergic and excitatory cortical neurons

Neuronal circuits in the cerebral cortex consist mainly of glutamatergic/excitatory and GABAergic/inhibitory neurons. In the visual cortex, the binocular responsiveness of neurons is modified by monocular visual deprivation during the critical period of postnatal development. Although GABAergic neurons are considered to play a key role in the expression of the critical period, it is not known whether their binocular responsiveness and ocular dominance plasticity are different from those of excitatory neurons. Recently, the end of the critical period was found to be not strict so that cortical neurons in the adult still have some ocular dominance plasticity. It is not known, however, which type of neurons or both maintain such plasticity in adulthood. To address these issues, we applied in vivo two-photon functional Ca(2+) imaging to transgenic mice whose GABAergic neurons express a yellow fluorescent protein called Venus. We found that GABAergic neurons are more binocular than excitatory neurons in the normal visual cortex, and both types of neurons show the same degree of modifiability to monocular visual deprivation during the critical period, but the modifiability of GABAergic neurons is stronger than that of excitatory neurons after the end of the critical period.

Bidirectional ocular dominance plasticity of inhibitory networks: recent advances and unresolved questions

Monocular visual deprivation (MD) produces profound changes in the ocular dominance (OD) of neurons in the visual cortex. MD shifts visually evoked responses away from the deprived eye and toward domination by the open-eye. Over 30 years ago, two different theories were proposed to account for these changes: either through effects on excitatory visual drive, thereby shifting the balance of excitation in favor of the open-eye, or through effects on intracortical inhibition, thereby suppressing responses from the deprived eye. In the intervening years, a scientific consensus emerged that the major functional effects of MD result from plasticity at excitatory connections in the visual cortex. A recent study by Yazaki-Sugiyama et al. (2009) in mouse visual cortex appears to re-open the debate. Here we take a critical look at these intriguing new data in the context of other recent findings in rodent visual cortex.

AMP-activated protein kinase mediates activity-dependent regulation of peroxisome proliferator-activated receptor gamma coactivator-1alpha and nuclear respiratory factor 1 expression in rat visual cortical neurons

Nuclear respiratory factor 1 (NRF-1) is one of the key transcription factors implicated in mitochondrial biogenesis by activating the transcription of mitochondrial transcription factor A (mtTFA) and subunit genes of respiratory enzymes. NRF-1 transactivation activity can be enhanced by interaction with transcription coactivator peroxisome proliferator-activated receptor gamma coactivator 1alpha (PGC-1alpha). The expression of PGC-1alpha, NRF-1 and mtTFA in neurons is known to be tightly regulated by neuronal activity. However, the coupling signaling mechanism is poorly understood. Here, we use primary cultures of rat visual cortical neurons and a rat model of monocular deprivation (MD) to investigate whether AMP-activated protein kinase (AMPK) is implicated in mediating activity-dependent regulation of PGC-1alpha and NRF-1 expression in neurons. We find that KCl depolarization rapidly activates AMPK and significantly increases PGC-1alpha, NRF-1, and mtTFA levels with increased ATP production in neuron cultures. Similarly, pharmacological activation of AMPK with 5'-aminoimidazole-4-carboxamide riboside (AICAR) or resveratrol also markedly increases PGC-1alpha and NRF-1 mRNA levels in neuron cultures. All these effects can be completely blocked by an AMPK inhibitor, Compound C. Conversely, 1 week of MD significantly reduces AMPK phosphorylation and activity, dramatically down-regulates PGC-1alpha and NRF-1 expression in deprived primary visual cortex. Administration of resveratrol in vivo significantly activates AMPK activity and attenuates the effects of MD on mitochondria by significant increase in PGC-1alpha and NRF-1 levels, mitochondria amount, and coupled respiration. These results strongly indicate that AMPK is an essential upstream mediator that couples neuronal activity to mitochondrial energy metabolism by regulation of PGC-1alpha-NRF-1 pathway in neurons.

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.

Local cell proliferation upon enucleation in Direct Retinal Brain Targets in the Visual system of the Adult Mouse

In this study we used incorporation of the DNA synthesis marker 5-bromo-2′-deoxyuridine or BrdU to visualize cell proliferation in the visual system of the adult mouse as a response to monocular enucleation. We detected new BrdU-labeled cells in different subcortical retinal target regions and we established a specific time frame in which this cell proliferation occurred. By performing immunofluorescent double stainings for BrdU and different vascular (glucose transporter type 1, collagen type IV), glial (thymosin β4, glial fibrillary acidic protein) and neuronal (Neuronal Nuclei, doublecortin) markers, we identified these proliferating cells as activated microglia. Additional immunohistochemical stainings for thymosin β4 and glial fibrillary acidic protein also revealed reactive astrocytes in the different retinorecipient nuclei and allowed us to delineate a time frame for microglial and astroglial activation. A PCR array experiment further showed increased levels of cytokines, chemokines, growth factors and enzymes that play an important role in microglial-astroglial communication during the glial activation process in response to the deafferentation.

Differential regulation of synapsin phosphorylation by monocular deprivation in juveniles and adults

The rodent visual cortex retains significant ocular dominance plasticity beyond the traditional postnatal critical period. However, the intracellular mechanisms that underlie the cortical response to monocular deprivation are predicted to be different in juveniles and adults. Here we show monocular deprivation in adult, but not juvenile rats, induced an increase in the phosphorylation of the prominent presynaptic effecter protein synapsin at two key sites known to regulate synapsin function. Monocular deprivation in adults induced an increase in synapsin phosphorylation at the PKA consensus site (site 1) and the CaMKII consensus site (site 3) in the visual cortex ipsilateral to the deprived eye, which is dominated by non-deprived eye input. The increase in synapsin phosphorylation was observed in total cortical homogenate, but not synaptoneurosomes, suggesting that the pool of synapsin targeted by monocular deprivation in adults does not co-fractionate with excitatory synapses. Phosphorylation of sites 1 and 3 stimulates the release of synaptic vesicles from a reserve pool and increases in the probability of evoked neurotransmitter release, which may contribute to the strengthening of the non-deprived input characteristic of ocular dominance plasticity in adults.

Nurturing brain plasticity: impact of environmental enrichment

Environmental enrichment (EE) is known to profoundly affect the central nervous system (CNS) at the functional, anatomical and molecular level, both during the critical period and during adulthood. Recent studies focusing on the visual system have shown that these effects are associated with the recruitment of previously unsuspected neural plasticity processes. At early stages of brain development, EE triggers a marked acceleration in the maturation of the visual system, with maternal behaviour acting as a fundamental mediator of the enriched experience in both the foetus and the newborn. In adult brain, EE enhances plasticity in the cerebral cortex, allowing the recovery of visual functions in amblyopic animals. The molecular substrate of the effects of EE on brain plasticity is multi-factorial, with reduced intracerebral inhibition, enhanced neurotrophin expression and epigenetic changes at the level of chromatin structure. These findings shed new light on the potential of EE as a non-invasive strategy to ameliorate deficits in the development of the CNS and to treat neurological disorders.

Co-regulation of ocular dominance plasticity and NMDA receptor subunit expression in glutamic acid decarboxylase-65 knock-out mice

Experience can shape cortical circuits, especially during critical periods for plasticity. In visual cortex, imbalance of activity from the two eyes during the critical period shifts ocular dominance (OD) towards the more active eye. Inhibitory circuits are crucial in this process: OD plasticity is absent in GAD65KO mice that show diminished inhibition. This defect can be rescued by application of benzodiazepines, which increase GABAergic signalling. However, it is unknown how such changes in inhibition might disrupt and then restore OD plasticity. Since NMDA dependent synaptic plasticity mechanisms are also known to contribute to OD plasticity, we investigated whether NMDA receptor levels and function are also altered in GAD65KO. There are reduced NR2A levels and slower NMDA currents in visual cortex of GAD65KO mice. Application of benzodiazepines, which rescues OD plasticity, also increases NR2A levels. Thus it appears as if OD plasticity can be restored by adding a critical amount of excitatory transmission through NR2A-containing NMDA receptors. Together, these observations can unify competing ideas of how OD plasticity is regulated: changes in either inhibition or excitation would engage homeostatic mechanisms that converge to regulate NMDA receptors, thereby enabling plasticity mechanisms and also ensuring circuit stability.

Neural mechanisms of recovery following early visual deprivation

Natural patterned early visual input is essential for the normal development of the central visual pathways and the visual capacities they sustain. Without visual input, the functional development of the visual system stalls not far from the state at birth, and if input is distorted or biased the visual system develops in an abnormal fashion resulting in specific visual deficits. Monocular deprivation, an extreme form of biased exposure, results in large anatomical and physiological changes in terms of territory innervated by the two eyes in primary visual cortex (V1) and to a loss of vision in the deprived eye reminiscent of that in human deprivation amblyopia. We review work that points to a special role for binocular visual input in the development of V1 and vision. Our unique approach has been to provide animals with mixed visual input each day, which consists of episodes of normal and biased (monocular) exposures. Short periods of concordant binocular input, if continuous, can offset much longer episodes of monocular deprivation to allow normal development of V1 and prevent amblyopia. Studies of animal models of patching therapy for amblyopia reveal that the benefits are both heightened and prolonged by daily episodes of binocular exposure.

Distinctive features of adult ocular dominance plasticity

Sensory experience profoundly shapes neural circuitry of juvenile brain. Although the visual cortex of adult rodents retains a capacity for plasticity in response to monocular visual deprivation, the nature of this plasticity and the neural circuit changes that accompany it remain enigmatic. Here, we investigate differences between adult and juvenile ocular dominance plasticity using Fourier optical imaging of intrinsic signals in mouse visual cortex. This comparison reveals that adult plasticity takes longer than in the juvenile mouse, is of smaller magnitude, has a greater contribution from the increase in response to the open eye, and has less effect on the hemisphere ipsilateral to the deprived eye. Binocular deprivation also causes different changes in the adult. Adult plasticity is similar to juvenile plasticity in its dependence on signaling through NMDA receptors. We propose that adult ocular dominance plasticity arises from compensatory mechanisms that counterbalance the loss of afferent activity caused by visual deprivation.

Age-dependent ocular dominance plasticity in adult mice

Background

Short monocular deprivation (4 days) induces a shift in the ocular dominance of binocular neurons in the juvenile mouse visual cortex but is ineffective in adults. Recently, it has been shown that an ocular dominance shift can still be elicited in young adults (around 90 days of age) by longer periods of deprivation (7 days). Whether the same is true also for fully mature animals is not yet known.

Methodology/Principal Findings

We therefore studied the effects of different periods of monocular deprivation (4, 7, 14 days) on ocular dominance in C57Bl/6 mice of different ages (25 days, 90–100 days, 109–158 days, 208–230 days) using optical imaging of intrinsic signals. In addition, we used a virtual optomotor system to monitor visual acuity of the open eye in the same animals during deprivation. We observed that ocular dominance plasticity after 7 days of monocular deprivation was pronounced in young adult mice (90–100 days) but significantly weaker already in the next age group (109–158 days). In animals older than 208 days, ocular dominance plasticity was absent even after 14 days of monocular deprivation. Visual acuity of the open eye increased in all age groups, but this interocular plasticity also declined with age, although to a much lesser degree than the optically detected ocular dominance shift.

Conclusions/Significance

These data indicate that there is an age-dependence of both ocular dominance plasticity and the enhancement of vision after monocular deprivation in mice: ocular dominance plasticity in binocular visual cortex is most pronounced in young animals, reduced but present in adolescence and absent in fully mature animals older than 110 days of age. Mice are thus not basically different in ocular dominance plasticity from cats and monkeys which is an absolutely essential prerequisite for their use as valid model systems of human visual disorders.

Tumor necrosis factor-alpha mediates one component of competitive, experience-dependent plasticity in developing visual cortex

Rapid, experience-dependent plasticity in developing visual cortex is thought to be competitive. After monocular visual deprivation, the reduction in response of binocular neurons to one eye is matched by a corresponding increase to the other. Chronic optical imaging in mice deficient in TNFalpha reveals the normal initial loss of deprived-eye responses, but the subsequent increase in response to the open eye is absent. This mutation also blocks homeostatic synaptic scaling of mEPSCs in visual cortex in vitro, without affecting LTP. In monocular cortex, thought not to be subject to competition, responses in TNFalpha mutants are as reduced as in the binocular zone. Pharmacological inhibition of endogenous TNFalpha in wild-type mice phenocopies the knockout. These findings suggest that experience-dependent competition in developing visual cortex is the outcome of two distinct, noncompetitive processes, a loss of deprived-eye responses followed by an apparently homeostatic increase in responses dependent on TNFalpha signaling.

Molecular and cellular mechanisms of memory allocation in neuronetworks

Determining how neuronal networks encode memories is a key goal of neuroscience. Although neuronal circuit processes involved in encoding, storing and retrieving memory have attracted a great deal of attention, the processes that allocate individual memories to specific neurons within a network have remained elusive. Recent findings unraveled the first insights into the processes that modulate memory allocation in neuronetworks. They showed that neurons in the lateral amygdala compete to take part in auditory fear conditioned memory traces and that the levels of the transcription factor CREB (cAMP-response element binding protein) can affect the probability of a neuron to be recruited into a given memory representation. CREB-mediated transcriptional regulation involves several signaling pathways, known to mediate nuclear responses to diverse behavioral stimuli, along with coordinated interactions with multiple other transcription activators, coactivators and repressors. Moreover, activation of CREB triggers an autoinhibitory feedback loop, a metaplastic process that could be used to allocate memories away from cells that have been recently involved in memory. Beyond CREB, there may be a host of other processes that dynamically modulate memory allocation in neuronetworks by shaping cooperation and competition among neurons.

Opposing functions of CREB and MKK1 synergistically regulate the geometry of dendritic spines in visual cortex

Most excitatory inputs onto pyramidal neurons are made on dendritic spines. The geometry of dendritic spines modulates synaptic function; yet we know little regarding the molecular signals that regulate spine geometry. Here we report that neurons coordinately regulate the geometry of spines to compensate for variability in spine number, by a process requiring the transcription factor CREB and the kinase MKK1. We find that CREB function is induced, whereas MKK1 is inhibited, by activity blockade. To obtain evidence that CREB and MKK1 regulate dendritic spine geometry in vivo, we coexpressed green fluorescent protein and dominant negative CREB or MKK1 in pyramidal neurons of the intact rat visual cortex. Spines on apical dendrites of layer 3 neurons were then characterized by confocal microscopy. We find that CREB and MKK1 regulate spine geometry in opposite ways. MKK1 is required to reduce spine head size when spine density is high, whereas CREB is required to enlarge spines when spine density is low. Our data suggest that CREB and MKK1 might function as complementary negative feedback mechanisms to maintain synaptic drive within bounds.

Sequential development of long-term potentiation and depression in different layers of the mouse visual cortex

Visual deprivation affects the responses of layer IV cells more prominently during early postnatal development, whereas responses in layer II/III remain modifiable until later ages. We examined whether these laminar differences correlate with changes in long-term potentiation (LTP) and long-term depression (LTD) of the ascending pathways to layers IV and II/III in the mouse visual cortex. Our analysis revealed that LTP and LTD in layer IV principal cells is lost shortly after the eyes open, but persists in layers II/III beyond puberty. These results suggest that plasticity proceeds sequentially through cortical layers in a manner that parallels the flow of information during sensory processing.

Adult visual experience promotes recovery of primary visual cortex from long-term monocular deprivation

Prolonged visual deprivation from early childhood to maturity is believed to cause permanent visual impairment. However, there have been case reports of substantial improvement of binocular vision in human adults following lifelong visual impairment or deprivation. These observations, together with recent findings of adult ocular dominance plasticity in rodents, led us to re-examine whether adult primary visual cortex (V1) is capable of any recovery following long-term monocular deprivation starting in development. Using mice as a model, we find that monocular deprivation from early development to mature ages (well past the critical period) severely impaired binocular vision by reducing the amplitude of responses elicited by stimulation of the deprived eye. Surprisingly, we find little effect on nondeprived eye responses. Restoration of binocular vision in mature adults yields modest but significant improvement of visual responses in V1. Remarkably, we find that when binocular vision is followed by occlusion of the nondeprived eye, visual responses in V1 recover almost fully, as measured by visual evoked potential amplitude, spatial frequency threshold, and single-unit activity. We conclude that adult V1 can recover from long-term deprivation when provided with an optimal regimen of visual experience.

Experience-dependent recovery of vision following chronic deprivation amblyopia

The shift in ocular dominance induced by brief monocular deprivation is greatest during a postnatal critical period and is thought to decline irreversibly thereafter. However, here we demonstrate that complete visual deprivation through dark exposure restores rapid ocular dominance plasticity in adult rats. In addition, the loss of visual acuity resulting from chronic monocular deprivation is reversed if dark exposure precedes removal of the occlusion in adulthood, suggesting a potential use for dark exposure in the treatment of adult amblyopia.

Persistence of experience-induced homeostatic synaptic plasticity through adulthood in superficial layers of mouse visual cortex

It is well established that sensory cortices of animals can be modified by sensory experience, especially during a brief early critical period in development. Theoretical analyses indicate that there are two synaptic plasticity mechanisms required: input-specific synaptic modifications and global homeostatic mechanisms to provide stability to neural networks. Experience-dependent homeostatic synaptic plasticity mechanisms have subsequently been demonstrated in the visual cortex of juvenile animals. Here, we report that experience-dependent homeostatic synaptic plasticity persists through adulthood in the superficial layers of the mouse visual cortex. We found that 2 d of visual deprivation in the form of dark rearing is necessary and sufficient to cause an increase in AMPA receptor-mediated miniature EPSC amplitude in layer 2/3 neurons. This increase was rapidly reversed by 1 d of light exposure. This reversible change in synaptic strength persisted in adult mice past the critical period for ocular dominance plasticity, which is reported to end at approximately 1 month of age in rodents. Interestingly, the mechanism of homeostatic synaptic modifications in 3-month-old mice differed from that in young mice (3 weeks old) in that the multiplicative nature of synaptic scaling is lost. Our results demonstrate that the superficial layers of adult mouse visual cortex retain the ability to undergo reversible experience-dependent homeostatic synaptic plasticity.

Screening mouse vision with intrinsic signal optical imaging

The introduction of forward genetic screens in the mouse asks for techniques that make rapid screening of visual function possible. Transcranial imaging of intrinsic signal is suitable for this purpose and could detect the effects of retinal degeneration, and the increased predominance of the contralateral eye in albino animals. We quantified visual response properties of the cortex by introducing a normalization method to reduce the impact of biological noise. In addition, the presentation of a 'reset'-stimulus shortly after the probing stimulus at a different visual location could reduce the interstimulus time necessary for the decay of the response. Applying these novel methods, we found that acuity of C57Bl/6J mice rises from 0.35 cycles per degree (cpd) at postnatal day 25 to 0.56 cpd in adults. Temporal resolution was lower in adults than in juvenile animals. There was no patchy organization of spatial or temporal frequency preference at the intrinsic signal resolution. Monocular deprivation, a model for amblyopia and critical period plasticity, led to a loss in acuity and a shift towards the nondeprived eye in juvenile animals. Short deprivation did not lead to increased acuity of the nondeprived eye. In adults, a small ocular dominance shift was detectable with urethane anaesthesia. This was not observed when the combination of the opiate fentanyl, fluanisone with a benzodiazepine was used, adding evidence to the hypothesis that enhancing GABA(A)-receptor function masks an adult shift. Together, these novel applications confirm that noninvasive screening of many functional properties of the visual cortex is possible.

Experience-dependent changes in NMDA receptor composition at mature central synapses

N-methyl-D-aspartate receptor (NMDAR) activation is obligatory for the induction of diverse forms of synaptic plasticity. The molecular composition and the function of NMDARs are themselves modified by synaptic activity, which, in turn, alters the ability of synapses to undergo subsequent plastic modification. This homeostatic control of synaptic plasticity is well-known for the experience-dependent development of sensory cortices. However, it is now becoming clear that NMDAR properties may not only be altered at juvenile, but also at mature synapses. Diverse types of behavioral manipulation, such as sensory experience, learning and sleep deprivation alter the NR2A/NR2B ratio of hippocampal or cortical NMDARs. As an additional facet to the dynamics of NMDAR function, NMDAR trafficking is regulated by G-protein-coupled neurotransmitter receptors implicated in learning and arousal, such as orexin and dopamine. These findings suggest that mature glutamatergic synapses may be modified by recent activity via alterations in synaptic NMDAR function. Rapid forms of NMDAR trafficking, perhaps controlled by the neurochemical environment featuring specific states of arousal and learning, may regulate plasticity and modulate cognitive abilities in adulthood.

Monocular deprivation in adult mice alters visual acuity and single-unit activity

It has been discovered recently that monocular deprivation in young adult mice induces ocular dominance plasticity (ODP). This contradicts the traditional belief that ODP is restricted to a juvenile critical period. However, questions remain. ODP of young adults has been observed only using methods that are indirectly related to vision, and the plasticity of young adults appears diminished in comparison with juveniles. Therefore, we asked whether the newly discovered adult ODP broadly reflects plasticity of visual cortical function and whether it persists into full maturity. Single-unit activity is the standard physiological marker of visual cortical function. Using a more optimized protocol for recording single-units, we find evidence of adult ODP of single-units and show that it is most pronounced in deep cortical layers. Furthermore, using visual evoked potentials (VEP), we find that ODP is equally robust in young adults and mature adults and is observable after just one day of monocular deprivation. Finally, we find that monocular deprivation in adults changes spatial frequency thresholds of the VEP, decreasing the acuity of the deprived pathway and improving the acuity of the non-deprived pathway. Thus, in mice, the primary visual cortex is capable of remarkable adaptation throughout life.

A Novel Phosphodiesterase Type 4 Inhibitor, HT-0712, Enhances Rehabilitation-Dependent Motor Recovery and Cortical Reorganization After Focal Cortical Ischemia

Rehabilitation-dependent motor recovery after cerebral ischemia is associated with functional reorganization of residual cortical tissue. Recovery is thought to occur when remaining circuitry surrounding the lesion is “retrained” to assume some of the lost function. This reorganization is in turn supported by synaptic plasticity within cortical circuitry and manipulations that promote plasticity may enhance recovery. Activation of the cAMP/CREB pathway is a key step for experience-dependent neural plasticity. Here we examined the effects of the prototypical phosphodiesterase inhibitor 4 (PDE4) rolipram and a novel PDE inhibitor (HT-0712), known to enhance cAMP/CREB signaling and cognitive function, on restoration of motor skill and cortical function after focal cerebral ischemia. Adult male rats were trained on a skilled reaching task to establish a baseline level of motor performance. Intracortical microstimulation was then used to derive high-resolution maps of forelimb movement representations within the caudal forelimb area of motor cortex contralateral to the trained paw. A focal ischemic infarct was created within approximately 30% of the caudal forelimb area. The effects of administering either rolipram or the novel PDE4 inhibitor HT-0712 during rehabilitation on motor recovery and restoration of movement representations within residual motor cortex were examined. Both compounds significantly enhanced motor recovery and induced an expansion of distal movement representations that extended beyond residual motor cortex. The expansion beyond the initial residual cortex was not observed in vehicle injected controls. Furthermore, the motor recovery seen in the HT-0712 animals was dose dependent. Our results suggest that PDE4 inhibitors during motor rehabilitation facilitate behavioral recovery and cortical reorganization after ischemic insult to levels significantly greater than that observed with rehabilitation alone.

Developmental Downregulation of Histone Posttranslational Modifications Regulates Visual Cortical Plasticity

The action of visual experience on visual cortical circuits is maximal during a critical period of postnatal development. The long-term effects of this experience are likely mediated by signaling cascades regulating experience-dependent gene transcription. Developmental modifications of these pathways could explain the difference in plasticity between the young and adult cortex. We studied the pathways linking experience-dependent activation of ERK to CREB-mediated gene expression in vivo. In juvenile mice, visual stimulation that activates CREB-mediated gene transcription also induced ERK-dependent MSK and histone H3 phosphorylation and H3-H4 acetylation, an epigenetic mechanism of gene transcription activation. In adult animals, ERK and MSK were still inducible; however, visual stimulation induced weak CREB-mediated gene expression and H3-H4 posttranslational modifications. Stimulation of histone acetylation in adult animals by means of trichostatin promoted ocular dominance plasticity. Thus, differing, experience-dependent activations of signaling molecules might be at the basis of the differences in experience-dependent plasticity between juvenile and adult cortex.

Dynamic changes in CREB phosphorylation and neuroadaptive gene expression in area V1 of adult monkeys after monocular enucleation

Our understanding of the molecular events that emerge after change in sensory input remains elusive, especially with regard to mature area V1. Here, we characterized P-CREB expression in area V1 of monkeys at multiple time-points after monocular enucleation (ME) to assess the possible contribution of CREB in visually deprived neocortex. Immunoblot assays and immunostainings showed that P-CREB is dynamically regulated in adult area V1, reaching a peak level between 5 and 30 days after ME, and becoming reduced at the 90-day post-ME time-point. This striking temporal increase in P-CREB level was paralleled by a concomitant increase of two CREB-regulated pro-survival effectors, namely Bcl-2 and Bcl-w. We present our results in the context of recent advances about adult visual neocortex and propose that ME induces a multifaceted CREB-mediated response that favors intrinsic stability of neurons and facilitates mature cortical networks to reorganize over a prolonged period.

Enhancement of vision by monocular deprivation in adult mice

Plasticity of vision mediated through binocular interactions has been reported in mammals only during a "critical" period in juvenile life, wherein monocular deprivation (MD) causes an enduring loss of visual acuity (amblyopia) selectively through the deprived eye. Here, we report a different form of interocular plasticity of vision in adult mice in which MD leads to an enhancement of the optokinetic response (OKR) selectively through the nondeprived eye. Over 5 d of MD, the spatial frequency sensitivity of the OKR increased gradually, reaching a plateau of approximately 36% above pre-deprivation baseline. Eye opening initiated a gradual decline, but sensitivity was maintained above pre-deprivation baseline for 5-6 d. Enhanced function was restricted to the monocular visual field, notwithstanding the dependence of the plasticity on binocular interactions. Activity in visual cortex ipsilateral to the deprived eye was necessary for the characteristic induction of the enhancement, and activity in visual cortex contralateral to the deprived eye was necessary for its maintenance after MD. The plasticity also displayed distinct learning-like properties: Active testing experience was required to attain maximal enhancement and for enhancement to persist after MD, and the duration of enhanced sensitivity after MD was extended by increasing the length of MD, and by repeating MD. These data show that the adult mouse visual system maintains a form of experience-dependent plasticity in which the visual cortex can modulate the normal function of subcortical visual pathways.

Experience-Dependent Plasticity in Adult Visual Cortex

Experience-dependent plasticity is a prominent feature of the mammalian visual cortex. Although such neural changes are most evident during development, adult cortical circuits can be modified by a variety of manipulations, such as perceptual learning and visual deprivation. Elucidating the underlying mechanisms at the cellular and synaptic levels is an essential step in understanding neural plasticity in the mature animal. Although developmental and adult plasticity share many common features, notable differences may be attributed to developmental cortical changes at multiple levels. These range from shifts in the molecular profiles of cortical neurons to changes in the spatiotemporal dynamics of network activity. In this review, we will discuss recent progress and remaining challenges in understanding adult visual plasticity, focusing on the primary visual cortex.

PirB Restricts Ocular-Dominance Plasticity in Visual Cortex

Experience can alter synaptic connectivity throughout life, but the degree of plasticity present at each age is regulated by mechanisms that remain largely unknown. Here, we demonstrate that Paired-immunoglobulin-like receptor B (PirB), a major histocompatibility complex class I (MHCI) receptor, is expressed in subsets of neurons throughout the brain. Neuronal PirB protein is associated with synapses and forms complexes with the phosphatases Shp-1 and Shp-2. Soluble PirB fusion protein binds to cortical neurons in an MHCI-dependent manner. In mutant mice lacking functional PirB, cortical ocular-dominance plasticity is more robust at all ages. Thus, an MHCI receptor is expressed in central nervous system neurons and functions to limit the extent of experience-dependent plasticity in the visual cortex throughout life. PirB is also expressed in many other regions of the central nervous system, suggesting that it may function broadly to stabilize neural circuits.

Virally mediated knock-down of NR2 subunits ipsilateral to the deprived eye blocks ocular dominance plasticity

NMDA receptors (NMDARs) are important in developmental plasticity in the visual cortex. The NR2A and NR2B subunits of this receptor develop with different time courses, suggesting that they play different roles in plasticity. To understand the role of the NR2B subunit, we knocked-down NR2B gene expression in visual cortex by injecting a recombinant adenovirus containing an antisense NR2B oligonucleotide. To assess knock-down, we injected the recombinant adenovirus into the right visual cortex of rats (p22) or mice (p30). Eight days later we perfused the animals and processed the visual cortex for NMDAR subunit immunoreactivity (IR). NR2B-IR was depleted dramatically in the neuropil near the injection. Depletion was more modest in the neuronal somata. Surprisingly, NR2A-IR was also reduced, but NR1-IR was not reduced. To assess the functional effects of depletion, we measured ocular dominance plasticity with monocular deprivation (MD). We compared mice receiving the NR2B antisense virus with mice receiving virus containing only the GFP sequence and mice receiving no injection. All injections were between p26 and p29 in the right cortex and bilateral recordings were performed 6-8 days later. Animals receiving the antisense virus lost plasticity if the right eye was deprived. If the left eye was deprived, the cortex was normally plastic bilaterally. Injection of control virus had no effect on plasticity. The data indicate that ocular dominance plasticity requires normal NMDARs in the hemisphere ipsilateral to the deprived eye but not in the hemisphere contralateral to the deprived eye.

Bidirectional modifications of visual acuity induced by monocular deprivation in juvenile and adult rats

Recent electrophysiological studies of rodent visual cortex suggest that, in addition to deprived-eye depression, monocular deprivation (MD) also shifts ocular dominance by potentiation of open-eye responses. We used computer-based, two-choice discrimination tasks to assess the behavioral significance of these findings in rats. As expected, prolonged MD, from postnatal day 21 until adulthood (>150 d) markedly decreased visual acuity through the deprived eye. However, we also found that the acuity through the nondeprived eye was significantly enhanced compared with normally reared controls. Interestingly, when the deprived eye was opened in adults, there was a gradual but incomplete recovery of acuity in the deprived eye preceded by a loss of the enhanced acuity in the nondeprived eye. These changes were reversed by again reclosing the eye. These findings suggest that the bidirectional changes in visually evoked responses after MD are behaviorally meaningful and that significant plasticity is exhibited well into adulthood.

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.

Common molecular mechanisms in explicit and implicit memory

Cellular and molecular studies of both implicit and explicit memory suggest that experience-dependent modulation of synaptic strength and structure is a fundamental mechanism by which these memories are encoded and stored within the brain. In this review, we focus on recent advances in our understanding of two types of memory storage: (i) sensitization in Aplysia, a simple form of implicit memory, and (ii) formation of explicit spatial memories in the mouse hippocampus. These two processes share common molecular mechanisms that have been highly conserved through evolution.

Visual deprivation modifies both presynaptic glutamate release and the composition of perisynaptic/extrasynaptic NMDA receptors in adult visual cortex

Use-dependent modifications of synapses have been well described in the developing visual cortex, but the ability for experience to modify synapses in the adult visual cortex is poorly understood. We found that 10 d of late-onset visual deprivation modifies both presynaptic and postsynaptic elements at the layer 4-->2/3 connection in the visual cortex of adult mice, and these changes differ from those observed in juveniles. Although visual deprivation in juvenile mice modifies the subunit composition and increases the current duration of synaptic NMDA receptors (NMDARs), no such effect is observed at synapses between layer 4 and layer 2/3 pyramidal neurons in adult mice. Surprisingly, visual deprivation in adult mice enhances the temporal summation of NMDAR-mediated currents induced by bursts of high-frequency stimulation. The enhanced temporal summation of NMDAR-mediated currents in deprived cortex could not be explained by a reduction in the rate of synaptic depression, because our data indicate that late-onset visual deprivation actually increases the rate of synaptic depression. Biochemical and electrophysiological evidence instead suggest that the enhanced temporal summation in adult mice could be accounted for by a change in the molecular composition of NMDARs at perisynaptic/extrasynaptic sites. Our data demonstrate that the experience-dependent modifications observed in the adult visual cortex are different from those observed during development. These differences may help to explain the unique consequences of sensory deprivation on plasticity in the developing versus mature cortex.

Visual deprivation reactivates rapid ocular dominance plasticity in adult visual cortex

Brief monocular deprivation (< or =3 d) induces a rapid shift in the ocular dominance of binocular neurons in the juvenile rodent visual cortex but is ineffective in adults. Here, we report that persistent, rapid, juvenile-like ocular dominance plasticity can be reactivated in adult rodent visual cortex when monocular deprivation is preceded by visual deprivation. Ocular dominance shifts in visually deprived adults are caused by a rapid depression of the response to stimulation of the deprived eye, previously only reported in juveniles, and a simultaneous potentiation of the response to stimulation of the nondeprived eye. The enhanced ocular dominance plasticity induced by visual deprivation persists for days, even if binocular vision precedes monocular deprivation. Visual deprivation also induces a significant decrease in the level of GABAA receptors relative to AMPA receptors and a return to the juvenile form of NMDA receptors in the visual cortex, two molecular changes that we propose enable the persistent reactivation of rapid ocular dominance plasticity.

Effects of visual experience on activity-dependent gene regulation in cortex

There are critical periods in development when sensory experience directs the maturation of synapses and circuits within neocortex. We report that the critical period in mouse visual cortex has a specific molecular logic of gene regulation. Four days of visual deprivation regulated one set of genes during the critical period, and different sets before or after. Dark rearing perturbed the regulation of these age-specific gene sets. In addition, a 'common gene set', comprised of target genes belonging to a mitogen-activated protein (MAP) kinase signaling pathway, was regulated by vision at all ages but was impervious to prior history of sensory experience. Together, our results demonstrate that vision has dual effects on gene regulation in visual cortex and that sensory experience is needed for the sequential acquisition of age-specific, but not common, gene sets. Thus, a dynamic interplay between experience and gene expression drives activity-dependent circuit maturation.

Prior experience enhances plasticity in adult visual cortex

The brain has a remarkable capacity to adapt to alterations in its sensory environment, which is normally much more pronounced in juvenile animals. Here we show that in adult mice, the ability to adapt to changes can be improved profoundly if the mouse has already experienced a similar change in its sensory environment earlier in life. Using the standard model for sensory plasticity in mouse visual cortex-ocular dominance (OD) plasticity-we found that a transient shift in OD, induced by monocular deprivation (MD) earlier in life, renders the adult visual cortex highly susceptible to subsequent MD many weeks later. Irrespective of whether the first MD was experienced during the critical period (around postnatal day 28) or in adulthood, OD shifts induced by a second MD were faster, more persistent and specific to repeated deprivation of the same eye. The capacity for plasticity in the mammalian cortex can therefore be conditioned by past experience.

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.

Reversible blockade of experience-dependent plasticity by calcineurin in mouse visual cortex

Numerous protein kinases have been implicated in visual cortex plasticity, but the role of serine/threonine protein phosphatases has not yet been established. Calcineurin, the only known Ca2+/calmodulin-activated protein phosphatase in the brain, has been identified as a molecular constraint on synaptic plasticity in the hippocampus and on memory. Using transgenic mice overexpressing calcineurin inducibly in forebrain neurons, we now provide evidence that calcineurin is also involved in ocular dominance plasticity. A transient increase in calcineurin activity is found to prevent the shift of responsiveness in the visual cortex following monocular deprivation, and this effect is reversible. These results imply that the balance between protein kinases and phosphatases is critical for visual cortex plasticity.

Multiple periods of functional ocular dominance plasticity in mouse visual cortex

The precise period when experience shapes neural circuits in the mouse visual system is unknown. We used Arc induction to monitor the functional pattern of ipsilateral eye representation in cortex during normal development and after visual deprivation. After monocular deprivation during the critical period, Arc induction reflects ocular dominance (OD) shifts within the binocular zone. Arc induction also reports faithfully expected OD shifts in cat. Shifts towards the open eye and weakening of the deprived eye were seen in layer 4 after the critical period ends and also before it begins. These shifts include an unexpected spatial expansion of Arc induction into the monocular zone. However, this plasticity is not present in adult layer 6. Thus, functionally assessed OD can be altered in cortex by ocular imbalances substantially earlier and far later than expected.

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.

Swept contrast visual evoked potentials and their plasticity following monocular deprivation in mice

The swept contrast visual evoked potential technique is a quasi-psychophysical method that can help bridge the gap between cell biology and visual performance in studies of ocular dominance plasticity. In mice we found that four days of monocular deprivation diminished the amplitude of evoked potentials from the deprived eye relative to the non-deprived eye. This ocular dominance plasticity was nearly as great in adult mice as in juveniles. The monocular deprivation effect was mediated, at least in part, by enhancement of responses evoked from the non-deprived eye, rather than by depression of responses from the deprived eye.