Requirement for the RIIbeta isoform of PKA, but not calcium-stimulated adenylyl cyclase, in visual cortical plasticity

Genetic mechanisms for impaired synaptic plasticity in schizophrenia revealed by computational modelling

Schizophrenia phenotypes are suggestive of impaired cortical plasticity in the disease, but the mechanisms of these deficits are unknown. Genomic association studies have implicated a large number of genes that regulate neuromodulation and plasticity, indicating that the plasticity deficits have a genetic origin. Here, we used biochemically detailed computational modelling of post-synaptic plasticity to investigate how schizophrenia-associated genes regulate long-term potentiation (LTP) and depression (LTD). We combined our model with data from post-mortem mRNA expression studies (CommonMind gene-expression datasets) to assess the consequences of altered expression of plasticity-regulating genes for the amplitude of LTP and LTD. Our results show that the expression alterations observed post mortem, especially those in anterior cingulate cortex, lead to impaired PKA-pathway-mediated LTP in synapses containing GluR1 receptors. We validated these findings using a genotyped EEG dataset where polygenic risk scores for synaptic and ion channel-encoding genes as well as modulation of visual evoked potentials (VEP) were determined for 286 healthy controls. Our results provide a possible genetic mechanism for plasticity impairments in schizophrenia, which can lead to improved understanding and, ultimately, treatment of the disorder.

FOXG1 Regulates PRKAR2B Transcriptionally and Posttranscriptionally via miR200 in the Adult Hippocampus

Rett syndrome is a complex neurodevelopmental disorder that is mainly caused by mutations in MECP2. However, mutations in FOXG1 cause a less frequent form of atypical Rett syndrome, called FOXG1 syndrome. FOXG1 is a key transcription factor crucial for forebrain development, where it maintains the balance between progenitor proliferation and neuronal differentiation. Using genome-wide small RNA sequencing and quantitative proteomics, we identified that FOXG1 affects the biogenesis of miR200b/a/429 and interacts with the ATP-dependent RNA helicase, DDX5/p68. Both FOXG1 and DDX5 associate with the microprocessor complex, whereby DDX5 recruits FOXG1 to DROSHA. RNA-Seq analyses of Foxg1^cre/+ hippocampi and N2a cells overexpressing miR200 family members identified cAMP-dependent protein kinase type II-beta regulatory subunit (PRKAR2B) as a target of miR200 in neural cells. PRKAR2B inhibits postsynaptic functions by attenuating protein kinase A (PKA) activity; thus, increased PRKAR2B levels may contribute to neuronal dysfunctions in FOXG1 syndrome. Our data suggest that FOXG1 regulates PRKAR2B expression both on transcriptional and posttranscriptional levels.

Postsynaptic localization and regulation of AMPA receptors and Cav1.2 by β2 adrenergic receptor/PKA and Ca2+/CaMKII signaling

The synapse transmits, processes, and stores data within its tiny space. Effective and specific signaling requires precise alignment of the relevant components. This review examines current insights into mechanisms of AMPAR and NMDAR localization by PSD-95 and their spatial distribution at postsynaptic sites to illuminate the structural and functional framework of postsynaptic signaling. It subsequently delineates how β2 adrenergic receptor (β2 AR) signaling via adenylyl cyclase and the cAMP-dependent protein kinase PKA is organized within nanodomains. Here, we discuss targeting of β2 AR, adenylyl cyclase, and PKA to defined signaling complexes at postsynaptic sites, i.e., AMPARs and the L-type Ca2+ channel Cav1.2, and other subcellular surface localizations, the role of A kinase anchor proteins, the physiological relevance of the spatial restriction of corresponding signaling, and their interplay with signal transduction by the Ca2+- and calmodulin-dependent kinase CaMKII How localized and specific signaling by cAMP occurs is a central cellular question. The dendritic spine constitutes an ideal paradigm for elucidating the dimensions of spatially restricted signaling because of their small size and defined protein composition.

GABAB receptor-dependent bidirectional regulation of critical period ocular dominance plasticity in cats

Gama amino butyric acid (GABA) inhibition plays an important role in the onset and offset of the critical period for ocular dominance (OD) plasticity in the primary visual cortex. Previous studies have focused on the involvement of GABA_A receptors, while the potential contribution of GABA_B receptors to OD plasticity has been neglected. In this study, the GABA_B receptor antagonist SCH50911 or agonist baclofen was infused into the primary visual cortex of cats concurrently with a period of monocular deprivation (MD). Using single-unit recordings we found that the OD shift induced by four days of MD during the critical period was impaired by infusion of the antagonist SCH50911, but enhanced by infusion of the agonist baclofen. In contrast, seven days of MD in adult cats did not induce any significant OD shift, even when combined with the infusion of SCH50911 or baclofen. Together, these findings indicate that an endogenous GABA_B receptor-mediated inhibition contributes to juvenile, but not adult, OD plasticity.

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

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

Balancing plasticity/stability across brain development

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

Routes to cAMP: shaping neuronal connectivity with distinct adenylate cyclases

cAMP signaling affects a large number of the developmental processes needed for the construction of the CNS, including cell differentiation, axon outgrowth, response to guidance molecules or modulation of synaptic connections. This points to a key role of adenylate cyclases (ACs), the synthetic enzymes of cAMP, for neural development. ACs exist as 10 different isoforms, which are activated by distinct signaling pathways. The implication of specific AC isoforms in neural wiring was only recently demonstrated in mouse mutants, knockout (KO) for different AC isoforms, AC1, AC3, AC5, AC8 and soluble (s)AC/AC10. These studies stressed the importance of three of these isoforms, as sensors of neural activity that could modify the survival of neurons (sAC), axon outgrowth (sAC), or the response of axons to guidance molecules such as ephrins (AC1) or semaphorins (AC3). We summarize here the current knowledge on the role of these ACs for the development of sensory maps, in the somatosensory, visual and olfactory systems, which have been the most extensively studied. In these systems, AC1/AC3 KO revealed targeting mistakes due to the defective pruning and lack of discrimination of incoming axons to signals present in target structures. In contrast, no changes in cell differentiation, survival or axon outgrowth were noted in these mutants, suggesting a specificity of cAMP production routes for individual cellular processes within a given neuron. Further studies indicate that the subcellular localization of ACs could be key to their specific role in axon targeting and may explain their selective roles in neuronal wiring.

Pain modulators regulate the dynamics of PKA-RII phosphorylation in subgroups of sensory neurons

Knowledge about the molecular structure of protein kinase A (PKA) isoforms is substantial. In contrast, the dynamics of PKA isoform activity in living primary cells has not been investigated in detail. Using a high content screening microscopy approach, we identified the RIIβ subunit of PKA-II to be predominantly expressed in a subgroup of sensory neurons. The RIIβ-positive subgroup included most neurons expressing nociceptive markers (TRPV1, NaV1.8, CGRP, IB4) and responded to pain-eliciting capsaicin with calcium influx. Isoform-specific PKA reporters showed in sensory-neuron-derived F11 cells that the inflammatory mediator PGE₂ specifically activated PKA-II but not PKA-I. Accordingly, pain-sensitizing inflammatory mediators and activators of PKA increased the phosphorylation of RII subunits (pRII) in subgroups of primary sensory neurons. Detailed analyses revealed basal pRII to be regulated by the phosphatase PP2A. Increase of pRII was followed by phosphorylation of CREB in a PKA-dependent manner. Thus, we propose RII phosphorylation to represent an isoform-specific readout for endogenous PKA-II activity in vivo, suggest RIIβ as a novel nociceptive subgroup marker, and extend the current model of PKA-II activation by introducing a PP2A-dependent basal state.

Long-term depression of synaptic transmission in the adult mouse insular cortex in vitro

The insular cortex (IC) is known to play important roles in higher brain functions such as memory and pain. Activity-dependent long-term depression (LTD) is a major form of synaptic plasticity related to memory and chronic pain. Previous studies of LTD have mainly focused on the hippocampus, and no study in the IC has been reported. In this study, using a 64-channel recording system, we show for the first time that repetitive low-frequency stimulation (LFS) can elicit frequency-dependent LTD of glutamate receptor-mediated excitatory synaptic transmission in both superficial and deep layers of the IC of adult mice. The induction of LTD in the IC required activation of the N-methyl-d-aspartate (NMDA) receptor, metabotropic glutamate receptor (mGluR)5, and L-type voltage-gated calcium channel. Protein phosphatase 1/2A and endocannabinoid signaling are also critical for the induction of LTD. In contrast, inhibiting protein kinase C, protein kinase A, protein kinase Mζ or calcium/calmodulin-dependent protein kinase II did not affect LFS-evoked LTD in the IC. Bath application of the group I mGluR agonist (RS)-3,5-dihydroxyphenylglycine produced another form of LTD in the IC, which was NMDA receptor-independent and could not be occluded by LFS-induced LTD. Our studies have characterised the basic mechanisms of LTD in the IC at the network level, and suggest that two different forms of LTD may co-exist in the same population of IC synapses.

Development and plasticity of the primary visual cortex

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

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

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

Critical period of experience-driven axon retraction in the pharmacologically inhibited visual cortex

Monocular deprivation (MD) during the critical period reduces the visual cortical response to the deprived eye and causes the geniculocortical axons serving the deprived eye to retract. When MD is combined with a pharmacological inhibition of the visual cortex, the cortical neurons weaken their response to an open eye and the input axons serving the open eye retract. To determine whether the 2 types of ocular dominance (OD) plasticity reflect an experience-driven modification of neural circuits sharing the same developmental time course, we analyzed the OD plasticity in an inhibited visual cortex using cats at different ages. MD did not affect the OD distribution in the inhibited cortex of adults, confirming that the OD plasticity in the inhibited cortex represents a developmental plasticity. In developing animals, the OD plasticity in the inhibited cortex was observed at the late phase of the critical period (P40-46) but not at the early phase (P22-26). We found a retraction of input axons serving an open eye at the late phase, whereas those at the early phase were comparable to the axons of normal animals. Therefore, the maturation of visual circuits might include an experience-driven rearrangement of thalamocortical projections during the late phase of development.

Candidate genes in ocular dominance plasticity

Many studies have been devoted to the identification of genes involved in experience-dependent plasticity in the visual cortex. To discover new candidate genes, we have reexamined data from one such study on ocular dominance (OD) plasticity in recombinant inbred BXD mouse strains. We have correlated the level of plasticity with the gene expression data in the neocortex that have become available for these same strains. We propose that genes with a high correlation are likely to play a role in OD plasticity. We have tested this hypothesis for genes whose inactivation is known to affect OD plasticity. The expression levels of these genes indeed correlated with OD plasticity if their levels showed strong differences between the BXD strains. To narrow down our candidate list of correlated genes, we have selected only those genes that were previously found to be regulated by visual experience and associated with pathways implicated in OD plasticity. This resulted in a list of 32 candidate genes. The list contained unproven, but not unexpected candidates such as the genes for IGF-1, NCAM1, NOGO-A, the gamma2 subunit of the GABA(A) receptor, acetylcholine esterase, and the catalytic subunit of cAMP-dependent protein kinase A. This demonstrates the viability of our approach. More interestingly, the following novel candidate genes were identified: Akap7, Akt1, Camk2d, Cckbr, Cd44, Crim1, Ctdsp2, Dnajc5, Gnai1, Itpka, Mapk8, Nbea, Nfatc3, Nlk, Npy5r, Phf21a, Phip, Ppm1l, Ppp1r1b, Rbbp4, Slc1a3, Slit2, Socs2, Spock3, St8sia1, Zfp207. Whether all these novel candidates indeed function in OD plasticity remains to be established, but possible roles of some of them are discussed in the article.

Phosphorylation of AMPA receptors is required for sensory deprivation-induced homeostatic synaptic plasticity

Sensory experience, and the lack thereof, can alter the function of excitatory synapses in the primary sensory cortices. Recent evidence suggests that changes in sensory experience can regulate the synaptic level of Ca²⁺-permeable AMPA receptors (CP-AMPARs). However, the molecular mechanisms underlying such a process have not been determined. We found that binocular visual deprivation, which is a well-established in vivo model to produce multiplicative synaptic scaling in visual cortex of juvenile rodents, is accompanied by an increase in the phosphorylation of AMPAR GluR1 (or GluA1) subunit at the serine 845 (S845) site and the appearance of CP-AMPARs at synapses. To address the role of GluR1-S845 in visual deprivation-induced homeostatic synaptic plasticity, we used mice lacking key phosphorylation sites on the GluR1 subunit. We found that mice specifically lacking the GluR1-S845 site (GluR1-S845A mutants), which is a substrate of cAMP-dependent kinase (PKA), show abnormal basal excitatory synaptic transmission and lack visual deprivation-induced homeostatic synaptic plasticity. We also found evidence that increasing GluR1-S845 phosphorylation alone is not sufficient to produce normal multiplicative synaptic scaling. Our study provides concrete evidence that a GluR1 dependent mechanism, especially S845 phosphorylation, is a necessary pre-requisite step for in vivo homeostatic synaptic plasticity.

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.

Protein kinase A signaling as an anti-aging target

Protein kinase A (PKA) is a multi-unit protein kinase that mediates signal transduction of G-protein-coupled receptors through its activation by adenyl cyclase (AC)-mediated cAMP. The vital importance of PKA signaling to cellular function is reflected in the widespread expression of PKA subunit genes. As one of its many functions, PKA plays a key role in the regulation of metabolism and triglyceride storage. The PKA pathway has become of great interest to the study of aging, since mutations that cause a reduction in PKA signaling have been shown to extend lifespan in yeast, and to both delay the incidence and severity of age-related disease, and to promote leanness and longevity, in mice. There is increasing interest in the potential for the inhibition or redistribution of adiposity to attenuate aging, since obesity is associated with impaired function of most organ systems, and is a strong risk factor for shortened life span. Its association with coronary heart disease, hypertension, type 2 diabetes, cancer, sleep apnea and osteoarthritis is leading to its accession as a major cause of global ill health. Therefore, gene signaling pathways such as PKA that promote adiposity are potential inhibitory targets for aging intervention. Since numerous plant compounds have been found that both prevent adipogenesis and inhibit PKA signaling, a focused investigation into their effects on biological systems and the corresponding molecular mechanisms would be of high relevance to the discovery of novel and non-toxic compounds that promote healthy aging.

Regulation of NMDA receptor Ca2+ signalling and synaptic plasticity

NMDARs (N-methyl-D-aspartate receptors) are critical for synaptic function throughout the CNS (central nervous system). NMDAR-mediated Ca(2+) influx is implicated in neuronal differentiation, neuronal migration, synaptogenesis, structural remodelling, long-lasting forms of synaptic plasticity and higher cognitive functions. NMDAR-mediated Ca(2+) signalling in dendritic spines is not static, but can be remodelled in a cell- and synapse-specific manner by NMDAR subunit composition, protein kinases and neuronal activity during development and in response to sensory experience. Recent evidence indicates that Ca(2+) permeability of neuronal NMDARs, NMDAR-mediated Ca(2+) signalling in spines and induction of NMDAR-dependent LTP (long-term potentiation) at hippocampal Schaffer collateral-CA1 synapses are under control of the cAMP/PKA (protein kinase A) signalling cascade. Thus, by enhancing Ca(2+) influx through NMDARs in spines, PKA can regulate the induction of LTP. An emerging concept is that activity-dependent regulation of NMDAR-mediated Ca(2+) signalling by PKA and by extracellular signals that modulate cAMP or protein phosphatases at synaptic sites provides a dynamic and potentially powerful mechanism for bi-directional regulation of synaptic efficacy and remodelling.

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.

Synaptic mechanisms for plasticity in neocortex

Sensory experience and learning alter sensory representations in cerebral cortex. The synaptic mechanisms underlying sensory cortical plasticity have long been sought. Recent work indicates that long-term cortical plasticity is a complex, multicomponent process involving multiple synaptic and cellular mechanisms. Sensory use, disuse, and training drive long-term potentiation and depression (LTP and LTD), homeostatic synaptic plasticity and plasticity of intrinsic excitability, and structural changes including formation, removal, and morphological remodeling of cortical synapses and dendritic spines. Both excitatory and inhibitory circuits are strongly regulated by experience. This review summarizes these findings and proposes that these mechanisms map onto specific functional components of plasticity, which occur in common across the primary somatosensory, visual, and auditory cortices.

AKAP-independent localization of type-II protein kinase A to dynamic actin microspikes

Regulation of the cyclic AMP-dependent protein kinase (PKA) in subcellular space is required for cytoskeletal dynamics and chemotaxis. Currently, spatial regulation of PKA is thought to require the association of PKA regulatory (R) subunits with A-kinase anchoring proteins (AKAPs). Here, we show that the regulatory RIIalpha subunit of PKA associates with dynamic actin microspikes in an AKAP-independent manner. Both endogenous RIIalpha and a GFP-RIIalpha fusion protein co-localize with F-actin in microspikes within hippocampal neuron growth cones and the leading edge lamellae of NG108-15 cells. Live-cell imaging demonstrates that RIIalpha-associated microspikes are highly dynamic and that the coupling of RIIalpha to actin is tight, as the movement of both actin and RIIalpha are immediately and coincidently stopped by low-dose cytochalasin D. Importantly, co-localization of RIIalpha and actin in these structures is resistant to displacement by a cell-permeable disrupter of PKA-AKAP interactions. Biochemical fractionation confirms that a substantial pool of PKA RIIalpha is associated with the detergent-insoluble cytoskeleton and is resistant to extraction by a peptide inhibitor of AKAP interactions. Finally, mutation of the AKAP-binding domain of RIIalpha fails to disrupt its association with actin microspikes. These data provide the first demonstration of the physical association of a kinase with such dynamic actin structures, as well as the first demonstration of the ability of type-II PKA to localize to discrete subcellular structures independently of canonical AKAP function. This association is likely to be important for microfilament dynamics and cell migration and may prime the investigation of novel mechanisms for localizing PKA activity.

Synaptic Mechanisms of Activity-Dependent Remodeling in Visual Cortex during Monocular Deprivation

It has long been appreciated that in the visual cortex, particularly within a postnatal critical period for experience-dependent plasticity, the closure of one eye results in a shift in the responsiveness of cortical cells toward the experienced eye. While the functional aspects of this ocular dominance shift have been studied for many decades, their cortical substrates and synaptic mechanisms remain elusive. Nonetheless, it is becoming increasingly clear that ocular dominance plasticity is a complex phenomenon that appears to have an early and a late component. Early during monocular deprivation, deprived eye cortical synapses depress, while later during the deprivation open eye synapses potentiate. Here we review current literature on the cortical mechanisms of activity-dependent plasticity in the visual system during the critical period. These studies shed light on the role of activity in shaping neuronal structure and function in general and can lead to insights regarding how learning is acquired and maintained at the neuronal level during normal and pathological brain development.

Bidirectional synaptic mechanisms of ocular dominance plasticity in visual cortex

As in other mammals with binocular vision, monocular lid suture in mice induces bidirectional plasticity: rapid weakening of responses evoked through the deprived eye followed by delayed strengthening of responses through the open eye. It has been proposed that these bidirectional changes occur through three distinct processes: first, deprived-eye responses rapidly weaken through homosynaptic long-term depression (LTD); second, as the period of deprivation progresses, the modification threshold determining the boundary between synaptic depression and synaptic potentiation becomes lower, favouring potentiation; and third, facilitated by the decreased modification threshold, open-eye responses are strengthened via homosynaptic long-term potentiation (LTP). Of these processes, deprived-eye depression has received the greatest attention, and although several alternative hypotheses are also supported by current research, evidence suggests that α-amino-3- hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor endocytosis through LTD is a key mechanism. The change in modification threshold appears to occur partly through changes in N-methyl-d-aspartate (NMDA) receptor subunit composition, with decreases in the ratio of NR2A to NR2B facilitating potentiation. Although limited research has directly addressed the question of open-eye potentiation, several studies suggest that LTP could account for observed changes in vivo. This review will discuss evidence supporting this three-stage model, along with outstanding issues in the field.

Sensory deprivation unmasks a PKA-dependent synaptic plasticity mechanism that operates in parallel with CaMKII

Calcium/calmodulin kinase II (CaMKII) is required for LTP and experience-dependent potentiation in the barrel cortex. Here, we find that whisker deprivation increases LTP in the layer IV to II/III pathway and that PKA antagonists block the additional LTP. No LTP was seen in undeprived CaMKII-T286A mice, but whisker deprivation again unmasked PKA-sensitive LTP. Infusion of a PKA agonist potentiated EPSPs in deprived wild-types and deprived CaMKII-T286A point mutants but not in undeprived animals of either genotype. The PKA-dependent potentiation mechanism was not present in GluR1 knockouts. Infusion of a PKA antagonist caused depression of EPSPs in undeprived but not deprived cortex. LTD was occluded by whisker deprivation and blocked by PKA manipulation, but not blocked by cannabinoid antagonists. NMDA receptor currents were unaffected by sensory deprivation. These results suggest that sensory deprivation causes synaptic depression by reversing a PKA-dependent process that may act via GluR1.

Age-dependent decline in supragranular long-term synaptic plasticity by increased inhibition during the critical period in the rat primary visual cortex

Supragranular long-term potentiation (LTP) and depression (LTD) are continuously induced in the pathway from layer 4 during the critical period in the rodent primary visual cortex, which limits the use of supragranular long-term synaptic plasticity as a synaptic model for the mechanism of ocular dominance (OD) plasticity. The results of the present study demonstrate that the pulse duration of extracellular stimulation to evoke a field potential (FP) is critical to induction of LTP and LTD in this pathway. LTP and LTD were induced in the pathway from layer 4 to layer 2/3 in slices from 3-wk-old rats when FPs were evoked by 0.1- and 0.2-ms pulses. LTP and LTD were induced in slices from 5-wk-old rats when evoked by stimulation with a 0.2-ms pulse but not by stimulation with a 0.1-ms pulse. Both the inhibitory component of FP and the inhibitory/excitatory postsynaptic potential amplitude ratio evoked by stimulation with a 0.1-ms pulse were greater than the values elicited by a 0.2-ms pulse. Stimulation with a 0.1-ms pulse at various intensities that showed the similar inhibitory FP component with the 0.2-ms pulse induced both LTD and LTP in 5-wk-old rats. Thus extracellular stimulation with shorter-duration pulses at higher intensity resulted in greater inhibition than that observed with longer-duration pulses at low intensity. This increased inhibition might be involved in the age-dependent decline of synaptic plasticity during the critical period. These results provide an alternative synaptic model for the mechanism of OD plasticity.

Developmental switch in requirement for PKA RIIbeta in NMDA-receptor-dependent synaptic plasticity at Schaffer collateral to CA1 pyramidal cell synapses

The cAMP/protein kinase A (PKA) signaling cascade is crucial for synaptic plasticity in a wide variety of species. PKA regulates Ca2+ permeation through NMDA receptors (NMDARs) and induction of NMDAR-dependent synaptic plasticity at the Schaffer collateral to CA1 pyramidal cell synapse. Whereas the role of PKA in induction of NMDAR-dependent LTP at CA1 synapses is established, the identity of PKA isoforms involved in this phenomenon is less clear. Here we report that protein synthesis-independent NMDAR-dependent LTP at the Schaffer collateral-CA1 synapse in the hippocampus is deficient, but NMDAR-dependent LTD is normal, in young (postnatal day 10 (P10)-P14) mice lacking PKA RIIbeta, the PKA regulatory protein that links PKA to NMDARs at synaptic sites. In contrast, in young adult (P21-P28) mice lacking PKA RIIbeta, LTP is normal and LTD is abolished. These findings indicate that distinct PKA isoforms may subserve distinct forms of synaptic plasticity and are consistent with a developmental switch in the signaling cascades required for LTP induction.

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.

Neuromodulators control the polarity of spike-timing-dependent synaptic plasticity

Near coincidental pre- and postsynaptic action potentials induce associative long-term potentiation (LTP) or long-term depression (LTD), depending on the order of their timing. Here, we show that in visual cortex the rules of this spike-timing-dependent plasticity are not rigid, but shaped by neuromodulator receptors coupled to adenylyl cyclase (AC) and phospholipase C (PLC) signaling cascades. Activation of the AC and PLC cascades results in phosphorylation of postsynaptic glutamate receptors at sites that serve as specific "tags" for LTP and LTD. As a consequence, the outcome (i.e., whether LTP or LTD) of a given pattern of pre- and postsynaptic firing depends not only on the order of the timing, but also on the relative activation of neuromodulator receptors coupled to AC and PLC. These findings indicate that cholinergic and adrenergic neuromodulation associated with the behavioral state of the animal can control the gating and the polarity of cortical plasticity.

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.

Effects of a two-diopter vertical prism on posture

Postural control in upright stance requires the central integration of visual, vestibular, somatosensory (as cutaneous receptors) and proprioceptive (as joint receptors) inputs. Clinical studies seem to indicate an association between vertical heterophoria (VH) and balance control. The purpose of the study was to simulate a VH and examine its influence on body stabilisation in quiet stance. We studied 15 healthy subjects (25.6+/-3.0 years). The postural stability was measured with a platform under the following conditions: normal viewing, with a two-diopter prism base down placed on the dominant eye (PDE) or the non-dominant eye (PNDE). Both eyes were open in all conditions. All conditions were run at two distances: 200 and 40 cm. The results showed: (i) PNDE increased the antero-posterior body sway for both distances; this result could be related to sensory processing of disparity and/or to inappropriate eye movement response to the disparity induced by the prism; (ii) PDE improved the postural stability only at far distance (reduction of the center-of-pressure excursion area and of the lateral body sway). Such positive result could be due to appropriate sensory processing of disparity and/or eye movement response; the latter would reduce vertical disparity and modify the dynamic and tonic eye muscle activity relative to normal viewing at far distance. We conclude that: (i) experimentally induced vertical phoria does indeed influence postural control; (ii) vertical prisms, even of small power, can have complex effects, positive or negative, depending on the eye wearing it and at the distance fixated.

Activity regulates the expression of AMPA receptor subunit GluR4 in developing visual cortex

In the developing visual cortex, the expression of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) subunit GluR4 precedes that of the other AMPAR subunits GluR1-3, and then declines to become almost absent in adults. The current study shows that the neuronal activity regulates the expression of GluR4 by a culture system in vitro and a dark-rearing (DR) system in vivo. Membrane depolarization by treatment of cultured neurons of the visual cortex with a high concentration of KCl (35 mm; HK) promoted a decline in the expression of GluR4. This effect of HK on the expression of GluR4 was significantly blocked by the addition of an N-methyl-d-aspartate receptor (NMDAR) antagonist, (D)-2-amino-5-phosphonovaleric acid (APV), but not by the voltage-sensitive calcium channel antagonist nifedipine. Moreover, the Ca(2+)-calmodulin-dependent kinase (CaMKII) inhibitor KN62 and the cAMP-dependent protein kinase A (PKA) inhibitor H-89 blocked this effect, which suggests the involvement of Ca(2+) influx via NMDAR and the subsequent activation of CaMKII and PKA. Conversely, the MAP kinase inhibitor PD98059 promoted the effect of HK on the expression of GluR4. Significantly, APV, KN62, H-89 and PD98059 either promoted or inhibited the expression of GluR4 even in normal KCl (5 mm) conditions. The developmental change in the expression of GluR4 was significantly attenuated in DR in vivo, and the results suggest that neuronal activity such as visual experience may be involved in the mechanism of the expression of GluR4, which is mediated by NMDAR and tuned by certain protein kinases at an early developmental stage in the visual cortex.

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.

Developmental synaptic plasticity at the thalamocortical input to barrel cortex: mechanisms and roles

The thalamocortical (TC) input to layer IV provides the major pathway for ascending sensory information to the mammalian sensory cortex. During development there is a dramatic refinement of this input that underlies the maturation of the topographical map in layer IV. Over the last 10 years our understanding of the mechanisms of the developmental and experience-driven changes in synaptic function at TC synapses has been greatly advanced. Here we describe these studies that point to a key role for NMDA receptor-dependent synaptic plasticity, a role for kainate receptors and for a rapid maturation in GABAergic inhibition. The expression mechanisms of some of the forms of neonatal synaptic plasticity are novel and, in combination with other mechanisms, produce a layer IV circuit that exhibits functional properties necessary for mature sensory processing.

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.

Distance impairs postural stability only under binocular viewing

Prior studies indicate that postural stability under binocular viewing is not better than under monocular viewing. This was tested at the distances of 145 cm [Fox, C.R. (1990). Some visual influences on human postural equilibrium: binocular versus monocular fixation. Perception and Psychophysics, 47 (5), 409-422] and 90 cm [Isotalo, E., Kapoula, Z., Feret, P.H., Gauchon, K., Zamfirescu, F., & Gagey, P.M. (2004). Monocular versus binocular vision in postural control. Auris Nasus Larynx, 31 (1), 11-17]. On the other hand, postural stability is known to decrease with distance increase. We re-examined the effect of binocular versus monocular viewing on postural stability at near and far distances (40 and 200 cm), and for both young (25.7+/-2.7 years), and old subjects (61.2+/-4.6 years). For both groups of subjects, proximity decreased the area of CoP, the standard deviation of antero-posterior sway (SDy) and the variance of speed. The group of elderly presented increased variance of speed at far distance in comparison with young subjects. The novel finding is the interaction between distance and viewing condition. Under binocular viewing, the area of CoP was significantly higher at far distance than at near; in contrast, monocular viewing produced similar CoP values at both distances. Increased instability at far distance when both eyes are viewing is attributed to decreased sensitivity to binocular disparity cues and to visual motion in depth resulting from body sway. Monocular viewing would provide similar stability at far and at near distance, because sensitivity to lateral visual motion, detected monocularly, decreases less with distance than sensitivity to binocularly detected motion in depth. Alternatively, such monocular viewing could increase subject's attention and lead to tighter postural control regardless of the distance.

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.

Involvement of protein kinase A in patterning of the mouse somatosensory cortex

Patterning of the mouse somatosensory cortex is unusually evident because of the presence of a "barrel field." Presynaptic serotonin and postsynaptic glutamate receptors regulate barrel formation, but little is known of the intracellular signaling pathways through which they act. To determine whether protein kinase A (PKA) plays a role in the development of the barrel field, we examined five viable PKA subunit-specific knock-out (KO) mouse lines for barrel field abnormalities. Barrels are present in these mice, but those lacking the RIIbeta subunit display significantly reduced contrast between the cell densities of barrel hollows and sides compared with wild-type animals. Thalamocortical afferent segregation in the posterior medial barrel subfield appeared normal, suggesting a postsynaptic site of gene action for the RIIbeta protein. Immunoelectron microscopy confirmed that RIIbeta was selectively localized to dendrites and dendritic spines. Mice lacking RIIbeta show reduced glutamate receptor A (GluRA) subunit insertion into the postsynaptic density in postnatal day 7 somatosensory cortex; however, GluRA KO mice developed normal barrels. Our results clearly demonstrate a role for postsynaptic PKA signaling pathways in barrel differentiation. They also demonstrate a clear dissociation between the regulation of GluRA trafficking by PKA and its role in barrel formation. Finally, although a role for PKA downstream of cAMP cannot be ruled out, these data suggest that PKA may not be the principle downstream target because none of the mutants showed a barrelless phenotype similar to that observed in adenylate cyclase type 1 KO mice. These results give insight into activity-dependent mechanisms that regulate barrel formation.

Barrel map development relies on protein kinase A regulatory subunit II beta-mediated cAMP signaling

The cellular and molecular mechanisms mediating the activity-dependent development of brain circuitry are still incompletely understood. Here, we examine the role of cAMP-dependent protein kinase [protein kinase A (PKA)] signaling in cortical development and plasticity, focusing on its role in thalamocortical synapse and barrel map development. We provide direct evidence that PKA activity mediates barrel map formation using knock-out mice that lack type IIbeta regulatory subunits of PKA (PKARIIbeta). We show that PKARIIbeta-mediated PKA function is required for proper dendritogenesis and the organization of cortical layer IV neurons into barrels, but not for the development and plasticity of thalamocortical afferent clustering into a barrel pattern. We localize PKARIIbeta function to postsynaptic processes in barrel cortex and show that postsynaptic PKA targets, but not presynaptic PKA targets, have decreased phosphorylation in pkar2b knock-out (PKARIIbeta(-/-)) mice. We also show that long-term potentiation at TC synapses and the associated developmental increase in AMPA receptor function at these synapses, which normally occurs as barrels form, is absent in PKARIIbeta(-/-) mice. Together, these experiments support an activity-dependent model for barrel map development in which the selective addition and elimination of thalamocortical synapses based on Hebbian mechanisms for synapse formation is mediated by a cAMP/PKA-dependent pathway that relies on PKARIIbeta function.

Synaptic basis for whisker deprivation-induced synaptic depression in rat somatosensory cortex

Whisker deprivation weakens excitatory layer 4 (L4) inputs to L2/3 pyramidal cells in rat primary somatosensory (S1) cortex, which is likely to contribute to whisker map plasticity. This weakening has been proposed to represent long-term depression (LTD) induced by sensory deprivation in vivo. Here, we studied the synaptic expression mechanisms for deprivation-induced weakening of L4-L2/3 inputs and assessed its similarity to LTD, which is known to be expressed presynaptically at L4-L2/3 synapses. Whisker deprivation increased the paired pulse ratio at L4-L2/3 synapses and slowed the use-dependent block of NMDA receptor currents by MK-801 [(5S,10R)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate], indicating that deprivation reduced transmitter release probability at these synapses. In contrast, deprivation did not alter either miniature EPSC amplitude in L2/3 neurons or the amplitude of quantal L4-L2/3 synaptic responses measured in strontium, indicating that postsynaptic responsiveness was unchanged. In young postnatal day 12 (P12) rats, at least 4 d of deprivation were required to significantly weaken L4-L2/3 synapses. Similar weakening occurred when deprivation began at older ages (P20), when synapses are mostly mature, indicating that weakening is unlikely to represent a failure of synaptic maturation but instead represents a reduction in the strength of existing synapses. Thus, whisker deprivation weakens L4-L2/3 synapses by decreasing presynaptic function, similar to known LTD mechanisms at this synapse.

Mitogen-activated protein kinase-mediated reinforcement of hippocampal early long-term depression by the type IV-specific phosphodiesterase inhibitor rolipram and its effect on synaptic tagging

Rolipram, a selective inhibitor of cAMP-specific phosphodiesterase 4 (PDE4), has been shown to reinforce an early form of long-term potentiation (LTP) to a long-lasting LTP (late LTP). Furthermore, it was shown that the effects of rolipram-mediated reinforcement of LTP interacts with processes of synaptic tagging (Navakkode et al., 2004). Here we show in CA1 hippocampal slices from adult rats in vitro that rolipram also converted an early form of long-term depression (LTD) that normally decays within 2-3 h, to a long-lasting LTD (late LTD) if rolipram was applied during LTD-induction. Rolipram-reinforced LTD (RLTD) was NMDA receptor- and protein synthesis-dependent. Furthermore, it was dependent on the synergistic coactivation of dopaminergic D(1) and D(5) receptors. This let us speculate that RLTD resembles electrically induced, conventional CA1 late LTD, which is characterized by heterosynaptic processes and synaptic tagging. We therefore asked whether synaptic tagging occurs during RLTD. We found that early LTD in an S1 synaptic input was transformed into late LTD if early LTD was induced in a second independent S2 synaptic pathway during the inhibition of PDE by rolipram, supporting the interaction of processes of synaptic tagging during RLTD. Furthermore, application of PD 98059 (2'-amino-3'-methoxyflavone) or U0126 (1,4-diamino-2,3-dicyano-1,4-bis[2-aminophenylthio]butadiene), specific inhibitors of mitogen-activated protein kinases (MAPKs), prevented RLTD, suggesting a pivotal role of MAPK activation for RLTD. This MAPK activation was triggered during RLTD by the synergistic interaction of NMDA receptor- and D(1) and D(5) receptor-mediated Rap/B-Raf pathways, but not by the Ras/Raf-1 pathway in adult hippocampal CA1 neurons, as shown by the use of the pathway-specific inhibitors manumycin (Ras/Raf-1) and lethal toxin 82 (Rap/B-Raf).

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.

A Comparison of Experience-Dependent Plasticity in the Visual and Somatosensory Systems

In the visual and somatosensory systems, maturation of neuronal circuits continues for days to weeks after sensory stimulation occurs. Deprivation of sensory input at various stages of development can induce physiological, and often structural, changes that modify the circuitry of these sensory systems. Recent studies also reveal a surprising degree of plasticity in the mature visual and somatosensory pathways. Here, we compare and contrast the effects of sensory experience on the connectivity and function of these pathways and discuss what is known to date concerning the structural, physiological, and molecular mechanisms underlying their plasticity.

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.

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.