Ipsilateral whiskers suppress experience-dependent plasticity in the barrel cortex

A latent pool of neurons silenced by sensory-evoked inhibition can be recruited to enhance perception

To investigate which activity patterns in sensory cortex are relevant for perceptual decision-making, we combined two-photon calcium imaging and targeted two-photon optogenetics to interrogate barrel cortex activity during perceptual discrimination. We trained mice to discriminate bilateral whisker deflections and report decisions by licking left or right. Two-photon calcium imaging revealed sparse coding of contralateral and ipsilateral whisker input in layer 2/3, with most neurons remaining silent during the task. Activating pyramidal neurons using two-photon holographic photostimulation evoked a perceptual bias that scaled with the number of neurons photostimulated. This effect was dominated by optogenetic activation of non-coding neurons, which did not show sensory or motor-related activity during task performance. Photostimulation also revealed potent recruitment of cortical inhibition during sensory processing, which strongly and preferentially suppressed non-coding neurons. Our results suggest that a pool of non-coding neurons, selectively suppressed by network inhibition during sensory processing, can be recruited to enhance perception.

Impact of somatostatin interneurons on interactions between barrels in plasticity induced by whisker deprivation

The activity of inhibitory interneurons has a profound role in shaping cortical plasticity. Somatostatin-expressing interneurons (SOM-INs) are involved in several aspects of experience-dependent cortical rewiring. We addressed the question of the barrel cortex SOM-IN engagement in plasticity formation induced by sensory deprivation in adult mice (2-3 months old). We used a spared vibrissa paradigm, resulting in a massive sensory map reorganization. Using chemogenetic manipulation, the activity of barrel cortex SOM-INs was blocked or activated by continuous clozapine N-oxide (CNO) administration during one-week-long deprivation. To visualize the deprivation-induced plasticity, [¹⁴C]-2-deoxyglucose mapping of cortical functional representation of the spared whisker was performed at the end of the deprivation. The plasticity was manifested as an extension of cortical activation in response to spared vibrissae stimulation. We found that SOM-IN inhibition in the cortical column of the spared whisker did not influence the areal extent of the cortex activated by the spared whisker. However, blocking the activity of SOM-INs in the deprived column, adjacent to the spared one, decreased the plasticity of the spared whisker representation. SOM-IN activation did not affect plasticity. These data show that SOM-IN activity is part of cortical circuitry that affects interbarrel interactions underlying deprivation-induced plasticity in adult mice.

Sensory Experience as a Regulator of Structural Plasticity in the Developing Whisker-to-Barrel System

Cellular structures provide the physical foundation for the functionality of the nervous system, and their developmental trajectory can be influenced by the characteristics of the external environment that an organism interacts with. Historical and recent works have determined that sensory experiences, particularly during developmental critical periods, are crucial for information processing in the brain, which in turn profoundly influence neuronal and non-neuronal cortical structures that subsequently impact the animals' behavioral and cognitive outputs. In this review, we focus on how altering sensory experience influences normal/healthy development of the central nervous system, particularly focusing on the cerebral cortex using the rodent whisker-to-barrel system as an illustrative model. A better understanding of structural plasticity, encompassing multiple aspects such as neuronal, glial, and extra-cellular domains, provides a more integrative view allowing for a deeper appreciation of how all aspects of the brain work together as a whole.

The Role of Interhemispheric Interactions in Cortical Plasticity

Despite the fact that there is a growing awareness to the callosal connections between hemispheres the two hemispheres of the brain are commonly treated as independent structures when peripheral or cortical manipulations are applied to one of them. The contralateral hemisphere is often used as a within-animal control of plastic changes induced onto the other side of the brain. This ensures uniform conditions for producing experimental and control data, but it may overlook possible interhemispheric interactions. In this paper we provide, for the first time, direct proof that cortical, experience-dependent plasticity is not a unilateral, independent process. We mapped metabolic brain activity in rats with 2-[¹⁴C] deoxyglucose (2DG) following experience-dependent plasticity induction after a month of unilateral (left), partial whiskers deprivation (only row B was left). This resulted in ∼45% widening of the cortical sensory representation of the spared whiskers in the right, contralateral barrel field (BF). We show that the width of 2DG visualized representation is less than 20% when only contralateral stimulation of the spared row of whiskers is applied in immobilized animals. This means that cortical map remodeling, which is induced by experience-dependent plasticity mechanisms, depends partially on the contralateral hemisphere. The response, which is observed by 2DG brain mapping in the partially deprived BF after standard synchronous bilateral whiskers stimulation, is therefore the outcome of at least two separately activated plasticity mechanisms. A focus on the integrated nature of cortical plasticity, which is the outcome of the emergent interactions between deprived and non-deprived areas in both hemispheres may have important implications for learning and rehabilitation. There is also a clear implication that there is nothing like “control hemisphere” since any plastic changes in one hemisphere have to have influence on functioning of the opposite one.

An automated homecage system for multiwhisker detection and discrimination learning in mice

Automated, homecage behavioral training for rodents has many advantages: it is low stress, requires little interaction with the experimenter, and can be easily manipulated to adapt to different experimental conditions. We have developed an inexpensive, Arduino-based, homecage training apparatus for sensory association training in freely-moving mice using multiwhisker air current stimulation coupled to a water reward. Animals learn this task readily, within 1–2 days of training, and performance progressively improves with training. We examined the parameters that regulate task acquisition using different stimulus intensities, directions, and reward valence. Learning was assessed by comparing anticipatory licking for the stimulus compared to the no-stimulus (blank) trials. At high stimulus intensities (>9 psi), animals showed markedly less participation in the task. Conversely, very weak air current intensities (1–2 psi) were not sufficient to generate rapid learning behavior. At intermediate stimulus intensities (5–6 psi), a majority of mice learned that the multiwhisker stimulus predicted the water reward after 24–48 hrs of training. Both exposure to isoflurane and lack of whiskers decreased animals’ ability to learn the task. Following training at an intermediate stimulus intensity, mice were able to transfer learning behavior when exposed to a lower stimulus intensity, an indicator of perceptual learning. Mice learned to discriminate between two directions of stimulation rapidly and accurately, even when the angular distance between the stimuli was <15 degrees. Switching the reward to a more desirable reward, aspartame, had little effect on learning trajectory. Our results show that a tactile association task in an automated homecage environment can be monitored by anticipatory licking to reveal rapid and progressive behavioral change. These Arduino-based, automated mouse cages enable high-throughput training that facilitate analysis of large numbers of genetically modified mice with targeted manipulations of neural activity.

The pial vasculature of the mouse develops according to a sensory-independent program

The cerebral vasculature is organized to supply the brain’s metabolic needs. Sensory deprivation during the early postnatal period causes altered neural activity and lower metabolic demand. Neural activity is instructional for some aspects of vascular development, and deprivation causes changes in capillary density in the deprived brain region. However, it is not known if the pial arteriole network, which contains many leptomeningeal anastomoses (LMAs) that endow the network with redundancy against occlusions, is also affected by sensory deprivation. We quantified the effects of early-life sensory deprivation via whisker plucking on the densities of LMAs and penetrating arterioles (PAs) in anatomically-identified primary sensory regions (vibrissae cortex, forelimb/hindlimb cortex, visual cortex and auditory cortex) in mice. We found that the densities of penetrating arterioles were the same across cortical regions, though the hindlimb representation had a higher density of LMAs than other sensory regions. We found that the densities of PAs and LMAs, as well as quantitative measures of network topology, were not affected by sensory deprivation. Our results show that the postnatal development of the pial arterial network is robust to sensory deprivation.

Defining a critical period for inhibitory circuits within the somatosensory cortex

Although experience-dependent changes in brain inhibitory circuits are thought to play a key role during the “critical period” of brain development, the nature and timing of these changes are poorly understood. We examined the role of sensory experience in sculpting an inhibitory circuit in the primary somatosensory cortex (S1) of mice by using optogenetics to map the connections between parvalbumin (PV) expressing interneurons and layer 2/3 pyramidal cells. Unilateral whisker deprivation decreased the strength and spatial range of inhibitory input provided to pyramidal neurons by PV interneurons in layers 2/3, 4 and 5. By varying the time when sensory input was removed, we determined that the critical period closes around postnatal day 14. This yields the first precise time course of critical period plasticity for an inhibitory circuit.

Associations of Unilateral Whisker and Olfactory Signals Induce Synapse Formation and Memory Cell Recruitment in Bilateral Barrel Cortices: Cellular Mechanism for Unilateral Training Toward Bilateral Memory

Somatosensory signals and operative skills learned by unilateral limbs can be retrieved bilaterally. In terms of cellular mechanism underlying this unilateral learning toward bilateral memory, we hypothesized that associative memory cells in bilateral cortices and synapse innervations between them were produced. In the examination of this hypothesis, we have observed that paired unilateral whisker and odor stimulations led to odorant-induced whisker motions in bilateral sides, which were attenuated by inhibiting the activity of barrel cortices. In the mice that showed bilateral cross-modal responses, the neurons in both sides of barrel cortices became to encode this new odor signal alongside the innate whisker signal. Axon projections and synapse formations from the barrel cortex, which was co-activated with the piriform cortex, toward its contralateral barrel cortex (CBC) were upregulated. Glutamatergic synaptic transmission in bilateral barrel cortices was upregulated and GABAergic synaptic transmission was downregulated. The associative activations of the sensory cortices facilitate new axon projection, glutamatergic synapse formation and GABAergic synapse downregulation, which drive the neurons to be recruited as associative memory cells in the bilateral cortices. Our data reveal the productions of associative memory cells and synapse innervations in bilateral sensory cortices for unilateral training toward bilateral memory.

Somatosensory map expansion and altered processing of tactile inputs in a mouse model of fragile X syndrome

Fragile X syndrome (FXS) is a common inherited form of intellectual disability caused by the absence or reduction of the fragile X mental retardation protein (FMRP) encoded by the FMR1 gene. In humans, one symptom of FXS is hypersensitivity to sensory stimuli, including touch. We used a mouse model of FXS (Fmr1 KO) to study sensory processing of tactile information conveyed via the whisker system. In vivo electrophysiological recordings in somatosensory barrel cortex showed layer-specific broadening of the receptive fields at the level of layer 2/3 but not layer 4, in response to whisker stimulation. Furthermore, the encoding of tactile stimuli at different frequencies was severely affected in layer 2/3. The behavioral effect of this broadening of the receptive fields was tested in the gap-crossing task, a whisker-dependent behavioral paradigm. In this task the Fmr1 KO mice showed differences in the number of whisker contacts with platforms, decrease in the whisker sampling duration and reduction in the whisker touch-time while performing the task. We propose that the increased excitability in the somatosensory barrel cortex upon whisker stimulation may contribute to changes in the whisking strategy as well as to other observed behavioral phenotypes related to tactile processing in Fmr1 KO mice.

Unbiased, High-Throughput Electron Microscopy Analysis of Experience-Dependent Synaptic Changes in the Neocortex

Neocortical circuits can be altered by sensory and motor experience, with experimental evidence supporting both anatomical and electrophysiological changes in synaptic properties. Previous studies have focused on changes in specific neurons or pathways-for example, the thalamocortical circuitry, layer 4-3 (L4-L3) synapses, or in the apical dendrites of L5 neurons- but a broad-scale analysis of experience-induced changes across the cortical column has been lacking. Without this comprehensive approach, a full understanding of how cortical circuits adapt during learning or altered sensory input will be impossible. Here we adapt an electron microscopy technique that selectively labels synapses, in combination with a machine-learning algorithm for semiautomated synapse detection, to perform an unbiased analysis of developmental and experience-dependent changes in synaptic properties across an entire cortical column in mice. Synapse density and length were compared across development and during whisker-evoked plasticity. Between postnatal days 14 and 18, synapse density significantly increases most in superficial layers, and synapse length increases in L3 and L5B. Removal of all but a single whisker row for 24 h led to an apparent increase in synapse density in L2 and a decrease in L6, and a significant increase in length in L3. Targeted electrophysiological analysis of changes in miniature EPSC and IPSC properties in L2 pyramidal neurons showed that mEPSC frequency nearly doubled in the whisker-spared column, a difference that was highly significant. Together, this analysis shows that data-intensive analysis of column-wide changes in synapse properties can generate specific and testable hypotheses about experience-dependent changes in cortical organization.

SIGNIFICANCE STATEMENT

Development and sensory experience can change synapse properties in the neocortex. Here we use a semiautomated analysis of electron microscopy images for an unbiased, column-wide analysis of synapse changes. This analysis reveals new loci for synaptic change that can be verified by targeted electrophysiological investigation. This method can be used as a platform for generating new hypotheses about synaptic changes across different brain areas and experimental conditions.

Central Gain Restores Auditory Processing following Near-Complete Cochlear Denervation

Sensory organ damage induces a host of cellular and physiological changes in the periphery and the brain. Here, we show that some aspects of auditory processing recover after profound cochlear denervation due to a progressive, compensatory plasticity at higher stages of the central auditory pathway. Lesioning >95% of cochlear nerve afferent synapses, while sparing hair cells, in adult mice virtually eliminated the auditory brainstem response and acoustic startle reflex, yet tone detection behavior was nearly normal. As sound-evoked responses from the auditory nerve grew progressively weaker following denervation, sound-evoked activity in the cortex-and, to a lesser extent, the midbrain-rebounded or surpassed control levels. Increased central gain supported the recovery of rudimentary sound features encoded by firing rate, but not features encoded by precise spike timing such as modulated noise or speech. These findings underscore the importance of central plasticity in the perceptual sequelae of cochlear hearing impairment.

CREB Regulates Experience-Dependent Spine Formation and Enlargement in Mouse Barrel Cortex

Experience modifies synaptic connectivity through processes that involve dendritic spine rearrangements in neuronal circuits. Although cAMP response element binding protein (CREB) has a key function in spines changes, its role in activity-dependent rearrangements in brain regions of rodents interacting with the surrounding environment has received little attention so far. Here we studied the effects of vibrissae trimming, a widely used model of sensory deprivation-induced cortical plasticity, on processes associated with dendritic spine rearrangements in the barrel cortex of a transgenic mouse model of CREB downregulation (mCREB mice). We found that sensory deprivation through prolonged whisker trimming leads to an increased number of thin spines in the layer V of related barrel cortex (Contra) in wild type but not mCREB mice. In the barrel field controlling spared whiskers (Ipsi), the same trimming protocol results in a CREB-dependent enlargement of dendritic spines. Last, we demonstrated that CREB regulates structural rearrangements of synapses that associate with dynamic changes of dendritic spines. Our findings suggest that CREB plays a key role in dendritic spine dynamics and synaptic circuits rearrangements that account for new brain connectivity in response to changes in the environment.

Stimulus intensity determines experience-dependent modifications in neocortical neuron firing rates

Although subthreshold inputs of neocortical sensory neurons are broadly tuned, the spiking output is more restricted. These subthreshold inputs provide a substrate for stimulus intensity-dependent changes their spiking output, as well as for experience-dependent plasticity to alter firing properties. Here we investigated how different stimulus intensities modified the firing output of individual neurons in layer 2/3 of the mouse barrel cortex. Decreasing stimulus intensity over a 30-fold range lowered the firing rates evoked by principal whisker stimulation and reduced the overall size of the responding ensemble in whisker-undeprived animals. We then examined how these responses were changed after single-whisker experience (SWE). After 7 days of SWE, the mean magnitude of response to spared whisker stimulation at the highest stimulus intensity was not altered. However, lower-intensity whisker stimulation revealed a more than 10-fold increase in mean firing output compared with control animals. Also, under control conditions, only ∽15% of neurons showed any firing at low stimulus intensity, compared with more than 70% of neurons after SWE. However, response changes measured in the immediately surrounding representations were detected only for the highest stimulus intensity. Overall, these data showed that the measurement of experience-dependent changes in the spike output of neocortical neurons was highly dependent upon stimulus intensity.

Abnormalities in whisking behaviour are associated with lesions in brain stem nuclei in a mouse model of amyotrophic lateral sclerosis

The transgenic SOD1(G93A) mouse is a model of human amyotrophic lateral sclerosis (ALS) and recapitulates many of the pathological hallmarks observed in humans, including motor neuron degeneration in the brain and the spinal cord. In mice, neurodegeneration particularly impacts on the facial nuclei in the brainstem. Motor neurons innervating the whisker pad muscles originate in the facial nucleus of the brain stem, with contractions of these muscles giving rise to "whisking" one of the fastest movements performed by mammals. A longitudinal study was conducted on SOD1(G93A) mice and wild-type litter mate controls, comparing: (i) whisker movements using high-speed video recordings and automated whisker tracking, and (ii) facial nucleus degeneration using MRI. Results indicate that while whisking still occurs in SOD1(G93A) mice and is relatively resistant to neurodegeneration, there are significant disruptions to certain whisking behaviours, which correlate with facial nuclei lesions, and may be as a result of specific facial muscle degeneration. We propose that measures of mouse whisker movement could potentially be used in tandem with measures of limb dysfunction as biomarkers of disease onset and progression in ALS mice and offers a novel method for testing the efficacy of novel therapeutic compounds.

Rapid adult experience-dependent anatomical plasticity in layer IV of primary somatosensory cortex

Sensory deprivation, such as whisker deprivation, is one of the most common paradigms used to examine experience-dependent plasticity. Many of these studies conducted during development have demonstrated anatomical and synaptic neocortical plasticity with varying lengths of deprivation (for review, see Holtmaat and Svoboda, 2009). However, to date, there have been few studies exploring brief periods of experience-dependent neocortical plasticity in adulthood, similar to that observed from learning and memory paradigms (Siucinska and Kossut, 1996, 2004; Galvez et al., 2006; Chau et al., 2013). Examining both synapsin I and Golgi-Cox stained neurons in primary somatosensory cortex of unilaterally whisker-deprived adult mice, the current study demonstrates that 5 days of whisker deprivation results in more synapses in spared barrels and reduced synapses in deprived barrels. To our knowledge, this is the first study to characterize anatomical changes in layer IV of primary somatosensory cortex after a brief period of sensory deprivation in adulthood. Furthermore, findings from the present study suggest that analyses from prolonged periods of either sensory deprivation or stimulation during adulthood are missing forms of plasticity that could provide better insight into various cognitive processes, such as learning and memory.

Initiation, labile, and stabilization phases of experience-dependent plasticity at neocortical synapses

Alteration of sensory input can change the strength of neocortical synapses. Selective activation of a subset of whiskers is sufficient to potentiate layer 4-layer 2/3 excitatory synapses in the mouse somatosensory (barrel) cortex, a process that is NMDAR dependent. By analyzing the time course of sensory-induced synaptic change, we have identified three distinct phases for synaptic strengthening in vivo. After an early, NMDAR-dependent phase where selective whisker activation is rapidly translated into increased synaptic strength, we identify a second phase where this potentiation is profoundly reduced by an input-specific, NMDAR-dependent depression. This labile phase is transient, lasting only a few hours, and may require ongoing sensory input for synaptic weakening. Residual synaptic strength is maintained in a third phase, the stabilization phase, which requires mGluR5 signaling. Identification of these three phases will facilitate a molecular dissection of the pathways that regulate synaptic lability and stabilization, and suggest potential approaches to modulate learning.

Time-lapse electrical recordings of single neurons from the mouse neocortex

The ability of the brain to adapt to environmental demands implies that neurons can change throughout life. The extent to which single neurons actually change remains largely unstudied, however. To evaluate how functional properties of single neurons change over time, we devised a way to perform in vivo time-lapse electrophysiological recordings from the exact same neuron. We monitored the contralateral and ipsilateral sensory-evoked spiking activity of individual L2/3 neurons from the somatosensory cortex of mice. At the end of the first recording session, we electroporated the neuron with a DNA plasmid to drive GFP expression. Then, 2 wk later, we visually guided a recording electrode in vivo to the GFP-expressing neuron for the second time. We found that contralateral and ipsilateral evoked responses (i.e., probability to respond, latency, and preference), and spontaneous activity of individual L2/3 pyramidal neurons are stable under control conditions, but that this stability could be rapidly disrupted. Contralateral whisker deprivation induced robust changes in sensory-evoked response profiles of single neurons. Our experiments provide a framework for studying the stability and plasticity of single neurons over long time scales using electrophysiology.

Synaptic lability after experience-dependent plasticity is not mediated by calcium-permeable AMPARs

Activity- or experience-dependent plasticity has been associated with the trafficking of calcium-permeable α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (CP-AMPARs) in a number of experimental systems. In some cases it has been shown that CP-AMPARs are only transiently present and can be removed in an activity-dependent manner. Here we test the hypothesis that the presence of CP-AMPARs confers instability onto recently potentiated synapses. Previously we have shown that altered sensory input (single-whisker experience; SWE) strengthens layer 4-2/3 excitatory synapses in mouse primary somatosensory cortex, in part by the trafficking of CP-AMPARs. Both in vivo and in vitro, this potentiation is labile, and can be depressed by N-Methyl-D-aspartate receptor (NMDAR)-activation. In the present study, the role of CP-AMPARs in conferring this synaptic instability after in vivo potentiation was evaluated. We develop an assay to depress the strength of individual layer 4-2/3 excitatory synapses after SWE, using a strontium (Sr⁺⁺)-replaced artificial cerebrospinal fluid (ACSF) solution (Sr-depression). This method allows disambiguation of changes in quantal amplitude (a post-synaptic measure) from changes in event frequency (typically a presynaptic phenomenon). Presynaptic stimulation paired with post-synaptic depolarization in Sr⁺⁺ lead to a rapid and significant reduction in EPSC amplitude with no change in event frequency. Sr-depression at recently potentiated synapses required NMDARs, but could still occur when CP-AMPARs were not present. As a further dissociation between the presence of CP-AMPARs and Sr-depression, CP-AMPARs could be detected in some cells from control, whisker-intact animals, although Sr-depression was never observed. Taken together, our findings suggest that CP-AMPARs are neither sufficient nor necessary for synaptic depression after in vivo plasticity in somatosensory cortex. This article is part of a Special Issue entitled “Calcium permeable AMPARs in synaptic plasticity and disease.”

Input-specific critical periods for experience-dependent plasticity in layer 2/3 pyramidal neurons

Critical periods for experience-dependent plasticity have been well characterized within sensory cortex, in which the ability of altered sensory input to drive firing rate changes has been demonstrated across brain areas. Here we show that rapid experience-dependent changes in the strength of excitatory synapses within mouse primary somatosensory cortex exhibit a critical period that is input specific and mechanistically distinct in layer 2/3 pyramidal neurons. Removal of all but a single whisker [single whisker experience (SWE)] can trigger the strengthening of individual glutamatergic synaptic contacts onto layer 2/3 neurons only during a short window during the second and third postnatal week. At both layer 4 and putative 2/3 inputs, SWE-triggered plasticity has a discrete onset, before which it cannot be induced. SWE synaptic strengthening is concluded at both inputs after the beginning of the third postnatal week, indicating that both types of inputs display a critical period for experience-dependent plasticity. Importantly, the timing of this critical period is both delayed and prolonged for layer 2/3-2/3 versus layer 4-2/3 excitatory synapses. Furthermore, plasticity at layer 2/3 inputs does not invoke the trafficking of calcium-permeable, GluR2-lacking AMPA receptors, whereas it sometimes does at layer 4 inputs. The dissociation of critical period timing and plasticity mechanisms at layer 4 and layer 2/3 synapses, despite the close apposition of these inputs along the dendrite, suggests remarkable specificity for the developmental regulation of plasticity in vivo.

An embedded subnetwork of highly active neurons in the neocortex

VIDEO ABSTRACT

Unbiased methods to assess the firing activity of individual neurons in the neocortex have revealed that a large proportion of cells fire at extremely low rates (<0.1 Hz), both in their spontaneous and evoked activity. Thus, firing in neocortical networks appears to be dominated by a small population of highly active neurons. Here, we use a fosGFP transgenic mouse to examine the properties of cells with a recent history of elevated activity. FosGFP-expressing layer 2/3 pyramidal cells fired at higher rates compared to fosGFP(-) neurons, both in vivo and in vitro. Elevated activity could be attributed to increased excitatory and decreased inhibitory drive to fosGFP(+) neurons. Paired-cell recordings indicated that fosGFP(+) neurons had a greater likelihood of being connected to each other. These findings indicate that highly active, interconnected neuronal ensembles are present in the neocortex and suggest these cells may play a role in the encoding of sensory information.

PICK1 regulates incorporation of calcium-permeable AMPA receptors during cortical synaptic strengthening

While AMPA-type glutamate receptors (AMPARs) found at principal neuron excitatory synapses typically contain the GluR2 subunit, several forms of behavioral experience have been linked to the de novo synaptic insertion of calcium-permeable (CP) AMPARs, defined by their lack of GluR2. In particular, whisker experience drives synaptic potentiation as well as the incorporation of CP-AMPARs in the neocortex. Previous studies implicate PICK1 (protein interacting with C kinase-1) in activity-dependent internalization of GluR2, suggesting one potential mechanism leading to the subsequent accumulation of synaptic CP-AMPARs and increased synaptic strength. Here we test this hypothesis by using a whisker stimulation paradigm in PICK1 knock-out mice. We demonstrate that PICK1 facilitates the surface expression of CP-AMPARs and is indispensable for their experience-dependent synaptic insertion. However, the failure to incorporate CP-AMPARs in PICK1 knock-outs does not preclude sensory-induced enhancement of synaptic currents. Our results indicate that synaptic strengthening in the early postnatal cortex does not require PICK1 or the addition of GluR2-lacking AMPARs. Instead, PICK1 permits changes in AMPAR subunit composition to occur in conjunction with synaptic potentiation.

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

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

Reliable and precise neuronal firing during sensory plasticity in superficial layers of primary somatosensory cortex

Neocortical neurons show astonishing variation in the presence and timing of action potentials across stimulus trials, a phenomenon whose function and significance has been the subject of great interest. Here we present data showing that this response variability can be significantly reduced by altered sensory experience. Removal of all but one whisker from the side of the mouse face results in the rapid (within 24 h) potentiation of mean firing rates within the cortical representation of the spared whisker in young postnatal animals (postnatal days 13-16). Analysis of single-unit responses from whisker-spared animals shows that this potentiation can be attributed to an enhancement of trial-to-trial reliability (i.e., reduced response failures), as well as an increase in the mean number of spikes evoked within a successful trial. Changes were confined to superficial layers 2/3 and were not observed in the input layer of the cortex, layer 4. In addition to these changes in firing rates, we also observed profound changes in the precise timing of sensory-evoked responses. Trial-to-trial temporal precision was enhanced and the absolute latency of responses was reduced after single-whisker experience. Enhanced spike-timing precision and trial-to-trial reliability could also be triggered in adolescent animals with longer periods (7 d) of single-whisker experience. These experiments provide a quantitative analysis of how sensory experience can enhance both reliability and temporal precision in neocortical neurons and provide a framework for testing specific hypotheses about the role of response variability in cortical function and the molecular mechanisms underlying this phenomenon.

Intrinsic signal imaging of deprivation-induced contraction of whisker representations in rat somatosensory cortex

In classical sensory cortical map plasticity, the representation of deprived or underused inputs contracts within cortical sensory maps, whereas spared inputs expand. Expansion of spared inputs occurs preferentially into nearby cortical columns representing temporally correlated spared inputs, suggesting that expansion involves correlation-based learning rules at cross-columnar synapses. It is unknown whether deprived representations contract in a similar anisotropic manner, which would implicate similar learning rules and sites of plasticity. We briefly deprived D-row whiskers in 20-day-old rats, so that each deprived whisker had deprived (D-row) and spared (C- and E-row) neighbors. Intrinsic signal optical imaging revealed that D-row deprivation weakened and contracted the functional representation of deprived D-row whiskers in L2/3 of somatosensory (S1) cortex. Spared whisker representations did not strengthen or expand, indicating that D-row deprivation selectively engages the depression component of map plasticity. Contraction of deprived whisker representations was spatially uniform, with equal withdrawal from spared and deprived neighbors. Single-unit electrophysiological recordings confirmed these results, and showed substantial weakening of responses to deprived whiskers in layer 2/3 of S1, and modest weakening in L4. The observed isotropic contraction of deprived whisker representations during D-row deprivation is consistent with plasticity at intracolumnar, rather than cross-columnar, synapses.

Experience--a double edged sword for restorative neural plasticity after brain damage

During the time period following damage, the brain undergoes widespread reorganizational processes. Manipulations of behavioral experience can be potent therapeutic interventions for shaping this reorganization and enhancing long-term functional outcome. Recovery of function is a major concern for survivors of central nervous system damage and management of post-injury rehabilitation is increasingly becoming a topic of chief importance. Animal research, the focus of this review, suggests that, in the absence of behavioral manipulations, the brain is unlikely to realize its full potential for supporting function. However, experiences also have the capacity to be maladaptive for brain and behavioral function. From a treatment perspective, it may be unwise to adopt the canon of "first, do no harm" because maladaptive experiences include behaviors that individuals learn to do on their own. A better understanding of how behavioral experience interacts with brain reorganization could result in rehabilitative therapies, individually tailored and optimized for functional outcome.

Maladaptive effects of learning with the less-affected forelimb after focal cortical infarcts in rats

It is common following stroke to focus early rehabilitation efforts on developing compensatory use of the less-affected body side. Here we used a rat model of focal cortical infarct to examine how motor skill acquisition with the less-affected ("intact") forelimb influences sensorimotor function of the infarct-impaired forelimb and neural activity in peri-infarct cortex. Rats proficient in skilled reaching with one forelimb were given focal ischemic lesions in the contralateral sensorimotor cortex (SMC). Recovery in this forelimb was tested following a period of reach training focused on the intact forelimb or control procedures. Quantitative measures of the cumulatively expressed transcription factor, FosB/DeltaFosB, were used to assay intact forelimb training effects on neuronal activity in remaining SMC of the infarcted hemisphere. Intact forelimb training worsened behavioral recovery in the impaired forelimb following unilateral focal ischemia. Furthermore, it decreased neuronal FosB/DeltaFosB expression in layer II/III of peri-infarct SMC. These effects were not found in sham-operated rats trained sequentially with both forelimbs or in animals receiving bilateral forelimb training after unilateral infarcts. Thus, focused use of the intact forelimb has detrimental effects on recovery of impaired forelimb function following a focal ischemic injury and this is linked to reduced neuronal activation in remaining cortex. These results suggest that peri-infarct cortex becomes vulnerable to early post-stroke experience with the less-affected forelimb and that this experience may drive neural plasticity here in a direction that is maladaptive for functional outcome.

Ongoing in vivo experience triggers synaptic metaplasticity in the neocortex

In vivo experience can occlude subsequent induction of long-term potentiation and enhance long-term depression of synaptic responses. Although a reduced capacity for synaptic strengthening may function to prevent excessive excitation, such an effect paradoxically implies that continued experience or training should not improve and may even degrade neural representations. In mice, we examined the effect of ongoing whisker stimulation on synaptic strengthening at layer 4-2/3 synapses in the barrel cortex. Although N-methyl-d-aspartate receptors were required to initiate strengthening, they subsequently suppressed further potentiation at these synapses in vitro and in vivo. Despite this transition, synaptic strengthening continued with additional sensory activity but instead required the activation of metabotropic glutamate receptors, suggesting a mechanism by which continued experience can result in increasing synaptic strength over time.