Experience-Dependent Plasticity in Adult Visual Cortex

Effectively reducing sensory eye dominance with a push-pull perceptual learning protocol

Much knowledge of sensory cortical plasticity is gleaned from perceptual learning studies that improve visual performance [1-7]. Although the improvements are likely caused by modifications of excitatory and inhibitory neural networks, most studies were not primarily designed to differentiate their relative contributions. Here we designed a novel push-pull training protocol to reduce sensory eye dominance (SED), a condition that is mainly caused by unbalanced interocular inhibition [8-10]. During the training, an attention cue presented to the weak eye precedes the binocular competitive stimulation. The cue stimulates the weak eye (push) while causing interocular inhibition of the strong eye (pull). We found that this push-pull protocol reduces SED (shifts the balance toward the weak eye) and improves stereopsis more so than the push-only protocol, which solely stimulates the weak eye without inhibiting the strong eye. The stronger learning effect with the push-pull training than the push-only training underscores the crucial involvement of a putative inhibitory mechanism in sensory plasticity. The design principle of the push-pull protocol can potentially lend itself as an effective, noninvasive treatment of amblyopia.

Social vision: sustained perceptual enhancement of affective facial cues in social anxiety

Heightened perception of facial cues is at the core of many theories of social behavior and its disorders. In the present study, we continuously measured electrocortical dynamics in human visual cortex, as evoked by happy, neutral, fearful, and angry faces. Thirty-seven participants endorsing high versus low generalized social anxiety (upper and lower tertiles of 2104 screened undergraduates) viewed naturalistic faces flickering at 17.5 Hz to evoke steady-state visual evoked potentials (ssVEPs), recorded from 129 scalp electrodes. Electrophysiological data were evaluated in the time-frequency domain after linear source space projection using the minimum norm method. Source estimation indicated an early visual cortical origin of the face-evoked ssVEP, which showed sustained amplitude enhancement for emotional expressions specifically in individuals with pervasive social anxiety. Participants in the low symptom group showed no such sensitivity, and a correlational analysis across the entire sample revealed a strong relationship between self-reported interpersonal anxiety/avoidance and enhanced visual cortical response amplitude for emotional, versus neutral expressions. This pattern was maintained across the 3500 ms viewing epoch, suggesting that temporally sustained, heightened perceptual bias towards affective facial cues is associated with generalized social anxiety.

Distinct trafficking and expression mechanisms underlie LTP and LTD of NMDA receptor-mediated synaptic responses

Although an increasing number of studies have demonstrated the plasticity of NMDA receptor-mediated synaptic transmission, little is known about the molecular mechanisms that underlie this neurologically important process. In a study of NMDAR-mediated synaptic responses in hippocampal Schaffer-CA1 synapses whose AMPA receptor (AMPAR) activity is totally blocked, we uncovered differences between the trafficking mechanisms that underlie the long-term potentiation (LTP) and long-term depression (LTD) that can be induced in these cells under these conditions. The LTP-producing protocol failed to induce a change in the amplitude of NMDAR-mediated postsynaptic currents (NMDAR EPSCs) in the first 5-10 min, but induced gradual enhancement of NMDAR EPSCs thereafter that soon reached a stable magnitude. This "slow" LTP of NMDAR EPSCs (LTP(NMDA)) was blocked by inhibiting exocytosis or actin polymerization in postsynaptic cells. By contrast, LTD of NMDAR EPSCs (LTD(NMDA)) was immediately inducible, and, although it was blocked by the actin stabilizer, it was unaffected by exocytosis or endocytosis inhibitors. Furthermore, concomitant changes in the decay time of NMDAR EPSCs suggested that differential switches in NR2 subunit composition accompanied LTP(NMDA) and LTD(NMDA), and these changes were blocked by the calcium buffer BAPTA or an mGluR antagonist. Our results suggest that LTP(NMDA) and LTD(NMDA) utilize different NMDAR trafficking pathways and express different ratios of NMDAR subunits on the postsynaptic surface.

How the timing and quality of early experiences influence the development of brain architecture

Early life events can exert a powerful influence on both the pattern of brain architecture and behavioral development. In this study a conceptual framework is provided for considering how the structure of early experience gets "under the skin." The study begins with a description of the genetic framework that lays the foundation for brain development, and then proceeds to the ways experience interacts with and modifies the structures and functions of the developing brain. Much of the attention is focused on early experience and sensitive periods, although it is made clear that later experience also plays an important role in maintaining and elaborating this early wiring diagram, which is critical to establishing a solid footing for development beyond the early years.

Neural dynamics of in vitro cortical networks reflects experienced temporal patterns

Learning ultimately relies on changes in the flow of activity within neural microcircuits. Plasticity of neural dynamics is particularly relevant for the processing of temporal information. Chronic stimulation of cultured rat cortical networks revealed ‘experience-dependent’ plasticity in neural dynamics. We observed changes in the temporal structure of activity that reflected the intervals used during training, suggesting that cortical circuits are inherently capable of temporal processing on short time scales.

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

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

Perceptual learning improves contrast sensitivity of V1 neurons in cats

BACKGROUND

Perceptual learning has been documented in adult humans over a wide range of tasks. Although the often-observed specificity of learning is generally interpreted as evidence for training-induced plasticity in early cortical areas, physiological evidence for training-induced changes in early visual cortical areas is modest, despite reports of learning-induced changes of cortical activities in fMRI studies. To reveal the physiological bases of perceptual learning, we combined psychophysical measurements with extracellular single-unit recording under anesthetized preparations and examined the effects of training in grating orientation identification on both perceptual and neuronal contrast sensitivity functions of cats.

RESULTS

We have found that training significantly improved perceptual contrast sensitivity of the cats to gratings with spatial frequencies near the "trained" spatial frequency, with stronger effects in the trained eye. Consistent with behavioral assessments, the mean contrast sensitivity of neurons recorded from V1 of the trained cats was significantly higher than that of neurons recorded from the untrained cats. Furthermore, in the trained cats, the contrast sensitivity of V1 neurons responding preferentially to stimuli presented via the trained eyes was significantly greater than that of neurons responding preferentially to stimuli presented via the "untrained" eyes. The effect was confined to the trained spatial frequencies. In both trained and untrained cats, the neuronal contrast sensitivity functions derived from the contrast sensitivity of the individual neurons were highly correlated with behaviorally determined perceptual contrast sensitivity functions.

CONCLUSIONS

We suggest that training-induced neuronal contrast gain in area V1 underlies behaviorally determined perceptual contrast sensitivity improvements.

Advances in visual perceptual learning and plasticity

Visual perceptual learning (VPL) is defined as a long-term improvement in performance on a visual task. In recent years, the idea that conscious effort is necessary for VPL to occur has been challenged by research suggesting the involvement of more implicit processing mechanisms, such as reinforcement-driven processing and consolidation. In addition, we have learnt much about the neural substrates of VPL and it has become evident that changes in visual areas and regions beyond the visual cortex can take place during VPL.

Plasticity between neuronal pairs in layer 4 of visual cortex varies with synapse state

In neocortex, the induction and expression of long-term potentiation (LTP) and long-term depression (LTD) vary depending on cortical area and laminae of presynaptic and postsynaptic neurons. Layer 4 (L4) is the initial site of sensory afference in barrel cortex and primary visual cortex (V1) in which excitatory inputs from thalamus, L6, and neighboring L4 cells are integrated. However, little is known about plasticity within L4. We studied plasticity at excitatory synaptic connections between pairs and triplets of interconnected L4 neurons in guinea pig V1 using a fixed delay pairing protocol. Plasticity outcomes were heterogeneous, with some connections undergoing LTP (n = 7 of 42), some LTD (n = 19 of 42), and some not changing (n = 16 of 42). Although quantal analysis revealed both presynaptic and postsynaptic plasticity expression components, reduction in quantal size (a postsynaptic property) contributing to LTD was ubiquitous, whereas in some cell pairs, this change was overridden by an increase in the probability of neurotransmitter release (a presynaptic property) resulting in LTP. These changes depended on the initial reliability of the connections: highly reliable connections depressed with contributions from presynaptic and postsynaptic effects, and unreliable connections potentiated as a result of the predominance of presynaptic enhancement. Interestingly, very strong, reliable pairs of connected cells showed little plasticity. Pairs of connected cells with a common presynaptic or postsynaptic L4 cell behaved independently, undergoing plasticity of different or opposite signs. Release probability of a connection with initial 100% failure rate was enhanced after pairing, potentially avoiding silencing of the presynaptic terminal and maintaining L4-L4 synapses in a broader dynamic range.

Characterization of the transcripts and protein isoforms for cytoplasmic polyadenylation element binding protein-3 (CPEB3) in the mouse retina

Background

Cytoplasmic polyadenylation element binding proteins (CPEBs) regulate translation by binding to regulatory motifs of defined mRNA targets. This translational mechanism has been shown to play a critical role in oocyte maturation, early development, and memory formation in the hippocampus. Little is known about the presence or functions of CPEBs in the retina. The purpose of the current study is to investigate the alternative splicing isoforms of a particular CPEB, CPEB3, based on current databases, and to characterize the expression of CPEB3 in the retina.

Results

In this study, we have characterized CPEB3, whose putative role is to regulate the translation of GluR2 mRNA. We identify the presence of multiple alternative splicing isoforms of CPEB3 transcripts and proteins in the current databases. We report the presence of eight alternative splicing patterns of CPEB3, including a novel one, in the mouse retina. All but one of the patterns appear to be ubiquitous in 13 types of tissue examined. The relative abundance of the patterns in the retina is demonstrated. Experimentally, we show that CPEB3 expression is increased in a time-dependent manner during the course of postnatal development, and CPEB3 is localized mostly in the inner retina, including retinal ganglion cells.

Conclusion

The level of CPEB3 was up-regulated in the retina during development. The presence of multiple CPEB3 isoforms indicates remarkable complexity in the regulation and function of CPEB3.

Plasticity of fixation in patients with central vision loss

The aim of this study was to explore the plasticity of fixation in patients with central vision loss. Most of these patients use preferred retinal loci (PRLs) in the healthy eccentric part of the retina to fixate, but fixation stability and retinal location are not always optimal for best visual performance. This study examined whether fixation stability and a new PRL location can be trained and whether these changes in ocular motor control transfer into better reading performance. Six patients with age-related macular degeneration participated in the study. Fixation stability measurements, microperimetry, and auditory biofeedback training were performed with the MP-1 microperimeter. The auditory biofeedback was used during five 1-h long training sessions to improve fixation and relocate the PRL. Fixation location and stability were recorded while viewing four different targets: a cross, a letter, a word, and a nine-cycle radial grating. Visual acuity was assessed with the Early Treatment Diabetic Retinopathy Study (ETDRS) chart and reading performance with the MNRead test. The results showed that all patients developed a new PRL in an optimal location for reading, and they were able to use it consistently while viewing different targets. Fixation stability improved 53% after training. Learning transferred to the old PRL even though fixation stability at this location was not trained. All these improvements in ocular motor control translated into better reading performance: reading speed improved 38% and reading acuity and critical print size gained two lines. We conclude that the ability of the ocular motor system to fixate is flexible in patients with central vision loss: a new PRL can be trained, fixation stability can be improved, and learning transfers to an untrained location. These gains in ocular motor control result in better visual performance. This property can be successfully used to optimize the residual vision of patients with central vision loss.

Visual cortex plasticity evokes excitatory alterations in the hippocampus

The integration of episodic sequences in the hippocampus is believed to occur during theta rhythm episodes, when cortico-hippocampal dialog results in reconfiguration of neuronal assemblies. As the visual cortex (VC) is a major source of sensory information to the hippocampus, information processing in the cortex may affect hippocampal network oscillations, facilitating the induction of synaptic modifications. We investigated to what degree the field activity in the primary VC, elicited by sensory or electrical stimulation, correlates with hippocampal oscillatory and synaptic responsiveness, in freely behaving adult rats. We found that the spectral power of theta rhythm (4–10 Hz) in the dentate gyrus (DG), increases in parallel with high-frequency oscillations in layer 2/3 of the VC and that this correlation depends on the degree of exploratory activity. When we mimic robust thalamocortical activity by theta-burst application to dorsal lateral geniculate nucleus, a hippocampal theta increase occurs, followed by a persistent potentiation of the DG granule field population spike. Furthermore, the potentiation of DG neuronal excitability tightly correlates with the concurrently occurring VC plasticity. The concurrent enhancement of VC and DG activity is also combined with a highly negative synchronization between hippocampal and cortical low-frequency oscillations. Exploration of familiar environment decreases the degree of this synchrony. Our data propose that novel visual information can induce high-power fluctuations in intrinsic excitability for both VC and hippocampus, potent enough to induce experience-dependent modulation of cortico-hippocampal connections. This interaction may comprise one of the endogenous triggers for long-term synaptic plasticity in the hippocampus.

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.

Acceleration of pentylenetetrazol seizure kindling associated with induction of sensitized visual responses evoked by strobe stimulation

Exposure of normal adult rats of a variety of species to trains of light flashes leads to acquisition of an enduring high amplitude visual cortical response [Uhlrich DJ, Manning KA, O'Laughlin ML, Lytton WW (2005) Photic-induced sensitization: acquisition of an augmenting spike-wave response in the adult rat through repeated strobe exposure. J Neurophysiol 94:3925-3937]. The photically-induced sensitized response exhibits epileptiform characteristics, including spike-wave morphology, tendency to generalize across the brain, and sensitivity to the anti-epileptic drug ethosuximide. These findings and anecdotal clinical reports raise the possibility that certain sensory stimulation could induce neural plastic changes that affect seizures in some individuals. We hypothesize that photic-induced sensitization can prime seizure-related neural circuitry, resulting in exacerbation of seizures. To test this we compared seizure kindling rates using the pentylenetetrazol (PTZ) model of epileptogenesis in sensitized and unsensitized adult Sprague-Dawley rats. Experimental group rats were sensitized by exposure to repetitive stroboscopic stimulation over 4-6 days until the sensitized photic response fully developed and response magnitude stabilized at its highest plateau. Rats then received a sub-convulsive injection of PTZ (24 mg/kg i.p.) every other day until they attained class 5 seizures. Control rats were not strobed or sensitized, but were otherwise treated identically. Chronic electrodes overlying the dura in occipital cortex recorded the primary visual response. Similar electrodes near the border of somatosensory and motor cortex (SM) were used to record spread of the sensitized response to a patently non-visual region. Rat behavior was monitored by direct observation and digital audio/video recording. All control rats and seven of 14 photically sensitized rats kindled seizures at rates consistent with those reported previously. However, the seven other photically sensitized rats displayed markedly accelerated seizure kindling. Rats with accelerated kindling showed greater spread of the sensitized visual response to somato-motor cortex and, when tested in a post hoc experiment, exhibited a higher likelihood of photo-triggered seizures. These results indicate that photic-induced sensitization in susceptible individuals can prime neural circuitry involved in the generation of PTZ-kindled seizures.

Sensory Experience and Cortical Rewiring

Adult primary sensory cortex is not hard wired, but adapts to sensory experience. The cellular basis for cortical plasticity involves a combination of functional and structural changes in cortical neurons and the connections between them. Functional changes such as synaptic strengthening have been the focus of many investigations. However, structural modifications to the connections between neurons play an important role in cortical plasticity. In this review, the authors focus on structural remodeling that leads to rewiring of cortical circuits. Recent work has identified axonal remodeling, growth of new dendritic spines, and synapse turnover as important structural mechanisms for experience-dependent plasticity in mature cortex. These findings have begun to unravel how rewiring occurs in adult neocortex and offer new insights into the cellular mechanisms for learning and memory.

Working memory representation in atypical language dominance

One of the most important factors controlling material specific processing in the human brain is language dominance, i.e. hemispheric specialization in semantic processes. Although previous studies have shown that lateralized long-term memory processes in the medial temporal lobes are modified in subjects with atypical (right) language dominance, the effect of language dominance on the neural basis of working memory (WM) has remained unknown. Here, we used functional MRI (fMRI) to study the impact of language dominance on the neural representation of WM. We conducted an n-back task in three different load conditions and with both verbal and nonverbal (spatial) material in matched groups of left and right language dominant subjects. This approach allowed us to investigate regions showing significant interactions between language dominance and material. Overall, right dominant subjects showed an increased inter-individual variability of WM-related activations. Verbal WM involved more pronounced activation of the left fusiform cortex in left dominant subjects and of the right inferior parietal lobule in the right dominant group. Spatial WM, on the other hand, induced activation of right hemispheric regions in left dominant subjects, but no specific activations in right dominant subjects. Taken together, these findings indicate that the neural basis of verbal WM processes depends on language dominance and is more mutable in right dominant subjects. The increased variability in right dominant subjects strongly suggests that a standard network of material-dependent WM processes exists in left dominant subjects, and that right dominant subjects use variable alternative networks.

Short-term (2 to 5 h) dark exposure lowers long-term potentiation (LTP) induction threshold in rat primary visual cortex

Up- and down-regulation of synaptic strength (i.e., long-term potentiation, LTP, long-term depression) is thought to be the primary mechanism mediating experience-dependent plasticity of cortical networks. Recent evidence indicates that the expression of plastic changes at synapses itself is dynamic and regulated, at least in part, by the recent history of synaptic activity, a concept termed metaplasticity. Here, adult, urethane-anesthetized rats were exposed to light or dark conditions for various durations (1, 2, and 5 h) to influence activity levels in the retinal-dorsal lateral geniculate nucleus (dLGN)-primary visual cortex (V1) pathway. Field potentials, recorded in layer IV of V1, were evoked by light flashes to the retina or single pulse electrical stimulation of the dLGN. Brief (60 s) periods of high frequency (50 Hz) retinal light stimulation results in an increase in visual evoked potential (VEP) amplitude in animals exposed to complete darkness for 2 h, while VEP amplitude failed to show potentiation in animals maintained in darkness for shorter periods. Similarly, weak theta burst stimulation of the dLGN failed to induce LTP in animals maintained under continuous light, but elicited robust LTP after 5 h of dark exposure. These data demonstrate that induction thresholds for sensory- and electrically-induced LTP in the retino-geniculo-cortical pathway of adult rats are dynamically regulated by levels of preceding sensory stimulation. Importantly, such metaplastic adjustments of plasticity in V1 can occur over time-scales significantly shorter than previously recognized.

State dependence of network output: modeling and experiments

Emerging experimental evidence suggests that both networks and their component neurons respond to similar inputs differently, depending on the state of network activity. The network state is determined by the intrinsic dynamical structure of the network and may change as a function of neuromodulation, the balance or stochasticity of synaptic inputs to the network, and the history of network activity. Much of the knowledge on state-dependent effects comes from comparisons of awake and sleep states of the mammalian brain. Yet, the mechanisms underlying these states are difficult to unravel. Several vertebrate and invertebrate studies have elucidated cellular and synaptic mechanisms of state dependence resulting from neuromodulation, sensory input, and experience. Recent studies have combined modeling and experiments to examine the computational principles that emerge when network state is taken into account; these studies are highlighted in this article. We discuss these principles in a variety of systems (mammalian, crustacean, and mollusk) to demonstrate the unifying theme of state dependence of network output.

State-dependent computations: spatiotemporal processing in cortical networks

A conspicuous ability of the brain is to seamlessly assimilate and process spatial and temporal features of sensory stimuli. This ability is indispensable for the recognition of natural stimuli. Yet, a general computational framework for processing spatiotemporal stimuli remains elusive. Recent theoretical and experimental work suggests that spatiotemporal processing emerges from the interaction between incoming stimuli and the internal dynamic state of neural networks, including not only their ongoing spiking activity but also their 'hidden' neuronal states, such as short-term synaptic plasticity.

Distinctive features of adult ocular dominance plasticity

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

JNK1 regulates histone acetylation in trigeminal neurons following chemical stimulation

Trigeminal nerve fibers in nasal and oral cavities are sensitive to various environmental hazardous stimuli, which trigger many neurotoxic problems such as chronic migraine headache and trigeminal irritated disorders. However, the role of JNK kinase cascade and its epigenetic modulation of histone remodeling in trigeminal ganglion (TG) neurons activated by environmental neurotoxins remains unknown. Here we investigated the role of JNK/c-Jun cascade in the regulation of acetylation of H3 histone in TG neurons following in vitro stimulation by a neuro-inflammatory agent, mustard oil (MO). We found that MO stimulation elicited JNK/c-Jun pathway significantly by enhancing phospho-JNK1, phospho-c-Jun expression, and c-Jun activity, which were correlated with an elevated acetylated H3 histone in TG neurons. However, increases in phospho-c-Jun and c-Jun activity were significantly blocked by a JNK inhibitor, SP600125. We also found that altered H3 histone remodeling, assessed by H3 acetylation in triggered TG neurons, was reduced by SP600125. The study suggests that the activated JNK signaling in regulation of histone remodeling may contribute to neuro-epigentic changes in peripheral sensory neurons following environmental neurotoxic exposure.

Spike timing-dependent plasticity: a Hebbian learning rule

Spike timing-dependent plasticity (STDP) as a Hebbian synaptic learning rule has been demonstrated in various neural circuits over a wide spectrum of species, from insects to humans. The dependence of synaptic modification on the order of pre- and postsynaptic spiking within a critical window of tens of milliseconds has profound functional implications. Over the past decade, significant progress has been made in understanding the cellular mechanisms of STDP at both excitatory and inhibitory synapses and of the associated changes in neuronal excitability and synaptic integration. Beyond the basic asymmetric window, recent studies have also revealed several layers of complexity in STDP, including its dependence on dendritic location, the nonlinear integration of synaptic modification induced by complex spike trains, and the modulation of STDP by inhibitory and neuromodulatory inputs. Finally, the functional consequences of STDP have been examined directly in an increasing number of neural circuits in vivo.

A method for chronic stimulation of cortical organotypic cultures using implanted electrodes

The neural mechanisms underlying some forms of learning and memory require hours or days to be expressed; however it has proven difficult to study these slowly developing forms of plasticity in reduced preparations due to the short-term nature of acute slice preparations and the fact that most culture preparations lack exposure to structured external input, which plays a critical role in normal cortical development and plasticity. To address this limitation, we developed a method for chronic stimulation of organotypic slice cultures using implanted microelectrodes. This method imparts the ability to apply patterned stimulation to cortical tissue for hours or days, and allows intracellular electrophysiological recordings before and after the stimulation. Importantly, the permanent implantation of the electrodes in the tissue assures that the same neuronal pathways are being excited both during the chronic stimulation while the cultures are in the incubator and while recording in the testing phase. This technique establishes a reduced model for studying experience-dependent plasticity.

Altered sensory experience induces targeted rewiring of local excitatory connections in mature neocortex

Experience-dependent plasticity in adulthood is slower than during development. Previous experience can accelerate adult cortical plasticity. However, the contributions of functional synaptic changes and modifications in neuronal structure to the acceleration of adult cortical plasticity remain unclear. If structural remodeling was important then it should be exhibited by neuronal connections that have altered during plasticity. We trimmed rodents' whiskers to induce experience-dependent plasticity and reconstructed pairs of layer 2/3 (L2/3) pyramidal neurons after electrophysiological recording. We reported recently that local excitatory connections strengthen without a change in synapse number in cortex with retained sensory input (spared) (Cheetham et al., 2007). Here, we show that strengthened connections are rewired. The rewiring involves remodeling of the axonal arbor of excitatory connections with only minor changes in postsynaptic dendritic trees. The axonal remodeling resulted in a greater length of presynaptic axon close to postsynaptic dendrites at existing local excitatory connections in spared cortex. In control cortex, the length of axon close to dendrite in unconnected pairs of L2/3 pyramidal neurons was similar to that in synaptically connected pairs of L2/3 pyramidal neurons. This finding suggests that the probability of forming a synapse and, therefore, establishing a connection, is not driven solely by the length of axon close to dendrite. The axonal remodeling that we describe is not associated with altered synapse number, but instead increases the number of sites where synapses could be formed between synaptically connected neurons with minimal structural changes. This enables rapid and cost-efficient rewiring of local excitatory connections when re-exposed to similarly altered sensory experience in adulthood.

Synaptic plasticity from visual cortex to hippocampus: systems integration in spatial information processing

The adult cerebral cortex possesses the remarkable ability to change its neuronal connectivity through experience, a phenomenon termed "synaptic plasticity." Synaptic plasticity constitutes a cellular mechanism that is thought to underlie information storage and memory formation in the brain, and represents a use-dependent long-lasting increase or decrease in synaptic strength. Recent findings, that the adult visual cortex undergoes dynamic synaptic plasticity that is driven by active visual experience, suggest that it may be involved in information processing that could contribute to memory formation. The visual cortex provides a crucial sensory input to the hippocampus, and is a key component for the creation of spatial memories. An understanding of how visual cortical neurons respond with synaptic plasticity to visual experience, and whether these responses influence the induction of hippocampal plasticity, is fundamental to our understanding of the neuronal mechanisms and functional consequences of visuospatial information processing. In this review, we summarize recent findings with regard to the expression of dynamic synaptic plasticity in the visual cortex and how this plasticity may influence information processing in the hippocampus.

Layer 2/3 synapses in monocular and binocular regions of tree shrew visual cortex express mAChR-dependent long-term depression and long-term potentiation

Acetylcholine is an important modulator of synaptic efficacy and is required for learning and memory tasks involving the visual cortex. In rodent visual cortex, activation of muscarinic acetylcholine receptors (mAChRs) induces a persistent long-term depression (LTD) of transmission at synapses recorded in layer 2/3 of acute slices. Although the rodent studies expand our knowledge of how the cholinergic system modulates synaptic function underlying learning and memory, they are not easily extrapolated to more complex visual systems. Here we used tree shrews for their similarities to primates, including a visual cortex with separate, defined regions of monocular and binocular innervation, to determine whether mAChR activation induces long-term plasticity. We find that the cholinergic agonist carbachol (CCh) not only induces long-term plasticity, but the direction of the plasticity depends on the subregion. In the monocular region, CCh application induces LTD of the postsynaptic potential recorded in layer 2/3 that requires activation of m3 mAChRs and a signaling cascade that includes activation of extracellular signal-regulated kinase (ERK) 1/2. In contrast, layer 2/3 postsynaptic potentials recorded in the binocular region express long-term potentiation (LTP) following CCh application that requires activation of m1 mAChRs and phospholipase C. Our results show that activation of mAChRs induces long-term plasticity at excitatory synapses in tree shrew visual cortex. However, depending on the ocular inputs to that region, variation exists as to the direction of plasticity, as well as to the specific mAChR and signaling mechanisms that are required.

A Drosophila gain-of-function screen for candidate genes involved in steroid-dependent neuroendocrine cell remodeling

The normal functioning of neuroendocrine systems requires that many neuropeptidergic cells change, to alter transmitter identity and concentration, electrical properties, and cellular morphology in response to hormonal cues. During insect metamorphosis, a pulse of circulating steroids, ecdysteroids, governs the dramatic remodeling of larval neurons to serve adult-specific functions. To identify molecular mechanisms underlying metamorphic remodeling, we conducted a neuropeptidergic cell-targeted, gain-of-function genetic screen. We screened 6097 lines. Each line permitted Gal4-regulated transcription of flanking genes. A total of 58 lines, representing 51 loci, showed defects in neuropeptide-mediated developmental transitions (ecdysis or wing expansion) when crossed to the panneuropeptidergic Gal4 driver, 386Y-Gal4. In a secondary screen, we found 29 loci that produced wing expansion defects when crossed to a crustacean cardioactive peptide (CCAP)/bursicon neuron-specific Gal4 driver. At least 14 loci disrupted the formation or maintenance of adult-specific CCAP/bursicon cell projections during metamorphosis. These include components of the insulin and epidermal growth factor signaling pathways, an ecdysteroid-response gene, cabut, and an ubiquitin-specific protease gene, fat facets, with known functions in neuronal development. Several additional genes, including three micro-RNA loci and two factors related to signaling by Myb-like proto-oncogenes, have not previously been implicated in steroid signaling or neuronal remodeling.

Synaptic plasticity: multiple forms, functions, and mechanisms

Experiences, whether they be learning in a classroom, a stressful event, or ingestion of a psychoactive substance, impact the brain by modifying the activity and organization of specific neural circuitry. A major mechanism by which the neural activity generated by an experience modifies brain function is via modifications of synaptic transmission; that is, synaptic plasticity. Here, we review current understanding of the mechanisms of the major forms of synaptic plasticity at excitatory synapses in the mammalian brain. We also provide examples of the possible developmental and behavioral functions of synaptic plasticity and how maladaptive synaptic plasticity may contribute to neuropsychiatric disorders.

A synaptic memory trace for cortical receptive field plasticity

Receptive fields of sensory cortical neurons are plastic, changing in response to alterations of neural activity or sensory experience. In this way, cortical representations of the sensory environment can incorporate new information about the world, depending on the relevance or value of particular stimuli. Neuromodulation is required for cortical plasticity, but it is uncertain how subcortical neuromodulatory systems, such as the cholinergic nucleus basalis, interact with and refine cortical circuits. Here we determine the dynamics of synaptic receptive field plasticity in the adult primary auditory cortex (also known as AI) using in vivo whole-cell recording. Pairing sensory stimulation with nucleus basalis activation shifted the preferred stimuli of cortical neurons by inducing a rapid reduction of synaptic inhibition within seconds, which was followed by a large increase in excitation, both specific to the paired stimulus. Although nucleus basalis was stimulated only for a few minutes, reorganization of synaptic tuning curves progressed for hours thereafter: inhibition slowly increased in an activity-dependent manner to rebalance the persistent enhancement of excitation, leading to a retuned receptive field with new preference for the paired stimulus. This restricted period of disinhibition may be a fundamental mechanism for receptive field plasticity, and could serve as a memory trace for stimuli or episodes that have acquired new behavioural significance.

NMDA receptor trafficking in synaptic plasticity and neuropsychiatric disorders

The number and subunit composition of synaptic N-methyl-D-aspartate receptors (NMDARs) are not static, but change in a cell- and synapse-specific manner during development and in response to neuronal activity and sensory experience. Neuronal activity drives not only NMDAR synaptic targeting and incorporation, but also receptor retrieval, differential sorting into the endosomal-lysosomal pathway and lateral diffusion between synaptic and extrasynaptic sites. An emerging concept is that activity-dependent, bidirectional regulation of NMDAR trafficking provides a dynamic and potentially powerful mechanism for the regulation of synaptic efficacy and remodelling, which, if dysregulated, can contribute to neuropsychiatric disorders such as cocaine addiction, Alzheimer's disease and schizophrenia.

Developmental Downregulation of Histone Posttranslational Modifications Regulates Visual Cortical Plasticity

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