Experience-dependent switch in sign and mechanisms for plasticity in layer 4 of primary visual cortex

Metastable dynamics of neural circuits and networks

Cortical neurons emit seemingly erratic trains of action potentials or "spikes," and neural network dynamics emerge from the coordinated spiking activity within neural circuits. These rich dynamics manifest themselves in a variety of patterns, which emerge spontaneously or in response to incoming activity produced by sensory inputs. In this Review, we focus on neural dynamics that is best understood as a sequence of repeated activations of a number of discrete hidden states. These transiently occupied states are termed "metastable" and have been linked to important sensory and cognitive functions. In the rodent gustatory cortex, for instance, metastable dynamics have been associated with stimulus coding, with states of expectation, and with decision making. In frontal, parietal, and motor areas of macaques, metastable activity has been related to behavioral performance, choice behavior, task difficulty, and attention. In this article, we review the experimental evidence for neural metastable dynamics together with theoretical approaches to the study of metastable activity in neural circuits. These approaches include (i) a theoretical framework based on non-equilibrium statistical physics for network dynamics; (ii) statistical approaches to extract information about metastable states from a variety of neural signals; and (iii) recent neural network approaches, informed by experimental results, to model the emergence of metastable dynamics. By discussing these topics, we aim to provide a cohesive view of how transitions between different states of activity may provide the neural underpinnings for essential functions such as perception, memory, expectation, or decision making, and more generally, how the study of metastable neural activity may advance our understanding of neural circuit function in health and disease.

Presynaptic NMDA Receptors Influence Ca2+ Dynamics by Interacting with Voltage-Dependent Calcium Channels during the Induction of Long-Term Depression

Spike-timing-dependent long-term depression (t-LTD) of glutamatergic layer (L)4-L2/3 synapses in developing neocortex requires activation of astrocytes by endocannabinoids (eCBs), which release glutamate onto presynaptic NMDA receptors (preNMDARs). The exact function of preNMDARs in this context is still elusive and strongly debated. To elucidate their function, we show that bath application of the eCB 2-arachidonylglycerol (2-AG) induces a preNMDAR-dependent form of chemically induced LTD (eCB-LTD) in L2/3 pyramidal neurons in the juvenile somatosensory cortex of rats. Presynaptic Ca²⁺ imaging from L4 spiny stellate axons revealed that action potential (AP) evoked Ca²⁺ transients show a preNMDAR-dependent broadening during eCB-LTD induction. However, blockade of voltage-dependent Ca²⁺ channels (VDCCs) did not uncover direct preNMDAR-mediated Ca²⁺ transients in the axon. This suggests that astrocyte-mediated glutamate release onto preNMDARs does not result in a direct Ca²⁺ influx, but that it instead leads to an indirect interaction with presynaptic VDCCs, boosting axonal Ca²⁺ influx. These results reveal one of the main remaining missing pieces in the signaling cascade of t-LTD at developing cortical synapses.

Genetic and activity dependent-mechanisms wiring the cortex: Two sides of the same coin

The cerebral cortex is responsible for the higher-order functions of the brain such as planning, cognition, or social behaviour. It provides us with the capacity to interact with and transform our world. The substrates of cortical functions are complex neural circuits that arise during development from the dynamic remodelling and progressive specialization of immature undefined networks. Here, we review the genetic and activity-dependent mechanisms of cortical wiring focussing on the importance of their interaction. Cortical circuits emerge from an initial set of neuronal types that engage in sequential forms of embryonic and postnatal activity. Such activities further complement the cells' genetic programs, increasing neuronal diversity and modifying the electrical properties while promoting selective connectivity. After a temporal window of enhanced plasticity, the main features of mature circuits are established. Failures in these processes can lead to neurodevelopmental disorders whose treatment remains elusive. However, a deeper dissection of cortical wiring will pave the way for innovative therapies.

Implications of Extended Inhibitory Neuron Development

A prolonged developmental timeline for GABA (γ-aminobutyric acid)-expressing inhibitory neurons (GABAergic interneurons) is an amplified trait in larger, gyrencephalic animals. In several species, the generation, migration, and maturation of interneurons take place over several months, in some cases persisting after birth. The late integration of GABAergic interneurons occurs in a region-specific pattern, especially during the early postnatal period. These changes can contribute to the formation of functional connectivity and plasticity, especially in the cortical regions responsible for higher cognitive tasks. In this review, we discuss GABAergic interneuron development in the late gestational and postnatal forebrain. We propose the protracted development of interneurons at each stage (neurogenesis, neuronal migration, and network integration), as a mechanism for increased complexity and cognitive flexibility in larger, gyrencephalic brains. This developmental feature of interneurons also provides an avenue for environmental influences to shape neural circuit formation.

Brief period of monocular deprivation drives changes in audiovisual temporal perception

The human brain retains a striking degree of plasticity into adulthood. Recent studies have demonstrated that a short period of altered visual experience (via monocular deprivation) can change the dynamics of binocular rivalry in favor of the deprived eye, a compensatory action thought to be mediated by an upregulation of cortical gain control mechanisms. Here, we sought to better understand the impact of monocular deprivation on multisensory abilities, specifically examining audiovisual temporal perception. Using an audiovisual simultaneity judgment task, we discovered that 90 minutes of monocular deprivation produced opposing effects on the temporal binding window depending on the eye used in the task. Thus, in those who performed the task with their deprived eye there was a narrowing of the temporal binding window, whereas in those performing the task with their nondeprived eye there was a widening of the temporal binding window. The effect was short lived, being observed only in the first 10 minutes of postdeprivation testing. These findings indicate that changes in visual experience in the adult can rapidly impact multisensory perceptual processes, a finding that has important clinical implications for those patients with adult-onset visual deprivation and for therapies founded on monocular deprivation.

Decreased reproducibility and abnormal experience-dependent plasticity of network dynamics in Fragile X circuits

Fragile X syndrome is a neurodevelopmental disorder associated with a broad range of neural phenotypes. Interpreting these findings has proven challenging because some phenotypes may reflect compensatory mechanisms or normal forms of plasticity differentially engaged by experiential differences. To help minimize compensatory and experiential influences, we used an ex vivo approach to study network dynamics and plasticity of cortical microcircuits. In Fmr1^−/y circuits, the spatiotemporal structure of Up-states was less reproducible, suggesting alterations in the plasticity mechanisms governing network activity. Chronic optical stimulation revealed normal homeostatic plasticity of Up-states, however, Fmr1^−/y circuits exhibited abnormal experience-dependent plasticity as they did not adapt to chronically presented temporal patterns in an interval-specific manner. These results, suggest that while homeostatic plasticity is normal, Fmr1^−/y circuits exhibit deficits in the ability to orchestrate multiple forms of synaptic plasticity and to adapt to sensory patterns in an experience-dependent manner—which is likely to contribute to learning deficits.

Synaptic and intrinsic mechanisms underlying development of cortical direction selectivity

Modifications of synaptic inputs and cell-intrinsic properties both contribute to neuronal plasticity and development. To better understand these mechanisms, we undertook an intracellular analysis of the development of direction selectivity in the ferret visual cortex, which occurs rapidly over a few days after eye opening. We found strong evidence of developmental changes in linear spatiotemporal receptive fields of simple cells, implying alterations in circuit inputs. Further, this receptive field plasticity was accompanied by increases in near-spike-threshold excitability and input-output gain that resulted in dramatically increased spiking responses in the experienced state. Increases in subthreshold membrane responses induced by the receptive field plasticity and the increased input-output spiking gain were both necessary to explain the elevated firing rates in experienced ferrets. These results demonstrate that cortical direction selectivity develops through a combination of plasticity in inputs and in cell-intrinsic properties.

Versatility and Flexibility of Cortical Circuits

Cortical circuits are known to be plastic and adaptable, as shown by an impressive body of evidence demonstrating the ability of cortical circuits to adapt to changes in environmental stimuli, development, learning, and insults. In this review, we will discuss some of the features of cortical circuits that are thought to facilitate cortical circuit versatility and flexibility. Throughout life, cortical circuits can be extensively shaped and refined by experience while preserving their overall organization, suggesting that mechanisms are in place to favor change but also to stabilize some aspects of the circuit. First, we will describe the basic organization and some of the common features of cortical circuits. We will then discuss how this underlying cortical structure provides a substrate for the experience- and learning-dependent processes that contribute to cortical flexibility.

Neurophysiology and Regulation of the Balance Between Excitation and Inhibition in Neocortical Circuits

Brain function relies on the ability of neural networks to maintain stable levels of activity, while experiences sculpt them. In the neocortex, the balance between activity and stability relies on the coregulation of excitatory and inhibitory inputs onto principal neurons. Shifts of excitation or inhibition result in altered excitability impaired processing of incoming information. In many neurodevelopmental and neuropsychiatric disorders, the excitability of local circuits is altered, suggesting that their pathophysiology may involve shifts in synaptic excitation, inhibition, or both. Most studies focused on identifying the cellular and molecular mechanisms controlling network excitability to assess whether they may be altered in animal models of disease. The impact of changes in excitation/inhibition balance on local circuit and network computations is not clear. Here we report findings on the integration of excitatory and inhibitory inputs in healthy cortical circuits and discuss how shifts in excitation/inhibition balance may relate to pathological phenotypes.

Developmental metaplasticity in neural circuit codes of firing and structure

Firing-rate dynamics have been hypothesized to mediate inter-neural information transfer in the brain. While the Hebbian paradigm, relating learning and memory to firing activity, has put synaptic efficacy variation at the center of cortical plasticity, we suggest that the external expression of plasticity by changes in the firing-rate dynamics represents a more general notion of plasticity. Hypothesizing that time constants of plasticity and firing dynamics increase with age, and employing the filtering property of the neuron, we obtain the elementary code of global attractors associated with the firing-rate dynamics in each developmental stage. We define a neural circuit connectivity code as an indivisible set of circuit structures generated by membrane and synapse activation and silencing. Synchronous firing patterns under parameter uniformity, and asynchronous circuit firing are shown to be driven, respectively, by membrane and synapse silencing and reactivation, and maintained by the neuronal filtering property. Analytic, graphical and simulation representation of the discrete iteration maps and of the global attractor codes of neural firing rate are found to be consistent with previous empirical neurobiological findings, which have lacked, however, a specific correspondence between firing modes, time constants, circuit connectivity and cortical developmental stages.

Early Cross-modal Plasticity in Adults

It is known that, after a prolonged period of visual deprivation, the adult visual cortex can be recruited for nonvisual processing, reflecting cross-modal plasticity. Here, we investigated whether cross-modal plasticity can occur at short timescales in the typical adult brain by comparing the interaction between vision and touch during binocular rivalry before and after a brief period of monocular deprivation, which strongly alters ocular balance favoring the deprived eye. While viewing dichoptically two gratings of orthogonal orientation, participants were asked to actively explore a haptic grating congruent in orientation to one of the two rivalrous stimuli. We repeated this procedure before and after 150 min of monocular deprivation. We first confirmed that haptic stimulation interacted with vision during rivalry promoting dominance of the congruent visuo-haptic stimulus and that monocular deprivation increased the deprived eye and decreased the nondeprived eye dominance. Interestingly, after deprivation, we found that the effect of touch did not change for the nondeprived eye, whereas it disappeared for the deprived eye, which was potentiated after deprivation. The absence of visuo-haptic interaction for the deprived eye lasted for over 1 hr and was not attributable to a masking induced by the stronger response of the deprived eye as confirmed by a control experiment. Taken together, our results demonstrate that the adult human visual cortex retains a high degree of cross-modal plasticity, which can occur even at very short timescales.

Selective Maturation of Temporal Dynamics of Intracortical Excitatory Transmission at the Critical Period Onset

Although the developmental maturation of cortical inhibitory synapses is known to be a critical factor in gating the onset of critical period (CP) for experience-dependent cortical plasticity, how synaptic transmission dynamics of other cortical synapses are regulated during the transition to CP remains unknown. Here, by systematically examining various intracortical synapses within layer 4 of the mouse visual cortex, we demonstrate that synaptic temporal dynamics of intracortical excitatory synapses on principal cells (PCs) and inhibitory parvalbumin- or somatostatin-expressing cells are selectively regulated before the CP onset, whereas those of intracortical inhibitory synapses and long-range thalamocortical excitatory synapses remain unchanged. This selective maturation of synaptic dynamics results from a ubiquitous reduction of presynaptic release and is dependent on visual experience. These findings provide an additional essential circuit mechanism for regulating CP timing in the developing visual cortex.

Role of GABAA-Mediated Inhibition and Functional Assortment of Synapses onto Individual Layer 4 Neurons in Regulating Plasticity Expression in Visual Cortex

Layer 4 (L4) of primary visual cortex (V1) is the main recipient of thalamocortical fibers from the dorsal lateral geniculate nucleus (LGNd). Thus, it is considered the main entry point of visual information into the neocortex and the first anatomical opportunity for intracortical visual processing before information leaves L4 and reaches supra- and infragranular cortical layers. The strength of monosynaptic connections from individual L4 excitatory cells onto adjacent L4 cells (unitary connections) is highly malleable, demonstrating that the initial stage of intracortical synaptic transmission of thalamocortical information can be altered by previous activity. However, the inhibitory network within L4 of V1 may act as an internal gate for induction of excitatory synaptic plasticity, thus providing either high fidelity throughput to supragranular layers or transmittal of a modified signal subject to recent activity-dependent plasticity. To evaluate this possibility, we compared the induction of synaptic plasticity using classical extracellular stimulation protocols that recruit a combination of excitatory and inhibitory synapses with stimulation of a single excitatory neuron onto a L4 cell. In order to induce plasticity, we paired pre- and postsynaptic activity (with the onset of postsynaptic spiking leading the presynaptic activation by 10ms) using extracellular stimulation (ECS) in acute slices of primary visual cortex and comparing the outcomes with our previously published results in which an identical protocol was used to induce synaptic plasticity between individual pre- and postsynaptic L4 excitatory neurons. Our results indicate that pairing of ECS with spiking in a L4 neuron fails to induce plasticity in L4-L4 connections if synaptic inhibition is intact. However, application of a similar pairing protocol under GABAARs inhibition by bath application of 2μM bicuculline does induce robust synaptic plasticity, long term potentiation (LTP) or long term depression (LTD), similar to our results with pairing of pre- and postsynaptic activation between individual excitatory L4 neurons in which inhibitory connections are not activated. These results are consistent with the well-established observation that inhibition limits the capacity for induction of plasticity at excitatory synapses and that pre- and postsynaptic activation at a fixed time interval can result in a variable range of plasticity outcomes. However, in the current study by virtue of having two sets of experimental data, we have provided a new insight into these processes. By randomly mixing the assorting of individual L4 neurons according to the frequency distribution of the experimentally determined plasticity outcome distribution based on the calculated convergence of multiple individual L4 neurons onto a single postsynaptic L4 neuron, we were able to compare then actual ECS plasticity outcomes to those predicted by randomly mixing individual pairs of neurons. Interestingly, the observed plasticity profiles with ECS cannot account for the random assortment of plasticity behaviors of synaptic connections between individual cell pairs. These results suggest that connections impinging onto a single postsynaptic cell may be grouped according to plasticity states.

Roles of Presynaptic NMDA Receptors in Neurotransmission and Plasticity

Presynaptic NMDA receptors (preNMDARs) play pivotal roles in excitatory neurotransmission and synaptic plasticity. They facilitate presynaptic neurotransmitter release and modulate mechanisms controlling synaptic maturation and plasticity during formative periods of brain development. There is an increasing understanding of the roles of preNMDARs in experience-dependent synaptic and circuit-specific computation. In this review we summarize the latest understanding of compartment-specific expression and function of preNMDARs, and how they contribute to synapse-specific and circuit-level information processing.

Synapse-type-specific plasticity in local circuits

Neuroscientists spent decades debating whether synaptic plasticity was presynaptically or postsynaptically expressed. It was eventually concluded that plasticity depends on many factors, including cell type. More recently, it has become increasingly clear that plasticity is regulated at an even finer grained level; it is specific to the synapse type, a concept we denote synapse-type-specific plasticity (STSP). Here, we review recent developments in the field of STSP, discussing both long-term and short-term variants and with particular emphasis on neocortical function. As there are dozens of neocortical cell types, there is a multiplicity of forms of STSP, the vast majority of which have never been explored. We argue that to understand the brain and synaptic diseases, we have to grapple with STSP.

Models of Metaplasticity: A Review of Concepts

Part of hippocampal and cortical plasticity is characterized by synaptic modifications that depend on the joint activity of the pre- and post-synaptic neurons. To which extent those changes are determined by the exact timing and the average firing rates is still a matter of debate; this may vary from brain area to brain area, as well as across neuron types. However, it has been robustly observed both in vitro and in vivo that plasticity itself slowly adapts as a function of the dynamical context, a phenomena commonly referred to as metaplasticity. An alternative concept considers the regulation of groups of synapses with an objective at the neuronal level, for example, maintaining a given average firing rate. In that case, the change in the strength of a particular synapse of the group (e.g., due to Hebbian learning) affects others' strengths, which has been coined as heterosynaptic plasticity. Classically, Hebbian synaptic plasticity is paired in neuron network models with such mechanisms in order to stabilize the activity and/or the weight structure. Here, we present an oriented review that brings together various concepts from heterosynaptic plasticity to metaplasticity, and show how they interact with Hebbian-type learning. We focus on approaches that are nowadays used to incorporate those mechanisms to state-of-the-art models of spiking plasticity inspired by experimental observations in the hippocampus and cortex. Making the point that metaplasticity is an ubiquitous mechanism acting on top of classical Hebbian learning and promoting the stability of neural function over multiple timescales, we stress the need for incorporating it as a key element in the framework of plasticity models. Bridging theoretical and experimental results suggests a more functional role for metaplasticity mechanisms than simply stabilizing neural activity.

Delayed in vitro development of Up states but normal network plasticity in Fragile X circuits

A broad range of neurophysiological phenotypes have been reported since the generation of the first mouse model of Fragile X syndrome (FXS). However, it remains unclear which phenotypes are causally related to the cognitive deficits associated with FXS. Indeed, because many of these phenotypes are known to be modulated by experience, a confounding factor in the interpretation of many studies is whether some phenotypes are an indirect consequence of abnormal development and experience. To help diminish this confound we first conducted an in vitro developmental study of spontaneous neural dynamics in cortical organotypic cultures. A significant developmental increase in network activity and Up states was observed in both wild-type and Fmr1(-/y) circuits, along with a specific developmental delay in the emergence of Up states in knockout circuits. To determine whether Up state regulation is generally impaired in FXS circuits, we examined Up state plasticity using chronic optogenetic stimulation. Wild-type and Fmr1(-/y) stimulated circuits exhibited a significant decrease in overall spontaneous activity including Up state frequency; however, no significant effect of genotype was observed. These results demonstrate that developmental delays characteristic of FXS are recapitulated during in vitro development, and that Up state abnormalities are probably a direct consequence of the disease, and not an indirect consequence of abnormal experience. However, the fact that Fmr1(-/y) circuits exhibited normal homeostatic modulation of Up states suggests that these plasticity mechanisms are largely intact, and that some of the previously reported plasticity deficits could reflect abnormal experience or the engagement of compensatory mechanisms.

Differential maturation of vesicular glutamate and GABA transporter expression in the mouse auditory forebrain during the first weeks of hearing

Vesicular transporter proteins are an essential component of the presynaptic machinery that regulates neurotransmitter storage and release. They also provide a key point of control for homeostatic signaling pathways that maintain balanced excitation and inhibition following changes in activity levels, including the onset of sensory experience. To advance understanding of their roles in the developing auditory forebrain, we tracked the expression of the vesicular transporters of glutamate (VGluT1, VGluT2) and GABA (VGAT) in primary auditory cortex (A1) and medial geniculate body (MGB) of developing mice (P7, P11, P14, P21, adult) before and after ear canal opening (~P11-P13). RNA sequencing, in situ hybridization, and immunohistochemistry were combined to track changes in transporter expression and document regional patterns of transcript and protein localization. Overall, vesicular transporter expression changed the most between P7 and P21. The expression patterns and maturational trajectories of each marker varied by brain region, cortical layer, and MGB subdivision. VGluT1 expression was highest in A1, moderate in MGB, and increased with age in both regions. VGluT2 mRNA levels were low in A1 at all ages, but high in MGB, where adult levels were reached by P14. VGluT2 immunoreactivity was prominent in both regions. VGluT1 (+) and VGluT2 (+) transcripts were co-expressed in MGB and A1 somata, but co-localization of immunoreactive puncta was not detected. In A1, VGAT mRNA levels were relatively stable from P7 to adult, while immunoreactivity increased steadily. VGAT (+) transcripts were rare in MGB neurons, whereas VGAT immunoreactivity was robust at all ages. Morphological changes in immunoreactive puncta were found in two regions after ear canal opening. In the ventral MGB, a decrease in VGluT2 puncta density was accompanied by an increase in puncta size. In A1, perisomatic VGAT and VGluT1 terminals became prominent around the neuronal somata. Overall, the observed changes in gene and protein expression, regional architecture, and morphology relate to-and to some extent may enable-the emergence of mature sound-evoked activity patterns. In that regard, the findings of this study expand our understanding of the presynaptic mechanisms that regulate critical period formation associated with experience-dependent refinement of sound processing in auditory forebrain circuits.

Competition, inhibition, and critical periods of cortical plasticity

Maturation of cortical inhibition just after eye opening is a necessary precedent for the emergence of competitive, experience-dependent ocular dominance plasticity in the visual cortex. What inhibition is doing in this context, though, is not clear. Here I outline new hypotheses on the roles of somatic and dendritic inhibition in the opening and closure of critical periods, and their roles in the competitive processes therein.

Target-specific properties of thalamocortical synapses onto layer 4 of mouse primary visual cortex

In primary sensory cortices, thalamocortical (TC) inputs can directly activate excitatory and inhibitory neurons. In vivo experiments in the main input layer (L4) of primary visual cortex (V1) have shown that excitatory and inhibitory neurons have different tuning properties. The different functional properties may arise from distinct intrinsic properties of L4 neurons, but could also depend on cell type-specific properties of the synaptic inputs from the lateral geniculate nucleus of the thalamus (LGN) onto L4 neurons. While anatomical studies identified LGN inputs onto both excitatory and inhibitory neurons in V1, their synaptic properties have not been investigated. Here we used an optogenetic approach to selectively activate LGN terminal fields in acute coronal slices containing V1, and recorded monosynaptic currents from excitatory and inhibitory neurons in L4. LGN afferents made monosynaptic connections with pyramidal (Pyr) and fast-spiking (FS) neurons. TC EPSCs on FS neurons were larger and showed steeper short-term depression in response to repetitive stimulation than those on Pyr neurons. LGN inputs onto Pyr and FS neurons also differed in postsynaptic receptor composition and organization of presynaptic release sites. Together, our results demonstrate that LGN input onto L4 neurons in mouse V1 have target-specific presynaptic and postsynaptic properties. Distinct mechanisms of activation of feedforward excitatory and inhibitory neurons in the main input layer of V1 are likely to endow neurons with different response properties to incoming visual stimuli.

Synapse-specific control of experience-dependent plasticity by presynaptic NMDA receptors

Sensory experience orchestrates the development of cortical circuitry by adaptively modifying neurotransmission and synaptic connectivity. However, the mechanisms underlying these experience-dependent modifications remain elusive. Here we demonstrate that visual experience suppresses a presynaptic NMDA receptor (preNMDAR)-mediated form of timing-dependent long-term depression (tLTD) at visual cortex layer (L) 4-2/3 synapses. This tLTD can be maintained during development, or reinstated in adulthood, by sensory deprivation. The changes in tLTD are mirrored by changes in glutamate release; visual deprivation enhances both tLTD and glutamate release. These effects require the GluN3A NMDAR subunit, the levels of which are increased by visual deprivation. Further, by coupling the pathway-specific optogenetic induction of tLTD with cell-type-specific NMDAR deletion, we find that visual experience modifies preNMDAR-mediated plasticity specifically at L4-L2/3 synapses.

A model of order-selectivity based on dynamic changes in the balance of excitation and inhibition produced by short-term synaptic plasticity

Determining the order of sensory events separated by a few hundred milliseconds is critical to many forms of sensory processing, including vocalization and speech discrimination. Although many experimental studies have recorded from auditory order-sensitive and order-selective neurons, the underlying mechanisms are poorly understood. Here we demonstrate that universal properties of cortical synapses-short-term synaptic plasticity of excitatory and inhibitory synapses-are well suited for the generation of order-selective neural responses. Using computational models of canonical disynaptic circuits, we show that the dynamic changes in the balance of excitation and inhibition imposed by short-term plasticity lead to the generation of order-selective responses. Parametric analyses predict that among the forms of short-term plasticity expressed at excitatory-to-excitatory, excitatory-to-inhibitory, and inhibitory-to-excitatory synapses, the single most important contributor to order-selectivity is the paired-pulse depression of inhibitory postsynaptic potentials (IPSPs). A topographic model of the auditory cortex that incorporates short-term plasticity accounts for both context-dependent suppression and enhancement in response to paired tones. Together these results provide a framework to account for an important computational problem based on ubiquitous synaptic properties that did not yet have a clearly established computational function. Additionally, these studies suggest that disynaptic circuits represent a fundamental computational unit that is capable of processing both spatial and temporal information.

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.

GABAergic synapses: their plasticity and role in sensory cortex

The mammalian neocortex is composed of a variety of cell types organized in a highly interconnected circuit. GABAergic neurons account for only about 20% of cortical neurons. However, they show widespread connectivity and a high degree of diversity in morphology, location, electrophysiological properties and gene expression. In addition, distinct populations of inhibitory neurons have different sensory response properties, capacities for plasticity and sensitivities to changes in sensory experience. In this review we summarize experimental evidence regarding the properties of GABAergic neurons in primary sensory cortex. We will discuss how distinct GABAergic neurons and different forms of GABAergic inhibitory plasticity may contribute to shaping sensory cortical circuit activity and function.

Inhibitory plasticity dictates the sign of plasticity at excitatory synapses

The broad connectivity of inhibitory interneurons and the capacity of inhibitory synapses to be plastic make them ideal regulators of the level of excitability of many neurons simultaneously. Whether inhibitory synaptic plasticity may also contribute to the selective regulation of single neurons and local microcircuits activity has not been investigated. Here we demonstrate that in rat primary visual cortex inhibitory synaptic plasticity is connection specific and depends on the activation of postsynaptic GABAB-Gi/o protein signaling. Through the activation of this intracellular signaling pathway, inhibitory plasticity can alter the state of a single postsynaptic neuron and directly affect the induction of plasticity at its glutamatergic inputs. This interaction is modulated by sensory experience. Our data demonstrate that in recurrent circuits, excitatory and inhibitory forms of synaptic plasticity are not integrated as independent events, but interact to cooperatively drive the activity-dependent rewiring of local microcircuits.

Cannabinoid-dependent potentiation of inhibition at eye opening in mouse V1

Cannabinoid (CB) signaling is a well established regulator of synaptic transmission. Recent work demonstrated that CB release is necessary for the induction of inhibitory synaptic plasticity. In primary visual cortex (V1) CB receptors are present throughout life, though their level of expression is developmentally regulated. In the input layer of V1 (layer 4, L4) these receptors show low levels of expression and colocalize with GABAergic terminals suggesting that they may play an important role in regulating GABAergic transmission. Here we show that in the developmental window extending from eye opening to the onset of the critical period for visual cortical plasticity L4 inhibitory inputs onto pyramidal neurons are highly sensitive to activation of CB release. More specifically, application of synthetic and endogenous CB receptors agonists led to a significant increase in the amplitude and frequency of both spontaneous inhibitory post-synaptic currents and miniature inhibitory post-synaptic currents. This form of inhibitory potentiation is activity-dependent, induced by repetitive bursting of pyramidal neurons and regulated by the time of eye opening. CB-dependent regulation of inhibitory drive may be a mechanism for the regulating L4 pyramidal neurons excitability and function at a time in which V1 transitions from being activated by spontaneous activity to being driven by visual inputs.

Target-cell-specific short-term plasticity in local circuits

Short-term plasticity (STP) denotes changes in synaptic strength that last up to tens of seconds. It is generally thought that STP impacts information transfer across synaptic connections and may thereby provide neurons with, for example, the ability to detect input coherence, to maintain stability and to promote synchronization. STP is due to a combination of mechanisms, including vesicle depletion and calcium accumulation in synaptic terminals. Different forms of STP exist, depending on many factors, including synapse type. Recent evidence shows that synapse dependence holds true even for connections that originate from a single presynaptic cell, which implies that postsynaptic target cell type can determine synaptic short-term dynamics. This arrangement is surprising, since STP itself is chiefly due to presynaptic mechanisms. Target-specific synaptic dynamics in addition imply that STP is not a bug resulting from synapses fatiguing when driven too hard, but rather a feature that is selectively implemented in the brain for specific functional purposes. As an example, target-specific STP results in sequential somatic and dendritic inhibition in neocortical and hippocampal excitatory cells during high-frequency firing. Recent studies also show that the Elfn1 gene specifically controls STP at some synapse types. In addition, presynaptic NMDA receptors have been implicated in synapse-specific control of synaptic dynamics during high-frequency activity. We argue that synapse-specific STP deserves considerable further study, both experimentally and theoretically, since its function is not well known. We propose that synapse-specific STP has to be understood in the context of the local circuit, which requires combining different scientific disciplines ranging from molecular biology through electrophysiology to computer modeling.

Inhibitory synaptic plasticity: spike timing-dependence and putative network function

While the plasticity of excitatory synaptic connections in the brain has been widely studied, the plasticity of inhibitory connections is much less understood. Here, we present recent experimental and theoretical findings concerning the rules of spike timing-dependent inhibitory plasticity and their putative network function. This is a summary of a workshop at the COSYNE conference 2012.

Layer-specific experience-dependent rewiring of thalamocortical circuits

Thalamocortical circuits are central to sensory and cognitive processing. Recent work suggests that the thalamocortical inputs onto L4 and L6, the main input layers of neocortex, are activated differently by visual stimulation. Whether these differences depend on layer-specific organization of thalamocortical circuits; or on specific properties of synapses onto receiving neurons is unknown. Here we combined optogenetic stimulation of afferents from the visual thalamus and paired recording electrophysiology in L4 and L6 of rat primary visual cortex to determine the organization and plasticity of thalamocortical synapses. We show that thalamocortical inputs onto L4 and L6 differ in synaptic dynamics and sensitivity to visual drive. We also demonstrate that the two layers differ in the organization of thalamocortical and recurrent intracortical connectivity. In L4, a significantly larger proportion of excitatory neurons responded to light activation of thalamocortical terminal fields than in L6. The local microcircuit in L4 showed a higher degree of recurrent connectivity between excitatory neurons than the microcircuit in L6. In addition, L4 recurrently connected neurons were driven by thalamocortical inputs of similar magnitude indicating the presence of local subnetworks that may be activated by the same axonal projection. Finally, brief manipulation of visual drive reduced the amplitude of light-evoked thalamocortical synaptic currents selectively onto L4. These data are the first direct indication that thalamocortical circuits onto L4 and L6 support different aspects of cortical function through layer-specific synaptic organization and plasticity.

Developmental regulation of spatio-temporal patterns of cortical circuit activation

Neural circuits are refined in an experience-dependent manner during early postnatal development. How development modulates the spatio-temporal propagation of activity through cortical circuits is poorly understood. Here we use voltage-sensitive dye imaging (VSD) to show that there are significant changes in the spatio-temporal patterns of intracortical signals in primary visual cortex (V1) from postnatal day 13 (P13), eye opening, to P28, the peak of the critical period for rodent visual cortical plasticity. Upon direct stimulation of layer 4 (L4), activity spreads to L2/3 and to L5 at all ages. However, while from eye opening to the peak of the critical period, the amplitude and persistence of the voltage signal decrease, peak activation is reached more quickly and the interlaminar gain increases with age. The lateral spread of activation within layers remains unchanged throughout the time window under analysis. These developmental changes in spatio-temporal patterns of intracortical circuit activation are mediated by differences in the contributions of excitatory and inhibitory synaptic components. Our results demonstrate that after eye opening the circuit in V1 is refined through a progression of changes that shape the spatio-temporal patterns of circuit activation. Signals become more efficiently propagated across layers through developmentally regulated changes in interlaminar gain.