Bidirectional plasticity in fast-spiking GABA circuits by visual experience

The role of dendritic inhibition in shaping the plasticity of excitatory synapses

Using computational tools we explored the impact of local synaptic inhibition on the plasticity of excitatory synapses in dendrites. The latter critically depends on the intracellular concentration of calcium, which in turn, depends on membrane potential and thus on inhibitory activity in particular dendritic compartments. We systematically characterized the dependence of excitatory synaptic plasticity on dendritic morphology, loci and strength, as well as on the spatial distribution of inhibitory synapses and on the level of excitatory activity. Plasticity of excitatory synapses may attain three states: “protected” (unchanged), potentiated (long-term potentiation; LTP), or depressed (long-term depression; LTD). The transition between these three plasticity states could be finely tuned by synaptic inhibition with high spatial resolution. Strategic placement of inhibition could give rise to the co-existence of all three states over short dendritic branches. We compared the plasticity effect of the innervation patterns typical of different inhibitory subclasses—Chandelier, Basket, Martinotti, and Double Bouquet—in a detailed model of a layer 5 pyramidal cell. Our study suggests that dendritic inhibition plays a key role in shaping and fine-tuning excitatory synaptic plasticity in dendrites.

In vivo cholinergic modulation of the cellular properties of medial entorhinal cortex neurons

Extensive in vitro data and modeling studies suggest that intrinsic properties of medial entorhinal cortex (MEC) neurons contribute to the spiking behaviour of functional cell types of MEC neurons, such as grid cells, recorded in behaving animals. It remains unclear, however, how intrinsic properties of MEC neurons influence cellular dynamics in intact networks in vivo. In order to begin to bridge the gap between electrophysiological data sets from brain slices and behaving animals, in the present study we performed intracellular recordings using sharp electrodes in urethane-anaesthetized rats to elucidate the cellular dynamics of MEC neurons in vivo. We focused on the h-current-dependent sag potential during hyperpolarizing current steps, subthreshold resonance in response to oscillatory frequency sweeps (chirp stimuli), persistent spiking in response to brief depolarizing inputs and the relationship between firing frequency and input (f-I curve), each of which is sensitive to cholinergic modulation in vitro. Consistent with data from in vitro studies, cholinergic activation by systemic application of the acetylcholinesterase inhibitor, physostigmine, resulted in decreased sag amplitude, increased sag time constant and a decrease of the peak resonance frequency. The f-I curve was also modulated by physostigmine in many neurons, but persistent spiking was not observed in any of our recordings, even when picrotoxin, a GABAA blocker, was included in the internal solution of the recording pipette to reduce possible effects of network inhibition. These results suggest that intrinsic oscillatory and rate-coding mechanisms, but not intrinsic bistability, are significantly modulated by acetylcholine in the intact entorhinal network.

Visual experience and subsequent sleep induce sequential plastic changes in putative inhibitory and excitatory cortical neurons

Ocular dominance plasticity in the developing primary visual cortex is initiated by monocular deprivation (MD) and consolidated during subsequent sleep. To clarify how visual experience and sleep affect neuronal activity and plasticity, we continuously recorded extragranular visual cortex fast-spiking (FS) interneurons and putative principal (i.e., excitatory) neurons in freely behaving cats across periods of waking MD and post-MD sleep. Consistent with previous reports in mice, MD induces two related changes in FS interneurons: a response shift in favor of the closed eye and depression of firing. Spike-timing-dependent depression of open-eye-biased principal neuron inputs to FS interneurons may mediate these effects. During post-MD nonrapid eye movement sleep, principal neuron firing increases and becomes more phase-locked to slow wave and spindle oscillations. Ocular dominance (OD) shifts in favor of open-eye stimulation--evident only after post-MD sleep--are proportional to MD-induced changes in FS interneuron activity and to subsequent sleep-associated changes in principal neuron activity. OD shifts are greatest in principal neurons that fire 40-300 ms after neighboring FS interneurons during post-MD slow waves. Based on these data, we propose that MD-induced changes in FS interneurons play an instructive role in ocular dominance plasticity, causing disinhibition among open-eye-biased principal neurons, which drive plasticity throughout the visual cortex during subsequent sleep.

Efficient associative memory storage in cortical circuits of inhibitory and excitatory neurons

Many features of synaptic connectivity are ubiquitous among cortical systems. Cortical networks are dominated by excitatory neurons and synapses, are sparsely connected, and function with stereotypically distributed connection weights. We show that these basic structural and functional features of synaptic connectivity arise readily from the requirement of efficient associative memory storage. Our theory makes two fundamental predictions. First, we predict that, despite a large number of neuron classes, functional connections between potentially connected cells must be realized with <50% probability if the presynaptic cell is excitatory and >50% probability if the presynaptic cell is inhibitory. Second, we establish a unique relation between probability of connection and coefficient of variation in connection weights. These predictions are consistent with a dataset of 74 published experiments reporting connection probabilities and distributions of postsynaptic potential amplitudes in various cortical systems. What is more, our theory explains the shapes of the distributions obtained in these experiments.

The spike-timing dependence of plasticity

In spike-timing-dependent plasticity (STDP), the order and precise temporal interval between presynaptic and postsynaptic spikes determine the sign and magnitude of long-term potentiation (LTP) or depression (LTD). STDP is widely utilized in models of circuit-level plasticity, development, and learning. However, spike timing is just one of several factors (including firing rate, synaptic cooperativity, and depolarization) that govern plasticity induction, and its relative importance varies across synapses and activity regimes. This review summarizes this broader view of plasticity, including the forms and cellular mechanisms for the spike-timing dependence of plasticity, and, the evidence that spike timing is an important determinant of plasticity in vivo.

Development and plasticity of the primary visual cortex

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

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

Neural circuits are extensively refined by sensory experience during postnatal development. How the maturation of recurrent cortical synapses may contribute to events regulating the postnatal refinement of neocortical microcircuits remains controversial. Here we show that, in the main input layer of rat primary visual cortex, layer 4 (L4), recurrent excitatory synapses are endowed with multiple, developmentally regulated mechanisms for induction and expression of excitatory synaptic plasticity. Maturation of L4 synapses and visual experience lead to a sharp switch in sign and mechanisms for plasticity at recurrent excitatory synapses in L4 at the onset of the critical period for visual cortical plasticity. The state of maturation of excitatory pyramidal neurons allows neurons to engage different mechanisms for plasticity in response to the same induction paradigm. Experience is determinant for the maturation of L4 synapses, as well as for the transition between forms of plasticity and the mechanisms they may engage. These results indicate a tight correlation between the effects of sensory drive and maturation on cortical neurons and provide a new set of cellular mechanisms engaged in the postnatal refinement of cortical circuits.

Division and subtraction by distinct cortical inhibitory networks in vivo

Brain circuits process information through specialized neuronal subclasses interacting within a network. Revealing their interplay requires activating specific cells while monitoring others in a functioning circuit. Here we use a new platform for two-way light-based circuit interrogation in visual cortex in vivo to show the computational implications of modulating different subclasses of inhibitory neurons during sensory processing. We find that soma-targeting, parvalbumin-expressing (PV) neurons principally divide responses but preserve stimulus selectivity, whereas dendrite-targeting, somatostatin-expressing (SOM) neurons principally subtract from excitatory responses and sharpen selectivity. Visualized in vivo cell-attached recordings show that division by PV neurons alters response gain, whereas subtraction by SOM neurons shifts response levels. Finally, stimulating identified neurons while scanning many target cells reveals that single PV and SOM neurons functionally impact only specific subsets of neurons in their projection fields. These findings provide direct evidence that inhibitory neuronal subclasses have distinct and complementary roles in cortical computations.

Otx2 binding to perineuronal nets persistently regulates plasticity in the mature visual cortex

Specific transfer of (orthodenticle homeobox 2) Otx2 homeoprotein into GABAergic interneurons expressing parvalbumin (PV) is necessary and sufficient to open, then close, a critical period (CP) of plasticity in the developing mouse visual cortex. The accumulation of endogenous Otx2 in PV cells suggests the presence of specific Otx2 binding sites. Here, we find that perineuronal nets (PNNs) on the surfaces of PV cells permit the specific, constitutive capture of Otx2. We identify a 15 aa domain containing an arginine-lysine doublet (RK peptide) within Otx2, bearing prototypic traits of a glycosaminoglycan (GAG) binding sequence that mediates Otx2 binding to PNNs, and specifically to chondroitin sulfate D and E, with high affinity. Accordingly, PNN hydrolysis by chondroitinase ABC reduces the amount of endogenous Otx2 in PV cells. Direct infusion of RK peptide similarly disrupts endogenous Otx2 localization to PV cells, reduces PV and PNN expression, and reopens plasticity in adult mice. The closure of one eye during this transient window reduces cortical acuity and is specific to the RK motif, as an Alanine-Alanine variant or a scrambled peptide fails to reactivate plasticity. Conversely, this transient reopening of plasticity in the adult restores binocular vision in amblyopic mice. Thus, one function of PNNs is to facilitate the persistent internalization of Otx2 by PV cells to maintain CP closure. The pharmacological use of the Otx2 GAG binding domain offers a novel, potent therapeutic tool with which to restore cortical plasticity in the mature brain.

Elimination of inhibitory synapses is a major component of adult ocular dominance plasticity

During development, cortical plasticity is associated with the rearrangement of excitatory connections. While these connections become more stable with age, plasticity can still be induced in the adult cortex. Here we provide evidence that structural plasticity of inhibitory synapses onto pyramidal neurons is a major component of plasticity in the adult neocortex. In vivo two-photon imaging was used to monitor the formation and elimination of fluorescently labeled inhibitory structures on pyramidal neurons. We find that ocular dominance plasticity in the adult visual cortex is associated with rapid inhibitory synapse loss, especially of those present on dendritic spines. This occurs not only with monocular deprivation but also with subsequent restoration of binocular vision. We propose that in the adult visual cortex the experience-induced loss of inhibition may effectively strengthen specific visual inputs with limited need for rearranging the excitatory circuitry.

GABA through the ages: regulation of cortical function and plasticity by inhibitory interneurons

Inhibitory interneurons comprise only about 20% of cortical neurons and thus constitute a clear minority compared to the vast number of excitatory projection neurons. They are, however, an influential minority with important roles in cortical maturation, function, and plasticity. In this paper, we will highlight the functional importance of cortical inhibition throughout brain development, starting with the embryonal formation of the cortex, proceeding by the regulation of sensory cortical plasticity in adulthood, and finishing with the GABA involvement in sensory information processing in old age.

Noradrenergic 'tone' determines dichotomous control of cortical spike-timing-dependent plasticity

Norepinephrine (NE) is widely distributed throughout the brain. It modulates intrinsic currents, as well as amplitude and frequency of synaptic transmission affecting the 'signal-to-noise ratio' of sensory responses. In the visual cortex, α₁- and β-adrenergic receptors (AR) gate opposing effects on long-term plasticity of excitatory transmission. Whether and how NE recruits these plastic mechanisms is not clear. Here, we show that NE modulates glutamatergic inputs with different efficacies for α₁- and β-AR. As a consequence, the priming of synapses with different NE concentrations produces dose-dependent competing effects that determine the temporal window of spike-timing dependent plasticity (STDP). While a low NE concentration leads to long-term depression (LTD) over broad positive and negative delays, a high NE concentration results in bidirectional STDP restricted to very narrow intervals. These results indicate that the local availability of NE, released during emotional arousal, determines the compound modulatory effect and the output of STDP.

Synaptotagmin-2 is a reliable marker for parvalbumin positive inhibitory boutons in the mouse visual cortex

BACKGROUND

Inhibitory innervation by parvalbumin (PV) expressing interneurons has been implicated in the onset of the sensitive period of visual plasticity. Immunohistochemical analysis of the development and plasticity of these inhibitory inputs is difficult because PV expression is low in young animals and strongly influenced by neuronal activity. Moreover, the synaptic boutons that PV neurons form onto each other cannot be distinguished from the innervated cell bodies by immunostaining for this protein because it is present throughout the cells. These problems call for the availability of a synaptic, activity-independent marker for PV+ inhibitory boutons that is expressed before sensitive period onset. We investigated whether synaptotagmin-2 (Syt2) fulfills these properties in the visual cortex. Syt2 is a synaptic vesicle protein involved in fast Ca(2+) dependent neurotransmitter release. Its mRNA expression follows a pattern similar to that of PV throughout the brain and is present in 30-40% of hippocampal PV expressing basket cells. Up to now, no quantitative analyses of Syt2 expression in the visual cortex have been carried out.

METHODOLOGY/PRINCIPAL FINDINGS

We used immunohistochemistry to analyze colocalization of Syt2 with multiple interneuron markers including vesicular GABA transporter VGAT, calbindin, calretinin, somatostatin and PV in the primary visual cortex of mice during development and after dark-rearing.

CONCLUSIONS/SIGNIFICANCE

We show that in the adult visual cortex Syt2 is only found in inhibitory, VGAT positive boutons. Practically all Syt2 positive boutons also contain PV and vice versa. During development, Syt2 expression can be detected in synaptic boutons prior to PV and in contrast to PV expression, Syt2 is not down-regulated by dark-rearing. These properties of Syt2 make it an excellent marker for analyzing the development and plasticity of perisomatic inhibitory innervations onto both excitatory and inhibitory neurons in the visual cortex.

α4* Nicotinic acetylcholine receptors modulate experience-based cortical depression in the adult mouse somatosensory cortex

The molecular mechanisms that mediate experience-based changes in the function of the cerebral cortex, particularly in the adult animal, are poorly understood. Here we show using in vivo voltage-sensitive dye imaging, that whisker trimming leads to depression of whisker-evoked sensory responses in primary, secondary and associative somatosensory cortical regions. Given the importance of cholinergic neurotransmission in cognitive and sensory functions, we examined whether α4-containing (α4*) nicotinic acetylcholine receptors (nAChRs) mediate cortical depression. Using knock-in mice that express YFP-tagged α4 nAChRs subunits, we show that whisker trimming selectively increased the number α4*-YFP nAChRs in layer 4 of deprived barrel columns within 24 h, which persisted until whiskers regrew. Confocal and electron microscopy revealed that these receptors were preferentially increased on the cell bodies of GABAergic neurons. To directly link these receptors with functional cortical depression, we show that depression could be induced in normal mice by topical application or micro-injection of α4* nAChR agonist in the somatosensory cortex. Furthermore, cortical depression could be blocked after whisker trimming with chronic infusions of an α4* nAChR antagonist. Collectively, these results uncover a new role for α4* nAChRs in regulating rapid changes in the functional responsiveness of the adult somatosensory cortex.

Central adaptation to odorants depends on PI3K levels in local interneurons of the antennal lobe

We have previously shown that driving PI3K levels up or down leads to increases or reductions in the number of synapses, respectively. Using these tools to assay their behavioral effects in Drosophila melanogaster, we showed that a loss of synapses in two sets of local interneurons, GH298 and krasavietz, leads to olfaction changes toward attraction or repulsion, while the simultaneous manipulation of both sets of neurons restored normal olfactory indexes. We show here that olfactory central adaptation also requires the equilibrated changes in both sets of local interneurons. The same genetic manipulations directed to projection (GH146) or mushroom body (201Y, MB247) neurons did not affect adaptation. Also, we show that the equilibrium is a requirement for the glomerulus-specific size changes which are a morphological signature of central adaptation. Since the two sets of local neurons are mostly, although not exclusively, inhibitory (GH298) and excitatory (krasavietz), we interpret that the normal phenomena of sensory perception, measured as an olfactory index, and central adaptation rely on an inhibition/excitation ratio.

Parallel regulation of feedforward inhibition and excitation during whisker map plasticity

Sensory experience drives robust plasticity of sensory maps in cerebral cortex, but the role of inhibitory circuits in this process is not fully understood. We show that classical deprivation-induced whisker map plasticity in layer 2/3 (L2/3) of rat somatosensory (S1) cortex involves robust weakening of L4-L2/3 feedforward inhibition. This weakening was caused by reduced L4 excitation onto L2/3 fast-spiking (FS) interneurons, which mediate sensitive feedforward inhibition and was partially offset by strengthening of unitary FS to L2/3 pyramidal cell synapses. Weakening of feedforward inhibition paralleled the known weakening of feedforward excitation. As a result, mean excitation-inhibition balance and timing onto L2/3 pyramidal cells were preserved. Thus, reduced feedforward inhibition is a covert compensatory process that can maintain excitatory-inhibitory balance during classical deprivation-induced Hebbian map plasticity.

Layer- and cell-type-specific subthreshold and suprathreshold effects of long-term monocular deprivation in rat visual cortex

Connectivity and dendritic properties are determinants of plasticity that are layer and cell-type specific in the neocortex. However, the impact of experience-dependent plasticity at the level of synaptic inputs and spike outputs remains unclear along vertical cortical microcircuits. Here I compared subthreshold and suprathreshold sensitivity to prolonged monocular deprivation (MD) in rat binocular visual cortex in layer 4 and layer 2/3 pyramids (4Ps and 2/3Ps) and in thick-tufted and nontufted layer 5 pyramids (5TPs and 5NPs), which innervate different extracortical targets. In normal rats, 5TPs and 2/3Ps are the most binocular in terms of synaptic inputs, and 5NPs are the least. Spike responses of all 5TPs were highly binocular, whereas those of 2/3Ps were dominated by either the contralateral or ipsilateral eye. MD dramatically shifted the ocular preference of 2/3Ps and 4Ps, mostly by depressing deprived-eye inputs. Plasticity was profoundly different in layer 5. The subthreshold ocular preference shift was sevenfold smaller in 5TPs because of smaller depression of deprived inputs combined with a generalized loss of responsiveness, and was undetectable in 5NPs. Despite their modest ocular dominance change, spike responses of 5TPs consistently lost their typically high binocularity during MD. The comparison of MD effects on 2/3Ps and 5TPs, the main affected output cells of vertical microcircuits, indicated that subthreshold plasticity is not uniquely determined by the initial degree of input binocularity. The data raise the question of whether 5TPs are driven solely by 2/3Ps during MD. The different suprathreshold plasticity of the two cell populations could underlie distinct functional deficits in amblyopia.

Persistent cortical plasticity by upregulation of chondroitin 6-sulfation

Cortical plasticity is most evident during a critical period in early life, but the mechanisms that restrict plasticity after the critical period are poorly understood. We found that a developmental increase in the 4-sulfation/6-sulfation (4S/6S) ratio of chondroitin sulfate proteoglycans (CSPGs), which are components of the brain extracellular matrix, leads to the termination of the critical period for ocular dominance plasticity in the mouse visual cortex. Condensation of CSPGs into perineuronal nets that enwrapped synaptic contacts on parvalbumin-expressing interneurons was prevented by cell-autonomous overexpression of chondroitin 6-sulfation, which maintains a low 4S/6S ratio. Furthermore, the increase in the 4S/6S ratio was required for the accumulation of Otx2, a homeoprotein that activates the development of parvalbumin-expressing interneurons, and for functional maturation of the electrophysiological properties of these cells. Our results indicate that the critical period for cortical plasticity is regulated by the 4S/6S ratio of CSPGs, which determines the maturation of parvalbumin-expressing interneurons.

Experience-dependent regulation of CaMKII activity within single visual cortex synapses in vivo

Unbalanced visual input during development induces persistent alterations in the function and structure of visual cortical neurons. The molecular mechanisms that drive activity-dependent changes await direct visualization of underlying signals at individual synapses in vivo. By using a genetically engineered Förster resonance energy transfer (FRET) probe for the detection of CaMKII activity, and two-photon imaging of single synapses within identified functional domains, we have revealed unexpected and differential mechanisms in specific subsets of synapses in vivo. Brief monocular deprivation leads to activation of CaMKII in most synapses of layer 2/3 pyramidal cells within deprived eye domains, despite reduced visual drive, but not in nondeprived eye domains. Synapses that are eliminated in deprived eye domains have low basal CaMKII activity, implying a protective role for activated CaMKII against synapse elimination.

Presynaptic gating of postsynaptically expressed plasticity at mature thalamocortical synapses

Thalamocortical (TC) projections provide the major pathway for ascending sensory information to the mammalian neocortex. Arrays of these projections form synaptic inputs on thalamorecipient neurons, thus contributing to the formation of receptive fields (RFs) in sensory cortices. Experience-dependent plasticity of RFs persists throughout an organism's life span but in adults requires activation of cholinergic inputs to the cortex. In contrast, synaptic plasticity at TC projections is limited to the early postnatal period. This disconnect led to the widespread belief that TC synapses are the principal site of RF plasticity only in neonatal sensory cortices, but that they lose this plasticity upon maturation. Here, we tested an alternative hypothesis that mature TC projections do not lose synaptic plasticity but rather acquire gating mechanisms that prevent the induction of synaptic plasticity. Using whole-cell recordings and direct measures of postsynaptic and presynaptic activity (two-photon glutamate uncaging and two-photon imaging of the FM 1-43 assay, respectively) at individual synapses in acute mouse brain slices that contain the auditory thalamus and cortex, we determined that long-term depression (LTD) persists at mature TC synapses but is gated presynaptically. Cholinergic activation releases presynaptic gating through M(1) muscarinic receptors that downregulate adenosine inhibition of neurotransmitter release acting through A(1) adenosine receptors. Once presynaptic gating is released, mature TC synapses can express LTD postsynaptically through group I metabotropic glutamate receptors. These results indicate that synaptic plasticity at TC synapses is preserved throughout the life span and, therefore, may be a cellular substrate of RF plasticity in both neonate and mature animals.

Multisensory integration of natural odors and sounds in the auditory cortex

VIDEO ABSTRACT

Motherhood is associated with different forms of physiological alterations including transient hormonal changes and brain plasticity. The underlying impact of these changes on the emergence of maternal behaviors and sensory processing within the mother's brain are largely unknown. By using in vivo cell-attached recordings in the primary auditory cortex of female mice, we discovered that exposure to pups' body odor reshapes neuronal responses to pure tones and natural auditory stimuli. This olfactory-auditory interaction appeared naturally in lactating mothers shortly after parturition and was long lasting. Naive virgins that had experience with the pups also showed an appearance of olfactory-auditory integration in A1, suggesting that multisensory integration may be experience dependent. Neurons from lactating mothers were more sensitive to sounds as compared to those from experienced mice, independent of the odor effects. These uni- and multisensory cortical changes may facilitate the detection and discrimination of pup distress calls and strengthen the bond between mothers and their neonates.

Age-dependent effect of hearing loss on cortical inhibitory synapse function

The developmental plasticity of excitatory synapses is well established, particularly as a function of age. If similar principles apply to inhibitory synapses, then we would expect manipulations during juvenile development to produce a greater effect and experience-dependent changes to persist into adulthood. In this study, we first characterized the maturation of cortical inhibitory synapse function from just before the onset of hearing through adulthood. We then examined the long-term effects of developmental conductive hearing loss (CHL). Whole cell recordings from gerbil thalamocortical brain slices revealed a significant decrease in the decay time of inhibitory currents during the first 3 mo of normal development. When assessed in adults, developmental CHL led to an enduring decrease of inhibitory synaptic strength, whereas the maturation of synaptic decay time was only delayed. Early CHL also depressed the maximum discharge rate of fast-spiking, but not low-threshold-spiking, inhibitory interneurons. We then asked whether adult onset CHL had a similar effect, but neither inhibitory current amplitude nor decay time was altered. Thus inhibitory synapse function displays a protracted development during which deficits can be induced by juvenile, but not adult, hearing loss. These long-lasting changes to inhibitory function may contribute to the auditory processing deficits associated with early hearing loss.

Orientation selectivity of synaptic input to neurons in mouse and cat primary visual cortex

Primary visual cortex (V1) is the site at which orientation selectivity emerges in mammals: visual thalamus afferents to V1 respond equally to all stimulus orientations, whereas their target V1 neurons respond selectively to stimulus orientation. The emergence of orientation selectivity in V1 has long served as a model for investigating cortical computation. Recent evidence for orientation selectivity in mouse V1 opens cortical computation to dissection by genetic and imaging tools, but also raises two essential questions: (1) How does orientation selectivity in mouse V1 neurons compare with that in previously described species? (2) What is the synaptic basis for orientation selectivity in mouse V1? A comparison of orientation selectivity in mouse and in cat, where such measures have traditionally been made, reveals that orientation selectivity in mouse V1 is weaker than in cat V1, but that spike threshold plays a similar role in narrowing selectivity between membrane potential and spike rate. To uncover the synaptic basis for orientation selectivity, we made whole-cell recordings in vivo from mouse V1 neurons, comparing neuronal input selectivity-based on membrane potential, synaptic excitation, and synaptic inhibition-to output selectivity based on spiking. We found that a neuron's excitatory and inhibitory inputs are selective for the same stimulus orientations as is its membrane potential response, and that inhibitory selectivity is not broader than excitatory selectivity. Inhibition has different dynamics than excitation, adapting more rapidly. In neurons with temporally modulated responses, the timing of excitation and inhibition was different in mice and cats.

Loss of visually driven synaptic responses in layer 4 regular-spiking neurons of rat visual cortex in absence of competing inputs

Monocular deprivation (MD) during development shifts the ocular preference of primary visual cortex (V1) neurons by depressing closed-eye responses and potentiating open-eye responses. As these 2 processes are temporally and mechanistically distinct, we tested whether loss of responsiveness occurs also in absence of competing inputs. We thus compared the effects of long-term MD in layer 4 regular-spiking pyramidal neurons (L4Ns) of binocular and monocular V1 (bV1 and mV1) with whole-cell recordings. In bV1, input depression was larger than potentiation, and the ocular dominance shift was larger for spike outputs. MD-but not retinal inactivation with tetrodotoxin-caused a comparable loss of synaptic and spike responsiveness in mV1, which is innervated only by the deprived eye. Conversely, brief MD depressed synaptic responses only in bV1. MD-driven depression in mV1 was accompanied by a proportional reduction of visual thalamic inputs, as assessed upon pharmacological silencing of intracortical transmission. Finally, sub- and suprathreshold responsiveness was similarly degraded in L4Ns of bV1 upon complete deprivation of patterned vision through a binocular deprivation period of comparable length. Thus, loss of synaptic inputs from the deprived eye occurs also in absence of competition in the main thalamorecipient lamina, albeit at a slower pace.

Brain plasticity and recovery from early cortical injury

Neocortical development represents more than a simple unfolding of a genetic blueprint: rather, it represents a complex dance of genetic and environmental events that interact to adapt the brain to fit a particular environmental context. Most cortical regions are sensitive to a wide range of experiential factors during development and later in life, but the injured cortex appears to be unusually sensitive to perinatal experiences. This paper reviews the factors that influence how normal and injured brains (both focal and ischemic injuries) develop and adapt into adulthood. Such factors include prenatal experiences in utero as well as postnatal experiences throughout life. Examples include the effects of sensory and motor stimulation, psychoactive drugs (including illicit and prescription drugs), maternal and postnatal stress, neurotrophic factors, and pre- and postnatal diet. All these factors influence cerebral development and influence recovery from brain injury during development.

Genetics and function of neocortical GABAergic interneurons in neurodevelopmental disorders

A dysfunction of cortical and limbic GABAergic circuits has been postulated to contribute to multiple neurodevelopmental disorders in humans, including schizophrenia, autism, and epilepsy. In the current paper, I summarize the characteristics that underlie the great diversity of cortical GABAergic interneurons and explore how the multiple roles of these cells in developing and mature circuits might contribute to the aforementioned disorders. Furthermore, I review the tightly controlled genetic cascades that determine the fate of cortical interneurons and summarize how the dysfunction of genes important for the generation, specification, maturation, and function of cortical interneurons might contribute to these disorders.

The role of GABAergic inhibition in ocular dominance plasticity

During the last decade, we have gained much insight into the mechanisms that open and close a sensitive period of plasticity in the visual cortex. This brings the hope that novel treatments can be developed for brain injuries requiring renewed plasticity potential and neurodevelopmental brain disorders caused by defective synaptic plasticity. One of the central mechanisms responsible for opening the sensitive period is the maturation of inhibitory innervation. Many molecular and cellular events have been identified that drive this developmental process, including signaling through BDNF and IGF-1, transcriptional control by OTX2, maturation of the extracellular matrix, and GABA-regulated inhibitory synapse formation. The mechanisms through which the development of inhibitory innervation triggers and potentially closes the sensitive period may involve plasticity of inhibitory inputs or permissive regulation of excitatory synapse plasticity. Here, we discuss the current state of knowledge in the field and open questions to be addressed.

The many forms and functions of long term plasticity at GABAergic synapses

On February 12th 1973, Bliss and Lomo submitted their findings on activity-dependent plasticity of glutamatergic synapses. After this groundbreaking discovery, long-term potentiation (LTP) and depression (LTD) gained center stage in the study of learning, memory, and experience-dependent refinement of neural circuits. While LTP and LTD are extensively studied and their relevance to brain function is widely accepted, new experimental and theoretical work recently demonstrates that brain development and function relies on additional forms of plasticity, some of which occur at nonglutamatergic synapses. The strength of GABAergic synapses is modulated by activity, and new functions for inhibitory synaptic plasticity are emerging. Together with excitatory neurons, inhibitory neurons shape the excitability and dynamic range of neural circuits. Thus, the understanding of inhibitory synaptic plasticity is crucial to fully comprehend the physiology of brain circuits. Here, I will review recent findings about plasticity at GABAergic synapses and discuss how it may contribute to circuit function.

Influence of a subtype of inhibitory interneuron on stimulus-specific responses in visual cortex

Inhibition modulates receptive field properties and integrative responses of neurons in cortical circuits. The contribution of specific interneuron classes to cortical circuits and emergent responses is unknown. Here, we examined neuronal responses in primary visual cortex (V1) of adult Dlx1(-/-) mice, which have a selective reduction in cortical dendrite-targeting interneurons (DTIs) that express calretinin, neuropeptide Y, and somatostatin. The V1 neurons examined in Dlx1(-/-) mice have reduced orientation selectivity and altered firing rates, with elevated late responses, suggesting that local inhibition at dendrites has a specific role in modulating neuronal computations. We did not detect overt changes in the physiological properties of thalamic relay neurons and features of thalamocortical projections, such as retinotopic maps and eye-specific inputs, in the mutant mice, suggesting that the defects are cortical in origin. These experimental results are well explained by a computational model that integrates broad tuning from dendrite-targeting and narrower tuning from soma-targeting interneuron subclasses. Our findings suggest a key role for DTIs in the fine-tuning of stimulus-specific cortical responses.

Visual tuning properties of genetically identified layer 2/3 neuronal types in the primary visual cortex of cre-transgenic mice

The putative excitatory and inhibitory cell classes within the mouse primary visual cortex V1 have different functional properties as studied using recording microelectrode. Excitatory neurons show high selectivity for the orientation angle of moving gratings while the putative inhibitory neurons show poor selectivity. However, the study of selectivity of the genetically identified interneurons and their subtypes remain controversial. Here we use novel Cre-driver and reporter mice to identify genetic subpopulations in vivo for two-photon calcium dye imaging: Wfs1(+)/Gad1(−) mice that labels layer 2/3 excitatory cell population and Pvalb(+)/Gad1(+) mice that labels a genetic subpopulation of inhibitory neurons. The cells in both mice were identically labeled with a tdTomato protein, visible in vivo, using a Cre-reporter line. We found that the Wfs1(+) cells exhibited visual tuning properties comparable to the excitatory population, i.e., high selectivity and tuning to the angle, direction, and spatial frequency of oriented moving gratings. The functional tuning of Pvalb(+) neurons was consistent with previously reported narrow-spiking interneurons in microelectrode studies, exhibiting poorer selectivity than the excitatory neurons. This study demonstrates the utility of Cre-transgenic mouse technology in selective targeting of subpopulations of neurons and makes them amenable to structural, functional, and connectivity studies.

Removing brakes on adult brain plasticity: from molecular to behavioral interventions

Adult brain plasticity, although possible, remains more restricted in scope than during development. Here, we address conditions under which circuit rewiring may be facilitated in the mature brain. At a cellular and molecular level, adult plasticity is actively limited. Some of these "brakes" are structural, such as perineuronal nets or myelin, which inhibit neurite outgrowth. Others are functional, acting directly upon excitatory-inhibitory balance within local circuits. Plasticity in adulthood can be induced either by lifting these brakes through invasive interventions or by exploiting endogenous permissive factors, such as neuromodulators. Using the amblyopic visual system as a model, we discuss genetic, pharmacological, and environmental removal of brakes to enable recovery of vision in adult rodents. Although these mechanisms remain largely uncharted in the human, we consider how they may provide a biological foundation for the remarkable increase in plasticity after action video game play by amblyopic subjects.

Structure and function relationships during ocular dominance plasticity in the visual cortex

Our ability to learn relies on the potential of the neocortex to change its neuronal circuits through experience. This change is mediated by the loss or formation of synaptic contacts or the adjustment of their synaptic strength. In recent decades, the primary visual cortex has proven an excellent system for studying structure/function relationships during plasticity in the neocortex. Here we describe current knowledge about the structural changes in inhibitory or excitatory synapses that accompany experience dependent plasticity in the visual cortex. We discuss unresolved issues and technical developments that will help to provide answers in the near future.

Altered expression of parvalbumin and calbindin in interneurons within the primary visual cortex of neonatal enucleated hamsters

In the present study, we tested the hypothesis that the expression of calcium binding proteins (CaBPs), parvalbumin (PV), calretinin (CR) and calbindin (CB), is dependent upon sensory experience as emphasized in visual deprivation and deafferentation studies. The expression of CaBPs was studied in interneurons within the primary and extrastriate visual cortices (V1, V2M, V2L) and auditory cortex (AC) of adult hamsters enucleated at birth. The effects of enucleation were mainly confined to area V1 where there was a significant volume reduction (26%) and changes in the laminar distribution of PV and CB immunoreactive (IR) cells. The density of PV-IR cell bodies was significantly increased in layer IV and reduced in layer V. Moreover, the density of CB-IR neurons was inferior in layer V of V1 in enucleated hamsters (EH) compared to controls. These results suggest that some features of the laminar distribution of specific CaBPs, in primary sensory cortices, are dependent upon or modulated by sensory input.

Experience-dependent changes in excitatory and inhibitory receptor subunit expression in visual cortex

Experience-dependent development of visual cortex depends on the balance between excitatory and inhibitory activity. This activity is regulated by key excitatory (NMDA, AMPA) and inhibitory (GABA_A) receptors. The composition of these receptors changes developmentally, affecting the excitatory–inhibitory (E/I) balance and synaptic plasticity. Until now, it has been unclear how abnormal visual experience affects this balance. To examine this question, we measured developmental changes in excitatory and inhibitory receptor subunits in visual cortex following normal visual experience and monocular deprivation. We used Western blot analysis to quantify expression of excitatory (NR1, NR2A, NR2B, GluR2) and inhibitory (GABA_Aα1, GABA_Aα3) receptor subunits. Monocular deprivation promoted a complex pattern of changes in receptor subunit expression that varied with age and was most severe in the region of visual cortex representing the central visual field. To characterize the multidimensional pattern of experience-dependent change in these synaptic mechanisms, we applied a neuroinformatics approach using principal component analysis. We found that monocular deprivation (i) causes a large portion of the normal developmental trajectory to be bypassed, (ii) shifts the E/I balance in favor of more inhibition, and (iii) accelerates the maturation of receptor subunits. Taken together, these results show that monocularly deprived animals have an abnormal balance of the synaptic machinery needed for functional maturation of cortical circuits and for developmental plasticity. This raises the possibility that interventions intended to treat amblyopia may need to address multiple synaptic mechanisms to produce optimal recovery.

Cerebellar molecular layer interneurons - computational properties and roles in learning

In recent years there has been an increased interest in the function of inhibitory interneurons. In the cerebellum this interest has been paired with successes in obtaining recordings from these neurons in vivo and genetic manipulations to probe their function during behavioral tasks such as motor learning. This review focuses on a synthesis of recent findings on the computational properties that these neurons confer to the cerebellar circuitry and on their recently discovered capacity for plasticity and learning in vivo. Since the circuitry of the cerebellar cortex is relatively well-defined, the specificity with which the potential roles of these interneurons can be described will help to guide new avenues of research on the functions of interneurons in general.

Response features of parvalbumin-expressing interneurons suggest precise roles for subtypes of inhibition in visual cortex

Inhibitory interneurons in the cerebral cortex include a vast array of subtypes, varying in their molecular signatures, electrophysiological properties, and connectivity patterns. This diversity suggests that individual inhibitory classes have unique roles in cortical circuits; however, their characterization to date has been limited to broad classifications including many subtypes. We used the Cre/LoxP system, specifically labeling parvalbumin(PV)-expressing interneurons in visual cortex of PV-Cre mice with red fluorescent protein (RFP), followed by targeted loose-patch recordings and two-photon imaging of calcium responses in vivo to characterize the visual receptive field properties of these cells. Despite their relative molecular and morphological homogeneity, we find that PV+ neurons have a diversity of feature-specific visual responses that include sharp orientation and direction-selectivity, small receptive fields, and band-pass spatial frequency tuning. These results suggest that subsets of parvalbumin interneurons are components of specific cortical networks and that perisomatic inhibition contributes to the generation of precise response properties.

Experience-driven plasticity in binocular vision

Experience-driven neuronal plasticity allows the brain to adapt its functional connectivity to recent sensory input. Here we use binocular rivalry, an experimental paradigm in which conflicting images are presented to the individual eyes, to demonstrate plasticity in the neuronal mechanisms that convert visual information from two separated retinas into single perceptual experiences. Perception during binocular rivalry tended to initially consist of alternations between exclusive representations of monocularly defined images, but upon prolonged exposure, mixture percepts became more prevalent. The completeness of suppression, reflected in the incidence of mixture percepts, plausibly reflects the strength of inhibition that likely plays a role in binocular rivalry. Recovery of exclusivity was possible but required highly specific binocular stimulation. Documenting the prerequisites for these observed changes in perceptual exclusivity, our experiments suggest experience-driven plasticity at interocular inhibitory synapses, driven by the correlated activity (and also the lack thereof) of neurons representing the conflicting stimuli. This form of plasticity is consistent with a previously proposed but largely untested anti-Hebbian learning mechanism for inhibitory synapses in vision. Our results implicate experience-driven plasticity as one governing principle in the neuronal organization of binocular vision.

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

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

Cell type-specific long-term plasticity at glutamatergic synapses onto hippocampal interneurons expressing either parvalbumin or CB1 cannabinoid receptor

Different GABAergic interneuron types have specific roles in hippocampal function, and anatomical as well as physiological features vary greatly between interneuron classes. Long-term plasticity of interneurons has mostly been studied in unidentified GABAergic cells and is known to be very heterogeneous. Here we tested whether cell type-specific plasticity properties in distinct GABAergic interneuron types might underlie this heterogeneity. We show that long-term potentiation (LTP) and depression (LTD), two common forms of synaptic plasticity, are expressed in a highly cell type-specific manner at glutamatergic synapses onto hippocampal GABAergic neurons. Both LTP and LTD are generated in interneurons expressing parvalbumin (PV+), whereas interneurons with similar axon distributions but expressing cannabinoid receptor-1 show no lasting plasticity in response to the same protocol. In addition, LTP or LTD occurs in PV+ interneurons with different efferent target domains. Perisomatic-targeting PV+ basket and axo-axonic interneurons express LTP, whereas glutamatergic synapses onto PV+ bistratified cells display LTD. Both LTP and LTD are pathway specific, independent of NMDA receptors, and occur at synapses with calcium-permeable (CP) AMPA receptors. Plasticity in interneurons with CP-AMPA receptors strongly modulates disynaptic GABAergic transmission onto CA1 pyramidal cells. We propose that long-term plasticity adjusts the synaptic strength between pyramidal cells and interneurons in a cell type-specific manner and, in the defined CA1 interneurons, shifts the spatial pattern of inhibitory weight from pyramidal cell dendrites to the perisomatic region.

Spike-timing dependent plasticity in inhibitory circuits

Inhibitory circuits in the brain rely on GABA-releasing interneurons. For long, inhibitory circuits were considered weakly plastic in the face of patterns of neuronal activity that trigger long-term changes in the synapses between excitatory principal cells. Recent studies however have shown that GABAergic circuits undergo various forms of long-term plasticity. For the purpose of this review, we identify three major long-term plasticity expression sites. The first locus is the glutamatergic synapses that excite GABAergic inhibitory cells and drive their activity. Such synapses, on many but not all inhibitory interneurons, exhibit long-term potentiation (LTP) and depression (LTD). Second, GABAergic synapses themselves can undergo changes in GABA release probability or postsynaptic GABA receptors. The third site of plasticity is in the postsynaptic anion gradient of GABAergic synapses; coincident firing of GABAergic axons and postsynaptic neurons can cause a long-lasting change in the reversal potential of GABA_A receptors mediating fast inhibitory postsynaptic potentials. We review the recent literature on these forms of plasticity by asking how they may be triggered by specific patterns of pre- and postsynaptic action potentials, although very few studies have directly examined spike-timing dependent plasticity (STDP) protocols in inhibitory circuits. Plasticity of interneuron recruitment and of GABAergic signaling provides for a rich flexibility in inhibition that may be central to many aspects of brain function. We do not consider plasticity at glutamatergic synapses on Purkinje cells and other GABAergic principal cells.

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

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

Relative contribution of feedforward excitatory connections to expression of ocular dominance plasticity in layer 4 of visual cortex

Brief monocular deprivation (MD) shifts ocular dominance (OD) in primary visual cortex by causing depression of responses to the deprived eye. Here we address the extent to which the shift is expressed by a modification of excitatory synaptic transmission. An OD shift was first induced with 3 days of MD, and then the influences of intracortical polysynaptic inhibitory and excitatory synapses were pharmacologically removed, leaving only "feedforward" thalamocortical synaptic currents. The results show that the rapid OD shift following MD is strongly expressed at the level of thalamocortical synaptic transmission.