Functional maturation of visual cortex is linked with dynamic changes in synaptic expression of GABAergic mechanisms. These include setting the excitation–inhibition balance required for experience-dependent plasticity, as well as, intracortical inhibition underlying development and aging of receptive field properties. Animal studies have shown that there is developmental regulation of GABAergic mechanisms in visual cortex. In this study, we show for the first time how these mechanisms develop in the human visual cortex across the lifespan. We used Western blot analysis of postmortem tissue from human primary visual cortex (n = 30, range: 20 days to 80 years) to quantify expression of eight pre- and post-synaptic GABAergic markers. We quantified the inhibitory modulating cannabinoid receptor (CB1), GABA vesicular transporter (VGAT), GABA synthesizing enzymes (GAD65/GAD67), GABAA receptor anchoring protein (Gephyrin), and GABAA receptor subunits (GABAAα1, GABAAα2, GABAAα3). We found a complex pattern of different developmental trajectories, many of which were prolonged and continued well into the teen, young adult, and even older adult years. These included a monotonic increase or decrease (GABAAα1, GABAAα2), a biphasic increase then decrease (GAD65, Gephyrin), or multiple increases and decreases (VGAT, CB1) across the lifespan. Comparing the balances between the pre- and post-synaptic markers we found three main transition stages (early childhood, early teen years, aging) when there were rapid switches in the composition of the GABAergic signaling system, indicating that functioning of the GABAergic system must change as the visual cortex develops and ages. Furthermore, these results provide key information for translating therapies developed in animal models into effective treatments for amblyopia in humans.
GABA; development; human; aging; visual cortex; inhibition; plasticity
AMPA-receptor trafficking plays a central role in excitatory plasticity, especially during development. Changes in the number of AMPA receptors and time spent at the synaptic surface are important factors of plasticity that directly affect long-term potentiation (LTP), long-term depression (LTD), synaptic scaling, and the excitatory-inhibitory (E/I) balance in the developing cortex. Experience-dependent changes in synaptic strength in visual cortex (V1) use a molecularly distinct AMPA trafficking pathway that includes the GluA2 subunit. We studied developmental changes in AMPA receptor trafficking proteins by quantifying expression of GluA2, pGluA2 (GluA2serine880), GRIP1, and PICK1 in rat visual and frontal cortex. We used Western Blot analysis of synaptoneurosome preparations of rat visual and frontal cortex from animals ranging in age from P0 to P105. GluA2 and pGluA2 followed different developmental trajectories in visual and frontal cortex, with a brief period of over expression in frontal cortex. The over expression of GluA2 and pGluA2 in immature frontal cortex raises the possibility that there may be a period of GluA2-dependent vulnerability in frontal cortex that is not found in V1. In contrast, GRIP1 and PICK1 had the same developmental trajectories and were expressed very early in development of both cortical areas. This suggests that the AMPA-interacting proteins are available to begin trafficking receptors as soon as GluA2-containing receptors are expressed. Finally, we used all four proteins to analyze the surface-to-internalization balance and found that this balance was roughly equal across both cortical regions, and throughout development. Our finding of an exquisite surface-to-internalization balance highlights that these AMPA receptor trafficking proteins function as a tightly controlled system in the developing cortex.
AMPA receptor; trafficking; critical period; GRIP; PICK1; visual cortex; frontal cortex; synaptic plasticity
Sensory experience, and the lack thereof, can alter the function of excitatory synapses in the primary sensory cortices. Recent evidence suggests that changes in sensory experience can regulate the synaptic level of Ca2+-permeable AMPA receptors (CP-AMPARs). However, the molecular mechanisms underlying such a process have not been determined. We found that binocular visual deprivation, which is a well-established in vivo model to produce multiplicative synaptic scaling in visual cortex of juvenile rodents, is accompanied by an increase in the phosphorylation of AMPAR GluR1 (or GluA1) subunit at the serine 845 (S845) site and the appearance of CP-AMPARs at synapses. To address the role of GluR1-S845 in visual deprivation-induced homeostatic synaptic plasticity, we used mice lacking key phosphorylation sites on the GluR1 subunit. We found that mice specifically lacking the GluR1-S845 site (GluR1-S845A mutants), which is a substrate of cAMP-dependent kinase (PKA), show abnormal basal excitatory synaptic transmission and lack visual deprivation-induced homeostatic synaptic plasticity. We also found evidence that increasing GluR1-S845 phosphorylation alone is not sufficient to produce normal multiplicative synaptic scaling. Our study provides concrete evidence that a GluR1 dependent mechanism, especially S845 phosphorylation, is a necessary pre-requisite step for in vivo homeostatic synaptic plasticity.
The developmental increase in the strength of inhibitory synaptic circuits defines the time window of the critical period for plasticity in sensory cortices. Conceptually, plasticity of inhibitory synapses is an attractive mechanism to allow for homeostatic adaptation to the sensory environment. However, a brief duration of visual deprivation that causes maximal change in excitatory synapses produces minimal change in inhibitory synaptic transmission. Here we examined developmental and experience-dependent changes in inhibition by measuring miniature IPSCs (mIPSCs) in layer 2/3 pyramidal neurons of mouse visual cortex. During development from postnatal day 21 (P21) to P35, GABAA receptor function changed from fewer higher-conductance channels to more numerous lower-conductance channels without altering the average mIPSC amplitude. Although a week of visual deprivation did not alter the average mIPSC amplitude, a subsequent 2 h exposure to light produced a rapid rebound potentiation. This form of plasticity is restricted to a critical period before the developmental change in GABAergic synaptic properties is completed, and hence is absent by P35. Visual experience-dependent rebound potentiation of mIPSCs is accompanied by an increase in the open channel number and requires activity-dependent transcription of brain-derived neurotrophic factor (BDNF). Mice lacking BDNF transcription through promoter IV did not show developmental changes in inhibition and lacked rebound potentiation. Our results suggest that sensory experience may have distinct functional consequences in normal versus deprived sensory cortices, and that experience-dependent BDNF expression controls the plasticity of inhibitory synaptic transmission particularly when recovering vision during the critical period.
activity-dependent; BDNF; critical period; mIPSC; potentiation; visual cortex
Inhibition from fast-spiking (FS) interneurons plays a crucial role in shaping cortical response properties and gating developmental periods of activity-dependent plasticity, yet the expression mechanisms underlying FS inhibitory plasticity remain largely unexplored. In layer 4 of visual cortex (V1), monocular deprivation (MD) induces either depression or potentiation of FS to star pyramidal neuron (FS→SP) synapses, depending on the age of onset (Maffei et al., 2004, 2006). This reversal in the sign (− to +) of plasticity occurs on the cusp of the canonical critical period (CP). To investigate the expression locus behind this switch in sign of inhibitory plasticity, mice underwent MD during the pre-CP [eye-opening to postnatal day (p)17] or CP (p22–p25), and FS→SP synaptic strength within layer 4 was assessed using confocal and immunoelectron microscopy, as well as optogenetic activation of FS cells to probe quantal amplitude at FS→SP synapses. Brief MD before p17 or p25 did not alter the density of FS→SP contacts. However, at the ultrastructural level, FS→SP synapses in deprived hemispheres during the CP, but not the pre-CP or in GAD65 knock-out mice, had larger synapses and increased docked vesicle density compared with synapses from the nondeprived control hemispheres. Moreover, FS→SP evoked miniature IPSCs increased in deprived hemispheres when MD was initiated during the CP, accompanied by an increase in the density of postsynaptic GABAA receptors at FS→SP synapses. These coordinated changes in FS→SP synaptic strength define an expression pathway modulating excitatory output during CP plasticity in visual cortex.
electron microscopy; fast-spiking; LTP; monocular deprivation; optogenetic; pyramidal
Monocular deprivation (MD) during development leads to a dramatic loss of responsiveness through the deprived eye in primary visual cortical neurons, and to degraded spatial vision (amblyopia) in all species tested so far, including rodents. Such loss of responsiveness is accompanied since the beginning by a decreased excitatory drive from the thalamo-cortical inputs. However, in the thalamorecipient layer 4, inhibitory interneurons are initially unaffected by MD and their synapses onto pyramidal cells potentiate. It remains controversial whether ocular dominance plasticity similarly or differentially affects the excitatory and inhibitory synaptic conductances driven by visual stimulation of the deprived eye and impinging onto visual cortical pyramids, after a saturating period of MD. To address this issue, we isolated visually-driven excitatory and inhibitory conductances by in vivo whole-cell recordings from layer 4 regular-spiking neurons in the primary visual cortex (V1) of juvenile rats. We found that a saturating period of MD comparably reduced visually–driven excitatory and inhibitory conductances driven by visual stimulation of the deprived eye. Also, the excitatory and inhibitory conductances underlying the synaptic responses driven by the ipsilateral, left open eye were similarly potentiated compared to controls. Multiunit recordings in layer 4 followed by spike sorting indicated that the suprathreshold loss of responsiveness and the MD-driven ocular preference shifts were similar for narrow spiking, putative inhibitory neurons and broad spiking, putative excitatory neurons. Thus, by the time the plastic response has reached a plateau, inhibitory circuits adjust to preserve the normal balance between excitation and inhibition in the cortical network of the main thalamorecipient layer.
Visual experience scales down excitatory synapses in the superficial layers of visual cortex in a process that provides an in vivo paradigm of homeostatic synaptic scaling. Experience-induced increases in neural activity rapidly up-regulates mRNAs of immediate early genes (IEGs) involved in synaptic plasticity, one of which is Arc (activity-regulated cytoskeleton protein or Arg3.1). Cell biological studies indicate that Arc/Arg3.1 protein functions to recruit endocytic machinery for AMPA receptor (AMPAR) internalization, and this action, together with its activity-dependent expression, rationalizes a role for Arc/Arg3.1 in homeostatic synaptic scaling. Here, we investigated the role of Arc/Arg3.1 in homeostatic scaling in vivo by examining experience-dependent development of layer 2/3 (L2/3) neurons in the visual cortex of Arc/Arg3.1 knockout (KO) mice. Arc/Arg3.1 KOs show minimal changes in basal and developmental regulation of excitatory synaptic strengths, but display a profound deficit in homeostatic regulation of excitatory synapses by visual experience. As further evidence of specificity, we found that the visual experience-induced regulation of inhibitory synapses is normal, although the basal inhibitory synaptic strength is increased in the Arc/Arg3.1 KOs. Our results demonstrate that Arc/Arg3.1 plays a selective role in regulating visual experience-dependent homeostatic plasticity of excitatory synaptic transmission in vivo.
sensory cortex; mEPSC; mIPSC; synaptic scaling; immediate early gene; development
Neuronal output is shaped by a balance of excitation and inhibition. How this balance is attained in the central nervous system during development is not well understood, and is complicated by the fact that, in vivo, GABAergic and glycinergic synaptogenesis precedes that of glutamatergic synapses. Here, we determined the distributions of inhibitory postsynaptic sites on the dendritic arbors of individual neurons, and compared their developmental patterns with that of excitatory postsynaptic sites. We focused on retinal ganglion cells (RGCs), the output neurons of the retina, which receive excitatory input from bipolar cells and inhibitory input from amacrine cells. To visualize and map inhibitory postsynaptic sites, we generated transgenic mice in which RGCs express fluorescently tagged Neuroligin 2 (YFP-NL2) under the control of the Thy1 promoter. By labeling RGC dendrites biolistically in YFP-NL2-expressing retinas, we were able to map the spatial distribution and thus densities of inhibitory postsynaptic sites on the dendritic arbors of individual large-field RGCs across ages.
We demonstrate that YFP-NL2 is present at inhibitory synapses in the inner plexiform layer by its co-localization with gephyrin, the γ2 subunit of the GABAA receptor and glycine receptors. YFP-NL2 puncta were apposed to the vesicular inhibitory transmitter transporter VGAT but not to CtBP2, a marker of presynaptic ribbons found at bipolar cell terminals. Similar patterns of co-localization with synaptic markers were observed for endogenous NL2. We also verified that expression of YFP-NL2 in the transgenic line did not significantly alter spontaneous inhibitory synaptic transmission onto RGCs. Using these mice, we found that, on average, the density of inhibitory synapses on individual arbors increased gradually until eye opening (postnatal day 15). A small centro-peripheral gradient in density found in mature arbors was apparent at the earliest age we examined (postnatal day 8). Unexpectedly, the adult ratio of inhibitory/excitatory postsynaptic sites was rapidly attained, shortly after glutamatergic synaptogenesis commenced (postnatal day 7).
Our observations suggest that bipolar and amacrine cell synaptogenesis onto RGCs appear coordinated to rapidly attain a balanced ratio of excitatory and inhibitory synapse densities prior to the onset of visual experience.
Excessive exposure to loud noise can damage the cochlea and create a hearing loss. These pathologies coincide with a range of CNS changes including reorganisation of frequency representation, alterations in the pattern of spontaneous activity and changed expression of excitatory and inhibitory neurotransmitters. Moreover, damage to the cochlea is often accompanied by acoustic disorders such as hyperacusis and tinnitus, suggesting that one or more of these neuronal changes may be involved in these disorders, although the mechanisms remain unknown. We tested the hypothesis that excessive noise exposure increases expression of markers of excitation and plasticity, and decreases expression of inhibitory markers over a 32-day recovery period. Adult rats (n = 25) were monaurally exposed to a loud noise (16 kHz, 1/10th octave band pass (115 dB SPL)) for 1-hour, or left as non-exposed controls (n = 5). Animals were euthanased at either 0, 4, 8, 16 or 32 days following acoustic trauma. We used Western Blots to quantify protein levels of GABAA receptor subunit α1 (GABAAα1), Glutamic-Acid Decarboxylase-67 (GAD-67), N-Methyl-D-Aspartate receptor subunit 2A (NR2A), Calbindin (Calb1) and Growth Associated Protein 43 (GAP-43) in the Auditory Cortex (AC), Inferior Colliculus (IC) and Dorsal Cochlear Nucleus (DCN). Compared to sham-exposed controls, noise-exposed animals had significantly (p<0.05): lower levels of GABAAα1 in the contralateral AC at day-16 and day-32, lower levels of GAD-67 in the ipsilateral DCN at day-4, lower levels of Calb1 in the ipsilateral DCN at day-0, lower levels of GABAAα1 in the ipsilateral AC at day-4 and day-32. GAP-43 was reduced in the ipsilateral AC for the duration of the experiment. These complex fluctuations in protein expression suggests that for at least a month following acoustic trauma the auditory system is adapting to a new pattern of sensory input.
Monocular deprivation (MD) is a classic paradigm for experience-dependent cortical plasticity. One form is known as homeostatic plasticity, in which neurons innervated by the deprived eye show a remarkable capacity to compensate for degraded visual signals in an attempt to stabilize network activity. While the evidence supporting homeostatic plasticity in visual cortex is extensive, it remains unclear whether neurons in subcortical visual structures respond to MD in a similar manner. Here we examined if cells in the dorsal lateral geniculate nucleus (dLGN), the thalamic relay between the retina and visual cortex, show similar forms of experience-dependent homeostatic plasticity following MD. Two-week old mice were monocularly deprived for a period of 5-7 days and miniature excitatory postsynaptic currents (mEPSCs) were obtained from cells located in dLGN regions receiving input from the deprived or nondeprived eye. We found that MD promotes increases in the frequency and amplitude of mEPSCs, and were restricted to the monocular segment contralateral to the deprived eye. These changes were accompanied by an increase in the probability of glutamate release at corticothalamic terminals that arise from the deprived visual cortex. Our findings indicate that homeostatic synaptic regulation from MD extends beyond cortical circuitry and shed light on how the brain modulates and integrates activity in the face of altered sensory experience.
thalamus; visual cortex; visual deprivation; ocular dominance plasticity
Ocular dominance plasticity (ODP) following monocular deprivation (MD) is a model of activity dependent neural plasticity that is restricted to an early critical period regulated by maturation of inhibition. Unique developmental plasticity mechanisms may improve outcomes following early brain injury. Our objective was to determine the effects of neonatal cerebral hypoxia ischemia (HI) on ODP. The rationale extends from observations that neonatal HI results in death of subplate neurons, a transient population known to influence development of inhibition. In rodents subjected to neonatal HI and controls, maps of visual response were derived from optical imaging during the critical period for ODP and changes in the balance of eye-specific response following MD were measured. In controls, MD results in a shift of the ocular dominance index (ODI) from a baseline of 0.15 to −0.10 (P<0.001). Neonatal HI with moderate cortical injury impairs this shift, ODI = 0.14 (P<0.01). Plasticity was intact in animals with mild injury and in those exposed to hypoxia alone. Neonatal HI resulted in decreased parvalbumin expression in hemispheres receiving HI compared with hypoxia alone: 23.4 vs. 35.0 cells/high power field (P=0.01), with no change in other markers of inhibitory or excitatory neurons. Despite abnormal inhibitory neuron phenotype, spontaneous activity of single units and development of orientation selective responses were intact following neonatal HI, while overall visual responses were reduced. Our data suggest that specific plasticity mechanisms are impaired following early brain injury and that the impairment is associated with altered inhibitory neuronal development and cortical activation.
brain injury; ocular dominance; inhibition; development; Brain Development; Imaging; Neonatal
Two theories have influenced our understanding of cortical development: the integrated network theory, where synaptic development is coordinated across areas; and the cascade theory, where the cortex develops in a wave-like manner from sensory to non-sensory areas. These different views on cortical development raise challenges for current studies aimed at comparing detailed maturation of the connectome among cortical areas. We have taken a different approach to compare synaptic development in rat visual, somatosensory, and frontal cortex by measuring expression of pre-synaptic (synapsin and synaptophysin) proteins that regulate vesicle cycling, and post-synaptic density (PSD-95 and Gephyrin) proteins that anchor excitatory or inhibitory (E-I) receptors. We also compared development of the balances between the pairs of pre- or post-synaptic proteins, and the overall pre- to post-synaptic balance, to address functional maturation and emergence of the E-I balance. We found that development of the individual proteins and the post-synaptic index overlapped among the three cortical areas, but the pre-synaptic index matured later in frontal cortex. Finally, we applied a neuroinformatics approach using principal component analysis and found that three components captured development of the synaptic proteins. The first component accounted for 64% of the variance in protein expression and reflected total protein expression, which overlapped among the three cortical areas. The second component was gephyrin and the E-I balance, it emerged as sequential waves starting in somatosensory, then frontal, and finally visual cortex. The third component was the balance between pre- and post-synaptic proteins, and this followed a different developmental trajectory in somatosensory cortex. Together, these results give the most support to an integrated network of synaptic development, but also highlight more complex patterns of development that vary in timing and end point among the cortical areas.
synapsin; synaptophysin; PSD-95; gephyrin; critical period; integrated network; E-I balance; cortical development
Monocular deprivation (MD) during the critical period (CP) shifts ocular dominance (OD) of cortical responsiveness toward the nondeprived eye. The synaptic mechanisms underlying MD-induced OD plasticity, in particular the contribution of cortical inhibition to the plasticity, have remained unsolved. In this study, using in vivo whole-cell voltage-clamp recordings, we revealed eye-specific excitatory and inhibitory synaptic inputs to layer 4 excitatory neurons in mouse primary visual cortex (V1) at a developmental stage close to the end of CP. We found in normally reared mice that ocular preference is primarily determined by the contralateral bias of excitatory input and that inhibition does not play an active role in shaping OD. MD results in a parallel reduction of excitation and inhibition driven by the deprived eye, while reducing the inhibition but preserving the excitation driven by the nondeprived eye. MD of longer periods causes larger changes in synaptic amplitude than MD of shorter periods. Furthermore, MD resulted in a shortening of onset latencies of synaptic inputs activated by both contralateral and ipsilateral eye stimulation, while the relative temporal relationship between excitation and inhibition driven by the same eye was not significantly affected. Our results suggest that OD plasticity is largely attributed to a reduction of feedforward input representing the deprived eye, and that an unexpected weakening of cortical inhibitory connections accounts for the increased responsiveness to the nondeprived eye.
The nervous system is highly sensitive to experience during early postnatal life, but this phase of heightened plasticity decreases with age. Recent studies have demonstrated that developmental-like plasticity can be reactivated in the visual cortex of adult animals through environmental or pharmacological manipulations. These findings provide a unique opportunity to study the cellular and molecular mechanisms of adult plasticity. Here we used the monocular deprivation paradigm to investigate large-scale gene expression patterns underlying the reinstatement of plasticity produced by fluoxetine in the adult rat visual cortex. We found changes, confirmed with RT-PCRs, in gene expression in different biological themes, such as chromatin structure remodelling, transcription factors, molecules involved in synaptic plasticity, extracellular matrix, and excitatory and inhibitory neurotransmission. Our findings reveal a key role for several molecules such as the metalloproteases Mmp2 and Mmp9 or the glycoprotein Reelin and open up new insights into the mechanisms underlying the reopening of the critical periods in the adult brain.
Amblyopia, a developmental disorder of the visual cortex, is one of the leading causes of visual dysfunction in the working age population. Current estimates put the prevalence of amblyopia at approximately 1-3%1-3, the majority of cases being monocular2. Amblyopia is most frequently caused by ocular misalignment (strabismus), blur induced by unequal refractive error (anisometropia), and in some cases by form deprivation.
Although amblyopia is initially caused by abnormal visual input in infancy, once established, the visual deficit often remains when normal visual input has been restored using surgery and/or refractive correction. This is because amblyopia is the result of abnormal visual cortex development rather than a problem with the amblyopic eye itself4,5 . Amblyopia is characterized by both monocular and binocular deficits6,7 which include impaired visual acuity and poor or absent stereopsis respectively. The visual dysfunction in amblyopia is often associated with a strong suppression of the inputs from the amblyopic eye under binocular viewing conditions8. Recent work has indicated that suppression may play a central role in both the monocular and binocular deficits associated with amblyopia9,10 .
Current clinical tests for suppression tend to verify the presence or absence of suppression rather than giving a quantitative measurement of the degree of suppression. Here we describe a technique for measuring amblyopic suppression with a compact, portable device11,12 . The device consists of a laptop computer connected to a pair of virtual reality goggles. The novelty of the technique lies in the way we present visual stimuli to measure suppression. Stimuli are shown to the amblyopic eye at high contrast while the contrast of the stimuli shown to the non-amblyopic eye are varied. Patients perform a simple signal/noise task that allows for a precise measurement of the strength of excitatory binocular interactions. The contrast offset at which neither eye has a performance advantage is a measure of the "balance point" and is a direct measure of suppression. This technique has been validated psychophysically both in control13,14 and patient6,9,11 populations.
In addition to measuring suppression this technique also forms the basis of a novel form of treatment to decrease suppression over time and improve binocular and often monocular function in adult patients with amblyopia12,15,16 . This new treatment approach can be deployed either on the goggle system described above or on a specially modified iPod touch device15.
Medicine; Issue 70; Ophthalmology; Neuroscience; Anatomy; Physiology; Amblyopia; suppression; visual cortex; binocular vision; plasticity; strabismus; anisometropia
Early sensory experience instructs the maturation of neural circuitry in cortex 1,2. This has been extensively studied in the primary visual cortex where loss of vision to one eye permanently degrades cortical responsiveness to that eye 3,4, a phenomenon known as ocular dominance plasticity (ODP). Cortical inhibition mediates this process 4-6, but the precise role of specific classes of inhibitory neurons in ODP is controversial. Here we report that evoked firing rates of binocular excitatory neurons in primary visual cortex immediately drop by half when vision is restricted to one eye, but gradually return to normal over the following 24 hours, despite the fact that vision remains restricted to one eye. This restoration of binocular-like excitatory firing rates following monocular deprivation results from a rapid, though transient reduction in the firing rates of fast-spiking, parvalbumin-positive (PV) interneurons, which in turn can be attributed to a decrease in local excitatory circuit input onto PV interneurons. This reduction in PV cell evoked responses following monocular lid suture is restricted to the critical period for ODP and appears to be necessary for subsequent shifts in excitatory ODP. Pharmacologically enhancing inhibition at the time of sight deprivation blocks ODP and, conversely, pharmaco-genetic reduction of PV cell firing rates can extend the critical period for ODP. These findings define the microcircuit changes initiating competitive plasticity during critical periods of cortical development. Moreover, they show that the restoration of evoked firing rates of L2/3 pyramidal neurons by PV-specific disinhibition is a key step in the progression of ocular dominance plasticity.
Two functionally distinct forms of synaptic plasticity, Hebbian long-term potentiation (LTP) and homeostatic synaptic scaling, are thought to cooperate to promote information storage and circuit refinement. Both arise through changes in the synaptic accumulation of AMPA receptors (AMPAR), but whether they use similar or distinct receptor trafficking pathways is unknown. Here we show that TTX-induced synaptic scaling in cultured visual cortical neurons leads to the insertion of GluR2-containing AMPARs at synapses. Similarly, visual deprivation with monocular TTX injections results in synaptic accumulation of GluR2-containing AMPARs. Unlike chemical LTP, synaptic scaling is blocked by a GluR2 C-tail peptide, but not by a GluR1 C-tail peptide. Knockdown of endogenous GluR2 with an shRNA also blocks synaptic scaling, but not chemical LTP. Scaling can be rescued with expression of exogenous GluR2 resistant to the shRNA, but a chimeric GluR2 subunit with the C-terminal domain swapped with the GluR1 C-terminal domain (GluR2/CT1) does not rescue synaptic scaling, indicating that regulatory sequences on the GluR2 C-tail are required for the accumulation of synaptic AMPARs during scaling. Taken together, our results suggest that synaptic scaling and LTP utilize different trafficking pathways, making these two forms of plasticity both functionally and molecularly distinct.
homeostasis; synaptic plasticity; visual deprivation; LTP; AMPA Receptors; GluR2
Electrophysiology-delivery of fluorescent viral vectors-and two-photon microscopy were used to demonstrate the rapidity of axonal restructuring of both excitatory and inhibitory neurons in rodent cortical layer II/III following alterations in sensory experience.
Cortical topography can be remapped as a consequence of sensory deprivation, suggesting that cortical circuits are continually modified by experience. To see the effect of altered sensory experience on specific components of cortical circuits, we imaged neurons, labeled with a genetically modified adeno-associated virus, in the intact mouse somatosensory cortex before and after whisker plucking. Following whisker plucking we observed massive and rapid reorganization of the axons of both excitatory and inhibitory neurons, accompanied by a transient increase in bouton density. For horizontally projecting axons of excitatory neurons there was a net increase in axonal projections from the non-deprived whisker barrel columns into the deprived barrel columns. The axon collaterals of inhibitory neurons located in the deprived whisker barrel columns retracted in the vicinity of their somata and sprouted long-range projections beyond their normal reach towards the non-deprived whisker barrel columns. These results suggest that alterations in the balance of excitation and inhibition in deprived and non-deprived barrel columns underlie the topographic remapping associated with sensory deprivation.
The adult brain is capable of learning new tasks and being shaped by new experiences. Evidence for experience-dependent plasticity of the adult cerebral cortex is seen in the functional rearrangement of cortical maps of sensory input and in the formation of new connections following alteration of sensory experience. The barrel cortex of the rodent receives sensory input from the whiskers and is an ideal model for examining the influence of experience on cortical function and circuitry. In the current study, we asked how experience alters cortical circuitry by examining excitatory and inhibitory axons within the adult whisker barrel cortex before and after plucking of a whisker and hence removal of its sensory input. By combining delivery of genes encoding fluorescent proteins, under the control of cell-type specific promoters, with two-photon imaging, we were able to directly examine subpopulations of axons and to determine when and to what extent experience altered specific connections in the adult living brain. Following whisker plucking we observed both the retraction of existing connections and an exuberant amount of growth of new axons. Axonal restructuring occurred rapidly and continued to undergo changes over the following weeks, with reciprocal sprouting of axons of excitatory neurons located in non-deprived cortex and of inhibitory neurons located in deprived cortex. The changes in the inhibitory circuits preceded those seen for excitatory connections.
Experience-dependent maturation of neocortical circuits is required for normal sensory and cognitive abilities, which are distorted in neurodevelopmental disorders. We have tested whether experience-dependent neocortical modifications require Ube3a, an E3 ubiquitin ligase whose dysregulation has been implicated in autism and Angelman syndrome (AS). Using visual cortex as a model, we demonstrate that experience-dependent maturation of excitatory cortical circuits is severely impaired in AS mice deficient in Ube3a. This developmental defect is associated with profound impairments in neocortical plasticity. Remarkably, normal plasticity is preserved under conditions of sensory deprivation, but rapidly lost by sensory experiences. The loss of neocortical plasticity is reversible, as late-onset visual deprivation restores normal synaptic plasticity. Further, Ube3a-deficient mice lack ocular dominance plasticity in vivo when challenged with monocular deprivation. These results show that Ube3a is necessary to maintain plasticity during experience-dependent neocortical development, and suggest that loss of neocortical plasticity contributes to deficits associated with AS.
In the cortex, NMDA receptors (NMDARs) play a critical role in the control of synaptic plasticity processes. We have previously shown in rat visual cortex that the application of a high frequency of stimulation (HFS) protocol used to induce long-term potentiation (LTP) in layer 2/3 leads to a parallel potentiation of excitatory and inhibitory inputs received by cortical layer 5 pyramidal neurons without changing the excitation/inhibition (E/I) balance of the pyramidal neuron, indicating a homeostatic control of this parameter.
We show here that the blockade of NMDARs of the neuronal network prevents the potentiation of excitatory and inhibitory inputs and this result opens to question the role of the NMDAR isoform involved in the induction of LTP, actually being strongly debated. In P18-P23 rat cortical slices, the blockade of synaptic NR2B-containing NMDARs prevents the induction of the potentiation induced by the HFS protocol, whereas the blockade of NR2A-containing NMDARs reduced the potentiation itself. In P29-P32 cortical slices, the specific activation of NR2A-containing receptors fully ensures the potentiation of excitatory and inhibitory inputs. These results constitute the first report of a functional shift in subunit composition of NMDARs during the critical period (P12-P36) which explains the relative contribution of both NR2B- and NR2A-containing NMDARs in synaptic plasticity processes. These effects of HFS protocol are mediated by the activation of synaptic NMDARs but our results also indicate that the homeostatic control of the E/I balance is independent of NMDARs activation and is due to specialized recurrent interactions between excitatory and inhibitory networks.
2-Amino-5-phosphonovalerate; pharmacology; Animals; Cerebral Cortex; cytology; growth & development; physiology; Electric Stimulation; Electrophysiology; Neuronal Plasticity; physiology; Patch-Clamp Techniques; Pyramidal Cells; drug effects; physiology; Rats; Receptors; N-Methyl-D-Aspartate; drug effects; physiology; Synapses; physiology; Visual Cortex; cytology; growth & development; physiology
Postnatal cortical circuit development is characterized by windows of heightened plasticity that contribute to the acquisition of mature connectivity and function. What drives the transition between different critical plasticity windows is not known. Here we show that a switch in sign of inhibitory plasticity correlates with the reported transition between pre-critical (pre-CP) and critical period (CP) for ocular dominance plasticity (ODP). In layer 4 of binocular visual cortex (V1b), depression of inhibitory synapses onto pyramidal neurons is induced when rats are monocularly deprived (MD) for 2 days at the end of the third postnatal week (pre-CP); whereas potentiation is induced if the MD is started in the fourth postnatal week (CP). The magnitude of potentiation increases with deprivations started close to the peak of the CP for ODP. The direction of inhibitory plasticity depends on the differential manipulation of circuits activated by the two eyes. During development, these two forms of plasticity shift the balance between excitation and inhibition of the circuit in opposite directions, while the excitatory synaptic drive remains unaffected. Inhibitory plasticity is thus fundamental in modulating cortical circuit refinement and might be one of the mechanisms promoting OD shifts.
visual cortex; synaptic plasticity; inhibitory plasticity; GABA; visual deprivation; microcircuitry
The maturation of the GABAergic system is a crucial determinant of cortical development during early postnatal life, when sensory circuits undergo a process of activity-dependent refinement. An altered excitatory/inhibitory balance has been proposed as a possible pathogenic mechanism of autism spectrum disorders (ASD). The homeobox-containing transcription factor Engrailed-2 (En2) has been associated to ASD, and En2 knockout (En2−/−) mice show ASD-like features accompanied by a partial loss of cortical GABAergic interneurons. Here we studied GABAergic markers and cortical function in En2−/− mice, by exploiting the well-known anatomical and functional features of the mouse visual system. En2 is expressed in the visual cortex at postnatal day 30 and during adulthood. When compared to age-matched En2+/+ controls, En2−/− mice showed an increased number of parvalbumin (PV+), somatostatin (SOM+), and neuropeptide Y (NPY+) positive interneurons in the visual cortex at P30, and a decreased number of SOM+ and NPY+ interneurons in the adult. At both ages, the differences in distinct interneuron populations observed between En2+/+ and En2−/− mice were layer-specific. Adult En2−/− mice displayed a normal eye-specific segregation in the retino-geniculate pathway, and in vivo electrophysiological recordings showed a normal development of basic functional properties (acuity, response latency, receptive field size) of the En2−/− primary visual cortex. However, a significant increase of binocularity was found in P30 and adult En2−/− mice, as compared to age-matched controls. Differently from what observed in En2+/+ mice, the En2−/− primary visual cortex did not respond to a brief monocular deprivation performed between P26 and P29, during the so-called “critical period.” These data suggest that altered GABAergic circuits impact baseline binocularity and plasticity in En2−/− mice, while leaving other visual functional properties unaffected.
plasticity; inhibition; monocular deprivation; critical period; neurodevelopmental disorder
It has been suggested that excitatory and inhibitory inputs to cortical cells are balanced, and that this balance is important for the highly irregular firing observed in the cortex. There are two hypotheses as to the origin of this balance. One assumes that it results from a stable solution of the recurrent neuronal dynamics. This model can account for a balance of steady state excitation and inhibition without fine tuning of parameters, but not for transient inputs. The second hypothesis suggests that the feed forward excitatory and inhibitory inputs to a postsynaptic cell are already balanced. This latter hypothesis thus does account for the balance of transient inputs. However, it remains unclear what mechanism underlies the fine tuning required for balancing feed forward excitatory and inhibitory inputs. Here we investigated whether inhibitory synaptic plasticity is responsible for the balance of transient feed forward excitation and inhibition. We address this issue in the framework of a model characterizing the stochastic dynamics of temporally anti-symmetric Hebbian spike timing dependent plasticity of feed forward excitatory and inhibitory synaptic inputs to a single post-synaptic cell. Our analysis shows that inhibitory Hebbian plasticity generates ‘negative feedback’ that balances excitation and inhibition, which contrasts with the ‘positive feedback’ of excitatory Hebbian synaptic plasticity. As a result, this balance may increase the sensitivity of the learning dynamics to the correlation structure of the excitatory inputs.
One of the longstanding enigmas in neuroscience is the origin of inherent neural noise. It has been suggested that this noise results from a careful balance of excitatory and inhibitory inputs to neurons. Obtaining this balance requires fine tuning of the relative strengths of the inhibitory and excitatory inputs to each cell. However the mechanism that enables this fine tuning of parameters remains unclear. We suggest that a balance of excitatory and inhibitory inputs can be achieved via a process of unsupervised learning of the inhibitory synaptic strengths. We find that whereas Hebbian learning induces strong positive feedback on excitatory inputs that acts as a force that pulls them towards their boundaries, Hebbian learning of inhibition induces negative feedback. This negative feedback acts to balance the average excitatory input to the cell and makes the learning process less sensitive to the statistics of the inhibitory inputs. Surprisingly, this balance increases the sensitivity of learning to the statistics of the excitatory inputs. Thus, the balance of feed-forward excitation and inhibition emerges as a natural outcome of Hebbian learning applied to the inhibitory inputs to the cell.
The mammalian visual system exhibits significant experience-induced plasticity in the early postnatal period. While physiological studies have revealed the contribution of the CB1 cannabinoid receptor (CB1) to developmental plasticity in the primary visual cortex (V1), it remains unknown whether the expression and localization of CB1 is regulated during development or by visual experience. To explore a possible role of the endocannabinoid system in visual cortical plasticity, we examined the expression of CB1 in the visual cortex of mice. We found intense CB1 immunoreactivity in layers II/III and VI. CB1 mainly localized at vesicular GABA transporter-positive inhibitory nerve terminals. The amount of CB1 protein increased throughout development, and the specific laminar pattern of CB1 appeared at P20 and remained until adulthood. Dark rearing from birth to P30 decreased the amount of CB1 protein in V1 and altered the synaptic localization of CB1 in the deep layer. Dark rearing until P50, however, did not influence the expression of CB1. Brief monocular deprivation for 2 days upregulated the localization of CB1 at inhibitory nerve terminals in the deep layer. Taken together, the expression and the localization of CB1 are developmentally regulated, and both parameters are influenced by visual experience.
Two days of monocular deprivation (MD) of kittens during a critical period of development is known to produce a loss of visual responses in the primary visual cortex to stimulation of the nondeprived eye, and 7 days of deprivation results in retraction of axon branches and loss of presynaptic sites from deprived-eye geniculocortical arbors. The rapid loss of responsiveness to deprived-eye visual stimulation could be due to a decrease in intracortical excitatory input to deprived-eye ocular dominance columns (ODCs) relative to nondeprived-eye columns. Alternatively, deprived-eye visual responses could be suppressed by an increase in intracortical inhibition in deprived columns relative to nondeprived columns. We tested these hypotheses in critical period kittens by labeling ODCs in layer IV of primary visual cortex with injections of the anterograde tracer Phaseolus vulgaris-leucoagglutinin (PHA-L) into lamina A of the lateral geniculate nucleus (LGN). After either 2 or 7 days of MD, densities of intracortical excitatory presynaptic sites within deprived relative to nondeprived ODCs were estimated by measuring synaptic vesicle protein (SVP) immunoreactivity (IR). Because most of the synapses within layer IV of primary visual cortex are excitatory inputs from other cortical neurons, levels of SVP-IR provide an estimate of the amount of intracortical excitatory input. We also measured levels of immunoreactivity of the inhibitory presynaptic terminal marker glutamic acid decarboxylase (GAD)65 in deprived relative to nondeprived ODCs. Monocular deprivation (either 2 or 7 days) had no effect on the distributions of either SVP- or GAD65-IR in deprived and nondeprived columns. Therefore, the rapid loss of deprived-eye visual responsiveness following MD is due neither to a decrease in intracortical excitatory presynaptic sites nor to an increase in intracortical inhibitory presynaptic sites in layer IV of deprived-eye ODCs relative to nondeprived columns.
presynaptic terminal; striate cortex; lateral geniculate body; GABA; synaptophysin