Persistent synaptic scaling independent of AMPA receptor subunit composition

Diverging from the Norm: Reevaluating What Miniature Excitatory Postsynaptic Currents Tell Us about Homeostatic Synaptic Plasticity

The idea that the nervous system maintains a set point of network activity and homeostatically returns to that set point in the face of dramatic disruption-during development, after injury, in pathologic states, and during sleep/wake cycles-is rapidly becoming accepted as a key plasticity behavior, placing it alongside long-term potentiation and depression. The dramatic growth in studies of homeostatic synaptic plasticity of miniature excitatory synaptic currents (mEPSCs) is attributable, in part, to the simple yet elegant mechanism of uniform multiplicative scaling proposed by Turrigiano and colleagues: that neurons sense their own activity and globally multiply the strength of every synapse by a single factor to return activity to the set point without altering established differences in synaptic weights. We have recently shown that for mEPSCs recorded from control and activity-blocked cultures of mouse cortical neurons, the synaptic scaling factor is not uniform but is close to 1 for the smallest mEPSC amplitudes and progressively increases as mEPSC amplitudes increase, which we term divergent scaling. Using insights gained from simulating uniform multiplicative scaling, we review evidence from published studies and conclude that divergent synaptic scaling is the norm rather than the exception. This conclusion has implications for hypotheses about the molecular mechanisms underlying synaptic scaling.

TNF-α Orchestrates Experience-Dependent Plasticity of Excitatory and Inhibitory Synapses in the Anterior Piriform Cortex

Homeostatic synaptic plasticity, which induces compensatory modulation of synapses, plays a critical role in maintaining neuronal circuit function in response to changing activity patterns. Activity in the anterior piriform cortex (APC) is largely driven by ipsilateral neural activity from the olfactory bulb and is a suitable system for examining the effects of sensory experience on cortical circuits. Pro-inflammatory cytokine tumor necrosis factor-α (TNF-α) can modulate excitatory and inhibitory synapses, but its role in APC is unexplored. Here we examined the role of TNF-α in adjusting synapses in the mouse APC after experience deprivation via unilateral naris occlusion. Immunofluorescent staining revealed that activity deprivation increased excitatory, and decreased inhibitory, synaptic density in wild-type mice, consistent with homeostatic regulation. Quantitative RT-PCR showed that naris occlusion increased the expression of Tnf mRNA in APC. Critically, occlusion-induced plasticity of excitatory and inhibitory synapses was completely blocked in the Tnf knockout mouse. Together, these results show that TNF-α is an important orchestrator of experience-dependent plasticity in the APC.

NGPF2 triggers synaptic scaling up through ALK-LIMK-cofilin-mediated mechanisms

Synaptic scaling is an extensively studied form of homeostatic plasticity critically involved in various brain functions. Although it is accepted that synaptic scaling is expressed through the postsynaptic accumulation of AMPA receptors (AMPARs), the induction mechanism remains elusive. In this study, we show that TTX treatment induces rapid but transient release of the neurite growth-promoting factor 2 (NGPF2), and this release is necessary and sufficient for TTX-induced scaling up. In addition, we show that inhibition of the anaplastic lymphoma kinase (ALK)-LIMK-cofilin signaling pathway blocks TTX- and NGPF2-induced synaptic scaling up. Furthermore, we show that TTX-induced release of NGPF2 is protein synthesis dependent and requires fragile X mental retardation protein 1 (FMRP1). These results indicate that activity blockade induces NGPF2 synthesis and release to trigger synaptic scaling up through LIMK-cofilin-dependent actin reorganization, spine enlargement, and stabilization of AMPARs at the synapse.

TNF-Mediated Homeostatic Synaptic Plasticity: From in vitro to in vivo Models

Since it was first described almost 30 years ago, homeostatic synaptic plasticity (HSP) has been hypothesized to play a key role in maintaining neuronal circuit function in both developing and adult animals. While well characterized in vitro, determining the in vivo roles of this form of plasticity remains challenging. Since the discovery that the pro-inflammatory cytokine tumor necrosis factor-α (TNF-α) mediates some forms of HSP, it has been possible to probe some of the in vivo contribution of TNF-mediated HSP. Work from our lab and others has found roles for TNF-HSP in a variety of functions, including the developmental plasticity of sensory systems, models of drug addiction, and the response to psychiatric drugs.

Low-Dose Perampanel Rescues Cortical Gamma Dysregulation Associated With Parvalbumin Interneuron GluA2 Upregulation in Epileptic Syngap1+/- Mice

BACKGROUND

Loss-of-function SYNGAP1 mutations cause a neurodevelopmental disorder characterized by intellectual disability and epilepsy. SYNGAP1 is a Ras GTPase-activating protein that underlies the formation and experience-dependent regulation of postsynaptic densities. The mechanisms that contribute to this proposed monogenic cause of intellectual disability and epilepsy remain unresolved.

METHODS

We established the phenotype of the epileptogenesis in a Syngap1^+/- mouse model using 24-hour video electroencephalography (vEEG)/electromyography recordings at advancing ages. We administered an acute low dose of perampanel, a Food and Drug Administration-approved AMPA receptor (AMPAR) antagonist, during a follow-on 24-hour vEEG to investigate the role of AMPARs in Syngap1 haploinsufficiency. Immunohistochemistry was performed to determine the region- and location-specific differences in the expression of the GluA2 AMPAR subunit.

RESULTS

A progressive worsening of the epilepsy with emergence of multiple seizure phenotypes, interictal spike frequency, sleep dysfunction, and hyperactivity was identified in Syngap1^+/- mice. Interictal spikes emerged predominantly during non-rapid eye movement sleep in 24-hour vEEG of Syngap1^+/- mice. Myoclonic seizures occurred at behavioral-state transitions both in Syngap1^+/- mice and during an overnight EEG from a child with SYNGAP1 haploinsufficiency. In Syngap1^+/- mice, EEG spectral power analyses identified a significant loss of gamma power modulation during behavioral-state transitions. A significant region-specific increase of GluA2 AMPAR subunit expression in the somas of parvalbumin-positive interneurons was identified.

CONCLUSIONS

Acute dosing with perampanel significantly rescued behavioral state-dependent cortical gamma homeostasis, identifying a novel mechanism implicating Ca²⁺-impermeable AMPARs on parvalbumin-positive interneurons underlying circuit dysfunction in SYNGAP1 haploinsufficiency.

Divergent Synaptic Scaling of Miniature EPSCs following Activity Blockade in Dissociated Neuronal Cultures

Neurons can respond to decreased network activity with a homeostatic increase in the amplitudes of miniature EPSCs (mEPSCs). The prevailing view is that mEPSC amplitudes are uniformly multiplied by a single factor, termed "synaptic scaling." Deviations from purely multiplicative scaling have been attributed to biological differences, or to a distortion imposed by a detection threshold limit. Here, we demonstrate in neurons dissociated from cortices of male and female mice that the shift in mEPSC amplitudes observed in the experimental data cannot be reproduced by simulation of uniform multiplicative scaling, with or without the distortion caused by applying a detection threshold. Furthermore, we demonstrate explicitly that the scaling factor is not uniform but is close to 1 for small mEPSCs, and increases with increasing mEPSC amplitude across a substantial portion of the data. This pattern was also observed for previously published data from dissociated mouse hippocampal neurons and dissociated rat cortical neurons. The finding of "divergent scaling" shifts the current view of homeostatic plasticity as a process that alters all synapses on a neuron equally to one that must accommodate the differential effect observed for small versus large mEPSCs. Divergent scaling still accomplishes the essential homeostatic task of modifying synaptic strengths in the opposite direction of the activity change, but the consequences are greatest for those synapses which individually are more likely to bring a neuron to threshold.SIGNIFICANCE STATEMENT In homeostatic plasticity, the responses to chronic increases or decreases in network activity act in the opposite direction to restore normal activity levels. Homeostatic plasticity is likely to play a role in diseases associated with long-term changes in brain function, such as epilepsy and neuropsychiatric illnesses. One homeostatic response is the increase in synaptic strength following a chronic block of activity. Research is focused on finding a globally expressed signaling pathway, because it has been proposed that the plasticity is uniformly expressed across all synapses. Here, we show that the plasticity is not uniform. Our work suggests that homeostatic signaling molecules are likely to be differentially expressed across synapses.

The AMPA Receptor Code of Synaptic Plasticity

Changes in the properties and postsynaptic abundance of AMPA-type glutamate receptors (AMPARs) are major mechanisms underlying various forms of synaptic plasticity, including long-term potentiation (LTP), long-term depression (LTD), and homeostatic scaling. The function and the trafficking of AMPARs to and from synapses is modulated by specific AMPAR GluA1-GluA4 subunits, subunit-specific protein interactors, auxiliary subunits, and posttranslational modifications. Layers of regulation are added to AMPAR tetramers through these different interactions and modifications, increasing the computational power of synapses. Here we review the reliance of synaptic plasticity on AMPAR variants and propose "the AMPAR code" as a conceptual framework. The AMPAR code suggests that AMPAR variants will be predictive of the types and extent of synaptic plasticity that can occur and that a hierarchy exists such that certain AMPARs will be disproportionally recruited to synapses during LTP/homeostatic scaling up, or removed during LTD/homeostatic scaling down.

Postsynaptic localization and regulation of AMPA receptors and Cav1.2 by β2 adrenergic receptor/PKA and Ca2+/CaMKII signaling

The synapse transmits, processes, and stores data within its tiny space. Effective and specific signaling requires precise alignment of the relevant components. This review examines current insights into mechanisms of AMPAR and NMDAR localization by PSD-95 and their spatial distribution at postsynaptic sites to illuminate the structural and functional framework of postsynaptic signaling. It subsequently delineates how β2 adrenergic receptor (β2 AR) signaling via adenylyl cyclase and the cAMP-dependent protein kinase PKA is organized within nanodomains. Here, we discuss targeting of β2 AR, adenylyl cyclase, and PKA to defined signaling complexes at postsynaptic sites, i.e., AMPARs and the L-type Ca2+ channel Cav1.2, and other subcellular surface localizations, the role of A kinase anchor proteins, the physiological relevance of the spatial restriction of corresponding signaling, and their interplay with signal transduction by the Ca2+- and calmodulin-dependent kinase CaMKII How localized and specific signaling by cAMP occurs is a central cellular question. The dendritic spine constitutes an ideal paradigm for elucidating the dimensions of spatially restricted signaling because of their small size and defined protein composition.

Human Autoantibodies against the AMPA Receptor Subunit GluA2 Induce Receptor Reorganization and Memory Dysfunction

AMPA receptors are essential for fast excitatory transmission in the CNS. Autoantibodies to AMPA receptors have been identified in humans with autoimmune encephalitis and severe defects of hippocampal function. Here, combining electrophysiology and high-resolution imaging with neuronal culture preparations and passive-transfer models in wild-type and GluA1-knockout mice, we analyze how specific human autoantibodies against the AMPA receptor subunit GluA2 affect receptor function and composition, synaptic transmission, and plasticity. Anti-GluA2 antibodies induce receptor internalization and a reduction of synaptic GluA2-containing AMPARs followed by compensatory ryanodine receptor-dependent incorporation of synaptic non-GluA2 AMPARs. Furthermore, application of human pathogenic anti-GluA2 antibodies to mice impairs long-term synaptic plasticity in vitro and affects learning and memory in vivo. Our results identify a specific immune-neuronal rearrangement of AMPA receptor subunits, providing a framework to explain disease symptoms.

Major Vault Protein, a Candidate Gene in 16p11.2 Microdeletion Syndrome, Is Required for the Homeostatic Regulation of Visual Cortical Plasticity

Microdeletion of a region in chromosome 16p11.2 increases susceptibility to autism. Although this region contains exons of 29 genes, disrupting only a small segment of the region, which spans five genes, is sufficient to cause autistic traits. One candidate gene in this critical segment is MVP, which encodes for the major vault protein (MVP) that has been implicated in regulation of cellular transport mechanisms. MVP expression levels in MVP^+/- mice closely phenocopy those of 16p11.2 mutant mice, suggesting that MVP^+/- mice may serve as a model of MVP function in 16p11.2 microdeletion. Here we show that MVP regulates the homeostatic component of ocular dominance (OD) plasticity in primary visual cortex. MVP^+/- mice of both sexes show impairment in strengthening of open-eye responses after several days of monocular deprivation (MD), whereas closed-eye responses are weakened as normal, resulting in reduced overall OD plasticity. The frequency of miniature EPSCs (mEPSCs) in pyramidal neurons is decreased in MVP^+/- mice after extended MD, suggesting a reduction of functional synapses. Correspondingly, upregulation of surface GluA1 AMPA receptors is reduced in MVP^+/- mice after extended MD, and is accompanied by altered expression of STAT1 and phosphorylated ERK, which have been previously implicated in OD plasticity. Normalization of STAT1 levels by introducing STAT1 shRNA rescues surface GluA1 and open-eye responses, implicating STAT1 as a downstream effector of MVP. These findings demonstrate a specific role for MVP as a key molecule influencing the homeostatic component of activity-dependent synaptic plasticity, and potentially the corresponding phenotypes of 16p11.2 microdeletion syndrome.SIGNIFICANCE STATEMENT Major vault protein (MVP), a candidate gene in 16p11.2 microdeletion syndrome, has been implicated in the regulation of several cellular processes including transport mechanisms and scaffold signaling. However, its role in brain function and plasticity remains unknown. In this study, we identified MVP as an important regulator of the homeostatic component of experience-dependent plasticity, via regulation of STAT1 and ERK signaling. This study helps reveal a new mechanism for an autism-related gene in brain function, and suggests a broader role for neuro-immune interactions in circuit level plasticity. Importantly, our findings might explain specific components of the pathophysiology of 16p11.2 microdeletion syndrome.

Homeostatic synaptic scaling: molecular regulators of synaptic AMPA-type glutamate receptors

The ability of neurons and circuits to maintain their excitability and activity levels within the appropriate dynamic range by homeostatic mechanisms is fundamental for brain function. Neuronal hyperactivity, for instance, could cause seizures.  One such homeostatic process is synaptic scaling, also known as synaptic homeostasis. It involves a negative feedback process by which neurons adjust (scale) their postsynaptic strength over their whole synapse population to compensate for increased or decreased overall input thereby preventing neuronal hyper- or hypoactivity that could otherwise result in neuronal network dysfunction. While synaptic scaling is well-established and critical, our understanding of the underlying molecular mechanisms is still in its infancy. Homeostatic adaptation of synaptic strength is achieved through upregulation (upscaling) or downregulation (downscaling) of the functional availability of AMPA-type glutamate receptors (AMPARs) at postsynaptic sites.  Understanding how synaptic AMPARs are modulated in response to alterations in overall neuronal activity is essential to gain valuable insights into how neuronal networks adapt to changes in their environment, as well as the genesis of an array of neurological disorders. Here we discuss the key molecular mechanisms that have been implicated in tuning the synaptic abundance of postsynaptic AMPARs in order to maintain synaptic homeostasis.

Control of Homeostatic Synaptic Plasticity by AKAP-Anchored Kinase and Phosphatase Regulation of Ca2+-Permeable AMPA Receptors

Neuronal information processing requires multiple forms of synaptic plasticity mediated by NMDARs and AMPA-type glutamate receptors (AMPARs). These plasticity mechanisms include long-term potentiation (LTP) and long-term depression (LTD), which are Hebbian, homosynaptic mechanisms locally regulating synaptic strength of specific inputs, and homeostatic synaptic scaling, which is a heterosynaptic mechanism globally regulating synaptic strength across all inputs. In many cases, LTP and homeostatic scaling regulate AMPAR subunit composition to increase synaptic strength via incorporation of Ca²⁺-permeable receptors (CP-AMPAR) containing GluA1, but lacking GluA2, subunits. Previous work by our group and others demonstrated that anchoring of the kinase PKA and the phosphatase calcineurin (CaN) to A-kinase anchoring protein (AKAP) 150 play opposing roles in regulation of GluA1 Ser845 phosphorylation and CP-AMPAR synaptic incorporation during hippocampal LTP and LTD. Here, using both male and female knock-in mice that are deficient in PKA or CaN anchoring, we show that AKAP150-anchored PKA and CaN also play novel roles in controlling CP-AMPAR synaptic incorporation during homeostatic plasticity in hippocampal neurons. We found that genetic disruption of AKAP-PKA anchoring prevented increases in Ser845 phosphorylation and CP-AMPAR synaptic recruitment during rapid homeostatic synaptic scaling-up induced by combined blockade of action potential firing and NMDAR activity. In contrast, genetic disruption of AKAP-CaN anchoring resulted in basal increases in Ser845 phosphorylation and CP-AMPAR synaptic activity that blocked subsequent scaling-up by preventing additional CP-AMPAR recruitment. Thus, the balanced, opposing phospho-regulation provided by AKAP-anchored PKA and CaN is essential for control of both Hebbian and homeostatic plasticity mechanisms that require CP-AMPARs.SIGNIFICANCE STATEMENT Neuronal circuit function is shaped by multiple forms of activity-dependent plasticity that control excitatory synaptic strength, including LTP/LTD that adjusts strength of individual synapses and homeostatic plasticity that adjusts overall strength of all synapses. Mechanisms controlling LTP/LTD and homeostatic plasticity were originally thought to be distinct; however, recent studies suggest that CP-AMPAR phosphorylation regulation is important during both LTP/LTD and homeostatic plasticity. Here we show that CP-AMPAR regulation by the kinase PKA and phosphatase CaN coanchored to the scaffold protein AKAP150, a mechanism previously implicated in LTP/LTD, is also crucial for controlling synaptic strength during homeostatic plasticity. These novel findings significantly expand our understanding of homeostatic plasticity mechanisms and further emphasize how intertwined they are with LTP and LTD.

Synaptic homeostasis requires the membrane-proximal carboxy tail of GluA2

Bidirectional scaling of synaptic transmission, expressed as a compensatory change in quantal size following chronic activity perturbation, is a critical effector mechanism underlying homeostatic plasticity in the brain. An emerging model posits that the GluA2 AMPA receptor (AMPAR) subunit may be important for the bidirectional scaling of excitatory transmission; however, whether this subunit plays an obligatory role in synaptic scaling, and the identity of the precise domain(s) involved, remain controversial. We set out to determine the specific AMPAR subunit required for scaling up in CA1 hippocampal pyramidal neurons, and found that the GluA2 subunit is both necessary and sufficient. In addition, our results point to a critical role for a single amino acid within the membrane-proximal region of the GluA2 cytoplasmic tail, and suggest a distinct model for the regulation of AMPAR trafficking in synaptic homeostasis.

GluA4 subunit of AMPA receptors mediates the early synaptic response to altered network activity in the developing hippocampus

Development of the neuronal circuitry involves both Hebbian and homeostatic plasticity mechanisms that orchestrate activity-dependent refinement of the synaptic connectivity. AMPA receptor subunit GluA4 is expressed in hippocampal pyramidal neurons during early postnatal period and is critical for neonatal long-term potentiation; however, its role in homeostatic plasticity is unknown. Here we show that GluA4-dependent plasticity mechanisms allow immature synapses to promptly respond to alterations in network activity. In the neonatal CA3, the threshold for homeostatic plasticity is low, and a 15-h activity blockage with tetrodotoxin triggers homeostatic upregulation of glutamatergic transmission. On the other hand, attenuation of the correlated high-frequency bursting in the CA3-CA1 circuitry leads to weakening of AMPA transmission in CA1, thus reflecting a critical role for Hebbian synapse induction in the developing CA3-CA1. Both of these developmentally restricted forms of plasticity were absent in GluA4(-/-) mice. These data suggest that GluA4 enables efficient homeostatic upscaling and responsiveness to temporal activity patterns during the critical period of activity-dependent refinement of the circuitry.

Synaptic Consolidation Normalizes AMPAR Quantal Size following MAGUK Loss

The mechanisms controlling synapse growth and maintenance are of critical importance for learning and memory. The MAGUK family of synaptic scaffolding proteins is abundantly expressed at glutamatergic central synapses, but their importance in controlling the synaptic content of glutamate receptors is poorly understood. Here, we use a chained RNAi-mediated knockdown approach to simultaneously remove PSD-93, PSD-95, and SAP102, the MAGUKs previously shown to be responsible for synaptic localization of glutamate receptors. We find that MAGUKs are specifically responsible for creating functional synapses after initial spine formation by filling functionally silent spines with glutamate receptors. Removal of the MAGUKs causes a transient reduction in AMPA receptor quantal size followed by synaptic consolidation resulting in a normalization of quantal size at the few remaining functional synapses. Consolidation requires signaling through L-type calcium channels, CaM kinase kinase, and the GluA2 AMPA receptor subunit, akin to a homeostatic process.

Activity-dependent synaptic GRIP1 accumulation drives synaptic scaling up in response to action potential blockade

Synaptic scaling is a form of homeostatic plasticity that stabilizes neuronal firing in response to changes in synapse number and strength. Scaling up in response to action-potential blockade is accomplished through increased synaptic accumulation of GluA2-containing AMPA receptors (AMPAR), but the receptor trafficking steps that drive this process remain largely obscure. Here, we show that the AMPAR-binding protein glutamate receptor-interacting protein-1 (GRIP1) is essential for regulated synaptic AMPAR accumulation during scaling up. Synaptic abundance of GRIP1 was enhanced by activity deprivation, directly increasing synaptic GRIP1 abundance through overexpression increased the amplitude of AMPA miniature excitatory postsynaptic currents (mEPSCs), and shRNA-mediated GRIP1 knockdown prevented scaling up of AMPA mEPSCs. Furthermore, knockdown and replace experiments targeting either GRIP1 or GluA2 revealed that scaling up requires the interaction between GRIP1 and GluA2. Finally, GRIP1 synaptic accumulation during scaling up did not require GluA2 binding. Taken together, our data support a model in which activity-dependent trafficking of GRIP1 to synaptic sites drives the forward trafficking and enhanced synaptic accumulation of GluA2-containing AMPAR during synaptic scaling up.

Calcineurin mediates synaptic scaling via synaptic trafficking of Ca2+-permeable AMPA receptors

Kim and Ziff examine the molecular mechanism of synaptic scaling, showing that inhibition of neuronal excitability reduces calcium influx into neurons, resulting in decreased calcineurin activity. This leads to increased surface expression of calcium-permeable AMPA receptors as a homeostatic response.

Homeostatic synaptic plasticity is a negative-feedback mechanism for compensating excessive excitation or inhibition of neuronal activity. When neuronal activity is chronically suppressed, neurons increase synaptic strength across all affected synapses via synaptic scaling. One mechanism for this change is alteration of synaptic AMPA receptor (AMPAR) accumulation. Although decreased intracellular Ca²⁺ levels caused by chronic inhibition of neuronal activity are believed to be an important trigger of synaptic scaling, the mechanism of Ca²⁺-mediated AMPAR-dependent synaptic scaling is not yet understood. Here, we use dissociated mouse cortical neurons and employ Ca²⁺ imaging, electrophysiological, cell biological, and biochemical approaches to describe a novel mechanism in which homeostasis of Ca²⁺ signaling modulates activity deprivation-induced synaptic scaling by three steps: (1) suppression of neuronal activity decreases somatic Ca²⁺ signals; (2) reduced activity of calcineurin, a Ca²⁺-dependent serine/threonine phosphatase, increases synaptic expression of Ca²⁺-permeable AMPARs (CPARs) by stabilizing GluA1 phosphorylation; and (3) Ca²⁺ influx via CPARs restores CREB phosphorylation as a homeostatic response by Ca²⁺-induced Ca²⁺ release from the ER. Therefore, we suggest that synaptic scaling not only maintains neuronal stability by increasing postsynaptic strength but also maintains nuclear Ca²⁺ signaling by synaptic expression of CPARs and ER Ca²⁺ propagation.

Synaptic scaling is a form of homeostatic plasticity that normalizes the strength of synapses (the structure that allows nerve cells to communicate) and is triggered by chronic inhibition of neuronal activity. Although extensive studies have been conducted, the molecular mechanism of this synaptic adaptation is not understood. Using cultured cortical neurons, we show that chronic inhibition of neuronal activity reduces calcium influx into neurons, which, in turn, decreases the activity of the calcium-dependent phosphatase calcineurin. These changes lead to an increase in GluA1-containing, calcium-permeable AMPA receptors, which mediate communication at the synapse. Newly inserted calcium-permeable AMPA receptors restore calcium currents, which enhance synaptic strength and recover calcium signaling. We also show that inhibition or activation of calcineurin activity is sufficient to induce or block synaptic scaling, respectively, suggesting that calcineurin is an important mediator of homeostatic synaptic plasticity. Taken together, our findings show that synaptic scaling is a homeostatic process that not only enhances synaptic transmission but also maintains calcium signaling in neurons under activity deprivation.

The role of GluA1 in ocular dominance plasticity in the mouse visual cortex

Ocular dominance plasticity is a widely studied model of experience-dependent cortical plasticity. It has been shown that potentiation of open eye responses resulting from monocular deprivation relies on a homeostatic response to loss of input from the closed eye, but the mechanisms by which this occurs are not fully understood. The role of GluA1 in the homeostatic component of ocular dominance (OD) plasticity has not so far been tested. In this study, we tested the idea that the GluA1 subunit of the AMPA receptor is necessary for open eye potentiation. We found that open eye potentiation did not occur in GluA1 knock-out (GluA1(-/-)) mice but did occur in wild-type littermates when monocular deprivation was imposed during the critical period. We also found that depression of the closed eye response that normally occurs in the monocular as well as binocular zone is delayed, but only in the monocular zone in GluA1(-/-) mice and only in a background strain we have previously shown lacks synaptic scaling (C57BL/6OlaHsd). In adult mice, we found that OD plasticity and facilitation of OD plasticity by prior monocular experience were both present in GluA1(-/-) mice, suggesting that the GluA1-dependent mechanisms only operate during the critical period.