Effects of junk-food on food-motivated behavior and nucleus accumbens glutamate plasticity; insights into the mechanism of calcium-permeable AMPA receptor recruitment

In rats, eating obesogenic diets increases calcium-permeable AMPA receptor (CP-AMPAR) transmission in the nucleus accumbens (NAc) core, and enhances food-motivated behavior. Interestingly, these diet-induced alterations in NAc transmission are pronounced and sustained in obesity-prone (OP) male rats and absent in obesity-resistant (OR) populations. However, effects of diet manipulation on food motivation, and the mechanisms underlying this NAc plasticity in OPs is unknown. Using male selectively-bred OP and OR rats, we assessed food-motivated behavior following ad lib access to chow (CH), junk-food (JF), or 10d of JF followed by a return to chow diet (JF-Dep). Motivation for food was greater in OP than OR rats, as expected. However, JF-Dep only produced enhancements in food-seeking in OP groups, while continuous JF access reduced food-seeking in both OPs and ORs. Additionally, optogenetic, chemogenetic, and pharmacological approaches were used to examine NAc CP-AMPAR recruitment following diet manipulation and ex vivo treatment of brain slices. Reducing excitatory transmission in the NAc was sufficient to recruit CP-AMPARs to synapses in OPs, but not ORs. In OPs, JF-induced increases in CP-AMPARs occurred in mPFC-, but not BLA-to-NAc inputs. Together results show that diet differentially affects behavioral and neural plasticity in obesity susceptible populations. We also identify conditions for acute recruitment of NAc CP-AMPARs; these results suggest that synaptic scaling mechanisms contribute to NAc CP-AMPAR recruitment. Overall, this work helps elucidate how diet interacts with obesity susceptibility to influence food-motivated behavior and extends our fundamental understanding of NAc CP-AMPAR recruitment.

A Bioluminescence Reporter Assay for Retinoic Acid Control of Translation of the GluR1 Subunit of the AMPA Glutamate Receptor

The present protocol describes a bioluminescence reporter assay developed to quantify the ability of synthetic agonists of retinoic acid receptors (RARs) to activate glutamate receptor subunit 1 (GluR1) translation. The reporter assay uses firefly luciferase under the control of the GluR1 5' untranslated region (5' UTR) which is bound by RARs to regulate its translation. This method is used to demonstrate the role of RARα in retinoic acid regulation of GluR1 translation. This method may also be used to screen drugs that influence RAR induction of GluR1 translation as an important mechanism controlling learning and memory in the brain.

Constrained plasticity reserve as a natural way to control frequency and weights in spiking neural networks

Biological neurons have adaptive nature and perform complex computations involving the filtering of redundant information. However, most common neural cell models, including biologically plausible, such as Hodgkin-Huxley or Izhikevich, do not possess predictive dynamics on a single-cell level. Moreover, the modern rules of synaptic plasticity or interconnections weights adaptation also do not provide grounding for the ability of neurons to adapt to the ever-changing input signal intensity. While natural neuron synaptic growth is precisely controlled and restricted by protein supply and recycling, weight correction rules such as widely used STDP are efficiently unlimited in change rate and scale. The present article introduces new mechanics of interconnection between neuron firing rate homeostasis and weight change through STDP growth bounded by abstract protein reserve, controlled by the intracellular optimization algorithm. We show how these cellular dynamics help neurons filter out the intense noise signals to help neurons keep a stable firing rate. We also examine that such filtering does not affect the ability of neurons to recognize the correlated inputs in unsupervised mode. Such an approach might be used in the machine learning domain to improve the robustness of AI systems.

Sleep Promotes Downward Firing Rate Homeostasis

Homeostatic plasticity is hypothesized to bidirectionally regulate neuronal activity around a stable set point to compensate for learning-related plasticity, but to date only upward firing rate homeostasis (FRH) has been demonstrated in vivo. We combined chronic electrophysiology in freely behaving animals with an eye-reopening paradigm to enhance firing in primary visual cortex (V1) and found that neurons bidirectionally regulate firing rates around an individual set point. Downward FRH did not require N-methyl-D-aspartate receptor (NMDAR) signaling and was associated with homeostatic scaling down of synaptic strengths. Like upward FRH, downward FRH was gated by arousal state but in the opposite direction: it occurred during sleep, not during wake. In contrast, firing rate depression associated with Hebbian plasticity happened independently of sleep and wake. Thus, sleep and wake states temporally segregate upward and downward FRH, which might prevent interference or provide unopposed homeostatic compensation when it is needed most.

Functional strengthening through synaptic scaling upon connectivity disruption in neuronal cultures

An elusive phenomenon in network neuroscience is the extent of neuronal activity remodeling upon damage. Here, we investigate the action of gradual synaptic blockade on the effective connectivity in cortical networks in vitro. We use two neuronal cultures configurations-one formed by about 130 neuronal aggregates and another one formed by about 600 individual neurons-and monitor their spontaneous activity upon progressive weakening of excitatory connectivity. We report that the effective connectivity in all cultures exhibits a first phase of transient strengthening followed by a second phase of steady deterioration. We quantify these phases by measuring GEFF, the global efficiency in processing network information. We term hyperefficiency the sudden strengthening of GEFF upon network deterioration, which increases by 20-50% depending on culture type. Relying on numerical simulations we reveal the role of synaptic scaling, an activity-dependent mechanism for synaptic plasticity, in counteracting the perturbative action, neatly reproducing the observed hyperefficiency. Our results demonstrate the importance of synaptic scaling as resilience mechanism.

Uniting homeostatic plasticity and exosome biology: A revision of the conceptual framework for drug discovery in neurodegenerative diseases?

Neurodegenerative diseases (NDDs) are in need of new drug discovery approaches. Our previous systematic analyses of Huntington's Disease (HD) literature for protein-protein interactors (PPIs) and modifiers of mutant Huntingtin-driven phenotypes revealed enrichment for PPIs of genes required for homeostatic synaptic plasticity (HSP) and exosome (EV) function and exosomal proteins, which in turn highly overlapped each other and with PPIs of genes associated with other NDDs. We proposed that HSP and EVs are linked to each other and are also involved in NDD pathophysiology. Recent studies showed that HSP is indeed altered in HD and AD, and that presynaptic homeostatic plasticity in motoneurons compensates for ALS pathology. Eliminating it causes earlier degeneration and death. If this holds true in other NDDs, drug discovery in animal models should then include elucidation of homeostatic compensation that either masks phenotypes of physiologically expressed mutant genes or are overridden by their overexpression. In this new conceptual framework, enhancing such underlying homeostatic compensation forms the basis for novel therapeutic strategies to slow progression of NDDs. Moreover, if EVs are linked to HSP, then their ability to penetrate the brain, target cell types, deliver miRNA and other molecules can be leveraged to develop attractive drug modalities. Testing this new framework is posed as four questions on model development and mechanistic studies progressing from higher throughput platforms to mouse models. Similar approaches may apply to other CNS disorders including schizophrenia, autism, Rett and Fragile X syndromes due to potential links of their susceptibility genes to HSP and EVs.

Excitability, synaptic balance, and addiction: The homeostatic dynamics of ionotropic glutamatergic receptors in VTA after cocaine exposure

Glutamatergic AMPA and NMDA receptors in the ventral tegmental area (VTA) are central for cocaine first exposure and posterior craving maintenance. However, the exact rules that coordinate the synaptic dynamics of these receptors in dopaminergic VTA neurons and behavioral outcomes are poorly understood. Additionally, synaptic homeostatic plasticity is present in response to chronic excitability changes in neuronal circuits, adjusting the strength of synapses to stabilize the firing rate. Despite having correspondent mechanisms, little is known about the relationship between continuous cocaine exposure and homeostatic synaptic changes in the VTA neurons. Here, we assess the role of homeostatic mechanisms in the neurobiology of cocaine addiction by providing a brief overview of the parallels between cocaine-induced synaptic potentiation and long-term synaptic adaptations, focusing on the regulation of GluA1- and GluN1- containing receptors.

Non-scaling regulation of AMPA receptors in homeostatic synaptic plasticity

Homeostatic synaptic plasticity (HSP) as an activity-dependent negative feedback regulation of synaptic strength plays important roles in the maintenance of neuronal and neural circuitry stability. A primary cellular substrate for HSP expression is alterations in synaptic accumulation of glutamatergic α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPAR). It is widely believed that during HSP, AMPAR accumulation changes with the same proportion at each synapse of a neuron, a process known as synaptic scaling. However, direct evidence on AMPAR synaptic scaling remains largely lacking. Here we report a direct examination of inactivity-induced homeostatic scaling of AMPAR at individual synapse by live-imaging. Surprisingly, instead of uniform up-scaling, a scattered pattern of changes in synaptic AMPAR was observed in cultured rat hippocampal neurons. While the majority of synapses showed up-regulation after activity inhibition, a reduction of AMPAR could be detected in certain synapses. More importantly, among the up-regulated synapses, a wide range of AMPAR changes was observed in synapses of the same neuron. We also found that synapses with higher levels of pre-existing AMPAR tend to be up-regulated by lesser extents, whereas the locations of synapses relative to the soma seem not affecting AMPAR scaling strengths. In addition, we observed strong competition between neighboring synapses during HSP. These results reveal that synaptic AMPAR may not be scaled during HSP, suggesting novel molecular mechanisms for information processing and storage at synapses.

Principles underlying the input-dependent formation and organization of memories

The neuronal system exhibits the remarkable ability to dynamically store and organize incoming information into a web of memory representations (items), which is essential for the generation of complex behaviors. Central to memory function is that such memory items must be (1) discriminated from each other, (2) associated to each other, or (3) brought into a sequential order. However, how these three basic mechanisms are robustly implemented in an input-dependent manner by the underlying complex neuronal and synaptic dynamics is still unknown. Here, we develop a mathematical framework, which provides a direct link between different synaptic mechanisms, determining the neuronal and synaptic dynamics of the network, to create a network that emulates the above mechanisms. Combining correlation-based synaptic plasticity and homeostatic synaptic scaling, we demonstrate that these mechanisms enable the reliable formation of sequences and associations between two memory items still missing the capability for discrimination. We show that this shortcoming can be removed by additionally considering inhibitory synaptic plasticity. Thus, the here-presented framework provides a new, functionally motivated link between different known synaptic mechanisms leading to the self-organization of fundamental memory mechanisms.

Denervated mouse dentate granule cells adjust their excitatory but not inhibitory synapses following in vitro entorhinal cortex lesion

Neurons adjust their synaptic strength in a homeostatic manner following changes in network activity and connectivity. While this form of plasticity has been studied in detail for excitatory synapses, homeostatic plasticity of inhibitory synapses remains not well-understood. In the present study, we employed entorhinal cortex lesion (ECL) of organotypic entorhino-hippocampal tissue cultures to test for homeostatic changes in GABAergic neurotransmission onto partially denervated dentate granule cells. Using single and paired whole-cell patch-clamp recordings, as well as immunostainings for synaptic markers, we find that excitatory synaptic strength is robustly increased 3 days post lesion (dpl), whereas GABAergic neurotransmission is not changed after denervation. Even under conditions of pharmacological inhibition of glutamatergic neurotransmission, which prevents neurons to compensate for the loss of input via excitatory synaptic scaling, down-scaling of GABAergic synapses does not emerge 3 days after denervation. We conclude that granule cells maintain structural and functional properties of GABAergic synapses even in the face of substantial changes in network connectivity. Hence, alterations in inhibitory neurotransmission, as seen in pathological brain states, may not simply reflect a homeostatic response to disconnection.

Phosphorylation of synaptic GTPase-activating protein (synGAP) by polo-like kinase (Plk2) alters the ratio of its GAP activity toward HRas, Rap1 and Rap2 GTPases

SynGAP is a Ras and Rap GTPase-activating protein (GAP) found in high concentration in the postsynaptic density (PSD) fraction from mammalian forebrain where it binds to PDZ domains of PSD-95. Phosphorylation of pure recombinant synGAP by Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) shifts the balance of synGAP's GAP activity toward inactivation of Rap1; whereas phosphorylation by cyclin-dependent kinase 5 (CDK5) has the opposite effect, shifting the balance toward inactivation of HRas. These shifts in balance contribute to regulation of the numbers of surface AMPA receptors, which rise during synaptic potentiation (CaMKII) and fall during synaptic scaling (CDK5). Polo-like kinase 2 (Plk2/SNK), like CDK5, contributes to synaptic scaling. These two kinases act in concert to reduce the number of surface AMPA receptors following elevated neuronal activity by tagging spine-associated RapGAP protein (SPAR) for degradation, thus raising the level of activated Rap. Here we show that Plk2 also phosphorylates and regulates synGAP. Phosphorylation of synGAP by Plk2 stimulates its GAP activity toward HRas by 65%, and toward Rap1 by 16%. Simultaneous phosphorylation of synGAP by Plk2 and CDK5 at distinct sites produces an additive increase in GAP activity toward HRas (∼230%) and a smaller, non-additive increase in activity toward Rap1 (∼15%). Dual phosphorylation also produces an increase in GAP activity toward Rap2 (∼40-50%), an effect not produced by either kinase alone. As we previously observed for CDK5, addition of Ca²⁺/CaM causes a substrate-directed doubling of the rate and stoichiometry of phosphorylation of synGAP by Plk2, targeting residues also phosphorylated by CaMKII. In summary, phosphorylation by Plk2, like CDK5, shifts the ratio of GAP activity of synGAP to produce a greater decrease in active Ras than in active Rap, which would produce a shift toward a decrease in the number of surface AMPA receptors in neuronal dendrites.

Scaling Synapses in the Presence of HIV

A defining feature of HIV-associated neurocognitive disorder (HAND) is the loss of excitatory synaptic connections. Synaptic changes that occur during exposure to HIV appear to result, in part, from a homeostatic scaling response. Here we discuss the mechanisms of these changes from the perspective that they might be part of a coping mechanism that reduces synapses to prevent excitotoxicity. In transgenic animals expressing the HIV proteins Tat or gp120, the loss of synaptic markers precedes changes in neuronal number. In vitro studies have shown that HIV-induced synapse loss and cell death are mediated by distinct mechanisms. Both in vitro and animal studies suggest that HIV-induced synaptic scaling engages new mechanisms that suppress network connectivity and that these processes might be amenable to therapeutic intervention. Indeed, pharmacological reversal of synapse loss induced by HIV Tat restores cognitive function. In summary, studies indicate that there are temporal, mechanistic and pharmacological features of HIV-induced synapse loss that are consistent with homeostatic plasticity. The increasingly well delineated signaling mechanisms that regulate synaptic scaling may reveal pharmacological targets suitable for normalizing synaptic function in chronic neuroinflammatory states such as HAND.

Stable Control of Firing Rate Mean and Variance by Dual Homeostatic Mechanisms

Homeostatic processes that provide negative feedback to regulate neuronal firing rates are essential for normal brain function. Indeed, multiple parameters of individual neurons, including the scale of afferent synapse strengths and the densities of specific ion channels, have been observed to change on homeostatic time scales to oppose the effects of chronic changes in synaptic input. This raises the question of whether these processes are controlled by a single slow feedback variable or multiple slow variables. A single homeostatic process providing negative feedback to a neuron's firing rate naturally maintains a stable homeostatic equilibrium with a characteristic mean firing rate; but the conditions under which multiple slow feedbacks produce a stable homeostatic equilibrium have not yet been explored. Here we study a highly general model of homeostatic firing rate control in which two slow variables provide negative feedback to drive a firing rate toward two different target rates. Using dynamical systems techniques, we show that such a control system can be used to stably maintain a neuron's characteristic firing rate mean and variance in the face of perturbations, and we derive conditions under which this happens. We also derive expressions that clarify the relationship between the homeostatic firing rate targets and the resulting stable firing rate mean and variance. We provide specific examples of neuronal systems that can be effectively regulated by dual homeostasis. One of these examples is a recurrent excitatory network, which a dual feedback system can robustly tune to serve as an integrator.

The DLGAP family: neuronal expression, function and role in brain disorders

The neurotransmitter glutamate facilitates neuronal signalling at excitatory synapses. Glutamate is released from the presynaptic membrane into the synaptic cleft. Across the synaptic cleft glutamate binds to both ion channels and metabotropic glutamate receptors at the postsynapse, which expedite downstream signalling in the neuron. The postsynaptic density, a highly specialized matrix, which is attached to the postsynaptic membrane, controls this downstream signalling. The postsynaptic density also resets the synapse after each synaptic firing. It is composed of numerous proteins including a family of Discs large associated protein 1, 2, 3 and 4 (DLGAP1-4) that act as scaffold proteins in the postsynaptic density. They link the glutamate receptors in the postsynaptic membrane to other glutamate receptors, to signalling proteins and to components of the cytoskeleton. With the central localisation in the postsynapse, the DLGAP family seems to play a vital role in synaptic scaling by regulating the turnover of both ionotropic and metabotropic glutamate receptors in response to synaptic activity. DLGAP family has been directly linked to a variety of psychological and neurological disorders. In this review we focus on the direct and indirect role of DLGAP family on schizophrenia as well as other brain diseases.

On learning time delays between the spikes from different input neurons in a biophysical model of a pyramidal neuron

Biological systems are able to recognise temporal sequences of stimuli or compute in the temporal domain. In this paper we are exploring whether a biophysical model of a pyramidal neuron can detect and learn systematic time delays between the spikes from different input neurons. In particular, we investigate whether it is possible to reinforce pairs of synapses separated by a dendritic propagation time delay corresponding to the arrival time difference of two spikes from two different input neurons. We examine two subthreshold learning approaches where the first relies on the backpropagation of EPSPs (excitatory postsynaptic potentials) and the second on the backpropagation of a somatic action potential, whose production is supported by a learning-enabling background current. The first approach does not provide a learning signal that sufficiently differentiates between synapses at different locations, while in the second approach, somatic spikes do not provide a reliable signal distinguishing arrival time differences of the order of the dendritic propagation time. It appears that the firing of pyramidal neurons shows little sensitivity to heterosynaptic spike arrival time differences of several milliseconds. This neuron is therefore unlikely to be able to learn to detect such differences.

Transcriptional and epigenetic regulation of Hebbian and non-Hebbian plasticity

The epigenome is uniquely positioned as a point of convergence, integrating multiple intracellular signaling cascades into a cohesive gene expression profile necessary for long-term behavioral change. The last decade of neuroepigenetic research has primarily focused on learning-induced changes in DNA methylation and chromatin modifications. Numerous studies have independently demonstrated the importance of epigenetic modifications in memory formation and retention as well as Hebbian plasticity. However, how these mechanisms operate in the context of other forms of plasticity is largely unknown. In this review, we examine evidence for epigenetic regulation of Hebbian plasticity. We then discuss how non-Hebbian forms of plasticity, such as intrinsic plasticity and synaptic scaling, may also be involved in producing the cellular adaptations necessary for learning-related behavioral change. Furthermore, we consider the likely roles for transcriptional and epigenetic mechanisms in the regulation of these plasticities. In doing so, we aim to expand upon the idea that epigenetic mechanisms are critical regulators of both Hebbian and non-Hebbian forms of plasticity that ultimately drive learning and memory.

NMDA-receptor inhibition increases spine stability of denervated mouse dentate granule cells and accelerates spine density recovery following entorhinal denervation in vitro

Neuronal networks are reorganized following brain injury. At the structural level this is in part reflected by changes in the spine turnover of the denervated neurons. Using the entorhinal cortex lesion in vitro model, we recently showed that mouse dentate granule cells respond to entorhinal denervation with coordinated functional and structural changes: During the early phase after denervation spine density decreases, while excitatory synaptic strength increases in a homeostatic manner. At later stages spine density increases again, and synaptic strength decreases back to baseline. In the present study, we have addressed the question of whether the denervation-induced homeostatic strengthening of excitatory synapses could not only be a result of the deafferentation, but could, in turn, affect the dynamics of the spine reorganization process following entorhinal denervation in vitro. Using a computational approach, time-lapse imaging of neurons in organotypic slice cultures prepared from Thy1-GFP mice, and patch-clamp recordings we provide experimental evidence which suggests that the strengthening of surviving synapses can lead to the destabilization of spines formed after denervation. This activity-dependent pruning of newly formed spines requires the activation of N-methyl-d-aspartate receptors (NMDA-Rs), since pharmacological inhibition of NMDA-Rs resulted in a stabilization of spines and in an accelerated spine density recovery after denervation. Thus, NMDA-R inhibitors may restore the ability of neurons to form new stable synaptic contacts under conditions of denervation-induced homeostatic synaptic up-scaling, which may contribute to their beneficial effect seen in the context of some neurological diseases.

Molecular mechanisms of homeostatic synaptic downscaling

Homeostatic synaptic downscaling is a negative feedback response to chronic elevated network activity to reduce the firing rate of neurons. This form of synaptic plasticity decreases the strength of individual synapses to the same proportion, or in a multiplicative manner. Because of this, synaptic downscaling has been hypothesized to counter the potential run-away excitation due to Hebbian type of long term potentiation (LTP), while preserving relative synaptic weight encoded in individual synapses and thus memory information. In this article, we will review the current knowledge on the signaling and molecular mechanisms of synaptic downscaling. Specifically, we focus on three general areas. First the functional roles of several immediate early genes such as Plk2, Homer1a, Arc and Narp are discussed. Secondly, we examine the current knowledge on the regulation of synaptic protein levels by ubiquitination and transcriptional repression in synaptic downscaling. Thirdly, we review the dynamics of signaling molecules such as kinases and phosphatases critical for synaptic downscaling, and their regulation of synaptic scaffolding proteins. Finally we briefly discuss the heterogeneity of homeostatic synaptic downscaling mechanisms. This article is part of the Special Issue entitled 'Homeostatic Synaptic Plasticity'.

Cell adhesion and homeostatic synaptic plasticity

At synapses, pre- and post-synaptic cells get in direct contact with each other via cell adhesion molecules (CAMs). Several CAMs have been identified at the neuromuscular junction and at central synapses, where they regulate synaptic strength, by recruiting scaffolding proteins, neurotransmitter receptors and synaptic vesicles in response to the binding of counter-receptors across the synaptic cleft. Many synapses are also surrounded by astrocytic processes and embedded in conspicuous extracellular matrix (ECM). It is now widely recognized that astrocytes play a central role in regulating the synaptic machinery by exchanging information with the neuronal elements via diffusible molecules and direct physical interactions; this has lead to the concept of the 'tri-partite synapse'. More recently, the term 'tetra-partite synapse' has been introduced to underlie the importance of ECM in shaping synaptic function by mediating interaction and signaling between neurons and astrocytes. Here, we will review how this integrated view of the synapse can help us understand homeostatic synaptic plasticity at the neuromuscular junction and in the central nervous system. We will explore how synaptic CAMs regulate two forms of homeostatic plasticity: (i) postsynaptic scaling of synaptic currents to counteract changes in neuronal network activity and (ii) the compensatory modulation of presynaptic neurotransmitter release in response to changes in postsynaptic efficacy. We will discuss recent findings on activity-dependent trans-synaptic signaling events and the role of cell adhesion in the feedback control of network activity. This article is part of the Special Issue entitled 'Homeostatic Synaptic Plasticity'.

Tuning into diversity of homeostatic synaptic plasticity

Neurons are endowed with the remarkable ability to integrate activity levels over time and tune their excitability such that action potential firing is maintained within a computationally optimal range. These feedback mechanisms, collectively referred to as "homeostatic plasticity", enable neurons to respond and adapt to prolonged alterations in neuronal activity by regulating several determinants of cellular excitability. Perhaps the best-characterized of these homeostatic responses involves the regulation of excitatory glutamatergic transmission. This homeostatic synaptic plasticity (HSP) operates bidirectionally, thus providing a means for neurons to tune cellular excitability in response to either elevations or reductions in net activity. The last decade has seen rapid growth in interest and efforts to understand the mechanistic underpinnings of HSP in part because of the theoretical stabilization that HSP confers to neural network function. Since the initial reports describing HSP in central neurons, innovations in experimental approaches have permitted the mechanistic dissection of this cellular adaptive response and, as a result, key advances have been made in our understanding of the cellular and molecular basis of HSP. Here, we review recent evidence that outline the presence of distinct forms of HSP at excitatory glutamatergic synapses which operate at different sub-cellular levels. We further present theoretical considerations on the potential computational roles afforded by local, synapse-specific homeostatic regulation. This article is part of the Special Issue entitled 'Homeostatic Synaptic Plasticity'.

Experience-dependent homeostatic synaptic plasticity in neocortex

The organism's ability to adapt to the changing sensory environment is due in part to the ability of the nervous system to change with experience. Input and synapse specific Hebbian plasticity, such as long-term potentiation (LTP) and long-term depression (LTD), are critical for sculpting the nervous system to wire its circuit in tune with the environment and for storing memories. However, these synaptic plasticity mechanisms are innately unstable and require another mode of plasticity that maintains homeostasis to allow neurons to function within a desired dynamic range. Several modes of homeostatic adaptation are known, some of which work at the synaptic level. This review will focus on the known mechanisms of experience-induced homeostatic synaptic plasticity in the neocortex and their potential function in sensory cortex plasticity. This article is part of the Special Issue entitled 'Homeostatic Synaptic Plasticity'.

Synaptic retinoic acid signaling and homeostatic synaptic plasticity

One of the defining features of the nervous system is its ability to modify synaptic strength in an experience-dependent manner. Chronic elevation or reduction of network activity activates compensatory mechanisms that modulate synaptic strength in the opposite direction (i.e. reduced network activity leads to increased synaptic strength), a process called homeostatic synaptic plasticity. Among the many mechanisms that mediate homeostatic synaptic plasticity, retinoic acid (RA) has emerged as a novel signaling molecule that is critically involved in homeostatic synaptic plasticity induced by blockade of synaptic activity. In neurons, silencing of synaptic transmission triggers RA synthesis. RA then acts at synapses by a non-genomic mechanism that is independent of its well-known function as a transcriptional regulator, but operates through direct activation of protein translation in neuronal dendrites. Protein synthesis is activated by RA-binding to its receptor RARα, which functions locally in dendrites in a non-canonical manner as an RNA-binding protein that mediate RA's effect on translation. The present review will discuss recent progress in our understanding of the novel role of RA, which led to the identification of RA as a critical synaptic signaling molecule that mediates activity-dependent regulation of protein synthesis in neuronal dendrites. This article is part of the Special Issue entitled 'Homeostatic Synaptic Plasticity'.

Combined effects of LTP/LTD and synaptic scaling in formation of discrete and line attractors with persistent activity from non-trivial baseline

In this article, we analyze combined effects of LTP/LTD and synaptic scaling and study the creation of persistent activity from a periodic or chaotic baseline attractor. The bifurcations leading to the creation of new attractors have been detailed; this was achieved using a mean field approximation. Attractors encoding persistent activity can notably appear via generalized period-doubling bifurcations, tangent bifurcations of the second iterates or boundary crises, after which the basins of attraction become irregular. Synaptic scaling is shown to maintain the coexistence of a state of persistent activity and the baseline. According to the rate of change of the external inputs, different types of attractors can be formed: line attractors for rapidly changing external inputs and discrete attractors for constant external inputs.