Homeostatic control of neural activity: from phenomenology to molecular design

Excitatory/inhibitory synaptic imbalance leads to hippocampal hyperexcitability in mouse models of tuberous sclerosis

Neural circuits are regulated by activity-dependent feedback systems that tightly control network excitability and which are thought to be crucial for proper brain development. Defects in the ability to establish and maintain network homeostasis may be central to the pathogenesis of neurodevelopmental disorders. Here, we examine the function of the tuberous sclerosis complex (TSC)-mTOR signaling pathway, a common target of mutations associated with epilepsy and autism spectrum disorder, in regulating activity-dependent processes in the mouse hippocampus. We find that the TSC-mTOR pathway is a central component of a positive feedback loop that promotes network activity by repressing inhibitory synapses onto excitatory neurons. In Tsc1 KO neurons, weakened inhibition caused by deregulated mTOR alters the balance of excitatory and inhibitory synaptic transmission, leading to hippocampal hyperexcitability. These findings identify the TSC-mTOR pathway as a regulator of neural network activity and have implications for the neurological dysfunction in disorders exhibiting deregulated mTOR signaling.

Correlations in ion channel expression emerge from homeostatic tuning rules

Experimental observations reveal that the expression levels of different ion channels vary across neurons of a defined type, even when these neurons exhibit stereotyped electrical properties. However, there are robust correlations between different ion channel expression levels, although the mechanisms that determine these correlations are unknown. Using generic model neurons, we show that correlated conductance expression can emerge from simple homeostatic control mechanisms that couple expression rates of individual conductances to cellular readouts of activity. The correlations depend on the relative rates of expression of different conductances. Thus, variability is consistent with homeostatic regulation and the structure of this variability reveals quantitative relations between regulation dynamics of different conductances. Furthermore, we show that homeostatic regulation is remarkably insensitive to the details that couple the regulation of a given conductance to overall neuronal activity because of degeneracy in the function of multiple conductances and can be robust to "antihomeostatic" regulation of a subset of conductances expressed in a cell.

Homeostatic plasticity at the Drosophila neuromuscular junction

In biology, homeostasis refers to how cells maintain appropriate levels of activity. This concept underlies a balancing act in the nervous system. Synapses require flexibility (i.e. plasticity) to adjust to environmental challenges. Yet there must also exist regulatory mechanisms that constrain activity within appropriate physiological ranges. An abundance of evidence suggests that homeostatic regulation is critical in this regard. In recent years, important progress has been made toward identifying molecules and signaling processes required for homeostatic forms of neuroplasticity. The Drosophila melanogaster third instar larval neuromuscular junction (NMJ) has been an important experimental system in this effort. Drosophila neuroscientists combine genetics, pharmacology, electrophysiology, imaging, and a variety of molecular techniques to understand how homeostatic signaling mechanisms take shape at the synapse. At the NMJ, homeostatic signaling mechanisms couple retrograde (muscle-to-nerve) signaling with changes in presynaptic calcium influx, changes in the dynamics of the readily releasable vesicle pool, and ultimately, changes in presynaptic neurotransmitter release. Roles in these processes have been demonstrated for several molecules and signaling systems discussed here. This review focuses primarily on electrophysiological studies or data. In particular, attention is devoted to understanding what happens when NMJ function is challenged (usually through glutamate receptor inhibition) and the resulting homeostatic responses. A significant area of study not covered in this review, for the sake of simplicity, is the homeostatic control of synapse growth, which naturally, could also impinge upon synapse function in myriad ways. This article is part of the Special Issue entitled 'Homeostatic Synaptic Plasticity'.

Homeostatic synaptic plasticity in developing spinal networks driven by excitatory GABAergic currents

Homeostatic plasticity refers to mechanisms that the cell or network engage in order to homeostatically maintain a preset level of activity. These mechanisms include compensatory changes in cellular excitability, excitatory and inhibitory synaptic strength and are typically studied at a developmental stage when GABA or glycine is inhibitory. Here we focus on the expression of homeostatic plasticity in the chick embryo spinal cord at a stage when GABA is excitatory. When spinal activity is perturbed in the living embryo there are compensatory changes in postsynaptic AMPA receptors and in the driving force for GABAergic currents. These changes are triggered by reduced GABAA receptor signaling, which appears to be part of the sensing machinery for triggering homeostatic plasticity. We compare and contrast these findings to homeostatic plasticity expressed in spinal systems at different stages of development, and to the developing retina at a stage when GABA is depolarizing. This article is part of the Special Issue entitled 'Homeostatic Synaptic Plasticity'.

Pitchfork bifurcation in a receptor theory-based model of the serotonergic system

Abnormalities in the serotonergic system are thought to be a potent cause of several mental diseases. Past research has shown that autoreceptors are the key component. It is thought that the autoreceptor constructs a negative feedback circuit on this system and realizes homeostatic control on its neural activity. This study is mainly organized from the above mentioned knowledge. In this paper, we construct two possible models of the serotonergic system based on receptor theory and provide some predictions for this system with each model. In the first model, we predict that the deficit of serotonin synthesis causes destabilization of the amount of autoreceptors; autoreceptors show an explosive increase if serotonin synthesis drops below a certain threshold value. In the second model, we indicate that the serotonergic system changes its stable property from a monostable one to a bistable one by certain factors. We clarify these factors and show that this changing process is named pitch-fork bifurcation. Additionally, we also suggest another notable phenomena which would appear when we consider a stochastic perturbation on the receptor expressions. Lastly, we suggest some experimental ideas towards the verification of the validity of these models.

Balance of calcineurin Aα and CDK5 activities sets release probability at nerve terminals

The control of neurotransmitter release at nerve terminals is of profound importance for neurological function and provides a powerful control system in neural networks. We show that the balance of enzymatic activities of the α isoform of the phosphatase calcineurin (CNAα) and the kinase cyclin-dependent kinase 5 (CDK5) has a dramatic influence over single action potential (AP)-driven exocytosis at nerve terminals. Acute or chronic loss of these enzymatic activities results in a sevenfold impact on single AP-driven exocytosis. We demonstrate that this control is mediated almost entirely through Cav2.2 (N-type) voltage-gated calcium channels as blocking these channels with a peptide toxin eliminates modulation by these enzymes. We found that a fraction of nerve terminals are kept in a presynaptically silent state with no measurable Ca(2+) influx driven by single AP stimuli attributable to the balance of CNAα and CDK5 activities because blockade of either CNAα or CDK5 activity changes the proportion of presynaptically silent nerve terminals. Thus, CNAα and CDK5 enzymatic activities are key determinants of release probability.

Rapid feedback regulation of synaptic efficacy during high-frequency activity at the Drosophila larval neuromuscular junction

High-frequency firing of neurons depresses transmitter release at many synapses. At the glutamatergic synapse of the Drosophila larval neuromuscular junction, we find that presynaptic depression is modulated by postsynaptic ionotropic glutamate receptor (iGluR) activity. Although basal release at low frequency was insensitive to postsynaptic iGluR activity, recovery from depression elicited by high-frequency presynaptic trains decreased with partial block of native iGluRs. Moreover, recovery from depression increased with optical activation of the light-gated mammalian iGluR6 (LiGluR) expressed postsynaptically. The enhancement of recovery from depression occurred within 2 min of optical activation of LiGluR and persisted for minutes after optical deactivation. This effect depended on cAMP-dependent presynaptic recruitment of vesicles from the reserve pool. Our findings reveal a unique dimension to postsynaptic iGluR activity: fast retrograde signaling that preserves transmission efficacy during high-frequency presynaptic firing.

Neurotransmitter switching in the adult brain regulates behavior

Neurotransmitters have been thought to be fixed throughout life, but whether sensory stimuli alter behaviorally relevant transmitter expression in the mature brain is unknown. We found that populations of interneurons in the adult rat hypothalamus switched between dopamine and somatostatin expression in response to exposure to short- and long-day photoperiods. Changes in postsynaptic dopamine receptor expression matched changes in presynaptic dopamine, whereas somatostatin receptor expression remained constant. Pharmacological blockade or ablation of these dopaminergic neurons led to anxious and depressed behavior, phenocopying performance after exposure to the long-day photoperiod. Induction of newly dopaminergic neurons through exposure to the short-day photoperiod rescued the behavioral consequences of lesions. Natural stimulation of other sensory modalities may cause changes in transmitter expression that regulate different behaviors.

Synaptopodin regulates denervation-induced homeostatic synaptic plasticity

Synaptopodin (SP) is a marker and essential component of the spine apparatus (SA), an enigmatic cellular organelle composed of stacked smooth endoplasmic reticulum that has been linked to synaptic plasticity. However, SP/SA-mediated synaptic plasticity remains incompletely understood. To study the role of SP/SA in homeostatic synaptic plasticity we here used denervation-induced synaptic scaling of mouse dentate granule cells as a model system. This form of plasticity is of considerable interest in the context of neurological diseases that are associated with the loss of neurons and subsequent denervation of connected brain regions. In entorhino-hippocampal slice cultures prepared from SP-deficient mice, which lack the SA, a compensatory increase in excitatory synaptic strength was not observed following partial deafferentation. In line with this finding, prolonged blockade of sodium channels with tetrodotoxin induced homeostatic synaptic scaling in wild-type, but not SP-deficient, slice cultures. By crossing SP-deficient mice with a newly generated transgenic mouse strain that expresses GFP-tagged SP under the control of the Thy1.2 promoter, the ability of dentate granule cells to form the SA and to homeostatically strengthen excitatory synapses was rescued. Interestingly, homeostatic synaptic strengthening was accompanied by a compensatory increase in SP cluster size/stability and SA stack number, suggesting that activity-dependent SP/SA remodeling could be part of a negative feedback mechanism that aims at adjusting the strength of excitatory synapses to persisting changes in network activity. Thus, our results disclose an important role for SP/SA in homeostatic synaptic plasticity.

Chronic inactivation of a neural circuit enhances LTP by inducing silent synapse formation

Chronic inactivation of a neural network is known to induce homeostatic upregulation of synaptic strength, a form of synaptic plasticity that differs from Hebbian-type synaptic plasticity in that it is not input-specific, but involves all synapses of an individual neuron. However, it is unclear how homeostatic and Hebbian synaptic plasticity interact in the same neuron. Here we show that long-term potentiation (LTP) at Schaffer collateral-CA1 synapses is greatly enhanced in cultured mouse hippocampal slices after chronic (60 h) network-activity blockade with tetrodotoxin (TTX). This increase in LTP is not due to an altered synaptic NMDA receptor composition or presynaptic function. Instead, we found that silencing neural network activity not only increases the abundance of both AMPA and NMDA receptors at existing synapses as previously described, but also promotes the presence of new glutamatergic synapses that contain only NMDA receptors-a class of synapses that are functionally silent due to the absence of AMPA receptors. Induction of LTP in TTX-treated neurons leads to insertion of AMPA receptors into the silent synapses, thereby "switching on" these silent synapses, which produces the observed enhancement of LTP magnitude. Our findings suggest that homeostatic synaptic plasticity manifests not only in the adjustment of the strength of existing synapses, but also in the modulation of new synapse formation/maintenance. Moreover, presence of new but functionally silent synapses enables more robust LTP to occur through rapid conversion of silent synapses to active synapses, resulting in a stronger input-specific modulation of synapses following prolonged network silencing.

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'.

Animal-to-animal variability in the phasing of the crustacean cardiac motor pattern: an experimental and computational analysis

The cardiac ganglion (CG) of Homarus americanus is a central pattern generator that consists of two oscillatory groups of neurons: "small cells" (SCs) and "large cells" (LCs). We have shown that SCs and LCs begin their bursts nearly simultaneously but end their bursts at variable phases. This variability contrasts with many other central pattern generator systems in which phase is well maintained. To determine both the consequences of this variability and how CG phasing is controlled, we modeled the CG as a pair of Morris-Lecar oscillators coupled by electrical and excitatory synapses and constructed a database of 15,000 simulated networks using random parameter sets. These simulations, like our experimental results, displayed variable phase relationships, with the bursts beginning together but ending at variable phases. The model suggests that the variable phasing of the pattern has important implications for the functional role of the excitatory synapses. In networks in which the two oscillators had similar duty cycles, the excitatory coupling functioned to increase cycle frequency. In networks with disparate duty cycles, it functioned to decrease network frequency. Overall, we suggest that the phasing of the CG may vary without compromising appropriate motor output and that this variability may critically determine how the network behaves in response to manipulations.

RIM controls homeostatic plasticity through modulation of the readily-releasable vesicle pool

Rab3 interacting molecules (RIMs) are evolutionarily conserved scaffolding proteins that are located at presynaptic active zones. In the mammalian nervous system, RIMs have two major activities that contribute to the fidelity of baseline synaptic transmission: they concentrate calcium channels at the active zone and facilitate synaptic vesicle docking/priming. Here we confirm that RIM has an evolutionarily conserved function at the Drosophila neuromuscular junction and then define a novel role for RIM during homeostatic synaptic plasticity. We show that loss of RIM disrupts baseline vesicle release, diminishes presynaptic calcium influx, and diminishes the size of the readily-releasable pool (RRP) of synaptic vesicles, consistent with known activities of RIM. However, loss of RIM also completely blocks the homeostatic enhancement of presynaptic neurotransmitter release that normally occurs after inhibition of postsynaptic glutamate receptors, a process termed synaptic homeostasis. It is established that synaptic homeostasis requires enhanced presynaptic calcium influx as a mechanism to potentiate vesicle release. However, despite a defect in baseline calcium influx in rim mutants, the homeostatic modulation of calcium influx proceeds normally. Synaptic homeostasis is also correlated with an increase in the size of the RRP of synaptic vesicles, although the mechanism remains unknown. Here we demonstrate that the homeostatic modulation of the RRP is blocked in the rim mutant background. Therefore, RIM-dependent modulation of the RRP is a required step during homeostatic plasticity. By extension, homeostatic plasticity appears to require two genetically separable processes, the enhancement of presynaptic calcium influx and a RIM-dependent modulation of the RRP.

Short-term synaptic plasticity compensates for variability in number of motor neurons at a neuromuscular junction

We studied how similar postsynaptic responses are maintained in the face of interindividual variability in the number of presynaptic neurons. In the stomatogastric ganglion of the lobster, Homarus americanus, the pyloric (PY) neurons exist in variable numbers across animals. We show that each individual fiber of the stomach muscles innervated by PY neurons received synaptic input from all neurons present. We performed intracellular recordings of excitatory junction potentials (EJPs) in the muscle fibers to determine the consequences of differences in the number of motor neurons. Despite the variability in neuron number, the compound electrical response of muscle fibers to natural bursting input was similar across individuals. The similarity of total synaptic activation was not due to differences in the spiking activity of individual motor neurons across animals with different numbers of PY neurons. The amplitude of a unitary EJP in response to a single spike in a single motor neuron also did not depend on the number of PY neurons present. Consequently, the compound EJP in response to a single stimulus that activated all motor axons present was larger in individuals with more PY neurons. However, when axons were stimulated with trains of pulses mimicking bursting activity, EJPs facilitated more in individuals with fewer PY neurons. After a few stimuli, this resulted in depolarizations similar to the ones in individuals with more PY neurons. We interpret our findings as evidence that compensatory or homeostatic regulatory mechanisms can act on short-term synaptic dynamics instead of absolute synaptic strength.

Input-specific homeostatic regulation of AMPA receptor accumulation at central synapses

Neurons are able to restore their activity to a set-point level when challenged by external or internal perturbations. This type of homeostatic plasticity is important in the maintenance of neuronal or network stability during development and normal brain function. One of the major cellular events underlying the expression of homeostatic regulation is the alteration of glutamatergic AMPA receptor (AMPAR) accumulation and thus, synaptic strength. Traditional global homeostatic plasticity is believed to adjust the input strength of all synapses. Since each individual synapse receives different input with varied levels of activity and distinct history of synaptic plasticity, an input-specific homeostatic regulation is necessary to restrain synaptic activity within a physiological range. Our studies suggest that at the single synapse level, homeostatic plasticity is expressed via input-specific alterations of AMPAR amounts. This homosynaptic homeostatic regulation is expected to play an important role in preventing the deleterious situations imposed by Hebbian plasticity to secure long-term synaptic stability.

Hyperpolarisation-activated cyclic nucleotide-gated channels regulate the spontaneous firing rate of olfactory receptor neurons and affect glomerular formation in mice

Olfactory receptor neurons (ORNs), which undergo lifelong neurogenesis, have been studied extensively to understand how neurons form precise topographical networks. Neural projections from ORNs are principally guided by the genetic code, which directs projections from ORNs that express a specific odorant receptor to the corresponding glomerulus in the olfactory bulb. In addition, ORNs utilise spontaneous firing activity to establish and maintain the neural map. However, neither the process of generating this spontaneous activity nor its role as a guidance cue in the olfactory bulb is clearly understood. Utilising extracellular unit-recordings in mouse olfactory epithelium slices, we demonstrated that the hyperpolarisation-activated cyclic nucleotide-gated (HCN) channels in the somas of ORNs depolarise their membranes and boost their spontaneous firing rates by sensing basal cAMP levels; the odorant-sensitive cyclic nucleotide-gated (CNG) channels in cilia do not. The basal cAMP levels were maintained via the standing activation of β-adrenergic receptors. Using a Tet-off system to over-express HCN4 channels resulted in the enhancement of spontaneous ORN activity and dramatically reduced both the size and number of glomeruli in the olfactory bulb. This phenotype was rescued by the administration of doxycycline. These findings suggest that cAMP plays different roles in cilia and soma and that basal cAMP levels in the soma are directly converted via HCN channels into a spontaneous firing frequency that acts as an intrinsic guidance cue for the formation of olfactory networks.

Loss of signal transducer and activator of transcription 3 (STAT3) signaling during elevated activity causes vulnerability in hippocampal neurons

Chronically altered levels of network activity lead to changes in the morphology and functions of neurons. However, little is known of how changes in neuronal activity alter the intracellular signaling pathways mediating neuronal survival. Here, we use primary cultures of rat hippocampal neurons to show that elevated neuronal activity impairs phosphorylation of the serine/threonine kinase, Erk1/2, and the activation of signal transducer and activator of transcription 3 (STAT3) by phosphorylation of serine 727. Chronically stimulated neurons go through apoptosis when they fail to activate another serine/threonine kinase, Akt. Gain- and loss-of-function experiments show that STAT3 plays the key role directly downstream from Erk1/2 as the alternative survival pathway. Elevated neuronal activity resulted in increased expression of a tumor suppressor, p53, and its target gene, Bax. These changes are observed in Kv4.2 knock-out mouse hippocampal neurons, which are also sensitive to the blockade of TrkB signaling, confirming that the alteration occurs in vivo. Thus, this study provides new insight into a mechanism by which chronic elevation of activity may cause neurodegeneration.

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'.

Time scales of memory, learning, and plasticity

After only about 10 days would the storage capacity of our nervous system be reached if we stored every bit of input. The nervous system relies on at least two mechanisms that counteract this capacity limit: compression and forgetting. But the latter mechanism needs to know how long an entity should be stored: some memories are relevant only for the next few minutes, some are important even after the passage of several years. Psychology and physiology have found and described many different memory mechanisms, and these mechanisms indeed use different time scales. In this prospect we review these mechanisms with respect to their time scale and propose relations between mechanisms in learning and memory and their underlying physiological basis.

Competition from newborn granule cells does not drive axonal retraction of silenced old granule cells in the adult hippocampus

In the developing nervous system synaptic refinement, typified by the neuromuscular junction where supernumerary connections are eliminated by axon retraction leaving the postsynaptic target innervated by a single dominant input, critically regulates neuronal circuit formation. Whether such competition-based pruning continues in established circuits of mature animals remains unknown. This question is particularly relevant in the context of adult neurogenesis where newborn cells must integrate into preexisting circuits, and thus, potentially compete with functionally mature synapses to gain access to their postsynaptic targets. The hippocampus plays an important role in memory formation/retrieval and the dentate gyrus (DG) subfield exhibits continued neurogenesis into adulthood. Therefore, this region contains both mature granule cells (old GCs) and immature recently born GCs that are generated throughout adult life (young GCs), providing a neurogenic niche model to examine the role of competition in synaptic refinement. Recent work from an independent group in developing animals indicated that embryonically/early postnatal generated GCs placed at a competitive disadvantage by selective expression of tetanus toxin (TeTX) to prevent synaptic release rapidly retracted their axons, and that this retraction was driven by competition from newborn GCs lacking TeTX. In contrast, following 3-6 months of selective TeTX expression in old GCs of adult mice we did not observe any evidence of axon retraction. Indeed ultrastructural analyses indicated that the terminals of silenced GCs even maintained synaptic contact with their postsynaptic targets. Furthermore, we did not detect any significant differences in the electrophysiological properties between old GCs in control and TeTX conditions. Thus, our data demonstrate a remarkable stability in the face of a relatively prolonged period of altered synaptic competition between two populations of neurons within the adult brain.

The kinase activity of EphA4 mediates homeostatic scaling-down of synaptic strength via activation of Cdk5

Neurons within a network have the ability to homeostatically scale-down their excitatory synaptic strength under conditions of persistent neuronal activity elevation, a process pivotal to neural circuit stability. How this homeostatic regulation is achieved at the molecular level in developing neural circuits, which face gradually elevated neuronal activity as part of circuit wiring, is not well-understood. Using dissociated hippocampal neuronal cultures, we identified a critical and cell autonomous role for the receptor tyrosine kinase EphA4 in mediating activity-induced homeostatic down-regulation of excitatory synaptic strength. Reducing the endogenous level of EphA4 in individual neurons by RNAi effectively blocked activity-induced scaling-down of excitatory synaptic strength, while co-transfection of RNAi resistant EphA4 rescued this effect. Furthermore, interfering with EphA4 forward signaling using EphA4-Fc blocked activity-induced homeostatic synaptic scaling-down, while direct activation of EphA4 with its ligand EphrinA1 weakened excitatory synaptic strength. Up- or down-regulating EphA4 function in individual neurons also did not affect the density of excitatory synapses. The kinase activities of EphA4 and its downstream effector Cdk5 were both required for homeostatic synaptic scaling, as overexpression of EphA4 with constitutively active kinase activity reduced excitatory synaptic strength, while interfering with either the kinase activity of EphA4 or Cdk5 blocked activity-induced synaptic scaling. Consistently, the activities of EphA4 and Cdk5 increased significantly during global and persistent activity elevation. Together, our work demonstrated that the kinase activity of EphA4, via activation of downstream Cdk5 activity, mediates the scaling-down of excitatory synaptic strength under conditions of global activity elevation.

Evolving a neural olfactorimotor system in virtual and real olfactory environments

To provide a platform to enable the study of simulated olfactory circuitry in context, we have integrated a simulated neural olfactorimotor system with a virtual world which simulates both computational fluid dynamics as well as a robotic agent capable of exploring the simulated plumes. A number of the elements which we developed for this purpose have not, to our knowledge, been previously assembled into an integrated system, including: control of a simulated agent by a neural olfactorimotor system; continuous interaction between the simulated robot and the virtual plume; the inclusion of multiple distinct odorant plumes and background odor; the systematic use of artificial evolution driven by olfactorimotor performance (e.g., time to locate a plume source) to specify parameter values; the incorporation of the realities of an imperfect physical robot using a hybrid model where a physical robot encounters a simulated plume. We close by describing ongoing work toward engineering a high dimensional, reversible, low power electronic olfactory sensor which will allow olfactorimotor neural circuitry evolved in the virtual world to control an autonomous olfactory robot in the physical world. The platform described here is intended to better test theories of olfactory circuit function, as well as provide robust odor source localization in realistic environments.

Functional adaptation of cortical interneurons to attenuated activity is subtype-specific

Functional neuronal homeostasis has been studied in a variety of model systems and contexts. Many studies have shown that there are a number of changes that can be activated within individual cells or networks in order to compensate for perturbations or changes in levels of activity. Dissociating the cell autonomous from the network-mediated events has been complicated due to the difficulty of sparsely targeting specific populations of neurons in vivo. Here, we make use of a recent in vivo approach we developed that allows for the sparse labeling and manipulation of activity within superficial caudal ganglionic eminence (CGE)-derived GABAergic interneurons. Expression of the inward rectifying potassium channel Kir2.1 cell-autonomously reduced neuronal activity and lead to specific developmental changes in their intrinsic electrophysiological properties and the synaptic input they received. In contrast to previous studies on homeostatic scaling of pyramidal cells, we did not detect any of the typically observed compensatory mechanisms in these interneurons. Rather, we instead saw a specific alteration of the kinetics of excitatory synaptic events within the reelin-expressing subpopulation of interneurons. These results provide the first in vivo observations for the capacity of interneurons to cell-autonomously regulate their excitability.

MSK1 regulates homeostatic and experience-dependent synaptic plasticity

The ability of neurons to modulate synaptic strength underpins synaptic plasticity, learning and memory, and adaptation to sensory experience. Despite the importance of synaptic adaptation in directing, reinforcing, and revising the behavioral response to environmental influences, the cellular and molecular mechanisms underlying synaptic adaptation are far from clear. Brain-derived neurotrophic factor (BDNF) is a prime initiator of structural and functional synaptic adaptation. However, the signaling cascade activated by BDNF to initiate these adaptive changes has not been elucidated. We have previously shown that BDNF activates mitogen- and stress-activated kinase 1 (MSK1), which regulates gene transcription via the phosphorylation of both CREB and histone H3. Using mice with a kinase-dead knock-in mutation of MSK1, we now show that MSK1 is necessary for the upregulation of synaptic strength in response to environmental enrichment in vivo. Furthermore, neurons from MSK1 kinase-dead mice failed to show scaling of synaptic transmission in response to activity deprivation in vitro, a deficit that could be rescued by reintroduction of wild-type MSK1. We also show that MSK1 forms part of a BDNF- and MAPK-dependent signaling cascade required for homeostatic synaptic scaling, which likely resides in the ability of MSK1 to regulate cell surface GluA1 expression via the induction of Arc/Arg3.1. These results demonstrate that MSK1 is an integral part of a signaling pathway that underlies the adaptive response to synaptic and environmental experience. MSK1 may thus act as a key homeostat in the activity- and experience-dependent regulation of synaptic strength.

Development of larval motor circuits in Drosophila

How are functional neural circuits formed during development? Despite recent advances in our understanding of the development of individual neurons, little is known about how complex circuits are assembled to generate specific behaviors. Here, we describe the ways in which Drosophila motor circuits serve as an excellent model system to tackle this problem. We first summarize what has been learned during the past decades on the connectivity and development of component neurons, in particular motor neurons and sensory feedback neurons. We then review recent progress in our understanding of the development of the circuits as well as studies that apply optogenetics and other innovative techniques to dissect the circuit diagram. New approaches using Drosophila as a model system are now making it possible to search for developmental rules that regulate the construction of neural circuits.

Snapin is critical for presynaptic homeostatic plasticity

The molecular mechanisms underlying the homeostatic modulation of presynaptic neurotransmitter release are largely unknown. We have previously used an electrophysiology-based forward genetic screen to assess the function of >400 neuronally expressed genes for a role in the homeostatic control of synaptic transmission at the neuromuscular junction of Drosophila melanogaster. This screen identified a critical function for dysbindin, a gene linked to schizophrenia in humans (Dickman and Davis, 2009). Biochemical studies in other systems have shown that Snapin interacts with Dysbindin, prompting us to test whether Snapin might be involved in the mechanisms of synaptic homeostasis. Here, we demonstrate that loss of snapin blocks the homeostatic modulation of presynaptic vesicle release following inhibition of postsynaptic glutamate receptors. This is true for both the rapid induction of synaptic homeostasis induced by pharmacological inhibition of postsynaptic glutamate receptors, and the long-term expression of synaptic homeostasis induced by the genetic deletion of the muscle-specific GluRIIA glutamate receptor subunit. Loss of snapin does not alter baseline synaptic transmission, synapse morphology, synapse growth, or the number or density of active zones, indicating that the block of synaptic homeostasis is not a secondary consequence of impaired synapse development. Additional genetic evidence suggests that snapin functions in concert with dysbindin to modulate vesicle release and possibly homeostatic plasticity. Finally, we provide genetic evidence that the interaction of Snapin with SNAP25, a component of the SNARE complex, is also involved in synaptic homeostasis.

The principle of coherence in multi-level brain information processing

Synchronisation has become one of the major scientific tools to explain biological order at many levels of organisation. In systems neuroscience, synchronised subthreshold and suprathreshold oscillatory neuronal activity within and between distributed neuronal assemblies is acknowledged as a fundamental mode of neuronal information processing. Coherent neuronal oscillations correlate with all basic cognitive functions, mediate local and long-range neuronal communication and affect synaptic plasticity. However, it remains unclear how the very fast and complex changes of functional neuronal connectivity necessary for cognition, as mediated by dynamic patterns of neuronal synchrony, could be explained exclusively based on the well-established synaptic mechanisms. A growing body of research indicates that the intraneuronal matrix, composed of cytoskeletal elements and their binding proteins, structurally and functionally connects the synapses within a neuron, modulates neurotransmission and memory consolidation, and is hypothesised to be involved in signal integration via electric signalling due to its charged surface. Theoretical modelling, as well as emerging experimental evidence indicate that neuronal cytoskeleton supports highly cooperative energy transport and information processing based on molecular coherence. We suggest that long-range coherent dynamics within the intra- and extracellular filamentous matrices could establish dynamic ordered states, capable of rapid modulations of functional neuronal connectivity via their interactions with neuronal membranes and synapses. Coherence may thus represent a common denominator of neurophysiological and biophysical approaches to brain information processing, operating at multiple levels of neuronal organisation, from which cognition may emerge as its cardinal manifestation.

Pathological plasticity in fragile X syndrome

Deficits in neuronal plasticity are common hallmarks of many neurodevelopmental disorders. In the case of fragile-X syndrome (FXS), disruption in the function of a single gene, FMR1, results in a variety of neurological consequences directly related to problems with the development, maintenance, and capacity of plastic neuronal networks. In this paper, we discuss current research illustrating the mechanisms underlying plasticity deficits in FXS. These processes include synaptic, cell intrinsic, and homeostatic mechanisms both dependent on and independent of abnormal metabotropic glutamate receptor transmission. We place particular emphasis on how identified deficits may play a role in developmental critical periods to produce neuronal networks with permanently decreased capacity to dynamically respond to changes in activity central to learning, memory, and cognition in patients with FXS. Characterizing early developmental deficits in plasticity is fundamental to develop therapies that not only treat symptoms but also minimize the developmental pathology of the disease.

Homeostatic plasticity in Drosophila central neurons, and implications in human diseases

Synaptic homeostasis is a form of neuronal plasticity that stabilizes activity of neural networks. Both presynaptic and postsynaptic effects are well documented in response to activity changes. The electrical signaling machinery of individual neurons, or intrinsic properties, have also been implicated in this plasticity. How synaptic and intrinsic changes are coordinated, however, is still a puzzle. A recent study by Ping and Tsunoda shows both synaptic and intrinsic changes in Drosophila central neurons in response to prolonged inactivity. ( 1) Changes include the upregulation of Dα7 nicotinic acetylcholine receptors (nAChRs) and Shal (Kv 4) potassium channels. This work has two noteworthy findings. First, although mediated by different receptors, synaptic homeostasis in the central nervous system (CNS) is conserved across species. This is perhaps the most direct demonstration that nAChRs mediate synaptic homeostasis. Changes in the expression of nAChRs have long been noted during development, as well as during pathological conditions, such as nicotine addiction ( 2) and Alzheimer disease. ( 3) The second interesting finding is the relationship between synaptic and intrinsic plasticity: nAChRs are upregulated immediately, subsequently triggering a rapid increase in Shal K (+) channels. This novel mechanism regulates synaptic homeostasis to stabilize synaptic potentials. This study sets the stage for Drosophila central neurons as a model for cholinergic synaptic homeostasis, its regulation and role in disease.

TOR is required for the retrograde regulation of synaptic homeostasis at the Drosophila neuromuscular junction

Homeostatic mechanisms operate to stabilize synaptic function; however, we know little about how they are regulated. Exploiting Drosophila genetics, we have uncovered a critical role for the target of rapamycin (TOR) in the regulation of synaptic homeostasis at the Drosophila larval neuromuscular junction. Loss of postsynaptic TOR disrupts a retrograde compensatory enhancement in neurotransmitter release that is normally triggered by a reduction in postsynaptic glutamate receptor activity. Moreover, postsynaptic overexpression of TOR or a phosphomimetic form of S6 ribosomal protein kinase, a common target of TOR, can trigger a strong retrograde increase in neurotransmitter release. Interestingly, heterozygosity for eIF4E, a critical component of the cap-binding protein complex, blocks the retrograde signal in all these cases. Our findings suggest that cap-dependent translation under the control of TOR plays a critical role in establishing the activity dependent homeostatic response at the NMJ.

Specific sets of intrinsic and extrinsic factors drive excitatory and inhibitory circuit formation

How are excitatory (glutamatergic) and inhibitory (GABAergic) synapses established? Do distinct molecular mechanisms direct differentiation of glutamatergic and GABAergic synapses? In the brain, glutamatergic and GABAergic synaptic connections are formed with specific patterns. To establish such precise synaptic patterns, neurons pass through multiple checkpoints during development, such as cell fate determination, cell migration and localization, axonal guidance and target recognition, and synapse formation. Each stage offers key molecules for neurons/synapses to obtain glutamatergic or GABAergic specificity. Some mechanisms are based on intrinsic systems to induce gene expression, whereas others are based on extrinsic systems mediated by cell-cell or axon-target interactions. Recent studies indicate that specific formation of glutamatergic and GABAergic synapses is controlled by the expression or activation of different sets of molecules during development. In this review, the authors outline stages critical to the determination of glutamatergic or GABAergic specificity and describe molecules that act as determinants of specificities in each stage, with a particular focus on the synapse formation stage. They also discuss possible mechanisms underlying glutamatergic and GABAergic synapse formation via synapse-type specific synaptic organizers.

Transsynaptic control of presynaptic Ca²⁺ influx achieves homeostatic potentiation of neurotransmitter release

Given the complexity of the nervous system and its capacity for change, it is remarkable that robust, reproducible neural function and animal behavior can be achieved. It is now apparent that homeostatic signaling systems have evolved to stabilize neural function. At the neuromuscular junction (NMJ) of organisms ranging from Drosophila to human, inhibition of postsynaptic neurotransmitter receptor function causes a homeostatic increase in presynaptic release that precisely restores postsynaptic excitation. Here we address what occurs within the presynaptic terminal to achieve homeostatic potentiation of release at the Drosophila NMJ. By imaging presynaptic Ca(2+) transients evoked by single action potentials, we reveal a retrograde, transsynaptic modulation of presynaptic Ca(2+) influx that is sufficient to account for the rapid induction and sustained expression of the homeostatic change in vesicle release. We show that the homeostatic increase in Ca(2+) influx and release is blocked by a point mutation in the presynaptic CaV2.1 channel, demonstrating that the modulation of presynaptic Ca(2+) influx through this channel is causally required for homeostatic potentiation of release. Together with additional analyses, we establish that retrograde, transsynaptic modulation of presynaptic Ca(2+) influx through CaV2.1 channels is a key factor underlying the homeostatic regulation of neurotransmitter release.

Wherefore art thou, homeo(stasis)? Functional diversity in homeostatic synaptic plasticity

Homeostatic plasticity has emerged as a fundamental regulatory principle that strives to maintain neuronal activity within optimal ranges by altering diverse aspects of neuronal function. Adaptation to network activity is often viewed as an essential negative feedback restraint that prevents runaway excitation or inhibition. However, the precise importance of these homeostatic functions is often theoretical rather than empirically derived. Moreover, a remarkable multiplicity of homeostatic adaptations has been observed. To clarify these issues, it may prove useful to ask: why do homeostatic mechanisms exist, what advantages do these adaptive responses confer on a given cell population, and why are there so many seemingly divergent effects? Here, we approach these questions by applying the principles of control theory to homeostatic synaptic plasticity of mammalian neurons and suggest that the varied responses observed may represent distinct functional classes of control mechanisms directed toward disparate physiological goals.

AMPA receptor trafficking in homeostatic synaptic plasticity: functional molecules and signaling cascades

Homeostatic synaptic plasticity is a negative-feedback response employed to compensate for functional disturbances in the nervous system. Typically, synaptic activity is strengthened when neuronal firing is chronically suppressed or weakened when neuronal activity is chronically elevated. At both the whole cell and entire network levels, activity manipulation leads to a global up- or downscaling of the transmission efficacy of all synapses. However, the homeostatic response can also be induced locally at subcellular regions or individual synapses. Homeostatic synaptic scaling is expressed mainly via the regulation of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) trafficking and synaptic expression. Here we review the recently identified functional molecules and signaling pathways that are involved in homeostatic plasticity, especially the homeostatic regulation of AMPAR localization at excitatory synapses.

Relay of retrograde synaptogenic signals through axonal transport of BMP receptors

Neuronal function depends on the retrograde relay of growth and survival signals from the synaptic terminal, where the neuron interacts with its targets, to the nucleus, where gene transcription is regulated. Activation of the Bone Morphogenetic Protein (BMP) pathway at the Drosophila larval neuromuscular junction results in nuclear accumulation of the phosphorylated form of the transcription factor Mad in the motoneuron nucleus. This in turn regulates transcription of genes that control synaptic growth. How BMP signaling at the synaptic terminal is relayed to the cell body and nucleus of the motoneuron to regulate transcription is unknown. We show that the BMP receptors are endocytosed at the synaptic terminal and transported retrogradely along the axon. Furthermore, this transport is dependent on BMP pathway activity, as it decreases in the absence of ligand or receptors. We further demonstrate that receptor traffic is severely impaired when Dynein motors are inhibited, a condition that has previously been shown to block BMP pathway activation. In contrast to these results, we find no evidence for transport of phosphorylated Mad along the axons, and axonal traffic of Mad is not affected in mutants defective in BMP signaling or retrograde transport. These data support a model in which complexes of activated BMP receptors are actively transported along the axon towards the cell body to relay the synaptogenic signal, and that phosphorylated Mad at the synaptic terminal and cell body represent two distinct molecular populations.

Barfly: sculpting membranes at the Drosophila neuromuscular junction

The ability of a cell to change the shape of its membranes is intrinsic to many cellular functions. Proteins that can alter or recognize curved membrane structures and those that can act to recruit other proteins which stabilize the membrane curvature are likely to be essential in cell functions. The BAR (Bin, amphiphysin, RVS167 homology) domain is a protein domain that can either induce lipidic membranes to curve or can sense curved membranes. BAR domains are found in several proteins at neuronal synapses. We will review BAR domain structure and the role that BAR domain containing proteins play in regulating the morphology and function of the Drosophila neuromuscular junction. In flies the BAR domain containing proteins, endophilin and syndapin affect synaptic vesicle endocytosis, whereas CIP4, dRich, nervous wreck and syndapin affect synaptic morphology. We will review the growing evidence implicating mutations in BAR domain containing proteins being the cause of human pathologies.

The Rett syndrome protein MeCP2 regulates synaptic scaling

Synaptic scaling is a form of homeostatic synaptic plasticity characterized by cell-wide changes in synaptic strength in response to changes in overall levels of neuronal activity. Here we report that bicuculline-induced increase in neuronal activity leads to a decrease in mEPSC amplitude and a decrease in expression of the AMPA receptor subunit GluR2 in rat hippocampal cultures. Bicuculline treatment also leads to an increase in the levels of the transcriptional repressor MeCP2, which binds to the GluR2 promoter along with the corepressors HDAC1 and mSin3A. Downregulation of MeCP2 by shRNA expression or genetic deletion blocks the bicuculline-induced decrease in GluR2 expression and mEPSC amplitude. These observations indicate that MeCP2 mediates activity-dependent synaptic scaling, and suggest that the pathophysiology of Rett syndrome, which is caused by mutations in MeCP2, may involve defects in activity-dependent regulation of synaptic currents.

Entorhinal denervation induces homeostatic synaptic scaling of excitatory postsynapses of dentate granule cells in mouse organotypic slice cultures

Denervation-induced changes in excitatory synaptic strength were studied following entorhinal deafferentation of hippocampal granule cells in mature (≥ 3 weeks old) mouse organotypic entorhino-hippocampal slice cultures. Whole-cell patch-clamp recordings revealed an increase in excitatory synaptic strength in response to denervation during the first week after denervation. By the end of the second week synaptic strength had returned to baseline. Because these adaptations occurred in response to the loss of excitatory afferents, they appeared to be in line with a homeostatic adjustment of excitatory synaptic strength. To test whether denervation-induced changes in synaptic strength exploit similar mechanisms as homeostatic synaptic scaling following pharmacological activity blockade, we treated denervated cultures at 2 days post lesion for 2 days with tetrodotoxin. In these cultures, the effects of denervation and activity blockade were not additive, suggesting that similar mechanisms are involved. Finally, we investigated whether entorhinal denervation, which removes afferents from the distal dendrites of granule cells while leaving the associational afferents to the proximal dendrites of granule cells intact, results in a global or a local up-scaling of granule cell synapses. By using computational modeling and local electrical stimulations in Strontium (Sr(2+))-containing bath solution, we found evidence for a lamina-specific increase in excitatory synaptic strength in the denervated outer molecular layer at 3-4 days post lesion. Taken together, our data show that entorhinal denervation results in homeostatic functional changes of excitatory postsynapses of denervated dentate granule cells in vitro.

SAM68 regulates neuronal activity-dependent alternative splicing of neurexin-1

The assembly of synapses and neuronal circuits relies on an array of molecular recognition events and their modification by neuronal activity. Neurexins are a highly polymorphic family of synaptic receptors diversified by extensive alternative splicing. Neurexin variants exhibit distinct isoform-specific biochemical interactions and synapse assembly functions, but the mechanisms governing splice isoform choice are not understood. We demonstrate that Nrxn1 alternative splicing is temporally and spatially controlled in the mouse brain. Neuronal activity triggers a shift in Nrxn1 splice isoform choice via calcium/calmodulin-dependent kinase IV signaling. Activity-dependent alternative splicing of Nrxn1 requires the KH-domain RNA-binding protein SAM68 that associates with RNA response elements in the Nrxn1 pre-mRNA. Our findings uncover SAM68 as a key regulator of dynamic control of Nrxn1 molecular diversity and activity-dependent alternative splicing in the central nervous system.

Extensive morphological divergence and rapid evolution of the larval neuromuscular junction in Drosophila

Although the complexity and circuitry of nervous systems undergo evolutionary change, we lack understanding of the general principles and specific mechanisms through which it occurs. The Drosophila larval neuromuscular junction (NMJ), which has been widely used for studies of synaptic development and function, is also an excellent system for studies of synaptic evolution because the genus spans >40 Myr of evolution and the same identified synapse can be examined across the entire phylogeny. We have now characterized morphology of the NMJ on muscle 4 (NMJ4) in >20 species of Drosophila. Although there is little variation within a species, NMJ morphology and complexity vary extensively between species. We find no significant correlation between NMJ phenotypes and phylogeny for the species examined, suggesting that drift alone cannot explain the phenotypic variation and that selection likely plays an important role. However, the nature of the selective pressure is still unclear because basic parameters of synaptic function remain uniform. Whatever the mechanism, NMJ morphology is evolving rapidly in comparison with other morphological features because NMJ phenotypes differ even between several sibling species pairs. The discovery of this unexpectedly extensive divergence in NMJ morphology among Drosophila species provides unique opportunities to investigate mechanisms that regulate synaptic growth; the interrelationships between synaptic morphology, neural function, and behavior; and the evolution of nervous systems and behavior in natural populations.

Lola regulates glutamate receptor expression at the Drosophila neuromuscular junction

Communication between pre- and post-synaptic cells is a key process in the development and modulation of synapses. Reciprocal induction between pre- and postsynaptic cells involves regulation of gene transcription, yet the underlying genetic program remains largely unknown. To investigate how innervation-dependent gene expression in postsynaptic cells supports synaptic differentiation, we performed comparative microarray analysis of Drosophila muscles before and after innervation, and of prospero mutants, which show a delay in motor axon outgrowth. We identified 84 candidate genes that are potentially up- or downregulated in response to innervation. By systematic functional analysis, we found that one of the downregulated genes, longitudinals lacking (lola), which encodes a BTB-Zn-finger transcription factor, is required for proper expression of glutamate receptors. When the function of lola was knocked down in muscles by RNAi, the abundance of glutamate receptors (GluRs), GluRIIA, GluRIIB and GluRIII, as well as that of p-21 activated kinase (PAK), was greatly reduced at the neuromuscular junctions (NMJs). Recordings of the synaptic response revealed a decrease in postsynaptic quantal size, consistent with the reduction in GluR levels. Lola appears to regulate the expression of GluRs and PAK at the level of transcription, because the amount of mRNAs encoding these molecules was also reduced in the mutants. The transcriptional level of lola, in turn, is downregulated by increased neural activity. We propose that Lola coordinates expression of multiple postsynaptic components by transcriptional regulation.

Correlated variations in the parameters that regulate dendritic calcium signaling in mouse retinal ganglion cells

The amplitude and time course of stimulus-evoked second messenger signals carried by intracellular changes in free calcium ([Ca](free)) depend on the total influx of Ca(2+), the fraction bound to endogenous buffer and the rate of extrusion. Estimates of the values of these three parameters in proximal dendrites of 15 mouse α retinal ganglion cells were made using the "added buffer" method and found to vary greatly from one experiment to the next. The variations in the measured parameters were strongly correlated across the sample of cells. This reduced the variability in the amplitude and time course of the dendritic Ca(2+) signal and suggests that the expression of Ca(2+) channels, binding proteins and extrusion mechanisms is homeostatically coordinated to maintain the amplitude and kinetics of the Ca(2+) signal within a physiologically appropriate range.

Homeostatic regulation of AMPA receptor trafficking and degradation by light-controlled single-synaptic activation

During homeostatic adjustment in response to alterations in neuronal activity, synaptic expression of AMPA receptors (AMPARs) is globally tuned up or down so that the neuronal activity is restored to a physiological range. Given that a central neuron receives multiple presynaptic inputs, whether and how AMPAR synaptic expression is homeostatically regulated at individual synapses remain unclear. In cultured hippocampal neurons we report that when activity of an individual presynaptic terminal is selectively elevated by light-controlled excitation, AMPAR abundance at the excited synapses is selectively downregulated in an NMDAR-dependent manner. The reduction in surface AMPARs is accompanied by enhanced receptor endocytosis and dependent on proteasomal activity. Synaptic activation also leads to a site-specific increase in the ubiquitin ligase Nedd4 and polyubiquitination levels, consistent with AMPAR ubiquitination and degradation in the spine. These results indicate that AMPAR accumulation at individual synapses is subject to autonomous homeostatic regulation in response to synaptic activity.

Decrease in calcium concentration triggers neuronal retinoic acid synthesis during homeostatic synaptic plasticity

Blockade of synaptic activity induces homeostatic plasticity, in part by stimulating synthesis of all-trans retinoic acid (RA), which in turn increases AMPA receptor synthesis. However, the synaptic signal that triggers RA synthesis remained unknown. Using multiple activity-blockade protocols that induce homeostatic synaptic plasticity, here we show that RA synthesis is activated whenever postsynaptic Ca(2+) entry is significantly decreased and that RA is required for upregulation of synaptic strength under these homeostatic plasticity conditions, suggesting that Ca(2+) plays an inhibitory role in RA synthesis. Consistent with this notion, we demonstrate that both transient Ca(2+) depletion by membrane-permeable Ca(2+) chelators and chronic blockage of L-type Ca(2+)-channels induces RA synthesis. Moreover, the source of dendritic Ca(2+) entry that regulates RA synthesis is not specific because mild depolarization with KCl is sufficient to reverse synaptic scaling induced by L-type Ca(2+)-channel blocker. By expression of a dihydropyridine-insensitive L-type Ca(2+) channel, we further show that RA acts cell autonomously to modulate synaptic transmission. Our findings suggest that, in synaptically active neurons, modest "basal" levels of postsynaptic Ca(2+) physiologically suppress RA synthesis, whereas in synaptically inactive neurons, decreases in the resting Ca(2+) levels induce homeostatic plasticity by stimulating synthesis of RA that then acts in a cell-autonomous manner to increase AMPA receptor function.

Neuronal activity regulates glutamate transporter dynamics in developing astrocytes

Glutamate transporters (GluTs) maintain a low ambient level of glutamate in the central nervous system (CNS) and shape the activation of glutamate receptors at synapses. Nevertheless, the mechanisms that regulate the trafficking and localization of transporters near sites of glutamate release are poorly understood. Here, we examined the subcellular distribution and dynamic remodeling of the predominant GluT GLT-1 (excitatory amino acid transporter 2, EAAT2) in developing hippocampal astrocytes. Immunolabeling revealed that endogenous GLT-1 is concentrated into discrete clusters along branches of developing astrocytes that were apposed preferentially to synapsin-1 positive synapses. Green fluorescent protein (GFP)-GLT-1 fusion proteins expressed in astrocytes also formed distinct clusters that lined the edges of astrocyte processes, as well as the tips of filopodia and spine-like structures. Time-lapse three-dimensional confocal imaging in tissue slices revealed that GFP-GLT-1 clusters were dynamically remodeled on a timescale of minutes. Some transporter clusters moved within developing astrocyte branches as filopodia extended and retracted, while others maintained stable positions at the tips of spine-like structures. Blockade of neuronal activity with tetrodotoxin reduced both the density and perisynaptic localization of GLT-1 clusters. Conversely, enhancement of neuronal activity increased the size of GLT-1 clusters and their proximity to synapses. Together, these findings indicate that neuronal activity influences both the organization of GluTs in developing astrocyte membranes and their position relative to synapses.

Activity-dependent neurotransmitter respecification

For many years it has been assumed that the identity of the transmitters expressed by neurons is stable and unchanging. Recent work, however, shows that electrical activity can respecify neurotransmitter expression during development and in the mature nervous system, and an understanding is emerging of the molecular mechanisms underlying activity-dependent transmitter respecification. Changes in postsynaptic neurotransmitter receptor expression accompany and match changes in transmitter specification, thus enabling synaptic transmission. The functional roles of neurotransmitter respecification are beginning to be understood and appear to involve homeostatic synaptic regulation, which in turn influences behaviour. Activation of this novel form of plasticity by sensorimotor stimuli may provide clinical benefits.