Compensatory changes in cellular excitability, not synaptic scaling, contribute to homeostatic recovery of embryonic network activity

Homeostatic Regulation of Spike Rate within Bursts in Two Distinct Preparations

Homeostatic plasticity represents a set of mechanisms thought to stabilize some function of neural activity. Here, we identified the specific features of cellular or network activity that were maintained after the perturbation of GABAergic blockade in two different systems: mouse cortical neuronal cultures where GABA is inhibitory and motoneurons in the isolated embryonic chick spinal cord where GABA is excitatory (males and females combined in both systems). We conducted a comprehensive analysis of various spiking activity characteristics following GABAergic blockade. We observed significant variability in many features after blocking GABAA receptors (e.g., burst frequency, burst duration, overall spike frequency in culture). These results are consistent with the idea that neuronal networks achieve activity goals using different strategies (degeneracy). On the other hand, some features were consistently altered after receptor blockade in the spinal cord preparation (e.g., overall spike frequency). Regardless, these features did not express strong homeostatic recoveries when tracking individual preparations over time. One feature showed a consistent change and homeostatic recovery following GABAA receptor block. We found that spike rate within a burst (SRWB) increased after receptor block in both the spinal cord preparation and cortical cultures and then returned to baseline within hours. These changes in SRWB occurred at both single cell and population levels. Our findings indicate that the network prioritizes the burst spike rate, which appears to be a variable under tight homeostatic regulation. The result is consistent with the idea that networks can maintain an appropriate behavioral response in the face of challenges.

Plasticity in Preganglionic and Postganglionic Neurons of the Sympathetic Nervous System during Embryonic Development

Sympathetic preganglionic neurons (SPNs) are the final output neurons from the central arm of the autonomic nervous system. Therefore, SPNs represent a crucial component of the sympathetic nervous system for integrating several inputs before driving the postganglionic neurons (PGNs) in the periphery to control end organ function. The mechanisms which establish and regulate baseline sympathetic tone and overall excitability of SPNs and PGNs are poorly understood. The SPNs are also known as the autonomic motoneurons (MNs) as they arise from the same progenitor line as somatic MNs that innervate skeletal muscles. Previously our group has identified a rich repertoire of homeostatic plasticity (HP) mechanisms in somatic MNs of the embryonic chick following in vivo synaptic blockade. Here, using the same model system, we examined whether SPNs exhibit similar homeostatic capabilities to that of somatic MNs. Indeed, we found that after 2-d reduction of excitatory synaptic input, SPNs showed a significant increase in intracellular chloride levels, the mechanism underlying GABAergic synaptic scaling in this system. This form of HP could therefore play a role in the early establishment of a setpoint of excitability in this part of the sympathetic nervous system. Next, we asked whether homeostatic mechanisms are expressed in the synaptic targets of SPNs, the PGNs. In this case we blocked synaptic input to PGNs in vivo (48-h treatment), or acutely ex vivo, however neither treatment induced homeostatic adjustments in PGN excitability. We discuss differences in the homeostatic capacity between the central and peripheral component of the sympathetic nervous system.

Homeostatic Regulation of Motoneuron Properties in Development

Homeostatic plasticity represents a set of compensatory mechanisms that are engaged following a perturbation to some feature of neuronal or network function. Homeostatic mechanisms are most robustly expressed during development, a period that is replete with various perturbations such as increased cell size and the addition/removal of synaptic connections. In this review we look at numerous studies that have advanced our understanding of homeostatic plasticity by taking advantage of the accessibility of developing motoneurons. We discuss the homeostatic regulation of embryonic movements in the living chick embryo and describe the spinal compensatory mechanisms that act to recover these movements (homeostatic intrinsic plasticity) or stabilize synaptic strength (synaptic scaling). We describe the expression and triggering mechanisms of these forms of homeostatic plasticity and thereby gain an understanding of their roles in the motor system. We then illustrate how these findings can be extended to studies of developing motoneurons in other systems including the rodents, zebrafish, and fly. Furthermore, studies in developing drosophila have been critical in identifying some of the molecular signaling cascades and expression mechanisms that underlie homeostatic intrinsic membrane excitability. This powerful model organism has also been used to study a presynaptic form of homeostatic plasticity where increases or decreases in synaptic transmission are associated with compensatory changes in probability of release at the neuromuscular junction. Further, we describe studies that demonstrate homeostatic adjustments of ion channel expression following perturbations to other kinds of ion channels. Finally, we discuss work in xenopus that shows a homeostatic regulation of neurotransmitter phenotype in developing motoneurons following activity perturbations. Together, this work illustrates the importance of developing motoneurons in elucidating the mechanisms and roles of homeostatic plasticity.

Prenatal exposure to nicotine disrupts synaptic network formation by inhibiting spontaneous correlated wave activity

Correlated spontaneous activity propagating over a wide region of the central nervous system is expressed during a specific period of embryonic development. We previously demonstrated using an optical imaging technique with a voltage-sensitive dye that this wave-like activity, which we referred to as the depolarization wave, is fundamentally involved in the early process of synaptic network formation. We found that the in ovo application of bicuculline/strychnine or d-tubocurarine, which blocked the neurotransmitters mediating the wave, significantly reduced functional synaptic expression in the brainstem sensory nucleus. This result, particularly for d-tubocurarine, an antagonist of nicotinic acetylcholine receptors, suggested that prenatal nicotine exposure associated with maternal smoking affects the development of neural circuit formation by interfering with the correlated wave. In the present study, we tested this hypothesis by examining the effects of nicotine on the correlated activity and assessing the chronic action of nicotine in ovo on functional synaptic expression along the vagal sensory pathway. In ovo observations of chick embryo behavior and electrical recording using in vitro preparations showed that the application of nicotine transiently increased embryonic movements and electrical bursts associated with the wave, but subsequently inhibited these activities, suggesting that the dominant action of the drug was to inhibit the wave. Optical imaging with the voltage-sensitive dye showed that the chronic exposure to nicotine in ovo markedly reduced functional synaptic expression in the higher-order sensory nucleus of the vagus nerve, the parabrachial nucleus. The results suggest that prenatal nicotine exposure disrupts the initial formation of the neural circuitry by inhibiting correlated spontaneous wave activity.

Differential neuropeptide modulation of premotor and motor neurons in the lobster cardiac ganglion

The American lobster, Homarus americanus, cardiac neuromuscular system is controlled by the cardiac ganglion (CG), a central pattern generator consisting of four premotor and five motor neurons. Here, we show that the premotor and motor neurons can establish independent bursting patterns when decoupled by a physical ligature. We also show that mRNA encoding myosuppressin, a cardioactive neuropeptide, is produced within the CG. We thus asked whether myosuppressin modulates the decoupled premotor and motor neurons, and if so, how this modulation might underlie the role(s) that these neurons play in myosuppressin's effects on ganglionic output. Although myosuppressin exerted dose-dependent effects on burst frequency and duration in both premotor and motor neurons in the intact CG, its effects on the ligatured ganglion were more complex, with different effects and thresholds on the two types of neurons. These data suggest that the motor neurons are more important in determining the changes in frequency of the CG elicited by low concentrations of myosuppressin, whereas the premotor neurons have a greater impact on changes elicited in burst duration. A single putative myosuppressin receptor (MSR-I) was previously described from the Homarus nervous system. We identified four additional putative MSRs (MSR-II-V) and investigated their individual distributions in the CG premotor and motor neurons using RT-PCR. Transcripts for only three receptors (MSR-II-IV) were amplified from the CG. Potential differential distributions of the receptors were observed between the premotor and motor neurons; these differences may contribute to the distinct physiological responses of the two neuron types to myosuppressin.NEW & NOTEWORTHY Premotor and motor neurons of the Homarus americanus cardiac ganglion (CG) are normally electrically and chemically coupled, and generate rhythmic bursting that drives cardiac contractions; we show that they can establish independent bursting patterns when physically decoupled by a ligature. The neuropeptide myosuppressin modulates different aspects of the bursting pattern in these neuron types to determine the overall modulation of the intact CG. Differential distribution of myosuppressin receptors may underlie the observed responses to myosuppressin.

Homeostatic Recovery of Embryonic Spinal Activity Initiated by Compensatory Changes in Resting Membrane Potential

When baseline activity in a neuronal network is modified by external challenges, a set of mechanisms is prompted to homeostatically restore activity levels. These homeostatic mechanisms are thought to be profoundly important in the maturation of the network. It has been shown that blockade of either excitatory GABAergic or glutamatergic transmission in the living chick embryo transiently blocks the movements generated by spontaneous network activity (SNA) in the spinal cord. However, the embryonic movements then begin to recover by 2 h and are completely restored by 12 h of persistent receptor blockade. It remains unclear what mechanisms mediate this early recovery (first hours) after neurotransmitter blockade, or even if the same mechanisms are triggered following GABAergic and glutamatergic antagonists. Here we find two distinct mechanisms that could underlie this homeostatic recovery. First, we see a highly robust compensatory mechanism observed shortly after neurotransmitter receptor blockade. In the first 2 h of GABAergic or glutamatergic blockade in vitro, there was a clear depolarization of resting membrane potential (RMP) in both motoneurons and interneurons. These changes reduced threshold current and were observed in the continued presence of the antagonist. Therefore, it appears that fast changes in RMP represent a key fast homeostatic mechanism for the maintenance of network activity. Second, we see a less consistent compensatory change in the absolute threshold voltage in the first several hours of in vitro and in vivo neurotransmitter blockade. These mechanisms likely contribute to the homeostatic recovery of embryonic movements following neurotransmitter blockade.

Canard-induced complex oscillations in an excitatory network

In Neuroscience, mathematical modelling involving multiple spatial and temporal scales can unveil complex oscillatory activity such as excitable responses to an input current, subthreshold oscillations, spiking or bursting. While the number of slow and fast variables and the geometry of the system determine the type of the complex oscillations, canard structures define boundaries between them. In this study, we use geometric singular perturbation theory to identify and characterise boundaries between different dynamical regimes in multiple-timescale firing rate models of the developing spinal cord. These rate models are either three or four dimensional with state variables chosen within an overall group of two slow and two fast variables. The fast subsystem corresponds to a recurrent excitatory network with fast activity-dependent synaptic depression, and the slow variables represent the cell firing threshold and slow activity-dependent synaptic depression, respectively. We start by demonstrating canard-induced bursting and mixed-mode oscillations in two different three-dimensional rate models. Then, in the full four-dimensional model we show that a canard-mediated slow passage creates dynamics that combine these complex oscillations and give rise to mixed-mode bursting oscillations (MMBOs). We unveil complicated isolas along which MMBOs exist in parameter space. The profile of solutions along each isola undergoes canard-mediated transitions between the sub-threshold regime and the bursting regime; these explosive transitions change the number of oscillations in each regime. Finally, we relate the MMBO dynamics to experimental recordings and discuss their effects on the silent phases of bursting patterns as well as their potential role in creating subthreshold fluctuations that are often interpreted as noise. The mathematical framework used in this paper is relevant for modelling multiple timescale dynamics in excitable systems.

Homeostatic Intrinsic Plasticity Is Functionally Altered in Fmr1 KO Cortical Neurons

Cortical hyperexcitability is a hallmark of fragile X syndrome (FXS). In the Fmr1 knockout (KO) mouse model of FXS, cortical hyperexcitability is linked to sensory hypersensitivity and seizure susceptibility. It remains unclear why homeostatic mechanisms fail to prevent such activity. Homeostatic intrinsic plasticity (HIP) adjusts membrane excitability through regulation of ion channels to maintain activity levels following activity perturbation. Despite the critical role of HIP in the maturation of excitability, it has not been examined in FXS. Here, we demonstrate that HIP does not operate normally in a disease model, FXS. HIP was either lost or exaggerated in two distinct neuronal populations from Fmr1 KO cortical cultures. In addition, we have identified a mechanism for homeostatic intrinsic plasticity. Compromising HIP function during development could leave cortical neurons in the FXS nervous system vulnerable to hyperexcitability.

Fragile X syndrome (FXS) is characterized by cortical hyperexcitability, but the mechanisms driving hyperexcitability are poorly understood. Homeostatic intrinsic plasticity (HIP) regulates ion channel function to maintain appropriate activity levels. Bülow et al. show that HIP is functionally altered in FXS neurons, which may leave cortical neurons vulnerable to hyperexcitability.

NALCN channels enhance the intrinsic excitability of spinal projection neurons

Spinal projection neurons convey nociceptive signals to multiple brain regions including the parabrachial (PB) nucleus, which contributes to the emotional valence of pain perception. Despite the clear importance of projection neurons to pain processing, our understanding of the factors that shape their intrinsic membrane excitability remains limited. Here, we investigate a potential role for the Na leak channel NALCN in regulating the activity of spino-PB neurons in the developing rodent. Pharmacological reduction of NALCN current (INALCN), or the genetic deletion of NALCN channels, significantly reduced the intrinsic excitability of lamina I spino-PB neurons. In addition, substance P (SP) activated INALCN in ascending projection neurons through downstream Src kinase signaling, and the knockout of NALCN prevented SP-evoked action potential discharge in this neuronal population. These results identify, for the first time, NALCN as a strong regulator of neuronal activity within central pain circuits and also elucidate an additional ionic mechanism by which SP can modulate spinal nociceptive processing. Collectively, these findings indicate that the level of NALCN conductance within spino-PB neurons tightly governs ascending nociceptive transmission to the brain and thereby potentially influences pain perception.

The Effects of GABAergic Polarity Changes on Episodic Neural Network Activity in Developing Neural Systems

Early in development, neural systems have primarily excitatory coupling, where even GABAergic synapses are excitatory. Many of these systems exhibit spontaneous episodes of activity that have been characterized through both experimental and computational studies. As development progress the neural system goes through many changes, including synaptic remodeling, intrinsic plasticity in the ion channel expression, and a transformation of GABAergic synapses from excitatory to inhibitory. What effect each of these, and other, changes have on the network behavior is hard to know from experimental studies since they all happen in parallel. One advantage of a computational approach is that one has the ability to study developmental changes in isolation. Here, we examine the effects of GABAergic synapse polarity change on the spontaneous activity of both a mean field and a neural network model that has both glutamatergic and GABAergic coupling, representative of a developing neural network. We find some intuitive behavioral changes as the GABAergic neurons go from excitatory to inhibitory, shared by both models, such as a decrease in the duration of episodes. We also find some paradoxical changes in the activity that are only present in the neural network model. In particular, we find that during early development the inter-episode durations become longer on average, while later in development they become shorter. In addressing this unexpected finding, we uncover a priming effect that is particularly important for a small subset of neurons, called the “intermediate neurons.” We characterize these neurons and demonstrate why they are crucial to episode initiation, and why the paradoxical behavioral change result from priming of these neurons. The study illustrates how even arguably the simplest of developmental changes that occurs in neural systems can present non-intuitive behaviors. It also makes predictions about neural network behavioral changes that occur during development that may be observable even in actual neural systems where these changes are convoluted with changes in synaptic connectivity and intrinsic neural plasticity.

Developmental roles of the spontaneous depolarization wave in synaptic network formation in the embryonic brainstem

One of the earliest activities expressed within the developing central nervous system is a widely propagating wave-like activity, which we referred to as the depolarization wave. Despite considerable consensus concerning the global features of the activity, its physiological role is yet to be clarified. The depolarization wave is expressed during a specific period of functional synaptogenesis, and this developmental profile has led to the hypothesis that the wave plays some roles in synaptic network organization. In the present study, we tested this hypothesis by inhibiting the depolarization wave in ovo and examining its effects on the development of functional synapses in vagus nerve-related brainstem nuclei of the chick embryo. Chronic inhibition of the depolarization wave had no significant effect on the developmental time course, amplitude, and spatial distribution of monosynaptic excitatory postsynaptic potentials in the first-order nuclei of the vagal sensory pathway (the nucleus of the tractus solitarius (NTS) and the contralateral non-NTS region), but reduced polysynaptic responses in the higher-order nucleus (the parabrachial nucleus). These results suggest that the depolarization wave plays an important role in the initial process of functional synaptic expression in the brainstem, especially in the higher-order nucleus of the cranial sensory pathway.

Synaptic up-scaling preserves motor circuit output after chronic, natural inactivity

Neural systems use homeostatic plasticity to maintain normal brain functions and to prevent abnormal activity. Surprisingly, homeostatic mechanisms that regulate circuit output have mainly been demonstrated during artificial and/or pathological perturbations. Natural, physiological scenarios that activate these stabilizing mechanisms in neural networks of mature animals remain elusive. To establish the extent to which a naturally inactive circuit engages mechanisms of homeostatic plasticity, we utilized the respiratory motor circuit in bullfrogs that normally remains inactive for several months during the winter. We found that inactive respiratory motoneurons exhibit a classic form of homeostatic plasticity, up-scaling of AMPA-glutamate receptors. Up-scaling increased the synaptic strength of respiratory motoneurons and acted to boost motor amplitude from the respiratory network following months of inactivity. Our results show that synaptic scaling sustains strength of the respiratory motor output following months of inactivity, thereby supporting a major neuroscience hypothesis in a normal context for an adult animal.

Neurons in the brain communicate using chemical signals that they send and receive across junctions called synapses. To maintain normal behavior over time, circuits of neurons must reliably process these signals. A variety of nervous system disorders may result if they are unable to do so, as may occur when neural activity changes as a result of disease or injury.

The processes underlying the stability of a neuron’s synapses is referred to as “homeostatic” synaptic plasticity because the changes made by the neuron directly oppose the altered level of activity. In one form of homeostatic plasticity, known as synaptic scaling, neurons modify the strength of all of their synapses in response to changes in neural activity.

There is substantial experimental evidence to show that in young animals, neurons that communicate using a chemical called glutamate undergo synaptic scaling in response to artificial changes in activity. It had not been directly shown that synaptic scaling protects the neural activity of adult animals in their natural environments, in part, because neural activity in most healthy animals generally only goes through small changes. However, the neurons in the brain that cause the breathing muscles of bullfrogs to contract are ideal for studying homeostatic plasticity because they are naturally inactive for several months when frogs hibernate in ponds during the winter. During this time, the bullfrogs do not need to use their lungs to breathe because enough oxygen passes through their skin to keep them alive.

Santin et al. have now observed synaptic scaling of glutamate synapses in individual bullfrog neurons that had been inactive for two months. Further experiments that examined the activity of the breathing control circuit in the brainstem provided evidence that synaptic scaling leads to sufficient amounts of neural activity that would activate the breathing muscles when frogs emerge from hibernation. Therefore neural activity after prolonged, natural inactivity relies on synaptic scaling to preserve life-sustaining behavior in frogs.

These results open up new questions: mainly, how do synaptic scaling and other forms of homeostatic plasticity operate in animals as they experience normal variations in neural activity? Determining how homeostatic plasticity works normally in an animal will help us to understand what happens when plasticity mechanisms go wrong, as is thought to occur in several human nervous system diseases including nervous system injury, Alzheimer’s disease, and epilepsy.

Developmental Disruption of Recurrent Inhibitory Feedback Results in Compensatory Adaptation in the Renshaw Cell-Motor Neuron Circuit

When activating muscles, motor neurons in the spinal cord also activate Renshaw cells, which provide recurrent inhibitory feedback to the motor neurons. The tight coupling with motor neurons suggests that Renshaw cells have an integral role in movement, a role that is yet to be elucidated. Here we used the selective expression of the nicotinic cholinergic receptor α2 (Chrna2) in mice to genetically target the vesicular inhibitory amino acid transporter (VIAAT) in Renshaw cells. Loss of VIAAT from Chrna2^Cre -expressing Renshaw cells did not impact any aspect of drug-induced fictive locomotion in the neonatal mouse or change gait, motor coordination, or grip strength in adult mice of both sexes. However, motor neurons from neonatal mice lacking VIAAT in Renshaw cells received spontaneous inhibitory synaptic input with a reduced frequency, showed lower input resistance, and had an increased number of proprioceptive glutamatergic and calbindin-labeled putative Renshaw cell synapses on their soma and proximal dendrites. Concomitantly, Renshaw cells developed with increased excitability and a normal number of cholinergic motor neuron synapses, indicating a compensatory mechanism within the recurrent inhibitory feedback circuit. Our data suggest an integral role for Renshaw cell signaling in shaping the excitability and synaptic input to motor neurons.SIGNIFICANCE STATEMENT We here provide a deeper understanding of spinal cord circuit formation and the repercussions for the possible role for Renshaw cells in speed and force control. Our results suggest that while Renshaw cells are not directly required as an integral part of the locomotor coordination machinery, the development of their electrophysiological character is dependent on vesicular inhibitory amino acid transporter-mediated signaling. Further, Renshaw cell signaling is closely associated with the molding of motor neuron character proposing the existence of a concerted maturation process, which seems to endow this particular spinal cord circuit with the plasticity to compensate for loss of the Renshaw cell in adult circuit function.

Elevated intracellular Na+ concentrations in developing spinal neurons

Over 25 years ago it was first reported that intracellular chloride levels (Cl^-_in ) were higher in developing neurons than in maturity. This finding has had significant implications for understanding the excitability of developing networks and recognizing the underlying causes of hyperexcitability associated with disease and neural injury. While there is some evidence that intracellular sodium levels (Na⁺_in ) change during the development of non-neural cells, it has largely been assumed that Na⁺_in is the same in developing and mature neurons. Here, using the sodium indicator SBFI, we test this idea and find that Na⁺_in is significantly higher in embryonic spinal motoneurons and interneurons than in maturity. We find that Na⁺_in reaches ~ 60 mM in mid-embryonic development and is then reduced to ~ 30 mM in late embryonic development. By retrogradely labeling motoneurons with SBFI we can reliably follow Na⁺_in levels in vitro for hours. Bursts of spiking activity, and blocking voltage-gated sodium channels did not influence observed motoneuron sodium levels. On the other hand, Na⁺_in was reduced by blocking the Na⁺ -K⁺ -2Cl^- cotransporter NKCC1, and was highly sensitive to changes in external Na⁺ and a blocker of the Na⁺ /K⁺ ATPase. Our findings suggest that the Na⁺ gradient is weaker in embryonic neuronal development and strengthens in maturity in a manner similar to that of Cl^- .

Clarithromycin increases neuronal excitability in CA3 pyramidal neurons through a reduction in GABAergic signaling

Antibiotics are used in the treatment and prevention of bacterial infections, but effects on neuron excitability have been documented. A recent study demonstrated that clarithromycin alleviates daytime sleepiness in hypersomnia patients (Trotti LM, Saini P, Freeman AA, Bliwise DL, García PS, Jenkins A, Rye DB. J Psychopharmacol 28: 697-702, 2014). To explore the potential application of clarithromycin as a stimulant, we performed whole cell patch-clamp recordings in rat pyramidal cells from the CA3 region of hippocampus. In the presence of the antibiotic, rheobase current was reduced by 50%, F-I relationship (number of action potentials as a function of injected current) was shifted to the left, and the resting membrane potential was more depolarized. Clarithromycin-induced hyperexcitability was dose dependent; doses of 30 and 300 μM clarithromycin significantly increased the firing frequency and membrane potential compared with controls (P = 0.003, P < 0.0001). We hypothesized that clarithromycin enhanced excitability by reducing GABA_A receptor activation. Clarithromycin at 30 μM significantly reduced (P = 0.001) the amplitude of spontaneous miniature inhibitory GABAergic currents and at 300 μM had a minor effect on action potential width. Additionally, we tested the effect of clarithromycin in an ex vivo seizure model by evaluating its effect on spontaneous local field potentials. Bath application of 300 μM clarithromycin enhanced burst frequency twofold compared with controls (P = 0.0006). Taken together, these results suggest that blocking GABAergic signaling with clarithromycin increases cellular excitability and potentially serves as a stimulant, facilitating emergence from anesthesia or normalizing vigilance in hypersomnia and narcolepsy. However, the administration of clarithromycin should be carefully considered in patients with seizure disorders.

NEW & NOTEWORTHY

Clinical administration of the macrolide antibiotic clarithromycin has been associated with side effects such as mania, agitation, and delirium. Here, we investigated the adverse effects of this antibiotic on CA3 pyramidal cell excitability. Clarithromycin induces hyperexcitability in single neurons and is related to a reduction in GABAergic signaling. Our results support a potentially new application of clarithromycin as a stimulant to facilitate emergence from anesthesia or to normalize vigilance.

Synergistic plasticity of intrinsic conductance and electrical coupling restores synchrony in an intact motor network

Motor neurons of the crustacean cardiac ganglion generate virtually identical, synchronized output despite the fact that each neuron uses distinct conductance magnitudes. As a result of this variability, manipulations that target ionic conductances have distinct effects on neurons within the same ganglion, disrupting synchronized motor neuron output that is necessary for proper cardiac function. We hypothesized that robustness in network output is accomplished via plasticity that counters such destabilizing influences. By blocking high-threshold K⁺ conductances in motor neurons within the ongoing cardiac network, we discovered that compensation both resynchronized the network and helped restore excitability. Using model findings to guide experimentation, we determined that compensatory increases of both G_A and electrical coupling restored function in the network. This is one of the first direct demonstrations of the physiological regulation of coupling conductance in a compensatory context, and of synergistic plasticity across cell- and network-level mechanisms in the restoration of output.

DOI:http://dx.doi.org/10.7554/eLife.16879.001

Neurons can communicate with each other by releasing chemicals called neurotransmitters, or by forming direct connections with each other known as gap junctions. These direct connections allow electrical impulses to flow from one neuron to another via pores in the membranes between the cells. Unlike communication via neurotransmitters, gap junctions are usually thought to be hard-wired and unchanging over the life of the animal.

Lane et al. recorded electrical activity in a network of neurons that generates rhythmic heart contractions in the Jonah crab. Neurons in this network usually all fire an electrical impulse at the same time, which is crucial to make sure that the whole heart contracts at the same time. The experiments show that drugs that block potassium channel pores in the membrane cause the neurons to fire too much and at different times to each other.

However, the network of neurons soon adapted to the changes caused by the drugs and returned to working as normal. Mimicking these changes in a computer model of the neuron network, together with experimental data, showed that changes to the gap junctions play a major role in restoring normal activity to the network.

The next step following on from this research is to understand how a network of neurons ‘senses’ that it is not working normally and changes its electrical activity.

DOI:http://dx.doi.org/10.7554/eLife.16879.002

Glycinergic Neurotransmission: A Potent Regulator of Embryonic Motor Neuron Dendritic Morphology and Synaptic Plasticity

Emerging evidence suggests that central synaptic inputs onto motor neurons (MNs) play an important role in developmental regulation of the final number of MNs and their muscle innervation for a particular motor pool. Here, we describe the effect of genetic deletion of glycinergic neurotransmission on single MN structure and on functional excitatory and inhibitory inputs to MNs. We measured synaptic currents in E18.5 hypoglossal MNs from brain slices using whole-cell patch-clamp recording, followed by dye-filling these same cells with Neurobiotin, to define their morphology by high-resolution confocal imaging and 3D reconstruction. We show that hypoglossal MNs of mice lacking gephyrin display increased dendritic arbor length and branching, increased spiny processes, decreased inhibitory neurotransmission, and increased excitatory neurotransmission. These findings suggest that central glycinergic synaptic activity plays a vital role in regulating MN morphology and glutamatergic central synaptic inputs during late embryonic development.

SIGNIFICANCE STATEMENT

MNs within the brainstem and spinal cord are responsible for integrating a diverse array of synaptic inputs into discrete contractions of skeletal muscle to achieve coordinated behaviors, such as breathing, vocalization, and locomotion. The last trimester in utero is critical in neuromotor development, as this is when central and peripheral synaptic connections are made onto and from MNs. At this time-point, using transgenic mice with negligible glycinergic postsynaptic responses, we show that this deficiency leads to abnormally high excitatory neurotransmission and alters the dendritic architecture responsible for coherently integrating these inputs. This study compliments the emerging concept that neurodevelopmental disorders (including autism, epilepsy, and amyotrophic lateral sclerosis) are underpinned by synaptic dysfunction and therefore will be useful to neuroscientists and neurologists alike.

Depolarizing GABA and developmental epilepsies

Early in development, GABA, which is the main inhibitory neurotransmitter in adult brain, depolarizes immature neurons and exerts dual--excitatory and shunting/inhibitory--effects in the developing neuronal networks. The present review discusses some general questions, including the properties of excitation at depolarizing GABAergic synapse and shunting inhibition by depolarizing GABA; technical issues in exploration of depolarizing GABA using various techniques and preparations, including the developmental aspects of traumatic injury and what is known (or rather unknown) on the actions of GABA in vivo; complex roles of depolarizing GABA in developmental epilepsies, including a contribution of depolarizing GABA to enhanced excitability in the immature networks, caused by repetitive seizures accumulation of intracellular chloride concentration that increases excitatory GABA power and its synchronizing proconvulsive effects, and correction of chloride homeostasis as a potential strategy to treat neonatal seizures.

Spinal NMDA receptor activation constrains inactivity-induced phrenic motor facilitation in Charles River Sprague-Dawley rats

Reduced spinal synaptic inputs to phrenic motor neurons elicit a unique form of spinal plasticity known as inactivity-induced phrenic motor facilitation (iPMF). iPMF requires tumor necrosis factor-α (TNF-α) and atypical protein kinase C (aPKC) activity within spinal segments containing the phrenic motor nucleus to stabilize early, transient increases in phrenic burst amplitude into long-lasting iPMF. Here we tested the hypothesis that spinal N-methyl-d-aspartate receptor (NMDAR) activation constrains long-lasting iPMF in some rat substrains. Phrenic motor output was recorded in anesthetized, ventilated Harlan (HSD) and Charles River (CRSD) Sprague-Dawley rats exposed to a 30-min central neural apnea. HSD rats expressed a robust, long-lasting (>60 min) increase in phrenic burst amplitude (i.e., long-lasting iPMF) when respiratory neural activity was restored. By contrast, CRSD rats expressed an attenuated, transient (∼15 min) iPMF. Spinal NMDAR inhibition with DL-2-amino-5-phosphonopentanoic acid (APV) before neural apnea or shortly (4 min) prior to the resumption of respiratory neural activity revealed long-lasting iPMF in CRSD rats that was phenotypically similar to that in HSD rats. By contrast, APV did not alter iPMF expression in HSD rats. Spinal TNF-α or aPKC inhibition impaired long-lasting iPMF enabled by NMDAR inhibition in CRSD rats, suggesting that similar mechanisms give rise to long-lasting iPMF in CRSD rats with NMDAR inhibition as those giving rise to long-lasting iPMF in HSD rats. These results suggest that NMDAR activation can impose constraints on TNF-α-induced aPKC activation after neural apnea, impairing stabilization of transient iPMF into long-lasting iPMF. These data may have important implications for understanding differential responses to reduced respiratory neural activity in a heterogeneous human population.

NMDA receptors and L-type voltage-gated Ca²⁺ channels mediate the expression of bidirectional homeostatic intrinsic plasticity in cultured hippocampal neurons

Homeostatic plasticity is engaged when neurons need to stabilize their synaptic strength and excitability in response to acute or prolonged destabilizing changes in global activity. Compared to the extensive studies investigating the molecular mechanisms for homeostatic synaptic plasticity, the mechanism underlying homeostatic intrinsic plasticity is largely unknown. Through whole-cell patch-clamp recording in low-density cultures of dissociated hippocampal neurons, we demonstrate here that prolonged activity blockade induced by the sodium channel blocker tetrodotoxin (TTX) leads to increased action potential firing rates. Conversely, prolonged activity enhancement induced by the A-type gamma-aminobutyric acid receptor antagonist bicuculline (BC) results in decreased firing rates. Prolonged activity enhancement also enhanced potassium (K(+)) current through Kv1 channels, suggesting that changes in K(+) current, in part, mediate stabilization of hippocampal neuronal excitability upon prolonged activity elevation. In contrast to the previous reports showing that L-type voltage-gated calcium (Ca(2+)) channels solely mediate homeostatic regulation of excitatory synaptic strength (Ibata et al., 2008; Goold and Nicoll, 2010), inhibition of N-Methyl-d-aspartate (NMDA) receptors alone mimics the elevation in firing frequency driven by prolonged TTX application, while the decrease in firing rates induced by prolonged BC treatment involves the activity of NMDA receptors and L-type voltage-gated Ca(2+) channels. These results collectively provide strong evidence that alterations in Ca(2+) influx through NMDA receptors and L-type voltage-gated Ca(2+) channels mediate homeostatic intrinsic plasticity in hippocampal neurons in response to prolonged activity changes.

Maintenance of the large-scale depolarization wave in the embryonic chick brain against deprivation of the rhythm generator

Widely correlated spontaneous activity in the developing nervous system is transiently expressed and is considered to play a fundamental role in neural circuit formation. The depolarization wave, which spreads over a long distance along the neuraxis, maximally extending to the lumbosacral cord and forebrain, is an example of this spontaneous activity. Although the depolarization wave is typically initiated in the spinal cord in intact preparations, spontaneous discharges have also been detected in the isolated brainstem. Although this suggests that the brainstem has the ability to generate spontaneous activity, but is paced by a caudal rhythm generator of higher excitability, a number of questions remains. Does brainstem activity simply appear as a passive consequence, or does any active change occur in the brainstem network to compensate for this activity? If the latter is the case, does this compensation occur equally at different developmental stages? Where is the new rhythm generator in the isolated brainstem? To answer these questions, we optically analyzed spatio-temporal patterns of activity detected from the chick brainstem before and after transection at the obex. The results revealed that the depolarization wave was homeostatically maintained, which was characterized by an increase in excitability and/or the number of neurons recruited to the wave. The wave was more easily maintained in younger embryos. Furthermore, we demonstrated that the ability of brainstem neurons to perform such an active compensation was not lost even at the stage when the depolarization wave was no longer observed in the intact brainstem.

Optogenetic-mediated increases in in vivo spontaneous activity disrupt pool-specific but not dorsal-ventral motoneuron pathfinding

Rhythmic waves of spontaneous electrical activity are widespread in the developing nervous systems of birds and mammals, and although many aspects of neural development are activity-dependent, it has been unclear if rhythmic waves are required for in vivo motor circuit development, including the proper targeting of motoneurons to muscles. We show here that electroporated channelrhodopsin-2 can be activated in ovo with light flashes to drive waves at precise intervals of approximately twice the control frequency in intact chicken embryos. Optical monitoring of associated axial movements ensured that the altered frequency was maintained. In embryos thus stimulated, motor axons correctly executed the binary dorsal-ventral pathfinding decision but failed to make the subsequent pool-specific decision to target to appropriate muscles. This observation, together with the previous demonstration that slowing the frequency by half perturbed dorsal-ventral but not pool-specific pathfinding, shows that modest changes in frequency differentially disrupt these two major pathfinding decisions. Thus, many drugs known to alter early rhythmic activity have the potential to impair normal motor circuit development, and given the conservation between mouse and avian spinal cords, our observations are likely relevant to mammals, where such studies would be difficult to carry out.

In vivo synaptic scaling is mediated by GluA2-lacking AMPA receptors in the embryonic spinal cord

When spiking activity within a network is perturbed for hours to days, compensatory changes in synaptic strength are triggered that are thought to be important for the homeostatic maintenance of network or cellular spiking activity. In one form of this homeostatic plasticity, called synaptic scaling, all of a cell's AMPAergic miniature postsynaptic currents (mEPSCs) are increased or decreased by some scaling factor. Although synaptic scaling has been observed in a variety of systems, the mechanisms that underlie AMPAergic scaling have been controversial. Certain studies find that synaptic scaling is mediated by GluA2-lacking calcium receptors (CP-AMPARs), whereas others have found that scaling is mediated by GluA2-containing calcium-impermeable receptors (CI-AMPARs). Spontaneous network activity is observed in most developing circuits, and in the spinal cord this activity drives embryonic movements. Blocking spontaneous network activity in the chick embryo by infusing lidocaine in vivo triggers synaptic scaling in spinal motoneurons; here we show that AMPAergic scaling occurs through increases in mEPSC conductance that appear to be mediated by the insertion of GluA2-lacking AMPA receptors at the expense of GluA2-containing receptors. We have previously reported that in vivo blockade of GABAA transmission, at a developmental stage when GABA is excitatory, also triggered AMPAergic synaptic scaling. Here, we show that this form of AMPAergic scaling is also mediated by CP-AMPARs. These findings suggest that AMPAergic scaling triggered by blocking spiking activity or GABAA receptor transmission represents similar phenomena, supporting the idea that activity blockade triggers scaling by reducing GABAA transmission.

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'.

Large-scale synchronized activity in the embryonic brainstem and spinal cord

In the developing central nervous system, spontaneous activity appears well before the brain responds to external sensory inputs. One of the earliest activities is observed in the hindbrain and spinal cord, which is detected as rhythmic electrical discharges of cranial and spinal motoneurons or oscillations of Ca(2+)- and voltage-related optical signals. Shortly after the initial expression, the spontaneous activity appearing in the hindbrain and spinal cord exhibits a large-scale correlated wave that propagates over a wide region of the central nervous system, maximally extending to the lumbosacral cord and to the forebrain. In this review, we describe several aspects of this synchronized activity by focusing on the basic properties, development, origin, propagation pattern, pharmacological characteristics, and possible mechanisms underlying the generation of the activity. These profiles differ from those of the respiratory and locomotion pattern generators observed in the mature brainstem and spinal cord, suggesting that the wave is primordial activity that appears during a specific period of embryonic development and plays some important roles in the development of the central nervous system.

Downregulation of GluA2 AMPA receptor subunits reduces the dendritic arborization of developing spinal motoneurons

AMPA receptors lacking the GluA2 subunit allow a significant influx of Ca(2+) ions. Although Ca(2+)-permeable AMPA receptors are a familiar feature at early stages of development, the functional significance of these receptors during the maturation of the nervous system remains to be established. Chicken lumbar motoneurons express Ca(2+)-permeable AMPA receptors at E6 but the Ca(2+) permeability of AMPA receptors decreases ∼3-fold by E11. Considering that activity-dependent changes in intracellular Ca(2+) regulates dendritic outgrowth, in this study we investigated whether downregulation of GluA2 expression during a critical period of development alters the dendritic arborization of spinal motoneurons in ovo. We use an avian replication-competent retroviral vector RCASBP (B) carrying the marker red fluorescent protein (RFP) and a GluA2 RNAi construct to downregulate GluA2 expression. Chicken embryos were infected at E2 with one of the following constructs: RCASBP(B)-RFP, RCASBP(B)-RFP-scrambled RNAi, or RCASBP(B)-RFP-GluA2 RNAi. Infection of chicken embryos at E2 resulted in widespread expression of RFP throughout the spinal cord with ≥60% of Islet1/2-positive motoneurons infected, resulting in a significant reduction in GluA2 protein expression. Downregulation of GluA2 expression had no effect on the dendritic arborization of E6 motoneurons. However, downregulation of GluA2 expression caused a significant reduction in the dendritic arborization of E11 motoneurons. Neither motoneuron survival nor maturation of network activity was affected by changes in GluA2 expression. These findings demonstrate that increased GluA2 expression and changes in the Ca(2+) permeability of AMPA receptors regulate the dendritic arborization of spinal cord motoneurons during a critical period of development.

Electrical activity as a developmental regulator in the formation of spinal cord circuits

Spinal cord development is a complex process involving generation of the appropriate number of cells, acquisition of distinctive phenotypes and establishment of functional connections that enable execution of critical functions such as sensation and locomotion. Here we review the basic cellular events occurring during spinal cord development, highlighting studies that demonstrate the roles of electrical activity in this process. We conclude that the participation of different forms of electrical activity is evident from the beginning of spinal cord development and intermingles with other developmental cues and programs to implement dynamic and integrated control of spinal cord function.

Age-dependent homeostatic plasticity of GABAergic signaling in developing retinal networks

Developing retinal ganglion cells fire in correlated spontaneous bursts, resulting in propagating waves with robust spatiotemporal features preserved across development and species. Here we investigate the effects of homeostatic adaptation on the circuits controlling retinal waves. Mouse retinal waves were recorded in vitro for up to 35 h with a multielectrode array in presence of the GABA(A) antagonist bicuculline, allowing us to obtain a precise, time-resolved characterization of homeostatic effects in this preparation. Experiments were performed at P4-P6, when GABA(A) signaling is depolarizing in ganglion cells, and at P7-P10, when GABA(A) signaling is hyperpolarizing. At all ages, bicuculline initially increased the wave sizes and other activity metrics. At P5-P6, wave sizes decreased toward control levels within a few hours while firing remained strong, but this ability to compensate disappeared entirely from P7 onwards. This demonstrates that homeostatic control of spontaneous retinal activity maintains specific network dynamic properties in an age-dependent manner, and suggests that the underlying mechanism is linked to GABA(A) signaling.

Mechanisms of GABAergic homeostatic plasticity

Homeostatic plasticity ensures that appropriate levels of activity are maintained through compensatory adjustments in synaptic strength and cellular excitability. For instance, excitatory glutamatergic synapses are strengthened following activity blockade and weakened following increases in spiking activity. This form of plasticity has been described in a wide array of networks at several different stages of development, but most work and reviews have focussed on the excitatory inputs of excitatory neurons. Here we review homeostatic plasticity of GABAergic neurons and their synaptic connections. We propose a simplistic model for homeostatic plasticity of GABAergic components of the circuitry (GABAergic synapses onto excitatory neurons, excitatory connections onto GABAergic neurons, cellular excitability of GABAergic neurons): following chronic activity blockade there is a weakening of GABAergic inhibition, and following chronic increases in network activity there is a strengthening of GABAergic inhibition. Previous work on GABAergic homeostatic plasticity supports certain aspects of the model, but it is clear that the model cannot fully account for some results which do not appear to fit any simplistic rule. We consider potential reasons for these discrepancies.

The magnitudes of hyperpolarization-activated and low-voltage-activated potassium currents co-vary in neurons of the ventral cochlear nucleus

In the ventral cochlear nucleus (VCN), neurons have hyperpolarization-activated conductances, which in some cells are enormous, that contribute to the ability of neurons to convey acoustic information in the timing of their firing by decreasing the input resistance and speeding-up voltage changes. Comparisons of the electrophysiological properties of neurons in the VCN of mutant mice that lack the hyperpolarization-activated cyclic nucleotide-gated channel α subunit 1 (HCN1(-/-)) (Nolan et al. 2003) with wild-type controls (HCN1(+/+)) and with outbred ICR mice reveal that octopus, T stellate, and bushy cells maintain their electrophysiological distinctions in all strains. Hyperpolarization-activated (I(h)) currents were smaller and slower, input resistances were higher, and membrane time constants were longer in HCN1(-/-) than in HCN1(+/+) in octopus, bushy, and T stellate cells. There were significant differences in the average magnitudes of I(h), input resistances, and time constants between HCN1(+/+) and ICR mice, but the resting potentials did not differ between strains. I(h) is opposed by a low-voltage-activated potassium (I(KL)) current in bushy and octopus cells, whose magnitudes varied widely between neuronal types and between strains. The magnitudes of I(h) and I(KL) were correlated across neuronal types and across mouse strains. Furthermore, these currents balanced one another at the resting potential in individual cells. The magnitude of I(h) and I(KL) is linked in bushy and octopus cells and varies not only between HCN1(-/-) and HCN1(+/+) but also between "wild-type" strains of mice, raising the question to what extent the wild-type strains reflect normal mice.

Non-cell-autonomous factor induces the transition from excitatory to inhibitory GABA signaling in retina independent of activity

During development, the effect of activating GABA(A) receptors switches from depolarizing to hyperpolarizing. Several environmental factors have been implicated in the timing of this GABA switch, including neural activity, although these observations remain controversial. By using acutely isolated retinas from KO mice and pharmacological manipulations in retinal explants, we demonstrate that the timing of the GABA switch in retinal ganglion cells (RGCs) is unaffected by blockade of specific neurotransmitter receptors or global activity. In contrast to RGCs in the intact retina, purified RGCs remain depolarized by GABA, indicating that the GABA switch is not cell-autonomous. Indeed, purified RGCs cocultured with dissociated cells from the superior colliculus or cultured in media conditioned by superior collicular cells undergo a normal switch. Thus, a diffusible signal that acts independent of local circuit activity regulates the maturation of GABAergic inhibition in mouse RGCs.

GABAergic synaptic scaling in embryonic motoneurons is mediated by a shift in the chloride reversal potential

Homeostatic synaptic plasticity ensures that networks maintain specific levels of activity by regulating synaptic strength in a compensatory manner. When spontaneous network activity was blocked in vivo in the embryonic spinal cord, compensatory increases in excitatory GABAergic synaptic inputs were observed. This homeostatic synaptic strengthening was observed as an increase in the amplitude of GABAergic miniature postsynaptic currents. We find that this process is mediated by an increase in chloride accumulation, which produces a depolarizing shift in the GABAergic reversal potential (E(GABA)). The findings demonstrate a previously unrecognized mechanism underlying homeostatic synaptic scaling. Similar shifts in E(GABA) have been described following various forms of neuronal injury, introducing the possibility that these shifts in E(GABA) represent a homeostatic response.

Compensation for variable intrinsic neuronal excitability by circuit-synaptic interactions

Recent theoretical and experimental work indicates that neurons tune themselves to maintain target levels of excitation by modulating ion channel expression and synaptic strengths. As a result, functionally equivalent circuits can produce similar activity despite disparate underlying network and cellular properties. To experimentally test the extent to which synaptic and intrinsic conductances can produce target activity in the presence of variability in neuronal intrinsic properties, we used the dynamic clamp to create hybrid two-cell circuits built from four types of stomatogastric neurons coupled to the same model Morris-Lecar neuron by reciprocal inhibition. We measured six intrinsic properties (input resistance, minimum membrane potential, firing rate in response to +1 nA of injected current, slope of the frequency-current curve, spike height, and spike voltage threshold) of dorsal gastric, gastric mill, lateral pyloric, and pyloric dilator neurons from male crabs of the species Cancer borealis. The intrinsic properties varied twofold to sevenfold in each cell type. We coupled each biological neuron to the Morris-Lecar model with seven different values of inhibitory synaptic conductance and also used the dynamic clamp to add seven different values of an artificial h-conductance, thus creating 49 different circuits for each biological neuron. Despite the variability in intrinsic excitability, networks formed from each neuron produced similar circuit performance at some values of synaptic and h-conductances. This work experimentally confirms results from previous modeling studies; tuning synaptic and intrinsic conductances can yield similar circuit outputs from neurons with variable intrinsic excitability.

Synaptic scaling and the development of a motor network

Neurons respond homeostatically to chronic changes in network activity with compensatory changes such as a uniform alteration in the size of miniature postsynaptic current (mPSC) amplitudes termed synaptic scaling. However, little is known about the impact of synaptic scaling on the function of neural networks in vivo. We used the embryonic zebrafish to address the effect of synaptic scaling on the neural network underlying locomotion. Activity was decreased during development by TTX injection to block action potentials or CNQX injection to block glutamatergic transmission. Alternatively TNFalpha was chronically applied. Recordings from spinal neurons showed that glutamatergic mPSCs scaled up approximately 25% after activity reduction and fortuitously scaled down approximately 20% after TNFalpha treatment, and were unchanged following blockade of neuromuscular activity alone with alpha-bungarotoxin. Regardless of the direction of scaling, immediately following reversal of treatment no chronic effect was distinguishable in motoneuron activity patterns or in swimming behavior. We also acutely induced a similar increase of glutamatergic mPSC amplitudes using cyclothiazide to reduce AMPA receptor desensitization or decrease of glutamatergic mPSC amplitudes using a low concentration of CNQX to partially block AMPA receptors. Though the strength of the motor output was altered, neither chronic nor acute treatments disrupted the patterning of synaptic activity or swimming. Our results show, for the first time, that scaling of glutamatergic synapses can be induced in vivo in the zebrafish and that synaptic patterning is less plastic than synaptic strength during development.

Homeostatic Plasticity and STDP: Keeping a Neuron's Cool in a Fluctuating World

Spike-timing-dependent plasticity (STDP) offers a powerful means of forming and modifying neural circuits. Experimental and theoretical studies have demonstrated its potential usefulness for functions as varied as cortical map development, sharpening of sensory receptive fields, working memory, and associative learning. Even so, it is unlikely that STDP works alone. Unless changes in synaptic strength are coordinated across multiple synapses and with other neuronal properties, it is difficult to maintain the stability and functionality of neural circuits. Moreover, there are certain features of early postnatal development (e.g., rapid changes in sensory input) that threaten neural circuit stability in ways that STDP may not be well placed to counter. These considerations have led researchers to investigate additional types of plasticity, complementary to STDP, that may serve to constrain synaptic weights and/or neuronal firing. These are collectively known as “homeostatic plasticity” and include schemes that control the total synaptic strength of a neuron, that modulate its intrinsic excitability as a function of average activity, or that make the ability of synapses to undergo Hebbian modification depend upon their history of use. In this article, we will review the experimental evidence for homeostatic forms of plasticity and consider how they might interact with STDP during development, and learning and memory.

Pharmacological manipulation of GABA-driven activity in ovo disrupts the development of dendritic morphology but not the maturation of spinal cord network activity

Background

In the adult nervous system, GABA acts as a major inhibitory neurotransmitter; however, at early stages of neurodevelopment, GABA receptor activation leads to membrane depolarization and accumulation of [Ca²⁺]_i. The role of excitatory GABAergic neurotransmission in the development of the nervous system is not fully understood. In this study, we investigated the role of excitatory GABA-driven activity in regulating the dendritic morphology and network function in the developing chicken spinal cord.

Results

Both bicuculline, a GABA receptor antagonist, and muscimol, a GABA agonist, inhibit the generation of spontaneous network activity in the isolated spinal cord at E8 or E10, indicating that altering GABA receptor activation disrupts the generation of spontaneous network activity in the chicken spinal cord. Treatment of chicken embryos with bicuculline or muscimol between E5 and E8 (or between E8 and E10), inhibits the dendritic outgrowth of motoneurons when compared to vehicle-treated embryos. The inhibitory effect of bicuculline or muscimol on the dendritic morphology of motoneurons was likely due to inhibition of GABA-driven network activity since a similar effect was also observed following reduction of network activity by Kir2.1 overexpression in the spinal cord. The inhibitory effect of bicuculline or muscimol was not caused by an adverse effect on cell survival. Surprisingly, chronic treatment of chicken embryos with bicuculline or muscimol has no effect on the shape and duration of the episodes of spontaneous activity, suggesting that maturation of network activity is not altered by disruption of the dendritic outgrowth of motoneurons.

Conclusions

Taken together, these findings indicate that excitatory GABA receptor activation regulates the maturation of dendritic morphology in the developing spinal cord by an activity-dependent mechanism. However, inhibition of dendritic outgrowth caused by disruption of GABA-driven activity does not alter the maturation of spontaneous electrical activity generated by spinal cord networks, suggesting that compensatory mechanisms can reverse any adverse effect of dendritic morphology on network function.

Learning to see: patterned visual activity and the development of visual function

To successfully interact with their environments, developing organisms need to correctly process sensory information and generate motor outputs appropriate to their size and structure. Patterned sensory experience has long been known to induce various forms of developmental plasticity that ultimately shape mature neural circuits. These same types of plasticity also allow developing organisms to respond appropriately to the external world by dynamically adapting neural circuit function to ongoing changes in brain circuitry and sensory input. Recent work on the visual systems of frogs and fish has provided an unprecedented view into how visual experience dynamically affects circuit function at many levels, ranging from gene expression to network function, ultimately leading to system-wide functional adaptations.

Characterization of rhythmic Ca2+ transients in early embryonic chick motoneurons: Ca2+ sources and effects of altered activation of transmitter receptors

In the nervous system, spontaneous Ca(2+) transients play important roles in many developmental processes. We previously found that altering the frequency of electrically recorded rhythmic spontaneous bursting episodes in embryonic chick spinal cords differentially perturbed the two main pathfinding decisions made by motoneurons, dorsal-ventral and pool-specific, depending on the sign of the frequency alteration. Here, we characterized the Ca(2+) transients associated with these bursts and showed that at early stages while motoneurons are still migrating and extending axons to the base of the limb bud, they display spontaneous, highly rhythmic, and synchronized Ca(2+) transients. Some precursor cells in the ependymal layer displayed similar transients. T-type Ca(2+) channels and a persistent Na(+) current were essential to initiate spontaneous bursts and associated transients. However, subsequent propagation of activity throughout the cord resulted from network-driven chemical transmission mediated presynaptically by Ca(2+) entry through N-type Ca(2+) channels and postsynaptically by acetylcholine acting on nicotinic receptors. The increased [Ca(2+)](i) during transients depended primarily on L-type and T-type channels with a modest contribution from TRP (transient receptor potential) channels and ryanodine-sensitive internal stores. Significantly, the drugs used previously to produce pathfinding errors altered transient frequency but not duration or amplitude. These observations imply that different transient frequencies may differentially modulate motoneuron pathfinding. However, the duration of the Ca(2+) transients differed significantly between pools, potentially enabling additional distinct pool-specific downstream signaling. Many early events in spinal motor circuit formation are thus potentially sensitive to the rhythmic Ca(2+) transients we have characterized and to any drugs that perturb them.

Mechanisms underlying spontaneous patterned activity in developing neural circuits

Patterned, spontaneous activity occurs in many developing neural circuits, including the retina, the cochlea, the spinal cord, the cerebellum and the hippocampus, where it provides signals that are important for the development of neurons and their connections. Despite there being differences in adult architecture and output across these various circuits, the patterns of spontaneous network activity and the mechanisms that generate it are remarkably similar. The mechanisms can include a depolarizing action of GABA (gamma-aminobutyric acid), transient synaptic connections, extrasynaptic transmission, gap junction coupling and the presence of pacemaker-like neurons. Interestingly, spontaneous activity is robust; if one element of a circuit is disrupted another will generate similar activity. This research suggests that developing neural circuits exhibit transient and tunable features that maintain a source of correlated activity during crucial stages of development.