Opposite changes in synaptic activity of organotypic rat spinal cord cultures after chronic block of AMPA/kainate or glycine and GABAA receptors

Homeostatic regulation of neuronal function: importance of degeneracy and pleiotropy

Neurons maintain their average firing rate and other properties within narrow bounds despite changing conditions. This homeostatic regulation is achieved using negative feedback to adjust ion channel expression levels. To understand how homeostatic regulation of excitability normally works and how it goes awry, one must consider the various ion channels involved as well as the other regulated properties impacted by adjusting those channels when regulating excitability. This raises issues of degeneracy and pleiotropy. Degeneracy refers to disparate solutions conveying equivalent function (e.g., different channel combinations yielding equivalent excitability). This many-to-one mapping contrasts the one-to-many mapping described by pleiotropy (e.g., one channel affecting multiple properties). Degeneracy facilitates homeostatic regulation by enabling a disturbance to be offset by compensatory changes in any one of several different channels or combinations thereof. Pleiotropy complicates homeostatic regulation because compensatory changes intended to regulate one property may inadvertently disrupt other properties. Co-regulating multiple properties by adjusting pleiotropic channels requires greater degeneracy than regulating one property in isolation and, by extension, can fail for additional reasons such as solutions for each property being incompatible with one another. Problems also arise if a perturbation is too strong and/or negative feedback is too weak, or because the set point is disturbed. Delineating feedback loops and their interactions provides valuable insight into how homeostatic regulation might fail. Insofar as different failure modes require distinct interventions to restore homeostasis, deeper understanding of homeostatic regulation and its pathological disruption may reveal more effective treatments for chronic neurological disorders like neuropathic pain and epilepsy.

Graphene Oxide Nanosheets Reduce Astrocyte Reactivity to Inflammation and Ameliorate Experimental Autoimmune Encephalomyelitis

In neuroinflammation, astrocytes play multifaceted roles that regulate the neuronal environment. Astrocytes sense and respond to pro-inflammatory cytokines (CKs) and, by a repertoire of intracellular Ca2+ signaling, contribute to disease progression. Therapeutic approaches wish to reduce the overactivation in Ca2+ signaling in inflammatory-reactive astrocytes to restore dysregulated cellular changes. Cell-targeting therapeutics might take advantage by the use of nanomaterial-multifunctional platforms such as graphene oxide (GO). GO biomedical applications in the nervous system involve therapeutic delivery and sensing, and GO flakes were shown to enable interfacing of neuronal and glial membrane dynamics. We exploit organotypic spinal cord cultures and optical imaging to explore Ca2+ changes in astrocytes, and we report, when spinal tissue is exposed to CKs, neuroinflammatory-associated modulation of resident glia. We show the efficacy of GO to revert these dynamic changes in astrocytic reactivity to CKs, and we translate this potential in an animal model of immune-mediated neuroinflammatory disease.

Hybrid Interfaces Made of Nanotubes and Backbone-Altered Dipeptides Tune Neuronal Network Architecture

Peptides constituted of backbone homologated α-amino acids combined with carbon materials offer interesting possibilities in the modulation of cellular functions. In this work, we have prepared diphenylalanine β- and γ-peptides and conjugated them to carbon nanotubes (CNTs). These hybrids were able to self-assemble into fibrillar dendritic structures enabling the growth of primary hippocampal cells and the modulation of their neuronal functions. In particular, following the deposition of the different nanomaterials on glass substrates, we have evaluated their effects on circuit function and geometry. The geometrical restrictions due to CNT nucleated nodes allowed growth of neuronal networks with control over network geometry, and exploring its functional impact. In diverse applications from basic neuroscience, the presence of CNT nodes may be exploited in brain interfaces able to convey highly localized electrical stimuli.

Chemically Cross-Linked Carbon Nanotube Films Engineered to Control Neuronal Signaling

In recent years, the use of free-standing carbon nanotube (CNT) films for neural tissue engineering has attracted tremendous attention. CNT films show large surface area and high electrical conductivity that combined with flexibility and biocompatibility may promote neuron growth and differentiation while stimulating neural activity. In addition, adhesion, survival, and growth of neurons can be modulated through chemical modification of CNTs. Axonal and synaptic signaling can also be positively tuned by these materials. Here we describe the ability of free-standing CNT films to influence neuronal activity. We demonstrate that the degree of cross-linking between the CNTs has a strong impact on the electrical conductivity of the substrate, which, in turn, regulates neural circuit outputs.

Cytokine inflammatory threat, but not LPS one, shortens GABAergic synaptic currents in the mouse spinal cord organotypic cultures

Background

Synaptic dysfunction, named synaptopathy, due to inflammatory status of the central nervous system (CNS) is a recognized factor potentially underlying both motor and cognitive dysfunctions in neurodegenerative diseases. To gain knowledge on the mechanistic interplay between local inflammation and synapse changes, we compared two diverse inflammatory paradigms, a cytokine cocktail (CKs; IL-1β, TNF-α, and GM-CSF) and LPS, and their ability to tune GABAergic current duration in spinal cord cultured circuits.

Methods

We exploit spinal organotypic cultures, single-cell electrophysiology, immunocytochemistry, and confocal microscopy to explore synaptic currents and resident neuroglia reactivity upon CK or LPS incubation.

Results

Local inflammation in slice cultures induced by CK or LPS stimulations boosts network activity; however, only CKs specifically reduced GABAergic current duration. We pharmacologically investigated the contribution of GABA_AR α-subunits and suggested that a switch of GABA_AR α1-subunit might have induced faster GABA_AR decay time, weakening the inhibitory transmission.

Conclusions

Lower GABAergic current duration could contribute to providing an aberrant excitatory transmission critical for pre-motor circuit tasks and represent a specific feature of a CK cocktail able to mimic an inflammatory reaction that spreads in the CNS. Our results describe a selective mechanism that could be triggered during specific inflammatory stress.

Electronic supplementary material

The online version of this article (10.1186/s12974-019-1519-z) contains supplementary material, which is available to authorized users.

Graphene Oxide Flakes Tune Excitatory Neurotransmission in Vivo by Targeting Hippocampal Synapses

Synapses compute and transmit information to connect neural circuits and are at the basis of brain operations. Alterations in their function contribute to a vast range of neuropsychiatric and neurodegenerative disorders and synapse-based therapeutic intervention, such as selective inhibition of synaptic transmission, may significantly help against serious pathologies. Graphene is a two-dimensional nanomaterial largely exploited in multiple domains of science and technology, including biomedical applications. In hippocampal neurons in culture, small graphene oxide nanosheets (s-GO) selectively depress glutamatergic activity without altering cell viability. Glutamate is the main excitatory neurotransmitter in the central nervous system and growing evidence suggests its involvement in neuropsychiatric disorders. Here we demonstrate that s-GO directly targets the release of presynaptic vesicle. We propose that s-GO flakes reduce the availability of transmitter, via promoting its fast release and subsequent depletion, leading to a decline ofglutamatergic neurotransmission. We injected s-GO in the hippocampus in vivo, and 48 h after surgery ex vivo patch-clamp recordings from brain slices show a significant reduction in glutamatergic synaptic activity in respect to saline injections.

3D Organotypic Spinal Cultures: Exploring Neuron and Neuroglia Responses Upon Prolonged Exposure to Graphene Oxide

Graphene-based nanomaterials are increasingly engineered as components of biosensors, interfaces or drug delivery platforms in neuro-repair strategies. In these developments, the mostly used derivative of graphene is graphene oxide (GO). To tailor the safe development of GO nanosheets, we need to model in vitro tissue responses, and in particular the reactivity of microglia, a sub-population of neuroglia that acts as the first active immune response, when challenged by GO. Here, we investigated central nervous system (CNS) tissue reactivity upon long-term exposure to GO nanosheets in 3D culture models. We used the mouse organotypic spinal cord cultures, ideally suited for studying long-term interference with cues delivered at controlled times and concentrations. In cultured spinal segments, the normal presence, distribution and maturation of anatomically distinct classes of neurons and resident neuroglial cells are preserved. Organotypic explants were developed for 2 weeks embedded in fibrin glue alone or presenting GO nanosheets at 10, 25 and 50 μg/mL. We addressed the impact of such treatments on premotor synaptic activity monitored by patch clamp recordings of ventral interneurons. We investigated by immunofluorescence and confocal microscopy the accompanying glial responses to GO exposure, focusing on resident microglia, tested in organotypic spinal slices and in isolated neuroglia cultures. Our results suggest that microglia reactivity to accumulation of GO flakes, maybe due to active phagocytosis, may trim down synaptic activity, although in the absence of an effective activation of inflammatory response and in the absence of neuronal cell death.

Loss of inhibitory synapses causes locomotor network dysfunction of the rat spinal cord during prolonged maintenance in vitro

The isolated spinal cord of the neonatal rat is widely employed to clarify the basic mechanisms of network development or the early phase of degeneration after injury. Nevertheless, this preparation survives in Krebs solution up to 24 h only, making it desirable to explore approaches to extend its survival for longitudinal studies. The present report shows that culturing the spinal cord in oxygenated enriched Basal Medium Eagle (BME) provided excellent preservation of neurons (including motoneurons), glia and primary afferents (including dorsal root ganglia) for up to 72 h. Using DMEM medium was unsuccessful. Novel characteristics of spinal networks emerged with strong spontaneous activity, and deficit in fictive locomotion patterns with stereotypically slow cycles. Staining with markers for synaptic proteins synapsin 1 and synaptophysin showed thoroughly weaker signal after 3 days in vitro. Immunohistochemical staining of markers for glutamatergic and glycinergic neurons indicated significant reduction of the latter. Likewise, there was lower expression of the GABA-synthesizing enzyme GAD65. Thus, malfunction of locomotor networks appeared related to loss of inhibitory synapses. This phenomenon did not occur in analogous opossum preparations of the spinal cord kept in vitro. In conclusion, despite histological data suggesting that cultured spinal cords were undamaged (except for inhibitory biomarkers), electrophysiological data revealed important functional impairment. Thus, the downregulation of inhibitory synapses may account for the progressive hyperexcitability of rat spinal networks despite apparently normal histological appearance. Our observations may help to understand the basis of certain delayed effects of spinal injury like chronic pain and spasticity.

Afferent Input Induced by Rhythmic Limb Movement Modulates Spinal Neuronal Circuits in an Innovative Robotic In Vitro Preparation

Locomotor patterns are mainly modulated by afferent feedback, but its actual contribution to spinal network activity during continuous passive limb training is still unexplored. To unveil this issue, we devised a robotic in vitro setup (Bipedal Induced Kinetic Exercise, BIKE) to induce passive pedaling, while simultaneously recording low-noise ventral and dorsal root (VR and DR) potentials in isolated neonatal rat spinal cords with hindlimbs attached. As a result, BIKE evoked rhythmic afferent volleys from DRs, reminiscent of pedaling speed. During BIKE, spontaneous VR activity remained unchanged, while a DR rhythmic component paired the pedaling pace. Moreover, BIKE onset rarely elicited brief episodes of fictive locomotion (FL) and, when trains of electrical pulses were simultaneously applied to a DR, it increased the amplitude, but not the number, of FL cycles. When BIKE was switched off after a 30-min training, the number of electrically induced FL oscillations was transitorily facilitated, without affecting VR reflexes or DR potentials. However, 90 min of BIKE no longer facilitated FL, but strongly depressed area of VR reflexes and stably increased antidromic DR discharges. Patch clamp recordings from single motoneurons after 90-min sessions indicated an increased frequency of both fast- and slow-decaying synaptic input to motoneurons. In conclusion, hindlimb rhythmic and alternated pedaling for different durations affects distinct dorsal and ventral spinal networks by modulating excitatory and inhibitory input to motoneurons. These results suggest defining new parameters for effective neurorehabilitation that better exploits spinal circuit activity.

A novel method of organotypic spinal cord slice culture in rats

This study was undertaken to establish a method for the culture of organotypic spinal cord slices. A long-term organotypic spinal cord slice culture was conducted from postnatal rats. Lumbar spinal cord was isolated, and meninges were removed from the spinal cord. The spinal cord was embedded in 4% agarose, and was sectioned by vibratome into slices. Then the slices were cultured on the surface of the membrane inserts, which were placed in six-well plates containing 1 ml of growth medium at 37°C in an incubator with 5% humidified carbon dioxide. The cultured organotypic spinal cord slices were examined by light microscopy and immunocytochemistry. The organotypic spinal cord slices were fully attached to the membrane inserts after 10 days in vitro. The general change in color and transparency from whitish to transparent gray appeared at the seventh and eighth day. Under the light microscope, the outgrowth of cells from the edge of the living slices arose from the second day of the culture, and arose to peak at the sixth and seventh day. The organotypic spinal cord slices were characterized as clear, semitransparent structures with bright and good refraction until the 14th day of culture. The viability of the slices was excellent as assessed by the trypan blue exclusion method at the 28th day, and they were positive for NeuN and GFAP. This culture technique, which does not require complex operation skills, might be a simple and efficient method for obtaining organotypic spinal cord slices in sufficient number, high viability, and contamination-free from postnatal rats.

Bridging pro-inflammatory signals, synaptic transmission and protection in spinal explants in vitro

Multiple sclerosis is characterized by tissue atrophy involving the brain and the spinal cord, where reactive inflammation contributes to the neurodegenerative processes. Recently, the presence of synapse alterations induced by the inflammatory responses was suggested by experimental and clinical observations, in experimental autoimmune encephalomyelitis mouse model and in patients, respectively. Further knowledge on the interplay between pro-inflammatory agents, neuroglia and synaptic dysfunction is crucial to the design of unconventional protective molecules. Here we report the effects, on spinal cord circuits, of a cytokine cocktail that partly mimics the signature of T lymphocytes sub population Th1. In embryonic mouse spinal organ-cultures, containing neuronal cells and neuroglia, cytokines induced inflammatory responses accompanied by a significant increase in spontaneous synaptic activity. We suggest that cytokines specifically altered signal integration in spinal networks by speeding the decay of GABA_A responses. This hypothesis is supported by the finding that synapse protection by a non-peptidic NGF mimetic molecule prevented both the changes in the time course of GABA events and in network activity that were left unchanged by the cytokine production from astrocytes and microglia present in the cultured tissue. In conclusion, we developed an important tool for the study of synaptic alterations induced by inflammation, that takes into account the role of neuronal and not neuronal resident cells.

Characterization of Superficial Dorsal Horn Neurons from "Tamamaki" Mice and Stability of their GAD67-EGFP Phenotype in Defined-Medium Organotypic Culture

Defined medium organotypic cultures (DMOTC) containing spinal dorsal horn neurons are especially useful in studying the etiology and pharmacology of chronic pain. We made whole-cell recordings from neurons in acutely isolated mouse spinal cord slices or from those maintained in DMOTC for up to 6 weeks. In acute slices, neurons in the substantia gelatinosa exhibited 7 different firing patterns in response to 800-ms depolarizing current commands; delay (irregular), delay (tonic), tonic, regular firing, phasic, initial bursting and single spiking. Initial bursting and regular firing neurons are not found in rat substantia gelatinosa. In acute slices from "Tamamaki" mice that express enhanced green fluorescent protein (EGFP) under the control of the glutamic acid decarboxylase 67 (GAD67) promotor, tonic, phasic and regular firing neurons exhibited the strongest GABAergic (GAD67-EGFP+) phenotype. Delay (tonic) and delay (irregular) neurons almost never expressed GAD67 (GAD67-EGFP-) and are likely glutamatergic. All seven phenotypes were preserved in mouse spinal cord neurons in DMOTC prepared from e12 embryos and the GAD67-EGFP+ phenotype continued to associate with phasic and regular firing neurons. Only 3 out of 51 GAD67-EGFP+ neurons exhibited a delay (tonic) firing pattern. Modifications to the mouse genome thus continue to be expressed when embryonic neurons develop in vitro in DMOTC. However, analysis of the amplitude and interevent interval of spontaneous EPSCs (sEPSCs) indicated substantial re-arrangement of synaptic connections within the cultures. Despite this, the characteristics and age-dependence of asynchronous oscillatory activity, as monitored by multiphoton Ca²⁺ imaging, were similar in acute slices and in DMOTC.

In Vitro Innervation as an Experimental Model to Study the Expression and Functions of Acetylcholinesterase and Agrin in Human Skeletal Muscle

Acetylcholinesterase (AChE) and agrin, a heparan-sulfate proteoglycan, reside in the basal lamina of the neuromuscular junction (NMJ) and play key roles in cholinergic transmission and synaptogenesis. Unlike most NMJ components, AChE and agrin are expressed in skeletal muscle and α-motor neurons. AChE and agrin are also expressed in various other types of cells, where they have important alternative functions that are not related to their classical roles in NMJ. In this review, we first focus on co-cultures of embryonic rat spinal cord explants with human skeletal muscle cells as an experimental model to study functional innervation in vitro. We describe how this heterologous rat-human model, which enables experimentation on highly developed contracting human myotubes, offers unique opportunities for AChE and agrin research. We then highlight innovative approaches that were used to address salient questions regarding expression and alternative functions of AChE and agrin in developing human skeletal muscle. Results obtained in co-cultures are compared with those obtained in other models in the context of general advances in the field of AChE and agrin neurobiology.

Formation and Maintenance of Functional Spines in the Absence of Presynaptic Glutamate Release

Dendritic spines are the major transmitter reception compartments of glutamatergic synapses in most principal neurons of the mammalian brain and play a key role in the function of nerve cell circuits. The formation of functional spine synapses is thought to be critically dependent on presynaptic glutamatergic signaling. By analyzing CA1 pyramidal neurons in mutant hippocampal slice cultures that are essentially devoid of presynaptic transmitter release, we demonstrate that the formation and maintenance of dendrites and functional spines are independent of synaptic glutamate release.

3D meshes of carbon nanotubes guide functional reconnection of segregated spinal explants

In modern neuroscience, significant progress in developing structural scaffolds integrated with the brain is provided by the increasing use of nanomaterials. We show that a multiwalled carbon nanotube self-standing framework, consisting of a three-dimensional (3D) mesh of interconnected, conductive, pure carbon nanotubes, can guide the formation of neural webs in vitro where the spontaneous regrowth of neurite bundles is molded into a dense random net. This morphology of the fiber regrowth shaped by the 3D structure supports the successful reconnection of segregated spinal cord segments. We further observed in vivo the adaptability of these 3D devices in a healthy physiological environment. Our study shows that 3D artificial scaffolds may drive local rewiring in vitro and hold great potential for the development of future in vivo interfaces.

Altered development in GABA co-release shapes glycinergic synaptic currents in cultured spinal slices of the SOD1(G93A) mouse model of amyotrophic lateral sclerosis

KEY POINTS

Increased environmental risk factors in conjunction with genetic susceptibility have been proposed with respect to the remarkable variations in mortality in amyotrophic lateral sclerosis (ALS). In vitro models allow the investigation of the genetically modified counter-regulator of motoneuron toxicity and may help in addressing ALS therapy. Spinal organotypic slice cultures from a mutant form of human superoxide dismutase 1 (SOD1G93A) mouse model of ALS allow the detection of altered glycinergic inhibition in spinal microcircuits. This altered inhibition improved spinal cord excitability, affecting motor outputs in early SOD1(G93A) pathogenesis.

ABSTRACT

Amyotrophic lateral sclerosis (ALS) is a fatal, adult-onset neurological disease characterized by a progressive degeneration of motoneurons (MNs). In a previous study, we developed organotypic spinal cultures from an ALS mouse model expressing a mutant form of human superoxide dismutase 1 (SOD1(G93A) ). We reported the presence of a significant synaptic rearrangement expressed by these embryonic cultured networks, which may lead to the altered development of spinal synaptic signalling, which is potentially linked to the adult disease phenotype. Recent studies on the same ALS mouse model reported a selective loss of glycinergic innervation in cultured MNs, suggestive of a contribution of synaptic inhibition to MN dysfunction and degeneration. In the present study, we further exploit organotypic cultures from wild-type and SOD1(G93A) mice to investigate the development of glycine-receptor-mediated synaptic currents recorded from the interneurons of the premotor ventral circuits. We performed single cell electrophysiology, immunocytochemistry and confocal microscopy and suggest that GABA co-release may speed the decay of glycine responses altering both temporal precision and signal integration in SOD1(G93A) developing networks at the postsynaptic site. Our hypothesis is supported by the finding of an increased MN bursting activity in immature SOD1(G93A) spinal cords and by immunofluorescence microscopy detection of a longer persistence of GABA in SOD1(G93A) glycinergic terminals in cultured and ex vivo spinal slices.

Delayed application of the anesthetic propofol contrasts the neurotoxic effects of kainate on rat organotypic spinal slice cultures

Excitotoxicity due to hyperactivation of glutamate receptors is thought to underlie acute spinal injury with subsequent strong deficit in spinal network function. Devising an efficacious protocol of neuroprotection to arrest excitotoxicity might, therefore, spare a substantial number of neurons and allow later recovery. In vitro preparations of the spinal cord enable detailed measurement of spinal damage evoked by the potent glutamate analogue kainate. Any clinically-relevant neuroprotective treatment should start after the initial lesion and spare networks for at least 24h when cell damage plateaus. Using this strategy, we have observed that the gas anesthetic methoxyflurane provided strong, delayed neuroprotection. It is unclear if this beneficial effect was due to the mechanism of action by methoxyflurane, or it was the consequence of anesthetic depression. To test this hypothesis, we investigated the effect by propofol (commonly injected i.v. for general anesthesia) after kainate excitotoxicity induced on organotypic spinal slices. At 5μM concentration, propofol significantly attenuated cell death, including neuronal losses and, especially, damage to the highly vulnerable motoneurons. The action by propofol was fully prevented when co-applied with the GABAA antagonist bicuculline, indicating that neuroprotection required intact GABAA receptor function. Although bicuculline per se was not neurotoxic, it largely enhanced the lesional effects of kainate, suggesting that GABAA receptor activity could limit excitotoxicity. Our data might offer an explanation for the beneficial clinical outcome of neurosurgery performed as soon as possible after spinal lesion: we posit that general anesthesia contributes to this outcome, regardless of the type of anesthetic used.

Hyperpolarization-independent maturation and refinement of GABA/glycinergic connections in the auditory brain stem

During development GABA and glycine synapses are initially excitatory before they gradually become inhibitory. This transition is due to a developmental increase in the activity of neuronal potassium-chloride cotransporter 2 (KCC2), which shifts the chloride equilibrium potential (ECl) to values more negative than the resting membrane potential. While the role of early GABA and glycine depolarizations in neuronal development has become increasingly clear, the role of the transition to hyperpolarization in synapse maturation and circuit refinement has remained an open question. Here we investigated this question by examining the maturation and developmental refinement of GABA/glycinergic and glutamatergic synapses in the lateral superior olive (LSO), a binaural auditory brain stem nucleus, in KCC2-knockdown mice, in which GABA and glycine remain depolarizing. We found that many key events in the development of synaptic inputs to the LSO, such as changes in neurotransmitter phenotype, strengthening and elimination of GABA/glycinergic connection, and maturation of glutamatergic synapses, occur undisturbed in KCC2-knockdown mice compared with wild-type mice. These results indicate that maturation of inhibitory and excitatory synapses in the LSO is independent of the GABA and glycine depolarization-to-hyperpolarization transition.

mGlu1 receptor mediates homeostatic control of intrinsic excitability through Ih in cerebellar Purkinje cells

Homeostatic intrinsic plasticity is a cellular mechanism for maintaining a stable neuronal activity level in response to developmental or activity-dependent changes. Type 1 metabotropic glutamate receptor (mGlu1 receptor) has been widely known to monitor neuronal activity, which plays a role as a modulator of intrinsic and synaptic plasticity of neurons. Whether mGlu1 receptor contributes to the compensatory adjustment of Purkinje cells (PCs), the sole output of the cerebellar cortex, in response to chronic changes in excitability remains unclear. Here, we demonstrate that the mGlu1 receptor is involved in homeostatic intrinsic plasticity through the upregulation of the hyperpolarization-activated current (Ih) in cerebellar PCs. This plasticity was prevented by inhibiting the mGlu1 receptor with Bay 36-7620, an mGlu1 receptor inverse agonist, but not with CPCCOEt, a neutral antagonist. Chronic inactivation with tetrodotoxin (TTX) increased the components of Ih in the PCs, and ZD 7288, a hyperpolarization-activated cyclic nucleotide-gated channel selective inhibitor, fully restored reduction of firing rates in the deprived neurons. The homeostatic elevation of Ih was also prevented by BAY 36-7620, but not CPCCOEt. Furthermore, KT 5720, a blocker of protein kinase A (PKA), prevented the effect of TTX reducing the evoked firing rates, indicating the reduction in excitability of PCs due to PKA activation. Our study shows that both the mGlu1 receptor and the PKA pathway are involved in the homeostatic intrinsic plasticity of PCs after chronic blockade of the network activity, which provides a novel understanding on how cerebellar PCs can preserve the homeostatic state under activity-deprived conditions.

Palmitoylation of gephyrin controls receptor clustering and plasticity of GABAergic synapses

Gephyrin, the principal scaffolding protein at inhibitory synapses, needs to be palmitoylated in order to cluster and to assemble functional synapses.

Postsynaptic scaffolding proteins regulate coordinated neurotransmission by anchoring and clustering receptors and adhesion molecules. Gephyrin is the major instructive molecule at inhibitory synapses, where it clusters glycine as well as major subsets of GABA type A receptors (GABA_ARs). Here, we identified palmitoylation of gephyrin as an important mechanism of strengthening GABAergic synaptic transmission, which is regulated by GABA_AR activity. We mapped palmitoylation to Cys212 and Cys284, which are critical for both association of gephyrin with the postsynaptic membrane and gephyrin clustering. We identified DHHC-12 as the principal palmitoyl acyltransferase that palmitoylates gephyrin. Furthermore, gephyrin pamitoylation potentiated GABAergic synaptic transmission, as evidenced by an increased amplitude of miniature inhibitory postsynaptic currents. Consistently, inhibiting gephyrin palmitoylation either pharmacologically or by expression of palmitoylation-deficient gephyrin reduced the gephyrin cluster size. In aggregate, our study reveals that palmitoylation of gephyrin by DHHC-12 contributes to dynamic and functional modulation of GABAergic synapses.

Efficient signal transmission at synapses is essential for higher brain functions. Inhibitory signaling in the brain takes place primarily at GABA (γ-aminobutyric acid)-ergic synapses. GABA type A receptors (GABA_ARs) are clustered at the postsynaptic side by a scaffold composed of the peripheral membrane protein gephyrin. We demonstrate that gephyrin is modulated by palmitoylation, a reversible posttranslational fatty acid modification. Palmitoylation facilitates the membrane association of gephyrin and is therefore essential for normal clustering of gephyrin at GABAergic synapses. Reciprocally, palmitoylation of gephyrin is regulated by GABA_AR activity. Of the 23 known palmitoyl transferases that catalyze the palmitoylation of proteins in human cells, we identified one enzyme, DHHC-12, to specifically modify gephyrin. Our results provide a new aspect to the posttranslational control of synaptic plasticity.

Activity blockade and GABAA receptor blockade produce synaptic scaling through chloride accumulation in embryonic spinal motoneurons and interneurons

Synaptic scaling represents a process whereby the distribution of a cell's synaptic strengths are altered by a multiplicative scaling factor. Scaling is thought to be a compensatory response that homeostatically controls spiking activity levels in the cell or network. Previously, we observed GABAergic synaptic scaling in embryonic spinal motoneurons following in vivo blockade of either spiking activity or GABA_A receptors (GABA_ARs). We had determined that activity blockade triggered upward GABAergic scaling through chloride accumulation, thus increasing the driving force for these currents. To determine whether chloride accumulation also underlies GABAergic scaling following GABA_AR blockade we have developed a new technique. We expressed a genetically encoded chloride-indicator, Clomeleon, in the embryonic chick spinal cord, which provides a non-invasive fast measure of intracellular chloride. Using this technique we now show that chloride accumulation underlies GABAergic scaling following blockade of either spiking activity or the GABA_AR. The finding that GABA_AR blockade and activity blockade trigger scaling via a common mechanism supports our hypothesis that activity blockade reduces GABA_AR activation, which triggers synaptic scaling. In addition, Clomeleon imaging demonstrated the time course and widespread nature of GABAergic scaling through chloride accumulation, as it was also observed in spinal interneurons. This suggests that homeostatic scaling via chloride accumulation is a common feature in many neuronal classes within the embryonic spinal cord and opens the possibility that this process may occur throughout the nervous system at early stages of development.

The cell-autonomous role of excitatory synaptic transmission in the regulation of neuronal structure and function

The cell-autonomous role of synaptic transmission in the regulation of neuronal structural and electrical properties is unclear. We have now employed a genetic approach to eliminate glutamatergic synaptic transmission onto individual CA1 pyramidal neurons in a mosaic fashion in vivo. Surprisingly, while electrical properties are profoundly affected in these neurons, as well as inhibitory synaptic transmission, we found little perturbation of neuronal morphology, demonstrating a functional segregation of excitatory synaptic transmission from neuronal morphological development.

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'.

From neural plate to cortical arousal-a neuronal network theory of sleep derived from in vitro "model" systems for primordial patterns of spontaneous bioelectric activity in the vertebrate central nervous system

In the early 1960s intrinsically generated widespread neuronal discharges were discovered to be the basis for the earliest motor behavior throughout the animal kingdom. The pattern generating system is in fact programmed into the developing nervous system, in a regionally specific manner, already at the early neural plate stage. Such rhythmically modulated phasic bursts were next discovered to be a general feature of developing neural networks and, largely on the basis of experimental interventions in cultured neural tissues, to contribute significantly to their morpho-physiological maturation. In particular, the level of spontaneous synchronized bursting is homeostatically regulated, and has the effect of constraining the development of excessive network excitability. After birth or hatching, this "slow-wave" activity pattern becomes sporadically suppressed in favor of sensory oriented "waking" behaviors better adapted to dealing with environmental contingencies. It nevertheless reappears periodically as "sleep" at several species-specific points in the diurnal/nocturnal cycle. Although this "default" behavior pattern evolves with development, its essential features are preserved throughout the life cycle, and are based upon a few simple mechanisms which can be both experimentally demonstrated and simulated by computer modeling. In contrast, a late onto- and phylogenetic aspect of sleep, viz., the intermittent "paradoxical" activation of the forebrain so as to mimic waking activity, is much less well understood as regards its contribution to brain development. Some recent findings dealing with this question by means of cholinergically induced "aroused" firing patterns in developing neocortical cell cultures, followed by quantitative electrophysiological assays of immediate and longterm sequelae, will be discussed in connection with their putative implications for sleep ontogeny.

Carbon nanotubes: artificial nanomaterials to engineer single neurons and neuronal networks

In the past decade, nanotechnology applications to the nervous system have often involved the study and the use of novel nanomaterials to improve the diagnosis and therapy of neurological diseases. In the field of nanomedicine, carbon nanotubes are evaluated as promising materials for diverse therapeutic and diagnostic applications. Besides, carbon nanotubes are increasingly employed in basic neuroscience approaches, and they have been used in the design of neuronal interfaces or in that of scaffolds promoting neuronal growth in vitro. Ultimately, carbon nanotubes are thought to hold the potential for the development of innovative neurological implants. In this framework, it is particularly relevant to document the impact of interfacing such materials with nerve cells. Carbon nanotubes were shown, when modified with biologically active compounds or functionalized in order to alter their charge, to affect neurite outgrowth and branching. Notably, purified carbon nanotubes used as scaffolds can promote the formation of nanotube-neuron hybrid networks, able per se to affect neuron integrative abilities, network connectivity, and synaptic plasticity. We focus this review on our work over several years directed to investigate the ability of carbon nanotube platforms in providing a new tool for nongenetic manipulations of neuronal performance and network signaling.

Spinal cord explants use carbon nanotube interfaces to enhance neurite outgrowth and to fortify synaptic inputs

New developments in nanotechnology are increasingly designed to modulate relevant interactions between nanomaterials and neurons, with the aim of exploiting the physical properties of synthetic materials to tune desired and specific biological processes. Carbon nanotubes have been applied in several areas of nerve tissue engineering to study cell behavior or to instruct the growth and organization of neural networks. Recent reports show that nanotubes can sustain and promote electrical activity in networks of cultured neurons. However, such results are usually limited to carbon nanotube/neuron hybrids formed on a monolayer of dissociated brain cells. In the present work, we used organotypic spinal slices to model multilayer tissue complexity, and we interfaced such spinal segments to carbon nanotube scaffolds for weeks. By immunofluorescence, scanning and transmission electronic microscopy, and atomic force microscopy, we investigated nerve fiber growth when neuronal processes exit the spinal explant and develop in direct contact to the substrate. By single-cell electrophysiology, we investigated the synaptic activity of visually identified ventral interneurons, within the ventral area of the explant, thus synaptically connected, but located remotely, to the substrate/network interface. Here we show that spinal cord explants interfaced for weeks to purified carbon nanotube scaffolds expand more neuronal fibers, characterized by different mechanical properties and displaying higher growth cones activity. On the other hand, exploring spontaneous and evoked synaptic activity unmasks an increase in synaptic efficacy in neurons located at as far as 5 cell layers from the cell-substrate interactions.

Carbon nanotube scaffolds tune synaptic strength in cultured neural circuits: novel frontiers in nanomaterial-tissue interactions

A long-term goal of tissue engineering is to exploit the ability of supporting materials to govern cell-specific behaviors. Instructive scaffolds code such information by modulating (via their physical and chemical features) the interface between cells and materials at the nanoscale. In modern neuroscience, therapeutic regenerative strategies (i.e., brain repair after damage) aim to guide and enhance the intrinsic capacity of the brain to reorganize by promoting plasticity mechanisms in a controlled fashion. Direct and specific interactions between synthetic materials and biological cell membranes may play a central role in this process. Here, we investigate the role of the material's properties alone, in carbon nanotube scaffolds, in constructing the functional building blocks of neural circuits: the synapses. Using electrophysiological recordings and rat cultured neural networks, we describe the ability of a nanoscaled material to promote the formation of synaptic contacts and to modulate their plasticity.

Pre & postsynaptic tuning of action potential timing by spontaneous GABAergic activity

Frequency and timing of action potential discharge are key elements for coding and transfer of information between neurons. The nature and location of the synaptic contacts, the biophysical parameters of the receptor-operated channels and their kinetics of activation are major determinants of the firing behaviour of each individual neuron. Ultimately the intrinsic excitability of each neuron determines the input-output function. Here we evaluate the influence of spontaneous GABAergic synaptic activity on the timing of action potentials in Layer 2/3 pyramidal neurones in acute brain slices from the somatosensory cortex of young rats. Somatic dynamic current injection to mimic synaptic input events was employed, together with a simple computational model that reproduce subthreshold membrane properties. Besides the well-documented control of neuronal excitability, spontaneous background GABAergic activity has a major detrimental effect on spike timing. In fact, GABA(A) receptors tune the relationship between the excitability and fidelity of pyramidal neurons via a postsynaptic (the reversal potential for GABA(A) activity) and a presynaptic (the frequency of spontaneous activity) mechanism. GABAergic activity can decrease or increase the excitability of pyramidal neurones, depending on the difference between the reversal potential for GABA(A) receptors and the threshold for action potential. In contrast, spike time jitter can only be increased proportionally to the difference between these two membrane potentials. Changes in excitability by background GABAergic activity can therefore only be associated with deterioration of the reliability of spike timing.

Attenuated glycine receptor function reduces excitability of mouse medial vestibular nucleus neurons

Spontaneous activity in medial vestibular nucleus (MVN) neurons is modulated by synaptic inputs. These inputs are crucial for maintaining gaze and posture and contribute to vestibular compensation after lesions of peripheral vestibular organs. We investigated how chronically attenuated glycinergic input affects excitability of MVN neurons. To this end we used three mouse strains (spastic, spasmodic, and oscillator), with well-characterized naturally occurring mutations in the inhibitory glycine receptor (GlyR). First, using whole-cell patch-clamp recordings, we demonstrated that the amplitude of the response to rapidly applied glycine was dramatically reduced by 25 to 90% in MVN neurons from mutant mice. We next determined how reduced GlyR function affected MVN neuron output. Neurons were classified using two schemas: (1) the shape of their action potential afterhyperpolarization (AHP); and (2) responses to hyperpolarizing current injection. In the first schema, neurons were classified as types A, B and C. The prevalence of type C neurons in the mutant strains was significantly increased. In the second schema, the proportion of neurons lacking post inhibitory rebound firing (PRF-deficient) was increased. In both schemas an increase in AHP amplitude was a common feature of the augmented neuron group (type C, PRF-deficient) in the mutant strains. We suggest increased AHP amplitude reduces overall excitability in the MVN and thus maintains network function in an environment of reduced glycinergic input.

The patterns of spontaneous Ca2+ signals generated by ventral spinal neurons in vitro show time-dependent refinement

Embryonic spinal neurons maintained in organotypic slice culture are known to mimic certain maturation-dependent signalling changes. With such a model we investigated, in embryonic mouse spinal segments, the age-dependent spatio-temporal control of intracellular Ca(2+) signalling generated by neuronal populations in ventral circuits and its relation with electrical activity. We used Ca(2+) imaging to monitor areas located within the ventral spinal horn at 1 and 2 weeks of in vitro growth. Primitive patterns of spontaneous neuronal Ca(2+) transients (detected at 1 week) were typically synchronous. Remarkably, such transients originated from widespread propagating waves that became organized into large-scale rhythmic bursts. These activities were associated with the generation of synaptically mediated inward currents under whole-cell patch-clamp. Such patterns disappeared during longer culture of spinal segments: at 2 weeks in culture, only a subset of ventral neurons displayed spontaneous, asynchronous and repetitive Ca(2+) oscillations dissociated from background synaptic activity. We observed that the emergence of oscillations was a restricted phenomenon arising together with the transformation of ventral network electrophysiological bursting into asynchronous synaptic discharges. This change was accompanied by the appearance of discrete calbindin immunoreactivity against an unchanged background of calretinin-positive cells. It is attractive to assume that periodic oscillations of Ca(2+) confer a summative ability to these cells to shape the plasticity of local circuits through different changes (phasic or tonic) in intracellular Ca(2+).

GABAergic and glycinergic interneuron expression during spinal cord development: dynamic interplay between inhibition and excitation in the control of ventral network outputs

A key objective of neuroscience research is to understand the processes leading to mature neural circuitries in the central nervous system (CNS) that enable the control of different behaviours. During development, network-constitutive neurons undergo dramatic rearrangements, involving their intrinsic properties, such as the blend of ion channels governing their firing activity, and their synaptic interactions. The spinal cord is no exception to this rule; in fact, in the ventral horn the maturation of motor networks into functional circuits is a complex process where several mechanisms cooperate to achieve the development of motor control. Elucidating such a process is crucial in identifying neurons more vulnerable to degenerative or traumatic diseases or in developing new strategies aimed at rebuilding damaged tissue. The focus of this review is on recent advances in understanding the spatio-temporal expression of the glycinergic/GABAergic system and on the contribution of this system to early network function and to motor pattern transformation along with spinal maturation. During antenatal development, the operation of mammalian spinal networks strongly depends on the activity of glycinergic/GABAergic neurons, whose action is often excitatory until shortly before birth when locomotor networks acquire the ability to generate alternating motor commands between flexor and extensor motor neurons. At this late stage of prenatal development, GABA-mediated excitation is replaced by synaptic inhibition mediated by glycine and/or GABA. At this stage of spinal maturation, the large majority of GABAergic neurons are located in the dorsal horn. We propose that elucidating the role of inhibitory systems in development will improve our knowledge on the processes regulating spinal cord maturation.

Reduced expression of the "fast" calcium transporter PMCA2a during homeostatic plasticity

In a model of homeostatic plasticity, hippocampal slice culture CA3 pyramidal neurons responded to excitatory synapse inactivity by enhancing glutamate release through an increased number of miniature excitatory post-synaptic currents, mEPSCs and excitatory pre-synaptic terminals. Also accompanying these changes was a specific reduction in the expression of a "fast" calcium transporter, the plasma membrane calcium ATPase, PMCA2a. This transporter normally influences glutamate release from excitatory terminals where it helps control calcium levels. The reduction in PMCA2a expression occurred within 2 days of synapse inactivity; it was specific and reversible in young and mature hippocampal slice cultures and required removal of NMDA receptor mediated activity. Furthermore, the enhanced mEPSCs in the model were resistant to pharmacological inhibition of PMCA transporter activity. Reduced expression of PMCA2a during homeostatic plasticity therefore provides a mechanism to remodel pre-synaptic Ca2+ dynamics as a flexible way to alter glutamate release.

GABAA transmission is a critical step in the process of triggering homeostatic increases in quantal amplitude

When activity levels are altered over days, a network of cells is capable of recognizing this perturbation and triggering several distinct compensatory changes that should help to recover and maintain the original activity levels homeostatically. One feature commonly observed after activity blockade has been a compensatory increase in excitatory quantal amplitude. The sensing machinery that detects altered activity levels is a central focus of the field currently, but thus far it has been elusive. The vast majority of studies that reduce network activity also reduce neurotransmission. We address the possibility that reduced neurotransmission can trigger increases in quantal amplitude. In this work, we blocked glutamatergic or GABA(A) transmission in ovo for 2 days while maintaining relatively normal network activity. We found that reducing GABA(A) transmission triggered compensatory increases in both GABA and AMPA quantal amplitude in embryonic spinal motoneurons. Glutamatergic blockade had no effect on quantal amplitude. Therefore, GABA binding to the GABA(A) receptor appears to be a critical step in the sensing machinery for homeostatic synaptic plasticity. The findings suggest that homeostatic increases in quantal amplitude may normally be triggered by reduced levels of activity, which are sensed in the developing spinal cord by GABA, via the GABA(A) receptor. Therefore, GABA appears to be serving as a proxy for activity levels.

Analysis of FK506-mediated protection in an organotypic model of spinal cord damage: heat shock protein 70 levels are modulated in microglial cells

Functional loss after spinal cord injuries is originated by primary and secondary injury phases whose underlying mechanisms include massive release of excitatory amino acids to cytotoxic levels that contribute to neural death. Attenuation of this excitotoxicity is a key point for improving the functional outcome after injury. One of the drugs with potential neuroprotective actions is FK506, a molecule widely used as an immunosuppressant. FK506 may exert neuroprotection via inhibition of calcineurin by binding the FKBP12, or by binding other immunophilins such as FKBP52, leading to modulation of heat shock proteins (Hsp) 90 and 70. In the present study, we used an in vitro model of organotypic culture of rat spinal cord slices to assess whether FK506 is able to protect them against glutamate excitotoxicity. The results showed that FK506 promoted a significant protective effect on the spinal cord tissue at concentrations of 50 and 100 nM. Hsp70 induction was restricted to microglial cells in spinal cord slices treated with either glutamate or FK506. In contrast, the combination of both agents led to a transient reduction in Hsp70 levels in parallel to a marked reduction in IL-1beta precursor production by glial cells. The use of geldanamycin, which promotes persistent induction of Hsp70 in these cells as well as in motoneurons, did not produce tissue neuroprotection. These observations suggest that FK506 might protect spinal cord tissue by targeting on microglial cells and that transient downregulation of Hsp70 on these cells after excitotoxicity is a relevant mechanism of action of FK506.

ERG conductance expression modulates the excitability of ventral horn GABAergic interneurons that control rhythmic oscillations in the developing mouse spinal cord

During antenatal development, the operation and maturation of mammalian spinal networks strongly depend on the activity of ventral horn GABAergic interneurons that mediate excitation first and inhibition later. Although the functional consequence of GABA actions may depend on maturational processes in target neurons, it is also likely that evolving changes in GABAergic transmission require fine-tuning in GABA release, probably via certain intrinsic mechanisms regulating GABAergic neuron excitability at different embryonic stages. Nevertheless, it has not been possible, to date, to identify certain ionic conductances upregulated or downregulated before birth in such cells. By using an experimental model with either mouse organotypic spinal cultures or isolated spinal cord preparations, the present study examined the role of the ERG current (I(K(ERG))), a potassium conductance expressed by developing, GABA-immunoreactive spinal neurons. In organotypic cultures, only ventral interneurons with fast adaptation and GABA immunoreactivity, and only after 1 week in culture, were transformed into high-frequency bursters by E4031, a selective inhibitor of I(K(ERG)) that also prolonged and made more regular spontaneous bursts. In the isolated spinal cord in which GABA immunoreactivity and m-erg mRNA were colocalized in interneurons, ventral root rhythms evoked by NMDA plus 5-hydroxytryptamine were stabilized and synchronized by E4031. All of these effects were lost after 2 weeks in culture or before birth in coincidence with decreased m-erg expression. These data suggest that, during an early stage of spinal cord development, the excitability of GABAergic ventral interneurons important for circuit maturation depended, at least in part, on the function of I(K(ERG)).

Activity-independent intracellular Ca2+ oscillations are spontaneously generated by ventral spinal neurons during development in vitro

Within the CNS, distinct neurons may rely on different processes to modulate cytosolic Ca2+ depending on the network developmental phase. In particular, in the immature spinal cord, synchronous electrical discharges are coupled with biochemical signals triggered by intracellular Ca2+ waves. Nevertheless, the presence of neuronal-specific Ca2+ elevations independent from synaptic activity within mammalian spinal networks has not yet been described. The present report is the first description of repetitive calcium events generated by discrete ventral spinal neurons maintained in organotypic culture during in vitro maturation stages crucial for network evolution. Ventral interneurons in one-third of slices displayed spontaneous intracellular calcium transients suppressed by calcium-free extracellular solution or by application of cobalt, and resistant to blockers of network activity like TTX, CNQX, APV, strychnine or bicuculline. Our data suggest a primary role for mitochondria in intracellular calcium oscillations, because CCCP, that selectively collapses the mitochondrial electrochemical gradient, eliminated the ability of these neurons to show activity-independent calcium oscillations. Likewise, CGP-37157, a blocker of mitochondrial Na+/Ca2+ exchanger, inhibited oscillations in the majority of neurons. We propose that spontaneous Ca2+ transients, dynamically regulated by mitochondria, occurred in a discrete cluster of interneurons possibly to guide the development of synaptic connections.

Spontaneous network activity in the embryonic spinal cord regulates AMPAergic and GABAergic synaptic strength

Spontaneous network activity (SNA) has been described in most developing circuits, including the spinal cord, retina, and hippocampus. Despite the widespread nature of this developmental phenomenon, its role in network maturation is poorly understood. We reduced SNA in the intact embryo and found compensatory increases in synaptic strength of spinal motoneuron inputs. AMPAergic miniature postsynaptic current (mPSC) amplitude and frequency increased following the reduction of activity. Interestingly, excitatory GABAergic mPSCs also increase in amplitude through a process of synaptic scaling. Finally, the normal modulation of GABAergic mPSC amplitude was accelerated. Together, these compensatory responses appear to increase the excitability of the cord and could act to maintain appropriate SNA levels, thus demonstrating a distinct functional role for synaptic homeostasis. Because spontaneous network activity can regulate AMPAergic and GABAergic synaptic strength during development, SNA is likely to play an important role in a coordinated maturation of excitatory and inhibitory synaptic strength.

Interneurons transiently express the ERG K+ channels during development of mouse spinal networks in vitro

During spinal cord maturation neuronal excitability gradually differentiates to meet different functional demands. Spontaneous activity, appearing early during spinal development, is regulated by the expression pattern of ion channels in individual neurons. While emerging excitability of embryonic motoneurons has been widely investigated, little is known about that of spinal interneurons. Voltage-dependent K+ channels are a heterogeneous class of ion channels that accomplish several functions. Recently voltage-dependent K+ channels encoded by erg subfamily genes (ERG channels) were shown to modulate excitability in immature neurons of mouse and quail. We investigated the expression of ERG channels in immature spinal interneurons, using organotypic embryonic cultures of mouse spinal cord after 1 and 2 weeks of development in vitro. We report here that all the genes of the erg family known so far (erg1a, erg1b, erg2, erg3) are expressed in embryonic spinal cultures. We demonstrate for the first time that three ERG proteins (ERG1A, ERG2 and ERG3) are co-expressed in the same neuronal population, and display a spatio-temporal distribution in the spinal slices. ERG immuno-positive cells, representing mainly GABAergic interneurons, were present in large numbers at early stages of development, while declining later, with a ventral to dorsal gradient. Patch clamp recordings confirmed these data, showing that ventral interneurons expressed functional ERG currents only transiently. Similar expression of the erg genes was observed at comparable ages in vivo. The role of ERG currents in regulating neuronal excitability during the earliest phases of spinal circuitry development will be examined in future studies.

Gamma protocadherins are required for synaptic development in the spinal cord

Fifty-eight cadherin-related protocadherin (Pcdh) genes are tandemly arrayed in three clusters (alpha, beta, and gamma) on mouse chromosome 18. The large number of clustered Pcdh family members, their presence at synapses, and the known binding specificities of other cadherin superfamily members all suggest that these Pcdhs play roles in specifying synaptic connectivity. Consistent with this idea, mice lacking all 22 genes of the Pcdh-gamma cluster have decreased numbers of spinal cord synapses and are nearly immobile. Interpretation of this phenotype was complicated, however, by the fact that Pcdh-gamma loss also led to apoptosis of many spinal interneurons. Here, we used two methods to circumvent apoptosis and neurodegeneration in Pcdh-gamma mutant mice. First, we analyzed mutants lacking both Pcdh-gamma proteins and the proapoptotic protein Bax. Second, we generated a hypomorphic allele of Pcdh-gamma in which apoptosis was minimal. In both cases, spinal interneurons were preserved but the mice bore dramatically decreased numbers of spinal cord synapses and exhibited profound neurological defects. Moreover, synaptic function was compromised in neurons cultured from the hypomorphs. These results provide evidence for a direct role of gamma-Pcdhs in synaptic development and establish genetic tools for elucidating their contribution to synaptic specificity.

Interneurone bursts are spontaneously associated with muscle contractions only during early phases of mouse spinal network development: a study in organotypic cultures

For a short time during development immature circuits in the spinal cord and other parts of the central nervous system spontaneously generate synchronous patterns of rhythmic activity. In the case of the spinal cord, it is still unclear how strongly synchronized bursts generated by interneurones are associated with motoneurone firing and whether the progressive decline in spontaneous bursting during circuit maturation proceeds in parallel for motoneurone and interneurone networks. We used organotypic cocultures of spinal cord and skeletal muscle in order to investigate the ontogenic evolution of endogenous spinal network activity associated with the generation of coordinate muscle fibre contractions. A combination of multiunit electrophysiological recordings, videomicroscopy and optical flow computation allowed us to measure the correlation between interneurone firing and motoneurone outputs after 1, 2 and 3 weeks of in vitro development. We found that, in spinal organotypic slices, there is a developmental switch of spontaneous activity from stable bursting to random patterns after the first week in culture. Conversely, bursting recorded in the presence of strychnine and bicuculline became increasingly regular with time in vitro. The time course of spontaneous activity maturation in organotypic slices is similar to that previously reported for the spinal cord developing in utero. We also demonstrated that spontaneous bursts of interneurone action potentials strongly correlate with muscular contractions only during the first week in vitro and that this is due to the activation of motoneurones via AMPA-type glutamate receptors. These results indicate the occurrence in vitro of motor network development regulating bursting inputs from interneurones to motoneurones.

Plasticity of GABAergic synapses in the neonatal rat hippocampus

While the development and plasticity of excitatory synaptic connections have been studied into detail, little is known about the development of inhibitory synapses. As proposed for excitatory synapses, recent studies have indicated that activity-dependent forms of synaptic plasticity, such as long-term potentiation and long-term depression, may play a role in the establishment of functional inhibitory synaptic connections. Here, I review these different forms of plasticity and focus on their possible role in the developing neuronal network.

Spontaneous synaptic activity is required for the formation of functional GABAergic synapses in the developing rat hippocampus

Here we examine the role of the spontaneous synaptic activity generated by the developing rat hippocampus in the formation of functional gamma-aminobutyric acid (GABA) synapses. Intact hippocampal formations (IHFs) were dissected at birth and incubated for 1 day in control or tetrodotoxin (TTX)-supplemented medium at 25 degrees C. After the incubation, miniature GABA(A)-mediated postsynaptic currents (mGABA(A)-PSCs) were recorded in whole-cell voltage-clamped CA3 pyramidal neurones from IHF-derived slices. After 1 day in vitro in control medium, the frequency of mGABA(A)-PSCs was similar to that recorded in acute slices obtained 1 day after birth, but significantly higher than the frequency recorded from acute slices just after birth. These results suggest that the factors required in vivo for the formation of functional GABAergic synapses are preserved in the IHFs in vitro. The frequency increase was prevented when IHFs were incubated for 1 day with TTX. TTX treatment affected neither the morphology of CA3 pyramidal neurones nor cell viability. The TTX effects were reproduced when IHFs were incubated in the presence of glutamatergic or GABAergic ionotropic receptor antagonists or in high divalent cationic medium. The present results indicate that the spontaneous synaptic activity generated by the developing hippocampus is a key player in the formation of functional GABAergic synapses, possibly via network events requiring both glutamatergic and GABAergic receptors.

Spinal circuits formation: a study of developmentally regulated markers in organotypic cultures of embryonic mouse spinal cord

In this study, we have addressed the issue of neural circuit formation using the mouse spinal cord as a model system. Our primary objective was to assess the suitability of organotypic cultures from embryonic mouse spinal cord to investigate, during critical periods of spinal network formation, the role of the local spinal cellular environment in promoting circuit development and refinement. These cultures offer the great advantage over other in vitro systems, of preserving the basic cytoarchitecture and the dorsal-ventral orientation of the spinal segment from which they are derived [Eur J Neurosci 14 (2001) 903; Eur J Neurosci 16 (2002) 2123]. Long-term embryonic spinal cultures were developed and analyzed at sequential times in vitro, namely after 1, 2, and 3 weeks. Spatial and temporal regulation of neuronal and non-neuronal markers was investigated by immunocytochemical and Western blotting analysis using antibodies against: a) the non-phosphorylated epitope of neurofilament H (SMI32 antibody); b) the enzyme choline acetyltransferase, to localize motoneurons and cholinergic interneurons; c) the enzyme glutamic acid decarboxylase 67, to identify GABAergic interneurons; d) human eag-related gene (HERG) K(+) channels, which appear to be involved in early stages of neuronal and muscle development; e) glial fibrillary acidic protein, to identify mature astrocytes; f) myelin basic protein, to identify the onset of myelination by oligodendrocytes. To examine the development of muscle acetylcholine receptors clusters in vitro, we incubated live cultures with tetramethylrhodamine isothiocyanate-labeled alpha-bungarotoxin, and we subsequently immunostained them with SMI32 or with anti-myosin antibodies. Our results indicate that the developmental pattern of expression of these markers in organotypic cultures shows close similarities to the one observed in vivo. Therefore, spinal organotypic cultures provide a useful in vitro model system to study several aspects of neurogenesis, gliogenesis, muscle innervation, and synaptogenesis.

Altered inhibitory synaptic transmission in superficial dorsal horn neurones in spastic and oscillator mice

The spastic (spa) and oscillator (ot) mouse have naturally occurring mutations in the inhibitory glycine receptor (GlyR) and exhibit severe motor disturbances when exposed to unexpected sensory stimuli. We examined the effects of the spa and ot mutations on GlyR- and GABAAR-mediated synaptic transmission in the superficial dorsal horn (SFDH), a spinal cord region where inhibition is important for nociceptive processing. Spontaneous mIPSCs were recorded from visually identified neurones in parasagittal spinal cord slices. Neurones received exclusively GABAAR-mediated mIPSCs, exclusively GlyR-mediated mIPSCs or both types of mIPSCs. In control mice (wild-type and spa/+) over 40% of neurones received both types of mIPSCs, over 30% received solely GABAAR-mediated mIPSCs and the remainder received solely GlyR-mediated mIPSCs. In spa/spa animals, 97% of the neurones received exclusively GABAAergic or both types of mIPSCs. In ot/ot animals, over 80 % of the neurones received exclusively GABAAR-mediated mIPSCs. GlyR-mediated mIPSC amplitude and charge were reduced in spa/spa and ot/ot animals. GABAAR-mediated mIPSC amplitude and charge were elevated in spa/spa but unaltered in ot/ot animals. GlyR- and GABAAR-mediated mIPSC decay times were similar for all genotypes, consistent with the mutations altering receptor numbers but not kinetics. These findings suggest the spastic and oscillator mutations, traditionally considered motor disturbances, also disrupt inhibition in a sensory region associated with nociceptive transmission. Furthermore, the spastic mutation results in a compensatory increase in GABAAergic transmission in SFDH neurones, a form of inhibitory synaptic plasticity absent in the oscillator mouse.

Epileptiform activity in hippocampal slice cultures exposed chronically to bicuculline: increased gap junctional function and expression

Chronic (18 h) exposure of cultured hippocampal slices to the type-A GABA receptor blocker, bicuculline methiodide (BMI) 10 micro m increased the levels of connexin 43 (Cx43) and connexin 32 (Cx32) mRNAs, but not connexin 26 and connexin 36, as demonstrated by RNase protection assays. The levels of Cx43 and Cx32 proteins in membrane fractions detected by western blotting were also significantly increased. Immunoblotting indicated that BMI also promoted a significant expression of the transcription protein c-fos. The rate of fluorescence recovery after photobleaching, an index of gap junctional coupling, was also significantly increased, whereas it was blocked by the gap junctional blocker, carbenoxolone (100 micro m). Extracellular recordings in CA1 stratum pyramidale, performed in BMI-free solution, demonstrated that BMI-exposed cultures possessed synaptic responses characteristic of epileptiform discharges: (i) significantly greater frequency of spontaneous epileptiform discharges, (ii) post-synaptic potentials with multiple population spikes, and (iii) significantly longer duration of primary afterdischarges. Carbenoxolone (100 micro m), but not its inactive analog, oleanolic acid (100 micro m), reversibly inhibited spontaneous and evoked epileptiform discharges. The findings of BMI-induced parallel increases in levels of gap junction expression and function, and the increase in epileptiform discharges, which were sensitive to gap junctional blockers, are consistent with the hypothesis that increased gap junctional communication plays an intrinsic role in the epileptogenic process.

Tissue PO2 and the effects of hypoxia on the generation of locomotor-like activity in the in vitro spinal cord of the neonatal mouse

The neonatal mouse en bloc spinal cord-brainstem preparation used in combination with advances in mouse genomics provides a novel strategy for studying the spinal control of locomotion. How well the mouse en bloc preparation is oxygenated however, is unknown. This is an important consideration given that (a) other superfused mammalian en bloc preparations have anoxic cores and (b) hypoxia can have profound effects on neuronal activity. Here we measure the level of tissue oxygenation in the mouse preparation and determine how neuronal activity within the spinal cord is influenced by poor superfusion and/or low oxygen. To measure tissue oxygenation, oxygen depth profiles were obtained (P0-1 and P2-3; Swiss Webster mice). At P0-1, spinal cords were oxygenated throughout under resting conditions. When fictive locomotor activity was evoked (5-HT 10 microM, dopamine 50 microM, NMA 5 microM), there was a substantial reduction in tissue PO(2) starting within 5 min of drug application. Following washout, the PO(2) slowly returned to control levels over a period of 30 min. The experiments described above were repeated using P2-3 preparations. In this older age group, the spinal cord preparations had a hypoxic/anoxic core that was exacerbated during metabolically demanding tasks such as drug-evoked rhythmic activity. To examine how an anoxic core affects neuronal activity within the spinal cord we either altered the flow-rate or manipulated superfusate PO(2). When the flow-rate was reduced a transient disruption in the rhythmicity of drug-induced locomotion occurred during the first 15 min (P0-1 preparations). However, the motor output adapted and stabilized. During prolonged superfusion with hypoxic artificial cerebrospinal fluid on the other hand, both the motor bursts in spinal nerves and the activity of most neurons near the center of the tissue were abolished.Overall, this study suggests that while oxygenation of P0-P1 preparations is adequate for studies of locomotor function, oxygenation of older preparations is more problematic. Our data also show that neonatal spinal neurons require oxygen to maintain activity; and the spinal locomotor rhythm generator continues to function providing the peripheral tissue of the cord is oxygenated. Together, these results are consistent with the results of a previous study which suggest that the locomotor pattern generator is located close to the surface of the spinal cord.

Chronic treatment with ionotropic glutamate receptor antagonist kynurenate affects GABAergic synaptic transmission in rat hippocampal cell cultures

It is well documented that prolonged treatment with antagonists of ionotropic glutamate receptors activates a number of homeostatic mechanisms including alteration of glutamatergic transmission. We studied whether this treatment can also affect GABAergic transmission. Using whole-cell voltage clamp recording and local extracellular stimulation we investigated evoked inhibitory postsynaptic currents (IPSCs) in cultured rat hippocampal neurons grown in the presence of ionotropic glutamate receptor antagonist kynurenate (1 mM) and in control conditions. Chronic kynurenate treatment did not significantly affect the amplitude of evoked IPSCs and IPSC reversal potentials. In contrast we found that the paired-pulse depression was increased by 67% in cultures treated with kynurenic acid. We conclude that additional mechanism(s), alteration of GABAergic synaptic transmission, may contribute to homeostatic plasticity induced by chronic block of ionotropic glutamate receptors.

Activity-dependent modulation of GABAergic synapses in developing rat spinal networks in vitro

The role of activity-dependent plasticity in modulating inhibitory synapses was investigated in embryonic rat spinal cord slice cultures, by chronic exposure to non-NMDA receptor blockers. GABAergic synaptic efficacy in control and chronic-treated cultures was investigated by patch-recordings from visually identified spinal interneurons. In both culture groups proximal stimulation induced the appearance of postsynaptic currents (PSCs), which were fully antagonized by 20 microM bicuculline application and reverse polarity at potential values close to those reported for spontaneous GABAergic PSCs. In chronically treated cells GABAergic evoked PSCs displayed a larger failure rate and a smaller coefficient of variation of mean PSC amplitude, when compared to controls. As opposed to controls, chronic GABAergic evoked PSCs did not facilitate upon paired-pulse stimulation. Facilitation at chronic synapses was observed when extracellular calcium levels were decreased below physiological values (< 2 mM). Kainate was used to disclose any functional differences between control and treated slices. In accordance with the presynaptic action of kainate, the application of this drug along with GYKI, an AMPA receptor selective antagonist, changed, with analogous potency, short-term plasticity of GABAergic synapses from control and treated cultures. Nevertheless, in chronic cultures, the downstream effects of such activation unmasked short-term depression. Ultrastructural analysis of synapses in chronically treated cultures showed a reduction both in symmetric synapses and in the number of vesicles at symmetric terminals. Thus, based on electrophysiological and ultrastructural data, it could be suggested that during the development of spinal circuits, GABAergic synapses are modulated by glutamatergic transmission, and thus implying that excitatory transmission regulates the strength of GABAergic synapses.