Riluzole is a potent drug to protect neonatal rat hypoglossal motoneurons in vitro from excitotoxicity due to glutamate uptake block

Respiratory neuropathology in spinocerebellar ataxia type 7

Spinocerebellar ataxia type 7 (SCA7) is an autosomal dominant neurological disorder caused by deleterious CAG repeat expansion in the coding region of the ataxin 7 gene (polyQ-ataxin-7). Infantile-onset SCA7 leads to severe clinical manifestation of respiratory distress, but the exact cause of respiratory impairment remains unclear. Using the infantile-SCA7 mouse model, the SCA7266Q/5Q mouse, we examined the impact of pathological polyQ-ataxin-7 on hypoglossal (XII) and phrenic motor units. We identified the transcript profile of the medulla and cervical spinal cord and investigated the XII and phrenic nerves and the neuromuscular junctions in the diaphragm and tongue. SCA7266Q/5Q astrocytes showed significant intranuclear inclusions of ataxin-7 in the XII and putative phrenic motor nuclei. Transcriptomic analysis revealed dysregulation of genes involved in amino acid and neurotransmitter transport and myelination. Additionally, SCA7266Q/5Q mice demonstrated blunted efferent output of the XII nerve and demyelination in both XII and phrenic nerves. Finally, there was an increased number of neuromuscular junction clusters with higher expression of synaptic markers in SCA7266Q/5Q mice compared with WT controls. These preclinical findings elucidate the underlying pathophysiology responsible for impaired glial cell function and death leading to dysphagia, aspiration, and respiratory failure in infantile SCA7.

Preferential potentiation of AMPA-mediated currents in brainstem hypoglossal motoneurons by subchronic exposure of mice expressing the human superoxide dismutase 1 G93A gene mutation to neurotoxicant methylmercury in vivo

The exact causes of Amyotrophic lateral sclerosis (ALS), a progressive and fatal neurological disorder due to loss of upper and/or lower motoneurons, remain elusive. Gene-environment interactions are believed to be an important factor in the development of ALS. We previously showed that in vivo exposure of mice overexpressing the human superoxide dismutase 1 (hSOD1) gene mutation (hSOD1G93A; G93A), a mouse model for ALS, to environmental neurotoxicant methylmercury (MeHg) accelerated the onset of ALS-like phenotype. Here we examined the time-course of effects of MeHg on AMPA receptor (AMPAR)-mediated currents in hypoglossal motoneurons in brainstem slices prepared from G93A, hSOD1wild-type (hWT) and non-carrier WT mice following in vivo exposure to MeHg. Mice were exposed daily to 3 ppm (approximately 0.7 mg/kg/day) MeHg via drinking water beginning at postnatal day 28 (P28) and continued until P47, 64 or 84, then acute brainstem slices were prepared, and spontaneous excitatory postsynaptic currents (sEPSCs) or AMPA-evoked currents were examined using whole cell patch-clamp recording technique. Brainstem slices of untreated littermates were prepared at the same time points to serve as control. MeHg exposure had no significant effect on either sEPSCs or AMPA-evoked currents in slices from hWT or WT mice during any of those exposure time periods under our experimental conditions. MeHg also did not cause any significant effect on sEPSCs or AMPA-currents in G93A hypoglossal motoneurons at P47 and P64. However, at P84, MeHg significantly increased amplitudes of both sEPSCs and AMPA-evoked currents in hypoglossal motineurons from G93A mice (p < 0.05), but not the sEPSC frequency, suggesting a postsynaptic action on AMPARs. MeHg exposure did not cause any significant effect on GABAergic spontaneous inhibitory postsynaptic currents (sIPSCs). Therefore, MeHg exposure in vivo caused differential effects on AMPARs in hypoglossal motoneurons from mice with different genetic backgrounds. MeHg appears to preferentially stimulate the AMPAR-mediated currents in G93A hypoglossal motoneurons in an exposure time-dependent manner, which may contribute to the AMPAR-mediated motoneuron excitotoxicity, thereby facilitating development of ALS-like phenotype.

In Vitro Models of Amyotrophic Lateral Sclerosis

Amyotrophic Lateral Sclerosis (ALS) is one of the commonest neurodegenerative diseases of adult-onset, which is characterized by the progressive death of motor neurons in the cerebral cortex, brain stem and spinal cord. The dysfunction and death of motor neurons lead to the progressive muscle weakness, atrophy, fasciculations, spasticity and ultimately the whole paralysis of body. Despite the identification of several genetic mutations associated with the pathogenesis of ALS, including mutations in chromosome 9 open reading frame 72 leading to the abnormal expansion of GGGGCC repeat sequence, TAR DNA-binding protein 43, fused in sarcoma/translocated in liposarcoma, copper/zinc superoxide dismutase 1 (SOD1) and TANK-binding kinase 1, the exact mechanisms underlying the specific degeneration of motor neurons that causes ALS remain incompletely understood. At present, since the transgenic model expressed SOD1 mutants was established, multiple in vitro models of ALS have been developed for studying the pathology, pathophysiology and pathogenesis of ALS as well as searching the effective neurotherapeutics. This review reviewed the details of present established in vitro models used in studying the pathology, pathophysiology and pathogenesis of ALS. Meanwhile, we also discussed the advantages, disadvantages, cost and availability of each models.

Functional up-regulation of the M-current by retigabine contrasts hyperexcitability and excitotoxicity on rat hypoglossal motoneurons

KEY POINTS

Excessive neuronal excitability characterizes several neuropathological conditions, including neurodegenerative diseases such as amyotrophic lateral sclerosis. Hypoglossal motoneurons (HMs), which control tongue muscles, are extremely vulnerable to this disease and undergo damage and death when exposed to an excessive glutamate extracellular concentration that causes excitotoxicity. Our laboratory devised an in vitro model of excitotoxicity obtained by pharmacological blockade of glutamate transporters. In this paradigm, HMs display hyperexcitability, collective bursting and eventually cell death. The results of the present study show that pharmacological up-regulation of a K⁺ current (M-current), via application of the anti-convulsant retigabine, prevented all hallmarks of HM excitotoxicity, comprising bursting, generation of reactive oxygen species, expression of toxic markers and cell death. ○Our data may have translational value to develop new treatments against neurological diseases by using positive pharmacological modulators of the M-current.

ABSTRACT

Neuronal hyperexcitability is a symptom characterizing several neurodegenerative disorders, including amyotrophic lateral sclerosis (ALS). In the ALS bulbar form, hypoglossal motoneurons (HMs) are an early target for neurodegeneration because of their high vulnerability to metabolic insults. In recent years, our laboratory has developed an in vitro model of a brainstem slice comprising the hypoglossal nucleus in which HM neurodegeneration is achieved by blocking glutamate clearance with dl-threo-β-benzyloxyaspartate (TBOA), thus leading to delayed excitotoxicity. During this process, HMs display a set of hallmarks such as hyperexcitability (and network bursting), reactive oxygen species (ROS) generation and, finally, cell death. The present study aimed to investigate whether blocking early hyperexcitability and bursting with the anti-convulsant drug retigabine was sufficient to achieve neuroprotection against excitotoxicity. Retigabine is a selective positive allosteric modulator of the M-current (I_M ), an endogenous mechanism that neurons (comprising HMs) express to dampen excitability. Retigabine (10 μm; co-applied with TBOA) contrasted ROS generation, release of endogenous toxic factors into the HM cytoplasm and excitotoxicity-induced HM death. Electrophysiological experiments showed that retigabine readily contrasted and arrested bursting evoked by TBOA administration. Because neuronal I_M subunits (Kv7.2, Kv7.3 and Kv7.5) were expressed in the hypoglossal nucleus and in functionally connected medullary nuclei, we suggest that they were responsible for the strong reduction in network excitability, a potent phenomenon for achieving neuroprotection against TBOA-induced excitotoxicity. The results of the present study may have translational value for testing novel positive pharmacological modulators of the I_M under pathological conditions (including neurodegenerative disorders) characterized by excessive neuronal excitability.

Propofol Protects Rat Hypoglossal Motoneurons in an In Vitro Model of Excitotoxicity by Boosting GABAergic Inhibition and Reducing Oxidative Stress

In brainstem motor networks, hypoglossal motoneurons (HMs) play the physiological role of driving tongue contraction, an activity critical for inspiration, phonation, chewing and swallowing. HMs are an early target of neurodegenerative diseases like amyotrophic lateral sclerosis that, in its bulbar form, is manifested with initial dysphagia and dysarthria. One important pathogenetic component of this disease is the high level of extracellular glutamate due to uptake block that generates excitotoxicity. To understand the earliest phases of this condition we devised a model, the rat brainstem slice, in which block of glutamate uptake is associated with intense bursting of HMs, dysmetabolism and death. Since blocking bursting becomes a goal to prevent cell damage, the present report enquired whether boosting GABAergic inhibition could fulfill this aim and confer beneficial outcome. Propofol (0.5 µM) and midazolam (0.01 µM), two allosteric modulators of GABA_A receptors, were used at concentrations yielding analogous potentiation of GABA-mediated currents. Propofol also partly depressed NMDA receptor currents. Both drugs significantly shortened bursting episodes without changing single burst properties, their synchronicity, or their occurrence. Two hours later, propofol prevented the rise in reactive oxygen species (ROS) and, at 4 hours, it inhibited intracellular release of apoptosis-inducing factor (AIF) and prevented concomitant cell loss. Midazolam did not contrast ROS and AIF release. The present work provides experimental evidence for the neuroprotective action of a general anesthetic like propofol, which, in this case, may be achieved through a combination of boosted GABAergic inhibition and reduced ROS production.

Effects of riluzole on spinal seizure-like activity in the brainstem-spinal cord preparation of newborn rat

Riluzole blocks persistent Na⁺ current, inhibits generation of neuronal bursts and decreases glutamate-induced excitotoxicity. In previous studies of respiratory activity, riluzole suppressed inspiratory-related burst generation activity in rat slice or en bloc preparations. We examined riluzole's effects on inspiratory burst generation and drug-induced seizure-like activity in newborn rat en bloc preparations. Medulla-spinal cord preparations from postnatal day 0-3 Wistar rats were isolated under deep isoflurane anesthesia and were superfused with artificial cerebrospinal fluid equilibrated with 95% O₂ and 5% CO₂, pH 7.4, at 25-26°C. Inspiratory activity was monitored from the fourth cervical ventral root. Seizure-like activity was induced by application of 20μM DL-threo-β-benzyloxyasparatate (TBOA, a glutamate uptake blocker preferentially acting on astrocytes) or coadministration of GABA_A antagonist bicuculline (10μM) and glycine antagonist strychnine (10μM). Pretreatment and co-application with 10μM riluzole abolished the seizure-like burst activity induced by TBOA or bicuculline/strychnine. N-methyl-d-aspartic acid receptor antagonist MK801 (10μM) also depressed this activity. Riluzole may attenuate excessive glutamate action involved in pathological hyperexcitability of motor neurons with no major effect on generation of respiratory activity. Riluzole at the optimal dose could be a potential treatment to protect drug-induced epileptic brain tissue from excitotoxic damage without inducing respiratory suppression.

The Astrocytic S100B Protein with Its Receptor RAGE Is Aberrantly Expressed in SOD1G93A Models, and Its Inhibition Decreases the Expression of Proinflammatory Genes

Neuroinflammation is one of the major players in amyotrophic lateral sclerosis (ALS) pathogenesis, and astrocytes are significantly involved in this process. The astrocytic protein S100B can be released in pathological states activating the receptor for advanced glycation end products (RAGE). Different indications point to an aberrant expression of S100B and RAGE in ALS. In this work, we observed that S100B and RAGE are progressively and selectively upregulated in astrocytes of diseased rats with a tissue-specific timing pattern, correlated to the level of neurodegeneration. The expression of the full-length and soluble RAGE isoforms could also be linked to the degree of tissue damage. The mere presence of mutant SOD1 is able to increase the intracellular levels and release S100B from astrocytes, suggesting the possibility that an increased astrocytic S100B expression might be an early occurring event in the disease. Finally, our findings indicate that the protein may exert a proinflammatory role in ALS, since its inhibition in astrocytes derived from SOD1^G93A mice limits the expression of reactivity-linked/proinflammatory genes. Thus, our results propose the S100B-RAGE axis as an effective contributor to the pathogenesis of the disease, suggesting its blockade as a rational target for a therapeutic intervention in ALS.

Nicotine protects rat hypoglossal motoneurons from excitotoxic death via downregulation of connexin 36

Motoneuron disease including amyotrophic lateral sclerosis may be due, at an early stage, to deficit in the extracellular clearance of the excitatory transmitter glutamate. A model of glutamate-mediated excitotoxic cell death based on pharmacological inhibition of its uptake was used to investigate how activation of neuronal nicotinic receptors by nicotine may protect motoneurons. Hypoglossal motoneurons (HMs) in neonatal rat brainstem slices were exposed to the glutamate uptake blocker DL-threo-β-benzyloxyaspartate (TBOA) that evoked large Ca2+ transients time locked among nearby HMs, whose number fell by about 30% 4 h later. As nicotine or the gap junction blocker carbenoxolone suppressed bursting, we studied connexin 36 (Cx36), which constitutes gap junctions in neurons and found it largely expressed by HMs. Cx36 was downregulated when nicotine or carbenoxolone was co-applied with TBOA. Expression of Cx36 was preferentially observed in cytosolic rather than membrane fractions after nicotine and TBOA, suggesting protein redistribution with no change in synthesis. Nicotine raised the expression of heat shock protein 70 (Hsp70), a protective factor that binds the apoptotic-inducing factor (AIF) whose nuclear translocation is a cause of cell death. TBOA increased intracellular AIF, an effect blocked by nicotine. These results indicate that activation of neuronal nicotinic receptors is an early tool for protecting motoneurons from excitotoxicity and that this process is carried out via the combined decrease in Cx36 activity, overexpression of Hsp70 and fall in AIF translocation. Thus, retarding or inhibiting HM death may be experimentally achieved by targeting one of these processes leading to motoneuron death.

Induced Pluripotent HD Monkey Stem Cells Derived Neural Cells for Drug Discovery

Huntington's disease (HD) is a neurodegenerative disease caused by an expansion of CAG trinucleotide repeat (polyglutamine [polyQ]) in the huntingtin ( HTT) gene, which leads to the formation of mutant HTT (mHTT) protein aggregates. In the nervous system, an accumulation of mHTT protein results in glutamate-mediated excitotoxicity, proteosome instability, and apoptosis. Although HD pathogenesis has been extensively studied, effective treatment of HD has yet to be developed. Therapeutic discovery research in HD has been reported using yeast, cells derived from transgenic animal models and HD patients, and induced pluripotent stem cells from patients. A transgenic nonhuman primate model of HD (HD monkey) shows neuropathological, behavioral, and molecular changes similar to an HD patient. In addition, neural progenitor cells (NPCs) derived from HD monkeys can be maintained in culture and differentiated to neural cells with distinct HD cellular phenotypes including the formation of mHTT aggregates, intranuclear inclusions, and increased susceptibility to oxidative stress. Here, we evaluated the potential application of HD monkey NPCs and neural cells as an in vitro model for HD drug discovery research.

Nicotinic receptor activation contrasts pathophysiological bursting and neurodegeneration evoked by glutamate uptake block on rat hypoglossal motoneurons

KEY POINTS

Impaired uptake of glutamate builds up the extracellular level of this excitatory transmitter to trigger rhythmic neuronal bursting and delayed cell death in the brainstem motor nucleus hypoglossus. This process is the expression of the excitotoxicity that underlies motoneuron degeneration in diseases such as amyotrophic lateral sclerosis affecting bulbar motoneurons. In a model of motoneuron excitotoxicity produced by pharmacological block of glutamate uptake in vitro, rhythmic bursting is suppressed by activation of neuronal nicotinic receptors with their conventional agonist nicotine. Emergence of bursting is facilitated by nicotinic receptor antagonists. Following excitotoxicity, nicotinic receptor activity decreases mitochondrial energy dysfunction, endoplasmic reticulum stress and production of toxic radicals. Globally, these phenomena synergize to provide motoneuron protection. Nicotinic receptors may represent a novel target to contrast pathological overactivity of brainstem motoneurons and therefore to prevent their metabolic distress and death.

ABSTRACT

Excitotoxicity is thought to be one of the early processes in the onset of amyotrophic lateral sclerosis (ALS) because high levels of glutamate have been detected in the cerebrospinal fluid of such patients due to dysfunctional uptake of this transmitter that gradually damages brainstem and spinal motoneurons. To explore potential mechanisms to arrest ALS onset, we used an established in vitro model of rat brainstem slice preparation in which excitotoxicity is induced by the glutamate uptake blocker dl-threo-β-benzyloxyaspartate (TBOA). Because certain brain neurons may be neuroprotected via activation of nicotinic acetylcholine receptors (nAChRs) by nicotine, we investigated if nicotine could arrest excitotoxic damage to highly ALS-vulnerable hypoglossal motoneurons (HMs). On 50% of patch-clamped HMs, TBOA induced intense network bursts that were inhibited by 1-10 μm nicotine, whereas nAChR antagonists facilitated burst emergence in non-burster cells. Furthermore, nicotine inhibited excitatory transmission and enhanced synaptic inhibition. Strong neuroprotection by nicotine prevented the HM loss observed after 4 h of TBOA exposure. This neuroprotective action was due to suppression of downstream effectors of neurotoxicity such as increased intracellular levels of reactive oxygen species, impaired energy metabolism and upregulated genes involved in endoplasmic reticulum (ER) stress. In addition, HMs surviving TBOA toxicity often expressed UDP-glucose glycoprotein glucosyltransferase, a key element in repair of misfolded proteins: this phenomenon was absent after nicotine application, indicative of ER stress prevention. Our results suggest nAChRs to be potential targets for inhibiting excitotoxic damage of motoneurons at an early stage of the neurodegenerative process.

Functional alterations of astrocytes in mental disorders: pharmacological significance as a drug target

Astrocytes play an essential role in supporting brain functions in physiological and pathological states. Modulation of their pathophysiological responses have beneficial actions on nerve tissue injured by brain insults and neurodegenerative diseases, therefore astrocytes are recognized as promising targets for neuroprotective drugs. Recent investigations have identified several astrocytic mechanisms for modulating synaptic transmission and neural plasticity. These include altered expression of transporters for neurotransmitters, release of gliotransmitters and neurotrophic factors, and intercellular communication through gap junctions. Investigation of patients with mental disorders shows morphological and functional alterations in astrocytes. According to these observations, manipulation of astrocytic function by gene mutation and pharmacological tools reproduce mental disorder-like behavior in experimental animals. Some drugs clinically used for mental disorders affect astrocyte function. As experimental evidence shows their role in the pathogenesis of mental disorders, astrocytes have gained much attention as drug targets for mental disorders. In this paper, I review functional alterations of astrocytes in several mental disorders including schizophrenia, mood disorder, drug dependence, and neurodevelopmental disorders. The pharmacological significance of astrocytes in mental disorders is also discussed.

Downregulation of solute carriers of glutamate in gliosomes and synaptosomes may explain local brain metastasis in anaplastic glioblastoma

Advanced grades of glioblastoma are highly aggressive, especially in terms of multisite spread within the brain or even to distant sites at the spinal cord. In advanced grades of glioblastoma, glutamate and glutamine are reported to be increased in concentration in the extracellular fluid. It has been reported that glutamate acts as an extracellular signaling molecule for facilitating local spread of advanced grades of glioblastoma. In the present study, we aimed to examine whether glutamate uptake mechanisms is impaired in advanced glioblastoma. The possible downregulated mechanisms of glutamate uptake would facilitate persistence of glutamate in the extracellular environment, rather than intracellular uptake. We obtained biobanked human specimens of glioblastoma and tested expression of proteins belonging to the solute carrier families of proteins that are known to function as membrane-located excitatory amino acid like glutamate transporters. The present study provides preliminary evidence of the downregulation of membrane expression of excitatory amino acid transporters solute carrier family 1 member 3 (SLC1A3) and its palmitoylated form in gliosomes, as well as SLC1A2 in the glio-synaptosomes. Compounds like riluzole used in the treatment of amyotrophic lateral sclerosis and the antibiotic ceftriaxone have the potential to facilitate glutamate uptake. These medications may be examined as adjunct chemotherapy in the massively aggressive tumor glioblastoma multiforme.

Amyotrophic lateral sclerosis: a focus on disease progression

Since amyotrophic lateral sclerosis (ALS) was discovered and described in 1869 as a neurodegenerative disease in which motor neuron death is induced, a wide range of biomarkers have been selected to identify therapeutic targets. ALS shares altered molecular pathways with other neurodegenerative diseases, such as Alzheimer's, Huntington's, and Parkinson's diseases. However, the molecular targets that directly influence its aggressive nature remain unknown. What is the first link in the neurodegenerative chain of ALS that makes this disease so peculiar? In this review, we will discuss the progression of the disease from the viewpoint of the potential biomarkers described to date in human and animal model samples. Finally, we will consider potential therapeutic strategies for ALS treatment and future, innovative perspectives.

Sumoylation of critical proteins in amyotrophic lateral sclerosis: emerging pathways of pathogenesis

Emerging lines of evidence suggest a relationship between amyotrophic lateral sclerosis (ALS) and protein sumoylation. Multiple studies have demonstrated that several of the proteins involved in the pathogenesis of ALS, including superoxide dismutase 1, fused in liposarcoma, and TAR DNA-binding protein 43 (TDP-43), are substrates for sumoylation. Additionally, recent studies in cellular and animal models of ALS revealed that sumoylation of these proteins impact their localization, longevity, and how they functionally perform in disease, providing novel areas for mechanistic investigations and therapeutics. In this article, we summarize the current literature examining the impact of sumoylation of critical proteins involved in ALS and discuss the potential impact for the pathogenesis of the disease. In addition, we report and discuss the implications of new evidence demonstrating that sumoylation of a fragment derived from the proteolytic cleavage of the astroglial glutamate transporter, EAAT2, plays a direct role in downregulating the expression levels of full-length EAAT2 by binding to a regulatory region of its promoter.

[Amyotrophic lateral sclerosis: update on etiological treatment]

Amyotrophic lateral sclerosis is a rare neurodegenerative disease. It is characterized by motoneurons progressive degeneration. Associated with a paralysis of the legs, arms and the respiratory muscles, its evolution is lethal. Riluzole is the only drug available with an marketing authorisation (autorisation de mise sur le marché [AMM]) in this indication. In the beginning stages of the disease it demonstrated a modest efficacy by prolonging survival for a few months. Although the physiopathological mechanisms of this disease have not been totally solved, the progression of knowledge in recent years in this area led to the development of a large number of neuroprotective agents which showed effective results in animal models of ALS and which could be good candidates for the treatment of ALS. Several clinical trials have been conducted about antiglutamatergic, antioxidant, antiapoptotic agents and growing cell factors but they failed to demonstrate efficacy on survival or quality of life. Therefore, clinical trials using innovative therapeutics and stem cells are ongoing and offer more distant hope.

Calcium channel agonists protect against neuromuscular dysfunction in a genetic model of TDP-43 mutation in ALS

TAR DNA binding protein (TDP-43, encoded by the TARDBP gene) has recently been shown to be associated with amyotrophic lateral sclerosis (ALS), but the early pathophysiological deficits causing impairment in motor function are unknown. Here we expressed the wild-type human gene (wtTARDBP) or the ALS mutation G348C (mutTARDBP) in zebrafish larvae and characterized their motor (swimming) activity and the structure and function of their neuromuscular junctions (NMJs). Of these groups only mutTARDBP larvae showed impaired swimming and increased motoneuron vulnerability with reduced synaptic fidelity, reduced quantal transmission, and more orphaned presynaptic and postsynaptic structures at the NMJ. Remarkably, all behavioral and cellular features were stabilized by chronic treatment with either of the L-type calcium channel agonists FPL 64176 or Bay K 8644. These results indicate that expression of mutTARDBP results in defective NMJs and that calcium channel agonists could be novel therapeutics for ALS.

Bicuculline- and neurosteroid-sensitive tonic chloride current in rat hypoglossal motoneurons and atypical dual effect of SR95531

Hypoglossal motoneurons (HMs) are known to be under 'permanent' bicuculline-sensitive inhibition and to show 'transient' synaptic γ-aminobutyric acid (GABA)(A) and glycine inhibitory responses. The present paper describes a permanent bicuculline-sensitive current that should contribute to their tonic inhibition. This current was recorded in brainstem slices superfused without any exogenous agonist and remained detectable with tetrodotoxin. It could also be blocked by the other GABA(A) antagonists picrotoxin (PTX) and 2-(3-carboxypropyl)-3-amino-6-(4 methoxyphenyl)pyridazinium bromide) (SR95531; gabazine), but persisted in the presence of a specific blocker of α5-containing GABA(A) receptors. Addition of 2 μm 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol hydrochloride (THIP), known to preferentially activate GABA(A) receptors devoid of a γ-subunit, induced a sustained anionic current that could be further enhanced by neurosteroids such as allopregnanolone (100 nm). Thus, HMs show a tonic inhibitory current carried by extrasynaptic γ-free GABA(A) receptors, highly sensitive to neurosteroids. A second result was obtained by using SR95531 at concentrations sufficiently high to rapidly block the tonic current above the chloride equilibrium potential (E(C) (l)). Surprisingly, below E(C) (l) , SR95531 (10-40 μm) activated a sustained inward current, associated with a conductance increase, and resistant to bicuculline or PTX (100 μm). Similarly, after blockade of the bicuculline-sensitive current, SR95531 activated an outward current above E(C) (l). The bicuculline-resistant anionic current activated by SR95531 could be blocked by a GABA(C) receptor antagonist. Thus, two types of inhibitory GABA receptors, belonging to the GABA(A) and GABA(C) families, are able to show a sustained activity in HMs and provide promising targets for neuroprotection under overexcitatory situations known to easily damage these particularly fragile neurons.

Postnatal developmental profile of neurons and glia in motor nuclei of the brainstem and spinal cord, and its comparison with organotypic slice cultures

In vitro preparations of the neonatal rat spinal cord or brainstem are useful to investigate the organization of motor networks and their dysfunction in neurological disease models. Long-term spinal cord organotypic cultures can extend our understanding of such pathophysiological processes over longer times. It is, however, surprising that detailed descriptions of the type (and number) of neurons and glia in such preparations are currently unavailable to evaluate cell-selectivity of experimental damage. The focus of the present immunohistochemical study is the novel characterization of the cell population in the lumbar locomotor region of the rat spinal cord and in the brainstem motor nucleus hypoglossus at 0-4 postnatal days, and its comparison with spinal organotypic cultures at 2-22 days in vitro. In the nucleus hypoglossus, neurons were 40% of all cells and 80% of these were motoneurons. Astrocytes (35% of total cells) were the main glial cells, while microglia was <10%. In the spinal gray matter, the highest neuronal density was in the dorsal horn (>80%) and the lowest in the ventral horn (≤57%) with inverse astroglia numbers and few microglia. The number of neurons (including motoneurons) and astrocytes was stable after birth. Like in the spinal cord, motoneurons in organotypic spinal culture were <10% of ventral horn cells, with neurons <40%, and the rest made up by glia. The present report indicates a comparable degree of neuronal and glial maturation in brainstem and spinal motor nuclei, and that this condition is also observed in 3-week-old organotypic cultures.

Riluzole

Amyotrophic lateral sclerosis (Lou Gehrig’s disease) is a devastating neurodegenerative disorder for which the only licensed treatment is riluzole. Although riluzole clinical efficacy is rather limited, its use has important implications for identifying those parameters that might improve its clinical benefits (dose, timing, disease stage) and for its off-label administration in other neurodegenerative diseases, such as spinal cord injury. Studies of riluzole also have an intrinsically heuristic value to unveil mechanisms regulating the excitability of brain and spinal neurons because this drug is a pharmacological tool to probe the function of certain ion channels, or to study neurotransmitter release processes, and intracellular neuroprotective pathways. The present review focuses on how riluzole acts on brain and spinal neurons within motor networks, what mechanisms can be deduced from its effects, and what conditions may favor its use to contrast neurodegeneration or to ameliorate late symptoms like spasticity. Taking as an example the experimental neurodegeneration caused by overactivation of glutamatergic synapses (excitotoxicity), it seems likely that protection of motor networks by riluzole involves selected administration timing and dosing to target processes for releasing glutamate from very active synapses or for dampening repetitive firing by hyperfunctional motor cells.

Neurotoxic injury pathways in differentiated mouse motor neuron-neuroblastoma hybrid (NSC-34D) cells in vitro--limited effect of riluzole on thapsigargin, but not staurosporine, hydrogen peroxide and homocysteine neurotoxicity

The neuroblastoma-spinal motor neuron fusion cell line, NSC-34, in its differentiated form, NSC-34D, permits examining the effects of riluzole, a proven treatment for amyotrophic lateral sclerosis (ALS) on cell death induction by staurosporine (STS), thapsigargin (Thaps), hydrogen peroxide (H(2)O(2)) and homocysteine (HCy). These neurotoxins, applied exogenously, have mechanisms of action related to the various proposed molecular pathogenetic pathways in ALS and are differentiated from endogenous cell death that is associated with cytoplasmic aggregate formation in motor neurons. Nuclear morphology, caspase-3/7 activation and high content imaging were used to assess toxicity of these neurotoxins with and without co-treatment with riluzole, a benzothiazole compound with multiple pharmacological actions. STS was the most potent neurotoxin at killing NSC-34D cells with a toxic concentration at which 50% of maximal cell death is achieved (TC(50)=0.01μM), followed by Thaps (TC(50)=0.9μM) and H(2)O(2) (TC(50)=15μM) with HCy requiring higher concentrations to kill at the same level (TC(50)=2200μM). Riluzole provided neurorescue with a 20% absolute reduction (47.6% relative reduction) in apoptotic cell death against Thaps-induced NSC-34D cell (p≤0.05), but had no effect on STS-, H(2)O(2)- and HCy-induced NSC-34D cell death. This effect of riluzole on Thaps induction of cell death was independent of caspase-3/7 activation. Riluzole mitigated a toxin that can cause intracellular calcium dysregulation associated with endoplasmic reticulum (ER) stress but not toxins associated with other cell death mechanisms.

Delayed neuroprotection by riluzole against excitotoxic damage evoked by kainate on rat organotypic spinal cord cultures

Kainate-mediated excitotoxicity of organotypic spinal cord cultures is an in vitro model advantageous to investigate basic mechanisms of acute spinal injury and its pharmacological neuroprotection. Using such cultures, the putative neuroprotective agent riluzole applied at 5 μM (plasma therapeutic concentration) was studied for its ability to prevent neurotoxicity evoked by 1 h administration of kainate. We monitored real-time release of glutamate, release of lactate dehydrogenase (LDH) (cell damage marker), occurrence of cell pyknosis, the number of surviving neurons and motoneurons, and cell culture metabolic activity. Co-applied riluzole strongly blocked the kainate-evoked early rise in extracellular glutamate (via calcium dependent or independent processes) and suppressed LDH release (limited to <20% of total). Although there were no significant cell losses within the first h after kainate washout, pyknosis, fewer neurons and motoneurons were observed 24 h later. MTT assay demonstrated that surviving cells were metabolically competent. Co-application of kainate and tetrodotoxin also failed to protect spinal cord slices 24 h later. When riluzole application begun at kainate washout and continued for 24 h, significant neuroprotection was observed for neurons in the central and dorsal regions, while ventral horn cells (including motoneurons) were not protected. Our data suggest that riluzole neuroprotection against excitotoxicity was feasible, although it paradoxically required delayed drug administration, and was not extended to the ventral horn. We propose that riluzole was acting on yet-unidentified processes downstream of glutamate release and receptor activation. Deciphering their identity and role in cell death mechanisms may be an important goal to develop neuroprotection.

Respiratory motoneurons and pathological conditions: lessons from hypoglossal motoneurons challenged by excitotoxic or oxidative stress

Hypoglossal motoneurons (HMs) are respiration-related brainstem neurons that command rhythmic contraction of the tongue muscles in concert with the respiratory drive. In experimental conditions, HMs can exhibit a range of rhythmic patterns that may subserve different motor outputs and functions. Neurodegenerative diseases like amyotrophic lateral sclerosis (ALS; Lou-Gehrig disease) often damage HMs with distressing symptoms like dysarthria, dysphagia and breathing difficulty related to degeneration of respiratory motoneurons. While the cause of ALS remains unclear, early diagnosis remains an important goal for potential treatment because fully blown clinical symptoms appear with degeneration of about 30% motoneurons. Using a simple in vitro model of the rat brainstem to study the consequences of excitotoxicity or oxidative stress (believed to occur during the onset of ALS) on HMs, it is possible to observe distinct electrophysiological effects associated with HM experimental pathology. In fact, excitotoxicity caused by glutamate uptake block triggers sustained bursting and enhanced synaptic transmission, whereas oxidative stress generates slow depolarization, augmented repeated firing, and decreased synaptic transmission. In either case, only a subpopulation of HMs shows abnormal functional changes. Although these two insults induce separate functional signatures, the consequences on HMs after a few hours are similar and are preceded by activation of the stress transcription factor ATF-3. The deleterious action of excitotoxicity is inhibited by early administration of riluzole, a drug currently employed for the symptomatic treatment of ALS, demonstrating that this in vitro model can be useful for testing potential neuroprotective agents.