Protective effect of riluzole on excitatory amino acid-mediated neurotoxicity in motoneuron-enriched cultures

Excitotoxic death of a subset of embryonic rat motor neurons in vitro

We have used cultures of purified embryonic rat spinal cord motor neurons to study the neurotoxic effects of prolonged ionotropic glutamate receptor activation. NMDA and non-NMDA glutamate receptor agonists kill a maximum of 40% of the motor neurons in a concentration- and time-dependent manner, which can be blocked by receptor subtype-specific antagonists. Subunit-specific antibodies stain all of the motor neurons with approximately the same intensity and for the same repertoire of subunits, suggesting that the survival of the nonvulnerable population is unlikely to be due to the lack of glutamate receptor expression. Extracellular Ca2+ is required for excitotoxicity, and the route of entry initiated by activation of non-NMDA, but not NMDA, receptors is L-type Ca2+ channels. Ca2+ imaging of motor neurons after application of specific glutamate receptor agonists reveals a sustained rise in intracellular Ca2+ that is present to a similar degree in most motor neurons, and can be blocked by appropriate receptor/channel antagonists. Although the lethal effects of glutamate receptor agonists are seen in only a subset of cultured motor neurons, the basis of this selectivity is unlikely to be simply the glutamate receptor phenotype or the level/pattern of rise in agonist-evoked intracellular Ca2+.

Antioxidative and proapoptotic effects of riluzole on cultured cortical neurons

Riluzole is used clinically in patients with amyotrophic lateral sclerosis. As oxidative stress, in addition to excitotoxicity, may be a major mechanism of motoneuron degeneration in patients with amyotrophic lateral sclerosis, we examined whether riluzole protects against nonexcitotoxic oxidative injury. Probably reflecting its weak antiexcitotoxic effects, riluzole (1-30 microM) attenuated submaximal neuronal death induced by 24-h exposure to 30 microM kainate or NMDA, but not that by 100 microM NMDA, in cortical cultures. Riluzole also attenuated nonexcitotoxic oxidative injury induced by exposure to FeCl3 in the presence of MK-801 and CNQX. Consistent with its antioxidative effects, riluzole reduced Fe3+-induced lipid peroxidation, and inhibited cytosolic phospholipase A2. By contrast, riluzole did not attenuate neuronal apoptosis induced by staurosporine. Rather unexpectedly, 24-48-h exposure to 100-300 microM riluzole induced neuronal death accompanied by nuclear and DNA fragmentations, which was attenuated by caspase inhibitor carbobenzyloxy-Val-Ala-Asp-fluoromethyl ketone but not by protein synthesis inhibitor cycloheximide. The present study demonstrates that riluzole has direct antioxidative actions, perhaps in part by inhibiting phospholipase A2. However, in the same neurons, riluzole paradoxically induces neuronal apoptosis in a caspase-sensitive manner. Considering current clinical use of riluzole, further studies are warranted to investigate its potential cytolethal effects.

Effects of depolarizing stimuli on calcium homeostasis in cultured rat motoneurones

Intracellular calcium concentrations in individual rat motoneurones in enriched primary cultures were measured by Indo-1 fluorimetry. Motoneurones in the cultures were characterized morphometrically and by cholineacetyltransferase immunocytochemistry. Depolarization of the cells with glutamic acid or veratridine increased intracellular calcium levels, which returned to baseline only slowly after removal of the depolarizing agent. The use of selective agonists (N-methyl-D-aspartic acid, AMPA, kainic acid, quisqualic acid and 1R-3S-ACPD) and antagonists (MK 801 and CNQX) showed that the excitatory amino acid-evoked responses were mediated by AMPA/kainate receptors rather than by NMDA receptors. Depolarization-evoked calcium transients in motoneurones are blocked by the neuroprotective drug riluzole Calcium transients reflected entry of calcium from without the cell, and their blockade by nitrendipine and lanthanum chloride suggested that this entry took place primarily through voltage-dependent calcium channels. These findings may be relevant for understanding the selective vulnerability of motoneurones to excitotoxicity in amyotrophic lateral sclerosis, and the therapeutic activity of riluzole in the treatment of this disease.

Glial cells potentiate kainate-induced neuronal death in a motoneuron-enriched spinal coculture system

AMPA/kainate receptor-mediated excitotoxicity is believed to play a pathogenic role in amyotrophic lateral sclerosis. To further characterize the mechanisms involved in AMPA/kainate receptor-mediated motoneuron injury, we investigated the influence of spinal glial cells on kainate-induced motoneuron death in vitro. A motoneuron-enriched neuronal population was obtained from embryonic mouse spinal cord by metrizamide density centrifugation. This population was cultured either on a pre-established glial feeder layer of ventral spinal origin (coculture) or in glia-free conditions (monoculture). Glial feeder layers significantly enhanced basal survival of neurons, and supported neuronal differentiation as judged by neuronal morphology and expression of the motoneuron markers peripherin and SMI-32. Neuronal vulnerability to kainate was two- to three-fold higher in coculture than in monoculture, and increased significantly with time in coculture. The effects of glial feeder layers on neuronal basal survival, differentiation and kainate vulnerability were not mimicked by conditioned medium from glial cells. The increase in neuronal kainate vulnerability with time in coculture was associated with a marked rise in the proportion of cocultured neurons possessing Ca2+-permeable AMPA/kainate receptors, as determined by kainate-activated Co2+-uptake. Neurons in monoculture were unstained by kainate-activated Co2+-uptake. Neurons were immunoreactive to specific antibodies against the AMPA receptor subunits GluR1 and GluR2 both in monoculture and coculture. This study indicates that motoneuron differentiation in coculture is associated with increased vulnerability to kainate and increased expression of Ca2+-permeable AMPA/kainate receptors. In this paradigm glial cells support basal survival and differentiation of neurons, but potentiate kainate-induced neuronal death.

Riluzole has no acute effect on motor unit parameters in ALS

Riluzole is the only drug to have been approved for the treatment of amyotrophic lateral sclerosis (ALS/MND). Its mechanism of action is complex and includes actions on NMDA and kainate receptors and modulation of voltage gated Na channels. In ALS, its effects on measurable parameters of the motor units utilising current neurophysiological techniques are unknown. In an acute randomized, double-blind, placebo-controlled, cross-over experiment, we serially assessed the effects of riluzole on motor units in muscles affected by ALS/MND using EMG. We discuss the results of our observations in the light of previous clinical trials, and their implications.

Electrophysiology of the neuroprotective agent riluzole on striatal spiny neurons

Striatal spiny neurons are selectively vulnerable in Huntington's disease (HD). No effective treatment is available to limit neuronal death in this pathological condition. In an experimental model of HD, a beneficial effect has recently been reported by the neuroprotective agent riluzole. We performed intracellular recordings in order to characterize the electrophysiological effects of this compound on striatal spiny neurons. Riluzole (0.1-100 microM) affected neither the resting membrane potential nor the input resistance/membrane conductance of the recorded cells. Bath application of this pharmacological agent produced a dose-dependent reduction of the number of spikes evoked by long-lasting depolarizing pulses. The EC50 value for this effect was 0.5 microM. Low doses of riluzole selectively reduced the firing frequency in the last part of the depolarizing pulse suggesting a use-dependent action at low concentrations of this compound. Riluzole produced a dose-dependent reduction of the amplitude of the corticostriatal glutamatergic excitatory post-synaptic potentials (EPSPs) with an extrapolated EC50 value of 6 microM. This effect was reversible and maximal at a concentration of 100 microM. Paired-pulse facilitation (PPF) was not affected by riluzole suggesting that the reduction of excitatory transmission was not only caused by a decrease of presynaptic release. Accordingly, riluzole also reduced the amplitude of membrane depolarization induced by exogenous glutamate. The modulatory action of riluzole on the activity of striatal spiny neurons might support the use of this drug in experimental models of excitotoxicity and in the neurodegenerative disorders involving the striatum.

Riluzole attenuates cortical lesion size, but not hippocampal neuronal loss, following traumatic brain injury in the rat

The neuroprotective effects of Riluzole, a compound with several mechanisms of action including the inhibition of sodium channel activity and glutamate release, were evaluated in a rat model of parasagittal fluid-percussion (FP) brain injury. Male Sprague-Dawley rats (350-400 g, n = 17) were anesthetized with sodium pentobarbital (60 mg/kg i.p.) and subjected to parasagittal FP brain injury of moderate severity (2.3-2.5 atm). Fifteen min following injury, animals randomly received an i.v. bolus of either Riluzole (8 mg/kg, n = 8) or vehicle (n = 9), followed by subcutaneous injections (identical dose) at 6 hr and 24 hr. Two weeks after injury and drug treatment, animals were sacrificed and a series of brain sections, stained with Hematoxylin and Eosin (H&E) or cresyl violet, were evaluated for quantitative cortical lesion volume and cell counts of hippocampal CA3 neurons, respectively, using a computerized image analysis system. Administration of Riluzole significantly reduced FP-induced tissue loss in the temporal/occipital cortices ipsilateral to the site of impact by 46%, compared to vehicle-treated, brain-injured animals (P = 0.01). In contrast, the selective neuronal loss observed in the CA3 region of the ipsilateral hippocampus was unaffected by Riluzole treatment. The present study demonstrates that Riluzole can attenuate cortical lesion size following brain trauma. These neuroprotective effects may be related to the synergy of the different mechanisms of action of Riluzole.

Effect of glutamate on dendritic growth in embryonic rat motoneurons

Glutamate is a major excitatory neurotransmitter for spinal motoneurons. We have investigated its effect on survival and neurite formation in cultures of highly enriched motoneurons from 15-d-old rat embryos. Whereas the survival of these neurons was not reduced by this treatment, a distinct and specific effect on dendrite outgrowth could be observed. Axon outgrowth was not affected by glutamate. Our data suggest that calcium influx via ionotropic AMPA/kainate (AMPA/KA) receptors is responsible for the regulation of dendrite outgrowth by excitatory neurotransmission. This was shown by the use of specific inhibitors for the different classes of glutamate receptors. The effect was reduced by continuous depolarization at 35 mM KCl and by treatment with joro spider toxin (JSTX-3, 3 microM), a blocker of Ca2+-conducting AMPA receptors. Removal of glutamate after 5 d of culture led to increased dendrite growth during the following culture period, and delayed addition resulted in a reduction in the length of already existing dendrites. Our observation that the effect is dose-dependent and reversible reflects a potential physiological function of excitatory neurotransmission on dendrite growth and morphology during a developmental period when synaptic contacts from afferent neurons to motoneurons are made in the spinal cord.

Riluzole does not attenuate increases in hippocampal glutamate concentrations in a rabbit model of repeated transient global cerebral ischemia

The aim of the present study was to examine the ability of riluzole to inhibit glutamate release during episodes of transient global cerebral ischemia. New Zealand White rabbits (n = 36) were anesthetized with halothane and mechanically ventilated to maintain normocarbia. Microdialysis catheters were inserted bilaterally into the dorsal hippocampus and perfused with artificial cerebrospinal fluid at 2 microL/min. Animals were randomly assigned to control, hypothermia (30 degrees C), or riluzole (2 or 8 mg/kg; R2 and R8) groups. Two episodes of transient global cerebral ischemia (each lasting 10 min) were produced by inflating the pneumatic tourniquet combined with induced hypotension. Dialysate was collected throughout the periischemic period, and glutamate concentrations were determined by using high-performance liquid chromatography. Peak levels were compared by using the Kruskal-Wallis test. Glutamate concentrations significantly increased by twofold to fourfold during the second ischemic period for the control, R2, and R8 groups. Glutamate concentrations in the hypothermic group were significantly lower than those in the other three groups and remained at baseline levels during the entire experiment. This study demonstrates that the sodium channel blocker riluzole does not inhibit ischemia-induced glutamate accumulation. The previously reported neuroprotective ability of riluzole may be caused by mechanisms other than the presynaptic inhibition of glutamate release during ischemia.

IMPLICATIONS

Glutamate, an excitatory neurotransmitter, is released in excessive amounts during brain ischemia and may result in neuronal injury and death. Riluzole, a neuronal sodium channel blocker, has neuroprotective properties in some animal models of brain ischemia, possibly because of its ability to inhibit the release of glutamate from synaptic vesicles. However, this microdialysis study failed to demonstrate any attenuation of glutamate release during transient global ischemia after the administration of either 2 mg/kg or 8 mg/kg of riluzole.

Riluzole protects from motor deficits and striatal degeneration produced by systemic 3-nitropropionic acid intoxication in rats

The putative neuroprotective effect of riluzole was investigated in a rat model of progressive striatal neurodegeneration induced by prolonged treatment (three weeks, intraperitoneal) with 3-nitropropionic acid, an irreversible inhibitor of succinate dehydrogenase. Quantitative analysis of motor behaviour indicated a significant protective effect (60%) of riluzole (8 mg/kg/day) on 3-nitropropionic acid-induced motor deficits as assessed using two independent motor tests. Furthermore, quantitative analysis of 3-nitropropionic acid-induced lesions indicated a significant 84% decrease in the volume of striatal damage produced by 3-nitropropionic acid, the neuroprotective effect apparently being more pronounced in the posterior striatum and pallidum. In addition, it was checked that this neuroprotective effect of riluzole against systemic 3-nitropropionic acid did not result from a decreased bioavailability of the neurotoxin or a direct action of riluzole on 3-nitropropionic acid-induced inhibition of succinate dehydrogenase. We found that riluzole significantly decreased by 48% the size of striatal lesions produced by stereotaxic intrastriatal injection of malonate, a reversible succinate dehydrogenase inhibitor. Furthermore, the inhibition of cortical and striatal succinate dehydrogenase activity induced by systemic 3-nitropropionic acid was left unchanged by riluzole administration. The present results, consistent with a beneficial effect of riluzole in amyotrophic lateral sclerosis, suggest that this compound may be useful in the treatment of chronic neurodegenerative diseases.

Riluzole promotes survival of rat motoneurons in vitro by stimulating trophic activity produced by spinal astrocyte monolayers

In the present study we have assessed whether riluzole stimulates the production of trophic activities for motoneurons by spinal astrocyte cultures. Astrocyte monolayers prepared from new-born rats were exposed to vehicle or riluzole (1-10 microM) for 30-36 h, then washed and further incubated without riluzole for 24 h in L15 medium to obtain the astrocyte conditioned media (ACM). Motoneuron-enriched cultures were used to test the ability of the ACM to support motoneuron viability. Astrocyte monolayers exposed to 1 microM riluzole did not show changes in morphology or in DNA or protein synthesis. However, the conditioned medium obtained from astrocyte monolayers after this treatment increased motoneuron survival compared to that from vehicle-treated cultures. A similar effect was found when astrocytes were exposed to a higher riluzole concentration (10 microM) but with greater dilutions of the conditioned medium. This trophic activity was abolished by boiling or after treatment with trypsin. These findings strongly suggest the existence of a new trophic mechanism, through which riluzole may exert motoneuron protection.

Glutamate, excitotoxicity and amyotrophic lateral sclerosis

The "glutamate hypothesis" is one of three major pathophysiological mechanisms of motor neurone injury towards which current research effort into amyotrophic lateral sclerosis (ALS) is directed. There is great structural and functional diversity in the glutamate receptor family which results from combinations of 14 known gene products and their splice variants, with or without additional RNA editing. It is possible that motor neurones express a unique molecular profile of glutamate receptors. Abnormal activation of glutamate receptors is one of five main candidates as a final common pathway to neuronal death. In classical acute excitotoxicity, there is influx of Na+ and CI-, and destabilisation of intracellular Ca2+ homeostasis, which activates a cascade of harmful biochemical events. The concept of secondary excitotoxicity, where cellular injury by glutamate is triggered by disturbances in neuronal energy status, may be particularly relevant to a chronic neurodegenerative disease such as ALS. Data are now beginning to emerge on the fine molecular structure of the glutamate receptors present on human motor neurones, which have a distinct profile of AMPA receptors. Two important molecular features of motor neurones have been identified that may contribute to their vulnerability to neurodegeneration. The low expression of calcium binding proteins and the low expression of the GluR2 AMPA receptor subunit by vulnerable motor neurone groups may render them unduly susceptible to calcium-mediated toxic events following glutamate receptor activation. Eight lines of evidence that indicate a disturbance of glutamatergic neurotransmission in ALS patients are reviewed. The links between abnormal activation of glutamate receptors and other potential mechanisms of neuronal injury, including activation of calcium-mediated second messenger systems and free radical mechanisms, are emphasised. Riluzole, which modulates the glutamate neurotransmitter system, has been shown to prolong survival in patients with ALS. Further research may allow the development of subunit-specific therapeutic targeting of glutamate receptors and modulation of "downstream" events within motor neurones, aimed at protecting vulnerable molecular targets in specific populations of ALS patients.

Riluzole, a novel neuroprotective agent, attenuates both neurologic motor and cognitive dysfunction following experimental brain injury in the rat

Several potential mechanisms are involved in mediating the pathophysiology of traumatic brain injury (TBI), including inflammatory processes and excitotoxicity. In the present study, we evaluated the ability of the use-dependent sodium channel inhibitor Riluzole to attenuate cognitive and neurologic motor deficits and reduce regional cerebral edema and histologic cell damage following lateral fluid-percussion (FP) brain injury in rats (n = 109). In study 1, 58 anesthetized male Sprague-Dawley rats (350-400 g) were subjected to FP brain injury of moderate severity (2.3-2.5 atm). Fifteen minutes following brain injury, animals randomly received an i.v. bolus of either Riluzole (4 mg/kg, n = 11), Riluzole (8 mg/kg, n = 11), or glycol vehicle (n = 20), followed by 6 h and 24 h s.c. injections (identical dose). Surgically prepared but uninjured animals received vehicle (n = 16) and served as controls. Animals were evaluated for cognitive deficits at 48 h postinjury and killed for assessment of regional brain edema. Administration of vehicle or Riluzole (4 mg/kg x 3) had no significant effect on memory or edema, whereas Riluzole (8 mg/kg x 3) significantly attenuated post-traumatic cognitive dysfunction (p < 0.05). In study 2, a second group of animals (n = 25) was injured, treated with Riluzole (8 mg/kg x 3 doses, n = 13) or vehicle (n = 12), and evaluated for neurologic motor function over 2 weeks. Animals treated with Riluzole demonstrated significantly improved motor scores beginning 1 week postinjury (p < 0.05). In study 3, brain-injured animals were treated with Riluzole (8 mg/kg x 3 doses, n = 10) or vehicle (n = 10), and posttraumatic lesion volume was assessed at 48 h postinjury using 2,3,5-triphenyltetrazolium chloride (TTC) staining. Treatment with Riluzole had no significant effect on posttraumatic lesion volume. The present study demonstrates that use-dependent sodium channel inhibitors, such as Riluzole, can attenuate both cognitive and neuromotor dysfunction associated with brain trauma.

Riluzole. A review of its pharmacodynamic and pharmacokinetic properties and therapeutic potential in amyotrophic lateral sclerosis

Riluzole, a benzothiazole, affects neurons by 3 mechanisms: by inhibiting excitatory amino acid release, inhibiting events following stimulation of excitatory amino acid receptors and stabilising the inactivated state of voltage-dependent sodium channels. It has demonstrated neuroprotective activity in vivo and in vitro. Results from 2 randomised double-blind placebo-controlled trials in patients with amyotrophic lateral sclerosis (ALS; motor neuron disease) have demonstrated that riluzole can extend survival and/or time to tracheostomy. After 18 months, the relative risk of death or tracheostomy with riluzole 100 mg/day was reduced by 21%. Although riluzole slowed the rate of deterioration in muscle strength in the first trial, this was not confirmed in the second, larger trial. Riluzole had no effect on any other functional or secondary variable. Gastrointestinal effects, anorexia, asthenia, circumoral paraesthesia and dizziness were reported more frequently with riluzole than placebo. Elevated alanine aminotransferase levels were observed in 10.6 versus 3.8% of patients treated with riluzole 100 mg/day versus placebo, leading to treatment withdrawal in 3.8 versus 2.1% of patients. In conclusion, riluzole is the first drug that has been shown to have an effect on survival in patients with ALS. Although the effect of riluzole was modest, it has allowed some insight into the pathogenesis of ALS from which future gains may be made.

Effect of riluzole on quinolinate-induced neuronal damage in rats: comparison with blockers of glutamatergic neurotransmission

Intrastriatal injection of quinolinate, an N-methyl-D-aspartate (NMDA) agonist, induces a local neuronal lesion, and provides an excitotoxic model of Huntington's disease. In this study, we investigated the effect of different agents acting at various levels of the glutamatergic neurotransmission: (i) dizocilpine (MK801) (0.5 mg/kg ip) significantly reduced the lesion by 74%; (ii) 6-(1-imidazolyl)-7-nitroquinoxaline-2,3(1H,4H)-dione (YM-90K) (3 x 10 and 3 x 20 mg/kg ip) and (iii) lamotrigine (50 mg/kg ip) had no effect; (iv) riluzole (4 and 8 mg/kg per os) significantly reduced the lesion by 35%. The inefficiency of YM-90K suggested that alpha-amino-3-hydroxy-5-methylisoxasole-4-propionate (AMPA) receptors do not participate to the quinolinate-induced excitotoxicity. The mechanism of action of riluzole may be related also to a combination of its different properties. This study indicates that riluzole may be useful for treatment of Huntington's disease.