Effects of depolarizing stimuli on calcium homeostasis in cultured rat motoneurones

The Effects of Riluzole on Cisplatin-induced Ototoxicity

Introduction  Riluzole (2-amino-6-trifluoromethoxy benzothiazole) is known as a neuroprotective, antioxidant, antiapoptotic agent. It may have beneficial effects on neuronal cell death due to cisplatin-induced ototoxicity.

Objective  To evaluate the effect of riluzole on cisplatin-induced ototoxicity in guinea pigs.

Methods  Twenty-four guinea pigs, studied in three groups, underwent auditory brainstem response evaluation using click and 8 kHz tone burst stimuli. Subsequently, 5 mg/kg of cisplatin were administered to all animals for 3 days intraperitoneally (i.p.) to induce ototoxicity. Half an hour prior to cisplatin, groups 1, 2 and 3 received 2 ml of saline i.p., 6 mg/kg of riluzole hydrochloride i.p., and 8 mg/kg of riluzole hydrochloride i.p., respectively, for 3 days. The auditory brainstem responses were repeated 24 hours after the last drug administration. The cochleae were analyzed by transmission electron microscopy (TEM).

Results  After drug administiration, for 8,000 Hz stimulus, group 1 had significantly higher threshold shifts when compared with groups 2 ( p  < 0.05) and 3 ( p  < 0.05), and there was no significant difference in threshold shifts between groups 2 and 3 ( p  > 0.05). Transmission electron microscopy findings demonstrated the protective effect of riluzole on the hair cells and the stria vascularis, especially in the group treated with 8 mg/kg of riluzole hydrochloride.

Conclusion  We can say that riluzole may have a protective effect on cisplatin- induced ototoxicity. However, additional studies are needed to confirm these results and the mechanisms of action of riluzole.

Choline and NMDG directly reduce outward currents: reduced outward current when these substances replace Na+ is alone not evidence of Na+-activated K+ currents

Choline chloride is often, and N-methyl-d-glucamine (NMDG) sometimes, used to replace sodium chloride in studies of sodium-activated potassium channels. Given the high concentrations used in sodium replacement protocols, it is essential to test that it is not the replacement substances themselves, as opposed to the lack of sodium, that cause any observed effects. We therefore compared, in lobster stomatogastric neurons and leech Retzius cells, the effects of applying salines in which choline chloride replaced sodium chloride, and in which choline hydroxide or sucrose was added to normal saline. We also tested, in stomatogastric neurons, the effect of adding NMDG to normal saline. These protocols allowed us to measure the direct effects (i.e., effects not due to changes in sodium concentration or saline osmolarity or ionic strength) of choline on stomatogastric and leech currents, and of NMDG on stomatogastric currents. Choline directly reduced transient and sustained depolarization-activated outward currents in both species, and NMDG directly reduced transient depolarization-activated outward currents in stomatogastric neurons. Experiments with lower choline concentrations showed that adding as little as 150 mM (stomatogastric) or 5 mM (leech) choline reduced at least some depolarization-activated outward currents. Reductions in outward current with choline chloride or NMDG replacement alone are thus not evidence of sodium-activated potassium currents. NEW & NOTEWORTHY We show that choline or N-methyl-d-glucamine (NMDG) directly (i.e., not due to changes in extracellular sodium) decrease outward currents. Prior work studying sodium-activated potassium channels in which sodium was replaced with choline or NMDG without an addition control may therefore be artifactual.

Riluzole But Not Melatonin Ameliorates Acute Motor Neuron Degeneration and Moderately Inhibits SOD1-Mediated Excitotoxicity Induced Disrupted Mitochondrial Ca2+ Signaling in Amyotrophic Lateral Sclerosis

Selective motoneurons (MNs) degeneration in the brain stem, hypoglossal motoneurons (HMNs), and the spinal cord resulting in patients paralysis and eventual death are prominent features of amyotrophic lateral sclerosis (ALS). Previous studies have suggested that mitochondrial respiratory impairment, low Ca²⁺ buffering and homeostasis and excitotoxicity are the pathological phenotypes found in mice, and cell culture models of familial ALS (fALS) linked with Cu/Zn-superoxide dismutase 1 (SOD1) mutation. In our study, we aimed to understand the impact of riluzole and melatonin on excitotoxicity, neuronal protection and Ca²⁺ signaling in individual HMNs ex vivo in symptomatic adult ALS mouse brain stem slice preparations and in WT and SOD1-G93A transfected SH-SY5Y neuroblastoma cell line using fluorescence microscopy, calcium imaging with high speed charged coupled device camera, together with immunohistochemistry, cell survival assay and histology. In our experiments, riluzole but not melatonin ameliorates MNs degeneration and moderately inhibit excitotoxicity and cell death in SH-SY5Y^WT or SH-SY5Y^G93A cell lines induced by complex IV blocker sodium azide. In brain stem slice preparations, riluzole significantly inhibit HMNs cell death induced by inhibiting the mitochondrial electron transport chain by Na-azide. In the HMNs of brainstem slice prepared from adult (14–15 weeks) WT, and corresponding symptomatic SOD1^G93A mice, we measured the effect of riluzole and melatonin on [Ca²⁺]_i using fura-2 AM ratiometric calcium imaging in individual MNs. Riluzole caused a significant decrease in [Ca²⁺]_i transients and reversibly inhibited [Ca²⁺]_i transients in Fura-2 AM loaded HMNs exposed to Na-azide in adult symptomatic SOD1^G93A mice. On the contrary, melatonin failed to show similar effects in the HMNs of WT and SOD1^G93A mice. Intrinsic nicotinamide adenine dinucleotide (NADH) fluorescence, an indicator of mitochondrial metabolism and health in MNs, showed enhanced intrinsic NADH fluorescence in HMNs in presence of riluzole when respiratory chain activity was inhibited by Na-azide. Riluzole’s inhibition of excitability and Ca²⁺ signaling may be due to its multiple effects on cellular function of mitochondria. Therefore formulating a drug therapy to stabilize mitochondria-related signaling pathways using riluzole might be a valuable approach for cell death protection in ALS. Taken together, the pharmacological profiles of the riluzole and melatonin strengthen the case that riluzole indeed can be used as a therapeutic agent in ALS whereas claims of the efficacy of melatonin alone need further investigation as it fail to show significant neuroprotection efficacy.

Pre- and postsynaptic mechanisms underlying inhibition of hypoglossal motor neuron excitability by riluzole

Riluzole is the sole treatment for amyotrophic lateral sclerosis (ALS), but its therapeutically relevant actions on motor neurons are not well defined. Whole cell patch-clamp recordings were made from hypoglossal motor neurons (HMs, n = 25) in brain stem slices from 10- to 23-day-old rats anesthetized with pentobarbital sodium to investigate the hypothesis that riluzole inhibits HMs by multiple mechanisms. Riluzole (20 μM) hyperpolarized HMs by decreasing an inward current, inhibited voltage-gated persistent Na(+) and Ca(2+) currents activated by slow voltage ramps, and negatively shifted activation of the hyperpolarization-activated cationic current (IH). Repetitive firing of HMs was strongly inhibited by riluzole, which also increased action potential threshold voltage and rheobase and decreased amplitude and maximum rise slope but did not alter the maximal afterhyperpolarization amplitude or decay time constant. HM rheobase was inversely correlated with persistent Na(+) current density. Glutamatergic synaptic transmission was inhibited by riluzole by both pre- and postsynaptic effects. Riluzole decreased activity-dependent glutamate release, as shown by decreased amplitude of evoked and spontaneous excitatory postsynaptic currents (EPSCs), decreased paired-pulse ratio, and decreased spontaneous, but not miniature, EPSC frequency. However, riluzole also decreased miniature EPSC amplitude and the inward current evoked by local application of glutamate onto HMs, suggesting a reduction of postsynaptic glutamate receptor sensitivity. Riluzole thus has a marked inhibitory effect on HM activity by membrane hyperpolarization, decreasing firing and inhibiting glutamatergic excitation by both pre- and postsynaptic mechanisms. These results broaden the range of mechanisms controlling motor neuron inhibition by riluzole and are relevant to researchers and clinicians interested in understanding ALS pathogenesis and treatment.

A review of the neural mechanisms of action and clinical efficiency of riluzole in treating amyotrophic lateral sclerosis: what have we learned in the last decade?

Amyotrophic lateral sclerosis (ALS) is a devastating and fatal neurodegenerative disease of adults which preferentially attacks the neuromotor system. Riluzole has been used as the only approved treatment for amyotrophic lateral sclerosis since 1995, but its mechanism(s) of action in slowing the progression of this disease remain obscure. Searching PubMed for "riluzole" found 705 articles published between January 1996 and June 2009. A systematic review of this literature found that riluzole had a wide range of effects on factors influencing neural activity in general, and the neuromotor system in particular. These effects occurred over a large dose range (<1 μM to >1 mM). Reported neural effects of riluzole included (in approximate ascending order of dose range): inhibition of persistent Na(+) current = inhibition of repetitive firing < potentiation of calcium-dependent K(+) current < inhibition of neurotransmitter release < inhibition of fast Na(+) current < inhibition of voltage-gated Ca(2+) current = promotion of neuronal survival or growth factors < inhibition of voltage-gated K(+) current = modulation of two-pore K(+) current = modulation of ligand-gated neurotransmitter receptors = potentiation of glutamate transporters. Only the first four of these effects commonly occurred at clinically relevant concentrations of riluzole (plasma levels of 1-2 μM with three- to four-fold higher concentrations in brain tissue). Treatment of human ALS patients or transgenic rodent models of ALS with riluzole most commonly produced a modest but significant extension of lifespan. Riluzole treatment was well tolerated in humans and animals. In animals, despite in vitro evidence that riluzole may inhibit rhythmic motor behaviors, in vivo administration of riluzole produced relatively minor effects on normal respiration parameters, but inhibited hypoxia-induced gasping. This effect may have implications for the management of hypoventilation and sleep-disordered breathing during end-stage ALS in humans.

Effect of riluzole on Ca2+ movement and cytotoxicity in Madin-Darby canine kidney cells

Riluzole is a drug used in the treatment of amyotrophic lateral sclerosis; however, its in vitro action is unclear. In this study, the effect of riluzole on intracellular Ca2+ concentration ([Ca2+]i) in Madin-Darby canine kidney (MDCK) cells was investigated using the Ca2+ -sensitive fluorescent dye, fura-2. Riluzole (100-500 microM) caused a rapid and sustained increase of [Ca2+]i in a concentration-dependent manner (EC50 = 150 microM). Some 40 and 50% of this [Ca2+]i increase was prevented by the removal of extracellular Ca2+ and the addition of La3+, respectively, but was unchanged by dihydropyridines, verapamil and diltiazem. In Ca2+ -free medium, thapsigargin - an inhibitor of the endoplasmic reticulum (ER) Caz+ -ATPase--caused a monophasic [Ca2+]i increase, after which the increasing effect of riluzole on [Ca2+]i was attenuated by 70%; in addition, pre-treatment with riluzole abolished thapsigargin-induced [Ca2+]i increases. U73122, an inhibitor of phospholipase C (PLC), abolished ATP (but not riluzole)-induced [Ca2+]i increases. At concentrations of 250 and 500 microM, riluzole killed 40 and 95% cells, respectively. The cytotoxic effect of riluzole (250 microM) was unaltered by pre-chelating cytosolic Ca2+ with BAPTA. Collectively, in MDCK cells, riluzole rapidly increased [Ca2+]i by stimulating extracellular Ca2+ influx via an La3+ -sensitive pathway and intracellular Ca2+ release from the ER via, as yet, unidentified mechanisms. Furthermore, riluzole caused Ca2+ -unrelated cytotoxicity in a concentration-dependent manner.

Glutamate modulators as novel interventions for mood disorders

Recent evidence suggests that critical molecules in neurotrophic signaling cascades are long-term targets for currently available monoaminergic antidepressants. As chronic and severe mood disorders are characterized by impairments in neuronal resilience, pharmacological strategies that subserve a neuroprotective function might alter disorder pathophysiology and modify disease progression. Several promising approaches involve modulation of the glutamate neurotransmitter system, via post-synaptic receptor blockade or potentiation and presynaptic vesicular release inhibition. A focused review of the extant scientific literature was conducted, with a discussion of 3 compounds or classes of drugs currently undergoing clinical investigation: ketamine, riluzole, and AMPA receptor potentiators. Recent investigations in mood disordered patients suggest that the NMDA receptor antagonist ketamine might demonstrate rapid antidepressant properties. Riluzole has been shown to reverse glutamate-mediated impairments in neuronal plasticity and to stimulate the synthesis of brain derived neurotrophic factor. Open-label trials in treatment-resistant depression have yielded promising results. Likewise, AMPA receptor potentiators favorably impact neurotrophic factors as well as enhance cognition.

CONCLUSIONS

Pharmacological approaches that modulate components of the glutamate system offer novel targets for severe, recurrent mood disorders. Controlled studies are necessary.

Optical assessment of motoneuron function in a “twenty-four-hour” acute spinal cord slice model from fetal rats

In acute slice preparations of most brain regions, neuronal functions are preserved for only few hours. Since the effects of growth factors or neurotoxic agents are often manifested beyond this time scale, corresponding studies are typically performed on cultured cells. However, cell cultures are generated and maintained under vastly different conditions that can grossly alter neuronal properties. For example, glutamate application to motoneuronal cultures has been reported to modulate neurite formation in some studies while in others it has been reported to kill cells. Here, we have examined whether acute spinal cord slices from rat fetuses can be used within a time window of 24 h for assessment of long-term effects of neuromodulators. In these slices, we have studied the action of glutamate on lumbar motoneurons loaded with fura-2 and rhodamine-123 to monitor intracellular Ca2+ ([Ca2+]i) and mitochondrial potential (Deltapsi), respectively. Further, loading with fura-2 or propidium iodide allowed for morphological assessment of cell viability and death, respectively. Pulses (15 s) or 1 h application of glutamate (300 microM) evoked a moderate (approximately 500 nM) [Ca2+]i rise, but no change of Deltapsi. Even after 24 h, no glutamate-induced cell death was observed and glutamate pulse-evoked [Ca2+]i transients were comparable to controls. The data demonstrate that glutamate does not deregulate [Ca2+]i homeostasis in fetal motoneurons in situ. We propose that acute spinal cord slices from perinatal rodents are a robust model that allows for analysis of neuronal properties and cell viability within a time window of at least 24 h.

Emerging experimental therapeutics for bipolar disorder: clues from the molecular pathophysiology

Bipolar affective disorder (manic-depressive illness) is a common, severe, chronic, and often life-threatening illness, associated with significant comorbidity. The recognition of the significant morbidity and mortality of patients with bipolar disorder, as well as the growing appreciation that a high percentage of patients respond poorly to existing treatments, has made the task of discovering new therapeutic agents, that are both efficacious and have few side effects increasingly more important. Most recent agents introduced into the pharmacopeia for the treatment of bipolar disorder have been anticonvulsants and atypical antipsychotics. We propose that novel treatments developed specifically for bipolar disorder will arise from (1) understanding more precisely the molecular mechanisms of treatments that are clearly efficacious or (2) developing medications based on the knowledge obtained of the underlying pathophysiology of bipolar disorder. Knowledge with regard to the underlying pathophysiology of bipolar disorder is increasing at a rapid pace, including alterations in intracellular signaling cascades as well as impairments of cellular plasticity and resilience in critical neuronal circuits. We propose that therapeutics designed to enhance cellular plasticity and resilience and that counter maladaptive stress-responsive systems may have considerable utility for the treatment of bipolar disorder. Therapeutic strategies designed to address cellular resilience and plasticity include the regulation of neurotrophic pathways, glucocorticoid signaling, phosphodiesterase activity, and glutamatergic throughput and mitochondrial function. While the task of developing novel medications for bipolar disorder is truly daunting, these and similar approaches will ultimately lead to better medications for the millions who suffer from this devastating illness.

Activation of P-type calcium channel regulates a unique thapsigargin-sensitive calcium pool in embryonic motoneurons

By regulating voltage-dependent Ca2+ influx and intracellular Ca2+ homeostasis, electrical activity plays a central role in motoneuron development. Dissociated cultures of purified embryonic rat motoneurons were used to explore the molecular mechanisms by which Ca2+ influx control [Ca2+]i transients in these neurons. Thapsigargin (250 nm) and cyclopiazonic acid (10 micro m), which deplete Ca2+ stores in the endoplasmic reticulum, decrease by 30% the depolarization-induced [Ca2+]i transients in motoneurons without affecting voltage-activated calcium currents. This thapsigargin-sensitive intracellular Ca2+ pool differs from other previous described Ca2+ stores that are sensitive to ryanodine or caffeine, inositol triphosphate, insulin and from mitochondrial Ca2+ pools. Thapsigargin affected the Cav2.1 P-type Ca2+ channel component of the depolarization-induced [Ca2+]i transient in motoneurons but spared [Ca2+]i transient induced by Cav1 L-type and Cav2.2 N-type Ca2+ channel components, suggesting a close functional relationship between Cav2.1 subunit and this unique thapsigargin-sensitive Ca2+ store. Altogether the present results demonstrate a new pathway, used by embryonic motoneurons, to regulate Ca2+ signalling through voltage-activated (Cav2.1) Ca2+ channels.

Regulation of cellular plasticity cascades in the pathophysiology and treatment of mood disorders: role of the glutamatergic system

There is increasing evidence from a variety of sources that mood disorders are associated with regional reductions in brain volume, as well as reductions in the number, size, and density of glia and neurons in discrete brain areas. Although the precise pathophysiology underlying these morphometric changes remains to be fully elucidated, the data suggest that severe mood disorders are associated with impairments of structural plasticity and cellular resilience. In this context, it is noteworthy that a growing body of data suggests that the glutamatergic system--which is known to play a major role in neuronal plasticity and cellular resilience--may be involved in the pathophysiology and treatment of mood disorders. Preclinical studies have shown that the glutamatergic system represents targets (often indirect) for the actions of antidepressants and mood stabilizers. There are a number of glutamatergic "plasticity enhancing" strategies that may be of considerable utility in the treatment of mood disorders. Among the most immediate ones are NMDA antagonists, inhibitors of glutamate-release agents, and AMPA potentiators; this research progress holds much promise for the development of novel therapeutics for the treatment of severe, refractory mood disorders.

AMPA-evoked acetylcholine release from cultured spinal cord motoneurons and its inhibition by GABA and glycine

The release of [(3)H]acetylcholine evoked by alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) and its inhibition mediated by GABA(A) and glycine receptors were studied in superfused cultured rat embryo spinal cord motoneurons prelabeled with [(3)H]choline. AMPA elicited tritium release, possibly representing [(3)H]acetylcholine release in a concentration-dependent manner. The release was external Ca(2+)-dependent and was sensitive to Cd(2+) ions, omega-conotoxin GVIA and omega-conotoxin MVIIC, but not to nifedipine, suggesting the involvement of N-, P/Q-, but not L-type Ca(2+) channels. The AMPA effect was insensitive to tetrodotoxin. The glutamate receptors involved are AMPA type since the AMPA-evoked [(3)H]acetylcholine release was blocked by LY303070 and was potentiated by the antidesensitizing agent cyclothiazide. Muscimol inhibited completely the AMPA effect on [(3)H]acetylcholine release; muscimol was potentiated by diazepam and antagonized by SR95531, indicating the involvement of benzodiazepine-sensitive GABA(A) receptors. Glycine, acting at strychnine-sensitive receptors, also inhibited the effect of AMPA, but only in part. The inhibitory effects of muscimol and glycine are additive. We conclude that glutamate can act at AMPA receptors sited on spinal motoneurons to evoke release of acetylcholine. GABA and glycine, possibly released as cotransmitters from spinal interneurons, inhibit glutamate-evoked acetylcholine release by activating GABA(A) and glycine receptors on motoneurons.

Intracellular activity of rat spinal cord motoneurons in slices

Using a modification of Aghajanian and Rasmussen's techniques, we have developed an adult rat cervical spinal cord slice preparation in which motoneurons remain viable. Key factors are replacement of all sodium ions in the perfusion medium with sucrose during cutting and incubation, and gentle manipulation of the tissues to prevent root damage during removal. Intracellular recordings were confirmed as motoneuronal by intracellular injection of Lucifer yellow, allowing visualization of dendrites and cell body, and showing an axonal bleb at the cut end in the ventral root. Over 50 neurons were recorded for periods of between 30 min and 4 h. Cervical motoneurons (n=10) had an average membrane potential of -62 mV, average input resistance of 24 M(Omega), and showed no spontaneous activity. Ionophoresis application of the glutamate agonists, AMPA and NMDA, revealed potent excitation by AMPA but little or no response to NMDA. While NMDA receptors reportedly are prominent in developing rodent motoneurons, these observations indicate otherwise in the adult. Upon prolonged ionophoresis, or bath application, depolarizing responses to AMPA led to depolarization and spike inactivation that was often irreversible. The apparent lack of desensitization of AMPA responses, usually seen in other neurons, may underlie the unique vulnerability of motoneurons to excitotoxic damage.

Effect of riluzole on the acquisition and expression of amygdala kindling

PURPOSE

Riluzole possesses various synaptic effects including an inhibitory action on glutamate release. The drug has been shown to inhibit kindled seizures, while its effect on the acquisition of kindling has not been reported. We investigated effects of riluzole on the kindling development in addition to effects on kindled seizures.

METHODS

A bipolar electrode was implanted in the right amygdala of rats. Riluzole was injected intraperitoneally 30 min before kindling stimulation. To investigate effects of riluzole on the kindling development, rats were stimulated once daily for the drug session of 14 days at a current of 200 microA, 60 Hz, 1 ms for 2 s and thereafter stimulated without drugs (drug-free session) until completion of kindling. Seizure ranks and after discharge duration were observed every day. To investigate effects of riluzole on kindled seizures, fully-kindled rats were stimulated at the current of generalized seizure threshold (GST) before and after the administration of riluzole. Seizure ranks and after discharge duration were measured. GST after the treatment was also determined.

RESULTS

The number of stimuli required for the first appearance of stage five seizure was significantly larger in rats treated with 8 mg/kg of riluzole than in vehicle controls. Riluzole at a dose of 8 mg/kg significantly retarded the development of seizure stages in the drug session. By comparison, effects on the duration of after discharge was relatively mild, though significantly different from the vehicle control. Riluzole at a dose of either 4 or 8 mg/kg markedly inhibited behavioral seizures and reduced the duration of after discharge in kindled seizures provoked by GST. The drug also significantly increased GST at both doses, suggesting that the anticonvulsant effects were attributed to the increase in GST.

CONCLUSION

It was demonstrated that inhibitory effects of riluzole on both kindled seizures and the development of behavioral seizures in kindling acquisition with relatively mild correlation to afterdischarge duration. These effects might be attributed to inhibitory actions of riluzole on glutamate release and NMDA-receptor mediated events.

The effect of riluzole treatment in rats on the survival of injured adult and grafted embryonic motoneurons

The effect of riluzole on the survival of injured motoneurons was studied. The L4 ventral root was avulsed and reimplanted into the spinal cord. Immediately after the operation, 4 animals were treated with riluzole for 3 weeks while another 4 animals received no treatment after the operation. Three months later the fluorescent dyes, Fast Blue and Diamidino Yellow, were applied to the cut ventral ramus of the L4 spinal nerve, for retrograde labelling of neurons. Three days later, the spinal cords were processed to reveal the retrograde-labelled cells. In untreated animals, there were 20 +/- 2.1 labelled neurons (+/- SEM), while in animals treated with riluzole there were 723 +/- 26. Thus, treatment with riluzole dramatically enhanced the survival of injured motoneurons. In another series of experiments, after avulsion of the L4 ventral root and its reinsertion, embryonic spinal cord pieces were grafted into the host cord. Five animals received riluzole treatment and 4 were left untreated. In the untreated animals, 125 +/- 5.1 retrograde-labelled cells of both graft and host origin were detected. In rats treated with riluzole, 645 +/- 35.7 retrograde-labelled cells were seen and almost all of these were of host origin. Thus, treatment with riluzole enhanced the survival of injured host motoneurons, and by doing so, (i) reduced the ability of grafted neurons to extend their axons into the reimplanted L4 ventral root, and (ii) reduced the survival of the grafted cells.

NMDA and AMPA/kainate receptors are involved in the anticonvulsant activity of riluzole in DBA/2 mice

The anticonvulsant activity of riluzole against sound-induced seizures was studied in the DBA/2 mouse model. Riluzole (0.1-4 mg kg(-1), intraperitoneal (i.p.)) produced dose-dependent effects with ED(50) values for the suppression of tonic, clonic and wild running phases of 0.72, 1.38 and 2.71 mg kg(-1), respectively. Riluzole also protected DBA/2 mice from seizures induced by an intracerebroventricular (i.c.v.) injection of N-methyl-D-aspartate (NMDA) with ED(50) values of 3.03 and 5.0 mg kg(-1) for tonus and clonus, respectively. Pretreatment with glycine, an agonist to the glycine/NMDA receptors, shifted the dose-response effect of riluzole to the right (ED(50)=6.53 against tonus and 9.34 mg kg(-1) vs. clonus). Similarly, D-serine, an agonist at the glycine site, shifted the ED(50) of riluzole against the tonic component of audiogenic seizures from 0.72 to 1.97, and that against clonus from 1.38 to 2.77 mg kg(-1). Riluzole was also potent to prevent seizures induced by administration of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), an AMPA/kainate receptor agonist (ED(50)=1.80 and 3.35 mg kg(-1), against tonus and clonus, respectively). Pretreatment with aniracetam, a positive allosteric modulator of AMPA/kainate receptors, shifted the dose-response curve of riluzole to the right (ED(50)=1.78 against tonus and 2.58 mg kg(-1) vs. clonus). The data indicate that riluzole is an effective anticonvulsant drug in the genetic model of seizure-prone DBA/2 mice. Our findings suggest that the anticonvulsant properties of riluzole depend upon its interaction with neurotransmission mediated by both the glycine/NMDA and the AMPA/kainate receptor complex.

Contribution of Ca(2+)-permeable AMPA/KA receptors to glutamate-induced Ca(2+) rise in embryonic lumbar motoneurons in situ

Intracellular Ca(2+) ([Ca(2+)](i)) was fluorometrically measured with fura-2 in lumbar motoneurons of acutely isolated spinal cord slices from embryonic rats. In ester-loaded cells, bath-applied glutamate (3 microM to 1 mM) evoked a [Ca(2+)](i) increase by up to 250 nM that was abolished by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) plus 2-amino-5-phosphonovalerate (APV). CNQX or APV alone reduced the response by 82 and 25%, respectively. The glutamatergic agonists kainate (KA), quisqualate (QUI), and S-alpha-amino-3-hydroxy-5-methyl-4-isoxalone (S-AMPA) evoked a similar [Ca(2+)](i) transient as glutamate. N-methyl-D-aspartate (NMDA) was only effective to increase [Ca(2+)](i) in Mg(2+)-free saline, whereas [1S,3R]-1-aminocyclopentane-1,3-dicarboxylic acid ([1S,3R]-ACPD) had no effect. The glutamate-induced [Ca(2+)](i) rise was suppressed in Ca(2+)-free superfusate. Depletion of Ca(2+) stores with cyclopiazonic acid (CPA) did not affect the response. Thirty-six percent of the [Ca(2+)](i) increase in response to membrane depolarization induced by a 50 mM K(+) solution persisted on combined application of the voltage-gated Ca(2+) channel blockers nifedipine, omega-conotoxin-GVIA and omega-agatoxin-IVA. In fura-2 dialyzed motoneurons, the glutamate-induced [Ca(2+)](i) increase was attenuated by approximately 70% after changing from current to voltage clamp. Forty percent of the remaining [Ca(2+)](i) transient and 20% of the concomitant inward current of 0.3 nA were blocked by Joro spider toxin-3 (JSTX). The results show that voltage-gated Ca(2+) channels, including a major portion of R-type channels, constitute the predominant component of glutamate-induced [Ca(2+)](i) rises. NMDA and Ca(2+)-permeable KA/AMPA receptors contribute about equally to the remaining component of the Ca(2+) rise. The results substantiate previous assumptions that Ca(2+) influx through JSTX-sensitive KA/AMPA receptors is involved in (trophic) signaling in developing motoneurons.

The role of excitotoxicity in neurodegenerative disease: implications for therapy

Glutamic acid is the principal excitatory neurotransmitter in the mammalian central nervous system. Glutamic acid binds to a variety of excitatory amino acid receptors, which are ligand-gated ion channels. It is activation of these receptors that leads to depolarisation and neuronal excitation. In normal synaptic functioning, activation of excitatory amino acid receptors is transitory. However, if, for any reason, receptor activation becomes excessive or prolonged, the target neurones become damaged and eventually die. This process of neuronal death is called excitotoxicity and appears to involve sustained elevations of intracellular calcium levels. Impairment of neuronal energy metabolism may sensitise neurones to excitotoxic cell death. The principle of excitotoxicity has been well-established experimentally, both in in vitro systems and in vivo, following administration of excitatory amino acids into the nervous system. A role for excitotoxicity in the aetiology or progression of several human neurodegenerative diseases has been proposed, which has stimulated much research recently. This has led to the hope that compounds that interfere with glutamatergic neurotransmission may be of clinical benefit in treating such diseases. However, except in the case of a few very rare conditions, direct evidence for a pathogenic role for excitotoxicity in neurological disease is missing. Much attention has been directed at obtaining evidence for a role for excitotoxicity in the neurological sequelae of stroke, and there now seems to be little doubt that such a process is indeed a determining factor in the extent of the lesions observed. Several clinical trials have evaluated the potential of antiglutamate drugs to improve outcome following acute ischaemic stroke, but to date, the results of these have been disappointing. In amyotrophic lateral sclerosis, neurolathyrism, and human immunodeficiency virus dementia complex, several lines of circumstantial evidence suggest that excitotoxicity may contribute to the pathogenic process. An antiglutamate drug, riluzole, recently has been shown to provide some therapeutic benefit in the treatment of amyotrophic lateral sclerosis. Parkinson's disease and Huntington's disease are examples of neurodegenerative diseases where mitochondrial dysfunction may sensitise specific populations of neurones to excitotoxicity from synaptic glutamic acid. The first clinical trials aimed at providing neuroprotection with antiglutamate drugs are currently in progress for these two diseases.