Association of c-fos mRNA expression and excitotoxicity in primary cultures of mouse neocortical and cerebellar neurons

Age-dependent neurological phenotypes in a mouse model of PRRT2-related diseases

Paroxysmal kinesigenic dyskinesia is an episodic movement disorder caused by dominant mutations in the proline-rich transmembrane protein PRRT2, with onset in childhood and typically with improvement or resolution by middle age. Mutations in the same gene may also cause benign infantile seizures, which begin in the first year of life and typically remit by the age of 2 years. Many details of PRRT2 function at the synapse, and the effects of mutations on neuronal excitability in the pathophysiology of epilepsy and dyskinesia, have emerged through the work of several groups over the last decade. However, the age dependence of the phenotypes has not been explored in detail in transgenic models. Here, we report our findings in heterozygous and homozygous Prrt2 knockout mice that recapitulate the age dependence of dyskinesia seen in the human disease. We show that Prrt2 deletion reduces the levels of synaptic proteins in a dose-dependent manner that is most pronounced at postnatal day 5 (P5), attenuates at P60, and disappears by P180. In a test for foot slippage while crossing a balance beam, transient loss of coordination was most pronounced at P60 and less prominent at age extremes. Slower traverse time was noted in homozygous knockout mice only, consistent with the ataxia seen in rare individuals with biallelic loss of function mutations in Prrt2. We thus identify three age-dependent phenotypic windows in the mouse model, which recapitulate the pattern seen in humans with PRRT2-related diseases.

Ameliorating effects of histamine H3 receptor antagonist E177 on acute pentylenetetrazole-induced memory impairments in rats

Histamine H3 receptors (H3Rs) are involved in several neuropsychiatric diseases including epilepsy. Therefore, the effects of H3R antagonist E177 (5 and 10 mg/kg, intraperitoneal (i.p.)) were evaluated on acute pentylenetetrazole (PTZ)-induced memory impairments, oxidative stress levels (glutathione (GSH), malondialdehyde (MDA), catalase (CAT), and superoxide dismutase (SOD)), various brain neurotransmitters (histamine (HA), acetylcholine (ACh), γ-aminobutyric acid (GABA)), and glutamate (Glu), acetylcholine esterase (AChE) activity, and c-fos protein expression in rats. E177 (5 and 10 mg/kg, i.p.) significantly prolonged step-through latency (STL) time in single-trial passive avoidance paradigm (STPAP), and shortened transfer latency time (TLT) in elevated plus maze paradigm (EPMP) (all P < 0.05). Moreover, and in the hippocampus of PTZ-treated animals, E177 mitigated abnormal levels of AChE activity, ACh and HA (all P < 0.05), but failed to modify brain levels of GABA and Glu. Furthermore, E177 alleviated hippocampal oxidative stress by significantly decreasing the elevated levels of MDA, and increasing the abnormally decreased level of GSH (all P < 0.05). Furthermore, E177 reduced elevated levels of hippocampal c-fos protein expression in hippocampal tissues of PTZ-treated animals (all P < 0.05). The observed results propose the potential of H3R antagonist E177 with an added advantage of avoiding cognitive impairment, emphasizing the H3Rs as a prospective target for future pharmacological management of epilepsy with associated memory impairments.

Atipamezole Attenuated Telazol/Xylazine-Induced Expression of C-Fos in Rat Thalamencephal and Cerebral Cortex

The aim of this study was to assess whether atipamezole can restrain telazol/xylazine induced expression of c-fos in the rat brain. Rats were injected with a mixture of 13.81 mg/kg telazol and 5.21 mg/kg xylazine, following 10 min later 0.522 mg/kg atipamezole. Thereon, the thalamencephal and cerebral cortex were removed one hour after the last injection. The level of Fos protein was measured in the brain tissue by Westernblot. The results revealed that atipamezole attenuates telazol/xylazine induction of c-fos expression in the thalamencephal and cerebral cortex. The results indicated that atipamezole is able to inhibit telazol/xylazine-induced c-fos expression in the rat brain, thus protecting it from nerve damage.

Suppression of c-Fos protein and mRNA expression in pentylenetetrazole-induced kindled mouse brain by isoxylitones

An early immediate gene c-fos has been proposed as the gene responsible for turning on molecular events that might underlie the long-term neural changes occurring during kindling. We have evaluated the effects of novel anticonvulsant isomeric compounds isoxylitones [(E/Z)-2-propanone-1,3,5,5-trimethyl-2-cyclohexen-1-ylidine] on the c-Fos protein and mRNA expression in the brain samples of kindled mice and compared it with the normal and untreated kindled groups. Kindling was induced in male NMRI mice by repeated administration of sub-convulsive dose (50 mg/kg) of pentylenetetrazole (PTZ) until a seizure score of 4-5 was achieved. The c-Fos expression was quantified by combination of immunohistochemistry and RT-PCR protocols. Both the immunohistochemical and RT-PCR analysis revealed a marked increase in the expression of c-fos mRNA and protein in the brain regions tested in case of PTZ-kindled control group compared to normal control. In contrast, the isoxylitone (30 mg/kg)-treated group demonstrated significant reduction of c-Fos expression compared to PTZ-kindled control animals. However, low expression of c-fos mRNA was only detected in the thalamus of the isoxylitone-treated brain samples. Based on these observations, we suggest that isoxylitones may have the capacity to control the seizure pattern by mechanism such as the suppression of c-Fos protein and mRNA levels in different regions of the brain. Further investigations to explore the mechanism of action of these compounds are under process.

Effect of γ-glutamylcysteine ethylester on the levels of c-fos mRNA expression, glutathione and reactive oxygen species formation in kainic acid excitotoxicity

Objectives

The aim of this study was to investigate the effect of γ-glutamylcysteine ethylester (GCEE), a precursor of glutathione biosynthesis, on the levels of glutathione, formation of reactive oxygen species and c-fos mRNA expression in rat hippocampus and cortex in kainic acid-induced excitotoxicity.

Methods

Sprague–Dawley rats were used and divided into four groups: control, kainic acid (10 mg/kg), GCEE (10 mg/kg) and kainic acid (10 mg/kg) + GCEE (10 mg/kg). Kainic acid and GCEE were administered to the rats intraperitoneally. The levels of glutathione and the expressions of c-fos mRNA in hippocampus and cortex tissues were determined using spectrophotometric and reverse transcription followed real-time PCR methods, respectively. Formation of reactive oxygen species was determined using dichlorofluorescin fluorescence in brain synaptosomes treated with kainic acid or GCEE in vitro.

Key findings

Kainic acid treatment significiantly upregulated the expression of c-fos mRNA in the hippocampus and cortex when compared to the control group. GCEE treatment significantly decreased the levels of c-fos mRNA in the cortex when compared to the kainic acid-treated group. GCEE treatment against kainic acid significantly increased the levels of glutathione in the cortex and hippocampus, and decreased the levels of formation of reactive oxygen species when compared to kainic acid-treated synaptosomes.

Conclusions

The increased levels of glutathione and the reduced levels of reactive oxygen species formation lead us to conclude that GCEE may be beneficial as a potential antioxidant against neurodegenerative processes where excitotoxicity is involved.

Transcriptomic responses in mouse brain exposed to chronic excess of the neurotransmitter glutamate

Background

Increases during aging in extracellular levels of glutamate (Glu), the major excitatory neurotransmitter in the brain, may be linked to chronic neurodegenerative diseases. Little is known about the molecular responses of neurons to chronic, moderate increases in Glu levels. Genome-wide gene expression in brain hippocampus was examined in a unique transgenic (Tg) mouse model that exhibits moderate Glu hyperactivity throughout the lifespan, the neuronal Glutamate dehydrogenase (Glud1) mouse, and littermate 9 month-old wild type mice.

Results

Integrated bioinformatic analyses on transcriptomic data were used to identify bio-functions, pathways and gene networks underlying neuronal responses to increased Glu synaptic release. Bio-functions and pathways up-regulated in Tg mice were those associated with oxidative stress, cell injury, inflammation, nervous system development, neuronal growth, and synaptic transmission. Increased gene expression in these functions and pathways indicated apparent compensatory responses offering protection against stress, promoting growth of neuronal processes (neurites) and re-establishment of synapses. The transcription of a key gene in the neurite growth network, the kinase Ptk2b, was significantly up-regulated in Tg mice as was the activated (phosphorylated) form of the protein. In addition to genes related to neurite growth and synaptic development, those associated with neuronal vesicle trafficking in the Huntington's disease signalling pathway, were also up-regulated.

Conclusions

This is the first study attempting to define neuronal gene expression patterns in response to chronic, endogenous Glu hyperactivity at brain synapses. The patterns observed were characterized by a combination of responses to stress and stimulation of nerve growth, intracellular transport and recovery.

Increased c-fos expression in the central nucleus of the amygdala and enhancement of cued fear memory in Dyt1 DeltaGAG knock-in mice

DYT1 dystonia is caused by a trinucleotide deletion of GAG (DeltaGAG) in DYT1, which codes for torsinA. A previous epidemiologic study suggested an association of DYT1 DeltaGAG mutation with early-onset recurrent major depression. However, another study reported no significant association with depression, but instead showed an association with anxiety and dystonia. In this study, we analyzed these related behaviors in Dyt1 DeltaGAG heterozygous knock-in mice. The knock-in mice showed a subtle anxiety-like behavior but did not show depression-like behaviors. The mutant mice also displayed normal sensorimotor gating function in a prepulse inhibition test. While normal hippocampus-dependent contextual fear memory and hippocampal CA1 long-term potentiation (LTP) were observed, the knock-in mice exhibited an enhancement in the formation of cued fear memories. Anatomical analysis indicated that the number of c-fos positive cells was significantly increased while the size of the central nucleus of the amygdala (CE) was significantly reduced in the knock-in mice. These results suggest that the Dyt1 DeltaGAG mutation increased the activity of the CE and enhanced the acquisition of the cued fear memory.

Sodium channel activation augments NMDA receptor function and promotes neurite outgrowth in immature cerebrocortical neurons

A range of extrinsic signals, including afferent activity, affect neuronal growth and plasticity. Neuronal activity regulates intracellular Ca(2+), and activity-dependent calcium signaling has been shown to regulate dendritic growth and branching (Konur and Ghosh, 2005). NMDA receptor (NMDAR) stimulation of Ca(2+)/calmodulin-dependent protein kinase signaling cascades has, moreover, been demonstrated to regulate neurite/axonal outgrowth (Wayman et al., 2004). We used a sodium channel activator, brevetoxin (PbTx-2), to explore the relationship between intracellular [Na(+)] and NMDAR-dependent development. PbTx-2 alone, at a concentration of 30 nM, did not affect Ca(2+) dynamics in 2 d in vitro cerebrocortical neurons; however, this treatment robustly potentiated NMDA-induced Ca(2+) influx. The 30 nM PbTx-2 treatment produced a maximum [Na(+)](i) of 16.9 +/- 1.5 mM, representing an increment of 8.8 +/- 1.8 mM over basal. The corresponding membrane potential change produced by 30 nM PbTx-2 was modest and, therefore, insufficient to relieve the voltage-dependent Mg(2+) block of NMDARs. To unambiguously demonstrate the enhancement of NMDA receptor function by PbTx-2, we recorded single-channel currents from cell-attached patches. PbTx-2 treatment was found to increase both the mean open time and open probability of NMDA receptors. These effects of PbTx-2 on NMDA receptor function were dependent on extracellular Na(+) and activation of Src kinase. The functional consequences of PbTx-2-induced enhancement of NMDAR function were evaluated in immature cerebrocortical neurons. PbTx-2 concentrations between 3 and 300 nM enhanced neurite outgrowth. Voltage-gated sodium channel activators may accordingly represent a novel pharmacologic strategy to regulate neuronal plasticity through an NMDA receptor and Src family kinase-dependent mechanism.

Valproic acid, but not lamotrigine, suppresses seizure-induced c-fos and c-Jun mRNA expression

Seizure-induced activity has been shown to increase the expression of immediate early genes (IEGs) c-fos and c-Jun in the CNS. Anti-epileptic drugs (AEDs) can suppress the induction of a seizure, but it is unknown if AEDs affect the expression of seizure-induced IEGs. We found that valproic acid (VPA), but not lamotrigine (LTG), was capable of suppressing seizure-induced c-fos and c-Jun mRNA expression in rats despite a similar anticonvulsant effect. LTG in some regions of the CNS enhanced seizure-induced IEG expression. These studies indicate that the older AED (VPA), as compared to the newer AED (LTG), can suppress seizure-induced IEG expression. The consequence of this suppression of IEGs following a generalized seizure may be viewed either as a neuroprotective or detrimental effect upon the brain.

Excitotoxic death induced by released glutamate in depolarized primary cultures of mouse cerebellar granule cells is dependent on GABAAreceptors and niflumic acid-sensitive chloride channels

Excitotoxic neuronal death has been linked to neurological and neurodegenerative diseases. Several studies have sought to clarify the involvement of Cl(-) channels in neuronal excitotoxicity using either N-methyl-D-aspartic acid (NMDA) or alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate/kainic acid agonists. In this work we induced excitotoxic death in primary cultures of cerebellar granule cells by means of endogenously released glutamate. Excitotoxicity was provoked by exposure to high extracellular K(+) concentrations ([K(+)](o)) for 5 min. Under these conditions, a Ca(2+)-dependent release of glutamate was evoked. When extracellular glutamate concentration rose to between 2 and 4 microM, cell viability was significantly reduced by 30-40%. The NMDA receptor antagonists (MK-801 and D-2-amino-5-phosphonopentanoic acid) prevented cell death. Exposure to high [K(+)](o) produced a (36)Cl(-) influx which was significantly reduced by picrotoxinin. In addition, the GABA(A) receptor antagonists (bicuculline, picrotoxinin and SR 95531) protected cells from high [K(+)](o)-triggered excitotoxicity and reduced extracellular glutamate concentration. The Cl(-) channel blockers niflumic acid and 5-nitro-2-(3-phenylpropylamino)benzoic acid also exerted a neuroprotective effect and reduced extracellular glutamate concentration, even though they did not reduce high [K(+)](o)-induced (36)Cl(-) influx. Primary cultures of cerebellar granule cells also contain a population of GABAergic neurons that released GABA in response to high [K(+)](o). Chronic treatment of primary cultures with kainic acid abolished GABA release and rendered granule cells insensitive to high [K(+)](o) exposure, even though NMDA receptors were functional. Altogether, these results demonstrate that, under conditions of membrane depolarization, low micromolar concentrations of extracellular glutamate might induce an excitotoxic process through both NMDA and GABA(A) receptors and niflumic acid-sensitive Cl(-) channels.

Improvements in an in-vitro assay for excitotoxicity by measurement of early gene (c-fos mRNA) levels

Quantitative, real-time reverse transcription-polymerase chain reaction (RT-PCR) measurements were made to investigate the levels of c-fos mRNA as one measure of the expression of the c-fos gene. Exposure of mouse cerebellar granule cells to excitotoxic concentrations of glutamate (Glu) and aspartate (Asp) led to a changed time profile for mRNA expression, from a transient c-fos expression at 15-30 min to a delayed, elevated and sustained expression at later time points which was prevented by selective antagonism of the NMDA receptor but not of the AMPA/kainate receptor demonstrating that this c-fos induction was mediated through the specific activation of the NMDA Glu receptor subtype. The question as to whether c-fos expression changes could be used to predict excitotoxicity was addressed by testing the c-fos response of the cultures to several compounds, at low (and therefore non-toxic) and high (toxic) concentrations at two suitable time-points of exposure (30 and 240 min), in the presence and absence of Glu receptor antagonists. The compounds were divided into four groups, excitotoxins, neurotoxic but non-excitotoxic compounds, neuroactive but non-toxic compounds, and compounds that were toxic to other target organelles. The results of this study, using real-time RT-PCR, support the proposal that c-fos mRNA can be used as a specific biomarker of excitotoxicity and moreover encourage further studies to employ this highly sensitive, quantifiable and reproducible technique in a high throughput screen, with minimal use of animals for primary culture set-up. Furthermore, this test has the potential for application in screening newly-designed excitatory amino acid receptor antagonists in the search for clinically relevant drugs to treat a variety of neuropathologies.

Corticotropin releasing factor enhances survival of cultured GABAergic cerebellar neurons after exposure to a neurotoxin

Corticotropin-releasing factor (CRF), in addition to its role as a hormone in the stress response, functions as a neuromodulator in the cerebellum, where it enhances both the spontaneous and amino acid induced firing rate of Purkinje cells. In the cerebellum, CRF and its two types of receptors (CRF-R(1) and CRF-R(2)) are present during cerebellar development at ages that precede the onset of afferent ingrowth and synaptogenesis, suggesting a distinct role during early cerebellar development. The present study was undertaken to determine whether CRF enhances the survival of cerebellar neurons, in particular GABAergic neurons. Primary cultures of cerebellar neurons obtained from embryonic day 18 mice were composed primarily, but not exclusively, of GABAergic neurons. Although CRF-R(1) is present in most neurons in this culture system, when CRF was added to the medium, no significant change in neuronal survival was observed when compared to control cultures. It is possible that a role for CRF is not seen in growth-promoting culture medium at the plating density chosen for this study and may only be evident when the cells have been exposed to conditions that reduce the likelihood of survival, such as exposure to neurotoxins such as AraC. We propose that, because AraC increases the number of cleaved caspase-3 positive cells, indicating apoptosis, it is possible that a CRF effect involves an inhibition of the apoptotic pathway. Cultures treated with AraC had a decrease in the total number of GABAergic neurons and an increase in apoptotic cells as measured with the apoptotic marker cleaved caspase-3. Co-treatment with CRF rescued many GABAergic neurons. It is interesting to note that apoptotic cells do not exhibit GABA or c-fos positive immunolabeling. Thus, these data support the concept that CRF plays a neuroprotective role in the survival of GABAergic cerebellar neurons in culture after exposure to a neurotoxin.

Kainate down-regulates a subset of GABAA receptor subunits expressed in cultured mouse cerebellar granule cells

The effect of kainate, an agonist selective for ionotropic AMPA/kainate type of glutamate receptors, on GABAA receptor subunit expression in cultured mouse cerebellar granule cells was studied using quantitative RT-PCR, ligand binding and electrophysiology. Chronic kainate treatment, without producing excitotoxicity, resulted in preferential, dose- and time-dependent down-regulation of alpha1, alpha6 and beta2 subunit mRNA expression, the expression of beta3, gamma2 and delta subunit mRNAs being less affected. The down-regulation was reversed by DNQX, an AMPA/kainate-selective glutamate receptor antagonist. A 14-day kainate treatment resulted in 46% decrease of total [3H]Ro 15-4513 binding to the benzodiazepine sites. Diazepam-insensitive [3H]Ro 15-4513 binding was decreased by 89% in accordance with very low amount of alpha6 subunit mRNA present. Diazepam-sensitive [3H]Ro 154513 binding was decreased only by 40%, contrasting >90% decrease in alpha1 subunit mRNA expression. However, this was consistent with lower potentiation of GABA-evoked currents in kainate-treated than control cells by the alpha1-selective benzodiazepine site ligand zolpidem, suggesting compensatory expression of alpha5 (and/or alpha2 or alpha3) subunits producing diazepam-sensitive but zolpidem-insensitive receptor subtypes. In conclusion, chronic kainate treatment of cerebellar granule cells selectively down-regulates oil, alpha6 and beta2 subunits resulting in altered GABAA receptor pharmacology.

Neurovirulence depends on virus input titer in brain in feline immunodeficiency virus infection: evidence for activation of innate immunity and neuronal injury

Lentiruses cause neurological disease depending on the virus strain and its neurotropism, yet it remains uncertain to what the impact of infectious virus quantity in the brain early in infection is on the subsequent development of neurological disease or neurovirulence. We investigated the relationship between infectious virus input titer and the resulting neurovirulence, using ex vivo and in vivo assays of feline immunodeficiency virus (FIV)-induced neurovirulence. FIV infection of cell cultures and neonatal cats was performed using 10(2.5) (low-titer) or 10(4.5) (high-titer) 50% tissue culture infectious doses (TCID(50))/ml of the neurovirulent FIV strain, V1CSF. Ex vivo neurotoxicity assays revealed that conditioned medium (CM) from feline macrophages infected with high-titer (P <.001) or low-titer (P <.01) V1CSF induced greater neuronal death than CM from mock-infected cells. In vivo, animals infected intracranially with high-titer V1CSF showed neurodevelopmental delays compared to mock-infected animals (P <.001) and animals infected with low-titer V1CSF (P <.02), concurrent with reduced weight gains and greater depletion of CD4+ cells over a 12-week period. Neuropathological changes, including astrogliosis, macrophage activation, and neuronal damage, were evident in V1CSF-infected animals and were viral titer dependent. In vivo magnetic resonance (MR) spectroscopy and proton nuclear magnetic resonance ((1)H-NMR) spectroscopy of tissue extracts revealed evidence of neuronal injury, including reduced N-acetyl aspartate/creatine (P <.05) and increased trimethylamine/creatine (P <.05) ratios, in the frontal cortex of high-titer V1CSF-infected animals compared to the other groups. T2-weighted MR imaging detected increased signal intensities in the frontal cortex and white matter of V1CSF-infected animals relative to controls, which was more evident as viral titer increased (P <.01). The present findings indicate that lentivirus infectious titers in the brain during the early stages of infection determine the severity of neurovirulence, reflected by neurobehavioral deficits, together with neuroradiological and neuropathological findings of activation of innate immunity and neuronal injury.

Effects of peroxisome proliferator-activated receptor agonists on LPS-induced neuronal death in mixed cortical neurons: associated with iNOS and COX-2

In neurodegenerative disease, the use of non-steroidal anti-inflammatory drugs (NSAIDs) has been regarded as beneficial. The NSAID, an inhibitor of cyclooxygenase (COX), has been also suggested as a ligand of the peroxisome proliferator-activated receptor (PPAR). In cortical neuron-glial co-cultures, we examined the effect of PPAR agonists on lipopolysaccharide(LPS)-induced neuronal death, which has been known to be NO-dependent. LPS induced iNOS expression and the release of nitric oxide in microglia, and COX-2 expression in neurons. PPAR-gamma agonists such as 15d-PGJ(2), ciglitazone and troglitazone prevented LPS-induced neuronal death and abolished LPS-induced NO and PGE(2) release, however PPAR-alpha agonists such as clofibrate and WY14,643 did not produce the same results. PPAR-gamma agonists also reduced LPS-induced iNOS and COX-2 expression, which suggested by interfering with the NF-kappaB signal pathway.

Envelope gene-mediated neurovirulence in feline immunodeficiency virus infection: induction of matrix metalloproteinases and neuronal injury

The release of neurotoxins by activated brain macrophages or microglia is one mechanism proposed to contribute to the development of neurological disease following infection by lentiviruses, including feline immunodeficiency virus (FIV). Since molecular diversity in the lentiviral envelope gene influences the expression of host molecules implicated in neuronal injury, the role of the envelope sequence in FIV neuropathogenesis was investigated by using the neurovirulent FIV strain V1CSF, the nonneurovirulent strain Petaluma, and a chimera (FIVCh) containing the V1CSF envelope gene in a Petaluma background. All three viruses replicated in primary feline macrophages with equal efficiency, but conditioned medium from V1CSF- or FIVCh-infected cells was significantly more neurotoxic than medium from Petaluma-infected cultures (P < 0.001) and could be attenuated in a dose-dependent manner by treatment with either the matrix metalloproteinase (MMP) inhibitor prinomastat (PMT) or function-blocking antibodies to MMP-2. Although FIV sequences were detectable by PCR in brain tissue from neonatal cats infected with each of the viral strains, immunohistochemistry revealed increased astrogliosis and macrophage activation in the brains of V1CSF- and FIVCh-infected cats relative to the other groups, together with elevated markers of neuronal stress that included morphological changes and increased c-fos immunoreactivity. Similarly, MMP-2, but not MMP-9, mRNA and protein expression was increased in brain tissues of V1CSF- and FIVCh-infected cats relative to Petaluma-infected animals (P < 0.01). Infection with V1CSF or FIVCh was also associated with greater CD4(+) cell depletion (P < 0.001) and neurodevelopmental delays (P < 0.005), than in Petaluma-infected animals; these deficits improved following PMT therapy. These findings indicated that diversity in the envelope gene sequence influenced the neurovirulence exhibited by FIV both in vitro and in vivo, possibly through a mechanism involving the differential induction of MMP-2.

Differential mechanisms of glutamate-stimulated perturbations in the kinetics of c-fos mRNA induction are associated with maturation of cerebellar granule cells in primary culture

In further exploring proposals for the measurement of early gene (c-fos mRNA) levels as a predictive index for in vitro excitotoxicity, this study, using immature (2 days in vitro) cultures of mouse cerebellar granule cells as an experimental model system, was undertaken to determine the effect of glutamate (Glu) i) in stimulating increases in intracellular free-calcium ([Ca2+]), ii) on cell viability and iii) on induction of steady-state c-fos mRNA levels. In parallel experiments the action of agents (viz. 55 mM KCl and the calcium ionophore, A23187) that mediate Ca2+ entry into cells via different routes was also evaluated. Glu was unable to induce excitotoxicity in granule cells at this stage of development in culture, but did stimulate a concentration-dependent and marked increase in [Ca2+], levels while also mediating a dramatic concentration-dependent perturbation in the kinetics of c-fos mRNA induction that appeared to arise solely from NMDA receptor-mediated Ca2+ influx. The results are presented in comparison to the actions of KCI and A23187 and considered in relation to earlier studies undertaken using mature (7 days in vitro) cultures of cerebellar granule cells.

Apoptosis and necrosis occurring in excitotoxic cell death in isolated chick embryo retina

Excitotoxic studies using isolated chick embryo retina indicated that such an in vitro model provides a valid tool to characterize the effect of different agonists for subtypes of glutamate ionotropic receptors. In retinas maintained for 24 h in a Krebs medium, after a brief exposure (30 min) to glutamate agonists, we compared the effects produced by NMDA and non-NMDA-agonists, such as kainic acid (KA) or alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA). Delayed retinal damage was assessed by measuring lactate dehydrogenase (LDH) present in the medium after exposure to the previously named agonists. Although at high concentrations, both KA and AMPA produced more relevant release than NMDA, 7-8% of total retinal LDH was released after exposure to a 50 microM concentration of non-NMDA agonists. These values were similar to those obtained after 100 microM NMDA. In this regard, retinal tissue appeared to be less sensitive to excitotoxicity based on the activation of NMDA receptor subtype. All three agents produced histopathological lesions typical for excitotoxic damage. A delayed form of excitotoxicity observed in retina segments was predominated by necrotic features. However, the activation of apoptotic machinery early during the incubation period subsequent to brief exposure to NMDA (100 microM) was also present. The activation of caspase enzymes was studied by a fluorometric protease activity assay as well as by western blot analysis. Caspase-3-like activity reached the highest value within 3 h of incubation after exposure to excitotoxin, then the level of enzyme activity declined to lower values. As confirmed by a time-related appearance of TUNEL-positive nuclei, apoptotic features appeared to be specific for retina response to NMDA. In contrast, the exposure to a 50 microM concentration of KA or AMPA induced necrotic cell damage which was evident through the incubation, leading to a delayed mechanism of excitotoxicity. These observations provide evidence that in the retinal model, with regard to agonist concentrations and subtype of glutamate receptors, the cascade of events leading to excitotoxicity may result in either apoptotic or necrotic neuronal cell damage.

Possible role for the FosB/JunD AP-1 transcription factor complex in glutamate-mediated excitotoxicity in cultured cerebellar granule cells

The potent excitatory and neurotoxic actions of glutamate are known to influence the expression of a variety of genes, including those encoding the AP-1 transcription factor, which comprises proteins belonging to the Fos and Jun families. However, the precise role of Fos- and Jun-like transcription factors in these events remains elusive. Here we demonstrate, using primary cultures of mouse brain cerebellar granule cells as an in vitro model system, a possible involvement of the FosB/JunD heterodimer in excitotoxicity. Granule cells were grown for either 2 or 7 days in vitro (DIV) before exposure to varying concentrations (1-3000 microM) of the excitotoxin glutamate. In 7-DIV cells, glutamate induced a concentration-dependent neuronal death, whereas, in 2-DIV cells, no glutamate-induced neuronal damage was seen. We were particularly interested in comparing the protein composition of the AP-1 transcription factor complex in cells exposed to excitotoxic and to nontoxic conditions. AP-1 DNA binding activity was demonstrated by gel shift analysis in nuclear extracts derived from 7-DIV cells following exposure to either a nontoxic (10 microM) or an excitotoxic (250 microM) dose of glutamate and was similarly observed in extracts of 2-DIV cells exposed to the same levels of glutamate. Gel supershift analysis using antibodies against the different Fos and Jun family members allowed differentiation between AP-1 DNA binding in nuclear extracts as a function of both 1) viability status and 2) the stage of development. Of major significance was the finding that FosB could be detected as a component of AP-1 in 7-DIV cells only under excitotoxic conditions, whereas c-Fos, Fra-2, and JunD proteins were detectable under both excitotoxic and nontoxic conditions in cells of this age. In 2-DIV cells (in which glutamate is nontoxic), AP-1 comprised combinations of only Fra-1, Fra-2, c-Jun, and JunD. Because Fos family members are unable to form homodimers, this finding raises the possibility that the FosB/JunD heterodimer may have special significance in the mechanism of excitotoxic neuronal death.

Mechanisms of L-cysteine neurotoxicity

We review here the possible mechanisms of neuronal degeneration caused by L-cysteine, an odd excitotoxin. L-Cysteine lacks the omega carboxyl group required for excitotoxic actions via excitatory amino acid receptors, yet it evokes N-methyl-D-aspartate (NMDA) -like excitotoxic neuronal death and potentiates the Ca2+ influx evoked by NMDA. Both actions are prevented by NMDA antagonists. One target for cysteine effects is thus the NMDA receptor. The following mechanisms are discussed now: (1) possible increase in extracellular glutamate via release or inhibition of uptake/degradation, (2) generation of cysteine alpha-carbamate, a toxic analog of NMDA, (3) generation of toxic oxidized cysteine derivatives, (4) chelation of Zn2+ which blocks the NMDA receptor-ionophore, (5) direct interaction with the NMDA receptor redox site(s), (6) generation of free radicals, and (7) formation of S-nitrosocysteine. In addition to these, we describe another new alternative for cytotoxicity: (8) generation of the neurotoxic catecholamine derivative, 5-S-cysteinyl-3,4-dihydroxyphenylacetate (cysdopac).

Preliminary evaluation of an in vitro test for assessment of excitotoxicity by measurement of early gene (c-fos mRNA) levels

Using primary cultures of mouse cerebellar granule cells as an in vitro model system, it has been demonstrated that different profiles of temporal expression of the c-fos proto-oncogene are observed under non-excitotoxic and excitotoxic conditions. A ratio has been derived previously for the steady-state level of c-fos mRNA after 30 min and 240 min which suggests that a 240 min/30 min ratio of greater than 1 correlates with excitotoxicity, whereas a ratio of less than 1 correlates with a non-excitotoxic outcome. Moreover, a positive correlation is seen with abrogation of excitotoxicity in response to selective excitatory amino acid receptor antagonists. This test, proposed as a specific biomarker for excitotoxicity is undergoing prevalidation. Excitotoxicity is defined as neuronal cell death mediated by hyperactivation of glutamate receptor subtypes and therefore might be expected to be prevented by selective glutamate receptor antagonists. In preliminary evaluation studies, we have conducted work under the direction of the European Center for Validation of Alternate Methods (ECVAM) using compounds specified by ECVAM that have been subdivided into four groups based on known or presumed actions. These groups comprise: Group 1-endogenous/synthetic excitotoxins; Group 2-environmental, putative excitotoxins; Group 3-neurotoxic but non-excitotoxic compounds, and Group 4-non-toxic compounds. The results of this study support the proposal that the c-fos mRNA time-ratio test is a specific biomarker of excitotoxicity. Just as importantly, this test has the potential for application in screening newly-designed EAA receptor antagonists in the search for clinically relevant drugs to treat a variety of neuropathologies.

CP-101,606, an NR2B subunit selective NMDA receptor antagonist, inhibits NMDA and injury induced c-fos expression and cortical spreading depression in rodents

(1S, 2S)-1-(4-hydroxyphenyl)-2-(4-hydroxy-4-phenylpiperidino)-1-propanol (CP-101,606) is a noncompetitive antagonist of N-methyl-D-aspartate (NMDA) receptors containing the NR2B subunit. This compound was used to investigate the role of NR2B containing receptors in three responses to NMDA receptor activation in vivo. In mouse, CP-101,606 completely inhibited increases in fos-like immunoreactivity in dentate gyrus caused by a subconvulsant intraperitoneal dose of NMDA. In rat, the compound completely blocked cortical c-fos mRNA induction following focal injury in parietal cortex and the initiation and propagation of electrically induced cortical spreading depression. Inhibition of these responses by CP-101,606 indicates that c-fos induction and cortical spreading depression are dependent on activation of NMDA receptors containing the NR2B subunit. Since NMDA receptor dependent c-fos induction and cortical spreading depression may contribute to neuron loss after focal CNS injury, inhibition of these responses by CP-101,606 may contribute to the neuroprotective efficacy of the compound.

Non-NMDA glutamate receptors modulate capsaicin induced c-fos expression within trigeminal nucleus caudalis

1. We examined the effects of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate receptor antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and 1,2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzol[f]quinoxaline-7-sulpho namide (NBQX), the kainate receptor antagonists gamma-(R-)-glutamylaminomethanesulphonic acid (GAMS) and 6,7,8,9-tetrahydro-5-nitro-1H-benz[g]indole-2,3-dione-3-oxime (NS-102), and the group III metabotropic glutamate receptor (mGluR) agonist 2-amino-4-phosphono-S-butanoic acid (L-AP4) on c-fos-like immunoreactivity (c-fos LI) in trigeminal caudalis (Sp5C), lateral reticular (LRt), medullary reticular (Md) and solitary tract (Sol) nuclei, after intracisternal injection of capsaicin in urethane anaesthetized Sprague-Dawley rats. 2. Few c-fos labelled cells were observed within Sp5C in capsaicin-vehicle treated animals. The number of positive c-fos cells increased by 17 fold after intracisternal capsaicin (5 nmol) administration. 3. Pretreatment with CNQX (0.02, 0.1, 0.6, 3 and 15 mg kg-1) or NBQX (0.01, 0.1 and 1 mg kg-1), administered intraperitoneally 15 min before capsaicin, significantly reduced labelled cells within Sp5C by a maximum of 45 and 34%, respectively. The number of c-fox LI cells within LRt, Md and Sol was not affected. Pretreatment with L-AP4 (1, 3 and 10 mg kg-1) decreased the number of Sp5C c-fos LI cells by a maximum of 30%, whereas GAMS (1 and 10 mg kg-1) and NS-102 (1 and 5 mg kg-1) did not show any significant effect. 4. These results suggest that blockade of AMPA receptors, but not kainate receptors, or the activation of group III mGluRs, decrease the response of Sp5C neurons to trigeminovascular activation. Thus, in addition to NMDA receptors, mGluRs and AMPA receptors may modulate cephalic pain and may provide a potential therapeutic target for antimigraine drugs.

Interactions of excitatory neurotransmitters and xenobiotics in excitotoxicity and oxidative stress: glutamate and lead

Increased glutamate release is associated with serious neurological disorders such as epilepsy, stroke, Alzheimer's disease and other brain injuries. Excessive glutamate release and subsequent glutamatergic neuronal stimulation increase the production of reactive oxygen species (ROS), which in turn induce oxidative stress, excitotoxicity and neuronal damage. A number of studies have shown that co-exposure of neuronal cells to glutamate, and an environmental toxin, lead, can greatly amplify glutamate excitotoxicity and cell death through apoptosis or necrosis. Even though the mechanisms of excitotoxicity or those of glutamate-lead interactions have not been exhaustively delineated, there is ample evidence to suggest that increased production of ROS may play an important role in both events. Subsequently, increased DNA binding of redox-regulated transcription factors, NF-kappaB and AP-1, seems to be associated with these events. Induction of an immediate early gene, c-fos, is seen in neuronal cells exposed to glutamate or lead. Immediate early genes are important in regulating the expression of other neuronal genes; Elevated expressions of the genes encoding Hsp70 or cyclo-oxygenase-2 seem to be involved in the apoptosis or necrosis induced by glutamate, and may be associated with induction of several of the genes in cells exposed to lead, or to the glutamate-lead combination. Further studies are required to clarify the mechanisms of glutamate-lead neurotoxicity.