Role of desensitization of AMPA receptors on the neuronal viability and on the [Ca2+]i changes in cultured rat hippocampal neurons

Calcium Ions Aggravate Alzheimer's Disease Through the Aberrant Activation of Neuronal Networks, Leading to Synaptic and Cognitive Deficits

Alzheimer’s disease (AD) is a neurodegenerative disease that is characterized by the production and deposition of β-amyloid protein (Aβ) and hyperphosphorylated tau, leading to the formation of β-amyloid plaques (APs) and neurofibrillary tangles (NFTs). Although calcium ions (Ca²⁺) promote the formation of APs and NFTs, no systematic review of the mechanisms by which Ca²⁺ affects the development and progression of AD has been published. Therefore, the current review aimed to fill the gaps between elevated Ca²⁺ levels and the pathogenesis of AD. Specifically, we mainly focus on the molecular mechanisms by which Ca²⁺ affects the neuronal networks of neuroinflammation, neuronal injury, neurogenesis, neurotoxicity, neuroprotection, and autophagy. Furthermore, the roles of Ca²⁺ transporters located in the cell membrane, endoplasmic reticulum (ER), mitochondria and lysosome in mediating the effects of Ca²⁺ on activating neuronal networks that ultimately contribute to the development and progression of AD are discussed. Finally, the drug candidates derived from herbs used as food or seasoning in Chinese daily life are summarized to provide a theoretical basis for improving the clinical treatment of AD.

Mechanistic perspectives on differential mitochondrial-based neuroprotective effects of several carnitine forms in Alzheimer's disease in vitro model

Mitochondrial deregulation has emerged as one of the earliest pathological events in Alzheimer's disease (AD), the most common age-related neurodegenerative disorder. Improvement of mitochondrial function in AD has been considered a relevant therapeutic approach. L-carnitine (LC), an amino acid derivative involved in the transport of long-chain fatty acids into mitochondria, was previously demonstrated to improve mitochondrial function, having beneficial effects in neurological disorders; moreover, acetyl-L-carnitine (ALC) is currently under phase 4 clinical trial for AD (ClinicalTrials.gov NCT01320527). Thus, in the present study, we investigated the impact of different forms of carnitines, namely LC, ALC and propionyl-L-carnitine (PLC) on mitochondrial toxicity induced by amyloid-beta peptide 1-42 oligomers (AβO; 1 μM) in mature rat hippocampal neurons. Our results indicate that 5 mM LC, ALC and PLC totally rescued the mitochondrial membrane potential and alleviated both the decrease in oxygen consumption rates and the increase in mitochondrial fragmentation induced by AβO. These could contribute to the prevention of neuronal death by apoptosis. Moreover, only ALC ameliorated AβO-evoked changes in mitochondrial movement by reducing the number of stationary mitochondria and promoting reversal mitochondrial movement. Data suggest that carnitines (LC, ALC and PLC) may act differentially to counteract changes in mitochondrial function and movement in neurons subjected to AβO, thus counteracting AD-related pathological phenotypes.

hiPSC-Based Model of Prenatal Exposure to Cannabinoids: Effect on Neuronal Differentiation

Phytocannabinoids are psychotropic substances ofcannabis with the ability to bind endocannabinoid (eCB) receptors that regulate synaptic activity in the central nervous system (CNS). Synthetic cannabinoids (SCs) are synthetic analogs of Δ⁹-tetrahydrocannabinol (Δ⁹-THC), the psychotropic compound of cannabis, acting as agonists of eCB receptor CB₁. SC is an easily available and popular alternative to cannabis, and their molecular structure is always changing, increasing the hazard for the general population. The popularity of cannabis and its derivatives may lead, and often does, to a child's exposure to cannabis both in utero and through breastfeeding by a drug-consuming mother. Prenatal exposure to cannabis has been associated with an altered rate of mental development and significant changes in nervous system functioning. However, the understanding of mechanisms of its action on developing the human CNS is still lacking. We investigated the effect of continuous exposure to cannabinoids on developing human neurons, mimicking the prenatal exposure by drug-consuming mother. Two human induced pluripotent stem cells (hiPSC) lines were induced to differentiate into neuronal cells and exposed for 37 days to cannabidiol (CBD), Δ⁹-THC, and two SCs, THJ-018 and EG-018. Both Δ⁹-THC and SC, at 10 μM, promote precocious neuronal and glial differentiation, while CBD at the same concentration is neurotoxic. Neurons exposed to Δ⁹-THC and SC show abnormal functioning of voltage-gated calcium channels when stimulated by extracellular potassium. In sum, all studied substances have a profound impact on the developing neurons, highlighting the importance of thorough research on the impact of prenatal exposure to natural and SC.

Maturation of Human Pluripotent Stem Cell-Derived Cerebellar Neurons in the Absence of Co-culture

The cerebellum plays a critical role in all vertebrates, and many neurological disorders are associated with cerebellum dysfunction. A major limitation in cerebellar research has been the lack of adequate disease models. As an alternative to animal models, cerebellar neurons differentiated from pluripotent stem cells have been used. However, previous studies only produced limited amounts of Purkinje cells. Moreover, in vitro generation of Purkinje cells required co-culture systems, which may introduce unknown components to the system. Here we describe a novel differentiation strategy that uses defined medium to generate Purkinje cells, granule cells, interneurons, and deep cerebellar nuclei projection neurons, that self-formed and differentiated into electrically active cells. Using a defined basal medium optimized for neuronal cell culture, we successfully promoted the differentiation of cerebellar precursors without the need for co-culturing. We anticipate that our findings may help developing better models for the study of cerebellar dysfunctions, while providing an advance toward the development of autologous replacement strategies for treating cerebellar degenerative diseases.

Sip-1 mutations cause disturbances in the activity of NMDA- and AMPA-, but not kainate receptors of neurons in the cerebral cortex

Smad-interacting protein-1 (Sip1) [Zinc finger homeobox (Zfhx1b), Zeb2] is a transcription factor implicated in the genesis of Mowat-Wilson syndrome (MWS) in humans. MWS is a rare genetic autosomal dominant disease caused by a mutation in the Sip1 gene (aka Zeb2 or Zfhx1b) mapped to 2q22.3 locus. MWS affects 1 in every 50-100 newborns worldwide. It is characterized by mental retardation, small stature, typical facial abnormalities as well as disturbances in the development of the cardio-vascular and renal systems as well as some other organs. Sip1 mutations cause abnormal neurogenesis in the brain during development as well as susceptibility to epileptic seizures. In the current study we investigated the role of the Sip1 gene in the activity of NMDA-, AMPA- and KA- receptors. We showed that a particular Sip1 mutation in the mouse causes changes in the activity of both NMDA- and AMPA- receptors in the neocortical neurons in vitro. We demonstrate that neocortical neurons that have only one copy of Sip1 (heterozygous, Sip1^fI/wt), are more sensitive to both NMDA- and AMPA- receptors agonists as compared to wild type neurons (Sip1^wt/wt). This is reflected in higher amplitudes of agonist induced Ca²⁺ signals as well as a lower half maximal effective concentration (ЕC50). In contrast, neurons from homozygous Sip1 mice (Sip1^fI/fI), demonstrate higher resistance to these respective receptor agonists. This is reflected in lower amplitudes of Ca²⁺-responses and so a higher concentration of receptor activators is required for activation.

The angiogenic factor angiopoietin-1 is a proneurogenic peptide on subventricular zone stem/progenitor cells

In the adult mammalian brain, the subventricular zone (SVZ) hosts stem cells constantly generating new neurons. Angiopoietin-1 (Ang-1) is an endothelial growth factor with a critical role in division, survival, and adhesion of endothelial cells via Tie-2 receptor activity. Expression of Tie-2 in nonendothelial cells, especially neurons and stem cells, suggests that Ang-1 may be involved in neurogenesis. In the present work, we investigated the putative role of Ang-1 on SVZ neurogenesis. Immature cells from SVZ-derived neurospheres express Ang-1 and Tie-2 mRNA, suggesting a role for the Ang-1/Tie-2 system in the neurogenic niche. Moreover, we also found that Tie-2 protein expression is retained on differentiation in neurons and glial cells. Ang-1 triggered proliferation via activation of the ERK1/2 (extracellular signal-regulated kinase 1/2) mitogen-activated protein kinase (MAPK) kinase pathway but did not induce cell death. Accordingly, coincubation with an anti-Tie-2 neutralizing antibody prevented the pro-proliferative effect of Ang-1. Furthermore, Ang-1 increased the number of NeuN (neuronal nuclear protein)-positive neurons in cultures treated for 7 d, as well as the number of functional neurons, as assessed by monitoring [Ca(2+)](i) rises after application of specific stimuli for neurons and immature cells. The proneurogenic effect of Ang-1 is mediated by Tie-2 activation and subsequent mTOR (mammalian target of rapamycin kinase) mobilization. In agreement, neuronal differentiation significantly decreased after exposure to an anti-Tie-2 neutralizing antibody and to rapamycin. Moreover, Ang-1 elicited the activation of the SAPK (stress-activated protein kinase)/JNK (c-Jun N-terminal kinase) MAPK, involved in axonogenesis. Our work shows a proneurogenic effect of Ang-1, highlighting the relevance of blood vessel/stem cell cross talk in health and disease.

Protective effect of salidroside against H2O2-induced cell apoptosis in primary culture of rat hippocampal neurons

Salidroside, a phenylpropanoid glycoside separated from a medicinal plant Rhodiola rosea, has been documented to have protective effects on neuronal cells in vitro. This study investigated whether salidroside was able to extend its unique neuroprotection to primary cultured rat hippocampal neurons against hydrogen peroxide (H(2)O(2))-induced cell damage. Cell viability tests and cell apoptosis assays confirmed that salidroside pretreatment attenuated H(2)O(2)-stimulated apoptotic cell death in primary culture of hippocampal neurons in a concentration-dependent manner. The measurements of caspase-3 activity, nitric oxide (NO) production, and NO synthase (NOS) activity suggest that the protection of salidroside, shown in this study, might be mediated by inhibiting caspase-3 activity, and antagonizing NO production and NOS activity during H(2)O(2) stimulation. Perhaps, this study might contribute to the development of salidroside as a broad-spectrum agent for preventing and/or treating neuronal damage in neurodegenerative disorders.

Neuropeptide Y inhibits [Ca2+]i changes in rat retinal neurons through NPY Y1, Y4, and Y5 receptors

Neuropeptide Y (NPY) and NPY receptors are widely distributed in the CNS, including the retina, but the role of NPY in the retina is largely unknown. The aim of this study was to investigate whether NPY modulates intracellular calcium concentration ([Ca(2+)](i)) changes in retinal neurons and identify the NPY receptors involved. As NPY decreased the [Ca(2+)](i) amplitudes evoked by 30 mM KCl in only 50% of neurons analyzed, we divided them in two populations: NPY-non-responsive neurons (Delta2/Delta1 > or = 0.80) and NPY-responsive neurons (Delta2/Delta1 < 0.80), being the Delta2/Delta1 the ratio between the amplitude of [Ca(2+)](i) increase evoked by the second (Delta2) and the first (Delta1) stimuli of KCl. The NPY Y(1)/Y(5), Y(4), and Y(5) receptor agonists (100 nM), but not the Y(2) receptor agonist (300 nM), inhibited the [Ca(2+)](i) increase induced by KCl. In addition, the inhibitory effect of NPY on evoked-[Ca(2+)](i) changes was reduced in the presence of the Y(1) or the Y(5) receptor antagonists. In conclusion, NPY inhibits KCl-evoked [Ca(2+)](i) increase in retinal neurons through the activation of NPY Y(1), Y(4), and Y(5) receptors. This effect may be viewed as a potential neuroprotective mechanism of NPY against retinal neurodegeneration.

Salidroside attenuates glutamate-induced apoptotic cell death in primary cultured hippocampal neurons of rats

Salidroside, a compound of natural origin, has displayed a broad spectrum of pharmacological properties. This study aimed to evaluate the inhibitory effects of salidroside on glutamate-induced cell death in a primary culture of rat hippocampal neurons as compared to brain-derived neurotrophic factor (BDNF), a usual positive control. MTT and LDH assays, together with Hoechst 33342 staining, terminal deoxynucleotidyl transferase dUTP-mediated nicked end labeling (TUNEL) assay and flow cytometric analysis using annexin-V and propidium (PI) label, indicated that salidroside pretreatment attenuated glutamate-induced apoptotic cell death in primary cultured hippocampal neurons, showing a dose-dependent pattern. Furthermore, caspase-3 activity assay and calcium measurements with Fluo 4-AM, respectively, revealed that salidroside pretreatment antagonized activation of caspase-3 and elevation of intracellular calcium level, both of which were induced by glutamate stimulation, thus adding to the understanding of how salidroside offered neuroprotection against glutamate excitotoxicity.

Tumor necrosis factor-alpha modulates survival, proliferation, and neuronal differentiation in neonatal subventricular zone cell cultures

Tumor necrosis factor (TNF)-alpha has been reported to modulate brain injury, but remarkably, little is known about its effects on neurogenesis. We report that TNF-alpha strongly influences survival, proliferation, and neuronal differentiation in cultured subventricular zone (SVZ) neural stem/progenitor cells derived from the neonatal P1-3 C57BL/6 mice. By using single-cell calcium imaging, we developed a method, based on cellular response to KCl and/or histamine, that allows the functional evaluation of neuronal differentiation. Exposure of SVZ cultures to 1 and 10 ng/ml mouse or 1 ng/ml human recombinant TNF-alpha resulted in increased differentiation of cells displaying a neuronal-like profile of [Ca2+](i) responses, compared with the predominant profile of immature cells observed in control, nontreated cultures. Moreover, by using neutralizing antibodies for each TNF-alpha receptor, we found that the proneurogenic effect of 1 ng/ml TNF-alpha is mediated via tumor necrosis factor receptor 1 activation. Accordingly, the percentage of neuronal nuclear protein-positive neurons was increased following exposure to mouse TNF-alpha. Interestingly, exposure of SVZ cultures to 1 ng/ml TNF-alpha induced cell proliferation, whereas 10 and 100 ng/ml TNF-alpha induced apoptotic cell death. Moreover, we found that exposure of SVZ cells to TNF-alpha for 15 minutes or 6 hours caused an increase in the phospho-stress-activated protein kinase/c-Jun N-terminal kinase immunoreactivity initially in the nucleus and then in growing axons, colocalizing with tau, consistent with axonogenesis. Taken together, these results show that TNF-alpha induces neurogenesis in neonatal SVZ cell cultures of mice. TNF-alpha, a proinflammatory cytokine and a proneurogenic factor, may play a central role in promoting neurogenesis and brain repair in response to brain injury and infection.

Differential contribution of L-, N-, and P/Q-type calcium channels to [Ca2+]i changes evoked by kainate in hippocampal neurons

We investigated the contribution of L-, N- and P/Q-type Ca(2+) channels to the [Ca(2+)](i) changes, evoked by kainate, in the cell bodies of hippocampal neurons, using a pharmacological approach and Ca(2+) imaging. Selective Ca(2+) channel blockers, namely nitrendipine, omega-Conotoxin GVIA (omega-GVIA) and omega-Agatoxin IVA (omega-AgaIVA) were used. The [Ca(2+)](i) changes evoked by kainate presented a high variability, and were abolished by NBQX, a AMPA/kainate receptor antagonist, but the N-methyl-D-aspartate (NMDA) receptor antagonist, D-AP5, was without effect. Each Ca(2+) channel blocker caused differential inhibitory effects on [Ca(2+)](i) responses evoked by kainate. We grouped the neurons for each blocker in three subpopulations: (1) neurons with responses below 60% of the control; (2) neurons with responses between 60% and 90% of the control, and (3) neurons with responses above 90% of the control. The inhibition caused by nitrendipine was higher than the inhibition caused by omega-GVIA or omega-AgaIVA. Thus, in the presence of nitrendipine, the percentage of cells with responses below 60% of the control was 41%, whereas in the case of omega-GVIA or omega-AgaIVA the values were 9 or 17%, respectively. The results indicate that hippocampal neurons differ in what concerns their L-, N- and P/Q-type Ca(2+) channels activated by stimulation of the AMPA/kainate receptors.

Changes in calcium dynamics following the reversal of the sodium-calcium exchanger have a key role in AMPA receptor-mediated neurodegeneration via calpain activation in hippocampal neurons

Proteolytic cleavage of the Na(+)/Ca(2+) exchanger (NCX) by calpains impairs calcium homeostasis, leading to a delayed calcium overload and excitotoxic cell death. However, it is not known whether reversal of the exchanger contributes to activate calpains and trigger neuronal death. We investigated the role of the reversal of the NCX in Ca(2+) dynamics, calpain activation and cell viability, in alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptor-stimulated hippocampal neurons. Selective overactivation of AMPA receptors caused the reversal of the NCX, which accounted for approximately 30% of the rise in intracellular free calcium concentration ([Ca(2+)](i)). The NCX reverse-mode inhibitor, 2-[2-[4-(4-nitrobenzyloxy)phenyl]ethyl]isothiourea (KB-R7943), partially inhibited the initial increase in [Ca(2+)](i), and prevented a delayed increase in [Ca(2+)](i). In parallel, overactivation of AMPA receptors strongly activated calpains and led to the proteolysis of NCX3. KB-R7943 prevented calpain activation, cleavage of NCX3 and was neuroprotective. Silencing of NCX3 reduced Ca(2+) uptake, calpain activation and was neuroprotective. Our data show for the first time that NCX reversal is an early event following AMPA receptor stimulation and is linked to the activation of calpains. Since calpain activation subsequently inactivates NCX, causing a secondary Ca(2+) entry, NCX may be viewed as a new suicide substrate operating in a Ca(2+)-dependent loop that triggers cell death and as a target for neuroprotection.

N-methyl-D-aspartate receptor-dependent regulation of the glutamate transporter excitatory amino acid carrier 1

The neuronal transporter excitatory amino acid carrier 1 (EAAC1) is enriched in perisynaptic regions, where it may regulate synaptic spillover of glutamate. In this study we examined potential interactions between EAAC1 and ionotropic glutamate receptors. N-Methyl-D-aspartate (NMDA) receptor subunits NR1, NR2A, and NR2B, but not the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor subunit GluR2, were co-immunoprecipitated with EAAC1 from neuron-enriched hippocampal cultures. A similar interaction was observed in C6 glioma and human embryonic kidney cells after co-transfection with Myc epitope-tagged EAAC1 and NMDA receptor subunits. Co-transfection of C6 glioma with the combination of NR1 and NR2 subunits dramatically increased (approximately 3-fold) the amount of Myc-EAAC1 that can be labeled with a membrane-impermeable biotinylating reagent. In hippocampal cultures, brief (5 min), robust (100 microM NMDA, 10 microM glycine) activation of the NMDA receptor decreased biotinylated EAAC1 to approximately 50% of control levels. This effect was inhibited by an NMDA receptor antagonist, intracellular or extracellular calcium chelators, or hypertonic sucrose. Glutamate, alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid with cyclothiazide, and thapsigargin mimicked the effects of NMDA. These studies suggest that NMDA receptors interact with EAAC1, facilitate cell surface expression of EAAC1 under basal conditions, and control internalization of EAAC1 upon activation. This NMDA receptor-dependent regulation of EAAC1 provides a novel mechanism that may shape excitatory signaling during synaptic plasticity and/or excitotoxicity.

Susceptibility of hippocampal neurons to Abeta peptide toxicity is associated with perturbation of Ca2+ homeostasis

Neuritic dystrophy, loss of synapses and neuronal death in the cerebral cortex and hippocampus are hallmarks of Alzheimer's disease. The aim of the present study was to investigate the differential susceptibility of cortical and hippocampal neurons to amyloid-beta (Abeta)-induced toxicity. For that, we have used primary neuronal cultures prepared from rat brain cortex and hippocampus which were treated with the synthetic peptides Abeta25-35 or Abeta1-40. Abeta-induced apoptotic cell death was analyzed by determining caspase-3-like activity. Neuritic dystrophy was evaluated by cobalt staining and MAP2 immunoreactivity. Perturbation of Ca(2+) homeostasis caused by exposure to Abeta was evaluated by determining basal cytosolic calcium levels in the whole neuronal population and by single cell calcium imaging under basal and KCl-depolarization conditions. Finally, levels of GluR2 subunit of glutamate AMPA (alpha-amino-3-hydroxy-5-methylisoxazole-4-proprionate) receptors were quantified by western blotting. Our results demonstrated that hippocampal neurons in culture are more susceptible than cortical neurons to Abeta-induced apoptosis and also that this mechanism involves the perturbation of Ca(2+) homeostasis. Accordingly, the exposure of hippocampal neurons to Abeta peptides decreases the protein levels of the GluR2 subunit of glutamate AMPA receptors that may be associated with a significant rise of cytosolic Ca(2+) concentration, leading to dendritic dystrophy and activation of apoptotic neuronal death.

High glucose and diabetes increase the release of [3H]-D-aspartate in retinal cell cultures and in rat retinas

Several evidences suggest that glutamate may be involved in retinal neurodegeneration in diabetic retinopathy (DR). For that reason, we investigated whether high glucose or diabetes affect the accumulation and the release of [(3)H]-D-aspartate, which was used as a marker of the glutamate transmitter pool. The accumulation of [(3)H]-D-aspartate did not change in cultured retinal neural cells treated with high glucose (30 mM) for 7 days. However, the release of [(3)H]-D-aspartate, evoked by 50 mM KCl, significantly increased in retinal cells exposed to high glucose. Mannitol, which was used as an osmotic control, did not cause any significant changes in both accumulation and release of [(3)H]-D-aspartate. In the retinas, 1 week after the onset of diabetes, both the accumulation and release of [(3)H]-D-aspartate were unchanged comparing to the retinas of age-matched controls. However, after 4 weeks of diabetes, the accumulation of [(3)H]-D-aspartate in diabetic retinas decreased and the release of [(3)H]-D-aspartate increased, compared to age-matched control retinas. These results suggest that high glucose and diabetes increase the evoked release of D-aspartate in the retina, which may be correlated with the hypothesis of glutamate-induced retinal neurodegeneration in DR.

Neuroprotection by BDNF against glutamate-induced apoptotic cell death is mediated by ERK and PI3-kinase pathways

Neurotrophins protect neurons against glutamate excitotoxicity, but the signaling mechanisms have not been fully elucidated. We studied the role of the phosphatidylinositol 3-kinase (PI3-K) and Ras/mitogen-activated protein kinase (MAPK) pathways in the protection of cultured hippocampal neurons from glutamate induced apoptotic cell death, characterized by nuclear condensation and activation of caspase-3-like enzymes. Pre-incubation with the neurotrophin brain-derived neurotrophic factor (BDNF), for 24 h, reduced glutamate-evoked apoptotic morphology and caspase-3-like activity, and transiently increased the activity of the PI3-K and of the Ras/MAPK pathways. Inhibition of the PI3-K and of the Ras/MAPK signaling pathways abrogated the protective effect of BDNF against glutamate-induced neuronal death and similar effects were observed upon inhibition of protein synthesis. Moreover, incubation of hippocampal neurons with BDNF, for 24 h, increased Bcl-2 protein levels. The results indicate that the protective effect of BDNF in hippocampal neurons against glutamate toxicity is mediated by the PI3-K and the Ras/MAPK signaling pathways, and involves a long-term change in protein synthesis.

Different roles of adenosine A1, A2A and A3 receptors in controlling kainate-induced toxicity in cortical cultured neurons

Adenosine is a neuromodulator that can control brain damage through activation of A(1), A(2A) and A(3) receptors, which are located in both neurons and other brain cells. We took advantage of cultured neurons to investigate the role of neuronal adenosine receptors in the control of neurotoxicity caused by kainate and cyclothiazide. Both A(1), A(2A) and A(3) receptors were immunocytochemically identified in cortical neurons. Activation of A(1) receptors with 100 nM CPA did not modify the extent of neuronal death whereas the A(1) receptor antagonist, DPCPX (50 nM), attenuated neurotoxicity by 28 +/- 5%, and effect similar to that resulting from the removal of endogenous adenosine with 2U/ml of adenosine deaminase (27 +/- 3% attenuation of neurotoxicity). In the presence of adenosine deaminase, DPCPX had no further effect and CPA now exacerbated neurotoxicity by 42 +/- 4%. Activation of A(2A) receptor with 30 nM CGS21680 attenuated neurotoxicity by 40 +/- 8%, an effect prevented by the A(2A) receptor antagonists, SCH58261 (50 nM) or ZM241385 (50 nM), which by themselves were devoid of effect. Finally, neither A(3) receptor activation with Cl-IB-MECA (100-500 nM) nor blockade with MRS1191 (5 microM) modified neurotoxicity. These results show that A(1) receptor activation enhances and A(2A) receptor activation attenuates neurotoxicity in cultured cortical neurons, indicating that these two neuronal adenosine receptors directly control neurodegeneration. Interestingly, the control by adenosine of neurotoxicity in cultured neurons is similar to that observed in vivo in newborn animals and is the opposite of what is observed in adult brain preparations where A(1) receptor activation and A(2A) receptor blockade are neuroprotective.

The AMPA-antagonist talampanel is neuroprotective in rodent models of focal cerebral ischemia

Cerebroprotection after administration of glutamate receptor antagonists has been well documented. The present study is intended to determine whether the non-competitive alpha-amino-3-hydroxy-methyl-4-isoxazolyl-propionic acid (AMPA) receptor antagonist talampanel, known as antiepileptic drug, has neuroprotective effects in stroke models in rodents. The infarct size was measured in three models of stroke by 2,3,5-triphenyltetrazolium chloride staining. Therapeutic time window was also examined in rats subjected to 1h middle cerebral artery occlusion. The degree of neuroprotection was tested in mice, using 1.5, 2 h or permanent middle cerebral artery occlusions. Effect on photochemically induced thrombosis was investigated in rats applying 30 min time window after brain irradiation. Talampanel reduced the infarct size by 47.3% (p<0.01) after a 30 min delay and 48.5% (p<0.01) after 2 h delay following middle cerebral artery occlusion in rats. In mice, talampanel reduced the extension of the infarcted tissue at the levels of striatum and hippocampus by 44.5% (p<0.05) and 39.3% (p<0.01) after 1.5 h transient ischemia and still caused 37.0% (p<0.05) and 37.0% (p<0.05) inhibitions when 2 h occlusion was applied. In photothrombosis talampanel showed a 40.1% (p<0.05) inhibition. Protective actions of talampanel in various stroke models, in rats and mice, suggest a possible therapeutic role of the compound in stroke patients.

Early calpain-mediated proteolysis following AMPA receptor activation compromises neuronal survival in cultured hippocampal neurons

In this work, we investigated the involvement of calpains in the neurotoxicity induced by short-term exposure to kainate (KA) in non-desensitizing conditions of AMPA receptor activation (cyclothiazide present, CTZ), in cultured rat hippocampal neurons. The calpain inhibitor MDL28170 had a protective effect in cultures treated with KA plus CTZ (p < 0.01), preventing the decrease in MTT reduction caused by exposure to KA (p < 0.001). Caspase inhibition by ZVAD-fmk was not neuroprotective against the toxic effect of KA. At 1 h after treatment, we could already observe significantly increased calpain activity, which was prevented by MDL 28170 and NBQX. Western blot analysis of calpain substrates, GluR1, neuronal nitric oxide synthase (nNOS) and nonerythroid spectrin (fodrin), showed a time-dependent and MDL 28170-sensitive proteolysis of these proteins. This effect was due to calpains, but not caspases, since ZVAD-fmk was ineffective in preventing proteolytic events. Breakdown products of fodrin (BDPs) were detected as early as 15 min after exposure to KA. Overall, these results show early activation of calpains following activation of AMPA receptors as well as compromise of neuronal survival, likely due to proteolytic events that affect proteins involved in neuronal signaling.

Nitric oxide inhibits complex I following AMPA receptor activation via peroxynitrite

We investigated the role of nitric oxide (NO) on mitochondrial complexes activity, following short-term non-desensitizing activation of AMPA receptors with kainate (KA) plus cyclothiazide (CTZ), in cultured rat hippocampal neurons. In these conditions, we observed a decrease in the activity of mitochondrial complexes I, II/III, and IV. A selective neuronal nitric oxide synthase inhibitor, 7-Nitroindazole, prevented the decrease in the activity of mitochondrial complex I, but not for the other complexes. Exposure to KA plus CTZ also increased cyclic GMP levels significantly, and led to increased levels of 3-nitrotyrosine, a biomarker for peroxynitrite production. Taken together, our results suggest that non-desensitizing activation of AMPA receptors causes inhibition of mitochondrial complex I via peroxynitrite.

Interactions of allosteric modulators of AMPA/kainate receptors on spreading depression in the chicken retina

The functional role of AMPA and kainate receptors in spreading depression (SD) was investigated in the isolated chicken retina. Competitive (NBQX) and non-competitive (GYKI 52466, GYKI 53405 and GYKI 53655) antagonists of the AMPA receptor inhibited AMPA-induced SD in a concentration-dependent manner. Concentrations of drugs caused 50% inhibition (IC(50) values) are 0.2, 16.6, 7.0 and 1.4 microM, respectively. AMPA receptor positive modulator cyclothiazide was more effective in the potentiation of SD evoked by AMPA than by kainate. Slight potentiation of either AMPA- or kainate-induced SD was observed only at high concentration (1 mg/ml) by the kainate receptor modulator concanavalin A. Compounds that positively modulate AMPA receptor function (cyclothiazide, IDRA-21, S 18986, 1-BCP and aniracetam) caused a concentration-dependent potentiation in SD. Concentrations of drugs that caused 50% potentiation (estimated EC(50) values) are 9, 135, 142, 450 and 1383 microM, respectively. Interaction between cyclothiazide, aniracetam or S 18986 administered with each other, or with GYKI 52466, respectively, was also investigated. When cyclothiazide and S 18986 were co-applied, their effects seemed to be additive. However, lack of additivity was obtained when S 18986 was added together with aniracetam. Positive modulators applied at equiactive concentrations reduced the inhibitory action of GYKI 52466 and differently shifted its concentration-response curve. In this respect, S 18986 was the most effective (IC(50) of GYKI 52466 changed from 16.6 to 51.9 microM). Our findings indicate the contribution of AMPA rather than kainate receptors in the mediation of retinal spreading depression. Our data further support the idea that multiple positive modulatory sites are present on the AMPA receptor complex in addition to a negative modulatory site.

Ursolic acid protects hippocampal neurons against kainate-induced excitotoxicity in rats

Ursolic acid is the major component of extracts of the Chinese herb, Souyang. This study determines whether and how ursolic acid protects against kainate-induced excitotoxicity in rat hippocampus. Primary neuronal cultures of cells isolated from the hippocampi of 7-day-old rats were treated with 150 microM kainate. After 2 h of treatment, free radicals were elevated and mitochondrial membrane potential was reduced significantly, and after 12 h, cell viability was decreased. Pretreatment with 5-15 microM ursolic acid dose-dependently and significantly attenuated the kainate-induced damage as well as alleviating the decrease in mitochondrial membrane potential and suppressing the increase in free radical generation. The results suggest that multiple mechanisms including modulation of AMPA receptor, protection of mitochondria, decrease in free radical generation, and scavenging of free radicals might be involved in ursolic acid protection against kainate-induced cell toxicity.

Neuroprotective effect of H. perforatum extracts on beta-amyloid-induced neurotoxicity

In the present study we assessed the neuroprotective role of a Hypericum perforatum ethanolic extract and obtained fractions in amyloid-beta peptide (Abeta)((25-35))-induced cell death in rat cultured hippocampal neurons. Lipid peroxidation was used as a marker of oxidative stress by following the formation of TBARS in rat cortical synaptosomes, after incubation with ascorbate/Fe2+, alone or in the presence of EC97 effective concentrations of H. perforatum fractions. Induced lipid peroxidation was significantly inhibited by fractions containing flavonol glycosides, flavonol and biflavone aglycones, and by a fraction containing several phenols, mainly chlorogenic acid-type phenolics (21%, 77% and 98%, respectively). Lipid peroxidation evaluated after incubation with 25 microM Abeta(25-35), was significantly inhibited by H. perforatum extract. Cell viability was assessed by use of the Syto-13/PI assay. The total ethanolic extract (TE) and fractions containing flavonol glycosides, flavonol and biflavone aglycones, reduced Abeta(25-35)-induced cell death (65%, 58% and 59%, respectively). These results were further supported by morphological analysis of cells stained with cresyl violet. Peptide beta-amyloid(25-35) induced a decrease in cell volume, chromatin condensation and nuclear fragmentation, alterations not evident in the presence of the TE and fractions containing hypericins (hypericin concentration = 11.02 microM), or fractions containing flavonoids (quercetin concentration = 21.13 microM). Dendritic lesion, an evidence of neurodegeneration, was observed by neuronal staining with cobalt following insult with Abeta(25-35), but prevented after exposure to the peptide plus the fractions referred above. The results of the present paper suggest that H. perforatum extracts may be endowed with neuroprotective compounds able to prevent Abeta(25-35)-induced toxicity.

Neuroprotective properties of Valeriana officinalis extracts

Valeriana officinalis have been used in traditional medicine for its sedative, hypnotic, and anticonvulsant effects. There are several reports in the literature supporting a GABAergic mechanism of action for valerian. The rationale of the present work is based on the concept that by decreasing neuronal network excitability valerian consumption may contribute to neuroprotection. The aim of our investigation was to evaluate the neuroprotective effects of V. officinalis against the toxicity induced by amyloid beta peptide 25-35 Abeta(25-35). Cultured rat hippocampal neurons were exposed to Abeta(25-35) (25 microM) for 24-48 h, after which morphological and biochemical properties were evaluated. The neuronal injury evoked by Abeta, which includes a decrease in cell reducing capacity and associated neuronal degeneration, was prevented by valerian extract. Analysis of intracellular free calcium (Ca(2+)i) indicated that the neuroprotective mechanisms may involve the inhibition of excess influx of Ca2+ following neuronal injury. Moreover, membrane peroxidation in rat hippocampal synaptosomes was evaluated, and our data indicate that valerian extract partially inhibited ascorbate/iron-induced peroxidation. In conclusion we show evidence that the signalling pathways involving Ca(2+)i and the redox state of the cells may play a central role in the neuroprotective properties of V. officinalis extract against Abeta toxicity. The novelty of the findings of the present work, indicating neuroprotective properties of valerian against Abeta toxicity may, at the long-term, contribute to introduction of a new relevant use of valerian alcoholic extract to prevent neuronal degeneration in aging or neurodegenerative disorders.

Understanding the physiology of glutamate receptors by use of a protocol for neuronal staining

Teaching students about the physiology of neurotransmitter receptors usually requires practical lessons with the use of sophisticated equipment and complex analysis of data. Here, we report our experience in teaching medical students with a simple, practical protocol that transforms the physiology of glutamate receptors into neuronal staining, observable under bright-field microscopy. Essentially, the students were challenged to selectively stain a subpopulation of cultured neurons expressing Ca(2+)-permeable alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptors (a subgroup of ionotropic glutamate receptors). Neurons expressing this type of receptors were loaded with Co(2+) (in substitution for Ca(2+)) after nondesensitizing activation of AMPA receptors. After precipitation, the Co(2+) was revealed after treatment with silver. At the end of the procedure, the neurons expressing Ca(2+)-permeable AMPA receptors were visually identified under bright-field microscopy. The procedure allowed the visualization of the complete dendritic network of the stained neurons and allowed the students to learn very efficiently about the physiology of glutamate receptors.

AMPA receptor-mediated neurotoxicity: role of Ca2+ and desensitization

Glutamate-induced neurodegeneration is the result of excessive stimulation of the different subtypes of glutamate receptors. With regard to the AMPA ((RS)-2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl)propionate) receptors it has been clear from numerous studies that in addition to the Ca2+ permeability of the receptor complexes, their desensitization properties may play a determining role in the neurodegeneration mediated by this subtype of the glutamate receptors. Recent studies have revealed important amino acid residues in the AMPA receptor subunits that control the desensitization kinetics and that may constitute important targets for drugs that may alter the desensitization of the AMPA receptor complexes.

The production of reactive oxygen species in intact isolated nerve terminals is independent of the mitochondrial membrane potential

Dependence on mitochondrial membrane potential (deltapsim) of hydrogen peroxide formation of in situ mitochondria in response to inhibition of complex I or III was studied in synaptosomes. Blockage of electron flow through complex I by rotenone or that through complex III by antimycin resulted in an increase in the rate of H2O2 generation as measured with the Amplex red assay. Membrane potential of mitochondria was dissipated by either FCCP (250 nM) or DNP (50 microM) and then the rate of H2O2 production was followed. Neither of the uncouplers had a significant effect on the rate of H2O2 production induced by rotenone or antimycin. Inhibition of the F0F1-ATPase by oligomycin, which also eliminates deltapsim in the presence of rotenone and antimycin, respectively, was also without effect on the ROS formation induced by rotenone and only slightly reduced the antimycin-induced H2O2 production. These results indicate that ROS generation of in situ mitochondria in nerve terminals in response to inhibition of complex I or complex III is independent of deltapsim. In addition, we detected a significant antimycin-induced H2O2 production when the flow of electrons through complex I was inhibited by rotenone, indicating that the respiratory chain of in situ mitochondria in synaptosomes has a substantial electron influx distal from the rotenone site, which could contribute to ROS generation when the complex III is inhibited.

Mitochondrial apoptotic cell death and moderate superoxide generation upon selective activation of non-desensitizing AMPA receptors in hippocampal cultures

In the present work we investigated the effect of selective stimulation of non-desensitizing alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors in the intracellular processes leading to hippocampal neuronal death and production of reactive oxygen species (ROS). Activation of AMPA receptors in the presence of cyclothiazide (CYZ), a blocker of AMPA receptor desensitization, resulted in the death of approximately 25% of neurones, which was prevented by 2,3-dihydroxy-6-nitro-7-sulphamoyl-benzo(f)quinoxaline (NBQX), an AMPA-preferring receptor antagonist. (+)-5-Methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine hydrogen maleate (MK-801) protected the neurones from necrotic death induced by AMPA or NMDA receptor activation. Neurodegeneration caused by selective activation of non-desensitizing AMPA receptors, in the presence of AMPA, CYZ and MK-801, significantly decreased the number of Co2+-positive neurones, used as a cytochemical marker of Ca2+-permeable AMPA receptors, but maintained intracellular ATP/ADP. The AMPA-mediated apoptotic cell death involved mitochondrial cytochrome c release and the activation of caspases-1 and -3, which was prevented by NBQX. Interestingly, although selective activation of AMPA receptors was not associated with production of intracellular peroxides, a moderate increase in superoxide production was observed upon exposure to antimycin A (AA). Furthermore, increased activity of Mn- superoxide dismutase (SOD) was observed on selective activation of non-desensitizing AMPA receptors. Taken together, these data make important contributions to the elucidation of the downstream pathways activated in AMPA receptor-mediated excitotoxicity in cultured rat hippocampal neurones.

Activation of neuropeptide Y receptors is neuroprotective against excitotoxicity in organotypic hippocampal slice cultures

Glutamate and NPY have been implicated in hippocampal neuropathology in temporal lobe epilepsy. Thus, we investigated the involvement of NPY receptors in mediating neuroprotection against excitotoxic insults in organotypic cultures of rat hippocampal slices. Exposure of hippocampal slice cultures to 2 microM AMPA (alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate) induced neuronal degeneration, monitored by propidium iodide uptake, of granule cells and CA1 pyramidal cells. For dentate granule cells, selective activation of Y1, Y2, or Y5 receptors with 1 microM [Leu31,Pro34]NPY, 300 nM NPY13-36 or 1 microM 500 nM NPY(19-23)-(Gly1,Ser3,Gln4,Thr6,Ala31,Aib32,Gln34)-PP, respectively, had a neuroprotective effect against AMPA, whereas only the activation of Y2 receptors was effective for CA1 pyramidal cells. When the slice cultures were exposed to 6 microM kainate, the CA3 pyramidal cells displayed significant degeneration, and in this case the activation of Y1, Y2, and Y5 receptors was neuroprotective. For the kainic acid-induced degeneration of CA1 pyramidal cells, it was again found that only the Y2 receptor activation was effective. Based on the present findings, it was concluded that Y1, Y2, and Y5 receptors effectively can modify glutamate receptor-mediated neurodegeneration in the hippocampus.

Cobalt staining of hippocampal neurons mediated by non-desensitizing activation of AMPA but not kainate receptors

Activation of calcium permeable glutamate receptors is likely to be important for neuronal death associated with brain trauma, stroke and neurodegenerative diseases. Cobalt uptake can be used to identify cells containing Ca2+-permeable non-NMDA ionotropic glutamate receptors. However, the relative contribution of AMPA and kainate receptors, and also the role of receptor desensitization on the influx of Co2+, remain to be established. We found that the selective non-desensitizing activation of AMPA receptors was efficient in promoting Co2+ staining. However, the selective activation of kainate receptors, even under non-desensitizing conditions, did not result in Co2+ staining. Taken together, our results show that non-desensitizing stimulation of AMPA, but not of kainate receptors, mediates the influx of Co2+ in cultured rat hippocampal neurons.

Inherent desensitisation-preventing properties of a novel, subtype-selective AMPA receptor agonist, (S)-CPW 399, as a possible explanation for its excitotoxic action in cultured cerebellar granule cells

The synthesis and pharmacological characterisation of (S)-CPW 399 as a novel, potent and subtype-selective agonist of the AMPA receptor was recently reported. Studies have been extended to investigate its excitotoxic action in primary cultures of mouse cerebellar granule cells. (S)-CPW 399 induced neuronal cell death in a time- and concentration-dependent manner (EC(50) approximately 70 microM) at 24-h exposure. (S)-CPW-induced neuronal death could be prevented by co-administration with either of the AMPA/kainate selective receptor antagonists 6-cyano-7-nitro-quinoxaline-2,3-dione (CNQX) and 6-nitro-7-sulphamoylbenzo[f]quinoxaline-2,3-dione (NBQX) or by the 2,3-benzodiazepine, GYKI 53655 (a selective AMPA receptor antagonist); while no protection was afforded by either the NMDA receptor antagonist D,L(+/-)-2-amino-5-phosphonopentanoate (APV) or by nifedipine (an L-type calcium channel antagonist) when used alone or in combination. Cyclothiazide, which blocks AMPA receptor desensitisation, caused minimal potentiation of (S)-CPW 399-induced neuronal death, supporting accumulating evidence that (S)-CPW 399 is a full AMPA receptor agonist that markedly prevents a receptor desensitised conformation. (S)-AMPA, (S)-willardiine (a naturally-occurring heterocyclic excitatory amino acid) and its halogenated derivative, (S)-5-fluorowillardiine, had no deleterious effect on neuronal viability when used alone but each, in the presence of cyclothiazide, induced a concentration-dependent excitotoxic cell death with a rank order of potency (fluorowillardiine>>AMPA=willardiine). (S)-CPW 399 stimulated an increase in intracellular free-calcium levels ([Ca(2+)](i)) in a concentration-dependent fashion (EC(50) approximately 5 microM) attaining a value of six-fold that of 'resting' cells at maximum stimulation; achieved at approximately 100 microM (S)-CPW 399. The (S)-CPW 399-stimulated increase in [Ca(2+)](i) was virtually abolished by GYKI 53655, NBQX, CNQX and by cobalt ions; markedly inhibited by nifedipine and marginally affected by D-APV. These results suggest that (S)-CPW 399 may be used as a pharmacological tool to aid in the investigation of the role of AMPA receptors in excitotoxicity and their molecular mechanisms of desensitisation.

Neuronal nitric oxide synthase proteolysis limits the involvement of nitric oxide in kainate-induced neurotoxicity in hippocampal neurons

In this work, we investigated the role of nitric oxide (NO) in neurotoxicity triggered by alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptor activation in cultured hippocampal neurons. In the presence of cyclothiazide (CTZ), short-term exposures to kainate (KA; 5 and 15 min, followed by 24-h recovery) decreased cell viability. Both NBQX and d-AP-5 decreased the neurotoxicity caused by KA plus CTZ. Long-term exposures to KA plus CTZ (24 h) resulted in increased toxicity. In short-, but not in long-term exposures, the presence of NO synthase (NOS) inhibitors (l-NAME and 7-NI) decreased the toxicity induced by KA plus CTZ. We also found that KA plus CTZ (15-min exposure) significantly increased cGMP levels. Furthermore, short-term exposures lead to decreased intracellular ATP levels, which was prevented by NBQX, d-AP-5 and NOS inhibitors. Immunoblot analysis revealed that KA induced neuronal NOS (nNOS) proteolysis, gradually lowering the levels of nNOS according to the time of exposure. Calpain, but not caspase-3 inhibitors, prevented this effect. Overall, these results show that NO is involved in the neurotoxicity caused by activation of non-desensitizing AMPA receptors, although to a limited extent, since AMPA receptor activation triggers mechanisms that lead to nNOS proteolysis by calpains, preventing a further contribution of NO to the neurotoxic process.

Negative allosteric modulation of AMPA-preferring receptors by the selective isomer GYKI 53784 (LY303070), a specific non-competitive AMPA antagonist

GYKI 53784 or LY303070 [(-)1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-4,5-dihydro-3-methylcarbamoyl-2,3-benzodiazepine] belongs to a new family of 2,3-benzodiazepine compounds (also called homophtalazines) selective and non-competitive antagonists at alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA) receptors. These compounds include the original GYKI-52466, its more potent derivative GYKI 53655 and the active isomer of the latter, GYKI 53784. This review summarizes current knowledge of this novel AMPA antagonist: GYKI 53784. GYKI 53784 is the most potent of the compounds in the 2,3-benzodiazepine class, blocking AMPA receptor-mediated responses. In contrast to the compounds of the quinoxalinedione family, that block AMPA as well as kainate receptors, GYKI 53784 does not block the activation of kainate receptors. Furthermore, GYKI 53784 does not act at the same receptor site as positive AMPA modulators (i.e., cyclothiazide, BDP-12, 1-BCP or aniracetam). GYKI 53784 is a powerful neuroprotective agent in both in vitro and in vivo models of AMPA receptor-mediated excitotoxicity. In contrast to NMDA receptor antagonists, whose favorable clinical actions are compromised by important side effects such as the impairment of memory functions, the selective AMPA antagonist, GYKI 53784, may be of potential clinical value, both in acute (stroke and trauma) and chronic (Alzheimer's disease, epilepsy) neurological disorders.

Protective effect of the antiepileptic drug candidate talampanel against AMPA-induced striatal neurotoxicity in neonatal rats

2,3-Benzodiazepines represent a family of specific, noncompetitive AMPA receptor antagonists with anticonvulsant and neuroprotective properties. In this study, the antiexcitotoxic potency of the clinical antiepileptic drug candidate, talampanel (4 x 2 mg/kg), and that of two related 2,3-benzodiazepines, 5-(4-aminophenyl)-8-methyl-9H-1,3-dioxolo[4,5-h][2,3]-benzodiazepine (GYKI 52466) (4 x 10 mg/kg) and GYKI 53784 (4 x 2 mg/kg), was investigated in 7-day-old rats. The AMPA antagonists were applied in four consecutive i.p. injections at 1-h intervals, the first dosage was given shortly after the intrastriatal injection of (S)-alpha-amino-3-hydroxy-5,7-methylisoxazole-4-propionic acid (AMPA) (2.5 nmol). All tested compounds protected animals from brain damage induced by AMPA as assessed 5 days later by using a tissue volume determination method based on computer-aided serial section reconstruction. GYKI 53784 (56.1 +/- 5.0% protection) and talampanel (42.5 +/- 5.3% protection) were more potent neuroprotective agents than GYKI 52466 (21.8 +/- 2.8% protection). Furthermore, the three compounds attenuated the unilateral AMPA injection-induced turning behavior and seizure-like events.Our present findings are in agreement with those of other investigators who found talampanel neuroprotective in various in vivo experimental models. These data indicate that besides being a promising antiepileptic drug candidate talampanel may have a value in the pharmacotherapy of acute and chronic neurodegenerative diseases, including perinatal ischemia/hypoxia-induced brain injuries, as well.

The pharmacological manipulation of glutamate receptors and neuroprotection

The overactivation of glutamate receptors is a major cause of Ca(2+) overload in cells, potentially leading to cell damage and death. There is an abundance of agents and mechanisms by which glutamate receptor activation can be prevented or modulated in order to control these effects. They include the well-established, competitive and non-competitive antagonists at the N-methyl-D-aspartate (NMDA) receptors and modulators of desensitisation of the alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) receptors. More recently, it has emerged that some compounds can act selectively at different subunits of glutamate receptors, allowing a differential blockade of subtypes. It is also becoming clear that a number of endogenous compounds, including purines, can modify glutamate receptor sensitivity. The kynurenine pathway is an alternative but distinct pathway to the generation of glutamate receptor ligands. The products of tryptophan metabolism via the kynurenine pathway include both quinolinic acid, a selective agonist at NMDA receptors, and kynurenic acid, an antagonist at several glutamate receptor subtypes. The levels of these metabolites change as a result of the activation of inflammatory processes and immune-competent cells, and may have a significant impact on Ca(2+) fluxes and neuronal damage. Drugs which target some of these various sites and processes, or which change the balance between the excitotoxin quinolinic acid and the neuroprotective kynurenic acid, could also have potential as neuroprotective drugs.

AMPA-induced Ca(2+) influx in cultured rat cortical nonpyramidal neurones: pharmacological characterization using fura-2 microfluorimetry

Immunocytochemical and Co(2+) uptake studies revealed that in primary cultures of rat cortical neurones, the majority of neurones are gamma-aminobutyric acid (GABA) immunopositive and can express Ca(2+)-permeable alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) receptors. By fura-2 microfluorimetry, it was shown that the stimulation with the selective agonist (S)-AMPA (0.3-300 microM) induced a concentration-dependent but cell-variable increase in intracellular Ca(2+) concentration ([Ca(2+)](i)) (EC(50) value 7.4 microM) in more than 80% of the medium-sized multipolar neurones studied. The AMPA-induced rise in [Ca(2+)](i) seems to be due to Ca(2+) entry through AMPA receptor channels, because the response was abolished in Ca(2+)-free solution and by AMPA receptor selective antagonists, but was not significantly influenced by cyclopiazonic acid, an inhibitor of the endoplasmatic Ca(2+)-ATPase, by selective N-methyl-D-aspartic acid (NMDA) receptor antagonists, as well as the Na(+) channel blocker tetrodotoxin and the majority of tested Ca(2+) channel blockers. In conclusion, the results indicate that the cerebral cortical neurones in culture represent mostly GABAergic interneurone-like cells and the majority of them possess Ca(2+)-permeable AMPA receptors, important for intracellular signal transduction and neuronal plasticity.

Kainic acid-induced neuronal cell death in cerebellar granule cells is not prevented by caspase inhibitors

1. We examined the role of non-NMDA receptors in kainic acid (KA)-induced apoptosis in cultures of rat cerebellar granule cells (CGCs). KA (1 - 500 microM) induced cell death in a concentration-dependent manner, which was prevented by NBQX and GYKI 52466, non-NMDA receptor antagonists. Moreover, AMPA blocked KA-induced excitotoxicity, through desensitization of AMPA receptors. 2. Similarly, KA raised the intracellular calcium concentration of CGCs, which was inhibited by NBQX and GYKI 52466. Again, AMPA (100 microM) abolished the KA (100 microM)-induced increase in intracellular calcium concentration. 3. KA-induced cell death in CGCs had apoptotic features, which were determined morphologically, by DNA fragmentation, and by expression of the prostate apoptosis response-4 protein (Par-4). 5. KA (500 microM) slightly (18%) increased caspase-3 activity, which was strongly enhanced by colchicine (1 microM), an apoptotic stimulus. However, neither Z-VAD.fmk, a pan-caspase inhibitor, nor the more specific caspase-3 inhibitor, Ac-DEVD-CHO, prevented KA-induced cell death or apoptosis. In contrast, both drugs inhibited colchicine-induced apoptosis. 5. The calpain inhibitor ALLN had no effect on KA or colchicine-induced neurotoxicity. 6. Our findings indicate that colchicine-induced apoptosis in CGCs is mediated by caspase-3 activation, unlike KA-induced apoptosis.

Death-associated protein (DAP) kinase plays a central role in ceramide-induced apoptosis in cultured hippocampal neurons

Treatment of cultured hippocampal neurons with high concentrations of short-chain acyl ceramide derivatives, such as N-hexanoyl-D-sphingosine (C(6)-Cer), results in apoptotic cell death. We now show that death-associated protein (DAP) kinase plays an important role in mediating this effect. Upon incubation with C(6)-Cer, DAP kinase levels are elevated as early as 1 h after treatment, reaching levels 2-3-fold higher than untreated cells after 4 h. Neurons cultured from DAP kinase-deficient mice were significantly less sensitive to apoptosis induced by C(6)-Cer or by ceramide generated by high concentrations of nerve growth factor. A peptide corresponding to the 17 amino acids at the C terminus of DAP kinase protected wild type neurons from C(6)-Cer-induced death and from death induced by the addition of exogenous bacterial neutral sphingomyelinase, whereas a scrambled peptide had no protective effect, implying that the DAP kinase C-terminal tail inhibits the function of DAP kinase. Together, these data demonstrate that DAP kinase plays a central role in ceramide-induced cell death in neurons, but the pathway in which DAP kinase is involved is not the only one via which ceramide can induce apoptosis.

Differential postreceptor signaling events triggered by excitotoxic stimulation of different ionotropic glutamate receptors in retinal neurons

The aim of this work was to investigate whether excitotoxicity induced by overstimulation of different ionotropic glutamate receptors could trigger different intracellular signaling cascades. Cultured chick neuronal retina cells, essentially amacrine-like, were particularly sensitive to the toxicity induced by non-NMDA glutamate receptor agonists. One hour stimulation with 100 microM kainate induced a reduction of cell viability of about 44%, as assessed by the MTT test 24 hr after stimulation. Kainate-induced toxicity was mediated through AMPA receptors. Glutamate (100 microM, 1 hr) reduced cell viability by 26%, essentially acting through N-methyl-D-aspartate receptors. Five hours after stimulation, neuronal retina cells had an apoptotic-like nuclear morphology. In retinal neurons, the excitotoxic stimulation, with either glutamate or kainate, induced a calcium-dependent enhancement of the DNA-binding activity of the activating protein-1 (AP-1) transcription factor, which was maximal 2 hr after stimulation. Glutamate induced a greater increase in the AP-1 DNA-binding activity than did kainate. Supershift assays using antibodies directed against different members of the Fos and Jun protein families showed that the AP-1 complex in retinal neurons includes proteins of the Fos family, namely, Fra-2, c-Jun, and Jun D. The DNA-binding activity of the nuclear factor-kappaB transcription factor was not significantly changed upon excitotoxic stimulation with any agonist. Stimulation of glutamate receptors with 100 microM kainate or 100 microM glutamate for 2 min was sufficient to induce the activation of the extracellular signal-regulated kinase (ERK). Inhibition of the ERK activation with the MEK inhibitors U 0126 and PD 98059 increased the toxicity induced by kainate but was without effect on the toxicity induced by glutamate. These results indicate that, although stimulation with both glutamate receptor agonists increased ERK phosphorylation, only kainate-induced ERK activation correlates with the activation of a survival signaling pathway. Our results suggest that, in chick embryo retinal neurons, the signaling pathways that mediate excitotoxic cell death and neuroprotection are stimulus specific.

Glutamate receptor desensitization block potentiates the stimulated GABA release through external Ca2+-independent mechanisms from granule cells of olfactory bulb

Glutamate stimulated release of [3H]GABA was studied, during receptor desensitization block and its modulation by voltage gated Ca2+ channels, internal Ca2+ mobilization and GABA transport inhibitors from olfactory bulb slices. Under control conditions, glutamate and agonists induced release was strongly inhibited by Mg/0 Ca2+ Krebs and Cd2+ and partially inhibited by Ni2+ and nifedipine. Cyclothiazide, which blocks desensitization of glutamate receptors, potentiated glutamate, kainate, AMPA and quisqualate induced release. This effect was less dependent of entry of external Ca2+, but was inhibited by trifluoperazine and thapsigargin, inhibitors of Ca2+-calmodulin and endoplasmatic Ca2+ ATPase respectively. Nipecotic acid and NO-711, inhibitors of the GABA transporter, were also able to reduce cyclothiazide potentiated release induced by the 4 secretagogues. Under control conditions, glutamate stimulates the release of GABA in cooperation with VDCC. However, during receptor desensitization block, glutamate stimulated GABA release is mainly modulated through mechanisms dependent on internal Ca2+ mobilization and reversal of the GABA transporter.

Modulation of intracellular calcium changes and glutamate release by neuropeptide Y1 and Y2 receptors in the rat hippocampus: differential effects in CA1, CA3 and dentate gyrus

In the present work, we investigated the role of pre- and post-synaptic neuropeptide Y1 (NPY1) and Y2 receptors on the calcium responses and on glutamate release in the rat hippocampus. In cultured hippocampal neurones, we observed that only NPY1 receptors are involved in the modulation of intracellular free calcium concentration ([Ca(2+)](i)). In 88% of the neurones analysed, the increase in the [Ca(2+)](i), in response to depolarization with 50 mM KCl, was inhibited by 1 microM [Leu31,Pro34]NPY, whereas 300 nM NPY13-36 was without effect. However, studies with hippocampal synaptosomes showed that both NPY1 and Y2 receptors can modulate the [Ca(2+)](i) and glutamate release. The pharmacological characterization of the NPY-induced inhibition of glutamate release indicated that Y2 receptors play a predominant role, both in the modulation of Ca(2+)-dependent and -independent glutamate release. However, we could distinguish between Y1 and Y2 receptors by using [Leu31,Pro34]NPY and NPY13-36. Active pre-synaptic Y1 receptors are present in the dentate gyrus (DG) as well as in the CA3 subregion, but its activity was not revealed by using the endogenous agonist, NPY. Concerning the Y2 receptors, they are present in the three subregions (CA1, CA3 and DG) and were activated by either NPY13-36 or NPY. The present data support a predominant role for NPY2 receptors in mediating NPY-induced inhibition of glutamate release in the hippocampus, but the physiological relevance of the presently described DG and CA3 pre-synaptic NPY1 receptors remains to be clarified.

Role of kainate receptor activation and desensitization on the [Ca(2+)](i) changes in cultured rat hippocampal neurons

We investigated the role of kainate (KA) receptor activation and desensitization in inducing the increase in the intracellular free Ca(2+) concentration ([Ca(2+)](i)) in individual cultured rat hippocampal neurons. The rat hippocampal neurons in the cultures were shown to express kainate receptor subunits, KA2 and GluR6/7, either by immunocytochemistry or by immunoblot analysis. The effect of LY303070, an alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate (AMPA) receptor antagonist, on the alterations in the [Ca(2+)](i) caused by kainate showed cell-to-cell variability. The [Ca(2+)](i) increase caused by kainate was mostly mediated by the activation of AMPA receptors because LY303070 inhibited the response to kainate in a high percentage of neurons. The response to kainate was potentiated by concanavalin A (Con A), which inhibits kainate receptor desensitization, in 82.1% of the neurons, and this potentiation was not reversed by LY303070 in about 38% of the neurons. Also, upon stimulation of the cells with 4-methylglutamate (MGA), a selective kainate receptor agonist, in the presence of Con A, it was possible to observe [Ca(2+)](i) changes induced by kainate receptor activation, because LY303070 did not inhibit the response in all neurons analyzed. In toxicity studies, cultured rat hippocampal neurons were exposed to the drugs for 30 min, and the cell viability was evaluated at 24 hr using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. The selective activation of kainate receptors with MGA, in the presence of Con A, induced a toxic effect, which was not prevented by LY303070, revealing a contribution of a small subpopulation of neurons expressing kainate receptors that independently mediate cytotoxicity. Taken together, these results indicate that cultured hippocampal neurons express not only AMPA receptors, but also kainate receptors, which can modulate the [Ca(2+)](i) and toxicity.