Kainate-induced retina amacrine-like cell damage is mediated by AMPA receptors

The endocannabinoid 2-arachidonoylglycerol and dual ABHD6/MAGL enzyme inhibitors display neuroprotective and anti-inflammatory actions in the in vivo retinal model of AMPA excitotoxicity

The endocannabinoid system has been shown to be a putative therapeutic target for retinal disease. Here, we aimed to investigate the ability of the endocannabinoid 2-arachidonoylglycerol (2-AG) and novel inhibitors of its metabolic enzymes, α/β-hydrolase domain-containing 6 (ABHD6) and monoacylglycerol lipase (MAGL), a) to protect the retina against excitotoxicity and b) the mechanisms involved in the neuroprotection. Sprague-Dawley rats, wild type and Akt2^-/- C57BL/6 mice were intravitreally administered with phosphate-buffered saline or (RS)-α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid hydrobromide (AMPA). 2-AG was intravitreally co-administered with AMPA in the absence and presence of AM251 or AM630 (cannabinoid 1 and 2 receptor antagonists, respectively) or Wortmannin [Phosphoinositide 3-Kinase (PI3K)/Akt inhibitor]. Inhibitors of ABHD6 and dual ABHD6/MAGL (AM12100 and AM11920, respectively) were co-administered with AMPA intravitreally in rats. Immunohistochemistry was performed using antibodies raised against retinal neuronal markers (bNOS), microglia (Iba1) and macroglia (GFAP). TUNEL assay and real-time PCR were also employed. The CB2 receptor was expressed in rat retina (approx. 62% of CB1 expression). 2-AG attenuated the AMPA-induced increase in TUNEL⁺ cells. 2-AG activation of both CB1 and CB2 receptors and the PI3K/Akt downstream signaling pathway, as substantiated by the use of Akt2^-/- mice, afforded neuroprotection against AMPA excitotoxicity. AM12100 and AM11920 attenuated the AMPA-induced glia activation and produced a dose-dependent partial neuroprotection, with the dual inhibitor AM11920 being more efficacious. These results show that 2-AG has the pharmacological profile of a putative therapeutic for retinal diseases characterized by neurodegeneration and neuroinflammation, when administered exogenously or by the inhibition of its metabolic enzymes.

Growth hormone protects against kainate excitotoxicity and induces BDNF and NT3 expression in chicken neuroretinal cells

There is increasing evidence to suggest a beneficial neuroprotective effect of growth hormone (GH) in the nervous system. While our previous studies have largely focused on retinal ganglion cells (RGCs), we have also found conclusive evidence of a pro-survival effect of GH in cells of the inner nuclear layer (INL) as well as a protective effect on the dendritic trees of the inner plexiform layer (IPL) in the retina. The administration of GH in primary neuroretinal cell cultures protected and induced neural outgrowths. Our results, both in vitro (embryo) and in vivo (postnatal), showed neuroprotective actions of GH against kainic acid (KA)-induced excitotoxicity in the chicken neuroretina. Intravitreal injections of GH restored brain derived neurotrophic factor (BDNF) expression in retinas treated with KA. In addition, we demonstrated that GH over-expression and exogenous administration increased BDNF and neurotrophin-3 (NT3) gene expression in embryonic neuroretinal cells. Thus, GH neuroprotective actions in neural tissues may be mediated by a complex cascade of neurotrophins and growth factors which have been classically related to damage prevention and neuroretinal tissue repair.

Neurodegeneration as a primary change and role of neuroprotection in diabetic retinopathy

Diabetic retinopathy (DR) was earlier recognized as a vascular disease, but nowadays, it is considered as a neurovascular disorder. Neuronal death is the primary change which leads to various vascular changes which are visible to an ophthalmologist. But these changes are feature of an advanced disease and can affect vision at any moment of time. There are various evidences which suggests that glutamate excitotoxicity, hyperhomocysteinemia, kynurenic acid, and erythro-poietin plays important role in causation of retinal ganglionic cell apoptosis in diabetic patients. Adaptive optics, a new imaging technique, also showed that loss of photoreceptors (specialized neurons) is the early change in diabetic retinopathy. These changes suggest DR as a neurovascular disorder. Neuroprotective agents also showed good results in delaying progression of DR especially memantine, insulin receptor activation, and neurotrophic factors. More research in this field will help us to find novel therapeutic measures for DR, which can delay or even stop progression of DR at a very early stage.

Role of the ubiquitin-proteasome system in brain ischemia: friend or foe?

The ubiquitin-proteasome system (UPS) is a catalytic machinery that targets numerous cellular proteins for degradation, thus being essential to control a wide range of basic cellular processes and cell survival. Degradation of intracellular proteins via the UPS is a tightly regulated process initiated by tagging a target protein with a specific ubiquitin chain. Neurons are particularly vulnerable to any change in protein composition, and therefore the UPS is a key regulator of neuronal physiology. Alterations in UPS activity may induce pathological responses, ultimately leading to neuronal cell death. Brain ischemia triggers a complex series of biochemical and molecular mechanisms, such as an inflammatory response, an exacerbated production of misfolded and oxidized proteins, due to oxidative stress, and the breakdown of cellular integrity mainly mediated by excitotoxic glutamatergic signaling. Brain ischemia also damages protein degradation pathways which, together with the overproduction of damaged proteins and consequent upregulation of ubiquitin-conjugated proteins, contribute to the accumulation of ubiquitin-containing proteinaceous deposits. Despite recent advances, the factors leading to deposition of such aggregates after cerebral ischemic injury remain poorly understood. This review discusses the current knowledge on the role of the UPS in brain function and the molecular mechanisms contributing to UPS dysfunction in brain ischemia with consequent accumulation of ubiquitin-containing proteins. Chemical inhibitors of the proteasome and small molecule inhibitors of deubiquitinating enzymes, which promote the degradation of proteins by the proteasome, were both shown to provide neuroprotection in brain ischemia, and this apparent contradiction is also discussed in this review.

Excitotoxic stimulation downregulates the ubiquitin-proteasome system through activation of NMDA receptors in cultured hippocampal neurons

Overactivation of glutamate receptors contributes to neuronal damage (excitotoxicity) in ischemic stroke but the detailed mechanisms are not fully elucidated. Brain ischemia is also characterized by an impairment of the activity of the proteasome, one of the major proteolytic systems in neurons. We found that excitotoxic stimulation with glutamate rapidly decreases ATP levels and the proteasome activity, and induces the disassembly of the 26S proteasome in cultured rat hippocampal neurons. Downregulation of the proteasome activity, leading to an accumulation of ubiquitinated proteins, was mediated by calcium entry through NMDA receptors and was only observed in the nuclear fraction. Furthermore, excitotoxicity-induced proteasome inhibition was partially sensitive to cathepsin-L inhibition and was specifically induced by activation of extrasynaptic NMDA receptors. Oxygen and glucose deprivation induced neuronal death and downregulated the activity of the proteasome by a mechanism dependent on the activation of NMDA receptors. Since deubiquitinating enzymes may regulate proteins half-life by counteracting ubiquitination, we also analyzed how their activity is regulated under excitotoxic conditions. Glutamate stimulation decreased the total deubiquitinase activity in hippocampal neurons, but was without effect on the activity of Uch-L1, showing that not all deubiquitinases are affected. These results indicate that excitotoxic stimulation with glutamate has multiple effects on the ubiquitin-proteasome system which may contribute to the demise process in brain ischemia and in other neurological disorders.

Activation of somatostatin receptor (sst 5) protects the rat retina from AMPA-induced neurotoxicity

In a recent study, we employed an in vivo model of retinal excitotoxicity to investigate the neuroprotective effect of somatostatinergic agents. Intravitreal administration of somatostatin and sst(2) selective agonists protected the retina from (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid hydrobromide (AMPA) induced excitotoxicity. The sst(1) and sst(4) selective ligands had no effect (Kiagiadaki and Thermos, 2008). The presence of sst(5) receptors in rat retina was only recently reported (Ke and Zhong, 2007). Synthetic agonists that activate sst(2) receptors also bind with high affinity to the sst(5) subtype. In the present study the putative neuroprotective effects of sst(5) receptor activation were investigated. Adult female and male Sprague-Dawley (250-350g) rats were employed. Groups of animals received intravitreally PBS (50mM) or AMPA (42 nmol/eye) alone or in combination with L-817,818 (sst(5), 10(-5), 10(-4)M). To exclude neuroprotective effects via the activation of sst(2) receptors, L-817,818 (10(-4)M) was coinjected with the sst(2) antagonist CYN-154806 (10(-4)M). Immunohistochemistry (IHC) studies using the anti-retinal marker choline acetyltransferase (ChAT) and TUNEL staining were employed to examine retinal cell loss and protection. IHC and Western blot analysis were also employed to assess whether the sst(5) receptors are viable in the AMPA treated tissue as compared to control retina. sst(5) receptors were not affected by AMPA. L-817,818 protected the retina from the AMPA insult in the dose of 10(-4)M, while CYN-154806 (10(-4)M) had no effect on the sst(5) neuroprotection. TUNEL staining confirmed the AMPA-induced retinal toxicity and the L-817,818 neuroprotection. These results demonstrate for the first time that sst(5) receptors are functional in the retina, and that sst(5) analogs administered intravitreally protect the retina from excitotoxicity. Further studies are essential to ascertain the therapeutic relevance of these results.

Domoic acid toxicologic pathology: a review

Domoic acid was identified as the toxin responsible for an outbreak of human poisoning that occurred in Canada in 1987 following consumption of contaminated blue mussels [Mytilus edulis]. The poisoning was characterized by a constellation of clinical symptoms and signs. Among the most prominent features described was memory impairment which led to the name Amnesic Shellfish Poisoning [ASP]. Domoic acid is produced by certain marine organisms, such as the red alga Chondria armata and planktonic diatom of the genus Pseudo-nitzschia. Since 1987, monitoring programs have been successful in preventing other human incidents of ASP. However, there are documented cases of domoic acid intoxication in wild animals and outbreaks of coastal water contamination in many regions world-wide. Hence domoic acid continues to pose a global risk to the health and safety of humans and wildlife. Several mechanisms have been implicated as mediators for the effects of domoic acid. Of particular importance is the role played by glutamate receptors as mediators of excitatory neurotransmission and the demonstration of a wide distribution of these receptors outside the central nervous system, prompting the attention to other tissues as potential target sites. The aim of this document is to provide a comprehensive review of ASP, DOM induced pathology including ultrastructural changes associated to subchronic oral exposure, and discussion of key proposed mechanisms of cell/tissue injury involved in DOM induced brain pathology and considerations relevant to food safety and human health.

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.

AMPA and Kainate Receptors in Turtle Retina: An Immunocytochemical Study

1. Glutamate is one of the main neurotransmitters in the retina. Its effects are mediated by a large number of ionotropic and metabotropic receptors.2. The distribution of ionotropic AMPA receptor subunits GluR1-4, kainate receptor subunits GluR5-7 and KA2, as well as delta receptors 1-2 was studied in turtle retina. Indirect immunofluorescence was used to localize the different receptor subunits viewed using light microscopy.3. Results show that all subunits, with exception of GluR1 and GluR5, are widely distributed in the turtle retina.4. They are mainly located in the both plexiform layers of the retina where punctate staining, a sign for synaptic localization, is observed.5. The vast majority of the subunits possess specific pattern of staining that allow to suppose that they are involved in different retinal circuits.6. It can be assumed that the GluR2/3 and GluR6/7 subunits are expressed on the dendrites of a subpopulation of bipolar cells that are immunopositive for alpha-isoform of protein kinase C (PKCalpha). The GluR2/3 and GluR6/7 subunits are most probably used by the same PKCalpha immunopositive bipolar cells in their synaptic contacts with the third-order retinal neurons, the amacrine and ganglion cells.

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.

Diltiazem-induced neuroprotection in glutamate excitotoxicity and ischemic insult of retinal neurons

PURPOSE

Cell death is often related to an abnormal increase in Ca(2+) flux. In the retina, Ca(2+) channels are mainly from the L-type that do not inactivate with time. Under excitotoxic and ischemic conditions, their continuous activation may therefore contribute significantly to the lethal Ca(2+) influx. To assess this hypothesis, the Ca(2+) channel blocker, diltiazem, was applied in excitotoxic and ischemic conditions.

METHODS

To induce excitotoxicity, retinal cell cultures from newborn rats were incubated with glutamate. The toxicity of glutamate was quantified by neuronal immunostaining with an antibody directed against the neuron specific enolase. Glutamate receptor function in vitro was assessed in pig retinal cell cultures by patch clamp recording. Retinal ischemia was induced by raising the intraocular pressure in adult rats. Retinal cell loss was quantified on retinal sections by measuring nuclear cell densities.

RESULTS

In retinal cell culture, glutamate application induced a major cell loss. This cell loss was attributed to glutamate excitotoxicity because glutamate receptor blockers like MK-801 and CNQX increased significantly neuronal survival. MK-801 and CNQX, which block NMDA and AMPA/Kainate receptors, respectively, had additive effects. Expression of AMPA/Kainate glutamate receptors in mixed adult retinal cell cultures was attested by patch clamp recording. In newborn rat retinal culture, glutamate excitotoxicity was significantly reduced by addition of the L-type Ca(2+) channel blocker, diltiazem. In in vivo experiments, the increase in ocular pressure induced a decrease in cell number in the inner nuclear and ganglion cell layers. When animals received diltiazem injections, the ischemic treatment induced a less severe reduction in retinal cells; this neuroprotection was statistically significant in the ganglion cell layer.

CONCLUSION

These results are consistent with previous studies suggesting that Ca(2+) channel activation contributes to retinal cell death following either glutamate excitotoxicity or retinal ischemia. Under both conditions, the L-type Ca(2+) channel blocker, diltiazem, can limit cell death. These results extend the potential application of diltiazem in retinal neuroprotection to retinal pathologies involving glutamate excitotoxicity and ischemia.

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.

Down-regulation of GluR2 is associated with Ca2+-dependent protease activities in kainate-induced apoptotic cell death in cultured [correction of culturd] rat hippocampal neurons

In the present study, the molecular mechanisms underlying kainate-induced neurotoxicity were characterized in cultured rat hippocampal neurons. Long-term exposure to kainate caused typically apoptotic cell death, which was accompanied by the accumulation of calcium, marked down-regulation of GluR2 subunit, and the activation of calpain and caspase-3. All these alterations were prevented by alpha-amino-3-hydroxy-5-methylisoxazole-4-proprionic acid (AMPA) receptor antagonist CNQX, but not by NMDA receptor antagonist MK801 and membrane L-type calcium channel antagonist nifedipine. In the presence of cyclothiazide, kainate-induced neurotoxicity was significantly enhanced. Inhibition of either caspases by zVAD-fmk or calpains by calpeptin protected neurons from neurotoxicity. These results suggest that long-term exposure of hippocampal neurons to kainate causes apoptosis, whose mechanisms involve multiple Ca(2+)-dependent cascades, in which AMPA receptor subunits may be targets for Ca(2+)-activated protease-mediated degradation during kainate-induced neuron apoptosis.

Low sensitivity of retina to AMPA-induced calcification

Glutamate is involved in most CNS neurodegenerative diseases. In particular, retinal diseases such as retinal ischemia, retinitis pigmentosa, and diabetic retinopathy are associated with an excessive synaptic concentration of this neurotransmitter. To gain more insight into retinal excitotoxicity, we carried out a dose-response study in adult rats using alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), a glutamate analogue. AMPA intraocular injections (between 0.27 and 10.8 nmol) caused no morphologic modification, but a 10.8 + 21 nmol double injection in a 10-day interval produced a lesion characterized by discrete neuronal loss, astroglial and microglial reactions, and calcium precipitation. Abundant calcium deposits similar to those present in rat and human brain excitotoxicity or hypoxia-ischemia neurodegeneration were detected by alizarin red staining within the retinal surface and the optic nerve. Glial reactivity, associated normally with astrocytes in the nerve fiber, was assessed in Müller cells. GABA immunoreactivity was detected not only in neuronal elements but also in Müller cells. In contrast to the high vulnerability of the brain to excitotoxin microinjection, AMPA-induced retinal neurodegeneration may provide a useful model of low central nervous system sensitivity to excitotoxicity.

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.

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.

Expression of AMPA/kainate receptors during development of chick embryo retina cells: in vitro versus in vivo studies

The activity and the subunit expression of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA)/kainate ionotropic glutamate receptors were studied in retina cells developing in chick embryos and in retina cells cultured as retinospheroids, at the same stages of development. In the retinospheroids, the activity of the AMPA/kainate receptors was monitored by following the changes in the intracellular free calcium concentration ([Ca(2+)](i)), in response to AMPA, kainate or to L-glutamate, and the expression of the receptor subunits GluR1, GluR2/3, GluR4 and GluR6/7 was determined in the retinospheroids and in chick retinas by immunodetection using polyclonal antibodies. The changes in [Ca(2+)](i) in response to 400 microM kainate increased from 5h in vitro to 3 days, and remained constant until day 14, whereas the [Ca(2+)](i) in response to 500 microM L-glutamate or 400 microM AMPA increased from 5h in vitro to 3 days, and thereafter decreased slightly until day 14. The [Ca(2+)](i) responses to kainate are mainly due to AMPA receptor stimulation, since the signals were abolished by LY303070, the AMPA receptor antagonist, and were not affected by MK-801, the NMDA receptor antagonist. In retinospheroids, the levels of expression of GluR1 subunit increased from 5h in vitro until day 7, then decreased until day 14. The levels of expression of GluR2/3 and GluR4 subunits increased from 5h in vitro until day 10, and remained constant until day 14. The levels of kainate receptor subunits GluR6/7 increased from 5h in vitro until day 3, and thereafter decreased slightly until day 14. In the retinas, the expression of GluR1 and GluR6/7 subunits increased from day 8 until day 15, and then decreased until day 22 (post-natal 1). The subunits GluR2/3 and GluR4 increased from day 8 until day 18, and remained constant until day 22. The results suggest that AMPA/kainate receptors are expressed at early embryonic stages, although at low levels and before synapse formation (E12). However, the AMPA receptors are not completely functional at the first stage studied since they do not respond to the agonist AMPA. Also, the patterns of AMPA/kainate receptor subunit expression in retinospheroids of chick embryo retina cells cultured in vitro and in retina cells developing in the embryo (in vivo) were similar, indicating that the AMPA/kainate receptor subunits expression in these primary cultures mimics their expression in the developing chick retina.

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.

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.

Glutamate regulates the viability of retinal cells in culture

In this study, we show that glutamate regulates the viability of cultured retinal cells upon transient glucose deprivation. At low concentrations (10-100 microM) glutamate decreased MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] reduction to about 50% of control and decreased intracellular ATP levels (about 4-fold) after transient glucose removal. Under these conditions, the decrease in MTT reduction was associated with the activation of NMDA (N-methyl-D-aspartate) receptors. Upon exposure to high (10 mM) glutamate and transient glucose deprivation, the intracellular levels of glutamate increased. High glutamate significantly counteracted the decrease in MTT reduction and ATP production observed in the presence of low glutamate concentrations. AOAA (aminooxyacetic acid), a non-specific inhibitor of mitochondrial transaminases, enhanced the intracellular glutamate levels, but did not largely affect glutamate-mediated changes in MTT reduction or ATP production. Furthermore, the intracellular levels of pyruvate were not significantly altered, suggesting that changes in ATP production were not due to an increase in glycolysis. Thus, the recovery from glucose deprivation seems to be facilitated in retinal neuronal cells that had been exposed to high glutamate, in comparison with low glutamate, suggesting a role for high glutamate and glucose in maintaining retinal cell function following conditions of glucose scarcity.

Guanine nucleotides block agonist-driven 45Ca2+ influx in chick embryo retinal explants

AMPA (EC50 = 1.0 x 10(-6) M) and NMDA (EC50 = 1.3 x 10(-4) M) trigger 45Ca2+ influx in 13-day chick embryonic retinal explants. This agonist-driven cationic flux is specifically inhibited by typical competitive antagonists, such as 6,7-dinitroquinoxaline-2,3-dione (DNQX) and 2-amino-7-phosphonoheptanoate (AP7), respectively. Guanine nucleotides, with different degrees of phosphorylation, namely 5'-GMP, guanosine 5'-O-(2-thiodiphosphate) (GDPbetaS), guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) and 5'-guanylyl-imidodiphosphate (GppNHp), are also efficient blockers of 45Ca2+ influx. These results confirm the antagonistic behavior of guanine nucleotides towards ionotropic glutamate receptors and suggest a convenient experimental approach for screening of novel agonists and antagonists.

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

We investigated the role of desensitization of alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate (AMPA) receptors on the neurotoxicity and on the [Ca2+]i changes induced by kainate or by AMPA in cultured rat hippocampal neurons. The neuronal viability was evaluated either by the 3-(4,5-dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium bromide (MTT) assay, or by analysis of cell morphology. Short-term exposure of the neurons to kainate or AMPA (30 min) was not toxic, but the exposure for 24 h to the excitotoxic drugs caused a concentration-dependent neurotoxic effect which was prevented by LY 303070, a noncompetitive AMPA receptor antagonist. In the presence of cyclothiazide (CTZ), kainate or AMPA was toxic (30 min exposure), or the toxic effect was significantly enhanced (24 h exposure), but in this case LY 303070 did not completely protect the cells against kainate-induced toxicity. The alterations in the [Ca2+]i caused by kainate or AMPA showed a great cell-to-cell variability. LY 303070 completely or partially inhibited the responses stimulated by kainate. CTZ differentially affected the responses evoked by kainate or AMPA. In the majority of hippocampal neurons, CTZ did not potentiate, or only slightly potentiated, the kainate-stimulated responses but in 11% of neurons there was a great potentiation. In AMPA-stimulated neurons, the responses were slightly or greatly potentiated in the majority of neurons, but not in all of them. The results show that AMPA and kainate may be toxic, depending on the time of exposure and on the blockade of the desensitization of the AMPA receptors. Overall, our results clearly show that there exist different populations of hippocampal neurons with different sensitivities to kainate, AMPA, CTZ and LY 303070. Moreover, the effects of CTZ on both [Ca2+]i alterations and neurotoxicity are not fully correlated.