Differential contribution of the guanylyl cyclase-cyclic GMP-protein kinase G pathway to the proliferation of neural stem cells stimulated by nitric oxide

Nitric oxide (NO) is an important inflammatory mediator involved in the initial boost in the proliferation of neural stem cells following brain injury. However, the mechanisms underlying the proliferative effect of NO are still unclear. The aim of this work was to investigate whether cyclic GMP (cGMP) and the cGMP-dependent kinase (PKG) are involved in the proliferative effect triggered by NO in neural stem cells. For this purpose, cultures of neural stem cells isolated from the mouse subventricular zone (SVZ) were used. We observed that long-term exposure to the NO donor (24 h), NOC-18, increased the proliferation of SVZ cells in a cGMP-dependent manner, since the guanylate cyclase inhibitor, ODQ, prevented cell proliferation. Similarly to NOC-18, the cGMP analogue, 8-Br-cGMP, also increased cell proliferation. Interestingly, shorter exposures to NO (6 h) increased cell proliferation in a cGMP-independent manner via the ERK/MAP kinase pathway. The selective inhibitor of PKG, KT5823, prevented the proliferative effect induced by NO at 24 h but not at 6 h. In conclusion, the proliferative effect of NO is initially mediated by the ERK/MAPK pathway, and at later stages by the GC/cGMP/PKG pathway. Thus, our work shows that NO induces neural stem cell proliferation by targeting these two pathways in a biphasic manner.

Effects of 3,4-methylenedioxymethamphetamine administration on retinal physiology in the rat

3,4-Methylenedioxymethamphetamine (MDMA; ecstasy) is known to produce euphoric states, but may also cause adverse consequences in humans, such as hyperthermia and neurocognitive deficits. Although MDMA consumption has been associated with visual problems, the effects of this recreational drug in retinal physiology have not been addressed hitherto. In this work, we evaluated the effect of a single MDMA administration in the rat electroretinogram (ERG). Wistar rats were administered MDMA (15 mg/kg) or saline and ERGs were recorded before (Baseline ERG), and 3 h, 24 h, and 7 days after treatment. A high temperature (HT) saline-treated control group was also included. Overall, significantly augmented and shorter latency ERG responses were found in MDMA and HT groups 3 h after treatment when compared to Baseline. Twenty-four hours after treatment some of the alterations found at 3 h, mainly characterized by shorter latency, tended to return to Baseline values. However, MDMA-treated animals still presented increased scotopic a-wave and b-wave amplitudes compared to Baseline ERGs, which were independent of temperature elevation though the latter might underlie the acute ERG alterations observed 3 h after MDMA administration. Seven days after MDMA administration recovery from these effects had occurred. The effects seem to stem from specific changes observed at the a-wave level, which indicates that MDMA affects subacutely (at 24 h) retinal physiology at the outer retinal (photoreceptor/bipolar) layers. In conclusion, we have found direct evidence that MDMA causes subacute enhancement of the outer retinal responses (most prominent in the a-wave), though ERG alterations resume within one week. These changes in photoreceptor/bipolar cell physiology may have implications for the understanding of the subacute visual manifestations induced by MDMA in humans.

Methamphetamine transiently increases the blood-brain barrier permeability in the hippocampus: role of tight junction proteins and matrix metalloproteinase-9

Methamphetamine (METH) is a powerful stimulant drug of abuse that has steadily gained popularity worldwide. It is known that METH is highly neurotoxic and causes irreversible damage of brain cells leading to neurological and psychiatric abnormalities. Recent studies suggested that METH-induced neurotoxicity might also result from its ability to compromise blood-brain barrier (BBB) function. Due to the crucial role of BBB in the maintenance of brain homeostasis and protection against toxic molecules and pathogenic organisms, its dysfunction could have severe consequences. In this study, we investigated the effect of an acute high dose of METH (30mg/kg) on BBB permeability after different time points and in different brain regions. For that, young adult mice were sacrificed 1h, 24h or 72h post-METH administration. METH increased BBB permeability, but this effect was detected only at 24h after administration, being therefore a transitory effect. Interestingly, we also found that the hippocampus was the most susceptible brain region to METH, comparing to frontal cortex and striatum. Moreover, in an attempt to identify the key players in METH-induced BBB dysfunction we further investigated potential alterations in tight junction (TJ) proteins and matrix metalloproteinase-9 (MMP-9). METH was able to decrease the protein levels of zonula occludens (ZO)-1, claudin-5 and occludin in the hippocampus 24h post-injection, and increased the activity and immunoreactivity of MMP-9. The pre-treatment with BB-94 (30mg/kg), a matrix metalloproteinase inhibitor, prevented the METH-induced increase in MMP-9 immunoreactivity in the hippocampus. Overall, the present data demonstrate that METH transiently increases the BBB permeability in the hippocampus, which can be explained by alterations on TJ proteins and MMP-9.

Diabetes induces early transient changes in the content of vesicular transporters and no major effects in neurotransmitter release in hippocampus and retina

Diabetes induces changes in neurotransmitter release in central nervous system, which depend on the type of neurotransmitter and region studied. In this study, we evaluated the effect of diabetes (two and eight weeks duration) on basal and evoked release of [(14)C]glutamate and [(3)H]GABA in hippocampal and retinal synaptosomes. We also analyzed the effect of diabetes on the protein content of vesicular glutamate and GABA transporters, VGluT-1, VGluT-2 and VGAT, and on the α(1A) subunit of P/Q type calcium channels, which are abundant in nerve terminals. The protein content of vesicular glutamate and GABA transporters, and of the α(1A) subunit, was differently affected by diabetes in hippocampal and retinal synaptosomes. The changes were more pronounced in the retina than in hippocampus. VGluT-1 and VGluT-2 content was not affected in hippocampus. Moreover, changes occurred early, at two weeks of diabetes, but after eight weeks almost no changes were detected, with the exception of VGAT in the retina. Regarding neurotransmitter release, no major changes were detected. After two weeks of diabetes, neurotransmitter release was similar to controls. After eight weeks of diabetes, the basal release of glutamate slightly increased in hippocampus and the evoked GABA release decreased in retina. In conclusion, diabetes induces early transient changes in the content of glutamate and/or GABA vesicular transporters, and on calcium channels subunit, in retinal or hippocampal synaptosomes, but only minor changes in the release of glutamate or GABA. These results point to the importance of diabetes-induced changes in neural tissues at the presynaptic level, which may underlie alterations in synaptic transmission, particularly if they become permanent during the later stages of the disease.

TNF-α signals through PKCζ/NF-κB to alter the tight junction complex and increase retinal endothelial cell permeability

OBJECTIVE

Tumor necrosis factor-α (TNF-α) and interleukin-1 beta (IL-1β) are elevated in the vitreous of diabetic patients and in retinas of diabetic rats associated with increased retinal vascular permeability. However, the molecular mechanisms underlying retinal vascular permeability induced by these cytokines are poorly understood. In this study, the effects of IL-1β and TNF-α on retinal endothelial cell permeability were compared and the molecular mechanisms by which TNF-α increases cell permeability were elucidated.

RESEARCH DESIGN AND METHODS

Cytokine-induced retinal vascular permeability was measured in bovine retinal endothelial cells (BRECs) and rat retinas. Western blotting, quantitative real-time PCR, and immunocytochemistry were performed to determine tight junction protein expression and localization.

RESULTS

IL-1β and TNF-α increased BREC permeability, and TNF-α was more potent. TNF-α decreased the protein and mRNA content of the tight junction proteins ZO-1 and claudin-5 and altered the cellular localization of these tight junction proteins. Dexamethasone prevented TNF-α-induced cell permeability through glucocorticoid receptor transactivation and nuclear factor-kappaB (NF-κB) transrepression. Preventing NF-κB activation with an inhibitor κB kinase (IKK) chemical inhibitor or adenoviral overexpression of inhibitor κB alpha (IκBα) reduced TNF-α-stimulated permeability. Finally, inhibiting protein kinase C zeta (PKCζ) using both a peptide and a novel chemical inhibitor reduced NF-κB activation and completely prevented the alterations in the tight junction complex and cell permeability induced by TNF-α in cell culture and rat retinas.

CONCLUSIONS

These results suggest that PKCζ may provide a specific therapeutic target for the prevention of vascular permeability in retinal diseases characterized by elevated TNF-α, including diabetic retinopathy.

Long-term exposure to high glucose induces changes in the content and distribution of some exocytotic proteins in cultured hippocampal neurons

A few studies have reported the existence of depletion of synaptic vesicles, and changes in neurotransmitter release and in the content of exocytotic proteins in the hippocampus of diabetic rats. Recently, we found that diabetes alters the levels of synaptic proteins in hippocampal nerve terminals. Hyperglycemia is considered the main trigger of diabetic complications, although other factors, such as low insulin levels, also contribute to diabetes-induced changes. Thus, the aim of this work was to evaluate whether long-term elevated glucose per se, which mimics prolonged hyperglycemia, induces significant changes in the content and localization of synaptic proteins involved in exocytosis in hippocampal neurons. Hippocampal cell cultures were cultured for 14 days and were exposed to high glucose (50 mM) or mannitol (osmotic control; 25 mM plus 25 mM glucose), for 7 days. Cell viability and nuclear morphology were evaluated by MTT and Hoechst assays, respectively. The protein levels of vesicle-associated membrane protein-2 (VAMP-2), synaptosomal-associated protein-25 (SNAP-25), syntaxin-1, synapsin-1, synaptophysin, synaptotagmin-1, rabphilin 3a, and also of vesicular glutamate and GABA transporters (VGluT-1 and VGAT), were evaluated by immunoblotting, and its localization was analyzed by immunocytochemistry. The majority of the proteins were not affected. However, elevated glucose decreased the content of SNAP-25 and increased the content of synaptotagmin-1 and VGluT-1. Moreover, there was an accumulation of syntaxin-1, synaptotagmin-1 and VGluT-1 in the cell body of some hippocampal neurons exposed to high glucose. No changes were detected in mannitol-treated cells. In conclusion, elevated glucose per se did not induce significant changes in the content of the majority of the synaptic proteins studied in hippocampal cultures, with the exception of SNAP-25, synaptotagmin-1 and VGluT-1. However, there was an accumulation of some proteins in cell bodies of hippocampal neurons exposed to elevated glucose, suggesting that the trafficking of these proteins to the synapse may be compromised. Moreover, these results also suggest that other factors, in addition to hyperglycemia, certainly contribute to alterations detected in synaptic proteins in diabetic animals.

Calcium dobesilate inhibits the alterations in tight junction proteins and leukocyte adhesion to retinal endothelial cells induced by diabetes

OBJECTIVE

Calcium dobesilate (CaD) has been used in the treatment of diabetic retinopathy in the last decades, but its mechanisms of action are not elucidated. CaD is able to correct the excessive vascular permeability in the retina of diabetic patients and in experimental diabetes. We investigated the molecular and cellular mechanisms underlying the protective effects of CaD against the increase in blood-retinal barrier (BRB) permeability induced by diabetes.

RESEARCH DESIGN AND METHODS

Wistar rats were divided into three groups: controls, streptozotocin-induced diabetic rats, and diabetic rats treated with CaD. The BRB breakdown was evaluated using Evans blue. The content or distribution of tight junction proteins (occludin, claudin-5, and zonula occluden-1 [ZO-1]), intercellular adhesion molecule-1 (ICAM-1), and p38 mitogen-activated protein kinase (p38 MAPK) was evaluated by Western blotting and immunohistochemistry. Leukocyte adhesion was evaluated in retinal vessels and in vitro. Oxidative stress was evaluated by the detection of oxidized carbonyls and tyrosine nitration. NF-κB activation was measured by enzyme-linked immunosorbent assay.

RESULTS

Diabetes increased the BRB permeability and retinal thickness. Diabetes also decreased occludin and claudin-5 levels and altered the distribution of ZO-1 and occludin in retinal vessels. These changes were inhibited by CaD treatment. CaD also inhibited the increase in leukocyte adhesion to retinal vessels or endothelial cells and in ICAM-1 levels, induced by diabetes or elevated glucose. Moreover, CaD decreased oxidative stress and p38 MAPK and NF-κB activation caused by diabetes.

CONCLUSIONS

CaD prevents the BRB breakdown induced by diabetes, by restoring tight junction protein levels and organization and decreasing leukocyte adhesion to retinal vessels. The protective effects of CaD are likely to involve the inhibition of p38 MAPK and NF-κB activation, possibly through the inhibition of oxidative/nitrosative stress.

Neuropeptide Y stimulates retinal neural cell proliferation--involvement of nitric oxide

Neuropeptide Y (NPY) is a 36 amino acid peptide widely present in the CNS, including the retina. Previous studies have demonstrated that NPY promotes cell proliferation of rat post-natal hippocampal and olfactory epithelium precursor cells. The aim of this work was to investigate the role of NPY on cell proliferation of rat retinal neural cells. For this purpose, primary retinal cell cultures expressing NPY, and NPY Y(1), Y(2), Y(4) and Y(5) receptors [Alvaro et al., (2007) Neurochem. Int., 50, 757] were used. NPY (10-1000 nM) stimulated cell proliferation through the activation of NPY Y(1), Y(2) and Y(5) receptors. NPY also increased the number of proliferating neuronal progenitor cells (BrdU(+)/nestin(+) cells). The intracellular mechanisms coupled to NPY receptors activation that mediate the increase in cell proliferation were also investigated. The stimulatory effect of NPY on cell proliferation was reduced by L-nitroarginine-methyl-esther (L-NAME; 500 microM), a nitric oxide synthase inhibitor, 1H-[1,2,4]oxadiazolo-[4, 3-a]quinoxalin-1-one (ODQ; 20 microM), a soluble guanylyl cyclase inhibitor or U0126 (1 microM), an inhibitor of the extracellular signal-regulated kinase 1/2 (ERK 1/2). In conclusion, NPY stimulates retinal neural cell proliferation, and this effect is mediated through nitric oxide-cyclic GMP and ERK 1/2 pathways.

Diabetes changes the levels of ionotropic glutamate receptors in the rat retina

PURPOSE

Diabetic retinopathy (DR) is a leading cause of vision loss and blindness among adults between the age 20 to 74. Changes in ionotropic glutamate receptor subunit composition can affect retinal glutamatergic neurotransmission and, therefore, contribute to visual impairment. The purpose of this study was to investigate whether diabetes leads to changes in ionotropic glutamate receptor subunit expression at the protein and mRNA level in the rat retina.

METHODS

Changes in the expression of ionotropic glutamate receptor subunits were investigated at the mRNA and protein levels in retinas of streptozotocin (STZ)-induced diabetic and age-matched control rats. Animals were euthanized one, four and 12 weeks after the onset of diabetes. Retinal protein extracts were prepared, and the receptor subunit levels were assessed by western blotting. Transcript levels were assessed by real-time quantitative PCR.

RESULTS

Transcript levels of most ionotropic glutamate receptor subunits were not significantly changed in the retinas of diabetic rats, as compared to age-matched controls but protein levels of alpha-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate (AMPA), kainate, and N-methyl-D-aspartic acid receptors (NMDA) receptors were found to be altered.

CONCLUSIONS

The results provide evidence that diabetes affects the retinal content of ionotropic glutamate receptor subunits at the protein level. The possible implications of these changes on retinal physiology and visual impairment in DR are discussed.

High glucose changes extracellular adenosine triphosphate levels in rat retinal cultures

Diabetic retinopathy (DR) is the leading cause of blindness in adults. In diabetes, there is activation of microglial cells and a concomitant release of inflammatory mediators. However, it remains unclear how diabetes triggers an inflammatory response in the retina. Activation of P2 purinergic receptors by adenosine triphosphate (ATP) may contribute to the inflammatory response in the retina, insofar as it has been shown to be associated with microglial activation and cytokine release. In this work, we evaluated how high glucose, used as a model of hyperglycemia, considered the main factor in the development of DR, affects the extracellular levels of ATP in retinal cell cultures. We found that basal extracellular ATP levels were not affected by high glucose or mannitol, but the extracellular elevation of ATP, after a depolarizing stimulus, was significantly higher in retinal cells cultured in high glucose compared with control or mannitol-treated cells. The increase in the extracellular ATP was prevented by application of botulinum neurotoxin A or by removal of extracellular calcium. In addition, degradation of exogenously added ATP was significantly lower in high-glucose-treated cells. It was also observed that, in retinal cells cultured under high-glucose conditions, the changes in the intracellular calcium concentrations were greater than those in control or mannitol-treated cells. In conclusion, in this work we have shown that high glucose alters the purinergic signaling system in the retina, by increasing the exocytotic release of ATP and decreasing its extracellular degradation. The resulting high levels of extracellular ATP may lead to inflammation involved in the pathogenesis of DR.

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.

Müller cells do not influence leukocyte adhesion to retinal endothelial cells

PURPOSE

Diabetic retinopathy is associated with inflammation. The authors investigated the influence of Müller cells on leukocyte adhesion to retinal endothelial cells.

METHODS

ICAM-1 levels were assessed by Western blotting and immunocytochemistry. Leukocyte adhesion was quantified using a fluorescence assay.

RESULTS

High glucose and oxidative/nitrosative stress conditions increased ICAM-1 levels in endothelial cells and leukocyte adhesion. Under the influence of Müller cells (co-cultures/conditioned medium), the effects were comparable to those found when endothelial cells were exposed, alone, to similar conditions.

CONCLUSIONS

These results show that Müller cells do not influence leukocyte adhesion under the in vitro conditions used in this study.

Diabetes changes ionotropic glutamate receptor subunit expression level in the human retina

Early diabetic retinopathy is characterized by changes in subtle visual functions such as contrast sensitivity and dark adaptation. The outcome of several studies suggests that glutamate is involved in retinal neurodegeneration during diabetes. We hypothesized that the protein levels of ionotropic glutamate receptor subunits are altered in the retina during diabetes. Therefore, we investigated whether human diabetic patients have altered immunoreactivity of ionotropic glutamate receptor subunits in the retina. In total, 12 donor eyes from subjects with diabetes mellitus were examined and compared to 6 eyes from non-diabetic subjects without known ocular disease, serving as controls. Immunohistochemical analysis was performed using specific antibodies directed against the ionotropic alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate (AMPA) receptor subunits GluR1, GluR2, GluR4, and against the N-methyl-d-aspartate glutamate receptor subunit NR1. In the inner plexiform and outer plexiform layers the immunoreactivity of GluR2 and NR1 subunits was significantly increased in subjects with diabetes when compared to the levels found in controls. No significant changes in GluR1 and GluR4 subunit expression were observed. These results suggest that early visual dysfunction in diabetic patients may be due, at least partially, to changes in glutamate receptor subunit expression or distribution.

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.

High glucose induces caspase-independent cell death in retinal neural cells

Diabetic retinopathy is a leading cause of blindness among adults in the western countries. It has been reported that neurodegeneration may occur in diabetic retinas, but the mechanisms underlying retinal cell death are poorly understood. We found that high glucose increased the number of cells with condensed nuclei and the number of TUNEL-positive cells, and caused an increase in the translocation of phosphatidylserine to the outer leaflet of the plasma membrane, indicating that high glucose induces apoptosis in cultured retinal neural cells. The activity of caspases did not increase in high glucose-treated cells, but apoptosis-inducing factor (AIF) levels decreased in the mitochondria and increased in the nucleus, indicating a translocation to the nucleus where it may cause DNA fragmentation. These results demonstrate that elevated glucose induces apoptosis in cultured retinal neural cells. The increase in apoptosis is not dependent on caspase activation, but is mediated through AIF release from the mitochondria.

Elevated glucose changes the expression of ionotropic glutamate receptor subunits and impairs calcium homeostasis in retinal neural cells

PURPOSE

Altered glutamatergic neurotransmission and calcium homeostasis may contribute to retinal neural cell dysfunction and apoptosis in diabetic retinopathy (DR). The purpose of this study was to determine the effect of high glucose on the protein content of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) and kainate glutamate receptor subunits, particularly the GluR2 subunit, because it controls Ca2+ permeability of AMPA receptor-associated channels. The effect of high glucose on the concentration of cytosolic free calcium ([Ca2+]i) was also investigated.

METHODS

The protein content of GluR1, GluR2, GluR6/7, and KA2 subunits was assessed by Western blot. Cobalt staining was used to identify cells containing calcium/cobalt-permeable AMPA receptors. The [Ca2+]i changes evoked by KCl or kainate were recorded by live-cell confocal microscopy in R28 cells and in primary cultures of rat retina, loaded with fluo-4.

RESULTS

In primary cultures, high glucose significantly decreased the protein content of GluR1 and GluR6/7 subunits and increased the protein content of GluR2 and KA2 subunits. High glucose decreased the number of cobalt-positive cells, suggesting a decrease in calcium permeability through AMPA receptor-associated channels. In high-glucose-treated cells, changes in [Ca2+]i were greater than in control cells, and the recovery to basal levels was delayed. However, in the absence of Na+, to prevent the activation of voltage-sensitive calcium channels, the [Ca2+]i changes evoked by kainate in the presence of cyclothiazide, which inhibits AMPA receptor desensitization, were significantly lower in high-glucose-treated cells than in control cultures, further indicating that AMPA receptors were less permeable to calcium. Mannitol, used as an osmotic control, did not cause significant changes compared with the control.

CONCLUSIONS

The results suggest that elevated glucose may alter glutamate neurotransmission and calcium homeostasis in the retina, which may have implications for the mechanisms of vision loss in DR.

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.

Modification of adenosine A1 and A2A receptor density in the hippocampus of streptozotocin-induced diabetic rats

Adenosine A(1) and A(2A) receptors are neuromodulatory systems that can control mnemonic behavior, which is modified by diabetes. Since the density of these adenosine receptors can change upon chronic noxious brain conditions, we now tested if the density of A(1) and A(2A) receptors was modified in the hippocampus of streptozotocin-induced diabetic rats. The binding density of the selective A(1) receptor antagonist, (3)H-DPCPX, was decreased by 36% in total hippocampal membranes 7 days after induction of diabetes and this down-regulation was maintained after 30 and 90 days, which was also confirmed by Western blot analysis of A(1) receptor immunoreactivity. In contrast, the binding density of the selective A(2A) receptor antagonist, (3)H-SCH 58261, was enhanced by 83% in total hippocampal membranes 7 days after induction of diabetes and this up-regulation persisted after 30 and 90 days. These results show that the balance between inhibitory A(1) and facilitatory A(2A) adenosine receptors is modified in the hippocampus of streptozotocin-induced diabetic rats. Thus, the most abundant A(1) receptors are down-regulated and there is an up-regulation of A(2A) receptors, suggesting a gain of function of hippocampal A(2A) receptors compared to A(1) receptors in diabetes, as has been observed in other chronic noxious brain conditions where A(2A) receptor blockade affords robust neuroprotection.

Old and new drug targets in diabetic retinopathy: from biochemical changes to inflammation and neurodegeneration

Diabetic Retinopathy (DR) is a major complication of diabetes and is a leading cause of blindness in western countries. DR has been considered a microvascular disease, and the blood-retinal barrier breakdown is a hallmark of this disease. The available treatments are scarce and not very effective. Despite the attempts to control blood glucose levels and blood pressure, many diabetic patients are affected by DR, which progresses to more severe forms of disease, where laser photocoagulation therapy is needed. DR has a huge psychological impact in patients and tremendous economic and social costs. Taking this into account, the scientific community is committed to find a treatment to DR. Understanding the cellular and molecular mechanisms underlying the pathogenesis of DR will facilitate the development of strategies to prevent, or at least to delay the progression of the disease. The involvement of the polyol pathway, advanced glycation end products, protein kinase C and oxidative stress in the pathogenesis of DR is well-documented, and several clinical trials have been conducted to test the efficacy of various drugs. More recent findings also demonstrate that DR has characteristics of chronic inflammatory disease and neurodegenerative disease, which increases the opportunity of intervention at the pharmacological level. This review presents past and recent evidences demonstrating the involvement of different molecules and processes in DR, and how different approaches and pharmacological tools have been used to prevent retinal cell dysfunction.

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.

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.

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.

Mechanisms of action of carbamazepine and its derivatives, oxcarbazepine, BIA 2-093, and BIA 2-024

Carbamazepine (CBZ) has been extensively used in the treatment of epilepsy, as well as in the treatment of neuropathic pain and affective disorders. However, the mechanisms of action of this drug are not completely elucidated and are still a matter of debate. Since CBZ is not very effective in some epileptic patients and may cause several adverse effects, several antiepileptic drugs have been developed by structural variation of CBZ, such as oxcarbazepine (OXC), which is used in the treatment of epilepsy since 1990. (S)-(-)-10-acetoxy-10,11-dihydro-5H-dibenz [b,f]azepine-5-carboxamide (BIA 2-093) and 10,11-dihydro-10-hydroxyimino-5H-dibenz[b,f] azepine-5-carboxamide (BIA 2-024), which were recently developed by BIAL, are new putative antiepileptic drugs, with some improved properties. In this review, we will focus on the mechanisms of action of CBZ and its derivatives, OXC, BIA 2-093 and BIA 2-024. The available data indicate that the anticonvulsant efficacy of these AEDs is mainly due to the inhibition of sodium channel activity.

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.

Inhibition of glutamate release by BIA 2-093 and BIA 2-024, two novel derivatives of carbamazepine, due to blockade of sodium but not calcium channels

We investigated the mechanism(s) of action of two new putative antiepileptic drugs (AEDs), (S)-(-)-10-acetoxy-10,11-dihydro-5H-dibenz[b,f]azepine-5-carboxamide (BIA 2-093) and 10,11-dihydro-10-hydroxyimino-5H-dibenz[b,f]azepine-5-carboxamide (BIA 2-024), by comparing their effects on the release of endogenous glutamate in hippocampal synaptosomes, with those of carbamazepine (CBZ) and oxcarbazepine (OXC). The AEDs inhibited the release of glutamate evoked by 4-aminopyridine (4-AP) or veratridine in a concentration-dependent manner, being CBZ more potent than the other AEDs. Using conditions of stimulation (30 mM KCl), where Na(+) channels are inactivated, the AEDs did not inhibit either the Ca(2+)-dependent or -independent release of glutamate. The results indicate that BIA 2-093 and BIA 2-024 have sodium channel-blocking properties, but CBZ and OXC are more potent than the new AEDs. Moreover, the present data also indicate that Ca(2+) channels coupled to the exocytotic release of glutamate and the activity of the glutamate transporter were not affected by the AEDs.

Neurotoxic/neuroprotective profile of carbamazepine, oxcarbazepine and two new putative antiepileptic drugs, BIA 2-093 and BIA 2-024

We investigated and compared the toxicity profile, as well as possible neuroprotective effects, of some antiepileptic drugs in cultured rat hippocampal neurons. We used two novel carbamazepine derivatives, (S)-(-)-10-acetoxy-10,11-dihydro-5H-dibenz[b, f]azepine-5-carboxamide (BIA 2-093) and 10, 11-dihydro-10-hydroxyimino-5H-dibenz[b,f]azepine-5-carboxamide (BIA 2-024), and compared their effects with the established compounds carbamazepine and oxcarbazepine. The assessment of neuronal injury was made by using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl (MTT) assay, as well as by analysing morphology and nuclear chromatin condensation (propidium iodide staining), after hippocampal neurons were exposed to the drugs for 24 h. The putative antiepileptic drugs, BIA 2-093 or BIA 2-024 (at 300 microM), only slightly decreased MTT reduction, whereas carbamazepine or oxcarbazepine were much more toxic at lower concentrations. Treatment with the antiepileptic drugs caused nuclear chromatin condensation in some neurons, which is characteristic of apoptosis, and increased the activity of caspase-3-like enzymes, mainly in neurons treated with carbamazepine and oxcarbazepine. The toxic effect caused by carbamazepine was not mediated by N-methyl-D-aspartate (NMDA) or by alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate (AMPA) receptors. Moreover, the antiepileptic drugs failed to protect hippocampal neurons from the toxicity caused by kainate, veratridine, or ischaemia-like conditions.

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.

Carbamazepine inhibits L-type Ca2+ channels in cultured rat hippocampal neurons stimulated with glutamate receptor agonists

In order to better understand the mechanism(s) of action of carbamazepine (CBZ), we studied its effects on the increase in [Ca2+]i and [Na+]i stimulated by glutamate ionotropic receptor agonists, in cultured rat hippocampal neurons, as followed by indo- or SBFI fluorescence, respectively. CBZ inhibited the increase in [Ca2+]i stimulated either by glutamate, kainate, alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate (AMPA), or N-methyl-D-aspartate (NMDA), in a concentration-dependent manner. In order to discriminate the effects of CBZ on the activation of glutamate receptors from possible effects on Ca2+ channels, we determined the inhibitory effects of Ca2+ channel blockers on [Ca2+]i changes in the absence or in the presence of CBZ. The presence of 1 microM nitrendipine, 0.5 microM omega-conotoxin GVIA (omega-CgTx GVIA), or of both blockers, inhibited the kainate-stimulated increase in [Ca2+]i by 51.6, 32.9 or 68.7%, respectively. In the presence of both 100 microM CBZ and nitrendipine, the inhibition was similar (54.1%) to that obtained with nitrendipine alone, but in the presence of both CBZ and omega-CgTx GVIA, the inhibition was greater (54%) than that caused by omega-CgTx GVIA alone. However, CBZ did not inhibit the increase in [Na+]i stimulated by the glutamate receptor agonists, but inhibited the increase in [Na+]i due to veratridine. Tetrodotoxin, or MK-801, did not inhibit the influx of Na+ stimulated by kainate, indicating that Na+ influx occurs mainly through the glutamate ionotropic non-NMDA receptors. Moreover, LY 303070, a specific AMPA receptor antagonist, inhibited the [Na+]i response to kainate or AMPA by about 70 or 80%, respectively, suggesting that AMPA receptors are mainly involved. Taken together, the results suggest that CBZ inhibits L-type Ca2+ channels and Na+ channels, but does not inhibit activation of glutamate ionotropic receptors.

Increase of the intracellular Ca2+ concentration mediated by transport of glutamate into rat hippocampal synaptosomes: characterization of the activated voltage sensitive Ca2+ channels

The changes in the intracellular free Ca2+ concentration, [Ca2+]i, mediated by glutamate and D-aspartate into rat hippocampal synaptosomes was studied. Glutamate increased the [Ca2+]i in a dose-dependent manner with an EC50 of 1.87 microM and a maximal increase of 31.5 +/- 0.9 nM. We also observed that stimulation of the synaptosomes with 100 microM alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), 100 microM kainate, or 100 microM D-aspartate increased the synaptosomal [Ca2+]i. The effect of either of these non-NMDA receptor agonists and of D-aspartate was additive, suggesting the activation of two different components (the ionotropic non-NMDA receptors or the glutamate transporters). Stimulation of synaptosomes with 100 microM glutamate increased the [Ca2+]i and prevented the effect of either non-NMDA receptor agonists and the effect of D-aspartate. We also observed that incubation of the synaptosomes with D-aspartate induced the Ca(2+)-independent release of glutamate, possibly through the reversal of the glutamate carrier. The aim of incubating the synaptosomes with D-aspartate was to avoid undesirable secondary activation of glutamate receptors. After incubating the synaptosomes with 100 microM D-aspartate (10 min at 37 degrees C), the subsequent stimulation with D-aspartate increased the [Ca2+]i due to glutamate transport. This increase in [Ca2+]i induced by 100 microM D-aspartate was insensitive to 1 microM nitrendipine, but was inhibited by about 50% by the presence of both 500 nM omega-CgTx GVIA and 100 nM omega-Aga IVA or by 500 nM omega-CgTx MVIIC. We clearly identified two different processes by which glutamate increased the [Ca2+]i in rat hippocampal synaptosomes: activation of non-NMDA receptors and activation of the glutamate transporters. We also characterized the voltage sensitive Ca2+ channels (VSCC) activated as a consequence of the glutamate transport, and determined that class B (N-type) and class A (P or Q-type) Ca2+ channels were responsible for about 50% of the signal.

Inhibition of N-,P/Q- and other types of Ca2+ channels in rat hippocampal nerve terminals by the adenosine A1 receptor

The effects of the adenosine A1 receptor agonist, N6-cyclopentyladenosine (CPA), on both the increase in intracellular free Ca2+ concentration ([Ca2+]i) and on the release of endogenous glutamate in rat hippocampal synaptosomes were studied. The inhibitory effect of CPA on the increase in [Ca2+]i stimulated with 4-aminopyridine was neutralized by the adenosine A1 receptor antagonist, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX). The inhibitory effect of CPA was greater in synaptosomes from the CA1 subregion than in whole hippocampal synaptosomes. The inhibitory effects of both CPA and of the Ca2+ channel blockers, omega-conotoxin GVIA, omega-conotoxin MVIIC or omega-conotoxin GVIA plus omega-conotoxin MVIIC, were greater than those caused by the Ca2+ channel blockers. The release of endogenous glutamate was inhibited by 41% by CPA. The inhibition observed when CPA and omega-conotoxin GVIA or CPA and omega-conotoxin MVIIC were present was also greater than the inhibition by the Ca2+ channel blockers alone. The presence of both omega-conotoxin GVIA and omega-conotoxin MVIIC did not completely inhibit the release of glutamate, and CPA significantly enhanced this inhibition. The membrane potential and the accumulation of [3H]tetraphenylphosphonium of polarized or depolarized synaptosomes was not affected by CPA, suggesting that adenosine did not increase potassium conductances. The present results suggest that, in hippocampal glutamatergic nerve terminals, adenosine A1 receptor activation partly inhibits P/Q- and other non-identified types of Ca2+ channels.

Modulation of glutamate release from rat hippocampal synaptosomes by nitric oxide

We used hippocampal synaptosomes to study the effect of NO originating from NO donors and from the activation of the NO synthase on the Ca2+-dependent release of glutamate due to 4-aminopyridine (4-AP) depolarization. We distinguished between the effects of NO on the exocytotic and on the carrier-mediated release of glutamate, which we found to be related to an increase in cGMP content and to a reduction of the ATP/ADP ratio, respectively. The NO donor hydroxylamine, at concentrations < or = 0.3 mM, inhibited the Ca2+-dependent glutamate release evoked by 4-AP, and addition of the NO donor, NOC-7, had a similar effect, which was reversed by the NO scavenger, carboxy-PTIO. Increasing the activity of NO synthase by addition of L-arginine also led to a decrease in the Ca2+-dependent release of glutamate induced by 4-AP, and this effect was reversed by inhibiting NO synthase with NG-nitro-L-arginine. This depression of the exocytotic release of glutamate was accompanied by an increase in cGMP levels due to the stimulation of soluble guanylyl cyclase by NO, produced either by the NO donors (hydroxylamine <0.3 mM) or by the endogenous NO synthase, but no significant decrease in ATP/ADP ratio was observed. However, at concentrations > or = 0.3 mM, hydroxylamine drastically increased the basal release and completely inhibited the Ca2+-dependent release of glutamate (IC50 = 168 microM). At these higher levels of NO, cGMP levels dropped to about 40% of the maximal values obtained at lower concentrations, and the ATP/ADP ratio decreased to about 50% (at 0.3 mM hydroxylamine). The large increase in the basal release could be partially inhibited by L-trans-2,4-PDC, previously loaded into the synaptosomes, suggesting that the nonexocytotic basal release occurred by reversal of the glutamate carrier. Therefore, the increase in cGMP induced by NO stimulation of the guanylyl cyclase decreases the exocytotic release of glutamate, but higher NO levels reduce the ATP/ADP ratio by inhibiting mitochondrial function, which therefore causes the massive release of cytosolic glutamate through the glutamate carrier.

Modulation of Ca2+ channels by activation of adenosine A1 receptors in rat striatal glutamatergic nerve terminals

We determined that activation of adenosine A1 receptors in striatal synaptosomes with 100 nM N6-cyclopentyladenosine (CPA) inhibited both the release of endogenous glutamate and the increase of intracellular free Ca2+ concentration ([Ca2+]i), due to 4-aminopyridine (4-AP) stimulation, by 28 and 19%, respectively. Furthermore, CPA enhanced the inhibition of endogenous glutamate release due to omega-conotoxin GVIA (omega-Cgtx GVIA), omega-Cgtx MVIIC or omega-Cgtx GVIA plus omega-Cgtx MVIIC. Similar effects were observed in the [Ca2+]i signal. The inhibitory effects of CPA and omega-Cgtx GVIA were additive, but the effects of CPA and omega-Cgtx MVIIC were only partially additive. These results suggest that P/Q-type Ca2+ channels and other type(s) of Ca2+ channel(s), coupled to glutamate release, are inhibited subsequently to activation of adenosine A1 receptors.

Involvement of class A calcium channels in the KCl induced Ca2+ influx in hippocampal synaptosomes

Using Ca2+ channel toxins, we determined the types of voltage-sensitive calcium channels activated by two levels of KCl depolarization in hippocampal synaptosomes. The increase in the intracellular Ca2+ concentration ([Ca2+]i) induced by 30 mM KCl was equally sensitive to either omega-agatoxin IVA (omega-Aga IVA) or to omega-conotoxin MVIIC (omega-CgTx MVIIC), and the inhibition produced by these two peptides was not additive. The present results indicate that omega-Aga IVA and omega-CgTx MVIIC do not distinguish between two different VSCC in hippocampal synaptosomes and that they both inhibit a channel with the alpha 1A subunit which is present in the rat hippocampus.

A functionally active presynaptic high-affinity kainate receptor in the rat hippocampal CA3 subregion

We studied the modulation of the intracellular free calcium concentrations ([Ca2+]i) by kainate/AMPA receptor activation in synaptosomes isolated from whole rat hippocampus, or from its CA1, CA3 or dentate gyrus subregions. The receptor was activated either by 100 microM S-alpha-amino-3-hydroxy-5-methyl-4-isoxazolopropionic acid (AMPA) (EC50 = 26.6 +/- 4.9 microM) or by 100 microM kainate (EC50 = 0.81 +/- 0.1 microM), but the effects of these agonists were not additive. The response to either AMPA or kainate was competitively inhibited by 10 microM 6-cyano-7-nitroquinoxaline-2,3-dioxine. Higher [Ca2+]i responses to 100 microM AMPA or to 100 microM kainate were observed in the CA3 subregion (43.2 +/- 2.5 nM or 42.8 +/- 2.3 nM, respectively) than in the whole hippocampus (22.4 +/- 1.1 nM or 22.4 +/- 1.6, respectively), in the CA1 subregion (26.4 +/- 1.1 nM or 26.6 +/- 2.6 nM, respectively) or in dentate gyrus (24.6 +/- 1.4 nM or 21.5 +/- 1.0 nM, respectively). These results indicate that the CA3 subregion of the hippocampus is enriched in a presynaptic high-affinity kainate receptor which modulates the [Ca2+]i in nerve terminals.