Stimulation of the N-methyl-D-aspartate receptor promotes the biochemical differentiation of cerebellar granule neurons and not astrocytes

Role of NF-κB in cytochrome P450 epoxygenases down-regulation during an inflammatory process in astrocytes

Cytochrome P450 (CYP) epoxygenases and their metabolic products, epoxyeicosatrienoic acids (EETs), have been proposed as important therapeutic targets in the brain. However, CYP expression can be modified by the presence of diverse pro-inflammatory cytokines and the subsequent activation of the NF-κB pathway. It has been indicated that CYP epoxygenases are down-regulated by inflammation in the heart, kidney and liver. However, up to this point, there has been no evidence regarding regulation of CYP epoxygenases during inflammation in the brain. Therefore, in order to explore the effects of inflammation and NF-κB activation in CYP2J3 and CYP2C11 regulation, rat primary astrocytes cultures were treated with LPS with and without IMD-0354 (selective NF-κB inhibitor). Cyp2j3 and Cyp2c11 mRNA expression was determined by qRT-PCR; protein expression was determined by immunofluorescence and by Western Blot and total epoxygenase activity was determined by the quantification of EETs by ELISA. NF-κB binding sites in Cyp2j3 and Cyp2c11 promoter regions were bioinformatically predicted and Electrophoretic Mobility Shift Assays (EMSA) were performed to determine if each hypothetic response element was able to bind NF-κB complexes. Results shown that LPS treatment is able to down-regulate astrocyte CYP2J3 and CYP2C11 mRNA, protein and activity. Additionally, we have identified NK-κB as the transcription factor involved in this regulation.

Diabetes Drug Discovery: hIAPP1-37 Polymorphic Amyloid Structures as Novel Therapeutic Targets

Human islet amyloid peptide (hIAPP_1–37) aggregation is an early step in Diabetes Mellitus. We aimed to evaluate a family of pharmaco-chaperones to act as modulators that provide dynamic interventions and the multi-target capacity (native state, cytotoxic oligomers, protofilaments and fibrils of hIAPP_1–37) required to meet the treatment challenges of diabetes. We used a cross-functional approach that combines in silico and in vitro biochemical and biophysical methods to study the hIAPP_1–37 aggregation-oligomerization process as to reveal novel potential anti-diabetic drugs. The family of pharmaco-chaperones are modulators of the oligomerization and fibre formation of hIAPP_1–37. When they interact with the amino acid in the amyloid-like steric zipper zone, they inhibit and/or delay the aggregation-oligomerization pathway by binding and stabilizing several amyloid structures of hIAPP_1–37. Moreover, they can protect cerebellar granule cells (CGC) from the cytotoxicity produced by the hIAPP_1–37 oligomers. The modulation of proteostasis by the family of pharmaco-chaperones A–F is a promising potential approach to limit the onset and progression of diabetes and its comorbidities.

Defects in the CAPN1 Gene Result in Alterations in Cerebellar Development and Cerebellar Ataxia in Mice and Humans

A CAPN1 missense mutation in Parson Russell Terrier dogs is associated with spinocerebellar ataxia. We now report that homozygous or heterozygous CAPN1-null mutations in humans result in cerebellar ataxia and limb spasticity in four independent pedigrees. Calpain-1 knockout (KO) mice also exhibit a mild form of ataxia due to abnormal cerebellar development, including enhanced neuronal apoptosis, decreased number of cerebellar granule cells, and altered synaptic transmission. Enhanced apoptosis is due to absence of calpain-1-mediated cleavage of PH domain and leucine-rich repeat protein phosphatase 1 (PHLPP1), which results in inhibition of the Akt pro-survival pathway in developing granule cells. Injection of neonatal mice with the indirect Akt activator, bisperoxovanadium, or crossing calpain-1 KO mice with PHLPP1 KO mice prevented increased postnatal cerebellar granule cell apoptosis and restored granule cell density and motor coordination in adult mice. Thus, mutations in CAPN1 are an additional cause of ataxia in mammals, including humans.

Delineating the factors and cellular mechanisms involved in the survival of cerebellar granule neurons

Cerebellar granule neurons (CGNs) constitute the most abundant neuronal population in the mammalian brain. Their postnatal generation and the feasibility to induce their apoptotic death in vitro make them an excellent model to study the effect of several neurotransmitters and neurotrophins. Here, we first review which factors are involved in the generation and proliferation of CGNs in the external granule layer (EGL) and in the regulation of their differentiation and migration to internal granule layer (IGL). Special attention was given to the role of several neurotrophins and the NMDA subtype of glutamate receptor. Then, using the paradigm of potassium deprivation in cultured CGNs, we address several extracellular factors that promote the survival of CGNs, with particular emphasis on the cellular mechanisms. The role of specific protein kinases leading to the regulation of transcription factors and recent data involving the small G protein family is also discussed. Finally, the participation of some members of Bcl-2 family and the inhibition of mitochondria-related apoptotic pathway is also considered. Altogether, these studies evidence that CGNs are a key model to understand the development and the survival of neuronal populations.

On the effects of CP 55-940 and other cannabinoid receptor agonists in C6 and U373 cell lines

Cannabinoid receptor (CBs) agonists affect the growth of tumor cells via activation of deadly cascades. The spectrum of action of these agents and the precise role of the endocannabinoid system (ECS) on oncogenic processes remain elusive. Herein we compared the effects of synthetic (CP 55-940 and WIN 55,212-2) and endogenous (anandamide or AEA) CBs agonists (10-20 μM) on morphological changes, cell viability, and induction of apoptosis in primary astrocytes and in two glioblastoma cell lines (C6 and U373 cells) in order to characterize their possible differential actions on brain tumor cells. None of the CBs agonist tested induced changes in cell viability or morphology in primary astrocytes. In contrast, CP 55-940 significantly decreased cell viability in C6 and U373 cells at 5 days of treatment, whereas AEA and WIN 55,212-2 moderately decreased cell viability in both cell lines. Treatment of U373 and C6 for 3 and 5 days with AEA or WIN 55,212-2 produced discrete morphological changes in cell bodies, whereas the exposure to CP 55-940 induced soma degradation. CP 55-940 also induced apoptosis in both C6 and U373 cell lines. Our results support a more effective action of CP 55-940 to produce cell death of both cell lines through apoptotic mechanisms. Comparative aspects between cannabinoids with different profiles are necessary for the design of potential treatments against glial tumors.

Cholesterol-induced astrocyte activation is associated with increased amyloid precursor protein expression and processing

Cholesterol is essential for maintaining lipid raft integrity and has been regarded as a crucial regulatory factor for amyloidogenesis in Alzheimer's disease (AD). The vast majority of studies on amyloid precursor protein (APP) metabolism and amyloid β-protein (Aβ) production have focused on neurons. The role of astrocytes remains largely unexplored, despite the presence of activated astrocytes in the brains of most patients with AD and in transgenic models of the disease. The role of cholesterol in Aβ production has been thoroughly studied in neurons and attributed to the participation of lipid rafts in APP metabolism. Thus, in this study, we analyzed the effect of cholesterol loading in astrocytes and analyzed the expression and processing of APP. We found that cholesterol exposure induced astrocyte activation, increased APP content, and enhanced the interaction of APP with BACE-1. These effects were associated with an enrichment of ganglioside GM1-cholesterol patches in the astrocyte membrane and with increased ROS production. GLIA 2015;63:2010-2022.

Effect of staurosporine in the morphology and viability of cerebellar astrocytes: role of reactive oxygen species and NADPH oxidase

Cell death implies morphological changes that may contribute to the progression of this process. In astrocytes, the mechanisms involving the cytoskeletal changes during cell death are not well explored. Although NADPH oxidase (NOX) has been described as being a critical factor in the production of ROS, not much information is available about the participation of NOX-derived ROS in the cell death of astrocytes and their role in the alterations of the cytoskeleton during the death of astrocytes. In this study, we have evaluated the participation of ROS in the death of cultured cerebellar astrocytes using staurosporine (St) as death inductor. We found that astrocytes express NOX1, NOX2, and NOX4. Also, St induced an early ROS production and NOX activation that participate in the death of astrocytes. These findings suggest that ROS produced by St is generated through NOX1 and NOX4. Finally, we showed that the reorganization of tubulin and actin induced by St is ROS independent and that St did not change the level of expression of these cytoskeletal proteins. We conclude that ROS produced by a NOX is required for cell death in astrocytes, but not for the morphological alterations induced by St.

Endogenous XIAP, but not other members of the inhibitory apoptosis protein family modulates cerebellar granule neurons survival

Programmed cell death plays a critical role during cerebellar development. In particular, it has been shown in vivo and in vitro that developing cerebellar granule neurons (CGN) die apoptotically. Apoptosis involves a series of morphological changes and the activation of caspases. Inhibitor of apoptosis proteins (IAPs) is implicated in negative regulation of caspase activation and apoptotic cell death. Although apoptotic death of CGN has been extensively studied, there is no information about the role of IAPs in the developing cerebellum. Here, we studied the participation of some members of IAPs in the survival of the developing rat CGN in culture and under physiological conditions. Under these conditions, we found a differential expression pattern of cIAP-1, cIAP-2, XIAP and survivin during cerebellar development in an age-dependent manner, highlighting the significant increase of XIAP levels. We also detected an interaction between XIAP and caspase 3 at postnatal day (P) 12 and 16. On the other hand, we found a significant decrease of XIAP levels in cultured CGN maintained in chronic potassium deprivation, an apoptotic condition, suggesting a possible relationship between XIAP levels and neuronal viability. Under these conditions, we also detected the interaction of XIAP with active caspase-3. The down-regulation of XIAP in CGN cultured under survival conditions (chronic potassium depolarization) induced a reduction of cell viability and an increment of apoptotic cells. These findings support the idea that IAPs could be involved in the survival of CGN and that XIAP might be critical for neuronal survival in cerebellar development and during chronic depolarization in cultured CGN through a mechanism involving caspase inhibition.

Role for apoptosis-inducing factor in the physiological death of cerebellar neurons

Apoptosis-inducing factor (AIF) is implicated in caspase-independent apoptotic-like death. AIF released from mitochondria translocates to the nucleus, where it mediates some apoptotic events such as chromatin condensation and DNA degradation. Here, the role of AIF in the neuronal death was studied under physiological conditions. When we analyzed the cellular localization of AIF during cerebellar development, we found a significant increase in the number of neurons with nuclear AIF localization in an age-dependent manner. On the other hand, cerebellar granule neurons (CGN) chronically cultured in low concentration of potassium (5 mM; K5) die with apoptotic-like characteristics after five days. In the present study we found that K5 induces a caspase-dependent apoptotic-like death of CGN as well as a late nuclear translocation of AIF. When CGN death induced by K5 was carried out in the presence of a general inhibitor of caspases, there was a slight decrement of cell death, but neurons eventually died by showing apoptotic-like features such as phosphatidylserine translocation and nuclear condensation. Besides, there was a significant increment of nuclear AIF translocation. These findings support the idea that AIF could be involved in apoptotic-like death of CGN and that it could be an alternative mechanism of neuronal death during cerebellar development.

Apoptosis and autophagy in rat cerebellar granule neuron death: Role of reactive oxygen species

Programmed cell death (PCD) has been defined as an active, controlled process in which cells participate in their own demise. Apoptosis, or type I PCD, has been widely characterized, both morphologically and biochemically. More recently, autophagy, the self-digesting mechanism involved in the removal of cytoplasmic long-lived proteins, has been involved in cell death, and type II PCD is defined as cell death occurring with autophagic features. Neurons can undergo more than one type of PCD as a backup mechanism when the traditional death pathway is inhibited or in response to a particular death-inducing stimulus. Reactive oxygen species (ROS) have been shown to be important signaling molecules in the execution of apoptosis and, more recently, in the autophagic pathway. In this work, we characterize apoptotic and autophagic cell death in rat cerebellar granule neuron (CGN) culture, a widespread model for the study of neuronal death. Potassium deprivation (K5) and staurosporine (STS) were used for death induction. We found apoptotic and autophagic features under both conditions. Caspase inhibition as well as autophagy inhibition by 3-methyl adenine decreased cell death. Moreover, CGN can undergo the alternative type of cell death when the other one is inhibited. An antioxidant or NADPH oxidase inhibitors delayed apoptosis and had no effect in autophagic features. Thus, we found that autophagy plays a role in cell death of CGN and that, when cells are treated with K5 or STS, both autophagy and ROS seem to promote apoptosis by independent mechanisms.

The effects of glutamate receptor antagonists on cerebellar granule cell survival and development

N-Methyl-d-aspartate (NMDA) receptor stimulation promotes neuronal survival and differentiation under both in vitro and in vivo conditions. We studied the effects of various NMDA receptor antagonists acting at different NMDA receptor binding sites and non-NMDA receptor antagonists on the development and survival of cerebellar granule cell (CGC) culture. Only three of the drugs tested induced neurotoxicity-MK-801 (non-competitive NMDA channel blocking antagonist), ifenprodil (an antagonist of the NR2B site and polyamine site of the NMDA receptor) and L-701.324 (full antagonist at glycine site), while CGP-37849 (a competitive NMDA antagonist), (+)-HA-966 (a partial agonist of the glycine site of the NMDA receptor), and NBQX (a competitively acting AMPA receptor antagonist) were not toxic at any concentration (1-100 microM) used. Among these drugs, only MK-801 was toxic for the immature CGC on second day in vitro (2DIV), and toxicity was diminished parallel to the neuronal maturation. In more mature neurons (7DIV), MK-801 demonstrated some neuroprotection, which diminished spontaneously occurring neuronal death in culture. Neither NMDA nor glutamate were able to prevent the neurotoxic effect of MK-801 at 2DIV. MK-801, ifenprodil and L-701.324 induced DNA fragmentation on 2DIV in CGC culture measured by the TUNEL method. The BOC-D-FMK, the universal caspase inhibitor, completely reversed MK-801-induced DNA fragmentation, suggesting an apoptotic pathway of MK-801-induced cell death. Neurite outgrowth as a characteristic feature of the development of CGC was diminished after treatment with MK-801, ifenprodil and L-701.324. In conclusion, the results of the present study demonstrate that only nonselective channel blocker MK-801 decreases cell viability, induces apoptosis and inhibits neurite outgrowth of CGC in a development-dependent manner.

Effect of NMDA antagonists on the death of cerebellar granule neurons at different ages

Cerebellar granule neurons (CGN) are the most abundant neuronal type in the cerebellum. During development, these cells migrate from the external to the internal granule layer (IGL), where they receive excitatory glutamatergic and cholinergic contacts from mossy fibers. During this period of development a large proportion of CGN are eliminated via apoptosis. In vitro studies have demonstrated that when CGN are obtained from rats at postnatal day 8 (P8), the sustained activation of N-methyl-D-aspartate (NMDA) receptor at 2-4 days in vitro rescues neurons from cell death. The NMDA action on cultured CGN could mimic the in vivo actions of the transient activation of the glutamate receptors by the transmitter released by mossy fibers by P12. However, some results suggest that glutamate stimulation could be relevant for CGN at earlier stages of development. In this study we evaluated the effect of NMDA receptor stimulation or blockade on the cell death of both in vivo and cultured CGN obtained from P2 to P8 rats. Our results showed that the blockade of NMDA receptors with the antagonists D,L-2-amino-5-phosphonovaleric acid or dizocilpine (MK-801) reduces cell survival to 20-40%, whereas NMDA treatment increases neuronal survival by approximately 50-60%. In vivo, the treatment with MK-801 reduced the number of apoptotic CGN in the molecular layer (ML) from P5 to P8. These results suggest that NMDA receptor stimulation plays a critical role in the regulation of CGN death during the first week of rat cerebellar development.

N-methyl-D-aspartate blocks activation of JNK and mitochondrial apoptotic pathway induced by potassium deprivation in cerebellar granule cells

During the postnatal development of cerebellum, lack of excitatory innervation from the mossy fibers results in cerebellar granule cell (CGC) apoptosis during the migration of the cells toward the internal granule cell layer. Accordingly, CGCs die by apoptosis when cultured in physiological KCl concentrations (5 mm; K5), and they survive in the presence of depolarizing conditions such as high KCl concentration (25 mm; K25) or N-methyl-D-aspartate (NMDA). We have recently shown that NMDA is able to exert a long lasting neuroprotective effect when added to immature (2 days in vitro) CGC cultures by inhibition of caspase-3 activity. Here we show that NMDA- and K25-mediated neuroprotection is associated with an increase in the levels of Bcl-2, an inhibition of K5-mediated increase in Bax, and the inhibition of the release of apoptogenic factors from mitochondria such as Smac/DIABLO and cytochrome c. Moreover, we have shown that similar effects are observed when c-Jun N-terminal kinases (JNKs) are inhibited and that treatment of CGC cultures with NMDA blocks K5-mediated JNK activation. These results allow us to postulate that the inhibition of JNK-mediated release of apoptogenic factors from mitochondria is involved in the NMDA protection from K5-mediated apoptosis of CGCs.

Characterization of the expression and function of N-methyl-D-aspartate receptor in keratinocytes

The N-methyl-D-aspartate (NMDA) receptor is expressed on neural tissue where it gates calcium ion entry upon stimulation. Using immunohistochemistry, it has been demonstrated in this study that the NMDAR1 receptor is also expressed on keratinocytes (KCs) in normal human skin and inflamed psoriatic skin in vivo. Furthermore, the NMDA receptor was functional as demonstrated by the ability of this receptor to trigger Ca++ influx in KCs. Incubation of cultured, human KCs with MK-801 decreases the cell growth and induces an increase in apoptosis. These findings demonstrate that the KC expression of NMDA receptor is a mechanism through which the influx of Ca++ into the cell can be regulated and suggest that the expression of this receptor may play a role in the regulation of KC growth and differentiation.

Regulation of glutamate-synthesizing enzymes by NMDA and potassium in cerebellar granule cells

The presence of 25 mm potassium (KCl) or N-methyl-d-aspartate (NMDA) in cultured cerebellar granule neurons (CGN) induces a trophic effect, including a specific regulation of the enzymes involved in the glutamate neurotransmitter synthesis. In this study we explored the effect of these conditions on the cytosolic and mitochondrial isoenzymes of aspartate aminotransferase (AAT), and phosphate-activated glutaminase (PAG) in CGN. We found that NMDA and KCl increased the AAT total activity by 40% and 70%, respectively. This effect was mediated by an augmentation in the protein levels (68% by NMDA, 58% by KCl). NMDA raised the Vmax and KCl raised both the maximol velocity (Vmax) and Michaelis constant (Km) of AAT. NMDA increased cytosolic AAT activity by 30% and mitochondrial activity by 70%; KCl increased cytosolic and mitochondrial AAT activity by 60% and 100%, respectively. This activation was also related to an increase in the protein levels. The effect of both conditions on the activity and protein levels were more pronounced in mitochondrial than cytosolic AAT and the increment elicited by KCl was higher in both isoforms than that produced by NMDA. The PAG and AAT mRNA levels were also regulated by incubation with NMDA and KCl similarly to the observed changes in the protein levels. These results suggest that NMDA receptor stimulation during CGN development differentially regulates the two AAT isoenzymes involved in the maturation of CGN and that the regulation of both AAT and PAG occurs also at the mRNA expression level, suggesting the involvement of a mechanism of gene expression regulation.

Mechanisms of cell death by deprivation of depolarizing conditions during cerebellar granule neurons maturation

Cerebellar granule cells (CGC) cultured under 5mM KCl (K5) undergo apoptosis after 5 days in vitro (DIV). CGC death is reduced by chronic treatment with 25 mM KCl (K25) or NMDA. Also, when CGC cultured for 6-8 DIV in K25 are transferred to a K5 medium, cells die apoptotically. Moreover, Bcl-2 and Bcl-xL protect neurons from apoptosis, while Bax and Bcl-xS may act as proapototic proteins. It is suggested that these members of the Bcl-2 family may be involved in the cytochrome-c (cyt-c) release to the cytosol. Cytochrome-c is able to form a complex with other proteins to activate a cascade of proteases. In this work, we found that Bcl-2 levels in K5 cells did not show any change during 2-7 days in vitro (DIV); but cells grown with NMDA and K25 displayed an increase (55% approximately) of Bcl-2 from 4 DIV, as compared to control. Under these conditions, Bax levels showed a tendency to decrease with age under control cells and NMDA/K25 induced a reduction of approximately 10% in Bax levels from 4 DIV. On the other hand, in cells maintained in K25 during 7 DIV and then switched to a K5 medium, the levels of Bax showed a consistent decrease (30% after 8h). Under these conditions, the Bcl-2 levels did not show any significant change after 24h. Cytochrome-c levels were unaffected under K5, NMDA and K25 and only a marginal increase of cytochrome-c in the cytosol was detected at 6h after switching. We also found that caspase-9 was only activated under K25-deprivation meanwhile caspase-3 was involved in both protocols. These results suggest that the Bcl-2 family members, caspases activation and cytochrome-c release are involved in CGC death induced by K5 and their participation in this process could be different depending on neuronal maturation in culture.

Blockade of ionotropic glutamate receptors produces neuronal apoptosis through the Bax-cytochrome C-caspase pathway: the causative role of Ca2+ deficiency

Blockade of ionotropic glutamate receptors induces neuronal cell apoptosis. We investigated if mitochondria-mediated death signals would contribute to neuronal apoptosis following administration of glutamate antagonists. The administration of MK-801 and CNQX (MK-801/CNQX), the selective antagonists of N-methyl-d-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate receptors, produced widespread neuronal death in neonatal rat brain and cortical cell cultures. MK-801/CNQX-induced neuronal apoptosis was prevented by zVAD-fmk, a broad inhibitor of caspases, but insensitive to inhibitors of calpain or cathepsin D. Activation of caspase-3 was observed within 6-12 h and sustained over 36 h after exposure to MK-801/CNQX, which cleaved PHF-1 tau, the substrate for caspase-3. Activation of caspase-3 was blocked by high K+ and mimicked by BAPTA-AM, a selective Ca2+ chelator. Reducing extracellular Ca2+, but not Na+, activated caspase-3, suggesting an essential role of Ca2+ deficiency in MK-801/CNQX-induced activation of caspases. Cortical neurons treated with MK-801/CNQX triggered activation of caspase-9, release of cytochrome c from mitochondria, and translocation of Bax into mitochondria. The present study suggests that blockade of ionotropic glutamate receptors causes caspase-3-mediated neuronal apoptosis due to Ca2+ deficiency that is coupled to the sequential mitochondrial death pathway.

Relationship Between Intracellular Free Calcium Concentration and NMDA-induced Cerebellar Granule Cell Survival In Vitro

The survival of cerebellar granule cells in culture is stimulated by activation of the N-methyl-d-aspartate (NMDA) class of glutamate receptors. Activation of these receptors at the key period for cell survival in vitro (3 days; 3DIV) resulted in a sustained elevation of intracellular free calcium concentration [Ca2+]i over the same concentration range of NMDA that led to granule cell survival. Agents that release Ca2+ from intracellular stores led to only small, transient elevations of [Ca2+]i and were unable to stimulate granule cell survival. Addition of the Ca2+ ionophore ionomycin to granule cell cultures at 3DIV resulted in increased granule cell number at 7DIV. The ability of ionomycin to stimulate granule cell survival was related to the [Ca2+]i elicited, indicating that a rise in [Ca2+]i is sufficient to activate the processes leading to granule cell survival and that the extent of the elevation in [Ca2+]i is crucially important in determining granule cell fate.

Endogenous activation of group-I metabotropic glutamate receptors is required for differentiation and survival of cerebellar Purkinje cells

We have applied subtype-selective antagonists of metabotropic glutamate (mGlu) receptors mGlu1 or mGlu5 [7-(hydroxy-imino) cyclopropa[b]chromen-1a-carboxylate ethyl ester (CPCCOEt) or 2-methyl-6-(phenylethynyl)pyridine (MPEP)] to mixed rat cerebellar cultures containing both Purkinje and granule cells. The action of these two drugs on neuronal survival was cell specific. Although CPCCOEt (1, 10, 30 microm) reduced the survival of Purkinje cells, MPEP (3 or 30 microm) selectively reduced the survival of granule cells. Both effects required an early exposure of cultures to antagonists [from 3 to 6 d in vitro (DIV) for CPCCOEt, and from 3 to 6 or 6 to 9 DIV for MPEP]. Addition of MPEP from 6 to 9, 9 to 13, or 13 to 17 DIV also induced profound morphological changes in the dendritic tree and dendritic spines of Purkinje cells, suggesting that endogenous activation of mGlu5 receptors is required for the age-dependent refinement of Purkinje cell phenotype. In in vivo studies, an early blockade of mGlu1 receptors induced in rats by local injections of LY367385 (20 nmol/2 microl), local injections of mGlu1 antisense oligonucleotides (12 nmol/2 microl), or systemic administration of CPCCOEt (5 mg/kg, s.c.) from postnatal day (P) 3 to P9 reduced the number and dramatically altered the morphology of cerebellar Purkinje cells. In contrast, mGlu5 receptor blockade induced by local injections of antisense oligonucleotides reduced the number of granule cells but also produced substantial morphological changes in the dendritic tree of Purkinje cells. These results provide the first evidence that the development of cerebellar neurons is under the control of mGlu1 and mGlu5 receptors, i.e., the two mGlu receptor subtypes coupled to polyphosphoinositide hydrolysis.

Isovolumetric regulation mechanisms in cultured cerebellar granule neurons

Cultured cerebellar granule neurons exposed to gradual reductions in osmolarity (-1.8 mOsm/min) maintained constant volume up to -50% external osmolarity (pi(o)), showing the occurrence of isovolumetric regulation (IVR). Amino acids, Cl-, and K+ contributed at different phases of IVR, with early efflux threshold for [3H]taurine, D-[3H]aspartate (as marker for glutamate) of pi(o) -2% and -19%, respectively, and more delayed thresholds of -30% for [3H]glycine and -25% and -29%, respectively, for Cl- (125I) and K+ (86Rb). Taurine seems preferentially involved in IVR, showing the lowest threshold, the highest efflux rate (five-fold over other amino acids) and the largest cell content decrease. Taurine and Cl- efflux were abolished by niflumic acid and 86Rb by 15 mM Ba2+. Niflumic acid essentially prevented IVR in all ranges of pi(o). Cl--free medium impaired IVR when pi(o) decreased to -24% and Ba2+ blocked it only at a late phase of -30% pi(o). These results indicate that in cerebellar granule neurons: (i) IVR is an active process of volume regulation accomplished by efflux of intracellular osmolytes; (ii) the volume regulation operating at small changes of pi(o) is fully accounted for by mechanisms sensitive to niflumic acid, with contributions of both Cl- and amino acids, particularly taurine; (iii) Cl- contribution to IVR is delayed with respect to other niflumic acid-sensitive osmolyte fluxes (osmolarity threshold of -25% pi(o)); and (iv), K+ fluxes do not contribute to IVR until a late phase (< -30% pi(o)).

The effects of forelimb deafferentation on the post-natal development of synaptic plasticity in the hippocampus

The effects of partial deafferentation of the forelimb on the development of long-term potentiation in the hippocampus of rats aged 13-18 days were studied. Long-term potentiation in hippocampus field CA1 was of greater amplitude and duration in control rats at 16-18 days of post-natal ontogenesis than in adult animals. Partial deafferentation by section of the median nerve in the forelimb on the 13th day of life led to the disappearance of this excess at 16-18 days. The peak in synaptic plasticity occurred later in operated animals--on day 17--and was much less marked than in controls. The decreases in the amplitude and duration of long-term potentiation in hippocampal field CA1 in operated animals provides evidence for a decrease in the sensitivity and/or number of NMDA receptors. This suggests that partial deafferentation of one limb may lead not to a decrease but to an increase in spike and synaptic activity in the hippocampus, which in normal conditions may affect the maturation of the plastic properties of synaptic transmission associated with the expression and positions of NMDA receptors. The level of long-term potentiation in sham-operated rats was significantly greater than in controls of the same age. This significant increase in NMDA-dependent long-term potentiation may be explained by a decrease in the level of activation due to anesthesia. It is suggested that the decrease in the spike activity of cells receiving signals from the median nerve may be compensated for by activation of other specific and non-specific inputs.

Reduction of phospholemman expression decreases osmosensitive taurine efflux in astrocytes

The role of phospholemman (PLM) in taurine and Cl(-) efflux elicited by 30% hyposmotic solution was studied in cultured cerebellar astrocytes with reduced PLM expression by antisense oligonucleotide (AO) treatment. PLM, a substrate for protein kinases (PK) C and A, is a protein that increases an anion current in Xenopus oocytes and forms taurine-selective channels in lipid bilayers. Taurine contributes as an osmolyte to regulatory volume decrease (RVD) and is highly permeable through PLM channels in bilayers. Two antisense oligonucleotides (AO1 and AO2) effectively decreased the expression of the PLM protein by 40% and 30%, respectively, and markedly reduced [(3)H]taurine efflux by 67% and 62%. AO treatment also decreased the osmosensitive release of Cl(-), followed as (125)I. The inhibition of Cl(-) efflux (23% for AO1 and 13% for AO2) was notably lower than for [(3)H]taurine. The contribution of PKC and PKA in the function of PLM was also evaluated in astrocytes. Pharmacological activation or inhibition of PKC and PKA revealed that the osmosensitive taurine efflux is essentially PKC-independent while (125)I efflux is reduced by the PKC blockers H-7 (21%) and Gö6983 (41%). The PKA activator forskolin and dbcAMP increased taurine efflux by 66-70% and (125)I efflux by 21-45%. Norepinephrine increased the osmosensitive taurine efflux at about the same extent as dbcAMP and forskolin, and this was reduced by PKA blockers. These results suggest that PLM plays a role in RVD in astrocytes by predominantly influencing taurine fluxes, which are modulated by PKA but not PKC.

Neuregulins and the shaping of synapses

In developing and mature neural circuits, neural electrical activity controls the correct formation of connections and their state. Neuregulins (NRGs) mediate between the electrical neural activity and molecular components by regulating the expression of ion channel receptors or transmitter release in synapses. Furthermore, NRGs may be signaling factors involved in tuning locomotion or other higher functions by coordinating excitatory and inhibitory neurons.

The regulatory connection between the activity of granule cell NMDA receptors and dendritic differentiation of cerebellar Purkinje cells

It is known that cerebellar granule cells are powerful inducers for the differentiation of Purkinje cells. However, the detailed mechanism of this regulation has not yet been clarified. Here, using cerebellar neuronal culture, we show that the activation of NMDA receptors expressed by granule cells triggers the signaling pathway for the dendritic differentiation of Purkinje cells. This signal has been shown to promote the granule cell survival through BDNF-mediated TrkB activation, leading to Purkinje cell differentiation by increasing the granule-Purkinje cell interaction. Among the possible signal molecules provided to the dendrites of Purkinje cells from granule cells, nitric oxide was found to have no effect on the dendritic outgrowth and branching, but electrical activity and the subsequent intracellular Ca(2+) increase were thought to play an important role in the branching and thickening of the dendrites, because blockade of both non-NMDA and metabotropic glutamate receptors caused a significant decrease in the number of branch points and the diameter of Purkinje dendrites without apparently affecting the dendrite extension and spine formation. Collectively, these results suggest that Purkinje cell differentiation is regulated by two successive steps. The first step is initiated by the NMDA receptor-mediated signal in granule cells, which acts as a trophic factor for granule cells. The second step involves the activation of granule-Purkinje synapses, providing neurotrophic substances and electrical activity essential for Purkinje cell differentiation.

Role of heat shock proteins in the effect of NMDA and KCl on cerebellar granule cells survival

Cerebellar granule cells (CGC) die apoptotically after five days in culture (DIV) at physiological concentrations of potassium (5 mM; K5). When CGC are depolarized (K25) or treated with NMDA (150 microM) cell survival is increased. CGC changed from K25 to K5 die after 24-48 h. It is known that heat shock protein (HSP) may protect from cell death. Here, we found that cells in K5 showed an increase in HSP-70 levels after 3 DIV. Similarly, in cells changed from K25 to K5, HSP-70 levels were increased after 6 h. Neither NMDA nor K25 treatment affected HSP-70 levels from 2-7 DIV. Ethanol or thermal stress induced HSP-70, but cell survival was not affected in K5 medium. These results suggest that HSP, particularly HSP-70, are not involved in the mechanisms by which NMDA and KCl promote cell survival.

Effect of NMDA antagonists on the activity of glutaminase and aspartate aminotransferase in the developing rat cerebellum

Chronic treatment of rats from postnatal day 6 to 25 with drugs that interact with the N-methyl-D-aspartate (NMDA) receptor induced a differential effect on the activity of some enzymes involved in neurotransmitter synthesis. Two of these drugs ((5R,10S)-(+)-5-methyl-10,11 -dihydro-5H-dibenzo(a,d)cyclohepten-5,10-imine hydrogen maleate (MK-801) and 3-(2-carboxypiperazin-4-yl)propyl-1phosphonic acid (CPP)) caused a marked reduction (20-40%) of glutaminase and aspartate aminotransferase activity in the cerebellum. These changes were observed only at a very precise time of development (i.e. 10 to 19 postnatal day). The competitive antagonist, amino phosphonovaleric acid (APV), did not affect any of the enzymes studied at all tested ages. When animals were treated with NMDA only a slight, but significant, increase in the activity of glutaminase was observed at 9-11 postnatal day only. Any of the agonists or antagonists tested significantly affected the activity of lactate dehydrogenase as compared to control animals. Histologic observations of cerebella treated with the indicated drugs showed that only MK-801, and CPP to a lesser extent, induced a small reduction in the width of the internal granule layer. The body weight of animals treated with MK-801 was clearly reduced, but only in more mature rats (> 16 postnatal day), when animals did not show any alteration in the enzymes tested. These results support the suggestion that presynaptic influences, particularly from glutamatergic neurons, are critical to promote cerebellar granule neurons differentiation during critical periods of the cerebellar development.

Involvement of three glutamate receptor epsilon subunits in the formation of N-methyl-D-aspartate receptors mediating excitotoxicity in primary cultures of mouse cerebellar granule cells

The N-methyl-D-aspartate receptors have been implicated in neuronal plasticity and their overactivation leads to neurotoxicity. Molecular cloning and co-expression of various glutamate receptor zeta and epsilon complementary DNAs support a heteromeric structural organization for N-methyl-D-aspartate receptors. In this study, we show that cerebellar granular neurons in primary culture of mouse express glutamate receptor zeta1 and at least three glutamate receptor epsilon (epsilon1, epsilon2, and epsilon3) protein subunits. In vitro, the temporal patterns of glutamate receptor epsilon1, epsilon2, and epsilon3 subunit expression depend on culture stages. By day 9, a somatic and neuritic immunolocalization for all N-methyl-D-aspartate subunits was clearly identified in most neuronal, but not glial cells. The role of particular subunits in N-methyl-D-aspartate-mediated excitotoxicity was probed by exposing the cerebellar granule cells to antisense oligodeoxynucleotides generated against specific N-methyl-D-aspartate receptor subunits. Antisense oligodeoxynucleotide treatments significantly down-regulated the amounts of the corresponding N-methyl-D-aspartate subunits. The decrease in N-methyl-D-aspartate subunit protein correlated with a reduction in N-methyl-D-aspartate-induced calcium influx and N-methyl-D-aspartate-mediated excitotoxicity in cerebellar cultures. In contrast, antisense oligodeoxynucleotide treatment failed to protect neurons from 1-methyl-4-phenylpyridinium-induced metabolic cell toxicity. Antisense oligodeoxynucleotide treatment targeted at N-methyl-D-aspartate glutamate receptor epsilon subunits demonstrate that glutamate receptor epsilon1, epsilon2, and epsilon3 proteins form N-methyl-D-aspartate receptors responsible for neurotoxic effects on cerebellar neurons. This study provides direct evidence for the existence of distinct N-methyl-D-aspartate receptor subunit proteins in cerebellar granule cells developing in vitro that may trigger N-methyl-D-aspartate-dependent excitotoxicity.

Chronic pre-explant blockade of the NMDA receptor affects survival of cerebellar granule cells explanted in vitro

Rat pups were treated with the competitive NMDA antagonist CGP 39551 with daily injections on postnatal days 1 to 8. Cultures of cerebellar granule cells were prepared from these pups as well as from control pups of the same age and body weight. Granule neurons explanted from CGP 39551-treated pups showed a decreased survival, both at short (2 days) or longer (8 days) time in vitro, irrespective of trophic (high K+) or non-trophic (low K+) culture conditions. Granule cells from control or treated animals underwent apoptotic death when shifted from high to low K+ after maturation in vitro and were rescued by lithium (5 mM). Under the same experimental conditions, the block of protein synthesis through cycloheximide only partially protected from apoptotic death granule neurons from control rats, whereas it was totally effective on cultures derived from CGP 39551-treated animals. This suggests that a different balance between apoptotic/necrotic cell death may be the result of the same experimental conditions in the two types of cultures. Finally, the acquisition of excitotoxic sensitivity to glutamate and the protection given by MK-801 were the same in both types of cultures. The present results demonstrate that the previous block of the NMDA receptor negatively affects the subsequent survival of granule cells once they are explanted in vitro, whereas some features related to the maturation of these neurons in vitro are not impaired.

Nerve growth factor up-regulates the N-methyl-D-aspartate receptor subunit 1 promoter in PC12 cells

The N-methyl-D-aspartate (NMDA) subtype of glutamate receptor plays important roles in synaptic plasticity, the induction of long term potentiation, and excitotoxicity. Mechanisms governing the regulation of expression of its subunit genes remain largely unknown. The promoter of the essential subunit of the NMDA receptor heteromer, NMDAR1, contains DNA binding elements recognized by the nerve growth factor-inducible/early growth reaction factor (NGFI/Egr) family of transcription factors that are rapidly induced by neurotrophins, such as nerve growth factor (NGF). This study examined the effect of NGF on the activity of the N-methyl-D-aspartate receptor subunit 1 (NMDAR1) promoter/luciferase reporter constructs in PC12 cells, which contain the high affinity TrkA receptor for NGF and the low affinity p75(NTR) receptor for neurotrophins. NGF up-regulated the activity of the NMDAR1 promoter by 3-4-fold in a time- and dose-dependent manner. 5' deletional analysis of the promoter indicated that the responsive element(s) resides in the proximal region containing GSG and Sp1 sites. Mutational analysis of these sites revealed that both were important for NGF regulation. Transient expression of Egr-1 increased activity of the wild type promoter but failed to increase activity of a GSG mutant promoter. Other neurotrophins did not activate the promoter, while K-252a inhibited the action of NGF. These results suggest that the NGF effect is mediated by the high affinity NGF receptor, Trk A and that neurotrophin binding to the low affinity neurotrophin receptor, p75(NTR), alone does not affect the promoter activity. Our results suggest that NGF is able to up-regulate the activity of the NMDAR1 promoter and may play a role in controlling the expression levels of NMDA receptors.

Regulatory volume decrease and associated osmolyte fluxes in cerebellar granule neurons are calcium independent

To investigate a possible role for Ca as a transduction signal for regulatory volume decrease (RVD), the effects of external Ca removal, Ca channel blockers (Cd, Co, La, Gd, verapamil, diltiazem, dihydropyridines) and inhibitors of endoplasmic reticulum Ca release (dantrolene, ryanodine, TMB-8) were examined on RVD and on the swelling-activated efflux of two main osmolytes: Cl (traced by 125I) and [3H]taurine. Omission of Ca plus EGTA did not affect RVD or osmolyte release but when BAPTA was the chelator, RVD decreased 20%, 125I fluxes were unaffected and taurine stimulated efflux decreased (20%) while the basal efflux slightly increased (<10%). Verapamil, diltiazem, Co, Cd, La and Gd did not affect RVD or osmolyte fluxes. Nimodipine and nitrendipine (25-50 microM) markedly decreased RVD and osmolyte fluxes (>90%) through a mechanism independent of extracellular Ca. Swelling elicited an increase in cytosolic Ca measured by fura-2, which was notably variable ranging 50-350 nM. However, RVD and osmolyte fluxes were not affected by the blockers of endogenous Ca release dantrolene, ryanodine and TMB-8 or by the permeable Ca chelator BAPTA-AM, even when the cytosolic Ca increase was abolished by the chelator. These results indicate that 1) RVD and osmolyte fluxes are independent of extracellular Ca 2) RVD, osmolyte release and cytosolic Ca raise are only coincident events. Consequently, Ca is unlikely to be a transducing signal for RVD in neurons.

Modification of NMDA receptor channels and synaptic transmission by targeted disruption of the NR2C gene

A novel strain of mutant mouse has been generated with a deletion of the gene encoding the NR2C subunit of the NMDA receptor, which is primarily expressed in cerebellar granule cells. Patch-clamp recordings from granule cells in thin cerebellar slices were used to assess the consequences of the gene deletion. In granule cells of wild-type animals, a wide range of single-channel conductances were observed (19-60 pS). The disruption of the NR2C gene results in the disappearance of low-conductance NMDA receptor channels ( < 37 pS) normally expressed in granule cells during developmental maturation. The NMDA receptor-mediated synaptic current is markedly potentiated in amplitude, but abbreviated in duration (with no net difference in total charge), and the non-NMDA component of the synaptic current was reduced. We conclude that the NR2C subunit contributes to functional heteromeric NMDA receptor-subunit assemblies at the mossy fiber synapse and extrasynaptic sites during maturation, and the conductance level exhibited by a given receptor macromolecule may reflect the stochiometry of subunit composition.

Changes of actin cytoskeleton during swelling and regulatory volume decrease in cultured astrocytes

Swelling of cultured astrocytes exposed to hyposmotic medium modified the organization of the filamentous actin (F-actin) cytoskeleton, making the actin network diffuse in the cell body but concentrated at foci corresponding to the tips of the cell projections retracted by swelling. This change was reversible, and, after 2 h, the actin cytoskeleton tended to recover, and cells regained their flat and stellate shape. Cytochalasins B and D (CB and CD, respectively), which disrupt the actin cytoskeleton, did not affect regulatory volume decrease (RVD) or the swelling-activated efflux of Cl- and inositol, although 10 microM CD increased the basal efflux of taurine. The mercurial p-chloromercuribenzenesulfonate (0.5-1 mM), known to disrupt the membrane cytoskeleton in isosmotic conditions, induced a 46, 50, and 38% release of [3H]taurine, 125I, and [3H]inositol, respectively, causing cell shrinkage and retraction of the cytoskeleton. Coincidently, the swelling-stimulated release of [3H]taurine and 125I was reduced by 60 and 30%, respectively. Results of this study do not exclude the possibility that changes in the actin cytoskeleton elicited by swelling are involved in mechanisms of RVD and only indicate that the disruption caused by cytochalasins is unrelated to that process.

Nitric oxide enhances amino acid release from immature chick embryo retina

Nitric oxide (NO) was investigated for its ability to induce amino acid release from immature chick retina. The production of endogenous NO by activation of NO synthase after stimulation of N-methyl-D-aspartate (NMDA) subtype of glutamate receptor caused a significant increase in basal release of gamma-aminobutyric acid (GABA) and glutamine, whereas a more modest increase in the glutamate release was also observed. The exposure of chick retina from 9-day-old embryos to NO-generating compounds, S-nitroso-N-acetylpe-nicillamine (SNAP) and sodium nitroprusside (SNP) produced a dose dependent increase in GABA, glutamine, and glutamate release. This effect was reduced by about 80% by haemoglobin. These results indicate that NO has a stimulatory effect on amino acid release from chick embryo immature retina. However, this effect does not appear to involve a cGMP-related mechanism because 8-bromo-cGMP, a stable analogue of cGMP, failed to affect spontaneous amino acid release and because zaprinast did not enhance NMDA-stimulated release. In conclusion, our present observations may account for a role of NMDA-mediated events in the biochemical maturation under depolarizing conditions.

Distinct N-methyl-D-aspartate receptor 2B subunit gene sequences confer neural and developmental specific expression

Expression of the N-methyl--aspartate (NMDA) receptor 2B (NR2B) subunit is neural-specific and differentially regulated. It is expressed in the forebrain and in cerebellar granule cells at early postnatal stages and selectively repressed in the cerebellum after the second postnatal week, where it is replaced by the NR2C subunit. This switch confers distinct properties to the receptor. In order to understand the molecular mechanisms that differentially regulate the NR2B gene in the forebrain and cerebellum during development, we have isolated and characterized the promoter region of the NR2B gene. Two 5' noncoding exons and multiple transcription start sites were identified. Transcriptional analysis in transgenic mice reveals that an upstream 800-base pair region, which includes the first exon, is sufficient to direct neural-specific transcription. Developmental repression of the gene in the cerebellum requires additional regulatory elements residing in the first intron or second exon. Sequence elements that may participate in the regulation of the NR2B gene were identified by comparison to other neural genes. These studies provide insight into the molecular mechanisms regulating the switch of NMDA receptor subunit expression in the cerebellum, which ultimately account for the physiological changes in receptor function during development.

Cl channel blockers inhibit the volume-activated efflux of Cl and taurine in cultured neurons

The effects of the Cl channel blockers 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB), 1,9-dideoxyforskolin (DDF), dipyridamole, and niflumic acid and of the polyunsaturated fatty acids arachidonic, linolenic, and linoleic acids on regulatory volume decrease (RVD) and associated 125I and [3H]taurine fluxes in cultured rat cerebellar granule neurons were examined. Dose-response curves of NPPB, DDF, and dipyridamole showed 20-100% inhibition of RVD and osmolyte fluxes. Niflumic acid was less potent, requiring 150-600 microM to show effects of this magnitude. The polyunsaturated fatty acids (5-20 microM) inhibited 80-90% RVD and osmolyte fluxes, with arachidonic acid exhibiting the most potent effect. The volume-associated taurine efflux was somewhat higher in younger neurons, but the pharmacological sensitivity was essentially the same in immature and mature cells. The effects of all tested drugs on 125I and [3H]taurine fluxes were remarkably similar, indicating a close pharmacological sensitivity of the transport mechanism for the two osmolytes. This is in line with the suggestion of a common pathway for the volume-associated release of Cl and amino acids functioning as osmolytes.

Developmental expression of endothelin receptors in cerebellar neurons differentiating in culture

We describe the identification and expression of endothelin (ET) receptor subtypes in differentiating cultured cerebellar neurons. Using [125I]ET-1 and the subtype-selective ligands BQ-123 and sarafotoxin 6c as selective ligands for the ETA and ETB receptors, respectively, we found that cerebellum from 8-day-old rats displayed only the ETB receptor subtype. We next cultivated cerebellar granule cell neurons to study ET receptor differentiation between 2 and 22 days in vitro. Using the above reagents, we found that while unlabeled ET-1 displayed monophasic competition curves, BQ-123 and sarafotoxin 6c displayed partial displacement curves, indicating the presence of both ETA and ETB receptors on these neurons. The proportion of ETB receptors gradually decreased from day 2 onwards the proportion of ETA receptors gradually increased. On days 2, 3, 4, and 5 of culture, the ETB:ETA receptor ratios were 90:10, 70:30, 60:40, and 40:60, respectively. There was no further change in receptor subtype ratio beyond day 5 and up to day 22. Northern blot analysis showed that ETB receptor message expression was 6.9-fold higher than that of ETA receptor expression on day 2, but steadily decreased with time, whereas ETA receptor message expression was minimal on day 2 and maximal by day 3 and 4. By day 7, receptor message was of equal abundance, which was in good agreement with the binding studies. This novel, developmentally regulated process predicts the existence of endogenous mediators of neuronal ET receptor expression.

Agonist-induced down-regulation of NMDA receptors in cerebellar granule cells in culture

In contrast to the acute toxic effect of NMDA on mature cerebellar granule cells, chronic treatment with NMDA (140 microM from 1 to 9 days in vitro) did not compromise cell survival. Such treatment markedly suppressed NMDA receptor activity: at 8 days in vitro NMDA-induced 45Ca2+ influx was reduced by approximately 60% and acute exposure to NMDA (highest concentration tested, 1 mM) at 9 days in vitro did not cause detectable toxicity. The reduction in NMDA receptor activity was accompanied by a significant decrease (approximately 80% at 9 days in vitro) in the level of the NR1 and the NR2A NMDA receptor subunit protein, detected using the selective photoaffinity ligand [125I]CGP55802A. It seems, therefore, that the agonist-induced decrease in NMDA receptor activity is due to receptor down-regulation. In contrast to the marked influence of chronic NMDA exposure on the cellular content of the NMDA receptor subunit proteins, mRNA levels of the different subunits (NR1, NR2A, NR2B and NR2C) were not significantly affected. It seems, therefore, that agonist-induced down-regulation of the NMDA receptor involves critically mRNA translation and/or post-translational regulation.

Inhibition by Cl- channel blockers of the volume-activated, diffusional mechanism of inositol transport in primary astrocytes in culture

[3H]Inositol accumulated by rat brain cultured astrocytes is released when cells swell by exposure to solutions of decreased osmolarity. Activation of inositol efflux was proportional to reductions in osmolarity from 30%-70%. This volume-activated inositol efflux pathway was increased (27%) in Na(+)-free medium and decreased (22%) in Cl(-)-free medium. It was independent of extracellular Ca2+ and was reduced (30%) in the presence of the intracellular chelator [1,2-bis(o-aminophenoxy) ethane-N,N,N',N'-tetraacetic acid tetra-(acetoxymethyl)-ester] (BAPTA-AM). The inositol efflux pathway was markedly inhibited by Cl- channel blockers, which at maximal inhibitory concentrations decreased inositol efflux by 70%-83%. The potency range of the drugs was: 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB) > 1-9, dideoxyforskolin > 4,4'-diisothiocyanatostilbene-2, 2'-disulfonic acid (DIDS) > niflumic acid. Inositol efflux was strongly inhibited by the SH blocker N-ethyl maleimide (NEM), which at 100 microM abolished inositol release. Inositol efflux can be reversed by increasing its extracellular concentration, suggesting that the efflux is mediated by a diffusional pathway whose direction is given by the concentration gradient. The inhibition of volume-associated fluxes of inositol by Cl- channel blockers supports the suggestion of an anion channel as the common pathway for inorganic and organic osmolytes in cultured astrocytes.

GABA attenuates the neurotoxic effects of ethanol in neuron-enriched cultures from 8-day-old chick embryo cerebral hemispheres

Neuron-enriched cultures were prepared from 8-day-old chick embryo cerebral hemispheres and exposed to ethanol (50 mM), GABA (10(-5) M) and ethanol (50 mM) + GABA (10(-5) M) from day 4 to 8 in culture. At day 8, control, ethanol, GABA and ethanol + GABA-treated cultures were examined morphologically and biochemically. Choline acetyltransferase (ChAT) and glutamic acid decarboxylase (GAD) activities were used as markers for cholinergic and GABAergic neuronal phenotypic expression, respectively. Control cultures showed more numerous and large neuronal aggregates as well as prominent neuritic bundles. Moreover, cultures treated with GABA depicted even more numerous neuronal aggregates with interconnecting neurites as compared to control. In contrast, ethanol-treated cultures exhibited smaller neuronal aggregates with less prominent neuritic bundles than control. However, cultures treated concomitantly with ethanol + GABA exhibited numerous and larger aggregates than cultures treated with ethanol alone. Neuritic bundles which were highly reduced in ethanol-treated cultures became prominent in the presence of GABA. As previously reported, ethanol alone enhanced ChAT and reduced GAD activities. GABA given alone enhanced the expression of both neuronal phenotypes. When GABA was given concomitantly with ethanol the decline in GAD and the rise in ChAT observed in ethanol-treated cultures was restored by GABA to almost control levels. Thus, ethanol-induced alterations in morphology and neuronal phenotypes were counteracted by the neurontrophic effect of GABA.

Effects of protein kinase C modulation on NMDA receptor mediated regulation of neurotransmitter enzyme and c-fos protein in cultured neurons

The role of protein kinase C (PKC) in N-methyl-D-aspartate (NMDA) receptor-mediated biochemical differentiation and c-fos protein expression was investigated in cultured cerebellar granule neurons. The biochemical differentiation of glutamatergic granule cells was studied in terms of the specific activity of phosphate-activated glutaminase, an enzyme treatment in the synthesis of the putative neurotransmitter pool of glutamate. When the partially depolarized cells were treated with NMDA for the last 1 to 3 days (between 2 and 5 days in vitro), it elevated the specific activity of glutaminase. In contrast, NMDA had little effect on the activity of aspartate aminotransferase or of lactate dehydrogenase. Treatment of 10-day old granule neurons with NMDA also resulted in a marked increase in the immunocytochemically measured expression of c-fos protein. The increases in both the activity of glutaminase and the steady state level of c-fos protein were specific to the activation of NMDA receptors, as they were completely blocked by D,L-2-amino-5-phosphonovaleric acid. The specific stimulation of NMDA receptors in PKC-depleted granule neurons or in the presence of reasonably specific PKC inhibitors also produced significant elevation in the activity of glutaminase and the expression of c-fos protein. These increases were similar in magnitude to those observed in the granule neurons of the respective control groups. Our findings demonstrate that PKC is not directly involved in the NMDA receptor-mediated signal transduction processes associated with biochemical differentiation and c-fos induction in cerebellar granule neurons.

Selective neurotoxicity of ruthenium red in primary cultures

The inorganic dye ruthenium red (RuR) has been shown to be neurotoxic in vivo when injected intracerebrally. In this work the toxicity of RuR was compared in primary cultures of rat cortical neurons, cerebellar granule neurons and cerebellar astroglia. Microscopic examination of the cultures revealed that RuR penetrates the somata of both types of neurons used and produces vacuolization and loss and fragmentation of neurites. In contrast, no RuR was seen inside cultured astrocytes and no morphological signs of damage were observed in these cells. RuR toxicity was also assessed by immunocytochemistry of alpha-tubulin and by biochemical measurement of the reduction of (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) by the cultured cells. The morphological alterations in the neurons were closely correlated with loss of tubulin immunoreactivity and particularly with a notable decrement in the ability to reduce MTT. Using the latter parameter, it was found that neuronal damage was independent of the age of the cultures, augmented progressively with time of incubation with RuR, from 8 to 24 h, and showed a clear dose-response curve from 20 to 100 microM RuR. Astrocytes showed only a slight decrease in MTT reduction after 24 h of incubation with 100 microM RuR. It is concluded that RuR seems to be toxic for neurons but not for astroglia, and that this selectivity is probably related to the ability of the neurons to internalize the dye. The possible mechanisms of RuR penetration and neuronal damage are discussed.

NMDA receptor responses in adult hippocampal CA1 region after neonatal treatment with MK-801: comparison with NMDA receptor responses in the immature rat

Neonatal MK-801 treatment from postnatal day 8-19 leads to long-term effects on brain function, suggesting that exposure to this drug leads to the development of a brain with immature network properties. One aspect of this hypothesis, that the NMDA receptors preserve their immature state after the treatment, has been tested by measuring the potency of the competitive antagonist D-AP5 in hippocampal slices. We have previously shown that an increased potency to D-AP5 is a characteristic property of NMDA receptors during early life. In the present study we measured field potentials in the CA1 region of rat hippocampal slices evoked by iontophoretic NMDA application in the Schaffer-commissural synaptic fields. Agonist dose-response curves were constructed, followed by bath applications of increasing concentrations of the antagonist D-AP5. The maximum NMDA evoked field response was the same in slices of mature control (PND70-90; 18.9 +/- 1.2 mV) and MK-801 treated rats (PND70-90; 19.3 +/- 0.9 mV), but significantly larger in immature slices (PND10-16; 24.0 +/- 0.6 mV). The sensitivity to NMDA in hippocampal slices from each group was estimated by quantifying the ionotophoretic ejection current (= dose) which evoked 50% of the maximum field response (EC50). A significantly higher sensitivity to NMDA was found in hippocampal slices obtained from MK-801-treated rats (EC50 = 3.6 +/- 0.2 nA) than in slices from control (EC50 = 6.1 +/- 0.7 nA) or immature (EC50 = 5.9 +/- 0.5 nA) animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Chronic mild acidosis specifically reduces functional expression of N-methyl-D-aspartate receptors and increases long-term survival in primary cultures of cerebellar granule cells

Previous studies suggest that chronic depolarization by addition of 25 mM KCl or N-methyl-D-aspartate to primary cultures of cerebellar granule cells promotes expression of the N-methyl-D-aspartate subtype of glutamate receptor, as determined by electrophysiological responsiveness and susceptibility to excitotoxicity. Recent studies have demonstrated that acute mild acidosis reduces N-methyl-D-aspartate receptor channel activity by a non-competitive action of H+ on an extracellular site of the receptor channel complex. Since the level of N-methyl-D-aspartate receptor expression in granule cell cultures is activity-dependent, we examined whether chronic mildly acidotic culture conditions would selectively diminish the level of N-methyl-D-aspartate responsiveness in granule cells, in effect producing a functional level of expression more comparable to that observed in vivo. To test this, cerebellar granule cells from eight-day neonatal rats were grown in an HCO3-buffered medium containing elevated K+ (25 mM KCl) either under standard conditions (95% air/5% CO2, pH 7.4), or under chronic mildly acidotic conditions (90% air/10% CO2, estimated pH of 7.1). Glutamate receptor subtype expression was subsequently assessed using standard neurotoxicity assays, a quantitative immunoblotting assay for N-methyl-D-aspartate receptors and whole cell patch clamp recordings. Cells grown in the 10% CO2 environment exhibited a significant reduction in susceptibility to L-glutamate neurotoxicity (at least 10-fold), but not kainate-induced neurotoxicity, relative to cells grown in 5% CO2. In both culture conditions, L-glutamate- and kainate-induced toxicity were mediated by activation of N-methyl-D-aspartate and non-N-methyl-D-aspartate receptors, respectively, as determined by the sensitivity of agonist-induced toxicity to specific receptor antagonists. Using polyclonal antibodies generated against a peptide sequence recognizing five of eight splice variants in the common "R1" subunit of N-methyl-D-aspartate receptors, a 31% reduction in the amount of immunoreactive protein was observed in membrane preparations from cells grown in 10% CO2, relative to the amount detected in cells grown in 5% CO2. Moreover, perfusion of cells with glutamate (50 microM) in a nominally Mg(2+)-free solution containing glycine (2 microM) elicited N-methyl-D-aspartate antagonist-sensitive inward currents in proportionately fewer cells cultured in 10% CO2, relative to cells cultured in 5% CO2. Long-term survival was also significantly enhanced in cells exposed chronically to mild acidotic culture conditions, relative to cells grown under standard pH conditions (22 days, 10% CO2 vs 16 days, 5% CO2).(ABSTRACT TRUNCATED AT 400 WORDS)

Plasticity of NMDA receptor expression during mouse cerebellar granule cell development

A period of hypersensitivity to N-methyl-D-aspartate (NMDA) has been described during the early development of different types of neuron. Since activation of NMDA receptors can also induce rapid neuron death, the hypersensitivity to NMDA may be tightly controlled. In the present study we show that mouse cerebellar granule neurons become transiently hypersensitive to NMDA between days 10 and 14 after plating in a culture medium containing 30 mM K+. The NMDA sensitivity is higher when cells are cultured in the presence of an NMDA receptor antagonist [30 mM K+ plus 100 microM 3-((+/-)-2-carboxypiperazine-4-yl)-propyl-1-phosphonic acid (CPP)], and no hypersensitivity is observed when cells are cultured in the continuous presence of NMDA (12.5 mM K+ plus 100 microM NMDA). The high NMDA sensitivity in control cells is associated with a higher density of NMDA receptors than that measured in NMDA-treated cells, suggesting that the sensitivity to NMDA may be partly controlled by activity-dependent NMDA receptor down-regulation. We also examined the level of NMDA-zeta 1 mRNA and found no correlation between this parameter and the transient pattern of NMDA sensitivity. Such NMDA receptor plasticity may be of importance in the central nervous system, protecting developing cells from excitotoxicity at critical developmental stages.

Sulfhydryl groups essential for the volume-sensitive release of taurine from astrocytes

The volume-sensitive [3H]taurine efflux from cultured astrocytes was found to be strongly inhibited by the sulfhydryl group-modifying reagent N-ethylmaleimide (NEM). This maleimide inhibited taurine efflux evoked by 50% hyposmotic solutions with an IC50 of 54 microM. The inhibition by NEM followed pseudo-first order reaction kinetics. A double log plot of the pseudo-first order constant against NEM concentration gave a linear relationship with a slope of 1.2. The data are consistent with a simple bimolecular reaction mechanism in which one molecule of NEM reacts with one sulfhydryl group per transport unit. The membrane-impermeant maleimide derivative 5-eosin maleimide did not affect the volume-stimulated taurine efflux. The sulfhydryl-modifying mercurial reagents mersalyl and p-chloro mercuribenzenesulfonate (0.5-1 mM) increased [3H]taurine efflux under isosmotic conditions and concomitantly decreased the hyposmolarity-evoked efflux. The results demonstrate an essential requirement for sulfhydryl groups for the volume-sensitive taurine efflux.

Seizures, brain damage and brain development

Recent evidence suggests that hippocampal damage can be both the result of seizure activity and the cause of further chronic epilepsy. A review of current models of status epilepticus-induced brain damage reveals that excitotoxic mechanisms probably mediate the lesions in most brain regions. NMDA receptors appear to play a dominant role, although non-NMDA glutamate receptors are important in several specific neuronal populations. In the immature brain, a number of unique metabolic features determine a different set of vulnerabilities, resulting in a brain which is more resistant than the adult's to certain mechanisms of brain damage, but quite vulnerable to others. The inhibition of growth by severe seizure activity has implications for the developing brain that have not yet been fully explored. The mechanisms by which seizure-induced hippocampal lesions cause chronic epilepsy have been explored in several recent animal models. A rearrangement of hippocampal circuits may result from death of selected populations of inhibitory neurons, or from misdirected regeneration by excitatory neurons. It could lead to chronic epilepsy through loss of normal inhibition, through sprouting of new excitatory connections, through conservation of excitatory connections which in a healthy brain would be pruned during development, or through facilitation of kindling by one of these mechanisms. These recent results are beginning to reconcile the pathology seen in human hippocampi ablated for intractable epilepsy with that observed in experimental animals, and offer the promise of even greater advances in the future. They suggest a mechanism for Gower's dictum that "seizures beget seizures" and highlight the importance of the interneurons of the dentate gyrus in epileptogenesis.

Properties of osmolyte fluxes activated during regulatory volume decrease in cultured cerebellar granule neurons

Efflux pathways for amino acids, K, and Cl activated during regulatory volume decrease (RVD) were characterized in cultured cerebellar granule neurons exposed to hyposmotic conditions. Results of this study favor diffusion pores (presumably channels) over energy-dependent transporters as the mechanisms responsible for the efflux of these osmolytes. The selectivity of osmolyte pathways activated by RVD was assessed by increasing the extracellular concentrations of cations, anions, and amino acids to such an extent that upon opening of the pathway, a permeable compound will enter the cell and block RVD by reducing the efflux of water carried by the exit of intracellular osmolytes. The cationic pathway was found selective for K (and Rb), whereas the anionic pathway was rather unselective being permeable to Cl, nitrate, iodine, benzoate, thiocyanate, and sulfate but impermeable to gluconate. Glutamate and aspartate as K but not as Na salts were permeable through the anion channel. RVD was slightly inhibited by quinidine but otherwise was insensitive to known K channel blockers. RVD was inhibited by 4,4'-diisothiocyanostilbene-2-2'-disulfonic acid (DIDS), niflumic acid, and dipyridamole. Gramicidin did not affect cell volume in isosmotic conditions but greatly accelerated RVD, suggesting that cell permeability to Cl is low in isosmotic conditions but increases markedly during RVD making K permeability the rate limit of the process. The permeability pathway for amino acids activated during RVD as permeable to short chain alpha- and beta-amino acids, but excluded glutamine and basic amino acids.

Regulatory volume decrease in cultured astrocytes. I. Potassium- and chloride-activated permeability

Regulatory volume decrease (RVD) in detached cerebellar astrocytes in culture after acute exposure to hyposmolarity was characterized in this and the accompanying paper [H. Pasantes-Morales, R. A. Murray, R. Sanches-Olea, and J. Moran. Am. J. Physiol. 266 (Cell Physiol. 35): C172-C178, 1994]. RVD was independent of extracellular calcium, was accelerated at pH 8-9 and retarded at pH 6, and was reduced at temperatures < 18 degrees C. The cationic pathway activated by hyposmolarity was specific for K+ and Rb+, since RVD was abolished and secondary swelling occurred when these ions replaced Na+. However, Li+, choline, tris(hydroxymethyl)aminomethane, and glucosamine, all as Cl- salts, did not affect RVD. The anion pathway was unselective, since RVD was inhibited when NaCl was replaced by anion K+ salts with a permeability rank of SCN- = I- > NO3- > Cl- > benzoate > acetate >> SO3- > gluconate. RVD was unaffected by bumetanide (50 microM) and weakly inhibited by furosemide (2 mM). Quinidine but not other K+ channel blockers inhibited RVD, and its effect was reversed by gramicidin. RVD was inhibited by 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid and dipyridamole but not by diphenylamine-2-carboxylate or anthracene-9-carboxylate. These results suggest that diffusion possibly via channels rather than cotransporters is involved in the swelling-activated K+ and Cl- fluxes. Gramicidin did not change astrocyte volume in isosmotic conditions, but greatly accelerated RVD, suggesting that low Cl- permeability in isosmotic conditions markedly increases by swelling, thus making K+ permeability the rate-limiting step for RVD.