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

Microglia in Glia-Neuron Co-cultures Exhibit Robust Phagocytic Activity Without Concomitant Inflammation or Cytotoxicity

A simple method to co-culture granule neurons and glia from a single brain region is described, and microglia activation profiles are assessed in response to naturally occurring neuronal apoptosis, excitotoxin-induced neuronal death, and lipopolysaccharide (LPS) addition. Using neonatal rat cerebellar cortex as a tissue source, glial proliferation is regulated by omission or addition of the mitotic inhibitor cytosine arabinoside (AraC). After 7-8 days in vitro, microglia in AraC(-) cultures are abundant and activated based on their amoeboid morphology, expressions of ED1 and Iba1, and ability to phagocytose polystyrene beads and the majority of neurons undergoing spontaneous apoptosis. Microglia and phagocytic activities are sparse in AraC(+) cultures. Following exposure to excitotoxic kainate concentrations, microglia in AraC(-) cultures phagocytose most dead neurons within 24 h without exacerbating neuronal loss or mounting a strong or sustained inflammatory response. LPS addition induces a robust inflammatory response, based on microglial expressions of TNF-α, COX-2 and iNOS proteins, and mRNAs, whereas these markers are essentially undetectable in control cultures. Thus, the functional effector state of microglia is primed for phagocytosis but not inflammation or cytotoxicity even after kainate exposure that triggers death in the majority of neurons. This model should prove useful in studying the progressive activation states of microglia and factors that promote their conversion to inflammatory and cytotoxic phenotypes.

Amiloride but not memantine reduces neurodegeneration, seizures and myoclonic jerks in rats with cardiac arrest-induced global cerebral hypoxia and reperfusion

It has been reported that both activation of N-methyl-D-aspartate receptors and acid-sensing ion channels during cerebral ischemic insult contributed to brain injury. But which of these two molecular targets plays a more pivotal role in hypoxia-induced brain injury during ischemia is not known. In this study, the neuroprotective effects of an acid-sensing cation channel blocker and an N-methyl-D-aspartate receptor blocker were evaluated in a rat model of cardiac arrest-induced cerebral hypoxia. We found that intracisternal injection of amiloride, an acid-sensing ion channel blocker, dose-dependently reduced cerebral hypoxia-induced neurodegeneration, seizures, and audiogenic myoclonic jerks. In contrast, intracisternal injection of memantine, a selective uncompetitive N-methyl-D-aspartate receptor blocker, had no significant effect on cerebral hypoxia-induced neurodegeneration, seizure and audiogenic myoclonic jerks. Intracisternal injection of zoniporide, a specific sodium-hydrogen exchanger inhibitor, before cardiac arrest-induced cerebral hypoxia, also did not reduce cerebral hypoxia-induced neurodegeneration, seizures and myoclonic jerks. These results suggest that acid-sensing ion channels play a more pivotal role than N-methyl-D-aspartate receptors in mediating cerebral hypoxia-induced brain injury during ischemic insult.

The ketogenic diet

BACKGROUND

The ketogenic diet is a low-carbohydrate, adequate-protein, and high-fat diet with a long history of use for the treatment of intractable seizures in children. This dietary therapy has been enjoying increasing popularity in recent years, despite the availability of increasing numbers of new antiepileptic drugs and surgical treatments.

REVIEW SUMMARY

The authors review the history of the ketogenic diet, the traditional protocol in initiating it, possible mechanisms of its action, evidence for efficacy, and side effects. In addition, they highlight some of the areas of active research in this field as well as future directions and unanswered questions.

CONCLUSION

The ketogenic diet is an efficacious and relatively safe treatment of intractable seizures. Despite its long history, however, much remains unknown about the diet, including its mechanisms of action, the optimal protocol, and the full range of its applicability. Investigations of the diet are providing new insight into the mechanisms behind seizures and epilepsy itself, as well as possible new therapies.

Roscovitine, olomoucine, purvalanol: inducers of apoptosis in maturing cerebellar granule neurons

Cyclin-dependent kinases (CDKs) mediate proliferation and neuronal development, while aberrant CDK activity is associated with cancer and neurodegeneration. Consequently, pharmacologic inhibitors, such as 2,6,9-trisubstituted purines, which potently inhibit CDKs 1, 2, and 5, were developed to combat these pathologies. One agent, R-roscovitine (CYC202), has advanced to clinical trials as a potential cancer therapy. In primary neuronal cultures, these agents have been used to delineate the physiologic and pathologic functions of CDKs, and associated signaling pathways. Herein we demonstrate that three 2,6,9-trisubstituted purines: olomoucine, roscovitine, and purvalanol, used at concentrations ascribed by others to potently inhibit CDKs 1, 2, and 5, are powerful triggers of death in maturing cerebellar granule neurons, assessed by loss of mitochondrial reductive capacity and differential staining with fluorescent indicators of living/dead neurons. Based on several criteria, including delayed time course and establishment of an irreversible commitment point of death, pyknotic cell and nuclear morphology, and caspase-3 cleavage, the death process is apoptotic. However, pharmacological and biochemical data indicate that apoptosis is independent of CDK 1, 2, or 5 inhibition. This is based on the pattern of changes in c-jun mRNA, c-Jun protein, and Ca(2+)/cAMP response element binding protein (CREB) phosphorylation, and also, the ineffectiveness of structurally distinct CDK 1, 2, and 5 inhibitors butyrolactone-1 and PNU112445A to induce apoptosis. Collectively, our results, and those of others, indicate that the CDK regulation of transcription (CDKs 7 and 9) should be examined as a target of these agents, and as an indirect mediator of neuronal fate.

Granule neurons in cerebellum express distinct splice variants of the inositol trisphosphate receptor that are modulated by calcium

Primary cultures of granule cells (GC) from rat cerebellar cortex were used to determine whether bioelectric activity, via a Ca(2+)/calmodulin-dependent kinase (CaMK) signaling cascade, modulates expression and exon selection in the inositol trisphosphate receptor type 1 (IP(3)R1). IP(3)R1 contains or lacks three exons (S1, S2, and S3) that are regulated in a regionally and temporally specific manner. The neuronal, or long, form of IP(3)R1 is distinguished from peripheral tissues by inclusion of the S2 exon. Although previous studies indicated that IP(3)R1 are undetectable in the cerebellar granular layer in vivo, receptor protein and mRNA are induced in cultured GC grown in medium supplemented with 25 mM KCl or NMDA, two trophic agents that promote long-term survival, compared with GC grown in 5 mM KCl. IP(3)R1 induction in response to 25 mM KCl or NMDA is attenuated by coaddition of voltage-sensitive calcium channel or NMDA receptor antagonists, respectively. Actinomycin D, CaMK, and calcineurin antagonists likewise suppress induction. Unlike the major variants of IP(3)R1 in Purkinje neurons, which lack S1 and S3, GC grown with trophic agents express mRNA containing these exons. Both neuronal types contain S2. Evidence obtained using mutant mice with Purkinje cell lesions, laser-microdissected GC neurons from slices, and explant cultures indicates that GC predominantly express the S1-containing variant of IP(3)R1 in vivo.

Trophic agents that prevent neuronal apoptosis activate calpain and down-regulate CaMKIV

CaMKIV is enriched in neuronal nuclei and mediates Ca2+-dependent survival via transcription factor phosphorylation. Cultured cerebellar granule neurons were used to examine whether distinct modes of Ca2+ signaling differentially modulate CaMKIV expression and function. For long-term survival, these neurons require 25 mm KCl or NMDA, which stimulates Ca2+ entry through voltage-sensitive Ca2+ channels or NMDA receptors (NRs). Lower levels of Ca2+ entry through NRs support survival of a neuronal subpopulation grown in 5 mm KCl media. Several effects were demonstrated: (i) sustained exposure to 25 mM KCl or 140 microM NMDA produced CaMKIV down-regulation, compared to 5 mM KCl cultures; (ii) CaMKIV down-regulation was attenuated by nifedipine, APV and CaM kinase inhibitors, indicating that it is Ca2+ dependent and reversible; (iii) down-regulation was both selective for nuclear substrates and calpain-mediated; (iv) proteolysis was exacerbated by leptomycin B, a nuclear export inhibitor. Although CaMKIV proteolysis by trophic agents seems paradoxical in light of evidence supporting its critical role in survival, the CaMKIV/CREB signal transduction pathway was preserved, as assessed by CaM kinase-mediated CREB phosphorylation, and the ability of CaM kinase inhibitors to interfere with KCl-mediated survival. We hypothesize that limited calpain-mediated proteolysis of CaMKIV is a negative feedback response to the sustained activation of a Ca2+ and CaMKIV signaling pathway by these agents.

Ca2+ and pH modulate alternative splicing of exon 5 in NMDA receptor subunit 1

RT-PCR and intracellular Ca2+ measurements were used to identify factors that modulate alternative splicing of exon 5 in the NMDA receptor transcript encoding NR1, in cultured cerebellar granule neurons. Although cells grown in media containing 5 mM KCl demonstrate compromised survival, they show the predicted developmental transition from NR1a (-exon 5) to NR1b (+exon 5) mRNA expression. This transition was blocked under culture conditions that promote survival; inclusion or exclusion of exon 5 is a reversible process that is sensitive to alterations in Ca2+ and pH. We conclude that alternative splicing of NR1 pre-mRNA transcripts may be regulated by developmental cues that modulate the degree of glutamate receptor activation.

Acetazolamide-induced increase in blood flow to rabbit organs is confirmed using colored microspheres

Inhibitors of carbonic anhydrase activity have been found to increase blood and organ PCO2 and to increase blood flow (BF) in individual organs. To determine whether carbonic anhydrase inhibition coordinately induces an increase in BF in several organs, we assayed the effect of the carbonic anhydrase inhibitor, acetazolamide (AZ), on BF in rabbit organs using the colored microsphere (CM) assay. Eight female white rabbits were anesthetized with ketamine and urethane, and administered three sequential doses of 4 mg/kg AZ. After each dose, the rabbits were injected with 9 x 10(5) CMs of different colors, and arterial blood was collected. We found that AZ had no effect on blood pressure, body temperature, hemoglobin concentration, or PaCO2. In contrast, 12 mg/kg AZ significantly increased PaO2 and significantly decreased base excess. When we measured organ BF, we observed, in response to 12 mg/kg AZ, an 82% increase in brain BF and a 55% increase in kidney BF, but no change in BF of the liver, stomach wall, or abdominal muscle. These findings suggest that the inhibition of carbonic anhydrase activity by AZ, which decreases the rate of CO2 conversion to HCO3-, causes the retention of CO2 in tissues and organs, and thus increases BF in specific organs. Administration of carbonic anhydrase inhibitors, such as AZ, may increase BF to the brain and kidney without reducing PaO2, thereby increasing the supply of oxygen in conditions involving hypoxia such as ischemia and shock.

Discrepant intracellular pH changes following intracellular Ca2+ increases induced by glutamate and Ca2+ ionophores in rat hippocampal neurons

We investigated changes in intracellular pH (pHi) in relation to intracellular Ca2+ concentration ([Ca2+]i) in primary cultured hippocampal neurons treated with glutamate. [Ca2+]i and pHi were imaged with fluorescent dyes and confocal microscopy. Exposure to 1 mM glutamate for 10 min increased [Ca2+]i and evoked acidosis. These changes persisted for at least 60 min, even after removal of glutamate. The increase in [Ca2+]i and the acidosis were not observed in Ca2+-free solution and were attenuated in the presence of MK-801, an NMDA receptor antagonist. We also found that the increase in [Ca2+]i and acidosis could be induced by addition of Ca2+ to the extracellular solution in the cells pretreated with glutamate in Ca2+-free solution, even if glutamate did not exist in the extracellular solution. On the other hand, ionomycin and Br-A23187, calcium ionophores, increased [Ca2+]i to almost the same level as glutamate and increased pHi. Extracellular Ca2+ was also indispensable for the increase in [Ca2+]i and the alkalosis. These results suggest the followings: 1) intracellular acidosis by glutamate is dependent on the presence of extracellular Ca2+; 2) the acidosis does not result from only the increase in [Ca2+]i; and 3) glutamate induces the irreversible disorder of regulatory mechanisms of [Ca2+]i not only by Ca2+-dependent process, but also by Ca2+-independent process.

NMDA receptor modulation by a conditioned medium derived from rat cerebellar granule cells

Our previous studies have shown that the response to the excitotoxic action of glutamate by cultured cerebellar granule cells depends upon the cell density or the volume of medium in which they have been grown: the higher the cell density or the lower the volume, the higher the response to glutamate. We have hypothesized that this variable response is due to the formation in culture of a glutamate-sensitizing activity GSA more abundantly in conditioned medium derived from high-density or low-volume cultures than that present in low-density or high volume cultures and capable of restoring sensitivity in previously resistant granule cells. In order to elucidate the mechanism of action of glutamate-sensitizing activity, we measured the extent and function of NMDA receptors in low- and high-volume cultures and assessed the effect of glutamate-sensitizing activity on the same receptors. We found that under high-volume conditions the extent of MK-801 binding, the amount of NMDA receptor type 1, the currents evoked in whole cells after an NMDA pulse and the response of cultured cells to this ligand were markedly reduced compared with low-volume cultures. Addition of glutamate-sensitizing activity to high-volume cultures increased their glutamate sensitivity, the NMDA-evoked currents, the extent of MK-801 binding and the amount of NMDA receptor type 1 protein present. The corresponding mRNA transcripts, on the contrary, were unchanged in high-volume, low-volume and high-volume GSA-treated cultures.

Neurodegeneration mediated by glutamate and beta-amyloid peptide: a comparison and possible interaction

In Alzheimer's disease, abnormal extracellular accumulations of beta-amyloid (a major component of the senile plaques) and of the excitatory amino acid glutamate are both thought to be associated with degeneration of nerve cells. In the present study, using cultured cortical or hippocampal neurons as an in vitro model, we compared the effects of various factors influencing neurodegeneration mediated by glutamate or by beta-amyloid peptide (A beta). We also asked the question: does long-term treatment with sublethal doses of A beta-(25-35) potentiate glutamate-mediated excitotoxicity? Neuronal cell death was quantified using the lactate dehydrogenase (LDH) method. Since extracellular LDH remains stable for days, the magnitude of relative afflux of LDH correlates in a linear fashion with the number of damaged neurons in cultures. When applied singly, both glutamate (for 15 min) and A beta-(25-35) or its parent peptide A beta-(1-40) (continuously) produced a dose-dependent neuronal degeneration. In the case of glutamate, the half-maximal effects were observed at about 0.08 mM glutamate for both cerebral cortical and hippocampal neurons (cultured for 13 days in vitro, DIV). The effect of A beta-(25-35) was also time-dependent, while neurons grown in a chemically defined medium showed relatively greater susceptibility to A beta-(25-35) than those cultured in a serum-containing medium. These differential effects were not related to the presence of different numbers of glial cells in the cultures. Treatment with different doses of the antimitotic inhibitor, cytosine arabinoside, for 24 h (6-7 DIV) produced at 13 DIV cortical neuronal cultures with varying numbers of astrocytes, as determined by the astrocyte-specific enzyme glutamine synthetase. The presence of astrocytes decreased the toxicity of glutamate for neurons. The modulation was due to uptake of glutamate by astrocytes, thereby reducing its effective concentration, as the effect was seen at 0.1 mM and not at 10 mM glutamate. Incorporation of an NMDA receptor mediated Ca2+ ion channel blocker, MK-801, together with glutamate completely inhibited degeneration of cortical neurons, and pretreatment of cultures with basic fibroblast growth factor for 2 days did so partially. However, these compounds had no effect on neurotoxicity mediated by A beta-(25-35). Lastly, the effect of glutamate interacted with that of A beta-(25-35). Pretreatment of cortical neurons for 2 days with 10 microM A beta-(25-35) by itself had no appreciable effect, but it potentiated significantly the degeneration of these neurons mediated by glutamate.(ABSTRACT TRUNCATED AT 400 WORDS)