Decreased G-protein-mediated regulation and shift in calcium channel types with age in hippocampal cultures

Variable bioenergetic sensitivity of neurons and astrocytes to insulin and extracellular glucose

Energy flow within cellular elements of the brain is a well-orchestrated, tightly regulated process, however, details underlying these functions at the single-cell level are still poorly understood. Studying hypometabolism in aging and neurodegenerative diseases may benefit from experimentation on unicellular bioenergetics. Here, we examined energy status in neurons and astrocytes using mixed hippocampal cultures and PercevalHR, an ATP:ADP nanosensor. We assessed exposures of several compounds including KCl, glutamate, FCCP, insulin, and glucose. A mitochondrial stress test was performed, and PercevalHR's fluorescence was corrected for pH using pHrodo. Results demonstrate that PercevalHR can reliably report on the energetic status of two cell types that communicate in a mixed-culture setting. While KCl, glutamate, and FCCP showed clear changes in PercevalHR fluorescence, insulin and glucose responses were found to be more subtle and sensitive to extracellular glucose. These results may highlight mechanisms that mediate insulin sensitivity in the brain.

Modulation of L-type calcium channels in Alzheimer's disease: A potential therapeutic target

Calcium plays a fundamental role in various signaling pathways and cellular processes in the human organism. In the nervous system, voltage-gated calcium channels such as L-type calcium channels (LTCCs) are critical elements in mediating neurotransmitter release, synaptic integration and plasticity. Dysfunction of LTCCs has been implicated in both aging and Alzheimer's Disease (AD), constituting a key component of calcium hypothesis of AD. As such, LTCCs are a promising drug target in AD. However, due to their structural and functional complexity, the mechanisms by which LTCCs contribute to AD are still unclear. In this review, we briefly summarize the structure, function, and modulation of LTCCs that are the backbone for understanding pathological processes involving LTCCs. We suggest targeting molecular pathways up-regulating LTCCs in AD may be a more promising approach, given the diverse physiological functions of LTCCs and the ineffectiveness of LTCC blockers in clinical studies.

Novel calcium-related targets of insulin in hippocampal neurons

Both insulin signaling disruption and Ca²⁺ dysregulation are closely related to memory loss during aging and increase the vulnerability to Alzheimer's disease (AD). In hippocampal neurons, aging-related changes in calcium regulatory pathways have been shown to lead to higher intracellular calcium levels and an increase in the Ca²⁺-dependent afterhyperpolarization (AHP), which is associated with cognitive decline. Recent studies suggest that insulin reduces the Ca²⁺-dependent AHP. Given the sensitivity of neurons to insulin and evidence that brain insulin signaling is reduced with age, insulin-mediated alterations in calcium homeostasis may underlie the beneficial actions of insulin in the brain. Indeed, increasing insulin signaling in the brain via intranasal delivery has yielded promising results such as improving memory in both clinical and animal studies. However, while several mechanisms have been proposed, few have focused on regulation on intracellular Ca²⁺. In the present study, we further examined the effects of acute insulin on calcium pathways in primary hippocampal neurons in culture. Using the whole-cell patch-clamp technique, we found that acute insulin delivery reduced voltage-gated calcium currents. Fura-2 imaging was used to also address acute insulin effects on spontaneous and depolarization-mediated Ca²⁺ transients. Results indicate that insulin reduced Ca²⁺ transients, which appears to have involved a reduction in ryanodine receptor function. Together, these results suggest insulin regulates pathways that control intracellular Ca²⁺ which may reduce the AHP and improve memory. This may be one mechanism contributing to improved memory recall in response to intranasal insulin therapy in the clinic.

A knowledge driven supervised learning approach to identify gene network of differentially up-regulated genes during neuronal senescence in Rattus norvegicus

Various approaches have been described to infer the gene interaction network from expression data. Several models based on computational and mathematical methods are available. The fundamental thing in the identification of the gene interaction is their biological relevance. Two genes belonging to the same pathway are more likely to affect the expression of each other than the genes of two different pathways. In the present study, interaction network of genes is described based on upregulated genes during neuronal senescence in the Cerebellar granule neurons of rat. We have adopted a supervised learning method and used it in combination with biological pathway information of the genes to develop a gene interaction network. Further modular analysis of the network has been done to identify senescence-related marker genes. Currently there is no adequate information available about the genes implicated in neuronal senescence. Thus identifying multipath genes belonging to the pathway affected by senescence might be very useful in studying the senescence process.

Analysis of gene expression during aging of CGNs in culture: implication of SLIT2 and NPY in senescence

Senescence is the major key factor that leads to the loss of neurons throughout aging. Cellular senescence is not the consequence of single cause, but there are multiple aspects which may induce senescence in a cell. Various causes such as gene expression, molecular interactions and protein processing and chromatin organization are described as causal factor for senescence. It is well known that the damage to the nuclear or mitochondrial DNA contributes to the aging either directly by inducing the apoptosis/cellular senescence or indirectly by altering cellular functions. The significant nuclear DNA damage with the age is directly associated with the continuous declining in DNA repair. The continuous decline in expression of topoisomerase 2 beta (Topo IIβ) in cultured cerebellar granule neurons over time indicated the decline in the repair of damage DNA. DNA Topo IIβ is an enzyme that is crucial for solving topological problems of DNA and thus has an important role in DNA repair. The enzyme is predominantly present in non-proliferating cells such as neurons. In this paper, we have studied the genes which were differentially expressed over time in cultured cerebellar granule neurons (CGNs) and identified potential genes associated with the senescence. Our results showed that the two genes neuropeptide Y (Npy) and Slit homolog 2 (Drosophila) (Slit2) gradually increase during aging, and upon suppression of these two genes, there was gradual increase in cell viability along with restoration of the expression of Topo IIβ and potential repair proteins.

Functional consequences of age-related morphologic changes to pyramidal neurons of the rhesus monkey prefrontal cortex

Layer 3 (L3) pyramidal neurons in the lateral prefrontal cortex (LPFC) of rhesus monkeys exhibit dendritic regression, spine loss and increased action potential (AP) firing rates during normal aging. The relationship between these structural and functional alterations, if any, is unknown. To address this issue, morphological and electrophysiological properties of L3 LPFC pyramidal neurons from young and aged rhesus monkeys were characterized using in vitro whole-cell patch-clamp recordings and high-resolution digital reconstruction of neurons. Consistent with our previous studies, aged neurons exhibited significantly reduced dendritic arbor length and spine density, as well as increased input resistance and firing rates. Computational models using the digital reconstructions with Hodgkin-Huxley and AMPA channels allowed us to assess relationships between demonstrated age-related changes and to predict physiological changes that have not yet been tested empirically. For example, the models predict that in both backpropagating APs and excitatory postsynaptic currents (EPSCs), attenuation is lower in aged versus young neurons. Importantly, when identical densities of passive parameters and voltage- and calcium-gated conductances were used in young and aged model neurons, neither input resistance nor firing rates differed between the two age groups. Tuning passive parameters for each model predicted significantly higher membrane resistance (R m ) in aged versus young neurons. This R m increase alone did not account for increased firing rates in aged models, but coupling these R m values with subtle differences in morphology and membrane capacitance did. The predicted differences in passive parameters (or parameters with similar effects) are mathematically plausible, but must be tested empirically.

Functional implications of a psychiatric risk variant within CACNA1C in induced human neurons

Psychiatric disorders have clear heritable risk. Several large-scale genome-wide association studies have revealed a strong association between susceptibility for psychiatric disorders, including bipolar disease, schizophrenia and major depression, and a haplotype located in an intronic region of the L-type voltage-gated calcium channel (VGCC) subunit gene CACNA1C (peak associated SNP rs1006737), making it one of the most replicable and consistent associations in psychiatric genetics. In the current study, we used induced human neurons to reveal a functional phenotype associated with this psychiatric risk variant. We generated induced human neurons, or iN cells, from more than 20 individuals harboring homozygous risk genotypes, heterozygous or homozygous non-risk genotypes at the rs1006737 locus. Using these iNs, we performed electrophysiology and quantitative PCR experiments that demonstrated increased L-type VGCC current density as well as increased mRNA expression of CACNA1C in iNs homozygous for the risk genotype, compared with non-risk genotypes. These studies demonstrate that the risk genotype at rs1006737 is associated with significant functional alterations in human iNs, and may direct future efforts at developing novel therapeutics for the treatment of psychiatric disease.

Ethanol alters calcium signaling in axonal growth cones

Calcium (Ca2+) channels are sensitive to ethanol and Ca2+ signaling is a critical regulator of axonal growth and guidance. Effects of acute and chronic exposure to ethanol (22, 43, or 87 mM) on voltage-gated Ca2+ channels (VGCCs) in whole cells, and KCl-induced Ca2+ transients in axonal growth cones, were examined using dissociated hippocampal cultures. Whole-cell patch-clamp analysis in neurons with newly-formed axons (Stage 3) revealed that rapidly inactivating, low-voltage activated (LVA) and non-inactivating, high-voltage activated (HVA) currents were both inhibited in a dose-dependent manner by acute ethanol, with relatively greater inhibition of HVA currents. When assessed by Fluo-4-AM imaging, baseline fluorescence and Ca2+ response to ethanol in Stage 3 neurons was similar compared to neurons without axons, but peak Ca2+ transient amplitudes in response to bath-applied KCl were greater in Stage 3 neurons and were decreased by acute ethanol. The amplitude of Ca2+ transients elicited specifically in axonal growth cones by focal application of KCl was also inhibited by acute exposure to moderate-to-high concentrations of ethanol (43 or 87 mM), whereas a lower concentration (22 mM) had no effect. When 43 or 87 mM ethanol was present continuously in the medium, KCl-evoked Ca2+ transient amplitudes were also reduced in growth cones. In contrast, Ca2+ transients were increased by continuous exposure to 22 mM ethanol. Visualization using a fluorescent dihydropyridine analog revealed that neurons continuously exposed to ethanol expressed increased amounts of L-type Ca2+ channels, with greater increases in axonal growth cones than cell bodies. Thus, acute ethanol reduces Ca2+ current and KCl-induced Ca2+ responses in whole cells and axonal growth cones, respectively, and chronic exposure is also generally inhibitory despite apparent up-regulation of L-type channel expression. These results are consistent with a role for altered growth cone Ca2+ signaling in abnormal neuromorphogenesis associated with fetal alcohol spectrum disorders.

Loss of Klotho during melanoma progression leads to increased filamin cleavage, increased Wnt5A expression, and enhanced melanoma cell motility

We have previously shown that Wnt5A-mediated signaling can promote melanoma metastasis. It has been shown that Wnt signaling is antagonized by the protein Klotho, which has been implicated in aging. We show here that in melanoma cells, expressions of Wnt5A and Klotho are inversely correlated. In the presence of recombinant Klotho (rKlotho), we show that Wnt5A internalization and signaling is decreased in high Wnt5A-expressing cells. Moreover, in the presence of rKlotho, we observe an increase in Wnt5A remaining in the medium, coincident with an increase in sialidase activity, and decrease in syndecan expression. These effects can be inhibited using a sialidase inhibitor. In addition to its effects on Wnt5A internalization, we also demonstrate that Klotho decreases melanoma cell invasive potential by a second mechanism that involves the inhibition of calpain and a resultant decrease in filamin cleavage, which we demonstrate is critical for melanoma cell motility.

Imaging of Ca2+ responses mediated by presynaptic L-type channels on GABAergic boutons of cultured hippocampal neurons

We have previously demonstrated that L-type Ca(2+) channels are involved in post-tetanic potentiation (PTP) of GABAergic IPSCs in cultured hippocampal neurons. Here we have used intracellular Fluo-3 to detect [Ca(2+)](i) in single GABAergic boutons in response to stimulation that evokes PTP. During control stimulation of the presynaptic GABAergic neuron at 40 Hz for 1-2 s, DeltaF/F(0) increased rapidly to a peak value and started to decline shortly after the train ended, returning to baseline within 10-20 s. The L-type channel blocker, isradipine (5 microM), had no significant effect on the amplitude or kinetics of the Ca(2+) signal. Following blockade of N- and P/Q-type Ca(2+)-channels, the amplitude was reduced by 52.9+/-3%. Isradipine caused a reduction of the remaining response (by 26.6+/-5%, P<0.01), that was fully reversible on washing. The L-type channel "agonist", BayK 8644 (8 microM), caused a significant enhancement of the peak (by 18.7%+/-7%, P<0.05). The rising phase of the Ca(2+) signal, which is related to the rate of entry of Ca(2+) into the bouton, was decreased by isradipine (by 25.5+/-6%, P<0.05) and enhanced by BayK 8644 (by 45.2%+/-16%, P<0.05). These Ca(2+) imaging experiments support the putative role of L-type channels in PTP of GABAergic synapses on cultured hippocampal neurons. We expect L-channels to be few in number, although they may couple strongly to intracellular signalling cascades that could amplify a signal that regulates synaptic vesicle turnover in the GABAergic boutons.

Increased vulnerability of hippocampal neurons with age in culture: temporal association with increases in NMDA receptor current, NR2A subunit expression and recruitment of L-type calcium channels

Excessive glutamate (Glu) stimulation of the NMDA-R is a widely recognized trigger for Ca(2+)-mediated excitotoxicity. Primary neurons typically show a large increase in vulnerability to excitotoxicity with increasing days in vitro (DIV). This enhanced vulnerability has been associated with increased expression of the NR2B subunit or increased NMDA-R current, but the detailed age-courses of these variables in primary hippocampal neurons have not been compared in the same study. Further, it is not clear whether the NMDA-R is the only source of excess Ca(2+). Here, we used primary hippocampal neurons to examine the age dependence of the increase in excitotoxic vulnerability with changes in NMDA-R current, and subunit expression. We also tested whether L-type voltage-gated Ca(2+) channels (L-VGCCs) contribute to the enhanced vulnerability. The EC(50) for Glu toxicity decreased by approximately 10-fold between 8-9 and 14-15 DIV, changing little thereafter. Parallel experiments found that during the same period both amplitude and duration of NMDA-R current increased dramatically; this was associated with an increase in protein expression of the NR1 and NR2A subunits, but not of the NR2B subunit. Compared to MK-801, ifenprodil, a selective NR2B antagonist, was less effective in protecting older than younger neurons from Glu insult. Conversely, nimodipine, an L-VGCC antagonist, protected older but not younger neurons. Our results indicate that enhanced excitotoxic vulnerability with age in culture was associated with a substantial increase in NMDA-R current, concomitant increases in NR2A and NR1 but not NR2B subunit expression, and with apparent recruitment of L-VGCCs into the excitotoxic process.

Overexpression of Na(+)/K (+)-ATPase parallels the increase in sodium transport and potassium recycling in an in vitro model of proximal tubule cellular ageing

Na(+)/K(+)-ATPase plays a key role in the transport of Na(+) throughout the nephron, but ageing appears to be accompanied by changes in the regulation and localization of the pump. In the present study, we examined the effect of in vitro cell ageing on the transport of Na(+) and K(+) ions in opossum kidney (OK) cells in culture. Cells were aged by repeated passing, and Na(+)/K(+)-ATPase activity and K(+) conductance were evaluated using electrophysiological methods. Na(+)K(+)-ATPase alpha(1)- and beta(1)-subunit expression was quantified by Western blot techniques. Na(+)/H(+) exchanger activity, changes in membrane potential, cell viability, hydrogen peroxide production and cellular proliferation were determined using fluorimetric assays. In vitro cell ageing is accompanied by an increase in transepithelial Na(+) transport, which results from an increase in the number of Na(+)/K(+)-ATPase alpha(1)- and beta(1)-subunits, in the membrane. Increases in Na(+)/K(+)-ATPase activity were accompanied by increases in K(+) conductance as a result of functional coupling between Na(+)/K(+)-ATPase and basolateral K(+) channels. Cell depolarization induced by both KCl and ouabain was more pronounced in aged cells. No changes in Na(+)/H(+) exchanger activity were observed. H(2)O(2) production was increased in aged cells, but exposure for 5 days to 1 and 10 microM: of H(2)O(2) had no effect on Na(+)/K(+)-ATPase expression. Ouabain (100 nM: ) increased alpha(1)-subunit, but not beta(1)-subunit, Na(+)/K(+)-ATPase expression in aged cells only. These cells constitute an interesting model for the study of renal epithelial cell ageing.

Calpains and human disease

Calpains, particularly conventional dimeric calpains, have claimed to be involved in the cell degeneration processes that characterize numerous disease conditions linked to dysfunctions of cellular Ca2+ homeostasis. The evidence supporting their involvement has traditionally been indirect and circumstantial, but recent work has added more solid evidence supporting the role of ubiquitous dimeric calpains in the process of neurodegeneration. The only disease condition in which a calpain defect has been conclusively involved concerns an atypical monomeric calpain: the muscle specific calpain-3, also known as p94. Inactivating defects in its gene cause a muscular dystrophy termed LGMD-2A. The molecular mechanism by which the absence of the proteolytic activity of calpain-3 causes the dystrophic process is unknown. Another atypical calpain, which has been characterized recently as a Ca2(+)-dependent protease, calpain 10, appears To be involved in the etiology of type 2 diabetes. The involvement has been inferred essentially from genetic evidence. Also in the case of type 2 diabetes the molecular mechanisms that could link the disease to calpain 10 are unknown.

Electrophysiological mechanisms of delayed excitotoxicity: positive feedback loop between NMDA receptor current and depolarization-mediated glutamate release

Delayed excitotoxic neuronal death after insult from exposure to high glutamate concentrations appears important in several CNS disorders. Although delayed excitotoxicity is known to depend on NMDA receptor (NMDAR) activity and Ca(2+) elevation, the electrophysiological mechanisms underlying postinsult persistence of NMDAR activation are not well understood. Membrane depolarization and nonspecific cationic current in the postinsult period were reported previously, but were not sensitive to NMDAR antagonists. Here, we analyzed mechanisms of the postinsult period using parallel current- and voltage-clamp recording and Ca(2+) imaging in primary hippocampal cultured neurons. We also compared more vulnerable older neurons [about 22 days in vitro (DIV)] to more resistant younger (about 15 DIV) neurons, to identify processes selectively associated with cell death in older neurons. During exposure to a modest glutamate insult (20 microM, 5 min), similar degrees of Ca(2+) elevation, membrane depolarization, action potential block, and increased inward current occurred in younger and older neurons. However, after glutamate withdrawal, these processes recovered rapidly in younger but not in older neurons. The latter also exhibited a concurrent postinsult increase in spontaneous miniature excitatory postsynaptic currents, reflecting glutamate release. Importantly, postinsult NMDAR antagonist administration reversed all of these persisting responses in older cells. Conversely, repolarization of the membrane by voltage clamp immediately after glutamate exposure reversed the NMDAR-dependent Ca(2+) elevation. Together, these data suggest that, in vulnerable neurons, excitotoxic insult induces a sustained positive feedback loop between NMDAR-dependent current and depolarization-mediated glutamate release, which persists after withdrawal of exogenous glutamate and drives Ca(2+) elevation and delayed excitotoxicity.

Normal brain ageing: models and mechanisms

Normal ageing is associated with a degree of decline in a number of cognitive functions. Apart from the issues raised by the current attempts to expand the lifespan, understanding the mechanisms and the detailed metabolic interactions involved in the process of normal neuronal ageing continues to be a challenge. One model, supported by a significant amount of experimental evidence, views the cellular ageing as a metabolic state characterized by an altered function of the metabolic triad: mitochondria-reactive oxygen species (ROS)-intracellular Ca2+. The perturbation in the relationship between the members of this metabolic triad generate a state of decreased homeostatic reserve, in which the aged neurons could maintain adequate function during normal activity, as demonstrated by the fact that normal ageing is not associated with widespread neuronal loss, but become increasingly vulnerable to the effects of excessive metabolic loads, usually associated with trauma, ischaemia or neurodegenerative processes. This review will concentrate on some of the evidence showing altered mitochondrial function with ageing and also discuss some of the functional consequences that would result from such events, such as alterations in mitochondrial Ca2+ homeostasis, ATP production and generation of ROS.

New Conotoxin SO-3 Targeting N-type Voltage-Sensitive Calcium Channels

Selective blockers of the N-type voltage-sensitive calcium (Ca_V) channels are useful in the management of severe chronic pain. Here, the structure and function characteristics of a novel N-type Ca_V channel blocker, SO-3, are reviewed. SO-3 is a 25- amino acid conopeptide originally derived from the venom of Conus striatus, and contains the same 4-loop, 6-cysteine framework (C-C-CC-C-C) as O-superfamily conotoxins. The synthetic SO-3 has high analgesic activity similar to ω-conotoxin MVIIA (MVIIA), a selective N-type Ca_V channel blocker approved in the USA and Europe for the alleviation of persistent pain states. In electrophysiological studies, SO-3 shows more selectivity towards the N-type Ca_V channels than MVIIA. The dissimilarity between SO-3 and MVIIA in the primary and tertiary structures is further discussed in an attempt to illustrate the difference in selectivity of SO-3 and MVIIA towards N-type Ca_V channels.

Dynamic regulation of glycinergic input to spinal dorsal horn neurones by muscarinic receptor subtypes in rats

Activation of spinal muscarinic acetylcholine receptors (mAChRs) inhibits nociception. However, the cellular mechanisms of this action are not fully known. In this study, we determined the role of mAChR subtypes in regulation of synaptic glycine release in the spinal cord. Whole-cell voltage-clamp recordings were performed on lamina II neurones in the rat spinal cord slices. The mAChR agonist oxotremorine-M significantly increased the frequency of glycinergic sIPSCs but not mIPSCs. Surprisingly, the effect of oxotremorine-M on sIPSCs was largely attenuated at a higher concentration. On the other hand, 1-10 microm oxotremorine-M dose-dependently increased the frequency of sIPSCs in rats pretreated with intrathecal pertussis toxin. Furthermore, oxotremorine-M also dose-dependently increased the frequency of sIPSCs in the presence of himbacine (an M2/M4 mAChR antagonist) or AF-DX116 (an M2 mAChR antagonist). The M3 mAChR antagonist 4-DAMP abolished the stimulatory effect of oxotremorine-M on sIPSCs. Interestingly, the GABA(B) receptor antagonist CGP55845 potentiated the stimulatory effect of oxotremorine-M on sIPSCs. In the presence of CGP55845, both himbacine and AF-DX116 similarly reduced the potentiating effect of oxotremorine-M on sIPSCs. Collectively, these data suggest that the M3 subtype is present on the somatodendritic site of glycinergic neurones and is mainly responsible for muscarinic potentiation of glycinergic input to spinal dorsal horn neurones. Concurrent stimulation of mAChRs on adjacent GABAergic interneurones attenuates synaptic glycine release through presynaptic GABA(B) receptors on glycinergic interneurones. This study illustrates a complex dynamic interaction between GABAergic and glycinergic synapses in the spinal cord dorsal horn.

SO-3, a new O-superfamily conopeptide derived from Conus striatus, selectively inhibits N-type calcium currents in cultured hippocampal neurons

Whole-cell currents in cultured hippocampal neurons were recorded to investigate the effects of SO-3, a new O-superfamily conopeptide derived from Conus striatus, on voltage-sensitive channels. SO-3 had no effect on voltage-sensitive sodium currents, delayed rectifier potassium currents, and transient outward potassium currents. Similar to the selective N-type calcium channel blocker omega-conotoxin MVIIA (MVIIA), SO-3 could concentration-dependently inhibit the high voltage-activated (HVA) calcium currents (I(Ca)). MVIIA(3 microM), 10 microM nimodipine, and 0.5 microM omega-agatoxin IVA (Aga) could selectively block the N-, L-, and P/Q-type I(Ca), which contributed approximately 32, approximately 38, and approximately 21% of the HVA currents in hippocampal neurons, respectively. About 31% of the total HVA currents were inhibited by 3 microM SO-3. SO-3 (3 microM) and 3 microM MVIIA inhibited the overlapping components of HVA currents, whereas no overlapping component was inhibited by 3 microM SO-3 and 10 microM nimodipine, or by 3 microM SO-3 and 0.5 microM Aga. Also, 3 microM SO-3 had no effect on R-type currents. SO-3 had less inhibitory effects on non-N-type HVA currents than MVIIA at higher concentrations (30 and 100 microM). The inhibitory effects of SO-3 and MVIIA on HVA currents were almost fully reversible. However, the recovery from block by MVIIA was more rapid than recovery from block by SO-3. It is concluded that SO-3 is a new omega-conotoxin selectively targeting N-type voltage-sensitive calcium channels. Considering the significance of N-type calcium channels for pain transduction, SO-3 may have therapeutic potential as a novel analgesic agent.

Modulation of voltage-gated Ca2+ current by 4-hydroxynonenal in dentate granule cells

Although recent studies have suggested that dentate granule cells play a key role in hippocampal functions, electrophysiological properties in these cells have not been sufficiently explored. In the present study, modification of voltage-gated Ca2+ channels by 4-hydroxynonenal (4HN), a major aldehydic product of membrane lipid peroxidation, in cultured dentate granule cells was examined using the whole-cell patch clamp technique. When whole-cell voltage clamp was applied, the cells exhibited a high-voltage-activated Ca2+ current, which was totally sensitive to 30 microM Cd2+ and partially sensitive to 2 microM nifedipine. 4HN enhanced the Ca2+ current in these cells. When L-type Ca2+ channels were blocked by application of nifedipine, the enhancement was completely canceled, whereas application of omega-conotoxin-GVIA or omega-agatoxin-IVA, blockers of N- and P/Q-type Ca2+ channels, respectively, had no effect. These results suggest that 4HN modulates L-type Ca2+ channels in the dentate granule cells, and thereby plays a role in the physiological and pathophysiological responses of these cells to oxidative stress.

Effects of age and spatial learning on adenylyl cyclase mRNA expression in the mouse hippocampus

Adenylyl cyclase (AC) subtypes have been implicated in memory processes and synaptic plasticity. In the present study, the effects of aging and learning on Ca2+/calmodulin-stimulable AC1, Ca2+-insensitive AC2 and Ca2+/calcineurin-inhibited AC9 mRNA level were compared in the dorsal hippocampus of young-adult and aged C57BL/6 mice using in situ hybridization. Both AC1 and AC9 mRNA expression were downregulated in aged hippocampus, whereas AC2 mRNA remained unchanged, suggesting differential sensitivities to the aging process. We next examined AC mRNA expression in the hippocampus after spatial learning in the Morris water maze. Acquisition of the spatial task was associated with an increase of AC1 and AC9 mRNA levels in both young-adult and aged groups, suggesting that Ca2+-sensitive ACs are oppositely regulated by aging and learning. However, aged-trained mice had reduced AC1 and AC9, but greater AC2, mRNA levels relative to young-trained mice and age-related learning impairments were correlated with reduced AC1 expression in area CA1. We suggest that reduced levels of hippocampal AC1 mRNA may greatly contribute to age-related defects in spatial memory.

Mitochondrial polarisation status and [Ca2+]i signalling in rat cerebellar granule neurones aged in vitro

Mitochondrial membrane potential is a major factor that controls, ultimately, the cellular energy supply. By use of a mitochondrial membrane potential dye (rhodamine 123, R123) and image analysis we show that during long-term (>3 weeks) culture of primary neurones (cerebellar granule neurones) there is a gradual and time-dependent depolarisation of neuronal mitochondria. This process was demonstrated by analysing the changes in the heterogeneity of the cytosolic rhodamine 123 fluorescent signal as a function of the age in culture and by measuring the amplitude of the rhodamine 123 fluorescence evoked by the addition of a mitochondrial protonophore (FCCP). The relationship between cytosolic [Ca(2+)](i) and mitochondrial membrane potential was assessed by recording both parameters simultaneously, in neurones loaded with fura-2 and rhodamine 123. Neuronal stimulation (KCl-evoked depolarisation) induced a mitochondrial depolarisation response resulting from the entry of cytosolic Ca(2+) into mitochondria. In young cultures (10 DIV), the mitochondrial membrane potential recovered fully within 30s from the start of the stimulation, despite the continuous presence of the depolarisation stimulus and the maintained cytosolic [Ca(2+)](i) signal. In contrast, in older neurones (DIV 22), the mitochondrial response was of smaller amplitude and displayed a much longer repolarization period. Also, in these older neurones, the threshold [Ca(2+)](i) level required for the initiation of the mitochondrial depolarisation response was increased by 50%. Thus, the present results indicate that neuronal maturation and ageing in conditions of long-term in vitro culture determine significant changes in the mitochondrial polarisation status that are manifest both in resting conditions and during stimulation and could explain some of the reported changes in neuronal homeostasis in long-term neuronal cultures.

In vitro development of P- and R-like calcium currents in insect (Periplaneta americana) embryonic brain neurons

Voltage-gated calcium currents are important for the survival and growth of embryonic cockroach brain neurons in primary culture. In the present experiments, we have studied, using the patch-clamp technique, the evolution with time in culture of the voltage-dependency and of the pharmacological properties of the calcium conductance of these neurons during the formation of a network. We have observed a progressive increase of the high-voltage-activated calcium conductance and a 10mV shift of the voltage-dependency of activation towards more negative potentials. The proportion of the R-like calcium current component increased during network formation. At the same time, the highly omega-AgaTxIVA-sensitive P-like component of the current is progressively replaced by a component which is less sensitive to the toxin. The origin and functional implications of these modifications are discussed.

Delay-dependent working memory impairment in young-adult and aged 5-HT1BKO mice as assessed in a radial-arm water maze

Serotonin (5-HT) plays a modulatory role in mnemonic functions, especially by interacting with the cholinergic system. The 5-HT1B receptor is a key target of this interaction. The 5-HT1B receptor knockout mice were found previously to exhibit a facilitation in hippocampal-dependent spatial reference memory learning. In the present study, we submitted mice to a delayed spatial working memory task, allowing the introduction of various delays between an exposure trial and a test trial. The 5-HT1BKO and wild-type mice learned the task in a radial-arm water maze (returning to the most recent presented arm containing the escape platform), and exhibited a high level of performance at delays of 0 and 5 min. However, at the delay of 60 min, only 5-HT1BKO mice exhibited an impairment. At a delay of 90 min, all mice were impaired. Treatment by scopolamine (0.8 mg/kg) induced the same pattern of performance in wild type as did the mutation for short (5 min, no impairment) and long (60 min, impairment) delays. The 22-month-old wild-type and knockout mice exhibited an impairment at short delays (5 and 15 min). The effect of the mutation affected both young-adult and aged mice at delays of 15, 30, and 60 min. Neurobiological data show that stimulation of the 5-HT1B receptor inhibits the release of acetylcholine in the hippocampus, but stimulates this in the frontal cortex. This dual function might, at least in part, explain the opposite effect of the mutation on reference memory (facilitation) and delay-dependent working memory (impairment). These results support the idea that cholinergic-serotonergic interactions play an important role in memory processes.

The calpains in aging and aging-related diseases

Calpains are a family of calcium-dependent cysteine proteases under complex cellular regulation. By making selective limited proteolytic cleavages, they modulate the activity of enzymes, including key signaling molecules, and induce specific cytoskeletal rearrangements, accounting for their roles in cell motility, signal transduction, vesicular trafficking and structural stabilization. Calpain activation has been implicated in various aging phenomena and diseases of late life, including cataract formation, erythrocyte senescence, diabetes mellitus type 2, hypertension, arthritis, and neurodegenerative disorders. The early and pervasive involvement of calpains in Alzheimer's disease potentially influences the development of beta-amyloid and tau disturbances and their consequences for neurodegeneration and neuronal cell loss.

Gene microarrays in hippocampal aging: statistical profiling identifies novel processes correlated with cognitive impairment

Gene expression microarrays provide a powerful new tool for studying complex processes such as brain aging. However, inferences from microarray data are often hindered by multiple comparisons, small sample sizes, and uncertain relationships to functional endpoints. Here we sought gene expression correlates of aging-dependent cognitive decline, using statistical profiling of gene microarrays in well powered groups of young, mid-aged, and aged rats (n = 10 per group). Animals were trained on two memory tasks, and the hippocampal CA1 region of each was analyzed on an individual microarray (one chip per animal). Aging- and cognition-related genes were identified by testing each gene by ANOVA (for aging effects) and then by Pearson's test (correlating expression with memory). Genes identified by this algorithm were associated with several phenomena known to be aging-dependent, including inflammation, oxidative stress, altered protein processing, and decreased mitochondrial function, but also with multiple processes not previously linked to functional brain aging. These novel processes included downregulated early response signaling, biosynthesis and activity-regulated synaptogenesis, and upregulated myelin turnover, cholesterol synthesis, lipid and monoamine metabolism, iron utilization, structural reorganization, and intracellular Ca2+ release pathways. Multiple transcriptional regulators and cytokines also were identified. Although most gene expression changes began by mid-life, cognition was not clearly impaired until late life. Collectively, these results suggest a new integrative model of brain aging in which genomic alterations in early adulthood initiate interacting cascades of decreased signaling and synaptic plasticity in neurons, extracellular changes, and increased myelin turnover-fueled inflammation in glia that cumulatively induce aging-related cognitive impairment.

Age-related changes in tolerance to the marine algal excitotoxin domoic acid

During an incident of toxic mussel poisoning, the epileptogenic excitotoxin domoic acid (DOM) was associated with lasting neurological deficits mainly in older patients (), suggesting supersensitivity to excitotoxins is a feature of brain aging. Here, hippocampal slices from young (3 months) and aged (26-29 months) Sprague Dawley rats were assessed by CA1 field potential analysis before and after preconditioning with DOM. In naïve slices from young animals, DOM produced initial hyperexcitability followed by significant dose-dependent reductions in population spike amplitude during prolonged application. Following toxin washout, only small changes in neuronal activity were evident during a second application of DOM, suggesting that a resistance to the effects of DOM occurs in hippocampal slices which have undergone prior exposure to DOM. This inducible tolerance was not antagonized by the NMDA receptor blockers APV or MK-801, nor was it diminished by the group I, II or III mGluR blockers AIDA, CPPG and EGLU. Likewise, neither the AMPA/KA blocker CNQX nor the VSCC blocker nifedipine were effective in blocking tolerance induction in young slices. Field potential analysis revealed significant age-related reductions in CA1 EPSP strength, population spike amplitude and paired-pulse inhibition, but aged slices did not differ in sensitivity to DOM relative to young. However, aged CA1 failed to exhibit any tolerance to DOM following preconditioning, suggesting that a loss of inducible neuroprotective mechanisms may account for increased sensitivity to excitotoxins during aging.

Age-dependent agonist-specific dysregulation of membrane-mediated signal transduction: emergence of the gate theory of aging

Although a general mechanism for the limited responsiveness of senescent cells has yet to be established, reduced responsiveness may in part be ascribed to deficits in the apparatus required for cell surface receptor-mediated signal transduction. Age-related changes of receptor-mediated signal transduction occur at many levels, and are known to include quantitative and qualitative changes in growth factor receptors, G-protein coupled receptors, and many other downstream signaling molecules. Here, we emphasize the prime role of the cellular surface in the perception and transmission of external stimuli in response to the aging process. As major means of cellular signal transduction, the receptor tyrosine kinase (RTK) system and the G protein-coupled receptor (GPCR) system of senescent cells were investigated. We observed that the RTK system was severely damaged, while the GPCR system was only partially inactivated by aging. These results suggest that the agonist-dependent dysregulation of and imbalance of signal transduction pathways might be responsible for the functional deterioration of senescent cells, and indicate a possibility of the functional recovery of senescent cells through agonist-specific signal system activation. Moreover, those data evoke the emerging concept that the senescent phenotype may be modulated by the membrance-associated signal system, implying the gate theory of aging.

Group I metabotropic glutamate receptor inhibition selectively blocks a prolonged Ca(2+) elevation associated with age-dependent excitotoxicity

It has been recognized for some years that a prolonged Ca(2+) elevation that is predictive of impending cell death develops in cultured neurons following excitotoxic insult. In addition, neurons exhibit enhanced sensitivity to excitotoxic insult with increasing age in culture. However, little is known about the processes that selectively regulate the post-insult Ca(2+) elevation and therefore, it remains unclear whether it is associated specifically with age-dependent toxicity.Here, we tested the hypothesis that a group I metabotropic glutamate receptor antagonist selectively modulates the prolonged Ca(2+) elevation in direct association with its protective effects against excitotoxicity. Rat hippocampal cultures of two ages (8-9 and 21-28 days in vitro) were exposed to a 5-min glutamate insult (400 microM in younger and 10 microM in older cultures) sufficient to kill >50% of the neurons, and were treated with vehicle or the specific group I metabotropic glutamate receptor antagonist 1-aminoindan-1,5-dicarboxylic acid (AIDA; 1 mM), throughout and following the insult. Neuronal survival was quantified 24 h after insult. In parallel studies, neurons of similar age in culture were imaged ratiometrically with a confocal microscope during and for 60 min after the glutamate insult. A large post-insult Ca(2+) elevation was present in older but not most younger neurons. The N-methyl-D-aspartate receptor antagonist, MK-801, blocked the Ca(2+) elevation both during and following the insult. In contrast, AIDA blocked only the post-insult prolonged Ca(2+) elevation in older neurons. Moreover, AIDA was neuroprotective in older but not younger cultures. From these results we suggest that the post-insult Ca(2+) elevation is regulated differently from the Ca(2+) elevation during glutamate insult and is modulated by group I metabotropic glutamate receptors. Further, the prolonged Ca(2+) elevation appears to be directly linked to an age-dependent component of vulnerability.

17beta-estradiol benzoate decreases the AHP amplitude in CA1 pyramidal neurons

Disruption of Ca(2+) homeostasis is hypothesized to mediate several electrophysiological markers of brain aging. Recent evidence indicates that estradiol can rapidly alter Ca(2+)-dependent processes in neurons through nongenomic mechanisms. In the current study, electrophysiological effects of 17beta-estradiol benzoate (EB) on the Ca(2+)-activated afterhyperpolarization (AHP) were investigated using intracellular sharp electrode recording in hippocampal slices from ovariectomized Fischer 344 female rats. The AHP amplitude was enhanced in aged (22-24 mo) compared with young (5-8 mo) rats and direct application of EB (100 pM) reduced the AHP in aged rats. The age-related difference was due, in part, to the increased AHP amplitude of aged animals, since an EB-mediated decrease in the AHP could be observed in young rats when the extracellular Ca(2+) was elevated to increase the AHP amplitude. In aged rats, bath application of EB occluded the ability of the L-channel blocker, nifedipine (10 microM), to attenuate the AHP. The results support a role for EB in modifying hippocampal Ca(2+)-dependent processes in a manner diametrically opposite that observed during aging, possibly through L-channel inhibition.

Sustained Ca2+-induced Ca2+-release underlies the post-glutamate lethal Ca2+ plateau in older cultured hippocampal neurons

Several studies have shown that a prolonged Ca(2+) elevation follows a glutamate-mediated excitotoxic insult in cultured neurons, and may be associated with impending cell death. Recently, we showed that the prolonged Ca(2+) elevation that emerges as neurons age in culture is specifically linked to an age-related increase in excitotoxic vulnerability. However, the multiple sources of Ca(2+) that contribute to Ca(2+) elevation during and after glutamate exposure are not well understood. Here, we examined the Ca(2+) sources of the age-related prolonged Ca(2+) elevation in cultured hippocampal neurons. Studies with caffeine showed that the ryanodine receptor-dependent releasable pool of Ca(2+) from intracellular stores was similar in older and younger neurons. Thapsigargin, which inhibits intracellular store refilling, did not mimic the age-related prolonged Ca(2+) elevation and, in fact, partially reduced it. Ryanodine, which blocks Ca(2+)-induced Ca(2+)-release (CICR) from stores, completely blocked the age-related prolonged Ca(2+) elevation following glutamate exposure but did not alter maximal Ca(2+) elevation during the glutamate exposure. Thus, we conclude that sustained CICR plays a selective and key role in generating the lethal, age-related, prolonged Ca(2+) elevation, and is the likely mechanism underlying age-related, enhanced vulnerability to excitotoxicity in neurons.

The lipid peroxidation product 4-hydroxynonenal facilitates opening of voltage-dependent Ca2+ channels in neurons by increasing protein tyrosine phosphorylation

Calcium influx through voltage-dependent calcium channels (VDCCs) mediates a variety of functions in neurons and other excitable cells, but excessive calcium influx through these channels can contribute to neuronal death in pathological settings. Oxyradical production and membrane lipid peroxidation occur in neurons in response to normal activity in neuronal circuits, whereas excessive lipid peroxidation is implicated in the pathogenesis of of neurodegenerative disorders. We now report on a specific mechanism whereby lipid peroxidation can modulate the activity of VDCCs. The lipid peroxidation product 4-hydroxy-2,3-nonenal (4HN) enhances dihydropyridine-sensitive whole-cell Ca2+ currents and increases depolarization-induced increases of intracellular Ca2+ levels in hippocampal neurons. Prolonged exposure to 4HN results in neuronal death, which is prevented by treatment with glutathione and attenuated by the L-type Ca2+ channel blocker nimodipine. Tyrosine phosphorylation of alpha1 VDCC subunits is increased in neurons exposed to 4HN, and studies using inhibitors of tyrosine kinases and phosphatases indicate a requirement for tyrosine phosphorylation in the enhancement of VDCC activity in response to 4HN. Phosphorylation-mediated modulation of Ca2+ channel activity in response to lipid peroxidation may play important roles in the responses of neurons to oxidative stress in both physiological and pathological settings.

Agonist-specific differential changes of cellular signal transduction pathways in senescent human diploid fibroblasts

Changes in the signal transduction efficiency of senescent cells led us to compare the signaling events induced by two mitogenic agonists, platelet-derived growth factor (PDGF) and lysophosphatidic acid (LPA) in presenescent and senescent or near-senescent human diploid fibroblasts. When the changes in intracellular [Ca(2+)](i) were analyzed, both PDGF and LPA generated a rhythmic increase in [Ca(2+)](i) in presenescent cells. The frequency of calcium response was reduced and desensitized in PDGF-stimulated senescent cells, while response to a LPA-induced calcium signal was also reduced in frequency, though its magnitude was unaltered. PDGF treatment increased the fibrous actin (F-actin) level in presenescent cells but not in senescent cells in contrast to a reduced but visible increase in F-actin in LPA-treated senescent cells. The effect of PDGF on phospholipase D (PLD) activation was also reduced significantly, as a ca. 60-80% reduction of PLD activity was observed in PDGF-stimulated cells but only a little reduction in LPA-induced cells. Agonist-specific differential changes of cellular signaling events caused a differential effect on DNA synthesis after growth factor stimulation. We observed a dramatic (80-90%) reduction of [3H]thymidine incorporation into DNA in the PDGF-stimulated near-senescent cells. LPA resulted in a 2-3-fold increase in thymidine incorporation even in the near-senescent cells. These differences in the responses of senescent or near-senescent cells to PDGF- and LPA-stimulation raised questions about the differential changes of the respective signaling apparatuses induced by aging. Since PDGF signaling event was affected greatly by aging, we further examined the protein contents involved in PDGF signal transduction pathway. PDGF receptor (PDGFR), protein kinase C-alpha (PKC-alpha), phospholipase C-gamma1 (PLC-gamma1), and PLD1 were examined by Western blot analysis. The protein levels of PKC-alpha and PLC-gamma1 were unchanged, but those of PLD1 and PDGFR were reduced with age. The reduced content of PDGFR protein may be one of the important contributors to the failure of PDGF-stimulated signal transduction in human senescent fibroblasts. Our results strongly suggest that age-dependent agonist-specific changes in signaling events might be in charge of the functional deterioration of senescent cells through imbalance of signal responses.

Depolarization-induced 65zinc influx into cultured cortical neurons

Toxic Zn(2+) influx may be a key mechanism underlying selective neuronal death after transient global ischemia in rats. To identify routes responsible for neuronal Zn(2+) influx, we measured the accumulation of (65)Zn(2+) into cultured murine cortical cells under depolarizing conditions (60 mM K(+)) associated with severe hypoxia-ischemia in brain tissue. Addition of 60 mM K(+) or 300 microM kainate substantially increased (65)Zn(2+) accumulation into mixed cultures of neurons and glia, but not glia alone. (65)Zn(2+) accumulation was attenuated by increasing concentrations of extracellular Ca(2+) or trypsin pretreatment, but not by late trypsinization, and corresponded to an increase in atomic Zn(2+). Confirming predominantly neuronal entry, K(+)-induced (65)Zn(2+) accumulation was reduced by prior selective destruction of neurons with NMDA. K(+)-induced (65)Zn(2+) influx was not sensitive to glutamate receptor antagonists, but was attenuated by Gd(3+) and Cd(2+) as well as 1 microM nimodipine; it was partially sensitive to 1 microM omega-conotoxin-GVIA, and insensitive to 1 microM omega-agatoxin-IVA. K(+)-induced, Gd(3+)-sensitive (45)Ca(2+) accumulation but not (65)Zn(2+) accumulation was sharply attenuated by lowering extracellular pH to 6.6.

Calcineurin enhances L-type Ca(2+) channel activity in hippocampal neurons: increased effect with age in culture

The Ca(2+)/calmodulin-dependent protein phosphatase, calcineurin, modulates a number of key Ca(2+) signaling pathways in neurons, and has been implicated in Ca(2+)-dependent negative feedback inactivation of N-methyl-D-aspartate receptors and voltage-sensitive Ca(2+) channels. In contrast, we report here that three mechanistically disparate calcineurin inhibitors, FK-506, cyclosporin A, and the calcineurin autoinhibitory peptide, inhibited high-voltage-activated Ca(2+) channel currents by up to 40% in cultured hippocampal neurons, suggesting that calcineurin acts to enhance Ca(2+) currents. This effect occurred with Ba(2+) or Ca(2+) as charge carrier, and with or without intracellular Ca(2+) buffered by EGTA. Ca(2+)-dependent inactivation of Ca(2+) channels was not affected by FK-506. The immunosuppressant, rapamycin, and the protein phosphatase 1/2A inhibitor, okadaic acid, did not decrease Ca(2+) channel current, showing specificity for effects on calcineurin. Blockade of L-type Ca(2+) channels with nimodipine fully negated the effect of FK-506 on Ca(2+) channel current, while blockade of N-, and P-/Q-type Ca(2+) channels enhanced FK-506-mediated inhibition of the remaining L-type-enriched current. FK-506 also inhibited substantially more Ca(2+) channel current in 4-week-old vs. 2-week-old cultures, an effect paralleled by an increase in calcineurin A mRNA levels. These studies provide the first evidence that calcineurin selectively enhances L-type Ca(2+) channel activity in neurons. Moreover, this action appears to be increased concomitantly with the well-characterized increase in L-type Ca(2+) channel availability in hippocampal neurons with age-in-culture.

The chemokine interleukin-8 acutely reduces Ca(2+) currents in identified cholinergic septal neurons expressing CXCR1 and CXCR2 receptor mRNAs

The chemokine IL-8 is known to be synthesized by glial cells in the brain. It has traditionally been shown to have an important role in neuroinflammation but recent evidence indicates that it may also be involved in rapid signaling in neurons. We investigated how IL-8 participates in rapid neuronal signaling by using a combination of whole-cell recording and single-cell RT-PCR on dissociated rat septal neurons. We show that IL-8 can acutely reduce Ca(2+) currents in septal neurons, an effect that was concentration-dependent, involved the closure of L- and N-type Ca(2+) channels, and the activation of G(ialpha1) and/or G(ialpha2) subtype(s) of G-proteins. Analysis of the mRNAs from the recorded neurons revealed that the latter were all cholinergic in nature. Moreover, we found that all cholinergic neurons that responded to IL-8, expressed mRNAs for either one or both IL-8 receptors CXCR1 and CXCR2. This is the first report of a chemokine that modulates ion channels in neurons via G-proteins, and the first demonstration that mRNAs for CXCR1 are expressed in the brain. Our results suggest that IL-8 release by glial cells in vivo may activate CXCR1 and CXCR2 receptors on cholinergic septal neurons and acutely modulate their excitability by closing calcium channels.

Epilepsy-induced decrease of L-type Ca2+ channel activity and coordinate regulation of subunit mRNA in single neurons of rat hippocampal 'zipper' slices

L-type voltage-sensitive Ca2+ channels (VSCCs) preferentially modulate several neuronal processes that are thought to be important in epileptogenesis, including the slow afterhyperpolarization (AHP), LTP, and trophic factor gene expression. However, little is yet known about the roles of L-type VSCCs in the epileptogenic process. Here, we used cell-attached patch recording techniques and single cell mRNA analyses to study L-type VSCCs in CA1 neurons from partially dissociated (zipper) hippocampal slices from entorhinally-kindled rats. L-type Ca2+-channel activity was reduced by >50% at 1.5-3 months after kindling. Following recording, the same single neurons were extracted and collected for mRNA analysis using a recently developed method that does not amputate major dendritic processes. Therefore, neurons contained essentially full complements of mRNA. For each collected neuron, mRNA contents for the L-type pore-forming alpha1D/Ca(v)1.3-subunit and for calmodulin were then analyzed by semiquantitative kinetic RT-PCR. L-type alpha1D-subunit mRNA was correlated with L-type Ca2+-channel activity across single cells, whereas calmodulin mRNA was not. Thus, these results appear to provide the first direct evidence at the single channel and gene expression levels that chronic expression of an identified Ca2+-channel type is modulated by epileptiform activity. Moreover, the present data suggest the hypothesis that down regulation of alpha1D-gene expression by kindling may contribute to the long-term maintenance of epileptiform activity, possibly through reduced Ca2+-dependent AHP and/or altered expression of other relevant genes.

Vitamin D hormone confers neuroprotection in parallel with downregulation of L-type calcium channel expression in hippocampal neurons

Although vitamin D hormone (VDH; 1,25-dihydroxyvitamin D(3)), the active metabolite of vitamin D, is the major Ca(2+)-regulatory steroid hormone in the periphery, it is not known whether it also modulates Ca(2+) homeostasis in brain neurons. Recently, chronic treatment with VDH was reported to protect brain neurons in both aging and animal models of stroke. However, it is unclear whether those actions were attributable to direct effects on brain cells or indirect effects mediated via peripheral pathways. VDH modulates L-type voltage-sensitive Ca(2+) channels (L-VSCCs) in peripheral tissues, and an increase in L-VSCCs appears linked to both brain aging and neuronal vulnerability. Therefore, we tested the hypothesis that VDH has direct neuroprotective actions and, in parallel, targets L-VSCCs in hippocampal neurons. Primary rat hippocampal cultures, treated for several days with VDH, exhibited a U-shaped concentration-response curve for neuroprotection against excitotoxic insults: lower concentrations of VDH (1-100 nm) were protective, but higher, nonphysiological concentrations (500-1000 nm) were not. Parallel studies using patch-clamp techniques found a similar U-shaped curve in which L-VSCC current was reduced at lower VDH concentrations and increased at higher (500 nm) concentrations. Real-time PCR studies demonstrated that VDH monotonically downregulated mRNA expression for the alpha(1C) and alpha(1D) pore-forming subunits of L-VSCCs. However, 500 nm VDH also nonspecifically reduced a range of other mRNA species. Thus, these studies provide the first evidence of (1) direct neuroprotective actions of VDH at relatively low concentrations, and (2) selective downregulation of L-VSCC expression in brain neurons at the same, lower concentrations.

Neuronal ageing in long-term cultures: alterations of Ca2+ homeostasis

Deficiencies of Ca2+ homeostasis are proposed to play an important role in neuronal ageing and/or neurodegeneration. The aim of this study was to investigate, in a defined neuronal population, primary cerebellar granule neuron culture, the time-dependent changes in Ca2+ homeostasis and compare them with data obtained in cerebellar brain slices from aged rats. In neurons aged in culture (DIV 23), a small decrease in the resting [Ca2+]i was associated with a decrease in the maximal rate of [Ca2+]i increase upon KCl-induced depolarization and in the amplitude of the [Ca2+]i response, when compared with mature neurons (DIV 9). The most significant change of [Ca2+]i signal parameters was a 50% decrease in the rate of [Ca2+]i recovery after the stimulation. These results were similar to those obtained in aged brain slices, and indicate that primary neuronal cultures could serve as a model for studying the age-related changes in Ca2+ homeostasis.

Expression of alpha 1D subunit mRNA is correlated with L-type Ca2+ channel activity in single neurons of hippocampal "zipper" slices

L-type voltage-sensitive Ca(2+) channels (L-VSCCs) play an important role in developmental and aging processes, as well as during normal function of brain neurons. Here, we tested a prediction of the hypothesis that membrane density of functional L-VSCCs is regulated by the level of gene expression for its alpha(1D) pore-forming subunit. If so, alpha(1D) mRNA and L-VSCC activity should be positively correlated within individual neurons. Conventional methods of aspiration and/or acute cell dissociation used in prior single-cell studies have generally yielded variable and incomplete recovery of intracellular mRNA. Thus, quantitative relationships between channel function and expression have been difficult to define. In this study, we used the partially dissociated ("zipper") hippocampal slice preparation as a method for collecting a single neuron's mRNA complement. This preparation, developed to expose neuronal somata for recording, also enables the extraction of a neuron with major processes largely intact. Thus, single-cell measures of gene/mRNA expression can be based on approximately the cell's full set of mRNA transcripts. In adult and aged rat hippocampal zipper slices, L-VSCC activity was first recorded in CA1 neurons in cell-attached patch mode. The same neurons were then extracted and collected for semiquantitative reverse transcriptase-PCR analysis of alpha(1D) and calmodulin A (CaM) mRNA content. Across multiple single neurons, a significant, positive correlation was found between the rank orders of L-VSCC activity and of alpha(1D), but not CaM, mRNA expression. Thus, these studies support the possibility that the level of alpha(1D) gene expression regulates the density of functional L-VSCCs.