Expression of herpes simplex virus type 2 protein ICP10 PK rescues neurons from apoptosis due to serum deprivation or genetic defects

Previous studies have shown that the herpes simplex virus type 2 protein kinase ICP10 PK activates the Ras/MEK/MAPK pathway in nonneuronal cells. Here we report that ectopically expressed ICP10 PK has anti-apoptotic activity in various paradigms of neuronal cell death. Neuronally differentiated PC12 cells and primary murine hippocampal cultures transfected with an expression vector for ICP10 PK were protected from cell death resulting from growth factor withdrawal. Protection from apoptosis was also seen in ICP10 PK-transfected hippocampal neurons from the trisomy 16 mouse, a naturally occurring genetic abnormality the human analog of which is Down syndrome. Cells transfected with an expression vector for a mutant that lacks kinase activity were not protected, although it was expressed as well as ICP10 PK. The data indicate that ICP10 PK has a broad anti-apoptotic activity in neuronal cells which depends on a functional PK.

Folic acid deficiency and homocysteine impair DNA repair in hippocampal neurons and sensitize them to amyloid toxicity in experimental models of Alzheimer's disease

Recent epidemiological and clinical data suggest that persons with low folic acid levels and elevated homocysteine levels are at increased risk of Alzheimer's disease (AD), but the underlying mechanism is unknown. We tested the hypothesis that impaired one-carbon metabolism resulting from folic acid deficiency and high homocysteine levels promotes accumulation of DNA damage and sensitizes neurons to amyloid beta-peptide (Abeta) toxicity. Incubation of hippocampal cultures in folic acid-deficient medium or in the presence of methotrexate (an inhibitor of folic acid metabolism) or homocysteine induced cell death and rendered neurons vulnerable to death induced by Abeta. Methyl donor deficiency caused uracil misincorporation and DNA damage and greatly potentiated Abeta toxicity as the result of reduced repair of Abeta-induced oxidative modification of DNA bases. When maintained on a folic acid-deficient diet, amyloid precursor protein (APP) mutant transgenic mice, but not wild-type mice, exhibited increased cellular DNA damage and hippocampal neurodegeneration. Levels of Abeta were unchanged in the brains of folate-deficient APP mutant mice. Our data suggest that folic acid deficiency and homocysteine impair DNA repair in neurons, which sensitizes them to oxidative damage induced by Abeta.

E-p-methoxycinnamic acid protects cultured neuronal cells against neurotoxicity induced by glutamate

1. We previously reported that four new phenylpropanoid glycosides and six known cinnamate derivatives isolated from roots of Scrophularia buergeriana Miquel (Scrophulariaceae) protected cultured cortical neurons from neurotoxicity induced by glutamate. Here, we have investigated the structure-activity relationships in the phenylpropanoids using our primary culture system. 2. The alpha,beta-unsaturated ester moiety and the para-methoxy group in the phenylpropanoids appeared to play a vital role in neuroprotective activity. This suggested that E-p-methoxycinnamic acid (E-p-MCA) might be a crucial component for their neuroprotective activity within the phenylpropanoid compounds. E-p-MCA significantly attenuated glutamate-induced neurotoxicity when added prior to an excitotoxic glutamate challenge. 3. The neuroprotective activity of E-p-MCA appeared to be more effective in protecting neurons against neurotoxicity induced by NMDA than from that induced by kainic acid. E-p-MCA inhibited the binding of [propyl-2,3-(3)H]-CGP39653 and [2-(3)H]-glycine to their respective binding sites on rat cortical membranes. However, even high concentrations of E-p-MCA failed to inhibit completely [propyl-2,3-(3)H]-CGP39653 and [2-(3)H]-glycine binding. 4. Indeed, E-p-MCA diminished the calcium influx that routinely accompanies glutamate-induced neurotoxicity, and inhibited the subsequent overproduction of nitric oxide and cellular peroxide in glutamate-injured neurons. 5. Thus, our results suggest that E-p-MCA exerts significant protective effects against neurodegeneration induced by glutamate in primary cultures of cortical neurons by an action suggestive of partial glutamatergic antagonism.

Functional expression of distinct NMDA channel subunits tagged with green fluorescent protein in hippocampal neurons in culture

We generated expression vectors for N-terminally green fluorescent protein -tagged NR2A and NR2B subunits (GFP-NR2A and GFP-NR2B). Both constructs expressed GFP and formed functional NMDA channels with similar properties to untagged controls when co-transfected with NR1 subunit partner in HEK293 cells. Primary cultured hippocampal neurons were transfected at five days in vitro with these vectors. Fifteen days after transfection, well-defined GFP clusters were observed for both GFP-NR2A and GFP-NR2B subunits being co-localized with endogenous NR1 subunit. Whole-cell recordings showed that the GFP-NR2A subunit determined the decay of NMDA-mediated miniature spontaneous excitatory postsynaptic currents (NMDA-mEPSCs) in transfected neurons. Live staining with anti-GFP antibody demonstrated the surface expression of GFP-NR2A and GFP-NR2B subunits that was partly co-localized a presynaptic marker. Localization of NMDA receptor clusters in dendrites was studied by co-transfection of CFP-actin and GFP-NR2 subunits followed by anti-GFP surface staining. Within one week after plating most surface NMDAR clusters were distributed on dendritic shafts. Later in development, a large portion of surface clusters for both GFP-NR2A and GFP-NR2B subunits were clearly localized at dendritic spines. Our report provides the basis for studies of NMDA receptor location together with dendritic dynamics in living neurons during synaptogenesis in vitro.

ASIC-like, proton-activated currents in rat hippocampal neurons

The expression of mRNA for acid sensing ion channels (ASIC) subunits ASIC1a, ASIC2a and ASIC2b has been reported in hippocampal neurons, but the presence of functional hippocampal ASIC channels was never assessed. We report here the first characterization of ASIC-like currents in rat hippocampal neurons in primary culture. An extracellular pH drop induces a transient Na(+) current followed by a sustained non-selective cation current. This current is highly sensitive to pH with an activation threshold around pH 6.9 and a pH(0.5) of 6.2. About half of the total peak current is inhibited by the spider toxin PcTX1, which is specific for homomeric ASIC1a channels. The remaining PcTX1-resistant ASIC-like current is increased by 300 microM Zn(2+) and, whereas not fully activated at pH 5, it shows a pH(0.5) of 6.0 between pH 7.4 and 5. We have previously shown that Zn(2+) is a co-activator of ASIC2a-containing channels. Thus, the hippocampal transient ASIC-like current appears to be generated by a mixture of homomeric ASIC1a channels and ASIC2a-containing channels, probably heteromeric ASIC1a+2a channels. The sustained non-selective current suggests the involvement of ASIC2b-containing heteromeric channels. Activation of the hippocampal ASIC-like current by a pH drop to 6.9 or 6.6 induces a transient depolarization which itself triggers an initial action potential (AP) followed by a sustained depolarization and trains of APs. Zn(2+) increases the acid sensitivity of ASIC channels, and consequently neuronal excitability. It is probably an important co-activator of ASIC channels in the central nervous system.

Focal stimulation of single GABAergic presynaptic boutons on the rat hippocampal neuron

Evoked inhibitory postsynaptic currents (eIPSCs) generated from a single GABAergic bouton were recorded and the functional properties were investigated. Native single boutons attached to mechanically dissociated rat hippocampal CA1 neurons, namely "synaptic bouton" preparation, were visualized with FM 1-43 dye and selectively stimulated by a glass pipette directed to a single bouton by focal stimulation. The GABAergic eIPSCs were elicited in like all-or-none fashion regarding both stimulus strength and pipette location, thus indicating that the eIPSCs result from the activation of a single bouton. The GABA release from the boutons was action potential dependent since eIPSCs were blocked in the presence of either voltage-dependent Na(+) or Ca(2+)channel blocker. Even in the presence of tetrodotoxin (TTX), eIPSCs could be elicited by additional application of a voltage-dependent K(+) channel blocker, 4-AP. The GABA release depended on external Ca(2+) concentration. Amplitude histogram of eIPSCs did not follow Poisson distribution or show discrete peaks. As a result, this new experimental approach using both focal stimulation and a synaptic bouton preparation allows for a detailed study of the native synaptic machinery in nerve terminals measuring smaller than 1 microm in size in the CNS.

The effects of temperature on vesicular supply and release in autaptic cultures of rat and mouse hippocampal neurons

Membrane fusion plays a central role in the synaptic vesicle cycle. While many of the pre- and postfusion events have been investigated at room temperature, few researchers have investigated these processes at more physiologically relevant temperatures. We have used autaptic cultures of hippocampal neurons to investigate changes in the size and refilling rate of the readily releasable pool (RRP) of synaptic vesicles brought about by an increase in temperature from 25 to 35 degrees C. We have also examined temperature-dependent changes in spontaneous and action potential (AP)-evoked release as well as the fraction of the RRP that is released during an AP. Although we found a threefold increase in the refilling rate of the RRP at the higher temperature, there was no apparent change in the size of the RRP with increased temperature. Moreover, we observed a slight but significant decrease in the quanta released during an AP. This increased refilling rate and decreased release probability resulted in a reduction of both the degree and time course of synaptic depression during high frequency stimulation at the higher temperature. This reduction in synaptic depression was accompanied by an increased maintenance of the synchronous component of release during high frequency stimulation. These findings indicate that the dynamics of vesicular supply and release in hippocampal neurons at room temperature are significantly different at near physiological temperatures and could affect our present understanding of the way in which individual neurons and networks of neurons process information.

A non-alpha7 nicotinic acetylcholine receptor modulates excitatory input to hippocampal CA1 interneurons

The nicotinic acetylcholine receptor (nAChR), particularly the alpha7 subtype, has received profound attention for its role in modifying excitatory postsynaptic currents (EPSCs) in hippocampal pyramidal neurons as well as in neurons from other brain regions. Here, we tested the possibility that an nAChR could affect EPSCs in the interneurons of rat hippocampal slices. Using whole-cell patch-clamp technique on CA1 stratum radiatum interneurons and U-tube application of agents, we show that nicotinic agonists enhance EPSC frequency in interneurons. Among the agents tested, cytisine and mecamylamine were the most effective agonist and antagonist, respectively, suggesting a role for alpha3beta4-containing nAChRs in the modulation of interneuron EPSCs. Ligands selective for the alpha7 nAChR had very little or no effect on interneuron EPSCs. Low concentrations of nicotine also enhanced EPSC frequency, implicating the involvement of non-alpha7 nAChRs in controlling interneuron excitability in smokers. We conclude that nAChR-dependent EPSC modulation in the hippocampus is both subtype- and neuron-specific and that a non-alpha7 nAChR, presumably alpha3beta4, controls glutamate transmission to CA1 interneurons.

Effect of depleting vesicular and cytoplasmic dopamine on methylenedioxymethamphetamine neurotoxicity

The mechanism by which 3,4-methylenedioxymethamphetamine (MDMA) produces serotonin (5-HT) neurotoxicity is unknown but considerable evidence suggests that endogenous brain dopamine (DA) is involved. However, it has recently become apparent that some of the data implicating brain DA in MDMA neurotoxicity may be confounded by drug effects on thermoregulation. The purpose of the present studies was to examine the role of DA in MDMA neurotoxicity, while controlling for possible confounding effects of drug- induced changes in core temperature. Rats were treated with reserpine, alone and in combination with alpha-methyl-p -tyrosine (AMPT), to deplete vesicular and cytoplasmic stores of DA. When drug-induced hypothermia was averted (by raising ambient temperature), the 5-HT neuroprotective effects of reserpine and AMPT were no longer apparent. The lack of neuroprotection by AMPT and reserpine, alone and in combination, in studies that control for the effects of these drugs on core temperature, suggests that DA per se is not essential for the expression of MDMA-induced 5-HT neurotoxicity.

Responses of hippocampal neurons at different stages of acquisition of conditioned reflex avoidance in rats

The electrophysiological properties of hippocampal slices from trained and untrained rats presented with identical numbers of combinations during acquisition of a conditioned two-sided avoidance reaction were compared. Associative learning (presentation of combinations of conditioned and unconditioned stimuli) was accompanied by gradual increases in the amplitude of the population spike in hippocampal field CA1. However, achievement of the learning criterion was accompanied by a decrease in the amplitude of the population spike. The same type of difference between groups was seen in field CA1 using low-intensity stimulation of Schaffer collaterals, while there was no change in the amplitude at maximum-intensity stimulation. After the rats achieved the learning criterion, the level of facilitation in conditions of paired stimuli was significantly greater than in untrained animals and also as compared with the passive and active controls. These data provide evidence supporting the hypothesis that changes in the efficiency of synaptic transmission occur during the learning process. However, the direction of changes depends on the extent of training.

Network interactions mediated by new excitatory connections between CA1 pyramidal cells in rats with kainate-induced epilepsy

Axon sprouting and synaptic reorganization in the hippocampus are associated with the development of seizures in temporal lobe epilepsy. Synaptic interactions among CA1 pyramidal cells were examined in fragments of hippocampal slices containing only the CA1 area from saline- and kainate-treated rats. Glutamate microapplication to the pyramidal cell layer increased excitatory postsynaptic current (EPSC) frequency, but only in rats with kainate-induced epilepsy. In bicuculline, action potentials evoked in single pyramidal cells increased the frequency of network bursts only in slices from rats with kainate-induced epilepsy. These data further support the hypothesis that excitatory connections between CA1 pyramidal cells increase after kainate-induced status epilepticus.

Synaptic plasticity in an altered state

In this issue of Neuron, record from synaptically coupled pairs of CA3 neurons to closely examine the induction of synaptic depression at a small number of identified synapses. The authors provide convincing evidence that the activation history of a synapse determines both the ability of a synapse to depress and the mechanism of depression.

Diverse trafficking patterns due to multiple traffic motifs in G protein-activated inwardly rectifying potassium channels from brain and heart

G protein-activated inwardly rectifying potassium channels (Kir3, GIRK) provide an important mechanism for neurotransmitter regulation of membrane excitability. GIRK channels are tetramers containing various combinations of Kir3 subunits (Kir3.1--Kir3.4). We find that different combinations of Kir3 subunits exhibit a surprisingly complex spectrum of trafficking phenotypes. Kir3.2 and Kir3.4, but not Kir3.1, contain ER export signals that are important for plasma membrane expression of Kir3.1/Kir3.2 and Kir3.1/Kir3.4 heterotetramers, the GIRK channels found in the brain and the heart, respectively. Additional motifs in Kir3.2 and Kir3.4 control the trafficking between endosome and plasma membrane. In contrast, the Kir3.3 subunit potently inhibits plasma membrane expression by diverting the heterotetrameric channels to lysosomes. Such rich trafficking behaviors provide a mechanism for dynamic regulation of GIRK channel density in the plasma membrane.

State-dependent heterogeneity in synaptic depression between pyramidal cell pairs

Paired recordings between CA3 pyramidal neurons were used to study the properties of synaptic plasticity in active and silent synapses. Synaptic depression is accompanied by decreases in both AMPAR and NMDAR function. The mechanisms of synaptic depression, and the potential to undergo activity-dependent plastic changes in efficacy, differ depending on whether a synapse is active, recently silent, or potentiated. These results suggest that silent and active synapses represent distinct synaptic "states," and that once unsilenced, synapses express plasticity in a graded manner. The state in which a synapse resides, and the states recently visited, determine its potential and mechanism for undergoing subsequent plastic changes.

Aging and neuronal plasticity: lessons from a model

In spite of many well-documented examples of age-related reductions in neuronal plasticity, the causes of such changes remain largely unknown. One example of age-reduced plasticity involves an aberrant sprouting response of mature rat sympathetic neurons into the CNS (hippocampal formation). This phenomenon has proven to be useful for exploring the relative contribution of target aging (extrinsic influences) versus neuronal aging (intrinsic influences) to reduced sprouting. Aged sympathetic neurons mount a robust growth response when confronted with young target tissue or when exposed to exogenous trophic factor in vivo. In contrast, the aged target tissue (the hippocampal formation in this example) exhibits reduced receptivity for sympathetic sprouting. This change in the target does not appear to be due to alterations in baseline levels of trophic or substrate support for axonal growth. Rather, aging appears to dampen the consequences of target denervation so that the aged target elicits less sprouting. Age-related reductions in neuronal sprouting are speculated to reflect increasing commitment to information storage at the expense of neuronal plasticity.

Transcriptional regulation of cortical neuron migration by POU domain factors

The identification of pathways mediated by the kinase Cdk5 and the ligand reelin has provided a conceptual framework for exploring the molecular mechanisms underlying proper lamination of the developing mammalian cerebral cortex. In this report, we identify a component of the regulation of Cdk5-mediated cortical lamination by genetic analysis of the roles of the class III POU domain transcription factors, Brn-1 and Brn-2, expressed during the development of the forebrain and coexpressed in most layer II-V cortical neurons. Brn-1 and Brn-2 appear to critically control the initiation of radial migration, redundantly regulating the cell-autonomous expression of the p35 and p39 regulatory subunits of Cdk5 in migrating cortical neurons, with Brn-1(-/-)/Brn-2(-/-) mice exhibiting cortical inversion.

Expression of glial cell line-derived neurotrophic factor mRNA in the human newborn and adult hippocampal formation

Glial cell line-derived neurotrophic factor (GDNF) mRNA-containing neurons were found in normal neonate and adult human hippocampus with a localization pattern consistently similar among different ages. They were numerous in proximal CA1 to CA3 pyramidal layer, granular layer and hilus, and sparse in oriens and molecular layers. The present data provide a map of GDNF-producing neurons in the human archicortex and suggest a role for GDNF in neuronal function throughout life.

Dynamic localization and clustering of dendritic Kv2.1 voltage-dependent potassium channels in developing hippocampal neurons

Dendritic excitability is modulated by the highly variable spatial and temporal expression pattern of voltage-dependent potassium channels. Somatodendritic Kv2.1 channels contribute a major component of delayed rectifier potassium current in cultured hippocampal neurons, where Kv2.1 is localized to large clusters on the soma and proximal dendrites. Here we found that dramatic differences exist in the clustering of endogenous Kv2.1 in cultured rat hippocampal GABAergic interneurons and glutamatergic pyramidal neurons. Studies on neurons developing in culture revealed that while a similar sequence of Kv2.1 localization and clustering occurred in both cell types, the process was temporally delayed in pyramidal cells. Localization and clustering of recombinant green fluorescent protein-tagged Kv2.1 occurred by the same sequence of events, and imaging of GFP-Kv2.1 clustering in living neurons revealed dynamic fusion events that underlie cluster formation. Overexpression of GFP-Kv2.1 accelerated the clustering program in pyramidal neurons such that the observed differences in Kv2.1 clustering in pyramidal neurons and interneurons were eliminated. Confocal imaging showed a preferential association of Kv2.1 with the basal membrane in cultured neurons, and electrophysiological recordings from neurons cultured on transistors revealed that Kv2.1 contributed the bulk of a previously described adherens junction delayed rectifier potassium conductance. Finally, Kv2.1 clusters were found spatially associated with ryanodine receptor intracellular Ca(2+) ([Ca(2+)](i)) release channels. These findings reveal a stepwise assembly of Kv2.1 potassium channels into membrane clusters during development, and an association of these clusters with Ca(2+) signaling apparatus. Together these data suggest that the restricted localization of Kv2.1 may play an important role in the previously observed contribution of this potassium channel in regulating dendritic [Ca(2+)](i) transients.

The brain-derived neurotrophic factor enhances synthesis of Arc in synaptoneurosomes

Protein synthesis in neurons is essential for the consolidation of memory and for the stabilization of activity-dependent forms of synaptic plasticity such as long-term potentiation (LTP). Activity-dependent translation of dendritically localized mRNAs has been proposed to be a critical source of new proteins necessary for synaptic change. mRNA for the activity-regulated cytoskeletal protein, Arc, is transcribed during LTP and learning, and disruption of its translation gives rise to deficits in both. We have found that selective translation of Arc in a synaptoneurosomal preparation is induced by the brain-derived neurotrophic factor, a neurotrophin that is released during high-frequency stimulation patterns used to elicit LTP. This effect involves signaling through the TrkB receptor and is blocked by the N-methyl-d-aspartate-type glutamate receptor antagonist, MK801. The results suggest there is a synergy between neurotrophic and ionotropic mechanisms that may influence the specificity and duration of changes in synaptic efficacy at glutamatergic synapses.

Physiological and morphological diversity of immunocytochemically defined parvalbumin- and cholecystokinin-positive interneurones in CA1 of the adult rat hippocampus

To investigate the electrophysiological properties, synaptic connections, and anatomy of individual parvalbumin-immunoreactive (PV-IR) and cholecystokinin-immunoreactive (CCK-IR) interneurones in CA1, dual intracellular recordings using biocytin-filled microelectrodes in slices of adult rat hippocampus were combined with fluorescence labelling of PV- and CCK-containing cells. Of 36 PV-IR cells, 29 were basket cells, with most of their axonal arbours in the stratum pyramidale (SP). Six were bistratified cells with axons ramifying throughout stratum oriens (SO) and stratum radiatum (SR). One was a putative axo-axonic cell with an axonal arbour confined to half of the SP and a narrow adjacent region of the SO. Of 27 CCK-IR neurones, 13 were basket cells, with most of their axonal arbours in the SP, and included basket cells with somata in the SP (6), SO (3), and SR (2) and at the border between the stratum lacunosum-moleculare (SLM) and the SR (2). In addition, several dendrite-targeting cell classes expressed CCK-IR: 4 of 9 bistratified cells with axons ramifying in the SO and SR; all five Schaffer-associated cells whose axons ramified extensively in the SR; both cells classified as quadrilaminar because their axons ramified in the SO, SP, SR, and SLM; one SO-SO cell whose dendritic and axonal arbours were contained within the SO; and one perforant path-associated cell with axonal and dendritic arbours within the distal SR and SLM. The majority (31 of 36) of PV-IR neurones recorded were fast-spiking, and most fast-spiking cells tested (25 of 29 basket, 1 axo-axonic, and 5 of 6 bistratified cells) were PV-IR. However, 1 of 6 regular-spiking basket, 1 of 4 regular-spiking bistratified, and 3 of 5 burst-firing basket cells were also PV-IR. In contrast, the majority (17 of 27) of the CCK-IR neurones recorded were regular-spiking, 3 were burst-firing, and 7 were fast-spiking. These data confirm that the majority of PV-IR and CCK-IR axon terminals innervate proximal portions of CA1 pyramidal cells. Most PV-IR cells are fast-spiking, whereas most CCK-IR cells are regular-spiking. In both neurochemical classes basket cells predominate, but both groups included subpopulations of dendrite-targeting cells. Despite these similarities, the two populations exhibited very different somatic distributions, and each contained cellular morphologies not represented in the other.

Differential expression of receptors for Shiga and Cholera toxin is regulated by the cell cycle

Cholera and Shiga toxin bind to the cell surface via glycolipid receptors GM1 and Gb3, respectively. Surprisingly, the majority of Vero cells from a non-synchronized population bind either Cholera or Shiga toxin but not both toxins. The hypothesis that the differential expression of toxin receptors is regulated by the cell cycle was tested. We find that Cholera toxin binds preferentially in G0/G1, with little binding through S-phase to telophase,whereas Shiga toxin binds maximally through G2 to telophase but does not bind during G0/G1 and S-phase. The changes result from the corresponding changes in Gb3 and GM1 synthesis, not from variations of receptor transport to the cell surface. The changes do not reflect competition of Gb3 and GM1 synthesis for lactosylceramide. Cells as diverse as Vero cells, PC12 cells and astrocytes show the same cell-cycle-dependent regulation of glycosphingolipid receptors,suggesting that this novel phenomenon is based on a conserved regulatory mechanism.

The core membrane fusion complex governs the probability of synaptic vesicle fusion but not transmitter release kinetics

Synaptic vesicle fusion is driven by the formation of a four-helical bundle composed of soluble N-ethylmaleimide sensitive factor (NSF) attachment protein receptors (SNAREs). Exactly how the structural interactions that lead to the formation of this complex relate to neurotransmitter release is not well understood. To address this question, we used a strategy to "rescue" synaptic transmission after proteolytic cleavage of the synaptosome-associated protein of 25 kDa (SNAP-25) by botulinum neurotoxin E (BoNtE). Transfection of CA3 hippocampal pyramidal cells with BoNtE-resistant SNAP-25 restored synaptic transmission. Additional mutations that alter the interaction between SNAP-25 C-terminal coil and the other SNARE coils dramatically reduce transmitter release probability but leave the kinetics of synaptic responses unaltered. These data indicate that at synapses, SNARE interactions are necessary for fusion but are not the rate-limiting step of neurotransmission.

Densin-180 interacts with delta-catenin/neural plakophilin-related armadillo repeat protein at synapses

Densin-180, a protein purified from the postsynaptic density fraction of the rat forebrain, is the founding member of a newly described family of proteins termed the LAP (leucine-rich repeats and PSD-95/Dlg-A/ZO-1 (PDZ) domains) family that plays essential roles in establishment of cell polarity. To identify Densin-180-binding proteins, we screened a yeast two-hybrid library using the carboxyl-terminal fragment of Densin-180 containing PDZ domain as bait, and we isolated delta-catenin/neural plakophilin-related armadillo repeat protein (NPRAP) as a Densin-180-interacting protein. delta-catenin/NPRAP, a member of the armadillo repeat family, is a nervous system-specific adherens junction protein originally discovered as an interactor with presenilin-1, a protein involved in Alzheimer's disease. Densin-180 PDZ domain binds the COOH terminus of delta-catenin/NPRAP containing the PDZ domain-binding sequence. Endogenous Densin-180 was co-immunoprecipitated with delta-catenin/NPRAP and N-cadherin. Although Densin-180 was reported to be a transmembrane protein, Densin-180 was not accessible to surface biotinylation in dissociated hippocampal neurons; hence Densin-180 may be a cytosolic protein. Densin-180 co-localized with delta-catenin/NPRAP at synapses in delta-catenin/NPRAP and may be involved in organization of the synaptic cell-cell junction through interaction with the delta-catenin/NPRAP-N-cadherin complex.

Differential effects of glucocorticoids and gonadal steroids on glutathione levels in neuronal and glial cell systems

The aim of the present study was to investigate the short- and long-term effects of glucocorticoids [corticosterone (CORT), dexamethasone (DEX), 6-methylprednisolone (6-MP)] and gonadal steroids [17beta-estradiol (E(2)), progesterone (PROG), testosterone (TEST)] on the levels of the antioxidant glutathione (GSH) in different cell systems of the CNS (neuronal hippocampal HT22 cells, primary hippocampal and neocortical brain cells, and C(6) glioma cells). In HT22 cells, steroids exerted mainly long-term effects. Significant increases of GSH levels were detectable after a 24 hr treatment with 10(-7) M of DEX (122% +/- 5%), 6-MP (208% +/- 32%), E(2) (134% +/- 10%), and TEST (155% +/- 17%). A significant decrease occurred after incubation with PROG for 24 hr (79% +/- 9%). In primary hippocampal cultures, a 24 hr treatment with DEX (140% +/- 8%), E(2) (123% +/- 6%), and PROG (118% +/- 5%) led to significant increases of the GSH levels, whereas, in neocortical primary cultures, only an incubation with E(2) increased GSH (149% +/- 8%). In C(6) cells, hormone treatment led to both significant short-term (1 hr: CORT 114% +/- 5%, DEX 90% +/- 3%, E(2) 88% +/- 3%; 3 hr: DEX 115% +/- 5%, E(2) 122% +/- 6%, TEST 78% +/- 4%) and significant long-term (24 hr: CORT 74% +/- 4%, 6-MP 84% +/- 5%, E(2) 115% +/- 6%, PROG 91% +/- 4%, TEST 116% +/- 5%) effects. In summary, we were able to demonstrate differential effects of steroids on GSH levels in different cellular CNS models, showing an important influence of steroids and especially E(2) on antioxidative cellular functions in neuronal and glial cells.

Differential desensitization of responses mediated by presynaptic and postsynaptic A1 adenosine receptors

G-protein-coupled receptors (GPCRs) often desensitize during continuous activation, but it is not known whether desensitization is influenced by subcellular location. In hippocampal neurons, activation of adenosine A1 receptors (A1Rs) or GABA(B) receptors on synaptic terminals inhibits neurotransmitter release, whereas activation of the same receptors on cell bodies and dendrites decreases excitability by activating inwardly rectifying potassium (GIRK) channels. Here we report that responses mediated by presynaptic A1Rs desensitize more slowly than responses mediated by postsynaptic (somatodendritic) A1Rs in cultured neurons. Agonist treatment for 2 hr has no effect on adenosine-induced presynaptic inhibition, whereas such treatment nearly abolishes adenosine-induced activation of postsynaptic GIRK channels. Agonist treatment for longer periods (>12 hr) eventually desensitizes A1R-mediated presynaptic inhibition. Presynaptic and postsynaptic responses both recover from desensitization after agonist removal, but recovery of presynaptic inhibition requires more time. Desensitization of postsynaptic responses apparently occurs at the level of the receptor, because postsynaptic G-proteins and GIRK channels appear to be fully functional. Inhibition of voltage-gated calcium channels by postsynaptic A1Rs also desensitizes rapidly, although this desensitization is less complete than is observed for activation of postsynaptic GIRK channels. Comparison of concentration-response curves for presynaptic and postsynaptic responses suggests that a receptor reserve exists for presynaptic inhibition, but that the magnitude of this reserve is insufficient to account for the absence of presynaptic desensitization after brief agonist exposure. These results suggest that agonist-induced desensitization of responses mediated by neuronal GPCRs may depend on the subcellular location of the receptors.