Inhibition of glutamatergic activation of extracellular signal-regulated protein kinases in hippocampal neurons by the intravenous anesthetic propofol

BACKGROUND

Intravenous anesthetics cause amnesia, but the underlying molecular mechanisms are poorly understood. Recent studies reveal a significant role of extracellular signal-regulated protein kinases (ERKs) in controlling synaptic plasticity and memory formation. As a major synapse-to-nucleus superhighway, ERK transmits N-methyl-D-aspartate (NMDA) receptor signals to inducible transcriptional events essential for NMDA receptor-dependent forms of synaptic plasticity and memory. This study investigated the role of the widely used intravenous anesthetic propofol in regulating NMDA receptor-dependent ERK phosphorylation.

METHODS

The possible effect of propofol on NMDA receptor-mediated ERK phosphorylation was detected in cultured rat hippocampal neurons with Western blot analysis.

RESULTS

The authors found that propofol at clinical relevant concentrations (1-10 microm) reduced NMDA receptor-mediated ERK phosphorylation. This reduction was independent of gamma-aminobutyric acid transmission. The inhibition of the NMDA receptor seems to contribute to the effect of propofol on NMDA-stimulated ERK phosphorylation, because propofol reduced constitutive NMDA receptor NR1 subunit phosphorylation and impaired NMDA receptor-mediated Ca influx. Furthermore, by inhibiting the ERK pathway, propofol blocked NMDA receptor-dependent activation of two key transcription factors, Elk-1 and cyclic adenosine monophosphate response element-binding protein (CREB), and, as a result, attenuated Elk-1/CREB-dependent reporter gene (c-Fos) expression.

CONCLUSIONS

These results suggest that propofol possesses the ability to inhibit NMDA receptor activation of the ERK pathway and subsequent transcriptional activities in hippocampal neurons. These findings indicate a new avenue to explore a transcription-dependent mechanism that may underlie anesthetic interference with synaptic plasticity related to amnesic properties of intravenous anesthetics.

A preliminary study of the levels of testis oxidative stress parameters after MK-801-induced experimental psychosis model: protective effects of CAPE

We evaluated the effects of caffeic acid phenethyl ester (CAPE) on antioxidant enzyme levels and histopathologic changes in dizocilpine (MK-801) induced schizophrenic rat testis. A total of 30 adult male Wistar-Albino rats were divided into three groups. Group-I was used as control. Rats in the Group-II were intraperitoneally injected with MK-801, whereas those in Group-III were intraperitoneally injected with CAPE in addition to MK-801. The testes were collected for biochemical and histopathological examinations. Antioxidant enzyme activities, malondialdehyde, protein carbonyl and nitric oxide levels in testicular tissues were analyzed with spectrophotometric methods. Induction of schizophrenia resulted in a significant oxidative stress by increasing the levels of antioxidant enzymes. Tissue malondialdehyde and protein carbonyl levels were also increased. Treatment with CAPE led to significant decrease in oxidative injury. Administration of CAPE reduced the detrimental histopathologic changes caused by MK-801. The results showed that experimentally induced schizophrenia caused oxidative stress in testes of rats and treatment with CAPE reduced these harmful effects.

Mesenteric Artery Clamping/Unclamping-Induced Acute Lung Injury Is Attenuated by N-Methyl-D-Aspartate Antagonist Dextromethorphan

Lung N-methyl-D-aspartate receptors (NMDAR) may cause excitotoxic pulmonary edema if activated. Acute lung injury may be mediated by oxidative stress, frequently generated by local or remote ischemia and reperfusion (IR). This experimental study assessed the effects of intravenous dextromethorphan, an NMDAR antagonist, on reperfusion lung injury following superior mesenteric artery (SMA) clamping/unclamping. SMA of 48 (12 per group) anesthetized adult male Wistar rats was clamped for 90 min (IR); 48 additional rats underwent a sham laparotomy (control). The experimental timeframe was identical in all groups. Ten minutes before unclamping, three dextromethorphan doses were administered intravenously in three IR and three control groups, followed by 3 h of respiratory and hemodynamic assessment and postexperimental assessment of survival. Intravenous 10 and 20 mg/kg dextromethorphan attenuated an 85% increase in peak ventilatory pressure, a 45% reduction in PO(2)/FiO(2), 4-12-fold increase in bronchoalveolar lavage-retrieved volume, and polymorphonuclear leukocytes/bronchoalveolar cells ratio, all associated with SMA unclamping in the IR-nontreated and the IR-40 mg/kg dextromethorphan-treated rats. Lung tissue polymorphonuclear leukocyte count, total xanthine oxidase activity, reduced glutathione, and wet-to-dry weight ratio were all within normal ranges in the two lower-dose-treated groups. These effective regimens were also associated with longer postexperimental animal survival. Dextromethorphan was not associated with changes in three control groups. Thus, Intravenous dextromethorphan mitigates lung reperfusion injury following SMA clamping/unclamping in a dose-dependent manner. This is a novel potential use of dextromethorphan in vivo.

Neuroprotectant FK506 inhibits glutamate-induced apoptosis of astrocytes in vitro and in vivo

Neuron-astrocyte interactions are critical for signalling, energy metabolism, extracellular ion and glutamate homeostasis, volume regulation and neuroprotection in the CNS. Glutamate uptake by astrocytes may prevent excitotoxic glutamate elevation and determine neuronal survival. However, an excess of glutamate can cause the death of astrocytes. FK506, an inhibitor of calcineurin, and an immunosuppressive drug, is neuroprotective in animal models of neurologic diseases, including focal and global ischaemia. In the present work, we demonstrate that a single injection of FK506 60 min after a transient middle cerebral artery occlusion (MCAo) significantly decreases the number of terminal deoxynucleotidyl transferase nick-end labelling (TUNEL)-positive cells in the ischaemic cortex and striatum. Using 3-D confocal microscopy we found that, 24 h after MCAo, many TUNEL-positive cells in the ischaemic striatum and cortex are astrocytes. Furthermore, we demonstrate that exposure of cultured cortical astrocytes to 50-100 mM Glu for 24 h induces apoptotic alterations in nuclear morphology, DNA fragmentation, dissipation of mitochondrial transmembrane potential (DeltaPsi) and caspase activation. FK506 (1 muM) efficiently inhibits Glu-induced apoptosis of cultured astrocytes, DNA fragmentation and changes in mitochondrial DeltaPsi. Our findings suggest that modulation of glutamate-induced astrocyte death early after reperfusion may be a novel mechanism of FK506-mediated neuroprotection in ischaemia.

Different effects of nonNMDA and NMDA receptor antagonists (NBQX and dizocilpine) on cortical epileptic afterdischarges in rats

Excitatory amino acids play an important role in generation of epileptic seizures. To study the participation of different types of their receptors in cortical epileptic afterdischarges, a noncompetitive NMDA receptor antagonist dizocilpine and a competitive AMPA receptor antagonist NBQX were used. Adult rats with implanted epidural stimulation and registration electrodes were pretreated either with NBQX (30 or 60 mg/kg i.p.) or with dizocilpine (0.1 or 0.5 mg/kg i.p.) and low-frequency stimulation of sensorimotor cortical area was repeatedly applied with stepwise increased current intensities. Lower dose of NBQX unexpectedly decreased thresholds for elicitation of spike-and-wave afterdischarges (ADs), clonic seizures accompanying this type of ADs and for transition into the second, limbic type of ADs. Lower dose of dizocilpine increased these three thresholds. Higher doses of either drug did not significantly change threshold intensities. Duration of ADs was also influenced by the two antagonists in opposite directions: higher dose of NBQX resulted in prolongation of ADs mainly due to an increased duration of the spike-and-wave part of ADs whereas dizocilpine shortened ADs in a dose-dependent manner affecting both types of ADs. In addition, NBQX did not influence interhemispheric responses meanwhile dizocilpine moderately suppressed these evoked potentials. According to our results, NMDA receptors are important for generation of cortical epileptic afterdischarges meanwhile the role of AMPA receptors is not clear and has to be analyzed.

Ivermectin inhibits AMPA receptor-mediated excitotoxicity in cultured motor neurons and extends the life span of a transgenic mouse model of amyotrophic lateral sclerosis

alpha-Amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor-mediated excitotoxicity contributes to the selective motor neuron death in amyotrophic lateral sclerosis (ALS). In this study, we investigated the effect of P2 receptor-influencing substances on kainate-induced motor neuron death in an in vitro model for AMPA receptor-mediated excitotoxicity. Complete protection was found after preincubation of the motor neurons with ivermectin or Cibacron Blue 3G-A. Preincubation with both P2X4 modulators did not influence the number or Ca2+ permeability of the AMPA receptors and addition during kainate stimulation alone had no effect. Preincubation with a low concentration of ATP, the natural agonist of the P2X4 receptor, also protected the motor neurons against a subsequent excitotoxic stimulation, while high concentrations of ATP were toxic. Moreover, ivermectin increased the toxicity of low ATP concentrations, indicating that ivermectin can potentiate the effect of ATP on its receptor. Ivermectin and ATP also protected against hypoxia/hypoglycemia. To further investigate the relevance of these findings for ALS, we treated SOD1(G93A)-mice, a transgenic animal model for familial ALS, with ivermectin. This resulted in an extension of the life span of these mice with almost 10%. We conclude that ivermectin induces a mechanism in motor neurons, in vivo and in vitro, that protects against subsequent excitotoxic insults. Our in vitro data indicate that this protective mechanism is due to the potentiation by ivermectin of an effect of ATP mediated by the P2X4 receptor.

Clozapine but not haloperidol treatment reverses sub-chronic phencyclidine-induced disruption of conditional discrimination performance

Abusers of phencyclidine (PCP) often present with a symptom profile similar to that exhibited by schizophrenic patients. Animal models utilising such psychotomimetics are currently informing research into the condition. Accumulating evidence suggests that a central cognitive deficit in schizophrenia is the inability to use task-setting cues to guide goal directed behaviour and that this ability is mediated by prefrontal dopamine (DA). The current study used the non-competitive NMDA antagonist phencyclidine (PCP) and Haloperidol (typical antipsychotic) and Clozapine (atypical antipsychotic) in order to further investigate the influence of DAergic manipulation on a task that requires the use of conditional information to inform goal-directed performance. An instrumental conditional discrimination task was employed in which rats learn to respond appropriately according to the presence of specific auditory conditional stimuli. Probe test 1 showed impaired conditional discrimination performance following sub-chronic PCP administration (seven twice-daily injection protocol) compared to control which was reversed by acute treatment with clozapine (5 mg/kg) but not haloperidol (0.1 mg/kg) both administered 60 min pre-test. Probe test 2 (8 days post-treatment) showed enduring deficits to conditional discrimination performance that were again reversed by clozapine but not haloperidol (injection procedures as above). These results show that tasks dependent upon conditional relationships are particularly sensitive to manipulation of DAergic systems as prolonged treatment with PCP has been shown to selectively reduce prefrontal cortex (PFC) DA activity and treatment with clozapine (known to ameliorate cognitive deficits) but not haloperidol has been shown to selectively restore PFC DA levels.

Acute D2/D3 dopaminergic agonism but chronic D2/D3 antagonism prevents NMDA antagonist neurotoxicity

BACKGROUND

Antagonists of the N-methyl-D-aspartate (NMDA) glutamate receptor, most likely by producing disinhibtion in complex circuits, acutely produce psychosis and cognitive disturbances in humans, and neurotoxicity in rodents. Studies examining NMDA Receptor Hypofunction (NRHypo) neurotoxicity in animals, therefore, may provide insights into the pathophysiology of psychotic disorders. Dopaminergic D2 and/or D3 agents can modify psychosis over days to weeks, suggesting involvement of these transmitter system(s).

METHODS

We studied the ability of D2/D3 agonists and antagonists to modify NRHypo neurotoxicity both after a one-time acute exposure and after chronic daily exposure.

RESULTS

Here we report that D2/D3 dopamine agonists, probably via D3 receptors, prevent NRHypo neurotoxicity when given acutely. The protective effect with D2/D3 agonists is not seen after chronic daily dosing. In contrast, the antipsychotic haloperidol does not affect NRHypo neurotoxicity when given acutely at D2/D3 doses. However, after chronic daily dosing of 1, 3, or 5 weeks, haloperidol does prevent NRHypo neurotoxicity with longer durations producing greater protection.

CONCLUSIONS

Understanding the changes that occur in the NRHypo circuit after chronic exposure to dopaminergic agents could provide important clues into the pathophysiology of psychotic disorders.

NMDA antagonist MK-801 impairs acquisition of place strategies, but not their use

Evidence that NMDA receptors contribute to synaptic plasticity in the hippocampus has stimulated research on their role in behavioral learning and memory. Recent studies indicate that NMDA antagonists decrease use of place strategies by rats in a T-maze task that can be solved using either a "place" or "response" strategy. In the present study, rats were given MK-801 before maze exposure and/or training on this redundant strategy T-maze task. MK-801 did not impair rats' ability to learn the task, but did change the strategies they used on a probe trial administered after learning. MK-801 decreased use of place strategies only when administered before both maze exposure and training; rats given MK-801 only before maze exposure or only before training tended to use place strategies on the probe trial. These results show that MK-801 does not prevent rats from utilizing previously acquired spatial information, but does appear to impair the acquisition of spatial information needed for place strategies.

In vitro synergistic cytotoxicity of gemcitabine and pemetrexed and pharmacogenetic evaluation of response to gemcitabine in bladder cancer patients

The present study was performed to investigate the capability of gemcitabine and pemetrexed to synergistically interact with respect to cytotoxicity and apoptosis in T24 and J82 bladder cancer cells, and to establish a correlation between drug activity and gene expression of selected genes in tumour samples. The interaction between gemcitabine and pemetrexed was synergistic; indeed, pemetrexed favoured gemcitabine cytotoxicity by increasing cellular population in S-phase, reducing Akt phosphorylation as well as by inducing the expression of a major gemcitabine uptake system, the human equilibrative nucleoside transporter-1 (hENT1), and the key activating enzyme deoxycytidine kinase (dCK) in both cell lines. Bladder tumour specimens showed an heterogeneous gene expression pattern and patients with higher levels of dCK and hENT1 had better response. Moreover, human nucleoside concentrative transporter-1 was detectable only in 3/12 patients, two of whom presented a complete response to gemcitabine. These data provide evidence that the chemotherapeutic activity of the combination of gemcitabine and pemetrexed is synergistic against bladder cancer cells in vitro and that the assessment of the expression of genes involved in gemcitabine uptake and activation might be a possible determinant of bladder cancer response and may represent a new tool for treatment optimization.

Acteoside and its aglycones protect primary cultures of rat cortical cells from glutamate-induced excitotoxicity

We have previously reported that acteoside isolated from the leaves of Callicarpa dichotoma has significant neuroprotective activity against glutamate-induced neurotoxicity in primary cultured rat cortical cells. To determine the essential structural moiety within this phenylethanoid glycoside needed to exert neuroprotective activity, acteoside was hydrolyzed with acid into its aglycones, caffeic acid and 3',4'-dihydroxylphenylethanol. Caffeic acid and 3',4'-dihydroxylphenylethanol also showed significant neuroprotective activities. Acteoside and its aglycones inhibited glutamate-induced intracellular Ca2+ influx resulting in overproduction of nitric oxide and reduced the formation of reactive oxygen species. These compounds preserved the mitochondrial membrane potential and the activities of antioxidative enzymes, such as superoxide dismutase, glutathione reductase and glutathione peroxidase reduced by glutamate. It was followed by the preservation of the level of glutathione and finally the inhibition of membrane lipid peroxidation.

Dopaminergic control of corticostriatal long-term synaptic depression in medium spiny neurons is mediated by cholinergic interneurons

Long-term depression (LTD) of the synapse formed between cortical pyramidal neurons and striatal medium spiny neurons is central to many theories of motor plasticity and associative learning. The induction of LTD at this synapse is thought to depend upon D(2) dopamine receptors localized in the postsynaptic membrane. If this were true, LTD should be inducible in neurons from only one of the two projection systems of the striatum. Using transgenic mice in which neurons that contribute to these two systems are labeled, we show that this is not the case. Rather, in both cell types, the D(2) receptor dependence of LTD induction reflects the need to lower M(1) muscarinic receptor activity-a goal accomplished by D(2) receptors on cholinergic interneurons. In addition to reconciling discordant tracts of the striatal literature, these findings point to cholinergic interneurons as key mediators of dopamine-dependent striatal plasticity and learning.

Effects of amino acid substitutions in transmembrane domains of the NR1 subunit on the ethanol inhibition of recombinant N-methyl-D-aspartate receptors

BACKGROUND

The N-methyl-D-aspartate (NMDA) subtype of glutamate receptor is involved in a variety of processes that regulate neuronal plasticity and is an important target for the acute and chronic effects of ethanol. However, the specific sites where ethanol interacts with the receptor protein have yet to be fully elucidated. We previously demonstrated that a phenylalanine to alanine mutation in the third transmembrane domain (TM3) of the NR1 subunit decreased the ethanol inhibition of NMDA receptors expressed in HEK293 cells. In this study, we characterized the ethanol inhibition of NMDA receptors containing additional mutations within the TM3 and TM4 domains of the NR1 subunit.

METHODS

Site-directed mutagenesis was used to alter specific amino acid residues in the TM3 and TM4 domains of the NR1 subunit. Mutant NR1 subunits were coexpressed with the NR2A subunit in HEK293 cells and examined for alterations in ethanol sensitivity using whole-cell voltage-clamp electrophysiology.

RESULTS

Replacing phenylalanine at TM3 position 639 in the NR1 subunit (F639) with 9 different amino acids produced functional receptors when coexpressed with the NR2A subunit. All mutants showed a concentration-dependent inhibition by ethanol (10-100 mM), with the alanine and serine mutants being significantly less sensitive to ethanol. Amino acid substitutions at the F639 site also produced variable changes in the concentration-response relationship to glycine. However, no significant correlation between glycine EC(50) values and the magnitude of ethanol inhibition was observed. Alanine mutations at TM4 positions 813 (M813A) and 819 (L819A), but not at 817 (F817A), of the NR1 subunit enhanced ethanol inhibition. Substitution of tryptophan for TM4 residues in the NR1 subunit (positions 820-822) that are homologous to a site in the NR2A subunit shown to reduce ethanol inhibition (A825W) had no effect on ethanol sensitivity. However, these NR1 TM4 tryptophan mutants restored the ethanol inhibition of the NR1 TM3 F639A mutant to wild-type levels in a stepwise fashion.

CONCLUSIONS

These results indicate that the ethanol sensitivity of NMDA receptors may be regulated by discrete sites within the TM3 and TM4 domains of the NR1 subunit.

Differential effects of classical neuroleptics and a newer generation antipsychotics on the MK-801 induced behavioural primitivization in mouse

Cognitive dysfunction plays an important role in mental disorders like schizophrenia and may involve inadequate glutamatergic signalling in different regions of the brain, mediated by e.g. glutamatergic N-methyl-D-aspartate (NMDA) receptors. In rodents, NMDA receptor antagonists often increase motor activity; in addition they induce a more primitive and undifferentiated behavioural pattern, which we believe may correspond to some of the cognitive defects seen in schizophrenia. In the present study, the movement pattern of mice treated with the uncompetitive NMDA receptor antagonist MK-801 in conjunction with six antipsychotic agents, some with reported clinical effects on cognition, was characterised and quantified. The classical neuroleptic drugs chlorpromazine and trifluoperazine, the atypical antipsychotic agents ziprasidone and olanzapine, the gamma-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)-receptor potentiator CX516 and the serotonin (5-HT)2A-antagonist M100907 were tested. In accordance with previous observations, MK-801 was found to induce a primitive and monotonous behavioural pattern dominated by forward locomotion; spatial movements, the number of switches between the states moving and stationary, and rearing frequency were reduced. All test substances counteracted MK-801-induced hyperactivity, but differed in their ability to improve behavioural quality. Chlorpromazine and trifluoperazine were unable to restore behavioural diversity while ziprasidone, olanzapine, CX516 and M100907 restored it to varying degrees. A striking similarity in movement pattern was seen between the hypoglutamatergic mice treated with the AMPA-receptor agonist CX516, and those receiving the 5HT2A-antagonist M100907.

Activation of the caspase 8 pathway mediates seizure-induced cell death in cultured hippocampal neurons

In response to harmful stresses, cells induce programmed cell death (PCD) or apoptosis. Seizures can induce neural damage and activate biochemical pathways associated with PCD. Since seizures trigger intracellular calcium overload, it has been presumed that the intrinsic cell death pathway mediated by mitochondrial dysfunction would modulate cell death following seizures. However, previous work suggests that the extrinsic cell death pathway may initiate the damage program. Here we investigate intrinsic versus extrinsic cell death pathway activation using caspase cleavage as a marker for activation of these pathways in a rat in vitro model of seizures. Hippocampal cells, chronically treated with kynurenic acid, had kynurenic acid withdrawn to induce seizure-like activity for 40 min. Subjecting rat hippocampal cultures to seizures increased cell death and apoptosis-like DNA fragmentation using TUNEL staining. Seizure-induced cell death was blocked by both MK801 (10 microM) and CNQX (40 microM), which suggests multiple glutamate receptors regulate seizure-induced cell death. Cleavage of the initiator caspases, caspase 8 and 12 were increased 4h following seizure, and cleavage of the quintessential executioner caspase, caspase 3 was increased 4h following seizure. In contrast, caspase 9 cleavage only increased 24h following seizure. Using an affinity labeling approach to trap activated caspases in situ, we show that caspase 8 is the apical caspase activated following seizures. Finally, we show that the caspase 8 inhibitor Ac-IETD-CHO was more effective at blocking seizure-induced cell death than the caspase 9 inhibitor Ac-LEHD-CHO. Taken together, our data suggests the extrinsic cell death pathway-associated caspase 8 is activated following seizures in vitro.

Mechanisms involved in the nociception produced by peripheral protein kinase c activation in mice

Protein kinase C (PKC) is able to phosphorylate several cellular components that serve as key regulatory components in signal transduction pathways of nociceptor excitation and sensitisation. Therefore, the present study attempted to assess some of the mechanisms involved in the overt nociception elicited by peripheral administration of the PKC activator, phorbol 12-myristate 13-acetate (PMA), in mice. The intraplantar (i.pl.) injection of PMA (16-1600 pmol/paw), but not its inactive analogue alpha-PMA, produced a long-lasting overt nociception (up to 45 min), as well as the activation of PKCalpha and PKCepsilon isoforms in treated paws. Indeed, the local administration of the PKC inhibitor GF109203X completely blocked PMA-induced nociception. The blockade of NK1, CGRP, NMDA, beta1-adrenergic, B2 or TRPV1 receptors with selective antagonists partially decreased PMA-induced nociception. Similarly, COX-1, COX-2, MEK or p38 MAP kinase inhibitors reduced the nociceptive effect produced by PMA. Notably, the nociceptive effect promoted by PMA was diminished in animals treated with an antagonist of IL-1beta receptor or with antibodies against TNFalpha, NGF or BDNF, but not against GDNF. Finally, mast cells as well as capsaicin-sensitive and sympathetic fibres, but not neutrophil influx, mediated the nociceptive effect produced by PMA. Collectively, the results of the present study have shown that PMA injection into the mouse paw results in PKC activation as well as a relatively delayed, but long-lasting, overt nociceptive behaviour in mice. Moreover, these results demonstrate that PKC activation exerts a critical role in modulating the excitability of sensory neurons.

Endogenous activation of mGlu5 metabotropic glutamate receptors supports self-renewal of cultured mouse embryonic stem cells

Cultured mouse embryonic stem (ES) cells maintained under undifferentiated conditions (i.e. grown in medium containing 15% FCS and leukemia inhibitory factor--LIF) expressed mGlu5 metabotropic glutamate receptors. Activation of these receptors with quisqualate increased [Ca2+]i but only when cultures were deprived of extracellular glutamate, indicating that the receptor was saturated by the endogenous glutamate. Pharmacological blockade of mGlu5 receptors with 2-methyl-6-(phenylethynyl)pyridine (MPEP) or antisense-induced knock-down of mGlu5 receptors decreased the expression of the two main transcription factors that sustain ES cell self-renewal, i.e. Oct-4 and Nanog, as assessed by real-time PCR and immunoblotting. Exposure of ES cell cultures to MPEP also reduced alkaline phosphatase activity, a marker of undifferentiated ES cells. These data support a critical role for mGlu receptors in early development showing that mGlu5 receptors are expressed by ES cells and their activation sustains ES cell self-renewal in culture.

Differential effects of 7-OH-DPAT and apomorphine on hyperactivity induced by MK-801 (dizocilpine) in rats

Recent experiments from this laboratory demonstrated synergistic locomotor depressant effects of AMPA/kainate receptor blockade and D(2/3) dopamine (DA) receptor stimulation. This study explored functional interactions between DA and glutamate (Glu) systems using the NMDA receptor antagonist MK-801 and the DA receptor agonists 7-OH-DPAT and apomorphine. Using photocell locomotor activity boxes, systemic effects of MK-801 in combination with 7-OH-DPAT (0.03 mgkg(-1) SC, n=8) or a pre-synaptically effective dose of apomorphine (0.05 mgkg(-1) SC, n=6) were measured in male Sprague-Dawley rats. Effects of bilateral applications of MK-801 and 7-OH-DPAT into the nucleus accumbens (NAS) shell subregion were also investigated (n=7). When given alone, MK-801 (0.13 mgkg(-1) or 0.66 microg intra-NAS shell) increased horizontal locomotor activity, while 7-OH-DPAT (0.03 mgkg(-1)) or apomorphine (0.05 mgkg(-1)) decreased this measure. Co-administration of 7-OH-DPAT (systemically or into the NAS shell) completely blocked MK-801 induced hyperactivity. In contrast, MK-801 and apomorphine demonstrated additive effects. Stimulation of D(3) DA receptors may therefore block the hyperactivity induced by NMDA receptor antagonism, and the NAS shell is an important site for this interaction. The differential effects of the DA agonists on hyperactivity induced by NMDA receptor blockade support the proposal that 7-OH-DPAT may induce hypoactivity by stimulation of postsynaptic D(3) DA receptors.

Nicotine normalizes increased prefrontal cortical dopamine D1 receptor binding and decreased working memory performance produced by repeated pretreatment with MK-801: a PET study in conscious monkeys

The effects of acute nicotine were determined on dopamine (DA) D(1) (D(1)R) and D(2) (D(2)R) receptor binding in the neocortex of conscious monkeys under control conditions as well as after chronic pretreatment with MK-801 (dizocilpine), a noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist. Extrastriatal neocortical D(1)R and D(2)R binding was evaluated with [(11)C]NNC112 and [(11)C]FLB457 with high-specific radioactivity using positron emission tomography (PET). Acute administration of nicotine bitartrate, given as an intravenous (i.v.) bolus plus infusion for 30 min at doses of 32 microg/kg+0.8 microg/kg/min or 100 microg/kg+2.53 microg/kg/min as base, induced slight but significant dose-dependent increases of DA in the extracellular fluid of prefrontal cortex (PFC) as determined by microdialysis. However, acute nicotine did not affect either [(11)C]NNC112 or [(11)C]FLB457 binding to D(1)R or D(2)R, respectively, in any cortical region. Chronic MK-801 (0.03 mg/kg, intramuscularly (i.m.), twice daily for 13 days) increased [(11)C]NNC112 binding to D(1)R in PFC. No significant changes were detected in [(11)C]FLB457 binding to PFC D(2)R. Although chronic MK-801 lowered baseline DA and glutamate levels in PFC, acute nicotine normalized reduced DA to control levels. Acute nicotine dose-dependently normalized the increased binding of [(11)C]NNC112 to D(1)R produced by chronic MK-801 but [(11)C]FLB457 binding to PFC D(2)R did not change. Working memory performance, impaired after chronic MK-801, was partially improved by acute nicotine. These results demonstrate that acute nicotine normalizes MK-801-induced PFC abnormality of D(1)R in PFC.

Effects of ketamine on prefrontal and striatal regions in an overt verbal fluency task: a functional magnetic resonance imaging study

RATIONALE

Glutamatergic dysfunction at N-methyl-D: -aspartate (NMDA) receptors has been proposed as a neurochemical model for schizophrenia. A key feature of this disorder is impairments in cognitive function.

OBJECTIVE

The present study sought to investigate the effects of ketamine, an NMDA antagonist, on the performance and neural correlates of verbal fluency, a task that engages executive function.

METHODS

Ten healthy dextral male volunteers received intravenous placebo normal saline or ketamine (bolus of 0.23 mg/kg and infusion of 0.65 mg/kg), administered in a double-blind, randomized order, during two functional magnetic resonance imaging sessions. During scanning, subjects performed a verbal fluency task. Two levels of cognitive load were examined in the task, and overt responses were acquired in order to measure subject performance on-line.

RESULTS

Ketamine induced symptoms in the healthy individuals comparable to an acute psychotic state. Although ketamine did not significantly impair task performance relative to placebo, an interaction of task demand with ketamine was observed in the anterior cingulate, prefrontal, and striatal regions.

CONCLUSIONS

The behavioural and functional effects of ketamine during verbal fluency in healthy individuals were comparable to those evident in patients with schizophrenia. The findings support a role for glutamatergic dysfunction in the pathophysiology of schizophrenia.

Antioxidant Treatment Inhibited Glutamate-Evoked NF-κB Activation in Primary Astroglial Cell Cultures

In glial cells, glutamate exposure causes alterations in cell redox status, mainly mediated by glutathione depletion and reactive oxygen species generation. These effects finally lead to astrocyte dysfunction which contributes to the pathogenesis of several neurological disorders. This study was aimed to investigate the involvement of the NF-kappaB pathway in oxidative stress induced by glutamate exposure in primary cultures of astrocytes. Further, we evaluated the power of the antioxidants genistein (0.1-10 microM) and IRFI 016 (20-80 microM), a synthetic tocopherol analogue, compared with glutathione ethyl ester (10-50 microM) and cysteamine-HCl (100-500 microM), to antagonize the effects elicited by glutamate (500 microM). Alterations of cell redox status were reduced, in a dose-dependent way, by antioxidants; in particular, 80 microM IRFI 016 and 10 microM genistein almost completely restored glutathione basal levels and significantly diminished ROS production, as well as 100 microM glutathione ethyl ester. These antioxidant effects were stronger than those caused by 500 microM cysteamine-HCl. Further, glutamate promoted the up-regulation of p50 and p65 NF-kappaB subunits and their nuclear translocation, as revealed by Western blot analysis and electrophoretic mobility shift assay of both subunits. The activation of p50 and p65 NF-kappaB subunits induced by glutamate exposure was significantly reduced by IRFI 016, acting in a dose-dependent manner. Altogether, these data confirm that the NF-kappaB pathway is involved in cell response to oxidative stress induced by glutamate injury in primary astrocyte cultures, and suggest that the use of antioxidants, such as IRFI 016, may be a helpful pharmacological strategy for neuroprotection.

The wake-promoting peptide orexin-B inhibits glutamatergic transmission to dorsal raphe nucleus serotonin neurons through retrograde endocannabinoid signaling

The wake-promoting neuropeptides orexins (hypocretins) play a crucial role in controlling neuronal excitability and synaptic transmission in the CNS. In this study, using whole-cell patch-clamp recordings in an acute dorsal raphe nucleus (DRN) slice preparation, we report that orexin B (Orx-B) depresses the evoked glutamate-mediated synaptic currents in DRN 5-HT neurons. The Orx-B-induced depression is accompanied by an increase in the paired-pulse ratio and the coefficient of variance, suggesting a presynaptic site of action. Orx-B also reduces the frequency but not the amplitude of miniature EPSCs, indicating that depression of glutamatergic transmission is mediated by a decrease in glutamate release. Surprisingly, the Orx-B-induced inhibition of glutamatergic transmission is abolished by postsynaptic inhibition of G-protein signaling with GDPbetaS, suggesting that this effect is signaled by postsynaptic orexin receptors and expressed presynaptically, presumably through a retrograde messenger. Interestingly, the Orx-B-induced depression of glutamate release is mimicked and occluded by the cannabinoid receptor agonist WIN 55,212-2, and is abolished by the CB1 cannabinoid receptor antagonist AM 251. These results imply that the Orx-B-induced depression of glutamatergic transmission to DRN 5-HT neurons is mediated by retrograde endocannabinoid release. Examination of downstream signaling pathways involved in this response indicates that the effect of Orx-B requires the activation of phospholipase C and DAG lipase enzymatic pathways but not a rise in postsynaptic intracellular calcium. Therefore, our findings reveal a previously unsuspected mechanism by which postsynaptic orexin receptors can modulate glutamatergic synaptic transmission to DRN 5-HT neurons.