Capsaicin protects cortical neurons against ischemia/reperfusion injury via down-regulating NMDA receptors

Capsaicin, the ingredient responsible for the pungent taste of hot chili peppers, is widely used in the study and management of pain. Recently, its neuroprotective effect has been described in multiple studies. Herein, we investigated the underlying mechanisms for the neuroprotective effect of capsaicin. Direct injection of capsaicin (1 or 3nmol) into the peri-infarct area reduced the infarct volume and improved neurological behavioral scoring and motor coordination function in the middle cerebral artery occlusion (MCAO)/reperfusion model in rats. The time window of the protective effect of capsaicin was within 1h after reperfusion, when excitotoxicity is the main reason of cell death. In cultured cortical neurons, administration of capsaicin attenuated glutamate-induced excitotoxic injury. With respect to the mechanisms of the neuroprotective effect of capsaicin, reduced calcium influx after glutamate stimulation was observed following capsaicin pretreatment in cortical neurons. Trpv1 knock-out abolished the inhibitory effect of capsaicin on glutamate-induced calcium influx and subsequent neuronal death. Reduced expression of GluN1 and GluN2B, subunits of NMDA receptor, was examined after capsaicin treatment in cortical neurons. In summary, our studies reveal that the neuroprotective effect of capsaicin in cortical neurons is TRPV1-dependent and down-regulation of the expression and function of NMDA receptors contributes to the protection afforded by capsaicin.

Anti-apoptotic effect of esculin on dopamine-induced cytotoxicity in the human neuroblastoma SH-SY5Y cell line

Dopamine (DA), as a neurotoxin, can elicit severe Parkinson's disease-like syndrome by elevating intracellular reactive oxygen species (ROS) levels and apoptotic activity. In this study, we examined the effect of esculin, which was extracted from Fraxinus sielboldiana blume, on DA-induced cytotoxicity and the underlying mechanism in human neuroblastoma SH-SY5Y cells. Our results suggest that the protective effects of esculin (10(-7), 10(-6) and 10(-5) M) on DA-induced cytotoxicity may be ascribed to its anti-oxidative properties by reducing ROS level, and its anti-apoptotic effect via protecting mitochondrion membrane potential (DeltaPsim), enhancing superoxide dismutaese (SOD) activity and reduced glutathione (GSH) levels, and regulating P53, Bax and Bcl-2 expression. In addition, esculin inhibited the release of cytochrome c and apoptosis-inducing factor (AIF), and the protein expression of activated caspase 3. These data indicate that esculin may provide a useful therapeutic strategy for the treatment of progressive neurodegenerative diseases such as Parkinson's disease (PD).

Cannabinoids inhibit sodium-dependent, high-affinity excitatory amino acid transport in cultured rat cortical astrocytes

Cannabinoids have been shown to increase the extracellular levels of glutamate in vivo and in vitro, but no studies have evaluated the possible involvement of glial glutamate reuptake system. The present study investigates whether cannabinoids and endocannabinoid, anandamide have an effect on astroglial excitatory amino acid (EAA) transport. The kinetics of glutamate transport was studied in rat cortical astrocytes, using the radiolabeled, non-metabolized amino acid, D-[3H] aspartate in the absence or presence of cannabinoid receptor agonists. The results show that in vehicle controls the uptake of d-aspartate was rapid, sodium-dependent and saturated within the first 5 min, resulting in a K(m) 7.365+/-1.16 micromol/L (n=5) and the maximum velocity (V(max)) 1207+/-51 nmol/mg protein/min. Addition of the synthetic cannabinoid analog R(+)-[2,3-dihydro-5-methyl-3-[(morpholinyl)methyl]pyrrolol][1,2,3de]-1,4-benzoxazinyl]-(1-naphthalenyl)methanone (WIN 55,212-2; 3 micromol/L) increased the K(m) (26.25+/-4.84 micromol/L) without affecting the V(max) (1122+/-77 nmol/mg protein/min), suggesting the inhibition was competitive and reversible. Various other cannabinoid agonists also inhibited D-aspartate uptake in a dose-dependent and stereospecific manner. The cannabinoid inhibition of EAA transport was partially blocked by the cannabinoid type-1 (CB1) receptor antagonist N-(piperidin-1-yl-5(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamidehydrochloride (SR141716A; 100 nmol/L). The inhibitory effects of WIN 55,212-2, or its endogenous counterpart anandamide were reversed by 98,059, an inhibitor of mitogen-activated kinase (MAPK) kinase (MEK). These results suggest that cannabinoids and endocannabinoids may constitute a novel class of inhibitors of astroglial glutamate transport system.

Inhibition of release of taurine and excitatory amino acids in ischemia and neuroprotection

Volume regulated anion channels (VRAC) have been extensively studied in purified single cell systems like cell cultures where they can be activated by cell swelling. This provides a convenient way of analyzing mechanisms and will likely lead to the holy grails of the field, namely the nature or natures of the volume sensor and the nature or natures of VRACs. Important reasons for such an understanding are that these channels are ubiquitous and have important physiological functions which under pathological conditions convert to deleterious effects. Here we summarize data showing the involvement of VRACs in ischemia-induced release of excitatory amino acids (EAAs) in a rat model of global ischemia. Using microdialysis studies we found that reversal of the astrocytic glutamate transporter and VRACs contribute about equally to the large initial release of EAAs and together account for around 80% of the total release. We used the very potent VRAC blocker, tamoxifen, to see if such inhibition of EAA release via VRACs led to significant neuroprotection. Treatment in the focal rat MCA occlusion model led to around 80% reduction in infarct size with an effective post initiation of ischemia therapeutic window of three hours. However, the common problem of other effects for even the most potent inhibitors pertains here, as tamoxifen has other, potentially neuroprotective, effects. Thus it inhibits nitrotyrosine formation, likely due to its inhibition of nNOS and reduction of peroxynitrite formation. Although tamoxifen cannot therefore be used as a test of the "VRAC-excitotxicity" hypothesis it may prove successful for translation of basic stroke research to the clinic because of its multiple targets.

[Research progress of neuroprotective mechanisms of Gastrodia elata and its preparation]

The effects of Gastrodia elata on preventing decrepitude and advancing memory are closely associated with its neuroprotective activity. Previous researches proved that G. elata, its active components and preparations played a neuroprotective role by affecting the excitotoxicity, nitric monoxide (NO) system, neuroglia, biomembrane, oxidative neurotoxicity, apoptosis et al. Recent researches also suggest that reducing energy metabolism impairment, anti-inflammatory and immune modulating function may be new research targets of neuroprotective mechanism of G. elata.

[Neuroprotection in brain ischemia--doubts and hopes]

In ischaemic stroke the two major potential therapeutic strategies are aimed at either improving cerebral blood flow or directly interacting with the cytotoxic cascade--a large body of evidence gained from animal studies is in support of them. In clinical trials direct neuroprotection by blocking the neurotoxic cascade remained ineffective, although there are several clinical trials still in progress. We summarize the experimental data and present the results of clinical trials and also discuss why so many drugs, which were effective in animal studies, failed in human trials. It is emphasized, that 1. in most animal studies the reduction of infarct size, i.e. the amount of saved penumbral tissue, was the outcome measure, whereas neurological function remained unassessed; 2. the recovery of intellectual performance and higher cortical functions are of major importance in the future quality of life in stroke victims; however, it is impossible to examine these parameters appropriately in animal studies; 3. in many clinical trials the patient population was rather heterogenous and low in number, the study protocol was not optimal and the critical analysis of the subacute and chronic phase was lacking or insufficient. We present the major experimental stroke models, discuss their similarities, differences and limitations as compared to the human pathophysiological processes. The pitfalls of extrapolating data from animal studies to clinical practice are also summarized. The complex network of functional and morphological intercellular connections, the long timescale of neurotoxic and reparative events and the lessons learned from clinical trials suggest, that the use of drug combinations (therapeutic cocktails) targeting multiple steps of the neurotoxic cascade would hopefully result in more effective treatment of ischaemic stroke. Strategies to facilitate brain plasticity and regeneration is an additional promising tool to enhance recovery in brain ischaemia.

"In vivo" monitoring of neuronal network activity in zebrafish by two-photon Ca(2+) imaging

The zebrafish larva is a powerful model for the analysis of behaviour and the underlying neuronal network activity during early stages of development. Here we employ a new approach of "in vivo" Ca(2+) imaging in this preparation. We demonstrate that bolus injection of membrane-permeable Ca(2+) indicator dyes into the spinal cord of zebrafish larvae results in rapid staining of essentially the entire spinal cord. Using two-photon imaging, we could monitor Ca(2+) signals simultaneously from a large population of spinal neurons with single-cell resolution. To test the method, Ca(2+) transients were produced by iontophoretic application of glutamate and, as observed for the first time in a living preparation, of GABA or glycine. Glycine-evoked Ca(2+) transients were blocked by the application of strychnine. Sensory stimuli that trigger escape reflexes in mobile zebrafish evoked Ca(2+) transients in distinct neurons of the spinal network. Moreover, long-term recordings revealed spontaneous Ca(2+) transients in individual spinal neurons. Frequently, this activity occurred synchronously among many neurons in the network. In conclusion, the new approach permits a reliable analysis with single-cell resolution of the functional organisation of developing neuronal networks.

Emerging molecular targets in the treatment of bacterial meningitis

The mortality and morbidity associated with bacterial meningitis have remained significant despite advances in antimicrobial chemotherapy and supportive care. A major contributing factor to this high mortality and morbidity is our incomplete understanding of the pathogenesis of this disease and its associated neurological sequelae. Most cases of bacterial meningitis develop as a result of haematogenous spread, but it is unclear how circulating bacteria cross the blood-brain barrier. Experimental animal studies indicate that two forms of neuronal injury, such as necrotic cortical injury and apoptotic hippocampal injury, are predominant in bacterial meningitis, but the mechanisms by which these two forms of injury occur are unclear. Recent studies have identified several bacteria-host determinants for bacterial translocation of the blood-brain barrier, and several host inflammatory markers that are associated with neuronal injury in animal models of experimental bacterial meningitis. These determinants/markers may provide important targets for the prevention and treatment of bacterial meningitis. This review focuses on representative steps in the pathogenesis of bacterial meningitis that are likely to be key targets in coming years, and summarises the status of current knowledge for each target.

Effects of acamprosate on excitatory amino acids during multiple ethanol withdrawal periods

BACKGROUND

Our previous studies on the effects of acamprosate on enhanced locomotion during repeated withdrawals are now extended to the effects of acamprosate on excitatory amino acids in the hippocampus during repeated ethanol withdrawals.

METHODS

In this study, Wistar rats were made ethanol dependent by 4 weeks of vapor inhalation. After this first cycle of chronic ethanol treatment, rats underwent repeated and alternate cycles of 24 hr withdrawals and 1 week of chronic ethanol treatment. The microdialysis technique was used together with high-performance liquid chromatography and electrochemical detection to quantify different amino acids such as aspartate and glutamate.

RESULTS

An intraperitoneal administration of acamprosate (400 mg/kg) to naïve rats did not alter aspartate or glutamate levels compared with the saline groups. During the first cycle of ethanol withdrawal, the administration of acamprosate (400 mg/kg, intraperitoneally) 2 hr after the commencement of ethanol withdrawal decreased both aspartate and glutamate microdialysate levels when compared with their respective saline group. Acamprosate administration also significantly decreased glutamate levels during the third withdrawal compared with the saline group, whereas no changes were seen in aspartate levels.

CONCLUSION

The results of this work demonstrate that acamprosate reduced the excitatory amino acid glutamate increase observed during repeated ethanol withdrawal. These effects of acamprosate may provide a protective mechanism against neurotoxicity by reducing excitatory amino acids, particularly glutamate.

Neuroprotective agents in acute ischemic stroke

The concept of neuroprotection relies on the principle that delayed neuronal injury occurs after ischemia. The phenomenon of the "ischemic cascade" has been described, and each step along this cascade provides a target for therapeutic intervention. A wide variety of drugs have been studied in humans. Ten classes of neuroprotective agents have reached phase III efficacy trials but have shown mixed results. They included calcium channel antagonists, NMDA receptor antagonists, lubeluzole, CDP-choline, the free radical scavenger tirilazad and ebselen, enlimomab, GABA agonist clomethiazole, the sodium channel antagonist fosphenytoin, magnesium, glycine site antagonist GV150526 and piracetam. Furthermore, the mechanisms that underlie the development of focal ischemic injury continue to be discovered, opening new therapeutic perspective for neuroprotection that might clinically be applicable in the future.

Neuronal injury in bacterial meningitis: mechanisms and implications for therapy

In bacterial meningitis, long-term neurological sequelae and death are caused jointly by several factors: (1) the systemic inflammatory response of the host, leading to leukocyte extravasation into the subarachnoid space, vasculitis, brain edema and secondary ischemia; (2) stimulation of resident microglia within the CNS by bacterial compounds; and (3) possible direct toxicity of bacterial compounds on neurons. Neuronal injury is mediated by the release of reactive oxygen intermediates, proteases, cytokines and excitatory amino acids, and is executed by the activation of transcription factors, caspases and other proteases. In experimental meningitis, dexamethasone as an adjunct to antibiotic treatment leads to an aggravation of neuronal damage in the hippocampal formation, suggesting that corticosteroids might not be the ideal adjunctive therapy. Several approaches that interfere selectively with the mechanisms of neuronal injury are effective in animal models, including the use of nonbacteriolytic protein synthesis-inhibiting antibiotics, antioxidants and inhibitors of transcription factors, matrix metalloproteinases, and caspases.

Neuromuscular blocking agents in neurointensive care

INTRODUCTION

Intensive care treatment of patients with severe head injury is aimed at preventing secondary injury. One of the cornerstones in this treatment is sedation and ventilation. Use of Neuromuscular Blocking Agents (NBA) has gained widespread use as part of the protocol for maintaining normal intracranial pressure values, without class 1 evidence for the efficacy of the treatment.

METHODS

We examined data of the use of NBA as infusion during ventilator treatment, and IntraCranial Pressure (ICP) measurements in the database from the international multicenter randomized double blind trial of the NMDA receptor antagonist Selfotel. No specific mode of sedation was recommended in the study protocol.

RESULTS

Of the 427 patients enrolled in the study 326 had a full data set, 138 received NBA during their stay in the ICU. There were no statistical difference in demographic data between the two groups. During their stay in the ICU, patients who received NBA had a median of 13.5 hours with a recorded ICP above 20 mm Hg, patients who did not receive NBA had a median of 6.5 hours with ICP above 20 mm Hg (p < 0.05).

CONCLUSION

Our data challenges the concept of using NBA as part of a routine sedation strategy in treatment of patients with severe head injury.

The role of MK-801 in sensitization to stimulants

Behavioral responses to stimulants can be progressively and persistently enhanced by their repeated administration. This phenomenon, called behavioral sensitization, may underlie substance abuse, psychosis, recurrence in bipolar disorder, or other psychiatric problems. A growing body of work has implicated excitatory amino acid systems in behavioral sensitization. Most of the evidence for a role of excitatory amino acids has come from experiments demonstrating prevention of sensitization by excitatory amino acid antagonists, especially the noncompetitive NMDA receptor antagonist MK-801. Results of studies with MK-801 have varied, however, leading to conflicting interpretations of its relationship to behavioral sensitization. This paper critically discusses the design of experiments that have used MK-801, and interprets data from these experiments in terms of the two leading explanations for the role of MK-801: 1) that sensitization is an example of the family of plastic events that require excitatory amino acid transmission or 2) that interoceptive cues associated with MK-801 lead to state-dependent learning that modifies sensitization because, in essence, the animal does not recognize the stimulant as the same drug if it is given in close association with MK-801. Based on conflicting reports on effects of MK-801, we propose 1) strategies for distinguishing components of MK-801's effects on responses to stimulants, 2) a model that is a hybrid of the two interpretations of its effects on sensitization, and 3) experimental strategies for testing this model.

Inhibitory effect of fangchinoline on excitatory amino acids-induced neurotoxicity in cultured rat cerebellar granule cells

Glutamate receptors-mediated excitotoxicity is believed to play a role in the pathophysiology of neurodegenerative diseases. The present study was performed to evaluate the inhibitory effect of fangchinoline, a bis-benzylisoquinoline alkaloid, which has a characteristic as a Ca2+ channel blocker, on excitatory amino acids (EAAs)-induced neurotoxicity in cultured rat cerebellar granule neuron. Fangchinoline (1 and 5 microM) inhibited glutamate (1 mM), N-methyl-D-aspartate (NMDA; 1 mM) and kainate (100 microM)-induced neuronal cell death which was measured by trypan blue exclusion test. Fangchinoline (1 and 5 microM) inhibited glutamate release into medium induced by NMDA (1 mM) and kainate (100 microM), which was measured by HPLC. And fangchinoline (5 microM) inhibited glutamate (1 mM)-induced elevation of intracellular calcium concentration. These results suggest that inhibition of Ca2+ influx by fangchinoline may contribute to the beneficial effects on neurodegenerative effect of glutamate in pathophysiological conditions.

[Medicinal and chemical approaches to focused search of agents for treatment and therapy of Alzheimer disease]

The analysis and justification of medicinal chemistry approaches for focused search of novel agents for Alzheimer's disease (AD) and related disorders treatment and prevention have been reviewed. The systematization of modern biochemical and structural date related to the action of physiologically active compounds on the nervous system apparatus engaged in the AD-like disorders pathogenesis was performed. The major attention was paid to the cholinomimetic, anti-amyloid and antimetabolic approaches, basing on the results published in scientific literature in 3-4 last years and results of preclinical and clinical trials, presented in the internet database in the fall of 2000.

Tonic excitatory input to the rostral ventrolateral medulla in Dahl salt-sensitive rats

The goal of the present study was to test the hypothesis that the balance of tonic excitation and inhibition of vasomotor neurons in the rostral ventrolateral medulla (RVLM) driven by excitatory amino acid (EAA)-mediated inputs to the RVLM is shifted toward excitation in Dahl salt-sensitive (DS) rats compared with Dahl salt-resistant (DR) rats. Glutamate and the EAA antagonist kynurenic acid were microinjected into the RVLM of chloralose-anesthetized DS and DR rats maintained on diets containing either 0.3% NaCl or 8.0% NaCl. DS rats had a higher arterial pressure than DR rats, and this difference was greatly exaggerated by high dietary salt intake. Bilateral injection of kynurenic acid (2.7 nmol) into the RVLM decreased mean arterial pressure by 16+/-2 mm Hg in DS rats fed a diet containing 0.3% NaCl, and this effect was significantly larger in DS rats fed the high-salt diet (40+/-2 mm Hg). In contrast, injections of kynurenic acid into the RVLM did not significantly decrease arterial pressure in DR rats fed either diet. In DR rats, the pressor response elicited by the injection of glutamate into the RVLM was potentiated in rats fed the high-salt diet. The glutamate-evoked pressor response was greater in DS rats compared with DR rats, and the response in DS rats was not influenced by the salt content of the diet. These data suggest that tonically active EAA inputs to the RVLM may contribute to salt-sensitive hypertension in the Dahl model.

Effects of excitatory amino acids and nimodipine on calcium currents in cultured rat cortical neurons

AIM

To study the effect of excitatory amino acid (EAA) and calcium channel blocker on neuronal calcium channels.

METHODS

With path-clamp technique (whole-cell recording), the effects of Bay-K-8644, cesium glutamate, potassium aspartate, and nimodipine (Nim) on calcium currents (ICa) in cultured cortical neurons of neonatal rats were studied.

RESULTS

ICa was raised obviously by Bay-K-8644 and glutamate. ICa was raised concentration-dependently by aspartate (0.5, 5, 50 mmol.L-1), with increasing rates 15% +/- 3%, 37% +/- 3%, and 53% +/- 6%, respectively. The inhibition of ICa was obvious while adding Nim in the bath solution. With Nim 10 mumol.L-1, the inhibitory rate was 46% +/- 4%.

CONCLUSION

EAA had increasing effects on neuronal calcium currents and Nim inhibited Ca2+ influx in neurons.

Role of zinc in blockade of excitotoxic action of quinolinic acid by picolinic acid

This study examined whether picolinic acid (PIC) inhibits quinolinic acid (QUIN)-induced excitotoxicity through zinc chelation. Injection of QUIN into the nucleus basalis magnocellularis significantly depleted cortical choline acetyltransferase activity 7 days post injection and PIC inhibited this response. Zinc augmented the QUIN- but not NMDA-induced response. When PIC was co-administered with zinc, PIC failed to attenuate the QUIN-induced response. The inhibition of QUIN-induced cholinergic toxicity by PIC may involve chelation of zinc.

Lubeluzole protects hippocampal neurons from excitotoxicity in vitro and reduces brain damage caused by ischemia

Previously reported effects of lubeluzole, such as inhibition of glutamate release, inhibition of nitric oxide (NO) synthesis and blockage of voltage-gated Na+- and Ca2+-ion channels, suggest a neuroprotective action of this drug. Here we report about the effects of lubeluzole and its R-isomer on glutamate-induced neuronal cell death in mixed hippocampal cultures. In addition, we studied the effect of lubeluzole in focal cerebral ischemia models in mice and rats. In hippocampal cultures exposed to 500 nM glutamate for 1 h, lubeluzole (0.1-100 nM), but not the R-isomer (1-100 nM), reduced the percentage of damaged neurons from 42 +/- 8% to 18 +/- 7% (P < 0.01). In mice and rats, lubeluzole reduced ischemic brain damage, when administered immediately after middle cerebral artery occlusion. Interestingly, the protective effect (reduction of the infarct volume in rats to 77% of control; P < 0.01) was also found when the lubeluzole treatment (2.5 mg/kg) was started 3 h after ischemia. Especially this latter effect suggests that lubeluzole will be a useful drug for stroke therapy.

The role of excitatory amino acids in airway reflex responses in anesthetized dogs

In these studies we examined the role of excitatory amino acids (EAAs) neurotransmission in communicating sensory inputs to the airway-related vagal preganglionic neurons, by examining the effects of either NMDA or AMPA/kainate receptor blockade on reflex and chemical responses of tracheal smooth muscle. Experiments were performed in chloralose anesthetized, paralyzed and mechanically ventilated beagle dogs (n = 18), under hyperoxic, normocapnic, and normohydric conditions. Topical application or microinjection of NMDA receptor blockers, into the region of the ventrolateral medulla where airway-related vagal preganglionic neurons are located, insignificantly decreased the reflex changes in tracheal tone. However, topical application or microinjection of AMPA/kainate subtype of glutamate receptor selective antagonists markedly reduced reflex increase in tracheal tone induced by (1) lung deflation, (2) stimulation of laryngeal cold receptors, and (3) activation of peripheral or central chemoreceptors. These effects were potentiated by prior NMDA receptor blockade. Findings indicate that an increase in central cholinergic outflow to the airways by a variety of excitatory afferent inputs is mediated via activation of EAA receptors, mainly AMPA/kainate subtype of glutamate receptors.

Peptide growth factors but not ganglioside protect against excitotoxicity in rat retinal neurons in vitro

Glutamate is the major excitatory neurotransmitter in the retina, but excessive stimulation of its receptors leads to widespread neuronal stress and death. Both growth factors and gangliosides display important influences on responses to neuronal injury and degeneration. In this study, we have investigated the potential protective effects of two well characterized growth factors, epidermal and basic fibroblast growth factor (EGF and bFGF respectively), and the monosialoganglioside GM1, on cultured rat retinal neurons submitted to toxic levels of excitatory amino acids. Application of 1 mM glutamic acid reduced global neuronal viability by 80% when compared to control untreated cultures, whereas treatment with the glutamic acid agonist kainic acid (1 mM) led to specific, large decreases (75% reduction) in amacrine cell numbers. 24 h pretreatment with either EGF or bFGF (500 pM each) prevented the majority of excitatory amino acid-induced neuronal death, whereas similar treatment with 10(-5) M GM1 did not block neuronal degeneration. These findings demonstrate that EGF and bFGF act as neuroprotective agents against retinal excitotoxicity in vitro, whereas ganglioside GM1 is not effective in this particular paradigm.

Role of excitatory amino acid receptors in cardiorespiratory coupling in ventrolateral medulla

The role of (+/-)-alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid (AMPA)-kainate and N-methyl-D-aspartate (NMDA) receptors in the rostral ventrolateral medulla (RVLM) and caudal ventrolateral medulla (CVLM) on the central respiratory drive (CRD)-related activity of splanchnic sympathetic nerve activity (SNA) was examined in rats. SNA increased during inspiration (I peak) and postinspiration (PI peak). Bilateral microinjections of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; AMPA-kainate antagonist) or DL-2-amino-5-phosphonovaleric acid (APV; NMDA antagonist) into RVLM abolished the PI, but not the I, peak. Blockade of all excitatory amino acid receptors in RVLM with kynurenate, or mixtures of APV and CNQX, also failed to eliminate the I peak. Somatosympathetic responses were abolished by CNQX injection into RVLM, but were unaffected by APV. CNQX, but not APV, injection into CVLM increased the PI peak of SNA. Our findings suggest the following. 1) Both NMDA and AMPA-kainate receptors in RVLM are involved in the coupling between the sympathetic nervous system and CRD, which generates the PI peak seen in SNA. 2) The I peak of SNA is independent of excitatory amino acid transmission within RVLM. 3) There are different relative amounts of NMDA and AMPA-kainate receptors at synapses where respiratory and somatic inputs converge onto RVLM neurons. 4) Glutamatergic inputs to CVLM neurons modulate the coupling between SNA and CRD in RVLM.

Serum affects the characteristics of excitatory amino acid-binding sites on Müller cells

The binding of [3H]L-aspartate to membranes obtained from primary cultures of chick retinal Müller cells (glia) was studied Cells seeded in low-serum-containing medium (1%) and maintained in this condition showed an increased number of binding site from 1 to 5 days in vitro (DIV), when compared with controls cultured in medium containing 10% serum; these changes were not reversed by the addition of 10% serum after 48 h in vitro. Increased binding at this age was due to the expression of a low affinity binding system, competitively inhibited by the glutamate uptake blocker L-aspartate-beta-hydroxamate, suggesting that high serum might inhibit the expression of uptake sites at precise maturation stages. Experiments showed the effect was due to a thermolabile serum component. The increase in binding sites is parallel in time to both an increase in aspartate uptake and the initiation of synaptogenesis in the whole retina. Our results suggest that the presence of serum at defined stages in retinal development, could result in the elevation of extracellular glutamate and the concomitant excitotoxic death of neuronal cells, due to a decreased glutamate uptake by glial cells.

Properties of AMPA receptors expressed in rat cerebellar granule cell cultures: Ca2+ influx studies

Cultured cerebellar granule cells become vulnerable to excitatory amino acids, especially to NMDA and kainate, by 9 days in vitro. In the same time, the sensitivity of cells to (RS)-alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate (AMPA), in terms of AMPA-induced toxicity or 45Ca2+ uptake, was very low. The low AMPA responsiveness was due to receptor desensitization, because agents known to block desensitization, cyclothiazide and the lectins concanavalin A and wheat germ agglutinin, rendered granule cells vulnerable to AMPA and produced a pronounced stimulation of 45Ca2+ accumulation. 45Ca2+ influx was induced specifically by AMPA-receptor stimulation, because it was blocked virtually completely by 2,3-dihydroxy-6-nitro-7-sulfamoylbenzoquinoxaline (NBQX) and the benzodiazepine GYKI 52466 (selective non-NMDA receptor antagonists). Nevertheless, indirect routes activated by cellular responses to AMPA-receptor stimulation contributed significantly to the overall 45Ca2+ influx. These included Ca2+ uptake through NMDA-receptor channels, voltage-sensitive Ca2+ channels, and via Na+/Ca2+ exchange. However, nearly one-fifth of the total 45Ca2+ influx remained unaccounted for and this estimate was similar to 45Ca2+ influx observed under Na(+)-free conditions. This observation suggested that a significant proportion of the Ca2+ flux passes through the AMPA-receptor channel proper, a view supported by Co2+ uptake into nearly all granule cells on exposure to AMPA in the presence of cyclothiazide. Results are discussed in light of the reported AMPA receptor-subunit composition of cerebellar granule cells in vitro.

Excitatory amino acid antagonists attenuate the effects of cocaine on extracellular dopamine in the nucleus accumbens

The effect of N-methyl-D-aspartate and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate/kainate excitatory amino acid antagonists on systemic cocaine-induced increases in extracellular dopamine was examined. The alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate/kainate receptor subtype antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), and N-methyl-D-aspartate receptor antagonists, 2-amino-5-phosphonovalerate and dizocilpine, were infused via a microdialysis probe placed in the nucleus accumbens. The local infusion of 2-amino-5-phosphonovalerate (500 microM), dizocilpine (50 microM) and CNQX (100 microM), started 80 min before cocaine injection, significantly inhibited the cocaine-induced increase in extracellular dopamine. The CNQX blockade was dose-dependent with respect to both CNQX concentration infused and dose of cocaine administered. Simultaneous infusion of the two antagonists (500 microM 2-amino-5-phosphonovalerate and 100 microM CNQX) did not lead to further reductions in the effects of cocaine when compared to either antagonist alone. Our results suggest that both N-methyl-Daspartate and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate/kainate subtypes of excitatory amino acid antagonist receptors may contribute to the stimulatory effect of cocaine on extracellular dopamine in the nucleus accumbens.

NMDA receptor blockade increases in vivo striatal dopamine synthesis and release in rats and mice with incomplete, dopamine-depleting, nigrostriatal lesions

The role of excitatory amino acids (EAAs) in the regulation of striatal dopamine (DA) synthesis and release in rats and mice with incomplete DA-depleting lesions of the nigrostriatal system was investigated using in vivo microdialysis in rats and estimates of striatal in vivo tyrosine hydroxylase activity in mice. Dopaminergic nigrostriatal input to the striatum was partially lesioned in rats with stereotactic perinigral 6-hydroxydopamine injections and in C57BL mice with systemic 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) treatment. In rats, addition of the NMDA receptor antagonist 2-amino-5-phosphonopentanoic acid (AP-5, 10 mM) to dialysate increased local striatal DA release and synthesis. In 6-hydroxydopamine partially lesioned rats, the effects of AP-5 on DA release were significantly diminished, and AP-5 had no significant effect on DA synthesis. Perfusion with the non-NMDA receptor antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX, 1 mM) alone increased DA synthesis slightly, whereas DNQX + AP-5 (10 mM) increased DA synthesis to levels comparable with those observed with AP-5 alone. Local striatal DA synthesis was also increased by addition of the NMDA receptor antagonist (+/-)-3-(2-carboxypiperizin-4-yl)propyl-1-phosphonic acid (CPP, 1 mM) to the perfusion buffer. Local inhibition of nitric oxide synthase with nitro-L-arginine (10 mM) perfused through the striatal microdialysis probe did not alter DA synthesis, suggesting that the effects observed with NMDA receptor blockade are not mediated by nitric oxide. In unlesioned mice, none of the systemically injected EAA receptor antagonists altered striatal DA synthesis.(ABSTRACT TRUNCATED AT 250 WORDS)

Osmotic stimulation of vasopressin mRNA content in the supraoptic nucleus requires synaptic activation

The role of synaptic input to the vasopressin neurons in hypertonicity-induced increase in vasopressin mRNA content was evaluated. Synaptic connection with the anterior hypothalamus is required for hypertonicity to increase vasopressin release. However, the potential for other mechanisms to induce the increase in vasopressin mRNA content is suggested by the fact that hypertonicity induces depolarization of supraoptic neurons independently of synaptic input. Explants of the hypothalamoneurohypophysial system were used to study the effect of depolarization and hypertonicity in the presence and absence of nonspecific synaptic blockade by 15 mM MgSO4 or blockade of excitatory amino acid receptors with kynurenic acid. Vasopressin release and mRNA content were increased by depolarization with 40 mM KCl and by exposure to hypertonicity (P < 0.05). Basal and osmotically stimulated vasopressin release was decreased by MgSO4 and by kynurenic acid. Both agents prevented the hypertonicity-induced increase in vasopressin mRNA content. Thus either synaptic input or increased VP release is required for hypertonicity to increase vasopressin mRNA, and excitatory amino acids are implicated in this response.

Effects of some excitatory amino acid antagonists on imipenem-induced seizures in DBA/2 mice

The behavioural and convulsant effects of imipenem (Imi), a carbapenem derivative, were studied after intraperitoneal (i.p.) or intracerebroventricular (i.c.v.) administration in DBA/2 mice, a strain genetically susceptible to sound-induced seizures. The anticonvulsant effects of some excitatory amino acid antagonists and muscimol (Msc), a GABAA agonist, against seizures induced by i.p. or i.c.v. administration of Imi were also evaluated. The present study demonstrated that the order of anticonvulsant activity in our epileptic model, after i.p. administration, was (+)-5-methyl-10,11-dihydro-5H-dibenzo(a,d)-cyclohepten-5,10-imine maleate (MK-801) > (+/-)(E)-2-amino-4-methyl-5-phosphono-3-pentenoate ethyl ester (CGP 39551) > 3-((+/-)-2-carboxypiperazin-4-yl)propenyl-1-phosphonic acid (CPPene) > 3-((+/-)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CCP) > 2,3-dihydroxy-6-nitro-7-sulfamoylbenzo(F)-quinoxaline (NBQX). Ifenprodil, a compound acting on the polyamine site of NMDA receptor complex was unable to protect against seizures induced by Imi, suggesting that the poliamine site did not exert a principal role in the genesis of seizures induced by Imi. In addition, the order of anticonvulsant potency in our epileptic model, after i.c.v. administration, was CPPene > MK-801 > Msc > (-)-2-amino-7-phosphonic acid (AP7) > gamma-D-glutamylaminomethylsulphonate (gamma-D-GAMS) > NBQX > kynurenic acid (KYNA) > 6-cyano-7-nitro-quinoxaline-2,3-dione (CNQX). The relationship between the different site of action and the anticonvulsant activity of these derivatives was discussed. Although the main mechanism of Imi induced seizures cannot be easily determined, potential interactions with the receptors of the excitatory amino acid neurotransmitters exists. In fact, antagonists of excitatory amino acids are able to increase the threshold for the seizures or to prevent the seizures induced by Imi. In addition, Imi acts on the central nervous system by inhibition of GABA neurotransmission and Msc, a selective GABAA agonist, was able to protect against seizures induced by Imi.

Seizures during ethanol withdrawal are blocked by focal microinjection of excitant amino acid antagonists into the inferior colliculus and pontine reticular formation

Physical dependence on ethanol can result in seizure susceptibility during ethanol withdrawal. In rats, generalized tonic-clonic seizures are precipitated by auditory stimulation during the ethanol withdrawal syndrome. Excitant amino acids (EAAs) are implicated as neurotransmitters in the inferior colliculus and the brain stem reticular formation, which play important roles in the neuronal network for genetic models of audiogenic seizures (AGSs). Ethanol blocks the actions of EAAs in various brain regions, including the inferior colliculus. In this study, dependence was produced by intragastric administration of ethanol for 4 days. During ethanol withdrawal, AGSs were blocked by systemic administration of competitive or noncompetitive NMDA antagonists 3-((+/-)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP) or dizocilpine (MK-801). Focal microinjections of NMDA or non-NMDA antagonists into the inferior colliculus or the pontine reticular formation also inhibited AGSs. MK-801 was the most potent anticonvulsant systemically. When injected into the inferior colliculus, CPP had a more potent anticonvulsant effect than either MK-801 or the non-NMDA antagonist 6-cyano-7-nitroquinoxaline-2,3-dione. The inferior colliculus was more sensitive than the pontine reticular formation to the anticonvulsant effects of both competitive NMDA and non-NMDA antagonists. The results of the present support the idea that continued ethanol administration may lead to development of supersensitivity to the action of EAAs in inferior colliculus and pontine reticular formation neurons. This may be a critical mechanism subserving AGS susceptibility during ethanol withdrawal.

The neuroprotective kappa-opioid CI-977 alters glutamate-induced calcium signaling in vitro

The effect of the neuroprotective kappa opioid agonist CI-977 on glutamate (GLU)-stimulated calcium signaling was studied in individual primary rat cortical neurons. Using laser scanning confocal microscopy and the fluorescent calcium probe fluo-3, both the sustained and biphasic intracellular calcium concentration [Ca2+]i changes induced by GLU (20-40 microM) were altered by CI-977 (25-100 nM), thereby shifting the neuronal population response from unbuffered to buffered patterns of [Ca2+]i flux. This effect was consistent with the previously demonstrated neuroprotective action of CI-977 against glutamate toxicity in vitro. The effect of CI-977 in altering GLU-induced [Ca2+]i signaling was attenuated by naloxone, consistent with a neuroprotective action of CI-977 at opioid receptors, presumably of the kappa subtype.

Neuroleptic malignant syndrome

Neuroleptic malignant syndrome (NMS) was first recognized as a life-threatening complication of dopamine receptor antagonists characterized by extrapyramidal disturbances, hyperthermia, and elevated serum creatine kinase levels. Concepts of NMS have changed because medications other than classic neuroleptic drugs have been implicated as triggering agents and because syndromes identical to NMS have been observed in patients with Parkinson's disease withdrawn from their medication or suffering akinetic hyperthermic parkinsonian crisis. The neurochemical key features in all these conditions are probably functional dopamine deficiency and ensuing hyperactivity of excitatory amino acid neurotransmission in the basal ganglia and hypothalamus. Recognition of NMS is the most important step in its management; the outcome is good if causative drugs are discontinued or if parkinsonian therapy is readjusted. Supportive care includes management of hyperthermia and fluid replacement. Controversial therapeutic measures include the application of dopamine receptor agonists, excitatory amino acid antagonists, or dantrolene. Psychiatric patients with a history of NMS and psychotic relapse necessitating neuroleptic drugs do not commonly redevelop NMS when reexposed to dopamine receptor antagonists but may be treated most safely with atypical neuroleptic drugs such as clozapine.

Neurotrophin-4/5 protects hippocampal and cortical neurons against energy deprivation- and excitatory amino acid-induced injury

Neurotrophin-4/5 (NT-4/5) is a recently discovered member of the neurotrophin family of neurotrophic factors which includes NGF, BDNF and NT-3. NT-4/5 is expressed in the brain where its function is unknown. We have found that NT-4/5 can protect cultured embryonic rat hippocampal and cortical neurons against glucose deprivation-induced injury. Significant protection was observed with NT-4/5 concentrations from 100-1000 ng/ml, with a dose-response curve similar to that of BDNF. Neuronal vulnerability to glutamate toxicity was significantly reduced in cultures pretreated with NT-4/5. Moreover, neurons pretreated with NT-4/5 were more resistant to toxicity induced by calcium ionophore A23187, demonstrating that NT-4/5 increases neuronal resistance to calcium-mediated injury. These data indicate that, as with other neurotrophins, NT-4/5 may serve a neuroprotective function in the brain.

Pharmacological characterization of the afterdischarge that precedes the onset of maximal dentate activation in the rat

Two types of afterdischarges have been recorded in the dentate gyrus after trains of electrical stimulation. The first afterdischarge contains predominantly broad positive potentials. The second afterdischarge contains bursts of large amplitude population spikes and has been termed maximal dentate activation. It has been proposed that the first afterdischarge is a precursor to maximal dentate activation and, therefore, it is termed a pre-MDA afterdischarge. The effects of several drugs on these afterdischarges were compared in adult male rats anesthetized with urethane. Stimulus trains (50 Hz) designed to elicit the pre-MDA afterdischarge were administered to the left CA3 region while recording in the ipsilateral dentate gyrus and contralateral CA1 cell layer. Phenytoin (80 mg/kg), phenobarbital (60 mg/kg), ethosuximide (300 mg/kg) and sodium valproate (300 mg/kg) had no effect on the duration or characteristics of the discharge. Carbamazepine (50-60 mg/kg) and ketamine (30 mg/kg) reversibly blocked the discharge. Diazepam (3 mg/kg) reduced the duration of the discharge and also reduced the amplitude of population spikes recorded in CA1. Baclofen (10 mg/kg) and bicuculline (0.5 mg/kg) increased the duration of the discharge. The effects of dizocilpine (MK-801, 2 mg/kg) were inconsistent. These results were compared to the effects of the same drugs on the time-to-onset and duration of maximal dentate activation. The pharmacological sensitivities of maximal dentate activation and pre-MDA discharges were found to be qualitatively different.

Excitatory amino acid release from contused brain tissue into surrounding brain areas

The EAA release from contused brain tissue and its effect on the extracellular EAA levels in brain areas surrounding the contusion were investigated with microdialysis technique in the rat. A significant increase in extracellular EAA levels was observed in the contused brain tissue. The EAA increase was significantly greater in the contused brain tissue than in the isolated but non-contused brain tissue. It was further demonstrated that EAAs were released from non-contused brain areas 1-2 mm distant from contused brain tissue. No such EAA release from surrounding brain areas was demonstrated when the cavity was filled with isolated but non-contused brain tissue. The increase in EAAs was attenuated by KYN administered through microdialysis, suggesting that the EAA release from the surrounding brain areas appears to be a consequence that is secondary to the EAA release from the contused brain tissue. Such a diffusion-reaction process is probably mediated by the neurotransmitter actions of EAAs. The results of the present study are of clinical importance, since surgical removal of contused brain tissue and administration of EAA antagonists may serve to protect the surrounding brain areas from EAA neurotoxicity.

Attenuation of hemispheric swelling associated with acute subdural hematomas by excitatory amino acid antagonist in rats

Acute subdural hematoma (ASDH) gives rise to a mass effect not only by itself but also through unilateral hemispheric swelling. The present study tested the hypothesis that hemispheric swelling is mediated by mechanisms which involve excitatory amino acids (EAAs). After removal of the subdural clot, introduced by homologous blood (0.1-0.2 ml), the % brain water was determined from the formula: ((wet weight--dry weight)/wet weight) x 100. The % brain water of the left hemisphere was significantly greater than that of the right hemisphere during the initial 6 hours after induction of ASDH in animals injected with 0.2 ml blood. A less marked but significant increase was observed in the animals injected with 0.1 ml blood. Systemic pretreatment with kynurenic acid (KYN; 800 mg/kg, i.p.), a broad-spectrum EAA antagonist, attenuated the increase in % brain water in the animals injected with 0.2 ml blood. In order to determine the changes in cerebral metabolism induced by the model of ASDH employed in the present study, we measured the cortical cytochrome oxidase (CYO) activity, a marker of mitochondrial respiration, in a separate group of animals. The CYO activity estimated densitometrically from the histochemical staining was not significantly altered in the animals injected with either 0.1 or 0.2 ml blood, suggesting absence of ischemia. These results indicated that the hemispheric swelling associated with thin ASDHs may be partially mediated by mechanisms other than ischemia, in which EAAs appear to be involved.