N-methyl-D-aspartate receptor-mediated increase in intracellular calcium is reduced by ketamine and phencyclidine

Neuroprotection mediated by natural products and their chemical derivatives

Neuronal injuries can lead to various diseases such as neurodegenerative diseases, stroke, trauma, ischemia and, more specifically, glaucoma and optic neuritis. The cellular mechanisms that regulate neuronal death include calcium influx and calcium overload, excitatory amino acid release, oxidative stress, inflammation and microglial activation. Much attention has been paid to the effective prevention and treatment of neuroprotective drugs by natural products. This review summarizes the neuroprotective aspects of natural products, extracted from Panax ginseng, Camellia sinensis, soy and some other plants, and some of their chemical derivatives. Their antioxidative and anti-inflammatory action and their inhibition of apoptosis and microglial activation are assessed. This will provide new directions for the development of novel drugs and strategies to treat neurodegenerative diseases.

Mechanistic studies on ketamine-induced mitochondrial toxicity in zebrafish embryos

Ketamine, a phencyclidine derivative, is an antagonist of the Ca²⁺-permeable N-methyl-d-aspartate (NMDA)-type glutamate receptors. It is a pediatric anesthetic and has been implicated in developmental neurotoxicity. Ketamine has also been shown to deplete ATP in mammalian cells. Our previous studies showed that acetyl l-carnitine (ALCAR) prevented ketamine-induced cardiotoxicity and neurotoxicity in zebrafish embryos. Based on our finding that ALCAR's protective effect was blunted by oligomycin A, an inhibitor of ATP synthase, we further investigated the effects of ketamine and ALCAR on ATP levels, mitochondria and ATP synthase in zebrafish embryos. The results demonstrated that ketamine reduced ATP levels in the embryos but not in the presence of ALCAR. Ketamine reduced total mitochondrial protein levels and mitochondrial potential, which were prevented with ALCAR co-treatment. To determine the cause of ketamine-induced ATP deficiency, we explored the status of ATP synthase. The results showed that a subunit of ATP synthase, atp5α1, was transcriptionally down-regulated by ketamine, but not in the presence of ALCAR, although ketamine caused a significant upregulation in another ATP synthase subunit, atp5β and total ATP synthase protein levels. Most of the ATP generated by heart mitochondria are utilized for its contraction and relaxation. Ketamine-treated embryos showed abnormal heart structure, which was abolished with ALCAR co-treatment. This study offers evidence for a potential mechanism by which ketamine could cause ATP deficiency mediated by mitochondrial dysfunction.

Developmental neurotoxicity of ketamine in pediatric clinical use

Ketamine is widely used as an anesthetic, analgesic, and sedative in pediatric clinical practice and it is also listed as an illicit drug by most countries. Recent in vivo and in vitro animal studies have confirmed that ketamine can induce neuronal cell death in the immature brain, resulting from widespread neuronal apoptosis. These effects can disturb normal development further altering the structure and functions of the brain. Our recent studies further indicate that ketamine can alter neurogenesis from neural stem progenitor cells in the developing brain. Taken together, these findings identify a novel complication associated with ketamine use in premature infants, term newborns, and pregnant women. Recent data on the developmental neurotoxicity of ketamine are reviewed with proposed future directions for evaluating the safety of ketamine in these patient populations.

The emerging use of ketamine for anesthesia and sedation in traumatic brain injuries

Traditionally, the use of ketamine for patients with traumatic brain injuries is contraindicated due to the concern of increasing intracranial pressure (ICP). These concerns, however, originated from early studies and case reports that were inadequately controlled and designed. Recently, the concern of using ketamine in these patients has been challenged by a number of published studies demonstrating that the use of ketamine was safe in these patients. This article reviews the current literature in regards to using ketamine in patients with traumatic brain injuries in different clinical settings associated with anesthesia, as well as reviews the potential mechanisms underlying the neuroprotective effects of ketamine. Studies examining the use of ketamine for induction, maintenance, and sedation in patients with TBI have had promising results. The use of ketamine in a controlled ventilation setting and in combination with other sedative agents has demonstrated no increase in ICP. The role of ketamine as a neuroprotective agent in humans remains inconclusive and adequately powered; randomized controlled trials performed in patients undergoing surgery for traumatic brain injury are necessary.

NEUROPROTECTIVE EFFECTS OF INTRAMUSCULAR KETAMINE IN RABBIT RETINAS AFTER PARS PLANA VITRECTOMY AND SILICONE OIL INJECTION

PURPOSE

To investigate potential retinal neuroprotective effects of intramuscular ketamine in rabbits after pars plana vitrectomy (PPV) and intravitreal silicone oil injection (SOI).

METHODS

Twelve New Zealand rabbits (weight, 2.0-2.5 kg) underwent PPV with SOI in the right eye. Postoperatively, six rabbits received a daily intramuscular injection of ketamine for 4 weeks (ketamine-operated eyes), and six rabbits received a daily intramuscular injection of saline (saline-operated eyes). The retina from the left eye of each rabbit served as a control (ketamine-control and saline-control eyes). The animals were euthanized at 4 weeks after surgery. Qualitative and quantitative analyses were performed using the Zeiss Axiophot microscope and KS 400 software.

RESULTS

Qualitative analysis using light microscopy demonstrated more extensive edema and cell disorganization in saline-operated retinas than in ketamine-operated, ketamine-control, and saline-control retinas. Quantitatively, the cell densities (cell/mm) in the outer nuclear layer (ONL), inner nuclear layer (INL), and ganglion cell layer (GCL) in saline-operated retinas were significantly (P < 0.05) lower than those in these layers in ketamine-operated, ketamine-control, and saline-control retinas. The cell density in the ONL in saline-operated retinas was 52% lower than that in ketamine-operated retinas, 55% lower than that in ketamine-control retinas, and 56% lower than that in saline-control retinas. The cell density in the INL in saline-operated retinas was 44% lower than that in ketamine-operated retinas, 48% lower than that in ketamine-control retinas, and 49% lower than that in saline-control retinas. The cell density in the GCL in saline-operated retinas was 60% lower than that in ketamine-operated retinas, 64% lower than that in ketamine-control retinas, and 64% lower than that in saline-control retinas.

CONCLUSION

PPV with SOI was associated with retinal cell death and disorganization in rabbit eyes. Intramuscular ketamine administration provided protection against these effects.

Effects of preoperative epidural administration of racemic ketamine for analgesia in sheep undergoing surgery

OBJECTIVE

To investigate the effects of preoperative epidural administration of racemic ketamine to provide analgesia in sheep undergoing experimental hind limb orthopedic surgery.

ANIMALS

12 adult sheep (weight range, 51.4 to 67.2 kg).

PROCEDURE

Sheep were anesthetized with guaifenesin, thiopental, and isoflurane; after induction of anesthesia, sheep received a lumbosacral epidural injection of ketamine (1 mg/kg; n = 6) or saline (0.9% NaCl) solution (1 mL/7 kg; 6 [control group]). Respiratory and cardiovascular variables were recorded before and at intervals during and for 6 hours after anesthesia. During that 6-hour postoperative period, analgesia was evaluated subjectively with a numeric ranking scale that included assessments of comfort, posture, movement, and response to wound palpation; buprenorphine was administered when a score > 3 (maximum score, 10) was achieved. Rectal temperature, heart and respiratory rates, and lameness were evaluated daily for 2 weeks after surgery.

RESULTS

At all evaluations, cardiovascular and respiratory variables were comparable between the 2 groups. Compared with control sheep, time to first administration of rescue analgesic was significantly longer and total dose of buprenorphine administered during the 6- hour postoperative period was significantly decreased for ketamine-treated sheep. During the second week following surgery, ketamine-treated sheep had significantly less lameness than control sheep.

CONCLUSIONS AND CLINICAL RELEVANCE

In sheep undergoing hind limb surgery, preoperative epidural administration of ketamine appears to provide analgesia in the immediate postoperative period and has residual analgesic effects, which may contribute to more rapid return of normal function in surgically treated limbs.

Injections of blood, thrombin, and plasminogen more severely damage neonatal mouse brain than mature mouse brain

The mechanism of brain cell injury associated with intracerebral hemorrhage may be in part related to proteolytic enzymes in blood, some of which are also functional in the developing brain. We hypothesized that there would be an age-dependent brain response following intracerebral injection of blood, thrombin, and plasminogen. Mice at 3 ages (neonatal, 10-day-old, and young adult) received autologous blood (15, 25, and 50 microl respectively), thrombin (3, 5, and 10 units respectively), plasminogen (0.03, 0.05, and 0.1 units respectively) (the doses expected in same volume blood), or saline injection into lateral striatum. Forty-eight hours later they were perfusion fixed. Hematoxylin and eosin, lectin histochemistry, Fluoro-Jade, and TUNEL staining were used to quantify changes related to the hemorrhagic lesion. Damage volume, dying neurons, neutrophils, and microglial reaction were significantly greater following injections of blood, plasminogen, and thrombin compared to saline in all three ages of mice. Plasminogen and thrombin associated brain damage was greatest in neonatal mice and, in that group unlike the other 2, greater than the damage caused by whole blood. These results suggest that the neonatal brain is relatively more sensitive to proteolytic plasma enzymes than the mature brain.

Anesthesia for intraoperative neurophysiologic monitoring of the spinal cord

Intraoperative neurophysiologic monitoring (INM) using somatosensory and motor evoked potentials (MEPs) has become popular to reduce neural risk and to improve intraoperative surgical decision making. Intraoperative neurophysiologic monitoring is affected by the choice and management of the anesthetic agents chosen. Because inhalational and intravenous anesthetic agents have effects on neural synaptic and axonal functional activities, the anesthetic effect on any given response will depend on the pathway affected and the mechanism of action of the anesthetic agent (i.e., direct inhibition or indirect effects based on changes in the balance of inhibitory or excitatory inputs). In general, responses that are more highly dependent on synaptic function will have more marked reductions in amplitude and increases in latency as a result of the synaptic effects of inhalational anesthetic agents and similar effects at higher doses of intravenous agents. Hence, recording cortical somatosensory evoked potentials and myogenic MEPs requires critical anesthetic choices for INM. The management of the physiologic milieu is also important as central nervous system blood flow, intracranial pressure, blood rheology, temperature, and arterial carbon dioxide partial pressure produce alterations in the responses consistent with the support of neural functioning. Finally, the management of pharmacologic neuromuscular blockade is critical to myogenic MEP recording in which some blockade may be desirable for surgery but excessive blockade may eliminate responses. A close working relationship of the monitoring team, the anesthesiologist, and the surgeon is key to the successful conduct and interpretation of INM.

The anesthetics propofol and ketamine inhibit cocaine-induced egr-1 gene expression in rat forebrain

Acute cocaine injection to rats is known to induce the expression of immediate early genes in the forebrain, the effect being primarily mediated by the dopaminergic system. We examined the effect of the anesthetics ketamine and propofol on cocaine-induced egr-1 mRNA expression. Using in situ hybridization, we show that both compounds did not induce egr-1 gene by themselves, but were able to dose-dependently reduce cocaine-induced egr-1 mRNA synthesis in the nucleus accumbens, caudate-putamen and cingulate cortex. Our data suggest that in addition to glutamate NMDA receptors, propofol may act via GABA(A) receptors or ion channels.

Anesthetics and the brain

The action of anesthetics on the nervous system can be understood by considering their possible interactions with neuronal function. Anesthesia may be produced by a change in the balance of inhibitory synapses (notable via GABAa receptors) and excitatory synapses (notably glutamate receptors). Our knowledge of the specific mechanisms of anesthetic drugs and the structures in the CNS remains inadequate to explain the anesthetic state by one mechanism. The action of anesthetics can also be considered based on the action of the drugs on cerebral physiology, notably CMR, CBF, metabolic coupling, and autoregulation. Some specific anesthetic recommendations can be made for certain neurosurgical procedures and pathology based on the effects on physiology.

The role of the N-methyl-D-aspartic acid receptor in the relaxant effect of ketamine on tracheal smooth muscle

Ketamine and magnesium (Mg2+), well known bronchodilators, have been used to treat patients with status asthmaticus. Both can block the N-methyl-D-aspartic acid (NMDA) receptor. NMDA receptors exist in the airway, and their activation seems to be linked to the release actions of sensory neuropeptides resulting in increased airway tone. We sought to determine whether ketamine relaxes the guinea pig trachea contracted by histamine by blocking the NMDA receptor. Female guinea pigs (250-400 g) were killed with an overdose of pentobarbital. The trachea was removed and cut spirally into strips 3 mm wide and 15 mm long. The strips were mounted in a 10-mL organ bath filled with Tyrode's solution bubbled through with 95% O2/5% CO2 at 37 degrees C. Strip contractions were measured isometrically with a force displacement transducer. We then studied the effect of NMDA receptor antagonists on histamine-induced tracheal contraction. In this protocol, we examined the effect of ketamine, Mg2+, zinc (Zn2+), or MK-801 (a noncompetitive NMDA receptor blocker) on strips contracted by 10(-5) M histamine. After full contraction was attained, ketamine (0.5-1.5 mM), MgSO4 (2-8 mM), ZnCl2(0.2-0.8 mM), or MK-801 (1.5-6 x 10(-5) M) was added, and the strip tension was measured again. We also studied the effect of NMDA on the relaxation by ketamine. After full contraction by 10(-5) M histamine, 0.5-1.5 mM KET was added alone or in combination with 0.1 mM NMDA, and the strip tension was measured again. Finally, we measured the effect of MK-801 on the relaxant effect of ketamine. After full contraction by 10(-5) M histamine, 0.5-2 mM ketamine was added alone or in combination with 0.75 or 1.5 x 10(-5) M MK-801, and the strip tension was measured again. All NMDA receptor antagonists tested reversed the tracheal contraction induced by histamine in a dose-dependent manner. However, neither the agonist NMDA nor the noncompetitive receptor blocker MK-801 affected tracheal relaxation induced by ketamine. We conclude that ketamine relaxes the tracheal smooth muscle contracted by histamine through a mechanism independent of NMDA receptors. The decreased bronchomotor tone induced by ketamine is probably due to interference with a Ca2+-requiring step necessary to maintain the contraction caused by histamine.

IMPLICATIONS

Stimulation of the N-methyl-D-aspartic acid (NMDA) receptor in the airway results in airway constriction. The bronchodilator ketamine blocks the NMDA receptor. However, ketamine relaxes the guinea pig trachea contracted by histamine through a mechanism independent of the NMDA receptor.

Glutamate dehydrogenase mRNA is immediately induced after phencyclidine treatment in the rat brain

To clarify the molecular mechanism of phencyclidine (PCP)-induced schizophreniform psychosis in humans and of behavioral abnormalities in experimental animals, we used differential screening of a cDNA library from the cerebral cortex of rats treated with PCP. We identified a PCP-induced cDNA clone as the gene encoding glutamate dehydrogenase (GDH), an enzyme central to glutamate metabolism. GDH mRNA levels significantly increased as early as 15 min following PCP administration in both the cerebral cortex and the cerebellum. This effect was observed even in the presence of a protein synthesis inhibitor, cycloheximide. In contrast to a transient increase in c-fos expression, the elevation of GDH mRNA levels lasted up to 8 days after a single PCP injection. These results suggest that GDH mRNA induction may be involved in the pathology of PCP-induced psychosis, and that GDH may be one of the candidate genes that are vulnerable in subjects with schizophrenia.

Presynaptic GABAB-and gamma-hydroxybutyric acid-mediated mechanisms in generalized absence seizures

gamma-Hydroxybutyric acid (GHB) is a naturally occurring compound which has the ability to induce generalized absence seizures when given to animals. This effect of GHB may be blocked by either GHB or GABAB receptor antagonists. We sought to test the hypothesis that pre-synaptic GHB- and GABAB-mediated mechanisms in thalamus and cortex are operative in the GHB model of generalized absence seizures. Presynaptic Ca(2+)-dependent K+ efflux was determined using Ca(2+)-stimulated Rb86 efflux in synaptosomes prepared from thalamus and cortex in the presence of GHB, a specific GHB receptor antagonist, the specific GABAB agonist (-)baclofen, or the specific GABAB antagonists, phaclofen and CGP 35348. The effect of these compounds was determined also on basal and K(+)-stimulated 45Ca2+ uptake and basal and K(+)-stimulated synaptosomal cytosolic Ca2+([Ca2]i) in synaptosomes prepared from thalamus and cortex and on [125I] omega-conotoxin binding in thalamus and cortex using autoradiographic binding techniques. There was no demonstrable change in Ca(2+)-stimulated Rb86 efflux in any experimental condition studied; however GHB and (-)baclofen both suppressed K(+)-stimulated 45Ca2+ uptake and [Ca2]i in synaptosomes and were associated with a decrease in [125I] omega-conotoxin binding which achieved statistical significance only in frontal cortex, a brain region selectively involved in the genesis of GHB-induced absence seizures. The effects of GHB and (-)baclofen on K(+)-stimulated 45Ca2+ uptake and [Ca2]i in synaptosomes were additive. The effects of GHB in this regard were attenuated by the GHB antagonist and phaclofen while that of (-)baclofen was attenuated by CGP 35348. These data do not support the hypothesis that the GHB and GABAB receptor are one and the same. Rather, they raise the possibility that a presynaptic GHB/GABAB receptor complex might be involved in the pathogenesis of GHB-induced generalized absence seizures.

AIDS-related dementia and calcium homeostasis

Approximately a third of adults and half of children with acquired immunodeficiency syndrome (AIDS) eventually suffer from neurological manifestations, including dysfunction of cognition, movement, and sensation. Among the various pathologies reported in the brain of patients with AIDS is neuronal injury and loss. A paradox arises, however, because neurons themselves are for all intents and purposes not infected by human immunodeficiency virus type 1 (HIV-1). This paper reviews evidence suggesting that at least part of the neuronal injury observed in the brain of AIDS patients is related to excessive influx of Ca2+. There is growing support for the existence of HIV- or immune-related toxins that lead indirectly to the injury or death of neurons via a potentially complex web of interactions between macrophages (or microglia), astrocytes, and neurons. Human immunodeficiency virus-infected monocytoid cells (macrophages, microglia, or monocytes), especially after interacting with astrocytes, secrete substances that potentially contribute to neurotoxicity. Not all of these substances are yet known, but they may include eicosanoids, that is, arachidonic acid and its metabolites, as well as platelet-activating factor. Macrophages activated by HIV-1 envelope protein gp120 also appear to release arachidonic acid and its metabolites. These factors can lead to increased glutamate release or decreased glutamate reuptake. In addition, gamma interferon (IFN-gamma) stimulation of macrophages induce release of the glutamate-like agonist quinolinate. Human immunodeficiency virus-infected or gp120-stimulated macrophages also produce cytokines, including tumor necrosis factor-alpha and interleukin-1 beta, which contribute to astrogliosis. A final common pathway for neuronal susceptibility appears to be operative, similar to that observed in stroke, trauma, epilepsy, neuropathic pain, and several neurodegenerative diseases, possibly including Huntington's disease, Parkinson's disease, and amyotrophic lateral sclerosis. This mechanism involves the activation of voltage-dependent Ca2+ channels and N-methyl-D-aspartate (NMDA) receptor-operated channels, and therefore offers hope for future pharmacological intervention. This review focuses on clinically tolerated calcium channel antagonists and NMDA antagonists with the potential for trials in humans with AIDS dementia in the near future.

HIV-related neuronal injury. Potential therapeutic intervention with calcium channel antagonists and NMDA antagonists

Perhaps as many as 25-50% of adult patients and children with acquired immunodeficiency syndrome (AIDS) eventually suffer from neurological manifestations, including dysfunction of cognition, movement, and sensation. How can human immunodeficiency virus type 1 (HIV-1) result in neuronal damage if neurons themselves are for all intents and purposes not infected by the virus? This article reviews a series of experiments leading to a hypothesis that accounts at least in part for the neurotoxicity observed in the brains of AIDS patients. There is growing support for the existence of HIV- or immune-related toxins that lead indirectly to the injury or demise of neurons via a potentially complex web of interactions among macrophages (or microglia), astrocytes, and neurons. HIV-infected monocytoid cells (macrophages, microglia, or monocytes), after interacting with astrocytes, secrete eicosanoids, i.e., arachidonic acid and its metabolites, including platelet-activating factor. Macrophages activated by HIV-1 envelope protein gp120 also appear to release arachidonic acid and its metabolites. In addition, interferon-gamma (IFN-gamma) stimulation of macrophages induces release of the glutamate-like agonist, quinolinate. Furthermore, HIV-infected macrophage production of cytokines, including TNF-alpha and IL1-beta, contributes to astrogliosis. A final common pathway for neuronal susceptibility appears to be operative, similar to that observed in stroke, trauma, epilepsy, neuropathic pain, and several neurodegenerative diseases, possibly including Huntington's disease, Parkinson's disease, and amyotrophic lateral sclerosis. This mechanism involves the activation of voltage-dependent Ca2+ channels and N-methyl-D-aspartate (NMDA) receptor-operated channels, and, therefore, offers hope for future pharmacological intervention. This article focuses on clinically tolerated calcium channel antagonists and NMDA antagonists with the potential for trials in humans with AIDS dementia in the near future.

Ketamine inhibition of cytoplasmic calcium signalling in rat pheochromocytoma (PC-12) cells

This study examines the mechanism of action of ketamine, a dissociative anesthetic, with a specific focus on its ability to inhibit changes in the concentration of intracellular free calcium, [Ca2+]i, in PC-12 cells. The resting [Ca2+]i as measured with the fluorescent probe Fura-2 AM in control cells is 184.8 +/- 8.6 nM (mean +/- SEM, n = 15). Changes in [Ca2+]i via influx through voltage-gated calcium channels after membrane depolarization with potassium chloride were monitored in the absence and presence of various concentrations of ketamine. Potassium-depolarization caused a dose-dependent rapid increase in [Ca2+]i, averaging 62 +/- 5%, 33 +/- 2% and 18 +/- 3% (n = 10 each) above control levels for 70 mM, 50 mM and 35 mM KCl, respectively. Ketamine, in the dosage range studied (5-500 microM), inhibited the increase in [Ca2+]i stimulated by potassium-depolarization in a dose-dependent manner. The computer-fitted dose-response curve of the pooled data yielded a half maximal suppression concentration, ED50, of 33 microM. In conclusion, this study demonstrates that ketamine inhibits Ca2+ influx through voltage-gated Ca2+ channels in PC-12 cells at clinically relevant doses, and may play a role in ketamine's action as a general anesthetic agent.

NMDA-induced increase in [Ca2+]i and45Ca2+ uptake in acutely dissociated brain cells derived from adult rats

A preparation of acutely dissociated brain cells derived from adult (3-month-old) rat has been developed under conditions preserving the metabolic integrity of the cells and the function of N-methyl-D-aspartate (NMDA) receptors. The effects of glutamate and NMDA on [Ca2+]i measured with fluo3 and 45Ca2+ uptake have been studied on preparations derived from hippocampus and cerebral cortex. Glutamate (100 microM) and N-methyl-DL-aspartate (200 microM) increased [Ca2+]i by 26-12 nM and 23-9 nM after 90 s in cerebral cortex and hippocampus, and stimulated 45Ca2+ uptake about 16-10% in the same regions. The increases in [Ca2+]i and 45Ca2+ uptake were inhibited by 40% in the presence of 1 mM MgCl2 and by 90-50% in the presence of MK-801. The results indicate (a) that a large fraction of the [Ca2+]i response to glutamate in freshly dissociated brain cells from the adult rat involves NMDA receptors, (b) when compared with results in newborn rats, there is a substantial blunting of the [Ca2+]i increase in adult age.

Adenosine and glutamate modulate each other's release from rat hippocampal synaptosomes

In rat hippocampal synaptosomes, adenosine decreased the K+ (15 mM) or the kainate (1 mM) evoked release of glutamate and aspartate. An even more pronounced effect was observed in the presence of the stable adenosine analogue, R-phenylisopropyladenosine. All these effects were reversed by the selective adenosine A1 receptor antagonist 8-cyclopentyltheophylline. In the same synaptosomal preparation, K+ (30 mM) strongly stimulated the release of the preloaded [3H]adenosine in a partially Ca(2+)-dependent and tetrodotoxin (TTX)-sensitive manner. Moreover, in the same experimental conditions, both L-glutamate and L-aspartate enhanced the release of [3H]adenosine derivatives ([3H]ADD). The glutamate-evoked release was dose dependent and appeared to be Ca2+ independent and tetrodotoxin insensitive. This effect was not due to metabolism because even the nonmetabolizable isomers D-glutamate and D-aspartate were able to stimulate [3H]ADD release. In contrast, the specific glutamate agonists N-methyl-D-aspartate, kainate, and quisqualate failed to stimulate [3H]ADD release, suggesting that glutamate and aspartate effects were not mediated by known excitatory amino acid receptors. Moreover, NMDA was also ineffective in the absence of Mg2+ and L-glutamate-evoked release was not inhibited by adding the specific antagonists 2-amino-5-phosphonovaleric acid or 6-7-dinitroquinoxaline-2,3-dione. The stimulatory effect did not appear specific for only excitatory amino acids, as gamma-aminobutyric acid stimulated [3H]ADD release in a dose-related manner. These results suggest that, at least in synaptosomal preparations from rat hippocampus, adenosine and glutamate modulate each other's release. The exact mechanism of such interplay, although still unknown, could help in the understanding of excitatory amino acid neurotoxicity.

Neurochemical aspects of the N-methyl-D-aspartate receptor complex

The N-methyl-D-aspartic acid (NMDA)-sensitive subclass of brain excitatory amino acid receptors is supposed to be a receptor-ionophore complex consisting of at least 3 different major domains including an NMDA recognition site, glycine (Gly) recognition site and ion channel site. Biochemical labeling of the NMDA domain using [3H]L-glutamic acid (Glu) as a radioactive ligand often meets with several critical methodological pitfalls and artifacts that cause a serious misinterpretation of the results. Treatment of brain synaptic membranes with a low concentration of Triton X-100 induces a marked disclosure of [3H]Glu binding sensitive to displacement by NMDA with a concomitant removal of other several membranous constituents with relatively high affinity for the neuroactive amino acid. The NMDA site is also radiolabeled by the competitive antagonist (+/-)-3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid that reveals possible heterogeneity of the site. The Gly domain is sensitive to D-serine and D-alanine but insensitive to strychnine, and this domain seems to be absolutely required for an opening of the NMDA channels by agonists. The ionophore domain is radiolabeled by a non-competitive type of NMDA antagonist that is only able to bind to the open but not closed channels. The binding of these allosteric antagonists is markedly potentiated by NMDA agonists in a manner sensitive to antagonism by isosteric antagonists in brain synaptic membranes and additionally enhanced by further inclusion of Gly agonists through the Gly domain. Furthermore, physiological and biochemical responses mediated by the NMDA receptor complex are invariably potentiated by several endogenous polyamines, suggesting a novel polyamine site within the complex. At any rate, activation of the NMDA receptor complex results in a marked influx of Ca2+ as well as Na+ ions, which subsequently induces numerous intracellular metabolic alterations that could be associated with neuronal plasticity or excitotoxicity. Therefore, any isosteric and allosteric antagonists would be of great benefit for the therapy and treatment of neurodegenerative disorders with a risk of impairing the acquisition and formation process of memories.

Glutamate-evoked release of endogenous adenosine from rat cortical synaptosomes is mediated by glutamate uptake and not by receptors

L-Glutamate (10 microM-1 mM) released endogenous adenosine from rat cortical synaptosomes. Studies with excitatory amino acid antagonists, (+)-5-methyl-16,11,dihydro-5H- dibenzo[a,d]cyclohepten-5,10-imine maleate (MK-801), 6,7-dinitroquinoxaline-2,3-dione (DNQX), Mg2+, and agonists N-methyl-D-aspartate (NMDA), kainate, and quisqualate, indicated that this release was not receptor mediated. D,L-2-Amino-4-phosphonobutanoic acid (APB) also did not affect glutamate-evoked adenosine release. Inhibition of glutamate uptake by dihydrokainate or replacement of extracellular Na+ blocked glutamate-evoked adenosine release. D-aspartate, which is a substrate for the glutamate transporter but is not metabolized, also released adenosine, suggesting that release was due to amino acid transport and not to its subsequent metabolism. D-Glutamate, a relatively poor substrate for the transporter, was correspondingly less potent than L-glutamate at releasing adenosine. Glutamate-evoked adenosine release was not Ca2+ dependent or tetrodotoxin sensitive and did not appear to occur on the bidirectional nucleoside transporter. Inhibition of ecto-5'-nucleotidase virtually abolished glutamate-evoked adenosine release, indicating that adenosine was derived from extracellular metabolism of released nucleotide(s). However, L-glutamate did not release ATP and did not appear to release cyclic AMP. Therefore, transport of glutamate into presynaptic terminals releases some other nucleotide which is converted extracellularly to adenosine. This adenosine could act at P1-purinoceptors to modulate glutamatergic neurotransmission.

Effects of NMDA antagonists on picrotoxin-, low Mg2+- and low Ca2+-induced epileptogenesis and on evoked changes in extracellular Na+ and Ca2+ concentrations in rat hippocampal slices

The anticonvulsant properties of ketamine and 2-APV were compared on 3 types of convulsant activity in hippocampal area CA1: the 'picrotoxin-epilepsy,' the 'low magnesium epilepsy' and the 'low calcium epilepsy.' In particular the spontaneous activity, the synaptically evoked responses and the changes in [Ca2+]0 were examined, since in many cases of epilepsy, Ca2+ uptake into cells is enhanced. In normal medium, ketamine and 2-APV have nearly no effect on stimulus evoked decreases in [Ca2+]0, although they clearly depress NMDA-induced ionic changes. However, ketamine and 2-APV prevent to some extent the augmentation of stimulus-induced changes in [Ca2+]0, observed after treating slices with picrotoxin or Mg2+-free medium. This extra Ca2+ uptake is probably mediated by NMDA operated channels. Our findings also show that ketamine, like 2-APV, has a stronger anticonvulsant effect on the low Mg-than on the picrotoxin-induced epileptiform activity. Responses to iontophoretically applied NMDA are facilitated in the 'low calcium epilepsy' and can be selectively blocked by ketamine. Spontaneous epileptiform activity occurring in low calcium can be blocked by ketamine only when some synaptic transmission is still present.

Ethanol inhibits NMDA-induced increases in free intracellular Ca2+ in dissociated brain cells

The effect of N-methyl-D-aspartate (NMDA) on free intracellular Ca2+ concentrations [( Ca2+]i) and the interaction of ethanol on the NMDA-mediated response was examined in freshly dissociated brain cells isolated from newborn rats. NMDA (25 microM) increased [Ca2+]i by approximately 70 nM, measured by fura-2 fluorometry, and this increase could be prevented or reversed by the NMDA antagonists Mg2+ (1.0 mM) and 2-amino-5-phosphonovalerate (AP5, 100 microM). Ethanol (25, 50, 100 mM) added 50 s before NMDA (25 microM) reduced the rise in [Ca2+]i when compared to the 25 microM NMDA response in the absence of ethanol. Thus, ethanol may have direct actions on NMDA-receptor activated increases in [Ca2+]i.

Lack of excitatory amino acid-induced effects on calcium fluxes measured with 45Ca2+ in rat cerebral cortex synaptosomes

Ca2+ uptake was measured in purified rat cerebral cortex synaptosomes (P3 pellets) using 45Ca2+ as a tracer. Ca2+ influx increased in time, and with an increase in external K+ concentration and temperature. The net (external K+-induced, depolarization-dependent) uptake follows a two-component course. The exponential term, due to the opening of voltage-operated calcium channels (VOC), has a rate constant which increases with an increase in the depolarization level (1.04 versus 0.54 nmol/s/mg protein for 50 mM - versus 15 mM [K+]-dependent net influx). The linear term, due to the Na+/Ca2+ exchange system, has a similar rate constant at all depolarization levels (0.16 +/- 0.05 and 0.11 +/- 0.02 nmol/s/mg protein). Excitatory amino acids (glutamate, kainate and n-methyl-d-aspartate-NMDA-) were tested on this preparation at doses ranging between 5 x 10(-5) M and 5 x 10(-3) M and at multiple incubation times, under resting conditions and under two depolarizing conditions (partial depolarization: 15 mM external K+ and maximal depolarization: 50 mM external K+). NMDA was also tested in the absence of Mg2+. No effect was detectable under any of these experimental conditions. Hypotheses to interpret these data are discussed. Further studies on other preparations are needed in order to directly investigate the presynaptic effects of excitatory amino acids.