The anti-excitotoxic effects of certain anesthetics, analgesics and sedative-hypnotics

The mechanistic functional landscape of retinitis pigmentosa: a machine learning-driven approach to therapeutic target discovery

BACKGROUND

Retinitis pigmentosa is the prevailing genetic cause of blindness in developed nations with no effective treatments. In the pursuit of unraveling the intricate dynamics underlying this complex disease, mechanistic models emerge as a tool of proven efficiency rooted in systems biology, to elucidate the interplay between RP genes and their mechanisms. The integration of mechanistic models and drug-target interactions under the umbrella of machine learning methodologies provides a multifaceted approach that can boost the discovery of novel therapeutic targets, facilitating further drug repurposing in RP.

METHODS

By mapping Retinitis Pigmentosa-related genes (obtained from Orphanet, OMIM and HPO databases) onto KEGG signaling pathways, a collection of signaling functional circuits encompassing Retinitis Pigmentosa molecular mechanisms was defined. Next, a mechanistic model of the so-defined disease map, where the effects of interventions can be simulated, was built. Then, an explainable multi-output random forest regressor was trained using normal tissue transcriptomic data to learn causal connections between targets of approved drugs from DrugBank and the functional circuits of the mechanistic disease map. Selected target genes involvement were validated on rd10 mice, a murine model of Retinitis Pigmentosa.

RESULTS

A mechanistic functional map of Retinitis Pigmentosa was constructed resulting in 226 functional circuits belonging to 40 KEGG signaling pathways. The method predicted 109 targets of approved drugs in use with a potential effect over circuits corresponding to nine hallmarks identified. Five of those targets were selected and experimentally validated in rd10 mice: Gabre, Gabra1 (GABARα1 protein), Slc12a5 (KCC2 protein), Grin1 (NR1 protein) and Glr2a. As a result, we provide a resource to evaluate the potential impact of drug target genes in Retinitis Pigmentosa.

CONCLUSIONS

The possibility of building actionable disease models in combination with machine learning algorithms to learn causal drug-disease interactions opens new avenues for boosting drug discovery. Such mechanistically-based hypotheses can guide and accelerate the experimental validations prioritizing drug target candidates. In this work, a mechanistic model describing the functional disease map of Retinitis Pigmentosa was developed, identifying five promising therapeutic candidates targeted by approved drug. Further experimental validation will demonstrate the efficiency of this approach for a systematic application to other rare diseases.

Evidence against involvement of kynurenate branch of kynurenine pathway in pathophysiology of Fuchs' dystrophy and keratoconus

Kynurenine aminotransferases (KAT) are enzymes catalyzing formation of kynurenic acid (KYNA) from kynurenine. KYNA is a Janus-faced molecule of high biological activity. On the one hand KYNA was identified as a UV filter and neuroprotectant with free radical scavenging properties, but on the other hand it may contribute to photodamage of lens proteins resulting in cataract formation. Fuchs endothelial corneal dystrophy (FECD) and keratoconus (KC) are common, vision threatening corneal dystrophies whose etiology is not fully understood. In our previous works, we confirmed the presence of KATs in the human cornea together with GPR35, a receptor for KYNA. This prompted us to investigate the potential changes in the expression of three isoforms: KAT I, KAT II, and KAT III in normal and FECD- and KC-affected corneas. Immunohistochemistry accompanied by gene expression data mining revealed that the levels of neither KAT I, KAT II, nor KAT III are affected in FECD and KC. This constitutes evidence against the involvement of KATs in the pathophysiology of FECD and KC.

Curcumin in Depression: Potential Mechanisms of Action and Current Evidence-A Narrative Review

Major depressive disorder (MDD) is one of the most prevalent and debilitating disorders. Current available treatments are somehow limited, so alternative therapeutic approaches targeting different biological pathways are being investigated to improve treatment outcomes. Curcumin is the main active component in the spice turmeric that has been used for centuries in Ayurvedic medicine to treat a variety of conditions, including anxiety and depressive disorders. In the past decades, curcumin has drawn researchers' attention and displays a broad range of properties that seem relevant to depression pathophysiology. In this review, we break down the potential mechanisms of action of curcumin with emphasis on the diverse systems that can be disrupted in MDD. Curcumin has displayed, in a number of studies, a potency in modulating neurotransmitter concentrations, inflammatory pathways, excitotoxicity, neuroplasticity, hypothalamic-pituitary-adrenal disturbances, insulin resistance, oxidative and nitrosative stress, and endocannabinoid system, all of which can be involved in MDD pathophysiology. To date, a handful of clinical trials have been published and suggest a benefit of curcumin in MDD. With evidence that is progressively growing, curcumin appears as a promising alternative option in the management of MDD.

Ketamine and phencyclidine: the good, the bad and the unexpected

The history of ketamine and phencyclidine from their development as potential clinical anaesthetics through drugs of abuse and animal models of schizophrenia to potential rapidly acting antidepressants is reviewed. The discovery in 1983 of the NMDA receptor antagonist property of ketamine and phencyclidine was a key step to understanding their pharmacology, including their psychotomimetic effects in man. This review describes the historical context and the course of that discovery and its expansion into other hallucinatory drugs. The relevance of these findings to modern hypotheses of schizophrenia and the implications for drug discovery are reviewed. The findings of the rapidly acting antidepressant effects of ketamine in man are discussed in relation to other glutamatergic mechanisms.

Pretreatment with group I metabotropic glutamate receptors antagonists attenuates lethality induced by acute cocaine overdose and expression of sensitization to hyperlocomotor effect of cocaine in mice

Cocaine abuse and dependence is a worldwide health problem. However, there are no currently approved medications to reduce cocaine abuse/relapse and toxicity. The aim of the present study was to test, whether group I metabotropic glutamate receptors (mGluRs) antagonists (mGluR1 and mGluR5) differentially regulate toxic versus behavioral effects of cocaine, both phenomena relevant to the psychopathology of cocaine addiction in humans. In the present study, we assessed the impact of mGluR1 antagonist-EMQMCM and mGluR5 antagonist-MTEP on the cocaine-induced lethality and the expression of sensitization to hyperlocomotor effect of cocaine in mice. Our study indicated that EMQMCM and MTEP, both substances at the doses of 5 and 10 mg/kg (but not 2.5 mg/kg), decreased cocaine-induced lethality produced by 75 mg/kg of cocaine, which was given acutely. The effect of EMQMCM was dose-dependent, and this compound at the dose of 10 mg/kg almost completely abolished the lethality induced by cocaine. MTEP reduced this cocaine effect at the doses of 5 and 10 mg/kg, equally. Furthermore, EMQMCM (1.25-5 mg/kg) at the doses of 2.5 and 5.0 mg/kg, and MTEP (2.5-10 mg/kg) only at the highest dose of 10 mg/kg, significantly reduced the expression of cocaine-induced (10 mg/kg) behavioral sensitization. Our results suggest that stimulation of mGluR1 and mGluR5 is involved in lethal effect of cocaine overdose and cocaine seeking behavior evaluated in behavioral sensitization test. However, the participation of mGluR1 in these cocaine effects seems to be dominant. Therefore, antagonists showing preferences towards mGluR1 might be useful in therapy of cocaine toxicity and abuse.

A zebrafish retinal graded photochemical stress model

INTRODUCTION

In order to develop a model for investigating the genes that contribute to retinal degeneration, we examined the early graded photochemical stress response in the adult zebrafish (Danio rerio) retina and investigated the role of an NMDA inhibitor, thiokynurenate.

METHODS

Following intravitreal injection of rose bengal (6 or 12 mg/mL), light (37x10(3) or 83x10(3) lx) was directed onto the central retina with and without 400 nM thiokynurenate. Histologic and electron microscopic analysis was performed at 2 and 4 h and gene expression analysis was carried out at 2, 4 and 6 h.

RESULTS

Light and electron microscopy demonstrated a graded photochemical response in photoreceptor, nuclear, and ganglion cell layer thickness. Increased vacuolation of the inner plexiform layer was also observed. The inhibitor produced a distinct lesion pattern. Cellular stress genes were elevated in low and high lesions, while some homeobox gene expression was reduced with thiokynurenate.

DISCUSSION

The phenotypic and genetic changes observed from this model can serve as a basis for understanding the pathology of retinal oxidative and cellular stress. These changes may aid our understanding of aging and macular degeneration.

Effect of oxcarbazepine pretreatment on convulsive activity and brain damage induced by kainic acid administration in rats

Temporal lobe epilepsy is one of the most common types of epilepsy. Progress in the understanding and treatment of this type of epilepsy would be greatly facilitated by the availability of an animal model, which reproduced the behavioral and electrographic features of this condition. In this context, kainic acid (KA, 2-carboxy-3-carboxymethyl-4-isopropenylpyrrolidine) administration causes a syndrome characterized by an acute status epilepticus and subsequent brain damage similar to that in temporal lobe epilepsy of humans. The aim of the present study was to investigate whether oxcarbazepine (10,11-dihydro-10-oxo-5 H -dibenz(b,f)azepine-5-carboxamide), an antiepileptic drug, protects against both epileptic activity and brain damage induced by KA administration. Chronically implanted adult male Wistar rats were polygraphically recorded during 10 continuous hours under 4 different conditions: a) control, b) after KA administration alone, c) after KA administration in oxcarbazepine pretreated animals and d) after the administration of oxcarbazepine alone. Animals treated with KA alone presented behavioral and electrophysiological convulsive activity as well as brain damage. Latency of seizure installation was lengthened significantly and convulsive activity was slightly reduced, however, brain damage was still present in oxcarbazepine pretreated animals. Administration of oxcarbazepine alone induced a hypnotic behavior and brain damage was also present.

Reduced early hypoxic/ischemic brain damage is associated with increased GLT-1 levels in mice expressing mutant (P301L) human tau

Mutations in tau proteins are associated with a group of neurodegenerative diseases, termed tauopathies. To investigate whether over-expressing human tau with P301L mutation also affects stroke-induced brain damage, we performed hypoxia/ischemia (H/I) in young adult P301L tau transgenic mice. Surprisingly, brain infarct volume was significantly smaller in transgenic mice compared to wild-type mice 24 h after H/I induction. TUNEL staining also revealed less brain apoptosis in transgenic mice following H/I. H/I resulted in a significant increase in tau fragments generated by caspase activation and a marked decrease in tau phosphorylation at residue T231 in cortex of wild-type but not transgenic mice. Activation of calpain and caspase-3 following H/I was also reduced in transgenic compared to wild-type mice, as reflected by lower levels of the specific spectrin breakdown products generated by calpain or caspase-3. Finally, basal levels of the glial glutamate transporter, GLT-1, were higher in brains of transgenic as compared to wild-type mice. These results support the idea that enhanced levels of GLT-1 in transgenic mice are responsible for reducing H/I-induced brain damage by decreasing extracellular glutamate accumulation and subsequent calpain and caspase activation.

Subtoxic N-methyl-D-aspartate delayed neuronal death in ischemic brain injury through TrkB receptor- and calmodulin-mediated PI-3K/Akt pathway activation

Previous studies have shown that subtoxic NMDA moderated the neuronal survival in vitro and vivo. We performed this experiment to clarify the precise mechanism underlie subtoxic NMDA delayed neuronal death in ischemic brain injury. We found that pretreatment of NMDA (100 mg/kg) increased the number of the surviving CA1 pyramidal cells of hippocampus at 5 days of reperfusion. This dose of NMDA could also enhance Akt activation after ischemia/reperfusion (I/R). Here, we examined the possible mechanism that NMDA induced Akt activation. On the one hand, we found NMDA receptor-mediated Akt activation was associated with increased expression of BDNF (brain-derived neurotrophic factor) and activation of its high-affinity receptor TrkB after I/R in the hippocampus CA1 region, which could be held down by TrkB receptor antagonist K252a. On the other hand, we found that NMDA enhanced the binding of Ca2+-dependent calmodulin (CaM) to p85 (the regulation subunit of PI-3K), which led to the activation of Akt. W-13, an active CaM inhibitor, prevented the combination of CaM and p85 and subsequent Akt activation. Furthermore, NMDA receptor-mediated Akt activation was reversed by combined treatment with LY294002, the specific blockade of PI-3K. Taken together, our results suggested that subtoxic NMDA exerts the neuroprotective effect via activation of prosurvival PI-3K/Akt pathway against ischemic brain injury, and BDNF-TrkB signaling and Ca2+-dependent CaM cascade might contribute to NMDA induced activation of PI-3K/Akt pathway.

Gender-related differences in recovery of locomotor function after spinal cord injury in mice

STUDY DESIGN

In order to study the role of gender in recovery, we induced a thoracic compression spinal cord injury (SCI) separately in 2-month-old male and female C57Bl/6 mice.

OBJECTIVES

We intended to assess effects of gender on recovery of hindlimb motor function and to correlate these with histomorphologic profiles of injured spinal cord tissue.

METHODS

Locomotor function was evaluated by three means: a modified locomotor scoring system for rodents, beam walking and computerized activity meter. Histology was analyzed by comparison of hematoxylin and eosin-stained perfused specimens.

RESULTS

Locomotor scores were 2.2+/-0.9 on day 1 in male mice, while, in contrast, they were significantly higher, 7.3+/-1.7, in females (P<0.02). On day 14 Basso, Beattie and Bresnahan scores were 9.5+/-2.2 in male mice and 16.0+/-2.2 in females (P<0.03). Terminal histology showed that the spinal cord architecture was relatively better preserved in female mice and that the extent of necrosis and infiltration of inflammatory cells was less compared to males.

SETTING

Neurobiology Research Laboratory of University of Kansas Medical School in US Department of Veterans Affairs Medical Center, Kansas City, Missouri.

CONCLUSION

We found that the severity of the initial injury as well as the ultimate recovery of motor function after SCI is significantly influenced by gender, being remarkably better in females. The mechanism(s) of neuroprotection in females, although not yet elucidated, may be associated with the effects of estrogen on pathophysiological processes (blood flow, leukocyte migration inhibition, antioxidant properties, and inhibition of apoptosis).

SPONSORSHIP

Medical Research, US Department of Veterans Affairs, the Christopher Reeve Paralysis Foundation and NIH.

Glutamate-induced excitotoxicity in retina: neuroprotection with receptor antagonist, dextromethorphan, but not with calcium channel blockers

The purpose of our studies was to evaluate different strategies for possible neuroprotection in glutamate-induced neurotoxicity in the retina. In a first set of experiments we attempted to determine if dextrorphan antagonism of glutamate action on NMDA receptors would protect against excitotoxic injury associated with secondary damage seen after surgical laser treatment in retina. In a second set of experiments, the effects of different calcium channel blockers in an in-vitro model of N-methyl-D-aspartate (NMDA)-induced retinal ganglion cell excitotoxicity that utilized rabbit retinal explants were evaluated. Dextrorphan infusion prior to laser treatment of rabbit retina produced a significant decrease in the area of neural retinal damage. We attribute the apparent dextrorphan protection to attenuation of glutamate mediated excitotoxicity secondary to laser induced cell death. Preincubation of rabbit retinal explants with verapamil, nimodipine or omega-conotoxin MVIIA did not cause a significant change in NMDA induced cell death in the ganglion cell layer.

Transient down-regulation of NMDA receptor subunit gene expression in the rat retina following NMDA-induced neurotoxicity is attenuated in the presence of the non-competitive NMDA receptor antagonist MK-801

Excitotoxic challenge has been thought to directly target NMDA-receptive neurons to undergo cell death. Recent evidence suggests that NMDA induced cell death is a selective process and that the specificity may be determined by the subunit composition of the NMDA receptor. Using a rat retinal model, we examined the effects of NMDA induced neurotoxicity on the regulation of NMDA receptor subunit gene and protein expression levels to determine if excitotoxic challenge preferentially regulates one or more of the NMDA receptor subunits. Following NMDA insult, the mRNA levels for NR1(com ), NR2A, NR2B and, to a lesser extent, the NR2C subunit were substantially reduced within 24 hr post-treatment (PT), and remained depressed for up to 48 hr. Levels for NR2D, although initially suppressed as early as 6 hr-PT, were least affected by NMDA insult and showed almost full recovery by 48 hr. By 10 days, the levels of gene expression for all five subunits recovered to levels that were indistinguishable from sham treated and untreated retinas. Co-administration of MK-801 with NMDA suppressed the effects of NMDA-induced down-regulation of all five genes. Protein levels for NR1(com ), NR2A and NR2B were also monitored at select time points following NMDA-insult. By 2 days-PT, protein levels for the three subunits were dramatically reduced. By day 10, the levels of protein expression for NR1(com)and NR2B remained suppressed despite the rise in gene expression for these two subunits, whereas protein for NR2A showed a substantial rise in expression. Of the five genes assayed, NR2A and NR2B showed the greatest reduction in expression following NMDA treatment, suggesting that one or both of these subunit may signal events leading to neuronal cell death in the retina. Conversely, gene expression of the NR2D subunit was least affected by NMDA exposure. In view of the evidence that the NR2D subunit is expressed by rod bipolar cells in the rat and that these neurons do not die following NMDA insult, it appears that inclusion of this subunit into functional receptors may provide protection against NMDA-induced cell death. Although the significance of the transient down-regulation of four out of the five NMDA receptor subunits is still not fully understood, the recovery of expression of these genes by day 10-PT indicates that not all of the NMDA receptive neurons are susceptible to NMDA-induced cell death. The preferential down-regulation of the NR2A and NR2B receptor subunits may implicate these subunits as key players in mediating the excitotoxic signal in the retina and possibly elsewhere in the brain.

Neuroprotective effects of MK-801 on L-2-chloropropionic acid-induced neurotoxicity

L-2-Chloropropionic acid is selectively toxic to the cerebellum in rats; the granule cell necrosis observed within 48 h can be prevented by prior administration of MK-801. Short-term treatment (2 h) with L-2-chloropropionic acid has also been shown to activate the mitochondrial pyruvate dehydrogenase complex in fasted adult rats. This study aimed to investigate the effect of prior exposure to MK-801 on the biochemical and neurotoxicological effects of L-2-chloropropionic acid. Extracts were prepared from the forebrain and cerebellum of animals that had been treated with L-2-chloropropionic acid, with and without prior treatment with MK-801, and were analysed using magnetic resonance spectroscopy and amino acid analysis. Glucose metabolism was studied by monitoring the metabolism of [1-(13)C]-glucose using GC/MS. L-2-Chloropropionic acid caused increased glucose metabolism in both brain regions 6 h after administration, confirming activation of the pyruvate dehydrogenase complex, which was not prevented by MK-801. After 48 h an increase in lactate and a decrease in N-acetylaspartate was observed only in the cerebellum, whereas phosphocreatine and ATP decreased in both tissues. MK-801 prevented the changes in lactate and N:-acetylaspartate, but not those on the energy state. These studies suggest that L-2-chloropropionic acid-induced neurotoxicity is only partly mediated by the NMDA subtype of glutamate receptor.

NPS 1506 attenuates cognitive dysfunction and hippocampal neuron death following brain trauma in the rat

Although several noncompetitive N-methyl-d-aspartate (NMDA) receptor antagonists have been shown to be substantially efficacious in experimental models of brain trauma, side effects associated with this class of compounds have impeded clinical application. Therefore, new noncompetitive NMDA receptor antagonists have been developed, including NPS 1506, that appear to be nontoxic but retain efficacy. In the present study, we evaluated the efficacy of NPS 1506 in a model of parasagittal fluid percussion brain trauma in the anesthetized rat. Administration of 1 mg/kg NPS 1506 at both 10 min and 4 h posttrauma induced no changes in brain temperature, mean arterial pressure, pulse, or arterial blood gasses. At 1 week postinjury, animals treated with the same dosing regimen of NPS 1506 demonstrated a dramatic attenuation of memory dysfunction evaluated by a water maze task (P < 0.02) and had greatly reduced neuron death in the CA3 subfield of the hippocampus (P < 0.01). However, NPS 1506 treatment did not significantly affect the extent of cortical tissue loss following injury. Since memory dysfunction and hippocampal damage are common and potentially related consequences of brain trauma in humans, our results suggest that NPS 1506 treatment may have clinical utility.

Pharmacological mechanisms mediating phencyclidine-induced apoptosis of striatopallidal neurons: the roles of glutamate, dopamine, acetylcholine and corticosteroids

Phencyclidine (PCP) has recently been shown to induce apoptosis of a subpopulation of striatopallidal neurons which lie in the dorsomedial caudate-putamen. The pharmacological mechanisms underlying this PCP-induced striatal death were investigated in a series of small experiments. Striatal silver-methenamine-stained sections from rats injected acutely with dizocilpine (MK-801; 1.5-5 mg/kg, i.p.) were analysed to determine whether other non-competitive N-methyl-D-aspartate (NMDA) receptor antagonists could induce apoptotic-like changes in striatal cells. The effects of amphetamine (3-12 mg/kg, i.p.) were similarly investigated as PCP can elevate extracellular dopamine levels and dopamine has the potential to be neurotoxic. The potential involvement of dopamine transmission in PCP-induced striatal apoptosis was also tested by determining the effect of co-administering SCH23390 (D1 dopamine receptor antagonist) and quinpirole (D2 dopamine receptor agonist) on PCP (80 mg/kg, s.c.)-induced striatal apoptotic-like cell death. Equivalent experiments were performed using scopolamine (cholinergic antagonist) as this drug blocks PCP-induced damage of the retrosplenial cortex and RU38486 (corticosteroid receptor antagonist) as a similar subpopulation of striatal neurons undergoes apoptosis following dexamethasone administration. Injection of neither MK-801 nor amphetamine induced elevations of apoptotic-like cells in the striatum nor did co-administration of SCH23390 or scopolamine affect the levels of PCP-induced striatal cell death. In contrast, quinpirole elevated the levels of PCP-induced apoptotic-like striatal cell death and RU38486 markedly reduced it. Within the retrosplenial cortex, scopolamine lowered PCP-induced apoptotic-like cell death whereas RU38486 was without effect. These results suggest that PCP-induced striatal apoptosis results from a corticosteroid-dependent mechanism. The results further demonstrate that different pathological mechanisms underlie PCP-induced neuronal damage in the striatum and the retrosplenial cortex.

The selective vulnerability of striatopallidal neurons

The different types of striatal neuron show a range of vulnerabilities to a variety of insults. This can be clearly seen in Huntington's disease where a well mapped pattern of pathological events occurs. Medium spiny projection (MSP) neurons are the first striatal cells to be affected as the disease progresses whilst interneurons, in particular the NADPH diaphorase positive ones, are spared even in the late stages of the disease. The MSP neurons themselves are also differentially affected. The death of MSP neurons in the patch compartment of the striatum precedes that in the matrix compartment and the MSP neurons of the dorsomedial caudate nucleus degenerate before those in the ventral lateral putamen. The enkephalin positive striatopallidal MSP neurons are also more vulnerable than the substance P/dynorphin MSP neurons. We review the potential causes of this selective vulnerability of striatopallidal neurons and discuss the roles of endogenous glutamate, nitric oxide and calcium binding proteins. It is concluded that MSP neurons in general are especially susceptible to disruptions of cellular respiration due to the enormous amount of energy they expend on maintaining unusually high transmembrane potentials. We go on to consider a subpopulation of enkephalinergic striatopallidal neurons in the rat which are particularly vulnerable. This subpopulation of neurons readily undergo apoptosis in response to experimental manipulations which affect dopamine and/or corticosteroid levels. We speculate that the cellular mechanisms underlying this cell death may also operate in degenerative disorders such as Huntington's disease thereby imposing an additional level of selectivity on the pattern of degeneration. The possible contribution of the selective death of striatopallidal neurons to a number of clinically important psychiatric conditions including obsessive compulsive disorders and Tourette's syndrome is also discussed.

Thiopental attenuates energetic impairment but fails to normalize cerebrospinal fluid glutamate in brain-injured patients

OBJECTIVES

Brain-injured patients are susceptible to secondary brain damage related to decreased cerebral perfusion pressure associated with edema formation and increased intracranial pressure (ICP). Whenever conventional therapy fails to reduce elevated ICP, barbiturate coma represents an additional intervention that may control ICP. In patients suffering from severe traumatic brain injury, cerebrospinal fluid levels of glutamate, hypoxanthine, and lactate were measured during barbiturate coma and correlated to electroencephalographic recordings and ICP.

DESIGN

Prospective, descriptive study.

SETTING

Ten-bed surgical intensive care unit in a university hospital.

PATIENTS

Twenty-one patients with severe traumatic brain injury (Glasgow Coma Scale score < or = 9); 11 required barbiturate coma because of refractory intracranial hypertension, and 10 were manageable with continuous administration of fentanyl and midazolam.

INTERVENTIONS

Thiopental was administered continuously for increased ICP within the first 24 hrs after trauma and adjusted to the burst-suppression pattern (four to six bursts per minute) on continuous electroencephalographic monitoring.

MEASUREMENTS AND MAIN RESULTS

Glutamate and hypoxanthine were analyzed using high-performance liquid chromatography, whereas lactate was measured enzymatically. Patients requiring thiopental presented with significantly higher ICP, glutamate, and hypoxanthine levels than patients receiving fentanyl and midazolam (p < .05). Within the first 24 hrs, thiopental significantly reduced cerebrospinal fluid glutamate and hypoxanthine levels in all patients, i.e., the burst-suppression pattern was successfully induced (p < .001). Interestingly, in five patients cerebrospinal fluid glutamate increased to initial values again despite unchanged neuronal activity. In these patients, ICP, hypoxanthine, and lactate remained significantly elevated compared with the six patients with steadily decreasing cerebrospinal fluid glutamate, hypoxanthine, lactate, and ICP values (p < .02).

CONCLUSIONS

Barbiturate coma does not unequivocally preserve energetic stability despite successful suppression of neuronal activity. Despite the use of barbiturate coma in patients with refractory intracranial hypertension, persistent release or impaired uptake of glutamate may be associated with continuous anaerobic metabolism, as shown by increases in cerebrospinal fluid hypoxanthine and lactate levels.

An experimental basis for implicating excitotoxicity in glaucomatous optic neuropathy

Most therapy for glaucoma is directed at the management of the intraocular pressure (IOP). Conventional wisdom holds that excessive pressure within the eye leads to the ganglion cell loss/optic nerve damage seen in this disease. Both glutamate and elevated IOP can selectively damage the retinal ganglion cells in the mammalian eye. We have identified an elevated level of glutamate in the vitreous humor of glaucoma patients (27 microM as compared to 11 microM in the control population). This concentration of glutamate suffices--on its own--to kill retinal ganglion cells. It is plausible that the IOP may represent an initial insult that precipitates the production of excessive glutamate. Therefore, even if glutamate elevation is an epiphenomenon associated with the course of the disease, it may contribute to ganglion cell loss in humans. Lowering the IOP may slow down glutamate production, but if nothing is done to block the toxic effects of glutamate as well, visual loss may result despite excellent IOP control. If interventions can be found to retard the production or toxic effects of glutamate, it may be possible to slow glaucomatous visual loss.

Barbiturates impair astrocyte glutamate uptake

Barbiturates are widely used as neuroprotective agents during status epilepticus and during surgical procedures that cause cerebral ischemia. The efficacy of this practice is unproved, however, and while barbiturates may counter neuronal excitotoxicity, they can also inhibit mitochondrial ATP production. Since glutamate uptake is energetically costly, mitochondrial inhibition could impair glutamate uptake. To examine this possibility, glutamate uptake was measured in primary rat astrocyte cultures in the presence of several barbiturates. Different barbiturates had differing effects on glutamate uptake at normal glucose concentrations, but all potentiated inhibition of glutamate uptake during glucose deprivation. Thiamylal and thiopental were the most potent barbiturates examined, with 0.3 mM causing approximately 40% reduction in glutamate uptake rates. Barbiturates also potentiated ATP depletion during glucose deprivation, supporting mitochondrial inhibition as the mechanism of these effects. These findings suggest that barbiturates can, under some conditions, impair glutamate uptake at concentrations relevant to their clinical use.

The intact isolated (ex vivo) retina as a model system for the study of excitotoxicity

Excitotoxicity is defined as a mode of neural cell death triggered by overactivation of receptors for the amino acid transmitter glutamate. There is considerable evidence that excitotoxicity is responsible for cell death in several neuropathological states, including some retinal diseases. The isolated retina, particularly from chick embryos, has been used extensively as an experimental system to characterize this process. This paper summarizes the use of isolated retina as a model system for studies of excitotoxicity from a theoretical and methodological point of view, and reviews results obtained from studies utilizing this system.

Isoflurane and pentobarbital reduce the frequency of transient ischemic depolarizations during focal ischemia in rats

Repetitive transient ischemic depolarizations (IDs) during focal cerebral ischemia are thought to contribute to ischemic damage. Isoflurane and pentobarbital reduce injury (versus the nonanesthetized state) after focal cerebral ischemia. The mechanism by which these drugs reduce injury is not known. This protective effect might be mediated by a reduction in the number of IDs. We measured the frequency of IDs during focal cerebral ischemia in animals anesthetized with isoflurane or pentobarbital and compared it with that in N2O/fentanyl anesthetized animals and in animals in which the N-methyl-D-aspartate receptor antagonist MK801 (dizocilpine) was given. Focal cerebral ischemia was induced by the occlusion of the middle cerebral artery for a period of 2 h. Cortical infarct volumes were determined after 3 h of reperfusion by image analysis of 2,3,5-triphenyl tetrazolium-stained coronal brain sections. The infarct volume was significantly greater in the N2O/fentanyl group than in the other three groups. Infarct volumes in the isoflurane, pentobarbital, and MK801 groups were similar. The frequency of IDs was significantly greater in the N2O/fentanyl group than in the other three groups, and was the least in the MK801 group. There was a direct correlation between the number of IDs and the volume of tissue injury. The data indicate that the protective effect of isoflurane and pentobarbital might, in part, be determined by their ability to reduce IDs during focal ischemia. However, the observation that the infarct volume was similar in the MK801, isoflurane, and pentobarbital groups, despite a greater frequency of IDs in the latter two groups, suggests that mechanisms other than a simple reduction in the number of IDs probably also play a role in anesthetic-mediated cerebral protection.

IMPLICATIONS

Transient ischemic depolarizations during focal ischemia contribute to brain injury. Both isoflurane and pentobarbital reduced the frequency of these depolarizations. Isoflurane- and pentobarbital-mediated reduction in the frequency of depolarizations might, in part, mediate the previously documented neuroprotective effect of these drugs.

Rigid phencyclidine analogues. Binding to the phencyclidine and sigma 1 receptors

Three phencyclidine (PCP) analogues possessing a highly rigid carbocyclic structure and an attached piperidine ring which is free to rotate were synthesized. Each analogue has a specific fixed orientation of the ammonium center of the piperidinium ring to the centrum of the phenyl ring. The binding affinities of the rigid analogues 1-piperidino-7,8-benzobicyclo[4.2.0]octene (14), 1-piperidinobenzobicyclo[2.2.1]heptene (16), and 1-piperidinobenzobicyclo[2.2.2]octene (13) for the PCP receptor ([3H]TCP) and th-receptor (NANM) were determined. The three analogues show low to no affinity for the PCP receptor but good affinity for the th-receptor and can be considered th-receptor selective ligands with PCP/th ratios of 13, 293, and 368, respectively. The binding affinities for the th-receptor are rationalized in terms of a model for the th-pharmacophore.

Excitability of neural elements within the rat corpus striatum

The excitability of cholinergic, glutamatergic and dopaminergic elements within the rat neostriatum was studied in both in vivo and in vitro preparations. In vivo, the microdialysis technique was used to measure the release of striatal acetylcholine and dopamine under basal and electrically evoked conditions. For comparison, acetylcholine, dopamine and glutamate release was assayed in media obtained from superfused rat striatal slices. Electrical stimulation was used to derive the strength-duration functions and their chronaxies of stimulated elements containing the three neurotransmitter types. The chonaxies for experiments in vitro and in vivo were similar: the chronaxy values for elements containing acetylcholine were the shortest, the values for glutamate were intermediate, and the values for those containing dopamine were the longest. Based on the chronaxy estimates, it is proposed that the elements containing acetylcholine are the large cholinergic interneurons of striatum, and the elements containing glutamate and dopamine are the terminals of corticostriatal and nigrostriatal neurons, respectively. These results indicate that electrical stimulation of neural elements surrounding a microdialysis probe can be an additional tool to examine the factors that regulate neurotransmitter release. Likewise, investigators can activate specific striatal elements by using pulse durations that coincide with their chronaxies.

The chicken retina as an experimental model for investigation of central nervous lesions

The aim of this study was to establish an experimental model to investigate neuronal lesions. The retina is an easy accessible model system to study central nervous system (CNS) disorders or neuronal effective drugs. It consists of only a few characteristic layers and is easy to prepare as an intact piece of tissue. In the retina the typical cell swelling of a developing lesion is accompanied by a very strong intrinsic optical signal (IOS) which is simultaneous with the electrical signal and is based on changes in light scattering. The IOS can be easily observed during the whole experiment and can be recorded with non-invasive optical methods for further quantification of damage. In the developed model, the lesions are elicited electrically with a tungsten microelectrode (0.1 M[omega]). The degree of damage depends on the magnitude of the stimulus. The parameters for the quantification of damage are the area and the brightness of the affected tissue. The growth of the lesions can be influenced with drugs added to the perfusion system. In the present study we tested, glycerol, mannitol and ketamine which are known to be neuroprotective in other animal models.

Ultrastructural and biochemical studies on the neuroprotective effects of excitatory amino acid antagonists in the ischemic rat retina

The effects of glutamate receptor agonists were evaluated, by utilizing the electron microscope, in a photothrombotic occlusion model of rat retinal vessels in order to study the ischemic damage and its antagonism in each morphologically identified population of retinal neurons. Rats were systemically injected with rose bengal fluorescein dye and one of their eyes was then exposed to bright light. This treatment caused neuronal damage and reduced the activities of the neuronal marker enzymes, choline acetyltransferase and glutamate decarboxylase, by approximately 75%. A single intravitreal injection of 2,3-dihydroxy-6-nitro-7-sulfamoylbenzoquinoxaline (NBQX, 10-50 nmol), an antagonist of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors, or of thiokynurenate (100-400 nmol), which also antagonizes N-methyl-D-aspartate (NMDA) receptors, performed immediately after the lesion, significantly reduced this loss. The electron microscope examination showed major damage in each type of retinal neuron, the pigment epithelium, and the microvessels. NBQX or thiokynurenic acid reduced, in a comparable manner, the effects of ischemia on the pigment epithelium, the photoreceptors, and the bipolar and the horizontal cells. NBQX was particularly efficient in reducing the damage to the amacrine cells located in the inner nuclear layer. The displaced amacrine and ganglion cells were not protected by NBQX but were almost completely spared in animals treated with thiokynurenate. These results show that antagonism of AMPA receptors is sufficient to reduce ischemic damage in a large number of retinal neurons, but that neuroprotection in the ganglion cell layer may be obtained only with agents which also antagonize NMDA receptors.

Disturbances in the distribution of neurotransmitters in the rat retina after ischemia

PURPOSE

Disturbances in neurotransmitter distribution have been observed in cerebral ischemia in the pathophysiologic process of excitotoxicity. The goal of this study was to examine the effect of pressure-induced retinal ischemia on the distribution of the retinal neurotransmitters glutamate and gamma-aminobutyric acid (GABA) within the rat retina.

METHODS

Animals were subjected to increased intraocular pressure of 110 mm Hg for 45 min using an intracameral hydrostatic pressure device. The distribution of glutamate and GABA immunoreactivity (IR) was determined at 0, 2, 4, 8 and 24 hrs after reperfusion by immunogold with silver intensification.

RESULTS

Three phases of neurotransmitter immunoreactivity patterns were discernible following retinal ischemia. Immediately following reperfusion (Phase I), a shift of GABA-IR from inner retinal neurons to the Mueller cells and their processes was noted. In contrast, despite marked decreases in neuronal glutamate-IR, a less pronounced shift of glutamate-IR to the Muller cells was simultaneously noted. This shift of neurotransmitter IR to the Mueller cells was transient with the gradual reappearance of IR within the inner retinal neurons noted 2-8 hrs after reperfusion (Phase II). Phase III began at 8 hrs after reperfusion with progressive loss of GABA-IR noted in the inner retina; by 24 hrs, secondary loss of inner retinal glutamate-IR was evident with corresponding dropout and pyknosis of inner retinal neurons apparent.

CONCLUSIONS

The distribution of glutamate-IR and GABA-IR was significantly altered following retinal ischemia. The alteration noted in Phase I suggested that the regulation of glutamate by Mueller cells was disrupted by this ischemic insult leading to glutamate excitotoxicity, and delayed neuronal cell degeneration as evidenced by the subsequent loss of inner retinal immunoreactivity in Phase III.

The effects of (-)- and (+)-beta-cyclazocine on NMDA-evoked responses and NMDA-mediated cell damage in cultured rat hippocampal neurons

Microspectrofluorimetric measurements of excitatory amino acid-evoked rises in intracellular free calcium concentration ([Ca2+]i), electrophysiological measurements of currents through single NMDA receptor-operated ion channels and estimates of cellular viability following NMDA challenge were employed to examine the interactions of (-)- and (+)-beta-cyclazocine with the NMDA receptor-channel complex in cultured rat hippocampal neurons. Rises in [Ca2+]i evoked by NMDA, but not those evoked by kainate, AMPA or 50 mM K+, were reduced by (-)-beta-cyclazocine in a concentration- and use-dependent manner with an estimated IC50 value of 272 nM. In outside-out patches, (-)-beta-cyclazocine did not change the magnitudes of unitary NMDA-evoked currents but diminished both the frequency of channel openings and their mean open time. The IC50 for (-)-beta-cyclazocine against NMDA channel open state probability was estimated at 84 nM. The actions of (-)-beta-cyclazocine were consistent with a voltage-dependent open channel block of the NMDA channel with a blocking rate constant of 7.03.10(7) M-1.s-1 at -40 mV. Neurons exposed to a high concentration of NMDA in vitro were protected from death by 1 and 10 microM (-)-beta-cyclazocine. In all of the above assays, (+)-beta-cyclazocine was considerably less potent an NMDA antagonist and neuroprotective agent than (-)-beta-cyclazocine; the IC50 for (+)-beta-cyclazocine against channel open state probability was estimated at 14 microM. The results demonstrate that (-)-beta-cyclazocine is a potent and selective inhibitor of NMDA-evoked responses in cultured rat hippocampal neurons and an effective neuroprotective agent in vitro.

A behavioral model of excitotoxicity: retinal degeneration, loss of vision, and subsequent recovery after intraocular NMDA administration in adult rats

To establish a new behavioral animal model of excitotoxicity, we injected adult rats intraocularly with a single dose of 2, 20, or 100 nmol of N-methyl-D-aspartate (NMDA). We quantified visual impairment by plotting the size of the visual field in which the rats successfully oriented towards a small, moving target. In comparison to the saline-injected (contralateral) control side, the side injected with 2 nmol of NMDA was not significantly impaired. When injected with higher doses, the rats were nearly blind immediately after surgery, with only about 20% (20 nmol NMDA) or 10% (100 nmol NMDA) of residual vision. Within about 3 weeks, however, visual performance returned to near-normal levels. Simultaneous intraocular administration of a non-competitive NMDA-antagonist, MK-801 (1 nmol), resulted in complete behavioral protection. NMDA administration led to a dose-dependent loss of cells within the ganglion cell layer, as assessed in whole-mounted retinae which were retrogradely labelled with horseradish peroxidase (HRP). Whereas 2 nmol of NMDA led to the loss of about 30% of retinal ganglion cells (RGCs), at higher NMDA doses only 13% of the RGCs survived. After the injection of 20 nmol of NMDA, large-diameter RGCs (> 22 microns) survived the lesion to a greater extent than small diameter cells (8-21 microns); at 100 nmol cells of all diameters were equally affected. The number of Nissl-stained cells with small diameters (< 11 microns), presumed to be displaced amacrine cells, was also affected by NMDA, although to a lesser degree. Analysis of behavioral performance (vision score) and the number of cells in the retina revealed a correlation of r = 0.76 between visual performance and the number of HRP-filled RGCs immediately after surgery. Lower correlations were found between visual performance and cells stained with Nissl of diameters smaller than 11 microns (presumed RGCs without retinofugal connections; r = 0.55 and r = 0.58, respectively). Because of the spontaneous recovery of vision, all correlations declined to values near 0 after 3 weeks. Thus, despite a dramatic loss of RGCs following NMDA administration, visual deficits recover significantly in adult rats within 2-3 weeks.

An ex vivo rat retinal preparation for excitotoxicity studies

Although the isolated chicken embryo retina has been a very useful in vitro preparation for studying mechanisms of excitotoxicity, it is an avian rather than mammalian tissue and its embryonic age makes it unsuitable for a full range of developmental and aging studies. Therefore, we have explored the feasibility of using the rat retina at various ages for in vitro excitotoxicity studies. In this model, retinal segments were isolated in artificial cerebrospinal fluid (CSF) at 5 degrees C then incubated under various conditions at 30 degrees C and assessed histologically for signs of neurodegenerative changes. Retinal segments from 7-, 30-, 120- and 660-day-old rats incubated in CSF for 3 h and from 30-day-old rats incubated for 24 h retained a normal histological appearance. Thus, this preparation is suitable for in vitro studies pertaining to either acute or delayed excitotoxic phenomena in the mammalian CNS at any age from infancy to old age. Excitotoxin agonist experiments in the 30-day-old rat retina revealed the surprising result that the non-NMDA agonists, kainate and AMPA, at a low concentration (100 microM) damaged a much larger number of retinal neurons than NMDA did at a very high concentration (10 mM).

6,7-Dinitroquinoxaline-2,3-dione but not MK-801 exerts a protective effect against kainic acid neurotoxicity in the goldfish retina

Recent findings indicated that the excitotoxicity of glutamate analogues was prevented in the mammalian nervous system by N-methyl-D-aspartate (NMDA) antagonists. The neurodegenerative effects of kainic acid, and the putative protection of MK-801 and 6,7-dinitroquinoxaline-2,3-dione (DNQX), were investigated by morphological studies showing the toxicity of kainic acid to the neurons of the inner nuclear layer, and measuring choline acetyltransferase and glutamate decarboxylase activities in the retina. In addition, the proliferation of Müller retinal cells was assumed as an index of neuronal degeneration and was quantified by counting glial fibrillary acidic protein immunopositive cells. Our observations suggest that the non-NMDA receptor antagonist DNQX exerted a protective effect on goldfish retinal neurons, while MK-801 did not prevent the neurotoxicity induced by kainic acid in the goldfish retina. This finding is in agreement with previous work on kainic acid toxicity in the goldfish optic tectum.

Glutamate receptor antagonists protect against ischemia-induced retinal damage

The effects of intravitreal injections of excitatory amino acid receptor antagonists have been studied on the ischemic neuronal damage induced by photochemical occlusion of the retinal vessels. Rats were systemically injected with rose bengal fluorescein dye and one of their eyes was exposed to bright light. The activities of the enzymes, choline-acetyltransferase and glutamate decarboxylase, were measured as an index of neuronal loss in the lesioned tissue. Lesioned retinas had a 75 +/- 5% reduction in choline-acetyltransferase activity and a 72 +/- 8% reduction in glutamate-decarboxylase activity, suggesting that the lesion causes a massive loss of retinal neurons, which use acetylcholine or gamma-aminobutyric acid (GABA) as neurotransmitter. A single intravitreal injection of excitatory amino acid receptor antagonists, performed immediately after the lesion, significantly reduced this loss. Both alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) and N-methyl-D-aspartate) (NMDA) types of ionotropic glutamate receptor antagonists were active in a dose-dependent manner. Almost complete protection was also obtained with relatively large doses of thiokynurenic acid (400 nmol), a non-selective antagonist of both AMPA and NMDA glutamate receptors, while 7-Cl-thiokynurenic acid, a potent and selective glycine receptor antagonist, was not active up to 200 nmol. These results strongly suggest that excitotoxic mechanisms are involved in ischemia-induced neuronal death in the retina and that appropriate treatments with antagonists of both AMPA and NMDA receptor types may significantly reduce this damage.

Transient cerebral ischemia disrupts performance on a one-trial passive avoidance task in the domestic chick and is associated with neuronal degeneration in the central nervous system

We have examined the effects of transient cerebral ischemia on performance of a one-trial passive avoidance task by chicks. Transient forebrain ischemia was induced by bilateral carotid artery occlusion for a period of 10 min. In one experimental group, ischemia was produced prior to training on the avoidance task whereas in the other group ischemic intervention was not made until 3 h after initial training. Sham-operated groups were matched to each of the experimental groups. All four groups were tested for retention of the avoidance response 24 h post-surgery. The sham-operated birds and those receiving post-training ischemia showed good retention of the avoidance response, whereas in birds which received ischemia prior to training there was significant amnesia. Neuronal damage, determined qualitatively using a silver impregnation method, was observed in several forebrain regions including the hippocampus, hyperstriatal regions, paleostriatum primitivum, ventral archistriatum, and lateral corticoid area. Damage was also observed in the Purkinje cells of the cerebellum. The behavioural and anatomical effects of transient forebrain ischemia have not been previously investigated in an avian species and the finding of significant amnesia for a learning task following ischemia is in good agreement with several behavioural studies in mammals.

Therapeutic time window and dose response of the beneficial effects of ketamine in experimental head injury

BACKGROUND AND PURPOSE

The aim of this study was to determine the time and dose response of the therapeutic effects of the N-methyl-D-aspartate receptor antagonist ketamine in experimental head injury.

METHODS

Sixty-six male Sprague-Dawley rats were divided into eight groups. Groups A, B, and C were surgically prepared but received no trauma. Groups D through H received a nonpenetrating impact to the left cranium. Group A (n = 7) received no treatment. Groups B (n = 4) and C (n = 5) received 60 and 120 mg/kg IP ketamine, respectively. Group D (n = 8) received no treatment. Groups E (n = 8) and F (n = 7) received 120 and 180 mg/kg IP ketamine, respectively, 1 hour after head trauma. Groups G (n = 7) and H (n = 9) were treated with 180 mg/kg IP ketamine 2 and 4 hours after head trauma, respectively. Neurological severity score (NSS, 0 through 25 from no injury to severe injury) was determined at 1, 24, and 48 hours after head trauma. After death at 48 hours, cortical slices were taken adjacent to the lesion on the traumatized hemisphere and from comparable sites in the contralateral hemisphere for determination of tissue specific gravity and water content. Brains were then placed in 4% formaldehyde, and the volume of hemorrhagic necrosis was measured 4 days later. NSS was compared within and between groups using the Kruskal-Wallis test for repeated measurements and Mann-Whitney U test for post hoc testing. Water content, specific gravity, and hemorrhagic necrosis were compared within and between groups using two-way ANOVA followed by Fisher's protected least significant difference procedure. A value of P < .05 was considered statistically significant.

RESULTS

Head trauma alone increased NSS, decreased specific gravity, increased water content, and caused cerebral infarction in the injured hemisphere. Ketamine given in two time-dose regimens, 180 mg/kg IP at 2 hours (group G) and 120 mg/kg IP at 1 hour (group F) after trauma, improved NSS from 11.6 +/- 1.7 and 14.4 +/- 0.8 at 1 hour to 4.4 +/- 1.3 and 8.0 +/- 1.4 (mean +/- SEM) at 48 hours, respectively (P < .03). Compared with the untreated group (group D), 180 mg/kg IP ketamine given at 2 and 4 hours after head trauma decreased the volume of hemorrhagic necrosis from 37.1 +/- 9.5 mm3 to 10.1 +/- 3.8 and 15.3 +/- 3.6 mm3, respectively (P < .05). Brain tissue specific gravity and water content at 48 hours were not significantly different between treated and untreated groups. There was no difference in rectal and temporalis muscle temperature between groups.

CONCLUSIONS

We conclude that 180 mg/kg IP ketamine was effective in ameliorating neurological dysfunction after head trauma in rats when the administration time was delayed for 1 hour to 2 hours but not after 4 hours. When given at 1 hour after head trauma, ketamine at 120 mg/kg but not 60 mg/kg is effective in reducing neurological damage after head trauma.

Excitatory amino acids: implications for psychiatric disorders research

The hyperdopaminergic theory of schizophrenia may account for some types of schizophrenia, but schizophrenia with negative symptoms or resulting in a chronic state of deterioration after repeated relapses cannot be explained by this theory. This minireview first discusses the interactions between dopamine and excitatory amino acid (EAA) neurons to produce abnormal behavior. Secondly, it deals with the influence of the psychotropic drugs on EAA, such as the relationship between phencyclidine and the hypoglutamate theory, the involvement of EAA in behavioral sensitization induced by amphetamines, the interactions between antipsychotic, antidepressant and antianxiety drugs and EAA, considering the possibility of developing newer psychotropic drugs related with EAA. Finally, glutamate receptors measured in postmortem schizophrenic brains are tabulated and the bases of the hypoglutamate hypothesis are discussed.

Ketamine alters calcium and magnesium in brain tissue following experimental head trauma in rats

We previously reported that the N-methyl-D-aspartate receptor antagonists dizocilpine maleate and ketamine improved the neurological severity score (NSS) after head trauma in rats. Other investigators have reported increased calcium and decreased magnesium following head trauma in untreated rats. The present study was designed to determine whether ketamine influences the concentrations of calcium and magnesium in brain tissue following head trauma. Eighty-six male Sprague-Dawley rats (180 +/- 15 g) were divided into eight groups. Groups A (no head injury) and C (head injury) received no treatment. Groups B (no head injury) and D-H (head injury) received ketamine. In groups D, E, and F, ketamine, 180 mg/kg i.p., was given 1, 2, and 4 h after head trauma, respectively. In groups G and H, ketamine, 120 and 60 mg/kg, respectively, was given 1 h after head trauma. After we killed the rats at 48 h, cortical slices were taken to measure tissue calcium and magnesium content by the inductively coupled plasma atomic emission spectroscopy method. In the contused hemispheres, calcium increased and magnesium decreased (p < 0.0001). Among the head-injured groups, the increase in brain tissue calcium was smaller in groups receiving 60 mg/kg of ketamine at 1 h or 180 mg/kg of ketamine at 1, 2, or 4 h than in the group not receiving ketamine. The decrease in brain tissue magnesium was smaller in the groups receiving 180 mg/kg of ketamine at 1 and 2 h than in the group not receiving ketamine.(ABSTRACT TRUNCATED AT 250 WORDS)

Secobarbital attenuates excitotoxicity but potentiates oxygen-glucose deprivation neuronal injury in cortical cell culture

We examined the effects of secobarbital and other sedative-hypnotic barbiturates on the neuronal death induced by exposure to excitatory amino acids or deprivation of oxygen or glucose in mouse cortical cell cultures. N-Methyl-D-aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-4- isoxazolepropionate, and kainate toxicities were attenuated in a concentration-dependent fashion by high concentrations of secobarbital or thiopental. Antagonism of NMDA toxicity was not overcome by increasing NMDA concentration and not mimicked by gamma-aminobutyrate. Despite these antiexcitotoxic actions, secobarbital exacerbated the neuronal death induced by deprivation of either glucose alone or oxygen and glucose together; death induced by oxygen deprivation alone was little affected. Thiopental and methohexital also increased oxygen-glucose deprivation injury. A possible explanation for this injury potentiation was provided by the observation that secobarbital enhanced the cellular ATP depletion induced by combined oxygen-glucose deprivation. Deleterious effects on ATP production may counterbalance the protective effects of barbiturates under some conditions.

Photochemically-induced lesion of the rat retina: a quantitative model for the evaluation of ischemia-induced retinal damage

The effects of ischemia-induced retinal damage were quantitatively evaluated in rats with the aim of obtaining a suitable model to study the pathogenesis of the loss of retinal neurons after ischemic episodes. Anaesthetized rats were injected with 80 mg/kg i.v. of the fluorescein rose bengal dye and one eye was exposed to cold light for different periods (from 5 to 30 min). The animals were sacrificed at different times (1 and 4 hr; 2 and 7 days) after the lesion and the photochemically-induced damage was evaluated. The damaged retinae appeared thicker, numerous neurons of the inner nuclear layers showed swelling of the perinuclear cytoplasm and the retinal vessels were enlarged. The activity of choline acetyltransferase (ChAT) and glutamic acid decarboxylase (GAD), two marker enzymes of the GABAergic and cholinergic neurons, significantly decreased, indicating a degeneration of GABAergic and cholinergic amacrine cells.

Acute, chronic and differential effects of several anesthetic barbiturates on glutamate receptor activation in neuronal culture

The acute and chronic effects of several anesthetic barbiturates, in therapeutic concentrations, on the excitatory amino acid (EAA)-induced elevation of intracellular calcium levels ([Ca2+]i) were examined in neuronal tissue culture. The ultrashort-acting barbiturate, thiamylal, was effective in blocking elevations of [Ca2+]i induced by kainate, N-methyl-D-aspartate (NMDA), and quisqualate or by membrane depolarization with 40 mM KCl. The structurally similar barbiturate, secobarbital which differs from thiamylal only by having an oxygen in place of a sulfur, was able to block elevations induced by the above EAAs but was less effective than thiamylal and did not significantly reduce [Ca2+]i that resulted from membrane depolarization with KCl. Pentobarbital, while differing from secobarbital by only a methyl group, was without effect on either the NMDA- or 40 mM KCl-induced elevations of [Ca2+]i. By contrast, cyproheptadine, a compound that has been shown to block Ca2+ channels, has a different profile from the above barbiturates in that cyproheptadine is more effective in blocking elevation of [Ca2+]i induced by membrane depolarization with KCl while the barbiturates are more effective in reducing [Ca2+]i induced by EAAs. An anticonvulsant barbiturate, phenobarbital, did not reduced elevations of [Ca2+]i induced by any EAA tested or by membrane depolarization with KCl. When cells were treated chronically with thiamylal for 4 days, 2-6 h after the abrupt drug withdrawal there was a hyperresponsiveness to the elevations of [Ca2+]i induced by both kainate and NMDA but not by quisqualate. A similar hyperresponsiveness was not seen after the chronic treatment with phenobarbital.(ABSTRACT TRUNCATED AT 250 WORDS)

Use-dependent pentobarbital block of kainate and quisqualate currents

The effects of pentobarbital on whole-cell excitatory amino acid-induced currents were studies in cultured rat cortical neurons. Currents evoked by 40 microM kainate were reversibly inhibited by pentobarbital with an IC50 value of 50 microM. The block of the kainate response by pentobarbital was use dependent, requiring kainate stimulation. In the absence of kainate activation, 10 min perfusions of 100 microM pentobarbital inhibited kainate-induced currents less than 10%. Recovery from pentobarbital block also exhibited use dependence, reversing in 5-10 s with kainate stimulation, while persisting 10 min or more in the absence of agonist. Pentobarbital inhibition of the kainate response was not voltage dependent. Responses evoked by 10 microM quisqualate consisted of a peak current desensitizing to a smaller steady-state current. The co-application of 100 microM pentobarbital reduced the steady-state current by 49 +/- 5%. The peak current before desensitization, however, was inhibited less than 10%. Currents evoked by 25 microM N-methyl-D-aspartate were not significantly inhibited by co-application of 100 microM pentobarbital. The results suggest that the pentobarbital-induced inhibition of kainate responses involves open channel block and that the block of quisqualate currents primarily involve non-desensitizing receptor channels that generate steady-state currents.

N-methyl-D-aspartate-induced neurotoxicity in the adult rat retina

The present study provides evidence that the adult mammalian retina is highly sensitive to the excitotoxic action of NMDA. In particular, we have investigated the effects of a single intravitreal injection of different doses of N-methyl-D-aspartate (NMDA) (2-200 nmoles) on the adult rat retina. Morphological evaluation of transverse sections of retinae demonstrated a dose-dependent loss of cells in the ganglion cell layer (GCL) and a reduction in the thickness of the inner plexiform layer. No obvious alterations were noted in the more distal retinal layers. Quantitative analyses of Nissl-stained whole-mounted retinae revealed that administration of 20 nmoles of NMDA resulted in a 70% loss of cells with a soma diameter greater than 8 microns (presumed retinal ganglion cells); a 20% loss of cells with a soma diameter smaller than 8 microns (presumed displaced amacrine cells) was also observed. In addition, NMDA produced a dose-dependent decrease of retinal choline acetyltransferase (ChAT) activity, suggesting that NMDA affects cholinergic amacrine cells as well. MK-801, a non-competitive NMDA antagonist, completely prevented the NMDA-induced loss of cells in the GCL and blocked, in a dose-dependent manner, the NMDA-induced decrease of ChAT activity. The excitotoxic action of NMDA observed in these experiments is thus likely mediated through the NMDA receptor subtype. This "in vivo" model may be utilized to identify potential drugs that antagonize or limit the deleterious effects consequent to NMDA receptor overstimulation in the central nervous system.

Neuroprotective effects of SKF 10,047 in cultured rat cerebellar neurons and in gerbil global brain ischemia

BACKGROUND AND PURPOSE

Excitatory amino acids and their receptors are involved in mediating ischemic neuronal damage. The sigma-agonists are believed to interact with the N-methyl-D-aspartate receptor. Therefore, we studied the neuroprotective, hypothermic, and motor deficit effects of the sigma-agonist SKF 10,047 and the N-methyl-D-aspartate antagonist MK-801.

METHODS

Neuroprotective effects were compared using an in vitro ischemia model of cultured rat cerebellar granule cells and the gerbil model of global brain ischemia induced by 5 minutes of bilateral carotid artery occlusion followed by 7 days of reperfusion.

RESULTS

In vitro, (+)MK-801 protected against 100 microM glutamate with a 50% protective concentration of 30 nM, followed by (-)MK-801 (150 nM), cyclazocine (0.5 microM), (+)SKF 10,047 (3.3 microM), pentazocine (5 microM), and (-)SKF 10,047 (10 microM). In vivo, (+)SKF 10,047 pretreatment (60 mg/kg) or multiple postischemic treatments provided neuroprotection comparable with MK-801 pretreatment (10 mg/kg). When ischemic animals were administered the multiple dosing regimen of (+)SKF 10,047, no hypothermic effect was noted in the temporalis muscle over 4 hours' postischemia. Motor deficits monitored by a swing grid test showed that 50% recovery from (+)SKF 10,047 was 5.5 times faster than recovery from MK-801.

CONCLUSIONS

These results are the first to report a hypothermia-free, in vivo neuroprotective effect of (+)SKF 10,047, a prototypical drug of the sigma-agonist class.

NMDA antagonist neurotoxicity: mechanism and prevention

Antagonists of the N-methyl-D-aspartate (NMDA) subtype of glutamate receptor, including phencyclidine (PCP) and ketamine, protect against brain damage in neurological disorders such as stroke. However, these agents have psychotomimetic properties in humans and morphologically damage neurons in the cerebral cortex of rats. It is now shown that the morphological damage can be prevented by certain anticholinergic drugs or by diazepam and barbiturates, which act at the gamma-aminobutyric acid (GABA) receptor-channel complex and are known to suppress the psychotomimetic symptoms caused by ketamine. Thus, it may be possible to prevent the unwanted side effects of NMDA antagonists, thereby enhancing their utility as neuroprotective drugs.

Effects of ketamine on outcome from temporary middle cerebral artery occlusion in the spontaneously hypertensive rat

This experiment evaluated the potential for ketamine HCl, a non-competitive glutamate antagonist, to minimize injury resulting from temporary focal cerebral ischemia. Male spontaneously hypertensive rats were randomly assigned to receive either ketamine (n = 13) or halothane anesthesia (n = 12) during 2 h of reversible middle cerebral artery occlusion (MCAO). Ketamine was administered as a 50 mg/kg i.v. loading dose followed by a continuous 1.25 mg/kg/min i.v. infusion beginning 25 min prior to ischemia and continued until 30 min after reperfusion. An additional group of rats (ketamine-shams, n = 8) underwent craniectomy and ketamine administration (as above) but the middle cerebral artery was not ligated. Physiologic values were similar between groups with the exception of plasma glucose which was elevated in the halothane-MCAO group. After 4 days recovery, rats in all groups were neurologically evaluated. There were no differences between the two groups undergoing MCAO for neurologic grading or open field behavior, although both groups performed worse than did ketamine-shams (P less than 0.05). In contrast, motor performance revealed more severe deficits in the ketamine-MCAO rats vs either the halothane-MCAO or ketamine-sham groups (P less than 0.05). Cerebral infarct volume was then planimetrically measured after triphenyl tetrazolium chloride (TTC) staining of fresh brain sections. Mean +/- S.D. infarct volume was not different between the halothane-MCAO (134 +/- 51 mm3) and ketamine-MCAO (131 +/- 64 mm3) groups. Seven of 8 sham rats were free of TTC demarcated injury and in the remaining rat injury was minimal.(ABSTRACT TRUNCATED AT 250 WORDS)

Mitogenic effects of excitatory amino acids in the adult rat retina

We studied the retinas of adult rats after the intravitreal injection of excitatory amino acids and ouabain. Kainic acid, domoic acid, N-methyl D-asparate and ouabain produced swelling and vacuolization of the outer plexiform, inner nuclear and inner plexiform layers and pyknosis. Mitoses were present in retinas treated with all agents other than N-methyl D-asparate. Rompun ketamine anesthesia blocked the mitogenic effects. Immunohistochemical labeling of both glial fibrillary acidic protein and S100 protein would indicate that the mitoses are occurring in glial cells. We suggest that the mitogenic effects are mediated through action on glial cationic channels, and might account for the reactive gliosis observed in some retinal lesions.

Reversible focal ischemia in the rat: effects of halothane, isoflurane, and methohexital anesthesia

Barbiturates and the volatile anesthetic isoflurane reduce CMR to similar values. If the mechanism of barbiturate protection against focal ischemic injury is due to a reduction in cellular energy requirements, then isoflurane should similarly reduce ischemic injury. To evaluate this, spontaneously hypertensive rats underwent 2 h of reversible middle cerebral artery occlusion (MCAO) while receiving deep methohexital, isoflurane, or halothane anesthesia. Ninety-six hours postischemia, neurologic deficits were present but without a difference between groups. Mean +/- SD infarct volume, as assessed by triphenyl tetrazolium chloride staining and computerized planimetry, was significantly less in the methohexital group (n = 8; 166 +/- 74 mm3) than in either the halothane (n = 9; 249 +/- 71 mm3; p less than 0.04) or the isoflurane (n = 9; 243 +/- 62 mm3; p less than 0.03) groups. One possible explanation for the lack of protective effect for isoflurane might be related to its vasodilative properties, which could result in a cerebral vascular steal. To examine this possibility, rats anesthetized with methohexital or isoflurane underwent autoradiographic determination of CBF with or without MCAO. In isoflurane-anesthetized sham rats (n = 5; no ischemia), CBF was approximately three times greater than in methohexital-treated (n = 5) sham rats. During ischemia, although a regional reduction in flow was noted in both anesthetic groups, mean flow remained greater in the isoflurane group.(ABSTRACT TRUNCATED AT 250 WORDS)