Cellular origins of endogenous amino acids released into the extracellular fluid of the rat striatum during severe insulin-induced hypoglycemia

Morphine physical dependence intensification by hypoglycemia: NMDA receptor involvement

The destruction of N-methyl-D-aspartate (NMDA) receptor-bearing neurons by insulin-induced hypoglycemia has long been known to be due to excessively released aspartate and glutamate. In this study, the effects of NMDA-bearing neuron destruction by insulin-induced hypoglycemia on the development of morphine (M) physical dependence, which was found related to functional states of NMDA receptors, were investigated. NMDA receptor antagonists CGP 39551 and MK-801 were used to see whether they could change intensity of precipitated abstinence syndrome by preventing destruction. Therefore, two groups of fasting rats injected IP with physiological saline, and another two groups given IP 10 mg/kg CGP 39551 and 0.5 mg/kg MK-801 received 15 IU/kg crystalline zinc insulin IP. After 2 h, the rats were orally given 2 x 4 ml of 5% glucose solution. On the third day, two pellets containing 75 mg base M were SC implanted to all rats. On the sixth day, they were IP given 2 mg/kg naloxone (NL). Then jumps, wet-dog shakes, and defecation were counted while diarrhea and ptosis were rated for 15 min. The rats given insulin manifested significantly more intense NL-precipitated abstinence syndrome than controls. The rats administered CGP 39551 showed a less intense physical dependence than those injected with only insulin. But, the intensity was still significantly higher than controls. In the rats that received MK-801, the abstinence syndrome was more or less equal to that in controls.(ABSTRACT TRUNCATED AT 250 WORDS)

45CaCl2 autoradiography in brain from rabbits with encephalopathy from acute liver failure or acute hyperammonemia

In experimental hepatic encephalopathy and hyperammonemia, extracellular levels of glutamate are increased in hippocampus and cerebral cortex. It has been suggested that overstimulation of glutamate receptors causes a pathological entry of calcium into neurons via receptor-operated (NMDA- and AMPA-type) or voltage-dependent calcium channels leading to calcium overload and cell death. Neurodegeneration as a result of exposure to excitotoxins, including glutamate, can be localized and quantified using 45CaCl2 autoradiography. This approach was used to study cerebral calcium accumulation in rabbits with acute liver failure and acute hyperammonemia. Acute liver failure was induced in 6 rabbits, acute hyperammonemia in 4 rabbits; 4 control rabbits received sodium-potassium-acetate. At the start of the experiment 500 microCi 45CaCl2 was given intravenously. After development of severe encephalopathy, the animals were killed by decapitation. All rabbits with acute liver failure or acute hyperammonemia developed severe encephalopathy, after 13.2 +/- 1.7 and 19.3 +/- 0.5 hours respectively (mean +/- SEM). Plasma ammonia levels were 425 +/- 46 and 883 +/- 21 mumol/l, respectively (p < 0.05). Control rabbits maintained normal plasma ammonia levels (13 +/- 5 mumol/l), demonstrated normal behaviour throughout the study and were sacrificed after 16 hours. 45Ca(2+)-autoradiograms of 40 microns brain sections were analyzed semiquantitatively using relative optical density and computerized image analysis. As compared to background levels 45Ca was not increased in hippocampus or any other brain area of rabbits with severe encephalopathy from acute liver failure or acute hyperammonemia. This suggests that, despite increased extracellular brain glutamate levels in these conditions, glutamate neurotoxicity was not important for the development of encephalopathy in these rabbits.

Interleukin-6 protects cultured rat hippocampal neurons against glutamate-induced cell death

We examined the effect of interleukin-6 (human recombinant) on glutamate-induced neuronal death of cultured 20-day fetal rat hippocampal neurons. After 7 days in culture, the hippocampal neurons were markedly degenerated by the addition of L-glutamate and also N-methyl-D-aspartate. The neuronal death was prevented by the addition of MK801, a potent N-methyl-D-aspartate antagonist. Interleukin-6 at the concentration of 50 ng/ml has a significant preventive effect on the glutamate-induced neuronal death. Basic fibroblast growth factor at the concentration of 100 ng/ml gave also significant protective effect on hippocampal neurons, but nerve growth factor was ineffective in preventing the toxicity. It has been postulated that glutamate plays an important role in the pathogenesis of neuronal death such as ischemia and the various neurological diseases. Interleukin-6 might have somewhat physiological or pathological role in these events.

Effects of ischaemia on neurotransmitter release from the isolated retina

The effects of "ischaemia" (glucose-free Krebs-bicarbonate medium gassed with N2/CO2) on the release of glutamate and other major neurotransmitters in the retina were examined using the isolated rat and rabbit retina. Amino acid transmitters, acetylcholine, and dopamine were measured by HPLC. The release of glutamate, aspartate, GABA, and glycine from ischaemic retinas was more than doubled after 30 min, and after 90 min of ischaemia the release of amino acids was approximately 15-20-fold that of control values. Ischaemia also produced large increases in the release of dopamine from both the rat and especially the rabbit retina. In contrast, the release of acetylcholine from the rat retina was significantly decreased by ischaemia, although the release of choline was increased. Because the ischaemia-induced release of glutamate, aspartate, and GABA from the rat retina was completely Ca independent, and exposure of the retina to high K (50 mM) did not stimulate amino acid release, it is concluded that the mechanisms underlying the ischaemia-induced release do not involve an initial release of K or an influx of calcium.

Selective excitotoxic pathology in the rat hippocampus

The pattern of cell loss and neuronal degeneration resulting from multiple microinjections of N-methyl-D-aspartate (NMDA), ibotenate (IBO), quisqualate (QUIS), and kainate (KA) into hippocampus was studied, together with the protection provided by the NMDA antagonist 3-(+/-)-2-carboxypiperazin-4-yl-propyl-1-phosphonate (CPP). Histological evaluation was carried out after 7 days of survival. NMDA and IBO resulted in an extensive loss of all cells in the hippocampus including dentate gyrus, hilar cells, and CA3-CA1 pyramidal cells, but there was an absence of damage to areas and structures outside hippocampus. After QUIS and KA injections the hippocampal damage was limited to hilar cells in the dentate gyrus, CA3 pyramidal cells, and partial loss of CA1 cells; there was extensive extrahippocampal damage including entorhinal cortex, amygdala, layers III, V, and VI of ventral neocortex, olfactory areas, and various thalamic nuclei. CPP provided almost complete protection from the effects of intrahippocampal injections of NMDA and IBO, but did not affect the hippocampal cell loss found after QUIS and KA (with the exception of minor protection of some CA1 cells). CPP protected most extrahippocampal sites from the damage resulting from QUIS and KA, indicating that such excitotoxic cell death is indirect and involves NMDA receptor activation by an endogenous agent. The use of multiple microinjections as opposed to single injections allows a clearer interpretation of selective excitotoxic vulnerability.

Glutamate stimulates glucagon secretion via an excitatory amino acid receptor of the AMPA subtype in rat pancreas

The effect of L-glutamate was studied on glucagon secretion from rat isolated pancreas perfused with 2.8 mM glucose. L-Glutamate (3.10(-5)-10(-4)M) induced an immediate, transient and concentration-dependent glucagon release. The three non-N-methyl-D-aspartate (NMDA) receptor agonists, kainate (3.10(-5)-3.10(-3)M), alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) (3.10(-5)-10(-4)M) and quisqualate (3.10(-6)-10(-5)M), all elicited a peak-shaped glucagon response. Compared to glutamate, AMPA and quisqualate exhibited a similar efficacy, whereas kainate caused a 4-fold higher maximal glucagon response. In contrast, NMDA (10(-3)M) was ineffective. The selective antagonist of non-NMDA receptors, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 5.10(-5)M), totally prevented the glucagon response to 10(-4) M glutamate (IC50 congruent to 0.8 +/- 0.3 10(-6)M) and 3.10(-4)M kainate. Furthermore, quisqualate at a maximal effective concentration (3.10(-4)M) inhibited the response to kainate (10(-3)M). This study showed that L-glutamate stimulates glucagon release in rat pancreas by activating a receptor of the AMPA subtype.

Stress preferentially increases extraneuronal levels of excitatory amino acids in the prefrontal cortex: comparison to hippocampus and basal ganglia

The technique of intracerebral microdialysis was used to assess the effect of stress on the extracellular concentrations of excitatory amino acids, glutamate and aspartate, in the rat medial prefrontal cortex, hippocampus, striatum, and nucleus accumbens. A 20-min restraint procedure led to an increase in extracellular glutamate in all regions tested. The increase in glutamate levels was significantly higher in the prefrontal cortex than that observed in other regions. With the exception of the striatum, extracellular levels of aspartate were increased in all regions. Furthermore, the increase in aspartate levels was significantly higher in prefrontal cortex compared to hippocampus and nucleus accumbens. Local perfusion of tetrodotoxin during the restraint procedure significantly decreased the stress-induced increase in extracellular excitatory amino acids. In order to ensure that the above results were not an artifact of restraint not associated with stress (e.g., decreased mobility), we also examined the effect of swimming stress on the extracellular levels of excitatory amino acids in selected regions, i.e., striatum and medial prefrontal cortex. Both regions displayed a significant increase in extracellular levels of aspartate and glutamate following 20 min of swimming in room temperature water. This study provides direct evidence that stress increases the neuronal release of excitatory amino acids in a regionally selective manner. The implications of the present findings for stress-induced catecholamine release and/or hippocampal degeneration are discussed.

Relationship between extracellular neurotransmitter amino acids and energy metabolism during cerebral ischemia in rats monitored by microdialysis and in vivo magnetic resonance spectroscopy

The time-course of changes in extracellular glutamate and energy metabolism during 30 or 60 min of complete cerebral ischemia and 60-90 min of reperfusion was investigated by microdialysis and magnetic resonance spectroscopy in parallel groups of rats. During the first 10 min of ischemia, adenosine triphosphate (ATP) was completely depleted, and lactate increased 10-fold; after 30 min, intracellular pH had decreased to 6.33 +/- 0.11. ATP and lactate did not change further between 30 and 60 min of ischemia. Glutamate increased 30-fold between 10 and 30 min of ischemia and continued to increase in the 60-min ischemia group. After 30 min of reperfusion, glutamate had returned to pre-ischemic levels in both groups. The cellular energy state recovered within 50-60 min after 30 min of ischemia but never returned to more than 60% of baseline values after 60 min of ischemia. The continued increase in extracellular glutamate after total depletion of ATP suggests that glutamate release during ischemia is not entirely energy dependent. Ca(2+)-independent glutamate release and failure of energy-dependent glutamate re-uptake mechanisms may result in continued increase in extracellular glutamate. The rapid normalization of extracellular glutamate after 30 and 60 min of ischemia despite differences in the recovery of energy metabolism suggests that the glutamate levels were reduced by an energy-independent mechanism, such as diffusion into the restored circulation.

Amino acid concentrations in cerebrospinal fluid and serum in Alzheimer's disease and vascular dementia

Cerebrospinal fluid (CSF) and serum levels of 22 amino acids were studied in 13 patients with dementia of the Alzheimer type (DAT), 13 patients with vascular dementia (VD) and 15 age-matched controls. We found significantly reduced levels of glutamate in CSF samples from DAT patients compared to VD and control subjects, but CSF levels of aspartate were found to be significantly elevated in the two groups of dementia studied. Moreover, CSF concentrations of tyrosine, leucine and phenylalanine were significantly increased in VD patients in comparison with those in DAT patients and control subjects. Our results showed a wide increase in CSF/serum amino acid ratios in DAT and VD groups compared to controls. However, no differences were found in CSF/serum ratios between dementia groups. These changes show evidence for a possible disorder of amino acid metabolism with different patterns in these two dementia types.

Effects of short-term hypoxia on [3H]glutamate release from preloaded hippocampal and cortical synaptosomes

The effect of short-term hypoxia on the release of [3H]glutamate from preloaded hippocampal and cortical synaptosomes was studied in a rapid superfusion system. The technique minimised the loss of released glutamate by reuptake. The results indicated that the effects of short term hypoxia were qualitatively similar to those reported in previous studies using more long-term hypoxia, but were significantly smaller. The non-Ca(2+)-dependent efflux of glutamate from cortical synaptosomes was increased by hypoxia as was the Ca(2+)-dependent release from hippocampal tissue. Possible mechanisms for these findings were discussed. The small amplitude of these changes in comparison to the effects seen in slowly perfused tissue in vitro and in vivo indicated that the contribution made by changes in neuronal efflux to the overall increase in extracellular glutamate seen in hypoxia is relatively minor.

An evaluation of the role of extracellular amino acids in the delayed neurodegeneration induced by quinolinic acid in the rat striatum

The effect of the N-methyl-D-aspartate receptor agonist quinolinic acid on extracellular levels of striatal amino acids, following its injection directly into the rat striatum, has been investigated using intracerebral dialysis in the attempt to elucidate the cellular mechanisms underlying delayed neurodegeneration. A neurotoxic dose (200 nmol) of quinolinic acid caused an elevation in the levels of aspartate (x 6), glutamate (x 2), asparagine (x 2), serine (x 2.5), glycine (x 3), and threonine (x 2) which peaked in the fractions 20-40 min after the injection and achieved statistical significance for aspartate and asparagine. The dialysate content of these amino acids returned to basal values within 1 h and no further changes were observed in the following 4 h. Injection of an equivalent dose of nicotinic acid did not mimic the effect of quinolinate, indicating that osmotic and/or mechanical damage was not responsible for the observed phenomena. Pretreatment with the N-methyl-D-aspartate receptor channel blocker dizocilpine (MK-801) completely blocked the quinolinate-induced increase of the amino acids, thus confirming that N-methyl-D-aspartate receptor activation is required for this effect to occur. Seven days after the injection of quinolinate, histological analysis showed an extensive loss of neuronal elements in the injected striatum, which was completely prevented in the dizocilpine-treated animals. Sections from striata of animals injected with nicotinic acid showed normal-appearing neurons and no differences were detectable from controls.(ABSTRACT TRUNCATED AT 250 WORDS)

Limiting ischemic injury by inhibition of excitatory amino acid release

Excitatory amino acids (EAAs) are important mediators of ischemic injury in stroke. N-Methyl-D-aspartate (NMDA) receptor antagonists have been shown to be very effective neuroprotective agents in animal models of stroke, but may have unacceptable toxicity for human use. An alternative approach is to inhibit the release of EAAs during stroke. BW1003C87 [5-(2,3,5-trichlorophenyl)-2,4-diaminopyrimidine], a drug that inhibits veratrine-induced release of the EAA glutamate in vitro, was tested in a rat model of proximal middle cerebral artery (MCA) occlusion. BW1003C87 significantly decreased ischemia-induced glutamate release in brain when given either 5 min before or 15 min following permanent MCA occlusion. Pretreated and posttreated rats had smaller infarct volumes and preserved glucose metabolism in the ischemic cortex at 24 h after MCA occlusion. BW1003C87 did not induce heat shock protein in the cingulate or retrosplenial cortex, suggesting that it does not injure neurons in these regions as do NMDA antagonists. These results demonstrate that drugs that inhibit glutamate release in ischemia may be nontoxic and show promise for the treatment of stroke.

Evidence that the loss of the voltage-dependent Mg2+ block at the N-methyl-D-aspartate receptor underlies receptor activation during inhibition of neuronal metabolism

In this study, the importance of the Mg2+ blockade of the N-methyl-D-aspartate (NMDA) receptor during metabolic stress was examined in embryonic day 13 chick retina. Retina exposed to mild conditions of metabolic stress (i.e., blockade of glycolysis with 1 mM iodoacetate for 30 min) underwent acute histological somal and neuritic swelling and an increase in gamma-aminobutyric acid (GABA) release into the medium. These acute signs of metabolic stress were eliminated by NMDA antagonists present during pharmacological blockade of glycolysis, occurred in the absence of a net increase in extracellular glutamate or aspartate, and were not affected by the presence or absence of Ca2+ in the incubation medium. One possible explanation for the activation of NMDA receptors in the absence of an increase in extracellular ligand is that NMDA sensitivity during metabolic stress may be governed at the receptor level. Depolarization of membrane potential during metabolic stress may result in the loss of the Mg2+ blockade from the NMDA receptor channel, resulting in an increased potency for glutamate. To test this, the dose-response characteristics for NMDA, glutamate, and kainate in the presence or absence of extracellular Mg2+ and the effects of Mg2+ on metabolic inhibition were examined. The potency for NMDA- or glutamate-mediated acute toxicity was enhanced two- to fivefold in the absence of Mg2+. Omission of Mg2+ greatly decreased the minimal concentration of agonist needed to produce acute excitotoxicity; 25 versus 5 microM for NMDA and 300 versus 10 microM for glutamate in 1.2 or zero Mg2+, respectively. Elevating external Mg2+ to 20 mM completely protected against NMDA-mediated acute toxic effects. In contrast, varying external Mg2+ had no effect on kainate-induced toxicity. Acute toxicity caused by inhibition of metabolism was not potentiated in the absence of Mg2+ but was attenuated by elevating extracellular Mg2+. The protective effect of Mg2+ during metabolic inhibition was not additive with NMDA antagonists, suggesting that the action of Mg2+ was at the level of the NMDA receptor. These findings are consistent with the hypothesis that the Mg2+ block is lifted during metabolic inhibition and may be the primary event resulting in NMDA receptor activation.

Regulatory Interactions Among Axon Terminals Affecting the Release of Different Transmitters from Rat Striatal Slices Under Hypoxic and Hypoglycemic Conditions

An in vitro model of ischemia was utilized to study the effects of both oxygen and glucose depletion on transmitter release from rat striatal slices. The spontaneous and stimulation-evoked releases of tritiated dopamine, gamma-aminobutyric acid, glutamate, and acetylcholine were measured. Hypoxia increased the evoked release of glutamate and dopamine without effect on the resting release. In contrast, hypoglycemia itself increased the resting release of dopamine. Hypoxia in combination with hypoglycemia provoked a massive release of glutamate, dopamine, and gamma-aminobutyric acid. The effect on acetylcholine release was less pronounced. Ca2+ withdrawal partly reduced the effect of hypoxia combined with hypoglycemia on dopamine release and application of tetrodotoxin (1 microM) abolished it. MK-801 (3 microM), an N-methyl-D-aspartate receptor antagonist, attenuated the effect of hypoxia and hypoglycemia on [3H]dopamine release. omega-Conotoxin (0.1 microM) had a similar effect on stimulation-evoked release under a hypoxic condition. The D2 receptor antagonist sulpiride (100 microM) failed to enhance the release of [3H]acetylcholine in hypoxia combined with hypoglycemia. It was suggested that in response to hypoxia combined with hypoglycemia there is a massive release of glutamate due to the increased firing rate which in turn releases dopamine from the axon terminals through stimulation of presynaptic N-methyl-D-aspartate receptors. Dopaminergic inhibitory control on ACh release seems not to be operative under conditions of hypoxia combined with hypoglycemia.

Inhibition of sodium-potassium-ATPase: a potentially ubiquitous mechanism contributing to central nervous system neuropathology

Direct and indirect evidence suggests that Na+/K(+)-ATPase activity is reduced or insufficient to maintain ionic balances during and immediately after episodes of ischemia, hypoglycemia, epilepsy, and after administration of excitotoxins (glutamate agonists). Recent results show that inhibition of this enzyme results in neuronal death, and thus a hypothesis is proposed that a reduction and/or inhibition of this enzyme contributes to producing the central neuropathy found in the above disorders, and identifies potential mechanisms involved. While the extent of inhibition of Na+/K(+)-ATPase during ischemia, hypoglycemia and epilepsy may be insufficient to cause neuronal death by itself, unless the inhibition is severe and prolonged, there are a number of interactions which can lead to a potentiation of the neurotoxic actions of glutamate, a prime candidate for causing part of the damage following trauma. Presynaptically, inhibition of the Na+/K(+)-ATPase destroys the sodium gradient which drives the uptake of acidic amino acids and a number of other neurotransmitters. This results in both a block of reuptake and a stimulation of the release not only of glutamate but also of other neurotransmitters which modulate the neurotoxicity of glutamate. An exocytotic release of glutamate can also occur as inhibition of the enzyme causes depolarization of the membrane, but exocytosis is only possible when ATP levels are sufficiently high. Postsynaptically, the depolarization could alleviate the magnesium block of NMDA receptors, a major mechanism for glutamate-induced neurotoxicity, while massive depolarization results in seizure activity. With less severe inhibition, the retention of sodium results in osmotic swelling and possible cellular lysis. A build-up of intracellular calcium also occurs via voltage-gated calcium channels following depolarization and as a consequence of a failure of the sodium-calcium exchange system, maintained by the sodium gradient.

Effect of depolarization on striatal amino acid efflux in perinatal rats: an in vivo microdialysis study

We used in vivo microdialysis to determine if infusion of depolarizing concentrations of potassium stimulated striatal excitatory amino acid (EAA) efflux in post-natal day (PND) 7 rats. Dialysis probes were perfused with 100 mM KCl for 60 min (n = 6); EAA efflux was unaffected until 40-60 min after onset of the infusion, when a trend towards increased EAA efflux was observed (glutamate 284 +/- 56% of baseline). In animals exposed to 8% oxygen (n = 7) before a more prolonged (100 min) KCl infusion, again over the first 40 min of KCl there were no changes in EAA efflux; subsequently, glutamate, aspartate and taurine efflux increased (peak values 682 +/- 187%, 228 +/- 32%, and 1208 +/- 437% of baseline). These data suggest that in PND 7 rats a substantial contribution to basal striatal EAA efflux may be derived from non-neurotransmitter pools.

N-methyl-D-aspartate releases excitatory amino acids in rat corpus striatum in vivo

There is a considerable amount of conflicting evidence from several studies as to the action of applied N-methyl-D-aspartate (NMDA) on the release of glutamate and aspartate in the brain. In the present study the effect of NMDA on extracellular levels of endogenous amino acids was investigated in conscious, unrestrained rats using intracerebral microdialysis. NMDA caused dose-related increases in extracellular levels of glutamate and aspartate; threonine and glutamine were unaffected. The NMDA-evoked release of glutamate and aspartate was significantly decreased by the specific NMDA receptor antagonist 3-[(+-)-2-carboxypiperazin-4-yl]-propyl-l-phosphonic acid. In addition, increasing the perfusate concentration (and therefore the extracellular concentration) of Ca2+ significantly enhanced the NMDA-evoked release of glutamate and aspartate, whereas removal of Ca2+ and addition of a high Mg2+ concentration to the perfusate caused a significant reduction in their NMDA-evoked release. Moreover, the NMDA-evoked release of glutamate and aspartate was reduced in decorticate animals. These results demonstrate that, in the striatum in vivo, NMDA causes selective release of endogenous glutamate and aspartate from neurone terminals and that this action occurs through an NMDA receptor-mediated mechanism. The ability of NMDA receptor activation to induce release of glutamate and aspartate, perhaps by a positive feedback mechanism, may be relevant to the pathologies underlying epilepsy and ischaemic and hypoglycaemic brain damage.

Hypoglycemia alters striatal amino acid efflux in perinatal rats: an in vivo microdialysis study

In adult brain, during insulin-induced hypoglycemia, striatal extracellular fluid concentrations of the excitatory amino acids glutamate and aspartate rise markedly (fourfold to tenfold). In this study, we used in vivo microdialysis to determine if insulin-induced hypoglycemia altered striatal amino acid efflux in similar fashion in the immature brain. Microdialysis probes were inserted into the right striatum of rats on postnatal day 7. After a 2-hour recovery period, in each animal a 30-minute baseline sample was obtained. Then insulin (0.6 mu/kg, intraperitoneal injection) was administered (n = 6) and dialysate sampling was continued over the next 210 minutes (terminal blood glucose level less than 5 mg/dl). Untreated control rats (n = 6) were sampled over the same time interval. After pre-column derivatization with o-phthaldialdehyde, dialysate samples were assayed by high-pressure liquid chromatography with electrochemical detection to measure their amino acid content; eight amino acids (glutamate, aspartate, taurine, glutamine, alanine, serine, glycine, and asparagine) were consistently detected. In controls, amino acid efflux did not change over 4 hours. In hypoglycemic animals, glutamate efflux increased (peak: 238 +/- 85% of baseline, p = 0.02, repeated measures analysis of variance [ANOVA]), glutamine efflux declined (to 44 +/- 5% of baseline, p = 0.002, ANOVA), and taurine efflux increased (up to 310 +/- 120% of baseline; p less than 0.06, ANOVA). In contrast with 9- to 12-fold increases in aspartate efflux reported in adult striatum, aspartate efflux increased only slightly (to 174 +/- 69% of baseline; not significant).(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of N-methyl-D-aspartate on [Ca2+]i and the energy state in the brain by 19F- and 31P-nuclear magnetic resonance spectroscopy

The effects of N-methyl-D-aspartate (NMDA) on the free intracellular Ca2+ concentration [( Ca2+]i) and the energy state in superfused cerebral cortical slices have been studied using 19F- and 31P-nuclear magnetic resonance spectroscopy. [Ca2+]i was measured using the calcium indicator 1,2-bis(2-amino-5-fluorophenoxy)ethane-N,N,N',N'-tetraacetic acid (5FBAPTA). NMDA (10 microM) in the absence of extracellular Mg2+ caused the expected rise in [Ca2+]i but produced an impairment of the energy state: the phosphocreatine (PCr) content was decreased by 42%, and the Pi/PCr ratio was increased by 55%. There was no detectable change in ATP or free intracellular Mg2+ concentration. Increasing the NMDA concentration in the superfusing medium to 100 or 400 microM caused no further increase in [Ca2+]i or further decrease in PCr content, but the Pi/PCr ratio continued to rise. The impairment of the energy state preceded the effect on [Ca2+]i, and these changes were irreversible on return to control conditions. Repeating the experiments in the presence of 1.2 mM extracellular Mg2+ resulted in similar changes in the energy state, with no change in [Ca2+]i. The possibilities that the effects were due to membrane depolarisation or to the presence of 5FBAPTA within the tissues were eliminated. The results suggest that low concentrations (10 microM) of NMDA produce an impaired energy state independent of the presence of extracellular Mg2+ and that the decreased energy state is not due to the changes in [Ca2+]i, which are seen only in the absence of extracellular Mg2+.

Effects of ethanol on hexose uptake by cultured rat brain cells

The effects of ethanol on hexose uptake by glial cells was investigated using primary cultures prepared from term rat fetuses. Specific 3H 2-deoxy-D-glucose (2DG) uptake was significantly reduced by a 4-hr exposure to ethanol at concentrations of 25, 50, and 100 mM, but not 200 or 300 mM. The inhibitory effect of 50 mM ethanol increased with the duration of exposure, with 2DG uptake inhibited by 36% after 18 hr. Astrocytes cultured from the brains of term fetuses of rats fed ethanol during pregnancy showed essentially the same 2DG uptake response to in vitro ethanol treatment. Kinetics of 2DG uptake showed a significant decrease of Vmax in the presence of ethanol. No interaction was found between ethanol and insulin, which stimulated 2DG uptake and protein content of the cultures. The data suggest that ethanol can modulate hexose uptake by astrocytes cultured from fetal rat brain. However, insulin actions on glial cells were not affected by ethanol.

Gangliosides attenuate the delayed neurotoxicity of aspartic acid in vitro

The neurotoxic effects of L-aspartate were evaluated in rat cerebellar granule cell cultures. Acute (15 min) exposure to L-aspartate produced a time-dependent, delayed degeneration of neuronal cell bodies and neurites (LD50 about 40 microM) over 24 h. Aspartate neurotoxicity was prevented by competitive and non-competitive N-methyl-D-aspartate (NMDA) antagonists, but not by non-NMDA antagonists, suggesting a major involvement of NMDA receptors in this neuronal injury. Gangliosides, including GM1, were also effective in attenuating the cytotoxicity of L-aspartate. The neurotoxic potential of L-aspartate may thus contribute to pathologies involving the action of endogenous excitatory amino acids.

Spinal cord ischemia-induced elevation of amino acids: extracellular measurement with microdialysis

Excitatory amino acids have been implicated in the production of calcium mediated neuronal death following central nervous system ischemia. We have used microdialysis to investigate changes in the extracellular concentrations of amino acids in the spinal cord after aortic occlusion in the rabbit. Glutamate, aspartate, glutamine, asparagine, glycine, taurine, valine, and leucine were measured in the microdialysis perfusate by high pressure liquid chromatography. The concentrations of glutamate, glycine, and taurine were significantly higher during ischemia and reperfusion than controls. Delayed elevations in the concentrations of asparagine and valine were also detected. The elevation of glutamate is consistent with the hypothesis that excitotoxins may mediate neuronal damage in the ischemic spinal cord. Increased extracellular concentrations of asparagine and valine may reflect preferential use of amino acids for energy metabolism under ischemic conditions. The significance of increased concentrations of inhibitory amino acid neurotransmitters is unclear.

Changes in extracellular amino acid neurotransmitters produced by focal cerebral ischemia

Excitatory amino acids (EAAs) have been implicated in the pathophysiology of cellular injury after brain ischemia. Changes in extracellular levels of amino acids in rat cerebral cortex after permanent proximal middle cerebral artery (MCA) occlusion were examined using microdialysis. Significant increases were found in dialysate concentrations of glutamate, aspartate and gamma-aminobutyric acid (GABA) from the ischemic cortex during the first 90 min after MCA occlusion compared to pre-ischemic concentrations and contralateral hemispheric controls. Total tissue levels of these amino acids in the infarcted hemisphere 90 min after onset of ischemia were not different from the contralateral hemisphere. These results are consistent with the hypothesis that the release of EAAs may contribute to tissue damage in focal cerebral ischemia.

Regional neuroprotective effects of the NMDA receptor antagonist MK-801 (dizocilpine) in hypoglycemic brain damage

Current evidence points to an important role of N-methyl-D-aspartate (NMDA) receptor activation in the pathogenesis of hypoglycemic neuronal death. MK-801 [dizocilpine maleate, (+)-5-methyl-10,11-dihydro-5H-di[a,d]cyclohepten-5,10-imine] is an anticonvulsant compound also known to be a potent noncompetitive antagonist at NMDA receptors, readily crossing the blood-brain barrier after parenteral administration. Treatment of rats with dizocilpine (1.5-5.0 mg/kg) injected intravenously during profound hypoglycemia (blood glucose levels 1.5-2.0 mM) at the stage of delta-wave (1-4 Hz) slowing of the EEG mitigated selective neuronal necrosis in the hippocampus and striatum, assessed histologically after 1-week survival. The degree of neuroprotection in the striatum and in the CA1 pyramidal cells of the hippocampus was dose dependent. Because of concern for a possible hypothermic mechanism of brain protection by MK-801, core temperature was closely monitored and was found not to decrease significantly. Since CBF is normal or increased in hypoglycemia, a fall in brain temperature during hypoglycemia is unlikely to play a role in the mechanism of the neuroprotection seen with the drug. The findings indicate that in profound hypoglycemia, intravenous administration of the NMDA antagonist dizocilpine, even after the appearance of delta-wave EEG slowing, can reduce the number of necrotic neurons in several brain regions and suggest that the neuroprotective effect of MK-801 is not related to hypothermia.

Extracellular levels of quinolinic acid are moderately increased in rat neostriatum following severe insulin-induced hypoglycaemia

Extracellular concentrations of the brain metabolite quinolinic acid, an endogenous excitotoxin, were monitored by microdialysis in rat neostriatum and hippocampus/cortex during and following a 30-min period of insulin-induced hypoglycaemia. During hypoglycaemia-induced isoelectricity, extracellular levels of quinolinic acid in the striatum (basal value, 1.1 +/- 0.3 pmol per 30-microliters fraction) were elevated 1.7 times as compared to the control period. Thirty to ninety minutes following hypoglycaemia a significant increase in extracellular quinolinic acid to 2.2 times basal level was noted. After 2 h recovery, the beginning of neuronal necrosis was observed in the dorsolateral striatum. Implantation of the dialysis probe did not influence the extent of neuronal damage. No changes in extracellular quinolinic acid levels were observed in the hippocampus/cortex. The data indicate that following a severe hypoglycaemic insult vulnerable striatal cells are exposed to hyperphysiological extracellular quinolinic acid concentrations over an extended period of time. Considering the pronounced susceptibility of rat striatal neurons to the toxin, the small but prolonged elevation in the extracellular levels of quinolinic acid could be of significance for the development of delayed neuronal death in hypoglycaemia.

Brain and plasma quinolinic acid in profound insulin-induced hypoglycemia

Profound insulin-induced hypoglycemia is associated with early-onset neuronal damage that resembles excitotoxic lesions and is attenuated in severity by antagonists of N-methyl-D-aspartate receptors. Hypoglycemia increases L-tryptophan concentrations in brain and could increase the concentration of the L-tryptophan metabolite quinolinic acid (QUIN), an agonist of N-methyl-D-aspartate receptors and an excitotoxin in brain. Therefore, we investigated the effects of 40 min of profound hypoglycemia (isoelectric EEG) and 1-2 h of normoglycemic recovery on the concentrations of QUIN in brain tissue, brain extracellular fluid, and plasma in male Wistar rats. Plasma QUIN increased 6.5-fold by the time of isoelectricity (2 h after insulin administration). Regional brain QUIN concentrations increased two- to threefold during hypoglycemia and increased a further two- to threefold during recovery. However, no change in extracellular fluid QUIN concentrations in hippocampus occurred during hypoglycemia or recovery as measured using in vivo microdialysis. Therefore, the increases in brain tissue QUIN concentrations may reflect elevations of QUIN in the intracellular space or be secondary to the increases in QUIN in the vascular compartment in brain per se. L-Tryptophan concentrations increased more than twofold during recovery only. Serotonin decreased greater than 50% throughout the brain during hypoglycemia, while 5-hydroxyindoleacetic acid concentrations increased more than twofold during hypoglycemia and recovery. In striatum, dopamine was decreased 75% during hypoglycemia but returned to control values during recovery, while striatal 3,4-dihydroxyphenylacetic acid and homovanillic acid were increased more than twofold during both hypoglycemia and recovery.(ABSTRACT TRUNCATED AT 250 WORDS)

Alteration in extracellular amino acids after traumatic spinal cord injury

It has recently been demonstrated that N-methyl-D-aspartate antagonists limit tissue damage after spinal cord trauma, implicating excitatory amino acids in the secondary injury response. To determine whether spinal cord trauma alters the concentrations of extracellular amino acids, microdialysis was conducted in spinal cord during and after administration of impact trauma. Extracellular concentrations of excitatory, inhibitory, and nontransmitter amino acids were elevated after trauma, with the degree of increase related to severity of injury. Moderate trauma resulted in an immediate but transient increase (200-400%) in the extracellular levels of all amino acids measured. Severe trauma produced a more prolonged and significant increase (400-630%) in the concentrations of extracellular amino acids, including aspartate and glutamate. These results are consistent with the hypothesis that excitatory amino acids may contribute to delayed tissue injury after central nervous system trauma.

Regulation of glutamate and aspartate release from the Schaffer collaterals and other projections of CA3 hippocampal pyramidal cells

Excitatory synaptic transmission in the CNS can be modulated by endogenous substances and metabolic states that alter release of the transmitter, usually glutamate and/or aspartate. To explore this issue, we have studied the release of endogenous glutamate and aspartate from synaptic terminals of the CA3-derived Schaffer collateral, commissural and ipsilateral associational fibers in slices of hippocampal area CA1. These terminals release glutamate and aspartate in about a 5:1 ratio. The release process is modulated by adenosine, by the transmitters themselves and by nerve terminal metabolism. Adenosine inhibits the release of both amino acids by acting upon an A1 receptor. The transmitters, once released, can regulate their further release by acting upon both an NMDA and a non-NMDA (quisqualate/kainate) receptor. Activation of the NMDA receptor enhances the release of both glutamate and aspartate, whereas activation of the non-NMDA receptor depresses the release of aspartate only. Superfusion of CA1 slices with a glucose-deficient medium increases the release of both amino acids and reduces the glutamate/aspartate ratio. These results have implications for the regulation of excitatory synaptic transmission in the CA1 area and for the mechanism of hypoglycemic damage to CA1 pyramidal cells.

Hypoglycemic neurotoxicity in vitro: involvement of excitatory amino acid receptors and attenuation by monosialoganglioside GM1

Rat cerebellar granule cells, when subjected to a glucose-free environment for 4 h, developed extensive degeneration of neuronal cell bodies and their associated neurite network over the following 24 h. This neuronal damage was quantitated with a colorimetric assay using the metabolic dye 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide. Hypoglycemic neuronal injury could be markedly reduced by the presence of both competitive (3-(+/-)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid) and non-competitive (phencyclidine) N-methyl-D-aspartate receptor antagonists, but not by kainate/quisqualate preferring antagonists 6-cyano-7-nitroquinoxaline-2,3-dione and 6,7-dinitroquinoxaline-2,3-dione. Glucose deprivation neuronal injury was also reduced by adding glutamate-degrading enzymes to the incubation medium. Monosialoganglioside GM1, but not its asialo derivative (lacking sialic acid), was also effective in protecting against hypoglycemic neurodegeneration when included during the period of glucose deprivation. These results suggest that the neuronal injury to cerebellar granule cells resulting from glucose deprivation is mediated predominantly by activation of the N-methyl-D-aspartate type of excitatory amino acid receptor, perhaps through the action of endogenously released glutamate. Furthermore, the monosialoganglioside GM1, a member of a class of naturally occurring sialoglycosphingolipids, is able to attenuate this neuronal injury--as already observed for glutamate neurotoxicity and anoxic neuronal death in cerebellar granule cells. Gangliosides may thus prove to be of therapeutic utility in excitatory amino acid-associated neuropathologies.

Effects of glucose deficiency on glutamate/aspartate release and excitatory synaptic responses in the hippocampal CA1 area in vitro

The effects of glucose deficiency on (1) the K+-evoked release of glutamate and aspartate and (2) excitatory synaptic transmission were studied in the Schaffer collateral-commissural-ipsilateral associational (SCCIA) projection to area CA1 of the rat hippocampal formation in vitro. Compared with 1 or 10 mM glucose, superfusion of CA1 slices with 0.1 mM glucose enhanced the K+-evoked release of both glutamate and aspartate, increased the ratio of aspartate release to glutamate release and did not affect the release of GABA. With both high and low glucose concentrations, the K+-evoked release of glutamate and aspartate originated predominantly from a Ca2+-sensitive store associated with the SCCIA projection. Superfusion with glucose-deficient medium abolished the inhibitory effect of adenosine on glutamate and aspartate release, but augmented the enhancing effect of the adenosine antagonist 8-phenyltheophylline. These results suggest that enough endogenous adenosine was released from the slices under these conditions to saturate the presynaptic A1 receptors. Despite its facilitatory effect on excitatory transmitter release, glucose-deficient medium inhibited transmission at Schaffer collateral-commissural synapses. Even when the postsynaptic response to a single electrical pulse was abolished, however, a substantial response could still be evoked through paired-pulse or frequency potentiation and the inhibition promptly reversed upon superfusion with 10 mM glucose. The increased ratio of aspartate release to glutamate release appears to reflect changes in the tissue content of these amino acids. The enhanced release of both excitants is suggested to result partly from a rise in intraterminal Ca2+ concentration and partly from inhibition of glutamate/aspartate uptake. Enhanced aspartate release may be particularly relevant to hypoglycemic damage in the CA1 area, because aspartate is a more potent hippocampal excitotoxin than glutamate.

Cerebral excess release of neurotransmitter amino acids subsequent to reduced cerebral glucose metabolism in early-onset dementia of Alzheimer type

A massive cerebral release of amino acids and ammonia was found in early-onset dementia of Alzheimer type. Aspartate and glycine were liberated in high concentrations, whereas glutamate remained rather unchanged. This excess cerebral protein catabolism is due to a 44% reduction in cerebral glucose metabolism. Whereas glutamate and other glucoplastic amino acids may substitute glucose, elevated aspartate may contribute to neuronal damage. The results are discussed with respect to a possible neuronal insulin/insulin receptor deficiency.

Changes in excitatory amino acid receptor binding in the intact and decorticated rat neostriatum following insulin-induced hypoglycemia

An involvement of excitatory amino acid (EAA) transmitter-receptor interactions in the development of hypoglycemia-induced neuronal damage has been suggested. We report here on the binding to EAA receptors in the rat caudate nucleus and cerebral cortex, during and following severe insulin-induced hypoglycemia with an isoelectric EEG of 10 or 30 min duration. The binding of alpha-[3H]amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid [( 3H]AMPA) to quisqualate receptors, [3H]kainic acid (KA) to kainate receptors, and [3H]glutamate to N-methyl-D-aspartate (NMDA)-sensitive sites was determined by quantitative autoradiography. During EEG isoelectricity, AMPA binding was reduced by approximately 40%, which could represent quisqualate receptor desensitization. One hour following glucose-induced recovery, AMPA binding was no longer different from control level. As the recovery period was prolonged to 1 or 4 weeks, AMPA binding decreased. The decrease was more pronounced in the dorsolateral than in the ventromedial part of the striatum. This correlates with the distribution of neuronal damage, and probably reflects loss of receptor binding sites due to cell death. During the period of EEG silence there was a tendency toward an increase in NMDA displaceable glutamate binding. Following 4 weeks of recovery, binding to NMDA receptors was significantly decreased. Glutamate binding to NMDA-sensitive sites was remarkably resistant to neuronal necrosis and was not significantly different from control values in the dorsolateral caudate 1 week following the hypoglycemic coma. No changes in KA binding were found until 1 week posthypoglycemia, when a significant reduction in binding was noted in the lateral striatum.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of impact trauma on neurotransmitter and nonneurotransmitter amino acids in rat spinal cord

N-Methyl-D-aspartate (NMDA) administration exacerbates neurological dysfunction after traumatic spinal cord injury in rats, whereas NMDA antagonists improve outcome in this model. These observations suggest that release of excitatory amino acids contributes to secondary tissue damage after traumatic spinal cord injury. To further examine this hypothesis, concentrations of free amino acids were measured in spinal cord samples from anesthetized rats subjected to various degrees of impact trauma to the T9 spinal segment. Levels of excitatory and inhibitory neurotransmitter amino acids [gamma-aminobutyric acid (GABA), glutamate, aspartate, glycine, taurine] and levels of nonneurotransmitter amino acids (asparagine, glutamine, alanine, threonine, serine) were determined at 5 min, 4 h, and 24 h posttrauma. Uninjured surgical (laminectomy) control animals showed modest but significant declines in aspartate and glutamate levels, but not in other amino acids, at all time points. In injured animals, the excitatory amino acids glutamate and aspartate were significantly decreased by 5 min posttrauma, and remained depressed at 4 h and 24 h as compared with corresponding laminectomy controls. In contrast, the inhibitory amino acids, glycine, GABA, and taurine, were decreased at 5 min postinjury, had partially recovered at 4 h, and were almost fully recovered at 24 h. The nonneurotransmitter amino acids were unchanged at 5 min posttrauma and significantly increased at 4 h, with partial recovery at 24 h. At 4 h postinjury, severe trauma caused significantly greater decreases in aspartate and glutamate than did either mild or moderate injury. These findings are consistent with the postulated role of excitatory amino acids in CNS trauma.

Glucose deprivation neuronal injury in cortical culture

Murine cortical cell cultures deprived of glucose for 6-8 h developed extensive neuronal degeneration, apparent both morphologically and by efflux of lactate dehydrogenase to the bathing medium. This neuronal damage could be substantially reduced by addition of D-2-amino-5-phosphonovalerate (D-APV), in a concentration-dependent (IC50 about 2 microM) and stereospecific (D-APV more potent than L-APV) fashion. A similar neuron-protective effect could also be obtained with several other NMDA antagonists, 2-amino-7-phosphonoheptanoate, phencyclidine, MK-801, ketamine, and (+)-SKF 10,047, as well as with the broad spectrum glutamine antagonist kynurenate. In contrast, little protection could be obtained with gamma-D-glutamylaminomethyl sulfonate and L-glutamate diethyl ester, compounds which have been reported to act primarily at non-NMDA receptors. These observations support the hypothesis that glucose deprivation-induced cortical neuronal injury is largely mediated by NMDA receptors, and suggest that cell culture methodology can be useful in the quantitative characterization of that injury.

GABA inhibits sexual behaviour in female rats

Ejaculation by male rats caused an abrupt and marked increase in the concentration of GABA in the cerebrospinal fluid and an equally abrupt and marked inhibition of sexual behaviour in female rats. The increase in the concentration of GABA in the cerebrospinal fluid and the inhibition of the behaviour was specifically mediated by the ejaculation of the male; sexual stimulation unaccompanied by ejaculation had no effect. The post-ejaculatory suppression of sexual receptivity in female rats was partially reversed by intracerebroventricular injection of the GABA antagonist bicuculline and the behaviour of receptive rats was inhibited by intracerebroventricular injection of the GABA agonist muscimol. Increasing the concentration of GABA in the cerebrospinal fluid by i.p. injection of the GABA transminase inhibitor gamma-vinyl GABA caused an increase of the concentration of GABA in the cerebrospinal fluid and inhibited the display of sexual receptivity. It is suggested that GABA mediates physiologically relevant inhibition of sexual behaviour in female rats.

Glucose and synaptosomal glutamate metabolism: studies with [15N]glutamate

The metabolism of [15N]glutamate was studied with gas chromatography-mass spectrometry in rat brain synaptosomes incubated with and without glucose. [15N]Glutamate was taken up rapidly by the preparation, reaching a steady-state level in less than 5 min. 15N was incorporated predominantly into aspartate and, to a much lesser extent, into gamma-aminobutyrate. The amount of [15N]ammonia formed was very small, and the enrichment of 15N in alanine and glutamine was below the level of detection. Omission of glucose substantially increased the rate and amount of [15N]aspartate generated. It is proposed that in synaptosomes (a) the predominant route of glutamate nitrogen disposal is through the aspartate aminotransferase reaction; (b) the aspartate aminotransferase pathway generates 2-oxoglutarate, which then serves as the metabolic fuel needed to produce ATP; (c) utilization of glutamate via transamination to aspartate is greatly accelerated when flux through the tricarboxylic acid cycle is diminished by the omission of glucose; (d) the metabolism of glutamate via glutamate dehydrogenase in intact synaptosomes is slow, most likely reflecting restriction of enzyme activity by some unknown factor(s), which suggests that the glutamate dehydrogenase reaction may not be near equilibrium in neurons; and (e) the activities of alanine aminotransferase and glutamine synthetase in synaptosomes are very low.

Glucose deprivation depolarizes plasma membrane of cultured astrocytes and collapses transmembrane potassium and glutamate gradients

Primary cultures of astrocytes were used to investigate the effects of glucose deprivation on plasma membrane potential, on the respiration and on the energy status of these cells. Plasma membrane potential, as monitored with a cyanine dye, 3,3'-diethylthiadicarbocyanine, hyperpolarized by about 100% when glucose was added to substrate-deprived cells. The effect of glucose was prevented by iodoacetate or ouabain. In the absence of glucose, cellular adenosine triphosphate/adenosine diphosphate ratio was extensively reduced and pyruvate was unable either to restore energy status or to hyperpolarize the plasma membrane of astrocytes, although it was the preferential substrate for mitochondria within the cells. Glucose deprivation and inhibition of glycolysis or respiration in the presence of glucose caused dramatic decrease in transmembrane potassium ion and L-glutamate gradients. The gradients were not restored in the presence of pyruvate. Thus, aerobic glycolysis, rather than oxidation of pyruvate, is required to maintain maximal plasma membrane potential, adenosine triphosphate/adenosine diphosphate ratios as well as K+ and L-glutamate gradients. This evidence, together with the unresponsiveness of astrocyte respiration to ouabain, indicates a functional dissociation between energy dissipation at the plasma membrane and mitochondrial synthesis of adenosine triphosphate. The results are discussed with regard to the vulnerability of glia at low levels of blood glucose and the contribution of glial dysfunction to development of hypoglycaemic encephalopathy.

Glutamate becomes neurotoxic via the N-methyl-D-aspartate receptor when intracellular energy levels are reduced

The N-methyl-D-aspartate (NMDA) subtype of glutamate receptor appears to play a pivotal role in enabling glutamate to express its neurotoxic potential in a variety of neurological disorders. Our results show that the transition of glutamate from neurotransmitter to neurotoxin is facilitated when cellular energy is limited in cultured cerebellar neurons. Omission of glucose, exclusion of oxygen, or inclusion of inhibitors of oxidative phosphorylation or of the sodium/potassium pump, enables the excitatory amino acids glutamate or NMDA to express their neurotoxic potential. We interpret these results as demonstrating that glucose metabolism, ATP production, and functioning Na+,K+-ATPases are necessary to generate a resting potential sufficient to maintain the voltage-dependent Mg2+ block of the NMDA receptor channel; relief of the Mg2+ block enables the excitatory amino acids to act persistently at the NMDA receptor, resulting in the opening of ion channels and subsequent neuronal damage. These findings are discussed in the context of perturbations or abnormalities which lead to decreased availability or utilization of glucose and oxygen in the brain which may trigger endogenous excitatory amino acids to become neurotoxic by this mechanism.

Increases in striatal and hippocampal impedance and extracellular levels of amino acids by cardiac arrest in freely moving rats

The time course of changes in the tissue impedance and the levels of extracellular transmitter and non-transmitter amino acids was studied in the striatum and hippocampus of the unanesthetized rat after cardiac arrest. Electrodes were implanted for the continuous measurement of tissue impedance so that a measure of the volume of extracellular space was provided. Alternatively, bilateral dialysis probes were used for monitoring levels of extracellular amino acids in subsequent 30-s samples using an automated precolumn derivatization technique for reversed-phase HPLC analysis and fluorimetric detection. The impedance started to rise approximately 1.2 min following cardiac arrest, increased rapidly during the first 5 min, and increased almost linearly thereafter. After 15 min, a decrease of approximately 50% in the extracellular space was calculated. The impedance rose more steeply in the striatum than in the hippocampus. The extracellular levels of taurine, which increased greater than 300% within 5 min after cardiac arrest, most closely resembled the time course of the change in impedance. Glutamate and aspartate levels did not increase until 5 min after circulatory arrest, and at 15 min they had risen to a level of 465 and 265% for the striatum and 298 and 140% for the hippocampus of the resting release, respectively. The release of gamma-aminobutyric acid (GABA) was multiphasic and did not resemble that of any of the other--putative--transmitter amino acids. Fifteen minutes after cardiac arrest, the levels of GABA were 617 and 774% of the resting release in the striatum and hippocampus, respectively. Glycine and alanine efflux substantially increased (232 and 151% in striatum and 141 and 154% in hippocampus, respectively) 15 min postmortem, whereas the glutamine level was slightly increased and levels of asparagine, histidine, threonine, ethanolamine, serine, arginine, and tyrosine were inconsistently higher in the two brain regions. At this time, the extracellular levels of glutamate, GABA, and aspartate were only slightly lower, as expected from the tissue levels and from levels of the other amino acids, an observation indicating that all the amino acids may diffuse through postmortem brain tissue to a nearly similar extent.(ABSTRACT TRUNCATED AT 400 WORDS)

2-Amino-5-phosphonovalerate attenuates the severe hypoglycemia-induced loss of perforant path-evoked field potentials in the rat hippocampus

The effects of severe hypoglycemia on perforant path-evoked field potentials were examined in the rat hippocampus. Although a complete loss of this response was noted when blood glucose concentration fell below 1 mM, this occurred before cessation of electroencephalogram (EEG) activity. Both spontaneous and evoked responses recovered partially following glucose readministration. D-2-Amino-5-phosphonovalerate, an NMDA-sensitive acidic amino acid receptor antagonist, facilitated this recovery from the hypoglycemic challenge when administered via a dialysis probe.

Blood flow and metabolism in heterotopic cerebellar grafts during hypoglycemia

Hypoglycemia-induced disturbances of brain metabolism and neuronal injury exhibit a distinct predilection for forebrain structures, in particular the caudate-putamen, hippocampus and cerebral cortex, whereas the cerebellum is remarkably resistant. In an attempt to assess the biological basis of this differential regional vulnerability, we have used a neural transplantation technique to compare hemodynamic and metabolic changes in cerebellum during severe hypoglycemia with those in heterotopic cerebellar grafts. To this end, the cerebellar anlage of fetal rat brain (day 15 of gestation) was stereotactically transplanted into the vulnerable caudate-putamen. Following a differentiation period of 8 weeks the grafts had developed into an organotypic population of mature cells with laminar histoarchitecture. Host animals were then subjected to insulin-induced hypoglycemia. After 15 min of isoelectric EEG, blood flow was increased throughout the brain but residual glucose consumption was significantly higher in cerebellum (0.29 mumol/g per min) and cerebellar grafts (0.22 mumol/g per min) as a result of increased glucose extraction. Hypoglycemia caused a depletion of ATP in all brain structures except cerebellum where normal levels were maintained. Correlation of local ATP content and glucose utilization revealed a threshold-like decline of ATP at a glucose utilization rate of 0.27 mumol/g per min. ATP, in consequence, was normal in cerebellum but partially depleted in cerebellar grafts. It is concluded that the resistance of cerebellum to hypoglycemia is due to its capacity for higher glucose extraction at low blood glucose levels, and that this unique intrinsic property is preserved after heterotopic transplantation.