Biomedical subjects

A Schurr

Publications and source records attributed to A Schurr.

At least 19 recordsLinked to original sources

PubMed1995

Synergism between diltiazem and MK-801 but not APV in protecting hippocampal slices against hypoxic damage.

In the present study, we investigated the possibility that MK-801 (dizocilpine), a noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist, owes its potent neuroprotective properties to calcium channel blocking ability rather than to its NMDA receptor antagonism. Rat hippocampal slices were exposed to a long hypoxic period (20 min) from which only 13.8% recovered their neuronal function after 30 min of reoxygenation. The recovery rate of neuronal function from 20-min hypoxia was increased to 100% when slices were pretreated with 5 microM MK-801. DL-2-amino-5-phosphonovalerate (APV), a competitive NMDA receptor antagonist, even at relatively high concentration (100 microM), provided only marginal protection against such severe hypoxic insult. The L-type calcium channel blocker diltiazem (DILT) was more effective than APV in protecting hypoxic slices against neuronal damage. Combining suboptimal concentrations of DILT and MK-801 produced a neuroprotective effect with significantly exceeded the calculated additive effect of the two drugs. Such synergism could not be demonstrated between DILT and APV, a combination that produced only the expected additive neuroprotective effect. The observed synergy between the calcium channel blocker (DILT) and MK-801, along with other studies that demonstrated interaction between these two drugs, led us to postulate that MK-801 possesses calcium channel blocking properties through which its neuroprotective effect is exerted. These calcium channels could either be of the L-type or otherwise, channels which are being activated only under stressful conditions, such as hypoxia or ischemia.

Animals↗

PubMed1995

Hypoxia, excitotoxicity, and neuroprotection in the hippocampal slice preparation.

The excitotoxic hypothesis postulates a central role for the excitatory amino acids (EAAs) and their receptors in the neuronal damage that ensues cerebral ischemia-hypoxia and numerous other brain disorders. A major premise of the excitotoxic hypothesis is that neuronal protection can be achieved via blockade of EAA receptors with specific antagonists. This paper describes the use of the rat hippocampal slice preparation in the evaluation of various EAAs and their analogues for their potency as excitotoxins (agonists) and antagonists of the NMDA and the kainate/AMPA glutamate receptor subtypes. The hypersensitivity of hypoxic hippocampal slices to the presence of excitotoxins provided us with an inexpensive, sensitive tool to distinguish between structurally similar compounds. Moreover, these studies indicate that hypoxic neuronal damage cannot solely result from an excitotoxic mechanism; the involvement of voltage-dependent calcium channels in such damage is likely, as is evident from experiments performed in calcium-depleted medium and with the non-competitive NMDA antagonist MK-801. At sub-toxic doses, quinolinate, a tryptophan metabolite implicated in Huntington's disease, appears to be a strong potentiator of the toxicity of all excitotoxins tested.

Animals↗

PubMed1995

Protection by MK-801 against hypoxia-, excitotoxin-, and depolarization-induced neuronal damage in vitro.

Exposure of rat hippocampal slices to 12-min hypoxia produced only mild neuronal damage, as 72% of all slices recovered their CA1-evoked population spike following a 30-min recovery period. However, when this hypoxic insult was administered in the presence of 2.5 microM kainate or AMPA, only 6 and 15% of the slices, respectively, recovered their neuronal function. This enhancement of hypoxic damage by kainate could be attenuated in a dose-dependent fashion by the kainate/AMPA antagonist GYKI 52466 but not by the competitive NMDA antagonist APV. Unexpectedly, the noncompetitive NMDA antagonist MK-801 also attenuated the kainate- and AMPA-enhanced hypoxic neuronal damage and was more efficacious than GYKI 52466. Considering (1) the ability of MK-801 to antagonize hypoxic neuronal damage in the absence or the presence of NMDA, kainate or AMPA; (2) the antihypoxic effect of MK-801 in the presence of APV + 7-chlorokynurenate, a pairing that supposedly blocks MK-801 binding to the NMDA receptor; (3) the ability of MK-801 to protect hippocampal slices against brain damage induced by depolarization + excitotoxin (50 mM KCl + mM glutamate for 60 min); and (4) the ability of diltiazem, an L-type calcium channel blocker, to protect hippocampal slices against hypoxic neuronal damage, we conclude that the mode of action of MK-801 cannot be explained by its NMDA receptor antagonistic properties alone. A possible blockade of Ca2+ channels, most likely of the L-type, by MK-801 should be considered along with other mechanisms.

2-Amino-5-phosphonovalerate↗

PubMed1993

Kainate toxicity in energy-compromised rat hippocampal slices: differences between oxygen and glucose deprivation.

The effects of kainate (KA) on the recovery of neuronal function in rat hippocampal slices after hypoxia or glucose deprivation (GD) were investigated and compared to those of (R,S)-alpha-amino-3-hydroxy-5-methyl-4- isoxazoleproprionate (AMPA). KA and AMPA were found to be more toxic than either N-methyl-D-aspartate (NMDA), quinolinate, or glutamate, both under normal conditions and under states of energy deprivation. Doses as low as 1 microM KA or AMPA were sufficient to significantly reduce the recovery rate of neuronal function in slices after a standardized period of hypoxia or GD. The enhancement of hypoxic neuronal damage by both agonists could be partially blocked by the antagonist kynurenate, by the NMDA competitive antagonist AP5, and by elevating [Mg2+] in or by omitting Ca2+ from the perfusion medium. The AMPA antagonist glutamic acid diethyl ester was ineffective in preventing the enhanced hypoxic neuronal damage by either KA or AMPA. The antagonist of the glycine modulatory site on the NMDA receptor, 7-chlorokynurenate, did not block the KA toxicity but was able to block the toxicity of AMPA. 2,3-Dihydroxyquinoxaline completely blocked the KA- and AMPA-enhanced hypoxic neuronal damage. The KA-enhanced, GD-induced neuronal damage was prevented by Ca2+ depletion and partially antagonized by kynurenate but not by AP5 or elevated [Mg2+]. The results of the present study indicate that the KA receptor is involved in the mechanism of neuronal damage induced by hypoxia and GD, probably allowing Ca2+ influx and subsequent intracellular Ca2+ overload.(ABSTRACT TRUNCATED AT 250 WORDS)

2-Amino-5-phosphonovalerate↗

PubMed1992

The excitotoxicity of heterocyclic dicarboxylic acids in rat hippocampal slices: structure-activity relationships.

The structural resemblance of certain heterocyclic dicarboxylates to aspartate and glutamate led investigators to study their potency as agonists and antagonists of the N-methyl-D-aspartate (NMDA) receptor. The sensitivity of hypoxic rat hippocampal slices to NMDA ligands is several fold greater than that of normoxic slices. In the present study, the excitotoxic potency of heterocyclic dicarboxylates was assessed electrophysiologically by measuring their ability to enhance hypoxic and hypoglycemic neuronal damage in the rat hippocampal slice preparation. Four compounds were tested: quinolinate (QUIN), 4,5-imidazole-dicarboxylate (IZDA), 1,2,3-triazole-4,5-dicarboxylate (TZDA), and 2,3-pyrazinedicarboxylate (PZDA). QUIN was the most toxic drug in enhancing both hypoxic and hypoglycemic neuronal damage. IZDA and TZDA were slightly less toxic than QUIN, while PZDA was innocuous. The effect of the 3 active drugs was blocked by the NMDA competitive antagonist DL-2-amino-5-phosphonovalerate. The sequence -N-CH(COOH)-CH(COOH)- appears to be a prerequisite for a heterocyclic dicarboxylate to exert NMDA-type agonistic properties. A 5-membered ring heterocyclic compound which contains more than one nitrogen atom in its ring retains its NMDA-type toxicity while a 6-membered ring with more than one nitrogen atom (PZDA) does not.

Animals↗

PubMed1992

The mechanism of cerebral hypoxic-ischemic damage.

The four most prominent hypotheses on the cellular processes leading to hypoxic-ischemic neuronal damage or death are (1) the lactacidosis hypothesis, (2) the calcium overload hypothesis, (3) the excitotoxic hypothesis, and (4) the oxygen-free radical hypothesis. The authors comment on the evidence in favor of and against each in an attempt to select the one hypothesis that best explains the mechanism of cerebral hypoxic-ischemic damage while withstanding the scrutiny of scientific testing. A major part of this inquiry is derived from in vitro studies that are suited to mechanistic exploration. They conclude that the calcium overload hypothesis is the best qualified in this respect. It is important to note, however, that some of the other hypothetical mechanisms may play a secondary role in exacerbating neuronal damage by accelerating calcium influx and overload.

Animals↗

PubMed1991

Neurotoxicity of quinolinic acid and its derivatives in hypoxic rat hippocampal slices.

The excitotoxicity of quinolinic acid (2,3-pyridinedicarboxylic acid), a potent endogenous N-methyl-D-aspartate (NMDA)-type agonist, was characterized in the hypoxic hippocampal slice preparation. A series of other pyridinedicarboxylic acids was also tested in this preparation in order to obtain information about the structural requirements for the interaction between the NMDA receptor and its agonists. Of the 7 pyridinedicarboxylic acids tested, only quinolinic acid and its anhydride exerted their excitotoxicity by enhancing hypoxic neuronal damage in rat hippocampal slices at a relatively low concentration (100 microM). Much higher concentration (1 mM) of 3,4-pyridinedicarboxylic acid was required to exhibit any enhancement of hypoxic neuronal damage. The rest of the derivatives were innocuous. The effect of quinolinic acid was blocked by DL-2-amino-5-phosphonovaleric acid, by elevated magnesium levels in the incubation medium or by perfusion with a medium depleted of calcium. Aglycemic damage was also enhanced by quinolinic acid. It appears from the present study that two adjacent carboxylic groups on the pyridine ring, preferably at positions 2 and 3, are a prerequisite for an interaction between the NMDA receptor and its agonist. However, other factors may have great influence on that interaction as was evident from the total impotency of 6-methyl-quinolinic acid. The hypoxic hippocampal slice preparation and its neuronal function is an inexpensive model system, sensitized to the neurotoxins, and thus, allows the easy screening and evaluation of potential ligands of the glutamate receptor and its subtypes.

2-Amino-5-phosphonovalerate↗

PubMed1991

Diabetes-induced bile acid composition changes in rat bile determined by high performance liquid chromatography.

The distribution of glycine and taurine conjugated bile acids in bile from streptozotocin-induced diabetic rats were determined by high performance liquid chromatography (HPLC). Biliary bile acid output in diabetic rats was significantly greater compared to control (p less than 0.001). The increase is not a generalized effect of diabetes, but is the preferential increased production of taurochenodeoxycholic acid. These observed changes in bile acid composition may represent greater capacity of bile from diabetic rats to solubilize cholesterol. In the absence of a gallbladder, however, rat bile undergo continuous enterohepatic circulation, and consequently is not subjected to modifications by gallbladder epithelial cells that would potentiate cholesterol precipitation.

Animals↗

PubMed1989

Cerebral ischemia revisited: new insights as revealed using in vitro brain slice preparations.

The elucidation of the pathophysiological mechanisms of cerebral ischemia/hypoxia dictates the use of experimental models which mimic this disabling brain condition. In vivo experimental models have been available for many decades and are responsible for the bulk of, though incomplete, knowledge we have about these mechanisms. Since study in isolation of each postulated mechanism is impossible in vivo, the need for an in vitro experimental model has intensified in recent years. Consequently, rat and guinea pig hippocampal slice preparations have emerged as the models of choice. This review attempts to highlight some of the results obtained using brain slices in the study of cerebral ischemia/hypoxia and compare them to those obtained in vivo. Both the biochemical and the physiological correlates of energy metabolism, ion homeostasis, neurotransmission and neuromodulation of this brain condition are reviewed. The agreements, and especially the disagreements, between the in vivo and in vitro findings are emphasized. Details are given of the possible roles of both lactic acid, Ca2+ and excitotoxins in the neuronal damage inflicted by cerebral ischemia/hypoxia. Recent attempts to protect brain slices against experimental cerebral ischemic/hypoxic damage are also reviewed here briefly.

Action Potentials↗

PubMed1989

Electrophysiology of energy metabolism and neuronal function in the hippocampal slice preparation.

The brain slice preparation offers a unique opportunity to study synaptic function in vitro. Employing electrophysiological methods to measure synaptic activity, we manipulated the extracellular environment of the rat hippocampal slice preparation: (1) by exposing it to different degrees of hypoxia, (2) by changing the levels of glucose, (3) lactate, and (4) H+, separately and in combination with each other. The lower the oxygen level during hypoxia and the longer its duration were, the lower was the recovery rate of synaptic function in the slice upon restoration of oxygenation. Reduction or complete depletion of glucose from the perfusion medium had similar effects, although synaptic function could recover after longer periods of glucose lack as compared with oxygen lack. Reduction in the levels of both oxygen and glucose had an additive effect on the recovery rate of synaptic function when compared with the effect of each of them alone. 'Hyperglycemic' concentration of glucose prolonged the hypoxic period slices could tolerate. Acidosis, induced either by lactic acid or HCl, had no adverse effect on hypoxic slices when the pH was held at or above 6.0 or when lactic acid concentration was below 20 mM. At 10 mM, lactic acid appeared to have a beneficial effect on hypoxic slices. Consequently, it was found that lactate can replace glucose as the sole aerobic energy substrate to support synaptic function in cerebral tissue in vitro.

Action Potentials↗

PubMed1989

Protection against cerebral hypoxia by local anesthetics: a study using brain slices.

The ability of the local anesthetics lidocaine, 2-chloroprocaine and cocaine to protect neuronal tissue against hypoxic damage was evaluated. Rat hippocampal slices were incubated with non-depressive doses of these agents 60 min prior to their exposure to 15 min hypoxia. The rate of recovery of synaptic function (evoked field potentials) following the hypoxic episode was used as an index of hypoxic damage. Slices treated with 0.1 mM of any of the three local anesthetics exhibited a significant increase in the recovery rate of synaptic function from hypoxia as compared to control, untreated slices. These results indicate that local anesthetics, by reducing neuronal sodium influx (and possibly its concomitant calcium influx) which occurs upon hypoxic depolarization, are able to prolong the hypoxic insult a cerebral tissue could tolerate.

Action Potentials↗

PubMed1989

Identification and function of brain stem neurons regulating rat ileal water absorption.

The central nervous system (CNS) regions regulating ileal water and ion absorption are unknown. We determined 1) the CNS origin of brain stem neurons that directly innervate the rat ileum, and 2) that these neurons influence intestinal water absorption. Horseradish peroxidase (HRP) was injected into the muscle layer of the rat ileum. The brains were examined for HRP reaction product (HRPRP) 3, 5, or 7 days later. Only cell bodies of the dorsal motor nucleus of the vagus (DMNV) were labeled. Unilateral cervical vagotomy prevented deposition in the ipsilateral DMNV. To determine whether the DMNV regulates ileal water absorption, electrical and chemical stimulation (30 microA, 4 Hz, 0.2 ms, and 300 pmol L-glutamate every 5 min, respectively) were used. Both the DMNV and the adjacent nucleus tractus solitarius (NTS) were stimulated, causing a reduction in water absorption. Bilateral vagotomy prevented the effect of bilateral electrical stimulation, but unilateral vagotomy did not prevent the decrease due to ipsilateral stimulation. These studies show that 1) the DMNV innervates the ileum, and 2) alteration of vagal efferent activity by stimulation of the DMNV and NTS reduces ileal water absorption.

Animals↗

PubMed1988

Lactate-supported synaptic function in the rat hippocampal slice preparation.

The present study was undertaken to examine the possibility that cerebral energy metabolism can be fueled by lactate. As a sole energy substrate, lactate supported normal synaptic function in rat hippocampal slices for hours without any sign of deterioration. Slices that were synaptically silent as a result of glucose depletion could be reactivated with lactate to show normal synaptic function. When slices were exposed to the glycolytic inhibitor iodoacetic acid, lactate-supported synaptic function was unaffected, whereas that supported by glucose was completely abolished. This indicated that lactate was metabolized directly via pyruvate to enter the tricarboxylic acid cycle. Thus, under conditions that lead to lactate accumulation (cerebral ischemia) this "end product" may be a useful alternative as a substrate for energy metabolism.

Animals↗

PubMed1988

Lactic acidosis and recovery of neuronal function following cerebral hypoxia in vitro.

The rat hippocampal slice preparation was used to study the combined effects of hypoxia and lactic acidosis on neuronal function. Control slices were exposed to a standard hypoxic insult while being perfused with normal artificial cerebrospinal fluid (ACSF). Experimental slices were perfused with ACSF containing 1.0, 2.0, 10.0 or 20.0 mM lactic acid, 30 min before and during the same standard hypoxic insult. Following at 30-min recovery period the ability of these slices to respond to orthodromic stimulation by displaying a population spike (synaptic function) was tested. No significant decreases in the recovery rate of synaptic function were found between control and experimental groups, excluding the combination of 20 mM lactic acid and 10 min hypoxia, where such a decrease was found. The combination of 10 mM lactic acid and 12 min hypoxia brought about an increase in the recovery rate of synaptic function. Thus, the adverse effects attributed to lactic acid in vivo were not seen in the present in vitro study. Neuronal tissue appears to be able to handle excess lactic acid by yet, unknown mechanism (high intracellular buffer capacity?). The suggested in vivo damage due to lactic acidosis could originate in the cerebrovascular system. On the other hand, the possibility that lactic acidosis is harmless under hypoxic conditions should also be considered.

Acidosis, Lactic↗

PubMed1988

Pitfalls in the use of brain slices.

In vitro brain slices are the preparation of choice for the detailed examination of local circuit properties in mammalian brain. However it is the investigator's responsibility to verify that the circuits under investigation are indeed confined within the boundaries of the functional region of the slice used. The medium in which the slice is maintained is under the full control of the investigator. This places the burden on the investigator to ensure that: (1) the properties of the medium are fully under control; (2) the effects of the medium on the slice are known; (3) the conditions under which the slice is being maintained bear some reasonable relation to those it enjoys (or endures) in vivo. Generalizations to in vivo conditions must be made with caution. If at all possible, similar studies (perhaps less extensive, due to the greater technical difficulties) should be done in vivo to provide a basis for comparison. Investigators using drugs should be aware of, and respect, the basic pharmacological principles cited in the text. In particular, the substantial freedom the investigator has in defining the extracellular medium should not be abused.

Animals↗

PubMed1988

The rat hippocampal slice preparation as an in vitro model of ischemia.

In vivo models of cerebral ischemia do not fully control for the interacting effects of many variables (e.g., anesthesia, temperature, cerebrovascular changes) and often do not clearly define the region affected. Numerous in vivo studies have indicated that hyperglycemia augments ischemic brain damage; this effect is often attributed to lactic acidosis. To separate the effects on neuronal tissue of ischemia from those due to actions on the cerebrovascular system, we used an in vitro blood-free system as an ischemic model. In our study we evaluated the effects of various combinations of oxygen and glucose levels on evoked synaptic activity in the CA1 region of the rat hippocampal slice preparation. A 50% inhibitory dose for both oxygen and glucose on neuronal synaptic function was determined. It is our intention to use this model for preliminary screening of antihypoxic/anti-ischemic drugs.

Animals↗

PubMed1987

The mechanism of neuronal resistance and adaptation to hypoxia.

In this work we provide a theoretical explanation for the observations that: (i) young animals are more resistant to hypoxia than adult ones and (ii) repeated exposure to a hypoxic insult increases the tolerance of young animals and isolated brain tissue to that insult. Considered here is the role of taurine, a putative Ca2+ transport modulator, in attenuating Ca2+ influx and overload in brain tissue upon hypoxia. It is proposed that the higher resistance of young animals to hypoxia stems from their higher brain content of taurine as compared with adults. The increased resistance to lack of oxygen upon re-exposure to hypoxia may occur as a result of protein and coenzyme A (CoA) breakdown which leads to the accumulation of products like cystine, cysteine, cysteamine and other sulfur-containing compounds. Upon reoxygenation, these compounds are oxidized to form taurine, which in turn attenuates neuronal Ca2+ accumulation. The sulfur-containing compounds are considered to be natural scavengers of oxygen-derived free radicals which are formed upon reoxygenation and have been implicated as a major component in the process leading to ischemic/hypoxic brain damage. Repeated hypoxic insults bring about the formation of higher levels of taurine and hence the observed adaptation to oxygen lack. The hypothesis presented here is supported by experimental observations in our laboratory and those of others.

Adaptation, Physiological↗

PubMed1987

Increased glucose improves recovery of neuronal function after cerebral hypoxia in vitro.

The rat hippocampal slice preparation was used to evaluate the effect of increasing glucose levels in the perfusion medium on the recovery of synaptic function after a standardized hypoxic insult. Slices exposed to low glucose (5 mM) did not recover from a standard hypoxic insult (10 min of 95% N2/5% CO2 atmosphere). Following the same insult, 39% of the control (10 mM glucose) slices recovered their synaptic function, while 93% of the slices provided with high glucose level (20 mM) exhibited recovery of synaptic function. Thus, a dose-dependent effect of glucose on recovery of neuronal function following an intermediate period (10 min) of oxygen deprivation was found. The high-glucose-treated slices could tolerate a severe hypoxic insult of 15 min or even 20 min from which 94% and 81% of them recovered, respectively. Only 21% of the control (10 mM glucose) slices recovered their synaptic activity following 15 min of hypoxia, and none survived 20 min of that insult. The adverse effects of hyperglycemia reported in vivo were not seen in our study. This may be due to the sustained perfusion of the brain slice preparation, which could limit accumulation of lactic acid during hypoxia. However, treatment of slices with lactic acid prior to and during the hypoxic insult did not worsen the outcome. Alternatively, glucose may protect against the damaging effects of oxygen free radicals formed during reoxygenation. Nevertheless, the antihypoxic effect of glucose appears to be a metabolic one, since L-glucose (the non-metabolic analog of D-glucose) was innocuous in this respect.

Animals↗