Enhancement of endogenous excitatory amino acids by theophylline does not modify the behavioral and histological consequences of forebrain ischemia

Cellular defenses against excitotoxic insults

The cellular events mediating necrotic neuron death are now reasonably well understood, and involve excessive extracellular accumulation of glutamate and free cytosolic calcium. When such necrotic neurological insults occur, neurons are not passively buffeted, but instead mobilize a variety of defenses in an attempt to decrease the likelihood of neuron death, or to decrease the harm to neighboring neurons (by decreasing the likelihood of inflammation). This review considers some of these defenses, organizing them along the lines of those which decrease neuronal excitability, decrease extracellular glutamate accumulation, decrease cytosolic calcium mobilization, decrease calcium-dependent degenerative events, enhance neuronal energetics, and bias a neuron towards apoptotic, rather than necrotic, death. Although these are currently perceived as a disparate array of cellular adaptations, some experimental approaches are suggested that may help form a more unified subdiscipline of cellular defenses against neurological insults. Such an advance would help pave the way for the rational design of therapeutic interventions against necrotic insults.

Adenosine: does it have a neuroprotective role after all?

A neuroprotective role for adenosine is commonly assumed. Recent studies revealed that adenosine may unexpectedly, under certain circumstances, have the opposite effects contributing to neuronal damage and death. The basis for this duality may be the activation of distinct subtypes of adenosine receptors, interactions between these receptors, differential actions on neuronal and glial cells, and various time frames of adenosinergic compounds administration. If these aspects are understood, adenosine should remain an interesting target for therapeutical neuroprotective approaches after all.

No effect of glutamate on metabolic disturbances in hippocampal slices of mature fetal guinea pigs after transient in vitro ischemia

The involvement of glutamate in the development of cerebral metabolic disturbances in mature fetuses after transient ischemia was studied using a hippocampal slice model. We investigated the effects of exogenously applied glutamate or glutamate antagonists on the recovery of energy metabolism and protein synthesis rate (PSR) in hippocampal slices of mature guinea pigs after in vitro ischemia. The slices were incubated in a thermostatically controlled flow-through chamber and gassed with carbogen (95% O2/5% CO2). In vitro ischemia was induced by transferring the slices to an aglycemic, artificial cerebrospinal fluid (aCSF) equilibrated with 95% N2/5% CO2. In a first set of experiments slices were exposed to 10 mM glutamate during a 20-40 min period of in vitro ischemia. In a second set slices were incubated in aCSF containing MK-801 (100 microM) or kynurenic acid (0.5 mM) 30 min before, during and 2 h after in vitro ischemia. After a 12 h recovery phase, the concentrations of adenylates in the slices were measured by HPLC after extraction with perchloric acid. PSR was calculated from the rate of incorporation of [14C]leucine into tissue proteins. Neither glutamate nor glutamate antagonists had any effect on the postischemic recovery of energy metabolism and PSR when applied during in vitro ischemia. It is therefore concluded that glutamate does not play a major role in the development of metabolic disturbances in hippocampal slices from mature guinea pig fetuses subjected to transient in vitro ischemia.

Adenosine A1 antagonism increases specific synaptic forms of glutamate release during anoxia, revealing a unique source of excitation

The role of the adenosine A1 receptor in the modulation of anoxia-induced synaptic glutamate release was examined in CA1 pyramidal neurons by whole-cell voltage-clamp recording in the rat hippocampal slice preparation. Anoxia leads to an increased action potential-independent synaptic glutamate release in the form of a higher frequency of miniature excitatory postsynaptic currents (mEPSCs). This increase is not significantly affected when slices are preincubated in the adenosine A1 receptor antagonist, 8-cyclopentyl-1, 3-dipropylxanthine (DPCPX). A second population of spontaneous inward currents, however, occurs in DPCPX-treated slices during a well-defined period following the onset of anoxia. Their suppression by glutamate antagonists, tetrodotoxin, or by the cutting of the Schaffer collateral pathway indicates that they represent action potential-dependent, glutamatergic excitatory postsynaptic currents (ap-EPSCs) originating from CA3 pyramidal neurons. CA3 neurons were examined in current-clamp whole-cell patch mode to determine the origin of this increased orthodromic excitation. After the onset of anoxia, CA3 cells initially exhibit a small depolarization or hyperpolarization associated with a decrease in input resistance. This is followed by transient depolarization (the depolarizing "nub"), which is associated with an increase in input resistance. The nub evoked single as well as bursts of action potentials in CA3 neurons. The occurrence of these CA3 nub-elicited action potentials coincides with that of ap-EPSCs recorded in the CA1 cells. Recording with cesium- rather than standard potassium-containing electrodes results in the suppression of the nub and its associated increase in input resistance. In conclusion we have shown that adenosine tone plays an important role in suppressing anoxia-induced spontaneous ap-EPSCs but not action potential-independent mEPSCs in CA1 neurons. These EPSCs originate from a depolarization in CA3 pyramidal neurons, which is associated with an increase in resistance. This previously undescribed phenomenon likely results from a decrease in the conductance of an unidentified potassium channel.

Effects of an A1 adenosine receptor agonist on the neurochemical, behavioral and histological consequences of ischemia

Untreated rats and rats given the A1 receptor adenosine agonist, R-phenylisopropyladenosine (R-PIA), were subjected to four vessel ischemia. The effect of R-PIA on hippocampal amino acid release, hippocampal neuronal damage, exploratory behavior, learning capacity and global neurological score were evaluated. R-PIA decreased by half the glutamate released during ischemia and improved the global neurological scores 3, 24, 48, 78 h and 7 days after ischemia. But R-PIA had no effect on taurine/GABA release (during ischemia), hippocampal neuronal damage (7 days post-ischemia), exploratory behavior (48 h post-ischemia) or learning capacity (7 days post-ischemia). Thus, a decrease in glutamate release by R-PIA is not systematically correlated with an improvement of histological damage or learning capacity. Reduced glutamate release is not therefore a sufficient criterion on which to evaluate the neuroprotective capacity of a drug.

Propentofylline administered by microdialysis attenuates ischemia-induced hippocampal damage but not excitatory amino acid release in gerbils

Systemic administration of propentofylline (PPF), an adenosine uptake inhibitor, has been demonstrated to protect CA1 pyramidal cells from death following transient cerebral ischemia in gerbils. In order to examine the direct effects of this inhibitor, we tested whether or not PPF administered into the hippocampus in situ through a microdialysis probe could attenuate ischemia-induced excitatory amino acid (EAA) release and prevent subsequent death of CA1 pyramidal cells in the gerbil. The EAA release and death of CA1 pyramidal cells observed in the hippocampus were compared with those in the contralateral hippocampus of the same animal into which vehicle alone was administered. The results indicated that pre- as well as post-treatments with PPF inhibited the death of CA1 pyramidal cells after 5-min ischemia in a dose-dependent manner, but did not significantly alter the EAA release during ischemia and reperfusion in the same animals. While the neuroprotective effect of PPF against ischemic damage has commonly been ascribed to attenuation of EAA release during ischemia, other actions of adenosine such as those influencing the synaptic responses, neuronal excitation, and local cerebral circulation, or as yet unidentified actions may be involved in the observed neuroprotective effects of PPF.

Adenosine modulation of amino acid release in rat hippocampus during ischemia and veratridine depolarization

This study was undertaken to determine whether endogenous adenosine modulates 'in vivo' neurotransmitter amino acid release via its presynaptic receptors. Two conditions were compared: neuronal depolarization by local infusion of veratridine (600 microM), and transient global ischemia by four-vessel occlusion. Both stimuli were applied for 20 min. Extracellular amino acid (glutamate, taurine/GABA, glycine) variations in concentration were determined in the rat hippocampus by microdialysis and HPLC. Modulation of adenosine receptor activity was objectified by continuous local infusion of an adenosine agonist (R-phenylisopropyladenosine R-PIA) or an antagonist (theophylline), starting one hour before stimulation of amino acid release. R-PIA (100 microM) significantly decreased the glutamate release (50%) evoked by veratridine, whereas it did not significantly modify the ischemia-induced glutamate release. In contrast, theophylline did not significantly affect veratridine-induced glutamate release, but it significantly potentiated glutamate efflux (400%) under ischemic conditions. Neither treatment altered the release of the other amino acids. These data suggest that endogenous adenosine appearing in the extracellular space during veratridine-induced depolarization cannot control glutamate release. In contrast, ischemia-induced glutamate release was strongly inhibited by the concomitant increase in extracellular adenosine.

Neuroprotective role of adenosine in cerebral ischaemia

Several lines of evidence suggest that adenosine may be an endogenous protective agent in cerebral ischaemia. Adenosine is normally present in the extracellular fluid in most tissues of the body, including the brain, and its level increases dramatically following hypoxia or ischaemia. The rate of adenosine production is enhanced when the energy demand is larger than the rate of energy supply. Adenosine acts on specific receptors that are present in most cells in the body and that produce cellular effects that tend to antagonize a number of pathological events thought to be instrumental for ischaemic nerve cell death. Karl Rudolphi and colleagues review evidence for the neuroprotective potential of adenosine and indicate some targets for drug development.

Inhibition of glutamate release in rat hippocampus by kynurenic acid does not protect CA1 cells from forebrain ischemia

We assessed the effect of a broad spectrum glutamatergic receptor antagonist, kynurenic acid (500 mg/kg) on ischemia-induced hippocampal glutamate release and neuronal damage. Kynurenic acid significantly decreased glutamate release during ischemia but had no effect on the hippocampal lesion. Some protection was observed in the cortex and in the striatum. These data suggested that the extracellular accumulation of glutamate during forebrain ischemia does not play a major role in the hippocampus.