Escape from inhibition of synaptic transmission during in vitro hypoxia and hypoglycemia in the hippocampus

Anoxic spreading depolarization in the neonatal rat cortex in vitro

Anoxic spreading depolarization (aSD) is a hallmark of ischemic injury in the cerebral cortex. In adults, aSD is associated with rapid and nearly complete neuronal depolarization and loss of neuronal functions. While ischemia also evokes aSD in the immature cortex, developmental aspects of neuronal behavior during aSD remain largely unknown. Here, using oxygen-glucose deprivation (OGD) ischemia model in slices of the postnatal rat somatosensory cortex, we found that immature neurons displayed much more complex behaviors: they initially moderately depolarized during aSD, then transiently repolarised (for up to tens of minutes), and only then passed to terminal depolarization. The ability to fire action potentials was maintained in neurons mildly depolarized during aSD without reaching the level of depolarization block, and these functions were regained in the majority of immature neurons during post-aSD transient repolarization. The amplitude of depolarization and the probability of depolarization block during aSD increased, whereas transient post-SD repolarization levels and duration, and associated recovery in neuronal firing decreased with age. By the end of the first postnatal month, aSD acquired an adult-like phenotype, where depolarization during aSD merged with terminal depolarization and the phase of transient recovery was lost. Thus, changes in neuronal function during aSD undergo remarkable developmental changes that may contribute to lower susceptibility of the immature neurons to ischemia.

Optimising the energetic cost of the glutamatergic synapse

As for electronic computation, neural information processing is energetically expensive. This is because information is coded in the brain as membrane voltage changes, which are generated largely by passive ion movements down electrochemical gradients, and these ion movements later need to be reversed by active ATP-dependent ion pumping. This article will review how much of the energetic cost of the brain reflects the activity of glutamatergic synapses, consider the relative amount of energy used pre- and postsynaptically, outline how evolution has energetically optimised synapse function by adjusting the presynaptic release probability and the postsynaptic number of glutamate receptors, and speculate on how energy use by synapses may be sensed and adjusted.

Synaptic and Network Contributions to Anoxic Depolarization in Mouse Hippocampal Slices

Ischemic stroke remains the third leading cause of death and leading cause of adult disability worldwide. A key event in the pathophysiology of stroke is the anoxic depolarization (AD) of neurons in the ischemic core. Previous studies have established that both the latency to AD and the time spent in AD prior to re-oxygenation are predictors of neuronal death. The present studies used hippocampal slices from male and female mice to investigate the electrophysiological events that affect latency to AD after oxygen deprivation. The results confirm that the epoch between AD and re-oxygenation largely determines the magnitude of synaptic recovery after anoxic challenge. Using a selective antagonist of adenosine A₁ receptors, we also confirmed that adenosine released during anoxia (ANOX) suppresses synaptic glutamate release; however, this action has no effect on AD latency or the potential for post-anoxic recovery of synaptic transmission. In contrast, antagonism of AMPA- and NMDA-type glutamate receptors significantly prolongs the latency to AD and alters the speed and synchrony of associated depolarizing waves. Experiments using slices with fields Cornu ammonis 3 (CA3) and Cornu ammonis 1 (CA1) disconnected showed that AD latency is longer in CA1 than in CA3; however, the early AD in CA3 is propagated to CA1 in intact slices. Finally, AD latency in CA1 was found to be longer in slices from female mice than in those from age-matched male mice. The results have implications for stroke prevention and for understanding brain adaptations in hypoxia-tolerant animals.

The Selective Antagonism of Adenosine A2B Receptors Reduces the Synaptic Failure and Neuronal Death Induced by Oxygen and Glucose Deprivation in Rat CA1 Hippocampus in Vitro

Ischemia is a multifactorial pathology characterized by different events evolving in time. Immediately after the ischemic insult, primary brain damage is due to the massive increase of extracellular glutamate. Adenosine in the brain increases dramatically during ischemia in concentrations able to stimulate all its receptors, A₁, A_2A, A_2B, and A₃. Although adenosine exerts clear neuroprotective effects through A₁ receptors during ischemia, the use of selective A₁ receptor agonists is hampered by their undesirable peripheral side effects. So far, no evidence is available on the involvement of adenosine A_2B receptors in cerebral ischemia. This study explored the role of adenosine A_2B receptors on synaptic and cellular responses during oxygen and glucose deprivation (OGD) in the CA1 region of rat hippocampus in vitro. We conducted extracellular recordings of CA1 field excitatory post-synaptic potentials (fEPSPs); the extent of damage on neurons and glia was assessed by immunohistochemistry. Seven min OGD induced anoxic depolarization (AD) in all hippocampal slices tested and completely abolished fEPSPs that did not recover after return to normoxic condition. Seven minutes OGD was applied in the presence of the selective adenosine A_2B receptor antagonists MRS1754 (500 nM) or PSB603 (50 nM), separately administered 15 min before, during and 5 min after OGD. Both antagonists were able to prevent or delay the appearance of AD and to modify synaptic responses after OGD, allowing significant recovery of neurotransmission. Adenosine A_2B receptor antagonism also counteracted the reduction of neuronal density in CA1 stratum pyramidale, decreased apoptosis at least up to 3 h after the end of OGD, and maintained activated mTOR levels similar to those of controls, thus sparing neurons from the degenerative effects caused by the simil-ischemic conditions. Astrocytes significantly proliferated in CA1 stratum radiatum already 3 h after the end of OGD, possibly due to increased glutamate release. A_2Breceptor antagonism significantly prevented astrocyte modifications. Both A_2B receptor antagonists did not protect CA1 neurons from the neurodegeneration induced by glutamate application, indicating that the antagonistic effect is upstream of glutamate release. The selective antagonists of the adenosine A_2B receptor subtype may thus represent a new class of neuroprotective drugs in ischemia.

Recovery of Syrian hamster hippocampal signaling following its depression during oxygen-glucose deprivation is enhanced by cold temperatures and by hibernation

Signal transmission over a hippocampal network of CA3 and CA1 neurons in Syrian hamsters (Mesocricetus auratus), facultative hibernators, has not been fully characterized in response to oxygen-glucose deprivation (OGD). We hypothesized that during OGD, hippocampal signal transmission fails first at the synapse between CA3 and CA1 pyramidal neurons and that recovery of signal processing following OGD is more robust in hippocampal slices at cold temperature, from hamsters vs. rats, and from hibernating vs. non-hibernating hamsters. To test these hypotheses, we recorded fEPSPs and population spikes of CA1 neurons at 25°C, 30°C, and 35°C in 400μm slices over a 15min control period with the slice in oxygenated aCSF containing glucose (control solution), a 10min treatment period (OGD insult) where oxygen was replaced by nitrogen in aCSF lacking glucose, and a 30min recovery period with the slice in the control solution. The initial site of transmission failure during OGD occurred at the CA3-CA1 synapse, and recovery of signal transmission was at least, if not more (depending on temperature), complete in slices from hibernating vs. non-hibernating hamsters, and from non-hibernating hamsters vs. rats. Thus, hamster neuroprotective mechanisms supporting functional recovery were enhanced by cold temperatures and by hibernation.

Effects of oxygen and glucose deprivation on synaptic transmission in rat dentate gyrus: role of A2A adenosine receptors

The hippocampus is comprised of two distinct subfields that show different responses to hypoxic-ischemic brain injury: the CA1 region is particularly susceptible whereas the dentate gyrus (DG) is quite resistant. Our aim was to determine the synaptic and proliferative response of the DG to severe oxygen and glucose deprivation (OGD) in acute rat hippocampal slices and to investigate the contribution of A(2A) adenosine receptor antagonism to recovery of synaptic activity after OGD. Extracellular recordings of field excitatory post-synaptic potentials (fEPSPs) in granule cells of the DG in brain slices prepared from male Wistar rats were used. A 9-min OGD is needed in the DG to always induce the appearance of anoxic depolarization (AD) and the irreversible block of synaptic activity, as recorded up to 24 h from the end of the insult, whereas only 7-min OGD is required in the CA1 region. Selective antagonism of A(2A) adenosine receptors by ZM241385 significantly prevents or delays the appearance of AD and protects from the irreversible block of neurotransmission induced by 9-min OGD in the DG. The effects of 9-min OGD on proliferation and maturation of cells localized in the subgranular zone of DG in slices prepared from 5-bromo-2'-deoxyuridine (BrdU) treated rats was investigated. Slices were further incubated with an immature neuronal marker, doublecortin (DCX). The number of BrdU(+) cells was significantly decreased 6 h after 9-min OGD and this effect was antagonized by ZM241385. After 24 h from the end of 9-min OGD, the number of BrdU(+) cells returned to that found before OGD and increased arborization of tertiary dendrites of DCX(+) cells was observed. The adenosine A(2A) antagonist ZM241385 protects from synaptic failure and from decreased proliferation of immature neuronal cells at a precocious time after OGD.

Cellular Links between Neuronal Activity and Energy Homeostasis

Neuronal activity, astrocytic responses to this activity, and energy homeostasis are linked together during baseline, conscious conditions, and short-term rapid activation (as occurs with sensory or motor function). Nervous system energy homeostasis also varies during long-term physiological conditions (i.e., development and aging) and with adaptation to pathological conditions, such as ischemia or low glucose. Neuronal activation requires increased metabolism (i.e., ATP generation) which leads initially to substrate depletion, induction of a variety of signals for enhanced astrocytic function, and increased local blood flow and substrate delivery. Energy generation (particularly in mitochondria) and use during ATP hydrolysis also lead to considerable heat generation. The local increases in blood flow noted following neuronal activation can both enhance local substrate delivery but also provides a heat sink to help cool the brain and removal of waste by-products. In this review we highlight the interactions between short-term neuronal activity and energy metabolism with an emphasis on signals and factors regulating astrocyte function and substrate supply.

The adenosine A2A receptor antagonist ZM241385 enhances neuronal survival after oxygen-glucose deprivation in rat CA1 hippocampal slices

BACKGROUND AND PURPOSE

Activation of adenosine A(2A) receptors in the CA1 region of rat hippocampal slices during oxygen-glucose deprivation (OGD), a model of cerebral ischaemia, was investigated.

EXPERIMENTAL APPROACH

We made extracellular recordings of CA1 field excitatory postsynaptic potentials (fepsps) followed by histochemical and immunohistochemical techniques coupled to Western blots.

KEY RESULTS

OGD (7 or 30 min duration) elicited an irreversible loss of fepsps invariably followed by the appearance of anoxic depolarization (AD), an unambiguous sign of neuronal damage. The application of the selective adenosine A(2A) receptor antagonist, ZM241385 (4-(2-[7-amino-2-{2-furyl}{1,2,4}triazolo{2,3-a}{1,3,5}triazin-5-ylamino]ethyl)phenol; 100-500 nmolxL(-1)) prevented or delayed AD appearance induced by 7 or 30 min OGD and protected from the irreversible fepsp depression elicited by 7 min OGD. Two different selective adenosine A(2A) receptor antagonists, SCH58261 and SCH442416, were less effective than ZM241385 during 7 min OGD. The extent of CA1 cell injury was assessed 3 h after the end of 7 min OGD by propidium iodide. Substantial CA1 pyramidal neuronal damage occurred in untreated slices, exposed to OGD, whereas injury was significantly prevented by 100 nmolxL(-1) ZM241385. Glial fibrillary acid protein (GFAP) immunostaining showed that 3 h after 7 min OGD, astrogliosis was appreciable. Western blot analysis indicated an increase in GFAP 30 kDa fragment which was significantly reduced by treatment with 100 nmolxL(-1) ZM241385.

CONCLUSIONS AND IMPLICATIONS

In the CA1 hippocampus, antagonism of A(2A) adenosine receptors by ZM241385 was protective during OGD (a model of cerebral ischaemia) by delaying AD appearance, decreasing astrocyte activation and improving neuronal survival.

Phenytoin and carbamazepine delay the initial depression of the population spike upon exposure to in vitro ischemia and promote its post-ischemic functional recovery in rat hippocampal slices

Antiepileptic drugs have been shown to reduce the severity of neurodegeneration resulting from stroke or brain injury. In the present study, we evaluated the effects of the antiepileptic drugs phenytoin and carbamazepine on the time course of changes in the population spike (PS) during brief oxygen/glucose deprivation (OGD) in the CA1 pyramidal region of rat hippocampal slices in vitro. After introducing simulated ischemia by OGD, the PS was initially inhibited, followed by transient recovery and subsequent reinhibition again concomitantly with disappearance of the presynaptic volley (PV). The slices were then reperfused with oxygen/glucose-containing solution. Both phenytoin and carbamazepine (30 and 100 muM each) concentration-dependently delayed the initial inhibition and the time to transient recovery of the PS during OGD, thus prolonging the time until disappearance of the PV. However, they significantly promoted restoration of the PS after reperfusion. These results suggest that treatment with phenytoin and carbamazepine increases the resistance of tissue to energy deprivation, as evidenced by the facilitated post-ischemic recovery of the PS, despite prolonged ischemia.

Depletion of ATP and release of presynaptic inhibition in the CA1 region of hippocampal slices during hypoglycemic hypoxia

Transient recovery (TR) of evoked synaptic potentials and ATP depletion during the late stage of hypoxic hypoglycemic insults were investigated in rat hippocampal slices. TR was observed not only in the late stage of insult, but also during recovery. The concentration of ATP corresponded to the appearance (27% of control) and disappearance (15% of control) of TR. Paired pulse studies showed the presynaptic nature of the release of inhibition of synaptic transmission during TR. Both N- and P/Q-type voltage-dependent calcium channels were involved in the appearance of TR. This evidence suggests that underlying mechanisms of TR appearance during hypoxic hypoglycemic insult might be related to ATP depletion and release of A1 adenosine receptor mediated inhibition of presynaptic voltage-dependent calcium channels.

Transient recovery of synaptic transmission is related to rapid energy depletion during hypoxia

Transient recovery (TR) of evoked synaptic potential during the late stage of hypoxic hypoglycemia (HH) insult was investigated in rat hippocampal slices using extracellular recording methods. TR was observed in association with a rapid deterioration of antidromic population spikes (aPSs) following HH insult. TR was not elicited in normoglycemic hypoxia (NH), in which a gradual and delayed deterioration of aPSs was noted. TR was not modulated by either Ca(2+)- or PKC-dependent processes. When a glycolytic inhibitor was added, NH resulted in a rapid deterioration of aPSs and prompted appearance of TR. TR was also seen in slices using lactate to generate energy via oxidative phosphorylation, when hypoxic conditions were subsequently created. Other pharmacological interventions that aimed to cause rapid deterioration of aPSs without depleting energy stores failed to reproduce TR. The evidence thus suggests that the underlying mechanisms of TR appearance during HH insult are highly correlated with rapid energy depletion.

A preferential role for glycolysis in preventing the anoxic depolarization of rat hippocampal area CA1 pyramidal cells

During brain anoxia or ischemia, a decrease in the level of ATP leads to a sudden decrease in transmembrane ion gradients [anoxic depolarization (AD)]. This releases glutamate by reversing the operation of glutamate transporters, which triggers neuronal death. By whole-cell clamping CA1 pyramidal cells, we investigated the energy stores that delay the occurrence of the AD in hippocampal slices when O2 and glucose are removed. With glycolytic and mitochondrial ATP production blocked in P12 slices, the AD occurred in approximately 7 min at 33 degrees C, reflecting the time needed for metabolic activity to consume the existing ATP and phosphocreatine, and for subsequent ion gradient decrease. Allowing glycolysis fueled by glycogen, in the absence of glucose, delayed the AD by 5.5 min, whereas superfused glucose prevented the AD for >1 h. With glycolysis blocked, the latency to the AD was 6.5 min longer when mitochondria were allowed to function, demonstrating that metabolites downstream of glycolysis (pyruvate, citric acid cycle intermediates, and amino acid oxidation) provide a significant energy store for oxidative phosphorylation. With glycolysis blocked but mitochondria functioning, superfusing lactate did not significantly delay the AD, showing that ATP production from lactate is much less than that from endogenous metabolites. These data demonstrate a preferential role for glycolysis in preventing the AD. They also define a hierarchy of pool sizes for hippocampal energy stores and suggest that brain ATP production from glial lactate may not be significant in conditions of energy deprivation.

Brief, repeated, oxygen-glucose deprivation episodes protect neurotransmission from a longer ischemic episode in the in vitro hippocampus: role of adenosine receptors

1. Ischemic preconditioning in the brain consists of reducing the sensitivity of neuronal tissue to further, more severe, ischemic insults. We recorded field epsps (fepsps) extracellularly from hippocampal slices to develop a model of in vitro ischemic preconditioning and to evaluate the role of A1, A2A and A3 adenosine receptors in this phenomenon. 2. The application of an ischemic insult, obtained by glucose and oxygen deprivation for 7 min, produced an irreversible depression of synaptic transmission. Ischemic preconditioning was induced by four ischemic insults (2 min each) separated by 13 min of normoxic conditions. After 30 min, an ischemic insult of 7 min was applied. This protocol substantially protected the tissue from the irreversible depression of synaptic activity. 3. The selective adenosine A1 receptor antagonist, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX, 100 nm), completely prevented the protective effect of preconditioning. The selective adenosine A2A receptor antagonist 4-(2-[7-amino-2-(2-furyl)[1,2,4]triazolo[2,3-a][1,3,5]triazin-5-ylamino]ethyl)phenol (ZM 241385, 100 nm) did not modify the magnitude of fepsp recovery compared to control slices. The selective A3 adenosine receptor antagonists, 3-propyl-6-ethyl-5[ethyl(thio)carbonyl]-2-phenyl-4-propyl-3-pyridinecarboxylate (MRS 1523, 100 nm) significantly improved the recovery of fepsps after 7 min of ischemia. 4. Our results show that in vitro ischemic preconditioning allows CA1 hippocampal neurons to become resistant to prolonged exposure to ischemia. Adenosine, by stimulating A1 receptors, plays a crucial role in eliciting the cell mechanisms underlying preconditioning; A2A receptors are not involved in this phenomenon, whereas A3 receptor activation is harmful to ischemic preconditioning.

Neuronal mechanisms of the anoxia-induced network oscillations in the rat hippocampus in vitro

1. A spindle of fast network oscillations precedes the ischaemia-induced rapid depolarisation in the rat hippocampus in vivo. However, this oscillatory pattern could not be reproduced in slices and the underlying mechanisms remain poorly understood. We have found that anoxia-induced network oscillations (ANOs, 20-40 Hz, lasting for 1-2 min) can be reproduced in the intact hippocampi of postnatal day P7-10 rats in vitro, and we have examined the underlying mechanisms using whole-cell and extracellular field potential recordings in a CA3 pyramidal layer. 2. ANOs were generated at the beginning of the anoxic depolarisation, when pyramidal cells depolarised to subthreshold values. Maximal power of the ANOs was attained when pyramidal cells depolarised to -56 mV; depolarisation above -47 mV resulted in a depolarisation block of pyramidal cells and a waning of ANOs. 3. A multiple unit activity in extracellular field recordings was phase locked to the negative and ascending phases of ANOs. Pyramidal cells recorded in current-clamp mode generated action potentials with an average probability of about 0.05 per cycle. The AMPA receptor-mediated EPSCs and the GABA receptor-mediated IPSCs in CA3 pyramidal cells were also phase locked with ANOs. 4. ANOs were prevented by tetrodotoxin and glutamate receptor antagonists CNQX and APV, and were slowed down by the allosteric GABA(A) receptor modulator diazepam. In the presence of the GABA(A) receptor antagonist bicuculline, ANOs were transformed to epileptiform discharges. 5. In the presence of the A1 adenosine receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), the anoxia induced an epileptiform activity and no ANOs were observed. 6. In normoxic conditions, a rise of extracellular potassium to 10 mM induced an epileptiform activity. Increasing extracellular potassium in conjunction with a bath application of the adenosine A1 receptor agonist cyclopentyladenosine induced oscillations similar to ANOs. 7. Multisite recordings along the septo-temporal hippocampal axis revealed that ANOs and anoxic depolarisation originate in the temporal part, and propagate towards the septal pole at a speed of 1.9 mm x min(-1). 8. ANOs were observed starting from P7, i.e. at a developmental stage when the effects of GABA change from depolarisation to hyperpolarisation. 9. These results suggest that the synchronisation of anoxia-induced oscillations relies on synaptic mechanisms; that the inhibition by GABA and adenosine sets the tune for a generation of oscillations and prevents an epileptiform activity; and that a synchronous GABAergic inhibition is instrumental in a phase locking neuronal activity similarly to other types of oscillatory activities in the gamma frequency range.

Hypoxia and neuronal function under in vitro conditions

Neurons in the mammalian CNS are highly sensitive to the availability of oxygen. Hypoxia can alter neuronal function and can lead to neuronal injury or death. The underlying changes in the membrane properties of single neurons have been studied in vitro in slice preparations obtained from various brain areas. Hypoxic changes of membrane potential and input resistance correspond to a decrease in ATP concentration and an increase in internal Ca2+ concentration. Functional modifications consisting of substantial membrane depolarization and failure of synaptic transmission can be observed within a few minutes following onset of hypoxia. The hypoxic depolarization accompanied by a hyperexcitability is a trigger signal for induction of neuronal cell death and is mediated mainly by activation of glutamate receptors. The mechanisms of the hypoxic hyperpolarization are more complex. Two types of potassium channels contribute to the hyperpolarization, the Ca(2+)- and the ATP-activated potassium channel. A number of neurotransmitters and neuromodulators is involved in the preservation of normal cell function during hypoxia. Therefore, hypoxia-induced cellular changes are unlikely to have a single, discrete pathway. The complexity of cellular changes implies that several strategies may be useful for neuroprotection and a successful intervention may be dependent upon drug action at more than one target site.

The effects of metabolic stress on glutamate receptor-mediated depolarizations in the in vitro rat hippocampal slice

A grease-gap preparation for the in vitro rat hippocampal slice has been used to record field excitatory postsynaptic potentials (fEPSPs), extracellular d.c. potential and depolarizations in response to glutamate receptor agonists before, during and after hypoxic/ischaemic episodes in the CA1 region. Synaptic transmission was depressed by hypoxia in a temperature-dependent manner (t1/2 at 28 degrees C, 1.9 +/- 0.2 min; t1/2 at 36 degrees C, 1.0 +/- 0.1 min) but was unaffected by the absence of D-glucose during hypoxia (ischaemia) at 28 degrees C. The reappearance of the fEPSP during hypoxic/ischaemic episodes was a prelude to severe disruptions of synaptic transmission if control conditions were not reinstated within 1 min of the secondary depression of the fEPSP. For a 10 min episode of hypoxia, recovery of synaptic transmission at 28 degrees C (96 +/- 1.5% of control) was significantly better than recovery following either hypoxia at 36 degrees C or ischaemia at 28 degrees C (41 +/- 17.2% and 55 +/- 21% of control, respectively). Chart recordings of the d.c. potential during hypoxia revealed a predominate (67% of all episodes) triphasic sequence of events (i, hyperpolarization; ii, depolarization; iii, post-hypoxic hyperpolarization on reoxygenation). Depolarizing responses to N-methyl-D-aspartate (NMDA, 20-40 microM; in 1 mM extracellular Mg2+), alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA, 2-10 microM) and L-glutamate (L-Glu, 2-5 mM) could be elicited at times when fEPSPs were completely depressed and up to 20 min into a hypoxic episode, the latest time-point examined. This implies, as others have suggested, that the hypoxic depression of excitatory synaptic transmission is presynaptic in origin. The application of AMPA or NMDA during the hypoxic depression of the fEPSP occasionally resulted in a short-lasting (12-45 min) potentiation (117-143% of control) of the fEPSP on return to normoxia. Furthermore, in other slices, which were exposed to severe metabolic stress, synaptic transmission was depressed to a significantly greater extent than AMPA depolarizations (mean depression; 76 +/- 5% and 28 +/- 8%, respectively).

Phorbol ester alters the electrophysiological responses to hypoxia and ischemic-like conditions in the rat hippocampal slice

The effect of incubation with the protein kinase C activator, 4 beta-phorbol 12,13-dibutyrate (beta-PDBu) on the electrophysiological responses to hypoxia and combined hypoxia and hypoglycemia was investigated in the rat hippocampal slice. Preincubation with beta-PDBu prevents adenosine-mediated inhibition of synaptic transmission under normoxic, normoglycemic conditions. beta-PDBu preincubation also reduces the adenosine-mediated hypoxia-induced depression of synaptic transmission revealing a substantial adenosine-independent hypoxia-induced depression of synaptic transmission. During combined hypoxia and hypoglycemia, slices preincubated in beta-PDBu display a significant shortening of the time of anoxic depolarization, an effect of beta-PDBu that is not mimicked by application of the adenosine antagonist cyclopentyltheophylline (8-CPT). It is concluded that the state of PKC activation may influence the electrophysiological responses to hypoxia and ischemia.

Purine release and inhibition of synaptic transmission during hypoxia and hypoglycemia in rat hippocampal slices

Evoked synaptic potentials and purine efflux were measured simultaneously from rat hippocampal slices. Slices were exposed to hypoxia, to glucose-free medium, and to in vitro ischemia consisting of glucose-free, hypoxic medium. During exposure to hypoxia or the glucose-free condition, radiolabelled purine efflux increased and the evoked population spike declined. Synaptic potentials and purine efflux returned to baseline values after reintroduction of normoxic and normoglycemic medium. During exposure to in vitro ischemia, purine and adenosine efflux were greatly increased with the appearance of the anoxic depolarization.