Postsynaptic Population Spike Research Articles

Effect of Riluzole, a Glutamate Release Inhibitor, on Synaptic Plasticity in the Intrahippocampal Aβ Rat Model of Alzheimer’s Disease

Alzheimer’s disease (AD) is associated with cognitive deficits of varying degrees and with impairment of the synaptic transmission-related tasks. Pathologically, AD is highlighted with accumulation of extracellular β-amyloid plaques and of neurofibrillary tangles. Glutamate-mediated neurotoxicity plays a pivotal role in the pathogenesis of AD. Deficits of long-term potentiation (LTP) and neuronal synaptic plasticity as an essential mechanism of the learning and memory disorders in AD has been ascribed to over-activation of glutamate receptors. We examined the effect of riluzole, a glutamate release inhibitor, on LTP impairment in the dentate gyrus (DG) in a rat model of AD provided by bilateral intrahippocampal amyloid β (Aβ 25-35) injections; riluzole was administered at a dose of 10 mg/kg. The LTP in perforant path-DG synapses was evaluated using measurements of the field excitatory postsynaptic potential (fEPSP) slope and population spike (PS) amplitude. We found that Aβ (25–34) significantly decreased the fEPSP slope and PS amplitude, as compared to those in the sham group; riluzole pretreatment in the Aβ-microinjected group significantly increased these parameters. Taken together, it is concluded that riluzole could noticeably improve synaptic plasticity and enhan ce LTP in the rat model of AD.

Electrophysiological evidence of biphasic action of carnosine on long-term potentiation in urethane-anesthetized rats

Carnosine is a dipeptide synthesized by the carnosine synthetase from β-alanine and l-histidine. The well-known effects of carnosine may be related with mechanisms producing long-term potentiation which is one of the electrophysiological signs of memory. In the present study we aimed to investigate the effect of four different doses of carnosine on long-term potentiation in urethane-anesthetized rat. A bipolar stimulating electrode was placed in the medial perforant path and a double-barrel glass micropipette was placed in the dentate gyrus as the recording electrode. Artificial cerebrospinal fluid (in the control group) or carnosine (0.1, 1, 10, and 100μg/μL) was infused into the dentate gyrus. Our results showed that the I/O curve of the excitatory postsynaptic potential slope or population spike amplitude was not significantly shifted by carnosine. We found that population spike amplitude increased to 244% and 287% at the dose of 100μg/μL in the post-tetanic and induction phases, respectively, but decreased to 163% and 186% at the dose of 0.1μg/μL and to 145% and 162% at the dose of 1μg/μL when compared with 203% and 232% of the control values. However, there were no significant differences for the slope of excitatory postsynaptic potential. Carnosine had no effect on the EPSP slope or PS amplitude recorded from the dentate gyrus in response to test stimuli when high-frequency stimulation was not delivered. In the present study, we speculated that the effects of carnosine in lower or higher doses could be explained by its effect on different processes, such as soluble guanylyl cyclase inhibition or the conversion of carnosine into histamine.

Hippocampal synaptic transmission and LTP in vivo are intact following bilateral vestibular deafferentation in the rat

Numerous studies in animals and humans have shown that damage to the vestibular system in the inner ear results in spatial memory deficits, presumably because areas of the brain such as the hippocampus require vestibular input to accurately represent the spatial environment. Consistent with this hypothesis, studies in animals have demonstrated that complete bilateral vestibular deafferentation (BVD) causes a disruption of place cell firing as well as theta activity. The aim of this study was to investigate whether BVD in rats affects baseline field potentials (field excitatory postsynaptic potentials and population spikes) and long-term potentiation (LTP) in CA1 and the dentate gyrus (DG) of awake freely moving rats up to 43 days post-BVD and of anesthetized rats at 7 months post-BVD. Compared to sham controls, BVD had no significant effect on either baseline field potentials or LTP in either condition. These results suggest that although BVD interferes with the encoding, consolidation, and/or retrieval of spatial memories and the function of place cells, these changes are not related to detectable in vivo decrements in basal synaptic transmission or LTP, at least in the investigated pathways.

Generation of excitatory postsynaptic potentials in the hippocampus after functional modification of glycosaminoglycans

Experiments on hippocampal slices form 4-week-old rats (n=28) showed that addition of lidase (1.0 and 10.0 U/ml) to the perfusion solution (artificial cerebrospinal fluid) was accompanied by the impaired generation or blockade of excitatory postsynaptic potentials and population spikes in the hippocampal CA1 region during stimulation of Schaffer collaterals. Removal of lidase from this solution normalized the amplitude of evoked responses. Hence, lidase in these concentrations produced a reversible effect on synaptic transmission. Our results indicate that structure and function of glycosaminoglycans in the extracellular matrix determine signal transduction in the nervous tissue.

Desflurane Improves the Recovery of the Evoked Postsynaptic Population Spike From CA1 Pyramidal Cells After Hypoxia in Rat Hippocampal Slices*

Desflurane is a volatile anesthetic that allows rapid induction and emergence, reduces cerebral metabolism, and enhances tissue perfusion. We studied the effect of treatment with 4%, 6%, and 12% desflurane on hypoxic neuronal damage by comparing the size of the postsynaptic evoked population spike recorded from the cornu ammonis 1 (CA1) pyramidal cell layer of rat hippocampal slices before and 2 hours after a hypoxic insult. When the tissue was treated with 6% desflurane before, during, and after 3.5 minutes of hypoxia, recovery was significantly better in slices exposed to desflurane (37% +/- 9%) compared with untreated hypoxic slices (15% +/- 5%). A lower (4%) or higher (12%) concentration of desflurane did not significantly improve recovery after 3.5 minutes of hypoxia. In the period before hypoxia, 12% and 6% desflurane significantly increased the latency and decreased the amplitude of the postsynaptic population spike; 4% desflurane had a similar but nonsignificant effect on latency and amplitude. We conclude that 6% desflurane, a clinically useful concentration (1 minimal alveolar concentration), improved the recovery of postsynaptic evoked responses in rat hippocampal slices after 3.5 minutes of hypoxia. In vivo studies must be conducted to assess the potential clinical significance of 6% desflurane's neuroprotective activity.

Status epilepticus in mice deficient for succinate semialdehyde dehydrogenase: GABAA receptor–mediated mechanisms

The epilepsy that occurs in SSADH deficiency has a seizure phenotype similar to that occurring in the SSADH(-/-) mouse. We examined the expression and function of the GABA(A) receptor (GABA(A)R) in SSADH-deficient mice. A selective decrease in binding of [(35)S]tert-butylbicyclophosphorothionate was observed in SSADH(-/-) mice at postnatal day 7 that was progressive until the third postnatal week of life when, at the nadir of the decreased [(35)S]tert-butylbicyclophosphorothionate binding, generalized convulsive seizures emerged that rapidly evolved into status epilepticus. We also observed a substantial downregulation of the beta(2) subunit of GABA(A)R, a reduction in GABA(A)-mediated inhibitory postsynaptic potentials, and augmented postsynaptic population spikes recorded from hippocampal slices. The SSADH(-/-) mouse model represents a powerful investigative tool for understanding the pathophysiology of the seizures associated with human SSADH deficiency. These data raise the possibility that progressive dysfunction of the GABA(A)R may be involved in the development of seizures in SSDAH-deficient mice. Elucidation of the precise fundamental mechanisms of the perturbation of the GABA(A)R-mediated function in SSADH(-/-) mice could lead to the development of novel treatment modalities designed to reduce the neurological morbidity in children with SSADH deficiency.

Sevoflurane improves electrophysiological recovery of rat hippocampal slice CA1 pyramidal neurons after hypoxia.

Sevoflurane is a volatile anesthetic agent that reduces cerebral metabolism and thereby may reduce neuronal damage during energy deprivation. We have examined the effect of sevoflurane on hypoxic neuronal damage in rat hippocampal slices. Slices were subjected to 0%, 2%, or 4% sevoflurane 10 minutes before, during, and 10 minutes after hypoxia. The Schaffer collateral pathway was stimulated every 10 seconds and the evoked population spike recorded in the CA1 pyramidal cell region throughout the experiment. During hypoxia, the postsynaptic evoked response was blocked. The time until the blockade of this response in the 0% sevoflurane group was 158 seconds. Sevoflurane (4%) significantly delayed the loss of the evoked response during hypoxia (242 seconds). The percent recovery of the postsynaptic population spike was calculated by dividing the size of the response 120 minutes after hypoxia by its prehypoxic, presevoflurane amplitude. There was no recovery of the population spike in the 0% sevoflurane group 120 minutes after the end of 5 minutes of hypoxia (6 +/- 6%); there was significantly better recovery after 5 minutes of hypoxia in the sevoflurane (4%) treated group (40 +/- 9%). A lower concentration of sevoflurane (2%) delayed the loss of evoked response during hypoxia (191 seconds), but it did not significantly affect recovery of the population spike after hypoxia (7 +/- 7%). Hypoxia irreversibly damages electrophysiologic activity. A high, but clinically usable, concentration of sevoflurane increases the time during hypoxia until the postsynaptic evoked response is blocked and improves recovery of this response after 5 minutes of hypoxia.

Protective effect of high glucose against ischemia-induced synaptic transmission damage in rat hippocampal slices.

Cerebral ischemic damage is an important cause of morbidity and mortality. However, there is conflicting evidence regarding the effect of the extracellular glucose concentration in focal and global ischemic injury. This study was designed to investigate this effect in ischemia-induced synaptic transmission damage in rat hippocampal slices. Slices were superfused with artificial cerebrospinal fluid (ACSF) containing various concentrations of glucose before and after ischemia. The evoked somatic postsynaptic population spike (PS) and dendritic field excitatory postsynaptic potential (fEPSP) were extracellularly recorded in the CA1 stratum pyramidal cell layer and s. radiatum after stimulation of the Schaeffer collaterals, respectively. The glucose concentration in ACSF before and after ischemia determined the duration of ischemia tolerated by synaptic transmission as demonstrated by complete recovery of the somatic PS and dendritic fEPSP. Specifically, the somatic PS and dendritic fEPSP completely recovered following 3, 4, and 5 min of ischemia only when slices were superfused with ACSF containing 4, 10, and 20 mM glucose before and after ischemia, respectively. The latencies of the somatic and dendritic ischemic depolarization (ID) occurrence in the CA1 s. pyramidal cell layer and s. radiatum were significantly longer with 10 than 4 mM glucose in ACSF before ischemia and significantly longer with 20 than 10 mM glucose in ACSF before ischemia. Regardless of the glucose concentration in ACSF before and after ischemia, the somatic PS and dendritic fEPSP only partially recovered when ischemia was terminated at the occurrence of ID. These results indicate that high glucose in ACSF during the period before and after ischemia significantly protects CA1 synaptic transmission against in vitro ischemia-induced damage through postponing the occurrence of ID.

Synaptic adaptation to repeated hypoglycemia depends on the utilization of monocarboxylates in Guinea pig hippocampal slices.

This report provides in vitro evidence that synaptic activity becomes resistant to repeated hypoglycemia, i.e., hypoglycemic synaptic adaptation occurs. Synaptic function was estimated by the amplitude of the postsynaptic population spike (PS) recorded in the granule cell layer of guinea pig hippocampal slices. ATP, phosphocreatine (PCr), glycogen, and glucose concentrations were measured to investigate energy metabolism homeostasis. Glucose deprivation produced a complete elimination of the PS amplitude, with a 50% inhibition by 10.6 min, and a approximately 15% reduction in ATP and PCr concentrations. Low-glucose (0.5-1 mmol/l) medium gradually depressed the PS. After recovery from glucose depletion, repeated glucose deprivation produced a slowly developing depression of PS, with a 50% inhibition by 36.5 min. However, ATP and PCr concentrations were maintained. Incubation in secondary low-glucose medium maintained PS amplitude. Hippocampal glycogen and glucose concentrations promptly decreased during repeated glucose deprivation, indicating that glycogenolysis does not fuel synaptic adaptation to repeated hypoglycemia. Synaptic function during repeated glucose depletion was reversibly depressed by addition of alpha-cyano-4-hydroxycinnamic acid or 3-isobutyl-1-methylxanthine, inhibitors of the monocarboxylate transporter. Replacement of extracellular glucose with Na-lactate or Na-pyruvate sustained synaptic transmission after transient glucose depletion. These results indicate that synaptic utilization of monocarboxylates sustains hypoglycemic synaptic adaptation.

Effect of lactate on the synaptic potential, energy metabolism, calcium homeostasis and extracellular glutamate concentration in the dentate gyrus of the hippocampus from guinea-pig.

Towards understanding the role of glycolysis on synaptic function, we examined the effect of lactate on synaptic potential, energy metabolism, Ca(2+) homeostasis and extracellular glutamate in the dentate gyrus of guinea-pig hippocampus. Postsynaptic population spikes were recorded from the granule cell layer of the dentate gyrus in guinea-pig hippocampal slices after replacing glucose with lactate in the perfusion medium. Population spikes were not maintained and spontaneously recovered around 35min after the replacement of glucose with lactate. However, ATP levels of the dentate gyrus remained unchanged while those during the glucose-free condition decreased to 73% of the initial levels at 60min. Intracellular Ca(2+) was measured with the calcium indicator dye fura-2 AM, and the population spike was recorded simultaneously. Ca(2+) levels increased concomitantly with the early decay of synaptic potentials, and recovered partially with the spontaneous recovery of synaptic potentials. The time course of decay of population spikes and the increase of Ca(2+) levels during lactate replacement were similar to those during glucose deprivation. Increase in Ca(2+) levels during lactate replacement was completely blocked by the ryanodine receptor/calcium release channel antagonist dantrolene. Glutamate was released more significantly in the medium during lactate replacement than with normal Ringer solution, and less than that during glucose deprivation. Addition of the N-methyl-D-aspartate blocker, D-(-)-2-amino-5-phosphonovaleric acid, and the L-type calcium channel blocker, nimodipine, but not dantrolene blocked spontaneous recovery of population spikes. The results indicate that lactate can maintain energy levels in hippocampal slices, but cannot maintain ion homeostasis in granule cells of the dentate gyrus. Glycolysis plays an important role in maintaining ion homeostasis, and activation of N-methyl-D-aspartate and L-type calcium channels is necessary for support of synaptic function by lactate utilization.

Effects of propofol on hippocampal synaptic transmission in behaving rats.

The action of propofol has been studied in vitro and in vivo, but the effects of intravenously administered propofol on synaptic transmission in freely behaving rats have not been studied before. Rats were implanted with recording electrodes in the dentate gyrus and with stimulation electrodes in the medial perforant path (MPP). Paired pulses at different interpulse intervals (IPIs) were delivered to the MPP, and average evoked potentials were recorded in the dentate gyrus before and after a bolus of propofol (10 or 20 mg/kg administered intravenously) or control vehicle was injected via femoral vein cannula. Because of the layered structure of the hippocampus, population excitatory postsynaptic potentials and population spikes could be distinguished and analyzed. Propofol has no significant effect on the population excitatory postsynaptic potentials or population spike evoked by a single MPP stimulus pulse. However, paired-pulse inhibition of the dentate population spikes was increased at IPI of 20 and 30 ms. Paired-pulse inhibition of the population spike was most prominent when tail pinch response was lost (deep and moderate anesthesia), but it persisted during light anesthesia. At 200 ms IPI, paired-pulse facilitation of population spikes was observed during moderate anesthesia in most rats. In freely behaving rats, intravenous propofol enhanced paired-pulse inhibition at < 50 ms IPI, likely by enhancing gamma-aminobutyric acid A receptor-mediated inhibition. Propofol also increased paired-pulse facilitation at 200 ms IPI through an unknown mechanism, which may contribute to the neuroexcitatory effect of propofol.

Effect of small changes in temperature on CA1 pyramidal cells from rat hippocampal slices during hypoxia: implications about the mechanism of hypothermic protection against neuronal damage

Small reductions in temperature have been shown to improve neurologic recovery after ischemia. We have examined the effect of temperature on biochemical and physiological changes during hypoxia using rat hippocampal slices as a model system. The postsynaptic population spike recorded from the CA1 pyramidal cell region of slices subjected to 7 min of hypoxia with hypothermia (34°C) recovered to 73% of its prehypoxic level; slices subjected to the same period of hypoxia at 37°C did not recover. After 7 min of hypoxia ATP fell to 48% of its prehypoxic concentration at 34°C and 30% at 37°C. Potassium fell to 86% during 7 min of hypoxia with hypothermia, this compares to a fall to 58% at 37°C. The increase in sodium after 7 min of hypoxia was also attenuated by hypothermia (133% vs. 163% of its prehypoxic concentration). When the hypoxic period was shortened to 3 min (37°C) the population spike recovered to 94%. If the temperature was increased to 40°C there was only 7% recovery of the population spike after 3 min of hypoxia. With hyperthermia (40°C), ATP fell to 33% after 3 min of hypoxia, this compares to 81% at normothermia. Potassium fell to 76% after 3 min of hypoxia with hyperthermia, this compares to 91% at 37°C. Sodium concentrations increased with hyperthermia before hypoxia, at 3 min of hypoxia there was no significant difference between the hyperthermic and normothermic tissue; there was a large increase in sodium with hyperthermia after 5 min of hypoxia (209% vs. 146%). We conclude that the improved recovery after hypothermic hypoxia is at least in part due to the attenuated changes in ATP, potassium and sodium during hypoxia and that the worsened recovery with hyperthermia is due to an exacerbation of the change in ATP, potassium and sodium concentrations during hypoxia.

Frequency- and structure-dependent inhibition of normal and epileptiform activity by 6-benzoyldeltamine in rat hippocampal slices

The present study investigated the effects of the Aconitum alkaloids 6-benzoyldeltamine and the structurally related eldeline on neuronal activity in rat hippocampal slices. 6-Benzoyldeltamine (1–30 μM) decreased the orthodromic field potentials recorded in area CA1 in a concentration-dependent manner. The inhibitory effect of eldeline (3-100 μM) was lower. The attenuation of the postsynaptic population spike was accompanied by a simultaneous decrease in the presynaptic fibre spike evoked by electrical stimulation of the Schaffer collaterals. The input-output relationship of the presynaptic fibre spike as function of the stimulation intensity, and for the postsynaptic population spike as function of the presynaptic fibre spike was shifted to the right. Thus, electrophysiologically, these alkaloids seem to inhibit predominantly the excitability of the afferent fibres and, in consequence, neurotransmission between Schaffer collaterals and the CA1 neurons, thereby suppressing the firing of the latter. The inhibitory action of 6-benzoyldeltamine revealed use-dependence as obvious by an enhanced attenuation of the antidromic spike when stimulation frequency was increased. 6-Benzoyldeltamine inhibited stimulus-triggered epileptiform population bursts in area CA1 elicited by omission of Mg 2+, as well as spontaneously occurring epileptiform discharges in area CA3 elicited by omission of Mg 2+ and elevation of K +. Complete suppression of spontaneous activity was observed at 1 μM 6-benzoyldeltamine, which reduced the population spike only by about 20% of control. It is concluded that the inhibitory and antiepileptiform effect of 6-benzoyldeltamine is mediated by a frequency-dependent decrease in excitability, which might be important for filtering high frequency bursts of action potentials characteristic for epileptiform activity in the hippocampus.

Effects of the Aconitum alkaloid songorine on synaptic transmission and paired-pulse facilitation of CA1 pyramidal cells in rat hippocampal slices.

1. The present study investigated the electrophysiological effects of songorine (1 100 microM), an alkaloid occurring in plants of the Aconitum genus, in rat hippocampal slices. 2. Songorine (10-100 microM) evoked a concentration-dependent increase in the amplitude of the orthodromic population spike and in the slope of the field e.p.s.p. The enhancement was long-lasting and was not reversed by up to 90 min of washout. Songorine failed to affect size and shape of the presynaptic fiber spike which represents the compound action potential of the Schaffer collaterals. This indicates that enhancement of the synaptic response is no consequence of an increased afferent excitability. 3. The antidromically evoked population spike was not affected by songorine at concentrations up to 100 microM suggesting that the enhancement of the orthodromic population spike and of the field e.p.s.p. was not due to an increase in pyramidal cell excitability. 4 The input-output curve for the postsynaptic population spike was shifted to the left implying that a presynaptic fiber spike of the same size elicited a larger postsynaptic response, indicating a decrease in threshold for generation of the population spike. 5. The songorine-evoked increase in excitability was not affected by the NMDA receptor antagonist, D-AP5. However, the effect of songorine was completely abolished by the selective dopamine D2 receptor antagonist sulpiride (0.1 microM) as well as by haloperidol (10 microM) and was mimicked by application of the dopamine releaser, amantadine (100 mM). In contrast, the selective D1 receptor antagonist, SCH23390, did not block the action of songorine. 6. The results indicate that the plant alkaloid songorine enhances excitatory synaptic transmission which may be due to an agonistic action at D2 receptors.

Effects of mesaconitine on [3H]noradrenaline uptake and neuronal excitability in rat hippocampus.

Mesaconitine, one of the main alkaloids contained in Aconiti tubers, is a centrally acting analgesic without affinity to opioid receptors. It has been reported that the antinociception is due to an interaction with the noradrenergic system. In the present study, the effect of mesaconitine on the uptake of noradrenaline and on neuronal activity was examined in rat hippocampus. Experiments were performed as a study of [3H]noradrenaline uptake into rat hippocampal synaptosomes. Mesoconitine inhibited [3H]noradrenaline uptake in a concentration-dependent manner with a Ki of 111.95+/-18 nM. In a further series of experiments, the effects of mesaconitine on the extracellularly recorded population spike were investigated in rat hippocampal slices. At a concentration of 10 nM, mesaconitine increased the amplitude of the postsynaptic population spike by 31.10%+/-6.7% of control and elicited one or two additional spikes. The presynaptic fiber spike and the field excitatory postsynaptic potential were not affected by this alkaloid. The enhancement of neuronal activity was abolished by 1 microM propranolol as well as by 1 microM timolol. It is concluded that mesoconitine increased the excitability in rat hippocampal pyramidal cells by an involvement of the noradrenergic system, with at least one mechanism being inhibition of noradrenaline uptake leading to an enhanced extraneuronal noradrenaline level.

The effect of fentanyl on electrophysiologic recovery of CA 1 pyramidal cells from anoxia in the rat hippocampal slice.

Fentanyl is widely used in conditions in which the brain is at risk of ischemic or anoxic injury. We evaluated the effect of fentanyl on anoxic injury to CA 1 pyramidal cells in the rat hippocampus. These neurons are extremely sensitive to anoxic injury and are densely populated with opioid receptors. We prepared hippocampal slices from adult Sprague-Dawley rats and evoked a postsynaptic population spike in the CA 1 pyramidal cell region by stimulating the Schaffer collateral pathway. The amplitude of this response was used to evaluate the effect of fentanyl on anoxic injury. Pretreatment with fentanyl (50 or 500 ng/mL) did not alter the amplitude of the CA 1 population spike before anoxia, nor did it alter the recovery of this response after 5,6, or 7 min of anoxia. After 5 min of anoxia, the population spike recovered to 76% of its preanoxic level in the control group and to 87% in the group treated with 500 ng/mL of fentanyl. After 6 min of anoxia, recovery was 45% in the control group, 57% in the group treated with 50 ng/mL of fentanyl, and 58% in the group treated with 500 ng/mL of fentanyl. After 7 min of anoxia, recovery was 5% in the control group and 4% in the group treated with 50 ng/mL of fentanyl. We conclude that fentanyl does not affect the recovery of the electrophysiological response in rat hippocampal neurons subjected to an anoxic insult. Because fentanyl is used in large doses during surgical procedures in which the brain is at increased risk of ischemic or anoxic injury, it is important to determine its effect on such injury. Using the rat hippocampal slice model, we found fentanyl to be neither neurotoxic nor protective against anoxic injury to neurons when used in concentrations comparable to those produced in clinical practice.

The effect of isoflurane on biochemical changes during and electrophysiological recovery after anoxia in rat hippocampal slices.

It is unclear whether isoflurane protects against neuronal damage. This study examines the extent and mechanism by which isoflurane might affect anoxic neuronal damage. The size of the evoked postsynaptic population spike recorded from the CA 1 pyramidal cell layer of the rat hippocampal slice 60 min after anoxia was compared with its preanoxic, preisoflurane level. Intracellular adenosine triphosphate (ATP), sodium, and potassium levels were measured in the dentate and CA 1 regions at the end of the anoxic period in similarly treated slices. Isoflurane increased the latency and reduced the amplitude of the evoked response before anoxia. Isoflurane (2%) did not significantly improve recovery of the evoked response after 5 min of anoxia (untreated slices = 6 +/- 2% (mean +/- SEM), isoflurane = 17 +/- 7%); 1.5% isoflurane also did not significantly improve recovery after 4 min of anoxia (untreated = 30 +/- 8% vs. 1.5% isoflurane = 47 +/- 12%). Isoflurane did not significantly attenuate the decrease in ATP levels in either the dentate or CA 1 regions of the hippocampal slice during 4 or 7 min of anoxia; however, there was a significant improvement in ATP levels after 10 min of anoxia in both regions of isoflurane-treated preparations (1.0 +/- 0.1 vs. 1.4 +/- 0.1, CA 1; 1.3 +/- 0.1 vs. 2.0 +/- 0.2 nM/mg dry weight, dentate). Sodium concentrations increased and potassium concentrations decreased during anoxia. Isoflurane did not significantly attenuate the changes in these ions during anoxia. In conclusion, isoflurane does not significantly improve recovery of CA 1 pyramidal cells during anoxia nor does it attenuate the anoxic changes in ATP, sodium, and potassium after 4 or 7 min of anoxia. With a more prolonged period of anoxia (10 min) isoflurane reduces the decrease in ATP levels.

Age-dependence of effects of A1 adenosine receptor antagonism in rat hippocampal slices.

1. Population excitatory postsynaptic potentials (EPSPs) and population spikes evoked in area CA1 of hippocampal slices from aged Fischer 344 rats were significantly smaller in amplitude than responses obtained in slices from young Fischer 344 rats. 2. The A1 adenosine receptor antagonist 8-cyclopentyltheophylline (8-CPT) produced a concentration-dependent increase in synaptic potentials in slices from both young and aged rats. Low concentrations (1 nM) of 8-CPT were effective in producing increases in both population spike amplitudes and population EPSP slopes in young and aged rat slices. Response increases were maximized by 100 nM 8-CPT in slices from rats of both age groups. 3. Adenosine antagonism produced greater average increases in synaptic responses in hippocampal slices from aged rats at all concentrations tested (1.0 nM-1.0 microM). A qualitative age-related difference in the response to 8-CPT was also observed; 8-CPT produced a late component, consisting of multiple population spikes, in evoked responses in slices obtained from aged but not young rats. 4. Adenosine antagonism significantly increased the maximum evocable response (both spike amplitude and EPSP slope) in slices from aged rats, relative to increases observed in slices from young rats. This suggested that smaller synaptic potentials seen in slices from aged rats were in part due to greater levels of "tonic" adenosinergic inhibition. 5. Slices from young and aged rats were incubated in the adenosine reuptake inhibitor soluflazine (R64719; 1.0, 10, and 100 microM) and the inhibition of population EPSPs was observed for 60 min. No difference was observed in the rate of inhibition or the maximal level of inhibition produced by soluflazine, in slices from rats of either age group. 6. Application of (+)-5-methyl-10,11-dihydro-5H-dibenzo-[a,d]cyclo-hepten- 5,10-imine hydrogen maleate (MK-801) and 2-amino-5-phosphonopentanoic acid (2-AP5), antagonists of N-methyl-D-aspartate (NMDA) excitatory amino acid (EAA) receptors, reduced the late multiple population spike component in slices from aged rats incubated in 8-CPT. A smaller direct effect of the NMDA antagonists was observed in slices from aged rats in the absence of 8-CPT treatment at maximal response levels. No effect of NMDA receptor antagonism was observed in slices from young rats under either condition. 7. Hippocampal tissue, from young and old rats utilized in the electrophysiological experiments, was assayed for A1 adenosine binding site density with a saturating concentration of radiolabeled agonist and antagonist. Guanine nucleotide modulation of agonist binding was also measured.(ABSTRACT TRUNCATED AT 400 WORDS)

The barbiturate thiopental reduces ATP levels during anoxia but improves electrophysiological recovery and ionic homeostasis in the rat hippocampal slice

The barbiturate anesthetic thiopental enhances recovery of the evoked population spike recorded from rat hippocampal slices after short periods of anoxia. Thiopental reduces changes in sodium, potassium and calcium but enhances the fall in ATP levels during anoxia. The postsynaptic population spike recorded from the CA1 pyramidal cell region of the slices treated with thiopental (600 μM) recovered to 67% of the preanoxic amplitude after 3.5 min of anoxia. There was less recovery (24%) when a lower concentration of thiopental (250 μM) was used. Untreated slices recovered to only 10% of their preanoxic amplitude after 3.5 min of anoxia. Other studies have demonstrated that maintaining ATP levels during anoxia may be an important mechanism of protection. In contrast to those studies, thiopental was protective although it enhanced the fall of ATP levels after 3.5 min of anoxia in the CA1 region and after 3.5 and 5 min in the dentate region. Thus enhanced recovery of the population spike with thiopental is not due to its preservation of ATP levels. This result allows a clear separation of improved ATP levels during anoxia from other mechanisms of protection. We therefore looked for other mechanisms of protection. Sodium and potassium levels were measured after 10 min of anoxia. In untreated tissue, sodium levels in the slice rose and potassium levels fell significantly. In thiopental-treated tissue, changes in sodium and potassium caused by anoxia and by veratridine under normoxic conditions were significantly reduced. During anoxia calcium-45 uptake increases; thiopental significantly reduces this uptake. We have found that high concentrations of thiopental protect against short periods of anoxia. Thiopental does not protect by attenuating the fall in ATP during anoxia. We conclude that thiopental blocks sodium, potassium and calcium changes during anoxia and we postulate that this may be a mechanism by which it improves recovery after anoxia.

Methods for studying the effect of anesthetics on anoxic damage in the rat hippocampal slice

The rat hippocampal slice was used as a model system to study the effects of anesthetics on anoxic damage. Thiopental, but not isoflurane, allowed recovery of the postsynaptic population spike evoked from the dentate granule cells. Creatine preincubation protected both dentate granule and CA1 pyramidal cells against anoxic damage. Calcium-free, 10 mM magnesium artificial cerebrospinal fluid (ACSF) was shown to protect CA1 pyramidal cells against anoxic damage. The presynaptic population spike recovered to its preanoxic amplitude after anoxia even under conditions where the postsynaptic population spike demonstrated no recovery. Thus the hippocampal slice is a useful system for studying the effects of anesthetics and other pharmacological agents on anoxic damage.