In vivo intracellular demonstration of an ischemia-induced postsynaptic potential from CA1 pyramidal neurons in rat hippocampus

Resveratrol Preconditioning Protects Against Ischemia-Induced Synaptic Dysfunction and Cofilin Hyperactivation in the Mouse Hippocampal Slice

Perturbations in synaptic function are major determinants of several neurological diseases and have been associated with cognitive impairments after cerebral ischemia (CI). Although the mechanisms underlying CI-induced synaptic dysfunction have not been well defined, evidence suggests that early hyperactivation of the actin-binding protein, cofilin, plays a role. Given that synaptic impairments manifest shortly after CI, prophylactic strategies may offer a better approach to prevent/mitigate synaptic damage following an ischemic event. Our laboratory has previously demonstrated that resveratrol preconditioning (RPC) promotes cerebral ischemic tolerance, with many groups highlighting beneficial effects of resveratrol treatment on synaptic and cognitive function in other neurological conditions. Herein, we hypothesized that RPC would mitigate hippocampal synaptic dysfunction and pathological cofilin hyperactivation in an ex vivo model of ischemia. Various electrophysiological parameters and synaptic-related protein expression changes were measured under normal and ischemic conditions utilizing acute hippocampal slices derived from adult male mice treated with resveratrol (10 mg/kg) or vehicle 48 h prior. Remarkably, RPC significantly increased the latency to anoxic depolarization, decreased cytosolic calcium accumulation, prevented aberrant increases in synaptic transmission, and rescued deficits in long-term potentiation following ischemia. Additionally, RPC upregulated the expression of the activity-regulated cytoskeleton associated protein, Arc, which was partially required for RPC-mediated attenuation of cofilin hyperactivation. Taken together, these findings support a role for RPC in mitigating CI-induced excitotoxicity, synaptic dysfunction, and pathological over-activation of cofilin. Our study provides further insight into mechanisms underlying RPC-mediated neuroprotection against CI and implicates RPC as a promising strategy to preserve synaptic function after ischemia.

The effects of epothilone D on microtubule degradation and delayed neuronal death in the hippocampus following transient global ischemia

Disruption of microtubule cytoskeleton plays an important role during the evolution of brain damage after transient cerebral ischemia. However, it is still unclear whether microtubule-stabilizing drugs such as epothilone D (EpoD) have a neuroprotective action against the ischemia-induced brain injury. This study examined the effects of pre- and postischemic treatment with different doses of EpoD on the microtubule damage and the delayed neuronal death in the hippocampal CA1 subfield on day 2 following reperfusion after 13-min global cerebral ischemia. Our results showed that systemic treatment with 0.5 mg/kg EpoD only slightly alleviated the microtubule disruption and the CA1 neuronal death, while treatment with 3.0 mg/kg EpoD was not only ineffective against the CA1 neuronal death, but also produced additional damage in the dentate gyrus in some ischemic rats. Since the pyramidal cells in the CA1 subfield and the granule neurons in the dentate gyrus are known to be equipped with dynamically different microtubule systems, this finding indicates that the effects of microtubule-disrupting drugs may be unpredictably complicated in the central nervous system.

Modulation of 4-aminopyridine-induced neuronal activity and local pO(2)in rat hippocampal slices by changing the flow rate of the superfusion medium

The brain slice preparation is the most frequently used tool for testing of pharmacological agents on the neuronal excitability. However in the absence of blood circulation in vitro, the tissue oxygenation strongly depends on the experimental conditions. It is well established that both hypoxia as well as hyperoxia can modulate the neuronal network activity. Thereby changes in tissue oxygen level during experiment may affect the final result. In the present study we investigated the effect of oxygenation on seizure susceptibility in the hippocampal slice preparation using 4-aminopyridine (4-AP) model of ictogenesis in inmature rats. We found that changing the medium perfusion rate in the range of 1-5 ml/min greatly affects the tissue oxygenation, amplitude and frequency of 4-AP-induced synchronous neuronal activity. The decrease in the flow rate as well as substitution of the oxygen in the extracellular medium with nitrogen causes a strong reduction of 4-AP-induced synchronous neuronal discharges. Our results demonstrate a significant linear correlation between the power of 4-AP-induced neuronal activity and the oxygen level in slice tissue. Also we demonstrated that the presence of medium flow is a necessary condition to support the constant level of the slice oxygenation. These data suggest that the oxygen supply of the brain slice strongly depends on experimental protocol and could modulate in vitro neuronal network excitability which should be taken into consideration when planning epilepsy-related studies.

Analysis of spatial and temporal protein expression in the cerebral cortex after ischemia-reperfusion injury

Background and Purpose

Hypoxia, or ischemia, is a common cause of neurological deficits in the elderly. This study elucidated the mechanisms underlying ischemia-induced brain injury that results in neurological sequelae.

Methods

Cerebral ischemia was induced in male Sprague-Dawley rats by transient ligation of the left carotid artery followed by 60 min of hypoxia. A two-dimensional differential proteome analysis was performed using matrix-assisted laser desorption ionization-time-of-flight mass spectrometry to compare changes in protein expression on the lesioned side of the cortex relative to that on the contralateral side at 0, 6, and 24 h after ischemia.

Results

The expressions of the following five proteins were up-regulated in the ipsilateral cortex at 24 h after ischemia-reperfusion injury compared to the contralateral (i.e., control) side: aconitase 2, neurotensin-related peptide, hypothetical protein XP-212759, 60-kDa heat-shock protein, and aldolase A. The expression of one protein, dynamin-1, was up-regulated only at the 6-h time point. The level of 78-kDa glucose-regulated protein precursor on the lesioned side of the cerebral cortex was found to be high initially, but then down-regulated by 24 h after the induction of ischemia-reperfusion injury. The expressions of several metabolic enzymes and translational factors were also perturbed soon after brain ischemia.

Conclusions

These findings provide insights into the mechanisms underlying the neurodegenerative events that occur following cerebral ischemia.

Hyperbaric hyperoxia and normobaric reoxygenation increase excitability and activate oxygen-induced potentiation in CA1 hippocampal neurons

Breathing hyperbaric oxygen (HBO) is common practice in hyperbaric and diving medicine. The benefits of breathing HBO, however, are limited by the risk of central nervous system O2 toxicity, which presents as seizures. We tested the hypothesis that excitability increases in CA1 neurons of the rat hippocampal slice (400 microm) over a continuum of hyperoxia that spans normobaric and hyperbaric pressures. Amplitude changes of the orthodromic population spike were used to assess neuronal O2 sensitivity before, during, and following exposure to 0, 0.6, 0.95 (control), 2.84, and 4.54 atmospheres absolute (ATA) O2. Polarographic O2 electrodes were used to measure tissue slice PO2 (PtO2). In 0.95 ATA O2, core PtO2 at 200 microm deep was 115±16 Torr (mean±SE). Increasing O2 to 2.84 and 4.54 ATA increased core PtO2 to 1,222±77 and 2,037±157 Torr, respectively. HBO increased the orthodromic population spike amplitude and usually induced hyperexcitability (i.e., secondary population spikes) and, in addition, a long-lasting potentiation of the orthodromic population spike that we have termed "oxygen-induced potentiation" (OxIP). Exposure to 0.60 ATA O2 and hypoxia (0.00 ATA) decreased core PtO2 to 84±6 and 20±4 Torr, respectively, and abolished the orthodromic response. Reoxygenation from 0.0 or 0.6 ATA O2, however, usually produced a response similar to that of HBO: hyperexcitability and activation of OxIP. We conclude that CA1 neurons exhibit increased excitability and neural plasticity over a broad range of PtO2, which can be activated by a single, hyperoxic stimulus. We postulate that transient acute hyperoxia stimulus, whether caused by breathing HBO or reoxygenation following hypoxia (e.g., disordered breathing), is a powerful stimulant for orthodromic activity and neural plasticity in the CA1 hippocampus.

Dendritic plasticity of CA1 pyramidal neurons after transient global ischemia

Dendrites and spines undergo dynamic changes in physiological conditions, such as learning and memory, and in pathological conditions, such as Alzheimer's disease and epilepsy. Long-term dendritic plasticity has also been reported after ischemia/hypoxia, which might be compensatory effects of surviving neurons for the functional recovery after the insults. However, the dendritic changes shortly after ischemia, which might be associated with the pathogenesis of ischemic cell death, remain largely unknown. To reveal the morphological changes of ischemia-vulnerable neurons after ischemia, the present study investigated the alteration of dendritic arborization of CA1 pyramidal neurons in rats after transient cerebral ischemia using intracellular staining technique in vivo. The general appearance of dendritic arborization of CA1 neurons within 48 h after ischemia was similar to that of control neurons. However, a dramatic increase of dendritic disorientation was observed after ischemia with many basal dendrites coursed into the territory of apical dendrites and apical dendrites branched into the region of basal dendrites. In addition, a significant increase of apical dendritic length was found 24 h after ischemia. The increase of dendritic length after ischemia was mainly due to the dendritic sprouting rather than the extension of individual dendrites, which mainly occurred in the middle segment of the apical dendrites. These results reveal a plasticity change in dendritic arborization of CA1 neurons shortly after cerebral ischemia.

Enhancement of Excitatory Synaptic Transmission in Spiny Neurons After Transient Forebrain Ischemia

Spiny neurons in the neostriatum are highly vulnerable to ischemia. Enhancement of excitatory synaptic transmissions has been implicated in ischemia-induced excitotoxic neuronal death. Here we report that evoked excitatory postsynaptic currents in spiny neurons were potentiated after transient forebrain ischemia. The ischemia-induced potentiation in synaptic efficacy was associated with an enhancement of presynaptic release as demonstrated by an increase in the frequency of miniature excitatory postsynaptic currents (mEPSCs) and a decrease in the paired-pulse ratio. The amplitude of inward currents evoked by exogenous application of glutamate did not show significant changes after ischemia, suggesting that postsynaptic mechanism is not involved. The ischemia-induced increase in mEPSCs frequency was not affected by blockade of voltage-gated calcium channels, but it was eliminated in the absence of extracellular calcium. Bath application of ATP P2X receptor antagonist pyridoxal-phosphate-6-azophenyl-2′,4′-disulfonic acid (PPADS) significantly reduced mEPSC frequency in ischemic neurons but had no effects on the control ones. Furthermore, the inhibitory effect of PPADS on ischemic neurons was abolished in Ca2+-free external solution. These results indicate that excitatory synaptic transmissions in spiny neurons are potentiated after ischemia via presynaptic mechanisms. Activation of P2X receptors and the consequent Ca2+ influx might contribute to the ischemia-induced facilitation of glutamate release.

Morphological heterogeneity of CA1 pyramidal neurons in response to ischemia

We have found, based on the electrophysiological properties, two subtypes of CA1 pyramidal neurons in the CA1 region of the normal hippocampus, late postsynaptic potential (L-PSP) neurons and non-L-PSP neurons. In addition, our previous study has shown that the electrophysiological properties of these two subtypes of pyramidal neurons were differentially modified after ischemia. In the present study, we hypothesized that ischemia might also induce different morphological alterations in these two subtypes of neuron. To test the hypothesis, we compared the changes in the dendritic arborization and soma volume of these two subtypes of neurons in rats subjected to transient global ischemia. We found a significant decrease in the basal dendritic length of L-PSP neurons at 12 hr after reperfusion, resulting mainly from a significant decrease in the dendrite terminal length. The apical dendritic length of L-PSP neurons markedly increased at 24 hr after ischemia, resulting mainly from an increase in the number of branching arbors in the middle part of the apical dendritic trees. The soma size of L-PSP neurons was significantly reduced at 12 hr, but they became slightly larger at 24 hr and 48 hr after reperfusion. In contrast to L-PSP neurons, non-L-PSP neurons showed slight modifications in the dendritic arborization but had persistent swelling of their soma after ischemia. These results indicate that pathological changes in these two subtypes of neurons are different after ischemia.

Depressed responses to applied and synaptically-released GABA in CA1 pyramidal cells, but not in CA1 interneurons, after transient forebrain ischemia

Transient cerebral ischemia kills CA1 pyramidal cells of the hippocampus, whereas most CA1 interneurons survive. It has been proposed that calcium-binding proteins, neurotrophins, and/or inhibitory neuropeptides protect interneurons from ischemia. However, different synaptic responses early after reperfusion could also underlie the relative vulnerabilities to ischemia of pyramidal cells and interneurons. In this study, we used gramicidin perforated patch recording in ex vivo slices to investigate gamma-aminobutyric acid (GABA) synaptic function in CA1 pyramidal cells and interneurons 4 h after a bilateral carotid occlusion accompanied by hypovolemic hypotension. At this survival time, the amplitudes of both miniature inhibitory postsynaptic currents (mIPSCs) and GABA-evoked currents were reduced in CA1 pyramidal cells, but not in CA1 interneurons. In addition, the mean rise time of mIPSCs was reduced in pyramidal cells. The reversal potential for the GABA current (E(GABA)) did not shift toward depolarizing values in either cell type, indicating that the driving force for chloride was unchanged at this survival time. We conclude that early during reperfusion GABAergic neurotransmission is attenuated exclusively in pyramidal neurons. This is likely explained by reduced GABAA receptor sensitivity or clustering and possibly also reduced GABA release, rather than by an elevation of intracellular chloride. Impaired GABA function may contribute to ischemic neuronal death by enhancing the excitability of CA1 pyramidal cells and facilitating N-methyl-D-aspartic acid channel opening. Therefore, normalizing GABAergic function might be a useful pharmacological approach to counter excessive, and potentially excitotoxic, glutamatergic activity during the postischemic period.

Synaptic plasticity in the ischaemic brain

Activity-dependent long-term potentiation (LTP) of excitatory neurotransmission underlies specific forms of associative learning and memory. A brief period of energy deprivation induces LTP in specific subsets of neurons; this synaptic plasticity might contribute to the delayed effects of brain ischaemia. In this review, we discuss the similarities and differences between LTP induced by energy deprivation and "physiological" LTP. On the basis of recent studies, we propose that pathological plasticity induced by energy deprivation can play a part in delayed neuronal death in the hippocampus and the striatum after global ischaemia and in the conversion of ischaemic penumbra to infarct core after focal ischaemia. We discuss evidence that ischaemia could also induce protective and reparative forms of neuronal plasticity that may play a part in ischaemic tolerance and poststroke recovery.

Depression of fast excitatory synaptic transmission in large aspiny neurons of the neostriatum after transient forebrain ischemia

Spiny neurons in the neostriatum die within 24 hr after transient global ischemia, whereas large aspiny (LA) neurons remain intact. To reveal the mechanisms of such selective cell death after ischemia, excitatory neurotransmission was studied in LA neurons before and after ischemia. The intrastriatally evoked fast EPSCs in LA neurons were depressed < or =24 hr after ischemia. The concentration-response curves generated by application of exogenous glutamate in these neurons were approximately the same before and after ischemia. A train of five stimuli (100 Hz) induced progressively smaller EPSCs, but the proportion of decrease in EPSC amplitude at 4 hr after ischemia was significantly smaller compared with control and at 24 hr after ischemia. Parallel depression of NMDA receptor and AMPA receptor-mediated EPSCs was also observed after ischemia, supporting the involvement of presynaptic mechanisms. The adenosine A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine blocked the inhibition of evoked EPSCs at 4 hr after ischemia but not at 24 hr after ischemia. Electron microscopic studies demonstrated that the most presynaptic terminals in the striatum had a normal appearance at 4 hr after ischemia but showed degenerating signs at 24 hr after ischemia. These results indicated that the excitatory neurotransmission in LA neurons was depressed after ischemia via presynaptic mechanisms. The depression of EPSCs shortly after ischemia might be attributable to the enhanced adenosine A1 receptor function on synaptic transmission, and the depression at late time points might result from the degeneration of presynaptic terminals.

Asymmetrical changes of excitatory synaptic transmission in dopamine-denervated striatum after transient forebrain ischemia

Spiny neurons in the neostriatum are highly vulnerable to cerebral ischemia. Recent studies have shown that the postischemic cell death in the right striatum was reduced after ipsilateral dopamine denervation whereas no protection was observed in the left striatum after dopamine denervation in the left side. In order to reveal the mechanisms of such asymmetrical protection, electrophysiological changes of dopamine-denervated striatal neurons were compared after ischemia between the left and right striatum using intracellular recording and staining techniques in vivo. No difference in cortically evoked initial excitatory postsynaptic potentials was found between the left and right striatum in intact animals after ipsilateral dopamine denervation. The initial excitatory postsynaptic potentials in the dopamine-denervated right striatum were suppressed after transient forebrain ischemia while no significant changes were found in the dopamine-denervated left striatum. Paired-pulse tests suggested that these changes involved presynaptic mechanisms. Although the incidence of a late depolarizing postsynaptic potential elicited by cortical stimulation increased after ischemia in both sides, the increase was greater in the left side. The analysis of current-voltage relationship of spiny neurons indicated that inward rectification in the left striatum transiently disappeared shortly after ischemia whereas that in the right side remained unchanged. The intrinsic excitability of spiny neurons in both sides were suppressed after ischemia, however, the suppression in the right side was stronger than in the left side. The above results demonstrate that after ipsilateral dopamine denervation, the depression of excitatory synaptic transmission and neuronal excitability in the right striatum is more severe than that in the left striatum following ischemia. The depression of excitatory synaptic transmission and neuronal excitability, therefore, might play an important role in neural protection after ischemic insult.

Differential changes of synaptic transmission in spiny neurons of rat neostriatum following transient forebrain ischemia

Spiny neurons in neostriatum are vulnerable to cerebral ischemia. To reveal the mechanisms underlying the postischemic neuronal damage, the spontaneous activities, evoked postsynaptic potentials and membrane properties of spiny neurons in rat neostriatum were compared before and after transient forebrain ischemia using intracellular recording and staining techniques in vivo. In control animals the membrane properties of spiny neurons were about the same between the left and right neostriatum but the inhibitory synaptic transmission was stronger in the left striatum. After severe ischemia, the spontaneous firing and membrane potential fluctuation of spiny neurons dramatically reduced. The cortically evoked initial excitatory postsynaptic potentials were suppressed after ischemia indicated by the increase of stimulus threshold and the rise time of these components. The paired-pulse facilitation test indicated that such suppression might involve presynaptic mechanisms. The inhibitory postsynaptic potentials in spiny neurons were completely abolished after ischemia and never returned to the control levels. A late depolarizing postsynaptic potential that was elicited from approximately 5% of the control neurons by cortical stimulation could be evoked from approximately 30% of the neurons in the left striatum and approximately 50% in the right striatum after ischemia. The late depolarizing postsynaptic potential could not be induced after acute thalamic transection. The intrinsic excitability of spiny neurons was suppressed after ischemia evidenced by the significant increase of spike threshold and rheobase as well as the decrease of repetitive firing rate following ischemia. The membrane input resistance and time constant increased within 6 h following ischemia and the amplitude of fast afterhyperpolarization significantly increased after ischemia. These results indicate the depression of excitatory monosynaptic transmission, inhibitory synaptic transmission and excitability of spiny neurons after transient forebrain ischemia whereas the excitatory polysynaptic transmission in neostriatum was potentiated. The facilitation of excitatory polysynaptic transmission is stronger in the right neostriatum than in the left neostriatum after ischemia. The suppression of inhibitory component and the facilitation of excitatory polysynaptic transmission may contribute to the pathogenesis of neuronal injury in neostriatum after transient cerebral ischemia.

Magnesium chloride alone or in combination with diazepam fails to prevent hippocampal damage following transient forebrain ischemia

In the central nervous system, magnesium ion (Mg2+) acts as an endogenous modulator of N-methyl-D-aspartate (NMDA)-coupled calcium channels, and may play a major role in the pathomechanisms of ischemic brain damage. In the present study, we investigated the effects of magnesium chloride (MgCl2, 2.5, 5.0 or 7.5 mmol/kg), either alone or in combination with diazepam (DZ), on ischemia-induced hippocampal cell death. Male Wistar rats (250-300 g) were subjected to transient forebrain ischemia for 15 min using the 4-vessel occlusion model. MgCl2 was applied systemically (sc) in single (1x, 2 h post-ischemia) or multiple doses (4x, 1, 2, 24 and 48 h post-ischemia). DZ was always given twice, at 1 and 2 h post-ischemia. Thus, ischemia-subjected rats were assigned to one of the following treatments: vehicle (0.1 ml/kg, N = 34), DZ (10 mg/kg, N = 24), MgCl2 (2.5 mmol/kg, N = 10), MgCl2 (5.0 mmol/kg, N = 17), MgCl2 (7.5 mmol/kg, N = 9) or MgCl2 (5 mmol/kg) + DZ (10 mg/kg, N = 14). Seven days after ischemia the brains were analyzed histologically. Fifteen minutes of ischemia caused massive pyramidal cell loss in the subiculum (90.3%) and CA1 (88.4%) sectors of the hippocampus (P<0.0001, vehicle vs sham). Compared to the vehicle-treated group, all pharmacological treatments failed to attenuate the ischemia-induced death of both subiculum (lesion: 86. 7-93.4%) and CA1 (lesion: 85.5-91.2%) pyramidal cells (P>0.05). Both DZ alone and DZ + MgCl2 reduced rectal temperature significantly (P<0.05). No animal death was observed after drug treatment. These data indicate that exogenous magnesium, when administered systemically post-ischemia even in different multiple dose schedules, alone or with diazepam, is not useful against the histopathological effects of transient global cerebral ischemia in rats.

Changes in membrane properties of CA1 pyramidal neurons after transient forebrain ischemia in vivo

We have previously identified three distinct populations of CA1 pyramidal neurons after reperfusion based on differences in synaptic response, and named these late depolarizing postsynaptic potential neurons (enhanced synaptic transmission), non-late depolarizing postsynaptic potential and small excitatory postsynaptic neurons (depressed synaptic transmission). In the present study, spontaneous activity and membrane properties of CA1 neurons were examined up to 48 h following approximately 14 min ischemic depolarization using intracellular recording and staining techniques in vivo. In comparison with preischemic properties, the spontaneous firing rate and the spontaneous synaptic activity of CA1 neurons decreased significantly during reperfusion; spontaneous synaptic activity ceased completely 36-48 h after reperfusion, except for a low level of activity which persisted in non-late depolarizing postsynaptic potential neurons. Neuronal hyperactivity as indicated by increasing firing rate was never observed in the present study. The membrane input resistance and time constant decreased significantly in late depolarizing postsynaptic potential neurons at 24-48 h reperfusion. In contrast, similar changes were not observed in non-late depolarizing postsynaptic potential neurons. The rheobase, spike threshold and spike frequency adaptation in late depolarizing postsynaptic potential neurons increased progressively following reperfusion. Only a transient increase in rheobase and spike threshold was detected in non-late depolarizing postsynaptic potential neurons and spike frequency adaptation remained unchanged in these neurons. The amplitude of fast afterhyperpolarization increased in all neurons after reperfusion, with the smallest increment in non-late depolarizing postsynaptic potential neurons. Small excitatory postsynaptic potential neurons shared similar changes to those of late depolarizing postsynaptic potential neurons. These results suggest that the enhancement and depression of synaptic transmission following ischemia are probably due to changes in synaptic efficacy rather than changes in intrinsic membrane properties. The neurons with enhanced synaptic transmission following ischemia are probably the degenerating neurons, while the neurons with depressed synaptic transmission may survive the ischemic insult.

Decreased hippocampal expression, but not functionality, of GABA(B) receptors after transient cerebral ischemia in rats

We have examined the effects of transient global ischemia on both the gene expression levels and the functionality of GABA(B) receptors in rat brain, using antisense in situ hybridization and electrophysiological evaluations. At the level of gene expression, no significant change in GABA(B) receptor expression was observed in any hippocampal subfield at either 6 or 12 h after challenge. At 24 h postchallenge, however, a significant decrease in GABA(B) receptor expression was observed in both the CA1 and CA3 subfields, whereas no change was observed in the dentate granule cell layer. Although expression in both the vulnerable CA1 and less vulnerable CA3 subfields was diminished at this time postchallenge, there was no significant difference in the degree of the diminished expression between these subfields. At the functional level, the dose-dependent ability of baclofen (1-100 microM) to inhibit an evoked excitatory postsynaptic potential (f-EPSP) in the CA1 subfield was evaluated at 24 h postischemia, in comparison with the dose-response observed in sham-operated subjects. No significant differences were observed in the efficacy of GABA(B) receptor-mediated inhibition of the elicited f-EPSP at any of the baclofen concentrations examined. These data demonstrate that although the mRNA expression levels for the GABA(B) receptor are diminished in both vulnerable and less vulnerable neurons of Ammon's horn at 24 h following transient global ischemia, the functionality of the GABA(B) receptor system is maintained at this time postchallenge.

Transient neurophysiological changes in CA3 neurons and dentate granule cells after severe forebrain ischemia in vivo

Transient neurophysiological changes in CA3 neurons and dentate granule cells after severe forebrain ischemia in vivo. J. Neurophysiol. 80: 2860-2869, 1998. The spontaneous activities, evoked synaptic responses, and membrane properties of CA3 pyramidal neurons and dentate granule cells in rat hippocampus were compared before ischemia and </=7 days after reperfusion with intracellular recording and staining techniques in vivo. A four-vessel occlusion method was used to induce approximately 14 min of ischemic depolarization. No significant change in spontaneous firing rate was observed in both cell types after reperfusion. The amplitude and slope of excitatory postsynaptic potentials (EPSPs) in CA3 neurons decreased to 50% of control values during the first 12 h reperfusion and returned to preischemic levels 24 h after reperfusion. The amplitude and slope of EPSPs in granule cells slightly decreased 24-36 h after reperfusion. The amplitude of inhibitory postsynaptic potentials in CA3 neurons transiently increased 24 h after reperfusion, whereas that in granule cells showed a transient decrease 24-36 h after reperfusion. The duration of spike width of CA3 and granule cells became longer than that of control values during the first 12 h reperfusion. The spike threshold of both cell types significantly increased 24-36 h after reperfusion, whereas the frequency of repetitive firing evoked by depolarizing current pulse was decreased during this period. No significant change in rheobase and input resistance was observed in CA3 neurons. A transient increase in rheobase and a transient decrease in input resistance were detected in granule cells 24-36 h after reperfusion. The amplitude of fast afterhyperpolarization in both cell types increased for 2 days after ischemia and returned to normal values 7 days after reperfusion. The results from this study indicate that the neuronal excitability and synaptic transmission in CA3 and granule cells are transiently suppressed after severe forebrain ischemia. The depression of synaptic transmission and neuronal excitability may provide protection for neurons after ischemic insult.

Intravenously administered cell-permeant calcium buffer decreases evoked synaptic potentials in rat dentate gyrus in vivo

We examined the effects of the neuroprotective cell-permeant Ca2+ buffer, 2-aminophenol-N,N,O-triacetic acid acetoxymethyl ester (APTRA-AM, 20-40 mg/kg), on synaptically evoked potentials in the dentate gyrus of awake rats. Intravenous APTRA-AM (20 mg/kg) decreased the evoked potentials with peak effects approximately 6 h after infusion, and recovery to control levels by 24 h. Peak decrease in the population spike (PS) amplitude was by 72+/-17% of control, and the excitatory postsynaptic potential (EPSP) slope was decreased by 31+/-12%. APTRA-AM (40 mg/kg), decreased the PS amplitude and EPSP slope by 58+/-7% and 31+/-6% of pre-drug levels, respectively. These effects were qualitatively similar to the presynaptically mediated decreases in synaptic potentials previously demonstrated in vitro with APTRA-AM. These results indicate that the cell-permeant Ca2+ buffer, APTRA-AM, attenuates hippocampal excitability in vivo, most likely by decreasing synaptic neurotransmission.

Prolonged enhancement and depression of synaptic transmission in CA1 pyramidal neurons induced by transient forebrain ischemia in vivo

Evoked postsynaptic potentials of CA1 pyramidal neurons in rat hippocampus were studied during 48 h after severe ischemic insult using in vivo intracellular recording and staining techniques. Postischemic CA1 neurons displayed one of three distinct response patterns following contralateral commissural stimulation. At early recirculation times (0-12 h) approximately 50% of neurons exhibited, in addition to the initial excitatory postsynaptic potential, a late depolarizing postsynaptic potential lasting for more than 100 ms. Application of dizocilpine maleate reduced the amplitude of late depolarizing postsynaptic potential by 60%. Other CA1 neurons recorded in this interval failed to develop late depolarizing postsynaptic potentials but showed a modest blunting of initial excitatory postsynaptic potentials (non-late depolarizing postsynaptic potential neuron). The proportion of recorded neurons with late depolarizing postsynaptic potential characteristics increased to more than 70% during 13-24 h after reperfusion. Beyond 24 h reperfusion, approximately 20% of CA neurons exhibited very small excitatory postsynaptic potentials even with maximal stimulus intensity. The slope of the initial excitatory postsynaptic potentials in late depolarizing postsynaptic potential neurons increased to approximately 150% of control values up to 12 h after reperfusion indicating a prolonged enhancement of synaptic transmission. In contrast, the slope of the initial excitatory postsynaptic potentials in non-late depolarizing postsynaptic potential neurons decreased to less than 50% of preischemic values up to 24 h after reperfusion indicating a prolonged depression of synaptic transmission. More late depolarizing postsynaptic potential neurons were located in the medial portion of CA1 zone where neurons are more vulnerable to ischemia whereas more non-late depolarizing postsynaptic potential neurons were located in the lateral portion of CA1 zone where neurons are more resistant to ischemia. The result from the present study suggests that late depolarizing postsynaptic potential and small excitatory postsynaptic potential neurons may be irreversibly injured while non-late depolarizing postsynaptic potential neurons may be those that survive the ischemic insult. Alterations of synaptic transmission may be associated with the pathogenesis of postischemic neuronal injury.

Electrophysiological changes of CA3 neurons and dentate granule cells following transient forebrain ischemia

The electrophysiological responses of CA3 pyramidal neurons and dentate granule (DG) cells in rat hippocampus were studied after transient forebrain ischemia using intracellular recording and staining techniques in vivo. Approximately 5 min of ischemic depolarization was induced using 4-vessel occlusion method. The spike threshold and rheobase of CA3 neurons remained unchanged up to 12 h following reperfusion. No significant change in spike threshold was observed in DG cells but the rheobase transiently increased 6-9 h after ischemia. The input resistance and time constant of CA3 neurons increased 0-3 h after ischemia and returned to control ranges at later time periods. The spontaneous firing rate in CA3 neurons transiently decreased shortly following reperfusion, while that of DG cells progressively decreased after ischemia. In CA3 neurons, the amplitude and slope of excitatory postsynaptic potentials (EPSPs) transiently decreased 0-3 h after reperfusion, and the stimulus intensity threshold for EPSPs transiently increased at the same time. No significant changes in amplitude and slope of EPSPs were observed in DG cells, but the stimulus intensity threshold for EPSPs slightly increased shortly after reperfusion. The present study demonstrates that the excitability of CA3 pyramidal neurons and DG cells after 5 min ischemic depolarization is about the same as control levels, whereas the synaptic transmission to these cells was transiently suppressed after the ischemic insult. These results suggest that synaptic transmission is more sensitive to ischemia than membrane properties, and the depression of synaptic transmission may be a protective mechanism against ischemic insults.

Assembly of proteins to postsynaptic densities after transient cerebral ischemia

Transient ischemia leads to changes in synaptic efficacy and results in selective neuronal damage during the postischemic phase, although the mechanisms are not fully understood. The protein composition and ultrastructure of postsynaptic densities (PSDs) were studied by using a rat transient ischemic model. We found that a brief ischemic episode induced a marked accumulation in PSDs of the protein assembly ATPases, N-ethylmaleimide-sensitive fusion protein, and heat-shock cognate protein-70 as well as the BDNF receptor (trkB) and protein kinases, as determined by protein microsequencing. The changes in PSD composition were accompanied by a 2.5-fold increase in the yield of PSD protein relative to controls. Biochemical modification of PSDs correlated well with an increase in PSD thickness observed in vivo by electron microscopy. We conclude that a brief ischemic episode modifies the molecular composition and ultrastructure of synapses by assembly of proteins to the postsynaptic density, which may underlie observed changes in synaptic function and selective neuronal damage.

Mild hypothermia protects rat hippocampal CA1 neurons from irreversible membrane dysfunction induced by experimental ischemia

In order to examine the effects of hypothermia on the changes in membrane potential induced by experimental ischemia (deprivation of oxygen and glucose), intracellular recordings were made from single CA1 pyramidal neurons in slice preparations of rat hippocampus. Application of ischemic medium caused irreversible changes in membrane potential consisting of an initial hyperpolarization, then a slow depolarization and a rapid depolarization. At temperatures of 35 degrees C and 37 degrees C, once the rapid depolarization occurred, readministration of oxygen and glucose failed to restore the membrane potential, a state referred to as irreversible membrane dysfunction. When the temperature was lowered to between 27 degrees C and 33 degrees C, the membrane potential returned to the control resting membrane potential in 75% of the neurons. The temperature coefficients (Q10) of the latency, the amplitude, and the maximal slope of the rapid depolarization were 2.5, 1.4 and 2.9, respectively. It is concluded that the critical neuroprotective temperature in ischemia-induced membrane dysfunction is found to be 33 degrees C in single CA1 neurons in vitro.