Internal Ca2+ stores involved in anoxic responses of rat hippocampal neurons

Calcium stores regulate excitability in cultured rat hippocampal neurons

Extracellular calcium ions support synaptic activity but also reduce excitability of central neurons. In the present study, the effect of calcium on excitability was explored in cultured hippocampal neurons. CaCl₂ injected by pressure in the vicinity of a neuron that is bathed only in MgCl₂ as the main divalent cation caused a depolarizing shift in action potential threshold and a reduction in excitability. This effect was not seen if the intracellular milieu consisted of Cs⁺ instead of K-gluconate as the main cation or when it contained ruthenium red, which blocks release of calcium from stores. The suppression of excitability by calcium was mimicked by caffeine, and calcium store antagonists cyclopiazonic acid or thapsigargin blocked this action. Neurons taken from synaptopodin-knockout mice show significantly reduced efficacy of calcium modulation of action potential threshold. Likewise, in Orai1 knockdown cells, calcium is less effective in modulating excitability of neurons. Activation of small-conductance K (SK) channels increased action potential threshold akin to that produced by calcium ions, whereas blockade of SK channels but not big K channels reduced the threshold for action potential discharge. These results indicate that calcium released from stores may suppress excitability of central neurons. NEW & NOTEWORTHY Extracellular calcium reduces excitability of cultured hippocampal neurons. This effect is mediated by calcium-gated potassium currents, possibly small-conductance K channels. Release of calcium from internal stores mimics the effect of extracellular calcium. It is proposed that calcium stores modulate excitability of central neurons.

Cytosolic calcium regulation in rat afferent vagal neurons during anoxia

Sensory neurons are able to detect tissue ischaemia and both transmit information to the brainstem as well as release local vasoactive mediators. Their ability to sense tissue ischaemia is assumed to be primarily mediated through proton sensing ion channels, lack of oxygen however may also affect sensory neuron function. In this study we investigated the effects of anoxia on isolated capsaicin sensitive neurons from rat nodose ganglion. Acute anoxia triggered a reversible increase in [Ca2+]i that was mainly due to Ca2+-efflux from FCCP sensitive stores and from caffeine and CPA sensitive ER stores. Prolonged anoxia resulted in complete depletion of ER Ca2+-stores. Mitochondria were partially depolarised by acute anoxia but mitochondrial Ca2+-uptake/buffering during voltage-gated Ca2+-influx was unaffected. The process of Ca2+-release from mitochondria and cytosolic Ca2+-clearance following Ca2+ influx was however significantly slowed. Anoxia was also found to inhibit SERCA activity and, to a lesser extent, PMCA activity. Hence, anoxia has multiple influences on [Ca2+]i homeostasis in vagal afferent neurons, including depression of ATP-driven Ca2+-pumps, modulation of the kinetics of mitochondrial Ca2+ buffering/release and Ca2+-release from, and depletion of, internal Ca2+-stores. These effects are likely to influence sensory neuronal function during ischaemia.

Impaired hippocampal Ca2+ homeostasis and concomitant K+ channel dysfunction in a mouse model of Rett syndrome during anoxia

Methyl-CpG-binding protein 2 (MeCP2) deficiency causes Rett syndrome (RTT), a neurodevelopmental disorder characterized by severe cognitive impairment, synaptic dysfunction, and hyperexcitability. Previously we reported that the hippocampus of MeCP2-deficient mice (Mecp2(-/y)), a mouse model for RTT, is more susceptible to hypoxia. To identify the underlying mechanisms we now focused on the anoxic responses of wildtype (WT) and Mecp2(-/y) CA1 neurons in acute hippocampal slices. Intracellular recordings revealed that Mecp2(-/y) neurons show only reduced or no hyperpolarizations early during cyanide-induced anoxia, suggesting potassium channel (K(+) channel) dysfunction. Blocking adenosine-5'-triphosphate-sensitive K(+) channels (K(ATP-)) and big-conductance Ca(2+)-activated K(+) channels (BK-channels) did not affect the early anoxic hyperpolarization in either genotype. However, blocking Ca(2+) release from the endoplasmic reticulum almost abolished the anoxic hyperpolarizations in Mecp2(-/y) neurons. Single-channel recordings confirmed that neither K(ATP)- nor BK-channels are the sole mediators of the early anoxic hyperpolarization. Instead, anoxia Ca(2+)-dependently activated various small/intermediate-conductance K(+) channels in WT neurons, which was less evident in Mecp2(-/y) neurons. Yet, pharmacologically increasing the Ca(2+) sensitivity of small/intermediate-conductance K(Ca) channels fully restored the anoxic hyperpolarization in Mecp2(-/y) neurons. Furthermore, Ca(2+) imaging unveiled lower intracellular Ca(2+) levels in resting Mecp2(-/y) neurons and reduced anoxic Ca(2+) transients with diminished Ca(2+) release from intracellular stores. In conclusion, the enhanced hypoxia susceptibility of Mecp2(-/y) hippocampus is primarily associated with disturbed Ca(2+) homeostasis and diminished Ca(2+) rises during anoxia. This secondarily attenuates the activation of K(Ca) channels and thereby increases the hypoxia susceptibility of Mecp2(-/y) neuronal networks. Since cytosolic Ca(2+) levels also determine neuronal excitability and synaptic plasticity, Ca(2+) homeostasis may constitute a promising target for pharmacotherapy in RTT.

Oxygen sensing in the body

This review is divided into three parts: (a) The primary site of oxygen sensing is the carotid body which instantaneously respond to hypoxia without involving new protein synthesis, and is historically known as the first oxygen sensor and is therefore placed in the first section (Lahiri, Roy, Baby and Hoshi). The carotid body senses oxygen in acute hypoxia, and produces appropriate responses such as increases in breathing, replenishing oxygen from air. How this oxygen is sensed at a relatively high level (arterial PO2 approximately 50 Torr) which would not be perceptible by other cells in the body, is a mystery. This response is seen in afferent nerves which are connected synaptically to type I or glomus cells of the carotid body. The major effect of oxygen sensing is the increase in cytosolic calcium, ultimately by influx from extracellular calcium whose concentration is 2 x 10(4) times greater. There are several contesting hypotheses for this response: one, the mitochondrial hypothesis which states that the electron transport from the substrate to oxygen through the respiratory chain is retarded as the oxygen pressure falls, and the mitochondrial membrane is depolarized leading to the calcium release from the complex of mitochondria-endoplasmic reticulum. This is followed by influx of calcium. Also, the inhibitors of the respiratory chain result in mitochondrial depolarization and calcium release. The other hypothesis (membrane model) states that K(+) channels are suppressed by hypoxia which depolarizes the membrane leading to calcium influx and cytosolic calcium increase. Evidence supports both the hypotheses. Hypoxia also inhibits prolyl hydroxylases which are present in all the cells. This inhibition results in membrane K(+) current suppression which is followed by cell depolarization. The theme of this section covers first what and where the oxygen sensors are; second, what are the effectors; third, what couples oxygen sensors and the effectors. (b) All oxygen consuming cells have a built-in mechanism, the transcription factor HIF-1, the discovery of which has led to the delineation of oxygen-regulated gene expression. This response to chronic hypoxia needs new protein synthesis, and the proteins of these genes mediate the adaptive physiological responses. HIF-1alpha, which is a part of HIF-1, has come to be known as master regulator for oxygen homeostasis, and is precisely regulated by the cellular oxygen concentration. Thus, the HIF-1 encompasses the chronic responses (gene expression in all cells of the body). The molecular biology of oxygen sensing is reviewed in this section (Semenza). (c) Once oxygen is sensed and Ca(2+) is released, the neurotransmittesr will be elaborated from the glomus cells of the carotid body. Currently it is believed that hypoxia facilitates release of one or more excitatory transmitters from glomus cells, which by depolarizing the nearby afferent terminals, leads to increases in the sensory discharge. The transmitters expressed in the carotid body can be classified into two major categories: conventional and unconventional. The conventional neurotransmitters include those stored in synaptic vesicles and mediate their action via activation of specific membrane bound receptors often coupled to G-proteins. Unconventional neurotransmitters are those that are not stored in synaptic vesicles, but spontaneously generated by enzymatic reactions and exert their biological responses either by interacting with cytosolic enzymes or by direct modifications of proteins. The gas molecules such as NO and CO belong to this latter category of neurotransmitters and have unique functions. Co-localization and co-release of neurotransmitters have also been described. Often interactions between excitatory and inhibitory messenger molecules also occur. Carotid body contains all kinds of transmitters, and an interplay between them must occur. But very little has come to be known as yet. Glimpses of these interactions are evident in the discussion in the last section (Prabhakar).

Early and simultaneous emergence of multiple hippocampal biomarkers of aging is mediated by Ca2+-induced Ca2+ release

Age-dependent changes in multiple Ca2+-related electrophysiological processes in the hippocampus appear to be consistent biomarkers of aging, and several also correlate with cognitive decline. These findings have led to the hypothesis that a common mechanism of Ca2+ dyshomeostasis underlies aspects of aging-dependent brain impairment. However, some key predictions of this view remain untested, including that multiple Ca2+-related biomarkers should emerge concurrently during aging and their onset should also precede/coincide with initial signs of cognitive decline. Moreover, blocking a putative common source of dysregulated Ca2+ should eliminate aging differences. Here, we tested these predictions using combined electrophysiological, imaging, and pharmacological approaches in CA1 neurons to determine the ages of onset (across 4-, 10-, 12-, 14-, and 23-month-old F344 rats) of several established biomarkers, including the increases in the slow afterhyperpolarization, spike accommodation, and [Ca2+]i rise during repetitive synaptic stimulation. In addition, we tested the hypothesis that altered Ca2+-induced Ca2+ release (CICR) from ryanodine receptors, which can be triggered by L-type Ca2+ channels, provides a common source of dysregulated Ca2+ in aging. Results showed that multiple aging biomarkers were first detectable at about the same age (12 months of age; approximately midlife), sufficiently early to influence initial cognitive decline. Furthermore, selectively blocking CICR with ryanodine slowed the Ca2+ rise during synaptic stimulation more in aged rat neurons and, notably, reduced or eliminated aging differences in the biomarkers. Thus, this study provides the first evidence that altered CICR plays a role in driving the early and simultaneous emergence in hippocampus of multiple Ca2+-related biomarkers of aging.

Effect of hypoxia on hyperpolarization-activated current in mouse dorsal root ganglion neurons

The properties of hyperpolarization-activated current (I(h)) in mouse dorsal root ganglion (DRG) neurons and the effect of hypoxia on the current have been studied using whole-cell configuration of the patch clamp technique. Under voltage-clamp mode, I(h), blocked by 1 mM extracellular CsCl, was present in 75.5% of mouse DRG neurons. The distribution rate increased as the neurons become larger, 5.3%, 79.8% and 94.2% in small, medium and large neurons, respectively. Both I(h) density and the rate of I(h) activation increased in response to more hyperpolarized potential. The activation of I(h) current in larger neuron was faster than in smaller neuron, there was a significant correlation between the time constant of I(h) activation and neuron's size. However, I(h) density did not show any correlation with neuron's size. Under current-clamp mode, 'depolarizing sag' was observed in all neurons with I(h) current. The reversal potential (V(rev)) and the maximal conductance density of I(h) (G(h.max-density)) were -31.0 +/- 4.8 mV and 0.17 +/- 0.02 nS/pF, with a half-activated potential (V(0.5) = -99.4 +/- 1.1 mV) and a slope factor (kappa = -10.2 +/- 0.3 mV). There was a correlation between neuron's size and G(h.max-density) only. According to the effect of hypoxia on resting membrane potential, there were hypoxia-sensitive and hypoxia-insensitive neurons. In the hypoxia-sensitive neurons, I(h) was fully abolished by hypoxia, although the resting membrane potential was hyperpolarized. V(0.5) and V(rev) were shifted about 30 mV toward hyperpolarization, whereas G(h.max-density) and kappa were not affected by hypoxia. We suggest that the kinetics and voltage-dependent characteristics of I(h) are varied in mouse DRG neurons with different size. Hypoxia inhibits I(h) in the hypoxia-sensitive neurons by shifting its activation potential to a more hyperpolarized level.

Dopamine-containing neurons are silenced by energy deprivation: a defensive response or beginning of cell death?

Metabolic stress associated to mitochondrial dysfunction has been put forward as an important factor causing degeneration of mesencephalic dopamine-containing neurons in Parkinson's disease (PD). Here we overview how these neurons react to acute hypoxia or hypoglycemia, that are conditions of energy deprivation causing a reduced production of ATP by mitochondria. These neurons, which show a tonic firing discharge under normal condition, undergo into membrane hyperpolarization during hypoxia or hypoglycemia that silence their spontaneous activity. We outline the cellular mechanisms causing membrane hyperpolarization and the accompanied disturbances of intracellular calcium and sodium homeostasis. A better understanding of the changes occurring during transient energy deprivation might contribute to understand the physiopathology of these neurons that derives from mitochondrial dysfunction.

Alterations of potassium currents in ischemia-vulnerable and ischemia-resistant neurons in the hippocampus after ischemia

CA1 pyramidal neurons in the hippocampus die 2-3 days following transient forebrain ischemia, whereas CA3 pyramidal neurons and granule cells in the dentate gyrus remain viable. Excitotoxicity is the major cause of ischemic cell death, and potassium currents play important roles in regulating the neuronal excitability. The present study compared the changes of potassium currents in acutely dissociated hippocampal neurons at different intervals after ischemia. In CA1 neurons, the amplitude of rapid inactivating potassium currents (I(A)) was significantly increased at 14 h and returned to control levels at 38 h after ischemia; the rising slope and decay time constant of I(A) were accordingly increased after ischemia. The activation curve of I(A) in CA1 neurons shifted to the depolarizing direction at 38 h after ischemia. In granule cells, the amplitude and rising slope of I(A) were significantly increased at 38 h after ischemia; the inactivation curves of I(A) shifted toward the depolarizing direction accordingly at 38 h after ischemia. The I(A) remained unchanged in CA3 neurons after ischemia. The amplitudes of delayed rectifier potassium currents (I(Kd)) in CA1 neurons were progressively increased after ischemia. No significant difference in I(Kd) was detected in CA3 and granule cells at any time points after reperfusion. These results indicated that the voltage dependent potassium currents in hippocampal neurons were differentially altered after cerebral ischemia. The up-regulation of I(A) in dentate granule cells might have protective effects. The increase of I(Kd) in CA1 neurons might be associated with the neuronal damage after ischemia.

Calcium signaling and neuronal vulnerability to ischemia in the striatum

Neurons express extremely different sensitivity to ischemic insults. The neuronal vulnerability is region-specific and the striatum is among the most susceptible areas to ischemic damage. Projecting GABAergic medium-sized neurons are very sensitive to energy metabolism impairment, whereas interneurons are selectively spared. However, the reasons for this differential vulnerability are largely unknown. Calcium ions (Ca2+) are important intracellular messengers enabling several physiological processes. However, excessive Ca2+ influx from the extracellular space or release from internal stores can elevate Ca2+ to levels that exceed the capacity of single neurons to appropriately buffer such overload. This capacity also appears to be a peculiar feature of single neuronal subtypes. This review will provide a brief survey of the ionic basis underlying the differential responses to in vitro ischemia of distinct striatal neuronal subtypes, mainly focusing on the role of Ca2+. The potential relevance of these findings in the development of therapeutic strategies for acute stroke will be discussed.

Hypoxia sensing and pathways of cytosolic Ca2+ increases

Oxygen-sensing and reactivity to changes in the concentration of oxygen is a fundamental property of cellular physiology. This central role is determined, mainly, by, to the fact that oxygen represents the final acceptor of electrons, derived from the normal cellular metabolism, at the end of the mitochondrial respiratory chain. Despite significant advances in molecular characterization of various oxygen-sensitive processes, the nature of the oxygen-sensor molecules and the mechanisms that link sensors to effects remains unclear. One such controversy is about the role and nature of reactive oxygen species (ROS) changes during hypoxia. Irrespective of the mechanisms of oxygen sensing, one of the constant early responses to hypoxia in almost all cell types is an increase in intracellular Ca2+ ([Ca2+]i). In many instances, this increase is mediated by the activation of various plasma membrane Ca2+ conductances. Some of these channels have specific Ca2+ permeability (e.g. voltage-operated Ca2+ channels), whereas others have non-specific cation conductances and are activated by a variety of ligands (ligand-operated channels). In the last decade, a large superfamily of channels with significant Ca2+ permeability has been progressively identified and characterised: the TRP channels. Through their properties, some groups of the TRP channels provide a link to the other hypoxia-activated mechanism of [Ca2+]i increase: the release of Ca2+ from intracellular Ca2+ stores. Since the [Ca2+]i signals, depending on their localization and intensity, are important regulators of the subsequent cellular responses to hypoxia, a deeper understanding of the mechanisms through which hypoxia regulate the activity of these pathways that increase intracellular Ca2+ could point the way towards the development of new therapeutic approaches to reduce or suppress the pathological effects of cellular hypoxia, such as those seen in stroke or myocardial ischemia.

Hypoxic excitability changes and sodium currents in hippocampus CA1 neurons

1. The objective of the present study was to distinguish if inhibition of neuronal activity by hypoxia is related to a block of voltage-gated Na+ channels. 2. The effect of chemical hypoxia induced by cyanide (0.5 mM, 10 min perfusion) was studied with patch-clamp technique in visualized intact CA1 pyramidal neurons in rat brain slices. Action potentials were elicited in whole cell current-clamp recordings and the threshold was estimated by current pulses of 50-ms duration and incremental amplitudes (n = 31). The effect of cyanide on the Na+ current and conductance was studied in voltage clamp recordings from cell-attached patches (n = 13). 3. Cyanide perfusion during 10 min increased the threshold for excitation by 73 +/- 79 pA (p = 0.001), which differed from the effect in control cells (11 +/- 41 pA, ns). The change in current threshold was correlated to a change in membrane potential (r = -0.88, p < 0.0001). Cyanide had no significant effect on the peak amplitude, duration, or rate of rise of the action potential. 4. Cyanide perfusion did not change the Na+ current size, but caused a small decrease in ENa (-17 +/- 22 mV, ns) and a slight increase in Na+ conductance (+14 +/- 26%, ns), which differed (p = 0.045) from controls (-19 +/- 23 %, ns). 5. In conclusion, chemical hypoxia does not cause a decrease in Na+ conductance. The decreased excitability during hypoxia can be explained by an increase in the current threshold, which is correlated with the effect on the membrane potential.

Chemical anoxia activates ATP-sensitive and blocks Ca(2+)-dependent K(+) channels in rat dorsal vagal neurons in situ

The contribution of subclasses of K(+) channels to the response of mammalian neurons to anoxia is not yet clear. We investigated the role of ATP-sensitive (K(ATP)) and Ca(2+)-activated K(+) currents (small conductance, SK, big conductance, BK) in mediating the effects of chemical anoxia by cyanide, as determined by electrophysiological analysis and fluorometric Ca(2+) measurements in dorsal vagal neurons of rat brainstem slices. The cyanide-evoked persistent outward current was abolished by the K(ATP) channel blocker tolbutamide, but not changed by the SK and BK channel blockers apamin or tetraethylammonium. The K(+) channel blockers also revealed that ongoing activation of K(ATP) and SK channels counteracts a tonic, spike-related rise in intracellular Ca(2+) ([Ca(2+)](i)) under normoxic conditions, but did not modify the rise of [Ca(2+)](i) associated with the cyanide-induced outward current. Cyanide depressed the SK channel-mediated afterhyperpolarizing current without changing the depolarization-induced [Ca(2+)](i) transient, but did not affect spike duration that is determined by BK channels. The afterhyperpolarizing current and the concomitant [Ca(2+)](i) rise were abolished by Ca(2+)-free superfusate that changed neither the cyanide-induced outward current nor the associated [Ca(2+)](i) increase. Intracellular BAPTA for Ca(2+) chelation blocked the afterhyperpolarizing current and the accompanying [Ca(2+)](i) increase, but had no effect on the cyanide-induced outward current although the associated [Ca(2+)](i) increase was noticeably attenuated. Reproducing the cyanide-evoked [Ca(2+)](i) transient with the Ca(2+) pump blocker cyclopiazonic acid did not evoke an outward current. Our results show that anoxia mediates a persistent hyperpolarization due to activation of K(ATP) channels, blocks SK channels and has no effect on BK channels, and that the anoxic rise of [Ca(2+)](i) does not interfere with the activity of these K(+) channels.

Alterations of single potassium channel activity in CA1 pyramidal neurons after transient forebrain ischemia

Selective neuronal injury in the CA1 zone of hippocampus following transient cerebral ischemia has been well documented. Extracellular potassium concentration markedly increases during ischemia/hypoxia. Accumulating evidence has indicated that the outward potassium currents, including delayed rectifier potassium current, not only influence membrane excitability but also mediate apoptosis. It has been shown that the amplitude of delayed rectifier potassium current in CA1 neurons significantly increased after cerebral ischemia. To elucidate the mechanisms underlying the changes of potassium currents following ischemia, single potassium channel activities of rat CA1 neurons were compared before and after transient forebrain ischemia. Using cell-attached configuration, depolarizing voltage steps activated outward single channel events. The channel properties, the kinetics and pharmacology of these events resemble the delayed rectifier potassium current. After ischemia, the unitary amplitude of single channels significantly increased, the open probability, mean open time and open time constant also significantly increased while the conductance remained unchanged. These data indicate that the increase of single channel activity is responsible, at least in part, for the increase of delayed rectifier potassium current in CA1 neurons after cerebral ischemia.

Pharmacological protection of synaptic function, spatial learning, and memory from transient hypoxia in rats

Hypoxia significantly reduced cholinergic theta activity in rat CA1 field and intracellular theta in the CA1 pyramidal cells, recorded in hippocampal slices. The hypoxic responses of the hippocampal CA1 pyramidal cells to a brief hypoxia consisted of a short period of "synaptic arrest", observed as an elimination of excitatory postsynaptic current under voltage clamp and recovered immediately as oxygenation was reinitiated. The hypoxic synaptic arrest was not associated with reduced postsynaptic responses of the pyramidal cells to externally applied L-glutamate, suggesting that the synaptic arrest might result from a presynaptic mechanism. The hypoxic synaptic arrest was abolished in the presence of 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), a specific adenosine A(1) receptor antagonist. Blocking adenosine A(1) receptors also eliminated effects of hypoxia on the hippocampal CA1 field theta activity and intracellular theta of the CA1 pyramidal cells. In behaving rats, brief hypoxia impaired their water maze performance in both the escape latency and probe tests. The impairment was prevented by intralateral cerebroventricular injections of DPCPX. These results suggest that hypoxia releases adenosine and produces an inhibition of synaptic transmission and intracellular signal cascade(s) involved in generation/maintenance of hippocampal CA1 theta activity. This protection of synaptic efficacy and spatial learning through adenosine A(1) receptor antagonism may represent an effective therapeutic strategy to eliminate functional interruption due to transient hypoxic episodes and/or chronic hypoxia secondary to compromise of respiratory function.

Chemical hypoxia in hippocampal pyramidal cells affects membrane potential differentially depending on resting potential

The aim of the present study was to analyze the effect of chemical hypoxia (cyanide) on the membrane potential of hippocampal CA1 neurons and to elucidate the reason for previously found differences in the reaction to hypoxia in these cells. Recordings were performed in brain slices from 8-19-day-old rats with whole-cell patch clamp on cells identified with near-infrared video microscopy. Cyanide (0.1-2.0 mM) caused different responses depending on the resting potential of the cells: hyperpolarization (or an initial depolarization followed by hyperpolarization) was generally seen in cells with less negative resting potential (-56+/-6.1 mV), and depolarization in cells with more negative resting potential (-62+/-3.4 mV). After 10 min in cyanide the membrane potential in all cells had reached approximately the same level (-62+/-5.8 mV), the direction and size of the voltage response having an inverse linear relation to the resting potential (k=-0.98, r=0.71). The direction of the cyanide response was not reversed by current injection (depolarization by 12 mV) in cells with more negative resting potential (-60+/-2.8 mV). Wash out of cyanide caused hyperpolarization in 70% of the cells. Presence of ouabain (2 microM) resulted in pronounced depolarization during cyanide perfusion, and potentiated the hyperpolarization during wash out indicating that this part of the effect is not dependent on a reactivation of the Na/K pump. In conclusion, chemical hypoxia with cyanide changes the membrane potential in CA1 cells in size and direction depending on the original resting potential of the cells. The present findings suggested that cyanide activated not only K+ channels but in addition increased a Na+ current which has a more positive equilibrium potential.

Differential changes of potassium currents in CA1 pyramidal neurons after transient forebrain ischemia

CA1 pyramidal neurons are highly vulnerable to transient cerebral ischemia. In vivo studies have shown that the excitability of CA1 neurons progressively decreased following reperfusion. To reveal the mechanisms underlying the postischemic excitability change, total potassium current, transient potassium current, and delayed rectifier potassium current in CA1 neurons were studied in hippocampal slices prepared before ischemia and at different time points following reperfusion. Consistent with previous in vivo studies, the excitability of CA1 neurons decreased in brain slices prepared at 14 h following transient forebrain ischemia. The amplitude of total potassium current in CA1 neurons increased approximately 30% following reperfusion. The steady-state activation curve of total potassium current progressively shifted in the hyperpolarizing direction with a transient recovery at 18 h after ischemia. For transient potassium current, the amplitude was transiently increased approximately 30% at approximately 12 h after reperfusion and returned to control levels at later time points. The steady-state activation curve also shifted approximately 20 mV in the hyperpolarizing direction, and the time constant of removal of inactivation markedly increased at 12 h after reperfusion. For delayed rectifier potassium current, the amplitude significantly increased and the steady-state activation curve shifted in the hyperpolarizing direction at 36 h after reperfusion. No significant change in inactivation kinetics was observed in the above potassium currents following reperfusion. The present study demonstrates the differential changes of potassium currents in CA1 neurons after reperfusion. The increase of transient potassium current in the early phase of reperfusion may be responsible for the decrease of excitability, while the increase of delayed rectifier potassium current in the late phase of reperfusion may be associated with the postischemic cell death.

In vivo electrical activity of brainstem neurons in fetal rats during asphyxia

To see changes in the activity and the sensitivity to glutamate of fetal brain neurons during asphyxia, the electrical activity of brainstem neurons was recorded extracellularly in fetal rats which were still connected with the dams by the intact umbilical cord. In urethan-anesthetized pregnant rats, fetal asphyxia (2-10 min) was induced by occluding the umbilical cord with a surgical clip, while reperfusion of the umbilical blood flow was performed by local application of a relaxant of blood vessels to the occlusion site. The spontaneous discharge of fetal brainstem neurons was suppressed for a long period of time by umbilical cord occlusion. The suppression of the firing occurred 48-150 (78+/-7) s after the start of umbilical cord occlusion, and lasted even after fetal cortical PO(2) recovered to control level after reperfusion. The changes occurred with a marked reduction in spike amplitude. A similar suppression was observed for the spikes induced by iontophoretic application of glutamate, although fetal brainstem neurons were extremely sensitive to glutamate before asphyxia. The suppression of the spontaneous spikes became more notable and longer when asphyxia was repeated. These findings suggest that the long-lasting suppression of fetal neurons during asphyxia may contribute to a reduction of cellular energy requirements in the fetal brain, thereby playing a role in the resistance of fetal neurons to brain damage caused by asphyxia. Furthermore, the reduced sensitivity of fetal neurons to glutamate during asphyxia may also contribute to prevent brain damage due to excitotoxity of glutamate.

Intracellular Ca(2+) stores in chemoreceptor cells of the rabbit carotid body: significance for chemoreception

The notion that intracellular Ca(2+) (Ca(i)(2+)) stores play a significant role in the chemoreception process in chemoreceptor cells of the carotid body (CB) appears in the literature in a recurrent manner. However, the structural identity of the Ca(2+) stores and their real significance in the function of chemoreceptor cells are unknown. To assess the functional significance of Ca(i)(2+) stores in chemoreceptor cells, we have monitored 1) the release of catecholamines (CA) from the cells using an in vitro preparation of intact rabbit CB and 2) the intracellular Ca(2+) concentration ([Ca(2+)](i)) using isolated chemoreceptor cells; both parameters were measured in the absence or the presence of agents interfering with the storage of Ca(2+). We found that threshold [Ca(2+)](i) for high extracellular K(+) (K(e)(+)) to elicit a release response is approximately 250 nM. Caffeine (10-40 mM), ryanodine (0.5 microM), thapsigargin (0.05-1 microM), and cyclopiazonic acid (10 microM) did not alter the basal or the stimulus (hypoxia, high K(e)(+))-induced release of CA. The same agents produced Ca(i)(2+) transients of amplitude below secretory threshold; ryanodine (0.5 microM), thapsigargin (1 microM), and cyclopiazonic acid (10 microM) did not alter the magnitude or time course of the Ca(i)(2+) responses elicited by high K(e)(+). Several potential activators of the phospholipase C system (bethanechol, ATP, and bradykinin), and thereby of inositol 1,4,5-trisphosphate receptors, produced minimal or no changes in [Ca(2+)](i) and did not affect the basal release of CA. It is concluded that, in the rabbit CB chemoreceptor cells, Ca(i)(2+) stores do not play a significant role in the instant-to-instant chemoreception process.

Direct measurement of adenosine release during hypoxia in the CA1 region of the rat hippocampal slice

We have used an enzyme-based, twin-barrelled sensor to measure adenosine release during hypoxia in the CA1 region of rat hippocampal slices in conjunction with simultaneous extracellular field recordings of excitatory synaptic transmission. When loaded with a combination of adenosine deaminase, nucleoside phosphorylase and xanthine oxidase, the sensor responded linearly to exogenous adenosine over the concentration range 10 nM to 20 microM. Without enzymes, the sensor when placed on the surface of hippocampal slices recorded a very small net signal during hypoxia of 40 +/- 43 pA (mean +/- s.e.m.; n = 7). Only when one barrel was loaded with the complete sequence of enzymes and the other with the last two in the cascade did the sensor record a large net difference signal during hypoxia (1226 +/- 423 pA; n = 7). This signal increased progressively during the hypoxic episode, scaled with the hypoxic depression of the simultaneously recorded field excitatory postsynaptic potential and was greatly reduced (67 +/- 6.5 %; n = 9) by coformycin (0.5-2 microM), a selective inhibitor of adenosine deaminase, the first enzyme in the enzymic cascade within the sensor. For 5 min hypoxic episodes, the sensor recorded a peak concentration of adenosine of 5.6 +/- 1.2 microM (n = 16) with an IC(50) for the depression of transmission of approximately 3 microM. In slices pre-incubated for 3-6 h in nominally Ca(2+)-free artificial cerebrospinal fluid, 5 min of hypoxia resulted in an approximately 9-fold greater release of adenosine (48.9 +/- 17.7 microM; n = 6). High extracellular Ca(2+) (4 mM) both reduced the adenosine signal recorded by the sensor during hypoxia (3.5 +/- 0.6 microM; n = 4) and delayed the hypoxic depression of excitatory synaptic transmission.

Differential fall in ATP accounts for effects of temperature on hypoxic damage in rat hippocampal slices

Intracellular recordings, ATP and cytosolic calcium measurements from CA1 pyramidal cells in rat hippocampal slices were used to examine the mechanisms by which temperature alters hypoxic damage. Hypothermia (34 degrees C) preserved ATP (1.7 vs. 0.8 nM/mg) and improved electrophysiologic recovery of the CA1 neurons after hypoxia; 58% of the neurons subjected to 10 min of hypoxia (34 degrees C) recovered their resting and action potentials, while none of the neurons at 37 degrees C recovered. Increasing the glucose concentration from 4 to 6 mM during normothermic hypoxia improved ATP (1.3 vs. 0.8 nM/mg) and mimicked the effects of hypothermia; 67% of the neurons recovered their resting and action potentials. Hypothermia attenuated the membrane potential changes and the increase in intracellular Ca(2+) (212 vs. 384 nM) induced by hypoxia. Changing the glucose concentration in the artificial cerebrospinal fluid primarily affects ATP levels during hypoxia. Decreasing the glucose concentration from 4 to 2 mM during hypothermic hypoxia worsened ATP, cytosolic Ca(2+), and electrophysiologic recovery. Ten percent of the neurons subjected to 4 min of hypoxia at 40 degrees C recovered their resting and action potentials; this compared with 60% of the neurons subjected to 4 min of normothermic hypoxia. None of the neurons subjected to 10 min of hypoxia at 40 degrees C recovered their resting and action potentials. Hyperthermia (40 degrees C) worsens the electrophysiologic changes and induced a greater increase in intracellular Ca(2+) (538 vs. 384 nM) during hypoxia. Increasing the glucose concentration from 4 to 8 mM during 10 min of hyperthermic hypoxia improved ATP (1.4 vs. 0.6 nM/mg), Ca(2+) (267 vs. 538 nM), and electrophysiologic recovery (90 vs. 0%). Our results indicate that the changes in electrophysiologic recovery with temperature are primarily due to changes in ATP and that the changes in depolarization and Ca(2+) are secondary to these ATP changes. Both primary and secondary changes are important for explaining the improved electrophysiologic recovery with hypothermia.

Responses to reversible anoxia of intracellular free and bound Ca(2+) in rat cortical slices

Severe anoxia induces destabilisation of intracellular calcium homeostasis in neurones. The mechanism of this effect, and particularly the interrelationship between changes in intracellular concentration of free Ca(2+) ions and the content of the intracellular Ca(2+) stores, during and after anoxia, is not clear. We used a superfusion system of rat olfactory cortical slices for the fluorimetric estimation of changes in the intracellular concentration of free Ca(2+) ions and in the level of bound Ca(2+), utilising the fluorescent indicators Fura-2 and chlortetracycline, respectively. It was found that 10-min normoglycaemic anoxia results in simultaneous decrease in bound and increase in free Ca(2+) levels, whereas during 60-min reoxygenation, we detected an increase in both indices. The NMDA receptor antagonists MK-801 and APV attenuated changes in free Ca(2+) level during anoxia and reoxygenation and intensified anoxia-evoked decrease in bound Ca(2+) content, whereas a late post-anoxic increase in bound Ca(2+) was abolished. These data suggest that the influx of extracellular Ca(2+) to neurones via NMDA receptors, plays a critical role in the rise of intracellular free Ca(2+) concentration during and after anoxia. Biphasic changes in bound Ca(2+) content during anoxia and reoxygenation may reflect an anoxia-induced release of Ca(2+) from intracellular stores, followed later by a neuronal calcium overload and refilling of intracellular Ca(2+) binding sites.

Involvement of intracellular calcium stores during oxygen/glucose deprivation in striatal large aspiny interneurons

Striatal large aspiny interneurons were recorded from a slice preparation using a combined electrophysiologic and microfluorometric approach. The role of intracellular Ca2+ stores was analyzed during combined oxygen/glucose deprivation (OGD). Before addressing the role of the stores during energy deprivation, the authors investigated their function under physiologic conditions. Trains of depolarizing current pulses caused bursts of action potentials coupled to transient increases in intracellular calcium concentration ([Ca2+]i). In the presence of cyclopiazonic acid (30 micromol/L), a selective inhibitor of the sarcoendoplasmic reticulum Ca2+ pumps, or when ryanodine receptors were directly blocked with ryanodine (20 [micromol/L), the [Ca2+]i transients were progressively smaller in amplitude, suggesting that [Ca2+]i released from intracellular stores helps to maintain a critical level of [Ca2+]i during physiologic firing activity. As the authors have recently reported, brief exposure to combined OGD induced a membrane hyperpolarization coupled to an increase in [Ca2+]i. In the presence of cyclopiazonic acid or ryanodine, the hyperpolarization and the rise in [Ca2+]i induced by OGD were consistently reduced. These data support the hypothesis that Ca2+ release from ryanodine-sensitive Ca2+ pools is involved not only in the potentiation of the Ca2+ signals resulting from cell depolarization, but also in the amplification of the [Ca2+]i rise and of the concurrent membrane hyperpolarization observed in course of OGD in striatal large aspiny interneurons.

Release of monoamines and nitric oxide is involved in the modulation of hyperpolarization-activated inward current during acute thalamic hypoxia

Using slices of the dorsal lateral geniculate nucleus, it has been shown that, in the presence of excitatory and inhibitory amino acid antagonists, brief periods of hypoxia (3-4 min of 95% N(2)/5% CO(2)) induce in thalamocortical neurons an increase in instantaneous input conductance (G(N)) accompanied by an inward shift in baseline holding current (I(BH)). These effects have been suggested to be mediated, at least in part, by a positive shift in the voltage-dependence of the hyperpolarization-activated, mixed Na(+)/K(+) current (I(h)) and a change in its activation kinetics which transforms it into an almost instantaneously activated current. In this study, using the whole-cell patch-clamp technique, the contribution of an increased Ca(2+)-dependent transmitter release to the hypoxic response of thalamocortical neurons was further investigated using (i) blockers of calcineurin, a Ca(2+)/calmodulin-activated phosphatase that selectively regulates Ca(2+)-dependent release, and (ii) antagonists of neurotransmitters that are known to modulate I(h). Thalamocortical neurons (n = 23) recorded with electrodes filled with calcineurin autoinhibitory fragment (30-250 microM), a membrane impermeable blocker of calcinuerin, showed no difference either in resting, or in the hypoxia-induced changes in, G(N), I(BH) and I(h), when compared to thalamocortical cells patched with electrodes that did not contain calcineurin autoinhibitory fragment. In contrast, in 18 of these neurons recorded with calcineurin autoinhibitory fragment-filled electrodes, bath application either of cyclosporin-A (20 microM) or tacrolimus (50-100 microM), two membrane permeable blockers of calcineurin, abolished the effects of hypoxia on G(N), I(BH), and I(h). Separate application of noradrenaline, serotonin, histamine and nitric oxide antagonists produced only a small depression of the hypoxic response, while concomitant bath application of these antagonists decreased the hypoxia-induced changes in G(N) and I(BH) by 55 and 42%, respectively (n = 12). Concomitant bath application of 8-bromo-adenosine-3'5'-cyclicmonophosphate and 8-bromo-guanosine-3'5'-cyclicmonophosphate (both 1mM), which are known to mediate the action of these transmitters on I(h), increased G(N) (40%), decreased I(h) time-constant of activation (30%) and significantly occluded (50%) the hypoxia-induced effect on G(N) and I(BH). Thalamocortical neurons (n = 6) patched with electrodes filled with 8-bromo-adenosine-3'5'-cyclicmonophosphate and 8-bromo-guanosine-3'5'-cyclicmonophosphate (both 1 mM) showed a larger G(N) than the one recorded with the standard internal solution, and a significant depression of the hypoxia-induced changes in G(N) and I(BH). These results indicate that during acute thalamic hypoxia an increased release of noradrenaline, serotonin, histamine and nitric oxide is responsible for transforming I(h) into an instantaneously activating current via cyclic AMP- and cyclic GMP-mediated mechanisms.

Pharmacological identification of the K(+) currents mediating the hypoglycemic hyperpolarization of rat midbrain dopaminergic neurones

Hypoglycemia (zero glucose) initially depolarized the membrane and increased the spontaneous firing of rat midbrain dopaminergic neurones (more than 50%) intracellularly recorded in an in vitro slice preparation. Under single-electrode voltage-clamp mode (V(h) -55 mV), this transient phase correlated with an inward current of -18 pA. In all the cells tested (n=30), an inhibition fully developed over 16.9 min of hypoglycemia and was associated with a hyperpolarization of the membrane (7.7 mV) or outward current (95.6 pA). Upon re-application of a control solution (glucose 10 mM) a rebound hyperpolarization/outward current developed. The depression of firing was only seen when the artificial cerebrospinal fluid (ACSF) contained less than 1 mM glucose. In addition, the period of time required to block the spontaneous activity decreased, by diminishing the extracellular concentration of glucose from 1 to 0 mM. The hypoglycemia-induced outward current was associated with an increase in membrane conductance and reversed polarity at -100.4 mV, close to the reversal potential of K(+). The post-hypoglycemic outward current was not associated with an increase in membrane conductance and did not reverse. The K(+)-ATP channel blockers, tolbutamide (300 microM-1 mM) and glibenclamide (3-30 microM) reduced the hypoglycemia-induced inhibition. In addition, the blocker of the Ca(++)-activated K(+)-channels, charybdotoxin (100-400 nM) partially counteracted the hypoglycemic hyperpolarization. Furthermore, barium (100-300 microM) fully antagonized the hypoglycemia-induced inhibition. The post-hypoglycemic hyperpolarization/outward current was not observed in cells treated with the Na(+)/K(+) ATPase pump inhibitor strophanthidin (1-3 microM). Our data suggest that midbrain dopaminergic cells respond to glucose deprivation with a hyperpolarization generated by the opening of several K(+) channels (sulphonylurea-sensitive, charybdotoxin-sensitive and sulphonylurea and charybdotoxin-insensitive) and by the activation of the Na(+)/K(+) ATPase pump after the hypoglycemic period.

Glycolysis prevents anoxia-induced synaptic transmission damage in rat hippocampal slices

Prolonged anoxia can cause permanent damage to synaptic transmission in the mammalian CNS. We tested the hypothesis that lack of glucose is the major cause of irreversible anoxic transmission damage, and that anoxic synaptic transmission damage could be prevented by glycolysis in rat hippocampal slices. The evoked population spike (PS) was extracellularly recorded in the CA1 pyramidal cell layer after stimulation of the Schaffer collaterals. When the slice was superfused with artificial cerebrospinal fluid (ACSF) containing 4 mM glucose, following 10 min anoxia, the evoked PS did not recover at all after 60 min reoxygenation. When superfusion ACSF contained 10 mM glucose with or without 0.5 mM alpha-cyano-4-hydroxycinnate (4-CIN), after 60 min reoxygenation the evoked PS completely recovered following 10 min anoxia. When superfusion ACSF contained 20 mM glucose with or without 1 mM sodium cyanide (NaCN), after 60 min reoxygenation the evoked PS completely recovered even following 120 min anoxia. In contrast, when superfusion ACSF contained 4 mM glucose, following 10 min 1 mM NaCN chemical anoxia alone, without anoxic anoxia, the evoked PS displayed no recovery after 60 min reoxygenation. Moreover, when 16 mM mannitol and 16 sodium L-lactate were added into 4 mM glucose ACSF, following 10 min anoxia the evoked PS failed to recover at all after 60 min reoxygenation. The results indicate that elevated glucose concentration powerfully protected the synaptic transmission against anoxic damage, and the powerful protection is due to anaerobic metabolism of glucose and not a result of the higher osmolality in higher glucose ACSF. We conclude that lack of glucose is the major cause of anoxia-induced synaptic transmission damage, and that if sufficient glucose is supplied, glycolysis could prevent this damage in vitro.

Apamin-sensitive conductance mediates the K(+) current response during chemical ischemia in CA3 pyramidal cells

Pyramidal cells typically respond to ischemia with initial transient hyperpolarization, which may represent a neuroprotective response. To identify the conductance underlying this hyperpolarization in CA3 pyramidal neurons of rat hippocampal organotypic slice cultures, recordings were obtained using the single-electrode voltage-clamp technique. Brief chemical ischemia (2 mM 2-deoxyglucose and 3 mM NaN(3), for 4 min) induced a response mediated by an increase in K(+) conductance. This current was blocked by intracellular application of the Ca(2+) chelator, bis-(o-aminophenoxy)-N,N,N', N'-tetraacetic acid (BAPTA), reduced with low external [Ca(2+)], and inhibited by a selective L-type Ca(2+) channel inhibitor, isradipine, consistent with the activation of a Ca(2+)-dependent K(+) conductance. Experiments with charybdotoxin (10 nM) and tetraethylammonium (TEA; 1 mM), or with the protein kinase C activator, phorbol 12,13-diacetate (PDAc; 3 microM), ruled out an involvement of a large conductance-type or an apamin-insensitive small conductance, respectively. In the presence of apamin (1 microM), however, the outward current was significantly reduced. These results demonstrate that in rat hippocampal CA3 pyramidal neurons an apamin-sensitive Ca(2+)-dependent K(+) conductance is activated in response to brief ischemia generating a pronounced outward current.

Thiopental attenuates hypoxic changes of electrophysiology, biochemistry, and morphology in rat hippocampal slice CA1 pyramidal cells

BACKGROUND AND PURPOSE

Thiopental has been shown to protect against cerebral ischemic damage; however, it has undesirable side effects. We have examined how thiopental alters histological, physiological, and biochemical changes during and after hypoxia. These experiments should enable the discovery of agents that share some of the beneficial effects of thiopental.

METHODS

We made intracellular recordings and measured ATP, sodium, potassium, and calcium concentrations from CA1 pyramidal cells in rat hippocampal slices subjected to 10 minutes of hypoxia with and without 600 micromol/L thiopental.

RESULTS

Thiopental delayed the time until complete depolarization (21+/-3 versus 11+/-2 minutes for treated versus untreated slices, respectively) and attenuated the level of depolarization at 10 minutes of hypoxia (-33+/-6 versus -12+/-5 mV). There was improved recovery of the resting potential after 10 minutes of hypoxia in slices treated with thiopental (89% versus 31% recovery). Thiopental attenuated the changes in sodium (140% versus 193% of prehypoxic concentration), potassium (62% versus 46%), and calcium (111% versus 197%) during 10 minutes of hypoxia. There was only a small effect on ATP (18% versus 8%). The percentage of cells showing clear histological damage was decreased by thiopental (45% versus 71%), and thiopental improved protein synthesis after hypoxia (75% versus 20%).

CONCLUSIONS

Thiopental attenuates neuronal depolarization, an increase in cellular sodium and calcium concentrations, and a decrease in cellular potassium and ATP concentrations during hypoxia. These effects may explain the reduced histological, protein synthetic, and electrophysiological damage to CA1 pyramidal cells after hypoxia with thiopental.

Changes in the calcium dependence of glutamate transmission in the hippocampal CA1 region after brief hypoxia-hypoglycemia

Using the model of hypoxia-hypoglycemia (HH) in rat brain slices, we asked whether glutamate transmission is altered following a brief HH episode. The HH challenge was conducted by exposing slices to a glucose-free medium aerated with 95% N2-5% CO2, for approximately 4 min, and glutamate transmission in the hippocampal CA1 region was monitored at different post HH times. In slices examined </=8 h post HH, CA1 synaptic field potentials are comparable in amplitude to controls, but are less sensitive to experimental manipulations designed to attenuate intracellular Ca2+ signals, as compared with controls. Reducing calcium influx, by applying a nonspecific calcium channel blocker Co2+ or lowering external Ca2+, attenuated CA1 synaptic potentials much less in challenged slices than in controls. Buffering intracellular Ca2+ by bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid-AM (BAPTA-AM) attenuated CA1 synaptic potentials in control but not in slices post HH. Furthermore, minimally evoked excitatory postsynaptic currents displayed a lower failure rate in post-hypoxic CA1 neurons compared with controls. Based on these convergent observations, we suggest that evoked CA1 glutamate transmission is altered in the first several hours after brief hypoxia, likely resulting from alterations in intracellular Ca2+ homeostasis and/or Ca2+-dependent processes governing transmitter release.

Calcium-induced calcium release contributes to action potential-evoked calcium transients in hippocampal CA1 pyramidal neurons

Calcium-induced calcium release (CICR) is a mechanism by which local elevations of intracellular calcium (Ca2+) are amplified by Ca2+ release from ryanodine-sensitive Ca2+ stores. CICR is known to be coupled to Ca2+ entry in skeletal muscle, cardiac muscle, and peripheral neurons, but no evidence suggests that such coupling occurs in central neurons during the firing of action potentials. Using fast Ca2+ imaging in CA1 neurons from hippocampal slices, we found evidence for CICR during action potential-evoked Ca2+ transients. A low concentration of caffeine enhanced Ca2+ transient amplitude, whereas a higher concentration reduced it. Simultaneous Ca2+ imaging and whole-cell recordings showed that membrane potential, action potential amplitude, and waveform were unchanged during caffeine application. The enhancement of Ca2+ transients by caffeine was not affected by the L-type channel blocker nifedipine, the phosphodiesterase inhibitor IBMX, the adenylyl cyclase activator forskolin, or the PKA antagonist H-89. However, thapsigargin or ryanodine, which both empty intracellular Ca2+ stores, occluded this effect. In addition, thapsigargin, ryanodine, and cyclopiazonic acid reduced action potential-evoked Ca2+ transients in the absence of caffeine. These results suggest that Ca2+ release from ryanodine-sensitive stores contributes to Ca2+ signals triggered by action potentials in CA1 neurons.

Effect of hypoxia on membrane potential and resting conductance in rat hippocampal neurons

The present patch-clamp study describes the effect of hypoxia at 30-31 degrees C on membrane potential and resting conductance in pyramidal cells from the hippocampal CA1 region in rat brain slices. The initial effect of hypoxia was a gradual hyperpolarization; the peak change in membrane potential measured over 15 min was -5.3 +/- 0.22 mV (P < 0.0001). After reoxygenation followed a transient hyperpolarization measuring -1.8 +/- 0.24 mV (P < 0.0001) and a subsequent normalization of the membrane potential, which after 5 min did not differ from its level prior to the hypoxic episode. Voltage-clamp analysis showed that the hypoxic hyperpolarization was related to an outward current at the holding potential (-60 mV) and an increase in resting conductance. The effect was not influenced by intracellular Cl- concentration, which indicated that it was not due to an inward flow of Cl- ions. The addition of tolbutamide, glibenclamide and dantrolene sodium did not affect the hypoxic hyperpolarization, neither did the presence of ATP in the pipette solution. The presence/absence of glucose in the perfusion medium did not influence the initial hyperpolarization during hypoxia; however, glucose seemed to prevent the subsequent depolarization under hypoxia. It was concluded that hypoxia caused an initial hyperpolarization of CA1 cells which was related to an increase in the resting conductance. The results did not suggest the involvement of ATP-sensitive K+ channels.

Calcium-induced calcium release contributes to action potential-evoked calcium transients in hippocampal CA1 pyramidal neurons

Calcium-induced calcium release (CICR) is a mechanism by which local elevations of intracellular calcium (Ca2+) are amplified by Ca2+ release from ryanodine-sensitive Ca2+ stores. CICR is known to be coupled to Ca2+ entry in skeletal muscle, cardiac muscle, and peripheral neurons, but no evidence suggests that such coupling occurs in central neurons during the firing of action potentials. Using fast Ca2+ imaging in CA1 neurons from hippocampal slices, we found evidence for CICR during action potential-evoked Ca2+ transients. A low concentration of caffeine enhanced Ca2+ transient amplitude, whereas a higher concentration reduced it. Simultaneous Ca2+ imaging and whole-cell recordings showed that membrane potential, action potential amplitude, and waveform were unchanged during caffeine application. The enhancement of Ca2+ transients by caffeine was not affected by the L-type channel blocker nifedipine, the phosphodiesterase inhibitor IBMX, the adenylyl cyclase activator forskolin, or the PKA antagonist H-89. However, thapsigargin or ryanodine, which both empty intracellular Ca2+ stores, occluded this effect. In addition, thapsigargin, ryanodine, and cyclopiazonic acid reduced action potential-evoked Ca2+ transients in the absence of caffeine. These results suggest that Ca2+ release from ryanodine-sensitive stores contributes to Ca2+ signals triggered by action potentials in CA1 neurons.

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.

Adenosine release mediates cyanide-induced suppression of CA1 neuronal activity

The rapid suppression of CNS function produced by cyanide (CN) was studied by field, intracellular, and whole-cell recording in hippocampal slices (at 33-34 degrees C). Population spikes and field EPSPs were depressed by 4-5 min bath applications of 50-100 microM CN (IC50 was 18 miroM for spikes and 72 microM for EPSPs). The actions of CN were reversibly suppressed by the adenosine antagonists 8-sulfophenyltheophylline (8-SPT; 10 microM) and 8-cyclopentyl-1,3-dipropylxanthine (DPCPX; 0.2 microM), potentiated by the adenosine transport inhibitor dipyridamole (0.5 microM), but unaffected by the KATP channel blocker glyburide (10 microM). Therefore the CN-induced reductions of synaptic efficacy and postsynaptic excitability-demonstrated by synaptic input:output plots-are mediated mainly by adenosine. In whole-cell or intracellular recordings, CN depressed EPSCs and elicited an increase in input conductance and an outward current, the reversal potential of which was approximately -90 mV (indicating that K+ was the major carrier). These effects also were attenuated by 8-SPT. In the presence of 1 mM Ba, CN had no significant postsynaptic action; Cs (2 mM) also prevented CN-induced outward currents but only partly blocked the increase in conductance. Another 8-SPT-sensitive action of CN was to depress hyperpolarization-activated slow inward relaxations (Q current). At room temperature (22-24 degrees C), although it did not change holding current and slow inward relaxations, CN raised the input conductance; this effect also was prevented by 8-SPT (10 microM), but not by glyburide (10 microM). Adenosine release thus appears to be the major link between acute CN poisoning and early depression of CNS synaptic function.

Potassium conductance causing hyperpolarization of CA1 hippocampal neurons during hypoxia

In experiments on slices (from 100- to 150-g Sprague-Dawley rats) kept at 33 degreesC, we studied the effects of brief hypoxia (2-3 min) on CA1 neurons. In whole cell recordings from submerged slices, with electrodes containing only KMeSO4 and N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid, and in the presence of kynurenate and bicuculline (to minimize transmitter actions), hypoxia produced the following changes: under current clamp, 36 cells were hyperpolarized by 2.7 +/- 0.5 (SE) mV and their input resistance (Rin) fell by 23 +/- 2.7%; in 30 cells under voltage clamp, membrane current increased by 114 +/- 22.3 pA and input conductance (Gin) by 4.9 +/- 0.9 nS. These effects are much greater than those seen previously with K gluconate whole cell electrodes, but only half those seen with "sharp" electrodes. The hypoxic hyperpolarizations (or outward currents) were not reduced by intracellular ATP (1-5 mM) or bath-applied glyburide (10 microM): therefore they are unlikely to be mediated by conventional ATP-sensitive K channels. On the other hand, their depression by internally applied ethylene glycol-bis-(beta-aminoethyl ether)-N,N, N',N'-tetraacetic acid (1.1 and 11 mM) and especially 1, 2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (11-33 mM) indicated a significant involvement of Ca-dependent K (KCa) channels. The beta-adrenergic agonist isoprenaline (10 microM) reduced hypoxic hyperpolarizations and decreases in Rin (n = 4) (and in another 11 cells corresponding changes in Gin); and comparable but more variable effects were produced by internally applied 3':5'-adenosine cyclic monophosphate (cAMP, 1 mM, n = 6) and bath-applied 8-bromo-cAMP (n = 8). Thus afterhyperpolarization-type KCa channels probably take part in the hypoxic response. A major involvement of G proteins is indicated by the near total suppression of the hypoxic response by guanosine 5'-O-(3-thiotriphosphate) (0. 1-0.3 mM, n = 23) and especially guanosine 5'-O-(2-thiodiphosphate) (0.3 mM, n = 26), both applied internally. The adenosine antagonist 8-(p-sulfophenyl)theophylline (10-50 microM) significantly reduced hypoxic hyperpolarizations and outward currents in whole cell recordings (with KMeSO4 electrodes) from submerged slices but not in intracellular recordings (with KCl electrodes) from slices kept at gas/saline interface. In further intracellular recordings, antagonists of gamma-aminobutyric acid-B or serotonin receptors also had no clear effect. In conclusion, these G-protein-dependent hyperpolarizing changes produced in CA1 neurons by hypoxia are probably initiated by Ca2+ release from internal stores stimulated by enhanced glycolysis and a variable synergistic action of adenosine.

Attenuation of hypoxic current by intracellular applications of ATP regenerating agents in hippocampal CA1 neurons of rat brain slices

Hypoxia-induced outward currents (hyperpolarization) were examined in hippocampal CA1 neurons of rat brain slices, using the whole-cell recording technique. Hypoxic episodes were induced by perfusing slices with an artificial cerebrospinal fluid aerated with 5% CO2/95% N2 rather than 5% CO2/95% O2, for about 3 min. The hypoxic current was consistently and reproducibly induced in CA1 neurons dialysed with an ATP-free patch pipette solution. This current manifested as an outward shift in the holding current in association with increased conductance, and it reversed at -78 +/- 2.5 mV, with a linear I-V relation in the range of -100 to -40 mV. To provide extra energy resources to individual neurons recorded, agents were added to the patch pipette solution, including MgATP alone, MgATP + phosphocreatine + creatine kinase, or MgATP + creatine. In CA1 neurons dialysed with patch solutions including these agents, hypoxia produced small outward currents in comparison with those observed in CA1 neurons dialysed with the ATP-free solution. Among the above agents examined, whole-cell dialysis with MgATP + creatine was the most effective at decreasing the hypoxic outward currents. We suggest that the hypoxic hyperpolarization is closely related to energy metabolism in individual CA1 neurons, and that the energy supply provided by phosphocreatine metabolism may play a critical role during transient metabolic stress.

Response of thalamocortical neurons to hypoxia: a whole-cell patch-clamp study

The effect of hypoxia (3-4 min of 95% N2, 5% CO2) on thalamocortical (TC) neurons was investigated using the whole-cell patch-clamp technique in rat dorsal lateral geniculate nucleus slices kept submerged at 32 degreesC. The predominant feature of the response of TC neurons to hypoxia was an increase in input conductance (DeltaGN = 117 +/- 15%, n = 33) that was accompanied by an inward shift in baseline holding current (IBH) at -65 and -57 mV (DeltaIBH = -45 +/- 6 pA, n = 18, and -25 +/- 8 pA, n = 33, respectively) but not at -40 mV. The hypoxia-induced increase in GN (as well as the shift in IBH) was abolished by procedures that are known to block Ih, i.e., bath application of 4-(N-ethyl-N-phenylamino)-1, 2-dimethyl-6-(methylamino)-pyrimidinium chloride (100-300 microM) (DeltaGN = 5 +/- 13%, n = 11) and CsCl (2-3 mM) (DeltaGN = 16 +/- 16%, n = 5), or low [Na+]o (DeltaGN = 10 +/- 10%, n = 5), whereas bath application of BaCl2 (0.1-2.0 mM) had no significant effect (DeltaGN = 128 +/- 14%, n = 8). The hypoxic response was also abolished in low [Ca+2]o (DeltaGN = 25 +/- 16%, DeltaIBH = -6 +/- 8 pA, n = 13), but was unaffected by recording with electrodes containing EGTA (10 mM), BAPTA (10-30 mM), Cs+, or Cl-, as well as in the presence of external tetraethylammonium and 4-aminopyridine. Furthermore, preincubation of the slices with botulinum toxin A (100 nM), which is known to reduce Ca2+-dependent transmitter release, blocked the hypoxic response (DeltaGN = -3 +/- 15%, DeltaIBH = 10 +/- 5 pA, n = 4). We suggest that a positive shift in the voltage-dependence of Ih and a change in its activation kinetics, which transforms it into a fast activating current, may be responsible for the hypoxia-induced changes in GN and IBH, probably via an increase in Ca+2-dependent transmitter release.

Response of thalamocortical neurons to hypoxia: a whole-cell patch-clamp study

The effect of hypoxia (3-4 min of 95% N2, 5% CO2) on thalamocortical (TC) neurons was investigated using the whole-cell patch-clamp technique in rat dorsal lateral geniculate nucleus slices kept submerged at 32 degreesC. The predominant feature of the response of TC neurons to hypoxia was an increase in input conductance (DeltaGN = 117 +/- 15%, n = 33) that was accompanied by an inward shift in baseline holding current (IBH) at -65 and -57 mV (DeltaIBH = -45 +/- 6 pA, n = 18, and -25 +/- 8 pA, n = 33, respectively) but not at -40 mV. The hypoxia-induced increase in GN (as well as the shift in IBH) was abolished by procedures that are known to block Ih, i.e., bath application of 4-(N-ethyl-N-phenylamino)-1, 2-dimethyl-6-(methylamino)-pyrimidinium chloride (100-300 microM) (DeltaGN = 5 +/- 13%, n = 11) and CsCl (2-3 mM) (DeltaGN = 16 +/- 16%, n = 5), or low [Na+]o (DeltaGN = 10 +/- 10%, n = 5), whereas bath application of BaCl2 (0.1-2.0 mM) had no significant effect (DeltaGN = 128 +/- 14%, n = 8). The hypoxic response was also abolished in low [Ca+2]o (DeltaGN = 25 +/- 16%, DeltaIBH = -6 +/- 8 pA, n = 13), but was unaffected by recording with electrodes containing EGTA (10 mM), BAPTA (10-30 mM), Cs+, or Cl-, as well as in the presence of external tetraethylammonium and 4-aminopyridine. Furthermore, preincubation of the slices with botulinum toxin A (100 nM), which is known to reduce Ca2+-dependent transmitter release, blocked the hypoxic response (DeltaGN = -3 +/- 15%, DeltaIBH = 10 +/- 5 pA, n = 4). We suggest that a positive shift in the voltage-dependence of Ih and a change in its activation kinetics, which transforms it into a fast activating current, may be responsible for the hypoxia-induced changes in GN and IBH, probably via an increase in Ca+2-dependent transmitter release.

Hypoxia activates ATP-dependent potassium channels in inspiratory neurones of neonatal mice

1. The respiratory centre of neonatal mice (4 to 12 days old) was isolated in 700 micro(m) thick brainstem slices. Whole-cell K+ currents and single ATP-dependent potassium (KATP) channels were analysed in inspiratory neurones. 2. In cell-attached patches, KATP channels had a conductance of 75 pS and showed inward rectification. Their gating was voltage dependent and channel activity decreased with membrane hyperpolarization. Using Ca2+-containing pipette solutions the measured conductance was lower (50 pS at 1.5 mM Ca2+), indicating tonic inhibition by extracellular Ca2+. 3. KATP channel activity was reversibly potentiated during hypoxia. Maximal effects were attained 3-4 min after oxygen removal from the bath. Hypoxic potentiation of open probability was due to an increase in channel open times and a decrease in channel closed times. 4. In inside-out patches and symmetrical K+ concentrations, channel currents reversed at about 0 mV. Channel activity was blocked by ATP (300-600 microM), glibenclamide (10-70 microM) and tolbutamide (100-300 microM). 5. In the presence of diazoxide (10-60 microM), the activity of KATP channels was increased both in inside-out, outside-out and cell-attached patches. In outside-out patches, that remained within the slice after excision, the activity of KATP channels was enhanced by hypoxia, an effect that could be mediated by a release of endogenous neuromodulators. 6. The whole-cell K+ current (IK) was inactivated at negative membrane potentials, which resembled the voltage dependence of KATP channel gating. After 3-4 min of hypoxia, K+ currents at both hyperpolarizing and depolarizing membrane potentials increased. IK was partially blocked by tolbutamide (100-300 microM) and in its presence, hypoxic potentiation of IK was abolished. 7. We conclude that KATP channels are involved in the hypoxic depression of medullary respiratory activity.

Effects of transient oxygen-glucose deprivation on G-proteins and G-protein-coupled receptors in rat CA3 pyramidal cells in vitro

The role of guanosine triphosphate-binding proteins (G-proteins) in the generation of the outward current during transient oxygen-glucose deprivation (OGD) was investigated in CA3 pyramidal cells in rat hippocampal organotypic slice cultures using the single-electrode voltage-clamp technique with KMeSO4-filled microelectrodes. To simulate ischaemia, brief chemical OGD (2 mM 2-deoxyglucose and 3 mM NaN3 for 4-9 min) was used, which induced an outward K+ current associated with an increase in input conductance. OGD failed to induce the outward current under conditions where G-protein function was disrupted by loading cells with guanosine 5'-O-(2-thiodiphosphate) [GDPbetaS] or after prolonged injection of guanosine 5'-O(3-thiotdphosphate) [GTPgammaS]. However, in slices treated with pertussis toxin (PTX), OGD still elicited the outward current, indicating that PTX-insensitive G-proteins are involved. Consistent with this insensitivity to PTX, neither adenosine receptors nor GABA(B) (gamma-aminobutyric acid) receptors, which operate via PTX-sensitive G-proteins, mediate the OGD-induced outward current. When adenosine receptors or GABA(B) receptors were blocked with 1,3-dipropyl-8-psulphophenylxanthine (DPSPX, 5 microM) or CGP 52 432 (10 microM), respectively, the OGD-induced response was not modified. The response also persisted following pretreatment of slice cultures with tetanus toxin to prevent vesicular release of neurotransmitters and neuromodulators from presynaptic terminals. Both PTX-sensitive and PTX-insensitive G-protein-mediated responses were suppressed during OGD. The inward current induced by the metabotropic glutamate receptor agonist 1 S, 3R-1-aminocyclopentane-1,3-dicarboxylate (1S,3R-ACPD) and the outward current elicited by adenosine or baclofen were strongly or completely attenuated. In contrast, the ionotropic alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) response was not affected. These findings suggest that during OGD there is a functional uncoupling of receptors from G-proteins, and a direct receptor-independent activation of PTX-insensitive G-proteins leading to an increase in membrane K+ conductance.

Changes in [Ca2+]i and membrane currents during impaired mitochondrial metabolism in dissociated rat hippocampal neurons

1. This study was designed to establish the basis for altered membrane excitability during the inhibition of mitochondrial metabolism in central mammalian neurons. Perforated whole-cell patch clamp and fluorimetric techniques were combined to examine changes in membrane currents, intracellular calcium ([Ca2+]i) and mitochondrial potential (DeltaPsim) in neurons dissociated from the CA1 subfield of the hippocampus of young rats. 2. On application of the mitochondrial inhibitor NaCN, or the uncoupler carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP), the membrane potential hyperpolarized and membrane conductance increased. Under voltage clamp, an outward current was seen. The reversal potential of the current at -83 mV and its dependence on extracellular [K+] confirmed that this was a K+ conductance. 3. Simultaneous recordings of [Ca2+]i and current showed a striking correlation between a rise in [Ca2+]i and the developed outward current. Flash photolysis of the caged Ca2+ chelator, diazo-2, reversed both the rise in [Ca2+]i and the outward current. The current was reduced by 80 % by charybdotoxin, was attenuated by 10 mM TEA+ but was unaffected by apamin or by the KATP channel blocker tolbutamide (400 microM-1 mM). These data suggest strongly that the current is carried by Ca2+-dependent K+ channels. 4. Simultaneous recordings of membrane current, DeltaPsim and [Ca2+]i revealed the sequence of events in response to impaired mitochondrial function (CN, FCCP or anoxia): DeltaPsim depolarized, followed rapidly by an increase in [Ca2+]i followed in turn by the outward current. [Ca2+]i and membrane current recovered only after mitochondrial repolarization. 5. The rise in [Ca2+]i appeared to result from an increased Ca2+ influx through voltage-gated Ca2+ channels. It was dependent on extracellular Ca2+ and was much reduced by methoxyverapamil (D600). The rate of Mn2+ quench of fura-2 fluorescence was increased by the inhibitors, and the inhibitors induced a small inward current when K+ channels were blocked that preceded the rise in [Ca2+]i. However, the increase in [Ca2+]i showed no obvious dependence on membrane potential in cells clamped at a range of holding potentials from -90 to -45 mV. 6. Thus, removal of oxygen, uncoupling mitochondrial oxidative phosphorylation or inhibition of respiration, all lead to mitochondrial depolarization, an increased Ca2+ influx through (voltage-gated) channels, even at hyperpolarized membrane potentials, raising [Ca2+]i which in turn drives an increased K+ conductance that modulates membrane excitability.

Activation of Ca(2+)-dependent K+ channels by cyanide in guinea pig adrenal chromaffin cells

The effects of cyanide (CN) on whole cell current measured with the perforated-patch method were studied in adrenal medullary cells. Application of CN produced initially inward and then outward currents at -52 mV or more negative. As the membrane potential was hyperpolarized, amplitude and latency of the outward current (Io) by CN became small and long, respectively. A decrease in the external Na+ concentration did not affect the latency for CN-induced Io but enhanced the amplitude markedly. The CN Io reversed polarity at -85 mV, close to the Nernst potential for K+, and was suppressed by the K+ channel blockers curare and apamin but not by glibenclamide, suggesting that Io is due to the activation of Ca(2+)-dependent K+ channels. Consistent with this notion, the Ca(2+)-mobilizing agents, muscarine and caffeine, also produced Io. Exposure to CN in a Ca(2+)-deficient medium for 4 min abolished caffeine- or muscarine-induced Io without development of Io, and addition of Ca2+ to the CN-containing solution induced Io. We conclude that exposure to CN produces Ca(2+)-dependent K+ currents in an external Ca(2+)-dependent manner, probably via facilitation of Ca2+ influx.

Ion transport and membrane potential in CNS myelinated axons. II. Effects of metabolic inhibition

Compound resting membrane potential was recorded by the grease gap technique (37 degrees C) during glycolytic inhibition and chemical anoxia in myelinated axons of rat optic nerve. The average potential recorded under control conditions (no inhibitors) was -47 +/- 3 (SD) mV and was stable for 2-3 h. Zero glucose (replacement with sucrose) depolarized the nerve in a monotonic fashion to 55 +/- 10% of control after 60 min. In contrast, glycolytic inhibition with deoxyglucose (10 mM, glucose omitted) or iodoacetate (1 mM) evoked a characteristic voltage trajectory consisting of four distinct phases. A distinct early hyperpolarizing response (phase 1) was followed by a rapid depolarization (phase 2). Phase 2 was interrupted by a second late hyperpolarizing response (phase 3), which led to an abrupt reduction in the rate of potential change, causing nerves to then depolarize gradually (phase 4) to 75 +/- 9% and 55 +/- 6% of control after 60 min, in deoxyglucose and iodoacetate, respectively. Pyruvate (10 mM) completely prevented iodoacetate-induced depolarization. Effects of glycolytic inhibitors were delayed by 20-30 min, possibly due to continued, temporary oxidative phosphorylation using alternate substrates through the tricarboxylic acid cycle. Chemical anoxia (CN- 2 mM) immediately depolarized nerves, and phase 1 was never observed. However a small inflection in the voltage trajectory was typical after approximately 10 min. This was followed by a slow depolarization to 34 +/- 4% of control resting potential after 60 min of CN-. Addition of ouabain (1 mM) to CN--treated nerves caused an additional depolarization, indicating a minor glycolytic contribution to the Na+-K+-ATPase, which is fueled preferentially by ATP derived from oxidative phosphorylation. Phases 1 and 3 during iodoacetate exposure were diminished under nominally zero Ca2+ conditions and abolished with the addition of the Ca2+ chelator ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA; 5 mM). Tetraethylammonium chloride (20 mM) also reduced phase 1 and eliminated phase 3. The inflection observed with CN- was eliminated during exposure to zero-Ca2+/EGTA. A Ca2+-activated K+ conductance may be responsible for the observed hyperpolarizing inflections. Block of Na+ channels with tetrodotoxin (TTX; 1 microM) or replacement of Na+ with the impermeant cation choline significantly reduced depolarization during glycolytic inhibition with iodoacetate or chemical anoxia. The potential-sparing effects of TTX were less than those of choline-substituted perfusate, suggesting additional, TTX-insensitive Na+ influx pathways in metabolically compromised axons. The local anesthetics, procaine (1 mM) and QX-314 (300 microM), had similar effects to TTX. Taken together, the rate and extent of depolarization of metabolically compromised axons is dependent on external Na+. The Ca2+-dependent hyperpolarizing phases and reduction in rate of depolarization at later times may reflect intrinsic mechanisms designed to limit axonal injury during anoxia/ischemia.

Effect of metabolic inhibitors on membrane potential and ion conductance of rat astrocytes

1. The aim of this study was to elucidate the effect of metabolic inhibition on the membrane potential and ion conductance of rat astrocytes. The metabolic inhibitors investigated were dinitrophenol (DNP), carbonyl cyanide p-trifluoromethoxyphenyl hydrazone (FCCP), cyanide, and oligomycin. 2. Primary cultures of astroglial cells from newborn rat cerebral cortex were cultivated for 13-20 days on chamber slides. The effect of metabolic inhibitors on the cellular ATP concentration was estimated from the decrease in peak chemiluminescence from the luciferin/luciferase reaction. The membrane potential and ion conductances were measured from whole-cell recordings with the patch-clamp technique. 3. After 2.0 min of incubation ATP decreased from the control level to 43% with cyanide (2 mM), 58% with DNP (1 mM), 47% with FCCP (1 microM), and 69% with oligomycin (10 microM). 4. Under normal conditions V was -74.4 +/- 1.0 mV. DNP and FCCP both caused a rapid and reversible depolarization equivalent to a shift in the I/V curve of 8.2 +/- 1.3 and 19.7 +/- 3.8 mV, respectively. DNP decreased the slope conductance (g) by 22.1% but FCCP had no significant effect on g. In contrast, neither oligomycin nor cyanide had any significant effect on the I/V curve. 5. Tetraethylammonium (TEA; 10 mM) depolarized the cells by 7.1 +/- 2.0 mV but had no significant effect on g. In the presence of TEA, DNP caused a depolarization of 52.8 +/- 3.5 mV and increased g by 45.5 +/- 9.6%. The action of FCCP was not affected by the presence of TEA. 6. Perfusion of the astrocytes with a Cl- free solution inhibited the action of DNP and FCCP. Thus the depolarization was only 4.2 +/- 1.5 mV in DNP and 3.7 +/- 0.3 mV in FCCP, which were significantly smaller effects than in the presence of a high intracellular [Cl-]. 7. Block of tentative KATP channels with tolbutamide (1 mM) or Cl- channels with Zn2+ (1 mM) did not inhibit the depolarization caused by DNP or FCCP. 8. In conclusion, DNP and FCCP have specific effects on the plasmalemma in rat astrocytes which may be due to opening of Cl- channels. This effect was not seen with cyanide or oligomycin and should be considered as a possible complication when DNP and FCCP are used for metabolic inhibition.

Adenosine release mediates cyanide-induced suppression of CA1 neuronal activity

The rapid suppression of CNS function produced by cyanide (CN) was studied by field, intracellular, and whole-cell recording in hippocampal slices (at 33-34 degrees C). Population spikes and field EPSPs were depressed by 4-5 min bath applications of 50-100 microM CN (IC50 was 18 miroM for spikes and 72 microM for EPSPs). The actions of CN were reversibly suppressed by the adenosine antagonists 8-sulfophenyltheophylline (8-SPT; 10 microM) and 8-cyclopentyl-1,3-dipropylxanthine (DPCPX; 0.2 microM), potentiated by the adenosine transport inhibitor dipyridamole (0.5 microM), but unaffected by the KATP channel blocker glyburide (10 microM). Therefore the CN-induced reductions of synaptic efficacy and postsynaptic excitability-demonstrated by synaptic input:output plots-are mediated mainly by adenosine. In whole-cell or intracellular recordings, CN depressed EPSCs and elicited an increase in input conductance and an outward current, the reversal potential of which was approximately -90 mV (indicating that K+ was the major carrier). These effects also were attenuated by 8-SPT. In the presence of 1 mM Ba, CN had no significant postsynaptic action; Cs (2 mM) also prevented CN-induced outward currents but only partly blocked the increase in conductance. Another 8-SPT-sensitive action of CN was to depress hyperpolarization-activated slow inward relaxations (Q current). At room temperature (22-24 degrees C), although it did not change holding current and slow inward relaxations, CN raised the input conductance; this effect also was prevented by 8-SPT (10 microM), but not by glyburide (10 microM). Adenosine release thus appears to be the major link between acute CN poisoning and early depression of CNS synaptic function.

Hypoxia in striatal and cortical neurones: membrane potential and Ca2+ measurements

Simultaneous measurements of membrane potential and intracellular Ca2+ were used to study the effects of hypoxia on striatal and cortical neurones. Striatal neurones responded to hypoxia with a reversible membrane depolarization coupled with a transient increase in intracellular Ca2+. Thirty minutes of hypoxia caused an irreversible membrane depolarization associated with a massive raise in Ca2+ levels, leading to cell death. Conversely, cortical neurones were more resistant to O2 deprivation. Hypoxia (4-10 min) induced minimal changes in both membrane potential and Ca2+ signals. Longer periods (20-30 min) caused an initial membrane hyperpolarization followed by a large but reversible depolarization coupled with a transient increase in Ca2+ signals. These results support the hypothesis of a differential sensitivity of central neurones to hypoxia, suggesting that striatal neurones are more vulnerable than cortical cells.