O2 deprivation in the central nervous system: on mechanisms of neuronal response, differential sensitivity and injury

Kynurenic acid in brain and cerebrospinal fluid of fetal, newborn, and adult sheep and effects of placental embolization

Concentrations of the endogenous glutamate receptor antagonist kynurenic acid (KA) were measured in various brain regions and in cisternal cerebrospinal fluid of fetal, newborn, and adult sheep. KA concentrations were significantly higher in the fetal brain and cerebrospinal fluid at 90 and 140 d gestation compared with postnatal ages. In fetuses of 132-139 d gestation, KA concentrations in cerebrospinal fluid collected by drainage from an indwelling cisternal catheter increased significantly after infusion of the organic acid transport inhibitor probenecid (100 or 200 mg/kg, i.v.) indicating active transport of KA out of the fetal brain. In fetuses in which the umbilical circulation had been chronically restricted from 120 to 140 d gestation by partial embolization of the placenta, plasma concentrations of the KA precursor kynurenine were significantly lower than in control fetuses, and KA concentrations in the hypothalamus and hippocampus were significantly reduced; other brain regions were not affected. These results indicate that the production of KA is higher in the fetal brain compared with the newborn and adult brain. Because KA diminishes the risk of excitotoxic neuronal damage under hypoxic-ischemic conditions, the high levels of KA in the brain before birth may have a neuroprotective function. The decrease of KA concentrations in the hypothalamus and hippocampus after umbilical embolization suggests that, after chronic hypoxia in utero, these regions of the brain may become more vulnerable to subsequent episodes of acute hypoxia or ischemia encountered in late gestation or during parturition.

Neuroprotective effects of riluzole: an electrophysiological and histological analysis in an in vitro model of ischemia

The protective effects of riluzole against the neuronal damage caused by O2 and glucose deprivation (ischemia) was investigated in rat cortical slices by recording electrophysiologically the cortico-cortical field potential and by evaluating histologically the severity of neuronal death. Five minutes of ischemia determined an irreversible depression of the amplitude of the field potential. In addition, this insult caused a clear enhancement of the number of death cells that were specifically colored with trypan blue (a vital colorant which stains altered cells). We found that riluzole, which by itself depressed the synaptic transmission, neuroprotected when perfused 15-20 min before and during ischemia. In fact, due to the treatment with riluzole, the ischemia-induced irreversible depression of the field potential recovered and less cells were stained with trypan blue. These findings demonstrate that riluzole prevents neuronal death in an in vitro model of ischemia and suggest a therapeutic use of this drug in order to reduce the pathophysiological outcomes of stroke.

Early changes in adenosine A1 receptors in cerebral cortex slices submitted to in vitro ischemia

The effects of brain ischemia on the maximum binding capacity (Bmax) and affinity (Kd) of A1 receptors were studied in the rat cerebral cortex, with an in vitro approach. The results were correlated with changes in 3H-adenosine release, studied under identical experimental conditions. Fifteen minutes of in vitro 'ischemia' (hypoxic, glucose-free medium) induced a significant increase in both Bmax (2398+/-132 fmol/mg protein, 151% of the control, P < 0.05) and in Kd (2.43+/-0.12 nM, 161% of the control, P < 0.01). At the same time, an increase in tritium efflux from [3H]-adenosine labeled cerebral cortex slices to 324% of the control was observed. A trend toward normalization was evident 5-15 min after 'reoxygenation' (restoring normal medium), but the binding parameters were still altered after 60 min (Bmax 2110+/-82 fmol/mg protein, Kd 2.26+/-0.14 nM, P < 0.01 vs the corresponding control) as was adenosine release (196% of the control). These findings suggest that the increased availability of adenosine and its receptors may be a defense mechanism against ischemic injury, while the reduced affinity of A1 receptors, possibly due to desensitization, may be a sign of ischemia-induced cellular damage.

Electrophysiological recordings and calcium measurements in striatal large aspiny interneurons in response to combined O2/glucose deprivation

Electrophysiological recordings and calcium measurements in striatal large aspiny interneurons in response to combined O2/glucose deprivation. The effects of combined O2/glucose deprivation were investigated on large aspiny (LA) interneurons recorded from a striatal slice preparation by means of simultaneous electrophysiological and optical recordings. LA interneurons were visually identified and impaled with sharp microelectrodes loaded with the calcium (Ca2+)-sensitive dye bis-fura-2. These cells showed the morphological, electrophysiological, and pharmacological features of large striatal cholinergic interneurons. O2/glucose deprivation induced a membrane hyperpolarization coupled to a concomitant increase in intracellular Ca2+ concentration ([Ca2+]i). Interestingly, this [Ca2+]i elevation was more pronounced in dendritic branches rather than in the somatic region. The O2/glucose-deprivation-induced membrane hyperpolarization reversed its polarity at the potassium (K+) equilibrium potential. Both membrane hyperpolarization and [Ca2+]i rise were unaffected by TTX or by a combination of ionotropic glutamate receptors antagonists, D-2-amino-5-phosphonovaleric acid and 6cyano-7-nitroquinoxaline-2, 3-dione. Sulfonylurea glibenclamide, a blocker of ATP-sensitive K+ channels, markedly reduced the O2/glucose-deprivation-induced membrane hyperpolarization but failed to prevent the rise in [Ca2+]i. Likewise, charybdotoxin, a large K+-channel (BK) inhibitor, abolished the membrane hyperpolarization but did not produce detectable changes of [Ca2+]i elevation. A combination of high-voltage-activated Ca2+ channel blockers significantly reduced both the membrane hyperpolarization and the rise in [Ca2+]i. In a set of experiments performed without dye in the recording electrode, either intracellular bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid or external barium abolished the membrane hyperpolarization induced by O2/glucose deprivation. The hyperpolarizing effect on membrane potential was mimicked by oxotremorine, an M2-like muscarinic receptor agonist, and by baclofen, a GABAB receptor agonist. However, this membrane hyperpolarization was not coupled to an increase but rather to a decrease of the basal [Ca2+]i. Furthermore glibenclamide did not reduce the oxotremorine- and baclofen-induced membrane hyperpolarization. In conclusion, the present results suggest that in striatal LA cells, O2/glucose deprivation activates a membrane hyperpolarization that does not involve ligand-gated K+ conductances but is sensitive to barium, glibenclamide, and charybdotoxin. The increase in [Ca2+]i is partially due to influx through voltage-gated high-voltage-activated Ca2+ channels.

Maturation of the mammalian respiratory system

In this review, the maturational changes occurring in the mammalian respiratory network from fetal to adult ages are analyzed. Most of the data presented were obtained on rodents using in vitro approaches. In gestational day 18 (E18) fetuses, this network functions but is not yet able to sustain a stable respiratory activity, and most of the neonatal modulatory processes are not yet efficient. Respiratory motoneurons undergo relatively little cell death, and even if not yet fully mature at E18, they are capable of firing sustained bursts of potentials. Endogenous serotonin exerts a potent facilitation on the network and appears to be necessary for the respiratory rhythm to be expressed. In E20 fetuses and neonates, the respiratory activity has become quite stable. Inhibitory processes are not yet necessary for respiratory rhythmogenesis, and the rostral ventrolateral medulla (RVLM) contains inspiratory bursting pacemaker neurons that seem to constitute the kernel of the network. The activity of the network depends on CO2 and pH levels, via cholinergic relays, as well as being modulated at both the RVLM and motoneuronal levels by endogenous serotonin, substance P, and catecholamine mechanisms. In adults, the inhibitory processes become more important, but the RVLM is still a crucial area. The neonatal modulatory processes are likely to continue during adulthood, but they are difficult to investigate in vivo. In conclusion, 1) serotonin, which greatly facilitates the activity of the respiratory network at all developmental ages, may at least partly define its maturation; 2) the RVLM bursting pacemaker neurons may be the kernel of the network from E20 to adulthood, but their existence and their role in vivo need to be further confirmed in both neonatal and adult mammals.

The reversible change of GluR2 RNA editing in gerbil hippocampus in course of ischemic tolerance

The ischemic tolerance is known to show protective effects on the neurons and the restricted Ca2+ influx through Ca2+ channels might be involved. In alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor, ribonucleic acid (RNA) editing of the GluR2 subunit determines receptor desensitization and Ca2+ permeability. The authors investigated the effect of ischemic tolerance on the messenger RNA editing of Q/R and R/G sites of GluR2 subunit in hippocampus. It was found that the rate of RNA editing in Q/R site showed no change (100% edited), whereas that in R/G site decreased significantly (83.3% normal editing level to 60.4%) at day 3 (preconditioning period) and returned to normal level at day 14 (after preconditioning period). Further investigation revealed that the decrease of editing rate in ischemic tolerance resulted mainly from the decrease of editing in CA1 area.

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.

Beta-alanine release from the adult and developing hippocampus is enhanced by ionotropic glutamate receptor agonists and cell-damaging conditions

The release of the inhibitory amino acid beta-alanine was investigated in hippocampal slices from adult (3-month-old) and developing (7-day-old) mice, using a superfusion system. The release was enhanced by beta-alanine itself and the structural analogs taurine and y-aminobutyrate. It was dependent on Na+, but independent of Ca2+ in both mature and immature hippocampus, being thus mostly mediated by uptake carriers operating in an outward direction. The release was potentiated in the developing mice, but not affected in the adults, by the ionotropic glutamate receptor agonists N-methyl-D-aspartate, kainate, 2-amino-3-hydroxy-5-methyl-4-isoxazolepropionate and tetrazolylglycine in a receptor-mediated manner. Cell-damaging conditions, including hypoxia, hypoglycemia, ischemia, oxidative stress and the presence of free radicals, greatly enhanced beta-alanine release at both ages, but more markedly in the adults. The great amounts of beta-alanine, together with the inhibitory amino acids taurine and gamma-aminobutyrate, released simultaneously with the excitatory amino acids in the hippocampus may constitute an important protective mechanism against excitotoxicity, which leads to neuronal death.

Neurotransmitters and neuromodulators controlling the hypoxic respiratory response in anaesthetized cats

1. The contributions of neurotransmitters and neuromodulators to the responses of the respiratory network to acute hypoxia were analysed in anaesthetized cats. 2. Samples of extracellular fluid were collected at 1-1.5 min time intervals by microdialysis in the medullary region of ventral respiratory group neurones and analysed for their content of glutamate, gamma-aminobutyric acid (GABA), serotonin and adenosine by high performance liquid chromatography. Phrenic nerve activity was correlated with these measurements. 3. Levels of glutamate and GABA increased transiently during early periods of hypoxia, coinciding with augmented phrenic nerve activity and then fell below control during central apnoea. Serotonin and adenosine increased slowly and steadily with onset of hypoxic depression of phrenic nerve activity. 4. The possibility that serotonin contributes to hypoxic respiratory depression was tested by microinjecting the 5-HT-1A receptor agonist 8-OH-DPAT into the medullary region that is important for rhythmogenesis. Hypoxic activation of respiratory neurones and phrenic nerve activity were suppressed. Microinjections of NAN-190, a 5-HT-1A receptor blocker, enhanced hypoxic augmentation resulting in apneustic prolongation of inspiratory bursts. 5. The results reveal a temporal sequence in the release of neurotransmitters and neuromodulators and suggest a specific role for each of them in the sequential development of hypoxic respiratory disturbances.

Gene regulation by O2 deprivation: an anoxia-regulated novel gene in Drosophila melanogaster

Organisms, across the animal kingdom, vary in their tolerance or susceptibility to cell injury from O2 deprivation. In this study we have taken advantage of the genetically well studied fruit fly to dissect basic mechanisms underlying their ability to tolerate lack of O2. Using differential display and molecular techniques, we cloned and characterized a novel gene, named fau, which is up-regulated considerably following anoxia in Drosophila melanogaster. Northern blot analysis revealed that the transcript of this gene is approximately 0.9 kb in length with an open reading frame encoding a small hydrophilic protein ( approximately 14.4 kDa). This protein has no homology to previously described gene products but has many potential phosphorylation sites. In situ hybridization showed that this gene is located in region 7C-D on the Drosophila X-chromosome and its transcript concentrated in the lamina and cortical neurons of the Drosophila central nervous system (CNS). Transgenic flies showed that over-expression of fau significantly reduced the recovery time of the flies from anoxia. We conclude that (1) this study provided a framework on which the mechanisms underlying anoxia tolerance can be dissected in the fruit fly and (2) fau gene plays an important role in the regulation of tissue responsiveness to O2 deprivation.

Ionic mechanisms underlying depolarizing responses of an identified insect motor neuron to short periods of hypoxia

Hypoxia can dramatically disrupt neural processing because energy-dependent homeostatic mechanisms are necessary to support normal neuronal function. In a human context, the long-term effects of such disruption may become all too apparent after a "stroke," in which blood-flow to part of the brain is compromised. We used an insect preparation to investigate the effects of hypoxia on neuron membrane properties. The preparation is particularly suitable for such studies because insects respond rapidly to hypoxia, but can recover when they are restored to normoxic conditions, whereas many of their neurons are large, identifiable, and robust. Experiments were performed on the "fast" coxal depressor motoneuron (Df) of cockroach (Periplaneta americana). Five-minute periods of hypoxia caused reversible multiphasic depolarizations (10-25 mV; n = 88), consisting of an initial transient depolarization followed by a partial repolarization and then a slower phase of further depolarization. During the initial depolarizing phase, spontaneous plateau potentials normally occurred, and inhibitory postsynaptic potential frequency increased considerably; 2-3 min after the onset of hypoxia all electrical activity ceased and membrane resistance was depressed. On reoxygenation, the membrane potential began to repolarize almost immediately, becoming briefly more negative than the normal resting potential. All phases of the hypoxia response declined with repeated periods of hypoxia. Blockade of ATP-dependent Na/K pump by 30 microM ouabain suppressed only the initial transient depolarization and the reoxygenation-induced hyperpolarization. Reduction of aerobic metabolism between hypoxic periods (produced by bubbling air through the chamber instead of oxygen) had a similar effect to that of ouabain. Although the depolarization seen during hypoxia was not reduced by tetrodotoxin (TTX; 2 microM), lowering extracellular Na+ concentration or addition of 500 microM Cd2+ greatly reduced all phases of the hypoxia-induced response, suggesting that Na influx occurs through a TTX-insensitive Cd2+-sensitive channel. Exposure to 20 mM tetraethylammonium and 1 mM 3,4-diaminopyridine increased the amplitude of the hypoxia-induced depolarization, suggesting that activation of K channels may normally limit the amplitude of the hypoxia response. In conclusion we suggest that the slow hypoxia-induced depolarization on motoneuron Df is mainly carried by a TTX-resistant, Cd2+-sensitive sodium influx. Ca2+ entry may also make a direct or indirect contribution to the hypoxia response. The fast transient depolarization appears to result from block of the Na/K pump, whereas the reoxygenation-induced hyperpolarization is largely caused by its subsequent reactivation.

Sodium influx plays a major role in the membrane depolarization induced by oxygen and glucose deprivation in rat striatal spiny neurons

BACKGROUND AND PURPOSE

Striatal spiny neurons are selectively vulnerable to ischemia, but the ionic mechanisms underlying this selective vulnerability are unclear. Although a possible involvement of sodium and calcium ions has been postulated in the ischemia-induced damage of rat striatal neurons, the ischemia-induced ionic changes have never been analyzed in this neuronal subtype.

METHODS

We studied the effects of in vitro ischemia (oxygen and glucose deprivation) at the cellular level using intracellular recordings and microfluorometric measurements in a slice preparation. We also used various channel blockers and pharmacological compounds to characterize the ischemia-induced ionic conductances.

RESULTS

Spiny neurons responded to ischemia with a membrane depolarization/inward current that reversed at approximately -40 mV. This event was coupled with an increased membrane conductance. The simultaneous analysis of membrane potential changes and of variations in [Na+]i and [Ca2+]i levels showed that the ischemia-induced membrane depolarization was associated with an increase of [Na+]i and [Ca2+]i. The ischemia-induced membrane depolarization was not affected by tetrodotoxin or by glutamate receptor antagonists. Neither intracellular BAPTA, a Ca2+ chelator, nor incubation of the slices in low-Ca2+-containing solutions affected the ischemia-induced depolarization, whereas it was reduced by lowering the external Na+ concentration. High doses of blockers of ATP-dependent K+ channels increased the membrane depolarization observed in spiny neurons during ischemia.

CONCLUSIONS

Our findings show that, although the ischemia-induced membrane depolarization is coupled with a rise of [Na+]i and [Ca2+]i, only the Na+ influx plays a prominent role in this early electrophysiological event, whereas the increase of [Ca2+]i might be relevant for the delayed neuronal death. We also suggest that the activation of ATP-dependent K+ channels might counteract the ischemia-induced membrane depolarization.

Contribution of GABA(A)-mediated conductances to anoxia-induced depolarization

CA1 pyramids were studied intracellularly in rat hippocampal slices to establish the contribution of excitatory amino acid (EAA) and GABA(A) receptors to the depolarizations induced by brief (< 10 min) anoxic episodes. An increase of the amplitude of the depolarizations evoked by successive anoxic episodes occurred with KCl (n=4 cells), not with K-acetate-filled (n=3) recording electrodes. Moreover, with K-acetate-filled electrodes the anoxic depolarization amplitude was reduced, but not abolished by EAA receptor antagonists (n=14). The residual anoxic depolarizations were blocked by a GABA(A) receptor antagonist (n=5) and decreased by the carbonic anhydrase inhibitor acetazolamide (n=4). We conclude that the anoxic depolarizations generated by CA1 pyramids are caused by the activation of EAA along with GABA(A) receptors leading to an increased membrane conductance to both Cl- and HCO3-.

Association of chronic sublethal hypoxia with ventriculomegaly in the developing rat brain

Bronchopulmonary dysplasia remains a major cause of neurodevelopmental handicap in preterm infants. Because bronchopulmonary dysplasia may be associated with prolonged hypoxemia without obvious changes in systemic blood pressure, we developed an animal model of chronic sublethal hypoxia to test the hypothesis that this insult results in significant alterations in corticogenesis in the developing brain. Three groups of newborn rats were placed in a chamber with FIO2 9.5% on postnatal day 3 (P3). One group was sacrificed at P13; a second group was sacrificed at P33, and the third group was removed at P33 and reared in normoxia until sacrifice at P63. Control rats were those raised in room air for the corresponding periods of time. Rats were transcardially perfused and the brains were embedded in celloidin and prepared for morphometric analysis using standard stereology methods. Although experimental rat pups in the third group demonstrated 'catch-up' of body weight following return to normoxia, these studies demonstrated both failure of brain growth (p<0.01) and progressive cerebral ventriculomegaly (p<0.01). Decreased subcortical white matter (p<0. 05) and corpus callosum size (p<0.01) were noted at P63 in pups reared under conditions of chronic hypoxia. Decreases in cortical volume (p<0.05) were noted at all three experimental time points for hypoxic-reared pups when compared to control animals. These data suggest that chronic sublethal hypoxia may lead to severe impairments in corticogenesis in an animal model of developing brain.

Hydroxyl radical production in the substantia nigra after 6-hydroxydopamine and hypoxia-reoxygenation

To study the involvement of oxidative stress in 6-OHDA neurotoxicity, we investigated the production of the hydroxyl free radical (OH.) in the substantia nigra (SN) and the striatum (CS) several moments after intranigral injection of the neurotoxin, with or without an added episode of hypoxia (30 min, 95% N2, 5% O2). We utilized the hydroxylation of salicylate to 2,3 dihydroxybenzoic acid (2,3 DHBA) as indication of OH. production. When 2.3 DHBA levels were not modified, the levels of 2,5 DHBA were taken as an indication of cytochrome P450 (CYP 450) metabolism. 6-OHDA alone did not increase the production of 2,3 DHBA in the SN. 2,5 DHBA increased significantly after 120 min and was high up to 24 h. An episode of hypoxia (60 min after 6-OHDA injection) significantly worsened the decrease of dopamine (DA) in the striatum assessed 8 days after injection of 6-OHDA in the SN. Hypoxia performed 60 min and 24 h before or 24 h after 6-OHDA did not show any additional effect on striatal DA levels. Contrary to results obtained after 6-OHDA alone, 2,3 DHBA increased significantly 120 min after the injection, when the hypoxia-reoxygenation was added to the 6-OHDA treatment. Our data are showing a relationship between the increase in OH. production and a concomitant worsening of neuronal degeneration. As a whole, the results support the idea that neurons undergoing 6-OHDA neurotoxicity have their antioxidant defences affected and that oxidative stress is actually an important eliciting factor in 6-OHDA dependant neurodegeneration. However, OH. may not be the main radical species involved in this process. Additionally, 6-OHDA also appeared to provoke a long-term increase in CYP 450 activity.

Inhibition of major cationic inward currents prevents spreading depression-like hypoxic depolarization in rat hippocampal tissue slices

Hypoxia-induced spreading depression-like depolarization (hypoxic SD, or anoxic depolarization) is accompanied by the near-loss of membrane potential, severe reduction of membrane resistance, and influx of Na+, Ca2+, Cl- and water into neurons. The biophysical nature of these membrane changes is incompletely understood. In the present study we applied a pharmacological mixture (10 microM DNQX, 10 microM CPP, 1 microM TTX, and 2 mM Ni2+) to rat hippocampal tissue slices to inhibit major Na+ and Ca2+ inward currents. This inhibitory cocktail slightly depolarized CA1 pyramidal neurons and completely blocked all evoked potentials. In its presence severe hypoxia of up to 20 min duration failed to induce hypoxic SD and the accompanying intrinsic optical signal. Instead, only moderate, very slow negative shifts of the extracellular DC potential were observed. Following 10 min hypoxia and 1 hour wash-out of the inhibitors antidromic and orthodromic responses were still blocked but hypoxic SD with markedly delayed onset could be induced in most slices. In current-clamped CA1 pyramidal cells hypoxia induced a rapid, near-complete depolarization and decreased the input resistance by 89%. In the presence of the cocktail, however, hypoxia caused a gradual, partial depolarization, to about -40 mV; the membrane resistance decreased by only 37%. We conclude that simultaneous blockade of the known major Na+ and Ca2+ channels consistently prevents hypoxic SD. The hypothesis that SD initiation is the consequence of general loss of selective permeability or general membrane breakdown becomes unlikely. Instead, influx of Na+ and Ca2+ might play a crucial role in the generation of the rapid SD-like depolarization.

Spreading depression determines acute cellular damage in the hippocampal slice during oxygen/glucose deprivation

During ischaemia neurons depolarize and release the neurotransmitter L-glutamate, which accumulates extracellularly and binds to postsynaptic receptors. This initiates a sequence of events thought to culminate in immediate and delayed neuronal death. However, there is growing evidence that during ischaemia the development of spreading depression (SD) can be an important determinant of the degree and extent of ischaemic damage. In contrast, SD without metabolic compromise (as occurs in migraine aura) causes no discernible damage to brain tissue. SD is a profound depolarization of neurons and glia that propagates like a wave across brain tissue. Brain cell swelling, an early event of both the excitotoxic process and of SD, can be assessed by imaging associated intrinsic optical signals (IOSs). We demonstrate here that IOS imaging clearly demarcates the ignition site and migration of SD across the submerged hippocampal slice of the rat. If SD is induced by elevating [K+]O, the tissue fully recovers, but in slices that are metabolically compromised at 37.5 degrees C by oxygen/glucose deprivation (OGD) or by ouabain exposure, cellular damage develops only where SD has propagated. Specifically, the evoked CA1 field potential is permanently lost, the cell bodies of involved neurons swell and their dendritic regions increase in opacity. In contrast to OGD, bath application of L-glutamate (6-10 mM) at 37.5 degrees C evokes a non-propagating LT increase in CA1 that reverses without obvious cellular damage. Moreover, application of 2-20 mM glutamate or various glutamate agonists fail to evoke SD in the submerged hippocampal slice. We propose that SD and OGD together (but not alone) constitute a 'one-two punch', causing acute neuronal death in the slice that is not replicated by elevated glutamate. These findings support the proposal that SD generation during stroke promotes and extends acute ischaemic damage.

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.

Intracellular Ca2+ during metabolic activation of KATP channels in spontaneously active dorsal vagal neurons in medullary slices

Intracellular Ca2+ ([Ca2+]i) and membrane properties were measured in fura-2 dialysed dorsal vagal neurons (DVN) spontaneously active at a frequency of 0.5-5 Hz. [Ca2+]i increased by about 30 nm upon rising spike frequency by more than 200% due to 20-50 pA current pulses or 10 micrometer serotonin. It fell by 30 nm upon block of spiking by current-injection, tetrodotoxin or Ni2+ and also during hyperpolarization due to gamma-aminobutyric acid or opening of adenosine triphosphate (ATP) -sensitive K+ (KATP) channels with diazoxide. KATP channel-mediated hyperpolarizations during anoxia or cyanide produced an initial [Ca2+]i decrease which reversed into a secondary Ca2+ rise by less than 100 nm. Similar moderate rises of [Ca2+]i were observed during block of aerobic metabolism under voltage-clamp as well as in intact cells, loaded with fura-2 AM. The magnitude of the metabolism-related [Ca2+]i transients did not correlate with the amplitude of the KATP channel-mediated outward current. [Ca2+]i did not change during diazoxide-induced or spontaneous activation of KATP outward current observed in 10% of cells after establishing whole-cell recording. Increasing [Ca2+]i with cyclopiazonic acid did not activate KATP channels. [Ca2+]i was not affected upon block of outward current with sulphonylureas, but these KATP channel blockers were effective to reverse inhibition of spike discharge and, thus, the initial [Ca2+]i fall upon spontaneous or diazoxide-, anoxia- and cyanide-induced KATP channel activation. A sulphonylurea-sensitive hyperpolarization and [Ca2+]i fall was also revealed in the early phase of iodoacetate-induced metabolic arrest, whereas after about 20 min, occurrence of a progressive depolarization led to an irreversible rise of [Ca2+]i to more than 1 micrometer. The results indicate that KATP channel activity in DVN is not affected by physiological changes of intracellular Ca2+ and the lack of a major perturbance of Ca2+ homeostasis contributes to their high tolerance to anoxia.

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.

Chemoreflex thresholds to CO2 in decerebrate cats

We used a modified rebreathing technique to measure chemoreflex thresholds to CO2 in decerebrate, paralyzed and ventilated cats. Cats were hyperventilated to neural apnea (PaCO2 < 15 mmHg) with one ventilator and then switched to a rebreathing circuit consisting of a balloon inside a bottle connected to a second ventilator. The volume of the circuit was approximately 110 ml. The balloon contained 5% CO2:95% O2 for hyperoxic rebreathing or approximately 5% CO2 with 11 or 6.5% O2 for moderately and severely hypoxic rebreathing. A plateau in CO2 concentration at the onset of rebreathing indicated equilibration of CO2 between the circuit, alveolar gas and venous and arterial blood. After rapid equilibration of CO2 between the cat and the circuit, CO2 increased linearly with time during rebreathing. Under hyperoxic conditions, phrenic activity began to increase at an end-tidal P(CO2) (PET(CO2)) of 35.1 +/- 6.1 (SD) mmHg (n = 8); during hypoxia, phrenic activity began to increase at a significantly lower PET(CO2) of 27.8 +/- 4.8 mmHg (P < 0.01, n = 6). We interpret these values as the central and peripheral chemoreflex thresholds to CO2, respectively. Persistent phrenic activity prevented determination of a threshold during severe hypoxic rebreathing. Our modified method of hyperoxic and hypoxic rebreathing allows detection of the effects of hypoxia on the central and peripheral chemoreflex thresholds and, within a cat, measurements of chemoreflex sensitivities.

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.

Norepinephrine efflux evoked by potassium chloride in cat sympathetic nerves: dual mechanism of action

Using a dialysis technique, prominent efflux of norepinephrine (NE) from cardiac sympathetic nerve endings was observed under local administration of potassium chloride (KCl, 100 mM). KCl induced NE efflux was suppressed by omega-conotoxin GVIA or desipramine but residual efflux of NE was still detectable. In the presence of omega-conotoxin GVIA, KCl induced efflux of NE was augmented by pretreatment with reserpine, indicating that this efflux of NE was derived from axoplasma with neurotransporter. These data suggest that a KCl induced brisk increase in dialysate NE levels might occur as a consequence of exocytotic NE release and carrier mediated outward NE transport from nerve endings.

Hypoxic and hypoglycaemic changes of intracellular pH in cerebral cortical pyramidal neurones

Intracellular pH and membrane potential were measured during hypoxia and/or hypoglycaemia in cortical pyramidal neurones of a rat cortical slice preparation. Intracellular pH (pHi) was calculated from ratiometric microfluorometry of the pH-sensitive dye BCECF injected via sharp recording microelectrodes into the neurones. Transient (5 min) hypoxia induced a fall of pHi (7.12 +/- 0.03) of -0.72 +/- 0.11 pH units while transient (10 min) hypoglycaemia induced an increase of 0.37 +/- 0.09 pH units. Hypoglycaemia did not prevent the hypoxic acidification. Lowering extracellular Na+ induced a membrane hyperpolarization and alkalinization by 0.29 +/- 0.12 pH units but did not affect the development or recovery of the hypoxic acidification. The alkalinization during hypoglycaemia suggested that there is some anaerobic glycolysis under normoglycemic conditions. The hypoxic acidification, however, is unlikely to result from anaerobic glycolysis or reversal of Na(+)-dependent H+ extrusion.

Acute neural damage in the rat neocortex in vitro induced by a combination of anoxia and mechanical stress

To elucidate the mechanisms of neural damage after brain ischemia, rat neocortical slices were exposed to anoxia at room temperature for 1 h, and other slices were prepared from the neocortical blocks exposed to anoxia at room temperature for 1 h. Field potentials elicited by the stimulation of layer IV were recorded in supragranular layers in these slices. No clear damage was observed electrophysiologically or morphologically in these slices. In contrast, a complete loss of the trans-synaptic field potentials and a decrease in the density of the cells stained with Neutral Red were elicited by injecting an anoxic medium into the neocortical blocks at room temperature for 1 h. In the slice preparations, the injection of the anoxic medium failed to reproduce clear neural damage, while a combination of mechanical stress and anoxia elicited a complete loss of trans-synaptic potentials; this was alleviated by Gd3+ (50 microM) and D(-)-2-amino-5-phosphonovaleric acid (100 microM). These results indicate that a combination of mechanical stress and anoxia produces acute and severe neural damage even at room temperature in vitro. The mechanism of the damage and the relationship between the neural damage in vitro and in vivo are discussed.

Upregulation of L-type Ca2+ channels in reactive astrocytes after brain injury, hypomyelination, and ischemia

Anti-peptide antibodies that specifically recognize the alpha1 subunit of class A-D voltage-gated Ca2+ channels and a monoclonal antibody (MANC-1) to the alpha2 subunit of L-type Ca2+ channels were used to investigate the distribution of these Ca2+ channel subtypes in neurons and glia in models of brain injury, including kainic acid-induced epilepsy in the hippocampus, mechanical and thermal lesions in the forebrain, hypomyelination in white matter, and ischemia. Immunostaining of the alpha2 subunit of L-type Ca2+ channels by the MANC-1 antibody was increased in reactive astrocytes in each of these forms of brain injury. The alpha1C subunits of class C L-type Ca2+ channels were upregulated in reactive astrocytes located in the affected regions in each of these models of brain injury, although staining for the alpha1 subunits of class D L-type, class A P/Q-type, and class B N-type Ca2+ channels did not change from patterns normally observed in control animals. In all of these models of brain injury, there was no apparent redistribution or upregulation of the voltage-gated Ca2+ channels in neurons. The upregulation of L-type Ca2+ channels in reactive astrocytes may contribute to the maintenance of ionic homeostasis in injured brain regions, enhance the release of neurotrophic agents to promote neuronal survival and differentiation, and/or enhance signaling in astrocytic networks in response to injury.

Release of endogenous glutamate, aspartate, GABA, and taurine from hippocampal slices from adult and developing mice under cell-damaging conditions

The releases of endogenous glutamate, aspartate, GABA and taurine from hippocampal slices from 7-day-, 3-, 12-, and 18-month-old mice were investigated under cell-damaging conditions using a superfusion system. The slices were superfused under hypoxic conditions in the presence and absence of glucose and exposed to hydrogen peroxide. In the adult hippocampus under normal conditions the basal release of taurine was highest, with a response only about 2-fold to potassium stimulation (50 mM). The low basal releases of glutamate, aspartate, and GABA were markedly potentiated by K+ ions. In general, the release of the four amino acids was enhanced under all above cell-damaging conditions. In hypoxia and ischemia (i.e., hypoxia in the absence of glucose) the release of glutamate, aspartate and GABA increased relatively more than that of taurine, and membrane depolarization by K+ markedly potentiated the release processes. Taurine release was doubled in hypoxia and tripled in ischemia but K+ stimulation was abolished. In both the mature and immature hippocampus the release of glutamate and aspartate was greatly enhanced in the presence of H2O2, that of aspartate particularly in developing mice. In the immature hippocampus the increase in taurine release was 10-fold in hypoxia and 30-fold in ischemia, and potassium stimulation was partly preserved. The release processes of the four amino acids in ischemia were all partially Ca2+-dependent. High concentrations of excitatory amino acids released under cell-damaging conditions are neurotoxic and contribute to neuronal death during ischemia. The substantial amounts of the inhibitory amino acids GABA and taurine released simultaneously may constitute an important protective mechanism against excitatory amino acids in excess, counteracting their harmful effects. In the immature hippocampus in particular, the massive release of taurine under cell-damaging conditions may have a significant function in protecting neural cells and aiding in preserving their viability.

Hypoxia-tolerant neonatal CA1 neurons: relationship of survival to evoked glutamate release and glutamate receptor-mediated calcium changes in hippocampal slices

Neurons in the neonatal mammalian brain survive greater degrees of hypoxic stress than those in the mature brain. To investigate how developmental changes in glutamate receptor-mediated neurotoxicity contribute to this difference, we measured hypoxia-evoked glutamate release, glutamate receptor contribution to hypoxia-evoked intracellular calcium changes, and survival of hypoxia-/ischemia-sensitive CA1 neurons in rat hippocampus. Glutamate release was measured by a fluorescence assay, calcium changes in CA1 neurons with fura-2, and cell viability using Nissl and fluorescence staining with calcein-AM/ethidium homodimer, all in 300-micron thick hippocampal slices from 3-30 post-natal day (PND) rats. Glutamate released from PND 3-7 slices during hypoxia (PO2 = 5 mmHg) was only one third that of PND 18-22 slices. In PND 3-7 slices, survival of CA1 neurons after 5 min of hypoxia and 6 h of recovery was significantly greater than in PND 18-22 slices (viability indices 0.60 and 0.28, respectively, (p < 0.05). Five min of anoxia significantly altered Nissl staining pattern and morphology of CA1 neurons in PND 18-22 but not PND 3-7 slices. Hypoxia (PO2 = 5 mm Hg) caused three to five times greater increases in [Ca2+]i in PND 18-22 slices than in PND 3-7 slices (p < 0.001). During re-oxygenation, [Ca2+]i returned to baseline in PND 3-7 slices, but remained elevated in PND 18-22 slices. Glutamate receptor-mediated calcium changes in CA1 during hypoxia were 33% and 62% of the total calcium change in PND 3-7 and PND 18-22 CA1, respectively. We conclude that survival of CA1 neurons in PND 3-7 slices following hypoxic stress is associated with smaller increases and enhanced recovery of [Ca2+]i, less accumulation of glutamate, and less glutamate receptor-mediated calcium influx than in PND 18-22 slices.

Differential effects of anoxia and glutamate on cultured neocortical neurons

That glutamate increases in the extracellular space of the brain during hypoxia or ischemia and that this amino acid, in high enough concentrations, kills neurons has led investigators to use glutamate and study the mechanisms underlying neuronal excitotoxicity as a model for acute cell death that occurs with low oxygen. However, there is some evidence that increased glutamate, on the one hand, and anoxia, on the other, may not be similar events. In this study we undertook experiments to determine whether glutamate, at various concentrations (20-500 microM), and anoxia induce similar changes in intracellular Ca2+ and in cell morphology as assessed by cell volume and eccentricity (degree of some ellipsoid shape). We found that glutamate was much more rapid in inducing a rise in Cai2+ and that the rise itself occurred at a faster rate than during anoxia. Anoxia produced more marked changes in cell volume and eccentricity. These results, which show major differences between glutamate and anoxia, indicate that while glutamate may play an important role in anoxic brain injury, glutamate excitotoxicity should not be used to mimic the effects of anoxia on nerve and brain function.

Preconditioning in vivo ischemia inhibits anoxic long-term potentiation and functionally protects CA1 neurons in the gerbil

Preconditioning with sublethal ischemia induces tolerance to subsequent lethal ischemia in neurons. We investigated electrophysiologic aspects of the ischemic tolerance phenomenon in the gerbil hippocampus. Gerbils were subjected to 2 minutes of forebrain ischemia (preconditioning ischemia). Some of them were subjected to a subsequent 5 minutes of forebrain ischemia 2 to 3 days after the preconditioning ischemia (double ischemia). Hippocampal slices were prepared from these gerbils subjected to the preconditioning or double ischemia, and field excitatory postsynaptic potentials were recorded from CA1 pyramidal neurons. Capacity for long-term potentiation triggered by tetanic stimulation (tetanic LTP) was transiently inhibited 1 to 2 days after the double ischemia but then recovered. Latency of anoxic depolarization was not significantly different between slices from preconditioned gerbils and those from sham-operated gerbils when these slices were subjected to in vitro anoxia. Postanoxic potentiation of N-methyl-D-aspartate (NMDA) receptor-mediated transmission (anoxic LTP) was inhibited in slices from gerbils 2 to 3 days after the preconditioning ischemia, whereas it was observed in slices from sham-operated gerbils and gerbils 9 days after the preconditioning ischemia. These results suggest that protection by induced tolerance is (1) not only morphologic but also functional, and (2) expressed in inhibiting postischemic overactivation of NMDA receptor-mediated synaptic responses.

Effects of K+ channel blockers on the anoxic response of CNS myelinated axons

Compound action potentials (CAPs) from adult rat optic nerves were recorded in vitro. The area under the CAP was compared before and after 1 h anoxia/reoxygenation. Resting compound membrane potential was measured using the cold grease-gap technique. The acute reduction of CAP magnitude by anoxia was unaffected by TEA (20 mM), 4-AP (300 microM), or the KATP blockers glibenclamide (300 microM) and tolbutamide (2 mM). Neither these K+ channel blockers, nor glipizide (100 microM) or the KATP activator diazoxide (500 microM) altered post-anoxic CAP area recovery. In contrast, although Cs+ (5 mM) accelerated anoxic CAP failure and membrane depolarization, this cation significantly increased CAP area recovery post-anoxia from 22+/-10% (s.d.) to 60+/-22% (p < 0.0001). The unique effects of Cs+ suggest that inward rectifier channels may play an important role in the induction of anoxic injury in optic nerve axons.

Role of the pons in hypoxic respiratory depression in the neonatal rat

The main purpose of this study was to evaluate the role of the pons in hypoxic respiratory depression (HRD) of the neonatal rat. Experiments were conducted using the isolated brainstem-spinal cord preparation of the neonatal rat (1-3 days old). The brainstem was transected at various levels. We found that ablation of the diencephalon decreased respiratory frequency (fR), and conversely, that ablation of the midbrain or pons increased fR. In the preparation with the pons intact (without the midbrain), hypoxia (superfusate PO2 = 56 mmHg) caused strong depression of respiratory activity, which was characterized by a steady decrease in fR and in integrated inspiratory burst amplitude (integral of Phr). In the preparation with the intact ventral pons (without midbrain and dorsal pons) we observed similar, though weaker, HRD. When the entire pons was ablated, integral of Phr was little depressed by hypoxia and thus, HRD was further attenuated. We conclude that the pons contributes importantly to the induction of hypoxic respiratory depression in the neonatal rat. Both the ventral and dorsal portions of the pons are involved in the control of hypoxic respiratory depression. In addition, we show that the respiratory modulatory functions of the diencephalon (facilitating) and midbrain (inhibitory) are already expressed at the time of birth.

Anoxic and ischemic injury of myelinated axons in CNS white matter: from mechanistic concepts to therapeutics

White matter of the brain and spinal cord is susceptible to anoxia and ischemia. Irreversible injury to this tissue can have serious consequences for the overall function of the CNS through disruption of signal transmission. Myelinated axons of the CNS are critically dependent on a continuous supply of energy largely generated through oxidative phosphorylation. Anoxia and ischemia cause rapid energy depletion, failure of the Na(+)-K(+)-ATPase, and accumulation of axoplasmic Na+ through noninactivating Na+ channels, with concentrations approaching 100 mmol/L after 60 minutes of anoxia. Coupled with severe K+ depletion that results in large membrane depolarization, high [Na+]i stimulates reverse Na(+)-Ca2+ exchange and axonal Ca2+ overload. A component of Ca2+ entry occurs directly through Na+ channels. The excessive accumulation of Ca2+ in turn activates various Ca(2+)-dependent enzymes, such as calpain, phospholipases, and protein kinase C, resulting in irreversible injury. The latter enzyme may be involved in "autoprotection," triggered by release of endogenous gamma-aminobutyric acid and adenosine, by modulation of certain elements responsible for deregulation of ion homeostasis. Glycolytic block, in contrast to anoxia alone, appears to preferentially mobilize internal Ca2+ stores; as control of internal Ca2+ pools is lost, excessive release from this compartment may itself contribute to axonal damage. Reoxygenation paradoxically accelerates injury in many axons, possibly as a result of severe mitochondrial Ca2+ overload leading to a secondary failure of respiration. Although glia are relatively resistant to anoxia, oligodendrocytes and the myelin sheath may be damaged by glutamate released by reverse Na(+)-glutamate transport. Use-dependent Na+ channel blockers, particularly charged compounds such as QX-314, are highly neuroprotective in vitro, but only agents that exist partially in a neutral form, such as mexiletine and tocainide, are effective after systemic administration, because charged species cannot penetrate the blood-brain barrier easily. These concepts may also apply to other white matter disorders, such as spinal cord injury or diffuse axonal injury in brain trauma. Moreover, whereas many events are unique to white matter injury, a number of steps are common to both gray and white matter anoxia and ischemia. Optimal protection of the CNS as a whole will therefore require combination therapy aimed at unique steps in gray and white matter regions, or intervention at common points in the injury cascades.

L-type Ca2+ channel blockers attenuate electrical changes and Ca2+ rise induced by oxygen/glucose deprivation in cortical neurons

BACKGROUND AND PURPOSE

Experimental evidence supports a major role of increased intracellular calcium [Ca2+]i levels in the induction of neuronal damage during cerebral ischemia. However, the source of Ca2+ rise has not been fully elucidated. To clarify further the role and the origin of Ca2+ in cerebral ischemia, we have studied the effects of various pharmacological agents in an in vitro model of oxygen (O2)/glucose deprivation.

METHODS

Pyramidal cortical neurons were intracellularly recorded from a slice preparation. Electrophysiological recordings and microfluorometric measurements of [Ca2+]i were performed simultaneously in slices perfused with a glucose-free physiological medium equilibrated with a 95% N2/5% CO2 gas mixture.

RESULTS

Eight to twelve minutes of O2/glucose deprivation induced an initial membrane hyperpolarization, followed by a delayed, large but reversible membrane depolarization. The depolarization phase was accompanied by a transient increase in [Ca2+]i levels. When O2/glucose deprivation exceeded 13 to 15 minutes, both membrane depolarization and [Ca2+]i rise became irreversible. The dihydropyridines nifedipine and nimodipine significantly reduced either the membrane depolarization or the [Ca2+]i elevation. In contrast, tetrodotoxin had no effect on either of these parameters. Likewise, antagonists of ionotropic and group I and II metabotropic glutamate receptors failed to reduce the depolarization of the cell membrane and the [Ca2+]i accumulation. Finally, dantrolene, blocker of intracellular Ca2+ release, did not reduce both electrical and [Ca2+]i changes caused by O2/glucose depletion.

CONCLUSIONS

This work supports a role of L-type Ca2+ channels both in the electrical and ionic changes occurring during the early phases of O2/glucose deprivation.

Hypoxic response of hypoglossal motoneurones in the in vivo cat

1. In current and voltage clamp, the effects of hypoxia were studied on resting and synaptic properties of hypoglossal motoneurones in barbiturate-anaesthetized adult cats. 2. Twenty-nine hypoglossal motoneurones with a mean membrane potential of -55 mV responded rapidly to acute hypoxia with a persistent membrane depolarization of about +17 mV. This depolarization correlated with the development of a persistent inward current of 0.3 nA at holding potentials close to resting membrane potential. 3. Superior laryngeal nerve (SLN) stimulation-evoked EPSPs were reduced in amplitude by, on average, 46% while IPSP amplitude was reduced by 31% SLN stimulation-evoked EPSCs were reduced by 50-70%. 4. Extracellular application of adenosine (10 mM) hyperpolarized hypoglossal motoneurones by, on average, 5.6 mV, from a control value of -62 mV. SLN stimulation-evoked EPSPs decreased by 18% and IPSPs decreased by 46% during adenosine application. 5. Extracellular application of the KATP channel blocker glibenclamide led to a blockade of a persistent outward current and a significant increase of SLN stimulation-evoked EPSCs. 6. We conclude that hypoglossal motoneurones have a very low tolerance to hypoxia. They appear to be under metabolic stress even in normoxia and their capacity to activate protective potassium currents is limited when compared with other brainstem neurones. This may help to explain the rapid disturbance of hypoglossal function during energy depletion.

Maturation of anoxia-induced gasping in the rat: potential role for N-methyl-D-aspartate glutamate receptors

After anoxia-induced apnea, gasping remains the last operative mechanism for survival. In developing rats, the gasping response to anoxia exhibits triphasic characteristics. Because anoxia is associated with enhanced release of glutamate, we hypothesized that N-methyl-D-aspartate (NMDA) glutamate receptors may underlie components of the gasping response. Rat pups aged 2 d (n = 50), 5 d (n = 43), 10 d (n = 42), and 15 d (n = 45) underwent anoxic challenges with 100% N2 in a whole body plethysmograph, 30 min after intraperitoneal administration of MK801 [(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine hydrogen maleate; dizocilpine] (3 mg/kg), a noncompetitive NMDA glutamate receptor channel antagonist, or normal saline. In control pups, after primary apnea onset, a triphasic gasping pattern was apparent at all postnatal ages and included two distinct types of gasps (I and II). In 2- and 5-d MK801-treated animals, phase 1 and type I gasps were absent, leading to marked prolongations of the gasp latency and phase 2, the latter displaying type II gasps only. In addition, phase 3 duration was also prolonged with increased type II gasp frequencies. In contrast, in some 10-d-old (40%) and in all 15-d-old MK801-treated pups, although overall gasping duration was prolonged, the triphasic gasping pattern seen in matched controls was also present. We conclude that NMDA glutamate receptors mediate particular phasic components of the gasping response during early postnatal life but not at later stages of development. We speculate that developmental changes occur in both function and expression of NMDA and other neurotransmitters within brainstem regions underlying the neural substrate for gasp generation.

Hypoxia-induced differential electrophysiological changes in rat locus coeruleus neurons

The effects of hypoxia on rat locus coeruleus (LC) neurons were investigated by intracellular recording from in vitro brain slices. In response to a brief exposure to hypoxic medium (2-5 min), equilibrated with 95% N2 - 5% CO2, two populations of cells could be distinguished, type 1 neurons (61%), showing hyperpolarization (9.3 +/- 0.4 mV, n = 125) and cessation of spontaneous action potentials, and type 2 neurons (39%), displaying gradual pure depolarization (6.0 +/- 0.3 mV, n = 80), instead of hypoxic hyperpolarization. Both types of response were associated with a reduction in membrane input resistance (34 +/- 1% for type 1 cells, n = 125, and 21 +/- 2% for type 2 cells, n = 68). While both types of neurons share similar electrophysiological properties, their membrane input resistance differ significantly (type 1 cells: 144 +/- 5 M omega, n = 125; type 2 cells: 183 +/- 9 M omega, n = 80, p < 0.001). These responses were compared to cyanide-induced chemical hypoxia. Cyanide (2 mM) induced the identical membrane response as effected by nitrogen hypoxia. All cells which responded to nitrogen-saturated hypoxic medium with a pure depolarizing response gave a similar response to cyanide and all neurons hyperpolarized by cyanide were also hyperpolarized by hypoxic medium. Moreover, the K(ATP) channel opener, diazoxide (1 mM), could mimic the hypoxia-induced hyperpolarization in type 1 neurons (10.6 +/- 0.9 mV, n = 18), but was unable to induce hyperpolarization in type 2 cells (n = 13). In addition, the N2-hypoxia-induced hyperpolarization was completely blocked by tolbutamide (200 microM, n = 8) or glibenclamide (3 microM, n = 9). These results indicate that a brief period of hypoxia evokes two different responses in LC neurons and this may be due to the heterogeneous distribution of K(ATP) channels among different LC neurons.

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.

Genetic basis of tolerance to O2 deprivation in Drosophila melanogaster

The ability to tolerate a low-O2 environment varies widely among species in the animal kingdom. Some animals, such as Drosophila melanogaster, can tolerate anoxia for prolonged periods without apparent tissue injury. To determine the genetic basis of the cellular responses to low O2, we performed a genetic screen in Drosophila to identify loci that are responsible for anoxia resistance. Four X-linked, anoxia-sensitive mutants belonging to three complementation groups were isolated after screening more than 10,000 mutagenized flies. The identified recessive and dominant mutations showed marked delay in recovery from O2 deprivation. In addition, electrophysiologic studies demonstrated that polysynaptic transmission in the central nervous system of the mutant flies was abnormally long during recovery from anoxia. These studies show that anoxic tolerance can be genetically dissected.

Hypoxia-induced dysfunction in developing rat neocortex

Neocortical slices from young [postnatal day (P) 5-8], juvenile (P14-18), and adult (>P28) rats were exposed to long periods of hypoxia. Field potential (FP) responses to orthodromic synaptic stimulation, the extracellular DC potential, and the extracellular Ca2+ concentration ([Ca2+]o] were measured simultaneously in layers II/III of primary somatosensory cortex. Hypoxia caused a 42 and 55% decrease in the FP response in juvenile and adult cortex, respectively. FP responses recorded in slices from young animals were significantly more resistant to oxygen deprivation as compared with the juvenile (P < 0.01) and adult age group (P < 0.001) and declined by only 3% in amplitude. In adult cortex, hypoxia elicited, after 7 +/- 4.5 min (mean +/- SD), a sudden anoxic depolarization (AD) with an amplitude of 14 +/- 6 mV and a duration of 0.89 +/- 0.28 min at half-maximal amplitude. Although the AD onset latency was significantly longer in P5-8 (12.5 +/- 4.9 min, P < 0.001) and P14-18 (8.7 +/- 3.2 min, P < 0.002) cortex, the amplitude and duration of the AD was larger in young (45.7 +/- 7.6 mV, 2.19 +/- 0.71 min, both P < 0.001) and juvenile animals (29.9 +/- 9.1 mV, P < 0.001, 0.96 +/- 0.26 min, P > 0.05) when compared with the adults. The hypoxia-induced [Ca2+]o decrease was significantly (P < 0.002) larger in young cortex (1,115 +/- 50 microM) as compared with the adult (926 +/- 107 microM). Prolongation of hypoxia after AD onset for >5 min elicited in young and juvenile cortex a long-lasting AD with an amplitude of 40.5 mV associated with a decrease in [Ca2+]o by >1 mM. On reoxygenation, only slices from these age groups showed spontaneous repetitive spreading depression in 3 out of 26 cases. In adults, the same protocol caused a significantly (P < 0.05) smaller and shorter AD and never a spreading depression. However, recovery in synaptic transmission after this long-term hypoxia was better in young and juvenile cortex, indicating a prolonged or even irreversible deficiency in synaptic function in mature animals. Application of ketamine caused a 49% reduction in the initial amplitude of the AD in juvenile cortex but did not significantly affect the AD in slices from adult animals. These data indicate that the young and juvenile cortex tolerates much longer periods of oxygen deprivation as compared with the adult, but that a sufficiently long hypoxia causes severe pathophysiological activity in the immature cortex. This enhanced sensitivity of the immature cortex is at least partially mediated by activation of N-methyl-D-aspartate receptors.

Anoxia-induced neuronal injury: role of Na+ entry and Na+-dependent transport

An important cause of anoxia-induced nerve injury involves the disruption of the ionic balance that exists across the neuronal membrane. This loss of ionic homeostasis results in an increase in intracellular calcium, sodium, and hydrogen and is correlated with cell injury and death. Using time-lapse confocal microscopy, we have previously reported that nerve cell injury is mediated largely by sodium and that removing extracellular sodium prevents the anoxia-induced morphological changes. In this study, we hypothesized that sodium enters neurons via specific mechanisms and that the pharmacologic blockade of sodium entry would prevent nerve damage. In cultured neocortical neurons we demonstrate that replacing extracellular sodium with NMDG+ prevents anoxia-induced morphological changes. With sodium in the extracellular fluid, various routes of sodium entry were examined, including voltage-sensitive sodium channels, glutamate receptor channels, and sodium-dependent chloride-bicarbonate exchange. Blockade of these routes had no effect. Amiloride, however, prevented the morphological changes induced by anoxia lasting 10, 15, or 20 min. At doses of 10 microM-1 mM, amiloride protected neurons in a dose-dependent fashion. We argue that amiloride acts on a Na+-dependent exchanger (e.g., Na+-Ca2+) and present a model to explain these findings in the context of the neuronal response to anoxia.

Enhanced taurine release in cell-damaging conditions in the developing and ageing mouse hippocampus

Taurine has been shown to be essential for neuronal development and survival in the central nervous system. The release of preloaded [3H]taurine was studied in hippocampal slices from seven-day-, three-month- and 18-22-month-old mice in cell-damaging conditions. The slices were superfused in hypoxic, hypoglycemic and ischemic conditions and exposed to free radicals and oxidative stress. The release of taurine was greatly enhanced in the above conditions in all age groups, except in oxidative stress. The release was large in ischemia, particularly in the hippocampus of aged mice. Potassium stimulation was still able to release taurine in cell-damaging conditions in immature mice, whereas in adult and aged animals the release was so substantial that this additional stimulus failed to work. Taurine release was partially Ca2+-dependent in all cases. The massive release of the inhibitory amino acid taurine in ischemic conditions could act neuroprotectively, counteracting in several ways the effects of simultaneous release of excitatory amino acids. This protection could be of great importance in developing brain tissue, while also having an effect in aged brains.

Anoxia-induced depolarization in CA1 hippocampal neurons: role of Na+-dependent mechanisms

We have previously shown that (1) removal of extracellular sodium (Na+) reduces the anoxia-induced depolarization in neurons in brain-slice preparations and (2) amiloride, which blocks Na+-dependent exchangers, prevents anoxic injury in cultured neocortical neurons. Since anoxia-induced depolarization has been linked to neuronal injury, we have examined in this study the role of Na+-dependent exchangers and voltage-gated Na+ channels in the maintenance of membrane properties of CA1 neurons at rest and during acute hypoxia. We recorded intracellularly from CA1 neurons in hippocampal slices, monitored Vm and measured input resistance (Rm) with periodic injections of negative current. We found that tetrodotoxin (TTX, 1 microM) hyperpolarized CA1 neurons at rest and significantly attenuated both the rate of depolarization (delta Vm/dt) and the rate of decline of Rm (delta Rm/dt) by about 60% during the early phase of hypoxia. The effect of TTX was dose-dependent. Amiloride (1 mM) decreased Vm and increased Rm in the resting condition but changed little the effect of hypoxia on neuronal function. Benzamil and 5-(N-ethyl-N-isopropyl)-2',4'-amiloride (EIPA), two specific inhibitors of Na+ dependent exchangers, were similar to amiloride in their effect. We conclude that neuronal membrane properties are better maintained during anoxia by reducing the activity of TTX-sensitive channels and not by the action of Na+-dependent exchangers.

Beneficial effects of TDN-345, a novel Ca2+ antagonist, on ischemic brain injury and cerebral glucose metabolism in experimental animal models with cerebrovascular lesions

The effects of TDN-345 on mortality and ischemic neurological deficit following transient global cerebral ischemia in Mongolian gerbils and also the rate of local cerebral glucose utilization (LCGU) in stroke-prone spontaneously hypertensive rats (SHRSP) with cerebrovascular lesions were investigated. In Mongolian gerbils, ischemia was produced by clamping the bilateral common carotid arteries for 15 min. TDN-345 (0.1-1.0 mg/kg) dose-dependently decreased the mortality and ischemic neurological deficit score when administered orally twice, 60 min before ischemia and 90 min after recirculation. Additionally, TDN-345 (0.2 or 1.0 mg/kg, p.o. once daily for 3 weeks after the onset of stroke) decreased the mortality and recurrence of stroke in SHRSP. To determine the site of action of TDN-345 in the brain, the rate of LCGU in various brain regions in SHRSP with stroke was examined using a [14C]2-deoxy-D-glucose method. The rate of LCGU decreased significantly in all the brain regions in SHRSP with stroke compared with Wistar-Kyoto (WKY) control rats, whereas the reduction in the rate of LCGU in SHRSP with stroke was prevented by TDN-345 treatment, especially in the sensorimotor cortex and locus coeruleus. These results suggest that TDN-345 has therapeutic efficacy in the treatment of cerebrovascular disease.

Expression and localization of Na+/H+ exchangers in rat central nervous system

Neurons in the central nervous system regulate their intracellular pH using particular membrane proteins of which two, namely the Na+-dependent Cl-/HCO3- exchanger and the Na+/H+ exchanger, are essential. In this study we examined messenger RNA expression and distribution of Na+/H+ exchanger in the newborn rat central nervous system and with maturation using Northern blot analysis and in situ hybridization. Our study clearly shows that each Na+/H+ exchanger has a different expression pattern in the rat central nervous system. As in non-excitable tissues, Na+/H+ exchanger 1 is by far the most abundant of all Na+/H+ exchangers in the rat central nervous system. Its expression is ubiquitous although its messenger RNA appears at higher levels in the hippocampus, in the 2nd/3rd layers of periamygdaloid cortex and in the cerebellum. The low level of messenger RNAs encoding Na+/H+ exchanger 2 and 4 is mainly expressed in the cerebral cortex and in the brainstem-diencephalon, while Na+/H+ exchanger 3 transcripts are found only in the cerebellar Purkinje cells. From a developmental point of view, Na+/H+ exchanger 1, 2 and 4 showed an increased level in their transcripts in the cerebral cortex while an opposite trend existed in the cerebellum from postnatal day 0 to postnatal day 30. The messenger RNA for Na+/H+ exchanger 3, however, increased its level with age in cerebellum. From our data we conclude that: i) the expression of the Na+/H+ exchanger is age-, region-, and subtype-specific, with Na+/H+ exchanger 1 being the most prevalent in the rat central nervous system; ii) specialization of groups of neurons with respect to the type of Na+/H+ exchanger is clearly illustrated by Na+/H+ exchanger 3 which is almost totally localized in cerebellar Purkinje cells; and iii) the developmental increase in the messenger RNA for Na+/H+ exchanger 1 in the cerebral cortex and hippocampus is consistent with our previous studies on intracellular pH physiology in neonatal and mature neurons. Together this study indicates that expression of each Na+/H+ exchanger messenger RNA is differentially regulated both during development and in the different regions of rat central nervous system.

Chronic postnatal hypoxia increases the numbers of cortical neurons

Premature infants have been shown to undergo prolonged periods of sublethal hypoxia. There is considerable evidence to link these hypoxic events with neurodevelopmental disorders. As an animal model for this clinical problem, rats were raised from the third day of life in a chamber where the O2 level was 9.5%. After 30 days of hypoxia the rats were sacrificed and their brains processed for determination of the number of cortical neurons. This work was performed to test the hypothesis that chronic hypoxia would result in increased cortical cell death. The hypoxic rats had lower body and brain weights as well as decreased cortical volumes. However, hypoxic rats had increased neuronal density and significantly more cortical neurons than controls (P < 0.05). The results of this study suggest that chronic sublethal hypoxia may lead to reduction in the amount of programmed cell death in the developing neocortex.

Anoxia regulates gene expression in the central nervous system of Drosophila melanogaster

We took advantage of the Drosophila melanogaster's extraordinary resistance to anoxia to study the molecular mechanisms underlying this phenomenon. We analyzed mRNA expression of heat shock proteins (HSP) (HSP26 and HSP70), ubiquitins (UB) (UB3 and UB4), cytochrome oxidase I (COXI) and superoxide dismutase (SOD) using slot blot analysis. The expression of HSP genes, especially HSP70, was remarkably up-regulated (up to a thousand-fold) while those of UB4 and COXI were down-regulated (10-60%) in response to the anoxic stress. The expression of UB3 gene was up-regulated by 1.5x and the expression of SOD gene was not significantly affected. In response to heat shock stress, the expression of HSP genes increased by up to several thousand-fold, the expression of UB genes increased modestly (23-91%) but the expression of SOD and COXI genes was reduced by 25%. Furthermore, the expression patterns of HSP genes under anoxia and heat shock were clearly different. The expression of HSP genes peaked by 15 min into anoxia and then declined but stayed above baseline. In contrast, their expression increased as a function of time during heat exposure. From these results, we conclude that: (1) different forms of stress regulates gene expression in different ways; (2) anoxia differentially regulates gene expression; and (3) the up-regulation of HSP70 and down-regulation of UB4 by anoxia are consistent with the idea that Drosophila melanogaster resist anoxia, at least in part, by protecting proteins against degradation.

Pre-synaptic effect of the ATP-sensitive potassium channel opener diazoxide on rat substantia nigra pars reticulata neurons

Spontaneous synaptic currents were recorded from visually identified substantia nigra pars reticulata (SNR) neurons in the rat brain slice preparation by whole-cell patch clamp technique. GABA neurons were distinguished from dopamine neurons by their electrophysiological characteristics. In the presence of 20 microM AP5 and CNQX, the spontaneous synaptic currents recorded from GABA neurons were sensitive to bicuculline and reversed polarity at a potential close to the equilibrium potential of Cl-, indicating that they were mediated by GABA(A) receptors. TTX at 1 microM eliminated action potential-dependent release of GABA from nerve terminals, revealing the miniature inhibitory post-synaptic currents (mIPSCs). The ATP-sensitive potassium channel (K(ATP) channel) opener diazoxide (30-300 microM) significantly reduced the frequency of the mIPSCs in a dose-dependent manner. However, diazoxide did not affect the average value and the distribution of the mIPSC amplitudes. Thus, this effect of diazoxide was pre-synaptic in nature. The K(ATP) channel blocker glibenclamide (300 microM) was able to restore the frequency of the mIPSCs. These data suggest that the striatonigral projection, which represents the major inhibitory input controlling SNR GABA neuron activities, possesses presynaptic K(ATP) channels on the nerve terminals.