Actions of brain-protecting substances against both oxygen and glucose deprivation in the guinea pig hippocampal neurons studied in vitro

Sodium channel blockade unmasks two temporally distinct mechanisms of striatal dopamine release during hypoxia/hypoglycaemia in vitro

Massive striatal dopamine release during cerebral ischaemia has been implicated in the resulting neuronal damage. Sodium influx is an early event in the biochemical cascade during ischaemia and blockade of sodium channels may increase resistance to ischaemia by reducing energy demand involved in compensation for sodium and potassium fluxes. In this study, we have determined the effects of opening and blockade of voltage-gated sodium channels on hypoxia/hypoglycaemia-induced dopamine release. Slices of rat caudate nucleus were maintained in a slice chamber superfused by an oxygenated artificial cerebrospinal fluid containing 4 mM glucose. Ischaemia (hypoxia/hypoglycaemia) was mimicked by a switch to a deoxygenated artificial cerebrospinal fluid containing 2 mM glucose and dopamine release was measured using fast cyclic voltammetry. In drug-free (control) slices, there was a 2-3 min delay after the onset of hypoxia/hypoglycaemia followed by a rapid dopamine release event which was associated with anoxic depolarization. In slices treated with the Na+ channel opener, veratridine (1 microM), the time to onset of dopamine release was shortened (101 +/- 20 s, compared with 171 +/- 8 s in controls, P < 0.05). Conversely, phenytoin (100 microM), lignocaine (200 microM) and the highly selective sodium channel blocker, tetrodotoxin (1 microM) markedly delayed and slowed dopamine release vs paired controls. In the majority of cases, dopamine release was biphasic after sodium channel blockade: a slow phase preceded a more rapid dopamine release event. The latter was associated with anoxic depolarization. Neither the fast nor the slow release events were affected by pretreatment with the selective dopamine uptake blocker GBR 12935 (0.2 microM), suggesting that uptake carrier reversal did not contribute to these events. In conclusion, sodium channel antagonism delays and slows hypoxia/hypoglycaemia-induced dopamine release in vitro. Furthermore, sodium channel blockade delays anoxic depolarization and its associated neurotransmitter release, revealing an earlier dopamine release event that does not result from reversal of the uptake carrier.

Free radical-induced elevation of ornithine decarboxylase activity in developing rat brain slices

OBJECTIVE

In developing brain, we have previously shown both in vivo [L.D. Longo, S. Packianathan, J.A. McQueary, R.B. Stagg, C.V. Byus and C.D. Cain, Acute hypoxia increases ornithine decarboxylase activity and polyamine concentrations in fetal rat brain, Proc. Natl. Acad. Sci. USA, Vol. 90 (1993) 692-696] and in vitro [S. Packianathan, C.D. Cain, B.H. Liwnicz and L.D. Longo, Ornithine decarboxylase activity in vitro in response to acute hypoxia: a novel use of newborn rat brain slices, Brain Res., Vol. 688 (1995) 61-71] that acute hypoxia is associated with a significant increase in ornithine decarboxylase (ODC) activity and polyamine concentrations. We tested the hypothesis that oxygen free radicals induce an increase in ODC activity similar to that of hypoxia and that both this and the hypoxia-induced response are inhibited by free radical scavengers.

MATERIALS AND METHODS

Slices of cerebrum, 300-500 microm thick, were made from P3 newborn Sprague-Dawley rat pups and equilibrated for 1 h in artificial cerebrospinal fluid continuously bubbled with 95% O2/5% CO2. Free radical-induced ODC activity response was measured beginning after a 1-h recovery period. Experiments were performed on slices treated with 5 X 10(-7) M xanthine (X) + 10 mU/ml xanthine oxidase (XO), with or without the free radical scavengers superoxide dismutase (SOD; 100 U/ml), catalase (CAT; 700 U/ml) or glutathione peroxidase (GPX; 3 U/ml). We also quantified slice malonaldehyde concentrations in response to hypoxia (21% O2/5% CO2/74% N2).

RESULTS

Under control conditions, ODC activity was stable during the 2-h post-recovery period. In response to X/XO treatment, ODC activity increased 2.3-fold at 1.5 h post-recovery. In examining ODC activity as a function of xanthine dose, we noted that ODC activity increased in response to 2.5 X 10(-7) M xanthine; however, it decreased in response to 7.5 X 10(-7) M or higher concentrations. Free radical-induced ODC activity was significantly decreased by addition of the free radical scavengers, SOD, CAT or GPX. In addition, the hypoxic-induced increases in ODC activity and malonaldehyde concentration was also eliminated by the addition of SOD with CAT.

CONCLUSIONS

(1) Oxygen free radicals, particularly hydroxyl radical (OH.), appear to trigger an induction of ODC activity in newborn rat cerebrum slices. (2) Oxygen free radicals also appear to mediate the hypoxic-induced increase in ODC activity. (3) Any consequent increase in polyamine synthesis may have profound effects on neurogenesis and neurodifferentiation in the developing brain.

Ornithine decarboxylase activity in vitro in response to acute hypoxia: a novel use of newborn rat brain slices

In fetal as well as newborn rats, acute hypoxic exposure results in significantly elevated brain ornithine decarboxylase (ODC) activity, polyamine concentrations, and ODC mRNA. The interpretations of these in vivo hypoxic-induced changes, however, are complicated by maternal confounding effects. To test the hypothesis that acute hypoxia will also increase ODC activity in vitro, we developed a brain slice preparation which eliminates such maternal effects. Sections of whole cerebrum, approximately 300-500 microns thick, were made from 3- to 4-day old Sprague-Dawley rat pups. The slices were equilibrated for 1 h in artificial cerebrospinal fluid (ACSF) continuously bubbled with 95% O2/5% CO2, prior to induction of hypoxia. We induced hypoxia by changing the oxygen concentration to 40%, 30%, 21%, 15%, 10%, or 0% O2, all with 5% CO2 and balance N2. In the normoxic control brain slices, low but stable basal ODC activity persisted for up to 5 h post-sacrifice. Slices in ACSF treated with bovine serum albumin (BSA), or both BSA and fetal bovine serum (FBS), however, showed stable ODC activity values 2- to 3-fold higher than slices in ACSF alone, for up to 5 h. In response to acute hypoxia (i.e., 15, 21, and 30% O2), ODC activity was elevated 1.5- to 2-fold above control values between 1 and 2 h after initiation of hypoxia. Qualitative light and electron microscopic examination of the neonatal brain slices following 2 h hypoxic exposure suggested that the great majority of cells did not show severe hypoxic damage or necrosis. It was concluded that: (1) in neonatal rat brain slices in vitro, stable ODC activity values approximating the whole brain ODC activity seen at sacrifice, can be maintained for several hours; (2) the in vivo hypoxic-induced increase in ODC activity can be approximated in vitro; (3) the neonatal rat brain slice preparation may be an alternative to other methods for studying hypoxic-induced ODC enzyme kinetics, or other brain enzymes, without maternal confounding effects; and (4) ODC activity may be an indicator of active metabolism within the newborn brain slice both in normoxia and hypoxia.

Phenytoin delays ischemic depolarization, but cannot block its long-term consequences, in the rat hippocampal slice

The anticonvulsant phenytoin has been reported to block anoxia-induced losses of synaptic activity in the rat hippocampal slice and experimental ischemia-induced losses of synaptic activity in the guinea pig hippocampal slice. We examined phenytoin in our rat hippocampal slice model of experimental ischemia (anoxia +2 mM D-glucose). In this model, ischemic depolarization (ID) occurs 4-5 min after the introduction of anoxic medium, and oxygen and D-glucose are restored 1 min after the onset of ID. In control slices, synaptic recovery is never observed following ID in 2 mM D-glucose. Phenytoin (30,100 and 300 microM), perfused for 20 min prior to, and for 10 min following anoxia, did not allow for synaptic recovery following ID. At the higher concentrations, however, it did increase the latency to ID. In addition, the presynaptic volley (PV), which normally disappears at the time of ID, was lost substantially earlier in the presence of phenytoin. These findings suggest that the anti-ischemic effects of phenytoin reported by others are due to delay of ID. This may suggest that phenytoin will be effective in preventing global ischemia-induced damage only when the ischemic insult is of short duration.

Effect of lactic and CO2 acidosis on neuronal function following glucose-oxygen deprivation in rat hippocampal slices

The present study was designed to determine whether lactate changes the critical pH point at which the recovery of rat population spike is inhibited following glucose-oxygen deprivation and second, which degree of lactic acidosis is similar to the effect of CO2 acidosis. The population spike was recorded from the hippocampal CA1 region after stimulation of the Schaffer collaterals. Slices were randomly perfused with various acidotic solutions for 30 min. During the last 15 min, glucose-oxygen deprivation was combined with the acidotic perfusion. Then the hippocampal slices were perfused with a standard solution of pH 7.4 for 60 min and recovery was compared to the control population spike and expressed as a percentage of the control value. In the control acidotic solution, the critical pH point was 5.0. When 15 mM or 30 mM lactate were added to the control solution, the critical pH point changed to 5.5 or 6.0, suggesting that the inhibition of the population spike was enhanced by lactate in a dose-dependent fashion. The recovery of the population spike was inhibited by exposing the slices to CO2 of 25% or above (pH was 5.76 or below) and this inhibition of recovery associated with CO2 acidosis was the same degree as occurred with 30 mM, namely severe lactic acidosis.

Phenytoin reduces neonatal hypoxic-ischemic brain damage in rats

We investigated the possible protective effect of phenytoin on hypoxic-ischemic brain damage in neonatal rats. Six-day-old rats underwent ligation of the left carotid artery followed by exposure to an 8% oxygen atmosphere for 2.5 hrs. We sacrificed the animals 72 hrs later and assessed the hypoxic-ischemic brain damage histologically. Phenytoin (50 mg/kg), administered intraperitoneally 1 hr before the hypoxia, reduced hypoxic-ischemic infarction in the cerebral cortex and striatum, and attenuated neuronal necrosis in the hippocampus. The plasma concentration of phenytoin after injection was 11.1 +/- 1.9 micrograms/ml (mean +/- S.E.M.) at 1 hr and 22.9 +/- 1.4 micrograms/ml at 4 hrs. Percent volumes of the infarction calculated by dividing the sum of damaged areas by the total area in serial coronal sections were 79 +/- 3% (mean +/- S.E.M.) in vehicle controls versus 13 +/- 6% in phenytoin-treated pups in the cerebral cortex, and 79 +/- 4% in vehicle controls versus 12 +/- 5% in phenytoin-treated pups in the striatum. We semiquantitatively investigated the hypoxic-ischemic change in 5 hippocampal areas: dentate gyrus, CA4, CA3, CA1, and subiculum, in the dorsal hippocampus. Pre-hypoxic treatment with phenytoin reduced hypoxic-ischemic damage in all areas examined. When phenytoin was administered immediately after the hypoxia, there was no difference between vehicle-injected controls and phenytoin-treated pups. These results demonstrate that phenytoin can reduce neonatal hypoxic-ischemic brain damage.

Effects of calcium and calcium antagonists against deprivation of glucose and oxygen in guinea pig hippocampal slices

To provide evidence to support the calcium hypothesis of cerebral ischemia, we examined the effects of extracellular calcium and calcium antagonists (verapamil, flunarizine, nicardipine) on in vitro 'ischemia' using guinea pig hippocampal slices. As a model of in vivo ischemia we used a state of both glucose and oxygen deprivation. Recovery of dentate antidromic field response and histological changes were used as indices of cell damage. After 10 min of deprivation in standard Krebs-Ringer solution, the field potentials exhibited minimum recovery and dentate neurons were severely damaged. Damaged neurons had pyknotic nuclei and swollen cytoplasms. Drugs were added and the calcium concentration was changed during 30 min of pre-deprivation and during deprivation. In the first experiment we demonstrated that pre-treated calcium antagonists protect the dentate granule cells against glucose and oxygen deprivation. The order of the protective potency was flunarizine greater than verapamil much greater than nicardipine. In the second experiment we also showed that neuronal damage caused by deprivation is dependent on the extracellular concentration of calcium. Our data show that extracellular calcium is partially responsible for 'ischemic' neuronal injury in the hippocampal slice. Both low calcium and voltage-gated calcium channel blockers can preserve an antidromic population spike. Conversely, high calcium in the bath can worsen the damage caused by in vitro 'ischemia' to hippocampal slices.