Free radicals, glutamate, dopamine, and serotonin after ischemia reperfusion insult in aging gerbil brain

Brain antioxidant regulation in mammals and anoxia-tolerant reptiles: balanced for neuroprotection and neuromodulation

Reactive oxygen species (ROS) generated by mitochondrial respiration and other processes are often viewed as hazardous substances. Indeed, oxidative stress, defined as an imbalance between oxidant production and antioxidant protection, has been linked to several neurological disorders, including cerebral ischemia-reperfusion and Parkinson's disease. Consequently, cells and organisms have evolved specialized antioxidant defenses to balance ROS production and prevent oxidative damage. Research in our laboratory has shown that neuronal levels of ascorbate, a low molecular weight antioxidant, are ten-fold higher than those in much less metabolically active glial cells. Ascorbate levels are also selectively elevated in the CNS of anoxia-tolerant reptiles compared to mammals; moreover, plasma and CSF ascorbate concentrations increase markedly in cold-adapted turtles and in hibernating squirrels. Levels of the related antioxidant, glutathione, vary much less between neurons and glia or among species. An added dimension to the role of the antioxidant network comes from recent evidence that ROS can act as neuromodulators. One example is modulation of dopamine release by endogenous hydrogen peroxide, which we describe here for several mammalian species. Together, these data indicate adaptations that prevent oxidative stress and suggest a particularly important role for ascorbate. Moreover, they show that the antioxidant network must be balanced precisely to provide functional levels of ROS, as well as neuroprotection.

NMDA receptor activation mediates hydrogen peroxide-induced pathophysiology in rat hippocampal slices

Endogenous reactive oxygen species (ROS) can act as modulators of neuronal activity, including synaptic transmission. Inherent in this process, however, is the potential for oxidative damage if the balance between ROS production and regulation becomes disrupted. Here we report that inhibition of synaptic transmission in rat hippocampal slices by H2O2 can be followed by electrical hyperexcitability when transmission returns during H2O2 washout. As in previous studies, H2O2 exposure (15 min) reversibly depressed the extracellular population spike (PS) evoked by Schaffer collateral stimulation. Recovery of PS amplitude, however, was typically accompanied by mild epileptiform activity. Inclusion of ascorbate (400 microM) during H2O2 washout prevented this pathophysiology. No protection was seen with isoascorbate, which is a poor substrate for the stereoselective ascorbate transporter and thus remains primarily extracellular. Epileptiform activity was also prevented by the N-methyl-D-aspartate (NMDA) receptor antagonist, DL-2-amino-5-phosphonopentanoic acid (AP5) during H2O2 washout. Once hyperexcitability was induced, however, AP5 did not reverse it. When present during H2O2 exposure, AP5 did not alter PS depression by H2O2 but did inhibit the recovery of PS amplitude seen during pulse-train stimulation (10 Hz, 5 s) in H2O2. Inhibition of glutamate uptake by l-trans-2,4-pyrrolidine dicarboxylate (PDC; 50 microM) during H2O2 washout markedly enhanced epileptiform activity; coapplication of ascorbate with PDC prevented this. These data indicate that H2O2 exposure can cause activation of normally silent NMDA receptors, possibly via inhibition of redox-sensitive glutamate uptake. When synaptic transmission returns during H2O2 washout, enhanced NMDA receptor activity leads to ROS generation and consequent oxidative damage. These data reveal a pathological cycle that could contribute to progressive degeneration in neurological disorders that involve oxidative stress, including cerebral ischemia.

Taurine in gerbil retina: changes during ischemia reperfusion/insult (I.R.I.) and aging

The retina contains a high amount of taurine which suggests a role in retinal function. The mongolian gerbil is well studied in stroke because of its incomplete circle of Willis. Two groups of gerbils were used: one served as control and the other was subjected to unilateral left carotid occlusion during 30 minutes, followed by 60 minutes of reperfusion. Gerbils were selected by ocular fundus and only sensitive gerbils were retained for the experimentation. We studied the level of taurine in gerbil retina of different ages: 3, 9, 15 and 24 months old (sham operated and ischemic groups). Level of taurine was determined by HPLC/electrochemical method. Compared to sham operated groups, level of taurine was significantly increased in ischemic groups for all ages studied. In the sham operated groups, level of taurine was low at birth, reached a plateau, and then decreased with aging. In the ischemic groups, level of taurine regularly increased from 3 to 24 months of age. With comparison evaluated for each age (modification ischemic versus sham operated groups expressed in percentage), level of taurine was quite equal at 3 and 9 months of age, but increased in 15 and 24 months old gerbils.

Age-related changes of oxygen free radical production in the rat brain slice after hypoxia: on-line measurement using enhanced chemiluminescence

Superoxide radical production in brain slices of 12-, 24- and 60-day-old rats was measured online during and after 15 min of hypoxia with lucigenin enhanced chemiluminescence. We found a typical radical burst after reoxygenation which developed with aging and is almost nonexistent in the youngest and most prominent in the oldest group. This cannot be explained by a decreasing tissue superoxide dismutase (SOD) concentration in the brain with aging, since the concentration of the enzyme, determined in the same age groups, increased with age.

Effect of piribedil, a D-2 dopaminergic agonist, on dopamine, amino acids, and free radicals in gerbil brain after cerebral ischemia

The excitatory amino acids (EAA) are involved in the pathogenesis of the cerebral ischemia. Moreover, several investigators have demonstrated that a considerable amount of dopamine (DA) is released in the striatum after ischemia reperfusion/insult (IRI). Recently, studies have demonstrated in vitro, that D-2 agonist, at the level of striatum and retina, may represent a powerful signal to inhibit release of excitatory amino acids implicated in cerebral ischemia. Therefore we have been incited to test, in vivo, the action of a D-2 agonist, piribedil, on gerbil brain after IRI. We have used the Stroke Index (SI); then to precise the mechanism of action, we have determined the levels of dopamine, EAA, and hydroxyl-free radicals (OH), in striatum, hippocampus, and hemisphere. Piribedil, administered at dose of 10 mg/kg, per os, 60 min before induction of transient cerebral ischemia in gerbils, presents a neuroprotective effect, as measured by SI and significantly reverses the increase of DA, EAA, and OH induced by IRI. The mechanism of action of piribedil could be related to its D-2 agonist property.

Age-related regional changes in hydroxyl radical stress and antioxidants in gerbil brain

The levels of hydroxyl radicals and oxidized GSH have been examined as indices of oxidative stress in young (3 months), middle-aged (15 months), and old (20-24 months) gerbil brain hippocampus, cortex, and striatum. The hydroxyl radical stress was estimated by measuring the salicylate hydroxyl radical trapping products 2,5- and 2,3-dihydroxybenzoic acid. The stress was significantly higher in all three brain regions in middle-aged and old gerbils versus young animals (< or = 66.0%). Regional comparisons showed that the stress was significantly higher in cortex than in either the hippocampus or striatum of the middle-aged and old gerbils (< or = 32.0%). The ratio of oxidized to total GSH also increased progressively in middle-aged and old animals in all three brain regions (p < 0.05, < or = 41.1%), further indicating a general age-related increase in oxidative stress. Parallel to this age-related increase in oxidative stress, a significant, albeit slight (8%), decrease in neuronal number in hippocampal CA1 region was observed in both the middle-aged and old animals. Possible differences in antioxidant levels were also examined. Total GSH levels were similar across age groups (variance < 12%). However, the regional comparison showed that it was highest in striatum in all age groups. The levels of alpha-tocopherol (vitamin E) were significantly higher in the middle-aged and old animals in all three regions (< or = 70.4%). Vitamin E was highest in the hippocampus and the differences between the hippocampus and the cortex and striatum increased with age.(ABSTRACT TRUNCATED AT 250 WORDS)