Neurophysiological consequences of nitroxide antioxidants.

Nitroxides are antioxidant compounds that have been shown to provide radioprotection in vivo and in vitro. Radioprotection in vivo is limited by toxicity, which appears to be neurologic in nature. To further evaluate the toxicity of these compounds, three representative nitroxides, Tempol, Tempamine, and Tempo, were examined in slices of guinea pig hippocampus. Each nitroxide increased the population spike and caused potentiation of excitatory postsynaptic potential--spike coupling. Repetitive activity and epileptiform activity were observed at the highest concentrations of Tempo and Tempamine. Tempol was the least toxic compound in this system, followed by Tempamine and Tempo. Additional studies are necessary to further define the effects of nitroxides on the central nervous system and to develop strategies to mitigate these effects.

Gamma radiation (5-10 Gy) impairs neuronal function in the guinea pig hippocampus.

Guinea pigs were exposed to 5 and 10 Gy gamma radiation. Hippocampal brain slices were isolated 30 min, 1 day, 3 days and 5 days after irradiation or sham irradiation and the electrophysiological characteristics of the neural tissue were evaluated. Both radiation doses elicited significant changes that were dependent on dose, dose rate and time. Synaptic efficacy decreased soon after exposure to 5 Gy at dose rates of both 1 and 20 Gy/min. Recovery occurred by 5 days. Ten grays at 20 Gy/min potentiated the postsynaptic potential 1 day after irradiation. By 3 days, synaptic efficacy was decreased and did not recover. The ability of the synaptic potentials to generate spikes was potentiated within 30 min after exposure to 5 Gy at 1 Gy/min and persisted through 3 days, with recovery at 5 days. At the 20 Gy/min dose rate, a similar potentiation did not result with 10 Gy and occurred only at 3 days after irradiation with 5 Gy. Rather, within 30 min and after 5 days, spike generation was significantly depressed by these exposures. Both synaptic efficacy and spike generation contribute to the net input-output relationship of the neuronal population. This relationship was profoundly decreased within 30 min with recovery at 1 day and subsequent decline with the higher dose rate in a dose-dependent manner. These persistent changes in neuronal function are likely to be a consequence of the actions of ionizing radiation on the physiological processes that influence the neuronal environment.

Role of glutathione in repair of free radical damage in hippocampus in vitro.

Depletion of glutathione (GSH), an intrinsic antioxidant, increases vulnerability to free radical damage in a number of cell systems. This study investigates the role of GSH in limiting electrophysiological damage and/or recovery from free radical exposure in slices of guinea pig hippocampus. Synaptic potentials (PSPs) and population spikes (PSs) were recorded from field CA1. Free radicals were generated from 0.006% peroxide through the Fenton reaction. Analysis of the input-output curves showed that peroxide treatment decreased PSPs and impaired ability of the PSPs to generate PSs as previously reported. Recovery was nearly total within a half hour. Treatment with 5 mM buthionine sulfoximine (BSO) for 2 h depleted hippocampal GSH to 79.2% of control values. The extent of free radical damage was not increased. Recovery, however, was only partial. GSH was further depleted by oxidation with diamide or covalent bonding with dimethyl fumarate (DMF) immediately before and during the peroxide treatment. Neither diamide nor DMF treatment in BSO-incubated tissue enhanced peroxide-induced electrophysiological deficits. Following these treatments, however, tissue showed little recovery from free radical damage. We conclude that glutathione is essential for repair processes in hippocampal neurons exposed to oxidative damage.

Electrophysiological consequences of exposure of hippocampal slices to dihydroxyfumarate, a generator of superoxide radicals.

In an effort to understand the damaging actions of free radicals to neuronal electrophysiology, the superoxide generator, dihydroxyfumarate (DHF), was evaluated in slices of guinea pig hippocampus. Using field potential recording techniques, population spikes and population synaptic potentials were recorded in field CA1. Slices were exposed to 3 mM DHF either alone or in the presence of a protectant. DHF did not alter the ability of the afferent volley to generate a synaptic potential, but it did impair the ability of the synaptic potential to elicit a population spike. In addition, DHF induced lipid peroxidation as measured by the thiobarbituric acid assay. Superoxide dismutase (SOD) provided no protection. Instead, SOD treatment promoted DHF damage to synaptic potentials. Catalase alone mitigated the actions of DHF, but only in SOD plus catalase was the DHF-induced electrophysiological deficit and lipid peroxidation completely antagonized. The iron chelator, Desferal, did not protect but promoted synaptic damage. Desferal may be ineffective because of the nitroxide radical formed upon its reaction with DHF. The hydroxyl radical scavenger, dimethylsulfoxide, prevented lipid peroxidation and reduced the DHF-induced deficit but did not completely prevent the impairment of spike generation. These data suggest that DHF exerts its actions through generation of hydrogen peroxide which would further react with tissue iron to produce hydroxyl radicals.