Epilepsy and adenosinetriphosphate (ATP): effect of electrical stimulation and high potassium perfusion on hippocampal ATP contents

Physiological bases of the K+ and the glutamate/GABA hypotheses of epilepsy

Epilepsy is a heterogeneous family of neurological disorders that manifest as seizures, i.e. the hypersynchronous activity of large population of neurons. About 30% of epileptic patients do not respond to currently available antiepileptic drugs. Decades of intense research have elucidated the involvement of a number of possible signaling pathways, however, at present we do not have a fundamental understanding of epileptogenesis. In this paper, we review the literature on epilepsy under a wide-angle perspective, a mandatory choice that responds to the recurrent and unanswered question about what is epiphenomenal and what is causal to the disease. While focusing on the involvement of K+ and glutamate/GABA in determining neuronal hyperexcitability, emphasis is given to astrocytic contribution to epileptogenesis, and especially to loss-of-function of astrocytic glutamine synthetase following reactive astrogliosis, a hallmark of epileptic syndromes. We finally introduce the potential involvement of abnormal glycogen synthesis induced by excess glutamate in increasing susceptibility to seizures.

Changes in hippocampal adenosine efflux, ATP levels, and synaptic transmission induced by increased temperature

Previous studies have demonstrated that when the temperature of hippocampal brain slices is increased, there is a corresponding depression of synaptic potentials mediated by an increased activation of presynaptic adenosine A(1) receptors. The present experiments demonstrate that when the temperature of hippocampal slices is raised from 32.5 degrees C to either 38.5 degrees C or 40.0 degrees C there is a marked, temperature-dependent increase in the efflux of endogenous adenosine and a corresponding decrease in excitatory synaptic responses. The increase in efflux is rapidly reversible on lowering the slice temperature and the temperature-induced efflux is repeatable. Control experiments suggest that this increased efflux of adenosine is not the result of hypoxia or ischemia secondary to a temperature-induced increase in the metabolic rate of the slice. The increase in adenosine efflux was not accompanied by any significant change in the ATP levels in the brain slice, whereas a hypoxic stimulus sufficient to produce a comparable depression of excitatory transmission produced an approximately 75% decrease in ATP levels. These experiments indicate that changes in brain slice temperature can alter purine metabolism in such a way as to increase the adenosine concentration in the extracellular space, as well as adenosine efflux from hippocampal slices, in the absence of significant changes in ATP levels.

Membrane Na+K+ATPase activity: changes using an experimental model of alcohol dependence and withdrawal

An alcohol dependent states was induced in rats via gastric intubation. Alcohol was given three times daily for seven days in increasing doses, the final dose and concentration varying from rat to rat, being adjusted according to weight loss and state of intoxication. After seven days dosing, alcohol was withdrawn and twenty hours later an audiogenic stimulus was given to induce convulsions. Na+K+ATPase activity was measured in the hippocampus of rats during alcohol administration and during withdrawal. Animals which received twenty-one doses of alcohol showed a significant increase in Na+K+ATPase activity compared with controls. On withdrawal of alcohol, the enzyme activity fell, but remained higher than control values. In the withdrawal groups, membrane Na+K+ATPase levels were increased significantly compared to control levels in the order: no convulsion less than with convulsion less than postconvulsion. It is concluded that Na+K+ATPase activity is modified during chronic alcohol administration and during seizures induced after alcohol withdrawal by audiogenic stimulation.