Temporal lobe epilepsy (TLE), the most common type of epilepsy, is characterized by destruction of specific hippocampal neurons. Recent evidence indicates that this neuronal damage is the result of excessive stimulation by excitotoxic amino acids (e.g., glutamic acid). Two stages of excitotoxic neuronal damage have been identified: and early phase, occurring in minutes, associated with osmotic imbalance, cellular swelling, and lysis; and a delayed phase, characterized by elevated intracellular calcium. One major homeostatic element which counteracts events underlying both phases of excitotoxicity is the sodium pump, Na,KATPase. Three different mammalian genes encode this family of structurally similar heterodimeric isoenzymes which are responsible for ionic and osmotic equilibrium. Although decreased Na,K-ATPase activity has been recognized in TLE and animal models of epilepsy, few studies have examined the distribution, regulation, or role of the sodium pump isoforms in epileptic hippocampal neuronal death. Our preliminary studies indicate a heterogeneous distribution of each isoform in rat hippocampal neurons. In TLE, we have discovered an inverse relationship between the level of one Na, K-ATPase isoform, and the susceptibility to neuronal death. This proposal outlines studies to explore the hypothesis that the activity of the sodium pump in individual hippocampal neurons is the major determinant of their susceptibility to excitotoxic damage. Using in-situ mRNA hybridization, monoclonal immunohistochemistry, in-vitro autoradiography, and a cytochemical assay, the amount of each isoform in the principal neurons of surgically obtained human hippocampus will be correlated with their susceptibility to epilepsy associated damage. Similar studies will be carried out in rats with seizures induced by graded electrical stimulation. Because the hippocampal somatostatin interneuron appears to be an early excitotoxic target, and thus of possible significance in the pathogenesis of TLE, we will particularly focus on the characteristics of its sodium pump. Finally, using hippocampal neurons in-vitro, we will regulate the activity of the NaKATPase isoforms and determine their role in glutamate-induced cell death. These studies will shed light on the complexity of action of the sodium pump, which is fundamentally critical to normal neuronal function. They will also provide new information about the larger issue of excitotoxic neuronal death, and possibly its prevention, which has now been implicated in the pathophysiological of many human neurodegenerative states, including stroke, Huntington's disease, hypoglycemic brain damage, and Parkinson's disease.
