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NEUROPHARMACOLOGY

Epileptogenesis Causes Acute and Chronic Increases in GABA_A Receptor Endocytosis That Contributes to the Induction and Maintenance of Seizures in the Hippocampal Culture Model of Acquired Epilepsy

Departments of Neurology (R.E.B., S.S., D.C.L., B.D.M., R.J.D.), Pharmacology and Toxicology (R.J.D.), and Molecular Biophysics and Biochemistry (R.J.D.), Virginia Commonwealth University, Richmond, Virginia

Altered GABAergic inhibitory tone has been observed in association with a number of both acute and chronic models of epilepsy and is believed to be the result, in part, of a decrease in function of the postsynaptic GABA_A receptor (GABA_AR). This study was carried out to investigate if alterations in receptor internalization contribute to the decrease in GABA_AR function observed with epilepsy, utilizing the hippocampal neuronal culture model of low-Mg²⁺-induced spontaneous recurrent epileptiform discharges (SREDs). Analysis of GABA_AR function in "epileptic" cultures showed a 62% reduction in [³H]flunitrazepam binding to the GABA_A receptor subunit and a 50% decrease in GABA currents when compared with controls. Confocal microscopy analysis of immunohistochemical staining of GABA_AR 2/3 subunit expression revealed approximately a 30% decrease of membrane staining in hippocampal cultures displaying SREDs immediately after low-Mg²⁺ treatment and in the chronic epileptic state. Low-Mg²⁺-treated cultures internalized antibody labeled GABA_A receptor with an increase in rate of 68% from control. Inhibition of GABA_AR endocytosis in epileptic cultures resulted in both a recovery to control levels of membrane GABA_A 2/3 immunostaining and a total blockade of SREDs. These results indicate that altered GABA_AR endocytosis contributes to the decrease in GABA_AR expression and function observed in this in vitro model of epilepsy and plays a role in causing and maintaining SREDs. Understanding the mechanisms underlying altered GABA_A R recycling may offer new insights into the pathophysiology of epilepsy and provide novel therapeutic strategies to treat this major neurological condition.

Address correspondence to: Dr. Robert J. DeLorenzo, Virginia Commonwealth University, School of Medicine, P.O. Box 980599, Richmond, VA 23298. E-mail: rdeloren{at}hsc.vcu.edu

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