Abstracts

Rationale: Nearly 30% of patients with epilepsy do not respond to currently available treatments; therefore novel therapies are being heavily investigated. Furthermore, the mechanism of onset of epilepsy is not fully understood and there is a need to further examine the disease process in order to find new therapeutic targets. The hippocampal structure and aberrant regulation of glutamate have been strongly implicated in epilepsy. The following studies illustrate the use of microelectrode arrays (MEA) to elucidate more information about glutamate dynamics in the hippocampus of different animal models of epilepsy. Acute recordings of glutamate were conducted in models of epilepsy including kindled, traumatic brain injured (TBI) and genetic models. In addition, chronic, awake recordings were completed in 4-aminopyridine (4-AP) treated rats. Methods: Microelectrode arrays for all studies were prepared for glutamate detection. Briefly, MEAs consist of platinum recording sites, arranged in side-by-side pairs. One pair of sites was coated with glutamate oxidase in a protein matrix, which catalyzed the breakdown of glutamate into a reporter molecule (peroxide), which was oxidized by the applied potential at the MEA surface and measured via the FAST system (Quanteon, LLC). The other pair of sites had the protein matrix, but no enzyme; it measured background current and noise/interferents for self-referencing. A single-barrel micropipette or a 26-gauge stainless steel guide cannula was attached to the ceramic paddle with sticky wax to allow for intracranial application of different solutions. The ceramic substrate of the MEA used for acute studies was modified for chronic implantation allowing glutamate measurements in awake rats. A shortened paddle design configured with the ceramic MEA tip was attached to a miniature connector along with a Ag/AgCl reference electrode. All electrical connections were covered with epoxy (Figure 1). Animal models studied were amygdalar kindled mice and rats, a 4-AP rat model, two genetic mouse models, and a TBI rat model. Results: Microelectrode arrays offer superior spatial ( m) and temporal (1-4 Hz) resolution, which allowed us to record glutamate dynamics in discrete areas of the hippocampus on a sub-second basis. All models showed unique and never-before-seen spontaneous and highly rhythmic bursts of glutamate we term transients (Figure 2). These transients were seen in all sub regions of the hippocampus. The amplitude of the transients was increased in anesthetized, kindled models, and increased during status epilepticus in awake animals. In chronically implanted rats, the transients coincided with behavioral signs of status epilepticus, and both transients and seizure behavior ceased upon administration of tetrodotoxin (TTX). Conclusions: Because glutamate neurotransmission in the hippocampus is dysregulated in the animal models studied, glutamate could prove to be a powerful target for future antiepileptic therapies.