Abstracts

Rationale: Absence seizures are generalized onset seizures that occur most commonly in children and are associated with a rhythmic spike-wave discharge (SWD) on EEG. Absence seizures can be extremely debilitating, occurring hundreds of times per day. Previous work in rats showed that there are variations in the activity of neurons in response to the absence seizure and these variations can be classified in four main categories (sustained increase, sustained decrease, onset peak and no change). Our goal in this project is to understand why these variations exist and how they can be useful in understanding the mechanism of SWDs.

Methods: C3H/HeJ mice of age 6-11 weeks were implanted with tripolar electrodes under ketamine/xylazine anesthesia. Intracranial EEG electrodes were placed at frontal and parietal cortex and a ground electrode placed mid-cerebellum. Following recovery, the awake mice were head-fixed, and EEG was recorded using an AM systems Model 1800 AC amplifier. A burr hole was created over the right barrel cortex for recording neuronal activity later. A glass electrode containing a bleached silver wire, immersed in a solution of neurobiotin tracer (dissolved in NaCl solution), was inserted into the burr hole. The electrode was gradually lowered to a maximum depth of 1.2mm (thickness of barrel cortex) until neuronal activity was seen. Voltage was measured using a MultiClamp 700B and signals were digitized at a sampling rate of 20 kHz with a CED 1401 A/D converter and analyzed using Spike 2. Neurons were labelled with the neurobiotin by passing 600 pA current pulses at 2 Hz.

Results: Recordings from 27 neurons showed a variety of neuronal activities in the cortex. These could broadly be classified into neurons with activity synchronized to the seizure and neurons with activity nonsynchronous to the seizure with most falling into the former category. The activity of the neurons synchronized to the seizure also showed significant variations and could be broadly classified into 4 categories. These categories are sustained increase (frequency of action potentials is elevated throughout seizure), sustained decrease (frequency of action potentials is decreased throughout seizure), onset peak (frequency of action potentials is elevated only at the beginning of the seizure) and no change (frequency of action potentials remains the same).

Conclusions: There are variations in the activity of neurons in response to absence seizures. These variations are not species specific and are found in both rats and mice. The variation in activity of neurons can be divided into 4 categories (sustained increase, sustained decrease, onset peak and no change) and these categories are found across both species. These findings are significant because they suggest that there is a characteristic neuronal response to absence seizures that is broadly preserved across species and could potentially be used as a model to study neuronal dynamics of absence seizures in humans.

Funding: NIH/NINDS R37NS10090, James G. Hirsch, M.D. Medical Student Fellowship – Yale School of Medicine