Abstracts

RATIONALE: Epilepsy is probably a network disorder. It requires hypersynchronous firing of a network that is comprised of interconnected neuronal clusters. To study the behavior of the epileptic neuronal network in a top-down manner, a research method that could detect the network activity with satisfactory combined temporal and spatial resolution is needed. Optical intrinsic signal (OIS) imaging, which has been shown to detect seizure activity with excellent sensitivity (Chen et. al, Neurology 2000 55: 312-315), is used in this study to investigate the hypersynchronous activity of neocortical neurons of rats during acute seizures induced by penicillin.
METHODS: In Spraque-Dawley rats (n = 12), surgery and OIS imaging were performed under anesthesia. The rat was placed in a stereotactic frame, and the skull bone was thinned with a scrapper. EEG depth electrode was mounted on a stereotactic manipulator, advanced and placed in the cortex through the burr hole at 4 mm posteriorly, 6 mm laterally relative to the bregma on the side of OIS imaging. The referential and ground electrodes were placed on the scalp and forelimb respectively. EEG activity was monitored with an oscilloscope. OIS imaging was obtained with a CCD camera (Princeton Instruments, EEV 0206, 192 X 144 pixel array) over one hemisphere with an optical filter of 850 or 610 nm. EEG and OIS signals were synchronized and collected on a PC computer simultaneously. The EEG sampling frequency was 1000 Hz, and the camera frame rate was either 2 frames per second or 1 frame per 3 seconds. The exposure time was 200 milliseconds or 2.5 seconds respectively. Acute seizures were induced by injecting or dripping 100 units of penicillin solution topically over the second burr hole 2 mm in front of the EEG electrode. The dura matter was removed in both burr holes during surgery by using the dental drill, or a sharp syringe needle. A ratio analysis was performed on the optical images by using the equation: [(data image - control image) / control image]. The control image was selected from images before penicillin application. Regions of interest (ROI) were selected manually from separate areas showing synchronus activity after visual inspection for correlation analysis.
RESULTS: Various brain regions were shown to have synchronized activity on OIS imaging during seizures. In the early phase of electrographical seizures, several separate but remote brain regions had shown synchronized activity. These areas probably represent a seizure network of hypersynchronized activity. With the progression of seizures, the areas of synchronized activity expanded and convened with each other. The correlation analysis of ROI, with a correlation coefficient of 0.96 [plusminus] 0.01 (mean [plusminus] s.d., n = 12), supports the notion that hypersynchronous activity could be demonstrated with OIS imaging in seizures.
CONCLUSIONS: 1)Hypersynchronous activity among remote brain regions during acute seizures could be demonstrated by OIS imaging.
2)OIS imaging could be employed to study the behavior of epileptic networks in the neocortex.
[Supported by: VA Career Development Award and NIMH grant MH52083]