Abstracts

Rationale: To study the role of heterotopic neuronal clusters in epilepsy, we studied a rat model of Periventricular Nodular Heterotopia (PNH). Specifically, we investigated the role of the PNH in initiation and propagation of epileptiform discharges (EDs), and examined the connectivity of PNH cells in brain slices from MAM rats, Methods: Pregnant Sprague-Dawley rats were injected (gestational day 15) with 25mg/kg Methyl-Azoxy-Methanol (MAM). This treatment reproducibly causes abnormal brain development in offspring, with pronounced abnormally located cell clusters (nodules). Parasagittal brain slices were prepared from the brains of MAM rats. Field potentials were recorded at three different sites (hippocampus, cortex and PNH); epileptiform discharges were elicited in the tissue by bath application of 50 µM bicuculline methiodide and 5 mM K+. To asses the role of the PNH in seizure discharge, mechanical cuts were made between: a) the PNH and hippocampus; b) the PNH and cortex; c) entorhinal cortex and neocortex; and d) cortex and hippocampus. Intracellular recordings were obtained from individual cells in the PNH, and responses recorded to electrical stimulation within the PNH and/or neighboring cortex and hippocampus. 20µM bicuculline was bath-applied to study GABA-mediated inhibition Recorded cells were filled with 2% biocytin for anatomical reconstruction. Results: In 18 experiments, bicuculline-induced EDs appeared first in the hippocampus. In intact slices, EDs were time-locked across recording sites, with onset order: hippocampus, PNH, cortex. Rarely, EDs, were seen within the PNH which were not time-locked with the neighboring tissues. In 10 experiments, after a cut was made between the PNH and the hippocampus, the sequence of discharge changed to; hippocampus, then cortex, and finally the PNH (Fig 1). In the remaining 8 experiments, PNH discharges were not time-locked to those of the hippocampus or cortex after the cut. In 37 experiments, intracellularly-recorded neurons in the PNH showed EPSPs evoked in response to stimulation of either cortex or hippocampus. In 7 experiments, stimulation at either site also elicited antidromic action potentials. In 24 neurons, stimulation within the PNH elicited IPSPs which were blocked by bicuculline (4 of 7 experiments). Anatomical reconstruction of biocytin-filled PNH neurons revealed an extensive dendritic arborization within the nodule (Fig 2); in some cells, an axon was seen entering the neighboring cortex. Conclusions: In vitro data from this model suggest that the PNH receives independent excitatory drives from both the hippocampus and cortex, with predominant input from hippocampus. When challenged with an epileptogenic stimulus, the PNH does not usually initiate EDs; rather, it follows bursts initiated in hippocampus (or cortex, when hippocampal connection are severed). Given this functional connectivity the PNH is unlikely to be the initiating zone of seizure activity, but may serve as bridge for the propagation of epileptiform discharges. Supported by NIH grant NS57209