Origin and propagation of spontaneous electrographic sharp waves in the in vitro turtle brain: a model of neuronal synchronization

Objectives: Neuronal synchronization is a basic feature in the generation of epileptiform discharges. Spontaneous large sharp waves (LSWs) can be recorded in the turtle brain in vitro, indicating the synchronous activation of large neuronal populations. The aim of this study was to analyze the spatial and temporal distribution of LSWs within the brain; the participation of glutamate in LSWs generation was also investigated. Methods: Extracellular field potentials were recorded in vivo (n=4) and in vitro (n=36). LSWs were recorded from cerebral cortex, optic tectum, and thalamus. Results: LSWs were recorded from cerebral cortex, optic tectum and thalamus. No LSWs were observed in cerebellum and brain stem. In some experiments, LSWs could be recorded only from medial cortex. Latency studies demonstrated that, within each hemisphere, medial cortex led the generation of LSWs; in addition, isolated medial cortex could sustain LSWs. Intracortical laminar field potentials in medial cortex indicated that LSWs generate mainly in the molecular layer, probably at pyramidal cell dendrites. Pharmacological experiments demonstrated that NMDA and non-NMDA glutamate receptors are involved in LWSs generation. Conclusions: These results suggest that turtle medial cortex is the pacemaker area for LSWs generation and it can be a useful model to study cellular and circuital mechanisms of neuronal synchronization.