DESCRIPTION: (from applicant's abstract) Epilepsy affects about 2 percent of the world population and is particularly frequent in children. Relatively little is known about how epileptic seizures propagate across and recruit apparently normal cortical circuits. Given the complexity of the neocortex, where dozens of classes of excitatory and inhibitory neurons are involved in different circuit functions, it is likely that the initiation and spread of epileptic discharges are differentially controlled by specific neuronal classes. Over the last decade, we have developed an optical approach using calcium imaging from population of neurons to study neocortical circuits and to image their activation in three dimensions with 2 photon excitations. Using this strategy, we can optically detect action potentials in the somata from dozens or hundreds of neurons, image epileptiform events with single cell resolution and detect which neurons participate in different types of epileptiform events. We propose a systematic effort to understand the role of different classes of neocortical neurons in the initiation and propagation of epilepsy. We will use calcium imaging of neuronal populations during pharmacological-induced epileptiform events in neocortical slices from juvenile (P9 - P20) rat somatosensory cortex, in order to better understand the circuit mechanisms responsible for juvenile epilepsy and at the same time image the transition from interictal to ictal events. The experiments will be carried out combining whole cell recordings and biocytin reconstructions with state-of-the-art imaging techniques, including two-photon microscopy, a photodiode array and a fast cooled CCD camera. Our first goal is to characterize morphologically and physiologically the neurons involved in spontaneous and evoked interictal and ictal epileptiform events. Our final goal is to apply a novel optical probing method to reconstruct the circuitry underlying epileptiform events by revealing the postsynaptic targets that are triggered by layer 5 IB neurons, or other potentially key cell classes. The answers to these questions could have therapeutic implications for targeting specific neurons or cell layers which play a critical role in epileptiform events. Also, our results will be particularly useful in identifying the cellular and circuit mechanism responsible for the lower seizure threshold of developing and juvenile neocortex and the transition from interictal to ictal events.