Abstracts

Rationale: Axon sprouting and rewiring of neural networks after brain injury underlie both functional recovery and, in some patients, epileptogenesis. However, the nature of these changes in network connectivity is poorly understood; for example, we do not know which changes support reconstitution of function, and which drive epilepsy.Methods: To investigate the progressive synaptic changes in a neural network during post-traumatic epileptogenesis, we combined dual whole-cell patch clamp recording with two-photon imaging in a well-established hippocampal organotypic slice culture model (Berdichevsky et al. J Neurosci 2013.). Recorded neuronal activities were detected and categorized by custom software (DClamp: available at www.ieeg.org/?q=node/34) into ictal, interictal and postsynaptic activities. Results: As we have previously reported, hippocampal organotypic slices became epileptic over a predictable time course of several days after traumatic injury (slice preparation), with interictal spikes preceding ictal activity. We analyzed the cross-correlation of activities in pairs of neurons versus the inter-neuron distance for 1) ictal activity 2) interictal activity, and 3) spontaneous postsynaptic currents at different stages of epileptogenesis (days in culture). We found that interictal and postsynaptic currents were synchronized locally shortly after slicing. The correlations of postsynaptic activities in distant pyramidal cell pairs (> 200 um apart) increased steadily with number of days in culture, and most closely paralleled the appearance of ictal activity. Pyramidal neurons morphological changes were also measured by two photon imaging after Alexa Fluor 594 Hydrazides was micro-injected through the recording pipette, showed that the dendritic arborization is continuously during epileptogenesis. Conclusions: During post-traumatic alterations in neuronal network connectivity, the appearance of increased long-distance correlations in synaptic activities is most closely associated with the appearance of ictal activity in this preparation. This suggests either that synaptic connections between distant neurons are necessary for seizure activity, or that ongoing seizure activity promotes the development of these longer-range synaptic connections.