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Characterization of spontaneous electrical dynamics in developing neuronal networks in vitro

The protocol for the preparation of dissociated cortical cell cultures has been standardized among the partners. From the onset of plating rat cortical nerve cells in a culture chamber the neurons start growing out dendritic and axonal arborisations and form synaptic connections. After several days in vitro the developing neuronal network starts to exhibit electrical activity from the spontaneous action potential firing of the individual neurons. This firing activity displays two typical phases, one of uncorrelated firing among the neurons at low firing rate, and one of highly synchronized firing and strong network interactions (called network bursts). In the project the spontaneous firing dynamics has been characterized for its firing rate and spatio-temporal pattern during network development. Although network bursts were highly variable to each other, the underlying probabilistic patterns of firing in space-time appeared to be highly stable during development. As during this period outgrowth still continues and synapses are being formed, this is a surprising and important outcome.

Electrical stimulation has been shown to have lasting effects on the spontaneous firing rates of individual neurons, ranging from increased or decreased firing rates up to the complete activation or silencing of neurons. Overall spatio-temporal patterns of firing within network bursts, however, remain highly stable. Conditions for maximal responses of electrical stimulation need to be optimised for each individual culture. Different electrical stimulation protocols have been explored in order to induce changes in the response patterns of the cultured networks. Tetanic (co-) stimulation was able to significantly potentiate responses, measured by the area of the post-stimulus time histogram. With the burst-phasing protocol the responsiveness of the network could also be altered. The one-site paired pulse stimulation protocol was found to generally enhance the responsiveness of the network. The two-site paired-pulse stimulation protocol was able to depress the response at one site of a pair, elicited by a stimulus at the other site. These experiments have demonstrated the sensitivity of the network to alter its responsiveness differentially after carefully designed stimulation protocols.

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