The spontaneous activity of organotypic and dissociated networks

In the absence of external stimuli, the nervous system exhibits a spontaneous electrical activity whose functions are not fully understood, and that represents the background noise of brain operations. In vitro models have long represented a simple and useful tool for studying the basic properties of neurons and networks. This study provides a detailed characterization of spontaneous activity of neuronal networks in different in vitro models. In particular, it clarifies the role of the extra-cellular environment and of the intrinsic architecture in shaping the spontaneous activity of networks by means of calcium imaging techniques. The results presented within this study come from three experimental works, each one addressing a particular feature of the network model: • Chemical composition of the extra-cellular environment: a comparison of dissociated hippocampal cultures grown in three different culturing media revealed that the use of an astrocyte-conditioned medium improves significantly the frequency and synchronization of neuronal signaling. • Mechanical and topographical properties of the extra-cellular environment: the design of a hybrid micro-nano substrate for dissociated hippocampal cultures revealed that nano-scaled patterns provide an improved artificial extra-cellular matrix for obtaining neuronal networks with a frequent spontaneous signaling. • Network architecture: synchronized events called Global Up states - involving the totality of neurons in the network - are observed in both organotypic and dissociated neurons; the duration of Global Up states increases by increasing the complexity of the network, while their frequency decreases. Simulations with simplified models of single- and multilayered networks confirm the experimental data. Taken together, these results show that the spontaneous synchronous activity of neurons is a result of their intrinsic biophysical properties, arising also after disruption of the original network architecture. However, dissociated neurons show different levels of synchrony depending on the chemical and topographical composition of the surrounding artificial extra-cellular matrix. Moreover, the specific architecture of the network and its single- or multilayered composition has an influence on the frequency and duration of spontaneous events, suggesting a potential explanation for the diversity of oscillatory rhythms observed in the brain.