Spontaneous Synchronous Network Activity Underlying The Development Of Immature Rodent Medial Prefrontal Cortex

Developing neurons and circuits exhibit spontaneous synchronous network activity during a restricted period of early brain development. These patterns of network activity enable a high degree of synchrony in immature neurons in spite of the small number of functionally mature synapses and are hypothesized to participate in activity-dependent growth, synapse formation, and generation of functional circuits in the developing CNS. Imbalances in excitatory/inhibitory synaptic maturation in mPFC are proposed to underlie behavioural alterations in neurodevelopmental and psychiatric disorders. However, little is known about the early formation and network activity of mPFC in the rodent during neurotypical development. The research here presented is aimed to identify the timeframe and major contributors to this type of activity and how it is altered in different models of neurodevelopmental disorders. In Chapter 1, we developed methods to detect cells marked with calcium-sensitive reporter dyes within developing neuronal networks, and then quantify and classify their activity. In Chapter 2, using acute coronal brain slices prepared from C57BL/6 WT pups, we used these methods with data from Ca2+ imaging, and whole cell patch-clamp to identify the critical period of SSA in the mPFC to the two first postnatal weeks. Pharmacological manipulations allowed us to recognise that the activity was largely driven by glutamatergic activity with a small contribution of gap junctions. Blockade of the GABAA receptor partially restored this activity at the end of the second postnatal week, pointing towards maturation of GABA functions as the main responsible for the cessation of this activity. In Chapter 3, by replicating the methods used in the WT animals, we show that a Fragile X mouse mode has an altered critical period with a significant delay and altered levels of activity at different time points. The model also displayed an altered response to pharmacological manipulation of GABAergic signalling, indicating early on alterations to this system. In Chapter 4, we evaluated this type of activity, and its critical period, in iPSC-derived neuronal cultures from Rett Syndrome patients. In comparison to their isogenic control, RTT neurons display a phenotype of immaturity with decreased neurite outgrowth and axonal branching. They also display a delay of SSA appearance and a reduction in spontaneous spiking frequency, suggesting a disturbance in neuronal network development. This could consequently lead to the alterations in connectivity and an impaired balance of inhibition and excitation presented by patients.