Lack of a key stage of hyper-connectivity between deep and superficial layers during barrel cortex development in a rat model of Absence Epilepsy

Abstract

The development of cortical neuronal wiring is precisely orchestrated and goes through several stages, some of which coincide with critical periods when sensory experience is most influential. In particular, although ascending excitatory and inhibitory projections from the deep layer 5 to upper layers are first strong, they recede by the end of the critical period of their targets cells located in upper layers. Alterations in these transient innervations impair the construction of the later circuits that remain at adulthood, but it is unknown whether they could lead to pathologies. Here, we address this question in a genetic model of Absence Epilepsy, a neuro-developmental disease, where epileptogenesis occurs during the postnatal maturation of barrel cortex, the seizure initiation site. Using functional mapping by laser scanning photostimulation with glutamate uncaging in slices, we investigated the pattern of projections onto layers 2/3 pyramidal cells from 2-week old rats. We found that its maturation skipped the key stage during which pyramidal cells received strong projections from both excitatory and inhibitory neurons located in deep layers. At the same age, neuronal activity recorded in vivo with two-photon functional imaging was organized in fewer clusters than in control rat pups during this transient hyper-innervation. Later, around the onset of typical absence seizures ([~]1 month old), over-excitability of cells was observed across layers. Using this genetic model of childhood epilepsy, we provide first evidence that failure to develop this transient hyper-innervation from deep cortical layers plays a role in pathological neural dysfunctions. Significance StatementDuring development of cortex, innervation from deep to upper layers is thought to provide a temporary scaffold for the construction of the circuits that remain at adulthood. Whether an alteration in this sequence causes brain malfunctions in neuro-developmental diseases is unknown. Using functional approaches, we investigated in a genetic model of Absence Epilepsy and control rats the maturation of innervation onto layer 2/3 pyramidal cells of barrel cortex and the cell organization into neuronal assemblies. We found that development in this model lacks this early surge of connectivity with deep layers and the concomitant structuring into multiple assemblies. Later on, at seizure onset, neurons in all layers are hyper-excitable, suggesting this feature of epilepsy develops from prior connectivity defects.