Network integration of neocortical Cajal-Retzius neurons shapes early cortical dynamics and contributes to their transient nature.

Cajal-Retzius neurons (CRN) represent an early-born transient neuronal population in the mammalian neocortex. They are best known for secreting reelin, which is essential for neuronal migration and layer formation in the developing cortex. However, their functional integration into developing cortical circuits and their contribution to early network dynamics have been poorly understood. In this study, we investigated the structural and functional integration of CRN into developing neocortical networks with immunohistochemistry, three-dimensional reconstruction of GABAergic inputs, extracellular electrophysiology and calcium imaging. In addition, multiplexed FISH was used to determine the expression levels of the chloride transporters NKCC1 and KCC2 in CRN at different postnatal stages. Finally, we employed optogenetic stimulation to assess the functional and developmental consequences of CRN activation. We found that CRN receive dense dendritic GABAergic synaptic inputs at early postnatal stages. Functional analyses revealed that a substantial fraction of CRN is spontaneously active, and that their activity is synchronized with both CRN and non-CRN neurons. At the molecular level, they display a persistent NKCC1-dominant expression profile. This profile can contribute to the excitatory GABA responses, but also affect their cell death, as chronic blockade of NKCC1 was associated with reduced CRN loss in vitro. Acute optogenetic activation of CRN at early stages increased network excitability in organotypic neocortical cultures and induced stimulus-evoked field responses, while chronic activation at later time points accelerated CRN loss. Overall, these findings demonstrate that, during development, dense GABAergic input connectivity, coupled with the putative excitatory action of GABA, can engage CRN in early network activity. Furthermore, not only do CRN actively participate in early immature circuits, but their activity also amplifies spontaneous network dynamics and contributes to their own transience, highlighting a potential novel functional role of CRN during neocortical development.