Extrinsic control of the early postnatal CA1 hippocampal circuits

The adult CA1 region of the hippocampus produces coordinated neuronal dynamics with minimal reliance on its extrinsic inputs. By contrast, neonatal CA1 is tightly linked to externally generated sensorimotor activity, but the circuit mechanisms underlying early synchronous activity in CA1 remain unclear. Here, using a combination of in vivo and ex vivo circuit mapping, calcium imaging, and electrophysiological recordings in mouse pups, we show that early dynamics in the ventro-intermediate CA1 are under the mixed influence of entorhinal (EC) and thalamic (VMT) inputs. Both VMT and EC can drive internally generated synchronous events ex vivo. However, movement-related population bursts detected in vivo are exclusively driven by the EC. These differential effects on synchrony reflect the different intrahippocampal targets of these inputs. Hence, cortical and subcortical pathways act differently on the neonatal CA1, implying distinct contributions to the development of the hippocampal microcircuit and related cognitive maps. [Display omitted] •Early CA1 spontaneous dynamics rely on extrinsic inputs from the EC and the VMT•EC inputs activate directly both GABAergic neurons and pyramidal cells•VMT inputs preferentially directly activate local GABAergic interneurons•Twitch-related synchronous activity in CA1 is exclusively driven by the EC Leprince et al. disentangle the respective contributions of the entorhinal and thalamic inputs to the coordination of local CA1 dynamics at the end of the first postnatal week in mice. Both inputs can significantly modulate the ex vivo and in vivo early CA1 dynamics but through different integration pathways.