Microcircuitry and afferent connectivity of the deep layers of the medial entorhinal cortex

The ability to navigate the environment and to remember and learn from past experiences is essential for an organism’s survival. In the mammalian brain, these functions rely on an interplay between the hippocampal formation and the medial entorhinal cortex (MEC) which use multimodal sensory information to generate internal representations of the external world. Detailed spatial maps of the environment are first created by neurons in superficial layers of MEC. Hippocampal circuits subsequently combine the spatial information into complex representations that correspond to specific memory episodes. The mnemonic representations are finally consolidated in neocortical networks, requiring signals to be transmitted through neuronal populations in MEC deep layers V (LV) and VI (LVI). Recent structural and functional insights have forced the classical view of MEC deep layers as a simple hippocampal-neocortical relay station to be greatly revised. LV can be divided into sublayers Va and Vb with contrasting roles – while cells in LVa mediate the canonical transfer of hippocampal representations to the neocortex, cells in LVb project locally to superficial MEC layers. Conversely, LVI neurons project back to all hippocampal subfields and help stabilize hippocampal representations. Although these findings suggest a capacity for sophisticated information processing within the MEC deep layer network, detailed understanding of this processing has remained incomplete due to poor knowledge of hippocampal innervation of individual deep layers and crosstalk between layers V and VI. The present study combines anatomical tracing and in vitro electrophysiology to systematically characterize the functional organization of the hippocampal output pathway to MEC deep layers and the integration of LVI neurons into the MEC deep layer network. We confirm direct hippocampal projections to both layers V and VI and demonstrate the preferential targeting of LV neurons. Importantly, we discover a novel ventral hippocampal projection to LVa cells that uniquely distributes along the entire MEC dorsoventral axis. Simultaneously, we verify that dorsal hippocampal outputs mainly target LVb neurons. These sublayer-specific connectivity patterns set important constraints for the flow of hippocampal information to different downstream networks. We further examine interlaminar connectivity of LVI neurons, finding minimal excitatory and relatively common inhibitory connections between cells in layers VI and V. Overall, our results establish organizational principles for the hippocampal-medial entorhinal output pathway and suggest that MEC deep layers process signals largely independently in parallel streams of activity.