Anatomical substrates for local connectivity of neurons in layers 5a and 5b of the medial entorhinal cortex

Data used to generate figures and statistical outputs for the upcoming publication Christina M. Brown, Saishree Badrinarayanan, Zuohang Wu, Martyna Rakowska, Derek L. F. Garden, Matthew F. Nolan (in submission). 'Anatomical substrates for local connectivity of neurons in layers 5a and 5b of the medial entorhinal cortex'. Deep layers of the medial entorhinal cortex (MEC) play key roles in spatial cognition and memory by integrating inputs from the hippocampus, neocortex and superficial MEC. The distinct molecular and projection identities of neurons in layer 5a (L5a) and layer 5b (L5b) suggests that deep layer computations involve multiple steps using specialised neuron types, but the axonal pathways involved remain unclear. Here, we investigate anatomical substrates for deep layer connectivity using reconstructions of neurons in L5a and L5b, and rabies virus tracing of inputs to L5a principal neurons. Local axons of L5a pyramidal neurons had greatest length in L5b, whereas axons of L5b pyramidal and polymorphic neurons were more prevalent in L3. Axons of L5b pyramidal and polymorphic neurons either bifurcated to project in dorsal and ventral directions, or favoured a single direction. Using rabies virus-mediated trans-synaptic tracing we found that cells in layers 6, 5b and 5a project to L5a principal cells, with 42.8% of trans-synaptically labelled cells in L5b co-labelling with parvalbumin, suggesting they are inhibitory neurons, whilst 24.1% were positive for Ctip2 suggesting they are principal cells. Our results support the idea that neurons in L5a and L5b have distinct influences on local computations in the MEC, identify dorsoventral variability in the output of L5b neurons, and suggest that L5a neurons receive relatively sparse input from principal neurons in L5b that is opposed by inhibitory input from PV interneurons in L5b. We consider implications of this organisation for signal processing within hippocampal-entorhinal-telencephalic pathways. ; The dataset contains: sholl_axon_2022.csv (used for ...