Circuitry and function of layer 1 interneurons in the medial entorhinal cortex

For mammals to navigate through a complex environment and remember paths to previous destinations, neural processing must occur in the hippocampal formation. In this region of the brain, a number of different cell populations exhibit spatially modulated firing patterns. One area in particular, called the medial entorhinal cortex (MEC), contains a high density of spatially-tuned cell types that are thought to be integral to a dynamic perception of location. These cells are mostly concentrated in the superficial layers of the MEC and have been well-studied since their discovery. Much of this research includes the examination of local interneurons and their tight control over the firing patterns of excitatory cells, such as layer 2 parvalbumin- and somatostatin-expressing interneurons. However, the contribution of interneurons in layer 1 has been overlooked, even though it likely contains dendritic branches from spatially-tuned neurons. To understand the functional role of layer 1 in the MEC, I sought to characterise the electrophysiology and morphology of layer 1 interneurons, how they fit within the MEC circuitry and their effects on behaviour. Recent research has uncovered a selective marker for neocortical L1 cells called neuron-derived neurotrophic factor (NDNF). This, in turn, has led to the creation of the NDNF-Cre mouse line, which grants genetic access to NDNF+ve cells, making it a potentially effective animal model for investigating layer 1 interneurons. Using this mouse line, I began investigating the anatomical features of layer 1 neurons in the MEC and evaluated the suitability of the NDNF-Cre line for targeting these interneurons. Using cre-dependent viral reporters and immunohistochemistry, I demonstrate that the NDNF-Cre mouse line grants selective access to the majority of layer 1 cells in the MEC, making it a suitable animal model for investigating the role of MEC layer 1. I next performed whole cell patch clamp recordings on NDNF+ve cells to establish their electrophysiological properties and filled them with biocytin to visualise their morphology. These neurons exhibited a typical neurogliaform cell electrophysiology and morphology, which is consistent with layer 1 cells in other cortices. I then examined the synaptic outputs of NDNF+ve cells by combining patch clamp recordings of principal neurons in layers 1–5 with the optogenetic activation of layer 1 NDNF+ve cells. This showed that principal neurons in all layers of the MEC receive direct GABAergic inhibition from NDNF+ve interneurons. With their main output connections mapped out, I sought to identify the origins of their inputs by using a cre-dependent rabies virus retrograde tracing approach. This revealed the labelling of input neurons in a variety of cortical structures, with most input cells localised in the hippocampal formation and in layer 3 of the MEC. Based on these experiments, I hypothesised that layer 1 NDNF+ve cell activation would suppress spatial learning. To test this hypothesis in the future, I evaluated strategies for the manipulation of L1 NDNF+ve interneurons during spatial learning and memory formation. Together, these results show that a significant portion of layer 1 cells in the MEC directly inhibit principal neurons in layers 1–5 and receive inputs primarily from cells within the hippocampal formation. Although this may imply that much of the communication in layer 1 is related to spatial information, its functional role remains unclear.

Grantor dc:publisher

The University of Edinburgh

Year dc:date.issued

2024

Author dc:creator

• Armstrong, Jack

Advisors dc:contributor.advisor

• Nolan, Matthew
• Surmeli, Gulsen
• Ainge, James
• Daw, Michael

• medial entorhinal cortex
• interneurons
• layer 1 interneurons
• spatial learning
• MEC circuitry
• NDNF

Language dc:language.iso

en

Identifier URI

http://dx.doi.org/10.7488/era/4471

OAI identifier oai:identifier

oai:era.ed.ac.uk:1842/41748