Sparser Representation of Experience in Aged Rat Lateral Entorhinal Cortex

Abstract

The hippocampus undergoes biological changes with age that mediate memory dysfunction. The hippocampus is highly connected to lateral entorhinal cortex (LEC), which is thought to represent non-spatial features of experience, including odors. We aimed to discover if LEC neuronal populations are selectively activated by distinct odors, and hypothesized that aging alters these activity patterns. After training adult and aged rats to run laps around a track, one behavioral group experienced two run sessions, 20 minutes apart, with the same set of odors (AA) added around the track each time, while another group had distinct odor sets (AB). mRNA of the immediate-early gene Arc is localized to discrete neuronal compartments based on the time since activation. We used fluorescence in situ hybridization and confocal microscopy to visualize the subcellular distribution of Arc mRNA to identify the neurons activated during each session. The behavioral experiences induced elevated LEC activity, but population activity failed to distinguish between distinct odor sets. This suggests that LEC populations stably represent higher order features of experience. Additionally, a lower proportion of LEC neurons participated during behavior in aged than in young rats. A decrease in neuron activation could reflect a reduction or refinement of LEC network function.