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Ready, steady, go! Development of spatial representations in the rat

Kruge, Ingvild Ulsaker
Doctoral thesis
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URI
http://hdl.handle.net/11250/2423621
Date
2016
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  • Kavliinstitutt for nevrovitenskap [177]
Abstract
In mice and men, the hippocampal region is the central brain network for declarative memory and dynamic

representation of location. Hippocampal place cells and entorhinal grid cells use head direction, border and

speed input to form an internal map of the local environment, and provides continuous updates of a rat's

position in relation to its movements in space. In particular, the crystalline readout of individual grid cells is

unique, with stable periodic firing fields that cover the available space with a metric grid. The vast majority

of knowledge about the architecture and function of the hippocampal region comes from experiments in

adult models, and currently little is known about the anatomical and functional development of these

circuits and cell types. In the work presented here, we study the development of anatomical connectivity

and functional maturation of circuits in the hippocampal region. Our results show that early development

in the region is precocious and largely independent of spatial learning. Grid cells, however, stand out in that

they rely on recent experience with vertical boundaries in order to stabilize an equidistant grid lattice at

adult ages, suggesting a highly plastic network.

In paper I we show that the first projections from hippocampal subiculum to entorhinal cortex (EC) are

already present and adult-like in their topography in the first postnatal week. Moreover, in the second

postnatal week the early projections mature and expand in line with the adult topography. This highly

organized development of connections between hippocampus and EC is strikingly different than what is

seen during development of connectivity in primary sensory cortices. Our results indicate that the

hippocampal-entorhinal system is in a ready-to-use state just before the onset of exploratory navigation in

rat pups.

In paper II we found functional head direction cells before eye-opening in the pre- and parasubiculum of

rat pups. Without visual inputs, the cells did not maintain a stable preferred head direction. However,

simultaneously recorded cells drifted coherently, similarly to what is seen during rotation experiments in

the adult head direction system. Within a few hours after eye-opening, we recorded stable head direction

cells in two-week old rat pups that displayed adult characteristics. In line with our findings in paper I, the

parahippocampal head direction cells are indeed ready to use before eyelids unseal in rat pups, and need

minimal visual inputs to stabilize.

Unlike the early adult-like representation of head-direction cells, it is already known that grid cells need the

first two weeks after eye-opening to mature. In paper III we show that grid cells are sensitive to the absence

of vertical boundaries for stable anchoring of the grid, in both young and adult rats. Our results indicate

that experience in itself is sufficient to change the properties of the grid network in varying ages.
Publisher
NTNU
Series
Doctoral theses at NTNU;2016:320

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