The neural representation of space is encoded by spatially-modulated
neurons including head direction cells (HD cells), place cells, and grid
cells. Previous work has shown that these cell types emerge sequentially in
rat postnatal development. Typically, spatial cognition experiments using
in vivo electrophysiology are made as a single animal forages in an openfield environment while tethered to an acquisition system. In developmental studies, this requires the removal of the rat pup from its homecage,
mother and littermates. Wireless technology is emerging as a promising
alternative: neural data loggers permit the recording of single-unit neuronal activity in an animal’s homecage, thereby tracking spatial cell development while minimising disruption of early sensory experiences.
Wireless recordings of spatial cells have not previously been conducted in the developing rat. The first experiment presented in this thesis
was therefore a feasibility study demonstrating that wireless recordings of
spatial cells in rat pups are comparable to standard techniques, focusing
on HD cells in the anterodorsal thalamic nucleus (ADN), and place cells in
the CA1 region of the hippocampus.
The second experiment of this thesis employed wireless homecage
recordings to investigate whether the activity patterns of HD cells during development differed from those observed in traditional open-field
recordings. To address the research question, HD cell ensembles in the
ADN were wirelessly recorded in the homecage from postnatal day (P)12
to P16. The directional modulation of HD cells was analysed by calculating both the mean resultant vector length (RV length) and directional information. Whilst RV length reflects the unidirectional tuning preference
of individual HD cells, directional information quantifies any directional
modulation of a cell, whether unidirectional or multidirectional. The results revealed that HD cells in the homecage exhibited lower RV lengths
compared to those in the open field, indicating poorer unidirectional tuning. However, this difference was not reflected in the directional information of recorded cells, which remained consistent between environments
until P16. This suggests that while heading direction in the open field is
primarily encoded in a unidirectional manner, in the homecage it may be
encoded through multiple preferred firing directions within a trial, indicating frequent resetting of HD cells. This phenomenon had the effect of
equalising the directional information of cells between the two environments.
The temporal and spatial coupling of recorded cells was also investigated on short-timescales. Strikingly, in contrast to previous reports (Bassett et al., 2018), the temporal and spatial synchronisation of HD cell ensemble firing did not precede the establishment of stable unidirectional
tuning in the homecage. The internal spatial organisation of the network
did also not appear to be maintained between recording environments,
which may be as a result of differing movement statistics between environments. This finding suggests that the attractor network properties of
HD cells in developing rats may be contingent on the context in which
they are studied.