Proceedings of The Physiological Society
University of Oxford (2011) Proc Physiol Soc 23, PC167
Dynamics of CA1 and subicular neurone firing during theta and slow-wave rhythm transitions in the anaesthetized rat
D. Squirrell1, J. Gigg1
1. Faculty of Life Sciences, University of Manchester, Manchester, United Kingdom.
Neurones within the hippocampus code for aspects of episodic memory such as encountered objects, the position and motion path of the animal, past and future behaviour (1). The processing of this hippocampal information is widely accepted to be orchestrated in a phase-dependent manner during oscillatory states such as theta and slow-wave rhythms (2,3). The major target for hippocampal CA1 output is the subiculum, however, despite its strategic position we know comparatively little regarding how subicular neuronal activity is organized with that of CA1. To address this we recorded simultaneous activity from CA1 and subiculum during theta and slow-wave in the urethane-anaesthetized Wistar rat (1.5g/kg; 30% w/v i.p.; n=6). Separate recording electrode arrays (each with thirty-two 413μm2 contacts over 4 shanks, spaced 50μm vertically and 200μm horizontally; A4x8-5-50-200-413, NeuroNexusTech, USA) were placed in pyramidal layers of dorsal CA1 and subiculum. Spontaneous spikes and local field potentials were recoded from each electrode for at least one hour (Recorder64, Plexon, USA). Recordings showed predominant theta activity (c.75% of recording period) with the occasional transition to slow-wave (c.25% of period). Spikes were sorted (OfflineSorter, Plexon) and field potentials down-sampled followed by filtering to reveal periods of theta or slow-wave. Phase-histograms for sorted spikes relative to theta peaks (nominally 0 degree phase) were computed (Spike2, CED) and analyzed (Oriana v3, KCS) to determine the strength of tuning and preferred firing phase. Results showed that CA1 neurones (n=33) could be separated into two populations based on their firing preference (mean vectors of 34 and 200 degrees). This is consistent with previous findings (4). Subicular neurones (n=75), however, fired as a single population with a mean vector angle of 15 degrees. Most neurones in CA1 and subiculum fired during both theta and slow-wave; however, subicular neurones shifted their firing pattern significantly from tonic to bursting as the ongoing rhythm moved from theta to slow-wave, with a tendency for an opposite shift in CA1. The major conclusions from these results are: (a) that, at least under urethane anaesthesia, transfer of information from dorsal CA1 to dorsal subicular targets may occur predominantly during the positive phase of theta (as measured in CA1 pyramidal layer), that is, not at theta 'trough'; and (b) that the potential 'read-out' of information from subiculum during slow-wave may be greatly facilitated by a switch in spike firing of individual subicular neurones from tonic to bursting patterns.
Where applicable, experiments conform with Society ethical requirements