Differential modulation of theta oscillations across the septotemporal axis of the hippocampus by spatial, non-spatial and pharmacological manipulations

Theta oscillations (6–12 Hz) have been theorized to provide a clocking mechanism that can bring physically separated neurons together in time, thus potentially allowing for the integration or segregation of distributed cell assemblies over time. Thus theta provides a potential mechanism for coordinating the information processing occurring across vast distances within the hippocampus. A large body of literature indicates anatomical, physiological and functional differences across the septotemporal axis of the hippocampus. Differences exist in the afferent input across the septotemporal axis, as well as in the distribution of receptors, such as N-methyl-d-aspartate receptors. The fine grained spatial map present in septal hippocampus systematically degrades into a coarser spatial representation in temporal hippocampus. Also, the influence of running speed on theta decreases across the septotemporal axis. Behaviorally, the septal portion of the hippocampus has been shown to be critical for spatially based tasks, while the temporal portion has been shown to be involved in the emotional and motivational aspects of tasks. ^ Despite these septotemporal differences, little work has focused on theta oscillations outside of septal hippocampus. As a result, the research presented in this dissertation sought to examine the differential responses in theta across the septotemporal axis to spatial, non-spatial and pharmacological manipulations. Local field potentials at sites across the septotemporal axis of the hippocampus were recorded while rats ran in highly familiar and entirely novel spatial environments, as well as upon exposing them to the novel, non-spatial experience of not receiving reinforcement for their maze running. Exposure to the novel spatial environment increased the amplitude of theta in both septal and temporal hippocampus, while the novel, non-spatial experience resulted in a preferential increase in theta amplitude in temporal hippocampus. Additionally, the effects of low doses of the NMDA receptor antagonist ketamine on theta across the septotemporal axis were investigated. Ketamine disrupted theta power in more temporal aspects of the hippocampus and decrease the coherence of theta between septal and temporal hippocampus. These findings support of idea of differentiation across the septotemporal axis and illustrate the fact that theta can change independently at different sites within the hippocampus.