Oscillatory patterns along the entire septo-temporal axis of the hippocampus in behaving rats

The hippocampus exhibits two major modes of activity: Theta oscillations during the “online” active behavior and sharp-wave associated ripples during the “offline” inactive state. The rodent hippocampus is a complex C- shaped structure with a longitudinal axis (LA) of approximately 10mm. While theta and ripples oscillations have been extensively studied over the last 4 decades with strong evidence of their role in hippocampal memory functions, the organization of these oscillatory patterns along the LA has never been studied before. This lacuna is striking because there are genomic, anatomic, cellular, synaptic, and functional differences along its LA. My dissertation investigated the organization of theta (5-10Hz) and sharp wave associated ripple (140 -200Hz) oscillations along the entire LA of the hippocampus in behaving rats. Using multiple drives I simultaneously recorded local field potentials and single unit activity from multiple locations along the LA. The first study examined traveling theta waves. While the frequency of theta oscillations remained similar, theta amplitude and coherence between recording sites decreased from the dorsal to the ventral hippocampus. Theta phase shifted monotonically along the LA, reaching a maximum of 180o. While the majority of single units were phase-locked to the local field at all dorso-ventral segments, approximately 25% of single units from the ventral segment were phase locked to the dorsal hippocampal theta. Ventral hippocampal theta was weakly correlated with the locomotion velocity and varied independently of dorsal hippocampal theta. Thus, theta oscillations can temporally integrate or segregate neocortical representations along the LA of the hippocampus. The second study examined the local generation and spread of ripples. Qualitatively similar ripples occurred at all levels of the LA and propagated in both directions: septally or temporally, with an average speed of 0.35m/sec. While ripples propagated smoothly in the septal 2/3rd of the hippocampal tissue, ripples in the ventral segment remained isolated. These findings suggest that the septal hippocampus can use ripples to combine the information before transferring integrated signals to downstream targets. However, because ripples occur at different times in the ventral and dorsal segments, they broadcast independent information to their non-overlapping cortical and subcortical targets.Ph. D.Includes bibliographical referencesIncludes vitaby Jagdish Pate