Mechanisms of Sleep-Dependent Memory Consolidation in the Rodent Prefrontal Cortex

Abstract

Animals learn over the course of a lifetime, and the ability to retrieve memories after long delays depends on memory consolidation. This process involves coordinated oscillatory and cellular activity in the cortex and hippocampus during sleep, but it is not clear how sleep activity after learning engages this mechanism. Studies of memory consolidation in rats have identified two types of oscillatory activity—sleep spindles in the prefrontal cortex and sharp wave-ripples in the hippocampus—as events that may guide cell activity that supports memory consolidation. This dissertation comprises studies of electrophysiological methods and behavioral paradigms used in the investigation of memory consolidation, and applies these to experimental work on oscillations and cell activity occurring during sleep. The methodological study develops a procedure for the selection of spindle detection parameters and extends it for application to experimental work. The study of behavioral tasks relevant to memory consolidation examines two existing tasks—one driven by cues, and the other designed to require new learning—and identifies differences in their behavioral profiles, but not their impact on broad sleep features, establishing that they constitute a well-matched pair of tasks for reasoning about the differential recruitment of memory consolidation mechanisms. In experimental work regarding the influence of these tasks on spindles and ripples, coordination is observed to occur between these two oscillations but no effect is found for either task on the oscillation density of ripples (per second) or spindles (per minute), or on the coordination between them. Further investigation of the coordination of cell activity during spindles shows that prefrontal cell activity may be influenced by co-occurring ripple activity, yet this influence does not vary with respect to task exposures. The possibility is discussed that the lack of task effects is due to higher task familiarity than in other memory consolidation research.