Event:

To study the interplay between hippocampus and medial prefrontal cortex (Pfc) and its importance for learning and memory consolidation, our team measured the coherence in theta oscillations between these two structures in rats learning new rules on a Y-maze. This coherence peaks at the choice point of the maze, and is further augmented there upon acquisition of the task rule. Simultaneously, Pfc pyramidal neurons reorganize their phase relation to hippocampal theta, concentrating at the theta trough. At the same time synchronously active Pfc cell assemblies emerge, also at the choice point. The formation of these cell assemblies during high coherence periods could result from the increased inhibition exerted by interneurons on pyramidal cells then, as measured by cross-correlation analyses. This may result from dopamine, because we find similar hippocampal-Pfc theta coherence increases and neuronal phase shifts following local administration of dopamine in Pfc of anesthetized rats. Pfc cell assemblies emerging during high coherence are preferentially replayed during subsequent sleep, concurrent with hippocampal sharp waves (when hippocampal cell assemblies replay previous experience). Since the formation of hippocampal theta modulated Pfc cell assemblies during high coherence periods would synchronize them with Hpc cell assemblies, this would lead to Hebbian LTP between the two cell assemblies, as evidenced by the reactivation of Pfc cell assemblies during Hpc ripples in sleep after the task. Thus hippocampal/prefrontal theta coherence could selectively lead to synchronization of reward predictive activity in prefrontal networks, tagging it for subsequent memory consolidation. Overall, this suggests that coherent oscillations may serve as a general brain mechanism for synchronization of cell assemblies between brain areas and then favoring timely facilitation of communication in specific functional pathways for processes such as decision making and mnesic consolidation.