Reuniens transiently synchronizes memory networks at beta frequencies

Using episodic memories to help guide decisions requires top-down medial prefrontal cortex interactions with the hippocampus. Mechanistically, this integrated prefrontal-hippocampal memory state is thought to be organized by synchronized network oscillations and mediated by connectivity with the nucleus reuniens of the thalamus. Here, we recorded local field potentials from the prefrontal-reuniens-hippocampal network while rats were engaged in a nonspatial sequence memory task which helps isolate memory-related activity from running-related oscillations. We found that synchronous prefrontal-hippocampal beta bursts (15-30 Hz) were dominant during memory trials, whereas synchronous theta (6-12 Hz) was highest during running. Beta rose during a trial and peaked just before a decision, whereas theta was highest during running. Beta bursts first appeared in reuniens and then in prefrontal and hippocampal sites simultaneously, suggesting beta could be directly driven by reuniens. To test this, we used an optogenetic approach (retroAAV-ChR2) to see if reuniens was capable of driving prefrontal-hippocampal beta synchrony. Reuniens activation induced prefrontal-hippocampal beta coherence, and reduced theta coherence, resembling the observed memory-driven network state. These findings demonstrate for the first time that reuniens contributes to memory by driving transient synchronous beta in the prefrontal-hippocampal system facilitating coherent interactions that sub-serve memory-based decision making.