The ventral midline thalamus coordinates prefrontal–hippocampal neural synchrony during vicarious trial and error

When faced with difficult choices, the possible outcomes are considered through a process known as deliberation. In rats, deliberation is thought to be reflected by pause-and-reorienting behaviors, better known as vicarious trial and errors (VTEs). While VTEs are thought to require medial prefrontal cortex (mPFC) and dorsal hippocampal (dHPC) interactions, no empirical evidence has yet demonstrated such a dual requirement. The nucleus reuniens (Re) of the ventral midline thalamus is anatomically connected with both the mPFC and dHPC, is required for HPC-dependent spatial memory tasks, and is critical for mPFC-dHPC neural synchronization. Currently, it is unclear if, or how, the Re is involved in deliberation. Therefore, by examining the role of the Re on VTE behaviors, we can better understand the anatomical and physiological mechanisms supporting deliberation. Here, we examined the impact of Re suppression on VTE behaviors and mPFC-dHPC theta synchrony during asymptotic performance of a HPC-dependent delayed alternation (DA) task. Pharmacological suppression of the Re increased VTE behaviors that occurred with repetitive choice errors. These errors were best characterized as perseverative behaviors, in which some rats repeatedly selected a goal arm that previously yielded no reward. We then examined the impact of Re suppression on mPFC-dHPC theta synchrony during VTEs. We found that during VTEs, Re inactivation was associated with a reduction in mPFC-dHPC theta coherence and mPFC-to-dHPC theta directionality. Our findings suggest that the Re contributes to deliberation by coordinating mPFC-dHPC neural interactions.