Ventral Midline Thalamus Is Necessary for Hippocampal Place Field Stability and Cell Firing Modulation.

The reuniens (Re) and rhomboid (Rh) nuclei of the ventral midline thalamus are reciprocally connected with the hippocampus (Hip) and the medial prefrontal cortex (mPFC). Growing evidence suggests that these nuclei might play a crucial role in cognitive processes requiring Hip-mPFC interactions, including spatial navigation. Here, we tested the effect of ReRh lesions on the firing properties and spatial activity of dorsal hippocampal CA1 place cells as male rats explored a familiar or a novel environment. We found no change in the spatial characteristics of CA1 place cells in the familiar environment following ReRh lesions. Contrariwise, spatial coherence was decreased during the first session in a novel environment. We then investigated field stability of place cells recorded across 5 d both in the familiar and in a novel environment presented in a predefined sequence. While the remapping capacity of the place cells was not affected by the lesion, our results clearly demonstrated a disruption of the CA1 cellular representation of both environments in ReRh rats. More specifically, we found ReRh lesions to produce (1) a pronounced and long-lasting decrease of place field stability and (2) a strong alteration of overdispersion (i.e., firing variability). Thus, in ReRh rats, exploration of a novel environment appears to interfere with the representation of the familiar one, leading to decreased field stability in both environments. The present study shows the involvement of ReRh nuclei in the long-term spatial stability of CA1 place fields. SIGNIFICANCE STATEMENT Growing evidence suggest that the ventral midline thalamic nuclei (reuniens and rhomboid) might play a substantial role in various cognitive tasks including spatial memory. In the present article, we show that the lesions of these nuclei impair the spatial representations encoded by CA1 place cells of both familiar and novel environments. First, reduced variability of place cell firing appears to indicate an impairment of attentional processes. Second, impaired stability of place cell representations could explain the long-term memory deficits observed in previous behavioral studies.