Projections from medial prefrontal cortex and hippocampus to midline thalamus and perirhinal cortex: a double retrograde tracing study in rats

The nucleus reuniens (RE), paraventricular nucleus of the thalamus (PVT), and perirhinal cortex (PRC), which have reciprocal connections with both the medial prefrontal cortex (mPFC) and hippocampus (HPC), could act as the relays to regulate the interplay between the mPFC and HPC. To better understand the anatomical structure of neurons in the mPFC and HPC subregions projecting to the peri-reuniens nucleus (periRE), posterior PVT (pPVT), and PRC, fluorescent retrograde tracers of CTB-AF488 or CTB-AF594 were unilaterally injected into the pPVT and PRC (Experiment 1) or into the pPVT and periRE (Experiment 2), respectively. The cell labeling was examined in the region of interest of the mPFC-HPC network, including the prelimbic cortex (PL), infralimbic cortex (IL), dorsal and ventral CA1 (dCA1 and vCA1), and dorsal and ventral subiculum (dSub and vSub). Our results indicated that mPFC projections to the periRE or pPVT were more robust on the ipsilateral side in general, with the exception of IL projections to the pPVT showing no lateralization, and the majority of the projections originated from the deep layer. There was no lateralization of mPFC projections to the PRC. As for the HPC, the majority of PRC-projecting cells were found on the ipsilateral side of the vCA1 and dSub. There was in general no lateralization of HPC projections to the periRE or pPVT, with the exception of vSub projections to the pPVT more robust on the ipsilateral side. Finally, collateral projecting cells were sparse in all regions examined of the two experiments.