Bifurcating Neurons In The Anterior Thalamic Nuclei

The anterior thalamic nuclei (ATN) form a nodal point within a distributed memory network. The conventional view of ATN function describes segregated efferents to different terminal regions. By contrast, we found that bifurcating neurons are common within the anteromedial nucleus (AM) of the ATN. A substantial proportion of AM neurons ([~]36% within a region of interest) showed collateral projections when one of two retrograde neurotracers, Cholera Toxin Subunit B (CTB) conjugated to either Alexa Fluor(R) 488 or 594, was placed in the medial prefrontal cortex (mPFC) or dorsal subiculum (dSub). A marked degree of collateralization ([~]20% AM neurons) was also found when neurotracers were placed in mPFC and caudal retrosplenial cortex (cRSC); about 10% showed collaterals when the cRSC was paired with either dSub or vHF; the fewest (6%) was found for mPFC paired with the ventral hippocampal formation (vHF). A generally similar range of percentages of bifurcating neurons was found in the adjacent nucleus reuniens (Re). Evidence that AM neurons project simultaneously to many distantly-located structures provides a new perspective on ATN function. These neurons would facilitate direct coordination among key neural structures to support memory and may explain the strong association between the ATN and diencephalic amnesia.