EVIDENCE OF PARALLEL AND DISTINCT CIRCUITS BETWEEN THE DORSAL SUBICULUM AND ANTERIOR THALAMIC NUCLEI

Spatial navigation and memory arise from interactions between multiple neural circuits. One mechanism through which the brain encodes the environment involves spatially modulated neurons, such as place cells, whose activity represents animal’s location. Originally observed in the hippocampus, place cells have recently been found in the anteromedial (AM) and anteroventral (AV) thalamic nuclei. However, since there is no direct projection from hippocampus proper to anterior thalamus, the source of spatial information in these nuclei remains unclear. Although segregation of AM and AV inputs has been reported in the subiculum, it is unknown whether there is any overlap in subicular neurons projecting to one or other thalamic nucleus. To address this, we performed pairwise injections of retrograde tracers CTb-488 and CTb-555 into the AM and AV, respectively, and examined the distribution of presynaptic partners. Dual-labelled cells, projecting to both AM and AV, were found across the brain, though they represented a small fraction of total population of labelled cells. In dorsal subiculum, collateral projections were scarce, and the populations were segregated: AM-projecting cells were found mostly in the proximal subiculum, while AV afferents were primarily present in the distal subiculum. Such segregation was not observed in other regions, suggesting the existence of distinct, parallel information streams from the subiculum – a major hippocampal output – to the anterior thalamus. This implies that the spatial information in the AM and AV may come from distinct subicular subregions. Future experiments will investigate functional differences between these circuits in supporting spatial navigation.