MAPPING EMOTIONAL AND CONTEXTUAL INPUTS ONTO DENDRITIC BRANCHES OF VENTRAL CA1 PYRAMIDAL NEURONS

The basolateral amygdala (BLA) is the main hub for emotional valence encoding. BLA is interconnected with the ventral hippocampus, which processes the emotional meaning of context and stimuli. Interestingly, BLA projections to ventral CA1 (vCA1) convey positive- and negative-valence information in similar proportions, suggesting the existence of subcircuits routing opposite emotional information. This raises the question of how vCA1 pyramidal neurons (PNs) integrate opposing emotional signals. One possibility is that the synaptic spatial organization shapes neuronal computation through local dendritic non-linearity.
In addition to BLA inputs, vCA1 integrates spatial and contextual information via Schaffer collaterals (SCs) projections from ventral CA3. SCs and BLA axons target common dendritic domains of vCA1 PNs, consistent with the possibility that the emotional-spatial integration may occur at individual dendritic branches. However, a comprehensive understanding of the spatial organization of excitatory synaptic inputs remains elusive.
We hypothesize that SCs and BLA excitatory synapses exhibit a non-random spatial organization across dendritic branches of vCA1 PNs, contributing to branch-specific integration. To test this, we activate BLA and SCs inputs with optogenetic and electrical stimulation, respectively. In addition, we use large-scale two-photon laser scanning microscopy to map active BLA and SCs inputs by detecting calcium transients at single-spine in vCA1 PNs in acute hippocampal slices. By expressing ChR2 in Fos+ BLA neurons of fear-conditioned mice, we achieve selective stimulation of BLA axons. This functional approach allows us to map the spatial organization of valence-specific BLA inputs and SCs inputs along vCA1 PN dendrites and compare it to untrained mice.