Functional mapping of BLA and CA3 Dendritic Synaptic Inputs in CA1 Pyramidal Neurons of the Ventral Hippocampus

Dendritic integration plays a crucial role in neuronal processing, enabling neurons to transform the spatio-temporal patterns of synaptic input activation into action potential output. Pioneering modeling studies and experimental work studies exploiting two-photon glutamate uncaging enabling the stimulation of individual glutamatergic synapses in specific dendritic subdomains demonstrated that specific spatial arrangements of activated dendritic glutamatergic synapses can promote non-linear inputs summation a phenomenon that profoundly affects the probability and the temporal structure of somatic spiking. However, despite the recognized importance of spatial synaptic organization in shaping dendritic integration, our knowledge of how synaptic inputs are arranged across the entire dendritic tree of pyramidal neurons remains limited. Indeed, while anatomical studies utilizing the technique of GFP reconstitution at synapses provide an excellent description of the spatial relationship among synaptic contacts across the whole neuronal dendritic arbor, they fail to identify which inputs are engaged during neuronal activity. On the contrary, functional studies analyzing synaptic activity with calcium or glutamate imaging typically restrict their focus on small dendritic portions. In the present work, we used two-photon laser scanning microscopy to perform calcium imaging at the level of single glutamatergic spines in the whole dendritic tree of pyramidal neurons located in the CA1 region of the ventral hippocampal (vCA1 PNs). In particular, we focused on spines formed by inputs from two specific projections, the basolateral amygdala (BLA) and CA3 (Schaffer Collaterals, SC) that converge onto the vCA1 PNs apical proximal and basal dendrites. By employing a home-written automated region-of-interest detection algorithm we analyzed approximately 1,300 spines per neuron and examined synaptic activation patterns induced by optogenetic and electrical stimulation of BLA and SC inputs, respectively. Our results reveal that BLA inputs are more evenly distributed between apical and basal dendrites, whereas CA3 inputs predominantly target apical compartments. Interestingly, spine density varies across dendritic branches, with third-degree apical dendrites exhibiting the highest density of both total and activated spines.