Title: Structural and functional characterization of dendritic arbors and GABAergic synaptic

35 The basolateral amygdala (BLA) is a complex brain region associated with processing 36 emotional states, such as fear, anxiety and stress. Some aspects of these emotional states are 37 driven by the network activity of synaptic connections, derived from both local circuitry and 38 projections to the BLA from other regions. Although the synaptic physiology and general 39 morphological characteristics are known for many individual cell types within the BLA, the 40 combination of morphologic, electrophysiological and distribution of neurochemical 41 GABAergic synapses in a three-dimensional neuronal arbor has not been reported for single 42 neurons from this region. The aim of this study was to assess differences in morphological 43 characteristics of BLA principal cells and interneurons, quantify the distribution of 44 GABAergic neurochemical synapses within the entire neuronal arbor of each cell type and 45 determine whether GABAergic synaptic density correlates with electrophysiological 46 recordings of inhibitory post-synaptic currents. We show that BLA principal neurons form 47 complex dendritic arborizations, with proximal dendrites having fewer spines, but higher 48 densities of neurochemical GABAergic synapses compared to distal dendrites. Furthermore, 49 we found that BLA interneurons exhibited reduced dendritic arbor lengths and spine 50 densities, but had significantly higher densities of putative GABAergic synapses compared to 51 principal cells, which was correlated with an increased frequency of spontaneous inhibitory 52 post-synaptic currents. The quantification of GABAergic connectivity, in combination with 53 morphological and electrophysiological measurements of the BLA cell types, is the first step 54 toward a greater understanding of how fear and stress lead to changes in morphology, local 55 connectivity and/or synaptic reorganisation of the BLA. 56