Distribution of dendritic spines and inhibitory inputs on layer 2 and layer 3 pyramidal neurons of the anterior cingulate cortex
Gilman, Joshua Paul
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The anterior cingulate cortex (ACC) plays an important role in reward-based decision-making, linking higher-order thinking and emotions. Because of this area's dense connectivity it is important to study the properties of the excitatory and inhibitory network that governs ACC output. The aim of this study was to characterize the morphology of dendritic excitatory postsynaptic sites and inhibitory inputs on layer 2 and layer 3 ACC pyramidal neurons, the principal intracortical projection neurons of the cortex. Using biocytin-filling and high-resolution confocal imaging, we quantified the distribution of dendritic spines, the major sites of excitatory input, on pyramidal cells. We visualized inhibitory inputs apposed to specific pyramidal cell compartments, including the axon initial segment, soma, dendrites, and dendritic spines, through immunohistochemical labeling of vesicular γ-aminobutyric acid transporter. Layer 2 and layer 3 cells had similar spine densities on their apical and basal dendritic compartments, with a maximum spine density occurring in their middle apical and middle basal compartments. Axon initial segments of layer 3 cells had a higher density of inhibitory input compared to the layer 2 cells (0.84 vs 0.66 apps/μm). The apical dendritic shaft had a higher apposition density than the basal dendritic shaft in an individual layer (layer 2, 0.50 vs 0.32; layer 3, 0.50 vs 0.28 apps/μm) with the majority of the innervation occurring on the proximal compartments of both apical and basal segments. Although located in different laminae, these cells showed similar inhibitory input distributions, with higher amounts of inhibition proximally. Finally, these inhibitory inputs also occurred on dendritic spines, with the highest density on thin spines. However, proportionally, mushroom spines had the highest level of innervation, with up to 44% of these spines receiving inhibitory input. These findings add to the understanding of how inhibition at the cellular level can affect the output of the ACC and begin to uncover important relationships between cellular structure and function in this brain region.