TARGET-SELECTIVE LOCAL SYNAPTIC CONNECTIVITY OF LAYER 2/3 PYRAMIDAL NEURONS IN THE MARMOSET PREFRONTAL CORTEX AREA 9

The prefrontal cortex (PFC) is a brain region dramatically expanded in primates and is considered to play a crucial role in higher cognitive functions. A recent study using marmosets (Watakabe et al., Neuron. 2023) introduced the existence of columnar structures in the PFC anatomically and their reciprocal interconnections. However, this analysis was mesoscale, leaving the synaptic-level connection patterns unexplored.
Here we examined the synaptic connection in the marmoset neocortex using volume Correlated Light and Electron Microscopy (vCLEM). We have co-injected anterograde and retrograde viral tracers labeled with different colors into Area 10 (A10). The fixed brain tissue was then observed using laser confocal microscopy and embedded in resin for electron microscopy. We processed Area 9 using ATUM-Blade-TEM, a large-volume electron microscopy (vEM) imaging system. With the help of vCLEM, we identified the corresponding neural elements in the vEM dataset and examined the synaptic connections of the labeled neural structures.
In this study we aimed to investigate the local connections of the retrogradely labeled pyramidal cells, which projected to the A10. The target structures were manually segmented to investigate the synaptic connections of the axons of the retrogradely labeled cells in A9. We found that they targeted pyramidal cell dendrites (about 63%), fast-spiking basket cell dendrite shafts (about 35%) or spines of interneurons (about 2%). Our findings show that synaptic architecture within columnar microcircuits is systematically organized, indicating highly specific and target-selective synaptic wiring in the PFC. Further studies may reveal fundamental principles of cortical microcircuit organization in the marmoset PFC.