Chronic abolition of evoked vesicle release from layer 5 projection neurons disrupts the laminar distribution of parvalbumin interneurons in the adult cortex

Establishing precisely built neuronal networks during cortical development requires appropriate proportions of glutamatergic and GABAergic neurons. Developmental disturbances in pyramidal neuron activity can impede the development of GABAergic neurons with long-lasting effects on inhibitory networks. However, the role of deep-layer pyramidal neurons in instructing the development and distribution of GABAergic neurons remains unknown. To unravel the role of deep-layer pyramidal neuron activity in orchestrating the spatial and laminar organisation of parvalbumin neurons, we selectively manipulated the activity of projection neurons in layer 5 of the cortex. By ablating SNAP25 from subsets of glutamatergic L5 projection neurons across the cortical mantle, we abolished Ca2+-dependent vesicle release from Rbp4-Cre+ pyramidal neurons. We explored the local (location of the cell bodies) and the global (subcortical projection sites) effects of chronically silencing cortical L5 neurons on parvalbumin interneurons. We found that the chronic cessation of vesicle release from L5 projection neurons left the density, distribution, and developmental trajectory of cortical and subcortical PV neurons intact during the second and third postnatal week; however, it resulted in the reorganisation of the laminar distribution of cortical PV neurons in layer 4 of S1 in the adult cortex. The abolition of evoked vesicle release from L5 also affected the perineuronal nets in the adult motor cortex and revealed a significant decrease in the density of VVA+ and PV- VVA+ cells in L5 of M1 at 3 months of age. The alterations in the laminar arrangement of VVA+ neurons may imply that Ca2+-dependent synaptic transmission from L5 may control PNN density in adult networks. We also discovered that the absence of L5 activity only had a transient effect on the morphology of striatal PV neurons. The correlation between PV and VVA neurons was contingent on brain regions and cortical layers, therefore the link between the perineuronal net and PV neurons is significantly more intricate than previously believed. The present study will aid our understanding of the bidirectional relationship between deep-layer pyramidal cells and GABAergic neurons while uncovering the long-term effects of chronically disrupting pyramidal neurons on inhibitory networks. ### Competing Interest Statement The authors have declared no competing interest.