Axons of cortical basket cells originating from dendrites develop higher local complexity than axons emerging from basket cell somata

Neuronal differentiation is regulated by neuronal activity. Here, we analyzed dendritic and axonal growth of Basket cells and non-Basket cells using sparse transfection of channelrhodopsin-YFP and repetitive optogenetic stimulation in slice cultures of rat visual cortex. Neocortical interneurons often display axon-carrying dendrites (AcDs). We found that the AcDs of Basket cells and non-Basket cells were on average their most complex dendrite. Further, the AcD configuration had an influence on Basket cell axonal development. Axons originating from an AcD formed denser arborizations with more terminal endings within the dendritic field of the parent cell. Intriguingly, this occurred already in unstimulated Basket cells, and complexity was not increased further by optogenetic stimulation. However, optogenetic stimulation exerted a growth-promoting effect on axons emerging from Basket cell somata. The axons of non-Basket cells neither responded to the AcD configuration nor to the optogenetic stimulation. The results suggest that the formation of locally dense Basket cell plexuses is regulated by spontaneous activity. Moreover, in the AcD configuration, the AcD and the axon it carries mutually support each others growth. TeaserTextbooks usually draw cortical neurons with axons emerging from the cell body. Axons emerging from dendrites have been considered a caprice of nature until recent work demonstrated that axon-carrying dendrites are neither a rare nor a random phenomenon. Rather, this morphological configuration awards specific functional properties. Here, we discovered a morphogenetic role of the AcD configuration in developing Basket neurons which deliver axo-somatic inhibition to pyramidal neurons. Driven by spontaneous activity, AcD and the axon mutually promote each others growth such that the axon rapidly forms a denser presynaptic terminal field. Basket cells with axons from the cell body achieve the same complexity with technical assistance via an optogenetic stimulation. This way, a dendritic origin accelerates Basket cell axonal development.