Title : Distal axotomy enhances retrograde presynaptic excitability onto 1 injured pyramidal neurons via trans - synaptic signaling

20 Injury of descending motor tracts remodels cortical circuitry and leads to enhanced 21 neuronal excitability, thus influencing recovery following injury. The neuron-specific 22 contributions remain unclear due to the complex cellular composition and connectivity of 23 the CNS. We developed a microfluidics-based in vitro model system to examine intrinsic 24 synaptic remodeling following axon damage. We found that distal axotomy of cultured 25 rat pyramidal neurons caused dendritic spine loss at synapses onto the injured neurons 26 followed by a persistent retrograde enhancement in presynaptic excitability over days. 27 These in vitro results mirrored hyper-activity of directly injured corticospinal neurons in 28 hindlimb motor cortex layer Vb following spinal cord contusion. In vitro axotomy-29 induced hyper-excitability coincided with elimination of inhibitory presynaptic terminals, 30 including those formed onto dendritic spines. We identified netrin-1 as downregulated 31 following axotomy and exogenous netrin-1 applied 2 days after injury normalized spine 32 density, presynaptic excitability, and the fraction of inhibitory inputs onto injured 33 neurons. These findings demonstrate a novel model system for studying the response of 34 pyramidal circuitry to axotomy and provide new insights of neuron-specific mechanisms 35 that contribute to synaptic remodeling. 36 37. CC-BY-NC-ND 4.0 International license peer-reviewed) is the author/funder. It is made available under a The copyright holder for this preprint (which was not .