Optogenetic interrogation of synapse formation
Once axons have regenerated, how do you know if they make effective synapses? We deliver channelrhodpsin to cortical neurons, then optogenetically stimulate the regenerated axons while performing single-unit recording in potential target cells. If the target cells respond to the pre-synaptic activity, that's good evidence for a functional synapse. The details of this approach can be found in a 2016 publication (Jayaprakash et al.).
Dr. Naveen Jayaprakash leads the team that drives the optogenetic and stem cell work
(Dec. 2017 update) We recently applied this strategy to corticospinal tract axons that were stimulated with KLF6 treatment to regenerate through stem cell grafts and back into host tissue beyond. We found clear post-synaptic responses when we optogenetically forced activity in CST axons - in fact spinal neurons just below the injury responded to CST stimulation at a rate that was almost 60% of the response rate above the injury. Thus the regenerated CST restored a substantial fraction of the lost synaptic connectivity, at least within a spinal segment below the injury. These results are summarized in our latest pre-print.