Functional Regrowth of Norepinephrine Axons in the Adult Mouse Brain Following Injury
It is widely believed that axons in the central nervous system of adult mammals do not regrow following injury. This failure is thought, at least in part, to underlie the limited recovery of function following injury to the brain or spinal cord. Some studies of fixed tissue have suggested that, counter to dogma, norepinephrine (NE) axons regrow following brain injury. Here, we have used in vivo two-photon microscopy in layer 1 of the primary somatosensory cortex in transgenic mice harboring a fluorophore selectively expressed in NE neurons. This protocol allowed us to explore the dynamic nature of NE axons following injury with the selective NE axon toxin N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP4). Following DSP4 treatment, NE axons were massively depleted and then slowly and partially recovered their density over a period of weeks. This regrowth was dominated by new axons entering the imaged volume. There was almost no contribution from local sprouting from spared NE axons. Regrown axons did not appear to use either the paths of previously lesioned NE axons nor NE axons that were spared and survived DSP4 treatment as a guide. To measure NE release, GCaMP8s was selectively expressed in neocortical astrocytes and startle-evoked, NE receptor-mediated Ca2+ transients were measured. These Ca2+ transients were abolished soon after DSP4 lesion but returned to pre-lesion values after 3-5 weeks, roughly coincident with NE axon regrowth, suggesting that the regrown NE axons are competent to release NE in response to a physiological stimulus in the awake mouse. Significance StatementIt is widely believed that axons in the central nervous system (CNS) of adult mammals are incapable of regrowth following injury. Counter to this notion, we describe the structural and functional regrowth of norepinephrine axons following brain injury in the adult mouse. These results extend previous studies describing the regenerative capacity of serotonin axons in the CNS by demonstrating axon regrowth of another neuronal subtype and the capacity of these regrown axons to respond normally to an external physiological stimulus. Taken together, these findings suggest that monoaminergic neurons share a common program for axon regrowth. Elucidation of this molecular and genetic program could inform therapies to promote axon regrowth and functional recovery following injury to the CNS.