Functional Regrowth of Norepinephrine Axons in the Adult Mouse Brain Following Injury.

Area of Science:

• Neuroscience
• Neurobiology
• Axon Regeneration

Background:

• Central nervous system (CNS) axon regrowth failure in adult mammals limits functional recovery after brain or spinal cord injury.
• Previous studies suggested norepinephrine (NE) axon regrowth after brain injury, challenging established dogma.

Purpose of the Study:

• To investigate the dynamic behavior and functional recovery of norepinephrine (NE) axons in the adult mammalian brain following injury.
• To determine if NE axons exhibit regrowth and functional NE release after targeted depletion.

Main Methods:

• Utilized in vivo two-photon microscopy in transgenic mice expressing a fluorophore in NE neurons.
• Administered the selective NE axon toxin N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP4) to induce injury.
• Measured NE release by expressing GCaMP8s in neocortical astrocytes to monitor NE receptor-mediated Ca2+ transients.

Main Results:

• DSP4 treatment massively depleted NE axons, followed by slow, partial recovery over weeks, primarily through new axon entry, not local sprouting.
• Regrown NE axons did not utilize paths of lesioned or spared axons as guidance.
• Startle-evoked NE release, indicated by Ca2+ transients, was abolished post-lesion but recovered within 3-5 weeks, coinciding with axon regrowth.

Conclusions:

• Norepinephrine (NE) axons in the adult mammalian brain demonstrate regrowth capacity following injury.
• Regrown NE axons are functionally competent, releasing NE in response to physiological stimuli in awake animals.
• The findings challenge the dogma of limited CNS axon regeneration and suggest potential for functional recovery.