The Future of Neuron Regeneration
My topic of choice is neuron regeneration, and how the process can be artificially promoted for the treatment of neuropathy or injury. In Connectome, Sebastian Seung says, ʺ...it appears that brain injury facilitates rewiring by releasing axonal growth mechanisms that are normally suppressed by certain molecules. Future drug therapies might target these molecules, enabling the brain to rewire in ways that are not currently possibleʺ (Seung 128). When I read this quote, I wondered what specific drug therapies were available, and how much was currently known about the mechanisms underlying neuronal regeneration. For this paper, I found an article entitled ‘Etifoxine improves peripheral nerve regeneration and functional recovery,” published by the National Academy of Sciences. In the experiment, rats underwent a surgery that involved exposing the right sciatic nerve and freezing it with a copper cryode. This process created a peripheral nerve injury. The study then investigated the effects of the drug etifoxine in promoting the healing process.
When peripheral nerves are faced with injury or neuropathy, recovery is often slow and incomplete. However, etifoxine, a ligand of the translocator protein TSPO, is believed to promote regeneration of damaged peripheral nerves. Freezing the sciatic nerve quickly destroyed axons without affecting connective tissues or basal lamina. Three days after the surgery, one group of rats received daily injections of etifoxine, while the other group received an equal volume of the vehicle substance, without the active ingredient. Each group was then monitored over the course of several days for indications of nerve regeneration. They were subjected to tests to assess improvements in motor control and sensory function (Girard 20505).
Diamindino yellow retrograde tracing was used to map the connectivity of regenerated axons. Seven to fifteen days after the surgery, the regeneration of myelinated axons was more advanced in the etifoxine-treated group than the control group. The treated rats also exhibited significantly more medium-sized nerve fibers, and these fibers were of regular shape and surrounded by myelin. Degenerated axons could still be seen in the control group even fifteen days after the surgery (Girard 20506).
These images are nerve fibers magnified by an electron microscope. Image G is from the control group, and the arrows indicate degenerated neurons. Image H is from the etifoxine group. There are no degenerated neurons.
To further assess the progress of axonal regeneration, the presence of certain substances was measured. STMN-2 is found in high concentration in growth cones, peripherin is expressed during nerve regeneration, and NF200 is indicative of larger, more mature axons. Therefore, the presence of these substances indicates progress in axon regeneration.
STMN-2-immunoreactive fibers were observed as early as three days in the etifoxine group. After seven days, linear peripherin-immunoreactive fibers were observed in the etifoxine group, and their numbers had doubled three days later. By ten days after the surgery, there was a remarkable increase in the number of ions expressing NF200 in the etifoxine group. On the other hand, after fifteen days small degenerated axons were still present in the control group. These observations prove that etifoxine promotes the regeneration of axons (Girard 20506).
To assess axonal growth, sciatic nerves were transected from the rats. Silicone nerve guide tubes were placed in the samples to guide axonal growth of STMN-2 immunoreactive axons. The axons of the etifoxine-treated group saw much more significant growth than the control group, as illustrated by Figure 2. This shows that etifoxine promotes axon growth after nerve damage.
This graph shows that the etifoxine group saw greater axonal growth than the vehicle group. The dashed lines in the images show axonal growth from day seven to day ten. The axons of the etifoxine group, indicated by + E, were much longer than the vehicle group.
The reduction in the number of macrophages at the site of injury is an important part of the regeneration process. Although macrophages clear debris, they can cause serious inflammation if present for too long. By seven days after the surgery, the number of macrophages per millimeter in the etifoxine group was less than half that of the control group.
The rats’ recovery of motor control was assessed through a series of tests that included both groups, as well as rats that had not been subjected to the surgery. Locomotion was evaluated with a walking track test. The test involved dipping the animal’s paws in ink, then allowing them to walk through a track that was covered in paper. Locomotion was assessed daily with the Sciatic Function Index. Twenty-one days after the surgery, the mean SFI of the etifoxine group was comparable to that of the normal rats, demonstrating that the etifoxine group had made an almost full recovery. Fine motor coordination was also tested with a Locotronic device. Thirteen days after the surgery, the etifoxine group matched the performance of the normal group, while the performance of the control group remained poor as late as twenty-nine days after the surgery (Girard 20508).
These findings show that etifoxine supports axonal regeneration and growth, as well as macrophage responses. In addition, functional tests revealed that the drug improves the rate and quality of functional recovery. While it the underlying mechanisms of this recovery is still unknown, this drug shows promise for the treatment of nerve injury and neuropathies. The drug is already used for long-term treatment of anxiety, and has minimal side effects. Hopefully, etifoxine will be a pioneer of drug treatment for neuron regeneration (Girard 20508).
Girard, C. et al. "Etifoxine Improves Peripheral Nerve Regeneration and Functional Recovery." Proceedings of the National Academy of Sciences 105.51 (2008): 20505-0510. JSTOR. Web. 3 Aug. 2012. <http://www.jstor.org/stable/25465854>.
Seung, Sebastian. Connectome: How the Brain's Wiring Makes Us Who We Are. Boston: Houghton Mifflin Harcourt, 2012. Print.