After a traumatic injury of the nervous system or in the course of a
neurodegenerative disease, the speed of axonal regeneration and the control of the inflammatory response are fundamental parameters of functional recovery. Spontaneous regeneration takes place in the peripheral nervous system, although the process is slow and often incomplete. There is currently no efficient treatment for enhancing axonal regeneration, including elongation speed and functional reinnervation.
Ligands of the translocator
protein 18 kDa (TSPO) are currently under investigation as therapeutic means for promoting neuroprotection, accelerating axonal regeneration and modulating
inflammation. The mechanisms of action of TSPO
ligands involve the regulation of mitochondrial activity and the stimulation of
steroid biosynthesis. In the peripheral nervous system, TSPO expression is strongly up-regulated after injury, primarily in Schwann cells and macrophages, but also in neurones. Its levels return to low control values when nerve regeneration is completed, strongly supporting an important role in regenerative processes. We have demonstrated a role for the
benzoxazine etifoxine in promoting axonal regeneration in the lesioned rat sciatic nerve, either after freeze-injury or complete transection.
Etifoxine is already clinically approved for the treatment of
anxiety disorders (
Stresam(®) , Biocodex, Gentilly, France). Daily treatment with
etifoxine resulted in a two-fold acceleration in axonal regeneration, as well as in a marked improvement of both the speed and quality of functional recovery. The neuroregenerative effects of
etifoxine are likely to be mediated by TSPO, and they may involve an increased synthesis of
pregnenolone and its metabolites, such as
progesterone. After freeze-injury of the sciatic nerve, administration of
etifoxine also strongly reduced the number of activated macrophages and decreased the production of the inflammatory
cytokines tumour
necrosis factor-α and interleukin-1β. Thus, this drug offers promise for the treatment of
peripheral nerve injuries and axonal neuropathies. It may also be used as a lead compound in the development of new TSPO-based neuroprotective approaches.