Progesterone (PROG), produced by the ovaries and placenta, is known to be a reproductive hormone involved in pregnancy. However, we have shown that PROG can also be synthesized from pregnenolone in the central and peripheral nervous systems and can thus be designated as a neurosteroid . In addition to its endocrine mode of action, PROG has paracrine and/or autocrine activities within the brain and peripheral nerves, which go beyond reproductive functions.Levels of PROG in the sciatic nerve of adult male mice and rats ( 10 ng/g) are about ten times higher than in plasma and remain elevated 5 days after removal of the steroidogenic endocrine glands by combined castration and adrenalectomy. Both Schwann cells and sensory neurons are a source of PROG in the peripheral nervous system: they express the 3β-hydroxysteroid dehydrogenase (3β-HSD), enzyme which converts pregnenolone to PROG, as shown by in situ hybridization, RT-PCR and immunocytochemistry. Whether isolated from embryonic or adult rat dorsal root ganglia, both Schwann cells and sensory neurons can convert [ 3 H]pregnenolone to [ 3 H]PROG and its 5α-reduced metabolites. However, cultured Schwann cells synthesize PROG and its metabolites only in response to a diffusible neuronal factor. This was shown by using a culture paradigm in which Schwann cells were grown in the presence of sensory neurons, but separated from them by a microporous membrane which does not allow direct contact between cells.In vivo, expression of the 3β-HSD is also dependent on an axonal signal. 3β-HSD mRNA was detected by RT-PCR in the intact rat sciatic nerve and was down-regulated to undetectable levels 3 days after cryolesion, when axons have degenerated. In response to this type of injury, axons and their myelin sheaths quickly degenerate within the distal part of the nerve, but Schwann cells survive and form a cellular tube through which the damaged axons can regenerate. By 6 days, when Schwann cells make new contact with the regenerating axons, 3β-HSD mRNA is again present.In the cryolesioned male mouse sciatic nerve, we showed that the local synthesis of PROG plays an important role in myelin formation. When Schwann cells come again in contact with the regenerating nerve fibers, they remyelinate them. Blocking either the local synthesis (trilostane) or the receptor-mediated action (RU486) of PROG impairs the formation of new myelin sheaths, analyzed on electron microscopic cross sections two weeks after lesions. Conversely, application of 100 μg of either PROG or its precursor pregnenolone close to lesioned nerves significantly enhances the thickness of the myelin sheaths. The structure and compaction of the myelin sheaths formed in response to either neurosteroid are morphologically normal when examined by electron microscopy. Myelination of axons is also increased when a physiological concentration of PROG (20 nM) is added to co-cultures of rat sensory neurons and Schwann cells.Progesterone promotes myelination by acting on Schwann cells and by increasing the expression of specific myelin proteins. Schwann cells contain an intracellular receptor for PROG (PROG-R), as shown by RT-PCR, immunocytochemistry and binding studies using the selective ligand [ 3 H]ORG 2058. The presence of a PROG-R was also demonstrated in vivo in both intact and regenerating sciatic nerves of adult male rats. In contrast to the PROG-R expressed in uterus and pituitary gland, the one present in Schwann cells and sciatic nerve is not inducible by estradiol.The expression of two important peripheral myelin genes, coding respectively for peripheral myelin protein 22 (PMP22) and for protein zero (P0), is increased by PROG. Ten days after cryolesion, application of PROG (100 μg) to the regenerating male rat sciatic nerve for 48 hrs increases PMP22 and P0 mRNA and protein levels, as determined by Northern analysis and immunocytochemistry. Schwann cells, isolated from neonatal rat sciatic nerves, were transiently transfected with transgenes in which the promoter regions of PMP22 or P0 were linked to the luciferase gene. When PROG was added to the culture medium, the activity of the P0 promoter and of the PMP22 promoter 1 were increased in a dose-dependent manner (1 nM-1 μM).Progesterone may facilitate the regeneration of peripheral nerves by a dual action: by promoting the formation of new myelin sheaths and by accelerating axonal growth. Indeed, PROG also exerts trophic effects on sensory neurons. Thus, PROG increases the outgrowth of neurites from embryonic rat dorsal root ganglia explants when added to the culture medium. A neurotrophic effect of PROG was also observed in vivo. When applied to the regenerating male rat sciatic nerve after cryolesion, PROG accelerates the regeneration of the damaged axons. Ten days after lesion, the number of axons stained with an antiserum against neurofilaments and observed by confocal microscopy was significantly increased by PROG.These findings show a function for locally produced progesterone on myelination and axonal regeneration and suggest a new pharmacological approach for peripheral nerve repair, after lesion or during peripheral neuropathies. We have found high levels of pregnenolone and PROG in human somatic nerves. In addition, Schwann cells isolated from human somatic nerves are able to convert [ 3 H]pregnenolone to [ 3 H]PROG.