Neurosteroids, with special reference to the effect of progesterone on myelination in peripheral nerves

Some steroids ore synthesized within the central and peripheral nervous system, mostly by glial cells. These ore known as neurosteroids. In the brain, neurosteroids have been shown to act directly on membrane receptors for neurotransmitters. For example, progesterone inhibits the neuronal nicotinic acetylcholine receptor, whereas its 3 alpha,5 alpha-reduced metabolite 3 alpha,5 alpha-tetrahydroprogesterone (allopregnanolane) activates the A gamma-aminobutyric acid receptor complex. Besides these effects, neurosteroids also regulate important glial functions, such as the synthesis of myelin proteins. Thus, in cultures of glial cells prepared from neonatal rot brain, progesterone increases the number of oligodendrocytes expressing the myelin basic protein (MBP) and the 2',3'-cyclic nucleotide-3'-phophodiesterose (CNPase). An important role for neurosteroids in myelin repair has been demonstrated in the rodent sciatic nerve, where progesterone and its direct Precursor pregnenolone ore synthesized by Schwann cells. After cryolesion of the mole mouse sciatic nerve, blocking the local synthesis or action of progesterone impairs remyelination of the regenerating axons, whereas administration of progesterone to the lesion site promotes the formation of new myelin sheaths.