Progesterone and its metabolites increase myelin basic protein expression in organotypic slice cultures of rat cerebellum

We have previously shown that progesterone (PROG) is synthesized by Schwann cells and promotes myelin formation in the peripheral nervous system (PNS). We now report that this neurosteroid also stimulates myelination in organotypic slice cultures of 7-day-old (P7) rat and mouse cerebellum. Myelination was evaluated by immunofluorescence analysis of the myelin basic protein (MBP). After 7 days in culture (7DIV), we found that adding PROG (2-5 x 10(-5) M) to the culture medium caused a fourfold increase in MBP expression when compared to control slices. The effect of PROG on MBP expression involves the classical intracellular PROG receptor (PR): the selective PR agonist R5020 significantly increased MBP expression and the PR antagonist mifepristone (RU486) completely abolished the effect of PROG on this MBP expression. Moreover, treatment of P7-cerebellar slice cultures from PR knockout (PRKO) mice with PROG had no significant effect on MBP expression. PROG was metabolized in the cerebellar slices to 5alpha-dihydroprogesterone (5alpha-DHP) and to the GABA(A) receptor-active metabolite 3alpha,5alpha-tetrahydroprogesterone (3alpha,5alpha-THP, allopregnanolone). The 5alpha-reductase inhibitor L685-273 partially inhibited the effect of PROG, and 3alpha,5alpha-THP (2-5 x 10(-5) M) significantly stimulated the MBP expression, although to a lesser extent than PROG. The increase in MBP expression by 3alpha,5alpha-THP involved GABA(A) receptors, as it could be inhibited by the selective GABA(A) receptor antagonist bicuculline. These findings suggest that progestins stimulate MBP expression and consequently suggest an increase in CNS myelination via two signalling systems, the intracellular PR and membrane GABA(A) receptors, and they confirm a new role of GABA(A) receptors in myelination.