Effects of progesterone synthesized de novo in the developing purkinje cell on its dendritic growth and synaptogenesis

De novo steroidogenesis from cholesterol is a conserved property of vertebr ate brains, and such steroids synthesized de novo in the brain are called n eurosteroids. The identification of neurosteroidogenic cells is essential t o the understanding of the physiological role of neurosteroids in the brain . We have demonstrated recently that neuronal neurosteroidogenesis occurs i n the brain and indicated that the Purkinje cell actively synthesizes sever al neurosteroids de novo from cholesterol in vertebrates. Interestingly, in the rat, this neuron actively synthesizes progesterone de novo from choles terol only during neonatal life, when cerebellar cortical formation occurs most markedly. Therefore, in this study, the possible organizing actions of progesterone during cerebellar development have been examined. In vitro st udies using cerebellar slice cultures from newborn rats showed that progest erone promotes dose-dependent dendritic outgrowth of Purkinje cells but dos e not affect their somata. This effect was blocked by the anti-progestin RU 486 [mifepristone; 17 beta -hydroxy-11 beta-(4-methylaminophenyl)-17 alpha -(1-propynyl) estra-4,9-dien-3 one-6-7]. In vivo administration of progeste rone to pups further revealed an increase in the density of Purkinje spine synapses electron microscopically. In contrast to progesterone, there was n o significant effect of 3 alpha ,5 alpha -tetrahydroprogesterone, a progest erone metabolite, on Purkinje cell development. Reverse transcription-PCR-S outhern and immunocytochemical analyses showed that intranuclear progestero ne receptors were expressed in Purkinje cells. These results suggest that p rogesterone promotes both dendritic outgrowth and synaptogenesis in Purkinj e cells through intranuclear receptor-mediated mechanisms during cerebellar development. Such organizing actions may contribute to the formation of th e cerebellar neuronal circuit.