It is becoming clear that steroids can be synthesized de novo by the brain of vertebrates. Such steroids synthesized de novo in the brain, as well as other areas of the nervous system, are called neurosteroids. To understand neurosteroid actions in the brain, we need data on the specific biosynthesis in particular sites of the brain at particular times. Therefore our studies for this exciting area of neuroscience research have focused on the biosynthesis and action of neurosteroids in the identified neurosteroidogenic cells underlying important brain functions. We have demonstrated that the Purkinje cell, a typical cerebellar neuron, is a major site for neurosteroid formation in the brain. This neuron actively synthesizes progesterone and estradiol de novo from cholesterol only during neonatal life, when cerebellar cortical formation occurs dramatically. This is the first observation of neuronal neurosteroidogenesis in the brain. Subsequently the actions of progesterone and estradiol during cerebellar development have become clear by a series of our studies using an excellent Purkinje cellular model. These neurosteroids promote dendritic growth, spinogenesis and synaptogenesis via each receptor in the Purkinje cell. Here we summarize the advances made in our understanding of organizing actions of neurosteroids in the Purkinje cell, an important brain neuron.
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