It is believed that neurosteroids are produced in the brain and other nervous systems. Here, we show that allopregnanolone (ALLO), a neurosteroid, is exceedingly produced in the pineal gland compared with the brain and that pineal ALLO acts on the Purkinje cell, a principal cerebellar neuron, to prevent apoptosis in the juvenile quail. We first demonstrated that the pineal gland is a major organ of neurosteroidogenesis. A series of experiments using molecular and biochemical techniques has further demonstrated that the pineal gland produces a variety of neurosteroids de novo from cholesterol in the juvenile quail. Importantly, ALLO was far more actively produced in the pineal gland than in the brain. Pinealectomy (Px) decreased ALLO concentration in the cerebellum and induced apoptosis of Purkinje cells, whereas administration of ALLO to Px quail chicks prevented apoptosis of Purkinje cells. We further found that Px significantly increased the number of Purkinje cells that expressed active caspase-3, a key protease in apoptotic pathway, and daily injection of ALLO to Px quail chicks decreased the number of Purkinje cells expressing active caspase-3. These results indicate that the neuroprotective effect of pineal ALLO is associated with the decrease in caspase-3 activity during the early stage of neuronal development. We thus provide evidence that the pineal gland is an important neurosteroidogenic organ and that pineal ALLO may be involved in Purkinje cell survival during development. This is an important function of the pineal gland in the formation of neuronal circuits in the developing cerebellum.steroid | melatonin | HPLC | gas chromatography/mass spectrometry T he cerebellar cortex has been used as an excellent model to study synaptic formation and transmission of neural networks because it forms relatively simple neuronal networks compared with those of other brain regions. The Purkinje cell is a principal cerebellar neuron that integrates the process of memory and learning. It is known that in birds and mammals pinealectomy (Px) induces cell loss in the brain including Purkinje cells during development (1, 2). This observation suggests that a certain component(s) in the pineal gland contributes to Purkinje cell survival during development.It is now established that steroids can be synthesized de novo in the central and peripheral nervous systems. Such steroids are called "neurosteroids," and de novo neurosteroidogenesis from cholesterol is a conserved property of the vertebrate brain (for reviews, see refs. 3-7). The Purkinje cell is known as a site of neurosteroidogenesis in the brain (for review, see ref. 8). This cerebellar neuron produces progesterone (PROG) and estradiol-17β (E2) de novo from cholesterol during neonatal life, when cerebellar neuronal circuit formation occurs. Both PROG and E2 promote dendritic growth, spinogenesis, and synaptogenesis via each cognate nuclear receptor in the developing Purkinje cell (9-11). Allopregnanolone (ALLO; 3α,5α-tetrahydroprogesterone), a progesterone metabol...