Melatonin regulates diurnal changes in locomotor activity in vertebrates, but the molecular mechanism for this neurohormonal regulation of behavior is poorly understood. Here we show that 7α-hydroxypregnenolone, a previously undescribed avian neurosteroid, mediates melatonin action on diurnal locomotor rhythms in quail. In this study, we first identified 7α- hydroxypregnenolone and its stereoisomer 7β-hydroxypregnenolone in quail brain. These neurosteroids have not been described previously in avian brain. We then demonstrated that 7α-hydroxypregnenolone acutely increased quail locomotor activity. To analyze the production of 7α-hydroxypregnenolone, cytochrome P4507α, a steroidogenic enzyme of this neurosteroid, was also identified. Subsequently, we demonstrated diurnal changes in 7α-hydroxypregnenolone synthesis in quail. 7α-Hydroxypregnenolone synthesis and locomotor activity in males were much higher than in females. This is the first demonstration in any vertebrate of a clear sex difference in neurosteroid synthesis. This sex difference in 7α-hydroxypregnenolone synthesis corresponded to the sex difference in locomotion. We show that only males exhibited marked diurnal changes in 7α-hydroxypregnenolone synthesis, and these changes occurred in parallel with changes in locomotor activity. Finally, we identified melatonin as a key component of the mechanism regulating 7α-hydroxypregnenolone synthesis. Increased synthesis of 7α-hydroxypregnenolone occurred in males in vivo after melatonin removal via pinealectomy and orbital enucleation (Px plus Ex). Conversely, decreased synthesis of this neurosteroid occurred after melatonin administration to Px plus Ex males. This study demonstrates that melatonin regulates synthesis of 7α-hydroxypregnenolone, a key factor for induction of locomotor activity, thus inducing diurnal locomotor changes in male birds. This is a previously undescribed role for melatonin.
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