Abstract
Birds show high levels of steroid synthesis in the brain. Steroid biosynthesis in the brain can provide a mechanism for local and rapid changes in steroid levels in particular brain regions. Neural steroidogenic enzymes are regulated on both short and long timescales by both endogenous and environmental factors. Here, we discuss the regulation of one steroidogenic enzyme, 3β-hydroxysteroid dehydrogenase/Δ5-Δ4 isomerase (3β-HSD), which activates the prohormone dehydroepiandrosterone by converting it to a potent and aromatizable androgen, androstenedione. First, brain 3β-HSD mRNA and activity are regulated by developmental stage in nestling Zebra Finches (Taeniopygia guttata). Second, in vitro estradiol rapidly (<10 min) inhibits Zebra Finch brain 3β-HSD activity. Third, restraint stress rapidly (<15 min) regulates brain 3β-HSD activity in adult Zebra Finches and Song Sparrows (Melospiza melodia). Fourth, brain 3β-HSD activity is upregulated during the non-breeding season in wild male Song Sparrows. Fifth, during the non-breeding season, brain 3β-HSD activity is rapidly (<30 min) elevated by territorial encounters in a region-specific manner in male Song Sparrows. Taken together, these studies suggest that brain 3β-HSD is a key factor in the control of avian neurophysiology and behavior.
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