Steroid 5α-reductases in the brain: From neurosteroidogenesis to therapeutic implications.

Steroid 5α-reductases (5αRs) catalyze the irreversible, NADPH-dependent reduction of Δ4-3-ketosteroids. This process regulates major steroid signaling pathways, including neurosteroid biosynthesis, androgen activation, and glucocorticoid metabolism. By governing these processes, 5αRs influence GABAergic and dopaminergic neurotransmission, and behavioral responses to stress and reward. Although the two principal isoenzymes, 5αR1 and 5αR2, have overlapping functions, they are increasingly recognized as functionally distinct. 5αR1 supports constitutive neurosteroid synthesis in several brain regions. Conversely, 5αR2, long known for converting testosterone to dihydrotestosterone during male sexual differentiation, has emerged as the enzyme that mediates rapid allopregnanolone production in the prefrontal cortex of males during acute stress. These advances may provide a framework for understanding the complex neurobehavioral profile of 5αR inhibitors such as finasteride and dutasteride. These drugs have been associated with depression, anxiety, suicidality, sexual dysfunction, and, in some individuals, persistent sexual, somatic, and neuropsychiatric symptoms after discontinuation. At the same time, 5αR inhibition may have therapeutic value in conditions characterized by excessive or maladaptive neurosteroid signaling, including tic disorders, impulse-control disorders, and substance use disorders. In this comprehensive review, we synthesize pharmacological, genetic, and behavioral evidence on the distinct contributions of 5αR1 and 5αR2 to steroid signaling across stress, sex, and development. Finally, we outline several open questions in the biology of 5αRs, the resolution of which will be essential to advancing toward mechanistically precise and clinically actionable interventions.