The adrenergic and serotonergic stimulations of rat C6 glioma cells have previously been shown to induce the activation of steroid 5alpha-reductase (5alpha-R) gene expression, resulting in their differentiation through the production of neuroactive 5alpha-reduced steroid metabolites. In addition, progesterone and histone deacetylase (HDAC) inhibitors have also been reported to promote the glial cell differentiation with the enhancement of serotonin-stimulated brain-derived neurotrophic factor gene transcription through the production of 5alpha-reduced neurosteroids, thus suggesting that glial cell differentiation is probably implicated in the protection and survival of neuronal cells in the brain. Therefore, the expression of 5alpha-R gene in glial cells seems physiologically important in maintaining the neural function in the brain, but little is known about the mechanism underlying the regulation of 5alpha-R gene transcription. In the present study, the effect of a HDAC inhibitor trichostatin A (TSA) on 5alpha-R gene transcription in the glioma cells was examined, and TSA was shown to induce the elevation of 5alpha-R mRNA levels through the activation of the 5alpha-R promoter via a mechanism involving Sp1 and Sp3 transcription factors in a time- and concentration-dependent manner. Thus, both Sp1 and Sp3 are considered to play a physiological role in the regulation of 5alpha-R gene expression, and hence the production of 5alpha-reduced neurosteroids in glial cells.