Characterization of brain neurons that express enzymes mediating neurosteroid biosynthesis

Allopregnanolone (ALLO) and tetrahydrodeoxycorticosterone (THDOC) are potent positive allosteric modulators of GABA action at GABA(A) receptors. ALLO and THDOC are synthesized in the brain from progesterone or deoxycorticosterone, respectively, by the sequential action of two enzymes: 5 alpha-reductase (5 alpha-R) type I and 3 alpha-hydroxysteroid dehydrogenase (3 alpha-HSD). This study evaluates 5 alpha-R type I and 3 alpha-HSD mRNA expression level in mouse brain by using in situ hybridization combined with glutamic acid decarboxylase 67/65, vesicular glutamate transporter 2, glial fibrillary acidic protein, and S100 ss immunohistochemistry. We demonstrate that 5 alpha-R type I and 3 alpha-HSD colocalize in cortical, hippocampal, and olfactory bulb glutamatergic principal neurons and in some output neurons of the amygdala and thalamus. Neither 5 alpha-R type I nor 3 alpha-HSD mRNAs are expressed in S100 ss- or glial fibrillary acidic protein-positive glial cells. Using glutamic acid decarboxylase 67/65 antibodies to mark GABAergic neurons, we failed to detect 5 alpha-R type I and 3 alpha-HSD in cortical and hippocampal GABAergic interneurons. However, 5 alpha-R type I and 3 alpha-HSD are significantly expressed in principal GABAergic output neurons, such as striatal medium spiny, reticular thalamic nucleus, and cerebellar Purkinje neurons. A similar distribution and cellular location of neurosteroi-dogenic enzymes was observed in rat brain. Taken together, these data suggest that ALLO and THDOC, which can be synthesized in principal output neurons, modulate GABA action at GABA(A) receptors, either with an autocrine or a paracrine mechanism or by reaching GABA(A) receptor intracellular sites through lateral membrane diffusion.