An isotope dilution based-targeted and non-targeted carbonyl neurosteroid/steroid profiling

Neurosteroids are brain-derived steroids, capable of rapidly modulating neuronal excitability in a non-genomic manner. Dysregulation of their synthesis, or metabolism has been implicated in many pathological conditions. Here, we describe an isotope dilution based targeted and non-targeted (ID-TNT) profiling of carbonyl neurosteroids/steroids. The method combines stable isotope dilution, hydroxylamine derivatization, high-resolution MS scanning and data dependent MS/MS analysis, allowing absolute quantification of pregnenolone, progesterone, 5α-dihydroprogesterone, 3α,5α-tetrahydroprogesterone and 3β,5α-tetrahydroprogesterone, and relative quantification of other carbonyl containing steroids. The utility and validity of this approach was tested in an acute stress mouse model and via pharmacological manipulation of the steroid metabolic pathway with finasteride. We report that brain levels of 3α,5α-tetrahydroprogesterone, a potent enhancer of GABAA receptor (GABAAR-mediated inhibitory function, from control mice is in the 5-20 pmol/g range, a value greater than previously reported. The approach allows the use of data from targeted analysis to guide the normalization strategy for non-targeted data. Furthermore, novel findings, including a striking increase of brain pregnenolone following finasteride administration were discovered in this study. Collectively, our results indicate that this approach has distinct advantages for examining targeted and non-targeted neurosteroid/steroid pathways in animal models, and could facilitate a better understanding of the physiological and pathological roles of neurosteroids as modulators of brain excitability.