Metabolism of neuroactive steroids in day-old chick brain

Metabolism of the neuroactive steroids pregnenolone (PREG), progesterone (PROG), dehydroepiandrosterone (DHEA) and dehydroepiandrosterone sulphate (DHEAS) was investigated in day-old chick brain following direct injection of the 3H-labelled compounds into the intermediate medial mesopallium and sampling at times known to be crucial for memory formation in this brain region. 3H-label from these steroids was cleared rapidly from the brain, decreasing to barely detectable levels within 5 h. Following extraction and fractionation, the 3H-labelled brain steroids were identified by TLC, coupled with acetylation and/or separation in different solvent systems. PREG and PROG were converted within 10 min mostly to 20β-dihydropregnenolone (20β-DHPREG) and 5β-dihydroprogesterone, respectively. There was no detectable metabolism of DHEA. Label from DHEAS persisted for longer (half-time 18.9 min) than the free steroid but with no detectable metabolism other than a small amount (4%) of desulphation to DHEA. Further investigation of chick brain steroid metabolism by incubation of subcellular fractions (1–3 h, 37°C) with PREG, PROG or DHEA plus NADPH led to the formation of the following compounds: 20β-DHPREG from PREG (particularly in cytosol); 5β-dihydroprogesterone and 3α,5β-tetrahydroprogesterone from PROG and no detectable metabolism of DHEA. Following incubation of the same brain fractions and labelled steroids with NAD+, there was no detectable metabolism of PREG or PROG but some conversion of DHEA to androstenedione, especially in the nuclear fraction. The results suggest direct actions of DHEA(S) on the early stages of memory formation in the chick and introduce the possibility that PREG may act indirectly via 20β-DHPREG.