NEUROSTEROIDS - 3-ALPHA-HYDROXY-5-ALPHA-PREGNAN-20-ONE AND ITS PRECURSORS IN THE BRAIN, PLASMA, AND STEROIDOGENIC GLANDS OF MALE AND FEMALE RATS

A RIA procedure for measuring progesterone (PROG), 5alpha-pregnane-3,20-dione (5alpha-DH PROG), and 3alpha-hydroxy-5alpha-pregnan-20-one (3alpha, 5alpha-TH PROG) has been developed and validated by GLC/mass spectrometry. Measurements were made in intact and adrenalectomized (ADX) male rats, in cyclic, pregnant, spayed, and spayed-ADX females, and in both males and spayed females injected with PROG. The predominant contribution of the ovary to the concentrations of 3alpha,5alpha-TH PROG in plasma and brain, was indicated by its larger levels in females, in particular during pregnancy, and by its presence in ovarian tissue and disappearance after ovariectomy. An additional adrenal origin in both males and females was shown. Neither PROG nor 5alpha-DH PROG disappeared from brain, contrary to plasma, after combined adrenalectomy and gonadectomy, thus suggesting that PROG might be synthetized de novo in brain. However, the concentrations of 3alpha,5alpha-TH PROG in plasma and brain of female rats were positively correlated with the concentrations of PROG in plasma, indicating that plasma PROG was the major precursor of 3alpha, 5alpha-TH PROG. The direct formation of 3alpha,5alpha-TH PROG from PROG in brain was strongly suggested by the increased 3alpha,5alpha-TH PROG/PROG ratios in brain vs. plasma, when measured in control females, and after injection of PROG to both males and OVX females. It was previously reported that 3alpha,5alpha-TH PROG is a sedative/anxiolytic steroid, as a result of its binding to gamma-aminobutyric acid (GABA)A receptors and allosteric potentiation of GABAcergic neurotransmission. Its concentrations in brain reach indeed the neuroactive range in cyclic and pregnant females, and are compatible with a physiological role of this neurosteroid.