Neurosteroids: 3 alpha-hydroxy-5 alpha-pregnan-20-one and its precursors in the brain, plasma, and steroidogenic glands of male and female rats.

Abstract

A RIA procedure for measuring progesterone (PROG), 5 alpha-pregnane-3,20-dione (5 alpha-DH PROG), and 3 alpha-hydroxy-5 alpha-pregnan-20-one (3 alpha,5 alpha-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 3 alpha,5 alpha-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 5 alpha-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 3 alpha,5 alpha-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 3 alpha,5 alpha-TH PROG. The direct formation of 3 alpha,5 alpha-TH PROG from PROG in brain was strongly suggested by the increased 3 alpha,5 alpha-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 3 alpha,5 alpha-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.

Unconventional ligands and modulators of nicotinic receptors

“…The concentrations at which steroid hormones modulate nAChR activity are in the tens of micromolar, i.e., three to five orders of magnitude higher than those found in the mammalian brain (Backstrom et al, 1976;Corpechot et al, 1993;Bixo et al, 1997). However, the actual concentrations of steroids at synapses or neuronal compartments bearing steroid-sensitive membrane receptors can be substantially higher than the CSF or tissue concentrations of the hormones.…”

Hypothalamic pituitary adrenal axis

“…These observations are consistent with findings from studies performed in the adult rat (Corpechot et al 1993). The plasma concentrations of these steroids in the intact fetuses were similar to those we have reported previously , Nguyen et al 2003a).…”

Steroid modulation of GABAA receptor-mediated transmission in the hypothalamus: Effects on reproductive function

“…Using this methodology, allopregnanolone concentrations in cortical homogenates were estimated to be ~1-2 ng/g in juvenile, ~1 ng/g in diestrous, 4-6 ng/g in estrous, and 4-8 ng/g in proestrous females, with levels reaching ~10-30 ng/g during pregnancy (Paul and Purdy, 1992;Corpéchot et al, 1993;Concas et al, 1998;Kellogg and Frye, 1999). These assessments have been used to estimate that allopregnanolone levels are diestrus females (Paul and Purdy, 1992), but can reach ~100 nM during the late stages of pregnancy (see for example; Brussaard et al, 1997;Lambert et al, 2003).…”

“…Corpéchot et al (1993) reported that allopregnanolone was below the limit of detection in the adult male rat brain. In contrast, Kellogg and Frye (1999) reported levels of allopregnanolone in brain extracts from adult males of 3.3 ng/g; a concentration only slightly lower than that they reported for estrous females and comparable to that which they found for 3α-diol in males (3.2 ng/g).…”