Effects of progesterone or neuroactive steroid?

Abstract

Smithet al.¹ reported some interesting results about the effects of progesterone treatment and withdrawal on the gene encoding the GABA_Areceptor α4 subunit, a constituent of receptors for the neurotransmitter GABA (γ-aminobutyric acid). However, we disagree with their conclusion that the effects of progesterone withdrawal are mediated by reduced levels in the brain of the GABA_Areceptor active neurosteroid 3α-OH-5α-pregnan-20-one (3α,5α-THP, or allopregnanalone). The ability of indomethacin to reverse the effects of progesterone was interpreted as evidence that the effects of progesterone were mediated by 3α,5α-THP. Smith et al.¹ did not provide direct evidence that levels of 3α,5α-THP were reduced by indomethacin (which reversed the effects of progesterone), or that progesterone levels were not altered by indomethacin. In some cases, indomethacin can increase progesterone levels directly², which may explain why indomethacin can reverse the effects of progesterone withdrawal.

Main

The effect of indomethacin blockade of 3α-hydroxysteroid oxidoreductase activity is dependent on the relative levels of 3α,5α-THP and its immediate precursor 5αdihydroprogesterone. When 3α,5α-THP concentrations are raised after progesterone infusion or stress, indomethacin would block the oxidation of 3α,5α-THP, increasing its concentration. Using the same dose regimen as ref. 1 we find that stress-induced elevation of 3α,5α-THP levels in the brain are increased by pretreatment with indomethacin (Fig. 1). Blockade of this enzyme with fluoxetine has also been shown to raise 3α,5α-THP levels in the rat brain³. The omission of 3α,5α-THP and progesterone levels in the report by Smith et al.¹ therefore raises concerns about the conclusion that 3α,5α-THP mediates the effects of progesterone withdrawal.

Figure 1: Indomethacin increases stress-induced 3α,5α-THP levels in rat cerebral cortex.

Male Sprague-Dawley rats were habituated to handling and injection. Indomethacin (0.1 mg per kg, intraperitoneal) was administered 20 minutes before ethanol injection stress (2 g per kg, intraperitoneal) and 3α,5α-THP levels were measured as described⁴. Indomethacin increased stress-induced 3α,5α-THP levels by 58.56 62.0 % (P<0.01, Student's t-test).

Manipulation of progesterone levels in female rats is likely to influence the expression of many genes as well as the levels of numerous steroids other than 3α,5α-THP. Therefore, even if 3α,5α-THP levels are reduced by indomethacin, this does not demonstrate that the effects of progesterone withdrawal on GABA_Areceptors are mediated by 3α,5α-THP. Correlational evidence, although suggestive, does not establish a causal relationship.

The effects of progesterone and ethanol withdrawal on GABA_Areceptor α4 subunit gene expression are similar. However, cross-tolerance between ethanol and 3α,5α-THP does not occur in vivo. Indeed, chronic ethanol administration produces cross-tolerance to benzodiazepines, but also results in sensitization to both behavioural and neurochemical responses to 3α,5α-THP⁴. Moreover, this effect is greater in female than male rats, suggesting that the higher progesterone levels in female rats are associated with greater sensitization to 3α,5α-THP⁵. This sensitization is probably due to intrinsic changes in GABA_Areceptors, as concentrations of 3α,5α-THP are not altered after chronic ethanol administration⁴. Although the effects of ethanol⁴,⁶ and progesterone on GABA_Areceptor α4 gene expression are known, there is no direct evidence that 3α,5α-THP alters the expression of this gene.

Reply - Smith et. al.