Physiological modulation of GABA_A receptor plasticity by progesterone metabolites

Abstract

The possible functional relation between changes in brain and plasma concentrations of neurosteroids and the plasticity of γ-aminobutyric acid type A (GABA_A) receptors in the brain during pregnancy and after delivery was investigated in rats. The concentrations in the cerebral cortex and plasma of pregnenolone as well as of progesterone and its neuroactive derivatives allopregnanolone (3α-hydroxy-5α-pregnan-20-one) and allotetrahydrodeoxycorticosterone (5α-hydroxy-3α,21-diol-20-one) increased during pregnancy, peaking around day 19, before returning to control (estrus) values immediately before delivery (day 21). In the postpartum period, steroid concentrations in plasma and brain did not differ from control values. The densities of [${}^3\text{H}$]GABA, [${}^3\text{H}$]flunitrazepam, and t-[${}^{35}\text{S}$]butylbiciclophosphorotionate (TBPS) binding sites in the cerebral cortex also increased during pregnancy, again peaking on day 19 and returning to control values on day 21; receptor density was decreased further 2 days after delivery and again returned to control values within 7 days. These changes were accompanied by a decrease in the apparent affinity of the binding sites for the corresponding ligand on day 19 of pregnancy. The amount of the γ2L subunit mRNA decreased progressively during pregnancy, in the cerebral cortex and hippocampus, returned to control value around the time of delivery and did not change in the postpartum period. On the contrary, the amount of α4 subunit mRNA was not modified during pregnancy both in the cerebral cortex and hippocampus whereas significantly increased 7 days after delivery only in the hippocampus. No significant changes were apparent for α1, α2, α3, β1, β2, β3 and γ2S subunit mRNAs. Administration of finasteride, a specific 5α-reductase inhibitor, to pregnant rats from days 12 to 18 markedly reduced the increases in the plasma and brain concentrations of allopregnanolone and allotetrahydrodeoxycorticosterone as well as prevented both the increase in the densities of [${}^3\text{H}$]flunitrazepam and [${}^{35}\text{S}$]TBPS binding sites and the decrease of γ2L mRNA normally observed during pregnancy. The results demonstrate that the changes in the plasticity of GABA_A receptors that occur in rat brain during pregnancy and after delivery are related to the physiological changes in plasma and brain concentrations of neurosteroids.

Introduction

Systemic administration of progesterone or its metabolites 3α-hydroxy-5α-pregnan-20-one (allopregnanolone) and 5α-hydroxy-3α,21-diol-20-one (allotetrahydrodeoxycorticosterone) share whit benzodiazepines and barbiturates the capability to induce anxiolytic (Crawley et al., 1986; Bitran et al., 1991), hypnotic (Mendelson et al., 1987) or anticonvulsant (Belelli et al., 1990; Kokate et al., 1994; Concas et al., 1996) effects and to enhance the function of GABA_A receptors (Majewska, 1992; Lambert et al., 1995). Accordingly, pharmacological treatments, such as the oral administration of progesterone to healthy female volunteers (Freeman et al., 1993; McAuley et al., 1995) and the intracerebroventricular or parental administration of progesterone, allopregnanolone, or allotetrahydrodeoxycorticosterone to rats (Crawley et al., 1986; Belelli et al., 1990; Bitran et al., 1991; Kokate et al., 1994; Concas et al., 1996) induced parallel alterations in the behavior related to GABA_A receptors function. These clinical and experimental data have suggested that fluctuations in plasma and brain concentrations of neuroactive steroids induced by physiological, pharmacological or pathological conditions might affect GABA_A receptor function and this may result in alterations in the emotional state, sleep pattern and seizure threshold. Thus, the marked fluctuations in the plasma and brain concentrations of these compounds associated with physiological conditions such as acute and chronic stress, pregnancy, postpartum, estrous cycle, menopause, etc. (Purdy et al., 1991; Barbaccia et al., 1994, Barbaccia et al., 1996, Barbaccia et al., 1997; Finn and Gee, 1994; Halbreich et al., 1996; Rapkin et al., 1997; Concas et al., 1998; Biggio et al., 1999) may play an important role for the development of some disorders (anxiety, depression, premenstrual syndrome, etc.) associated to such conditions.

To clarify the role of these neuroactive steroids in the modulation of the GABA_A receptor activity, we have now evaluated the possible functional relation between the plasticity of GABA_A receptors and the variations in plasma and brain concentrations of progesterone, allopregnanolone and allotetrahydrodeoxycorticosterone associated with a physiological condition in which both GABA_A receptors and neurosteroids undergo marked and dynamic changes. Thus, we investigated in rats whether the increase in the concentrations of neuroactive steroids that occurs in plasma and brain during pregnancy may exert a tonic modulatory action on the density and gene expression of GABA_A receptors in the cerebral cortex and hippocampus. Moreover, we also evaluated whether the sudden decrease in the concentrations of these compounds immediately before parturition and their low abundance during lactation may represent a withdrawal-like phenomenon. Finally, to prove the physiological role of brain levels of allopregnanolone and allotetrahydrodeoxycorticosterone in the modulation of GABA_A receptor function and plasticity during pregnancy, we used finasteride, a selective blocker of 5α-reductase (Rittmaster, 1994), to abolish the marked increase of brain allopregnanolone and allotetrahydrodeoxycorticosterone during the last week of pregnancy.