NEUROSTEROIDS MODULATE GABAA RECEPTOR GENE EXPRESSION IN ASTROCYTES FROM RAT BRAIN

The expression of GABAA receptors was studied in mesencephalic astrocytes after prolonged exposure to positive allosteric modulators of the GABAA receptors, such as the 5a-reduced, 3a-hydroxylated progesterone derivatives. Post natal 3 days old rats were used to prepare astrocytes. This cell preparation has been demonstrated to be more than 99% pure as determined by immunocytochemical analysis of glial fibrillary protein (GFAP) which is a specific marker for astrocytes. We also show that this cell preparation have both [3H]GABA (1038 fmol/mg of protein) and [3H]PK 11195 (1102 fmole/mg of protein) binding. The glial peripheral benzodiazepine receptors (PBR) seem to be functional as demonstrated by stimulation of the synthesis of neuroactive steroids by the selective (1) PBR ligand CB34 at concentration of 10mM for 30 minutes. In fact, the exposure to CB34 increases the concentration of both, allopregnanolone (3a,5a-TH PROG) and tetrahydroxycorticosterone (3a,5a-TH DOC) in astrocytes. In addition, astrocytes are able to convert exogenous progesterone in 3a,5a-TH PROG and 3a,5a-TH DOC. Our results also show that chronic progesterone treatment (10mM; 5 days) and its withdrawal (6 hours) can modify the gene expression of discrete GABAA receptor subunit mRNAs with a different pattern respect to what observed in neurons (2). In fact, chronic progesterone treatment increased (55±8%) the abundance of the a4 subunit mRNA and this increase was still evident after 6 hours progesterone withdrawal (22±5%). The results of our study suggest that long term exposure of GABAA receptors to neuroactive steroids can regulate the receptor gene expression. The effects on specific GABAA receptor subunits differ in neurons and astrocytes. This diversity may reflect different roles of neuronal and glial GABAA receptors in the brain.