Regulation of rat mineralocorticoid receptor expression in neurons by progesterone

We have studied the effects of progesterone on the transcription of the mineralocorticoid receptor (MR) gene in neurons in vitro and in vivo. Progesterone treatment caused a 2.5-fold increase in activity of the MR promoter in transiently transfected N2A neuroblastoma cells. Similarly, MR promoter activity in GH3 pituitary cells was increased 2-fold after treatment with the specific progesterone receptor agonist R5020, with an even greater induction after priming with 17 beta-estradiol. Progesterone treatment also produced a dose-dependent increase in MR messenger RNA (mRNA) levels in primary hippocampal neuron cultures. In vivo, chronic administration of progesterone to estrogen-primed adrenalectomized/ovariectomized rats significantly increased MR mRNA levels in all hippocampal subfields, as determined by semiquantitative in situ hybridization histochemistry. Whereas chronic estradiol treatment decreased MR mRNA levels in the hippocampus, progesterone administration in the absence of estradiol priming was without any effect. These results indicate that 1) progesterone increases MR mRNA levels in vitro and in vivo; 2) the stimulatory effects of progesterone are at least partially mediated by induction of MR promoter activity; and 3) estrogen priming is essential for the effect of progesterone upon MR mRNA in vivo. Further, they suggest the possibility of heterologous regulation of corticosteroid receptors in the brain, whereby the responsiveness of the limbic-hypothalamo-pituitary-adrenal system to corticosteroids may be modulated.