Distribution of the mineralocorticoid and the glucocorticoid receptor mRNAs in the rat hippocampus.

The cellular localization of mineralocorticoid receptor (MR) and glucocorticoid receptor (GR) gene expression in the rat hippocampus was studied by in situ hybridization using 35S-labeled RNA-probes, complementary to either 513 bases of the rat brain mineralocorticoid receptor (MR)-mRNA or 500 bases of the rat liver glucocorticoid receptor (GR)-mRNA. Neurons in CA1, CA2, and the dentate gyrus expressed both receptor genes at high levels. The MR-mRNA was demonstrated in all pyramidal cell fields (CA1-4) of the hippocampal formation and the granular neurons of the dentate gyrus. In contrast, GR-mRNA was mainly restricted to CA1 and CA2 pyramidal cell fields and the dentate gyrus. This pattern of hybridization was found to agree with the cellular distribution of the two types of corticosteroid receptors detected previously in the hippocampus by autoradiography of the radio-labeled receptors and by immunocytochemistry of the receptor protein. These observations suggest that the corticosteroid receptors described previously as type 1 and type 2 are encoded by MR- and GR-mRNA, respectively. Although both the MR and GR genes are co-expressed in some hippocampal neurons, the unique patterns of distribution of the two receptor mRNAs in the hippocampal formation suggest that the genes for these receptors are differentially regulated. Moreover, the microanatomy of MR and GR expression provides insight into molecular mechanisms underlying the characteristic action of various steroids on behaviors involved in stress and circadian regulation.