Unexpected effects of metyrapone on corticosteroid receptor interaction with the genome and subsequent gene transcription in the hippocampus of male rats

Glucocorticoid hormones (GCs) play a pivotal role in many stress-related biological processes. In the hippocampus, GCs act through mineralocorticoid (MRs) and glucocorticoid receptors (GRs) to modify gene transcription. Involvement of GCs in biological processes has been studied using the corticosterone (CORT)-synthesis blocker metyrapone. How metyrapone affects GC action at the genomic level is however still unclear. Therefore, we studied the effects of this enzyme blocker on plasma CORT levels and hippocampal MR and GR binding to GC responsive elements (GREs) within the GC target genes Fkbp5 (FK506-binding protein 5), Per1 (Period 1) and Sgk1 (Serum- and glucocorticoid-activated kinase 1), as well as transcriptional responses of these genes under control and acute stress conditions in rats. For comparison, we also studied these endpoints in adrenalectomized (ADX) rats. Although metyrapone had no effect on baseline levels of CORT, the drug increased MR and GR to GRE binding within the GC target genes and the transcriptional activity of these genes. As expected, acute forced swim (FS) stress strongly increased plasma CORT levels, hippocampal MR and GR to GRE binding within Fkbp5, Per1 and Sgk1, and transcriptional activity (mainly hnRNA levels) of these genes. Metyrapone attenuated, but not abolished, these effects of stress on plasma CORT and MR and GR to GRE binding. The drug effects on FS-induced transcriptional activity were gene-dependent with a reduction seen in Fkbp5 hnRNA (but not Fkbp5 mRNA), an enhancement in Per1 hnRNA (but not Per1 mRNA), and no effect on both Sgk1 hnRNA and mRNA levels. ADX however completely abrogated the effects of FS on plasma CORT as well as hippocampal MR and GR to GRE binding and transcriptional responses.
Thus, in contrast to ADX, metyrapone produced inconsistent effects on GC-sensitive genomic endpoints that question its suitability as a tool in neuroendocrine and other research.