CORTISOL UP-REGULATES CORTICOTROPIN-RELEASING FACTOR GENE-EXPRESSION IN THE FETAL OVINE BRAIN-STEM AT 0.70 GESTATION

Glucocorticoids are important for the development of the central nervo us system. In the ovine fetus, increased levels of plasma cortisol at term provide a stimulus to initiate parturition. CRF is central to thi s event in that it is one of the main modulators of the hypothalamic-p ituitary-adrenal (HPA) axis. The purpose of the present study was to d etermine the effect of physiological increases in fetal plasma cortiso l levels on corticotropin-releasing factor (CRF) gene expression in th e developing ovine brain. Fetal plasma cortisol levels were chronicall y elevated at 0.70 gestation (100 days) to physiological levels found at 0.90 gestation (130 days; term 145 +/- 2 days) when glucocorticoid- induced maturational changes are known to occur in the HPA axis. The 3 ' end of the ovine CRF gene encodes 4 putative polyadenylation (poly(A )) signals that may post-transcriptionally regulate gene expression th rough stability, translation and localization of the mRNA in a tempora l and spatial manner. To determine whether CRF mRNA levels or poly(A) site usage are differentially regulated by cortisol in a region-specif ic manner, we used an RNase protection assay with an antisense CRF RNA probe from the 3' coding and untranslated regions of the gene to quan tify changes in mRNA levels in the hypothalamus (Hypo), hippocampal-am ygdala complex (H and A), frontal cerebral cortex (FCC) and brainstem. Our novel finding was a 3.5-fold increase in CRF mRNA levels in the m edulla oblongata of fetuses from the cortisol group compared to those from the saline group (P = 0.001). CRF mRNA levels in the Hypo, H and A and FCC did not change significantly in fetuses from the cortisol gr oup. CRF mRNA. transcripts derived from alternative poly(A) site usage were observed in all brain regions examined; however, cortisol admini stration did not change the ratio of mRNAs polyadenylated at site 1 ve rsus sites 2-4. These results indicate that changes in the environment (e.g. physiological increases in fetal plasma cortisol levels at an e arlier time during development) have regional effects on CRF gene expr ession in the developing ovine brain.