In situ hybridization detection of estradiol-induced changes in ribosomal RNA levels in rat brain

In this study, quantitative assessment of estradiol (E₂)-induced changes in levels of ribosomal RNA within brain regions concentrating the hormone was accomplished by in situ hybridization with nick-translated tritiated ribosomal DNA probes and use of a computer-based image analysis system. Ovariectomized rats were either implanted with estradiol capsules for 6 h, 24 h, or 15 days, or sham-implanted under the same time course to serve as controls. The mean number of grains, somal area, and grain density of neurons within three E₂-concentrating brain regions, the ventrolateral portion of the ventromedial and the arcuate nuclei of the hypothalamus (VL-VMN and ARC, respectively) and the corticomedial nucleus of the amygdala (AMY) were determined. In the VL-VMN and ARC, levels of rRNA were significantly increased after 6 h of E₂ treatment (70% and 30%, respectively) and after 24 h of E₂ treatment (60% and 62%, respectively). However, these effects on rRNA levels in VL-VMN and ARC were not observed after prolonged exposure of 15 days to the hormone. Neuronal hypertrophy was present only after 24 h of E₂ treatment in the VL-VMN and ARC (32% and 14%, respectively). No changes were found in the AMY. As an additional internal control, measurements were also collected from the dorsomedial portion of the VMN (DM-VMN), a region with few E₂-concentrating neurons. No changes in any of the parameters were found in DM-VMN at any time after exposure to the hormone. By extending the in situ hybridization technique to the quantitative level, these findings demonstrate differential estrogenic regulation of a known gene product, rRNA, in rat brain that is temporally and regionally specific.