Morphological changes in the hypothalamic neurons of female rats exposed to continuous illumination

The morphological changes of neurons in the paraventricular nucleus (PVN), suprachiasmatic nucleus (SCN), ventromedial nucleus (VMN), preoptic area (POA), supraoptic nucleus (SON) and arcuate nucleus (ARN) were studied in 16-18 month-old Long-Evans female rats which had been exposed to continuous illumination (LL) for 11 months. Most neurons in those hypothalamic areas showed compact cytoplasm with abundant Nissl substances and evenly distributed chromatin. These morphological features were not found in the same hypothalamic regions of rats placed under a regular alternative light-dark regimen (14h light and 10h dark). These findings suggest that decreased melatonin and elevated estrogen levels induced by long-term exposure to LL might be involved in the morphological changes in hypothalamic neurons of female rats.