Effects of continuous environmental illumination on the albino rat hypothalamo-neurohypophysial system

Continuous environmental illumination or constant light (LL) exposure causes a suppression of daily water intake, and long-term exposure of greater than 19 days produces a hypertrophy of magnocellular neuroendocrine cells (MNCs) in the hypothalamus. These findings led Glantz to hypothesize that LL increases the secretion of vasopressin (VP). We wanted to determine whether LL could trigger morphological changes within the hypothalamo-neurohypophysial system (HNS) seen with other manipulations that result in enhanced hormone release. The posterior pituitary of male albino rats that were exposed to LL for 24 or 48 h were examined ultrastructurally for evidence of enhanced hormone release. In addition, water intake, plasma VP levels, and MNC size within the supraoptic nucleus (SON) were measured. After LL exposure, the posterior pituitary morphology was different, suggesting enhanced hormone release. LL exposure did not affect plasma VP or the size of SON MNCs, but did suppress drinking behavior. These data show that posterior pituitary morphology is affected rapidly by LL exposure. The HNS response to LL exposure may consist of changes within the first 24 h of LL found within the posterior pituitary followed later by hypertrophy of the SON MNCs.