EFFECTS OF CONTINUOUS ENVIRONMENTAL ILLUMINATION ON THE ALBINO-RAT HYPOTHALAMONEUROHYPOPHYSEAL 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 neuroendo crine cells (MNCs) in the hypothalamus. These findings led Glantz to h ypothesize that LL increases the secretion of vasopressin (VP). We wan ted to determine whether LL could trigger morphological changes within the hypothalamo-neurohypophysial system (HNS) seen with other manipul ations that result in enhanced hormone release. The posterior pituitar y of male albino rats that were exposed to LL for 24 or 48 h were exam ined ultrastructurally for evidence of enhanced hormone release. In ad dition, water intake, plasma VP levels, and MNC size within the suprao ptic nucleus (SON) were measured. After LL exposure, the posterior pit uitary 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 m orphology is affected rapidly by LL exposure. The HNS response to LL e xposure may consist of changes within the first 24 h of LL found withi n the posterior pituitary followed later by hypertrophy of the SON MNC s.