Effects of long-term exposure to light or darkness and return to normal light-dark cycle on serum melatonin levels in rats

Abstract

Increasing evidence suggests that the presence of
 constant light or darkness have diverse effects on circadian physiology. The
 aim of this study is to explore serum levels of melatonin upon return to a
 normal light-dark cycle (LDC) in rats exposed to constant light (LL) or
 darkness (DD). Results showed the different profiles of melatonin levels after
 exposure to LL or DD. Similarly, the restoration of the LDC (12L:12D) modified
 the endogenous melatonin levels. In the LL group, serum melatonin remained at
 levels similar to control values, and when normal LDC was restored, melatonin
 levels of the rats decreased but without significant difference compared to
 control. In the DD group, serum melatonin increased significantly (22%), and
 upon switching to normal LDC, serum melatonin level was significantly decreased
 compared to constant dark condition. Even 15 days later, a significant 76% drop
 in serum melatonin level was still observed in LDC condition. The results
 suggested that prolonged exposure to LL or DD, especially to DD, had profound
 effects on the serum melatonin. LL has little influence and this result can be
 explained by the potential compensation of extrapineal melatonin generated by
 other tissues and organs.