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

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.