Circadian time-dependent antioxidant and inflammatory responses to acute cadmium exposure in the brain of zebrafish

Up to date, little information is available on effects of circadian rhythm on metal-induced toxicity in fish. In this study, zebrafish were acutely exposed to 0.97 mg L-1 cadmium for 12 h either at ZTO (the light intensity began to reached maximum) or at ZT12 (light intensity began to reached minimum) to evaluate the temporal sensitivity of oxidative stress and inflammatory responses in the brain of zebrafish. Profiles of responses of some genes at mRNA, protein and activity levels were different between ZTO and ZT12 in the normal water. Exposure to Cd induced contrary antioxidant responses and similar inflammatory responses between ZTO and ZT12. However, the number of inflammatory genes which were up-regulated was significantly greater at ZT12 than at ZTO. And, the up-regulated inflammatory genes were more responsive at ZT12 than at ZTO. At ZT12, antioxidant genes were down-regulated at mRNA, protein and activity levels. Contrarily, antioxidant genes were not affected at mRNA levels but activated at the protein and/or activity levels at ZTO. Reactive oxygen species (ROS) sharply increased and remained relatively stable when fish were exposed to Cd at ZT12 and ZTO, respectively. Positive correlations between ROS levels and mRNA levels of nuclear transcription factor kappa B (NF-kappa B) and between mRNA levels of NE-kappa B and its target genes were observed, suggesting that ROS may play an essential role in regulating the magnitude of inflammatory responses. Taken together, oxidative stress and immunotoxicity in the brain were more serious when fish were exposed to Cd in the evening than in the morning, highlighting the importance of circadian rhythm in Cd-induced neurotoxicity in fish. (C) 2016 Elsevier B.V. All rights reserved.