Martinez-Chavez Carlos Christian, Al-Khamees Sammi, Campos-Mendoza Antonio, Penman David James, Migaud Herve
Institute of Aquaculture, University of Stirling, Stirling, UK.
Chronobiol Int. 2008 Feb;25(1):31-49. doi: 10.1080/07420520801917547.
The purpose of this work was to investigate the circadian melatonin system in two tropical teleost species characterized by different behavioral habits, Nile tilapia (diurnal) and African catfish (nocturnal). To do so, fish were subjected to either a control photoperiod (12L:12D), continuous light (LL) or darkness (DD), or a 6L:6D photoperiod. Under 12L:12D, plasma melatonin levels were typically low during the photophase and high during the scotophase in both species. Interestingly, in both species, melatonin levels significantly decreased prior to the onset of light, which in catfish reached similar basal levels to those during the day, demonstrating that melatonin production can anticipate photic changes probably through circadian clocks. Further evidence for the existence of such pacemaker activity was obtained when fish were exposed to DD, as a strong circadian melatonin rhythm was maintained. Such an endogenous rhythm was sustained for at least 18 days in Nile tilapia. A similar rhythm was shown in catfish, although DD was only tested for four days. Under LL, the results confirmed the inhibitory effect of light on melatonin synthesis already reported in other species. Finally, when acclimatized to a short photo-cycle (6L:6D), no endogenous melatonin rhythm was observed in tilapia under DD, with melatonin levels remaining high. This could suggest that the circadian clocks cannot entrain to such a short photocycle. Additional research is clearly needed to further characterize the circadian axis in teleost species, identify and localize the circadian clocks, and better understand the environmental entrainment of fish physiology.
这项工作的目的是研究两种具有不同行为习性的热带硬骨鱼——尼罗罗非鱼（昼行性）和非洲鲶鱼（夜行性）的昼夜褪黑素系统。为此，将鱼置于对照光周期（12小时光照：12小时黑暗）、持续光照（LL）或黑暗（DD），或6小时光照：6小时黑暗的光周期条件下。在12小时光照：12小时黑暗条件下，两种鱼的血浆褪黑素水平在光照期通常较低，在暗期较高。有趣的是，在这两种鱼中，褪黑素水平在光照开始前显著下降，在鲶鱼中降至与白天相似的基础水平，这表明褪黑素的产生可能通过生物钟预测光照变化。当鱼暴露于DD条件下时，获得了这种起搏器活动存在的进一步证据，因为维持了强烈的昼夜褪黑素节律。这种内源性节律在尼罗罗非鱼中至少持续了18天。鲶鱼也表现出类似的节律，尽管DD只测试了四天。在LL条件下，结果证实了光照对褪黑素合成的抑制作用，这在其他物种中已有报道。最后，当适应短光周期（6小时光照：6小时黑暗）时，在DD条件下罗非鱼未观察到内源性褪黑素节律，褪黑素水平保持较高。这可能表明生物钟无法适应如此短的光周期。显然需要进一步的研究来进一步表征硬骨鱼物种的昼夜轴，识别和定位生物钟，并更好地理解鱼类生理学的环境调节。
