Hannibal Jens, Fahrenkrug Jan
Department of Clinical Biochemistry, Bispebjerg Hospital, University of Copenhagen, Denmark.
Ann Med. 2002;34(5):401-7. doi: 10.1080/078538902320772151.
The brain's biological clock located in the suprachiasmatic nucleus (SCN) generates circadian rhythms of physiology and behaviour of approximately 24 hours. The clock needs, however, like a watch that runs too fast or too slow, daily adjustment and the most important stimulus for this adjustment is the environmental light/dark cycle, a process know as photoentrainment. It is well established that the eye contains a separate anatomical and functional system mediating light information to the clock. Until recently, the photopigment responsible for light entrainment of the circadian system has been elusive but recent studies have provided evidence that melanopsin, a recently identified opsin, could be the circadian photopigment. This conclusion is based on the observation that melanopsin is expressed exclusively in retinal ganglion cells projecting to the SCN, a projection known as the retinohypothalamic tract (RHT) and that these ganglion cells are intrinsically photosensitive. Melanopsin is present in the plasma membrane of soma, dendrites and axons forming an extensive photoreceptive network in the entire retina. Although these findings make melanopsin a strong candidate as a circadian photopigment, a number of functional experiments are needed before the role of melanopsin is finally proven.
位于视交叉上核(SCN)的大脑生物钟产生约24小时的生理和行为昼夜节律。然而,就像走得太快或太慢的手表一样,生物钟需要每日进行调整,而这种调整的最重要刺激因素是环境光/暗周期,这一过程称为光同步化。众所周知,眼睛包含一个独立的解剖和功能系统,将光信息传递给生物钟。直到最近,负责昼夜节律系统光同步化的光色素一直难以捉摸,但最近的研究提供了证据表明,黑视蛋白(一种最近发现的视蛋白)可能是昼夜节律光色素。这一结论基于以下观察结果:黑视蛋白仅在投射到SCN的视网膜神经节细胞中表达,这一投射称为视网膜下丘脑束(RHT),并且这些神经节细胞本身具有光敏感性。黑视蛋白存在于胞体、树突和轴突的质膜中,在整个视网膜中形成广泛的光感受网络。尽管这些发现使黑视蛋白成为昼夜节律光色素的有力候选者,但在最终证明黑视蛋白的作用之前,还需要进行一些功能实验。