Guido M E, de Guido L B, Goguen D, Robertson H A, Rusak B
Department of Psychology, Dalhousie University, Halifax, Nova Scotia, Canada.
J Biol Rhythms. 1999 Aug;14(4):275-80. doi: 10.1177/074873099129000687.
Nocturnal light induces the expression of various immediate-early genes (IEGs) in the suprachiasmatic nucleus (SCN), the primary pacemaker of the circadian system of mammals, and causes phase shifts of behavioral rhythms. In the hamster SCN, some IEGs show both sensitivity to light induction at night and a daily peak of spontaneous expression near dawn in different regions of the nucleus. To investigate whether both patterns of IEG expression are observed in the rat SCN, the authors studied the expression of NGFI-A, junB, c-fos, and fosB near the time of subjective dawn in rats entrained to a light-dark 12:12 cycle and then maintained in constant total darkness for approximately 48 h. They found that there were two independent rhythms of expression for junB and c-fos mRNAs in the SCN: (1) a rhythm of photic sensitivity expressed throughout the night and (2) a spontaneous rhythm of expression triggered around dawn and persisting for at least 2 h into the day. By contrast, fosB and NGFI-A transcripts were expressed only after light exposure at night and did not exhibit significant levels of spontaneous expression in the absence of photic input. These observations demonstrate that the circadian clock gates expression of two independent rhythms related to IEG expression in the rat SCN. The rhythm of sensitivity to nocturnal light exposure is expressed more strongly in the ventral SCN and may be related to photic entrainment. The second rhythm is triggered spontaneously in darkness around subjective dawn and is expressed in more dorsal parts of the SCN.
夜间光照可诱导哺乳动物昼夜节律系统的主要起搏器——视交叉上核（SCN）中多种即刻早期基因（IEGs）的表达，并导致行为节律的相位偏移。在仓鼠的SCN中，一些即刻早期基因在夜间对光诱导敏感，且在核的不同区域黎明前出现自发表达的日峰值。为了研究大鼠SCN中是否也观察到即刻早期基因表达的这两种模式，作者研究了在12:12明暗循环中饲养、然后在持续完全黑暗中维持约48小时的大鼠，在主观黎明时刻附近NGFI-A、junB、c-fos和fosB的表达。他们发现，SCN中junB和c-fos mRNA有两种独立的表达节律：（1）整夜表达的光敏感性节律；（2）黎明前后触发并持续至白天至少2小时的自发表达节律。相比之下，fosB和NGFI-A转录本仅在夜间光照后表达，在无光照输入时未表现出显著的自发表达水平。这些观察结果表明，昼夜节律时钟控制着大鼠SCN中与即刻早期基因表达相关的两种独立节律的表达。对夜间光照的敏感性节律在腹侧SCN中表达更强，可能与光诱导有关。第二种节律在主观黎明前后的黑暗中自发触发，在SCN的更背侧部分表达。
