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Daily Rhythm of Spontaneous Immediate-Early Gene Expression in the Rat Suprachiasmatic Nucleus

Departments of Psychology, Pharmacology, Dalhousie University, Halifax B3H 4J1, Nova Scotia, Canada; CIQUIBIC-Departamento de Química Biológica, Facultad de Ciencias Químicas, Universidad Nacional de Córdoba, 5000 Córdoba, Argentina

Liliana B. de Guido

Departments of Psychology, Pharmacology, Dalhousie University, Halifax B3H 4J1, Nova Scotia, Canada

Donna Goguen

Harold A. Robertson

Department of Pharmacology, Dalhousie University, Halifax B3H 4J1, Nova Scotia, Canada

Benjamin Rusak

Departments of Pharmacology, and Psychiatry, Dalhousie University, Halifax B3H 4J1, Nova Scotia, Canada

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 fosBnear the time of subjective dawn in rats entrained to a light-dark 12:12 cycle and then maintained in constant total darkness for ~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.

Key Words: suprachiasmatic nucleus • circadian rhythm • junB • c-fos • fosB • NGFI-A • immediate-early gene • spontaneous expression • transcription regulation

Journal of Biological Rhythms, Vol. 14, No. 4, 275-280 (1999)
DOI: 10.1177/074873099129000687


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