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Minireview: The Neuroendocrinology of the Suprachiasmatic Nucleus as a Conductor of Body Time in Mammals

Departments of Psychology (I.N.K., R.S.), Columbia University, New York, New York 10027; Department of Psychology (R.S.), Barnard College, New York, New York 10027; and Department of Anatomy and Cell Biology (R.S.), Columbia University, New York, New York 10032

Address all correspondence and requests for reprints to: Rae Silver, Ph.D., Department of Psychology, Columbia University, 406 Schermerhorn Hall, MC5501, 1190 Amsterdam Avenue, New York, New York 10027. E-mail: qr{at}columbia.edu.

Circadian rhythms in physiology and behavior are regulated by a master clock resident in the suprachiasmatic nucleus (SCN) of the hypothalamus, and dysfunctions in the circadian system can lead to serious health effects. This paper reviews the organization of the SCN as the brain clock, how it regulates gonadal hormone secretion, and how androgens modulate aspects of circadian behavior known to be regulated by the SCN. We show that androgen receptors are restricted to a core SCN region that receives photic input as well as afferents from arousal systems in the brain. We suggest that androgens modulate circadian behavior directly via actions on the SCN and that both androgens and estrogens modulate circadian rhythms through an indirect route, by affecting overall activity and arousal levels. Thus, this system has multiple levels of regulation; the SCN regulates circadian rhythms in gonadal hormone secretion, and hormones feed back to influence SCN functions.