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ToxSci Advance Access published online on October 4, 2006
Toxicological Sciences, doi:10.1093/toxsci/kfl126
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© The Author 2006. Published by Oxford University Press on behalf of the Society of Toxicology. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org
Received September 1, 2006
Accepted October 2, 2006

Neurotoxicology

Effects of Tryptophan Photoproducts in the Circadian Timing System: Searching for a Physiological Role for Aryl Hydrocarbon Receptor

Motoko Mukai 1 and Shelley A. Tischkau 2 *

1 Department of Veterinary Biosciences, College of Veterinary Medicine, University of Illinois at Urbana-Champaign, Urbana, IL 61802
2 Department of Veterinary Biosciences, College of Veterinary Medicine, University of Illinois at Urbana-Champaign, Urbana, IL 61802; Neuroscience Program, University of Illinois at Urbana-Champaign

* To whom correspondence should be addressed.
Shelley A. Tischkau, E-mail: tischkau{at}uiuc.edu


  Abstract

The aryl hydrocarbon receptor (AhR) mediates adverse effects of dioxins, but its physiological role remains ambiguous. The similarity between AhR and canonical circadian clock genes suggests potential involvement of AhR in regulation of circadian timing. Photoproducts of tryptophan (TRP), including 6-formylindolo[3,2-b]carbazole (FICZ), have high affinity for AhR and are postulated as endogenous ligands. Although TRP photoproducts activate AhR signaling in vitro, their effects in vivo have not been investigated in mammals. Because TRP photoproducts may act as transducers of light, we examined their effects on the circadian clock. Intraperitoneal injection of TRP photoproducts or FICZ to C57BL/6J mice dose-dependently induced AhR downstream targets, CYP1A1 and 1B1 mRNA expression, in liver. c-fos mRNA, a commonly used marker for light responses, was also induced with FICZ and all responses were AhR dependent. A rat immortalized suprachiasmatic nucleus (SCN) cell line, SCN 2.2, was used to examine the direct effect of TRP photoproducts on the molecular clock. Both TRP photoproducts and FICZ increased CYP1A1 expression and prolonged FICZ incubation altered the circadian expression of clock genes (Per1, Cry1, Cry2) in SCN 2.2 cells. Furthermore, FICZ inhibited glutamate-induced phase shifting of the mouse SCN electrical activity rhythm. Circadian light entrainment is critical for adjustment of the endogenous rhythm to environmental light cycle. Our results reveal a potential for TRP photoproducts to modulate light-dependent regulation of circadian rhythm through triggering of AhR signaling. This may lead to further understanding of toxicity of dioxins and the role of AhR in circadian rhythmicity.

Keywords: dioxin; suprachiasmatic nucleus; circadian rhythm; light entrainment; phase shift.
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Home page
 J Biol RhythmsHome page
M. Mukai, T.-M. Lin, R. E. Peterson, P. S. Cooke, and S. A. Tischkau
Behavioral Rhythmicity of Mice Lacking AhR and Attenuation of Light-Induced Phase Shift by 2,3,7,8-Tetrachlorodibenzo-p-Dioxin
J Biol Rhythms, June 1, 2008; 23(3): 200 - 210.
[Abstract] [PDF]


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