Figure 4. A clock output pathway regulating circadian xenobiotic detoxification. The SCN master pacemaker synchronizes circadian oscillators in peripheral organs, such as liver, kidney and small intestine. The molecular signaling pathways involved in this process are still poorly understood, but they are related to feeding fasting cycles, body temperature rhythms, cyclic hormone secretion (eg, glucocorticoids), and possibly the rhythmic accumulation of metabolites. The molecular clocks in liver, kidney and small intestine govern the circadian expression of the PAR bZip transcription factors DBP, HLF, and TEF, which in turn modulate the rhythmic expression of regulators and enzymes involved in detoxification (see ref 122 for details). SCN, suprachiasmatic nuclei.