Most of the physiological and behavioral processes in mammals exhibit a daily rhythm, maintained by a central 'clock' in the brain, but the molecules that connect the brain clock with the body remain unknown. In 23 May Nature, Michelle Cheng and colleagues from University of California, Irvine, show that prokineticin 2, a cysteine-rich secreted protein, functions as an output molecule from the suprachiasmatic nucleus to the rest of the body.

Cheng et al. studied mice under dark and light conditions and found that expression of PK2 mRNA in the SCN oscillates with very high amplitude in a circadian fashion. In addition, inhibition of nocturnal locomotor activity in rats by intracerebroventricular delivery of recombinant PK2 during subjective night (when the endogenous PK2 mRNA level is low) "further supports the hypothesis that PK2 is an output molecule that transmits behavioral circadian rhythm," claim the authors.

These results "offer enormous...

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