Two Clocks Are Better Than One

Science Signaling  27 May 2008:
Vol. 1, Issue 21, pp. ec200
DOI: 10.1126/stke.121ec200

The circadian clock of mammals resides within the hypothalamus, in the suprachiasmatic nucleus, and it is entrained by light. There is evidence that another circadian clock--entrained by food--also exists in the brain, possibly elsewhere in the hypothalamus. By replacing a missing clock component (Bmal) in selected hypothalamic nuclei of mice lacking Bmal1, Fuller et al. identified the dorsomedial nucleus of the hypothalamus as the site of the food-entrained clock. When Bmal1 was reintroduced selectively into the suprachiasmatic nucleus, the previously unresponsive mice regained the ability to entrain both locomotor activity and body temperature rhythms to a 12:12 light:dark cycle. Reintroduction of Bmal into the dorsomedial nucleus restored the ability of the animals to entrain to a restricted period of food availability, but not to a light cycle. Thus, the dorsomedial nucleus contains a second circadian clock that seems to be induced when food is restricted to take over control of functions such as activity levels that increase the chances of the animal successfully locating additional food resources.

P. M. Fuller, J. Lu, C. B. Saper, Differential rescue of light- and food-entrainable circadian rhythms. Science 320, 1074-1077 (2008). [Abstract] [Full Text]