Dinnertime
What do the seals in the zoo and obese people eating at night have in common? The me altime determines their behavior. The seal feeding in the Heidelberg Zoo is worth a visit: the marine mammals always come up with new, circus-worthy tricks, for which they are rewarded with delicious fish. Not only the big and small visitors know the time at which the spectacle begins and look for a good spot at the edge of the pool in good time. The seals also know exactly when their big hour has come. At least half an hour before the show starts, they swim excitedly around the pool and look expectantly in the direction the attendant will come from.
Life rhythm and food intake are closely related, so closely in fact that me altime turns genes in a body clock on and off, as Masashi Yanagisawa of the Howard Hughes Medical Institute at the University of Texas and his team have now discovered. The working group searched for the timer that directs the eating behavior of mice. A superordinate internal clock in the suprachiasmatic nucleus in the anterior hypothalamus controls the normal rhythm of life in rodents – as in all mammals. This biological clock is adjusted via daylight and coordinates several of the body's subordinate clocks.
However, if animals are regularly served food at a certain time, they – like the zoo seals mentioned at the beginning – show clear foraging behavior shortly before the feeding time. However, they also do this when their superordinate internal clock in the suprachiasmatic nucleus has been switched off - so there must be a timer for the search for food that is independent of this clock. But where is this hidden?
Somewhere in her body they remembered that time
(Masashi Yanagisawa) To track this clock, Yanagisawa's research group fed mice living on a regular 12-hour daylight schedule once every 24 hours within a four-hour period-but not at night, as would correspond to the natural behavior of rodents, but in the middle of the day. The mice reacted as expected: Nine days after the introduction of the unusual feeding time, when they were supposed to be sound asleep at this time, they became nervous and ran around two hours before the meal was served. They continued to search for food even if there had been nothing to eat for two days at the usual time. The animals had clearly adjusted to the me altimes. "Somewhere in their bodies they remembered that time," says Masashi Yanagisawa.
Now the scientists searched the brains of the animals for the areas that directed this behavior. To do this, they cut the brains of their mice into fine slices at different points in time after the change in feeding time and analyzed the activity of the genes for the protein Period (short per), which is crucially involved in regulating the inner clock.
In the suprachiasmatic nucleus, nothing special happened due to the unusual meal times. In another brain area, however, in the dorsomedial hypothalamic nucleus, the per genes suddenly became active, even though there had been no food for two days by then. This brain area coordinates information from the suprachiasmatic nucleus about the time of day with various behaviors and physiological processes.
Yanagisawa and his team had thus uncovered a timer in the dorsomedial hypothalamic nucleus that is adjusted by me altimes, and which in turn influences the behavior of the mice.
Obese people often have a nightly meal – it is possible that, like in the mice in this experiment, their inner clock, which is controlled by the meal time, has lost its rhythm and is therefore no longer synchronized with the sleep-wake cycle. In this case, the midnight snack would not only be a habit, but actually the reaction to an expectation of the brain - that would of course make it very difficult to do without it.
