Study: hungry mice are happy mice

BEIJING, July 15 (Xinhuanet) -- Unhappy and want to
be happy? A new study says you should get hungry.

When our bodies notice we need more calories, levels
of a hormone called ghrelin increase. Ghrelin is known to trigger hunger, but
new research suggests this may be a side effect of its primary job as a
stress-buster.

Researchers manipulated ghrelin levels in mice
through a variety of methods, including prolonged calorie restriction, ghrelin
injection and a genetic modification rendering the mice numb to ghrelin's
effect.

Mice who had limited ghrelin activity seemed
depressed. If pushed into deep water they made no effort to swim. When
introduced to a maze, they clung to the entryway. And when placed with other
mice, they tended to keep to themselves. (These behaviors were reversed when the
mice were given a low-dose antidepressant commonly prescribed to humans.)

In contrast, mice with high levels of ghrelin swam
energetically in deep water, looking for escape. They eagerly explored new
environments. And they were much more social.

The researchers think that hunger-induced happiness
is an adaptive measure. Getting food, especially in the wild, requires
concentration, clear-headed perception and often cooperation.

Hunger is not the only stressor that causes ghrelin
to rise. Social anxiety can stimulate it as well. When mice were exposed to an
older "bully" mouse, ghrelin levels rose and stayed high for weeks.

Elevated ghrelin could be why some people overeat
when under pressure. If the stress-induced snack is avoided, the research
suggests, ghrelin levels will remain high and help us confront the stressor in a
calm, effective way.

(Agencies)