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Biology Articles » Chronobiology » Researchers Find Biological Clock For Smell In Mice
Biologists at Washington University in St. Louis have discovered a large
biological clock in the smelling center of mice brains and have
revealed that the sense of smell for mice is stronger at night, peaking
in evening hours and waning during day light hours.
A team led by Erik Herzog, Ph.D., Washington University associate
professor of Biology in Arts & Sciences, discovered the clock in the
olfactory bulb, the brain center that aids the mouse in detecting
The olfaction biological clock is hundreds of times larger than the
known biological clock called the suprachiasmatic nucleus (SCN), located
at the base of the brain right on top of where the optic nerves cross
each other. Cells in both the SCN and the olfactory bulb keep 24-hour
time and are normally highly sychronized to each other and environmental
cycles of day-night.
"It's been a question for some time whether the SCN functions as the
only biological clock," said Herzog. "One wouldn't think that the
ability to smell would cycle, but that's what we show.
" I think now that the SCN is like the atomic clock, important for
keeping central time, and then there are all of these peripheral clocks -
for timing tasks like sleep-wake, vigilance, digestion, olfaction,
hearing, touch and vision, though not all yet found. It may be that the
peripheral clocks are like individual wristwatches that we must
Perhaps most surprising is the observation that the olfactory bulb
clock can run independent of daily rhythms in sleep-wake or the SCN,
making it the Big Ben of the mammalian circadian rhythm world.
"It seems to be one of those biological clocks that can keep running
itself for a long time, even without the SCN," Herzog said.
Results were published in the Nov. 22, 2006, issue of the Journal of
Herzog and collaborators Daniel Granados-Fuentes, Ph.D., Washington
University postdoctoral researcher, and undergraduate student researcher
Alan Tseng, put a little cedar oil on a Q-tip and allowed mice to sniff
it for five minutes.
"We then preserved their brains and counted the number of olfactory
bulb cells that had been activated by the odorant," Herzog said. "The
gene cFOS is a marker for cells that were activated by the stimulus. We
recorded the expression of that gene. All of the data came from in vivo
They saw more of those cells light up in the olfactory bulb at night
than in the day.
"The olfactory bulb might be more sensitive at night when the
creatures are active than when they are resting in the day," Herzog
speculated. "This might help them find food or mates when they are
hungry for food or for love."
Do the results suggest that women should splash on the Estee Lauder
during the night so that men can notice all the more and shun the bottle
during the day?
"There are anecdotes in the literature about humans liking certain
perfumes more during the evening than the morning, and there is some
evidence that we also have daily rhythms in olfaction," Herzog said.
Herzog said that it is rare to find someone missing their SCN, so
it's tricky to study the human olfactory clock by itself. For this
reason, his lab plans to study olfactory behavior in mice.
"We can say that this (olfactory bulb) clock has a functional
consequence, and now we're setting up to do olfactory behavior," he
said. "We'll ask the mice to tell us when they can smell odors of
different concentrations, and we hope to learn more about how and how
much the clock modulates their sense of smell, and which cells and genes
The olfactory bulb biological clock study opens up many questions, a
key one of which is: Why are there multiple clocks?
"This idea of multiple biological clocks is new," Herzog said. "We
might need now to consider ourselves a clock shop. It appears that
disrupting the coordination between these clocks is bad for our health,
like in jet lag or shift work."
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