First evidence that serotonin plays a role in
guiding food choices and may influence lifespan
Newswise, September 12, 2016 — ANN ARBOR, Mich. — If you’re a
human who’s really hungry, a handful of nuts, a piece of cheese or a nice juicy
steak may really hit the spot. If you’re a fruit fly, a nibble of yeast will do
the trick.
Why do we – and those flies that sometimes inhabit our
kitchens – seek out protein-full foods when we’re running on empty? And what
does that preference mean for the odds of living a longer life, whether it’s
measured in decades for a human, or days for a fly?
New research from a University of Michigan Medical School team
suggests for the first time that a brain chemical may have a lot to do with
both questions.
In a new paper in the journal eLife, U-M scientist Scott
Pletcher, Ph.D., and his team demonstrate the key role that the chemical called
serotonin plays in the feeding habits and life spans of fruit flies. The
paper’s first author is Jennifer Ro, Ph.D., now at Harvard Medical School.
Reward in
the brain
Serotonin is a “reward” chemical, which means when it’s
released in the brain in response to an action, it travels between brain cells
and produces a sense of reward or even pleasure.
Pletcher and his team report that it appears to play a key role
in fruit flies’ strong tendency to seek out protein, not sugars, when they’ve
been deprived of food for a while.
In other words, it affects the value that flies place on
protein at that time -- which means that it’s somehow tied to how the flies
figure out which foods contain protein in the first place.
Not only that, but the brain-based reward that the flies got
from eating protein appears to influence how quickly the flies aged.
When that reward was blocked, the flies ate just as much food
as before in their normal diets – but lived far longer.
In fact, they lived nearly twice as long – just from blocking
a single serotonin receptor found on the surface of only about 100 neurons in
their brains.
While it’s far too soon to apply their findings to our understanding
of human feeding patterns or longevity, Pletcher notes that the serotonin
reward system in fruit flies is very similar to that in mammals including
humans.
So are many other basic systems, which is what makes fruit
flies such an important species to study because one scientific team can study
hundreds of generations of them.
A choice
of entrees
The researchers made their discovery by manipulating the genes
involved in the serotonin system, as well as manipulating the flies’ access to
different types of food using a special chamber they developed.
Called the FLIC, or Fly Liquid-food Interaction Counter, this
device allowed them to continuously monitor food preferences for each
micro-meal and to identify how and when flies were rewarded by a protein-rich
diet. Armed with information, they designed experiments to examine whether such
nutritional rewards affect health and lifespan by providing flies just a sugary
diet, just a protein-focused diet, or the choice of three options: those two
single-nutrient diets and a mixed diet throughout their life.
“This work builds on previous findings that the perception of
food modulates aging in much the same way as dietary intake, but the brain
regions and systems involved in this have been unknown,” says Pletcher.
“We found that the serotonin pathway is important for
interpreting the composition of the food, as well as the reward that drives
consumption of the food.”
Protein-rich diets have previously been found to lead to
shorter lifespans, he notes. “These results suggest that serotonin is directly
involved in this process, though we have not yet found the mechanism,” he adds.
The new results add to a changing scientific view of how food
affects health and lifespan. The way animals respond to nutrients, including detecting
them in their environment and seeking out certain ones during different times,
goes far beyond simply seeking calories of any kind. Protein, which is crucial
for building and maintaining cells in the body, serves a different function
from sugars and other carbohydrates, which are sources of energy.
Next
steps
The brain’s ability to register that an animal has eaten
enough of a certain nutrient is key to its ability to signal -- via reward
pathways -- that an earlier hunger has been satisfied, Pletcher explains.
Even when that reward pathway was blocked in the fruit fly
experiments, the flies stopped eating for other reasons -- they didn’t stuff
themselves dangerously.
But the inability to sense the special reward that they
usually would have gotten from eating protein did something to influence their
lifespan. Now, the Pletcher group is working to determine just what that might
be.
In the meantime, humans whose stomachs are rumbling and brains
are sending a message of serious hunger should feel free to satisfy that
craving for a protein-rich snack or meal. Just don’t bank on it having any
particular impact on your lifespan – after all, human lives are much more
complex than those of fruit flies.
But more research in fruit flies may help us understand just
why protein seems the most appealing or causes a unique sense of reward. Says
Pletcher,
“This paves the way for future work to understand how the
brain mechanisms that allow animals to perceive and evaluate food act to
control lifespan and aging.”
In addition to Ro and Pletcher, the research team included
Gloria Pak, Paige A. Malec, Yang Lyu, David B. Allison, and Robert T. Kennedy.
The research was funded by the National Institutes of Health (AG030593,
GM102279, AG023166, AG043972, DK046960, GM007315, AG000114 and AG047696), the
Ellison Medical Foundation and a Glenn/American Federation for Aging Research
Scholarship for Research in the Biology of Aging. Reference: eLife, http://dx.doi.org/10.7554/eLife.16843
