Study explores the neuroscience of overindulging

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By Tim Newman Fact 


checked by Jasmin Collier


In a recent study in mice,
scientists found a particular brain circuit that helps explain why it is so
easy to overindulge in unhealthful foods.
There is a large bag of chips
in our lap; we are not hungry, but we manage to eat every last one of them.
Many of us will be familiar
with this scenario, but humans are not the only mammal with a drive to overeat
high-calorie foods.
In evolutionary terms, if an
animal finds a food source high in nutrients, it makes sense to eat as much as
possible; in the wild, starvation is an ever-present danger.
Today, we have access to
energy-dense foods wherever we look; in fact, it can be challenging to find
foods that are not packed with sugar and fat.
We have evolved to find these
types of food delicious — and food companies know it.
Homeostatic vs. hedonic
feeding


So-called homeostatic feeding
occurs when an animal eats until it has satiated its hunger and restored its
energy levels.
Hedonic feeding, on the other
hand, describes an animal’s drive to eat more than it needs if the food source
is particularly nutrient-dense and delicious.
Although our highly evolved
brains can usually wield enough self-control to override these primal urges, we
are not always successful.
This life-saving mechanism
might now play a part in the rise of obesity and associated conditions.
As new study co-author Prof.
Thomas Kash, Ph.D., points out, “There’s just so much calorically dense
food available all the time now, and we haven’t yet lost this wiring that
influences us to eat as much food as possible.”
Recently, researchers from the
University of North Carolina Health Care in Chapel Hill took a detailed look at
this phenomenon in rodents’ brains. They recently published their findings in
the journal Neuron.
In recent years, researchers
looking for ways to reduce obesity have investigated the mechanisms involved in
homeostatic feeding. To date, this approach has not led to successful
interventions.
More recently, however,
scientists — including those involved in the new study — have looked to hedonic
feeding for answers.
Nociceptin and overeating

Previous studies have shown
that nociceptin, a peptide consisting of 17 amino acids that functions as a
neurotransmitter, might play a part in hedonic feeding.
Other research has
demonstrated that nociceptin receptors make little difference to homeostatic
feeding, but that they appear to play a role in hedonic feeding. Drug companies
are of course interested in creating “anti-binging drugs,” but
scientists are a long way from achieving that.
However, Prof. Kash and team
have moved one step closer by pinning down the neural circuit that seems to be
most heavily involved in hedonic eating in mice.
To drill down to a specific
circuit, they engineered mice that produce a fluorescent marker-tagged
nociceptin. This made it easier to visualize the cells involved in nociceptin
circuits.
Many circuits in the brain
utilize nociceptin, but the researchers identified one particular circuit that
lit up when the mice binged on energy-dense foods. This circuit has projections
to other parts of the brain that help regulate feeding, so it seems to be a
strong candidate.
This particular circuit
originates in the central nucleus of the amygdala, a part of the brain that
plays a vital role in an animal’s response to emotional stimuli.
The authors believe that
“this is the first study to ascribe specific hedonic feeding actions to a
subpopulation of [central amygdala] neurons.”
Removing the overeating
circuit


In follow-up experiments, the
scientists deleted around half of the neurons that produce nociceptin in the
circuit. They found that this reduced levels of binge eating.
They gave the mice access to
standard chow and high-calorie food, alternatively. With these neurons
silenced, the mice significantly reduced their intake of high-calorie food and
resisted diet-induced obesity. Their consumption of standard chow remained
consistent.
“Scientists have studied
the amygdala for a long time, and they’ve linked it to pain and anxiety and
fear, but our findings here highlight that it does other things too, like
regulate pathological eating.”
Prof. Thomas Kash, Ph.D.

This is an intriguing
discovery, but it is just the beginning of a long process; scientists will need
to carry out much more research to fully understand how this new mechanism fits
into the bigger picture.
“Our study is one of the
first to describe how the brain’s emotional center contributes to eating for
pleasure,” explains first study author J. Andrew Hardaway, Ph.D.
“It adds support to the
idea that everything mammals eat is being dynamically categorized along a
spectrum of good/tasty to bad/disgusting, and this may be physically
represented in subsets of neurons in the amygdala.”
“The next major step and
challenge,” he adds, “is to tap into these subsets to derive new
therapeutics for obesity and binge eating.”
The complex story of
nociceptin


Since its discovery in 1995,
nociceptin has received a great deal of attention from researchers.
Aside from its potential to
moderate hedonic eating, scientists are investigating it for the treatment of
depression and alcohol abuse and testing its potential use as a pain reliever.
Although this molecule has
great potential to moderate a range of behaviors and states, this variety also
produces difficulties: Nociceptin is prevalent in the central nervous system,
so how does one make a treatment specific enough to alter only the behavior of
interest?
It is also worth mentioning the
drawbacks of using a mouse model to study binge eating. Although the rodent
model has provided a wealth of information relevant to the control of feeding
in humans, binge eating is a different case.
For instance, a review titled
‘Food addiction and binge eating: Lessons learned from animal models’ states
that “[rodent] models cannot reproduce all social context that influence
human eating behavior; neither some psychological aspects, such as sense of
lack of self-control, blame, or guilt.”
The author of another review
on the topic writes that “there is currently not a general consensus in
terms of which criteria a rodent model should fulfill to be considered accurate
for the study of neurobiological aspects of binge eating episodes.”
As ever, scientists will
continue delving into the world of brain circuit-induced overeating until they
reach an answer or the trail runs cold. For now, watch this space.







SOURCE: MEDICAL NEWS TODAY
Kayode Ojo
Kayode Emmanuel Ojo is the Co-Founder and Managing Director at SHEFFA Limited. He is presently studying Computer Science at the National Open University, Victoria Island, Lagos.