Duke University
By now, you might have discovered that taming your sweet tooth
as a New Year's resolution is harder than you think.
New research by Duke University scientists suggests that a habit
leaves a lasting mark on specific circuits in the brain, priming us to feed our
cravings.
Published online Jan. 21 in the journal Neuron, the
research deepens scientists' understanding of how habits like sugar and other
vices manifest in the brain and suggests new strategies for breaking them.
"One day, we may be able to target these circuits in people
to help promote habits that we want and kick out those that we don't want,"
said the study's senior investigator Nicole Calakos, M.D., Ph.D., an associate
professor of neurology and neurobiology at the Duke University Medical Center.
Their groups trained otherwise healthy mice to form sugar habits
of varying severity, a process that entailed pressing a lever to receive tiny
sweets. The animals that became hooked kept pressing the lever even after the
treats were removed.
The researchers then compared the brains of mice that had formed
a habit to the ones that didn't. In particular, the team studied electrical
activity in the basal ganglia, a complex network of brain areas that controls
motor actions and compulsive behaviors, including drug addiction.
In the basal ganglia, two main types of paths carry opposing
messages: One carries a 'go' signal which spurs an action, the other a 'stop'
signal.
Experiments by Duke neurobiology graduate student Justin O'Hare
found that the stop and go pathways were both more active in the sugar-habit
mice. O'Hare said he didn't expect to see the stop signal equally ramped up in
the habit brains, because it has been traditionally viewed as the factor that
helps prevent a behavior.
The team also discovered a change in the timing of activation in
the two pathways. In mice that had formed a habit, the go pathway turned on
before the stop pathway. In non-habit brains, the stop signal preceded the go.
These changes in the brain circuitry were so long-lasting and
obvious that it was possible for the group to predict which mice had formed a
habit just by looking at isolated pieces of their brains in a petri dish.
Scientists have previously noted that these opposing basal
ganglia pathways seem to be in a race, though no one has shown that a habit
gives the go pathway a head start.
O'Hare said that's because the go and stop
signals had not been studied in the same brain at the same time. But new
labeling strategies used by the Duke scientists allowed researchers to measure
activity across dozens of neurons in both pathways simultaneously, in the same
animal.
"The go pathway's head start makes sense," said
Calakos. "It could prime the animal to be more likely to engage in the
behavior." The researchers are testing this idea, as well as investigating
how the rearrangements in activity occur in the first place.
Interestingly, the group observed that changes in go and stop
activity occurred across the entire region of the basal ganglia they were
studying as opposed to specific subsets of brain cells.
O'Hare said this may
relate to the observation that an addiction to one thing can make a person more
likely to engage in other unhealthy habits or addictions as well.
To see if they could break a habit, the researchers encouraged
the mice to change their habit by rewarding them only if they stopped pressing
the lever. The mice that were the most successful at quitting had weaker go
cells.
But how this might translate into help for humans with bad habits is
still unclear. Because the basal ganglia is involved in a broad array of
functions, it may be tricky to target with medicines.
Calakos said some researchers are beginning to explore the
possibility of treating drug addiction using transcranial magnetic stimulation
or TMS, a noninvasive technique that uses magnetic pulses to stimulate the
brain.
"TMS is an inroad to access these circuits in more severe
diseases," she said, in particular targeting the cortex, a brain area that
serves as the main input to the basal ganglia.
For more ordinary bad habits "simpler, behavioral
strategies many of us try may also tap into similar mechanisms," Calakos
added. "It may be just a matter of figuring out which of them are the most
effective."
Meanwhile, Calakos and her team are studying what distinguishes
ordinary habits from the problematic ones that can be seen in conditions like
obsessive-compulsive disorder.
The study was supported by the National Institutes of Health
(R01 NS064577, T32 NS051156, GM008441-23, AA021074) McKnight Foundation, The
Brain and Behavior Foundation, and the Ruth K. Broad Foundation.
