It’s a recognizable experience – you’re on your way to grab a healthy salad
when you pass that familiar burger joint sign that elicits memories of greasy
cheeseburgers, and suddenly you’re in line at the counter, adding fries to that
order.
How does that happen? How does the sight of a sign drive a behavior? That’s
the question behind recent research published in Nature Neuroscience by University of
Minnesota Medical School neuroscientist Benjamin Saunders, PhD.
“I’m interested in how cues in the environment – sights, sounds, and places
– can guide behavior,” said Saunders. “If you think about something like a fast
food sign, by itself, it’s just some colors and symbols and letters with no
innate value or meaning. We want to know how an otherwise neutral signal can
acquire complex motivational value to promote complex behaviors.”
Saunders and his team focused their research on dopamine neurons, cells in
the brain that turn on when experiencing a reward, like a special food. They
are also the neurons that degenerate in Parkinson’s disease.
“Dopamine neurons signal reward – they turn on when you eat food, for
example – so wanted to know if dopamine neurons can actually assign “reward”
value to these transient environmental cues, like signs,” said Saunders, said
Saunders, who conducted some of his research as a postdoctoral fellow in the
laboratory of Patricia Janak, PhD, at Johns Hopkins University.
Their research studies asked two core questions: Do dopamine neurons have a
role in causing cues in our environment to acquire value? And, if so, do
different groups of dopamine neurons serve different functions within this
process?
The study used a Pavlovian model of conditioning to see if turning on a
light – a simple cue – just before dopamine neurons were activated could
motivate action. The classic Pavlov model combined the ringing of a bell with
providing a tasty steak to a dog that, over time, conditioned a dog to drool
when the bell rang with or without a steak. In this research, however, there
was no “real” reward like food or water, in order to allow researchers to
isolate the function of dopamine neuron activity.
“We learned that dopamine neurons are one way our brains give the cues
around us meaning,” said Saunders. “The activity of dopamine neurons alone –
even in the absence of food, drugs, or other innately rewarding substances –
can imbue cues with value, giving them the ability to motivate actions.”
To answer the second core question, the researchers targeted specific
segments of dopamine neurons – those located in the substantial nigra (SNc) and
those located in the ventral tegmental area (VTA). These two types of neurons
have historically been studied in different disease research fields – SNc
neurons in Parkinson’s disease, and VTA neurons in addiction studies.
Scientists learned that cues predicting activation of the two types of
neurons drove very different responses – those predicting the SNc neurons led
to a sort of “get up and go” response of invigorated rapid movement. The cue
predicting VTA neuron activation, however, became enticing on it own, driving
approach to the cue’s location, a sort of “where do I go?” response.
“Our results reveal parallel motivational roles for dopamine neurons in
response to cues. In a real world situation, both forms of motivation are
critical,” said Saunders. “You have to be motivated to move around and behave,
and you have to be motivated to go to the specific location of things you want
and need.”
These results provide important understanding of the function of dopamine
neurons related to motivations triggered by environmental cues. And this work
contributes to the understanding of relapse for those struggling with
addictions.
“If a cue – a sign, an alley, a favorite bar – takes on this powerful
motivational value, they will be difficult to resist triggers for relapse,”
said Saunders. “We know dopamine is involved, but an essential goal for future
studies is to understand how normal, healthy cue-triggered motivation differs
from dysfunctional motivation that occurs in humans with addiction and related
diseases.”
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