TOYING WITH EMOTIONSResearchers made lab rats experience desire or dread by tampering with the same brain chemicals, located in slightly different regions of the brain. A video camera recorded it all.
The Berridge Lab
Dopamine conducts a frenzied song of craving at one end of a
tiny brain region and a panic-stricken hymn at the other. Depending on where
along the length of the region the neurotransmitter is triggered, it elicits
emotions ranging from desire to disgust, a new study shows.
“The roles [of dopamine] may be partitioned, and perhaps
defined, by anatomy,” comments Emily Hueske, a neuroscientist at the
Massachusetts Institute of Technology.
With the recent study, researchers have come one step closer
to explaining how dopamine performs a spectrum of functions. Dopamine interacts
with spatially coded signals so that its output varies from one end of a brain
region to the other, the team reports in the July 9 Journal of Neuroscience.
In the long-term, drugs might be developed to locally treat
various dopamine-mediated disorders such as drug addiction, obsession, obesity
and anxiety.
Kent Berridge, a neuroscientist and psychologist at the University of Michigan
in Ann Arbor,
and his colleagues set out to understand how dopamine could lead to desire for
a reward, and then turn around and cause fear, pain and stress.
Berridge’s team focused on the area of the nucleus accumbens
known as the pleasure center in all mammals. The researchers report the effects
of tampering with dopamine and another chemical messenger, the glutamate
neurotransmitter, along the length of the nucleus accumbens of rats.
A tiny, localized injection at the front end disrupted
glutamate and turned normal rats into binge-eaters. But when researchers
injected the same glutamate blocker at the back end of the nucleus accumbens,
the rats stopped eating and became fearful — kicking up sand at the bottom of
their cages, as wild rodents are wont to do when a snake or a scorpion is in
their midst, Berridge says.
When both dopamine and glutamate were blocked, the rats did
not display the extreme behaviors. In nature, the interaction between the two
may guide how a rat responds to signals from the environment. Glutamate may
bring in information from the outside world, and dopamine may act on that
information, Berridge suggests.
Because the injections only blocked glutamate or dopamine in
tiny bits of the nucleus accumbens, the researchers were able to map out a
millimeter-by-millimeter gradient of reactions over the region. “The brain
cares where you are exactly,” Berridge says.
“This is perhaps a
surprise,” says behavioral neuroscientist Richard Palmiter of the University of Washington
in Seattle.
He’s not shocked about the gradient of dopamine-mediated reactions because
desire and dread aren’t completely unrelated. Regardless of how the rat
responds to a stimulus, “dopamine is basically saying: ‘Hey, pay attention to
your environment’,” he says.
Still, this study shows how motivation for a reward can turn
to fear within a single structure, Berridge says.
The researchers describe the gradient as a keyboard, with
keys going from desire to fear. The minute keyboard gradient found in the rats
may translate into a slightly larger, centimeter-by-centimeter keyboard in
humans’ nucleus accumbens. Berridge speculates that the boundaries of “keys”
are skewed in people with certain disorders, such that a sensation produces
more pleasure than it should in an addict or too much fear in schizophrenic patients.
Once scientists know what underlies the front-to-back
gradient, drugs could be refined to more accurately treat separate disorders,
says Charlotte Boettiger, a behavioral neuroscientist at the University of North Carolina
at Chapel Hill. It may be years before those
treatments are developed, however. “We don’t presently have a way to target
drugs to one part or the other.”
In this video, a squirrel defensively kicks up sand when it spots a
rattlesnake. Other rodents instinctively kick in the face of fear. In
the lab, rats kicked up their cage bedding when researchers tampered
with certain brain chemicals.
Video courtesy of BBC & National Geographic, provided to Kent Berridge by Profs. D.H. Owings & R.G. Coss
This explains a lot of experience with what till now seemed strange, odd and unwarranted responses to normal life activity. In past relationships I saw little blips of such responses that in usually a period of two years became full blown. This behavioral sequence was observed repeatedly, and became predictable and recognizable from changes in voice, speed of speech and attitudes which best can be described as unprovoked.
Faure, A. . . . and K.C. Berridge. 2008. Mesolimbic dopamine in desire and dread: Enabling motivation to be generated by localized glutamate disruptions in nucleus accumbens. Journal of Neuroscience 28(July 9):7184-7192. doi:10.1523/JNEUROSCI.4961-07.2008
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