However, a team of MIT neuroscientists has
now discovered a circuit in this structure that responds to rewarding events.
In a study of mice, activating this circuit with certain stimuli made the
animals seek those stimuli further. The researchers also found a circuit that
controls responses to fearful events, but most of the neurons in the central
amygdala are involved in the reward circuit, they report.
"It's surprising that
positive-behavior-promoting subsets are so abundant, which is contrary to what
many people in the field have been thinking," says Susumu Tonegawa, the
Picower Professor of Biology and Neuroscience and director of the RIKEN-MIT
Center for Neural Circuit Genetics at the Picower Institute for Learning and
Tonegawa is the senior author of the study,
which appears in the March 22 issue of the journal Neuron. The paper's lead
authors are graduate students Joshua Kim and Xiangyu Zhang.
The paper builds on a study published last
year in which Tonegawa's lab identified two distinct populations of neurons in
a different part of the amygdala, known as the basolateral amygdala (BLA).
These two populations are genetically programmed to encode either fearful or
In that study, the researchers found that
the neurons encoding positive and negative memories relay information to
different parts of the central amygdala. In their new work, they set out to
further clarify the connections from the two BLA populations to the central
amygdala, and to determine the functions of the central amygdala cells that receive
information from the BLA.
First, the researchers analyzed the genetic
profiles of the central amygdala neurons and divided them into seven groups
based on the genetic markers they express and their anatomical location. They
then used optogenetics, a technique that allowed them to control neuron
activity with light, to investigate the functions of each population.
The researchers found that five of these
populations stimulate reward-related behavior: When the mice were exposed to
light, the mice repeatedly sought more light exposure because these neurons
were driving a reward circuit. These same populations all receive input from
the positive emotion cells in the BLA.
Another population of neurons underlies
fear-related innate and memory behaviors, and the last population was not
required for either fear- or reward-related behavior.
This finding contradicts the consensus that
the central amygdala is involved primarily in fear-related behavior, the
"Classically people have generalized
the central amygdala as a fear-related structure. They think it's involved in
anxiety and fear-related responses," Kim says. "However, it looks
like the structure as a whole mainly seems to participate in appetitive
The researchers cannot rule out the
possibility that some yet-to-be-discovered cells in the central amygdala
control negative behavior, they say. "However, the cells that we have
identified so far represent more than 90 percent of the central amygdala,"
Tonegawa says. "If there are some other cells for negative behavior, it's
a small fraction."
In another surprising finding, the
researchers discovered that the fear-linked neurons they identified in the
central amygdala do not send messages directly to the part of the brain that is
believed to receive fear-related input from the central amygdala. This part of
the brain, the periaqueductal gray (PAG), is located in the brainstem and plays
a role in responding to pain, stress, and external threats.
Still unknown is where those central
amygdala cells send their output, and whether it eventually gets to the PAG
after stopping somewhere else. Tonegawa's lab is now trying to trace these
circuits further to find out where they go.
The researchers are also studying the role
of BLA neurons in fear extinction, which is the process of rewriting fearful
memories so that they are associated with more positive feelings. This approach
is often used to treat disorders such as depression and posttraumatic stress
原文連結： Massachusetts Institute of
Technology. " Scientists
identify brain circuit that drives pleasure-inducing behavior. "
ScienceDaily, 22 March, 2017.
參考文獻：Joshua Kim et al., Basolateral to Central Amygdala
Neural Circuits for Appetitive Behaviors. Neuron,
March 2017. DOI: 10.1016/j.neuron.2017.02.034