Scientists identify brain circuit that drives pleasure-inducing behavior

March 22, 2017, Massachusetts Institute of Technology
Credit: Wikimedia Commons

Scientists have long believed that the central amygdala, a structure located deep within the brain, is linked with fear and responses to unpleasant events.

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 Memory.

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.

Driving behavior

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 (BLA). These two populations are genetically programmed to encode either fearful or happy memories.

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 researchers say.

"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 behaviors."

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 , it's a small fraction."

Surprising circuits

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 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 further to find out where they go.

The researchers are also studying the role of BLA 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 disorder.

Explore further: Neuroscientists identify two neuron populations that encode happy or fearful memories

Related Stories

Neuroscientists identify two neuron populations that encode happy or fearful memories

October 18, 2016
Our emotional state is governed partly by a tiny brain structure known as the amygdala, which is responsible for processing positive emotions such as happiness, and negative ones such as fear and anxiety.

Scientists discover a new pathway for fear deep within the brain

February 12, 2014
Fear is primal. In the wild, it serves as a protective mechanism, allowing animals to avoid predators or other perceived threats. For humans, fear is much more complex. A normal amount keeps us safe from danger. But in extreme ...

A new neural circuit controls fear in the brain

January 19, 2015
Some people have no fear, like that 17-year-old kid who drives like a maniac. But for the nearly 40 million adults who suffer from anxiety disorders, an overabundance of fear rules their lives. Debilitating anxiety prevents ...

Horror movie scenes help team identify key brain circuits for processing fear

February 8, 2017
Researchers at the University of California, Irvine have identified a key neural pathway in humans that explains how the brain processes feelings of fear and anxiety, a finding that could help scientists unlock new ways to ...

Positive and negative memories and behaviors are split up in the brains of mice

October 17, 2016
Like broccoli and ice cream on a toddler's plate, the brain also keeps nice and nasty information in separate places. Within the amygdala, an important memory center in the brain, pleasant experiences, tastes, and smells ...

Neuroscientists pinpoint location of fear memory in amygdala

January 28, 2013
A rustle of undergrowth in the outback: it's a sound that might make an animal or person stop sharply and be still, in the anticipation of a predator. That "freezing" is part of the fear response, a reaction to a stimulus ...

Recommended for you

New neurons in the adult brain are involved in sensory learning

February 23, 2018
Although we have known for several years that the adult brain can produce new neurons, many questions about the properties conferred by these adult-born neurons were left unanswered. What advantages could they offer that ...

Study in mice suggests personalized stem cell treatment may offer relief for multiple sclerosis

February 22, 2018
Scientists have shown in mice that skin cells re-programmed into brain stem cells, transplanted into the central nervous system, help reduce inflammation and may be able to help repair damage caused by multiple sclerosis ...

Nolan film 'Memento' reveals how the brain remembers and interprets events from clues

February 22, 2018
Key repeating moments in the film give viewers the information they need to understand the storyline. The scenes cause identical reactions in the viewer's brain. The results deepen our understanding of how the brain functions, ...

Biomarker, clues to possible therapy found in novel childhood neurogenetic disease

February 22, 2018
Researchers studying a rare genetic disorder that causes severe, progressive neurological problems in childhood have discovered insights into biological mechanisms that drive the disease, along with early clues that an amino ...

A look at the space between mouse brain cells

February 22, 2018
Between the brain's neurons and glial cells is a critical but understudied structure that's been called neuroscience's final frontier: the extracellular space. With a new imaging paradigm, scientists can now see into and ...

Schizophrenia a side effect of human development

February 21, 2018
Schizophrenia may have evolved as an "unwanted side effect" of the development of the complex human brain, a new study has found.

0 comments

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.