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

Forty percent of people have a fictional first memory, says study

July 17, 2018
Researchers have conducted one of the largest surveys of people's first memories, finding that nearly 40 per cent of people had a first memory which is fictional.

Protein found to be key component in irregularly excited brain cells

July 17, 2018
In a new study in mice, researchers have identified a key protein involved in the irregular brain cell activity seen in autism spectrum disorders and epilepsy. The protein, p53, is well-known in cancer biology as a tumor ...

Insight without incision: Advances in noninvasive brain imaging offers improvements to epilepsy surgery

July 17, 2018
About a third of epilepsy sufferers require treatment through surgery. To check for severe epilepsy, clinicians use a surgical procedure called electrocorticography (ECoG). An ECoG maps a section of brain tissue to help clinicians ...

New drug target for remyelination in MS is identified

July 17, 2018
Remyelination, the spontaneous regeneration of the fatty insulator in the brain that keeps neurons communicating, has long been seen as crucial to the next big advance in treating multiple sclerosis (MS). However, a lack ...

Artificial neural networks now able to help reveal a brain's structure

July 17, 2018
The function of the brain is based on the connections between nerve cells. In order to map these connections and to create the connectome, the "wiring diagram" of a brain, neurobiologists capture images of the brain with ...

Convergence of synaptic signals is mediated by a protein critical for learning and memory

July 16, 2018
Inside the brain, is a complex symphony of perfectly coordinated signaling. Hundreds of different molecules amplify, modify and carry information from tiny synaptic compartments all the way through the entire length of a ...

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.