Scientists say they have identified a brain switch that gets people hooked on alcohol, drugs and cigarettes.
The finding opens the door to developing treatments to fight addictions — like the opioid epidemic gripping the US.
Researchers in New York exposed mice to a particular sound before giving them either a reward — sugar water — or ‘punishment’ — water.
Scans showed that key neurons in the amygdala started to fire in response to the noises, teaching the animals to associate them with a specific reward. But when these neurons were inhibited, the mice were unable to learn what the sounds meant.
The above scan shows an amygdala inside the brain of a mouse (red dot on the left). Humans and mice carry this brain structure
The amygdala are parts of the brain responsible for processing emotional stimuli.
Each person has two of the parts in their brain matter, located in the lower portion of the brain below the ears.
It is responsible for how the brain reacts to certain stimuli. For example, it will release dopamine, a hormone that triggers a sense of euphoria, as a reward for what it determines to be a positive experience.
The brain region has long been speculated to play a key role in addiction, as it will eventually become trained to release rewards when a person uses their vice.
This makes a person seek out the poor behavior or substance even more. It also leads to cravings for drugs, alcohol and other goods to trigger the dopamine release.
The research team from Cold Spring Harbor Laboratory, on Long Island, New York, looked into the role this region played in addiction by testing the reward system of mice.
Published Wednesday in the journal Nature, mice in the study were exposed to specific sounds before being administered a reward or punishment.
The rewards would be a light electric shock while the rewards were being allowed to drink water after long restrictions or receiving sugar or food
Using brain scans, researchers found that the amygdala was being stimulated in response to the sounds after the mice had gotten used to the reward and punishment cycle.
Responses changed depending on the sound, though. This signaled to researchers that the mice were learning what each byte of audio meant.
In the next stage of the experiment, the scientists inhibited the neurons in the amygdala.
They then repeated the tests using the same sounds and rewards or punishments.
But they found that the mice could not be trained to associate a sound with a particular reward or punishment without the neurons.
They also found that the brain’s normal dopamine response was suppressed.
Lead author Dr Bo Li, a neurologist who led the research, said: ‘This is entirely new to us.
‘These neurons really care about the nature of each individual stimulus. It’s almost like a sensory area.’
He added: ‘While previous research had linked the central amygdala to dopamine neurons, it was unclear exactly how they were connected.
‘We found those neurons are required for normal function for dopamine neurons, and therefore are important for reward learning.
‘That is direct evidence of how they regulate the function of dopamine neurons.’
Dr Li and colleagues now plan to examine the relationship between the amygdala and addiction.
Their trials are currently in mice but could eventually advance to other animals and humans.
The research could one day lead to better treatments for opioid and methamphetamine addiction.
For the experiment, mice heard a signal before either being offered water or sucrose. Scans showed they had learned to associate specific sounds with specific rewards or punishments
Dr Li said: ‘Our study provides a basis for developing more specific ways to regulate these neurons in different disease conditions.’
The US is currently in the grips of an opioid epidemic, with a record 108,000 Americans having died from overdoses.
The rising deaths are being attributed to fentanyl, which is up to 100 times more potent than morphine, being mixed with other drugs with people being unaware that the drug they are using contains heroin.
There are also mounting concerns over drugs being mixed with xylazine, an animal tranquilizer, which overdose medication naloxone does not work against.