The researchers suggest the findings could lead to ways that help people moderate addictive behavior like binge eating.
The researchers report their findings in the journal Neuron. Lead author Dr. Jocelyn M. Richard, who researches psychological and brain sciences at Johns Hopkins University in Baltimore, MD, says:
"External cues - anything from a glimpse of powder that looks like cocaine or the jingle of an ice cream truck - can trigger a relapse or binge eating. Our findings show where in the brain this connection between environmental stimuli and the seeking of food or drugs is occurring."
The area of the brain that the study focuses on is called the ventral pallidum (VP), a structure within the basal ganglia - a collection of brain cell clusters located deep beneath the cerebral cortex, the outer layer of the brain that is often referred to as gray matter.
While it is thought that the VP is involved in reward seeking, note the authors, very little is known about its role in triggering behavior in response to incentive cues.
To investigate this further, the team trained rats to learn that if they pushed a lever when they heard a particular sound - such as a siren or series of sharp beeps - they would be rewarded with a drink of sugar water.
The researchers then got the trained rats to respond to the triggers while they monitored neuron activity within their VP areas.
Switching off VP neurons slowed reward-seeking
The team saw that many more neurons than expected became active - and quite vigorously so - as the rats heard the cues they had been trained to respond to.
The researchers also noted that the stronger the neuron activity, the more rapidly the rats responded and sought their reward. The team could could even predict how fast the rats would move toward their reward just from observing the size of the neuron response to the cue, as Dr. Richard explains:
"We were surprised to see such a high number of neurons showing such a big increase in activity as soon as the sound played."
In another part of the study, Dr. Richards and colleagues used optogenetics to temporarily turn off the VP neurons when the rats were exposed to the sound cues.
Optogenetics is a method where animals like rats and mice are genetically engineered to have brain cells that can be selectively switched on or off using light pulses.
When they switched the trained rats' VP neurons off, the researchers found they were less likely to pull the levers to get the sugar water. Also, when the rats did pull the levers, they did so much more slowly.
The researchers suggest the findings could lead to treatments that help people moderate addictive behavior by toning down or calming brain reactions to the relevant triggers.
"We don't want to make it so that people don't want rewards. We want to tone down the exaggerated motivation for rewards."
Dr. Jocelyn M. Richard