Researchers at Johns Hopkins University discovered that rats responding to cues for sugar with the speed and excitement of binge-eaters were less motivated for the treat when certain neurons were suppressed.
The findings suggest these neurons, in a largely unstudied region of the brain called the ventral pallidum, are deeply connected to the tendency to overindulge in response to external triggers, a problem faced by people addicted to food, alcohol and drugs.
Lead author Jocelyn M. Richard, a Johns Hopkins post-doctoral fellow in psychological and brain sciences, said:
“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."
First, researchers trained rats to realize that if they heard a certain sound, either a siren or staccato beeps, and a pushed a lever, they would get a drink of sugar water. Then, as the rats performed the task, researchers monitored neurons within the ventral pallidum area of the rats' brains, a subcortical structure near the base of the brain.
Surprisingly High Number of Neurons
When the rats heard the cue linked to their treat, a much larger-than-expected number of neurons reacted, and vigorously, researchers found. They also found that when the neuron response was particularly robust, the rats were extra quick to go for the sugar.
[caption id=“attachment_79801” align=“aligncenter” width=“680”] A view of a rat’s brain; the ventral pallidum, where researchers showed that binge behavior can be suppressed, is in the red-stained roughly triangular areas toward the bottom. Credit: Jocelyn Richard/JHU[/caption]
The researchers were able to predict how fast the rats would move for the sugar just by observing how excited the neurons became at the sound of the cue.
“We were surprised to see such a high number of neurons showing such a big increase in activity as soon as the sound played,” Richard said.
Next, the researchers used optogenetics, a technique that allows the manipulation of cells through targeted beams of light, to temporarily suppress the activity of ventral pallidum neurons while the rats heard the sugar cues. With those neurons inactive, the rats were less likely to pull the sugar lever; when they did pull it, they were much slower to do so.
That ability to slow and calm the reaction to cues or triggers for binges could be key for people trying to moderate addictive behaviors, Richard said.
“We don’t want to make it so that people don’t want rewards,” Richard said. “We want to tone down the exaggerated motivation for rewards."
The ventral pallidum is a structure within the basal ganglia of the brain. It is an output nucleus whose fibres project to thalamic nuclei, such as the ventral anterior nucleus, the ventral lateral nucleus, and the medial dorsal nucleus. The ventral pallidum is a component of the limbic loop of the basal ganglia, a pathway involved in the regulation of motivation, behaviour, and emotions.
Jocelyn M. Richard et al Ventral Pallidum Neurons Encode Incentive Value and Promote Cue-Elicited Instrumental Actions Neuron (2016). DOI: 10.1016/j.neuron.2016.04.037
Top Image: Neurons react to the cue for sugar during the experiment. Credit: Jocelyn Richard/JHU