Researchers at the University of Maryland School of Medicine (UMSOM) have discovered a circuit between the hippocampus and nucleus accumbens in the brains of mice that has a crucial role in regulating goal oriented behaviors and rewarding stimuli. It was discovered that the strength of the signals travelling between these two regions can change. This is also referred to as plasticity. It was found that over strengthening of the signals can lead to addiction, while weakening could lead to depression.
“These two parts of the brain are known to be important in processing rewarding experiences,” said lead researcher Dr. Scott Thompson. “The communication between these regions is stronger in addiction, although the mechanisms underlying this were unknown. We also suspected that opposite changes in the strength of this communication would occur in depression. A weakening of their connections could explain the defect in reward processing that causes the symptom of anhedonia in depressed patients.”
The UMSOM researchers introduced special light sensitive proteins to specific neurons in the brains of mice to activate or inhibit the hippocampus-nucleus accumbens connection. Four seconds of light exposure activated the pathway, persistently reinforcing the strength of the signals from one brain region to the other. This created an artificial reward memory.
The researchers observed that the next day the mice returned to the same place where the artificial memory was created. Researchers then used light to silence the same pathway in mice with the light-sensitive protein that inhibited the neurons and found that this pathway is required for associating a reward with its location. This led to the mice losing their attachment to this location.
This circuit was also examined in depressed mice, however, the hippocampus-nucleus accumbens pathway could only be activated with light sensitive proteins after they had been administered anti-depressants.