Drs. Susana Peciña and Kent Berridge of the University of Michigan have traced rats' liking for sweets to a 1-cubic millimeter site in the medial shell of the nucleus accumbens. Using fine-grained brain mapping, the researchers correlated mu-opioid activation of this area [by D-Ala2-N-Me-Phe4-Glycol5-enkephalin (DAMGO)] with the facial reactions rats exhibit upon receiving infusions of sweet tastes into the mouth. Enhancing mu-opioid activity in this hedonic "hot spot" produced two to four times the number of positive reactions (e.g., licking) to sucrose relative to other regions of the medial shell. Stimulating the hot spot with DAMGO also reduced the rats' negative reactions to a bitter taste by 25 percent. The findings suggest that opioid circuits in the medial shell involved in liking (e.g., positive facial expressions in reaction to a taste) and wanting (e.g., pressing a lever for a substance) are related but not identical, as activating mu-opioid circuits in widely distributed areas of the medial shell increased food intake.
The Journal of Neuroscience 25(50):11777-11786, 2005. [Full Text, PDF, 643KB]