The U-M study reports that the brain divides wanting and liking into separate circuits for the same sweet reward. Natural heroin-like chemicals (opioids) in a few brain "pleasure hotspots" make individuals want to eat more of a tasty sweet food, and make them like its sweet taste more when they eat it, the study says. The same thing happens with addictions to drugs, sex, gambling and other pursuits involving "brain reward" circuits. The research is featured in the Journal of Neuroscience.
U-M psychology researchers Kyle Smith and Kent Berridge show that two different brain circuits carry out the wanting and liking for the sweet reward, even when both are triggered in the same brain pleasure hotspots.
"We typically want what we like, and like what we want," Smith said. "But these results suggest that wanting and liking are processed by distinct brain circuits and may not always go hand-in-hand."
Experimenters put an opioid drug (Damgo) into a pleasure hotspot in the brains of rats?in the front base of the brain?using a painless microinjection technique to deliver tiny chemical droplets to the brain target without disturbing the rats.
The opioid made the rats want to eat three times more food than normal, and to show double the normal number of "liking" expressions when they tasted the sugar. "Liking" expressions are positive facial lip licking expressions that are similar in rats, monkeys, apes and even human infants.
"The brain seems to be more stingy with mechanisms for pleasure than for desire," Berridge said.
To turn off a particular brain circuit, the experimenters simultaneously made another microinjection of an opioid-suppressing chemical?in a different pleasure hotspot of the brain in some rats.
The opioid-suppressing chemical in that second hotspot completely prevented any increase in liking for the sugar taste from being caused by the first opioid-activating drug in the nucleus accumbens.
But the opioid-activation in nucleus accumbens still caused the rats to want to eat triple the normal amount of food, even though the extra "liking" for it was gone.
Finally, the experimenters examined the brain circuits involved using a technique called Fos mapping, based on changes in color of particular neural circuits that had been stimulated to manufacture proteins by the opioid drugs, which become visible if the brain tissue is chemically treated later.
A single looping circuit between hotspots was found to be always activated by microinjections that caused pleasure liking. On the other hand, a different outgoing circuit from nucleus accumbens appeared to cause the wanting by going to the hypothalamus instead.
The findings suggest that liking and wanting for tasty treats can either change together or change separately, depending on which brain circuits are involved. For example, various eating disorders might involve different activation patterns in the two brain circuits, possibly dissociating liking from wanting in some cases but not in others.
"It's relatively hard for a brain to generate pleasure, because it needs to activate different opioid sites together to make you like something more," Berridge said. "It's easier to activate desire, because a brain has several 'wanting' pathways available for the task. Sometimes a brain will like the rewards it wants. But other times it just wants them."
For more information on the psychology department, visit: http://www.lsa.umich.edu/psych/flash.asp.
Affective Neuroscience & Biopsychology Lab