The researchers found that not only does the NAc decide whether stimuli--in this case sweet sucrose or bitter quinine--are rewarding or aversive, but the center's neurons also encode learning associated with the stimuli.
The NAc is located in the brain's limbic system, which generates feelings and emotions. It is the key brain center involved in reinforcing the taking of drugs of abuse.
In their experiments, researchers led by Mitchell F. Roitman and Regina M. Carelli at Dr. Carelli's laboratory at University of North Carolina in Chapel Hill used recording microelectrodes to measure the electrophysiological response of neurons in the NAc of rats when they fed the rats small squirts of sucrose or quinine. The rats actively responded to the two tastes. For sucrose, they immediately licked and moved their mouths to ingest the sugar. In response to quinine, they gaped their mouths and rubbed their chins--the rat equivalent of "ptui."
The researchers found that neurons in the NAc fired in response to both the sucrose and the quinine, showing that the brain center played a role in judging both rewarding and aversive stimuli. Importantly, they found that different sets of NAc neurons responded to the sucrose than to the quinine, revealing different circuitry for processing rewarding and aversive stimuli. They also found that activity in the NAc neurons was associated with the animals' behavioral response to the tastes.
The researchers also tested whether the NAc was involved in learning to discriminate such stimuli. They gave the rats specific cues--changes in lighting and tones or clicks--associated with either the sucrose or quinine squirts. The researchers found that NAc neurons did respond to the cues and that the neurons did not respond in the same way for the two substances. The researchers found that many of the NAc neurons that responded to the cues predicting the taste stimuli also responded to the taste stimuli themselves.
"The findings presented here continue to support the idea that the NAc comprises part of the brain's reward circuit," wrote the researchers. "However, the data clearly indicate that individual NAc neurons play a role in aversion. Neural responses are organized on a microcircuit level--that is, rewarding and aversive responses are segregated." The researchers also concluded that their findings show that individual NAc neurons play a role in Pavlovian learning.
The researchers concluded that, "The results presented here provide valuable insight into how the NAc is organized for 'natural' hedonic stimuli and their predictors."
The researchers include Mitchell F. Roitman, Robert A. Wheeler, and Regina M. Carelli of University of North Carolina at Chapel Hill. This work was supported by NS11152 (M.F.R.) and DA14339 (R.M.C.).
Mitchell F. Roitman, Robert A. Wheeler, and Regina M. Carelli: "Nucleus Accumbens Neurons Are Innately Tuned for Rewarding and Aversive Taste Stimuli, Encode Their Predictors, and Are Linked to Motor Output"
Publishing in Neuron, Volume 45, Number 4, February 17, 2005, pages 587-597.
Journal
Neuron