ack of dopamine, a neurotransmitter long thought to play a central role in pleasure and motivation, doesn't interfere with the appetite for food but does affect how hard creatures will work for it.

In a study published in the May 28 issue of Neuroscience, John Salamone, a professor of psychology, reports that when rats were required to press a lever multiple times to receive food, those without dopamine pressed the lever fewer times than those with the chemical. When the rats only had to press the lever either once or four times for each food pellet, rats in both groups pressed the lever with about the same frequency.

Salamone's results indicate that rats without the neurotransmitter will work as hard as those with it to receive food when the required task is minimal. When the task requires greater effort, however, rats without dopamine are less willing to exert themselves.

According to a prevalent scientific theory, dopamine is the neurotransmitter that controls pleasure and motivation for everything from food to illicit drugs. That theory, which Salamone rejects, has been used to explain the basis for drug addiction.

To conduct the study, Salamone depleted dopamine in the nucleus accumbens, an area of the forebrain, from 29 rats by injecting them with a chemical. Twenty-nine other rats did not receive the chemical. Rats in each group were trained to press a lever to receive food and then randomly assigned to one of four schedules that required them to press the lever one, four, 16 or 64 times in order to receive food pellets.

After testing the rats for four weeks, Salamone found that the depletion of accumbens dopamine altered the relation between the number of lever pushes required to receive food and the number of times the rats pressed the lever. It also suppressed responding when the rats were required to press the lever 16 or 64 times to receive pellets.

As part of the same study, Salamone gave another group of rats food before testing them to compare their responses to the group without dopamine. Pre-feeding rats suppressed responding regardless of the number of lever pushes required while effects of accumbens dopamine depletion were determined by the number of times the lever had to be pushed. Thus, interfering with dopamine does not produce the same effects as pre-feeding to reduce food motivation.

The findings of Salamone's recent study fit into a growing body of research that is not consistent with popular theory and could lead scientists to rethink the link between dopamine and pleasure.

"Although dopamine is involved in aspects of substance abuse," Salamone says, "it does not appear as though drugs of abuse simply turn on the 'reward' system for natural stimuli such as food."

Allison Thompson