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Addiction and apathy share one circuit
The circuit runs from the basal ganglia to the frontal lobes. When it stops working, as it did in a man whose motivation was wiped out by two tiny strokes, the result is profound apathy. When drugs push it into overdrive, the result is states of hyper motivation. Dopamine levels aren't the only factor in motivation, yet they are a major one in setting how strongly someone pursues a particular outcome.
Researchers who study apathy rarely read the addiction literature, and finding the overlap there came as a surprise. In addiction research the question is what a person finds incentively salient enough to act on to get an outcome. The circuit underneath is the same one: underactive, it produces apathy, and overactive, it produces addictive states.
How do different drugs reach the same circuit?
Cocaine blocks the transporter that clears dopamine from the gap between neurons. Amphetamine makes that transporter run in reverse so that dopamine pours out. Nicotine excites dopamine neurons directly through nicotinic receptors, and heroin removes the brake that inhibitory neurons place on them. Four routes in, and all of them raise dopamine in the same pathway from the midbrain to the nucleus accumbens.
How was the brain's reward circuit discovered?
In 1954, James Olds and Peter Milner at McGill University implanted electrodes in rats' brains. They found that rats would press a lever again and again to stimulate certain regions, sometimes forgoing food. The sites were later traced to dopamine pathways running from the midbrain to the nucleus accumbens.
Can too much dopamine action cause compulsive behavior?
Parkinson's patients treated with dopamine-stimulating drugs sometimes develop impulse control disorders such as pathological gambling and compulsive shopping. A 2010 study by Daniel Weintraub and colleagues in Archives of Neurology found these problems in about 14 percent of patients on such drugs. Most of the patients lost the behavior when the dose was lowered or the drug was stopped.
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