There's a lot of conflicting information out there. But it's all good because "Forewarned is forearmed"!
http://www.dana.org/news/Brainwork/detail.aspx?id=23406
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Recent studies of people with Parkinson’s suggest that newer and more powerful dopamine “agonists,” which bind to dopamine receptors on brain cells, are more likely to produce this “ventral overdose” effect. A review in Movement Disorders in 2007 found that pathological gambling behavior has been diagnosed two to eight times more frequently among Parkinson’s patients who take these agonists than among the general population. Yet it is not easy to reconcile such an effect with current models of dopamine signaling.
According to these models, the perception of an unexpected reward stimulus leads to a short-term burst of excitement among dopamine-producing neurons in the midbrain, near the brainstem. The brief episode of increased firing results in a quick but sizeable surge of dopamine along these neurons’ output fibers and into target regions in the striatum. In the ventral striatum, this dopamine surge is widely thought to be a primary learning signal, effectively telling the brain how much to value the relevant stimulus.
However, this signal is known to be mediated mainly by dopamine “D1” receptors on striatal neurons. And as Dagher points out, “dopamine agonists that are used clinically don’t act on the D1 receptor. They act only on dopamine D2 and D3 receptors.”
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How would dopamine agonists block the influence of these prefrontal areas and thereby impair reversal learning? According to one prominent hypothesis, based on work in rodents and published in 2005 in Nature Neuroscience, the influence of prefrontal signals on striatal neurons is regulated by striatal dopamine levels, via D2 receptors. “Dopamine can act as a traffic cop to decide which input has the most influence” over striatal neurons, Dagher says.
In this view, too much of a D2-agonist Parkinson’s medication would dial down the prefrontal influence, allowing behavior to become less restrained and less thoughtful. Understanding how this prefrontal-limbic balancing act can be disrupted is an area of intense research, Clark says.
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