Never Enough: The Neuroscience and Experience of Addiction

by Judith Grisel

If lesions of the mesolimbic circuit lead to anhedonia, what happens if the nigrostriatal pathway is eliminated? This produces a fairly common condition, especially in the aged. Deficits of dopamine in the nigrostriatal pathway are responsible for Parkinson’s disease. Parkinson’s sufferers have extreme difficulty enacting their intentions. For instance, people with Parkinson’s describe an incredible mental effort required to do a simple motor task such as buttoning a shirt. Parkinsonian deficits occur between the desire to move and the movement circuitry, which are both intact. How does the nigrostriatal lesion in Parkinson’s patients occur? Dopamine in both pathways naturally declines with age, partly accounting for general decreases in the eagerness to explore new things and the ability to quickly move toward them. But even before we get old, there are individual differences in dopamine activity, distributed normally, as shown in a bell curve, with those on the low end generally at greater risk for Parkinson’s. Besides being slower to enact intentions, low dopamine is also associated with higher-than-average orderliness, conscientiousness, and frugality. In other words, it confers a tendency toward rigidity in areas other than movement.