Dopamine is implicated in risky behaviors such as drug addiction and gambling. First, dopamine manipulations can make animals and humans impulsive. Second, dopamine has been identified as a learning signal that encodes reward prediction errors. Here we try to reconcile these two findings by measuring dopamine receptor levels in healthy subjects using positron emission tomography, and by reducing dopamine levels using a dietary manipulation (tyrosine depletion), in combination with behavioral tasks.
We show that striatal dopamine D1 receptor levels predict reward learning while striatal D2 receptor levels predict punishment learning. This is consistent with the known roles of D1 and D2 receptors in the direct excitatory D1-bearing pathway and the indirect inhibitory D2-bearing pathway. Reducing dopamine levels leads to an improvement in punishment learning.
In sum, loss aversion may be under the control of dopamine tone acting via D2 receptors. This explains why tonic stimulation of D2 receptors with dopamine agonists, as in Parkinson's disease, may lead to addictive and impulsive disorders that rapidly resolve upon discontinuation of the medication. Thus, impulsivity can result from impaired inhibition secondary to tonic stimulation of the indirect basal ganglia pathway.
Finally, studies using functional MRI in Parkinson's patients suffering from gambling disorders will be shown. They suggest that an inherent reduction in loss aversion makes some patients vulnerable to impulse control disorders induced by dopamine agonist therapy.