Figure 3.

Recasting the direct (GO) and indirect (NOGO) corticostriatal pathways in terms of thrift and affordability. Cortical activities associated with different behavioral options send afferents to both the direct and indirect corticostriatal pathways. D1 and D2 expressed on direct and indirect pathway MSNs, respectively, both facilitate behavioral activation by facilitating and disinhibiting the associated cortical activity. Holding D1 constant for illustration, an increase in D2 expression confers greater disinhibition in response to DA, decreasing the threshold of facilitation necessary in the direct pathway to engage a behavior. Conversely, reduction in D2 confers a smaller disinhibition in response to DA, requiring greater facilitation from the direct pathway to overcome the inhibition. In the hypothesis proposed here, we suggest that the D1-expressing, striatonigral GO pathway effectively encodes utility/benefit while the D2-expressing, striatopallidal pathway implements cost constraints by up- and down-regulating D2 expression and function in response to general economic conditions, which effectively constrains facilitation by utility/benefit, implementing affordability.