Which parts of the brain are affected by dopamine? Does dopamine spread and interact across the whole brain? If not, which areas are affected most and which least?
Quoting my thesis:

CNS Dopamine/DA projections primarily emerge from two subcortical basal ganglia nuclei in the brain stem, travelling along three major pathways (Purves et al., 2004).
The nigrostriatal pathway, connecting the substantia nigra and the striatum, is mainly implicated in higher motor control. The mesolimbic pathway contains DA projections from the Ventral Tegmental Area/VTA and the pars compacta of the Substantia Nigra (SNc) to limbic systems including hip- pocampus, amgydala and thalamus, and is critical in learning and memory in a mechanism that is well-investigated (Lisman &amp; Grace, 2005). The mesocortical pathway connects the VTA and SNc to the cortex. In contrast to the extensive noradrenergic innervation of the cortex, DA therefore reaches only selected areas (Foote &amp; Morrison, 1987). DA density is extensive in prefrontal and anterior cingulate areas, and falls off rapidly across an rostro-caudal gradient. The temporal lobe is only weakly innvervated (including the enthorinal cortex), and little to no innervation reaches parietal and especially occipital lobes, though DA fibers in monkey area 7 have been reported (Foote &amp; Morrison, 1987).

So basically, subcortical innervation is rich, but cortical innervation is focused on frontal areas. DA is often depicted as having zero cortical presence outside of frontal areas, but this is false; while DA innervation density falls off sharply outside of the frontal lobes, at least in humans and other apes, there is some DA reaching at least as far back as the temporal lobes.
Of course, this is only the direct effect of DA. In principle, all of the brain is strongly affected by the effects of DA, because frontal cortices, who are strongly influenced by DA, in turn strongly influence the rest of the bain.
Sources (obviously): Foote, S. L., &amp; Morrison, J. H. (1987). Extrathalamic modulation of cortical function. Annual review of neuroscience, 10, 67–95.