Here, 39% of NAc neurons displayed phasic activity; all but four cells also had simultaneous dopamine fluctuations around the lever press for cocaine. Of the cells that did not show phasic activity (61%), dopamine fluctuations were observed at only two locations. These sites supported stimulated dopamine release although it was smaller than found at phasic locations. Stimulated release measured with carbon-fiber electrodes has been shown to depend on the specific terminal region in the NAc (Wightman et al., 2007), with release amounts differing by ~ 2-fold in most locations. Thus, even at the length scale of our electrode (~ 100 μm) the NAc appears heterogeneous within each sub-region. This is consistent with anatomical studies showing neurochemical heterogeneity in the NAc, perhaps corresponding to a ‘patch-matrix’ organization (Gerfen et al., 1987; Voorn et al., 2004). Alternatively, it may be that the dopaminergic nerve terminals at nonphasic cells simply did not receive signals to cause release during lever-pressing perhaps due to presynaptic modulation of terminals (Howland et al., 2002; Britt & McGehee, 2008). Although this hetereogeneity in dopamine release is evident in
