al., 2007; Gerfen and Surmeier, 2011). In contrast, D2 signaling inhibits the up-state transition, thereby preventing increased spiking, via reduction of Cav1 L-type Ca2+ channel activity and Nav1 Na+ channel activity while increasing K+ channel currents (Surmeier et al., 2007; Gerfen and Surmeier, 2011; Figure 1). Such opposite alterations in the two MSNs suggest that increased dopamine signaling elicited by drugs of abuse should enhance glutamatergic activation of D1+ MSNs and reduce glutamatergic activation of D2+ MSNs. In reality, such responses are far more varied and complex for reasons that remain poorly understood. This topic will be addressed further below.
