The nucleus accumbens is key for encoding reward/aversion and associative learning, being the limbic-motor interface of the brain. This encoding occurs through activity of medium spiny neurons (MSNs) that express either dopamine receptor D1 or D2. Here, we discuss evidence supporting a complex and complementary role of D1- and D2-MSNs in encoding both appetitive and aversive cue–outcome associative learning.

The nucleus accumbens (NAc) has been considered a key brain region for encoding reward/aversion and cue–outcome associations. These processes are encoded by medium spiny neurons that express either dopamine receptor D1 (D1-MSNs) or D2 (D2-MSNs). Despite the well-established role of NAc neurons in encoding reward/aversion, the underlying processing by D1-/D2-MSNs remains largely unknown. Recent electrophysiological, optogenetic and calcium imaging studies provided insight on the complex role of D1- and D2-MSNs in these behaviours and helped to clarify their involvement in associative learning. Here, we critically discuss findings supporting an intricate and complementary role of NAc D1- and D2-MSNs in associative learning, emphasizing the need for additional studies in order to fully understand the role of these neurons in behaviour.