Reward expectation selectively boosts the firing of accumbens D1+ neurons during motivated approach.

The nucleus accumbens (NAc) helps govern motivation to pursue rewards. Two distinct sets of NAc projection neurons-expressing dopamine D1 versus D2 receptors-are thought to promote and suppress motivated behaviors respectively. However, support for this conceptual framework is limited: in particular the spiking patterns of these distinct cell types during motivated behavior have been largely unknown. We monitored identified D1+ and D2+ neurons in the NAc Core, as unrestrained rats performed an operant task in which motivation to initiate work tracks recent reward rate. D1+ neurons preferentially increased firing as rats initiated trials, and fired more when reward expectation was higher. By contrast, D2+ cells preferentially increased firing later in the trial especially in response to reward delivery - a finding not anticipated from current theoretical models. Our results provide new evidence for the specific contribution of NAc D1+ cells to self-initiated approach behavior, and will spur updated models of how we learn from rewards.