Reinforcement learning models postulate that neurons that release dopamine encode information about action and action outcome, and provide a teaching signal to striatal spiny projection neurons in the form of dopamine release(1). Dopamine is thought to guide learning via dynamic and differential modulation of protein kinase A (PKA) in each class of spiny projection neuron(2). However, the real-time relationship between dopamine and PKA in spiny projection neurons remains untested in behaving animals. Here we monitor the activity of dopamine-releasing neurons, extracellular levels of dopamine and net PKA activity in spiny projection neurons in the nucleus accumbens of mice during learning. We find positive and negative modulation of dopamine that evolves across training and is both necessary and sufficient to explain concurrent fluctuations in the PKA activity of spiny projection neurons. Modulations of PKA in spiny projection neurons that express type-1 and type-2 dopamine receptors are dichotomous, such that these neurons are selectively sensitive to increases and decreases, respectively, in dopamine that occur at different phases of learning. Thus, PKA-dependent pathways in each class of spiny projection neuron are asynchronously engaged by positive or negative dopamine signals during learning. The net PKA activities in each class of spiny projection neuron in the nucleus accumbens of the mouse are dichotomously modulated by asynchronous positive and negative dopamine signals during different phases of learning.