A rat is trained to press a lever for a food reward. Initially, dopamine neurons fire strongly when the food is delivered. After weeks of training with a reliable tone that precedes food delivery, researchers record dopamine activity. Which pattern best describes the learned response?
ADopamine neurons fire strongly to food delivery and weakly to the tone, as dopamine encodes pleasure
BDopamine neurons fire strongly to the tone and show little or no response to food delivery itself
CDopamine neurons fire equally to both tone and food delivery, since both are rewarding
DDopamine neurons are suppressed by the tone because it creates anticipatory anxiety
Phasic dopamine release encodes reward prediction error, not pleasure. Early in training, the unpredicted food triggers a dopamine burst. As the tone becomes a reliable predictor, the dopamine response shifts to the tone (the earliest predictor of reward). Food delivery, now fully predicted, produces no additional dopamine signal — it is 'accounted for.' If the expected reward is then omitted, dopamine activity is suppressed below baseline, encoding a negative prediction error.
Question 2 True / False
The motor deficits characteristic of Parkinson's disease (tremor, rigidity, bradykinesia) are primarily caused by dopamine loss in the mesolimbic pathway projecting from the VTA to the nucleus accumbens.
TTrue
FFalse
Answer: False
Parkinson's disease motor symptoms result from the loss of dopamine neurons in the substantia nigra pars compacta (SNc) of the nigrostriatal pathway, which projects to the dorsal striatum (caudate and putamen). The mesolimbic pathway (VTA → nucleus accumbens) is more involved in reward, motivation, and addiction. The mesolimbic pathway is relatively spared in early Parkinson's, which is why motivational deficits emerge later in the disease than motor symptoms.
Question 3 Short Answer
Distinguish between phasic and tonic dopamine release: how do they differ in their temporal dynamics and what distinct functional roles does each serve?
Think about your answer, then reveal below.
Model answer: Phasic dopamine refers to brief, high-amplitude bursts of dopamine release (milliseconds to seconds) triggered by unexpected rewards or reward-predicting cues; it encodes reward prediction errors and drives learning about which actions and stimuli predict reward. Tonic dopamine refers to the low-level, sustained baseline dopamine concentration in the synapse maintained by ongoing spontaneous firing; it sets the general motivational state, modulates working memory in prefrontal cortex, and regulates the responsiveness of striatal circuits to phasic signals.
The distinction matters clinically and conceptually. Drugs of abuse (cocaine, amphetamine) primarily amplify phasic signals by blocking reuptake or forcing vesicular release, hijacking the prediction error system. Antipsychotics primarily reduce tonic D2 receptor signaling. Parkinson's treatment (L-DOPA) partly restores tonic dopamine, though it imperfectly mimics the phasic signals lost with SNc neuron death. The two modes of release are regulated differently and have distinct effects on learning versus motivational state.