Dopamine tone increases similarly during predictable and unpredictable administration of rewarding brain stimulation at short inter-train intervals.

Unpredicted rewards, but not predicted ones, trigger strong phasic changes in the firing rates of midbrain dopamine (DA). In contrast, neurochemical measurements of DA tone have failed to reveal an influence of reward predictability. However, the subjects of the neurochemical experiments were asked to predict reward onset over longer intervals (12s, on average) than the subjects of the electrophysiological studies (typically, 2s). Thus, the contrasting effects of reward predictability could reflect the difference in the duration of the interval separating the predictor from the reward rather than a difference in the influence of reward predictability on phasic and tonic DA signaling. This hypothesis was tested in rats receiving trains of rewarding electrical brain stimulation with either a predictable or unpredictable onset. The mean inter-train interval was 1.5s, a value close to the 2-s CS-US interval that has been used in electrophysiological studies demonstrating the dependence of phasic DA responses on reward predictability. Despite the shortened inter-train interval, the time courses of the observed stimulation-induced elevations in DA levels were very similar, regardless of whether train onset was predictable. This finding is consistent with the idea that tonic DA signaling is insensitive to the predictability of rewards.