Unpredicted rewards trigger more vigorous phasic responses in midbrain dopamine (DA) neurons than predicted rewards. However, recent evidence suggests that reward predictability may fail to influence DA signaling over longer scales: In rats passively receiving rewarding electrical brain stimulation, the concentration of DA in dialysate obtained from nucleus accumbens probes was similar regardless of whether reward onset was predictable (G. Hernandez et al., 2006). The present experiment followed up on these findings by requiring the rats to work for the rewarding stimulation, thus confirming whether they indeed learned the timing and predictability of reward delivery. Performance under fixed-interval and variable-interval schedules was compared, and DA levels in the nucleus accumbens were measured by means of in vivo microdialysis. The observed patterns of operant responding indicate that the rats working under the fixed-interval schedule learned to predict the time of reward availability, whereas the rats working under the variable-interval schedule did not. Nonetheless, indistinguishable changes in DA concentration were observed in the 2 groups. Thus, reward predictability had no discernable effect on a measure believed to track the slower components of DA signaling.
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