Striatal dopamine release in the rat during a cued lever-press task for food reward and the development of changes over time measured using high-speed voltammetry

Abstract

Substantia nigra dopamine neuronal activity in the primate is thought to be related to the error in predicting reward delivery. Dopamine release in rat nucleus accumbens has been shown to increase in relation to drug/food-seeking behaviour. It is not known how the release of dopamine in the striatum corresponds to the many distinct steps of a rewarded, cued task (e.g. recognizing the cue, executing the behaviour, anticipating the reward, receiving the reward) and how dopamine release then changes over time as task performance improves. To investigate dopamine release during a rewarded, cued task and the development of changes in dopamine release over time, changes in extracellular striatal dopamine concentration during a rewarded, cued lever-press task were measured a few days every week for 5 months using high-speed in vivo voltammetry. Rats were trained to press a lever after a tone to obtain a food reward. The reaction time for the lever press decreased gradually as training continued. Changes in dopamine concentration were measured in the anterior striatum (ventral portion) during the task performance after an initial 6-day familiarization period, in which the animals learned that a lever press yielded food, and a 5-week period for surgery, recovery, and electrode preparation. During the task performance, dopamine concentration started to increase just after the cue, peaked near the time of the lever press, and returned to basal levels 1-2 s after the lever press. This pattern of changes in dopamine concentration was observed over the 5 months of testing, the peak dopamine concentration increasing steadily until peaking at week 7, at which time the task performance had not yet improved significantly from week 2. By week 13, task performance had significantly improved and peak dopamine concentration had begun to subside. Thus, the increase in dopamine concentration after the cue was highest while the task was not yet perfected and subsided toward the end of the learning process. It was concluded that striatal dopamine release during a cued lever-press task is relevant to the novelty of the conditions.