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Voltammetry was used to monitor dopamine (DA) transmission in nucleus accumbens (NAcc) of rats lever-pressing for food. Under standard conditions, animals responded on a fixed ratio 1 (FR1) schedule for 0.2 ml of milk delivered over 30 sec; milk delivery was paired with a 30 sec cue light. Consumption of the initial few milk rewards of the session caused DA signal increases. These initial signal increases were typical of the first and, at times, the second test days. On subsequent days, the most pronounced initial signal increases coincided with presentation of conditioned stimuli that marked the start of the session. Biphasic changes in DA signal that were time-locked to each reinforced lever-press were also observed; responses were preceded by increases and were followed, during milk consumption, by decreases in DA signal. At the end of milk delivery, the signal increased again in apparent anticipation of the next lever-press. Delaying milk delivery caused a corresponding delay in DA signal decreases, and the amount of time signals remained depressed was bound by the duration of milk consumption. Greater decreases in DA signal were observed when the rate of milk delivery was doubled or tripled, and such increases in reward value were associated with more pronounced signal increases during the period that preceded each lever-press. In contrast, DA signal increases were seen when milk was delivered at half the usual rate or was withheld altogether or when animals were denied access to the lever. Under partial reinforcement conditions, reinforced lever-presses were preceded by more pronounced signal increases and decreases of comparable magnitude accompanied milk consumption. These results suggest that meso-NAcc DA neurons are activated primarily in response to the incentive rather than to the reinforcing properties of rewards.
Richardson et al. (Sun,) studied this question.