Changes in nucleus accumbens dopamine transmission associated with fixed- and variable-time schedule-induced feeding.

We examined the changes in nucleus accumbens (NAcc) dopamine (DA) transmission associated with non-contingent meal presentations under conditions of high (fixed time-, FT-schedule) and low (variable time-, VT-schedule) predictability. Of interest were the changes in NAcc DA transmission associated with discrepancies between the time food is expected and when it is actually presented. We used in vivo voltammetry to monitor NAcc DA levels as rats received, on the first and second test days, 30-s meals of condensed milk on a VT-52 schedule (inter-meal intervals of 32, 35, 40, 45, 52, 64, and 95 s). On the third and subsequent days meals were presented first on a VT-52 s schedule and then on an FT-52 s schedule. On day 1, monotonic increases in NAcc DA signals were observed during both meal consumption and the intervals between VT meal presentations. By day 2, however, meal presentations on the VT schedule elicited biphasic DA signal fluctuations; DA signals increased prior to each meal presentation but then started to decline during the feeding bout that followed. Fixed-time meal presentations on day 3 disrupted this pattern, resulting in a concurrent escalation of DA signal fluctuations upon subsequent VT meal presentations. These findings provide further evidence that, in trained animals, NAcc DA transmission is activated by conditioned incentive cues rather than by primary rewards. They also suggest that the increases in NAcc DA transmission associated with reward expectancy are sensitive to temporal cues (e.g. interval timing) and to discrepancies between expected and actual outcomes.