Fig. 4. Substitution with water to rats trained to drink glucose results in refusal of drinking and different dynamics of NAc glutamate and glucose.

Graphs in the left column show mean (±SEM) changes in currents detected by glutamate, Null and glucose sensors (a, e; 2-s bins) and mean (±SEM) changes in [glutamate] and [glucose] (b, f; 4-s bins) after presentation of the water-filled cup. Graphs in right columns show mean (±SEM) changes in currents detected by glutamate, Null and glucose sensors (c, g; 2-s bins) and mean (±SEM) changes in glucose and glucose concentrations (d, h; 16-s bins) after the start of drinking. Water presentation resulted in a difference between glutamate and Null currents (Current × Time interaction F_38,418=1.87 p<0.05), indicating a rapid and significant rise in [glutamate] (b, F_6,272=4.10, p<0.05). Glucose currents also rapidly changed with water presentation (Current × Time interaction, F_38,380=3.27 p<0.05) showing a rapid, significant, and unimodal rise in [glucose] (f, F_5,233=2.83, p<0.05). After the rats tasted the water, glutamate currents dynamically changed (Current × Time interaction F_76,836=1.96 p<0.05) indicating [glutamate] levels began to increase steadily (d, F_6,531=3.12 p<0.05), reaching levels <200 μM 20 min after this event. Concurrently, the glucose current changed versus the Null (Current × Time interaction F_76,760=2.11 p<0.05), signifying a significant [glucose] change (h, F_5,455=2.18, p<0.05), initially showing a greater and more persistent decrease when compared to glucose drinking, although due to variability no individual bin was significantly different from baseline. Water drinking also lacked the robust tonic increase seen after glucose consumption. n defines number of tests in each group and filled symbols in b and d show values significantly different from the initial, quiet-rest baseline (p<0.05).