Figure 5.

Intragastric (IG) infusions of glucose and serine induce differential brain dopamine responses. A, B, An infusion pump holding a syringe containing a given nutrient was connected to the gastric catheter and placed under control of the behavioral chamber, so that every lick for water detected by the lickometers immediately triggered a 3-s-long infusion of the nutrient into the stomach (at a rate of 10 μl/min). The positions of water bottles in the cage were associated with infusion of one specific nutrient. A, Across conditioning sessions, animals produced significantly more licks for water during glucose infusion sessions compared with serine infusion sessions (*p < 0.04). B, The effect depended on conditioning day, because the difference in water intake between glucose and serine infusion sessions was significant for sessions 2 and 3 combined but not for session 1 (*p < 0.02). C, Overall percentage changes produced by glucose and serine intragastric infusions. In the nucleus accumbens, glucose infusions were associated with significantly higher levels of extracellular dopamine when directly compared with serine infusions (*p < 0.03, two-sample t test). In addition, significant decreases in dopamine levels were observed after serine infusions (**p < 0.02, one-sample t test against 100%). D, Whereas glucose infusions were consistently associated with increased dopamine levels across samples, relative decreases produced by serine infusions were marked in particular at the third sample (***p < 0.04). E, In dorsal striatum, significant increases in dopamine levels were associated with glucose (*p = 0.009, one-sample t test against 100%), but not serine (p < 0.09), infusions. F, Across samples, significant increases in dopamine levels were observed only during glucose infusions at 30 min after infusion onset (**p < 0.04, one-sample t test against 100%).