Underweight rats have enhanced dopamine release and blunted acetylcholine response in the nucleus accumbens while bingeing on sucrose

Abstract

The present study tested whether rats release more accumbens dopamine (DA) during a sugar binge when they are underweight vs. normal weight. Since acetylcholine (ACh) in the nucleus accumbens (NAc) normally increases as a meal progresses and satiety ensues, we also tested whether ACh release is altered when an animal has lost weight. Rats were maintained on daily 8-h access to chow, with 10% sucrose solution available for the first 2 h. Microdialysis performed on day 21, at normal body weight, revealed an increase in extracellular DA to 122% of baseline in response to drinking sucrose. Extracellular ACh peaked at the end of the meal. Next, the rats were food and sucrose restricted so that by day 28 they were at 85% body weight. When retested, these animals released significantly more DA when drinking sucrose (179%), but ACh release failed to rise. A control group was tested in the same manner but given sugar only on days 1, 21 and 28. At normal body weight, control animals showed a non-significant rise in DA when drinking sucrose on day 21. On day 28, at 85% body weight, the controls showed a small increase (124%) in DA release; however, this was significantly lower than the 179% observed in the underweight rats with daily sugar access. These findings suggest that when an animal binges on sugar and then loses weight, the binge releases significantly more DA and less ACh than when animals are at a normal body weight.

Fig. 1

Daily sugar intake during the 21 days at a normal body weight. Intake increased significantly over time for the rats with 2 h of access to sugar each day. The control group drank approximately the same amount on days 1 and 21.

Fig. 2

Accumbens DA and ACh release when rats binge on sugar at a normal body weight and then again at 85% body weight. (A) DA is released in response to drinking sugar on day 21 of access at a normal body weight, and (B) this release is enhanced (to 179% of baseline) when animals binge on sugar at a reduced body weight. Rats with access to sugar just two times do not show this neurochemical effect. (C) ACh rises as the sugar meal progresses for both groups when at normal body weight. (D) This effect is blunted for the daily bingeing group when at a reduced body weight. Error bars represent standard error of the mean. * P<0.05 from baseline. White squares represent the 2-h sugar twice group, and black squares represent the 2-h sugar daily group. Figure reprinted with permission (Avena, 2007).

Fig. 3

Histology revealed that microdialysis samples were drawn primarily from the medial NAc shell. AcbC=accumbens core, CPu=caudate, aca=anterior commissure.