Junk food diet-induced obesity increases D2 receptor autoinhibition in the ventral tegmental area and reduces ethanol drinking

Abstract

Similar to drugs of abuse, the hedonic value of food is mediated, at least in part, by the mesostriatal dopamine (DA) system. Prolonged intake of either high calorie diets or drugs of abuse both lead to a blunting of the DA system. Most studies have focused on DAergic alterations in the striatum, but little is known about the effects of high calorie diets on ventral tegmental area (VTA) DA neurons. Since high calorie diets produce addictive-like DAergic adaptations, it is possible these diets may increase addiction susceptibility. However, high calorie diets consistently reduce psychostimulant intake and conditioned place preference in rodents. In contrast, high calorie diets can increase or decrease ethanol drinking, but it is not known how a junk food diet (cafeteria diet) affects ethanol drinking. In the current study, we administered a cafeteria diet consisting of bacon, potato chips, cheesecake, cookies, breakfast cereals, marshmallows, and chocolate candies to male Wistar rats for 3-4 weeks, producing an obese phenotype. Prior cafeteria diet feeding reduced homecage ethanol drinking over 2 weeks of testing, and transiently reduced sucrose and chow intake. Importantly, cafeteria diet had no effect on ethanol metabolism rate or blood ethanol concentrations following 2g/kg ethanol administration. In midbrain slices, we showed that cafeteria diet feeding enhances DA D2 receptor (D2R) autoinhibition in VTA DA neurons. These results show that junk food diet-induced obesity reduces ethanol drinking, and suggest that increased D2R autoinhibition in the VTA may contribute to deficits in DAergic signaling and reward hypofunction observed with obesity.

Fig 1. Cafeteria diet access results in elevated caloric intake and an obese-like phenotype.

Caloric intake and the source of calories were assessed over 3 weeks. (A) Rats with daily access to cafeteria diet consumed significantly more calories over the 3 weeks of feeding than the chow only group (n = 14-19/group). (B) The cafeteria diet group consumed significantly more calories from cafeteria diet food items than from chow pellets (n = 19). (C) The chow only group consumed more calories from chow pellets than the cafeteria diet group (n = 14-19/group). (D) Cafeteria diet access resulted in increased weight gain over the 3 weeks of feeding (n = 14-19/group). (E) Four weeks of cafeteria diet feeding significantly increased body weight, compared to chow only fed controls (main effect of diet, p < 0.001, two-way ANOVA, n = 44-46/group). (F) Throughout the 4 weeks of cafeteria diet access, the cafeteria diet group consumes significantly less chow than the chow only group (main effect of diet, p < 0.0001, two-way ANOVA, n = 10-11/group). * p < 0.05, ** p < 0.01, *** p < 0.001, Bonferroni post hoc test.

Fig 2. Prior cafeteria diet feeding reduces ethanol drinking with no effect on ethanol metabolism rate or BECs.

(A) Mean baseline ethanol drinking (g/kg) over the 7 days prior to cafeteria diet feeding was similar between groups (p = 0.7480, Student’s t-test, n = 6-7/group). (B) Prior cafeteria diet feeding (4 weeks) reduced the total volume of ethanol (10%, v/v, 2hr/day) consumed during the 2 weeks of testing (main effect of diet, p < 0.05, two-way ANOVA, n = 6-7/group), (C) with no effect on total water consumption (n = 6-7/group). (D) There was no difference in the slopes of BECs (30–120 min following a 2g/kg administration, i.p.) between groups (p = 0.6535, linear regression, n = 4-5/group). BECs were similar between groups at 30, 60, and 120 min post-ethanol administration. BEC, blood ethanol concentration; i.p., intraperitoneal.

Fig 3. Prior cafeteria diet feeding transiently reduced sucrose drinking and chow intake.

(A) Mean baseline sucrose drinking (mL/kg) over the 7 days prior to cafeteria diet feeding was similar between groups (p = 0.6489, Student’s t-test, n = 15-16/group). (B) Prior cafeteria diet feeding (4 weeks) transiently altered sucrose (5%, w/v, 2hr/day) consumption (diet x time interaction, p < 0.005, two-way ANOVA, n = 15-16/group). (C) There was no difference in water consumption between groups (n = 15-16/group). (D) Prior cafeteria diet feeding transiently reduced chow intake (n = 10/group). *** p < 0.001, Bonferroni post hoc test.

Fig 4. Cafeteria diet feeding had no effect on basal tonic pacemaker firing frequency of VTA DA neurons.

(A) Basal tonic firing frequency of VTA DA neurons was similar between groups (p = 0.4681, Student’s t-test, n = 36-38/group). (B) Representative traces of VTA DA neuron firing following 4 weeks of chow only (blue) or cafeteria diet (red) feeding. DA, dopamine; VTA; ventral tegmental area.

Fig 5. Cafeteria diet feeding increases D2R-mediated autoinhibition of VTA DA neurons.

(A) Cafeteria diet increased the mean peak amplitude of quinpirole-mediated (100 nM) inhibitory outward GIRK currents compared to chow only controls. Quinpirole was bath applied for 10 min and sulpiride (1 μM) rapidly reversed the quinpirole-mediated current. Examples of quinpirole-mediated outward currents (V_h = -62 mV) for chow only (blue) or cafeteria diet fed (red) rats (n = 16-25/group). (B) Cafeteria diet feeding increased the inhibitory effects of 10 nM quinpirole on VTA DA neuron firing frequency over 10 min of quinpirole bath application (main effect of diet, p < 0.0005, two-way ANOVA, n = 13-16/group) and (C) quinpirole-mediated percent inhibition of firing frequency (p < 0.001, Student’s t-test). Representative traces of DA neuron firing frequency during baseline or 10 nM quinpirole application following chow only (blue) or cafeteria diet (red) feeding. (D-E) Inhibition of DA neuron firing frequency by 30 nM quinpirole was similar between groups (n = 9/group). Following 10 min of 30 nM quinpirole bath application, sulpiride (1 μM) was applied to the bath to rapidly reversed quinpirole-mediated inhibition of firing frequency. Representative traces of DA neuron firing frequency during baseline or 30 nM quinpirole application following chow only (blue) or cafeteria diet (red) feeding. *** p < 0.001, Student’s t-test. DA, dopamine; D2R, dopamine D2 receptor; GIRK, G protein-gated inwardly rectifying potassium channels; VTA; ventral tegmental area.