Altered taste sensitivity in obese, prediabetic OLETF rats lacking CCK-1 receptors

Dopamine D2 receptors contribute to increased avidity for sucrose in obese rats lacking CCK-1 receptors

Accumulating evidence has indicated a link between dopamine signaling and obesity in both animals and humans. We have recently demonstrated heightened avidity to sapid sweet solutions in the obese cholecystokinin (CCK)-1 receptor deficient Otsuka Long Evans Tokushima fatty (OLETF) rat. To investigate the dopamine dependence and the respective contribution of D1 and D2 receptor subtypes in this phenomenon, real and sham intake of 0.3 M sucrose solution was compared between prediabetic, obese OLETF and age-matched lean Long-Evans Tokushima Otsuka (LETO) cohorts following peripheral (i.p.) administration of equimolar doses (50-800 nmol/kg) of the D1 (R-(+) 7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine, SCH23390) and D2 (raclopride) selective receptor antagonists. Both antagonists were potent in reducing sucrose intake in both strains with both drugs suppressing sham intake starting at lower doses than real intake (200 nmol/kg vs. 400 nmol/kg for SCH23390, and 400 nmol/kg vs. 600 nmol/kg for raclopride, respectively). Furthermore, when percent suppression of intake, a measure that controlled for the higher baseline sucrose intake by obese rats was analyzed, OLETF rats expressed an increased sensitivity to raclopride in reducing ingestion of sucrose with a 1.7- and 2.9-fold lower inhibitory dose threshold (ID50) for real and sham intake conditions, respectively, compared with LETO controls. In contrast, SCH23390 caused no differential strain effect with respect to dosage whether sucrose was real or sham fed. These findings demonstrate that D2 receptors are involved in heightened increased consumption of sucrose observed in the OLETF obese rat.

Altered basal and stimulated accumbens dopamine release in obese OLETF rats as a function of age and diabetic status

The Otsuka Long-Evans Tokushima Fatty (OLETF) rat lacking the CCK-1 receptor is hyperphagic, prefers palatable and high-calorie meals, and gradually develops obesity and type 2 diabetes. To determine dopamine levels in this strain, we used in vivo quantitative (no net flux) microdialysis at three different ages representing nondiabetic (8 wk), prediabetic (18 wk), and diabetic (56 wk) stages in OLETF and age-matched lean Long-Evans Tokushima Otsuka (LETO) controls. Results showed significantly elevated basal dopamine levels in the caudomedial nucleus accumbens of OLETF rats compared with LETO at younger ages (8 wk: 20.10 +/- 5.61 nM vs. 15.85 +/- 5.63 nM; 18 wk: 7.37 +/- 3.71 nM vs. 4.75 +/- 1.25 nM, means +/- SD). In contrast, at 56 wk of age, a profound decline in extracellular dopamine concentrations was seen in both strains with a tendency for a greater effect in OLETF rats (1.78 +/- 0.40 nM vs. 2.39 +/- 0.42 nM). Further, extracellular fraction, an index for reuptake, was higher in 56-wk-old OLETF compared with LETO (0.648 +/- 0.049 vs. 0.526 +/- 0.057). Potassium-stimulated dopamine efflux revealed an increased capacity of vesicular pool in OLETF rats compared with LETO across all age groups with an accentuated strain difference at 56 wk. These findings demonstrate altered striatal dopamine functions (i.e., increased stimulated release and uptake) in obese OLETF rat. This could be due to the lack of functional CCK-1 receptors, or metabolic and hormonal factors associated with the development of obesity and insulin resistance, or both.

Interoceptive "satiety" signals produced by leptin and CCK

The present studies assessed the extent to which the adiposity signal leptin and the brain-gut hormone cholecystokinin (CCK), administered alone or in combination, give rise to interoceptive sensory cues like those that are produced by a low (1h) level of food deprivation. Rats were trained with cues arising from 1 to 24-h food deprivation as discriminative stimuli. For one group, 24-h food deprivation predicted the delivery of sucrose pellets, whereas 1-h food deprivation did not. Another group received the reversed deprivation level-sucrose contingency. After asymptotic performance was achieved, the effects of leptin and CCK on food intake and on discrimination performance were tested under 24-h food deprivation. In Experiment 1a, leptin administered into the third cerebroventricle (i3vt) at 3.5 or 7.0 microg doses had little effect, compared to saline on food intake or discriminative responding. In Experiment 1b, leptin (7.0 microg, i3vt) combined with CCK-8 (2 microg/kg, i.p.) reduced food intake significantly, but the findings indicated that CCK-8 alone produces interoceptive discriminative cues more like those produced by 1- than 24-h food deprivation. Experiment 2a tested rats with i.p. leptin (0.3 and 0.5mg/kg). Although neither dose suppressed intake, the 0.3mg/kg dose produced interoceptive cues like 1-h food deprivation. Experiment 2b tested two doses of CCK-8 (2 and 4 mg/kg, i.p.) and found significant intake suppression and generalization of discrimination with both doses of CCK-8. These findings suggest a role for both leptin and CCK in the production of sensory consequences that correspond to "satiety".

Conditioned preference for sweet stimuli in OLETF rat: effects of food deprivation

The Otsuka Long-Evans Tokushima Fatty (OLETF) rat, an outbred strain of Long- Evans Tokushima Otsuka rat (LETO) that lacks CCK-1 receptor expression, is hyperphagic and develops obesity and type-2 diabetes. The present study sought to assess how OLETF rats alter intake, preference, and conditioned preference of palatable solutions after acute food deprivation. Our results show that after 24 h chow restriction, LETO rats increase both sucrose intake and two-bottle sucrose preference relative to their free-fed baseline, whereas OLETF rats do not increase sucrose intake (0.3 M or 1.0 M sucrose) or preference (1.0 M vs. 0.3 M sucrose) when they are food deprived. In contrast, OLETF rats exhibit a higher conditioned flavor preference when sucrose is used as unconditioned stimulus (US) relative to LETO rats, whether overnight food restricted (81% vs. 71% for OLETF and LETO rats, respectively) or free fed (82% vs. 54% for OLETF and LETO rats, respectively) during the test. When a noncaloric saccharin solution is used as US, OLETF rats show a higher preference for the saccharin-associated flavor relative to LETO rats when nondeprived (76% vs. 58% for OLETF and LETO rats, respectively); however, neither strain shows differential conditioned flavor preference for saccharin in the deprivation state during the test. These findings suggest that OLETF rats fail to integrate postabsorptive and orosensory effects of sucrose in a conditioning setting to influence intake. Thus, it appears that OLETF rats form preferences for sucrose based largely on orosensory and hedonic properties of the solution, rather than caloric value.

Decreased gastric mechanodetection, but preserved gastric emptying, in CCK-1 receptor-deficient OLETF rats

Obese CCK-1 receptor-lacking Otsuka Long Evans Tokushima fatty (OLETF) rats are hyperphagic relative to control, nonmutant Long Evans Tokushima Otsuka (LETO) rats. This study sought to assess whether the overeating observed in OLETF rats is associated with changes in gastric emptying rates or detection of gastric volume. We performed experiments in both 12- and 29-wk-old OLETF and LETO rats to address possible alterations in gastric functions during the development of increased body weight and blood glucose abnormalities in OLETF rats. Gastric emptying of a 5-g solid chow test meal was not significantly different between strains at either 1, 2, or 4 h postmeal. When rats with ad libitum access to chow were tested, there were no significant differences in gastric emptying between strains at any time period despite OLETF rats consuming significantly more chow than LETO rats. Similar to solid food, 5-min gastric emptying of a 5-ml isosmotic and hyperosmotic saline or glucose load was not significantly different between strains. When the stomach was distended with a 15-ml semisolid chow load, there was no significance difference in emptying at either 1 or 2 h. No significant differences in gastric emptying were detected between 12- and 29-wk-old rats under any conditions. Both young and old OLETF rats, however, reduced sham intake significantly less compared with LETO rats during a brief period of gastric distension by 5- or 10-ml balloon inflation. Finally, OLETF rats showed decreased Fos expression in the nucleus of the solitary tract relative to LETO rats after an 8-ml gastric distension. These findings demonstrate that OLETF rats do not express deficits in controlling gastric emptying rates; however, they exhibit decreased behavioral and vagal responsiveness to gastric distension that may contribute to the increased meal size in these animals.