Feeding microstructure in diet-induced obesity susceptible versus resistant rats: central effects of urocortin 2

Abstract

With one billion people overweight worldwide, the need to identify risk factors and treatments for obesity is urgent. The present study determined whether rats genetically prone to diet-induced obesity (DIO) show preexisting differences in meal microstructure and are sensitive to central anorectic effects of corticotropin-releasing factor type 2 (CRF(2)) receptor stimulation. Male, selectively bred DIO rats and their diet resistant (DR) counterparts (n = 9/genotype) were weaned onto low-fat chow and compared as young adults for spontaneous or intracerebroventricular urocortin 2 administration-induced (0, 0.3, 1, 3 microg) differences in ingestion. DIO rats were hyperphagic selectively at the dark cycle onset, showing shorter latencies to initiate feeding, faster returns to eating following meal completion, and a lower satiety ratio than DR rats. At other times, DIO rats had briefer postmeal intervals, but ate smaller and briefer meals, resulting in normal intake. DIO rats also ate faster than DR rats. Urocortin 2 was less potent in DIO rats, ineffective at the 0.3 microg dose, but produced CRF(2) antagonist-reversible anorexia at higher doses. Though heavier, chow-maintained DIO rats were proportionately as or more lean than DR rats. Thus, DIO rats showed signs of a preexisting, heritable deficit in the maintenance of postmeal satiety and a reduced sensitivity to anorectic CRF(2) agonist stimulation. The meal patterns of DIO rats temporally resemble human 'snacking' behaviour, which predicts adult obesity. Because central CRF(2) stimulation retains full anorectic efficacy at higher doses in the DIO model, manipulating this neuropeptidergic system might yield new therapeutic approaches for diet-induced obesity.

Figure 1. Cumulative nocturnal (left panel) and diurnal (right) food intake of non-deprived genetically selected diet-induced obesity-resistant (DR) and susceptible (DIO) male rats (n = 8–9/genotype)

Data represent the mean ± s.e.m. from the average of two consecutive 23 h sampling periods measured at 77–78 days of age. The symbol denotes a significant genotype difference, *P < 0.05 (between-subject Student's t test).

Figure 2. Spontaneous meal microstructure differences between genetically selected diet-induced obesity-resistant (DR) and susceptible (DIO) male rats

Data represent the mean (+s.e.m.) average meal size for food, meal frequency, average intermeal interval, and average meal duration for food during the dark (left panels) and light (right) phases and were calculated from the average of two consecutive 23 h sampling periods measured at 77–78 days of age (n = 8–9 rats/genotype). Note ln scale of y-axis for intermeal interval duration, reflecting their time scale. Symbols denote significant genotype differences, *P < 0.05, **P < 0.01 (between-subjects Student's t test).

Figure 3. Meal microstructure differences between genetically selected diet-induced obesity-resistant (DR) and susceptible (DIO) rats within early and later portions of the dark cycle

Data represent the mean +s.e.m. of average meal size and meal duration for food and intermeal interval duration during 0–2 h versus 2–12 h time intervals of the dark cycle and were calculated from the average of two consecutive 23 h sampling periods measured at 77–78 days of age (n = 8–9 male rats/genotype). Note ln scale of y-axis for intermeal interval duration, reflecting their time scale. Symbols denote significant differences: **P < 0.01 and ***P < 0.001 from the DR genotypes; #P < 0.05, ##P < 0.01 and ###P < 0.001 from the genotype's respective 0–2 h condition (Student's t test).

Figure 4. Relative frequency histogram of the ln-transformed duration of consecutive, within-meal interfeeding intervals (IFIs) in male genetically selected diet-induced obesity-resistant (DR) (top panel) and susceptible (DIO) rats (middle panel) during the dark cycle. The bottom panel overlays the summary Gaussian fits of each genotype's relative frequency histogram

The frequency histogram shows consecutive interfeeding intervals that were between e¹ and e⁴ s in duration (2.7–54.6) with a bin width of e^0.1. This time scale focuses on the intervals of sustained eating, as represented in the peak. The tail that extends to the right of the distribution putatively represents within-meal pauses. Data represent consecutive nocturnal interfeeding intervals measured during two 12 h sampling intervals in 77- to 78-day-old genetically selected DR and DIO rats (n = 8–9 male rats/genotype). Note ln-scale of x-axis.

Figure 5. Dose-dependent effects of third ventricle Ucn 2 administration on the mean (+s.e.m.) cumulative nocturnal food intake of genetically selected diet-induced obesity-resistant (DR) (left panel) and susceptible (DIO) (right) rats

Adult male rats (n = 8–9/genotype) were pretreated (−10 min) with Ucn 2 in a balanced Latin square design with test sessions beginning at the onset of the dark cycle. Insets depicts the mean of the cumulative difference from vehicle condition. Scale in insets differs from that of main panel. Letters denote significant differences of the vehicle condition from (a) 0.3 μg, (b) 1 μg, (c) 3 μg doses (P < 0.05, within-subjects Newman–Keuls test).

Figure 6. Cotreatment with the selective CRF₂ receptor antagonist astressin₂-B prevents anorectic effects of central Ucn 2 administration in genetically selected diet-induced obesity-resistant (DR) (left panel) and susceptible (DIO) (right) rats

Data represent mean (+s.e.m.) cumulative 2 h and 7 h food intake in adult male DR and DIO rats (n = 9/genotype) following treatment with vehicle (Veh), Ucn 2 (1 μg), astressin₂-B (A₂-B, 4 μg), or Ucn 2 + A₂-B in a balanced Latin square design. Treatments were given as a single third ventricle injection (2 μl) 15 min before testing, which began at the dark cycle onset. Symbols denote significant differences: *P < 0.05 and **P < 0.01 from vehicle condition, #P < 0.05 and ##P < 0.01 from Ucn 2 condition (Student's t test).