Comparison of C57BL/6 and DBA/2 mice in food motivation and satiety

Abstract

Demand functions describe the relationship between the consumption of a commodity and its mean or unit price. In the first experiment, we analyzed food demand in two strains of mice (C57BL/6 and DBA/2) that differ on several behavioral dimensions, but have not been examined extensively for differences in feeding and meal patterns. Mice worked for food pellets in a continuous access closed economy in which total intake and meal patterns could be measured. A series of fixed (FUP), variable (VUP), and progressive (PUP) unit price schedules were imposed. Under all schedules, DBA/2 mice consumed significantly more food than C57BL/6, a difference that was not attributable to disparity in body weight or weight gain. The higher intake of DBA/2 mice was due predominantly to larger meal size compared with C57BL/6, with no strain difference in meal frequency. In a second experiment, strain differences in meal size were not found to correlate with anorectic sensitivity to cholecystokinin (CCK) administration, or with c-Fos expression induced by CCK in PVN, AP and NTS. Thus, DBA/2 mice were motivated to sustain a higher daily food intake and meal size than C57BL/6 under the range of demand costs employed in the present work, but this strain difference is unlikely to be due to CCK action or responsiveness.

Figure 1

Mean (±SE; Ns=8) performance of C57BL/6 (B6, closed circles) and DBA/2 (D2, open circles) working for food according to various fixed unit price (FUP) ratios (left panels), variable unit prices (VUP, middle panels) and progressive unit prices (PUP, right panels). Note that the FUP and VUP abscissae are scaled logarithmically. The top row shows the food demand in pellets consumed per day. The second row shows the number of nose pokes emitted in obtaining that food (logarithmic ordinate). The third row shows the mean meals per day, and the fourth row the corresponding mean meal size. All differences between B6 and D2 were significant (Ps<0.05) except for the meals per day.

Figure 2

Mean (±SE; Ns=8) 30 min intakes of food in groups of B6 and D2 mice following injection of either vehicle or CCK-8 (6μg/kg). The left panel shows intakes of Crunchies treat by non-deprived mice and the right panel shows intakes of chow after 18 h food deprivation. In all cases, intakes after CCK were significantly lower than after the corresponding vehicle injection (Ps<0.01). The intake of Crunchies was lower in D2 than B6 mice both after vehicle (P<0.01) and CCK (P<0.05), but both the absolute and fractional suppression by CCK did not differ between strains. The intake of chow did not differ between strains after vehicle but was lower in B6 than D2 after CCK (P<0.05). The fractional suppression by CCK was thus greater in B6 than D2 (P<0.01).