Profound differences in fat versus carbohydrate preferences in CAST/EiJ and C57BL/6J mice: Role of fat taste

Differential patterns of opioid and dopamine D1 receptor antagonism on nutritive and non-nutritive sweetener intakes in C57BL/6:129 hybrid mice relative to inbred C57BL/6 and 129 mice

Opioid and dopamine (DA) D1 receptor antagonists differentially reduce nutritive and non-nutritive sweetener intakes in inbred mouse strains. Sucrose intake was more effectively reduced by naltrexone in C57BL/6 (B6) mice relative to 129P3 (129) mice, but more effectively reduced by SCH23390 in 129 mice relative to B6 mice. Opioid and DA D1 antagonists differentially reduced saccharin intakes in B6 mice relative to other strains. Given these differential patterns in sweetener intake in B6 and 129 mice, the present study examined whether systemic naltrexone (0.01-5 mg/kg) and SCH23390 (50-1600 nmol/kg) reduced intakes of 10 % sucrose or 0.2 % saccharin solutions over a 120 min time course in first-generation hybrid mice (B6:129) of B6 and 129 parents and reduced low-nutritive sweetener intakes in 129 mice. Naltrexone (5 mg/kg) significantly reduced 10 % sucrose intake in B6:129 hybrid mice more like that of 129 than B6 mice. In contrast, SCH23390 (400-1600 nmol/kg) reduced 10 % sucrose intake in B6:129 hybrid mice more effectively than that observed in B6 or 129 parental strains. Because 129 mice consumed relatively low amounts of 0.2 % saccharin, they were tested with a more attractive low-nutritive solution containing 0.2 % saccharin and 2 % sucrose. Naltrexone failed to reduce saccharin intake in B6:129 hybrid mice but suppressed saccharin+sucrose intake in 129 mice more like that observed in B6 mice. SCH23390 similarly inhibited saccharin or saccharin+sucrose intakes in hybrid B6:129, 129, and B6 mice with B6 mice more resistant to the lowest SCH23390 dose. Thus, whereas sucrose intake in B6:129 hybrid mice exhibited similar sensitivity to opioid and to a lesser degree DA D1 antagonism to their 129, but not B6 parents, opioid and DA D1 mediation of low- and non-nutritive sweet intake produced unique profiles among B6:129 hybrid and B6 and 129 strains which does not support a simple heritability explanation.

The neural basis of sugar preference

When it comes to food, one tempting substance is sugar. Although sweetness is detected by the tongue, the desire to consume sugar arises from the gut. Even when sweet taste is impaired, animals can distinguish sugars from non-nutritive sweeteners guided by sensory cues arising from the gut epithelium. Here, we review the molecular receptors, cells, circuits and behavioural consequences associated with sugar sensing in the gut. Recent work demonstrates that some duodenal cells, termed neuropod cells, can detect glucose using sodium-glucose co-transporter 1 and release glutamate onto vagal afferent neurons. Based on these and other data, we propose a model in which specific populations of vagal neurons relay these sensory cues to distinct sets of neurons in the brain, including neurons in the caudal nucleus of the solitary tract, dopaminergic reward circuits in the basal ganglia and homeostatic feeding circuits in the hypothalamus, that alter current and future sugar consumption. This emerging model highlights the critical role of the gut in sensing the chemical properties of ingested nutrients to guide appetitive decisions.

Administration of Exendin-4 but not CCK alters lick responses and trial initiation to sucrose and intralipid during brief-access tests

Administration of cholecystokinin (CCK) or the glucagon-like peptide 1 (GLP-1) receptor agonist Exendin-4 (Ex-4) reduces food intake. Findings in the literature suggest CCK reduces intake primarily as a satiety signal whereas GLP-1 may play a role in both satiety and reward-related feeding signals. Compounds that humans describe as âsweetâ and âfattyâ are palatable yet are signaled via separate transduction pathways. Here, unconditioned lick responses to sucrose and intralipid were measured in a brief-access lick procedure in food-restricted male rats in response to i.p. administration of Ex-4 (3 h before test), CCK (30 min before test), or a combination of both. The current experimental design measures lick responses to water and varying concentrations of both sucrose (0.03, 0.1, and 0.5 M) and intralipid (0.2%, 2%, and 20%) during 10-s trials across a 30-min single test session. This design minimized postingestive influences. Compared with saline-injected controls, CCK (1.0, 3.0, or 6.0 µg/kg) did not change lick responses to sucrose or intralipid. Number of trials initiated and lick responses to both sucrose and intralipid were reduced in rats injected with 3.0 µg/kg, but not 1.0 µg/kg Ex-4. The supplement of CCK did not alter lick responses or trials initiated compared with Ex-4 administration alone. These findings support a role for GLP-1 but not CCK in the oral responsiveness to palatable stimuli. Furthermore, Ex-4-induced reductions were observed for both sucrose and intralipid, compounds representing âsweetâ and âfat,â respectively.

Reduced acceptance of saccharin solutions by rats previously consuming more highly palatable solutions

Rats first given 24-h access to 10% sucrose for 4 or 12 days (Stage 1) were then switched to a saccharin solution for a 12-day Stage 2. The initial result of this switch was that these Sucrose groups drank less saccharin than Water groups that had been given only water to drink in Stage 1. This difference was maintained throughout Stage 2 by the females that served in Experiments 1 and 4 and by the males that served in Experiment 3. Experiment 1 also found that access to 10% glucose in Stage 1 produced an essentially identical decrease in subsequent saccharin acceptance as that produced by giving 10% sucrose in Stage 1. The impact on subsequent acceptance of saccharin was also tested in rats given two types of maltodextrin solution. The first type of maltodextrin (Myopure brand) was used with the males in Experiment 2; this failed to find any difference between the Maltodextrin and the Water group. However, when a second type of maltodextrin (SolCarb brand) was given to males in Stage 1 of Experiment 3, the results for this group were similar to those from a group given sucrose in Stage 1. The final experiment confirmed that prior exposure to maltodextrin solutions can reduce saccharin acceptance by female rats. Overall, the results suggest that acceptance of saccharin is sensitive to a contrast effect, in that it is reduced by prior exposure to a solution that is more palatable but not necessarily sweet.