Opioid mediation of starch and sugar preference in the rat.

In our prior studies, administration of the opioid receptor antagonist naltrexone did not block conditioned preferences for a flavor paired with a preferred sugar solution over a flavor paired with saccharin. This may be because both training solutions were sweet, and their attractiveness was reduced by naltrexone. The present study compared the effects of naltrexone on preferences for flavors paired with sugar or starch drinks that have distinctive tastes to rats. Experiment 1 assessed naltrexone's effect on the preference for unflavored 8% cornstarch and 8% sucrose aqueous solutions/suspensions. The food-restricted rats displayed a significant sucrose preference which increased following systemic treatment with naltrexone (1 or 3mg/kg) even though total intake of both solutions declined. In Experiment 2, rats were trained to drink flavored (cherry or grape) starch and sucrose solutions in separate one-bottle sessions. In a two-bottle choice test with both flavors presented in a sucrose-starch mixture, the rats significantly preferred the starch-paired flavor. Naltrexone treatment blocked the expression of this starch-conditioned preference. In Experiment 3, rats were treated with saline or naltrexone throughout one-bottle training with flavored sucrose and starch solutions. In a subsequent choice test, both the saline and naltrexone groups displayed significant preferences for the starch-paired flavor, indicating that opioid antagonism failed to alter the acquisition of this conditioned preference. In summary, novel outcomes of this study included the increased rather than the predicted decrease in sucrose preference produced by naltrexone. Also, starch unexpectedly conditioned the stronger flavor preference, although this can be explained by the differential post-oral reinforcing actions of starch and sucrose, and naltrexone blocked the expression, but not the acquisition, of this preference. These findings suggest that the reward value of starch in liquid form is more dependent upon opioid signaling than is that of sugar.

Sucrose taste but not Polycose taste conditions flavor preferences in rats.

Rats have an inborn preference for sweet taste and learn to prefer flavors associated with sweetness. They are also strongly attracted to the taste of glucose polymers (e.g., Polycose). This "poly" taste differs in quality from the sweet taste of sugar. To determine if poly taste, like sweet taste, conditions flavor preferences rats were trained with a distinctive flavor (CS+) added to 2% Polycose solution and a different flavor (CS-) added to plain water. In a subsequent two-bottle test the rats did not prefer the CS+ to CS- when both flavors were presented in water. In contrast, other rats significantly preferred a CS+ flavor that had been paired with 2% sucrose. Adding saccharin to a flavored Polycose solution did not improve CS+ flavor learning; rather, Polycose appeared to overshadow saccharin-induced conditioning. Flavor conditioning by a 16% Polycose solution was assessed using a sham-feeding procedure to prevent post-oral reinforcement. Although the rats sham-fed substantial amounts of the CS+ flavored Polycose solution, they failed to prefer the CS+ to the CS- flavor. This contrasts with the preference other rats displayed for a CS+ paired with sham-fed sucrose. Why attractive sweet and poly tastes differ in their ability to condition flavor preferences is not certain, although some findings suggest that they differentially activate dopamine and/or serotonin circuits involved in flavor learning.

Obesity by choice revisited: effects of food availability, flavor variety and nutrient composition on energy intake.

Recent work suggested that the energy intake and weight gain of rats maintained on chow and 32% sucrose solution could be increased by simply offering more sources of sucrose [Tordoff M.G. Obesity by choice: the powerful influence of nutrient availability on nutrient intake. Am J Physiol 2002;282:R1536-R1539.]. In Experiment 1 this procedure was replicated but the effect was not: rats given one bottle of sucrose and five bottles of water consumed as much sucrose as those given five bottles of sucrose and one of water. Adding different flavors to the sucrose did not increase intakes further in Experiment 2. The relative potency of sucrose and other optional foods was studied in Experiment 3. Sucrose solution stimulated more overeating and weight gain than fat (vegetable shortening), and offering both sucrose and shortening did not generate further increases in energy intake. Finally, foods commonly used to produce overeating and weight gain were compared. Sucrose was less effective than a high-fat milk diet, and offering cookies in addition to the milk did not increase energy intake further. The nature of optional foods (nutrient composition and physical form) was markedly more important than the number of food sources available to the animals, and is a better contender as the reason for "obesity by choice".