To develop and use a behavioral paradigm for assessments of what nutrient properties are detected by intestinal chemoreceptors, we combined features of the "electronic esophagus" preparation (Elizalde G and Sclafani A. Physiol Behav 47: 63-77, 1990) and the conditioned taste aversion protocol (Garcia J and Koelling RA. Psychon Sci 4: 123-124, 1966). In four experiments, separate groups of food-deprived rats with gastric (experiments 1-4) or duodenal (experiment 4) catheters were infused with either carbohydrates (maltodextrin) or fats (corn oil) into their stomachs or small intestines, either while they consumed nonnutritive flavored solutions (experiments 1 and 2) or in the absence of any intake (experiments 3 and 4). For some animals, one of the macronutrient infusions was paired with lithium chloride injections shown to support conventional conditioned aversions. After training, in various oral preference test trials, animals were given opportunities to taste and consume the nonnutritive solutions that had served as oropharyngeal conditioned stimuli as well as the nutrients that had been infused intragastrically, with or without poisoning, but never sampled by mouth. As previously established, preferences for the nonnutritive flavors were enhanced by association with intragastric infusions of macronutrients, with carbohydrates producing the greater preference. On first exposure to the two macronutrients for oral consumption, animals reduced their intake of the nutrient that had been previously poisoned when it was infused into the gastrointestinal tract. These results, along with additional controls, suggest that nutrient tastes detected in the intestines can be recognized centrally based on oropharyngeal gustatory stimulation.

No abstract

The mouse W/Wv mutation of the c-Kit receptor causes extensive loss of gastrointestinal interstitial cells of Cajal and vagal intramuscular arrays (IMAs; one of the two putative mechanoreceptors in gastrointestinal smooth muscle). To characterize the behavioral phenotype of the c-Kit mouse and to evaluate the roles of these mechanoreceptors in controlling food intake, meal patterns and daily intakes of W/Wv mice and controls were examined using solid (20-mg pellets) and liquid (Isocal) maintenance diets. After the meal pattern experiments, CCK (0.5, 1, 2, 4, 8, and 16 microg/kg ip) was administered to examine the role of the interstitial cells and vagal IMA mechanoreceptors in relaying peripheral signals of satiety activated by CCK-A receptors, whereas the specificity of the response was assessed with the antagonist devazepide (300 microg/kg ip). On both diets, the W/Wv mice ate smaller meals for shorter durations, with a compensatory increase in meal number, resulting in daily intakes and body weights similar to the controls. After CCK injections, the mutant mice consistently suppressed intake more ( approximately 2x) in 30-min tests, regardless of the test diet (12.5% glucose, chow, pellets, and Isocal). The increased sensitivity of W/Wv mice to CCK reflected an increased potency of the hormone (c-Kit mouse ED50 = 2.4 microg/kg; control ED50 = 6.4 microg/kg) and a shift of the dose-response curve to the left. Devazepide blocked the CCK suppression of ingestion. These results indicate that the selective loss of the interstitial cells and IMAs disrupts short-term feeding of the W/Wv mice by inducing an earlier satiety, possibly by altering gastric accommodation and/or emptying, without affecting the long-term mechanisms controlling overall intake or body weight. The results also suggest that the reduction of interstitial cells and IMAs augments the sensitivity to or increases the efficiency of exogenous CCK.

Since mice with a deletion of the neurotrophin-4 (NT-4) gene exhibit a loss of both nodose ganglion neurons and vagal afferent terminals in the small intestines, we hypothesized that the reduced intestinal innervation of the NT-4 knockout (NT-4KO) mouse would lead to a corresponding reduction in the preabsorptive feedback from macronutrients. To explore this prediction, we measured meal patterns in NT-4KOs and controls, while, on different days, intragastric infusions of either lipids (Intralipid; 10%, 20%) or glucose (12.5%, 25%) were yoked to each animal's spontaneous feeding of a pelleted diet (approximately 1 kcal infused/1 kcal ingested). NT-4KO mice were relatively, though not completely, insensitive to the lipid infusions, whereas they were as sensitive as controls to glucose infusions. More specifically, the regulatory deficits of NT-4KOs included 1) attenuated satiation from the lipid infusions, as measured by smaller intrameal reductions of both meal size and meal duration, 2) defects in satiety associated with the fat infusions, as measured by smaller intermeal increases of both satiety ratio and intermeal interval, and (3) losses in daily compensatory responses for lipid calories. These results support the hypothesis that NT-4KO mice have deficits in macronutrient feedback from the gastrointestinal tract, indicate that the defects are specific insofar as they do not include impairments in the feedback of glucose infusions on feeding, and suggest that early feedback about dietary lipids is important in the regulation of satiation, satiety, and longer-term compensation of daily caloric intake.