Role of portal and plasma glucose elevations in taste-to-postingestive consequence learning

Abstract

<p>Animals are able to associate the taste of a food with its positive nutritive consequences and will modify their feeding behavior when re-exposed to tastes on the basis of this association. However, the specific nutritive consequence(s) that animals detect and associate with a food's taste are unknown. This thesis tested the ability of two postingestive events, elevations of plasma or portal glucose, to support taste-to-postingestive consequence learning. Two strategies were used to evaluate the relevance of elevations in plasma glucose in taste-to-postingestive consequence learning. First, since real, but not sham, feeding supports taste-to-postingestive consequence learning, I recorded plasma glucose levels during sham and real feeding (Experiment 1). For the first 15 minutes of the meal, both sham and real feeding produced similar elevations in plasma glucose. Thereafter, real feeding produced sustained elevations in plasma glucose, whereas sham feeding did not. Next, I paired tastes during sham feeding with a glucose infusion into the jugular vein, calibrated (Experiment 2) to mimick the plasma glucose profile of real feeding. Since rats that experienced tastes paired with plasma glucose elevations characteristic of real feeding, did not demonstrate taste-to-postingestive consequence learning (Experiment 3), plasma glucose elevations must not support taste-to-postingestive consequence learning. In Experiment 4 I also demonstrated that sham feeding produces elevations in plasma glucose because nutrient is absorbed. To assess the relevance of portal elevations in taste-to-postingestive consequence learning, I paired flavoured saccharin or flavoured glucose solutions with glucose infusions in the portal vein (Experiments 6 & 7). Only rats that received portal infusions of glucose paired with flavoured glucose solutions demonstrated taste-to-postingestive consequence learning (Experiment 7). Since oral glucose solutions, alone, elevate glucose levels in the portal vein, portal elevations of glucose may only support taste-to-postingestive consequence learning at suprathreshold levels. Alternatively, signal(s) arising from the stomach or gatrointestinal tract may potentiate the relevance of elevations of portal glucose since oral glucose solutions also stimulate these sites.</p>