Post-oral infusion sites that support glucose-conditioned flavor preferences in rats

Abstract

Rats learn to prefer a flavored solution (CS+) paired with a gastrointestinal glucose infusion over an alternate flavor (CS-) paired with a non-caloric infusion. Prior work implicates a post-gastric site of glucose action, which is the focus of this study. In Exp. 1, male rats (8-10/group) were infused in the duodenum (ID), mid-jejunum (IJ), or distal ileum (II) with 8% glucose or water as they drank saccharin-sweetened CS+ and CS- solutions, respectively, in one-bottle 30-min sessions. Two-bottle tests (no infusions) were followed by a second train-test cycle. By the second test, the ID and IJ groups preferred the CS+ (69%, 67%) to the CS- but the II group did not (48%). Satiation tests showed that ID and IJ infusions of glucose reduced intake of a palatable solution similarly, while II infusions were ineffective. In Exp. 2, rats (10/group) drank CS solutions in one-bottle, 30-min sessions and were given 2-h ID or hepatic portal vein (HP) infusions. The CS+ and CS- were paired with 10 ml infusions of 10% glucose and 0.9% saline, respectively. Following 8 training sessions, the ID group preferred the CS+ (67%) to the CS- but the HP group did not (47%) in a two-bottle test. The similar CS+ preferences displayed by ID and IJ, but not II groups implicate the jejunum as a critical site for glucose-conditioned preferences. A pre-absorptive glucose action is indicated by the CS+ preference displayed by ID but not HP rats in Exp. 2. Our data were obtained with non-nutritive CS solutions. HP glucose infusions are reported to condition preferences for a flavored food that itself has pre- and post-absorptive actions. Thus, there may be multiple sites for glucose conditioning with the upper or mid-intestines being the first site of action.

Figure 1

Mean (+SEM) intakes of CS+ and CS− solutions by the intraduodenal (ID), intrajejunal (IJ) and intraileal (II) groups during one-bottle training (top panel) and two-bottle testing (bottom panel) in cycles 1 and 2 of Experiment 1. Numbers atop the columns in the bottom panel indicate the percentage preference for the CS+; underlined values denote significantly greater intake of CS+ than CS− in the two-bottle tests.

Figure 2

Mean (+SEM) intakes of 2% maltodextrin + 0.2% saccharin solution by the intraduodenal (ID), intrajejunal (IJ) and intraileal (II) groups during the satiation tests in Experiment 1. Responses to 8% glucose are shown in the top panel and to 16% glucose in the bottom panel. Numbers atop the columns indicate the percentage reduction in intake during glucose infusion relative to water infusion; significant reductions are underlined.

Figure 3

Mean (+SEM) intakes of CS+ and CS− solutions by the one -hour (ID-1H) and two-hour (ID-2H) groups during training (top panel) and testing (bottom panel) in Experiment 2B. Numbers atop the columns in the bottom panel indicate the percentage preference for the CS+; underlined values denote significantly greater intake of CS+ than CS− in the two -bottle tests.