Role of gustatory and postingestive actions of sweeteners in the generation of positive affect as evaluated by place preference conditioning

Either an 18% sucrose or a 0.1% saccharin solution was used as reward in a conditioned place preference procedure. The respective solution was consumed immediately before the subject was confined to the non-preferred compartment of a place preference box. In this way only the affective reaction produced by the rewarding stimulus participated in place conditioning. Non-deprived rats consumed similar quantities of the two solutions, suggesting that they were equally rewarding. Yet only sucrose induced place preference. The lack of effect of saccharin may have been due to weaker positive affect or to an inability to induce an affective state that outlasted consummatory behavior. The first possibility was evaluated in the second experiment. Food-deprived rats were subjected to a discrimination learning task in a runway with plain water, sucrose or saccharin as reinforcer. Sucrose sustained running responses and produced improvement of discrimination behavior. Saccharin had a small effect at the first sessions only. It was concluded that saccharin is less rewarding than sucrose. In a final experiment, one group of rats was injected with glucose (2 g/kg) 17 min before being introduced in the non-preferred compartment of a place preference box. Another group was given the same dose of glucose just before drinking saccharin for 15 min. Immediately thereafter place conditioning was performed. Glucose alone had no effect, while a place preference was established when glucose injection was combined with saccharin drinking. These results show that the positive affect produced by sweet substances does not outlast consummatory behavior in the absence of postabsorptive actions. (C) 1997 Academic Press Limited.