Behavioral and neural characterization of conditioned flavor-taste preferences

Many animals, including humans, choose their source of nutrition based on the nutritive value and the flavor (i.e., odor, taste and texture) of available foods. Sweet taste is one of the more potent orosensory stimuli that contributes to food choice. Laboratory animals develop preferences for neutral or aversive tastes and flavors by associating them with the taste of sugars and non-caloric sweeteners. Learning how these preferences develop would aid in understanding how and why specific foods are selected over others. Given the wide availability and variety of sweetened foods and beverages in modern society, the formation and persistence of learned food preferences by consumers may contribute to health issues such as diabetes and obesity.; Conditioned flavor-taste preference (CFTP) learning is a form of associative learning in which a rat comes to prefer a neutral flavor paired with a preferred taste. Experimentally, one flavor (the conditioned stimulus or CS+; e.g., cherry or grape Kool-Aid) is paired with the sweet and highly preferred taste of fructose (F; the unconditioned stimulus or US) while a second flavor (the CS-) is paired with the less preferred taste of saccharin (S) on 1-bottle conditioning days (CS+/F or CS-/S). The acquisition of the learned preference is then assessed with a 2-bottle preference test in which both flavors mixed with saccharin (CS+/S and CS-/S) are presented simultaneously. While CFTP learning is well known, it has not been well characterized. The olfactory and gustatory associative brain regions necessary for CFTP learning are unknown. Dopamine receptors have been implicated, but otherwise it is not known which neurotransmitters or receptors mediate CFTP.; In order to identify the associative neural substrates that are involved in CFTP learning, three approaches were taken; behavioral, pharmacological and molecular assays.; To precisely characterize the behavioral acquisition and expression of a CFTP, lickometers were used to determine the pattern of drinking in rats. During 1 or 2-bottle preference tests, total intake, bout size, bout number, lick rate and first minute licks were analyzed. The pattern of drinking was examined under 3 conditions: (1) During expression of unconditioned preferences for 8% fructose over 0.2% saccharin. The unconditioned preference for fructose over saccharin was slow to develop, but was seen in significantly greater total intake, bout size, and first minute licks for fructose by the fourth preference test. (2) During the pairing of Kool-Aid flavors with either 8% fructose or 0.2% saccharin. CS-/S total intake and bout size was significantly greater than CS+/F during conditioning, but a preference for the CS+ flavor was seen in the third and fourth 2-bottle preference test days with significantly greater total intake and bout size of CS+/S vs. CS-/S. (3) During long-term presentations of Kool-Aid mixed with two different concentrations of saccharin (0.2% vs 0.05%) as the unconditioned stimuli. Total intake, bout size and bout number were significantly greater for the flavor mixed with the high concentration of saccharin over the low concentration of saccharin during conditioning. During 2-bottle preference tests when both flavors were mixed with the low concentration of saccharin, total intake and bout size were significantly greater for the CS+.; The increases in lick rate and bout size observed in all 3 experiments suggest that fructose is more palatable than saccharin, and a high concentration of saccharin is more palatable than a low concentration. A change in relative palatability of the Kool-Aid flavors is conditioned by association with the high palatability tastes; greater intake of the conditioned flavor is mediated by increased bout size. These results suggest that flavor preference learning interacts with both orosensory processes and satiety processes (i.e. prolonged bout size) to elevate intake of the preferred flavor.; The N-methyl-D-aspartate (NMDA)...