The effect of multisensory context and experience on flavor preference decisions in rats

Flavor is perceived through multiple senses, including gustation and olfaction. Previous studies have shown that different sensory qualities that make up flavor are integrated to inform perceptual judgements. Psychophysical work in humans further suggests a prominent role for congruency (i.e., the learnt correspondence between taste and odor components of flavor through eating experience) in shaping multisensory interactions underlying perceptual judgments of flavor. However, eating experience cannot be controlled in humans, and depending on the type of judgement, these studies yielded mixed findings. Here, we used rats to test how experimentally-controlled experience with specific flavor mixtures (OdorA+TasteA and OdorB +TasteB) from weaning to adulthood affects subsequent flavor preference judgements in a series of two-bottle preference tests. In unisensory conditions, animals made odor or taste preference decisions (i.e., OdorA versus OdorB and TasteA versus TasteB, respectively). In multisensory conditions, animals made identical decisions, but the addition of the other modality rendered one solution congruent; the other one incongruent (e.g., OdorA+TasteA versus OdorB+TasteA). The results show that animals effectively learned congruency associations between the taste and smell components of experienced flavor mixtures. Comparing unisensory and multisensory conditions revealed no systematic effect of congruency on the magnitude of flavor preference, but preferences were less variable in multisensory compared to unisensory conditions. Results from a second group of naïve animals further demonstrate that increased reliability of preference judgements in multisensory conditions was independent of experience.

Separate functions for responses to oral temperature in thermo-gustatory and trigeminal neurons

Oral temperature is a component and modifier of taste perception. Both trigeminal (V) and taste-sensitive cells, including those in the nucleus of the solitary tract (NTS), can respond to oral temperature. However, functional associations in thermal sensitivity between V and gustatory neurons are poorly understood. To study this we recorded electrophysiological responses to oral stimulation with cool (9, 15, 25, 32, and 34 °C) and warm (40 and 45 °C) temperatures from medullary V (n = 45) and taste-sensitive NTS (n = 27) neurons in anesthetized mice. Results showed temperatures below 34 °C activated the majority of V neurons but only a minority of NTS units. V neurons displayed larger responses to cooling and responded to temperatures that poorly stimulated NTS cells. Multivariate analyses revealed different temperatures induced larger differences in responses across V compared with NTS neurons, indicating V pathways possess greater capacity to signal temperature. Conversely, responses to temperature in NTS units associated with gustatory tuning. Further analyses identified two types of cooling-sensitive V neurons oriented toward innocuous or noxious cooling. Multivariate analyses indicated the combined response of these cells afforded distinction among a broad range of cool temperatures, suggesting multiple types of V neurons work together to represent oral cooling.

Flavor-independent maintenance, extinction, and reinstatement of fat self-administration in mice

BACKGROUND

Mounting evidence suggests that overeating may be conceptualized within the same behavioral and neurobiological framework as drug addiction. One potentially important difference between overeating versus drug abuse refers to the sensory stimulation of oral receptors by palatable foods, a feature that may be required for reinforcement during intake. Likewise, postingestive effects and caloric content of food also contribute to reinforcing behavior and might influence the development of compulsive eating behavior. The purpose of the current study was to establish whether intragastric self-administration of fat emulsions, that is, bypassing the oral cavity, recapitulates some of the behavioral and neurobiological hallmarks of psychostimulant self-administration.

METHODS

We used behavioral assays in mice to assess acquisition, maintenance, extinction, and reinstatement of intragastric self-administration of lipid emulsions to determine the extent to which postoral fat self-administration recapitulates psychostimulant self-administration. Striatal dopamine efflux during behavioral tasks was determined by brain microdialysis coupled to chromatographic-electrochemical analyses.

RESULTS

We show that in direct analogy to drug self-administration, 1) decreases in fat dose concentration were met with compensatory increases in response rates aimed at maintaining constant hourly caloric intake; 2) rates of responding markedly increased during both extinction and progressive ratio schedules of reinforcement; and 3) elevations in striatal dopamine levels observed during maintenance were markedly attenuated during extinction sessions, only to be restored on reinstatement.

CONCLUSIONS

Our data thus support the contention that stimulation of oral receptors by caloric foods may not be required for the expression of certain addiction-related neurobehavioral markers.