Appetitive and consummatory ingestive behavior : Role of taste, dopamine and NPY

Sammanfattning: The present thesis has examined the role of taste, dopamine and neuropeptide Y (NPY) in the regulation of the two phases of ingestive behavior: appetitive (approaching and handling of food) and consummatory ingestive behavior (chewing and swallowing). Behavioral studies were performed in male and female rats. THE ROLE OF TASTE: Background. A sweet stimulus loses its pleasantness to humans preloaded with glucose, but not if the subjects are food deprived. Thus, physiological state modulates the perception of taste. We hypothesized that taste perception should change during intraoral intake (consummatory ingestive behavior) because physiological state does. Hence, a series of experiments was performed to study aversive taste-related behavior during intraoral intake of a sucrose solution. Results. Behavioral markers of an aversive taste present in the oral cavity, gapes and chin rubs, were found to increase at the end of intraoral intake. This indicates that die taste perceived by the rat is turning aversive towards the end of the infusion and that this contributes to the termination of ingestive behavior. THE ROLE OF DOPAMINE: Background. Previous work in this laboratory has suggested that dopamine, cholecystokinin and glutamate interact in the inhibition of ingestive behavior at the level of the brainstem. Subsequently, it was discovered that dopamine D2 receptors are present in the rostral part of the nucleus of the solitary tract (NTS), a region in the brainstem that receives primary taste input. In addition to the brainstem, the nucleus accumbens has been implicated in the regulation of ingestive and taste-related behavior. A series of experiments including peripheral and central administration of dopamine D l and D2 receptor drugs was performed to examine the role of the brainstem and the nucleus accumbens in intake inhibition and aversive responses. Results. It was found that brainstem D2 receptors mediate suppression of consummatory ingestive behavior. Behaviors related to an aversive taste were also found to be mediated by D2 receptors but at a different neural site, the shell nucleus accumbens. However, none of these receptor populations are involved in taste evaluation. THE ROLE OF NPY: Background. NPY is often referred to as a powerful stimulant of food intake and leptin is referred to as an inhibitor. A series of experiments studying the effect of central administration of NPY and leptin on ingestive, taste-related and sexual behavior was performed. Results. NPY was found to stimulate appetitive ingestive behavior and, interestingly, decrease consummatory ingestive behavior. Leptin had the opposite effects. NPY suppressed sexual behavior in the presence of a sucrose-filled bottle, but had only a minor effect in the absence of a bottle and leptin facilitated sexual behavior. The suppression of consummatory ingestive behavior after NPY treatment was not blocked by an aversive taste that suppressed appetitive behavior. The NPY-induced suppression of sexual behavior was unaffected by blocking appetitive ingestive behavior. Furthermore, the effects of NPY on ingestive behavior is independent of taste evaluation since NPY treatment did not affect taste responses to a p at c or an aversive taste. SUMMARY. The taste of an ingested stimulus elicits aversive responses at the end of an intraoral infusion of sucrose. This increase in aversiveness contributes to the termination of ingestive behavior. Stimulation of D2 receptors in the brainstem inhibits ingestive behavior and stimulation of accumbal D2 receptors enhances the display of aversive taste-related behavior. While different consummatory behaviors can be displayed simultaneously, appetitive ingestive behavior interferes with the display of sexual behavior. NPY selectively stimulates appetitive ingestive behavior. Fat depletion and the ensuing decrease in leptin levels may be a peripheral stimulus for directing the attention of a starving animal (or human patient) towards food and to develop strategies to search for, although not necessarily eat, the food.