Orbitofrontal cortex function: the behavioural and molecular consequences of exposure to sugar and/or caffeine in rats

Caffeine is a plant-derived psychostimulant that is frequently consumed with high levels of sugar. Almost all children will be exposed to caffeinated products with the highest intakes observed in adolescence, when daily consumption of caffeinated products generally begins. This research was prompted by the possibility that drinking sugar sweetened-beverages as well as the increased availability of highly caffeinated, high sucrose ‘energy’ drinks could place some people at risk of being exposed to high levels of both caffeine and sugar. There is a considerable body of research that has examined how caffeine and sugar consumption influences physical health; however few studies have specifically compared the separate and combined effects of these commonly consumed substances. Furthermore it remains to be determined whether the age at which regular consumption begins has different consequences for future behavioral and neural function. This thesis presents a series of behavioral and proteomic studies that explored the consequences of drinking sugar-sweetened and or caffeinated water in adult and adolescent Sprague Dawley rats. Locomotor behavior was used as a measure of the psychostimulatory effect of treatment and as an indicator of persistent neurological change. Drugs of abuse disrupt natural reward responses with early exposure to psychostimulants modifying the risk of developing a substance abuse disorder, thus several key behaviors associated with addiction were examined. The molecular studies focused on the orbitofrontal cortex (OFC) as this brain area mediates the hedonistic response to sweet and bitter tastes; moreover it integrates gustatory and olfactory information to facilitate taste reward learning. Additionally the OFC plays a critical role in a number of cognitive and decision making processes that are known to mediate the development of substance abuse disorders. This research found extended exposure to caffeine and or sugar results in age and treatment specific modification to behavior, locomotor response to an acute methamphetamine (METH) challenge and OFC protein expression. When treatment free the adult treated caffeine and sugar treated rats were hyperactive, additionally the caffeine treated animals did not habituate to testing conditions. These behavioral differences could reflect reduced OFC function, as rats with lesions to this brain area have similar behavioral deficits. Interestingly when sugar and caffeine are combined this hyperactivity was prevented. However label free shotgun proteomic analysis found unique patterns of protein expression in the OFC were produced by sugar and/or caffeine treatment. Pathway analysis revealed that each treatment, via different mechanisms, may have implications for mitochondrial function and response to cellular stress and hence for the pathophysiology associated with neurological disease. Intriguingly there was evidence of some potential mechanisms that may explain the epidemiological findings that regular caffeine consumers have a lower risk of developing certain neurological diseases. A comparison of consumption behavior in the adult and adolescent animals provided with high and low dose caffeine, alone or in combination with sugar, suggests early exposure to caffeinated sugar-sweetened beverages could increase the risk of excess consumption later in life. Adolescent exposure to sugar and or caffeine had different consequences for the OFC mediated behaviors when measured treatment free during adulthood. Exposure to sugar during adolescence increased novelty seeking. Caffeine exposure decreased social interaction however adding sugar to caffeine increased this behavior. Finally, drinking caffeinated sugar-sweetened water through adolescence increased cue reactivity in adulthood. This suggests early regular exposure to caffeinated and or sugar sweetened beverages may have treatment specific long-term consequences for adult behavioral control. One of the key findings of this study was that sugar exposure during adolescence can increase sensitivity to a low dose methamphetamine (METH) challenge, however this effect was found only after an extended treatment free period. None of the treatments changed sensitivity to METH when the animals were challenged after a short washout regardless of age at the time of treatment. Proteomic analysis of the OFC of the adolescent treated animals at 6 weeks treatment free and 24-hours after the METH challenge found evidence that prior exposure to sugar can alter expression of proteins that are implicated in the development of behavioral sensitization to psychostimulants. There were treatment specific differences to proteins that mediate glutamatergic and GABAergic signalling. These different patterns of expression may have consequences for excitatory and inhibitory signalling with implications for OFC activation and connectivity to other brain areas, including mediating the sensitized locomotor response to METH measured in the sugar treated animals. The research of this thesis found that sugar and or caffeine had age and treatment specific consequences for behavior. The proteomics studies are exploratory by nature thus cannot make definitive statements about how caffeine and or sugar might modify the risk of developing any specific neurological disease or alter resilience to addiction, rather these results highlight the importance of diet in maintaining neural function, and reveal important pathways and proteins that can be affected by a diet high in sugar and or caffeine.