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UBC Theses and Dissertations

An investigation of the effect of restraint stress, carbohydrate restriction and serotonin on macronutrient intake in the female rat Price, Ingrid

Abstract

Serotonin (5-HT) influences food intake in humans and other species. Low levels of brain 5-HT increase carbohydrate (CHO) intake, whereas elevated levels of 5-HT decrease CHO intake. Stress influences both 5-HT activity and food intake. Two forms of stress were implemented in this dissertation to determine their effect on macronutrient intake (CHO, protein, fat) as well as the possible role of 5-HT in these effects. Research has revealed that varying the duration of restraint stress differentially influences food intake. These effects may be at least partially mediated through serotonergic mechanisms. Further, the deprivation state of the animal has been shown to alter 5-HT activity, and elevated brain 5-HT levels decrease CHO and to some extent fat intake in the rat. Experiments 1-3 (Chapter 2) investigated the effect of three different durations of restraint stress (20 min, 2 h and 2 h/day chronically for 5 days) on macronutrient intake in the female rat. Results indicated that all restraint durations inhibit fat and CHO intake following 4 h of food deprivation. However, 24 h of food deprivation attenuates the inhibition of CHO intake produced by either 20 min or chronic restraint but not that produced by 2 h of restraint. These data reveal an important interaction between the effect of restraint stress and food deprivation on the intake of specific macronutrients. Experiment 4 (Chapter 3) attempted to determine if the 5-HT1A agonist, 8-hydroxy-dipropylaminotetralin (DPAT), could counteract the inhibitory effect of 2 h of restraint stress on CHO intake. Rats were tested after 4 or 24 h of food deprivation. Restraint significantly inhibited CHO and fat intake after either 4 or 24 h of food deprivation, and protein intake was inhibited after 4 h but not 24 h of food deprivation. DPAT was able to counteract the effect of restraint stress on CHO intake when rats were food-deprived for 24 h prior to testing. DPAT had no effect on fat or protein intake, nor on CHO intake after 4 h of food deprivation. These results have implications for the effect of restraint stress on macronutrient intake and neurochemical systems. Experiments 5-8 (Chapter 4) investigated the impact of chronic CHOrestriction on macronutrient preference. Given the relationship between brain 5- HT and CHO, it is reasonable to assume that chronic CHO-restriction would reduce 5-HT activity, which could result in a preferential increase in CHO intake when a choice diet of CHO, protein or fat is offered. Initial experiments showed CHO over-consumption when rats were maintained on a diet restricted to 10% CHO for a 14 day period. CHO-restricted rats were injected with 0, 3 or 6 mg/kg fluoxetine (a 5-HT reuptake inhibitor) to determine whether an increase in 5-HT activity could attenuate the effect of chronic CHO-restriction on subsequent CHO intake. Results indicated that chronic CHO-restriction increased CHO intake while having no effect on protein or fat intake. Furthermore, 6 mg/kg fluoxetine attenuated the effect on CHO intake. The results of this study extend current research through determining the effect of restraint stress on macronutrient intake. Further, this study extends current findings in the dietary stress literature through employing a CHOrestriction procedure that does not restrict overall caloric intake. In addition, the experiments in this study were conducted with female rats. The vast majority of stress research to date has been conducted in male rats. Results from studies employing female rats may allow for stronger indications of the effect of stress on food intake in women that may have implications for eating disorders. The findings from these experiments are discussed in terms of potential mechanisms and implications for animal models of eating disorders and directions for future research.

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