Influence of early life and positive affect on feeding behaviour and food choice in the rat
Warnock, Amy Louise
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In recent years, worldwide obesity rates have risen dramatically, putting major strain on public health systems and the economy. Obesity is a multifaceted disease and its development can be influenced by a variety of factors including genetic, psychological and environmental influences. One area of current focus in obesity research is that of early life programming. It has been well-established that certain early life factors can impact the physiology and behaviour of the offspring. Because of this, early life programming has become increasingly well studied in order to develop a deeper understanding of how early life can influence obesity development. Another area of interest lies in positive mood. While there has been much research into the effects of negative states such as stress and anxiety on feeding behaviour, there is still very little known about how positive states can influence food choice. Using rat models of prenatal stress, neonatal overnutrition and positive affect, this thesis aimed to investigate the effects of early life and mood factors on feeding behaviour and food choice. Prenatal stress has been extensively studied and is characterised by an enhanced stress response in the offspring. Using two rat models of prenatal stress- social and restraint stress, the effects of prenatal stress on feeding behaviour and food choice in the offspring were examined. In both models, no effects of prenatal stress on either food intake or food choice were observed. However, in both cases the expected alterations to the offspring’s stress responses when exposed to an acute stressor were not replicated. This may suggest that models of prenatal stress are not as robust as often cited in the literature. As well as the prenatal environment, the early postnatal environment is also able to influence physiology and behaviour. In terms of obesity, a well-studied model is that of small litter size. Rats from small litters are over-nourished as neonates and because of this illustrate an increased body weight that persists throughout life. While this increase in weight gain has been well-established, there is no evidence examining the impact of neonatal overnutrition on long-term food choice. Therefore, food intake and food choice were measured in small and control litter rats over a 10-week period. When placed on an ad lib diet of bland chow, sucrose and lard, small litter rats consumed significantly more chow than control litter rats, whilst maintaining similar consumption of lard and sucrose. However, when offered a high-fat high-sugar (HFHS) pellet for two hours a day alongside ad lib chow, small litter rats illustrated increased consumption of the HFHS pellet compared to controls. This suggests that small litter rats may be programmed to adjust their food choices to enable them to maintain their increased body weight in comparison to controls. To examine the effects of positive affect on feeding behaviour, ultrasonic vocalisations (USVs, specifically those at 50 kHz) were used as a measure of positive affect in rats. In order to examine whether access to a food reward could induce a positive affect (as measured by an increase in 50 kHz USVs), rats were schedule-fed sweetened condensed milk and USVs measured before, during and after consumption. No differences in 50 kHz USVs were observed suggesting that a palatable food, whilst rewarding, does not alter affective state in the rat. Using heterospecific social contact (a tickling interaction simulating rough and tumble play) to induce positive affect, rats were presented with an hour-long sucrose preference test following social contact in order to examine the impact of positive affect on food choice. While no differences in sucrose consumption were found, a reduced sucrose preference was observed in rats receiving social contact compared to controls, suggesting that positive affect may play a role in mediating food choice. Finally, the effects of fasting (a negative stimulus thought to reduce 50 kHz USVs) and a food reward on motivation for social contact were examined. Both fasting and access to a food reward resulted in no differences in conditioned place preference to receive social interaction. Overall, the results obtained in this thesis implicate both neonatal overnutrition and, for the first time, positive affect as possible mediators of food choice, although further studies are required to fully establish these effects. Importantly, these results also raise questions regarding the reproducibility of some early life models, such as prenatal stress, and highlights the importance of sharing precise experimental protocols across laboratories. Through further investigation of the effects of early life and affective states on food consumption and choice, and the mechanisms behind these, this may enable the development of therapeutic interventions and preventative measures that can help slow, or even reverse, the global obesity epidemic.