Studies of food preferences in infants and children have shown that early exposure to different flavors can lead to increased acceptance of and preferences for these flavors in later life (Liem and Mennella 2002
; Mennella and Beauchamp 2002
). As children are increasingly exposed to foods high in fat during early life, it is important to determine how exposure to certain diets during this time may affect food preferences during adulthood and be a possible contributing factor to the increased intake of energy-dense palatable foods. In the current study, we examined how exposure to a high fat diet during the periweaning period (3-4 wks of age), when mice are consuming solid food and are no longer dependent on the dam for nutrition, would affect adult macronutrient preferences, food intake, and weight gain.
In a 10-day macronutrient choice preference test, high fat diet early-exposed mice showed a significantly greater preference for a high fat diet as adults, measured as the proportion of total daily caloric intake. As a control for diet familiarity, mice exposed to the high carbohydrate diet during early life showed no differences in adult macronutrient preferences, suggesting that changes in adult preference are not simply a result of prior experience with the diet. Changes in the maternal diet have been associated with altered preferences for macronutrients, with both low protein and high fat diets increasing preference for high fat diet at early ages, although these differences lessen with age(Bellinger et al. 2004
; Kozak et al. 2005
). However, these manipulations occur during gestation and lactation when the brain is still developing and thus are unlikely to be responsible for the effects observed here. Interestingly, exposure to a novel sweet treat (Froot Loops cereal) from P22-27 has been shown to increase consumption of this item in adulthood (Silveira et al. 2008
). However, conclusions from this work further suggested that the changes in consumption were due more to the limited access provided and the novel environment in which the food was presented than to any change in the rats' inherent preference for it. By using a nutritionally complete, macronutrient-rich diet presented ad libitum in the home cage environment, we were able to assess changes in global dietary preferences. Because the timing of the diet presentation occurred to late in development, it is less likely that changes in neural wiring in feeding and reward circuits are responsible for the observed changes in behavior, and that other mechanisms, such as epigenetic changes, may be present.
Despite the increased proportional intake of the high fat diet observed in the early-exposed mice, there were no differences in total daily caloric intake or weight gain during the macronutrient choice preference period. Mice consuming more of the high fat diet compensated for the excess calories by reducing their intake of the other macronutrient-enriched diets, particularly the high carbohydrate diet. Overall, these results suggest that the impact of the early exposure is on preference alone, and not overall food intake or metabolism. It is possible that had the length of the macronutrient choice preference test been increased, differences in body weight and caloric efficiency would have emerged due to the more prolonged increase in intake of dietary fat. However, during the chronic high fat diet exposure, we did not observe differences between groups in intake, weight gain, or adiposity, further supporting an effect of early life exposure specific to dietary preference.
Mechanistically, we investigated the possible contributing factors for the increased dietary fat preference. The timing of the diet exposure in the current study made it unlikely that direct effects on the hypothalamus were responsible for the phenotype. The circuitry of the arcuate nucleus, the primary center governing food intake, is formed largely during the second week of life, with the connections resembling that of the adult animal by P18 (Bouret et al. 2004
). Expression of the main orexigenic and anorexigenic peptides, neuropeptide Y (NPY) and pro-opiomelanocortin (POMC), also change over the course of early postnatal development, reaching adult levels around the third week of life (Ahima and Hileman 2000
; Grove et al. 2003
; Leibowitz et al. 2005
). Arcuate neurons become responsive to leptin and ghrelin between two and four weeks after birth (Mistry et al. 1999
; Proulx et al. 2002
). Most studies on the effects of early nutrition in rodents involve dietary manipulations during gestation and/or lactation, in order to capitalize on this period of plasticity in the rodent hypothalamus. By the fourth week of life, when our high fat diet exposure was initiated, hypothalamic development is largely complete. However, there is some evidence for limited plasticity in the adult hypothalamus (Horvath 2005
; Kokoeva et al. 2005
). We cannot rule out the possible contribution of such changes to our end phenotype.
Preferences for palatable diets have been closely linked with reward systems, with intake of preferred foods having profound effects on dopamine (DA) release in the nucleus accumbens, and alterations in DA function leading to changes in feeding behavior(Blum et al. 2000
; Colantuoni et al. 2001
; Colantuoni et al. 2002
; Cagniard et al. 2006
). In addition, early nutritional manipulations or exposure to rewarding stimuli in rodents have been shown to affect the long-term functioning of the DA system (Sato et al. 1991
; Zippel et al. 2003
; Kelley and Rowan 2004
). We have previously reported that withdrawal from a high fat diet can have profound and long-lasting effects on the DA system (Teegarden and Bale 2007
; Teegarden et al. 2008
). Thus, in the current study we hypothesized that reward signaling might be altered in mice exposed to high fat diet during early life. To test this hypothesis, mice were sacrificed following chronic high fat diet exposure and markers of reward signaling in the ventral striatum were examined. We found that mice exposed to high fat diet during early life had significantly higher levels of the transcription factor ΔFosB in the ventral striatum following chronic high fat diet exposure in adulthood. ΔFosB is induced in the nucleus accumbens following chronic exposure to drugs of abuse and natural rewards (Nestler et al. 2001
; Teegarden and Bale 2007
; Wallace et al. 2008
). Mice overexpressing ΔFosB in dynorphin-positive accumbal medium spiny neurons show an increased motivation to obtain a food reward due to a basal dysregulation of DA signaling (Olausson et al. 2006
; Teegarden et al. 2008
). Our own work has shown that these mice are more vulnerable to high fat diet withdrawal and show dramatic changes in markers of DA signaling following high fat diet exposure (Teegarden et al. 2008
). We also observed a significant increase in cyclin-dependent kinase 5 (Cdk5) and dopamine and cAMP-regulated phosphoprotein, molecular weight 32 kDa (DARPP-32) phosphorylated at threonine 75, as well as a trend for a corresponding reduction of pDARPP-32 Thr 34. In the progression of signaling following reward experience and elevation of ΔFosB, levels of Cdk5 begin to rise (Bibb et al. 2001
). As a negative regulator of DA neurotransmission and neuronal excitability (Chergui et al. 2004
; Benavides et al. 2007
), Cdk5 phosphorylates DARPP-32 at threonine 75 (Bibb et al. 1999
). Interestingly, phosphorylation of DARPP-32 at this site attenuates D1 DA receptor activity via direct inhibition of protein kinase A and inhibits phosphorylation at Thr 34 (Benavides and Bibb 2004
). Overall, these biochemical measures are highly suggestive of a reduction in DA signal transduction in the striatum during high fat diet exposure in mice previously exposed to and then withdrawn from a high fat diet during early life. We hypothesize that the reduced DA signaling observed during high fat diet exposure likely contributes to the increased preference for high fat diet during macronutrient choice preference. During chronic high fat diet exposure, it is likely that intake is limited by total caloric consumption, and thus no behavioral differences were observed. Our data is in line with clinical reports that suggest reduced DA signaling in obese patients (Wang et al. 2001
). The increase in preference for high fat diet in adulthood may be a compensatory response by the organism to normalize dopaminergic tone (Blum et al. 2000
; Wang et al. 2004
; Teegarden et al. 2008
The mechanism behind these changes in dopamine signaling remains to be elucidated. It is important to note that changes in opioid signaling in the ventral striatum have also been closely linked to changes in palatable feeding and dopaminergic signaling. In particular, stimulation of the mu opioid receptor leads to a robust increase in intake of a diet high in fat (Zhang et al. 1998
), and exposure to a high fat diet can alter opioid signaling (Blendy et al. 2005
; Jain et al. 2004
). However, we observed no differences in levels of the mu opioid receptor in the striatum between control and early high fat diet exposed mice. While this does not rule out a role for mu receptor signaling or other opiodergic factors, our data indicate that the change in dietary preference is due to changes in dopamine signaling that are unrelated to changes in mu opioid receptor levels.
In the rat, dopamine neurons are born around embryonic day 12 (E12) and begin to extend processes at E13. Innervation of the striatum extends into the first postnatal week, and reorganization continues at least until the third postnatal week (Van den Heuvel and Pasterkamp 2008
). Thus, the dietary manipulation paradigm in the current study is not likely to alter the initial formation of the mesolimbic dopamine system. Changes in fatty acid levels during development and later life can also affect DA and DA receptor levels in the frontal cortex of adult rats (Delion et al. 1994
; Delion et al. 1996
; Zimmer et al. 1998
), and maternal consumption of high fat diet can alter the functioning of the DA system in adult offspring, possibly leading to desensitization of dopamine receptors (Naef et al. 2008
). Although the diets used in our present study contained a balanced variety of fatty acids, the possibility remains that subtle variations in dietary fat content may alter long-term DA signaling. In addition, direct developmental effects that may be observed in models of maternal diet manipulation are unlikely to be responsible for the current results due to the late timing of the diet exposure, suggesting that epigenetic mechanisms may play a role. Plasticity in the nucleus accumbens is also observed following treatment with drugs of abuse. Cocaine, nicotine and amphetamine increase spine density in this area (Robinson and Kolb 2004
). These changes last for months after the last drug exposure, and can be induced by only a single experience (Kolb et al. 2003
). We have previously shown that withdrawal from a high fat diet in adults produces changes in stress and reward pathways in mice (Teegarden and Bale 2007
). Therefore, it is possible that the brief exposure and withdrawal of this diet during early life produces similar effects that reprogram these circuits. Finally, another candidate for mediating long-term changes in gene expression is epigenetic regulation. Dietary manipulation could also lead to long-term programming of gene expression via changes in DNA methylation or histone acetylation. Changes in methylation of genes in the DA system have been linked to psychiatric and mood disorders as well as addiction (Abdolmaleky et al. 2008
; Hillemacher et al. 2008
). While these studies do not directly address the effects of a high fat diet on DA system plasticity, they raise the intriguing possibility that the functioning of this system may be altered long-term by a natural reward during early life. These mechanisms may be further investigated in future studies.
In conclusion, the present study demonstrates that a brief exposure to a palatable, high fat diet during early life programs an increased preference for this diet during adulthood that is not based on diet familiarity. Mechanistically, reduced DA signal transmission in the ventral striatum in these mice may result in an increased preference for the high fat diet in an attempt to normalize DA levels. The data then suggest that exposure to a palatable, high fat diet during early life may lead to long-term reprogramming of the reward system, leaving the organism at risk not just for maladaptive eating habits but perhaps also to other disorders of the reward system.