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. 2008 Oct 20;586(Pt 24):5901–5910. doi: 10.1113/jphysiol.2008.159566

High-fat feeding alters the clock synchronization to light

Jorge Mendoza 1, Paul Pévet 1, Etienne Challet 1
PMCID: PMC2655413  PMID: 18936083

Abstract

High-fat feeding in rodents leads to metabolic abnormalities mimicking the human metabolic syndrome, including obesity and insulin resistance. These metabolic diseases are associated with altered temporal organization of many physiological functions. The master circadian clock located in the suprachiasmatic nuclei controls most physiological functions and metabolic processes. Furthermore, under certain conditions of feeding (hypocaloric diet), metabolic cues are capable of altering the suprachiasmatic clock's responses to light. To determine whether high-fat feeding (hypercaloric diet) can also affect resetting properties of the suprachiasmatic clock, we investigated photic synchronization in mice fed a high-fat or chow (low-fat) diet for 3 months, using wheel-running activity and body temperature rhythms as daily phase markers (i.e. suprachiasmatic clock's hands). Compared with the control diet, mice fed with the high-fat diet exhibited increased body mass index, hyperleptinaemia, higher blood glucose, and increased insulinaemia. Concomitantly, high-fat feeding led to impaired adjustment to local time by photic resetting. At the behavioural and physiological levels, these alterations include slower rate of re-entrainment of behavioural and body temperature rhythms after ‘jet-lag’ test (6 h advanced light–dark cycle) and reduced phase-advancing responses to light. At a molecular level, light-induced phase shifts have been correlated, within suprachiasmatic cells, with a high induction of c-FOS, the protein product of immediate early gene c-fos, and phosphorylation of the extracellular signal-regulated kinases I/II (P-ERK). In mice fed a high-fat diet, photic induction of both c-FOS and P-ERK in the suprachiasmatic nuclei was markedly reduced. Taken together, the present data demonstrate that high-fat feeding modifies circadian synchronization to light.

Current knowledge concerning the rhythmic aspects of energy homeostasis and food intake is rising (Mendoza, 2007). This is an area of great importance to human health because metabolic diseases like obesity and diabetes are associated with altered temporal organization of many physiological functions (Van Cauter et al. 1997; Bray & Young, 2007; Hastings et al. 2007). Energy metabolism and circadian rhythmicity are two systems influencing one another (Rutter et al. 2002; Kennaway et al. 2007; Kohsaka et al. 2007; Lavialle et al. 2008). On the one hand, the master circadian clock located in the suprachiasmatic nuclei of the hypothalamus controls a number of physiological functions and metabolic processes (Hastings et al. 2007). The daily light–dark cycle is the dominant synchronizer of the suprachiasmatic clock which receives photic information directly from the retinohypothalamic tract (Meijer & Schwartz, 2003). Such a temporal regulation of the suprachiasmatic clock is now considered to imply the synchronization of circadian oscillators contained in most peripheral organs like liver, heart or white adipose tissue. These peripheral oscillators are thought to play a critical role in tissue-specific physiology (Schibler et al. 2003). On the other hand, nutritional and hormonal cues are potent synchronizers of peripheral oscillators (Schibler et al. 2003). Under certain conditions of feeding (hypocaloric diet), metabolic cues are capable of altering the master circadian clock, as well as its circadian responses to light (Challet et al. 1997; Mendoza et al. 2005; Resuehr & Olcese, 2005; Mendoza et al. 2007). Altered day–night patterns of behaviours and hormones in high-fat-fed rodents exposed to light–dark cycles (Kohsaka et al. 2007; Cano et al. 2008) raises the possibility that high-fat feeding (hypercaloric diet) affects the mechanisms of photic synchronization. To test this hypothesis, the rate of re-entrainment after shifted light–dark cycles as well as the behavioural and cellular responses to light pulses were studied in mice fed with high-fat or chow (i.e. low-fat) diet.

Methods

Animals, housing and diet

Male 4-week-old C57BL/6J mice (Charles River Laboratories, L’arbresle, France) were housed in individual cages with running wheels, kept at 21 ± 1°C under a 12h : 12 h light–dark cycle (LD, lights on at 07:00 h) with food ad libitum (low-fat diet, 105, SAFE, Augy, France) and tap water for 2 weeks after surgery (see below). Mice were then divided into two groups (n = 16): the first group was maintained on the control, low-fat, pelleted diet (105; 12.6 kJ g−1; SAFE; distribution of metabolizable energy content as percentage: 23% protein, 65% carbohydrate and 12% fat), while the second group received a high-fat, pelleted diet (19.7 kJ g−1, SAFE; energy content distribution as percentage: 17% protein, 30% carbohydrate and 53% fat, including 6% from corn oil and 47% saturated fat from lard). This high-fat diet enriched in saturated fat has been previously used as an obesogenic food in rats (Sinitskaya et al. 2007), and is very close in composition to many other high-fat diets known to produce abdominal obesity and insulin resistance in C57BL/6J mice (e.g. Williams et al. 2003; Winzell & Ahrén, 2004; Kohsaka et al. 2007). Body mass and food intake were measured weekly. All experiments were performed in accordance with the rules of the European Committee Council Directive of November 24, 1986 (86/609/EEC) and the French Department of Agriculture (licence no. 67-88 to E.C.). Telemetry recording, E-mitter telemetry devices (MiniMitter Co., Sunriver, OR, USA) measuring body temperature and general motor activity were implanted intraperitoneally under gaseous anaesthesia (2% isoflurane in O2/N2O (50 :50)). Data were recorded every 5 min (Vitalview, MiniMitter).

Experimental design

Two weeks after surgery, the diet was changed to high-fat food for half of the mice as mentioned above. During 3 weeks of baseline, high-fat- and low-fat- (control) fed mice were maintained under a fixed LD (lights on at 07:00 h). Then mice were exposed to two ‘jet-lag tests’ in each direction (advance and delay). Thereafter, mice were challenged with light pulses in constant darkness. Finally, mice were re-entrained to LD before sampling after light exposure at night (Fig. 1).

Figure 1.

Figure 1

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Time schedule of feeding and lighting conditions over the 3 months of the experiment.

‘Jet-lag’ tests

After baseline, mice were exposed to a 6 h phase advance of LD (first ‘jet-lag’ test). On the day of the shift, time of lights-on was phase-advanced by 6 h, so that the dark period was shortened to 6 h. After 2 weeks under the new LD (12 : 12; lights on at 01:00 h), mice were exposed to a 6 h phase delay of LD (second ‘jet-lag’ test). On the day of the shift, time of lights-on was delayed by 6 h, so that the dark period was lengthened to 18 h. Mice were thus exposed again to the initial LD (12 : 12; lights on 07:00 h) for 2 weeks (Fig. 1).

Light pulses

Thereafter, mice were maintained under constant darkness conditions. Ten days later, animals were exposed to a 30 min fluorescent white light pulse (200 lx recorded at the level of the animals) at circadian time (CT) 13 (CT12 being defined as the activity onset) or CT22 (n = 8 per group and CT; Fig. 1), these two CTs being known to produce subsequent phase delays and advances, respectively, in mice (Schwartz & Zimmerman, 1990; Mendoza et al. 2005).

Ten days later, animals were re-entrained to LD for 2 weeks (Fig. 1). At light offset, food was removed to avoid any immediate effect of feeding on blood glucose or insulin. At projected CT13 or CT22, subgroups of mice were exposed to a 30 min light pulse. Light-exposed mice (4 low-fat-fed mice and 4 high-fat-fed mice per time point) were killed in darkness 1 h after the beginning of the light pulse. Dark control animals (4 low-fat-fed mice and 4 high-fat-fed mice per time point) were killed at the same time in darkness. After cervical dislocation and decapitation, trunk blood was collected, and brains were removed and fixed in 4% paraformaldehyde.

Acute expression of c-FOS, the phosphoprotein product of the immediate early gene c-fos, can be induced within suprachiasmatic cells in response to light pulses at night. Interestingly, light-induced phase shifts have been correlated with light-induced stimulation of c-FOS immunoreactivity in the suprachiasmatic clock (Aronin et al. 1990; Kornhauser et al. 1990; Vindlacheruvu et al. 1992). Moreover, the ERK-MAPK pathway, which leads to phosphorylation of the extracellular signal-regulated kinases I/II (P-ERK), is activated in the suprachiasmatic clock in response to exposure to light at night, suggesting its involvement in the mechanisms of photic resetting (Butcher et al. 2003; Coogan & Piggins, 2003; Hainich et al. 2006). For those reasons, we studied light-induced expression of c-FOS and P-ERK in the suprachiasmatic nuclei (SCN) of low- and high-fat- fed mice.

Glucose and hormonal assays

Blood glucose was immediately determined (Glucotrend, Roche Diagnostics, Meylan, France).

Plasma insulin was determined by an ELISA kit for mice and rats (EZRMI-13K, Linco Research, Inc., St Charles, MO, USA). The limit of sensitivity of insulin assay was 0.2 ng ml−1.

Plasma leptin was determined by an ELISA kit for mice (EZML-82K, Linco Research). The limit of sensitivity of leptin assay was 0.05 ng ml−1.

Plasma corticosterone was assayed with a commercial 125I RIA kit for mice and rats (ImmuChem Double Antibody, MP Biomedicals, Orangeburg, NY, USA). The limit of sensitivity of corticosterone assay was 7.7 ng ml−1.

Homeostatic model assessment (HOMA) was used to assess β-cell function and insulin resistance (ir), and calculated as follows: HOMA-ir = (Fasting blood glucose × Fasting plasma insulin)/22.5 as described by Matthews et al. (1985).

Immunohistochemistry

Brains were removed and fixed in 4% paraformaldehyde solution in 0.1 m phosphate buffer for 24 h, then cryoprotected in 30% sucrose for 48–72 h. Brains were quickly frozen with dry ice, and 30 μm coronal sections were cut through the extent of the SCN. Free-floating sections were processed for c-FOS (rabbit polyclonal antibody, dilution 1 :5000; Santa Cruz Biotechnology, Santa Cruz, CA, USA) and P-ERK (rabbit polyclonal antibody, dilution: 1 :20 000; Cell Signaling Technology, Inc., Beverly, MA, USA) immunostaining followed by biotinylated goat anti-rabbit (90 min; 1 :500; Vector Laboratories, Burlingame, CA, USA), then ABC solution (90 min; 1 :300; Vectastain Elite ABC Kit, Vector) before substrate visualization using 3,3′-diaminobenzidine (Sigma). The staining optical density of immunoreactivity was counted using NIH ImageJ software.

Data analysis

Daily rhythms of wheel-running activity and body temperature under LD were analysed with ClockLab software (Actimetrics, Evanston, IL, USA) using the ‘Activity profile’ function, and graphs were generated with SigmaPlot (Jandel Scientific, Chicago, IL, USA). In constant darkness, endogenous period and circadian onsets of locomotor activity and circadian increases in body temperature were calculated using, respectively, the ‘Periodogram’ and ‘Actogram’ functions of ClockLab. The phase shift was calculated as the difference between two lines fitted to circadian onsets before and after treatment, considering, respectively, 8 cycles before and after light exposure.

The rate of re-entrainment to a new LD, assessed by two observers blind to the treatment, was defined as the number of days necessary for the animal to present activity onsets/offsets at fixed phases relative to lights offset/onset, respectively, and a duration of activity period similar to baseline values.

Statistical analysis

Data are presented as mean ±s.e.m. Student's t test or ANOVA followed by post hoc Scheffé's test were used to compare two or more groups, respectively.

Results

Metabolic changes

Besides a significant body mass gain (Fig. 2A), the physiological abnormalities of lipid metabolism induced by chronic high-fat feeding included increased abdominal fat stores and greater body mass index, as well as hyperleptinaemia, compared with respective values in animals fed a low-fat diet (Fig. 2B–D). In addition, high-fat-fed mice displayed altered carbohydrate metabolism, as evidenced by higher blood glucose, increased insulinaemia and large values of homeostatic model assessment and insulin resistance (HOMA-ir), indicative of compromised β-cell function (Fig. 2E–G). Besides early late night variations, only a trend for reduced plasma corticosterone was visible after 3 months of high-fat feeding (Fig. 2H).

Figure 2.

Figure 2

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Pooled data of body mass gain (A), body mass index (B), mass of white adipose (epididymal) tissue (C) and homeostatic model assessment of insulin resistance (HOMA-ir; D) in mice fed with a low- or high-fat diet (n = 16 per feeding condition). Blood glucose (E), plasma insulin (F), leptin (G) and corticosterone (CORT; H) in mice fed with low- or high-fat diet sampled at projected circadian times (CT) 13 or 22 (n = 7–8 per CT and feeding condition), data for ‘dark’ and ‘light’ groups being pooled. *P < 0.05 between feeding conditions for a given CT.

In mice held under light–dark conditions, high-fat feeding led to altered temporal organization of behaviour and physiology. First, levels of locomotor activity and thermogenesis during the early night were decreased in high-fat-fed mice compared with the group fed with a control diet containing low fat, while nocturnal offsets still occurred before dusk in both groups (Fig. 3A and B). As a consequence, nocturnal values of wheel-running activity were significantly lower after chronic high-fat feeding in comparison to data from the low-fat diet (6156 ± 865 versus 3440 ± 443 rev, respectively; P < 0.05). Second, there were higher daytime values of body temperature in mice fed a high-fat diet compared with individuals fed a low-fat diet (Fig. 3C and D). Thus, the diurnal rhythms of locomotor activity and temperature were dampened in high-fat-fed mice.

Figure 3.

Figure 3

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Daily profiles of wheel-running activity (A and B) and body temperature (C and D) in mice during baseline (low-fat diet; A and C) and during high-fat (◊; n = 16) or low-fat feeding (•; n = 16). *P < 0.05 between feeding conditions for the time points below the horizontal lines.

‘Jet-lag’ tests

Mice were exposed to two ‘jet-lag’ protocols, involving first a 6 h phase advance and then a 6 h phase delay of LD. Both high- and low-fat groups display similar transient cycles to be resynchronized after a delay shift of LD (6.7 ± 0.3 versus 7.0 ± 0.3 days, respectively, for body temperature rhythm; not significant (NS); and 5.9 ± 0.2 versus 6.4 ± 0.3 days, respectively, for locomotor activity rhythm; NS; Fig. 4A–D, F and H). By contrast, high-fat feeding slowed by 20% the resynchronization of temperature rhythms to an advanced LD shift, compared to mice fed a control diet (8.1 ± 0.4 versus 6.7 ± 0.4 days, respectively; P < 0.05; Fig. 4C, D and G). Albeit a trend for a longer resynchronization period of wheel-running activity rhythm in the high-fat group compared to control diet group was visible, the difference did not reach the threshold of significance (7.9 ± 0.4 versus 6.9 ± 0.3 days, respectively; P = 0.07; Fig. 4A, B and E). Altered rate of resynchronization can be due to changes in the endogenous period and/or in phase-shifting responses of the light-entrainable master clock.

Figure 4.

Figure 4

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Daily double-plotted rhythms (A–D) and rate of re-entrainment (E–H) of wheel-running activity (A and B, and E and F) and body temperature (C and D, and G and H) in mice fed a low-fat (A and C, and black bars in panels E–H; n = 16) or high-fat diet (B and D, and grey bars in panels E–H; n = 16) and successively challenged by 6 h advance (1st arrow) and 6 h delay of the light–dark cycle (2nd arrow). *P < 0.05 between feeding conditions.

Endogenous period and light-induced phase shifts

To determine the endogenous period, mice were housed in constant darkness. In these conditions, the daily onset of locomotor activity defined the circadian time (CT) 12 for each animal. Free-running rhythms of body temperature and wheel-running behaviour in high-fat fed mice were longer than those in control mice fed with the low-fat diet (24.0 ± 0.02 versus 23.9 ± 0.01, 24.0 ± 0.2 versus 23.8 ± 00.02 hours, respectively; P < 0.05; Fig. 5A–D).

Figure 5.

Figure 5

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Light-induced phase shifts in circadian rhythms of wheel-running activity (A–D and I) and body temperature (E–H and J) in low-fat- (A, C, E and G, and black bars in I and J; n = 16) or high-fat-fed mice (B, D, F and H, and grey bars in I and J; n = 16) housed in constant darkness. Circadian time (CT) 12 being defined as the activity onset, mice were exposed to a light pulse at CT13 (A and B, E and F, and I and J) or CT22 (C and D, G and H, and I and J). *P < 0.05 between feeding conditions for a given time point.

To investigate mechanisms of photic resetting, mice held in constant darkness were exposed to single pulses of light in the early and late parts of the active period, at two different circadian times (i.e. CT13 and CT22). Of particular relevance is that light-induced phase advances after light exposure at CT22 were significantly reduced in high-fat-fed mice compared with control animals. This significant effect was detected not only for wheel-running activity (−41%), but also for the body temperature rhythm (−35%; Fig. 5I and J). Irrespective of the circadian rhythm considered, namely wheel-running activity or body temperature, light-induced phase delays after light exposure at CT13 were comparable in high- or low-fat-fed mice (Fig. 5I and J).

Cellular responses to light

Induction of c-FOS expression was observed in the SCN of both low- and high-fat-fed mice exposed to nocturnal light pulses. At projected CT13 and CT22, however, the magnitude of this molecular response in high-fat-fed mice was, respectively, 63 and 46% lower than in mice fed with the control diet (P < 0.05; Fig. 6). Interestingly, these reductions were more marked in ventral (i.e. retino-recipient) suprachiasmatic cells.

Figure 6.

Figure 6

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c-FOS and P-ERK responses to light exposure at night in the suprachiasmatic nuclei of mice fed a low- or high-fat diet. Mice were exposed (‘Light’; n = 4) or not exposed (‘Dark’; n = 4) to a light pulse at CT13 or CT22. *P < 0.05 between feeding conditions, #P < 0.05 between dark and light conditions. Scale bar = 500 μm. ROD: relative optical density.

In mice fed with the low-fat diet, light at CT13 or CT22 markedly activated P-ERK expression in the whole SCN. In sharp contrast, high-fat-fed mice showed no significant induction of P-ERK after light exposure at CT13 and a 50% reduction of photic induction at CT22, compared with mice fed with the low-fat diet (P < 0.05; Fig. 6).

Discussion

The functional relationships linking energy metabolism and circadian rhythmicity start to be elucidated (Bray & Young, 2007; Hastings et al. 2007; Mendoza, 2007). An important role of the circadian timing system is to maintain an adequate phase adjustment to daily changes in ambient light. Metabolic cues under certain (i.e. hypocaloric) feeding conditions, as induced by chronic food shortage, have been shown to modulate circadian synchronization to light (Mendoza et al. 2005, 2007). At the opposite end of the scale, we demonstrate here that in mice exposed to hypercaloric conditions for 3 months, the photic regulation of the circadian system can be altered by eating a diet enriched in saturated fat. As expected, this high-fat feeding was obesogenic (i.e. it led to abdominal adiposity and hyperleptinaemia) and diabetogenic (i.e. it led to hyperglycaemia and hyperinsulinaemia).

The present findings confirm changes in the circadian system in response to high-fat feeding, as functionally illustrated by a lengthening of the free-running period (Kohsaka et al. 2007; this study). Furthermore, the present data reveal other functional alterations of the main circadian clock due to chronic high-fat feeding, all related to impaired adjustment to local time by photic resetting. These alterations include slower rate of re-entrainment after jet-lag, reduced resetting responses to light, and lower photic induction of two regulatory proteins in the suprachiasmatic clock: c-FOS and P-ERK.

In the present context, it is interesting to mention that mutation of the clock gene Clock has been associated with metabolic disturbances leading eventually to obesity, although the direction and amplitude of the effects largely differ according to the genetic background of the mice (Turek et al. 2005; Oishi et al. 2006; Kennaway et al. 2007). Genetic (e.g. Zucker) and dietary types of obesity share fat overload and hyperleptinaemia. Genetically obese Zucker (fa/fa) rats have a missense mutation in the leptin receptor gene (Cusin et al. 1996). Because obese Zucker rats display free-running periods similar to lean littermates (Mistlberger et al. 1998), the longer endogenous periods in mice fed with high-fat diet (Kohsaka et al. 2007; this study) may not be linked directly to leptin receptor signalling. Nevertheless, this circadian change highlights the fact that functioning of the suprachiasmatic clock in constant darkness is sensitive to nutritional cues mediated by high-fat feeding. In vitro binding of CLOCK/BMAL1 to DNA is modulated by the redox state (Rutter et al. 2001). Furthermore, the cellular redox state can be changed by glucose availability and possibly by insulin signalling (Abou-Seif & Youssef, 2001; Xu et al. 2002; Shi & Liu, 2006). The present study shows increased blood glucose levels in high-fat fed mice, as compared to animals fed with a low-fat diet. If high-fat feeding modifies the redox state of suprachiasmatic cells, daily changes in binding activity of CLOCK/BMAL1 to E-boxes of clock genes may result in slower circadian oscillations and lengthening of the endogenous period.

High-fat feeding in mice under LD does not result in clear effects on daily timing of rhythms of temperature and locomotor activity (i.e. the nocturnal onsets matched well in the two, high- or low-fat-feeding conditions), albeit that it alters their day–night organization. Importantly, when challenged by a 6 h advanced LD (i.e. jet-lag protocol), mice fed with high-fat diet took one full day more to be resynchronized to the new LD compared to animals fed with low-fat pellets. The significant but small increase in the free-running period (i.e. 12 min per day) due to high-fat feeding cannot explain by itself such a difference. In the present study, we used a classical protocol of jet-lag (e.g. Reddy et al. 2002) that has the advantage of mimicking transmeridian flights (in our study, across 6 times zones). On the other hand, it does not allow us to fully discriminate between altered synchronization to light and alterations of masking responses to light (i.e. direct inhibitory effects of bright light on motor activity in mice). To do so, further experiments will use acute shifts of the LD followed by transfer to constant darkness to avoid masking to light, as previously described (Vansteensel et al. 2003).

Here we also investigated the phase-shifting effects of light pulses applied in constant darkness. In keeping with comparable rates of re-entrainment to a 6 h delayed LD, mice chronically fed with a low- or high-fat diet displayed light-induced phase delays of similar magnitude. Moreover, high-fat feeding was associated with a clear reduction in light-induced phase advances, thus providing a likely explanation for the slower rate of re-entrainment to a 6 h advance of LD.

It is interesting to note that a hypocaloric diet speeds up re-entrainment to shifted LD (Resuehr & Olcese, 2005) while high-fat feeding slows it (this study). Moreover, the metabolic modulation of circadian responses to light produces opposite effects between chronic hypo- and hyper-caloric diets, leading to larger and lower light-induced phase advances, respectively, while light-induced phase delays are unaffected in both feeding conditions (Mendoza et al. 2005; this study). A hypercaloric diet may lead to a chronically decreased sensitivity of the suprachiasmatic clock to light resetting properties. If this were the case, one would then expect both light-induced phase delays and advances to be affected. Alternatively, because these two categories of circadian shifts rely on different signalling pathways (Gillette & Mitchell, 2002), the intracellular cascades underlying light-induced phase advances might be more sensitive to high-fat-induced changes. The fact that only light-induced phase advances are reduced prompts us to speculate that some chronobiotic molecules may specifically modulate photic resetting in late night. Among putative circulating molecules, metabolic hormones such as leptin or insulin are possible candidates, since receptors to both hormones are expressed in the suprachiasmatic clock (Unger et al. 1989; Hakansson et al. 1998).

Leptin, which is secreted from adipose tissue, plays a critical role in the regulation of energy balance (Ahima & Osei, 2004). In accordance with increased abdominal fat stores, levels of plasma leptin are high in high-fat-fed mice. Furthermore, leptin displays the unusual property of inducing phase advances of the suprachiasmatic clock in vitro during most of the circadian cycle (Prosser & Bergeron, 2003). Considering additive shifting effects during photic resetting at night, light-induced phase delays would be smaller or absent (due to the sum with ‘leptinergic’ phase advances), while light-induced phase advances would be larger (due to the sum with ‘leptinergic’ phase advances). The observed resetting responses to light during high-fat feeding do not support these predictions, making it unlikely that leptin plays a critical role in this context.

Insulin secreted by the pancreas is another putative chronomodulating hormone, as its injection in the suprachiasmatic region in vivo has functional consequences (Sakaguchi et al. 1988), and insulin-induced hypoglycaemia reduces photic resetting (Challet et al. 1999). In mice fed with a high-fat diet, we observed large levels of plasma insulin. In comparison with shifting responses to light during high-fat feeding, it is worth mentioning that a very close phase dependence in the modulating effects of insulin on photic phase resetting was obtained previously in mice fed with low-fat diets, including no modulatory effect on light-induced phase delays and significant attenuation of light-induced phase advances (Challet et al. 1999). Further studies are therefore needed to test whether behavioural responses to light in high-fat-fed mice could be normalized by lowering plasma insulin to control levels. Moreover, the possible roles of other metabolic molecules (e.g. free fatty acids, ghrelin or neuropeptide Y) in the reduction of photic entrainment in the high-fat-diet-fed mice remain to be tested.

To investigate the molecular mechanisms associated with altered synchronization of the suprachiasmatic clock during high-fat feeding, we quantified the expression of c-FOS and P-ERK, two functional markers of light-activated signal transduction pathways (e.g. Aronin et al. 1990; Coogan & Piggins, 2003). In high-fat-fed animals, photic induction of P-ERK expression during late night was significantly reduced in the SCN while light induction in early light was similar to low-fat conditions. These effects were closely mirrored with light-induced shifts of locomotor activity rhythm. For c-FOS expression, light-induced expression was reduced in both early and late night, while the shifting responses to light were only significantly decreased in late night. Under certain circumstances (i.e. photic–non-photic interactions), dissociation between light-induced c-FOS expression in the SCN and behavioural phase shifts has been already described (Edelstein et al. 2003), suggesting that P-ERK expression may be a more reliable cellular marker of phase resetting. With respect to c-FOS expression, it is interesting to highlight the fact that the reduced molecular response to light during high-fat feeding was more obvious in the ventral (i.e. retino-recipient) region than in the dorsal region of the SCN. Thus, the ventral suprachiasmatic region in which vasoactive intestinal polypeptide (VIP) is expressed (Hastings et al. 2007) could be the specific target for the chronobiotic effect of high-fat feeding.

Diet-induced obesity in humans is one of the major current health diseases. In mice, high-fat feeding for only 3 months led to abdominal obesity and altered glucose metabolism, confirming that feeding this diet in animals closely mimics the human metabolic syndrome. Circadian disturbances can have deleterious impacts on metabolic health, and vice versa (Bray & Young, 2007; Hastings et al. 2007). As a consequence, we suggest that reduced circadian responses to light in mice fed with a high-fat diet can be clinically relevant. Disruption of circadian timing during jet-lag and rotational shift work can impair both performance and trigger chronic health problems. In particular, it has been proposed that the prevalence of metabolic disturbances can be increased by repeated shift work (Karlsson et al. 2003), leading to the concept of ‘chronobesity’. In view of the altered synchronization to light in high-fat-fed animals exposed to a fixed LD cycle (present study) and the putative obesogenic environment due to repeated LD shifts (Tsai et al. 2005; Bartol-Munier et al. 2006), management of circadian disorders such as jet-lag and rotational shift work might be even more important in obese people. Therefore, efficient preventive and/or chronotherapeutic actions, such as chronobiotic drugs, should be developed to maintain/restore a normal synchronization to local time.

Acknowledgments

We thank Dr Dominique Sage and Stéphanie Dumont for expert help with hormonal assays and Dr David Hicks for English revision. This work was supported by CNRS and University Louis Pasteur (E.C. and P.P.), and postdoctoral fellowships from the Fondation pour la Recherche Médicale (J.M.) and the Institut Servier (J.M.).

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