The food entrainable oscillator studied by DNA microarrays: What is the liver doing during food anticipatory activity?

Diurnal and nutritional adjustments of intracellular Ca2+ release channels and Ca2+ ATPases associated with restricted feeding schedules in the rat liver

Background

Intracellular calcium is a biochemical messenger that regulates part of the metabolic adaptations in the daily fed-fast cycle. The aim of this study was to characterize the 24-h variations of the liver ryanodine and IP₃ receptors (RyR and IP₃R) as well as of the endoplasmic-reticulum and plasma-membrane Ca²⁺-ATPases (SERCA and PMCA) in daytime restricted feeding protocol.

Methods

A biochemical and immunohistochemical approach was implemented in this study: specific ligand-binding for RyR and IP₃R, enzymatic activity (SERCA and PMCA), and protein levels and zonational hepatic-distribution were determined by immunoblot and immunohistochemistry respectively under conditions of fasting, feeding, and temporal food-restriction.

Results

Binding assays and immunoblots for IP₃R1 and 2 showed a peak at the light/dark transition in the ad-libitum (AL) group, whereas in the restricted-feeding (RF) group the peak shifted towards the food-access time. In the case of RyR binding experiments, both AL and RF groups showed a modest elevation during the dark period, with the RF rats exhibiting increased binding in response to feeding. The AL group showed 24-h rhythmicity in SERCA level; in contrast, RF group showed a pronounced amplitude elevation and a peak phase-shift during the light-period in SERCA level and activity. The activity of PMCA was constant along day in both groups; PMCA1 levels showed a 24-h rhythmicity in the RF rats (with a peak in the light period), meanwhile PMCA4 protein levels showed rhythmicity in both groups. The fasted condition promoted an increase in IP₃R binding and protein level; re-feeding increased the amount of RyR; neither the activity nor expression of SERCA and PMCA protein was affected by fasting–re-feeding conditions. Histochemical experiments showed that the distribution of the Ca²⁺-handling proteins, between periportal and pericentral zones of the liver, varied with the time of day and the feeding protocol.

Conclusions

Our findings show that RF influences mainly the phase and amplitude of hepatic IP₃R and SERCA rhythms as well as discrete zonational distribution for RyR, IP₃Rs, SERCA, and PMCA within the liver acinus, suggesting that intracellular calcium dynamics could be part of the rheostatic adaptation of the liver due to diurnal meal entrainment/food entrained oscillator expression.

Restricted feeding improves postischemic recovery of Langendorff-perfused rat hearts

The goal of the present study was to assess the effects of a restricted feeding schedule (RFS) on postischemic contractile recovery in relation to triacylglycerol (TAG), glycogen, and ATP content. Glucose-6-phosphate dehydrogenase (G6PDH) activity, reduced/oxidized glutathione ratio (GSH/GSSG), and thiobarbituric acid reactive substances (TBARS) levels were also determined. Isolated rat hearts entrained to daily RFS (2 h food access starting at 1200) or fed ad libitum (FED) for 3 weeks were Langendorff-perfused (25 min ischemia, 30 min reperfusion) with Krebs-Ringer bicarbonate solution (10 mmol/L glucose). RFS improved the recovery of contractility and reduced creatine kinase (CK) release upon reperfusion. Further, at the end of reperfusion, RFS hearts exhibited increased G6PDH activity and repletion of tissue glycogen, TAG, and ATP that was not observed in the FED hearts. GSH/GSSG at the end of reperfusion fell to the same value in both nutritional states, and TBARS levels were higher in the RFS hearts. In conclusion, RFS improved postischemic functional recovery, which was accompanied by a reduction in CK release and a striking energy recovery. Although enhanced G6PDH activity was displayed, RFS was unable to reduce lipid peroxidation, supporting a clear dissociation between protection against mechanical dysfunction and CK release on the one hand and oxidative damage on the other.

Daytime food restriction alters liver glycogen, triacylglycerols, and cell size. A histochemical, morphometric, and ultrastructural study

Background

Temporal restriction of food availability entrains circadian behavioral and physiological rhythms in mammals by resetting peripheral oscillators. This entrainment underlies the activity of a timing system, different from the suprachiasmatic nuclei (SCN), known as the food entrainable oscillator (FEO). So far, the precise anatomical location of the FEO is unknown. The expression of this oscillator is associated with an enhanced arousal prior to the food presentation that is called food anticipatory activity (FAA). We have focused on the study of the role played by the liver as a probable component of the FEO. The aim of this work was to identify metabolic and structural adaptations in the liver during the expression of the FEO, as revealed by histochemical assessment of hepatic glycogen and triacylglycerol contents, morphometry, and ultrastructure in rats under restricted feeding schedules (RFS).

Results

RFS promoted a decrease in the liver/body weight ratio prior to food access, a reduction of hepatic water content, an increase in cross-sectional area of the hepatocytes, a moderate reduction in glycogen content, and a striking decrease in triacylglyceride levels. Although these adaptation effects were also observed when the animal displayed FAA, they were reversed upon feeding. Mitochondria observed by electron microscopy showed a notorious opacity in the hepatocytes from rats during FAA (11:00 h). Twenty four hour fasting rats did not show any of the modifications observed in the animals expressing the FEO.

Conclusions

Our results demonstrate that FEO expression is associated with modified liver handling of glycogen and triacylglycerides accompanied by morphometric and ultrastructural adaptations in the hepatocytes. Because the cellular changes detected in the liver cannot be attributed to a simple alternation between feeding and fasting conditions, they also strengthen the notion that RFS promotes a rheostatic adjustment in liver physiology during FEO expression.

The rabbit pup, a natural model of nursing-anticipatory activity

Mother rabbits nurse their young once a day with circadian periodicity. Nursing bouts are brief (ca. 3 min) and occur inside the maternal burrow. Despite this limited contact mother rabbits and their pups are tuned to each other to ensure that the capacities of each party are used efficiently to ensure the weaning of a healthy litter. In this review we present behavioral, metabolic and hormonal correlates of this phenomenon in mother rabbits and their pups. Research is revealing that the circadian rhythm of locomotion shifts in parallel to the timing of nursing in both parties. In pups corticosterone has a circadian rhythm with highest levels at the time of nursing. Other metabolic and hormonal parameters follow an exogenous or endogenous rhythm which is affected by the time of nursing. In the brain, clock genes and their proteins (e.g. Per1) are differentially expressed in specific brain regions (e.g. suprachiasmatic nucleus, paraventricular nucleus) in relation to providing or ingesting milk in mothers and young, respectively. These findings suggest that circadian activities are modulated, in the mothers, by suckling stimulation and, in the young, by the ingestion of milk and/or the perception of the mammary pheromone. In conclusion, the rabbit pup is an extraordinary model for studying the entraining by a single daily food pulse with minimal manipulations. The mother offers the possibility of studying nursing as a non-photic synchronizer, also with minimal manipulation, as suckling stimulation from the litter occurs only once daily.

Food restricted schedules promote differential lipoperoxidative activity in rat hepatic subcellular fractions

Restricted access to food (from 12:00 to 14:00 h) produces a behavioral activation known as food anticipatory activity (FAA), which is a manifestation of the food entrained oscillator (FEO). Peripheral oscillators, especially in the liver, are thought to be part of the FEO. A variety of metabolic adaptations have been detected in the liver during the expression of this oscillator, including activation of mitochondrial respiration and changes in the cytoplasmic and mitochondrial redox states. Biological clocks are regulated by redox-sensitive factors. The present study explored the lipoperoxidative activity (LP) in the liver during the activity of the FEO. Conjugated dienes (CD) and thiobarbituric acid reactive substances (TBARS), with and without Fe2+-supplementation, were quantified in six subcellular fractions: whole homogenate, plasma membrane, mitochondria, microsomes, nucleus, and cytosol. The experimental protocol involved control groups of ad libitum fed and 24-h fasted rats, and groups under the restricted food schedule (RFS) which were sampled before FAA (08:00 h), during FAA (11:00 h) and after feeding (14:00 h). Clear differences in pro-oxidant activity was observed between ad libitum fed and 24-h fasted rats in almost all the subcellular fractions studied. RFS rats presented: CD levels more similar to the fasted rats, even at 14:00 h, after food presentation, and basal and Fe2+-supplemented TBARS levels tended to be lower than both controls, suggesting an increased antioxidant capacity associated with food restriction. In addition, a microarray analysis showed that several isoforms of peroxiredoxins, a family of antioxidant and hydrogen peroxide-catabolizing enzymes, were consistently up-regulated in each and every condition in which RFS was applied. Together, these data indicate a rheostatic adaptation of the liver in the handling of pro-oxidant reactions during the activity of the FEO.

Metabolic adaptations of liver mitochondria during restricted feeding schedules

Food anticipatory activity (FAA) is an output of the food-entrained oscillator (FEO), a conspicuous biological clock that expresses when experimental animals are under a restricted food schedule (RFS). We have shown that the liver is entrained by RFS and exhibits an anticipatory response before meal time in its oxidative and energetic state. The present study was designed to determine the mitochondrial oxidative and phosphorylating capacity in the liver of rats under RFS to further support the biochemical anticipatory role that this organ plays during the food entrainment (9). Metabolic and functional parameters of liver mitochondria were characterized before (0800 h), during (1100 h), and after (1400 h) FAA. The main results were as follows. First, there was an enhancement during FAA (1100 h) in 1) oxidative capacity (site I of the electron transport chain), 2) phosphorylating ability (estimated by ATP synthesis), and 3) activities of NADH shuttles. Second, after rats were fed (1400 h), the phosphorylating capacity remained high, but this was not the case for the respiratory control ratio for site I. Finally, in the three experimental conditions before, during, and after FAA, an increment was detected in the H(+) electrochemical potential, due to an elevation in mitochondrial membrane potential, and in mitochondrial yield. Most of the changes in mitochondrial properties related to RFS were also present when results were compared with those from the 24-h fasted group. In conclusion, the results support the notion that a distinctive rheostatic state is installed in the metabolic activity of the liver when FEO is being expressed.