The liver-clock coordinates rhythmicity of peripheral tissues in response to feeding.

The mammalian circadian system consists of a central clock in the brain that synchronizes clocks in the peripheral tissues. Although the hierarchy between central and peripheral clocks is established, little is known regarding the specificity and functional organization of peripheral clocks. Here, we employ altered feeding paradigms in conjunction with liver-clock mutant mice to map disparities and interactions between peripheral rhythms. We find that peripheral clocks largely differ in their responses to feeding time. Disruption of the liver-clock, despite its prominent role in nutrient processing, does not affect the rhythmicity of clocks in other peripheral tissues. Yet, unexpectedly, liver-clock disruption strongly modulates the transcriptional rhythmicity of peripheral tissues, primarily on daytime feeding. Concomitantly, liver-clock mutant mice exhibit impaired glucose and lipid homeostasis, which are aggravated by daytime feeding. Overall, our findings suggest that, upon nutrient challenge, the liver-clock buffers the effect of feeding-related signals on rhythmicity of peripheral tissues, irrespective of their clocks.

Food in synchrony with melatonin and corticosterone relieves constant light disturbed metabolism.

Circadian disruption is associated with metabolic disturbances such as hepatic steatosis (HS), obesity and type 2 diabetes. We hypothesized that HS, resulting from constant light (LL) exposure is due to an inconsistency between signals related to food intake and endocrine-driven suprachiasmatic nucleus (SCN) outputs. Indeed, exposing rats to LL induced locomotor, food intake and hormone arrhythmicity together with the development of HS. We investigated whether providing temporal signals such as 12-h food availability or driving a corticosterone plus melatonin rhythm could restore rhythmicity and prevent the metabolic disturbances under LL conditions in male rats. Discrete metabolic improvements under these separate treatments stimulated us to investigate whether the combination of hormone treatment together with mealtime restriction (12-h food during four weeks) could prevent the metabolic alterations. LL exposed arrhythmic rats, received daily administration of corticosterone (2.5 µg/kg) and melatonin (2.5 mg/kg) in synchrony or out of synchrony with their 12-h meal. HS and other metabolic alterations were importantly ameliorated in LL-exposed rats receiving hormonal treatment in synchrony with 12-h restricted mealtime, while treatment out of phase with meal time did not. Interestingly, liver bile acids, a major indication for HS, were only normalized when animals received hormones in synchrony with food indicating that disrupted bile acid metabolism might be an important mechanism for the HS induction under LL conditions. We conclude that food-elicited signals, as well as hormonal signals, are necessary for liver synchronization and that HS arises when there is conflict between food intake and the normal pattern of melatonin and corticosterone.

Role of the Suprachiasmatic and Arcuate Nuclei in Diurnal Temperature Regulation in the Rat.

In mammals, daily changes in body temperature (Tb) depend on the integrity of the suprachiasmatic nucleus (SCN). Fasting influences the Tb in the resting period and the presence of the SCN is essential for this process. However, the origin of this circadian/metabolic influence is unknown. We hypothesized that, not only the SCN but also the arcuate nucleus (ARC), are involved in the Tb setting through afferents to the thermoregulatory median preoptic nucleus (MnPO). Therefore, we investigated by neuronal tracing and microdialysis experiments the possible targeting of the MnPO by the SCN and the ARC in male Wistar rats. We observed that vasopressin release from the SCN decreases the temperature just before light onset, whereas α-melanocyte stimulating hormone release, especially at the end of the dark period, maintains high temperature. Both peptides have opposite effects on the brown adipose tissue activity through thermoregulatory nuclei such as the dorsomedial nucleus of the hypothalamus and the dorsal raphe nucleus. The present study indicates that the coordination between circadian and metabolic signaling within the hypothalamus is essential for an adequate temperature control. SIGNIFICANCE STATEMENT: When circadian and metabolic systems are not well synchronized, individuals may develop metabolic diseases. The underlying mechanisms are unknown. Here, we demonstrate that the balance between the releases of neuropeptides derived from the biological clock and from a metabolic sensory organ as the arcuate nucleus, are essential for an adequate temperature control. These observations show that brain areas involved in circadian and metabolic functions of the body need to interact to produce a coherent arrangement of physiological processes associated with temperature control.

Food entrains clock genes but not metabolic genes in the liver of suprachiasmatic nucleus lesioned rats.

Hepatic circadian transcription, considered to be driven by the liver clock, is largely influenced by food even uncoupling it from the suprachiasmatic nucleus (SCN). In SCN lesioned rats (SCNx) we determined the influence of a physiological feeding schedule on the entrainment of clock and clock-controlled (CCG) genes in the liver. We show that clock genes and the CCG Rev-erbα and peroxisome proliferator-activated receptor alpha (PPARα) in food-scheduled intact and SCNx have a robust diurnal differential expression persisting after a 24h fast. However, hepatic nicotinamide phosphoribosyl transferase (Nampt) shows time dependent changes that are lost in intact animals under fasting; moreover, it is unresponsive to the nutrient status in SCNx, indicating a poor reliance on liver clock genes and highlighting the relevance of SCN-derived signals for its metabolic status-related expression.

Peripheral circadian oscillators: time and food.

The suprachiasmatic nucleus (SCN) provides timing to the brain and to the whole organism. Its rhythmic signal to mainly hypothalamic structures results in a synchronized hormonal and autonomic output to the body that coordinates behavior and physiology. As a result of this, the expression of clock genes in all organs has a rhythm that is dictated by the SCN. Together with these clock genes, a number of cellular processes follow a similar rhythm, whereby it has been proposed that these events are driven at least, in part, by clock genes. Together, this forms a multiple oscillating system that interacts and under normal conditions is synchronized by the SCN. The autonomic and hormonal outputs from the SCN are examples of messages that are clearly targeted; the behaviors driven by the SCN are examples of messages that may have more diffuse targets. For example, food intake and locomotor activity, which are normally driven by the SCN, have the capacity to drive the rhythm of clock genes in cells of the liver. The influence of food has been shown by offering food outside the normal activity-food intake period. If such a condition persists, desynchronization follows between centrally and peripherally dictated rhythms because the SCN keeps transmitting temporal signals according to the day-night cycle. These circumstances promote pathologies such as the metabolic syndrome, which is characterized by the progressive onset of hypertension, insulin resistance, and diabetes. As clock genes are proposed to drive the rhythms of metabolic genes, it is very attractive to give the clock genes a central place in this desynchronization and pathology picture. Therefore, in this chapter, we pay special attention to the question of how the SCN is able to transmit its message to the cells of the body and focus on the liver, because of its essential role in metabolism. Here, we review recent evidence that shows how desynchronization may lead to the uncoupling of cellular processes within the liver cells. The basis for this cellular dissociation, we argue, is the fact that the network of brain-body interaction is desynchronized, leading also to an uncoupling of normally coupled systems within the cell.

ß-Adrenergic receptors in the insular cortex are differentially involved in aversive vs. incidental context memory formation.

The goal of this research was to determine the effects of ß-adrenergic antagonism in the IC before or after inhibitory avoidance (IA) training or context pre-exposure in a latent inhibition protocol. Pretraining intra-IC infusion of the ß-adrenergic antagonist propranolol disrupted subsequent IA retention and impaired latent inhibition of IA, but had no effect on formation of memory for an inert context (termed incidental memory). These results indicate that IC ß-adrenergic receptors are necessary for memory acquisition of an aversive, but not an inconsequential, context. Nevertheless, subsequent association of a familiar and hitherto inconsequential context with an unconditioned stimulus (US) does require activation of these receptors during its initial acquisition.

Blockade of nucleus basalis magnocellularis or activation of insular cortex histamine receptors disrupts formation but not retrieval of aversive taste memory.

Recent research, using several experimental models, demonstrated that the histaminergic system is clearly involved in memory formation. This evidence suggested that during different associative learning tasks, histamine receptor subtypes have opposite functions, related to the regulation of cortical cholinergic activity. Given that cortical cholinergic activity and nucleus basalis magnocellularis (NBM) integrity are needed during taste memory formation, the aim of this study was to determine the role of histamine receptors during conditioned taste aversion (CTA). We evaluated the effects of bilateral infusions of 0.5 microl of pyrilamine (100 mM), an H(1) receptor antagonist, into the NBM, or of R-alpha-methylhistamine (RAMH) (10 mM), an H(3) receptor agonist, into the insular cortex of male Sprague-Dawley rats 20 min before acquisition and/or retrieval of conditioned taste aversion. The results showed that blockade of H(1) receptors in NBM or activation of H(3) receptors in the insular cortex impairs formation but not retrieval of aversive taste memory. These results demonstrated differential roles for histamine receptors in two important areas for taste memory formation and suggest that these effects could be related with the cortical cholinergic activity modulation during CTA acquisition.