Gastrointestinal Vagal Afferents and Food Intake: Relevance of Circadian Rhythms

Chronobiotics, satiety signaling, and clock gene expression interplay

The biological clock regulates the way our body works throughout the day, including releasing hormones and food intake. Disruption of the biological clock (chronodisruption) may deregulate satiety, which is strictly regulated by hormones and neurotransmitters, leading to health problems like obesity. Nowadays, using bioactive compounds as a coadjutant for several pathologies is a common practice. Phenolic compounds and short-chain fatty acids, called "chronobiotics," can modulate diverse mechanisms along the body to exert beneficial effects, including satiety regulation and circadian clock resynchronization; however, the evidence of the interplay between those processes is limited. This review compiles the evidence of natural chronobiotics, mainly polyphenols and short-chain fatty acids that affect the circadian clock mechanism and process modifications in genes or proteins resulting in a signaling chain that modulates satiety hormones or hunger pathways.

Principles of synaptic encoding of brainstem circadian rhythms

Circadian regulation of autonomic tone and reflex pathways pairs physiological processes with the daily light cycle. However, the underlying mechanisms mediating these changes on autonomic neurocircuitry are only beginning to be understood. The brainstem nucleus of the solitary tract (NTS) and adjacent nuclei, including the area postrema and dorsal motor nucleus of the vagus, are key candidates for rhythmic control of some aspects of the autonomic nervous system. Recent findings have contributed to a working model of circadian regulation in the brainstem which manifests from the transcriptional, to synaptic, to circuit levels of organization. Vagal afferent neurons and the NTS possess rhythmic clock gene expression, rhythmic action potential firing, and our recent findings demonstrate rhythmic spontaneous glutamate release. In addition, postsynaptic conductances also vary across the day producing subtle changes in membrane depolarization which govern synaptic efficacy. Together these coordinated pre- and postsynaptic changes provide nuanced control of synaptic transmission across the day to tune the sensitivity of primary afferent input and likely govern reflex output. Further, given the important role for the brainstem in integrating cues such as feeding, cardiovascular function and temperature, it may also be an underappreciated locus in mediating the effects of such non-photic entraining cues. This short review focuses on the neurophysiological principles that govern NTS synaptic transmission and how circadian rhythms impacted them across the day.

Effect of pregnancy on the expression of nutrient-sensors and satiety hormones in mice

Small intestinal satiation pathways involve nutrient-induced stimulation of chemoreceptors leading to release of satiety hormones from intestinal enteroendocrine cells (ECCs). Whether adaptations in these pathways contribute to increased maternal food intake during pregnancy is unknown. To determine the expression of intestinal nutrient-sensors and satiety hormone transcripts and proteins across pregnancy in mice. Female C57BL/6J mice (10-12 weeks old) were randomized to mating and then tissue collection at early- (6.5 d), mid- (12.5 d) or late-pregnancy (17.5 d), or to an unmated age matched control group. Relative transcript expression of intestinal fatty acid, peptide and amino acid and carbohydrate chemoreceptors, as well as gut hormones was determined across pregnancy. The density of G-protein coupled receptor 93 (GPR93), free fatty acid receptor (FFAR) 4, cholecystokinin (CCK) and glucagon-like peptide1 (GLP-1) immunopositive cells was then compared between non-pregnant and late-pregnant mice. Duodenal GPR93 expression was lower in late pregnant than non-pregnant mice (P < 0.05). Ileal FFAR1 expression was higher at mid- than at early- or late-pregnancy. Ileal FFAR2 expression was higher at mid-pregnancy than in early pregnancy. Although FFAR4 expression was consistently lower in late-pregnant than non-pregnant mice (P < 0.001), the density of FFAR4 immunopositive cells was higher in the jejunum of late-pregnant than non-pregnant mice. A subset of protein and fatty acid chemoreceptor transcripts undergo region-specific change during murine pregnancy, which could augment hormone release and contribute to increased food intake. Further investigations are needed to determine the functional relevance of these changes.

Circadian rhythms in colonic function

A rhythmic expression of clock genes occurs within the cells of multiple organs and tissues throughout the body, termed "peripheral clocks." Peripheral clocks are subject to entrainment by a multitude of factors, many of which are directly or indirectly controlled by the light-entrainable clock located in the suprachiasmatic nucleus of the hypothalamus. Peripheral clocks occur in the gastrointestinal tract, notably the epithelia whose functions include regulation of absorption, permeability, and secretion of hormones; and in the myenteric plexus, which is the intrinsic neural network principally responsible for the coordination of muscular activity in the gut. This review focuses on the physiological circadian variation of major colonic functions and their entraining mechanisms, including colonic motility, absorption, hormone secretion, permeability, and pain signalling. Pathophysiological states such as irritable bowel syndrome and ulcerative colitis and their interactions with circadian rhythmicity are also described. Finally, the classic circadian hormone melatonin is discussed, which is expressed in the gut in greater quantities than the pineal gland, and whose exogenous use has been of therapeutic interest in treating colonic pathophysiological states, including those exacerbated by chronic circadian disruption.

Restricted Feeding Resets the Peripheral Clocks of the Digestive System

All organisms maintain an internal clock that matches the Earth's rotation over a period of 24 h, known as the circadian rhythm. Previously, we established Period1 luciferase (Per1::luc) transgenic (Tg) mice in order to monitor the expression rhythms of the Per1 clock gene in each tissue in real time using a bioluminescent reporter. The Per1 gene is a known key molecular regulator of the mammalian clock system in the autonomous central clock in the suprachiasmatic nucleus (SCN), and the peripheral tissues. Per1::luc Tg mice were used as a biosensing system of circadian rhythms. They were maintained by being fed ad lib (FF) and subsequently subjected to 4 hour (4 h) restricted feeding (RF) during the rest period under light conditions in order to examine whether the peripheral clocks of different parts in the digestive tract could be entrained. The peak points of the bioluminescent rhythms in the Per1::luc Tg mouse tissue samples were analyzed via cosine fitting. The bioluminescent rhythms of the cultured peripheral tissues of the esophagus and the jejunum exhibited phase shift from 5 to 11 h during RF, whereas those of the SCN tissue remained unchanged for 7 days during RF. We examined whether RF for 4 h during the rest period in light conditions could reset the activity rhythms, the central clock in the SCN, and the peripheral clock in the different points in the gastrointestinal tract. The fasting signals during RF did not entrain the SCN, but they did entrain each peripheral clock of the digestive system, the esophagus, and the jejunum. During RF for 7 days, the peak time of the esophagus tended to return to that of the FF control, unlike that of the jejunum; hence, the esophagus was regulated more strongly under the control of the cultured SCN compared to the jejunum. Thus, the peripheral clocks of the digestive system can entrain their molecular clock rhythms via RF-induced fasting signals in each degree, independently from the SCN.

GPR-160 Receptor Signaling in the Dorsal Vagal Complex of Male Rats Modulates Meal Microstructure and CART-Mediated Hypophagia

The g-protein coupled receptor GPR-160, recently identified as a putative receptor for the cocaine and amphetamine-regulated transcript (CART) peptide, shows abundant expression in the energy-balance control nuclei, including the dorsal vagal complex (DVC). However, its physiological role in the control of food intake has yet to be fully explored. Here, we performed a virally mediated, targeted knockdown (KD) of Gpr160 in the DVC of male rats to evaluate its physiological role in control of feeding. Our results indicate that DVC Gpr160 KD affects meal microstructure. Specifically, DVC Gpr160 KD animals consumed more frequent, but shorter meals during the dark phase and showed decreased caloric intake and duration of meals during the light phase. Cumulatively, however, these bidirectional effects on feeding resulted in no difference in body weight gain. We next tested the role of DVC GPR-160 in mediating the anorexigenic effects of exogenous CART. Our results show that DVC Gpr160 KD partially attenuates CART's anorexigenic effects. To further characterize Gpr160+ cells in the DVC, we utilized single-nucleus RNA sequencing data to uncover abundant GPR-160 expression in DVC microglia and only minimal expression in neurons. Altogether, our results suggest that DVC CART signaling may be mediated by Gpr160+ microglia, which in turn may be modulating DVC neuronal activity to control food intake.

Role of circadian rhythms and melatonin in bladder function in heath and diseases

The circadian system modulates all visceral organ physiological processes including urine storage and voiding. The "master clock" of the circadian system lies within suprachiasmatic nucleus of the hypothalamus while "peripheral clocks" are found in most peripheral tissue and organs, including the urinary bladder. Disruptions of circadian rhythms can cause organ malfunction and disorder or exacerbate pre-existing ones. It has been suggested that nocturia, which develops mostly in the elderly, could be a circadian-related disorder of the bladder. In the bladder, many types of gap junctions and ion channels in the detrusor, urothelium and sensory nerves are likely under strict local peripheral circadian control. The pineal hormone, melatonin, is a circadian rhythm synchroniser capable of controlling a variety of physiological processes in the body. Melatonin predominantly acts via the melatonin 1 and melatonin 2 G-protein coupled receptors expressed in the central nervous system, and many peripheral organs and tissues. Melatonin could be beneficial in the treatment of nocturia and other common bladder disorders. The ameliorating action of melatonin on bladder function is likely due to multiple mechanisms which include central effects on voiding and peripheral effects on the detrusor and bladder afferents. More studies are warranted to determine the precise mechanisms of circadian rhythm coordination of the bladder function and melatonin influences on the bladder in health and diseases.

Enteric VIP-producing neurons maintain gut microbiota homeostasis through regulating epithelium fucosylation

Interactions between the enteric nervous system (ENS) and intestinal epithelium are thought to play a vital role in intestinal homeostasis. How the ENS monitors the frontier with commensal and pathogenic microbes while maintaining epithelial function remains unclear. Here, by combining subdiaphragmatic vagotomy with transcriptomics, chemogenetic strategy, and coculture of enteric neuron-intestinal organoid, we show that enteric neurons expressing VIP shape the α1,2-fucosylation of intestinal epithelial cells (IECs). Mechanistically, neuropeptide VIP activates fut2 expression via the Erk1/2-c-Fos pathway through the VIPR1 receptor on IECs. We further demonstrate that perturbation of enteric neurons leads to gut dysbiosis through α1,2-fucosylation in the steady state and results in increased susceptibility to alcohol-associated liver disease (ALD). This was attributed to an imbalance between beneficial Bifidobacterium and opportunistic pathogenic Enterococcus faecalis in ALD. In addition, Bifidobacterium α1,2-fucosidase may promote Bifidobacterium adhesion to the mucosal surface, which restricts Enterococcus faecalis overgrowth and prevents ALD progression.

Regulation of Circadian Genes Nr1d1 and Nr1d2 in Sex-Different Manners during Liver Aging

Background: Circadian rhythm is associated with the aging process and sex differences; however, how age and sex can change circadian regulation systems remains unclear. Thus, we aimed to evaluate age- and sex-related changes in gene expression and identify sex-specific target molecules that can regulate aging. Methods: Rat livers were categorized into four groups, namely, young male, old male, young female, and old female, and the expression of several genes involved in the regulation of the circadian rhythm was confirmed by in silico and in vitro studies. Results: Gene Ontology and the Kyoto Encyclopedia of Genes and Genomes pathway enrichment analyses showed that the expression of genes related to circadian rhythms changed more in males than in females during liver aging. In addition, differentially expressed gene analysis and quantitative real-time polymerase chain reaction/western blotting analysis revealed that Nr1d1 and Nr1d2 expression was upregulated in males during liver aging. Furthermore, the expression of other circadian genes, such as Arntl, Clock, Cry1/2, Per1/2, and Rora/c, decreased in males during liver aging; however, these genes showed various gene expression patterns in females during liver aging. Conclusions: Age-related elevation of Nr1d1/2 downregulates the expression of other circadian genes in males, but not females, during liver aging. Consequently, age-related upregulation of Nr1d1/2 may play a more crucial role in the change in circadian rhythms in males than in females during liver aging.

Homo sapiens May Incorporate Daily Acute Cycles of “Conditioning–Deconditioning” to Maintain Musculoskeletal Integrity: Need to Integrate with Biological Clocks and Circadian Rhythm Mediators

Human evolution required adaptation to the boundary conditions of Earth, including 1 g gravity. The bipedal mobility of Homo sapiens in that gravitational field causes ground reaction force (GRF) loading of their lower extremities, influencing the integrity of the tissues of those extremities. However, humans usually experience such loading during the day and then a period of relative unloading at night. Many studies have indicated that loading of tissues and cells of the musculoskeletal (MSK) system can inhibit their responses to biological mediators such as cytokines and growth factors. Such findings raise the possibility that humans use such cycles of acute conditioning and deconditioning of the cells and tissues of the MSK system to elaborate critical mediators and responsiveness in parallel with these cycles, particularly involving GRF loading. However, humans also experience circadian rhythms with the levels of a number of mediators influenced by day/night cycles, as well as various levels of biological clocks. Thus, if responsiveness to MSK-generated mediators also occurs during the unloaded part of the daily cycle, that response must be integrated with circadian variations as well. Furthermore, it is also possible that responsiveness to circadian rhythm mediators may be regulated by MSK tissue loading. This review will examine evidence for the above scenario and postulate how interactions could be both regulated and studied, and how extension of the acute cycles biased towards deconditioning could lead to loss of tissue integrity.

Circadian Control of Gastrointestinal Motility

With the earth's 24-h rotation cycle, physiological function fluctuates in both diurnal and nocturnal animals, thereby ensuring optimal functioning of the body. The main regulator of circadian rhythm is the suprachiasmatic nucleus (SCN), which is considered the main pacemaker or "central clock" of the body. Located in the anterior hypothalamus, the SCN influences the activity of other brain regions, as well as peripheral organs, through the release of melatonin and corticosteroids. The SCN can be entrained by several cues, with light being the major cue. Light information from the retina is received by the SCN via the retinohypothalamic tract. Non-photic cues such as temperature and exercise can also entrain the SCN, while feeding time can entrain the "molecular clock" contained within peripheral tissues. This enables organs such as the gastrointestinal (GI) tract to coordinate function with environmental factors, such as food availability.The GI tract, which has the main functions of receiving and digesting food, and expelling waste, also shows oscillations in its activity during the circadian cycle. While these changes are evident under normal conditions, GI function is affected when normal circadian rhythm is disrupted. Recent reviews have assessed interactions between the central clock and gut clock; as such, this review aims to focus on the presence of endogenous circadian rhythms in the GI tract, with particular focus to changes to gastrointestinal motility.

Metabolic Homeostasis: It's All in the Timing

Cross-talk between peripheral tissues is essential to ensure the coordination of nutrient intake with disposition during the feeding period, thereby preventing metabolic disease. This mini-review considers the interactions between the key peripheral tissues that constitute the metabolic clock, each of which is considered in a separate mini-review in this collation of articles published in Endocrinology in 2020 and 2021, by Martchenko et al (Circadian rhythms and the gastrointestinal tract: relationship to metabolism and gut hormones); Alvarez et al (The microbiome as a circadian coordinator of metabolism); Seshadri and Doucette (Circadian regulation of the pancreatic beta cell); McCommis et al (The importance of keeping time in the liver); Oosterman et al (The circadian clock, shift work, and tissue-specific insulin resistance); and Heyde et al (Contributions of white and brown adipose tissues to the circadian regulation of energy metabolism). The use of positive- and negative-feedback signals, both hormonal and metabolic, between these tissues ensures that peripheral metabolic pathways are synchronized with the timing of food intake, thus optimizing nutrient disposition and preventing metabolic disease. Collectively, these articles highlight the critical role played by the circadian clock in maintaining metabolic homeostasis.

Appetite and Satiety Control-Contribution of Gut Mechanisms

The prevalence of obesity, and its comorbidities, particularly type 2 diabetes, cardiovascular and hepatic disease and certain cancers, continues to rise at an alarming rate worldwide [...].