Fonar, Gennadiy

ABSTRACT Background The circadian rhythm regulates gastrointestinal motility. In humans and preclinical models, such as rodents, whole gut transit (WGT) is slower during the rest phase compared to the active phase. Investigators typically study GI transit in rodents during the day, which is their rest phase, rather than during the night, which is their active phase. A circadian rhythm reversal in which mice are in a dark room during the working day (reverse light) allows studies on nocturnal animals during their active phase and has been previously shown to reduce WGT time. GI motility is often disrupted in individuals with disorders of gut–brain interaction (DGBI), which are female predominant. However, the effect of circadian rhythm on regional transit and sex dependence of the differences is not known, as most motility studies looking at circadian rhythm reversal are done in male mice. Methods We tested C57BL/6 wild‐type male and female mice in rest (12 h of light during the day) and active (reverse cycle for 2 weeks: 12 h of dark during the day) phases. We noted female estrous cycle by visual inspection. We performed carmine WGT by monitoring time‐lapse videos of pellet production. We performed fluorescence imaging of excised intestines 30 min after gavage to assess percent fluorescence for each GI region and then examined small intestinal transit (SIT) by measuring geometric center and leading edge. For colonic transit, we monitored bead expulsion time from distal colon to anus. Key Results Compared to rest phase, in the active phase, like male mice, female mice had (1) faster WGT, (2) increased frequency of pellet expulsion in the first 3 h of transit, (3) and greater total pellet production. Both male and female mice in their active phase exhibited (4) more contrast emptied from the stomach and they had (5) further leading edge of fluorescence and (6) geometric center, in SIT, and (7) faster colonic bead expulsion times. There were no significant sex differences in the active phase of WGT. In SIT, male mice had further leading edge in the rest phase than female mice, but this difference was not seen in the active phase, and in colonic transit, male mice in both the active and rest phases had faster bead expulsion than female mice. Conclusions Mice in the active phase have faster regional transit in small and large bowel than mice in the rest phase that collectively contributes to faster WGT times in the active phase of both male and female mice. These findings highlight the importance of circadian biology in sex‐dependent rodent GI transit.

The gastrointestinal (GI) tract plays a crucial role in nutrient absorption, motility, and metabolism, while serving as a barrier against pathogens. The intestinal epithelial layer, particularly enteroendocrine cells (EEC), is pivotal in sensing luminal stimuli due to its strategic position. Although comprising only 1% of the GI epithelium, in total EEC comprises perhaps the largest endocrine organ that couples GI luminal stimuli with release of hormones and neurotransmitters such as serotonin, glucagon-like peptide-1 (GLP-1), and peptide–tyrosine–tyrosine (PYY), which regulate local and systemic physiology. In this review, we use EECs as a model to explore critical intracellular signaling pathways—centering around calcium (Ca²⁺) and cyclic adenosine monophosphate (cAMP). EECs utilize ionotropic and metabotropic receptors to detect luminal stimuli, leading to increased intracellular cytoplasmic Ca²⁺ and altered intracellular cAMP dynamics. Ca²⁺ influx is critical for the exocytosis of signaling molecules. The interplay between Ca²⁺ and cAMP signaling pathways enhances the release of these molecules, affecting both local and systemic responses. We make a point to discuss the conventional and emerging methodologies for measuring Ca²⁺ and cAMP dynamics in EECs, which is a rapidly expanding toolbox, containing genetically encoded indicators and resonance energy transfer techniques. Understanding these signaling cascades is essential for deciphering the complex roles of EECs in GI physiology and their impact on systemic health.