Circadian rhythms in colonic function

PUBLISHED 30 August 2023A 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.Timothy J. Hibberd, Stewart Ramsay, Phaedra Spencer-Merris, Phil G. Dinning, Vladimir P. Zagorodnyuk and Nick J. Spence

Translating peripheral bladder afferent mechanosensitivity to neuronal activation within the lumbosacral spinal cord of mice

Primary afferent neurons transduce distension of the bladder wall into action potentials that are relayed into the spinal cord and brain, where autonomic reflexes necessary for maintaining continence are coordinated with pathways involved in sensation. However, the relationship between spinal circuits involved with physiological and nociceptive signalling from the bladder has only been partially characterised. We used ex vivo bladder afferent recordings to characterise mechanosensitive afferent responses to graded distension (0-60 mm Hg) and retrograde tracing from the bladder wall to identify central axon projections within the dorsal horn of the lumbosacral (LS) spinal cord. Labelling of dorsal horn neurons with phosphorylated-MAP-kinase (pERK), combined with labelling for neurochemical markers (calbindin, calretinin, gamma aminobutyric acid, and parvalbumin) after in vivo bladder distension (20-60 mm Hg), was used to identify spinal cord circuits processing bladder afferent input. Ex vivo bladder distension evoked an increase in primary afferent output, and the recruitment of both low- and high-threshold mechanosensitive afferents. Retrograde tracing revealed bladder afferent projections that localised with pERK-immunoreactive dorsal horn neurons within the superficial laminae (superficial dorsal horn), dorsal gray commissure, and lateral collateral tracts of the LS spinal cord. Populations of pERK-immunoreactive neurons colabelled with calbindin, calretinin, or gamma aminobutyric acid, but not parvalbumin. Noxious bladder distension increased the percentage of pERK-immunoreactive neurons colabelled with calretinin. We identified LS spinal circuits supporting autonomic and nociceptive reflexes responsible for maintaining continence and bladder sensations. Our findings show for the first time that low- and high-threshold bladder afferents relay into similar dorsal horn circuits, with nociceptive signalling recruiting a larger number of neurons.Luke Grundy, Andrea M. Harrington, Ashlee Caldwell, Joel Castro, Vasiliki Staikopoulos, Vladimir P. Zagorodnyuk, Simon J.H. Brookes, Nick J. Spencer, Stuart M. Brierle

Disengaging spinal afferent nerve communication with the brain in live mice

Our understanding of how abdominal organs (like the gut) communicate with the brain, via sensory nerves, has been limited by a lack of techniques to selectively activate or inhibit populations of spinal primary afferent neurons within dorsal root ganglia (DRG), of live animals. We report a survival surgery technique in mice, where select DRG are surgically removed (unilaterally or bilaterally), without interfering with other sensory or motor nerves. Using this approach, pain responses evoked by rectal distension were abolished by bilateral lumbosacral L5-S1 DRG removal, but not thoracolumbar T13-L1 DRG removal. However, animals lacking T13-L1 or L5-S1 DRG both showed reduced pain sensitivity to distal colonic distension. Removal of DRG led to selective loss of peripheral CGRP-expressing spinal afferent axons innervating visceral organs, arising from discrete spinal segments. This method thus allows spinal segment-specific determination of sensory pathway functions in conscious, free-to-move animals, without genetic modification.Melinda A. Kyloh, Timothy J. Hibberd, Joel Castro, Andrea M. Harrington, Lee Travis, Kelsi N. Dodds, Lukasz Wiklendt, Stuart M. Brierley, Vladimir P. Zagorodnyuk and Nick J. Spence