No abstract

The enteric nervous system (ENS) is an extensive network comprising millions of neurons and glial cells contained within the wall of the gastrointestinal tract. The major functions of the ENS that have been most studied include the regulation of local gut motility, secretion, and blood flow. Other areas that have been gaining increased attention include its interaction with the immune system, with the gut microbiota and its involvement in the gut–brain axis, and neuro-epithelial interactions. Thus, the enteric circuitry plays a central role in intestinal homeostasis, and this becomes particularly evident when there are faults in its wiring such as in neurodevelopmental or neurodegenerative disorders. In this review, we first focus on the current knowledge on the cellular composition of enteric circuits. We then further discuss how enteric circuits detect and process external information, how these signals may be modulated by physiological and pathophysiological factors, and finally, how outputs are generated for integrated gut function.

Summary. In the platelet, it is well established that many G-protein-and tyrosine kinase-coupled receptors stimulate phospholipase-C-dependent Ca 2+ mobilization; however, the extent to which secondary activation of adenosine 5¢-triphosphate (ATP)-gated P2X 1 receptors contributes to intracellular Ca 2+ responses remains unclear. We now show that selective inhibition of P2X 1 receptors substantially reduces the [Ca 2+ ] i increase evoked by several important agonists in human platelets; for collagen, thromboxane A 2 , thrombin, and adenosine 5¢-diphoshate (ADP) the maximal effect was a reduction to 18%, 34%, 52%, and 69% of control, respectively. The direct contribution of P2X 1 to the secondary Ca 2+ response was far greater than that of either P2Y receptors activated by co-released ADP, or via synergistic P2X 1 :P2Y interactions. The relative contribution of P2X 1 to the peak Ca 2+ increase varied with the strength of the initial stimulus, being greater at low compared to high levels of stimulation for both glycoprotein VI and PAR-1, whereas P2X 1 contributed equally at both low and high levels of stimulation of thromboxane A 2 receptors. In contrast, only strong stimulation of P2Y receptors resulted in significant P2X 1 receptor activation. ATP release was detected by soluble luciferin:luciferase in response to all agonists that stimulated secondary P2X 1 receptor activation. However, P2X 1 receptors were stimulated earlier and to a greater extent than predicted from the average ATP release, which can be accounted for by a predominantly autocrine mechanism of activation. Given the central role of [Ca 2+ ] i increases in platelet activation, these studies indicate that ATP should be considered alongside ADP and thromboxane A 2 as a significant secondary platelet agonist.

Coordination of gastrointestinal function relies on joint efforts of enteric neurons and glia, whose crosstalk is vital for the integration of their activity. To investigate the signaling mechanisms and to delineate the spatial aspects of enteric neuron‐to‐glia communication within enteric ganglia we developed a method to stimulate single enteric neurons while monitoring the activity of neighboring enteric glial cells. We combined cytosolic calcium uncaging of individual enteric neurons with calcium imaging of enteric glial cells expressing a genetically encoded calcium indicator and demonstrate that enteric neurons signal to enteric glial cells through pannexins using paracrine purinergic pathways. Sparse labeling of enteric neurons and high‐resolution analysis of the structural relation between neuronal cell bodies, varicose release sites and enteric glia uncovered that this form of neuron‐to‐glia communication is contained between the cell body of an enteric neuron and its surrounding enteric glial cells. Our results reveal the spatial and functional foundation of neuro‐glia units as an operational cellular assembly in the enteric nervous system.

Cholera toxin (CT) is well established to produce diarrhea by producing hyperactivity of the enteric neural circuits that regulate water and electrolyte secretion. Its effects on intestinal motor patterns are less well understood. We examined the effects of luminal CT on motor activity of guinea-pig jejunum in vitro. Segments of jejunum were cannulated at either end and mounted horizontally. Their contractile activity was video-imaged and the recordings were used to construct spatiotemporal maps of contractile activity with CT (1.25 or 12.5 μg/ml) in the lumen. Both concentrations of CT induced propulsive motor activity in jejunal segments. The effect of 1.25 μg/ml CT was markedly enhanced by co-incubation with granisetron (5-HT3 antagonist, 1 μM), which prevents the hypersecretion induced by CT. The increased propulsive activity was not accompanied by increased segmentation and occurred very early after exposure to CT in the presence of granisetron. Luminal CT also reduced the pressure threshold for saline distension evoked propulsive reflexes, an effect resistant to granisetron. In contrast, CT prevented the induction of segmenting contractions by luminal decanoic acid, so its effects on propulsive and segmenting contractile activity are distinctly different. Thus, in addition to producing hypersecretion, CT excites propulsive motor activity with an entirely different time course and pharmacology, but inhibits nutrient-induced segmentation. This suggests that CT excites more than one enteric neural circuit and that propulsive and segmenting motor patterns are differentially regulated.