Abstract
The enteric nervous system (ENS) regulates the motor, secretory and defensive functions of the gastrointestinal tract. Enteric neurons integrate mechanical and chemical inputs from the gut lumen to generate complex motor outputs. How intact enteric neural circuits respond to changes in the gut lumen is not well understood. We recorded intracellular calcium in live‐cell confocal recordings in neurons from intact segments of mouse intestine in order to investigate neuronal response to luminal mechanical and chemical stimuli. Wnt1‐, ChAT‐ and Calb1‐GCaMP6 mice were used to record neurons from the jejunum and colon. We measured neuronal calcium response to KCl (75 mM), veratridine (10 μM), 1,1‐dimethyl‐4‐phenylpiperazinium (DMPP; 100 μM) or luminal nutrients (Ensure®), in the presence or absence of intraluminal distension. In the jejunum and colon, distension generated by the presence of luminal content (chyme and faecal pellets, respectively) renders the underlying enteric circuit unresponsive to depolarizing stimuli. In the distal colon, high levels of distension inhibit neuronal response to KCl, while intermediate levels of distension reorganize Ca2+ response in circumferentially propagating slow waves. Mechanosensitive channel inhibition suppresses distension‐induced Ca2+ elevations, and calcium‐activated potassium channel inhibition restores neuronal response to KCl, but not DMPP in the distended colon. In the jejunum, distension prevents a previously unknown tetrodotoxin‐resistant neuronal response to luminal nutrient stimulation. Our results demonstrate that intestinal distension regulates the excitability of ENS circuits via mechanosensitive channels. Physiological levels of distension locally silence or synchronize neurons, dynamically regulating the excitability of enteric neural circuits based on the content of the intestinal lumen.
Key points
How the enteric nervous system of the gastrointestinal tract responds to luminal distension remains to be fully elucidated.
Here it is shown that intestinal distension modifies intracellular calcium levels in the underlying enteric neuronal network, locally and reversibly silencing neurons in the distended regions.
In the distal colon, luminal distension is integrated by specific mechanosensitive channels and coordinates the dynamics of neuronal activation within the enteric network.
In the jejunum, distension suppresses the neuronal calcium responses induced by luminal nutrients.
Physiological levels of distension dynamically regulate the excitability of enteric neuronal circuits.
figure legend Live, three‐dimensional calcium (Ca2+) recordings were made in intact segments of the jejunum, proximal and distal colon in mice expressing a genetically encoded Ca2+ indicator (GCaMP6) in the enteric nervous system (ENS). In the undistended region of the colon (as well other segments), intracellular Ca2+ is low and neurons respond simultaneously to a depolarizing stimulus (KCl). When distension increases, intracellular Ca2+ increases and neurons respond in circumferential waves. In the fully distended colon, the neuronal response is lost. Distension is integrated by mechanosensitive ion channels, including the calcium‐activated potassium channel KCa1.1 that regulates intracellular Ca2+ in the ENS. In presence of distension, responses to depolarizing stimuli (KCl and veratridine), a nicotinic receptor agonist (DMPP) or luminal nutrients (jejunum only) are lost.