Activity in varicosities within the myenteric plexus between and during the colonic migrating motor complex in the isolated murine large intestine

Background Neuronal communication within the myenteric plexus occurs when action potentials along nerve fibers produce Ca2+ transients in varicosities leading to exocytosis of vesicles and neurotransmitters release. We used Ca2+ transients in varicosities to monitor action potential activity in myenteric nerve pathways both between and during the colonic migrating motor complex (CMMC) in the isolated murine colon. Methods Strips of longitudinal muscle were removed to reveal the myenteric ganglia, which were then loaded with Fluo-4. Key Results Many varicosities, including synaptotagmin 1 labeled varicosities, exhibited ongoing Ca2+ transients (duration of unitary Ca2+ transient 3.9 s). Between CMMCs, varicosities fired at a frequency of 0.6 Hz, which correlated with spontaneous inhibitory junction potentials in the circular muscle, suggesting they were mainly in inhibitory nerve pathways. During a CMMC other previously quiescent varicosities fired at 1.3 Hz (max. 2.0 Hz) for the duration (24 s) of the CMMC, suggesting they were on excitatory nerve pathways. Activity in varicosities was correlated with Ca2+ transient responses in a number of neurons. Some varicosities appeared to release an inhibitory neurotransmitter that reduced activity in nNOS-positive neurons. Varicosities along the same nerve fiber exhibited identical patterns of activity that allowed nerve fibers to be traced throughout the myenteric plexus and internodal strands. Activity in varicosities was reduced by hexamethonium (100 lmol L) 1), and blocked by x-conotoxin GVIA (200 nM) and tetrodotoxin (1 lmol L) 1; TTX). Conclusions & Inferences Ca2+ imaging of varicosities allows for a determination of activity in neural pathways within the enteric nervous system.