A rhythmic motor pattern activated by circumferential stretch in guinea-pig distal colon

Abstract

Simultaneous intracellular recordings were made from pairs of circular muscle (CM) cells, at the oral and anal ends of a segment of guinea-pig distal colon, to investigate the neuronal mechanisms underlying faecal pellet propulsion. When a minimum degree of circumferential stretch was applied to sheet preparations of colon, recordings from CM cells revealed either no ongoing junction potentials, or alternatively, small potentials usually < 5 mV in amplitude. Maintained circumferential stretch applied to these preparations evoked an ongoing discharge of excitatory junction potentials (EJPs) at the oral recording site (range: 1-25 mV), which lasted for up to 6 h. The onset of each large oral EJP was time-locked with the onset of an inhibitory junction potential (IJP) at an anal recording electrode, located 2 cm from the oral recording. Similar results were obtained in isolated intact tube preparations of colon, when recordings were made immediately oral and anal of an artificial faecal pellet. The amplitudes of many large (> 5 mV) oral EJPs were linearly related to the amplitudes of anal IJPs occurring 20 mm apart. In the absence of an L-type Ca2+ channel blocker, action potentials occurred on each large oral EJP. Synchronized discharges of stretch-activated EJPs and IJPs were preserved following pretreatment with capsaicin (10 μm), were unaffected by nifedipine (1 μM) and did not require the mucosa or submucous plexus. EJPs and IJPs were abolished by hexamethonium (300 μM) or tetrodotoxin (1 μM), but persisted in the presence of pyridoxal phosphate-6-azophenyl-2′,4′-disulphonic acid (PPADS; 10, μM) or an NK3 tachykinin receptor antagonist (Neurokinin A 4-10; 100 nM to 5 μM). In summary, maintained circumferential stretch of the distal colon activates a population of intrinsic mechanosensory neurons that generate repetitive firing of ascending excitatory and descending inhibitory pathways to CM. These mechanosensory neurons, which may be interneurons, are stretch sensitive, rather than muscle tension sensitive, since they are resistant to muscular paralysis. We suggest the synchrony in onset of oral EJPs and anal IJPs over large regions of colon is due to synchronous synaptic activation of ascending and descending interneurons.