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Volume 09
Journal of Gastrointestinal & Digestive System
ISSN: 2161-069X
Gastro Congress 2019
July 08-09, 2019
July 08-09, 2019 | Zurich, Switzerland
14
th
Euro-Global Gastroenterology Conference
The importance of neuronal serotonin in colonic motor patterns
Terence K Smith
University of Nevada, USA
5
-hydroxytryptamine (5-HT, serotonin) has two sources in the gut, descending myenteric 5-HT interneurons
(~5% of all 5-HT) and enterochromaffin (EC) cells (~95% of 5-HT). 5-HT is synthesized by the rate limiting
enzyme tryptophan hydroxylase (TPH); TPH1 in EC cells and TPH2 in EC cells. In the colon, these two pools of
5-HT appear to be connected by intrinsic primary afferent neurons (IPANs) that have sensory endings in the mucosa
and synapse with 5-HT interneurons. The prominent inhibitory effects of 5-HT1A/2B/3/4 and 7 receptor antagonists
on colonic motility suggests that many of these receptors are on various cell types including glia that are largely
activated by 5-HT interneurons. Major motility patterns in the colon involving 5-HT include tonic inhibition of the
muscle layers interrupted by rhythmic peristaltic waves called colonic migrating motor complexes (CMMCs) and
secretomotor activity and possibly blood flow. In large mammals (e.g. Dog, pig and human colon) 5-HT neurons
occur in both the myenteric plexus and the extensive Henle’s plexus, that synapse with secretomotor neurons in
Meissner’s plexus. Henle’s plexus also provides a motor innervation to submucosal pacemaker cells that generate
slow waves. Some 5-HT neurons also project to prevertebral ganglia suggesting they are viscerofugal neurons that
they can activate postganglionic sympathetic neurons, whose terminals surround 5-HT neurons. We propose that
asynchronous firing in 5-HT neurons excite inhibitory motor neurons (IMNs) to generate tonic inhibition and
suppress pacemaker activity. In contrast, 5-HT released from EC cells activates IPANs that synchronizes 5-HT
neurons. Synchronized firing of 5-HT neurons likely activates glial cells, which release PGE2, that switch off IMNs
and remove tonic inhibition. Synchronized 5-HT neurons also likely generate a slow excitatory postsynaptic potential
(sEPSP) in IPANS by activating 5-HT7 receptors that generate CMMCs that in turn excite excitatory motor neurons
and pacemaker cells.
tksmith@med.unr.eduJ Gastrointest Dig Syst 2019, Volume 09