Characterisation of noradrenergic inputs in the myenteric plexus of the human colon

Author: Dominic Parker

Parker, Dominic, 2021 Characterisation of noradrenergic inputs in the myenteric plexus of the human colon, Flinders University, College of Medicine and Public Health

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The sympathetic nervous system inhibits human colonic motility largely by releasing noradrenaline in the myenteric plexus, causing presynaptic inhibition of the release of acetylcholine. It is unclear whether noradrenergic inputs preferentially target specific cell types in a “hard-wired” fashion, or release noradrenaline diffusely by volume transmission. The objective of these studies was to characterise the relationship of noradrenergic nerve terminals (varicosities) with intrinsic myenteric neurons.

Healthy colonic tissue was retrieved from colorectal resection specimens and small squares were dissected and fixed to expose the myenteric plexus. Multiple layer immunohistochemistry, confocal microscopy and 3D reconstruction identified noradrenergic varicosities (labelled with tyrosine hydroxylase, TH) close to nerve cell bodies labelled for HuC/D, choline acetyl transferase (ChAT) and nitric oxide synthase (NOS). Enkephalin (Enk) varicosities were used as a positive control because they have recently been shown to have an association with cholinergic neurons. Subsequent calculation of varicosity density within ±1µm of individual cells enabled a statistical comparison between cell types using a mixed effects model.

From 7 patients, 486 myenteric nerve cell bodies were analysed. The density of noradrenergic varicosities was significantly greater close to ChAT+/NOS- (cholinergic) compared to ChAT-/NOS+ (nitrergic) cells (median density ratio 1.35 varicosities/1000µm3, 95% credible interval 1.04-1.69). This suggests that noradrenergic inputs preferentially target cholinergic neurons in the myenteric plexus. Enkephalin varicosities also showed preferential targeting of ChAT+/NOS- compared to ChAT-/NOS+ neurons (median density ratio 1.51 varicosities/1000µm3, 95% credible interval 1.23-1.83) in keeping with results of previous studies. TH and Enk varicosity baskets were found surrounding cholinergic neurons, supporting the quantitative analysis and validating the novel method. A small population of catecholaminergic (likely dopaminergic) TH-immunoreactive myenteric neurons accounted for approximately 1% of all neurons in the myenteric plexus; these cells were ChAT-immunoreactive and hence likely to be cholinergic.

This study has further defined the pathway of sympathetic inhibition of colonic motility via the myenteric neurons of the enteric nervous system, demonstrating preferential connectivity between noradrenergic inputs and intrinsic cholinergic cells. Further research is required to determine the specific neurochemical code and functional class of cells targeted by sympathetic axons.

Keywords: enteric nervous system, myenteric plexus, sympathetic nervous system, colon, human, gastrointestinal motility

Subject: Surgery thesis

Thesis type: Masters
Completed: 2021
School: College of Medicine and Public Health
Supervisor: Phil Dinning