Innervation of intestinal arteries by axons with immunoreactivity for the vesicular acetylcholine transporter (VAChT)
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The presence of a cholinergic innervation of arterioles within the gut wall is suggested by pharmacological studies of nerve mediated vasodilatation, but attempts to identify nerve cells that give rise to cholinergic vasodilator fibres have yielded discrepant results. In the present work, antibodies to the vesicular acetylcholine transporter protein (VAChT) were used to investigate the relationships of immunoreactive nerve fibres to submucosal arterioles. Comparison was made with cerebral arteries, which are known to be cholinergically innervated. Double labelling immunohistochemical techniques revealed separate VAChT and tyrosine hydroxylase (TH) immunoreactive (IR) fibres innervating all sizes of arteries of the submucosa of the stomach, ileum, proximal colon, distal colon and rectum as well as the cerebral arteries. Arterioles of all digestive tract regions had greater densities of TH-IR innervation than VAChT-IR innervation. In the ileum, double labelling for VAChT-IR and VIP-IR or calretinin-IR showed more VAChT-IR than either VIP-IR or calretinin-IR fibres. Calretinin-IR and VAChT-IR were colocalised in a majority of calretinin-IR axons, but VIP-IR and VAChT-IR were not colocalised. All calretinin-IR nerve cells in submucous ganglia were immunoreactive for choline acetyltransferase, but only 1-2% of VIP-IR nerve cells were immunoreactive. Extrinsic denervation of the ileum did not alter the distribution of VAChT-IR fibres, but it eliminated TH-IR fibres. Removal of myenteric ganglia (myectomy) did not alter the distribution of fibres with VAChT or TH-IR. This work thus provides evidence for cholinergic innervation of intrinsic arterioles throughout the digestive tract and indicates that the fibres in the small intestine originate from submucosal nerve cells.
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