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.