J. Fickel et al., OPIOID RECEPTOR EXPRESSION IN THE RAT GASTROINTESTINAL-TRACT - A QUANTITATIVE STUDY WITH COMPARISON TO THE BRAIN, Molecular brain research, 46(1-2), 1997, pp. 1-8
The present study was undertaken to analyze the expression of two opio
id receptor genes (mu and kappa) in different gastrointestinal regions
of the rat. A combination of mRNA quantification and immunohistochemi
cal visualization was used to characterize their expression. Using nai
ve animals, RNA was extracted from tissues and used in RNase protectio
n assays: both receptor mRNAs were expressed in all investigated areas
but displayed different expression profiles across the Various region
s of the digestive tract. Stomach and proximal colon appeared to have
the highest expression levels of both receptors, whereas the lowest ex
pression levels were found in the duodenum. Expression levels for both
receptors were always lower in the gastrointestinal tract compared to
the brain. However, the kappa-receptor expression in the proximal col
on represented 40% of the amount found in the brain, which is almost 4
times as high as the respective mu-receptor expression. In contrast t
o smooth muscle cells, myenteric plexus perikarya of the rat stomach a
nd colon were immunoreactive with antibodies raised against the C-term
ini of both kappa- and mu-opioid receptors. Numerous nerve fibers were
also immunoreactive for both mu- and kappa-receptors and distributed
in the longitudinal and circular muscle layers. Small perikarya immuno
reactive for mu-receptor were localized around the myenteric plexus an
d at the submucosal border of the circular muscle, whereas only few pe
rikarya were immunoreactive for the kappa-receptor. We conclude that a
t least in rat stomach and colon, mu- and kappa-opioid receptors may d
irectly control neuronal communication but seem to have no direct infl
uence on smooth muscle cells.