OPIOID RECEPTOR EXPRESSION IN THE RAT GASTROINTESTINAL-TRACT - A QUANTITATIVE STUDY WITH COMPARISON TO THE BRAIN

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.