mu-Opioid receptor-knockout mice: the role of mu-opioid receptor in gastrointestinal transit.

The role of mu-opioid receptor in gastrointestinal transit was investigated using mu-opioid receptor knockout mice (MOR-KO). Our result establishes unequivocally that inhibition of GI transit by morphine is a mu-opioid receptor mediated function. In addition, we show that neither delta nor kappa receptor agonist given supraspinally or peripherally are able to inhibit GI transit in MOR-KO animals. It was interesting to observe that basal GI motility was lower in MOR-KO (-/-) compared to heterozygous (+/-) and wild type (+/+) animals.