Mice homozygous for the mu-opioid receptor mutant allele Oprm1tm1Kff (or MOR-) are viable and fertile. MOR-selective ligand binding is absent on brain membranes from homozygous mice. Homozygous mice exhibit a lack of morphine analgesia, reward, and withdrawal. This is accompanied by decreased mechanical, thermal, and chemical pain thresholds. Homozygous mice also show decreased ethanol self-administration and decreased THC-conditioned place aversion. In contrast to mutant mice deficient of delta- or kappa-opioid receptors (Stock Nos. 007557 or 007558, respectively), Oprm1tm1Kff homozygotes exhibit hypolocomotive spontaneous stress responses. Indeed, the reduced anxiety and depressive-like behavior observed in Oprm1tm1Kff mutants is in stark contrast to kappa-opioid receptor deficient mice. These knock-out mice may be useful in studying the biological activity of opioids, analgesics, and responses to mechanical, chemical and thermal nociception at a supraspinal level.
In an attempt to offer alleles on well-characterized or multiple genetic backgrounds, alleles are frequently moved to a genetic background different from that on which an allele was first characterized. These mutant mice were originally published on a mixed genetic background. It should be noted that the phenotype could vary from that originally described. We will modify the strain description if necessary as published results become available.
