Autoradiographic mapping of the opioid receptor-like 1 (ORL1) receptor in the brains of mu-, delta- or kappa-opioid receptor knockout mice

The opioid receptor-like 1 (ORL1) receptor shares a high degree of sequence homology with the classical mu-, delta- and kappa-opioid receptors and a functional mutual opposition between these receptors has been suggested. To further address this possible interaction we have used mu-, delta- and kappa-opioid receptor knockout mice to determine autoradiographically if there are any changes in the number or distribution of the ORL1 receptor, labelled with [(3)H]nociceptin, in the brains of mice deficient in each of the opioid receptors. An up-regulation of ORL1 expression was observed across all brain regions in delta-knockouts with cortical regions typically showing a 15-30% increase in binding that was most marked in heterozygous mice. In contrast, ORL1 receptor expression was down-regulated in virtually all brain structures in heterozygous kappa-knockouts although the magnitude of this change was not as great as for the delta-knockouts. No significant alterations in ORL1 receptor expression were observed across brain regions in mu-receptor knockout mice and there were no qualitative differences in ORL1 receptor expression in any groups. These data suggest there are interactions between the ORL1 system and the classical opioid receptors and that the interactions are receptor-specific. The greater differences observed in heterozygous mice suggest that these interactions might be most relevant when there is only partial loss of receptor function.

Quantitative autoradiography of mu-,delta- and kappa1 opioid receptors in kappa-opioid receptor knockout mice

Mice deficient in the kappa-opioid receptor (KOR) gene have recently been developed by the technique of homologous recombination and shown to lack behavioural responses to the selective kappa1-receptor agonist U-50,488H. We have carried out quantitative autoradiography of mu-, delta- and kappa1 receptors in the brains of wild-type (+/+), heterozygous (+/-) and homozygous (-/-) KOR knockout mice to determine if there is any compensatory expression of mu- and delta-receptor subtypes in mutant animals. Adjacent coronal sections were cut from the brains of +/+, +/- and -/- mice for the determination of binding of [3H]CI-977, [3H]DAMGO (D-Ala2-MePhe4-Gly-ol5 enkephalin) or [3H]DELT-I (D-Ala2 deltorphin I) to kappa1-, mu- and delta-receptors, respectively. In +/- mice there was a decrease in [3H]CI-977 binding of approximately 50% whilst no kappa1-receptors could be detected in any brain region of homozygous animals confirming the successful disruption of the KOR gene. There were no major changes in the number or distribution of mu- or delta-receptors in any brain region of mutant mice. There were, however some non-cortical regions where a small up-regulation of delta-receptors was observed in contrast to an opposing down-regulation of delta-receptors evident in mu-knockout brains. This effect was most notable in the nucleus accumbens and the vertical limb of the diagonal band, and suggests there may be functional interactions between mu- and delta-receptors and kappa1- and delta-receptors in mouse brain.

Quantitative autoradiographic mapping of mu-, delta- and kappa-opioid receptors in knockout mice lacking the mu-opioid receptor gene

Mice lacking the mu-opioid receptor (MOR) gene have been successfully developed by homologous recombination and these animals show complete loss of analgesic responses to morphine as well as loss of place-preference activity and physical dependence on this opioid. We report here quantitative autoradiographic mapping of opioid receptor subtypes in the brains of wild-type, heterozygous and homozygous mutant mice to demonstrate the deletion of the MOR gene, to investigate the possible existence of any mu-receptor subtypes derived from a different gene and to determine any modification in the expression of other opioid receptors. Mu-, delta-, kappa1- and total kappa-receptors, in adjacent coronal sections in fore- and midbrain and in sagittal sections, were labelled with [3H]DAMGO (D-Ala2-MePhe4-Gly-ol5 enkephalin), [3H]DELTI (D-Ala2 deltorphinI), [3H]CI-977 and [3H]bremazocine (in the presence of DAMGO and DPDPE) respectively. In heterozygous mice, deficient in one copy of the MOR gene, mu-receptors were detectable throughout the brain at about 50% compared to wild-type. In brains from mu-knockout mice there were no detectable mu-receptors in any brain regions and no evidence for mu-receptors derived from another gene. Delta-, kappa1- and total kappa-receptor binding was present in all brain regions in mutant mice where binding was detected in wild-type animals. There were no major quantitative differences in kappa- or delta-binding in mutant mice although there were some small regional decreases. The results indicate only subtle changes in delta- and kappa-receptors throughout the brains of animals deficient in mu-receptors.