Quantitative autoradiography of μ-,δ- and κ 1 opioid receptors in κ-opioid receptor knockout mice

Abstract

Mice deficient in the κ-opioid receptor (KOR) gene have recently been developed by the technique of homologous recombination and shown to lack behavioural responses to the selective κ 1-receptor agonist U-50,488H. We have carried out quantitative autoradiography of μ-, δ- and κ 1 receptors in the brains of wild-type (+/+), heterozygous (+/−) and homozygous (−/−) KOR knockout mice to determine if there is any compensatory expression of μ- and δ-receptor subtypes in mutant animals. Adjacent coronal sections were cut from the brains of +/+, +/− and −/− mice for the determination of binding of [ 3 H ]CI-977, [ 3 H ]DAMGO ( d-Ala 2-MePhe 4-Gly-ol 5 enkephalin) or [ 3 H ]DELT-I ( d-Ala 2 deltorphin I) to κ 1-, μ- and δ-receptors, respectively. In +/− mice there was a decrease in [ 3 H ]CI-977 binding of approximately 50% whilst no κ 1-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 μ- or δ-receptors in any brain region of mutant mice. There were, however some non-cortical regions where a small up-regulation of δ-receptors was observed in contrast to an opposing down-regulation of δ-receptors evident in μ-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 μ- and δ-receptors and κ 1- and δ-receptors in mouse brain.