Region specific increase of dopamine receptor D1/D2 mRNA expression in the brain of μ-opioid receptor knockout mice

Previous pharmacological studies have indicated the possible existence of functional interactions between opioidergic and dopaminergic neurons in the CNS. In this study, the expression of mRNAs encoding dopamine receptor D1/D2 was examined to investigate whether there is a change in the dopamine pathway of mice lacking the μ-opioid receptor by in situ hybridization technique. In the μ-opioid receptor knockout mice, the expression of dopamine receptor D1 mRNA was increased in the olfactory tubercle, nucleus accumbens, caudate putamen, and the layer VI of the neocortex compared with that of wild-type mice. The expression of dopamine receptor D2 mRNA was also increased in the olfactory tubercle, caudate putamen, and the nucleus accumbens of μ-opioid receptor knockout mice. These results indicate that there are compensational changes in the dopaminergic systems of μ-opioid receptor knockout mice.