Abstract
BACKGROUND: The actions of endogenous opioid peptides are mediated by 3 main classes of opioid receptors; mu (MOR), kappa (KOR), and delta (DOR). METHODS: We developed an absolute quantitative real-time reverse transcriptase PCR (AQ-rt-RT-PCR) assay to quantify MOR, DOR, and KOR mRNA in 22 human tissues. RESULTS: MOR mRNA was greatly enriched (12-20×10(6)copies/μg) in the cerebellum, nucleus accumbens, and caudate nucleus; moderate (6×10(6)copies/μg) in the dorsal root ganglion, spinal cord, and adrenal gland; low (2×10(4)copies/μg) in the pancreas and small intestine; and absent in the lung, spleen, kidney, heart, skeletal muscle, liver, and thymus. High levels (>8.8×10(6)copies/μg) of DOR mRNA were expressed in the brain and dorsal root ganglion; moderate (1.5×10(6)copies/μg) in the adrenal gland and pancreas; low (2×10(4)-6.5×10(5)copies/μg in the cerebellum, spinal cord, small intestine, skeletal muscle, thymus, lung, and kidney); and very low (3.8×10(3)copies/μg) in the heart. DOR mRNA was not detected in the spleen or liver. KOR mRNA was moderate (1×10(6)copies/μg) in brain regions and dorsal root ganglion, but low (1.6-7×10(5)copies/μg) in the cerebellum, temporal lobe and all other peripheral tissues. CONCLUSIONS: Our data demonstrate that the AQ-rt-RT-PCR is a highly reproducible and precise method to study the expression of opioid receptors in various tissues and under different disease conditions.