Opioid receptor expression in human brain and peripheral tissues using absolute quantitative real-time RT-PCR

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 x 10(6) copies/mu g) in the cerebellum, nucleus accumbens, and caudate nucleus; moderate (6 x 10(6) copies/mu g) in the dorsal root ganglion, spinal cord, and adrenal gland; low (2 x 10(4) copies/mu g) in the pancreas and small intestine; and absent in the lung, spleen, kidney, heart, skeletal muscle, liver, and thymus. High levels (>8.8 x 10(6) copies/mu g) of DOR mRNA were expressed in the brain and dorsal root ganglion; moderate (1.5 x 10(6) copies/mu g) in the adrenal gland and pancreas; low (2 x 10(4)-6.5 x 10(5) copies/mu g in the cerebellum, spinal cord, small intestine, skeletal muscle, thymus, lung, and kidney); and very low (3.8 x 10(3) copies/mu g) in the heart. DOR mRNA was not detected in the spleen or liver. KOR mRNA was moderate (1 x 10(6) copies/mu g) in brain regions and dorsal root ganglion, but low (1.6-7 x 10(5) copies/mu g) 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. (C) 2012 Elsevier Ireland Ltd. All rights reserved.