Immunohistochemical evidence of down-regulation of μ-opioid receptor after chronic PL-017 in rats

In a previous study, μ-opioid receptor binding was decreased by chronic treatment of rats with a μ-opioid receptor-selective agonist [CH₃Phe³, D- Pro⁴]morphiceptin (PL-017) [Tao, P.L., Lee, H.Y., Chang, L.R., Loh, H.H., 1990. Decrease in μ-opioid receptor binding capacity in rat brain after chronic PL-017 treatment. Brain Res. 526, 270-275]. However, there was a lack of correlation between the time course of receptor down-regulation and the loss of pharmacological effects of the drug. In the current study, we used immunohistochemistry to reinvestigate this issue. Male Sprague-Dawley rats were chronically treated with PL-017 i.c.v. for 1, 3 or 5 days, using an escalating dosage paradigm (0.75-6.0 μg), which resulted in a 1.4 to 32- fold increase in the AD₅₀. Rat brains were removed, frozen, coronally sectioned (14 μm) and processed for μ-, δ- or κ-opioid receptor immunohistochemistry by the avidin-biotin complex (ABC) method. Significant decreases in OP3 immunodensity were found in many brain regions which are enriched with OP3 after chronic treatment of PL-017, Time-dependent decreases in OP3 were detected and reached a plateau around 3 days of PL-017 treatment. No significant change in OP1 or OP2 immunodensity after chronic treatment with PL-017 was found. Our conclusion is that chronic treatment with PL-017 of rats selectively down-regulates μ-opioid receptors in the brain. This may be an important mechanism for PL-017 tolerance.