Decrease in μ-opioid receptor binding capacity in rat brain after chronic PL017 treatment

In previous studies, we have demonstrated that chronic treatment of rats with either etorphine or d-Ala², d-Leu⁵-enkephalin (DADLE) resulted in the reduction of opioid receptor binding activities during the course of tolerance development. In both cases, μ-opioid receptor binding capacity was attenuated together with the δ-opioid receptor binding capacity. Because both etorphine and DADLE are relatively non-specific opioid ligands, interacting with both μ and δ receptors, these studies could not determine whether down-regulation of a specific receptor type is possible. Therefore, in the current studies, animals were rendered tolerant to the μ-opioid receptor-selective ligand PL017 and the receptor binding capacity was measured afterwards. Treating Sprague-Dawley rats with increasing doses of PL017 (2.5-20 μg/kg) i.c.v. for 5 days resulted in a 30- to 40-fold increase in the AD₅₀ of the peptide to elicit the antinociceptive response and about 14-fold increase in the ED₅₀ of the peptide to elicit the catatonic effect. When μ- and δ-binding was determined using [³H]diprenorphine in the presence of morphiceptin or DPDPE respectively, a significant decrease (20-30%) in the μ-opioid receptor binding but not in δ-opioid receptor binding was observed in all the brain areas tested after 5 days of PL017 treatment. Scatchard analysis of the [³H]DAMGO saturation binding data revealed a decrease in B_max values and no change in the K_d values. Hence, μ-opioid receptors can be specifically regulated by ligand in the brain as δ-receptors are in neuroblastoma × glioma NG 108-15 cells. Chronic activation of the μ-opioid receptor in the brain would result in the down-regulation of the binding sites.