Chronic fentanyl treatments induce the up-regulation of mu opioid receptormRNA in rat pheochromocytoma cells

Chronic activation of adenylate cyclase-cAMP-cAMP-dependent protein kinase (PKA) systems by administration of opioid receptor agonists has been consid ered as one of the mechanisms of opioid tolerance and dependence. Although analysis of the CL opioid receptor (MOR) gene suggests that cAMP-related si gnal transduction systems regulate the expression of this gene, which trans cription factors affect the MOR gene expression in brain and neural cells h as not been clarified. This study deals with the effects of fentanyl on MOR mRNA levels in the rat pheochromocytoma cell line (PC12 cells). PC12 cells were cultured in medium with clinically relevant concentrations of fentany l. The quantitative reverse transcription and polymerase chain reaction (RT -PCR) method was used for determination of MOR mRNA. Treatment of PC12 cell s with fentanyl induced the MOR mRNA up-regulation in a concentration- and time-dependent manner. A cAMP analogue also up-regulated MOR mRNA. The intr acellular cAMP level increased after fentanyl treatment. A PKA inhibitor bl ocked the MOR mRNA up-regulation by fentanyl and the cAMP analogue. Express ion of a dominant inhibitory Ras also inhibited the MOR mRNA up-regulation. Fentanyl-induced up-regulation of MOR mRNA via activation of cAMP signalin g may be important in compensating for the MOR reduction during long-term t reatment of PC12 cells with fentanyl. The present st;dy could be relevant t o understanding the molecular mechanisms of opioids in a state of drug tole rance or dependence, and in patients under anesthesia or being treated for pain. (C) 2000 Elsevier Science B.V. All rights reserved.