It was reported that nicotine-induced dopamine release in the rat pheochromocytoma cell line, PC12 cells, was inhibited by κ-opioid. However, it is not known whether inhibition of catecholamine biosynthesis is involved in the inhibitory mechanisms of κ-opioids in PC12 cells. U-69593 (a κ-opioid agonist: ≥100 nM) significantly inhibited the nicotine-induced increase of tyrosine hydroxylase (TH, a rate-limiting enzyme in biosynthesis of catecholamine) enzyme activity and TH mRNA levels. These inhibitory effects were completely reversed by naloxone and nor-binaltorphimine dihydrochloride (nor-BNI), a specific κ-antagonist, whereas pertussis toxin (PTX) only partially reversed this inhibitory effect. Also, U-69593 (≥100 nM) significantly inhibited the nicotine-induced increase of cAMP production. This inhibitory effect was completely reversed by naloxone and nor-BNI, whilst only partially reversed by PTX. Moreover, U-69593 (≥100 nM) significantly inhibited the nicotine-induced increase of both the TH protein level and intracellular catecholamine levels. These results indicate that the anti-cholinergic actions of κ-opioid can be explained partially by its inhibition of both TH enzyme activity and TH synthesis, through suppression of the cAMP/protein kinase A pathway. It would also appear that the PTX-sensitive G-protein mediates the inhibitory effect of this pathway, at least in part.
