Attenuation of opioid receptor activity by phorbol esters in neuroblastoma x glioma NG108-15 hybrid cells

Chronic opioid treatment of neuroblastoma x glioma NG108-15 cells induces desensitization of the opioid receptor and this may involve a change in membrane protein phosphorylation. In an attempt to mimic this possible mechanism, we studied effects of phorbol ester activation of protein kinase C on opioid receptor activity. Incubation of NG108-15 hybrid cells with the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) abolished up to 45% of opioid inhibition of cyclic AMP accumulation in intact cells, while basal accumulation and prostaglandin E₁-stimulated cyclic AMP accumulation were unaltered. This decrease of opioid inhibition was dose- and time-dependent and the potency order of phorbol esters and apparent K(activation) (90 nM) for TPA were consistent with phorbol esters acting through the stimulation of protein kinase C. TPA also decreased the inhibition of cyclic AMP accumulation mediated through muscarinic and alpha-2 adrenergic receptors. These effects of TPA were best explained by a TPA-induced alteration of the inhibitory nucleotide-binding protein (G(i)), the common transducer protein of these receptors. Impairment of G(i) by TPA treatment was evidenced by a reduction in agonist-stimulated GTP hydrolysis and activation by GTP. Quantification of G(i) by pertussis toxin-catalyzed ADP-ribosylation revealed that TPA decreased maximal labeling. In summary, phorbol esters appeared to attenuate opioid receptor activity by altering the activity of the transducer protein G(i).