Receptor mechanisms of opioid tolerance in SH-SY5Y human neural cells

In differentiated SH-SY5Y human neuroblastoma cells, various opioids exhibited a wide range of potencies (Ki) in acutely inhibiting adenylate cyclase to different extents (Imax). After exposure of the cells to opioids for 24 hr, the initially reduced cAMP content of the cells recovered toward pre-exposure levels. Withdrawal of agonist from, or addition of antagonist to, the tolerant cells rapidly increased the cAMP content to 1.5 times the basal value. Long term treatment of the cells with agonists of high acute potency, such as Tyr-D-Ala-Gly-(Me)Phe-Gly-ol and levorphanol, decreased the Bmax of the antagonist [3H]naltrexone by 80-95%, increased the Ks for GTPase stimulation 10-14-fold, and increased the Ki for adenylate cyclase inhibition 2-3-fold. On the other hand, these parameters were only marginally affected by agonists of lower acute potency, such as profadol and morphiceptin, regardless of their Imax in inhibiting adenylate cyclase. The reduction in the level of receptor binding was experimentally not dissociable from effector desensitization. Tyr-D-Ala-Gly-(Me)Phe-Gly-ol retained the characteristics of a potent agonist in inducing tolerance even under conditions of submaximal signal, produced by lower concentrations of the peptide or by pretreatment with pertussis toxin. Alkylation of receptors by beta-chlornaltrexamine, although it reduced [3H]naltrexone binding by 50%, did not significantly alter the rank order of opioid agonists based on their ability to acutely inhibit adenylate cyclase. These results show that in opioid-tolerant SH-SY5Y cells the concurrently occurring down-regulation of receptor and shifts in the concentration dependence of effector response correlate with the potency of a given opioid in producing its acute effect but not with the maximum extent of that effect.