Opiate receptor down-regulation and desensitization in neuroblastoma X glioma NG108-15 hybrid cells are two separate cellular adaptation processes

Abstract

Chronic treatment of neuroblastoma X glioma NG108-15 hybrid cells with the opiate agonist etorphine resulted in a decrease in both opiate receptor density (receptor down-regulation) and opiate ability to inhibit prostaglandin E1 (PGE1)-stimulated increases in cyclic AMP levels (receptor desensitization). Opiate receptor down-regulation and desensitization were homologous as indicated by the lack of apparent change in muscarinic, alpha 2-adrenergic, and PGE1 receptor binding and also retention, albeit modulation, of the ability of carbachol and norepinephrine to inhibit PGE1-stimulated increases in cyclic AMP levels after 24 hr of etorphine treatment. PGE1-stimulated increases in cyclic AMP levels remained identical in etorphine-treated and control cells. Several lines of evidence indicate that receptor desensitization and receptor down-regulation in NG108-15 cells are two separate cellular adaptation processes. (a) With an agonist which appears to be efficiently coupled, i.e., an agonist whose apparent Kd value is much larger than its apparent IC50 value for regulation of cyclic AMP levels (Ki), the concentration of ligand required to produce half-maximal down-regulation is analogous to its Ki value, whereas the concentration of ligand required to produce half-maximal desensitization is related to its Kd value; (b) receptor desensitization precedes receptor down-regulation; (c) only opiate agonists could produce receptor down-regulation, whereas both opiate agonists and partial agonists could desensitize post-receptor occupancy events. Still further evidence for dissociability of these processes was obtained by incubating NG108-15 cells with etorphine at 30 degrees for 2 hr. Under these conditions, there was a decrease in etorphine's ability to regulate adenylate cyclase while [3H]diprenorphine binding remained unaltered. IC50 values of D-Ala2-D-Leu5-enkephalin's competition for [3H]diprenorphine binding to intact cells increased 19.6-fold after etorphine treatment for 90 min, while naloxone IC50 values remained unaltered. This apparent increase in IC50 values was much lower, about 2-fold, when receptor binding was carried out in membranes isolated from cells treated with etorphine chronically. Furthermore, analysis of [3H]etorphine binding to such membranes in the presence of 10 mM Mg2+ indicated a loss of receptor binding sites with no change in apparent affinity, whereas [3H]diprenorphine binding revealed no significant alteration in either Bmax or Kd values. Therefore, during opiate receptor desensitization, a reduction of agonist high-affinity site occurs with no apparent alteration in total receptor number.