The question of whether short- and long-term opioid agonist activities could be affected by receptor density is being addressed with Chinese hamster ovary (CHO) cell lines stably expressing delta-opioid receptors. CHO cells expressing different levels of delta-opioid binding sites were isolated and characterized. The opioid binding sites in these cell lines have stereoselective high affinity for 3H-diprenorphine (0.18 to 0.91 nM), with agonist binding sensitive to both Na+ and GTPγS, and are of the delta-2- opioid receptor subtype. This is concluded from observations that the delta- 2-opioid receptor-selective agonist naltriben (NTB) has higher affinity than the delta-1-opioid receptor-selective agonist 7-benzylidenenaltrexone (BNTX). It also could be demonstrated that delta-opioid agonists inhibited forskolin- stimulated intracellular 3H-cAMP production and that agonist inhibition could be blocked by pretreating cells with pertussis toxin. Again, NTB is more potent than BNTX or naloxone in reversing DPDPE inhibition of intracellular 3H-cAMP production. When the ability of [D-Ala2,D- Leu5]enkephalin (DADLE) to regulate adenylyl cyclase activity was examined in three separate CHO cell lines: CHODORX1-15, CHODORX1-8 and CHODORX1-4, which express 1.42 ± 0.08, 0.74 ± 0.07 and 0.27 ± 0.04 pmol/mg-protein of receptor, respectively, maximal inhibitory levels in clones X1-15 and X1-8 were similar, whereas maximal inhibitory level for clone X1-4 was 66% of the other two clones. However, when maximal inhibitory levels of other opioid receptor-selective agonists were examined, different levels of inhibition were observed among these three CHO clones. Mu- and kappa-opioid receptor- selective agonists exhibited partial agonistic properties in X1-4. Furthermore, the potencies of various agonists in inhibiting adenylyl cyclase activity in these three clones were dissimilar and were related to affinity for receptor only in the case of clone X1-4. Therefore, observed agonist effect is not simply a function of receptor density in these CHO clones expressing delta-opioid receptor. Long-term exposure of these CHO cell lines to DADLE resulted in a time-dependent down-regulation of opioid binding sites and loss in agonist ability to regulate intracellular 3H-cAMP level. Receptor down-regulation in these cell lines did not involve changes in opioid receptor mRNA levels as demonstrated by quantitative polymerase chain reaction analysis. The percentage of receptor being down-regulated in clone X1-4 was 38% compared with 60% to 67% in the other two clones. In contrast, receptor desensitization was maximal in clone X1-4, whereas receptor desensitization was minimal in clone X1-8. Interestingly, compensatory increase in adenylyl cyclase activity was maximal in clone X1-15 (360 ± 4.5% above the control level), which had minimal receptor desensitization, whereas in clone X1-4, which had complete receptor desensitization, only a 21 ± 5.0% increase from control adenylyl cyclase level was observed. These data suggest a receptor concentration dependency in long-term opioid responses and also suggest likely problems in interpretation of future receptor mutagenesis studies on short- and long-term action of delta-opioid agonists.
|Original language||English (US)|
|Number of pages||9|
|Journal||Journal of Pharmacology and Experimental Therapeutics|
|State||Published - 1994|