Abstract

Mu and delta opiate receptor regulation by opiate agonists and antagonists was studied in the human neuroblastoma cell line SH-SY5Y. Morphine down-regulated both mu and delta receptors, but its effects on each subtype could be dissociated by use of specific antagonists. The selective mu antagonist D-Phe-Cys-Tyr-D-Trp-Arg-Pen-Thr-NH2 (CTAP) blocked the down-regulation of mu, but not delta receptors. Conversely, the delta antagonist (N,N-diallyl-Tyr-Aib-Aib-Phe-Leu-OH([N,N-diallyl-Tyr1, Aib2,3]Leu- enkephalin)] ICI 174,864 blocked morphine-induced down-regulation of delta but not mu receptors. These selective antagonists also were studied alone for their effects on both receptors. CTAP alone at doses of 0.1 microM and higher up-regulated mu receptors. CTAP did not affect delta receptors at 0.3 microM or less, but it down-regulated them at doses of 1 microM or more, apparently due to its delta agonist activity at higher doses, which was reversed by ICI 174,864. ICI 174,864 alone also showed complex effects on the two subtypes, up-regulating both mu and delta sites. Its effects were most selective at a low dose (0.1 microM), which upregulated delta sites with minimal effects on mu sites. The nonselective antagonist naloxone provided a more robust upregulation (> 40%) of both mu and delta receptors than either selective antagonist alone or in combination. The mu-to-delta ratio (1.4 to 1) was not altered by differentiation of the cells with retinoic acid, which up-regulated both mu and delta receptors. Differentiation with the phorbol agent 12-O-tetradecanoyl-phorbol-13-acetate, however, up-regulated mu, but not delta receptors. The selective mu agonist Tyr-Pro-MePhe-D-Pro-NH2 (PL017) down-regulated mu receptors with a half-maximal effect at 180 nM, but was without effect on delta receptors at concentrations up to 10 microM. Conversely, the selective delta agonist Tyr-D-Pen-Gly-Phe-D-Pen([D-Pen2,5]-enkephalin) (DPDPE) potently down-regulated delta receptors, producing half-maximal decreases at 0.5 nM. At doses above those that reduced the maximum binding of [3H]pCl-DPDPE binding to the delta site, DPDPE also induced an apparent loss of affinity (increased Kd) at the delta site. It was without effect on mu receptors, however, at doses up to 10 microM. Thus, down-regulation of mu and delta receptors was homologous, because selective agonists down-regulated their respective receptors without effect on the heterologous opiate receptor.(ABSTRACT TRUNCATED AT 400 WORDS)