MODULATION OF OPIOID SIGNAL-TRANSDUCTION IN SH-SY5Y NEURAL CELLS BY DIFFERENTIATING AGENTS - CONCURRENT MU-RECEPTOR UP-REGULATION AND EFFECTOR DESENSITIZATION BY PHORBOL ESTER

Long-term exposure of SH-SY5Y human neural cells to retinoic acid (RA) increased the binding of [H-3]DAMGO and [H-3]DPDPE by up to 3-fold co mpared to membranes of untreated cells. In contrast, incubation of the cells with phorbol ester (TPA) selectively up-regulated the binding o f [H-3]DAMGO. RA enhanced the maximal inhibition of cAMP formation in SH-SY5Y cells by both DAMGO and DPDPE from 20% for both opioids in con trol cells to 75% and 50%, respectively. On the other hand, the effect of TPA was limited to a marginal increase in the inhibition of cAMP f ormation by DAMGO. Parameters of GTP gamma(35)S binding as well as low -Km GTPase activity revealed no deterioration in the structure or func tion of total G protein in the TPA-treated cells, and initial Western blots showed no difference in the cell content of G(o). Ongoing experi ments are focusing on the covalent modification of specific G protein subtypes involved in mu and delta opioid signal transduction in SH-SY5 Y cells differentiated by TPA.