Modulation of opioid binding associated with nuclear matrix and nuclear membranes of NG108-15 cells

Opioid binding sites were found in nuclear matrix preparations from NG108-15 neurohybrid cells. Binding parameters of delta-specific radioligands indicated that high-affinity binding sites discovered in purified nuclei were present in nuclear membranes and nuclear matrix fractions. Agonists bind with low affinity, if at all, to nuclear matrix preparations. Neither sensitivity of agonist binding to the GTP analog 5-guanylylimidodiphosphate nor adenylyl cyclase activity were detected in this fraction, suggesting the presence of guanine nucleotide binding regulatory protein/effector uncoupled sites. Opioid inhibition of basal and forskolin-stimulated adenylyl cyclase activity was found in nuclear membrane preparations. Cycloheximide treatment of cells inhibited opioid binding to nuclear membrane fractions to a greater extent than that associated with membranes sedimenting at 20,000 x g (P20) or nuclear matrix. Colchicine, a microtubule disrupter and inhibitor of receptor internalization, caused up-regulation of nuclear membrane and P20 opioid receptors and a loss in nuclear matrix associated sites. Taxol, a microtubule stabilizing agent, prevented the effect of colchicine. Etorphine-elicited down-regulation increased nuclear matrix associated binding while diminishing that in nuclear membranes and P20 fractions. Agonist-induced desensitization completely abolished nuclear matrix binding. In vitro preincubation of nuclear matrix preparations with protein kinase A catalytic subunit mimicked the desensitization effect. Forskolin treatment of cells potentiated nuclear matrix and P20 binding. These data suggest that nuclear membrane opioid receptors represent newly synthesized molecules en route to the cell surface, whereas nuclear matrix contains internalized delta sites.