ACUTE AND CHRONIC EFFECTS OF OPIOIDS ON DELTA-RECEPTOR AND MU-RECEPTOR ACTIVATION OF G-PROTEINS IN NG108-15 AND SK-N-SH CELL-MEMBRANES

To compare activation of G proteins by opioid receptors, opioid agonis t-stimulated guanosine 5'-O-(3-[S-35]thiotriphosphate) ([S-35]GTP gamm a S) binding in the presence of excess GDP was assayed in membranes fr om NG108-15 (delta) and SK-N-SH (primarily mu) cells. Basal [S-35]GTP gamma S binding consisted of a single class of low-affinity sites (K-D 400-500 nM). Addition of agonists produced a high-affinity site 100-3 00-fold higher in affinity than the basal site. The receptor/transduce r amplification factor (ratio of activated G protein B-max to opioid r eceptor B-max) was 10-fold higher for SK-N-SH mu receptors than for NG 108-15 delta receptors. Chronic delta agonist ([D-Ser(2)]-Leu-enkephal in-Thr; DSLET) treatment of NG108-15 cells resulted in an 80% loss of DSLET-stimulated [S-35]-GTP gamma S binding within 1 h. Morphine treat ment of SK-N-SH cells decreased mu agonist ([D-Ala(2),N-Me-Phe(4),Gly( 5)-ol]-enkephalin; DAMGO)-stimulated [S-35]GTP gamma S binding by 45% after 16 h, with no effect after 1 h. Loss of agonist response was due to a decrease in the B-max of activated G proteins with no change in the K-D. These results provide a quantitative description of G protein activation occurring on acute and chronic exposure to opioid agonists .