RESUMO
The effects of fentanyl isothiocyanate (FIT) and pertussis toxin on the binding of [3H]D-Ala2, D-Leu5-enkephalin ([3H]DADLE) to rat brain membranes were compared. The site of action of pertussis toxin was confirmed by the labeling of a 41,000 dalton protein in the presence of [alpha-32P]NAD. Both reagents produced inhibition of [3H]DADLE binding when binding was assayed in 10 mM Tris-HCl buffer alone. FIT inhibited binding 91% whereas pertussis toxin treatment resulted in 27% inhibition. However, when binding was assayed in 10 mM Tris-HCl containing SMG (100 mM NaCl, 3 mM manganese acetate, and 2 microM guanosine triphosphate), inhibition due to both reagents was attenuated markedly: 66% for FIT and 5% for toxin. In addition, both reagents markedly potentiated enhancement of binding by SMG. Thus, the effects of FIT and pertussis toxin on [3H]DADLE binding were qualitatively similar. These results suggest that FIT and pertussis toxin affect binding of [3H]DADLE to the same population of delta receptors. This was further supported by the observation that treatment of membranes with FIT prior to pertussis toxin treatment blocked the effect of toxin on [3H]DADLE binding. FIT selectively eliminates the SMG-insensitive, mu-competitive [3H]DADLE binding site [Rothman et al., Neuropeptides 4, 201 (1984); Rothman et al., Molec. Pharmac. 27, 399 (1985)]. These results indicate that this site is coupled to G protein substrates for pertussis toxin and that it mediates the inhibitory effects of delta ligands on adenylate cyclase. The FIT-insensitive, SMG-sensitive mu-noncompetitive [3H]DADLE site appears not to be coupled to G protein substrates for pertussis toxin and may mediate some other biochemical effects of delta ligands.
