ADP‐Ribosylation with Pertussis Toxin Modulates the GTP‐Sensitive Opioid Ligand Binding in Digitonin‐Soluble Extracts of Rat Brain Membranes

Pertussis toxin‐catalyzed ADP‐ribosylation of the guanine nucleotide‐binding proteins G_i and G_o is shown to proceed in Mg²⁺‐digitonin extracts from rat brain; the M_r 41,000 and M_r 39,000 peptides are labelled there as in the membranes. The ADP‐ribosylation in detergent solution retains the differential sensitivity to guanine nucleotide analogues. This reaction also removes the partial inhibition by the guanine nucleotides of the binding of opioid agonists, as does the same treatment in the membranes. The partial inhibition of agonist binding by Na⁺, however, is left unchanged. The binding of the antagonist naloxone is little affected by Na⁺ or by guanine nucleotides in the treated membranes, but the treated soluble receptors show an enhanced binding in high‐Na⁺ medium, although still guanine nucleotide insensitive. The data suggest that the toxin reaction in the absence of guanine nucleotides and agonist stabilizes the opioid receptor in a receptor‐G‐protein coupled state which is no longer sensitive to guanine nucleotides but retains its sensitivity to the Na⁺ ions.