Opioid peptide receptor studies. 12. Buprenorphine is a potent and selective mu/kappa antagonist in the [S-35]-GTP-gamma-S functional binding assay

We utilized the [S-35]-GTP-gamma-S functional binding assay to determine th e selectivity of opioid receptor agonists in guinea pig caudate membranes. The study focused on two opioid agonists used for treating opioid-dependent patients: methadone and buprenorphine. Selective antagonists were used to generate agonist-selective conditions: TIPP + nor-BNI to measure mu recepto rs, CTAP + nor-BNI to measure gamma receptors and TIPP + CTAP to measure ka ppa receptors. The assay was first validated with opioid agonists of known subtype specificity (DAMGO for mu, SNC80 for delta, and U69,593 for kappa r eceptors). Methadone-stimulated [S-35]-GTP-gamma-S binding was mu-specific and less potent and efficacious than etorphine (K-d = 1,537 nM vs. K-d = 7. 8 nM). Buprenorphine failed to stimulate [S-35]-GTP-gamma-S binding but inh ibited agonist-stimulated [S-35]-GTP-gamma-S binding. The antagonist-K-i va lues (nM) of buprenorphine at mu, delta, and kappa receptors were 0.088 nM, 1.15 nM, and 0.072 nM, respectively. The antagonist-K-i values (nM) of nal oxone at mu, delta, and kappa receptors were 1.39 nM, 25.0 nM, and 11.4 nM, respectively. Autoradiographic studies showed that buprenorphine failed to stimulate [S-35]-GTP-gamma-S binding in caudate-level rat brain sections b ut blocked DAMGO-stimulated [S-35]-GTP-gamma-S binding. In cells expressing the cloned rat mu receptor, buprenorphine was a partial agonist and potent mu antagonist. Administration of buprenorphine to rats produced a long-las ting (>24 h) decrease in mu and kappa 2 receptor binding and attenuated mu- stimulated [S-35]-GTP-gamma-S binding. Viewed collectively, these data indi cate that, in this assay system, buprenorphine is a potent mu and gamma rec eptor antagonist. The clinical implications remain to be elucidated. Publis hed 1999 Wiley-Liss, Inc.(dagger).