BINDING-AFFINITY AND SELECTIVITY OF OPIOIDS AT MU-RECEPTOR, DELTA-RECEPTOR AND KAPPA-RECEPTOR IN MONKEY BRAIN MEMBRANES

The binding parameters of radiolabeled DAMGO (mu), DPDPE and pCl-DPDPE (delta) and 5 alpha, 7 alpha, 8 beta-N-methyl-N-[7-(1-pyrrolidinyl)-1 - oxaspiro(4,5)dec-8-yl]benzeneacetamide (also known as U69593, kappa) and the affinity and selectivity profiles of various opioid agonists and antagonists at the three opioid receptor types were determined in membranes from brain cortex of rhesus monkey. Among the 10 opioids wit h established mu-selective actions, etonitazene inhibited the binding of [H-3]DAMGO with a K-i of 0.02 nM (0.01 nM without sodium) and exhib ited mu/delta and mu/kappa selectivities of 8800 and 11,650, respectiv ely. DAMGO had a K-i of 1.23 nM and was about 500-fold more selective at mu receptors compared with delta and kappa sites. Other mu opioids with higher than 100-fold binding selectivity were fentanil and sufent anil. Highly selective delta opioids were DPDPE, deltorphin II and nal trindole. With the exception of N,N-diallyl-Tyr-Aib-Aib-Phe-Leu-OH, al l investigated putative delta opioids bound to delta sites with low K( i)s, i.e., 0.04 nM, 0.13 nM and 1.4 nM for naltrindole, (+/-)-4-[(alph a-R)-alpha-{(2S*, -piperazinyl)3-hydroxybenzyl]-N,N-diethylbenzamide and DPDPE, respectively. In this series, the displacement of [H-3]pCl- DPDPE yielded results similar to those obtained with [H-3]DPDPE. With nanomolar K(i)s of 0.70, 0.89, 0.25 and 0.06, respectively, the highes t kappa selectivity was displayed by (trans)-(+/-)-3,4-dichloro-N-meth yl-N-[2-(1 -pyrrolidinyl)-cyclohexyl]benzeneacetamide and U69593, foll owed by dynorphin 1-13 and norbinaltorphimine. Both ethylketocyclazoci ne and bremazocine bound with high affinity to all three receptor type s, showing a 15- to 127-fold preference for the kappa receptor. The bi nding of mu-, delta-, and kappa-selective agonists and antagonists exh ibited distinct sensitivities to sodium. The results of this study, wh ich revealed picomolar binding affinity and receptor selectivity up to 11,600-fold in the primate brain, should aid in interpreting opioid a ctions in vivo and in selecting receptor specific ligands to character ize opioid receptor mechanisms in vitro.