Differentiation of kappa opioid agonist-induced antinociception by naltrexone apparent pA2 analysis in rhesus monkeys

Naltrexone (NTX) exhibited approximately 3-fold higher affinity for sites labeled by [H-3]U69,593 (putative kappa(1)-selective ligand) than [H-3]bremazocine (non-selective ligand) in the presence of mu and delta receptor blockade in monkey brain membranes. This led us to test an hypothesis that NTX could display in vivo antagonist selectivity for kappa(1)- versus non-kappa(1)-mediated effects. Six opioid agonists were characterized by NTX apparent pA(2), analysis in a 50 degrees C water tail-withdrawal assay in rhesus monkeys. Constrained NTX pA(2) values (95% confidence limits) were: alfentanil, 8.66 (8.47-8.85); ethylketocyclazocine, 7.97 (7.93-8.01); U69,593, 7.64 (7.49-7.79); U50,488, 7.55 (7.42-7.67); bremazocine, 6.92 (6.73-7.12); enadoline, 6.87 (6.69-7.05). Pretreatment with clocinnamox, an irreversible mu antagonist, confirmed that mu receptors were not involved in the antinociception produced by the kappa agonists, U69,593, U50,488, bremazocine and enadoline; however, both mu and kappa receptors mediated the antinociceptive effects of ethylketocyclazocine. The apparent NTX pA(2) profile of opioid agonists correlated highly with the radioligand binding studies, which indicates that U69,593 and U50,488 produced antinociception by acting on kappa-1 receptors, whereas bremazocine and enadoline probably acted via non-kappa-1 receptors. This study provides further functional evidence of kappa opioid receptor multiplicity in primates and suggests that NTX may be a useful tool to study this phenomenon in vivo.