Nor-binaltorphimine: a potent and selective kappa-opioid receptor antagonist with long-lasting activity in vivo

The kappa-receptor selectivity of nor-binaltorphimine (nor-BNI), a highly selective kappa-opioid receptor antagonist in vitro, was examined in vivo by measuring the time course of the antagonistic action of nor-BNI (5 and 20 mg/kg, s.c.) against the responses to U-50488H (10 mg/kg, s.c.), morphine (10 mg/kg, s.c.) and fentanyl (50 micrograms/kg, s.c.) in mice. In the tail pinch test, nor-BNI partially antagonized morphine and fentanyl analgesia, but not U-50488H analgesia in the first 30 min after s.c. administration. However, the kappa-antagonistic action gradually increased, reaching a plateau at 2 hr. This antagonistic action was maintained for at least 4 days. In contrast, the mu-antagonistic action declined to the control level at 2 or 4 hr after nor-BNI administration. In the acetic acid-induced writhing test, nor-BNI also exerted a more potent and selective kappa-antagonistic action at 24 hr than at 1 hr after its s.c. administration. Nor-BNI also showed a long-lasting kappa-antagonism against the hyperthermic response induced by U-50488H (5 mg/kg, s.c.). Thus, we found that nor-BNI is a slow-onset, long-lasting, selective kappa-antagonist in vivo.