Comparison of naloxonazine and beta-funaltrexamine antagonism of mu 1 and mu 2 opioid actions.

beta-Funaltrexamine (beta-FNA) irreversibly blocks morphine analgesia, lethality and its inhibition of gastrointestinal transit, confirming that these actions involve mu receptors. In dose-response studies, beta-FNA antagonized all the actions with similar potencies (ID50 values of 12.1, 11.3 and 12.3 mg/kg, respectively). beta-FNA also reduced intra-cerebroventricular and intrathecal DAMGO analgesia equally well (ID50 values of 6.09 and 7.7 mg/kg, respectively). Naloxanazine blocked systemic morphine analgesia (ID50 value 9.5 mg/kg) and supraspinal DAMGO analgesia (ID50 value 6.1 mg/kg) as potently as beta-FNA. However, against spinal DAMGO analgesia, morphine's inhibition of gastro-intestinal transit or lethality, naloxonazine (ID50 values 38.8, 40.7 and 40.9 mg/kg, respectively) was significantly less active than beta-FNA (p less than 0.05). beta-FNA remains a valuable tool in the classification of mu opioid actions. Within the mu category, actions can be defined as either mu 1 (naloxonazine-sensitive) or mu 2 (naloxonazine-insensitive).