Role of enterohepatic circulation in the analgesic action of 1-alpha-acetylmethadol in the rat.

The enterohepatic circulation of 1-alpha-acetylmethadol (LAAM) was investigated in the rat. Bile containing 3H-LAAM metabolites was collected from a biliary cannulated donor rat after administration of 3H-LAAM (5 mg/kg, 15 muCi/kg, sc) and infused into the intestine of a double biliary-cannulated recipient rat for 1 hr, and tritium excreted into the bile of the recipient rat was monitored. Within 1 hr after the end of infusion significant radioactivity was found in the bile of the recipient rat and by 10 hr 50% of the infused dose of 3H had been re-excreted into bile. The contribution of enterohepatic circulation of LAAM metabolites to the analgesic action of LAAM was also assessed. Pretreatment of rats with neomycin sulfate was used as a method of decreasing enterohepatic circulation of biliary glucuronide conjugates of LAAM metabolites, and such pretreatment had no effect on LAAM analgesia (6 mg/kg) measured by the hot-plate method. In rats with a biliary fistula, a situation in which enterohepatic circulation was completely eliminated, the analgesic response to a dose of LAAM (6 mg/kg, sc) was not different from sham-operated control group. The above findings indicate that enterohepatic circulation of LAAM metabolites does not contribute to the intensity or duration of LAAM analgesia.