HEPATOBILIARY EFFECTS OF MORPHINE ARE MEDIATED IN THE BRAIN

Morphine slows hepatobiliary elimination of sulfobromophthalein in rodents, raising dye levels in plasma and liver. Earlier studies showed these effects to be independent of other opiate effects such as bile duct spasm, hypothermia or blood gas changes resulting from respiratory depression. Because opiate receptors are distributed throughout the body, within the central nervous system and at peripheral sites including the gastrointestinal tract, experiments were performed to ascertain whether central or peripheral sites mediate the hepatobiliary effects of morphine. Sulfobromophthalein was administered intravenously to mice and its levels were measured in plasma and liver. Tail-flick latency indicated centrally mediated analgesia. Inhibited intestinal transit of India ink reflected an opiate effect with a significant peripheral component. When injected into a cerebral ventricle morphine was much more potent in producing analgesia and raising sulfobromophthalein levels than when administered intravenously or intraperitoneally. An intravenous dose of naloxone that reversed morphine analgesia also prevented sulfobromophthalein elevation but did not prevent gut slowing. Naltrexone injected in a cerebral ventricle also reversed analgesia and sulfobromophthalein elevation but not intestinal slowing. The polar opiate agonist N-methylmorphine did not cause analgesia or raise sulfobromophthalein levels at peripheral intraperitoneal doses to 100 mg/kg. When given in a central ventricle at 4 x 10(-3) mg/kg, this agent produced analgesia and raised sulfobromophthalein but did not slow intestinal transit. After spinal cord transection, intravenous morphine did not retard the tail-flick response or affect sulfobromophthalein disposition, but peripherally mediated intestinal transit was slowed as it was in intact mice. These experiments demonstrate parallel opiate effects on analgesia and on BSP disposition but not on intestinal transit. Hepatobiliary effects of morphine are mediated by interaction with opiate receptors in the brain.