Morphine-mediated effects on rat hepatic heme and cytochrome P-450 in vivo: antagonism by naloxone in the liver

Effects of toxicants on endoplasmic reticulum stress and hepatic cell fate determination

Toxicant-induced injury is a significant global health issue. However, the mechanisms through which toxicants such as carbon tetrachloride, acetaminophen, dimethylformamide, cocaine, and morphine induce the death of multiple cell types and contribute to liver toxicity are highly complex. This phenomenon involves intricate signaling pathways in association with oxidative stress, inflammation, and activation of death receptors, which are closely linked to endoplasmic reticulum (ER) stress. ER stress initially triggers the unfolded protein response, which either promotes cell survival or causes cell death at later times, depending on the severity and duration of the stress. Thus, comprehending the molecular basis governing cell fate determination in the context of ER stress may provide key insights into the prevention and treatment of toxicant-induced injury. This review summarizes our current understanding of agents that trigger different forms of ER stress-mediated cell death, necroptosis, ferroptosis, pyroptosis, and apoptosis, and covers the underlying molecular basis of toxicant-induced ER stress, as well as potential target molecules.

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.(ABSTRACT TRUNCATED AT 250 WORDS)

Codeine-mediated hepatotoxicity in isolated rat hepatocytes

Administration of codeine to freshly isolated rat hepatocytes resulted in cytotoxicity characterized by a dose- and time-dependent leakage of lactate dehydrogenase (LDH) out of the cells. Codeine also caused a decrease in hepatic reduced sulfhydryl content. Cytochrome P-450 content and NADPH levels were not changed. Induction and inhibition studies of several potential pathways of codeine biotransformation were carried out in order to determine if codeine must be metabolized to a reactive intermediate to elicit these hepatotoxic effects. Codeine hepatotoxicity as measured by LDH release was not changed after induction of cytochrome P-450 by phenobarbital and was decreased after cytochrome P-448 induction by beta-naphthoflavone. However, codeine hepatotoxicity was inhibited when an inhibitor of cytochrome P-450 metabolism, metyrapone, was added. Inhibition of the other major hepatic oxidative enzyme system, flavin adenine dinucleotide (FAD)-containing monooxygenase, increased the cytotoxicity of codeine. Inhibition of alcohol dehydrogenase had no effect on codeine hepatotoxicity. These results indicate that codeine hepatotoxicity is caused by a cytochrome P-450-generated intermediate of codeine, whereas FAD-containing monooxygenase may metabolize codeine to a nontoxic intermediate.

Morphine metabolism in isolated rat hepatocytes and its implications for hepatotoxicity

Isolated rat hepatocytes metabolized morphine to its glucuronide conjugate, morphinone-glutathione conjugate, normorphine and morphinone. Addition of morphine to the isolated hepatocytes induced a marked decrease in the level of glutathione in the cells and resulted in cell death. The formation of glutathione conjugate was correlated well with the loss of intracellular glutathione. The cytotoxicity of morphinone was higher than that of morphine. Naloxone and normorphine showed no cytotoxic effect on the cells. Naloxone inhibited the formation of morphinone-glutathione conjugate and prevented the morphine-induced cytotoxicity. Naloxone also blocked morphine-induced liver damage in vivo. In contrast, the morphinone-induced hepatotoxicity was not prevented by naloxone. It is concluded that morphine has a hepatotoxic effect, that the morphine-induced hepatotoxicity is due to its metabolic activation, and that naloxone acts as an inhibitor of an enzyme converting morphine to morphinone.

Morphine metabolism revisited: I. Metabolic activation of morphine to a reactive species in rats

Treatment of fasted rats with relatively high doses of morphine rapidly results in depletion of hepatic glutathione (GSH) content and marked elevation of serum transaminase activity. Such morphine-induced response has been generally attributed to central nervous system mediated effects of the drug. We now report that this response might be due to a direct effect of the drug in the liver. That is, its metabolic activation to reactive electrophilic metabolite(s), by the hepatic cytochrome P-450-dependent mixed function oxidase system. Structure-activity relationships of morphine and its congeners indicate that the (-)-3-hydroxy-N- methylmorphinan moiety is linked with the potential of these opioids to deplete hepatic GSH and to raise serum transaminases in rats.

L-alpha-acetylmethadol-induced tissue alterations in mice

Single oral dosages of the synthetic narcotic analgesic, L-alpha-acetylmethadol (LAAM) increased serum glutamic-pyruvic transaminase (SGPT) levels throughout a two-day observation period and produced a persistent depletion of hepatic and renal glutathione (GSH) levels. These LAAM-induced changes demonstrated dose- and time-dependence within that dosage range producing mortality. Histological evaluation of livers from LAAM-treated mice revealed cytoplasmic and nuclear changes in centrilobular hepatocytes. Interestingly, neither the LAAM-induced histopathological changes nor the depression of hepatic GSH were altered by the induction of hepatic metabolism following pretreatment with either phenobarbital or 3-methylcholanthrene; however, the induction of hepatic drug metabolism did abate the four-day mortality and SGPT elevations.

Does naloxone always act as an opiate antagonist?

Evidence for the ability of the opiate antagonist naloxone to block a variety of metabolic effects exerted by morphine and non-opiate drugs is reviewed. Naloxone prevents or reverses the following effects in the rat: (a) the chronic morphine-induced increase in liver [NADPH]; (b) the consequent chronic morphine-induced inhibition of liver tryptophan pyrrolase activity; (c) the resultant chronic morphine-induced enhancement of brain 5-hydroxytryptamine synthesis; (d) the similar effects on liver and brain tryptophan metabolism exerted chronically by other drugs of dependence (ethanol, nicotine and phenobarbitone); (e) the acute ethanol-induced increase in the hepatic [NADH]/[NAD] ratio. Naloxone also (f) inhibits basal and stimulated lipolysis in fed and 24hr-starved rats. This leads to prevention of (g) the consequent increase in the availability of circulating free tryptophan, and (h) the resultant tryptophan-mediated decrease in liver 5-aminolaevulinate synthase activity. The question of how many of these effects involve changes in endogenous opiates or at opiate receptors is not clearly understood at present and thus merits investigation. However, because most of the above effects are explained on biochemical grounds, and in view of evidence from behavioural and pharmacological studies [see (1)], the possibility must be considered that many of the actions of naloxone may be unrelated to its opiate-receptor-antagonistic properties.

Tryptophan and tryptophan pyrrolase in haem regulation. The role of lipolysis and direct displacement of serum-protein-bound tryptophan in the opposite effects of administration of endotoxin, morphine, palmitate, salicylate and theophylline on rat liver 5-aminolaevulinate synthase activity and the haem saturation of tryptophan pyrrolase

1. The increase in the haem saturation of rat liver tryptophan pyrrolase caused by tryptophan administration was previously shown to be associated with a decrease in 5-aminolaevulinate synthase activity. 2. It is now shown that similar reciprocal effects are caused by palmitate and salicylate, both of which increase tryptophan availability to the liver by direct displacement of the serum-protein-bound amino acid. 3. The reciprocal effects on the former two parameters caused by endotoxin and morphine are associated with an increase in liver tryptophan concentration produced by a lipolysis-dependent, non-esterified fatty acid-mediated, displacement of the serum-protein-bound amino acid. 4. All these changes and those caused by another lipolytic agent, theophylline, are prevented by the beta-adrenoceptor-blocking agent propranolol and by the opiate-receptor antagonist naloxone, whose anti-lipolytic nature is demonstrated. 5. High correlation coefficients have been obtained for one or more pairs of the following parameters: serum non-esterified fatty acid concentration, free serum tryptophan concentration, liver tryptophan concentration, liver 5-aminolaevulinate synthase activity, liver holo-(tryptophan pyrrolase) activity and the haem saturation of liver tryptophan pyrrolase. 6. It is suggested that liver tryptophan concentration may play an important role in the regulation of 5-aminolaevulinate synthase synthesis, and that the latter may be subject to control by changes in lipid metabolism and may be influenced by pharmacological agents that affect tryptophan disposition. 7. Preliminary evidence suggests that tryptophan may be bound in the liver and that such a possible binding may control its availability for its hepatic functions.