Effect of repeated administrations of heroin, naltrexone, methadone, and alcohol on morphine glucuronidation in the rat.
BACKGROUND
Heroin is rapidly metabolized to morphine that in turn is transformed in morphine-3-glucuronide (M3G), an inactive metabolite, and morphine-6-glucuronide (M6G), a potent mu-opioid receptor (MOR) agonist. We have found that heroin addicts exhibit higher M6G/M3G ratios relative to morphine-treated control subjects. We have also shown that heroin-treated rats exhibit measurable levels of M6G (which is usually undetectable in this species) and reduced levels of M3G.


OBJECTIVE
We investigated the role of MOR in these effects of heroin, by examining the effects of methadone, a MOR agonist, and of naltrexone, a MOR antagonist, on morphine glucuronidation. We also investigated the effects of alcohol, which is known to alter drug metabolism and is frequently coabused by heroin addicts.


METHODS
Morphine glucuronidation was studied in liver microsomes obtained from rats exposed daily for 10 days to saline, heroin (10 mg/kg, i.p.), naltrexone (20-40 mg/kg, i.p.), heroin + naltrexone (10 mg/kg+20-40 mg/kg, i.p.), methadone (5-20 mg/kg, i.p.), or 10% ethanol.


RESULTS
Heroin induced the synthesis of M6G and decreased the synthesis of M3G. Naltrexone exhibited intrinsic modulatory activity on morphine glucuronidation, increasing the synthesis of M3G via a low-affinity/high-capacity reaction characterized by positive cooperativity. The rate of M3G synthesis in the heroin + naltrexone groups was not different from that of the naltrexone groups. Methadone and ethanol induced a modest increase in M3G synthesis and had no effect on M6G synthesis.


CONCLUSIONS
The effects of heroin on morphine glucuronidation are not shared by methadone or alcohol (two drugs that figure prominently in the natural history of heroin addiction) and do not appear to depend on the activation of MOR.
Department of Human Physiology and Pharmacology Vittorio Erspamer, University of Rome La Sapienza, Piazzale Aldo Moro 5, 00185 Rome, Italy.
Letizia
L
Antonilli
Emma
E
Petecchia
Daniele
D
Caprioli
Aldo
A
Badiani
Paolo
P
Nencini
D000818:Animals; D000431:Ethanol / Q000494:pharmacology*; D003932:Heroin / Q000494:pharmacology*; D006556:Heroin Dependence / Q000378:metabolism*; D008297:Male; D008691:Methadone / Q000494:pharmacology*; D008862:Microsomes, Liver / Q000187:drug effects; D009020:Morphine / Q000494:pharmacology*; D009022:Morphine Derivatives / Q000378:metabolism*; D009271:Naltrexone / Q000494:pharmacology*; D051381:Rats; D017207:Rats, Sprague-Dawley; D017450:Receptors, Opioid, mu / Q000819:agonists
D009022:Morphine Derivatives; D017450:Receptors, Opioid, mu; D000431:Ethanol; D009271:Naltrexone; C035349:morphine-6-glucuronide; D003932:Heroin; D009020:Morphine; C018108:morphine-3-glucuronide; D008691:Methadone