Clinical and experimental pharmacology & physiology
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Clin. Exp. Pharmacol. Physiol. · May 1997
Concentration-effect relationships of morphine and morphine-6 beta-glucuronide in the rat.
1. The aims of the present study were to determine the relationship between the antinociceptive effect and concentrations of morphine and morphine-6 beta-glucuronide (M6G) in plasma and in the brain. 2. Morphine (14.0 and 28.0 mumol/kg) or M6G (8.67 and 17.3 mumol/kg) were administered s.c. to male Hooded-Wistar rats. ⋯ Concentrations of M6G in the brain were below the limit of quantification. The relationship between antinociceptive effect and plasma morphine or M6G were characterized by counter-clockwise hysteresis loops, probably reflecting a delay in crossing the blood-brain barrier. 4. Morphine-6 beta-glucuronide was approximately equipotent to morphine on the basis of dose, but substantially more potent on the basis of brain concentration.