Role of the Liver in Tolerance to Morphine in the Rat

Abstract

Gross and Thompson and Oberst discovered that dogs and also man excrete morphine partly in a combined form. Since then, attempts have been made to study morphine metabolism in tolerant and nontolerant animals with a view of finding whether habituation was accompanied by a difference in metabolism.,, Oberst noticed that the glucuronic acid concentration of human urine increases with increasing dosages of morphine. Gross, Plant and Thompson and Gross found that liver damage causes an increase in the excretion of free morphine in both tolerant and nontolerant dogs, and that liver damage inhibits conjugation of morphine to a certain extent. Bernheim and Bernheim,, discovered that liver slices conjugate morphine. Kidney and brain tissues were inactive. It seemed of interest to investigate whether liver slices from tolerant rats are capable of binding more morphine than slices from normal rats. Experimental Methods and Results. Essentially the technique of Bernheim and Bernheim was followed except that double the amount of morphine was added to the slices. We have not used the silicomolydic acid reagent because the depth of color was far from Proportional to the concentration of morphine. We have utilized instead the Folin-Denis reagent according to Oberst.10 Blank experiments were run which consisted of slices in Krebs-Henseleit buffer without added morphine. Morphine added to such suspension followed after 2 minutes by precipitation with trichloroacetic acid can be recovered quantitatively. Rats were made tolerant by repeated daily injections of morphine hydrochloride, starting with 2 mg/100 g and reaching a dose of 20 mg/100 g in a period of 20 days. Preliminary results obtained with 9 normal rats and 4 tolerant rats showed a binding of 2.16 mg morphine per g wet slices per 3 hours for normal rats, as compared with 1.86 mg for tolerant rats.