Persistence of morphine in central nervous system of rats after a singel injection and its bearing on tolerance.

Abstract

TOLERANCE of a very high grade and specificity is known to develop to morphine and its congeners. Although no precise information is available on the biochemical and anatomical sites of morphine–receptor interaction, it is generally believed that the matrix of the central nervous system is directly involved in many parameters of the response due to morphine-like analgesics. Morphine, however, has no special affinity for the central nervous system and only minute amounts are needed in brain to elicit the typical pharmacologic effects. Further, morphine concentrations in brain have been shown1,2 not to be necessarily related to the analgesic effect. Failure to demonstrate changes in the rate of excretion, distribution, increased metabolic degradation or transformation of morphine3–5 has suggested biochemical alteration of specific neural structures in the development of tolerance. Intracellular distribution studies6,7 with morphine administered in vivo showed it to be localized primarily in the soluble microsomal fraction of rat brain homogenate either in the free or lightly bound form and 10–25% of drug in crude nuclear brain fraction. This distribution pattern did not change with the development of tolerance. Tolerance has also been shown8 to develop apparently at the very first dose administered and some degree of tolerance in rats lasting as long as a year resulted from a single injection of morphine9. These observations are difficult to explain in view of the known disappearance of free morphine from the brain within 48 h of a single injection. All the previous studies, however, have centred chiefly on the presence of free drug in the central nervous system. This study provides evidence that at all periods of time up to 3 weeks or more (periods longer than 3 weeks not investigated) after a single subcutaneous injection of 10 mg/kg of 14C-morphine-N-methyl (synthesized by a method previously described10 and its chemical and radiochemical purity established by thin-layer chromatography in different solvent systems) in the rat there was a consistently higher radioactivity present in brain than could be accounted for on the basis of free morphine alone. Measurable and significant amounts of drug were present as polar “conjugated” drug and the ratio of “conjugated” morphine to total radioactivity increased with time up to 24 h. Thereafter all the radioactivity was wholly due to “conjugate”. Further, radioactivity amounting to 10–20 ng/g of brain (approximately 2–4 × 1013 molecules) could be detected in brain 3 weeks after a single subcutaneous injection of 10 mg/kg of morphine. Up to 2 h after injection of the drug, part of the radioactivity due to “conjugate” could be acid-hydrolysed to free morphine but at later times the “conjugate” was not hydrolysable with dilute acid. The nature of this conjugation of morphine and its possible bearing on the development of tolerance is discussed.