Enkephalin levels in rat brain after various regimens of morphine administration.

Rats implanted with a single morphine pellet for a short period of 72 h have been shown to have a lower concentration of enkephalin in the brain, as determined by a radioimmunoassay (RIA) detecting Leu- and Met-enkephalin as well as another endogenous substance. Two other paradigms of morphine implantations did not cause any change in enkephalin levels. Various protocols of morphine injections failed to elicit changes in enkephalin content in the rat brain. The significant decrease observed after the short regimen implantation suggests that at a certain stage in the development of physical dependence a reduction in the amount of enkephalin occurs, but returns to normal values within a short period.