Stability of Met-enkephalin content in brain structures of morphine-dependent or foot shock-stressed rats

Abstract

THE discovery of enkephalin has suggested one more theory to explain the mechanisms of opiate tolerance and addiction. Kosterlitz and Hughes¹ proposed that a reduced enkephalin release might explain tolerance and physical dependence to opiates. Simantov and Snyder² supported this view by showing that the content of enkephalin was changed in brain of morphine-dependent rats. Others^3–5 have reported that Met-enkephalin is rapidly metabolised by brain tissue. Because of this in vivo lability of Met-enkephalin, particular precaution must be taken in the preparation of brain tissue samples for enkephalin assay. In fact, we have shown that by killing the rats with a microwave beam focused to the head and by taking particular precautions in preparing the brain samples we could measure Met-enkephalin concentrations which were higher (by several orders of magnitude) than the concentrations measured by other authors who failed to take such precautions⁶. Since the changes in the brain enkephalin content reported to occur either after a stress induced by foot shock⁷ or during morphine dependence², were measured by a binding assay which lacks specificity for enkephalin, we decided to repeat these experiments with a technique that specifically measures Met-enkephalin⁶ to decide whether the brain enkephalin content changes during stress⁷ or opiate dependence².