Alteration of brain chromatin and nuclear synthetic activity in morphine-tolerant rats.

3H-UTP incorporation by endogenous RNA-polymerase of intact, isolated rat brain nuclei was enhanced by chronic morphine treatment. Analgesic tolerance using the hot-plate assay was also evident. Fractionation proflies for brain chromatin on hydroxylapatite from morphine and vehicle-treated rats was different. These results suggest that morphine-tolerance in the rat may be accompanied by enhanced nuclear synthesis of a new species of RNA.