Morphine lowering of self-stimulation thresholds: lack of tolerance with long-term administration

Science  14 Jan 1977:
Vol. 195, Issue 4274, pp. 189-191
DOI: 10.1126/science.831268


Rats were given increasing amounts of morphine over a period of weeks in order to achieve tolerance. Doses of the drug which initially reduced the threshold for self-stimulation behavior continued to do so after long-term administration. These results demonstrate a persistent central effect of morphine which may be related to the opiate "high."