Failure of morphine to increase striatal 3,4-dihydroxyphenylacetic acid in fasted rats

Morphine given to rats fed ad libitum increased the striatal levels of DA and DOPAC while naloxone had no effect. Conversely, after prolonged fasting, morphine failed to increase both DA and DOPAC, while naloxone markedly decreased striatal DOPAC concentration. Intermediate sensitivity to the DOPAC increasing effect of morphine and to the DOPAC decreasing effect of naloxone was present in rats trained to eat their daily meal within 2 h, at different times after feeding. It is concluded that striatal DA synthesis in fasted rats is sustained by an increased endorphin-like activity.