Development of morphine-induced changes of activity in the mouse

The development of spontaneous activity in response to morphine administration was studied at different postnatal ages in C57BL/6 mice. Morphine induced hyperactivity at all ages except in 3-week-old mice, in which a catatonic effect is evident. The results indicate that the neurophysiological and inhibitory systems responsible for the non-analgesic effects of morphine are not characterized by the same developmental pattern.